Sonication mediated hydrothermal process – an efficient method for the rapid synthesis of DDR zeolite membranes

Abstract

The formation and growth of a DDR zeolite membrane was developed on the low cost indigenous clay–alumina substrate for separation of H₂ from H₂–CO₂ mixture by selective deposition of oriented seed crystals, followed by secondary growth method with sonication mediated hydrothermal technique. The formation of free radicals by ultrasonic irradiation in the sonochemical method enhances the rate of nucleation which ultimately reduces the DDR zeolite crystallization time. Surface seeding not only accelerates the zeolite crystallization on the support surface but also enhances the formation of an homogenous zeolite membrane layer. The DDR seeds were synthesized by a sonication mediated hydrothermal technique within a short crystallization time i.e. 2 days and used to provide nucleation for the membrane growth. Accordingly DDR zeolite membranes were synthesized on seeded substrate within 5 days. The membrane thickness was found to be ∼26 μm. The synthesized membranes along with seed crystals were characterized by XRD, FTIR, FESEM and EDAX analysis. The performance of the membrane formed was evaluated by single gas as well as mixture gas permeation measurement for H₂ and CO₂. The H₂–CO₂ separation selectivity of the membrane increased up to 3.7 at room temperature which is more than the reported values. To the best of our knowledge, there is no report on the synthesis of a DDR zeolite membrane within 7 (2 days for seed crystal and 5 days for membrane synthesis) days by a secondary growth technique.

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1. Introduction

The increasing demand for “clean” and efficient energy has resulted in an increased global willingness to embrace the proposed “hydrogen economy” as a potential long term solution to the growing energy crisis. Hydrogen has been considered as the next generation clean fuel. But it is not available in pure form in nature.¹ Usually hydrogen can be produced from SMR process which consists primarily of the highly endothermic SMR (steam methane reforming) reaction (CH₄ + H₂O → CO + 3H₂) followed by the Water–Gas Shift reaction (WGS) (CO + H₂O → CO₂ + H₂).² Ultimately a mixture of H₂, CO₂, and CO is formed. Consequently H₂ should be removed from the reaction mixture mainly CO₂ to increase the ultimate efficiency of the process. Currently H₂ can be purified by pressure swing adsorption (PSA),^3,4 fractional/cryogenic distillation,^5,6 membrane based separation process^7,8 etc. Membranes can be an alternative to energy demanding separation processes such as distillation and absorption and can enhance conversion of equilibrium limited reactions. Inorganic microporous membranes are highly promising for high temperature H₂ separation because of their high thermal and chemical stabilities. Dense Pd membrane and silica membranes are currently used for separation of H₂ and CO₂ and other gases.^9,10

Zeolite membranes are a special class of porous inorganic membranes with well-defined intra-crystalline pores. They have superior thermal, mechanical, chemical and high pressure stability. Small pore zeolite membranes have the potential to separate light gases based on molecular sieving effects. Decadodecasil 3R (DDR) framework, having pore diameter 0.36 × 0.43 nm, a small pore sized member of the zeolite family, has been studied for various applications and most notably has shown excellent performance in H₂ gas separations from other light gases like CO₂, N₂, CH₄ etc.^2,11,12

The secondary growth method has been established as the most suitable method for the preparation of zeolite membranes. In this method the support surface is coated with zeolite seeds, followed by the synthesis of the membrane on seeded support by hydrothermal technique. According to the formation mechanism of zeolite membranes on a porous support, the nucleation of zeolite on the support–gel interface and in the bulk synthesis mixture are competitive processes.¹³ Thus to develop a continuous membrane layer, the nucleation of zeolite in the bulk synthesis mixture must be inhibited, while the nucleation of zeolite on the support surface must be increased. It is an effective approach to develop a high quality zeolite membrane. The main advantages are the control of thin oriented layers, improved zeolite growth on supports, higher reproducibility and so on.^14,15 From literature, zeolites can be synthesized from various precursor materials and methods. Generally, the zeolite synthesis is performed in the temperature range of 90–200 °C for periods of time between several hours to several weeks in autoclaves. Up to now, a few publications deal with the difficult and long synthesis of (up to 24 days) DDR zeolites.^16–20

