The gas phase hydrogenation of 2-butanone over supported nickel catalysts: introduction of enantioselectivity

Abstract

The gas phase hydrogenation of 2-butanone to 2-butanol at 343 K promoted using a Y zeolite-supported nickel catalyst (2.2% w/w Ni) prepared by ion exchange and a range of Ni/SiO 2 catalysts (1.5–20.3% w/w Ni) prepared by precipitation/deposition and impregnation has been studied. The freshly activated catalysts generated racemic products and enantioselectivity was introduced to a negligible degree by in situ and to an appreciable degree by ex situ treatment with a methanolic solution of l-tartaric acid. The effectiveness of the latter modification was strongly dependent on modifier concentration and an optimum enantiomeric excess (ee) of 31% was achieved at an intermediate tartaric acid concentration (8×10 −3 mol dm −3) where the latter also served to raise the fractional conversion of the 2-butanone feedstock when compared with the unmodified catalyst under the same reaction conditions. Treatment with l-valine or l-glutamic acid did not result in the introduction of any appreciable level of product optical activity. Conversion of 2-butanone over Ni/SiO 2 was subject to a short term loss of catalytic activity which was however readily restored by heating in flowing hydrogen at 673 K. Deactivation of the Ni/Na-Y catalyst was more severe and was not reversed by such a heat treatment; loss of activity in this case is attributed to an irreversible pore blocking. Non-selective hydrogenation is approximated by pseudo-first-order kinetics, the possibility of thermodynamic limitations is considered and the effects of varying the hydrogen partial pressure and gas space velocity are addressed. The reaction exhibits a high degree of structure sensitivity and the relationship between specific rate constant and nickel particle diameter is presented wherein an optimum particle size of ca. 3 nm is identified.