Liquid-phase synthesis of isoprene from MTBE and formalin using cesium salts of silicotungstic acid

Abstract

A single-stage synthesis of isoprene from methyl tertiary-butyl ether (MTBE) and formalin in an organic-aqueous two-phase system were studied by using Cs-exchanged silicotungstic acid catalysts (CsXH4-XSiW12O40, abbreviated as CsX-STA). The CsX-STA catalysts (x=2.0, 2.5, 3.0, and 3.5) were prepared using Cs2CO3 as Cs source and water as solvent. The catalysts were characterized by using XRF, FTIR, FT-Raman, XRD, N2 sorption, light scattering, TGA, and potentiometric titration techniques to relate catalytic activities to catalyst properties. The characterization results showed that the Keggin structure and cubic unit cell of all Cs-STA catalyst samples were well preserved. The increase in acid strength and total acid sites with decreasing Cs content (x) resulted in increasing formaldehyde conversion and isoprene selectivity. However, at low Cs content (x≤3.0), the high contribution of homogeneous catalysis also played a role in the catalytic activities. The highest activity (isoprene formation normalized with total acid sites) of Cs2.5-STA catalyst was found to be attributed to its high surface area. For the organic solvent effect, the Cs2.5-STA catalyst activity increased with the increasing solvent polarity in the order: n-hexane < cyclohexane < toluene.