Microporous Zeolite Membrane Reactors

Microporous Zeolite Membrane Reactors

CHAPTER 9 Microporous Zeolite Membrane Reactors Asma Ghorbani, Behrouz Bayati Chemical Engineering Department, Ilam University, Ilam, Iran 1 Introdu...

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CHAPTER 9

Microporous Zeolite Membrane Reactors Asma Ghorbani, Behrouz Bayati Chemical Engineering Department, Ilam University, Ilam, Iran

1 Introduction Membrane reactor (MR) as an evolving technology, which dates back to 1960s, offers new and important opportunities for increasing the production efficiency in the chemical and biotechnology industries. MRs combine a membrane separation and the reaction process (the reactor). These membranes are applied as an active contributor within reactions for raising the reaction rate, efficiency and selectivity, and resistance to high temperatures (Julbe et al., 2001; Drioli and Romano, 2001; Kong et al., 2007). Zeolite membranes with a high separation capacity are often employed in MR processes (Jeong et al., 2003). Zeolite MRs have micropore structure, excellent performance, resistance, and hydrothermal stability to extreme chemical environments for application process intensification and molecular separation (Kim et al., 2016). These MRs are usually formed of a thin microporous or mesoporous film zeolite on a macroporous support based on a-Al2O3, carbon, or stainless steel (McLeary et al., 2006).

2 Zeolite Membrane Reactors Configurations MRs have a variety of configurations to combine the membrane separation and reactor in a single unit. Types of these configurations include catalytic MR (CMR), catalytic nonpermselective MR (CNMR), packed-bed MR (PBMR), and packed-bed catalytic MR (PBCMR). CMR usually consists of a permselective membrane that also acts as a catalyst. In CMR, conversion and selectivity are increased by selective product removal and selective reactant supply, respectively. CNMR is an active catalytic and not permselective, which acts as a reaction front, facilitating stoichiometric rates of reactants. The CNMR is composed of a nonpermselective catalytic layer and a support layer (Harold et al., 2013). Features of CNMR include interphase contractor, short contact time, and exclusion of solvents that renders an environmentally attractive process (McLeary et al., 2006). The PBMR consists of a permselective membrane that is not catalytic and in the presence of a convectional catalyst forms a packed bed of extrudates/pellets in the flow (McLeary et al., 2006). Microporous Membranes and Membrane Reactors. https://doi.org/10.1016/B978-0-12-816350-4.00009-X # 2019 Elsevier Inc. All rights reserved.

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3 Separation Mechanism of Membrane Reactor The transport mechanisms within porous MRs can be dependent on the average free path length of molecules and the pore size. The transport mechanisms within porous MRs include viscous flow and molecular diffusion (mesopores and macropores), Knudsen diffusion (mesopores), capillary condensation (mesopores), micropore-activated diffusion, and surface diffusion (micropores and mesopores). The contribution of these mechanisms is affiliated to the properties of the gas molecules and the membranes, and the operating conditions of pressure and temperature (Julbe et al., 2001).

3.1 Catalytic Membrane Reactor CMR contains a permselective layer (membrane) and a support layer with a catalytic function (Harold et al., 2013). High temperature CMRs that combine chemical conversion (reaction) and separation in a single unit can solve the yield problems of thermodynamically limited reactions (Bobrov et al., 2005; Chang et al., 2002a; Drioli et al., 2008; Gobina and Hughes, 1994). Catalytic reactions are anxiously involved in wastewater treatments, chemical industry, and other processes. Important points to be considered for the production of an ideal catalytic membrane include high stability under reaction conditions, high selectivity, low cost, nontoxicity, the possibility of recovery, and reuse of the catalyst (Drioli et al., 2008). The use of a permselective MR provided the selective removal of hydrogen from a hydrogen-producing reaction and tended to shift the thermodynamic equilibrium toward conversion enhancement and product yield (Chang et al., 2002a; Gobina and Hughes, 1994). On the other hand, the zeolite membranes (composite or crystalline) were found to have an important application in CMRs to improve selectivity and yield of reactions, which are limited by thermodynamic equilibrium. One of the important characteristics of these types of MR is the probability of removing one of the products selectively and therefore affecting the equilibrium of the reaction. In zeolite MR, the products of reaction are continuously removed, which can lead to an increased conversion. (Tavolaro and Drioli, 1999). In zeolite MRs, the membrane is generally composed of a thin film of a microporous or mesoporous zeolite on a macroporous support (usually stainless steel, α-Al2O3, or carbon). A thin film of zeolite can simultaneously serve (1) as a permselective membrane and a catalyst, (2) as a permselective diffusion hindrance, and/or (3) as an inert nonselective reactant distributor (McLeary et al., 2006). Bernardo et al. (2006) used a Pt-Y zeolite CMR (FAU (Na-Y) zeolite membranes) prepared by ion exchange on the support of tubular α-Al2O3 for the removal of H2 by CO selective oxidation (Selox). Flow-through MR configuration used in the reactions is presented in Fig. 1. A conversion and a selectivity by a Pt-Y zeolite membrane for a mixture of H2-rich containing FFeed

