A review on additive manufacturing and materials for catalytic applications: Milestones, key concepts, advances and perspectives

A review on additive manufacturing and materials for catalytic applications: Milestones, key concepts, advances and perspectives O.H. Laguna ⇑ , P.F. Lietor, F.J. Iglesias Godino, F.A. Corpas-Iglesias Higher Polytechnic School of Linares, University of Jaén, Av. de la Universidad s/n, 23700 Linares (Jaén), Spain highlights Process intensification: the link for merging catalysis and additive manufacturing. Since 2015 patents and scientific papers on this topic grows exponentially. Printed devices used in catalysis may be classified in two main families. Monoliths (wood-pile configuration) are the most studied printed catalytic devices. There is room for improvement in terms of the geometry efficiency of the prints. graphical abstract article info Article history: Received 31 January 2021 Revised 14 June 2021 Accepted 21 June 2021 Available online 22 June 2021 Keywords: Printed catalysts Printed structured catalysts Reaction ware Monoliths Additive manufacturing and catalysis abstract Catalysis, a driving force of the chemical industry is increasingly being influenced by additive manufacturing. The link between them is based on the need to intensify catalytic processes in order to make them more efficient and sustainable. Additive manufacturing can satisfy such a need, generating devices with an advanced design, easy production, and great adaptation, in addition to their catalytic functionality. The exponential growth of examples reported on the application of 3D-printing in catalysis has led to the need to compile and analyse these cases and thus establish, through this review, the most in-depth analysis done to date. The manuscript includes a brief background of the history of additive manufacturing and the classification of the different printing techniques. Subsequently, it identifies the intensification of processes, among other aspects, as the key for understanding the union of additive manufacturing and catalysis. Then, it explores in detail how such a combination occurs, establishing the most comprehensive classification to date between the two large groups of printable devices with catalytic properties. Finally, a series of perspectives are proposed in which the most probable courses of new advances in this field of research are identified. Ó2021 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/). Contents 1. Introduction . . . ........................................................................................................ 2 2. General background of additive manufacturing. . . . . . . . . . . . . .................................................................. 3 3. Why does catalysis needs additive manufacturing? . . . . . . . . . .................................................................. 4 https://doi.org/10.1016/j.matdes.2021.109927 0264-1275/Ó2021 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). ⇑ Corresponding author. E-mail address: [email protected] (O.H. Laguna). Materials & Design 208 (2021) 109927 Contents lists available at ScienceDirect Materials & Design journal homepage: www.elsevier.com/locate/matdes 4. How can catalysis and additive manufacturing be integrated?. .................................................................. 5 4.1. Printed catalytic reaction ware. . . . . . . . . . . . .......................................................................... 10 4.1.1. Micro/milli and tubular reactors . . . . . . . . . . . . . . . . . ............................................................ 11 4.1.2. Hydrothermal reactors . . . . . . ............................................................................... 13 4.1.3. Flow devices . . . . . . . . . . . . . . ............................................................................... 13 4.1.4. Electrochemical cell components . . . . . . . . . . . . . . . . ............................................................ 13 4.1.5. Miscellaneous catalytic reaction ware. . . . . . . . . . . . . ............................................................ 14 4.2. Structured catalysts. . . . . .......................................................................................... 14 4.2.1. Monoliths ............................................................................................... 15 4.2.2. Electrodes ............................................................................................... 21 4.3. Integration of the catalyst in printed structured devices . . . . . . . . . . ....................................................... 21 4.3.1. Direct printing of structured catalysts approach . . . . ............................................................ 22 4.3.2. Printing of structured supports approach . . . . . . . . . . ............................................................ 24 4.3.3. Indirect printing of structured catalysts approach . . . ............................................................ 25 5. Patents with applications that merge catalysis and additive manufacturing. . . . . . . . . .............................................. 26 6. Perspectives of the union between catalysis and additive manufacturing . . . . . . . . . . . .............................................. 28 6.1. Simulation tools for enhancing the union between catalysis and additive manufacturing . . .................................... 29 6.2. Advances in the printing technologies. . . . . . .......................................................................... 30 6.3. Novel and advanced materials . . . . . . . . . . . . .......................................................................... 30 6.4. Catalytic bio-printing . . . .......................................................................................... 31 6.5. 4D printing . . . . . . . . . . . .......................................................................................... 31 7. Concluding remarks . . . . . . . . . . . . . . . . .................................................................................... 31 Declaration of Competing Interest . . . . .................................................................................... 31 Acknowledgements. . . . . . . . . . . . ........................................................................................ 31 References . . . . ....................................................................................................... 31 1. Introduction Catalysis is one of the pillars of the chemical industry that has promoted its evolution since the 20th century, and today new trends are being observed that confirm its modernization. One of them is the application of additive manufacturing, one of the key elements of the new industrial revolution that is taking place today and is known as Industry 4.0. The appearance of additive manufacturing dates back to the 80 s, but its application in catalysis has only begun to be appreciated during the last decade. The growth of pieces of work that address this combination is exponential and this determines the birth of a special field of research, in which there are different strategies to obtain devices with catalytic properties. For this reason, it is necessary to thoroughly evaluate as much as possible what has been studied to date in order to establish trends in this field, recognize their strengths, weaknesses and future prospects. Furthermore, it is important to identify and define concepts that allow an understanding of the strategies that are followed in order to incorporate additive manufacturing into Nomenclature Abbreviations Acronym Meaning 3DFD 3D fibber deposition ABS Acrylonitrile butadiene styrene ASP Acetoxy-silicone polymer CDLP Continuous direct light processing CLIP Continuous liquid interface production CMC Carboxymethylcellulose CO-PROX Preferential oxidation of CO in presence of H 2 D-ABS Diamond - ABS DED Direct Energy Deposition DIW Direct ink writing DLP Direct light processing DMLS Direct metal laser sintering DOD Drop on demand EBAM Electron beam additive manufacturing EBM Electron beam melting FDM Fused deposition modeling FFF Fused filament fabrication G-PLA Graphene - PLA HIPS High impact polystyrene HP Hewlett-Packard IPA Isopropylalcohol LENS Laser Engineered Net Shape MCS-30 MCS-30 activated carbon MJF MultiJet Fusion MOF Metal-organic framework MS Maraging Steel NAPFFK Acrylic hydrogelators (NAPFFK-acrylic acid) NPJ Nano particle jetting ONPG o-Nitrophenyl-beta-D-galactopyranoside PA12 Polyamide 12 PAD Prokaryotic phenacrylate decarboxylases PBF Powder Bed Fusion PCL Polycaprolactone PDA Polydiacetylene PEG-DA Poly(ethylene glycol) diacrylate PLA Polylactic acid PMMA Polymethilmetacrylate PP Polypropylene SEBM Selective electron beam melting SLA Stereolithography SLM Selective laser melting SLS Selective laser sintering SS (316L) Stainless steel (316L) TAR Transparent acrylic resin TPGDA Bisphenol A epoxy acrylate TPU Thermoplastic polyurethane O.H. Laguna, P.F. Lietor, F.J. Iglesias Godino et al. Materials & Design 208 (2021) 109927 2 catalysis. This is why, the following review has been carried out, which aims to be an important reference to better understand the basis from which this field of research has been created. To achieve this, we start from important previous works [1,2] that have addressed this issue, but in this case, a greater volume of information accumulated after the publication of the aforementioned reviews it is provided, including a large number of patents not analysed until today, which allows to raise other types of perspectives to analyse this topic. The review starts by a brief summary of the additive manufacturing background, showing in a general way the different types of printing technologies and highlighting which ones are used nowadays to produce devices with application in catalysis. This leads to the question of why catalysis needs additive manufacturing, which will be answered by exploring the key aspects that have motivated such a union. In this exploration, it will be possible to establish that the main link between these two technologies is the need to incorporate the concept of process intensification into as many catalytic reactions as possible and to be able to take advantage of it at an industrial level. The intensification of processes is highly dependent on aspects such as shape or geometry, dimensions and the type of material from which the catalytic devices are made, and these concepts are addressed from the design. Therefore, if the intensification of processes creates the link between catalysis and manufacturing, design is the tool that consolidates this link because it starts from the needs of catalytic processes and allows the consolidation of solutions through additive manufacturing. The large number of works cited in this review, including scientific papers and patents, allow to establish diagnoses on the materials and printing technologies most commonly used in the manufacture of devices for catalytic applications. Then, the classification of these types of devices is presented in detail, grouping them into two main families (catalytic reaction ware and printed structured catalysts), which in turn are subdivided depending on the printing strategy. In the case of printed structured catalysts, a classification is proposed that covers everything that has been published to date and can even be applied to future work. Afterwards, a complementary section devoted to the perspectives on this topic is included. In this section, the innovative items that may positively affect in the near future the synergy between catalysis and additive manufacturing are identified and developed, and finally, a brief summary of the main aspects that can be extracted from the review is presented in the concluding remarks section. 