In this paper, the authors summarized the findings of extracting essential oils and polyphenols from wet immature citrus peels using the supercritical carbon dioxide (SCO₂) extraction and the Soxhlet one. Monoterpenes were the main components in the Soxhlet extraction (solvent: ethanol), while the selectivity of monoterpenes drastically decreased and that of oxygenated compounds greatly increased in SCO₂ extraction. Additionally, the total polyphenol content in the SCO₂ extraction reached about 80% of that obtained by the Soxhlet extraction, indicating that polar organic compounds, which are normally hard to extract, can be efficiently extracted in the presence of water content in peels.

The technique of the deposition of polyimide inside the micro-scale spaces was developed using supercritical carbon dioxide (scCO₂). The present paper explains solubilities of polyimide monomers in scCO₂, phase behaviors during the polymerization of monomers to polyamic acid in scCO₂, and the deposition of polyimide inside the micro-scale trenches on the silicon wafer. As for the monomers of 4,4'-diaminodiphenyl ether and pyromellitic dianhydride, the trenches were filled with the polymers at relatively low deposition temperature. On the other hand, the trenches were filled with polyimide that consisted of fluorinated monomers of 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl and 4,4'

(hexafluoroisopropylidene)diphthalic anhydride even at high deposition temperature.

A new foaming apparatus, which can depressurize the vessel containing polymer sample and CO₂ from 100 MPa to atmospheric pressure in the order of milliseconds, was developed to generate a high degree of supersaturation. The effects of the high saturation pressure and rapid depressurization in polymer foaming process were studied using the apparatus with PMMA as a model polymer. Under the conditions studied, the cell nucleation density increased by more than an order of magnitude over the standard foaming process. Moreover, transparent nanocellular foams were obtained by combining the conditions and two-step foaming process.

To build the effective carbonation process for social implementation, the improving the yield and crystal quality of carbonates are essential during the reactive crystallization between CO₂ in exhaust gas and Ca･Mg in industrial wastes.

In the case where CO₂ fine bubbles are applied to the reactive crystallization of Ca･Mg carbonates, Ca²⁺ and Mg²⁺ accumulate because of the negative charge on the fine bubble surface, and the CO₃^2– concentration becomes higher due to the acceleration of the CO₂ mass transfer caused by minimizing the bubble diameter. Hence, the local supersaturation regions near the minute gas-liquid interfaces can be utilized to the enhanced production and the quality control of Ca･Mg carbonates. Additionally, the Ca･Mg carbonates which controlled crystal quality are convertible to inorganic phosphors with superior luminescent properties for upgrading.

CO₂ hydrates are a class of gas hydrates as well as natural methane hydrates vastly exist in subsea area. Recently, CO₂ hydrates are attracted much attention as a key technology for hydrate based CCS since there are a lot of options for CCS sites. In this commentary, we provide basic information for gas hydrates especially for CO₂ hydrates and introductory of CO₂ hydrate based applications including CO₂ capture, CO₂ sequestration and their processes.