Sustainable routes to alkenes: applications of homogeneous catalysis to the dehydration of alcohols to alkenes

Abstract

With the growing scientific and social awareness of environmental issues, there is an increasing demand for renewable alkene feedstocks used to make the products we rely on. Alkenes are typically synthesised through high temperature ‘cracking’ processes, but a more sustainable option is available through the dehydration of alcohols. This minireview compares the dehydration of alcohols to alkenes using homogeneous catalysts with well-established Brønsted acid and heterogeneous catalysts. Metal triflates M(OTf)₄ (M = Ti, Hf) are catalysts for the dehydration of primary, secondary and tertiary alcohols at decreasing temperatures with increasing substitution of the alcohol. The rhenium catalysts methylrhenium trioxide (MTO) and Re₂O₇ are efficient catalysts at 100 °C for dehydrating benzylic, allylic and tertiary alcohols. A rhodium catalyst has been developed that utilises a catalytic quantity of HI to generate reactive alkyl halides that can undergo oxidative addition and β-H elimination. These expensive precious metal catalysts are contrasted by recent results with iron triflate catalysts that promise more cost-effective processes in the near future.