2 10% CO at 200°C (λ¼ 2FOFeed ¼1.66) were measured at 98% and 62%, respectively. The CO CO

conversion and selectivity region with Pt-Y catalytic membrane rather than traditional reactors

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Fig. 1 Flow-through MR configuration used in the reactions.

at 200°C were expanded even with low space-time values. They found that, in the proposed operating conditions of temperature, feed concentration, O2/CO ratio in the feed, and pressure, the use of Pt-Y zeolite MRs for CO Selox after the low-temperature water-gas shift (WGS) unit in operating in the same temperature conditions were useful. •

The Selox using the Pt/Na-Y zeolite catalytic membranes in a continuous flow MR were investigated by Bernardo et al. (2008). CMR tests were studied with a dry reformate-shifted gas mixture with a low O2/CO feed ratio of 2.0 at pressure up to 600 kPa and temperature of 200°C–220°C. The results showed outlet CO concentration using the Pt/Na-Y zeolite catalytic membranes reduced from 10,000 ppm (1%) to 10–50 ppm. The selectivity for complete CO removal was 50%. They concluded that the high removal of CO was achieved by a Pt-Na-Y catalytic membrane at low pressure. Also, this CMR has a high stability for over many hours (200 h) in operation.

3.2 Packed-Bed Membrane Reactor PBMR is the most commonly used CMR, which provides only the separation function; the reaction function in catalytic applications is provided by catalyst particles placed inside or outside of the membrane. In PBMR, the catalyst is adjusted externally to the supported permselective membrane (Harold et al., 2013). PBMR is an important technique because it can combine chemical reaction and product separation in one step (Dangwal et al., 2017). In PBMR, the selective products of reaction are removed, which can lead to increased conversion, as well as selectivity increases through the selective reactant supply (McLeary et al., 2006). •

The H-ZSM-5 catalytic zeolite membrane has been used for reaction and removal of water during ethanol esterification by Pilar Bernal et al. (2002). According to XRD patterns, the H-ZSM-5 zeolite was stable under reaction conditions, and the XRD patterns were the same before and after the reaction (Fig. 2). They compared the performance of three reactor configurations: the H-ZSM-5 catalyst packed as powder inside an impervious tube (FBR),

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Fig. 2 XRD patterns of H-ZSM-5 zeolite, before and after the reaction at 348 K for 70.5 h. Table 1 Comparison of performances of three different membrane reactors with the same amount of catalyst Reactant conversion (%). Binary (ethanol/acetic acid) equimolar feed; catalyst load: 433 mg in all cases; temperature: 359 K; WHSV: 0.3 h1; He sweep gas flow rate: 360 N mL/min. ZMR: Na-ZSM5-1, AZMR: H-ZSM5-2 Equilibrium displacement (%). Quaternary feed (ethanol/acetic acid/ethyl acetate/water) in equilibrium; catalyst load: 987 mg in all cases; temperature: 338 K; WHSV: 1.8 h1; He sweep gas flow rate: 80 N mL/min. ZMR: Na-ZSM5-1, AZMR: H-ZSM5-3

