Molybdenum oxide model catalysts and vanadium phosphates as actual catalysts for understanding heterogeneous catalytic partial oxidation reactions: A contribution by Jean-Claude Volta

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Review

Molybdenum oxide model catalysts and vanadium phosphates as actual catalysts for understanding heterogeneous catalytic partial oxidation reactions: A contribution by Jean-Claude Volta Jacques C. Védrine a,∗ , Graham J. Hutchings b , Christopher J. Kiely c a

Laboratoire de Réactivité de Surface, Université P. & M. Curie-Paris VI, 4 place Jussieu, F-75252 Paris, France Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Cardiff CF10 3AT, UK c Department of Materials Science and Engineering, Lehigh University, 5 East Packer Avenue, Bethlehem, PA 18015-3195, USA b

a r t i c l e

i n f o

Article history: Received 21 November 2012 Received in revised form 24 January 2013 Accepted 25 January 2013 Available online xxx Keywords: Structure sensitivity Metal oxides Catalytic selective oxidation Volta’s contributions

a b s t r a c t This review summarises and expands part of the work that Jean Claude Volta carried out during his scientific career at the Institut de Recherches sur la Catalyse (IRC), CNRS, University of Lyon in France. The first part deals with the structure sensitivity of molybdenum oxide (MoO3 ) for oxidation reactions. The second part concerns the development of vanadium phosphate catalysts for butane oxidation which encompasses intensive studies of catalyst preparation, activation, and characterisation, including some of the first real in situ studies of a complex oxide catalytic system. J.-C. Volta was one the first scientists to develop real in situ studies with on-line GC analysis of the reactants and products under working conditions and their simultaneous characterisation by Raman spectroscopy. He also developed the spinecho mapping technique for determining the oxidation state of V through the chemical shift of 31 P NMR peak and used this to help unravel the complexity of transformations in VPO materials during preparation, activation and reaction allowing him to make meaningful structure–activity relationships. He contributed extensively to our understanding of how selective oxidation heterogeneous catalysts function. © 2013 Elsevier B.V. All rights reserved.

Contents 1. 2. 3. 4. 5. 6.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Structure sensitivity of oxidation reactions on molybdenum trioxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Vanadyl phosphate (VPO) catalysts for butane oxidation to maleic anhydride . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preparation of high area VPO catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preparation of active amorphous vanadium phosphates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Heterogeneous selective (partial) oxidation reactions have been largely studied in the literature since the pioneering work of Grasselli [1] at Sohio in the 1960s. These reactions generally proceed via a Mars and van Krevelen mechanism which involves (i) insertion of lattice oxygen into the organic reactant molecule and (ii) a redox

process involving electron transfer in the solid or at its surface. This mechanism is presented schematically below: 2[CatO] + R CH → 2[Cat] + R C O + H2 O 2[Cat] + O2 (gas) → 2[CatO] with rred = kred pR 

∗ Corresponding author. Tel.: +33 144275514; fax: +33 144276033. E-mail addresses: [email protected] (J.C. Védrine), [email protected] (G.J. Hutchings), [email protected] (C.J. Kiely).

00 00 00 00 00 00 00

and

rox = kox pnO (1 − ) 2

where [CatO] represents the oxidised catalyst surface and [Cat] its reduced state, rred is the reactant reduction rate and rox rate of reoxidation by co-fed oxygen, R CH and R C O the reactant and the product. The kinetic equation involves the relative concentration of

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The role of the MoO3 facet identity was also observed later on by Gaigneaux et al. [8] for the oxidation of isobutene CH2 = C(CH3 )2 to methacrolein CH2 = C(CH3 ) CHO at 420 ◦ C on MoO3 and MoO3 doped with Sb2 O4 , in which the latter was shown to favour the formation of (1 0 0) MoO3 surface facets and thus enhance methacrolein formation. A similar type of structure sensitivity was observed subsequently by Germain and Tatibouët [9] for MoO3 in alcohol oxidation of methanol to formaldehyde and ethanol to acetaldehyde. However, in this case the basal (0 1 0) facet was found to be the most selective for aldehyde formation. 3. Vanadyl phosphate (VPO) catalysts for butane oxidation to maleic anhydride Fig. 1. Structure sensitivity of selective oxidation reactions on various crystal facets of MoO3 catalysts.

