Theoretical study of fructose adsorption and conversion to trioses on metal-organic frameworks.

The conversion of chemically modified biomass into more valuable chemicals has recently gained significant attention from industry. In this study, we investigate the adsorption of fructose and its conversion into two trioses, glyceraldehyde (GLA) and dihydroxyacetone (DHA), on metal-organic frameworks using density functional theory calculations. The reaction mechanism proceeds through two main steps: first, the opening of the fructose ring; second, the retro-aldol fragmentation, which is favored over intramolecular hydrogen shifts. The substitution of a tetravalent metal in the metal-organic framework leads to different adsorption strengths in the order Hf-NU-1000 > Zr-NU-1000 > Ti-NU-1000. The catalytic activities of Hf-NU-1000 and Zr-NU-1000 are found to be similar. Both are more active than Ti-NU1000, corresponding to their relative Lewis acidity. It was found that functionalization of the organic linkers of the Hf-NU-1000 MOF does not improve its catalytic activity. The catalytic activity follows the order Hf-MOF-808 > Hf-NU-1000 > Hf-UIO-66 when based on either the overall activation energy or the turnover frequency (TOF).

A polarizable valence electron density based force field for high-energy interactions between atoms and molecules.

High-accuracy molecular force field models suited for hot gases and plasmas are not as abundant as those geared toward ambient pressure and temperature conditions. Here, we present an improved version of our previous electron-density based force field model that can now account for polarization effects by adjusting the atomic valence electron contributions to match ab initio calculated Mulliken partial charges. Using a slightly modified version of the Hohenberg-Kohn theorem, we also include an improved theoretical formulation of our model when applied to systems with degenerate ground states. We present two variants of our polarizable model, fitted from ab initio reference data calculated at CCSD(T)/cc-pVTZ and CCSD(T)/CEP-31G levels of theory, that both accurately model water dimer interaction energies. Further improvements include the additional interaction components with fictitious non-spherically symmetric, yet atom-centered, electron densities and fitting the exchange and correlation coefficients against analytical expressions. The latter removes all unphysical oscillations that are observed in the previous non-polarizable variant of our force field.

Understanding the interactions between lithium polysulfides and anchoring materials in advanced lithium-sulfur batteries using density functional theory.

Lithium-sulfur batteries (LSBs) are promising energy storage devices because of their high theoretical capacity and energy density. However, the "shuttle" effect in lithium polysulfides (LiPSs) is an unresolved issue that can hinder their practical commercial application. Research on LSBs has focused on finding appropriate materials that suppress this effect by efficiently anchoring the LiPSs intermediates. Quantum chemical computations are a useful tool for understanding the mechanistic details of chemical interaction involving LiPSs, and they can also offer strategies for the rational design of LiPSs anchoring materials. In this perspective, we highlight computational and theoretical work performed on this topic. This includes elucidating and characterizing the adsorption mechanisms, and the dominant types of interactions, and summarizing the binding energies of LiPSs on anchoring materials. We also give examples and discuss the potential of descriptors and machine learning approaches to predict the adsorption strength and reactivity of materials. We believe that both approaches will become indispensable in modelling future LSBs.

Sensing the ortho Positions in C6Cl6 and C6H4Cl2 from Cl2- Formation upon Molecular Reduction.

The geometrical effect of chlorine atom positions in polyatomic molecules after capturing a low-energy electron is shown to be a prevalent mechanism yielding Cl2-. In this work, we investigated hexachlorobenzene reduction in electron transfer experiments to determine the role of chlorine atom positions around the aromatic ring, and compared our results with those using ortho-, meta- and para-dichlorobenzene molecules. This was achieved by combining gas-phase experiments to determine the reaction threshold by means of mass spectrometry together with quantum chemical calculations. We also observed that Cl2- formation can only occur in 1,2-C6H4Cl2, where the two closest C-Cl bonds are cleaved while the chlorine atoms are brought together within the ring framework due to excess energy dissipation. These results show that a strong coupling between electronic and C-Cl bending motion is responsible for a positional isomeric effect, where molecular recognition is a determining factor in chlorine anion formation.

Density Functional Investigation of the Conversion of Furfural to Furfuryl Alcohol by Reaction with i-Propanol over UiO-66 Metal-Organic Framework.

