Functionalization of mesoporous silica nanoparticles and their applications in organo-, metallic and organometallic catalysis

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Huang, Yulin
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Victor S. Lin
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Mesoporous silica nanoparticles (MSN) with high surface area, tunable pore size and very narrow pore size distribution were functionalized by organic acid, organic base, metallic nanoparticles and organometallic complexes through co-condensation methods and/or post-synthesis grafting methods. And these surface-functionalized mesoporous materials were applied as heterogeneous catalysts in organocatalysis, metallic catalysis and organometallic catalysis.

Organocatalysis. First, Brynsted acid and base were confined into the mesoporous channels of MSN and they could co-exist as compatible catalysts for one-pot reaction cascades without neutralizing each other because they were confined in different MSN particles' channels. Brynsted acid and base also were site-separated by MSN's internal surface and external surface through co-condensation method to functionalize MSN's internal surface followed by grafting method to functionalize MSN's external surface. These internal and external surface-separated Brynsted acid and base could co-exist as compatible catalysts too.

Metallic catalysis. Water-soluble rhodium nanoparticles with well defined particle size were synthesized and immobilized on MSN during in situ MSN's synthesis. The obtained material (MSNRhNPs) had homogeneous rhodium nanoparticle size, homogeneous rhodium nanoparticle distribution in MSN, typical MSN's highly ordered structure and surface area and narrow pore size distribution as well. After MSNRhNPs were modified by manganese oxide, it could catalyze the hydrogenation of CO to produce the renewable energy alternative - ethanol with high selectivity and high activity. Additionally, after MSNRhNPs were functionalized by some chiral agents such as (-)-cinchonidine, it can used as a solid chiral catalyst which can be recycled and reused without any loss of reactivity and enantioselectivity.

Organometallic catalysis. Wilkinson-type rhodium phosphine complex was homogeneously immobilized on MSN surface by co-condensation method. This MSN- immobilized rhodium-phosphine complex (RhPMSN) had a new and total different catalytic performance: RhPMSN could enantioselectively catalyze the hydrogenation of pyruvate when (-)-cinchonidine was adsorbed on RhPMSN surface. However, RhCl(TPP)3 (TPP: triphenylphosphine) and (-)-cinchonidine could not enantioselectively catalyze the same reaction in homogeneous system.

An in-depth solid-state NMR study of RhPMSN has been presented. Functionalization of the ligand was confirmed by the presence of T sites in the 29Si CPMASNMR spectrum and quantification of these sites was achieved via integration of the 29Si DPMAS NMR spectrum. Both 1D and 2D SSNMR experiments showed that covalent attachment of the rhodium- phosphine ligand to the MSN surfaces was successful. Both 13C-1H and 31P-1H idHETCOR experiments provided structural details of oxidized and non-oxidized phosphine ligands, otherwise indiscernible in a conventional 1D CPMAS NMR experiments.

Organometallic complex (salen)Cr on MSN was synthesized and applied in the oxidation of tetramethylbenzidine (TMB) with iodosobenzene. MSN-(salen)CrIII as a heterogeneous catalyst exhibited both similarities and differences with the analogous (salen)CrIII(H2O)+ as catalyst in aqueous acetonitrile (10% H2O). It was shown that the covalently attached catalyst in mesoporous channels of MSN was still easily accessible to the reactants without diffusion problem.

Aminopropyl-functionalized MSN was synthesized and applied in the selective sequestration of carboxylic acids from biomass fermentation. Aminopropyl-functionalized MSN with a designed loading of functional groups could have a very high selectivity for carboxylic acid instead for ethanol, glucose, and protein. The regeneration of aminopropyl-functionalized MSN could be done easily by increasing pH to 10.5 because the adsorption reaction between carboxylic acids and aminopropyl-functionalized MSN was pH-dependent. And the regenerated aminopropyl-functionalized MSN showed adsorption capacity equivalent to the original.

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