FBR

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5.6

the H-ZSM-5 catalyst packed as powder inside a tubular Na-ZSM-5 membrane (ZMR), and no catalyst other than the H-ZSM-5 membrane (AZMR) with the same total amount of catalyst (Table 1). The result was that the conversion obtained at the same conditions was larger than in the conventional Na-ZSM-5 (ZMR) membrane and H-ZSM-5 catalyst packed as powder onto the fixed bed reactor. •

The removal of phenol discharged from various industries such as pharmaceutical, petrochemical, coal, and refineries has become a major concern, because of its toxicity to

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Membrane reactor

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Pump Water bath

Fig. 3 Schematic of experimental set-up used.

animals and humans. Yan et al. (2015) investigated catalytic wet peroxide oxidation (CWPO) of phenol by Fe-ZSM-5 zeolite catalyst MRs produced from a ZSM-5 MR with a Si/Al ratio of 80 and Fe loading of 15%, 25%, and 35%. The Fe-ZSM-5 catalyst MRs were fabricated by a ZSM-5 zeolite membrane on the surface (thickness of 6 μm) of paper-like sintered stainless steel fibers (PSSF) (20 mm i.d., 100 mm length) and Fe element with a form of Fe2O3, as well as packed among glassy beads (d ¼ 2–3 mm), to obtain further distribution and to keep the Fe-ZSM-5 membrane catalysts in the middle of the MR. They found that when Fe loading reached 35%, it caused the accumulation of a high concentration of Fe2O3 component on the ZSM-5 zeolite support, which is undesirable for catalytic activity. The set-up applied for the oxidation and removal of phenol by CWPO is presented in Fig. 3. The conversion of TOC, phenol, H2O2, and Fe leaching concentration in the solution during reaction at feed flow rate of 2.0 mL/min and temperature of 80°C are shown in Fig. 4. Their results indicated that the CWPO of phenol by Fe-ZSM-5 zeolite membrane catalyst for a Fe loading of 25% presents the highest conversion and activity (Xphenol ¼ 95% and XTOC ¼ 45%) for 7 h. Also, loss of concentration of Fe species was lower than 7 mg/L for all catalysts. •

Wang et al. (2014) designed and synthesized Pd–Ti silicalite zeolite composite membranes by using diverse Ti-containing zeolite catalysts to improve the reaction for benzene hydroxylation to phenol with H2 and O2 as reactants. Their results showed that the reaction was very affected by the porosity of titanium-containing zeolite film and the bonding of Ti atoms in the titanosilicate. The framework Ti can adsorb O2 species to form titanium peroxo

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(D) Fig. 4 Peroxide oxidation of phenol by Fe-ZSM-5 zeolite catalyst membrane reactors: (A) phenol conversion (Xphenol), (B) H2O2 conversion (XH2O2), (C) TOC conversion (XTOC), and (D) Fe leaching concentration. Time (h)

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species, which would repress the decomposition, whereas the extra-framework Ti promoted the decomposition of active O2 species, leading to higher water production. Large inter- and intracrystalline pores and mesopores obtained during the reactive species offers more opportunities to contact directly with the framework Ti resulting in high reaction activity and H2 selectivity (according to the phenol production). Also, these intraparticle pores helped the reactants acquire a more desirable approach to the active site than intercrystalline pores. On the other hand, the compact Ti silicalite zeolite film with smaller pore size was harmful to the reaction due to the slower diffusion of the reactants and products in the zeolite layer. VOCs, which come from industrial processes, various transport vehicles, and household products, are very dangerous for the environment and human health. The zeolite membrane catalyst reactor, using the Cu–Mn (1:6)/ZSM-5 membrane/PSSF catalyst, was prepared for removal of VOCs through catalytic combustion by Chen et al. (Chen et al., 2013). They examined the catalytic combustion of VOCs (ethyl acetate or isopropyl alcohol) in single and binary components over these zeolite MRs at different gas hourly space velocity (GHSV) and inlet concentrations. Catalytic combustion of isopropyl alcohol and ethyl acetate as single components over the zeolite MR at different concentrations and GHSV are shown in Figs. 5 and 6. The results indicated that the complete conversion of ethyl acetate or isopropyl alcohol as a single VOC component can be obtained at temperatures below 300°C.

