| 1. |
- Omojola, Oluwatoyin, 1988, et al.
(författare)
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2D CFD simulations of flow and reaction during carbon dioxide methanation: A spatially resolved channel plate reactor study
- 2023
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Ingår i: Chemical Engineering Science. - 0009-2509. ; 282
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Tidskriftsartikel (refereegranskat)abstract
- Carbon dioxide methanation is a way of storing excess electrical energy as grid compatible gas. Spatially resolved channel plate reactor experiments were used to validate competing reactor (1D, 2Dx-z) models. Parallel exothermic carbon dioxide methanation and endothermic reverse water gas shift reactions were considered. The kinetic model, where the rate determining step is between an oxygenated complex (HCOO*) and an active site (*), was used in 2Dx-z CFD simulations for six laminar inflow conditions and variations in pressure, temperature, H2/CO2 ratio, methane, and steam co-feeds. The performance is improved by decreasing flowrate, and increasing H2/CO2 ratio, pressure, and temperature. Co-feeding methane has a negligible effect on reactor performance. However, co-feeding steam significantly reduces performance. At relatively high conversions, differential rates are obtained. This is due to the negligible dependence of the rate of carbon dioxide conversion with the equilibrium term of the reverse water gas shift reaction. With these studies, a link between the reaction mechanism and reactor performance is established at conditions relevant to power-to-gas applications.
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| 2. |
- Omojola, Oluwatoyin, 1988, et al.
(författare)
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A quantitative multiscale perspective on primary olefin formation from methanol
- 2021
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Ingår i: Physical Chemistry Chemical Physics. - 1463-9084 .- 1463-9076. ; 23:38, s. 21437-21469
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Tidskriftsartikel (refereegranskat)abstract
- The formation of the first C–C bond and primary olefins from methanol over zeolite and zeotype catalysts has been studied for over 40 years. Over 20 mechanisms have been proposed for the formation of the first C–C bond. In this quantitative multiscale perspective, we decouple the adsorption, desorption, mobility, and surface reactions of early species through a combination of vacuum and sub-vacuum studies using temporal analysis of products (TAP) reactor systems, and through studies with atmospheric fixed bed reactors. These results are supplemented with density functional theory calculations and data-driven physical models, using partial differential equations, that describe the temporal and spatial evolution of species. We consider the effects of steam, early degradation species, and product masking due to the inherent autocatalytic nature of the process, which all complicate the observation of the primary olefin(s). Although quantitative spectroscopic determination of the lifetimes, surface mobility, and reactivity of adspecies is still lacking in the literature, we observe that reaction barriers are competitive with adsorption enthalpies and/or activation energies of desorption, while facile diffusion occurs in the porous structures of the zeolite/zeotype catalysts. Understanding the various processes allows for quantitative evaluation of their competing energetics, which leads to molecular insights as to what governs the catalytic activity during the conversion of methanol to primary olefins over zeolite/zeotype catalysts.
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| 3. |
- Omojola, Oluwatoyin, 1988, et al.
(författare)
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Competitive adsorption of oxygenates and aromatics during the initial steps of the formation of primary olefins over ZSM-5 catalysts
- 2020
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Ingår i: Catalysis Communications. - : Elsevier BV. - 1566-7367. ; 140
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Tidskriftsartikel (refereegranskat)abstract
- Aromatics present during the early stages of MTO conversion compete for active sites with the educts, oxygenates (methanol, DME), in primary olefin formation over ZSM-5 catalysts. This competitive adsorption is investigated using TPD and TPSR in a TAP reactor. DME induces methanol desorption at lower temperatures. Diphenylethane desorbs at higher temperatures than toluene. Microkinetic simulations show that toluene, methanol, diphenylethane and DME have activation energies of desorption of 107, 112, 119 and 121 kJmol−1 on strongest binding sites respectively. DME has the highest surface coverage and is suggested as the key surface oxygenate responsible for direct primary olefin formation.
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| 4. |
- Omojola, Oluwatoyin, 1988, et al.
