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In this context, Zhang et al. [161] tested silicalite-1/α-Al2O3 zeolite membranes packed together with HZSM-5 catalysts in the isomerization of m-xylene to p-xylene, applying different packing methods: (i) by depositing the catalyst on the Al2O3 support on the opposite side of the membrane; and (ii) by packing it in the tube in contact with the membrane layer. The latter packing method turned out to be more effective for the studied reaction since it gave rise to higher p-xylene yield and selectivity than in conventional reactor ascribed to the immediate removal of p-xylene from the reaction zone due to molecular sieving effects. However, since the observed enhancement was found to be strongly dependent on the membrane flux, the authors pointed out the need of membranes exhibiting higher permeation fluxes as a crucial requirement for the desired improvement of the membrane reactor efficiency. Moreover, in order to overcome the resistance diffusion through the catalytic bed, thus enhancing the permeation and intensifying the xylene isomerization, the authors suggested the use of catalytic membranes combining high-catalytic and separation efficiency. The performance of such catalytically active zeolite membranes will be discussed in more details in Section 5. However, already with the intention to overcome the drawbacks of the supported zeolite films typically suffering from defects occurred during thermal stress causing diminished selectivity, van Dyk et al. [158] used a zeolite/alumina nanocomposite membrane of the pore-plugging type (zeolite crystals grown as film inside the pores of the porous tubular support) for the m-xylene isomerization in an extractor type zeolite membrane reactor. The tubular membrane used was equipped with the commercial xylene isomerization Pt catalyst, ISOXYL. The selectivity and the p-xylene yield of the conventional reactor were 58% and 21% respectively, demonstrating higher selectivity that the equilibrium value of 46% at the expense of the lower yield in comparison to the equilibrium yield of 24.9%. In contrast, by combining the retentate and permeate fractions para-selectivity of 65% and p-xylene yield of 23% were reported for the membrane reactor. More interestingly, operating in permeate-only mode, the authors managed to obtain p-xylene selectivity of 100%, however at the expense of low productivity. Recently, Daramola et al. [162,163] attempted to further reveal the advantages of the nanocomposite architecture over the “film-like” zeolite membranes, applying nanocomposite MFI-alumina membrane tubes prepared via pore-plugging synthesis packed with Pt-HZSM-5 catalyst for m-xylene isomerization. The effect of different reactor configurations, namely membrane reactor with catalyst bed packed either in the membrane lumen of the tube or between the membrane tube and the module shell, all operating in the temperature range of 523–673 K was evaluated [162]. Decreasing the operating temperature resulted in linear increase in the p-xylene yield for the PBMR configuration due to the effective extraction of p-xylene from the reaction zone. On the contrary, packing the catalyst bed outside the membrane layer reduced the effect of the membrane 33PDF Image | Zeolite Catalysis
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