The impact of zeolite pore structure on the catalytic behavior of CuZnAl/zeolite hybrid catalysts for the direct DME synthesis

Abstract

[EN] In this work, the influence of the pore structure of 10-ring zeolites used as the methanol dehydration func-tion in CuZnAl(CZA)/zeolite hybrid catalysts was studied for the direct dimethyl ether (DME) synthesis. Tothis purpose, six different 10-ring H-zeolites (ZSM-5, FER, IM-5, TNU-9, MCM-22, ITQ-2) with alike bulkSi/Al ratios (in the 9 14 range) were employed. Additionally, the effect of crystallite size (for ZSM-5) andselective surface dealumination by treatment with oxalic acid (for MCM-22) was also investigated. Whilethe initial activity of the zeolites for methanol dehydration was driven by the concentration of strongBrønsted acid sites, the extent of decay was dictated by the pore structure, which determined the amountand nature of the formed carbon species. When evaluated for direct DME synthesis under methanolsynthesis-controlled conditions, all CZA/zeolite hybrid catalysts (prepared by grinding, CZA:zeolite massratio of 2:1) experienced a decline of CO conversion (and DME yield) with time-on-stream (TOS) due toa gradual loss of the methanol synthesis activity of the Cu-based component. Interestingly, the stabilitywith TOS was the lowest for the hybrid catalysts comprising zeolites with large external surface areas(Sext) such as ITQ-2 and MCM-22. Moreover, for zeolites with similar Sext, the deactivation extent of thehybrid catalysts increased with the concentration of surface Al species (from XPS) in the zeolite. Thus,the delaminated ITQ-2 zeolite (Si/Alsurf= 10.6, Sext= 324 m2/g) produced the less stable hybrid while thatcomprising zeolite TNU-9 (Si/Alsurf= 17.9, Sext= 12 m2/g) displayed the highest stability during the syngas-to-DME experiments. These results suggest that the deterioration of the methanol synthesis activity ofthe CZA catalyst in the hybrid catalysts prepared by grinding is produced by detrimental interactionsbetween zeolitic Al species and Cu sites at the surface-contact between zeolite and CZA particle