Selective conversion of methanol to aromatics with superior catalytic stability by relay catalysis over quadruple ZSM-5 sequence beds with gradient-increasing acidity

Abstract

Methanol to aromatics (MTA) process is an attractive non-petroleum route for producing high-valued aromatics, but simultaneously achieving high catalytic stability and high aromatics selectivity remains a challenging research target. Herein, we present a highly efficient MTA process by quadruple tandem relay catalysis containing four layers of nanosized ZSM-5 with different acidity features in series. Specifically, four ZSM-5 catalysts with gradient-increasing acid density were sequentially packed in the tubular fixed bed reactor for reaction. Among them, ZSM-5 with a high SiO2/Al2O3 ratio as 480 was packed in top layer to convert methanol into light alkanes and alkenes intermediates, then these reaction intermediates and a small amount of unreacted methanol were transported on other two ZSM-5 layers with lower SiO2/Al2O3 ratio as 240 and 120 respectively for stepwise converting to aromatics. In particular, Zn doped ZSM-5 with a low SiO2/Al2O3 ratio of 60 was packed in the bottommost layer which further promoted aromatization of upstream light hydrocarbons. Accordingly, aromatics selectivity was significantly increased to 32.6% using the sequence beds, much higher than 17.6% for the widely recognized two-step MTA. Based on deliberate analysis of methanol conversion and coke formation behavior in different beds, we demonstrate that a methanol concentration gradient existed along the sequence beds which remarkably decreased alkylation of aromatics induced by methanol. So, the catalyst stability of the ZSM-5 located in the bottommost layer was significantly enhanced to 238 h, which was 3-folds compared with the traditional two-step methanol conversion. The present work offers a promising strategy for MTA via tandem ZSM-5 catalysis systems, which has an active impact on the solving of the energy and environmental problems existing in the production process of aromatics.