Osmium-catalyzed selective oxidations of methane and ethane with hydrogen peroxide in aqueous medium

Article abstract of DOI:10.1002/adsc.200600438

Various transition metal chlorides including FeCl₃, CoCl ₂, RuCl₃, RhCl₃, PdCl₂, OsCl ₃, IrCl₃, H₂PtCl₆, CuCl₂ and HAuCl₄ were studied for the selective oxidations of methane and ethane with hydrogen peroxide in aqueous medium. Among the metal chlorides investigated, osmium(III) chloride (OsCl₃) exhibited the highest turnover frequency (TOF) for the formation of organic oxygenates (mainly alcohols and aldehydes) from both methane and ethane. For the OsCl ₃-catalyzed oxidation of methane with hydrgen peroxide, methyl hydroperoxide was also formed together with methanol and formaldehyde. The effects of various kinetic factors on the catalytic behavior of the OsCl ₃-H₂O₂ system were investigated, and TOF values of 12 and 41 h^-1 could be obtained for oxygenate formation during the oxidations of methane and ethane, respectively. In the presence of OsCl ₃, NaClO, NaClO₄ or NaIO₄ as oxidant was incapable of oxidizing methane and ethane to the corresponding oxygenates, and the use of tert-butyl hydroperoxide (TBHP) instead of H₂O₂ provided remarkably lower rates of formation of oxygenates. UV-Vis spectroscopic measurements suggested that OsCl₃ was probably oxidized into an Os(IV) species by H₂O₂ in aqueous medium, and the Os(IV) species might be involved in the oxygenation of methane or ethane. The result that the conversions of both methane and ethane to oxygenates were suppressed by the addition of a radical scavenger suggested that the reactions proceeded via a radical pathway.

Full text of DOI:10.1002/adsc.200600438

FULL PAPERS
DOI: 10.1002/adsc.200600438
Osmium-Catalyzed Selective Oxidations of Methane and Ethane
with HydrogenPeroxide inAqueous Medium
Qiang Yuan,a Weiping Deng,a Qinghong Zhang,a and Ye Wanga,*
a
State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen
University, Xiamen 361005, Peopleꢀs Republic of China
Fax : (+86)-592-218-3047; e-mail: wangye@xmu.edu.cn
Received: August 27, 2006; Revised: February 11, 2007
Abstract: Various transition metal chlorides includ- of OsCl3, NaClO, NaClO4 or NaIO4 as oxidant was
ing FeCl3, CoCl2, RuCl3, RhCl3, PdCl2, OsCl3, IrCl3, incapable of oxidizing methane and ethane to the
H2PtCl6, CuCl2 and HAuCl4 were studied for the se- corresponding oxygenates, and the use of tert-butyl
lective oxidations of methane and ethane with hydro- hydroperoxide (TBHP) instead of H2O2 provided re-
gen peroxide in aqueous medium. Among the metal markably lower rates of formation of oxygenates.
chlorides investigated, osmium(III) chloride (OsCl3) UV-Vis spectroscopic measurements suggested that
G
exhibited the highest turnover frequency (TOF) for OsCl3 was probably oxidized into an Os(IV) species
the formation of organic oxygenates (mainly alcohols by H2O2 in aqueous medium, and the Os(IV) species
and aldehydes) from both methane and ethane. For might be involved in the oxygenation of methane or
the OsCl3-catalyzed oxidation of methane with hy- ethane. The result that the conversions of both meth-
drgen peroxide, methyl hydroperoxide was also ane and ethane to oxygenates were suppressed by
formed together with methanol and formaldehyde. the addition of a radical scavenger suggested that the
The effects of various kinetic factors on the catalytic reactions proceeded via a radical pathway.
behavior of the OsCl3-H2O2 system were investigat-
ed, and TOF values of 12 and 41 hÀ1 could be ob-
tained for oxygenate formation during the oxidations Keywords: ethane; homogeneous catalysis; hydrogen
of methane and ethane, respectively. In the presence peroxide; methane; oxygenation; water solvent
Introduction
Shilov and co-workers[2,3] found that methane could
be converted into methanol and methyl chloride by a
Activation and catalytic oxidation of lower alkanes PtCl42À/PtCl6
system in an aqueous medium in
2À
into useful oxygenates have long been attractive and which Pt(II) was believed to function as an electro-
challenging research fields.[1–10] It is highly desirable phile for C H bond activation while Pt(IV) worked
À
to transform the inexpensive and abundant lower al- as an oxidant.[4,9] This earlier system has resulted in a
kanes, especially methane and ethane, which are the number of subsequent studies on the selective oxida-
main constituents of natural gas, into useful chemicals. tion of alkanes including methane via a route involv-
À
The activation of lower alkanes usually requires ing electrophilic C H activation and subsequent oxi-
severe conditions because of their chemical inertness, dative functionalization.[4,9,10] Sen and co-workers[11]
but carbon oxides (CO and CO2) may be formed reported that methane was oxidized and converted to
more easily than valuable oxygenates under such con- methyl trifluoroacetate (CF3COOCH3) by H2O2 in tri-
ditions. As compared with a heterogeneous catalytic fluoroacetic acid anhydride solvent in the presence of
system, which generally needs a much higher temper- Pd(II). The use of highly electrophilic Pd(II) and
ature (>5008C) to oxidize methane or ethane,[7,8]
homogeneous system with a highly active catalyst or withdrawing
a
Cu(II) complexes, which contained a strong electron-
ligand (hexafluoroacetylacetonate
reactive species may operate under milder reaction anion), further raised the efficiency for CF3COOCH3
conditions (<2008C),[9,10] and thus may be promising formation, and the best turnover frequency (TOF)
for the selective oxidations of methane and ethane to values reached 19.9 and 3.3 hÀ1 over the Pd- and Cu-
the corresponding oxygenates such as alcohols and al- based catalysts, respectively.[12] A Pd(II)–N-heterocy-
dehydes.
clic carbene complex provided a TOF of ca. 2 hÀ1 for
the conversion of methane to CF3COOCH3 in tri-
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Qiang Yuan et al.