reduced ( red ) and oxidised ( ox ) sites of the catalyst, and at steady state these two quantities will be equal (i.e. rred = rox ). The relative values of rred and rox parameters are important for determining the product selectivity. This mechanism involves lattice oxygen anions, which may be incorporated in the reactant while the corresponding lattice vacancies created are then replenished by gaseous oxygen in the re-oxidation step. The catalysts that have been really useful which engender this type of mechanism typically contain transition metal ions, and in particular Mo and V. Since this early work there have been many studies devoted to designing catalysts that can function as effective selective oxidation catalysts. A large number of these have concentrated on the study of propene/propane partial oxidation and butane activation and it is in this area that Volta conducted many seminal studies, and is therefore the focus of this article. Jean Claude Volta was born in Givors, France on 3rd March 1946, retired in 2006 [2] and passed away on 18th June 2011.

2. Structure sensitivity of oxidation reactions on molybdenum trioxide Structure sensitivity of catalytic reactions was well established for metal catalysts in the 1960–70s [3]. It was shown that some reactions were sensitive to the surface atom arrangements, i.e. on the geometry of the crystalline facet exposed to the reactants, while others were dependent on electronic effects, i.e. electron transfer within the solid catalyst. Structure sensitivity of metal oxides for oxidation reactions was demonstrated for the first time by Volta [4] for propene partial oxidation when using a novel method to prepare MoO3 having specific crystal morphologies by oxy-hydrolysis of MoCl5 intercalated between layers of graphite. It was shown that propene gives almost exclusively acrolein on the (1 0 0) lateral face and CO2 on ¯ (1 0 1) and basal (0 1 0) planes. For the oxidation the apical (1 0 1), of but-1-ene to butadiene or the oxidation of isobutene, the selectivities observed as a function of the surface facets exposed were completely different, namely (0 1 0) MoO3 was best for generating 1–3 butadiene from but-1-ene, whereas (1 0 0) MoO3 was the most effective for methacrolein and COx from isobutene, etc. These studies clearly demonstrated how the geometry of the reacting molecule and atomic arrangements at the oxide surface are important factors in oxidation reactions [5] as summarised in Fig. 1. A more general view on but-1-ene selective oxidation on large crystals of MoO3 was published later by Tatibouët et al. [6] while a review paper by Volta et al. [7] summarised the main findings at that time.

