Synthesis And Oxidative Desulfurization Performance Of Timodified Hierarchical Porous Materials

Recently,due to the stringent demand of the environmental regulations,the sulfur level in fuels had to be reduced below 10 or 15 ppmw in many countries.This means that the refractory sulfur-containing molecules like dibenzothiophene(DBT)and 4,6-dimethyldibenzothiophene(4,6-DMDBT)had to be desulfurized.Oxidative desulfurization(ODS)technique is one of the most promising nonhydrodesulfurization processes for deep desulfurization because of the low energy consumption and capability to remove the refractory sulfur compounds.This dissertation focused on prepatation,characterization and catalytic performance of Timodified hierarchical porous materials catalysts for deep oxidative desulfurization.Catalysts were tested by X-ray diffraction,electron microscope technique,nitrogen adsorption-desorption test,spectroscopic analysis and ODS reaction of DBT and 4,6-DMDBT.Ti-modified hierarchical mordenites were prepared using tetrabutyl orthotitanate(TBOT)as the precursor of Ti species by incipient wetness impregnation technique.The introduced Ti species were located outside the micropores in mordenite due to the large molecular size of TBOT.As a result,the active Ti species over hierarchical mordenites are easily accessible.The catalytic results indicated that Ti/MMs were highly active catalysts in the ODS of dibenzothiophene and mesoporosity contributes to enhanced the ODS ability.Based on this,the sulfur removal over a Ti-modified hierarchical mordenite(Ti/MM-0.5)with relatively large mesopore volume reached 98.6% after a reaction time of 120 min at 60 oC by using H2O2 as oxidant,which is nearly four times as that over Ti-modified conventional mordenite.Accordingly,the sulfur content in model fuel was reduced from 1000 ppmw to 14 ppmw.However,the ODS ability for 4,6-DMDBT over this catalyst still needs to be further enhanced.To improve the ODS ability for 4,6-DMDBT,Ti-containing hierarchical Beta prepared by post-synthetic method was utilized as the catalyst for deep ODS of fuels for the first time.It was found that the dealumniation of Beta before the introduction of Ti species could greatly enhance the ODS activity of Ti-containing hierarchical Beta in the ODS reactions of both DBT and 4,6-DMDBT.The Ti-containing hierarchical Beta(Ti-B-M-DA)prepared by desilication-dealumination processes exhibited much better ODS activity than that obtained by direct desilication or dealunination.The TOF number over Ti-B-M-DA reached 58.8 h-1 in ODS of DBT at 60 oC and ambient pressure.More importantly,Ti-B-M-DA exhibited highly active catalytic performance in ODS of 4,6-DMDBT.The TOF number reached 27.1 h-1.Accordingly,the sulfur content in model fuel was reduced from 500 ppmw to almost zero.To further improve the catalytic performance of catalysts,MIL-101(Cr)-1.5-4 with MIL-101(Cr)-like structure was rapidly synthesized in 4 h without the addition of solvent and hydrofluoric acid.Then Ti-modified MIL-101(Cr)-1.5-4(Ti/ MIL-101(Cr)-1.5-4)was prepared by post-treatment method.This catalyst exhibited high ODS activity in the ODS reaction of DBT.DBT removal reached nearly 100 % in 20 min at 60 oC by using H2O2 as the oxidant.The sulfur compound in model fuel can be completely removed.Such activity is nearly six times as that over Ti-modified hierarchical mordenite and trebel as that over Ti-modified hierarchical Beta under the same reaction conditions.Additionally,this catalyst can be reused,after three cycles,Ti/MIL-101(Cr)-1.5-4 almost maintained the initial catalytic activity in the ODS reaction of DBT.