Knoevenagel condensation of various aliphatic, aromatic and heterocyclic aldehydes with malononitrile was carried out in the liquid phase in one step by employing a sulfate-ion promoted zirconia solid acid catalyst. This catalyst facilitates the reaction under solvent-free conditions at moderate temperatures providing excellent yields of the products. The sulfate-ion promoted zirconia catalyst was synthesized by immersing a finely powdered hydrous Zr(OH)(4) into the 1 M H2SO4 solution and subsequent drying and calcination at 923 K. The Zr(OH)(4) was prepared from an aqueous ZrCCl2.8H(2)O solution by hydrolysis with dilute aqueous ammonia. The bulk and surface properties of the prepared catalyst was examined by X-ray powder diffraction, BET surface area, temperature programmed desorption of ammonia and Raman spectroscopy techniques. All characterization results revealed that the incorporated sulfate-ions show a significant influence on the surface and bulk properties of the ZrO2.In particular, the impregnated sulfate-ions stabilize the metastable zirconia tetragonal phase at ambient conditions and enhance the total number and strength of acid sites. (c) 2006 Elsevier B.V. All rights reserved.