Theoretical Investigation of the Selective Oxidation of Methane to Methanol on Nanostructured Fe-ZSM-5 by the ONIOM Method

K T, Ismail TM, Sajith PK. Theoretical investigation into the effect of water on the N2O decomposition reaction over Cu-ZSM-5 catalyst

Copper exchanged zeolites are an admirable catalyst for the direct decomposition reaction of harmful N2O gas. However, the inhibition of the decomposition reaction in the presence of water vapor greatly...

Methane Conversion over C2N-Supported Fe2 Dimers

Methane is a vast hydrocarbon resource around the globe that has the potential to replace petroleum as a raw material and energy source. Therefore, the catalytic conversion of methane into high value-added chemicals is significantly important for the utilization of this hydrocarbon resource. However, this is a great challenge due to the high-energy input required to overcome the reaction barrier. Herein, a highly active catalytic conversion process of methane on an iron dimer anchored on a two-dimensional (2D) C2N monolayer (Fe2@C2N) is reported. Density functional theory calculations reveal that the superior properties of Fe2@C2N can be attributed to the formation of the Fe-O-Fe intermediate with H2O2 as the O-donor molecule, which facilitates the formation of methyl radicals and promotes the conversion of methane. This finding could pave the way toward highly efficient non-precious metal catalysts for methane oxidation reactions.

Dehydrogenation of ethanol to acetaldehyde with nitrous oxide over the metal-organic framework NU-1000: a density functional theory study

The conversion of ethanol to more valuable hydrocarbon compounds receives great attention in chemical industries because it could diminish the dependency on petroleum as raw material. We investigate the catalytic performance of Fe-supported MOF NU-1000 for the dehydrogenation of ethanol to acetaldehyde with nitrous oxide (N2O) by deriving the relevant reaction profiles with density functional theory calculations. In the proposed mechanism, the activation barrier of the rate-determining step is almost four times lower in the presence of N2O than without it. The supported NU-1000 framework plays also important role since it facilitates electron transfers and stabilizes all species along the reaction coordinate. When considering the catalytic activity of tetravalent metal centers (Zr, Hf and Ti) substituted into NU-1000 it is found that their activity decreases in the order Hf ≥ Zr > Ti, based on activation energies and turnover frequencies (TOF). Concerning MOF linkers, we show that the catalytic activity is not further improved by functionalizing NU-1000 with either electron-donating or electron-withdrawing organic groups.

Ethylene Epoxidation with Nitrous Oxide over Fe-BTC Metal-Organic Frameworks: A DFT Study

The epoxidation of ethylene with N₂ O over the metal-organic framework Fe-BTC (BTC=1,3,5-benzentricarboxylate) is investigated by means of density functional calculations. Two reaction paths for the production of ethylene oxide or acetaldehyde are systematically considered in order to assess the efficiency of Fe-BTC for the selective formation of ethylene oxide. The reaction starts with the decomposition of N₂ O to form an active surface oxygen atom on the Fe site of Fe-BTC, which subsequently reacts with an ethylene molecule to form an ethyleneoxy intermediate. This intermediate can then be selectively transformed either by 1,2-hydride shift into the undesired product acetaldehyde or into the desired product ethylene oxide by way of ring closure of the intermediate. The production of ethylene oxide requires an activation energy of 5.1 kcal mol^-1 , which is only about one-third of the activation energy of acetaldehyde formation (14.3 kcal mol^-1 ). The predicted reaction rate constants for the formation of ethylene oxide in the relevant temperature range are approximately 2-4 orders of magnitude higher than those for acetaldehyde. Altogether, the results suggest that Fe-BTC is a good candidate catalyst for the epoxidation of ethylene by molecular N₂ O.

Modification of the catalytic properties of the Au4 nanocluster for the conversion of methane-to-methanol: synergistic effects of metallic adatoms and a defective graphene support

By means of the density functional theory calculations, enhanced catalytic activity of Au₄ cluster for the partial oxidation of methane with the N₂O oxidant is observed when the cluster is deposited on top of the Pd/graphene.

Methane activation on Fe- and FeO-embedded graphene and boron nitride sheet: role of atomic defects in catalytic activities

The influence of supporting materials, graphene and boron nitride sheets, on the reactivity of Fe and FeO active species have been unravelled by using a dispersion-corrected DFT (PBE-D2) method.

Across the board: Gabriele Centi

In this series of articles, the board members of ChemSusChem discuss recent research articles that they consider of exceptional quality and importance for sustainability. In this entry, Prof. Gabriele Centi comments on recent results on non-oxidative conversion of methane to ethylene and aromatics. The discussion takes into account technical, economical, and sustainability perspectives, and briefly comments on the role of shale gas in future chemical production scenarios.

Direct oxidation of methane to methanol on Fe–O modified graphene

The reaction mechanisms of the partial oxidation of methane to methanol over FeO/graphene are unraveled using an advanced DFT approach.

