Enhanced Catalytic Activity of (DMSO)2PtCl2 for the Methane Oxidation in the SO3-H2SO4 System

Article abstract of DOI:10.1021/acscatal.8b04101

Among the various methane activation reactions, oleum-mediated (SO₃-H₂SO₄) methane oxidation to methyl bisulfate (MBS), a methanol precursor, is one of the best methods in terms of methane conversion and product selectivity. In this report, we investigated the effect of the catalyst concentration on the MBS yield using the catalyst systems (bpym)PtCl₂, K₂PtCl₄, and (DMSO)₂PtCl₂ at 180 °C for 3 h. (bpym)PtCl₂ showed a very high stability as well as a high MBS yield of over 84%, but its catalytic activity expressed in terms of turnovers for 3 h was in the range 50-500. K₂PtCl₄ showed very high catalyst turnovers of over 17 000 at a low catalyst concentration; however, it deactivated rapidly to PtCl₂ which prevents achieving a high MBS yield that was achieved by (bpym)PtCl₂. However, (DMSO)₂PtCl₂ showed an enhanced catalytic performance for both the MBS yield and the turnovers. An MBS yield of over 84% with a selectivity of 94% was obtained at a catalyst concentration of 3.0 mM, and its turnovers reached over 19 000 at a low catalyst concentration. This higher catalytic performance of (DMSO)₂PtCl₂ compared to the other chloride-ligated Pt compounds is due to the DMSO ligand on the Pt, which increases the solubility of the Pt species in oleum. Furthermore, as the DFT study revealed, the low dissociation energy of DMSO from the Pt center can facilitate the coordination of methane on the Pt reaction center. Although (DMSO)₂PtCl₂ was deactivated to PtCl₂ after the reaction like the other Pt compounds with the chloride ligand, it can be reactivated to some extent by adding DMSO.

Full text of DOI:10.1021/acscatal.8b04101

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Article
Enhanced catalytic activity of (DMSO)2PtCl2 at
the methane oxidation in SO3-H2SO4 system
Huyen Tran Dang, Hee Won Lee, Jieon Lee, Hyunah Choo,
Soon Hyeok Hong, Minserk Cheong, and Hyunjoo Lee
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04101 • Publication Date (Web): 08 Nov 2018
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Enhanced catalytic activity of (DMSO)2PtCl2 at the
methane oxidation in SO3-H2SO4 system
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Huyen Tran Dang†,‡,¶, Hee Won Lee†,‡,¶, Jieon Lee§, Hyunah Choo§, Soon Hyeok Hongǁ, Minserk
Cheong⊥, Hyunjoo Lee*,†,‡
† Clean Energy Research Center, Korea Institute of Science and Technology, 39-1 Hawolgok
dong, Sungbuk-gu, Seoul 136-791, Republic of Korea
‡ Division of Energy & Environment Technology, KIST School, Korea University of Science and
Technology, Seoul 02792, Republic of Korea
§Center for Neuro-Medicine, Korea Institute of Science and Technology, Seoul 02792, Korea
ǁ Department of Chemistry, College of Natural Sciences, Seoul National University, Seoul 08826,
Republic of Korea
⊥Department of Chemistry and Research Institute of Basic Sciences, Kyung Hee University, 26
Kyungheedae-ro, Dondaemun-gu, Seoul 02447, Korea
* To whom correspondence should be addressed:
E-mail: hjlee@kist.re.kr; Fax: +82-2-958-5809; Tel.: +82-2-958-5868
¶ These authors contributed equally to this work.
