69. Gonen, Mehmet; Balkose, Devrim; Gupta, Ram B.; Ulku, Semra. Supercritical Carbon Dioxide Drying of Methanol-Zinc Borate Mixtures. Industrial & Engineering Chemistry Research (2009), 48(14), 6869-6876.
Supercrit. carbon dioxide (CO2) drying of zinc borate species was investigated to evaluate possible chem. alterations in the product during the drying. Methanol-wetted zinc borates produced either from borax decahydrate and zinc nitrate hexahydrate (2ZnO3B2O37H2O) or from zinc oxide and boric acid (2ZnO3B2O33H2O) were dried by both conventional and supercrit. carbon dioxide drying methods. Zinc borate samples dried by both techniques were characterized using anal. titrn., X-ray powder diffraction (XRD), thermo gravimetric anal. (TGA), Fourier transform IR (FTIR) spectroscopy, elemental anal., and SEM. It was found that while zinc borate obtained from zinc oxide and boric acid did not have any chem. interaction with CO2, carbonates were formed on the surface of zinc borate obtained from borax decahydrate and zinc nitrate hexahydrate. The main factor for the carbonate formation during supercrit. CO2 drying is anticipated as the structural differences of zinc borate species. CO2 is a nonpolar solvent, and it does not usually react with polar substances unless water is present in the medium. While 2ZnO3B2O33H2O had three bound OH groups, 2ZnO3B2O37H2O had five bound OH groups and one mole of water of crystn. It is proposed that the water of crystn. reacts with CO2 forming carbonic acid. Then, carbonic acid, which is stronger than boric acid, substitutes borate ions from their zinc salts.