Hydrogenolysis/hydrogenation of diphenyl ether as a model decomposition reaction of lignin from biomass in pressurized CO2/water condition

Article abstract of DOI:10.1016/j.cattod.2016.02.050

Catalytic hydrogenolysis of the C[sbnd]O bond of diphenyl ether (a lignin model compound) was investigated as a function of hydrogen pressure in scCO₂ medium in the presence of water. Using commercially available Rh/C catalyst, the C[sbnd]O bond cleavage of diphenyl ether mainly results phenolic monomer at 80 °C. Hydrogen pressure is one of the key parameters because (i) C[sbnd]O bond cleavage and the hydrogenation of aromatic rings are two competitive reactions; very sensitive to hydrogen pressure and (ii) hydrogen has complete solubility in scCO₂. Therefore, a critical control of hydrogen pressure was essential to reach the targeted cleavage of the C[sbnd]O bond when the reaction was conducted in scCO₂ medium under pressurized condition. Depending on the hydrogen pressure, a significant change in the ratio of monocyclic:bicyclic products from 91:9 (0.2 MPa) to 58:42 (2 MPa) was revealed in the shortest reaction time of 5 min. Thus, low hydrogen pressure was the effective choice for the scission of the C[sbnd]O bond, whereas higher hydrogen pressure hydrogenate the aromatic ring due to the higher coverage of hydrogen on the catalytic surface. Amount of the catalyst (catalyst:substrate ratio) displayed a subtle effect on the breakage of the C[sbnd]O bond. A threshold ratio of 1:5 was preferred under the present reaction condition as the increased amount hampered the substrate:water ratio and hydrogenation of the aromatic ring occurred. In addition, as the change in temperature is associated with the change in the physical properties of scCO₂, hence, the effect on the transformation of DPE was complicated and difficult to explain. Furthermore, different organic solvents as neat, along with CO₂ and with water also has substantial impact on the rapture of C[sbnd]O bond. The obtained results from the solvent studies again proved that scCO₂ along with water was the best choice for C[sbnd]O bond breakage and water is the driving force to mediate the reaction. In addition, a combination catalyst (Ni+Rh) was also tested for the same reaction under the similar working condition. Preliminary results suggested a synergistic effect in terms of the selectivity of monocyclic compounds.

Full text of DOI:10.1016/j.cattod.2016.02.050

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Catalysis Today
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Hydrogenolysis/hydrogenation of diphenyl ether as a model
decomposition reaction of lignin from biomass in pressurized
CO2/water condition
Maya Chatterjeea,∗, Takayuki Ishizakaa, Hajime Kawanamia,b,∗
a Microflow Chemistry Group, Research Institute for Chemical Process Technology AIST Tohoku, 4-2-1, Nigatake, Miyagino-ku, Sendai, 983-8551, Japan
b CREST, Japan Science and Technology (JST), 4-1-8, Honcho, Kawaguchi, Saitama, 332-0012, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Catalytic hydrogenolysis of the C O bond of diphenyl ether (a lignin model compound) was investigated
as a function of hydrogen pressure in scCO2 medium in the presence of water. Using commercially avail-
able Rh/C catalyst, the C O bond cleavage of diphenyl ether mainly results phenolic monomer at 80 ◦C.
Hydrogen pressure is one of the key parameters because (i) C O bond cleavage and the hydrogenation of
aromatic rings are two competitive reactions; very sensitive to hydrogen pressure and (ii) hydrogen has
complete solubility in scCO2. Therefore, a critical control of hydrogen pressure was essential to reach the
targeted cleavage of the C O bond when the reaction was conducted in scCO2 medium under pressurized
condition. Depending on the hydrogen pressure, a significant change in the ratio of monocyclic:bicyclic
products from 91:9 (0.2 MPa) to 58:42 (2 MPa) was revealed in the shortest reaction time of 5 min. Thus,
low hydrogen pressure was the effective choice for the scission of the C O bond, whereas higher hydro-
gen pressure hydrogenate the aromatic ring due to the higher coverage of hydrogen on the catalytic
surface. Amount of the catalyst (catalyst:substrate ratio) displayed a subtle effect on the breakage of the
Received 29 October 2015
Received in revised form 23 January 2016
Accepted 25 February 2016
Available online xxx
Keywords:
Heterogeneous catalyst
C
O bond cleavage
Pressurized CO2/water
Hydrogen pressure
C
O bond. A threshold ratio of 1:5 was preferred under the present reaction condition as the increased
amount hampered the substrate:water ratio and hydrogenation of the aromatic ring occurred. In addition,
as the change in temperature is associated with the change in the physical properties of scCO2, hence,
the effect on the transformation of DPE was complicated and difficult to explain. Furthermore, different
organic solvents as neat, along with CO2 and with water also has substantial impact on the rapture of C
O
bond. The obtained results from the solvent studies again proved that scCO2 along with water was the
best choice for C O bond breakage and water is the driving force to mediate the reaction. In addition, a
combination catalyst (Ni+Rh) was also tested for the same reaction under the similar working condition.
Preliminary results suggested a synergistic effect in terms of the selectivity of monocyclic compounds.
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
4, 4-O-5 etc. with strength and stability [5]. Among them 4-O-5 is
the strongest one with bond dissociation energy of 75.1 Kcal/mol.
