Hierarchic zeolites studied by IR spectroscopy : acid properties of zeolite ZSM-5 desilicated with NaOH and NaOH/tetrabutylamine hydroxide

Acid properties of zeolites ZSM-5 desilicated in pure NaOH and in mixtures of NaOH/tetrabutylamine hydroxide (TBAOH) of various concentrations of basic solutions as well as of different proportions between NaOH and TBAOH were investigated. IR studies of pyridine sorption informed on the concentration of both Bronsted and Lewis acid sites and on the acid strength of SiOHAl groups. The nature of Lewis sites was followed by CO sorption. The accessibility of acid sites was detected by the sorption of hindered pivalonitrile molecules. It has been shown that the concentration of Bronsted sites increased as the result of the decrease of Si/Al due to desilication in diluted basic solutions (0.1 M and 0.2 M). The concentration of protonic sites in parent zeolite and zeolites treated with diluted bases determined in IR studies was approximately the same as the Al content determined by chemical analysis. The desilication in more concentrated bases (0.5 M) resulting in partial destruction of zeolite led to generation of weakly acidic protonic sites (not SiOHAl groups). The acid strength of the SiOHAl groups decreased with the progress of desilication. It is interpreted as the result of the extraction the Al atoms generating the strongest SiOHAl groups. The IR experiments of CO sorption confirmed that Lewis acid sites were formed by the dehydroxylation. IR studies of pivalonitrile sorption showed that desilication, which produced mesopores, improved the accessibility of acid sites. This effect was more distinct when desilication was done with NaOH/TBAOH mixture.