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

In this work, the influence of the pore structure of 10-ring zeolites used as the methanol dehydration function in CuZnAl(CZA)/zeolite hybrid catalysts was studied for the direct dimethyl ether (DME) synthesis. To this purpose, six different 10-ring H-zeolites (ZSM-5, FER, IM-5, TNU-9, MCM-22, ITQ-2) with alike bulk Si/Al ratios (in the 9-14 range) were employed. Additionally, the effect of crystallite size (for ZSM-5) and selective surface dealumination by treatment with oxalic acid (for MCM-22) was also investigated. While the initial activity of the zeolites for methanol dehydration was driven by the concentration of strong Bronsted acid sites, the extent of decay was dictated by the pore structure, which determined the amount and nature of the formed carbon species. When evaluated for direct DME synthesis under methanol synthesis-controlled conditions, all CZA/zeolite hybrid catalysts (prepared by grinding, CZA:zeolite mass ratio of 2:1) experienced a decline of CO conversion (and DME yield) with time-on-stream (TOS) due to a gradual loss of the methanol synthesis activity of the Cu-based component. Interestingly, the stability with TOS was the lowest for the hybrid catalysts comprising zeolites with large external surface areas (S-ext) such as ITQ-2 and MCM-22. Moreover, for zeolites with similar S-ext, the deactivation extent of the hybrid catalysts increased with the concentration of surface Al species (from XPS) in the zeolite. Thus, the delaminated ITQ-2 zeolite (Si/Al-surf = 10.6, S-ext = 324 m(2)/g) produced the less stable hybrid while that comprising zeolite TNU-9 (Si/Al-surf = 17.9, S-ext = 12 m(2)/g) displayed the highest stability during the syngas-to-DME experiments. These results suggest that the deterioration of the methanol synthesis activity of the CZA catalyst in the hybrid catalysts prepared by grinding is produced by detrimental interactions between zeolitic Al species and Cu sites at the surface-contact between zeolite and CZA particles. (C) 2013 Elsevier B.V. All rights reserved.