B.Sc.(Hons), 1982, National University of Singapore; Ph.D., 1987, Texas A&M University
Department of Chemistry, NUS
3 Science Drive 3
Heterogeneous Catalysis, Nanostructured Materials, Green Chemistry, Environmental Catalysis, Improved Functional Materials, Biorenewables
Our research interests are focused on heterogeneous catalysis applied in various areas such as the synthesis of fine chemicals, biorenewables, and environmental catalysis. Catalysis plays an important part in our lives as many of the products and energy-related activities are derived from its application. It involved in the processing of a large fraction of products and is key to the efficiency of chemical reactions. In the last decade, catalysis has played a crucial role in the protection of our environment. Catalysts are used to reduce and minimize pollutants emitted during the processing of fuels and chemicals and from automobiles and power plants. They are vital to the production of new, environmentally friendlier fuels and chemicals.
One of our research interests is in improving the properties of existing materials. Both crystalline and amorphous materials are investigated. By studying the chemistry involved in the processing, we try to tailor materials with the suitable properties. We are able to make high surface area support materials like zirconia, alumina, titania, and ceria-zirconia that are also thermally stable. The research focuses on the parameters influencing the properties of these materials such as pore size, surface area and redox capability. The sorption properties of materials have important applications in wastewater treatment and carbon dioxide capture. In addition, the utilization and transformation of CO2 into useful chemicals is an area of interest.
One area of research interest is in the development and use of zeolites and mesoporous materials for the synthesis of fine and specialty chemicals. Brønsted and Lewis acids are required for many reactions such as alkylation, isomerization, oligomerization, hydration/dehydration, esterification, hydrolysis, and condensation The use of solid acids offers several advantages, namely (i) inherent safety and ease of handling due their less corrosive nature as compared to mineral acids (ii) facilitation of separation and recycling of the catalyst (iii) obviation of hydrolytic workup to obtain the product and (iv) avoidance of product contamination by trace amounts of neutralized acids. These benefits translate into savings in expensive equipment costs necessary to withstand corrosion as well as minimized waste stream contamination. Reactions in the microporous channels of zeolites confer shape selectivity in the formation of products. Using ordered mesoporous materials extends the range of materials to include bulky substrates.
The solid acids are good candidates for cascade reactions as they can be used in combination with other catalysts or modified to become multifunctional. Catalytic cascade conversions are increasingly considered for organic syntheses due to savings in time, energy and materials. Such conversions eliminate the recovery of intermediates, while the presence of a catalyst minimizes the need for stoichiometric amounts of reagents, thereby reducing the wastes generated. We have successfully carried out the one-pot transformation of citral to menthols, which involves a triple cascade of hydrogenation, isomerization and hydrogenation steps.
Due to the side products that can be formed, the first step in this route requires a highly selective route where the conjugated C=C bond in citral is first hydrogenated to form (±)-citronellal. In another cascade reaction, involving catalytic hydrogenation and Meerwein-Ponndorf Veley reduction, the hydrogenation of 4-tert-butylphenol and p-cresol to the corresponding alkylcyclohexanol was successfully carried out in one pot and with high yields.
Oxidation reactions are of fundamental importance in organic synthesis for introducing functionalities. The oxidation typically involves the use of stoichiometric amounts of reagents such as potassium permanganate, bromine, chromic acid/sulfuric acid in water, dipyridine Cr(VI) oxide, manganese dioxide, etc. As large amounts of toxic wastes are generated, there is intense research in find effective solutions. We investigate the use of oxidants such as molecular oxygen and hydrogen peroxide as they are inexpensive and produce water as the only byproduct. Besides heterogeneous catalysts involving precious metals, we investigate the use of less expensive metals such as silver and iron for the oxidation of alkenes and alcohols.
Nanostructured metal oxides are synthesized and used as supports for active substrates. The pore size of the material play a very important role in directing the selectivity of reactions based on steric constraints. We are interested in the synthesis of mesoporous metal oxides with tunable pore sizes to suit different reactant molecules. In a study on the direct esterification of glycerol with fatty acids, we showed that pore size constraints resulted in a high yield of monolaurin. The surface chemistry can be manipulated through grafting techniques and by framework incorporation.