Glycine peptide bond formation catalyzed by faujasite

The catalysis of peptide bond formation between two glycine molecules on H-FAU zeolite was computationally studied by the M08-HX density functional. Two reaction pathways, the concerted and the stepwise mechanism, starting from three differently adsorbed reactants, amino-bound, carboxyl-bound, and hydroxyl-bound, are studied. Adsorption energies, activation energies, and reaction energies, as well as the corresponding intrinsic rate constants were calculated. A comparison of the computed energetics of the various reaction paths for glycine indicates that the catalyzed reaction proceeds preferentially via the concerted reaction mechanism of the hydroxyl-bound configuration. This involves an eight-membered ring of the transition structure instead of the four-membered ring of the others. The step from the amino-bound configuration to glycylglycine is the rate-determining step of the concerted mechanism. It has an estimated activation energy of 51.2 kcal mol(-1). Although the catalytic reaction can also occur via the stepwise reaction mechanism, this path is not favored.

ONIOM study of isomerization reactions of aromatic hydrocarbons in acidic mordenite zeolite

Two different mechanisms: Shift 1,2-isomerization and isomerization via the disproportionation reaction are investigated for aromatic hydrocarbons over acidic mordenite zeolite by using our own n-layered integrated molecular orbital and molecular mechanics (ONIOM) scheme. The picture shows a schematic energy profile for the isomerization of toluene catalyzed by acidic mordenite.The geometrical structures and energies of isomerization reactions of aromatic hydrocarbons catalyzed by the 128T cluster model of acidic mordenite zeolite are studied by using our own n-layered integrated molecular orbital and molecular mechanics (ONIOM) scheme. The aromatic hydrocarbons investigated are toluene, xylenes, and trimethylbenzenes. Two proposed intramolecular reaction mechanisms, that is, shift 1,2-isomerization and isomerization via the disproportionation reaction, are studied and discussed in detail. Both of the mechanisms firstly proceed by protonation of aromatic hydrocarbons to form the aromatic-based carbenium ions. Attempts to determine the prevailing reaction mechanism of the isomerization are evaluated. In most cases, the isomerization reactivity sequence is: trimethylbenzenes>xylenes>toluene. The conclusions obtained using the ONIOM scheme agree with the available periodic DFT and experimental results on acidic mordenite zeolite.

Structure and nuclearity of active sites in Fe-zeolites: comparison with iron sites in enzymes and homogeneous catalysts

Fe-ZSM-5 and Fe-silicalite zeolites efficiently catalyse several oxidation reactions which find close analogues in the oxidation reactions catalyzed by homogeneous and enzymatic compounds. The iron centres are highly dispersed in the crystalline matrix and on highly diluted samples, mononuclear and dinuclear structures are expected to become predominant. The crystalline and robust character of the MFI framework has allowed to hypothesize that the catalytic sites are located in well defined crystallographic positions. For this reason these catalysts have been considered as the closest and best defined heterogeneous counterparts of heme and non heme iron complexes and of Fenton type Fe(2+) homogeneous counterparts. On this basis, an analogy with the methane monooxygenase has been advanced several times. In this review we have examined the abundant literature on the subject and summarized the most widely accepted views on the structure, nuclearity and catalytic activity of the iron species. By comparing the results obtained with the various characterization techniques, we conclude that Fe-ZSM-5 and Fe-silicalite are not the ideal samples conceived before and that many types of species are present, some active and some other silent from adsorptive and catalytic point of view. The relative concentration of these species changes with thermal treatments, preparation procedures and loading. Only at lowest loadings the catalytically active species become the dominant fraction of the iron species. On the basis of the spectroscopic titration of the active sites by using NO as a probe, we conclude that the active species on very diluted samples are isolated and highly coordinatively unsaturated Fe(2+) grafted to the crystalline matrix. Indication of the constant presence of a smaller fraction of Fe(2+) presumably located on small clusters is also obtained. The nitrosyl species formed upon dosing NO from the gas phase on activated Fe-ZSM-5 and Fe-silicalite, have been analyzed in detail and the similarities and differences with the cationic, heme and non heme homogeneous counterparts have been evidenced. The same has been done for the oxygen species formed by N(2)O decomposition on isolated sites, whose properties are more similar to those of the (FeO)(2+) in cationic complexes (included the [(H(2)O)(5)FeO](2+)"brown ring" complex active in Fenton reaction) than to those of ferryl groups in heme and non heme counterparts.

Relationship Between 1H Chemical Shifts of Deuterated Pyridinium Ions and Brønsted Acid Strength of Solid Acids

Deuterated pyridine (pyridine-d5) is one of the NMR probe molecules widely used for determination of acid strength of solid catalysts. However, the correlation between the 1H chemical shift of adsorbed pyridine-d5 and the Brønsted acid strength of solid acids has rarely been investigated. Here, an 8T zeolite model with different Si-H bond lengths is used to represent the Brønsted acid sites with different strengths (from weak, strong, to superacid) and to predict the pyridine adsorption structure as well as the 1H chemical shift. The theoretical calculation suggests that a smaller 1H chemical shift of the pyridinium ions on the solid acids indicates a stronger acid strength. On the basis of the results of theoretical calculations, a linear correlation between the pyridine-d5 1H chemical shift and the proton affinity (PA) of the Brønsted acid site has been derived. In combination with the available 1H MAS NMR experimental data, we conclude that pyridine-d5 can be used as a scale to characterize the solid acid strength.

