| Plant has been used in the preparation of nanocatalysts,as a natural reducing agent and protective agent,Improtantly,plant-mediated synthesis of nanaostructured metal catalysts is gaining more and more attention due to their low-cost and environmental-friendly advantage,and the as-synthesized catalysts demonstrate high catalytic activity and stability for some important industrial reactions.However,due to the diversity of plant and the complexity of biomolecule structure,the mechanism that the role of plants on the nanocatalysts is still challenging.At present,reaearchers mainly focused on the role of active biomolecules on the synthesis of nanoparticles by analyzing the content variation of active components or spectra shift on Fourier transform infrared spectroscopy(FTIR)before and after the synthesis.But the role of biomolecules in subsequent catalyst preparation(loading and calcination)and catalytic reaction was rarely mentioned.In this study,we adopted silica gel column chromatography to separate the active biomolecules in Cinnamomum camphora(CC)aqueous extracts,and then the obtained separated components were used to prepare a series of Au/TS-1 catalysts towards vapor-phase epoxidation of propylene.By comparing the catalytic activity with the aid of multiple characterizations,we tried to reveal the influence of biomolecules on the catalytic activity and stability of propylene epoxidation over Au/TS-1 catalyst.Firstly,according to the polarity difference of active components,Cinnamomum camphora(CC)extract were separated into three groups(SLo-S1,S2-S3 and S4-S6)through C18 reversed phase silica gel column,and were identified as carbohydrates,polyphenols and flavonoids respectively with the aid of FTIR?1HNMR?UV-Vis?HPLC.Then the separated components were used to react with HAuCl4.It was found that the reducing ability of flavonoids was the strongest,and that of carbohydrates was the weakest.Furthermore,a series of Au/TS-1 catalysts towards propylene epoxidation was prepared by bioreduction-ionic liquid-enhanced immobilization(BR-ILEI)method.And catalyst evaluation demonstrated that the Au/TS-1-S5 catalyst is the most active with the PO formation rate of 219.68 gpo kgcat-1 h-1.Its high catalytic activity can be explained as following:XPS spectra show that the catalyst prepared by flavonoids is only loaded with Au0 due to the strongest reducing power among the three components;XRD and TEM results indicate no obvious size variation of AuNPs after calcination resulting from the presence of-COOH.In addition,high surface hydrophobicity of the catalyst is helpful to improve the catalytic activity towards vapor-phase epoxidation of propylene.The Au/TS-1-S2 catalyst shown superior PO selectivity for propylene epoxidation,because polyphenols can effectively reduce the acidity of the catalyst surface and improve its diffusion.Finally,we investigated the stability of catalyst prepared by three separated components during 36 h at 300 ?.The PO formation rate of catalysts prepared by carbohydrates rapidly decreases with the function of time-on-stream,while that of catalysts prepared by polyphenolss and flavonoids nearly constant,suggesting remarkable catalytic stability for the propylene epoxidation.In-situ FT-IR spectra show that the deactivation of catalyst is not caused by the blocking of active sites with carbonaceous deposits.BET results indicate that no propylene oxide oligomers block the microspores.However,TEM results demonstrate that the sintering of gold particles are contributing to the deactivation of catalysts prepared by carbohydrates.The catalyst prepared by polyphenols and flavonoids components demonstrated remarkable stability because the-COOH in these components can prevent Au NPs from agglomeration.Based on the above results,the Au/meso-Ti-Si catalyst prepared by chemical method,which is easy to be deactivated,was modified by CC extracts and the stability of the catalyst was effectively improved. |
PDF Full Text Request