Direct Stepwise Oxidation of Methane to Methanol over Cu-SiO2

Article abstract of DOI:10.1021/acscatal.8b01021

Cu supported on SiO₂ can be used to directly convert methane to methanol in a stepwise process with no intrinsic need for a zeolite support. Effects of parameters such as the O₂ activation temperature, activation time, CH₄ reaction temperature, CH₄ partial pressure (p_CH4), and Cu wt % on methanol yield were investigated. Increasing the O₂ activation temperature in the 200-800 °C range significantly improved the methanol yield, and when carried out at 800 °C, a methanol yield of 11.5 μmol/g_catalyst was obtained after reaction with methane at 200 °C for the sample with 2 wt % Cu. Yield per mole of Cu increased exponentially from 1.0 to 59.1 mmol with decreased Cu wt % from 30 to 1, respectively. The increase in the O₂ activation time also strongly influenced the yield which corresponded to the increase in yield by a factor of >2 between 1 and 8 h. Increasing p_CH4 from 0.05 to 1 atm resulted in a 5-fold increase in yield after activation at 450 °C; however, it resulted in at least 20% lower yields after activation at 800 °C showing that active sites of different nature were formed at different activation temperatures. The increase in yield with ramped O₂ activation temperature correlated with the dehydration of the samples as evidenced by X-ray absorption near-edge spectroscopy (XANES) and via mass spectroscopy (MS) traces of H₂O during the O₂ activation step.

Full text of DOI:10.1021/acscatal.8b01021

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Article
Direct Stepwise Oxidation of Methane to Methanol over Cu-SiO2
Selmi Erim Bozbag, Petr Šot, Maarten Nachtegaal, Marco
Ranocchiari, Jeroen Anton van Bokhoven, and Carl Mesters
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01021 • Publication Date (Web): 15 May 2018
Downloaded from http://pubs.acs.org on May 15, 2018
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Direct Stepwise Oxidation of Methane to Methanol
over CuꢀSiO2
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,2,†
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Selmi E. Bozbag , Petr Sot , Maarten Nachtegaal , Marco Ranocchiari , Jeroen A. van
1,2,*
3,*
Bokhoven , Carl Mesters
1
Paul Scherrer Institute, Villigen, CHꢀ5232 Switzerland.
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ETH Zurich, Institute for Chemical and Bioengineering, WolfgangꢀPauliꢀStrasse 10, CHꢀ8093
Zurich, Switzerland.
3
Shell Technology Center Houston, 3333 Highway 6 South, Houston, TX 77083, USA
KEYWORDS: Methane to methanol, silica, copper, activation temperature, XRD, XAS.
ABSTRACT: Cu supported on SiO can be used to directly convert methane to methanol in a
2
stepꢀwise process with no intrinsic need for a zeolite support. Effects of parameters such as the
O activation temperature, activation time, CH reaction temperature, CH partial pressure (pCH4)
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and Cu wt. % on methanol yield were investigated. Increasing the O activation temperature in
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°
°
the 200ꢀ800 C range significantly improved the methanol yield and when carried out at 800 C, a
°
methanol yield of 11.5 µmol/gcatalyst was obtained after reaction with methane at 200 C for the
sample with 2 wt. % Cu. Yield per mole of Cu increased exponentially from 1.0 to 59.1 mmol
with decreased Cu wt. % from 30 to 1, respectively. The increase in the O activation time also
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strongly influenced the yield which corresponded to the increase in yield by a factor of >2
between 1h and 8h. Increasing pCH4 from 0.05 to 1 atm resulted in a 5ꢀfold increase in yield after
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activation at 450 C however it resulted in at least 20% lower yields after activation at 800 C
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showing that active sites of different nature were formed at different activation temperatures. The
increase in yield with ramped O activation temperature correlated with the dehydration of the
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samples as evidenced by Xꢀray absorption near edge spectroscopy (XANES) and via mass
spectroscopy (MS) traces of H O during the O activation step.
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1
. Introduction
Direct conversion of methane into methanol using molecular oxygen remains one of the
greatest challenges in catalysis. Although this reaction is thermodynamically possible, it is
difficult to omit the kinetically controlled overꢀoxidation of methane since the desired product
methanol is more reactive. This problem can be addressed by forming a stable complex of the
reaction intermediate thereby preventing the total oxidation of methane to CO and then
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ꢀ3
converting the intermediate to methanol in a stepꢀwise process .
Over the last decade a number of reports showed the potential of Cu loaded zeolites such as
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ZSMꢀ5 , mordenite , smallꢀpore zeolites and others for the conversion of methane to
methanol. The stepwise (sub)stoichiometric process consists of the activation of O on active
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copperꢀoxo sites
followed by the reaction of methane in which the oxidized copper sites
react with methane to form a chemisorbed stable intermediate which allows the production of
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, 15
methanol via the subsequent hydration step either at process conditions
or via room
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temperature solvent extraction . Processes using oxidants other than O
and a catalytic
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continuous process were also recently reported . At ambient pressure, the formation of the
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active copperꢀoxo species requires the dehydration of the sample in a high temperature treatment
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typically >400 C
. At high temperature, the chemisorbed methane intermediate decomposes
°
and therefore the subsequent reaction step is usually carried out at lower temperatures (<250 C).
