Highly Porous Zirconium Aryldiphosphonates and Their Conversion to Strong Bronsted Acids
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Porous inorganic-organic hybrids have been prepared by the reaction of 4,4′-biphenylbis(phosphonic acid) with Zr(IV) in organic solvents. The resultant products consist of α-zirconium phosphate-type layers crosslinked by biphenyl pillars. By using an excess of Zr in the synthesis, surface areas of ∼400 m2/g have been obtained and the pores can be controlled to be the micro-type with diameters of 10-20 Å and a relatively narrow pore size distribution. The aromatic rings are readily sulfonated by SO3 under pressure to produce very strong Bronsted acid catalysts. An NMR procedure, utilizing the shift of the carbonyl carbon of acetone-2-13C sorbed onto the sulfonated products, indicated an acid strength for the Bronsted acid sites equal to that of 100% sulfuric acid. Unpillared Zr(O3PC6H4SO3H)2 has a somewhat lower acid strength, but still has a higher acid strength than zeolites HX and HY. The particles of this layered acid exfoliate in water and light-scattering data show that they are in 5 nm size range. They also exhibit high proton conductivity as solid membranes. These sulfonated materials have a potential as strong acid catalysts for a variety of reactions at a low temperature. © 2002 Elsevier Science (USA).
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