Mechanistic Insights into the Desorption of Methanol and Dimethyl Ether Over ZSM-5 Catalysts

Toyin Omojola,Nikolay Cherkasov,André C Van Veen,Evgeny V Rebrov,Dmitry B Lukyanov,Andrew I Mcnab,James A Anderson

doi:10.1007/s10562-017-2249-4

Toyin Omojola, Nikolay Cherkasov + Show 5 more

Open Access

https://doi.org/10.1007/s10562-017-2249-4

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Abstract

The desorption of methanol and dimethyl ether has been studied over fresh and hydrocarbon-occluded ZSM-5 catalysts with Si/Al ratios of 25, 36 and 135 using a temporal analysis of products reactor. The catalysts were characterized by XRD, SEM, N2 physisorption and pyridine FT-IR. The crystal size increases with Si/Al ratio from 0.10 to 0.78 µm. The kinetic parameters were obtained using the Redhead method and a plug flow reactor model with coupled convection, adsorption and desorption steps. ZSM-5 catalysts with Si/Al ratios of 25 and 36 exhibit three adsorption sites (low, medium, and high temperature sites), while there is no difference between medium and high temperature sites at a Si/Al ratio of 135. Molecular adsorption on the low temperature site and dissociative adsorption on the medium and high temperature sites give a good match between experiment and the plug flow reactor model. The DME desorption activation energy was systematically higher than that of methanol. Adsorption stoichiometry shows that methanol and DME form clusters onto the binding sites. When non-activated re-adsorption is accounted for, a local equilibrium is reached only on the low and medium temperature binding sites. No differences were observed, other than in site densities, when extracting the kinetic parameters for fresh and activated ZSM-5 catalysts at full coverage.Graphical

Highlights

Alternative carbon sources such as biomass are vital for the secure and sustainable production of fuels and chemicals in the twenty-first century
To verify the source of the methylating species, this paper provides a site-specific behaviour of the desorption properties of methanol and dimethyl ether (DME)
Our results suggest that methanol desorption follows a first order kinetics (Fig. 5a) while

Summary

Introduction

Alternative carbon sources such as biomass are vital for the secure and sustainable production of fuels and chemicals in the twenty-first century. The channel intersections have a critical diameter of 0.9 nm [3] This 3D pore structure is responsible for its high selectivity and catalyst stability. Blaszkowski and van Santen [10,11,12] showed that the simultaneous adsorption and activation of two methanol molecules towards the formation of DME and H 2O excluding surface methoxy group formation is the preferred pathway. Surface methoxy groups have been readily observed with stopped flow NMR studies over ZSM-5 catalysts [13]. These surface methoxy groups can be formed by the adsorption of methanol or DME. The presence or absence of surface methoxy groups, necessary to validate the computational studies, can be linked to the dissociative or associative adsorption behaviour of oxygenates respectively

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