The aforementioned stepwise process limits the overꢀoxidation of the chemisorbed intermediate
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and therefore results in highly selective methanol yields, ranging from 9 to 180 µmol/gcatalyst
Differences in the yield were associated with the concentration of the active sites and the nature
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of their speciation
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Controllable synthesis of the active copper sites on the zeolite surface is difficult since beside
the aforementioned sites a variety of copper species including isolated ions, oligomers, clusters
and nanoparticles could form depending on the copper loading, zeolite morphology, Al location
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within the zeolite and the synthesis method
. Careful manipulation of the synthesis conditions
is necessary for higher yield; Grunder et al. suggested that formation of unselective Cuꢀoxo
clusters can be avoided by limiting the pH during the ionꢀexchange procedure below the point of
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zero charge of the surface silanol groups in the zeolite
. The formation of the active sites in
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Cuꢀzeolites has been a subject of many spectroscopic
and computational studies
.
A definite agreement about the structure of the active site or sites has not been reached so far.
For the methane to methanol reaction, the nature and the reactivity of the active site is also
highly influenced by the process parameters such as O activation temperature and CH reaction
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pressure. All of the aforementioned treatments could result in different active site speciation.
Based on spectroscopic and computational studies, when the process is carried out at ambient
°
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pressure with an O activation step >400 C, mono(ꢁꢀoxo)dicopper in CuꢀZSM5 and CuꢀSSZꢀ
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tetraneutral clusters in CuꢀMOR were proposed as active sites. Papas et al. showed that the
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hydrated Z[Cu OH] monocopper sites were the precursors to the active sites in CuꢀSSZꢀ13 and
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Alayon et al. proposed an additional waterꢀstable active Cu species for CuꢀMOR formed upon O2
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activation at 450 C followed by hydration at 200 C . When the stepꢀwise process was carried
°
out isothermally at 200 C and the subsequent CH reaction step was carried out at high pressure,
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higher methanol yields were obtained and nanoparticles were proposed as the active sites based
on ultravioletꢀvisible spectroscopy (UVꢀVis) and transmission electron microscopy (TEM)
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data
. The complicated formation mechanisms of active copperꢀoxo sites, which can result in
a variety of active or inactive Cu species on zeolites, prompted us to investigate supports other
than zeolites. For the partial oxidation of methane, CuꢀSBA15 catalyst showed appreciable
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formaldehyde selectivity at low CH conversions at 625 C . Among other supports, amorphous
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SiO could be an attractive alternative to synthetic zeolites. CuꢀSiO using a modified SiO with
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an Al O content of 0.6 wt% (Si/Al = 141) did not show appreciable activity for the conversion
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of methane to methanol most probably due to the absence of active sites which did not form on
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SiO at O activation temperatures below 500 C. Active Cuꢀoxo sites might be formed on the
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silica surface at different conditions i.e. at higher O activation temperatures.
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In this study, we show that the direct stepꢀwise conversion of methane to methanol is
°
possible on CuꢀSiO subsequent to the O activation at higher temperatures (> 500 C) without
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the necessity of a zeolite support. The effects of activation temperature, activation duration, CH4
partial pressure during reaction and Cu wt. % on the methanol yield were investigated. CuꢀSiO2
catalysts were characterized using N adsorption, Xꢀray diffraction (XRD), temperature
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programmed desorptionꢀmass spectroscopy (TPDꢀMS), UVꢀVis , in-situ Xꢀray absorption
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spectroscopy (XAS) and Scanning transmission electron microscopy (STEM) during or after
various stages of the catalytic process.
2. Experimental
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.1.Catalyst Preparation and Activity
CuꢀSiO with varying Cu loadings (1,2,5,15,30 wt. %) were prepared via poreꢀvolume
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impregnation of a silica support (PQ Silica PD44011) with an ammonium copper carbonate stock
solution. The ammonium copper carbonate solution was prepared by adding 250 g of (NH ) CO
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to 690 g of an ammonia solution (25%). After stirring for 30 min at 30 C, a clear solution was
obtained. Subsequently, 300 g of copper hydroxy carbonate was added, in portions of 60 g within
5 min to the vigorously stirred ammonia carbonate solution. The solution was then diluted to the
°
desired metal content and impregnated to the silica support followed by drying at 150 C and
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calcination at 400 C. The drying and calcination steps were carried out in air.
The activity tests were carried out in a continuousꢀflow fixedꢀbed reactor. The catalyst
powder (0.7 g) was placed in the middle of a quartz tube reactor which was fitted into a tubular
furnace. Gas flows were regulated using calibrated mass flow controllers (Bronkhorst). Figure 1
shows the two different reaction protocols employed. All gas flows were kept at 30 mL/min (1
°
atm, 25 C). The first reaction protocol is hereafter referred to as R1: A stream of O was
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introduced into the reactor and the reactor was heated to the desired activation temperature with
a ramp of 1 °C/min and remained at this temperature for a desired period. This was followed by
cooling and removing the excess O with a He stream for 30 min. A CH stream (5% in He) was
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then introduced for 30 min. Finally, the system was cooled down to room temperature under He
flow for methanol quantification by extraction (described below). The second reaction protocol
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(
hereafter referred to as R2) was similar to that of the first one except methanol was extracted
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using steam at 200 C just after the CH reaction step (Step G2 in Figure 1). Steam was supplied
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using a controlled evaporator mixer (CEM W202A, Bronkhorst) with a rate of 10 g/h where He
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(30 ml/min) was used as carrier gas. The evaporator temperature was set to 120 C. All the lines
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between the evaporator and reactor were heated to 120 C to avoid condensation. A condenser,
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cooled down to 15 C, was connected at the outlet of the reactor where the extract was
condensed. Liquid samples containing a mixture of water and methanol were taken from the
bottom of the condenser at desired intervals. Once the extraction was completed (as analyzed by
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gas chromatography (GC), Agilent 6890), a second cycle was initiated with heating from 200 C
in O to the desired activation temperature. Reactor downstream concentrations were monitored
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using a quadrupole mass spectrometer (Pfeiffer, SEM voltage 1050 V, CHꢀTRON detector).
