AlPO 4-supported nickel catalysts : VI. Support effects on the individual and competitive hydrogenation of allyl alcohol and its α and β methyl derivatives

Abstract

Individual and competitive liquid-phase hydrogenation of allyl alcohols (2-propen-1-ol, 2-methyl-2-propen-1-ol, and 2-buten-1-ol) has been carried out at 293–313 K and 0.41 MPa of initial hydrogen pressure on nickel catalysts at 20 wt% supported on Al 2O 3, SiO 2, and on three different types of AlPO 4. Furthermore, unsupported bulk nickel was employed as catalyst. The independence of the relative reactivities, R, with respect to the temperature and the Arrhenius-type law obtained for the relative adsorption constants, K, from ln K vs T −1, in all studied catalysts, was associated with the existence of a linear free-energy relationship (LFER) which also manifested itself in a linear relationship between the differential adsorption heat, Δω, and differential entropy factors, Δ S. According to the values of both kinds of parameters (Δω and Δ S) for every catalyst, it was concluded that the steric effects on the CH 3 adsorption decreased in the order Ni A1PO 4 -P > Ni AlPO 4 -F > Ni SiO 2 > Ni Al 2O 3 > Ni AlPO 4 -B > Ni bulk ; which is exactly the opposite of the inductive effect. -P, -F, and -B refer to the precipitation agent employed in the synthesis of AlPO 4: propylene oxide, ammonia, and ethylene oxide, respectively. Such LFER also manifested itself in the existence of a linear correlation between log A and E a (Arrhenius constant and apparent activation energy, respectively) known as “compensation effect.” The existence of this LFER is adscribed to the independence of the true activation energy from substituent and catalyst effects in these hydrogenation processes.