Gies et al.¹⁸ has reported the synthesis of a DDR zeolite for the first time in 1986 and later den Exter et al. optimized the composition of the initial sol and scaled up for better characterization of the material.¹⁹ However, this process required long synthesis time, typically from 25 to 42 days. Later on Gascon et al. has reported the synthesis of DDR crystals within 2 days but in the first phase they have added seed crystals for rapid crystallization which were synthesized for 25 days at 160 °C.²⁰

During the past decade low temperature and environmental friendly preparation procedures are used for the preparation of solid state compounds.^21,22 For preparation of nano materials, sonochemical methods has been proven as a potential tool for synthesis of various inorganic materials and fine ceramic powders.^23–26 In our previous work we have reported the formation of DDR zeolite powders by sonochemical method.²⁷

In this work, we have reported the synthesis of a DDR zeolite membrane within 5 days. A substantial reduction of the synthesis time combined with full conversion of the sol into zeolite material at short crystallisation time would make such synthesis very useful for DDR zeolite membranes. The seed crystals were synthesized within 2 days. After that the zeolite seed monolayer was applied on a clay–alumina tubular support by dip coating method. The good adherence of the seed crystals on the surface of the substrate controls the ultimate uniformity of the membrane properties. To ensure a good adherence of the zeolite seed monolayers on the clay–alumina tubular support, a cationic polymer, poly-dimethyl diallyl ammonium chloride (PDADMAC) was used as an intermediate linker. DDR membranes were synthesized on the seeded support within 5 days by sonication mediated hydrothermal technique. In the case of the sonication mediated method, the application of ultrasound to chemical reactions lead to homogeneous nucleation and a substantial reduction in crystallization time at room temperature compared to conventional chemical methods.^24–26 In this method, high energy acoustic cavitations are formed when liquids are subjected to ultrasonic irradiation. Cavitation is the formation, growth and implosive collapse of bubbles in liquid. Cavitational collapse produces extremely high temperature (>5000 K), pressure (>20 MPa) and very high cooling rates (>107 K s⁻¹). The reactivity of chemical species is stimulated by this process. It accelerates the reaction between liquid and solid reactants. Ultrasound seems to influence the physicochemical phenomena related to nucleation and crystal growth, occurring during crystallization.²⁶ To our best knowledge, there are still no reports on the preparation of a DDR zeolite membrane on a clay–alumina support by sonication assisted hydrothermal method within 5 days (total 7 days as 2 days are for seed synthesis) for separation of hydrogen. The synthesized membrane was characterized using XRD, FESEM and EDAX analysis. The ultimate performance of the membrane was characterized by single gas as well as mixture gas permeation measurements for H₂ and CO₂ at different feed pressures and different feed compositions.

2. Experimental

2.1. Synthesis of DDR zeolite seeds

The chemical reagents used are colloidal silica (Ludox HS-30 Sigma Aldrich), structure directing agent (SDA) 1-adamantanamine (Aldrich, India) and ethylene diamine (Merck, India). Two reactant mixtures were prepared respectively by suspending measured amount of colloidal silica and deionised water (DI water) in a glass beaker (mixture-1). The solution was stirred with a magnetic stirrer (SCHOTT Instruments GmbH, Germany) at 200 rpm for 30 minutes. In another mixture (mixture-2), 1-adamantanamine was mixed with ethylene diamine and the calculated amount of water was added to the reaction mixture. After stirring the mixture-2 for 1 h, it was mixed slowly to mixture-1 with constant and vigorous stirring. All these procedures were carried out at room temperature. The resulting mixture was sonicated for 3 h. The energy input for sonication was 250 W. The molar composition of the sol used for the synthesis was 1 silica : 0.5 1-adamantanamine : 4 ethylene diamine : 100 water.

For comparison, the DDR samples were synthesized by an hydrothermal process similar to the method described by den Exter. The molar composition of the initial sol was 1 silica : 0.5 1-adamantanamine : 4 ethylene diamine : 100 water.

The powdered products were recovered through centrifugation, washed with DI water until pH < 8.

2.2. Seeding of the substrate

DDR seed crystals were dispersed in deionised water under ultrasonication for 2 h. Indigenous clay alumina tube of diameter 10 mm, thickness 3 mm and 60 mm length was used as support for synthesis of the DDR membrane. Before seeding, the substrates were cleaned with acetone in ultrasonic cleaner (vibracell, USA) for 10–15 minutes just to remove the dust particles and oily matter. The outer surface of the support tubes was wrapped with Teflon tape so that the zeolite layer was formed inside the tube. To increase the orientation of the seed crystals on support surface, an intermediate PDADMAC layer was applied onto the support surface. Polydiallyldimethylammonium chloride (Sigma Aldrich) 2% (w/v) solution was prepared by dissolving PDADMAC in aqueous solution. Dissolved solution was filtered through a filter paper (Whatman, no. 42) to remove bubbles and any undissolved impurities, and then coated onto the inner side of support surface.

