A facile O-alkylation of 2-naphthol over zeolites Hβ, HY, and HZSM5 using dimethyl carbonate and methanol

Alkylation of 2-naphthol using dimethyl carbonate (DMC) and methanol (MeOH) as alkylating agents over zeolite Hβ, HY, and HZSM5 with different pore architecture is studied. The efficacies of the two alkylating agents are compared. The products of 2-naphthol (A) alkylation are 2-methoxynaphthalene (B), 1-methyl-2-naphthol (C), and 1-methyl-2-methoxynaphthalene (D). Zeolites β and Y are more active than ZSM5, and the selectivity toward 2-methoxynaphthalene (B) is always higher than C and D, irrespective of the zeolite type and conversion levels. DMC and MeOH are both equally effective methylating agents for the reaction, even though MeOH seems to have a slight edge over DMC with respect to C-alkylation. A mechanistic pathway for the formation of the products is suggested. The study also suggests that the active sites responsible for O-alkylation and C-alkylation are not necessarily the same and O-alkylation is more facile. The product formation is discussed from the point of view of zeolite pore structure and the presence of weak to moderate acid sites, which are mostly available within the pores. The mechanism of 2-naphthol alkylation on a zeolite surface involving Bronsted acid sites is the same for both alkylating agents, viz. DMC and MeOH. From the kinetic studies it is suggested that the reaction follows a Langmuir–Hinshelwood mechanistic pathway.