Selective hydrogenation of lignin-derived compounds under mild conditions

Lu Chen , Antoine P. van Muyden , Xinjiang Cui , Gabor Laurenczy * and Paul J. Dyson *
Institute of Chemical Sciences and Engineering, école Polytechnique Fedérale de Lausanne (EPFL), 1015 Lausanne, Switzerland. E-mail: pjd@epfl.ch; gabor.laurenczy@epfl.ch

Received 10th January 2020 , Accepted 19th March 2020

First published on 27th March 2020


A key challenge in the production of lignin-derived chemicals is to reduce the energy intensive processes used in their production. Here, we show that well-defined Rh nanoparticles dispersed in sub-micrometer size carbon hollow spheres, are able to hydrogenate lignin derived products under mild conditions (30 °C, 5 bar H2), in water. The optimum catalyst exhibits excellent selectivity and activity in the conversion of phenol to cyclohexanol and other related substrates including aryl ethers.


Chemicals and fuels are mainly fossil based and consequently are the major contributor of CO2 emissions and, hence, the development of renewable alternatives is urgently required. The utilization of inedible lignocellulosic biomass should help to decrease dependency on fossil resources while avoiding competition with food production.1,2 Lignin, one of the three main components of lignocellulosic biomass, is the only renewable polymer composed of phenolic units (an important component in petrochemicals). Lignin depolymerization typically affords a large number of products, although recently approaches that afford 2–4 main monomeric products and broad range of dimeric and oligomeric products have been reported.3–7 Nevertheless, the phenolic mixture usually needs to be further upgraded into fuels or platform chemicals,2 and selective hydrogenation of the aromatic rings present in the compounds is one such approach.8,9 Such an approach afford products with a large range of applications, e.g. solvents, fuels, polymer precursors, etc.10,11

Nanoparticles (NPs) based on noble metals such as Pd,12,13 Pt,14,15 Rh16,17 and Ru18,19 have been extensively explored as hydrogenation catalysts.20,21 For the hydrogenation of phenols, these catalysts tend to operate under forcing conditions (i.e. above 80 °C and/or 10 bar H2) in environmentally detrimental solvents.22,23 Such harsh conditions hamper the viability of the process and, therefore, the development of catalysts that operate under comparatively mild conditions and in an environmentally benign solvent are of considerable interest. Small, uniformly dispersed noble metal NPs are generally the most active,24–26 and by employing N-doped, carbon-based supports the size and dispersion of the NPs can be modulated.27,28 Another approach is to use well-defined sub-micrometer sized supports, such as hollow carbon spheres, to increase the dispersion of deposited NPs.29 Therefore, we decided to combine these strategies and evaluate different N-dopants in the synthesis of hollow carbon spheres (N-HCSs), and study their influence on the size distribution and dispersion of rhodium NPs. The resulting catalysts were evaluated as hydrogenation catalysts and exhibit high activity in the hydrogenation of aromatic compounds, including substrates derived from lignin.

The catalysts were prepared by a hydrothermal polymerization process followed by a pyrolysis step (Scheme 1). Sodium oleate (SO) and the block co-polymer (P123, EO20-PO70-EO20) were used as soft templates, 2,4-dihydroxybenzoic acid (DA) as a carbon source, dicyandiamide (DCDA), melamine, hexamethylentetramine (HMT) or urea as the N source and RhCl3 as the nanoparticle precursor. The resulting catalysts were named according to the N-dopant used, i.e. Rh-D (DCDA), Rh-M (melamine), Rh-H (HMT) and Rh-U (urea).


image file: d0gc00121j-s1.tif
Scheme 1 Synthesis of the catalysts, Rh-D, Rh-M, Rh-H and Rh-U.

The thickness and the size of the spheres differ depending upon the N-dopant used. HMT (Fig. 1c) affords the smallest spheres (363 nm of diameter) with the thinnest shell (50 nm), whereas DCDA (Fig. 1a) and urea (Fig. 1d) afford the largest spheres (diameter = 492 and 474 nm, respectively). The spheres obtained with melamine (Fig. 1b) are of intermediate size (diameter = 409 nm). The characteristics of the Rh NPs are also modified by the dopant (Fig. 1). The particles form small aggregates mostly on the inner layer of the sphere in Rh-D and Rh-U. In Rh-H they are more homogeneously dispersed with some of the NPs also observed on the outside of the sphere. The NPs in Rh-M form larger aggregates and are located throughout the sphere, possibly due to the low solubility of melamine that may interfere with the formation of the micelles. Previously, the solubility of dopants was shown to influence the characteristics of catalysts,30 and here the N-dopant strongly influences the size distribution of Rh NPs, varying from a mean value of 1.0 nm in Rh-H to 6.1 nm in Rh-M (Fig. 1c and b, respectively). The NPs in Rh-U and Rh-D (Fig. 1a and d, respectively) are small, but with a relatively large size distribution.