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ABSTRACT
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Among the various methane activation reactions, oleum (SO3-H2SO4) mediated methane oxidation
to methyl bisulfate (MBS), a methanol precursor, is one of the best methods in terms of methane
conversion and product selectivity. In this report, we investigated the effect of the catalyst
concentration on the MBS yield using the catalyst systems, (bpym)PtCl2, K2PtCl4, and
(DMSO)2PtCl2 at 180 oC for 3 h. (bpym)PtCl2 showed a very high stability as well as a high MBS
yield of over 84%, but its catalytic activity expressed in terms of turnovers for 3 h was in the ranges
of 50 – 500. K2PtCl4 showed very high catalyst turnovers of over 17,000 at a low catalyst
concentration; however, it deactivated rapidly to PtCl2 which prevents achieving a high MBS yield
that was achieved by (bpym)PtCl2. However, (DMSO)2PtCl2 showed an enhanced catalytic
performance for both the MBS yield and the turnovers. An MBS yield of over 84% with a
selectivity of 94% was obtained at a catalyst concentration of 3.0 mM, and its turnovers reached
over 19,000 at a low catalyst concentration. This higher catalytic performance of (DMSO)2PtCl2
compared to the other chloride-ligated Pt compounds is due to the DMSO ligand on the Pt, which
increases the solubility of the Pt species in oleum. Furthermore, as the DFT study revealed, the
low dissociation energy of DMSO from the Pt center can facilitate the coordination of methane on
Pt reaction center. Although (DMSO)2PtCl2 was deactivated to PtCl2 after the reaction like the
other Pt compounds with chloride ligand, it can be reactivated to some extent by adding DMSO.
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KEYWORDS: methane, oxidation, Pt, oleum, oxidation, methylbisulfate, methanol
INTRODUCTION
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Catalytic activation of methane to get more valuable chemical such as methanol is an important
issue from both a practical and academic standpoint.1-6 For the conversion of methane to methanol,
one of the four C-H bonds in the methane should be selectively oxidized. Due to its abundance
and innocuous property, molecular oxygen-mediated direct oxidation of methane using a
heterogeneous catalyst has been studied extensively.7-10 However, a high reaction temperature
above 600 oC and the trade-off between conversion and selectivity limit this method. Other kinds
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2-
of oxidants such as H2O2, SO3, S2O8 , SeO3 and iodates, which can oxidize methane at a milder
11-23
condition with high selectivity, have also been used.
In particular, SO3 is attracting much
attention because it can be regenerated using oxygen from the reduced state, SO2, and is used in
the commercialized process for sulfuric acid production.
The first pioneering work, in which oleum (SO3-H2SO4) was used as an oxidant for methane
oxidation, was carried out by Periana et al. who used Hg(SO4) and sulfuric acid as a catalyst and
a solvent, respectively.15 In this reaction, methane is oxidized to methyl bisulfate (MBS) which
can be converted to methanol by hydrolysis with water in a separate process (Scheme 1). This SO3-
H2SO4 oxidation system was further evolved by the development of the Catalytica/Periana system,
(bpym)PtCl2, which showed a methane conversion of 90%, a selectivity of over 80% and a catalyst
turnover of over 20.16 The prominent feature of this catalyst is the bipyrimidyl ligand which
stabilizes the Pt species so as not to be precipitated in the form of polymeric PtCl2. Since then, Pt-
catalyzed methane oxidation reactions to MBS have been studied in various ways.24-29 Cheng et
al. used an ionic liquid such as 1-methylimidazolium chloride to dissolve less-soluble Pt catalysts
such as PtCl2 and PtO2 in a sulfuric acid system.30 The use of ionic liquid was found to increase
the catalytic activities of the Pt compounds by coordination and/or intermolecular interaction with
Pt as well as by increasing the Pt solubility in the reaction system. Palkovits et al. and Soorholtz
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et al. had immobilized PtCl2 on a covalent triazine-based framework (CTF) and nitrogen-doped
carbon (NDC), respectively, and showed that the PtCl2-immobilized solid catalysts had very high
catalytic activities, which were superior to the homogeneous Pt(bpym)2Cl2.31-32 However, the
catalytic activities of the solid catalysts decreased during reuse due to the leaching of the Pt
component on the solid catalyst, suggesting the probability of a homogeneous reaction. This
assumption was further supported by the same research group that showed that a very small amount
of homogeneous Pt species can act as a catalyst.
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Zimmermann et al. reported that the performance of Pt compounds in this oleum-mediated
oxidation of methane was on the order of large-scale commercialized processes.33 They showed
that the turnover frequency (TOF, h-1) of platinum compounds including (NH3)2PtCl2, Pt black,
and Pt(acac)2 reached over 20,000, which was measured in a reaction condition with a very low
catalyst concentration ~ 0.6 mM and a short reaction time less than 5 min. In that report, K2PtCl4
showed the highest turnovers (TOs) of 940 with a product selectivity of 98% for 2.5 min, while,
(bpym)PtCl2 showed TOs of 650 for 30 min, which is a much higher value compared to the
previously reported value of 20 obtained with a higher catalyst concentration of 50 mM and a
longer reaction time of 2.5 h.33 Zimmermann et al. ascribed the difference of these two results to
(i) the effect of the SO3 concentration, (ii) a different catalyst solubility at a high catalyst loading,
and (iii) the difference in the reaction time. They also showed that PtCl2 had a comparable activity
to (bpym)PtCl2.
It seems that the formation of PtCl2 from the chloride-ligated Pt catalyst is inevitable during the
reaction, which leads to catalyst deactivation for any chloride-ligated Pt catalyst in a sulfuric acid
system. Because PtCl2 precipitates in the form of a polymer, the catalyst concentration is one of
the important factors that affect the MBS yield. To prevent catalyst deactivation in the form of
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PtCl2, one solution could be to conduct the reaction at a low catalyst concentration as reported by
Zimmermann.33 Another method might be to introduce a ligand that can stabilize the chloride-
ligated platinum catalyst like in the Catalytica system, (bpym)PtCl2. However, the strong bidentate
ligand does not appear to increase the catalytic activity compared to its precursor K2PtCl4. These
results motivated us to investigate a new chloride-ligated Pt compound with a ligand that can not
only increase the stability of the Pt species but also increase the catalytic activity under these harsh
reaction conditions.33
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Herein, we investigated the effect of the catalyst concentration on the catalytic activities of Pt
compounds including K2PtCl4, PtCl2, and (bpym)PtCl2. Understanding the catalytic behavior of
the conventional system according to the catalyst concentration prompted us to introduce a new
catalyst system, (DMSO)2PtCl2, which is widely used as an intermediate for the synthesis of a Pt-
based organometallic catalyst because of its good solubility in various solvents as well as easy
synthesis method 34-36. As expected, (DMSO)2PtCl2 exhibited the highest catalytic performance
among the tested catalysts. A DFT calculation was conducted to understand the high activity of
(DMSO)2PtCl2, and a potential energy profile for the catalytic cycle of this chloride-ligated Pt(II)
was presented.
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EXPERIMENTAL SECTION
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General Information. All chemicals were of analytical reagent grade and used without further
purification. K2PtCl4 was obtained from STREM Chemicals Inc. Dimethyl sulfoxide DMSO,
oleum (20%wt SO3), PtCl2 and other chemicals for the synthesis of catalyst were purchased from
Sigma Aldrich Chemical Co. High-purity methane containing 1% of Ar was supplied from
Shinyang Gas Co (Korea).
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Oxidation reaction. Partial oxidation of methane was conducted using bomb reactor (SUS316,
100 ml) equipped with glass liner, thermocouple, and heating jacket. Glass liner containing catalyst
and 30 g of 20%wt oleum (SO3 75 mmol) was positioned into the reactor and pressurized with 25
bar of CH4 at room temperature. With the solution stirred by using teflon-coated magnetic stirrer
with 800 rpm, reactor was heated to 180 oC and the resulting mixture was stirred for 3 h. During
the reaction, the pressure was maintained to 35 bar. After the reaction, the reactor was cooled to
room temperature and the remaining gas was vented in the hood.
Catalyst synthesis. Periana catalyst, (bpym)PtCl2 (η2-(2,2’-Bipyrimidyl)dichloroplatinum(II))
shown in Figure 1, was synthesized according to the previous method.16 Potassium
tetrachloroplatinate (II) K2PtCl4 (0.223 g, 0.54 mmol) was dissolved in water (2 mL), then 2, 2’-
bipyrimidine (0.085 g, 0.54 mmol) was added. This solution was stirred for 12 h at room
temperature. An orange precipitation was filtered, washed with acetone and dried in a drying
vacuum system at 60 oC. Yield: 92%, 1H NMR (D2SO4), δ 9.81 (d, J = 6.0 Hz, 2H), 9.34 (dd, J =
5.4, 1.8 Hz, 2H), 8.22 (t, J = 5.8 Hz, 2H).
