Spectroscopic Identification of Incorporated Metal Heteroatoms in Beta Zeolites by Diffuse Reflectance IR Spectroscopy

Metal-containing Beta (M-Beta) zeolites are excellent catalysts for a variety of industrially relevant reactions including several oxidation reactions, glucose conversion processes, catalytic transfer hydrogenation, and aldol condensation. The reactivity of these materials derives from the presence of metal atoms that replace silicon atoms inside the zeolite framework. Thus, being able to verify the incorporation of the metal ions into the framework is an important task in evaluating successful material syntheses. However, no widely available laboratory technique is currently capable of confirming the substitution of Si for a broad range of different heteroatoms, thereby resulting in reliance on hard-to-access facilities and/or continued reliance on indirect methods of material characterization. Herein, we demonstrate a simple and efficient method for identifying isomorphous substitution across a range of hydrothermal M-Beta zeolites by means of Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). Specifically, following dehydration of the materials, new metal-specific vibrational bands can be observed between 940 and 1005 cm–1 for Hf-, Sn-, and Zr-Beta zeolites, the fundamental frequency of which is specific for each heteroatom. Such vibrations are comparable to the asymmetric Si–O–Ti stretching vibrations previously identified for Ti-containing silicates in both hydrated and dehydrated states, which we also detected in this study. Through a variety of methods, we propose that the new vibrational bands relate to antisymmetric Si–O–M stretching vibrations of the metal heteroatom and that the appearance of these modes in the dehydrated regime reflects the restoration of the (IR-active) tetrahedral geometry of the metal as coordinated water ligands are lost. Taken together, these findings result in a facile method of directly confirming the isomorphous substitution of Hf, Sn, Ti, and Zr heteroatoms in Beta zeolites by means of IR spectroscopy.