Green dipolar aprotic solvents for the dynamic polycondensation of high-performance polyimide membranes

Green Chemistry PAPER Cite this: Green Chem., 2024, 26, 11984 Received 27th August 2024, Accepted 14th October 2024 DOI: 10.1039/d4gc04279d rsc.li/greenchem Green dipolar aprotic solvents for the dynamic polycondensation of high-performance polyimide membranes† E. San José, a,b M. R. de la Viuda, * a,b F. J. Carmona, a,b C. Soto, a,b L. Palacio, a,b P. Prádanos, a,b A. Hernández a,b and A. Tena * a,b The legislation is limiting the use of harmful organic solvents in industrial processes. The establishment of clear guidelines for minimizing solvent residues and the development and implementation of circular methodologies, together with growing environmental and health awareness, will promote the replacement of traditional solvents by more sustainable alternatives. In general, high-performance polymers, such as polyimides, are synthesized under specific reaction conditions. This work defines and develops clear guidelines, integrating them into a decision map to evaluate the potential of an alternative solvent for application in the synthesis of polyimides. Since every industrial application demands explicit criteria, our study focused on the development of polyimides for the membrane industry. More than 130 solvents were evaluated, and 10 solvents were found to have the potential to be employed in the synthesis of polyimides. The outcome was verified with 7 of those solvents, namely, γ-valerolactone (GVL), cyrene (Cy), dimethyl carbonate (DMC), dimethyl isosorbide (DMI), dimethyl sulfoxide (DMSO), 3-methoxy-N,N-di- methylpropanamide (commercially known as KJCMPA®-100), and the reference N-methyl-2-pyrrolidone (NMP), and tested for the synthesis of 3 polyimides (6FDA-HAB, 6FDA-6FpDA and 6FDA-DAM), obtaining extremely consistent outcomes. This work found four solvents, GVL, DMI, DMSO, and KJCMPA, that could substitute NMP, and other harmful solvents, in the synthesis of high-performance polymers. GVL provided even better results in terms of the molecular weight of the polyimides than the reference NMP, showing a realistic potential for its direct substitution. This work also reports more than 40 alternative solvents derived from the identified solvents. Finally, the key action points that should be taken into account for imminent advances in the subject were recognized. 1. Introduction There is a continuously growing awareness of the need to replace conventional toxic chemicals with more environmentally friendly chemicals in conventional processes. The consequences of using certain types of chemicals raise concerns worldwide, necessitating their regulation. The EU’s Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) legislation offers protection to chemical manufacturers, distributors and users. Main manufacturing and producer countries in Asia-Pacific regions, such as China, Korea and Japan, have increased their restrictions in terms of safety during the use and manipulation of chemicals. 1 Future steps for the regulation of chemical substances such as solvents will probably evolve towards more rigorous control in terms of risk assessments, as well as the promotion of actions in recycling and circularity. Therefore, academia and industry face challenges in transforming conventional industrial chemistry while maintaining product cost, material performance, and process competitiveness. In this sense, dipolar aprotic solvents are ubiquitous in chemistry owing to their ability to dissolve a wide variety of materials (often including salts), their versatility as reaction media, and often their low cost. 2 Dipolar solvents present unique features such as their ability to separate regions of high and low electronegativity and the absence of any ability to donate hydrogen bonds. Their extensive use is also favoured by the abundance of existing literature supporting their use in countless transformations and processes. 2 Dipolar aprotic solvents such as N-methyl pyrrolidone (NMP), dimethylformamide (DMF), and N,N-dimethylacetamide †Electronic supplementary information (ESI) available. See DOI: https://doi.org/ 10.1039/d4gc04279d a Surfaces and Porous Materials (SMAP), CSIC-Associated Research Unit, Universidad de Valladolid, 47011 Valladolid, Spain. E-mail: monica[email protected], [email protected] b Institute of Sustainable Processes (ISP), 47011 Valladolid, Spain 11984 |Green Chem.,2024,26, 11984–12007 This journal is © The Royal Society of Chemistry 2024 Open Access Article. Published on 18 October 2024. Downloaded on 2/17/2025 8:18:36 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online View Journal | View Issue (DMAC) are heavily used in the polymer industry, for instance, in the synthesis of common and high-performance polymers such as polyamides or polyimides. Nevertheless, these solvents pose serious hazards to the human health or environmental safety, and their use is restricted or prioritized for future restriction as Substances of Very High Concern (SVHC) under the REACH (CE No. 1907/2006) regulation. 3 The limited number of green dipolar aprotic solvents provides very few opportunities to replace traditional dipolar solvents, resulting in a substantial absence of appropriate substitutes. Additionally, the alternative solvent must present specific and appropriate characteristics to be employed in the specific process. For instance, NMP is generally used as the preferred solvent for extraction processes in the oil and gas industry, for the synthesis of cathodes in the battery production, for the coatings of semiconductor materials in electronics, for the production of fibers in the safety clothing, or for the production and processing of materials in the coating and membrane industry. 4 This work is focused on the development of an effective methodology to find greener solvents, which can be employed in the polymer-based membrane industry. The membrane industry is recognized as an environmentally friendly and modern technology that requires low energy consumption and has simple operational conditions compared to other traditional separation technologies. An ideal strategy for the membrane processing would be based on minimizing or even eliminating the use of solvents for the membrane production. 5 However, the materials are generally synthesized and processed from solvents such as NMP or DMF. The necessity to find alternative solvents has boosted the number of publications related to the search for greener ones in the membrane field. The polymeric membrane production workflow process generally implies two key steps: the synthesis of the polymeric material and the processing of the material as the membrane, which in an ideal situation would directly occur from the synthesis avoiding precipitation of the polymer, together with cleaning and drying. The main part of the research activities has been mainly focussed in the processing of the materials based on phase inversion processes, while very little attention has been paid to the synthetic protocol of the more or less dense polymer materials. Cyrene has been one of the most studied bio-derived solvents for the membrane fabrication processes. Polymers such as cellulose acetate (CA), poly(vinylidene fluoride) (PVDF), polyethersulfone (PES), polysulfone (PSf) and polyimide (PI) membranes have been processed as porous membranes 6 using solvents such as dihydrolevoglucosenone (Cyrene™) and derivatives from this solvent. Other greener solvents such as dimethyl isosorbide (DMI), 7 methyl lactate, 8 N,N-dimethyl lactamide and succindiamide, 9 γ-valerolactone, 10 γ-butyrolactone, 11 methyl-5-(dimethylamino)-2-methyl-5-oxo- pentanoate (Rhodiasolv®PolarClean), 12 and derivatives of dialkyl carbonates (DACs) have been used. Deep eutectic solvents (DESs) 13–15 will have an important impact in the coming years in the chemical industry. 16 By the moment, the use of DESs in the polymer and membrane industry is still limited to their use as an element to change the membrane formation behaviour as well as the hydrophobicity of the materials. Hong et al. had quantified the total environmental impact of membrane production, which could be subdivided into polymer synthesis (32–34%), polymer washing (16–17%), membrane casting (23–26%), and membrane drying (21–23%). 