SYNTHESIS, CHARACTERIZATION, CHEMISORPTION AND THERMODYNAMIC DATA OF UREA IMMOBILIZED ON SILICA

Urea has been covalently immobilized onto silica gel by reaction of ur ea with a silanized agent attached to silica or by reaction of the pre viously synthesized molecule containing both urea and silanizing agent with the support, giving 0.55 and 0.92 mmol of urea per gram of silic a, respectively. The physical surface area and infrared, proton and so lid-state carbon NMR spectra are similar for both surfaces, which extr act MCl(2), (M = Ni, Co, Zn, Cu, Cd, Hg) from acidic water, ethanol an d acetone solutions at 298 K. The retention of cations in aqueous solu tion increases the pH, with Ni and Hg being the most strongly adsorbed . The chemisorption isotherms obtained in non-aqueous media are simila r and fit the Langmuir equation. The sequence of maximum retention cap acity is Zn > Gu > Hg > Ni > Co and Cu > Co > Zn for ethanol and aceto ne, respectively. The integral heats of adsorption of cations by the i mmobilized surface at 298.2 K were larger in ethanol. With the excepti on of Co (Delta H = +25.51 +/- 0.08 kJ mol(-1)), the cation interactio ns are exothermic in acetone. The value for Cd (Delta H = -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 o f the solid suspended in carbon tetrachloride, collected after batchwi se adsorption, are in agreement with a tetrahedral geometry for Ni, Co and Cu.