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.
Abstract
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.
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