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Original scientific paper

Precipitation and Characterization of Two Novel Aluminum Hydroxo Phthalates Formed in the Presence of Silicic Acid

László Horváth ; Department of Physical Chemistry, Ruđer Bošković Institute, P.O. Box 180, HR-10002 Zagreb, Croatia
Halka Bilinski ; Department of Physical Chemistry, Ruđer Bošković Institute, P.O. Box 180, HR-10002 Zagreb, Croatia
Lajos Radics ; Central Research Institute for Chemistry, Hungarian Academy of Sciences, Budapest, Hungary
Nils Ingri ; Department of Inorganic Chemistry, University of Umeå, Sweden

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The present work, performed at constant ionic strength of 0.6 mol dm–3 NaCl at 25 °C, is a continuation of long-term studies of the 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 a monoclinic unit cell, which was formed in the absence of Si. Addition of Si induced precipitation of two novel compounds (B and C) of the orthorhombic unit cell. Compound B was formed at a low L/Al ratio (R ≤ 2) and was of variable composition: AlxSi1.450–0.725x(OH)3.8+0.1xL • 6H2O. It was found by calculation that at R ≤ 2 the end member with x = 2, Al2(OH)4L • 6H2O, determined the precipitation boundary. Compound C formed at R > 2 had the constant composition Al(OH)L • 3H2O. Formation constants βp,q,r were determined for the general equation pH+ + qAl3+ + rL2- → HpAlqLr. The obtained values for compounds B and C are: logβ(–4, 2, 1) = –7.40±0.10 and logβ(–1, 1, 1) = 1.55±0.05, respectively. In addition to chemical analysis, thermogravimetry (TG), X-ray diffraction (XRD), infrared spectroscopy (IR), solid state 27Al MAS NMR, 29Si MAS NMR and 23Na MAS NMR techniques were used for better characterization of solids and to detect possible impurities in them. Aluminum was found to be hexa-coordinated in all compounds. The IR spectra suggest that the phthalate ion acted as a bidendate ligand, bridging two aluminum atoms in solids B and C.


aluminum hydroxo phthalates, formation constants, effect of silicic acid, model clay formations, solid state NMR, <sup>27</sup>Al MAS NMR, <sup>29</sup>Si MAS NMR, <sup>23</sup>Na MAS NMR

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