Highly selective production of 2-methylnaphthalene by CO2 hydrogenation and naphthalene alkylation

Many researchers have extensively studied the alkylation of naphthalene with methanol for the preparation of high-value-added chemicals, especially methylnaphthalenes (MNs). Studies indicated that methanol generates a large number of methanol-to-olefin by-products on zeolites, resulting in low selectivity of the target product and poor stability of the catalyst, limiting its application in the industrial field. In this study, CO 2 hydrogenation was coupled with naphthalene alkylation, and the H 3 CO* active intermediate generated in situ was alkylated with naphthalene by using metal oxide-zeolite (OX-ZEO) bifunctional catalysts, which exhibited the ‘slow release’ phenomenon of the methylation reagent. It improved naphthalene conversion, suppressed deep alkylation and coke deposition side reactions and kept the catalyst stable for up to 500 h. A high-density and medium-strength Brønsted acid center facilitated the efficient migration of H 3 CO* active intermediates with the desorption of 2-methylnaphthalene (2-MN), which was particularly important for improving naphthalene conversion and 2-MN selectivity. The in situ IR showed that more Brønsted acid sites of Ga-S-1 effectively pulled the migration process of H 3 CO* species to the zeolitic acid center and increased the efficiency of C-C coupling compared to ZSM-5, which promoted the alkylation of H 3 CO* species with naphthalene. The results of chemisorption experiments showed that the medium-strength Brønsted acid site, which promoted the desorption of 2-MN, inhibited the deep alkylation reaction. The 2,6-dTBPy-IR results showed that the presence of fewer Brønsted centers on the outer surface of Ga-S-1 further optimized the product distribution and enhanced the selectivity of 2-MN in MNs to more than 90 %. Under optimal conditions, 80.1 % selectivity for MNs was obtained via ZnZrO x &Ga-S-1 bifunctional catalysts, with 97.4 % selectivity for 2-MN in MNs.