The support substrate was dipped in a 3 wt% DDR zeolite seed suspension in deionized water 5 times for duration of 15 s. After the dipping procedure, the seeded supports were dried at 100 °C for 24 h.

2.3. DDR membrane preparation

The seeded substrates were placed vertically in an autoclave. The autoclave was filled with the reaction mixture having the above mentioned composition and same synthesis method. After synthesis, the zeolite coated membrane was washed thoroughly with deionized water until the pH of the washing liquid became neutral. The synthesized membrane was calcined in air at 650 °C for 5 h to remove the organic compounds. Fig. 1 describes the different steps involved in the preparation of the DDR zeolite membrane.

Fig. 1 Schematic diagram of the different steps involved in the DDR membrane synthesis process.

2.4. Membrane characterization

The crystalline structure of the as synthesized membrane was determined by XRD patterns in the 2θ range of 0–60 °C which were collected at ambient temperature. XRD was carried out on a Philips 1710 diffractometer using CuKα radiation (α = 1.541 Å). The FTIR spectra of the DDR zeolite crystals were recorded in the diffuse reflectance mode using a Nicolet 380 FTIR spectrophotometer for detecting characteristics vibration band.

Microstructure and morphology of the membrane layer was examined using field emission scanning electron microscopy (FESEM: model Leo, S430i, UK). Elemental analyses of the samples were conducted by energy dispersive X-ray spectrometer (EDXS) attached to a Cambridge Stereo scan S440 microscope. The specific surface area of the particles was measured using the BET surface area analyzer (model: Autosorb 1, Quantachrome Corporation, USA) at liquid nitrogen temperature (77 K).

Single gas and mixture gas permeation for H₂ and CO₂ were measured by a specially designed permeation cell developed in our laboratory. The gas permeance of the membranes was measured by soap film flow meter under the feed pressure of 100 to 300 kPa at room temperature. The permselectivity of two gases G₁/G₂ was defined as the permeance ratio of gas G₁ and gas G₂. The gas permeation measurement of each single gas was repeated until the permeance data for the successive 10 tests were closed. The single gas permeance was the average of 10 successive tests. For the mixture gas, separation selectivity was measured by gas chromatography (model – Trace GC ultra, serial no. 20092814, Germany).

2.5. Characterization

The crystalline structure of the as synthesized powder was determined by X-ray diffraction pattern. XRD was carried out on a Philips 1710 diffractometer using CuKα radiation (α = 1.541 Å). The characteristics vibration bands for DDR seed crystal were investigated by FTIR (Nicolet 5 PC, Nicolet analytical instrument, Madison, WI). Microstructure and morphology of the seed and membrane layer, elemental mapping with EDAX, and cross sectional line scanning were examined using field emission scanning electron microscopy (FESEM: model Leo, S430i, UK).

In case of gas permeation studies, single gas and mixture gas permeation were measured by a specially designed permeation cell developed in our laboratory. For the permeation experiment the membrane was mounted on a stainless steel permeation cell, sealed between two silicon O-rings and mechanically supported by metal plate. The single gas permeance of the membrane was measured by soap film flow meter under the feed pressure of 100–300 kPa at room temperature. The permeability through the membrane was measured by using the formula,

where ΔP is the pressure difference between feed side and permeate side.

The permselectivity of two gases G₁/G₂ was defined as the permeance ratio of gas G₁ and G₂. The gas permeation measurement of each single gas was repeated until the permeance data for the successive 10 tests were closed. The single gas permeance was the average of 10 successive tests. For mixture gas (H₂–CO₂), separation selectivity was measured by gas chromatography (model – Trace GC ultra, serial no. 20092814, Thermo SCIENTIFIC, Germany) equipped with a thermal conductivity detector and HAYESEP N packed column. The oven temperature was kept at 50 °C. The separation factor α_A/B of the binary mixture permeation was calculated from eqn (1)

where x and y are the mole fractions of each component in the feed and permeate sides, respectively. Subscripts A and B refer to components A and B, respectively.

