Synergistic activating effect of promoter and oxidant in single step conversion of methane into methanol over a tailored polymer-Ag coordination complex

Article abstract of DOI:10.1039/c7ra02700a

Single-step conversion of methane to its oxygenated derivatives, such as methanol, is a challenging topic in C1 chemistry. The presence of Br?nsted-acidic sites, N- and O-type chelating ligands, and noble metals are demonstrated to be essential criteria for effective catalysis of this reaction. Considering these criteria, a catalytic complex was tailored herein. Poly-d-glucosamine (Ch) was used as chelating ligand for Ag, to incorporate the robust redox properties of Ag(i). The prepared AgCh complex was characterized by techniques including solid-state ¹H-NMR, FE-TEM, XANES, and XPS. Besides highlighting the utility of chelate complexation for providing new materials, this study elucidates the effects of the oxidant and promoters on the methane oxidation. The catalytic activity was tested for different oxidant combinations, including hydrogen peroxide, oxygen, and carbon dioxide. Of all of them, a mixture of hydrogen peroxide and oxygen showed the highest selectivity for oxidation of methane to methanol. Further, it was observed that the addition of 1-butyl-3-methylimidazolium chloride [BMIM]⁺Cl^- as a promoter to the hydrogen peroxide and oxygen-containing AgCh system could enhance methanol production. The methanol yield reached up to 3166 μmol, representing an 18-fold yield increase and an 8-fold methane conversion increase when compared to the results (175 μmol) without a promoter.

Full text of DOI:10.1039/c7ra02700a

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Synergistic activating effect of promoter and
oxidant in single step conversion of methane into
Cite this: RSC Adv., 2017, 7, 24168
methanol over a tailored polymer-Ag coordination
complex†
b
Raghavendra Shavi,a Vishwanath Hiremath,a Aditya Sharma,
Sung Ok Wonb
a
*
and Jeong Gil Seo
Single-step conversion of methane to its oxygenated derivatives, such as methanol, is a challenging topic in
C1 chemistry. The presence of Brønsted-acidic sites, N- and O-type chelating ligands, and noble metals are
demonstrated to be essential criteria for effective catalysis of this reaction. Considering these criteria,
a catalytic complex was tailored herein. Poly-D-glucosamine (Ch) was used as chelating ligand for Ag, to
incorporate the robust redox properties of Ag(I). The prepared AgCh complex was characterized by
techniques including solid-state 1H-NMR, FE-TEM, XANES, and XPS. Besides highlighting the utility of
chelate complexation for providing new materials, this study elucidates the effects of the oxidant and
promoters on the methane oxidation. The catalytic activity was tested for different oxidant combinations,
including hydrogen peroxide, oxygen, and carbon dioxide. Of all of them, a mixture of hydrogen
peroxide and oxygen showed the highest selectivity for oxidation of methane to methanol. Further, it
was observed that the addition of 1-butyl-3-methylimidazolium chloride [BMIM]+Clꢀ as a promoter to
the hydrogen peroxide and oxygen-containing AgCh system could enhance methanol production. The
methanol yield reached up to 3166 mmol, representing an 18-fold yield increase and an 8-fold methane
conversion increase when compared to the results (175 mmol) without a promoter.
Received 6th March 2017
Accepted 27th April 2017
DOI: 10.1039/c7ra02700a
rsc.li/rsc-advances
remains a challenge. The inertness of CH4, caused by its high
Introduction
C–H bond dissociation energy (DHC–H ¼ 439.57 kJ molꢀ1), is
a key problem that has prevented the use of so reaction
conditions for CH4 conversion, such as low acidity, low
temperature, and low pressure. Therefore, catalysts that are
tailored to increase the C–H bond activity and thereby enable
faster reaction kinetics for CH4 conversion, especially to
oxygenates, are urgently required.
Global climate change is an increasingly serious problem
resulting from increasing global industrialization and the
associated output of greenhouse gases, such as methane (CH4).
