(678d) Composition Function Relations for Methanol Dehydration and Oxidation On Acid-Redox Bifunctional Polyoxometalate Clusters

Keggin polyoxometalate (POM) clusters are oxides with known structure and diverse compositions, which makes them suitable for developing rigorous composition-function relations via theoretical calculation of relevant descriptors of redox and acid properties. Here, we use bifunctional acid-redox Mo and W-based POM clusters with different central atoms (H₃PMo₁₂O₄₀, H₄SiMo₁₂O₄₀) and addenda atoms (H_3+nPV_n=1-3Mo_12-nO₄₀, H₄PV₁W₁₁O₄₀), and their salts, supported on silica, to explore the effect of local redox properties, measured as H-atom addition energies (HAE), on methanol oxidative dehydrogenation (ODH)  rates and to extend previous studies of the effect of acid strength of charge-balancing protons, measured as deprotonation energy (DPE), on methanol dehydration turnover rates [1] to Mo-POM systems.

Methanol oxidation involves Mars van Krevelen (MVK) cycles and kinetic coupling of elementary steps that reduce and oxidize metal centers in oxide catalysts with H-abstraction by lattice oxygen from the substrate as the commonly observed rate limiting step. Measured H-abstraction rate constants were slightly lower for HSiMo than HPMo, but were insensitive to V-substitution of addenda atoms on acid form Mo-POM.  These results are inconsistent with about 0.3 eV lower UV-vis edge energies measured on HPV_n=1-3Mo_12-n than HPMo, and contradict the general expectation that V-susbtitution would lead to improved redox activity, but are consistent with previous results for unsupported and silica supported POM [2].  These findings were further examined using DFT based exploration of redox pathways which show that direct H-abstraction form a molecularly adsorbed methanol is more favorable on POM than dissociative pathway. H-abstraction occurs vial late TS and their calculated kinetic isotope effects are consistent with the measured values. The calculated TS energy, as expected, changes linearly with the local HAE value of the abstracting POM O-atom, because HAE reflects redox properties of POM by introducing electrons at metal centers through lattice O-atom (O*) via processes similar to those required for rate limiting H-abstraction steps. Consistent with the measured ODH rates, the HAE values were insensitive to V-substitution.  Spin-density maps of unpaired d-electron introduced due to reduction via H-addition shows that V atom in acid-form V-substituted POM is not reduced since the d-electron is located at a Mo atom. Similar calculations on anionic POM without the protons, however, show that V-atom is reduced and show more negative HAE values compared to HPMo.  These results indicate that commonly observed improved redox properties of VO_x groups can largely disappear in acid-form POM because of the additional proton introduced along with V-substitution stays with the VO_x groups as a V⁵⁺ replacing the Mo⁶⁺ needs this additional proton to satisfy its surrounding O ligands and the electronic effects of this acidic proton diminishes redox capability of VO_x.  Such effects are likely to be less significant in liquid-phase reactions with dissociated protons and anions or for salt-form POM which do not contain such protons.  HAE values emerge as relevant and accurate descriptor of redox property.

Re-oxidation steps were not rate limiting under conditions used in this work and the corresponding rate constant could not be obtained directly through steady-state experiments. Relative differences in re-oxidation rate constants for different compositions were assessed via UV-vis measurements of extents of reduction of catalysts during catalysis, and via time-scales of transients in extent of reduction due to changes in reactant concentrations [3]. The observed differences in re-oxidation were examined theoretically by exploring molecular O₂ adsorption and plausible subsequent O₂ activation routes involving O-atom migration, direct re-oxidization of vacancy pair, and reactions of one oxygen atom in bound peroxide structures with the organic substrate.

First-order rate constant (k_mono) for methanol dehydration decreases exponentially with increasing DPE due to decrease in stability of ion-pair transition states (TS) involved in acid catalyzed reactions which recover only a fraction of the electrostatic part of DPE [1]. The measured values of k_mono for Mo-POM lie below the k_mono versus DPE correlation previously developed for W-POM because they undergo reduction under reaction conditions which increases DPE relative to the values obtained using DFT calculations of intact clusters and, because of a larger covalent part of DPE in Mo-POM that is not recovered in ion pair transition states.  Contribution of these two effects to the difference between the trends for the two addenda atoms is estimated, by calculation DPE for reduced clusters, and by calculations of H⁺POM^- and TS⁺POM^- classical electrostatic interactions using integration over of DFT derived cation and anion charge distributions along with rigorous assessment of interaction energies within the framework of thermochemical cycle relating DPE to TS energy [1].

References

1. Carr R., Neurock M., Iglesia E., J. Catal. 278, 78 (2011).
2. Liu H., Iglesia E., J Phys. Chem. B, 107, 10840 (2003);   Brückman K., Tatibouët, J.M., Che, M., Serwicka E., Haber J., J. Catal., 139, 455 (1993).
3. Argyle, M.D., Chen, K., Iglesia, E., Bell, A.T., J. Phys. Chem. B, 109, 2414 (2005).