Evidence for strong acidity of the molecular sieve cloverite

ZEOLITES derive their catalytic activity from the strong acidity of protons attached to the negatively charged aluminosilicate frame-work, which makes the materials excellent proton donors. Unlike zeolites, the aluminophosphate molecular sieves^1,2 are built from alternating AlO^- ₄ and PO⁺₄ tetrahedra and are thus electrically neutral. Much attention has therefore been devoted to the generation of Bronsted acidity in these materials by introducing heteroatoms, such as Si, Mg, Fe, Co or Zn, to produce negatively charged frameworks^3-6. Similar arguments apply to gallophosphate molecular sieves^7-10, of which cloverite^9,10 is a remarkable example. This extra-large-pore material contains pore openings in the form of a four-leafed clover, defined by a ring of 20 gallium and phosphorus atoms, some of which are linked to terminal hydroxyl groups. Here we use NMR, X-ray photoelectron spectroscopy (ESCA) and infrared spectroscopy to show that the P-OH groups in cloverite are localized versions of those in solid phosphoric acid, H₃PO₄. Cloverite is thus a strong Bronsted acid even though no heteroatoms are present in its framework.