Catalytic Hydrogenolysis of Substituted Diaryl Ethers by Using Ruthenium Nanoparticles on an Acidic Supported Ionic Liquid Phase (Ru@SILP-SO3H)

Article abstract of DOI:10.1055/s-0037-1611678

Catalytic hydrogenolysis of diaryl ethers is achieved by using ruthenium nanoparticles immobilized on an acidic supported ionic liquid phase (Ru@SILP-SO ₃ H) as a multifunctional catalyst. The catalyst components are assembled through a molecular approach ensuring synergistic action of the metal and acid functions. The resulting catalyst is highly active for the hydrogenolysis of various diaryl ethers. For symmetric substrates such as diphenyl ether, hydrogenolysis is followed by full hydrodeoxygenation producing the corresponding cycloalkanes as the main products. For unsymmetric substrates, the cleavage of the C-O bond is regioselective and occurs adjacent to the unsubstituted phenyl ring. As hydrogenation of benzene is faster than hydrodeoxygenation over the Ru@SILP-SO ₃ H catalyst, controlled mixtures of cyclohexane and substituted phenols are accessible with good selectivity. Application of Ru@SILP-SO ₃ H catalyst in continuous-flow hydrogenolysis of 2-methoxy-4-methylphenoxybenzene is demonstrated with use of commercial equipment.

Full text of DOI:10.1055/s-0037-1611678

SYNLETT
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Georg Thieme Verlag Stuttgart · New York
2019, 30, 405–412
letter
405
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S. Rengshausen et al.
Letter
Synlett
Catalytic Hydrogenolysis of Substituted Diaryl Ethers by Using
Ruthenium Nanoparticles on an Acidic Supported Ionic Liquid
Phase (Ru@SILP-SO3H)
Simon Rengshausena,b
Fabian Etscheidtb
Johannes Großkurthb
Kylie L. Luskab
Alexis Bordeta
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Walter Leitner*a,b
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a Max-Planck-Institut für Chemische Energiekonversion, Stiftstraße
34-36, 45470 Mülheim an der Ruhr, Germany
b Institut für Technische und Makromolekulare Chemie, RWTH Aachen
University, Worringerweg 2, 52074 Aachen, Germany
walter.leitner@cec.mpg.de
Published as part of the 30 Years SYNLETT – Pearl Anniversary Issue
Received: 18.11.2018
Accepted after revision: 04.02.2019
Published online: 15.02.2019
sis of fine chemicals1 and the production of fuels from bio-
mass.2 Breaking aromatic C–O bonds remains challenging,
however, because of their high stability especially in diaryl
ethers (bond dissociation energy = 82.5 kcal mol–1).3 As a
result, significant efforts have been dedicated to the devel-
opment of homogeneous1c,4 and heterogeneous5 catalysts
for efficient hydrogenolysis of C–O linkages in diaryl ethers.
Multifunctional catalytic systems are required to control
the complex reaction network comprising the primary
cleavage with parallel or secondary hydrogenation and de-
oxygenation processes (Scheme 1).
In the field of heterogeneous catalysis, third-row metal
catalysts based on Ni,5f,6 Co,7 and Fe8 were proved to be ac-
tive for the hydrogenolysis of aromatic C–O bonds, provid-
ing high selectivities towards the production of aromatics
and phenols. However, these catalysts typically possess lim-
ited activities and stabilities under the reaction conditions
required for the transformation.6b,7 Ru-based catalysts are
also known to be highly active for the cleavage of aromatic
C–O bonds, typically associated with hydrogenation of the
aromatic moieties.5e,9 Generally, the catalytically active ma-
terials for hydrogenolysis of diaryl ethers comprise a com-
bination of metal and acid functionalities.5g,9b,d,10 The bi-
functional materials can lead to further hydrodeoxygen-
ation of the primary aromatics produced to form
DOI: 10.1055/s-0037-1611678; Art ID: st-2018-b0755-l
License terms:
Abstract Catalytic hydrogenolysis of diaryl ethers is achieved by using
ruthenium nanoparticles immobilized on an acidic supported ionic liq-
uid phase (Ru@SILP-SO3H) as a multifunctional catalyst. The catalyst
components are assembled through a molecular approach ensuring
synergistic action of the metal and acid functions. The resulting cata-
lyst is highly active for the hydrogenolysis of various diaryl ethers. For
symmetric substrates such as diphenyl ether, hydrogenolysis is followed
by full hydrodeoxygenation producing the corresponding cycloalkanes
as the main products. For unsymmetric substrates, the cleavage of the
C–O bond is regioselective and occurs adjacent to the unsubstituted
phenyl ring. As hydrogenation of benzene is faster than hydrodeoxy-
genation over the Ru@SILP-SO3H catalyst, controlled mixtures of cyclo-
hexane and substituted phenols are accessible with good selectivity.
