Microporous Zeolite Membrane: Structure, Preparation, Characterization, and Application

Microporous Zeolite Membrane: Structure, Preparation, Characterization, and Application

CHAPTER 7 Microporous Zeolite Membrane: Structure, Preparation, Characterization, and Application Xiuxiu Ren*, Yanshuo Li† * School of Petrochemical...

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Microporous Zeolite Membrane: Structure, Preparation, Characterization, and Application Xiuxiu Ren*, Yanshuo Li† *

School of Petrochemical Engineering, Changzhou University, Changzhou, China, †School of Material Science and Chemical Engineering, Ningbo University, Ningbo, China

1 Introduction Zeolite membrane is a type of aluminosilicate crystal with various topology growth on porous supports. The zeolite consists of order channel frameworks that are formed by varied ring numbers commonly based on silicon-oxygen and aluminum-oxygen tetrahedron connected in different ways and matrices. The aluminum content in zeolite structures plays a major role in the properties of membrane hydrophilicity and surface charge. Zeolite membrane thicknesses ranging from tens of nanometers to hundreds of micrometers have been reported (Rangnekar et al., 2015a, b). Zeolite membranes have great potential applications in gas separation, pervaporation, desalination, membrane reactors, sensors, protection or insulation layer, etc. The pore diameter of zeolite is usually smaller than 2 nm, which is in the microporous range according to the definition of the International Union of Pure and Applied Chemistry (IUPAC) in terms of material pore size. With the advantages of microchannels and cavities in the range of molecules, zeolite membranes have excellent ability in small specific gas or liquid separation. They also have the advantages of adsorption or catalytic sites with high surface area and excellent ion exchange capabilities, providing high performance in the catalysis era. Compared with polymer membranes, they can be utilized at high operating temperature, high pressure, and even in the presence of aggressive solvents. Hundreds of zeolite types are obtained, but not all of these are designed to prepare membranes. The zeolite structures synthesized as membranes include aluminophosphate five (AFI), beta polymorph A (BEA), linde type A (LTA), mordenite (MOR), zeolite socony mobil-five (MFI), faujasite (FAU), chabazite (CHA), and deca- and dodecahedra, 3 layers, rhombohedral (DDR), Microporous Membranes and Membrane Reactors. https://doi.org/10.1016/B978-0-12-816350-4.00007-6 # 2019 Elsevier Inc. All rights reserved.


158 Chapter 7 etc. (Koros, 2004; Fard et al., 2018). In recent years, zeolite membranes with large areas have been developed by different preparation techniques. Nowadays, LTA zeolite membranes are utilized in industries for dehydration of organic solvents by means of pervaporation and vapor permeation, which are low energy-consuming processes when compared to conventional thermal separation, for example, distillation. Zeolite membranes of different topologies (FAU, MFI, CHA, and DDR) have been developed to separate light gases and thoroughly researched with interesting results (Algieri et al., 2011). Intensive research efforts have been made on zeolite membranes in the last decades. It is expected that more types of zeolite membrane plants will soon be setting up to satisfy both process safety and superior product quality at a low cost using various technologies.

2 Zeolite Membrane Structure Zeolites cannot be prepared as self-supported membranes in a practical way due to low mechanical strength. It is commonly grown on porous supports. Thus, zeolite membranes include two parts: a useful thin zeolite layer and a support layer, as shown Fig. 1. A thin zeolite layer is chemically or texturally fit to grow on the support (Morigami et al., 2001). The zeolite layer can act as a selective transport, barrier, or catalyst with regular structure and active sites. The remarkable properties of the zeolite membrane are closely related to the zeolite structural features. The support is commonly a macroporous inorganic material with a certain porosity to give mechanical and thermal stability and reduce resistance of molecule transport.

2.1 Zeolite Structure Zeolites are composed of TO4 tetrahedra coordinated framework, and T stands for Si and Al or other heteroatoms (including P, Ge, B, Mg, Zn, Ga, Be, N, or S). Up to now, there are 228 kinds of zeolite topologies with an additional 7 partial disorder zeolite materials that have been so far reported. Every zeolite is given a unique three-letter code by the Structural Commission of the International Zeolite Association (IZA; http://www.iza-structure.org/). There were only 213 different framework structures accepted in 2013, with about three new zeolite types invented per year in the past 5 years.

Fig. 1 The structure of zeolite membranes.

Microporous Zeolite Membrane: Structure, Preparation, Characterization, and Application 159 Table 1 Representative zeolite structures

Zeolite Code


Dimensionality Separation (Molecular ˚) Cross-Section >0.26 A



















Channels [001] 6-ring ˚  2.65 A ˚ 2.65 A [001] 12-ring ˚  7.1 A ˚ 7.1 A [001] 12-ring ˚7A ˚ 6.5 A [001] 8-ring ˚  4.4 A ˚ 3.6 A [001] 10-ring ˚  5.5 A ˚ 4.0 A [100] 8-ring ˚  4.1 A ˚ 4.1 A [100] 10-ring ˚  5.5 A ˚ 5.1 A [111] 12-ring ˚  7.4 A ˚ 7.4 A [001] 8-ring ˚  3.8 A ˚ 3.8 A

Maximum Diameter of Surface Along (a × b × c) ˚  2.53 A ˚  2.53 A ˚ 2.53 A ˚  2.08 A ˚  7.5 A ˚ 2.08 A ˚  2.95 A ˚  6.45 A ˚ 1.57 A ˚  3.65 A ˚  2.63 A ˚ 3.65 A ˚  4.92 A ˚  2.6 A ˚ 4.92 A ˚  4.21 A ˚  4.21 A ˚ 4.21 A ˚  4.46 A ˚  4.46 A ˚ 4.7 A ˚  7.35 A ˚  7.35 A ˚ 7.35 A ˚  3.72 A ˚  3.72 A ˚ 3.72 A

In IZA, SOD and MOR dimensionality sorption is 0 and 1, respectively.

