NMR Signatures and Electronic Structure of Ti Sites in Titanosilicalite-1 from Solid-State ^47/49Ti NMR Spectroscopy

Although titanosilicalite-1 (TS-1) is among the mostsuccessfuloxidation catalysts used in industry, its active site structure isstill debated. Recent efforts have mostly focused on understandingthe role of defect sites and extraframework Ti. Here, we report the Ti-47/49 signature of TS-1 and molecular analogues [Ti(OTBOS)(4)] and [Ti(OTBOS)(3)((OPr)-Pr- i )] using novel MAS CryoProbe to enhance the sensitivity. Whilethe dehydrated TS-1 displays chemical shifts similar to those of molecularhomologues, confirming the tetrahedral environment of Ti consistentwith X-ray absorption spectroscopy, it is associated with a distributionof larger quadrupolar coupling constants, indicating an asymmetricenvironment. Detailed computational studies on cluster models highlightsthe high sensitivity of the NMR signatures (chemical shift and quadrupolarcoupling constant) to small local structural changes. These calculationsshow that, while it will be difficult to distinguish mono- vs dinuclearsites, the sensitivity of the Ti-47/49 NMR signature shouldenable distinguishing the Ti location among specific T site positions.