Highly Porous Zirconium Aryldiphosphonates and Their Conversion to Strong Bronsted Acids

Abstract

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 m 2/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 SO 3 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(O 3PC 6H 4SO 3H) 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.