Lignocellulosic biomass is the most abundantly available plant
materials on earth and has various sources such as hardwood, soft-
wood and herbaceous plants. It is cheaper than starch and sucrose
based materials. Lignocellulosic biomass is a composite material,
contains 15–30% of lignin by weight and ∼40% by energy [1–4].
Lignin is a three dimensional network polymer comprised vari-
ously linked phenylpropane units. The phenylpropane units are
connected by C C and ether linkages such as ␤-O-4, ␤-1, ␤-5, ␣-O-
Depolymerisations of lignin is an important strategy because it
generates valuable chemicals or low molecular mass feedstocks
[6] and challenging because of the stability of the ethereal bond
[7–14]; generally conducted as acid and base catalysed reactions
[15]. Ethereal compound is one of the main components of lignin,
different strategies were developed to break the C O bond, albeit
different catalysts (noble metals or non-noble metals) or methods
which can selectively target the functionality of the aryl ether bond
for removing oxygenated group from aryl ring [16–20] in the pres-
ence of hydrogen. However, during the cleavage of C O bond main
competitor is the hydrogenation of the aromatic ring as the dissoci-
ation energy of C O bond is very high of 218–314 kJ mol−1 [21] and
it is difficult to break down the ether bond without hydrogenating
∗
Corresponding authors.
E-mail addresses: c-maya@aist.go.jp (M. Chatterjee), t-ishizaka@aist.go.jp
(T. Ishizaka), h-kawanami@aist.go.jp (H. Kawanami).
http://dx.doi.org/10.1016/j.cattod.2016.02.050
0920-5861/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: M. Chatterjee, et al., Hydrogenolysis/hydrogenation of diphenyl ether as a model decomposition
reaction of lignin from biomass in pressurized CO2/water condition, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.02.050

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the aromatic ring. Therefore, to achieve desired cleavage of C
bond, application of harsh reaction condition was the only choice,
which resulted a mixture of several compounds. Different noble and
O
which require a careful control. In this work we attempted to clarify
the role of hydrogen in terms of product distribution and studied
the effect of temperature, the amount of substrate as well as differ-
non-noble metal catalysts were attempted for the breakage of C
O
ent organic solvents instead of water along with CO2 on the C O
bond. Among the non-noble metal supported catalysts [22,23], Ni
was used mostly in the hydrogenation reactions due to its low cost
and moderate performance. In 2011, a remarkable work of Sergeev
and Hartwig suggested a Ni based homogeneous catalyst Ni(COD)2
for successful cleavage of aryl C O bond in the presence of a spe-
cific carbene ligand and NaOBu as base [24]. In addition, the same
group also investigated the selective hydrogenolysis of aryl ethers
using homogeneous catalyst formed in-situ from the well-defined
soluble nickel precursor Ni(COD)2 or Ni-(CH2TMS)2(TMEDA) and a
base additive (tBuONa) in m-xylene at 120 ◦C within the reaction
time of 96 h without any additional dative ligand [25]. Following
their footsteps, heterogeneous catalysts, which are considered as
not very selective and require higher temperature for the cleav-
age of the C O bond was the subject of study. Zhao and Lercher
successfully introduced Ni/SiO2 catalyst containing 60% Ni for effi-
cient cleavage of aryl ethers in the presence of hydrogen in aqueous
medium [26]. Ni/ZSM-5 was adopted for the hydrogenolysis of real
lignin as well as model compounds under a comparatively mild
condition (250 ◦C, 5 MPa) [27]. Moreover, iron (III) acetylacetonate
[28], integrated Ni nanoparticles supported on porous SiC compos-
bond cleavage. In addition, the same reaction was also explored in
the presence of a Ni-Rh combination catalyst.
2. Experimental
2.1. Materials
All chemicals were obtained commercially. Diphenyl ether and
phenol were from Wako Pure Chemicals. 5 wt.% Rh/C and cyclo-
hexyl phenyl ether were obtained from Sigma-Aldrich. Carbon
dioxide (>99.99%) from Nippon Sanso Co. Ltd. was used for the
reaction.
2.2. Catalytic activity
The conversion of the model compound was conducted in a
50 ml stainless steel batch reactor containing required amount of
the catalyst, substrate (wt. ratio of catalyst:substrate = 1:5) and
water. The reactor was placed in an oven with fan heater to
maintain the constant temperature. After reaching the desired tem-
perature, hydrogen of required pressure was first introduced into
the reactor. Liquid CO2 was charged into the reactor using a high
pressure liquid pump (JASCO). The reaction mixture was stirred
continuously with a Teflon coated magnetic bar during the reac-
tion. After the reaction, the reactor was cooled with ice-water
and depressurized carefully by the back-pressure regulator. The
product mixture was separated from the catalyst simply by filtra-
tion. GC/MS (Varian Saturn 2200) was used for identification and
quantitative analysis of the product mixture. Quantification of the
products was obtained by a multi-point calibration curve for each
ites [29] were also reported for the desired cleavage of the C
O
bond of aryl ether to improve the catalytic activity or reusability
of the catalyst. Hydrolysis of lignin model compound was another
approach to cleave the C O bond conducted in supercritical water
using heterogeneous catalysts Ni/ZrO2/K2CO3, which still required
high temperature of 240–400 ◦C and 25–32 MPa of hydrogen [30].