image file: d0gc00121j-f1.tif
Fig. 1 TEM images of (a) Rh-D, (b) Rh-M, (c) Rh-H and (d) Rh-U prepared at 160 °C. The insert shows the corresponding size distribution and mean diameter of the Rh NPs as well as the structure of the N-dopant. The blue circle represents the average outer diameter of the carbon support. The pink scale indicates the average thickness of the shell.

X-ray photoemission spectroscopy (XPS) of Rh-H (Fig. S2) confirms the presence of carbon, rhodium, oxygen and nitrogen. The Rh NPs are difficult to detect by XPS, which indicates that they are mostly present on the inside of the sphere. The narrow scan Rh 3d spectrum (Fig. S2a) reveals the presence of metallic Rh(0) (83%) and a small amount of Rh2O3 phase (17%) due to surface oxidation,31 and the N spectrum (Fig. S2b) shows that the N atoms are linked to both Rh and C atoms.32,33 The XRD patterns of all the catalysts (Fig. S3) contain a characteristic peak for graphitic carbon at 23°. The shape of the BET adsorption isotherm of the Rh-H catalyst corresponds to that of a mesoporous material, with a surface area of 540 m2 g−1 and an average pore diameter of 4 nm (Fig. S4), i.e. a pore size sufficient size to allow a variety of substrates to enter the shell.

The catalytic activity of Rh-D, Rh-M, Rh-H and Rh-U was assessed in the hydrogenation of phenol in H2O at 30 °C under H2 (5 bar), see Fig. 2a. The N-doped catalysts are more active than the non-doped control catalyst (Rh-C, Fig. S1e), with the activity correlating with the average size of the NPs present in the different catalysts, i.e. the Rh-H catalyst which has the smallest NPs (Rh NP mean diameter = 1.0 nm) leads to a conversion of 100% and a yield of cyclohexanol of 99%, demonstrating high selectivity for ring hydrogenation over hydrogenolysis. The least active catalyst is Rh-M with the largest NPs (Rh NP mean diameter = 6.1 nm) with a conversion of only 47% and a poorer selectivity.


image file: d0gc00121j-f2.tif
Fig. 2 (a) Product distributions for the different catalysts in H2O. Reaction conditions: Phenol (1.0 mmol), cat. (10 mg), H2O (2.0 g), 5 bar H2, 30 °C, 6 h. (b) Influence of solvent on the reaction. Reaction conditions: Phenol (1.0 mmol), Rh-H (10 mg, Rh 3.5 wt% in Rh-H), solvent (2.0 g) 5 bar H2, 30 °C, 12 h. (c) Effect of H2 pressure on the reaction. Reaction conditions: Phenol (1.0 mmol), Rh-H (10 mg), H2O (2.0 g), 30 °C, 2 h. (d) Product distribution as a function of reaction time. Reaction conditions: Phenol (1.0 mmol), Rh-H (10 mg), H2O (2.0 g), 5 bar H2, 30 °C. Colour bars refer to the colour of the products shown in the scheme.

The solvent influences the Rh-H catalyst (Fig. 2b), with the activity correlating well with the solubility of phenol in the solvent, i.e. with highest conversion in water, ethyl acetate and isopropanol (100% conversion). The selectivity to cyclohexanol is lower in ethyl acetate and isopropanol compared to water, which might be due, at least in part, to the higher solubility of the product in these two solvents, leading to further reduction. Moreover, electrophilic aromatic substitution products, e.g. [1,1′-bi(cyclohexan)]-2-one or 1,1′-bi(cyclohexane), which are often detected with reactions operating at higher temperatures,34,35 were not observed. Previously, Shirai et al. showed that using 5 wt% Rh/C commercial catalyst the hydrogenation of phenol is incomplete under 30 bar of H2 in supercritical carbon dioxide at 80 °C.36 The Rh-H catalyst is considerably more active than commercial Rh/C which can be attributed to the small size of the Rh NPs and the N-dopant.

