Catalytic behaviour of a bifunctional system for the one step synthesis of DME by CO2 hydrogenation

Article abstract of DOI:10.1016/j.cattod.2013.11.017

One-step CO₂ hydrogenation reaction to dimethyl ether (DME) on a bifunctional system constituted by Cu-ZnO-ZrO₂ (ZCZ) methanol synthesis catalyst and HZSM-5 zeolite was studied. The influence of the reactor configuration in terms of combination procedure of the admixed samples on activity, selectivity and DME yield has been evaluated. The results obtained at T_R = 513 K, P_R = 3.0 MPa and GHSV = 9000 NL g _cat ^-1 h^-1 show a superior DME productivity (4.4 mmol kg_ZCZ ^-1 s^-1) using the bifunctional catalyst prepared by physical mixing. A combined effect of active sites located at metal/oxide(s)-acid interface was claimed as the main factor affecting the CO₂ conversion and DME productivity. The strong surface acidity of ZCZ catalyst is not suitable to drive DME synthesis through a consecutive mechanism, while DME formation effectively proceeds on weak and medium acid sites of HZSM-5.

Full text of DOI:10.1016/j.cattod.2013.11.017

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Catalytic behaviour of a bifunctional system for the one step synthesis
of DME by CO2 hydrogenation
G. Bonuraa, M. Cordarob, C. Cannillaa, A. Mezzapicaa, L. Spadaroa, F. Arenab, F. Frusteria,∗
a CNR-ITAE, Istituto di Tecnologie Avanzate per l’Energia “Nicola Giordano”, Via S. Lucia sopra Contesse, 5, 98126 Messina, Italy
b Dip. di Ingegneria Elettronica, Chimica e Ingegneria Industriale, Università di Messina, V.le F. Stagno D’Alcontres, 31, 98166 Messina, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 10 July 2013
Received in revised form 24 October 2013
Accepted 11 November 2013
Available online xxx
One-step CO2 hydrogenation reaction to dimethyl ether (DME) on a bifunctional system constituted
by Cu–ZnO–ZrO2 (ZCZ) methanol synthesis catalyst and HZSM-5 zeolite was studied. The influence
of the reactor configuration in terms of combination procedure of the admixed samples on activ-
ity, selectivity and DME yield has been evaluated. The results obtained at TR = 513 K, PR = 3.0 MPa and
GHSV = 9000 NL gcat−1 h−1 show a superior DME productivity (4.4 mmol kgZCZ−1 s−1) using the bifunctional
catalyst prepared by physical mixing. A combined effect of active sites located at metal/oxide(s)–acid
interface was claimed as the main factor affecting the CO2 conversion and DME productivity. The strong
surface acidity of ZCZ catalyst is not suitable to drive DME synthesis through a consecutive mechanism,
while DME formation effectively proceeds on weak and medium acid sites of HZSM-5.
Keywords:
CO2 hydrogenation
DME synthesis
Cu–ZnO–ZrO2 catalysts
HZSM-5
© 2013 Elsevier B.V. All rights reserved.
Bifunctional catalysts
1. Introduction
Fundamentally, there are two routes for the production of DME:
(i) a two-steps process, in which methanol is first produced on
The direct conversion of carbon dioxide into useful chemicals,
like methanol (MeOH) and/or dimethyl ether (DME), is an attractive
way of recycling CO2 and thereby controlling its emission into the
atmosphere [1–5]. In particular, DME is considered a prospective
“future fuel” as a diesel substitute, due to its high cetane number
[5,6]. In comparison to other synthetic fuels, DME has a very high
thermal efficiency of combustion, equivalent to that of traditional
fuels. It also does not emit sulphur oxides or soot and there is a
considerable reduction of nitrous oxides in the exhaust gases. DME
can be used as fuel for diesel engines as a cooking gas for household
applications and running turbines in a power generation facility.
