High pressure synthesis of novel materials
One of the most remarkable outcomes of physics and chemistry at extreme conditions is the synthesis of novel materials, which can significantly update our view of the periodic table and also of potential practical interest. In our case, we obtain a variety of composite materials by reacting simple molecules in the micro-pores of zeolites. Indeed, zeolites are complex materials exhibiting an impressive range of applications, including molecular sieve, gas storage, catalysis, electronics and photonics. We use these materials, particularly non catalytic zeolites in an entirely different fashion. In fact, we perform high pressure (0.5-30 GPa) chemical reactions of simple molecules on a sub-nanometer scale in the channels of a pure SiO2 zeolite, silicalite to obtain unique nano-composite materials with drastically modified physical and chemical properties. Our studies are based on a combination of X-ray diffraction, Raman, and IR spectroscopy techniques in the diamond anvil cell (DAC).
We have been first showing how silicalite can be easily filled by simple molecules such as Ar, CO2 and C2H4 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 . A silicon carbonate crystalline phase was then synthesized by reacting silicalite and molecular CO2 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 . On the other hand, a stable at room conditions crystalline CO2-SiO2 solid solution with average chemical formula of C0.6Si0.4O4 was obtained by reacting nano-confined CO2 and silicalite at P=16-22 GPa, and T~5000 K, in laser heated DACs . This is a new hard material with bulk modulus of 240 GPa, which is about one order of magnitude higher than the one of quartz, and it is also 30-40% lighter then quartz.
A spectacular crystalline nano-composite is then obtained by photo-polymerizing ethylene at 0.5-1.5 GPa under UV (351-364 nm) irradiation in the channels of silicalite  (figure 1). For this composite, also recovered at ambient conditions, 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 and thermo-mechanical properties may thus be tuned by varying the amount of polymerized ethylene. For instance, a null thermal expansion material could be obtained in principle.
Also, a unique crystalline nano-composite is obtained by polymerizing acethylene at ~4 GPa in the channels of silicalite , and recovered at ambient conditions (figure 2). This composite is made of conjugated chains embedded in the silicalite, and can be considered as the first step toward the formation of a perfect composite made of endless, conductive polyacetylene (PA) chains embedded in an inorganic framework, one possessing an all 1D channel system, which will protect the polymer from atmospheric moisture. We are presently trying the high pressure synthesis of the ideal 1D PA, exploiting the 1D channel system of a different siliceous zeolite, ZSM-22. This composite is predicted to exhibit unique, quantum properties, due to the sub-nano confinement and the 1D character of PA, which could lead to design new quantum computing devices.
The polymerization of CO in both silicalite and ZSM-22 is also under study, which would bring to new energetic materials. Finally, we are now planning the study of similar polymerization protocols, along with the functionalization of the nano-walls of well targeted zeolites for synthesizing novel composite materials for applications as resistive and chemi-fluorescent gas sensors with huge effective surface of the order of hundreds of square meters per grams and, correspondingly, unrivalled sensitivity.
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.
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