Bacterial cellulose assisted synthesis of hierarchical pompon-like SAPO-34 for CO2 adsorption

In the present work, a biosynthesis route for the preparation of hierarchical pompon-like SAPO-34 was developed. Commercially available bacterial cellulose aerogel was used as template. SiO2 loaded bacterial cellulose aerogel was used as silica source and a simple hydrothermal treatment was used for crystallization.

Microporous and Mesoporous Materials 331 (2022) 111664 Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso Bacterial cellulose assisted synthesis of hierarchical pompon-like SAPO-34 for CO2 adsorption Jie Gong a, Fei Tong a, Chunyong Zhang a, Mojtaba Sinaei Nobandegani b, Liang Yu b, *, Lixiong Zhang c, ** a b c College of Chemical and Environmental Engineering, Jiangsu University of Technology, Changzhou, 213001, Jiangsu, PR China Chemical Technology, Luleå University of Technology, SE-971 87 Luleå, Sweden State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing Tech University, Nanjing, 210009, PR China A R T I C L E I N F O A B S T R A C T Keywords: Hierarchical pompon-like SAPO-34 Biosynthesis route Bacterial cellulose CO2 adsorption In the present work, a biosynthesis route for the preparation of hierarchical pompon-like SAPO-34 was devel­ oped Commercially available bacterial cellulose aerogel was used as template SiO2 loaded bacterial cellulose aerogel was used as silica source and a simple hydrothermal treatment was used for crystallization XRD, FT-IR, SEM, TEM, N2 adsorption-desorption and TG techniques were employed to characterize the obtained samples The hierarchical pompon-like SAPO-34 showed a spherical morphology that was comprised of nanosheets with a thickness less than 30 nm The specific surface area of the hierarchical pompon-like SAPO-34 was 498 m2/g that was higher than the trigonal SAPO-34 crystals of 465 m2/g The ultrasonic treatment experiment indicated a high stability of the pompon-like structure In addition, the hierarchical pompon-like SAPO-34 exhibited a CO2 adsorption capacity of 2.26 mmol/g at 100 kPa and 298K and the corresponding CO2/CH4 ideal separation factor was 5.7, which was higher than that of trigonal SAPO-34 crystals The saturated adsorption capacity and b-value were estimated using single site Langmuir, Toth and Sips adsorption isotherm models and the observed results were constant Compared with trigonal SAPO-34, hierarchical pompon-like SAPO-34 displayed a higher satu­ rated adsorption capacity, but a lower b-value Introduction Biogas and natural gas with the main components of CO2 and CH4 are environmentally friendly fuels and feedstocks The CO2 concentration in raw biogas and natural gas can be as high as 45% and 25%, respectively [1,2] CO2 will reduce energy density, causing it to miss the pipeline quality requirement for minimum heating value [3] and react with water to form a corrosive acid, leading to pipeline corrosion [4] Therefore, to reach the required fuel and grid specifications, removal of CO2 from biogas and natural gas is necessary Silicoaluminophosphate (SAPO) zeolite SAPO-34 with a CHA framework structure has eight-membered rings defined threedimensional pore system The pore size is about 3.8 Å [3], which is larger than the molecular size of CO2 (3.3 Å) and similar to that of CH4 (3.8 Å) [3] Consequently, SAPO-34 could be used to separate CO2 from natural gas and biogas by molecular sieving Besides, due to the small pores, moderate acidity and high thermal/hydrothermal stability, SAPO-34 has also been used as catalyst in various reactions [5–7] However, the sole presence of micropores in SAPO-34 slows down the mass transfer to and from active sites located within the micropores, which would limit the reaction rate and lead to the rapid deactivation of the catalyst This restricts the use of SAPO-34 for practical applications To overcome the limitation and increase the mass transfer, many efforts have been taken to fabricate hierarchical SAPO-34 in recent years [3–20] Moreover, hierarchical zeolites contain more than one types of pores with different sizes and the larger pores could improve the mass transfer The hard and soft template methods have been considered as efficient methods for the preparation of hierarchical SAPO-34 [9–20] Carbon materials are the most common hard template [9–11] However, carbon materials are usually hydrophobic and may lead to the weak interaction with the synthesis gel Compared with