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Catalysis: Volume 18

Catalysis: Volume 18

by James J Spivey (Editor)

There is an increasing challenge for chemical industry and research institutions to find cost-efficient and environmentally sound methods of converting natural resources into fuels chemicals and energy. Catalysts are essential to these processes and the Catalysis Specialist Periodical Report series serves to highlight major developments in this area. This series


There is an increasing challenge for chemical industry and research institutions to find cost-efficient and environmentally sound methods of converting natural resources into fuels chemicals and energy. Catalysts are essential to these processes and the Catalysis Specialist Periodical Report series serves to highlight major developments in this area. This series provides systematic and detailed reviews of topics of interest to scientists and engineers in the catalysis field. The coverage includes all major areas of heterogeneous and homogeneous catalysis and also specific applications of catalysis such as NOx control kinetics and experimental techniques such as microcalorimetry. Each chapter is compiled by recognised experts within their specialist fields and provides a summary of the current literature. This series will be of interest to all those in academia and industry who need an up-to-date critical analysis and summary of catalysis research and applications. Catalysis will be of interest to anyone working in academia and industry that needs an up-to-date critical analysis and summary of catalysis research and applications. Specialist Periodical Reports provide systematic and detailed review coverage in major areas of chemical research. Compiled by teams of leading experts in their specialist fields, this series is designed to help the chemistry community keep current with the latest developments in their field. Each volume in the series is published either annually or biennially and is a superb reference point for researchers.

Editorial Reviews

Specialist Periodical Reports provide systematic and detailed review coverage of progress in the major areas of chemical research, written by experts in their specialist fields, for the benefit of active research chemists. This volume presents five topics of current interest in the field of catalysis: toward supported oxide catalysts via solid-solid wetting; model catalyst studies of supported metal sintering and redispersion kinetics; techniques for measuring zeolite acidity; applications of raman spectroscopy to heterogeneous catalysis; and oxidative coupling of methane. The detailed table of contents substitutes for an index. Annotation c. Book News, Inc., Portland, OR (booknews.com)

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Royal Society of Chemistry, The
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Specialist Periodical Reports Series , #18
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Read an Excerpt

Catalysis Volume 18

A Review of Recent Literature

By J.J. Spivey

The Royal Society of Chemistry

Copyright © 2005 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85404-234-0


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


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]:


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]:


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]:


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:


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 aliphatic or benzylic alcohol) [16, 19], (ii) the synthesis P/V ratio [16], (iii) the time and temperature of reduction [19], and (iv) the amount of water present during synthesis [16].

Currently only organic VPO catalysts are employed industrially in n-butane oxidation as the most active and selective. The following steps have been proposed in the formation of the VOHPO4•0.5H2O in organic medium [19]: (i) the formation of colloidal V2O5 at the water-alcohol interface, (ii) the dissolution of V2O5 through the formation of VV-alcoholate species, (iii) the reduction of the dissolved VV-alcoholate species in the liquid phase to solid V2O4, and (iv) the reaction of V2O4 with H3PO4 to form VOHPO4•0.5H2O at the solid-liquid interface. The type of aliphatic alcohol influences the temperature of reduction of VV which is kinetically controlled and complete only at long reduction times upon addition of benzyl alcohol and orthophosphoric acid [19].

In the reduction by benzyl alcohol, many studies reported the formation of VOHPO4•0.5H2O platelets possessing stacking faults of the (001) planes seen in the preferential broadening of the (001) reflection. The stacking faults develop due to the trapping of alcohol molecules between the (001) layers of the precursor and their release during precursor transformation to (VO)2P2O7 [4, 10, 16, 19]. The effect of the above synthesis parameters on the properties of VOHPO4•0.5H2O is to vary the exposure of the (001) plane, create the stacking fault strain in the crystallites and influence the degree of VV reduction.

In both aqueous and organic syntheses, the VOHPO4•0.5H2O precursor has a P/V ratio higher than the stoichiometric value [10, 16, 19]. The maximum value corresponds to P/V=1.1, while the excess phosphate remains in synthesis solution. The X-ray photoelectron (XPS) analysis indicates that the excess phosphate is localized at the surface of the vanadyl pyrophosphate catalysts (surface P/V = 1.5-3.0) [23].

The well-known redox chemistry of VV provides important insights into the mechanism of the VOHPO4•0.5H2O precursor formation in organic medium. Waters and Littler [24] have shown that most VV reductions proceed via a free-radical mechanism where complexation of VV to alcohol precedes the one-electron transfer step, i.e. an inner sphere electron transfer. Waters and Littler [24] proposed ternary tetrahedral complex formation between VO2+ ,H3O+ and R2CHOH to yield [V(OH)3OHCHR2]2+ and observed the following kinetic expression


corresponding to slow decomposition of this species to VIV and an alcohol radical, R2C•-OH. The formation of the protonated complex should be assisted by the more acidic medium which was observed experimentally in numerous studies [24-26]. The inertness of simple tertiary alcohols toward VV indicated that the α-C-H bond is involved in the reaction which was confirmed by measuring the primary isotope effect produced by deuterium substitution at this position in cyclohexanol [24]. A mechanism involving cyclic transfer of the α-H atom to a coordination sphere of the metal ion has been proposed:


This mechanism shows that the VV reduction during the VPO precursor synthesis is slow at low H3O+concentration. The rate of reduction is slow even in the presence of anhydrous H3PO4, since [H3O+] is low in the absence of water. The reaction is likely autocatalytic: the water evolved during the reduction is protonated and the resultant H3O+ accelerates the reaction rate.

