SOLID ACIDITY OF MONTMORILLONITE CONTAINED IN BLEACHING EARTH EVALUATED USING SOLID-STATE ³¹P MAS NMR

Abstract

For the purpose of investigating the solid acidities of montmorillonite (MMT) for use as a catalyst, various MMT samples were obtained by purifying several bleaching earths from different localities. MMT has an interlamellar cation exchange capacity (CEC_i) originating in the isomorphous substitution in its layers, and MMT whose interlayer cations were exchanged with Al³⁺, Fe³⁺, Cr³⁺, Zn²⁺, NH₄⁺ and H⁺ showed solid acidity, while MMT whose interlayer cations were exchanged with Mg²⁺, Ca²⁺ and Na⁺ did not. Since the solid-state ³¹P NMR spectrum with magic angle spinning (MAS) of trimethylphosphine oxide (TMPO)-adsorbed H⁺-type MMT showed signals assignable to interlayer H⁺ at 58–54 ppm and signals assignable to H⁺ located in holes in the octahedral layers after migrating from the interlayer space at 69–64 ppm, the Brønsted acid site concentration in H⁺-type MMT was correlated with its corrected CEC_i, whose molar mass was corrected by the MMT content. The aspect ratios of dehydrated MMT particles were in the range from 1.7 to 11 and less than one-tenth compared with hydrated MMT, indicating that the effect of the edge surface area in H⁺-type MMT crystallite cannot be ignored. None of the Si–OH, Al–OH and Si–OH–Al groups present in the edge surface of H⁺-type MMT acted as solid acid sites. The Zn²⁺-type MMT possessed weak Lewis acid sites, and the bleaching earth possessed Lewis acid sites after calcining at above 600 K. After acid treatment, TMPO interacting with the bleaching earth surface showed a solid-state ³¹P MAS NMR signal at 67 ppm, which was identical to the signal position of TMPO interacting with H⁺-type MMT, and another at 71 ppm, which was similar to that of TMPO interacting with the silica–alumina catalysts. The Brønsted acid site concentration was larger than those of the silica–alumina catalysts.