Mechanistic Details and Reactivity Descriptors in Oxidation and Acid Catalysis of Methanol

Abstract

Acid and redox reaction rates of CH 3 OH-O 2 mixtures on polyoxometalate (POM) clusters, together with isotopic, spectroscopic, and theoretical assessments of catalyst properties and reaction pathways, were used to define rigorous descriptors of reactivity and to probe the compositional effects for oxidative dehydrogenation (ODH) and dehydration reactions. 31 P-MAS NMR, transmission electron microscopy and titrations of protons with di-tert-butylpyridine during catalysis showed that POM clusters retained their Keggin structure upon dispersion on SiO 2 and after use in CH 3 OH reactions. The effects of CH 3 OH and O 2 pressures and of Dsubstitution on ODH rates show that C−H activation in molecularly adsorbed CH 3 OH is the sole kinetically relevant step and leads to reduced centers as intermediates present at low coverages; their concentrations, measured from UV−vis spectra obtained during catalysis, are consistent with the effects of CH 3 OH/O 2 ratios predicted from the elementary steps proposed. First-order ODH rate constants depend strongly on the addenda atoms (Mo vs W) but weakly on the central atom (P vs Si) in POM clusters, because C−H activation steps inject electrons into the lowest unoccupied molecular orbitals (LUMO) of the clusters, which are the d-orbitals at Mo 6+ and W 6+ centers. H-atom addition energies (HAE) at O-atoms in POM clusters represent the relevant theoretical probe of the LUMO energies and of ODH reactivity. The calculated energies of ODH transition states at each O-atom depend linearly on their HAE values with slopes near unity, as predicted for late transition states in which electron transfer and C−H cleavage are essentially complete. HAE values averaged over all accessible O-atoms in POM clusters provide the appropriate reactivity descriptor for oxides whose known structures allow accurate HAE calculations. CH 3 OH dehydration proceeds via parallel pathways mediated by late carbenium-ion transition states; effects of composition on dehydration reactivity reflect changes in charge reorganizations and electrostatic forces that stabilize protons at Brønsted acid sites.

Performance of density functional theory for describing hetero‐metallic active‐site motifs for methane‐to‐methanol conversion in metal‐exchanged zeolites

“…Importantly, it is recognized that HAEs can be used as a descriptor for alkane C‐H activation barriers . Functionals like OLYP and τ‐HCTH that do poorly for predicting the HAE will also likely do poorly for predicting the C‐H activation barriers.…”

“…The energies associated with 1a & 1b is the hydrogen addition energy (HAE). The HAE has been shown to be a good descriptor for the CH activation barriers of alkanes and methanol on a variety of systems . As such accurate prediction of the HAEs with DFT is useful for future investigation of MMC by hetero‐metallic oxide clusters.…”

Multifunctional Photostable Nanocomplex of ZnO Quantum Dots and Avobenzone via the Promotion of Enolate Tautomer

“…TD‐DFT calculations were used to determine excitation energies and oscillator strengths for 200 excited states based on 800 initial guesses. Calculated absorption spectra were obtained by broadening each excitation delta function using a Gaussian function of width 0.2 eV followed by summation over all Gaussians (Figure b) . Vibrational frequencies and intensities were obtained from frequency calculations within Gaussian 09 program.…”

“…Calculated absorption spectra were obtained by broadening each excitation delta function using a Gaussian function of width 0.2 eV followed by summation over all Gaussians (Figure 2b). [43] Vibrational frequencies and intensities were obtained from frequency calculations within Gaussian 09 program. IR spectra were obtained by broadening the IR intensity delta functions using Lorentzian functions of width 20 cm −1 .…”

Developments in the Atomistic Modelling of Catalytic Processes for the Production of Platform Chemicals from Biomass

“…The most relevant catalytic properties are related to the ability of Mo centers to cycle between different oxidation states, which can be modulated by the lattice, or the scaffold in molecular catalysts. Thus, reducibility, calculated as hydrogen addition energy (HAE),, was found as the single descriptor of the activity, which shows a volcano‐shape dependence on reducibility, see Figure . Finally, a better catalyst able to perform the reaction four times faster was identified.…”