Low temperature activation of methane over a zinc-exchanged heteropolyacid as an entry to its selective oxidation to methanol and acetic acid

Article abstract of DOI:10.1039/c4cc04950k

A Zn-exchanged heteropolyacid supported onto silica (Zn-HPW/SiO₂) activates methane at 25 °C into Zn-methyl. At higher temperatures and with CH₄/O₂ or CH₄/CO₂, it gives methanol and acetic acid respectively. This journal is

Full text of DOI:10.1039/c4cc04950k

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Low temperature activation of methane over a
zinc-exchanged heteropolyacid as an entry to its
selective oxidation to methanol and acetic acid†
Cite this: Chem. Commun., 2014,
50, 12348
Received 28th June 2014,
Accepted 8th August 2014
Umesh Patil, Youssef Saih, Edy Abou-Hamad, Ali Hamieh, Jeremie D. A. Pelletier
´ ´
and Jean Marie Basset*
DOI: 10.1039/c4cc04950k
www.rsc.org/chemcomm
A Zn-exchanged heteropolyacid supported onto silica (Zn-HPW/SiO2) Zinc-incorporated materials have been a subject of recent
activates methane at 25 8C into Zn–methyl. At higher temperatures interest for the catalytic transformation of light alkanes.17–21
and with CH4/O2 or CH4/CO2, it gives methanol and acetic acid The dissociative adsorption of hydrogen and light alkanes has
respectively.
been addressed in the literature for bulk zinc oxide.17,22–26 Zinc
is of particular interest as it has been proved recently that it
Transformation of methane to oxygenates (i.e. methanol, form- accelerates the heterolytic dissociation of the C–H bond of CH4
aldehyde or acetic acid) under economically attractive conditions by a reaction between CH4 and Zn2+ O2ꢀ which is not the case
remains an open challenge for the chemical industry.1–4 New for most other metal ions.27
catalytic processes must be developed to overcome all the con-
In the present study, we aimed to study the reactivity of Zn
straints due to the chemical inertness of the substrates especially containing a heteropolyacid for methane activation. To our
with respect to the high reactivity of the products and thermo- knowledge, we have been the first to report the formation of
dynamic limitations. Despite the fact that many efforts have been a Zn–CH3 species at room temperature on the heteropolyacid
made in the past decades, the commercial utilization of methane is surface with dual reactivity to transform into methanol (with
still largely dependent on the multi-step syn-gas (CO + H2) strategy, oxygen) or acetic acid (with carbon dioxide) under milder
which is energy and cost intensive.2 Although the homogenous reaction conditions.
catalysis system (such as Pd/H2SO4) is the most efficient system to
A Zn exchanged heteropolyacid (Zn-HPW; 1) was synthesized
date for the direct conversion of methane under mild conditions,5,6 via ion exchange from the parent phosphotungstic acid (HPW) in
the environmental concerns and the utilization of precious metals water and using ‘‘filter paper’’ as a ‘‘template’’ (see ESI†). After
set drawbacks for its potential use. Some breakthroughs were calcination treatment this material is insoluble in water or in other
achieved for the low temperature activation of methane7,8 to common organic solvents. A suspension of 1 in water was mixed
methanol or acetic acid using the environmentally benign with silica (Aerosil 200) to form a slurry; removal of water in vacuo
heterogeneous catalysts in which transition metal modified yielded silica supported Zn-HPW (Zn-HPW/SiO2; 2). 2 was then
zeolites catalyze the reaction.9,10
submitted to screening experiments using either pure CH4 or
Key to this development is the close examination of structure– mixtures such as CH4–CO2 or CH4–O2 under relatively mild condi-
reactivity relationships of already existing catalysts. In this context, tions. A variety of techniques including elemental analysis, thermal
metal oxides-based catalysts have been abundantly documented gravimetric analysis, mass spectrometry, SSNMR, BET, and infrared
but suffer from the disadvantage of being structurally poorly spectroscopy were employed for characterization.
