Recent development on the effect of water/moisture on the performance of zeolite membrane and MMMs containing zeolite for gas separation; review

Abstract

Understanding the effects of water vapour on gas permeation and separation properties of zeolite membranes especially at lower temperatures is important for the applications of these zeolite membranes for gas separations involving water vapour. Zeolite membranes have high potential for the purification of natural gas, biogas and syn-gas to produce the pipe-line gas and often those gases contain moisture. The separation of gases via zeolite is quite complex, particularly in the presence of water vapour in the feed stream. The increase of the fluxes of other gaseous components has been reported in some articles while the decrease is reported in the others. As well, both increase and decrease of selectivity are reported. Despite many works conducted during the past 15 years the details on the effect of the water vapour remain unknown. In this paper attempts are made to summarize the results of the experimental investigations with an emphasis on five types of zeolites, e.g. ZSM (MFI), DDR (deca-dodecasil 3R), SAPO-34 (silicoaluminophosphate), FAU (fauzasite) and 4A zeolite. As well, interpretation of the experimental observations is attempted based on a simple transport mechanism.

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Introduction

Gas separation is of vital environmental concern nowadays due to the issue of global climate change. The phenomenon of increasing greenhouse gas concentration, especially carbon dioxide (CO₂), in the atmosphere has drawn increasing interest among the researchers toward finding efficient methods for CO₂ capture before being emitted to atmosphere. Separation of CO₂ is one such essential step in the natural gas processing plant. Polymeric membranes are widely applied for gas separation in view of their characteristics such as low cost and ease of fabrication into desired gas separator modules. However, their application for gas separation is limited due to the instability at high temperature and occurrence of plasticizing effect in the presence of high CO₂ pressure, which results in reduced CO₂ separation performance of the polymeric membranes. The deficiencies of polymeric membranes have prompted the development of zeolite membranes, which could overcome the problems for CO₂ gas separation.

Zeolites

The term “zeolite” was originally coined in 1756 by Swedish mineralogist Axel Fredrik Cronstedt, who observed that rapid heating of the material stilbite produced large amounts of steam from water that had previously been adsorbed into the material. Zeolites are microporous, aluminosilicalite or silicate minerals. As of November 2010, 194 unique zeolite frameworks have been identified, and over 40 naturally occurring zeolite frameworks are known. Some of the more common mineral zeolites are analcime, chabazite, clinoptilolite, heulandite, natrolite, phillipsite, and stilbite.¹

Zeolite membranes can be used at high temperatures and with organic solvents, when polymeric membranes cannot operate. Another advantage of thin, supported zeolite membranes is well-defined pores of molecular dimensions. Thus zeolites have great potential to exhibit both high selectivity and high permeability. One of the applications of zeolite membranes is separation of small molecules from gas streams. The latter can be addressed in dehydration of natural gas (NG) in which the water vapour should be removed from the gas stream prior to its entrance to the pipeline. Natural gas contains methane (CH₄) as the main component, and other impurities such as CO₂, hydrogen sulfide and water vapour. Water vapour is the most common impurity in natural gas. Separation of water vapour from NG decreases its potential for corrosion, hydrate formation and freezing along the pipeline. Among different processes for dehydration of NG, two processes are used at industrial scales including absorption by liquid desiccants and adsorption by solid desiccants.²

Generally zeolites are tridimensional microporous crystalline aluminosilicates. The crystalline aluminosilicates consist of Si and Al tetrahedral units (TO_4/2, where T = Si or Al) linked through bridging oxygen atoms giving rise to the so-called secondary building units (SBUs) that are constituted by rings and prisms of various sizes.³ These units combine to generate frameworks with a regular distribution of molecular-sized pores and cavities. Zeolites have the chemical formula M_2/n OAl₂O₃·xSiO₂·yH₂O, where the charge-balancing nonframe work cation M (M cation usually from groups I or II) has valence n, x is 2.0 or more, and y is the moles of water in the voids.⁴

The Si/Al ratio of the zeolite structure and amount of cations control the surface properties of zeolites (e.g., hydrophobicity and acidity), and determine their adsorbent, catalytic and ion-exchange properties. Pores of zeolites are uniform in sizes determined by their crystal structure.^3,5 Generally, zeolites can be classified into small, medium, large, and ultra large pore materials. Small pore structures have pore apertures consisting of six, eight or nine tetrahedral (6-, 8-, and 9-membered rings), medium pore frameworks have 10-membered rings, large pore zeolites have 12-membered rings, and ultra large structures have 14-, 18- or 20 membered rings.⁶

The possibility of zeolite properties in a membrane configuration is an attractive one, with many potential applications. Their potential molecular sieving action, large surface areas and controlled host–sorbate interactions, especially at low vapour pressure, are the primary motivations in microporous zeolite membranes.⁶ Zeolite membranes are completely different from simple zeolite powders.

Zeolite membranes are the microporous inorganic membranes with higher thermal, mechanical and chemical stabilities compared to organic membranes. Its uniform and molecular-sized pore structure with controlled host–sorbate interactions makes it attractive as a shape-selective material for gas separation. Separation of CO₂ is one such essential step in the natural gas processing plant. Natural gas, biogas, syn-gas contain water and water absorbs in zeolite. Synthesis gas produced from biomass usually contains more CO₂ than desired, and it also contains hydrogen sulphide and moisture/water. It is necessary to study the effect of water/moisture on CO₂ separation by zeolite membrane when the pipeline gas is produced from natural gas, biogas or syn-gas. The effects of water vapour on gas permeation and separation properties of zeolite membranes are important to the applications of these zeolite membranes for separation involving water vapour. It is well known that water/vapour lowers H₂ and CO₂ permeation especially at low temperature.

The decrease in H₂ permeance could be attributed to the adsorption of water vapour and blocking of zeolite pores. This phenomenon should be more significant at lower temperature where the mobility of H₂O molecules decreases and the amount of adsorbed H₂O molecules increase due to the stronger interaction.

At high temperatures, the interaction between water vapour and surface material causes the densification or damage of pore structure, while at low temperatures, the permeance decreases significantly due to the presence of water vapour, even without destroying the pore structure.^5,7

It was observed that the transport of gas through zeolite membrane as a function of temperature goes through a maximum, then decreases, and rises again at higher temperature.⁸ The separation via zeolite membrane is based on molecular size and polarity among others. Molecules with higher molecular weight permeate better at low temperature while at high temperature lighter molecules show better permeability. This is the reason why H₂ permeates better at higher temperatures (about 500 °C) while CO₂ does at lower temperature. In general these membranes show high permeability and low selectivity.⁹

The particular affinity a species to an internal zeolite cavity depends on electronic considerations. The strong electrostatic field within a zeolite cavity results in very strong interaction with polar molecules such as water. Non-polar molecules are also strongly adsorbed due to the polarizing power of these electric fields. Thus, excellent separations can be achieved by zeolites even when no steric hindrance occurs.¹⁰

The structure and chemistry of support materials also play an important role in zeolite membrane formation and have a significant influence on the separation property of the membrane material. Surface roughness and pore size define the optimum zeolite crystal size and membrane thickness. The chemical nature of the support surface influences zeolite nucleation, crystal growth and membrane adhesion. It has been demonstrated that chemical modification of the support surface can significantly alter the zeolite membrane formation.¹¹

The separation of a gas mixture by zeolite is affected by:¹²

(1) Size or steric exclusion of certain components of a gas mixture prevents them from entering the zeolite pores, whereas the other components enter the pores and are adsorbed.

(2) Thermodynamic selectivity, which is a quantitative measure of the preferential adsorption of certain components over others when all components can enter the pores.

(3) Kinetic selectivity, which is a quantitative measure of the ability of certain components to enter the pores (and become adsorbed) faster than other components.

The suppression effect of an adsorbing component on the permeation of anon-adsorbing component is well known for zeolite membranes.¹³ Several articles on the suppression effect of water vapour on gas permeation in MFI membrane at low temperatures have been reported in literature.^14–16 Aluminum atoms in the zeolite framework significantly affect the physicochemical properties of zeolites. Water molecules adsorb on zeolite surface through dipole–field interactions as well as hydrogen bonds with residual hydroxyl groups.¹⁷ The number of the latter also depends on the population of Al atoms in the framework. Therefore, the affinity of zeolites with water molecules depends on their Si/Al ratio.^18,19 During the last fifteen years the effect of water/moisture on the separation of gases via zeolite membranes, mainly ZSM (Zeolite Socony Mobil–5) (MFI), SAPO-34 (silicoaluminophosphate), DDR (deca-dodecasil 3R), and FAU (fauzasite) has been reported.

