Direct sulfonation of methane to methanesulfonic acid by sulfur trioxide catalyzed by cerium(IV) sulfate in the presence of molecular oxygen

Article abstract of DOI:10.1002/adsc.200404060

Direct sulfonation of methane with SO₃ to methanesulfonic acid (MSA) is accomplished in sulfuric acid with almost 100% selectivity in the presence of a catalyst, namely, Ce and Rh salts and molecular oxygen as the catalyst regenerator. In the absence of O₂, the catalyst remains effective but the selectivity to MSA decreases to 53% and byproducts, principally CH₃OSO₃H, are formed. The effects of O ₂ pressure, catalyst concentration, temperature, SO₃ concentration, and methane pressure have been examined on the rate of SO ₃ conversion to MSA. The conversion of SO₃ to MSA was the same when CF₃SO₃H was used as the solvent instead of H₂SO₄.

Full text of DOI:10.1002/adsc.200404060

COMMUNICATIONS
Direct Sulfonation of Methane to Methanesulfonic Acid by
Sulfur Trioxide Catalyzed by Cerium(IV) Sulfate in the Presence
of Molecular Oxygen
Sudip Mukhopadhyay, Alexis T. Bell*
Department of Chemical Engineering, University of California, Berkeley, CA 94720-1460, USA
Fax: (þ1)-510-642-4778, e-mail: bell@cchem.berkeley.edu
Received: March 1, 2004; Accepted: June 12, 2004
Abstract: Direct sulfonation of methane with SO to
Identification of a catalytic process for the sulfonation
of methane would highly desirable, since this would
eliminate the need for a free-radical initiator. While Ish-
ii and co-workers have reported success in the vanadi-
um-catalyzed sulfonation of adamantane to the corre-
sponding sulfonic acids using SO and O , methane did
3
methanesulfonic acid (MSA) is accomplished in sul-
furic acid with almost 100% selectivity in the pres-
ence of a catalyst, namely, Ce and Rh salts and mo-
lecular oxygen as the catalyst regenerator. In the ab-
sence of O , the catalyst remains effective but the se-
2
2
2
[
4]
lectivity to MSA decreases to 53% and byproducts,
principally CH OSO H, are formed. The effects of
not undergo sulfonation to MSA. Hg-based catalysts
have been used at elevated temperature (300±4508C)
3
3
O2 pressure, catalyst concentration, temperature,
SO3 concentration, and methane pressure have
for methane sulfonation with SO to MSA; however
3
this process exhibits a low yield and cannot be imple-
mented due to problems in isolating MSA from a mix-
ture of byproducts, mainly esters and disulfonic acids
been examined on the rate of SO conversion to
3
MSA. The conversion of SO3 to MSA was the
same when CF SO H was used as the solvent instead
[
5]
formed during this reaction. In subsequent studies,
3
3
[
6]
of H SO .
Periana and co-workers have reported the selective
2
4
sulfonation of CH to MSA in fuming sulfuric acid at
4
[
6a]
Keywords: CÀH activation; metal catalyst; methane;
methanesulfonic acid; molecular oxygen; sulfona-
tion; sulfur trioxide
180 to 2208C using Hg salts or Pt complexes as the cat-
[6b]
alyst. In both the cases, the metal cations undergo re-
[
6]
duction and are reoxidized by sulfuric acid. We have
recently shown that Hg(CF SO H) can be used in the
3
3
2
presence of O to synthesize MSA by the reaction of
2
[
7]
methane and SO in fuming sulfuric acid. While Hg
3
salts are effective catalysts, their toxicity motivates the
Methanesulfonic acid (MSA, 70 wt %) is widely used in search for alternative catalysts. This work was undertak-
electrochemical systems and is an excellent catalyst for en to assess the catalytic activity of various metal salts
the esterification, alkylation, and condensation of or- (Scheme 1). It is shown that salts of Ce and Rh are par-
[
1]
ganic compounds. Lower reaction temperatures are ticularly attractive for the sulfonation of methane to
required when using MSA rather than titanate catalysts, MSA.
and purer, more colorless products can be obtained us-
Table 1 shows the effect of different catalysts on the
ing MSA than those produced using sulfuric or para-tol- rate of the sulfonation reaction. In a typical reaction
uenesulfonic acid as the catalyst. Anhydrous MSA is (see Experimental Section), CH and SO were reacted
4
3
also particularly well suited for pharmaceutical applica- in fuming sulfuric acid in a high-pressure, glass-lined au-
tions and as a catalyst for aromatic alkylation. The cur- toclave in the presence of a catalyst and O . Reactions
2
rent commercial process for the synthesis of MSA in- were carried out for 4±6 h at 1308C and the MSA thus
[
2]
1
[3b]
volves chlorine oxidation of methylmercaptan. While formed was identified and quantified by H NMR.
this process is highly productive, it produces six moles The conversion is reported on the basis of the limiting
of HCl per mole of MSA, resulting in a coupling of the reagent, SO , and is defined as the ratio of the moles
3
demand for the primary product and the byproduct.
As an alternative it is interesting to consider a direct
methane sulfonation route using SO or SO and O as
3
2
2
[3a]
the sulfonating agent. Sen and co-workers
have shown that a compound
such as K S O can be used as a free radical initiator
and,
[
3b, c]
more recently, we
[3d]
2
2
8
Scheme 1. Direct reaction of methane with SO in the pres-
ence of O2.
3
to sulfonate methane with SO in fuming sulfuric acid.
3
Adv. Synth. Catal. 2004, 346, 913±916
DOI: 10.1002/adsc.200404060
¹ 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
913