understood.11
FTIR of 1 revealed a series of vibrations (1090 cmꢀ1 for n(P–Oa),
In contrast, heteropolyacids (HPA) are structurally better under- 980 cmꢀ1 for n(WQOa), 890 cmꢀ1 for n(W–Ob–W), and 790 cmꢀ1
stood but have shown lower performances, selective yet not very for n(W–Oc–W).28 These four IR peaks are characteristic of the main
active, for light alkane oxidation.4,12–16 They also can be easily Keggin unit of HPW (see Fig. S1, ESI†). Interestingly, the structure
modified by the inclusion of metals (for instance in the addenda is intact even after partial substitution (70%) of the protons of
or as counter cations) with a significant effect on their reactivity. HPW with Zn2+. SSNMR experiments were recorded for 1, 2 and
HPW/SiO2 (see Fig. S2, ESI†). The 31P spectrum of HPW is shifted
from ꢀ11 to ꢀ4 ppm upon exchange of H+ with Zn2+. The
KAUST Catalysis Center (KCC), King Abdullah University of Science & Technology,
nitrogen adsorption desorption isotherm (Fig. S3a and b, ESI†)
Thuwal 23955-6900, Saudi Arabia. E-mail: jeanmarie.basset@kaust.edu.sa
is clear evidence of the formation of mesoporosity in Zn-HPW
in contrast to the parent acid, HPW, which is non-porous.
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4cc04950k
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Partial substitution of protons of HPW by zinc results in a
higher thermal stability as demonstrated by TGA-DSC analysis
(Fig. S4a and b, ESI†). With isopropyl amine as the probe
molecule in TPD, the acid strength of Zn-HPW revealed a profile,
which was different than for its parents HPW (Fig. S5, ESI†): two
types of acidity were observed, a moderate one (desorption peak
centred around 330 1C) and a strong one (desorption peak
centred around 473 1C). It is known that the incorporation of
Zn introduces some Lewis acidity in heteropolyacid molecules.29
The Zn exchanged heteropolyacids prepared in this work did
not reveal a well-defined powder X-ray diffraction pattern (see
Fig. S6, ESI†). The approximate elemental composition was deter-
mined using a combination of thermal and elemental analysis
techniques (Table S1, ESI†), viz., Zn1.4H0.6PW40.30 It is assumed
that since the Keggin structure remains intact, the zinc atoms
are located outside this Keggin structure.
Fig. 2 Mass spectrometer signals (left hand side) and GC-FID peaks (right
hand side) obtained during the reaction of Zn-HPW with different mixtures
containing CH4/He (bottom), CH4/O2 (middle), and CH4/CO2 (top) after
heating from 25 up to 500 1C. The GC retention time for MeOH is about
11.2 min, whereas acetic acid is eluted at 17.8 min. The mass spectrometer
signals in red are for m/z = 32 (MeOH) and the one in black is for m/z = 60
(acetic acid).
A few experiments were carried out to see if this new
material could activate methane. To gain mechanistic insight,
batch experiments were designed (A–C, vide supra) with a view
to characterise the samples using SSNMR. Experiments were
conducted for a combination of 0.5 mmol 13CH4 (at 25 1C) (A),
0.5 mmol 13CH4 + 0.5 mmol O2 (heated to 250 1C) (B) and
Upon heating methane in the presence of 13CO2 at 300 1C,
0.5 mmol 13CH4 + 0.5 mmol 13CO2 (heated to 300 1C) (C). All samples acetic acid (13CH313COOH) is formed on the surface that
(A, B and C) were characterized using 13C SSNMR (see Fig. 1).
exhibits 13C chemical shifts at 20 ppm (–CH3) and 182 ppm
It should be noted that the Zn-HPW/SiO2 precursor does not (–COO). The two-dimensional (2D) CP/MAS 1H–13C hetero-
show any detectable carbon signal using 13C SSNMR spectroscopy. nuclear correlation (HETCOR) spectra (contact time 0.2 ms)
For A, a new signal is observed at ꢀ23 ppm. It can be assigned to the (Fig. 2b) display a correlation between the carbon signal at
zinc methyl species (Zn–CH3),19 hence demonstrating that methane 20 ppm (–CH3) and a proton signal at 3.1 ppm (–CH3) and a
is being activated at room temperature on Zn-HPW/SiO2. For B, correlation between the carbon signal at 182 ppm (–COOH) and
a strong surface methanol (CH3OH) signal is evident at 49 ppm. a proton signal at 8.2 ppm (–COOH). In the CP/MAS HETCOR
This resonance is quite intense and represents the condensation of spectra acquired with a longer contact time (5 ms) (Fig. 2c), the
gas phase methanol on the silica surface. This demonstrates that 1H signals at 3.1 ppm (–CH3) also correlate with the 13C signals
methane is oxidized to methanol starting at around 250 1C.
at approximately 180 ppm (–COO), consistent with the –CH3
and –COO functions belonging to the same molecule.