Carbonyl C═O bond reduction via catalytic transfer hydrogenation (CTH) is one of the essential processes for biomass conversion to valuable chemicals and fuels. Here, we investigate the CTH of furfural to furfuryl alcohol with i-propanol on UiO-66 metal-organic frameworks using density functional theory calculations and linear scaling relations. Initially, the reaction over two defect sites presented on Zr-UiO-66, namely, dehydrated and hydrated sites, have been compared. The hydrated active site is favored over that on the dehydrated active site since the activation free energy of the rate-determining reaction step occurring on the hydrated active site is lower than that occurring on the dehydrated active site (14.9 vs 17.9 kcal/mol). The catalytic effect of exchanged tetravalent metals (Hf and Ti) on Zr-UiO-66 is also considered. We found that Hf-UiO-66 (13.5 kcal/mol) provides a lower activation energy than Zr-UiO-66 (14.9 kcal/mol) and Ti-UiO-66 (19.4 kcal/mol), which corresponds to it having a higher Lewis acidity. The organic linkers of UiO-66 MOFs play a role in stabilizing all of the species on potential energy surfaces. The linear scaling relationship also reveals the significant role of the UiO-66 active site in activating the carbonyl C═O of furfural, and strong relationships are observed between the activation free energy, the charge of the metal at the MOF active sites, and the complexation energies in reaction coordinates.

Effects of single and double active sites of Cu oxide clusters over the MFI zeolite for direct conversion of methane to methanol: DFT calculations.

In this work, we investigate the effect of various species of Cu oxide clusters including single and double active sites incorporated in the MFI zeolite framework for the direct conversion of methane to methanol. An M06-2X density functional calculation is employed to fine-tune the suitable number and species of active sites and to provide insights into the effect of the active sites on the reaction mechanism of methane to methanol. Two models, single and double active sites of Cu oxide clusters, have been chosen, in which the single active site of Cu oxide clusters, (mono(µ-oxo)dicopper(ii)), is located at the Al1'-Al12' pair ([Cu(µ-O)Cu]2+@Al1'-Al12'/MFI) or at the Al6-Al7 pair ([Cu(µ-O)Cu]2+@Al6-Al7/MFI) in the MFI framework. For the double active sites of Cu oxide clusters, two species of double active sites of Cu oxide are considered. The first one is the double active site of mono(µ-oxo)dicopper(ii) containingtwo Al-Al pairs (Al1'-Al12' and Al6-Al7 pairs) in the MFI framework (2[Cu(µ-O)Cu]2+/MFI) and the other is the double active site of trans-µ-1,2-peroxo dicopper(ii), which occupies two Al-Al pairs (Al1'-Al12' and Al6-Al7 pairs) in the MFI framework (2[Cu(µ-1,2-peroxo)Cu]2+/MFI). Furthermore, the activation energy for C-H bond dissociation in direct methane conversion to methanol is considered. Compared with the single active site of [Cu(µ-O)Cu]2+/MFI, the double active sites, in particular (2[Cu(µ-O)Cu]2+/MFI), exhibited the lowest activation energy, approximately 12.5 kcal mol-1. The high charge transfer between activated methane and Cu oxide active sites and also the high negative partial charge at the bridging oxygen of Cu oxide active sites, which directly interact with the methane molecule and abstracts its H atom, are considered as the important factors which affect the catalytic activity of Cu oxide clusters for direct methane conversion to methanol. These findings strongly support that the number and species of Cu oxide active sites incorporated in the MFI framework can highly affect the reaction mechanism of methane to methanol.

Effect of Intercalants inside Birnessite-Type Manganese Oxide Nanosheets for Sensor Applications.

