CO2 promoted hydrogenolysis of benzylic compounds in methanol and water

Article abstract of DOI:10.1039/c3ra41717d

This study successfully demonstrated the hydrogenolysis of benzylic alcohols and their derivatives over a Pd/C catalyst by using CO ₂-expanded methanol (CX-methanol) and compressed CO₂/water as two green reaction media. It was found that with the addition of low-pressure CO₂ (1 MPa), the reaction conversions of benzylic alcohols and their derivatives could all be increased in the CO₂ promoted systems. For example, the conversion of 1-phenylethanol could be elevated from 23% to 63% in CX-methanol and the conversion of benzyl alcohol could be elevated from 75% to 92% in compressed CO₂/water. The positive effects of CO₂ could be attributed to the decrease of mass transfer resistance and the increase of hydrogen solubility. CO₂ could also form methylcarbonic acid and carbonic acid in methanol and water, respectively. Therefore, it could enhance the departing ability of the protonated hydroxyl leaving groups by providing a more acidic environment. In addition, the hydrogenolysis (or deoxygenation) of aromatic aldehyde and ketone by using compressed CO₂/water as the solvent were also studied in this work. The results suggested that both CX-methanol and compressed CO ₂/water could be used as two efficient solvents for the hydrogenolysis reactions.

Full text of DOI:10.1039/c3ra41717d

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CO2 promoted hydrogenolysis of benzylic compounds
in methanol and water3
Cite this: RSC Advances, 2013, 3,
17222
Hsin-Wei Lin, Clive H. Yen, Han Hsu and Chung-Sung Tan*
This study successfully demonstrated the hydrogenolysis of benzylic alcohols and their derivatives over a
Pd/C catalyst by using CO2-expanded methanol (CX-methanol) and compressed CO2/water as two green
reaction media. It was found that with the addition of low-pressure CO2 (1 MPa), the reaction conversions
of benzylic alcohols and their derivatives could all be increased in the CO2 promoted systems. For example,
the conversion of 1-phenylethanol could be elevated from 23% to 63% in CX-methanol and the
conversion of benzyl alcohol could be elevated from 75% to 92% in compressed CO2/water. The positive
effects of CO2 could be attributed to the decrease of mass transfer resistance and the increase of hydrogen
solubility. CO2 could also form methylcarbonic acid and carbonic acid in methanol and water, respectively.
Therefore, it could enhance the departing ability of the protonated hydroxyl leaving groups by providing a
more acidic environment. In addition, the hydrogenolysis (or deoxygenation) of aromatic aldehyde and
ketone by using compressed CO2/water as the solvent were also studied in this work. The results
suggested that both CX-methanol and compressed CO2/water could be used as two efficient solvents for
the hydrogenolysis reactions.
Received 17th January 2013,
Accepted 12th July 2013
DOI: 10.1039/c3ra41717d
www.rsc.org/advances
alcohols hydrogenolysis. Felpin and Fouquet also reported the
CLC and CMC hydrogenation and C–O hydrogenolysis of
Introduction
Hydrogenolysis is the bond cleavage between carbon–carbon
or carbon–heteroatom by using hydrogen and it is considered
as a related reaction of hydrogenation. Catalytic hydrogeno-
lysis is an important synthetic reaction, which has been widely
used in petroleum chemistry, pharmaceutical industry and
biofuel conversion.1,2 During total synthesis, due to the high
chemical stability and easy preparations, benzylic compounds
are usually used to protect certain functional groups such as
alcohols, amines and acids.3,4 After the main reaction,
hydrogenolysis or in this case debenzylation could act as a
work-up process in order to obtain the desired products.
