Vanadium-Phosphorus-Oxides: from Fundamentals of n-Butane Oxidation to Synthesis of New Phases

BY VADIMV. GULIANTS AND MOISES A. CARREON

1 Introduction

The abundance and low cost of light alkanes have generated in recent years considerable interest in their oxidative catalytic conversion to olefins, oxygenates and nitriles in the petroleum and petrochemical industries [1-4]. Rough estimates place the annual worth of products that have undergone a catalytic oxidation step at $20-40 billion worldwide [4]. Among these, the 14-electron selective oxidation of n-butane to maleic anhydride (2,5-furandione) on vanadium-phosphorus-oxide (VPO) catalysts is one of the most fascinating and unique catalytic processes [4,5]:

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It is the only industrial process of a selective vapor-phase oxidation of an alkane that uses dioxygen [5]. The demand for maleic anhydride comes principally from the manufacture of unsaturated polyester resins, agricultural chemicals, food additives, lubricating oil additives, and pharmaceuticals [6].

Bergman and Frisch [7] disclosed in 1966 that selective oxidation of n-butane was catalyzed by the VPO catalysts, and since 1974 n-butane has been increasingly used instead of benzene as the raw material for maleic anhydride production due to lower price, high availability in many regions and low environmental impact [8]. At present more than 70% of maleic anhydride is produced from n-butane [6]. However, productivity from n-butane is lower than in the case of benzene due to lower selectivities to maleic anhydride at higher conversions and somewhat lower feed concentrations (< 2 mol. %) used to avoid ?ammability of a process stream. Under typical industrial conditions (2 mol. % n-butane in air, 673-723K, and space velocities of 1100-2600 h-1) the selectivities [9] for fixed- bed production of maleic anhydride from n-butane are 67-75 mol. % at 70-85 % n-butane conversion [10]. Another unique feature of the VPO catalysts is that no support is used in partial oxidation of n-butane. Many studies of n-butane oxidation on the VPO catalysts indicated that crystalline vanadyl(IV) pyrophosphate, (VO)2P2O7, is present in the most selective catalysts, e.g. [10-12]. However, the VPO system is characterized by facile formation and interconversion of a number of crystalline and amorphous VIII, VIV and VV phosphates [10]. Various research groups detected these phases in the VPO catalysts and proposed different models of the active and selective VPO phase and surface sites in n-butane oxidation [10-13].

The VPO catalysts are prepared by thermal dehydration of its precursor, vanadyl(IV) hydrogen phosphate hemihydrate, VOHPO4•0.5H2O. The catalytic performance of the VPO catalysts depends on (i) the method of VOHPO4•0.5H2O synthesis (types and concentrations of reagents, reducing agents and solvents, the reduction temperature and synthesis duration), (ii) the procedures for activation and conditioning of the precursor at high temperature and (iii) the nature of metal promoters. These factors important for understanding the catalytic behavior of the VPO system in n-butane oxidation were discussed previously in a number of excellent early reviews [10-14]. Therefore, in this chapter we briefly go over the conclusions of early studies and discuss in greater detail recent Fndings with emphasis on fundamental aspects of VPO catalysis, such as the mechanism of VOHPO4•0.5H2O formation and its transformation to active and selective VPO catalysts, the mechanism of n-butane oxidation, the role of promoters and the synthesis of new VPO phases. It is expected that new fundamental insights into molecular structure and catalytic function of this unique catalytic system will lead to the design of improved mixed metal oxide catalysts for selective oxidation of light alkanes.

2 Synthesis of VOHPO4•0.5H2O Precursor

There is a general agreement in the VPO literature [4, 10, 14-21] that the necessary synthesis conditions to obtain an optimal catalyst are the following: (i) synthesis of microcrystalline VOHPO4•0.5H2O in an alcohol characterized by the preferential exposure of the basal (001) planes, (ii) the presence of defects in the stacking of the (001) planes and (iii) a slight excess of phosphate with respect to the stoichiometric amount employed in the synthesis (P/V=1.01-1.10). This excess phosphate is strongly bound to the surface and cannot be removed by simple washing of the precursor in polar solvents.

Three major synthesis methods were reported for preparation of the VOHPO4•0.5H2O precursor:

1. In aqueous synthesis, VV compounds (e.g. V2O5) are reduced to VIV in aqueous solutions of orthophosphoric acid, followed by evaporation of the solvent to dryness [22]:

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2. In organic synthesis, VV compounds are reduced by an anhydrous alcohol, followed by the addition of anhydrous orthophosphoric acid dissolved in the same alcohol and precipitation of VOHPO4•0.5H2O [16, 19]:

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3. In model organic synthesis, VV orthophosphate dihydrate, VOPO4•2H2O, is first synthesized from V2O5 and H3PO4 in aqueous medium and then reduced to VOHPO4•0.5H2O by an alcohol in a separate step:

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The organic synthesis usually provides the most active and selective catalysts [4,10,14,16,19]. All three methods may also lead to various hydrated vanadyl(IV) hydrogen phosphate phases, VOHPO4•nH2O (n=0.5, 1, 2, 3, and 4), which are all precursors of the VPO catalysts. The precursor with n=0.5 (VOHPO4•0.5H2O) produces the best VPO catalysts [10]. Another phase, VO(H2PO4)2, is observed when a considerable excess of phosphate is used in the organic synthesis (P/V>2) [16]. The main differences observed in the VPO precursors obtained by various methods is the morphology of the VOHPO4e0.5H2O crystallites. The XRD patterns of VOHPO4•0.5H2O synthesized by aqueous and organic methods and, accordingly, referred to as organic and aqueous VPO precursors and catalysts are shown in Figure 1. These patterns indicate that organic precursors are less crystalline and preferentially expose the (001) planes [14], as manifested in a broader (001) reflection and its lower relative intensity as compared to the intensity of the in-plane (130) reflection (Figure 1).

The morphology of organic precursors depends on many factors, e.g. (i) the nature of the solvent/reducing agent (an...