The Role of Non-Framework Lewis Acidic Al Species of Alkali-Treated HZSM-5 in Methanol Aromatization

Mesoporous HZSM-5 prepared by alkaline treatment (also termed desilication) has drawn significant attention due to its potential in large-scale production and in versatile applications, such as separation and catalysis. Alkali-treated HZSM-5 contains considerable amounts of non-framework (amorphous) Lewis acidic Al species on the external surface, and is deemed to be essential in affecting its catalytic performances. This study intends to clarify the catalytic nature of amorphous Al species of alkali-treated HZSM-5 in methanol aromatization. Physicochemical characterizations, including N2 adsorption, scanning electron microscopy (SEM), X-ray diffraction (XRD), magic-angle-spinning nuclear magnetic resonance (MAS NMR), inductively coupled plasma (ICP) analysis, NH3 temperature-programmed desorption (TPD), and methanol-TPD, were performed. The outcomes showed that non-framework Al promotes the hydride transfer in mesoporous HZSM-5, thereby facilitating the aromatization reaction. Among aromatic products, durene can be promoted by non-framework Al through methylation/transalkylation of other aromatics, particularly xylenes, instead of being promoted by reduced space confinement in mesoporous HZSM-5.