Silver Modification On SBA-15 And MCM-41 Zeolites And Their Adsorption Mechanism For Thiophene

Sulfur compounds in gasoline would be converted into SO_x,a harmful environmental pollution,during the subsequent ultilization of gasoline as a fuel.It is of great importance to deeply remove sulfur in gasoline.However,for the industrialized hydrodesulfurization technology,it is difficult to achieve deep removal of thiophenic sulfurs because of their stable conjugated structures.Among many alternative desulfurization methods,adsorption desulfurization has advantages of easy operation,high desulfurization precision,and low environmental pollution,which has attracted the attention of researchers.Lots of studies shown that zeolites modified by silver have an excellent desulfurization performance for thiophenic sulfurs.Generally,on the surface of silver loaded sorbents,the silver always exists in three forms,Ag-O-Si,Ag-O,and Ag⁰.However,the interation machnisms between thiophenic sulfurs and different forms of silver still need to be further investigated.In this thesis,two pure silica zeolites,SBA-15 and MCM-41,with weak acidity and good stability were selected and loaded with different concentrations of silver ammonia solution and different calcination conditions to prepare the Ag/SBA-15 and Ag/MCM-41 sorbents with different silver contents and silver forms.XRD,N₂ adsorption,ICP,SEM,XPS and UV-Vis were used to characterize the structure,silver content and forms of the sorbents.The desulfurization efficiency of sorbents in thiophene-benzene,thiophene-cyclohexane,tetrahydrothiophene-benzene and tetrahydrothiophene-cyclohexane solutions was measured,and the effect of preparation conditions on the desulfurization performance of sorbents was investigated.In order to further study the adsorption mechanism,the saturated adsorption amounts of Ag5/SBA-15 and Ag5/MCM-41sorbents for cyclohexane,benzene,thiophene,and tetrahydrothiophene were determined,and the release sulfide process of sorbents adsorbed thiophene or tetrahydrothiophene was revealed at a certain heating rate and a specified temperature.Thiophene,2-methylthiophene,and 3-methylthiophene with different electronegative sulfur atoms were selected,and the desulfurization efficiency of sorbents in thiophene-cyclohexane,2-methylthiophene-cyclohexane,3-methylthiophene-cyclohexane solutions were analysed.The adsorption properties of sulfur-containing compounds with different electronegativities of sulfur atoms were examined.The main conclusions are lised as follows:?1?During the calcination process,silver ammonia decomposes and the contents of silver oxides increase,and has agglomeration potential at 450°C,which makes the adsorption capacities of Ag/SBA-15 and Ag/MCM-41 sorbents for thiophene and tetrahydrothiophene increase first and then decrease with temperature increase from 80°C to 450°C.The silver ammonia solution could enter into the pores of zeolites,resulting in a decrease in the specific surface area(S_BET)and pore volume?V?.After calcination,the decomposition of silver ammonia leads to an increase of specific surface area and pore volume.Whereas when the concentration of silver ammonia solution is higher than 0.037 mol/L,the specific surface area and pore volumeof the sorbents would decrease.?2?Compared with SBA-15 zeolite,MCM-41 zeolite has a larger specific surface area and more silicon hydroxyl group,which results higher silver content in Ag/MCM-41 sorbents,and higher desulfurization efficiency of these series of sorbents.When the concentration of silver ammonia solution is higher,the silver content of Ag10/SBA-15 sorbents is higher.?3?The saturated adsorption capacities of Ag5/SBA-15 for four adsorbates follow the order:tetrahydrothiophene>thiophene>benzene?cyclohexane,and these of Ag5/MCM-41 sorbents follow the order:tetrahydrothiophene>thiophene?benzene>cyclohexane.Ag₂O in Ag5/MCM-41 sorbents is the active site for thiophene by?-complexation,while Ag-O-Si in Ag/SBA-15 sorbents is the active site for thiophene adsorption by S-Ag bond.?4?The adsorption performance of sorbents for thiopheic sulfides is positively correlated with the electronegativity of sulfur atoms contained.When the methyl group is introduced into the thiophene ring,the adsorption capacity of the sulfur would increase.The adsorption capacity of the sorbents for sulfide is:tetrahydrothiophene>2-methylthiophene>3-methylthiophene>thiophene.The sulfur atomic charge increases when the methyl group is on?position of thiophene ring,which is more conducive to chemical adsorption.