The varied membered rings in zeolites are interconnected in a number of different ways forming different channels. Based on the number of opening channels for molecular transport, the zeolite structure can be classified into one-, two- and three-dimensional channels. The representative structures are shown in Table 1. Here, the dimensionality classification is ˚ , which is the based on the number of channel directions with a pore opening larger than 2.6 A ˚ smallest dynamic diameter of molecules. The pore size of 2.6 A is to provide a guide as to the smallest gas or vapor molecule that can diffuse, which is not totally consistent with the ˚ IZA definition based on sorption dimensionality of a channel pore opening larger than 3.4 A using an organic molecule as base for diffusion. One-dimensional channel zeolite: Zeolite LTL has an aluminosilicate three-dimensional topology but only uses one-dimensional large-pore channels parallel to the c-axis of the crystal for application. The large unit cell structure features hexagonal symmetry formed by 12-ring undulating channels, as shown in Fig. 2. Zeolite LTL was first synthesized by Breck and Acara (1965), and Bernard (1978) reported on using Pt-K-LTL zeolite as a selective catalyst for aromatization of hexane to benzene. White et al. (2008) reported on preparation of several LTL membranes by hydrothermal secondary growth method on porous alumina supports with a molar composition of 10 K2O: Al2O3: 20 SiO2: 2000 H2O. LTL layers with an average thickness of 2–7 μm were formed at 110°C by using nanocrystalline LTL zeolites (particle size: 20–60 nm) as seeds.

160 Chapter 7

Fig. 2 Framework images of LTL (A) along [001] (B) and normal to [001] with 12-channel including CAN-cages (from web http://www.iza-structure.org).

Two-dimensional channel zeolite: The MOR zeolite is usually recognized as a twodimensional structure, with large channels of 0.67 nm  0.70 nm and small channels of 0.26 nm  0.56 nm. In IZA, it has one-dimensionality, and the small channel was neglected. The typical material-mordenite membrane can be synthesized on a porous silica-alumina plate or alumina tubular supports, and has applications in catalysis, pervaporation, and gas separation with high acid-resistant stability. Three-dimensional channel zeolite: Most of zeolites have three-dimensional channels, such as LTA and MFI, which are most researched membranes. An LTA-type material is called NaA. The NaA zeolite membranes with three similar channels (0.41 nm) have been successfully used in industries for dehydration of organics. The Al-rich structure provides the NaA membrane with high water affinity. An MFI-type zeolite has two representative materials: ZSM-5 and silicalite-1. They have the same three-dimensional topology, but ZSM-5 contains Si and Al elements in its framework, whereas silicate-1 includes only Si in its structure. With the same b-direction of 0.54  0.56 nm channels, ZSM-5 and silicalite-1 membranes both present excellent performance on the separation of xylene isomers, as shown in Fig. 3 (Lai et al., 2004; O’Brien-Abraham et al., 2008). The pore size of MFI zeolite is close to the kinetic diameter of p-xylene (0.58 nm) but smaller than that of o- and m-xylene (0.68 nm both). Thus, p-xylene is expected to diffuse faster within the MFI framework, allowing perfect separation of p-xylene from isomer mixtures (Daramola et al., 2009). However, the difference of Al element for ZSM-5 and silicalite-1 results in different chemical adsorption and separation behavior. In pervaporation, ZSM-5 membranes with hydrophilic properties are used to separate water from organic/water mixtures to obtain pure solvents, whereas silicalite-1 membranes are preferred for the adsorption of organics and usually applied in the separation of low content of alcohols from water.

Microporous Zeolite Membrane: Structure, Preparation, Characterization, and Application 161

Fig. 3 The framework of zeolite MFI. Reproduced from Lai, Z., Tsapatsis, M., Nicolich, J., 2004. Siliceous ZSM-5 membranes by secondary growth of b-oriented seed layers. Adv. Funct. Mater. 14(7), 716–729 with permission from John Wiley & Sons.

2.2 Support Structure of Zeolite Membrane The support materials used to prepare zeolite membranes include α-alumina (or γ-), stainless steel, clay, mullite, yttria, calcium silicate, titanium, and polymer or other composite materials. The support shapes reported include disk, tube, multichannel tube, plate, or hollow fiber (HF). The structures of supports can be either symmetric (less expensive) or asymmetric pore distribution. The supports now primarily used are porous alumina supports, and symmetric alumina tubular supports have been used in industrial applications. Planar supports are easy to test and used for concept demonstration (Yan et al., 1995). The growth of a zeolite membrane is influenced by the nature of the support. A support with a smooth surface, active sites, and proper pores is expected to grow a thin and defect-free zeolite layer. To obtain high performance membranes, the supports are pretreated or modified with metal oxide, kaolin, and molecular linker for zeolite growth (Tanaka et al., 2005). For example, 1,4-phenylene diisocyanate (PDI) has ever been used as a molecular linker to link LTA zeolite and Al2O3 support, as shown in Fig. 4 (Xu et al., 2018a). The Al2O3 support was modified by PDI and formed functional support with the introduction of -NH2 groups. Then, aluminosilicate gel formed an oriented LTA zeolite membrane with a thickness of about 4.0 μm on this modified support. The membrane displayed high performance in the dehydration of alcohols. By pervaporation of 95 wt% ethanol/water mixtures at 90°C, the separation factor was 4480 with water flux of 3.4 kgm2h1.

162 Chapter 7

Fig. 4 A schematic diagram of supports modified by using PDI as a covalent linker in the synthesis of LTA zeolite membranes. Reproduced from Xu, K., Jin, H., Wang, L., Liu, Y., Zhou, C., Caro, J., et al., 2018. Seeding-free synthesis of oriented zeolite LTA membrane on PDI-modified support for dehydration of alcohols. Sep. Sci. Technol. 53(11), 1741–1751 with permission from Elsevier.

3 Preparation In the preparation process of zeolite membranes, the crystal nucleus of the amorphous materials is formed first, and then it grows into ordered crystals on supports from a few hours up to a few days at a certain temperature and pressure. A large number of methods have been tried and used to synthesize compact, intergrown, and defect-free membranes with a thin zeolite layer. An oriented zeolite layer with narrow zeolite structures for molecular sieving is also preferred. In addition, the bulky shape of crystals with a small aspect ratio is favorable which leaves no space at crystal boundaries. The preparation process is complicated, and the methods are various in their different steps. Based on the heating effect, it can be divided into conventional heating (oven heating) and microwave heating, both of which are the most common heating sources used for preparation of membranes. Based on the nutrient conditions in the liquid water phase or solid phase for crystal growth, it can be divided into either hydrothermal synthesis and dry-gel conversion (DGC) method, of which hydrothermal method is more commonly used. Based on whether the crystal nucleation and growth mechanism occurs in one or two steps, the method is called either an in situ crystallization method or a secondary growth method (the support is precoated with zeolite as seed). Thus, in a complete preparation process, these methods are always combined to synthesize zeolite membranes. For example, the secondary conventional hydrothermal synthesis method is most widely used for synthesis of defect-free zeolite membranes. This combination method process is sealing water phase nutrients and seeding support in a high-pressure autoclave, then heating by oven to induce nutrient solutions into crystals based on the zeolite seeds as nucleation sites on the support, then forming into membranes. The ultrathin oriented zeolite membranes, which is preferred in molecule

Microporous Zeolite Membrane: Structure, Preparation, Characterization, and Application 163 separations but difficult to prepare, is discussed in detail. Other methods for preparation zeolite membranes are also simply summarized.

