In situ studies of structural changes in DME synthesis catalyst with synchrotron powder diffraction

Article abstract of DOI:10.1016/j.apcata.2014.08.027

Structural changes in a bi-functional dimethyl ether synthesis catalyst (CuO-ZnO-Al₂O₃-MgO + γ-Al₂O₃, BFC), a methanol synthesis catalyst (CuO-ZnO-Al₂O₃-MgO, MSC) and a methanol dehydration catalyst (γ-Al₂O₃, MDC), were studied using X-ray synchrotron powder diffraction. The catalysts were first reduced in 10% H₂/He and then treated in a gas containing CO, H₂ and CO₂. Measurements were taken at temperatures between 50 °C and 500 °C. These measurements were complemented by ex-situ techniques-thermogravimetry (TG) and scanning electron microscopy (SEM)/energy-dispersive X-ray spectroscopy (EDS). The X-ray diffraction (XRD) results showed that the copper oxide phase, present in methanol synthesis and bi-functional catalysts, reduced to form Cu⁰ after reduction. No further chemical changes were observed for these catalysts. γ-Al₂O₃ was resistant to structural and chemical changes. The copper crystallite sizes of the methanol and bi-functional catalysts were found to increase with temperature. The extent of deactivation was higher for CuO-ZnO-Al₂O₃-MgO + γ-Al₂O₃ compared to CuO-ZnO-Al₂O₃-MgO.

Full text of DOI:10.1016/j.apcata.2014.08.027

Applied Catalysis A: General 486 (2014) 49–54
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
In situ studies of structural changes in DME synthesis catalyst with
synchrotron powder diffraction
Kazi Bayzid Kabir , Helen E. Maynard-Caselyb,1, Sankar Bhattacharyaa,∗
a
a
Department of Chemical Engineering, Monash University, Clayton, Victoria -3800, Australia
Australian Synchrotron, 800 Blackburn Road, Clayton, Victoria -3168, Australia
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 June 2014
Received in revised form 14 August 2014
Accepted 22 August 2014
Available online 29 August 2014
Structural changes in a bi-functional dimethyl ether synthesis catalyst (CuO-ZnO-Al O -MgO + ␥-Al O ,
2
3
2
3
BFC), a methanol synthesis catalyst (CuO-ZnO-Al2O3-MgO, MSC) and a methanol dehydration cata-
lyst (␥-Al2O3, MDC), were studied using X-ray synchrotron powder diffraction. The catalysts were
first reduced in 10% H2/He and then treated in a gas containing CO, H2 and CO2. Measurements were
◦
◦
taken at temperatures between 50 C and 500 C. These measurements were complemented by ex-situ
techniques—thermogravimetry (TG) and scanning electron microscopy (SEM)/energy-dispersive X-ray
spectroscopy (EDS). The X-ray diffraction (XRD) results showed that the copper oxide phase, present in
methanol synthesis and bi-functional catalysts, reduced to form Cu0 after reduction. No further chemical
changes were observed for these catalysts. ␥-Al2O3 was resistant to structural and chemical changes.
The copper crystallite sizes of the methanol and bi-functional catalysts were found to increase with
temperature. The extent of deactivation was higher for CuO-ZnO-Al2O3-MgO + ␥-Al2O3 compared to
CuO-ZnO-Al2O3-MgO.
Keywords:
Dimethyl ether
Bi-functional catalyst
Synchrotron radiation
X-ray diffraction
Structural changes
©
2014 Elsevier B.V. All rights reserved.
1
. Introduction
catalyst [10]. A combination of these two catalysts, usually called
a bi-functional catalyst [11], is a relatively new concept and merits
extensive investigation for optimal integration of the two conven-
tional steps.
Single-step conversion of syngas to dimethyl ether (DME) has
received significant attention in the synthetic fuel research area
since 1991 [1]. Conventional two-step synthesis involves produc-
tion of methanol from syngas, purification of methanol and then
dehydration of methanol in a separate reactor to produce DME.
