Molecular sieving silica membranes

In summary, the main goal of the present work is the development of a hydrothermally stable microporous silica membrane with prescribed transport properties. Preferably, these steam stable membranes should have very high permselectivities. Because the permselectivity of a molecular sieving silica membrane will drop to the Knudsen value of the y-alumina supporting membrane when the silica membrane deteriorates under steam reforming conditions, a selectivity of the silica layer higher than the Knudsen selectivity is sufficient. In this way the measurement of the permselectivity is a powerful tool to assess the hydrothermal stability of a supported microporous membrane. 

The improvements that have been made in the preparation of molecular sieving silica membranes started with the development of high quality membrane supports, because quality of the supporting system is of crucial importance for the quality of the final molecular sieving membrane. To this end, the synthesis of the supports was performed by means of colloidal proc-... 

Finally, in chapter 9, conclusions are drawn and suggestions made for further research on (steam-stable) molecular sieving silica membranes or mesoporous y-alumina membranes. Though not all of the project objectives were obtained, progress was made in the synthesis of micro- and mesoporous membranes. Especially the development of steam stable membranes may be a large step forward in the development of ceramic membranes. 

N.K. Raman and C.J. Brinker, Organic Template Approach to Molecular Sieving Silica Membranes , J. Membrane Sci., 105 273-79 (1995). 

Nair BN, Yamagushi T, Okubo T, Suematsu H, Keizer K, and Nakao S-I. Sol-gel synthesis of molecular sieving silica membranes. J. Membr. Sci. 1997 135 237-243. 

Nair BN, Keizer K, Suematsu H, Suma Y, Ono S, Okubo T, et al. Synthesis of gas and vapor molecular sieving silica membranes and analysis of pore size and connectivity. Langmuir. 2000 16(10) 4558-662. 

Raman NK, Brinker CJ. Organic template approach to molecular sieving silica membranes. J Memb Sci. 1995 105 273-9. 

Recently, investigation of silica membranes was also reported in the literature. Giessler et al. [32] were the first who investigated molecular sieve silica membrane for WGS reaction. They prepared membrane by using tetra ethyl... 

Ballinger, B., Motuzas, J., Smart, S., Costa, J. C. D. (2014). Palladium cobalt binary doping of molecular sieving silica membranes. Journal of Membrane Science, 451, 185—191. 

Duke, M., Rudolph, V., Lu, G. Q. and Dinizda Costa, J.C. (2004a) Scale-up of molecular sieve silica membranes for reformate purification. AIChE Journal, 50, 2630-2634. 

Duke, M.C., da Costa, JC. D., Lu, G. Q., Petch, M. and Gray, P. (2004b) Carbonised template molecular sieve silica membranes in fuel processing systems permeation, hydrostability and regeneration. Journal of Membrane Science, 241, 325-333. 

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. 

Packed-bed conversion. Membrane reactor conversion. Molecular Sieve Silica. 

With appropriate membrane pore size and a narrow distribution, membrane selectivity for smaller gas molecules can be high but the overall permeability is generally low due to a high flow resistance in fine pores. Several studies are being conducted to develop molecular sieve-type membranes using different inorganic materials, for example, those based on carbon (Liu, 2007), silica (Pex and van Delft, 2005), and zeolites (Lin, 2007). 

The book explores various examples of these important materials, including perovskites, zeolites, mesoporous molecular sieves, silica, alumina, active carbons, carbon nanotubes, titanium dioxide, magnesium oxide, clays, pillared clays, hydrotalcites, alkali metal titanates, titanium silicates, polymers, and coordination polymers. It shows how the materials are used in adsorption, ion conduction, ion exchange, gas separation, membrane reactors, catalysts, catalysts supports, sensors, pollution abatement, detergency, animal nourishment, agriculture, and sustainable energy applications. 

Dinis da Costa JC, Lu GQ, Rudolph V, and Lin YS. Novel molecular sieve silica (MSS) membranes characterisation and permeation of single-step and two-step sol-gel membranes. J. Membr. Sci. 2002 198 9-21. 

Abdel-Jawad, M. M., Gopalakrishnan, S., Duke, M. C., Macrossan, M. N., Schneider, P. S., Diniz da Costa, J. C. (2007). Flow fields on feed and permeate sides of tubular molecular sieving silica (MSS) membranes. Journal of Membrane Science, 299, 229—235. 

Inorganic membranes, such as purely molecular sieving zeohte membranes, carbon membranes, " alumina membranes," and silica membranes," have high thermal and chemical stabUities. Over the past 25 years, extensive work has been reported on the synthesis, characterization, and apphcation of inorganic membranes. 

Battersby, S.,Teixeira, P. W., Beltramini, J., Duke, M.C., Rudolph, V. and Diniz da Costa, J.C. (2006) An analysis of the Peclet and Damkohler numbers for dehydrogenation reactions nsing molecular sieve silica (MSS) membrane reactors. Catalysis Today, 116,12-17. 

Molecular sieving silica (MSS) membranes are amorphous, microporous ceramic membranes that separate gas species based on their adsorption... 

