Theoretical Insights into the Conversion of y-Valerolactone to Butene Over y-Alumina

The catalytic conversion of γ-valerolactone (GVL) over γ-alumina which is
comprised of Lewis acid sites results in the formation of 1-butene with a high initial selectivity but drops as a function of residence time and due to the formation of 2-butene. First principle density functional theory (DFT) calculations were carried out to examine the nature of the active sites on the (100) surface of γ-alumina, specifically the tricoordinated or tetrahedral aluminum atoms and the elementary steps involved in the general reaction pathways. The reaction mechanism is thought to follow a sequence of
Lewis acid catalyzed elementary steps involving the adsorption and ring opening of GVL to form a γ-carbenium intermediate and subsequent deprotonation to form adsorbed 3-pentenoic acid. The reprotonation of the bound 3-pentenoic acid at the γ-carbon leads to the formation a stable β-carbenium intermediate. The β-carbenium ion intermediate can readily undergo decarboxylation via retro-Diels-Alder (rDA) reaction to form 1-butene. The reaction proceeds with a very high selectivity to form 1-butene. This is due to high selectivity of activating GVL to form adsorbed 3-pentenoic acid over that of the 4- or 2-pentenoic intermediates which would result in the formation of 4-pentenoic acid and 2-pentenoic acid intermediates as well as to the elimination of olefin isomerization over γ-Al2O3 in the presence of water.