2. General background of additive manufacturing Additive manufacturing (AM) is a unique manufacturing approach that enables the flexible preparation of highly complex and precise 3D geometries that are difficult to realize using traditional fabrication methods such as casting and machining [3]. For this review, the term additive manufacturing will be used mainly, which is the most appropriate to refer to this manufacturing method according to ASTM (ISO/ASTM 52900:2015). However, it is important to consider that 3D printing or rapid prototyping expressions are often used in the same context, so it is possible that they appear sporadically throughout the document. During a typical additive manufacturing process, layers of a material (polymer, metal, ceramic or composites) are built up to create a solid object. The structure of the final printed part is achieved by three main stages, which have also been defined in a previous review [1]: a) The desired 3D geometry first is designed through computer-aided design (CAD) b) The structure is converted to a G-code file via a slicing program c) The printer is then prepared using the G-code and the specific material to get the final part [4]. Fig. 1. Additive manufacturing technologies: classification and main technology developer brands. O.H. Laguna, P.F. Lietor, F.J. Iglesias Godino et al. Materials & Design 208 (2021) 109927 3 Besides prototyping, the unique capability of transforming materials into functional devices with specific geometry using additive manufacturing technologies has stimulated a lot of interest in various fields to provide custom-made designs for tailored applications, such as product development, manufacturing aids, final parts amongst others [1]. For this, the main industrial sectors, such as aeronautical, automotive, medical, and also nowadays in the educational field [5,6] remain interested in this new manufacturing method. Moreover, it should also be noted that the concept of additive manufacturing is framed within the new industrial movement known as Industry 4.0, and that this movement involves a significant transformation regarding the way products are produced thanks to the digitization [7]. The additive manufacturing concept was developed by Kodama in the early 1980 0 s[8]. He was able to generate a complete 3D object from 2D cross sections, successively formed on top of one another, using photohardening polymers and UV light [8,9]. Later, in 1984, Charles Hull patented this technology called stereolithography (SLA) [10], and created 3D Systems Ò , one of the most recognized additive manufacturing companies worldwide. This technology is growing everyday due to its high accuracy and possibility of obtaining additively manufactured parts of all sizes with a high level of detail that generated interest within the industry and academic fields, since new materials were appearing. Over the next years, Deckard and Crump patented two new manufacturing technologies based on layer by layer deposition: Selective Laser Sintering (SLS) that uses powders as printable materials [11], and Fused Deposition Modeling (FDM) [12] that uses polymer wire based printable materials. From these first technologies, in the last 30 years different alternatives have been developed that allow today, to have a wide spectrum of additive manufacturing techniques, which can be classified into six main groups or families of printing techniques schematized in Fig. 1. This classification is based on aspects such as the presentation of the material (filaments, powder or in suspension), the methodology for the consolidation of the layers during printing, or the feeding of the printing material [13–17]. Regarding the applicability of the different additive manufacturing technologies, the field of catalytic processes is not usually included since this scenario is relatively new. However, there is increasing evidence that catalysts or catalytic materials, which are the essence of catalytic processes, may see their manufacture and performance improved by the incorporation of additive manufacturing in their production [2]. This is why it was sought to highlight in a special way in the classification of Fig. 1, what type of techniques are applied in the production of 3D objects applicable in catalysis nowadays. From the broad classification presented in Fig. 1, it is observable that despite the great versatility that the different manufacturing techniques can offer only some of them seem to be capable of or at least are used in most of the cases for generating 3D objects applicable in catalysis, as will be analyzed in more detail in section 3. Therefore, it is essential to analyze why and how the fusion between catalysis and additive manufacturing occurs, to understand why certain techniques are applied so far and not others. 3. Why does catalysis needs additive manufacturing? The outstanding evolution of the chemical industry during the 20th century was brought about by a series of relevant geopolitical and global events combined with the most prolific decades in terms of technological advances in the history of man. Consequently, without a doubt, the Heterogeneous Catalysis is a fundamental pillar in the rapid growth of large-scale energy, fuel, chemical and pharmaceutical industries [1]. The development of knowledge on Heterogeneous Catalysis began with a purely empirical approach, but a real understanding of the principles of the catalytic processes began only after the mid-twentieth century. This was thanks, amongst other things, to the discovery of characterization techniques with which to study phenomena on the surface of materials such as infrared spectroscopy or X-ray photoemission spectroscopy (XPS), which in fact motivated the field of study of surface science. At present, the gap between empirical results and their understanding from a theoretical point of view remains important, as was stated by Chorkendorff and Niemantsverdriet (see Fig. 2)[18]. However, the current scenario of a new industrial revolution (Industry 4.0) will surely contribute significantly to reducing this gap, since such a revolution is expected to change the paradigms within many of the traditional productive sectors. Additionally, the essence of this revolution, which is artificial intelligence, favours feedback between practice and theory [19]. Additive manufacturing is one of the pillars of industry 4.0 and its application in catalysis has aroused great interest over the last Fig. 2. Gap between knowledge in practical catalysis and theoretical understanding. Adapted from reference. O.H. Laguna, P.F. Lietor, F.J. Iglesias Godino et al. Materials & Design 208 (2021) 109927 4 decade. Thanks to this, you can find very complete reviews [1,2,9,17] that give a detailed overview of a series of very promising applications of additive manufacturing in catalytic systems. However, the growth of this field is such that in 2019 there was a significant increase in work which focused on this theme, with new contributions that could not have been considered in these reviews, and this has opened new horizons in the integration of catalysis and additive manufacturing. This can be confirmed in Fig. 3, where the amount of already published scientific papers per year is presented, based on a bibliographic search using keywords such as catalysis,printed catalysts,printed reactors,additive manufacturing,3D printing, amongst others. The search was filtered to include pieces of work that effectively combined additive manufacturing and catalysis, and, even though additive manufacturing emerged in the 1980 s, an exponential growth is clearly observed in the last decade of research in this field as presented in Fig. 3. Therefore, the question arises as to why catalysis needs additive manufacturing. To answer this question, the global context cited above has to be considered again, since the technological revolution scenario (Industry 4.0) that has been highlighted, demands the modernization of industry that must be transformed and adapted to enhance the use of renewable energy sources and find viable and profitable alternatives to oil. This is not only because it is a finite resource, but also because its use as the main source of energy has a considerably negative effect on the environment. Consequently, it has been defined as a priority for the next 30 years to reduce greenhouse gas emissions and thus to begin to slow down the effects of climate change [20,21]. The problem of alternative energy sources is that their efficiency is still low, compared to that of traditional sources, which affects their profitability and thus slows their incorporation into the market. One of the main challenges of industry today, therefore, is to achieve highly energy efficient processes, minimizing the demand for fuel (regardless of its origin) and reducing losses due to dissipation, that is, promoting adiabatic processes in those that maximize the reuse of energy. The concept of reusing energy is in accordance with one of the strategies that has been proposed today to adapt industry to a less polluting scenario. This is to transform the linear productive model into a circular one that maximizes the efficiency with which all resources (not just energy) are used to reduce generation of waste. In this strategy, known as the Circular Economy, the recycling of raw materials is greatly enhanced to reduce the exploitation of resources [22]. From the point of view of chemical processes, what is then sought is to produce renewable fuels and value-added products, through sustainable processes, with maximum productivity, thanks to a minimum energy and economic investment, and minimal CO 2 footprint. This is what is known as process intensification. In this concept, miniaturization is also one of the main pillars because the smaller the site where the process occurs, the greater the control that can be done on its efficiency and this does not have to assume a reduction in production volume [23]. One of the clearest examples of process intensification is the used of microchannel reactors to carry out processes, both on a laboratory scale [24,25] and on an industrial scale[26]. Microchannel reactors promote both heat and mass transport phenomena thanks to the submillimeter dimensions of their channels, making reactions occur faster, more selectively and with greater energy efficiency, in a very small volume. However, the widespread application of microchannel reactors technology is hampered by the complex design of the reactor, the high fabrication cost, and the low volume of catalyst per unit volume of reactor as was recently highlighted by Konarova et al. [23] The same authors point out, however, that such obstacles could be overcome with the incorporation of additive manufacturing. The link between catalysis and additive manufacturing as an alternative to the search for process intensification was also established by Parra-Cabrera et al. [2] in their comprehensive review about additive manufacturing technologies applied to chemical engineering and catalysis. They proposed that the foremost opportunities for integrating process intensification and additive manufacturing primarily lie in continuous flow reactions that use heterogeneous catalysts and/or present transport limitations. Then, more complex scenarios of non-isothermal reactions and multi-phase environments would determine the advances in this field [2]. In the case of Hurt et al. [1], who also presented a very complete review, they propose that there is a driving force to implement additive manufacturing in heterogeneous catalysis (although it could well be extrapolated to catalysis in general) based on two fundamental motivations: i) to scale down the reactors as a sustainable approach to producing chemicals on demand on-site. This is based on the fact that miniaturization does not have to mean a decrease in conversion, as demonstrated by Tubío et al. [27] with their printed monoliths with Cu-based catalyst tested in different Ullmann reactions; And ii) secondly, it is necessary to adapt the large scale chemical manufacturing units to a mode of operation that minimizes the environmental impact while maximizing energy efficiency. Despite this being very difficult challenge and one which is considerably expensive, the great versatility that additive manufacturing offers may allow the achievement of the gradual change that large industries require, without this entailing designing and building their facilities from scratch. As can be inferred, these two motivations are in full agreement with the principles of process intensification. Therefore, the idea that additive manufacturing can bring multiple benefits to the chemical industry through its integration with catalysis is clear. 4. How can catalysis and additive manufacturing be integrated? There is no defined strategy for the integration of catalysis and additive manufacturing. In fact, this depends on several aspects such as the principles of the printing method, the nature of the printing materials, the conditions in which the printed device will be evaluated, the design of the catalytic solution, amongst others. For this, it is reasonable that one of the first reviews on the subject classified the types of catalytic applications according to the additive manufacturing method used [17]. Nevertheless, Parra et al.