STUDIES OF THE SOLID SOLUTION INTERFACIAL DEALUMINATION OF KAOLINITE IN HCL(AQ) USING SOLID-STATE H-1 CRAMPS AND SP/MAS SI-29 NMR-SPECTROSCOPY/

The dealumination of kaolinite by the solid/solution ''interfacial'' r eaction with HCl(aq) at 98 degrees C was investigated using solid-stat e H-1 CRAMPS and MAS Si-29 NMR techniques. The single-pulse (SP)/MAS S i-29 NMR spectra of kaolinite-derived solids that are 2-83% dealuminat ed are observed to be dependent upon the degree of dealumination, show ing three resolvable resonances at -89, -100, and -109 ppm. The -89 pp m resonance is due to silicon Q(3)-type Si(OSi)(3)(OAl2) sites in unre acted kaolinite, while the latter two resonances are assigned to new Q (3)-type silica-alumina (Si-OH+-Al) and to Q(4)-type amorphous silica (Si(OSi)(4)) sites contained in dealuminated kaolinite solids followin g partial dealumination. The H-1 CRAMPS spectra of these dealuminated solids are also dependent upon the degree of removal of Al3+ ions from the kaolinite Al-OH-Al layer. In addition to a broad proton peak at 4 .0 ppm due to structural protons of the intact Al-OH-Al layer in unrea cted kaolinite, two new resonances at 0.4 and 7.0 ppm are assigned to protons of the new silanol (Si-OH) and alumina-silica (Si-OH+-Al) site s. The spectral intensities of the SP/MAS Si-29 and H-1 CRAMPS NMR res ults are correlated with the progress of dealumination, permitting the development of a structural model for partially dealuminated kaolinit e. This structural model is consistent with a kinetic model involving a chemically-controlled ''heterogeneous'' reaction process whereby Hion attack of the Si-O-Al linkages at the edges of the mineral surface leads to liberation of aluminum ions into the solution medium.