Modulation Of Active Ti Species Of Hierarchical Titanosilicates And Their Catalytic Performance In Alkenes Epoxidation Reaction

Zeolites are microporous crystalline materials with well-defined pores and high surface area,which are widely used as catalysts in the petroleum refining,petrochemical process and fine chemicals production.The discovery of titanosilicate TS-1(MFI-type)in 1983 has been regarded as a milestone in the field of zeolite catalysis.As an environmentally benign catalytic system,titanosilicates/H2O2 can effectively catalyze some important reactions with high selectivity,such as alkenes epoxidation,aromatic hydroxylation,ketones ammoximation,etc.After nearly four decades of development,several titanosilicates with different topologies have been successfully synthesized.However,with the widening of application of tianosilicate zeolites,the shortcomings of such catalytic materials appear.The sole microporosity of conventional titanosilicates greatly inhibit the intracrystalline diffusion and adsorption of bulky molecules on the accessible active sites,thus limiting their applications.Therefore,the introduction of auxiliary meso-/macroporous networks besides the intrinsic microporosity to form hierarchical structures in Ti-containing zeolites have proven to be an effective strategy to overcome the diffusion problem.Unlike the conventional aluminosilicate or pure silica zeolites,the synthesis of hierarchical titanosilicates is relatively complicated.During this process,the introduction of mesoporogens(soft or hard templates)can aggravate the mismatch between the incertion rate of Ti and formation rate of zeolitic framework;the non-framework Ti will deposit on the surface of zeolite crystals and condense with each othe to form anatase Ti O2,which could cause the inefficient decomposition of H2O2 and lower the catalytic activity of such catalysts.In addition,to date,most of the reported synthetic works on hierarchical titanosilicates were focused on MFI-type.The fabrication of hierarchical Ti-MWW,Ti-MOR,and Ti-beta remains to be further investigated.On the other hand,the intrinsic activity of titanosilicate zeolites depends on the Ti active sites.So,the proper modification of Ti active sites can directly improve the catalytic performance of such catalysts.Unfortunately,up to now,the high catalytic Ti species can only be obtained through complicated post treatment,and the harmful Ti species will be easily formed during this process.In this thesis,we focus on the synthesis of hierarchical titanosilicate zeolites,particularly the one-step rapid synthesis of TS-1 zeolites with highly active mononuclear Ti O6 species;their characterizationand catalytic performance in alkenes epoxidation reactions.The main research results of this thesis are shown as follows:1.Nano-sized hierarchical TS-1 catalyst has been successfully synthesized by using Triton X-100 as a mesopore directing template under rotational crystallization conditions.UV-Vis,UV-Raman and IR spectroscopy analyses prove that the formation rate of the MFI structure and the insertion rate of titanium could match well when the hierarchical TS-1 is synthesized under rotational crystallization conditions.The Ti ions are fully incorporated into the framework of TS-1-HR,and no anatase Ti O2 exists in it.Thanks to the hierarchical structure and absence of anatase Ti O2,TS-1-HR shows high catalytic performance in the epoxidation of linear and cyclic alkenes.2.The one-step rapid synthesis of TS-1 zeolite with highly catalytic active Ti species has been achieved by introducing active seeds and microwave irradiation.Based on the UV-vis and UV-Raman spectra,the novel octahedral-coordinated Ti species(Ti O6)is presented in resultant TS-1 zeolite.The mononuclear state of Ti O6 species is further determined by X-ray absorption spectroscopy.Experimental studies reveal that adding active seeds can induce the formation of mononuclear Ti O6;on the other hand,the microwave irradiation will boost the generation of such Ti O6.The mononuclear Ti O6 species in the prepared TS-1 remain stable upon calcination,without the formation of anatase Ti O2.The TS-1 catalyst with mononuclear Ti O6 species exhibits outstanding catalytic activity and stability in 1-hexene epoxidation.3.Slite-like mesopores can be introduced into Ti-MWW crystals by the post-treatment of microporous Ti-MWW zeolite in CTAB and TPAOH mixed solution.According to the characterization results,the long chain CTAB molecules cannot enter into the inner channels due to the steric inhibition;such surfactant will adsorb on the external surface of Ti-MWW.Thus,the OH-mainly attacks the framework silicon along with the interlayer space,and the silit-like mesopores finally form in Ti-MWW crystals.The resultant material remains high crystallinity,well-defined morphology and the sole distribution of framework Ti O4;on the other hand,such hierarchical Ti-MWW processes large external surface area,exposing more Ti active sites.Therefore,hierarchical Ti-MWW zeolites exhibit excellent catalytic activity in alkenes epoxidation reaction.In summary,several effective strategies have been developed toward the synthesis of hierarchical titanosilicate zeolites,in particular those with highly active Ti species(e.g.,mononuclear state of Ti O6 species).This work may open new perspectives for the designed synthesis and applications of titanosilicate zeolite catalysts in various important selective oxidation reactions.