Pd Nanoparticles-Polyethylenemine-Lipase Bionanohybrids as Heterogeneous Catalysts for Selective Oxidation of Aromatic Alcohols

A new kind of bionanohybrids, synthesized by using 63 kDa-lipase from Candida rugosa (CRL) in the presence of polyethyleneimine (PEI) by in situ formation of Pd (0) nanoparticles (PdNPs) in aqueous media at room temperature were obtained. Addition of Triton X-100 as additive was also used. XRD confirmed the formation of crystalline Pd (0) as metal species. TEM analyses revealed PdNPs ranging from 6 to 8 nm, whereas using Triton X-100 the size range of the nanoparticles was 9 to 24 nm (mainly being 13 nm). PdNPs of biohybrid created only with Triton X-100 ranged from 8 to 21 nm. The different heterogeneous nanobiohybrids were applied as catalyst in the oxidation of benzyl alcohol under mild conditions. In all cases, they were selective to the synthesis of alcohol to benzaldehyde without traces of carboxylic acid. More than >99 % conversion was obtained in THF at 50 degrees C under air as oxidant. The addition of hydrogen peroxide improved the conversion initially but the reaction was stopped at around 50-68 % conversion after 24 h in the best of cases. These new nanocatalysts synthesized using PEI showed a very high recyclability, maintaining >95 % activity after 5 cycles of use. Also successful results were obtained in the oxidation of other aromatic primary and secondary alcohols. The new PEI-nanobiohybrids conserved the enzymatic activity and showed excellent results in metallic-enzymatic domino reaction.