While carbon dioxide (CO2) is the major contributor to the
greenhouse effect by volume, CH4 is a much more potent
greenhouse gas. Thus, strategies to increase the use of waste
CH4 gas as a feedstock for the production of industrially
important chemicals, such as methanol and acetic acid, are an
effective means to reduce harmful CH4 emissions and may
reduce the greenhouse effect. Many attempts have previously
been made to utilize CH4 in the preparation of syngas,1–3 which
is a major feedstock for methanol production. However, the
formation of the nal methanol product requires multiple high-
temperature steps and the use of different catalysts of type Cu/
ZnO/Al2O3.4,5 Consequently, the direct single-step conversion of
CH4 to its oxygenates, such as methanol and acetic acid,
It is important to understand which properties of catalysts
are advantageous for the conversion of CH4 to its oxygenates, as
previously investigated by several research groups.6,7 Metal
complexes have been extensively tested as catalysts for the
conversion of CH4 because of their ability to undergo inorganic
nucleophilic reactions. Kirillova et al.6 tested vanadium-based
metal complexes as catalysts in the homogeneous catalytic
conversion of CH4 to acetic acid, and found that complexes with
more N,O- or O,O-polydentate groups in their ligands showed
the best catalytic activity. The use of these ligands enabled
a 50% improvement in the yield of acetic acid. Their report was
consistent with previous reports of alkane oxidation reactions
in which the N,O-type ligands (N-heterocyclic carboxylic acids)
were recognized for their involvement in the proton transfer
steps.6 Therefore, catalysts with these favorable ligand envi-
ronments are suitable for further tailoring. The role of the noble
aDepartment of Energy Science and Technology, Myongji University, Nam-dong,
Yongin-si, gyeonggi-do 449-728, South Korea. E-mail: jgseo@mju.ac.kr
bX-ray Open Laboratory, Advanced Analysis Center, Korea Institute of Science and
Technology, Seoul-02792, South Korea
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra02700a
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metal atoms in these catalyst complexes may be understood by competitive performance results solely from the active Ag sites,
considering Pt(bpym)Cl2 and HAuCl4 ref. 7 and 8 complexes, with no contribution from the support. We show that the total
which were previously employed for the production of methanol TON can be directly correlated to the number of moles of Ag
from CH4 in homogeneous systems with a highly acidic solvent contained in the catalyst. These observations prove that our
medium. While these complexes showed the best catalytic tailored catalyst is highly active for the CH4 conversion reaction.
activity for CH4 conversion (TON > 500), the conversion reaction We also investigated the effect of halide ion-containing ionic
did not reach completion, yielding methyl bisulfate (CH3- liquids on the reaction, nding that the inclusion of these ionic
OSO3H). The harsh conditions employed in these reaction liquids created a stable intermediate structure that gave ꢁ13
systems meant they were not investigated further. Since then, times greater CH4 conversion. The interaction between the ionic
trends in the eld have shied towards mimicry of the CH4 liquids and the catalyst is discussed later in this article. We also
monooxygenase (MMO) enzyme,9 which oxidizes CH4 to meth- prepared CuCh and FeCh using the same method and
anol in nature, and the replacement of hard solvents by compared their performance to that of AgCh to prove the effect
hydrogen peroxide (H2O2). As an example, the complex [CuI- of the noble metal in the AgCh complex.
CuICuI(7-N-Etppz)]1+, where 7-N-Etppz stands for the organic
ligand 3,30-(1,4-diazepane-1,4-diyl)bis[1-(4-ethylpiperazine-1-yl)
Experimental section
Materials and methods
propan-2-ol], was reported by Liu et al. for methanol produc-
tion.10 Based on previous reports of increased yields from
zeolite-supported Cu, Fe catalysts,11 the [CuICuICuI(7-N-Etppz)]
Silver nitrate (AgNO3, >99%), copper(II) nitrate pentahydrate
(CuNO3$5H2O, 98%), iron(III) nitrate nonahydrate (FeNO3-
1+ complex was supported on silica nanoparticles resulting in
an increased yield of methanol. However, while the harsh
conditions of the earlier reaction systems were successfully
replaced, the central aim of achieving a higher CH4 conversion
was not addressed. Noble metal complexes that function under
milder conditions are yet to be tested and understood. The few
reported examples of heterogeneous catalyst systems for CH4
conversion also reveal specic properties of the catalysts. For
example, Sun et al. replaced the acidic solvent (sulfuric acid)
used in a homogeneous reaction system with a solid acidic
catalyst, i.e., a heteropolyacid chemisorbed on silica.12 Their
study demonstrated and generalized the requirement for cata-
lysts with acidic properties by showing that acidic catalysts
convert CH4 to acetic acid under mild conditions. Brønsted-
acidic groups also play a considerable role in the proton
exchange steps involved in producing methanol and acetic
acid.13 Thus, to reduce the high bond dissociation energy of CH4
and thereby achieve a higher conversion of CH4 to its oxygen-
ates, catalysts should have several N,O-type ligands, high
acidity, Brønsted-acidic sites, and be heterogeneously sup-
ported. Further, previous reports7,14 have shown that the use of
inorganic chloride salts as promoters can increase the CH4
conversion. Though activity increased aer using promotor, the
total conversion of CH4 by mass did not meet the expectations.