Application of Ru@SILP-SO3H catalyst in continuous-flow hydrogenoly-
sis of 2-methoxy-4-methylphenoxybenzene is demonstrated with use
of commercial equipment.
Key words ruthenium nanoparticles, acidic SILP, multifunctional ca-
talysis, diaryl ether cleavage, hydrogenolysis, deoxygenation, lignin
The catalytic reductive cleavage of C–O bonds in aryl
ethers is an important transformation in synthetic organic
chemistry, with possible applications including the synthe-
HO
HO
O
O
hydrogenation
H2, catalyst
hydrodeoxygenation
H2, catalyst
hydrogenolysis
H2, catalyst
hydrogenation
H2, catalyst
+
+
+
+
O
H2O
Scheme 1 Catalytic cleavage of diaryl ethers through hydrogenolysis and possible parallel and subsequent hydrogenation and hydrodeoxygenation,
exemplified for diphenyl ether
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completely saturated cycloalkanes.11 To date, the influence
of the substrate’s substitution pattern on the product selec-
tivity of the hydrogenolysis remains, however, fairly unex-
plored.1c
tion of 1 under hydrogen pressure can lead to the formation
of several products, such as benzene (1a), phenol (1b), cy-
clohexane (1c), cyclohexanol (1d), and uncleaved mono-
and disaturated ethers (1e and 1f) (Table 1). Gratifyingly,
the material proved very active leading to full conversion at
a substrate/Ru loading of 500:1 under a standard set of re-
action conditions [170 °C, H2 (100 bar), 2 h; Table 1,
entry 1]. Cyclohexane (1c) was obtained with a selectivity
(S) of 62% indicating a significant contribution of hydrode-
oxygenation from cyclohexanol (1d, S = 28%). The only ob-
servable side products were the saturated ethers 1e + 1f
(S = 10%). Hydrogenolysis of dicyclohexyl ether (1f) under
similar reaction conditions gave only 37% conversion, con-
firming that the main pathway for ether cleavage originates
from the aromatic level (Table S3). Extending the reaction
time for the conversion of 1 to 16 hours led to almost com-
plete transformation into the fully saturated cyclic product
1c (S = 94%; Table 1, entry 2).
Several control experiments were carried out in order to
validate the synergistic action of the metal-acid functional-
ities of the supported material (Table 1, entry 3–7). The re-
sults show that the Ru-free acidic support SILP-SO3H is not
active for the cleavage of 1 under the reaction conditions
considered, as expected (Table 1, entry 3). While the acid-
free Ru@SILP is active for the hydrogenolysis of 1, large
amounts of saturated ethers (1e + 1f) (S = 17%) are pro-
duced and cyclohexanol (1d) (S = 46%) is not efficiently de-
oxygenated to cyclohexane (1c) (S = 37%) (Table 1, entry 4).
