Speciation of Copper in Spherical Mesoporous Silicates: From the Microscale to Angstrom

Abstract

MCM-41 type mesoporous silicate particles with spherical morphology containing copper nanospecies were synthesized by conventional hydrothermal synthesis. A narrow distribution of silicate particle sizes between 2 and 3 µm and a well-defined mesoporosity were evidenced by electronic microscopy, even for loadings of copper of about 10 wt %. Moreover, CuO nanoparticles ∼30 nm in diameter occluded in the amorphous silica, possibly filling some secondary mesoporosity, were also detected. By Fourier transform infrared spectroscopy with pyridine adsorption and UV-vis diffuse reflectance spectroscopy measurements together with reduction experiments under H 2 flow, a detailed study about the relationship between copper loading degree and the acid properties with the local isolated Cu δ+ species surroundings was performed. The incorporation of copper into the silica framework generated surface Lewis acid sites which increase with copper content. The origin of these acid sites could be attributed to the presence of isolated Cu δ+ species possibly coordinated with the framework oxygen atoms. It is noteworthy that Brønsted acid sites were not detected in our materials. By X-ray absorption near edge structure spectroscopy, we determine the coexistence of copper +1 and +2 states. The average coordination numbers calculated from extended X-ray absorption fine structure experiments are consistent with a mixture of nanosized species of copper: CuO nanoparticles, besides isolated Cu δ+ cations located in framework positions with oligonuclear clusters like [Cu δ+ • • • O δ-• • • Cu δ+ ] n in its surroundings, confined within the pores.

Coordination variation of hydrated Cu2+/Br1− ions traversing the interfacial water in mesopores

“…In the latter case the cation-anion pair is absent, [31][32][33][34] and Cu 2+ → Cu 1+ reduction begins to be measurable at ∼300…”

Selectively Obtaining ε-Caprolactam from Cyclohexanone Oxime Over Al-MCM-41 Catalysts

“…The chemisorption of pyridine followed by IR studies is usually a useful probe to detect the presence and nature of acid sites on a catalyst [27]. Figure 5 show the FT-IR spectra of the samples recorded after the adsorption of pyridine at room temperature and subsequent evacuation at 25, 50, 100 and 200°C.…”

“…The solid state nuclear magnetic resonance (NMR) spectra were conducted on a Bruker Avance II 300 spectrometer operating at 78.2 MHz for 27 Al. The sample was spun at the magic angle at a rate of 5 kHZ.…”

“…Experiments were carried out at ambient probe temperature. The 27 Al spectrum was recorded using direct polarization with pulses of 1 ls duration and a relaxation delay of 2 s. Aluminium chemical shifts are quoted with respect to 1 M aluminium nitrate solution.…”

“…The patterns XRD exhibit a main (100) reflection peak and weak two ascribed to (110) and (200) reflections, which is typical of highly ordered MCM-41 structures and consistent with the high surface area values obtained (above 1000 m 2 /g). 27 Al NMR measurements were conducted to distinguish between tetrahedral and octahedral aluminum coordination in order to establish the degree of aluminium substitution (tetrahedral sites) in the silica framework. The spectra for our samples (Al-M (20) and Al-M(60)), shown in Fig.…”