The magnetic and electronic structure of vanadyl pyrophosphate from density functional theory

We have studied the magnetic structure of the high symmetry vanadyl pyrophosphate ((VO)_(2)P_(2)O)7, VOPO), focusing on the spin exchange couplings, using density functional theory (B3LYP) with the full three-dimensional periodicity. VOPO involves four distinct spin couplings: two larger couplings exist along the chain direction (a-axis), which we predict to be antiferromagnetic, J_(OPO) = −156.8 K and J_O = −68.6 K, and two weaker couplings appear along the c (between two layers) and b directions (between two chains in the same layer), which we calculate to be ferromagnetic, J_layer = 19.2 K and J_chain = 2.8 K. Based on the local density of states and the response of spin couplings to varying the cell parameter a, we found that J_(OPO) originates from a super-exchange interaction through the bridging –O–P–O– unit. In contrast, J_O results from a direct overlap of 3d_(x^2 − y^2) orbitals on two vanadium atoms in the same V_(2)O_8 motif, making it very sensitive to structural fluctuations. Based on the variations in V–O bond length as a function of strain along a, we found that the V–O bonds of V–(OPO)_(2)–V are covalent and rigid, whereas the bonds of V–(O)_(2)–V are fragile and dative. These distinctions suggest that compression along the a-axis would have a dramatic impact on J_O, changing the magnetic structure and spin gap of VOPO. This result also suggests that assuming J_O to be a constant over the range of 2–300 K whilst fitting couplings to the experimental magnetic susceptibility is an invalid method. Regarding its role as a catalyst, the bonding pattern suggests that O_2 can penetrate beyond the top layers of the VOPO surface, converting multiple V atoms from the +4 to +5 oxidation state, which seems crucial to explain the deep oxidation of n-butane to maleic anhydride

A homolytic oxy-functionalization mechanism: intermolecular hydrocarbyl migration from M–R to vanadate oxo

A new mechanism for generating C–O bonds from metal-hydrocarbyls involving homolytic, intermolecular migration of the hydrocarbyl group to a vanadium oxo is reported. Responsible for the C–O bond in phenol formed by the reaction of OVCl_3 with HgPh_2, it may provide air-regenerable metal oxos a role in aerobic alkane oxidations

The Dual-Phase Mechanism for the Catalytic Conversion of n-Butane to Maleic Anhydride by the Vanadyl Pyrophosphate Heterogeneous Catalyst

Industrial production of maleic anhydride (MA) from n-butane relies on the vanadyl pyrophosphate (VPO) catalyst. Improving VPO’s selectivity and activity could have enormous economic and environmental impact, but efforts have been impeded by uncertainties regarding the active phases and atomistic mechanism of the VPO catalyst. We report here a plausible 15-step mechanism taking n-butane to MA with energetics computed using hybrid density functional theory calculations on periodic models of the surface layers. We find that the P═O group on the X1 phase is solely responsible for butane activation. The P═O group is made active by the reduction of a nearby vanadium atom, a so-called reduction-coupled oxo-activation. However, we show that a catalyst consisting only of the X1 phase would not be selective because of several highly exothermic steps. Instead, we show that the more stable α1 phase can catalyze the formation of MA after initial activation, thus proposing and validating a dual-phase mechanism that takes butane to MA. Our new mechanism inspires the development of a more selective VPO catalyst containing small X1 regions highly separated by α_1 surfaces

The importance of grand-canonical quantum mechanical methods to describe the effect of electrode potential on the stability of intermediates involved in both electrochemical CO_2 reduction and hydrogen evolution

The rational design of electrocatalysts to convert CO_2 to fuel requires predicting the effect of the electrode potential (U) on the binding and structures of the intermediates involved in CO_2 electrochemical reduction (CO2ER). In this study, we used grand-canonical quantum mechanics (GC-QM) to keep the potential constant during the reactions (rather than keeping the charge constant as in standard QM) to investigate the effect of Uon adsorption free energies (ΔGs) of 14 CO_2ER intermediates on Cu(111) as well as the intermediates involved in the competitive hydrogen evolution reaction (HER). In contrast to most previous theoretical studies where ΔGs were calculated under constant charge (= 0, neutral), we calculated ΔGs under constant potential (U = 0.0, −0.5, −1.0, and −1.5 V_(SHE)). By comparing the ΔGs calculated under constant U (= 0.0 V_(SHE)) to those calculated under constant charge, we found differences up to 0.22 eV which would change the rates at 298 K by a factor of about 5300. In particular we found that the adsorption of species with a C O functional group (i.e., *COOH, *CO, and *CHO) strengthened by up to 0.16 eV as U became more negative by 1 V, whereas the adsorption of –O– species (i.e., *OH, *OCH3, *COH, and *CHOH) weakened by up to 0.20 eV. For the (111) index surfaces of Cu, Au, Ag, Ir, Ni, Pd, Pt and Rh, we investigated the effect of U on the reaction free energy (ΔG) at pH = 0 for the crucial elementary steps for CO_2ER (*CO + (H+/e−) → *CHO, ΔG = (ΔG_(*CHO) – ΔG_(*CO)) + eU) and HER (* + (H+/e−) → *H, ΔG = ΔG_(*H) + eU. Our results indicated that the influence of U on (ΔG_(*CHO) – ΔG_(*CO)) was metal dependent. In contrast, the energy for converting a proton in solution to H* on the surface, ΔG_(*H), was barely affected by U(for the studied metals). Overall we found substantial differences (MAD > 0.18 eV) between the ΔGs calculated under U = −1.0 V_(SHE) (relevant to experiments) and those calculated under constant charge (= 0, neutral) common to most theoretical investigations. Therefore, we strongly recommend application GC-QM to obtain accurate energetics for CO_2ER

