Crystal structure analysis of chromium modified zeolite catalysts during trichloroethylene oxidation

Chromium cation exchanged Y- and ZSM5 zeolites (termed sorbent/catalysts) were developed and tested for ambient adsorption and deep oxidation capabilities with an environmentally hazardous chlorinated volatile organic compound, trichloroethylene (TCE). The primary objective of the study was to characterize these sorbent/catalysts before and after prolonged aging under reaction conditions and to relate any changes in their performance to loss or migration of active cations which occurred within the zeolite during the aging process.; Experiments were conducted in a continuous flow integral reactor to monitor the changes in the oxidative conversions of TCE ({dollar}sim{dollar}1100 ppm) in the presence of water vapor ({dollar}sim{dollar}13000 ppm) and excess air at about 2400 hr{dollar}sp{lcub}-1{rcub}{dollar} space velocity before and after aging. The aging tests were carried out over a period of 12 days in the presence of ({dollar}sim{dollar}1100 ppm of TCE) at 600{dollar}spcirc{dollar}C to accelerate the deactivation process. The activities and selectivities of the fresh and aged sorbent/catalyst media were monitored in the temperature range of 250 to 325{dollar}spcirc{dollar}C. The zeolite properties that could influence this cause-and-effect relationship (active metal content, surface area, acidity, Si/Al ratio, relative crystallinity and cation location/migration) were measured at the beginning and end of the aging process. The recently developed use of powder X-ray diffraction data combined with Rietveld refinement were used to locate the chromium cations in the crystalline zeolite matrix.; The fresh catalysts were found to be highly active ({dollar}>{dollar}90% TCE conversion at {dollar}{lcub}ge{rcub}300spcirc{dollar}C), while the selectivities towards CO{dollar}sb2{dollar} and Cl{dollar}sb2{dollar} were significant. The sorbent/catalyst activities for TCE oxidation decreased and selectivities towards CO{dollar}sb2{dollar} and Cl{dollar}sb2{dollar} changed due to the aging process. It was found that the TCE physisorption capacities of Y-zeolite samples could be improved by dealuminating them and the sorption capacities of majority of the sorbent/catalysts increased due to aging.; The metal cations were at the entrance to the supercages in most of the fresh Y-zeolite based sorbent/catalysts while they were close to the straight channels in the fresh ZSM5 based sorbent/catalysts. As the sorbent/catalysts were aged, some of the metal cations were lost from the zeolite structure and some migrated farther into the supercage and sodalite cage sites in the Y-zeolite structure and into sinusoidal channels in the ZSM5 structure. The loss in active metal content and its migration seemed to be the primary causes for catalyst deactivation. It is proposed that TCE feed molecules react with the active metal cation species to form volatile metal oxychloride species which migrate within the zeolite structure. Some of these species are redistributed into different locations in the zeolite structure much as in the chemical vapor deposition (CVD) process, while some species leave the structure completely resulting in metal cation loss. This loss and migration of metal cations appeared to decrease the sorbent/catalyst activity, change their selectivity trends and increase their physisorption capacities.