The single-step process eliminates the intermediate step, by incor-
porating both the reactions in the same reactor. This process also
has higher yield than the conventional process, since it removes
the thermodynamic limitation of CO-hydrogenation [2] by in situ
dehydration of methanol to DME. A hydrogenation catalyst (usu-
ally copper based [2]) and a dehydration catalyst (usually ␥-Al O
The properties of the catalyst under reaction conditions can be
different from ambient conditions. Therefore, ex situ characteri-
sation of catalysts does not always provide useful data on how
the catalyst would perform in realistic reaction conditions. The
1
10
high-flux of synchrotron based radiation sources (10 to 10 times
brighter than conventional laboratory X-ray sources [12]) enables
the study of solid catalysts in situ within high-pressure and high-
temperature environment [13].
Advantageous features of synchrotron radiation (SR), such as
beam stability and continuous tunability, have resulted in immense
progress in the application of SR based techniques for catalyst char-
acterisation over the past 20 years [12]. Combination of different
techniques also enables acquisition of complementary data from
the same set of experiments. Conversion of aurichalchite precur-
sor to copper catalyst (for reverse water gas shift) was studied by
recording diffraction and absorption sequentially with a very short
time delay [14]. Time-resolved XRD was combined with Raman
and mass spectroscopy to investigate the behaviour of SAPO-34
under methanol-to-olefin (MTO) condition [15]. XRD identified the
chemical and structural changes in the catalyst. The nature of coke
formed during the MTO condition was observed in the Raman
2
3
[
3]) placed in a single reactor are responsible for one-step con-
version of syngas to DME. Copper-based catalysts have various
industrial applications (i.e. methanol synthesis, water-gas shift,
methanol reforming) and hence extensively studied over the years
[
4–9]. Meanwhile, a wide range of solid acids (e.g. alumina, silica-
alumina, zeolites) have also been studied as methanol dehydration
∗ Corresponding author.
E-mail address: sankar.bhattacharya@monash.edu (S. Bhattacharya).
Current address: Bragg Institute, Australian Nuclear Science and Technology
1
Organisation, Kirrawee DC, NSW-2232, Australia.
http://dx.doi.org/10.1016/j.apcata.2014.08.027
0
926-860X/© 2014 Elsevier B.V. All rights reserved.

5
0
K.B. Kabir et al. / Applied Catalysis A: General 486 (2014) 49–54
Fig. 1. (a) Flow cell with sample inside the quartz capillary, (b) flow cell-hot air blower configuration.
Table 1
6
5
4
3
00
00
00
00
Bulk composition of the MSC and MDC catalysts (wt%).
Temperature
Diffractrograms taken
12
CuO
ZnO
Al2O3
MgO
C
1
1
CuO-ZnO-Al2O3-MgO
-Al2O3
63.6
–
24.8
–
10.3
96.5
1.30
–
–
3.50
␥
1
0
9
spectra while mass spectra provided information regarding the
gaseous product [15]. XRD study of hydrothermally synthesised
nickel succinate was also studied for dehydration–rehydration
8
3
4
5
6
7
↑
Gas Changed from H −He to CO−H −CO
◦
◦
2
2
2
(
40–200 C) and decomposition (up to 600 C) [16].
200
Changes in phase structures of binary (Cu, Zn) and ternary (Cu,
Zn, Al) catalysts were studied in situ using a reactor-X-ray powder
2
1
00
0
diffraction (XRD) cell during activation and synthesis conditions
1
0
[
17]. The CuO phase present in the catalyst reduced to Cu . The
activity also steadily decreased with on-stream time. This investi-
gation was further developed using a combination of XRD and X-ray
absorption fine structure spectroscopy (XAFS) [18]. Alloying of Cu
0
50
100
150
200
250
300
350
400
450
Time, min
◦
and Zn was observed when the catalysts were heated to 300 C or
Fig. 2. Temperature profile for XRD measurements (measurement times are indi-
above.
cated by x).
