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 CuI molecular precursors ([CuOSi(OtBu)3]4 and [CuOtBu]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(OtBu)3]4 (CuOSi/SBA (x.x), where x.x refers to the Cu weight %) exhibit CuI site isolation (by EXAFS and XANES). In contrast, EXAFS and XANES studies of the freshly prepared materials from the nonaqueous grafting of [CuOtBu]4 (CuOtBu/SBA (x.x)) suggest that the Cu−O−Cu linkages of the molecular precursor remain intact upon interacting with the support. Isolated CuI 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 1H 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 CuI sites, whereas the CuOtBu/SBA (x.x) materials only exhibit Cu as Cu0 nanoparticles of ca. 7 Å in diameter. After calcination under O2 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 CuOtBu/SBA (x.x) materials exhibit more amorphous CuO character after an identical oxidative treatment.