17O NMR Gives Unprecedented Insights into the Structure of Supported Catalysts and Their Interaction with the Silica Carrier

Flame silica was surface-labeled with 17O, through isotopic enrichment of both siloxanes and silanols. After heat treatment at 200 and 700 °C under vacuum, the resulting partially dehydroxylated silica materials were investigated by high-field solid-state 1H and 17O NMR. More specifically, MQ MAS and HMQC sequences were used to probe the 17O local environment. In a further step, these 17O-tagged supports were used for the preparation of supported catalysts by reaction with perhydrocarbyl transition metal derivatives (zirconium tetraalkyl, tantalum trisalkyl–alkylidene, and tungsten trisalkyl–alkylidyne complexes). Detailed 17O 1D and 2D MQ and HMQC MAS NMR studies demonstrate that signals in the Si–OH, Si–O–Si, and Si–O–metal regions are highly sensitive to local structural modifications, thanks to 17O wide chemical shift and quadrupolar constant ranges. Experimental results were supported by DFT calculations. From the selective surface labeling, unprecedented information on interactions between supported...