Preparation and catalytic performance of tungstophosphoric acid anchored to SiO₂@graphene aerogel 3D porous catalysts for the synthesis of ethyl levulinate biofuel

As fuel additives, ethyl levulinate (EL) can be used up to 5 wt% directly in the regular diesel engines, which can overcome the limited stock of fossil fuels and reduce the environment pollutions to some extent. In this work, the three-dimensional porous hybrids consisting of SiO₂ and graphene aerogel, which are denoted as SiO₂@GA, are facilely assembled and used as supports for H₃PW₁₂O₄₀ (HPW)-based solid acid catalysts. Structural analysis confirms that the resultant HPW/SiO₂@GA catalysts possess unique porous structure (S_BET ≥ 257 m² g⁻¹, V_p ≥ 0.450 cm³ g⁻¹) and exhibit excellent catalytic performance in the synthesis of EL by the esterification of levulinic acid (LA) with ethanol. The conversion of LA can be as high as 92.4% under the reaction conditions. Furthermore, various catalytic reaction parameters are also optimized over the 10 wt.% HPW/SiO₂@GA catalysts, which exhibit the highest turnover frequency (TOF = 83.91 mmol g⁻¹ h⁻¹) among the resultant catalysts. The results confirm the promising application of the HPW/SiO₂@GA heterogeneous catalysts in the synthesis of biofuel.