Selective hydrogenation of lignin-derived compounds under mild conditions

Article abstract of DOI:10.1039/d0gc00121j

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.

Full text of DOI:10.1039/d0gc00121j

Green Chemistry
View Article Online
View Journal
COMMUNICATION
Selective hydrogenation of lignin-derived
compounds under mild conditions†
Cite this: DOI: 10.1039/d0gc00121j
Lu Chen, Antoine P. van Muyden,
Paul J. Dyson
Xinjiang Cui, Gabor Laurenczy * and
Received 10th January 2020,
Accepted 19th March 2020
*
DOI: 10.1039/d0gc00121j
rsc.li/greenchem
A key challenge in the production of lignin-derived chemicals is to the process and, therefore, the development of catalysts that
reduce the energy intensive processes used in their production. operate under comparatively mild conditions and in an envir-
Here, we show that well-defined Rh nanoparticles dispersed in onmentally benign solvent are of considerable interest. Small,
sub-micrometer size carbon hollow spheres, are able to hydrogen- uniformly dispersed noble metal NPs are generally the most
ate lignin derived products under mild conditions (30 °C, 5 bar H2), active,24–26 and by employing N-doped, carbon-based supports
in water. The optimum catalyst exhibits excellent selectivity and the size and dispersion of the NPs can be modulated.27,28
activity in the conversion of phenol to cyclohexanol and other Another approach is to use well-defined sub-micrometer sized
related substrates including aryl ethers.
supports, such as hollow carbon spheres, to increase the dis-
persion of deposited NPs.29 Therefore, we decided to combine
these strategies and evaluate different N-dopants in the syn-
thesis 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 aro-
matic compounds, including substrates derived from lignin.
The catalysts were prepared by a hydrothermal polymeriz-
ation 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, hexa-
methylentetramine (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).
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 typi-
cally 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
The thickness and the size of the spheres differ depending
upon the N-dopant used. HMT (Fig. 1c) affords the smallest
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
Institute of Chemical Sciences and Engineering, école Polytechnique Fedérale de
Lausanne (EPFL), 1015 Lausanne, Switzerland. E-mail: pjd@epfl.ch,
gabor.laurenczy@epfl.ch
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0gc00121j
Scheme 1 Synthesis of the catalysts, Rh-D, Rh-M, Rh-H and Rh-U.
This journal is © The Royal Society of Chemistry 2020
Green Chem.

View Article Online
Communication
Green Chemistry
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.
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
Fig. 1 TEM images of (a) Rh-D, (b) Rh-M, (c) Rh-H and (d) Rh-U pre-
pared 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.
spheres (363 nm of diameter) with the thinnest shell (50 nm), temperatures,34,35 were not observed. Previously, Shirai et al.
whereas DCDA (Fig. 1a) and urea (Fig. 1d) afford the largest showed that using 5 wt% Rh/C commercial catalyst the hydro-
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.
X-ray photoemission spectroscopy (XPS) of Rh-H (Fig. S2†)
confirms the presence of carbon, rhodium, oxygen and nitro-
gen. 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. 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 con-
(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 ditions: 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
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.
bar H2, 30 °C. Colour bars refer to the colour of the products shown in
the scheme.
Green Chem.
This journal is © The Royal Society of Chemistry 2020

View Article Online
Green Chemistry
Communication
Table 2 Evaluation of the substrate scope of the Rh-H catalyst in H2O
genation of phenol is incomplete under 30 bar of H2 in super-
critical carbon dioxide at 80 °C.36 The Rh-H catalyst is con-
siderably 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 opti-
mized (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 commer-
cial 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 hydro-
genates phenol to cyclohexanol at 80 °C using 2-PrOH as a
hydrogen donor.41
Entry
1a
Substrate
Con. %
100
Products (Yield %)
2a
3a
100
100
4a
5a
6
100
100
100
100
7
8
100
—
9
99
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 : Rh molar ratio is increased, i.e. 588 : 1, 1470 : 1,
2940 : 1 (Table 1, entries 8–10), full converted to cyclohexanol
is achieved, albeit at extended reaction times, and the system
becomes biphasic.
10
100
11
12
13
14
99
95
94
94
Table 1 Comparison of the Rh-H catalyst with other widely used
carbon supported metal NP hydrogenation catalysts
15
16
88
Entry
Catalyst
Conversion %
(Yield %)
(Yield %)
1
Rh-H
Rh-H
100
93
90
43
66
0
0
100
100
100
100
92
69
27
44
0
0
100
100
100
0
1
21
16
22
0
0
0
0
0
2a
3
100
5 wt% Rh/C
5 wt% Ru/C
5 wt% Pt/C
5 wt% Pd/C
5 wt% Ni/C
Rh-H
4
5
6b
7
17
18
100
100
8c
9d
10e
Rh-H
Rh-H
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 : Rh molar ratio 294 : 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.37 c Phenol (2.0 mmol, phenol : Rh molar ratio 588 : 1),
12 h. d Phenol (5.0 mmol, phenol : Rh molar ratio 1470 : 1), 30 h.
19
100
e Phenol (10.0 mmol, phenol : Rh molar ratio 2940 : 1), 60 h. The Reaction conditions: Substrate (1.0 mmol), Rh-H (10 mg), H2O (2.0 g)
rhodium content in the Rh-H catalyst is 3.5 wt% according to induc- 10 bar H2, 60 °C, 24 h. Conversion and yields determined by GC.
a 30 °C, 5 bar H2, 12 h.
tively coupled plasma (ICP) analysis, see ESI† for further details.
This journal is © The Royal Society of Chemistry 2020
Green Chem.

