Methylation of Naphthalene with Methanol over the Al-Modified SiO₂ Zeolite Catalysts

Abstract

2,6-Dimethylnaphthalene (2,6-DMN) is a crucial intermediate in the synthesis of poly(ethylene naphthalate), a heatresistant insulating material. The key to synthesizing 2,6-DMN is the development of highly active catalysts, which still remains challenging. Herein, bare SiO 2 and a series of Al-modified SiO 2 catalysts (denoted as Al-SiO 2 ) were successfully synthesized through a hydrothermal method using sodium metaaluminate as the Al source. The prepared catalysts were used for catalytic alkylation of naphthalene, and it was found that the Al-doped SiO 2 exhibited better catalytic activity than bare SiO 2 . The main reason is ascribed to the increased acid sites for Bronsted and Lewis acids with the incorporation of Al. Among the Al-modified SiO 2 samples, the amount of weak and total acidities was the highest for the 30Al-SiO 2 sample. In addition, the reaction conditions over this prepared catalyst were studied and optimized. The results showed that it exhibited the highest activity when the temperature was 300 °C and the weight hourly space velocity was 1 h −1 . Furthermore, density functional theory calculations showed that the doping of Al into SiO 2 enhanced the adsorption of methanol, resulting in a faster rate of formation of methoxylated material in the first step. Besides, methylation at the 6 position was slightly preferred compared to that at the 7 position, in line with the experimental results.