2
.2. Methanol quantification
In the first reaction protocol the amount of produced methanol was quantified by extraction
with liquid water (Step G1). After the reaction with CH and cooling, the catalyst was transferred
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into a vial along with a magnetic stirring bar and 2 mL MilliQ water was added. The vial was
closed and the mixture was stirred for 2 h. The suspension was filtered through a glassꢀfiber
filter. Afterwards, 10 µl acetonitrile (internal standard, 10% V/V solution in water) was added
and the mixture was analyzed on an Agilent 6890 GC equipped with a Restek Rtx®ꢀ5 column
(30 m, ID = 0.25 mm, film thickness 0.25 µm) and a flame ionization detector. The oven
o
°
°
temperature was held at 38 C for 20 min and subsequently raised to 200 C at 50 C/min and
held there for 7 min. Solvent extraction was repeated at least twice until no more methanol was
obtained. The experimental error in methanol yield was
± 1 µmol/gcatalyst. In the second reaction
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protocol, after the liquid samples were taken from the outlet of the condenser (Step G2),
acetonitrile was added as internal standard to the sample and the mixture was analyzed by GC.
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.3.Characterization of the catalysts
Textural properties of the samples were determined by N adsorption using a Micromeritics
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Tristar II 3020 instrument. Approximately 100 mg of each material was transferred into the
measurement tube, degassed at 50 mTorr at 150 °C overnight and analyzed. The total pore
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volume (Vtotal) was obtained using the uptake amount at P/P = 0.986 and micropore volume
(Vmicro) was obtained using the tꢀplot method. The Brunauer, Emmett and Teller (BET) surface
areas of the samples were also measured. XRD measurements were carried out using a Cu Kα
source in a Bruker D8 Advance diffractometer. UVꢀVis measurements were carried out in a
measurement cell which was placed inside a glove bag filled with Ar. After treating the samples
at various reactive steps described in Fig. 1, Ar was led to flow through the catalyst bed. Then,
the quartz tubular reactor containing the catalyst bed was quickly removed from the test rig and
transferred into the glove bag. Catalyst was then transferred into PMMA cuvettes and spectra
were measured. The UV probe (reflection/backscattering, QR400ꢀ7ꢀSRꢀ125F, OD=3.18mm),
connected by an optical fiber to the spectrometer, was positioned at the center of the PMMA
cuvette which was fitted on a holder to obtain a good signal to noise ratio. The measurement cell
was placed in a dark box to minimize stray light. UVꢀVis spectra were recorded by a Maya 2000
Pro (Ocean Optics, Spectra Suite 2000) spectrometer at a scan rate of 200 ms per spectrum from
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5000 cm to 10000 cm . A thousand scans were averaged to produce a single spectrum. A
spectrum of the bare SiO was used as reference. The morphologies of the catalysts were
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characterized by STEM on the aberrationꢀcorrected HDꢀ2700CS (Hitachi; coldꢀfield emitter),
operated at an acceleration potential of 200 kV. A probe corrector (CEOS) is incorporated in the
microscope column between the condenser lens and the probeꢀforming objective lens providing
excellent highꢀresolution capability (beam diameter ca. 0.1 nm in the selected ultraꢀhigh
resolution mode). Images (1024 x 1024 pixels) were recorded with a highꢀangle annular dark
field (HAADF) detector with frame times of ca 15 s. These imaging conditions give rise to
atomic number (Z) contrast, a highly sensitive method to detect even atoms of strongly scattering
elements (high Z) on light supports. TEM was performed on a F30 microscope (FEI; field
emission gun), operated at an acceleration potential of 300 kV. XAS spectra around the Cu Kꢀ
edge were collected at the SuperXAS beamline, Swiss Light Source (SLS), of the Paul Scherrer
Institute in Villigen, Switzerland. The SLS ring is operated in top up mode at 400 mA and 2.4
GeV. The polychromatic beam from the 2.9 Tesla superbend source was collimated with a Siꢀ
coated collimating mirror at 2.5 mrad. This also served to remove higher harmonics. The Xꢀray
beam was subsequently monochromatized by a channelꢀcut monochromator using a Si(111)
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crystal pair. Focusing of the beam to a spot size of 500 x 100 (H x V) micrometer was achieved
using a Rhꢀcoated toroidal mirror. Spectra were measured in transmission mode using 15 cm
long ion chambers filled with 20% He/ 80% N mixture resulting in about 8% absorption in both.