Thus, from the existing literature, it was evident that Ni-based cat-
alysts (mainly homogeneous) are potential candidate with high
efficiency for the rapturing of C O bond under a comparatively mild
reaction condition. However, homogeneous Ni catalyst suffers dif-
ficulties of product separation, requirement of expensive ligands,
and reusability problems. On the other hand, some of the Ni based
heterogeneous catalysts are difficult to handle because of their air
sensitivity and in other cases large amount of metal was necessary
to achieve best performance [31].
product. The selectivity to each product was calculated by the fol-
ꢀ
lowing expression Si = Ci/ Cp, where Ci is the concentration of the
ꢀ
product ‘i’ and
Cp is the total concentration of the product.
3. Results and discussion
The offerings of heterogeneous catalysts tempted us to develop
a simple and convenient method for hydrogenolysis of aryl C
O
Scheme 1 represent possible reaction pathways of the conver-
sion of DPE under the studied reaction condition (PCO2 = 10 MPa,
P
bond of diphenyl ether. Noble metal catalysts are widely studied
for their excellent activity in various reactions also used in the
conversion of lignin into different phenolic compounds [32,33].
Thus, hydrogenolysis of diphenyl ether (DPE), which mimic the
4-O-5 linkages was conducted over the commercially available
Rh/C as heterogeneous catalyst in supercritical carbon dioxide
(scCO2)/water system [34,35]. The use of diaryl ether model com-
pound represents a firm test of the effectiveness of the catalyst on
the cleavage of ether linkages. The use of scCO2 as reaction medium
has several unique advantages such as clean product separation,
high product selectivity, non-toxicity, non-flammability etc. and
most importantly, in the presence of water, CO2 creates an acidic
environment [36,37] would be the responsible factor behind the
cleavage of the C O bond [38]. Instead of expensive metal hydride,
molecular H2 was used as the source of hydrogen. In the studied
process, under a mild reaction condition activated hydrogen breaks
aryl C O bond at 80 ◦C within the reaction time of 5 h.
H2 = 0.5 MPa, temperature = 80 ◦C, time = 5 h and water = 4 ml).
There are three different routes of conversion: (i) hydrogenolysis,
which results an equimolecular mixture of monocyclic com-
pounds such as phenol (PhOH) and benzene (Bz) followed by
the hydrogenation to cyclohexanone (CHO)/cyclohexanol (CHOH)
and cyclohexane (CH), respectively. (ii) Hydrolysis generates two
molecules of PhOH and subsequently hydrogenated to CHOH and
(iii) hydrogenation of aromatic rings; DPE can be converted to a par-
tial aromatic ring hydrogenated product cyclohexyl phenyl ether
(CHPE), followed by the formation of a completely hydrogenated
dicyclohexyl ether (DCHE). Presence of hydrogen was necessary for
the conversion of DPE, because there was no reaction in the absence
of hydrogen.
3.1. Dependence of the reaction path on hydrogen pressure
In our previous report [35], we observed that the transforma-
tion of DPE involves C O bond hydrogenolysis/hydrolysis and the
hydrogenation of aromatic ring, where water played a significant
role. When using only water or CO2, hydrogenolysis of C O was
hampered, hence a combined effect was required. Similarly, hydro-
gen is another important parameter to achieve successful cleavage
of the C O bond because hydrogen has complete solubility in scCO2
and increased concentration in the reaction medium is inevitable,
To investigate the effect of hydrogen pressure, the reaction
was conducted at different hydrogen pressure (0.2–2 MPa) keeping
other parameters constant (Fig. 1). Except the very low pressure of
0.2 MPa, DPE was completely converted to the corresponding reac-
tion products within the reaction time of 5 h. The effect of hydrogen
pressure on product distribution was significantly prominent. For
instance, the selectivity of CHOH was increased from 78.4% to 96%
Please cite this article in press as: M. Chatterjee, et al., Hydrogenolysis/hydrogenation of diphenyl ether as a model decomposition
reaction of lignin from biomass in pressurized CO2/water condition, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.02.050

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3
HO
HO
Hydrogenation
Hydrogenolysis
+
+
Cyclohexane
Cyclohexanol
Benzene
Phenol
Hydrogenation
HO
HO
Hydrolysis
2
2
O
H2
Rh/C
Cyclohexanol
Phenol
Diphenyl ether
O
O
Hydrogenation
Hydrogenation
Cyclohexylphenyl ether
Dicyclohexyl ether
HO
2
Cyclohexanol
Scheme 1. Possible reaction path of DPE conversion in scCO2/water.
as the pressure changes from 0.2 to 0.5 MPa and then decreased
to 31% when the pressure reached to 2 MPa. On the other hand,
the selectivity of DCHE was <5% at 0.2 MPa and increased to 68.5%
at the higher pressure of 2 MPa. According to the Scheme 1 DPE
could follow three routes such as hydrogenolysis, hydrolysis and
hydrogenation to transform into different products. Monocyclic
products such as CHOH, CHO and CH/Bz were produced through
the hydrogenolysis of C O bond or via hydrolysis, whereas, bicyclic
products (CHPE and DCHE) could be generated via hydrogenation.
Therefore, low hydrogen pressure was required for the breaking
of C O bond to generate monocyclic products. Contrarily, bicyclic
product such as DCHE was observed at higher pressure (>0.5 MPa)
in scCO2/water medium. No partially hydrogenated product (CHPE)
was detected in the final products. The solubility of hydrogen in
the reaction medium is main concern when the reaction was con-
ducted in scCO2 because hydrogen compete with the substrate.
