Session H7: CH.1 Chemistry: High Pressure Synthesis

High pressure synthesis of novel, zeolite based nano-composite materials

Meso/micro-porous solids such as zeolites are complex materials exhibiting an impressive range of applications, including molecular sieve, gas storage, catalysis, electronics and photonics. We used these materials, particularly non catalytic zeolites in an entirely different fashion. In fact, we performed high pressure (0.5-30 GPa) chemical reactions of simple molecules on a sub-nanometer scale in the channels of a pure SiO$_{\mathrm{2}}$ zeolite, silicalite to obtain unique nano-composite materials with drastically modified physical and chemical properties. Our material investigations are based on a combination of X-ray diffraction and optical spectroscopy techniques in the diamond anvil cell. I will first briefly show how silicalite can be easily filled by simple molecules such as Ar, CO$_{\mathrm{2}}$ and C$_{\mathrm{2}}$H$_{\mathrm{4}}$ among others from the fluid phase at high pressures, and how this efficient filling removes the well known pressure induced amorphization of the silica framework (Haines et al., JACS 2010). I will then present on a silicon carbonate crystalline phase synthesized by reacting silicalite and molecular CO$_{\mathrm{2}}$ that fills the nano-pores, at 18-26 GPa and 600-980 K; after the synthesis the compound is temperature quenched and it results to be slightly metastable at room conditions (Santoro et al., PNAS 2011). On the other hand, a stable at room condition spectacular crystalline nano-composite is obtained by photo-polymerizing ethylene at 0.5-1.5 GPa under UV (351-364 nm) irradiation in the channels of silicalite (Santoro et al., Nat. Commun,, in press 2013). For this composite we obtained a structure with single polyethylene chains adapting very well to the confining channels, which results in significant increases in bulk modulus and density, and the thermal expansion coefficient changes sign from negative to positive with respect to the original silicalite host. Mechanical properties may thus be tuned by varying the amount of polymerized ethylene. We then think our findings could allow the high pressure, catalyst free synthesis of a unique generation of technological, functional materials based on simple hydrocarbons polymerized in confining meso/micro-porous solids.   

Structures and Gas Storage Performance of Metal-organic Framework Materials at High Pressures

Metal Organic Frameworks (MOFs), are crystalline nanoporous materials comprised of small metal clusters connected three-dimensionally by polyfunctional organic ligands. MOFs have been widely studied due to their high porosity, surface area and thermal stability, which make them promising candidates for gas capture and storage. In the MOF family, Zeolitic Imidazolate Frameworks (ZIFs) have attracted much attention because of their promising applications for CO$_{\mathrm{2}}$ storage. In contrast to the extensive studies under ambient conditions, most ZIFs have only been studied under pressure in a very limited range. It is known that pressure can provide an effective driving force to achieve structural modification which includes changes in pore size, opening and geometry, channel shape and internal surface area. Subsequently, these pressure-induced changes will affect the sorption selectivity, capacity and access to the binding sites of the porous materials. Here, we report the first in situ high-pressure investigation of several ZIFs by FTIR spectroscopy. We observed rich pressure-induced transformations upon compression in different pressure ranges. Furthermore, the reversibilities of these transformations upon decompression were also examined. Finally, the performance of CO$_{\mathrm{2}}$ storage of selected ZIFs at high pressures will be addressed. Our observation and analyses contribute to the understanding of chemical and mechanical properties of ZIFs under high-pressure conditions and provide new insight into their storage applications.   

Growth of rectangular hollow tube single crystals with rutile-type structure in supercritical fluids

Super critical fluid is known as a suitable solvent in the dissolution and extraction process, due to its extreme high solubility and reactivity. On the other hand, further experimental approaches using supercritical fluid would offer new insights, especially in the field of novel material synthesis and crystal growth. We here report on the successful growth of single crystals with the rutile-type structure ($M$O$_{2}$ ; $M=$Ti, Si, Ge and Sn) in the supercritical fluids (water or oxygen) by using laser heated diamond-anvil cell at above 5 GPa. The resultant product showed the rectangular hollow tube with several tens of microns in length and the wall thickness of less than 500 nm. TEM analyses demonstrated that this rectangular hollow tube single crystal is surrounded by the (110) face and grown along the [001] direction. The preferential growth of (110) face is consistent with the lowest surface energy of (110) in the rutile-type structure. In addition, the rapid cooling rate of LHDAC and the high-solubility of oxides into the supercritical fluids also play an important role for the formation of the rectangular hollow tube. The details of the experiments will be discussed in the presentation.   

High-pressure synthesis of new materials via formation of new bonding patterns and unusual stoichiometries

The search for new materials synthesized under extreme conditions of high pressure and high pressure is currently actively pursued. There are multiple theoretical predictions for superior material properties, such as ultra-hardness, superior transport properties such as electrical and thermal conductivity, high energy-density, high-temperature superconductivity, ability to storage hydrogen, etc. Synthesis of new materials at high pressures is based on changes in the equilibrium chemical bonding. Moreover, materials with ``unusual'' stoichiometries have been predicted to become thermodynamically stable at high pressures. Implications of this novel extreme chemistry for synthesis of new materials for practical applications remain challenging because high-pressure bonding patterns are often thermodynamically unstable at ambient pressure. Search for a recovery mechanisms or attempts of synthesis in nominally metastable conditions require detailed knowledge of the energy landscape; extensive collaborative efforts of experiment and theory are needed for its determination. Here, I emphasize the importance for this task of \textit{in situ} fast diagnostic methods. I will present new results on synthesis of materials with new bonding patterns and unusual stoichiometries containing hydrogen, nitrogen, carbon, and halogens. This work has been performed in collaboration with M. Somayazulu, V. V. Struzhkin, V. Prakapenka, E. Stavrou, T. Muramatsu\textbf{, }A. Oganov, W. Zhang, Q. Zhu, S. E. Boulfelfel, A. O. Lyakhov, Z. Konopkova, H.-P. Liermann, D.-Y. Kim.