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

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, N 2 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 AbstractElectronic supplementary material The online version of this article (https://doi

Transient kinetic studies and microkinetic modeling of primary olefin formation from dimethyl ether over ZSM‐5 catalysts

“…They are suppressed due to negligible intramolecular collisions under vacuum conditions . In a previous contribution, we also showed that the rates of methanol and DME adsorption and desorption are limiting compared to intraparticle mass transport contributions. In Section 4, we show that these rates compete with the rate of transformation of intermediates during the conversion of DME to primary olefins.…”

“…The ammonium form of the fresh ZSM‐5 catalyst with a Si/Al ratio of 25, purchased from Zeolyst International, has a crystallite size of 0.10 ± 0.02 μm and an average particle size of 30 μm . The ZSM‐5 catalysts are here referred to as ZSM‐5 (25).…”

“…Brønsted acid site density and Brunauer‐Emmett‐Teller (BET) surface area are obtained from a previous communication …”

“…The ammonium form of the fresh ZSM-5 catalyst with a Si/Al ratio of 25, purchased from Zeolyst International, has a crystallite size of 0.10 ± 0.02 m and an average particle size of 30 m. 35 The ZSM-5 catalysts are here referred to as ZSM-5 (25). The ZSM-5 (25) catalysts were pressed, crushed, and sieved to obtain pellet sizes in the range of 250-500 m. Anhydrous DME (99.999%) and argon (99.999%) were purchased from CK Special Gases Ltd.…”

“…Recently, we observed that higher temperatures are required to desorb DME in comparison to methanol providing evidence that DME stays longer on the catalysts 35 and is the key oxygenate. This was also evidenced by Liu et al 18 and Wei et al 21 In this paper, we investigate the induction period during the formation of primary olefins from DME at 300 • C by conducting novel step response experiments in a temporal analysis of products (TAP) reactor.…”