Spectroscopic Evidence for Open and Closed Lewis Acid Sites in ZrBEA Zeolites

The Lewis acid properties of a series of isolated and well-defined Zr centers in a series of ZrBEA zeolites with different Si/Zr ratio and ZrO2 supported on silicious BEA have been investigated by means of IR spectroscopy of adsorbed probe molecules. Different types of Zr centers have been detected and assigned to (i) 4-fold coordinated Zr atoms in framework positions that weakly interact with Lewis bases (closed Lewis site), and (ii) stronger Lewis acid sites associated with 4-fold coordinated Zr centers with one Zr-OH hydroxyl group resulting from the hydrolysis of Zr-O-Si bond (open Lewis site). It has been shown that deuterated acetonitrile specifically probes open Lewis sites in ZrBEA, while the use of CO allows distinguishing between the two types Lewis sites. The relative amount of open and closed sites does not linearly depend on the Si/Zr ratio in the sample, which can point to different mechanisms of Zr incorporation from Zr-rich and Zr-poor gels.