Characterization of the Porosity in SBA-15 Silicas by Hyperpolarized 129Xe NMR

Supramolecular Organization in Confined Nanospaces

Empty spaces are abhorred by nature, which immediately rushes in to fill the void. Humans have learnt pretty well how to make ordered empty nanocontainers, and to get useful products out of them. When such an order is imparted to molecules, new properties may appear, often yielding advanced applications. This review illustrates how the organized void space inherently present in various materials: zeolites, clathrates, mesoporous silica/organosilica, and metal organic frameworks (MOF), for example, can be exploited to create confined, organized, and self-assembled supramolecular structures of low dimensionality. Features of the confining matrices relevant to organization are presented with special focus on molecular-level aspects. Selected examples of confined supramolecular assemblies - from small molecules to quantum dots or luminescent species - are aimed to show the complexity and potential of this approach. Natural confinement (minerals) and hyperconfinement (high pressure) provide further opportunities to understand and master the atomistic-level interactions governing supramolecular organization under nanospace restrictions.

NMR Hyperpolarization Techniques of Gases

Nuclear spin polarization can be significantly increased through the process of hyperpolarization, leading to an increase in the sensitivity of nuclear magnetic resonance (NMR) experiments by 4-8 orders of magnitude. Hyperpolarized gases, unlike liquids and solids, can often be readily separated and purified from the compounds used to mediate the hyperpolarization processes. These pure hyperpolarized gases enabled many novel MRI applications including the visualization of void spaces, imaging of lung function, and remote detection. Additionally, hyperpolarized gases can be dissolved in liquids and can be used as sensitive molecular probes and reporters. This Minireview covers the fundamentals of the preparation of hyperpolarized gases and focuses on selected applications of interest to biomedicine and materials science.

3D Study of the Morphology and Dynamics of Zeolite Nucleation

The principle aspects and constraints of the dynamics and kinetics of zeolite nucleation in hydrogel systems are analyzed on the basis of a model Na-rich aluminosilicate system. A detailed time-series EMT-type zeolite crystallization study in the model hydrogel system was performed to elucidate the topological and temporal aspects of zeolite nucleation. A comprehensive set of analytical tools and methods was employed to analyze the gel evolution and complement the primary methods of transmission electron microscopy (TEM) and nuclear magnetic resonance (NMR) spectroscopy. TEM tomography reveals that the initial gel particles exhibit a core-shell structure. Zeolite nucleation is topologically limited to this shell structure and the kinetics of nucleation is controlled by the shell integrity. The induction period extends to the moment when the shell is consumed and the bulk solution can react with the core of the gel particles. These new findings, in particular the importance of the gel particle shell in zeolite nucleation, can be used to control the growth process and properties of zeolites formed in hydrogels.

Probing framework–guest interactions in phenylene-bridged periodic mesoporous organosilica using spin-probe EPR

Spin-probe EPR reveals significant differences in the adsorption of probe molecules in benzene-bridged periodic mesoporous silica with crystal-like or amorphous walls.

High-resolution electron microscopy study of mesoporous dichalcogenides and their hydrogen storage properties

In this work, we report a detailed investigation on the surface topology of ordered mesoporous WS(2) nanoarrays nanocast from the silica SBA-15 template. Using a high-resolution scanning electron microscopy (HRSEM) technique, we acquire distinguishably clear images of the WS(2) surface. A large number of small nanorods are found to be evenly distributed among the nanowire arrays, supporting their ordered mesostructures. Based on these observations, tunnel-like connecting pores are proposed to have a similar distribution in the mother mesoporous silica SBA-15 template. Interestingly, we observe the atomic crystal lattices of the layered WS(2) on the HRSEM image. To the best of our knowledge, this is the first time that atomic crystal lattices have been directly observed using a SEM technique. In addition, both materials show good adsorption-desorption capabilities with hydrogen, and the maximum amount of hydrogen that can be taken up is 0.34 wt% for mesoporous WS(2) and 0.52 wt% for mesoporous MoS(2) at a pressure of 10 bar, indicating that both are potential hydrogen storage materials.

Ordered cubic mesoporous silicas with large pore sizes synthesized via high-temperature route

Ordered cubic mesoporous silicas with large pore sizes (LP-SBA-16) have been successfully synthesized at high temperatures (180-220 degrees C) using polymer surfactant of F127 as a template. Compared with conventional SBA-16 with entrance size at 3.6 nm, LP-SBA-16 samples synthesized at high temperatures show large entrance sizes at 11.8-24.7 nm, confirmed by the pore size distribution and TEM images. The formation of these large pore sizes in the samples is attributed to the increase of surfactant hydrophobicity and the merging of mesopores at high temperatures. The results obtained from NMR and XRD techniques show that ordered mesostructure in LP-SBA-16 samples basically remained even if the surfactant (F127) is fully decomposed at high temperatures, indicating that F127 surfactant becomes unimportant when the mesoporous walls of silica have been condensed. Furthermore, we have compared the adsorption capacity of myoglobin over conventional SBA-16 and LP-SBA-16 samples, and it is found that LP-SBA-16 samples exhibit much higher adsorption capacity than conventional SBA-16, which is potentially important for immobilization of enzymes on ordered mesoporous materials.

Is there any microporosity in ordered mesoporous silicas?

The porous structure of nanostructured silicas MCM-41 and SBA-15 has been characterized using N2 adsorption at 77 K, before and after n-nonane preadsorption, together with immersion calorimetry into liquids of different molecular dimensions. Selective blocking of the microporosity with n-nonane proves experimentally that MCM-41 is exclusively mesoporous while SBA-15 exhibits both micro- and mesopores. Additionally, N2 adsorption experiments on the preadsorbed samples show that the microporosity on SBA-15 is located in intrawall positions, the micropore volume accounting for only approximately 7-8 % of the total pore volume. Calorimetric measurements into n-hexane (0.43 nm), 2-methylpentane (0.49 nm), and 2,2-dimethylbutane (0.56 nm) estimate the size of these micropores to be < or = 0.56 nm.