Hydrazine is a common reducing agent widely used in many industrial and chemical applications; however, its high toxicity causes severe human diseases even at low concentrations. To detect traces of hydrazine released into the environment, a robust sensor with high sensitivity and accuracy is required. An electrochemical sensor is favored for hydrazine detection owing to its ability to detect a small amount of hydrazine without derivatization. Here, we have investigated the electrocatalytic activity of layered birnessite manganese oxides (MnO2) with different intercalants (Li+, Na+, and K+) as the sensor for hydrazine detection. The birnessite MnO2 with Li+ as an intercalant (Li-Bir) displays a lower oxidation peak potential, indicating a catalytic activity higher than the activities of others. The standard heterogeneous electron transfer rate constant of hydrazine oxidation at the Li-Bir electrode is 1.09- and 1.17-fold faster than those at the Na-Bir and K-Bir electrodes, respectively. In addition, the number of electron transfers increases in the following order: K-Bir (0.11 mol) < Na-Bir (0.17 mol) < Li-Bir (0.55 mol). On the basis of the density functional theory calculation, the Li-Bir sensor can strongly stabilize the hydrazine molecule with a large adsorption energy (-0.92 eV), leading to high electrocatalytic activity. Li-Bir also shows the best hydrazine detection performance with the lowest limit of detection of 129 nM at a signal-to-noise ratio of ∼3 and a linear range of 0.007-10 mM at a finely tuned rotation speed of 2000 rpm. Additionally, the Li-Bir sensor exhibits excellent sensitivity, which can be used to detect traces of hydrazine without any effect of interference at high concentrations and in real aqueous-based samples, demonstrating its practical sensing applications.

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.

Theoretical study of methane adsorption and C─H bond activation over Fe-embedded graphene: Effect of external electric field.

The effect of an external electric field (EF) on the methane adsorption and its activation on iron-embedded graphene (Fe-GPs) are investigated by using the M06-L density functional method. The EF is applied in the perpendicular direction to the graphene in the range of -0.015 to +0.015 a.u. with the interval of 0.005 a.u. The effects of EF on the adsorption, transition state and product complexes of the methane activation reaction are revealed. The binding energies of methane on Fe site in Fe-GPs are increased from -12.9 to -15.3, -18.1 and -21.5 kcal/mol for the negative EF of -0.005, -0.010 and -0.015, respectively. By applying positive EF, the activation barriers for methane activation are reduced in range of 3-8 kcal/mol (around 12-31%) and the reaction energies are more exothermic. The positive EF kinetically favors the reaction compared to the system without EF. The adsorption and activation of methane on Fe-GPs can be easily tuned by adjusting the external electric field for various applications. © 2019 Wiley Periodicals, Inc.

Phenol Tautomerization Catalyzed by Acid-Base Pairs in Lewis Acidic Beta Zeolites: A Computational Study.

We investigate the tautomerization of phenol catalyzed by acid-base active pair sites in Lewis acidic Beta zeolites by means of density functional calculations using the M06-L functional. An analysis of the catalytic mechanism shows that hafnium on the Beta zeolite causes the strongest absorption of phenol compared to zirconium, tin, and germanium. This can be rationalized by the highest delocalization of electron density between the Lewis site and the oxygen of phenol which can in turn be quantified by the perturbative E(2) stabilization energy. The reaction is assumed to proceed in two steps, the phenol O-H bond dissociation and the protonation of the intermediate to form the cyclohexa-2,4-dien-1-one product. The rate determining step is the first one with a free activation energy of 26.3, 25.0, 22.1 and 22.7 kcal mol-1 for Ge-Beta, Sn-Beta, Zr-Beta, and Hf-Beta zeolites, respectively. The turnover frequencies follow these reaction barriers. Hence, the intrinsic catalytic activity of the Lewis acidic Beta zeolites studied here is in the order of Hf-Beta≈Zr-Beta>Sn-Beta> Ge-Beta for the tautomerization of phenol.

Computational study of the carbonyl-ene reaction between formaldehyde and propylene encapsulated in coordinatively unsaturated metal-organic frameworks M3(btc)2 (M = Fe, Co, Ni, Cu and Zn).

The carbonyl-ene reaction between encapsulated formaldehyde and propylene over the coordinatively unsaturated metal-organic frameworks M3(btc)2 (M = Fe, Co, Ni, Cu and Zn) has been investigated by means of density functional calculations. Zn3(btc)2 adsorbs formaldehyde strongest due to electron delocalization between Zn and the oxygen atom of the reactant molecule. The reaction is proposed to proceed in a single step involving proton transfer and carbon-carbon bond formation. We find the relative catalytic activity to be Zn3(btc)2 > Fe3(btc)2 ≥ Co3(btc)2 > Ni3(btc)2 > Cu3(btc)2, based on activation energy and turnover frequencies (TOF). The low activation energy for Zn3(btc)2 compared to the others can be explained by the delocalization of electron density between the carbonyl bond and the catalyst active sites, leading to a more stable transition state. The five MOFs are used to propose a descriptor for the relationship between activation energy on one side and electronic properties or adsorption energies on the other side in order to allow a quick screening of other catalytic materials for this reaction.

