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

Abstract

Industrial production of maleic anhydride (MA) from n-butane relies on the vanadyl pyrophosphate (VPO) catalyst. Improving VPO's selectivity and activity could have enormous economic and environmental impact, but efforts have been impeded by uncertainties regarding the active phases and atomistic mechanism of the VPO catalyst. We report here a plausible 15 step mechanism taking n-butane to MA with energetics computed using hybrid density functional theory calculations on periodic models of the surface layers. We find that the P=O group on the X1 phase is solely responsible for butane activation. The P=O group is made active by the reduction of a nearby vanadium atom, a so-called reduction-coupled oxo-activation.However, we show that a catalyst consisting only of the X1 phase would not be selective due to several highly exothermic steps. Instead, we show that the more stable α 1 phase can catalyze the formation of MA after initial activation, thus proposing and validating a dual-phase mechanism that takes butane to MA. Our new mechanism inspires the development of a more selective VPO catalyst containing small X1 regions highly separated by α 1 surfaces.

Operando Electrical Conductivity and Complex Permittivity Study on Vanadia Oxidation Catalysts

“…The fact that the real part of the permittivity (Figure 2) does not change with the flow rate is taken as indication that the catalyst has retained the ability to buffer the increased demand for electrons from its bulk without a respective change of its surface, where one expects the location of the unchanged polarization. Such behavior requires inhomogeneity both lateral at the surface 38 (locations of adsorption/activation of n-butane and of attaching of activated oxygen) and as gradient into the bulk (electron withdrawal from the V 4+ reservoir of the VPP structure to the V x O y /P sites of n-butane conversion).…”

“…The two catalysts were chosen such as to allow also the elucidation of a possible role of the main group element phosphorus and thus to verify the simplification that for fundamental insight into selective oxidation the binary oxide may be sufficient. Recent work suggests that this simplification may be incorrect and that the main group element has a critical function in catalysis other than creating ,, the structural motif of a nondensely packed crystal structure.…”

Selective Oxidation of n-Butane over Vanadium Phosphate Based Catalysts: Reaction Network and Kinetic Analysis

“…If the product pattern and kinetic aspects of a catalyst under certain reaction conditions can be regarded as a sensitive fingerprint for the nanostructure of the active site of a catalyst, then the apparent similarities between VWPO and VPP point at a similarly structured active domain. This aspect should be discussed in terms of the novel mechanistic reaction model suggested recently by Goddard et al − The ROA mechanism is based on the redox-transformation of reduced (VO) 2 P 2 O 7 and oxidized X1-VOPO 4 phases. Such transformation cannot easily be transferred to the α II phase of our VWPO catalyst, although the surface composition and structure of both VPP and VWPO systems are still under debate and unknown, respectively.…”

“…A Mars–van Krevelen mechanism as proven by the participation of bulk oxygen in the reaction led to the concept of a riser reactor, which can further increase the yield of MAN, however, on the cost of complex engineering and attrition or friction issues to be solved . Recent theoretical studies by Goddard et al introduce a novel reaction model, i.e., a reduction-coupled oxo activation (ROA) mechanism responsible for the catalytic selective activation and functionalization of n -butane to MAN over a P-based active site. − …”

Selective Oxidation of n‐Butane over Vanadium‐Phosphorus Oxide: Oxygen Activation and Dynamics

“…Furthermore, several orthophosphate by‐phases were discussed to be involved into the reaction mechanism. The most recent proposal for a reaction mechanism involves the participation of P=O bonds …”