Preparation Of Titanium-containing Mesoporous Molecular Sieves And Their Catalytic Performances

Selective catalytic oxidation is an important method for the synthesis of oxygencontaining compounds,which is widely used in industrial processes.Most of the traditional catalysts are soluble metal salts,such as chromate,permanganate,etc.,which make a large number of toxic wastes as a result of these oxidants in the production processes.The discovery of titanium silicalite-1(TS-1)molecular sieve catalys overcame the serious pollution in traditional oxidation process to realize the green oxidation processes by combining hydrogen peroxide as an oxidant,open a new era of green oxidation production industry.Although TS-1 catalyst has excellent catalytic performance,its small pore size(about 0.55 nm)limits its application in bulky-molecule catalytic oxidation.In view of the demand of bulky-molecule catalysis,a series of large-micropore and extra-largemicropore titanosilicates were prepared,but the pore size was still less than 1.0 nm.Subsequently,titanium-containing mesoporous molecular sieves were prepared.Due to the advantages of regular channels,adjustable pore size at the range of 2-50 nm,high specific surface area,titanium-containing mesoporous molecular sieves showed superior properties in bulky-molecule catalysis.The reported preparation methods generally achieved titanium-containing mesoporous molecular sieves with large particle size and low titanium content,greatly affecting their catalytic performance.It is still a challenge to prepare nano-sized,high-titanium-content and high-specific-surface-area titaniumcontaining mesoporous molecular sieves.This thesis will mainly be divided into three parts:(1)A series of titanium-containing mesoporous molecular sieves with different sizes,morphologies and titanium contents were synthesized by using amino acids as additives.Through XRD,SEM,ICP and nitrogen adsorption characterization,the size of the prepared titanium-containing mesoporous molecular sieves is adjustable from 30 nm to500 nm,and the titanium content is adjustable from 0.08% to 100%.It is found that the content of titanium in the synthesis system significantly affects the mesoporous structure,morphology,coordination state of titanium and the catalytic performance of synthesized titanium-containing mesoporous molecular sieves(1)With the increase of titanium content,the order of mesopores will increase,and then decrease until the mesopores disappear.In the range of Si / Ti ≥ 1,uniform mesoporous distribution is at the range of 1.7 ～ 2.9 nm mesopore,while in the range of Si / Ti ≤ 1,there are no regular mesopores.(2)The morphology of the prepared samples,in the range of Si / Ti ≥ 30,belongs to descrete nanospheres,in the range of Si / Ti = 20-10,belongs to micro spherical morphology,when Si / Ti ≤ 2,it is irregular particle morphology.(3)The samples with Si / Ti of 1000,most of titanium species are four-coordinated active titanium species;Si / Ti in the range of 200 ～ 20,mainly four-coordinated titanium species,with a small number of six-coordinated titanium species,Si / Ti in the range of10 ～ 2,mainly four-coordinated and six-coordinated titanium species,with a small amount of titanium dioxide tiny particles;in the range of Si / Ti ≤ 1,three states of titanium species exist at the same time.(4)With the increase of titanium content,the conversion of cyclohexene is increasing.The highest conversion is observed as Si / Ti = 2,the conversion is 30.2%,and then the conversion drops sharply.With the increase of titanium content,the selectivity of epoxycyclohexane decreases gradually,and most of the products are 1,2-cyclohexanediol.About 70-80% of 1,2-cyclohexanediol,～ 10% of allylic oxidation products and ～ 10% of over oxidation products are found in the product.At the same time,it is found that the sample with titanium dioxide tiny particles will lead to the decomposition of hydrogen peroxide and reduce the utilization efficiency of hydrogen peroxide.(2)The effects of ammonium carbonate on the titanium-containing mesoporous molecular sieves were investigated based on the above synthetic system by using amino acids as additives.Through XRD,SEM,ICP and nitrogen adsorption characterization,it is found that ammonium carbonate significantly affects the mesoporous structure,morphology,coordination state of titanium and the catalytic performance of synthesized titanium-containing mesoporous molecular sieves.(1)Addition ammonium carbonate can decrease the extraframework Ti species and increase the BET surface area of the titanium-containing mesoporous molecular sieves,leading to the increase of the catalytic activity in olefin epoxidation reaction.(2)Ammonium carbonate can lower down the p H value,increase the size of the particles,and lead to aggregation between the particles.This will result in the low catalytic activity due to the low diffusion rate of reactants.(3)The synthesized titanium-containing mesoporous molecular sieves showed a high conversion(>90%)and a high selectivity(>80%)in β-caryophyllene epoxidation reaction by using TBHP as oxidant.(3)Catalyst of Ti-HSMS with hierarchical pore structure was synthesized.It is found that Ti-HSMS contains disordered secondary nanopores with larger pore diameters(about 20 nm)and ordered and regular mesoporous pores(3 nm).Among them,the irregular larger pores are secondary nanopores formed by acidic polypropionic acid electrolyte as a template.The regular smaller pores are ordered mesoporous channels formed by the assembly of cationic surfactant as a template.The introduced secondary nanopores can strengthen the material exchange capacity of the catalyst to a certain extent,facilitate efficient binding and catalysis between the reaction substrate and the active site of the catalyst,and improve the catalytic performance.In terms of application,Ti-HSMS,traditional Ti-MCM-41 and microporous TS-1molecular sieve in the catalytic epoxidation of large molecular olefins are explored.The results of the catalytic experiment data show:(1)The catalyst that catalyzes the epoxidation of olefins needs to be in the organic phase to exert a good catalytic effect,and the use of aqueous hydrogen peroxide as the oxidant will cause the conversion rate and selectivity of the catalyst to drop sharply.(2)The main active sites for catalytic conversion of titanium exist in the pores of the catalyst,and only a small amount are distributed on the surface of the catalyst.(3)The titanium-containing mesoporous molecular sieve with larger pore size had a good catalytic performance in epoxidation of bulky olefins.(4)The difference in catalytic activity of the catalysts with different titanium content proves that the skeleton titanium is the key factor to the catalytic reaction for olefin epoxidation.