The focus of the current study is on dimethyl ether synthesis
catalyst. Study on the effect of the addition of the dehydration com-
ponent to the methanol synthesis catalyst using in situ techniques
has not been reported in the literature. Therefore, in this study
dynamic changes in the structure of the bi-functional catalyst for
DME synthesis during activation and synthesis reaction were inves-
tigated using in situ synchrotron XRD. Studies only on the methanol
synthesis catalyst and dehydration catalyst were also performed
to examine the effect of the dehydration component on the per-
formance of methanol synthesis catalyst. Complementary analysis
was undertaken using thermogravimetry (TG) and scanning elec-
tron microscopy (SEM) with energy dispersive X-ray spectroscopy
capillary was then inserted into the flow cell, as shown in Fig. 1 (a).
The flow cell was then installed on to the beamline end station, as
shown in Fig. 1(b). The beam size at the sample was 1 mm (horizon-
tal) × 1.2 mm (vertical). The beam was focused at the centre of the
quartz capillary. The catalyst was reduced by flowing 10% H in He
2
◦
gas through the flow cell from room temperature to 250 C using
a gas manifold. The sample holder and the flow cell were allowed
to oscillate by 10 degrees to ensure maximum interaction between
the bulk sample and the X-ray source. A Cyberstar hot air blower
was used to heat up the sample inside the capillary at a heating
◦
rate of 10 C/min. Diffraction patterns from the capillary were col-
(
EDS).
lected with the MYTHEN-II strip detector [21] at a wavelength of
0
.7743 A˚ (approximately 16,000 keV). The wavelength was deter-
2
. Materials and methods
mined from the refinement of a NIST LaB -660b standard. Before
6
collecting the diffraction patterns the catalyst was held at a given
2.1. Materials
◦
temperature for 20 min. At 250 C the gas was changed from H -
2
He mixture to CO-H -CO2 mixture. Fig. 2 shows the temperature
profile and times for the 12 diffraction spectra collected during
the course of the experiment. Each diffractogram was collected for
2
A commercial methanol synthesis catalyst (CuO-ZnO-Al O -
2
3
MgO) and a commercial methanol dehydration catalyst (␥-Al O ),
2
3
were used in the current study. The samples will be called MSC
and MDC from here onwards. Bulk compositions of the catalysts
are shown in Table 1. The bi-functional catalyst for syngas to DME
synthesis was prepared by physically mixing the commercial cata-
lysts. The ratio of the MSC to MDC in the bi-functional catalyst was
1
0 min. No further analysis of the exhaust gas from the flow cell
was performed.
Structural parameters of the crystalline copper were obtained
after Reitveld refinement of the XRD data using TOPAS [22]. Copper
crystallite size in (1 1 1) direction was calculated using the Scherrer
equation [23]:
3
–1 (mass basis). Two gas mixtures (10% H2 in He; CO 40%, H2 40%
and CO2 20%) were also used during the XRD measurements.
Kꢀ
L =
2.2. Methods
B cos ꢁ
The catalysts were studied by the powder diffraction beam
line end station (10-BM-1) at the Australian Synchrotron [19,20].
In a typical experiment, ca. 50 mg of catalyst was loaded into
where B is the full width at half maximum (FWHM) of the peak
profile, L is the volume average of crystal thickness in direction
normal to the reflection plane, K is the constant of proportionality,
ꢁ is the angle of the reflection and ꢀ is the wavelength.
0
.7 mm OD (0.5 mm ID) quartz capillary. The catalyst loaded quartz

K.B. Kabir et al. / Applied Catalysis A: General 486 (2014) 49–54
51
Fig. 3. Diffraction pattern for bi-functional catalyst. (a) 3D plot (only copper oxide and copper phases are shown; ꢂ—CuO; O—Cu). (b) 2D projection (phase reflections are
shown as tick marks at the bottom of the figure).
Fig. 4. Diffraction pattern for MDC. (a) 3D plot (ꢂ—␥-Al2O3; O—graphite). (b) 2D projection (phase reflections are shown as tick marks at the bottom of the figure).
The surface morphology and point compositions were studied
using JEOL 7001F scanning electron microscope (SEM) equipped
with energy dispersive X-ray spectrometer (EDS). Thermogravi-
peaks at lower 2ꢁ values. In the diffraction patterns, only the peaks
corresponding to Cu0 and CuO are highlighted.