FULL PAPERS
fluoroacetic acid anhydride when potassium peroxodi- acid anhydride solvent played a quite important role,
sulfate (K2S2O8) was used as an oxidant.[13] Periana and no products were observed when water was used
et al.[14,15] reported a highly efficient oxidative conver- as the solvent.[31] A diiron-substituted silicotungstate
sion of methane to methyl bisulfate with concentrated was capable of catalyzing the selective oxidation of
H2SO4 (SO3) in the presence of an Hg(II) or Pt(II) methane with H2O2 in aqueous medium to useful oxy-
complex. Periana et al.[16] recently also showed that genates dominated by methyl formate with a TON of
methane could be converted to methanol or methyl 8.4 after 48 h (TOF, 0.175 hÀ1).[33] Using trifluoroacetic
bisulfate in a strong acid medium such as triflic or sul- acid as the solvent, Yamanaka et al.[34,35] succeeded in
furic acid in the presence of Au
N
N
could not act as a catalyst and was converted to with Zn(0) in the presence of EuCl3 catalyst. Metha-
Au(0) when sulfuric acid was used as the oxidant. To nol was produced from methane with a TOF of ca.
avoid the formation of Au(0) and to raise the turn- 4 hÀ1 but the decomposition of trifluoroacetic acid to
over number (TON) to >1, a stronger oxidant, i.e., CO2 could not be avoided.[35]
H2SeO4 (or SeO3) was required. It is clear that, in all
these systems, the combination of an electrophile an environmentally benign oxidant such as H2O2 but
such as Pd(II), Hg(II) or Au
(III) [Au(I)] and a proper also a non-toxic solvent.[17,18] As solvent, none can
oxidant such as H2O2 or concentrated H2SO4 is very match water for many obvious reasons such as cheap-
important.
ness, safety and cleanness.[36] Moreover, several stud-
It is clear that “green” oxidation requires not only
AHCTREUNG
On the other hand, there is another major route for ies have shown that the organic solvents such as tri-
the selective oxidation of lower alkanes in which the fluoroacetic acid and acetonitrile may take part in the
metal compounds or complexes initially activate the reaction or be converted under the conditions used
oxidant to generate a reactive species for the transfor- for oxidation or oxidative functionalization of meth-
mation of lower alkanes into organic oxygenates.[2,3] ane.[30,35,37–39] Thus it is highly desirable to develop ef-
As environmentally benign and economically attrac- fective homogeneous catalysts for the selective oxida-
tive oxidants, H2O2 and O2 have attracted particular tion of methane or ethane with H2O2 in aqueous
attention,[17,18] and a large number of studies on cata- medium. To our knowledge, reports on such studies
lytic oxidation of alkanes with H2O2 or O2 have been are very scarce and the TOF for the reported catalyst
reported.[1–4,17–35] However, efficient homogeneous sys- is very low.[33] Recently, we have investigated the cata-
tems for the catalytic oxidation of methane or ethane lytic properties of a series of transition metal chlor-
with H2O2 or O2 are not so numerous.[26–35] Shul’pin ides for the oxidations of methane and ethane with
and co-workers[26] found that methane could be oxi- H2O2 in water medium, and have found that OsCl3 ex-
dized into methyl hydroperoxide, formaldehyde and hibits the best catalytic performances in the oxida-
formic acid with air and H2O2 catalyzed by [n- tions of both methane and ethane. In the present
Bu4N]VO3 combined with pyrazine-2-carboxylic acid paper, we report the details of this aqueous OsCl3-
in acetonitrile at 25–758C, and the reaction was pro- H2O2-based homogeneous catalytic system for the
posed to proceed through a radical mechanism.[27] The oxygenations of methane and ethane. The possible re-
n-Bu4N
salts
of
polyphosphomolybdates active species and the reaction mechanism for this
[PMo11VO40]4À and [PMo6V5O39]12À could catalyze the system are also discussed.
selective oxidation of ethane by H2O2 in acetoni-
trile.[28] At 608C, the TON for the oxidation of ethane
to ethyl hydroperoxide, acetaldehyde and ethanol Results and Discussion
after 10 h of reaction was 14 (TOF, 1.4 hÀ1), but it de-
creased significantly when water was used to replace Oxidations of Methane and Ethane with Hydrogen
acetonitrile, indicating the crucial role of the acetoni- Peroxide Catalyzed by Various Transition Metals
trile solvent.[28] The same research group[29,30] also in- Chlorides
vestigated the catalytic behavior of several iron com-
pounds and complexes in the selective oxidations of Table 1 shows the amounts of products after 1 h of ox-
methane and ethane by H2O2 in acetonitrile at mild idation of CH4 with H2O2 in the presence of various
temperatures, and obtained TOF values of 4.3 hÀ1 and transition metal chlorides at 908C. Under the reaction
23.3 hÀ1 for methane and ethane oxidations, respec- conditions shown in Table 1, C1 oxygenates (CH3OH,
tively. Mizuno and co-workers[31,32] found that the HCHO and CH3OOH) and CO2 were also formed in
Keggin-type heteropoly acids, especially vanadium- the absence of metal chloride (entry 1, blank reac-
substituted polyphosphomolybdate (H4PV1Mo11O40), tion), and the total amount of C1 oxygenates was 24
were active for the selective oxidation of methane by mmol, corresponding to a concentration of 2.4 mmol
H2O2 in trifluoroacetic acid anhydride. Methyl for- dmÀ3. The presence of RuCl3, PdCl2, IrCl3 or H2PtCl6
mate and formic acid were produced as the main did not provide a higher total amount of C1 oxygen-
products with a TOF of ca. 11 hÀ1. The trifluoroacetic ates than the blank reaction. In the presence of RuCl3
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Osmium-Catalyzed Selective Oxidations of Methane and Ethane
Table 1. Selective oxidation of CH4 with H2O2 in the presence of various metal chlorides in aqueous medium.[a]
FULL PAPERS
Entry
Catalyst
H2O2 conv. [%]
Product amount (mmol)
TOF for C1 oxygenates [hÀ1]
CH3OH
HCHO
CH3OOH
CO2
1
2
3
4
5
6
7
8
None
FeCl3
23
88
62
100
70
44
52
50
32
80
35
6.0
18
18
0
28
18
59
14
7.0
29
39
13
39
11
0
16
5.0
58
3.0
8.0
16
14
5.0
14
27
0
2.0
0
3.0
0
0
10
None
7.2
5.6
0
4.6
2.3
12.0
1.7
1.5
4.7
10.1
177
22
CoCl2
RuCl3
RhCl3
PdCl2
OsCl3
IrCl3
H2PtCl6
CuCl2
HAuCl4
n.d.[b]
n.d.
