UC Davis

Metal organic frameworks (MOFs) are a novel class of solid materials, comprising inorganic metal nodes and multidentate organic ligands. These materials have drawn wide attention as potential catalysts. Microscopic characterization of these structures has become routine with x-ray diffraction (XRD) crystallography and determination of surface area/pore volume data by analysis of adsorption isotherms, analyzed with Brunauer-Emmett-Teller (BET) theory. However, competition for the nodes in synthesis between the organic linkers, acid modulators, and the solvent matrix results in structural defects that are not resolved by these microscopic techniques. These defects are capped with inhibiting ligands that block access to catalytically active sites. Alcohol dehydration reactions offer opportunities to investigate these active sites as reactant alcohol will remove the inhibiting capping species, and (more slowly) the organic linker. Revealed vacancies will expose different active sites capable of catalyzing alcohol dehydration reactions. MOF UiO-66, which incorporates Zr6O8 nodes and 1,4-benzene dicarboxylate linkers and is known for its stability, was investigated with methanol and isopropanol dehydration as test reactions. Catalyst performance was determined with a once-through flow reactor at atmospheric pressure and temperatures of 473–573 K. The products were analyzed by on-line gas chromatography, and catalyst samples removed from the reactor after various times on stream were characterized by X-ray diffraction crystallography, surface area measurements, scanning electron microscopy, and 1H NMR spectroscopy of samples digested in NaOH characterizing the collapse of the MOF structure and deactivation of the catalyst. The deactivation was caused by alcohol reacting to form node alkoxy ligand and to break node–linker bonds, unzipping the MOF and creating amorphous material preferentially near the MOF particle surfaces. The methods implemented in this work are suggested to be of value for assessing the strengths and limitations of MOFs as practical catalysts.