The catalytic combustion of a binary mixture of isopropyl alcohol and toluene over the zeolite MR was studied using variant concentration ratios and flow rates as a function of temperature as shown in Figs. 7 and 8. The conversion of ethyl acetate and isopropyl alcohol was more in the binary mixture than that of toluene, due to their smaller kinetic diameter, linear molecule type, and nucleophilic property. They also found that the zeolite MR had excellent contacting efficiency, lower bed pressure drop, excellent reaction stability, and reasonable mass/heat transfer efficiency as well as lower diffusion resistance. •



A tubular LTA membrane was prepared by a synthesis method called multi-in-situ crystallization inside a porous ceramic tube and used to synthesize methanol from hydrogen and carbon dioxide in a zeolite MR by Tavolaro and Tavolaro (2007). They compared the results obtained by the zeolite MR with results obtained with a traditional reactor (TR) under similar operating conditions. Conversion of CO2 obtained by the ZMR at 210°C was 17%, but the equilibrium value without TR was about 6%. Chang et al. (2002b) studied the dehydrogenation of propane to propylene by an isothermal high-temperature MR in the form of a shell-and-tube consisting of a Pt/K/Sn/Al2O3-packed catalyst and a Pd-coated γ-Al2O3 membrane. They found that the use of a CMR for dehydrogenation of propane, which selectively separates the hydrogen from the reaction mixture, can significantly increase the conversion and decrease the operating temperature of the reaction. Therefore, the Pd/γ-Al2O3 MR causes the conversion to be twice as high

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Isopropyl alcohol conversion (%)

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Fig. 5 Catalytic performance for VOCs combustion over a zeolite membrane reactor at variant concentrations: (A) isopropyl alcohol (GHSV of 7643 h1 in all cases and 3.7–6.8 mg/L of isopropyl alcohol) and (B) ethyl acetate (GHSV of 7643 h1 in all cases and 3.1–11.1 mg/L of ethyl acetate in the feed gas).



than the equilibrium conversion, and six times higher than the conversion of a TR system at 500°C (Fig. 9). Ethylene is important for the chemical industry as it is used in alkylation, polymerization, and oxidation. The increasing demand for ethylene production has led to significant

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Fig. 6 Catalytic performance for VOCs combustion over a zeolite membrane reactor at variant GHSV: (A) isopropyl alcohol (4.7 mg/L of isopropyl alcohol in the feed gas in all cases, GHSV of 3822–11466 h1) and (B) ethyl acetate (5.3 mg/L of ethyl acetate in the feed gas in all cases, GHSV of 3822–11466 h1).

research into the development of new processes to decrease energy consumption. Ethylene is typically produced by dehydrogenation and cracking of light alkanes. The ethane dehydrogenation reaction (EDH) by MRs is an attractive technique due to its selective removal of H2 and overcoming the equilibrium limit in favor of ethylene. Therefore,

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Fig. 7 Conversion of binary VOCs mixtures (toluene and isopropyl) over a zeolite membrane reactor (GHSV of 7643 h1 in all cases, 3.7–14 mg/L of toluene and 6.6 mg/L of isopropyl alcohol in the feed mixture gas).

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Fig. 8 Conversion of binary VOCs mixtures (toluene and isopropyl alcohol) over a structured zeolite membrane reactor (6.6 mg/L of isopropyl alcohol, and 6.7 mg/L of toluene in the feed mixture gas in all cases GHSV of 3822–11466 h1).

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Fig. 9 Propane dehydrogenation using Pd/γ-Al2O3 membrane based on the reaction temperature: packed-bed catalyst, Pt/Sn/γ-Al2O3; partial pressure of C3H8, 20 kPa; contact time, 2 s; flow rate of sweep gas (Ar), 100 mL/min.



