Robust Microporous Monoliths with Integrated Catalytically Active Metal Sites Investigated by Hyperpolarized 129Xe NMR

Robust microporous nanocomposites (specific surface area approximate to 400 m(2)/g) containing nickel nanoparticles have been synthesized and characterized by thermogravimetric analysis (TGA), differential thermal analysis (DTA), Fourier transform infrared (FT-IR) spectroscopy, nitrogen physisorption, powder X-ray diffraction (XRD), transmission electron microscopy (TEM), and C-13 and Si-29 solid-state nuclear magnetic resonance (NMR) spectroscopy. The commercially available polysilazane (HTT-1800) is chemically modified using an N-ligand-stabilized nickel complex that catalyzes the cross-linking of the polymer via hydrosilylation at room temperature. Upon pyrolysis at 600 degrees C under an inert atmosphere, nickel nanoparticles and micropores are generated in a concerted process. The specific surface area, pore volume, and size of the nickel particles can be tuned. The materials show excellent shape retention upon pyrolysis providing the possibility to fabricate monoliths. The composites are stable in the presence of moisture and are both thermally and solvothermally robust, as indicated by the nitrogen adsorption, FT-IR, and TGA measurements. Continuous-flow, hyperpolarized Xe-129 NMR methods were used in tandem to evaluate the effects of the nickel content and annealing time on the pore structure of the microporous nanocomposite. The adsorption enthalpy is rather independent of nickel particle inclusion. The interior adsorption sites are lined with methyl groups and the nickel particles seem to be located near the external surface of the composites and within the internal voids. The nickel nanoparticles were used to catalyze selective hydrogenation reactions indicating applications of the nanocomposites as catalyst itself or as catalyst support.