3.1 Conventional Hydrothermal Synthesis First, an Si-containing source, Al-containing source, or other tetrahedral framework atoms such as phosphorus or water, with or without the addition of a structure-directing organic template and mineralizing agent, were mixed and aged for a certain time to form the mother solution/gel. Then the solution/gel was crystallized on a support in a sealed autoclave at a specified temperature in the oven for a certain time. In the hydrothermal crystallization stage, well-defined and intergrown crystalline zeolite with several micrometer thickness layers were formed by the nucleation and grain growth. In situ synthesis or secondary growth method is always combined with hydrothermal treatment. For in situ synthesis, the mother solution/gel in liquid phase directly attaches to the support, and an organic template and long time are usually needed for the crystal nucleus growth into order pure zeolite crystals. MFI zeolite films were successfully synthesized by an in situ crystallization method on porous α-alumina and yttria-doped zirconia (YZ) substrates using tetrapropylammonium hydroxide (TPAOH) as a template (Dong et al., 2000). After the growth of zeolites on the support, the organic templates are removed to create channel pores. Calcination membranes at high temperature are usually used to remove templates, but it is likely to create or enlarge intercrystalline gap defects due to the compressive stress during the heating and cooling process. Thus, many researchers make great efforts to minimize the defects induced by template removal (Heng et al., 2004; Choi et al., 2009; Korelskiy et al., 2017; Chang et al., 2018). Zeolitic analcime (ANA) membrane was successfully prepared without an organic template on porous ceramic supports by the in situ synthesis method. The mother solution was aged for 3 days and crystallized time for over 24 h in hydrothermal conditions (Khumbudda et al., 2016). The aging process could increase the number of nuclei or nuclei precursors of mother solutions. The membranes created with the aging process formed a thinner ANA layer with 50 μm thickness compared to that of about 220 μm without aging. The hydrothermal time extended to 24 h led to sufficient internal growth of crystals on the support. Xu et al. (2017b) prepared template-free NaA zeolite membrane onto the inner side of porous α-alumina tubular support in a Teflon-lined stainless-steel crystallization autoclave by the in situ synthesis method. It was carried out at different temperatures for 3–9 h by 3 cycles to improve the zeolite quality. In the template-free synthesis of zeolite membranes by the in situ method, higher synthetic temperature and longer crystallization time are usually used. Secondary growth method is zeolite seed precoated on the support surface and then synthesized in hydrothermal conditions. This method separates the nucleation and crystal growth in two steps. The crystal seeds on the support can act as nucleation sites and more easily induce mother

164 Chapter 7 solution/gel growth into pure-phase and better-quality of zeolite membranes. It is also more effective to better control over the membrane’s microstructure and obtain higher reproducibility than in situ synthesis (Boudreau et al., 1999). The growth of thin and defect-free molecular sieve layers is mostly based on seeding conditions. Various seeding techniques such as dip-coating (Wang et al., 2014), vacuum seeding (Huang et al., 2004), or rub-seeding (Liu et al., 2011) are commonly used. By electrostatic forces (zeta potential differences), covalent chemical anchoring or capillary forces, the nucleation site location and density can be well controlled, and a homogeneous thin film can be formed based on these seed crystallite sites (Caro and Noack, 2008). The seed layer can also prevent the mother solution/gel from filtrating into the support, which may form unfavorable multilayers in the membrane. Pilot-scale zeolite NaA membranes have been successfully prepared by using a seeding method called varying temperature hot dip-coating (VTHD) (Li et al., 2013). Large and small NaA seeds were manipulated to form asymmetric seed layers on a coarse macroporous support surface. The large seeds acted as fillers to cover the large pores of the support, and the small ones acted as nuclei sites to grow crystals. The morphology of seeding support and the corresponding membrane is shown in Fig. 5. The NaA membrane prepared by the VTHD method showed high water flux of 2.85 kg m2 h1 with a separation factor over 10,000 in dehydration of ethanol/water (90/10 wt%) mixture at 343 K. Steam-assisted conversion (SAC) seeding method was another way in the preparation of SAPO-34 membrane by their groups (Zhou et al., 2014b). This seeding technique included depositing a seed-containing paste on the support and transforming the paste into a continuous seeding layer. The paste could serve as the binder to prevent small seeds from penetrating into the large pores of α-Al2O3 supports. A high-quality of SAPO-34 membrane with a thickness about 4 μm exhibited a high H2 permeance of 6.96  106 molm2 s1 Pa1 at room temperature. NaA zeolite membranes on ceramic HF supports was developed by a similar dip-coating, wiping, seeding method (Wang et al., 2009a). After dip-coating the seed on supports, wiping appears to make seeds more uniform and possibly cover the defects of the support. PV performance with a flux of 9.0 kg m2 h1 and separation factor of 10,000 for water/ethanol separation was achieved by this seeding method with good reproducibility. A new rubbingseed-paste (RSP) technique was reported by the same groups on the synthesis of NaA layers on large pores of supports (Wang et al., 2011). The alumina tubular supports were wetted with water, and then rubbed with a seed paste (solid, c.50 wt%), which was composed of seed crystals and synthesis hydrogel. Finally, by shoving the seed layer with a Teflon ring, surface roughness was reduced and uniformity of the zeolite membrane was improved. They are still making progress in the synthesis of MFI zeolite membranes by researching wetting agents of the supports and rubbing dry crystals (Peng et al., 2013; Xia et al., 2016). Interfacial polymerization (IP) technique combined with a dip-coating operation was also used to prepare NaA zeolite membranes onto a micrometer-sized α-Al2O3 HF support (Cao et al., 2016). The support was dip-coated into a seed suspension (seed dissolved in aqueous phase), and then into

Microporous Zeolite Membrane: Structure, Preparation, Characterization, and Application 165

Fig. 5 SEM images of surface and cross-section of (A, B) only hot dip-coating with 2 μm NaA seeds at 150oC; (C, D) rubbing the hot dip-coating seed in (A, B) steps, then dip-coating again at 80°C with 0.4 μm NaA seeds (VTHD methods); and (E, F) corresponding NaA zeolite membranes synthesized at 80°C for 5 h. Reproduced from Li, H., Wang, J., Xu, J., Meng, X., Xu, B., Yang, J., et al., 2013. Synthesis of zeolite NaA membranes with high performance and high reproducibility on coarse macroporous supports. J. Membr. Sci. 444(444), 513–522 with permission from Elsevier.

