H-FER-Catalyzed Conversion of Methanol to Ethanol and Dimethyl Ether: a First-Principles DFT Study

Abstract

Methanol adsorption and dehydration reactions within zeolites represent important steps in the catalytic conversion process to form long-chain  hydrocarbons. Herein, first-principles density functional theory (DFT) is employed in the determination of methanol adsorption and conversion in  ferrierite (FER), where we predict the fundamental adsorption geometries and energetics of methanol adsorption. The methanol molecule is shown to  physisorb at all explored binding sites, stabilized through hydrogen-bonded interactions with the acid site at O_meth—H_fram bond distances ranging from  1.33–1.51 Å. We demonstrate that the zeolites’ adsorption capability is affected by the silicon/aluminium ratio, with stronger adsorptions predicted in the  material with silicon to aluminium fractions of 5 than 8. The adsorption strength is also found to vary depending on the tetrahedral binding site, with the  T1O2 site yielding the most stable methanol adsorption structure in the Si/Al ratio=5(Eads = –22.5 kcal mol^–1), whereas the T1O1 site yields the most  stable adsorption geometry (E_ads = –19.2 kcal mol^–1) in the Si/Al ratio = 8. Upon translational and rotational motion, methanol is protonated resulting in  the breaking of its C-O bond to form a methoxy species bound to the framework oxygen (O–CH3 distance of 1.37 Å), whereas the water molecule is  stabilized at the acid site through H-bonding (O_wat-H = 2.0 Å). Further reaction between the methoxy species and a second methanol molecule results in  the formation of ethanol and protonated dimethyl ether, with adsorption energies of –42 and –25 kcal mol^–1, respectively. The results in this study  provide atomistic insight into the effect of acidity of the FER zeolite on the adsorption and conversion of methanol.