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

Abstract

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.

Influence of Precursors on the Induction Period and Transition Regime of Dimethyl Ether Conversion to Hydrocarbons over ZSM-5 Catalysts

“…47 . Transient microkinetic modelling studies 48 show that the transformation of the first C-C bond is rate-limiting in the induction period in accordance with previous studies by Qi et al. 32 .…”

“…Experiments involving multiple step response cycles were conducted and presented in a previous publication. 48 .…”

“…TPD was carried out after every step response experiment by supplying argon at similar flow rates to the DME feed for 20 min to remove weakly adsorbed species from the ZSM-5 Figure 2, which has been reported before 48 , shows the results of a step response experiment with 5 vol% DME at 300 °C. Propylene is the major olefin and exhibits an S-shaped profile.…”

“…The evolution of the hydrocarbon pool can be described not only by the induction-period chemistry, but also by the transition-regime and dual-cycle chemistry. 7,48 . Here, we report the behaviour of precursors during the induction period and transition regime.…”

“…We showed previously that the transformation of the first C-C bond is rate-limiting in the conversion of DME to primary olefins. 48 . This was observed by Qi et al 32 for MTO conversion.…”