A high-yield, liquid-phase approach for the partial oxidation of methane to methanol using SO3 as the oxidant

Article abstract of DOI:10.1002/adsc.200404394

A direct approach for producing methanol from methane in a three-step, liquid phase process is reported. In the first step, methane is reacted with SO₃ to form methanesulfonic acid (MSA) at 75°C using a free-radical initiator and MSA as the solvent. Urea-H₂O₂ in combination with RhCl₃ is found to be the most effective initiator (57% conversion of SO₃; 7.2% conversion of CH₄). MSA is then oxidized by SO₃ at 160°C in a second step to produce a mixture containing methyl bisulfate and some methyl methanesulfonate (87% conversion of MSA). In the third step, the mixture of methyl bisulfate and methyl methanesulfonate is hydrolyzed in the presence of an organic solvent, to produce an organic phase containing methanol and an aqueous phase containing sulfuric acid and some MSA (63% conversion of methyl bisulfate; 72% conversion of methyl methanesulfonate). Overall, 58% of the MSA (of which 23% is derived from methane) is converted to methanol.

Full text of DOI:10.1002/adsc.200404394

COMMUNICATIONS
A High-Yield, Liquid-Phase Approach for the Partial Oxidation
of Methane to Methanol using SO as the Oxidant
3
a, b,
a
a,
Sudip Mukhopadhyay, * Mark Zerella, Alexis T. Bell *
a
Department of Chemical Engineering, University of California, Berkeley, CA 94720-1462, USA
Fax: (þ1)-510-642-4778, e-mail: bell@cchem.berkeley.edu
b
Present address: Honeywell International Inc., Buffalo, NY 14210, USA
Fax: (þ1)-716-568-4451, e-mail: Sudip.Mukhopadhyay@honeywell.com
Received: December 27, 2004; Accepted: April 4, 2005
Abstract: A direct approach for producing methanol
from methane in a three-step, liquid phase process is
reported. In the first step, methane is reacted with
carried out in fuming sulfuric acid at temperatures of
180–2208C, and bisulfate yields of up to 43% based on
methane can be achieved. Since methyl bisulfate cannot
be readily separated from sulfuric acid, it is hydrolyzed
to methanol by addition of water to the methyl bisul-
fate-sulfuric acid solution. Separation of methanol
from the resulting solution is difficult, and the dilute sul-
furic acid solution remaining must be concentrated be-
fore it can be recycled. Here, we show that pure metha-
SO to form methanesulfonic acid (MSA) at 758C
3
using a free-radical initiator and MSA as the solvent.
Urea-H O in combination with RhCl is found to be
2
2
3
the most effective initiator (57% conversion of SO3;
.2% conversion of CH ). MSA is then oxidized by
7
4
SO at 1608C in a second step to produce a mixture
3
containing methyl bisulfate and some methyl meth-
anesulfonate (87% conversion of MSA). In the third
step, the mixture of methyl bisulfate and methyl
methanesulfonate is hydrolyzed in the presence of
an organic solvent, to produce an organic phase con-
taining methanol and an aqueous phase containing
sulfuric acid and some MSA (63% conversion of
methyl bisulfate; 72% conversion of methyl meth-
anesulfonate). Overall, 58% of the MSA (of which
nol can be produced from methane and SO in a three-
3
step process that avoids the use of sulfuric acid as a sol-
vent and enables the facile separation of methanol. Con-
centrated sulfuric acid is produced as a by-product,
which can either be sold or reprocessed to elemental sul-
fur.
23% is derived from methane) is converted to meth-
anol.
Keywords: catalytic oxidation; CÀH activation;
methane; methanol; rhodium; sulfur trioxide
In a typical reaction, methanesulfonic acid (MSA) is
formed in a high-pressure, glass-lined autoclave by re-
1
3
acting CH with SO dissolved in CH SO H containing
4
3
3
3
[
17–22]
a free radical initiator.
h at 758C. CH SO H is then oxidized in the presence of
Reactions are carried out for
6
3
3
Attempts to carry out the partial oxidation of meth- a catalyst to CH OSO H and CH SO CH by reaction
3
3
3
3
3
[1–12]
ane
to methanol in the gas phase have met with lim- with SO at temperatures of 130–1858C. Products are
3
1 13
ited success because of difficulties in controlling the free identified and quantified by H and C NMR spectro-
radical processes that are involved. Since methanol is scopy. In the last step, CH OSO H is hydrolyzed in the
3
3
more reactive than methane, the formation of CO and presence of an organic solvent. The methanol is then dis-
CO via secondary combustion is unavoidable. While a tilled from the organic phase and identified by NMR,
2
variety of catalysts, mostly metal oxides, have been re- GC-MS, and Raman spectroscopy.
ported for the partial oxidation of methane to methanol,
Table 1 shows the effects of initiator composition on
all require high temperatures and the reported metha- the sulfonation of methane to MSA with SO at 758C.
