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Micro- and mesoporous zeolite membranes enclosing V-Mg-O catalyst beds have been employed to control the oxygen partial pressure in order to enhance the selectivity towards propene in the oxidative dehydrogenation of propane [177]. The microporous zeolite membrane evinced to be an effective gas barrier since only small amounts of the reactant propane diffused through the membrane and all the oxygen permeating was consumed in the dehydrogenation reaction causing remarkable increase in the propene yields at low C3H8/O2 ratios under separate feeding configuration. Julbe et al. [178] studied MFI and vanadium-loaded MFI membranes for the same reaction using them in two configurations either as oxygen distributors or as flow-through contactors. The V-MFI membrane outperformed the MFI and in the flow-through configuration a propene yield of nearly 8% at selectivity of 40%–50% was obtained. As an alternative to the conventional fixed-bed reactors, methanol oxidative dehydrogenation to yield formaldehyde was studied in a NMR with either methanol or oxygen as permeating species [179]. The non-permselective stainless steel membrane was packed with Fe-Mo oxide catalyst. The configuration with oxygen feeding though the membrane outperformed the membrane reactor with methanol as permeating component and that of the traditional reactor, assuring lower oxygen concentration and thus selectivity improvement and higher formaldehyde yields. Later on, the same authors optimized the oxygen feed distribution on the basis of productivity of the desired product formaldehyde [180]. Modified MFI membranes were utilized in membrane reactors of the distributor type to run butadiene hydrogenation in the vapor phase [181]. By the use of the zeolite membrane to distribute hydrogen in a controlled manner along the catalyst bed, it was possible to overcome the selectivity drawback, and to greatly improve the butadiene conversion. 3.3. Contactor Type Membrane Reactors The last membrane reactor principle considering the membrane function is the contactor type, where catalytic active and non-permselective membranes can be employed. The use of contactors is usually announced when a reaction front should be set in order to intensify the contact between the catalyst and the reactants. Torres et al. reported on i-butene oligomerization with catalytic BEA zeolite membranes in the temperature range from 373 to 423 K [182,183]. The non-permselective membrane was used as a contact medium demonstrating high activity towards i-butene dimers. Furthermore, Pt-ZSM-5 membranes prepared on tubular supports were applied for the combustion of volatile organic compounds (VOCs) at low concentrations [184]. The authors fed hexane as representative for VOC at the one side of the membrane and O2 at the other side. Interestingly, a membrane with intermediate concentration of defects performed better due to 19PDF Image | Zeolite Catalysis
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