Palladium on carbon-catalyzed gentle and quantitative combustion of hydrogen at room temperature

Article abstract of DOI:10.1002/adsc.201300710

A quantitative and gentle oxidation (combustion) of hydrogen in the presence of oxygen in a variety of solvents was achieved under palladium on carbon (Pd/C)-catalyzed conditions at ordinary pressures and temperatures. A quantitative generation of water accountable for the consumed oxygen was observed. While hydrogen peroxide (H₂O₂), which would form as an intermediate, was extremely unstable under the Pd/C-catalyzed conditions to be converted into water with accompanying oxygen generation, the stability of H₂O₂ was found to be increased in cold (0 °C) trifluoroacetic acid, and the formation of 64% H₂O₂ based on the consumed oxygen could be detected. A mechanistic elucidation study revealed that the single electron transfer and generation of the hydroxyl radical are involved in the gentle combustion process. The reactive oxygen species generated during the process was effectively utilized for the chemical oxidation of sulfides and phosphines to afford the corresponding sulfoxides and phosphine oxides, respectively.

Full text of DOI:10.1002/adsc.201300710

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DOI: 10.1002/adsc.201300710
Palladium on Carbon-Catalyzed Gentle and Quantitative
Combustion of Hydrogen at Room Temperature
Yasunari Monguchi,a Takashi Ida,a Toshihide Maejima,a Takayoshi Yanase,a
Yoshinari Sawama,a Yasushi Sasai,b Shin-ichi Kondo,b and Hironao Sajikia,*
a
Laboratory of Organic Chemistry, Gifu Pharmaceutical University, 1-25-4 Daigaku-nishi, Gifu 501-1196, Japan
Fax : (+81)-58-230-8109; phone: (+81)-58-230-8109; e-mail: sajiki@gifu-pu.ac.jp
Laboratory of Pharmaceutical Physical Chemistry, Gifu Pharmaceutical University, 1-25-4 Daigaku-nishi, Gifu 501-1196,
b
Japan
Received: August 16, 2013; Revised: November 6, 2013; Published online: January 31, 2014
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300710.
Abstract: A quantitative and gentle oxidation (com-
bustion) of hydrogen in the presence of oxygen in
a variety of solvents was achieved under palladium
on carbon (Pd/C)-catalyzed conditions at ordinary
pressures and temperatures. A quantitative genera-
tion of water accountable for the consumed oxygen
was observed. While hydrogen peroxide (H2O2),
which would form as an intermediate, was extreme-
ly unstable under the Pd/C-catalyzed conditions to
be converted into water with accompanying oxygen
generation, the stability of H2O2 was found to be in-
creased in cold (08C) trifluoroacetic acid, and the
formation of 64% H2O2 based on the consumed
oxygen could be detected. A mechanistic elucida-
tion study revealed that the single electron transfer
and generation of the hydroxyl radical are involved
in the gentle combustion process. The reactive
oxygen species generated during the process was ef-
fectively utilized for the chemical oxidation of sul-
fides and phosphines to afford the corresponding
sulfoxides and phosphine oxides, respectively.
the sequential hydrogenation and oxidation of the 2-
alkylanthraquinone derivative consumes huge
a
amount of energy.[28] Most methods for the oxidation
of hydrogen gas in the literature require flowing (bub-
bling) or highly pressurized hydrogen and oxygen
gases in aqueous or alcoholic media in the presence
of a palladium catalyst,[1–22] and no quantitative con-
version of hydrogen or oxygen under ordinary pres-
sures and temperatures has been reported. Further-
more, the electron transfer process during the com-
bustion of hydrogen gas was never elucidated.
In this paper, we demonstrate an effective and
quantitative gentle combustion of hydrogen gas in or-
ganic and aqueous solvents using commercially avail-
able palladium on carbon (Pd/C) under ordinary pres-
sures and temperatures, involving the hydroxyl radical
during the reaction for the generation of H2O2 based
on the single electron transfer (SET) process, and an
effective in-situ application of the resulting reactive
oxygen species to organic synthesis as mild and transi-
ent oxidizing reagents.
A 17-mL test tube containing a suspension of 10%
palladium on carbon (Pd/C) in deuterated methanol
(CD3OD, 1 mL) was filled with hydrogen gas (16 mL)
and connected to an oxygen gas burette through flexi-
ble Tygon tubing (Figure 1). The smooth absorption
of oxygen gas through the gas burette under ordinary
pressures and temperatures was observed, and the
Keywords: heterogeneous catalysis; hydrogen; oxi-
dation; oxygen; palladium
The usual hydrogen-oxygen chemical reaction, i.e., nearly quantitative generation of water relative to the
combustion, is induced by ignition associated with the quantity of consumed oxygen was confirmed by the
liberation of an enormous amount of energy, such as 1H NMR analysis in CD3OD (Figure 2a). The amount
an explosion and flames, based on common sense. of generated water (solid line, closed diamond) was
The gentle chemical uniting of hydrogen with oxygen virtually identical to the theoretical quantity of water
has recently attracted much attention in various fields production (dashed line, closed square), which was
of chemistry due to the connection with the direct calculated based on the assumption that 2 equiv. of
synthesis of hydrogen peroxide (H2O2),[1–27] which is water are formed from the consumed oxygen, indicat-
widely used as an oxidant, bleach, and disinfectant, ing that hydrogen gas in CD3OD was effectively and
since its industrial manufacturing method based on quantitatively united with the oxygen in the presence
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of 10% Pd/C under ordinary temperatures and pres-
sures.[29,30] The gentle combustion of hydrogen gas
with oxygen gas could be achieved in a variety of sol-
vents, such as hydrocarbons, alcohols, trifluoroacetic
acid (TFA), and water; Figure 2b shows that the
oxygen gas was smoothly consumed in each sol-
vent.[31,32] Since hydrogen peroxide (H2O2), which
would be a possible intermediate leading to water,
was not detected in the filtrates of all samples after
the removal of the 10% Pd/C by iodometric analysis,
H2O2 would be quantitatively converted to water.
The stability of H2O2 in water, MeOH, and TFA in
the presence of the 10% Pd/C was then evaluated. A
mixture of 10% Pd/C and commercially obtained
aqueous H2O2, the concentration (24%, 26.9 mg in
100 mL) of which was determined by iodometry prior
to use, in each solvent was stirred at 08C, 258C and
408C for 5 min under an argon atmosphere. The re-
sidual quantity of H2O2 in the filtrates after removal
of the Pd/C and the consumed amount of 0.1M
Na2S2O3 for iodometric titration are shown in Table 1.
Although H2O2 was stable in all the solvents at 258C
without the 10% Pd/C (entries 1, 4, and 7), its stability
in the presence of the 10% Pd/C depended on the
type of solvents and temperatures.[33] While the H2O2
was readily decomposed at 258C in water and MeOH
(entries 3 and 6), the stability of H2O2 was somewhat
improved at 08C (entries 2 and 5). Furthermore, the
H2O2 was relatively stable in TFA (entries 8–10), and
almost no decomposition was observed at 08C for at
least 5 min (entry 8).
Figure 1. Reaction apparatus using a gas burette.
Table 1. Stability of H2O2 in the presence of 10% Pd/C
based on iodometry.
Entry
Solvent
T
[8C]
0.1M Na2S2O3
[mL][a]
Quantity of
H2O2 [mg]
1[b]
2
H2O
H2O
H2O
MeOH
MeOH
MeOH
TFA
TFA
TFA
25
0
25
25
0
25
25
0
25
40
14.5
8.3
0.5
15.3
3.2
0.7
16.6
15.9
10.2
5.5
24.95
14.28
0.86
26.33
5.51
3
4[b]
5
6
1.20
7[b]
8
28.57
27.36
17.55
9.46
9
10
TFA
[a]
The consumed amount of 0.1M Na2S2O3 (f=1.012)
during the iodometric titration.
A mixture of 24% H2O2 in each solvent was stirred at
258C without Pd/C.
Figure 2. Consumption of oxygen gas during the 10% Pd/C-
catalyzed oxidation of hydrogen gas. a) Generation of water
in CD3OD. b) Solvent effect.
[b]
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Palladium on Carbon-Catalyzed Gentle and Quantitative Combustion
strongly suggest the involvement of an SET process
during the reaction. Furthermore, it was clarified that
the oxygen atom in the generated water was derived
from the molecular oxygen by the detection of the in-
corporated 18O in the generated water when 18O-la-
belled oxygen gas (18O2) was used instead of the unla-
belled oxygen gas (see the Supporting Information).
Based on these experiments, the hydroxyl radical
and H2O2 are obviously generated during the reac-
tion. We then investigated the use of such reactive
oxygen species in organic synthesis (Table 2). When
1.8 equiv. of oxygen gas were introduced into a mix-
ture of 10% Pd/C and 4-methylthioacetophenone in
TFA at room temperature under a hydrogen atmos-
phere, the sulfide was selectively monooxidized to
give the corresponding sulfoxide in good yield
Figure 3. Detection of H2O2 as an intermediate.
The gentle Pd/C-catalyzed hydrogen–oxygen chemi- (entry 1).[35,36] The use of 18O2 instead of O2 led to
cal reaction was then carried out using TFA as a sol- smooth incorporation of an 18O atom into the 4-meth-
vent at 08C.[34] The resulting reaction mixture was ylthioacetophenone, indicating that the oxygen gas
quickly passed through the membrane filter under N2 functions as the oxygen source (entry 2, see also the
gas pressure to remove the Pd/C after the consump- Supporting Information). Either electron-sufficient
tion of 0.35 mL of oxygen during a 5 min period, and aryl alkyl sulfides (entries 3 and 4) or dialkyl sulfides
64% H2O2 based on the consumed oxygen gas was de- (entries 5 and 6) readily underwent the selective S-
tected in the filtrate by iodometry (Figure 3).
monooxidation to afford the corresponding sulfoxides
We next investigated the reaction mechanism of the without any formation of sulfones. Furthermore, the
gentle hydrogen combustion leading to the generation oxygenation method utilizing in-situ reactive oxygen
of water via H2O2 (Scheme 1) specifically about the species could be used for the synthesis of phosphine
involvement of the single electron transfer (SET) pro- oxides from trisubstituted phosphines (entries 7 and
cess, generation of the hydroxyl radical, and identifi- 8). Therefore, the present oxidation method would be
cation of the oxygen source. The uptake of oxygen expected to be a safe and effective protocol for the
gas to a 17-mL test tube containing a suspension of oxygenation of sulfides and phosphines using a short-
10% Pd/C (2 mmol, 0.3 mol% vs. H2) in CD3OD lived reactive oxygen species generated from only
(1 mL) under a hydrogen atmosphere (16 mL) at 1.8 equiv. of oxygen gas during the Pd/C-catalyzed
258C was significantly suppressed by the addition of gentle combustion of hydrogen gas under the appar-
tetracyanoquinodimethane (TCNQ, 20 mmol, 3 mol% ent hydrogen gas-dominated hydrogenation condi-
vs. H2) as a single electron scavenger, and only 2.5 mL tions.
of oxygen were absorbed during 11.5 min, although
In conclusion, hydrogen was gently and quantita-
16 mL oxygen were smoothly absorbed without tively reacted by the consecutive supply of oxygen in
TCNQ (see the Supporting Information). The genera- various solvents in the presence of commercially
tion of the hydroxyl radical was also confirmed by the available 10% Pd/C as a catalyst under ordinary pres-
ESR spectroscopy-spin trapping method using 5,5-di- sures and temperatures. Although the intermediary
methyl-1-pyrroline N-oxide (DMPO) as a trapping re- H2O2 in the reaction mixture was quite instable and
agent (see the Supporting Information). These results readily decomposed to water with the release of
Scheme 1. Proposed mechanism for the Pd/C-catalyzed combustion of hydrogen.
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Yasunari Monguchi et al.
Table 2. Oxygenation of sulfides and phosphines.
atmosphere (16 mL, 0.655 mmol, headspace) was stirred at
258C, and oxygen gas was introduced into the mixture
through a gas burette at 258C and ordinary pressures. After
a specific amount of oxygen was introduced (2.5, 5, 10, 16,
and 22 mL), the mixture was filtered by a well-dried and
small cotton-packed Pasteur pipette, then the amount of
1
water was measured by the H NMR analysis of the filtrate.
Solvent Effect of the 10% Pd/C-Catalyzed Oxidation
of Hydrogen Gas (Figure 2b)
A suspension of 10% Pd/C (2.1 mg, 2 mmol) in a variety of
solvents (1 mL) in an oven-dried 17-mL test tube under hy-
drogen atmosphere (16 mL, 0.655 mmol, headspace) was
stirred at 258C, then oxygen gas was introduced into the
mixture through a gas burette at 258C and ordinary pres-
sures. The time for the introduction of a specific amount of
oxygen was recorded.
Stability of Hydrogen Peroxide (H2O2) in the
Presence of 10% Pd/C (Table 1)
The concentration of commercially available aqueous 30%
H2O2 was determined to be 24% by iodometry using 0.1M
Na2S2O3 (f=1.012) for the titration. The 24% H2O2
(26.90 mg, 100 mL) was added to a suspension of 10% Pd/C
(2.1 mg, 2 mmol) in water, MeOH, or TFA (1 mL) in an
oven-dried 17-mL test tube under an argon atmosphere. The
mixture was stirred at 0, 25, or 408C for 5 min, then filtered
using a 0.45 mm membrane filter. The remaining H2O2 in the
resulting filtrate was measured by iodometry using 0.1M
Na2S2O3 (f=1.012) for the titration.
[a]
A small balloon of 18O2 was used instead of 1.8 equiv. of
Detection of H2O2 as an Intermediate (Figure 3)
O2.
[b]
10 mol% of 10% Pd/C were used.
