Selective Hydrogenation of 5-Acetoxymethylfurfural over Cu-Based Catalysts in a Flow Reactor: Effect of Cu-Al Layered Double Hydroxides Synthesis Conditions on Catalytic Properties

Abstract

Cu-containing layered double hydroxides (LDHs) were synthesized by a co-precipitation method at different reaction conditions, such as aging time, pH, precipitation rate and synthesis temperature. The effect of these parameters on the structure and chemical composition of the catalysts were investigated using a set of physical methods, including thermogravimetric analysis (TGA), X-ray diffraction (XRD), H2-TPR and in situ X-ray photoelectron spectroscopy (XPS). It allowed for checking of the reducibility of the samples. 5-Acetoxymethylfurfural was catalytically hydrogenated to 5-(acetoxymethyl)-2-furanmethanol (AMFM) over Cu-containing catalysts synthesized from layered double hydroxides so as to investigate its catalytic properties in flow reaction. It was shown that synthesis pH decreasing from 10 to 8 resulted in rise of AMF conversion that coincided with the higher surface Cu/Al ratio obtained by XPS. Preferable aging time of LDH materials for obtaining the most active catalyst was 2 h, an amount of time that favored the production of the catalyst with high surface Cu/Al ratio up to 0.38. Under optimized reaction conditions, the AMFM yield was 98%. Finally, a synthesis strategy for the preparation of highly efficient Cu-based hydrogenation catalyst with optimized characteristics is suggested.

Synthesis of CuAl-LDHs by Co-Precipitation and Mechanochemical Methods and Selective Hydrogenation Catalysts Based on Them

“…Further experiments revealed that at the CO 3 2− /Al 3+ ratio of 0.5 and 0.76 and pH in the range of 9-10.5, the single-phase CuAl-LDH product is formed reproducibly regardless of the order of addition of reagents, the precipitation temperature, and the aging temperature of the mother liquor. At the same time, an increase in the aging temperature from 25 to 70 • C contributes to an increase in the crystallinity of the target CuAl-LDH, which is reflected in an increase in the intensity of diffraction peaks with a decrease in their width (Figure S1, [11]). Subsequent synthesis of CuAl-LDH in the carbonate form (containing CO3 2− anions in the interlayer space) was carried out using a typical co-precipitation procedure reported in [30,31]; solutions of aluminum and copper nitrates were added to an excess of Na2CO3 solution.…”

“…Further experiments revealed that at the CO3 2− /Al 3+ ratio of 0.5 and 0.76 and pH in the range of 9-10.5, the single-phase CuAl-LDH product is formed reproducibly regardless of the order of addition of reagents, the precipitation temperature, and the aging temperature of the mother liquor. At the same time, an increase in the aging temperature from 25 to 70 °C contributes to an increase in the crystallinity of the target CuAl-LDH, which is reflected in an increase in the intensity of diffraction peaks with a decrease in their width (Figure S1, [11]).…”

“…However, the temperature conditions of dehydroxylation of brucite-like layers and the removal of interlayer anions differ markedly depending on the synthesis method of CuAl-LDHs. In the case of the sample synthesized by the mechanochemical method, the formation of the oxide phase is almost complete at 300 • C. The sample obtained by co-precipitation contains a considerable amount of strongly bound carbonates [11], the maximum removal rate of which is observed at 550 • C. Thus, the temperature required for the formation of the oxide phase differs depending on the method of LDH synthesis used. However, for a more correct comparison of the samples, the same calcination temperature of 550 • C was used for all precursors.…”

“…Of particular interest are copper-containing catalysts of this type due to the mild conditions of metal reduction, low price, availability, and good selectivity when used for the reduction of nitroarenes, reductive amination of aldehydes or ketones, selective hydrogenation of the formyl group in furan aldehydes, and n-methylation of p-anisidine [11][12][13]. In addition, copper-containing catalysts based on polymetallic compositions CuFe-, CuZnGa-, CuMgAl-, and CuNiSn-LDH demonstrate good catalytic performance in the oxidation of 5-hydroxymethylfurfural to 5-formyl-2-furanic acid [14], production of methanol from syngas [15], single-step liquid-phase hydroxylation of phenol to catechol and hydroquinone [16], and decomposition of phenol [17].…”

Editorial: Special Issue on “Advances on Catalysts Based on Copper”

“…Bukhtiyarova et al [1] studied Cu-Al mixed oxides in the flow hydrogenation of 5-acetoxymethylfurfural into 5-(acetoxymethyl)-2-furanmethanol, focusing on the effect of synthesis conditions, such as pH, temperature, aging time and precipitation rate, on Cu-Al layered double hydroxides. Under optimized reaction conditions, the 2 h Cu-Al catalyst, which was prepared at a constant temperature of 70 • C, a pH of 9 and an aging time of 2 h, provided a 98% yield of 5-(acetoxymethyl)-2-furanmethanol.…”