Heterogenized Molybdovanadophosphoric Acid on Amine-Functionalized SBA-15 for Selective Oxidation of Alkenes
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Catalysis Letters Vol. 98, No. 4, December 2004 ( 2004)
Heterogenized molybdovanadophosphoric acid on amine-functionalized SBA-15 for selective oxidation of alkenes N.K. Kala Raj, S.S. Deshpande, Rohit H. Ingle, T. Raja and P. Manikandan Catalysis and Inorganic Chemistry Division, National Chemical Laboratory, Dr. Homibhabha Road, Pune 411 008, India
Received 21 August 2004; accepted 26 August 2004
Mono-vanadium substituted molybdovanadophosphoric acid has been immobilized on amine-functionalized SBA-15 and characterized by different techniques. Small angle X-ray scattering (SAXS) analysis and SEM technique provide evidence for the structural integrity of the amine-functionalized SBA-15 and for the molybdovanadophosphoric acid immobilized sample. Diffuse reflectance UV–vis, 31 P, 51 V-NMR and FT-IR data confirm the incorporation of molybdovanadophosphoric acid onto the aminefunctionalized SBA-15. The textural properties of these materials were studied by nitrogen sorption studies. Oxidation of selected substrates, norbornene, cyclooctene, cyclohexene and styrene with aqueous hydrogen peroxide was carried out with the immobilized molybdovanadophosphoric acid catalyst (heterogenized) and the results were compared with that of the neat molybdovanadophosphoric acid catalyst (homogeneous). The selectivity of the desired products was higher with the immobilized catalyst. The immobilized catalyst could be separated after the reaction and the reusability of the catalyst has also been demonstrated. KEY WORDS: SBA-15, amine-functionalized SBA, mesoporous material, heteropolyacid, molybdovanadophosphoric acid, catalyst characterization (XRD, 31 P -NMR, diffuse reflectance UV–vis, SEM).
1. Introduction Heterogenization of homogeneous catalytic materials has been an attractive strategy to overcome the difficulties involved in the separation and reusability of homogeneous catalysts [1,2]. Many strategies have been adopted in the literature to heterogenize the homogeneous catalysts. Conventionally they are immobilized/ anchored on polymeric organic materials such as resins [3,4], supported on inert porous solids such as alumina and silica [5,6] or encapsulated in the pores and cavities of microporous and mesoporous materials such as zeolites, MCM-41 and SBA-15 [7–9]. There are certain disadvantages with polymeric supports due to their vulnerability of their organic part to some chemicals and solvents, and due to leaching of catalysts from the immobilized material over the period of reactions [10]. On the other hand, porous inorganic material supports are structurally stable and more resistant to organic chemicals and solvents. However, mere encapsulation of the homogeneous catalytic molecules into the porous materials also leads to partial leaching of catalysts. Moreover, controlling the amount of loading of catalytic molecule is limited when encapsulation technique is adopted [11]. To overcome the leaching problem, efforts have been made to immobilize the catalytic molecules on a functionalized silica surface, for example, micro
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