Aerogels supercritical fluids

Cao Y, Hsu JC, Hong ZS, Deng JF, Fan KN (2002) Characterization of high-surface-area zirconia aerogel synthesized from alcohothermal and supercritical fluid diying techniques. Catal Lett 81 107-112... 

The PCA process uses supercritical fluid drying to help preserve fine microstructures in the material. Supercritical fluid drying is a technique that has been used for many years to dry biological materials and, more recently, aerogels (qv). The original solvent is replaced by exchange with a supercritical fluid, such as C02, and the system is depressurized above the critical temperature of the SCF. SCFs have no vapor—liquid interface. Thus fine microstructures are... 

Sunol, S.G., Sunol, A.K., Keskin, O., Guney,0., Supercritical fluid aided preparation of aerogels and their characterization , in Innovations in Supercritical Fluids Science and Technology Chapter 17, pp 258-268, edited by K. W. Hutchenson and N. R. Foster, ACS Symposium Series 608 (1995). 

While this patent does report on an improved process for making aerogels, the concept of displacing the reaction solvent with a supercritical fluid has been reported as early as 1983 by Schmitt, Grieger-B lock, and Chapman as described in chapter 12. In the Schmitt process, the aqueous solvent used to form aerogels of silica is displaced by methanol, the system is heated and pressurized above the critical point of the methanol, and the methanol is vented from the system. The benefit is that methanol has a critical temperature that is 135 °C less than that of water. It is not possible to use CO2 to displace the water in the gel without the occunence of a vapor-liquid interface since water and CO2 are not miscible unless the temperature is raised to greater than 300 °C. 

Schmitt, W. J., R. A. Grieger-Block, and T. W. Chapman. 1983. The preparation of acid-catalyzed silica aerogel. In Chemical engineering at supercritical fluid conditions, ed. M. E. Paulaitis, J. M. L. Penninger, R. D. Gray, Jr., and P. Davidson. Ann Arbor, MI Ann Arbor Science. 

Erkey and co-workers [59-61] prepared Pt- and Ru-doped carbon aerogels using a supercritical deposition method. This involved dissolution of an organometallic precursor in a supercritical fluid and the exposure of a solid substrate to this solution. After impregnation of the support with the metal precursor, it was converted to the metal form by different methods. Dimethyl(l,5-cyclooctadiene) platinum(ll) was used as a precursor for Pt [59,60], and two different Ru complexes, trisacetylacetonate Ru(lll) and Ru(cod)(tmhd)2, were used for Ru [61], Monolithic organic and carbon aerogels... 

The 1990s witnessed the spread of supercritical techniques around the world very large industrial units were built in the United States and Australia for the treatment of coffee, tea, and hops. By the end of the 1990s, the discovery of new applications for supercritical fluids, such as for the production of microparticles, aerogels, and impregnation of polymers, strengthened the growing importance of supercritical CO2. 

Takishima, S., Suzuki, Y., Konishi, Y., Ohmori, T., Sato, Y, and Masuoka, H., Production of silica aerogels by supercritical CO2 drying. The Fourth International Symposium on Supercritical Fluids, Sendai, Japan, pp. 11-14, May 1997. 

Garcfa-Gonz ez, C.A., Camino-Rey, M.C., Alnaief, M., Zetzl, C., and Smirnova, L, Supercritical drying of aerogels using COj Effect of extraction time on the end material textural properties, Journal of Supercritical Fluids, 66, 297-306, 2012. 

W. (2010) Adsorptive crystallization of benzoic acid in aerogels from supercritical solutions./. Supercrit. Fluids, 52 (3), 249-257. 

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