A simple procedure has been developed for the syntheses of functionalized mesoporous materials with sulfonic groups involving the co-condensation of tetraethoxysilane and mercaptopropyltrimethoxysilane in the presence of block copolymers and hydrogen peroxide under acidic conditions. The modified SBA-15 materials show hexagonal mesoscopic order and pore sizes up to 60 Å, with acid exchange capacities ranging from 1 to 2 mequiv of H+/g of SiO2, surface areas up to 800 m2/g, and excellent thermal and hydrothermal stabilities. The formation of the sulfonic groups during co-condensation of the silica species coincides with enhanced mesoscopic ordering and changes in the adsorption properties of the final materials. 31P MAS NMR measurements of chemically adsorbed triethylphosphine oxide confirm the presence of Brönsted acid centers that are stronger that those found in Al-MCM-41. Finally, this procedure has been generalized to prepare functionalized mesoporous solids containing sulfonic groups and other organic moieties.
Surfactant-templated layered silicates are shown to possess complex compositional, structural, and dynamic features that manifest rich and interrelated order and disorder at molecular length scales. Temperature-dependent 1D and 2D solid-state 29Si NMR measurements reveal a chemical-exchange process involving the surfactant headgroups that is concomitant with reversible broadening of 29Si NMR line shapes under magic-angle-spinning (MAS) conditions at temperatures in the range 205−330 K. Specifically, the temperature-dependent changes in the 29Si transverse dephasing times T2′ can be quantitatively accounted for by 2-fold reorientational dynamics of the surfactant headgroups. Variable-temperature analyses demonstrate that the temperature-dependent 29Si shifts, peak broadening, and 2D 29Si{29Si} correlation NMR line shapes are directly related to the freezing of the surfactant headgroup dynamics, which results in local structural disorder within the silicate framework.
A versatile synthetic protocol is reported that allows high concentrations of functionally active membrane proteins to be incorporated in mesostructured silica materials. Judicious selections of solvent, surfactant, silica precursor species, and synthesis conditions enable membrane proteins to be stabilized in solution and during subsequent coassembly into silica–surfactant composites with nano- and mesoscale order. This was demonstrated by using a combination of nonionic (n-dodecyl-β-d-maltoside or Pluronic P123), lipid-like (1,2-diheptanoyl-sn-glycero-3-phosphocholine), and perfluoro-octanoate surfactants under mild acidic conditions to coassemble the light-responsive transmembrane protein proteorhodopsin at concentrations up to 15 wt % into the hydrophobic regions of worm-like mesostructured silica materials in films. Small-angle X-ray scattering, electron paramagnetic resonance spectroscopy, and transient UV–visible spectroscopy analyses established that proteorhodopsin molecules in mesostructured silica films exhibited native-like function, as well as enhanced thermal stability compared to surfactant or lipid environments. The light absorbance properties and light-activated conformational changes of proteorhodopsin guests in mesostructured silica films are consistent with those associated with the native H+-pumping mechanism of these biomolecules. The synthetic protocol is expected to be general, as demonstrated also for the incorporation of functionally active cytochrome c, a peripheral membrane protein enzyme involved in electron transport, into mesostructured silica–cationic surfactant films.
In numerous applications involving fluid flow through a porous structure, performance improvements, such as better flow characteristics and increased fluid−solid contact, are obtained when multiple length scales of porosity are present in the porous medium. To this end, we have developed a route to hierarchically porous rutile titania that obviates the need for preformed or preassembled structure-directing agents. The preparation involves the successive leaching of phases and components from a dense composite of wurtzite ZnO and inverse spinel Zn2TiO4. The resulting monoliths are composed of grains of highly crystalline rutile titania and possess a hierarchy of interconnected pores with length scales of 5 μm and 100 nm. Ex situ solid-state 67Zn and 47,49Ti NMR has helped probe local environments around these elements in both the mixed and pure crystalline materials that are formed during the leaching processes.