Furthermore, DME can also be used as a starting material for the
production of a range of chemicals, such as oxygenates, olefins, and
hydrocarbon fuels (gasoline, jet fuel) or as an efficient H2 carrier for
fuel cells and other applications [5–7].
a metallic catalyst and then dehydrated to DME on an acid cata-
lyst located in a second reactor [30–32]; (ii) a single step process,
in which bifunctional catalysts constituted by metallic and acidic
functionalities are used in only one reactor to perform the reactions
simultaneously [30–32]. The main advantage of the single-step pro-
cess is the elimination of thermodynamic restriction of methanol
synthesis. Whichever is the approach, two consecutive steps or one
single step, three independent reactions are always involved in the
DME synthesis process from CO2/H2 mixtures, being other possible
reactions easily obtained by linear combination of the followings:
Methanol synthesis reaction (MS):
CO2 + 3H2 ꢀ CH3OH + H2O ꢀH◦ = −49.4 KJ mol−1
Methanol dehydration reaction (MD):
(2)
(3)
(4)
2CH3OH ꢀ CH3OCH3 + H2O ꢀH◦ = −23.4 KJ mol−1
Reverse water gas shift reaction (RWGS):
Notwithstanding an extensive literature documents the feasibil-
ity of a process for the production of DME via syngas [8–19], only
few papers deal with the DME synthesis via CO2 hydrogenation
[5,20–29]:
CO2 + H2 ꢀ CO + H2O ꢀH◦ = +41.2 KJ mol−1
Catalysts for the direct CO2-to-DME process should be able to
efficiently catalyse both MS and MD reactions, while the yield of
CO, formed via the RWGS side reaction [27,33], should be kept to a
minimum.
CO2 + 6H2 ꢀ CH3OCH3 + 3H2O ꢀH298 = −122.2 KJ mol−1
(1)
Typically, MS reaction is carried out on Cu–ZnO–Al2O3 cata-
lysts, although Cu–ZnO–ZrO2 catalysts are recently claimed as more
active catalytic systems due to a better water tolerance of zirconia
∗
Corresponding author.
E-mail address: francesco.frusteri@itae.cnr.it (F. Frusteri).
0920-5861/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cattod.2013.11.017
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Table 1
an appropriate amount of HZSM-5 finely dispersed, under vig-
Physicochemical characteristics of ZCZ catalyst.
orous stirring in an ultrasound bath maintained at 310 K. The
hybrid catalyst was dried at 383 K for 16 h and further calcined
at 623 K for 4 h. Then, the sample was crushed and sieved and
the 40–70 mesh particle size fraction was used for characteri-
zation and testing measurements;
CuO (wt.%)
ZnO (wt.%)
ZrO2 (wt.%)
41.2
14.8
44.0
174
0.310
60.8
29.1
4
Surface area (m2 g−1
)
1
Pore volume (cm3 g−1
)
Copper specific areaa (m2 g−1
Copper dispersiona (%)
)
(2) ZZ-D, dual-bed of same-sized (40–70 mesh) ZCZ methanol cat-
alyst (first bed) and HZSM-5 catalyst (second bed);
Copper crystallite sizea (nm)
(3) ZZ-G, homogeneous solid mixture, prepared by grinding in an
agate mortar of powdered single catalysts and pelletization
(40–70 mesh) of the resulting bifunctional system; and
(4) ZZ-M, homogeneous physical mixture of same-sized (40–70
mesh) ZCZ and HZSM-5 catalysts.
a
From “single-pulse” (363 K) N2O titration measurements (see Ref. [37]).
2.2. Catalysts characterization
Surface area (SA) and pore volume (PV). Nitrogen adsorp-
tion/desorption isotherms for SA and PV calculations were obtained
by using a Micromeritics ASAP 2020 gas adsorption device. Before
analysis, all the samples were outgassed at 453 K under vacuum
(3 h).
X-ray diffraction (XRD) analyses of the samples in the 2ꢁ range
5–100◦ were performed using a Philips X-Pert diffractometer, oper-
´
˚
ating with Ni ˇ-filtered Cu K˛ radiation (ꢂ = 1.5406 A) at 40 kV and
30 mA and a scan step of 0.05◦ s−1. Identification of XRD patterns
was made on the basis of the JCPDS database [41].