Calero and Gómez-Alvarez¹⁸ made detailed studies of the H bonded structure of water adsorbed in zeolites using Monte Carlo simulation with validated atomistic descriptions and intermolecular interactions. The calculated hydrogen bonding properties were in close relation to the adsorption behavior. The adsorption mechanism proceeds through nucleation of water molecule in zeolite. At saturated states, it was proved by simulations that water is significantly perturbed from its bulk behavior. More extended and complex hydrogen bonding networks were found in FAU and MFI zeolites.

A number of methods have been reported for the preparation of zeolite membranes. These methods are generally differing in the pre-treatments of the stable or temporary supports, impregnation techniques for these supports, or in their post-synthesis treatments. In principle, synthesis mixtures characterized by high percentages of water and low alkalinity are optimal for the preparation of the zeolitic membranes.⁶ It is well known that the process for the preparation of membranes affects the performance of finished membranes. Zeolite membranes were often prepared in conditions similar to zeolite powder synthesis. Several methods for the synthesis of zeolite membranes have been developed in order to control the thickness of the membrane, compactness, and orientation of atoms or molecules. Beside these properties, the hydrophilic/hydrophobic nature of zeolites can be tuned by changing the Si/Al ratio in the framework of zeolites. In general all methods described in the literature involve either the use of zeolite crystals previously synthesized or the crystallization of zeolite layers. Zeolite membranes are usually prepared by in situ hydrothermal synthesis on porous stainless steel, alpha-alumina, or alpha-alumina support tubes or disks for the gas permeation studies.

There are many methods for the preparation of zeolite membranes which are as follows:^19–24

(1) Embedded method: Barrer and James²⁵ were the first to fabricate zeolite membranes in which microcrystalline ion-exchanging zeolites are bonded by inert polymeric fillers in such a way that the electrochemical behavior is determined by the crystals and by crystal contacts. The embedded method is not a good choice as in the final membrane there will be many defects and the performance is not reliable.

(2) In situ hydrothermal synthesis method: this method is commonly used. In this method bring a porous support in direct contact with the synthesis solution or gel, and then to allow the growth of a zeolite film on the surface of the support under hydrothermal conditions. The formation of the zeolite membrane under hydrothermal conditions involves the formation of the supersaturation region adjacent to the substrate surface, nucleation, aggregation, crystallization and crystal growth.

(3) Seeding technique (secondary growth method): to enable better control of nucleation and crystal growth steps, seeding technique is better than other techniques. In this method, first a colloidal zeolite suspension of sub-μm-sized seed crystals prepared. These crystals will be coated as a seed layer on the surface of the substrate. Hydrothermal synthesis is followed to grow zeolite film on the seed layer. The preparation of zeolite sol as crystal seeds is the key step in the secondary method.

(4) Microwave method: the microwave method for the preparation of zeolite membranes is similar to the conventional hydrothermal method except that the autoclave is placed in a microwave field. The synthesis time reduce greatly.

(5) DGC method was first proposed by Xu et al.,²⁶ which is a novel method to synthesize zeolite membrane on porous supports. The zeolite ZSM-5 has been synthesized from amorphous aluminosilicate gels in a vapour of ethylenediamine, triethylamine, and water. DGC has the benefit of better thickness control compared with liquid phase synthesis, since the amount of nutrient for growing zeolite is directly controlled by the amount of gel applied.

Hedlund et al.^27,28 discussed a method to prepare an ultra-thin (less than 1 μm) membrane without defects on an open support.

ZSM/MFI

ZSM-5, Zeolite Socony Mobil-5, (framework type MFI from ZSM-5) is an aluminosilicate zeolite belonging to the pentasil family of zeolites. Its chemical formula is Na_nAl_nSi_96−nO·16H₂O (0 < n < 27). Several articles are reported on the suppression effect of water vapour on gas permeation in MFI zeolite membranes at low temperature (<572 K).^11,12 Gas permeation through MFI zeolite membranes in the presence of water vapour at high temperatures (above 300 °C) is technically important to apply MFI zeolite membranes for chemical reactions and separation involving water vapour, such as water gas shift reaction for hydrogen production.²⁹

Highly siliceous ZSM-5 zeolites with low Al₂O₃ content are known to have highly hydrophobic surfaces.³⁰ It is well known that the surfaces of the siliceous zeolites (e.g., silicalite, dealuminated mordenite, USY-zeolites) show higher hydrophobic properties than Al₂O₃-containing zeolites.³¹ Takeuchi et al.³² studied the adsorption of the water molecules on various ZSM-5 zeolites (different SiO₂/Al₂O₃ ratio, Table 1).

Table 1 SiO₂/Al₂O₃ ratio of various ZSM-5 zeolite³²

Zeolite	SiO2/Al2O3
ZSM-5	1880
H-ZSM	220
H-ZSM-5	68

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HZSM zeolite (SiO₂/Al₂O₃ ratio of 1880), with a low Al₂O₃ content, exhibited the highest hydrophobicity among the ZSM-5 zeolites with different SiO₂/Al₂O₃ ratios. As the Al₂O₃ content increased to ca. 4 wt%, corresponding to a SiO₂/Al₂O₃ ratio of 23.8, the amount of H₂O adsorbed increased three fold more than the highly siliceous H-ZSM-5 (1880).³² Desorption of the H₂O molecules can be seen at 373–573 K for H-ZSM-5 zeolites with low SiO₂/Al₂O₃ ratios of 23.8 and 68. However, very small amounts of H₂O desorbed from H-ZSM-5 with low Al₂O₃ content (SiO₂/Al₂O₃ = 220 and 1880). Moreover, as the SiO₂/Al₂O₃ ratio decreased, the desorption temperature shifted slightly toward higher-temperature regions. Generally, H₂O molecules weakly adsorb on surface hydroxyl groups by the hydrogen bonds and strongly adsorb on the cationic sites of oxide surfaces by electrostatic interactions. Maybe this phenomenon is also true for other zeolites.

A detailed study of the effect of moisture on the CO₂/N₂ permeation and separation performance of Mobile Five (MFI) zeolite membranes was done by Sublet et al.³³ At high water concentrations, beyond the value at which capillary condensation occurs in small mesoporous defects, MFI membranes show the gas permeance reduction of about 40% at the steady state when the water concentration is 10 v/v% at 373 K. The incorporation of water promotes however CO₂/N₂ separation factors, most probably due to defective pore blockage and formation of bicarbonate stable intermediates. The CO₂ permeation and separation properties of nanocomposite MFI-alumina membranes in the presence of water, combined with their hydrothermal stability and optimal reproducibility, make these materials potentially competitive for post combustion CO₂ capture applications, compared with other zeolite membrane materials (e.g., FAU-type membranes).

Hill and Seddon³⁴ investigated the hygroscopic nature of H-ZSM-5 and reported that under ambient conditions the zeolite H-ZSM-5 contains 7–9% water and anhydrous zeolite can be obtained by prolonged heating over 200 °C (473.1 K). On the other hand, silicates typically contain much less water (around 1%) under ambient conditions. The sorption of water into H-ZSM-5 obeys Henry's law. The surface of the zeolite is hydrophilic, and water is able to displace hydrocarbon from the surface. Moreover sorption of water inside the channels requires the presence of acid centers which can break up surface bound water. The apparent anhydrous nature of silicalites is seen as a result of surface bound water being unable to penetrate the channels because of hydrogen bonding between water moieties and the hydrophilic surface favors the structuring of the adsorbed water and militates against individual water molecules diffusing down the channels. The very strong a desorption of water on the zeolites is caused by interaction of the permanent and large dipole moment of water with a zeolite cation. The Henry's law constant for adsorption of water on KA zeolite is much larger than that for the NaX zeolite due to the smaller cavity of the A zeolite and the higher charge density of the K ion.

