Please use this identifier to cite or link to this item: http://scholarbank.nus.edu.sg/handle/10635/27942
Title: PARTIAL OXIDATION OF CYCLOHEXANE OVER GOLD NANOPARTICLES SUPPORTED ON FUNCTIONALIZED MESOPOROUS SILICA: TOWARDS AN ENVIRONMENTALLY FRIENDLY CHEMICAL TECHNOLOGY
Authors: WU PINGPING
Keywords: heterogeneous catalysis, gold nanoparticle, cyclohexane oxidation, mesoporous silica.
Issue Date: 16-Dec-2010
Source: WU PINGPING (2010-12-16). PARTIAL OXIDATION OF CYCLOHEXANE OVER GOLD NANOPARTICLES SUPPORTED ON FUNCTIONALIZED MESOPOROUS SILICA: TOWARDS AN ENVIRONMENTALLY FRIENDLY CHEMICAL TECHNOLOGY. ScholarBank@NUS Repository.
Abstract: Cyclohexanone and cyclohexanol (also known as K/A oil) are the intermediates in the manufacture of Nylon 6 and Nylon 6, 6. Presently, a homogeneous catalytic process with transition metal salts as the catalysts is the main route to producing K/A oil in the industry. As the process generates waste solids and toxic liquids, it has become more and more desirable to develop an environmentally friendly chemical technology to replace the currently used process. Catalysis by gold has become one of the most intensively studied topics in recent years. Very recently, Au nanoparticles (NPs) supported on mesoporous silica have been found to display a high catalytic activity in oxidation reactions. Mesoporous SBA-15 silica has been proved to be the most appropriate support for dispersing Au NPs in comparison with other mesoporous silica materials. A facile one-pot preparation method was developed in this work. A series of Au NPs supported on vinyl-functionalized, thioether-functionalized and thiol-functionalized mesoporous silica catalysts were prepared, characterized and evaluated for cyclohexane oxidation with molecular oxygen. It was found that the presence of functionality in the one-pot synthesis system played an important role in the immobilization and dispersion of Au NPs on the silica support, which determines the overall catalytic properties. Au NPs with diameters in the range of 3-7 nm supported on mesoporous silica were obtained by using the one-pot method in the presence of 1,4-bis(triethoxysily)propane tetrasulfide (TESPTS) with thioether functionality. The ratio of TESPTS to tetraethyl orthosilicate (TEOS) directly determined the mesoporous structure and the dispersion of Au NPs on the support. The one-pot preparation chemistry was studied by using different techniques and it was observed that the complexation of thioether groups with gold precursor (AuCl4-) occurred in the synthesis mixture. These complexes evolved to form thioether-stabilized Au clusters during hydrothermal treatment, decomposed and deposited as Au NPs upon high-temperature calcination or H2 reduction. A sample prepared using the one-pot synthesis method with a TESPTS/TEOS ratio of 1/40 and followed by calcination displayed the highest catalytic activity with a turn over frequency (TOF) of 11653 h-1. Fine Au NPs (2-4 nm) were obtained on thiol-functionalized mesoporous silica with the assistance of 3-mercaptopropyltrimethoxysilane (MPTMS) as a functionality, which was found to not only complex Au precursor via thiol functional groups but also covalently bond to the silica matrix. The highest catalytic activity with a TOF of 59951 h-1 for cyclohexane oxidation was realized on an optimized catalyst. In summary, MPTMS was found to be the most effective functionality to immobilize Au NPs (with a size in the range of 2-4 nm) on the silica support.
URI: http://scholarbank.nus.edu.sg/handle/10635/27942
Appears in Collections:Ph.D Theses (Open)

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