(författare)
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Influence of Precursors on the Induction Period and Transition Regime of Dimethyl Ether Conversion to Hydrocarbons over ZSM-5 Catalysts
- 2019
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Ingår i: Industrial & Engineering Chemistry Research. - : American Chemical Society (ACS). - 1520-5045 .- 0888-5885. ; 58:36, s. 16479-16488
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Tidskriftsartikel (refereegranskat)abstract
- ZSM-5 catalysts were subjected to step response cycles of dimethyl ether (DME) at 300 °C in a temporal analysis of product (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. The 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 toward 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.
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| 5. |
- Omojola, Oluwatoyin, 1988, et al.
(författare)
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Mechanistic insights into the conversion of dimethyl ether over ZSM-5 catalysts: A combined temperature-programmed surface reaction and microkinetic modelling study
- 2021
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Ingår i: Chemical Engineering Science. - : Elsevier BV. - 0009-2509. ; 239
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Tidskriftsartikel (refereegranskat)abstract
- The rates of adsorption, desorption, and surface reaction of dimethyl ether (DME) to olefins over fresh and working ZSM-5 catalysts with different Si/Al ratios (36 and 135) were decoupled using a combination of temperature-programmed surface reaction experiments and microkinetic modelling. Transient reactor performance was simulated by solving coupled 1D nonlinear partial differential equations accounting for elementary steps during the induction period based on the methoxymethyl mechanism on the zeolite catalyst and axial dispersion and convection in the reactor. Propylene is the major olefin formed, and site-specific scaling relations between the activation energies of DME desorption and barriers to the formation of methoxymethyl and methyl propenyl ether are observed. Six ensembles of sites are observed with a maximum of three adsorption/desorption sites and three adsorption/desorption/reaction sites. Barriers are generally higher for working catalysts than fresh catalysts. Activation energies of propylene formation of ca. 200 kJ mol−1 are obtained, corroborating direct mechanistic proposals.
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| 6. |
- Omojola, Oluwatoyin, 1988, et al.
(författare)
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Mechanistic Insights into the Desorption of Methanol and Dimethyl Ether Over ZSM-5 Catalysts
- 2018
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Ingår i: Catalysis Letters. - : Springer Science and Business Media LLC. - 1572-879X .- 1011-372X. ; 148:1, s. 474-488
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Tidskriftsartikel (refereegranskat)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, N2 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.
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| 7. |
- Omojola, Oluwatoyin, 1988
(författare)
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Mechanistic Insights into the Induction Period of Methanol-to-Olefin Conversion over ZSM-5 Catalysts: A Combined Temperature-Programmed Surface Reaction and Microkinetic Modeling Study
- 2023
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Ingår i: Industrial & Engineering Chemistry Research. - 1520-5045 .- 0888-5885. ; 62:36, s. 14244-14265
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Tidskriftsartikel (refereegranskat)abstract
- The induction period of propylene formation from methanol is compared to that of dimethyl ether (DME) over ZSM-5 catalysts of different compositions using a combination of temperature-programmed surface reaction experiments and microkinetic modeling. Transient reactor performance is simulated by solving coupled 1D nonlinear partial differential equations accounting for elementary steps based on the methoxy methyl mechanism and axial dispersion and convection in the reactor. Three binding site ensembles and three active site ensembles are observed. These sites constitute up to 3% (binding sites similar to 70%, active sites similar to 30%) of the total acid sites during methanol conversion and up to 1% (binding sites similar to 30%, active sites similar to 70%) of the total acid sites during DME conversion. Over the binding sites, during the induction period of methanol, and DME conversion, the acid site density is the key descriptor. Over the active sites, acid site density is the key descriptor with higher site densities correlating with lower barriers of propylene formation during the induction period of methanol conversion. The barrier to DME desorption is the key descriptor, with lower desorption barriers correlating with lower barriers of propylene formation during the induction period of DME conversion. Barriers to propylene formation are lower during the induction period of methanol conversion (up to 141 kJ mol(-1)) compared to that of DME conversion (up to 200 kJ mol(-1)) over ZSM-5 catalysts.