COMMUNICATIONS
Sudip Mukhopadhyay, Alexis T. Bell
[a]
Table 1. Effect of different catalysts on the rate of sulfonation of methane to MSA.
Entry
Catalyst
t [h]
% SO to
MSA
MSA
[mmol]
% SO to
% Selectivity to
MSA
3
3
CH OSO H
3
3
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
VOSO4
Sn(SO4)2
RhCl3
5
5
6
6
6
5
5
5
5
4
5
5
4
5
5
5
5
1.2
1
7
11
10
0.26
0.21
1.49
2.34
2.13
0
±
0.015
0
1
1
1.5
0.01
0.09
0
0.03
0.06
1
1
0
0
1
55
50
82
99.9
99
±
25
54
0
Ce(SO4)2
Ce (SO )
2
4
3
CaCl2
0
TiCl4
FeCl3
RuCl3
Co(II)phthalocyanine
AgCl
0.01
0.07
0
1
1
1
1
1
1
1
1
0
0
0
0.12
0.3
0.3
0.25
0.6
0.3
0.04
0.026
0.06
0.06
0.053
0.13
0.06
0.008
100
100
23
83
50
6
Zn(CH COO)
3
2
Ga (SO )
2
4 3
In(CF SO )
3 3
0.05
0.6
5
3
Tl(CF CO )
3
2 3
Pd(CF COO)
3
2
PtCl4
10
0.4
[
a]
Reaction conditions unless otherwise stated: CH , 360 psig (101 mmol); SO , 1.7 g (21.26 mmol); molar ratio of CH to SO ,
4
3
4
3
4
.75; O , 40 psig (14.8 mmol); catalyst, 0.09 mmol; solvent, fuming sulfuric acid, 5.67 g; temperature, 1308C.
2
of SO converted to MSA to the moles of SO taken ini- sion of SO to MSA from 0 to 11% without affecting
3
3
3
tially in to the reactor.
the MSA selectivity, which remained at ca. 100%. Rais-
VOSO and Sn(SO ) exhibited limited activity and ing the temperature from 95 to 1308C increased the con-
4
4 2
RhCl was moderately active; however, all three cata- version of SO to MSA from 4 to 11% (Table 2, en-
3
3
lysts showed high selectivity to MSA (Table 1, en- tries 11±14). However, the conversion of SO to CH3
3
tries 1±3). Ce(IV) or Ce(III) sulfates showed activities OSO H remained constant at 0±0.01%. The conversion
3
superior to RhCl3 and, more importantly MSA to MSA reached a maximum of 16% at 1508C; however,
selectivities>99% (Table 1, entries 4 and 5). Notably, the conversion of SO to CH OSO H at this temperature
3
3
3
almost no MSA formation was detected when CaCl2, also rose to 9%, and 1% methanedisulfonic acid was de-
TiCl , FeCl , RuCl , Co(II) phthalocyanine, AgCl, tected as well.
4
3
3
Zn(CH COO) , Ga (SO ) , In(CF SO ) , or Tl(CF
The conversion of SO to MSA observed after 6 h of
3
2
2
4
3
3
3
3
3
3
COO) were used as the catalyst (Table 1, entries 6± reaction increased with increasing initial SO concentra-
3
3
1
5). Pd(CF COO) and PtCl exhibited modest activity tion up to a concentration of 30 wt %, at which point the
