LTA zeolite membranes

For a packed-bed membrane reactor (PBMR) the membrane is permselective and removes the product as it is formed, forcing the reaction to the right. In this case, the membrane is not active and a conventional catalyst is used. Tavolaro et al. [45] demonstrated this concept in their work on CO2 hydrogenation to methanol using a LTA zeolite membrane. The tubular membrane was packed with bimetallic Cu/ZnO where CO2 and H2 react to form EtOH and H2O. These condensable products were removed by LTA membrane which increased the reaction yield when compared to a conventional packed bed reactor operating under the same conditions [45]. 

Zeolite membranes show high thermal stability and chemical resistance compared with those of polymeric membranes. They are able to separate mixtures continuously on the basis of differences in the molecular size and shape [18], and/or on the basis of different adsorption properties [19], since their separation ability depends on the interplay of the mixture adsorption equilibrium and the mixture. Different types of zeolites have been studied (e.g. MFI, LTA, MOR, FAU) for the membrane separation. They are used still at laboratory level, also as catalytic membranes in membrane reactors (e.g. CO clean-up, water gas shift, methane reforming, etc.) [20,21]. The first commercial application is that of LTA zeolite membranes for solvent dehydration by pervaporation [22], Some other pervaporation plants have been installed since 2001, but no industrial applications use zeolite membranes in the GS field [23]. The reason for this limited application in industry might be due to economical feasibility (development of higher flux membranes should reduce both costs of membranes and modules) and poor reproducibility. 

Zeolite membranes represent a new branch of inorganic membranes. Significant progress has been made so far in the synthesis of thin, high-flux, defect-free zeolite membranes with new techniques of preparation and modification. In fact, the hydrophilic LTA zeolite membranes have already been applied in industry for dehydration from organic solutions... 

Pretreatment of the support by 3-aminopropyltriethoxysilane (APTES) as a covalent linker between LTA zeolite membrane and the alumina support. From HuangA, UangFY, Steinbach F, GiroJ. Preparation and separation properties of LTA membrane by using APTES as covalent linker. J MembrSci 2010 350 5-9. 

Tavalaro, A. and Tavolaro, P. (2007) LTA zeolite composite membrane preparation, characterization and application in a zeohtic membrane reactor. Catal. Commun., 8, 789-794. 

A survey of recent literature on zeolite membrane preparation reveals that synthesis processes, even for well-known zeolite structures (i.e., MFl, LTA), are still carried out batchwise, using a hydrothermal route to produce a thin layer from hydrogels or sols containing the corresponding nutrients. As a general rule, the reactant mixture in contact with the support changes in composition with time provoking a reduction of the membrane quality. 

In general, when carrying out a new separation, the kinetic diameter and the heat of adsorption of the gases, which compose the mixture, are the main variables used to select the most adequate zeolite. MFI, FAU, LTA, SOD, ANA, DDR, MOR, BEA, CHA, FER, KFI are zeolite structures widely used as membranes for different separations. In gas separation, MFI zeolite membranes (silicalite-1, ZSM-5, and with Al, Fe, B, and Ge isomorphously substituted into their stmctures) are the most commonly used membranes because their pores (-0.55 nm diameter) are in the size range of many industrial mixtures furthermore, their synthetic chemistry is well established in the literature. 

The multistep synthesis is classically used to repair defects or to increase the zeolite membrane thickness. This technique also allows to superimpose layers with different zeolite structures. We have also to notice that for certain zeolite structures such as FAU or LTA, when the synthesis duration is too long, other phases can appear by transformation of the initial zeolite structure. 

Industrial applications of zeolite membranes can be considered only for separations where they offer some unique advantage in terms of flux, selectivity, or thermal and chemical stability. The very high fluxes obtained with LTA membranes (typically 100 times higher than those obtained with a polyacrylonitrile membrane at the same FLO/alcohol selectivity) explain the rapid expansion of this type of application at the end of the 90s [8J. 

