First-principles microkinetic analysis of Lewis acid sites in Zn-ZSM-5 for alkane dehydrogenation and its implication to methanol-to-aromatics conversion

Abstract

Zn-Containing ZSM-5 is a typical Lewis acid zeolite for C–H bond activation and exhibits high activity in methanol-to-aromatics (MTA) conversion. However, the nature of active Zn sites and the associated reaction mechanism still remain elusive. Using dehydrogenation of butane and cyclohexane as a model reaction, the structures and catalytic activity of four different Zn species (divalent Zn²⁺ and ZnOZn²⁺ and univalent ZnOH⁺ and ZnH⁺) in ZSM-5 were investigated using periodic density functional theory calculations combined with microkinetic modeling. The ZnOH⁺ species exhibits less stability and may be converted to other three Zn species considering the effects of proximity, atmosphere and temperature based on thermodynamics analysis. The dehydrogenation reaction involves the Zn–alkyl fragment and the concerted pathway is kinetically favored over the stepwise pathway. It is the framework oxygen of Zn²⁺, hydroxyl oxygen of ZnOH⁺ or bridging oxygen of ZnOZn²⁺ sites that initiates the first C–H bond dissociation. Microkinetic analysis reveals that Zn²⁺ and ZnOH⁺ species are more active than ZnOZn²⁺ and ZnH⁺ species for alkane dehydrogenation, while the dominant active Zn site for each reaction varies with temperature and pressure. The univalent ZnOH⁺ motif is identified as the most active site for dehydrogenation at low temperature and/or high pressure, and the contribution of the divalent Zn²⁺ site is promoted at higher temperature and/or lower pressure. The catalytic activity of the ZnOH⁺ site increases with the sequence of alkane dehydrogenation, while the reverse trend was observed on the Zn²⁺ site. The results in this work classify the reaction condition relevance of prevailing active Zn species for alkane dehydrogenation and offer some insights on the dilemma between the stability and activity of catalysts in the MTA reaction.