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A Cu foil mounted in front of a third ionization chamber filled with 100% N at 1.5 bar was
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measured simultaneously and used for absolute energy calibration. Around 15 mg of CuꢀSiO2
was fixed between two plugs of quartz wools and placed in a 3 mm diameter, thinꢀwalled quartz
capillary reactor. This was affixed to an air blower setꢀup for controlled heating. The temperature
inside the capillary reactor was preꢀcalibrated against the readout temperature of the blowing hot
air with a thermocouple inserted in the reactor until the thermocouple's tip touched the middle of
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the catalyst bed. Subsequently, the thermocouple was removed during the experimental
measurements and the air blower was set against the calibrated settings.
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0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 1. Schematic diagram of the thermal and chemical treatments on CuꢀSilica samples
during one catalytic cycle. A–D: activation; E: He purge; F: CH reaction; G: hydration and
4
methanol production. (TO2 and TCH4 represent the O activation and CH reaction temperatures,
2
4
respectively).
3
. Results and Discussion
3
.1.Catalytic Activity
The samples screened in this study were 1, 2, 5, 15 and 30 wt. % Cu containing CuꢀSiO which
2
are hereafter referred to as CuꢀSiO ꢀ1, CuꢀSiO ꢀ2, CuꢀSiO ꢀ5, CuꢀSiO ꢀ15 and CuꢀSiO ꢀ30,
2
2
2
2
2
respectively. All of the methanol yields reported were obtained using reaction protocol R1 unless
otherwise mentioned. The formation of active copperꢀoxo species in zeolites is significantly
influenced by the O activation temperature. Therefore, the effect of the activation temperature
2
on the methanol yield was studied (Fig. 2). For both CuꢀSiO ꢀ2 and CuꢀSiO ꢀ5 very low
2
2
methanol yields (0.7 µmol/gcatalyst, measured via G1) were obtained up to an activation
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°
°
temperature of 450 C followed by exposure to CH (5% CH in He) at 200 C. Methanol yield
4
4
°
increased significantly as the O activation temperature increased up to 800 C reaching 11.5 and
2
7.9 µmol/gcatalyst for CuꢀSiO ꢀ2 and CuꢀSiO ꢀ5, respectively. The yield obtained from the Cuꢀ
2
2
4
, 9
0
1
2
3
4
5
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7
8
9
0
1
2
3
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9
0
SiO ꢀ2 is even higher than that reported for many Cuꢀzeolite systems . The reusability of the
2
CuꢀSiO ꢀ2 in a subsequent catalytic cycle was also tested using reaction protocol R2 and resulted
2
2
in a very similar methanol yield (Fig. 2). When a reaction cycle using CuꢀSiO ꢀ5 with O2
o
o
activation at 800 C is followed by a reaction cycle with O activation at 450 C, the methanol
2
st
nd
yield drops from 7.9 µmol/gcatalyst in the 1 cycle to 1.9 µmol/g
in the 2 cycle showing the
catalyst
importance of high temperature activation. Data in Fig. 2 also shows the effect of methane partial
pressure on the methanol yield. The increase of methane partial pressure from 0.05 to 1 atm
resulted in an increase of the methanol yield from 0.73 and 0.5 to 4.8 µmol/gcatalyst for both
°
catalysts after O activation at 450 C for the CuꢀSiO ꢀ2 and CuꢀSiO ꢀ5, respectively. For the Cuꢀ
2
2
2
°
SiO ꢀ2, the increase of partial pressure increased the yield after O activation at 650 C but
2
2
°
decreased the yield after O activation at 800 C.
2
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Figure 2. Effect of O activation temperature (for 8 hours) on methanol yield (CH reaction was
2
4
°
carried out at 200 C and the p
catalytic cycle).
denotes the partial pressure of CH during Step F of the
4
CH4
Figure 3. Effect of (a) activation time and (b) CH reaction temperature on the methanol yield.
4
°
Activation was carried out in O at 800 C.
2
Fig. 3a shows the effect of the activation time on the methanol yield when the activation was
°
carried out at 800 C. Data clearly indicate an increase in methanol yield with activation time up
to 8 h where the yield increased from 3.5 to 9.5 µmol/gcatalyst for CuꢀSiO ꢀ5. The yield did not
2
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change considerably after an activation time of 12h. CuꢀSiO ꢀ1 and CuꢀSiO ꢀ2 showed a similar
2
2
trend. Thus, with increased O exposure time, the number of reactive sites increased. The
2
temperature of the CH reaction step has also an effect on the yield (Fig. 3b). The yield increased
4
°
0
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from 0.2 to 11.5 µmol/gcatalyst as the CH reaction temperature was raised from 50 to 200 C for
4
°
CuꢀSiO ꢀ2. As the CH reaction temperature was further increased to 300 C, the yield dropped
2
4
to 5.6 µmol/gcatalyst due to overꢀoxidation and the decomposition of the reacted methane
intermediate. Similar results were also obtained for Cuꢀchabazite where CH reaction
4
o
8
temperatures higher than 200 C resulted in decreased methanol yield and decreased selectivity .
