Applied Catalysis A: General 226 (2002) 323–327

Research communication

Acetic acid stability in the presence of oxygen over vanadiumphosphate catalysts: comments on the design of catalysts

for the selective oxidation of ethane

Jose Antonio Lopez-Sanchez, Richard Tanner, Paul Collier,Richard P.K. Wells, Colin Rhodes, Graham J. Hutchings∗

Department of Chemistry, Cardiff University, P.O. Box 912, Cardiff CF10 3TB, UK

Received 28 June 2001; accepted 27 September 2001

Abstract

The stability of acetic acid in the presence of excess oxygen is evaluated for a range of vanadium phosphate and oxidecatalysts. In the temperature range 300–400◦C most oxides catalyse the total oxidation of acetic acid to carbon dioxide andwater. However, over molybdenum oxide acetic acid oxidation is not significant. This is consistent with Mo being a majorcomponent of oxide catalysts that have, to date, been identified for the selective oxidation of ethane to acetic acid at thistemperature. Interestingly, VO(H2PO4)2, a phase that is very non-selective for the partial oxidation ofn-butane, exhibits thehighest stability for acetic acid under oxidising conditions. It is proposed that catalysts based on VO(H2PO4)2 may providethe basis of improved ethane partial oxidation catalysts. © 2002 Elsevier Science B.V. All rights reserved.

Keywords: Vanadium phosphate catalysts; Acetic acid decomposition

1. Introduction

The partial oxidation of ethane to acetic acid re-mains one of the greatest challenges facing scientistsinvolved in the partial oxidation of alkanes. To date, anumber of catalysts have been shown to catalyse thisreaction, but only very low yields have been reportedsince the reaction has to be carried out at low ethaneconversion to retain the high selectivity to acetic acidor other partial oxidation products [1–10]. It is appar-ent that catalysts giving the highest selectivities con-tain vanadium in combination with molybdenum andphosphorus, e.g. V-Mo-P(1) and V-Mo-Sb [9]. Vana-

∗ Corresponding author. Fax:+44-29-2087-4075.E-mail address: hutch@cf.ac.uk (G.J. Hutchings).

dium phosphates, which have been extensively studiedfor n-butane oxidation [11,12] have been recently stud-ied for the partial oxidation of ethane at relatively hightemperatures [13,14] and under these conditions onlyethene and carbon oxides are observed. Under the re-action conditions selected, partial oxidation products,such as acetic acid, would be expected to be oxidised,although they might have been formed initially.

Previously, we have proposed an approach for thedesign of oxidation catalysts for reactions where theproducts are considerably less stable than the reactants[15–18]. The key aspect of this approach concerns se-lecting catalyst supports and reaction conditions thatdo not catalyse the oxidation of the required product,as noted also by Sokolovskii [19]. In particular, theoxidative stability of potential products needs to be

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324 J.A. Lopez-Sanchez et al. / Applied Catalysis A: General 226 (2002) 323–327

evaluated in the presence of excess oxidant, since thisis the relevant oxidant/product ratio if the product isformed in low yields on the catalyst surface. We ex-plored this approach in the design of Ga2O3/MoO3catalysts for the selective oxidation of methane tomethanol [15]. In this research note, we present a studyof the decomposition of acetic acid over vanadiumphosphate catalysts in the presence of excess oxygenand comment on the potential design of catalysts forthe selective oxidation of ethane to acetic acid.

2. Experimental

V2O3, MoO2 and �-Ga2O3 were obtained com-mercially from Aldrich. VOPO4·2H2O was preparedby refluxing V2O5 (23 g) with H3PO4 (160 ml, 85%)and the resulting precipitate was washed with wa-ter. VOHPO4·0.5H2O was prepared by refluxingVOPO4·2H2O (1 g) with iso-butanol (50 ml) for 16 h,the resulting solid was washed with water. (VO)2P2O7was prepared by heating VOHPO4·0.5H2O in ni-trogen at 750◦C for 8 h. �-VOPO4 was preparedaccording to the method of Abdelouahab et al. [20].VO(H2PO4)2 was prepared according to the methodof Bordes [11]. An amorphous vanadium phosphatecatalyst, denoted VPOSC, was prepared using super-critical CO2 as an antisolvent as previously described[21]. All the materials were characterised by pow-der X-ray diffraction and laser Raman spectroscopyand the analytical data were consistent with literaturedata [20]. All the materials were pelleted and sievedto give particles (250–1000�m) and stored underdesication prior to use.