34
77
30
n.d.
162
75
9
10
11
2.0
48
[a]
Reaction conditions: metal chloride, 1.0 mmol dmÀ3; CH4, 3MPa; H2O2, 0.5 mol dmÀ3; H2O solvent, 10 cm3; temperature,
908C; reaction time, 1 h.
Not detected.
[b]
(entry 4), no formation of oxygenates was detected,
The results in Table 2 reveal that OsCl3 is also an
but H2O2 was completely consumed, indicating the efficient catalyst for the selective oxidation of C2H6 to
occurrence of rapid unproductive decomposition of CH3CH2OH and CH3CHO (entry 4). The TOF and
H2O2 catalyzed by RuCl3. On the other hand, FeCl3, the selectivity for the formation of C2 oxygenates
CoCl2, RhCl3, OsCl3, CuCl2 and HAuCl4 enhanced reach ~41 hÀ1 and 85%, respectively. It is of interest
the formation of C1 oxygenates. Among these transi- to note that, in the presence of a fixed catalyst, the
tion metal chlorides, OsCl3 exhibited the highest ac- conversions of H2O2 are similar during the CH4 and
tivity for the formation of C1 oxygenates, giving a C2H6 oxidations. This suggests that the reaction may
TOF for the formation of C1 oxygenates of 12 hÀ1. proceed not via the electrophilic C H activation (fol-
À
CO2 was also formed from CH4 in the presence of lowed by oxidative functionalization) mechanism,[9,10]
OsCl3, and the selectivity of C1 oxygenates was 61%. but via a mechanism in which the Os catalyst acti-
The formation of O2 with an amount of 1.12 mmol vates H2O2 to generate a reactive species for the oxy-
was detected in the gas phase in the case of using genation of methane or ethane. By assuming this, we
À
OsCl3, and the oxygen balance was estimated to be can understand that the difference in C H bond
~98%. HAuCl4 also showed a good enhancing effect energy between CH4 and C2H6 does not significantly
for the oxidation of CH4 to C1 oxygenates, providing affect the conversion of H2O2 but determines the rate
a TOF higher than 10 hÀ1 and a selectivity of 57%, of oxygenation of CH4 or C2H6 to C1 or C2 oxygen-
but precipitation possibly due to the formation of ates.
Au(0) powder took place after the reaction. In other
It is noteworthy that, although osmium compounds,
words, Au(III) is not a stable catalyst under our reac- especially OsO4, are well-known catalysts for the se-
G
tion conditions. On the other hand, such a phenomen- lective oxidation of alkenes with various oxidants,[40,41]
on has not been observed for the Os-catalyzed Os-catalyzed oxidations of alkanes are scarce. Shul’
system.
pin and co-workers once reported in a short commu-
Table 2. Selective oxidation of C2H6 with H2O2 in the presence of several metal chlorides in aqueous medium.[a]
Entry
Catalyst
H2O2 conv. [%]
Product amount (mmol)
TOF for C2 oxygenates [hÀ1]
CH3CH2OH
CH3CHO
CO2
1
2
3
4
5
6
None
FeCl3
PdCl2
OsCl3
H2PtCl6
HAuCl4
23
77
36
57
30
36
23
55
46
101
18
55
58
13
60
110
140
n.d.[b]
81
None
27.7
24.8
40.8
5.4
222
202
307
36
225
28.0
[a]
Reaction conditions: metal chloride, 1.0 mmol dmÀ3; C2H6, 3MPa; H2O2, 0.5 mol dmÀ3; H2O solvent, 10 cm3; temperature,
908C; reaction time, 1 h.
Not detected.
