A Study On The Boron-assisted Regulation Of Active Centers In MWW-type Titanosilicates And Its Catalytic Performance

As is well-known,the structure-activity relationship between the active center of zeolite and its catalytic performance has always been valued.Controllable adjustment of the nature of active center?type,quantity,strength,location and accessibility?is the key to give full play to the catalytic performance of molecular sieves.Considering the special role of element boron?B?in the synthesis of molecular sieves,this paper focused on the regulation and control of the properties of Ti active center in MWW-type titanosilicate.We try to figure out the role of boron species in influencing the structure,distribution and accessibility of Ti active centers,and expect the gained scientific knowledge will provide a strong support for the construction of efficient TS/H₂O₂catalytic system.The main research work consists of the following four parts.?1?Firstly,based on the acid treatment with the lamellar Ti-MWW?P?precursor,the function and origin of boron species in the transformation of the structure of Ti active centers in Ti-MWW were explored.Combined with the characterizations of UV-Vis,UV-Raman and FT-IR spectra,it was suggested that the extensive extra-framework“TiO₆”species in Ti-MWW?P?can bond to the framework“TiO₄”with a weak atomic binding forces,shielding the Lewis acidity of zeolite.These extra-framework“TiO₆”species can be removed completely by the nitric acid treatment,followed by the exposure of zeolite Lewis acidity.As the acid treatment proceeded,a large amount of skeleton“BO₄”and silicon atoms were successively extracted,accelerating the formation of open Ti?OSi?₃OH species transformed from the close Ti?OSi?₄ species.Thereby,the strength of Lewis acidity in Ti-MWW zeolite was enhanced,which was accompanied by the improvement of zeolite catalytic performance.?2?Secondly,based on the acid treatment with the lamellar Ti-MWW?P?precursor,the function and origin of boron species in the construction of the interlayer-expanded MWW-type zeolites were explored.Combined with the characterizations of XRD?TEM and argon adsorption isotherm,it was shown that with the prolonging of acid treatment time,some silicon species enriched in the filtrate can re-inserted to the space between the layers and serve as the pillars,therefore widening the interlayer distance.The overall analysis indicated that the removal of boron species can result in the loss of silicon species,providing the important pillars required for interlayer expansion.Further catalytic tests showed that the interlayer 12 member-ring pore window can enhance the diffusion and mass transfer of bulky cyclic olefin substrates,thus improving the accessibility of Ti active center in supercages and increasing the catalytic oxidation activity.?3?Thirdly,based on the difference in the distribution of heteroatoms?B,Al,Ti?at the crystallographic T sites of the MWW structure,the Al-containing Ti-MWW molecular sieves was synthesized,and the function and origin of boron species in regulating the distribution of Ti active centers were also explored.XPS,ICP and ²⁷Al MAS NMR characterization results showed that the competitive occupancy between the heteroatom B and Al on the T₂,T₈,T₃,T₇ positions happened,thereby promoting the distribution of B and its adjacent Ti active sites outside the crystal surface.Further catalytic tests showed that the diffusion path of olefin substrates was shortened,and the accessibility of Ti active center was enhanced,thereby the catalytic activities in the cyclohexene epoxidation and cyclohexanone ammoximation were increased.?4?Finally,based on the above B-assisted construction of efficient MWW-type titanosilicates and their catalytic performances,the products in the selective oxidation of cyclohexene catalyzed by TS/H₂O₂ system was explored.Taking the selectivity of different oxidation products as the evaluation standard,six important factors including the structure of Ti active sites,solvent,oxidant,additive and reaction temperature as well as the reaction time were investigated.Results showed that the key to regulating the ratio of cyclohexene oxidation products was to control the cracking mode?homolytic cleavage and heterolytic cleavage?of O-O bond in the active intermediate Ti-OOH.By increasing the electropositivity of active Ti atoms,the heterolytic cleavage of O-O bond in Ti-OOH can be promoted,thereby improving the proportion of epoxidation products.The presence of alkaline additives can accelerate the homolytic cleavage of O-O bond in Ti-OOH,thereby improving the selectivity of allyl oxidation products.