Diffuse reflectance i.r. spectroscopic study on hydroxyl groups of H-ZSM-5s having different sizes and properties

The stretching vibration of hydroxyl groups on H-ZSM-5 samples with different crystallite size and Al content was investigated by diffuse reflectance i.r. spectroscopy. Good linear relations for the intensity of the 3,740-cm⁻¹ for the region versus the external surface area and the intensity of the 3,610-cm⁻¹ for the region versus Al content were obtained. It is proven that the absorbance at 3,740 cm⁻¹ is attributed to the terminal (Si-) O-H groups, which are located completely on the external surface, and an absorbance at 3,610 cm⁻¹ is ascribed to (Al-)O-H groups, which are distributed on both the external and internal surface of H-ZSM-5.