Studying porous materials with krypton-83 NMR spectroscopy

This report is the first review of (83)Kr nuclear magnetic resonance as a new and promising technique for exploring the surfaces of solid materials. In contrast to the spin I = 1/2 nucleus of (129)Xe, (83)Kr has a nuclear spin of I = 9/2 and therefore possesses a nuclear electric quadrupole moment. Interactions of the quadrupole moment with the electronic environment are modulated by surface adsorption processes and therefore affect the (83)Kr relaxation rate and spectral lineshape. These effects are much more sensitive probes for surfaces than the (129)Xe chemical shielding and provide unique insights into macroporous materials in which the (129)Xe chemical shift is typically of little diagnostic value. The first part of this report reviews the effect of quadrupolar interactions on the (83)Kr linewidth in zeolites and also the (83)Kr chemical shift behavior that is distinct from that of its (129)Xe cousin in some of these materials. The second part reviews hyperpolarized (hp) (83)Kr NMR spectroscopy of macroporous materials in which the longitudinal relaxation is typically too slow to allow sufficient averaging of thermally polarized (83)Kr NMR signals. The quadrupolar-driven T(1) relaxation times of hp (83)Kr in these materials are sensitive to surface chemistry, surface-to-volume ratios, coadsorption of other species on surfaces, and surface temperature. Thus, (83)Kr T(1) relaxation can provide information about surfaces and chemical processes in macroscopic pores and can generate surface-sensitive contrast in hp (83)Kr MRI.

2D Multinuclear NMR, Hyperpolarized Xenon and Gas Storage in Organosilica Nanochannels with Crystalline Order in the Walls

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.

Distribution of Gallium Nanocrystals in Ga/MCM-41 Mesocomposites by Continuous-Flow Hyperpolarized 129Xe NMR Spectroscopy

The distribution of gallium nanocrystals in mesoporous MCM-41 host were analyzed by continuous-flow hyperpolarized 129Xe NMR spectroscopy. In contrast to unclear TEM images for the high metal contents, laser-polarized 129Xe probe can detect the whole distribution of gallium in the MCM-41 host. It is found that gallium nanocrystals are included in the mesochannels of MCM-41; a part of them also remains in the interparticle voids. The distribution of gallium metal in MCM-41 is heterogeneous. Not all the mesochannels host metallic gallium even at a high gallium loading of 65.1 wt %. Variable temperature measurements can provide information on the xenon adsorption parameters. This approach opens a sensitive way to probe the distribution of high content species in porous host materials.

Adsorption of hydrocarbons on mesoporous SBA-15 and PHTS materials

Plugged hexagonal templated silica (PHTS) materials are synthesized using a high TEOS/EO(20)PO(70)EO(20) ratio in the SBA-15 synthesis. This generates internal microporous nanocapsules or plugs in part of the channels, which could be inferred from the two-step desorption branch. These materials exhibit a tunable amount of open and plugged pores and a very high micropore volume (up to 0.24 mL/g) and are more stable than the conventional micellar templated structures known so far. In this study the adsorption properties of PHTS are investigated and compared to those of its plug-free analogue SBA-15. For this purpose nitrogen, n-hexane, n-heptane, c-hexane, 3-methylpentane, 1-hexene, and water were adsorbed on SBA-15 and PHTSs with a different ratio of open and plugged mesopores. The adsorption of n-hexane, c-hexane, n-heptane, and 3-methylpentane on SBA-15 and PHTS-A demonstrated that the presence of the plugs had an effect on the uptake of adsorbate in the low relative pressure region, the position of the capillary condensation step, and the total adsorbed amount of adsorbate. The results showed that n-heptane and 3-methylpentane cannot access part of the micropore system of SBA-15 and PHTS-A. Adsorption of c-hexane and n-hexane on PHTS-A indicated that not only the kinetic diameter but also the shape of the molecule is an important factor for being able to be adsorbed into the micropores or past the plugs. Moreover, these two adsorbates were the most efficient in filling up the available pore volume. From the adsorption of n-hexane on PHTSs with a different ratio of open and plugged pores, it was concluded that the size of the plugs differed, which depends on the synthesis conditions. Water adsorption isotherms proved SBA-15 and PHTS-B to be more hydrophobic than PHTS-A. n-Hexane, 1-hexene, and toluene were adsorbed on SBA-15 and the PHTSs to investigate the influence of the polarity of the adsorbate. The isotherms showed higher uptakes for polar adsorbates on more hydrophobic materials and vice versa.

Influence of the Al source and synthesis of ordered Al-SBA-15 hexagonal particles with nanostairs and terraces

We investigated the effect of various aluminum sources on the morphology and microstructure of nanoporous Al-SBA-15. With aluminum tri-sec-butoxide (Al-TSB), very large, ordered hexagonal rods of Al-SBA-15 covered by nanostairs and terraces were synthesized. Such a morphology was not obtained with an inorganic Al source (sodium aluminate) or with organic sources seemingly similar to Al-TSB, such as aluminum tri-tert-butoxide, aluminum tri-n-butoxide, or aluminum isopropoxide. The results obtained from X-ray diffraction, scanning electron microscopy/transmission electron microscopy, N(2) adsorption/desorption, and high-performance (129)Xe NMR suggest that preparing an organic Al-Si precursor by premixing liquid organic Al and Si sources (Al-TSB and tetraethyl orthosilicate) is crucial in obtaining highly ordered mesoporous Al-SBA-15 materials with a well-defined morphology.