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'' reaction with HCl(aq) at 98 degrees C was investigated using solid-state H-1 CRAMPS and MAS Si-29 NMR techniques. The single-pulse (SP)/MAS Si-29 NMR spectra of kaolinite-derived solids that are 2-83% dealuminated are observed to be dependent upon the degree of dealumination, showing three resolvable resonances at -89, -100, and -109 ppm. The -89 ppm resonance is due to silicon Q(3)-type Si(OSi)(3)(OAl2) sites in unreacted 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 following 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 unreacted 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) sites. The spectral intensities of the SP/MAS Si-29 and H-1 CRAMPS NMR results are correlated with the progress of dealumination, permitting the development of a structural model for partially dealuminated kaolinite. This structural model is consistent with a kinetic model involving a chemically-controlled ''heterogeneous'' reaction process whereby H+ ion attack of the Si-O-Al linkages at the edges of the mineral surface leads to liberation of aluminum ions into the solution medium.