Synthesis and Catalytic Application of Robust Grafted Tripodal (Phenoxy) Ti-POSS Complexes in Crosslinked Hyperbranched
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ynthesis and Catalytic Application of Robust Grafted Tripodal (Phenoxy) Ti-POSS Complexes in Crosslinked Hyperbranched Poly(siloxysilane) Matrix E. H. Aisha,* and S. A. Darwishb a Department b Organometallic
of Chemistry, Faculty of Science, Menoufia University, Menoufia, 32511 Egypt and Organometalloid Chemistry Department, National Research Centre, Giza, 12622 Egypt *e-mail: [email protected] Received August 8, 2020; revised September 10, 2020; accepted October 6, 2020
Abstract—We have studied synthesis, activity, epoxide selectivity, H2O2 efficiency, and recyclability of new robust heterogeneous alkene epoxidation catalysts of grafted tripodal (phenoxy) Ti-polyhedral oligomeric silsesquioxane (Ti-POSS) complexes in hyperbranched poly(siloxysilane) matrix. Crosslinked hyperbranched (CHB) poly(siloxysilane)-grafted [Ti(OC6H4CN-p){(HSiMe2(CH2)3)(i-C4H9)6Si7O12}] (5) and CHB-poly(siloxysilane)grafted [Ti(OC6H5){(HSiMe2(CH2)3)(i-C4H9)6Si7O12}] (6) have demonstrated high activity, epoxide selectivity (≥99%) and H2O2 efficiency (≥98%) in cyclohexene and 1-octene epoxidation with aqueous H2O2 higher than titanosilicate catalysts such as TS-1 and Ti-β, due to high Lewis acidity of the Ti-center and to the use of CHBpoly(siloxysilane) matrix. The catalysts are readily recyclable with retained activity. Keywords: epoxidation, phenoxy-Ti-POSS, cyclohexene, 1-octene, H2O2
DOI: 10.1134/S1070363220100230 INTRODUCTION Tripodal Ti-POSS complexes are the promising precursors for the synthesis of well-defined highly active heterogeneous epoxidation catalysts [1–3]. Loading of selected Ti-POSS complexes on a silica-polymer support has been carried out [4, 5], but controlling of 3D geometry of the polymer was met some obstacles. Under heterogeneous conditions bipodal and tetrapodal Ti sites [6–8] were less efficient in alkene epoxidation than tripodal Ti sites [1–3]. Therefore, modification of the synthesis and immobilization of tripodal Ti-POSS complexes were the decisive points for achieving optimum catalytic activity and durability of heterogeneous epoxidation catalysts. We have performed synthesis of selective heterogeneous catalysts [1] via physical adsorption of some tripodal Ti-POSS complexes on a PDMS. The prepared catalysts have been used in epoxidation of cyclohexene and 1-octene with aqueous H2O2. However, diffusion time into the PDMS slowed down the alkene conversion rates and the catalysts turnover frequencies. With extended use of physically encapsulated Ti-POSS in PDMS and recyclability, some detaching of Ti-POSS complexes from the PDMS was detected. Therefore we switched to chemical grafting of some tripodal Ti-POSS catalysts into a CHB-polymer [2]. Application of H2O2 as an oxidant
was favorable due to its low cost, high content of oxygen and environmental friendliness as water was the only byproduct of the process [1, 2]. Herein, we describe a simple and efficient method of synthesis of novel heterogeneous alkene epoxidation catalysts by grafting of incompletely condensed-POSS trisilanol in hyperbranched poly(sil
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