[2] 1995 2000 2005 2010 2015 2020 -5 0 5 10 15 20 25 30 35 40 45 50 55 Number of published papers Year Fig. 3. Search for articles published in the last 25 years where additive manufacturing and catalysis are combined (sources: Web of Science and Scopus / Updated to December 2020). O.H. Laguna, P.F. Lietor, F.J. Iglesias Godino et al. Materials & Design 208 (2021) 109927 5 introduced two criteria to the classification of the already published works that allowed identification of which of them had focused on the manufacture of reactors and / or mixers and which on obtaining heterogeneous structured catalysts. Hurt et al. [1], presented a first attempt to address the classification of the work depending on the strategy used for the catalysis / additive manufacturing integration, and this classification was made possible by identifying the advantages and challenges of integration. The advantages are mainly those of traditional structured systems (low pressure drops, ease of removal from the reaction medium), maximized by the possibility of designs with advanced shapes. Although, perhaps most interesting is the technological challenges they pointed out. One such challenge is a limited materials palette, since the number of materials that can be used for additive manufacturing as ‘‘printable materials” is low compared with the huge number of catalytic processes and this makes it difficult to cater to the specific needs of temperature or solvent compatibilities of different chemical processes [1]. Such limited availability of materials is also exacerbated by the strategy of most of the additive manufacturing providers. This strategy is one of locking their users into using only their own-brand products, making the process more expensive and slowing down the advances on new printable materials. On the other hand, taking into account that the catalytic properties of the printable materials, by themselves, are limited, it is necessary to combine them with real catalysts and the interaction between these two systems (before, during or after printing) can be inefficient or even adversely effect the performance of the catalyst [1]. Moreover, the printing processes often results in solids being made through a layer-by-layer procedure, with this generating solids whose properties tend to be anisotropic since the setting processes to solidify through the union of layers depend on direction[1]. This can be a drawback when the parts are subjected to mechanical stress in an unfavorable direction during consolidation, such as in pressurized reactions. Finally, they pointed out the cases in which the combination of the catalyst with the printable material may have generated a part that is not yet in thermodynamic equilibrium [1]. Therefore, additional treatments are required, generally thermal and / or chemical that do not affect the catalyst and this is somewhat difficult to achieve. Taking into account the validity of the criteria of the reviews presented up to now to classify the works that combine catalysis and additive manufacturing [1,2,17,28,29], merging them to obtain a more comprehensive diagnosis of the advances in this field it is proposed, including the incorporation of more up-to-date reports. To begin with, the updated statistics in Fig. 4 on the printing methods and materials most used so far in studies that integrate additive manufacturing and catalysis are presented, based on bibliographic research that includes more than a hundred papers from 1997 to present. A first look at the statistics in Fig. 4 would suggest that there is an acceptable diversification in the number and types of printing methods used, although DIW, FDM, SLA, SLM and SLS are clearly more used than others are. This is why, information about most used materials and the layer thickness is included for these cases. This last parameter is closely related with the resolution of the obtained 3D objects thus as lower the layer thickness higher the resolution. The greater application of these techniques is mainly due to the fact that there is already a certain maturity in their development (both in printing devices and in printable materials) and this is reflected in the market. Despite this, the supposed increasing diversification of printing methods reaches a bottleneck when analyzing printing materials, because the properties of the different families of systems that can be used for additive manufacturing set the limits of the applicability of the printing. However, there are not as many satisfactorily-tested alternatives as may be assumed, either on the market, or in the scientific scenario. This makes it clear that expanding the range of printable materials is one of the main areas to be developed in additive manufacturing. Prior to take a general look at the most relevant groups of printing materials that have been used in printing devices for catalytic applications, it is important to highlight the low, medium or high qualitative estimate, which is presented in Fig. 4 for each family of materials of a series of parameters. The first one is Print Tem, that is, the range of temperatures at which the pieces are manufactured and in this case, low means below 250 °C; medium from 250 to 400 °C and high above 400 °C. The second criterion (Difficulty) estimates the complexity of the printing process. The third parameter (Shrinkage) estimates the shrinkage of the final pieces during post-treatment. The fourth (Soluble) estimates how easily the material can be solubilized, but it does not differ whether in an organic solvent or not and finally, the fifth (Work Temp.) Estimates three temperature ranges in which the final pieces of each material are stable. Again low means below 250 °, medium from 250 to 400 °C and high, above 400 °C. In all cases, the qualitative estimate was built based on the information reported in technical catalogs of each material, combining it with the data extracted from scientific articles in which the use of these materials has been reported to obtain devices with catalytic applications. The first group of materials is polymeric ones. These are the group most widely used in additive manufacturing since these can be used in extrusion, light polymerization, power bed, and almost all other mainstream printing technologies as stated by Zhou el al.[17]. These may offer a considerable range of properties based on their plasticity and durability. However, their polymeric nature mainly limits their application in thermal processes above 200 °C and their stability in operating environments where there are certain organic solvents. In addition, these materials present low surface areas and poor surface properties, which hinders their incorporation into catalytic solutions regardless of the strategy. One way to overcome such drawbacks is the incorporation of active ingredients into the polymeric matrix aiming to produce hybrid materials such as TiO 2 -ABS (presented in Fig. 3B), and this would extend the use of doped polymers in the printing of systems for catalysis. Nevertheless, additives are required for the preservation of the suitable rheology for an accurate printing process, and controlling the effects of those additives is difficult, especially in the nature of the catalyst if those additives are present during the printing process. In addition, the presence of additives may require further treatments after the printing process with the aim of removing possible contaminants that have influence on the activity of the catalytic phase. In view of the above, the printing of catalysts or catalyst supports using polymeric materials does not appear to be advantageous at all. However, there are other ways to bring additive manufacturing of polymers to the field of catalysis and it is by printing reaction ware with advanced designs that it is possible to facilitate the catalytic process. For this reason, a series of cases in which additive manufacturing has been applied to obtain useful reaction ware for catalytic processes is included, and which will be seen in detail later when specifically at printed reaction ware for catalytic reactions are looked. Regarding ceramic materials, the number of examples drawn from our review probably seems high. However, what is actually being considered are more or less complex combinations of oxides such as SiO 2 ,Al 2 O 3 , CeO 2 , ZrO 2 and TiO 2 , some of these containing the presence of doping agents, although in low proportion. There are also aluminosilicates such as clays or zeolites and SiC cases. It is true that these materials are also the most widely used systems as supports for powder catalysts. Nevertheless, there are also O.H. Laguna, P.F. Lietor, F.J. Iglesias Godino et al. Materials & Design 208 (2021) 109927 6 Fig. 4. Additive manufacturing technologies and materials used for the manufacturing of catalytic devices: A) Used technologies; B) Polymeric material including doped formulations, C) Ceramic materials; D) Metal materials; E) Carbon materials, MOF’s and bioactive materials. O.H. Laguna, P.F. Lietor, F.J. Iglesias Godino et al. Materials & Design 208 (2021) 109927 7 families of materials such as carbides, nitrides or even oxides of both transition metals and rare earths, which are practically unexplored and which could be studied further to take advantage of their properties in the generation of printed systems with catalytic abilities. To print with ceramic materials, these must be combined with other organic and inorganic agents (binders) to obtain a printable formulation that meets the appropriate rheological conditions, since reaching the melting temperature of a pure ceramic system (to apply the principle FDM) would require special equipment with injectors capable of working above 1000 °C. Therefore, the prints made of ceramic formulations that include binders, are really raw parts that must be sintered by means of a thermal process. During this type of treatment, the pieces undergo significant changes in their dimensions and their physical and chemical properties can also be altered. Furthermore, these processes make the production of ceramic systems slower and more expensive. In addition, the modification of the dimensions of the prints in the post-treatment of sintering makes the transfer of the digitally designed model not so perfect in reality, generating margins of error in the generation of parts. This is undoubtedly a point that needs improvement when printing with these types of materials. Although powder-based strategies for ceramic printing are being explored, slurry-based with photopolymerization methods based on the DLP strategy are definitely the most widely used and generate fewer operating costs. However, there are key aspects which still need to be improved, such as the scattering effect of the ceramic material present in the printable formulation that absorbs the incident energy that promotes Photopolymerization, thus decreasing its efficiency and generating heterogeneities in the extension of the printed part. Therefore, the future of this procedure relies on a better understanding of the interaction between the laser and the printing material and how this influences the layer deposition mechanism. Accordingly, the printing of ceramic systems for catalytic applications with new materials is only a matter of time, given the emerging state of this technological field. In fact, there are already reports of ceramic systems with complex designs and high resolutions, such as those recently reviewed by Chen et al.[30]. In this summary, you can find reports of the additive manufacturing of structured monoliths, scaffolds and parts with materials such as B 4 C[31], CaCO 3 [32],Al 2 O 3 [33], bio-active glass [34,35],Al 2 O 3 +- ZrO 2 [32], SiC [36], hydroxyapatite [37], SiO 2 [38], TiO 2 [39], SiCN [40], SiOC [41],Si 3 N 4 [42], 3YSZ [43,44],Ti 3 SC 2 [45], amongst others. However, the vast majority of these studies focus on the mechanical properties of prints with applications in the engineering field, the generation of biocompatible parts, but not with catalysis. This is, for instance, the case of Al-Ketan et al. [46] that although the title of their work: ‘‘additive manufacturing of architected catalytic ceramic substrates” suggests that they deeply analyzed design and production, and studied the mechanical and flow properties of triply periodic minimal surfaces made with ceramic precursors, in fact they omitted to study any catalytic property. This does not mean that the quality of the structured systems obtained is not high. In fact, the high resolution with which they have been able to obtain periodical designs of high complexity (see Fig. 5) is very promising in the field of catalysis with structured systems, since the geometry of the structures plays a crucial role in the transport phenomena during most catalyzed reactions. There is another way to incorporate additive manufacturing into the generation of ceramic structures and that is to use printed molds with advanced designs, made of polymeric material. The ceramic material is mixed with additives to generate a paste that is embedded within the template and by calcination, the mold material is gasified and the ceramic piece is consolidated. This process is known as indirect printing and has been used in the generation of structured systems for catalytic purposes as in the case of Davo-Quinonero et al.