Production of high-energy-density fuels by catalytic β-pinene dimerization: Effects of the catalyst surface acidity and pore width on selective dimer production

[Display omitted] •Selective dimerization of β-pinene to high-energy-density fuels.•Silica–alumina aerogel catalysts modified by the condensation of methyltrimethoxysilane with surface hydroxyls.•2.5-Times more selective production of dimers by the modified aerogel catalysts.•Reaction pathway to understand the condensation of pinenes.•Brønsted acid sites responsible for the condensation of pinenes. High carbon number hydrocarbon fuels were produced by catalytic β-pinene dimerization. These fuels can be used as high-energy-density fuels, including diesel and jet fuels. To suppress the production of oligomers composed of more than two pinene units, which are undesirable in liquid fuels because of their high boiling temperatures and viscosities, the pore width of the silica–alumina aerogel catalysts was modified and their surfaces were treated chemically. The chemical treatment of the catalyst surface improved the selectivity of fuel-grade dimer production and suppressed oligomer formation. The reaction pathways for dimer production from pinenes were also studied. It was found that Brønsted acids are responsible for pinene condensation, although earlier works reported that Lewis acids are responsible for condensate formation.