Time-resolved Operando DRIFTS-MS Study of the Moisture Tolerance of Small-pore SAPO-34 Molecular Sieves During CH4/CO2 Separation

Time-resolved operando DRIFTS study of the moisture tolerance of small-pore SAPO-34 molecular sieves during CH4/CO2 separation Marta Romero, Juan C. Navarro, Luis F. Bobadilla*, María I. Domínguez, Svetlana Ivanova, Francisca Romero-Sarria, Miguel A. Centeno and José A. Odriozola Instituto de Ciencia de Materiales de Sevilla – Dpto. Química Inorgánica, Centro mixto CSIC – Universidad de Sevilla, Av. Américo Vespucio 49, 41092 Sevilla (Spain) (*) Corresponding autor: [email protected] (Luis F. Bobadilla) Abstract This study offers reliable information for understanding the effect of the presence of moisture during CO2/CH4 separation on small-pore SAPO-34 molecular sieves. Two SAPO-34 samples with different physicochemical properties (chemical composition, crystal size and textural properties) were prepared by hydrothermal synthesis using a unique template or mixing-templates, respectively. By means of a transient operando DRIFTS study switching adsorption/desorption cycles, it was proven that the presence of larger Si islands in the 8-membered ring cavities increases the hydrophobic character and enhances the moisture tolerance of the SAPO-34. This knowledge is fundamental for achieving the rational design of SAPO-34 membranes more efficient under realistic conditions. Keywords: SAPO-34; CO2/CH4 separation; moisture tolerance; operando DRIFTS 1. Introduction Climate change has been an active topic for both scientific and political debate during the last decades. International Panel on Climate Change (IPCC) predicts that, by the year 2100, the concentration of CO2 in the atmosphere can achieve a value of 570 ppmv provoking an increase of the mean global temperature of 2 ºC and elevating the sea level above 38 m [1]. Currently, the perception of public opinion on global warming is becoming more generalized and exist a genuine concern in the society to reduce the CO2 emissions [2, 3]. The utilization of technologies based on the transformation of residual biomass into energy such as the production of biogas is believed to be one of the best options to satisfy the high global energy demand and mitigate the CO2 emissions [4]. Biogas is a promising renewable resource which is mostly comprised of methane (60%) and carbon dioxide (35–40%), and can be produced from a process known as biomass anaerobic digestion [5]. Biogas upgrading is required to separate the CO2 from CH4 and to obtain an enriched biomethane suitable for energetic uses as substitute of the natural gas. In this regard, several technologies such as absorption [6, 7] , cryogenic distillation [8, 9], membrane separation [10, 11] , and adsorption [12, 13] are commercially available for biogas upgrading. The separation of CO2 without large energetic costs is highly desirable, and thus the utilization of membranes that preferentially permeate CO2 at high selectivities can significantly reduce the costs of biogas purification [14]. In comparison to conventional polymeric membranes, materials based on microporous inorganic materials such a zeolites are preferably used for CO2/CH4 separation due to their superior thermal, mechanical and chemical strength as well as high pressure stability [15]. Zeolites are aluminosillicate crystalline solids with uniform-sized pores below 1 nm. Numerous studies have demonstrated the potential of zeolite-based materials as membranes for gas permeation and separation [16-18]. The zeolite membranes typically prepared and used for gas separation can be distinguished into three categories: largepore (Y-type, X-type, MOR, and beta), medium-pore (ZSM-5, ZSM-11, and FER), and small-pore (SAPO-34, A-type, SSZ-13, ALPO-18, and SSZ-13) [15, 19]. Meanwhile, the small-pore zeolites are typically preferred for separating selectively mixtures of CO2 and methane due to a combination of differences in molecular diffusivity and competitive adsorption [20, 21]. SAPO-34 membranes have been potentially used for CO2/CH4 mixtures separation [21-27]. The SAPO-34 is a crystalline microporous silicoaluminophosphate with chabazite (CHA) framework structure which possesses a three dimensional system of 8-membered rings pores of 0.38 nm diameter [28]. This diameter is essentially the same as the kinetic diameter of methane (0.38 nm) but higher than CO2 (0.33 nm), thereby CO2 can diffuse towards the channels and adsorb on the pores. From the viewpoint of industrial applications, the presence of impurities in the stream can negatively affect the performance of SAPO-34 membranes in the separation process. This effect is