The reaction pressure and reaction time were further optimized (Fig. 2c and d, respectively), with the Rh-H catalyst even able to slowly hydrogenate phenol at 30 °C and an atmospheric pressure of H2. The recyclability and stability of the catalyst was assessed and with product yield decreasing slightly over five batches (Fig. S5). No rhodium was detected in the product phase.

The activity of Rh-H was compared with a series of commercial catalysts (Table 1), demonstrating the superiority of the Rh-H catalyst. With the commercial Rh/C, the lower activity may be attributed to the larger mean diameter of the Rh NPs (5.7 nm, Fig. S6),27,28 which is considerable larger than the Rh NPs in the Rh-H catalyst (mean diameter 1.0 nm). Moreover, the Rh-H catalyst operates under lower H2 pressures and temperatures than reported noble metal catalysts.9,38–40 Ni/C with mean diameter 4.3 nm (Fig. S7) are inactive under the mild reaction conditions whereas RANEY® nickel hydrogenates phenol to cyclohexanol at 80 °C using 2-PrOH as a hydrogen donor.41

Table 1 Comparison of the Rh-H catalyst with other widely used carbon supported metal NP hydrogenation catalysts
Entry Catalyst Conversion %

image file: d0gc00121j-u1.tif

image file: d0gc00121j-u2.tif

(Yield %) (Yield %)
Reaction conditions: Phenol (1.0 mmol), Rh-H (10 mg) (or 5 wt% Rh/C (7 mg), or 5 wt% Ru/C (7 mg), or 5 wt% Pt/C (13 mg), or 5 wt% Pd/C (7 mg), or 5 wt% Ni/C (4 mg)), H2O (2.0 g) 5 bar H2, 30 °C, 6 h. Phenol[thin space (1/6-em)]:[thin space (1/6-em)]Rh molar ratio 294[thin space (1/6-em)]:[thin space (1/6-em)]1 in all experiments.a 1 bar H2, 48 h.b 5 wt% Ni/C was synthesized using a impregnation method described in the literature.37c Phenol (2.0 mmol, phenol[thin space (1/6-em)]:[thin space (1/6-em)]Rh molar ratio 588[thin space (1/6-em)]:[thin space (1/6-em)]1), 12 h.d Phenol (5.0 mmol, phenol[thin space (1/6-em)]:[thin space (1/6-em)]Rh molar ratio 1470[thin space (1/6-em)]:[thin space (1/6-em)]1), 30 h.e Phenol (10.0 mmol, phenol[thin space (1/6-em)]:[thin space (1/6-em)]Rh molar ratio 2940[thin space (1/6-em)]:[thin space (1/6-em)]1), 60 h. The rhodium content in the Rh-H catalyst is 3.5 wt% according to inductively coupled plasma (ICP) analysis, see ESI† for further details.
1 Rh-H 100 100 0
2a Rh-H 93 92 1
3 5 wt% Rh/C 90 69 21
4 5 wt% Ru/C 43 27 16
5 5 wt% Pt/C 66 44 22
6b 5 wt% Pd/C 0 0 0
7 5 wt% Ni/C 0 0 0
8c Rh-H 100 100 0
9d Rh-H 100 100 0
10e Rh-H 100 100 0


Note that the solubility of phenol and cyclohexanol in H2O at 30 °C is 0.084 and 0.043 g ml−1, respectively, and therefore under our conditions the system is monophasic. However, as the phenol[thin space (1/6-em)]:[thin space (1/6-em)]Rh molar ratio is increased, i.e. 588[thin space (1/6-em)]:[thin space (1/6-em)]1, 1470[thin space (1/6-em)]:[thin space (1/6-em)]1, 2940[thin space (1/6-em)]:[thin space (1/6-em)]1 (Table 1, entries 8–10), full converted to cyclohexanol is achieved, albeit at extended reaction times, and the system becomes biphasic.

The substrate scope of the Rh-H catalyst was evaluated (Table 2), with substrates containing electron withdrawing groups (i.e.Table 2, entries 2, 18 and 19) most efficiently hydrogenated, whereas substrates with two or three electron donating groups (i.e.Table 2, entries 13–15) less efficiently hydrogenated. The presence of two alcohol groups leads to cleavage of one of the C–O bonds in the substrate, which potentially reduces the complexity of the products when mixtures of phenolic compounds are transformed simultaneously (Table 2, entries 3–6). Notably, as the temperature is increased to 60 °C, hydroquinone is fully converted to cyclohexanol, i.e. the same product obtained from phenol hydrogenation (Table 2, entry 6). The hydrogenation of lignin derived compounds guaiacol and 4-propylguaiacol (Table 2, entries 16 and 17), was also efficiently achieved.