Dangwal et al. (2017) studied the EDH reaction by a PBMR that operates with a Pt/Al2O3 catalyst. They investigated the effects of MFI zeolite PBMR and operating conditions on ethylene selectivity, ethylene yield, and ethane conversion. The conversion, selectivity, and yield of ethylene with the MFI zeolite PBMR were greater than in a packed-bed reactor. The model results are shown that high ethane conversion (more than 98%) can be obtained under significantly operating temperature, space velocity, and pressure, even for membranes with moderate H2 permeance and selectivity. WGS reaction is a significant step for conversion of CO to H2 (CO + H2O $ CO2 + H2, 4 H298 K ¼ 41.2 kJ/mol) from biomass and fossil fuels via the gasification route (Kim et al., 2012, 2013; Zhang et al., 2012; Dong et al., 2015). Kim et al. (2012) constructed WGS MR using a MFI zeolite membrane packed with a Ce-doped ferrite (Fe) catalyst to investigate the effect of reaction pressure (2–6 atm) at high temperatures (400–550°C) and the permeate swept through nitrogen at 1 atm. Increasing pressure and temperature increased the reaction rate and the rate of H2 permeation, resulting in a significant increase in CO conversion. Increased pressure also significantly reduces the adverse reactions of methanization. The increase of CO conversion and reduction of methanation in the WGS MR led to more removal of H2 from the catalyst at higher pressure. The zeolite

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membrane and the Ce/Fe catalyst provided good stability and resistance to high concentrations of H2S in the high-temperature WGS reaction. Arvanitis et al. (2018) reported on an experimental CO conversion in a single HTP WGS MR that uses a CCD-modified MFI zeolite membrane supported on a commercially available alumina tube. They found that, when using the tubular zeolite membrane, after preparation by the catalytic cracking deposition (CCD) method, the values of H2/CO2 separation selectivity and H2 permeance were 38 and 1.07  107 mol/m2 s Pa, respectively, at 500°C. They made WGS MR by packing a nanocrystalline iron oxide-based catalyst inside the membrane tube. The MR indicated the capability to obtain almost complete conversion of CO and total H2 recovery with concurrent generation of high pressure CO2 in retentate at a GHSV of 15,000 h1, 500°C, and 20 bar. Zhang et al. (2012) used H2-selective MFI-type zeolite membranes modified with CuO/ ZnO/Al2O3 catalysts for low-temperature WGS reaction (LT-WGS) for separation of CO2/ H2 mixture. The membrane was prepared by loading MFI zeolite pores with CCD of methyldiethoxysilane. Generally, they found that the CO conversion for LT-WGS reaction improved when they applied the MR instead of a packed-bed reactor. The results of H2/CO2 mixture separation showed a separation factor of 42.6 with H2 permeance of 2.82  107 mol/m2 s Pa at 500°C. They investigated both the membrane and packed-bed reactor’s performance for LT-WGS reaction by variation of space velocity, H2O-to-CO feed ratio, temperature, and sweep gas flow rate. CO conversion was enhanced significantly for the MR at low H2O-to-CO ratio, high space velocity, and rather low temperature due to a large deviation of the equilibrium state. High CO conversion was 95.4% in MR, which was larger than the conversion the equilibrium state. CO conversion for MR versus PBR was variable under operating conditions such as H2O-to-CO ratio and space velocity, which is mainly due to the deviation of the reaction from equilibrium state. These results showed that the MR versus packed-bed reactor had better performance for low-temperature WGS reaction in the CO conversion. Kim et al. (2016) described a one-step in-situ synthesis of zeolite membranes combining metal nanoclusters. In summary, they showed the formation of a hybrid pure-silica MFI/Pt membrane, which is obtained via one-step hydrothermal synthesis by addition of a Pt precursor and MPS to the solution. They studied the membrane morphology and composition with high-resolution imaging and mapping techniques, and they also specified the catalytic properties of the zeolite membrane through propane dehydrogenation (PDH) at a temperature of 600°C. Their results showed that Pt is uniformly distributed in the zeolite, and also did not show any notable difference in crystal structure. HAADF-STEM, TEM, and imaging showed that Pt is present as small clusters of size distribution in the range of 0.5-2 nm after calcination of the zeolite. X-ray absorption spectroscopy also showed that the hybrid MFI-Pt zeolite included a combination of reduced Pt0 and Pt2+ species. Permeation results indicated that the hybrid MFI-Pt membranes have high permeance of H2