especially notable in presence of water, which even in very small quantities is strongly adsorbed in SAPO-34 and supress drastically the CO2 uptake. Some studies have explored the effect of the presence of water in the CO2/CH4 separation. Poshusta et al. [29] found that the presence of water almost completely blocks the pores of the zeolite inhibiting the entrance of the other gases. In other work, Li et al. [30] studied the effect of a mixture of impurities composed by water, nitrogen, propane, butane, and ethylene on the CO2/CH4 separation over a SAPO-34 membrane supported on stainless steel. They found that after exposure to water and hydrocarbons in the feed, the permeance and selectivity of SAPO-34 membrane were both decreased although reversibly restored by thermal treatment. It is well known that by varying Si/Al ratio of zeolites is possible to fine-tune the hydrophobic/hydrophilic nature of the zeolite [31]. High-silica zeolites are typically more hydrophobic and resistant to the presence of moisture. Furthermore, zeolites with high silicon content are found to be less defective and thus the gases transport inhibition associated to structural defects are reduced. Recently, Lee et al. [32] investigated the role of hydrophobicity and defect structure on rich-siliceous chabazite-type zeolites membranes and they found that the CH4/CO2 separation factor is enhanced in presence of water vapour due to the defects are blocked by physisorbed water molecules. Although several works have been reported analysing the effect of the presence of moisture and other impurities on CH4/CO2 separation over SAPO-34 membranes, up to now limited studies have been focused on investigating simultaneously the surface properties of SAPO-34 and its functionalities under operation conditions. Wuttke et al. [33] reported for the first time an infrared spectroscopic characterisation study during a separation process by using the operando methodology. The Latin term operando is referred to the spectroscopic characterization of the material when is “working” and simultaneously the performance is analysed. Hence, operando measurements allow to establish directly a structure-performance relationship and can guide the rational design of more efficient materials. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) is a very sensitive technique to get insight into the surface species involved in a chemical process under working conditions. This report concerns the use of DRIFTS for monitoring the dynamic surface structure of SAPO-34 during CO2/CH4 separation and how can be affected the surface by the presence of trace amounts of moisture. With the aim of understanding the origin of moisture tolerance, two SAPO-34 molecular sieves with different crystal size and Si/Al molar ratio were investigated. 2. Experimental section 2.1. Chemicals Aluminum isopropoxide (Sigma-Aldrich, 98%), colloidal silica (Ludox AS-40, 40 wt.%, Sigma-Aldrich) and orthophosphoric acid (Sigma-Aldrich, 85%) were used as precursors of the framework elements. The tetraethyl ammonium hydroxide (TEAOH, Sigma- Aldrich, 35 wt.% aqueous solution) and dipropylamine (DPA, Sigma-Aldrich, 99%) were used as structure-directing agent for SAPO-34 synthesis. All chemicals purchased were used as received without any further purification. 2.2. Preparation of SAPO-34 molecular sieves Two SAPO-34 molecular sieves denoted as S1 and S2, respectively, were prepared via hydrothermal synthesis using two different organic templates at 200 ºC as described Bai et al. [34]. The template agent used for S1 sample was TEAOH whereas a combination of TEAOH and DPA was used as mixed templates for S2 sample. The seed gel molar composition for each sample is given in Table 1. In a typical synthesis procedure, S1 sample was prepared using TEAOH as template agent. First, a seed precursor solution was prepared by stirring a mixture of aluminum isoproxide, TEAOH, and deionized water for 2 h to achieve a homogenous solution. Then a colloidal silica suspension was added and the resulting solution was kept in agitation for another 2 h. Finally, an aqueous solution of ortophosphoric acid was added slowly and the solution was stirred for 3 days at room temperature to form a gel with a molar composition of 1.0 Al2O3 : 2.0 P2O5: 0.6 SiO2:4.0 TEAOH : 75 H2O. This gel was poured in a Teflon autoclave and the hydrothermal treatment was performed at 200 ºC for 24 h. The obtained seeds crystals were recovered by centrifugation, washed three times with deionized water, dried overnight, and then calcined in air at 550 ºC for 6 h with a heating rate of 1 ºC min-1 for removing the template. The synthesis of S2 sample was carried out using a combination of two templates (TEAOH and DPA) as structure-directing agent. The gel molar composition of this SAPO- 34 was 1.0 Al2O3 : 1.0 P2O5 : 0.3 SiO2 : 1.0 TEAOH : 1.6 DPA : 77 H2O. In this procedure, an aqueous solution of ortophosphoric acid and aluminium isopropoxide was initially kept in agitation for 3 h. Afterwards, Ludox colloidal silica was added and stirred for other 3 h. Then, TEAOH (35 wt% aqueous solution) and DPA were added and the resulting solution was stirred for six days at room temperature. From here, the procedure used to obtain the S2 sample was identical to that used for S1 sample. 