Study On Catalytic Oxidation Performance Of HTS-1

As a type of titanium-silicon catalytic material with excellent catalytic oxidation performance and environmental friendliness,the synthesis of TS-1 zeolite and its successful application as selective oxidation catalyst for hydrocarbon organic compounds are considered as a milestone in the field of zeolite catalysis in the 1980s.In this paper,a novel hollow titanium silicate molecular sieve(HTS-1)was used as catalyst to comprehensive study its catalytic oxidation performance and related work.The main results are shown as follows:1.Diversity of HTS-1 catalytic epoxidation reactions with H₂O₂were investigated using various olefins as substrates(monoene,dienes,cycloolefins,etc.).The results showed that HTS-1 with special hollow structure exhibited better catalytic properties than TS-1 in this system,and the catalytic epoxidation activity of HTS-1 for alkenes was higher than that of cycloolefins with the same carbon number.The effects of reaction conditions on the catalytic performance of HTS-1 were investigated by using cyclopentene and 4-vinylcyclohexene oxidation as probes.The results show that the selectivity of epoxy compounds can be significantly improved by adding appropriate alkaline additives.Under the optimum conditions,the conversion of cyclopentene and 4-vinylcyclohexene can reach 95.85%and 25.80%respectively,and the selectivity of target epoxides can reach 98.79%and 52.17%respectively.2.The catalytic activity of HTS-1 for alcohols oxidation was investigated using 2,3-butanediol oxidation as a probe.The influencing factors of the reaction were investigated from seven aspects:reaction temperature,time,solvent type,H₂O₂ concentration,H₂O₂ dosage,HTS-1dosage and alkaline additives dosage.The experimental results show that HTS-1/H₂O₂system has excellent catalytic properties for 2,3-butanediol oxidation and can be almost completely converted under certain conditions.If the carbon chain cleavage of 2,3-butanediol can be inhibited,acetaldehyde or acetic acid can be reduced,and the selectivity of target products 3-hydroxy-2-butanone and 2,3-butanedione can be improved,thus,a new green synthetic process of 2,3-butanedione can be opened up.At the same time,by investigating the conversion of 2,3-butanediol isomers,it is found that dl-type isomers are easier to oxidize than meso-type isomers.Under certain conditions,the conversion of dl-type isomers can be about 1.5 times of meso-type isomers.It is proved that the spatial structure of 2,3-butanediol isomers has a significant impact on their chemical properties.3.According to DFT calculations at the B3LYP level,the Fokker constants of different enols were calculated.By comparing nucleophilic properties,it was concluded that the epoxidation products and hydroxyl oxidation products of enols exhibited different competition rules with different carbon chain lengths or different positions of hydroxyl.At the same time,the catalytic activity of HTS-1 for allyl alcohol oxidation under mild conditions was investigated with H₂O₂ as green oxidant.The results showed that HTS-1 exhibited high catalytic activity and shape-selective catalytic effect.The reaction products are mainly epoxy compounds oxidized by double bonds,and a small number of aldehydes oxidized by hydroxyl groups are formed.The effect of reaction conditions on the conversion of allyl alcohol in water was investigated.Under the optimum conditions,the conversion of allyl alcohol and the selectivity of propylene oxide can reach 77.27%and 90.36%respectively.This is a green process for the efficient synthesis of propylene oxide.4.Catalytic oxidative cleavage of alkenes to aldehydes was firstly found in the oxidation of alkenes catalyzed by HTS-1/H₂O₂ system.The effects of solvents,temperatures and auxiliaries on the conversion and distribution of oxidation products were further investigated using 1-hexene as substrate.The results showed that ethyl acetate as solvent was the most favorable for oxidative cleavage of alkenes to aldehyde.The yield of n-valeraldehyde could be increased by increasing temperature.The addition of basic auxiliaries greatly reduced the selectivity of n-valeraldehyde.It is speculated that the oxidative cleavage mechanism of 1-hexene in HTS-1/H₂O₂ system is mainly an addition and cleavage oxidation process in acidic environment.Upon contacting HTS-1 with H₂O₂,Ti(IV)–OOH formed at the titanium centers,which added to the double bond to form an unstable intermediate.The lower aldehydes were then obtained by hydrolysis.This provides support for inhibiting the side reaction of olefin epoxidation in HTS-1/H₂O₂ system and improving the quality of epoxy compounds.At the same time,it opens up a new path for the mild oxidation of olefins to aldehydes.5.The catalytic performance of HTS-1 has significant difference in the systems with different oxidants.Among them,HTS-1/H₂O₂ system has the most prominent catalytic effect.The conversion of cyclohexene can reach 89.11%,and the selectivity of cyclohexane oxide can reach 92.17%.With cyclohexene oxidation as probe and oxygen as oxidant,HTS-1catalyzed cyclohexene allyl oxidation is more advantageous,the main product is cyclohexanone,while with hydrogen peroxide as oxidant,cyclohexene is mainly epoxidized.The selectivity of epoxides and the catalytic activity of HTS-1 can be improved by adding appropriate alkaline additives under different oxygen sources.6.The active sites of catalytic oxidation in HTS-1/H₂O₂ system were studied by various characterization methods.It was found that tetra-coordinated titanium in the framework was directly related to the catalytic performance.The six-coordinated Ti-OOH(?²)intermediate was found to be the active site of olefin epoxidation in HTS-1/H₂O₂ system by ultraviolet Raman spectroscopy.Using cyclohexene epoxidation as probe,the catalytic performance of HTS-1 for olefin continuous reaction was investigated.It was found that the catalyst almost deactivated after 60 h reaction.There were two main reasons for deactivation in this system:temporary deactivation caused by blockage of catalyst channel by macromolecule reactants;permanent deactivation caused by dissolution of Ti on catalyst skeleton by alkaline substances.