VPO catalysts were also intensively studied by J.-C. Volta. He demonstrated the important role of surface structure on the reactivity for the various polymorphic phases present in the VPO system, which is an active catalyst in C4 alkane and alkene oxidation to maleic anhydride. The reaction of butane oxidation to maleic anhydride [10] involves 14 electrons, the abstraction of 8 H atoms, and the insertion of 3 O atoms according to the reaction: C4 H10 + 3.5·O2 → C4 H2 O3 + 4·H2 O VPO has been applied industrially to n-butane oxidation in replacement of benzene or butene oxidation, which has permitted the upgrading of a rather abundant and thus inexpensive raw material, which would normally just have fuel value and in some cases was essentially wasted by flaring. Many techniques have been employed [11] to characterise the catalyst precursor phase (VOHPO4 –0.5H2 O), the effect of activation on microstructure and the catalyst evolution during catalytic reaction. The contributions of the groups led by Volta and Bordes-Richard were especially important, who placed special emphasis on the role of the different precursor phases, their structural transformation upon activation, their characterisation and their catalytic properties. A particular aspect was the transformation of hydrated precursors into the active phases, i.e. VOPO4 –2H2 O → ␣,␤-VOPO4 for the oxidation of butenes [12] and VOHPO4 –0.5H2 O → (VO)2 P2 O7 for n-butane oxidation [13]. Such transformations (illustrated in Fig. 3) allow one to gain control of the crystalline morphology and, for instance, to favour formation of (VO)2 P2 O7 crystallites preferentially exposing (1 0 0) faces which have been shown to be the most selective for butane oxidation to maleic anhydride. Using unusual physical techniques at that time, such as in situ Raman spectroscopy [14] and spin-echo mapping of 31 P MAS-NMR spectra [15], Volta et al. showed that the most active catalysts contain low amounts of V5+ and suggested that there is a local intimate mixture of VOPO4 and (VO)2 P2 O7 phases, that are related by topotactic transformations, which at their interfacial sites facilitate the redox mechanism of the reaction. Spin-echo mapping of 31 P MAS-NMR [15], which had been developed specifically by Volta to unravel the structural complexities of VPO catalysts, permits the facile discrimination of V5+ (−40 to 10 ppm), V4+ (a very broad signal at ∼2500 ppm) and V3+ (a very broad signal at ∼4650 ppm) coordinated to P and these entities could be dynamically followed during catalyst activation and reaction studies [16]. In addition, ESR spectroscopy was used to characterise V4+ (d1 ) cations, and 31 P-1 H cross polarisation in NMR [17] in order to show that butane is preferentially adsorbed at surface sites associated with ␦-VOPO4 type phases. XRD identified different VPO structures such as the basic pyrophosphate phase (VO)2 P2 O7 and other V4+ and V5+ hydroxy-phosphate polymorphs such as ␣I -,␣II -,␤,␥- and ␦-VOPO4 , while XPS/LEIS techniques [18] showed that catalyst surface is enriched in P by a factor of more

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Fig. 2. n-Butane oxidation to maleic anhydride and tetrahydrofuran.

than three with respect to the nominal chemical composition of ca. 1.05. Furthermore, XRD, in situ Raman spectroscopy [19], 18 O2 isotopic exchange, TAP and transient techniques were used to follow the catalyst behaviour and proved that the reaction mechanism is of the Mars and van Krevelen-type [20], while electrical conductivity measurements, showed the implications of electrical semi-conductivity in these materials. It is this deep level of characterisation, using such a broad range of techniques, which allowed the development by DuPont of a circulating moving bed technology [21] in Asturias, Spain for the synthesis of tetrahydrofuran (THF) via (i) maleic anhydride formation on a VPO catalyst and (ii) its subsequent reduction to THF by hydrogen on a Pd/Al2 O3 catalyst (see Fig. 2). Unfortunately due to technical problems production at the unit was recently stopped. Nevertheless, the intricacy of the catalysis understanding gained from Volta’s classic studies helped underpin the much deeper understanding we now have of VPO catalysts. Volta was a pioneer and visionary in developing in situ characterisation [14] of solid catalysts under real catalytic conditions and using a GC analysis of the reactants and products on-line. It is interesting to recall that at the University of Lille in the 1970s, the first experiments by in situ electron microscopy were carried out on V2 O5 –MoO3 catalysts in the presence of propene, which permitted the study of the morphological changes that were occurring in the catalyst during the reaction [22]. But in this particular case no on-line analysis was performed. The terms in situ [23] and

operando [24], although originally precisely defined, have unfortunately often been misused, as discussed elsewhere [25], because characterisation and catalytic reaction were not performed in the same cell and with simultaneous on-line analysis of reactants and products. To obtain truly meaningful catalytic data, it is worth recalling that the characterisation and analysis of reactants and products during catalytic reaction have to be specifically designed so as to avoid any diffusion limitations as this will mask the real effects that one seeks to study; in addition, this requires careful attention to the reactor size and the form of the catalysts (e.g. powder or pellets). VPO compounds exhibit many possible crystalline phases, for example (VO)2 P2 O7 , ␣I , ␣II , ␤, ␥, ␦, metastable ␻ polymorphs of VOPO4 , and other disordered phases [26]. The relative volume fraction of these phases in the final catalyst depends on the method of preparation, the nature of the catalyst precursor and the precise reaction/activation conditions. The ␻-VOPO4 phase is stable only at elevated temperatures and appears to be very sensitive to reactants and products of butane oxidation; it transforms rapidly to the ␦-VOPO4 phase upon butane exposure under reaction conditions. VPO catalysts also tend to contain considerable amounts of disordered phases, which makes it difficult to understand the true nature of VPO catalysts during catalytic processes and to pin down the exact nature of the active sites. VPO catalysts contain only V4+ and V5+ species in perfectly crystalline (VO)2 P2 O7 and VOPO4 phases, respectively. Vanadium (V5+ ) is probably responsible for

