Hierarchical Porous MOFs With Hydrophobic Surface Properties For Lipase Immobilization

Metal-organic frameworks(MOFs)have the characteristics of rich porous structure and easy functional modification,which have attracted extensive attention of researchers in the field of immobilized enzymes.However,the poor matching of the microporous structure and surface properties of MOFs with enzyme largely limits their application in the field of immobilized enzymes.Lipase is a widely used biocatalyst in industry,whose size is about 5 nm,and there is an inner hydrophobic "lid" structure above the active site,so the mesoporous structure and hydrophobic surface of the support can improve its immobilized lipase performance.In this paper,lipase was used as a model,MOFs were used as substrate materials,and the geometric structure and surface properties of MOFs were regulated to match the properties of lipase,so as to improve the performance of immobilized lipase.Firstly,the geometric structure and lipase were matched to synthesize hierarchical porous MOFs with both microporous and mesoporous structures.Then,the surface properties were matched with lipase to construct MOFs with surface hydrophobicity.Finally,MOFs with dual characteristics of hydrophobicity and hierarchical porous were synthesized to verify the improvement of lipase immobilized perference,and the immobilized enzyme with the best performance was selected to synthesize functional oil 1,3-Diglyceride.The main research contents are as follows:First,because the porous structures of most MOFs are micropores smaller than 2nm,it is impossible to immobilize the enzyme inside its pores and provide better protection.In order to enable lipase to be immobilized in the pores of MOFs,three kinds of hierarchical porous MOFs with mesopores were synthesized in this work.They were Ui O-66(coordination of Zr and terephthalic acid)synthesized by post-treatment,ZIF-8(coordination of Zn and 2-methylimidazole)synthesized by template method,and MIL-101(Fe)synthesized by acid etching(coordination of Fe and terephthalic acid).The hierarchical porous Ui O-66 had a particle size of about 300 nm,and the mesopore size can be adjusted between 2 and 20 nm,and the stability was excellent.The hierarchical porous ZIF-8 had a particle size of about 500 nm,a mesopore diameter of about 4 nm with narrow mesopore distribution,and good stability.The particle size of MIL-101(Fe)was 1 μm,and the maximum mesoporous pore size can reach 34.6 nm,but its mesoporous content was small and its stability was relatively poor.Above all,the synthesized hierarchical porous Ui O-66 was suitable for immobilized lipase because of its regulatable mesoporous size and good stability.It can be selected as the research object of surface property regulation to carry out the next work.Further,lipase has a unique “lid” structure,so the hydrophobic surface facilitates the adsorption and interfacial activation of lipase.In order to study of interfacial properties on the performance of the immobilized lipase,Ui O-66 was taken as the research object,the surface of Ui O-66 was hydrophobically modified by using three modifiers Octadecyltrimethoxysilane(TMODS),Octadecylphosphonic acid(OPA)and Dowcoring 1-2577(DC 1-2577)in different ways,and a series of hydrophobically modified Ui O-66 were prepared.Infrared spectrum,X-ray photoelectron spectrum and element distribution map proved that the three modifiers successfully modified the surface of Ui O-66 and promoted the water contact angle of Ui O-66 from 63°to 100°-140°.After modification,the specific surface area of Ui O-66 decreased slightly,but the crystal structure was not affected.The obtained hydrophobic Ui O-66 was dispersed in ethanol for 8 h ultrasound and the water contact angle of Ui O-66@TMODS-100 decreased from 113° to 93°,while the water contact angles of Ui O-66@OPA-4 and Ui O-66@DC-20 were almost unchanged,presumably because TMODS was bound to the surface of Ui O-66 by physical adsorption,and the binding force was not strong and easy to fall off.Finally,combining the regulation of the carrier geometry and the surface properties,the surface hydrophobic hierarchical porous MOFs with dual properties were synthesized,to verify the results of directional regulation of MOFs.First,optimize the immobilization conditions,and then explored the effects of the surface properties of immobilized lipase.Compared the performance of different hydrophobic Ui O-66 immobilized enzymes,Ui O-66@DC-20 had the highest activity recovery rate which was the best hydrophobic Ui O-66.Afterwards,the surface hydrophobic hierarchical porous Ui O-66 was prepared by the optimal hydrophobic modification scheme,and the performance indexes of different hierarchical porous Ui O-66 immobilized lipases were systematically investigated.The results showed that Ui O-66-16@DC-20 had the highest enzyme load,while Ui O-66-08@DC-20 had the best protection against lipase.Finally,Ui O-66-08@DC-20@CALB was selected as the catalyst for the synthesis of 1,3-Diglyceride.The content of 1,3-Diglyceride reached the maximum value at 53% after 2 h of reaction.Compared with the commercial immobilized enzyme Novozyme 435,the reaction time was shortened and had fewer by-product.The self-made immobilized enzyme showed better catalytic efficiency and selectivity than Novozyme 435.