Synthesis Of Hierarchical Porous TS-1 Molecular Sieves And Their Application In Oxidative Desulfurization

With the development of technology and economy, environmental pollution becomes more serious. The presence of high concentration of sulphur-containing organic compounds in liquid fuels has caused the greenhouse effect, haze and acid rain. In order to minimize the environmental effects, rigid legislations in controlling the sulphur concentrations in transports fuels have been established in many countries of the world. Oxidative Desulphurization(ODS) is considered as one of the most promising methods for fuel deep desulphurization. Titanium silicalite-1(TS-1),an important crystalline microporous material formed by the isomorphous substitution of a certain amount of silicon in a tetrahedral site of the MFI silicalite lattice by titanium, has attracted much attention due to its excellent catalytic oxidation properties in oxidative desulfurization(ODS). However, a main shortcoming of this zeolite is that its intrinsic micropores strongly inhibit the diffusion of bulky reactants and products, which brings limitation for its applications in the oxidation removal of bulky sulphur compounds from liquid fuels. To overcome this drawback, intensive research has been focused on the synthesis of hierarchical TS-1 zeolites consisting of the typical MFI micropores and additional mesoporosity. Nowadays, a series of hierarchical TS-1 zeolites have beensynthesized by using different methods. However, some problems are still required to be solved, such as the complexity of the synthesis procedure, the high cost or the toxicity of the mesoporous templates. Besides, the mesopores in hierarchical TS-1are commonly formed by the piled particles, which are not very stable in hydrothermal conditions. Therefore, it is of great significance to develop a simple,green and cost-effective method to synthesize hierarchical TS-1 zeolites with uniform intracrystalline mesopores, which are efficient catalysts for the oxidative removal of bulky sulphur compounds. The primary contents of this dissertation are listed as follows:(1) hierarchically porous TS-1 zeolites(HTS-1) with uniform intracrystalline mesopores(2-3 nm) have been successfully synthesized through a hydrothermal method by using Triton X-100 as assisted surfactant. The textural parameters of the HTS-1 zeolites can be adjusted to a certain degree by changing the introduced amounts of Triton X-100 in the synthesis system. With the increasing amounts of Triton X-100 in the synthesis system, the mesoporous volumes and the external specific surface areas of the HTS-1 increase. The resulting HTS-1 zeolites show remarkably enhanced activity in the catalytic oxidation of bulky organosulfur compounds(Th, BT, DBT and 4, 6-DMDBT) compared to the conventional TS-1zeolites, which can efficiently convert Th(or DBT) to thiophene sulfoxide and thiophene sulfone(or dibenzothiophene sulfone). This work demonstrates a green surfactant-assisted approach to synthesize hierarchical TS-1 zeolites with potential application for catalytic oxidation of bulky organic compounds.(2) hierarchically porous TS-1 zeolites have been prepared by mild etching at room temperature using NH4F-HF solution or consecutive treatments with Na OH and NH4F-HF solutions. Under the employed conditions a secondary system of meso- and macro-pores is generated without substantial changes in the framework composition. The resulting hierarchical TS-1 zeolites exhibit remarkable activity in the catalytic oxidation of bulky organosulfur compounds(DBT) compared to theparent TS-1 zeolite. This study demonstrates that the proper combination of etching compounds and treatment conditions is of paramount importance to tune the properties of hierarchical TS-1 zeolite catalysts.(3) hierarchically porous TS-1 zeolites with larger and regular mesopore size distributions(5-40 nm) have been successfully synthesized through the hydrothermal method by introducting PDADMAC as assisted surfactant mesopore template. TEM images reveal that a large amount of intracrystalline mesopores are present in the single crystallinite of TS-1 zeolites. With increasing the amount of PDADMAC, the mesoporous distribution is becoming more regular(the average pore size is 10 nm),and the textural parameters of the hierarchical TS-1 zeolites can be adjusted to a certain degree by changing the introduced amounts of PDADMAC in the synthesis system. 13 C NMR spectra demonstrate that the PDADMAC acts as mesoporous template during the synthesis process for the formation of hierarchical TS-1 zeolites.The resulting hierarchical TS-1 zeolites show superior activity in the catalytic oxidation of bulky organosulfur compounds compared to the HTS-1 zeolites synthesized by other approaches. Moreover, the hierarchical TS-1 catalysts have high hydrothermal stability and can be easily recycled without any post-treatment(for removing the adsorbed products). This work demonstrates a simple synthetic route to preapare hierarchical TS-1 zeolites with intracrystalline mesopores by using a cheap and easy avaliable mesoporous template, which will open new perspectives for potential application in catalytic oxidation of bulky organic compounds.