The mechanism of n-pentane partial oxidation on VPO and VPBiO catalysts

The paper addresses the mechanism of maleic (MA), citraconic (CA) and phthalic (PhA) anhydride formation in the oxidation of n-pentane on VPO catalysts. In the key experiments oxidation of n-butane and n-pentane and reaction of its products with 1,3-butadiene were studied in the two reactor system working on-line. A large quantity of PhA appeared in the products when 1,3-butadiene was introduced into the n-butane–air reaction mixture at the inlet of the second reactor. This study clearly shows that the basic route of the PhA formation in n-pentane oxidation on VPMeO catalyst is the Diels–Alder reaction between diolefin C₄ and MA.     

Influence of Rare-Earth and Bimetallic Promoters on Various VPO Catalysts for Partial Oxidation of n-Butane

Vanadium phosphorous oxide (VPO) catalyst was prepared using dihydrate method and tested for the potential use in selective oxidation of n-butane to maleic anhydride. The catalysts were doped by La, Ce and combined components Ce + Co and Ce + Bi through impregnation. The effect of promoters on catalyst morphology and the development of acid and redox sites were studied through XRD, BET, SEM, H₂-TPR and TPRn reaction of n-butane/He. Addition of rare-earth element to VPO formulation and drying of catalyst precursor by microwave irradiation increased the fall width at half maximum (FWHM) and reduced the crystallite size of the Vanadyl hydrogen phosphate hemihydrate (VOHPO₄ · 1/2 H₂O, VHP) precursor phase and thus led to the production of final catalysts with larger surface area. The Ce doped VPO catalyst which, assisted by the microwave heating method, exhibited the highest surface area. Moreover, the addition of promoters significantly increased catalyst activity and selectivity as compared to undoped VPO catalyst in the oxidation reaction of n-butane. The H₂-TPR and TPRn reaction profiles showed that the highest amount of active oxygen species, i.e., the V⁴⁺–O^? pair, was removed from the bimetallic (Ce + Bi) promoted catalyst. This pair is responsible for n-butane activation. Furthermore, based on catalytic test results, it was demonstrated that the catalyst promoted with Ce and Bi (VPOD1) was the most active and selective catalyst among the produced catalysts with 52% reaction yield. This suggests that the rare earth metal promoted vanadium phosphate catalyst is a promising method to improve the catalytic properties of VPO for the partial oxidation of n-butane to maleic anhydride.     

How the Yield of Maleic Anhydride in n-Butane Oxidation,Using VPO Catalysts,was Improved Over the Years

Key patents for the oxidation of n-butane to maleic anhydride (MA) using V/P mixed metal oxides (VPO) have been analyzed to evidence the important parameters needed to optimize the catalytic behaviour. The important aspects for the optimization of the catalyst performance are the preparation of the precursor (VO)HPO₄·0.5H₂0, its activation to form the active catalyst VO₂P₂O₇ and a small amount of VOPO₄, the methodology of the addition of promoters, and the shaping of the catalyst. Even small, sustainable improvements in the MA yield (>1 %) achieved by improved, modified or optimized catalysts are significant on an industrial scale with the world production of MA being a respectable 2.7 Million tons/year. Although substantial improvements in MA yield have been achieved industrially over the past 40 years by improving the VPO catalyst composition and by optimizing process operations; the MA process remains, as practiced today, one of the least efficient industrial selective oxidation processes. Therefore, a huge incentive exists in the field to improve the MA catalyst and process, which led us to search for clues towards this end by analyzing the pertinent patent literature, as reported here.     