2.3. Materials characterization The chemical composition was analysed by X-ray fluorescence (XRF) spectrometry on an AXIOS PANalytical spectrometer with Rh tube as source of radiation. The measurements were carried out onto pressed pellets (sample including 6 wt.% of wax). The textural properties of the samples were determined by nitrogen adsorptiondesorption measurements at 77 K using a Micromeritics Tristar II instrument. Prior to each measurement, the zeolites were degassed at 400 °C for 2 h in vacuum. The specific surface area (SBET), micropore surface area (Smicro) and micropore pore volume (Vmicro) for each sample were estimated by the BET (Brunauer–Emmett–Teller) equation and tplot method, respectively. X-ray powder diffraction (XRD) patterns were recorded on an X’Pert Pro Powder Diffractometer by PANalytical employing Cu-Kα radiation (λ = 0.154 nm) at 30 mA and 40 kV. Data were collected in the range of 5° ≤ 2θ ≤ 40° with a step size of 0.05° every 240 s. The obtained patterns were identified using as reference the JCPDS (Joint Committee of Powder Diffraction Standards) index. The surface morphology of the materials was analysed by scanning electron microscopy (SEM) on a JEOL 5400 microscope operating at 20 kV. The samples were previously coated with a thin layer of gold to prevent charging problems. CO2 adsorption isotherms were tested at 25 ºC on an ASAP 2420 model instrument from Micromeritics. Prior to adsorption isotherm measurement, the samples were pretreated at 450 ºC for 2 h in vacuum for removing the adsorbed species. 2.4. Operando DRIFTS experiments Operando DRIFTS measurements were performed in a high-temperature environmental reaction chamber supported in a Praying Mantis (Harrick) optical system with ZnSe windows. The spectra were recorded with an average of 64 scans per spectrum at 4 cm-1 of resolution in a Thermo Nicolet iS50 FTIR spectrometer equipped with a liquid nitrogen cooled MCT detector. Prior to the measurements, the sample was in situ pretreated at 550 ºC during 1 h under a flow rate of 100 mL min-1 of argon for removing the adsorbed water. For the time-resolved experiments of CO2/CH4 separation, a stream of 100 mL min-1 of 3.6% CO2 and 5.4% CH4 balanced in Ar (molar ratio of CH4/CO2 = 1.5) and a flow of 100 mL min-1 of Ar both including traces of water were switched with a 4-way valve and introduced into the IR cell every 300 s under isothermal conditions at 30 ºC. A background spectrum was collected without sample using an aluminium mirror, and all spectra of the adsorbed species on the solid surface were obtained by subtracting the spectrum of the sample after pretreatment. 3. Results and discussion 3.1. Chemical composition, structural analysis and morphology The molar composition of the starting synthesis gel used to prepare both samples and the final crystal composition determined by XRF on both SAPO-34 powders are listed in Table 1. As can be observed, the Si/Al ratios of the crystal powders obtained are slightly higher than the initial gel mixture in each case. This indicates that the molar percent of silicon in both final SAPO-34 was higher than that of the starting gel and consequently Si is incorporated into the framework by substitution of Al and/or P atoms. It has been established that the incorporation of Si into the AlPO4 framework can proceeds by two different substitution mechanisms [35, 36]: (i) the substitution of phosphorous atoms by atoms of silicon to form Si(4Al) entities which are known as “isolated Si”, and (ii) the double substitution of neighbouring Al and P by two silicon atoms to Si(nAl) with n = 0 – 3 entities designated as “Si islands”. The two substitution mechanisms are influenced by the synthesis conditions and the simultaneous occurrence of both mechanisms can also takes place resulting the presence of different silicon environments in which Si(nAl) structures coexist at the border of silicon islands [37]. Sastre et al. [36] used the topological density concept (TDC), which is related to the number of isolated