17 On a life cycle basis, nearly half of the environmental burden was attributed to membrane casting and drying, while another one-third of the impacts came from the synthesis of polymers. By the approach of processing the membranes directly from the polymer synthesis, it will be possible to save unnecessary steps reducing up to 50% of the possible environmental impact of the process. 17 Therefore, more efforts should be devoted to the search for greener solvents which can be used at any step of the process. This is especially critical for the polymer synthesis and membrane formation. Therefore, the use of greener solvents in the polymer synthesis and membrane processing will boost the transition toward a more sustainable membrane production process. In this work, we developed a tool which can be easily implemented by any researcher or industry for the selection and substitution of the regulated toxic solvents by more sustainable solvents which will come to the market in the coming years. For that purpose, we analysed the key-factors involved in the polymer synthesis and membrane formation process. We selected three well-reported polyimides which are generally tested as gas separation membranes. These, or variations in them, are often synthesized by a very large number of research groups in the topic. In this way, a decision tree for the selection of the currently suitable solvents was developed. As a result of the selection procedure, three selected polyimides were synthesized in an ensemble of 6 of the more suitable solvents. The polymers resulting from these experiments were compared with the three reference polymers synthesized in NMP. 2. Definition of the necessary criteria to be met by the alternative solvents As general methodology, the selection of alternative solvents was developed taking into account the generally accepted criteria for the harmonized system of classification and labelling of chemicals in terms of safety, health and environmental criteria. 18 In addition, different solvent guidelines, 19–24 the Globally Harmonized System (GHS) 18 and European regulations attending to the classification used in REACH 25,26 were considered. In this sense, this type of general preliminary classification of any chemical or solvent allows a simple and quick evaluation of its sustainability and capacity to be used in a process. However, the polymer synthesis and membrane manufacture contain specific characteristics which must be taken into account. In this selection tool, up to 138 solvents will be analyzed. Green Chemistry Paper This journal is © The Royal Society of Chemistry 2024 Green Chem.,2024,26, 11984–12007 | 11985 Open Access Article. Published on 18 October 2024. Downloaded on 2/17/2025 8:18:36 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online 2.2.1 General safety, health, and environmental criteria. The safety criterion should consider the risk involved in handling the solvents, attending to their flammability. According to the GHS (Globally Harmonised System), 18 integrated into the Classification, Labelling and Packaging (CLP) regulation, 27 the risk is mainly based on the flash point (FP), combined with the boiling point (BP). If a solvent has FP < 24 °C and BP < 85 °C, it has been discarded here. For example, tetrahydrofuran, with a flash point of −14 °C and a boiling point of 66 °C should not be used in a process. However, N-methyl-2-pyrroli- done, with a flash point of 96 °C and a boiling point of 202 °C complains this criterion. 28 Therefore, at this point, only the flash point and the boiling point of the solvents are considered. To be an option, the solvents should present a flash point ≥24 °C. If the solvent has a flash point <24 °C but a boiling point ≥85 °C, the solvent can still be used. However, solvents presenting a flash point <24 °C and a boiling point <85 °C will be discarded from the list of sustainable chemicals. At this point, only the solvents meeting the safety criteria will be evaluated for the health criterion. The health criterion considers the risk of exposure. In this case, relating this criterion to the exposure limits imposed by authorities would be the most appropriate. However, these limits are only established for common solvents and their values are not unified, 29–31 which would drastically reduce the applicability and scope of this, and any other proposed study. To simplify the analysis, this criterion was related to the hazard statements in the GHS/CLP system and a scoring system developed within the project CHEM21, 20 where the health score of any solvent is equal to the figure corresponding to the highest hazard according to Fig. 1. In this regard, any substance with a score above 5 or that fits with the pictogram of mortals or highly toxic, shown in Fig. 1, would be directly discarded. According to the Safety Data Sheet (SDS), the solvent NMP holds multiple health hazards such as acute toxicity (category 4 –harmful if swallowed, in contact with skin, inhaled), skin corrosion/irritation (category 2 –Causes skin irritation), eye damage/irritation (category 2A –causes eye irritation), aspiration hazard (category 2 –may be harmful if swallowed and enters airways), carcinogen (group 2B –the agent (mixture) may damage fertility or the unborn child), toxicity single exposure (category 3: respiratory tract irritation (sore throat and cough)), and toxicity repeated exposure (category 2: may cause damage to organs through prolonged or repeated exposure in organs such as the kidney, liver, spleen, and blood). These health hazards, attending to the GHS classification determines that the codes associated with them are: H315, H319, H335 and H360. Attending to Fig. 1, we will select the hazard with the highest score. In this case, it is H360, which scores 9 points in the health criteria, and that will be the qualification for the solvent, NMP in this case. In the elaboration of the decision tree, it has been considered that any solvent presenting acute toxicity (fatal or toxic) or a score higher than 5 will be discarded from the list of sustainable chemicals. The environmental impact of solvents was judged here on the basis of a life cycle analysis that pays attention to both the production and disposal methods. Actually, the source of the solvents needs an in-depth analysis because sometimes even the biodegradable solvents might come totally or partially from the petrochemical industry (e.g. methanol and n-butanol). Moreover, the boiling point also plays an important role in the environmental impact. A solvent with a low BP leads to the generation of volatile organic compounds (VOCs), while a solvent with high BP is not easily recyclable. Therefore, 85 °C–139 °C (ref. 20) has been accepted as the ideal tempera- Fig. 1 Score for the chemicals according to the health criteria. One extra point is added if BP < 85 °C. Adapted from ref. 20. Paper Green Chemistry 11986 |Green Chem.,2024,26, 11984–12007 This journal is © The Royal Society of Chemistry 2024 Open Access Article. Published on 18 October 2024. Downloaded on 2/17/2025 8:18:36 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online ture range when a solvent has to be disposed. However, the tendency in the industry is to design processes which tend to have partial or complete circularity. For instance, solvents such as DMF, DMAc, and NMP can be biologically degraded by using bacteria. 32 Therefore, although the solvents such as NMP are categorized as H402 –harmful to aquatic life, the existence of effective methodologies to degrade it will eventually allow the minimization or elimination of the possible environmental impact of this solvent. Then, even if the boiling point of the solvent exceeds the established temperature range, the possibility of using separation processes or reutilization of the solvent must be considered. This consideration leads to an urgent need of developing efficient technologies to recover the solvents. In general, the solvent recovery by distillation techniques is complex and requires high energy consumption to be carried out. Water extraction can be used instead; however, the resulting solvent/water mixture is difficult and costly to separate, leading to a very poor quality of the fuel for incineration. Membrane technology is facing this challenge by the development of processes such as organic solvent nanofiltration (OSN), 33 which is going to be a key technology in all the chemical sectors in the coming years. Biologically driven processes, by bacteria or algae, leading to the production of valuable compounds which, finally, can be integrated in the process loop, 34 can also be suggested. The development of an easily adaptable, energy-efficient and environmentally friendly technology that could recover or create value from the solvents is critical for the incorporation of green solvents into the chemical industry. Additionally, the degradation of the solvents must be considered. 