Determination of the hydroxyl group content in silica by thermogravimetry and a comparison with 1H MAS NMR results

The hydroxyl group content in nine different types of silicas was determined by thermogravimetry (TG). The TG measurements were performed by heating the silica samples (15–30 mg) at a rate of 5°C min−1 from room temperature to 1250°C in an argon flow. The dehydration and dehydroxylation phenomena (two mass loss steps) were distinguished with the help of the differential TG curve (DTG). The temperature where physisorbed water was assumed to be totally released was dependent on the silica and was found to range from 100 to 130°C. The OH group content was calculated from the second mass loss step by assuming that there were only siloxane bridges left in silica at 1250°C. The total hydroxyl group content was calculated from the entire mass loss of the second step. The remaining amount of OH groups at any temperature below 1250°C could also be calculated and was done in this study at 550 and 1000°C. When comparing the total OH group content and the OH group content at 550°C determined by the TG method with the results of calcined samples measured by 1H MAS NMR, they were found to be in a good agreement with each other. However, a significant difference was observed between the methods regarding the remaining OH group content at 1000°C.