Fig. 1. Combination procedures between methanol and acid catalyst (᭹ ZCZ; ꢀ
Ammonia temperature programmed desorption (NH3-TPD). NH3-
TPD measurements in the range 373–973 K were performed in a
conventional flow apparatus by using ca. 100 mg sample in a linear
quartz micro-reactor (i.d., 4 mm; l., 200 mm), with a helium carrier
flow rate of 25 STP mL min−1. Prior to each measurement, the sam-
ple was pretreated at 573 K with hydrogen flowing at 100 mL min−1
for 1 h and then cooled down to 423 K and saturated at a flow
rate of 25 mL min−1 for 1.5 h with a 5% NH3/He stream. Then, the
samples were purged in He atmosphere for ca. 1 h until baseline
stabilization.
Transmission electron microscopy (TEM) analysis. A Philips CM12
instrument equipped with a high-resolution camera was used to
acquire and elaborate TEM images. Powdered samples were dis-
persed in 2-propanol under ultrasound irradiation and the resulting
suspension put drop-wise on a holey carbon-coated support grid.
HZSM-5).
in respect of alumina [34–37]. MD reaction, instead, fast takes place
over a solid acid component.
In the present work, a Cu–ZnO–ZrO2 catalyst and a mesoporous
HZSM-5 zeolite are used as methanol synthesis and dehydra-
tion component, respectively. Although mesoporous zeolites suffer
from a lack of size and shape selectivity, contrarily to microporous
zeolites, their use have been proved to exhibit better activity and
higher resistance to deactivation due to hierarchical structure,
which improves the mass transfer process during the reactions
[38–40]. Four different combination procedures between the two
components have been adopted, in order to maximize the contact
between CO2 activation, hydrogenation and methanol dehydration
sites. The influence of the preparation method in the one step syn-
thesis of DME by CO2 hydrogenation is discussed.
2.3. Catalyst testing
2. Experimental
Catalyst testing was performed in a fixed-bed stainless steel
reactor (i.d., 4 mm; l., 200 mm), loaded with 0.25 g of hybrid catalyst,
diluted with inert same-sized SiC (0.25 g). A CO2/H2/N2 reaction
mixture with a molar ratio of 23/69/8 was fed at a temperature
ranging from 443 to 563 K, pressure of 0.1–5.0 MPa and gas hourly
2.1. Catalyst preparation
The Cu–ZnO–ZrO2 (ZCZ) methanol synthesis catalyst, with a
Zn/Cu atomic ratio of 0.4, was prepared by “reverse co-precipitation
method under ultrasound irradiation”, according to the procedure
elsewhere reported [37]. The list of physicochemical properties of
such a catalyst is reported in Table 1.
space velocity (GHSV) between 4500 and 36,000 NL kgcat−1 h−1
.
Prior to each test, the catalyst was reduced in situ at 573 K for 1 h in
pure hydrogen flow at atmospheric pressure. The reaction stream
was analysed by a GC equipped with a two-column separation sys-
tem connected to a flame ionized detector and thermal conductivity
detector, respectively. CO2 conversion along with DME, MeOH and
CO selectivity were obtained from the GC data and calculated as
elsewhere reported [33].
To form the bifunctional system, the ZCZ catalyst was admixed
to a commercial mesoporous NH4-ZSM-5 zeolite (Zeolyst Int.
CBV3024E), characterized by a SiO2/Al2O3 molar ratio of 30.
