Generation of highly dispersed Cu nanoparticles on SiO₂ support using impregnation protocol assisted with sugar alcohol for selective vapor-phase hydrogenation of citral

Abstract

The impregnation protocol, assisted with sugar alcohol such as sorbitol, effectively generated SiO₂-supported Cu (Cu/SiO₂) containing highly dispersed Cu nanoparticles, i.e. high Cu surface area. The resulting Cu/SiO₂ catalyst efficiently hydrogenated citral to citronellol, which proceeded mainly through citronellal formation. The sugar alcohols used in the impregnation generated small-size Cu precursors, and the small size was maintained even after calcination and reduction protocol. The Cu/SiO₂ catalyst prepared with sugar alcohol also exhibited high stability at a prolonged time on stream.

Graphical Abstract

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Hydrogenation of unsaturated ketones/aldehydes to their corresponding unsaturated alcohols is among the important chemical transformations, owing to various applications of unsaturated alcohols in pharmaceuticals, beverages, cosmetics, and agriculture.^1–4 Citral (1) is a biomass-derived C10 α, β-unsaturated aldehyde with an isolated C = C bond. Many valuable chemicals, namely, citronellal (2), geraniol or nerol (3), citronellol (4), isopulegol (5), menthone (6), menthol (7), and 3,7-dimethyloctanal (8), can potentially be generated from 1.⁵ This variety of products makes 1 a valuable commodity; consequently, the efficient catalytic transformation of 1 is highly desired.

Among numerous derivatives of 1, 4 has been reported as an important chemical in the perfume industry.^5,6  4 can be obtained by hydrogenating the conjugated C = C and C = O bonds in 1, while keeping the isolated C = C bond intact. Due to this reason, the choice of metal catalyst must be tailored to facilitate the hydrogenation of conjugated C = C and C = O bonds while at the same time avoiding over-hydrogenation of the isolated C = C bond.

Copper-based catalysts have been reported as efficient heterogeneous catalysts in a variety of hydrogenation in both liquid^7–10 and vapor phases.^11–14 For the aromatic unsaturated aldehyde or ketone, Cu-based catalysts have been reported to efficiently hydrogenate the C = O bond, while keeping the C = C in the aromatic ring intact.^15–17 Supported Cu catalysts, in particular, have gained considerable interest as their performance can be tuned by reducing the size of Cu species and increasing its dispersion on the support.^18–21 In addition, the type of support can also be altered to suit the need of the catalytic transformation.^22–24 The above-mentioned findings showcase the potential of supported Cu catalysts for the hydrogenation of 1 to 4.

In this work, we employed SiO₂-supported Cu (Cu/SiO₂) catalysts prepared with organic additive impregnation protocol for the hydrogenation of 1. We have demonstrated that this strategy effectively enhances the performance of Cu/SiO₂ catalysts in numerous catalytic transformations.^25–29 In addition, we have reported that biomass-derived chemicals, such as citric acid (CA), glucose (Glc), and mannitol (Man), act as efficient organic additives to boost the performance of Cu/SiO₂ catalysts.^30–32 Thus, it is important to explore the potential of other biomass-derived chemicals to enhance the performance of Cu/SiO₂ catalysts for the hydrogenation of 1.

Initially, the activity of Cu/SiO₂ catalysts prepared with various sugar alcohols, such as glycerol (Gly), erythritol (Ery), xylitol (Xyl), Man, and sorbitol (Sor), was investigated in the hydrogenation of 1. The catalysts in this work were expressed as R-A-xCu/SiO₂, where A, R, and x represent the organic additive (OA) used, the molar ratio of A to Cu, and the Cu content in wt%. For example, 1/4-Sor-20Cu/SiO₂ means the Cu/SiO₂ catalyst prepared using Sor with the molar ratio of Sor to Cu of 1/4, and it contained 20 wt% of Cu.

