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New Insights into the Reaction Mechanism of Sodium Vanadate for an Aqueous Zn Ion Battery
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Reaction Mechanism for the Conversion of γ-Valerolactone (GVL) over a Ru Catalyst: A First-Principles Study
Conversion of biomass-derived γ-valerolactone (GVL) to valuable chemicals has been studied extensively, and understanding the reaction mechanism is very valuable for improving turnover rates and selectivities. Here, we report first-principles density functional calculations, through which we show in detail the reaction pathways of GVL conversion on a Ru(0001) surface, in good agreement with recent experimental results performed on supported Ru catalysts. We find that (i) GVL undergoes a ring-opening reaction rather easily, and (ii) the rate-limiting step toward the formation of 1,4-pentanediol (1,4-PDO) and 2-pentanol (2-PeOH) is the hydrogenation step. The high energy barrier for this step is caused by a strong interaction between Ru and the unsaturated acyl intermediate that is formed after opening the ring. Among all the primary products, formation of 2-butanol (2-BuOH) has the smallest activation barrier, while the slowest step is C–C bond cleavage in the decarbonylation step. We further show that the same acyl intermediate after ring opening of GVL can also be formed by dehydrogenation of 1,4-PDO with a moderate activation barrier, which suggests that both 2-PeOH and 2-BuOH can also be produced in secondary steps.
Kinetics and Reaction Mechanisms of Acetic Acid Hydrodeoxygenation over Pt and Pt–Mo Catalysts
In this work, kinetic measurements for silica-supported Pt and Pt-Mo catalysts were collected in vapor-phase acetic acid hydrodeoxygenation by varying the hydrogen partial pressure between 18 and 72 kPa and acetic acid partial pressure between 7 and 18 kPa at 423-473 K. At all testing conditions, the Pt-Mo catalyst was more active and selective. In addition, the apparent activation energy for Pt-Mo of 76±3 kJ/mol was lower than that of 84±4 kJ/mol for Pt. The apparent reaction orders were also different. The order in hydrogen of 0.8±0.1 for Pt-Mo changed to zero at higher hydrogen partial pressures while that for Pt remained constant at 0.6±0.1. A near-zero order in acetic acid for Pt-Mo changed to -2.1 at higher acetic acid pressures while that for Pt remained constant at -2.9±0.3. These differences in reaction kinetics as well as in selectivity trends with changes in temperature and feed composition indicated a change in the reaction mechanism for Pt-Mo. The catalysts were characterized with hydrogen temperature programmed desorption, oxygen temperature programmed oxidation and transmission electron microscopy with energy-dispersed X-ray spectroscopy elemental mapping. Mo was present in the form of subnanometer-size clusters on the surface of Pt nanoparticles. Both Pt and Pt-Mo catalysts were stable under the reaction conditions for 10 h, and the size and structure of Pt and Pt-Mo particles remained mostly unchanged, without coke accumulation. Density functional theory calculations show that surface acetate is not a major reaction intermediate on both Pt and Pt-Mo and, instead, C-OH bond splitting with the formation of acetyl is the first reaction step in hydrodeoxygenation. The activation energy for this step is dramatically lower on Pt-Mo. Furthermore, the activity of acetyl on Pt-Mo is different. Unlike on pure Pt, the reaction of C-O bond splitting becomes exothermic with a lower activation energy on Pt-Mo, and the reaction of C-C bond splitting, in contrast, becomes endothermic with a higher activation energy, explaining the experimentally observed higher activities and selectivities of Pt-Mo. In addition, the calculations demonstrate that a pair of Pt-Mo surface atoms acts as a single active site where Mo serves as a preferential binding anchor for O atoms. The presence of Mo atoms changes the structure and energy of adsorbed and reacting surface species.
Elucidation of reaction mechanism for m-cresol hydrodeoxygenation over Fe based catalysts: A kinetic study
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Vacuum Pyrolysis of Hybrid Poplar Milled Wood Lignin with Fourier Transform-Ion Cyclotron Resonance Mass Spectrometry Analysis of Feedstock and Products for the Elucidation of Reaction Mechanisms
The pyrolysis of lignocellulosic materials is a promising technique to produce fuels and chemicals. It is well known that the most abundant products of lignin pyrolysis are oligomeric molecules, known as pyrolytic lignin (PL). The chemical composition of PL has been extensively studied; however, there is still an important debate whether these oligomers are produced directly from the lignin or from the recombination of monomeric pyrolytic products. Existing theories are unable to describe the effect of vacuum on the distribution of pyrolysis products. Hybrid poplar milled wood lignin (MWL) was initially isolated and thoroughly characterized by Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS). Chemical formulas were assigned to each oligomeric compound detected. The MWL was also subjected to vacuum pyrolysis in a modified pyroprobe at 250, 750, and 1000 mbar (absolute pressure), and the resulting liquid products were analyzed by FT-ICR MS. A new strategy to assign structural representations to the oligomeric PL products is proposed, based on the plausible pyrolysis reaction mechanisms of depolymerization/fragmentation applied to original MWL oligomer formulas. Our results support the hypothesis that PL is formed from the removal of moieties from primary lignin pyrolysis products with between three and five aromatic rings. This depolymerization/fragmentation allows the oligomers to reduce their molecular weights to the point where they can be removed from the reaction zone by direct vaporization. Furthermore, this phenomenon highlights the importance of pressure on removal mechanisms and their impact on the molecular weight of the resulting products from lignin pyrolysis.