Hence, an effective control of hydrogen pressure is necessary to
generate monocyclic compounds. Otherwise, increased hydrogen
pressure will enhanced the amount of hydrogen in the reaction
medium. As a result, hydrogenation of the aromatic ring could be
easier instead of the cleavage of C O bond. Therefore, it is clear that
hydrogen pressure has significant influence on the reaction path of
DPE conversion.
Fig. 2a evidences the transformation of DPE at the lower pressure of
0.2 MPa within the reaction time of 5 min to 5 h leaving other condi-
tions similar (temperature = 80 ◦C, PCO2 = 10 MPa and water = 4 ml).
Results revealed that 25.2% of DPE was converted within the reac-
tion time of 5 min and the maximum conversion of 77.8% was
reached in 5 h. Considering the product distribution, in the begin-
ning, the product mixture was consisted of monocyclic compounds
such as CHOH (41.9%), CHO (40.2%) and CH (3.5%). No phenol was
detected even in the shortest reaction time (5 min) because of the
faster conversion to CHOH and CHO via two parallel reactions.
As the time progressed the conversion of DPE was increased to
77.8% followed by the enhancement of the selectivity of CHOH
from 41.9% to 78.4%. However, the selectivity of CHO was dropped
from 40.2% to 14.7% with the increased selectivity of CHOH. This
result suggested that the hydrogenation of CHO to CHOH was favor-
able under the present reaction condition. Final product mixture
after 5 h of reaction contains 78.4% of CHOH, 14.7% of CHO, 2.9%
of CH/Bz. Noteworthy, the selectivity of deoxygenated monocyclic
compound remain unaltered. On the other hand, bicyclic products
(4.2% DCHE and 10.2% CHPE) were also detected in the product mix-
ture after 5 min. Interestingly, CHPE was completely diminished
within 30 min, and the selectivity of DCHE slightly increased to 10%.
There was almost no change in the selectivity of DCHE even after
5 h of reaction. To interpret this result, it could be suggested that at
very low pressure of hydrogen cleavage of the ethereal C O bond
was the preferred route because substrate has better coverage on
the catalyst surface rather than hydrogen.
From the scenario presented above it is difficult to get an insight
about the actual reaction trend occurring in different hydrogen
pressure. A time dependent course of the reaction was studied at
different hydrogen pressure and the results are shown in Fig. 2a–d.
At 0.5 MPa, the conversion of DPE was 32% within the reac-
tion time of 5 min, and complete transformation required 30 min.
(Fig. 2b). Main components of the product mixture consisted of
CHOH with the selectivity of 52.9% along with 31.6% of CHO and
3.2% of CH after 5 min of reaction. Noticeably, the selectivity of
CHOH was improved compared to 0.2 MPa. Within the reaction
time of 30 min, CHO was completely converted to CHOH because
of the increased hydrogen pressure and consequently there was
an increased selectivity of CHOH, which reached to 96%. In addi-
tion, partially hydrogenated CHPE and DCHE were also detected
in the product mixture with very low selectivity of 5.1 and 6.4%,
respectively after 5 min but undetected in the final mixture. A care-
ful observation revealed that even at 0.5 MPa, cleavage of the C
O
bond was the main route of conversion as confirmed by the calcu-
lation of the molar ratio of monocyclic:bicyclic compounds in the
beginning (90:10) and after completion of the reaction (100:0).
Fig. 1. Effect of hydrogen pressure on the cleavage of C O bond and hydrogena-
tion of aromatic rings. Reaction condition: PCO2 = 10 MPa; catalyst: substrate = 1:5;
temp. = 80 ◦C, reaction time = 5 h and water = 4 ml.
Please cite this article in press as: M. Chatterjee, et al., Hydrogenolysis/hydrogenation of diphenyl ether as a model decomposition
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Fig. 2. Time course of the reaction depending on hydrogen pressure: (a) 0.2 MPa, (b) 0.5 MPa, (c) 1 MPa and (d) 2 MPa. Reaction condition: PCO2 = 10 MPa; catalyst: sub-
strate = 1:5; temp. = 80 ◦C and water = 4 ml.
Fig. 2c revealed the change in the catalytic performance of Rh/C
catalysts after applying 1 MPa of hydrogen pressure in the reac-
tion time of 5 min to 5 h. The reaction was very fast and 96% of
DPE was converted within 5 min. As the concentration of hydro-
gen increased the hydrogenation reaction was the main route of
the conversion, reflected from the composition of the product mix-
ture, which contains mainly DCHE with the selectivity of 60.2%. In
(TOF O) was higher than the TOFH suggesting the hydrogenoly-
sis and hydrolysis were two major routes in the transformation
of DPE in the present system. Furthermore, TOFs after 5 h of reac-
C
tion reduces significantly for each cases (TOF
and TOFH). The
C
O
maxima of TOF
and TOFH were dropped to 102 h−1 (0.5 MPa)
C
O
and 15 h−1 (2 MPa), respectively, which might be attributed to the
scarcity of the substrate in the system. Moreover, the catalyst in the
system was still active and can be restarted again by the addition
of the reactant.
addition, 30% of CHOH and 9.8% of CHO were also observed as C
O
bond cleaved product. It is intriguing that after 5 h of reaction, the
selectivity of CHOH was improved to 50% while the selectivity of
DCHE was decreased to 49%, thus, a possibility of the conversion of
DCHE to CHOH might be speculated during the course of the reac-
tion. However, when we conducted a separate reaction with CHPE,
except DCHE no other products were detected even after extend-
ing the reaction time of 18 h as fully saturated ethers are difficult to
breakdown and the possibility of the conversion of DCHE to CHOH
remain unexplained in the studied working condition.