Nanocavity effects of various zeolite frameworks on n-pentane cracking to light olefins: combination studies of DFT calculations and experiments.

Better control of the product selectivity of light olefins (e.g., ethylene and propylene) obtained from the n-pentane catalytic cracking process has attracted considerable attention from both scientific and petrochemical industrial points of view. In this context, we report insights into the effects of the nanocavities of various zeolite frameworks, including H-FER, H-ZSM-5, and H-FAU, representing small, medium, and large cavities, on the reaction mechanism of n-pentane cracking to light olefins by using M06-2X/6-31G(d,p) density functional calculations, eventually leading to fine-tuning the product distribution of light olefins. The reaction mechanism consists of the following two main steps: (i) the protolytic cracking of n-pentane to form a pentonium intermediate; and (ii) the subsequent dissociation of the intermediate to either ethane-propylene or ethylene-propane. The key reaction pathways controlling the product distribution of light olefins relate to the dissociation of the pentonium intermediate, which can produce selectively either propylene (P) or ethylene (E), resulting in a controllable P/E ratio. The differences in the activation energies for ethylene production compared with those of propylene production over H-FER, H-ZSM-5, and H-FAU are 6.7, 5.0, and 0.5 kcal mol-1, respectively. Compared with H-ZSM-5 and H-FAU, the higher difference in the activation energy of these two pathways over H-FER implies that the preferable production of ethane-propylene compared with ethylene-propane is more pronounced. It is therefore reasonable to conclude that a smaller pore zeolite such as H-FER eventually leads to a high ratio of production of propylene to ethylene, in accordance with experimental observations.

Insights into the reaction mechanism of n-hexane dehydroaromatization to benzene over gallium embedded HZSM-5: effect of H2 incorporated on active sites.

The catalytic dehydroaromatization of alkanes to aromatics has attracted considerable attention from the scientific community, because it can be used for the upgrading of low-cost alkanes into high added-value aromatics, such as benzene, toluene, and xylene (BTX). In this context, we report the reaction mechanism of n-hexane dehydroaromatization to benzene over two different reduced gallium species embedded in HZSM-5, including univalent Ga+ embedded in HZSM-5 (Ga/HZSM-5) and dihydrido gallium complex (GaH2+) embedded in HZSM-5 (GaH2/HZSM-5) by using the M06-2X/6-31G(d,p) level of calculation. The reaction proceeds by following two main steps: (i) the dehydrogenation of hexane to haxa-1,3,5-triene; (ii) the dehydroaromatization of haxa-1,3,5-triene to benzene. For the univalent Ga+ embedded in HZSM-5, the first step of the hexane dehydrogenation is considered to be the rate-determining step, which requires a high activation energy of 76.6 kcal mol-1. In strong contrast to this, in the case of the GaH2/HZSM-5 catalyst the rate determining step is found to be the second hydrogen abstraction from n-hexane with a lower activation barrier of 11.1 kcal mol-1. The reaction is therefore preferentially taking place over the GaH2/HZSM-5 catalyst. These observations clearly confirm the existence of a dihydrido gallium complex (GaH2+) as one of the most active species for the dehydroaromatization of alkanes and it is obtained in the presence of hydrogen in the catalytic system. This example opens up perspectives for a better understanding of the effect of active species on the catalytic reaction.

Furfural to Furfuryl Alcohol: Computational Study of the Hydrogen Transfer on Lewis Acidic BEA Zeolites and Effects of Cation Exchange and Tetravalent Metal Substitution.

