Hydrodeoxygenation of lignin-derived phenols into alkanes by using nanoparticle catalysts combined with Brensted acidic ionic liquids

Article abstract of DOI:10.1002/anie.201001531

Oxy-gone in a tandem: A catalytic system composed of metal nanoparticles (NPs) and a functionalized Brensted acidic ionic liquid (IL), both of which are immobilized in a nonfunctionalized IL, is highly efficient in upgrading lignin-derived phenolic compounds into alkanes; the hydrogenation and dehydration reactions take place in tandem. Figure Presented

Full text of DOI:10.1002/anie.201001531

Angewandte
Chemie
DOI: 10.1002/anie.201001531
Nanoparticle Catalysis
Hydrodeoxygenation of Lignin-Derived Phenols into Alkanes by Using
Nanoparticle Catalysts Combined with Brønsted Acidic Ionic
Liquids**
Ning Yan, Yuan Yuan, Ryan Dykeman, Yuan Kou,* and Paul J. Dyson*
Lignin is a promising feedstock for the production of biofuels
due to its availability (15–30 wt% of wood-based biomass)
[1]
and markedly lower oxygen content than polysaccharides.
Considerably more efforts have been paid to the conversion
[
2]
of polysaccharides; nevertheless, the development of lignin-
based biofuels is attracting increased attention. Current
strategies to produce biofuel from lignin are typically based
on two-step processes, in which lignin is first depolymerized
into a mixture of simple aromatic compounds (mostly
[
3]
[4]
[5]
phenols) either by hydrogenation, alkaline or acid
[
6]
hydrolysis, or fast pyrolysis, which is then followed by
upgrading into fuels, preferably by hydrodeoxygenation into
alkanes. Many advances have been made in the degradation
of lignin into phenols; for example, phenolic fractions can be
Scheme 1. Key steps involved in the formation of cyclohexane from
phenol, a lignin-derived model compound used as an example.
[
3c,d,6a]
readily obtained by fast pyrolysis from pure lignin
or
[6b–e]
directly from wood biomass,
the latter now being a
commercialized process. The second step, that is, the
transformation of phenolic compounds into hydrocarbon
fuels or other chemicals remains a challenge. The conven-
tional hydrodeoxygenation process based on NiMo and
CoMo sulfite catalysts is potentially problematic due to
sulfur contamination, coke accumulation, and water-induced
catalyst deactivation. Recently, aqueous-phase catalytic
systems that convert phenolic compounds into alkanes in a
series of hydrogenation and dehydration reactions have been
[
7]
dramatically to 2% when the water content in the system is
[
11]
reduced to 10%. Consequently, high reaction temperatures
exceeding 2508C are required in aqueous systems, which not
only implies demanding process engineering, but also high
energy consumption.
[
8]
The use of ionic liquids (ILs) could, in principle, overcome
these problems as well as maintain the advantages of a water-
based system (high efficiency, phase separation, etc.). Indeed,
ILs have been shown to be promising solvents in biofuel
[9]
[
10,3e]
[12]
reported (Scheme 1),
which overcome the problems
production, especially in the transformation of cellulose
[
13]
encountered with conventional catalytic systems. While the
new system is ideally suited for lignin-based phenolic
substrates, it contains several intrinsic limitations—the most
obvious is that a dehydration reaction takes place in water. In
fact, a previous case study on the dehydration reaction of
cyclohexanol to cyclohexene showed that at 1008C the
equilibrium is > 50% cyclohexanol in water and decreases
and in the production of biodiesel. Nevertheless, examples
[
14]
of lignin-based fuel production in ILs remain scarce. Here,
we describe the development of a bifunctional catalytic
system based on metal nanoparticles (NPs) and ILs, which can
effectively convert lignin-derived phenols into alkanes under
mild conditions.