The time course of the DPE conversion at 2 MPa again presents
the same scenario of 1 MPa (Fig. 2d). Within 5 min. DPE was com-
pletely converted mainly into DCHE (59.4%). The selectivity of
monocyclic and bicyclic products were unaltered after 5 h of reac-
tion. Interestingly, exclusive formation of DCHE was not observed
even after application of higher hydrogen pressure indicating not
enough hydrogen around the aromatic ring for its conversion and
expect a competition between the C O bond cleavage and the
hydrogenation of the aromatic ring [27].
3.2. Effect of the amount of substrate
Accumulating the above-mentioned experimental results it
could be inferred that after complete consumption of DPE selectiv-
ity of monocyclic and bicyclic products remain almost same even
after extending the reaction time and the rate of the reaction (TOF)
was decreased considerably. Thus, we consider to study the impact
of the amount of substrate on the catalytic performances and the
results are shown in Fig. 4. Maintaining the other operating con-
ditions constant, catalyst/substrate ratio was varied from 1:3 to
1:50. Result shows complete conversion of DPE was achieved up to
the catalyst:substrate ratio of 1:30 within the reaction time of 5 h,
but conversion dropped to 48% when the ratio of 1:50 was used.
Moreover, the catalyst:substrate ratio also influenced the selectiv-
Depending on the hydrogen pressure, we investigated the
turnover frequency (TOF) of C O bond cleavage (TOF O) and
C
hydrogenation of aromatic ring (TOFH) within the shortest reaction
time of 5 min and after 5 h of reaction (Fig. 3). Results reveal that
TOFs depend heavily on the hydrogen pressure. In the beginning of
the reaction, an extremely high TOFs should be noted. For instance,
when the hydrogen pressure was increased from 0.2 to 2 MPa, the
TOF
was changed from 365 h−1 to 641 h−1, which reaches a
C
O
maximum of 1054 h−1 at 0.5 MPa. On the other hand, the TOFH was
significantly enhanced from 68 to 527 h−1 along with the hydrogen
pressure of 0.2–2 MPa. Noticeably, for the breakage of C O bond,
TOF reaches a maximum and then decreased indicating the pref-
erence for low hydrogen pressure, whereas, TOFH was increased
constantly reflects the higher pressure favored hydrogenation of
the aromatic rings. However, in any case, rate of C O bond cleavage
Fig. 3. Variation of the amount of substrate. Reaction condition: PCO2 = 10 MPa,
PH2 = 0.5 MPa, time = 5 h, temp. = 80 ◦C and water = 4 ml.
Please cite this article in press as: M. Chatterjee, et al., Hydrogenolysis/hydrogenation of diphenyl ether as a model decomposition
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5
1200
1000
800
600
400
200
0
0
0.5
1
1.5
2
PH2 (MPa)
TOFC-O5
Fig. 4. Comparison of TOF C
TOFH5
TOFC-O
TOFH
and TOFH at 5 min and 5 h of reaction. TOF C
and
O5
Fig. 5. Effect of temperature on
C
O
bond cleavage. Reaction condition:
O
TOFH5 represents 5 min of reaction time; TOF C O and TOFH = reaction time 5 h; Reac-
PCO2 = 10 MPa, PH2 = 0.5 MPa, time = 5 h, catalyst:substrate = 1:5 and water = 4 ml.
tion condition: PCO2 = 10 MPa, PH2 = 0.5 MPa, catalyst: substrate = 1:5, temp. = 80 ◦
and water = 4 ml.
C
DPE. The role of water is critical here because DPE is completely
insoluble in water. To understand the role of water an on-water
reaction can be proposed in which the reaction occurred on the
water organic interface and the relative interfacial hydrogen bond-
ing is the key factor for the reaction as the reactant is insoluble in
water [40–42].
ity of monocyclic and bicyclic compounds. Very high selectivity
of CHOH (96%) was detected until catalyst:substrate ratio of 1:5
and then dropped sharply to 30.9% for 1:50, whereas the selectiv-
ity of CHO increased from 0 to 25.2% under the selected condition.
This result implied the very fast conversion of PhOH formed by the
breakage of the C O bond even in the highest catalyst:substrate
ratio, which can be converted to CHOH and CHO simultaneously.
