| In this paper,porous polyurea(PPU)without any modification was prepared through precipitation polymerization in acetonitrile,using toluene diisocyanate and diethylenetriamine as the monomers.PPU material was obtained without need for any surfactants or porogens on the resultant porous polymer.The morphology,pore size and size distribution of PPU were characterized by scanning electron microscope,mercury intrusion and BET nitrogen adsorption,respectively.Results demonstrate that PPU is of irregular granular form with the size of the granules varied from 30 ?m to 50 μm.The pore size distribution of PPU with size varying from 30 nm to 300 ?m through intrusive mercury test was carried out.The specific surface area and pore volume of PPU were 36.4 m2/g and 4.2 cm3/g,respectively.Its amine group amount of 0.631 mmol/g was determined through spectrophotometric absorbance based on its reaction with p-nitrobenzaldehyde(NBA).Amine groups on PPU surface are converted to aldehyde via a treatment with glutaraldehyde,followed by immobilization of lipase from pseudomonas fluorescenson(PFL)onto the surface of the activated PPU by covalent bonding of the aldehyde groups with the primary amine of the enzyme.Immobilization conditions were optimized with regard to glutaraldehyde concentration,reaction time,temperature and amount of lipase used.It was found that a maximal PFL immobilization of 108.8 mg/g with a high activity of the immobilized PFL of 253 U/mg were obtained at pH 8.0,GA concentration of 0.66 mol/L followed by PFL immobilization with PFL concentration of 2.6 g/L.The activity of the immobilized PFL was also studied at different temperature and in aqueous medium of different pH,and compared with that of the free PFL.Results revealed that both lipases achieved maximal activity at pH 8.0 and at temperature 50 oC,and the immobilized enzyme provided significant stability over the free enzyme,especially at a longer storage time.The immobilized PFL was then used as catalyst in the kinetic resolution of(R,S)-1-PEOH through its transesterification with vinyl acetate.The effects of organic solvent,amount of enzyme used,reaction temperature,molar ratio of substrates and acyl donor on 1-PEOH resolution were studied.The reusability of the immobilized PFL was also tested under the optimized conditions.Using n-hexane as the solvent,0.2 mg of immobilized PFL was used as enantioselective catalyst in kinetic resolution of racemic 1-PEOH under the optimized condition(eep>99.5%,E>200).The conversation of the substrate reached 49.3% at 54 h of the reaction.After 5 reuse cycles,the substrate conversion decreased to 46.3% and the enantioselectivity was practically kept the same obviously.Acetaldehyde was reported to inhibit enzyme catalytic activity and enantioselectivity.Knowing that acetaldehyde was formed in the kinetic resolution,different effects including acetaldehyde concentration,reaction solvent and temperature were investigated.It was seen that both the activity and the enantioselectivity of the catalyst were inhibited in the presence of acetaldehyde.At acetaldehyde concentration of 0.025 mol/L,the catalytic activity of PFL was least inhibited in n-hexane at 40 oC. |
