Catalytic hydrogenation of CO2 into methanol and dimethyl ether over Cu-X/V-Al PILC (X?=?Ce and Nb) catalysts

Article abstract of DOI:10.1016/j.cattod.2016.08.007

The production of methanol (MeOH) and dimethyl ether (DME) through CO₂ hydrogenation is an attractive route to recycle CO₂ and control its emission to the atmosphere. In the present work, the hydrogenation of CO₂ into MeOH and DME over Al-pillared clays impregnated with different metals (Cu-X/V-AI PILC, where X?=?Ce and Nb) was investigated in the temperature range 200–300?°C. The influence of additives on the physico-chemical properties of the catalysts was evaluated by N₂ adsorption, ex situ XRD, TPR analysis, and in situ XANES and XRD. The acid sites were analyzed by FTIR analysis after adsorption of pyridine and NH₃-TPD. The basic and metallic sites were analyzed by CO₂-TPD and N₂O-TPD, respectively. In situ XANES was used to investigate the Cu⁺ and CuO species. MeOH and DME selectivities were directly related to the presence of acid, basic and metallic sites and with the reaction temperature. The results indicated that the CuCe/V-Al PILC catalyst was the most promising catalyst for DME synthesis from CO₂.

Full text of DOI:10.1016/j.cattod.2016.08.007

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Contents lists available at ScienceDirect
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Catalytic hydrogenation of CO into methanol and dimethyl ether
2
over Cu-X/V-Al PILC (X = Ce and Nb) catalysts
a
b
a,∗
Francielle C.F. Marcos , José M. Assaf , Elisabete M. Assaf
a
University of São Paulo, Av. Trabalhador São-carlense, 400-13560-970, São Carlos, SP, Brazil
Federal University of São Carlos, Rod. W. Luiz, km 235-13565-905, São Carlos, SP, Brazil
b
a r t i c l e i n f o
a b s t r a c t
Article history:
The production of methanol (MeOH) and dimethyl ether (DME) through CO2 hydrogenation is an
attractive route to recycle CO2 and control its emission to the atmosphere. In the present work, the
hydrogenation of CO2 into MeOH and DME over Al-pillared clays impregnated with different metals (Cu-
Received 15 May 2016
Received in revised form 19 July 2016
Accepted 2 August 2016
Available online xxx
◦
X/V-AI PILC, where X = Ce and Nb) was investigated in the temperature range 200–300 C. The influence
of additives on the physico-chemical properties of the catalysts was evaluated by N2 adsorption, ex situ
XRD, TPR analysis, and in situ XANES and XRD. The acid sites were analyzed by FTIR analysis after adsorp-
tion of pyridine and NH3-TPD. The basic and metallic sites were analyzed by CO2-TPD and N2O-TPD,
Keywords:
CO2 hydrogenation
Pillared clay
Methanol
DME
Copper catalyst
+
respectively. In situ XANES was used to investigate the Cu and CuO species. MeOH and DME selectivities
were directly related to the presence of acid, basic and metallic sites and with the reaction temperature.
The results indicated that the CuCe/V-Al PILC catalyst was the most promising catalyst for DME synthesis
from CO2.
©
2016 Published by Elsevier B.V.
1
. Introduction
involved in the synthesis of MeOH and DME are represented by
Eqs. (1)–(3) [12–15].
Nowadays, the relationship between carbon dioxide (CO2)
emission and environmental impact is well recognized and the
development of technologies for converting CO2 into useful com-
pounds has been of high interest. Various studies on catalysis have
been aimed at obtaining methanol (MeOH) and dimethyl ether
CO + 3H2 ꢀ CH OH + H O ꢀH298K = −49.4 kJ mol−
1
2
3
2
ꢀG298K
+ 3.4 kJ mol−1
=
(1)
(
DME) by CO hydrogenation [1–10].
The synthesis of MeOH (MS) from catalytic CO hydrogenation
2
CO2 + H2 ꢀ CO + H O ꢀH298K = +41.2 kJ mol−1 ꢀG298K
2
2
is an environmentally friendly process, whose the main product is
methanol, (Eq. (1)) [6,11]. Other products of parallel reactions can
+28.6 kJ mol−1
=
(2)
also be formed, such as DME, H O, CO, hydrocarbons and long chain
alcohols [11].