Employing a physical mixture of the two individual materi-
als Ru@SILP and SILP-SO3H resulted in only marginal im-
provement of the cleavage and deoxygenation activities (Ta-
ble 1, entry 5). Use of a dispersion of Ru NPs in the nonsup-
ported IL-SO3H resulted in formation of 1c as the major
product, but the reaction reached only 34% conversion ac-
companied by agglomeration of the nanoparticles, in agree-
ment with previous reports12b (Table 1, entry 6 and Figures
S1, S2). The catalytic activity and selectivity of Ru@SiO2 un-
der these conditions were very low (Table 1, entry 7). Even
after 16 hours the yield of the cleavage products could not
In the present work, ruthenium nanoparticles embed-
ded in sulfonic acid-functionalized supported ionic liquid
phases (Ru@SILP-SO3H) are used as catalysts for the hydrog-
enolysis of substituted symmetric and unsymmetric diaryl
ethers. A molecular approach to the preparation of bifunc-
tional Ru@SILP-SO3H catalysts was developed previously in
our group and synergistic effects of the metal and acid
components were demonstrated.12 We now report that the
materials are highly active for the hydrogenolysis of aro-
matic C–O bonds in various substrates, allowing controlled
tandem integration with hydrodeoxygenation or hydroge-
nation of the resulting aromatic moieties. The substrate
scope includes substitution patterns as found in lignin feed-
stock, and controlled mixtures of cyclohexane and phenol
derivatives can be produced under continuous-flow operation.
The synthesis of Ru@SILP-SO3H was achieved following
a procedure previously reported by our group.12a,b In brief,
the sulfonic acid-functionalized supported ionic liquid
phase (SILP-SO3H) was prepared through the condensation
of the acidic ionic liquid [1-(4-sulfobutyl)-3-(3-triethoxysi-
lyl-propyl)imidazolium]NTf2 on dehydroxylated SiO2 with a
loading of 0.6 mmolIL gSiO2–1. The generation of ruthenium
nanoparticles on the SILP-SO3H phase involved the impreg-
nation and in situ reduction of [Ru(2-methylallyl)2(cod)]
under an atmosphere of H2 (100 bar) at 100 °C. A Ru loading
of 0.27 wt% was determined by ICP, matching well the theo-
retical value of 0.32 wt%. TEM analysis of the resulting
Ru@SILP-SO3H material evidenced the formation of well-
dispersed Ru NPs with a mean particle size of 1.7 ± 0.4 nm
(Figure 1). Characterization details are summarized in Ta-
bles S1 and S2 in the Supporting Information.
The catalytic performance of Ru@SILP-SO3H for the hy-
drogenolysis of diaryl ethers was assessed starting with di-
phenyl ether (1) as model substrate. According to the reac-
tion network shown in Scheme 1, the catalytic transforma-
Figure 1 TEM micrograph of ruthenium nanoparticles embedded on acid-functionalized supported ionic liquid phase (Ru@SILP-SO3H). The illustration
on the right hand side is intended to indicate the close proximity of the metal and acid components, but does not reflect the actual size of the NPs.
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Table 1 Hydrogenolysis of Diphenylether (1) with Use of Ru@SILP-SO3H and Control Experimentsa
OH
OH
O
O
O
catalyst
+
+
+
+
+
H
2 (100 bar), 170 °C
decalin
1
1a
1b
1c
1d
1e
1f
Entry
Catalyst
X (%)
Selectivity S (%)
1a
1b
1c
1d
1e
1f
10
1
Ru@SILP-SO3H
Ru@SILP-SO3H
SILP-SO3H
>99
>99
0
0
0
0
0
0
0
3
0
62
94
0
28
1
0
0
0
0
0
0
0
2b
3
0
4
0
0
0
4c
5
Ru@SILP
>99
>99
34
0
37
46
86
17
46
33
0
17
21
0
Ru@SILP + SILP-SO3H
Ru@IL-SO3H
0
6
11
0
7
Ru@SiO2
>99
18
65
a Ru@SILP-SO3H (37.5 mg, 0.0012 mmol Ru), 2 h, 170 °C, H2 (100 bar at rt), substrate (0.6 mmol, 500 equiv), decalin (0.5 mL).
b 16 h.
c Scale-up by factor 4. Conversion and selectivity determined by GC-FID by using hexadecane as an internal standard.
be further increased (Table S4). In addition, TEM analysis of
Ru@SiO2, Ru@SILP and Ru@SILP-SO3H before and after ca-
talysis (16 hours reaction time) evidenced severe aggrega-
tion in the case of Ru@SiO2, whereas Ru@SILP and Ru@SILP-
SO3H were stable (Figure S3–S6). This demonstrates the im-
portance of the ionic liquid layer in the SILP to stabilize the
Ru NPs under these reaction conditions. Taken together,
these results clearly demonstrate the synergistic bifunc-
tional catalysis of metal and acid sites and the stabilizing
effect of the IL moiety on the silica support.