3
nol yields based on methane have generally been less Conversions are expressed in terms of SO since it is
3
than 10%.
the limiting reagent and CH is present in excess.
4
An indirect approach for the conversion of methane K P O is the least effective initiator, whereas K S O ,
4
2
8
2
2
8
[
13–16]
to methanol has been reported by Periana et al.
urea-H O , and CaO are moderately effective. Howev-
2 2 2
These authors have demonstrated that methane can be er, urea-H O in combination with RhCl is the most ef-
2
2
3
converted to methyl bisulfate in a single-step using fective initiator, enabling 57% of the SO to be convert-
3
2
þ
þ
2
Hg , Pt-complexes, or I as catalysts. The reaction is ed to MSA using MSA as the solvent. Previous studies
Adv. Synth. Catal. 2005, 347, 1203–1206
DOI: 10.1002/adsc.200404394
ꢀ 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1203

COMMUNICATIONS
Sudip Mukhopadhyay et al.
[a]
Table 1. Effect of initiator composition on the yield of methanesulfonic acid (MSA).
#
Initiator [mmol]
MSA produced,
% Conversion of
Volumetric productivities to MSA
À3 À1
[
mmol]
SO to MSA
[mol cm
s ]
3
À8
À8
À7
À7
À7
1
2
3
4
5
K S O , 0.07
5
3
9
7
24
14
43
33
57
7.71Â10
4.62Â10
1.38Â10
1.05Â10
1.85Â10
2
2
8
K P O , 0.07
4
2
8
CaO , 0.09
2
Urea-H O , 0.4
2
2
Urea-H O , 0.4/RhCl , 0.1
12
2
2
3
[
a]
13
Reaction conditions: CH , 600 psig (166 mmol); SO , 21 mmol; MSA, 3.96 g; time, 6 h; temperature, 758C.
4
3
[a]
Table 2. RhCl -catalyzed oxidation of MSA by SO to methyl bisulfate.
3
3
#
T [8C]
Oxidant
mmol]
% Conversion of
MSA [mmol]
% Conversion
of MSA to
% Conversion
of MSA to
% Selectivity
Mol of
[
b]
[b]
À3 À1
[
to CH X
CH X cm
s
3
3
CH OSO H
CH SO CH
3
3
3
3
3
[
mmol]
[mmol]
À8
À8
À8
À8
À7
À7
À7
À7
1
2
3
4
4
5
6
7
8
9
0
130
150
160
185
160
160
160
160
120
160
160
SO , 15
8 (4.2)
12 (6.3)
15 (8)
18 (9.5)
33 (17.5)
48 (25.4)
88 (47)
100 (53)
0 (0)
8 (4.2)
9 (5)
10 (5.5)
4 (2)
26 (14)
30 (16)
55 (29)
49 (26)
0 (0)
0 (0)
100
100
94
84
93
94
93
87
0
2.92Â10
3
SO , 15
3 (0.7)
4 (1.0)
11 (3.0)
5 (1.2)
15 (4.0)
28 (7.5)
38 (10)
0 (0)
3.96Â10
3
SO , 15
4.51Â10
3
c
SO , 15
2.31Â10
3
SO , 21
1.05Â10
3
SO , 30
1.38Â10
3
SO , 55
2.53Â10
3
SO , 64
2.50Â10
3
d
e
H O , 30
0
2
2
À9
À7
O , 33
2 (1)
100 (53)
2 (1)
96 (51)
–
50
96
6.94Â10
2
1
SO , 65
0 (0)
5.9 Â10
3
[
[
[
[
[
a]
o
Reaction conditions: MSA, 5.11 g, (53 mmol); T, 160 C; time, 10 h. RhCl , 0.1 mmol.
CH X is methanol equivalent (CH OSO H and CH SO CH ).
5 h.
3
b]
3
3
3
3
3
3
c]
d]
e]
1
6
h.
H SO is the solvent.
2
4
utilizing the initiators listed in Table 1 have shown that
7
58C is an optimal temperature, since at higher temper-
[22]
atures the initiator rapidly decomposes, producing O2.
Since the reaction is thought to occur via a free radical
[17–22]
mechanism,
the presence of O inhibits the sulfona-
2
tion of methane to MSA. Thus, for example, sulfonation
at 1608C in fuming sulfuric acid results in a 4% conver-
sion of SO to CH OSO H after 6 h of reaction, but no
3
3
3
formation of MSA.
Table 2 shows the effects of different reaction condi-
tions on the oxidation of MSA to CH X (CH OSO H
3
3
3
or CH SO CH ) by SO . When a mixture containing
3
3
3
3
1
3
1
1
5 mmol of SO and 53 mmol MSA was heated at Figure 1. (a) C NMR of the mixture after 10 h of reaction
3
13
308C for 10 h, 8% of the MSA charged into the reactor between MSA and SO
, T¼1308C; SO
NMR of the mixture after 10 h of reaction between MSA
with SO , T¼1608C; SO , 55 mmol.
, 15 mmol. (b)
C
3
3
was transformed to CH OSO H with 100% selectivity
3
3
(
Figure 1a). Increasing the temperature to 1508C raised
3
3
the conversion of MSA to CH X to 12%, of which
3
7
1
5% was CH OSO H and 25% was CH SO CH . At product at 1608C (Figure 1b), CH SO CH became the
3 3 3 3 3 3 3 3
608C, 15% of the MSA was converted to products, of major product when the reaction temperature was
these products 94% appeared as CH X and the balance raised to 1858C.
3
as a mixture of CH OSO OCH , CH (SO H) , and
The effect of the amount of SO on the oxidation of
3
2
3
2
3
2
3
CH (SO ) H. While CH OSO H remained as the major MSA to CH X was also studied. Raising the amount of
3
3
n
3
3
3
1204
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Adv. Synth. Catal. 2005, 347, 1203–1206

Partial Oxidation of Methane to Methanol using SO3
COMMUNICATIONS
Table 3. Single-pass extractive hydrolysis of CH OSO H to CH OH in a solvent.
3
3
3
#
Mol ratio of
Solvent
T [ 8C]
Percent of MeOH extracted
H SO in the aqueous phase
2
4
H O/CH OSO H
into the solvent phase [mol %]
[wt %]
2
3
3
1
2
3
4
5
6
7
8
9
0
1
2
3
1
3
5
7
10
5
5
5
5
5
CH Cl
25
25
25
25
25
40
61
61
61
61
61
83
83
6
17
29
32
39
46
62
58
53
61
53
63
72
87
65
54
44
36
54
54
49
45
54
54
54
–
2
2
2
2
2
2
2
CH Cl
2
CH Cl
2
CH Cl
2
CH Cl
2
CH Cl
2
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
b
c
d
e
1
1
1
1
5
5
5
C H Cl
2 4
2
2
f
C H Cl
2 4
[
a]
1
Reaction conditions: CH OSO H, 0.375 g; solvent, 7 g; time, 2 h; N , 300 psig. The reaction progress is monitored by H
3
3
2
NMR.
[
[
[
[
[
b]
1
2
5
3
0
0 wt %; MSA was added in the feed.
0 wt %; MSA was added in the feed.
g CHCl were used.
g CHCl were used.
.375 g of CH SO CH were used instead of CH OSO H.
c]
d]
e]
f]
3
3
3
3
3
3
3
SO added to the reaction mixture from 15 to 64 mmol nol, experiments were performed in which 10 and 20%
3
increased the conversion of MSA from 15% to 100%. MSA was present in the feed. The extraction efficiency
While the selectivity to CH X was independent of the decreased by a small amount as the amount of MSA in-
3
amount of SO , the distribution between CH OSO H creased. Reducing the ratio of organic solvent to
3
3
3
and CH SO CH shifted from 85% CH OSO H to 72%. CH OSO H from 20 to 13 had little effect on the meth-
3
3
3
3
3
3
3
MSA oxidation by H O , O , and H SO was also con- anol extraction efficiency;however, decreasing this ratio
2
2
2
2
4
sidered. H O was inactive at 1308C and only a small to 10 had a noticeable detrimental effect. The hydrolysis
2
2
amount of CH OSO H was observed when O was of CH SO CH to CH OH was carried out under condi-
3
3
2
3
3
3
3
used as the oxidant. A 12% conversion of MSA to tions identical to those used for CH OSO H (entry 12 in
3
3
CH OSO H was achieved at 1608C after 10 h of reaction Table 3). A methanol extraction of 72% could be ach-
3
3
in 100% H SO and no CH SO CH was observed. ieved starting from CH SO CH , as apposed to 63% in
2
4
3
3
3
3
3
3
When H SO was used as the solvent and SO as the the case of CH OSO H.
2
4
3
3
3
oxidant, 96% of the initial MSA charged into the reac-
As seen in Table 3, the final sulfuric acid concentra-
tor was converted to CH OSO H and, again, no tion in the aqueous phase remained close to 60 wt %.