[47] that printed templates of monoliths with a commercial ultraviolet curable polymeric resin (Visijet FTX green), which were subsequently filled by extrusion with a commercial cordierite paste (COR-MIK-MP). The templates were removed by combustion in air at 500 °C for 2 h and then cordierite was sintered in air at 1250 °C for 2 h. As for metals as impression material for catalytic applications, the options found in literature are drastically reduced as can be observed in Fig. 3D. Only some stainless steels (specifically 316L) and copper such as those developed by Danaci et al. [48–50], have been explored in a way to obtain compact catalytic systems and metals such as aluminum, titanium or iron are notable for their absence. This is because metal printing is a major technological challenge due to some disadvantages such as slow build rates, limited component size (restricted by the size of the build chamber), the considerable effort required in application design and for setting process parameters, dimension accuracy, required posttreatment methods such as surface finishing and the stability and quality of the metal used, as summarized by Duda et al. [51].Up to now, the most widely used approach for printing with metals is the powder-based one. Nevertheless, not all metals (in powder form) behave similarly making some of them not easily printable. Furthermore, the interaction of the laser with the piece may not be homogeneous throughout the process, creating some zones with higher stress than others due to the heterogeneous thermal incidence. For this reason, alternative routes are being explored such as the development of metal wires to print by means of FDM technologies. However, this implies the mixing of metal with additives, which requires further post-treatment procedures such as those in the case of printing with ceramics. In this case, metal loading is a crucial issue and the main objective is to achieve filaments as rich as possible in metal, in order to minimize the effect of additives on the final finish of the piece. Moreover, new metal printing techniques are also being explored, based on novel principles such as electrochemical 3D metal printing, in which several printing materials can even be combined to generate what is known as Fig. 5. Additively manufactured ceramic substrates after sintering: 1–3) CLP; 4–6) Gyroid sheet-based with 4, 5, 6 mm cell size and 0.6 mm sheet thickness; 7–9) Gyroid strut-based at different macroporosities; 10–12) Primitive sheet-based with 4, 5, 6 mm cell size and 0.6 mm sheet thickness; 13–15) Primitive strut-based at different macroporosities. Reprinted from reference [46]. O.H. Laguna, P.F. Lietor, F.J. Iglesias Godino et al. Materials & Design 208 (2021) 109927 8 multi-metal 4D printing [52]. The Evolution to 4D occurs by the generation of printed systems capable of changing their shape in response to stimuli such as thermal or acoustic changes, although further developments are required to ensure a mature design and production of 4D objects [53]. The motivation to develop technologies for the printing of metals relies on the fact that in principle, these materials can offer unique functional features, such as high stiffness to weight ratio, heat dissipation and heat transfer control, which could be very useful in catalytic applications, as well as enhanced mechanical energy absorption [54]. Regarding the geometry, although there are some examples of printing structured systems made of metal with complex designs and good resolution [46], systems that have actually been applied in catalysis have basic designs made of stainless steel or copper such as that reported by Danaci et al. [49,50] presented in Fig. 6. Although these exhibited good catalytic performance in reactions such as the CO 2 methanation, the level of resolution and complexity of these sorts of monoliths is clearly improvable, as will be discussed in more detail below. Carbon materials present inherent advantages such as a high surface area, allotropy, electrical conductivity and chemical stability that make them attractive as catalytic supports and therefore as printing materials [17]. However, given their characteristics, they must be treated in a similar way to ceramic materials because they require additives to generate a matrix with an adequate rheology for the printing process. In this sense, carbon materials are often part of compound matrixes such as that prepared by Regufe et al. [55], which aims to generate a printable material with high CO 2 adsorption/desorption capacity tunable by means of electric field [55]. However, it has to be remarked that the printing of pure carbon-based (graphene-based) structures has also been achieved through the dispersion of the raw material in water or organic solvents to make printing inks for DIW [17,56–58]. Another alternative for obtaining carbon printed materials is the carbonization of thermosetting resins since these materials have a high degree of crosslinking, which can prevent deformation of the prints during the carbonization process. Resorcinolformaldehyde (R-F) solutions are good example of a pre-polymer solution with the suitable rheological behavior (non-Newtonian fluid with high viscosity) that allows a chain growth reaction that can be controlled in an alkaline medium [17,59]. These prepolymer solutions can be directly injected through a nozzle following a DIW process, or they could also be embedded in a printed template that determines the shape of the final structure after a carbonization process. This would be achieved using the indirect printing approach described above for the ceramic systems. Carbon aerogels that have been previously studied as catalytic materials[60], can be also considered for additive manufacturing through a extrusion process. However, special precautions have to be taken to avoid the material drying out during the extrusion as was pointed out by Zhou et al. [17], since the evaporation of the solvent at atmospheric conditions can cause drastic changes in the surface tension of the solvent at the vapor–liquid interface. This may result in the shrinkage of the material and the production of systems with poor mesoporous structure, since mesopores are destroyed during drying. Therefore, the extrusion of the ink directly into a bath of liquid that is immiscible with solvent was proposed by other authors [17]. For instance Zhu et al. [61] reported such a strategy for preparing graphene aerogels with macroscopic architectures where isooctane was used to prevent solvent evaporation during the printing and then, supercritical CO 2 was used to dry the prepared GO gel. One of the interesting features of carbon is that it can be extracted from renewable sources, such as residual biomass among others. Therefore, the research on the valorization of these raw materials through their transformation into advanced materials is a hot topic. This has motivated, for instance, the use of starch as an interesting material for the production of carbon printed structures. The properties of the starch molecules allow them to be partially dissolved in water, which can be favorable for the extrusion process as was observed by Zhou et al. [17] in a recent study. Along with carbon materials, bioactive materials have been included in Fig. 3E since there are so few studies in this field. However, the field of printing with bioactive materials (bio-printing) seeks to introduce the advantages of process intensification generating systems with controlled forms to carry out catalytic processes in which the catalysts are enzymes. In this sense, the immobilization of the bio-active species in a structured system, would facilitate their extraction from the reaction medium, which is one of the great challenges in enzymatic processes. Nevertheless, bio-active printing require much more delicate manufacturing and post-treatment conditions than those generated with the materials described so far in order to keep the bio-active agent alive and fully active during the process. Moreover, the resolution of the generated systems is clearly improvable and this will be achieved not only by exploring new printing methods, but also through the use of new additives that allow better control of the printing process without intervening with the properties of the bioactive principle. The most relevant examples of bio-printing for catalytic applications [62,63] will be presented in more detail later. After discussing the particularities of the families of materials used to print devices with catalytic applications, it can be said that although there are very general aspects that are more or less common for all, additive manufacturing with each of the families of materials should be addressed almost as a separate technology in order to achieve significant breakthroughs in a shorter period of time. This would make it possible to delve into the advances with each type of material more clearly and would broaden the range of printable materials, to focus them specifically on the field of catalysis, where thermal properties, specific area, chemical stability and surface activity are key factors. In this sense, the need to promote the development of new printable materials with fewer restrictions by suppliers, as mentioned above [1], becomes even more important. This does not imply ending the market for printable materials. On the contrary, it could be expanded through greater interaction of 3D technology providers with specialized users so that together they could expand the advanced printable materials that are offered. Another key aspect to understanding how to integrate catalysis and additive manufacturing is to define which additively manufactured objects can be useful in a catalytic process, both on a laboratory scale and on an industrial scale. Therefore, after a review of what, in our opinion, is everything published to date, not only Fig. 6. Additively manufactured structured reactors made with copper for the CO2 methanation. Reprinted from reference [50]. O.H. Laguna, P.F. Lietor, F.J. Iglesias Godino et al. Materials & Design 208 (2021) 109927 9 it is easy to think that these advantages could be transferred to the development of a new generation of monoliths. And yet how much has been achieved in the design of new monoliths with additive manufacturing? The answer is that it is a stage that is still in its exploratory stage. Although there are already several records about printed monoliths with catalytic applications, the complexity of Fig. 11. Graphos diagram that correlates additive manufacturing techniques, printable materials, catalysts and reactions where different monolith type structured catalysts are applied in most cases reported to date. O.H. Laguna, P.F. Lietor, F.J. Iglesias Godino et al. Materials & Design 208 (2021) 109927 16 the structures is still highly unprovable. In fact, for this review it was established that there are around sixty examples in which structured monolith type catalysts were designed and manufactured and three configurations have been identified (woodpile type, iso-reticular foam type and monolith with channels type), which are shown in Fig. 13. In Fig. 13A, the frequency of each configuration is included, thus confirming that most of the cases applied the woodpile configuration (87%), while monolith with channels or iso-reticular foam configurations also known as periodic open cell structures (POCS) have been applied just a few times. Woodpile is a simple configuration in which monoliths with different densities of cavities and channels are obtained, playing with variables such as the diameter of the bars with which the scaffold is built. The spacing between the bars can also be modified, which will later mean a different degree of stacking in the final block, or even a variation in the orientation of the bar planes that overlap each other with angles ranging from 45 to 90°. From the woodpile configuration, scaffoldingtype monoliths with often cylindrical or cubic shapes are generated. Therefore, this again confirms the emerging nature of the link between catalysis and additive manufacturing because the advantages of design are not yet reflected in devices with complex shapes. It is particularly important to point out that the simplicity of the geometric models printed and used in catalysis does not mean that it has not been possible to print systems of high geometric complexity. In fact, the number of reports grows progressively [9]. The point is that the implementation of many of them is still yet to come. Leaving geometric complexity aside, the important thing is that printed monoliths with a woodpile configuration which are used in catalysis, have improved the performance of powder catalysts in most of the cases, and their performance is comparable with or sometimes superior to that of traditional manufacturing devices, Fig. 12. Printed microreactor for the Fischer-Tropsch synthesis: A) Design of the channels; B) Printed reactor; C) SEM micrograph of the some channels coated with catalyst prior the catalytic reaction. Reproduced from Ref. [106] Fig. 13. Configurations used for the design and manufacturing of 3D printed monoliths: A) Amount of papers that apply every specific design; B) Woodpile configuration; C) Monolith with channels configuration D) Iso-reticular foam configuration or POCS. O.H. Laguna, P.F. Lietor, F.J. Iglesias Godino et al. Materials & Design 208 (2021) 109927 17 such as ceramic monoliths obtained by extrusion or molding. In this sense, knowledge is already being gathered on the aspects to be improved in all critical aspects (the development of printable materials, more precise and reproducible printing methods with increasingly advanced printers and new post-printing treatments, amongst others). 