To gain further insight into the influence of the substi-
tution pattern at aromatic moieties for the hydrogenolysis
with Ru@SILP-SO3H, reactions of symmetric (Table 2) and
unsymmetric (Table 3) diaryl ethers were performed under
slightly adjusted reaction conditions (200 °C, 100 bar H2,
3 h). Under these conditions, diphenyl ether (1) reacted
nearly quantitatively to cyclohexane (1c) (S = 97%) through
the hydrogenolysis/hydrodeoxygenation sequence, with
only minor amounts of cyclohexanol (1d) (S = 1%) and ben-
zene (1a) (S = 2%) as the side products (Table 2, entry 1).
The presence of methyl groups in para position of the phe-
nyl groups (4,4'-dimethyldiphenyl ether, 2) did not affect
the activity and selectivity of the catalyst, producing meth-
ylcyclohexane (2c) as sole product (S = 99%) (Table 2, entry
2). The introduction of methoxyl groups in ortho position
(2,2'-dimethoxydiphenyl ether, 3) also led to nearly full
conversion of the substrate (96%). However, a mixture of
products including aromatics [anisol (3a, S = 3%), guaiacol
(3b, S = 12%)] and saturated compounds (3c + 3d + 1c + 1d)
was formed (Table 2, entry 3). These results indicate that
the cleavage of the diaryl ether group was efficiently
achieved, but the resulting aromatic monomers 3a and 3b
were only partially hydrogenated to their saturated ana-
logues 3c and 3d. This indicates the hindrance of the hydro-
genation of the aromatic products by the methoxy substitu-
ents. In line with previous findings, the production of 1c as
the main product occurred through the hydrogenolysis of
the methoxy group in 3c and 3d to form the corresponding
alcohols and methane, followed by hydrodeoxygenation of
the hydroxyl group.12b
On the basis of this observation, we further investigated
the influence of the substitution pattern focusing on un-
symmetric substrates possessing a methoxy group in ortho
position. Several unsymmetric diaryl ethers were synthe-
sized (see experimental procedures for details13) and sub-
jected to hydrogenolysis by using Ru@SILP-SO3H as catalyst
under the previously established reaction conditions (Table
3). In all cases, cyclohexane was obtained as the major com-
ponent together with phenols and their secondary prod-
ucts. The hydrogenolysis of the monosubstituted 2-me-
thoxyphenoxybenzene (4) led to significant hydrodeoxy-
genation of the phenolic compounds, leading to over-
stoichiometric amounts of cyclohexane and only minor
amounts of the aromatic products (Table 3, entry 1). The
amount of aromatic products increased significantly to
reach synthetically useful values for substrates 5 and 6,
which possess an additional methyl group in meta and para
position, respectively (Table 3, entries 2 and 3). High con-
versions were achieved and the selectivities for creosol 5a
(37% selectivity, 41% isolated yield) and of iso-creosol 6a (S
= 42%) were nearly equivalent to those of cyclohexane. This
trend continued for the sterically hindered substrates 7 and
8, for which the substituted phenols 7a and 8a were ob-
tained in high selectivities in an approximate 1:1 ratio with
cyclohexane (Table 3, entries 4 and 5). Only small amounts
of other possible side products were formed with these
substrates.
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Table 2 Hydrogenolysis of Symmetric Diaryl Ethers with Ru@SILP-SO3Ha
R1
R1
R1
R1
R1
R1
R1
R1
R1
R1
OH
OH
O
O
O
Ru@SILP-SO3H
+
+
+
+
+
H2 (100 bar), 200 °C
decalin
R2
R2
R2
R2
R2
R2
R2
R2 R2
R2
1: R1 = H, R2 = H
2: R1 = H, R2 = Me
3: R1 = OMe, R2 = H
Entry Substrate
X (%)
Selectivity S (%)
O
OH
O
O
OH
1
-
-
1c
1a
1f
1
1d
1e
1b
98
94
2
0
97
1
0
0
OH
O
O
O
OH
2
2c
2a
2f
2
2b
2e
2d
0
0
99
0
0
0
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OH
O
OH
OH
O
3
3e
1d
3
3d
3c
3b
3e
3a
96
3
12
20
22
33
5
5
a Ru@SILP-SO3H (37.5 mg, 0.0012 mmol Ru), 3 h, 200 °C, H2 (100 bar at rt), substrate (0.6 mmol, 500 equiv), decalin (0.5 mL). Conversion and selectivity deter-
mined by GC-FID with use of hexadecane as the internal standard.
