chapter  34
32 Pages

Multinuclear NMR Studies of Silica Surfaces

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 425

Underivatized Silica . . . . . . . . . . . . . . . . . . . . . . . . . 428 29Si NMR Patterns . . . . . . . . . . . . . . . . . . . . . . . . 428

Linewidth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 428

Dehydration of Silica Gel . . . . . . . . . . . . . . . . 429

Fumed Silica . . . . . . . . . . . . . . . . . . . . . . . . . . . 431 1H NMR Studies . . . . . . . . . . . . . . . . . . . . . . . . . 431 17O NMR Experiments . . . . . . . . . . . . . . . . . . . . 435

The Interplay between 1H and 29Si Spins in

Silica Systems . . . . . . . . . . . . . . . . . . . . . . . . . . 436

Structural Models . . . . . . . . . . . . . . . . . . . . . . . . . 439

Derivatized Silicas . . . . . . . . . . . . . . . . . . . . . . . . 440

Silylation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 440

Alkylsilane-Modified Silica . . . . . . . . . . . . . . . . . 442

Other Surface Modifications . . . . . . . . . . . . . . . . 443

Substitutionally-Modified Silicas . . . . . . . . . . . . 450

Summary and Conclusions . . . . . . . . . . . . . . . . . . . 453

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . 453

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453

The solid-state NMR revolution of the past quarter century has had an enormous impact of the state of

knowledge of silica and modified silicas. 1H ! 29Si cross polarization (CP) with magic-angle spinning (MAS) is an invaluable approach for observing local silicon environments on the silica surface. 1H NMR

approaches distinguish clustered (hydrogen-bonded) and isolated surface silanols. Correlations between 29Si and 1H NMR behaviors in silicas have led to detailed structural models of the silica surface based

on intersections of OH-terminated 100 and 111 faces of b-cristobalite. Other nuclides (e.g., 1H, 2H, 13C, and 15N), along with 29Si, provide valuable approaches for the characterization of local structure and

motion in derivatized (e.g., silylated) silicas. 17O NMR shows great promise for structure elucidation in

modified silicas in which some of the Si sites of silica are replaced by other metal centers.