Synthesis Of TS-1 Zeolites And Metal Supported Zeolites With High Catalytic Performance

The combustion of conventional liquid fuels suffers from serious environment problems,such as acid rain,water pollution,and vegetation destruction,resulting from the emission of sulphur oxidates,which badly affect our physical health.As a result,many tough legislations have been published all over the word to limit the concentration of sulphur compounds.Compared to hydrodesulfurization(HDS),oxidative desulfurization(ODS)is safer,which has mild reaction conditions,i.e.,reactions are carried out at 60?80°C,under atmosphere with H₂O₂ as oxidant.Zeolites with high specific surface area,uniformly distributed micropores,and superior stability,which are the most important solid catalysts in chemical industry,have been wildly applied in kinds of valuable catalytic reactions.Among them,titanosilicate zeolite TS-1 are efficient active sites in catalytic ODS.However,limited by the intrinsic microporous structure in conventional zeolites,reactants,especially molecules with large size,have trouble in diffusion in the zeolite crystal,and catalytic active Ti sites in the micropores were hardly accessible to reactant molecules,result in the inferior catalytic activity.Therefore,there is some trouble in application in industrial catalysis of TS-1.Synthesizing hierarchical TS-1 zeolite by introducing mesopores or/and macropores,can increase the diffuse rate,expose more Ti active species,and remarkably improve the catalytic activity.Preparation of hierarchical zeolite always needs expensive or well-designed mesoporogen,and there may be pollution caused by removing of mesoporogen.Another common approach to synthesize hierarchical zeolite is post-treatment,i.e.,removing framework atoms by acid/alkali etching.However,there is extra experiment step but mass loss of zeolite crystal.Based on the above situation,it is significant for ODS catalytic application of TS-1 zeolites to prepare hierarchical TS-1zeolite with cost-effective,environment-friendly,and efficient strategy.Hydrogen(H₂) as an environment friendly renewable energy,has attracted a lot of attention.Storage,transportation,and release safely are major problem and scientific challenge.In the past few years,chemical hydrogen storage based on ammonia borane(AB)and other compounds has been the most promising hydrogen storage approach.Therefore,it is a significant subject in the field of energy to explore efficient catalysts for AB hydrolysis.Many catalytic materials have been used to boost H₂ release from AB,among them metal nanocatalysts such as Pt,Ru,Rh,show excellent catalytic performance.However,metal nanocatalysts always suffer from the unsatisfactory stability,sinter in harsh reaction conditions,and lose catalytic activity.Take high price of the noble metal into consideration,it is an urgent necessary for industry application that seeking new synthesis method to optimize catalyst structure,modify the dispersion and stability of metal species,thus improve catalytic performance.In this doctoral thesis,we carried out research into synthesis and catalytic performance of hierarchical TS-1 zeolites and metal supported zeolite catalysts.At first,we synthesis hierarchical TS-1 zeolite catalysts without the utilization of mesoporogen by temperature-based kinetic-regulation approach.The obtained catalysts showed excellent catalytic performance in ODS reaction of dibenzothiophene(DBT)due to the abundant mesoporous structure and high-activity framework Ti sites.Furthermore,zeolites can be used as supports to metal species in many valuable catalytic reactions.Pt/TS-1 zeolite catalysts have prepared by wetness impregnation method using TS-1zeolite crystal as supports.There is electron transfer between Pt and framework Ti atoms,which stabilizes Pt nanoparticles from aggregation in high-temperature reduction and reaction process,improving the activities in AB hydrolysis.Rh Hf clusters were supported on MFI zeolite through an incipient wetness impregnation.Formed Rh-[O]_x-Hf structure played an important role in the stabilization of Rh nanoclusters,thus avoiding the leaching and sinter.The obtained Rh Hf/S-1 zeolite catalyst has good catalytic performance in H₂ generation from AB hydrolysis and tandem hydrogenation of various nitroarenes.Besides,the excellent thermal/hydrothermal stability made it possible to apply Rh Hf/S-1 in industrial catalytic reaction processes with harsh conditions.The main research contents are summarized as follows:1.We have synthesized anatase-free hierarchical TS-1 zeolite catalysts by kinetic-regulation construction based on temperature.More specifically,tetrabutylammonium hydroxide(TBAOH)was used as organic structure agent without the utilization of mesoporogen,and a two-step crystallization with low temperature followed by high temperature was carried out to boost more catalytic active Ti species substituted into zeolite framework,and construct layer stacking-resultant hierarchical structure at the same time.Abundant mesopores and active framework Ti atoms were observed in obtained samples.The obtained hierarchical TS-1 zeolite catalysts showed excellent ODS activity of large-size DBT,with remarkably enhanced conversion rate compared to the conventional microporous counterpart(conversion 100%vs.12.3%at10 min).Temperature-based kinetic-regulation strategy provides an effective and feasible approach for construction of hierarchical titanosilicate zeolites with controllable active Ti species.2.TS-1 zeolite was utilized to support noble metal(such as Pt,Ru,and Rh)species by impregnation method.Framework Ti atoms in TS-1 zeolites could optimize the distribution of Pt species and inhibit the sintering,resulting in uniformly dispersed ultra-small metal Pt nanoparticles or nanosized cluster on zeolite supports.According to the characterization of XPS,Pt/TS-1 sample has electron transfer between framework Ti atoms and supported Pt.Compared to Pt/S-1,Pt nanoparticle size in Pt/TS-1 zeolite catalysts was smaller(2.2 nm vs.4.7 nm);complete hydrolysis of1 mmol AB with Pt/TS-1 could be realized in 14 min,at 25°C,dramatically less than the Pt/S-1 sample(47 min).The TS-1 zeolite-supported strategy is suitable for Ru and Rh,as well;other heteroatom framework-substituted zeolites can also stabilize the supported Pt species.3.We introduced Hf/Zr element to stabilize Rh species supported on zeolite by formation of Rh-[O]_x-Hf structure.The obtained Rh Hf/S-1 sample exhibited excellent H₂ generation rate from AB hydrolysis with a high turnover frequency(TOF)of363 mol _H2 mol _Rh^-1 min^-1 at 25°C,which is about 3-fold improvement than Hf-free counterpart(119 mol _H2 mol _Rh^-1 min^-1).High catalytic performances and selectivities were also achieved in the tandem reactions that coupled the AB hydrolysis with nitroarenes hydrogenation.Rh-[O]_x-Hf structure played an important role in stabilizing noble metal Rh species,thus inhibiting the sintering of metal at high-temperature(600–700°C)redox atmosphere or hydrothermal condition.It is in prospect that Rh Hf/S-1zeolite catalysts will be applied in the harsh reaction conditions in industry.The sinter-resistant strategy can be successfully applied in other noble metal,such as Pt,Ru,and other supports,such as Si O₂,as well,which paves a way to synthesis of stable sinter-resisted metal nanocatalysts.