ISOBUTANE DEHYDROGENATION ON PT-SN SIO2 CATALYSTS - EFFECT OF PREPARATION VARIABLES AND REGENERATION TREATMENTS/

The dehydrogenation of isobutane was studied under severely deactivati ng conditions, i.e., high temperatures and in the absence of added Hz, over silica-supported Pt-Sn catalysts. Several preparation methods we re investigated. It was found that the impregnation method employed ha s a strong influence on the degree of Pt-Sn interaction and the fracti on of Pt that remains unalloyed after the calcination and reduction pr ocess. The co-impregnation methods investigated were significantly sup erior to the sequential method. It was found that it is important to m inimize the amount of unalloyed Pt left on the catalyst because this f raction rapidly forms coke and deactivates. At the same time, the frac tion of unalloyed Pt, rather than the one alloyed with Sn, is responsi ble for most of the CO and hydrogen adsorbed at room temperature in ty pical chemisorption measurements. As a consequence, the TOF values bas ed on this type of measurements are in error because they are not rela ted to the density of sites that are responsible for long term activit y. It was also found that the high-temperature reduction/oxidation tre atments usually employed to regenerate spent catalysts can have a detr imental effect on the activity and selectivity of the Pt-Sn/SiO2 catal ysts. It is postulated that such thermal treatments lead to the disrup tion of the Pt-Sn alloys causing an increase in the fraction of unallo yed surface Pt. As a result, the rates of coke formation and deactivat ion drastically increase. The monometallic (Pt only) catalysts are als o affected by the high-temperature reduction/oxidation processes. The oxidation treatment results in an increased rate of coke formation and deactivation, while the regeneration process results in a much smalle r effect. This difference may be due to carbon residues left on the su rface. These residues may disrupt Pt ensembles and cause a decrease in the rate of undesired reactions, such as hydrogenolysis and coking, t hat require a large ensemble of Pt atoms. (C) 1997 Academic Press.