hard templates, soft templates with three-dimensional network structures, abundant func­ tional groups and designed molecular sizes demonstrate great signifi­ cance on the synthesis of hierarchical SAPO-34 [8] So far, polymers, * Corresponding author ** Corresponding author E-mail addresses: liang.yu@ltu.se (L Yu), lixzhang@njtech.edu.cn (L Zhang) https://doi.org/10.1016/j.micromeso.2021.111664 Received 26 October 2021; Received in revised form 18 December 2021; Accepted 27 December 2021 Available online 29 December 2021 1387-1811/© 2021 The Authors Published by Elsevier Inc This is an open access (http://creativecommons.org/licenses/by-nc-nd/4.0/) article under the CC BY-NC-ND license J Gong et al Microporous and Mesoporous Materials 331 (2022) 111664 like PEG [12], PEI [13] and PAM [14], and surfactants, like DMOD [15], CTAB [16], PDADMAC [17] and TPOAC [18,19] have been used to synthesize hierarchical SAPO-34 Liu [20] et al designed a new sur­ factant (DPHAB) for the synthesis of hierarchical SAPO-34 The obtained SAPO-34 displayed a hierarchical structure with pore size in the range of 2–46 nm, and the surface area reached to 673 m2/g However, such uncommon surfactants are expensive and non-renewable, restricting their applications [8,20] Our group has developed efficient methods to prepare hierarchical structure zeolitic materials, by using biomaterial e.g chitosan [21,22], gelatin [23], rapeseed pollen [24] and its extract [25] as templates For instance, by using pollen as a template, we have successfully prepared hollow hierarchical ZSM-5 as well as the other porous inorganic mate­ rials [24] Biomass chitosan has also been used for the preparation of zeolite A/CS xerogel hybrid films due to its unique hydrogel network structure [22] SAPO-34 with a unique morphology of curly nanosheets was synthesized when rapeseed pollen extract was used to prepare the synthesis solution, and showed a higher CO2 adsorption capacity and efficiency [25] The SAPO-34 also displayed a hierarchical structure Bacterial cellulose (BC) with high porosity, water retention capa­ bility, mechanical properties, formability, and biocompatibility, has been used as additive of food and medical materials, as well as in optical, electronic, and optoelectronic devices [26] Due to the interconnected networks of BC nanofibers with diameters ranging from to 100 nm [26], they have been used as templates to synthesize various inorganic hollow fiber materials or nanoparticles, such as oxide [27–29], MOFs [30], zeolite NaA [31], MFI [32,33], etc So far, however, its application in SAPO-34 synthesis has rarely been reported Besides, BC is an economical template that produced in large amounts in a microbial fermentation process In the present work, we report a hydrothermal synthesis route to simply prepare hierarchical pompon-like SAPO-34 using BC as template SiO2 loaded BC aerogel were prepared and used as silicon source to prepare SAPO-34 The obtained hierarchical SAPO-34 was deeply characterized and compared with the conventional SAPO-34 crystals The hierarchical pompon-like SAPO-34 crystals were composed of zeolite nanosheets with a thickness of less than 30 nm To the best of our knowledge, hierarchical SAPO-34 comprised of 2D zeolite nanosheets structure has not been reported by other groups The adsorption per­ formance of the SAPO-34 for CO2 and CH4 was also studied In addition, preparation of 2D zeolite nanosheets has gained much attention due to the application of nanosheets in membrane preparation [34–36] 2D zeolite nanosheets are mainly achieved by exfoliating layered zeolite precursors [35] Zeolite nanosheets, including MWW and MFI structure types have been successfully fabricated by Tsapatsis’s group [37,38] The zeolite nanosheets could be coated on porous sub­ strates to form well-packed thin deposits layer that exhibited excellent molecular sieving properties [37] These results displayed a great po­ tential of zeolite nanosheets Therefore, the preparation of SAPO-34 crystals comprising of nanosheets is of great interest phosphoric acid (85 wt%, Sinopharm Chemical Reagent Co., Ltd.) and 0.79 g of aluminum isopropoxide (Aladdin) were added into 10.0 g of deionized water and stirred until dissolved completely Afterwards, 0.077 g of SiO2/BC was added into the solution and followed by stirring for h Finally, 0.97 g of triethylamine (TEA, 99 wt%, Sinopharm Chemical Reagent Co., Ltd.) was added The molar composition