Supported Mono- and Bimetallic Gold Nanoparticle Catalysts for Different Organic Transformations

Abstract: Supported gold nanoparticles are emerging as an important class of catalysts for various organic transformations due to their tunable properties. This thesis embraces the fundamental studies of supported gold nanoparticles for different organic reactions from the viewpoint of understanding the role of synthesis methods, nature of carriers, and addition of second metal on the properties and functions of gold catalysts. The catalysts are synthesised by different routes, namely: deposition-precipitation, incipient wetness impregnation, and sol-immobilisation, and characterised ex situ by techniques such as X-ray fluorescence, N2 sorption, powder X-ray diffraction, X-ray photoelectron spectroscopy, and various types of electron microscopy. The reactions investigated include: (i) Sonogashira coupling, which is a unique method for the cross coupling between sp and sp2 or sp3 carbon atoms;(ii) epoxidation of styrene to styrene oxide; (iii) oxidation of ethylbenzene to acetophenone; and (iv) oxidative cross coupling of non-activated arenes. All these transformations lead to important intermediates which have numerous applications in polymer, perfume and/or pharmaceutical industries. In Sonogashira coupling of phenylacetylene and iodobenzene over Au/CeO2, Au/TiO2, and Au/Al2O3 catalysts, impact of synthesis routes, nature of support and Au particle size was studied. It is found that the catalysts prepared by deposition-precipitation lead to higher activity than their analogues obtained by incipient impregnation method due to a better dispersion and smaller size (4-15 nm) of gold particles on the former than the latter. In addition, redox (CeO2, TiO2) supports are more active than non-redox (Al2O3) carrier. Evaluation of gold nanoparticles supported on mesoporous materials (MCM-41, SBA-15, KIT-6 and fumed SiO2) in styrene epoxidation demonstrates Au/MCM-41 as the best system, due to its smallest Au size (ca. 3 nm), leading to full styrene conversion in 10 h with 96% styrene oxide selectivity. This is the best performance reported for these types of catalysts to date. Still, the attainment of full selectivity was not possible by above systems and thus the bimetallic Au-Pd nanoalloy supported in TiO2 was investigated in styrene oxidation, which enables full conversion with 99% selectivity to styrene oxide in 12 h due to electronic interactions between Au and Pd. This study marks the first application of Au-Pd in styrene epoxidation. Besides, the Au-Pd/TiO2 catalyst also shows good activity in ethylbenzene oxidation. Oxidative cross coupling of bromoanisole with benzene is studied over Au/ZrO2, Au/TiO2, and Au/Al2O3, where the former leads to the best productivity of cross coupled product due to the combination of smaller gold nanoparticles and redox nature of support. Overall, these studies suggest that the attainment of superior performance on supported gold nanoparticle catalysts relies on the appropriate choice of support, catalyst preparation route, and/or addition of second metal that could interact withgold.