The para-substituent effect and pH-dependence of the organometallic Baeyer–Villiger oxidation of rhenium–carbon bonds

We studied the Baeyer–Villiger (BV) type oxidation of phenylrhenium trioxide (PTO) by H2O2 in the aqueous phase using Quantum Mechanics (density functional theory with the M06 functional) focusing on how the solution pH and the para-substituent affect the Gibbs free energy surfaces. For both PTO and MTO (methylrhenium trioxide) cases, we find that for pH > 1 the BV pathway having OH− as the leaving group is lower in energy than the one involving simultaneous protonation of hydroxide. We also find that during this organometallic BV oxidation, the migrating phenyl is a nucleophile so that substituting functional groups in the para-position of phenyl with increased electron-donating character lowers the migration barrier, just as in organic BV reactions. However, this substituent effect also pushes electron density to Re, impeding HOO− coordination and slowing down the reaction. This is in direct contrast to the organic analog, in which para-substitution has an insignificant influence on 1,2-addition of peracids. Due to the competition of the two opposing effects and the dependence of the resting state on pH and concentration, the reaction rate of the organometallic BV oxidation is surprisingly unaffected by para-substitution

The Mechanism of Alkane Selective Oxidation by the M1 Phase of Mo–V–Nb–Te Mixed Metal Oxides: Suggestions for Improved Catalysts

We report here first principles predictions (density functional theory with periodic boundary conditions) of the structures, mechanisms, and activation barriers for the catalytic activation and functionalization of propane by the M1 phase of the Mitsubishi-BP America generation of Mo–V–Nb–Te–O mixed metal oxide (MMO) catalysts. Our calculations show that the reduction-coupled oxo activation (ROA) principle, which we reported at Irsee VI to play the critical role for the selective oxidation of n-butane to maleic anhydride by vanadium phosphorous oxide, also plays the critical role for the MMO activation of propane, as speculated during Irsee VI. However for MMO, this ROA principle involves Te=O and V rather than P=O and V. The ability of the Te=O bond to activate the propane CH bond depends sensitively upon the number of V atoms that are coupled through a bridging O to the Te=O center. Based on this ROA mechanism, we suggest synthetic procedures aimed at developing a single phase MMO catalyst with dramatically improved selectivity for ammoxidation. We also suggest a modified single phase composition suitable for simultaneous oxidative dehydrogenation of ethane and propane to ethene and propene, respectively, which is becoming more important with the increase in petroleum fracking. Moreover, we also suggest some organometallic molecules that activate alkane CH bonds through the ROA principle

The Critical Role of Phosphate in Vanadium Phosphate Oxide for the Catalytic Activation and Functionalization of n-Butane to Maleic Anhydride

We used density functional theory to study the mechanism of n-butane oxidation to maleic anhydride on the vanadium phosphorus oxide (VPO) surface. We found that O(1)═P on the V^(V)OPO_4 surface is the active center for initiating the VPO chemistry through extraction of H from alkane C–H bonds. This contrasts sharply with previous suggestions that the active center is either the V–O bonds or else a chemisorbed O_2 on the (V^(IV)O)_(2)P_(2)O_7 surface. The ability of O(1)═P to cleave alkane C–H bonds is due to its strong basicity coupled with large reduction potentials of nearby V^V ions. We examined several pathways for the subsequent functionalization of n-butane to maleic anhydride and found that the overall barrier does not exceed 21.7 kcal/mol

Functionalization of Rhenium Aryl Bonds by O-Atom Transfer

Aryltrioxorhenium (ArReO_3) has been demonstrated to show rapid oxy-functionalization upon reaction with O-atom donors, YO, to selectively generate the corresponding phenols in near quantitative yields. (18)^O-Labeling experiments show that the oxygen in the products is exclusively from YO. DFT studies reveal a 10.7 kcal/mol barrier (Ar = Ph) for oxy-functionalization with H_2O_2 via a Baeyer-Villiger type mechanism involving nudeophilic attack of the aryl group on an electrophilic oxygen of YO coordinated to rhenium

In Silico Design of Highly Selective Mo-V-Te-Nb-O Mixed Metal Oxide Catalysts for Ammoxidation and Oxidative Dehydrogenation of Propane and Ethane

We used density functional theory quantum mechanics with periodic boundary conditions to determine the atomistic mechanism underlying catalytic activation of propane by the M1 phase of Mo-V-Nb-Te-O mixed metal oxides. We find that propane is activated by Te═O through our recently established reduction-coupled oxo activation mechanism. More importantly, we find that the C–H activation activity of Te═O is controlled by the distribution of nearby V atoms, leading to a range of activation barriers from 34 to 23 kcal/mol. On the basis of the new insight into this mechanism, we propose a synthesis strategy that we expect to form a much more selective single-phase Mo-V-Nb-Te-O catalyst

Computational design of a pincer phosphinito vanadium ((OPO)V) propane monoxygenation homogeneous catalyst based on the reduction-coupled oxo activation (ROA) mechanism

We propose the vanadium bis(2-phenoxyl)phosphinite pincer complex, denoted (OPO)V, as a low temperature water-soluble catalyst for monoxygenation of propane to isopropanol with functionalization and catalyst regeneration using molecular oxygen. We use DFT study to predict that the barrier for (OPO)V to activate the secondary hydrogen of propane is ΔG‡ = 25.2 kcal/mol at 298K, leading to isopropanol via the new reduction-coupled oxo activation (ROA) mechanism. We then show that reoxidation by dioxygen to complete the cycle is also favorable with ΔG‡ = 6.2 kcal/mol at 298K. We conclude that (OPO)V represents a promising homogeneous catalyst for the monoxygenation of propane and other alkanes (including ethane), warranting experimental validation