View Article Online
Communication
Green Chemistry
The substrate scope of the Rh-H catalyst was evaluated
The selectivity for hydrogenation (relative to hydrogenolysis
(Table 2), with substrates containing electron withdrawing and hydrolysis) was assessed using various aryl ether sub-
groups (i.e. Table 2, entries 2, 18 and 19) most efficiently strates (Table 3). The Rh-H catalyst mostly provides hydrogen-
hydrogenated, whereas substrates with two or three electron ation products without (or with only limited) cleavage of C–O
donating groups (i.e. Table 2, entries 13–15) less efficiently bonds either via hydrogenolysis or hydrolysis. Only hydrolysis
hydrogenated. The presence of two alcohol groups leads to of benzyl phenyl ether (Tables 3 and S1† entry 6) and aryl-
cleavage of one of the C–O bonds in the substrate, which ethers (Tables 3 and S1† entries 1–4) were detected without
potentially reduces the complexity of the products when mix- any hydrogenolysis products (i.e. cyclic compounds without
tures of phenolic compounds are transformed simultaneously oxygen atoms). Only xanthene (Tables 3 and S1,† entry 5)
(Table 2, entries 3–6). Notably, as the temperature is increased showed 2% of a hydrogenolysis product. Therefore, our catalyst
to 60 °C, hydroquinone is fully converted to cyclohexanol, i.e. has a remarkable selectivity for hydrogenation over hydrogeno-
the same product obtained from phenol hydrogenation lysis with only limited hydrolysis, leading to the high catalytic
(Table 2, entry 6). The hydrogenation of lignin derived com- activity for hydrogenation. (Note that it was shown that hydro-
pounds guaiacol and 4-propylguaiacol (Table 2, entries 16 and lysis of aryl ethers takes place in two steps via an enol ether
17), was also efficiently achieved.
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†)
Conclusions
In summary, the Rh-H catalyst, comprising finely dispersed Rh
NPs immobilized within N-doped hollow carbon spheres, exhi-
bits 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 cata-
lyst operates in water without additives and under mild reac-
tion conditions.
Entry
1
Substrate
Con. %
100
Products (Yield %)
2
3
4
100
100
95
Conflicts of interest
5
6
100
83
There are no conflicts to declare.
Notes and references
7a
98
1 J. Zakzeski, P. C. A. Bruijnincx, A. L. Jongerius and
B. M. Weckhuysen, Chem. Rev., 2010, 110, 3552–3599.
2 W. Schutyser, T. Renders, S. Van Den Bosch,
S.-F. Koelewijn, G. T. Beckham and B. F. Sels, Chem. Soc.
Rev., 2018, 47, 852–908.
3 N. Yan, Y. Yuan, R. Dykeman, Y. Kou and P. J. Dyson,
Angew. Chem., Int. Ed., 2010, 49, 5549–5553.
4 L. Chen, J. Xin, L. Ni, H. Dong, D. Yan, X. Lu and S. Zhang,
Green Chem., 2016, 18, 2341–2352.
8
84
80
9a
10a
11a
79
5 M. P. Pandey and C. S. Kim, Chem. Eng. Technol., 2011, 34,
29–41.
100
6 M. Kleinert and T. Barth, Chem. Eng. Technol., 2008, 31,
736–745.
7 Z. Sun, B. Fridrich, A. De Santi, S. Elangovan and K. Barta,
Chem. Rev., 2018, 118, 614–678.
12
13
100
100
8 S. C. Qi, L. Zhang, H. Einaga, S. Kudo, K. Norinaga and
J. Ichiro Hayashi, J. Mater. Chem. A, 2017, 5, 3948–3965.
9 X. Cui, A.-E. Surkus, K. Junge, C. Topf, J. Radnik,
C. Kreyenschulte and M. Beller, Nat. Commun., 2016, 7,
11326.
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.
Green Chem.
This journal is © The Royal Society of Chemistry 2020