50 mol% of 10% Pd/C and 9.2 equiv. of O2 were used.
[c]
A suspension of 10% Pd/C (2.1 mg, 2 mmol) in TFA (1 mL)
in
a 10-mL plastic syringe, which was connected to
a 0.45 mm membrane filter, under a hydrogen atmosphere
(12.1 mL, 0.4952 mmol, headspace) was stirred at 08C, then
oxygen gas (0.70 mL) was introduced into the mixture
through a gas burette in 5 min at 258C and ordinary pres-
sures. Half of the amount of oxygen (0.35 mL,
0.01432 mmol) out of the introduced oxygen should be con-
sumed for the reaction with hydrogen gas in the ratio of 1 to
1 for the production of H2O2. The mixture was filtered
(0.45 mm, Milipore Corporation, Billerica, MA, USA), and
the generated H2O2 was measured by iodometry of the re-
sulting filtrate. 1.81 mL of 0.01M Na2S2O3 (f=1.007) were
used for titration; the amount of the generated H2O2 was
calculated to be 0.3099 mg [9.113 mmol (1.007ꢁ0.01ꢁ1.81ꢁ
0.5)]. Since the amount of the consumed oxygen was
0.01432 mmol (1ꢁ0.35Ä0.082Ä298), the yield of the gener-
ated H2O2 based on the consumed oxygen was calculated to
be 64% (0.009113Ä0.01432ꢁ100).
oxygen, H2O2 was detected when the reaction was
carried out in cooled (08C) TFA. The involvement of
SET in the reaction process was clarified by the sig-
nificant suppression of the reaction progress in the
presence of a single electron scavenger (TCNQ), and
generation of the hydroxyl radical was directly dem-
onstrated by the use of DMPO as a spin trapping re-
agent in the ESR analysis. The reactive oxygen spe-
cies was utilized for the in-situ oxygenation of sulfides
and phosphines, and the corresponding sulfoxides and
phosphine oxides were obtained in good yields.
Experimental Section
General Procedure for the Oxygenation of Sulfides
and Phosphines (Table 2)
Generation of Water in CD3OD during the 10% Pd/
C-Catalyzed Oxidation of Hydrogen (Figure 2a)
A suspension of 10% Pd/C (2.1 mg, 2 mmol) and sulfide or
A suspension of 10% Pd/C (2.1 mg, 2 mmol) in CD3OD
phosphine (0.1 mmol) in TFA (1 mL) in an oven-dried 17-
(1 mL) in an oven-dried 17-mL test tube under a hydrogen
mL test tube under
a
hydrogen atmosphere (16 mL,
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Palladium on Carbon-Catalyzed Gentle and Quantitative Combustion
0.655 mmol, headspace) was stirred at 258C, then oxygen
gas (4.5 mL, 0.184 mmol) in a 10-mL plastic syringe was
connected to the test tube and introduced into the mixture.
After a specific time, CH2Cl2 (10 mL) was added, and the
mixture was filtered using a 0.45 mm membrane filter. The
filtrate was concentrated under reduced pressure, and the
residue was extracted with CH2Cl2 (10 mLꢁ2) and saturated
aqueous NaHCO3 (1 mL). The combined organic layers
were dried over MgSO4, filtered, and concentrated under re-
duced pressure, and the residue was purified by flash
column chromatography on silica gel (CHCl3:MeOH, 20:1)
to afford the corresponding oxygenated product.
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from the amount of produced water, which was mea-
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2 equiv. of water are formed from the consumed
oxygen. Theoretical amounts of consumed oxygen and
produced water were also calculated from the amount
of the introduced oxygen; one-third and two-third of
the amount of the introduced oxygen should be the
theoretical amounts of consumed oxygen and produced
water, respectively. Also see the Supporting Informa-
tion.
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presumably due to the low distribution of the active
palladium species on the activated carbon.
[35] The effect of the kinds of the palladium catalyst on the
oxygenation of 4-methylthioacetophenone in TFA was
closely parallel to that on the H2O2 generation men-
tioned in the ref.[32] 10% Pd/C was the most effective.
See the Supporting Information.
[36] The oxygenation of 4-methylthioacetophenone hardly
proceeded in cyclohexane or toluene, although H2O2
was relatively stable in both solvents as noted in the
ref.[33]
[33] H2O2 was more stable in less polar solvents, such as cy-
clohexane and toluene; a half amount of H2O2 re-
mained in cyclohexane or toluene even at 408C under
an argon atmosphere in the presence of 10% Pd/C. See
the Supporting Information.
[34] The H2O2 detection was carried out in cooled TFA in
a short reaction period in view of the stability of gener-
ated H2O2 in the presence of Pd/C.
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