One Step Synthesis of Isoprene by Vapor Phase Condensation of Formaldehyde with Isobutylene over Boric Acid Cataiysts

Abstract

Isoprene synthesis by the vapor phase condensation of form aldehyde with isobutylene over the boric acid catalysts was studied, and the useful carriers for boric acid were investigated. The effective carrier had to possess on its surface a functional group which was able to interact with boric acid. It was found that silica gel and silica-alumina were the excellent carriers of boric acid for the synthesis of isoprene (Fig.1).It was postulated that the active center of these catalysts was formed by the condensation of boric acid with the surface hydroxyl group of silica gel or silica-alumina.On the silica gel carrier, ortho-boric acid was the most active among other forms of boric acid (Fig.2).It was found that the one pass yield of isoprene was more than 70 mol % (basedon formaldehyde) in the initial stage of the raction. However, the activity of the catalyst considerably decreased with process time (Fig.2), because the boricacid separated from catalyst system during the reaction.On the silica-alumina carrier, boric acid anhydride was the most active in this reaction (Table 2).It wasfound that on this catalyst there was an apparent induction period for isoprene synthesis at the initial stage of the reaction. After this period, however, the one pass yield and selectivity of isoprene attained 58 mol% and 80 mol% based onformaldehyde, respectively. And these values were provided for the relatively long process time (Fig. 3-1, 3-2).