Synthesis Of Mesoporous Silica With Special Morphology And Utilization Of Support For Catalyst

Mesoporous silica （MS） material, featured with regular packing pores, highsurface area (500-2000m²g^-1, adjustable pore size （2-50nm）, etc, has been recognizedas one of the most important materials. It can be widely applied in the adsorption,separation, catalysis, drug delivery, etc.Generally, the mesoporous structure of mesoporous materials is derived from thesuper-molecular templates introduced during synthesized process, such as surfactant orpolymer. Thus, the template plays a curial role in the formation of special morphologyand structure of MS. In addition, due to lack of active acid/base centers, the pure silicabased mesoporous material is totally inactive, almost no catalytic activity at all.Therefore, introducing hetero elements into the framework, or supporting activecomponents nanoparticles on the exterior and interior surface of the mesoporousmaterials, becomes important solution to make mesoporous materials usable incatalysis area.In this thesis, a series of mesoporous materials with various specialmorphologies and structures, including solid/hollow silica sphere, silica nanoparticlesand sponge-like silica, has been synthesized successfully by using one or dualtemplates. By characterizations of SEM/TEM, XRD, TG/DSC, FTIR, N2adsorption,etc., the synthesized materials were characterized, and the possible mechanism oftemplate-silica self-assemble was investigated. Furthermore, using the MS as supportand depositing metal nanoparticles on the support, we prepared a series of noble metals（Pd, Au, Pt, Ru and Ir） supported catalysts. Comprehensive structure characterizationsand performance evaluation tests, including selective hydrogenation of phenol orcinnamaldehyde and selective oxidation of benzyl alcohol, were conducted in order toexplore the intrinsic “structure-activity relation”. It’s of important significance forextending practical application of MS in chemical industry production. Someimportant achievements are summered as following:I), With dodecyl amine （DDA） and naturely occurred gelatin （GE） as dualtemplates, a sponge-like hybrid meso and macro materials （SMS）, with both mesopores of3.2nm and macropores of50-80nm, were synthesized. It’s found thatthe addition of gelatin as co-template can significantly influence on both themorphology and porosity of SMS. Once the mass ratio of M_GE:M_DDAincreased from0.05to0.6, the morphologies of synthesized silicas changed from sponge-like silica,hollow silica sphere to popcorn-like hollow silica; meanwhile the added gelatin canmodulate the porosity either. Due to its unique bimodal porosity, the synthesized SMSshowed higher adsorption capacity (540mg g^-1) and faster lysozyme adsorption ratethan the conventional2D packing pores of MCM-41, making SMS potentiallyapplicable in the immobilization of enzymes with high capacity and bio-activity. Basedon these experiment results, a possible self-assemble model among the DDA, GE andTEOS was proposed to explain the unique effect of gelatin.II), Using a facile emulsion-template method, in which the DDA acted asmesopores template, TEOS acted as both silica precursor and emulsion template,mesoporous hollow silica sphere （MHSS） with high surface area of961m²g^-1wassynthesized. The effect of synthesized conditions, including temperature, solventcomposite (V_alcohol/V_water), concentration of DDA and TEOS, on the morphology andporosity of MHSS was investigated, and the optimized conditions are: temperature of30-45oC, V_alcohol/V_waterof20/50, CDDAof0.053molL^-1and CTEOSof0.014molL^-1. Bychanging conditions, a series of silica with morphologies of flake-like, solid sphere andhollow sphere were obtained. With the same synthesized route, we have alsosynthesized titania-silica composite sphere （TS） with various Ti/Si ratio. Due to theincorporation of titanium in the framework, the TS showed good activity towards forthe oxidation of benzyl alcohol. The optimal Ti/Si ratio is in the range of6^-12. Theselectivity toward benzyl aldehyde is ca75%.III), Using MHSS as support, mono-Pd and Au, and Au promoted Pd bimetalcatalysts were prepared via organic impregnation-hydrogen reduction route. It’sshowed that the addition of trace gold to Pd （molar ratio Au/Pd=0.1） not only enhancedthe ability of sintering-resistant of metal and improve the metal dispersion, but alsoenhanced significantly its catalytic performance of PdAu/MHSS due to the synergiceffect between the palladium and gold. The catalyst presented10times and6times higher activities than those of commercial Pd/C and Pd/MHSS catalysts respectively,towards hydrogenation of phenol to cyclohexanone under mild condition （50oC, and1MPa of pH₂）. At reaction temperature of80oC, the instant reaction rate achieved ashigh as180g h^-1g^-1PdAu. In addition, the Pt/MHSS and PtM0.1/MHSS （M=Pd, Au, Ru, Irand NM/NPt=0.1） were prepared and the corresponding performance in hydrogenationof phenol were investigated either. It’s found that catalysis behavior of PtM0.1/MHSScatalysts concerned intimately with pre-treatment method （air calcination or hydrogenreduction） and kind of second metal modified. Finally the reaction path of phenolhydrogenation was discussed, in which the dissociative adsorption of hydrogen on themetal surface was considered as rate-determined step, the adsorption model of phenoldetermine product distribution.IV), Using CTABr as template, monodispersed mesoporous silica nanoparticles（MSN） were synthesized and applied as support for a series of Pd promoted Au catalyst,denoted as AuPdn/MSN (molar ratio NPd/Au=0.05^-1.0). Through structurecharacterizations of catalyst, it’s found interestingly that part of the mono-golddispersed as “nano-wires” confined in the meso-channel, and the Pd addition cansignificantly decrease the metal particles size to2-5nm. Via characterizations of XRD,DRUV-vis and H₂-TPR measurements, it’s revealed that the metal composition,pre-treatment method can strongly influence the structure of catalysts, includingcrystallite, electronic propteries and surface composition. The base-free selectivebenzyl alcohol oxidation was used as model reaction test, and it showed that theactivity of catalyst related intimately to the particle size, Pd/Au ratio and surfacecomposition of active component. The AuPd0.2/MSN presented the best activity, whichshowed8times and3times higher activity than that of mono Au/MSN and Pd/MSNcatalysts respectively. In addition, the effects of oxygen pressure, temperature, solventand so on, on the catalytic performance of AuPd0.2/MSN were investigated.Furthermore, we measured the activation energies （Ea） for various catalysts, and itpresented as E_aAu> Ea_Pd> E_aAuPd0.2, indicating that the synergic effect in the Pd-Aucatalyst could decrease the energy barrier, enhance the activity consequently.V), With MSN as support, a high-performance bimetallic catalyst, PdAun/MSN （molar ratio NAu/Pd=0.1-0.4）, were prepared successfully and applied for selectivehydrogenation of cinnamal dehyde to hydrocinnamal dehyde. It’s found that theaddition of Au can improve the metal dispersion either. The highly dispersedPdAu_0.2/MSN showed a homogeneous size of2.5nm with the highest hydrogenationactivity, which was4times and8times higher than that of mono-Pd/MSN and Pd/Crespectively. Based on the structure characterization of catalyst, the enhanced activityof PdAu_0.2/MSN should be attributed to the synergic effect of Pd with the added Auand the high dispersed active components. The ultra-high activity, as well as its novelstructure with controllable compositions, makes this catalyst very attractive for bothfundamental research and practical applications.