In general, hydrogenolysis reactions for C–C, C–O and C–N
bonds cleavage can be carried out in the presence of H2 and
catalyst. In conventional protocols, palladium on carbon (Pd/
C) is the most efficient catalyst for hydrogenolysis due to its
high activity and selectivity toward bond cleavage, and there-
fore, the reaction can be achieved at mild temperatures and
pressures.5,6 Kieboom et al. reported the kinetic study of the
substituent effects in the liquid-phase hydrogenolysis of
benzylic alcohols over Pd/C catalyst.7 The mechanism was
proposed as an SN2 type for primary alcohols, whereas an SN1
type for tertiary alcohol. Their work showed great importance
and provided a sounder basis for further research in benzylic
benzylic groups over a self-prepared Pd/C catalyst using Pd
acetate as a metal precursor.8 The results showed the
possibilities of hydrogenolysis in methanol under mild
conditions, which were successfully demonstrated with
numerous examples. In other research, a solvent screening
of the selective hydrogenolysis of benzylic compounds over
SiliaCat Pd(0) was studied by Pandarus and coworkers.9 Their
results showed that the reaction rates were influenced strongly
by the solvents and in declining order were methanol >
ethanol > tetrahydrofuran > hexane & ethyl acetate. However,
there is still lack of information regarding using green solvents
for the hydrogenolysis reactions.
Nowadays, to dispose and recycle solvents could be a
serious problem due to some of their properties including
volatile, flammable, explosive, harmful to the environment
and toxic to human beings. For those reasons, searching green
alternatives for traditional organic solvents becomes a top
priority in sustainable chemistry. In addition, several chemical
reaction rates were found to be significantly enhanced in green
solvent systems including CO2-expanded liquids (CXLs) and
compressed CO2/water.10–16 The physical and chemical proper-
ties of solvents could be adjusted by the addition of CO2. By
adjusting the temperature and pressure to control the amount
of dissolved CO2, the physical properties such as volume,
density, viscosity, mass transfer resistance and solvating power
of CXLs can be varied. Hao et al. had demonstrated the
hydrogenation of styrene, citral and nitrobenzene by using
Department of Chemical Engineering, National Tsing Hua University, No. 101,
Section 2, Kuang-Fu Road, Hsinchu 30013, Taiwan. E-mail: cstan@mx.nthu.edu.tw
Electronic supplementary information (ESI) available. See DOI: 10.1039/
3
c3ra41717d
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solvents has never been reported before. Therefore, the
purpose of this work is to provide a systematic study on the
superiority of CXLs and compressed CO2/water in the
hydrogenolysis of benzylic alcohols and their derivatives.
Three types of benzylic alcohols including primary, secondary
and tertiary were used for comparison in this study. Moreover,
the corresponding aldehyde and ketone of the benzylic
compounds were also selected for the reaction as further
applications.
Scheme 1 The formation of (a) methylcarbonic acid from methanol and CO2
and (b) carbonic acid from water and CO2.
CO2-expanded cyclohexane as the solvent.10 Their work
showed that the reaction rate could be accelerated and the
product selectivity could be adjusted in the CXLs. Akien et al.
also reported the debenzylation of 1,4-dibenzyloxybenzene in
CO2-expanded tetrahydrofuran.11 Although the reaction rate of
debenzylation was decreased by the addition of CO2, the
selectivity of intermediate could be enhanced and the yield of
by-products could be avoided in CXL.
Experimental
Materials
Palladium on activated carbon (5% Pd/C, Sigma-Aldrich),
benzyl alcohol (C7H8O; 99%, Sigma-Aldrich), 1-phenylethanol
(C8H10O; 97%, Alfa Aesar), 2-phenyl-2-propanol (C9H12O;
98%+, Alfa Aesar), benzaldehyde (C7H6O; 99%+, Alfa Aesar),
acetophenone (C8H8O; 99%, Sigma-Aldrich), diphenylmetha-
nol (C13H12O; 99%, Alfa Aesar), methanol (CH4O; >99.5%,
ECHO Chemical, Taiwan), tetrahydrofuran (C4H8O; >99%,
Macron), cyclohexane (C6H12; 99.9%, TEDIA), diethyl ether
(C4H10O; 99%, ECHO Chemical, Taiwan) were all used as
received without further purification. Carbon dioxide (99.5%)
and hydrogen (99.98%) were purchased from Boclh Industrial
Gases, Taiwan.