As an example of the selective removal of products, Foley et al. [36] anticipated a selective formation of dimethylamine over a catalyst coated with a carbon molecular sieve layer. Nishiyama et al. [37] demonstrated the concept of the selective removal of products. A silica-alumina catalyst coated with a silicalite membrane was used for disproportionation and alkylation of toluene to produce p-xylene. The product fraction of p-xylene in xylene isomers (para-selectivity) for the silicalite-coated catalyst largely exceeded the equilibrium value of about 22%. 

The preferred choice of a water-selective membrane up to now has been hydrophilic membranes because of their high water affinity. However, recently Kuhn et al. reported an all-silica DDR membrane for dehydration of ethanol and methanol with high fluxes (up to 20kg m h ) and high selectivities (H20/ethanol 1500 and H20/methanol 70 at 373 K) in pervaporation operation. The separation is based on molecular sieving with water fluxes comparable to well-performing hydrophilic membranes [51]. 

Membranes with extremely small pores ( < 2.5 nm diameter) can be made by pyrolysis of polymeric precursors or by modification methods listed above. Molecular sieve carbon or silica membranes with pore diameters of 1 nm have been made by controlled pyrolysis of certain thermoset polymers (e.g. Koresh, Jacob and Soffer 1983) or silicone rubbers (Lee and Khang 1986), respectively. There is, however, very little information in the published literature. Molecular sieve dimensions can also be obtained by modifying the pore system of an already formed membrane structure. It has been claimed that zeolitic membranes can be prepared by reaction of alumina membranes with silica and alkali followed by hydrothermal treatment (Suzuki 1987). Very small pores are also obtained by hydrolysis of organometallic silicium compounds in alumina membranes followed by heat treatment (Uhlhom, Keizer and Burggraaf 1989). Finally, oxides or metals can be precipitated or adsorbed from solutions or by gas phase deposition within the pores of an already formed membrane to modify the chemical nature of the membrane or to decrease the effective pore size. In the last case a high concentration of the precipitated material in the pore system is necessary. The above-mentioned methods have been reported very recently (1987-1989) and the results are not yet substantiated very well. 

Up to now, a variety of non-zeolite/polymer mixed-matrix membranes have been developed comprising either nonporous or porous non-zeolitic materials as the dispersed phase in the continuous polymer phase. For example, non-porous and porous silica nanoparticles, alumina, activated carbon, poly(ethylene glycol) impregnated activated carbon, carbon molecular sieves, Ti02 nanoparticles, layered materials, metal-organic frameworks and mesoporous molecular sieves have been studied as the dispersed non-zeolitic materials in the mixed-matrix membranes in the literature [23-35]. This chapter does not focus on these non-zeoUte/polymer mixed-matrix membranes. Instead we describe recent progress in molecular sieve/ polymer mixed-matrix membranes, as much of the research conducted to date on mixed-matrix membranes has focused on the combination of a dispersed zeolite phase with an easily processed continuous polymer matrix. The molecular sieve/ polymer mixed-matrix membranes covered in this chapter include zeolite/polymer and non-zeolitic molecular sieve/polymer mixed-matrix membranes, such as alu-minophosphate molecular sieve (AlPO)/polymer and silicoaluminophosphate molecular sieve (SAPO)/polymer mixed-matrix membranes. 

The chemical composihons of the zeolites such as Si/Al ratio and the type of cation can significantly affect the performance of the zeolite/polymer mixed-matrix membranes. MiUer and coworkers discovered that low silica-to-alumina molar ratio non-zeolitic smaU-pore molecular sieves could be properly dispersed within a continuous polymer phase to form a mixed-matrix membrane without defects. The resulting mixed-matrix membranes exhibited more than 10% increase in selectivity relative to the corresponding pure polymer membranes for CO2/CH4, O2/N2 and CO2/N2 separations [48]. Recently, Li and coworkers proposed a new ion exchange treatment approach to change the physical and chemical adsorption properties of the penetrants in the zeolites that are used as the dispersed phase in the mixed-matrix membranes [56]. It was demonstrated that mixed-matrix membranes prepared from the AgA or CuA zeolite and polyethersulfone showed increased CO2/CH4 selectivity compared to the neat polyethersulfone membrane. They proposed that the selectivity enhancement is due to the reversible reaction between CO2 and the noble metal ions in zeolite A and the formation of a 7i-bonded complex. 

Mixed matrix membranes with low silica-to-alumina ratio molecular sieves and methods for making and using these membranes. US Patent 7138006 B2. 

Fig. 1.5.3 Aerosol generator assembly for the production of composite or coated particles (a) carrier gas (e.g., helium) tank, (b) drying column containing silica gel and molecular sieve, (c) Millipore membrane of 0.1 p.m pore size, (d) flow meters, (e) nuclei (e.g., NaCI, AgCI) generator, (f) and (m) boilers containing different reactant liquids, (g) and (n) condensation chambers, (h) heater, (i) chamber for recondensation of droplets, (q) and (e) coreactant containers, (k) and (p) vapor injection ports, (I) and (g) reaction chambers, (r) powder collector. (From Ref. 39.)...

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