Table 1 summarizes the results of hydrogenation of 1 over R-A-20Cu/SiO₂ catalysts, while Supplementary Fig. S1 shows the time course of the reaction. The Cu/SiO₂ catalysts prepared with sugar alcohols (entries 2 to 6) were substantially superior to the ones without any OA (entry 1). We have previously reported that 12-crown-4-ether (12C4) and Glc also act as efficient OA in the dehydration of glycerol and hydrogenation of citronellal.^28,32 However, the performance of Cu/SiO₂ catalysts prepared with Glc and 12C4 (entries 7 and 8) was inferior to those of Cu/SiO₂ catalysts prepared with sugar alcohols.

Figure 1a and Supplementary Table S1 show the effect of reaction temperature on the catalytic activity of 1/4-Sor-20Cu/SiO₂ in the hydrogenation of 1 at a W/F of 0.041 h, while Supplementary Fig. S2 shows the time course of the reaction. The conversion of 1 increased with the increase in temperature. In contrast, the selectivity to 2 and 3 decreased with the rise in temperature. Meanwhile, the selectivity to 4, which can be generated from the consecutive hydrogenation of 2 and 3, increased as the temperature increased. These results imply that the initial step in the hydrogenation of 1 was the hydrogenation of the conjugated C = C bond to generate 2 or the hydrogenation of the C = O bond to generate 3. Considering that the selectivity to 2 was higher than the selectivity to 3, the hydrogenation of the conjugated C = C bond was preferable to the C = O bond. Even though several reports have suggested that aldehyde can be adsorbed on Cu metal using its C = O site,^33,34 the proximity of the conjugated C = C to C = O allows the hydrogenation of C = C bond to proceed efficiently.³⁵ Furthermore, the hydrogenation of the C = C bond has been known to be thermodynamically more favorable than the C = O bond.^35,36

To unravel the relative activities of the conjugated C = C and C = O bonds, several α, β-unsaturated carbonyl compounds were hydrogenated over 1/8-12C4-20Cu/SiO₂ catalyst. Supplementary Table S2 shows the that hydrogenation of α, β-unsaturated carbonyl compounds, such as 4-methyl-3-penten-2-one, 5-methyl-3-hexen-2-one, and 3-hepten-2-one, which mainly produced their respective saturated carbonyls, implying that the hydrogenation of the conjugated C = C bond was preferable to the C = O bond. A similar behavior might also occur in the hydrogenation of 1. It is then reasonable that the hydrogenation of 1 produces 2 as the major intermediate, while 3 acts as the minor intermediate.

We also explored the reactivity of the nonconjugated unsaturated carbonyls, such as 6-methyl-5-hepten-2-one and 5-hexen-2-one, to compare the reactivity of the unconjugated C = C with C = O bonds. Supplementary Table S3 (entry 1) shows that a high selectivity (96.3 mol%) to unsaturated alcohol (UOL) was observed with negligible selectivity to saturated ketone (SON) and saturated alcohol (SOL) in the hydrogenation of 6-methyl-5-hepten-2-one, where a methyl group is located next to the C = C bond. However, the hydrogenation of 5-hexen-2-one gave a different product distribution (entry 2). The selectivity to UOL of 38.8% was accompanied by a substantial selectivity to SOL (30.3%) and SON (19.4%). It can be said that a rapid consecutive hydrogenation of UOL proceeded in the absence of a methyl group next to the C = C bond, giving a substantial yield of SOL. This result implies that the hydrogenation of the C = O bond is more favorable than the unconjugated C = C bond. Catalytic results in Supplementary Tables S2 and S3 corroborate the high selectivity to 4 in the hydrogenation of 1, since the Cu/SiO₂ catalyst prepared with OA could easily hydrogenate the conjugated C = C and C = O bonds while maintaining the isolated C = C bond in 1.