On the other hand, when CO2 is dissolved in methanol and
water, methylcarbonic acid and carbonic acid could be
formed, respectively (Scheme 1).12 It could provide an acidic
environment for the reaction. Since CO2 is a reversible
acidification agent, the acid could be easily removed simply
by releasing the pressure. Surprisingly, the effective CO2
pressure is much lower than the critical pressure of CO2
(7.38 MPa) which means supercritical condition is not
required. Eckert’s group used CO2-expanded methanol (CX-
methanol) in the cyclohexanone acetal formation.13 Their
results indicated that methylcarbonic acid could act as a self-
neutralizing acid catalyst. A maximum enhancement factor of
reaction rate was observed under a CO2 pressure between 1
and 4 MPa. Fan et al. found that the negative effect of nitrogen
compounds could be eliminated by using compressed CO2/
water as the solvent in the hydrocracking of anthracene.14 The
acidic environment of compressed CO2/water could neutralize
the basic nature of compounds, therefore enhanced the
hydrocracking efficiency. Arai’s and Shirai’s groups found
that using compressed CO2/water as the solvent could
magnificently enhance the hydrogenation rate of nitrobenzene
and acetophenone, respectively.15,16 Their results also showed
that the compressed CO2/water could be more efficient as
compared to CXLs. Until now, the utilization of CXLs and
compressed CO2/water is still very limited in the literature.
Thus, to discover more extended examples of using CXLs and
compressed CO2/water as the solvents in reaction is our
primary goal.
Catalytic hydrogenolysis
The experiments for the hydrogenolysis in compressed CO2/
water of benzylic alcohols and their derivatives were imple-
mented in a semi-batch autoclave. In a typical trial, a mixture
of 50 mg of the Pd/C catalyst, 10 g of the benzyl alcohol and 50
g of deionized water were loaded into the high pressure
autoclave. A CO2 pressure of 1 MPa was first introduced into
the cell and then followed by a H2 pressure of 1 MPa for a total
pressure of 2 MPa. The reaction was set at a temperature of 323
K for a reaction time of 3 h. After the reaction, the system was
cooled in an ice bath and the samples were extracted with
diethyl ether. The products were then analyzed by a gas
chromatography-mass spectrometer (GC-MS; HP 5890 II/
Micromass Trio-2000) and gas chromatography-flame ioniza-
tion detector (GC-FID; Agilent GC6890). A dimethylpolysilox-
ane based GC capillary column (Agilent HP-5MS) was used.
The injector and the detector temperatures were set at 523 K
and 533 K, respectively. A temperature program was employed
for analysis starting at 323 K (hold for 5 min), followed by a 10
K min21 program rate to 473 K (hold for 10 min).
In our previous report, it was found that the hydrogenation
of benzyl alcohol had two reaction pathways, which include
the hydrogenation of aromatic ring to yield cyclohexenemetha-
nol and cyclohexanemethanol, and the hydrogenolysis of C–O
bond to yield toluene and methylcyclohexane.17,18 When using
Ru metal as the catalyst, C–O bond cleavage was suppressed,
and therefore, the hydrogenation of benzyl alcohol could
obtain high product selectivity toward cyclohexyl products. To
the best of our knowledge, the hydrogenolysis of benzylic
alcohols using CXLs and compressed CO2/water as the
Results and discussion
Hydrogenolysis of benzylic alcohols in CO2-expanded
methanol
In this work, in order to discuss the possibility of C–O bond
cleavage by using CXLs and compressed CO2/water as the
solvents, Pd/C catalyst was used in all of the following
experiments. Since Pd metal under mild condition is more
suitable for bond cleavage instead of ring hydrogenation,
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responsible for the significant improvement of the reaction
conversion. For comparison, different CXL systems were also
tested and the results are provided in the ESI, Table S1. The
3
promotion effect of CO2 was not observed in CO2-expanded
tetrahydrofuran and CO2-expanded cyclohexane. The acidity of
these solvents would remain the same since CO2 could not
react with these solvents.
