A stable and practical nickel catalyst for the hydrogenolysis of C-O bonds

Article abstract of DOI:10.1039/c6gc01955b

The selective hydrogenolysis of C-O bonds constitutes a key step for the valorization of biomass including lignin fragments. Moreover, this defunctionalization process offers the possibility of producing interesting organic building blocks in a straightforward manner from oxygenated compounds. Herein, we demonstrate the reductive hydrogenolysis of a wide variety of ethers including diaryl, aryl-alkyl and aryl-benzyl derivatives catalyzed by a stable heterogeneous NiAlO_x catalyst in the presence of a Lewis acid (LA). The special feature of this catalyst system is the formation of substituted cyclohexanols from the corresponding aryl ether.

Full text of DOI:10.1039/c6gc01955b

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A stable and practical nickel catalyst for the
hydrogenolysis of C–O bonds†
Cite this: DOI: 10.1039/c6gc01955b
Xinjiang Cui,a Hangkong Yuan,b Kathrin Junge,a Christoph Topf,a Matthias Beller*a
and Feng Shi*b
The selective hydrogenolysis of C–O bonds constitutes a key step for the valorization of biomass including
lignin fragments. Moreover, this defunctionalization process offers the possibility of producing interesting
organic building blocks in a straightforward manner from oxygenated compounds. Herein, we demonstrate
the reductive hydrogenolysis of a wide variety of ethers including diaryl, aryl–alkyl and aryl–benzyl derivatives
catalyzed by a stable heterogeneous NiAlOx catalyst in the presence of a Lewis acid (LA). The special feature
of this catalyst system is the formation of substituted cyclohexanols from the corresponding aryl ether.
Received 18th July 2016,
Accepted 23rd November 2016
DOI: 10.1039/c6gc01955b
www.rsc.org/greenchem
Considering the general applicability of heterogeneous cata-
lysts, the development of robust solid catalysts for the cleavage
Introduction
Reductive C–O bond cleavage is an underrated process for the of C–O bonds represents a worthwhile scientific endeavor.
utilization of oxygenated compounds, especially for the upgrad- Indeed, noble metal catalysts including Ru,11–14 Pd,15–17
ing of biomass, e.g. lignin-derived compounds.1–3 In this regard, Pt,18–21 Rh22 and Cu23,24 immobilized on various supports were
the selective hydrogenolysis of aromatic ethers has attracted used in the catalytic splitting of the C–O bond thereby forming
major interest in biorefinery concepts for the sustainable pro- mixtures of aromatic and aliphatic compounds upon hydro-
duction of fuels and bulk intermediates for the chemical indus- genolysis and hydrolysis.25 However, the low selectivity caused
try. For example, cyclohexanol might be obtained by specific by high reaction temperatures (>200 °C) and high pressures
hydrogenolysis of different aromatic ethers. Currently this com- (>4 MPa) as well as the necessity of adding corrosive Brønsted
pound, which is a key intermediate in the multi-million ton- acids prevented a widespread application. Interestingly, a series
scale synthesis of industrial products, such as nylon 6 and nylon of Ni-based catalysts, such as RANEY® Ni 26–29 and supported
66,4 is mainly produced by oxidation of cyclohexane.
Ni(0) particles,30–37 have been investigated by other groups for
In a seminal report, Hartwig and Sergeev reported a homo- the hydrogenolysis of C–O bonds. In spite of their impressive
geneous carbene-supported Ni(COD)2 complex that allows for developments, there is a need for more general catalysts for
the selective hydrogenation and cleavage of aromatic C–O such reactions. In fact, the hydrogenolysis of oxygenated com-
bonds affording arenes and alcohols in the presence of pounds in the presence of different functional groups to give
NatOBu.5 In addition, a series of well-defined molecular cata- aliphatic alcohols has not been addressed.
lysts based on Ru,6 Ir,7,8 V9 and Fe10 complexes have been
Complementary to all the discussed work, herein we
developed and were successfully applied in the selective clea- describe the preparation, characterization and catalytic appli-
vage of C–O bonds. However, most of these homogeneous cata- cation of heterogeneous NiAlOx catalysts.38 The activity of the
lyst systems suffer from costly preparation, product separation, material is readily tuned by varying the Ni/Al ratio in the precur-
reusability and handling as well as in some cases they require sor solution. The resulting optimal catalyst allows for the selec-
a reducing agent, e.g. borohydride.
tive hydrogenolysis of alkyl aryl and diaryl ethers including
lignin derived substrates to form the corresponding cleavage
products. Meanwhile, hydrogenolysis of C–O bonds in alkyl aryl
ethers was achieved using a heterogeneous Ni catalyst.
aLeibniz-Institut für Katalyse e.V., Albert-Einstein-Str. 29a, 18059 Rostock, Germany.