Weak attraction between the catalyst and the added salt
promoter still remains as a drawback in this case. Thus making
use of promoting salts that interact strongly with the catalyst,
forming an intermediate structure which boosts the kinetics of
CH4 conversion is preferable.
$9H2O), poly-D-glucosamine, and the ionic liquids 1-butyl-3-
methylimidazolium chloride (>98%) and 1-butyl-3-methy-
limidazolium bromide (>97%) were purchased from Sigma
Aldrich. Sodium hydroxide (NaOH) and hydrogen peroxide
(H2O2) (30%) were purchased from Sanchun chemicals, Korea.
High purity methane (99.95%) and carbon dioxide (99.999%)
were used for reactions, and high purity hydrogen (99.99%) and
oxygen (99.999%) were used as standards for gas chromatog-
raphy (GC) analysis. Double-distilled water was used in all
reactions.
Catalyst preparation
Poly-D-glucosamine (500 mg) was dispersed in double-distilled
water (250 mL) contained in a beaker (500 mL capacity). The
ꢂ
dispersion was heated to 80 C on a hot plate and stirred for
20 min at 600 rpm. Then, the required metal nitrate (Ag, Fe, or
Cu) (5 mmol) was dissolved in water (100 mL) in a separate
beaker. This solution was added dropwise to the poly-D-
glucosamine dispersion. Aer a further 10 min, the pH of the
solution was adjusted to 9.3 by the addition of sodium
hydroxide with careful stirring, forming the poly-D-
glucosamine-metal complex as a solid precipitate. The complex
was ltered, washed several times with water, and air-dried.
Catalyst with Ag, Fe and Cu are abbreviated as AgCh, FeCh
and CuCh respectively.
Sample characterization
Our strategy is to use a biopolymer, poly-D-glucosamine (Ch), Fourier transform infrared (FTIR) spectroscopy was recorded
to ligate Ag ions with its N- and O-based chelating groups. This using a Varian 2000 Fourier transforms infrared spectropho-
biopolymer also contains highly acidic Brønsted acid sites, and tometer by accumulating 32 scans between 2000 and 4000
the regular arrangement of the chelating atoms in Ch enables cmꢀ1. The samples were pelletized with KBr and put in an FTIR
a systematic arrangement of active Ag sites for the CH4 chamber for analysis. X-ray photoelectron spectroscopy (XPS)
conversion reaction. Therefore Ag coordinated Ch (AgCh) analyses (ESCALAB 250, UK) were performed under a vacuum of
complex meets all the criteria mentioned above for producing ꢁ1.3 ꢃ 10ꢀ7 Pa. A monochromated X-ray Al Ka source (energy:
a much higher yield of methanol and is competitive with the 15 kV, 150 W) with hg of 1486.6 eV was used for all measure-
performance of other zeolite-based catalysts. We highlight that, ments. The analyzed area was 400 mm2. The binding energy
unlike with zeolite-supported catalysts, the AgCh catalyst's values were charge-corrected to the C 1s signal (284.5 eV). Peak
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deconvolution was performed with XPS peaks 41 soware. X-ray
diffraction (XRD) spectra were recorded between 2q of 10 and
90ꢂ. A Cu anode was used, and continuous scans were taken for
each sample. Solid-state 1H magic angle spinning (MAS) nuclear
magnetic resonance (NMR) spectra were recorded on a Bruker
Avance II+ 400 MHz spectrometer. The instrument was operated
at a rate of 400 MHz with a spin rate of 13 kHz. X-ray absorption
spectroscopy (XAS) was used to collect Ag, Fe, and Cu K-edge X-
ray absorption near edge structure (XANES) data, using the lab-
source R-XAS Looper (Rigaku R-XAS) with a 3 kW X-ray gener-
ator and a Mo–LaB6 target-lament combination. A Johannson-
type curved Ge (220) crystal with a Rowland circle of radius of
320 mm was used in the monochromator section. The incident
X-ray intensity (I0) was monitored using a semi-transmission
type proportional counter lled with Ar gas, with Be windows
on both sides. Transmitted X-ray intensity (It) was measured
with a high-speed scintillation counter. Thin layered samples
(sandwiched between Kapton tape) were mounted between the
I0 and It detectors, in the transmission geometry. The Fe and Cu
K-edge energies were calibrated using standard Fe and Cu foils,
that were measured under the above conditions. Thermogravi-
metric analysis (TGA), using a TGA N 1000 (SCINCO) thermog-
ravimetric analyser was used to study the degradation patterns
of the catalysts. Thꢂe samples were heated to 600 ꢂC with
a ramping rate of 5 C minꢀ1. Elemental analysis was carried
out using a Flash EA 1112 elemental analyzer. Samples were
dissolved in acetic acid (0.1 M) for analysis. Transition electron
microscopy (TEM) images were recorded using a JEOL-200CX.