Scheme 2 Hydrogenolysis of benzylic alcohols.
At CO2 pressure of 3 MPa, the dilution effect of CO2 would
start to be more dominating. Since the volume expansion of
methanol increased with the CO2 pressure, the catalyst
concentration in the liquid phase should be diluted. Also,
the (PR) of hydrogen decreased from 50% to 25%. These
physical changes could cause the decrease of the gas-to-liquid
mass transfer rate.17,19 Moreover, dissolving CO2 into the
solvent could manipulate the polarity of the solvent. In this
case, the highly polar methanol (dielectric constant = 30)
would become less polar and, therefore, causing a decrease of
the dissociation constant of methylcarbonic acid.13 It is
speculated that the pH value at CO2 pressure of 3 MPa would
be much higher than that at CO2 pressure of 1 MPa. Therefore,
the conversions at CO2 pressure of 3 MPa were all significantly
lower than those at CO2 pressure of 1 MPa. This up-and-down
behaviour of the reaction conversion in CX-methanol was also
observed in the literature.13 Hence, it may suggest that low-
pressure CO2 (1 MPa) was an effective additive for the
hydrogenolysis reaction in methanol.
hydrogenolysis was the main reaction (Scheme 2). Fig. 1 shows
the influence of CO2 pressure on the hydrogenolysis of
benzylic alcohols in CX-methanol. Benzyl alcohol, 1-pheny-
lethanol and 2-phenyl-2-propanol were chosen as the reactant
substrate, and the corresponding products were toluene,
ethylbenzene and cumene, respectively. In the absence of
CO2, it was found that the reaction rates in declining order
were primary, secondary and tertiary benzylic alcohol, which is
in good agreement with the literature.7 Besides, the results
also showed that adding 1 MPa CO2 into methanol could
adequately enhance the reaction rate of the benzylic alcohols.
Note that in the case of 1-phenylethanol, the reaction
conversion had nearly tripled from 23% to 63%. When
doubling the hydrogen pressure to 2 MPa, the reaction
conversions remained very similar suggesting that the effect
of CO2 pressure was much more sensitive than that of
hydrogen pressure (see ESI, Fig. S1). Surprisingly, when
3
increasing the CO2 pressure from 1 to 3 MPa, the conversions
of the benzylic alcohols all decreased. The detailed reasons are
given below. At CO2 pressure of 1 MPa, the viscosity and
density of methanol would decrease since the volume
expanded.19–21 These physical changes could cause the
enhancement of mass transfer rate and hydrogen solubility,
and the reduction of diffusion resistance between the gas–
liquid interphase.22,23 In addition, when CO2 is dissolved in
methanol, methylcarbonic acid could be formed.12 In our case,
the proton dissociated from methylcarbonic acid could
protonate the hydroxyl group of the alcohol, resulting in a
better leaving group (water leaving group –OH2+) for C–O bond
cleavage.24 This acidic property of CX-methanol would be
Hydrogenolysis of benzylic alcohols in compressed CO2/water
Recently, water has been pointed out that it could accelerate
chemical reactions in which hydrogen was acted as
a
reactant.25 Therefore, water was also chosen as a basis in this
section to examine the promotion effect of CO2 in hydro-
genolysis. Fig. 2 provides the results of benzyl alcohol
hydrogenolysis in compressed CO2/water. The conversion of
benzyl alcohol increased steadily with the addition of CO2
pressure from 0 MPa to 1 MPa, and maintained consistently
after 1 MPa. The results showed a clear and strong relation-
ship between the conversion and the CO2 pressure. It is well-
known that carbonic acid could be formed in compressed CO2/
Fig. 2 Influence of CO2 pressure on the conversion of benzyl alcohol in
compressed CO2/water. Conditions: benzyl alcohol 10 g, Pd/C 50 mg, 3 h, water
50 g, H2 1 MPa, 323 K, stirring 1000 rpm. (The pH values of compressed CO2/
water were estimated according to ref. 26.)
