Effect of pretreatment on the acidity of heteropoly compounds in butene isomerization

Abstract

On a variety of heteropoly compounds as catalysts the isomerization of but-1-ene and cis-but-2-ene was performed with the aim of examining the relationship between the acidic properties of the compounds and the method of pretreatment. Neither of the central atoms, P and Si, in the anions in these compounds affected the isomerization activity of but-1-ene, but the counter-cations and MoO₆ or WO₆ octahedra surrounding the central atom did affect the activity. The cis-but-2-ene to trans-but-2-ene ratio was greater than 3 on the heteropolymolybdate catalysts pretreated at temperatures lower than 150 °C, and decreased with increasing pretreatment temperature until reaching a value of 1. In cis-but-2-ene isomerization Cu, Ni, Co and Mn salts of the heteropoly molybdates pretreated at 300 °C showed an increase in the ratio of trans-but-2-ene to but-1-ene, which corresponds to the formation of strongly acidic sites, while Ba and Sr salts did not show a pronounced increas in the ratio. A linear correlation between this ratio and the electronegativity of the salt counter-cation was seen in the pretreatment at 300 °C; the strongly acidic property is produced by the interaction of the counter-cation with the structural water, which is eliminated in the high-temperature range (200–250 °C) used in d.t.a. But-2-enes containing deuterium were produced in the but-1-ene isomerization on the catalysts pretreated with D₂O, indicating that the structural water also correlated with the formation of acidic sites on the catalysts.