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and exhibited selectivity of H2 over propane; also, the hybrid MFI-Pt catalyst MR demonstrated desirable catalytic stability in high-temperature PDH for a time of 12 h. Experimental and modeling analysis on WGS reaction using tubular ZSM-5/silicalite bilayer membrane supported on α-alumina were studied by Dong et al. (2015). Their experimental and modeling evaluations indicated that feed pressure, temperature, H2O/CO ratio, and GHSV were key factors on the WGS reaction performance in the zeolite MR. They concluded that, to obtain high CO conversion and H2 recovery, the MR should be operated at high feed pressure and high temperature. Both high pressure and high temperatures help to remove H2 rapidly through the zeolite MR, which results in an increase in the conversion of CO and H2 recovery. The conversion of CO and H2 recovery were 89.8% and 28.5%, respectively, at 5 atm and 500°C with GHSV of 72,000 h1 and the H2O/CO ratio of 3.0. Appropriate pressure, temperature, GHSV, and H2O/CO ratio are crucial factors to reach desirable reaction performance. The modeling results indicated both high CO conversion and H2 recovery were >95% and >90% for WGS in the zeolite MR, respectively. This result showed that MFI-type zeolite MR has high potential for the WGS reaction process. A Na-Y zeolite membrane was synthesized on the inner surface of a porous α-alumina support tube by a hydrothermal process, and then PtY zeolite catalyst membrane was prepared by ion exchange with an aqueous solution of [Pt(NH3)4]Cl2 and, as a CMR for selective oxidation of CO, which was included in hydrogen at a concentration of 104 ppm, was used by Hasegawa et al. (2001) (Fig. 10). The pressure on the feed side and permeate side was maintained at 1 atm, and the permeate side was swept with N2 or He. The CO and H2 fluxing through the PtY zeolite catalyst MR were independent on the temperature in the state without O2 in the feed, and the flux of CO was approximately 104 mol m2 s1. Also, the flux of H2 through the PtY catalyst membrane was not

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Fig. 10 Flow-through MR configuration used in the reactions.

218 Chapter 9 dependent on the O2 concentration in the feed. The mean residence time of CO and O2 crossing the membrane was approximately 10 times lengthy than H, which was resulted in the selective oxidation of CO through the membrane. Although there was a presence of steam at a concentration of 2%, the H2 and CO fluxes did not change the activity of the PtY zeolite catalyst MR. Tang et al. (2010) modified the porous α-alumina tube supporting MFI zeolite membrane by situ CCD method and characterized by permeation of CO2 and H2 single gas, as well as CO2/H2 mixture separation at high temperature, gas permeation also separated CO2/ H2 mixture at high temperature. They used a modified membrane as an MR to study the WGS reaction in a temperature range of 400–550°C. Their results show that the MFI-type zeolite membrane with partial modification of the zeolitic channels offers high selectivity and permeance for H2 separation from the WGS reaction at high temperature. They also found that the modified membrane has been indicated to have a sensible stability in the WGS reaction conditions at 400–550°C, although a test for a longer term is required. They investigated the WGS reaction in the zeolite MR using a Ce-doped ferrite catalyst. Their results demonstrated that the increment of CO conversion in the MR is achieved more effectively at >500°C where kinetic resistance reduces, and for low H2O-to-CO ratios (RH2O/CO), where the pH 2 in the reaction side is high. At 550°C and a low RH2O/CO of 1.0 with a weight hourly space velocity (WHSV) of 60,000 h1, the modified zeolite MR achieved XCO of 81.7%, which was much higher than that in the traditional packed-bed reactor and also higher than the equilibrium limit. The results of MR WGS reaction in this work also indicated that, at 500°C and 550°C, the insufficiently of H2 removal from the catalyst bed is the main factor limiting the increment of the CO conversion at low RH2O/CO and high WHSV. •



