Distribution of Aluminum over the Tetrahedral Sites in ZSM-5 Zeolites and Their Evolution after Steam Treatment

Abstract

Zeolite ZSM-5 is one of the most widely used zeolites, in particular in heterogeneous catalysis. This work investigates the incorporation of Al in the silica framework of monoclinic ZSM-5 and the Al speciation during steam treatment for four ZSM-5 samples with different Si/Al ratios, using ultrahigh field 27Al NMR (22.3 T), 29Si NMR, X-ray diffraction, and IR spectroscopy. 27Al MQMAS NMR at 22.3 T allows identification and quantification of 10 distinct tetrahedral framework resonances which are assigned to the crystallographic sites by their average T–O–T angles, determined from powder XRD patterns for the same samples. A clear Al site preference is observed and found to be dependent on the Si/Al ratio of the parent zeolite. The framework sites facing the intersections in ZSM-5 are found to be most prone to dealumination, whereas Al in the straight and sinusoidal-shaped channels are more stable toward steam treatment at high temperatures. Extra-framework Al species with 5- and 6-fold coordination and two distorted tetrahedral AlO4 sites have also been identified. The spectroscopic data are supported by catalytic activities from α-test cracking experiments which are in accordance with a reinsertion of Al in the framework at mild steaming and the onset of dealumination at higher temperatures.