4.2.1.1. Application examples of monoliths with a woodpile configuration. Regarding the catalytic reactions where printed monoliths are applied, as inferred from Fig. 11, the CO 2 methanation has been the subject of the largest number of reports at present, and all these studies have been led by Danaci et al. [48–50,107]. These authors have produced monoliths with a woodpile configuration by means of DIW, using different printable materials (Cu, Al 2 O 3 , SS(316L) and a Ni/Al 2 O 3 catalyst in all cases, which resist high temperature applications such as the cited reaction that which generally occurs between 150 and 550 °C. The authors have established a comparison of the size and distribution of the channels in the structure systems by the modification of the nozzle size during the printing process and the stacking of the layers. With this, they found that methane productivity depends on the nature and geometry of the structure, since in this reaction heat and mass transport limitations play a crucial role in the total CO 2 conversion and the yield to CH 4 . This is why the superior heat transfer ability of monoliths made by Cu or SS(316L) compared to ceramic systems, offers promising results for the manufacturing of metal monoliths. Moreover, the scalability of the process by the addition of different subunits containing printed monoliths was successfully achieved, which is a remarkable result that confirms not only the possibility of intensifying a process by incorporating structured catalysts, but also that of applying this strategy more easily, thanks to additive manufacturing. Woodpile monoliths like those presented in Fig. 14., have also been studied in processes that aim towards the generation of larger organic molecules from smaller ones such as CO or methanol, by means of the Fischer-Tropsch synthesis [23] or the methanol-toolefin approach [111,115] respectively. In these kinds of processes, the porosity of the structured catalyst is essential to ensuring a suitable control of the reaction pathways for the generation of the desired products. For instance, Konarova et al. [23] observed that their monoliths made of PVA, that uses Co-Mo alloys as catalysts, presented further microstructural features (structural porosity) useful for the Fischer-Tropsch reaction, thanks to thermal treatments carried out over the printed monoliths. In the case of the methanol-to-olefin reaction, Lefevere et al. [111] related the control of the geometry of the impression (made by DIW) with the design of the porosity of the material (ZSM-5 zeolite), although in their approach, they delved further into the effect of different binders to enrich the material of the structured system with the catalyst that has high porosity and thus favors the methanol-toolefin reaction. While, Li et al. [115], who also printed monoliths with ZSM-5 zeolite, doped such material with different elements (Zn, Cr, Mg, Cu, La, Ga and Y) and observed a clear difference in the selectivity towards ethylene and propylene, which was higher for the printed monoliths than for the powder catalysts, due to a macro-meso-microporous network promoted during the printing process [115]. Oxidation reactions, processes that require relatively high temperatures, have been also carried out with printed monoliths. For instance Leclerc et al. [108] who studied the partial oxidation of CH 4 , reported the first printed monolith (by means of DIW) entirely composed of ceria (CeO 2 ). This device, that was subsequently coated with Rh, presented a superior performance than its alumina manufactured counterpart, thanks to the oxygen exchange that CeO 2 possesses. This highlights the importance of testing new printing materials, with properties that contribute directly to the catalytic process. Zhu et al. [119] developed different structured catalysts, not only cubic monoliths, but also structures with spiral, honeycomb grid, hollow pillar array, and circular scaffolding shapes. One of the remarkable features of this study is the inclusion of Au and Ag within the printing ink that was used to generate the structured systems by means of DIW. Monoliths have a reasonable performance, but above all, the authors highlight the potential of manufacturing custom designs that minimize the amount of catalyst required to manufacture the device, which in the case of a formulation that includes precious metals, is an important issue from an economic point of view. Liquid phase oxidation experiments have also been carried out, such as the phenol oxidation (that requires mild conditions: 75 °C Fig. 14. Examples of monoliths with woodpile configuration: A) 3D-printed (DIW with copper and SS (316L)) monoliths for the CO2 methanation. Reprinted from Ref.[50];B) 3D-printed (DIW with ZSM-5 zeolite) monoliths for the methanol-to-olefins process. Reprinted from Ref.[111]; C) 3D-printed (DIW with Fe/SiC) monoliths for the oxidation of phenol. Reprinted from Ref.[116]; D) 3D-printed (DIW with carbon materials) monoliths for the oxidation of benzyl alcohol. Reprinted from Ref. [117] O.H. Laguna, P.F. Lietor, F.J. Iglesias Godino et al. Materials & Design 208 (2021) 109927 18 and atmospheric pressure) reported by Quintanilla et al. [116] using monoliths printed with Fe/SiC by means of DIW. The performance of the devices in the reaction seems comparable with that of other structured catalysts used for the same reaction, although in this case, the treatment of the monoliths by means of spark plasma sintering after the printing process decreases the leaching of Fe during the reaction. However, this is generated by a block in the accessibility of the iron available in the material, which implies a reduction in catalytic activity. Therefore, additional studies are required to find a compromise between these two aspects. Recently, Zhou et al. [117] studied the selective benzyl alcohol oxidation using monoliths printed in structured porous carbon materials. What is interesting in this study is the strategy used to promote meso and macro-porosity in the carbon material, which consisted of adding silica spheres to the carbon matrix and this mixture was used as printing material. Subsequently, by alkaline chemical attack, the silica was removed, thus leaving only the carbonaceous material, but with the porosity left by the silica spheres. In this case, not only the shape of the monoliths was designed, but also their porosity, which resulted in devices with high catalytic activity and that were highly selective towards benzaldehyde production. The high potential of printed structured catalysts has also been analyzed in enzymatic processes with printed monoliths and has been reported recently, such as the chemoenzymatic synthesis of 4-hydroxystilbene studied by Peng et al. [63], and the glucose fermentation by Qian et al. [62] Similarly, these types of devices have been tested in organic reactions. For instance, Azuje et al. [118] studied the Biginelli and Hantzsch reactions using monoliths made with Al 2 O 3 by means of DIW. They observed not only the high catalytic activity of the devices but also remarked on their recyclability and scalability. Furthermore, the authors highlighted the short reaction time and the easy extraction of the catalyst from the reaction medium once the process ends. On the other hand, there was a study carried out by Sánchez Díaz-Marta et al. [69] that printed a monolith, which was then inserted into a catalytic reaction ware also generated by additive manufacturing cited above. The structured monolith was designed for carrying out the Chan-Lam azidation/copper alkyne-azide cycloaddition/Suzuki reaction and is one of the first examples of a successful one-pot approach, where additive manufacturing is applied. This is a remarkable result since the entire design of the monolith assembled with the capsule allows to control of the order in which a series of processes occur. 4.2.1.2. Application example of monolith with channel configuration. So far, only two cases have been found in which the direct manufacture of a monolith with channels is reported. Firstly, Yang et al. [120] printed (by means of SLM) cubic monoliths with hollow channels using metallic glass (Zr 55 Cu 30 Ni 5 Al 10 ) as the printable material. The devices were tested in the degradation of methyl orange and presented a superior performance than a cubic structure without channels, although below that of a printed isoreticular foam structure. These results greatly demonstrated the strong effect of the monolith’s shape in the performance during a catalytic process. Secondly, Chaparro-Garnica et al. [104,151] studied printed devices for the preferential CO oxidation in presence of H 2 (PROX). Their monoliths were printed on a commercial UV curable polymeric resin (VisiJet FTX Green) by means of SLA, and their channels were subsequently coated with a CuO/CeO 2 . The fact of using a polymeric resin, greatly limits the operating temperature range of this device. Therefore, temperatures above 180 °C are not used. On the other hand, the low chemical affinity between the polymeric surface of the monolith and the catalyst makes it difficult to anchor it. For this reason, the design of flat channels was changed to one that consisted of channels with cavities so that catalyst deposits were generated. The catalytic results of the device are comparable to those of other structured systems developed with metallic or ceramic materials for this reaction. Therefore, beyond the catalytic performance, this study opens the possibility of expanding the applications of polymeric resins as printing material for useful devices in relatively mild temperature conditions. 4.2.1.3. Applications examples of iso-reticular foam configuration or POCS. Nowadays, the formulation of new printing methods and shapes of complex designs are being launched. However, only a few cases of successful printing have been directly related to catalysts and some of them do not present any catalytic result [152,153], beyond the mere establishment of their suitable potential as a structured catalytic system. This does not diminish the contribution of this type of work. It simply shows that it is a developing scenario based on the hypothesis that an iso-reticular foam configuration may be highly promising to favor catalytic processes based on the advantages of well controlled shapes. Iso-reticular foam configurations or POCS are based on a threedimensional projection of a unit cell to form a superior structure (as seen in the example in Fig. 13D), which, in the case of monoliths, determines the shape of these devices. These are inspired by shapes found in nature, such as the Bravais lattice types or superior structures generated from combinations of these such as those studied by Al-Ketan et al. [54]. The modification of the unit cell dimensions allows generating systems with greater or lesser pore density. The pore size can be regular throughout the entire length of the structure, but it is also possible to design systems with a variable porosity oriented towards one of the axes in which the structure projects, by means of a progressive modification of the unit cell dimensions, as observed in the example studied by Sheithauer et al. [154] presented in Fig. 15. Currently, there are very few studies in which experimental results have been obtained from catalytic reactions, using structured catalysts with complex and well controlled designs. The starting designs and the respective prints of said works are presented in Fig. 16. The first case is that reported by Essa et al. [102], where a nodal diamond-shape lattice design (Fig. 16A) satisfied the requirements of additive manufacturing over-hanging structures, which was established as an optimal configuration by means of CFD simulations. The devices were printed in Al 2 O 3 by means of SLM and a surface coating with MnOx was generated afterwards. The devices were tested in the H 2 O 2 decomposition reaction and exhibited a better performance when compared with that of alumina pellets coated with MnOx. Furthermore, low