The zeolite was converted in its protonic form by calcination
in air at 773 K (SABET, 317 m2 g−1; PVmeso, 0.200 cm3 g−1; PVmicro
,
0.115 cm3 g−1). Considering that previous results dealing with the
influence of zeolite loading on a hybrid catalyst shed light on an
optimal ratio of 1:1 (w/w) between methanol synthesis catalyst
and zeolite [33], four different combination procedures (see Fig. 1)
were adopted to get a bifunctional system, as follows:
3. Results and discussion
3.1. Textural properties
According to previous evidences on the optimum composition
of the methanol Cu–ZnO–ZrO2 system [37], it was seen that a
high content of hydrophobic zirconia carrier (≈44 wt.%) promotes
(1) ZZ-C, reverse co-precipitation of ZCZ metallic precursors under
ultrasound irradiation in a pH 8 buffered solution containing
Please cite this article in press as: G. Bonura, et al., Catalytic behaviour of a bifunctional system for the one step synthesis of DME by CO2
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Table 2
Textural properties of the investigated bifunctional catalysts.
´
SA (m2 g−1
)
PV (cm3 g−1
)
APDb (A)
˚
Sample
HZSM-5
ZZ-G
ZZ-C
251
223
245
0.266
0.173
0.313
42
31
51
ZZ-G
ZZ-Ma
a
*
Averaged values between ZCZ and H-ZSM5 samples.
Average pore diameter (APD, 4PV/SA).
*
b
structure and texture (see Table 1), enhancing the activity of the
Cu–ZnO based system in the presence of water [42]. Moreover,
synergistic effects of the ZrO2 carrier and ZnO promoter on Cu
exposure determine a significantly high dispersion (29%) and MSA
(60.8 m2 g−1) of the ZCZ system, as a consequence of high surface
area (174 m2 g−1) induced by the use of the reverse co-precipitation
procedure under ultrasound irradiation [36].
ZZ-C
ZCZ
5
10
15
20
25
30
35
40
45
Table 2 shows the physicochemical properties of the prepared
bifunctional systems. The co-precipitated ZZ-C sample exhibits
textural properties similar to ZZ-M, that is the system obtained
by physical mixing of ZCZ catalyst and HZMS-5 zeolite. In par-
ticular, ZZ-C presents SA of 251 m2 g−1, very close to SA of ZZ-M
(245 m2 g−1), the value of which was calculated as an average
Fig. 2. XRD patterns of methanol synthesis catalyst (ZCZ), methanol dehydration
catalyst (HZSM-5) and bifunctional catalysts (ZZ-C and ZZ-G).
randomly distributed over zeolite matrix, missing the typical clus-
ter structure of the ZCZ catalyst. As previously reported [33], this
is the result of a mechanical stress induced by grinding procedure,
which determines a detrimental interaction between the two func-
tionalities (ZCZ and HZSM-5). On the contrary, the co-precipitated
ZZ-C sample also shows highly dispersed metal-oxide(s) particles
on zeolite matrix, with a rather uniform size distribution. ZZ-M,
being a combination of the two mechanically mixed functionali-
ties, merely features the isolated structure of methanol ZCZ catalyst
and HZSM-5 zeolite, without any evidence of intimate chemical
interaction.
between ZCZ and HZSM-5 samples. Instead, lower SA (223 m2 g−1
)
value is recorded for the ZZ-G sample, as a consequence of the
grinding procedure of the admixed catalysts that causes a poros-
ity shrinkage (PV, 0.173 cm3 g−1), with a decreasing of APD to 31 A.
˚
Then, the decrease in SA and PV observed for such catalyst should
be mostly attributed to a pore blockage of the zeolite mesopores by
Cu–ZnO–ZrO2 particles [1].
3.2. Structural properties
Fig. 2 shows XRD patterns of methanol synthesis catalyst (ZCZ),
dehydration catalyst (HZSM-5) and bifunctional systems (ZZ-C
and ZZ-G). As it is possible to observe, ZCZ exhibits an amor-
phous pattern with a lack of “long range” crystalline order, which
accounts for the formation of nanosized oxide domains [36,37], in
which a high copper dispersion (29%) reflects copper crystallite
size of 4 nm (see Table 1). On the other hand, in the bifunctional
Cu–ZnO–ZrO2/HZSM-5 systems, the characteristic reflections of
HZSM-5 zeolite (JPCDS 44-0002) are always present, independently
of different catalyst preparation methods employed. Furthermore,
ZZ-C system shows an amorphous pattern in the 2ꢁ range of 25–40◦,
similar to that observed in the “bare” ZCZ, which signals high dis-
persion of methanol synthesis catalyst on the zeolite matrix. On
the contrary, ZZ-G system shows two little signals at 35.4◦ and
38.7◦, ascribable to diffraction planes −1 1 1 and 1 1 1 of the
crystalline tenorite CuO phase (JPCDS 5-661). By Scherrer’s equa-
tion applied to the −1 1 1 line, a mean CuO crystallite size of
6.2 nm was obtained, diagnostic of a significant increase of CuO
particle size upon the grinding procedure. Such findings are con-
firmed by TEM analysis of the studied samples, as shown in Fig. 3.
