Green synthesis of ester base oil with high viscosity – Part Ⅰ: Catalyst preparation, characterization, evaluation, and mechanism analysis

•Eco-friendly Sn/Zr mixed oxide catalysts were prepared by hydrothermal synthesis.•Catalytic activity and hydrophobicity of catalyst were improved by doping Zr.•Mechanism of catalyst preparation was studied and proposed.•Mixed esters were synthesized as an eco-friendly base oil with high viscosity.