166 Chapter 7 organic phase to polymerize the polyamide (PA). The nanosized seed crystals were frozen and fixed at the proper position by PA so that the seed layer could be accomplished. All membranes showed high performance in pervaporation applications. In conclusion, using crystals as seed on supports plays two key roles: covering or reducing the defects of the support surface, and forming a uniform and thin layer to support nucleation for secondary growth. Most of the research focuses on two directions of seeding methods to prepare defect-free membranes with thin layers.

3.2 Microwave Heating Synthesis Instead of conventional heat conduction in an oven, the crystallization of zeolite on supports can also be realized in a shorter time by microwave heating, as shown in Fig. 6A. According to the microwave heating method, broader synthesis composition, narrower particle size range, and higher purity can be achieved. Until now, LTA, MFI, FAU, CHA, MOR, etc. type zeolite membranes have been synthesized by using this heating method and showed excellent performance in separation applications. In microwave heating method, in situ and secondary growth are also combined for membrane preparation. Thin and compact CHA zeolite membranes were successfully synthesized on seeded symmetric stainless-steel tubular supports using microwave-aided secondary growth method (Hu et al., 2016). Li et al. investigated the synthesis of the LTA zeolite membrane by microwave synthesis method without seeding (Li et al., 2006; Li and Yang, 2008). They called it “in situ aging-microwave synthesis,” which is more preferable for industrial mass production.

Fig. 6 (A) Comparison of microwave heating and conventional heating; (B) the diagram of NaA zeolite membrane synthesis process by in situ aging-microwave synthesis. Reproduced from Li, Y., Chen, H., Liu, J., Yang, W., 2006. Microwave synthesis of LTA zeolite membranes without seeding. J. Membr. Sci. 277(1–2), 230–239; Li, Y., Yang, W.S., 2008. Microwave synthesis of zeolite membranes. J. Membr. Sci. 316(1), 3–17 with permission from Elsevier.

Microporous Zeolite Membrane: Structure, Preparation, Characterization, and Application 167 The schematic process is shown in Fig. 6B. The synthesis solutions first formed an amorphous gel layer on the support surface in the drying oven, aging for certain time. Then the supports were put into a microwave oven for the crystallization with frequency of 2450 MHz at 90°C for 25 min. The aging process contains the needed germ nuclei and overcomes the nucleationrelated bottleneck. The membrane was tested for pervaporation and gas separation, which showed a highly reproducible manner. The research group later synthesized FAU and T zeolite membranes using the same method as NaA membranes, which showed high separation performance for the water/alcohol liquid mixtures (Zhu et al., 2008; Zhou et al., 2009). Acid-resistant mordenite membranes were prepared on the porous mullite supports by microwave-assisted hydrothermal synthesis (Zhu et al., 2014). It showed good dehydration performance and reproducibility for dehydration of acetic acid/water mixtures. The types of allsilica DDR zeolite membranes could be synthesized in 30 min by nonseeded microwave synthesis using 1-adamantane amine and tetraethylammonium hydroxide as templates, which is the fastest synthesis method up to now (Bai et al., 2016a). The microwave-heating method not only reduced crystallization time with lower energy consumption, it also easily resulted in a thin layer that is expected to obtain high performance for separation. Including microwave heating, they reported the synthesis of SAPO-34 zeolite membrane by oil bath heating (Bai et al., 2016b). The synthesis time was significantly shortened from 5–8 h to 1 h, and the thickness of the synthesized membrane was reduced from 3–5 μm to 0.8 μm compared with oven heating. The heating source is an important factor on zeolite growth and membrane formation. Microwave energy is the electromagnetic field directly acting on the material, which eliminates thermal gradients, especially useful in scale-up preparation.

3.3 Oriented Ultrathin Zeolite Membrane Synthesis Great efforts have been made to improve flux or permeance in mixture separation, which greatly determines membrane applications in industries. Synthesis of orientated and thin layers of zeolite membranes are expected to present high performance. Oriented zeolite is where the channels grow vertically with respect to the substrate planes from the top to the bottom of the films, which maximizes the permeance of molecules. The thin films can reduce the diffusion resistance for molecules and promote permeance. The thickness of zeolite membranes is commonly greater than 3 μm prepared by conventional method (Choi et al., 2006). Thus, ultrathin zeolite membrane prepared with thickness of less than 1 μm is potential in the enhancement of the membrane flux. Using nanosized zeolite crystals as seeds is one of the developed approaches to prepare ultrathin membranes (Liu et al., 2009, 2010). Small crystals as seed could form a very thin seeding monolayer, then induced nutrients grow into thin films by hydrothermal, gel-free, or microwave-heating methods. Thus, very small sizes of zeolite crystals or even molecules (such as fragments of zeolites, even smaller than the crystal unit cell) as seed are pursued in the