Table 2 Evaluation of the substrate scope of the Rh-H catalyst in H2O
Entry Substrate Con. % Products (Yield %)
Reaction conditions: Substrate (1.0 mmol), Rh-H (10 mg), H2O (2.0 g) 10 bar H2, 60 °C, 24 h. Conversion and yields determined by GC.a 30 °C, 5 bar H2, 12 h.
1a image file: d0gc00121j-u3.tif 100 image file: d0gc00121j-u4.tif image file: d0gc00121j-u5.tif
2a image file: d0gc00121j-u6.tif 100 image file: d0gc00121j-u7.tif image file: d0gc00121j-u8.tif
3a image file: d0gc00121j-u9.tif 100 image file: d0gc00121j-u10.tif image file: d0gc00121j-u11.tif
4a image file: d0gc00121j-u12.tif 100 image file: d0gc00121j-u13.tif image file: d0gc00121j-u14.tif
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18 image file: d0gc00121j-u53.tif 100 image file: d0gc00121j-u54.tif
19 image file: d0gc00121j-u55.tif 100 image file: d0gc00121j-u56.tif


The selectivity for hydrogenation (relative to hydrogenolysis and hydrolysis) was assessed using various aryl ether substrates (Table 3). The Rh-H catalyst mostly provides hydrogenation products without (or with only limited) cleavage of C–O bonds either via hydrogenolysis or hydrolysis. Only hydrolysis of benzyl phenyl ether (Tables 3 and S1 entry 6) and aryl-ethers (Tables 3 and S1 entries 1–4) were detected without any hydrogenolysis products (i.e. cyclic compounds without oxygen atoms). Only xanthene (Tables 3 and S1, entry 5) showed 2% of a hydrogenolysis product. Therefore, our catalyst has a remarkable selectivity for hydrogenation over hydrogenolysis with only limited hydrolysis, leading to the high catalytic activity for hydrogenation. (Note that it was shown that hydrolysis of aryl ethers takes place in two steps via an enol ether intermediate that is rapidly hydrolysed.42)

Table 3 Hydrogenations of lignin dimer model compounds using Rh-H in H2O (only the two major products are shown, details of all products are provided in Table S1 in the ESI†)
Entry Substrate Con. % Products (Yield %)
Reaction conditions: Phenol (1.0 mmol), Rh-H (20 mg), H2O (2.0 g) 15 bar H2, 80 °C, 24 h. Conversion and yields determined by GC.a 100 °C.
1 image file: d0gc00121j-u57.tif 100 image file: d0gc00121j-u58.tif image file: d0gc00121j-u59.tif
2 image file: d0gc00121j-u60.tif 100 image file: d0gc00121j-u61.tif image file: d0gc00121j-u62.tif
3 image file: d0gc00121j-u63.tif 100 image file: d0gc00121j-u64.tif image file: d0gc00121j-u65.tif
4 image file: d0gc00121j-u66.tif 95 image file: d0gc00121j-u67.tif image file: d0gc00121j-u68.tif
5 image file: d0gc00121j-u69.tif 100 image file: d0gc00121j-u70.tif image file: d0gc00121j-u71.tif
6 image file: d0gc00121j-u72.tif 83 image file: d0gc00121j-u73.tif image file: d0gc00121j-u74.tif
7a image file: d0gc00121j-u75.tif 98 image file: d0gc00121j-u76.tif image file: d0gc00121j-u77.tif
8 image file: d0gc00121j-u78.tif 84 image file: d0gc00121j-u79.tif image file: d0gc00121j-u80.tif
9a image file: d0gc00121j-u81.tif 80 image file: d0gc00121j-u82.tif image file: d0gc00121j-u83.tif
10a image file: d0gc00121j-u84.tif 79 image file: d0gc00121j-u85.tif image file: d0gc00121j-u86.tif
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Conclusions

In summary, the Rh-H catalyst, comprising finely dispersed Rh NPs immobilized within N-doped hollow carbon spheres, exhibits excellent selectivity and activity in the conversion of phenol to cyclohexanol under ambient conditions. High activity was also observed for a wide and diverse range of related substrates including aryl ethers. Importantly, the catalyst operates in water without additives and under mild reaction conditions.

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/d0gc00121j

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