This material was also tested in a fixed bed reactor with 3
experimental gaseous conditions: (A0) methane/helium (1 : 19),
(B0) methane/oxygen/helium (1 : 1 : 18) or (C0) methane/carbon
dioxide/helium (1 : 1 : 18). Flow through reactor control experi-
ments in the absence of any sample were run to ensure that no
products were formed by heating the A0 or B0 or C0 gas mixtures
in the reactor. In the pure methane flow experiment (A0),
the only gases observed in addition to methane were hydrogen
and carbon dioxide, both at a temperature of around 230 1C.
It indicates that methane is oxidized non-selectively to CO2.
The evolution of hydrogen strongly suggests that methane is
activated by the proton of the heteropolyacid, in a similar
fashion to alkane activation by acid catalysts.13,31
Mixing methane with oxygen (B0) resulted in the observation
of methanol signals at temperatures between 250 and 400 1C
with a maximum at 300 1C (see Fig. 2a). Carbon dioxide, the only
by-product, is formed at the same temperature (230 1C) as for A0.
Complementary experiments were conducted using 13CH4 and
O2 as feeds. Labelled methanol 13CH3OH was formed (32 + 1 m/z)
at the same temperature and with almost similar signal inten-
sity; hence, confirming methane as the precursor of methanol.
At this stage, any further proposal relating to the detailed
Fig. 1 (A) One-dimensional (1D) 13C CP/MAS NMR spectrum of -enriched
methane (13CH4) on Zn-HPW/SiO2 catalyst at reaction temperatures of
25 1C (CH4), 250 1C (CH4 + O2), 300 1C (CH4 + CO2) acquired at 400 MHz
with 10 kHz MAS frequency with a repetition delay of 4 s, and 2 ms contact
time. Exponential line broadening of 80 Hz was applied prior to Fourier
transformation. (B) 2D 1H–13C HETCOR spectrum with 0.2 ms contact
time (B) and 5 ms (C) of Zn-HPW/SiO2 catalyst 300 1C (CH4 + CO2) with
10 KHz MAS frequency, 5000 scans per t1 increment, a 4 s repetition delay,
32 individual t1 increments).
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mechanism is speculative. We tentatively speculate that, to methane. This implies that the conversion of methane to
similar to the classic carbanion reactions in organometallic the zinc methyl species is a reversible process on Zn-HPW
chemistry,32,33 the zinc methyl species may be reacting with depending upon the temperature. It has been proposed that
oxygen to form the methyl peroxy species (this species was not the zinc methyl species originates from activation of methane
observed by 13C NMR spectroscopy) which can further decompose from Zn2+ cations possibly delocalized over oxygen atoms of a
to form methanol. This mechanistic proposal is also supported heteropolyacid similar to the observation with Zn exchanged
by Han et al.,34,35 who observed the formation of methanol in zeolites.37 Indeed, the dissociative adsorption of hydrogen, methane
the direct partial oxidation (DPO) of methane on Zn-modified and ethane over acid–base Znd+–Odꢀ pairs was investigated
ZSM-5 catalysts.
by theoretical studies.17 Other calculation investigations on
Under similar conditions to B0 but using a mixture of methane Zn doped La2O3 have favoured the existence of fragmented
and carbon dioxide (C0), the gas phase acetic acid signal was Zn–methyl adjacent to a hydroxyl group as generated by the
observed using a mass spectrometer. Acetic acid starts to form at reaction of methane with the metal oxide surface.38
250 1C and the signal reaches its maximum intensity at around
In conclusion, Zn-HPW/SiO2 (2) was shown to activate
350 1C. Separate experiments conducted with either 13CH4 or methane for stoichiometric conversion to methanol (with O2) and
13CO2 confirmed that the source of carbons in acetic acid viz. acetic acid (with CO2). Remarkably, the activation of methane is
methyl carbon derived from methane while the carboxyl moiety achieved already at room temperature as evidenced by the first time
was from carbon dioxide. No carbon monoxide or hydrogen is detection of Zn–CH3 on a Zn modified heteropolyacid.
observed in the gaseous phase confirmed using both GC and
We gratefully acknowledge King Abdullah University of
mass analyzer. All points out to the synthesis of acetic acid Science and Technology (KAUST) and Saudi Basic Industries
directly from carbon dioxide and methane and not from syngas Corporation (SABIC) for financial support.
(although there are serious thermodynamic limitations for the
reaction between methane and CO2 into acetic acid). The zinc
methyl species can effectively react with carbon dioxide through
the insertion mechanism that has well been established in
organometallic chemistry in homogeneous media for the reac-
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