silicon atoms that can be found in a SAPO-34, to determine the maximum Si content at which all the silicon atoms can be present into the framework as isolated Si entities. Accordingly, this value is given by the ratio Si/(Si + Al + P) = 0.108 and represent the topological limit above which some non-isolated SiO4 entities or Si islands will be formed in the structure. Consequently, a large amount of silicon islands are formed when Si content is high, whereas only isolated Si domains exists when Si content is low [38]. As can be observed in Table 1, these values are higher than 0.108 in both samples suggesting the presence of Si islands. Comparing both samples, it should be noted that S2 sample present a ratio only slightly above than 0.108 and although the presence of Si islands is unavoidable the number of isolated silicon atoms will be considerably superior in this sample. The relative proportion between Si islands and isolated Si affects directly to the variation in acidic properties of the resulting materials. Sastre et al. [36] established a direct correlation between acidity and Si island size, suggesting that the number of stronger acid sites is directly related to the concentration of Si islands and that the strength of the acid sites generated at the border of the Si islands will increase with Si island size. Nonetheless, it must be remarked that although the number of strong acid sites increases with the concentration of Si islands, the Si incorporation also increases the Si/Al ratio and, consequently, diminishes the total acidity of SAPO-34. Therefore, it can be expected that S1 sample present lower total acidity but higher number of strong acid sites associated to Si islands than S2 sample. The crystal structure and morphology of both SAPO-34 molecular sieves were analysed by XRD and SEM, respectively. Figure 1a shows the XRD patterns of the prepared powder materials and clearly demonstrates the formation of the typical chabazite (CHA) structure with high degree of crystallinity in both samples. It should be mentioned that any additional peak associated to impurities phases was detected. The average crystallite sizes (nm) were calculated by Scherrer equation using the full width of diffraction peak at half maximum intensity (peak at 2θ = 9.58 º). As shown Table 1, it is noteworthy that the S2 sample present an average crystalline size larger than S1 sample. Figure 1b shows SEM micrographs of the S1 and S2 samples. Both resulting materials exhibit a similar morphology based on cubic-like aggregated crystals. As illustrated SEM images, it is obvious that the crystal size of S2 sample became bigger than that of S1 sample in agreement with the values determined by using the Scherrer equation. It is well stablished that the crystal size of a SAPO-34 molecular sieve may change with the use of different templates. For instance, Nishiyama et al. [39] reported that the crystallization processes in the synthesis of SAPO-34 depend on the sort of template used, because both the nucleation and crystal-growth rates depend significantly on the interaction between the template and the inorganic species in the medium. Hence, the presence of a mixture of TEAOH and DPA in the synthesis of S2 SAPO-34 leads to larger crystals evidencing that the crystal growth step is intimately dependent on the type of template. 3.2. Textural properties and CO2 adsorption capacity Figure 2 shows the N2 adsorption-desorption isotherms at 77 K for both SAPO-34 samples, S1 and S2. According to the IUPAC classification [40], the two zeolites exhibit typical type I isotherms, which is indicative of microporous solids. Data of surface area SBET, micropore surface area and micropore volume estimated from the isotherms are included in Table 2. It is evident that the S1 SAPO-34 possess higher values of SBET, micropore surface area and micropore volume than S2 sample. These differences are expected in base to the smaller crystal size of S1 sample. Figure 3 provides the CO2 adsorption isotherms at 25 ºC for S1 and S2 samples, respectively. In both samples, the adsorption isotherms satisfied the dual-site Langmuir- Freundlich model implying that the CO2 adsorption corresponds to exothermic physisorption [41]. As shown Fig. 3 inlet, a gradual increase is observed in the lowpressure adsorption isotherm region for S2 SAPO-34 whereas an abrupt change in the slope is evidenced for S1 SAPO-34. This slight variation on the adsorption behaviour at changes are related to the formation of three new bands at 3676, 3650 and 3567 cm-1 at the same time which decline the bands assigned to bridged hydroxyls (3630 and 3599 cm-1). In agreement with Bordiga