Ethane Aromatization over Zn-HZSM-5: Early-Stage Acidity/Performance Relationships and Deactivation Kinetics

Catalytic conversion of ethane to aromatics (BTX) over metal/HZSM-5 catalysts involves significant catalyst deactivation due to coking. Consequently, true acidity/performance relationships are escaped if the intrinsic catalyst acidity was correlated to the “steady-state” performance. Here, the effect of acidity on the early-stage performance and time-dependent deactivation kinetics has been investigated. The early-stage ethane conversion and BTX selectivity both increased with decreasing Si/Al 2 ratios. Specifically, the space–time yields of BTX increase linearly with increasing Brønsted acidity, indicating Brønsted acids as the main active sites for BTX. Further evidence can be found from the transient experiment (C 2 H 6 /Ar ↔ C 2 H 6 /NH 3 ) and C 2 H 4 -TPSR. A promotion effect of the Zn(II) sites (mainly responsible for ethane dehydrogenation) on BTX formation was also observed. With time-on-stream, the catalytic performance attenuated due to coking, which can be modeled as “ r ( t ) = r 0 /(1 + kt α )” kinetically, and the parameters (for aromatics) k decreased and α increased with decreasing acidity.