The hydrogen transfer of furfural to furfuryl alcohol with i-propanol as the hydrogen source over cation-exchanged Lewis acidic BEA zeolite has been investigated by means of density functional calculations. The reaction proceeds in three steps. First the O-H bond of i-propanol is broken to form a propoxide intermediate. After that, the furylmethoxy intermediate is formed via hydrogen transfer process, and finally furylmethoxy abstracts the proton to form the furfuryl alcohol product. The second step is rate-determining by requiring the highest activation energy (23.8 kcal/mol) if the reaction takes place on Li-Sn-BEA zeolite. We find that the catalytic activity of various cation-exchanged Sn-BEA zeolites is in the order Li-Sn-BEA > Na-Sn-BEA > K-Sn-BEA. The lower activation energy for Li-Sn-BEA compared to Na-Sn-BEA and K-Sn-BEA can be explained by the larger charge transfer from the carbonyl bond to the catalyst, leading to its activation and to the attraction of the hydrogen being transferred. The larger charge transfer in turn is due to the smaller gap between the energies of furfural HOMO and the zeolite LUMO in Li-Sn-BEA, compared to both Na-Sn-BEA and K-Sn-BEA. In a similar way, we also compare the catalytic activity of tetravalent metal centers (Sn, Zr, and Hf) substituted into BEA and find in the order Zr ≥ Hf > Sn, based on activation energies. Finally we investigate statistically which property of the reactants is a suitable descriptor for an approximative prediction of the reaction rate in order to be able to quickly screen promising catalytic materials for this reaction.

A computational study of the catalytic aerobic epoxidation of propylene over the coordinatively unsaturated metal-organic framework Fe3(btc)2: formation of propylene oxide and competing reactions.

The aerobic epoxidation of propylene over the metal-organic framework Fe3(btc)2 (btc = 1,3,5-benzentricarboxylate) as catalyst has been investigated by means of density functional calculations. The mechanisms of the reaction towards propylene oxide, carbonylic products (acetone and propanal) and a pi-allyl radical were investigated to assess the efficiency of Fe3(btc)2 for the selective formation of propylene oxide. Propylene oxide and carbonylic products are formed on Fe3(btc)2 by proceeding via propyleneoxy intermediates in the first step. Subsequently, the intermediates can then either be transformed to propylene oxide by way of ring closure of the intermediate or to the carbonylic compounds of propanal and acetone via 1,2-hydride shift. The results show that the formation of propylene oxide is favored over the formation of carbonylic products mainly due to the activation barriers being 2-3 times smaller. The activation barriers for the formation of the propyleneoxy intermediates on the Fe3(btc)2 catalyst for the first and second reaction cycle are also lower than the barriers obtained for the formation of the pi-allyl radical that acts as the precursor to combustion products. On the basis of these computational results, we therefore expect a high catalytic selectivity of the Fe3(btc)2 catalyst with respect to the formation of propylene oxide. We also compared the catalytic activities of Fe3(btc)2 and Cu3(btc)2. The activation energy of the rate-determining step is almost 2 times lower for Fe3(btc)2 than that for Cu3(btc)2, due to a larger charge transfer from the catalytic site to the O2 molecule in the case of Fe3(btc)2.

Theoretical study of CO2 hydrogenation into formic acid on Lewis acid zeolites.

Conversion of carbon dioxide (CO2) to more valuable chemicals is nowadays receiving increasing attention from an environmental and industrial point of view. Herein, we computationally investigated CO2 hydrogenation to formic acid on Lewis acid zeolites by means of density functional theory (DFT) with the M06-L functional. The reaction proceeds in two steps, hydrogenation of CO2 to form the formate intermediate and hydrogen-abstraction to form formic acid. A defect zeolite seems to be favored over a perfect one, leading to its low rate determining step barrier of 5.2 kcal mol-1. We also considered the effect of the zeolite frameworks and found that the catalytic activities are in the order Sn-ZSM-5 > Sn-BEA > Sn-FAU. Finally, we performed catalytic activity screenings of tetravalent metals (Ge, Zr and Hf) substituted into the defect Sn-ZSM-5 zeolite. The order Hf > Zr > Sn > Ge was found based on the rate determining step activation energy. The difference in activation energy can be explained by the difference in charge transfer from the catalytic site to the reacting molecules.

Coordinatively Unsaturated Metal-Organic Frameworks M3(btc)2 (M = Cr, Fe, Co, Ni, Cu, and Zn) Catalyzing the Oxidation of CO by N2O: Insight from DFT Calculations.