of the mixture was Al2O3:P2O5:0.6SiO2:5TEA:300H2O The mixture was further stirred for 24 h at room temperature, and then transferred into a 40 ml autoclave for hydrothermal synthesis The hydrothermal synthesis was performed at 200 ◦ C for 72 h The as-synthesized products were recovered by centrifuge, and washed with deionized water for several times The sample was dried at 80 ◦ C overnight SAPO-34 without using BC aerogel was also prepared Silica powder that was prepared by freeze-drying silica sol (30 wt%) was used as the silica source The other procedures were the same as described above In addition, SAPO-34 was also synthesized by using 30 wt% silica sol directly Finally, all the products were calcined at 550 ◦ C for h under air atmosphere The heating rate was ◦ C/min The obtained samples were noted as S (prepared with SiO2/BC), S’ (prepared with silica sol) and S’’ (prepared with silica powder) 2.2 Characterizations X-ray diffraction (XRD) patterns were collected at 40 kV and 40 mA on a Bruker D8 Advance powder diffractometer using a Cu Kα radiation source A Braun position sensitive detector was used and the scan rate was 5◦ /min and the step size was 0.05◦ The Fourier transform infrared spectra (FT-IR) were obtained on a Nexus 870 FT-IR spectrometer in the wavenumber range of 4000–400 cm− The samples were mixed with KBr (in a mass ratio of 1:10) and pressed to pellets for FT-IR measure­ ment Scanning electron microscope (SEM, Hitachi S-4800) and trans­ mission electron microscope (TEM, JEL-200CX) were used to investigate the particle size, morphology and microstructure of the samples The Si, Al and P contents of SAPO-34 were analyzed by an energy dispersive Xray analyzer (EDX, Sigma) attached to the SEM (Quanta 200) N2 adsorption-desorption isotherms were measured at 77 K on a Micro­ meritics ASAP 2020 instrument The samples were all degassed at 300 ◦ C for 10 h before measurement Thermogravimetric (TG) analysis was carried out in an air atmosphere using a Netzsch STA 409 instrument The sample was heated to 800 ◦ C from room temperature with a heating rate of 10 ◦ C/min The sample was maintained at the final temperature for 20 Alumel calibration standard was used to calibrate the TGA instrument 2.3 Adsorption experiments Gas adsorption measurements of CO2 and CH4 were performed at 298 K and pressures up to 100 kPa on a Micromeritics ASAP 2020 in­ strument The samples were all degassed at 300 ◦ C for 10 h before the measurement The ideal separation factor of CO2 over CH4 was calcu­ lated as the ratio of the molar adsorption amount of CO2 and CH4 measured at the same pressure and temperature Experimental 2.1 Preparation of hierarchical pompon-like SAPO-34 2.4 Modeling of gas adsorption over SAPO-34 crystals Firstly, purified BC pellicles with a fiber content of about 1% (vol/ vol) (Hainan Guangyu Biotechnology Co., Ltd.) were cut into cm × cm pieces and washed with deionized water several times before use BC aerogel was obtained after freeze drying for 24 h To prepare silica/BC aerogel, the BC aerogel were immersed into an excessive silica sol (30 wt %, Zhejiang Yuda Chemical Industry Co., Ltd.) and maintained for 12 h Afterwards, the obtained silica sol/BC aerogel was freeze dried to remove water The processes were repeated several times until the content of silica in silica/BC aerogel was about 98 wt% The silica/BC aerogel was grounded (noted as SiO2/BC) and used as silicon source For the preparation of hierarchical pompon-like SAPO-34, 0.44 g of The measured adsorption isotherms were fitted by single site Lang­ muir adsorption isotherm using equation (1): C = Csat bP + bP (1) Where C and Csat represent the adsorbed amount and saturated adsorption capacity, respectively b is the affinity constant, and P is the pressure The saturated adsorption capacity was estimated by fitting the Langmuir model to the adsorption data The R-squared values, R2, were used to show the goodness-of-fit Meanwhile, b-values and saturated J Gong et al Microporous and Mesoporous Materials 331 (2022) 111664 adsorption capacities were obtained for different samples and gases, respectively In addition, Toth (equation (2)) and Sips (equation (3)) adsorption isotherms were also employed to fit the adsorption data For Toth Model, the adsorption concentration is estimated as: C= bP / Csat [ t] t + (bP) nanosheets are stacked with irregular pores of about 30 nm in diameters in between the nanosheets (Fig 