Figure 1b and Supplementary Table S4 demonstrate the effect of W/F on catalytic activity at 160 °C, while Supplementary Fig. S3 shows the typical time course of the reaction. A rapid increment of conversion of 1 was observed when the W/F was prolonged from 0.014 to 0.068 h, with full conversion reached at a W/F of 0.068 h or higher. Prolonging the W/F to 0.41 h maintained the full conversion of 1. Similarly, the selectivity to 4 followed the same trend of conversion of 1. Notably, prolonging the W/F from 0.068 to 0.41 h only gave a slight increment of selectivity to 4. The rapid increase in selectivity to 4 was accompanied by the rapid decrease in selectivity to 2 and 3, which further substantiates the claim that 2 and 3 acted as the intermediates for generating 4. Previously, we have reported that the hydrogenation of 2 can potentially generate several side products, such as 5, 6, 7, and 8.³² However, the selectivity to 4 remained high at a moderate reaction temperature of 160 °C even at the highest W/F of 0.41 h. This result demonstrates that the high dispersion of Cu nanoparticles did not facilitate significant side reactions at an optimum temperature of 160 °C.

The studies of reaction parameters (Fig. 1) and different types of carbonyls (Supplementary Tables S2 and S3) also explained the differences in product distribution in Table 1. The 20Cu/SiO₂ prepared with OA gave a notable selectivity to 4, indicating that the catalysts efficiently hydrogenated 2 and 3, originating from the hydrogenation of 1 (Scheme 1). Nevertheless, None-20Cu/SiO₂ predominantly produced 2 and 3 without 4, implying that consecutive hydrogenation did not proceed. Thus, the improvement of catalytic activity by the use of OA amplified the intrinsic nature of C = C and C = O bonds in 1 while maintaining the selective nature of Cu metal, leading to different product distributions. This assumption is further corroborated by the conversion-selectivity plot shown in Supplementary Fig. S4, which shows that the increase in selectivity to 4 occurred with the decrease in selectivity to 2 and 3.

A series of analyses were conducted to unravel the properties of Cu/SiO₂ catalysts. According to the XRD patterns in Fig. 2a, the presence of OA, especially sugar alcohols, reduced the size and dispersed the Cu₂(NO₃)(OH)₃ species, which was the precursor for Cu nanoparticles on SiO₂ support. This highly dispersed and small Cu₂(NO₃)(OH)₃ species transformed into small CuO after calcination (Fig. 2b), which further turned into small Cu⁰ after the reduction protocol. A similar behavior has also been reported in the dehydration of glycerol and the hydrogenation of 2.^28,32

The effect of OA on the dispersion of Cu nanoparticles was also reflected in the reducibility of the Cu species, recorded in H₂-TPR profiles. Figure3 reveals that the use of sugar alcohols generated Cu/SiO₂ catalysts, which contained more reducible Cu species. Previous reports have suggested that this behavior indicates the presence of Cu nanoparticles with better dispersion.²⁰ Furthermore, the presence of Cu nanoparticles on SiO₂ support was also corroborated by the TEM image, shown in Fig. 4a. Cu nanoparticles with an average particle size (d_Ave) of 4.4 nm were visibly dispersed on SiO₂ support. Notably, the particle size of Cu/SiO₂ catalysts prepared with sugar alcohol was smaller than the one prepared with 12C4 and Glc: d_Ave of 1/8-12C4- and 1/4-Glc-20Cu/SiO₂ catalysts were 4.96 and 6.9 nm, respectively.^28,32

The dispersion of Cu nanoparticles, i.e. Cu surface area, SA_Cu, was also quantitatively measured using the N₂O titration technique.³⁷ Figure 5a demonstrates that the 20Cu/SiO₂ catalysts prepared with sugar alcohol had larger SA_Cu than the other Cu/SiO₂ catalysts prepared with other OA. To further investigate the effect of SA_Cu on the efficiency of the catalysts, the quantitative correlation between SA_Cu with formation rate was investigated. The formation rate of 4 in the hydrogenation of 2 was used as a reaction model. The formation rate of 4 from the hydrogenation of 1 could not be performed as this transformation is a stepwise process. Figure 5a exhibits a proportional correlation between the formation rate of 4 and SA_Cu of several Cu/SiO₂ catalysts in the hydrogenation of 2. The detailed values of Fig. 5a are summarized in Supplementary Table S5. This behavior suggests that the efficiency of Cu/SiO₂ catalysts prepared with sugar alcohol originates from the presence of highly dispersed Cu nanoparticles on SiO₂ support.

