RELATIONSHIP BETWEEN THE SCALING OF THE ACID STRENGTH OF LEWIS SITES BY EPR AND NMR PROBES

While numerous techniques have been successful for scaling the acid strength of Brønsted sites, the situation is not satisfactory at all, for the Lewis acid sites. This is most unfortunate, since Lewis sites are present in most acid catalysts. This contribution aims to show that the simultaneous use of EPR and NMR probes suggests solutions to the problem of scaling the acid strength of Lewis sites. As shown previously, the hyperfine splitting of the EPR spectrum of the aniline radical cation or of the O-2 superoxide ion is a measurement of the strength of the electron acceptor site. Other researchers have suggested the shielding of the 31P nucleus and the shift of its resonance lines in chemisorbed trimethylphosphine (TMP) as a measurement of the Lewis acid strength. The comparison of the scaling obtained on a set of superacids, namely, and in the decreasing order of acidity, the sulfated derivatives of ZrO2, HfO2, Al2O3 and TiO2, by either the EPR or the NMR probes gives interesting information. While very strong Lewis acid centers in sulfated ZrO2 and HfO2 are revealed by 31P downfield shifted resonance lines, no such lines are observed in sulfated Al2O3 or TiO2 or zeolites (such as dealuminated mordenite) which contains strong Lewis acid centers. In the latter samples, the most downfleld shifted line corresponds to that generally assigned to TMPH+. Measurement of 31P relaxation rates suggests that TMP has a more restricted mobility on strong than on weak Lewis sites. They also show that the origin of the relaxation in TMPH+ is ambiguous: either the mobility of TMPH+ is very restricted and/or the extraproton is not as close to the phosphorus as anticipated. © 1993 Academic Press, Inc.