Previous reports on Cuꢀzeolites showed that the Cu loading is an important parameter that
affects the methanol yield. Cu loading in zeolites affect the formation of active sites i.e. at lower
48
loadings (typically Cu/Al<0.2 ) monoatomic species exist and as the loading is increased diꢀ or
triꢀcopper species or clusters may form. In zeolites active site formation is dictated by the
location of Al within the framework. Fig. 4 depicts the variation of the methanol yield with Cu
wt. % in CuꢀSiO . Different trends in the methanol yield with Cu wt. % were observed for
2
°
different activation temperatures (Fig. 4a). When the O activation was carried out at 800 C
2
°
followed by CH reaction at 200 C with a CH partial pressure of 0.05 atm, methanol yield per
4
4
gram catalyst increased with increasing Cu wt. % where the highest value of 11.5 µmol/gcatalyst
was reached with CuꢀSiO ꢀ2. After this point, the yield decreased with increasing Cu wt. %. A
2
bare silica sample treated with the same reactive steps of the catalytic cycle as the other catalysts
°
did not show any methanol yield (Fig. 4a). When the activation was carried out at 450 C, the
yield was considerably smaller for all the samples independent of the metal loading. CuꢀSiO ꢀ1
2
did not show any appreciable yield whereas as the metal loading was further increased, the yield
was increased and a plateau was reached for CuꢀSiO ꢀ2. When the CH partial pressure was 1
2
4
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atm during the CH reaction step (step F), at least four times higher yields (4.8 µmol/gcatalyst)
4
were obtained as compared to the case of 0.05 atm for both CuꢀSiO ꢀ2 and CuꢀSiO ꢀ5. Lower
2
2
°
yields were obtained when CH partial pressure was 1 atm after O activation at 800 C as
4
2
1
1
1
1
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0
compared to the same case with a CH partial pressure of 0.05 atm. This suggests a difference in
4
the nature and the concentration of the active sites for the O activation temperatures of 450 and
2
o
8
00 C. Fig. 4b illustrates the variation of the methanol yield per mole of copper in the catalyst.
°
°
After the O activation at 800 C followed by a CH reaction step at 200 C (p
= 0.05 atm), the
2
4
CH4
methanol yield per mole of Cu increased with decreasing metal loading in the 1ꢀ30 wt. % range
with CuꢀSiO ꢀ1 resulting in a yield of 59.1 mmol/mol Cu. This shows that the concentration of
2
the active Cu sites is higher for samples with a lower Cu content. However, the trend upto 2 wt.
°
%
is different when pCH4 = 1 atm (O activation at 800 C). Here, the highest yield (per mol of
2
Cu) was obtained for the 2 wt. % sample. This again shows that the nature of the active sites
differs at different CH reaction partial pressures. On the other hand, these different active sites
4
may be present for each partial pressure value but with different reaction orders in CH4
activation resulting in different methanol yields.
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Figure 4. Effect of Cu wt. % on methanol yield. (a) micromoles methanol /gcatalyst (b) mmol
methanol / mol Cu. pCH4 denotes the partial pressure of CH during Step F of the catalytic cycle.
4
3
.2.Catalyst Characterization
Table 1 shows the textural properties of the samples investigated in this study. The fresh silica
3
2
sample had a pore volume of 1.65 cm /g with BET surface area of 260 m /g with low
3
concentration of micropores (0.013 cm /g) as compared to the total pore volume. The pore
3
2
3
volume and thus the BET surface area decreased from 1.31 cm /g and 276 m /g to 0.70 cm /g
2
and 201 m /g when the Cu wt. % increased from 1 to 30, respectively. At the end of one catalytic
cycle, the micropore volume for all samples decreased slightly.
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Table 1 Textural properties of the samples
Fresh samples
Samples
BET 2surface tꢀplot
Micropore Pore volume
area (m /g)
volume
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
(
cm /g)
3
(
cm /g)
0
(bare silica)
260
276
269
213
200
201
0.013
0.012
0.012
0.027
0.035
0.028
1.65
1.31
1.34
0.98
0.77
0.70
CuꢀSiO ꢀ1
2
CuꢀSiO ꢀ2
2
CuꢀSiO ꢀ5
2
CuꢀSiO ꢀ15
2
CuꢀSiO ꢀ30
2
Postꢀcycle* samples
CuꢀSiO ꢀ1
240
0.009
0.008
0.011
0.006
0.005
1.12
1.07
1.24
0.85
0.70
2
CuꢀSiO ꢀ2
229
200
204
184
2
CuꢀSiO ꢀ5
2
CuꢀSiO ꢀ15
2
CuꢀSiO ꢀ30
2
*
As depicted in Fig. 1.
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Figure 5. (a) XRD of CuꢀSiO ꢀ2 after calcination at various temperatures in O . (b) Room
2
2
temperature (RT) UVꢀVis spectra of CuꢀSiO ꢀ2 after calcination at various temperatures in O2
2
°
and after CH reaction (O activated at 800 C). Arrows indicate changes in the particular feature
4
2
with respect to previous temperature increment. Inset shows the difference spectrum between O2
°
°
activated (800 C) and CH reacted (200 C) sample. (c) Zꢀcontrast STEM image after O
4
2
°
activation at 450 and at (d) 800 C.