35 For instance, although 36 the boiling point of a identified green solvent such as the Cyrene™, 227 °C, surpasses the proposed temperature range, it satisfies the environmental criterion because of its safe handling and since it is environmentally friendly and decompose in water and carbon dioxide. In addition, this solvent is produced from lignocellulosic biomass in a two-step solvent-free process, with more than 90% efficiency. 37 Until this part of the development of the solvent selection diagram, the concept could be applied to any chemical process where a new solvent is aimed to be introduced. In fact, several research papers have tackled similar questions 24 organizing the answer in different tools to support a consistent decision. However, the targeted chemical industry has precise features which depend on the limiting conditions of the explicit processes. In this case, the targeted procedure is the synthesis and processing of polymeric membranes applied in gas separation processes. 2.2.2 Specific criteria: polymer synthesis and membrane manufacturing. The membrane manufacturing process contains two main steps which are the polymer synthesis and the membrane formation. These two steps might occur independently of each other, yet the processing of the membranes directly from the polymer synthesis will be possible to save unnecessary steps allowing the reduction of up to 50% of the possible environmental impact of the process. 17 Polyimides (PI) are well recognized as high-performance materials and in general present good separation performance balance. Polyimides play a crucial role in various industries due to their exceptional properties and versatility. These polymers exhibit remarkable thermal stability, chemical resistance, and mechanical strength, making them a cornerstone material in advanced technological innovation across diverse sectors such as aerospace. In 2024, the yearly market size of polyimides will grow to more than 10.000 million dollar (annual growth of 2.1%) 38,39 whose largest market, by end-user industry, is electrical and electronics sector (39.8%). In this sector PIs are generally used due to their insulation power and their resistance to wear and friction. The second larger PI consumer is the aerospace industry which is also where the largest growth is expected in the coming 5 years (7.5%) especially due to their insulation and high-temperature resistance properties. It is expected that the PI market will grow up to 17.200 million dollar in the coming years (by 2031), which suppose a massive increase in the production driven by the rapidly increasing demand for lightweight, high-performance materials in the mentioned industries. With respect to the polyimide production, there are several methodologies to obtain the corresponding material including the two-step reaction, anhydride-isocyanate reaction, transimidization reaction, aromatic nucleophilic substitution, and even synthesis in aqueous media. 40 However, a vast majority of commercial polyimides are synthesized by the conventional twostep polycondensation reaction. 41 Generally, this reaction occurs between a diamine monomer and dianhydride monomer in the presence of an aprotic solvent such as N,N-di- methylformamide (DMF), N,N-dimethylacetamide (DMAC) or, N-methyl pyrrolidone (NMP) as a main solvent. Dipolar aprotic solvent is used in order to promote the nucleophilic substitution reactions (S N 2 and S N Ar). This type of aprotic solvents favor the S N 2 reactions as well as increase the reaction rate as they cannot experience a hydrogen bond with the nucleophiles as a protic solvent would do. 42 This great rate enhancement effect for S N A r reactions 43 by using dipolar aprotic solvents has been widely studied. 44 Therefore, a greener solvent that aims to substitute undesired solvents must be adequate to carry out a typical polycondensation reaction. Apart from the solubility and reactivity, the solvent characteristics such as viscosity, polarity or boiling point can also affect the final polymer characteristics such as molecular weight, glass transition temperature, or degradation stability. At this point, in order to process the material as membrane, two paths can be taken into account for the polymer: precipitation of the material; or, processing of the polymer directly as a membrane. Processing the polymer directly from the synthesis will eliminate all these steps, and it will be much more efficient from the point of view of cost, energy consumption, and environmental issues. Phase inversion is the most common method employed for the preparation of polymeric membranes. The membrane is obtained from a solution which is cast into a desired shape, i.e., a flat sheet or hollow fiber membrane, and turned into a porous solid via non-solvent-induced phase separation (NIPS), thermally Green Chemistry Paper This journal is © The Royal Society of Chemistry 2024 Green Chem.,2024,26,11984–12007 | 11987 Open Access Article. Published on 18 October 2024. Downloaded on 2/17/2025 8:18:36 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online induced phase separation (TIPS), vapor-induced phase separation (VIPS) or their combinations. 45 In order to carry out the process, the membranes are generally processed from highviscous solutions formed by the polymer, the solvent and, eventually, certain additives, which can go from a non-solvent, in order to increase the viscosity of the solution, to a pore former such as polypyrrolidone (PVP), 46–48 which is added in order to promote the formation of a porous structure. In any case, the polymer solution usually contains polymers at a concentration greater than 15 wt% (up to 25 wt% or even higher polymer contents). The great majority of the membranes are processed by the NIPS method. When the polymer cannot be solubilized under this concentration condition, the material can be processed as a thin-film composite membrane (TFCM). 49 For simplicity, environmental and economic reasons, water is usually selected as the non-solvent phase in the phase inversion process. Therefore, a suitable alternative solvent must present certain miscibility with water. The first stage of the process occurs at the interlayer formed between the polymer solution and the water, where the concentration of polymer is much higher. 50 A low solvent miscibility would hinder the solvent exchange and slow down the kinetics of the process, leading to an ineffective formation of the selective layer. Attending to it, alternative solvents should show a miscibility of, or greater than, 100 milligrams of solvent per 1 milliliter of water. Although on a commercial scale, a very large excess of water vs. solvent in the polymer solution (<0.001 wt%) is guaranteed. 