Rocek and Aylward [27] used substituted cyclobutanols as a probe into the mechanism of VV reduction by alcohols. Oxidation of cyclobutanols is a widely accepted method for discerning one- and two-electron processes, as they lead to cyclobutanones and γ-hydroxyaldehydes, respectively [28-30]. Rocek and Ayl-ward [27] observed the formation of γ-hydroxyaldehydes in VV reductions providing evidence of a one-electron process. The VV oxidation of cyclobutanols is first order in both the cyclobutanols and the protonated monomeric HVO2+ (aq.) species. HVO2+ (aq.) is a stronger oxidant than VO22+ (aq.), and at the acidic conditions employed in these oxidations, VV was predominantly in its monomeric form (HVO2+), despite its pronounced tendency toward the formation of polymeric ions [31]. Methyl cyclobutyl ether was found to be 104 times less reactive than cyclobutanol. This striking difference in reactivity indicated that the OH bond plays a vital role in the oxidation process and is broken either prior to or during the rate-determining step, as explained by formation of an ester of vanadic acid intermediate suggested as ROV(OH)2 (OH2)n2+(n=1, 2, or 3).

Tracey and Gresser [32] confirmed spontaneous formation of vanadate esters in aqueous solutions of alcohols in V NMR spectra (pH=7-11, 1-50 mM, 328K) where the vanadate occurs as the tetrahedral mono- or diprotonated anions, HVO42- and H2VO4- denoted as Vi, dimeric (HO3VOVO3H2- or V2), and tetrameric (H4V4O144- or V4) species. The complexity of esterification of vanadate arises in part from the ability of vanadate (VO43-) to undergo protonation and oligomerization as pH and concentration are changed [31,33,34]. Tracey and Gresser showed that vanadate complexes with monodentate hydroxylic ligands are tetrahedral analogs of phosphate esters. Equilibrium constants for the formation of monoanionic alkyl vanadate esters from Vi monoanion and alcohols, Kf=[ROVO3H- ]/[(H2VO4-][ROH]), are about 0.2 M-1, and relatively insensitive to the pKa of the alcohol and to whether the alcohol is primary, secondary, or tertiary [35-38]. Since the pKa values of Vi and alkyl vanadates are above 8.0, this Kf value can be used to estimate the concentration of a given vanadate ester in solution at a neutral pH containing known concentrations of Vi and alcohol. In studies of Vi in aqueous methanol Tracey and co-workers [38] found that methyl esters of divanadate can also form spontaneously with equilibrium constants similar to those for formation of esters of monovanadate: V2 + MeOH [RIGHT ARROW]V2(OMe) + H2O, K= 3.0 M-1.

Gresser et al. [39] observed formation of mixed anhydrides of vanadate with phosphate and pyrophosphate in aqueous solutions by 51V NMR, which may be considered as molecular precursors of vanadium phosphate phases. The formation of the mixed anhydrides was in fact more favorable than that of phosphate anhydrides (i.e. pyrophosphate) by more than 106-fold in the Kf for the phosphovanadate as compared to pyrophosphate at neutral pH. Formation of the divanadate species (V2) was 108 times more favored over formation of pyrophosphate. The reasons for the preferred formation of phosphovanadates may have to do with an interaction of the lone pairs of electrons on the bridging oxygen atom with orbitals on the adjacent vanadium and phosphorus atoms. If this occurred, it would cause the bridging oxygen to be more sp-like than sp3, with the result that the P-O-V or V-O-V bond angle in the anhydride would be larger than the P-O-P angle in the pyrophosphate. If the V-O-V and P-O-V angles are large, then the corresponding anhydrides in contrast to the pyrophosphate [40] may not be able to chelate Mg2+ which has been confirmed experimentally [41].

Some of the ways the vanadate esters and phosphovanadates [32-39] may be involved in the reduction of VV during the VOHPO4•0.5H2O precursor formation are:

A. Redox decomposition of monoalkyl esters of monomeric vanadate via a mechanism similar to that of Waters and Littler [24]. In this case, free radical species will be generated which can be detected by an ESR spin trapping technique.

B. Redox decomposition of monoalkyl ester of dimeric vanadate, ROVO2-O-VO33-, accompanied by two rapid successive one-electron transfers. Then, no free radical species are expected, and the products should be the carbonyl compound and VO2+.This mechanism has not yet been established for vanadate oxidations.


Excerpted from Catalysis Volume 18 by J.J. Spivey. Copyright © 2005 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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Meet the Author

Professor Spivey is the McLaurin Shivers Professor of Chemical Engineering at Louisiana State University and Director of the DOE Energy Frontier Research Center at LSU. Professor Spivey's research interests include the application of the principles of heterogeneous catalysis to catalytic combustion, control of sulfur and nitrogen oxides from combustion processes, acid/base catalysis (e.g., for condensation reactions), hydrocarbon synthesis, and the study of catalyst deactivation.

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