Unravelling the properties of supported copper oxide: can the particle size induce acidic behaviour?

Abstract

There is a renewed interest in designing solid acid catalysts particularly due to the significance of Lewis acid catalyzed processes such as Friedel-Crafts acylation and alkylation and cellulose hydrolysis for the development of sustainable chemistry. This paper reports a new focus point on the properties of supported CuO on silica, a material that up to now has been considered only as the precursor of an effective hydrogenation catalyst. Thus, it deals with a re-interpretation of some of our results with supported copper oxide aimed to unveil the root of acidic activity exhibited by this material, e.g. in alcoholysis reactions. Several techniques were used to highlight the very high dispersion of the oxide phase on the support allowing us to ascribe the acidic behavior to coordinative unsaturation of the very small CuO particles. In turn this unsaturation makes the CuO particles prone to coordinate surrounding molecules present in the reaction mixture and to exchange them according to their nucleophilicity.

Aminopropyl-silica-supported Cu nanoparticles: An efficient catalyst for continuous-flow Huisgen azide-alkyne cycloaddition (CuAAC)

“…3a) shows a narrow peak with a maximum at 260°C, which further confirms the presence of very uniform and small particles that are easy reduce under H 2 flow. The reduction temperature is in agreement with the presence of a Cu(II) oxide, as detected by HRTEM [52].…”

“…The data showed that the starting small Cu nanoparticles prepared in their reduced form are readily oxidized after exposition in air. Temperature-programmed reduction (TPR) was carried out in order to get detailed information about the nature, oxidation state, and reducibility of supported Cu catalyst [52]. The analysis consists in a reductive treatment of the catalyst under controlled temperature program and hydrogen concentration conditions.…”

Hydrogenolysis of glycerol to 1,2-propanediol on copper core-porous silica shell-nanoparticles

“…1, glycerol conversion and acetol yield increase with increasing Cu/Si atomic ratio, suggesting that the catalyst acid site number also increases with increasing Cu/Si atomic ratio. Zaccheria et al [34] prepared a CuO/SiO 2 catalyst using a chemisorptions hydrolysis technique and found that the highly dispersion catalyst exhibited acidic activity for catalyzing epoxide alcoholysis reaction. Fig.…”

An environmentally benign and low-cost approach to synthesis of thermally stable industrial catalyst Cu/SiO2 for the hydrogenation of dimethyl oxalate to ethylene glycol

“…For the sample 20Cu-AS30-AE it shows only one peak at 240 • C, corresponding to reduction of the finely disseminated CuO [10,26], while the sample containing half the metal coverage, but having the same quantity of metal (∼10 mg Cu) in the sample introduced in the test equipment, shows also one relatively sharp peak but at lower temperature (226 • C) In the case of 20Cu-AS30-AC there are two features, one peak representing a maximum of H 2 consumption at Signal (a.u.) 232 • C, corresponding to the reduction of finely disseminated CuO on support [10,26], and a shoulder at 248 • C, corresponding to the reduction of bulk CuO in the structure [10,26] while 10Cu-AS30-AC has one peak at 242 • C. The sample 20Cu-AM30-AC shows one peak centered at 249 • C with a possible small shoulder at 236 • C while 10Cu-AM30-AC has the main peak at 226 • C and a shoulder at 244 • C. XPS/XAES tests results illustrated in Fig. 3 give more information about the composition to an analysis depth of a maximum 20 nm [40].…”

“…-Sol-gel methods give active catalysts, but they tend to be slow, sensitive to other influences [24,25], and preparation methods are complex, reducing reproducibility [18]. -Cationic exchange methods [17] and chemisorption hydrolysis [26] leading to only 11-12 wt.% Cu loading. -An environmentally friendly method appears to be precipitation using urea decomposition.…”