Quantum Chemical Study of Mechanisms for Oxidative Dehydrogenation of Propane on Vanadium Oxide

C-H bond activation in light alkanes: a theoretical perspective

Alkanes are the major constituents of natural gas and crude oil, the feedstocks for the chemical industry. The efficient and selective activation of C-H bonds can convert abundant and low-cost hydrocarbon feedstocks into value-added products. Due to the increasing global demand for light alkenes and their corresponding polymers as well as synthesis gas and hydrogen production, C-H bond activation of light alkanes has attracted widespread attention. A theoretical understanding of C-H bond activation in light hydrocarbons via density functional theory (DFT) and microkinetic modeling provides a feasible approach to gain insight into the process and guidelines for designing more efficient catalysts to promote light alkane transformation. This review describes the recent progress in computational catalysis that has addressed the C-H bond activation of light alkanes. We start with direct and oxidative C-H bond activation of methane, with emphasis placed on kinetic and mechanistic insights obtained from DFT assisted microkinetic analysis into steam and dry reforming, and the partial oxidation dependence on metal/oxide surfaces and nanoparticle size. Direct and oxidative activation of the C-H bond of ethane and propane on various metal and oxide surfaces are subsequently reviewed, including the elucidation of active sites, intriguing mechanisms, microkinetic modeling, and electronic features of the ethane and propane conversion processes with a focus on suppressing the side reaction and coke formation. The main target of this review is to give fundamental insight into C-H bond activation of light alkanes, which can provide useful guidance for the optimization of catalysts in future research.

Single Vanadium Atoms Anchored on Graphitic Carbon Nitride as a High-Performance Catalyst for Non-oxidative Propane Dehydrogenation

In comparison with oil-based cracking technologies, the on-purpose dehydrogenation of propane (PDH) is a more eco-friendly and profitable approach to produce propylene. By means of density functional theory calculations, the present work reveals that the single vanadium (V) atom anchored on graphitic carbon nitride (V₁/g-C₃N₄) may serve as a promising single-atom catalyst for non-oxidative PDH with industrially practical activity, selectivity, and thermal stability. The high activity of V₁/g-C₃N₄ for PDH is attributed to the low-coordinated 3d orbitals of single V atoms, while the propylene selectivity is originated from the inhibition of the di-σ binding mode of propylene on the single V atoms. This work provides a guideline to design and screen out promising single-atom catalysts for selective dehydrogenation of alkanes.

Tools for Prescreening the Most Active Sites on Ir and Rh Clusters toward C-H Bond Cleavage of Ethane: NBO Charges and Wiberg Bond Indexes

B3LYP calculations were carried out to study the insertion of iridium (Ir) and rhodium (Rh) clusters into a C-H bond of ethane, which is often the rate-limiting step of the catalytic cycle of oxidative dehydrogenation of ethane. Our previous research on Ir catalysis correlates the diffusivity of the lowest unoccupied molecular orbital of the Ir clusters and the relative activities of the various catalytic sites. The drawback of this research is that the molecular orbital visualization is qualitative rather than quantitative. Therefore, in this study on C-H bond activation by the Ir and Rh clusters, we conducted analyses of natural bond orbital (NBO) charges and Wiberg bond indexes (WBIs), both of which are not only quantitative but also independent of the basis sets. We found strong correlation between the NBO charges, the WBIs, and the relative activities of the various catalytic sites on the Ir and Rh clusters. Analyses of the NBO charges and the WBIs provide a fast and reliable means of prescreening the most active sites on the Ir and Rh clusters and potentially on other similar transition-metal clusters that activate the C-H bonds of ethane and other light alkanes.

Quantum Tunneling Tautomer of N,N-Dimethyl-p-toluidine Dehydrogenates Identified by Deep-UV Laser Ionization Mass Spectroscopy

Utilizing customized deep-ultraviolet laser ionization mass spectroscopy, here we report a finding of remarkable dehydrogenation product of N,N-dimethyl-p-toluidine (DMT). The DMT dehydrogenates find comparable mass abundance with the DMT molecule ions showing decent stability at the loss of one electron and one H atom from the DMT molecule. First-principles calculation reveals that the dehydrogenation most readily occurs at the N-connected methyl group. Furthermore, at the removal of a hydrogen atom, a neighboring hydrogen atom on the other methyl come close and interact with the dehydrogenated methylene group, pertaining to C-H···C weak interactions which give rises to a resonant structure (C···H-C) on a basis of hydrogen atom quantum tunneling effect. The quantum tunneling tautomer of DMT dehydrogenates displays reversible donor-acceptor charge-transfer interactions as demonstrated by natural bonding orbital analysis and vibrational spectroscopy. It is worth noting that the novel dehydrogenation product was also observed for another small organic molecule o-phenylenediamine, which bears two neighboring amino groups and the subsequent dehydrogenation product pertains to resonant structures of N-H···N and N···H-N. The deep ultraviolet laser not only produces fragmentation-free mass spectra for such small organic molecules but also tailors the interesting quantum tunneling tautomer from such specific molecules.