Table 3 Hydrogenolysis of Unsymmetric Diaryl Ethers with Use of Ru@SILP-SO3Ha
R1
R1
R1
R1
R1
R1
O
OH
+
OH
+
OH
R4
O
Ru@SILP-SO3H
+
R2
+
+
+
+
+
R2
+
H2 (100 bar)
200 °C
decalin
R2
R4
R2
R2
R2
R2
R2
R2
R4
R3
R3
R3
R3
R3
4: R1 = OMe, R2,3,4 = H
5: R1 = OMe, R2 = Me, R3,4 = H
6: R1 = OMe, R3 = Me, R2,4 = H
7: R1 = OMe, R2 = Pr, R3,4 = H
8: R1,4 = OMe, R2 = Me, R3 = H
Entry Substrate
X (%)
Selectivity S (%)
OMe
OMe
OMe
OMe
OMe
OMe
OH
OH
O
OH
O
1
1c
1a
1d
4
3d
4a
3b
3a
3d
90
93
3
21
3
62
7
1
3
1
OMe
OMe
OMe
OMe
OH
O
O
OH
OH
2
2a
1a
1c
2c
2d
5a
5b
5
5c
2
37 (45%)c
1
46
1
3
8
1
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Table 3 (continued)
Entry Substrate
X (%)
Selectivity S (%)
OMe
O
OMe
OH
OMe
OMe
OH
OH
O
3
2c
2a
1c
1a
6a
6c
6d
6b
6
82
56
85
5
42
1
43
1
2
5
1
OMe
OMe
OMe
OH
O
4
1c
7c
1a
7
7a
7b
8
40
2
48
3
OMe
OMe
OMe
OH
OH
O
5b
OMe
OMe
1c
1a
5a
8
8a
6
47 (49%)c
1
45
a Ru@SILP-SO3H (37.5 mg, 0.0012 mmol Ru), 3 h, 200 °C, H2 (100 bar at rt), substrate (0.6 mmol, 500 equiv), decalin (0.5 mL).
b Substrate (0.3 mmol, 250 equiv). Conversion and selectivity determined by GC-FID by using hexadecane as an internal standard.
c Isolated yield.
The selectivities shown in Table 3 provide valuable in-
formation on the reaction network of the sequential cata-
lytic transformations. The initial hydrogenolysis of unsym-
metric diaryl ethers can proceed in principle through two
different routes, which are illustrated in Scheme 2. The pro-
duction of cyclohexane and substituted phenols in compa-
rable quantities strongly indicates that the ether cleavage
occurs preferentially adjacent to the nonsubstituted arene
ring with high regioselectivity (Scheme 2, Route 2).
After cleavage of the diaryl ethers, the Ru@SILP-SO3H
catalyst hydrogenates quickly the benzene produced to
form cyclohexane. The substituted phenols are hydrogenat-
ed significantly slower, leaving the substituted phenols as
the second main products. The conservation of the phenols’
substituent pattern in the aromatic products is consistent
with previous literature reports, which suggest that the de-
functionalization of phenols requires, first, their hydroge-
nation to the corresponding substituted cyclohexanols.12b
OMe
OMe
[Ru]
H2
Route 1
HO
HO
[Ru]
H2
OMe
O
[Ru]
H2
Route 2
OMe
OMe
OH
OH
[Ru]
H2
Scheme 2 Reaction pathway for the hydrogenolysis of unsymmetric diarylethers with use of Ru@SILP-SO3H: example of 2-methoxy-4-methylphenoxy-
benzene
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Once the hydrogenation occurs, subsequent hydrodeoxy-
genation reduces the oxygen content until, finally, the fully
saturated cyclic products are obtained very slowly.