3 2 4
and high selectivity to CH OSO H (Table 1, entries 16 conversion of SO to MSA was 11%. However, raising
3
3
3
and 17).
the SO concentration to 56 wt % decreased the amount
3
Given the high activity of Ce(SO ) , the effects of the of MSA formed and increased the amounts of CH OSO3
4
2
3
reaction conditions on the conversion of SO and the se- H (7%) and methanedisulfonic acid [CH (SO H) ] (1%)
3
2
3
2
lectivity to MSA were investigated. The results of these formed (Table 2, entries 15 and 16). For a given set of re-
experiments are presented in Table 2. The O partial action conditions, replacing H SO by trifluorometh-
2
2
4
pressure (Table 2, entries 1±5) affects both the catalyst anesulfonic acid resulted in an identical conversion of
activity and selectivity. In the absence of O , 8% of the SO to MSA.
2
3
initial SO is converted to MSA and 7% of the SO is
The mechanism by which Ce catalyzes the sulfonation
3
3
[9]
converted to CH OSO H after 6 h of reaction at of methane is not known. A free radical mechanism
3
3
1
308C. Raising the O partial pressure to 40 psig lowered can be ruled out since it is known that in the presence
2
the overall sulfonation rate; however, the selectivity to of O free-radical initiated sulfonation is strongly inhib-
MSA increased from 53 to 99.9%. Further increasing ited.
2
[
3a±c]
An alternative idea is that methane undergoes
the O partial pressure to 60 psig decreased the MSA se- oxidative addition to Ce(IV) in a manner similar to that
2
[
6a]
lectivity to 88%. The major byproduct detected in this proposed for Hg(II). If this were the case, then Ce(IV)
case was CH OSO H. Thus, with 40 psig O , a 23% con- cations would be expected to be reduced to Ce(III) cat-
3
3
2
version of SO to MSAwas achieved in 42 h with 99.9% ions. The function of O in such a scheme would be to re-
3
2
selectivity. The reaction rate depends on the catalyst oxidize Ce(III) back to Ce(IV). Consistent with this
loading (Table 2, entries 6±10). Increasing the amount hypothesis, the conversion of SO to MSA is observed
3
of Ce(SO ) from 0 to 0.09 mmol increased the conver- to increase with increasing O partial pressure. Howev-
4
2
2
914
¹ 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
Adv. Synth. Catal. 2004, 346, 913±916

