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

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

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 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 or 1,5-hexadiene reduces 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. A phenomenological evaluation of multiple parameters is presented

Zeolite minilith: A unique structured catalyst for the methanol to gasoline process

Structured microchannel H-ZSM-5 catalysts containing up to 80 wt% zeolite (balance bentonite) were fabricated by unit operations of paste preparation, extrusion, drying and firing. The structured catalysts, called miniliths due to their micrometre-range dimensions, were composed of parallel cylindrical channels with a wall thickness of 200–300 μm, density of 2.1 channels/mm2 and a channel diameter of 300 μm. These miniliths were characterised by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis, N2 physisorption and thermogravimetric analysis. For the first time, these miniliths were tested for the conversion of methanol to gasoline at 370 °C, 3 bar and a weight hourly space velocity (WHSV) of up to 1170 h−1. A gasoline product yield of 53% was obtained at a methanol conversion of 74% over the ZSM-5 miniliths. The pressure drop at the same conversion over a packed-bed reactor of equal ZSM-5 content was 2 orders of magnitude higher than that of the minilith. Reducing the amount of ZSM-5 catalyst in the packed bed, to obtain similar inlet pressure as the ZSM-5 minilith gave the same product yield at a much higher conversion (81%) demonstrating the potential of these structured microchannel reactors

A quantitative multiscale perspective on primary olefin formation from methanol

Our quantitative multi-scale perspective on the formation of the first C–C bond decouples the adsorption, desorption, reaction, and mobility of species and provides new insights that could guide rational catalyst design