CATALYTIC ACTIVITY OF LOW-VALENT CHROMIUM SUPPORTED ON Y-ZEOLITES

The catalytic activity and selectivity of low-valent chromium species supported on Y-zeolites were studied for the dehydrogenation of cyclohexane and for the hydrogenation of 1,3-butadiene. Chromium species having average oxidation numbers (AON) of 0.02-0.37 were prepared by decomposition of Cr(CO)6 adsorbed on HNa-Y zeolites with various values of proton content. The activity of the low-valent chromium species in the dehydrogenation of cyclohexane into benzene was high compared with Cr(II)- and Cr(III)-exchanged Y-zeolites and was also higher than that of the conventional chromia/silica and chromia/alumina catalysts. The activity decreases with increasing AON of chromium. The Cr(0) species was assigned as the active site. A sharp drop in the activity was observed when the AON of chromium was increased above 0.2. For the hydrogenation of 1,3-butadiene, the activity also decreased with increasing AON. Cr(0) was again thought to be the active site. The selectivity for the formation of butene isomers changed when the AON exceeded 0.2, that is, 1,2-addition of hydrogen was dominant at AON below 0.2 and 1,4-addition began to increase above 0.2. These results indicate that the changes in the electron density and in the morphology of Cr(0) species occur within a small range of the AON around 0.2. Simple models for the low-valent chromium species are proposed.