168 Chapter 7 preparation of membranes. Considering zeolite rarely grows below a particle size of 50 nm, an alternative approach is to prepare hierarchical zeolites or their precursors, which consists of one or more dimensions in the 1–10 nm range of zeolite domains connected to each other (Tsapatsis, 2011). A very precise replication scheme starting from ordered mesoporous-imprinted silicalite-1 to spherical elements as small as 10–40 nm was demonstrated by a fragmentation method involving sonication and dissolution at certain pH ranges (Lee et al., 2011). By depositing these fragments as seeds on porous α-alumina disks, MFI-type membranes were formed with continuous thin zeolite films (300–400 nm). The membranes exhibited both high permeances (3.5  107 mol m2 s1 Pa1) and separation factors (94–120) at 150°C for p- and o-xylene mixture separation. Even two-dimensional MFI zeolite nanosheets with a unit cell dimension were achieved (Rangnekar et al., 2015b; Zhang et al., 2016). It was prepared by exfoliation of melt compounding, and then the template was removed by acid treatment. The nanosheets formed a 3-nm (1.5 unit cell) thick MFI seed layer by using Langmuir-Schaefer deposition on supports. Then the seed layers resulted in preferentially thin films of MFI with a sub-12-nm thickness. Until now, ZSM-5 zeolite membrane is the most synthesized oriented membrane because of its multidimensional channel network with opening pores near the sizes of many industrially organic molecules. Sinusoidal channels of circular cross-sections in a-direction is interconnected with straight channels of elliptical cross-sections in b-direction, and a tortuous path is present along the c-direction (Fig. 3). A type of b-oriented monolayer with their b axes perpendicular to the support surface is suggested as the fastest diffusion pathway in ZSM-5 crystals by experimental and simulation results (Caro et al., 1993; Kaerger, 1991). The use of monomer, dimer, or trimer of TPAOH as a structure-directing agent resulted in a crystal growth rate in a different ratio between in-plane and out-of-plane growth. The b-oriented ZSM-5 membranes were synthesized by trimer TPAOH with small seed particles, achieving high flux and high selectivity in xylene isomer separation throughout a thin film with a thickness of 1 μm (Lai et al., 2003). Most zeolite membranes were synthesized in an alkaline condition, for example, by using NaOH as a mineralizing agent. Recently, fluoride acids and even neutral solutions were used to synthesize oriented zeolite membranes by the hydrothermal method (Zhou et al., 2014a; Peng et al., 2015). Straight channels of silica MFI zeolite were vertically aligned on a graded alumina support in a fluoride media resulting in membrane b-orientation and a thickness of 500 nm, as shown in Fig. 7. The membranes showed a high CO2 permeance and a super high selectivity of CO2/H2 of 109 at 35°C. Ultradilute synthesis solution was used to synthesize b-oriented MFI zeolite films by microwave synthesis. The membrane layers showed a thickness of 600 nm (Wang et al., 2013). Oriented MFI zeolite membranes are not only synthesized by the hydrothermal method, but gel-free or gel-less methods are also used as reported by Kyung Byung Yoon and Michael Tsapatsis’s groups (Pham et al., 2013; Elyassi et al., 2016). The b-oriented silicalite-1 zeolite membranes prepared by gel-free secondary growth could be

Microporous Zeolite Membrane: Structure, Preparation, Characterization, and Application 169

Fig. 7 Top and side view of SEM images for (A) MFI seed layers (B, C) after growth into membranes on aluminum support. Reproduced from Zhou, M., Korelskiy, D., Ye, P., Grahn, M., Hedlund, J., 2014a. A uniformly oriented MFI membrane for improved CO2 separation. Angew. Chem. Int. Ed. 126(13), 3560–3563 with permission from John Wiley & Sons.

well-formed on polymeric modified support, silica-coated quartz support, amorphous silica-coated silicon wafer supports, etc. By gel-less secondary growth of sub-100 nm MFI nanosheet seeding layers, zeolite layers grow to 100–250 nm thickness, which is much thinner than that by hydrothermal synthesis method (Agrawal et al., 2015). Oriented zeolite membranes with ultrathin layers are preferred in the separation application, but the synthesis conditions are harsh and difficult to prepare on porous ceramic or a metal support for scale up. For these methods, very flat surfaces and abundant functional groups of substrates are strictly required. Environmental-unfriendly organic solvents are always used as a clean or reaction medium between the seed and support. Moreover, the key to successful growth of a uniformly oriented thin film is to find an appropriate gel composition and reaction temperature for appropriate crystallization time, which need numerous trials to discover optimum conditions.

3.4 Other Synthesis Methods The DGC method was discussed in a few studies on the synthesis of zeolite membranes (Zhao et al., 2000; Yang et al., 2018). This method deposits a gel layer with structure-directing agents and other constituents of zeolites on a substrate. Then, the precursor gel layer proceeds with nucleation and crystal growth via osmosis of the structure-directing agents and water steams into the gel layer in a sealed autoclave (Matsuda, 2018). This route shows the advantages of lower template agent consumption, less pollution, and higher product yield, especially avoiding expensive metal precursor precipitation or agglomerate in the alkaline media (Miyake et al., 2017).

170 Chapter 7 SAC is a similar simple method that was used to prepare silicalite-1 membranes on a seeded silica support under gel-free conditions (Ueno et al., 2017). The seeded support was first covered by TPAOH aqueous solution and dried, and then converted into a silicalite-1 membrane layer with a small amount of water at the bottom of autoclave by oven heating. In this method, silica support was not just used as the support but also as the silica source for zeolite growth. The synthesized membranes showed performance with a high flux of 4.47 kg m2 h1 and separation factor of 66 for pervaporation of 10 wt% ethanol/water mixtures at 323 K. These preparation methods all have advantages and disadvantages. Some researchers try to use more than one method at different stages to achieve high performance of zeolite membranes. An obtained SAPO-34 zeolite membrane thickness was reduced from 7 μm by single hydrothermal synthesis to 2 μm by a combination of hydrothermal-DGC method (Li et al., 2016b). Recently, ionic liquids were utilized to synthesize inorganic materials. They can act as templates and precursors as well as solvents instead of conventional synthesis of inorganic materials using water and organics (Ma et al., 2010). High-silica MOR zeolites were obtained using ionic liquids as the structure-direct-agent (SDA) by the DGC method, which showed high thermal stability. Besides, ZSM-5 could also be prepared by this route (Ma et al., 2016). There are also many other new methods in the synthesis of zeolite membranes, such as a dissolution-recrystallization process, vacuum-inhalation repair method, pore-plugging synthesis, etc. (Liu et al., 2018; Xu et al., 2017a; Li et al., 2008). Most of these methods are built in the basement of hydrothermal and secondary growth, which are still popularly used today. Including synthesis methods, the influences of porous support, synthesis solution compositions, crystallization temperature, and time are all important factors and should be optimized to prepare zeolite membranes with high performance.

4 Characterization Zeolites prepared as membranes are expected to achieve molecule-selective separation based on the size and shape of the zeolite structure. Besides, chemical properties such as hydrophilic or organophilic properties, and acid or basic properties also affect performance. To better understand and control the membrane’s quality, the process and mechanism of zeolite membrane formation, including their crystal purity, order rate, morphology, thickness, and defects as well as physical and chemical properties, should be characterized and analyzed.