The stability of acetic acid was determined using astandard laboratory microreactor. The catalyst (0.5 g)was placed in a pyrex glass reactor and supported ona bed of glass wool (0.5 g). Air was passed through asaturator containing acetic acid and a controlled flowof 20 ml min−1 of air containing 1.3 vol.% acetic acidwas fed to the reactor. The air flow was maintainedusing a mass flow controller. The exit lines from thereactor were heated to ensure no condensation of theexit gases occurred, and analysis was carried out us-ing on-line gas chromatography. A blank reaction wascarried out in the absence of catalyst using only theglass wool (0.5 g) in the reactor and following the sameprocedure.

In addition, the acid base nature of the vanadiumphosphates was investigated using the decompositionof 2-methylbut-3-yn-2-ol (MBOH) [22], as a probereaction. MBOH (2.5 g h−1, Aldrich) was fed usinga calibrated syringe pump to a vaporiser and mixedwith helium (15 ml min−1). The reactants were fed viaheated lines to a laboratory microreactor containingcatalyst (1.0 g) diluted with glass beads. The productswere analysed by gas chromatography.

3. Results and discussion

An initial set of experiments were carried out to in-vestigate the oxidation of acetic acid using�-Ga2O3and MoO2 in the temperature range 200–500◦C andthe results are shown in Fig. 1. Following reaction, theV2O3 was found to have oxidised to V2O5 and simi-lar data were obtained using a sample of V2O5. It isclear that the blank reaction using silica wool is negli-gible below 300◦C, but increases in significance andabove 400◦C increases rapidly with increasing tem-perature. In these experiments the O2:CH3COOH molratio was 16:1. Clearly, this represents a massive ex-cess of the oxidant and this contrasts with the previ-ous studies for the oxidation of ethane to acetic acidwhich are typically carried out with a molar excess ofethane [1–10]. However, in these studies the mol ra-tio of O2:CH3COOH will also be significantly higherthan 1.0, as only very low yields of acetic acid areobtained at low ethane conversion. Hence, it is con-sidered important to investigate the stability of, in thiscase, acetic acid, at elevated temperatures in the pres-ence of excess oxygen. However, these data shouldonly be used to produce a relative order of stability ofacetic acid for the catalysts.

Acetic acid oxidation is observed to be significantlyenhanced when the oxide catalysts were used. In par-ticular, vanadium and gallium oxides give total oxida-tion, under these conditions. In contrast, acetic acid ismuch more resistant to total oxidation on molybdenumoxide. This observation is consistent with molybde-num being a preferred component in many of the cat-alysts that have been found to be selective for ethaneoxidation to acetic acid. However, above 400◦C, aceticacid oxidation becomes significant.

A second set of experiments were conducted withthe vanadium phosphate catalysts and the data are

J.A. Lopez-Sanchez et al. / Applied Catalysis A: General 226 (2002) 323–327 325

Fig. 1. Conversion of acetic acid as a function of temperature. Key: (×) blank reaction (silica wool); (�) MoO2; (�) �-Ga2O3; (�)V2O3/V2O5.

shown in Fig. 2. The data for the blank reactionare also shown for comparison. VOHPO4·0.5H2Owas the most active material for acetic acid oxida-tion. During the reaction the material transformedto �-VOPO4 and (VO)2P2O7, but separate studies

Fig. 2. Conversion of acetic acid as a function of temperature. Catalyst key: (�) blank reaction (silica wool); (�) VO(H2PO4)2; (�)VPOSC; (�) VOPO4·2H2O; (�) (VO)2 P2O7; (�) �-VOPO4; (�) VOHPO4·0.5H2O.

showed this only occurred at temperatures in excessof 350◦C, and by the time this transformation hadoccurred almost total acetic acid oxidation was ob-served. Characterisation of the catalysts followingreaction using powder X-ray diffraction and laser