3. Results and discussion

Fig. 2 depicts the XRD pattern of DDR seed crystals synthesized in this work. The synthesis temperature and time was 170 °C for 2 days to 5 days. The characteristics peaks of the seed crystals are designed by their (hkl) values. The XRD pattern of the sample synthesized by the process corresponds to be most similar to that of the DDR structure and the d-values are in agreement with those reported in JCPDS file no. 71-0962. The intensity and peak positions are well matched with previously reported DDR XRD spectra. The XRD patterns confirm the formation of the DDR zeolite within 2 days and no extra peak appears after 2 days of synthesis. So DDR zeolite seeds can be produced within 2 days which is an appreciably short time of synthesis compared to the literature values.

Fig. 2 XRD pattern of DDR seed crystals synthesized for (a) 2 days (b) 3 days and (c) 5 days.

In the sonochemical reaction at room temperature, two Si(OH)₄ species combines to form Si–O–Si through a transition state with a penta co-ordinated species with a high activation energy in presence of free radicals formed under ultrasonic cavitation followed by the removal of water molecules.²⁸ The formation steps may be as follows:

In the alkaline environment, the dominant silicate species is anionic. The condensation reaction occurs through two steps. The first step is the formation of a Si–O–Si bond between two molecules and the second step is the removal of H₂O molecules from the dimer species. In another mechanism, in strong basic solution, the most stable species are Si(OH)₄⁻. The two Si(OH)₄ combine to form Si–O–Si bond. Because the silicon acquires a formal negative charge in the intermediate penta co-ordinate state, the anionic species OH⁻ should help in stabilizing the negative charges so that it can cross-link and interact with the cationic organic structure directing agent to form a porous structure. This assumption can be confirmed by the fact that some part of the structure directing agent 1-ADA may exist in the protonated form¹⁶ and then it should be stabilized by negatively charged SiO framework. In case of DDR synthesis, the reaction takes place under basic conditions where the silica species are present as anions and a cationic quaternary ammonium compound such as 1-adamantanamine is used as structure directing agent. The probable detail mechanism of the formation of DDR zeolite within short crystallization time described in our previous work²⁷ is shown in Fig. 3.

Fig. 3 Schematic presentation of interaction between silica and 1-adamantamine.

The IR spectra of DDR zeolite seed crystals are shown in Fig. 4, left. The sample showed strong vibration at 1377, 883, 767, 647, and 437 cm⁻¹. The characteristics band at 437 and 767 cm⁻¹ were assigned to O–T–O (T = Si) bending and Si–O tetrahedral vibration respectively. The appearance of the peaks at 647 and 1654 cm⁻¹ were attributed to the double ring external linkage and vibration of water molecule respectively. The peaks at about 2915 and 2860 cm⁻¹ correspond to the stretching vibration of 1-adamantanamine.²⁹ The symmetric stretching vibration of internal tetrahedron was shown at 747 cm⁻¹. The corresponding FESEM micrograph of DDR seed crystals is shown in Fig. 4, right. The surface area of the synthesized powder was 212 m² g⁻¹. The FESEM micrograph showed that the synthesized DDR seeds are nanosized powder having size ∼20 nm. In order to form a continuous zeolite membrane on the support surface, the coverage of the seed crystals must be high. Polycrystalline zeolite membrane contains defects i.e. inter-crystalline pathway which is also called non-zeolitic pores. Usually non-zeolitic pores are larger than zeolitic pores. The presence of non-zeolitic pores reduces the selectivity of the gases. The main sources of non-zeolitic pores are resulted from cracks and defects of the membrane layer. The major cause of the crack is due to the lack of good adherence between the zeolite layer and the substrate layer. To increase the adherence of the zeolite seed layer to the substrate surface, the polymer PDADMAC was used as intermediate linker between seed layer and support surface. Poly-dimethyldiallylammonium chloride is a widespread high charge density homopolymer and it readily interacts with various solid materials with negative surface charges.³⁰ So it was suggested that the PDADMAC adsorbs on the alumina support primarily due to electrostatic attraction between the negatively charged support surface and the positively charged PDADMAC. As the charge density of the PDADMAC polymer is high, it might also attract the negatively charged zeolite particles by electrostatic attraction. It is expected that because of the electrostatic interaction, the negatively charged DDR zeolite particles were formed homogeneously and easily deposited to the positively charged support surface, facilitating the formation of a uniform and dense zeolite DDR membrane. Fig. 5 shows the schematic presentation of the zeolite seed layer on the support surface by using PDADMAC as intermediate linker. Fig. 6a–c describe the FESEM micrograph of the surface morphology of bare substrate (Fig. 6a), substrate with polymer coating (Fig. 6b) and PDADMAC modified seeded substrate (Fig. 6c). The total elemental mapping of the seeded support is depicted by Fig. 7b–e. From Fig. 7a, it is shown that polymer and seed concentration was more in the middle portion of the membrane compared to the other part. We have deliberately chosen this portion to get the appreciable signal from that part of the membrane (as the thickness of the polymer layer is less) and also clear distribution of the elements. From the figure, the distribution of elements like Si (Fig. 7c) and Cl (Fig. 7d) confirms the presence of the PDADMAC polymer layer along with the DDR seed layer which is distributed to the whole support surface. The corresponding EDAX analysis as shown in Fig. 7f confirms the qualitative analysis of the constituent elements.