View Article Online
Green Chemistry
Communication
10 J. Dijkmans, W. Schutyser, M. Dusselier and B. F. Sels, 27 Z. Xu, F.-S. Xiao, S. K. Purnell, O. Alexeev, S. Kawi,
Chem. Commun., 2016, 52, 6712–6715.
11 W. Schutyser, S. Van Den Bosch, J. Dijkmans, S. Turner,
S. E. Deutsch and B. C. Gates, Nature, 1994, 372,
346–348.
M. Meledina, G. Van Tendeloo, D. P. Debecker and 28 B. C. Gates, Chem. Rev., 1995, 95, 511–522.
B. F. Sels, ChemSusChem, 2015, 8, 1805–1818.
12 Q. Meng, M. Hou, H. Liu, J. Song and B. Han, Nat.
Commun., 2017, 8, 14190.
29 G.-H. Wang, J. Hilgert, F. H. Richter, F. Wang, H. Bongard,
B. Spliethoff, C. Weidenthaler and F. Schüth, Nat. Mater.,
2014, 13, 293–300.
13 H. Liu, T. Jiang, B. Han, S. Liang and Y. Zhou, Science, 30 W. R. Crowell and O. König, Mikrochem. Ver. Mikrochim.
2009, 326, 1250–1252. Acta, 1948, 33, 303–309.
14 C. Zhao, J. He, A. A. Lemonidou, X. Li and J. A. Lercher, 31 C. V. Rode, U. D. Joshi, T. Sato, O. Sato and M. Shirai,
J. Catal., 2011, 280, 8–16. J. Catal., 2004, 48, 385–388.
15 Y. Yoon, R. Rousseau, R. S. Weber, D. Mei and J. A. Lercher, 32 T. Kondo, D. Guo, T. Shikano, T. Suzuki, M. Sakurai,
J. Am. Chem. Soc., 2014, 136, 10287–10298. S. Okada and J. Nakamura, Sci. Rep., 2015, 5, 1–8.
16 C. V. Rode, U. D. Joshi, O. Sato and M. Shirai, Chem. 33 M. Xu, G. He, Z. Li, F. He, F. Gao, Y. Su, L. Zhang, Z. Yang
Commun., 2003, 1960. and Y. Zhang, Nanoscale, 2014, 6, 10307–10315.
17 H. Wang, F. Zhao, S. Fujita and M. Arai, Catal. Commun., 34 P. Giannoccaro, M. Gargano, A. Fanizzi, C. Ferragina and
2008, 9, 362–368. M. Aresta, Appl. Catal., A, 2005, 284, 77–83.
18 F. Lu, J. Liu and J. Xu, Mater. Chem. Phys., 2008, 108, 369– 35 J. He, C. Zhao and J. A. Lercher, J. Catal., 2014, 309, 362–
374. 375.
19 J. Wildschut, M. Iqbal, F. H. Mahfud, I. M. Cabrera, 36 C. V. Rode, U. D. Joshi, O. Sato and M. Shirai, Chem.
R. H. Venderbosch and H. J. Heeres, Energy Environ. Sci.,
2010, 3, 962.
Commun., 2003, 1960–1961.
37 S. Wang and G. Q. Lu, Carbon, 1998, 36, 283–292.
20 N. Yan, C. Zhao, P. J. Dyson, C. Wang, L. Liu and Y. Kou, 38 V. Vinokurov, A. Glotov, Y. Chudakov, A. Stavitskaya,
ChemSusChem, 2008, 1, 626–629.
21 Z. Li, R. S. Assary, A. C. Atesin, L. A. Curtiss and
T. J. Marks, J. Am. Chem. Soc., 2014, 136, 104–107.
E. Ivanov, P. Gushchin, A. Zolotukhina, A. Maximov,
E. Karakhanov and Y. Lvov, Ind. Eng. Chem. Res., 2017, 56,
14043–14052.
22 H. Liu, T. Jiang, B. Han, S. Liang and Y. Zhou, Science, 39 N. Singh, M.-S. Lee, S. A. Akhade, G. Cheng,
2009, 326, 1250–1252.
23 Q. Meng, M. Hou, J. Song, H. Liu and B. Han, Nat.
Commun., 2017, 8, 1–8.
D. M. Camaioni, O. Y. Gutiérrez, V.-A. Glezakou,
R. Rousseau, J. A. Lercher and C. T. Campbell, ACS Catal.,
2019, 9, 1120–1128.
24 R. Nie, H. Jiang, X. Lu, D. Zhou and Q. Xia, Catal. Sci. 40 W.-Y. Lu, S. Bhattacharjee, B.-X. Lai, A.-B. Duh, P.-C. Wang
Technol., 2016, 6, 1913–1920. and C.-S. Tan, Ind. Eng. Chem. Res., 2019, 58, 16326–16337.
25 Z. Dong, C. Dong, Y. Liu, X. Le, Z. Jin and J. Ma, Chem. Eng. 41 X. Wang and R. Rinaldi, Energy Environ. Sci., 2012, 5, 8244–
J., 2015, 270, 215–222. 8260.
26 H. Jiang, X. Yu, R. Nie, X. Lu, D. Zhou and Q. Xia, Appl. 42 M. Wang, H. Shi, D. M. Camaioni and J. A. Lercher, Angew.
Catal., A, 2016, 520, 73–81.
Chem., Int. Ed., 2017, 56, 2110–2114.
This journal is © The Royal Society of Chemistry 2020
Green Chem.