Journal of Molecular Catalysis A: Chemical 303 (2009) 84–89 Contents lists available at ScienceDirect Journal of Molecular Catalysis A: Chemical journal homepage: www.elsevier.com/locate/molcata Room temperature selective oxidation of toluene over vanadium substituted polyoxometalate catalysts K.T. Venkateswara Rao, P.S.N. Rao, P. Nagaraju, P.S. Sai Prasad, N. Lingaiah ∗ Catalysis Laboratory, I&PC Division, Indian Institute of Chemical Technology, Hyderabad 500607, India article info Article history: Received 1 September 2008 Received in revised form 29 December 2008 Accepted 6 January 2009 Available online 9 January 2009 Keywords: Selective oxidation Toluene Vanadium Molybdophosphoric acid Niobia abstract Selective oxidation of toluene at room temperature was carried on vanadium containing molybdophos- phoric acid supported on niobia catalysts using t-butyl hydrogen peroxide (TBHP) as oxidant. The catalysts were characterized by FT-IR, X-ray diffraction (XRD), 31 P MAS NMR, Laser Raman spectroscopy and N 2 adsorption. The characterization data reveals the incorporation of vanadium and retention of intact Keggin ion on the support. The catalyst activity is related to the presence of vanadium in Keggin ion. The effect of TBHP–toluene concentration on the conversion and selectivity during toluene oxidation was also studied. A plausible reaction mechanism for the selective formation of benzaldehyde was proposed. The present catalyst is highly active, selective towards benzaldehyde and reusable with out any appreciable loss in activity and selectivity. © 2009 Published by Elsevier B.V. 1. Introduction The catalytic oxidation of C–H bonds of hydrocarbons to oxygenated products under mild reaction condition is a major challenge in view of industrial aspects [1,2]. The oxygenated prod- ucts are the required intermediates for many chemical feedstocks, agrochemicals, fragrances, pharmaceuticals and polymers. The tra- ditional homogeneous catalytic oxidation methods often require stiochiometric amounts of metal complexes such as Cr(VI), Mn(VII) or Os(VIII) that generate large quantities of toxic metal waste [3]. These wastes can be eliminated if the vapor phase oxidation using a heterogeneous catalyst is employed. However, this requires excess temperature or pressure and also need specialized equipment [4]. Moreover, the selectivities to the desired oxygenated products are always less. Thus, the selective catalytic oxidation of inactive hydro- carbons to industrially important oxygenated derivatives is still remains a major challenge [5–7]. Among various hydrocarbons, toluene is an important one that can be converted into oxidation products such as benzyl alcohol, benzaldehyde and benzoic acid. Among these products, benzaldehyde is the most desirable prod- uct. However, benzaldehyde is easily over oxidized to benzoic acid. Traditionally, benzaldehyde is produced by side-chain chlorination of toluene and saponification of the resulting dichloromethyl group to form the aldehyde. The product still contains the chlorinated ∗ Corresponding author. Tel.: +91 40 27193163; fax: +91 40 27160921. E-mail address: nakkalingaiah@iict.res.in (N. Lingaiah). impurities, does not meet food and drug grade specifications. The vapor phase oxidation of toluene with oxygen generally carried at high pressure (0.5–2.5 MPa) and seems too harsh for improving the selectivity of benzaldehyde [8]. There exists lot of difficulties in vapor phase oxidation of toluene such as control of over oxi- dation or combustion that leads to the formation of carboxylic acids, phenols or decomposition to carbon oxides (CO 2 and CO) and tar [9]. There are only few reports on the selective oxida- tion of toluene is carried out at low temperatures using different oxidants like tert-butyl hydrogen peroxide, O 2 , etc. Peroxides are very attractive, relatively cheap and waste-avoiding oxidants only when it is used in a controlled manner without organic solvents and other toxic compounds [10]. Brutchey et al. have reported the toluene oxidation on Co-SBA-15 catalyst by using TBHP as an oxidant [11]. This catalytic system showed maximum toluene con- version of 8% with 64% benzaldehyde selectivity at 80 ◦ C in 24h. Ruthenium-based catalysts are extensively studied for oxidation reactions with TBHP [12–14]. However, these catalysts are expen- sive. In this regard, a more environmentally benign, economically fea- sible and selective catalytic process is desirable. Metal supported heterogeneous catalysts that can operate in combination with an acquiescent oxidant in liquid phase are advantageous, due to mild reaction conditions, catalyst recovery and have minimal environ- mental impact [15,16]. In this context, Keggin type hetropoly acids (HPAs) are widely used in various oxidation reactions [17,18]. HPAs like 12-molybdophosphoric acid and vanadium containing molyb- dophosphoric acid are used for oxidation of alcohols. These catalysts 1381-1169/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.molcata.2009.01.006