Catalytic performance of Nafion/SiO2 nanocomposites for the synthesis of α-tocopherol Original Research Article

Synthesis of α-tocopherol starting from trimethylhydroquinone (TMHQ) and isophytol (IP) was performed over Nafion/SiO2 nanocomposite catalysts (Nafion content: 5–20 wt.%) and Nafion® NR50 resin. The nanocomposites were made by in situ hydrolysis of tetraethoxysilane in the presence of home-made Nafion solution. Nitrogen adsorption and catalytic dehydration of 2-propanol were used, respectively, to characterize the texture and acid properties of the nanocomposites. It is found that acid strength of the Nafion-based acidic sites was weakened in the nanocomposites and the weakening disappeared when the amount of Nafion exceeded 30 wt.% of the nanocomposite. The pore structure and accessibility of the Nafion-based acidic sites in the nanocomposite catalysts showed pronounced effects on the catalytic efficiency toward the desired α-tocopherol. In comparison with Nafion resin in the condensed phase (Nafion® NR50), 5 and 13 wt.% Nafion/SiO2 catalysts showed tenfold higher catalytic activities by turnover frequency for α-tocopherol formation owing to increased Nafion dispersion and accessibility of the Nafion-based acid sites. Though the acid sites in the 20 wt.% Nafion/SiO2 catalyst had similar accessibility to those in the 5 and 13 wt.% Nafion/SiO2 catalysts by the dehydration of 2-propanol, smaller pore sizes of the 20 wt.% Nafion/SiO2 catalyst induced severe side reactions of the IP reactant, such as dehydration to form phytadienes and furan derivatives, which resulted in much lower yield (or selectivity) and turnover frequency for α-tocopherol.