◦
When the sample was heated to 100 C, no structural changes
◦
metric analysis of the catalyst was performed using a NETZCH STA
were observed. As the temperature was increased to 250 C in flow-
®
0
4
49 F3 Jupiter thermobalance. The analysis conditions were sim-
ing H /He, the CuO was reduced to Cu . The reduction of copper
2
ilar to the XRD measurements, as shown in Fig. 2.
A fixed-bed reactor (Parr 5403 continuous flow tubular reactor)
was used for the conversion studies. The reactor was designed for
pressures up to 34 MPa and temperatures up to 550 C. The reactor
has a brass liner to avoid methanation reaction. For determining
catalyst activities, the reactor was loaded with catalyst particles
oxide to copper is believed to be a single step process according to
the following reaction:
◦
CuO + H2 → Cu + H2O
At the end of the reduction, CuO or Cu2O phases were not
detected, and only metallic phase of copper was observed.
between 425 and 850 ␮m (20–40 mesh) size. The catalyst was
◦
◦
reduced at 190 C (5% H in nitrogen). The reactor temperature was
At 250 C, the gas was switched from H2-He to CO-H2-CO2 mix-
2
then raised to the reaction temperature. Synthesis reaction was car-
ture for the rest of the experiment. No new phases evolved during
the course of the reaction. However, the copper peak intensities
gradually increased with longer exposure of the catalyst to the syn-
−1
−1
ried out at a space velocity of 2000 mL gcat
h . Online analysis
of the outlet gas was carried out using Agilent 7890A (G3440A) GC
equipped with four columns (HayeSepN, MolSieve 5A, Porapak Q
and Sulfinert) and three detectors (TCD, FID and FPD).
◦
thesis gas at 250 C or higher temperatures. The crystalline graphite
◦
phase showed directional growth on plane (1 0 0) at 2ꢁ = 21.4 and
◦
plane (2 2 1) at 2ꢁ = 30.8 . These peaks indicate the coke forma-
tion by condensation of the hydrocarbons on the catalyst surface at
higher temperatures.
3
. Results and discussion
The XRD patterns for the MSC catalyst showed similar phase
changes as the bi-functional catalyst at different temperatures.
Hence, the spectrogram is not shown here.
The bulk structure of methanol synthesis catalyst under reduc-
tion and DME synthesis conditions was studied by in situ
synchrotron XRD. Bi-functional catalyst for DME synthesis was pre-
pared by physically mixing three parts MSC and one part MDC by
weight. The XRD patterns for bi-functional catalyst are shown in
Fig. 3. The first diffractogram for the fresh catalyst indicates pres-
ence of CuO, ZnO, and ␥-alumina. The catalyst also showed the
presence of a small amount of malachite as indicated by the small
Diffraction patterns for MDC are shown in Fig. 4. The catalyst
is composed of amorphous ␥-Al O3 with small amount of car-
2
bon (Rhombohedral graphite). The XRD pattern did not show any
change in the crystalline phase of the ␥-Al O3 during the experi-
2
ment. However, directional growth of the graphite phase, similar
to that of BFC, was observed. In fact, CO hydrogenation can occur
on ␥-Al O3 surface, in absence of MSC-1. The CO conversion is
2

5
2
K.B. Kabir et al. / Applied Catalysis A: General 486 (2014) 49–54
.65
3
MSC
Bi−functional catalyst
3
3
3
.645
3
.64
.635
3
.63
.625
3
.62
2
50
300
350
400
450
500
Fig. 5. Thermogravimetric analysis of the bi-functional catalyst.
o
Temperature, C
Table 2
Summary of Rietveld refinement.
Fig. 6. Lattice parameter of Cu for MSC and bi-functional catalysts.
Temperature
Methanolsynthesiscatalyst
Bi-functional catalyst
16
MSC
G
Rexp
G
Rexp
1
5
Bi−functional catalyst
250
300
350
400
450
500
2.585
2.491
2.496
2.657
2.839
3.061
4.009
4.076
4.166
4.261
4.29
2.576
2.586
2.584
2.617
2.714
2.75
4.236
4.26
4.285
4.272
4.227
4.191
14
13
12
4.28
1
1
1
0
9
8
usually low and the products are predominantly hydrocarbons [24].
