Study of forms of adsorption for methanol on chromium oxide and their role in complete and selective oxidation of alcohol

Abstract

Study of adsorption of alcohols on Cr203 by iR spectroscopy was begun by Roev and Terenin [i], who observed the formation of aicohoiates (C-O stretching vibrations in the 1200-i000 cm -l region) and suggested the possibility of formation of surface structures containing unsaturated bonds (C=C stretching vibrations at 1580 cm -l and =C-O- stretching vibrations at 1250 cm-l). However, the evidence for formation of the latter was insufficient and further investigations [2, 3] refuted the existence of olefin groups, showing that the absorption band at 1580 cm -l upon adsorption of CH~OH characterizes the asymmetric vibrations of CO0- in the formate structure. The absorption band at 1250 cm -I is observed in the spectrum of methanol vapor [4] and in our opinion belongs to OCH bending vibrations, it was established in [5] that methanol is adsorbed on the surface of chromium oxide in the following forms: aicohoi hydrogen-bonded with the surface (vCO 1045 cm-1); chromium methoxide (Vs,asCH 2840, 2940 cm -1, vCO 1090 cm-l); chromium formate (~s,asCO0 - 1360, 1565 cm-l). Most of the attention in the cited papers is focused on the reaction of the reactive forms of adsorbed oxygen with alcohols [2, 3], determination of the differences in the forms of adsorption of CI-C ~ alcohols on Cr203 [5]. Moreover, in order to determine the role of surface forms in complete and selective oxidation of the alcohol, we need a detailed study of the forms of adsorption for methanol, the mechanisms for their formation and conversion on the surface. With this goal, we studied the adsorption and desorption of methanol at temperatures of 298-673 K on the Cr20 ~ surface with different degrees of oxidation by IR spectroscopy. We performed experiments on complete oxidation of surface methoxides on the reduced sample. in order to determine the mechanism of formation of surface esters, we carried out adsorption of NH 3 and CO with subsequent adsorption and desorption of methanol upon elevation of the temperature. The catalyst was prepared according to the technique described in [6], calcination temperature 748 K, specific surface area 44 m~/g. The IR spectra in the 900i800 and 2700-3700 cm - region were recorded on the UR-20 spectrometer. The conditions under which the spectra were recorded and the design of the cuvette are described in [7, 8]. The sample surface was oxidized with oxygen at 743 K with subsequent evacuation at 293 K. The partially oxidized surface was obtained upon adsorption and desorption of oxygen at 743 K. The reduced surface was obtained from the oxidized surface by adsorption of alcohol at 300 K with subsequent evacuation at 743 K. Each operation of reduction of the surface corresponded to removal of 10% of the monolayer.