The magnetic and electronic structure of vanadyl pyrophosphate from density functional theory

Abstract

We have studied the magnetic structure of the high symmetry vanadyl pyrophosphate ((VO) 2 P 2 O 7 , VOPO), focusing on the spin exchange couplings, using density functional theory (B3LYP) with the full three-dimensional periodicity. VOPO involves four distinct spin couplings: two larger couplings exist along the chain direction (a-axis), which we predict to be antiferromagnetic, J OPO = À156.8 K and J O = À68.6 K, and two weaker couplings appear along the c (between two layers) and b directions (between two chains in the same layer), which we calculate to be ferromagnetic, J layer = 19.2 K and J chain = 2.8 K. Based on the local density of states and the response of spin couplings to varying the cell parameter a, we found that J OPO originates from a super-exchange interaction through the bridging -O-P-O-unit. In contrast, J O results from a direct overlap of 3d x 2 À y 2 orbitals on two vanadium atoms in the same V 2 O 8 motif, making it very sensitive to structural fluctuations. Based on the variations in V-O bond length as a function of strain along a, we found that the V-O bonds of V-(OPO) 2 -V are covalent and rigid, whereas the bonds of V-(O) 2 -V are fragile and dative. These distinctions suggest that compression along the a-axis would have a dramatic impact on J O , changing the magnetic structure and spin gap of VOPO. This result also suggests that assuming J O to be a constant over the range of 2-300 K whilst fitting couplings to the experimental magnetic susceptibility is an invalid method. Regarding its role as a catalyst, the bonding pattern suggests that O 2 can penetrate beyond the top layers of the VOPO surface, converting multiple V atoms from the +4 to +5 oxidation state, which seems crucial to explain the deep oxidation of n-butane to maleic anhydride.

The Critical Role of Phosphate in Vanadium Phosphate Oxide for the Catalytic Activation and Functionalization of n-Butane to Maleic Anhydride

“…7 We found that the band structures, projected density of states, and Mulliken charge distribution are very similar for the two forms, and hence we expect the surface chemistry to be similar. 8 Additionally since all the chemistry is initiated by the O(1)=P active site, we calculated the binding energy (D H ) of a free hydrogen atom to O(1)=P using the low symmetry, ambient pressure form of oxidized VOPO (X 1 -VOPO 4 ) and compared the result to that using the high symmetry, high pressure form of oxidized VOPO. We found the two D H 's are quite similar (86.1 kcal/mol vs. 84.3 kcal/mol).…”

The Mechanism of Alkane Selective Oxidation by the M1 Phase of Mo–V–Nb–Te Mixed Metal Oxides: Suggestions for Improved Catalysts

“…The nature of chemical bonds in VOPO was explored by investigating the variations in V-O bond lengths as a functional of strain along the a axis through compressing and stretching the cell parameter a by 3 and 5%, respectively. [1] Figure 2. Polyhedral views of the crystal structure of the high-symmetry form of VOPO.…”

“…Based on these asymmetric variations of the V-O bond lengths, we concluded that V-O(2) is a rigid covalent single bond while V-O(3) is a fragile dative bond (Scheme 1(b)). [1] This valence bond description plays an important role in understanding the catalysis. …”

“…[1][2][3] In section 4, we use quantum mechanics (QM) to examine the activation of propane by the M1 phase, where we use the improved experimental analysis of the partial occupations of the various sites. [4] Here we examine how the activation energy for propane depends on the specific distributions over the sites.…”

A novel one-dimensional chain built of vanadyl ions and pyrazine-2,5-dicarboxylate

“…There are a few vanadyl chain compounds of vanadium-phosphate type which are relevant to catalytic applications. [26][27][28][29][30] Examples of carboxylatebridged vanadyl chains are scarce, and they include formate, 31 acetate, 32,33 glycolate 32,33 and benzoate 34 as bridges. Theoretical calculations have suggested that the band gap in such systems may be tuned from semiconducting to metallic states, broadening the application possibilities of hybrid inorganicorganic compounds.…”