Fig. 3. Pseudomorphism between primary particles (top) and secondary particles (bottom) VOHPO4 –0.5H2 O precursor (left) and (VO)2 P2 O7 catalyst (right) corresponding a topotactic reaction driven by calcination. By controlling the morphology of the precursor, one can control the morphology of the final catalyst (adapted from Ref. [12]).

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formation of maleic anhydride and can be involved in the selective oxidation reaction of n-butane, while vanadium (V4+ ) is active in the formation of by-products [27]. However, the vanadyl phosphate phase is detectable by X-ray photoelectron spectroscopy in vanadium hydroxide oxide phosphate, a commercial VPO precursor; the only detectable crystalline phase in commercial catalysts is vanadyl pyrophosphate containing V4+ cations, which is a major active component of the n-butane oxidation reaction [28]. The presence of the VOPO4 (V5+ ) phase was also detectable by in situ studies. A small amount of VOPO4 phase present in conjunction with (VO)2 P2 O7 appears to be convenient for increasing the selectivity to maleic anhydride in the n-butane oxidation reaction. Non-crystallinity of VPO phases is related to the redox properties of these catalysts and facilitates the activation process of alkanes. Abon et al. [29]. reported that an amorphous catalyst is not able to form maleic anhydride but Wachs et al. [28]. showed that both isolated and adjacent surface vanadia species are able to oxidise butane to maleic anhydride. Ballarini et al. [30] described that an active surface made of vanadium oxide and polyphosphoric acids forms on VPO catalysts having P/V ratios >1.0, whereas at P/V ratios <1.0 a VOPO4 phase forms and the catalyst becomes very active but unselective. The precise role of the surface P/V atomic ratio remains unknown for propane ammoxidation, thus there still exists a strong need to explore P/V ratios above and below 1:1. Bulk VPO catalysts are also used for the conversion of propane into acrylonitrile [31], which is the main raw material for the production of synthetic fibres, plastics, elastomers and polymers such as styrene–acrylonitrile and acrylonitrile–butadiene–styrene. Copolymerization of acrylonitrile with butadiene produces synthetic rubber. Acrylonitrile is produced by an ammoxidation reaction of propylene using Fe–Bi–Mo–O and Fe–Sb–O catalysts, but this process should ideally be replaced by ammoxidation of propane due to the lower cost and great abundance of propane. This process includes multi-functional molybdates and antimonates as catalysts. However, propane conversion is about ten times lower than that of propene under the same conditions because the activation energy of the C H bond is considerably higher. Propene and acetonitrile form directly in this reaction; acrylonitrile, and hydrogen cyanide form as secondary products [32]. Recently a new method has been discovered for producing acrylonitrile from glycerol in a one-step reaction process [33]. The structure of the VPO-based catalysts during propane ammoxidation depends on reaction conditions and is affected not only by the presence of the hydrocarbon and its interaction with oxygen, but also by the presence of ammonia and water in the feed. The presence of steam has a profound effect on VPO structures. In the case of the propane or propylene oxidation reactions, steam addition improves activity and selectivity to acrylic acid, but propane conversion decreases due to (i) an increase of the crystallinity and (ii) the loss of acid sites at the surface of the catalyst. Centi et al. suggested that incorporating ammonia into the gas flow during the propane oxidation reaction might cause competitive adsorption phenomena [34]. These catalysts were evaluated for the selective ammoxidation reaction of alkanes [35] and alkylaromatics to the corresponding nitriles. Subsequently, Centi et al. proposed two pathways for the formation of acrylonitrile on bulk VPO catalysts [36], an anaerobic one, and a more rapid process that occurs in the presence of O2 .

