ISOBUTANE DEHYDROGENATION OVER SULFIDED NICKEL-CATALYSTS

We have studied the dehydrogenation of isobutane to isobutylene over heavily sulfided Ni catalysts at 873 K. In the absence of sulfur but in the presence of hydrogen, supported metallic nickel catalyzes the rapid hydrogenolysis of alkanes, resulting in exceedingly poor selectivity toward dehydrogenation products. However, the hydrogenolysis activity can be essentially eliminated by proper sulfidation, resulting in high selectivity toward isobutylene production. Due to thermodynamic limitations, elevated temperatures are necessary to attain reasonable conversions to isobutylene. Under these conditions, the formation of coke is rapid and is the main cause of catalyst deactivation. Treatment of nickel catalysts with relatively large amounts of sulfur-containing reagents results not only in dramatically improved selectivity but also in a decreased rate of coke formation. The catalysts exhibited activation periods of several hours until a maximum was reached when they were placed in the reaction environment at 873 K. This activation period did not depend upon the previous reduction treatment but did depend upon the time on stream. A correlation was found between the increase in activity and the growth of a carbidic phase on the catalyst. This species, characterized by TPO and XPS, reached a saturation value at a C/Ni ratio of unity, which was coincidental with the amount of carbon on the catalyst when it achieved its maximum activity. Two alternative explanations for the activation process are discussed. The first considers the participation of a nickel-carbidic-carbon complex in the reaction scheme. The second considers the creation of sulfur vacancies during the initial coke deposition period which would increase the exposure of a catalytically active Ni-S moiety. (C) 1994 Academic Press, Inc.