3. Elaboration of the decision tree for a gas separation membrane process 3.1. Discussion on the elaboration of the tree Including all general and specific criteria discussed previously allow us to define the conditions of the new solvents to be considered as possible candidates to substitute the traditional toxic solvents employed in the synthesis of polyimide membranes for gas separation applications. In this sense, the decision tree has been organized in four different sections: (a) safety criteria; (b) health criteria; (c) environmental criteria; and (d) Polymer synthesis and membrane production criteria. (a) Safety criteria: as described before, this section will be based on the flash point and boiling point of the solvents. Solvents with a flash point ≥24 °C or/and boiling point ≥85 °C will be considered safe and analysed in the next section. The rest of the solvents will be directly dismissed. (b) Health criteria: this section is critical with any solvent labelled as acute toxicity, which will be directly rejected. Additionally, any solvent ranked with more than 5 points according to Table 1 will not continue to the next section. (c) Environmental criteria: this criterion focuses on the solvent production protocol. Only solvents produced from natural resources or waste will be considered. The disposal methodology will be analyzed as a function of the boiling point of the solvent. Solvents presenting a boiling point in the range between 85 and 139 °C will continue to the next analysis. Solvents with a boiling point >139 °C will be eliminated from the selection unless a specific recovery or treatment methodology exists. However, the continuous advances in the recovery of solvents via separation technologies as well as the progress in biological treatments of the waste allow us to guess that future development of green solvents should be conducted together with the analysis of their circularity cycle. (d) Polymer synthesis and membrane production criteria: this section is very specific to the polymeric membrane topic despite the possibility of application to any process where polyimides, or other polymers, are synthesized. Polycondensation reactions, type S N 2, as well as other types of reactions must be possible in order to obtain the required polymer. Additionally, the solvent should be able to solubilize the polymer at elevated amounts of solid contents (>15 wt%). Under these conditions, the membrane will be formed by coagulation of the mixture solvent and polymer in water. Therefore, there must be certain miscibility between the solvent and the water (>100 mg mL −1 ). For lower concentrations in the solutions (>0.5 wt%) still the creation of TFCM would be possible. Fig. 2 shows the outcome of the defined criteria in the shape of a diagram for the search of more sustainable solvents applied in a membrane formation process, as well as in the synthesis of polymers via polycondensation reactions, i.e., polyimides. Skipping the synthetic part of the process, the diagram will directly go from the environmental impact section to the minimum polymer concentration needed in a solution to carry out the phase inversion process. Adopting the established criteria will lead to the selection of the correct solvent for a specific membrane formation process. 3.2. Selection of solvents The criteria for the classification of solvents are very broad. For instance, M. Durand et al., proposed up to 10 different types of solvents attending to the solvent molecular structure. 51 Similar approach has been carried out in order to clas- Table 1 Number of solvents as a function of the last point reached after evaluation using the solvent decision diagram Paper Green Chemistry 11988 |Green Chem.,2024,26, 11984–12007 This journal is © The Royal Society of Chemistry 2024 Open Access Article. Published on 18 October 2024. Downloaded on 2/17/2025 8:18:36 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online sify green solvents by different sustainability criteria. 52 In this work, the authors define up to 7 types of green solvents, according to their nature and production methodology. An extensive screening process following the criteria established in the decision tree shown in Fig. 2 was conducted encompassing 132 solvents. The solvents analyzed are presented in Table 1, according to their nature (apolar, polar protic, and polar aprotic). Therefore, the evaluation of the suitability of green solvents was carried out for 36 apolar solvents, 31 polar protic solvents and 65 polar aprotic solvents. Table 1 shows that 73 (36 from safety and 37 from health criteria →first two light blue columns) out of 132 solvents analyzed have already been excluded after evaluation of the first two stages of the diagram. A thoughtful analysis of the production methodologies for these solvents, which is included in the third criteria of the diagram, was performed. However, this point is very controversial for the consideration of a solvent as green. Synthetic methodologies are continuously explored for the production of greener and more efficient solvents. However, the existence of a more sustainable synthetic route does not warrant that the solvent will be produced in this way. Chemical processes with relatively low and disrupted solvent necessity will benefit from green synthetic methodologies, while processes with very high solvent consumption will, most likely, still be mainly fed by solvents produced from traditional methodologies or a mixture of both, green and traditional. Fig. 2 Decision diagram for sustainable solvents for the production of gas separation membranes. Green Chemistry Paper This journal is © The Royal Society of Chemistry 2024 Green Chem.,2024,26,11984–12007 | 11989 Open Access Article. Published on 18 October 2024. Downloaded on 2/17/2025 8:18:36 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online Referring to the study, only 16 out of the 59 solvents analyzed at the third stage were excluded at this point (darker blue column). Advances in the utilization of waste to generate value as well as the creation of new sustainable synthetic routes for chemicals, for instance, from carbon dioxide, will most likely evolve to the discovery of novel procedures to synthesize these and many other chemicals. At this point, as it can be seen in Table 1, only 43 solvents out of 132 evaluated (around 32%) have met the 3 first criteria stages of the diagram. The specific criteria of the process are now introduced into the evaluation. Despite showing all the general criteria, 33 out of the 43 have been rejected due to the specific synthetic criteria (grey column). Hence, every specific feature of a particular chemical process should be considered. In the polyimide synthesis, the synthetic protocol involves a polycondensation reaction where the reaction mechanism is S N 2. For this type of reaction, the solvent must be polar aprotic. As an example, the considered greener solvent N,N′-dimethyl lactamide (DML), commercialized by BASF under the trade name Agnique® AMD 3L, is a bio-based, biodegradable, labelfree polar solvent. This solvent has been tested for the membrane formation process with various polymers. 53–55 However, DML is a protic solvent, and this type of solvent cannot be used in the synthetic process, which offers a possible solution for membrane formation but not for the entire process. Then, only 23 of those 43 solvents (grey and green columns), due to their polar aprotic nature, can actually be applied in the synthetic process. In a more detailed way, the solvent must be able to dissolve the polymer. From those solvents, only 10 are able to dissolve the polymers (green column). Therefore, 10 out of the 132 solvents analyzed, less than 8%, are susceptible to be applied in membrane applications. The list of the aprotic solvents as well as the criteria to be excluded is included in Table S1 in the ESI.†These types of solvents, as just mentioned, are the preferred type of media to carry out the polycondensation reaction. The list of analyzed non-polar and protic solvents, which, in principle, cannot be used for these reactions, can be found in the ESI.†As mentioned before in Table 1, 10 out the 65 polar aprotic solvents analyzed have initially met the required conditions to transform the membrane process into a more sustainable one. These solvents are: γ-valerolactone, cyrene™, dimethyl carbonate, N-octyl pyrrolidone, diethyl carbonate, dimethyl isosorbide, γ-butyrolactone, N-butyl pyrrolidone, methyl-5-(dimethylamino)-2-methyl-5-oxopentanoate, and dimethyl sulfoxide. Some of the physico-chemical properties of the selected (and non-selected) polar aprotic solvents analyzed in the decision tree are included in Table S2.† Some of these solvents have been already tested in the membrane field. The focus of these studies searching greener solvents have been carried out in the processing of the membranes from polymers such as cellulose acetate (CA), poly(vinylidene fluoride) (PVDF), polyethersulfone (PES), polysulfone (PSf) and polyimide (PI). 