4. Preparation of high area VPO catalysts Having introduced the complex topic of VPO catalysts that Volta worked on in the early part of his career, this section will deal with a facet of the collaborative studies carried out between Volta, Kiely and Hutchings between 1992 and 2004. During this time much

Fig. 4. Summary of preparation routes for VPO, VPA and VPD catalysts.

emphasis was placed on understanding the preparation of active VPO catalysts and three specific synthesis methods were studied in great detail (Fig. 4) [37]. The VPA method (A stands for acid) uses water as the solvent. In this method V2 O5 is refluxed with hydrochloric acid and in this step V5+ is reduced to V4+ . H3 PO4 is then added to the solution (P:V molar ratio ≥1.0) and following a further reflux and evaporation, a blue green precursor solid is obtained that comprises mainly the hemihydrate phase VOHPO4 –0.5H2 O. However, significant amounts of an undesirable impurity VO(H2 PO4 )2 are also obtained. This VO(H2 PO4)2 impurity phase transforms to VO(PO3 )2 and amorphous vanadium phosphates on heat treatment which are associated with low activity catalysts. However, VO(H2 PO4 )2 is soluble in water, whereas VOHPO4 –0.5H2 O is insoluble, and so it can be readily removed from catalyst precursors by extraction with hot water and this results in a much more active catalyst. The VPO method (O stands for alcohol) is a variant of the VPA method and is based on the observation that an alcohol can act as a reducing agent and consequently the HCl is superfluous. In this method V2 O5 is refluxed with H3 PO4 (P/V molar ratio ≥1.0) with an alcohol (alcohol: V molar ratio ≥50) and a blue precursor is obtained as a precipitate that comprises almost exclusively the hemihydrate phase, VOHPO4 –0.5H2 O. Many alcohols have been tried in this respect, but isobutanol is one of those most widely used. In this reaction the alcohol reduces V5+ to V4+ and is in turn oxidised. The VPD method (D stands for dried) is based on the observation that the reaction of V2 O5 with H3 PO4 when water is used as the solvent, (i.e. in the absence of the alcohol used in the VPO method), leads to the formation of the V5+ dihydrate phase, VOPO4 ·2H2 O. The dihydrate is recovered and dried and then refluxed in a second step with an alcohol to form the hemihydrate. The structure of the alcohol determines the morphology of the hemihydrate precursor: primary alcohols tend to produce rosette-like clusters of thin hemihydrate platelets, whereas secondary alcohols generally produce thicker platelets having a lower surface area. With primary alcohols, materials with surface areas in excess of 40 m2 g−1 can be prepared [38]. All three preparation methods can be used to produce relatively pure samples of VOHPO4 –0.5H2 O although there are differences in their surface areas and morphologies. Subsequent heat treatment of the three precursors in butane–air mixtures give active catalysts for the oxidation of butane to maleic anhydride for which there is a linear relationship between specific n-butane conversion (i.e. mol butane converted per g per h) with the catalyst surface area [35,36]. This implies that the surface structure of all three of the activated catalysts are very similar and the activity differences are just due to the higher surface area, which results in VPD and VPO catalysts having a higher number of active sites per unit mass of catalyst. In these studies Volta and co-workers expected that the structures of the final active catalysts would be very similar. It was, therefore, very surprising that the bulk structure of the three activated catalysts are very different (Fig. 5) as determined

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Fig. 5. Characterisation of final catalysts prepared using the VPA, VPO and VPD methods using scanning electron microscopy, powder X-ray diffraction and [32] P NMR spectroscopy by spin echo mapping [40].

