Polyphenol Assisted Preparation Of Au-Ti Based Catalysts Toward Vapor Phased Propylene Epoxidation With H₂/O₂

Vapor phased propylene epoxidation with H2/O2 bears significant potential to produce propylene oxide(PO)at industrial scale.This process usually utilizes titaniumsilicalite-1 zeolite supported Au(Au/TS-1)as catalyst,in which Au-Ti catalytically active sites cooperate to generate HOO*and Ti-OOH species.Fundamentally,the TiOOH species plays a decisive role in the performance of the catalytic reaction.Therefore,the construction of Au-Ti integrated catalyst could strengthen the Au-Ti interaction,which facilitates to promote the generation and transfer of active species thereby increasing the performance of Au-Ti catalysts as a whole.In view of this,the polyphenol mediated coordination-reduction and solvent-free synthesis were adopted to prepare Au-Ti based catalysts,with the aim of strengthening Au-Ti interaction and regulating their electronic structures,thereby significantly improving the catalytic performances.First of all,polyphenol-enriched bio-extract was introduced to coordinate with the Ti4+ precursor,followed by solvent-free synthesis to prepare bio-TS-1 zeolite.Comprehensive structural characterizations indicate that the involvement of bio-extract contributes to improve the structure regularity and inhibit the framework defect.Based on the investigation of crystal structures and framework Ti states under different crystallization times,the regulation effect of bio-extract on the solvent-free synthesis process could be further described as:the addition of bio-phenol helps to slow down the hydrolysis of Ti source,which overall coordinates the crystallization rates of Si and Ti sources and thereby enables Ti(Ⅳ)to embed into framework sites.Meanwhile,the vibrational measurements confirm that the bio-phenol coordination remarkably increases the Lewis acidity of framework Ti,which exerts a stabilization effect on TiOOH species as a result.Furthermore,in light of DFT simulations,the positive effect of bio-phenol coordination could be speculated to that it promotes Ti(Ⅳ)insertion into in zeolitic architecture,especially to occupy those sites with enhanced Lewis acidity.Such an enhancement of acidity not only helps to delocalize the charge density of O-O moiety to prevent homolysis cleavage,but also promotes the reactivity of α-O toward double bond of propylene,which significantly facilitates the stabilization and transmission of Ti-OOH active species within Au-Ti interface.As expected,the resultant Au/bio-TS-1 supported catalyst shows propylene conversion of 10.7%,PO selectivity of 82.9%and H2 efficiency of 21.7%.Afterwards,to further promote the PO selectivity as well as H2 efficiency,Au-Ti integrated catalyst was prepared from the view of Au-Ti synergy,in which the Lewis acidity of framework Ti was regulated by Au-Ti interaction.Tannic acid was exploited to prepare Au-Ti pre-integrate precursor Au-Ti/SiO2,serving as Ti and Si sources for zeolite growth,followed by solvent-free synthesis to generate an-Au@TS-1 integrated catalyst.Comprehensive structural characterizations confirm that an-Au@TS-1 catalyst displays zeolite encapsulation structure with uniform distribution of Ti sites.Furthermore,the tannic acid mediated coordination-reduction process enables the AuTi proximity,namely realizing the integration of Au-Ti site at the geometric level.XPS and Raman results confirm the occurrence of Au-Ti electronic interaction in anAu@TS-1 catalyst.Combining with the results of XANES and acetonitrile adsorption IR,it could be found that the Lewis acidity of the Ti site is improved as a result of enhancing Au-Ti interaction.Furthermore,with the help of DFT and EXAFS,the above regulation mechanism could be explained as:strengthening the Au-Ti interaction contributes to weaken the polarization of the Si-Oδ--Tiδ+bond,thereby enhancing the Lewis acidity of the framework Ti.In such way,the transfer of Ti-OOH and HOO*species is promoted,whereas the decomposition of Ti-OOH is inhibited.Correspondingly,the PO selectivity and H2 efficiency of an-Au@TS-1 have been effectively improved,reaching 90.2%and 26.7%,respectively.Finally,to improve the overall catalytic performances,another Au-Ti integrated catalyst was prepared,in which the electronic structure of Au was regulated by Au-Ti interaction.Bio-extract was incorporated to prepare Au-Ti pre-integrate precursor AuTi/S-1,serving as Ti source and crystal seed for zeolite growth,followed by solventfree synthesis to generate Au-Ti@MFI integrated catalyst.Comprehensive structural characterizations suggest that Au-Ti@MFI catalyst displays zeolite encapsulation structure with uniform distribution of Au NPs.Furthermore,the bio-extract mediated coordination-reduction process realizes the integration of Au-Ti site at the geometric level.XPS and CO adsorption IR characterizations confirm the occurrence of Au-Ti electronic interaction in Au-Ti@MFI catalyst.Based on DFT calculations,it could be found that the d ban center of Au has been down-shifted,as a consequence of enhancing Au-Ti interaction.The resulted Au-Ti@MFI catalyst manifests excellent performances,of which propylene conversion,PO selectivity and PO yield reach 14.8%,83.9%and 205.0 gPO/kgcat·h,respectively.Above all,from the view of HOO*generation and transfer,the structure-activity relationship of Au-Ti@MFI could be proposed as:Integration Au-Ti active sites at the geometric level enables the enhancement of Au-Ti electronic interaction,which not only regulates the electronic structure of Au to effectively generate HOO*，but also increases the Au-Ti geometric intimacy to quickly transfer HOO*to framework Ti sites.Herein,the resulted integrated catalyst displays significantly enhanced conversion as well as selectivity.In summary,with great environment-benignity and operability,the methodology proposed here would provide reference for the rational synthesis of other integrated catalyst.Meanwhile,the structure-reactivity relationship proposed from the view of active species generation-transfer could hold theoretical support for related research on vapor phased propylene epoxidation with H2/O2.