The Research Of Catalysts And The Catalytic Mechanism For Selective Oxidation Of Glycerol

Biodiesel is a renewable resource that has great potential application in future.Glycerol is a by-product during the production of biodiesel,and the rapidly rising production of biodiesel has led to a serious surplus of glycerol.Therefore,catalytic oxidation of glycerol to valuable products has become a hot research topic.It was reported that the oxidation rate of glycerol over Pt catalysts decreased during time on stream,which might be caused by over-oxidation of surface metal NPs to oxides,and/or the decarboxylation of aldehyde intermediates to form CO that strongly adsorbed on Pt.And the performance of Au depended strongly on the basicity of reaction mixture.Herein,in order to overcome the deactivation of Pt-based catalysts in selective oxidation of glycerol,the surface chemistry of support was adjusted by doping heteroatom or alloying Pt with Co,so as to change the interaction of metal-support,mediate the dispersion or electronic property of Pt NPs.First of all,we used crbon nanofibers(CNFs)as support.The CNFs were pretreated by nitration and sulfuration in order to bring rich amount of defect sites.Comparatively,thiol groups(-SH)seemed to be more effective than O species for improving the dispersion of Pt NPs.It was found that highly dispersed Pt NPs were beneficial for the oxidation of glycerol.In addition,the selective oxidation of glycerol on Pt NPs was also affected by reaction condition.For example,oxygen pressure and substrate concentration had a little influence for reaction over Pt/S-CNFs,higher glycerol conversion but severe cleavage of C-C could be observed when reaction temperature increased.In order to solve the problem of Pt deactivation,reduced graphene oxide(RGO)was used as support and supported monometallic Pt and bimetallic PtCo NPs via a facile routine under microwave irradiation.It was concluded that the strong electrical interaction between Pt and Co in PtCo/RGO might facilitate the stabilization of Pt NPs,accelerate the oxidation of glycerol,and hinder the further oxidation of intermediates to Cl byproducts,thus improving the selectivity toward GLYA.N-doped MWCNTs(N-MWCNTs)with different N content was prepared via a facile calcinations of mixed multi-walled carbon nanotube(MWCNTs)and urea under different temperature.It was found that N-MWCNTs could improve the dispersion of Pt through strengthened metal-support interactions and donate its electron to metallic Pt.This electron-enriched Pt NPs on the surface of N-MWCNTs were active and stable for the selective oxidation of glycerol.The deactivation of Pt during the glycerol oxidation process can be partially retarded.Also,MWCNTs-pillared nitrogen-doped graphene(NG)was prepared by direct pyrolysis of melamine on MWCNTs,and the synthesized NG-MWCNTs composite was used as the support for Pt.It was found that NG-MWCNTs composite possessed higher surface area(173 m2/g)and bigger pore volume than that of bare MWCNTs and separated pyrolysis product of melamine(CNX).Pt/NG-MWCNTs was highly active and selective for selective oxidation of glycerol to glyceric acid in base-free aqueous solution.The unique catalytic activity of Pt/NG-MWCNTs was not only related to the well dispersed Pt clusters on NG-MWCNTs,but also electron-sufficient Pt due to the electron donating effect of N-dopants in NG-MWCNTs.But the surface area decreased with the increasing of nitrogen content,which was not conducive to the dispersion of Pt and adsorption of substrate.Preparation the hollow carbon nanosphere(HCS)via hydrothermal method and their supported Pt catalyst(Pt/HCS)for selective oxidation of glycerol.It was found that the shell thickness,surface area and oxygen content of HCS could be changed by adjusting the dosage of precursor.This catalyst gave excellent performance for selective oxidation of glycerol under base-free condition.Further study indicated that higher oxygen content was beneficial for improving the dispersion and stability of Pt.This study can provide efficient blue-prints for fabrication of versatile carbon materials for solving Pt deactivation in selective oxidation of glycerol.