Supported gold catalysis: from small molecule activation to green chemical synthesis

With diminishing natural resources, there is an ever-increasing demand for cost-effective and sustainable production of fine and commodity chemicals. For this purpose, there is a need for new catalytic methods that can permit efficient and targeted conversion of fossil and biorenewable feedstocks with lower energy requirements and environmental impact. A significant number of industrial catalytic processes are performed by platinum-group-metal (PGM)-based heterogeneous catalysts capable of activating a range of important small molecules, such as CO, O2, H2, and N2. In contrast, there is a general feeling that gold (Au) cannot act as an efficient catalyst because of its inability to activate most molecules, which is essential to any catalytic processes. As a consequence, researchers have long neglected the potential for use of gold as a catalyst. In recent years, however, chemists have put forth tremendous effort and progress in the use of supported gold catalysts to facilitate a variety of useful synthetic transformations. The seminal discovery by Haruta in 1987 that suitably prepared Au-based catalysts were surprisingly active for CO oxidation even at 200 K initiated rapid development of the field. Since then, researchers have widely employed Au-based catalysts in many types of mild chemical processes, with special focus on selective reactions involving small molecules (for example, CO, H2O, O2, or H2) as a reactant. That gold in the form of tiny nanoparticles (NPs, generally less than 5 nm in diameter) can subtly activate the reactant molecules under mild conditions has been evoked to explain the superior effectiveness of gold compared with conventional PGMs. In this context, Au-based catalysts are gaining great significance in developing new green processes with improved selectivity and energy minimization. In this Account, we describe our efforts toward the development of a range of green and selective processes largely through the appropriate choice of Au catalysts coupled with the coactivation of a plethora of simple small molecules. We have focused on developing new mild and selective reductive transformations that can offer efficient alternatives to conventional Au-catalyzed hydrogenation processes. We have demonstrated Au-catalyzed selective transformation involving HCOOH activation, Au-catalyzed selective reduction involving CO and H2O activation, and Au-catalyzed C-N/C-C bond formation via alcohol activation with high selectivity. The interplay between the support and gold plays a critical role in the success of these transformations, thus highlighting the crucial importance of support in tuning the performance of supported Au NPs. Most of the reactions can tolerate a range of functional groups, and some can occur under ambient conditions. Depending on the specific process, we propose several mechanistic scenarios that describe the plausible small-molecule-mediated reaction pathways. Additionally, we have observed an unusual reactant-promoted H2O or H2 activation over supported Au NPs, thus offering new strategies for green and facile synthesis of diverse amides and heteroaromatic nitrogen compounds. We anticipate that key insights into how simple small molecules are activated for further reaction over Au NPs should lead to a better understanding of gold catalysis and the development of new innovative PGM-free technologies.