Chen³⁰ hypothesized the role of defects to the water adsorption and studied adsorption of water vapour gravimetrically in mordenite type zeolites from pressure ranging from vacuum to saturation. It was reported by Chen that each additional alumina site allowed for a coordination (or adsorption) of four molecules of water. Matsukata et al.¹⁵ studied the effect of water adsorption on MFI-type zeolite during the permeation of H₂, n-C₄H₁₀ and equimolecular mixed gas H₂/n-C₄H₁₀. It was observed that the permeation properties of a MFI-type zeolite membrane were influenced by the water adsorbed in the membrane. Even 500 ppb water stream influences the single gas permeation properties of the membrane. These properties of the membranes might be caused by strong hydrophilicity of highly aluminated MFI-type zeolite formed in the in situ crystallization method.

Noack et al.¹⁴ studied the water vapour permeation fluxes through MFI zeolite membranes with different Si/Al ratios at low temperatures 378 K (105 °C). In pervapouration and steam permeation measurements of water/iso-propanol and water/methanol mixtures, the silicate membrane showed a hydrophobic and the ZSM-5 membrane a hydrophilic separation behavior. Funke et al.³⁵ found no permanent effect of water vapour on the pure gas permeation performance of ZSM-5 membranes. Though the pure N₂ and SF₆ permeances decreased after humidification of the feed stream (61% for N₂and 16% for SF₆), the initial values were recovered after water removal.

Bernal et al.³⁶ investigated the separation of equimolecular CO₂/N₂saturated with water vapour at room temperature using B-ZSM-5 membrane. The CO₂ permeation fluxes decreased about 54% and 29% for feed pressure of 1.7 and 2.2 bars, respectively, while the temperature of the membrane were increased from 300 K to 473 K. The existence of water in the feed did not change the separation factor significantly, even at low temperatures. In summary, the effect of humidity was small, due to the limited hydrophilicity of ZSM-5 membranes.

Shin et al.³⁷ studied ZSM-5 zeolite membranes for CO₂ separation from CO₂/N₂ mixture. ZSM-5 membranes were synthesized via hydrothermal-treating of various porous α-alumina tubes in template reaction mixtures. The ZSM-5 zeolite membranes were evaluated by the CO₂/N₂ separation factor and permeance measured as a function of the stage cut, the helium sweeping rate, the feed pressure, and the permeation temperature. The maximum separation factor of the ZSM-5 zeolite membrane for a CO₂/N₂ (50% CO₂) gas mixture was about 54.3 at 25 °C and its permeance was 3.6 × 10⁻⁸ mol m⁻² s⁻¹ Pa⁻¹. Fig. 1 shows the separation factor of the ZSM-5 zeolite membrane in moisture-saturated feed gas system. Separation factor increases as the permeation test time increases. In presence of humid gas, selectivity went down to even less than 1. With the increase in experiment time, the selectivity increased with time and reached >50, which was almost the same as obtained with dry feed. Shin et al. suggested that at the beginning, permeate N₂ flow is more than CO₂ flow because CO₂ adsorption on the zeolite pore wall was hindered by moisture. As the permeation time increases, moisture occupies the large pores through which N₂ flows mainly. In these circumstances the flow of nitrogen decreased. Fig. 1 shows that after 50 min experiment time, the CO₂/N₂ separation factor in moisture-saturated feed gas system is slightly higher (<55) than that in dry feed gas system (around 55). It is possible because ZSM-5 zeolite has a high hydrophobic quality.

Fig. 1 CO₂/N₂ separation factor vs. gas permeation test time for the surface modified ZSM-5 zeolite membrane in the moisture-saturated feed gas system. Reproduced from ref. 37 with permission from the Elsevier.

It was suggested by Shin et al.³⁷ that the permeation of CO₂ through ZSM-5 zeolite membranes was mainly governed by both mechanisms of the adsorption/surface diffusion and the activated micropore diffusion, while that of N₂ was governed by the activated micropore diffusion only.

Wang and Lin³⁸ studied the effects of water vapour on H₂ and CO₂ permeation and separation properties of ZSM-5 (Si/Al ∼ 80) zeolite and aluminum free silicalite membranes. The experiments were carried out with equimolar H₂/CO₂ binary and H₂/CO₂/H₂O ternary mixtures at temperatures 300–550 °C. Fig. 2 shows the temperature dependence of selectivity. The H₂/CO₂ separation factor for the mixture with and without water vapour is essentially the same for both membranes indicating that the presence of water vapour suppresses gas permeation for H₂ and CO₂ to the same extent. It suggests that at high temperatures water vapour has the same effects on lowering the permeation rate of other gases, regardless of gas type and zeolite surface-gas molecular gas interactions.

Fig. 2 H₂/CO₂ separation factor with equimolar H₂/CO₂ binary mixture (square) and H₂/CO₂/H₂O ternary mixture (circle) as feed for silicate (a) and ZSM-5 zeolite (b) membranes. Reproduced from ref. 38 with permission from the John Wiley and Sons.

Wang and Lin³⁸ also reported that both ZSM-5 and aluminum-free silicalite membranes adsorb water vapour even at temperatures in the range of 572–782 K and reduce the permeability of H₂ and CO₂ in the pores of zeolite membranes. It was also reported that water vapour exhibits negligible effects on gas permeation through silicate membranes at even high temperatures (>822 K). The suppression effect of water vapour on H₂ and CO₂ permeation was larger for the less hydrophobic ZSM-5 zeolite membrane than for the hydrophobic silicate membrane. For both membranes the suppression effect was stronger at lower temperatures and higher water vapour partial pressures.

Lindmark and Hedlund³⁹ studied the CO₂ removal from CO₂/H₂ in absence of water and in presence of water at the temperatures between 22 and 150 °C using MFI membranes. Different types of MFI membranes (LiZSM-5, BaZSM-5) were used and they were prepared from NaZSM-5. It was observed that the Si/Al ratio and the counter ions in the membrane had a significant effect on both single gas permeation and mixture separation by modifying both the effective pore size and the adsorption properties of the membrane. Three feed system (Table 2) were used to study the water/moisture effect on the performance of MFI membranes for the separation of CO₂/H₂ mixture.

Table 2 Compositions of the three feeds used in the separation experiments. The partial pressure of each component (kPa) is given in the table³⁹

Feed	CO2	He	H2	H2O
A	50.65	50.65	—	—
B	—	49.6	49.6	2.1
C	49.6	49.6	—	2.1

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Fig. 3 shows the results of experiments with feed A. The binary CO₂/H₂ permeances (a) and separation factors (b) for a silicalite-1 membrane (S1) and three ZSM-5 membranes with different counter ions (LiZ1, NaZ1 and BaZ1) are shown. Even though adsorbed CO₂ is not expected to block the permeation of hydrogen significantly, or vice versa, the hydrogen and CO₂ permeances at 25 °C are 5–6 times lower than the permeances observed in single gas measurements at the same temperature. This is probably explained by a combination of concentration polarization and back diffusion of helium from the sweep gas in the mixture separation experiments. The hydrogen permeance is lowest at 22 °C and increases by a factor of 1.5–2.0 with increasing temperature up to 150 °C for all membrane types.

Fig. 3 Binary CO₂/H₂ permeances (a) and separation factor (b) for a silicalite-1 membrane (S1) and three ZSM-5 membranes with different counter ions (LiZ1, NaZ1 and BaZ1). The feed consists of 50.65 kPa H₂ and 50.65 kPa CO₂. Sweep gas, He at 1000 ml min⁻¹ and atmospheric pressure. Reproduced from ref. 39 with permission from the Elsevier.

Fig. 4 shows the results of experiments where B was used as feed. Only the binary H₂O/H₂ permeation and separation factor are shown as the function of temperature. The hydrogen permeance is about 5–6 times lower in the presence of water (feed B) than in the presence of CO₂ (feed A), as suggested by Lindmark and Helund³⁹ it is possible that water is blocking the permeation of hydrogen quite effectively, whereas CO₂ seems to block hydrogen permeance to a minor extent. As the adsorption of water decreases with increasing temperature, hydrogen permeance increases.