Figure 5 presents the effect of O activation temperature on the structural and spectroscopic
2
features in CuꢀSiO ꢀ2. Fig. 5a shows the XRD patterns after O activation at various
2
2
temperatures. No crystallinity was observed for either fresh CuꢀSiO (not shown) or after the O2
2
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°
activation at 200 C. Monoclinic CuO with sharp peaks were clearly observed after O activation
2
°
at 450 C with an average diameter of 30 nm as calculated using the Scherrer equation. An
exemplar STEM image which is shown in Fig. 5c displays well dispersed CuO nanoparticles on
1
1
1
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1
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2
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SiO with average nanoparticle size of 1ꢀ2 nm. The small nanoparticles most probably formed
2
during the prolonged exposure to the electron beam as illustrated in Fig. 6. Here, one can clearly
observe the sintering of a CuO nanoparticle indicated with the white arrow until it reaches a size
of 2.9 nm. This suggests that there is a mixture of bulk CuO crystallites with larger size that were
absent in the regions analyzed by the STEM and highly dispersed Cu on the surface of SiO after
2
the O activation. Surface mixtures of highly dispersed and bulk CuO was also previously
2
5
0ꢀ51
reported on SiO supported catalysts
. Further increase in the O activation temperature to 650
2
2
°
°
C resulted in the complete disappearance of the CuO reflections in the XRD pattern. At 650 C,
a small peak assigned to cristobalite phase was observed in line with the literature on CuꢀSiO2
5
2ꢀ53
treated at similar temperatures
. We hypothesize that the disappearance of CuO crystal peaks
might be due a number of phenomena including the formation of dispersed Cu species, doping of
Cu into the SiO matrix due to partial crystallization of the support or the formation of
2
°
amorphous islands. At 800 C, peaks associated with (002) and (111) planes of CuO at 2θ angles
of 35.4 and 38.7, respectively were observed with a very small intensity indicating that a small
portion of copper formed CuO crystals. Since the XRD was performed subsequent to the cooling
°
down subsequent to the 800 C O activation, some crystallization might have occurred leading
2
°
to some small CuO reflections. STEM images taken after O activation at 800 C did not show
2
any nanoparticles (Fig 5d). This indicates that CuO nanoparticle concentration is very low and
the rest of Cu is either in amorphous form or in very small particles that are not detected.
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Figure 6. Bright Field TEM image of fresh CuꢀSiO ꢀ15 taken during exposure to the electron
2
beam. The white arrow indicates the growth of a copper oxide nanoparticle on the SiO surface
2
with time.
UVꢀvis spectroscopy is often used to elucidate the nature of the active copperꢀoxo sites in Cuꢀ
4
ꢀ5, 32, 54ꢀ57
zeolites
. Fig 5b shows the UVꢀVis spectra of the CuꢀSiO samples after O activation
2
2
°
at various temperatures along with the spectrum taken after the CH reaction at 200 C
4
°
ꢀ1
(
subsequent to O activation at 800 C). The charge transfer (CT) band at 22700 cm was
2
observed for all samples and its intensity increased with increasing activation temperature. The
shape of the band became much more distinct as the O activation temperature was raised above
2
°
ꢀ1
43
4
50 C. The feature at 22700 cm associated with mono(µꢀoxo)dicopper on CuꢀZSM5 and was
5
, 32, 54
also observed in CuꢀMOR
was previously suggested as the active site for the direct
ꢀ1
conversion of methane to methanol. The dꢀd band at 20000 cm was associated with other active
34
(
diꢀ)copper species in zeolites . The formation of these active (diꢀ)copper species in zeolites
was closely associated with the dehydration of the sample therefore we monitored the MS traces
of H O during the O activation step of CuꢀSiO ꢀ2 (Fig. 7). Here, two broad H O desorption
2
2
2
2
°
peaks could clearly be observed with peak centers at 315 and 608 C with distinct shoulders at
°
°
1
30 C, and 800 C, respectively. The dehydration of CuꢀSiO continued with the onꢀset of an
2
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°
isothermal dehydration at 800 C. The dehydration behavior seems to be in line with UVꢀVis
ꢀ1
behavior since the relative intensity of the 22700 cm feature reached its highest value after
°
activation at 800 C. Dehydrationꢀdriven formation of active copperꢀoxo sites seems to be the
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
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0
case for CuꢀSiO as well since the methanol yield increased with increasing O activation
2
2
4, 9,
temperature (Fig. 2). Formation of the active dicopper is also associated with the Cu/Al ratio
4
8
49
and the proximity of copper ions to the Al ions . Here, we show that at higher O activation
2
ꢀ1
temperatures, the 22700 cm feature could manifest on the SiO surface without requiring Al.
2
ꢀ1
°
The feature at 22700 cm observed after O activation at 650 and 800 C disappeared after
2
°
°
subsequent CH reaction at 200 C (O activated at 800 C) as shown in Fig 5b and also in the
4
2
difference spectrum given in the inset of Fig 5b along with other CT features at 25800, 20270
ꢀ1
ꢀ1
and 13380 cm as indicated with blue lines. Gradual disappearance of the 22700 cm feature
4
ꢀ5, 32
35
upon CH reaction
or upon NO reaction was also observed on Cuꢀzeolites. Therefore, this
4
feature along with the features observed in the difference spectrum could be ascribed to active
site(s) for methane activation on CuꢀSiO2.
Figure 7. MS traces of H O (m/z=18) during the O activation step of CuꢀSiO ꢀ2.
2
2
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Figure 8. (a) XRD of Cu on silica with various loadings after calcination at 800 C in O . (b)
2
°
RT UVꢀVis spectra of Cu on silica with various loadings after calcination at 800 C in O .
