Molecular and Electronic Structures, Brönsted Basicities, and Lewis Acidities of Group VIB Transition Metal Oxide Clusters

Abstract

The structures and properties of transition metal oxide (TMO) clusters of the group VIB metals, (MO(3))(n) (M = Cr, Mo, W; n = 1-6), have been studied with density functional theory (DFT) methods. Geometry optimizations and frequency calculations were carried out at the local and nonlocal DFT levels with polarized valence double-zeta quality basis sets, and final energies were calculated at nonlocal DFT levels with polarized valence triple-zeta quality basis sets at the local and nonlocal DFT geometries. Effective core potentials were used to treat the transition metal atoms. Two types of clusters were investigated, the ring and the chain, with the ring being lower in energy. Large ring structures (n > 3) were shown to be fluxional in their out of plane deformations. Long chain structures (n > 3) of (CrO(3))(n) were predicted to be weakly bound complexes of the smaller clusters at the nonlocal DFT levels. For M(6)O(18), two additional isomers were also studied, the cage and the inverted cage. The relative stability of the different conformations of M(6)O(18) depends on the transition metal as well as the level of theory. Normalized and differential clustering energies of the ring structures were calculated and were shown to vary with respect to the cluster size. Brönsted basicities and Lewis acidities based on a fluoride affinity scale were also calculated. The Brönsted basicities as well as the Lewis acidities depend on the size of the cluster and the site to which the proton or the fluoride anion binds. These clusters are fairly weak Brönsted bases with gas phase basicities comparable to those of H(2)O and NH(3). The clusters are, however, very strong Lewis acids and many of them are stronger than strong Lewis acids such as SbF(5). Brönsted acidities of M(6)O(19)H(2) and M(6)O(18)FH were calculated for M = Mo and W and these compounds were shown to be very strong acids in the gas phase. The acid/base properties of these TMO clusters are expected to play important roles in their catalytic activities.

Pt/Al2O3 Catalyzed 1,3‐Propanediol Formation from Glycerol using Tungsten Additives

“…Possibly, a glycerol tungstic acid ester is formed as an intermediate, which, in combination with tungsten's favorable acidic properties, leads to 13PD selectivity. [23][24][25] Interestingly, 20 to 40 % 13PD formation is observed if a tungsten containing acidic additive is used (Figure 2 c-e) in combination with a Pt based catalyst. Moreover, only very small amounts of 12PD were observed.…”

Cluster reactivity experiments: Employing mass spectrometry to investigate the molecular level details of catalytic oxidation reactions

“…Studies of small tungsten oxide clusters, therefore, should provide direct insight into the behavior of the bulk-phase material. The structures and acidic properties of (WO 3 ) n (n ϭ 1-6) were examined computationally, revealing that these clusters are weak Brønsted bases and strong Lewis acids (51). The high Lewis acidity of tungsten oxide clusters should facilitate the strong bonding of nucleophilic reactants such as alcohols and CO.…”

First principles study of electronic structure for cubane-like and ring-shaped structures of M4O4, M4S4 clusters (M = Mn, Fe, Co, Ni, Cu)

“…1,2 Such fullerene-like or graphene-like structures have also been synthesized recently using nanoparticles of transition metal (TM) oxides or TM dichalcogenide compounds. [3][4][5][6][7][8] Note that bulk TM oxides have already been studied extensively because of their wide range of applications from catalysis, organometallic, surface science to high temperature superconductivity, magnetic materials and so on. Furthermore, the TM oxides and chalcogenides being of semiconducting type in general, also lead to some obvious applications in sensors, electronics and solar cells.…”