cis-Dichlorobis(dimethyl sulfoxide)platinum(II) (DMSO)2PtCl2 shown in Figure 1 was
synthesized from K2PtCl4 (0.83g, 2.00mmol) and dimethyl sulfoxide (0.44 mL, 6.00 mmol) in
water (2 mL) at room temperature with a gentle stirring for 12 h. The white solid was filtered off,
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washed with ethanol and diethyl ether several times and dried under the vacuum condition. Yield:
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92%, 1H NMR (CDCl3), δ 3.54 (t, J = 11.0 Hz, 6H)
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Product Analysis. Liquid product containing methyl bisulfate was analyzed using H NMR
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spectroscopy (400 MHz, Varian). Calibration curve was made using sodium bisulfate
(CH3OSO3Na) and methane sulfonic acid (CH3SO3H) as an external standard in D2SO4. In order
to evaluate the CO2 formation, the gas phase product was captured, washed with distilled water,
and analyzed using GC-MS (HP 6890 GC with a 5973 Mass spectrometer selective detector)
equipped with capillary column (Poraplot Q 30 m x 25 um). The peak areas of Ar which was
included in methane (1%) was used as a reference to determine the concentration of CO2. The
Platinum concentration in the liquid phase was also examined by ICP-OES technique (Thermo
scientifiv iCAP 700 series-ICP spectrometer). The X-ray diffraction patterns (XRD) were recorded
on a Shimazu X-ray diffractometer (XRD-6000, Japan) using nickel-filtered CuKα radiation with
2θ angle from 10 to 90°.
After the reaction, yield and selectivity of MBS were calculated as follows. According to the
eq. (3) in Scheme 1. Catalyst turnovers (TOs) were obtained from the synthesized amount of MBS
and used catalyst amount after 3 h reaction.
MBS produced (mmol) × 2
Yield (MBS, %) = 100 ×
SO3 used (mmol)
MBS produced (mmol)
Selectivity (MBS, %) = 100 ×
MBS produced (mmol) + CO2 produced (mmol)
Turnovers (TOs, MBS) = MBS produced (mmol)/catalyst used (mmol)
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The mass balance of the reaction was calculated by conducting the reaction in a closed
condition. The pressured drop by the methane conversion to MBS and CO2 was measured and the
consumed amount of methane was calculated using the Benedic-Webb-Rubin equation.33 The
results in Table S3 shows that the produced amounts of MBS and CO2 were almost equal to the
consumed amount of methane which was calculated from the pressure decrease.
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Computational Details. All calculations were carried out using Gaussian 16 (Revision A.03.
Gaussian, Inc., Wallingford CT). All ground-state and transition-state geometries were optimized
at the B3LYP level of theory. Basis sets selected for these calculations are the 6-31G(d,p) for C,
H, N, O, S, and Cl and LANL2DZ for Pt. Geometries were confirmed as minima or transition-
state structures by normal-mode vibrational frequency analysis. Exhaustive conformational
searching was performed for all ground-state and transition state structures, but only the lowest-
energy structures are reported. Calculations were performed for reactions in the liquid phase, so
the SMD polarizable continuum solvent model37 augmented with parameters for sulfuric acid
(static (eps = 101) and optical (epsinf = 2.042) dielectric constants)38 was used.
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RESULTS AND DISCUSSION
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When methane is oxidized to methyl bisulfate (MBS) with SO3/H2SO4, two reactions can happen
in the reaction system (Scheme 1). The first reaction is the oxidation of methane to MBS in the
presence of a catalyst and oxidant/solvent, SO3/H2SO4 (eq. (1) in Scheme 1). At this reaction, water
is co-produced, which rapidly reacts with SO3 to form H2SO4 (eq. (2) in Scheme 1). Therefore, the
net reaction of the oxidation of methane to MBS can be expressed as eq. (3) in Scheme 1; one mole
of methane reacts with 2 moles of SO3 to make 1 mole of MBS and SO2, respectively. MBS can
be hydrolyzed to methanol and sulfuric acid as eq. (4) in Scheme 1. Besides MBS, CO2 is also
formed by this oxidation (eq. (5) in Scheme 1). Irrespective of the formation route, 6 equiv. of SO3
are consumed for the formation of CO2 from CH4. Therefore, the yield of MBS, the selectivity to
MBS, and the turnovers (TOs) for MBS were calculated as described in the experimental part.