2
The Cu loading is an important parameter affecting the speciation of the actives sites and thus
4
the methanol yield in Cuꢀzeolites . Therefore, we investigated the effect of metal loading on the
°
structural and spectroscopic features of the CuꢀSiO samples after O activation at 800 C.
2
2
°
Samples treated at 800 C were investigated since they resulted in the highest methanol yields.
XRD patterns shown in Fig. 8a indicate the presence of monoclinic CuO in all samples, with the
exception of the 30 wt% sample, however the concentration of CuO nanocrystals in the samples
varied with metal content. CuꢀSiO ꢀ1 and CuꢀSiO ꢀ2 had a low CuO concentration whereas the
2
2
concentration of nanoꢀcrystals was much higher for CuꢀSiO ꢀ15 which has a bimodal crystal size
2
distribution. The UVꢀvis spectra illustrated in Fig. 8b again indicates the presence of the 22700
ꢀ1
cm feature for all of the samples. However, for samples with loadings higher than 2 wt. % it is
difficult to ascribe this feature to a single site due to the bulky shape of the CT region between
ꢀ1
ꢀ1
3
5000 and 22000 cm . For the CuꢀSiO ꢀ1, a broad CT band at 32200 cm with a shoulder at
2
ꢀ1
29000 cm could also be observed. Similar CT features were ascribed to copper clusters in Cuꢀ
34
zeolites . Since all the samples had some activity towards the conversion of methane to
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methanol, isolated dicopper sites or even CuO clusters with sizes below the detection limit of
XRD could be active sites. We performed XRD after all of the relevant subsequent steps of the
catalytic cycle for the CuꢀSiO ꢀ2 (Fig. 9). All of the peaks associated with the crystal planes of
2
°
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CuO formed after the O activation at 800 C, shifted to higher 2θ values upon reaction with CH
2
4
°
58
at 200 C. This could be associated with the contraction of the crystal lattice by the
chemisorption of reaction intermediates on CuO. Upon subsequent extraction in H O (Step G1)
2
which removed the chemisorbed intermediates by the formation of methanol, the XRD showed
that the peaks shifted back to their original values obtained after the O activation step however
2
with lower intensity. This indicates that CuO nanoꢀcrystals might also be involved in the
activation of methane for the production of methanol on CuꢀSiO2
Figure 9. Section of the XRD pattern of CuꢀSiO ꢀ2 after various steps of the catalytic cycle.
2
Dotted line illustrates the 2θ angle of CuO (111) plane.
Fig. 10 compares the XANES and Fourier transformed (FT) extended xꢀray absorption fine
structure (EXAFS) spectra of CuꢀSiO ꢀ5 (Fig. 10a & c) and CuꢀSiO ꢀ30 (Fig. 10b & d) during
2
2
progressive heating in O flow. XANES recorded for the CuꢀSiO ꢀ5 sample showed a
2
2
progressive decrease of the maximum intensity feature at around 9000 eV which indicates a
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gradual change in local coordination of Cu with increasing temperature. The distinct shoulder
observed at around 8986 eV and a preꢀedge feature at around 8977 eV are the indications of the
2
+
Cu oxidation state. The intensity of this feature slightly increased with increasing temperature
2
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indicating that the concentration of the Cu species increased in agreement with observations of
15, 23
Cu species in Cuꢀzeolites
. The loss of the whiteline intensity can be ascribed to the
2
4
dehydration of the Cu(II) hydroxide species as previously demonstrated with the TPDꢀH O
2
experiment (Fig. 7). The XANES recorded of the CuꢀSiO ꢀ30 sample did not show any
2
appreciable change in the intensities of neither the whiteline nor the shoulder around 8986 eV
°
with increasing temperature from 30 to 800 C indicating that all Cu was in the form of Cu(II).
The FT EXAFS spectra recorded for the CuꢀSiO ꢀ5 and CuꢀSiO ꢀ30 (Fig. 10c&d) during
2
2
progressive heating in O flow show a first coordination shell centered around 1.9 Å (spectra
2
were corrected for phase shifts) and a second coordination shell centered around 2.8 Å. For the
CuꢀSiO ꢀ5, the increase in the temperature resulted in the change of the shape and the intensity
2
of the feature around 1.9 Å most probably due to the loss of coordinated oxygen atoms most
likely caused by dehydration. FTꢀEXAFS spectra taken of the CuꢀSiO ꢀ30 show no significant
2
change with increasing temperature similar to the behavior observed during the measurement of
XANES.
°
The EXAFS spectra recorded during the O activation at 800 C at 5, 60 and 120 min for Cuꢀ
2
2
SiO ꢀ5 and CuꢀSiO ꢀ30 are presented in Fig. 11. No apparent changes were observed in the k
2
2
weighted oscillatory functions for both catalysts with changes in O activation time. Given the
2
fact that EXAFS is a bulk technique and considering the data in Fig. 3a which shows that the O2
activation time is an important parameter for increased methanol yield, indicate that the active
sites might consist of only a small fraction of the Cu in CuꢀSiO2.