Indeed, in the ZZ-G catalyst, isolated metallic copper particles are
3.3. Surface properties
As known, although by NH3-TPD measurements is not possi-
ble to discriminate between Brønsted and Lewis acidity, ammonia
desorbed as a function of temperature can give a reliable measure
of acid strength. As reported in Table 3, ZCZ is characterized by a
low adsorption capacity (0.38 mmol NH3 gcat−1), totally associated
to ammonia desorbed at high temperature (Td, 823 K) from strong
acid sites. Instead, the acid distribution of the HZSM-5 zeolite is the
result of weak (27%) and medium acid sites (73%) spanned in the
range 506–685 K,−w1 ith a cumulative amount of desorbed ammonia
of 0.88 mmol gcat
.
All bifunctional systems show similar adsorption capacity
(1.01–1.20 mmol NH3 gcat−1), with a far higher number of acid sites
than the pure materials. This is likely caused by the water formed
during catalyst activation/reduction that induces dealumination
phenomena on zeolite, so increasing the total acidity of the systems
[43–45]. As also displayed in Fig. 4, both ZZ-M and ZZ-D samples
exhibit a low concentration of weak sites (14–19%) and an overall
population of medium and strong sites of 81% and 86%, respec-
tively. In spite of a similar surface acidity (1.06 mmol gcat−1), the
Table 3
Quantitative data of NH3-TPD patterns and acid sites distribution.a
−1
Sample
Amount desorbed (mmol gcat
)
Weak sites
Medium sites
Strong sites
Td1 (K)
x1
Td2 (K)
x2
Td3 (K)
x3
ZCZ
ZZ-C
ZZ-D
ZZ-G
ZZ-M
HZSM-5
0.38
0.37
1.20
1.06
1.01
0.88
–
–
506
498
505
506
–
–
0.14
0.34
0.19
0.27
–
636
676
695
684
685
–
823
826
823
857
824
–
1.00
0.82
0.26
0.39
0.32
–
0.18
0.60
0.27
0.49
0.73
a
Tdn, temperature of desorption maximum; xn, fractional population of sites.
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Fig. 3. TEM micrographs of the prepared bifunctional catalysts.
ZZ-G sample features a more balanced distribution of acid sites
in the range of temperature investigated, with a homogenization
of sites having different strength (0.27 < xn < 0.39) with respect to
the other systems. At last, with a fairly low amount of ammo-
nia desorbed (0.37 mmol NH3 gcat−1), the ZZ-C catalyst outlines the
predominance of strong acid sites (82%) and only a little popula-
tion of medium sites (18%), without any appearance of weaker acid
sites at low temperature. Evidently, the co-precipitation of ZCZ in an
aqueous medium in which HZSM-5 was dispersed caused a loss of
weak-medium acidity of zeolite, accounting for an almost complete
(82%) fraction of NH3 retained above 673 K.
