Metal-support effects on the intramolecular selectivity of crotonaldehyde hydrogenation over platinum

Abstract

The vapor-phase hydrogenation of crotonaldehyde (CROALD) was studied at low temperature and low conversions over Pt powder and Pt dispersed on SiO 2, η)-Al 2O 3, and TiO 2 after either a high or a low-temperature reduction, HTR and LTR, respectively. The hydrogenation of butyraldehyde (BUTALD) and crotyl alcohol (CROALC) to butanol (BUTNOL) and the isomerization of CROALC to BUTALD were also investigated to obtain additional information about this network of reactions. The typical Pt catalysts produced 100% BUTALD, as expected from previous studies of CROALD and the awareness of the high hydrogenation activity of Pt for CC double bonds. However, the TiO 2-supported Pt produced both CROALC and BUTNOL, and the best catalyst, sol (HTR) Pt TiO 2 , gave a selectivity of 37% CROALC with no BUTNOL formation. In addition to this marked enhancement in selectivity, the turnover frequencies on the sol (HTR) Pt TiO 2 samples, based on sites counted by hydrogen chemisorption near reaction temperatures, were more than an order of magnitude higher than the catalysts displaying normal adsorption behavior. Activation energies were somewhat higher on the sol Pt TiO 2 catalysts, however. This behavior is very similar to that observed previously for CO and acetone hydrogenation over these same catalysts, and it is consistent with the proposal that sites created at the Pt-titania interface are responsible for the activation of carbonyl bonds. Previous studies of liquid-phase hydrogenation of CROALD and other molecules with conjugated CC and CO double bonds support this model and indicate that a 1,4 diadsorbed species, rather than 1,2 and 3,4 diadsorbed species (counting the oxygen atom as 1), is formed in polar solvents. By analogy, defect sites on the titania at the metal-support interface may interact with the O atom, polarize the CO bond, and favor this intermediate which can react with hydrogen to give CROALC.