Fig. 4 Binary H₂ permeance (a) and H₂O/H₂ separation factor (b) for a silicalite-1 membrane (S1) and three ZSM-5 membranes with different counter ions (LiZ1, NaZ1 and BaZ1). The feed consists of 49.6 kPa H₂, 49.6 kPa He and 2.1 kPa H₂O. Sweep gas, He at 1000 ml min⁻¹ and atmospheric pressure. Reproduced from ref. 39 with permission from the Elsevier.

SAPO-34 (silicoaluminophosphate)

The crystal structure of SAPO, a micro pore zeolite, is similar to that of chabazite and has a special water absorbing capacity. Its formula is

(SiO₂)_x(Al₂O₃)_y(P₂O₅)_z.

Thermogravimetric studies on the adsorption of water on SAPO-34 powder have shown that significant adsorption only occurs below 373 K, and heating above this temperature is sufficient to desorb most of the water.⁴⁰ Poshusta et al.⁴¹ fabricated a SAPO-34 (silicoaluminophosphate) membrane and noted a CO₂/CH₄ separation factor of 30 at 300 K and 4.7 at 470 K. They also reported the effects of water content on gas permeation and suggested that adsorbed water appeared to completely block the SAPO pores, but permeation through non-SAPO pores increased with water content. Further, Poshusta et al.⁴² investigated the effects of humidity on gas permeation of five SAPO-34 membranes synthesized by the same procedure, but their initial properties were different. These differences could be due to uncontrollable factors in the synthesis (e.g. support defects, dust in the synthesis solution, and stress fractures in the membrane) or humidity induced changes in the membranes between the initial calcination and when the first permeances were measured. The effect of water on single gas permeances depends on membrane quality.

Membranes with high CO₂/CH₄ separation selectivities (>20) were stable in humidified gases, but degradation was seen for some membranes after months of exposure to the laboratory atmosphere. Once the membranes started to degrade, the rate of degradation appeared to accelerate. The degradation created non-SAPO pores that were larger than the SAPO-34 pores, as noticed by i-C₄H₁₀ permeance. The effect of humidity on gas permeance correlated with these indicators of non-SAPO pores. Adsorbed water appeared to completely block the SAPO pores, but permeation through non-SAPO pores increased with humidity. It was concluded by Li et al.⁴³ that water adsorption can be used to determine membrane quality and the fraction of transport through non-SAPO pores. The CO₂ recovered the original permeances and selectivity after calcination at 573 K for 24 h.⁴³ Water had a strong effect on gas permeation through SAPO-34 membranes. An important drawback of silicoaluminophosphates was their hydrophilicity and, as a consequence, low stability under humid conditions.⁴⁴ Thus, the separation performance of SAPO-34 membranes is greatly disturbed by the presence of water⁴² and reported that CO₂ flux and selectivity decrease in the presence of water since water has a strong affinity to the hydrophilic SAPO-34 membrane. Therefore, hydrophobic narrow pore zeolite membranes are more appropriate to separate CO₂ from humid gases. Consequently DDR membranes show high CO₂ flux and selectivity and an only negligible water influence on the separation performance in the CO₂ separation from natural gas^45,46 as shown below.

DDR type zeolites

Among the various zeolite materials, DDR (Deca-dodecasil 3R) is a pure silica (SiO₂) zeolite. The pore system comprises relatively large (19-hedral) cages interconnected through 8-ring windows with aperture approximately 3.6 × 4.4 Å. Due to its relatively small pore size, DDR can be used to separate light gases, such as CO₂ (kinetic diameter = 3.3 angstroms) from CH₄ (diameter = 3.8 angstroms). Other advantages of DDR zeolites include high thermal stability and chemical resistance due to the pure silica composition. Zeolite DDR can be synthesized from various precursor materials and methods in the temperature range 363–473 K for periods of time between several hours to several weeks in autoclaves with different sol composition.

Five new methods for synthesizing DDR zeolite nanocrystals (200–2000 nanometers in size) are disclosed using hydrothermal secondary growth (seeded growth). By changing the ratio of silica to water, the synthesis temperature, and the mineralizing agents, the morphology and size of the crystals can be manipulated.¹

DD3R membrane showed high selectivity of CO₂ over methane. The DD3R zeolite is also considered suitable for use in separation of biogases in which moisture is abundant.² The permeation and separation characteristics of light gases through DD3R membranes can be explained by taking into account: (1) steric effects introduced by the window opening of DD3R leading to molecular sieving and activated transport, (2) competitive adsorption effects, as observed for mixture involving strongly adsorbing gases, and (3) interaction between diffusing molecules in the cages of the zeolite.⁴⁷ DDR type zeolite is essentially comprised of silicon and oxygen atoms, and is expected to be affected less by water adsorption.

The effects of H₂O on CO₂/CH₄ separation were investigated for DDR type zeolite membranes by Himeno et al.⁴⁸ The system was maintained at a steady state with a CO₂/CH₄ mixture at 298 K for 4 h; a CO₂/CH₄ mixed gas that contained water vapour was then introduced to the feed. The CO₂ permeance through the DDR-type zeolite membrane decreased rapidly from 2.5 × 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ to 1.1 × 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹, although the CH₄ permeance was almost equal to that for a dry mixture gas. The CO₂/CH₄ separation selectivity decreased to half that of the dry CO₂/CH₄ mixture. However, the CO₂ permeance and CO₂/CH₄ selectivity were restored rapidly afterward when dry gas mixture was introduced. The CO₂ permeance and the CO₂/CH₄ selectivity with saturated water vapour were retained respectively as 1.1 × 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ and 100.

Tomita et al.⁴⁵ fabricated the molecular-sieve DDR type zeolite membrane with an aperture of 0.36 × 0.44 nm on a porous alumina substrate by hydrothermal process and studied the performances for CO₂/CH₄ separation in presence of water. At 301 K the permeance of CO₂ decreased from 7 × 10⁻⁸ mol m⁻² s⁻¹ Pa⁻¹ in the dry mixed gas feed to 4 × 10⁻⁸ mol m⁻² s⁻¹ Pa⁻¹ in the mixed gas feed with 3.3% water. The separation factors of dry and wet gas feed were 280 and 330, respectively. On the other hand, at 373 K the permeance was 4 × 10⁻⁸ mol m⁻² s⁻¹ Pa⁻¹ and the separation factor was 120. No changes were observed between dry and wet gas feed. It was suggested by Tomita et al.⁴⁵ that water adsorbed into the micropores and blocked the gas permeation of both CO₂ and CH₄ at 301 K to some extent, while at 373 K little effect of water adsorption on the gas permeation was noticed. The DDR type zeolite membrane consists only of silicon and oxygen atoms and is classified as hydrophobic among other zeolites. It was suggested by Tomita et al. that DDR type zeolite membrane is not perfectly hydrophobic as they observed some water adsorption effects at 301 K. It was concluded by Tomita et al., that DDR type membranes worked as molecular-sieving membrane. Himeno et al.⁴⁸ reported a comprehensive study on high-quality DD3R membrane, pointing out a remarkable decrease of CO₂ permeance (ca. 40%) in the presence of a moisture saturated stream, the CO₂/CH₄ separation factor being enhanced about 50%.

Himeno et al.⁴⁸ also studied the adsorption isotherm of water vapour on DD3R crystal. Fig. 5 shows the adsorption isotherm of water vapour on DD3R crystal at 298 K.⁴⁷ The upper limit of the relative pressure of water vapour was 0.92, and the maximum amount adsorbed was ∼19 mg g⁻¹ at 298 K at a water vapour pressure of 2.93 kPa. The maximum amount of adsorbed water vapour was compared to the accessible volume of the intra-crystalline space of the DDR. The accessible volume was estimated to be 0.14 ml g⁻¹, using liquid nitrogen adsorption, and the degree of volume filling was 0.13. A unit cell of DDR is comprised of six decahedra, nine dodecahedra, and six 19-hedra. Only the 19-hedra can absorb adsorbate molecules.^49,50 19-Hedron cage can theoretically accommodate around 10 water molecules. The number of adsorbed water molecules per 19-hedron cage was around 1. Therefore, water vapour was considered to influence permeation through DDR-type zeolite membranes only slightly. Indeed, the DDR-type zeolite membranes are only slightly affected by water vapour: the CO₂ permeance and the CO₂/CH₄ selectivity with saturated water vapour at 298 K remained 1.1 × 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ and 100, respectively.