According to a recent conference given by Prof. Kita [162], the classical synthesis method currently used by Mitsui allows to produce about 250 zeolite membranes per day. Both the LTA and T types (Na K) membranes are now commercial and more than 80 pervaporation and vapor permeation plants are operating in Japan for the dehydration of organic liquids [163]. A typical pervaporation system, similar to the one described in [8], is shown in Fig. 11. One of the most recent applications concerns the production of fuel ethanol from cellulosic biomass by a vapour permeation/ pervaporation combined process. The required heat is only 1 200 kcal per liter of product, i.e. half of that of the classical process. Mitsui has recently installed a bio-ethanol pilot plant based on tubular LTA membranes in Brazil (3 000 liters/day) and a plant with 30 000 liters/day has been erected in India. The operating temperature is 130 °C, the feed is 93 % ethanol, the permeate is water and the membrane selectivity is 10 000. 

Microwave heating has become one of the most successful approaches in the synthesis of zeolite membranes in terms of energy efficiency by reducing the synthesis times. LTA, MFI, AFl, FAU, SOD, and ETS-4 types of zeolite membranes have been successfully synthesized by microwave heating [5,20-24]. 

Figure 11.19b. The most studied structure is the MFI, followed by LTA, and a similar number of publications could be found about FAU and MOR. The section others corresponds to FER, BETA, MEL, ZSM-11, and related materials like ETS-10 or ETS-4. The distribution of the zeotypes studied in the period 2(X)5-2011 does not change that much, although the proportion of mixed matrix membranes or composites decreases to approximately 10%. The number of publications referred to MFI, FAU, LTA, or MOR is multiplied by three compared to the period 1995-2(X)5 and CHA structure has also been introduced. After the excellent review published by Falconer and Noble in 2004 [158], the work on pervaporation and zeolites has been reviewed in specific or general reviews of zeolite membranes and pervaporation [1,3,159].

Wang Z, Qinqin G, Shao J, Yan Y. High performance zeolite LTA pervaporation membranes on ceramic hollow fibers by dipcoating-wiping seed deposition. J Am Chem Soc 2009 131 6910-6911. 

Zeolite membranes have gained much interest recently. Zeolites are crystalline microporous aiuminasilicates. It is built up by a three dimensional network of SiO and -MO. tetrathedra [61 - 63], Zeolites have a veiy defined pore structure and figure III - 60 gives a schematic drawing of the structures of zeolite LTA (type A) and silicaliie-l. Due to... 

When zeolites are grown as films, zeolite membranes are formed. Efforts to prepare polycrystalline zeolite membranes started in the late 1980s, but not until the early 1990s were MFI-type zeolite membranes (ZSM-5 and silicalite-1) successfully prepared with very good permeation and separation properties [3]. Since then, zeolite membranes have constantly attracted considerable attention because of their unique properties in terms of size uniformity, shape selective separation behavior, and good thermal/chemical stabilities. So far, more than 20 different types of zeolite membranes have been prepared - such as LTA, FAU, MOR, FER, MEL, CHA, DDR, and AFI - with significant separation interest [4, 5]. Table 3.1 lists a few typical zeolite membranes and their potential applications for separation of fluid mixtures. 

Using a hydrophilic zeolite membrane such as LTA and zeolite T to remove the water from the reaction system by sweep gas or vacuum (pervaporation), the esterification reaction would drive toward the product side with the result of enhanced yields. Beneficial aspects of PVMRs also include low energy consumption and the possibility of carrying out esterification at a selected temperature. 

Four-channel alumina tubes, inside coated with an LTA zeolite layer in the Na state (4A) as used in Lithuania and four other plants recently built by GFT Membrane Systems GmbH Homburg/ Saar and equipped with LTA membranes made by Fraunhofer IKTS Dresden/Hermsdorf, Germany (former HITK/inocermic). Ethanol in feed 85wt.%, ethanol in retentate 99.8wt.%. [7]. 

By the consistent application of these concepts, any zeolite membrane can be prepared. As an example, recently different molecular sieve membranes with the structure LTA could be... 

Supported LTA membranes have been successfully installed during the last 10 years worldwide in about 25 plants for the dewatering of (bio)ethanol and i-propanol. Whereas there are outstanding separation performances of a new generation of zeolite membranes type SAPO-34 and DD3R on the lab scale, no short-term large-scale industrial application of zeolite membranes for gas separation can be predicted. 

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