ИСТИНА

Tin-containing zeolites, especially Sn-BEA, are the objects of high scientific and industrial interest due its outstanding catalytic properties in many industrially important reactions. 119Sn MAS NMR spectroscopy appears to be one of the most informative analytical tools for detailed characterization of active sites in the Sn-containing zeolites. However, a low intensity of signal and long T1 relaxation time of tin atoms makes 119Sn NMR applicable only for the samples with isotope enrichment. We have shown that the application of Carr-Purcell-Meiboom-Gill (CPMG) echo-train acquisition makes the direct detection of tin-sites in Sn-containing zeolites by 119Sn MAS NMR spectroscopy possible, without using any isotope enrichment or state-of-art NMR equipment. In the case of pure as-synthesized and hydrated calcined Sn-BEA zeolite a 5−40-fold decrease in the experimental time required to obtain a similar spectrum by Hahn-echo was achieved. The dehydration of the calcined Sn-BEA under vacuum increased the T2 relaxation times of tin incorporated in zeolite framework by two orders (from 3.7-25.5 to 800-2200 ms). The obtained gain in SNR for dehydrated sample was equal to 33.7 and made possible the registration of 119Sn MAS NMR spectra of Sn-BEA sample with 0.98 wt % of natural isotope abundance tin in several hours. The pulse sequences with improved sensitivity were used for characterization of tin state both in individual Sn-BEA zeolite and during its interaction with different probe molecules (acetonitrile, methanol, iso-pronanol, iso-butanol and water). The 119Sn CPMG MAS NMR spectra for individual Sn-BEA obtained typically contain the signals in the range of −600 to −800 ppm for as-synthesized and hydrated samples and in the range of −420 - −445 ppm for dehydrated samples. The lines observed correspond to Sn(IV) in hexa- and tetracoordinated states, respectively. The adsorption of probe molecules resulted in the observation of pentacoordinated tin species, due to the formation of 1:1 adsorption complexes with Sn sites. Water adsorption led first to formation of pentacoordinated tin species, which were further converted into hexacoordinated species at longer reaction times. The latter transformation was found to be kinetically limited and was attributed to chemical interaction of tin sites with water, such as hydrolysis of Si−O−Sn bonds. Y.G.K., A.V.Y., and I.I.I. thank the Russian Science Foundation for the financial support (Grant 14-23-00094). A.V.Y. also gratefully acknowledges Haldor Topsøe A/S for a Ph.D. fellowship.