Characterization of transient response curves in heterogeneous catalysis—II Estimation of the reaction mechanism in the oxidation of ethylene over a silver catalyst from the mode of the transient response curves

Int J of Chemical Kinetics

Lukyanov

Veen

2019

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.

Section: Resultsmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

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

Int J of Chemical Kinetics

Lukyanov

Veen

2019

“…Indeed, Thomas and Thomas 12 provided extensive details on dynamic modelling during catalyst deactivation. Ertl 30 , Kobayashi [31][32][33][34] , Dauenhauer 35,36 , Carroll 37,38 and Boudart 39,40 studied reaction dynamics. Warshel [41][42][43][44][45] investigated the dynamic phenomena during enzyme catalysis and observed that the duration of the kinetics of catalytic transformation is much smaller than conformational dynamics.…”

Section: Introductionmentioning

confidence: 99%

Multiscale Models in Heterogeneous Catalysis: Incorporating Dynamics into Reaction Kinetics

2021

Preprint

Modern operando spectroscopy and microscopy, and kinetic investigations have provided qualitative evidence for active site dynamics, catalyst surface dynamics, and charge transport. On the macroscale, intraparticle and interparticle mass and heat transfer can be tuned to optimise selectivity over heterogeneous catalysts. On the microscale, adsorbate-induced restructuring, adsorbate mobility, surface composition, oxidation states, charge transport, bandgap, and the degree of coordination of the active site have been identified for controlling product selectivity. There exist, however, limited physics-based and data-driven multiscale models that can assimilate these qualitative descriptors in an integrated manner to extract quantitative catalyst activity, stability, and product selectivity descriptors. A multiscale model, which describes the evolution of gas species, adspecie accumulation due to reactivity, stability, lifetime, and mobility, charge transport involving electrons and holes, heat transfer for non-isothermal conditions due to reaction exothermicity, and the changing catalyst states is provided. Dynamical effects are included in these models to bridge the gap between laboratory-scale studies and industrial technical reactors.

“…Temkin distinguished two types of relaxation onto steady state: (a) intrinsic relaxation, which is caused by the mechanism of the reaction itself and (b) extrinsic relaxation, which is caused by modifications of the mechanism as a result of subsurface chemistry. During the evolution from fresh to working state, intrinsic and extrinsic relaxations can be readily distinguished. , Kobayashi et al − described various shapes and mechanisms underlying specie relaxation onto steady state including (1) the S-shaped profile where effluents form with an induction period, (2) overshoot profile where effluents initially exceed steady-state values, and (3) monotonic profiles where effluents begin to form immediately.…”

Section: Introductionmentioning

confidence: 99%

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

Lukyanov

Cherkasov

et al. 2019

Ind. Eng. Chem. Res.

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.