4.1 Framework Characterization X-ray diffraction (XRD) is the most used instrument to characterize crystal frameworks, including zeolite phase identification, random/oriented channel direction, and relative crystallinity. It is an electromagnetic wave (the X-ray) impinging on a repeating arrangement of

Microporous Zeolite Membrane: Structure, Preparation, Characterization, and Application 171

Fig. 8 XRD patterns of (A) standard MFI powder, (B) MFI- seed layer, and (C) MFI film after secondary growth on glass substrates. Reproduced from Liu, Y., Li, Y., Yang, W., 2010. Fabrication of highly b-oriented MFI film with molecular sieving properties by controlled in-plane secondary growth. J. Am. Chem. Soc. 132(6), 1768–1769 with permission from American Chemical Society.

atoms in the crystals, producing data of the density of electrons within the crystal through measuring the angles θ and intensities of these diffracted beams based on Bragg’s law. The ordered zeolite phases can be identified when referenced to standard data. A complete XRD incident angle is from near 0 to 90 degrees (2θ) to detect powders or membranes, and zeolites are usually given peaks from 10 to 50 degrees. Fig. 8 shows the XRD patterns of commercial MFI powder and MFI zeolite film synthesized by Liu et al. (2010). The diffraction pattern for the synthesized membrane was identical with MFI powder, indicating that the crystals formed on the support surface have MFI-topology structure. The diffraction peaks from the (020), (040), (060), (080), and (0100) planes can illustrate that the synthesized film is a highly b-oriented MFI zeolite. If the support used for membranes is also ordered crystals, such as alumina and zirconia materials, strong peaks corresponding to the support will appear in XRD patterns (Dong et al., 2000). When identifying the MFI zeolite layers on these crystal supports by XRD, the peaks corresponding to supports should be excluded.

4.2 Morphology Characterization Zeolite morphology including crystal size, surface compactness/continuity, and layer thickness in a certain degree can characterize the quality of membranes. It can be illustrated by scanning electron microscope (SEM), which produces images of samples by scanning the surface with a focused beam of electrons. The microstructures of zeolite crystal or zeolite thickness ranging from hundreds of nanometers to approximately 500 μm has been reported by SEM

172 Chapter 7 (Bowen et al., 2004). A qualitative measure of the continuity of the membrane layer can also be examined by SEM. Examples of SEM images of the MFI zeolite seed layer and membrane are shown in Figs. 5 and 7. Depending on the SEM images, we can find the synthesizing mechanism and deduce rules to guide membrane preparation. The large defects can also be detected by SEM scanning. A crack is obvious in the SEM image, which appeared in the template removal process (Dong et al., 2000). To further investigate the atom array conditions, a transmission electron microscope (TEM) can be used. The resolution of the SEM is not high enough to image individual atoms due to large spot size and the interaction volume compared to the distance between atoms, so the TEM offers supplemented characterization for SEM images. Fig. 9 shows the TEM images of product morphology obtained from 40 nm silicalite-1 at two different pH ranges. Intact spherical elements can be obtained at pH 9.0–10.0, whereas isolated spherical elements with mesopores and small irregular fragments were observed at pH 11.0–12.0. The TEM images indicated that the morphology was etched at higher alkaline (Lee et al., 2011).

4.3 Pore Size Characterization The molecular sieving property of microporous zeolite membranes is derived from their unique order pore structures. The separation is realized by molecules in mixtures with smaller diameters than the zeolite pores that could pass through the membrane to permeate side, prohibiting larger ones. Thus, the evaluation of pore structures of zeolite membranes becomes important for separation applications.

Fig. 9 TEM images of products obtained from 40 nm silicate-1 nanocrystal at (A, B) pH 9.0-10.0 and (C, D) pH 11.0-12.0 solutions over 7 days. Reproduced from Lee, P.S., Zhang, X., Stoeger, J.A., Malek, A., Fan, W., Kumar, S., et al., 2011. Sub-40 nm zeolite suspensions via disassembly of three-dimensionally ordered mesoporous-imprinted silicalite-1. J. Am. Chem. Soc. 133(3), 493–502 with permission from the American Chemical Society.

Microporous Zeolite Membrane: Structure, Preparation, Characterization, and Application 173 Nitrogen adsorption is a method to characterize the pore size distribution as well as surface area and micropore/mesopore volume of zeolite membranes. The samples could be the powder of the zeolite precipitation at the bottom of the autoclave or a zeolite layer scratched off from the support. N2 adsorption is usually described through adsorption/desorption isotherms with the amount of N2 adsorptions in the zeolite as a function of its relative pressure, and then calculated by model simulation to evaluate the pore diameters. Ar adsorption can also be used as probe molecules to evaluate pores smaller than the N2 kinetic diameter (Na et al., 2011). However, the sample in this method is powder. The pores of zeolite membrane evaluated by scratching off a zeolite layer from the support and/or deducing from the powders is destructive and not accurate, especially in the existence of grain-boundary or intercrystalline gaps. These defects usually are micropores but larger than the zeolitic pore size, providing nonselective pathways for molecular transport. Unlike morphology defects (such as discontinuous surface or low intergrown of zeolite layers) that can be characterized by SEM, direct observation of these microporous defects in zeolite membranes remains a major challenge because the pores are in the micropore range (<2 nm) and in highly irregular topologic structure. Recently, some researchers reported the use of positron annihilation spectroscopy to nondestructively characterize the pore structure of porous material or membranes (Huang et al., 2008; Cabral-Prieto et al., 2013). Lin et al. investigated four MFI zeolite membranes on alumina supports that varied in their synthesis method by positron annihilation spectroscopy, which includes positron annihilation lifetime spectroscopy (PALS) and doppler broadening energy spectroscopy (DBES) (Ma et al., 2015). Fig. 10 presents the transport of PALS for randomly oriented MFI zeolite membrane (TR), randomly oriented MFI zeolite membrane prepared without an organic template (TFR), h0h-oriented MFI zeolite membrane (TH), and

Fig. 10 Transport of positron of PALS in the TR, TFR, TH, and TF zeolite membranes. Reproduced from Ma, X., Wang, H., Wang, H., Brien-Abraham, J.O., & Lin, Y.S., 2015. Pore structure characterization of supported polycrystalline zeolite membranes by positron annihilation spectroscopy. J. Membr. Sci. 477, 41–48 with permission from Elsevier.