3b) It has been reported that the chemical formula of SAPO-34 is (SixAlyPz)O2 (x = 0.01–0.98, y = 0.01–0.60, z = 0.01–0.52, and x + z = y) [39] The EDX results (Table 1) show that sample S has a decent Si:Al:P ratio as described in the chemical formula This result indicates that there is no amorphous in the sample and the SAPO-34 has high crystallinity This is in line with XRD results Moreover, sample S shows the highest Si content in the three samples It resulted from using silica/BC as a silicon source In silica/BC, silica particles are fixed on or between the BC nanofibers through hydrogen bonding between hydroxyl groups of BC and silica particles [40], and phosphoric acid and aluminum isopropoxide diffuse into it to form gel for zeolite synthesis In this case, the percentage of silica in the gel will be higher than that in the synthesis mixture Therefore, higher silicon content in pompon-like SAPO-34 This process is similar to the preparation of zeolite A/chitosan microspheres by diffusing sodium aluminate into the silica/chitosan microspheres that could maximize the conversion of silica [41] Sample S′ and sample S′′ prepared with silica sol and silica powder showed the same structure as indicated by XRD and FT-IR (Fig 1a and b) results However, the morphology is completely different as compared to sample S It seems like that only classical pseudo-cubic crystals were observed in S’ (Fig 3d) and some amorphous was observed in sample S’’ (Fig 3c) TEM images of sample S are shown in Fig The network structure and the pores between the nanosheets can be observed clearly (Fig 4a) The ultra-thin nanosheet can be seen clearly from the edge of the sample at high magnification (see Fig 4b) The internal structure of sample S was also investigated by SEM Before investigation, the sample was treated by ultrasonic for h and h, respectively The broken part of the zeolite after ultrasonic treatment for h shows that the internal of the zeolite is also composed of nanosheets (Fig 4c) The sample after ul­ trasonic treatment for h still shows a nanosheets comprised structure (Fig 4d), which indicated a high stability Fig 5a shows a TG curve of uncalcined sample S and S’ A three-stage weight loss was observed for both samples The first one occurred before 150 ◦ C with a weight loss of 5.72 wt% for samples S It can be ascribed to water desorption However, sample S’ shows a less weight loss of 3.70 wt% The difference in weight loss could be ascribed to that BC may absorb water from the air The second weight loss of sample S occurs at temperatures between 150 and 440 ◦ C with a large weight loss of 8.83 wt%, which mainly contributed by thermal decomposition of BC The third weight loss occurs at temperatures between 440 and 650 ◦ C with a weight loss of 4.62 wt%, which was ascribed to the further decompo­ sition/oxidation of TEA Fig 5b shows the TG curve of BC The results indicate that BC could be easily removed (at temperature >300 ◦ C) from SAPO-34 zeolite (2) where, t is the Toth heterogeneity parameter Sips model is defined as: / C= n (bP) / Csat 1 + (bP) (3) n where n is a dimensionless parameter that accounts the heterogeneity of the system However, when t and n approach one, Toth and Sips adsorption isotherms show similar behavior as single site Langmuir adsorption isotherm Results and discussion XRD pattern of the sample S (Fig 1a) prepared with SiO2/BC exhibits characteristic diffraction peaks of SAPO-34 as compared with the reference pattern [39] No extra peak was observed The relative in­ tensity of (100) peak at 9.6◦ was stronger compared to that of sample S′ and sample S′′ , which probably resulted from the unique structure Fig 1b shows the FT-IR spectrum of sample S, S′ and S’’ The absorption bands at 480, 530, 565 and 635 cm− indicate a CHA structure [14] However, the SiO4 band at 480 cm− and PO4 band at 565 cm− slightly shift to 495 and 555 cm− for sample S, which may be affected by the introduction of BC Fig shows the SEM images of BC aerogels and SiO2/BC The SEM image of BC aerogels exhibits a porous, interconnected and sophisti­ cated three-dimensional porous network structure (Fig 2a) that are composed of numerous nanofibers The diameter of the nanofibers is about 20–80 nm (Fig 2b) After being saturated with SiO2, the SEM clearly shows that the nanofibers are embodied by SiO2 (Fig 2c), and the morphology of BC nanofiber was preserved in SiO2/BC (Fig 2d) Fig shows the SEM images of SAPO-34 