2
32
H.W. Lee et al. / Journal of Catalysis 374 (2019) 230–236
2
.4. Vacuum distillation
1
00
100
80
60
40
20
0
The oxidation product containing MBS was transferred to the
8
0
apparatus shown in Fig. S1 in the Supporting information, and vac-
uum distillation was conducted as described in the Supporting
information. Distillation proceeded until no more product was col-
lected in the receiver. After the distillation, distillation residue and
distillated product were analyzed using H NMR and Raman spec-
troscopy to calculate the amount of MBS in the solution and to esti-
mate the purity of the product, respectively.
60
MBS yield
MBS selec.
1
CO selec.
2
4
0
0
0
CH4 conv.
SO3 conv.
2
2
.5. Computational detail
0
2
4
6
8
30 40 50 60
All calculations were carried out using Gaussian 16 (Revision
Catalyst conc.(mM)
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. Cal-
culations were performed for reactions in the liquid phase, so the
SMD polarizable continuum solvent model augmented with
parameters for sulfuric acid (static (eps = 101) and optical
Fig. 1. Effect of Pt-black concentrations on the Pt-black catalyzed methane
oxidation to methyl bisulfate (MBS). Conditions: oleum (20%wt SO ), 35 bar of
CH , 180 °C, 3 h.
3
4
As can be seen in Fig. 1, the yield of MBS at a low catalyst con-
centration of 0.31 mM was 26.4%. When the catalyst concentration
increased, the MBS yield reached maximum at 1.6 mM with a yield
2 2
of 82.0%, which is a very similar value obtained at (DMSO) PtCl -
catalyzed reaction [25]. At catalyst concentrations between 0.94
and 6.30 the change in MBS yield was marginal. However, when
the Pt black concentration increased to 31.5 mM, the formation
of MBS decreased to 51.7% and further decreased to 36.5% at
(
epsinf = 2.042) dielectric constants) was used.
6
3.2 mM Pt black concentration. The analysis of CO
after the reaction revealed the decrease in MBS formation could be
attributed to the formation of CO . At a catalyst concentration less
than 2 mM, the formation of CO was not significant, but its forma-
tion increased substantially at the higher catalyst concentrations.
When the selectivity to CO was calculated with respect to the
amount of MBS, it reached to 22.9% and 36.6% at 31.5 mM and
2
concentration
3
. Results and discussion
2
Methane oxidation to methanol via methyl bisulfate (MBS) in
2
oleum (H
hydrolysis, as shown in Eqs. (1)–(4) in Scheme 1. First, methane
oxidizes to MBS in oleum with the formation of SO and water.
Because the water reacts with SO to form H SO , methane con-
sumes 2 equiv. of SO at the oxidation. MBS hydrolyzes to metha-
nol and sulfuric acid as in Eq. (4). Accordingly, the net reaction
from methane to methanol can be expressed as Eq. (5) in Scheme 1;
one mole of methane reacts with two moles of SO
water to make one moles of methanol, SO and sulfuric acid. In the
case of CO , 6 equiv. of SO is consumed from methane to CO
2 4 3
SO -SO ) is comprised of two reactions, oxidation and
2
2
3
2
4
6
3.2 mM catalyst concentration, respectively.
3
The advantage of this oleum mediated methane oxidation is the
high conversion of methane and high selectivity to a methanol
intermediate, MBS, due to the stability of MBS in this oxidation
condition [2,10]. However, this benefit was lost at high concentra-
tion of Pt black catalyst. Fig. 2 and Table S3 show, at 1.6 mM, the
MBS yields for 1 h and 3 h were 57.3% and 82.1%, respectively.
After 12 h, the MBS yield was 82.4%. However, at 63.2 mM, the
MBS yield of 54.9% after 1 h oxidation fell to 36.5% after 3 h reac-
3
and one mole of
2
2
3
2
.
3.1. Effect of catalyst concentration
tion and decreased to 10.1% after 12 h with an increase in CO
2
for-
T. Zimmerman et al investigated the effect of catalyst concen-
tration on the absolute rate of MBS formation ( MBS) and reported
that K PtCl had the highest MBS for the methane oxidation at
.6 mM catalyst concentration. Similarly, we also found that
PtCl had the highest MBS yield of 62% at a catalyst concentra-
mation. These results indicate that MBS is very stable at 1.6 mM
c
2
4
c
0
K
100
100
80
60
40
20
0
2
4
tion of 0.77 mM and the yield decreased to 40–50% at the catalyst
concentration higher than that. In both reports, the concentration
8
0
0
effect was attributed to the formation of less active PtCl
PtCl and the co-released chloride anion aggravated the
catalytic activity of PtCl even more.
2
from
6
K
2
4
,
1
.6 mM
3 mM
2
6
For this Pt black-catalyzed reaction, we found the yield of MBS
also reached a maximum value at 1.6 mM catalyst concentration,
but the reason for the decreased yield at a higher catalyst concen-
tration was found not to be caused by catalyst deactivation, but
40
MBS yield
CO select.
2
20
because of the over- oxidation of MBS to CO
Methane oxidation using Pt black in the presence of oleum (20%
wt SO ) was conducted at 180 °C for 3 h under 35 bar of methane
pressure. In Table S2 and Fig. 1, the effect of catalyst amount on
2
.
0
0
2
4
6
8
10
12
3
Reaction Time (h)
the yield of MBS and selectivities to MBS and CO
together with the conversion of methane and SO
determined as described in the experimental section.
2
were marked
, which were
Fig. 2. Effects of reaction time on the MBS yield and CO
catalyzed reaction of different Pt concentration. Conditions: oleum (20%wt) 30 g,
35 bar of CH , 180 °C.
2
selectivity at the Pt black
3
4