(275b) Understanding Mechanisms in Heterogeneous Catalysis: Fischer-Tropsch and Acid Chemistries Revisited

Recent advances in unraveling the mechanistic details and site requirements for CO activation and the formation of C-C bonds in the Fischer-Tropsch synthesis and for transformations mediated by carbenium ions in solid acids are described from a perspective that emerged three decades ago within the Corporate Research Laboratories of Exxon. Theory, experiment, and spectroscopy have brought to us more precise assessments of the identity and kinetic relevance of elementary reaction steps and of descriptors of reactivity and selectivity based on the intrinsic properties of solid surfaces. In acid catalysis, polyoxometalates and zeotypes provided diverse compositions and known structures and thus essential benchmarking of theory and experiment; in doing so, they made possible an unequivocal dissection of the consequences of acid strength and confinement within voids of molecular dimensions for transition state stability. In the Fischer-Tropsch synthesis, this toolbox resolved long-standing mechanistic puzzles by accepting, and then describing, the essential role of the densely-covered surfaces that prevail in relevant practice. The dense chemisorbed CO adlayers present during Fischer-Tropsch synthesis turnovers lead to CO activation through assistance by coadsorbed H-atoms, to the observed effects of water on C-O activation rates, and to rates and chain growth selectivities that would be unattainable on sparsely-covered surfaces. These examples intend to illustrate how ideas and approaches in catalysis ultimately transcend the traditional industry-academia boundaries of inquiry, aided by tools and conceptual frameworks that have become increasingly competent at describing chemistry on surfaces, and to do so, for realistic solids at the conditions relevant to their use in catalysis.