Solid–Solid Catalysis by Inorganic Solid Acids: Pinacol Rearrangement over a Heteropoly Compound Consisting of Fine Particles

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Abstract Solid-state pinacol rearrangement of 1,1,2-triphenyl-1,2-ethanediol was performed over various solid acids at room temperature. The reaction was initiated by grinding a mixture of the solid reactant and catalyst for 5 min at room temperature. Heteropoly compounds (Cs2.5H0.5PW12O40 and H3PW12O40) and polymer resins (Amberlyst®-15 and Nafion®/SiO2), all which possess strong acidities, were active, whereas H-ZSM-5, SiO2–Al2O3, and p-toluenesulfonic acid were inactive. Cs2.5H0.5PW12O40 showed the highest activity (per unit catalyst weight). The turnover number, which is defined as the number of molecules reacted divided by the number of protons on the surface, exceeded 20 for 9 h, indicating that the solid–solid reaction proceeded catalytically. This activity of Cs2.5H0.5PW12O40 was higher than that for the solid–liquid reaction carried out in chloroform at room temperature, demonstrating the superiority of the solid–solid catalysis. Powder X-ray diffraction measurements indicated that the reactant crystals became nearly amorphous by grinding with Cs2.5H0.5PW12O40, while the crystallinity of Cs2.5H0.5PW12O40 was unchanged. The microstructure built up from fine particles of Cs2.5H0.5PW12O40 would be favorable to intimate contact of the reactant with the catalyst.