Hydrodesulfurization Performances Of Noble Metal Catalysts Supported Over Mesoporous Materials

The deep hydrodesulfurization (HDS) of diesel fuels has been a new challenge to develop catalysts for reduction of sulfur content, in which the more active catalysts are demanded to desulfurize the more refractory organosulfur compounds in diesel fuels, such as dibenzothiophene (DBT) and 4,6-dimethyl-dibenzothiophene (4,6-DMDBT).Because of steric hindrance of methyl groups neighboring S atom, the removal of sulfur from 4,6-DMDBT depends strongly on the hydrogenation activity of the catalyst. Due to superior hydrogenation ability, the supported noble metal catalysts with better thioresistance are good candidates for deep HDS.In the dissertation, several series of acidic mesoporous Al-MCM-41, strong acidic composite mesoporous materials and micro-and meso-porous materials were prepared to be the supports for noble metals.Their HDS performances were evaluated using DBT and 4,6-DMDBT as model sulfur-containing compounds. XRD,N2 adsorption, XRF, TEM, STEM, H2-chemisorption, pyridine adsorbed FTIR were performed to characterize the structure of the catalysts.The effects of support on the HDS performances were studied systemically, and the active phases of noble metal catalysts in HDS reaction were discussed.γ-Al2O3,SiO2, Si-MCM-41 and Al-MCM-41 were chosen as the supports for Pd and Pt catalysts.By comparing their activities and selectivities in the HDS of DBT and 4,6-DMDBT, the HDS performances of noble metal catalysts were strongly dependent on the properties of the support. The HDS activity and the hydrogenation (HYD) route were both improved significantly by depositing Pd or Pt on the acidic supports.Aluminum sulfate, aluminum isopropoxide,α-Al2O3·5H2O, were chosen to synthesis the Al containing Al-MCM-41.Their acidities were investigated by pyridine adsorbed FTIR, NH3-TPD, and isopropylbenzene (IPB) cracking activity test. Without any diffusion limitation for DBT and 4,6-DMDBT, the effects of support acidity were systemically studied. The promoting effects of acid supports were as followed:(1)The hydrogenation ability of the aromatic backbones of DBT and 4,6-DMDBT over noble metals were improved significantly; (2) The acid sites can be anchors for noble metals, preventing the coalescence of noble metal particles in HDS;(3)The weaker metal-sulfur interaction makes the working catalysts retain more metal-like active sites.A series of highly acidic mesoporous aluminosilicates (ZM) were hydrothermally synthesized by assembling zeolite subunits, which were generated by the hydrolysis of different amounts of HZSM-5 zeolite in Na2Si03 aqueous solution. Because the zeolite subunits were assembled into the framework, ZM showed high ratio of B/L acid sites and acid-catalyzed activity in IPB cracking. The HDS of DBT over ZM-supported Pd and Pt catalysts were studied. Their HDS activities depended strongly on the density of surface acid sites.When the SiO2/Al2O3 ratio of the mother liquid was 60 [denoted as ZM(60)],the highest HDS activity was observed. ZM(60)-supported Pd and Pt catalysts showed better catalytic activities than AlM(60)-supported counterparts in the HDS of DBT and 4,6-DMDBT, due to the higher hydrogenation activity.Overgrowth of MCM-41 on HY zeolite was performed using different templates [cetyltrimethylammonium bromide (CTABr) and cetyltrimethylammonium chloride (CTACl)].TEM and STEM images revealed that zeolite HY overgrown with an MCM-41 layer (less than 20 nm) was obtained using CTABr as the template [denoted as MY(Br)],and the MCM-41 pore channels were assumed to be oriented outward from the inner HY core. Nevertheless, for the samples synthesized using CTACl [denoted as MY(Cl)],the HY crystals were buried independently in the MCM-41 phase. The effective bonding by CTA+-Br--H+-Y route was proposed to be the key parameter to synthesize this type of composite materials.The HDS performances of Pd catalysts supported on MY(Br) and the mixed mixture of Si-MCM-41 and HY zeolite [denoted as MY(M)]were evaluated with DBT and 4,6-DMDBT as the model organosulfur compounds.Pd/MY(Br) exhibited the higher HDS activity and superior long-term stability. Its excellent HDS performances may result from the unique core/shell structure of MY(Br) and the resulting "auto-clean" mechanism involving spillover hydrogen in HDS.The HDS of DBT and its tetrahydro intermediates (TH-DBT),and the further conversion of their desulfurized products were determined at 300℃and 5.0 MPa. A new DBT HDS network was proposed, and the hydrocracking and isomerization products in DBT HDS were proven to be from the further conversion of cyclohexylbenzene (CHB).