Catalytic function of VOx/Al2O3 for oxidative dehydrogenation of propane: support microstructure-dependent mass transfer and diffusion

The intrinsic effect of the support microstructure on the catalytic function of VO_x/Al₂O₃ in the oxidative dehydrogenation of propane (ODHP) was studied.

SOMC grafting of vanadium oxytriisopropoxide (VO(OiPr)3) on dehydroxylated silica; analysis of surface complexes and thermal restructuring mechanism

VO(OⁱPr)₃ was grafted on highly dehydroxylated silica by a surface organometallic chemistry approach and its thermal evolution was analyzed with support of DFT calculations.

Constrained-Orbital Density Functional Theory. Computational Method and Applications to Surface Chemical Processes

We present a method for performing density-functional theory (DFT) calculations in which one or more Kohn-Sham orbitals are constrained to be localized on individual atoms. This constrained-orbital DFT (CO-DFT) approach can be used to tackle two prevalent shortcomings of DFT: the lack of transparency with regard to the governing electronic structure in large (planewave based) DFT calculations and the limitations of semilocal DFT in describing systems with localized electrons or a large degree of static correlation. CO-DFT helps to address the first of these issues by decomposing complex orbital transformations occurring during elementary chemical processes into simpler and more intuitive transformations. The second issue is addressed by using the CO-DFT method to generate configuration states for multiconfiguration Kohn-Sham calculations. We demonstrate both of these applications for elementary reaction steps involved in the oxygen evolution reaction.

Insights into the effects of steam on propane dehydrogenation over a Pt/Al2O3 catalyst

Steam and its dissociated species can be involved in the elementary steps of propane dehydrogenation through two mechanisms.

Size-dependent catalytic activity of supported vanadium oxide species: oxidative dehydrogenation of propane

Possible reaction pathways for the oxidative dehydrogenation of propane by vanadium oxide catalysts supported on silica are examined by density functional theory. Monomeric and dimeric vanadium oxide species are both considered and modeled by vanadyl-substituted silsesquioxanes. The reaction proceeds in two subsequent steps. In a first step, hydrogen abstraction from propane by a vanadyl (O═V) group yields a propyl radical bound to a HOV(IV) surface site. Propene is formed by a second hydrogen abstraction, either at the same vanadia site or at a different one. V(V)/V(IV) redox cycles are preferred over V(V)/V(III) cycles. Under the assumption of fast reoxidation, microkinetic simulations show that the first step is rate-determining and yields Arrhenius barriers that are lower for dimers (114 kJ/mol at 750 K) than for monomers (124 kJ/mol). The rate constants predicted for a mixture of monomers and dimers are 14% larger (750 K) than for monomers only, although the increase remains within experimental uncertainty limits. Direct calculations of energy barriers also yield lower values for dimeric species than for monomeric ones. Reactivity descriptors indicate that this trend will continue also for larger oligomers. The size distribution of oligomeric species is predicted to be rather statistical. This, together with the small increase in the rate constants, explains that turnover frequencies observed for submonolayer coverages of vanadia on silica do not vary with the loading within the experimental uncertainty limits.

Subnanometre platinum clusters as highly active and selective catalysts for the oxidative dehydrogenation of propane

Small clusters are known to possess reactivity not observed in their bulk analogues, which can make them attractive for catalysis. Their distinct catalytic properties are often hypothesized to result from the large fraction of under-coordinated surface atoms. Here, we show that size-preselected Pt(8-10) clusters stabilized on high-surface-area supports are 40-100 times more active for the oxidative dehydrogenation of propane than previously studied platinum and vanadia catalysts, while at the same time maintaining high selectivity towards formation of propylene over by-products. Quantum chemical calculations indicate that under-coordination of the Pt atoms in the clusters is responsible for the surprisingly high reactivity compared with extended surfaces. We anticipate that these results will form the basis for development of a new class of catalysts by providing a route to bond-specific chemistry, ranging from energy-efficient and environmentally friendly synthesis strategies to the replacement of petrochemical feedstocks by abundant small alkanes.