Synthesis of Nanosized ZSM-5 Zeolites by Different Methods and Their Catalytic Performance in the Alkylation of Naphthalene

Abstract

Abstract Three nanosized ZSM-5 zeolites were successfully prepared from reactive gels with the same Si/Al ratios by different synthetic procedures that included the use of tetrapropylammonium hydroxide or n-butylamine as a template and a seeding method that did not use an organic additive. The effect of the synthetic method on the physicochemical properties of the prepared samples was investigated by XRD, XRF, XPS, N2 physisorption, SEM, TEM, 27Al MAS NMR, NH3-TPD, and Py-FTIR. The catalytic performance of the nanosized ZSM-5 zeolites in the alkylation of naphthalene with methanol was compared. The prepared samples were phase-pure, highly crystalline ZSM-5 zeolites, but they had different bulk and surface Si/Al ratios as well as textural and acidic properties. The study of the prepared catalysts in naphthalene methylation revealed that both the acid characteristics of the ZSM-5 nanosized zeolites and their textural properties were responsible for their activity in the reaction. A difference in the composition of monomethylnaphthalenes and dimethylnaphthalenes was attributed to the ability of the catalyst to isomerize the primary reaction products on acid sites located on the external surface of the zeolite crystals. 2,7-DMN was found to be the preferred reaction product over 2,6-DMN when formed at pore entrances to ZSM-5 channels due to the differences in their dimensions. In contrast, 2,6-dimethylnaphthalene could be produced on weaker external Brønsted acid sites, which are hydroxyls attached to octahedral Al atoms. The presented results show that the method used to synthesize nanoscale ZSM-5 zeolites is a critical factor that determines the physicochemical properties and catalytic performance of the resulting crystals.