174 Chapter 7 c-oriented MFI zeolite membrane (TC). PALS analysis reveals a bimodal pore structure consisting of around 0.6 nm diameter of zeolitic micropores and irregular intercrystalline micropores in size from 1.4 to 1.8 nm. DBES results illustrate intercrystalline gaps of zeolites during the growth of the zeolite layer along the membrane thickness direction. The TFR membrane exhibited about 6–13 times higher p/o-xylene ideal selectivity than the other three membranes synthesized with the template, which was well explained by the more desirable microstructure as examined by PALS and DBES analysis. The pore structure data obtained by PALS are consistent with xylene isomer separation performance of these membranes. Permporometry is a relatively simple method to evaluate non-zeolitic pores (>2 nm) and the proportion of defects. A noncondensable and less adsorbing gas (such as He, H2, or N2) and a vapor (such as water or hexane) are sent as feed through the membrane (Hedlund et al., 2009). The gas will be blocked by the vapor, which prefers to fill the micropores of membranes. Then the radius of condensation pore can be calculated according to the Kelvin equation. For large non-zeolite defect pores, the vapor could not condense and the gas will pass through. Through the remaining flux of the gas, the defect distribution can be estimated. The maximal pore size and defect distribution is well agreement with the gas separation performance (Tsuru et al., 2003; Wang et al., 2009b). It is an effective way to evaluate the quality of zeolite membranes before applications. The gas separation performance is another way to evaluate the pore size and intercrystalline pores in the membranes (Choi et al., 2009). Although several methods have been developed to characterize pore structure of supported zeolite membranes, it remains a major challenge to develop a simple and accurate method in studying complex pore structures of microporous membranes.

4.4 Other Characterizations There are many other methods as supplements of XRD, TEM, and N2 adsorptions, to evaluate the physical and chemical properties of membranes, analyzing the growth process of zeolite and transport mechanism of membranes. (1) Chemical properties: Element types and their composition of zeolites can be measured using electron probe microanalysis (EPMA), which could qualitatively and quantitatively detect solid materials, even elements at extremely low content without destroying the materials (Kondo et al., 1997). The chemical state of the silicon and aluminum in zeolites can be investigated using 29Si MAS NMR and 27Al MAS NMR spectroscopy (Kuhn et al., 2009). (2) Surface roughness: The global uniformity of zeolite could be performed by quantitative and nondestructive X-ray microcomputed tomography (l-CT) (Ou et al., 2017). Atomic force microscopy (AFM) is a very high-resolution type of scanning probe microscopy on the order of fractions of a nanometer. It can be applied to form an image of the three-

Microporous Zeolite Membrane: Structure, Preparation, Characterization, and Application 175 dimensional shape of a sample surface in the nanometer range (Vilaseca et al., 2004). In addition, the mechanical properties such as the Young’s modulus of the sample also can be detected by AFM. Laser scan confocal microscope (LSCM) was used to characterize the cracks in silicalite layer (SL) films (Pham et al., 2013). LSCM pictures of randomly oriented SL film treated by the fluorescein showed crack formation on calcination at 500° C compared with the perfect b-oriented SL film.

5 Applications Zeolite membranes showed high performance in liquid and gas separation, which are greatly researched. So far, NaA zeolite membrane has been commercialized in the dehydration of different solvents due to their strong hydrophilicity and suitable pore size. A gas separation process using zeolite membranes is not yet realized, but excellent lab results for xylene isomer and butene isomer separation using MFI membranes, and CO2/CH4 separation using SAPO-34 and DDR membranes will lead to potential scale-up of industrial installation. The zeolite membranes used in other applications such as membrane reactor, reverse osmosis, and sensor is under review.

5.1 Pervaporation in Acid Conditions Mature techniques on pervaporation have been realized by using NaA zeolite membranes in neutral conditions. However, there are still many mixtures required for separation in acid conditions, such as dehydration of acetic acid, acrylic acid, or separation water from esterification. The industrial NaA zeolite membrane is limited in these acid conditions due to the degradation of Al in the Al-rich framework. For preparation of acid-resistant membranes, the Si/Al ratio of zeolite is very important. Higher Al content results in high water flux but low acid resistance, whereas higher Si content results in high acid resistance but low water permeance in pervaporation applications (Wenten et al., 2017). The suitable Si/Al ratios became the most factor for pervaporation in acid conditions. T zeolite membranes with Si/Al ratios of 3–4 have been developed by Zhou et al. (2013), which obtained water/i-propanol separation factor of 13,000 with a flux of 2.50 kg m2 h1 at 348 K. The T zeolite membranes were stable at a moderated acidic solution with pH ¼ 3. MOR zeolite membranes with Si/Al ratios of 5-6 were more stable in acetic acid solutions for nearly 1 year (Chen et al., 2012). In the pervaporation of acetic acid/H2O mixtures, the membranes showed a water flux of 0.7 and 0.87 kg m2 h1 by hydrothermal and microwave synthesis methods, respectively (Ren et al., 2012; Li et al., 2016a). CHA-type zeolite membranes are now greatly researched with strong acid resistance. The highsilica membranes unexpectedly demonstrated high pervaporation performances in water/acetic acid mixtures. The permeate flux and separation factor were about 8 kg m2 h1 and 2500 by

176 Chapter 7 the dehydration of 50 wt% acetic acid aqueous solution at 75°C (Yamanaka et al., 2012). Furthermore, the membranes can be kept well in mineral acids, such as HCl, H2SO4, and HNO3. Jiang et al. (2017) synthesized a CHA-type membrane composed of flake-like grains. In the pervaporation dehydration of 90 wt% ethanol/water mixtures, the membrane exhibited a water flux of 13.3 kg m2 h1 with a separation factor of 6000 at 75°C. The performance is much higher than most of MOR and T zeolite membranes reported. There are also many other membranes such as MFI-type that have been reported in acidic conditions. These membranes have suitable Si/Al ratios and are expected for use in industrial applications in the near future.