[b]
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nication[22] that OsCl3 could catalyze the oxidation of lyzed by OsCl3 under the reaction conditions of
alkanes to alcohols and aldehydes with H2O2 in aceto- Table 1 and Table 2 were estimated to be 0.15% and
nitrile in the presence of nitrogen-containing hetero- 0.52%, respectively.
cyclic compounds such as pyridine. They mentioned
that CH4 and C2H6 could also be oxidized with this
system, but no details were reported. The same re- Oxidations of Methane and Ethane with Other
search group recently reported that a carbonyl Os(0) Oxidants
complex with p-coordinated olefin showed high effi-
ciency for the oxygenation of alkanes such as cyclo- We have examined several other oxidants for the se-
hexane and n-heptane with H2O2 in acetonitrile.[42] lective oxidations of methane and ethane in the pres-
Recently, Mayer and co-workers[43] showed interest- ence of OsCl3 in aqueous medium. Table 3 shows
ingly that OsO4 could act as a catalyst for the oxida- that, in addition to H2O2, tert-butyl hydroperoxide
tion of isobutane with NaIO4 oxidant in aqueous solu- (TBHP) is also useful for the selective oxidations of
tion, giving a TON of ca. 4 to tert-butyl alcohol, acetic CH4 and C2H6 catalyzed by OsCl3 (entries 4 and 9),
acid and isobutyric acid after 168 h of reaction at while NaIO4, NaClO4 and NaClO cannot oxidize
858C. More recently, the same group reported that either CH4 or C2H6 into the corresponding oxygen-
aqueous solutions of OsO4 and NaIO4 could even oxi- ates. However, the activity for the formation of oxy-
dize CH4 to CH3OH under very mild conditions genates was lower with TBHP than that with H2O2 in
(508C and 9.5 atm). The final CH3OH concentration both CH4 and C2H6 oxidations. Moreover, a differ-
was 0.6% of the starting OsO4 and NaIO4 concentra- ence in the product distribution was observed be-
tions, and 2.7% of the starting CH4 concentration tween using TBHP and H2O2 in the case of C2H6 oxi-
after 120 h of reaction.[44] However, Mayer and co- dation (entries 9 and 10). The ratio of CH3CH2OH/
workers have shown that OsO4 cannot work using CH3CHO was significantly lower when TBHP was
H2O2 because of the rapid oxidation of H2O2 itself.[43] used instead of H2O2, suggesting that the nature of
Thus, our present result that OsCl3 can catalyze the the reactive species might be different.
selective oxidations of CH4 and C2H6 to the corre-
We have also investigated the catalytic behavior of
sponding C1 and C2 oxygenates with H2O2 in aqueous several metal chlorides for the oxidation of C2H6 with
solution is of significance. Furthermore, the TOF different oxidants in aqueous medium. Similar to the
values obtained in our work, i.e., 12 and 41 hÀ1 for result observed for OsCl3, no formation of oxygenates
the selective oxidations of CH4 and C2H6, respectively, was observed when NaIO4, NaClO4 or NaClO was
are higher than those reported in other related sys- used for FeCl3, PdCl2, H2PtCl6 and HAuCl4, whereas
tems such as the diiron-substituted silicotungstate-cat- the use of TBHP caused the occurrence of C2H6 oxi-
alyzed CH4 oxidation with H2O2 in aqueous solution dation (Table 4). However, the order of catalytic ac-
described above.[33] The conversions of CH4 and C2H6 tivity for oxygenate formation by changing catalyst in
with respect to the total amounts of CH4 and C2H6 the case of using TBHP, i.e., FeCl3 >H2PtCl6 ꢀ
present in the system to C1 and C2 oxygenates cata- PdCl2 >HAuCl4 ꢀOsCl3 was much different with that
Table 3. Selective oxidations of CH4 and C2H6 with different oxidants catalyzed by OsCl3 in aqueous medium.[a]
Entry
Oxidant
Product amount (mmol)
TOF (hÀ1)
CH4 oxidation
CH3OH
0
0
0
28
59
HCHO
0
0
0
26
58
CH3OOH
0
0
0
0
3.0
CO2
n.d.[b]
n.d.
n.d.
67
C1 oxygenates
0
0
0
5.4
12.0
1
2
3
4
5
NaIO4
NaClO4
NaClO
TBHP
H2O2
77
C2H6 oxidation
CH3CH2OH
0
0
0
5.0
101
CH3CHO
0
0
0
105
307
CO2
n.d.
n.d.
n.d.
38
C2 oxygenates
0
0
0
11.0
40.8
6
7
8
9
NaIO4
NaClO4
NaClO
TBHP
H2O2
10
140
[a]
Reaction conditions: OsCl3, 1.0 mmol dmÀ3; CH4 or C2H6, 3MPa; oxidant, 0.5 mol dmÀ3; H2O solvent, 10 cm3; tempera-
ture, 908C; reaction time, 1 h.
Not detected.
[b]
1202
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Osmium-Catalyzed Selective Oxidations of Methane and Ethane
FULL PAPERS
Table 4. Selective oxidation of C2H6 with TBHP in the pres-
C2H6 oxidations (entries 2, 3 and 6, 7). A further in-
crease in the concentration of catalyst from 1.0 to
2.0 mmol dmÀ3 only slightly increased the amount of
oxygenates in C2H6 oxidation (entries 7 and 8) and
rather decreased that in CH4 oxidation (entries 3 and
4). At the same time, the formation of CO2 increased
remarkably in both cases. Thus, the high concentra-
tion of catalyst can cause over-oxidation to CO2 and
is detrimental to the selective formation of oxygen-
ates in both CH4 and C2H6 oxidations.
ence of several metal chlorides in aqueous medium.[a]
Entry Catalyst
Product amount (mmol)
TOF for
C2 oxygen-
CH3CH2OH CH3CHO CO2
ates [hÀ1]
1
2
3
4
5
FeCl3
PdCl2
OsCl3
H2PtCl6 38
HAuCl4 32
20
31
5.0
189
112
105
108
82
90
36
38
20.9
14.3
11.0
n.d.[b] 14.6
35
11.4
The effects of the concentration of H2O2 on perfor-
mance during the oxidations of CH4 and C2H6 cata-
lyzed by OsCl3 are summarized in Table 6. No prod-
ucts could be detected without H2O2 (entries 1 and 5),
confirming that H2O2 is the only oxygen donor in the
present system. The increase in the concentration of
H2O2 up to 0.5 mol dmÀ3 caused significant increases
[a]
Reaction conditions: metal chloride, 1.0 mmol dmÀ3
;
C2H6, 3MPa; TBHP, 0.5 mol dmÀ3; H2O solvent, 10 cm3;
temperature, 908C; reaction time, 1 h.