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| 8. |
- Omojola, Oluwatoyin, 1988, et al.
(författare)
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Molecular behaviour of methanol and dimethyl ether in H-ZSM-5 catalysts as a function of Si/Al ratio: a quasielastic neutron scattering study
- 2020
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Ingår i: Catalysis Science and Technology. - 2044-4753 .- 2044-4761. ; 10:13, s. 4305-4320
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Tidskriftsartikel (refereegranskat)abstract
- The dynamical behaviour of methanol and dimethyl ether in H-ZSM-5 catalysts of differing Si/Al ratios (36 and 135) was probed using quasielastic neutron scattering to understand the effect of catalyst composition (Brønsted acid site concentration) on the behaviour of species present during the initial stages of the H-ZSM-5 catalysed methanol-to-hydrocarbons process. At room temperature in H-ZSM-5(36) isotropic methanol rotation was observed (rotational diffusional coefficient, DR = 2.6 × 1010 s-1), which contrasted qualitatively with H-ZSM-5(135) in which diffusion confined to a sphere matching the 5.5 Å channel width was observed, suggesting motion is more constrained in the lower Si/Al catalyst. At higher temperatures, confined methanol diffusion is exhibited in both catalysts with self-diffusion coefficients (Ds) measured in the range of 8-9 × 10-10 m2 s-1. However, the population of molecules immobile over the timescale probed by the instrument is significantly larger in H-ZSM-5(36), consistent with the far higher number of Brønsted acid adsorption sites. For dimethyl ether, diffusion confined to a sphere at all temperatures is observed in both catalysts with Ds measured in the range of 9-11 × 10-10 m2 s-1 and a slightly smaller fraction of immobile molecules in H-ZSM-5(135). The larger Ds values obtained for dimethyl ether arise from the sphere of confinement being larger in H-ZSM-5(36) (6.2 Å in diameter) than the 5.5 Å width of the pore channels. This larger width suggests that mobile DME is sited in the channel intersections, in contrast to the mobile methanol which is sited in the channels. An even larger confining sphere of diffusion was derived in H-ZSM-5(135) (∼8 Å in diameter), which we attribute to a lack of Brønsted sites, allowing for a larger free volume for DME diffusion in the channel intersections.
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| 9. |
- Omojola, Oluwatoyin, 1988
(författare)
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Site-specific scaling relations observed during methanol-to-olefin conversion over ZSM-5 catalysts
- 2022
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Ingår i: Chemical Engineering Science. - : Elsevier BV. - 0009-2509. ; 251
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Tidskriftsartikel (refereegranskat)abstract
- The conversion of methanol over ZSM-5 catalysts was studied using step response and temperature-programmed desorption and surface reaction analyses in a temporal analysis of products reactor, as well as quasi-elastic neutron scattering and Fourier transform infrared investigations, buttressed by archived 13C magic angle spinning nuclear magnetic resonance studies. The results were combined with micro-kinetic models that simulated the formation of the first C–C bond and primary olefin(s) from methanol. Dimethyl ether was the major surface oxygenate and a source of surface methoxy species. Propylene, the major olefin produced from dimethyl ether, was formed with a reaction barrier of ∼200 kJ mol-1, in agreement with archived density functional theory calculations. Propylene was formed from dimethyl ether via a methoxymethyl mechanism under intrinsic kinetic conditions. Site-specific scaling relations between the barriers to methyl propenyl ether and methoxy methyl species formation and dimethyl ether desorption were observed. The active sites of the ZSM-5 catalysts can be locally optimised and selectively tuned to improve their activity during the methanol-to-olefin conversion.
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| 10. |
- Omojola, Oluwatoyin, 1988, et al.
(författare)
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Transient kinetic studies and microkinetic modeling of primary olefin formation from dimethyl ether over ZSM-5 catalysts
- 2019
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Ingår i: International Journal of Chemical Kinetics. - : Wiley. - 1097-4601 .- 0538-8066. ; 51:7, s. 528-537
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Tidskriftsartikel (refereegranskat)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.
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