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•A microkinetic model was developed for DME synthesis over H-zeolite catalyst.•The optimized structures and reaction energetics were obtained by MP2 calculations.•The pre-exponential ...factors were estimated by fitting the experimental data.•The dominant reaction pathway and the rate-determining step were found.
In this study, the reaction pathways of DME synthesis by methanol dehydration over a H-zeolite catalyst were analyzed through both computational chemistry and microkinetic modeling methods. The reaction mechanisms consisted of nine forward and backward elementary-step reactions for both associative and dissociative pathways. Based on the second-order Møller–Plesset perturbation theory (MP2), to determine the effects of dispersion forces that were important in our reaction system, the structures of all related reaction species were optimized, and the transition states of the associative and dissociative pathways were elucidated. Also, the energies and activation barriers of the optimized structures and transition states were calculated. Then, a microkinetic model was developed using the energies and activation barriers obtained from the MP2 calculations. Meanwhile, the pre-exponential factors of the kinetic parameters were not calculated theoretically but estimated by fitting the experimental data, which enhanced the reliability of the microkinetic model. By comparing the relative elementary-step reaction rates calculated using the developed model, the dissociative pathway was suggested as a dominant pathway of DME synthesis, while the DME formation reaction of the dissociative pathway (CH3OH-CH3-Z → CH3OCH3-H-Z) was found to be the rate-determining step. The developed model was also used to evaluate the effects of temperature on the site fractions over the catalyst.
The crystallite size effects of Pt nanoparticles on the CeO2 (Pt/CeO2) prepared with four different Pt precursors were investigated in terms of their thermal stability and catalytic activity for a ...water-gas shift (WGS) reaction using the compositions of reformates after a typical steam reforming of propane. The Pt/CeO2 prepared with a diamine dinitroplatinum (Pt(NO2)2(NH3)3) precursor, which forms the cationic Pt(NH3)22+ species on the negatively-charged CeO2 surfaces, revealed a superior catalytic activity and thermal stability by forming the partially oxidized smaller Pt nanoparticles decorated with metallic Pt surfaces as well as by forming the strongly interacted PtOx-CeO2 interfaces. The stable preservation of the pristine smaller Pt nanoparticles with small aggregations even under the hysteresis test from 250 to 400 °C was mainly attributed to the strong metal-support interactions. The optimized Pt/CeO2 was further studied to obtain kinetic equations derived by Langmuir-Hinshelwood (LH) model, and the optimal operating conditions of WGS reaction were found to be ~280 °C and H2O/CO molar ratio of 9 with the activation energy of ~78.4 kJ/mol.
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•Effects of Pt precursors on CeO2 were studied for their thermal and catalytic stability during WGS reaction.•Cationic Pt (NH3)22+ on negatively-charged CeO2 revealed a higher stability with larger PtOx-CeO2 interfaces.•Proposed Langmuir-Hinshelwood models were well matched with experimental data.
Steam reforming of a light hydrocarbon mixture, which might be produced during the pyrolysis of waste resources, was experimentally conducted over a commercial Ni-based catalyst. Two different gas ...compositions were considered as simulated gases to utilize waste resources, and kinetic data were generated at a variety of temperatures, space velocities, and steam-to-carbon ratios to develop a kinetic model by fitting the experimental data. Experimental observations indicated that the inclusion of higher chains resulted in low methane conversion compared to those of the other chains, and the model demonstrated that the increased hydrogen production rate by higher chains enhanced methanation, resulting in a decreased methane conversion. The increased fraction of higher chains decreased the steam-to-carbon ratio for methane, resulting in a decreased methane conversion. Further analysis showed that despite the decrease in methane fraction, hydrogen production could be increased by 4% with a 10% methane fraction in the light hydrocarbon mixture, indicating that the direct reforming of a light hydrocarbon mixture from waste resources with no pre-reforming might be feasible.
•Steam reforming of a light hydrocarbon mixture was experimentally conducted.•Two different gas compositions were considered as simulated gases to utilize waste resources.•A kinetic model was developed by fitting the experimental data.•The inclusion of higher chains resulted in low methane conversion.•The increased fraction of higher chains in the light hydrocarbon mixture could increase hydrogen production.
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•An optimization framework based on a surrogate model was developed to optimize MWD.•Framework integrates MWD calculation, reactor dynamics, and nonideal mixing behaviors.•Neural ...network applied to global optimization problems to solve computational issues.
