Deactivation of Mg/HZSM-5 Catalysts for the Synthesis of Lower Olefins from Dimethyl Ether in a Slurry Reactor

Under slurry reactor conditions, the products of condensation are generrally formed on strong Mg/HZSM-5 acid sites independently of the SiO₂/Al₂O₃ molar ratio in the zeolite. The composition of condensation products remains virtually unchanged as the molar ratio grows and basically consists of trimethyl and tetramethyl benzenes, but their content falls as the volume of mesopores grows with increasing SiO₂/Al₂O₃. This lowers the hindrances to diffusion and the contribution from secondary reactions to improve the removal of coke precursors from the zeolite’s surface and favorably affect the catalyst’s activity (DME conversion doubles). The composition of reaction products changes slightly with an increase in the SiO₂/Al₂O₃ molar ratio, and the total selectivity toward lower olefins is ~70 wt %. A rapid loss in Mg/HZSM-5 activity upon extending the period of operation under slurry reactor conditions is not due to coking, but to the catalyst being clogged by dispersion medium (polydimethylsiloxane) decomposition products.