Preparation of Novel Composite VPO/Fumed Silica Catalyst for Partial Oxidation of n-Butane

By applying fumed SiO₂ and the deposition–precipitation method based on organic medium, the composite VPO/fumed SiO₂ catalysts were first prepared and tried for partial oxidation of n-butane to maleic anhydride. In the temperature range of 653–693 K the fumed SiO₂-based catalysts not only showed good activity but also maintained sufficiently high MA selectivity in comparison to some conventional supported VPO catalysts. As an example, the catalyst with 30% VPO content showed butane conversion of 60% and MA selectivity of 58 mol% at 673 K. The turnover rates of low loading samples are found to be higher than that of high loading sample as well as unsupported catalyst. Besides the unique interaction which may exist between VPO component and the fumed SiO₂ material, co-existence of dominant (VO)₂P₂O₇ and minor VOPO₄ in these non-equilibrated catalysts may be favorable for MA formation. Moreover, introducing the additive of polyethylene glycol (PEG) in the preparation medium can obviously enhance the dispersion of VPO component and hence lead to a more selective catalyst.     

Effect of Dehydration of VOPO4⋅2H2O on the Preparation and Reactivity of Vanadium Phosphate Catalyst for the Oxidation of n-Butane

The effect of drying of VOPO₄2H₂O on the preparation of vanadium phosphate catalysts for the selective oxidation of n-butane to maleic anhydride is described and discussed. It is found that partially dehydrated samples of the dihydrate containing small amounts of _I-VOPO₄ are formed when the material is initially dried. The presence of this impurity leads to a final catalyst containing trace amounts of _I-VOPO₄ in combination with (VO)₂P₂O₇ and this combination leads to a catalyst with a higher activity but with a lower selectivity to maleic anhydride. The drying stage is also found to influence the surface area and intrinsic activity of the activated catalyst.     

Dependence of n-Butane Activation on Active Site of Vanadium Phosphate Catalysts

The nature and the role of oxygen species and vanadium oxidation states on the activation of n-butane for selective oxidation to maleic anhydride were investigated. Bi–Fe doped and undoped vanadium phosphate catalysts were used a model catalyst. XRD revealed that Bi–Fe mixture dopants led to formation of α_II-VOPO₄ phase together with (VO)₂P₂O₇ as a dominant phase when the materials were heated in n-butane/air to form the final catalysts. TPR analysis showed that the reduction behaviour of Bi–Fe doped catalysts was dominated by the reduction peak assigned to the reduction of V⁵⁺ species as compared to the undoped catalyst, which gave the reduction of V⁴⁺ as the major feature. An excess of the oxygen species (O^2?) associated with V⁵⁺ in Bi–Fe doped catalysts improved the maleic anhydride selectivity but significantly lowering the rate of n-butane conversion. The reactive pairing of V⁴⁺-O^? was shown to be the centre for n-butane activation. It is proposed that the availability and appearance of active oxygen species (O^?) on the surface of vanadium phosphate catalyst is the rate determining step of the overall reaction.     

Ammoxidation of 3-picoline to nicotinonitrile over vanadium phosphorus oxide-based catalysts

Ammoxidation of 3-picoline to nicotinonitrile was investigated on vanadium phosphorus oxide (VPO), VPO/SiO₂ and additive atom (Cu, Zr, Mn and Co) incorporated VPO catalysts under atmospheric pressure and at 673 K. For the purpose of comparison a conventional V₂O₅–MoO₃/Al₂O₃ catalyst was also studied under identical conditions. These catalysts were characterized by means of X-ray diffraction, electron spin resonance, infrared, ammonia chemisorption and BET surface area methods. The VPO-based catalysts show better performance than the V₂O₅–MoO₃/Al₂O₃ catalyst. Further, the VPO/SiO₂ and VPO catalysts exhibit better conversion and product selectivities than the additive-containing VPO catalysts. Better activity of VPO and VPO/SiO₂ catalysts was related to their high active surface area, higher surface acidity and lower oxidation state of vanadium. The redox couple between (VO)₂P₂O₇ (V⁴⁺) and α_I-VOPO₄ (V⁵⁺) phases appears to be responsible for the ammoxidation activity of VPO catalysts. © 1998 SCI.     

