Characterization of surface active sites of catalysts with high-resolution solid-state NMR

Abstract

Results obtained in studies of the surface active sites of heterogeneous catalysts by use of high-resolution solid-state NMR are presented. The 1H NMR MAS (magic angle spinning) data on chemical shifts of surface OH groups and their interaction with the supported catalytically active complexes are discussed. The relation between the 1H chemical shift values of OH groups, on the one hand, and characteristics of their acid strength, such as the proton affinity PA OH of the acid residue formed on heterolytic dissociation of the OH groups, on the other hand, is presented. Changes in the intensity of 1H NMR lines for OH groups of commonly used supports (Al 2O 3, TiO 2, SiO 2 etc.) on interaction with supported oxides suggest the selective interaction of surface OH groups with supported complexes. For the solid heteropolyacid H 3PW 12O 40 (HPA) supported on SiO 2, the binding of the HPA molecule with the SiO 2 surface is most probably multicentred and results in the formation of supported species with different proton structures. Studies of 1H NMR MAS spectra of zirconium hydrides supported on SiO 2 have revealed the formation of four types of zirconium hydride complex. Solid-state high-resolution 1H NMR spectra of H 2S adsorbed on Al 2O 3 modified with KOH, K 2CO 3 and HF and on zeolites NaX, NaY and H-ZSM indicate dissociative adsorption of H 2S which occurs on the coordinatively unsaturated aluminium sites and Na + cations in zeolite cavities. The 1H and 13C NMR spectra indicate that adsorbed CH 3OH reacts more readily with SH − species than with H 2S adsorbed in the molecular form. Also presented are results of studies on Lewis acid sites in Al 2O 3, and zeolites Y and ZSM-5 using the NMR ( 1H, 13C and 15N) of weak adsorbed bases. The formation of Lewis acid sites in HY zeolites during their hydrothermal treatment has been studied by 15N NMR of adsorbed N 2O. The strongest Lewis sites are the isolated or weakly associated extralattice Al atoms. The 1H, 13C and 15N NMR chemical shifts of CH 4, N 2 and N 2O molecules, as well as 1H and 15N relaxation rates of CH 4 and N 2O adsorbed on a series of HY and H-ZSM zeolites, SiO 2 and Al 2O 3, containing controlled amounts of iron impurities, show that unlike SiO 2, γ-Al 2O 3 and HY zeolites, the H-ZSM-5 zeolites have strong electron-accepting sites formed by Fe 3+ and probably Al 3+ cations. The possible structure of these sites and the geometry of complexes formed during N 2O and CH 4 adsorption are discussed. In the final section the results obtained with 109Ag NMR of supported silver catalysts are presented. The potential of 109Ag NMR for studying catalysts of this type is discussed.