Effects of Precursor Composition on the Local Structure of Cu Dispersed on Mesoporous Silica:  A Detailed X-ray Absorption Spectroscopy Study

A low-temperature grafting approach using two Cu I molecular precursors ([CuOSi(O t Bu) 3 ] 4 and [CuO t Bu] 4 ) and a high-temperature exchange reaction using CuCl were utilized with a mesoporous silica support (SBA-15) to investigate the effects of catalyst preparation on the nature of copper−support interactions and site speciation. Detailed X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine-structure studies (EXAFS) studies were performed to characterize the nature of the Cu sites and the Cu−support interactions. The freshly prepared materials from the nonaqueous grafting of [CuOSi(O t Bu) 3 ] 4 (CuOSi/SBA ( x . x ), where x . x refers to the Cu weight %) exhibit Cu I site isolation (by EXAFS and XANES). In contrast, EXAFS and XANES studies of the freshly prepared materials from the nonaqueous grafting of [CuO t Bu] 4 (CuO t Bu/SBA ( x . x )) suggest that the Cu−O−Cu linkages of the molecular precursor remain intact upon interacting with the support. Isolated Cu I sites are observed as the major species in the freshly prepared material from the high-temperature exchange reaction using CuCl (CuCl/SBA (3.0)) (by XANES and EXAFS). Treatment of the materials under He at 573 K leads to loss of the organic species from the grafted materials (by 1 H NMR spectroscopy, thermogravimetric analysis, EA, and IR spectroscopy). EXAFS and XANES studies revealed that CuCl/SBA (3.0) and the CuOSi/SBA ( x . x ) materials still exhibit up to 95% isolated Cu I sites, whereas the CuO t Bu/SBA ( x . x ) materials only exhibit Cu as Cu 0 nanoparticles of ca. 7 Å in diameter. After calcination under O 2 at 573 K, residual chloride from the high-temperature preparation of CuCl/SBA (3.0) leads to formation of crystalline CuO particles, whereas the CuOSi/SBA ( x . x ) and CuO t Bu/SBA ( x . x ) materials exhibit more amorphous CuO character after an identical oxidative treatment.