Fig. 5 Adsorption isotherm of water vapour on a DDR crystal at 298 K. Reproduced from ref. 48 with permission from the Royal Society of Chemistry.

FAU type zeolite

Studies were made on CO₂ removal from low pressure flue gas using naturally occurring zeolites such as X and Y faujasite systems⁵¹ including synthetic zeolites such as 5A and 13X.⁵² Faujasite has a 3-dimensional pore structure with pores running perpendicular to each other in the x, y, and z planes similar to LTA and has a void volume fraction of 0.48, with a Si/Al ratio of 2.43. Gu et al.⁵³ prepared FAU (Y-type) zeolite membranes by secondary growth of NaY seed layers on alpha alumina substrate and studied the separation of dry and humidified CO₂/N₂ mixtures. The separation results for the dry and humid CO₂/N₂ mixtures are shown in Fig. 6. With the humidified CO₂/N₂ mixture, the permeance values of all three components, including H₂O, CO₂ and N₂, increased with the temperature in the entire tested range, while α_CO2/N2 experienced a maximum at about 140 °C. At room temperature, the CO₂ selectivity was about 31.2 for CO₂/N₂ dry gas mixture with a CO₂ permeance of 2.1 × 10⁻⁸ mol m⁻² Pa⁻¹ s⁻¹. The addition of water vapour to the feed stream decreased the performance for both CO₂ and N₂ in a temperature range of 23–200 °C. In presence of water vapour the selectivity was increased in the 110–200 °C range. On the other hand the selectivity of CO₂ drastically reduced blow 80 °C. In the entire range of tested temperature, both CO₂ and N₂ permeance of the moist mixture were below those for the dry mixture due to the adsorbed H₂O molecule as it reduced the zeolite pore volume.

Fig. 6 Gas permeance and CO₂ factor as functions of temperature for equimolecular CO₂/N₂ mixtures under dry and moist conditions. Reprinted with permission from⁵³ American Chemical Society.

The results of CO₂ separation at 200 °C are shown as a function of the water partial pressure in Fig. 7. At 200 °C, with increasing water partial pressure, the CO₂ selectivity showed a maximum of 4.6 at water partial pressure of 12.3 kPa. It was explained by the closure of the zeolite pore at high water partial pressure, which limited the entry and transport of CO₂ molecules. The N₂ permeance barely changed while the CO₂ permeance continued to drop after α_CO2/N2 reached the maximum, suggesting that N₂ permeation was mainly through large intercrystal pores.

Fig. 7 Effect of the water partial pressure on CO₂/N₂ separation at 200 °C. Reprinted with permission from ref. 53 American Chemical Society.

Sawamura et al.⁵⁴ demonstrated that FAU zeolite membranes have superior stability in comparison with a conventional LTA-type zeolite membrane in a separation of mixture containing large water content. Shirazian and Ashrafizadeh⁵⁵ studied the water vapour and methane permeation through NaA zeolite membrane. Zeolite NaA seeds for preparation of membranes were synthesized using hydrothermal treatment. NaA membranes were also prepared via secondary growth method on porous alpha alumina as substrate. The results showed that increasing the temperature enhances the permeation of water vapour more than that of methane. Ideal selectivities increased from 14 to 18 for temperatures of 303 and 343 K, respectively. A mass transfer model was developed based on Maxwell–Stefan approach for the prediction of vapour transfer through zeolitic pores of the membranes. Unilan isotherm was utilized for description of adsorption behavior of water vapour on NaA zeolite. The deviation of the model from experimental data was reasonable, while more satisfactory predictions are observed for temperatures higher than 333 K.

SSZ-13

SSZ-13 (structure type CHA) is an aluminosilicalite zeolite mineral belonging to the ABC-6 family of zeolites. Its chemical formula is

RN_aNa_bAl_2.4Si_33.6O₇₂·wH₂O (1.4 < a < 27) (0.7 < b < 4.3) (1 < w < 7),

where RN is N,N,N-1-trimethyladamantammonium. The material was patented by Chevron research Company in 1985,⁵⁶ and has a potential use as a solid catalyst for the methanol-to-olefins (MTO) process.

Kosinov et al.⁵⁷ fabricated high-silica (gel Si/Al = 100) SSZ-13 membranes by hydrothermal secondary growth on the surface of α-alumina hollow fiber supports. The membranes were evaluated for their performance in the separation of CO₂ from equimolar mixtures with CH₄ or N₂. They tried to compare the separation of gas mixtures in dry and humid conditions to determine the potential of the SSZ-13 membrane for separation of CO₂ from wet gas mixtures. The feed gas mixture was humidified by adding 2.2 kPa of water. Fig. 8 shows the results for the separation of dry and humidified CO₂–CH₄ and CO₂–N₂ as a function of temperature. The presence of water affects the permeance of all the gases, which should be predominantly due to partial blockage of the zeolite pores. At 293 K the gas permeance is substantially lower under humid conditions compared to dry conditions (Table 3). Due to the decreasing water coverage, the permeance of all of the gases increases with increasing separation temperature. Already at 393 K, the permeance under humid conditions is close to the permeance under dry conditions.

Fig. 8 Separation of equimolar mixtures of (a) CO₂–CH₄ and (b) CO₂–N₂ open symbols correspond to dry conditions, closed symbols to humid conditions, i.e. in the presence of 2.2 kPa of water; conditions: 0.6 MPa feed pressure, atmospheric pressure on permeate side, total feed gas flow rate 200 ml min⁻¹, 200 ml min⁻¹ of sweep gas He. Reproduced from ref. 57 with permission from the Royal Society of Chemistry.

Table 3 Permeance of gases at dry and humid conditions in separations of corresponding mixtures⁵⁷

Gas	Permeation (10^−7 mol m^−2 s^−1 Pa^−1)
	293 K, dry	293 K, wet	% of dry	393 K, dry	393 K, wet	% of dry
CO2(N2)	2.886	0.71	25	1.9	1.6	84
N2(CO2)	0.27	0.04	15	0.36	0.31	86
CH4(CO2)	0.76	0.16	21	0.11	0.098	89

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To understand the effect of humidity Kosinov et al. also studied adsorption of water on SSZ-13 crystals. The results of adsorption are shown in Fig. 10. From Fig. 9 it seems that the amount of adsorbed water decreases from 3.1 mmol g⁻¹ at 298 K to 0.32 mmol g⁻¹ at 393 K. At 473 K the adsorbed amount of water was below the detection limit (∼0.02 mmol g⁻¹). These results suggested that water partially blocks permeation of gases by adsorption into zeolite pores at low temperature. The hydrophobicity of high-silica SSZ-13 ensures nearly complete elimination of water adsorption at modest temperatures (473 K). It was concluded by Kosinov et al. that water will adsorb on and block some of non zeolitic pores at low temperature and, accordingly, improve the overall selectivity.

Fig. 9 Adsorption of water on SSZ-13 (Si/Al = 100) crystals measured N by TGA. During the experiment the sample was first completely dehydrated in dry He at 673 K, cooled down to 298 K and then subjected to a humidified (3 kPa H₂O) He flow. After saturation the temperature was gradually increased to 473 K in the presence of the humidified He flow. Reproduced from ref. 57 with permission from the Royal Society of Chemistry.

Due to their hydrophobicity, water in the feed mixture has only a small influence on the permeance at temperatures above 393 K. Hunger et al.⁵⁸ ensured that the hydrophobicity of high-silica SSZ-13 would eliminate water adsorption almost completely at modest temperatures (at around 473 K). Water improves the CO₂–N₂ and CO₂–CH₄ selectivity when the temperature is low, which is attributed to preferential blocking of the hydrophilic, non-zeolitic defect pores. The hydrothermal stability of the high-silica SSZ-13 membrane was evaluated by a long (220 h) CO₂–N₂ separation test with a humidified (9.5 kPa H₂O) feed mixture at 393 K and 0.6 MPa feed pressure. The permeance and selectivity were stable during this endurance test, undermining the promise of high silica SSZ-13 membranes for application in the separation of hot and humid gas mixtures.