The optimization of polymerization reactors with nonideal mixing has been greatly hindered by a tremendous computation burden. In this study, a global optimization framework to integrate full MWD calculations and detailed reactor dynamics with nonideal mixing behaviors was developed by reducing the considerable computation time required for implementing full MWD. A combination of surrogate-model-based optimization and a computational fluid dynamics (CFD)-compartment reactor model was applied to produce the desired MWD in a nonideal reactor. A neural network (NN) surrogate model was trained using 3,000 MWD data produced from the mechanistic reactor model. The results showed that the model successfully described asymmetric MWD with bimodality resulting from the nonideal mixing behaviors in the reactor. The surrogate model was used to construct operation maps for the asymmetry and bimodality factors, with SHapley Additive exPlanations (SHAP) values showing that the flow rate of the chain transfer agent had the largest impact on the asymmetry factor, whereas temperature significantly influenced the bimodality factor. Finally, the optimization strategy based on the asymmetric and bimodality factors successfully determined operating conditions to produce four tailored MWDs, reported in actual polymerization reactors with R scores of 0.988–0.997.
The higher catalytic activity and stability for a gas-phase carbonylation of dimethyl ether (DME) to methyl acetate (MA) on the seed-derived ferrierite (FER) were attributed to its higher ...crystallinity with small amounts of defect sites by recrystallization methods without using any organic structure directing agent. The recrystallized FER (FER-S1) with its smaller amount of Lewis acidic extraframework Al sites (EFAl) possessed proper number of Brønsted acidic sites in the eight-membered-ring (8-MR) channels in comparison to the pristine FER, which was responsible for an enhanced CO insertion rate to methoxy intermediates formed by dissociated DME molecules by referentially forming acetyl intermediates or to the highly active Brønsted acidic sites. The most active tetrahedral T2 sites with two adjacent Al atoms in the 8-MR channels having next-nearest Al–O–Si–O–Al configurations on the FER-S1 revealed the stronger adsorption of the stably adsorbed DME molecules as confirmed by DFT calculations. FER-S1 containing the proper amounts of Al atoms in the 8-MR with appropriate locations with optimal acidic properties was responsible for its higher activity and stability for the gas-phase carbonylation of DME, where the Al distributions were confirmed by Rietveld refinement XRD analysis, FT-IR, and DFT calculations. In addition, the acidic sites on the outer surfaces and larger cavity including 10-MR channels were responsible for an accelerated formation of aromatic coke precursors.
In this study, the development of kinetic rate equations for catalytic methane chlorination over pellet-type catalysts was addressed. Experimental data at various temperatures, feed compositions, ...space velocities, and pressures were used to estimate the kinetic parameters. The average errors for methane conversion and methyl chloride (MCM) selectivity were 15.9 and 4.4%, respectively, validating the effectiveness of the developed kinetics. A process model was developed by considering a commercial-scale methane chlorination reactor and separation train. HCl, which was responsible for half of the chlorine gas in the feed, might reduce the economics of the process. The MeOH hydrochlorination reaction was combined with methane chlorination, and three variations of the combined process were suggested, depending on the configuration of the separation sequence. The MeOH hydrochlorination reduced the energy requirement by more than 80% compared to methane chlorination. Techno-economic analysis showed that the feeding of pure HCl had the lowest purchase and annual production costs, resulting in the lowest minimum selling price (MSP) of 0.90 $/kg-MCM. The sensitivity analysis of the MSP concerning the MeOH and HCl price showed that the proposed process is the most economical up to 1.8 $/kg-MeOH and 160 $/ton-HCl, respectively.
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•The Ni2P/SiO2 catalyst exhibits the superior performance in ethylene oligomerization.•The particle size of Ni2P greatly influences the oligomerization activity.•The catalytic ...performance is affected by the Brønsted acid sites of Ni2P catalysts.•Ni2P/SiO2 is able to easily generate and transfer hydrogen species via POH groups.•The characteristics of Ni2P/SiO2 are beneficial for ethylene oligomerization.
Although homogeneous catalytic systems have been central in ethylene oligomerization, the SiO2-Al2O3-supported nickel phosphide (Ni2P) catalyst was recently examined to be capable of transforming ethylene into higher olefins. Herein, better catalytic performance in ethylene oligomerization is achieved with Ni2P/SiO2 than Ni2P/SiO2-Al2O3 and Ni2P/Al2O3. As a factor influencing ethylene oligomerization, the particle size of Ni2P is the smallest over Ni2P/SiO2 due to strong metal-support interaction. While the oligomer productivity is affected by the density of Brønsted acid site present in the supported Ni2P catalysts, Ni2P/SiO2 has a relatively high ratio of Brønsted to Lewis acid sites. Moreover, Ni2P/SiO2 is able to easily generate and transfer hydrogen species via the Brønsted acid POH groups, which would affect formation of nickel hydride species that is important in oligomerizing ethylene. Therefore, the above features of Ni2P/SiO2 are beneficial for ethylene oligomerization into higher olefins.