A Study on VPO Specimen Supported on Aluminum-Containing MCM-41 for Partial Oxidation of n-Butane to MA

Dispersed vanadium–phosphorus oxide species supported on Al-MCM-41 with different vanadium loadings have been synthesized for the first time for partial oxidation of butane to MA. It was found that the VPO species was dispersed over the Al-MCM-41 support material, both in the internal channel and on the external surface. With increasing vanadium loading, n-butane conversion increased but MA selectivity decreased considerably under the same reaction conditions. At lower conversions (<30%), rather high MA selectivity (ca. 70%) can be achieved on the low loading sample. Compared with the amorphous structure of large pore SiO₂ support, the unique structure of the MCM-41 and the incorporated Al³⁺ in the framework do have an impact on the reaction behavior of the supported VPO specimen. The chemical nature of the supported VPO species and the interaction between the applied VPO species and the support was found to vary notably with the content of vanadium in the sample and likewise affected the related physico-chemical characteristics and their reaction behaviors.     

In situ laser raman spectroscopy during sequential oxidizing and reducing conditions for a vanadium-phosphorous-oxide catalyst

A VPO catalyst prepared in an organic medium has been studied by in situ laser Raman spectroscopy for n-butane oxidation to maleic anhydride. Data could be obtained at low laser power and brief collection times. Raman characterization during continuous flow (steady-state) studies revealed that (VO)₂P₂O₇ was present. Sequential oxidizing (10% O₂ in N₂) and reducing (2% n-butane in N₂) conditions were explored at 350°C and 400°C. These cycling (unsteady-state) operations revealed that formation of V(5⁺) phases was enhanced during oxidizing conditions. The intensity of Raman bands due to (VO)₂P₂O₇ increased during reducing conditions.     

Selective oxidation of n-butane to maleic anhydride in fluid bed reactors: detailed kinetic investigation and reactor modelling

The kinetics of n-butane oxidation to maleic anhydride over a commercial V-P-O catalyst for fluid bed reactors was investigated systematically and modelled according to a scheme of eight reactions, involving also the combustion of both n-butane and maleic anhydride to CO and CO₂, as well as the formation and the combustion of acetic and acrylic acids. A one-dimensional, heterogeneous (two-phase) fluid bed reactor model incorporating the independently derived kinetic scheme was successfully validated on a predictive basis against conversion, selectivity and steam production data collected in a full-scale MA production plant.     

Effect of promoters for n-butane oxidation to maleic anhydride over vanadium–phosphorus‐oxide catalysts: comparison with supported vanadia catalysts

The oxidation of n-butane to maleic anhydride was investigated over model Nb‐, Si‐, Ti‐, V‐, and Zr‐promoted bulk VPO and supported vanadia catalysts. The promoters were concentrated in the surface region of the bulk VPO catalysts. For the supported vanadia catalysts, the vanadia phase was present as a two‐dimensional metal oxide overlayer on the different oxide supports (TiO₂, ZrO₂, Nb₂O₅, Al₂O₃, and SiO₂). No correlation was found between the electronegativity of the promoter or oxide support cation and the catalytic properties of these two catalytic systems. The maleic anhydride selectivity correlated with the Lewis acidity of the promoter cations and oxide supports. Both promoted bulk VPO and supported vanadia catalysts containing surface niobia species were the most active and selective to maleic anhydride. These findings suggest that the activation of n-butane on both the bulk and supported vanadia catalysts probably requires both surface redox and acid sites, and that the acidity also plays an important role in controlling further kinetic steps of n-butane oxidation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Redox processes of surface of vanadyl pyrophosphate in relation to selective oxidation ofn-butane

Oxidation and reduction processes of the surface of vanadyl pyrophosphate ((VO)₂P₂O₇) have been studied by means of Raman spectroscopy, X-ray photoelectron spectroscopy, thermogravimetry, and “micro-pulse” reaction ofn-butane in relation to the selective oxidation ofn-butane. Oxidation of (VO)₂P₂O₇ by O₂ was carried out to controlled extents by changing the reaction temperature and period. When (VO)₂P₂O₇ was oxidized at 733 K for 2 h, about one layer of the surface of (VO)₂P₂O₇ was oxidized to the X₁ phase, which was reported previously by the authors, as deduced from the amount of O₂-uptake and spectroscopic changes. Reductions of the surface X₁ phase and β-VOPO₄ were carried out by a “micro-pulse” reaction with n-butane at 733 K. The reduction byn-butane changed the surface X₁ phase to (VO)₂P₂O₇ and, at the same time, gave maleic anhydride with selectivities ranging from 13 to 40%. The selectivity was less than 10% in the case of β-VO)₂PO₄. These results suggest that the redox cycle between the X₁ phase and (VO)₂P₂O₇ in the surface layer is involved in the catalytic oxidation ofn-butane to maleic anhydride.     

