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

2,6-Dimethylnaphthalene (2,6-DMN) is a crucial intermediate in the synthesis of poly(ethylene naphthalate), a heat-resistant insulating material. The key to synthesizing 2,6-DMN is the development of highly active catalysts, which still remains challenging. Herein, bare SiO2 and a series of Al-modified SiO2 catalysts (denoted as Al-SiO2) 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 SiO2 exhibited better catalytic activity than bare SiO2. The main reason is ascribed to the increased acid sites for Brönsted and Lewis acids with the incorporation of Al. Among the Al-modified SiO2 samples, the amount of weak and total acidities was the highest for the 30Al-SiO2 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 SiO2 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.