Coke formation on the surface occurs by condensation of these
hydrocarbons on the surface [25].
The XRD patterns for all three samples also showed the pres-
ence of structures below the XRD cutoff, which is usually less than
2
–3 nm [26]. The presence of nano-crystallites (particles below XRD
7
2
50
300
350
400
450
500
cutoff) is indicated by the first peak in each XRD, at approximately
o
2
ꢁ of 2, indicated by the red pattern in Figs. 3(a) and 4(a).
Temperature, C
A complementary thermogravimetric analysis was performed
on the bi-functional catalyst under the same conditions as the
in situ XRD studies. The results are plotted in Fig. 5. The catalyst
Fig. 7. Variation of copper crystallite size in [111] direction with temperature.
was heated from room temperature in a 5% H in He environment.
Change in copper crystallite size as a function of temperature
is shown in Fig. 7. For both MSC and bi-functional catalysts, size
of the copper crystal increased with temperature and on-stream
time. Variation in the crystal size is insignificant between 250 and
2
◦
◦
The sample was heated to 100 C and held at 100 C for 30 min.
At this temperature, the sample lost approximately 2% mass due
to moisture loss. After this holding period, the sample was heated
◦
◦
again to 250 C. The caralyst exhibited a sudden drop in mass from
300 C. However, rapid growth in crystal size was observed above
◦
◦
1
80 C due to the reduction of the copper oxide phase to Cu by H2
300 C.
present in the reducing gas. The reduction was completed before
Activity of the catalyst is dependent on the crystallite size,
in this instance copper. The activity of the catalyst is inversely
related to the copper crystallite size. The change in the metal-
lic Cu crystallite size as a function of temperature is shown in
Fig. 7. The results show that the crystallite size of both MSC and
the bi-functional catalyst on freshly reduced catalyst is 7.2 nm
◦
2
50 C beyond which there was no significant change in the mass
of the sample. The TGA results complements well with XRD results,
where diffraction patterns show the formation of metallic Cu peaks
◦
at 250 C and disappearance of CuO peaks.
The XRD measurements were not continuous and only taken at
temperatures shown in Fig. 2, start of the reduction could not be
approximated from the XRD measurements. However, the TG curve
provides both start and end temperatures for the reduction.
Total weight loss observed during the reduction step, according
to the TG curve, corresponds to the loss of oxygen atom from copper
oxide. No further changes in the weight were observed for the rest
of the experiment.
◦
and 8.5 nm, respectively, at 250 C. The introduction of syngas at
◦
◦
250 C and heating up to 300 C did not show a significant effect
on the Cu-crystallite size, suggesting the stability of the catalysts
in this temperature range. However, when the reaction tempera-
◦
ture increased beyond 300 C, the crystallite size of Cu increased
rapidly and reached 11.1 nm and 14.2 nm at 500 C for MSC-1 and
◦
bi-functional catalysts, respectively. The results suggest agglomer-
ation of the crystallites or thermal sintering at higher temperatures.
Lattice parameters of Cu were determined using Rietveld refine-
ment [27]. The weighted profile R-factor (Rexp) and goodness of fit
Thermal sintering occurs above the Tammann temperature of
◦
(
G) for the refinement is shown in Table 2. Changes in the lattice
copper (≈0.5T
= 409.5 C). Sintering by release of atomic or
melting
parameter for MSC and bi-functional catalyst are shown in Fig. 6.
The lattice parameters increased with temperature in line with the
expected thermal expansion.
molecular species from crystallites can occur above the Hütting
◦
temperature of copper (≈0.33T
= 182 C) [28]. Presence of
melting
moisture or high CO2 in the gas phase also accelerates sintering

K.B. Kabir et al. / Applied Catalysis A: General 486 (2014) 49–54
53
Table 3
Point composition of the fresh and used catalysts.
