Influence of the metal and of the support on the activity and stability ofbifunctional catalysts for toluene hydrogenation

Gas phase hydrogenation of toluene, diluted in inert n-hexane in order to m inimise the thermal effects was investigated in a fu;ed bed reactor at 110 degreesC over series of bifunctional catalysts in which Pt or Pd was deposi ted on alumina or on an acidic HFAU zeolite. Practically no deactivation wa s observed when the metals were deposited on alumina, even in mixture with the HFAU zeolite, whereas there was a rapid initial deactivation when Pd an d especially Pt were deposited on the acidic zeolite. However, even in this latter case, the activity of the fresh catalysts can be obtained with a go od accuracy thanks to the use of a multiple loop value. For given metal and support, a linear correlation was found between the hydrogenating activity and the number of accessible metal atoms. However, Pt was found to be 20-6 0 times more active than Pd, both metals being more active on HFAU zeolite than on alumina. This acidity effect could be explained by the hydrogenatio n of aromatic molecules adsorbed on acidic sites by hydrogen spilled over f rom the metal surface. The initial deactivation of Pt- and Pd-HFAU zeolite catalysts was due to the rapid formation of 'coke' molecules inside the zeo lite micropores. 'Coke' was mainly constituted by C-14 and C-21 products re sulting from the acid alkylation of toluene molecules by the olefinic and d ienic intermediates of their hydrogenation. While the composition of 'coke' as well as its deactivating effect did not depend on the metal (Pt or Pd), the rate of 'coke' formation at isoconversion of toluene was found to be l ower on Pd than on Pt zeolite catalysts, which explains their slower deacti vation. (C) 2001 Elsevier Science B.V. All rights reserved.