Nature of Reactive\nSites in TS‑1 from <sup>15</sup>N Solid-State NMR and Ti K‑Edge\nX‑Ray Absorption\nSpectroscopic Signatures Upon Pyridine Adsorption

Ti-containing zeotypes, notably titanosilicalite-1 (TS-1),\nare\nprominent examples of heterogeneous catalysts that have found applications\nin selective oxidation processes with hydrogen peroxide. Despite extensive\ncharacterization studies including using various probe molecules to\ninterrogate the nature and the local environment of Ti sites, their\ndetailed structure (as well as reactivity) remains elusive. Here,\nwe demonstrate that using low temperature <sup>15</sup>N magic angle\nspinning (MAS) ssNMR spectroscopy of adsorbed pyridine on TS-1 combined\nwith Ti K-edge XANES on a range of samples (dehydrated, hydrated,\ncontacted with H<sub>2</sub>O<sub>2</sub> and pyridine) provides unique\ninformation regarding the Ti sites, highlighting their reactivity\nand dynamic nature. While dehydrated TS-1 shows only Lewis acid sites,\nthe presence of H<sub>2</sub>O generates Brønsted acid sites,\nwhose amount correlates with water loading. Moreover, the methodologybased\non <sup>15</sup>N ssNMR and Ti K-edge XANESapplied to a library\nof samples with various Ti-loadings and absence of extraframework\nTiO<sub>2</sub> also enables quantification of the amount of Lewis\nacid sites and to establish a structure–activity descriptor\n(ratio of pyridine adsorbed on silanols vs titanium). Complementary\nanalysis including computational modeling reveals that the reaction\nof Ti sites with H<sub>2</sub>O generates an acidic bridging silanol\nTi-(OH)-Si, upon hydrolysis of one Ti–O–Si linkage,\nwhere Ti expands its coordination from four to pentacoordinated according\nto XAS.