(Table 3, entry 2). In addition, small amounts of hydroge-
nated ethers (5c) (S = 11%), alcoholic intermediates (1d + 2d
+ 5b) (S = 8%) and benzene (1a) (S = 2%) were detected.
The conversion stayed within 86–96% for the period be-
tween two and six hours time on stream. ICP characteriza-
tion of the catalyst after six hours on stream did not show
any leaching of the Ru or the ionic liquid, and changes in the
catalyst’s textural properties were not detected by BET sur-
face area analysis. Characterization of the catalyst before
and after six hours on stream by using DRIFT-IR did not
show any significant change in the regions corresponding
to the characteristic absorption bands of the IL moiety
(2900–3200 cm–1: C–H stretches of the imidazolium ring
and N-alkyl chain; 1450–1570 cm–1: symmetric ring
stretches of the imidazolium moieties; Supporting Informa-
tion, Figures S7–9). In combination with the absence of
leaching, this suggests that the ionic liquid layer is stable
under the reaction conditions considered. In addition, the
size and dispersion of the Ru NPs did not change during the
reaction, as evidenced by TEM analysis (see Supporting In-
formation, Tables S1, S2, and Figure S10 for characterization
details). The Ru@SILP-SO3H catalyst thus appears to be
The hydrogenolysis of 2-methoxy-4-methylphenoxy-
benzene (5) by using the Ru@SILP-SO3H catalyst was inves-
tigated under continuous-flow conditions in order to evalu-
ate the product distribution as a function of time. In order
to demonstrate the compatibility of the procedure with
commercial equipment, the experiments were carried out
in an H-Cube ProTM equipped with a Phoenix Flow Reac-
torTM oven for high temperature experiments. After optimi-
zation of the reaction conditions towards high yields of cre-
osol (5a), (Supporting Information, Tables S5–S7 for de-
tails), the performance of Ru@SILP-SO3H was tested in a
six hours run (Scheme 3). A temperature of 170 °C, a pres-
sure of 40 bar H2 (90 NmL min–1), and a substrate flow of
0.3 mL min–1 were chosen corresponding to a residence
time of τ = 12 s (see Supporting Information for further in-
formation). After an induction period (1–2 hours), the reac-
tion reached a maximum conversion of 96% with good se-
lectivity of creosol (5a) (34%) and alkanes (1c + 2c) (43%),
similar to what was obtained under batch conditions
Scheme 3 Continuous flow hydrogenolysis of 2-methoxy-4-methylphenoxybenzene (5) by using Ru@SILP-SO3H in an H-CubeTM reactor. Reaction con-
ditions: Ru@SILP-SO3H (439 mg, 0.014 mmol Ru), 0.3 mL min–1 substrate (0.025 mol L–1), 90 NmL min–1 H2 (40 bar), 170 °C, decalin. Conversion and
selectivity were determined by GC-FID with use of hexadecane as an internal standard and based on the molar amount of the substrate used.
Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, 405–412

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chemically stable under continuous flow conditions. Never-
theless, a small decrease in conversion and alkane yield af-
ter four hours suggests a loss of hydrogenolysis and hydro-
genation activity with time. On the basis of previous find-
ings, the loss of catalytic activity observed may be
attributed to the accumulation of water on the catalyst
during the reaction.12b,14
thank Norbert Pfänder and Adrian Schlüter (MPI für Kohlenforschung,
Mülheim an der Ruhr) for TEM analysis.
Supporting Information
Supporting information for this article is available online at
https://doi.org/10.1055/s-0037-1611678.
S
u
p
p
orit
n
gInformati
o
n
S
u
p
p
orit
n
gInformati
o
n
The hydrogenolysis of diaryl ethers was accomplished
by using a bifunctional catalytic system consisting of ruthe-
nium nanoparticles embedded on an acidic supported ionic
liquid phase (Ru@SILP-SO3H). The molecular design of the
catalyst allowed for a close proximity between the metal
and acid sites, which was shown to be a requirement for
high catalytic activity. Various diaryl ethers with different
substitution patterns were used as the substrates to study
the catalytic activity and selectivity of Ru@SILP-SO3H.