Transient kinetics of n‐butane partial oxidation at elevated pressure

A transient Mars‐van Krevelen type kinetic model was developed for n‐butane partial oxidation over vanadyl pyrophosphate (VPP) catalyst. The model validity was verified over a relatively wide range of redox feed compositions as well as higher reactor pressure (410 kPa). Oxygen and n‐butane conversion increased with higher pressure while maleic anhydride (MA) selectivity decreased by as much as 20%. However, the overall MA yield was enhanced by up to 30%. High pressure maintains the catalyst in a higher oxidation state (as long as there is sufficient oxygen in the gas phase) and as a consequence, the catalytic activity is improved together with MA yield. High pressure also affects the redox reaction rates and activation energies. © 2012 Canadian Society for Chemical Engineering     

Ordered Mesostructured Mixed Metal Oxides: Microporous VPO Phases for n-Butane Oxidation to Maleic Anhydride

High surface area (～250 m²/g) microporous VPO phases were prepared from the VPO/surfactant mesophases by a two-step postsynthesis treatment, consisting of a Soxhlet extraction and thermal activation in reducing atmosphere at 400 °C. The resultant microporous VPO phases were studied in the partial oxidation of n-butane to maleic anhydride.     

Anisotropic oxidation of crystallites of vanadyl pyrophosphate

Anisotropic oxidation of crystallites of vanadyl pyrophosphate ((VO)₂P₂O₇) has been demonstrated by Raman spectroscopy with samples having different microstructures. Oxidation of these samples by O₂ produced X₁ phase,- and-VOPO₄ phases. The relative peak intensity of the X₁ phase to the other phases correlated well with the ratio of the (100) plane to the side planes (surface area-basis). This correlation showed that the (100) plane was oxidized to X₁ phase and the side planes to- and-VOPO₄. For example, thin plate-like (VO)₂P₂O₇, of which the (100) fraction is 98%, was oxidized almost exclusively to X₁ phase. But when it was fractured into small plates to increase the side planes and then oxidized,- and-VOPO₄ were detected in addition to the X₁ phase. These results are consistent with our previous conclusion that the (100) plane of (VO)₂P₂O₇ is selective, but side planes are non-selective for catalytic oxidation ofn-butane.     

Influence of structural properties of catalysts at various stages of selective oxidation: from catalyst preparation to catalytic reactors

Solid oxides selective in mild oxidation exhibit a definite crystal morphology. Each crystal face displays a surface crystal field which rules the transformation of the reactant to a given product. The catalytic metastability of the surface which has to be maintained in the operating conditions prevailing in reactors depends on the reactivity of the bulk, because of oxygen diffusion process. Application to VPO, the catalyst for oxidation of n-butane to maleic anhydride, gives the opportunity to examine these properties in the series preparation–activation–catalysis–ageing, in relation to the type of catalytic reactor. This revised version was published online in August 2006 with corrections to the Cover Date.     