The synthesis of DME is usually performed according to two
approaches: (i) two reactors: MeOH synthesis on metallic sites fol-
lowed by dehydration on solid acid catalysts (Eqs. (1) and (3)), or
2 CH OH ꢀ CH OCH + H O ꢀH298K = − 24.0 kJ mol−1
2
3
3
3
2
(3)
G298K = −12.1 kJ mol−1
ꢀ
Typically, Cu–ZnO–Al O3 is the most employed catalysts for
2
(
ii) single step, which comprises bifunctional catalysts constituted
industrial MeOH synthesis, in which metallic copper is the active
phase. ZnO is used to increase the Cu dispersion on the support,
thus providing a high number of active sites exposed to gaseous
by acid and metallic sites. The one-step DME synthesis involves
numerous competing reaction pathways, for example, the reverse
water-gas-shift (RWGS), which can occur in parallel with MeOH
synthesis, Eqs. (2) and (1), respectively [12]. The main reactions
reactants [7,8,16]. On the other hand, acid-catalysts such as Al O3
2
and HZSM-5 are responsible for methanol dehydration into DME
[
14,17–20]. Pillared clays (PILCs) have been studied as acid-solid
catalysts due to their valuable applicability and low cost. The
inclusion of aluminum polycations in the clays leads to significant
modifications in their acidity, which is due to the increase of specific
surface area and formation of new acid sites [21–24].
∗ Corresponding author.
E-mail address: eassaf@iqsc.usp.br (E.M. Assaf).
http://dx.doi.org/10.1016/j.cattod.2016.08.007
0
920-5861/© 2016 Published by Elsevier B.V.
Please cite this article in press as: F.C.F. Marcos, et al., Catalytic hydrogenation of CO into methanol and dimethyl ether over Cu-X/V-Al
2
PILC (X = Ce and Nb) catalysts, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.08.007

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2
Therefore, the resulting product distribution obtained from CO2
conversion is directly related to the catalyst composition and reac-
tion conditions. Many types of catalysts have been tested in the CO2
hydrogenation into MeOH and DME, for instance Cu-based catalysts
with different promoters such as Zn, Zr, Ce, Al, Si, V, Ti, Ga, B and Cr
was measured in-line with a TCD. 100 mg of sample were placed in
the TPR reactor and reduced with a H /Ar mixture (10% v/v) flowing
2
−
1
◦
at 30 mL min . The temperature was raised to 1000 C at a heating
◦
−1
rate of 5 C min
.
The metallic surface area of copper (SCu) was determined using a
[
4,11].
Cu-CeO -based catalysts have displayed promising perfor-
N O decomposition method [31] in an analytical multipurpose sys-
tem. Firstly, 100 mg of sample were reduced in a mixture of 1.96%
2
2
−
1
◦
mance on MeOH synthesis because of their redox properties and
ability to store oxygen [9]. According to Liu et al. [25], the inter-
action between Ce and Cu may promote electron transfer effects
between CeO and Cu particles, leading to formation of Cu species,
Eq. (4).
H /Ar (flow 30 mL min ) at 300 C for 1 h. After, the sample was
2
◦
cooled to 60 C and a gas mixture (10% N O/He) was carried out
at flow rate of 30 mL min for 30 min to oxidize the surface CuO
atoms producing Cu O. Physically adsorbed N O was removed by
2
−
1
+
2
2
2
−
1
N2 purging (flow 30 mL min ) for 1 h. After this step, the sample
was cooled to room temperature under N flow and a new reduction
Ce4+ + CuO ꢀ Ce3+ + Cu+
2
(4)
cycle was performed to reduce the surface Cu O to CuO.
2
Gao et al. [26] studied copper catalysts and observed that the
The chemical composition of the catalysts was determined by
energy dispersive X-ray spectroscopy (EDS) using a LEO 440 scan-
ning electron microscope equipped with a tungsten filament and
coupled to an energy dispersive X-ray detector.
formation of Cu+ and CuO species was essential to the synthesis
+
of MeOH, and the Cu /CuO relationship determined the specific
activity of the catalysts.
Nb-based catalysts have exhibited high catalytic activity and
selectivity for dehydration of alcohols. This behavior has been asso-
ciated to the presence of strong Lewis and Brønsted acid sites
The specific surface area was calculated by the B.E.T method. The
area and volume of micropores were determined by t-plot analysis.