Exploring the reaction mechanism in self-propagating Al/Ni multilayers by adding inert material
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Reaction Mechanism of the Sn2Fe Anode in Lithium-Ion Batteries
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A Theoretical Study on Reaction Mechanisms and Kinetics of Thiophene Hydrodesulfurization over MoS 2 Catalysts
In the present work, thiophene hydrodesulphurization (HDS) over the Mo-edge, the S-edge, and the Mo-S connection edge of MoS 2 catalyst with 50% sulfur coverage was studied using first-principles based microkinetic modeling. Two parallel HDS routes, i.e., direct desulfurization (DDS) and hydrogenation (HYD) were taken into account. It has been found that the major reaction route of thiophene HDS on the Mo- and the Mo-S edges is temperature dependent. In the low temperature range of 500~600 K, the HYD route is dominant, leading to the C 4 H 8 formation. As the temperature increases, the DDS route becomes competitive with the HYD route. At the temperature above 650 K, the DDS route will be the dominant HDS reaction route on the Mo- and the Mo-S edges. While the DDS route that leads to the formation of C4H6 is the major thiophene HDS reaction route on the S-edge in the entire temperature range of 500~750 K. The microkinetic modeling results show the overall HDS activity on the S-edge is lower than it on the Mo- and the Mo-S edges. The Mo-S edge also provides a preferential reaction pathway, which facilitates 2-hydrothiophene migration from the Mo-edge to the S-edge, followed by remaining elementary steps with lower activation barriers in the DDS route. The work was financially supported by the National Natural Science Foundation of China (No. 21476012, 21571012, 21407007 and 91534201). The computing time was granted by a scientific theme user proposal in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), which is a U.S. Department of Energy national scientific user facility located at Pacific Northwest National Laboratory (PNNL) in Richland, Washington. Part of computing time was provided the National Energy Research Scientific Computing Center (NERSC). D. Mei is supported by the US Department of Energy, Office of Sciences, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences.
Application of quantum mechanics/molecular mechanics methods in the study of enzymatic reaction mechanisms
Quantum mechanics/molecular mechanics ( QM / MM ) methods offer a very appealing option for the computational study of enzymatic reaction mechanisms, by separating the problem into two parts that can be treated with different computational methods. Hence, in a QM / MM formalism, the part of the system in which catalysis actually occurs and that involves the active site, substrates and directly participating amino acid residues is treated at an adequate quantum mechanical level to describe the chemistry taking place. For the remaining of the enzyme, which does not participate directly in the reaction, but that typically involves a much larger number of atoms, molecular mechanics is employed, traditionally through the application of a biomolecular force field. When applied with care, QM / MM methods can be used with great advantage in comparing, at a structural and energetic level, different mechanistic proposals, discarding mechanistic alternatives and proposing new mechanistic pathways that are consistent with the available experimental data. With time, diverse flavors within the QM / MM methods have emerged, differing in a variety of technical and conceptual aspects. Hence present alternatives differ between additive and subtractive QM / MM schemes, the type of boundary schemes, and the way in which the electrostatic interactions between the two regions are accounted for. Also, single‐conformation QM / MM , multi‐ PES approaches, and QM / MM Molecular Dynamics coexist today, each type with its own advantages and limitations. This review focuses on the application of QM / MM methods in the study of enzymatic reaction mechanisms, briefly presenting also the most important technical aspects involved in these calculations. Particular attention is dedicated to the application of the single‐conformation QM / MM , multi‐ PES QM / MM studies, and QM / MM‐FEP methods and to the advantages and disadvantages of the different types of QM / MM . Recent breakthroughs are also introduced. A selection of hand‐picked examples is used to illustrate such features. WIREs Comput Mol Sci 2017, 7:e1281. doi: 10.1002/wcms.1281 This article is categorized under: Structure and Mechanism > Computational Biochemistry and Biophysics Structure and Mechanism > Reaction Mechanisms and Catalysis Electronic Structure Theory > Combined QM/MM Methods