using a combination powder X-ray diffraction and 31 P MAS-NMR spectroscopy by spin echo mapping. The VPA activated material comprises mainly VOPO4 phases, the VPO activated material is mainly amorphous to X-rays but by the NMR method is found to comprise a mixture of VOPO4 and (VO)2 P2 O7 together with non-crystalline vanadium phosphates, whereas the VPD activated material is principally highly crystalline (VO)2 P2 O7 . Based on these findings Volta and co-workers concluded that the activity for these VPO catalysts is mainly a function of the surface area, i.e. the surfaces exposed on these different materials must all be the same even though their bulk and facet structures are completely different [39]. Two important review papers by Volta summarise the main aspects of these findings and parameters [7,40]. As described earlier in this paper, these studies led to the discovery that the surfaces of the active catalysts comprised an amorphous overlayer and that the vanadyl pyrophosphate was a highly crystalline support for this active surface phase (Fig. 6) [40]. This phenomenon is quite frequently observed. It was, for example, also demonstrated [41] for the V2 O5 –WO3 /TiO2 catalyst that is used industrially for the selective catalytic reduction of NOx .

observed, the crystalline structure of the vanadium phosphate surface was lost. This led us to ponder the possibility that amorphous vanadium phosphate phases could in fact be important in the selective butane oxidation reaction. Volta was at the forefront of preparing active VPO material using new and novel methods of catalyst preparation and it was

5. Preparation of active amorphous vanadium phosphates The importance of amorphous phases in vanadium phosphate catalysts was first noted in the seminal study of Volta [14] using in situ Raman spectroscopy coupled with online analysis showing how a crystalline VPA precursor was transformed to an active catalyst on heating with butane/air (Fig. 7). This work clearly demonstrated that as the first glimmers of selective oxidation were

Fig. 6. HREM micrograph showing the amorphous overlayer observed commonly on (VO)2 P2 O7 crystallites in activated VPO catalysts [40].

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Fig. 7. In situ laser Raman spectra and catalytic performance for butane oxidation to maleic anhydride during the transformation from crystalline VOHPO4 –0.5H2 O (prepared by the VPA method) to a largely amorphous material. T denotes the sample temperature in flowing 1.5% butane–air; CBut denotes the butane conversion and SMA denotes the selectivity to maleic anhydride; the bands in the Raman spectra have been assigned wherever possible: H,VOHPO4 .0.5H2 O; P, (VO)2 P2 O7 ; ␣II , ␣II -VOPO4 ; ␥, ␥-VOPO4 ; ␦, ␦-VOPO4 .[14].

this arena that he was involved in the preparation of amorphous vanadium phosphates. In this respect, the quest to prepare wholly amorphous vanadium phosphate catalysts required the use of supercritical CO2 as an anti-solvent as this enables a very rapid precipitation stage which can lead to the generation of amorphous phases. This is indeed the case for vanadium phosphates. Volta and co-workers found that rapid precipitation of the vanadium phosphate from an alcohol solution using supercritical CO2 as an anti-solvent provides a convenient preparation route for an amorphous vanadium phosphate material. [42] In this method, a solution of H3 PO4 in isopropanol was refluxed with VOCl3 for 16 h to give a blue solution. The resulting isopropanol solution was processed using supercritical CO2 to precipitate a vanadium phosphate using the apparatus shown schematically in Fig. 8. A material denoted VPOSCP1 was prepared using supercritical CO2 as the anti-solvent (PCO2 = 11 MPa, 60◦ C). Another catalyst was also prepared using the same methodology, but utilising liquid CO2 (PCO2 = 6 MPa, 15◦ C) and this was denoted VPOLP . In addition, a third solid was prepared by allowing the isopropanol solution to evaporate slowly under reduced pressure and this was denoted VPOEP . Electron microscopy and electron diffraction studies showed that VPOSCP1 comprised discrete amorphous spheroidal particles ranging from 75 nm to 5 ␮m in diameter. These particles showed no diffraction contrast (only thickness contrast), and no lattice fringes or nanocrystalline order. These three precursors were then evaluated as catalysts for the partial oxidation of butane to maleic anhydride and the results

are shown in Fig. 9 for the first 72 h of operation. For comparative purposes, the results for typical catalysts prepared by the standard VPA, VPO and VPD methods are also shown. It is apparent that none of the three catalysts requires an activation period to establish the steady state catalyst performance which is generally associated with vanadium phosphate catalysts, as is shown by the VPA, VPO and VPD catalysts (Fig. 8). During this activation period the