36 However, as it has been previously discussed, the option of processing the material directly from the synthesis will potentially reduce up to 50% of the environmental impact derived from the solvent utilization. Therefore, the goal is to find possible solvents which can be used for the entire membrane process, which includes the polymer synthesis and membrane formation. In this sense, the selected polymeric membrane material will define which type of solvent will be needed for the process. 3.3. Selection of the polymers As mentioned, polyimides are considered high-performance polymers with superior properties for multiple applications including their application as membranes for gas separation processes. Therefore, finding more sustainable solvents for this type of polymers will have a significant impact in multiple industrial sectors such as electronics or aerospace. The reactivity of the monomers will determine some of the properties of the final materials such as molecular weight or glass transition temperature. Therefore, in this work, three different polyimides, which have been often tested for gas separation applications, have been synthesized. The selected polyimides are: 6FDA-HAB-Ac, 6FDA-6FpDA and 6FDA-DAM (Fig. 3). These polymers present a common dianhydride structure, 6FDA and various diamine monomers with different reactivities (HAB, 6FDA and DAM). The selection is based on testing the solvents for the synthesis of polyimides presenting different reactivities. The synthetic methodology selected has been the classical twostep polycondensation reaction (as shown in the general reaction schedule in Fig. S1†). This method is well-extended, studied and applied in the main part of the industrial polymer production. In this specific case, the polycondensation reaction occurs between a dianhydride, 6FDA, and the corresponding diamine. More details about the synthetic protocol can be found in the ESI.†The dianhydride 6FDA is often selected for the design of polymers applied in the gas separation field due to the presence of –CF 3 groups which considerably increase the chain stiffness, 56 which usually leads to polymeric materials presenting a good balance between selectivity Fig. 3 Chemical structure of the monomers and selected final polyimides. Paper Green Chemistry 11990 |Green Chem.,2024,26, 11984–12007 This journal is © The Royal Society of Chemistry 2024 Open Access Article. Published on 18 October 2024. Downloaded on 2/17/2025 8:18:36 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online and permeability for several separations. 57 Fig. 3 shows the structure of the monomers and polymers synthesized in this work and the separation performances associated with the corresponding polyimide. In order to avoid any disparity on the results due to the synthetic process conditions, the synthetic protocol will be exactly the same for the materials and solvents employed in this work. The synthetic process will involve the formation of poly(amic acid) (PAA) in the first step, where the molecular weight of the polymers is built. The formation of this PAA involves the nucleophilic attack of the amino group on one of the carboxyl groups of the anhydride, causing the opening of the anhydride ring, by means of an acid-based reaction. Several factors might affect the PAA formation such as the chemical solubility, polymer concentration, temperature, and time, among the monomer’s reactivity and purity. At this point, the presence of impurities, reactive gases or even water might drastically affect the outcome of the reaction. The second step of the process is the transformation of PAA into the final polyimide (PI). This process can be carried out by three different methodologies: azeotropic imidization, thermal imidization, or chemical imidization. The first two processes depend on the chemical structure of the polymers as well as the solubility of the polymer, for instance, in the mixture between the solvent and the azeotropic solvent employed. The thermal imidization often leads to crosslinked structures due to the presence of secondary reactions between different polymer chains, which finally affects the solubility of the polymers and adds complexity to the structural characterization of the materials. The last one, the chemical imidization, has been selected for this study. This methodology is more standardized and is more independent of the structure and is more reproducible. This selected procedure for the second step of the reaction, the chemical imidization of PAA, involves closing the ring, while water is released, in order to obtain the corresponding polyimides (PIs). 3.4. Solubility parameter evaluation The polymer synthesis in solution as well as the membrane formation process involves, minimum, a binary polymer– solvent system. In polymer chemistry, predicting and understanding the polymer–solvent phase equilibrium in binary systems is quite important. The thermodynamic properties of polymeric solutions are generally expressed in terms of a polymer–solvent interaction parameter called the Flory– Huggins χparameter. 58 This χparameter depends on several factors such as polymer molecular weight, reaction temperature, or volume fractions. The experimental characterization of such parameters still present certain technical complexity. Computational modelling is simplifying the process by applying empirical models, which are basically based on the distance between the polymer and the solvent solubility parameters. 59,60 One of the most often employed is the Hansen solubility parameter (HSP) δ T . δT¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðδ2 dþδ2 pþδ2 hÞ qð1Þ Ra¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 4ðδd2 δd1Þ2þðδp2 δp1Þ2þðδh2 δh1Þ2 qð2Þ RelativeEnergy DifferenceðREDÞ¼Ra R0ð3Þ δ T is the total energy of vaporization of components considering the different molecular interactions arising from: δ d , dispersive forces; δ p , polar (dipole–dipole) forces; and δ h , hydrogen bonding, which can represent the x,y,zcoordinates in a Hansen sphere model. 61 The potential solubility factor of a target solvent is given based on the distance, R a , between the HSP vectors of the polymer and the solvent molecule. In general, any chemical substance or polymer will be soluble in a given solvent when they have similar energies of vaporization. The parameter R a can be calculated, according to eqn (2), by the distance between the polymer (or substance) 1 and a specific solvent 2, which is given by their respective solubility parameter components (δ d ,δ p , and δ h ). In this case, the solubility sphere is defined as the zone of solvent–polymer solubilization, where the radius of this sphere is defined as “interaction radius”R 0 . The R 0 value is based on experimental information resulting from the observation of the interaction between similar selected polymers and known solvents. Combining R a and R 0 , the relative energy difference (RED) is obtained as expressed in eqn (3). In this sense, when the value of RED is ≤1, the polymer would be solubilized in the selected solvent. For RED > 1, the materials will be considered as nonsoluble. Therefore, an efficient determination of the solubility parameters is crucial to have an optimal decision on the ideal solvent for a specific process. Empirical methods such as the group contribution methods have been extensively studied and widely used for the calculations of the molecular interactions. 62 Nevertheless, different methodologies including models such as UNIFAC-FV and entropic-FV, and advanced statistical-associating fluid theory (SAFT) have been developed in the last years. 63 The predictive capacity has evolved, especially by the contribution of Klamt and collaborators, by developing the COSMO-RS method. 64,65 This method is based on rapid calculations by quantum chemistry, which can be applied to a diverse variety of polymer–solvent systems. 66 However, these atomistic simulations are still facing some challenges in the topic, 67 leading to a very large variability in reported results (as it is shown in Table S3†for the solvent DMSO). In recent years, high speed predictions can be achieved by employing the machine learning approach, which is fed by the growing free accessible data and using continuously improving computational resources. In any case, experimental polymer–solvent interaction parameters are always required in order to train the model. In general, the studied techniques proposed to obtain experimental χparameter values, such as vapor pressure, osmotic pressure, or inverse chromatography, are still presenting some limitations besides being technically difficult and costly. 