3.4. Catalytic results
Initially, the effect of the preparation method of the
CuO–ZnO–ZrO2/HZSM-5 bifunctional systems on CO2 conver-
sion and MeOH/DME formation rates was studied. Fig. 5 shows
the results obtained at 513 K, 3.0 MPa and space velocity of
9000 NL kgcat−1 h−1. As a reference, the activity pattern of the
10
CO2 converted
MeOH produced
DME produced
9
1.0
0.9
0.8
0.7
0.6
0.5
8
7
6
5
4
3
2
1
ZCZ
0.4
ZZ-C
ZZ-G
0.3
ZZ-D
0.2
ZZ-M
HZSM-5
0.1
0.0
373
473 573
673
773
873
973
0
ZCZ ZZ-C ZZ-D ZZ-G ZZ-M
Temperature (K)
Fig. 4. Fraction of ammonia retained on catalyst surface as a function of evacuation
temperature.
Fig. 5. Influence of mixing methodology on the reaction rate (TR, 513 K; PR, 3.0 MPa;
GHSV, 9000 NL kgcat−1 h−1).
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5
60
90
75
60
45
30
15
0
50
40
30
20
10
0
100
80
60
40
20
0
XCO2
SDME
SMeOH
SCO
XCO2
SDME
SMeOH
SCO
SDME,eq
50
SDME,eq
40
30
20
10
0
X
CO2,eq
CO2,eq
X
443
463
483
503
523
543
563
0.1
1.0
3.0
5.0
Temperature (K)
Pressure (MPa)
Fig. 6. Influence of reaction temperature. Catalyst: ZZ-M (PR, 3.0 MPa; GHSV,
9000 NL kgcat−1 h−1). Dotted lines refer to equilibrium values of CO2 conversion (—)
and DME selectivity (···).
Fig. 7. Influence of reaction pressure. Catalyst: ZZ-M (TR, 493 K; GHSV,
9000 NL kgcat−1 h−1). Dotted lines refer to equilibrium values of CO2 conversion (—)
and DME selectivity (···).
ZCZ methanol catalyst is also reported. No hydrocarbons or coke
were found under the adopted experimental conditions. Notwith-
standing the presence of strong acid sites evidenced by NH3-TPD
measurements, the rate of DME production on ZCZ is close to zero.
On the whole, the ZZ-M catalyst presents the highest rate, both in
terms of CO2 conversion (8.4 mmol kgZCZ−1 s−1) and DME produc-
tion (4.4 mmol kgZCZ−1 s−1), in consequence of faster mass transfer
phenomena occurring between neighbouring metal/oxide(s)–acid
sites, so favouring the shifting of methanol equilibrium towards
DME with an increased CO2 conversion. Instead, the dual bed ZZ-
D system, in which methanol formed in the first step on ZCZ is
converted into DME on acid sites of zeolite in the second step,
attains only an enhancement of CO2 conversion rate over 20%
with respect to the reference system, without reaching the pro-
ductivity of the ZZ-M catalyst. Similarly to ZZ-D, an interesting CO2
conversion value was observed on the ZZ-C sample, prepared by
co-precipitation of metallic precursors on powdered zeolite. Nev-
ertheless, a quasi-zero DME production was achieved on such a
system, due to a surface acidity too strong, not suitable to effec-
tively drive the dehydration of methanol to DME [33]. Indeed, the
disappearance of weak-medium acid sites of zeolite is likely due
to the preparation method that, during the co-precipitation step,
causes an ionic exchange of acid sites of zeolite with metal precur-
sors of ZCZ catalyst. At last, although the attempt to realize a more
intimate contact between metal/oxide(s) and acid sites at interface,
the ZZ-G sample exhibits a lower CO2 conversion and DME pro-
duction (3.2 mmol kgZCZ−1 s−1), being ascribable to a mechanical
stress induced by grinding in mortar of the two admixed catalysts
(ZCZ and HZSM-5) that caused a detrimental interaction among the
active sites [33].
15
12
9
75
60
45
30
15
0
XCO2
SDME
SMeOH
SCO
6
3
0
4,500
9,000
18,000
36,000
GHSV (NL kgcat-1 h-1)
Fig. 8. Influence of space velocity. Catalyst: ZZ-M (TR, 493 K; PR, 3.0 MPa).
contribution of the reverse water gas shift (RWGS) reaction [33],
which is an endothermic process (see reaction (4)). Moreover,
MeOH selectivity maintains an almost constant level (12–17%)
along the investigated temperature range, as the result of the com-
petition between exothermic MS and MD reactions (reactions (2)
and (3)).