The data were obtained on a Quantachrome Nova 1000 gas adsorp-
◦
[
27–29]. Ladera et al. [27] studied a series of NbOx-based catalysts
tion analyzer. All samples were previously degassed at 250 C for
and observed that their catalytic performance on MeOH dehydra-
tion into DME was favored by high Nb contents.
In light of the above, the goal of this study was to evaluate the
performance of Cu-X/V-Al PILC (X = Ce or Nb) catalysts on the pro-
4 h.
XANES spectra at the Cu K-edge were recorded in situ by
transmission mode and under activation condition in the XAFS2
beamline of the LNLS. Initially, XANES spectra were obtained
at 25 C and then from 25 C to 300 C at 5 C min
in a hydrogen-rich atmosphere (5% H /He) with a flow rate of
◦
◦
◦
◦
−1
for 1 h
duction of MeOH and DME from catalytic CO hydrogenation.
2
2
−
1
2
. Experimental
100 mL min
.
The quantification of copper species in the Cu K-edge was
performed by linear combination analysis (LCA) and principal com-
ponent analysis (PCA) of XANES spenctra, including three model
compounds, namely, CuO, Cu2O and CuO. All analyses were carried
out in the MAX-StraightNoChaser XAFS data analysis code [32–34].
The acidity of catalysts was characterized by means of temper-
ature programmed desorption of ammonia (NH3-TPD) and Fourier
transform infrared spectroscopy after pyridine adsorption (Py-
FTIR). NH3-TPD analyses were conducted on an AutoChem II 2920
Micromerictics chemisorption analyzer. 100 mg of sample were
2
.1. Preparation of catalysts
A V-Al PILC (Volclay Al-pillared) support was prepared by pil-
laring montmorillonite (V) with Keggin ion (Al13), as reported in
details elsewhere [30]. Chemical analysis of the support by EDS
showed the presence of O, Mg, Al, Si, K and Fe at contents of 52.9,
1
.4, 15.5, 26.8, 0.4 and 3.0 wt.%, respectively [30].
The Cu-X/V-Al PILC catalysts were prepared by simultaneous
impregnation of the support with aqueous solutions of copper and
cerium nitrates, or copper and niobium nitrate. The Cu and addi-
tives (Ce and Nb) loadings in the support were adjusted to 10 wt.%
and 5 wt.%, respectively. The resulting precursors were dried at
◦
−1
pretreated at 550 C in a He atmosphere (50 mL min ) for 30 min,
◦
cooled to room temperature and reduced at 300 C for 1 h under
◦
flow of 10% H2/He. Next, the sample was cooled to 120 C under He
◦
◦
8
0 C for 24 h and then calcined in air at 500 C for 3 h. Addition-
flow for 30 min. NH3 adsorption was carried out at 20 kPa for 30 min
with a 15% NH3/He mixture. Physically adsorbed NH3 was removed
ally, one catalyst sample containing only Cu on pillared clay was
synthesized under similar conditions. The catalysts samples were
designated as CuCe/V-Al PILC, CuNb/V-Al PILC and Cu/V-Al PILC.
◦
−1
by He purging for 1 h, and the sample was heated at 10 C min
◦
◦
−1
from 50 C to 900 C under a He flow (30 mL min ).
The Py-FTIR spectra were recorded on a Prestigi-21 spec-
−
1
2.2. Characterizations
trophotometer in the range of 1800–1400 cm . For acidity
determinations by FTIR, 50 mg of sample were pretreated in a
◦
−1
The crystalline structures of catalysts were analyzed by pow-
tubular furnace at 300 C under N2 flow of 100 mL min for 1 h.
