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Grafted Calixarenes for Rational Design of Catalysts and Adsorbents

Justin Notestein, Andrew Solovyov, Namal De Silva, Enrique Iglesia, and Alexander Katz. Department of Chemical Engineering, University of California at Berkeley, Berkeley, CA 94720-1462

Ligand-metal-surface assemblies allow the construction of well-defined transition metal-based catalytic reaction or adsorption sites. These assemblies enable us to achieve our research goal of atomic scale control of connectivity and composition within solid supported active sites with the precision of their homogeneous counterparts. Such control over atomic connectivity in turn allows for rational design of the electronic properties of a catalytic metal or adsorbing surface. Our approach involves the careful grafting of macrocycle-metal complexes onto oxide supports, such that the grafted form resembles the molecular precursor, whose structure is known from single crystal X-ray diffraction and whose electronic properties are known from UV-visible spectroscopy and can be inferred from high-resolution NMR spectroscopy. In the active, heterogeneous state, the coordination sphere consists of ligands from the original homogeneous precursor as well as the oxide surface, all of which are proven to be relevant towards the function of the catalyst. This methodology enables incorporation of multi-dentate ligands for greater robustness and definition of active sites during ligand exchange processes inherent to catalysis and the creation of convergent binding sites for cooperative van der Waals interactions in adsorption. Ultimately, with this level of active site control comes the ability to make discrete alterations of the electronic and steric environment surrounding a catalyst active site, thereby enabling study of the behavior of heterogeneous catalysts, and may allow for the creation of novel catalysts with activity and selectivity not possible with traditional homogeneous or heterogeneous catalysts.

The approach above is exemplified by the creation of uniform populations of hydrophobic binding sites on an otherwise hydrophilic surface using grafted calixarenes as a scaffold for arranging van der Waals-rich molecular surfaces around a binding site.[1] We have shown that these materials display identical adsorption behavior regardless of calixarene surface density, proving that adsorption occurs at an individual calixarene macrocycle and that these sites are present during the adsorption of aromatic from vapor or aqueous solution. Because of the single-site nature of these materials, we have shown that large groups on the calixarene upper rim enhance the strength of the adsorption of aromatics due to the increased size of an individual adsorption site, without altering the number or availability of the effective surface area of the adsorbent, presenting a route to rational design and testing of adsorption sites. Adsorption of aromatic guests, when considered relative to their saturation vapor pressures, display identical isotherms, arguing against the presence of specific or directional interactions between calixarene and guest suggested by earlier single-crystal studies. Moreover, comparison of the same guest on the same adsorption site from vapor and aqueous phases presents an experimental route to determine the magnitude of the favorable enthalpic interactions between solvent water and an ostensibly hydrophobic surface.

We have also synthesized a novel family of ligand-modified, single-site heterogeneous catalysts using calixarenes as macrocyclic oxo-ligands for grafted transition metals, leading, in particular, to highly active alkene epoxidation catalysts based on titanium on silica.[2] These catalysts can display turnover rates in excess of 1000/h at >95% selectivity to cyclohexene epoxide at >98% conversion of the oxidant (alkyl hydroperoxide), are stable against leaching and ligand exchange during catalysis and display rigorous first order kinetics in hydroperoxide irrespective of co-product alcohol, often implicated as an inhibitor. Such catalysts are also single-site, meaning that the titanium site behavior is independent of the local Ti-active site density, in both spectroscopic and catalytic epoxidation characterization. These results bear important implications for rational catalyst design since intrinsic catalyst activity is not obscured by deactivation, inhibition, or medium-range structures that can be difficult to control. We have recently embarked on a program to control the steric and electronic character of the Ti active site by design of the calixarene ligand and evaluation by NMR, CD and UV-visible spectroscopies, X-ray absorption, and catalytic competence in Lewis acid-catalyzed epoxidation. We have introduced the use of oxygen containing calixarenes as a method to control the effective coordination number around the Ti, as probed by Ti K-edge XANES, without the need to create extended Ti oxide structures. We have also synthesized calixarenes bearing chiral groups in close proximity to the calixarene macrocycle and the metal center binding sites and have shown these stereocenters to induce chirality at the calixarene macrocycle and the bound metal. Finally, we have developed a family of electron withdrawing calixarenes coordinated to Ti and grafted to silica surfaces and correlated their structure, spectroscopic properties and catalytic behavior. We present these results as a comprehensive route to design and control the Lewis acidity and steric environment of a heterogeneous active site using organic ligands.

[1] Notestein, J.M.; Katz, A.; Iglesia, E. Langmuir 2006, 22, 4004.

[2] Notestein, J.M.,; Iglesia, E.; Katz, A. J. Am. Chem. Soc. 2004, 126, 16478.