While diphenyl ether and 4,4'-dimethyldiphenyl ether were
readily converted to the corresponding cycloalkanes in high
yields, the presence of methoxy groups as substituents hin-
dered the hydrodeoxygenation of the cleaved moieties. In
case of unsymmetric diaryl ethers, the cleavage of the C–O
bond occurred adjacent to the unsubstituted phenyl ring
regioselectively. This allows to achieve controlled formation
of mixtures of methoxy-substituted phenols and cyclohex-
anes starting from substrates with substitution patterns as
found in lignin-type feedstocks. Application of the
Ru@SILP-SO3H catalyst under continuous-flow conditions
was demonstrated for the hydrogenolysis of 2-methoxy-4-
methylphenoxybenzene. Production of a mixture of cyclo-
hexane and creosol as the main products was achieved over
six hours time on stream with conversions and yields simi-
lar to those observed under batch conditions. These results
further substantiate the potential of molecular approaches
for the design and the synthesis of multifunctional catalytic
systems based on metal nanoparticles immobilized on
functionalized supported ionic liquid phases (M@SILP-
func) to control complex reaction networks involving con-
secutive and parallel hydrogenation and hydrogenolysis
steps.
References and Notes
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Funding Information
This work was supported by the Cluster of Excellence ‘Tailor-Made Fu-
els from Biomass’, which is funded under contract EXC 236 by the Ex-
cellence Initiative by the German federal and state governments to
promote science and research at German universities.()
Acknowledgment
The authors would like to acknowledge Cornelia Broicher (ITMC,
RWTH Aachen) for BET absorption measurements, Hannelore
Eschmann (ITMC, RWTH Aachen) and Elke Wilden (ITMC, RWTH
Aachen) for GC measurements, and Wolfgang Falter (ITMC, RWTH
Aachen) for GC-MS measurements. Furthermore, we would like to
Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, 405–412

412
S. Rengshausen et al.
Letter
Synlett
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with a 1:1 mixture of a saturated aqueous solution of NH4Cl
(200 mL) and H2O (200 mL). After extraction with CH2Cl2 (3
× 200 mL) the combined organic phases were washed with a 5%
aqueous solution of KOH (300 mL) and brine (300 mL). After
drying the mixture with Na2SO4 the solvent was removed and
the crude reaction mixture was preadsorbed on silica gel. The
purification of the crude mixture was achieved with use of
silica gel. Remaining iodine compounds caused the deactivation
of the catalyst in subsequent catalytic reactions and therefore
had to be separated carefully. This afforded in some cases puri-
fication by two subsequent chromatography columns. Full char-
acterization of all substrates can be found in the Supporting
Information.
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2,6-Dimethoxy-4-methylphenoxybenzene: Prepared accord-
ing to the general procedure by using iodobenzene (5.00 g,
24.0 mmol)
and
2,6-dimethoxy-4-methylphenol
(4.8 g,
28.8 mmol). The crude product was purified by flash column
chromatography (eluent: ethyl acetate/n-pentane 1:8). 2,6-
Dimethoxy-4-methylphenoxybenzene was obtained as
a
slightly off-white solid (4.46 g, 18.3 mmol) in 76% yield. 1H NMR
(500 MHz, CDCl3):  = 7.29–7.26 (m, 2 H), 7.02–6.98 (m, 1 H),
6.93–6.90 (m, 2 H), 6.51 (s, 2 H), 3.79 (s, 6 H), 2.42 (s, 3 H) ppm.
13C NMR (125.7 MHz, CDCl3):  = 158.77 (C), 153.23 (C), 135.65
(C), 129.84 (C), 129.39 (CH), 121.52 (CH), 114.87 (CH), 106.28
(CH), 56.33 (CH3), 22.24 (CH3) ppm. HRMS (EI): m/z 244.11.
