HYDROTHERMAL SYNTHESIS (250-DEGREES-C) OF COPPER-SUBSTITUTED KAOLINITES

To obtain Cu-kaolinites with a controlled range of chemical compositio ns, syntheses were performed by hydrothermally ageing gels with kaolin ite stoichiometric compositions. Gels were prepared with sodium metasi licate and nitrates of octahedral cations. Temperature of synthesis wa s 250 degrees C with a corresponding equilibrium water pressure of 38 bars. Three samples with copper contents ranging from 0.1 to 7% and an other one with the chemical composition of the Cu end-member were synt hesized. While this fourth sample led to tenorite after the hydrotherm al treatment, the three others crystallized well into kaolinite. Up to almost 1% CuO was measured by TEM in some isolated 'clean' and hexago nal kaolinite particles. EPR and XPS spectroscopies were consistent wi th an octahedral position of Cu2+. In IR spectra, vAl-OH-Cu absorption bands were not observed, but vAl(2)OH bands appeared more and more bl urred when Cu content of samples increased. Weak bands situated at 868 cm(-1) and 840 cm(-1) are tentatively attributed to delta AlCuOH. By differential thermal analysis, a downward shift of 20 degrees C in tem perature of the endothermic peak from the less Cu-rich sample to the m ost Cu-rich one, argued for the existence of some Al-OH-Cu bonds, whos e binding energies are presumed to be less than the Al-OH-Al ones. In view of these results, Cu2+ appears incorporated in the octahedral she et of kaolinite. Moreover, this incorporation is made without major pe rturbation of the kaolinite structure.