Isoprene Synthesis Using MIL-101(Cr) Encapsulated Silicotungstic Acid Catalyst

Abstract

A single-stage synthesis of isoprene from methyl tert-butyl ether (MTBE) and formalin in an organic-aqueous two-phase system was studied by using solid acid catalysts, i.e., USY zeolite, silicotungstic acid (STA) 25 wt% encapsulated in MIL-101(Cr) (STA25@MIL-101), and STA 25 wt% encapsulated in SBA-15 (STA25@SBA-15). From preliminary experiments, the catalytic activity decreased in the order: STA25@MIL-101 > SBA25@SBA-15 > USY zeolite. This suggested that isoprene formation was favored with high surface area and high acid strength of catalyst. Then, the porous hybrid material of a MIL-101 metal organic framework and STA was studied in more detail. MIL-101 was not efficient for isoprene synthesis at mild reaction condition. On increasing STA loading, which was well correlated with the Brӧnsted acid property, the catalyst activity increased in the order: MIL-101 < STA30@MIL-101 < STA60@MIL-101. The high acidity catalyst gave high isoprene yield at optimum low temperature and low side reaction products. For the STA30@MIL-101 and STA60@MIL-101 catalysts, the isoprene yield could be sustained at 18.5% (0.4% SD) and 30.0% (1.5% SD), respectively over three recycling runs. It is apparent that no STA leaching from the low STA loading catalyst occurred.