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Raman spectroscopy confirmed that the VPOSC sam-ple remained amorphous and�-VOPO4 remained un-changed. (VO)2P2O7 was slightly oxidised and traceamounts of�-VOPO4 were observed to be present.VOPO4·2H2O was dehydrated in the temperature re-gion 100–200◦C and formed�I -VOPO4 initially andsubsequently�-VOPO4 and�-VOPO4 at higher tem-peratures. VO(H2PO4)2 was converted to VO(PO3)2at ca. 350◦C together with some amorphous material.The order of stability of acetic acid on these vana-dium phosphate catalyst materials in the presenceof excess oxygen is as follows at 400◦C: in paren-thesis are the phases present in the final catayst at400◦C if this is different from the initial material. Nocatalyst/silica wool> VO(H2PO4)2 [VO(PO3)2] >

VOPO4 · 2H2O [�-VOPO4 + �-VOPO4] > VPOSC[amorphous VPO]> (VO)2P2O7) > �-VOPO4 >

VOHPO4 ·0.5H2O [�-VOPO4+ traces(VO)2P2O7] ∼V2O5.

On the basis of this study, it is interesting to note thatVO(H2PO4)2 and VO(PO3)2 are not particularly activefor the oxidation of acetic acid, particularly at tem-peratures<350◦C. These vanadium phosphates havea similar activity to molybdenum oxide. This is a sig-nificant contrast to the effectiveness of VO(H2PO4)2and VO(PO3)2 for the oxidation ofn-butane, and thesecatalysts are known to be non-selective for the for-mation of maleic anhydride even at low conversion[23].

If effective catalysts are to be designed for the selec-tive oxidation of ethane to acetic acid then the resultsof this initial study show that materials that can acti-vate both oxidant and ethane at temperatures<350◦Care required. In addition, this study also shows thatcatalysts based on VO(H2PO4)2 and similar structurescould provide a useful basis for initial catalyst design.In addition, the amorphous vanadium phosphate pre-pared using supercritical CO2 as an antisolvent [21] isalso relatively inactive for acetic acid oxidation underthese conditions. This material is known to be more ac-tive for n-butane oxidation than crystalline vanadiumphosphates [21] and hence, this material may also beof value as a starting point for catalyst design.

It is interesting to comment further as to possi-ble reasons for the improved stability of acetic acidover VO(H2PO4)2 and VO(PO3)2. Previous studies[23] have shown that the surfaceP:V mol ratio ispossibly twice as high as that found for (VO)2P2O7

derived catalysts. This implies that the active vana-dium centres for oxygen activation and insertion aremore isolated on VO(H2PO4)2 and VO(PO3)2 sur-faces. Forn-butane oxidation, it has been suggested[24] that two vanadium centres are required for the14 electron oxidation (removal of 8H and insertion of3O2−) to selectively form maleic anhydride. When thevanadium active centres are diluted on the surface, thecombination of sites will be depleted and consequentlyVO(H2PO4)2 derived catalysts generally show verypoor maleic anhydride selectivity. Conversely, ethaneoxidation to acetic acid is a six electron oxidation (re-moval of 2H and insertion of 2O2−) and this is prob-ably achievable at a single active centre. In this case,site isolation would be an advantage.

The acid–base nature of the vanadium phosphateswas probed using the decomposition of MBOH at180◦C [22]. Three of the vanadium phosphates stud-ied for acetic acid oxidation were investigated, namely,VOHPO4·0.5H2O, (VO)2P2O7 and VO(H2PO4)2,since these are representative of vanadium phosphateswhich show high, medium and low reactivity forthe oxidation of acetic acid. All three materials gaveonly products of the acid catalysed decompositionof MBOH. However, the striking observation is thedifference in the initial rates of reaction observed,taking into account the surface areas of the material(VOHPO4 · 0.5H2O = 13 m2 g−1; (VO)2P2O7 =8 m2 g−1; VO(H2PO4)2 = 1 m2 g−1):

VO(H2PO4)2 1 × 10−2 mol m−2 h−1

(VO)2P2O7 4.7 × 10−4 mol m−2 h−1

VOHPO4·0.5H2O 1.7× 10−4 mol m−2 h−1

It is clear that VO(H2PO4)2 has a significantlyhigher density of acid sites than the other two vana-dium phosphates. This enhanced acid site densitymay also be an important factor with respect to theenhanced stability of acetic acid over VO(H2PO4)2.

