Desorption-dominated synthesis of CuO/SBA-15 with tunable particle size and loading in supercritical CO₂

Here, we present a novel method to control the size of CuO nanoparticles (NPs) and Cu loading in SBA-15 via fast desorption of supercritical CO₂ (scCO₂). After calcination, the average size of the CuO NPs (6.47 ± 2.89∼2.18 ± 0.97 nm) decreased with the increase of the depressurization rate (20-14 MPa, 50 °C) from transmission electron microscopy, and the x-ray diffraction results also indicated the decrement of the average particle size (8.6∼4.3 nm by a Scherrer equation). Two reduction peaks situated at 195 °C and 220 °C were found from the temperature-programmed reductions with H₂ profiles, and the intensity of the low-temperature peak increased with increasing the rate for a profile. The hydrogenation of dimethyl oxalate (DMO) to ethylene glycol (EG) was selected to evaluate the catalytic activity of the as-prepared sample. The reaction was conducted at p = 3.0 MPa, T = 200 °C, H₂/DMO = 120, the weight hourly space velocity  = 1.2 h^-1, and the EG selectivity remained at about 90% for over 100 h. The fast desorption of scCO₂ caused mechanical perturbations and crystallization of the adsorbed salt ions on the supports, decreasing the particle size and increasing Cu loading (8∼12 wt%).