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Generally, the hybrid catalysts usually utilized in such consecutive reactions comprise two types of active sites, namely for the methanol synthesis and acid sites for the dehydration of methanol to DME. Nevertheless, the core-shell catalysts were proposed as a better alternative to the conventional hybrid catalysts [261]. The main advantage of the capsule catalysts, pointed by the authors, is the feasibility to better control the sequential reactions. In detail, the syngas conversion to methanol takes place on the active sites in the core and the further methanol dehydrogenation to produce DME occurs in the zeolite shell. Therefore, zeolite membranes were coated on Cu/Zn/Al2O3 applying two different hydrothermal techniques, namely H-ZSM-5 zeolite synthesis containing aluminum sources in the precursor solution and close-to-neutral silicalite-1 zeolite synthesis where the core catalyst was used as aluminum source. The so prepared zeolite capsuled catalysts exhibited extraordinary dimethyl ether selectivity compared to the physical mixture of core catalyst and zeolite powder. Moreover, no further dehydrogenation of the desired product to alkane or alkene occurred on the capsule catalysts. It should be noted that the hybrid catalyst experienced significantly higher conversion than the capsule catalyst. However, the hybrid catalyst provides random contact between methanol and the active sites of zeolite catalyst so that the two consecutive reactions occur independently resulting in moderate selectivity for the desired dimethyl ether. On the other hand, the zeolite shell offers better control of the reaction order since it affords unavoidable contact for methanol formed in the core while passing the membrane and as a result enhances the probability for its further conversion into dimethyl ether leading to the desired high selectivity. Although the H-ZSM-5 coated catalyst exhibited excellent dimethyl ether selectivity, indication was given that the core catalyst structure was damaged during the acidic hydrothermal synthesis resulting in much lower CO conversion. In this context, Yang et al. [262,263] discussed the challenges and strategies for preparation of H-ZSM-5 zeolite shells on either bimetallic or silica-based catalyst. It has been demonstrated that the dual-layer method is an effective tool for the preparation of defect-free and compact zeolite shells. A silicalite-1 membrane was first synthesized as intermediate layer acting as core catalyst protection, further facilitating the growth of the H-ZSM-5 under the stronger alkaline conditions. The double-layer capsule catalysts with Cr/ZnO or Pd/SiO2 core achieved a sharply controlled reaction with excellent DME selectivity and no formation of C4+ hydrocarbons. Next to H-ZSM-5, silico-aluminophosphate molecular sieves (SAPO) are considered as attractive catalysts for the dehydration of methanol to DME [294,295]. Respectively, Pinkaew et al. [243] proposed a physical coating procedure using SiO2 as an adhesive for the preparation of a defect-free SAPO-46 shell over Cr/ZnO catalyst core. According to the authors, the developed method, performed under normal ambient conditions, is an effective way to overcome the disadvantages of the 54PDF Image | Zeolite Catalysis
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