Nature and Location of Copper Nanospecies in Mesoporous Molecular Sieves

Abstract

Copper-mesoporous molecular sieves type MCM-41 have been successfully prepared by different direct incorporation methods of the metal in the initial synthesis gel. Various techniques including XRD, AAS, adsorption/desorption of N 2 , UV-vis-DR, TPR, and XPS were employed for the materials characterization. All of the materials exhibited a good structural regularity, even for loadings of copper of ∼10 wt %, besides high specific surface areas and pore volumes. An extended and detailed study about the nature of copper oxidation state as well as of the copper species distribution on the siliceous nanostructure has been made. It was found that copper is present on the mesoporous silica in the form of various species: isolated mononuclear Cu δ+ cations possibly in coordination with the lattice oxygen; linear oligonuclearprobably inserted into mesoporous channels; and bulky CuO oxide segregated of the structure. The distribution of copper species in the final solid depends on the preparation method used as well as of the copper content in the initial gel. Moreover, we infer the coexistence of the both copper oxidation states +1 and +2 (+δ) in the major species of these materials: isolated and oligonuclear copper species.

One-Pot Synthesis and Characterization of Cu-SBA-16 Mesoporous Molecular Sieves as an Excellent Catalyst for Phenol Hydroxylation

“…ions, highly dispersed CuO clusters and aggregated CuO nanospecies, respectively [36]. And, the band at about 410 nm is also related to the presence of Cu oxide nanoclusters [18]. Furthermore, compared with the spectra of CuO reference sample (Fig.…”

“…But, the H 2 -TPR profile of Cu-SBA-16 is more complex and shows three obvious reduction peaks at 269, 295 and 442°C, respectively. According to the literature [18,32,33], the low-temperature peak at 269 and 273°C can be attributed to the reduction of highly dispersed CuO nanocluster which may be inside the pores coating the inner walls of mesoporous Cu-SBA-16. Compared with the single peak of CuO reference sample (Fig.…”

“…XPS is a powerful technique at superficial level to research the nature of transition metal compounds [18,37]. The Cu2p XPS of the Cu-SBA-16 is shown in Fig.…”

“…The copper nanospecies including copper site in silica framework and highly dispersed amorphous copper oxide are expected to exhibit behavior different from that of the bulk copper species [16,17]. Although, copper nanospecies have received considerable attentions in many scopes, only a few efforts have been devoted to the study on the nature of copper species in Cu-MMS prepared under basic condition [18,19]. The coordination, dispersion and aggregation degree of copper species in mesoporous silica materials are still not well understood [18].…”

“…Although, copper nanospecies have received considerable attentions in many scopes, only a few efforts have been devoted to the study on the nature of copper species in Cu-MMS prepared under basic condition [18,19]. The coordination, dispersion and aggregation degree of copper species in mesoporous silica materials are still not well understood [18]. In particular, the nature of copper species on the Cu-MMS of type SBAn synthesized under acidic condition has not been discussed so far, even though the synthesis of Cu-SBA-15 and Cu-SBA-16 mesoporous materials by hydrothermal method have been reported [8,16].…”