However, increased selectivity of CHO reflected that there was
not enough surface hydrogen to convert CHO to CHOH [39]. On
the other hand, Generation of bicyclic products such as CHPE and
DCHE were boosted along with the increased amount of substrate
in the system; hydrogenated products CHPE and DCHE appeared
with highest selectivity of 26.4% (1:50) and 38.9% (1:30), respec-
tively. From the experimental reactivity trend it is clear that with
increasing the amount of substrate, the reaction time for complete
3.3. Reaction temperature vs. C O bond cleavage
To investigate the effect of temperature, the reaction was con-
ducted at various temperature of 35, 50, 80, 100 and 130 ◦C and
the results are shown in Fig. 5. Unlike the reactions in liquid sol-
vent, there was no straight forward effect on the catalytic activity
when the reaction was conducted in scCO2. Noticeably, there was
no effect of temperature on the conversion. Independent to the
temperature, DPE was completely converted to the corresponding
products. However, the change in temperature manifested an inter-
esting result in terms of product distribution. At lower temperature,
hydrogenation and C O bond breakage co-existed. As the tem-
perature increased the selectivity of monocyclic product (CHOH,
CHO and CH/Bz) increased and reached a maximum of 100% at
80 ◦C; dominating the C O bond cleavage. No hydrogenated prod-
uct was detected in this condition. To explain this result it could be
suggested that hydrogenation of aromatic ring was the most favor-
able process at the lower temperature due to the presence of large
amount of hydrogen, but C O bond has higher energy, thus, higher
temperature has beneficial effect on the cleavage of C O bond. Fur-
thermore, it is reported that lower temperature and high hydrogen
pressure prefers hydrogenation instead of hydrogenolysis [43,44].
Therefore, one could expect a better performance regarding the
generation of monocyclic compounds after further elevation of
temperature. Unfortunately, the increased temperature of >80 ◦C
causes hydrogenation of the aromatic ring rather than C O bond
rapturing, consequently the selectivity of monocyclic product was
dropped substantially from maximum of 96–10% with the change
in temperature from 80 to 130 ◦C. As the reaction was conducted in
scCO2 medium under the fixed pressure condition when the tem-
perature increases there is every possibility that it could disturb
the solubility of CO2 in water and then the system behaves like no
water system which favored hydrogenation.
consumption of DPE varied and restricted the formation of C
O
bond cleavage product. It is noteworthy that the conversion was
low of 48% and the product mixture still contains CHO and CHPE
even after 5 h, which suggested the deficiency of hydrogen in the
medium due to the higher coverage of the substrate on the catalyst
surface.
From the above results it seems monocyclic products still main-
tained higher selectivity in comparison with the bicyclic one even
with the very high catalyst:substrate ratio. However, if we com-
pare the TOFs between the cleavage of C O bond (TOF O) and
C
the hydrogenation (TOFH), using the catalyst:substrate ratio of
1:50, the TOFs are turned out as comparable: TOF C O = 7.2 h−1 and
TOFH = 5.8 h−1. Hence, an increased amount of substrate restricted
that C O bond cleavage and enhance the possibility of hydro-
genation of aromatic ring. When the catalyst:substrate ratio was
increased it hampered the CO2:substrate (1:0.005 to 1:0.04),
H2:substrate (1:10 to 1:1) as well as H2O:substrate (1:0.004 to
1:0.036) ratio. After a series of experiments it could be inferred
that changing the CO2 and the H2 pressure has negligible effect on
the increased amount of substrate. From our previous work [35], it
has been found that presence of water is essential for the scission
of the C O bond and a threshold amount of water was necessary
to achieve monocyclic products with very high selectivity. Hence,
an extended experiment was conducted using catalyst:substrate
ratio of 1:50 by increasing the amount of water from 4 to 8 ml
at 80 ◦C and the reaction time of 5 h. Interestingly, the conver-
sion was increased from 48% to 77% with the addition of water.
Additional water also changes the product distribution, which con-
sists of 92.9% (CHOH = 52.3%, CHO = 40%, CH/Bz = 0.6%) and 7. 1%
(DCHE = 2.4%, CHPE = 4.7%) of monocyclic and bicyclic compounds,
respectively (Fig. 4; marked part). These findings again proved that
water is the main reason behind the cleavage of the C O bond of
3.4. Effect of solvent
As a part of the catalytic system, reaction medium also needs
attention to the cleavage of the C O bond. Wang and Rinaldi
[45] first described the effect of different organic solvents on the
hydrogenolysis of DPE using Raney Ni catalyst and suggested that
solvent is an important part of the reaction, which strongly influ-
enced the catalytic activity. To study the effect of the reaction
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M. Chatterjee et al. / Catalysis Today xxx (2016) xxx–xxx
Table 1
Impact of different organic solvents on C O bond cleavage.
Entry
Selectivity (%)
CHOH
Solvent
Conv. (%)
DCHE
CHPE
CHO
CH/Bz
Solvent (neat)
1
2
3
4
5
EtOH
14.0
1.5
17.9
43.4
23.9
65.1
91.3
66.1
–
–
–
–
0.8
6.7
–
–
–
–
–
MeOH
Propanol
THF
100.0
100.0
100
22.5
30.2
26.6
8.7
0/11.6
8.3/0
–
Hexane
100.0
Solvent + CO2
6
7
8
9
10
EtOH
3
5
–
–
19.3
23.4
3.9
–
100
89.0
2.1
61.2
61.5
–
–
10.8
10.9
3.9
–
–
MeOH
Propanol
THF
11.0
57.3
–
94
100.0
18.0
9.7/0
5/0.3
1.8/1.7
Hexane
26.9
Solvent + H2O
11
12
13
14
15
EtOH
100.0
57.0
100.0
95.5
54.2
38.9
100.0
44.9
27.7
2.4
18.2
–
34.9
70.6
–
11.5
–
–
–
13.0
29.0
–
9.9
1.1
0.3
1.6/0.8
–
4.5/0
2.3/0
MeOH
Propanol
THF
Hexane
100.0
Reaction condition: PCO2 = 10 MPa, PH2 = 0.5 MPa, time = 5 h, temp. = 80 ◦C, catalyst:substrate = 1:5 and water = 4 ml.
