External surface phenomena in dealumination and desilication of large single crystals of ZSM-5 zeolite synthesized from a sustainable source

Zeolite dealumination and desilication are common methods, which improve accessibility of the active sites located inside the zeolite crystallites and tune the zeolite acidity. The exact mechanism of these post-synthesis zeolite treatments remains under discussion. In this paper, a series of dealuminated and desilicated ZSM-5 zeolites are prepared using single or combined post-synthesis dealumination and desilication of large zeolite single crystals with oxalic acid and sodium hydroxide. The ZSM-5 zeolite has been synthesized using silica extracted from fly ash. Zeolite desilication results in the potholes, leaf-type structures and other clearly visible defects on the surface of zeolite crystallites, while the effect of dealumination is less pronounced. The low temperature nitrogen adsorption suggests the formation of mesopores in the zeolites formed by the voids between and in the irregular zeolite crystallites produced during post-synthesis treatments. Al-27 and Si-29 MAS NMR in combination with XRD data are indicative of only minor modifications of the bulk zeolite structure during dealumination and desilication. The total Bronsted acidity only slightly decreases and is not affected by the type of zeolite post-synthesis treatment. After desilication of large single crystals of ZSM-5, noticeable enhancement of the Bronsted acidity on the zeolite external surface has been detected by FTIR of adsorbed collidine. The zeolites with higher acidity on the external surface have shown higher activity in the anisole acylation with hexanoic acid. A combination of characterization techniques suggests that the dealumination and desilication of the large crystals of ZSM-5 zeolite using post-synthesis treatments respectively with acid and alkali selectively starts at the zeolite external surface.