Synthesis, characterization, chemisorption and thermodynamic data of urea immobilized on silica

Abstract

Urea has been covalently immobilized onto silica gel by reaction of urea with a silanized agent attached to silica or by reaction of the previously synthesized molecule containing both urea and silanizing agent with the support, giving 0.55 and 0.92 mmol of urea per gram of silica, respectively. The physical surface area and infrared, proton and solid-state carbon NMR spectra are similar for both surfaces, which extract MCl₂(M = Ni, Co, Zn, Cu, Cd, Hg) from acidic water, ethanol and acetone solutions at 298 K. The retention of cations in aqueous solution increases the pH, with Ni and Hg being the most strongly adsorbed. The chemisorption isotherms obtained in non-aqueous media are similar and fit the Langmuir equation. The sequence of maximum retention capacity is Zn < Cu < Hg < Ni < Co and Cu < Co < Zn for ethanol and acetone, respectively. The integral heats of adsorption of cations by the immobilized surface at 298.2 K were larger in ethanol. With the exception of Co (ΔH=+25.51 ± 0.08 kJ mol^–1), the cation interactions are exothermic in acetone. The value for Cd (ΔH=–35.91 ± 0.04 kJ mol^–1) contrasts with the results found in ethanol, which are endothermic and about one-third of the Cd value. Electronic spectra of the solid suspended in carbon tetrachloride, collected after batchwise adsorption, are in agreement with a tetrahedral geometry for Ni, Co and Cu.