drops in pressure were detected. Then, Chong et al. [120] reported the printing of different structures (not only iso-reticular foam configurations) using metallic glass and SLM technology. In this case, the authors established that depending on the designs manufactured, different amorphous phase fractions were achieved after the printing processes, and that the complex structures (Fig. 16B) were suitable to be dealloyed in order to obtain a milli/nano hierarchical porous structure, that exhibited excellent catalytic properties in the degradation of methyl orange. More recently, Manzano et al. [113] reported a comprehensive study where a commercial printer based on SLA principle (FormLabs Ò Form 1 was fully adapted in order to test their own printable formulations. For this, the authors modified the build platform and resin tanks with the aim of maximizing the efficiency of the analysis of the different formulations. Several structures printed in PEG-DA-based resins were successfully achieved (Fig. 16C), and some of them that included Cu within the formulation, were tested in the benzyl alcohol oxidation. The experimental results O.H. Laguna, P.F. Lietor, F.J. Iglesias Godino et al. Materials & Design 208 (2021) 109927 19 allowed linear relationships between 3D printed surface area, surface hydrophobicity, and catalyst performance to be established. Therefore, this is the first case where a strict control of geometry allows the comparison of catalytic activity with other variables that are shape dependent. Catalytic devices with advanced designs have also been reported for catalytic applications of immobilized enzymes. This is the case of Ye et al. [105] that used C-PLA for producing scaffolds (by means of FDM) with different shapes (Fig. 16D), and after functionalization treatments with piranha solution, peracetic acid and a silane coupling agent, the structures presented high specific surface areas and an abundance of active groups. This surface modification allowed the anchoring of four kinds of enzymes and the modified devices were tested in the amoxicillin and lactosucrose synthesis. From these results, a special scenario called bioprinting is out-lined, confirming that 3D-printed enzyme immobilization carriers offer a promising solution to building a simple platform that is low cost and flexible enough to accommodate various enzymes and reactors for industrial applications. Additionally, the authors highlighted that the next steps would include improving the number of surface-active groups on the scaffolds in order to achieve higher enzyme loading and increasing enzymatic stability by adding protecting agents. Moreover, they remarked that use of 3D bio-printing for the immobilization of thermostable enzymes has a huge potential, especially for the use of cascade reactions tuning the reaction sequence by staking modules with different catalytic activities [105]. Finally, Lind et al. [112] presented a pioneering report on the production of a cubic iso-reticular foam (Fig. 16E) by means of DMLS, using AlSi 10 Mg as printable material and Pt as catalyst. The high chemical stability of such ceramic substrate was suitable for the testing of the monolith in the NO oxidation to NO 2 , which implies being submitted to a very corrosive media. The main advantage of the applied methodology is the preparation of a Fig. 15. Graded porous structure of a cuboid where the pores size increases with increasing component height: A) side view; B) perspective view. Reprinted from Ref. [154] Fig. 16. Examples of monoliths with iso-reticular foam configuration already used as structured catalysts: A) 3D-printed (SLM with Al2O3) monolith with nodal diamondshape lattice. Reprinted from Ref. [102]; B) 3D-printed (SLM with Zr55Cu30Al10Ni5) monolith with lattice structure. Reprinted from Ref. [120]; C) Different 3D-printed (SLA with PEG-DA-based resins) monoliths with scaffolding structures. Reprinted from Ref. [113]; D) 3D-printed (FDM with C-PLA) monoliths with scaffolding structure. Reprinted from Ref. [105]; E) 3D-printed (DMLS with AlSi10Mg) monolith with cubic iso-reticular foam configuration. Reprinted from Ref. [112] O.H. Laguna, P.F. Lietor, F.J. Iglesias Godino et al. Materials & Design 208 (2021) 109927 20 structured catalyst with hybrid properties. On the one hand, the anodization procedure (carried out after the printing process) allowed the modification of the surface of the printed structure by means of the creation of an alumina layer that reduced spalling problems since it is chemically linked to the support. On the other hand, the created core–shell approach, combined the chemical properties of the surface Al 2 O 3 layer with the high thermal conductivity of the core section of the structure. The examples cited, including two recently published papers with similar design approaches for structured systems applied in other reactions [134,137] show that, so far, the most complex structures are achieved with technologies other than DIW, which until now has been the technique most used to generate monoliths. Therefore, future advances are expected from the exploration of new printing materials and new strategies with alternative techniques, which allow greater resolution and detail in the prints. Probably a bottleneck is being caused by the complication of making printable materials such as metals or ceramics in technologies that do not use such demanding manufacturing conditions, but at the same time achieve high resolutions. Another aspect that exacerbates this bottleneck is access to printing techniques for ceramic, metallic or composite materials that combine the properties of both families of materials. Leading-edge technologies are just taking off and are highly expensive. These two factors mean that it still takes time for them to reach different research centers and thus more strategies are being explored that allow a more efficient union between catalysis and additive manufacturing. However, one of the aspects that can help to overcome these barriers is the easy connection between studies that combine catalysis and additive manufacturing, with simulation models. Thanks to the current computational tools, the development of fluid dynamics models allows to pave the way towards better devices through simulations that consider different variables addressed from theoretical and experimental perspectives, even without catalytic experiments. For instance, the team led by E. Tronconi (from the Politecnico di Milano - Italy), which has a recognized track record in studying structured catalytic systems, is developing studies with printed foams that analyze phenomena such as pressure drop [155,156] or heat transfer [157] in 3D-printed open-cell foams for different reactions [158,159]. These types of studies are extremely important to understanding much more quickly the experimental results in future research, where the combination of catalysis and additive manufacturing is applied in any type of process and will facilitate its scaling-up. 4.2.2. Electrodes These structured systems can be applied as platforms for different electrochemical devices [145] such as pseudo-capacitors, pH sensors and catalytic setups for the electrogeneration of O 2 and H 2 , or the electrochemical degradation of pollutants. In fact, the fastest growing scenario today is the application of additive manufacturing to obtain electrodes as sensors for different species [160,161]. In the design of electrodes, the geometry and the exposed area are determinant since they establish the efficiency of the current flow and consequently the successful achievement of the electrochemical process [28,162]. For this reason, additive manufacturing is a tool that allows, on the one hand, the production of the electrodes to be easier and quicker and, on the other hand, to explore alternative designs, new materials and new treatments in order to improve the performance of these devices. Nowadays, there are not many reports on electrode printing. Additionally, some of these only focus on the manufacturing process and present general characterizations (mass transport, mechanical stability, thermal conduction among others) that show the potential of the printed electrodes to be used in electrocatalytic processes, but without catalytic evaluation results [163,164]. Nevertheless, some recent reports show both manufacturing and catalytic testing of electrodes, such as those exhibited in Fig. 17. The research team led by Pumera, leaders in the research on printed electrodes [144,145], presented what they named a ‘‘breakthrough in on-site prototyping and fabrication of highly tailored electrochemical devices with complex 3D shapes” [98].In this study, they showed the applicability of metal additive manufacturing as a valid fabrication technique of custom-shaped electrochemical electrode systems (helical shaped – Fig. 17A), that were tested in capacitance measurements, the oxygen evolution reaction and as pH sensors. The electrodes showed excellent capacitive and catalytic properties in alkaline solutions and Nernstian behavior as a pH sensor. Pumera et al. [70] also reported the printing of an electrode with a flat honeycomb grid configuration (Fig. 17B) by means of SLM with SS(316L), assembled within an electrochemical tested in the water splitting process. One of the relevant features of this study is that it combines in the same device different printed components, not only the electrode but also the other electrolyzer cell components, and this is why this paper was also cited in the section devoted to catalytic reaction ware (Fig. 9D). Consequently, different 3D-printing techniques and printable materials were used, so this is one of the few examples where additive manufacturing is applied in a multi-material project. Regarding the performance of the cell, it was possible to establish not only that it works correctly, but also the improvement in the performance of the electrodes by means of modifying their surface with the deposition of NiFe double-hydroxide film onto the anode and Ni-MoS 2 composite film onto the cathode. Following a similar approach that combines printed monoliths and electrolyzer cell components, Pumera et al. [82] also introduced a new design of basket-shaped electrodes (Fig. 17C), which were also tested in the water splitting process. In this case, the authors focused on the possibility of modifying the catalyst ´s surface by coating it with different catalysts (IrO 2 , Ni or Pt) by electrodeposition. More recently, Brown et al. [96] explored a rapid and versatile procedure for the production of tailorable electro-photocatalytic devices. In this case, the printed electrodes made by means of SLM with SS (316L), can be used as is, or modified with postfabrication processes in photo-electrochemical energy applications. In this study, the atomic deposition layer was used for the first time for the modification of the electrodes with the aim of enhancing their performance in photo-electrochemical water oxidation. Finally, Zhang et al. [97] presented the design and manufacturing of hierarchical nanoporous structures made with Cu or the alloy Cu-Mn by means of SLM (see Fig. 17D) for the electrochemical methanol oxidation. This work highlights a facile, low-cost, and alternative strategy for the design of structures with complex shapes that can be applied to binary, ternary and quaternary metal alloys for various functional applications. The printed electrodes, which presented structures similar to the iso-reticular foam configuration described above for monoliths, exhibited high efficiency during the electro-oxidation of methanol, based on enhanced mass transport properties, thanks to the control of the macroand nanoscaled porosity from the design itself. 