Efficient selective oxidation of propylene by dioxygen on mesoporous-silica-nanoparticle-supported nanosized copper

Sol-Gel Synthesis and Characterization of the Cu-Mg-O System for Chemical Looping Application

A sol-gel technique was applied to prepare the two-component oxide system Cu-Mg-O, where MgO plays the role of oxide matrix, and CuO is an active chemical looping component. The prepared samples were characterized by scanning electron microscopy, low-temperature nitrogen adsorption, and X-ray diffraction analysis. The reduction behavior of the Cu-Mg-O system was examined in nine consecutive reduction/oxidation cycles. The presence of the MgO matrix was shown to affect the ability of CuO towards reduction and re-oxidation significantly. During the first reduction/oxidation cycle, the main characteristics of the oxide system (particle size, crystallization degree, etc.) undergo noticeable changes. Starting from the third cycle, the system exhibits a stable operation, providing the uptake of similar hydrogen amounts within the same temperature range. Based on the obtained results, the two-component Cu-Mg-O system can be considered as a prospective chemical looping agent.

Direct Propylene Epoxidation with Molecular Oxygen over Cobalt-Containing Zeolites

Direct propylene epoxidation with molecular oxygen is a dream reaction with 100% atom economy, but aerobic epoxidation is challenging because of the undesired over-oxidation and isomerization of epoxide products. Herein, we report the construction of uniform cobalt ions confined in faujasite zeolite, namely, Co@Y, which exhibits unprecedented catalytic performance in the aerobic epoxidation of propylene. Propylene conversion of 24.6% is achieved at propylene oxide selectivity of 57% at 773 K, giving a state-of-the-art propylene oxide production rate of 4.7 mmol/g_cat/h. The catalytic performance of Co@Y is very stable, and no activity loss can be observed for over 200 h. Spectroscopic analyses reveal the details of molecular oxygen activation on isolated cobalt ions, followed by interaction with propylene to produce epoxide, in which the Co²⁺-Co^δ+-Co²⁺ (2 < δ < 3) redox cycle is involved. The reaction pathway of propylene oxide and byproduct acrolein formation from propylene epoxidation is investigated by density functional theory calculations, and the unique catalytic performance of Co@Y is interpreted. This work presents an explicit example of constructing specific transition-metal ions within the zeolite matrix toward selective catalytic oxidations.

Synthesis of rice husk-based ion-imprinted polymer for selective capturing Cu(II) from aqueous solution and re-use of its waste material in Glaser coupling reaction

With the deepening of the concept of recycling economy and green chemistry, selective capture of Cu(II) from wastewater by biosorbent and reuse of the spent Cu(II)-loaded adsorbent are of great significance. Herein, we synthesized composite of rice husk (RH) with mesoporous silica MCM-41 (RH@MCM-41) modified by organosilane containing amino and schiff groups as functional monomer and cross-linking agent. The silica modified RH@MCM-41 was employed as supporter to fabricate copper ion-imprinted polymers as absorbents (named as RM-CIIPs) via surface ion imprinting technique. Adsorption isotherms, kinetics, selectivity and mechanism of RM-CIIPs to remove Cu(II) were investigated with respect to different adsorption condition. Furthermore, we explored the catalytic activity of spent Cu(II)-loaded adsorbent in Glaser coupling reaction. Batch adsorption studies revealed that RM-CIIP-3 prepared with functional monomer shows the best adsorption capacity (91.4 mg/g) for Cu(II), and adsorption equilibrium could be reached within 30 min. RM-CIIP-3 exhibited an excellent selectivity for capturing Cu(II) and reusability in six adsorption/desorption cycles. More importantly, the spent Cu(II)-loaded adsorbent could be used as bio-heterogeneous catalyst and afford the desired product (1,4-diphenylbutadiyne) in 99.1% yield. Our research indicates an eco-friendly systematic strategy to utilize the waste material as an adsorbent for removing heavy metals and catalyst for industry.

High-performance and long-term stability of mesoporous Cu-doped TiO2 microsphere for catalytic CO oxidation

Although the low-temperature reaction mechanism of catalytic CO oxidation reaction remains unclear, the active sites of copper play a crucial role in this mechanism. One-step aerosol-assisted self-assembly (AASA) process has been developed for the synthesis of mesoporous Cu-doped TiO₂ microspheres (CuTMS) to incorporate copper into the TiO₂ lattice. This strategy highly enhanced the dispersion of copper from 41.10 to 83.65%. Long-term stability of the as-synthesized CuTMS materials for catalytic CO oxidation reaction was monitored using real-time mass spectrum. Isolated CuO and Cu-O-Ti were formed as determined by X-ray photoelectron spectroscopy (XPS). The formation of the Cu-O-Ti bonds in the crystal lattice changes the electron densities of Ti(IV) and O, causing a subsequent change in Ti(III)/Ti(IV) and O_non/O_Total ratio. 20CuTMS contained the highest lattice distortion (0.44) in which the O_non/O_Total ratio is lowest (0.18). This finding may be attributed to the absolute formation of the Cu-O-Ti bonds in the crystal lattice. However, the decrease of Ti(III)/Ti(IV) ratio to about 0.35 of 25CuTMS was caused by the CuO cluster formation on the surface. N₂O titration-assisted H₂ temperature-programmed reduction and in-situ Fourier transform infrared spectroscopy revealed the properties of copper and effects of active sites.