Hydrocarbon Selective Oxidation on Vanadium Phosphorus Oxide Catalysts: Insights from Electronic Structure Calculations

The ability of the vanadium phosphorus oxide (VPO) catalyst to selectively activate n-butane and then perform subsequent selective oxidation to maleic anhydride was investigated using electronic structure calculations. Both active site cluster models and periodic surface models, including explicit consideration of surface relaxation and hydration, led to the same qualitative conclusions about the reactivity of the (VO)₂P₂O₇ (1 0 0) surface in substrate adsorption and oxidation. Density functional theory (DFT) reactivity indices and Density of States (DOS) plots show that, whether stoichiometric or phosphorus-enriched, strained or relaxed, bare or hydrated, covalent reactivity at the (1 0 0) surface is controlled by vanadium species, their dual acid–base attack giving selective activation of n-butane via methylene C–H bond cleavage. 1-butene is predicted to chemisorb at the surface using a π-cation complex, the strength of which makes 1-butene an unlikely intermediate in the production of maleic anhydride from n-butane. Coordinatively-unsaturated surface P–O and in-plane P–O–V oxygen species are the most nucleophilic surface oxygens, which may explain the surface-enrichment in phosphorus always seen in industrial catalysts for maleic anhydride synthesis and also recent in situ microscopy images of surface oxygen transfer to n-butane. The resistance of the maleic anhydride selective oxidation product to further transformation was shown to be dependent on its orientation in the active site, and simulation of surface hydration indicated that dissociative adsorption of water may serve to regenerate the catalyst, replenishing its supply of selective nucleophilic oxygen species for mild oxidation.     

A generalized kinetic model for the partial oxidation of n-butane to maleic anhydride under aerobic and anaerobic conditions

The kinetics of n-butane oxidation for the production of maleic anhydride (MA) over a commercial VPO catalyst has been investigated under aerobic and anaerobic conditions and at a temperature range of 400-. A kinetic model that can be applied under both aerobic and anaerobic conditions is presented. The model considers three types of oxygen: adsorbed oxygen, surface lattice oxygen and sub-surface lattice oxygen. Both adsorbed and surface lattice oxygen are considered active for the production of maleic anhydride and CO_x.     

VPO催化剂上正丁烷选择氧化制顺酐反应动力学

对一种工业钒磷复合氧化物 (VPO)催化剂上正丁烷选择氧化制顺酐的反应动力学特性进行了系统的实验研究 .在空速大于 30 0 0h^-1、温度 35 0～ 44 5℃ ,进口总压约为 0 .2MPa和原料气组成为丁烷 0 .5 %～ 3.2 %、氧 5 .5 %～ 2 6 .5 %的条件下测得 45套本征反应动力学数据 .按照三角形反应网络处理实验数据 ,经参数估计、模型识别和统计检验得到了诸反应速率的表达式及相关参数数值     

The Reduction-Coupled Oxo Activation (ROA) Mechanism Responsible for the Catalytic Selective Activation and Functionalization of n-Butane to Maleic Anhydride by Vanadium Phosphate Oxide

We report here the results of density functional theory quantum mechanical (QM) studies of the detailed chemical mechanism underlying the n-butane selective oxidation to form maleic anhydride (MA) on vanadyl pyrophosphate [(VO)₂P₂O₇] and vanadyl phosphate [VOPO₄] surfaces. This QM-derived mechanism differs substantially from previous suggestions but is in excellent agreement with key experimental observations. We find that the O(1)=P bond of the oxidized X1 phase of the VOPO₄ surface is the active site for initiating the VPO chemistry, by extracting the H from the n-butane C–H bond. This contrasts sharply with previous suggestions, all of which involved the V=O bonds. The ability of O(1)=P to cleave alkane C–H bonds arises from a new unique mechanism that decouples the proton transfer and electron transfer components of this H atom transfer reaction. We find that the juxtaposition of a highly reducible V⁺⁵ next to the P=O bond but coupled via a bridging oxygen dramatically enhances the activity of the P=O bond to extract the proton from an alkane, while simultaneously transferring the electron to the V to form V⁺⁴. This Reduction-Coupled Oxo Activation (ROA) mechanism had not been known prior to these QM studies, but we believe that it may lead to a new strategy in designing selective catalysts for alkane activation and functionalization, and indeed it may be responsible for the selective oxidation by a number of known mixed metal oxide catalysts. To demonstrate the viability of this new ROA mechanism, we examine step by step the full sequence of reactions from n-butane to MA via two independent pathways. We that find that every step is plausible, with a highest reaction barrier of 21.7 kcal/mol.