Ion chromatography (IC) was used to analyse products in
solution. The method specications used were: column: Aminex
87H (Bio-rad), high performance liquid chromatography
(HPLC): Ultimate 3000 (Dionex, USA), eluent: H2SO4 (0.01N),
ow: 0.5 mL minꢀ1, oven temperature: 40 ꢂC, and detector:
refractive index (RI) (ERC refractoMAX520).
Amount of CO2 (mol) ¼ concentration of CO2
ꢃ volume of reaction vessel (mL), (2)
where Rf is the response factor.
Results
Characterization of poly-D-glucosamine-based metal
complexes
The solid-state 1H MAS NMR spectra for the poly-D-glucosamine
catalyst and its Ag coordination complex, AgCh, are shown in
Fig. 1. A wide and broad peak was observed from 0 to 20 ppm,
which clearly indicates the presence of highly acidic Brønsted
acid groups with strong hydrogen bonding. The large width of
the peak also conrms the presence of several hydrogen atom
environments with different electropositivity, which affects the
degree of hydrogen bonding. The hydrogen atoms of amine
groups also contribute to the peak broadening as the ppm range
for N–H coupling lies within the range of observed broad peak.
Aer the formation of the AgCh complex, a small shi to low-
er ppm values over the entire peak range and a decrease in peak
intensity were observed.
FTIR spectra were recorded for both Ch and the AgCh, CuCh,
and FeCh complexes (Fig. 2). The characteristic broad band
between 2900 and 3600 cmꢀ1 corresponds to the combined
absorptions of N–H and O–H vibrations.15 The peaks in the
range 1567–1651 cmꢀ1 are assigned to N–H deformational
vibrations. Aer the addition of the metals to Ch, a decrease in
the intensity of the absorbance between 3400 and 3600 cmꢀ1
,
assigned to N–H vibrations, was observed. This decrease indi-
cates the complex of the metal since the donation of electrons
from nitrogen to metal weakens the N–H bonds. Complexation
of the metal is further conrmed by the disappearance of the
weak band at 1567 cmꢀ1, associated with N–H vibrations, and
a shi in the peak at 1651 cmꢀ1. A sharp peak at 524 cmꢀ1
,
usually assigned to ligand-to-metal bonding, was also observed
in all metal complexes, further conrming the complexation of
Ch with metals. No marked changes in the absorbance of O–H
bonds were observed. The crystallization behavior of Ch and the
Catalytic activity test
The catalyst was added to an ice-cooled reactor vessel (150 mL
capacity) containing double-distilled water (15 mL) and H2O2 (5
mL, 30%). Thus charged, the vessel's lid was tted tightly and
the vessel was degassed by purging with N2 for 5 min. Then, the
reactor was pressurized to the required pressure with CH4 (g).
The system was heated at 60 ꢂC and electrically stirred at a rate
of 900 rpm. Aer attaining the correct temperature, the reaction
was allowed to proceed for the required time. Aer this time, the
system was simply allowed to cooldown to room temperature
and then cooled with ice to condense the gaseous oxygenated
products. The liquid was ltered and the products were ana-
lysed by IC to determine the product contents in units of
milligrams, from which amount of the product in units of moles
was calculated. All results were compared to standards for
conrmation. The gas-phase products were analysed by GC
(Agilent 7890A TCD/FID). The number of moles of CO2 was
calculated from a calibration curve. The CO2 content (mol) was
calculated by eqn (1) and (2),
1
Fig. 1 Solid-state H MAS NMR spectra for Ch (black line) and AgCh
Concentration of CO2 ¼ peak area ꢃ Rf
(1) (red line).