5.2 Gas Separation The mixture types of inorganic/inorganic, inorganic/organic, and organic/organic as gas or vapor pairs could be separated by zeolite membranes. Based on the sorption and diffusion properties of gas, a proper zeolite membrane can give much higher performance at high temperature and pressure when compared with polymers (Kosinov et al., 2016). In most cases, mixture separation through zeolite membranes is through molecular sieving due to the well-fine and uniform pore systems. The component of a mixture smaller than the pore size of the zeolite membrane can pass whereas the larger molecules are prohibited. Several different pore sizes of zeolite membranes and kinetic diameter of gas molecules are shown in Fig. 11 (Mcleary et al., 2006). Zeolite membranes of CHA, NaA, MFI, and MOR with relatively large pore sizes can obtain high permeance for small molecules, such as He, H2, CO2, CH4, etc. The rather large molecules with nonadsorption ability like SF6 can be taken as a direct measurement for the defects of zeolite membranes. A specific separation can be realized by choosing a zeolite with pore size between the mixtures, such as natural gas purification. DDR-type membranes have been reported with high CO2/CH4 selectivity and high flux due to the narrow window in the cage framework (Himeno et al., 2007). SSZ-13 and SAPO-34 zeolite both display chabazite topology with a pore system size of 0.38 nm, which is close to the kinetic diameter of CH4. Corresponding membranes with this suitable pore size have been reported with excellent molecular-sieving performance for separation of CO2/CH4, N2/CH4, and CO2/butane mixtures (Carreon, 2018; Li et al., 2010). Zeolite SSZ-13 membrane synthesized via secondary growth method displayed relatively high N2/CH4 separation selectivity of 13 with N2 permeances (66 GPU) at 20°C (Wu et al., 2015). A higher N2 permeance of 2591 GPUs with a N2/CH4 selectivity of 11.3 was obtained for SAPO-34 zeolite membrane for binary gas separation (Zong and Carreon, 2017). A few examples are opposite in their shape-selective separation. A mixture of n/i-C4H10, olefin/nitrogen, and heavier hydrocarbon (C3+)/CH4 separations have been developed by ZSM-5 membranes (Yu et al., 2018). The membranes were selective toward the heavier

Microporous Zeolite Membrane: Structure, Preparation, Characterization, and Application 177

Fig. 11 The effective pore sizes of zeolite membranes and the kinetic diameters of gas molecules. Reproduced from Mcleary, E.E., Jansen, J.C., Kapteijn, F., 2006. Zeolite based films, membranes and membrane reactors: progress and prospects. Microporous Mesoporous Mater. 90(1), 198–220 with permission from Elsevier.

hydrocarbons over small kinetic diameter gas (N2, CH4). At room temperature, the n-C4H10 permeance was 31  107 mol m2 s1 Pa1, and the selectivity of n-C4H10/CH4 was 25 for a 10/90 n-C4H10/CH4 binary feed. The selectivities of C3H8/CH4 were 9.5 and 19 at 297 K and 271 K for 10/90 C3H8/CH4 binary mixture, respectively. Gas permeation not only depends on diffusion but also on adsorption properties of the permeating gas associated with the zeolite. In the above case, the adsorption of hydrocarbons on ZSM-5 zeolite sites played a major role, resulting in an opposite shape selection. The adsorption mechanism can be neglected at high temperature by gas permeation through microporous zeolite membranes.

5.3 Other Applications (1) Membrane reactors: It combines a chemical reaction with an in situ separation in one unit, offering improved conversion for equilibrium limited reactions. Zeolite membrane applied in membrane reactors can provide high separation ability and stability in most chemicals at high temperatures as an extractor, distributor, or contactor. There are

178 Chapter 7 numerous examples of increases in yield of an equilibrium-controlled reaction such as dehydrogenation and esterification if the product molecules can be removed selectively from the product mixture (Tan and Li, 2015). FAU-LTA zeolite dual-layer membrane was reported for the synthesis of dimethyl ether (DME) in a catalytic membrane reactor (Zhou et al., 2016). A combination of mild acidity of the top H-FAU layer and hydrophilic Na-LTA layer applied in a continuous removal of water resulted in high methanol conversion and essentially 100% DME selectivity. (2) Reverse osmosis: Desalination using zeolite membranes was first appeared in 2008 when water contaminated with radioactive material was separated by an NaA zeolite membrane (Malekpour et al., 2008). With their cation exchange ability, zeolites can be competitive options for removal of dissolved ions, including ZSM-5, zeolite A, MOR, and zeolite Y (Fard et al., 2018). However, zeolite membranes are mostly expensive, which limits its application in the desalination industry. (3) Sensor: Combining sensors with highly selective zeolite membranes is attractive. They are suitable as a reactive layer or nonreactive filter to improve the selectivity and sensitivity of gas sensors by removing undesired species from mixtures, such as monitoring and controlling nitrogen oxide, carbon dioxide, etc. (Fong et al., 2007). A combination of a highly selective zeolite MFI/Al2O3 membrane and a highly sensitive but nonselective Pd-doped SnO2 sensor exhibited extremely high selectivity (>100) of formaldehyde over NH3, ethanol, methanol, acetone, and isoprene (G€ untner et al., 2018). The contents of formaldehyde decreased down to 30 ppb at 90% relative humidity. The permeance is outperforming and will have a promising future in detectors.

6 Conclusion and Future Trends In this chapter, we have presented general and leading-edge research in the development of zeolite membranes. In last decades, great improvements have been made by various techniques for zeolite membranes applied in pervaporation, gas separation, membrane reactors, etc. A facile technique of membrane preparation, high reproducibility, long lifetime of more than 10 years, and low cost in devices remain significant challenges. In addition, most zeolites have not been prepared as membranes, which may have potential applications. We anticipate using ever-growing computational resources to design and predict zeolite materials or combinations suited for specific applications. It is necessary as well as a challenge to understand the factors contributing to the synthesis of microporous zeolite membranes and molecular transport with new analytical techniques. The combination of zeolite with other materials such as polymer and silica, as well as new emerging materials like covalent organic frameworks (COFs), graphene, or other materials, may form a crucial toolset to overcome the obstacles on the implementation of membranes in industry.

Microporous Zeolite Membrane: Structure, Preparation, Characterization, and Application 179

List of Acronyms AFI, BEA, CHA, DDR FAU, LTA, LTL, MFI, MOR, SOD, Zeolite T Zeolite framework type code with different topology according to the name of International Zeolite Association AFI Aluminophosphate five (AIPO5) BEA Beta polymorph A CHA Chabazite (type material: SAPO-34, SSZ-13) DDR Deca- and dodecahedra, 3 layers, rhombohedral (DD3R) FAU Faujasite LTA Linde Type A (type material: NaA) LTL Linde Type L (type material: L) MFI Zeolite Socony Mobil-five (type material: ZSM-5, silicalite-1) MOR Mordenite SOD Sodalite Zeolite T Erionite and Offretite (ERI-OFF) structural intermediate

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