63 Additionally, the determination of the solubility parameter of polymers is complex because most polymers, like most polyimides or other high-performance polymers, cannot be vapor- Green Chemistry Paper This journal is © The Royal Society of Chemistry 2024 Green Chem.,2024,26, 11984–12007 | 11991 Open Access Article. Published on 18 October 2024. Downloaded on 2/17/2025 8:18:36 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online ized without decomposition. The HSP for polymers are generally calculated simply and conveniently by using typical contribution methods. This makes it difficult to create highly generalizable predictive models of χparameters that can be applied to a wide variety of polymeric systems. In any case, it can be guessed that the continuous evolution in the topic as well as the interactions between international researchers all over the globe, such as the massive collaborative effort allowing the continuous development of the Q-Chem quantum chemistry program, 68 will contribute to the refining of the predictive tools. It is clear that, although concepts and criteria are clear, still some variability can be found. Then, more uniform criteria, in particular development of really reliable methodology to determine experimentally and computationally the solubility parameters associated with a solvent or substance, are to be developed. Table 2 shows all the solvents that have met all the criteria as well as their solubility parameters. The list of solvents was ordered according to their total solubility parameter δ T calculated according to eqn (1). Two additional solvents have been added to the list: NMP, which will be used here as reference; and 3-methoxy-N,N-dimethylpropanamide (commercially known as KJCMPA®-100), which, at the beginning if this study, was considered as a sustainable solvent particularly based on the synthetic path. KJCMPA®-100 is a biodegradable linear amphiphilic amide solvent employed at electronic component degreasing and inkjet printing. 69 However, the solvent has been recently qualified, among other hazard statements, as suspected of damaging fertility or the unborn child. In any case, and having this in mind, we will include the obtained analysis of the use of this solvent in the synthesis of the different polymers. Besides, 5 solvents from the list have been selected to carry out this study based on their availability, type of solvent, and δ T values. Based on this, the solvents selected are as follows: γ-Valerolactone (GVL), Cyrene™(Cy), Dimethyl Carbonate (DMC), Dimethyl Isosorbide (DMI), and Dimethyl Sulfoxide (DMSO). This selection involves somehow the rest of the structures, as it will be discussed in section 4. Table 2 also includes the current cost of these solvents. The most usual solvents such as NMP or DMSO will generally present the lower cost while less traditional or novel solvents such as Cyrene™ or N-butyl pyrrolidone show more elevated costs. Solvents such as carbonate-based solvents generally present very moderate prices favored by the production simplicity and cost. It is important to highlight that the cost shown in Table 2 is for low amount of solvents. Ordering higher solvent amounts will rapidly drop the cost. For instance, NMP shows a cost of around 15 €per kg on a kilogram scale, however ordering the solvent on a ton scale will drop to cost to 1–2€per kg. In any case, the cost will be determined by the supply and demand law. Great industrial demand of a chemical will favor the search for more efficient and cost-effective synthetic routes. Therefore, the initial cost of a novel solvent will always be much higher than the cost of traditional solvents. Nevertheless, this fact should not be an initial stopper for its implementation in a process since a radical reduction in the cost can be expected. Similarly, as it was discussed for solvents, in polymers, the Hansen Solubility Parameters can also be estimated. As mentioned above, R a , defined in eqn (2), determines the distance between the HSP vectors of the solvent and the molecule. Then, the polymer will be soluble when similar energy of vaporization is shown. In other words, when the value of R a is equal to or less than 1. For R a > 1, the materials will be considered as non-soluble. Therefore, the appropriate HSP should be calculated for solvents and polymers. However, the precision of the method depends on the number of different solvents tested. 70 In general, a limited number of HSPs are accessible for polymers such as PVDF, 71 polycarbonate, 72 poly (ether sulfone), 73 or PMMA. 74 However, the HSP of the polymers will differ depending on the chemical nature of the structure. The list of HSP for polyimides, reported in literature, can be found in Table 3. The polymers are divided into 2 groups: (a) 6FDA-based polyimides and (b) other polyimides. Table 2 List of the 10 solvents meeting all the criteria to be employed in a membrane process as well as 2 more solvents added as reference for this study. The solvents selected to carry out the experimental part of this work are highlighted in bold. The cost for kilogram of high-purity solvent (> 99%) is also included Solvent δ d (Mpa 1/2 )δ p (Mpa 1/2 )δ h (Mpa 1/2 )δ T (Mpa 1/2 ) Cost €per kg γ-Valerolactone 15.5 4.7 6.6 17.5 40–50 Cyrene™17.2 4.5 5.4 18.6 320 Dimethyl carbonate 15.5 3.9 9.7 18.7 24–28 N-Octyl pyrrolidone 17.3 5.2 5.7 18.9 70–85 Diethyl carbonate 17.6 5.6 7.7 20 30–40 Dimethyl isosorbide 16.8 8.2 8.1 20.4 48–80 γ-Butyrolactone 17.7 7 8.1 20.7 58 N-Butyl pyrrolidone 17.5 9.9 5.8 20.9 420–540 c Methyl-5-(dimethylamino)-2-methyl-5-oxopentanoate a 15.8 10.7 9.2 21.2 — c,d Dimethyl sulfoxide 18.4 10.1 11.1 23.7 24–30 3-Methoxy-N,N-dimethylpropanamide b 17.2 10.9 9.5 22.5 120–140 c N-Methyl pyrrolidone 18 12.3 7.2 23 15–25 a Rhodiasolv® PolarClean. b KJCMPA®-100. c Purity between 95–98%. d Price not provided by the producer. Paper Green Chemistry 11992 |Green Chem.,2024,26, 11984–12007 This journal is © The Royal Society of Chemistry 2024 Open Access Article. Published on 18 October 2024. Downloaded on 2/17/2025 8:18:36 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online especially as a source of –Me, –SMe, or –SO 2 Me. 126–129 DMSO is normally recognised for the smell which makes the work with it in big quantities unkind. Arkema and other companies developed a new grade of DMSO renamed as DMSO EVOL™ which basically present the same characteristics and performances with a more pleasant fruit-like odor. 130 One isotopologue of dimethyl sulfoxide, concretely DMSO-d 6 , produced by heating DMSO in heavy water (D 2 O) in the presence of a base, is one of the most common solvents used in NMR spectroscopy. Following the same criteria as those for the previous solvents, and as it is shown in Fig. 10, the substitution of the methyl groups by other alkyl functionalities will lead to symmetrical and unsymmetrical dialkylsulfoxides. 131 Then, the structure design might lead to symmetrical derivatives such as diethyl sulfoxide (DESO), used as preservatory of cells, tissues and organs; 132 or unsymmetrical derivatives such as the methyl phenyl sulfoxide (PMSO) which shows special ability to differentiate oxidizing species like, for instance, the characterization of specific iron oxides [Fe IV O] 2+ . 133 Derivatives such as the sulforaphane can be found naturally in vegetables such as broccoli, cabbage or Brussels sprouts. 3-Methoxy-N,N-dimethylpropanamide, commercialized under the trade name KJCMPA™-100®, can be synthesized from the combination of levulinic acid (LA) and dimethylamine. The LA can be extracted from a very large variety of agricultural waste (lignocellulosic biomass) such as cotton, sugar cane bagasse, Cicer arietinum, rice straw, corn stover, sweet sorghum bagasse and miscanthus; marine and fresh Fig. 8 Dimethyl carbonate and some other examples of aprotic dialkyl carbonate derivatives. Fig. 9 Dimethyl isosorbide, its epimers and some examples of derivatives of isosorbide. Green Chemistry Paper This journal is © The Royal Society of Chemistry 2024 Green Chem.,2024,26, 11984–12007 | 11999 Open Access Article. Published on 18 October 2024. Downloaded on 2/17/2025 8:18:36 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online water algae; and bacterial sources like cyanobacteria. 