Novel alkoxysilane pentacoordinate O [dbnd]V(IV) complexes as supported catalysts for cyclohexane oxidation with dioxygen

Abstract

A variety of newly synthesized and well characterized alkoxysilane pentacoordinate Oxovanadium(IV) complexes, VO[Sal(PMeOSi)DPTA] 3[a], VO[Cl-Sal(PMeOSi)DPTA] 3[b], VO[Sal(PMeOSi)DETA] 6[a] and VO[Cl-Sal(PMeOSi)DETA] 6[b], (Sal = salicylaldehyde, DPTA = bis(aminopropyl)amine, DETA = diaminoethylamine), have been anchored by covalent bond into the surface of SiO 2 and/or Al 2O 3 via silicon-alkoxide route by a condensation process as supported catalysts. These solid supported catalysts (abbreviated as catalysts A to H) showed high catalytic efficiency in the selective oxidation reaction of cyclohexane using molecular oxygen under relatively mild condition in a micro-batch reactor. The Catalyst C (SiO 2/ 3[a] complex) system exhibits best activity, overall yield 38.5% (TONs, ca. 5.0 × 10 3) as well as high selectivity 98% (cyclohexanol 74%, cyclohexanone 24%). Notably, cyclohexane shows significantly improved yield 44.0%, by the addition of pyrazinecarboxylic acid as a co-catalyst. The TGA indicates these catalysts are stable up to maximum reaction temperature, ca. 473 K and ICP analysis shows there is negligible vanadium loss from the supported catalyst after the reaction, allowing further use of the V-catalyst. The various factors influences ( i.e. temperature, O 2 pressure, reaction time, catalyst amount) were also investigated in the systematic way, to optimize the reaction processes. The impact of radical traps and detection of intermediate peroxy radical were also investigated to establish a radical mechanism.