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-1 molecular precursors ([CuOSi((OBu)-Bu-t)(3)](4) and [(CuOBU)-B-t](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((OBu)-Bu-t)(3)](4) (CuOSi/ SBA (x.x), where x.x refers to the Cu weight %) exhibit Cu-1 site isolation (by EXAFS and XANES). In contrast, EXAFS and XANES studies of the freshly prepared materials from the nonaqueous grafting of [(CuOBu)-Bu-t](4) ((CuOBu)-Bu-t/SBA (x.x)) suggest that the Cu-O-Cu linkages of the molecular precursor remain intact upon interacting with the support. Isolated Cu-1 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 H-1 NMR spectroscopy, themogravimetric 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-1 sites, whereas the CuO'Bu/SBA (x.x) materials only exhibit Cu as Cu-0 nanoparticles of ca. 7 Angstrom 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 (CuOBu)-Bu-t/SBA (x.x) materials exhibit more amorphous CuO character after an identical oxidative treatment.