Hasegawa et al. (2002) in their other work prepared PtY, RuY, NiY, CoY, RhY, AgY, and CuY zeolite catalyst membrane by a hydrothermal process and ion-exchange methods, used for the oxidation of CO in a H2 mixture (CO:CO2:H2 ¼ 1:10:89 on a molar basis). The fluxes of CO for the NiY, AgY, and CoY zeolite catalyst membranes were not impressed in the presence of O2. However, the fluxes of CO reduced with an increasing concentration of O2, for cases of the CuY, PtY, RhY, and RuY zeolite MRs, when a small value of O2 was added to the feed. Ejtemaei et al. (2018) studied isomerization of nC5/iC5 at temperatures of 180–260°C and atmospheric conditions in a BZSM-5 zeolite MR packed with platinum containing sulfated zirconia fixed-bed (Pt-SZ/Al) catalyst that was synthesized by precipitation technique. They investigated the effects of sweep gas flow rate, reaction temperature, and a comparison between the performance of the PBMR and PBR. XRD and SEM images (Figs. 11 and 12) indicated SZ nanopowders with mostly tetragonal crystalline phase were synthesized, and a continuous B-ZSM-5 uniform layer was formed on the support surface. They found that nC5 conversion increased with incrementing temperature and iC5 selectivity reduced (Fig. 13). By increasing the flow rate of sweep gas, the conversion of nC5 decreased and iC5 selectivity increased due to the decrease in the contact time catalyst

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Fig. 11 XRD pattern of sulfated zirconia.





and reactant. Also, they found that nC5 conversion and iC5 selectivity for PBMR was always better than the PBR in the same conditions. Charchi Aghdam et al. (2016) investigated nC5 isomerization in a system by combining reaction and separation using a tublar Pt/ZSM-5 packed nanocatalyst MR. The feed is passed through the BZSM-5 membrane, and linear n-C5 molecules were separated from branched before contact with the catalyst bed. The catalytic performance of MR was compared with that of a conventional packed-bed reactor performance at similar operation conditions and with the same dimensions. The results of the comparison of a conventional PBR with MR showed that MR significantly increased hydroisomerization of nC5 and iC5 yield. The MR obtained a maximum enhance iC5 yield compared to a conventional PBR of 6300% at 220°C. They also found that the operating conditions had a significant effect on MR performance. Gora and Jansen (2005) studied the hydroisomerization of C6 in a single-pass operation with the use of a 30 cm2 silicalite-1 membrane in a tubular configuration around a packed bed of Pt-chlorinated catalyst, and also the MR performance for hydroisomerization of C6 as a function of sweep gas flow rate and temperature. Their results show that the MR selectively can separate linear hydrocarbons from branched. Results obtained from

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Fig. 12 SEM image of sulfated zirconia.



“single-passage mode” showed that the membrane and catalyst under similar conditions have high potential for upgrading low-octane hydroisomerization feed streams. The n-Hexane with a purity of  99% (1% 2-MP, selectivity 24) was supplied selectively through silicalite-1 membrane to the catalyst, where it was 100% selectively hydroisomerized, and also they found silicalite-1 membrane protects the catalyst against poisoning by impurities (H2O, benzene). Dehydrogenation of ethylbenzene to styrene using a novel MR containing a porous, tubular stainless-steel (PTSS) supported zeolite silicalite-1 membrane and packed with an iron oxide catalyst was studied by Kong et al. (2007). They investigated the dehydrogenation of ethylbenzene to styrene using both the fixed-bed reactor and the zeolite silicalite-1 MR at 0.8 atm pressure and temperatures of 580–640°C with a water/ethylbenzene volumetric ratio of 2.0. They concluded that, with the increasing purge gas flow rate, ethylbenzene conversion in the MR increased and then decreased slightly. About 7% higher ethylbenzene conversion was obtained for the zeolite silicalite-1 MR than the fixed-bed reactor at temperatures above 600°C without decreasing styrene selectivity.

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nC5 conversion in PBMR nC5 conversion in PBR iC5 selecvity in PBMR iC5 selecvity in PBR

60 50 40

80

30

70

20

iC5 selectivity (%)

sweep gas=0

80

10 0

150

170

190

210

(A)

230

250

270

Temperature (˚C) 100

100 90

sweep gas=3 cc min -1

70

nC5 conversion in PBMR

60

nC5 conversion in PBR

90

iC5 selecvity in PBR

50

80

iC5 selecvity in PBMR

40 30

70

20

iC5 selectivity (%)

80

nC5 conversion (%)

60

10 0

(B)

150

170

190

210

230

250

270

60

Temperature (˚C)

Fig. 13 Conversion variation of nC5 and selectivity of iC5 in PBR and PBMR. (A) Carrier gas ¼ 10 cc min1 (for PBR), carrier gas ¼ 10 cc min 1 (for PBMR) and sweep gas ¼ 0), (B) carrier gas ¼ 13 cc min1 (for PBR), carrier gas ¼ 10 cc min1 (for PBMR) and sweep gas ¼ 3.





