Aging and structure activity characteristics of CR-ZSM-5 catalysts during exposure to chlorinated VOCs

This study investigates the aging and partial deactivation process for four chromium exchanged ZSM-5 catalysts during oxidative destruction of 1% viny l chloride or trichloroethylene, each in humid (1.35% water) air at 500 deg rees C. After 51 h on stream under these harsh conditions, all catalysts ar e found to undergo partial loss of exchanged chromium, which correlates wit h the chlorine content of the chlorinated VOC (CVOC) feed molecule. Convers ely, changes in catalytic activity are not entirely correlated with residua l chromium levels in the zeolite, suggesting that the chromium cation locat ion is also important. A deactivation pathway is proposed involving slow ge neration of volatile CrO2Cl2 to explain the process of cation migration and loss. These results are utilized to derive relationships between catalyst structure (cation site locations, accessibilities, and framework bond stren gths) and catalytic properties (activity, selectivity, and site stability). The analysis shows that Cr cations, which are closely associated with a ma ximum number of zeolite framework oxygen atoms, are most resistant to migra tion or loss from the catalyst during CVOC oxidation. Chromium sites (S-II, S-III) within the straight channels or near channel intersections are foun d to be most active, but are also most prone to migration and/or loss. This enhanced activity is believed to occur because of the high accessibility o f S-II and S-III sites to incoming CVOC molecules. Rapid loss of chromium f rom these sites is believed to be due to their weak association with the ze olite framework. Conversely, chromium sites (S-I, S-IV) located in the sinu soidal channels are determined to be less active, but more persistent durin g deactivation because of their increased coordination with zeolite framewo rk atoms and reduced accessibility to incoming CVOC molecules. (C) 1999 aca demic Press.