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Gold has untapped potential in terms of selectivity in the reduction of targeted chemical functions and substituents. In this chapter, the selective action of supported gold in the hydrogenation of R-NO2, R–CH=O and R–C≡CH is examined, with an analysis of the pertinent literature. Hydrogenation activity requires the formation of gold particles at the nanoscale where the support is critical in determining ultimate catalytic performance. The crucial catalyst structural and surface properties required to achieve enhanced hydrogenation are discussed. The chapter examines in turn the chemoselective hydrogenation of chloronitrobenzene, dinitrobenzene, nitrobenzonitrile, nitrocyclohexane, benzaldehyde, nitrobenzaldehyde, phenylacetylene and furfural. Catalytic gold use in hydrogenolysis is also considered, focusing on hydrodechlorination as a progressive approach to the transformation and recycle of toxic chloro-compounds. The catalytic response is related to possible thermodynamic constraints with an examination of process variables, notably temperature, contact time and H2 partial pressure. Process sustainability is evaluated in terms of mode of operation/productivity, solvent usage, the application of bimetallic catalysts, hydrogen utilisation and the viability of dehydrogenation–hydrogenation coupling. The chapter ends with an assessment of the current state-of-the-art and a consideration of possible future research directions.

Exclusive hydrogenation of benzaldehyde to benzyl alcohol in gas phase continuous operation (393–413 K, 1 atm) was achieved over Au/Al2O3, Au/TiO2 and Au/ZrO2. Synthesis of Au/Al2O3 by deposition–precipitation generated a narrower distribution (2–8 nm) of smaller (mean = 4.3 nm) Au particles relative to impregnation (1–21 nm, mean = 7.9 nm) with increased H2 uptake under reaction conditions and higher benzaldehyde turnover. Switching reactant carrier from ethanol to water resulted in a significant enhancement of selective hydrogenation rate over Au/Al2O3 with 100% benzyl alcohol yield, attributed to increased available reactive hydrogen. This response extends to reaction over Au/TiO2 and Au/ZrO2.

The current study describes the catalytic ozonation of toluene using MCM-41 catalysts modified by different transition metals (Cu and Co) and methods (in situ synthesis and impregnation). The characteristic hexagonal channel array of the MCM-41 pore system was not destroyed by the transition metal modification, but the order and surface area were decreased. Large particles of copper oxide exposed on the (1 1 1) lattice plane, which were indicative of severe sintering, were found on the catalyst modified by Cu via in situ synthesis. Such particles were not observed on the other catalysts. Using the modified catalysts in toluene ozonation revealed that with increased reaction temperature, toluene conversion under a steady state increased, whereas ozone conversion decreased. The transition metal-modified MCM-41s also had largely improved catalytic activities compared with pure MCM-41. Catalytic performance was found to depend on the metal type and modification method. The method that resulted in improved catalytic performance was in situ synthesis for the Co-modified MCM-41, and was impregnation for the Cu-modified MCM-41. The relatively superior performance of the transition metal-modified catalysts over pure MCM-41 is attributed to two main features. One is the well dispersion of metal oxides, and the other is the strong capacity to decompose built-up organic by products on the catalyst surface. (C) 2011 Elsevier B.V. All rights reserved.