Afterwards, gaseous pyridine (Py) was adsorbed on the sample for
der X-ray diffraction (XRD). Ex situ diffraction patterns (XRD)
were obtained on a Rigaku Multiflex diffractometer using Cu K␣
◦
−1
1 h at 150 C using N2 gas carrier flowing at 100 mL min . The
◦
(
1.5406 Å) radiation source. The scan was performed from 10 to
temperature and N flow were maintained for 1 h to eliminate the
2
◦
◦
−1
8
0
at 2 min . In situ X-ray diffraction patterns were recorded
physically adsorbed pyridine molecules [35].
in a D10B-XPD beamline at the National Laboratory of Synchrotron
Light (LNLS) in Campinas-Brazil using a Huber diffractometer, Arara
furnace and Cyberstar detector. The parameters were: 2␪ range
Temperature programmed desorption of CO (CO -TPD) proce-
2
2
dure was similar to the NH -TPD analysis. Firstly, the sample was
3
◦
reduced at 300 C for 1 h in a flowing of mixture 10% H /He and
then cooling down to 50 C under He flow (30 mL min ). After that,
the catalyst was saturated with CO (30 mL min ) during 30 min.
2
◦
◦
◦
◦
−1
from 30 to 55 , step of 2.51 and 10 s acquisition time per point,
−
1
with radiation of ␭ = 1.54996 Å selected with a Si (111) monochro-
2
◦
mator. After performing XRD measurements at 25 C, the samples
Afterward, the CO -TPD experiment was performed with a heating
2
◦
−1
−1
◦
were heated in a hydrogen-rich atmosphere (5% H /He) at flow rate
rate of 10 C min under He flow (30 mL min ) until 900 C.
2
−
1
◦
◦
−1
of 100 mL min –300 C (reduction temperature) at 5 C min for
1
h and new XRD pattern acquisitions were performed.
2.3. Catalytic evaluation
Temperature programmed reduction (TPR) tests were per-
formed in a Micromeritics Pulse ChemSorb 2750 equipped with
a thermal conductivity detector (TCD). The hydrogen consumption
The CO2 hydrogenation reactions were carried out in a high-
pressure reactor (Parr Instruments) at 40 bar containing 0.2 g of
Please cite this article in press as: F.C.F. Marcos, et al., Catalytic hydrogenation of CO into methanol and dimethyl ether over Cu-X/V-Al
2
PILC (X = Ce and Nb) catalysts, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.08.007

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Table 3
3
Table 1
Average mass percentage of metal elements measured by EDS of catalysts.
XANES- Quantification of copper species on Cu K-edge by linear combination results
of XANES experimental spectra under H2 flow at 300 C.
◦
Catalysts
Average mass percentages
Catalyst
Linear composition
%
Cu
%Ce
%Nb
CuO (%)
1.1 ± 1.2
–
7.1 ± 1.1
Cu2O (%)
Cu0 (%)
QF
CuCe/V-Al PILC
CuNb/V-Al PILC
Cu/V-Al PILC
10.2 ± 0.2(10)
10.1 ± 0.7(10)
10.3 ± 0.1(10)
5.4 ± 0.4(5)
–
CuCe/V-Al PILC
CuNb/V-Al PILC
Cu/V-Al PILC
27.4 ± 2.0
28.9 ± 2.0
22.8 ± 1.8
73.5 ± 1.6
71.1 ± 2.0
67.0 ± 1.4
3.1
5.0
2.6
–
–
4.4 ± 0.17(5)
–
Table 2
and Nb oxides reduced the specific surface area, micropore area and
micropore volume of the catalysts. This decrease of specific surface
area can be attributed to the coverage of the V-Al PILC support by
small fine oxide particles, which were deposited over the pores of
the matrix. The CuCe/V-Al PILC catalyst showed the largest surface
area, micropore volume and copper metallic area in comparison
with other catalysts samples.
Specific surface area (SB.E.T ), micropore area (St-plot ), micropore volume (Vt-plot ) and
specific copper metallic area (SCu) of CuX/V-Al PILC catalysts.
SB.E.T(m2
g
−1
)
St-plot (m2
g
−1
)
Vt-plot (cm3
g
−1
)
SCu(m2
−1
g )
Catalysts
CuCe/V-Al PILC 121
CuNb/V-Al PILC 67
Cu/V-Al PILC
66
30
21
0.035
0.016
0.011
38
32
17
52
Fig. 2 displays the TPR profiles of samples. The V-Al PILC support
showed two reduction peaks related to iron present in natural clay
(V). This behavior was assessed by EDS analysis in a previous report
[30]. The first peak corresponds to reduction of hematite into mag-
netite phase, whereas the second peak is ascribed to the reduction
of magnetite into metallic iron, Eq. (7) [40].