Identifying how small molecular acceptors pack with polymer donors in thin and thick (bulk) films is critical to understanding the nature of electrical doping by charge transfer. In this study, the packing structure of the molecular acceptor tetrafluorotetracyanoquinodimethane (F4TCNQ) with the semiconducting polymer poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT-C14) is examined. A combination of solid-state NMR, synchrotron X-ray scattering, and optical spectroscopy was used to determine the packing motif for blends of PBTTT-C14 and F4TCNQ in thin and bulk films. These results indicate that F4TCNQ and PBTTT-C14 order in a cofacial arrangement where charge transfer is near 100% efficient in the solid state. These results provide crucial insights into the structures and compositions of ordered domains in doped semiconducting polymers and suggest a model for the microstructure where the location of the molecular acceptors are correlated rather than randomly dispersed.
To understand the nature and structure of N-doping centers in carbon materials, we combine two-dimensional (2D) solid-state NMR experiments and chemical shift calculations for 15N, 13C, and 1H nuclei from density functional theory (DFT). Comparisons of predicted chemical shifts with experimental 2D 13C–15N spectra show good agreement and the calculations explain the spectral broadening seen in the experiments. The major differences between the chemical shifts of graphitic/pyridinic/pyrrolic N-moieties are understood by comparing the electronegativities of the various environments. Moreover, the signal broadening is explained using four different factors: (1) the standalone N/C geometry, (2) the effect of a second N atom nearby, (3) the first or second neighbor C atom difference, and (4) the influence of residual water, which is important to understand the electrocatalytic environment. An intuitive correlation between the charge of the probed atom and the chemical shift is validated: the smaller the charge, i.e., higher electron density, the more shielded the nucleus is, and hence the smaller the associated chemical shift. These results can improve the understanding of the nature of heteroatom sites in nitrogen–carbon materials and contribute to the rational design of these materials with desired electronic properties and improved electrochemical performance.
A general protocol is demonstrated for determining the structures of molecularly ordered but noncrystalline solids, which combines constraints provided by X-ray diffraction (XRD), one- and two-dimensional solid-state nuclear magnetic resonance (NMR) spectroscopy, and first-principles quantum chemical calculations. The approach is used to determine the structure(s) of a surfactant-directed layered silicate with short-range order in two dimensions but without long-range periodicity in three-dimensions (3D). The absence of long-range 3D molecular order and corresponding indexable XRD reflections precludes determination of a space group for this layered silicate. Nevertheless, by combining structural constraints obtained from solid-state 29Si NMR analyses, including the types and relative populations of distinct 29Si sites, their respective 29Si–O–29Si connectivities and separation distances, with unit cell parameters (though not space group symmetry) provided by XRD, a comprehensive search of candidate framework structures leads to the identification of a small number of candidate structures that are each compatible with all of the experimental data. Subsequent refinement of the candidate structures using density functional theory calculations allows their evaluation and identification of "best" framework representations, based on their respective lattice energies and quantitative comparisons between experimental and calculated 29Si isotropic chemical shifts and 2J(29Si–O–29Si) scalar couplings. The comprehensive analysis identifies three closely related and topologically equivalent framework configurations that are in close agreement with all experimental and theoretical structural constraints. The subtle differences among such similar structural models embody the complexity of the actual framework(s), which likely contain coexisting or subtle distributions of structural order that are intrinsic to the material.
Mesostrukturierte MFI-Nanoschichten kristallisieren in der Hydrothermalsynthese nicht-topotaktisch über eine schichtartige Silicatzwischenstufe. Festkörper-2D-NMR-Analysen mit dynamischer Kernpolarisation (DNP) liefern direkte Belege, dass die intermediären schichtförmigen Silicatbausteine und das kristallisierende Netzwerk von MFI-Nanoschichten die gleichen kovalenten 29Si-O-29Si-Bindungen aufweisen. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.