Jeong et al. (2003) used FAU-type zeolite membrane on a porous α-Al2O3 support tube (MR packed with a Pt/Al2O3 catalyst), for the selective separation of hydrogen and benzene by the catalytic dehydrogenation of cyclohexane. By increasing the sweep flow rate, the cyclohexane conversion using the zeolite catalyst MR increased due to the simultaneous removal of benzene and hydrogen from the reaction site. Also, cyclohexane conversion enhanced with a decrease in cyclohexane feed rate. Mendes et al. (2011) studied the values with experimental evaluation and model analysis of a self-supported finger-like MR exclusively conceived for ultrapure hydrogen production. The WGS reactions were carried on a Pd-Ag MR under operating conditions such as GHSV 1 (1200–10, 800 LN kg1 cat h ) and temperature (200°C–300°C), using different operating modes (vacuum and sweep gas). They also experimentally tested the permeability of the metal membrane and the kinetics of the reaction. A finger-like MR was particularly used for

222 Chapter 9 ultrapure hydrogen production. The experimental and simulation conversion results of carbon monoxide indicated a suitable agreement for the MR in both vacuum and sweep gas operating modes. The comparison between the experimental results and the predicted compositions, at the exit of the MR, also confirm the validity of the presented MR model. Apart from two parameters related to reduce of H2 permeability due to exist of CO in the reaction mixture, all other parameters of model were specified from independent studies, namely membrane permeability for H2 and reaction kinetics.

4 Conclusion and Future Trends Zeolite MRs include the separator and catalyst. Zeolite MRs have the advantages of selectivity, stability with higher temperatures, and permeability. They can improve the selectivity and yield of the reactions that are limited by the thermodynamic equilibrium. Comparing the results of zeolite MRs with traditional reactors showed the zeolite MR improves conversion and yield of reaction. They are usually used for removal of hydrogen in dehydrogenation of hydrocarbons and sometimes in synthesis gas production or hydrogen-producing decomposition. The removal of hydrogen by CO Selox, as well as the WGS reactions for conversion of CO to H2, and removal of VOCs by the zeolite catalyst MRs, were investigated by authors. Their results showed that the zeolite MRs provide desirable conversion, selectivity, and yield. One of the biggest problems in the development of zeolite MRs is the issue of thermal shock, because this type of membrane is cracked by rapid temperature changes. To avoid these problems in the future, the method of synthesis of zeolite membranes should be modified or use of membrane support that has the same thermal expansion as zeolite.

List of Acronyms MR CMR CNMR PBMR PBCMR Selox WGS XRD CWPO PSSF VOC GHSV TR EDH

membrane reactor catalytic membrane reactor catalytic nonpermselective membrane reactor packed-bed membrane reactor packed-bed catalytic membrane reactor CO selective oxidation water-gas shift X-ray diffraction catalytic wet peroxide oxidation paper-like sintered stainless steel fibers volatile organic compound gas hourly space velocity traditional reactor ethane dehydrogenation reaction

Microporous Zeolite Membrane Reactors 223 CCD LT-WGS PDH WHSV HAADF-STEM TEM SEM SZ

catalytic cracking deposition low-temperature water gas shift reaction propane dehydrogenation weight hourly space velocity high angle annular dark-field scanning electron microscopy transmission electron microscopy scanning electron microscope sulfated zirconia

List of Symbols λ FO2Feed FCOFeed d Xphenol XTOC XH2O2 XCO GHSV RH2O/CO

the O2/CO stoichiometric equivalent feed molar ratio O2 feed flow rate CO feed flow rate diameter phenol conversion TOC conversion H2O2 conversion CO conversion gas hourly space velocity (h1) H2O-to-CO ratios

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