Biosynthetic fuels: A marriage of renewable resources and chemical thermodynamics

Abstract

Terpenes represent a highly diverse group of natural products with wide applications. Terpenoid structures can be derived from different renewable sources and utilized as fuels. Application as fuels requires hydrogenation of the double bonds in the respective molecules to ensure clean combustion. This hydrogenation can be performed either directly with elemental hydrogen or indirectly via transfer hydrogenation with a Liquid Organic Hydrogen Carrier. In this study, the thermochemical fundamentals of these reaction has been evaluated. For this task, for a first time a combination of experimental thermochemistry and quantum-chemical molecular modelling has been applied. The results show the hydrogenation of the respective structures is thermodynamically highly favourable. Correspondingly, dehydrogenation would be comparatively unfavorable. Yet, for a fuel no dehydrogenation is required. As an alternative to conventional hydrogenation, transfer hydrogenation, which is often subject to partial conversions due to limitations by the reaction equilibrium, can be realized with a high degree of conversion. While the entropy of reaction for hydrogen transfer from cyclohexyl-structures to terpene structures is close to zero, the enthalpy of reaction is highly negative. This leads to a Gibbs energy of reaction which is clearly negative and ensures a large thermodynamic driving force for the reaction. As a consequence, downstream processing, which is usually very demanding for transfer hydrogenations, is comparatively easy. These finding underline the great potential of these bio-derived substances for conversion into sustainable fuels.