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Figure 10. Cu K edge XANES spectra of CuꢀSiO samples taken during the O activation period
2
2
(a) CuꢀSiO ꢀ5, (b) CuꢀSiO ꢀ30. Magnitude of the FT EXAFS spectra and the imaginary part of
2 2
(
c) CuꢀSiO ꢀ5 (d) CuꢀSiO ꢀ30.
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Figure 11. EXAFS spectra of (a) CuꢀSiO ꢀ5, (b) CuꢀSiO ꢀ30 taken during the O activation
2
2
2
°
period at 800 C.
An important number of articles have appeared for the stepꢀwise direct conversion of methane
4
to methanol since the report of Groothaert et al. with most of them focusing on the reactivity of
4
transition metal impregnated zeolites. However, Groothaert et al. had also reported the activity
of CuꢀSiO with 0.6 wt. % Al O and 2 wt. % Cu with a yield of 1 µmol methanol/gcatalyst. After
2
2
3
°
activation at 450 C in O a value slightly higher than our current results (Fig. 2) at the same O
2
2
activation temperature and Cu wt. % was obtained. This is most probably due to the presence Al
4
in the SiO used which might have induced the formation of more active sites. The formation of
2
the active Cuꢀoxo species in zeolites requires the presence of framework Al sites. Addition of Al
2+
sites to SiO can create vacancies for the formation of highly dispersed Cu species. Reactive
2
copperꢀoxo species may form on pure silica based supports. For example, Patel et al. suggested
that the reduction of NO by CO on copperꢀoxide supported on mesoporous silica is mostly
59
60
affected by the presence of dispersed copper species based on H ꢀTPR data . An et al.
2
°
showed that after calcination at 650 C, Cu grafted SBAꢀ15 had a mixture of copperꢀoxo species
in the form of dispersed species and clusters, where the increased concentration of dispersed
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species correlates with the formaldehyde selectivity. Higher temperature treatment of CuꢀSiO2
during O activation might be inducing the formation of dispersed copperꢀoxo species which are
2
able to activate CH and results in a significantly increased methanol yield. Methanol yields
4
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obtained after high temperature O activation on CuꢀSiO are higher or similar to those of Cuꢀ
2
2
5
4, 32
MOR and CuZSMꢀ5
with similar Cu metal weight percentages. This shows that the surface
concentration of the active sites is similar. On the other hand, high temperature treatment of the
CuꢀSiO might also have caused changes in the microstructure of the silica support (as indicated
2
by XRD in Fig. 5) as it is very well known that transformations from amorphous silica or from
6
1ꢀ64
tridymite to cristobalite phase may occur at high temperature in the presence of a catalyst
The presence of cristobalite phases in the MnꢀNaꢀW/SiO catalysts was advantageous both for
.
2
methane conversion and the selectivity for the C species in the oxidative coupling of methane as
2
6
3ꢀ64
compared to amorphous silica support
. A similar process may also occur in CuꢀSiO catalyst
2
where the crystalline domains could potentially accommodate Cuꢀoxo species. Using different
1
5, 25, 29
45
synthesis routes
or conducting the CH reaction at high pressure resulted in much
4
higher methanol yields in Cuꢀzeolites. This might also be expected for CuꢀSiO if different
2
preparation routes and different reactions conditions such as high pressure CH treatment are
4
applied.
4
. Conclusion
CuꢀSiO is an effective catalyst for the direct conversion of methane to methanol in a stepꢀ
2
wise manner. The O activation temperature and methane partial pressure affected the methanol
2
°
yield significantly. When the activation was carried out at 800 C, yields similar to that of Cuꢀ
5
4
°
MOR and CuꢀZSM5 activated at around 400 C were obtained. The methanol yield was also
affected by the metal content. The yield per mole of copper exponentially increased as the weight
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percentage of copper decreased from 30 % to 1 %. Increasing pCH4 from 0.05 to 1 atm resulted in
°
a 5ꢀfold increase in yield after activation at 450 C, however it resulted in at least 20% lower
°
yields after activation at 800 C showing that different active sites were formed at different
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conditions with the possibility of having different reaction orders towards CH activation. UVꢀ
4
ꢀ1
Vis suggested that the copperꢀoxo site associated with the 22700 cm CT band could be one of
the active sites. For the CuꢀSiO ꢀ2, XRD indicated the formation of CuO nanoparticles after
2
°
°
activation at 450 C which were absent at 650 C and appeared again with a much lower
°
concentration at 800 C. MS signals during the O activation together with the XANES data
2
showed that the formation of the active sites requires the dehydration of the sample. The data
presented here showed that formation of active copper species that were previously thought to be
formed only on crystalline zeolite supports can be formed on a support material such as SiO if
2
proper conditions are met. Therefore, many other supports might be of use for supporting active
Cu species for methane to methanol conversion.
AUTHOR INFORMATION
Corresponding Author
*Corresponding author; eꢀmail: Carl.Mesters@shell.com; jeroen.vanbokhoven@chem.ethz.ch;
Tel.: + 41 44 632 5542
Present Addresses
†
: Koç University, Chemical and Biological Engineering Department, Sarıyer, 34450, Istanbul,
Turkey.
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Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval
to the final version of the manuscript.
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ACKNOWLEDGMENT
SEB acknowledges the financial support of Turkish Scientific and Technological Research
Council (TÜBĐTAK) for the international postdoctoral scholarship (2219). Electron microscopy
work was performed at ScopeM (ETH Zurich).
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