samples prepared from different silica sources The SEM image at low magnification (Fig 3a) shows that sample S is of pompon-like spherical morphology with an average particle size of about μm The high magnification SEM image (see Fig 3b) shows the as-synthesized SAPO-34 are composed of nano­ sheets The thickness of the nanosheets is less than 30 nm The Fig (a) XRD patterns and (b) FT-IR spectra of sample S, S′ and S’‘ J Gong et al Microporous and Mesoporous Materials 331 (2022) 111664 Fig SEM images of BC (a, b) and SiO2/BC (c, d) at different magnifications Fig SEM images of sample S at different magnifications (a, b), and sample S’ (c) and sample S’’ (d) J Gong et al Microporous and Mesoporous Materials 331 (2022) 111664 [25] This is in line with the observation of SEM, where nanosheets structure was obtained and the closely compacted nanosheets generated slit shaped mesopores The surface area of sample S is 498 m2/g, higher than 465 m2/g of sample S’’ Fig 6b shows the pore size distribution that calculated by the Barrett-Joyner-Halenda (BJH) method using desorp­ tion branches of isotherms The curve shows a narrow pore size distri­ bution with the peak centered at 3.7 nm The results confirm the mesopores in the sample, which indicates a hierarchical structure of the pompon-like SAPO-34 The adsorption isotherms of sample S and S′ for CO2 and CH4 at 298 K are presented in Fig The adsorption isotherms follow the type I isotherms based on Brunauer classification [42], which is a typical class of isotherm for the adsorption over microporous materials Apparently, sample S exhibits a higher CO2 adsorption capacity and the CO2 and CH4 adsorption capacities were 2.26 and 0.40 mmol/g at 100 kPa (Fig 7a), Table Al, P, and Si contents in sample S, S′ and S′′ measured by EDX Sample S S′ S′′ Al P Si Atom % Atom % Atom % 48.2 49.6 53.0 28.8 36.6 37.1 23.0 13.8 9.9 Fig 6a shows the N2 adsorption-desorption isotherms of the calcined sample S and S’ Both isotherms exhibit high uptake at the initial pres­ sure range, which results from the microporous structure of SAPO-34 zeolite The hysteresis loop at P/P0 = 0.45–0.99 indicates a meso­ porous structure in sample S Moreover, the hysteresis loop belongs to H4 type and H4 type isotherm mainly results from slit shaped mesopores Fig TEM images of sample S at low (a) and high (b) magnification SEM images of sample S with ultrasonic treatment for h (c) and for h (d) Fig TG curves of (a) sample S and S′ , (b) TG curve of BC J Gong et al Microporous and Mesoporous Materials 331 (2022) 111664 Fig (a) Nitrogen adsorption-desorption isotherms of sample S and S’ (b) BJH pore size distribution of sample S Fig Adsorption isotherms of CO2 and CH4 on sample S (a) and sample S’ (b) at 298 K Points show the measured data and curves represent the fitted model respectively The corresponding CO2/CH4 ideal separation factor was 5.7 It has been reported that the heats of adsorption of CO2 and CH4 on CHA zeolites are about − 25 and − 17 kJ/mol, respectively [43–45] The higher heat of adsorption for CO2 results from the basicity of the zeolite framework [46] and larger polarity of CO2 molecules, which increases the affinity between CO2 and zeolite, thereby more CO2 selective At the same conditions, sample S′ displayed a CO2 and CH4 adsorption capacity of 2.04 and 0.46 mmol/g (Fig 7b), respectively, which corresponds to a CO2/CH4 ideal separation factor of 4.4 The hierarchical pompon-like SAPO-34 showed a higher ideal separation factor of CO2/CH4, which could be ascribed to the nanosheet structure that generates more accessible pores and a higher surface area [47–49] Furthermore, as indicated by EDX analysis (see Table 1), the content of silicon was higher in sample S compared to that of sample S’, which increases Brønsted acid sites and provides more energetic adsorption sites for CO2 adsorption [50,51] Consequently, sample S showed a high CO2 adsorption capac­ ity The results are consistent with the results of CO2 adsorption on SAPO-34 reported by Liu [3] and Maurin [52] Fig shows that the single site Langmuir adsorption isotherm could fit the adsorption data well The R-squared values (see Table 2) were higher than 0.998 for all cases The estimated saturated adsorption ca­ pacity for CO2 was 4.05 mmol/g for sample S that was much higher than that for samples S’ The estimated b-values of CO2 was higher than that of CH4 because of the large polarizability of CO2, i.e CO2 