Highly selective self-condensation of cyclic ketones using MOF-encapsulating phosphotungstic acid for renewable high-density fuel

Abstract

Transferring biomass-derived cyclic ketones such as cyclopentanone and cyclohexanone to a mono-condensed product through aldol self-condensation has great potential for the synthesis of a renewable high-density fuel. However, the selectivity is low for numerous catalysts due to the rapid formation of di-condensed by products. Herein, MIL-101-encapsulating phosphotungstic acid is synthesized to catalyze the self-condensation with selectivity of more than 95%. PTA clusters are uniformly dispersed in MOF cages and decrease the empty space (pore size), which provides both acidic sites and shape-selective capability. The optimal PTA amount decreases corresponding to the increase of reactant size. The shape-selectivity is also realized by changing the pore size of MOF such as from MIL-101 to MIL-100. Moreover, the catalyst is resistant to PTA leaching and performs stably after 5 runs. After hydrodeoxygenation of the mono-condensed product, high-density biofuels with densities of 0.867 g ml⁻¹ and 0.887 g ml⁻¹ were obtained from cyclopentanone and cyclohexanone, respectively. This study not only provides a promising route for the production of high-density biofuel but also suggests the advantage of MOF-based catalysts for shape-selective catalysis involving large molecular size.