Bifunctional catalysts based on PdZn/ZnO and hierarchical zeolites in the direct synthesis of dimethyl ether from CO-rich syngas: influence of the support and the Zn/Pd molar ratio†

Abstract

Following the ‘precursor’ concept, a series of Pd/Zn-based, colloidal nanoparticles (NPs) with different Zn/Pd molar ratios were synthesized by reductive stabilization and used as precursors for the methanol active component in bifunctional catalysts. The bifunctional catalysts for the single-step dimethyl ether synthesis from CO-rich syngas were obtained by immobilizing the NPs on a microporous or hierarchical HZSM-5 zeolite, which were used as dehydration catalysts. The catalysts were characterized, e.g., by (in situ) powder X-ray diffraction, scanning and transmission electron microscopy with energy-dispersive X-ray analysis, N₂ physisorption, and NH₃ temperature programmed desorption. This study demonstrates the influence of the Zn/Pd molar ratio on the size of PdZn particles formed under reaction conditions, which correlates with the catalytic performance in the STD process. The introduction of mesopores in the hierarchical zeolite by desilication of HZSM-5 increased the DME yield while decreasing the selectivity to hydrocarbons.