Aging and Structure/Activity Characteristics of CR–ZSM-5 Catalysts during Exposure to Chlorinated VOCs

Abstract

This study investigates the aging and partial deactivation process for four chromium exchanged ZSM-5 catalysts during oxidative destruction of 1% vinyl chloride or trichloroethylene, each in humid (1.35% water) air at 500°C. After 51 h on stream under these harsh conditions, all catalysts are found to undergo partial loss of exchanged chromium, which correlates with the chlorine content of the chlorinated VOC (CVOC) feed molecule. Conversely, changes in catalytic activity are not entirely correlated with residual chromium levels in the zeolite, suggesting that the chromium cation location is also important. A deactivation pathway is proposed involving slow generation 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 strengths) and catalytic properties (activity, selectivity, and site stability). The analysis shows that Cr cations, which are closely associated with a maximum number of zeolite framework oxygen atoms, are most resistant to migration or loss from the catalyst during CVOC oxidation. Chromium sites (SII, SIII) within the straight channels or near channel intersections are found 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 of SII and SIII sites to incoming CVOC molecules. Rapid loss of chromium from these sites is believed to be due to their weak association with the zeolite framework. Conversely, chromium sites (SI, SIV) located in the sinusoidal channels are determined to be less active, but more persistent during deactivation because of their increased coordination with zeolite framework atoms and reduced accessibility to incoming CVOC molecules.