Polyoxometalate-based hybrid mesostructured catalysts for green epoxidation of olefins Z. Karimi, A.R. Mahjoub ⇑ , S.M. Harati Department of Chemistry, Tarbiat Modares University, 14115-175 Tehran, Iran article info Article history: Received 8 December 2010 Received in revised form 8 May 2011 Accepted 20 May 2011 Available online 6 June 2011 Keywords: Hybrid polyoxometalate Hexamethylphosphoramide SBA-15 Amine-functionalized Heterogeneous oxidation catalyst abstract Novel hybrid polyoxometalates (POM) of a-H 3 PW 12 O 40 ÁnHMPA and a-H 3 PMo 12 O 40 ÁnHMPA composed of a-H 3 PW 12 O 40 and H 3 PMo 12 O 40 heteropoly acides (HPAs) and hexamethylphosphoramide (HMPA) organic substrate has been synthesized and purified. SBA-15 mesoporous silica is synthesized, using P123 surfac- tant via hydrothermal method, and functionalized with aminopropyl functional groups via grafting method. The synthesized mesostructured supports are used for intercalation of the hybrid POMs. The par- ent Keggin HPAs are also immobilized within the supports to perform closer and more efficient investi- gation. After characterization, effect of functional groups on immobilization pattern and quality is taken into consideration. The mesostructured organic–inorganic hybrid materials are characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic absorption, and FT-IR analysis. The newly designed hybrid catalysts are investigated for heterogeneous epoxidation of olefins. Effects of temperature, oxidant, and catalyst amount are studied and the reaction conditions are optimized. An interpretation of the differences in the catalytic activity of the precursors is put forward and their catalytic activity is compared with their HPA counterparts. Furthermore, effects of functionalization on catalyst activity, stability, and reusability are taken into consideration. Results reveal that the designed mesostructured POM based hybrid catalysts can selectively and efficiently epoxidize olefins in presence of hydrogen peroxide as oxidant. The catalysts are shown to be heterogeneous and reusable without significant loss of activity in the proceeding rounds. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction Production of epoxides is of growing interest in chemical and petrochemical industries due to their applications as organic inter- mediates in pharmaceutical synthesis and as monomers in produc- tion of various functional polymers [1–5]. However, epoxidation reaction sometimes requires hazardous chemicals which leave acid wastes. Therefore, designation of efficient heterogeneous catalysts is an important step to going green, which involves both rigorous control of the surface chemistry and geometry at micro, meso, and macroscales. Besides, high active phase dispersion is inevitable for fast mass transfer of reactants to the active catalytic sites. Thus, much interest has been driven to production of unique mesostruc- tured systems with uniform and tailored pore structure. Polyoxometalates (POMs) are an immense class of oxygen bridged metal cluster anions of mainly tungsten and molybdenum [6,7]. Some POMs, such as phosphotungstic acid (H 3 PW 12 O 40 , PW) and phosphomolybdic acid (H 3 PMo 12 O 40 , PMo), have a soccer ball- shaped Keggin structure [8] with a molecular size of ca. 1 nm [9] in which a central PO 4 tetrahedron is surrounded by a W 12 O 36 or Mo 12 O 36 shell [6,10]. They have attracted much attention as envi- ronmentally benign acids and efficient and highly selective electro- philic and oxidation catalysts [10,11]. Besides, versatility and accessibility of Keggin series have led to synthesis of various hy- brid materials based on the Keggin structure [12,13]. In the last few years there have been a rapidly growing number of reports in the literature addressing the use of polyoxometalate as both homogeneous and heterogeneous catalysts [6,14–16]. However, to the best of our knowledge, only few reports are involved with catalytic applications of organic–inorganic hybrid Keggin POMs [13]. Since the polyoxometalates were found highly efficient for epoxidation of olefins by Venturello et al. [17,18] and Ishii et al. [19,20] in 1980s, various H 2 O 2 -based epoxidation systems have been efficiently catalyzed by POM based systems [21,23]. However, there exist some drawbacks about POMs catalysts: their thermal stability is relatively low and they are extremely soluble in polar solvents, which thus results in separation problems. Moreover, sur- face area of unsupported POMs is usually low (1–10 m 2 /g À1 ) which is an important restriction for catalytic systems. All these limitations may be satisfied by grafting POMs within proper 0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2011.05.024 ⇑ Corresponding author. Tel.: +98 21 82883443; fax: +98 21 88009730. E-mail address: mahjouba@modares.ac.ir (A.R. Mahjoub). Inorganica Chimica Acta 376 (2011) 1–9 Contents lists available at ScienceDirect Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica