Noncatalytic Cannizzaro-type reaction of acetaldehyde in supercritical water

Article abstract of DOI:10.1246/cl.2003.310

In supercritical water at 400°C and 0.5 g/cm³ (37 MPa), acetaldehyde was found to be transformed without catalysts into ethanol and acetic acid through a Cannizzaro-type disproportionation reaction. No aldol-type condensation products were detected, and the disproportionation competed with the decomposition into methane. Ethanol was generated in excess to acetic acid, and carbon dioxide appeared as a product only in the presence of supercritical water. This reaction behavior is explained by considering that carbon monoxide is provided by the acetaldehyde decomposition and leads to the formation of formic acid as a reducing intermediate for acetaldehyde.

Full text of DOI:10.1246/cl.2003.310

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Chemistry Letters Vol.32, No.3 (2003)
Noncatalytic Cannizzaro-type Reaction of Acetaldehyde in Supercritical Water
Ã
Yasuharu Nagai, Chihiro Wakai, Nobuyuki Matubayasi, and Masaru Nakahara
Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011
(Received January 6, 2003; CL-030001)
In supercritical water at 400 C and 0.5 g/cm3 (37 MPa),
ꢀ
the reaction system was homogeneous in supercritical water
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acetaldehyde was found to be transformed without catalysts into
ethanol and acetic acid through a Cannizzaro-type disproportio-
nation reaction. No aldol-type condensation products were
detected, and the disproportionation competed with the decom-
position into methane. Ethanol was generated in excess to acetic
acid, and carbon dioxide appeared as a product only in the
presence of supercritical water. This reaction behavior is
explained by considering that carbon monoxide is provided by
the acetaldehyde decomposition and leads to the formation of
formic acid as a reducing intermediate for acetaldehyde.
whose density was adjusted to 0.5 g/cm by controlling the ratio
of the sample volume to the vessel volume at room temperature.
At the reaction condition, the initial concentration of acetalde-
hyde corresponds to 0.25 M. After 4 h of reaction 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 liquid and gas phases coexist in the sample vessel. The
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liquid and gas phases were separately subjected to H and
C
NMR measurements at room temperature (JEOLECA-400); a
solution of 1,3,5-trioxane in D2O was sealed in a capillary and
used as an external reference. The mass balance was confirmed to
be maintained from the total peak areas of both the liquid- and
gas-phase spectra. For comparison, the neat pyrolysis was also
Hot water including supercritical attracts much attention as a
novel solvent for chemical processes and as a green and earth-
friendly alternative to harmful organic solvents. Water in
hydrothermal conditions mixes well with organic compounds,
and often induces noncatalytic chemical reactions that proceed
only in the presence of acidic or basic catalysts at ambient
conditions. In a previous paper, we showed a simple chemical
reaction that the undissociated form of solvent water involved an
effective acid character under hydrothermal conditions. To
understand and control new and diverse types of hydrothermal
reactions, a systematic investigation about the behaviors of
functional groups is necessary. Aldehydes are important in
laboratorial and industrial processes as solvent and synthetic
starting materials, and are abundant in chemical products. In a
series of papers, we focus on the noncatalytic reaction behavior of
aldehydes in hot water including supercritical.
At ambient conditions, it is well-known that aldehydes
without a-hydrogen exhibit a disproportionation reaction in the
presence of a large amount of base catalyst like OH . In this
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examined at 400 C. In this case, no solvent was added and the
density of acetaldehyde was set to 0.25 M at the initial reaction
condition.
Figure 1 shows the proton spectra in the liquid phase before
and after the reaction. Before the reaction, both acetaldehyde and
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its hydrated form 1,1-ethanediol were detected. After the
reaction, three new peaks emerged and those of aldehyde and
diol disappeared; one peak at 2.0 ppm represents acetic acid, and
the others ethanol. This reaction corresponds to the disproportio-
nation of acetaldehyde. According to the classical Cannizzaro
reaction, the disproportionation reaction proceeds in the presence
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of a large amount of base catalyst. In supercritical water, on the
other hand, the disproportionation of acetaldehyde is seen to take
place without catalysts. No products of an aldol-type condensa-
tion reaction were observed. Thus, a Cannizzaro-type reaction,
rather than an aldol-type, is induced under our experimental
condition. This observation is in marked contrast to the well-
known Cannizzaro disproportionation at ambient conditions
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reaction, one molecule of aldehyde activated by OH reduces
another to primary alcohol and the former is oxidized to
carboxylic acid; the yields of the alcohol and acid are equal and
cannot exceed 50%. In contrast, Tsujino et al. found that a
Cannizzaro-type disproportionation of formaldehyde, which does
not have an a-hydrogen, proceeds in hydrothermal conditions
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without any base catalyst. In this communication, we report a
reaction of acetaldehyde in supercritical water. At ambient
conditions, an aldehyde with a-hydrogen is not accompanied by
the Cannizzaro reaction because the aldol reaction is dominant in
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the presence of base catalyst. The purpose of this work is to
clarify whether the dominant noncatalytic reaction of acetalde-
hyde having a-hydrogen is disproportionation or aldol condensa-
tion, or other reactions in supercritical water.
Acetaldehyde was used as received from Nacalai. Water was
distilled 3 times after ion-exchange. The solution of acetaldehyde
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in H2O at 0.5 M (M = mol/dm ) filled a quartz tube of 1.5 mm i.d.
The sample was sealed after the air in the reactor was replaced by
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Figure 1. The liquid-phase H-NMR spectra of acetaldehyde
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argon. It was then put into an electric furnace kept at 400 C; the
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in H2O. The reaction was performed for4 h at400 C anda water
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temperature was controlled within Æ1 C. In the sample vessel,
density of 0.5 g/cm .
Copyright ꢀ 2003 The Chemical Society of Japan

Chemistry Letters Vol.32, No.3 (2003)
311
which generally needs a large amount of base catalyst and is
overwhelmed by the aldol condensation for aldehyde with a-
hydrogen. The conversion of acetaldehyde after 4 h of thereaction
was 78%. The peak area of methyl proton of ethanol is ꢁ2:5 times
as large as that of acetic acid as shown in Figure 1, although the
acid/alcohol ratio by the classical Cannizzaro reaction is 1:1. The
excess production of ethanol is discussed below.
In Figures 2(a) and (b), are shown the proton and carbon
spectra for the gas phase of the sample system, respectively, after
the reaction. Figure 2(c) shows the carbon spectrum for the neat
Figure 3. Noncatalytic reaction pathway of acetaldehyde in
supercritical water.
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pyrolysis of acetaldehyde at 400 C. When no solvent water is
added, methane and CO were detected. It was substantiated,
furthermore, by the carbon NMR measurement without proton
irradiation that the yields of the two products are equal. This
means that methane and CO are produced through the thermal
decarbonylation of acetaldehyde.9 In supercritical water, in
contrast, methane and CO2 were detected as shown in Figures 2(a)
and (b) and CO and H2 were not. Methane was also seen in the
liquid-phase spectrum in Figure 1. In a separate experiment, we
confirmed that acetic acid was stable under the present experi-
mental condition. In other words, methane and carbon dioxide are
not the products of decarboxylation of acetic acid. The methane
production thus shows that decarbonylation of acetaldehyde
proceeds in the present supercritical condition. It is then expected
that carbon monoxide generated by decarbonylation is further
converted through a reaction with water to such a hydration
product as formic acid.10 According to Tsujino et al., formic acid
reduces formaldehyde to methanol and the former is oxidized to
pathway of acetaldehyde in supercritical water. This scheme is
also consistent with the excess ethanol formation in Figure 1. The
disproportionation reaction competes with the decarbonylation,
and ethanol is generated through two pathways.
In this communication, we demonstrated that acetaldehyde,
which is a representative aldehyde with a-hydrogen, exhibits a
Cannizzaro-type disproportionation in supercritical water. Fur-
thermore, we observed that the disproportionation reaction
proceeds in supercritical water for the aldehydes with longer
hydrocarbon chain, such as propion-, butyr-, and valeraldehyde.
This shows that the noncatalytic Cannizzaro-type disproportio-
nation is common to aldehyde in hydrothermal conditions. In
subcritical water, on the other hand, we found that acetaldehyde
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exhibits only an aldol-type reaction at temperature below 250 C.
The crossover temperature at which the Cannizzaro-type reaction
becomes more important than the aldol-type was seen to be about
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350 C. In subsequent papers, using in situ NMR measurements,
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carbon dioxide and H2O. The production of carbon dioxide and
we present detailed kinetic analysis of Cannizzaro, decarbonyla-
tion, and aldol reaction of acetaldehyde and other aldehydes in hot
water.
the absence of carbon monoxide can thus explain that formic acid
is a reaction intermediate and reduces acetaldehyde to ethanol.
Moreover, H2 was not detected. This shows that CO2 is not
produced through the water gas shift reaction of CO. In
consequence, Figure 3 is presumed to be the noncatalytic reaction
This work is supported by the Research Grant-in-Aid from
the Ministry of Education, Culture, Sports, Science, and
Technology (No. 10304047, 13440179, and 13640509) and by
the CREST (Core Research for Evolutional Science and
Technology) of Japan Science and Technology Corporation
(
JST).
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Figure 2. The gas-phase spectra at a reaction time of 4 h for
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acetaldehyde. (a) and (b) represent the H and C spectra,
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