It should be noted that many previous studies havemodelled catalyst design for the selective oxidationof ethane to acetic acid on the pyrophosphate struc-ture and our study suggests that other surfaces whichare significantly enriched in phosphorus could be pre-ferred for selective oxidation of ethane to oxygenates.However, it should be stressed that in studies of ethaneconversion to acetic acid, the stability of acetic acidshould be determined for catalyst formulations in or-der to select the optimum reaction conditions.

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Acknowledgement

We thank the EPSRC for financial support.

References

[1] M. Roy, H. Ponceblanc, J.C. Volta, Top. Catal. 11 (2000) 101.[2] M. Roy, M. Gubelmann-Bonneau, H. Ponceblanc, J.C. Volta,

Catal. Lett. 42 (1996) 93.[3] L. Tersier, E. Bordes, M. Gubelmann-Bonneau, Catal. Today

24 (1995) 335.[4] K. Nowinska, M. Sopa, D. Szuba, Catal. Lett. 39 (1996) 275.[5] P. Barthe, C. Blanchard, French Patent no. 92-116407/15.[6] Y. Wang, K. Otsuka, J. Catal. 171 (1997) 106.[7] Z. Zhao, Y. Yamada, Y. Teng, A. Ueda, K. Nakagawa, T.

Kobayashi, J. Catal. 190 (2000) 215.[8] M. Merzouki, B. Taouk, L. Monceaux, E. Bordes, P. Courtine,

Stud. Suf. Sci. Catal. 72 (1992) 165.[9] W. Ueda, K. Oshihara, Appl. Catal. A 200 (2000) 135.

[10] N.F. Chen, K. Oshihara, W. Ueda, Catal. Today 64 (2001)121.

[11] E. Bordes, Catal. Today 1 (1987) 499.[12] G.J. Hutchings, Appl. Catal. 72 (1991) 1.

[13] P.M. Michalakos, M.C. Kung, I. Jahan, H.H. Kung, J. Catal.140 (1993) 226.

[14] P. Cianbelli, P. Galli, L. Lisi, M.A. Massuci, P. Patrano, R.Pirone, G. Ruoppolo, G. Russo, Appl. Catal. A 203 (2000)133.

[15] J.S.J. Hargreaves, G.J. Hutchings, R.W. Joyner, S.H. Taylor,Chem. Commun. (1996) 523.

[16] S.H. Taylor, J.S.J. Hargreaves, G.J. Hutchings, R.W. Joyner,Appl. Catal. A 126 (1995) 287.

[17] S.H. Taylor, J.S.J. Hargreaves, G.J. Hutchings, R.W. Joyner,C.W. Lembacher, Catal. Today 42 (1998) 217.

[18] G.J. Hutchings, S.H. Taylor, Catal. Today 49 (1999) 105.[19] V.D. Sokolovskii, Catal. Today 24 (1995) 377.[20] F.B. Abdelouahab, R. Olier, N. Guillaume, F. Lefebvre, J.C.

Volta, J. Catal. 134 (1994) 151.[21] G.J. Hutchings, J.K. Bartley, J.M. Webster, J.A.

Lopez-Sanchez, D.J. Gilbert, C.J. Kiely, A.F. Carley, S.M.Howdle, S. Sajip, S. Caldarelli, C. Rhodes, J.C. Volta, M.Poliakoff, J. Catal. 197 (2001) 232.

[22] H. Lauren-Pernot, F. Luck, J.M. Popa, Appl. Catal. 78 (1991)213.

[23] J.K. Bartley, C. Rhodes, C.J. Kiely, A.F. Carley, G.J.Hutchings, Phys. Chem. Chem. Phys. 2 (2000) 4999.

[24] G. Centi, Catal. Today 16 (1994) 1.