Selectivity zeolite membranes

Table 15.2 Performances of water selective zeolite membranes...

Zhang Y, Wu Z, Hong Z, Gu X, Xu N. Hydrogen-selective zeolite membrane reactor for low temperature water gas shift reaction. Chem Eng J 2012 197 314—321. 

Zerva, C., Philippopoulos, C. J. (2006). Ceria catalysts for water gas shift reaction influence of preparation method on their activity. Applied Catalysis B Environmental, 67, 105—112. Zhang, Y., Wu, Z., Hong, Z., Gu, X., Xu, N. (2012). Hydrogen-selective zeolite membrane reactor for low temperature water gas shift reaction. Chemical Engineering Journal, 197, 314-321. 

The separation factors are relatively low and consequently the MR is not able to approach full conversion. With a molecular sieve silica (MSS) or a supported palladium film membrane, an (almost) absolute separation can be obtained (Table 10.1). The MSS membranes however, suffer from a flux/selectivity trade-off meaning that a high separation factor is combined with a relative low flux. Pd membranes do not suffer from this trade-off and can combine an absolute separation factor with very high fluxes. A favorable aspect for zeoHte membranes is their thermal and chemical stability. Pd membranes can become unstable due to impurities like CO, H2S, and carbonaceous deposits, and for the MSS membrane, hydrothermal stability is a major concern [62]. But the performance of the currently used zeolite membranes is insufficient to compete with other inorganic membranes, as was also concluded by Caro et al. [63] for the use of zeolite membranes for hydrogen purification. 

Three different ways in which a zeolite membrane can contribute to a better sensor performance can be distinguished (i) the add-on selective adsorption or molecular sieving layer to the sensor improves selectivity and sensitivity, (ii) the zeolite layer acts as active sensing material and adds the selective adsorption and molecular sieving properties to this, and (iii) the zeohte membrane adds a catalytically active layer to the sensor, improving the selectivity by specific reactions. 

Zeolite membranes Resistant to contaminants Intermediate hydrogen flux and selectivity Intermediate hydrogen flux and selectivity Laboratory scale Need to increase hydrogen flux as well as selectivity, for example, by thinner yet defect-free zeolite membrane layers... 

Hong, M., R.D. Noble, and J.L. Falconer, Highly selective H2 Separation Zeolite Membranes for Coal Gasification Membrane Reactor Applications, Annual Technical Progress Report, U.S. DOE Contract DE-FG26-02NT41536, December 2005. 

Zeolite membranes have also been employed for organic-organic separations where selectivity is based on adsorption and diffusion differences of non-aqueous mixtures. NaX and NaY zeolite were used in the separation of methanol from MTBE and benzene (800 < a< 10000) exploiting the more polar nature of methanol which is attracted to the electrostatic poles of the high A1 content zeolites [38]. Other separations include (i) separation of n-hexane from 2,2-DMB using ZSM5, (ii) benzene from p-xylene using MOR/FER and (iii) xylene isomers [34]. 

The Maxwell model can also guide the selection of a proper polymer material for a selected zeolite at a given volume fraction for a target separation. For most cases, however, the Maxwell model cannot be applied to guide the selection of polymer or zeolite materials for making new mixed-matrix membranes due to the lack of permeabihty and selectivity information for most of the pure zeolite materials. In addition, although this Maxwell model is well-understood and accepted as a simple and effective tool for estimating mixed-matrix membrane properties, sometimes it needs to be modified to estimate the properties of some non-ideal mixed-matrix membranes. 

During the last few years, ceramic- and zeolite-based membranes have begun to be used for a few commercial separations. These membranes are all multilayer composite structures formed by coating a thin selective ceramic or zeolite layer onto a microporous ceramic support. Ceramic membranes are prepared by the sol-gel technique described in Chapter 3 zeolite membranes are prepared by direct crystallization, in which the thin zeolite layer is crystallized at high pressure and temperature directly onto the microporous support [24,25],... 

The ceramic and zeolite membranes described above have been shown to have exceptional selectivities for a number of important separations. However, the membranes are not easy to make and consequently are prohibitively expensive for many separations. One solution to this problem is to prepare membranes from materials consisting of zeolite particles dispersed in a polymer matrix. These membranes are expected to combine the selectivity of zeolite membranes with the low cost and ease of manufacture of polymer membranes. Such membranes are called mixed-matrix membranes. 

Recent developments demonstrate possibilities for inorganic C02 selective membranes. Microporous membranes with strong C02 adsorption show C02 selectivity if other gas species are hindered in accessing the pores. For instance, at intermediate temperatures, limited C02 selectivity to N2 (to about 400 °C) and H2 (to about 200 °C) is reported for MFI zeolite membranes [96]. Also, at high pressure (10-15 bars) C02 selectivity has been demonstrated in MFI membranes (C02/N2 separation factor ... 

Improved selectivity in the liquid-phase oligomerization of i-butene by extraction of a primary product (i-octene C8) in a zeolite membrane reactor (acid resin catalyst bed located on the membrane tube side) with respect to a conventional fixed-bed reactor has been reported [35]. The MFI (silicalite) membrane selectively removes the C8 product from the reaction environment, thus reducing the formation of other unwanted byproducts. Another interesting example is the isobutane (iC4) dehydrogenation carried out in an extractor-type zeolite CMR (including a Pt-based fixed-bed catalyst) in which the removal of the hydrogen allows the equilibrium limitations to be overcome [36],... 

The esterification of acetic acid with ethanol has been investigated using zeolite membranes grown hydrothermally on the surface of a porous cylindrical alumina support (the catalyst used was a cation exchange resin) [37]. The conversion exceeded the equilibrium limit, by the selective removal through the membrane of water and reached to almost 100% within 8h [37]. 

Bernardo, P., Algieri, C., Barbieri, G. and Drioli, E. (2008) Hydrogen purification from carbon monoxide by means of selective oxidation using zeolite membranes. Separation and Purification Technology, 62, 631—637. 

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