Figure 5b exhibits the long-run performance of 1/4-Sor-20Cu/SiO₂ catalyst in the hydrogenation of 1 at 160 °C and a W/F of 0.14 h. The 1/4-Sor-20Cu/SiO₂ catalyst maintained its high performance for a TOS of 10 h, showcasing its catalytic stability. Meanwhile, the 1/8-12C4-20Cu/SiO₂ catalyst, which gives high catalytic activity in glycerol dehydration, was rapidly deactivated within a TOS of 5 h. Supplementary Fig. S5 shows the XRD pattern of the spent 1/4-Sor- and 1/8-12C4-20Cu/SiO₂ catalysts. No substantial aggregation of Cu particles was observed, as the peaks associated with Cu species were hardly visible in the XRD pattern of the spent catalyst. This finding was corroborated by the TEM image (Fig. 4b), which clearly shows that the particle size of the spent 1/4-Sor-20Cu/SiO₂ catalyst was 5.9 nm. Supplementary Fig. S5 also shows that the Cu⁰ was partially oxidized as a small peak corresponding to Cu₂O was visible in the spent 1/4-Sor- and 1/8-12C4-20Cu/SiO₂ catalysts. In addition, Supplementary Fig. S6 reveals that coke was accumulated on the catalysts. The high intrinsic activity of 1/4-Sor-20Cu/SiO₂ catalyst allows it to withstand these two issues. However, partial oxidation of Cu⁰ and coke deposition were proven to severely deactivate 1/8-12C4-20Cu/SiO₂ due to lower intrinsic activity. Thus, it is apparent that the 1/4-Sor-20Cu/SiO₂ catalyst gave better catalytic stability than 1/8-12C4-20Cu/SiO₂.

Various reports have shown the hydrogenation of 1 in the liquid phase using pressurized hydrogen gas as the hydrogen source. The Ni-, Pt-, Ru-, and Pd-based catalysts mainly generated 2 as the intermediate product, which is hydrogenated to 4.^5,38,39 However, total hydrogenation to 3,7-dimethyloctan-1-ol also proceeds, exhibiting the challenge of controlling the selectivity. In addition, the selectivity to 4 has been reported to be affected by the type of solvent. In contrast to the liquid-phase system, vapor-phase hydrogenation of 1 was hardly found. Bentonite-supported Pd has been employed in vapor-phase hydrogenation of 1.⁴⁰ However, the selectivity to 4 was low, showcasing the superiority of Cu/SiO₂ in this work with reported literature.

In summary, the use of sugar alcohols, such as sorbitol, mannitol, and xylitol, as OA during impregnation protocol generated Cu/SiO₂ catalysts containing highly dispersed Cu nanoparticles, which was proven vital in the hydrogenation of 1 to 4. Considering that citral is also a biomass-derived chemical, the catalytic strategy in this work demonstrates the promising aspect of a green and sustainable process using biomass-derived chemicals to generate valuable intermediates and boost the performance of Cu/SiO₂ catalysts. This work also showcases the importance of further exploration of other biomass-derived chemicals in enhancing the performance of Cu/SiO₂ catalysts. A more detailed interaction between metal, OA, and support can also be explored. In addition, the investigation of the catalytic performance of our catalyst for catalytic transformation in a liquid phase is also worth doing. Thus, broader applications of the OA-impregnation protocol can be realized.

Supplementary data

Supplementary material is available at Chemistry Letters online.

Funding

This research received no external funding.

References

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