AlPO 4-supported nickel catalysts : VIII. Support effects on the gas-phase dehydrogenation of alkylbenzenes

Abstract

The nonoxidative dehydrogenation of alkylbenzenes (ethylbenzene, n-propylbenzene, and isopropylbenzene) has been carried out at 625–850 K on nickel catalysts at 20 wt% supported on Al 2O 3, SiO 2, and two different types of AlPO 4. Furthermore, unsupported bulk nickel was employed as the catalyst. The acidic and basic properties and the surface area of the supports and supported nickel catalysts were measured as well. Unlike what is described for TiO 2ZrO 2 catalysts, our observation was that the acid and basic sites do not exhibit catalytic activity in the nonoxidative dehydrogenation of alkylbenzenes. However, it appears that the acid-basic nature of the supports, through the effects of metal-support interaction, plays an important part in determining the specific catalytic activity of the supported nickel catalysts. These metal-support interaction effects, as well as the substituent effects, can be evaluated through several isokinetic parameters obtained from the existence of a linear correlation between the δH ≠ and δS ≠ activation parameters obtained from the Eyring equation (and between In A and E a, from the Arrhenius equation) known as the compensation effect. This is ascribed to the existence of a linear free-energy relationship (LFER) in a set of gas-phase reactions similar to that previously described for the liquid-phase hydrogenation of allyl alcohols on the same nickel-supported catalysts.