RESEARCH
◥
initiator affords MSA (Eq. 3) in 99% yield with
99% selectivity
REPORT
INDUSTRIAL CHEMISTRY
CH4(g) + SO3(l) → CH3SO3H(l)
(3)
+
We studied the reaction in a batch system to
optimize the reaction conditions and gain fur-
ther insight into the reaction mechanism. The
electrophilic initiator contains various sulfonyl
peroxide derivatives prepared as a mixture of
H2O2, MSA, and H2SO4 (19); monomethylsulfo-
nylperoxide sulfuric acid [HOS(O)2OOS(O)2CH3;
1 (MMSP)] is the reactive species (fig. S1). Pre-
viously, we reported that employing bis(meth-
ylsulfonyl) peroxide [H3CS(O)2OOS(O)2CH3;
2 (DMSP)] as the initiator led to substantial
yields of MSA (20, 21); however, this work dem-
onstrates that MMSP outperforms DMSP in
terms of rates and technical feasibility. We
observed lower productivities when using the
400-ml reactor compared with the 4-liter reactor,
presumably due to the larger quantity of CH4
in the headspace of the larger reactor (Table 1,
entries 1 and 2). Maintaining a constant amount
of CH4 throughout the reaction, we obtained
higher yields of MSA (entry 3) than those ob-
tained via regular batch experiments (entry 2).
The solubility of CH4 increases with increasing
pressure, enabling higher concentrations of CH4
in the liquid phase (fig. S8). We then varied the
reaction temperature from 25° to 85°C. More
than 99.9% selectivity toward MSA was achieved
at 50°C, whereas a more complex mixture of
products (MBS, SO2, and methanesulfonic acid
anhydride) typical of a radical pathway was ob-
tained at 85°C (entry 4), presumably due to the
thermal decomposition of the sulfonyl peroxide
(16, 17). Consistently, low-temperature experi-
ments (entry 5) afforded high conversions and
selectivity toward MSA; however, long reaction
times were needed (e.g., 70.2% yield at 25°C for
720 hours; fig. S5).
Experimentally, four major insights support-
ing a nonradical mechanism are evident from
the data in Table 1. First, high yields and selec-
tivities for MSA (entry 1) indicate a nonradical
selectivity pattern without the formation of higher
alkanes (e.g., radical recombination) or other
sulfonated hydrocarbons in the liquid or gas
phase, as observed by gas chromatography anal-
ysis (fig. S10). In contrast, previous studies showed
that radical reactions tend to have lower yields
and selectivities, generating a mixture of sulfo-
nated alkanes with concomitant evolution of
CO2 (10–12). Second, ultraviolet (UV) light ir-
radiation of the reaction mixture containing
the electrophilic initiator is known to prompt
the homolytic decomposition of the –O–O– bond.
Indeed, sulfonylperoxides are well known to
easily photodecompose to sulfate radical anions
(e.g., S2O82−/SO52− → SO4•−) (16). After the reac-
tion mixture was exposed to UV light (~190 nm)
for 8 hours, no changes in the pressure of CH4
were observed, indicating the inability of the
initiator (–O–O–) to form radicals under these
conditions (entry 6). Third, the use of O2 as a
Activation of methane to CH3 :
A selective industrial route
to methanesulfonic acid
Christian Díaz-Urrutia and Timo Ott*
Direct methane functionalization to value-added products remains a challenge
because of the propensity for overoxidation in many reaction environments.
Sulfonation has emerged as an attractive approach for achieving the necessary
selectivity. Here, we report a practical process for the production of methanesulfonic
acid (MSA) from only two reactants: methane and sulfur trioxide. We have achieved
>99% selectivity and yield of MSA. The electrophilic initiator based on a sulfonyl
peroxide derivative is protonated under superacidic conditions, producing a highly
electrophilic oxygen atom capable of activating a C–H bond of methane. Mechanistic
+
studies support the formation of CH3 as a key intermediate. This method is readily
scalable with reactors connected in series for prospective production of up to 20 metric
tons per year of MSA.
idespread application of fracking tech-
niques and biogas production has provided
access to large quantities of inexpensive
methane. Methane’s largest chemical trans-
formations remain confined to the highly
materials and the potential for rapid integra-
tion into current industrial chemical processes
(4). MSA is biodegradable and nonoxidizing,
with current uses in the pharmaceutical and
electroplating industries (50 metric kilotons/
year), as well as prospective applications in
metal recycling, energy storage, and biodiesel
(5). Early work by Snyder et al. demonstrated
oxidation of CH4 to mixtures of oxygenated and
sulfonated products (6), employing fuming sul-
furic acid (oleum) at high temperatures (325°C)
with HgSO4 as a catalyst. Later, Periana et al.
further developed the Hg-mediated electrophilic
C–H activation of CH4 in H2SO4 at 180°C, pro-
ducing SO2 and methyl bisulfate (CH3OSO3H;
MBS) (7). Later work by Periana and Schüth
and colleagues using Pt complexes (8) and Pt
salts (9) in H2SO4 and oleum also produced high
yields of MBS and SO2, respectively, at high
temperatures. Bell (10, 11) and Sen (12) and col-
leagues focused on methanesulfonation in oleum
using peroxo salts and different metal and
nonmetal additives as radical initiators. The
sulfonation is initiated in this case by the ther-
mal decomposition of peroxosulfates, generat-
ing sulfate radical anions (16, 17). Nevertheless,
these reactions suffered from low yields and
conversions as a result of free-radical recom-
bination (e.g., ethane formation) and undesired
side reactions (10–13), rendering them unsuit-
able for large-scale production. In this regard,
superacid chemistry (e.g., Hammett acidity
function H0 < −12) provides the balance be-
tween reactivity and selectivity that an industrial
process requires (18). We report here that treat-
ment of oleum (e.g., 20 to 60% SO3) with CH4 at
~100 bar (50°C) using <1 mol % of electrophilic
W
energy demanding steam-reforming and Fischer-
Tropsch processes (Eqs. 1 and 2). However, CH4
functionalization to more complex molecules is
limited because of overoxidation
CH4(g) + H2O(g) → CO(g) + 3H2(g) (1)
(2n + 1)H2(g) + nCO(g) → CnH(2n + 2)(g) + nH2O(g)
(2)
and higher reactivity of the potential products
than that of the starting materials (1, 2). Major
efforts to overcome this challenge have been
made (1, 3–14); however, the industrial applica-
bility of these processes is restricted by economic
constraints, scalability challenges, and low selec-
tivity. The only industrial process for the direct
valorization of CH4 to high–value-added chem-
icals is the oxidative coupling of CH4 using het-
erogeneous metal catalysts (14). More recently,
environmental concerns have boosted the search
for new applications of CH4 beyond its current
use as a heating and hydrogen source (15). In
this context, sulfonation of CH4 to methane-
sulfonic acid (MSA) has received substantial
attention owing to the abundance of both raw
R&D Department, Chemicals Division, Grillo-Werke AG,
Weseler Strasse 1, 47169 Duisburg, Germany.
*Corresponding author. Email: t.ott@grillo.de
Díaz-Urrutia et al., Science 363, 1326–1329 (2019)
22 March 2019
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