Carbon dioxide-expanded liquid substrate phase: An effective medium for selective hydrogenation of cinnamaldehyde to cinnamyl alcohol

Article abstract of DOI:10.1039/b408434a

It has been shown that CO₂-expanded cinnamaldehyde liquid phase is a unique and effective medium for cinnamaldehyde hydrogenation to cinnamyl alcohol, due to interactions between the C=O group of the substrate and CO ₂ molecules and increased solubility of H₂.

Full text of DOI:10.1039/b408434a

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Carbon dioxide-expanded liquid substrate phase: an effective medium
for selective hydrogenation of cinnamaldehyde to cinnamyl alcohol
Fengyu Zhao,a,b Shin-ichio Fujita,a Jianmin Sun,a Yutaka Ikushimab,c and Masahiko Arai*a,b
a Division of Materials Science and Engineering, Graduate School of Engineering, Hokkaido University,
Sapporo 060-8628, Japan. E-mail: marai@eng.hokudai.ac.jp; Fax: 181-11-7066594;
Tel: 181-11-7066594
b CREST, Japan Science and Technology Agency (JST), Saitama 332-0012, Japan
c Supercritical Fluid Research Center, National Institute of Advanced Industrial Science and Technology,
Sendai 983-8551, Japan
Received (in Cambridge, UK) 4th June 2004, Accepted 29th July 2004
First published as an Advance Article on the web 7th September 2004
It has been shown that CO2-expanded cinnamaldehyde liquid
and that of Ru complex 0.075 or 0.15 mmol, the reaction system
was in a single phase and the selectivity of COL was smaller than
30% (entries 1, 2). When 0.68 mmol of Ru complex was used at a
higher CO2 pressure of 16 MPa, the total CAL conversion
increased to 55% while the COL selectivity did not change (entry 3).
Under the conditions for this run, the system was in a two-phase
mixture including a small volume of CAL-rich liquid phase. In
addition, CAL hydrogenation was also examined under solvent-
less conditions and in high pressure N2 and CO2 under two-phase
conditions with using a large amount of CAL of 7.5 mmol (entries
4–7). Under the solvent-less conditions, a high COL selectivity of
88% was obtained (entry 4). When the CAL liquid phase was
pressurized by 8.5 MPa N2 (entry 5), the total conversion and COL
selectivity did not change compared to these obtained under
solvent-less conditions. However, 8.5 MPa CO2 was used instead of
N2, the total conversion was enhanced to 24% from 11% and the
COL selectivity to 91% from 88% (entry 6). When CO2 pressure
was further raised, the total conversion was further enhanced and
the COL selectivity was also improved, which was 98% at 16 MPa
(entry 7) in which the reaction mixture included a CO2-rich gas
phase and a large volume of CAL liquid phase dissolving CO2
(CO2-expanded liquid phase). Thus, the influence of phase
phase is a unique and effective medium for cinnamaldehyde
hydrogenation to cinnamyl alcohol, due to interactions
between the CLO group of the substrate and CO2 molecules
and increased solubility of H2.
The use of supercritical carbon dioxide (scCO2) as an environmen-
tally benign solvent for organic synthetic reactions is currently
receiving much attention. However, a drawback of scCO2 prevents
its utilization in homogeneous catalytic synthesis, due to its low
ability to dissolve organometallic complexes. Some attention has
been paid to enhancing the solubility of metal complexes in scCO2
by using ligands with an affinity for CO2.1,2 Is the homogeneous
phase really efficient for all the catalytic reactions in scCO2? It was
reported that insoluble rhodium complexes exhibited high activity
and selectivity for hydroformylation of 1-hexene in scCO2,3 and a
faster reaction rate was obtained in two-phase than in single-phase
for a-pinene hydrogenation in scCO2 with Pd/C catalysts.4
Moreover, scCO2 was reported to be dissolved in organic liquid
substances and, in some cases, the solubility of CO2 can approach a
value much higher than 80 mol% in the vicinity of mixture critical
point. The dissolution of CO2 in an organic solvent is also
accompanied by a significant volumetric expansion of the liquid
phase. Subramaniam and Busch reported that CO2-expanded
CH3CN solvent is an effective reaction medium for cyclohexene
oxidation with TPP/FeCl catalyst.5,6 And an increase in enantios-
electivity has recently been reported for asymmetric hydrogenation
of atropic acid in CO2-expanded [bmim]PF6 phase.7 In the present
communication, we will show that CO2-expanded cinnamaldehyde
(CAL) liquid phase using no organic solvent is a significantly
effective medium for its selective hydrogenation to cinnamyl
alcohol (COL) (Scheme 1) and that this is due to interactions
between the substrate and CO2 molecules, confirmed by in situ
FTIR, and increased H2 concentration in the liquid phase.
Table 1 shows the results of CAL hydrogenation and phase
behaviour observations.{ When the quantity of CAL was 0.2 mmol
Scheme 1 Hydrogenation of cinnamaldehyde in compressed CO2.
Table 1 Results of CAL hydrogenation with Ru complex in compressed CO2
Selectivity (%)
Entry
Ru/mmol
CAL/mmol
CO2 pressure/MPa
Conversion (%)
COL
HCAL
HCOL
Phase presenta
1
2
3
4
5
6
7
0.075
0.15
0.68
50
50
50
0.2
0.2
0.2
7.5
7.5
7.5
7.5
10
10
16
—
8.5 N2
8.5
16
21
22
55
11
12
24
54
25
23
24
88
88
91
98
