Encapsulation of vanadium complexes in inorganic or hybrid matrices via the sol-gel method: application to the epoxidation of allylic alcohols

Abstract

Complexos de vanádio foram encapsulados em matrizes inorgânicas (baseadas na hidrólise/ condensação de tetrametilortossilicato -TMOS) e híbridas (TMOS e um agente de co-condensação) pelo método sol-gel. Os sólidos resultantes foram testados na epoxidação de álcoois alílicos na presença de hidroperóxido de terc-butila (TBHP) como agente oxidante, a 70 o C. Para o catalisador baseado em [VO(salen)], a matriz híbrida levou a maiores freqüências de turnover iniciais do que a matriz inorgânica para todos os álcoois testados (trans-3-fenil-2-metil-2-propen-1-ol, trans-2-hexen-1-ol, cis-2-hexen-1-ol, 1-octen-3-ol), o que pode estar relacionado com seu maior diâmetro de poros. Embora não tenha sido observada lixiviação de vanádio nas reações catalíticas, os experimentos de reciclagem com o catalisador híbrido e trans-2-hexen-1-ol mostraram perda de vanádio em cada lavagem com Soxhlet (efetuada após cada corrida, usando CH 2 Cl 2 ), totalizando 1,5% em massa após 3 reações. Em 4 corridas sucessivas, mantendo as relações molares iniciais, as conversões decresceram de 100% a 85%, 65% e 40% (em 5h).Vanadium complexes have been encapsulated in inorganic (based on hydrolysis/condensation of tetramethylorthosilicate -TMOS) and hybrid matrices (TMOS plus a co-condensation agent) using the sol-gel method. The resulting solids were tested in the epoxidation of allylic alcohols in the presence of tert-butyl hydroperoxide (TBHP) as the oxidant agent at 70 o C. When the catalyst was based on [VO(salen)], the hybrid matrix led to higher initial turnover frequencies than the inorganic one for all tested alcohols (trans-3-phenyl-2-methyl-2-propen-1-ol, trans-2-hexen-1-ol, cis-2-hexen-1-ol, 1-octen-3-ol), which might be related to its higher pore diameter. Although no vanadium leaching was observed during the catalytic reactions, recycling experiments with the hybrid catalyst and trans-2-hexen-1-ol showed that some loss of vanadium took place at every Soxhlet washing (performed after every run, using CH 2 Cl 2 ): a total of 1.5 wt.% after three reactions. In four successive runs, keeping the initial molar ratios, conversions decreased from 100% to 85%, 65% and 40% (in 5h).Keywords: vanadium, epoxidation, allylic alcohols, sol-gel method Introduction[VO(acac) 2 ] is one of the best and most extensively studied catalyst for the epoxidation of unsaturated alcohols.1 The combination [VO(acac) 2 ]/tert-butyl hydroperoxide (TBHP) has proved to produce a highly regio-and stereoselective homogeneous system for the epoxidation of allylic 2-4 and homoallylic alcohols. 5Homogeneous catalysis, however, presents several drawbacks, in particular the recovering of the catalyst at the end of the process, warranting a search for supportedvanadium catalysts. There are several reports about the immobilization of [VO(acac) 2 ] and VO 3 -onto functionalized polystyrene resins. [6][7][8][9][10] The vanadium complexes have also been attached to ion-exchange resins. 11,12 More recently, the microencapsulation of [VO(acac) 2 ] in polystyrene was reported to pro...

Pd complexes based on phosphine-linked cyclophosphazenes: synthesis, characterization and application in Suzuki coupling reactions

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The Co‐Entrapment of a Homogeneous Catalyst and an Ionic Liquid by a Sol–gel Method: Recyclable Ionogel Hydrogenation Catalysts

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Up-converter nanophosphor Y2O2S:Er,Yb aminofunctionalized containing or not spherical silica conjugated with BSA

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