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Methane Oxidation using Conventional Pt(II) Catalysts. First, we did a methane oxidation
reaction using the previously studied chloride-ligated catalysts, (bpym)PtCl2, K2PtCl4, and PtCl2
in the presence of 30 g of 20wt% oleum at 180 oC for 3 h under a methane pressure of 35 bar. The
effects of the catalyst concentrations were investigated at ranges between 0.15 to 50 mM, which
cover the experimental condition of the previous reports by Periana and Zimmermann.16, 33 Figure
2 and Tables S4 – S7 showed that the tested catalysts had very different yields of the methane
oxidation product, methyl bisulfate (MBS) according to the catalyst concentration.
At a very low catalyst concentration of 0.015 mM, K2PtCl4 showed a quite high MBS yield of
11.5% corresponding to near 18,000 turnovers (TOs) during a 3 h reaction. With an increasing
catalyst concentration up to 0.77 mM, the yield also rapidly increased to 61.6%; however, the yield
of MBS was maximum at this concentration and decreased thereafter. At a concentration between
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1.54 and 50 mM, the yields were nearly 50%. The comparison reaction depicted in Figure 3 shows
that the MBS yield for a 0.77 mM K2PtCl4 solution is always higher than that of for a 50 mM
K2PtCl4 solution for a reaction from 1 – 5 h, and, after 12 h, the MBS yields reached about 74% in
both cases. This unexpected phenomenon can be ascribed to the formation of PtCl2 under a high
catalyst concentration condition. It was already reported that the chloride-ligated Pt complex is
transformed to α-PtCl2, a polymeric form, which is known to be less soluble in oleum thereby
comparatively less active.
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However, α-PtCl2 showed a very interesting behavior according to the concentration (Figure 2
and Table S4). It had almost constant MBS yields at about 38% with a catalyst concentration from
0.77 ~ 4.8 mM, indicating that the concentration of the active catalytic component is similar in
these concentration ranges. However, at a high catalyst concentration of 24.4 mM, it had quite a
high MBS yield of 58.2%, and the yield was increased to 71.7% when 50 mM catalyst was used,
which was undoubtedly higher compared to the K2PtCl4-catalyzed reaction.
Compared to K2PtCl4 and PtCl2, (bpym)PtCl2 showed a typical pattern as a homogeneous
catalyst concentration (Figure 2 and Table S6); the MBS yield almost linearly increased with the
increasing catalyst concentration. At 0.77 M, the MBS yield was 11.8%, which is the lowest value
among the catalysts tested. With increasing catalyst concentrations, the yield of the product
increased gently and reached the highest MBS yield of 84.2% at 12.6 mM in which the TOs was
158.
Overall, in terms of the catalytic activity, with TOs during a 3 h reaction, K2PtCl4 showed a very
prominent result, especially at the low catalyst concentration domain. However, in terms of the
MBS yield, it has a limitation in reaching the highest MBS yield regardless of the catalyst
concentration due to the instability of the catalyst. Although (bpym)PtCl2 had the highest MBS
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yield at a high catalyst concentration, it was clear that the catalytic activity is much lower compared
to those of K2PtCl4 and PtCl2.
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Methane Oxidation using (DMSO)2PtCl2. At this point, we propose and demonstrate a new
methane oxidation catalyst system, (DMSO)2PtCl2 showing the highest catalytic performance in
the production of MBS. Figure 2 and Table S6 show, at 0.015 mM of (DMSO)2PtCl2, the MBS
yield was 12.2%, and the yield rapidly increased to 67.8% when the concentration of the catalyst
reached 0.77 mM, which is almost similar with the value of K2PtCl4, 61.6%, obtained at the same
reaction condition. However, unlike K2PtCl4, the MBS yield increased more with the increasing
catalyst concentration and reached 84.6% at 3.0 mM. Under this condition, based on the amount
of CO2, the MBS selectivity was 94.7% and the SO3 conversion reached 98.7%. Further increases
in the catalyst concentration, however, resulted in slight decreases of the MBS yield to 81.3% and
the selectivity to 92.3% at 50.1 mM due to the increased formation of CO2. The highest turnovers
of over 19,000 was reached at 0.015mM and the values decreased with increasing catalyst
concentrations.
