Density Functional Theory Study of Deoxydehydration Reaction by TiO₂-Supported Monomeric and Dimeric Molybdenum Oxide Catalysts

Abstract

The development of efficient heterogeneous catalysts for converting biomass into value-added chemical compounds remains at the forefront of catalysis research. Deoxydehydration (DODH) reaction that can transform vicinal hydroxy groups with the cis-configuration to the corresponding CC bond in a single step is one of the promising techniques, and molybdenum oxide catalysts supported on TiO2 have been reported as an effective catalyst using hydrogen as a reducing agent. Here, using density functional theory calculations, structures of monomeric and dimeric molybdenum oxide catalysts supported on anatase TiO2(101) have been determined, and we decipher the reaction mechanisms of the conversion of 1,4-anhydroerythritol to 2,5-dihydrofuran over these catalysts as a model reaction. We have found that MoO3 and Mo2O5 are the most stable structures for monomeric and dimeric species that exhibit the oxidation states of MoVI and MoV–MoVI, respectively, under the experimental conditions. For monomeric species, it is rather difficult to catalyze DODH reaction due to the instability for MoIV species and also the higher barrier for the C–O bond scission for MoV or MoVI species. For dimeric species, structures with the oxidation state of MoIV–MoV or MoV–MoV that is found in the form of Mo2O4 exhibit promising energy profiles in terms of stability and energy barrier (∼1.0 eV) for the C–O bond dissociation. Considering the experimental facts that MoIV species is responsible for the DODH reaction and Mo–Mo bond is present, the MoIV–MoV structure could be the plausible active species. Our findings would provide useful information for the catalyst design using earth-abundant and less-expensive metal-based catalysts for the DODH reaction.

Kinetic Modeling of Erythritol Deoxydehydration and Consecutive Hydrogenation over the ReOx-Pd/CeO2 Catalyst

“…Thus, the reason for the inactivity of the aggregated Re species is important. In fact, for Mo-based catalysts, all monomeric Mo complexes, , polyoxomolybdate, and dimeric MoO x species on TiO 2 support , can work as active catalysts in DODH. There has been no convincing theoretical explanation for the low activity of aggregated Re species.…”

Insights into the Molecular Structure of MoOx Catalysts via Static and Transient Raman Experimentation

“…Additionally, a careful look at the Raman spectra reveals the presence of a broad, but rather weak, band at around 900 cm –1 . This feature has been the focus of several literature reports due to its distinct behavior under the reaction conditions, and it is believed that it is associated with the stretching modes of Mo–O–X configurations, where X can be either another molybdenum atom (bridging bond in oligomers) and/or the cation of the support (anchoring bond). ,,,,,,,,,,,,,, Structural implications of this band are discussed in more detail in later sections.…”

“…Furthermore, the experimental results also hint that the MoO peak, which is associated with polymeric species, should locate at a higher frequency than the monomers. It is noteworthy that previous DFT studies , also conclude the existence of stable Mo (V) and Mo (VI) species on the TiO 2 surface even though the Mo (VI) structures are embedded in polymeric forms. In this study, it is found that despite being much more distorted, the structure of monomeric Mo (VI) has a MoO vibration frequency well in line with one of the component peaks under MoO, observed experimentally.…”

“…Particularly, both Raman and infrared (IR) spectroscopies have revealed that the arrangement of dispersed oxo-molybdenum species can vary greatly, especially when deposited on different metal oxide supports. 18 O isotopic labeling has also been applied in conjunction with in situ Raman and IR studies to distinguish between mono-oxo, dioxo, or trioxo configurations on various surfaces. ,− The results from the isotopic exchange experimental framework, along with density functional theory (DFT) calculations, have shown that the dioxo species are favored on MoO 3 /SiO 2 , ,− the mono-oxo tetragonal pyramidal configuration is dominant on γ-alumina, − while the mono-oxo tetrahedral arrangement is the most stable configuration on TiO 2 . − …”

Theoretical studies on the mechanism of molybdenum-catalysed deoxydehydration of diols

“…in homogeneous and heterogeneous systems. [31][32][33] The DODH of diols catalysed by ammonium heptamolybdate was investigated by Fristrup and co-workers using density functional theory (DFT). 25,26 In these studies, the Mo species was simplified as MoO 3 to maintain the neutrality of the computational model system, and the reductants were the diol itself (1,2-propanediol or erythritol) or i PrOH, while phosphanes were not considered.…”