Not detected.
[b]
in the case of using H2O2 (Table 2), i.e., OsCl3 > in the amount of oxygenates (entries 3 and 7). The
HAuCl4 ꢀFeCl3 >PdCl2 @H2PtCl6. Such considerable formation of oxygenates would drop with a further in-
differences may result from the different ability of the crease in H2O2 concentration, whereas that of CO2 in-
catalyst toward activation of TBHP and of H2O2. Al- creased significantly. H2O2 conversions rose to >70%
though a different oxidant requires a proper catalyst, at the same time (entries 4 and 8). This result suggests
the combination of OsCl3-H2O2 exhibits the best per- that a too high concentration of H2O2 may also cause
formances for the selective oxidation of C2H6.
the over-oxidation to CO2 and the decrease in the for-
mation of oxygenates.
The effects of CH4 and C2H6 pressures on catalytic
results are plotted in Figure 1 and Figure 2. Both fig-
ures show that, in the absence of CH4 or C2H6, the de-
composition of H2O2 occurs rather rapidly, suggesting
that OsCl3 also catalyzes the unproductive decomposi-
Effects of Kinetic Factors on Oxidations of Methane
and Ethane with Hydrogen Peroxide Catalyzed by
OsCl3
Table 5 shows the effect of the concentration of OsCl3 tion of H2O2 under the present conditions. However,
on the formation of products during the oxidations of it is of interest to note that the presence of CH4 or
CH4 and C2H6 with H2O2 in aqueous solution. As de- C2H6 has inhibited the conversion of H2O2. The de-
scribed above, the reaction could also proceed with a composition of H2O2 must proceed via radical path-
slow rate without catalyst (entries 1 and 5), but the ways, and the presence of CH4 or C2H6 may affect the
presence of OsCl3 significantly accelerated the reac- radical reactions and thus has exerted influences on
tion. The increase in the concentration of OsCl3 from H2O2 conversions. In the case of CH4 oxidation
0.5 to 1.0 mmol dmÀ3 remarkably increased the (Figure 1), the increase in CH4 pressure from 1 to
amount of oxygenates in the cases of both CH4 and 5MPa slightly raised the H2O2 conversion. The total
Table 5. Effect of the concentration of OsCl3 on the selective oxidations of CH4 and C2H6 with H2O2 in aqueous medium.[a]
Entry
OsCl3 conc. (mmol dmÀ3)
H2O2 conv. (%)
Product amount (mmol)
CH4 oxidation
CH3OH
HCHO
13
11
58
24
CH3OOH
CO2
10
59
77
200
Total oxygenates
24
44
120
61
1
2
3
4
0
25
35
52
78
6.0
32
59
37
5.0
1.0
3.0
0
0.5
1.0
2.0
C2H6 oxidation
CH3CH2OH
23
CH3CHO
58
CO2
13
Total oxygenates
81
5
6
7
8
0
23
43
57
72
0.5
1.0
2.0
66
101
124
123
307
343
130
140
210
189
408
467
[a]
Reaction conditions: CH4 or C2H6, 3MPa; H2O2, 0.5 mol dmÀ3; H2O solvent, 10 cm3; temperature, 908C; reaction time,
1 h.
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Qiang Yuan et al.
FULL PAPERS
Table 6. Effect of the concentration of H2O2 on the selective oxidations of CH4 and C2H6 in the presence of OsCl3 in aque-
ous medium.[a]
Entry
H2O2 conc. (mol dmÀ3)
H2O2 conv. (%)
Product amount (mmol)
CH4 oxidation
CH3OH
0
HCHO
0
CH3OOH
0
CO2
0
Total oxygenates
0
1
2
3
4
0
-
0.3
0.5
0.8
40
52
82
32
59
50
22
58
54
19
3.0
0
86
77
140
73
120
104
C2H6 oxidation
CH3CH2OH
0
CH3CHO
0
CO2
0
Total oxygenates
0
5
6
7
8
0
-
0.25
0.5
0.8
45
57
76
69
101
78
91
307
184
36
140
470
160
408
262
[a]
Reaction conditions: OsCl3, 1.0 mmol dmÀ3; CH4 or C2H6, 3MPa; H2O solvent, 10 cm3; temperature, 908C; reaction time,
1 h.
Figure 2. Effect of C2H6 pressure on catalytic performance
of OsCl3 for the selective oxidation of C2H6 with H2O2 in
aqueous solutions. Reaction conditions: OsCl3, 1.0 mmol
dmÀ3; H2O2, 0.5 mol dmÀ3; H2O solvent, 10 cm3; tempera-
ture, 908C; reaction time, 1 h.
Figure 1. Effect of CH4 pressure on catalytic performance of
OsCl3 for the selective oxidation of CH4 with H2O2 in aque-
ous solutions. Reaction conditions: OsCl3, 1.0 mmol dmÀ3
;
H2O2, 0.5 mol dmÀ3; H2O solvent, 10 cm3; temperature,
908C; reaction time, 1 h.
CH4 pressure would raise the concentration of CH4
amount of all oxygenates (Figure 1A) as well as that solubilized in water and thus increases the conversion
of each oxygenate (Figure 1B) increased significantly of CH4 into products. An estimation using solubility
with increasing CH4 pressure to 3MPa, and was then data of CH4 in pure water[45] revealed that the concen-
almost saturated. The formation of CO2 still went up tration of CH4 at 908C would increase almost linearly
as the CH4 pressure exceeded 3MPa. The increase in from 0.0105 mol dmÀ3 to 0.0525 mol dmÀ3 as the CH4
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Osmium-Catalyzed Selective Oxidations of Methane and Ethane
FULL PAPERS
pressure was raised from 1 to 5MPa. As shown in
Figure 2, in the case of C2H6 oxidation, the amounts
of CH3CH2OH and CH3CHO increased remarkably
with increasing C2H6 pressure from 1 to 3MPa.