The catalytic activity and stability of (DMSO)2PtCl2 could also be confirmed from the effect of
the reaction time shown in Figure 3. At an early reaction time less than 3 h, the MBS yields of
(DMSO)2PtCl2 and K2PtCl4 were almost similar at the catalyst concentration of 0.77mM,
indicating that the intrinsic catalytic activities of these two catalyst are similar. However, when the
reaction time was increased to 14 h, the MBS yield increased more and reached 90.6% with the
(DMSO)2PtCl2, while the K2PtCl4 had a 71.3% MBS yield after the same reaction time. Therefore,
it can be concluded that (DMSO)2PtCl2 is more active than (bpym)PtCl2 and is more stable than
K2PtCl4.
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Extra Ligand Effect. This unprecedented high catalytic performance of (DMSO)2PtCl2 in this
oleum-mediated methane oxidation reaction can be ascribed to the DMSO ligand on the Pt
complex, which can stabilize the Pt species so as not to be converted rapidly to PtCl2 polymer at
the reaction condition thus increasing the single molecular homogeneous Pt species in the oleum
solution. Although the real concentration at a reaction condition of 180 oC in the presence of 20%
oleum, is unknown, measurement of the dissolved Pt concentration in 20wt% oleum at room
temperature revealed that the PtCl2 solubility increased 600 times when DMSO was dissolved
together with it (Table S3).
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The other role of DMSO is presumed to be to make a vacant site for methane coordination by
dissociation from the Pt center during the reaction. With (bpym)PtCl2, its stability is much higher
than any other chloride-ligated Pt complex; no precipitated PtCl2 was observed even at a highly
concentrated condition of 50 mM after the reaction. Nevertheless, the lower catalytic activity
compared to (DMSO)2PtCl2 might be due to the strong binding of the chloride ligand on the Pt.
To confirm this explanation, a DFT calculation was conducted for the dissociation energy of the
Pt-ligand. Figures 4 and 5 show that the dissociation energies of the Cl ligand for (bmpy)PtCl2 and
(DMSO)2PtCl2 are 22.1 kcal/mol and 20.7 kcal/mol, respectively, which are very similar to each
other, while the dissociation energy of the DMSO ligand from the (DMSO)2PtCl2 was calculated
to have a much lower value of 13.6 kcal/mol. Therefore, it is very reasonable to assume that,
instead of chloride dissociation, DMSO dissociation occurs in the (DMSO)2PtCl2-catalyzed
reaction for the methane coordination. The result shown in Table 1 that the addition of extra DMSO
or chloride to (DMSO)2PtCl2 resulted in a decreased yield of MBS also supports the dissociation
of the ligand from the Pt center thereby making a vacant site for methane coordination as one of
the important steps in this Pt-catalyzed reaction.
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The DMSO dissociation from (DMSO)2PtCl2 in oleum condition could be observed at the 1H-
NMR study. Figure S5 shows when (DMSO)2PtCl2 was dissolved in oleum, two peaks were
appeared at 3.61 ppm and 4.04 ppm ((b) in Figure S5), which assumed to free DMSO that was
dissociated from Pt center and Pt-bonded DMSO. Interestingly, adding DMSO to PtCl2 reagent
also show a similar NMR pattern ((d) in Figure S5), indicating DMSO can also activate PtCl2 in
oleum condition. This DMSO addition effect can be confirmed in the PtCl2-catalyzed reaction.
Table 1 shows when 2 equiv. of DMSO are added to the PtCl2-catalyzed reaction, the MBS yield
increased from 30.7% to 58.2%, whereas the addition of extra Cl ligand resulted in the
deterioration of the MBS yield. The addition of 2 and 4 equiv. of KCl to a 2.43 mM PtCl2-catalyzed
reaction reduced the MBS yield to 18.3% and 13.3% from 30.7%, respectively. Furthermore, an
excess amount of KCl completely suppresses oxidation. This extra chloride effect explains why
K2PtCl4 showed a maximum MBS yield at 0.77 mM and a lower MBS yields than those of PtCl2
at a concentration higher than 10 mM.