has higher affinity to the zeolite compared to CH4 Interestingly, the estimated bvalues of sample S′ were higher than that of samples S It is due to the high aluminum content in sample S′ as shown in Table The modeling further confirmed that the higher CO2 adsorption capacity for Table Fitted parameters by Single site Langmuir, Toth, and Sips adsorption isotherms for the adsorption of CO2 and CH4 on samples S and S’ Zeolites Gas components Adsorption isotherms Csat (mmol/g) Heterogeneity parameter b-value (Pa− 1) S CO2 Langmuir Toth Sips Langmuir Toth Sips Langmuir Toth Sips Langmuir Toth Sips 4.05 4.07 4.06 2.25 2.22 2.23 2.95 3.11 2.91 1.91 1.98 1.95 n/a 0.9810 0.9996 n/a 0.9435 0.9953 n/a 0.9126 0.9963 n/a 0.9969 0.9798 1.30 × 10− 1.28 × 10− 1.26 × 10− 2.20 × 10− 2.30 × 10− 2.20 × 10− 2.20 × 10− 2.20 × 10− 2.26 × 10− 3.20 × 10− 3.00 × 10− 3.20 × 10− CH4 S′ CO2 CH4 5 6 5 6 R-Square 0.9999 0.9999 0.9999 0.9991 0.9986 0.9991 0.9980 0.9988 0.9981 0.9989 0.9999 0.9999 Microporous and Mesoporous Materials 331 (2022) 111664 J Gong et al hierarchical pompon-like SAPO-34 was a result of the higher surface area Table shows the fitted parameters for the adsorption of CO2 and CH4 on samples S and S’ using single site Langmuir adsorption isotherm Table also summarizes the fitted parameters by Toth and Sips adsorption isotherms The results show that the Toth and Sips adsorption isotherms could also fit the adsorption well (the curves of the fitted model are not shown here) The obtained saturated adsorption capac­ ities and b-values were similar as the ones from the single site Langmuir adsorption isotherm All heterogeneity parameters were also close to one, which indicates a homogeneous surface of the samples [7] [8] [9] [10] Conclusions In summary, hierarchical pompon-like SAPO-34 with a particle size of about μm has been successfully synthesized by a simple hydro­ thermal treatment Use SiO2 loaded BC as a silica source is crucial for the synthesis The method is simple, and the BC template is economical and environmentally friendly The hierarchical pompon-like SAPO-34 par­ ticles were comprised of nanosheets with a thickness less than 30 nm The hierarchical pompon-like SAPO-34 exhibited a higher CO2 adsorp­ tion capacity and a higher ideal CO2/CH4 separation factor compared to the conventional trigonal SAPO-34 crystals due to the higher surface area In addition, due to the unique structure, the nanosheet can prob­ ably be exfoliated and used for the preparation of ultra-thin CHA membranes, which could be a new insight into CHA nanosheet membranes [11] [12] [13] [14] [15] CRediT authorship contribution statement Jie Gong: Investigation, Methodology, Project administration, Writing – original draft Fei Tong: Investigation Chunyong Zhang: Data curation, Resources Mojtaba Sinaei Nobandegani: Formal analysis Liang Yu: Formal analysis, Supervision, Writing – review & editing Lixiong Zhang: Supervision, Project administration, Funding acquisition [16] [17] [18] Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper [19] [20] Acknowledgements We are grateful for financial support from the National & Local Joint Engineering Research Center for Deep Utilization Technology of Rocksalt Resource (SF201804) and Jiangsu University of Technology (11610412042) We also thank Hainan Guangyu Biotechnology Co., Ltd for voluntary bacterial cellulose [21] [22] References [23] [1] H.G Katariya, H.P Patolia, Advances in Biogas Cleaning, Enrichment, and Utilization Technologies: a Way Forward, Biomass Conv Bioref, 2021, https://doi org/10.1007/s13399-021-01750-0 [2] R.W Baker, Future directions of membrane gas separation Technology, Ind Eng Chem Res 41 (2002) 1393–1411, https://doi.org/10.1021/ie0108088 [3] D.H Wang, P Tian, M Yang, S.T Xu, D 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https://doi.org/10.1021/jp711470c  ... To the best of our knowledge, hierarchical SAPO-34 comprised of 2D zeolite nanosheets structure has not been reported by other groups The adsorption per­ formance of the SAPO-34 for CO2 and CH4... been used to synthesize hierarchical SAPO-34 Liu [20] et al designed a new sur­ factant (DPHAB) for the synthesis of hierarchical SAPO-34 The obtained SAPO-34 displayed a hierarchical structure... factor of CO2 over CH4 was calcu­ lated as the ratio of the molar adsorption amount of CO2 and CH4 measured at the same pressure and temperature Experimental 2.1 Preparation of hierarchical pompon-like