As can be seen in Scheme 1, the reaction pathway includes
catalyzed hydrogenations and a dehydration step that is
catalyzed by a Brønsted acid. Following Dupontꢀs pioneering
[*] Dr. N. Yan, Dr. Y. Yuan, R. Dykeman, Prof. Dr. P. J. Dyson
[
15]
work, many notable examples of hydrogenation reactions
Institut des Sciences et Ingꢀnierie Chimiques
Ecole Polytechnique Fꢀdꢀrale de Lausanne (EPFL)
[16]
using nanocatalysts dispersed in ILs have been reported.
Indeed, we have developed a series of NP catalysts that are
1015 Lausanne (Switzerland)
E-mail: paul.dyson@epfl.ch
[17]
[18]
highly efficient for the hydrogenation of C=C, C=O, and
[
19]
Prof. Dr. Y. Kou
PKU Green Chemistry Center
Beijing National Laboratory for Molecular Sciences
College of Chemistry and Molecular Engineering
Peking University, Beijing 100871 (China)
E-mail: yuankou@pku.edu.cn
aromatic compounds in ILs under mild conditions. How-
ever, to the best of our knowledge, the dehydration reaction
shown in Scheme 1 has not been achieved in ILs, although
[20]
many other reactions catalyzed by acidic ILs are known.
Consequently, we thought it would be interesting to use
Brønsted acidic ILs to catalyze the dehydration reaction
shown in Scheme 1 and then include soluble metal NPs to
afford a system capable of converting phenol into cyclohex-
ane in a one-pot process.
[**] This research was performed in the framework of Sino–Swiss
Science & Technology Cooperation Program (No. 2010DFA42110).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201001531.
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The transformation of cyclohexanol into cyclohexene was
(top) the acidity of the ILs decreases in the following order,
1 > 2 > 5 > 6 (see the Supporting Information for further
details). The Hammett functions of ILs 1–12 are compiled in
Table S1 in the Supporting Information. The relative acidity
selected as a model reaction to evaluate the dehydration
reaction in various ILs. The application of classic mineral
acids with ILs was not successful: in [bmim][BF ], H SO
4
2
4
resulted in the formation of a tar and H PO reacted to form
of the SO H-functionalized ILs (1–7) is stronger than that of
3
4
3
an ester with cyclohexanol. Consequently, Brønsted acidic ILs
were evaluated. A series of Brønsted acidic ILs, with a
the anion-functionalized ILs (8 and 9). Moreover, the acidity
of 1–7 depends on the nature of the anion, with the triflate salt
being the most acidic. ILs 1–9 were evaluated as catalysts in
the dehydration reaction of cyclohexanol into cyclohexene in
[21]
SO H group covalently linked through an alkyl chain to the
3
cation, were prepared including those with acid-derived
anions (1–12 in Scheme 2). The Hammett acidity functions
[bmim][BF ] with a good correlation between the strength of
4
the acid and the yield of cyclohexene observed (Figure 2). The
Scheme 2. Brønsted acidic ILs used in this study.
Figure 2. Relationship between acidity H (
) and yield (*) of cyclo-
&
0
(
H ) of the cations were determined spectrophotometri-
0
hexene (for 3, 7, 8, and 9 H was greater than 3.0). Conditions for the
0
[
22]
cally,
by evaluating the protonation of an uncharged
conversion of cyclohexanol to cyclohexene: [bmim][BF ] (1 mL),
4
indicator (various anilines, termed I) in acetonitrile, in
terms of the measurable ratio of [I]/[IH ]. Representative
Brønsted acidic IL catalyst (0.4m), cyclohexanol (0.1 g, 1 mmol),
temperature 1308C, reaction time 2 h.
+
UV/Vis spectra are provided in Figure 1, in which the
unprotonated form of the dye shows an absorbance maximum
in the selected range. Essentially, the lower the absorbance
maximum in the presence of the acid, the greater the
dissociation constant of the acidic IL. Thus, from Figure 1
strongest acid 1 gave the highest yield of cyclohexene,
whereas considerably lower conversions were achieved
using ILs 4 and 7–9, which are the weakest acids. Exception-
ally, 6 resulted in the second best yield of cyclohexene,
probably due to the dehydration power and the poor
À
nucleophilicity of the HSO4 ion. It is noteworthy that
under the same conditions, but in the presence of water,
both 1 and H PO afford cyclohexene in low yield, confirming
3
4
that the IL system is superior to aqueous-based systems for
the dehydration reaction.