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Two kinds of PtCl2 forms are known to exist. 39-41 One is the α form, a polymeric structure, and
the other is the β form, a hexameric PtCl2. After the K2PtCl4-calalyzed reaction with a highly
concentrated solution of 50.1 mM, we could isolate the β-PtCl2 from the solid catalyst (supporting
information) and confirmed the structure using XRD. Figure 6 shows that the β-PtCl2 isolated from
the reaction had a very similar XRD pattern to the synthesized β-PtCl2. This result indicates that
PtCl2 exists not only in an insoluble polymeric form but also in a soluble monomer ~ oligomer
structure in oleum media, which could act as a homogeneous oxidation catalytic species in the
PtCl2-catalyzed reaction. Therefore, we can assume that, at a relatively low catalyst concentration
range of 0.77 ~ 4.8 mM, irrespective of the added catalyst amount, the total active catalytic species
might be similar in concentration due to the formation of various PtCl2 oligomers. Over a 20 mM
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concentration range, the dissolved oligomeric Pt species could increase with an increasing catalyst
concentration. At the same time, a heterogeneous catalytic reaction on the surface of the solid PtCl2
cannot be excluded at the high catalyst concentration condition.
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On the other hand, at a low catalyst concentration lower than 5 mM, the solubility of K2PtCl4
would be higher than that of PtCl2. Furthermore, a low concentration of Pt as well as the presence
of four chloride ligands on PtCl42- can prevent the immediate aggregation to the PtCl2 oligomer or
polymer. While, at a high concentration range, the PtCl2 formation could be accelerated. In
2-
addition, the existence of two equiv. of free chloride ions dissociated from PtCl4 during the
conversion to PtCl2 can block the active catalytic site on PtCl2.
Reactivation of deactivated (DMSO)2PtCl2. Although (DMSO)2PtCl2 exhibited the highest
catalytic activity among the tested Pt catalysts, it was not stable enough like (bpym)PtCl2; when a
large amount of (DMSO)2PtCl2 was used, PtCl2 formation was also observed after the reaction ((c)
in Figure S6). However, this limitation could be overcome to some extent by adding DMSO to the
deactivated system. After the oxidation of methane using (DMSO)2PtCl2 at 180 oC for 3 h, more
oleum was added, and methane oxidation was carried out again. Table 2 shows that the yield of
MBS decreased to 53.2% in the second reaction, indicating that (DMSO)2PtCl2 deactivated to
PtCl2 during the reaction. However, the addition of 2 equiv. of DMSO to the deactivated
(DMSO)2PtCl2 system increased the yield of MBS up to 78.4%, which is about 90% of the fresh
catalyst yield. The reason for the decreased activity in the second reaction mainly due to the
insufficient activation of the deactivated catalytic species and partially due to the increased volume
of the solution thereby decreasing the catalyst concentration as well as the contact area between
the liquid phase and gas phase.
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Reaction mechanism. Based on the above discussion, it seems that the (DMSO)2PtCl2 turns into
cationic sulfuric acid-coordinated Pt(II) species like I upon protonation of a facile ligand (Scheme
2). For the coordination of methane, a vacant coordination site has to be available through the
dissociation of a coordinated solvent or DMSO. After the coordination of methane, the C-H bond
of methane will be activated by oxidative addition to Pt, resulting in the formation of species III.
Reductive elimination of sulfuric acid leading to species IV and successive addition of protonated
SO3 forms species V. From species V, OH of the pronated SO3 is oxidatively added, and methyl
bisulfate, sulfuric acid, SO2, and water are eliminated to form the starting species I. The energy
diagram based on the DFT calculation is shown in Figure S7.
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CONCLUSION
o
Chloride-ligated Pt-catalyzed methane oxidation in oleum conducted at 180 C for 3 h using
various catalyst concentrations revealed that the ligand on the Pt catalysts controls the stability of
the catalysts as well as the catalytic activity. The Catalytica system, (bpym)PtCl2, showed the
lowest turnovers at low concentrations below 10 mM, but due to its stability, it had very high MBS
yields over 84% at 12.6 mM. K2PtCl4, a recently reported highly active methane oxidation catalyst,
showed a very high activity at a low catalyst concentration less than 1 mM; however, it showed a
maximum MBS yield of 61.6% at 0.77 mM and decreased thereafter, indicating that catalyst
deactivation in the form of PtCl2 accelerated with the increasing catalyst concentration. This
instability of K2PtCl4 and the low catalyst activity of Pt(bmpy)Cl2 could be overcome by
introducing the DMSO ligand to chloride-ligated Pt catalysts; (DMSO)2PtCl2 had a very high
turnovers close to 20,000 and the highest MBS yield of 84.6% at 3.0 mM. The role of DMSO in
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(DMSO)2PtCl2 seems to be to increase the catalyst stability preventing deactivation to PtCl2 during
the reaction and to dissociate from the Pt center more easily than chloride in (bpym)PtCl2 thereby
facilitating methane coordination on the active center. We observed that (DMSO)2PtCl2 was also
deactivated to PtCl2 after the reaction, but its activity could be restored to 90% of the activity of
the fresh catalyst by adding 2 equiv. of DMSO.
