Shape-selective methylation of naphthalene with methanol over SAPO-11 molecular sieve modified with hydrochloric acid and citric acid

Abstract

Herein, a series of SAPO-11 molecular sieves were modified by citric acid and hydrochloric acid. They were characterized by ICP, XRD, SEM, N₂ adsorption–desorption, NH₃-TPD, ²⁹Si MAS NMR, and Py-IR and evaluated by the methylation of naphthalene with methanol to 2,6-dimethylnaphthalene (2,6-DMN). According to the XRD results, hydrochloric acid at a high concentration not only removed extra-framework aluminum but also deleted framework silicon; this resulted in the formation of crystal defects. N₂ adsorption–desorption results showed that all the samples possessed micropores and secondary mesopores. The SAPO-11 sample modified with 4 mol L⁻¹ hydrochloric acid for 30 min exhibited the largest secondary mesopore size distributions. NH₃-TPD and ²⁹Si MAS NMR showed that hydrochloric acid at a high concentration could eliminate more acid sites. SAPO-11 modified with 4 mol L⁻¹ hydrochloric acid for 30 min presented high catalytic performance for the methylation of naphthalene; this was mainly attributed to the amount of secondary mesopores in the SAPO-11 molecular sieves.