Based on these findings the acidity of the ILs was
increased through the synthesis of cations with shorter alkyl
chains connecting the SO H group to the electron-withdraw-
3
ing imidazolium cation (ILs 10–12, Figure 2). The three new
ILs, together with 1, were tested as catalysts for cyclohexanol
dehydration at lower temperatures in both [bmim][BF ] and
4
[
bmim][TF N], and as anticipated, ILs 10–12 are more
2
efficient than IL 1, with 10 being the most effective
Table 1). A crystal structure of the 1-methyl-3-methanesul-
(
fonate imidazolium zwitterion derived from 10 has been
obtained (Figure 3). Notably, the nitrogen–carbon bond (N1–
C5) is significantly shorter [1.454(2) ꢁ] than in the 1-butyl-3-
[
23]
butanesulfonate imidazolium system [1.473(3) ꢁ].
Other
ILs were tested as solvent for the reaction (Table S2,
entries 12–15), and [bmim][TF N] resulted in highest con-
2
versions, probably due to its hydrophobicity allowing the
water to be removed from the system once generated.
Figure 1. Absorption spectra of 4-nitroaniline (top) and 2-nitroaniline
(
bottom) for selected Brønsted acidic ILs in acetonitrile.
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Table 1: Dehydration of cyclohexanol to cyclohexene under various
conditions.
XRD pattern of the Pd NPs contained several peaks due to
their larger size. In the presence of 1 the NP solutions can be
stored for months without any signs of degradation.
[
a]
Catalyst
Hammett function
Yield of cyclohexene
in [bmim][BF4]
in [bmim][TF N]
The activities of the NP–Brønsted acid–IL systems were
evaluated in the conversion of phenol to cyclohexane under
hydrogen (40 atm) at 1308C for 4 h. With the Rh NPs and IL 1
in water the predominant product was cyclohexanol instead of
cyclohexane (Table S3, entry 1). Instead, using [bmim][BF4]
2
10
11
12
1
À1.03
À0.96
0.41
94%
74%
49%
39%
96%
95%
76%
69%
0.80
or [bmim][TF N] as solvent very high conversions with
[a] Conditions: solvent (1 mL), Brønsted acidic IL catalyst (0.4m in
2
solvent), cyclohexanol (0.1 g, 1 mmol), 1108C, 2 h.
excellent selectivity to cyclohexane could be obtained. The
Pt NPs gave the best selectivity to cyclohexane (98%) at 48%
conversion in [bmim][BF ] (Table 2, entry 3) and in [bmim]-
4
[
TF N] the Rh NP containing system resulted in a conversion
2
of 98% with a selectivity to cyclohexane of 84% (Table 2,
entry 4). Using the stronger acid IL 10 instead of 1, the
reaction proceeds at 1108C (Table S3, entries 5 and 6). The Pd
NPs were unstable forming metallic precipitates (Table S3,
entry 2). whereas the other NPs appear to be stable and can
even be recycled. Leaching of the Ru and Rh NPs was not
observed (see the Supporting Information).
The bifunctional system may be used to transform a
variety of phenolic compounds in both [bmim][BF ] and
4
[
1
bmim][TF N] (Table 2, entries 5–7 and Table S3, entries 7–
2
4). For the branched phenols only the Rh NP containing
Figure 3. Molecular structure of the 1-methyl-3-methanesulfonate imi-
dazolium zwitterion. Ellipsoids are shown at 50% probability levels.
