In situATR-IR study of prochiral 2-methyl-2-pentenoic acidadsorption on Al2O3 and Pd/Al2O3

Abstract

Adsorption and desorption of trans-2-methyl-2-pentenoic acid (MPeA) in dichloromethane (CH₂Cl₂) were investigated by using in situ attenuated total reflection infrared (ATR-IR) spectroscopy. A liquid flow-through spectroscopic cell allowed for high quality spectra to be obtained from deposited thin films of Al₂O₃ and 1 wt% Pd/γ-Al₂O₃ on a ZnSe internal reflection element. The MPeA molecules adsorb on both Al₂O₃ and Pd surfaces molecularly and dissociatively under the concentration range examined (2–16 mM). In the case of molecular adsorption, both monomer (ν_CO ∼ 1720 cm⁻¹) and dimer (ν_CO ∼ 1685 cm⁻¹) species are observed to adsorb, with the relative amount of monomer to dimer dependent on the surface and the liquid phase acid concentration. In the case of dissociative adsorption, the acid adsorbs predominantly in a bridged bidentate configuration, as adjudged by the ca. 150–220 cm⁻¹ separation between asymmetric and symmetric vibrational bands. All of these species are found to be strongly adsorbed on both Al₂O₃ and 1 wt% Pd/γ-Al₂O₃ surfaces, even under pure solvent flow after adsorption.