Structural and acidic properties of copper-silica catalysts 1. A differential scanning calorimetry and Fourier transform-infrared/photoacoustic study

Abstract

Adsorption of pyridine on copper-silica catalysts is studied by differential scanning calorimetry (DSC) and Fourier transform-infrared/photoacoustic spectroscopy (FT-IR/PA), in the region of mid-frequency vibrations as well as in the region of hydroxyl groups. The adsorption of pyridine on copper-silica catalysts shows three modes of sorption in the 1650-1400 cm −1 region: physisorption or hydrogen-bonded (HPY), and two sites of Lewis acid sorption (LPY(I) and LPY(II)). The Lewis band at 1609 cm −1 was very strong compared with the 1450 cm −1 band since the former band showed a shift to higher wavenumbers whereas the latter showed a shift to lower wavenumbers. A relationship was found between the DSC scans, in the low temperature range (240-140°C), and the 1450-1443 cm −1 band. On the other hand, the high temperature DSC scans (400-360°C) were correlated with the behaviour of the 1609–1612 cm −1 band. The volumetric results of pyridine adsorption on 9 and 15 wt.% Cu catalysts show a constant behaviour; however, this constancy was also reflected by carbon monoxide adsorption, demonstrated by a similar behaviour of both 2128 and 2259 cm −1 bands. This was attributed to the blocking of the small pores of silica earlier at the 9 wt.% Cu loading. A new broad band at 2248–2259 cm −1, not identified in the ion-exchange catalysts, was obtained and attributed to CO adsorbed on agglomerated copper clusters.