A Variable-Temperature Diffuse Reflectance Infrared Fourier Transform Spectroscopy Study of the Binding of Water and Pyridine to the Surface of Acid-Activated Metakaolin

Four metakaolins were prepared by heating a Spanish kaolin at 600, 700, 800, and 900 °C for 10 h. Following preliminary optimization, these metakaolins were acid activated in 6 M hydrochloric acid at 90 °C for 6 h; the samples calcined at 600, 700, and 800 °C produced the highest surface area solids and were selected for further study. Variable-temperature diffuse reflectance infrared Fourier transform spectroscopy analysis of the resulting acid-activated metakaolins (AAMKs) identified a wide range of hydrogen bond strengths in adsorbed water at room temperature. Above 300 °C it was possible to fit the broad hydroxyl stretching band to seven contributing components at 3730, 3700, 3655, 3615, 3583, 3424, and 3325 cm^-1. As the sample temperature was increased, the 3730 cm^-1 band increased in intensity as the water hydrogen bonded to AlOHAl was thermally desorbed. The other six bands decreased in intensity. The spectra of adsorbed pyridine indicated the presence of both Brönsted and Lewis acid sites on the surface of the air-dried AAMKs. Preheating the AAMK at 200 °C prior to pyridine sorption reduced the number of Brönsted acid sites and increased the number of thermally stable Lewis acid sites. A reduction in the amount of adsorbed pyridine after pretreating the AAMK at 400 °C was tentatively attributed to a reduction in surface area. This was reflected in fewer thermally stable Lewis acid sites in the AAMK pretreated at 400 °C compared to the number present in the sample pretreated at 200 °C.