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followed by CuCh. Fig. 3(b) shows the Ag L1-edge XANES of the
AgCh catalyst. The peak at ꢁ3808.7 eV corresponds to the
transition from core 2s orbital to the unoccupied p-type orbitals,
which occurs at a higher energy in the AgCh than the corre-
sponding peak in metallic Ag foil (ꢁ2.6 eV). Based on this
difference in energy, we tentatively suggest that the complexed
Ag is present in the +1 oxidation state, as Ag+ ions, although the
low Ag content in the AgCh sample and low spectral resolution
introduce noise into the presented spectra which may slightly
affect the reported energies. This XAS evidence for the presence
of Ag+ in the AgCh complex is complemented by the XPS results
for Ag (see below), which also suggest the presence of Ag(I) in
the sample. Fig. 3(c) shows the Fe K-edge XANES spectra for
both the reference foil and FeCh. The pre-edge feature, the
energy shoulder at 7122 eV, and the white line intensity of the
FeCh spectrum indicate that the electronic structure of FeCh is
different to that of the Fe foil. The diverse spectral features of
FeCh are similar to those previously reported for iron oxide,
especially Fe(II) oxide,16,17 indicating the presence of Fe2+ ions in
the FeCh samples. The Cu K-edge XANES spectra for both CuCh
and Cu foil are shown in Fig. 3(d). The CuCh XANES spectrum
exhibits pre-edge spectral features indicative of 1s–3d transi-
tion.18 The spectral features of the CuCh sample match previ-
ously reported spectra of CuO and CuS compounds,19
suggesting that Cu2+ ions are present in the sample. Each of our
samples showed a shi in the white line to a slightly higher
energy, compared to their respective metallic foils, which
previous reports attribute to the fractional oxidation of Fe20 and
Cu21 atoms in the samples. Fractional oxidation is expected to
Fig. 2 FTIR spectra for Ch and the different catalyst species.
prepared catalysts was studied using XRD, and the spectra are
shown in Fig. 3(a). The pristine Ch showed a broad reection at
2q ¼ 22.9ꢂ, which implies that pristine Ch is amorphous. Aer
the complexation, this broad reection sharpened and shied
to 2q ¼ 20ꢂ, indicating higher crystallinity aer complexation of
Ch with metals. No reections corresponding to the metals were
observed in the complexed Ch, suggesting that the metal atoms
are only present as chelates, rather than metal particles. The
crystalline structure varied between each of the three
complexes, with the FeCh complex showing the sharpest
reection, with the sharpness decreasing in the order of AgCh
Fig. 3 (a) XRD spectra and (b–d) XANES spectra for AgCh, FeCh, and CuCh respectively.
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Fig. 4 High-resolution XPS spectra for (a) O 1s, (b) N 1s, and (c) Ag regions of the AgCh complex.
occur in the complexes prepared in the present study because of morphology, from brillar to uniform cube-like structures.
the substantial presence of both hydroxyl ions and different These cubes with varying size between 100 to 200 nm, are
ligating functional groups in the chelating polymer.
observed for AgCh only; no cube-like structures are observed for
The surface composition of the catalyst complexes was ana- AgCh only; no cube-like structures are observed for the other
lysed by XPS. The changes observed in the spectrum of the Ch metal–Ch complexes (Fig. S3, ESI†). The cubes are suggested to
polymer aer the formation of complexes can be seen for AgCh in form because of the tendency of the polymer to chelate metal
Fig. 4. XPS before and aer complexation comparison for Fe and atom in three-dimensional congurations.
Cu catalysts is shown in ESI, Fig. S2.† The intensity of the O 1s
and N 1s peak of Ch decreased aer AgCh formation. In both
regions, the peak shied to slightly higher values of binding
Discussion
energy, which clearly indicates the transfer of electron density
from Ch to Ag. As in the FTIR spectra, where the strong N–H
absorbance diminishes aer complexation, the XPS spectrum
The activity of all the prepared catalysts is shown in Fig. 6 and
Table 1. Zero conversion of the CH4 was observed when reaction
was carried out without catalyst. Calculated moles, as present in
reveals that the lone pair of electrons from the nitrogen atom of
their respective complex catalyst, of metal (Ag, Fe, Cu) nitrates
amine groups contributes to coordination of Ag. Fig. 4(c)
employed as catalysts were able to show a moderate activity.
However, no methanol was produced showing complete selec-
tivity towards formic acid. The pristine Ch produced a negli-
gible amount of methanol (12 mmol) indicating possibility of
methanol formation with this material when employed as
catalyst. Adding AgNO3 (which showed best activity among the
conrms the presence of Ag, and the two characteristic peaks at
368 and 373.9 eV conrm the oxidation state of Ag to be +1.
The TEM images of the Ch polymer presented in Fig. 5(a)
show that the pristine polymer has a loose, brillar structure,
indicating the presence of free polymer chains. The formation
of the AgCh complex [Fig. 5(b) and (c)] induces a change in the
tested metal nitrates) and Ch into the reaction mixture
produced formic acid, surprisingly, but not methanol. This
observation conrms metal nitrates dominates the reaction
path way to formic acid. Aer metal complexation, there was an
abrupt increase in methanol formation, along with other
Fig. 6 CH4 oxidation activity comparison of AgCh, FeCh and CuCh
Fig. 5 TEM images of (a) Ch and (b–d) AgCh at different magnification. catalysts.
