Synthesis of Mesoporous Silica-supported Catalysts and Their HDS Catalytic Performances

Author:Jiao Jin Qing

Supervisor:zhao zhen duan ai jun


Degree Year:2018





With regard to the stricter environmental regulations and decline in quality of petroleum feedstocks,the production of high-quality fuels has become one of the urgent tasks.Currently,hydrodesulfurization(HDS)technology is the most efficient technique to reach the goal to upgrade and update fuels,in which S-containing molecules react with hydrogen in the presence of the heterogeneous catalysts under high temperatures and pressures.Therefore,it is crucial to develop the effective HDS catalysts for the clean fuels production.This thesis research firstly focuses on the design and synthesis of novel support with open pore channel,suitable metal-support interaction,i.e.the preparation of mesoporous SBA-15 with long-range ordered channels,the synthesis of DMSNs with center-radial pore structure and the design of DBSNs with densely branching structure.Secondly,highly dispersed novel noble metals-based catalysts with good high hydrogenation property and supported NiMo catalysts were systematically prepared accordingly.Finally,the materials and the related catalysts were characterized by means of SEM,TEM,BET,ET,27Al NMR,NH3-TPD,UV-Vis DRS,XPS,H2-TPR,Raman and Py-IR.The catalytic activitives of the as-prepared catalysts were evaluated by the hydrodesulfurization(HDS)of dibenzothiophene(DBT).The relationship among the preparation method,the textile structure,the physiochemical properties of catalysts and their catalytic performcances were studied in detail.Furthermore,the catalytic nature of active phases and the reaction mechanisms were discussed and deeply understood.The main research contents and findings are summarized as follows:(1)Mesoporous SBA-15 materials with short rod morphology were successfully synthesized by hydrothermal method using pluronic P123 triblock copolymer as structure-directing agent.The Al-SBA-15 support with Si/Al molar ratio of 10 was synthesized by using aluminum isopropoxide as the aluminum source.Subsequently,the amino-functionalized mesoporous Al-SBA-15 supports were prepared by using3-aminopropyl triethoxysilane(APTES).The bimetallic PtPd/Al-SBA-15 catalyst was synthesized via a one-pot gas bubbling-assisted membrane reduction(GBMR)method with the poly(N-vinyl-2-pyrrolidone)(PVP)as the protecting agent and sodium borohydride(NaBH4)as the reducing agent.The characterization results of TEM revealed that SBA-15 materials possessed highly ordered hexagonal(p6mm)symmetry and well-ordered mesoporous structure with the mean pore diameter of 8.9nm.The introduced amino groups(-NH2)were successfully confirmed by the analysis of FTIR spectra and thermogravimetric analysis(TGA),which provided more binding sites for covalently adsorbing/immobilizing noble metal ions and noble metal nanoparticles(NPs).The supported PtPd alloy nanoparticles(NPs)were highly dispersed with the uniform size of 2.2 nm.The high dispersion and small size of noble metal NPs were conducive to creating more accessible and high-activity sites,which may improve the catalytic activity.The bimetallic PtPd/Al-SBA-15 catalyst exhibited the highest HDS conversion compared with Pt/Al-SBA-15,Pd/Al-SBA-15 and PtPd/SBA-15 catalysts.The synergetic effect of noble metal-acidic support and PtPd alloy NPs improved its HDS catalytic activity and surfur-resistence ability.(2)Dendritic mesoporous silica nanospheres(DMSNs)with center-radial pore structure were successfully fabricated by using cetyltrimethylammonium bromide(CTAB)and sodium salicylate(NaSal)as a structure directing agent,tetraethyl orthosilicate(TEOS)as a silica source and triethanolamine(TEA)as a catalyst.Aluminum-containing DMSNs with different molar ratios of Si/Al were synthesized by a post synthetic method and the Al-modified DMSNs-supported NiMo catalysts were prepared.The characterization results revealed that the average pore diameter of DMSNs was around 26.4 nm,which was helpful for eliminating the diffusion resistance of S-containing compounds.The center-radial pore structure of DMSNs confirmed by ET measurement,was beneficial for enhancing the accessibility of active sites in the pore channels.The incorporation of aluminum in the framework of DMSNs could improve the acidity amount and modulate the metal-support interaction.Consequently,the catalytic results showed that NiMo/Al-DMSNs catalysts exhibited higher HDS conversions(97.9%)for dibenzothiophene compared with commercial NiMo/γ-Al2O3 and NiMo/DMSNs catalysts.The best performance of NiMo/Al-DBSNs catalyst could be attributed to that the large center-radial pore channels eliminated diffusion resistance of DBT and enhanced the accessibility of active sites in the pore channels.Furthermore,the enhanced Br?nsted/Lewis(B/L)acid sites were helpful for the hydrogenolysis of C-S bonds and the hydroisomerization ability of DBT.Besides,the suitable metal-support interaction is beneficial to the well dispersion and sulfidation of active phases.(3)The Pd/Al-DMSNs and Pd/Al-SBA-15 catalysts were prepared by the GBMR method and the incipient-wetness impregnation method in order to compare the dispersion states of the supported Pd NPs and their hydrorefining catalytic performance.The characterization results of HRTEM images revealed that the Pd/Al-DMSNs and Pd/Al-SBA-15 prepared by the GBMR method had better dispersions of Pd NPs and no agglomeration of noble metals.It indicated that GBMR method could effectively control the nucleation and growth of noble metal NPs,which consequently achieved the controllable synthesis of supported noble metals.The hydrorefining catalytic results showed that Pd/Al-DMSNs catalyst exhibited higher catalytic activity in comparison with Pd/Al-SBA-15 catalyst under the same conditions.That was contributed to that the large center-radial pore structure of DMSNs eliminate the diffusion resistance of large size S-containing compounds and enhanced the accessibility of active phases to reactant molecules.(4)Monodispersed mesoporous densely branching silica nanoparticles(DBSNs)were firstly synthesized through a one-pot,surfactant-free approach.The as-prepared DBSNs were monodispersed spherical particles and uniform in sizes.Moreover,DBSNs had dense central-radial branching structure,and the average pore size was around 9 nm.In addition,the nanoparticle sizes and the porous structures of the DBSNs were tunable by simply adjusting the synthesis temperatures.Moreover,the related formation mechanism was studied,and a polymer assisted diffusion limited aggregation mechanism was proposed.The Al-DBSNs support with Si/Al molar ratio of 10 was synthesized via a post-synthesis method by using aluminum isopropoxide as the aluminum source.And the NiMo/Al-DBSNs catalyst was prepared with a two-step incipient-wetness impregnation method.The catalytic results showed that NiMo/Al-DBSNs catalyst exhibited higher HDS conversions in comparison with NiMo/γ-Al2O3 catalyst under the same conditions.The superior catalytic activity of NiMo/Al-DBSNs was derived from the perfect combination of the densely branching pore structure,the increased acidity,the suitable metal-support interaction and the good dispersion/accessibility of active metals.