Sulfonation of Methane to Methanesulfonic Acid
COMMUNICATIONS
[a]
Table 2. Effect of different process parameters on the rate of sulfonation of methane to MSA.
Entry
Parameter
t
[
% SO to
MSA
% SO to
MSA
[mmol]
% Selectivity to
MSA
3
3
h]
CH OSO H
3
3
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
O , 0 psig
20 psig
40 psig
40 psig
6
6
6
8
8
11
23
12
0
4
8
11
12
4
7
1.2
0.01
0.1
1.6
0
0
0
0.01
0.5
0
0
0.01
9
0.01
7
1.7
1.7
2.34
4.9
2.55
0
53
87
99.9
99.9
88
2
42
6
6
6
6
6
6
6
6
6
6
6
6
60 psig
Ce(SO ) , 0 mmol
±
4
2
0.03 mmol
0.06 mmol
0.09 mmol
0.12 mmol
Temp, 958C
1208C
0.85
1.7
100
100
99.9
96
100
100
99.9
61
2.34
2.55
0.85
1.49
2.34
3.4
1
1
1
1
1
1
1
7
1308C
1508C
11
16
11
9
[
[
b]
b]
SO , 30 wt %
2.34
1.91
99.9
53
3
SO , 56 wt %
3
[
a]
Reaction conditions unless otherwise stated : CH , 360 psig (101 mmol); O , 40 psig; catalyst, Ce(SO ) , 0.09 mmol; solvent,
4
2
4 2
fuming sulfuric acid, 5.67 g; T¼1308C.
[
b]
Ca.0.6 to 1% methanedisulfonic acid was also formed as a byproduct.
er, when the O partial pressure exceeds 40 psig, oxida- tached to a gas handling system. The reactor was pressurized
2
tion of MSA to CH OSO H occurs and the selectivity to with 40 psig O
(Matheson, 99.99%) and then with 360 psig
CH (Matheson, ultra high purity). The reactor was heated to
3
3
2
MSA is lost. The decrease in SO conversion to MSA
4
3
1
308C, a process which took 15±20 min, and this temperature
with increasing temperature is attributable to the oxida-
tion of MSA to CH OSO H, which occurs more rapidly
was then maintained for up to 6 h, unless stated otherwise. Fol-
lowing reaction, the reactor was cooled in an ice bath to room
temperature (ꢀ0.35 h), and the reactor was vented. The gases
exiting the reactor were passed through scrubbers containing
NaOH and Carbusorb to remove sulfur(II) compounds. The
3
3
with increasing temperature. While SO is required for the
3
formation of MSA, above a certain critical SO concentra-
3
tion sulfonation of MSA to methanedisulfonic occurs.
In conclusion, we have demonstrated a catalytic proc-
system was then purged with N purified by passage through
2
ess for the direct, liquid-phase sulfonation of methane oxysorb, ascarite, and molecular sieve traps. The liquid product
with SO in the presence of molecular O at relatively was removed from the glass liner and added slowly to 0.6 g of
3
2
low temperature and methane pressure. Under the
H O to convert any unreacted SO to H SO . Reaction prod-
2 3 2 4
1
ucts were characterized by H NMR. All spectra were acquired
using a Bruker AMX-400 MHz FT-NMR spectrometer. A ca-
best reaction conditions, 23% conversion of SO , 33%
3
conversion of O , and 4.85% conversion of methane to
2
pillary containing D O, immersed within the NMR tube con-
2
MSA was achieved in fuming sulfuric acid with 99.9%
product (MSA) selectivity, using Ce(SO ) as the cata-
taining the sample, was used as a lock reference, and integra-
tion standard. All other chemicals were purchased from Al-
drich Chemical Company and used without further purifica-
4
2
lyst. The production rate of MSAwas 1.3 mole/mole cat-
alyst-h. Besides Ce salts, RhCl emerged as an effective
1
3
tion. Products were identified by comparison of their H
catalyst in sulfonating CH with SO in the presence
4
3
NMR chemical shifts to standard samples. The corresponding
chemical shifts for MSA was 2.93 ppm to 3.18 ppm, depending
on the concentration of MSA in the reaction mixture.
of O2.
Experimental Section
Acknowledgements
3
Reactions were carried out in a 100 cm high-pressure auto-
clave (Parr Instruments ± 3000 psig maximum) constructed
of Hastelloy B. Unless otherwise stated, the following proce-
dure was used for all experiments. 0.09 mmol of Ce(SO ) (Al-
This work was supported by a grant from ATOFINAChemicals,
Inc.
4
2
drich Chemical Company), and 5.67 g fuming sulfuric acid (Al-
drich Chemical Company, 27±33% SO ) were added to a glass
3
liner containing a Teflon-encased stirring bar. For some experi- References and Notes
ments, additional SO (Aldrich Chemical Company, 99%) was
added to the liquid in the liner. The glass liner was then trans-
ferred to the reactor, after which the reactor was sealed and at-
3
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Adv. Synth. Catal. 2004, 346, 913±916
asc.wiley-vch.de
¹ 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
915

COMMUNICATIONS
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3
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916
¹ 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
Adv. Synth. Catal. 2004, 346, 913±916