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Table 1 Product quantification and percentage of CH4 conversion for different catalysts used in this worka
Product formed (mmol)
Entry
Catalyst
Oxidant
CH3OH
HCOOH
CH3COOH
CO2
CH4 conversionb (%)
1
No
H2O2
—
—
—
—
—
2
3
AgNO3 + Ch
Ch
H2O2
H2O2
—
12
65
24
—
2
—
0
—
0.03
0.43
0.37
0.37
0.21
0.13
0.19
4.50
4.00
2.16
2.02
4
5
6
7
8
9
10
11
12
13
AgCh
CuCh
FeCh
AgCh
CuCh
FeCh
H2O2
H2O2
H2O2
H2O2 + O2
H2O2 + O2
H2O2 + O2
H2O2
H2O2 + O2
H2O2
175
95
117
156
87
97
0
3166
0
952
363
387
380
77
41
38
25
50
30
44
414
287
436
312
47.57
27.86
24.49
19.52
13.31
16.5
47.92
37.27
38.40
33.55
64
133
6152
2370
2710
1680
AgCh + [BMIM]+Clꢀ
AgCh + [BMIM]+Clꢀ
AgCh + [BMIM]+Brꢀ
AgCh + [BMIM]+Brꢀ
H2O2 + O2
a
Reaction conditions – catalyst: 0.15 g, solvent: 15 mL DI H2O + 5 mL 30% H2O2, P(CH ): 3 ꢃ 106 Pa, temperature: 60 ꢂC, time: 3 h. b % conversion
4
calculated using formula ꢀ total moles of oxygenates formed/initial moles of CH4 ꢃ 100.
byproducts including formic acid and acetic acid. These results 0.15 g. When a larger amount of AgCh was used, the products
imply that the metal–Ch complex synergistically makes an contained more formic acid than methanol. Higher amount of
active component and thus, an active catalyst for the conversion catalysts produces more methanol which may over oxidised to
of CH4 to its oxygenates. Among the prepared AgCh, FeCh, and formic acid with the time and hence the more formic acid
CuCh catalysts, AgCh showed the best performance, producing production suggesting that the catalyst was highly active.
175 mmol of methanol and 227 mmol of total oxygenated prod- Investigations of the effect of reaction time, shown in Fig. 7(b),
ucts for 0.15 g of the catalyst at a CH4 pressure of 3 ꢃ 106 Pa. found that the maximum yield of liquid-phase products was
CuCh was more active than FeCh, nevertheless both these obtained aer 3 h, aer which the production of liquid-phase
catalysts showed activity that was ꢁ20 times lower than that of products decreased. These results suggest that reaction times
AgCh when the turnover number (TON) was considered. The longer than 3 h lead to deactivation of the catalyst. The over-
concentration of over-oxidized products was higher than the oxidation of CH4 to produce gaseous products may also cause
concentration of methanol in all cases. Higher stability of Ag- the observed decrease in the content of liquid products at
complexes in peroxide medium might be responsible for best reaction times longer than 3 h.
activity shown by AgCh compared to FeCh and CuCh. Ag-
complexes reported to form a strong ionized intermediates
where charge is normally a radical.22 These radical species,
Effects of O2 and CO2 on CH4 oxidation
which are generated by homolytic cleavage nature of peroxide's
O–O bond, are known to interact strongly with organic species
to form chemicals. High activity of AgCh in this study also can
be explained under similar mechanism in which radical species
formed by AgCh complex in H2O2 medium can interact with
hydrocarbon CH4 to form organic molecules like methanol,
formic acid and etc. Though the formic acid and acetic acid are
over oxidized products of methanol in this reaction, the AgCh
complex, which shows high stability of radical species, was able
to retain higher concentration of methanol in given time. Cu
shows different oxidation states (CuI and CuII) and so do Fe (FeII
and FeIII). Interconversion of these oxidation states reacting
with OH. Produced by H2O2 during the reaction,23 causes easy
over oxidation of methanol thereby giving more selectivity
towards formic and acetic acid in case of these two metal
complexes.
We studied in detail the effect of an additional oxidant, O2 (in
the presence and absence of H2O2), on the single-step conver-
sion of CH4 to methanol by our Ch catalysts. The results of the
activity comparisons for each of the different catalysts are
shown in Fig. 8(a) and Table 1, and showed trends that followed
our earlier results. The AgCh performed well in the presence of
O2, compared to CuCh and FeCh. Interestingly, the yield of
oxygenates was comparatively less than in reactions conducted
without O2. This nding suggests that when the reactor was
purged with O2, the primary source of O atoms for use in the
conversion of CH4 changes from H2O2 to O2. The observation of
H2O2 consumption in both systems conrms that when H2O2
alone was used, it acts as an oxidant for CH4 conversion and
when O2 was purged into reactor, H2O2 involves in the reaction
as O atom active exchanger with O2 thereby minimizing its
participation in over oxidizing side-reaction. This explanation
was further supported by investigations of the amount of over-
oxygenated products generated compared to the amount of
methanol, which showed that more methanol was produced in
the O2-containing systems. The lower selectivity towards over-
oxidized products observed for systems that contained O2
The activity of the AgCh catalyst was further studied by
varying the amount catalyst from 0.03 to 0.15 g (Fig. 7(a)).
