Noncatalytic disproportionation and decarbonylation reactions of benzaldehyde in supercritical water

Article abstract of DOI:10.1246/cl.2004.622

In supercritical water at 400°C and 0.5 g/cm³ (37 MPa), benzaldehyde is decomposed into benzene and CO at a yield of 16% in 4 h; the latter is considered to be further converted into formic acid in the reaction condition. The decomposition competes against two types of disproportionation reactions. One is cross-disproportionation between benzaldehyde and formic acid, leading to the formation of benzyl alcohol and CO₂. The other is self-disproportion of benzaldehyde where benzyl alcohol and benzoic acid are equally generated. The weight of the cross-disproportionation is larger than that of the self-disproportionation. As a result, the yield (6.6%) of benzyl alcohol is ≈2.5 times as large as that of benzoic acid.

Full text of DOI:10.1246/cl.2004.622

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Chemistry Letters Vol.33, No.5 (2004)
Noncatalytic Disproportionation and Decarbonylation Reactions
of Benzaldehyde in Supercritical Water
ꢀ
Yasuharu Nagai, Nobuyuki Matubayasi, and Masaru Nakahara
Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011
(Received February 18, 2004; CL-040187)
ꢁ
3
In supercritical water at 400 C and 0.5 g/cm (37 MPa),
lytic effect is present for quartz. The sample was sealed after
the air in the reactor was replaced by argon. It was then put into
benzaldehyde is decomposed into benzene and CO at a yield
of 16% in 4 h; the latter is considered to be further converted into
formic acid in the reaction condition. The decomposition com-
petes against two types of disproportionation reactions. One is
cross-disproportionation between benzaldehyde and formic acid,
leading to the formation of benzyl alcohol and CO2. The other is
self-disproportion of benzaldehyde where benzyl alcohol and
benzoic acid are equally generated. The weight of the cross-dis-
proportionation is larger than that of the self-disproportionation.
As a result, the yield (6.6%) of benzyl alcohol is ꢂ2:5 times as
large as that of benzoic acid.
ꢁ
an electric furnace kept at 400 C; the temperature was control-
ꢁ
led within ꢃ1 C. In the sample vessel, the reaction system was
homogeneous in supercritical water whose density was set to
3
0.5 g/cm by controlling the ratio of the sample volume to the
vessel volume at room temperature. Under the reaction condi-
tion, the initial concentration of benzaldehyde corresponds to
0.25 M. After a reaction time of 4 h, the sample was removed
from the furnace quickly and quenched in a cold water bath; it
took less than 30 s for the sample to cool down. At this stage,
the aqueous, organic, and gaseous phases coexist in the sample
vessel. The aqueous and gaseous phases were separately subject-
1
13
ed to H and C NMR measurements at room temperature using
ECA-400 (JEOL). A solution of 1,3,5-trioxane in D2O was
sealed in a capillary and used as an external reference. The sam-
ple was then opened and the aqueous and organic phases were
recovered in a single phase in acetone-d6. The acetone solution
was used for the NMR measurement to quantify the products and
residual benzaldehyde. For comparison, the neat pyrolysis was
Hot water including supercritical one attracts much attention
recently as a green alternative to harmful organic solvents for
chemical processes.1 To control hydrothermal reactions of a
variety of organics in a earth-friendly manner, we need a system-
atic investigation on each functional group. Aldehydes are im-
portant in laboratorial and industrial processes as solvent and
synthetic starting materials. It exhibits the Cannizzaro (dispro-
portionation) reaction at ambient conditions; only in the pres-
ence of a large amount of base catalyst, it transforms into alcohol
and acid at the ratio of 1:1. Recently, we have found form-
aldehyde and acetaldehyde undergo two types of noncatalytic
disproportionation reactions with hot water; one is the self-dis-
proportionation between two aldehydes of the same kind, and
the other is the cross-disproportionation between formic acid
and formaldehyde or acetaldehyde. They are expressed as fol-
lows:
–10
ꢁ
also examined at 400 C. In this case, no solvent was added
ꢁ
and the density of benzaldehyde was set to 0.25 M at 400 C.
Figure 1 shows the proton spectrum for the aqueous phase
after the reaction of 4 h. Before the reaction, three peaks as-
signed to benzaldehyde were detected; the peaks at 7.59, 7.73,
and 7.92 ppm represent meta, para, and ortho protons of the
phenyl ring, respectively. After the reaction, new peaks of ben-
zyl alcohol (7.31–7.43 ppm), benzoic acid (7.50, 7.64, and 7.98),
and benzene (7.40 ppm) emerged. Toluene was also detected as a
minor product at 2.22 ppm for the methyl proton. The production
of benzyl alcohol and benzoic acid indicates that the self-dispro-
portionation reaction of benzaldehyde takes place without cata-
lyst in supercritical water. In Table 1, we summarize the yields
of the products. The conversion of benzaldehyde is ꢂ40%, and
the main product is benzene. It should be noted that the yield of
benzyl alcohol is ꢂ2:5 times as large as that of benzoic acid, al-
though the alcohol/acid ratio by the self-disproportionation re-
action is 1. This implies that benzyl alcohol production involves
1
1
3,6
2
R{CHO þ H2O ! R{CH2OH þ R{COOH
ð1Þ
R{CHO þ HO{CHO þ H2O ! R{CH2OH þ H2CO3 ð2Þ
Formic acid, HO–CHO, is considered to be produced via CO
generated by the thermal decarbonylation of aldehydes. Alcohol
