Features of the deactivation of Mg/HZSM-5 catalysts for the synthesis of lower olefins from dimethyl ether in a slurry reactor

Abstract

In a slurry reactor, the formation of compaction products on Mg/HZSM-5 catalysts, irrespective of the SiO2 /Al2O3 molar ratio in the zeolite, proceeds predominantly on strong acidic sites. The composition of compaction products (mostly trimethyl- and tetramethylbenzenes) virtually does not change with a growth of the molar ratio; however, their amount decreases, which is related to a growth of the mesopore volume with an increase in the SiO2 /Al2O3 ratio. This leads to a decrease in diffusion limitations and contribution of secondary reactions and enhances the removal of coke precursors from the zeolite surface, thus promoting the catalyst activity (the conversion of DME increases twofold). The composition of reaction products changes only slightly with a growth of the SiO2 /Al2O3 molar ratio, the total selectivity to lower olefins is ca. 70 wt.%. A rapid loss of the Mg/HZSM-5 activity with extending the operation time in a slurry reactor is caused not by coking, but rather by «clogging» of the catalyst with decomposition products of the dispersion medium (polydimethylsiloxane).