Figure 3 and Figure 4 show the effects of reaction
temperature on catalytic performances of OsCl3 for
the oxidations of CH4 and C2H6 with H2O2. In both
Figure 4. Effect of reaction temperature on catalytic perfor-
mance of OsCl3 for the selective oxidation of C2H6 with
H2O2 in aqueous solutions. Reaction conditions: OsCl3,
1.0 mmol dmÀ3; H2O2, 0.5 mol dmÀ3; H2O solvent, 10 cm3;
C2H6, 3MPa; reaction time, 1 h.
perature region. The total amount of C2 oxygenates
also arrived at a maximum at 908C. For both CH4 and
C2H6 oxidations, as the temperature exceeded 908C,
the formation of CO2 increased sharply, whereas that
of oxygenates began to decrease, indicating that the
oxygenates might undergo consecutive oxidation to
CO2 at higher temperatures.
Figure 3. Effect of reaction temperature on catalytic perfor-
mance of OsCl3 for the selective oxidation of CH4 with
H2O2 in aqueous solutions. Reaction conditions: OsCl3,
1.0 mmol dmÀ3; H2O2, 0.5 mol dmÀ3; H2O solvent, 10 cm3;
CH4, 3MPa; reaction time, 1 h.
Time courses for the oxidations of methane and
ethane with H2O2 catalyzed by OsCl3 at 908C are
cases, the conversion of H2O2 went up steadily. For given in Figure 5 and Figure 6. Figure 5 shows that
CH4 oxidation (Figure 3), the total amount of C1 oxy- the amounts of both CH3OH and HCHO increase
genates increased with increasing reaction tempera- almost linearly with reaction time in the initial 1 h,
ture up to 908C, and then underwent a decrease with and then undergo decreases. However, a larger
a further increase in temperature. The same tendency amount of CH3OOH was obtained at a shorter reac-
was observed for the formation of CH3OH and tion time. This further suggests that CH3OOH is an
HCHO (Figure 3B), whereas the amount of intermediate product, which may be transformed to
CH3OOH reached a maximum at a lower tempera- CH3OH and HCHO. The TOF value for oxygenate
ture (708C), and then decreased. The result that rela- formation was almost the same in the initial 1 h and
tively larger amount of CH3OOH can only be ob- began to decrease then. On the other hand, the for-
tained at lower temperatures may indicate that mation of CO2 increased steeply after the initial 1 h.
CH3OOH is an intermediate product. On the other This indicates that the oxygenates undergo consecu-
hand, the formation of CO2 showed a steep rise as the tive oxidation to CO2 at longer reaction times. Similar
reaction temperature rose from 90 to 1008C. In the tendencies were observed for the oxidation of C2H6
case of C2H6 oxidation (Figure 4), no formation of except that the formation of alkyl hydroperoxide was
ethyl hydroperoxide was observed in the whole tem- not observed (Figure 6). The result that both alcohol
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FULL PAPERS
Figure 6. Time courses for the selective oxidation of C2H6
with H2O2 catalyzed by OsCl3 in aqueous medium. Reaction
Figure 5. Time courses for the selective oxidation of CH4
with H2O2 catalyzed by OsCl3 in aqueous medium. Reaction
conditions: OsCl3, 1.0 mmol dmÀ3; H2O2, 0.5 mol dmÀ3; CH4,
3MPa; H2O solvent, 10 cm3; temperature, 908C.
conditions: OsCl3, 1.0 mmol dmÀ3; H2O2, 0.5 mol dmÀ3
C2H6, 3MPa; H2O solvent, 10 cm3; temperature, 908C.
;
and aldehyde increase almost linearly in the initial 1 h OsCl3 aqueous solution before and after the addition
suggests that they may be formed in parallel to each of H2O2, and further after the CH4 oxidation at 908C.
other. In other words, aldehyde may not be the con- The spectra of reference compounds of Na2OsCl6 and
secutive oxidation product of alcohol for both CH4 OsO4 with Os in the +IV and +VIII states, respec-
and C2H6 oxidations in the initial 1 h.
tively, are also shown in Figure 7. The OsCl3 aqueous
solution exhibited a strong absorption band at 371 nm
with a shoulder at 298 nm and a weak band at ca.
500 nm (spectrum a). After the addition of H2O2, the
spectrum changed significantly, and two bands at 337
Characterizationof Os States for the Oxidation
OsO4 is a well-known oxidant and also a catalyst for and 370 nm were observed (spectrum b). Although
dihydroxylation of alkenes to the corresponding the intensity was lower, these two bands were close to
diols,[40,41] and it has recently been utilized for stoi- those mainly observed for the reference compound
2À
chiometric and catalytic oxidations of alkanes includ- with Os in the +IV state, i.e., OsCl6 (spectrum e).
ing CH4 with NaIO4 as oxidant.[43,44] On the other On the other hand, the aqueous OsO4 showed a very
hand, Shul’pin et al.[42] proposed an Os(II)/Os
ACHTREUNG
AHCTREUNG
droxyl radicals for the oxidation of alkanes such as cy- Os(IV) in the presence of H2O2 in aqueous medium.
clohexane catalyzed by an Os(0) complex in acetoni- It is noteworthy that, in an early study, several
trile. In our experiments, when H2O2 was introduced Os(IV) ammine complexes were reported to give UV-
into OsCl3 aqueous solution, we observed a color Vis bands at lower energy positions with higher ex-
change of the solution from dark brown to bright tinction coefficients as compared with the correspond-
yellow, which may indicate a change in Os valence ing Os
A
state after the interaction with H2O2. for the reference compound with Os in the +IV state
We have characterized the possible state of Os in Figure 7 is not consistent with this early report. We
during the oxidation of CH4 with H2O2 by UV-Vis speculate that this inconsistency might arise from the
spectroscopy. Figure 7 shows UV-Vis spectra of the ammine ligands, which do not exist in our case. Our
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Osmium-Catalyzed Selective Oxidations of Methane and Ethane
FULL PAPERS
tion. This allows us to further speculate that the
Os(IV) species may be involved in the selective oxi-
dations of CH4 and C2H6 with H2O2.