3
3
CH SO CH was observed.
This level is sufficiently high for direct use in the dyes,
3
3
3
Table 3 shows the effectiveness of the single-pass ex- paint, and pharmaceutical industries. 100% H SO4
2
tractive hydrolysis of CH OSO H in the presence of a could be produced by solar concentration or by addition
3
3
water immiscible organic solvent. Low boiling halogen- of SO3.
ated solvents such as CH Cl , CHCl , and C H Cl were To confirm the formation of methanol as the final
used. For a fixed set of hydrolysis and extraction condi- product, the extract obtained using ClCH CH Cl was
2
2
3
2
4
2
2
2
tions, CHCl and C H Cl showed comparable effective- fractionated. The product that was condensed was iden-
3
2
4
2
1
3
ness, and both solvents were superior to CH Cl . Since tified as pure methanol on the basis of C NMR
2
2
the extent of CH OSO H hydrolysis depends on the mo- (49 ppm) and Raman spectroscopy (1039, 1459, 2840,
3
3
À1
lar ratio of H O/CH OSO H, the value of this ratio was 2949, and 3341 cm ).
2
3
3
varied. Table 3 shows that as the ratio increases from 1 to
In conclusion, we have demonstrated a high-yield ap-
1
0, the percent extraction of CH OH increases rapidly at proach for the selective conversion of methane to meth-
3
first but then more slowly. At the same time, the weight anol. The proposed scheme could readily be incorporat-
percent of H SO in the aqueous phase decreases mo- ed into a refinery, since the SO required for the process
2
4
3
notonically. A molar ratio of 5 to 1 H O to CH OSO H could be produced by oxidation of H S, the product of
2
3
3
2
[
23–26]
resulted in a 29% extraction of methanol in one step petroleum hydrodesulfurization.
The SO formed
2
and an aqueous phase containing 54 wt % H SO . To de- during the oxidation of MSA can be recovered readily
2
4
termine the effect of MSA on the extraction of metha- and reoxidized to SO . The concentrated sulfuric acid
3
Adv. Synth. Catal. 2005, 347, 1203–1206
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COMMUNICATIONS
Sudip Mukhopadhyay et al.
produced as a by-product of CH OSO H hydrolysis
could be sent to the Claus process where it would react
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3
[
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2
ly be transformed to SO by catalytic oxidation with air.
3
3
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[
[
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autoclave reactor was charged with 0.4 mmol of urea-H O ,
1
590–1591.
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, 3193–3196.
2
2
0
.1 mmol of RhCl , 3.76 g of MSA, and 21 mmol of SO . A
3
3
5
small Teflon-coated magnetic stir bar was used to stir the reac-
[
10] G. A. Foulds, B. F. Gray, Fuel Proc. Tech. 1995, 42, 129–
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2
1
3
and pressurized with 600 psig of CH . The reactor was then
4
[
[
heated to 758C under stirring and kept at that temperature
for 6 h. After the stipulated period of time, the reactor was
cooled, purged with N , and opened to add extra SO to the re-
2
004, 211, 59–65.
2
3
action mixture. The autoclave was then closed, purged with N2
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Wentrcek, G. Voss, T. A. Masuda, Science 1993, 259,
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2
o
to 160 C. The reaction mixture was stirred at 1608C for 10 h.
After this period, the reactor was cooled, the pressure was re-
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leased, and the reactor was purged with N gas. The reaction
2
1
13
mixture was analyzed H and C NMR. D O was used in a ca-
pillary as the lock reference. The H NMR chemical shift was
2
1
2.87 ppm to 3.04 ppm for MSA, 3.76 ppm to 3.98 ppm for
[
16] R. A. Periana, G. Bhalla, W. J. Tenn, III, K. J. H. Young,
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CH OSO H, 3.13 ppm and 3.83 ppm for CH SO CH , the
3
3
3
3
3
range of chemical shifts in each case depending on the product
1
concentration. A H NMR chemical shift of 3.27 ppm was ob-
[
[
[
[
[
[
17] N. Basickes, T. E. Hogan, A. Sen, J. Am. Chem. Soc.
1
3
served for CH OH. The corresponding C NMR chemical
3
1
996, 118, 13111–13112.
shifts are: 39.5–40.5 ppm for MSA, 60–60.5 ppm for
CH OSO H, and 35.5–36.0 ppm and 58.0–58.5 ppm for
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CH SO CH .
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Acknowledgements
2
This work was supported by a grant from BP as a part of the
Methane Conversion Cooperative.
1
2
[
[
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ꢀ 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
Adv. Synth. Catal. 2005, 347, 1203–1206