ИСТИНА

31P MAS NMR spectroscopy of adsorbed alkyl-substituted phosphine oxides has witnessed tremendous progress during the last years and has become one of the most informative and sensitive methods of heterogeneous catalyst acidity investigation. However, quantitative evaluation of the number of sites is still a challenge. ๠is study clarifies the main origin of errors occurring during NMR experiments, introduces the appropriate standards (both internal and external), determines the relaxation parameters and the conditions for spectra acquisition and integration. As a result, the methodology for quantitative measurement of the acid sites’ content and the amount of internal and external silanol groups is established. The application of probe molecules different in size (namely, trimethylphosphine (TMPO), tri-n-butylphosphine (TBPO) and tri-n-octylphosphine (TOPO) oxides) is shown to be a good tool for distinguishing between the active sites inside the zeolite pores, mesopores and on the outer crystal surface. The methodology proposed is verified on BEA zeolites different in composition, texture and morphology.