Further Kosinov et al.⁵⁷ studied the permeation data of a bulky SF₆molecule in dry and wet conditions. The results are collected in Fig. 10. In dry condition SF₆ permeance gradually decreases with increasing temperature, which is characteristic for Knudsen (pore size10–100 nm) and Poiseuille (pore size > 50 nm) flows. The presence of water reduces the SF₆ permeation by ca., 50% at low temperature. Their work suggested that the high-silica SSZ-13 membranes are promising candidates for separation of different CO₂-containing mixtures at elevated pressure and temperature and in the presence of water vapour due to the low polarity of SSZ-13. The separation mechanism is based on a combination of adsorption and diffusion selectivity.

Fig. 10 Influence of water on SF₆ single gas permeance (open symbols – dry gas, closed symbols – 2.2 kPa of water; conditions: 0.6 MPa feed pressure, atmospheric pressure on permeate side, total flow rate 200 ml min⁻¹, 200 ml min⁻¹ of sweep gas He). Reproduced from ref. 57 with permission from the Royal Society of Chemistry.

Zeolite NaA (zeolite 4A)

Zeolite 4A molecular sieve is a kind of alkali metal silicic acid salt and can absorb water and other molecules with a critical diameter less than 4 Å.⁵⁹ Its chemical formula is Na₂O·Al₂O₃·2SiO₂·9/2H₂O and silica–alumina ratio is SiO₂/Al₂O₃ ≈ 2.

Okamoto et al.⁶⁰ prepared zeolite NaA membranes via hydrothermal synthesis with a short reaction time of 3 h at 373 K using a gel with the composition Al₂O₃ : SiO₂ : Na₂O : H₂O = 1 : 2 : 2 : 120 (in moles) and porous α-alumina support tubes seeded with zeolite NaA crystals. It was reported that zeolite NaA membranes were highly permeable to water vapour. Gases (H₂, O₂, N₂, CO₂, CH₄, SF₆) were permeable only when the membranes were completely dry. The gas permeances of zeolite NaA membranes varied rather significantly from sample to sample. The completely dried membranes displayed gas permeation behavior, which was attributed to Knudsen diffusion, indicating the presence of interstitial spaces between the zeolite crystal particles, or non zeolitic pores. However, membranes displayed excellent water permselective performance in pervapouration (PV) and vapour permeation (VP) toward water/organic liquid mixtures. A mechanism of PV and VP based on the capillary condensation of water in the zeolitic and nonzeolitic pores and the blocking of other molecules from entering the pores was proposed.

The permeation of H₂O vapour, CO₂, H₂, and CH₄ and their binary mixtures through a thin zeolite-4A membrane has been investigated in the temperature range between 30 and 100 °C by Zhu et al.⁶¹ The zeolite-4A membrane was synthesized with pre-treatment of the supporting TiO₂ by UV-photons. The membrane exhibited a high permeance property and maintained a high selectivity for water. The permeance of the water vapour in the binary mixtures was almost the same as its unary permeance and only weakly temperature-dependent. The permeances of the gaseous components were strongly suppressed by water, resulting in high selectivities for water removal from these gases. The suppression effect was ascribed to the strong adsorption affinity of H₂O inside the zeolite-4 Å pores, which blocked the permeation of the second gas component through the membrane.

Zeolite – MMMs and MOF (ZIF)

Separation performances (selectivity, flux) of polymeric membranes are usually quite low. However, polymeric membranes can be produced in high quantities at low cost. Inorganic membranes (zeolites, amorphous oxides, carbon) have very good separation performance but they are too expensive. The positive properties of those membrane materials can be combined by embedding inorganic active components into a polymeric matrix creating a composite membrane called mixed matrix membrane (MMM). Compared to original polymeric membranes, significant improvement in separation properties can be obtained.

The performance of the MMMs is influenced by several factors which are as follows;

(i) Properties of polymer and fillers.

(ii) Polymer filler compatibility.

(iii) Process of membrane formation.

In the development of MMMs proper selection for polymer as continuous phase and inorganic filler as dispersed phase is important since they can affect membrane morphology and separation performance. MMMs have higher selectivity compared to continuous polymer matrix.

Most of MMMs with modified zeolite were engineered to reduce the interphase void and incompatibility. The most difficult task is to make MMMs defect free. Defects are formed generally due to incompatible interfaces between the polymer and the filler particle. A good adhesion between filler and polymer in MMMs is essential to avoid non-selective voids. Koros group^62,63 summarized some non-ideal factors which were always present in the interfaces and modified the Maxwell equation accordingly to illustrate their effects on the gas transport properties of MMMs. The use of MOFs as fillers in MMMS might result a breakthrough in the MMM field, since compatibility issues can eventually be overcome by optimizing the MOF linker–polymer interactions.^64,65 Li et al.⁶⁶ used silane coupling agent, (3-aminopropyl)-diethoxymethyl silane (APDEMS) to modify zeolite surface for mixed matrix membranes (MMMs). Both permeability and selectivity of MMMs made from APDEMS modified zeolite are higher than those of MMMs made from unmodified zeolite at 20 wt% zeolite loading because of a decrease in the degree of partial pore blockage of zeolites.

Kim et al.⁶⁷ showed that the permeability of polysulfone nanocomposite membrane increased by introducing mesoporous materials whereas the selectivity did not change significantly and that was because of suitable compatibility between nanoparticles and polymer matrix. Ridzuana and Musab⁶⁸ showed the use of silane coupling agent is an excellent approach to increase the compatibility of the polymer and zeolite. It was concluded that surface modification of zeolite plays an important role to reduce the incompatibility of zeolite and polymer.

The use of MOFs as fillers in MMMS might result a breakthrough in the MMM field, since compatibility issues can eventually be overcome by optimizing the MOF linker–polymer interactions.⁶⁴

Following are the different techniques suggested to eliminate the unselective gaps between the polymer and zeolite in the fabrication of MMMs⁶⁹

(1) Surface modification of the zeolite external surface.

(2) Use of a silane coupling agent to introduce favorable interaction between polymer and zeolite.

(3) Addition of a plasticizer to increase the flexibility of the polymer or chemically linking the two components together.

(4) High temperature casting and removal of the dense films from the casting surface at a particular temperature.

MMMs which are comprised of inorganic materials distributed in polymeric phase are based on solid–solid system. When inorganic nanoparticles are added to the polymeric phase, it is predictable that resulting membrane separation properties become better than customary polymeric membranes due to the high gas permeance and selectivity of nanoparticles. Zeolites and many polymeric materials offer attractive transport properties for CO₂ separation. By mixing membrane materials, excellent membrane with high performance for CO₂ separation (selectivities of CO₂/N₂ = 17.8–39.6) can be prepared.^70–72

A variety of especially small pore zeolites have been used for their application in mixed matrix membranes (MMMs). While early studies were focused on LTA zeolite, this hydrophilic zeolite turned out to be less attractive for gas separation from humid feeds because of the pore blocking by water.⁷³ Recent studies have focused, therefore, on more hydrophobic zeolites with a high molar SiO₂/Al₂O₃ ratio. Moore et al.⁷³ investigated the effect of humidified feeds on oxygen permeability of MMMs (zeolite + polymer). It was noted that MMMs composed of zeolite 4A that was dispersed in poly(vinyl acetate) were adversely affected by water adsorbed in the pores of zeolite 4A. Water adsorbed into the pores of hydrophilic molecular sieves can cause a decrease in the permeability of oxygen, and this will likely reduce the overall selectivity of MMMs prepared with such a sieve. As expected, the permeability reduction was more for the hydrophilic zeolite 4A than in the hydrophobic SSZ-13. Hydrophobic SSZ-13 MMMs were less affected by adsorbed water.

Metal–organic frameworks (MOFs) are hybrid organic–inorganic nanoporous materials that exhibit regular crystalline lattices with relatively well-defined pore structures. MOFs membranes can be constructed from metal ions or metal ion clusters and bridging organic linkers. It exhibits regular crystalline lattices with relatively well-defined pore structures and interesting properties. The porosity of MOFs is, in general, much higher than that of their inorganic counterpart, zeolites, justifying the designation ‘framework’. MOFs (ZIF) can be fabricated in many different ways, making it possible to change their pore size and chemical properties.