•A methanol synthesis process using a mixed reforming reactor was modeled.•Kinetic models for the mixed reforming and methanol synthesis were developed.•Various effects of operating conditions on the ...MeOH production rate were evaluated.•An analysis was conducted with respect to both the overall and local CO2 conversions.•A trade-off existed between the maximum MeOH production and the maximum CO2 utilization.
In this study, kinetic models were developed for the mixed reforming and synthesis of methanol (MeOH). The effectiveness of the reforming model in our previous work was proven in an experimental study using a bench-scale reactor, while the intrinsic rate model and effectiveness factors were developed to represent the MeOH synthesis. For a 10-ton-per-day production of MeOH, the rate model was used to determine the size of a reforming reactor so that the supplied heat could be used exclusively to engage the reaction (not for the heat-up of the reactant), while the MeOH reactor was specified using the reported values. The process model was then used to evaluate various effects of the following factors on the MeOH production rate: (1) reaction temperature, (2) CO2 fraction in the feed, and (3) the recycle route of the unreacted gas either to the feed or to the MeOH reactor. Additionally, an analysis was conducted with respect to both the overall and local CO2 conversions in each reactor, and it was shown that a trade-off existed between the maximum MeOH production rate and the maximum CO2 utilization, regardless of the existence of a recycle stream.
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•Spatially confined NiO with Al2O3 overlayer on Ni/m-Al2O3 showed a superior stability.•Optimal amount of Al2O3 layers was <3 wt% on mesoporous Ni/m-Al structures.•Al2O3 protective ...overlayers increased long-term stability with small cokes depositions.
To enhance catalytic and thermal stability of Ni nanoparticles for a combined steam and CO2 reforming with CH4 (CSCR), the utilization of an ordered mesoporous Al2O3 support with ∼6 nm of average pore diameter (Ni/m-Al) was proposed in terms of spatial confinement effects of the Ni nanoparticles with the help of successive Al2O3 overlayer protective layers (Ni/m-Al@Al). At an optimal amount of Al2O3 coating layers less than ∼3 wt%, the much higher catalytic activity and stability were observed on the Ni/m-Al@Al(3). The synergy effects of Al2O3 overlayers on the ordered mesoporous Ni/m-Al were mainly attributed to the formation of strongly interacted Ni-Al2O3 interfaces as supported by its higher XPS binding energy in the spatially restricted mesoporous m-Al channels with the protective metal oxide overlayers. Those structures were also responsible for the suppressed migrations of the spatially confined Ni nanoparticles to the outer m-Al surfaces by Al2O3 protective overlayers, which resulted in an excellent long-term stability with small coke depositions by preserving the original Ni nanoparticle sizes.
In this study, a practical strategy to develop a microkinetic model for methanol synthesis from syngas over a Cu-based catalyst is described. The comprehensive model consists of forward and backward ...reactions of 28 possible elementary-step reactions for CO and CO2 hydrogenation and the water–gas shift reaction. A combination of ab initio density functional theory (DFT) and semiempirical unity bond index–quadratic exponential (UBI-QEP) methods was used to determine the heat of adsorption and activation energies. DFT calculations confirmed that formate (HCOO**) adsorbs in a bidentate fashion and provided the enthalpies and adsorption energies of gas and surface intermediates for subsequent UBI-QEP calculations. The pre-exponential factors were estimated from the order of magnitude of the transition state theory as the initial values and by fitting the experimental data, thus reducing the computational load by not calculating the vibrational frequencies and partition functions for translational, rotational, and vibrational motions. For the reactor model, partial equilibrium ratios were used to reduce the stiffness of the microkinetic model. The most plausible reaction pathways were suggested by considering relatively fast step reactions, while the surface reaction of H3CO* and H* was found to be the rate-determining step by the degree of rate control. The developed model was also used to evaluate the effects of the temperature, pressure, and H2 fraction in the feed on the methanol synthesis rate to elucidate the suitable operating conditions. The model effectiveness was validated by comparison with other reported works. The proposed approach can be further exploited for the efficient development of other microkinetic models.