Fig. 4 (Left) IR spectroscopy and (right) FESEM micrograph of DDR seed crystals.

Fig. 5 Schematic of the binding mechanism between zeolite seed crystals and the support surface via PDADMAC polymer as an intermediate linker.

Fig. 6 FESEM micrograph of (a) clay alumina support, (b) polymer modified support, and (c) zeolite seed crystals on the polymer modified support.

Fig. 7 (a) FESEM micrograph of DDR zeolite seed crystals on polymer modified clay–Al₂O₃ support, (b–e) consequent elemental mapping (Al, Si, Cl and O) of zeolite seed coated polymer modified support, and (f) the corresponding EDAX spectra of the DDR layer.

Fig. 8a and b show the XRD pattern of the clay–Al₂O₃ support along with DDR zeolite seeded support respectively. The formation of the DDR membrane with a high degree of crystallinity on the seeded support was confirmed by XRD pattern as shown in Fig. 8c. It shows that all peaks matched well with those of DDR powder besides the signals from the support. The XRD pattern proves that a crystalline DDR zeolite membrane has been successfully synthesized within 5 days.

Fig. 8 XRD pattern of (a) clay alumina support (b) DDR zeolite seeded support and (c) DDR zeolite membrane on seeded support.

Fig. 9a and b describe the surface morphology and cross sectional view of the synthesized DDR zeolite membrane. It shows that well crystalline highly interlocked dense structured membrane was formed. The cross sectional view of the membrane shows the uniform thickness of about 20–25 μm. Elemental mapping with energy-dispersive X-ray spectroscopy (EDAX) associated with FESEM is highly useful for studying compositional analysis of the membrane. EDAX analysis (inset of Fig. 9a) confirms the formation of phase pure DDR membrane layer where the atomic ratio of the Si and O was 1 : 2 which is desirable for DDR formation. The corresponding line scanning view of the particular membrane layer and the spectra were shown in Fig. 9c and d respectively. From the cross sectional line scanning view (Fig. 9c), it indicates that the DDR membrane layer was formed mainly on the clay–Al₂O₃ surface. The Si and O peak intensity is higher when the scanning was started at the membrane layer and the absence of alumina confirms that no alumina was leached out from the support surface to the zeolite layer (Fig. 9d). So the above mentioned results confirm the formation of a phase pure DDR zeolite membrane within 7 days (2 days for seed and 5 days for membrane). However XRD and FESEM can only indicate whether a continuous membrane was formed or not on the support, but cannot confirm the quality of the zeolite membrane. The quality of the zeolite membrane can only be evaluated by gas permeation properties of the membrane.

Fig. 9 FESEM micrograph of (a) DDR membrane layer on the PDADMAC modified along with EDAX analysis (as inset) (b) cross sectional view of DDR membrane layer on the support indicating the thickness by the arrow, (c) the line scanning of the membrane layer with compositional element scan and (d) the corresponding spectra of O, Al and Si (distance in micron).