4.3. Integration of the catalyst in printed structured devices As it has been possible to verify, the large number of variables that can be combined in the development of structured catalysts generates different approaches when classifying this type of device. Up to this point, the printing methods and materials, the type and configuration of the printed devices, as well as their functionality, and also the catalysts and the reactions in which they are applied, have been taken into account. However, the different O.H. Laguna, P.F. Lietor, F.J. Iglesias Godino et al. Materials & Design 208 (2021) 109927 21 examples cited so far lead to infer that there is no single strategy for structuring. Therefore, the question arises as to how and when the catalyst is incorporated in the structured system? Prior to, during or after the printing process? Amongst the excellent reviews devoted to the analysis of the union between catalysis and additive manufacturing, only that carried out by Hurt et al. [1] considered this question in any depth. In fact, they identified three main groups where the classification of the strategies for the catalyst incorporation followed the different cases reported up to the beginning of 2017. The first group is called ‘‘integration strategy” that considers the cases where catalytic phases have been included within an already-valid printable material prior to the printing process, or were used as the building material outright. Secondly, the group called ‘‘functionalization strategy” that implies the coating of the printed device during a post-treatment process. Finally, the last group which consisted of a combined approach of integration and functionalization In principle, these classification results are very useful. However, there are some cases that could be reclassified depending on the criteria applied to decide if the printed device is reaction ware or a structured catalyst. Additionally, new examples with alternative strategies for the use of additive manufacturing in catalysis have appeared and they do not quite fit into any of the cited groups. These are the cases where the printed device is a template into which a colloidal matrix is embedded and then, by means of a thermal and/or chemical treatment, it is transformed into a structured system with a geometry inverse to that of the template. The said template disappears thanks to the thermal and/or chemical treatment, and regarding the structured system, this can be made of the catalyst or not, in which case it would require a subsequent treatment of coating. Therefore, the strategy where the advantages of additive manufacturing are transferred to catalysis by means of the use of a template can be labelled as indirect printing. Considering this, an alternative classification of the studies devoted to the printing of structured catalysts it is proposed depending on the applied strategy, as presented in Fig. 18. Firstly, the approach labelled Direct Printing, where the catalyst constitutes all or an important part of the printable material, and thus the printed object can be subjected almost directly to the catalytic process. Despite this, in most cases activation post-treatments are required, generally of the thermal type in controlled atmospheres. The second one is the Printing of Support approach, which is made up of printed objects that are not made of the catalyst, but which will later be coated with it. This means that the incorporation of the catalyst involves additional procedures, apart from the subsequent activation of the catalyst before the reaction, which may involve heat treatments in controlled atmospheres. Finally, the Indirect Printing strategy it is proposed, where the printed devices are templates for the manufacturing of structured catalysts. These may or may not be made of materials that contain the catalyst in the colloidal matrix, and in the latter case, an additional process would be needed in which the device is coated with the catalyst. 4.3.1. Direct printing of structured catalysts approach The inclusion of a catalyst or the precursors that will be subsequently transformed into a catalyst, within the formulation of the printable material, is not simple and the procedure may differ, depending on the catalyst’s chemical nature and the printability requirements of the printing techniques used. In most of the cases included, the direct printing approach DIW is the most widely used technique (Fig. 18) and the main products are monolith type structured catalysts. However, electrodes, grids, and stir bar covers have been produced by means of other printing techniques such as SLS, SLM and SLA that provide a superior resolution than DIW. There are cases in which the printable material is fully developed and this is much easier to do when applying the DIW printing technique, because no photo-curable agents must be added and, in principle, the operating conditions require additives that allow the consolidation of the piece at room temperature without additional treatments. For instance Tubío et al. [27] reported their own printable material for obtaining monoliths with a woodpile Fig. 17. Examples of 3D-printed electrodes applied in electrocatalytic processes: A) Electrodes with helical strip shape (made by means of SLM with SS CL 20ES) tested in the oxygen evolution reaction. Reprinted from Ref. [98]; B) Electrodes with honeycomb grid shape (made by means of SLM with SS (316L)) tested in the water splitting. Reprinted from Ref. [70]; C) Electrodes with basket shape (made by means of SLM with SS (316L)) tested in the water splitting. Reprinted from Ref. [82]; D) Electrodes with iso-reticular foam configurations (made by means of SLM with Cu-Mo alloy) tested in the methanol oxidation. Reprinted from Ref. [97]. O.H. Laguna, P.F. Lietor, F.J. Iglesias Godino et al. Materials & Design 208 (2021) 109927 22 configuration by DIW. Their printable material consisted of Al 2 O 3 powder dispersed in aqueous Cu(NO 3 ) 2 solution (the precursors of the Cu/Al 2 O 3 catalyst) a viscosity modifier (hydroxypropyl methylcellulose) and a chelating agent (cationic polyelectrolyte poly(ethylenimine) to bind the matrix during the printing process. These authors optimized a composition for the printable material that enabled a homogeneous distribution of the catalyst throughout the monolith. Following a house-produced printable material approach, recently Middelkoop et al. [107] reported the production of monoliths for the CO 2 methanation reaction. In this case, the Ni/Al 2 O 3 catalysts were prepared prior to the printable matrix, and bentonite as well as powder Al 2 O 3 were included as binders. These authors also reported graphene-supported CeZrLa mixed-oxide catalysts for printing monoliths for the synthesis of propylene carbonate[110]. In this case, the catalysts were previously obtained by continuous hydrothermal flow synthesis, which implies mixing a flow of water-soluble precursors to give rapid, controlled and continuous production (within seconds) of nanomaterials. These powder nanomaterials (42 wt%) were mixed with an aqueous solution of methyl cellulose (54 wt%) and a small portion of lubricant additive (Thinky Mixer ARE-250) to ensure a suitable rheology during the process of printing by DIW. Wang et al. [165] recently presented the direct production of structured WO 3 -based catalysts by additive manufacturing using the DLP method. In this case, structures with complex configurations were printed using suspensions that contained the catalyst precursors in the form of salts dissolved in the medium. This implies that dispersed catalyst particles are not used, but rather that the catalyst is consolidated in the structured itself by calcination after printing. This favors the homogeneous distribution of the catalyst precursors throughout the volume of the printed device and promotes greater control of the resolution of the printing process. In some cases, catalysts that are part of the printed structured devices, require further treatments for the enhancement of an specific catalytic feature. For example, Quintanilla et al. [116] studied the densification of the printed structures that already contained the catalyst used for the phenol oxidation, using nonpressurized Spark Plasma Sintering, which involves submitting the printed monoliths to temperatures ranging from 1000 to 1500 °C in an argon atmosphere (~6 kPa), and the results obtained create a dilemma. On the one hand, the densification process is effective because it increases the hardness of the monoliths and reduces the leaching of iron. However, the accessibility to the Fe atoms, which are the active sites of the catalyst, decreases. Therefore, a balance must be found between the different properties that are sought in the final structure. Amongst the novel strategies used to control the properties of printed monoliths that include components with catalytic features, it is interesting to cite the one recently reported by Zhou et al. [117], who presented the manufacturing of monoliths with carbon of tailorable pore size for the benzyl alcohol oxidation. This control of the pore size of carbon was achieved by adding SiO 2 monodisperse spheres within the printable matrix that included starch and gelatin as binder and carbon sources. Once the mixture was completely gelatinized, the printing was carried out and the structures obtained were carbonized at 1100 °C for 2 h with a heating rate of 5 °C/min in an N 2 atmosphere. Afterwards, the monoliths were washed with NaOH (2 M) to remove the SiO 2 spheres resulting in pure carbon replicas with an additional porous structure provided by the no longer present SiO 2 spheres. The printing of structures that incorporate enzymes is another example that must be highlighted because it involves the creation of a separate branch within the union between additive manufacturing and catalysis, which is the printing of bio-active catalytic systems (bio-printing). This is the case of Wei et al. [127] who prepared supramolecular hydrogel via pH triggered molecular Fig. 18. Classification of the different strategies for the inclusion of the catalyst in most structured systems additively manufactured to date. O.H. Laguna, P.F. Lietor, F.J. Iglesias Godino et al. Materials & Design 208 (2021) 109927 23 self-assembly in water. Then, dual enzyme-catalyzed reagents, including glucose, among others, were added to the hydrogel until an extrudable rheology was obtained, to be used in the generation of structures through a pseudo DIW process with a syringe as an extruder. In this case, the authors remarked that the hydrogel is cytocompatible and can be used for in situ 3D cell culture, which makes this a promising material for bio-catalysis and tissue engineering. Another example of the printing of structures for the intensification of bioprocesses is the one reported by Quian et al. [62] that combines nanocellulose crystals and Baker’s yeast (Saccaromyces cerevisiae) to obtain different structures through DIW. The authors noted an unprecedented cell loading in the structured system, and it was discovered that the cells exerted a substantial impact on ink rheology, and the cell inks had the shear-tinning behavior desired for extrusion-based printing. Therefore, the living material itself influences the printability. Furthermore, a superior ethanol production was achieved during the glucose fermentation due to the design of the printed lattices compared to bulk counterparts. However, despite the considerable advances in biomaterials printing for catalysis, the immobilization of enzymes within hydrogels supposes mass transfer limitations due to the reduction of access to the active sites of enzymes and thus requiring the increment of the intrinsic enzyme activity. This means that more active systems may overcome the loss of volumetric activity in structured biocatalysts[99]. Regarding the introduction of catalyst on other type of structured catalysts, recently, Zhang et al. [97] presented the production of electrodes by means of SLM using a Cu-Mo alloy. In this case, the active componants are the metal species Cu or the Cu-Mn in the electrode. This is perhaps the easiest way to incorporate a catalyst into a structured system and this is when the printable material is entirely made up of the catalyst. Although in this case, a copper enriched electrode system was also generated on the surface through a pickling process. The absence of additives facilitates the prompt application of the device and reduces the possible effects on the properties of the material due to constant postprinting treatments. Another relatively simple strategy to include the catalyst is to modify the active component of formulations whose printability has been previously confirmed. For example, Manzano et al. [68,113] analyzed printed devices (made with PEG-DA by means of SLA) for the Mannich reaction and the Aldol and Huisgen cycloaddition reactions. In this case, modified bifunctional molecules were included within the printable formulation, providing accessible carboxylic acid, amine, and copper carboxylate functionalities that were transferred to the printed structured system and exhibited catalytic activity in the cited reactions. Furthermore, an enhancement of the reaction conversion upon increasing the complexity of the additively manufactured objects was observed, highlighting the successful functionalization of commercial resins for anchoring the catalyst prior to the printing process. 