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