One of the methods of IR studies of the heterogeneity of Si-OH-Al groups in zeolites is the investigation of the frequency shift of the band of free OH bands restored upon the adsorption of ammonia and subsequent desorption at increasing temperatures. We extended this method by following the shift of the band of the OH group interacting by hydrogen bonding with nitrogen. The advantage of nitrogen, compared with CO, which has been commonly used as a probe molecule in studies on hydrogen bonding, is that for nitrogen the frequency shift is smaller than for CO and therefore there is no overlapping of shifted OH band with the bands of ammonium ions. For zeolites NaHY, HMFI, and HBEA, the frequency shift of IR bands of both free and hydrogen-bonded Si-OH-Al with the increase of ammonia desorption temperature evidences the heterogeneity of these hydroxyls. On the other hand, in zeolite HFAU of Si/Al = 31, Si-OH-Al were found to be homogeneous. Heterogeneity of OH groups may be explained both by the presence of Si-OH-Al of various number of Al near the bridge and of Si-OH-Al of various geometry.
Keywords: acidity; heterogeneity of OH groups; nitrogen; probe molecules; zeolites.