E-mail: matthias.beller@catalysis.de
bState Key Laboratory for Oxo Synthesis and Selective Oxidation, Centre for Green
Chemistry and Catalysis, Lanzhou Institute of Chemical Physics, Chinese Academy of
Sciences, No. 18, Tianshui Middle Road, Lanzhou, 730000, China.
E-mail: fshi@licp.cas.cn
Results and discussion
Material synthesis
In a typical procedure for the preparation of NiAlOx catalysts,
Ni(NO3)2·6H2O and Al(NO3)3·9H2O were dissolved in deionized
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c6gc01955b
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water. Then, Na2CO3 solution was added dropwise and the
mixture was further stirred for 4 h at room temperature. The
reaction mixture was centrifuged and washed with water to
remove the base until the pH value of the aqueous solution
was ∼7. Subsequently, the solid sample was dried at 100 °C for
4 h, calcined in static air at 450 °C for 5 h and finally reduced
under a hydrogen flow at 450 °C for 2 h. Catalyst samples with
different Ni/Al ratios were prepared according to the same
procedure.
Characterization
In order to understand the structures of these materials,
detailed spectroscopic investigations were performed. Initially,
eight different samples were characterized by XRD and the
corresponding diffraction patterns are given in Fig. 1. It
suggests that the Ni1.0Al0O0 catalyst contains finely crystallized
metallic Ni without any oxygen present. However, in the pres-
ence of small quantities of AlOx, the formation of
Ni0.92Al0.08Ox is observed. Here, diffraction peaks of Ni(111),
Ni(200) and Ni(220) are significantly weakened and signals for
NiO(222), NiO(400) and NiO(440) are detected. The NiO diffrac-
tion signals gradually intensify when the Ni/Al ratio was varied
from 10 : 1 to 6 : 1 or 4 : 1, whereas the diffraction peaks for
metallic Ni almost disappeared. Noteworthily, on changing the
Ni/Al ratio to 2 : 1 the main peak of NiO, i.e. NiO(400) shifted
to a higher 2θ diffraction angle, which might indicate the for-
mation of a new crystalline phase (AlNi3(111)).
To obtain more information about the micro-structure of
the potential catalysts with different Ni/Al ratios, STEM-EDS
elemental analysis by line scan and STEM-EDS elemental
mapping of three typical catalyst samples, i.e. Ni0.92Al0.08Ox,
Ni0.87Al0.13Ox and Ni0.67Al0.33Ox (the ratios of Ni to Al were
determined by ICP-AES) were performed. The results are
depicted in Fig. 2 and S2,† respectively. Obviously, the Ni and
Al atoms are well mixed and uniformly distributed. According
to ICP-AES analysis, the Ni content in these three samples is
found to be 94.0%, 85.1% and 62.3% whereas the Al content
Fig. 2 STEM-EDS elemental analysis by line scan across the catalyst
samples Ni0.91Al0.09Ox, Ni0.86Al0.14Ox and Ni0.67Al0.33Ox (from the
topside). Left: HAADF-STEM images of the catalyst samples; right:
elemental line scan indicating the presence and counts of Ni, Al and O
elements.
amounted to 3.9%, 5.7% and 14.5%, respectively. As Al2O3 is
difficult to be reduced under the reaction conditions, we con-
clude that the oxygen is bound mainly to Al. Based on this
hypothesis, the NiO/Ni ratios are found to be about 0%, 17.8%
and 60.0% in these three samples. Finally, nitrogen adsorp-
tion–desorption analysis suggested that the BET surface areas
of Ni1.0Al0Ox, Ni0.92Al0.08Ox, Ni0.87Al0.13Ox and Ni0.67Al0.33Ox
were 0.50, 49.6, 93.9 and 128.9 m2 g−1, respectively, indicating
that the addition of AlOx results in an enhancement of the
BET surface area.
Catalytic experiments
The selective hydrogenolysis of alkylphenols to the corres-
ponding cyclohexanols can be an important step in the valori-
zation of lignin-derived compounds. Due to more facile ana-
lysis, we used initially butyl phenyl ether 1a as a model system.
However, it should be noted that the C–O cleavage in this
system is more demanding compared to benzylic ethers and
diarylethers. A key challenge for the valorization of alkyl-
phenols for the production of bulk chemicals as well as fuels
is the development of stable and robust non-noble metal
based heterogeneous catalysts. Due to metal price and avail-
ability, the use of non-noble metal catalysts is highly desired.