To confirm that the Os(IV) species may play an im-
portant role in our system, we have carried out oxida-
tions of CH4 and C2H6 with H2O2 using Na2OsCl6 and
OsO4. As shown in Table 7, in the case of using OsO4
(entries 3 and 6), no formation of oxygenates was ob-
served for both CH4 and C2H6 oxidations, but all of
the H2O2 molecules added were consumed, indicating
the rapid occurrence of unproductive decomposition
of H2O2 to O2. Mayer and co-workers[43] also reported
the rapid oxidation of H2O2 by OsO4 at a similar tem-
perature (858C) in aqueous solution, and they con-
cluded that H2O2 was unsuitable for the oxidation of
alkanes using OsO4 as a catalyst. On the other hand,
when Na2OsCl6 was used (entries 2 and 5), CH4 and
C2H6 could be oxidized to the corresponding alcohols
and aldehydes with H2O2. Na2OsCl6 exhibited lower
conversions of H2O2 than OsCl3 for both CH4 and
C2H6 oxidations. Simultaneously, the total amount of
oxygenates for either CH4 or C2H6 oxidation was also
lower by using Na2OsCl6 than that by using OsCl3.
However, the product distribution patterns (the ratio
of alcohol to aldehyde) by using Na2OsCl6 and OsCl3
were similar. Thus it is likely that the oxidation cata-
lyzed by OsCl3 proceed with a similar mechanism to
that by Na2OsCl6. As regards the higher activity of
OsCl3 as compared with Na2OsCl6, we speculate that
this may result from the difference in the ligands of
osmium, which may cause the difference in the coor-
Figure 7. UV-Vis spectra of osmium-containing aqueous sol-
utions. (a) OsCl3, (b) OsCl3 + H2O2, (c) OsCl3 + H2O2
after CH4 oxidation, (d) OsCl3 + NaIO4, (e) Na2OsCl6, (f)
OsO4. Reaction conditions for CH4 oxidation: OsCl3,
1 mmol dmÀ3; H2O2; 0.5 mol dmÀ3; solvent, water; CH4,
3MPa, temperature, 908C, time 1 h.
speculation that the Os(IV) species are formed after dination and activation of H2O2.
the addition of H2O2 to OsCl3 is also reasonable if
one considers that the standard electrode potential
for the H2O2/H2O redox pair (E0 =1.78 V) is much Possible ReactionMechanism
higher than that for the Os(IV)/Os(III) redox pair
G
(E0 =0.45 V for OsCl62À/OsCl63À) in aqueous solution. Mayer and co-workers have proposed an interesting
À
After the oxidation of CH4, the two bands became non-radical [3+2] addition of one C H bond of the
more distinct (spectrum c). Therefore, we conclude alkane molecule to two oxo groups of OsO4 for the
that osmium exists in the +IV state during the reac- selective oxidation of alkanes by OsO4.[43] For the oxi-
Table 7. Selective oxidations of CH4 and C2H6 with H2O2 in the presence of several osmium compounds in aqueous medi-
um.[a]
Entry
Catalyst
H2O2 conv. (%)
Product amount (mmol)
CH4 oxidation
CH3OH
HCHO
CH3OOH
3.0
0
CO2
77
50
0
Total oxygenates
1
2
3
OsCl3
Na2OsCl6
OsO4
52
33
100
59
28
0
58
21
0
120
49
0
0
C2H6 oxidation
CH3CH2OH
CH3CHO
CO2
140
30
Total oxygenates
4
5
6
OsCl3
Na2OsCl6
OsO4
57
36
100
101
55
0
307
175
0
408
230
0
0
[a]
Reaction conditions: osmium compound, 1.0 mmol dmÀ3; CH4 or C2H6, 3MPa; H2O2, 0.5 mol dmÀ3; H2O solvent, 10 cm3;
temperature, 908C; reaction time, 1 h.
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Qiang Yuan et al.
FULL PAPERS
dation of CH4, an unknown but very active oxidant corresponding alcohols and aldehydes with H2O2 in
might be formed from the OsO4/NaIO4 mixture be- aqueous medium. The turnover frequency values for
cause OsO4 or NaIO4 alone could not oxidize CH4 to oxygenate formation in methane and ethane oxida-
CH3OH.[44] As described above, it has become clear tions could reach 12 and 41 hÀ1, respectively. The for-
that OsO4 with Os(VIII) oxo is not the active species mation of a small amount of methyl hydroperoxide
G
in our system with H2O2 as the oxidant. The active was also observed during the oxidation of methane at
species is speculated to be an Os(IV) species although the very initial stage or lower reaction temperatures.