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ASSOCIATED CONTENT
AUTHOR INFORMATION
Corresponding Author
*E-mail: hjlee@kist.re.kr
Author Contributions
¶
H. T. Dang and H. W. Lee contributed equally to this work.
Notes
The authors declare no competing financial interest.
Supporting information
The Supporting Information is available free of charge on the ACS Publications website at DOI:
Figures S1-S2 shows NMR spectra of synthesized (bpym)PtCl2 and (DMSO)2PtCl2, respectively.
More detailed explanation about experimentals was added, which showed reactor system,
quantification and qualification for product, estimated error ranges. Procedures for the
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determination of Pt in sulfuric acid, isolation method for β-PtCl2 from product solid, and synthesis
of β-PtCl2 according to the previous work were also contained. Table S4-S7 are tabulated values
of Figure 2 showing TOs according to catalyst concentration.
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ACKNOWLEDGMENT
This work was supported by C1 Gas Refinery Program through the National Research
Foundation of Korea (NRF) (NRF-2015M3D3A1A01065435).
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Thiele, G. Structure and Bonding of the Hexameric Platinum(II) Dichloride, Pt6Cl12 (β-PtCl2). Z.
Anorg. Allg. Chem. 2003, 629, 516-522.
(40) Brodersen, V. K.; Thiele, G.; Schnering, H. G. Strukturuntersuchungen an Platin(II)-
Chloride. Z. Anorg. Allg. Chem. 1965, 337, 120-127.
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(41) Vorob’ev-Desyatovskii, N. V.; Smorodina, T. P.; Paramonov, V. A.; Postnikova, E. S.;
Sibirskaya, V. V.; Marchenko, A. I.; Kukshkin, Yu. N. Interaction of Adducts of Boron Trifluoride
with Platinum (II) Complexes (New Method of Producing β-Platinum Dichloride and Clatharates
from It). Zhurnal Prikl Him. 1985, 58, 977-984.
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SCHEMES, FUGURES, AND TABLES
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Step 1. Oxidation
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(1)
(2)
CH4 + SO3 + H2SO4
CH3OSO3H + SO2 + H2O
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H2O + SO3
H2SO4
CH3OSO3H + SO2
(3)
CH4 + 2SO3
Step 2. Hydrolysis
CH3OSO3H + H2O
CH3OH + H2SO4
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CO2 formation
CH4 + 6SO3
CO2 + 4SO2 + 2H2SO4
Scheme 1. Reaction steps of methane oxidation to methanol via methylbisulfate (CH3OSO3H,
MBS) using SO3-H2SO4.
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O
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Cl
Cl
N
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Cl
Cl
Pt
Pt
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O
(bpym)PtCl2
(DMSO)2PtCl2
Figure 1. The structures of (bpym)PtCl2 and (DMSO)2PtCl2.
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(DMSO)2PtCl2
(bpym)PtCl2
K2PtCl4
PtCl2
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20 30 40 50
Catalyst Concentration (mM)
Figure 2. Effects of catalyst concentration of various chloride-ligated Pt catalysts on methane
oxidation. Condition: oleum (20wt%) 30g, CH4 35 bar, 180oC, 3 h.
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(DMSO) PtCl 0.77mM
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K PtCl 0.77mM
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Figure 3. Effect of reaction time and catalyst concentration at (DMSO)2PtCl2 and K2PtCl4-
catalyzed methane oxidation. Condition: oleum (20wt%) 30g, CH4 35 bar, 180oC
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Figure 4. Optimized structures showing the interactions of (bypm)PtCl2 with H2SO4: (a) reactant
((bypm)PtCl2 + H2SO4), (b) transition state, and (c) product.
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Figure 5. Optimized structures showing the interactions of (DMSO)2PtCl2 with H2SO4: (a) reactant
((DMSO)2PtCl2+ H2SO4), (b1) transition state for chloride dissociation, and (c1) product, (b2)
transition state for DMSO dissociation, and (c2) product.
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