Al Distribution of MWW Zeolites and Its Correlation with Catalytic Performance in Direct Syngas Conversion

MWW zeolites are widely used heterogeneous catalysts, which have two-dimensional 10-membered ring (10MR) sinusoidal channels and 12MR supercages connected by 10MR openings. We study herein the Al distribution and its correlation with the catalytic performance by taking MCM-22 and MCM-49 as models since they exhibit the same three-dimensional MWW structure and can be prepared with different ratios of organic template to inorganic cations. ²⁷Al MAS NMR, ²⁹Si MAS NMR, and infrared (IR) spectroscopy show that HMCM-22 has more Al atoms at the T₇ sites, which generate more Bro̷nsted acid sites in the sinusoidal channels, while HMCM-49 has more Bro̷nsted acid sites in the supercages. As a result, HMCM-49 in combination with ZnCrO_x oxide (ZnCrO_x-HMCM-49) as a composite catalyst exhibits a longer induction period than ZnCrO_x-HMCM-22 in syngas conversion to gasoline range hydrocarbons. It needs more than 120 h to reach a stable gasoline selectivity for ZnCrO_x-HMCM-49. In contrast, it reduces to less than 20 h over ZnCrO_x-HMCM-22. This could be attributed to the narrow opening between supercages which limits the diffusion of hydrocarbons generated in the supercages and hence accumulation to form carbon deposits.