Joseph T Hupp
Northwestern University, Department of Chemistry, Evanston, IL 60208, USA
A grand challenge in the field of heterogeneous catalysis is to identify and fully characterize sites that are competent for catalysis of desired chemical transformations, ideally with high selectivity, high activity, and high stability. We have found that automated AIM (ALD-like chemistry in MOFs, where ALD is atomic layer deposition and MOF is metal-organic framework) can be used for chemically clean vapor-phase installation of uniform arrays of identically structured catalysts on the reactive nodes of suitably chosen MOFs (typically 6- or 8-connected hexa-zirconium(IV)oxo,hydroxo,aqua nodes of high-stability, high-area (>1,000 m2/g) mesoporous MOFs such as MOF-808, PCN-222, or NU-1000). This methodology, along with a solution-phase analogue termed SIM, can yield metal-oxygen, metal-sulfur, or metal(0) clusters of predetermined size, shape, and chemical composition, as well as desired monometallic catalytic complexes. The uniformity of the obtained catalysts reflects the crystallographic uniformity of the nodes to which the catalysts are grafted.
Thus, MOF nodes can be usefully viewed as uniquely well-defined nanoscopic supports for similarly well-defined catalysts. This presentation will outline the approach to synthesis and characterization, including operando synchrotron-based characterization, of MOF-supported arrays of well-defined clusters, and then illustrate, via one or two brief, unpublished, case studies, their application as catalysts for desirable, but challenging gas-phase chemical transformations. Together with input from computational modeling, these kinds of experiments can be used to address fundamental questions in contemporary, materials-based, Catalysis Science that require atomically precise knowledge of the siting and composition of pre-catalysts, activated catalysts, co-catalysts, reactants and products, i.e. questions of chemical selectivity, chemical confinement, and modulation of activity via control over catalyst metal-atom nuclearity, and general questions regarding emergent complexity in catalytic systems.