Vapor-Phase Beckmann Rearrangement of Oxime Molecules over H-Faujasite Zeolite

The Beckmann rearrangement (BR) plays an important role in a variety of industries. The mechanism of this reaction rearrangement of oximes with different molecular sizes, specifically, the oximes of formaldehyde (H(2)C=NOH), Z-acetaldehyde (CH(3)HC=NOH), E-acetaldehyde (CH(3)HC=NOH) and acetone (CH(3))(2)C=NOH, catalyzed by the Faujasite zeolite is investigated by both the quantum cluster and embedded cluster approaches at the B3LYP level of theory using the 6-31G (d,p) basis set. To enhance the energetic properties, single point calculations are undertaken at MP2/6-311G(d,p). The rearrangement step, using the bare cluster model, is the rate determining step of the entire reaction of these oxime molecules of which the energy barrier is between 50-70 kcal mol(-1). The more accurate embedded cluster model, in which the effect of the zeolitic framework is included, yields as the rate determining step, the formaldehyde oxime reaction rearrangement with an energy barrier of 50.4 kcal mol(-1). With the inclusion of the methyl substitution at the carbon-end of formaldehyde oxime, the rate determining step of the reaction becomes the 1,2 H-shift step for Z-acetaldehyde oxime (30.5 kcal mol(-1)) and acetone oxime (31.2 kcal mol(-1)), while, in the E-acetaldehyde oxime, the rate determining step is either the 1,2 H-shift (26.2 kcal mol(-1)) or the rearrangement step (26.6 kcal mol(-1)). These results signify the important role that the effect of the zeolite framework plays in lowering the activation energy by stabilizing all of the ionic species in the process. It should, however, be noted that the sizeable turnover of a reaction catalyzed by the Brønsted acid site might be delayed by the quantitatively high desorption energy of the product and readsorption of the reactant at the active center.

Theoretical Study of N2O Reduction by CO in Fe-BEA Zeolite

Quantum mechanical (QM) and QM/molecular mechanics (MM) studies of the full catalytic cycle of N(2)O reduction by CO in Fe-BEA zeolite, that is, oxidation of BEA-Fe by N(2)O and reduction of BEA-Fe-alphaO by CO, is presented. A large QM cluster, representing half of the channel of the BEA zeolite, is used. The contribution of the MM embedding to the calculated activation energies is found to be negligible. The minimum-energy paths for N(2)O decomposition and reduction with CO are calculated using the nudged elastic band (NEB) method. Calculated and experimental activation energies are in good agreement. The two possible orientations for the gaseous molecules adsorbing on the Fe site that are found lead to different activation energies.

Structures and Reaction Mechanisms of Cumene Formation via Benzene Alkylation with Propylene in a Newly Synthesized ITQ-24 Zeolite: An Embedded ONIOM Study

The cumene formation via benzene alkylation with propylene on the new three-dimensional nanoporous catalyst, ITQ-24 zeolite, has been investigated by using the ONIOM2(B3LYP/6-31G(d,p):UFF) method. Both consecutive and associative reaction pathways are examined. The contributions of the short-range van der Waals interactions, which are explicitly included in the ONIOM2 model, and an additional long-range electrostatic potential from the extended zeolite framework to the energy profile are taken into consideration. It is found that benzene alkylation with propylene in the ITQ-24 zeolite prefers to occur through the consecutive reaction mechanism. The benzene alkylation step is the reaction rate-determining step with an estimated activation energy of 35.70 kcal/mol, comparable with an experimental report in beta-zeolite of 34.9 kcal/mol. The electrostatic potential from the extended zeolite framework shows a much more significant contribution to the transition state selectivity than the van der Waals interactions.

Investigation of Ethylene Dimerization over Faujasite Zeolite by the ONIOM Method

Ethylene dimerization was investigated by using an 84T cluster of faujasite zeolite modeled by the ONIOM3(MP2/6-311++G(d,p):HF/6-31G(d):UFF) method. Concerted and stepwise mechanisms were evaluated. In the stepwise mechanism, the reaction proceeds by protonation of ethylene to form the surface ethoxide and then C--C bond formation between the ethoxide and the second ethylene molecule to give the butoxide product. The first step is rate-determining and has an activation barrier of 30.06 kcal mol(-1). The ethoxide intermediate is rather reactive and readily reacts with another ethylene molecule with a smaller activation energy of 28.87 kcal mol(-1). In the concerted mechanism, the reaction occurs in one step of simultaneous protonation and C--C bond formation. The activation barrier is calculated to be 38.08 kcal mol(-1). Therefore, the stepwise mechanism should dominate in ethylene dimerization.