we still do not exactly know the coordination struc- For both methane and ethane oxidations, aldehydes
ture of this Os(IV) species. As shown in Figure 7, might mainly be formed in parallel to alcohols but
OsCl3 can also be oxidized to the Os(IV) species in not via the consecutive oxidation of alcohols. Carbon
the presence of NaIO4 (spectrum d), but NaIO4 dioxide was also formed in both cases, and the forma-
cannot function as an efficient oxidant for the oxida- tion of carbon dioxide became serious at higher tem-
tion of CH4 or C2H6 using OsCl3 catalyst (Table 3). peratures and longer reaction times. TBHP could be
Therefore, it is hard to consider that an Os(IV) oxo used for the oxidations of methane and ethane in the
species may function for the activation and oxidation presence of OsCl3 catalyst but the activity for oxygen-
of CH4 or C2H6, while H2O2 only acts for the oxida- ate formation was remarkably lower. On the other
tion of the reduced osmium [Os(II) or Os(III)] spe- hand, NaIO4, NaClO4 or NaClO could not oxidize
U
cies to regenerate the Os(IV) oxo species. All the re- methane and ethane into the corresponding oxygen-
sults obtained in this work allow us to speculate that ates by OsCl3 catalyst. UV-Vis spectroscopic measure-
the role of the osmium catalyst is to activate H2O2 to ments indicated that osmium existed in the +IV state
generate active oxygen species, which are responsible during the oxidation reactions with H2O2. We con-
for the oxidations of CH4 and C2H6.
firmed that OsO4 only catalyzed the unproductive de-
To further uncover whether the reaction proceeds composition of H2O2 and was not the active phase in
via a radical mechanism, we have investigated the in- our system. The Os(IV) species was proposed for the
fluence of the addition of a radical scavenger (0.1 mol activation of H2O2, generating an active oxygen spe-
dmÀ3), hydroquinone, to the reaction system under cies for the selective oxidations of methane and
the conditions of Table 1 and Table 2 for OsCl3. We ethane probably via radical pathways.
found that the oxidations of both CH4 and C2H6 with
H2O2 catalyzed by OsCl3 ceased after the addition of
hydroquinone. This result suggests that the oxidation
with H2O2 catalyzed by OsCl3 proceeds through radi-
cal pathways. We assume that the redox of Os(IV)/
Experimental Section
Catalytic Reactions
Os(III) may take part in the activation of H2O2 to
G
generate active oxygen species, which may be hydrox-
yl (COH) or hydroperoxide (COOH) radical. The active
oxygen species may then activate methane or ethane
The catalytic oxidations of methane and ethane were carried
out with a stainless-steel autoclave containing a Teflon
vessel (75 cm3). In a typical run, a measured amount (0.1
by abstracting a hydrogen atom, forming methyl or mol dmÀ3 aqueous solution, 0.1 cm3) of catalyst was added
into the Teflon vessel, which was pre-charged with ultra-
pure water. Then a certain amount of aqueous H2O2 (30%)
was added into the reactor. The total volume of the reaction
solution was 10 cm3. The system was pressured with meth-
ane or ethane to a fixed pressure after air in the reactor was
removed with the reactant. The autoclave was heated to the
reaction temperature in an oil bath. The reaction solution
was vigorously stirred and maintained at the reaction tem-
perature for a fixed time. After the reaction, the autoclave
was cooled with ice to room temperature.
ethyl radicals. The subsequent reactions between the
radicals and/or between the radical with methane or
ethane may provide the products. It is worthy of men-
tion that an iron-based catalyst may carry out the
same reactions through Fe
(III)/Fe(II).[30] Under our
A
conditions, we really found that FeCl3 was also a rela-
tively better catalyst for the oxidations of CH4 and
C2H6 than several other transition metal chlorides
(Table 1 and Table 2). However, the unproductive de-
composition of H2O2 was more serious in the pres-
ence of FeCl3, and this resulted in lower concentra-
tions of oxygenate products from both CH4 and C2H6
as compared with OsCl3.
The liquid-phase products were analyzed by gas chroma-
tography (GC) after the addition of a known amount of
liquid standards (propanol for CH4 oxidation and acetone
for C2H6 oxidation). Two GCs (Model SP-2100) with a capil-
lary column (DB-5mS) connected with an FID detector and
a packed column (Porapak T) connected with a TCD detec-
tor were applied for the analyses of the products from CH4
oxidation, and another GC (Model GC-960) equipped with
a Porapak T column and an FID detector was used for the
analyses of the products from C2H6 oxidation. Alkyl hydro-
peroxide was evaluated using a method proposed by Shul’
Conclusions
Among the various transition metal chlorides investi-
gated, OsCl3 was found to be the most efficient cata-
lyst for the oxidations of methane and ethane to the pin et al.,[30] i.e., by comparing the amount of alcohol (meth-
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Adv. Synth. Catal. 2007, 349, 1199 – 1209

Osmium-Catalyzed Selective Oxidations of Methane and Ethane
FULL PAPERS
anol or ethanol) before and after the addition of NaBH4.
The titration of H2O2 was carried out with 0.01 mol dmÀ3
KMnO4 solution. Gas phase products such as O2, CO and
CO2 were analyzed by a gas chromatograph equipped with
Porapak Q and Molecular Sieve 5 A columns and TCD de-
tectors. The gas chromatograph was directly connected to
the outlet of the autoclave. The formation of CO2 and O2
was confirmed but no CO could be detected in all runs. We
checked the oxygen balance in some runs based on the
amounts of H2O2 consumed and products (including oxygen-
ates, CO2 and O2) formed, and the oxygen balance was
always better than 90%.
[15] R. A. Periana, D. J. Taube, S. Gamble, H. Taube, T.
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diluted aqueous solutions of Os compounds, mixtures of Os
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This workwas supported by the Natural Science Foundation
of China (grants 20433030 and 20625310), the National Basic
Research Program of China (grants 2005CB221408 and
2003CB615803), the Key Scientific Project of Fujian Province
(2005HZ01–3), the Scientific Research Foundation for the
Returned Oversea Chinese Scholars (grant to Q.Z.), and the
Program for New Century Excellent Talents at the University
of China (grant NCET-040602 to Y.W.).
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