ZIF-8 is highly attractive for gas separation applications and it is chemically stable in the presence of water and some aromatic hydrocarbons such as benzene, which are typical impurities in natural gas, making this particular ZIF composition potentially useful for the separation of CO₂ from CH₄.⁷⁴ Yang and Chung⁷⁵ fabricated two types of advanced nano-composite materials by incorporating as-synthesized wet-state zeoliticimidazolate frameworks-8 (ZIF-8) nano-particles into a polybenzimidazole (PBI) polymer. Mixed gas (CO₂/H₂) data show that the presence of water vapour impurity in the feed gas stream does not significantly influence the membrane performance at 230 °C. Thus, the newly developed H₂-selective membranes may have bright prospects for hydrogen purification and CO₂ capture in realistic industrial applications such as syngas processing, integrated gasification combined cycle (IGCC) power plant and hydrogen recovery.

Mottillo and Friščić⁷⁶ first reported that in zeoliticimidazolate frameworks of Zn, Co and Cd, including ZIF-8, exhibited sensitivity to CO₂ under mild conditions. The frameworks chemically reacted with CO₂ in the presence of moisture or liquid water to form carbonates, which was not reported previously. This phenomenon provides an explanation for conflicting reports on ZIF-8 stability to water and is of outstanding significance for evaluating the potential applications of metal–organic frameworks. This phenomenon may be applicable for other zeolite or zeolitc membranes.

Nguyen et al.⁷⁷ studied the hydrophobic zeoliticimidazolate frameworks (ZIFs) with the chabazite (CHA) topology. All of these structures were hydrophobic as confirmed by water adsorption isotherms. All studied ZIFs were equally effective at the dynamic separation of CO₂ from N₂ under both dry and humid conditions without any loss of performance and can be regenerated simply by using a N₂ flow at ambient temperature.

Junaidi et al.⁷⁸ fabricated water-resistant MMMs by incorporating fluorocarbon functionalized SAPO-34 zeolite into polysulfone (PSf), to convert the hydrophilic SAPO-34 zeolite into hydrophobic SAPO-34 zeolite by using. 1H,1H,2H,2H-Perflourodecyltriethoxysilane (HFDS). PSf-10/HFDS 1.0 (10 wt% of modified SAPO-34 zeolite) MMM showed great enhancement in ideal gas separation (ideal selectivity CO₂/CH₄ of 38.9 with CO₂ permeance of 278 GPU) compared to the neat PSf membrane and MMMs with unmodified SAPO-34 zeolite. On comparing the dry mixed-gas results, MMM incorporated with 10 wt% of unmodified SAPO-34 (PSf-10) suffered more than 90% reduction in separation performance during wet mixed-gas test. The actual selectivity of CO₂/CH₄ for all MMMs was slightly lower than the corresponding ideal selectivity, either at dry or wet conditions. The gas separation performance of PSf-10/HFDS 1.0 MMM could be well sustained in both dry and wet gaseous system even after 24 h operation period. The only drawback of this membrane was the reduction in CO₂ permeance, about 7%, if comparing with humid single gas permeation. The presence of hydrophobic SAPO-34 in MMM established extra resistant towards water penetration, causing the dissolved CO₂ to exist as the retentate. Junaidi et al.⁷⁸ work suggested that the future work could be to study the potential of PSf-10/HFDS 1.0 MMM in the removal CO₂ and moisture simultaneously for downstream processing.

Interpretation of experimental observations by a transport mechanism

Even though the presence of water in the feed stream displays different effects on the zeolite membrane performance as summarized in Table 4, some common patterns can be recognized among those effects. The factors that govern the effects are (1) hydrophilicity/hydrophobicity of the membrane (2) pore size and pore size distribution of the membrane (3) temperature and (4) partial vapour pressure of water in the feed stream.

Table 4 Separation of gases in presence of moisture/water

Membrane	Preparation	Feed	Temperature range	Results	Ref.
Porous silica	Sol–gel, method, coated on alumina tube	H2, He, CO2, N2 + moisture	35 °C	In humid conditions CO2 and N2 permeances decreased drastically	79
FAU membrane	Hydrothermal seeded growth	CO2/H2, CO2/N2 and CO2/CH4	60 °C	FAU membranes are CO2 selective. Presence of water vapour enhances the CO2 flux significantly	80
MFI	In situ	H2 + 500 ppb H2O	303–673 K	At room temp (T). Permeation was low in comparison with dried H2 permeation and then increased with increase in T and then leveled off around 100 °C	15
MFI (ZSM-5)	Hydrothermally on alumia tubes	CO2/N2 + moisture	Room temperature −140 °C	A steady state reduction of the gas permeation of about 40% for a water concentration of 10 v/v% at 373 K. Promotes CO2/N2 separation due to defective pore blockage and formation of (bi)carbonate stable intermediates	33
MFI (ZSM-5, (Si/Al ∼ 80) and aluminium –free silicalite)	Dip coating on alpha alumina	H2/CO2, H2/CO2/H2O	300–350 °C	Water vapour lowers H2 and CO2 permeation to the same extent, resulting in negligible effect on the H2/CO2 separation factor. The suppression effects of water vapour on H2 and CO2 permeation increase as temperature decreases and water vapour pressure increases. Water confined in MFI zeolite pores has the same suppression effect on the permeation of H2 and CO2 through the MFI zeolite membrane as observed on silicalite membrane	38
MFI (different type ZSM-5)	Gel method on alumina and stainless steel support	CO2/N2 bubbled through water	300–475 K	The CO2 permeation fluxes decreased about 54% and 29% for a feed pressure of 1.7 and 2.2 bar, respectively, without use of a sweep gas in the permeate stream, with the separation factor remaining essentially unchanged	36
ZSM-5				Although the pure N2 and SF6 permeances decreased after humidification of the feed stream (61% for N2 and 16% for SF6), the initial values were recovered after water removal	35
MFI (three different counter ions) and silicalite	Hydrothermally (alumina disc.)	CO2/H2 bubbled through water	Room temp. −150 °C	Counter ions had a significant effect. For the H2/H2O system, H2 permeation is 5–6 times lower than pure H2. For the CO2/H2O system CO2 permeation decreases to a minor extent. For the H2/CO2/H2O system permeation of both gases decrease	39
ZSM-5	Hydrothermally on α-alumina (4 types)	Moisture saturated CO2/N2	Room temperature	An increase of the membrane selectivity in the separation of a moisture-saturated equimolar CO2/N2 mixture by a surface-modified ZSM-5 membrane. At room-temperature CO2/N2 separation factor was about 50 for dry mixture and about 60 for wet mixture steady-state separation. It was believed that water blocked partially both the MFI adsorption sites and the intercrystalline non-zeolite pores, overcoming N2 permeation and thus increasing the CO2/N2 separation factor	37
NaA	Hydrothermally on α alumina	Different gases	Room temp.	Impermeable to every gas unless dried completely, Knudsen diffusion	61
NaA	Hydrothermally	CH4 and H2O vapour	303–342 K	Ideal selectivity of water vapour was more than methane. A mass transfer model was developed. Based on Maxwell–Stefan approach	55
Defect free FAU-type (Y)	Hydrothermally on α alumina	CO2/N2 dried and moist	23–200 °C	Decreased the P of CO2 and N2, H2O vapour enhanced the CO2 selectivity at 110–200 °C, but drastically lowered below 80 °C. At 200 °C with increasing H2O permeation. CO2 selectivity increased and then decrease with increasing water partial pressure. N2 permeation through large intercrystal pores	53
SAPO-34	Hydrothermal synthesis	CO2/CH4	Room temperature	Humidity has strong effect on permeation of gases and the effect depends on non-SAPO pores. Water almost blocks the SAPO pores in high quality membranes. Low quality membranes have larger pores and thus permeation through these pores increases with humidity. The CO2 recovered the original permeances and selectivity after calcination at 573 K for 24 h. Water had a strong effect on gas permeation through SAPO-34 membranes: water almost completely blocked the SAPO pores	42
DDR type zeolite	Hydro thermally zeolite crystal on alumina	CO2/CH4	280–373 K	Permeation of CO2 decreased in the wet gas feed at 280 K, separation factor increased. At 373 K no changes were observed between dry and wet feed in separation factor and permeation	45
DDR type zeolite	Hydrothermally on porous alumina tube	CO2/CH4	298 K	While the permeance of CH4 was not affected by water vapour, the presence of 3% water in the gas stream reduced the CO2 permeance to half of that for a dry feed stream, resulting in a 50% reduction in the CO2/CH4 selectivity	48
MMMs (fluorocarbon + SAPO-34)	Dry-wet phase inversion	CO2/CH4 bubbled through water	30 °C	Huge reduction in gas separation performance due to the blockage of SAPO-34 pores	78
MMMs membranes
Zeolite 4A/acetate and SSZ-13/PVAC	Mixing the components in dichloro-methane and dried	Humidified O2	35 °C	Water can affect both the polymer and sieve phases. Decrease in permeability of O2	73
ZIF-8/PBI	Two types. By mixing both components in solvent and dried	Humidified CO2/H2	230 °C	Impurities such as H2O vapour existing in the model syngas have no significant impact on the H2/CO2 separation	75