Increasing the amount of the catalyst resulted in increased
methanol formation. A catalyst loading of 0.03 g produced 48
mmol of methanol, which increased to 175 mmol at a loading of
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Fig. 7 The effect of (a) catalyst amount and (b) reaction time on the conversion of CH4 to its different oxygenate products.
C–H bond. Halide anions are well known as CH4 activators, and
scientists have made use of alkyl halides and halide salts for
this purpose. Although the nal CH4 conversion was increased
when halide salts were used, it was still either relatively low or
showed low selectivity towards oxygenates.8,11 This lower
performance was possibly because of a weaker attraction
between the formed halide ions (from the halide salt) in the
system and the catalyst. Halide ions remain in solution rather
than associating strongly with the catalyst and therefore do not
act efficiently during the catalytic cycle involved in CH4 activa-
tion. Hence, we suggest that the promoter's halide anion and
the associated cations must bind strongly to the catalyst by
means of a chemical bond. Thus, the formation of a suitable
intermediate structure (as shown in Fig. 9(b and c)) can occur,
providing a nearby source of promoter ions during the conver-
sion reaction.
CH4 monooxygenase (MMO) is an enzyme that converts CH4
to methanol and oxidizes C1 to C5 hydrocarbons in nature.9
This enzyme is also highly active towards the oxidation of
halogenated alkanes.25 Its structure includes Fe centers, which
are coordinated with the N atoms of histidine and the O atoms
of carboxylates and water. Our catalyst provides a similar envi-
ronment to MMO, and the use of imidazolium-based halide ion-
containing ionic liquids as promoters provides easily accessible
halide ions for triggering the formation of halogenated CH4. By
analogy to the high activity of MMO for halogenated alkane
oxidation and the similarity in catalyst environment between
our Ch-based catalysts and MMO, we expect that the haloge-
nated CH4 will be immediately oxidized. Another advantage of
this system is the presence of [BMIM]+, which can act similarly
to a reductase in some organic reactions.26 We suggest that this
Fig. 8 The effect of (a) O2 and (b) CO2 on the conversion of CH4 to its
different oxygenated products.
certainly suggests that there is a shi in source of O atoms from
H2O2 to O2 in activating the catalytic system for the CH4
conversion reaction. When the reaction was carried out with O2
and no H2O2, a low amount of methanol was obtained. Hence,
a combination of O2 and H2O2 is the optimal choice for higher
selectivity towards methanol.
The effect of CO2 on the reaction is shown graphically in
Fig. 8(b), which shows the amount of methanol obtained was
low in all cases. These results are as expected since CO2 is
a carboxylating agent. The acetic acid concentration was high
when CO2 was present, suggesting that the direct carboxylation
of methanol to acetic acid can occur. A comparison of the
activity of the different catalysts again shows AgCh to be the best
catalyst in all cases.
Halide ion-containing ionic liquids as reaction promoters
A recent study sought a suitable reaction promoter for the reductase action may provide a similar driving force for acti-
conversion of CH4 to its oxygenates.11 A previous report vation of CH4 as provided for MMO by nicotinamide adenine
demonstrates that the addition of Cu to an Au–Pd/TiO2 catalyst dinucleotide (NADH). Our experiments showed that 11-fold
(AuPdCu/TiO2) led to a 5-fold increase in the CH4 conversion improvement in the conversion of CH4 to its oxygenates was
productivity, in line with other reports.24 The promoter's role is achieved using these promoters. [BMIM]+Clꢀ and [BMIM]+Brꢀ
to activate CH4 by reducing the bond dissociation energy of the were employed as reaction promoters with AgCh, which had the
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Fig. 9 (a) The effect of the different ionic liquids used as reaction promoters. (b and c) Possible structures of AgCh after interacting with ionic
liquids of different size.
best performance in earlier tests compared to the other reaction with the AgCh catalyst, the [BMIM]+Clꢀ promoter and
prepared catalyst complexes (entry no. 10 in Table 1). The CH4 the H2O2 co-oxidant. The results, shown in Fig. 10, indicate that
conversion increased abruptly, with 11-fold and 5.5-fold the total conversion was lower when this catalyst system was
increases for [BMIM]+Clꢀ and [BMIM]+Brꢀ respectively. This used without O2 or CO2. However, the addition of O2 resulted in
nding suggests that Clꢀ ions are better halide promoters for the production of 3166 mmol of methanol, (entry no. 9 in Table
the reaction with the AgCh catalyst. To conrm the improve- 1) which is ꢁ18 times higher than the yield from AgCh alone.
