| Summary: | The selective oxidation of alcohols to the corresponding aldehydes or ketones has involved many efforts from industrial research owing to the irreplaceable importance of these carbonyl compounds in the pharmaceuticals, agrochemicals, and perfumery industries. Concerning the atom economy and environmental demand, heterogeneous catalyst is preferred as a greener alternative to the conventional stoichiometric procedures. Choosing an appropriate support with well-defined structure and suitable surface chemistry is a feasible and effective approach to obtain metal nanoparticles with specific size, shape, structure, and to avoid agglomeration that leading to catalytic deactivation. In this PhD work, the selective oxidation of aromatic alcohols to aldehydes and ketones, particularly benzyl alcohol to benzaldehyde, was tackled as model reaction with the intention of successfully synthesizing more efficient heterogeneous catalysts. Our primary objective is to elucidate the effect of metal nanoparticle confinement on the catalytic activity over supported noble metal catalysts with particular highlights on the interactions between both metal-metal and metal-support.
Highly dispersed noble metal nanoparticles with substantially similar mean particle size were effectively confined in the mesopores of SBA-16 with the unique “super-cage” structure. To study the chemistry of metal-support interactions, the surface of SBA-16 silicas was decorated with different functional groups, which leaded to great impact on the catalytic performance. Amine-functionalization remarkably improved the selectivity towards benzaldehyde. Au-Pd bimetallic catalysts showed enhanced catalytic performance due to the synergetic effect of Au and Pd, while the size-dependent effect was eliminated. The bimetallic nanoparticles were uniformly alloyed with Pd cluster-on-Au cluster structure.
Nonetheless, considering the limited enhancement of catalytic performance of Au-Pd/SBA16 catalysts due to the mass-transfer restriction in the mesopores of SBA-16, TUD-1 mesoporous silicas were further employed as support. Pd catalysts supported on TUD-1 functionalized with several organic functional groups, especially amino groups, was evaluated for the aerobic oxidation of benzyl alcohol without any inorganic base. TUD-1 outperformed other mesoporous silicas due to its unique open 3-D sponge-like mesostructure which can effectively confine Pd nanoparticles and suppress the mass-diffusion resistance. The type and content of grafted functional groups greatly affected the catalytic performance by tuning the surface basicity, metal particle size and size distribution, and metal-support interaction.
The particle size, morphology, structure and electron properties of metal nanoparticles may be varied by different preparation procedures. Successfully prepared via Ar glow discharge plasma reduction, Au-Pd bimetallic nanoparticles was highly active in the selective benzyl alcohol oxidation, giving a rate constant of 0.50 h-1, which was 12.5-fold of Au catalyst and 2-fold of Pd catalyst, respectively. Characterization analysis attributed this enhancement to a Pd-rich shell/Au-rich core structure with abundant surface coordination-unsaturated Pd atoms of those effectively confined and well dispersed Au-Pd nanoparticles. As a green, efficient and safe protocol, plasma reduction outperformed the conventional H2 thermal reduction due to a different particle nucleation and growth mechanism which afforded modified morphology and surface chemistry of metal nanoparticles. Further oxidation and re-reduction of plasma reduced Au-Pd catalyst resulted in the atomic re-arrangement of nanoparticles, leading to an inferior catalytic performance with decreased rate constant.
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