Fig. 1 XRD diffraction patterns of NiAlOx catalysts. The catalyst samples
are Ni1.0Al0Ox, Ni0.92Al0.08Ox, Ni0.89Al0.11Ox, Ni0.87Al0.13Ox, Ni0.80Al0.20Ox
and Ni0.67Al0.33Ox from the topside.
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Recently, the groups of Lercher33 and Esposito36 described for cyclohexanol 1c was only 25% (Table 1, entry 3). The catalytic
the first time the use of Ni/SiO2 and Ni/TiN for reductive C–O performance is significantly improved upon addition of AlOx
cleavage of aryl ethers. While both catalysts are efficient for and the highest activity is obtained with a Ni/Al molar ratio of
simpler model compounds, often the C–O dissociation is not approximately 6 : 1 (Table 1, entries 4 and 5). Interestingly, less
selective and mixtures of aliphatic and aromatic oxygen- active catalysts were obtained when the AlOx content was
containing products are formed, which renders subsequent further increased (Table 1, entries 6–8). Based on the charac-
purification processes difficult. Additionally, the preparation terization vide supra, we suppose that the incorporation of the
of the active Ni/TiN catalyst is demanding and its application NiO species facilitates the hydrogenolysis of the C–O bond. In
is limited. In this context, we prepared NiAlOx catalysts by a addition, we assume that a Lewis acid might interact with the
simple co-precipitation method. More importantly, their surface hydroxyl group41 and C–O bond,17,39 which makes the
activity can be tuned by controlling the ratio of Ni : Al. In pre- substrate close to the active site and leads to improved catalytic
liminary experiments, Ni0.87Al0.13Ox was used in the test reac- activity. Performing the reaction in different solvents showed
tion and although full conversion of 1a is observed, unfortu- higher activity in isopropanol compared with heptane and
nately no hydrogenolysis products are formed (Table 1, entry 1). H2O (Table 1, entries 9 and 10). However, no transfer hydro-
In order to promote this latter transformation, various genation was observed using isopropanol under hydrogen-free
Lewis acids were added to the reaction mixture. In agreement conditions. Then, the effect of different Lewis acids was
with other C–O cleavage reactions,17,39,40 increased activity is explored. Here, the highest activity for the hydrogenolysis of 1a
observed. For example, in the presence of La(OTf)3 significant was achieved in the presence of La(OTf)3. By contrast, almost
amounts of C–O cleaved products such as cyclohexanol (59% no desired alcohol but an aliphatic ether is observed in the
selectivity) are observed. In addition, minor amounts of cyclo- absence of the additive, which demonstrates the pronounced
hexane 1b and butoxycyclohexane 1d are detected (6% and influence of Lewis acids in this catalytic transformation.
35% selectivity, respectively) (Table 1, entry 2). Next, a series of Similarly, other triflates with different cations gave rise to
nickel-based catalysts with the Ni–Al ratio ranging from 2 : 1 to lower activities compared to lanthanum triflate (Table 1,
10 : 1 was explored. Among all the catalysts tested, Ni1.0Al0O0 entries 11–17). Therefore, we assume that the presence of Al
displayed only low activity and the selectivity to the desired promotes the adsorption of the aromatic ether onto the cata-
lyst surface, which leads to hydrogenation of the aromatic
ring. However, the acidity of the corresponding aluminum site
is too weak to promote the cleavage of the ether bond.
Table 1 Ni-catalyzed hydrogenation of butyl phenyl ether: optimiz-
Therefore, La(OTf)3 was added as a co-catalyst. The variation of
the Ni/Al ratio doesn’t influence the ratio of the individual pro-
ducts but affects the activity of the NiAlOx catalyst for aromatic
ring hydrogenation. The catalyst and Lewis acid can be
recycled by simply evaporating the products and solvent. To
make the catalytic system more sustainable, solid acids such
as phosphotungstic acid and silicotungstic acid were checked.
However, there is no improved result for the catalytic
hydrogenolysis of phenyl ether observed.
In addition, the model reaction was performed at different
temperatures. The highest selectivity was observed at 130 °C
(Table 1, entries 18–20), which is similar to the reactivity of
molecular-defined nickel catalysts. Comparing this material
with the classic RANEY® nickel as a benchmark catalyst, cyclo-
hexanol 1c is obtained in considerably lower yield (36%)
(Table 1, entry 21).
The reusability of Ni0.87Al0.13Ox in the hydrogenolysis of
biphenyl ether was tested. For this purpose the active material
and Lewis acids were separated from the reaction mixture via
simple rotary evaporation, and reused directly 8 times. To our
delight no obvious deactivation was observed. The selectivity
of the desired cyclohexanol and benzene amounted to 97 and
83% in the 8th cycle (Fig. S3†).