ment in activity was a result of the presence of halide anions, we This comparison reveals that the use of O2 reduces the over-
also comparedꢀthe performance of ionic liquid promoters with oxidation of methanol, and hence a combination of H2O2 and
ꢀ
PF6 and NTf2 anions (shown in Fig. 9(a)), which are exten- O2 is the best oxidant for methanol synthesis. However, the
sively used in many gas phase reactions.27,28 Even though these ionic liquid cation was responsible for the production of ꢁ1500
anions were effective promoters, their performance was mmol of formic acid. The selectivity towards methanol was
considerably lower than that of the halide ions. These results found to be 54% in this case as highest selectivity for methanol
also demonstrate the high possibility of cation activity in the detected in this work. The methanol selectivity comparison with
conversion reaction. The PF6ꢀ and NTf2ꢀ anions are larger than the literature is shown in Table S3† which explains the 54%
the halide anions, which affects the coordination structure and selectivity was worthy when compared previous reports.
stability of catalyst, as shown in Fig. 9(b and c). A control When CO2 was added, no methanol was observed. Instead,
experiment was done using only [[BMIM]+Clꢀ. The only 3832 mmol of formic acid was produced, which is more than
observed product was formic acid (2085 mmol), which indicated double compared to the amount of formic acid formed in O2
that the [BMIM]+Clꢀ was three times less effective when used purged system (ꢁ1500 mmol). This nding suggests that the
alone than when it was used as a promoter with the catalyst.
These results provide evidence that the ionic liquids form
a structure with the catalyst that favors higher CH4 conversion.
Hence, our attempt to establish an intermediate structure
between catalysts and promoter was effective in achieving
a higher conversion of CH4. The maximum conversion of CH4
observed aer using ionic liquids as promotors is 4.5%. When
compared to traditional catalysis, the achieved conversion is
very low. However for CH4 conversion into its oxygenates via
one-step, the 4.5% conversion is still noteworthy. A tabulated
comparison of CH4 conversion and methanol selectivity from
the literature is shown in Table S3.† It can be seen that the
performance of our catalyst at lower temperature is comparable
to other reports which are done at higher temperatures.
Finally, we combined the ndings of our optimizations by
testing the inuence of oxygen and CO2 on the CH4 conversion
Fig. 10 CH4 conversion activity of AgCh and [BMIM]+Clꢀ combina-
tions with different gaseous oxidants (O2 and CO2).
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combination of catalyst, promoter and CO2 presents a strong
CH4 carbonylation environment.
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Conclusions
A catalytic AgCh complex for conversion of CH4 to its oxygenates
was prepared and fully characterized. The complex contained
Ag(I), which provides the redox properties necessary to undergo
the inorganic nucleophilic reactions involved in the conversion
of CH4 to methanol. The suitability of Ag for this conversion was
demonstrated by comparing AgCh with catalysts prepared using
various metal atoms (Fe & Cu). Efforts to identify the best
oxidant revealed that H2O2 alone led to a higher CH4 conversion
percentage than a combination of O2 and H2O2. This result was
suggested to occur because of the shiing of O utilization
source from H2O2 to O2, which renders rate of reaction. We
determined the best environment to maximize methanol
selectivity in our study of ionic liquids with halide anions as
promoters. We found that a combination of AgCh with
[BMIM]+Clꢀ and H2O2 increased the CH4 conversion 11-fold,
but only over oxidized products were observed. The increased
conversion conrmed the utility of our strategy to establish an
interaction between the catalysts and the promoter. Finally,
a system using a combination of H2O2, O2, AgCh and
[BMIM]+Clꢀ showed the best selectivity for methanol and a 13-
fold improvement in the yield of methanol compared to the
system with H2O2 alone, producing 3166 mmol of methanol. As
an extended work, experiments are ongoing in our laboratory to
establish a cyclic radicle mechanism among the added halide
anions and CH4 in order to invent a new green feasible method
for this oxidation reaction.
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Acknowledgements
This work was supported by the Energy Efficiency & Resources
(No. 20163010092210) of the Korea Institute of Energy Tech-
nology Evaluation and Planning (KETEP) grant funded by the
Korea government Ministry of Trade, Industry & Energy and this
work was supported by Nano-Material Fundamental Technology
Development (2016M3A7B4909370) through National Research
Foundation of Korea (NRF) funded by the Ministry of Science,
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