Elements
Fresh MSC
Spectrum1
Used MSC
Spectrum2
Spectrum3
Spectrum4
Spectrum1
Spectrum2
Spectrum3
Spectrum4
Cu
Zn
Al
Mg
O
47.4
24.5
1.33
0.99
25.8
47.3
24.2
1.51
1.01
26.1
48.8
25.0
1.11
–
48.3
23.8
1.42
0.99
25.5
36.1
24.6
7.84
1.08
30.4
36.8
24.7
7.61
1.06
29.8
38.8
24.2
6.39
1.00
29.6
33.5
26.2
8.61
1.37
30.4
25.1
Fig. 8. Secondary electron images of the fresh (left) and used (right) catalyst.
as these gases hinder stabilisation of Cu/ZnO by the alumina phase.
The crystallite sizes calculated from the XRD data clearly indicate
that crystallite sizes are larger for the bi-functional catalyst. When
MDC is used in conjunction with MSC for syngas to DME synthe-
other hydrocarbons (methanation and Fisher-Tropsch reactions)
and CO2 (through water gas shift reaction). The selectivity of the
products depends on the temperature, as indicated by Fig. 9 and
◦
Table 4. At lower temperature (190 C), the CO conversion was low
sis, the dehydration and water gas shift reaction produces H O and
(2.61%) and the oxygenate (DME + methanol) selectivity was ∼99%,
2
CO , respectively. Therefore, the extent of deactivation was greater
as the rates of methanation and water gas shift reactions were very
2
◦
◦
for the bi-functional catalyst than that of the MSC.
low. At 240 C and 300 C, increase in the oxygenate yield can be
attributed to the increase in the rates of methanol synthesis and
methanol dehydration reactions. At these temperatures, copper-
based catalyst also acts as a good water gas shift catalyst. Therefore,
a significant increase in CO2 yield was observed which also resulted
Changes in the surface morphology for MSC are shown in Fig. 8.
The catalyst before reduction showed surface morphology without
any significant features. The surface was found to be nonporous.
In contrast, cracks developed on the surface of the catalyst during
thermal treatment in reducing environments. The point compo-
sition measurements using EDS technique for the fresh and used
catalyst also showed significant changes for Cu and Al. The surface
of the used catalyst is found to have more Al than the fresh catalyst,
while the fresh catalyst had more copper than the used one. Zinc
and magnesium concentration on the surface found to be similar
for both fresh and used catalyst. The variation in surface composi-
tion is shown in Table 3. The final temperature was high enough to
make the atoms or ions present in the bulk phase migrate to the
surface of the catalyst. Mobility of the bulk phase therefore can be
attributed to the change in the surface composition.
◦
in a decrease in the oxygenate selectivity. At 340 C, the oxygenate
selectivity went further down to ∼35%. This can be attributed to
a combination of factors: decrease in the catalyst activity (from
rapid growth in Cu crystallite size, Fig. 7), decomposition of DME
into hydrocarbons in the acidic alumina sites and coke formation
on the catalyst surface (observed during the X-ray diffraction mea-
surements, Figs. 3 and 4).
The DME yield and selectivity of the bi-functional catalyst at dif-
ferent temperatures are presented in Fig. 9. Effect of temperature
on the selectivity of other product is shown in Table 4. Conver-
sion of CO in presence of this catalyst can lead to the formation
of methanol and DME (via hydrogenation reaction), methane and
Table 4
Product selectivity at various temperatures.
◦
Products
Reaction temperature ( C)
190
240
290
340
DME + methanol
CO2
CH4 and other hydrocarbons
99.02
0.46
0.52
64.01
35.23
0.76
61.77
35.87
2.36
34.51
45.16
20.33
Fig. 9. DME yield and selectivity for bi-functional catalyst at different temperatures.
Feed: 50% CO + 50% H2; Pressure: 5 MPa; space velocity: 2000 mL gcat
−
1
−1
h .

5
4
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. Conclusions
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changes were observed. However, the crystalline copper phase
grew at the expense of microstructural features of the catalyst.
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[
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the bi-functional catalyst than that of the MSC indicating acceler-
ated deactivation of the bi-functional catalyst. The deactivation can
be due to the formation of water during dehydration of methanol
which when adsorbed onto the alumina support aids in the growth
of copper crystallites. Surface composition of the catalysts also
changed during thermal treatment.
4
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The work is supported by the Brown Coal Innovation
Australia (BCIA) and the Australian Research Council (ARC) LIEF
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