et al. [42], we speculate that these bands are associated to νOH stretching modes of slightly perturbed H2O molecules interacting with the bridged hydroxyl sites and they are typically observed at low coverages of water. It is reasonable to assume that the bands at 3676 and 3650 cm-1 are associated to the interaction of water molecules with the bridging hydroxyls located on the eight-membered rings (3630 cm-1) and six-membered rings (3599 cm-1), respectively. This assumption is in agreement with the attribution reported by Smith et al. [56]. Meanwhile, the band at 3567 cm-1 could be ascribed to the stretching mode of hydroxyls of H2O molecules forming a single hydrogen bond with the adjacent oxygen atom of the framework in the six-membered cavities [58]. In Fig. 7 is clearly evidenced that the presence of these bands is minor in S1 SAPO-34 sample than in S2 sample pointing out that S1 is more tolerant to moisture than S2. The main reason for explaining this observation could be ascribed to the larger crystal size of S2 sample, in which more Si(OH)Al fragments are exposed to be attacked by water molecules. Moreover, the interaction of water with SAPO-34 depends on the distribution of silicon atoms into the framework and different studies pointing to the higher stability of SAPO-34 materials incorporating silicon islands in comparison to SAPO-34 with isolated Si atoms [59]. Thus, the major presence of Si islands in S1 SAPO-34 is other of the main reasons to explain the detrimental effect on the water adsorption properties of this sample. Figure 8a provides a sketch of the chabazite (CHA) structure with space group R-3m and Figure 8b includes a schematic representation of the corresponding vibration modes associated to the interaction of water molecules with the Brønsted acid sites in SAPO- 34. It is noteworthy to mention that as the pore size of SAPO-34 is determined by the diameter of the eight-membered rings (3.8 Å) only the molecules with a smaller kinetic diameter will be accessible to the structural hydroxyl groups of SAPO-34. Therefore, only H2O (2.65 Å) and CO2 (3.3 Å) molecules can penetrate into the cavities of SAPO-34 while that methane (3.8 Å) molecules only will be perturbed by those hydroxyl groups which are located on the external surface of the crystallites. Furthermore, due to their small kinetic diameter, water molecules will be able to diffuse through the both 8- membered and 6-membered ring cavities. As shown Fig. 8c, the chabazite (CHA) topology involves four geometrically differently positions for Brønsted acid sites depending on to which framework oxygen atom the proton is attached. The usual nomenclature designates these four different oxygen sites as O1, O2, O3 and O4, respectively. The protons attached to O3 are not exposed to the eight-membered ring and consequently are distinctive from those attached to O1, O2, and O4 framework positions. Accordingly, Bordiga et al. [57] reported that the three protons attached to O1, O2 and O4 positions are all located on the 8-membered rings and they are represented by the fundamental stretching mode at 3630 cm-1. In contrast, the protons attached to O3 sites are isolated inside of the d6r cages and are associated to the lower wavenumber of νOH (3599 cm-1). This interpretation is supported by our experimental data and, as shown Fig. 7, it is obvious in both SAPO-34 samples that during adsorption of water molecules the band at 3630 cm-1 decays more rapidly than the band at 3599 cm-1, revealing that the hydroxyls located on the great cavities are more accessible than the hydroxyls located inside of the d6R cages. Despite this, it is essential to know that by means of theoretical calculations have been demonstrated that exist energetic differences between the four oxygen sites and a fine analysis would not be limited exclusively to two sites [55, 60]. However, the energetic differences existing between O1, O2 and O4 are minor and our approach can be considered valid for explaining the experimental data. In order to assess the evolution of the most relevant species involved during the cycles of adsorption/desorption over both SAPO-34 samples, Figure 9 displays the evolution of the IR peak heights plotted against time-on-stream. As can be observed, the time evolution of physisorbed CO2 (3500 cm-1), adsorbed water at low coverages (3676 cm-1) and molecular water occluded in the cavities (5248 cm-1) present important differences. Here it becomes evident that the efficiency of CO2 adsorption in S2 SAPO- 34 is minor and decreases more rapidly than that of S1 sample. The evolution of the peak at 3676 cm-1 reveals that in S2 sample all the Brønsted