medium on the conversion of DPE under the present reaction
condition, the reaction was conducted in the neat organic sol-
vents (methanol, ethanol, propanol, tetrahydrofuran and hexane),
organic solvents + CO2 and organic solvents + H2O at 80 ◦C and
hydrogen pressure of 0.5 MPa (Table 1). From the results it is
evident that except ethanol (14%) complete conversion of DPE
was achieved for all other solvents in neat condition (Table 1:
Entry 1–5). These results indicate that solvent has strong impact
indeed. Considering the selectivity of targeted monocyclic prod-
ucts (CHOH, CHO and CH) varies from 2.3 to 41.6%, however,
the selectivity range of bicyclic products (DCHE and CHPE) were
23.1–91.3%. The selectivity orders for monocyclic and bicyclic
compounds are: propanol > THF > methanol > hexane > ethanol and
hexane > THF > methanol > propanol > ethanol, respectively. Thus, it
could be predicted that hydrogenation of the aromatic ring was the
major route of DPE transformation because of the possible change
in the mode of adsorption of the substrate. In the presence of CO2,
the conversion was very poor in methanol (5%), ethanol (3%) as
well as in hexane (18%), but propanol and THF provide very high
conversion of 94% and 100%, respectively (Table 1: Entry 6–10).
The solubility of CO2 in ethanol and methanol were comparatively
lower than in propanol and THF and accordingly CO2 in presence of
THF offers best environment in terms of mass transfer limitation,
consequently higher activity [46,47]. The organic solvents in asso-
ciation with CO2, shows very high selectivity of bicyclic products
especially CHPE (61.5%). Analyzing the obtained data on product
distribution evidences that hydrogenation of aromatic ring was
the main route of DPE conversion in the solvent + CO2 and the
selectivity order was same for both of mono and bicyclic com-
pounds (THF > Propanol > hexane > methanol ≈ ethanol). It should
be addressed that in the presence of organic solvents CO2
can form CO2-expanded liquid, which improved the trans-
port properties and increased the solubility of reactant gasses
such as hydrogen, which facilitate the hydrogenation reac-
tion [48–51]. Clearly THF offers better performance in terms
of activity and selectivity (Table 1; Entry 9). The scenario
changed when those solvents were used along with H2O.
Higher conversion was detected in each solvent and fol-
lows the order of ethanol ≈ propanol ≈ hexane > THF > methanol
(Table 1: Entry 11–15). Regarding the product distribution,
propanol shows highest selectivity of 100% for CHOH and
except hexane in each case monocyclic products were the
Fig. 6. Ni-Rh combination catalysts for DPE conversion in scCO2/water medium.
Reaction condition: PCO2 = 10 MPa, PH2 = 0.5 MPa, temp. = 80 ◦C, time = 5 h, cata-
lyst:substrate = 1:5 and water = 4 ml.
major part of the mixture and the selectivity order was:
propanol > methanol ≈ ethanol > THF > hexane. Among the selected
organic solvents, alcohols are miscible in water, THF has compar-
atively low solubility and hexane shows lowest solubility. From
the selectivity order it is clear that miscibility of the organic sol-
vent is an important part of the reaction. The developed method
for the conversion of DPE to monocyclic compound using Rh/C
catalyst revealed that the presence of water changes the mode of
reaction from hydrogenation to hydrogenolysis. Thus, water is not
only a spectator for this system, it could dictate the product selec-
tivity. Mechanism behind the cleavage of the C O bond of DPE in
scCO2/water medium is critical. Previously, we observed that water
in the presence of CO2 influenced the pH of the medium and con-
sequently the C O bond cleavage. However, water in combination
with organic solvent (miscible) also results monocyclic compounds.
Thus, it could be predicted that solvents like water cannot absorb
directly on the Rh surface, but it can modify the surface by creat-
ing a hydrophilic environment and thereby a favorable situation
for the adsorption of DPE via C O bond followed by the enhanced
selectivity of monocyclic compounds [52,53].
3.5. Reactivity of combination catalyst
We already proved that the use of Rh/C as a supported noble
metal catalyst is an effective choice for the cleavage of the C
O
bond under a mild reaction condition and there was no reaction
without any catalyst. Among the non-noble metal catalysts, Ni is
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the most preferred and efficient for hydrogenolysis of C O bond
in the form of homogeneous or heterogeneous way [54–59]. How-
ever, under the present working condition, Ni catalyst performed
well on the conversion of DPE, but not in the hydrogenolysis of the
4. Conclusion
In conclusion, we have demonstrated a successful application
of Rh/C catalyst in the transformation of DPE to monocyclic com-
pounds via hydrogenolysis of C O bond under a mild reaction
condition. In the present reaction condition, the role hydrogen is
critical, and could have substantial impact on the cleavage of the
C
O bond. Bimetallic catalysts are sometimes reported as better
performer in terms of hydrogenolysis of C O bond. There are sev-
eral reports on bimetallic catalysts on different supports [54–60].