Fig. 1 Influence of CO2 pressure on the conversion of benzylic alcohols in CO2-
expanded methanol. Conditions: substrate 2 g, Pd/C 25 mg, 1 h, methanol 50 g,
H2 1 MPa, 323 K, stirring 1000 rpm.
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Table 1 Hydrogenolysis of 1-phenylethanol and 2-phenyl-2-propanol in compressed CO2/water
Entry
Substrate
Product
CO2 pressure (MPa)
Time (hr)
Conv. (%)
Selectivity (%)
1a
2a
0
1
3
3
57
78
100
100
3b
4b
5c
0
1
0
5
5
5
74
93
88
64, 32, 4
91, 6, 3
74, 19, 7
a
b
1-Phenylethanol 10 g, Pd/C 50 mg, water 50 g, 323 K, H2 1 MPa, stirring 1000 rpm. 2-Phenyl-2-propanol 1 g, Pd/C 25 mg, water 50 g, 323 K,
c
H2 1 MPa, stirring 1000 rpm. 2-Phenyl-2-propanol 1 g, Pd/C 25 mg, acetic acid 50 g (0.005 M, pH = 3.6), 323 K, H2 1 MPa, stirring 1000 rpm.
water. According to a report of Roosen and coworkers, the pH
value of 1 MPa CO2 dissolved into water at 323 K was
approximately 3.6.26 However, increasing CO2 pressure from 1
MPa to 3 MPa did not increase the reaction conversion further.
This phenomenon may due to that no significant changes in
pH value upon increasing CO2 pressure over 1 MPa.26 To
further examine the effect of pH value, an additional
experiment of benzyl alcohol hydrogenolysis was performed
desorption behaviors of substrate on the metal active site.27
Due to the high resonance of aromatic ring, ring hydrogena-
tion was considered as the most difficult type of hydrogena-
tion. When the substrate was adsorbed on the active site, it
would be more favorable for hydrogenolysis than ring
hydrogenation. However, the hydrogenolysis products could
be adsorbed on the catalyst with sufficient time, and the deep
hydrogenation of aromatic ring would possibly take place. In
this situation, introducing CO2 might help the hydrogenolysis
products to desorb from the active site and avoid deep
hydrogenation. On the other hand, acetic acid could only serve
as a reaction promoter but lack the ability to adjust the
desorption rate. Hence, an increase of cumene selectivity
could only be observed in compressed CO2/water instead of
acetic acid.
in
a compressed CO2/NaHCO3 buffer solution (0.8 M).
According to the same reference, this buffer solution could
maintain the pH value above 6 while 3 MPa CO2 was added
into the system. However, no reaction occurred in the
compressed CO2/buffer solution system. This phenomenon
was not very surprising since the hydrogenolysis reaction is
mainly enhanced by the acidic environment. Thus, the
decrease in pH value should be the main positive factor.