is actually yielded in excess to acid, and for the case of acetalde-
hyde, moreover, the cross-disproportionation is dominant in the
alcohol production. The competing disproportionation reaction
scheme represented by Eqs 1 and 2 is expected to be common
to other aldehydes in hot water. Thus, the natural, next target
to disclose the two disproportionation reactions is benzaldehyde,
the simplest member of the aromatic aldehydes. Nevertheless,
the competitive reaction mechanism is not yet established by
4
,5
previous studies on benzaldehyde. The key parameter, the ra-
tio of alcohol to acid, was not determined there. The competitive
disproportionation reactions are confirmed here also for benzal-
dehyde.
Benzaldehyde (Nacalai Co., 98%) was used without further
purification. Water, used as solvent and reactant, was distilled 3
times after ion-exchanged. Benzaldehyde and water, in molar ra-
tio 1:111, were loaded in a quartz tube of 1.5-mm i.d. No cata-
1
Figure 1. The liquid-phase H spectrum for the products of benzalde-
ꢁ
3
hyde reaction for 4 h at 400 C and a water density of 0.5 g/cm .
Copyright Ó 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.5 (2004)
623
Table 1. Product yields of benzaldehyde reaction without and with
ꢁ
Ph CHO + H O
2
HCOOH addition; 400 C, reaction time of 4 h, and [Ph–CHO]0 =
0
Ph CH2OH + Ph COOH
self-
.25 M
disproportionation
Yield/%
Ph CHO
HCOOH
not added
HCOOH
addeda
Products
pyrolysis
CO
H O
Ph CHO + H2O
cross-
disproportionation
2
HCOOH
Ph CH OH + CO + H O
2 2 2
Ph–CHO
Ph–CH2OH
Ph–COOH
Ph–H
Ph–CH3
H2
61.7
6.6
2.6
15.8
0.3
42.1
10.3
1.5
13.5
2.7
Ph H
CO2 + H2
Figure 3. Reaction pathways of benzaldehyde in supercritical water.
ꢂ7
The concentration of added HCOOH is 0.25 M at initial
ꢂ112
acid. This is a clear indication of the larger contribution of the
cross-disproportionation than that of the self-disproportionation.
The importance of the cross-disproportionation is confirmed
by the increase in the yield of benzyl alcohol induced by the ex-
cess addition of formic acid. The addition of formic acid of
0.25 M increased the production ratio of alcohol/acid to ꢂ7
from ꢂ2:5. This means that formic acid acts as a reducing re-
agent to benzaldehyde. Thus, it is explained that the excess ben-
zyl alcohol in Figure 1 is generated through the cross-dispropor-
tionation reaction between benzaldehyde and formic acid as
a
condition.
another reaction pathway in addition to the self-disproportiona-
tion of benzaldehyde.
Figures 2a and 2b illustrate the proton and carbon spectra for
the gas phase of the reacted system, respectively. In Figure 2c,
the carbon spectrum for the neat pyrolysis products of benzalde-
ꢁ
hyde at 400 C is shown. When no solvent water is added, only
benzene and CO are produced equally by the decarbonylation re-
action. In supercritical water, in contrast, H2 and CO2 were de-
tected in Figures 2a and 2b and CO was not within our precision.
In a separate experiment, we confirmed that benzoic acid was
stable in the present experimental condition. In other words, nei-
ther benzene nor CO2 are produced by the decarboxylation of
benzoic acid. The benzene production thus arises exclusively
from the decarbonylation of benzaldehyde; as referred to in
Table 1, the decomposition to benzene and CO is dominant in
supercritical water. This suggests that the CO generated through
the decarbonylation is further converted by a reaction with water
represented by Eq 2. On the other hand, H is formed by the in-
2
1
0
dependent decarboxylation reaction of formic acid. In conclu-
sion, the reaction pathways of benzaldehyde in supercritical wa-
ter in the absence of catalyst can be shown in Figure 3.
Although the reaction of benzaldehyde in supercritical water
has ever been examined, there exist discrepancies with respect to
4
the reaction products and pathways. Funazukuri et al. reported
such products as benzyl alcohol, toluene, and benzene without
detecting benzoic acid and assumed the presence of CO. Ikush-
5
ima et al. considered only the self-disproportionation. In this
6
to formic acid. As shown in the previous communications, for-
communication, we have demonstrated that benzaldehyde ex-
hibits the decarbonylation and two types of disproportionation
reactions in supercritical water.
mic acid reduces formaldehyde and acetaldehyde to methanol
and ethanol, respectively, and is itself oxidized to carbon dioxide
and H2O.3 This is the cross-disproportionation with formic
acid, the hydroxyl ‘‘aldehyde’’. As can be seen in Table 1, the
yield of benzyl alcohol is ꢂ2:5 times larger than that of benzoic
,6
This work is supported by the Grant-in-Aid for Scientific
Research (Nos. 14540531, 15205004, and 15076205) from the
Japan Society for the Promotion of Science and the Grant-in-
Aid for Creative Scientific Research (No. 13NP0201) from the
Ministry of Education, Culture, Sports, Science and Technology,
Japan.
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Figure 2. The gas-phase spectra at a reaction time of 4 h for benzal-
1
13
dehyde. (a) and (b) represent the H and C spectra, respectively, for
ꢁ
0
3
13
the reaction in water at 400 C and 0.5 g/cm , and (c) stands for the
C
ꢁ
ꢁ
spectrum taken at 180 C for the pyrolysis at 400 C without solvent.
The external reference employed is 1,3,5-trioxane in D2O, and the
magnetic susceptibility is corrected.
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Published on the web (Advance View) April 24, 2004; DOI 10.1246/cl.2004.622