W.V. Gerasimowicz V.M. Stockette D. D. Eberl D.M. Byler 1991 <p><span>Infrared spectroscopy has shown for the first time that the naturally occurring zeolite clinoptilolite can absorb urea, (NH</span>₂<span>)</span>₂<span>CO, under ambient conditions from either aqueous or ethanolic solutions. The two strongest NH stretching bands at 3441 and 3344 cm</span>⁻¹<span>&nbsp;in pure, solid urea shift to higher frequency (about 3504 and 3401 cm</span>⁻¹<span>) after absorption. Two of the four urea bands in the 1800-1300 cm</span>⁻¹<span>&nbsp;range (at 1683 and 1467 cm</span>⁻¹<span>) undergo marked downward shifts to about 1670 and 1445 cm</span>⁻¹<span>. The other two bands show little change in frequency. The strong band at 1602 cm</span>⁻¹<span>, however, diminishes in intensity to little more than an ill-defined shoulder on the 1626-cm</span>⁻¹<span>&nbsp;peak. When clinoptilolite is heated to 450 °C and then treated with molten urea (ca. 140 °C) for several minutes, and finally washed twice with ethanol to remove excess unreacted urea, further changes become apparent in the spectrum of the urea-treated clinoptilolite. The two NH stretching bands broaden without significant change in frequency. Two new bands appear in the midfrequency range at 1777 (weak) and 1719 (medium strong) cm</span>⁻¹<span>. Of the four original midfrequency peaks, the one at 1602 cm</span>⁻¹<span>&nbsp;is now absent. Two others (1627 and 1440 cm</span>⁻¹<span>) exhibit little change, while the fourth has broadened and shifted down to 1663 cm</span>⁻¹<span>, where it appears as a shoulder on the band at 1627 cm</span>⁻¹<span>. Both treatments clearly induce interaction between urea and the zeolite which seems to result in significant modifications in the nature of the hydrogen bonding of the substrate.</span></p> application/pdf 10.1016/0026-265X(91)90090-C en Elsevier Infrared spectroscopic examination of the interaction of urea with the naturally occurring zeolite clinoptilolite article