Coke Formation on ZSM-5 Zeolite Catalysts for 1-Hexene Isomerization

The coke evolution and catalyst deactivation during 1-hexnene isomerization catalyzed by ZSM-5 zeolite catalysts are investigated by combining catalytic experiments and characterization techniques. The formed coke in micropores should be small-sized monophenyl ring compounds (coke precursors) due to the micropore confinement, and the coke precursors on the external surface of ZSM-5 crystals can be turned into polycyclic aromatic hydrocarbons (hard coke). The coke in high-acidity ZSM-5 zeolite almost blocks all the micropores at the early reaction stage (<1 h), while that in low-acidity ZSM-5 only plugs a part of micropores and some acid sites in micropores are still available for isomerization and coking reactions. Thus, the conversion of the low-acidity ZSM-5 is higher, but its selectivity toward branched hexenes is lower. These understandings about coke evolution and catalyst deactivation should serve to guide the optimal design of zeolite catalysts for 1-hexnene isomerization and other reactions.