As regards the influence of reaction pressure on catalytic activ-
ity, experiments carried out at 493 K and 9000 NL kgcat−1 h−1 are
shown in Fig. 7. As it is possible to observe, the CO2 conversion
only slightly increases with pressure, from 3.1% (at PR, 0.1 MPa) to
Considering the interesting result obtained over the ZZ-M sam-
ple, the influence of experimental conditions on CO2 conversion
(XCO ) and product selectivity (Sx) was studied on such a system
in m2ore detail, in terms of reaction temperature, reaction pressure,
space velocity and catalyst particle size, as shown in Figs. 6–9.
Thermodynamically, being an exothermic reaction, it is eas-
ier to synthesize DME from CO2 at low reaction temperature (see
reaction (1)). So, the activity–selectivity pattern in a range of reac-
tion temperatures between 443 and 563 K was studied. At 3.0 MPa
and flow rate of 9000 NL kgcat−1 h−1 (Fig. 6), the CO2 conversion
increases from 1.9% to 23.0%, until reaching a plateau at higher tem-
perature (543–563 K), in correspondence of the thermodynamic
equilibrium. On the other hand, DME selectivity shows a decreasing
trend with reaction temperature, from a maximum of 86% at 443 K
up to a minimum value of 20% at 563 K. On the contrary, CO
selectivity rapidly increases with reaction temperature, attaining a
maximum value of 69% at 563 K. Indeed, such a significant CO pro-
duction at higher temperature is mainly ascribable to a prevailing
15
12
9
75
60
45
30
15
0
XCO2
SDME
SMeOH
SCO
6
3
0
40-70
70-100
16-25
Catalyst particle size (mesh)
Fig. 9. Influence of catalyst particle size. Catalyst: ZZ-M (TR, 493 K; PR, 3.0 MPa;
GHSV, 9000 NL kgcat−1 h−1).
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G. Bonura et al. / Catalysis Today xxx (2013) xxx–xxx
10.2% (at PR, 5.0 MPa), without any significant improvement from
3.0 to 5.0 MPa. So, it is confirmed that, being a molar decreasing
reaction (see reaction (1)), DME synthesis by CO2 hydrogenation is
favoured as the pressure increases. Moreover, at atmospheric pres-
sure CO represents the main product, with a selectivity of 92.5%
which rapidly decreases with pressure until to reach a value of
35.7% at 5.0 MPa. On the contrary, as also expected from ther-
modynamic predictions, the DME formation takes advantage of
pressure increasing and selectivity reaches the maximum value of
48.6% at 5.0 MPa. Nevertheless, the MeOH selectivity also increases
with pressure, although the values do not exceed 15.7% under the
investigated experimental conditions. Indeed, since both MD and
RWGS reactions are only slightly affected (if at all) by a rise in
pressure (according to the negligible volume variation of reactions
(3) and (4)), a lower dependence of CO2 conversion on high-
pressure conditions (PR > 3.0 MPa) likely reflects a more extensive
wetting of the catalyst surface, in consequence of the partial pres-
sure of water which, rising with conversion and pressure, causes
an incipient oxidation of Cu sites depressing the catalyst activity
[36,37].
300
250
200
150
100
50
513K
493K
473K
453K
0
Considering that the reaction under investigation is character-
ized by multiple (parallel and/or consecutive) reaction paths [33],
occurring on different surface active sites (metal/oxide(s)–acid)
at different reaction rates, experiments at different flow rates
have been performed with the aim to optimize the yield to DME
production. In this respect, the effect of space velocity on the
activity–selectivity pattern of the ZZ-M catalyst was investigated by
operating at TR, 493 K and PR, 3.0 MPa. As shown in Fig. 8, between
4500 and 36,000 NL kgcat−1 h−1, a regular decreasing in CO2 conver-
sion from 13.6% to 2.4% is observed, while, in terms of selectivity,
a progressive increase of DME production, from 42% to 59%, is
recorded. On the contrary, CO selectivity decreases from 42% to 24%,
while the MeOH selectivity shows only a slight variation (14–19%)
in the whole range of space velocity investigated. Since at low flow
rates the occurrence of external mass transfer constraints begins
to be evident, operation at a space velocity of 9000 NL kgcat−1 h−1
is resulted suitable to enhance the mass transfer phenomena [46].