Before gas permeation studies, the membranes were calcined to remove all the structure directing agents and organic compounds present in the zeolitic pore. The permeation of CO₂ and H₂ was measured at room temperature. The permeance is expressed as the flux rate through all the pores (zeolite and non-zeolitic) present in the membrane. The diffusion rate becomes significantly smaller where the kinetic diameter of the gas becomes larger than the pore size of the zeolite. The molecular kinetic diameters of H₂ and CO₂ are 0.29 and 0.33 nm respectively, which are close to the pore size of DDR zeolite. The configurational diffusion and the difference in molecular size between H₂ and CO₂ results in the difference in the rate of diffusion through the DDR zeolite channels. The diffusion rate of H₂ is faster than that of CO₂ and therefore, H₂ and CO₂ can be separated by DDR zeolite membrane. Fig. 10 describes the change of permeated flux of both H₂ and CO₂ through DDR zeolite membrane with varying trans-membrane pressure difference. It shows that the rate of change of permeating flux with pressure is less for both CO₂ and H₂. In case of molecular sieving through zeolitic pores, the rate of change of flux is pressure independent. But here in this work, the increase of flux with pressure shows that the membrane is associated with low concentration of non-zeolitic pores. As CO₂ has high quadruple moment, at high pressure, CO₂ adsorbs more strongly on the DDR zeolite membrane surface than H₂ and the rate of desorption of CO₂ from the membrane surface also decreases as a result, permeating flux also decreases compared to hydrogen.

Fig. 10 Single gas permeated flux of H₂ and CO₂ as a function of trans-membrane pressure difference.

The real performance of the membranes can be enlightened by their mixture gas separation ability. Fig. 11 assigns the H₂–CO₂ separation factor of the mixture gas at room temperature as a function of CO₂ feed concentration at 200 kPa feed pressure for the DDR membrane. The selectivity for H₂–CO₂ mixture gases decreases with increasing CO₂ concentration. This phenomenon can be explained by an adsorption–diffusion method. CO₂ possesses 20 times more adsorption capacity than H₂ on the DDR membrane pore surface.³¹ So, CO₂ has been preferentially adsorbed on the DDR pore surface and with increasing CO₂ concentration, the extent of pore coverage has also increased. Accordingly, the rate of adsorption and desorption increases and thus, permeation of CO₂ increases. Moreover, due to competitive adsorption of CO₂, it slows the H₂ permeation and finally the overall separation factor decreases. The synthesized DDR membrane on the clay–alumina support shows good hydrogen gas separation capacity and the results obtained were also in good agreement with the previous reported values as described in Table 1.^31–33

Fig. 11 H₂–CO₂ separation factor of the mixture gas at room temperature as a function of CO₂ concentration in feed at 200 kPa feed pressure for DDR membrane.

Table 1 Comparison of selectivity values of H₂–CO₂ to literature values

Membrane/support	Temperature (°C)	Permeance (mol m^−2 s Pa)		Selectivity	Ref.
		H2	CO2
DDR/ceramic support	30	0.5 × 10^−7	1 × 10^−7	0.5	31
DDR/disk shaped α alumina support	30	0.3 × 10^−7	0.42 × 10^−7	0.7	32
DDR/alumina support	30	1.52 × 10^−6	4.07 × 10^−6	0.37	33
Low cost clay alumina support	30	4.04 × 10^−7	1.102 × 10^−7	3.7	This study

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4. Conclusion

In summary, DDR zeolite membranes were synthesized on a clay alumina tubular support within 7 days. DDR zeolite powders were successfully synthesized within 2 days by a sonochemical mediated hydrothermal method. Sonication energy facilitates the formation of active radicals which are responsible for rapid crystallization of the zeolite phase. The XRD, IR, FESEM and EDAX analysis confirms the formation of the DDR zeolite within short crystallization period. The sonochemical treatment shows a significant effect on crystallization time and phase formation of the DDR zeolite. Application of ultrasonic irradiation to the ultimate sol composition enhances the formation rate of DDR zeolite seed and membrane. As a result, the synthesis time of the DDR zeolite membrane has been reduced to 5 days instead of 25 days as described in literature. The power cost is very low compared to other processes and the synthesis period is very short, the sonochemical process is beneficial for the synthesis of DDR zeolites. Single gas permeation studies showed that selectivity (permeability ratio) of the permeance increases with increasing feed pressure. The selectivity of the H₂–CO₂ slightly increases with increasing pressure. At higher pressure, competitive adsorption and difference in diffusivity due to molecular kinetic diameter are responsible for separation of H₂–CO₂.

Acknowledgements

The authors would like to thank CSIR, India, for financial support and are also thankful to Mr Kamal Dasgupta, Acting Director, CSIR CGCRI, for his kind permission to publish the research work.

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