The oxidation of CO by N2O over metal-organic framework (MOF) M3(btc)2 (M = Fe, Cr, Co, Ni, Cu, and Zn) catalysts that contain coordinatively unsaturated sites has been investigated by means of density functional theory calculations. The reaction proceeds in two steps. First, the N-O bond of N2O is broken to form a metal oxo intermediate. Second, a CO molecule reacts with the oxygen atom of the metal oxo site, forming one C-O bond of CO2. The first step is a rate-determining step for both Cu3(btc)2 and Fe3(btc)2, where it requires the highest activation energy (67.3 and 19.6 kcal/mol, respectively). The lower value for the iron compound compared to the copper one can be explained by the larger amount of electron density transferred from the catalytic site to the antibonding of N2O molecules. This, in turn, is due to the smaller gap between the highest occupied molecular orbital (HOMO) of the MOF and the lowest unoccupied molecular orbital (LUMO)  of N2O for Fe3(btc)2 compared to Cu3(btc)2. The results indicate the important role of charge transfer for the N-O bond breaking in N2O. We computationally screened other MOF M3(btc)2 (M = Cr, Fe, Co, Ni, Cu, and Zn) compounds in this respect and show some relationships between the activation energy and orbital properties like HOMO energies and the spin densities of the metals at the active sites of the MOFs.

Halogen substitutions leading to enhanced oxygen evolution and oxygen reduction reactions in metalloporphyrin frameworks.

The oxygen evolution and oxygen reduction reactions (OER and ORR, respectively) are important in the field of renewable and clean energy, particularly for hydrogen production and fuel cells. These applications have so far been limited because of the high price of the catalysts and the energy loss due to overpotentials. Hence, non-precious metal catalysts with high efficiency toward the OER/ORR are desirable. In this work, we employ density functional theory (DFT) calculations to study the OER/ORR on metalloporphyrin and halogenated metalloporphyrin frameworks. The free energies of the reaction intermediates, including OH, O and OOH, were measured on 14 metal sites (Cr, Mn, Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd, Ag, Ir, Pt and Au) of the metalloporphyrin frameworks. Adsorption free energy relations were found and used to establish the reaction trend. The group 9 metals, namely Co, Rh and Ir, turn out to be potential candidates for both the OER and ORR because they provide intermediate free energies close to those of an ideal catalyst. The substitution of halogen atoms at the beta-positions of the metalloporphyrins of group 9 metals modifies the adsorption free energies of the intermediates so that they become closer to the ideal values and in turn reduce the OER and ORR overpotentials. After functionalization, Co-Por-F provides the lowest ORR overpotential and reduces the OER overpotential, approaching the value for an expensive Ir catalyst. Analysis of the electronic structure shows that controlling the d-band splitting by chemical manipulation of the single active site catalyst can be the key to enhancing the efficiency of these reactions.

Theoretical study on the reaction mechanism of hydrogenation of furfural to furfuryl alcohol on Lewis acidic BEA zeolites: effects of defect structure and tetravalent metals substitution.

Furfural acquired from agricultural sources is receiving extensive attention in the petrochemical industry as it offers an alternative route to generate more valuable hydrocarbon compounds. Herein, we investigate the furfural hydrogenation to furfuryl alcohol catalyzed by Lewis acidic BEA zeolites at the molecular level by means of the M06-L density functional theory. The mechanistic pictures in the catalytic procedure are revealed. The possible reaction pathways are considered to proceed via either concerted or stepwise mechanisms. With the contribution of zeolite oxygen bridging for the H-H splitting, the rate determining step activation barrier for the stepwise mechanism is 14.7 kcal mol-1 lower than that for the concerted mechanism. The stepwise reaction therefore seems to be favored compared to the concerted one. The catalytic effect of the defect zeolite framework on the stepwise mechanism is also investigated. The activation energy for the stepwise rate-determining step over this site is significantly lower than the corresponding step over the perfect one by 14.1 kcal mol-1. Finally, the catalytic activity of tetravalent metal centers (Sn, Ge, Zr and Hf) substituted in BEA is also preliminarily compared and it is found to follow the order of Hf > Zr > Sn > Ge based on activation energies and the reaction rate. The difference in the activation energy can be traced back to the difference in the charge transfer from the catalytic site to the adsorbed molecules.

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.