The experiments were also carried out for the hydrogeno-
lysis of 1-phenylethanol and 2-phenyl-2-propanol by using
compressed CO2/water as the solvent. The results are
presented in Table 1. It shows that the reaction conversions
followed the same trend with benzyl alcohol. Nevertheless, two
ring-hydrogenated by-products, 2-cyclohexyl-2-propanol and
isopropylcyclohexane, were detected by GC-MS during the
hydrogenolysis of 2-phenyl-2-propanol, which is a tertiary
benzylic alcohol, in water and compressed CO2/water. Note
that this is not the case when using methanol and CX-
methanol as the solvents in the hydrogenolysis of 2-phenyl-2-
propanol. Moreover, it was observed that the ring hydrogena-
tion was suppressed while the system contained pressurized
CO2. Similar results of inhibition towards ring hydrogenation
in CXL were reported in the literature. Unfortunately, the
explanation of inhibition is still unclear.11 Therefore, it
deserves further discussion. An additional run was designed
for the examination of CO2 effect. The reaction environment
was controlled in an acetic acid solution with the same pH
value of compressed CO2/water. The results showed that the
reaction conversions of 2-phenyl-2-propanol could also be
elevated in acetic acid. It could confirm the aforementioned
statement that CO2 could act as an acid in the hydrogenolysis
of benzylic alcohols. Meanwhile, it should be noticed that the
selectivity of cumene was much lower in acetic acid than in
compressed CO2/water. In this situation, it is reasonable to
propose that CO2 might interfere with the adsorption and
Effect of solubility in compressed CO2/water
Since benzylic alcohols are miscible in methanol but less
soluble in water, the solubility of the substrates needs to take
into consideration while using compressed CO2/water as the
solvent. The solubility of the benzylic compounds in water
were reported as 47.9 g L21 for benzyl alcohol (323 K),28 14.7 g
L
21 for 1-phenylethanol (298 K)29 and 6.1 g L21 for 2-phenyl-2-
propanol (298 K).30 It was observed that the reaction mixture
could form a four-phase (water/substrate/CO2 + H2/catalyst)
system in the above experimental runs and up to 30% of
benzylic alcohols were dissolved in water. To realize the effect
of solubility, saturated solutions of benzylic alcohols were also
tested in this study. With all the substrate/catalyst molar ratio
and operating conditions maintained, the reaction mixtures
were precisely controlled at a three-phase system (aqueous
solution/CO2 + H2/catalyst). The results are given in Table 2,
which shows a similar trend as compared to the four-phase
systems in compressed CO2/water. It should be noted that all
the conversions were lower than those of the four-phase
conditions since the catalysts were diluted in the three-phase
conditions. These outcomes suggested that the excess amount
of substrate may not have negative effects and possible on-
water reactions may occur.31,32 In other words, the solubility of
benzylic alcohols in compressed CO2/water may not be the
limitation for hydrogenolysis. Similar results were also
discovered in the literature referring to the hydrogenation of
acetophenone.14 On the other hand, since the hydrogenolysis
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Table 2 Hydrogenolysis of saturated concentration of benzylic alcohols in compressed CO2/water (three-phase condition)a
Substrate
weight (g)
Water
weight (g)
Substrate/catalyst
molar ratio
CO2 pressure
(MPa)
Entry
Substrate
Pd/C (mg)
Time (hr)
Conv. (%)
1
2
3
4
5
6
Benzyl alcohol
Benzyl alcohol
1-Phenylethanol
1-Phenylethanol
2-Phenyl-2-propanol
2-Phenyl-2-propanol
2.4
2.4
2.0
2.0
0.4
0.4
50
50
100
100
60
12
12
10
10
10
10
3925
3925
3475
3475
311
0
1
0
1
0
1
3
3
3
3
5
5
52
54
23
34
48
79
60
311
a
3 h, 323 K, H2 1 MPa, stirring 1000 rpm.
products are insoluble in water, they could be easily separated
out and therefore the reaction system could be considered as
an ideal green reaction system. Under these circumstances,
increasing the amount of substrates could meet the require-
ment for process scale-up. And for sustainable and green
chemistry, reducing the ratio of water would also be
appreciated since water is a valuable natural resource.
a series reaction, which involved with a CLO reduction
followed by a C–O bond cleavage.34 Although Hiyoshi et al.
had reported the effective hydrogenation of acetophenone in
compressed CO2/water, the occurrence of hydrogenolysis was
not discussed in their study.14 Therefore, this work provides a
further demonstration on hydrogenolysis of aromatic aldehyde
and aromatic ketone in compressed CO2/water. The results
shows that the yield of toluene and ethylbenzene from
benzaldehyde and acetophenone, respectively, could be both
increased in compressed CO2/water as compared to water.
