The combination of 2D 1H-13C and 1H-29Si solid state NMR, hyperpolarized 129Xe NMR, synchrotron X-ray diffraction, together with adsorption measurements of vapors and gases for environmental and energetic relevance, was used to investigate the structure and the properties of periodic mesoporous hybrid p-phenylenesilica endowed with crystalline order in the walls. The interplay of 1H, 13C, and 29Si in the 2D heteronuclear correlation NMR measurements, together with the application of Lee-Goldburg homonuclear decoupling, revealed the spatial relationships (<5 angstroms) among various spin-active nuclei of the framework. Indeed, the through-space correlations in the 2D experiments evidenced, for the first time, the interfaces of the matrix walls with guest molecules confined in the nanochannels. Organic-inorganic and organic-organic heterogeneous interfaces between the matrix and the guests were identified. The open-pore structure and the easy accessibility of the nanochannels to the gas phase have been demonstrated by highly sensitive hyperpolarized (HP) xenon NMR, under extreme xenon dilution. Two-dimensional exchange experiments showed the exchange time to be as short as 2 ms. Through variable-temperature HP 129Xe NMR experiments we were able to achieve an unprecedented description of the nanochannel space and surface, a physisorption energy of 13.9 kJ mol-1, and the chemical shift value of xenon probing the internal surfaces. These results prompted us to measure the high storage capacity of the matrix towards benzene, hexafluorobenzene, ethanol, and carbon dioxide. Both host-guest, CH...pi, and OH...pi interactions contribute to the stabilization of the aromatic guests (benzene and hexafluorobenzene) on the extended surfaces. The full carbon dioxide loading in the channels could be detected by synchrotron radiation X-ray diffraction experiments. The selective adsorption of carbon dioxide (ca. 90 wt %) vs that of oxygen and hydrogen, together with the permanent porosity, high thermal stability, and high degree of order, makes this a suitable matrix for purifying hydrogen in clean-energy generation.
Rotor speed regulation in the hybrid walls of porous p‐phenylenesilica can be achieved by guest uptake and removal. A graduation from slow to ultrafast motional regimes (k=104–108 s−1) can be experienced by the molecular rotors depending on guest nature and temperature, as detected by solid‐state spin‐echo 2H NMR spectroscopy (see picture).
Diphenylene moieties, molecularly ordered in the framework of periodic mesoporous organosilicas, behave as molecular rotors and show a mobility with correlation times as short as a few nanoseconds.
Interlayer nanoporosity of hectorite pillared by tetraethylammonium ions is explored by hyperpolarized xenon NMR and relevant gases such as carbon dioxide revealing the adsorption capacity of the open galleries.
Nanocomposite micro-objects of mesoporous silica with polymers have been obtained by inclusion polymerization of vinyl monomers (styrene and methylmethacrylate) via a radical process. The intimacy between the silica scaffold and the grown polymer has been addressed by a multi-technique approach and the extended interface has been recognized. In particular, phase-modulated Lee-Goldburg 2D heterocorrelated NMR was exploited to study the heterogeneous interfaces and the micro-adhesion between the inorganic matrix and the organic phase. By the in-depth characterization, it was possible to achieve a model in which polymer nanofibrils interact with the walls of the nanochannels in the mesoporous silica, resulting in interdigitated nanophases. A material consisting of two distinct phases so intimately entangled can explain the success of a replication process in which the morphology of the original material is entirely transposed to a polymeric material that fully retains its shape. The obtained micrometric shapes as well as their nanometric structure were directly observed, respectively, by scanning and transmission electron microscopies.
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