134 Until few months ago, this solvent was considered as a very promising green alternative to conventional polar aprotic solvents. In fact, at the beginning of this work, even at the beginning of this year, the solvent was identified as a safe solvent, 69,135 compared to traditional ones. In the last months, the solvent has been classified with the hazard statements H319, H361, and H373. The statement H361, according to the health criteria, obtains a score of 6 (higher than 5), which discards it from the list of sustainable chemicals. Similarly to previous examples, the modification of the structure of LA in, for instance, length, type of substituents or configuration would lead to different derivatives. The extension of the chain to 3-butoxy-N,N-di- methylpropanamide, also known under the trade name KJCBPA™-100, leads to a lower toxicity where the GHS Hazard Statements for this compound are H302 and H319. Attending to similar statement, the structure of the molecule has been functionalized and some examples are shown in Fig. 11. One of these derivatives is the commercially available Solvay known as Rhodiasolv® PolarClean, for which the chemical structure is methyl-5-(dimethylamino)-2-methyl-5-oxopentanoate 136 (as shown in Fig. 11). PolarClean is a solvent produced from 2-methyleglutaronitrile. 137 This solvent is also placed in the list of solvents meeting all the criteria to be applied in the membrane industry. Then, it is clear that there is a need to understand which type of chemical structure will lead to undesirable features trying to avoid their use. N-Methyl-2-pyrrolidone (NMP) is probably one of the most employed polar aprotic solvents in the polymer and membrane field. The solvent structure entails a 5-membered ring lactam. The solvent can be obtained from multiple sources including the reaction between the γ-lactone, which, as mentioned above, can be produced from various bio-based chemicals, and the methylamine. It can be considered that sustainable methods to produce it have been industrially developed. In this way, more than 150 000 tons of NMP are produced every year. This solvent has a list of hazard statements such as H315 –causes skin irritation, H319 –serious eye irritation, H335 – specific target organ toxicity (single exposure) may cause respiratory irritation, and H360D –reproductive toxicity may damage the unborn child, which greatly limit its use. One consequence is that the NMP shall not be placed on the market as a material on its own or in mixtures in a concentration equal to or greater than 0.3 wt%. 138 Furthermore, the specific properties of the NMP, such as excellent solubility and coalescence capacity, low volatility, and outstanding miscibility with Fig. 11 3-Methoxy-N,N-dimethylpropanamide and some commercially available derivatives. Fig. 10 Dimethyl sulfoxide and some commercially available derivatives. Paper Green Chemistry 12000 |Green Chem.,2024,26, 11984–12007 This journal is © The Royal Society of Chemistry 2024 Open Access Article. Published on 18 October 2024. Downloaded on 2/17/2025 8:18:36 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online numerous solvents, including water, make use of this solvent critical in multiple industries including paints, batteries, coatings, wiring, polymers or membranes. 139 Of course, multiple efforts have been made to minimize the hazardous character of this chemical. The strategy is similar to the previous solvents, by using structurally homologues substances with different lengths of carbon side chains. For instance, one of the first industrially offered replacements was N-butyl-2-pyrro- lidone (NBP), commercially known as TamiSolve™NxG, which contains very similar physical properties 140,141 to those of the NMP, showing good results in the initial toxicity analysis. As expected, the solvent has been tested for the critical applications mentioned before including membrane formation 142–144 or polyimide synthesis. 75 Paradoxically, derivatives with shorter, the N-ethyl-2-pyrrolidone (NEP), and longer side chains, the N-octyl-2-pyrrolidone (NOP), have been classified with the statements H360D –reproductive toxicity may damage the unborn child and H314 –causes severe skin burns and eye damage, respectively, which, according to Fig. 1, is enough to eliminate them from the selection list. The same situation occurs for bicycle-type compounds such as 1-cyclo- hexyl-2-pyrrolidone, which is labelled as H314, while 1-benzyl- 2-pyrrolidinone is labelled as a safer chemical. Similar to the lactone-based derivatives, cycles with different ring members can be formed with the same functionality (as shown in Fig. 12). Interestingly enough, changing the number of ring members results in very diverse hazard statements. For more than 5-membered ring structures, the compounds show hazards which would pass the analysis of the decision tree. Other emerging polar aprotic solvents derived or extracted from natural sources will slowly start gaining interest in the field as soon as their production methodologies gain reliability. Some examples to mention might be p-cymene or limonene. Both compounds are derivatives from natural sources. However, they both present immiscibility with water, which limits their application at the last step of the membrane formation process. Triethyl phosphate (TEP) might be an efficient alternative for traditional solvents. TEP was obtained by the combination between ethanol and phosphoric acid. This solvent has been already tested in the membrane formation process, especially in the case of the PVDF polymer, with positive results. 145,146 Sulfolane has been identified as a sustainable reaction medium replacing other solvents, particularly in reactions occurring at very high temperatures due to its excellent high boiling point (285 °C). 147 However, the solvent has been recently labelled as H360 (which scores the maximum in Fig. 1). Identifying other families of solvents, probably ionic liquids (ILs) and deep eutectic solvents (DESs) 148–150 are presenting sufficient versatility and tuned capabilities to be, at least, tested for the entire membrane process. Both types of compounds have been heavily tested for a large variety of chemical reactions as well as common characteristics such as non-volatility, non-flammability, or high viscosity. ILs present an almost unlimited number of combinations increasing their synthetic flexibility and tunability for a specific reaction. 151 Aspects such as biodegradability or aquatic toxicity remain open in the IL topic. 152 The use of nontoxic and naturally derived precursors would minimize those risks. In the case of DESs, the preparation is simpler than ILs, presenting lower toxicity and cost. However, the testing on a polymer synthesis process, especially high-performance polymers such as polyimides, is still limited 153 for both types of compounds. Alternative solutions such as the use of supercritical or subcritical fluids are often employed in extraction processes especially involving food products. The chemical characteristics of a solvent might radically change under super- or subcritical conditions. The supercritical CO 2 has been employed for the synthesis as well as modification of aromatic polyimides. 154,155 Supercritical water has been widely used for the synthesis of polyimides and other polymers. 156,157 An innovative use of supercritical CO 2 has been used for the formation of very thin membranes. 