Moreover, as the reaction takes place over a bifunctional system,
which also involves an intraphase mass transfer among different
sites, the effect of catalyst particle size on CO2 conversion and prod-
uct selectivity (see Fig. 9) has been also investigated. Ranging from
70–100 to 16–25 mesh, in all tests performed no difference was
observed either in terms of CO2 conversion or products distribution.
Based on these results, it is possible to draw that, under isothermal
conditions, the internal mass transfer rate of methanol from metal-
oxide(s) sites of ZCZ to the acid sites of HZSM-5 does not affect the
activity–selectivity pattern, likely because the concentration pro-
files (of reactants and products) within the catalytic particle do not
change with the grain size. In any case, the 40–70 mesh fraction
was chosen for further investigations as the suitable compromise
between the pressure drop along the reactor and the mechanical
resistance of catalyst particle.
0
20
40
60
80
100
zeolite loading (wt. %)
Fig. 10. Effect of zeolite loading on DME productivity at different reaction temper-
ature (PR, 3.0 MPa; GHSV, 9000 NL kgcat−1 h−1).
4. Conclusions
In the present study, the performance of
a
novel
Cu–ZnO–ZrO2/HZSM-5 catalytic formulation for the one-step
DME production by CO2 hydrogenation has been investigated.
Different combination procedures between methanol synthesis
catalyst and methanol dehydration catalyst have been adopted, so
obtaining bifunctional systems characterized by different textural,
structural and surface properties.
Reaction temperature and pressure play a fundamental role in
driving the reaction towards DME formation. According to ther-
modynamics, the DME formation is favoured at low temperature
(<513 K), whereas at high pressure (>3.0 MPa) the negative role of
water on catalyst activity is envisaged.
Catalytic data in the absence of zeolite disclose that strong acid
sites of Cu–ZnO–ZrO2 catalyst are not suitable for DME synthesis,
while the bifunctional ZZ-M sample, prepared by physical mixing of
the pre-pelletized catalysts, exhibits a superior performance with
respect to the other investigated systems, mainly when a zeolite
loading of 50 wt.% is used.
Acknowledgements
This work was financially supported by the Italian
Research Fund (PON R&C 2007–2013, DD 713/Ric. 29.10.2010,
PON02 00451 3362376) through the “BIO4BIO” project, Biomolec-
ular and Energy valorization of residual biomass from Agroindustry
and Fishing Industry.
The authors would like to thank the scientific advisory board of
10th Natural Gas Conversion Symposium 2013, Doha, Qatar, for the
“First place Poster Award” received as an official acknowledgement
for the results presented at the conference.
At last, in Fig. 10 the influence of the zeolite loading on the space-
time yield of DME (STY) was also investigated. As it is possible
to observe, the rate of DME production in the absence of zeolite
results near to zero, clearly demonstrating that strong acid sites
(totally associated to ZCZ, as seen in Table 2) are not suitable to pro-
mote the dehydration reaction of methanol to DME. On the other
hand, no conversion of CO2 was observed over the “bare” zeolite,
because the lack of a specific functionality for CO2 and H2 activa-
tion. Overall, at any investigated reaction temperature, the specific
DME productivity follows a volcano-shape trend with a maximum
in correspondence of a zeolite loading of 50 wt.%.
The authors also thank Prof. Girolamo Giordano and Dr. Alfredo
Aloise from University of Calabria for performing the BET measure-
ments.
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Please cite this article in press as: G. Bonura, et al., Catalytic behaviour of a bifunctional system for the one step synthesis of DME by CO2
hydrogenation, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.11.017