Spectroscopic investigations on deactivation of zeolite catalysts during reactions of olefins

Coke formation over acidic zeolite catalysts upon reaction of olefins is investigated using in-situ IR spectroscopy, i.e. an IR cell which simultaneously serves as a fixed-bed flow reactor and is attached to a gas chromatograph. Zeolites HY, dealuminated H-MOR with two different Si/Al ratios and H-ZSM-5 are compared. The effects of the number of active sites and the pore volume of the zeolites on the initial rate of coke formation and the final coke level are studied.

The final level of coke deposition reflects to some extent the pore volume of the zeolite catalyst. In all cases a considerable amount of coke formed remained inside the pore structure but another fraction was deposited onto the external surface. Both findings were confirmed by sorption measurements on samples before and after loading with coke.

Possible mechanisms of deactivation through coke formation, viz, poisoning, consumption or blockage of sides are briefly considered. In-situ IR spectroscopy revealed that at most a small fraction of the active sites of HY, HMOR or HZSM-5 disappeared during coke deposition suggesting that deactivation is caused by site blocking rather than by poisoning or consumption of the acidic centres.

Finally, in-situ IR spectroscopy turned out to be a useful tool for discriminating between low-temperature coke (formed below about 500 K) and high-temperature coke (deposited above 500 K).