Zhang et al. reported that instead of pure Ni, a combination of Ni
with other metals such as Au [61] and Ru, Rh and Pd gave better
results [62]. It is well-known that a combination of two differ-
ent metals significantly change the catalytic activity in comparison
with their individual counterpart [63]. Here, our intention is to try a
simple approach to investigate the effect of two different catalysts
after mixing them together. If mechanical mixing results syner-
gism it could be a guide to the probable effectiveness of bimetallic
catalyst. Hence, a series of catalysts were prepared by the mechan-
ical mixing (wt./wt.) of Rh/C and Ni/MCM-41 of different ratio
to perform the reaction under the selected condition (tempera-
ture = 80 ◦C, PCO2 = 10 MPa, PH2 = 0.5 MPa, water = 4 ml, time = 5 h)
and the results are shown in Fig. 6. To avoid the complexity arises
from Ni salt and support material, Ni supported on inert material
like MCM-41 was used. Fig. 6 shows a comparison of only Rh and
Ni catalysts with the different combination of Ni and Rh catalysts
for the conversion of DPE. Individually, 50.9% DPE was converted
to bicyclic (CHPE) and monocyclic (CHOH) products with the selec-
tivity of 78.9% and 11.2%, respectively, using Ni/MCM-41. On the
contrary, DPE was completely converted to 96% of CHOH and 4%
of CH/Bz via hydrogenolysis/hydrogenation over Rh/C. The exper-
imental data obtained under identical reaction condition shows a
significant difference in the behavior of the catalytic activity when
Rh/C and Ni catalyst were mixed together. We conducted the reac-
tion using Rh/C and Ni/MCM-41 mixed in different ratios of 1:1
to 1:10 under the same reaction condition. Starting the reaction
with 1:1 (Rh/C:Ni/MCM-41) mixture; complete conversion and a
very high selectivity to CHOH (98%) was evidenced along with
the formation of CHPE (2%) indicating the predominate effect of
Rh. Now, if the ratio was changed from 1:1 to 1:5, the conversion
dropped to 89% and then further reduced to 45.2% as the ratio was
changed to 1:10 shows that the absence of a synergistic effect on the
conversion of DPE after combining the two catalysts. Considering
the product distribution, the product mixture contains only CHOH
(monocyclic) and CHPE (bicyclic). No deoxygenated monocyclic
products and completely hydrogenated products were detected.
Mixing gave better selectivity of CHOH compared to only Ni (11.2%),
which changes from 54.5% to 98% as the ratio changed from 1:1 to
1:5. However, a further increase of the ratio led to a decrease of
the selectivity in the reaction time of 5 h. If one takes into account
on the product mixture, in the presence of Ni, it could be sug-
gested that hydrolysis and hydrogenation were two reactions to
convert DPE. The improved selectivity of CHOH might be consid-
ered as a synergistic effect and can be explained by the bifunctional
mechanism which required an acid support and a metal func-
tion [64]. When the ratio was changed to 1:10, the selectivity of
CHOH was dropped to 26%, with an increase in the selectivity of
CHPE (74%). In order to verify the influence of reaction time on the
1:10 catalyst, the reaction was extended for 18 h (Fig. 4; marked
part). Surprisingly, the conversion was increased to 77.2% and an
improved selectivity of CHOH (53.3%) was also detected. It is too
early to predict the actual reason behind this observation. Prelim-
inary results of the addition of Ni with Rh/C emerged out as an
interesting one for further studies as there are the lot of open-
ings to modify this process to achieve a simple and easy to handle
catalyst for the hydrogenolysis of DPE under a mild reaction con-
dition,
C
O bond of DPE. A time course of the reaction based on the hydro-
gen pressure gave an insight on the reaction pathway. From the
beginning of the reaction, a strong influence of hydrogen pressure
on the conversion and product distribution was evident. A lower
hydrogen pressure was preferable for the rapturing of C O bond,
because of the preferable adsorption of DPE through the C O bond.
Higher hydrogen pressure resulted fully saturated bicyclic prod-
ucts but further hydrogenolysis of the C O bond of the bicyclic
product did not occur under the applied condition. A prominent
effect of temperature on the product selectivity suggested that
at lower temperature the transformation of DPE was occurred
via hydrogenation as higher temperature required for C O bond
cleavage. However, very high temperature again favored hydro-
genation because it could affect the solubility of CO2 in water.
Due to the high coverage of the catalyst active site, large excess
of substrate also hampered the hydrogenolysis of C O bond. Stud-
ies on different solvents in the neat condition or in the presence
of CO2 and water strongly suggested that a rational choice of the
solvents is extremely important part as they affect activity and con-
trol the selectivity. Solvents as neat and in the presence of CO2,
mainly produces bicyclic product whereas monocyclic products
were detected in the presence of water. A combination catalyst of
Rh/C and Ni/MCM-41 was prepared by mixing in different ratios
and attempted for this reaction. Interesting preliminary results
could serve as the starting point of further systematic studies to
achieve better performances on solid catalysts for DPE hydrogenol-
ysis.
Acknowledgements
H. Kawanami is funded by the Japan-U.S. cooperation project
for research and standardization of Clean Energy Technologies, The
Ministry of Economy, Trade and Industry (METI), Japan and CREST,
Japan Science and Technology (JST).
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Please cite this article in press as: M. Chatterjee, et al., Hydrogenolysis/hydrogenation of diphenyl ether as a model decomposition
reaction of lignin from biomass in pressurized CO2/water condition, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.02.050