Fig. 8. Schematic diagram of the apparatus used for the precipitation of vanadium phosphates using supercritical or liquid CO2 . BPR, back pressure regulator; PV, precipitation vessel; P, pump. Taken from Ref. [43].

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Fig. 9. The intrinsic activity for maleic anhydride (mol h−1 m−2 ) with timeon-stream. For VPA/VPO/VPD catalysts GHSV = 1200 h−1 , for all other catalysts GHSV = 2400 h−1 . Key: X VPO, VPA,  VPD, 䊉VPOSCP1 , VPOLP , VPOEP . Taken from Ref. [43].

VPA, VPO and VPD catalysts, derived from crystalline hemihydrate VOHPO4 ·0.5H2 O, undergo a structural transformation to (VO)2 P2 O7 and VOPO4 phases as described previously [9,43]. The catalysts derived from VPOSCP1 , VPOLP and VPOEP unfortunately all have low surface areas of 4, 6 and 8 m2 g−1 , respectively, when compared with the standard VPO (14 m2 g−1 ) and VPD (43 m2 g−1 ) materials. Interestingly, the catalyst derived from VPOSCP1 has a significantly higher intrinsic activity for the production of maleic anhydride than any of the other catalysts. 6. Concluding remarks From this brief resume of the work carried out by Jean Claude Volta, both individually and through the varied collaborations he engendered, it is clear that his efforts led to a greatly enhanced understanding of the two key catalyst systems that he chose to study, namely molybdates and vanadium phosphates. In particular, the use of such a varied range of characterisation techniques, many of which Volta pioneered, was really transformative in the field of oxidation catalysis. The main legacies of Volta’s various researches and discoveries include (i) the sensitivity of partial oxidation reactions to the structure of metal oxide catalysts; (ii) the importance of catalyst preparation parameters (e.g. choice of precursors, activation conditions, reactions conditions, etc.) on the resultant catalytic performance; (iii) the development of new characterisation techniques such as spin echo mapping in NMR; (iv) pioneering the use of physical characterisation techniques in situ, i.e. under real working conditions with simultaneous on-line analyses of reactants and products, in particular Raman spectroscopy. All these researches have had a tremendous influence on the catalytic community and in the field of selective oxidation in heterogeneous catalysis. For many of us who had the great privilege to collaborate with Jean Claude Volta, we remember him as a really warm, enthusiastic and intently energised person who lived life at a rate well above many of the rest of us. It was for all of us great fun and a privilege to work alongside him. References [1] J.L. Callahan, R.K. Grasselli, AIChE Journal 9 (1963) 755; R.K. Grasselli, Topics in Catalysis 15 (2001) 93. [2] J.M.M. Millet, B.K. Hodnett, J.C. Védrine, Applied Catalysis A: General 325 (2007) 197 (special issue in honour of J.C. Volta). [3] M. Boudart, Advances in Catalysis 20 (1969) 153; M. Boudart, in: G.C. Bond, P.B. Wells, F.C. Tompkins (Eds.), Proc. 6th Int. Congress on Catalysis, The Chemical Society, London, 1977, pp. 1–9. [4] J.C. Volta, W. Desquesnes, B. Moraweck, G. Coudurier, Reaction Kinetics and Catalysis Letters 12 (1979) 241.

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Please cite this article in press as: J.C. Védrine, et al., Molybdenum oxide model catalysts and vanadium phosphates as actual catalysts for understanding heterogeneous catalytic partial oxidation reactions: A contribution by Jean-Claude Volta, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.01.004