Catalytic performance of Nafion/SiO 2 nanocomposites for the synthesis of α-tocopherol

Abstract

Synthesis of α-tocopherol starting from trimethylhydroquinone (TMHQ) and isophytol (IP) was performed over Nafion/SiO 2 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/SiO 2 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/SiO 2 catalyst had similar accessibility to those in the 5 and 13 wt.% Nafion/SiO 2 catalysts by the dehydration of 2-propanol, smaller pore sizes of the 20 wt.% Nafion/SiO 2 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.