Since Ni0.87Al0.13Ox exhibited the highest activity for the
hydrogenolysis of butyl phenyl ether 1a, which gave 75% yield
of cyclohexanol and less than 5% yield of butanol (Table 2,
entry 1), it was deployed to study the substrate scope under the
optimized conditions. As shown in Table 2, the hydrogenolysis
ation of the reaction conditionsa
Sel. (%)
Entry
Catalyst
LA
Con. (%)
1b
1c
1d
1
2
3
4
5
6
7
8
Ni0.87Al0.13Ox
Ni0.87Al0.13Ox
Ni1.0Al0O0
—
98
95
20
51
88
95
75
69
30b
5c
37
58
30
0
0
0
41
93d
95e
100f
100
0
6
2
4
6
6
8
7
9
0
7
8
6
0
0
0
10
5
3
2
0
0
59
25
27
49
52
50
42
50
1
54
55
57
0
0
0
41
52
79
76
36
100
35
73
69
45
43
42
51
41
99
39
37
37
0
0
0
49
43
18
22
64
La(OTf)3
La(OTf)3
La(OTf)3
La(OTf)3
La(OTf)3
La(OTf)3
La(OTf)3
La(OTf)3
La(OTf)3
Al(OTf)3
Sc(OTf)2
Ga(OTf)3
Fe(OTf)2
Sn(OTf)3
Bi(OTf)3
Hf(OTf)4
La(OTf)3
La(OTf)3
La(OTf)3
La(OTf)3
Ni0.92Al0.08Ox
Ni0.89Al0.11Ox
Ni0.80Al0.20Ox
Ni0.67Al0.33Ox
Ni0.50Al0.50Ox
Ni0.87Al0.13Ox
Ni0.87Al0.13Ox
Ni0.87Al0.13Ox
Ni0.87Al0.13Ox
Ni0.87Al0.13Ox
Ni0.87Al0.13Ox
Ni0.87Al0.13Ox
Ni0.87Al0.13Ox
Ni0.87Al0.13Ox
Ni0.87Al0.13Ox
Ni0.87Al0.13Ox
Ni0.87Al0.13Ox
RANEY® Ni
9
10
11
12
13
14
15
16
17
18
19
20
21
a Reaction conditions: 0.5 mmol butyl phenyl ether, 20 mg catalyst,
2 mL isopropanol, 40 bar H2, 5 mol% LA, 120 °C, 6 h. The conversion
and selectivity were determined by GC-FID using dodecane as a stan-
dard. b Heptane. c H2O. d 100 °C. e 130 °C. f 150 °C.
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Table 2 Hydrogenolysis of alkyl aryl ethersa
Table 3 Ni-catalyzed hydrogenolysis of diaryl ethersa
Entry Substrates
1
Con. (%) Products
Yield (%) Entry Substrates Products/yields (%)
100
100
100
100
100
75 (5c)
66 (51d)
68
1
2
3
4
5
2
3
4
69
5b
45
6b
7b
70
65
55
20
a Reaction conditions: 0.5 mmol substrate, 20 mg catalyst, 2 mL isopro-
panol, 40 bar H2, 5 mol% La(OTf)3, 130 °C, 6 h. Yields were deter-
mined by GC-FID using dodecane as a standard. b 140 °C, 50 bar H2,
12 h. c Yield of butanol. d Yield of methane.
6b
7c
8
of inexpensive aryl alkyl ethers such as anisole, ethoxybenzene
and n-octylphenyl ether readily occurred and in all cases cyclo-
hexanol 1c is afforded in yields ranging from 66% to 69%
(Table 2, entries 2–4). The hydrogenolysis of 2-ethylanisole pro-
ceeded with full conversion affording the corresponding
alcohol as a mixture of diastereoisomers in a moderate yield of
45%. Finally, transformation of 4-methoxybiphenyl and
1-methoxynaphthalene proceeded at a slightly higher reaction
temperature and H2 pressure and the corresponding products
are obtained in 55% and 20% selectivity, respectively (Table 2,
entries 6 and 7).