48
51
50
9
10
7
27
26
26
3
2
2
G
G
Gb, L
Gc, L
Gd, L
G, L
G, L
50
1
1
a G: CO2-rich gas phase, L: CO2 expanded CAL liquid phase. b Very small droplets of CAL and/or catalyst were observed. c H2-rich gas phase.
d N2-rich gas phase. Reaction conditions: bis(pentafluorophenyl)phenylphosphine/Ru ~ 3; H2 pressure, 4 MPa; temperature, 50 uC; reaction
time, 2 h. The Ru complex is completely soluble in CAL under these conditions used.
2 3 2 6
C h e m . C o m m u n . , 2 0 0 4 , 2 3 2 6 – 2 3 2 7
T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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Fig. 1 FTIR spectra of pure CAL (gas) and CAL dissolved in compressed
CO2 as well as in H2 and CO2 mixture (dotted line) at 50 uC.
Fig. 2 FTIR spectra of pure CO2 and mixture of CO2 and CAL at 50 uC.
2) and CAL–CO2 interactions (factor 3) for the high COL
selectivity. The interactions may make the carbonyl group of CAL
molecules more polar and reactive. It still remains unsolved
whether CO2 molecules have effects on the specific activity of the
Ru complex in the CO2-expanded CAL liquid phase (and in the
CO2-rich gas phase). For a catalyst of supported metal particles,
direct effects of CO2 on their properties were previously
suggested.14
behaviour was significant and the presence of high-pressure CO2
was requisite for selective CAL hydrogenation.
The present results demonstrate that larger conversion and
larger COL selectivity can be obtained for CAL hydrogenation in
CO2-expanded CAL liquid phase. A few factors should be taken
into account to explain these results. (1) CAL concentration.
Although CO2 and H2 are dissolved in the CAL phase, the CAL
concentration is still large in this phase. High CAL concentration
was previously reported to benefit the formation of COL with
heterogeneous catalysts.9,10 (2) H2 concentration. The concentra-
tion of H2 in the CO2-expanded CAL phase should be larger
compared with that in the absence of CO2 because a large quantity
of CO2 dissolved in the CAL phase favors the dissolution of H2. (3)
Compressed CO2. The mass transfer resistance and substrate
inhibition, if existed, should be reduced in CO2-expanded CAL and
scCO2 phases. Note that compressed CO2 increases the CAL
conversion and the COL selectivity but compressed N2 does not;
hence, hydrostatic pressure to the CAL liquid phase is insignificant.
So, direct effects of CO2 may be assumed, and this was examined
by FTIR.{ Fig. 1 shows FTIR spectra from 1800 to 1600 cm21 for
CAL molecules dissolved in different gaseous mixtures at 50 uC. The
spectrum (a) is a reference of gaseous pure CAL, indicating n(CLO)
at 1740 cm21. n(CLC) was too weak under the conditions used. The
absorbance in N2 was very weak due to a less solubility of CAL in
N2. In contrast, CAL can be more soluble in dense CO2, leading to
stronger absorption (b–e). The absorption of n(CLO) in dense CO2
red-shifted compared with gaseous CAL and n(CLC) appeared at
1630 cm21 in 8.5 MPa CO2. Further it was red-shifted a little with
increasing CO2 pressure. The dotted lines show the results in the
presence of 4.0 MPa H2, which has only a marginal effect.
Moreover, the absorbance of n(CLO) of CO2 split into two peaks at
8.5 MPa from a single peak at 1.5 MPa, and these peaks split
further to four peaks in the presence of CAL (Fig. 2), indicating the
existence of interactions between CAL and CO2 molecules in a gas
In summary, the CO2-expanded substrate liquid phase has
potential as a green reaction medium that does not need additional
organic solvent.
Notes and references
{ Hydrogenation of CAL in scCO2 was carried out at 50 uC in a 50 mL
high-pressure stainless steel reactor and a catalyst prepared from RuCl3
and bis(pentafluorophenyl) phenylphosphine was used. This ligand was
selected due to its high effectiveness in scCO2 and the method of Ru
complex preparation was described elsewhere.8 The phase behavior and
solubility of Ru complex and CAL in pure CO2 and in H2 1 CO2 were
examined by visual observations with a 10 mL high-pressure sapphire-
windowed view cell.8 The reactionwas conducted under different conditions,
at which the reaction system was homogeneous or heterogeneous.
{ The FTIR spectra were measured with 0.01 mL CAL using a 1.5 mL high
pressure cell in a path length of 4 mm at 50 uC.
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state. For a CO2 molecule, the carbon atom is partially positive and
…
the oxygen atoms are partially negative. Thus, a C–H
hydrogen bond should exist between CO2 molecules and carbonyl
O
11
…
groups even though it is weaker than O–H O bonds. The
specific interaction of CO2 molecules with Lewis base groups,
…
especially carbonyl groups, through a C–H O hydrogen bond has
been reported.11,12 In addition, interactions are known to exist
between CO2 and transition metal complexes, as reviewed by
Leitner.13 We may assume that the above mentioned interactions
exist in the CO2-expanded CAL liquid phase as well.
When the reaction occurs in the CO2-expanded CAL phase, the
high H2 concentration (factor 1) is chiefly responsible for the
increased CAL conversion and the high CAL concentration (factor
C h e m . C o m m u n . , 2 0 0 4 , 2 3 2 6 – 2 3 2 7
2 3 2 7