The unlabeled atoms are hydrogen atoms. Selected bond lengths [ꢁ]
and angles [8]: N1–C5 1.454(2), S1–C5 1.800(1), N2–C4 1.470(2), S1–
O1 1.455(1); N1-C5-S1 110.95(9).
system was able to achieve a high alkane yield, suggesting that
hydrogenation of the aromatic ring is the limiting factor for
the reaction, since Rh is the most active metal in benzene
[
24]
hydrogenation. The combination of Ru and Rh NPs affords
a nearly quantitative yield of the alkane product for a
challenging substrate (Table 2, entry 8). It is also noteworthy
that substrates containing methoxy groups produced meth-
anol.
In conclusion, the transformation of lignin-derived phe-
nolic compounds to alkanes has been achieved in ILs. The
catalytic system is composed of metal NPs and a functional-
ized Brønsted acidic IL immobilized in a nonfunctionalized
IL, allowing hydrogenation and dehydration reactions to
occur in tandem. Compared to previous systems that are
either performed with metal sulfite or with mineral acid/
supported metal catalysts in water, this system allows lignin
derivatives to be upgraded in an efficient and less energy-
demanding process.
In the next step metal NP catalysts were combined with
the Brønsted acidic IL system with the aim of transforming
phenols to their corresponding alkanes in a one-pot process.
Four different metals were evaluated with Rh NPs prepared
directly in ILs and Ru, Pd, and Pt NPs immobilized in the ILs
[17]
following preparation in water using a literature method.
Typically, H was used as the reductant and poly(1-vinyl-3-
2
butylimidazolium chloride-co-N-vinyl-2-pyrrolidone), an NP
[
18,19]
stabilizer that is highly soluble in ILs,
was used as a
capping agent. TEM analysis indicated that the size of the Pd
NPs was around 5 nm and the size of the other three NPs was
smaller, ranging from 2 to 3 nm (Figure 4 and Figure S3 for
TEM and HRTEM images). XRD demonstrated the amor-
phous feature of the Rh, Pt, and Ru NPs (Figure S4). The
Experimental Section
Metal nanoparticles were prepared by H reduction using poly(1-
2
vinyl-3-butylimidazolium chloride-co-N-vinyl-2-pyrrolidone) as the
stabilizer. The hydrodeoxygenation reaction was performed in an
autoclave equipped with a heating/cooling system. Detailed synthetic
procedures and characterization of the Brønsted acidic ILs and metal
nanoparticles, as well as experimental protocol for conversion of
phenonlic compounds, are provided in the Supporting Information.
Received: March 14, 2010
Revised: May 11, 2010
Published online: June 30, 2010
Figure 4. TEM (left) and HRTEM (right) images of the Ru NPs
Keywords: biofuels · ionic liquids · lignin · nanoparticle catalysis ·
tandem catalysis
prepared from RuCl by H reduction stabilized by poly(1-vinyl-3-
3
2
.
butylimidazoliumchloride-co-N-vinyl-2-pyrrolidone) in water at 1308C.
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Communications
[
a]
Table 2: Evaluation of the NP–Brønsted acid–IL systems for the conversion of phenols to alkanes.
Entry
Metal
Solvent
Substrate
Conversion
Selectivity
MeOH
1
2
3
4
Rh NPs
Ru NPs
Pt NPs
Rh NPs
[bmim][BF4]
[bmim][BF4]
[bmim][BF4]
95%
77%
48%
98%
23%
0%
0%
10%
3%
2%
0%
67%
97%
98%
84%
–
–
–
–
[bmim][TF N]
16%
2
5
6
Rh NPs
Rh NPs
[bmim][TF N]
99%
98%
25%
14%
2%
0%
73%
86%
15%
–
2
[bmim][TF N]
2
7
8
Rh NPs
Rh/Ru NPs
[bmim][TF N]
99%
99%
16%
1%
0%
0%
84%
99%
–
–
2
[bmim][TF N]
2
[
a] Conditions: solvent (1 mL), substrate (1 mmol), metal/substrate 300:1, 1 (0.2m in solvent), H (40 atm), 1308C, 4 h.
2
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