acid sites were saturated with molecules of water whereas in S1 SAPO-34 the saturation was not reached. Consequently, the S2 SAPO-34 sample retains high surface coverage of water which occlud rapidly the cavities avoiding that CO2 molecules pass through the channels. As was claimed above, the amount of adsorbed water depends on the amount of silicon introduced into the framework, which at the same time also determines the chemical composition of the environment of the Si(OH)AI fragments. The S1 sample is characterized by a minor concentration of Si(OH)Al acid sites in the 8-membered ring cavities and contains major amount of Si islands incorporated into the framework. It is expected that the presence of Si-rich regions favours the hydrophobicity of the material and, consequently, enhances the moisture tolerance. In summary, we have monitored the evolution of the SAPO-34 surface to understand as the adsorption sites are blocked by physisorbed water molecules and we can conclude that an increase of the hydrophobicity by incorporating more Si into the framework is desirable for using SAPO-34 as membranes in realistic stream conditions containing water-vapor. This approach is very useful to achieve a rational design of materials which can be applied as membranes in large-scale separation processes. 4. Conclusions In the present article, we have performed an operando DRIFT spectroscopic study to provide evidences on the effect of the chemical composition and the crystal size of SAPO-34 materials in the moisture tolerance during CO2/CH4 separation processes. It was found that the presence of larger silicon island in the 8-membered ring cavities increases the hydrophobicity of the SAPO-34 material and enhances the efficiency of CO2/CH4 separation. Furthermore, these results provide new found potential for the use of SAPO-34 molecular sieves as membranes in the CO2/CH4 separation processes, which is often neglected as an industrially relevant material due to its low water tolerance. Acknowledgments Financial support for this work has been obtained from the Spanish Ministerio de Ciencia, Innovación y Universidades co-financed by FEDER funds from the European Union (RTI2018-096294-B-C33). Moreover, Luis F. Bobadilla thanks the contracts of Spanish Government financed in part by the European Regional Development Fund through the “Juan de la Cierva incorporación” program (IJCI-2015-26348). References [1] H. Yang, Z. Xu, M. Fan, R. Gupta, R.B. Slimane, A.E. Bland, I. Wright, Progress in carbon dioxide separation and capture: A review, J. Environ. Sci. 20 (2008) 14-27. [2] A.T. Abeles, L.C. Howe, J.A. Krosnick, B. MacInnis, Perception of public opinion on global warming and the role of opinion deviance, J. Environ. 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Chem. C 119 (2015) 24046-24055. Table 1. Chemical composition and average crystalline size of the SAPO-34 samples prepared Sample (Template) SiO2:Al2O3:P2O5 Gel molar ratio Crystal composition (*) Si/Al molar ratio (Gel) (Crystal) Si/(Si + Al + P) molar ratio in crystal Crystalline size / nm (**) S1 (TEAOH) 0.3:0.5:1.0 Si0.21Al0.56P0.23O2 0.30 0.38 0.21 600 S2 (TEAOH/DPA) 0.3:1.0:1.0 Si0.13Al0.72P0.15O2 0.15 0.18 0.13 1000 (*) Analysed by X-ray fluorescence (**) Estimated using the Scherrer equation Table 2. Textural properties and CO2 adsorption capacity for both SAPO-34 samples Sample (Template) Surface area (SBET) m2 g-1 t-plot micropore surface area / m2 g-1 t-plot micropore volume / cm3 g-1 CO2 uptake mmol g-1 S1 (TEAOH) 611 580 0.27 4.8 S2 (TEAOH/DPA) 568 525 0.25 4.3 Table 3. Assignment of the vibrational modes of hydroxyl groups (*) Calculated by subtraction from the combination band (ν + δ) (**) From ref. [49] Nature of OH group ν / cm-1 ν + δ / cm-1 δ / cm-1 ν0-2 overtones / cm-1 Si(OH)Al type I 3630 4670 1040 (*) 7090 Si(OH)Al type II 3599 4648 1049 (*) 7044 Si-OH 3745 4540 (weak) 795 (*) 7320 (weak) P-OH 3673 not observed - 7220 (weak) Al-OH 3776 not observed 900-750 (**) 7430 (weak) H2O molecular 3300-3500 5000-5400 1630 6900-7400 Figure 1. XRD patterns (a) and SEM micrographs (b) for both SAPO-34 samples Figure 8. (a) Schematic representation of the chabazite (CHA) topology, (b) drawing scheme of the interaction of water with the Si(OH)Al acid sites and the associated stretching vibrational modes, and (c) unit cell of the CHA structure with the four different crystallographic positions of the oxygen into the framework Figure 9. Evolution of selected IR peaks typical of physisorbed CO2 (3500 cm-1), adsorbed water at low coverage (3676 cm-1) and molecular water occluded inside the cavities (5248 cm-1) in both S1 (a) and S2 (b) SAPO-34 samples