9
10
Our Ni0.87Al0.13Ox catalyst is also active for the selective
hydrogenolysis of aryl–aryl ethers under comparable con-
ditions. For example, hydrogenolytic cleavage of biphenyl
ether, di-p-tolyl ether and 3-phenoxytoluene produces the
corresponding arenes and cyclohexanol in good to excellent
yields (Table 3, entries 1–3). Notably, in the case of different
arenes concomitant formation of substituted cyclohexanes is
observed due to the facile hydrogenation of the C–O cleavage
products. In addition, multiple dissociation of C–O bonds is
possible as demonstrated by the hydrogenation of 1,3-diphe-
noxybenzene (Table 3, entries 4–8). The desired aliphatic
alcohols are obtained in excellent yields, albeit a mixture of
regioisomeric diols (1–3 : 1) is observed. Interestingly, the
hydrogenation of oxygenated heterocycles such as xanthene
and dibenzofuran gives the corresponding reduced alcohols as
the main products in 51% and 76% yields, respectively
(Table 3, entries 9 and 10).
a Reaction conditions: 0.5 mmol substrate, 20 mg catalyst, 2 mL isopro-
panol, 20 bar H2, 5 mol% La(OTf)3, 130 °C, 12 h. Yields were deter-
mined by GC-FID using dodecane as a standard based on the molar
amount of substrate used. b 150 °C, 8 h. c 150 °C, 12 h. For entries
8–10, no LA was added.
Noteworthily, the Ni0.87Al0.13Ox catalyst allows also for the
efficient hydrogenolysis of benzylic C–O bonds (Scheme 1).
Full conversion is achieved in the case of benzyl phenyl ether
2a and cyclohexanol 1c is observed with 99% yield at lower
temperature compared to conditions shown in Tables 2 and 3,
which indicated that 2a showed a relatively higher reactivity to
Scheme 1 Hydrogenolysis of benzylic and lignin-derived aromatic
Ar–OAr (Table 3, entry 1) and Ar–OMe displayed the poorest ethers. Con = conversion and Y = yield, IPA = isopropanol.
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activity (Table 2, entry 2). To our delight the catalyst system desired temperature and the reaction mixtures were stirred for
consisting of Ni0.87Al0.13Ox and La(OTf)3 displays also high the desired time. After completion of the reaction, the auto-
activity for reductive C–O cleavage of lignin-derived fragments, clave was cooled to room temperature, dodecane was added to
such as 3a–5a. In the case of phenethoxybenzene full conver- the mixture as an internal standard and the mixture was
sion and excellent selectivity to cyclohexanol (99%) were diluted with ethyl acetate, followed by filtration and analysis of
obtained (Scheme 1). Moreover, the hydrogenolysis of 4a and a sample by GC and GC-MS.
5a proceeded smoothly for both substrates (76% and 86% con-
version, respectively) with cyclohexanol derivatives as the
major products.
Based on the results of catalytic activity measurements and
control experiments (Scheme S1†), a possible reaction mechan-
General procedure for hydrogenolysis of biphenyl ether
Into a 4 mL glass vial, the catalyst Ni0.87Al0.13Ox (20 mg), Lewis
acid (5 mol%), ether (0.5 mmol), solvent (isopropanol, 2 mL)
and a magnetic stirring bar were added. The reaction vials
were fitted with a cap and needle and then were placed into a
ism of the NiAlOx catalyzed hydrogenolysis of aromatic ethers
is presented in Scheme S2.† Initially, the metallic Ni species
300 mL autoclave. The autoclave was purged three times with
are responsible for the molecular adsorption of the substrates
H2 (10 bar) and was then pressurized to 20 bar H2. The auto-
clave was placed into an aluminum block, heated to the
and the activation to form Ni–H. Then, the activation of the
corresponding ether bond by the present La(OTf)3 constitutes
desired temperature and the reaction mixtures were stirred for
a crucial step in the catalytic cycle. Subsequently, phenol is
12 h. After completion of the reaction, the autoclave was
cooled to room temperature, dodecane was added to the
formed by the catalytic C–O cleavage. This step seems to be
rate determining and the following arene hydrogenation is
mixture as an internal standard and the mixture was diluted
suppressed in the presence of La(OTf)3. Finally, phenol is
with ethyl acetate, followed by filtration and analysis of a
adsorbed onto the catalyst surface and reduced by the hydride
sample by GC and GC-MS.
rapidly.
Conclusions
Notes and references
In summary, we demonstrate the hydrogenolysis of highly oxy-
genated fragments in the presence of a stable and robust
NiAlOx material and Lewis acid. After appropriate tuning of
the molar ratios of Al to Ni, Ni0.87Al0.13Ox was found to be the
optimal catalyst. Compared to the reported heterogeneous cat-
alysts for C–O cleavage, which typically operate at high temp-
eratures and pressure, this catalyst (Ni0.87Al0.13Ox) displays
good to excellent activity for the hydrogenolysis of aromatic
ethers to cyclohexanol derivatives. To the best of our knowl-
edge, all the substrates in Table 2 and most shown in Table 3
and Scheme 1 (4a and 5a) underwent hydrogenolysis in the
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