4.3.2. Printing of structured supports approach In this case, the catalyst is coupled with the structures after they have been manufactured, therefore there is a certain freedom in the printing process that is no longer limited by the possible negative effects on the properties of the catalyst. However, the need arises to work with materials that are capable of anchoring the catalyst [166]. In this sense, combining the information in Fig. 7,8,10 and 11, it can be said that a considerable variety of printing materials are beginning to be generated, which include ceramic, metallic and polymeric materials. In the same way, the use of hybrid systems such as TiO 2 -ABS, which combines a polymeric part and a ceramic part, is growing. However, regarding the type of catalyst incorporated, it can be observed that the metallic and ceramic structures are related to catalysts of a similar nature, while the polymer-type structures tend to be combined with catalysts based on carbon or bioactive agents such as enzymes. This is an expected result because the coupling between the catalyst and the printed structure is mainly due to chemical affinity. The incorporation of the catalyst occurs mainly on the surface of the structured system and can be beneficial in two important ways. Firstly, it reduces the amount of catalyst used to manufacture the entire structure, and this becomes highly relevant when it comes to catalysts made up of high-value elements such as precious metals. Furthermore, accessibility to active catalyst phases is improved because the vast majority are exposed on the surface of the framework and there are no catalyst parts hidden within the core of the structured system. Therefore, the phenomena of matter transfer during the catalytic process are much more efficient and the specific activity per mass of catalyst is much higher. Despite these advantages there are drawbacks. On the one hand the printing of structured supports involves a series of additional stages after the printing process that can increase the cost of this type of structure. On the other hand, achieving a stable anchoring of the catalytic layer to the surface of the catalyst becomes one of the greatest challenges for this type of system because the leaching of the catalyst must be avoided as much as possible during its use in the catalytic process, especially in catalytic reactions occurring in liquid media. This, in principle, would involve not only a suitable procedure to anchor the catalyst to the structured system that acts as support, but also protect the integrity of such structured system during the catalytic process. This is due to the fact that there is evidence in unprinted structured systems coated with catalyst that the structured material itself suffered alterations due to the environment of the working atmosphere, generating a progressive deterioration of the catalytic layer [24]. Until now, the methods used to anchorthe catalyst to the printed structures (Fig. 18), have been the same as those commonly used in the coating of non-printed structured systems. The most widely used one is dip-coating [48,50,69,102,104,105,121,123], perhaps because it applies a simple and accessible strategy, in which the surface of the structure is covered with a slurry of the catalyst. It does not require expensive technology, it is reproducible if the rheology of the catalyst suspension can be controlled and it can be carried outundermildconditions,guaranteeingtoagreatextentthepreservation of the integrity of the catalyst. In addition, it is a versatile technology that can be adapted to catalysts of different natures. In some non-printed structured systems that have been coated with catalyst through the dip-coating process, treatments are usually used on the support to increase the anchorage of the catalyst. For example, in micromonoliths made with FeCrAlloy Ò , a heat treatment at 900 °C is usually applied, which allows the evolution of a surface layer of aluminum whiskers, which, thanks to their morphology and ceramic character, protect the metal core of the structure and favor the anchoring of catalysts based on metal oxides. Nevertheless, in most of the cases included in Fig. 18 that reported the use of dip-coating, no pretreatments of the printed structures beyond a mere alcohol and ultrasound cleaning have been reported. This may be due to the fact that, in most cases, the chemical affinity between the support and the catalyst is high, which facilitates its anchoring. However, the case reported by Chaparro-Garnica et al. [104] is special because it poses the challenge of coating the channels of monoliths made of a commercial polymeric resin (Visijet FTX green), with a ceramic catalyst (CuO/ CeO 2 ). In addition to the alternative channel design to promote catalyst build-up pointed out in section 4.2.1.2., the authors proposed the surface be attacked with solvents before or during the active phase incorporation, but this was not entirely effective. What worked better was the incorporation of silica in the resin before printing since the presence of this oxide in the printed monolith O.H. Laguna, P.F. Lietor, F.J. Iglesias Godino et al. Materials & Design 208 (2021) 109927 24 favored the subsequent adhesion of the catalyst. Along the same lines, Ye el al. [105] applied both; the modification of the printing material prior to the generation of the structured ones and the subsequent chemical edging of their surface with piranha solution, paracetic acid, and a silane-coupling agent, resulting in the successful immobilization of enzymes as presented above in Fig. 16D. With respect to other methods for the incorporation of the catalyst, such as dropwise addition [108], wet impregnation [109,112], electrodeposition [70,82,98,167], atomic layer deposition [96] and cold spraying and electropainting[95], it can be established as a common factor that the catalyst load generated in the structures is less than that usually generated with dip-coating. For this reason, they focus mainly on the incorporation of metallic active phases, since their specific activity is quite high. However, these are also used to deposit some oxide-based catalysts in systems that do not need such a high load in order to achieve adequate catalytic activity results, as in the case of electrodes. Despite this, it can be said that the use of alternative methods to dip-coating is still in an exploratory phase, and there are no cases that allow the identification of strategies for modifying the printing material, or for the treatment of the printed structures, specifically designed to improve the performance of the catalyst anchorage by means of said methods. 4.3.3. Indirect printing of structured catalysts approach This is a strategy for the manufacture of structured catalysts that has not been clearly differentiated until today and, although currently there are few articles published on the matter, it clearly differs from the two strategies presented so far. In fact, in other fields where the additive manufacturing is applied, the relevance of this strategy is becoming observable. For instance, Tijing et al. [168] recently presented a review on the use of additive manufacturing for membrane separation, desalination and water treatment, and they included the principles of this strategy within one of the future prospects of this field where additive manufacturing is combined with subtractive manufacturing and formative manufacturing. In the indirect printing approach, additive manufacturing is used to generate molds that are then sacrificed after chemical and/or thermal treatments [159,169]. This reduces the importance of the chemical affinity between the catalyst and the printed structure because what is sought is that the mold simply drives the consolidation of the final structure through a physical process, which in principle is more practical. However, the template removal stage incorporates other types of factors to be taken into account, such as possible alterations to the final structure, either due to the atmosphere in which the template is removed, or due to diffusion problems during the removal of sacrificial material. Once the mold is printed, it is filled with a paste that is then cured and consolidated with a thermal or chemical treatment. The release of the material from the mold can occur simultaneously to the consolidation of the paste or afterwards, and in this case, the unit stages of the process would be increased. Taking into account this general operating procedure, two types of structures can be obtained in this way. Firstly, those in which the paste matrix incorporates the catalyst or its precursors and, after consolidation, a structure with catalytic activity is obtained. Once consolidated, these systems are similar to those obtained through the direct printing strategy (section 4.3.1), but to differentiate them, to label them indirect structured catalysts it is suggested. Secondly are those in which the matrix that is introduced into the mold does not contain the catalyst. Therefore, in the consolidation process, a structure will be generated that must be subsequently coated with the respective catalyst. This type of structure is similar to that generated by the printing of structured supports (section 4.3.2) and the same coating methods with the suitable catalysts could be applied. However, to differentiate them, naming them indirect structured supports it is proposed. As mentioned above, there are currently few articles that present the application of indirect printing for obtaining structured catalysts[159]. In fact, there are cases such as that of Hereijgers et al. [170] that presented just the manufacturing and characterization without catalytic tests of electrode mixers, which are 3D flow-through structures with mixing properties that at the same time act as electrodes. In this study, printed polymeric templates (made of LimoSolve polymer purchased from Formfutura) were filled with a paste consisting of Ni powder (61.2 wt%), epoxy (14.3 wt%) and glycerol (24.5 wt%). After filling the mold, epoxy was cured for 3 h at 40 °C. The mold was dissolved in toluene and then the structure was sintered by means of a heat treatment, with different stages up to 1000 °C. The first study that combines the manufacture of an indirect structured catalyst with catalytic activity measurements is that presented by Li et al. [171], whose reported cylinder, tetrahedron, and tetrakaidecahedron phenol–formaldehyde-based hierarchical monoliths coated with Ni/Al 2 O 3 catalyst for the CO methanation reaction. The phenol–formaldehyde paste was used for filling the templates (previously printed by means of FDM with PLA), and through a solvothermal polymerization process at 160 °C for 8 h under hypersaline conditions (ZnCl 2 )[171], subsequently brown monoliths were obtained. The monoliths were then washed with ethanol for removing the residual polymer templates and ZnCl 2 , dried in vacuum 105 °C, and finally calcined under N 2 atmosphere at 900 °C. These monoliths can be classified as indirect structured supports, since they require the subsequent coating with a catalyst. Therefore, with a first impregnation with aluminium isopropoxide and a calcination process at 550 °C, an alumina layer was generated on the surface of the monolith. Afterwards, the system was further impregnated with Ni(NO 3 ) 2 solution and then calcined under nitrogen atmosphere at 550 °C, resulting in the generation of the Ni/Al 2 O 3 catalyst. Regarding the catalytic activity measurements, not only was the high performance of the indirectly printed structured catalysts observed, but also a correlation between such performance and the tailored geometry of the devices was noted. Particularly, the control of the tortuosity of macro-channels into the monoliths resulted in high yields towards CH 4 due to the decrease of mass and heat transport limitations. Another example of indirect structure support production is that recently reported by He˛drzak et al. [172] where templates made of a commercial polymeric resin (B9R-4-Yellow) and printed by FDM, were used to obtain monoliths made of a -Al 2 O 3 . The monoliths were immersed in a hydrothermal reactor with the precursors for the ZSM-5 zeolite synthesis and thus a surface layer was generated with said catalyst (see Fig. 19B). These devices were tested in the gas-phase isomerization of a -pinene. The performance of the device is closely related to the tailoring of the Al/Si molar ratio during the zeolite synthesis over the surface of the monoliths. In general, the success of this work lies in the fact that the monoliths generated by this technique are catalytically active and comparable with other types of structured systems. Furthermore, they can regenerate in oxidizing atmospheres at high temperatures and do not deteriorate. Therefore, it is anticipated that in future studies, it will be possible to delve into the analysis of the influence of geometry on the total conversion and, even more importantly, on the distribution of products. Davó-Quiñonero et al. [47]obteined printed templates for obtaining monoliths tested in the CO oxidation and the CO-PROX reactions. In this case, templates were made of commercial polymeric resin Visijet FTX Green, and these were filled with a commercial cordierite paste (COR-MIK-MP provided by VICAR S.A.). 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