5 The formation of porous substrate and selective layer at once will be, no doubt, in the coming years very employed since it avoids the use of solvent in the last part of the membrane formation process. Redefinition of Fig. 12 NMP and other derivatives. Green Chemistry Paper This journal is © The Royal Society of Chemistry 2024 Green Chem.,2024,26, 11984–12007 | 12001 Open Access Article. Published on 18 October 2024. Downloaded on 2/17/2025 8:18:36 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online the technical and engineering parts of the entire process has to be carried out in order to be able to covert the synthesis and processing of the materials from the same supercritical process. As it can be seen from the discussion in this section, here we describe more than 60 alternative polar aprotic solvents, selected solvents and derivatives from those selected solvents, apart from other emerging solvents and alternative solutions, which have the potential of substitute the traditional solvents. This list can be extended by following the strategies describes for the formation of derivatives such as changing the length of carbon side chain, inclusion of functionalities or variation in the member rings in cycles. Research evolution has proven that the processes can be tuned until a sustainable synthetic protocol is found. In the same way, the cost of the solvents is not discussed here since the offer and demand law will finally push the price of any chemical significantly down if the substance is needed on an industrial scale. However, the possibility and design for the scaling up of the processes should be taken into account immediately after the concept has been proven. 5. Conclusions While the research criteria on the selection of greener solvent media are generally focused on the safety, health and environmental impacts of the solvents, the specific criteria for the process must be introduced into the discussion. In this work, the specific case of the synthesis of polyimides for their processing as a gas separation membrane has been developed. A simple decision diagram has been developed attending to the general as well as specific criteria of the polymer synthesis and membrane formation processes. As a result of the analysis of more than 130 solvents, ten commercially available greener solvents have been identified: γ-valerolactone, cyrene, dimethyl carbonate, N-octyl pyrrolidone, diethyl carbonate, dimethyl isosorbide, γ-butyrolactone, N-butyl pyrrolidone, methyl-5-(dimethylamino)-2-methyl-5-oxopentanoate, and dimethyl sulfoxide. Five of them selected and tested for the synthesis of polyimides present differences in their reactivity. Other two solvents were tested as reference: N-methyl pyrrolidone (NMP) and 3-methoxy-N,N-dimethylpropanamide (known by the trade name KJCMPA®-100). The synthetic protocol for the three polyimides was optimized by using NMP. The solvent was just replaced by the alternative solvents. In all the cases, the polycondensation reaction was possible, independent of the solvent and polymer, and the final polymer structure was identified by 1 H-NMR spectroscopy. By using the defined synthetic protocol, the solvents cyrene and dimethyl carbonate presented some encounters, indicating that the reaction protocol should be adapted to these solvents. In general, for the three polymers, the highest molecular weight was consistently obtained when the synthesis was performed in GVL, with the molecular weight values in the following order: GVL > NMP > KJCMPA > DMI > DMSO > Cyrene > DMC. Therefore, for the selected polyimides, the GVL could directly substitute NMP in a synthetic process. This work has also identified more than 60 possible alternative solvents following the chemical structure of the selected solvents obtained through the decision diagram as well as other emerging solvents. This list can be extended by simple strategies such as changing the length of carbon side chain, inclusion of functionalities or variation in the member rings in cycles. 6. Future directions and key action points 1. Development of solvents derived from green platforms. Ensuring a continuous and stable source of starting biobased molecules 158 will lead to a large number of possible novel solvents. An existing list 158,159 of the 10 key future compounds obtained from biorefineries includes: ethanol, furans, glycerol and derivatives, bio-hydrocarbons, lactic acid, succinic acid, hydroxy propionic acid/aldehyde, levulinic acid, D-sorbitol and xylitol. In our opinion, many other such as compounds obtained from the exploitation of the urban and industrial waste treatments will join this and other lists. 160,161 2. Development of simple, well-documented, reproducible and scalable synthetic protocols for the novel solvents. Rather than the solvent cost, which, at the end, will be dominated by the supply and demand law, ensuring a fast and continuous accessibility to the solvents will determine their implementation at the industrial level. 3. Development of simple methodologies for the recycling and reutilization of solvents. In this point, the membrane technology, as well as other separation technologies, will be fundamental to reach a high recovery degree of the solvents. Several membranes are already commercialized for the organic solvent nanofiltration. 82 However, significantly increasing the effort to improve the performance and knowledge of separations involving solvents and water will be very important for the reutilization of solvents in the membrane production process. In this way, the solvent can be integrated again into the synthetic process for a number of cycles, which will depend on the degree of recovery and purity (separation performances and process design) obtained from the separation process. 4. Development of simple tools for the determination of a preliminary idea of the environmental impact of the solvents such as carbon foot print. Several solvents have already been presented as alternative solvents and many more will appear in the coming years. However, sometimes researchers and chemical companies should work to avoid the temptation to use the absence of environmental and health information to keep the solvents ahead of legislation. 5. Biodegradability studies of solvents and solvent mixtures and their optimization to obtain valuable products (as the one expressed in the first point). When reutilization is not poss- Paper Green Chemistry 12002 |Green Chem.,2024,26, 11984–12007 This journal is © The Royal Society of Chemistry 2024 Open Access Article. Published on 18 October 2024. Downloaded on 2/17/2025 8:18:36 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online ible, the effective methodology for obtaining valuable compounds that can be integrated in a cycle will minimize the generation of wastes. 6. Study of the biodegradability of the polymeric materials will promote an entire circularity. The use of easily degradable materials will simply not be logic in certain applications where the materials must stand very aggressive media 162 in terms of contaminants, pH, or temperatures. However, in certain applications, the separation can occur under relatively mild conditions where a very wide range of materials can be employed and the biodegradability of the membranes, after use, will be highly beneficial. 7. More experiences should be carried out covering the entire process in order to show to the industry the real outcomes and benefits of the approach. In the membrane technology, this would mean to explore the connection of synthesis with the processing of the material as well as testing the performances. 8. Machine learning and artificial intelligence for the learning of production, reutilisation and biodegradability of the novel green-based solvents. The anticipation of the actions will allow us to have a better perspective of the opportunities offered by the use of certain chemicals and compounds. Data availability The data supporting this article have been included as part of the ESI.† Conflicts of interest There are no conflicts to declare. Acknowledgements This research was funded by the Project TED2021-131170A-I00 financed by MCIN/AEI/10.13039/501100011033 and the European Union Next Generation EU/PRTR. We are also grateful to the ECLOSION project MIG-202111071 (Mission2, Spanish “Plan Estatal I + D + I”), funded by Next Generation EU. Authors are thankful to the Spanish Research Agency for the funding of the project PID2023-149594OB. We are grateful as well to the Regional Government of Castilla y León and the EU-FEDER initiative for their programs CL-EI-2021-07 and UIC082. References 1 P. R. C. 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