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

Abstract

We used density functional theory to study the mechanism of n-butane oxidation to maleic anhydride on the vanadium phosphorus oxide (VPO) surface. We found that O(1)═P on the V(V)OPO4 surface is the active center for initiating the VPO chemistry through extraction of H from alkane C-H bonds. This contrasts sharply with previous suggestions that the active center is either the V-O bonds or else a chemisorbed O2 on the (V(IV)O)2P2O7 surface. The ability of O(1)═P to cleave alkane C-H bonds is due to its strong basicity coupled with large reduction potentials of nearby V(V) ions. We examined several pathways for the subsequent functionalization of n-butane to maleic anhydride and found that the overall barrier does not exceed 21.7 kcal/mol.

Dual-Phase Mechanism for the Catalytic Conversion of n-Butane to Maleic Anhydride by the Vanadyl Pyrophosphate Heterogeneous Catalyst

“…A long-standing puzzle was how a surface site could be active enough to break the strong butane C-H bond. Our previous two studies, denoted in this paper as CG 2,3 , used the Perdew-Burke-Ernzerhof 4 (PBE) flavor of density functional theory (DFT) with periodic boundary conditions (PBC) to investigate butane activation on (VO) 2 P 2 O 7 and on α 1 -, α 2 -, and X1-VOPO 4 . CG found that only the P=O motif on (VO) 2 P 2 O 7 α 2 -VOPO 4 α 1 -VOPO 4 X1-VOPO 4 5 the X1 phase could activate butane with a low activation barrier, E a = 13.6 kcal/mol.…”

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

“…[3] This leads to Ea,min = 35.5 kcal/mol, better than those for the αI-and αII-VOPO4 surfaces, but still not reactive enough to lead to explain the experiments. [15] This leaves us in the untenable position that no V site on any of the reduced or oxidized surfaces of VPO can explain the observed activation of n-butane.…”

“…[2,3] We first focused on identifying the reaction center responsible for activating the methylene C-H bond of n-butane. Only the [001] surface of VOPO is considered, since experiments have suggested that it plays an important role in the catalysis.…”

“…[3] This means that the electron and proton of the transferred hydrogen atom are separated to be hosted by two very different motifs: O=P for the proton and V for the electron, which are separated by at least 4 Å in the X1-VOPO4 structure. This is a unique new mechanism for activating C-H bonds, never previously suggested.…”

“…[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.…”