Note that the yield of ethylbenzene could be almost doubled by
the addition of 1 MPa CO2 (see entry 4 and entry 6 of Table 3).
The results suggested that compressed CO2/water could be a
suitable solvent for the hydrogenolysis of aromatic aldehyde
and ketones.
On the other hand, from the given results, it could be seen
that the secondary benzylic alcohol was the most susceptible
substrate in the compressed CO2/water systems. Since the
hydrogenolysis reaction rate of the primary benzyl alcohol was
rapid enough, the promoting effect of CO2 was limited.
Moreover, tertiary benzylic alcohol is nearly insoluble in water.
Therefore, the interaction between the substrate and the
solvent is also limited. By summarizing these results, we could
Hydrogenolysis of other benzylic compounds in compressed
CO2/water
Additional examples were demonstrated for the hydrogeno-
lysis of benzylic derivatives in compressed CO2/water, and they
are listed in Table 3. Benzaldehyde, acetophenone and
diphenylmethanol were chosen as the model compounds in
this section. First, for the hydrogenolysis of aromatic aldehyde
and aromatic ketone, the Clemmensen reduction, which
requires mineral acid and amalgamated zinc, was usually
applied for the reduction to alkanes.33 However, the usage of
strong acid and mercury compounds is not popular nowadays
since it could cause environmental issues. In this case, using
CO2 as a weak acidic source in hydrogenolysis could provide
an alternative route for aldehyde and ketone reduction. The
hydrogenolysis of aldehyde and ketone could be considered as
Table 3 Hydrogenolysis of other derivatives in compressed CO2/watera
Entry
1
Substrate
Product
CO2 pressure (MPa)
0
Time (hr)
13
Yield (%)
25,75
2
3
4
5
6
1
0
0
1
1
13
16
48
16
48
7,93
75,25
61,39
54,46
25,75
7b
8b
0
1
1
1
48
71
a
b
Substrate 10 g, Pd/C 50 mg, water 50 g, 323 K, H2 1 MPa, stirring 1000 rpm. Substrate 1 g, Pd/C 25 mg, water 50 g, 343 K, H2 1 MPa,
stirring 1000 rpm.
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8 F.-X. Felpin and E. Fouquet, Chem.–Eur. J., 2010, 16,
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9 V. Pandarus, F. Beland, R. Ciriminna and M. Pagliaro,
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point out that secondary benzylic alcohols could benefit the
most in reaction conversions with the addition of CO2. To
support this statement, another secondary benzylic alcohol,
diphenylmethanol, was verified and the results are in good
agreement which are shown in entries 7 and 8 of Table 3.
11 G. R. Akien, J.-C. Legeay, A. Wells and M. Poliakoff, Org.
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Conclusions
The hydrogenolysis of benzylic alcohols and their derivatives
were successfully demonstrated in CX-methanol and com-
pressed CO2/water. With the addition of low-pressure CO2 (1
MPa), the reaction conversions of primary, secondary and
tertiary benzylic alcohols could be adequately increased in the
CO2 promoted systems. The formation of the methylcarbonic
acid and carbonic acid helped catalyzed these hydrogenolysis
reactions. It is noteworthy that secondary benzyl alcohol was
the most susceptible substrate in the CO2 promoted systems.
Furthermore, the CO2 promoted systems could also be applied
in the hydrogenolysis/deoxygenation of aromatic aldehyde and
aromatic ketone. With the encouraging results in this study,
we believed that applying these green solvent systems into
refinery process for bio and petroleum chemicals could be very
promising.
20 X. Xie, J. S. Brown, D. Bush and C. A. Eckert, J. Chem. Eng.
Data, 2005, 50, 780–783.
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