Journal of Catalysis 206, 339–348 (2002) doi:10.1006/jcat.2001.3494, available online at http://www.idealibrary.com on Effect of Sb/V Ratio and of Sb + V Coverage on the Molecular Structure and Activity of Alumina-Supported Sb–V–O Catalysts for the Ammoxidation of Propane to Acrylonitrile M. O. Guerrero-P´ erez, ∗ J. L. G. Fierro, ∗ M. A. Vicente,† and M. A. Ba ˜ nares ∗,1 ∗ Instituto de Cat ´ alisis y Petroleoqu´ ımica, CSIC, Campus UAM-Cantoblanco, E-28049-Madrid, Spain; and †Departamento de Qu´ ımica Inorg ´ anica, Universidad de Salamanca, Pza. Merced s/n, E-37008-Salamanca, Spain Received September 14, 2001; revised December 14, 2001; accepted December 14, 2001 The effect of the total coverage of Sb + V oxides and the ef- fect of the Sb/V atomic ratio on the ammoxidation of propane to acrylonitrile on alumina-supported V and Sb oxide catalysts is re- ported. The fresh and used catalysts are characterized by XRD and in situ Raman spectroscopy. Comparison with binary V–Al–O and Sb–Al–O catalysts shows that the presence of both Sb and V oxides strongly enhances the rate of propane ammoxidation to acry- lonitrile on alumina-supported Sb–V oxide catalysts. The stability and structural changes during on-stream operation prior to reach steady-state operation originates from a close interaction between Sb and V oxides. The Sb–V interaction depends on total Sb + V coverage on alumina. Below the dispersion limit, SbVO 4 phases are not stable under reaction and break into the individual oxides. At Sb + V loading beyond dispersion limit, SbVO 4 phases are stable un- der reaction conditions while Sb and V oxides that did not combine during calcination of the precursor recombine into SbVO 4 phases. This solid-state reaction accounts for a higher propane conversion and selectivity to acrylonitrile. Comparison of the performance and molecular structures of fresh and used catalysts further suggests that Sb–V–O phases are necessary for this reaction. The specific formation of acrylonitrile per vanadium site reaches a maximum at an atomic Sb/V ratio of 2. It is likely that a moderate excess of an- timony may be necessary for an efficient ammoxidation of propane to acrylonitrile. c  2002 Elsevier Science (USA) Key Words: V–Sb–Al-oxides; oxidation; ammoxidation; propane; acrylonitrile; structure–activity relationship; coverage; in situ Raman; XRD. INTRODUCTION The direct conversion of propane into acrylonitrile by re- action with oxygen and ammonia is an alternative route to the conventional propylene ammoxidation since propylene is several times more expensive than propane. The eco- nomic implications of this new route are very important. Thus, in 1997 British Petroleum started a demonstration 1 To whom correspondence should be addressed. E-mail: mbanares@ icp.csic.es. plant to make acrylonitrile, using propane, and estimated to decrease production costs ca. 20% compared with con- ventional propylene-based technology (1). In this reaction, the activation of propane is the limiting step. Since the ad- sorption rate of propane is near ten times smaller than that of propylene (2), the conversion of propane is at least ten times smaller than that of propylene (3). The reaction condi- tions to activate the C–H bond in propane are more energy demanding, which has a negative effect on selectivity. The use of homogeneous–heterogeneous processes to promote propane to propylene conversion upstream the catalyst bed is an option (4). However, other side products may be gen- erated in the gas phase, like ammonia oxidation to nitrogen in the presence of molecular oxygen (5). The low activity of propane has also led to the use of gas phase additives (e.g., H 2 S or CH 3 Br) as radical generators. However, en- vironmental concerns do not make this option attractive. Therefore, the efforts focus on a catalyst for the direct am- moxidation of propane into acrylonitrile with no additives in the gas phase. Efficient catalysts for propane ammoxidation are based on antimony or molybdenum (6). Most of the reported work is concentrated on antimony-based catalysts, like vanadium antimonates with rutile structure (7–10) or on molybdenum (11–15). The molybdates can be promoted with Bi (11). Some molybdates may possess scheelite structure (12–14). Mo–V catalysts modified with Nb and Te may afford near 50% yield to acrylonitrile (15). On Sb–V–O based catalysts, the presence of alumina endows the system with better per- formance (14, 16). Despite the large relevance of these cata- lysts, the nature of the active site is still not fully understood. Sb–V–O catalysts with an excess of V are highly active and selective for propane oxidative dehydrogenation (ODH) while an excess of Sb affords Sb–V–O catalysts more effi- cient for propane ammoxidation (16). The catalysts show adsorption of both ammonia and propane (3, 17–19). However, different catalyst formula- tions may lead to different limiting steps in the reaction route. Molybdate catalysts appear to be limited by propane 339 0021-9517/02 $35.00 c  2002 Elsevier Science (USA) All rights reserved.