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

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The formation of primary olefins from dimethyl ether (DME) was studied over ZSM‐5 catalysts at 300°C using a novel step response methodology in a temporal analysis of products (TAP) reactor. For the first time, the TAP reactor framework was used to conduct single‐ and multiple‐step response cycles of DME (balance argon) over a shallow bed with the continuous flow panel. Propylene is the major primary olefin and portrays an S‐shaped profile with a preceding induction period when it is not observed in the gas phase. Methanol and water portray overshoot profiles due to their different rates of generation and consumption. DME effluent shows a rapid rise halfway to its steady‐state value leading to a slow rise thereafter because of its high desorption rates followed by subsequent reactions involving DME in further steps during the induction period. To analyze the experimental data quantitatively, nine reaction schemes were compared, and kinetic parameters were obtained by solving a transient plug flow reactor model with coupled dispersion, convection, adsorption, desorption, and reaction steps. The methoxymethyl pathway involving dimethoxyethane and methyl propenyl ether gives the closest match to experimental data in agreement with recent density functional theory studies. Gaseous dispersion coefficients of ca. 10−9 m2 s−1 were obtained in the TAP reactor. The novel experimental data validated against the transient kinetic model suggests that after the formation of initial species, the bottleneck in propylene formation is the transformation of the initial C–C bond, that is dimethoxyethane formed initially from DME and methoxymethyl groups. DME adsorption on ZSM‐5 catalyst generates surface methoxy groups, which further react with the feed to give methoxymethyl groups. These methoxymethyl groups are regenerated through a series of reactions involving intermediates such as dimethoxymethane and methyl propenyl ether before propylene formation.

A kinetic model for ethene and propene aromatization over is developed. This model describes olefin oligomerization and cracking on zeolite catalytic sites (ZCS), diene formation via hydrogen transfer on ZCS and via dehydrogenation on gallium species, diene cyclization on ZCS, and formation of cyclic diolefins and aromatics via hydrogen transfer on ZCS and via dehydrogenation on gallium species. The rate constants of various reaction steps are compared, and the contribution of gallium in formation of dienes and aromatics is estimated. It is shown that aromatics formation accelerates olefin conversion due to the olefin consumption, on one hand, and inhibits olefin conversion due to the partial blocking of the zeolite catalytic sites, on the other hand. Because of this, both the increase and the decrease in olefin conversion over GaHZSM-5 can be observed (in comparison with HZSM-5), depending on the feed olefin and on the reaction conditions.

ZSM-5 catalysts were subjected to step response cycles of dimethyl ether (DME) at 300 °C in a temporal analysis of products (TAP) reactor. Propylene is the major olefin and displays an S-shaped profile. A 44-min induction period occurs before primary propylene formation and is eliminated reduced upon subsequent step response cycles. The S-shaped profile was interpreted according to induction, transition-regime and steady-state stages to investigate hydrocarbon formation from DME. The influence of precursors (carbon monoxide, hydrogen, dimethoxymethane, and 1,5-hexadiene) was studied using a novel consecutive step response methodology in the TAP reactor. Addition of dimethoxymethane, carbon monoxide, hydrogen and 1,5-hexadiene reduce the induction period of primary olefin formation. However, while dimethoxymethane, carbon monoxide and hydrogen accelerate the transition-regime towards hydrocarbon pool formation, 1,5-hexadiene attenuates it. Heavier hydrocarbons obtained from 1,5-hexadiene compete for active sites during secondary olefin formation from the aromatic dealkylation chemistry. A phenomenological evaluation of multiple parameters is presented.