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

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 VOPO4 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 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 ions. We examined several pathways for the subsequent functionalization of nbutane to maleic anhydride and found that the overall barrier does not exceed 21.7 kcal/mol. V phosphorus oxide (VPO) is a unique catalyst that converts n-butane to maleic anhydride (MA) with high selectivity (60−70%). Because of the industrial importance of this catalyst (used to produce 500 kton of MA annually), numerous efforts have been devoted to understanding this unique catalytic reaction in order to improve the yield of MA from the current value of 50%. Such improvements in yield would bring enormous economic and environmental benefits. We believe that a major impediment here is the lack of atomistic-level reaction mechanisms to focus attention on the atomistic origins of the critical barriers for the selective and unselective processes. Here we report progress toward this goal. Despite numerous experimental and a few theoretical studies, the mechanism of the catalytic oxidation reaction remains under debate. Some suggest that the oxidation follows an olefinic route, during which reaction intermediates desorb, whereas others suggest that the reaction follows an alkoxide route, in which n-butane is anchored on the surface after the initial C−H activation and desorbs only when MA has been formed. Besides the uncertainty in the mechanism, such basic questions as which sites on the surface are active centers for the initial n-butane C−H bond cleavage remain unanswered. Here we report density functional theory (DFT) calculations aimed at elucidating the critical steps for this catalytic system. We propose that the first step of the reaction is the oxidation of (VO)2P2O7 (denoted as VOPO, the major component of VPO) by gaseous O2 to form a metastable phase of VOPO4. The surface of VOPO4 contains highly reactive phosphine−oxo bonds [PO(1)] that extract H from the methylene groups of n-butane with a barrier of only 13.5 kcal/mol, consistent with the experimental data. Our findings contrast with previous studies suggesting that the active centers are either surface V− O bonds or adsorbed O2, 5 with the assumption that pyrophosphate or phosphate is an innocent linked ligand. Moreover, we propose a full sequence of reactions for the conversion of n-butane to MA involving the consecutive oxidation of n-butane to 2-butene, butadiene, 2,5-dihydrofuran, crotonlactone, and MA with an overall barrier of only 21.7 kcal/mol. This constitutes the first computational investigation to identify the active sites as the O(1)P bonds of VOPO4 and to propose a full reaction mechanism for the VPOcatalyzed oxidation of n-butane all the way to MA. This mechanism should provide valuable hints for optimization of these catalysts. We first carried out a full optimization of the VOPO bulk structure (Figure 1), including atomic positions and cell parameters under orthorhombic symmetry [see the Supporting Information (SI)]. The VOPO surface model used in this work was prepared by cutting the DFT-optimized bulk structure parallel to the ab plane, cleaving the P−O−P interlayer bonds. It is generally agreed that the active sites involved in n-butane oxidation to MA are located on this surface. We capped the cleaved sites with either H atoms or OH groups to maintain V and P in the +4 and +5 oxidation states, respectively, while keeping the surface unit cell neutral. The total number of atoms in the unit cell is 58, which includes eight V, eight P, four OH, Received: November 26, 2012 Published: February 24, 2013 Figure 1. (a, c) Top and (b, d) side views of (a, b) bulk and (c, d) surface VOPO. Blue, yellow, red, and white balls represent V, P, O, and H, respectively. Communication