engleski

Precipitation and characterization of two novel aluminum hydroxo phthalates formed in the presence of silicic acid

The present work performed at constant ionic strength of 0.6 M NaCl at 25 0C is a continuation of long term studies of conditions necessary for initial clay formation in model systems, containing organic acids, Al and Si. Solid phases and the precipitation boundary characterizing the system H+-Al3+-phthalic acid (H2L) - silicic acid were determined and compared with the previously found compound Al2(OH)4L. 4H2O (A) of monoclinic unit cell, which was formed in the absence of Si. . Addition of Si induced precipitation of two novel compounds (B and C) of orthorhombic unit cell. Compound B formed at low L/Al ratio (N ? 2) and was of variable composition: Alx Si1.450-0.725x (OH)3.8+0.1x L .6 H2O. It was found by calculation that at N ? 2 the end member with X=2, of Al2(OH)4L 6H2O, determines the precipitation boundary. Compound C formed at N > 2 had the constant composition Al(OH)L.3H2O. Formation constants ßp, q, r were determined for the general equation pH++qAl3++rL2- › HpAlqLr. The obtained values for compounds B and C are: logß(-4, 2, 1) = -7.40ą0.10 and logß (-1, 1, 1) = 1.55ą0.05, respectively. In addition to chemical analysis, thermogravimetry (TG), powder diffraction (XRD), infrared spectroscopy (IR), solid state 27Al MAS NMR, 29Si MAS NMR and 23Na MAS NMR techniques were used for better characterization od solids and to detect possible impurities in them. Aluminum was found to be hexa-coordinated in all of them. From IR spectra could be suggested that phthalate ion acts as a bidendate ligand, bridging two aluminium atoms in solids B and C.

aluminum hydroxo phthalates; formation constants; effect of silicic acid; model clay formations; solid state NMR; 27Al MAS NMR; 29Si MAS NMR; 23Na MAS NMR

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