Assembling photoluminescent silicon nanocrystals into periodic mesoporous organosilica

A contemporary question in the intensely active field of periodic mesoporous organosilica (PMO) materials is how large a silsesquioxane precursor can be self-assembled under template direction into the pore walls of an ordered mesostructure. An answer to this question is beginning to emerge with the ability to synthesize dendrimer, buckyball, and polyhedral oligomeric silsesquioxane PMOs. In this paper, we further expand the library of large-scale silsesquioxane precursors by demonstrating that photoluminescent nanocrystalline silicon that has been surface-capped with oligo(triethoxysilylethylene), denoted as ncSi:(CH 2CH 2Si(OEt) 3) nH, can be self-assembled into a photoluminescent nanocrystalline silicon periodic mesoporous organosilica (ncSi-PMO). A comprehensive multianalytical characterization of the structural and optical properties of ncSi-PMO demonstrates that the material gainfully combines the photoluminescent properties of nanocrystalline silicon with the porous structure of the PMO. This integration of two functional components makes ncSi-PMO a promising multifunctional material for optoelectronic and biomedical applications. © 2012 American Chemical Society

Assessing the performance of commercial and biological gas hydrate inhibitors using nuclear magnetic resonance microscopy and a stirred autoclave

The formation kinetics of methane/ethane/propane hydrate in the presence of kinetic inhibitors was investigated using 1H nuclear magnetic resonance imaging (MRI) as well as a more-traditional method using a stirred autoclave. These studies were facilitated by fabricating a multi-drop insert for 1H NMR micro-imaging, which allowed the comparison of the performance of microliter quantities of several inhibitors simultaneously and under the same conditions. Both methods showed that hydrate nucleation and growth were delayed significantly in the presence of inhibitors, which included two biological inhibitors (antifreeze proteins) and a commercial inhibitor. The results demonstrate that MRI is a useful tool for the visualization and evaluation of the performance of kinetic inhibitors on mixed gas hydrate formation. The MRI technique should prove especially valuable in the case of analysis of potential inhibitors, pre-commercialization, which are available in only limited quantities, such as biological inhibitors. This technique may also find utility in the exploration of differences in inhibitor performance, which may suggest distinct mechanisms of inhibitor action.Peer reviewed: YesNRC publication: Ye

A New Approach to Characterizing Sorption in Materials with Flexible Micropores

Microporous dipeptides, also known as organic zeolites or biozeolites, as examples of small-pore peptide nanotubes provide a convenient set of materials for developing a systematic approach based on 129Xe NMR spectroscopy for the derivation of thermodynamic and molecular scale information on temperature dependent pore filling. The sorption of xenon in the isolated 1D chiral nanochannels of eight microporous dipeptides Ala-Val (AV), Val-Ala (VA), Leu-Ser (LS), Ala-Ile (AI), Val-Val (VV), Ile-Ala (IA), Ile-Val (IV), Val-Ile (VI) (all LL isomers) was monitored in situ with continuous-flow 129Xe NMR spectroscopy over a temperature range of 173-343 K. The materials all showed strongly anisotropic signals, with isotropic chemical shift changing from 95 to 281 ppm depending on the dipeptide used and/or temperature. The isosteric heats of sorption (qst) and entropy factors were determined from two independent models. The sorption process was complicated by reversible phase transformations of some dipeptides and irreversible changes due to aging of samples, both of which may be of considerable importance in applications of soft materials. The interpretation of the line shapes and chemical shift anisotropy as a function of temperature provided information on the structure of the xenon-cavity complex and made it possible taking into account the helicity and flexibility of the nanochannels and the dynamics of xenon. The approach illustrates a powerful way of analyzing pore space in soft microporous materials, yielding a quantitative thermodynamic description of sorption and the characteristics of the pore space and sorption events that occur on molecular-scale level during pore filling

Methanol incorporation in clathrate hydrates and the implications for oil and gas pipeline flow assurance and icy planetary bodies

One of the best-known uses of methanol is as antifreeze. Methanol is used in large quantities in industrial applications to prevent methane clathrate hydrate blockages from forming in oil and gas pipelines. Methanol is also assigned a major role as antifreeze in giving icy planetary bodies (e.g., Titan) a liquid subsurface ocean and/or an atmosphere containing significant quantities of methane. In this work, we reveal a previously unverified role for methanol as a guest in clathrate hydrate cages. X-ray diffraction (XRD) and NMR experiments showed that at temperatures near 273 K, methanol is incorporated in the hydrate lattice along with other guest molecules. The amount of included methanol depends on the preparative method used. For instance, single-crystal XRD shows that at low temperatures, the methanol molecules are hydrogen-bonded in 4.4% of the small cages of tetrahydrofuran cubic structure II hydrate. At higher temperatures, NMR spectroscopy reveals a number of methanol species incorporated in hydrocarbon hydrate lattices. At temperatures characteristic of icy planetary bodies, vapor deposits of methanol, water, and methane or xenon show that the presence of methanol accelerates hydrate formation on annealing and that there is unusually complex phase behavior as revealed by powder XRD and NMR spectroscopy. The presence of cubic structure I hydrate was confirmed and a unique hydrate phase was postulated to account for the data. Molecular dynamics calculations confirmed the possibility of methanol incorporation into the hydrate lattice and show that methanol can favorably replace a number of methane guests.Peer reviewed: YesNRC publication: Ye