Batch catalysis: In a typical experiment, Ru@SILP (37.5 mg,
0.0012 mmol Ru), substrate (0.6 mmol, 500 equiv), and decalin
(0.5 mL) were combined in a glass insert and placed in a high-
pressure autoclave. After purging the autoclave with H2, the
reaction mixture was stirred at 170 °C in an aluminium heating
cone under 120 bar H2 (pressurized at 100 bar H2 at rt). After
the reaction, the autoclave was cooled in an ice bath, carefully
vented, and the reaction mixture filtered before GC analysis
with use of hexadecane as an internal standard. In some cases a
gap in mass balance was observed because of the loss of cyclo-
hexane in the headspace of the autoclave (scale-up decreased
this error).
(13) Experimental Procedures
Safety warning: High-pressure experiments with compressed
H2 must be carried out only with appropriate equipment and
under rigorous safety precautions.
General: If not otherwise stated, the syntheses of the ionic
liquids (ILs), the supported ionic liquid phases (SILPs), and the
nanoparticles immobilized on SILPs (Ru@SILP and Ru@SILP-
SO3H) were carried out under an inert atmosphere by using
standard Schlenk techniques or inside a glovebox. After synthe-
sis, ILs, SILPs, Ru@SILP, and Ru@SILP-SO3H were stored under an
inert atmosphere. If not otherwise stated solvents were used
after distillation without any further purification. For reactions
under an inert atmosphere, solvents were additionally dried
with molecular sieves (4 Å) and degassed by flushing solvents
with argon. For catalysis decalin (from ACROS, 99% anhydrous)
was used without purification. The precursor [Ru(2-methylal-
lyl)2(cod)] was commercially available from Umicore. All other
chemicals and solvents were purchased from commercial sup-
pliers and used without purification.
Isolated yield (5a): The catalysis was scaled up by a factor of 5.
After reaction the decalin phase was isolated and the catalyst
washed with decalin (1 × 5 mL). The combined organic phases
were extracted with aqueous KOH (3 × 10 mL, 0.3 M). The KOH
phase was washed with n-pentane (3 × 10 mL) and neutralized
with aqueous HCl, subsequently. After neutralization, the
aqueous phase was extracted with CH2Cl2 (4 × 20 mL). After
drying the CH2Cl2 phase with MgSO4 and removal of the solvent
a yellow-brownish oil (186 mg, 1.35 mmol) was obtained in 45%
yield (See Supporting Information, Figure S11 for characteriza-
tion).
Continuous flow catalysis: A 70 mm CatCart® (Vpacked reactor
=
Synthesis of catalysts: The Ru@SILP, Ru@SILP-SO3H, and Ru@IL-
SO3H were synthesized as previously reported and Ru@SiO2 was
synthesized accordingly.12a
0.5126 mL) was filled with Ru@SILP-SO3H (439 mg, 0,014 mmol
Ru) and installed into the H-Cube ProTM. Prior to catalysis the
catalyst was flushed with decalin (1 mL min–1 for 30 min). Then,
the reactor was pressurized to 40 bar H2 (90 NmL min–1) and
heated to 170 °C. When reaching stable reaction conditions, the
substrate solution (0.025 M in decalin) was introduced into the
system (0.3 mL min–1). The reaction was allowed to equilibrate
for 30 min before the first samples were collected. The reaction
mixture was analysed by GC analysis with use of hexadecane as
an internal standard.
Synthesis of substrates: The synthesis of diarylethers was
carried out as previously reported.1cA round-bottom Schlenk-
flask (250 mL) was equipped with a magnetic stir bar, copper
iodide (456 mg, 2.40 mmol, 10.0 mol%), picolinic acid (590 mg,
4.80 mmol, 20.0 mol%), and potassium phosphate (10.2 g,
48.0 mmol, 2.00 equiv). A second Schlenk flask (100 mL) was
charged with the aryl iodide (24.0 mmol, 1 equiv), the phenol
(28.8 mmol, 1.2 equiv), and anhydrous DMSO (50 mL). The solu-
tion of flask 2 was transferred into flask 1 under flowing Argon
and the reaction mixture was stirred for 20 h at 100 °C, subse-
quently. After cooling down, the reaction mixture was diluted
(14) Mellmer, M. A.; Sener, C.; Gallo, J. M. R.; Luterbacher, J. S.;
Alonso, D. M.; Dumesic, J. A. Angew. Chem. Int. Ed. 2014, 53,
11872.
Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, 405–412