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Among many models proposed for the gas and vapour transport through the zeolite membrane, e.g. those based on surface diffusion^55,69,81,82 Stephan Maxwell and Knudsen approach,^69,83 Monte-Carlo and first-principles Car–Parrinello molecular dynamics simulations,⁸⁴ the model proposed by Coronas and Santamaria⁸ for the transport of ternary gas/vapour mixtures seems to be the most appropriate, in a slightly modified form, to explain the experimental observations, such as summarized in Table 4, in the presence of water vapour in the feed stream.

As shown in Fig. 11, the pores are classified into three groups according to the size, small (0.5–0.5 nm), intermediate (<2 nm) and large (2–5 nm). Let us now combine intermediate and large in one and call them large pores. The small pores may be considered as “intracrystalline pores” and large pores as “intercrystalline” pores, according to the designation by Nomura et al.⁸⁵ At present, these pores are assumed to be hydrophilic.

Fig. 11 Influence of temperature and partial pressure on the selectivity of a zeolite film. Reproduced from ref. 8 with permission from the Taylor & Francis.

The gas mixture is composed of three components. Component A is a small polar molecule (i.e. water), that is preferentially adsorbed to the pore wall. Component B is a small, essentially non-adsorbing permanent gas and component C is a large size molecule that is also non-adsorbing and can only permeate the membrane through large pores. Then, the transport of component B and C will take place in the following way.

(1) At high temperature or low water vapour pressure, both small and large pores are open. Large pores do not contribute to selectivity. B over C selectivity is primarily due to the sieving (and diffusion) effect of small pores.

(2) When the temperature is lowered or water vapour pressure is increased the small pores are blocked while the large pores remain open. Permeation of B decreases considerably but that of C does not. As a result the selectivity of B over C decreases.

(3) As the temperature is further lowered or the water vapour pressure is further increased, adsorption of water vapour is significant and both small and large pores are blocked. As a result the permeation of both B and C decreases.

(3.1) If B is soluble in A, selectivity of B over C increases. Furthermore, when the solubility effect of B in A surpasses the pore blocking effect, permeation of B may even increase. When there is no water vapour in the feed stream or if the membrane is hydrophobic, all pores are open and the transport of B and C is the same as in case (1).

The observations summarized in Table 4 can then be explained as follows.

Case (1) will explain the observations made in ref. 75.

Case (2) will explain the observations made in ref. 34 and 47.

Case (3) will explain the observations made in ref. 35, 37, 41, 60, 73, 78 and 79.

Case (3.1) will explain the observations made in ref. 32, 36, 38 and 80.

The combination of cases (1) and (3.1) will explain the observations made in ref. 45.

The combination of cases (1) (2) and (3) will explain the observations made in ref. 15.

Although the above model does not involve detailed mechanisms of the molecular transport through the zeolite pores, the model is simple but sufficient to explain many experimental results.

Effect of grafting a functional group on zeolite

Grafting a functional group on zeolite can change water absorption in zeolite. Zeolite's chemical composition can be tuned by introducing other heteroatoms (Ti, Sn, Fe, Cu, Co etc.), both in framework or extra-framework positions, and consequently, the nature of the zeolite active sites will change (acid–base or redox centers). Those “tailor-made” physicochemical abilities, together with their high hydrothermal stability and “non toxic” character, have allowed their wide application in industry.⁸⁶

Amine-containing adsorbents are often tolerant to the occurrence of moisture in the feed.⁸⁷ Because of the very nature of the amine–CO₂ interactions, the CO₂ uptake is actually enhanced by the presence of moisture. Under dry adsorption conditions, surface amine groups interact with CO₂ to form carbamate with a stoichiometric CO₂/N ratio of 0.5, whereas under proper humidity conditions, bicarbonate with a stoichiometric ratio CO₂/N = 1 may be formed. In contrast, in the presence of other adsorbents such as zeolites, CO₂ adsorption is severely inhibited by moisture because of unfavorable competition.⁸⁸

On introducing the amine functional group in Y-type zeolite the adsorption of CO₂ in zeolite is enhanced.⁸⁹ Sanaeepur et al.⁹⁰ used 3-aminopropyl(diethoxy)methylsilane (APDEMS) for silylation of a NaY zeolite surface to investigate the effect of surface modified zeolite (NaY-sm) in cellulose acetate (CA) gas separation membranes. Better gas permeation results were obtained using NaY-sm instead of NaY in the membranes.

Faria et al.⁹¹ demonstrated that a combination of Lewis and Brønsted acid sites, together with a metal function on a hydrophobic support, exhibits improved hydrothermal stability and resistance to leaching of the active sites. Conventional NaX faujasite zeolites, in which the sodium ions act as weak Lewis acid sites for the keto-aldose isomerization of glucose, are rapidly deactivated in liquid water at 383 K. In contrast, functionalized MWCNT-NaX zeolite keeps its crystalline structure in this harsh environment.

Jadhav et al.⁹² impregnated monoethanol amine (MEA) on zeolite 13X and examined the hybrid material for CO₂ adsorption at different temperatures. The MEA-impregnated adsorbent showed higher CO₂ capture capacity than zeolite 13X at 120 °C, and exhibited improved CO₂ selectivity, which was enhanced further in the presence of moisture.

Taib et al.⁹³ demonstrated that grafting the functionalized group on the surface of adsorbents (such as zeolite) did not modify the original structure but altered their original hydrophilic nature to hydrophobic.

Jin et al.⁹⁴ modified an asymmetric silica membrane supported on a porous alumina tube substrate with hexamethyldisilazane (HMDS) via dip-coating. The membrane was used for the separation of ethanol/water mixture by pervaporation. The surface of the silica membrane chemically bonded with trimethylsilyl groups (–Si(CH₃)₃) of HMDS showed hydrophobicity.

Summary

Zeolite membranes are thermally stable with good chemical resistance and mechanical strength in comparison to pure polymer membranes but they are very fragile and brittle. Furthermore, the technology for fabricating commercial zeolite membranes is still in the developmental stage while the technology for commercial production of polymeric membranes is very mature with applications in a series of industrial processes.

A common problem in zeolite membrane synthesis is that, despite the use of a pre-defined methodology, it is difficult to obtain membranes with consistent and predictable properties. In the past decade not much research has been done on the moisture effect on the gas separation via zeolite membranes, since zeolite membranes are relatively new in comparison to polymeric membranes. However, it is known that there is moisture effect on the gas separation for a number of zeolite membranes such as SAPO-34, MFI and DDR. The property of zeolite membranes (performances etc.) depends on the synthesis methods and it could also be possible that the effect of moisture/water is different for different zeolite membranes. Despite many unknown factors one thing is certain that hydrophobicity or hydrophilicity of zeolite, which is tunable by grafting, introduction of heteroatoms and changing Al/Si ratio, temperature and water vapour pressure of the feed gas stream play significant roles on the effect of water on zeolite membrane separation of gases. The mechanism for the CO₂ separation from other mixed gases in presence of moisture is very complicated. More study on this subject is necessary as zeolite membranes seem to become very important in future for the chemical, petroleum, gas processing industries.

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