catalyst diluted with same amount of inert SiC (0.2 g). The cat-
−
1
alysts were previously reduced with hydrogen (30 mL min ) at
◦
3
00 C for 1 h. A gaseous mixture of H2 and CO2 at a 3:1 molar
ratio was injected into the reactor through mass flow controllers
−
1
−1
at gas hourly space velocity (GHSV) of 20.000 L kgcat
h . The
experiments were performed at temperatures ranging from 200
to 300 C. The catalysts were reused for the three times in differ-
0
◦
␣-Fe2O3 → Fe3O4 → Fe
(7)
ent temperatures. The gaseous products were analyzed in-line on
a 7890A gas chromatograph (Agilent Technologies) equipped with
thermal conductivity (TCD) and Flame ionization (FID) detectors,
which analyzed the effluents from parallel HP pona, Plot-Q and HP
molesiev columns.
For all catalysts, the major peak is due to CuO reduction. The
CuCe/V-Al PILC and Cu/V-Al PILC catalysts presented similar onset
◦
reduction temperatures (∼300 C) for CuO, Eq. (8). The CuCe/V-
◦
Al PILC catalyst also presented another peak between 445 C and
6
◦
50 C, which may be attributed to the reduction of surface ceria
The CO conversion (XCO ) was calculated as Eq. (5):
2
2
CeO2, corresponding to the formation of non-stoichiometric Ce O3
2
species [37,41]. Furthermore, it is observed that the presence of
Ce does not favor the reduction of CuO at lower temperatures. This
indicates that Cu2+ ions strongly interact with Ce ions, hampering
the Cu redox process. The TPR profile of CuNb/V-AL PILC showed
two peaks at 270 C and 300 C, which can be attributed to the
reduction of copper oxide to metallic copper (Eq. (8)). The first peak
can be assigned to more structurally dispersed CuO species, while
Mols of CO2 in − Mols of CO2 out
Mols of CO2 in
XCO2 (%) =
(5)
4+
The selectivity (S) of each product (i = CH OH, CH OCH CO or
3
3
3,
CH ) was calculated as Eq. (6):
◦
◦
4
mols of i produced
Si (%) =
x100
(6)
moles of CO2 converted
the second peak to CuO present in alloy (CuxNb1 ), Fig. 2 B [38,39].
−x
These results are in close agreement with XRD analysis, Fig. 1B.
3
. Results and discussion
◦
Also, another peak at 430 C may be attributed to the superficial
reduction of Nb O5 [42].
2
3.1. Characterization
CuO(s) + H2(g) → CuO(s) + H O
(8)
2
(aq)
XRD patterns of catalysts registered at room temperature are
presented in Fig. 1. Diffraction peaks corresponding to the mono-
clinic CuO phase are clearly visible in all patterns (JCPDS 89-2530)
Fig. 3 shows the in situ XRD patterns recorded at several temper-
atures over the H reduction process. Initially at room temperature,
2
◦
◦
[
(
36]. Other peaks characteristic of montmorillonite (Mt) and quartz
Qz) phases are also observed. Furthermore, the presence of crys-
peaks at 2␪ = 35.5 and 38.7 due to CuO monoclinic reflections are
visible (JCPDS 89-2530) [36]. With increasing temperature, these
peaks gradually disappeared, and others related to CuO face cen-
talline Cu O phase in the catalysts was not identified by XRD. The
2
◦
◦
XRD pattern of CuCe/V-Al PILC showed the peaks described above
and other peaks ascribed to the fluorite face-centered cubic CeO2
phase, (JCPDS 81-0792) [37]. The XRD pattern of CuNb/V-Al PILC
exhibited only the peaks described above (Mt, Qz and CuO phases).
It was not possible to identify any crystalline niobium oxide phase,
possibly due to Nb oxide (ionic radius 0.69 Å) inclusion within the
copper oxide (ionic radius 0.79 Å) lattice, and consequent formation
tered cubic (fcc) phase appeared at 2␪ ∼ 42.1 and 50.9 (JCPDS
4-0835) [43]. However, after H2 reduction, it was also observed
◦
◦
one diffraction peak at 2␪ range between 35 and 39 . This peak
is related to the structure of montmorillonite present in the cata-
lysts. These progressive transformations are consistent with those
observed by TPR analyses (Fig. 2). The presence of Cu O phase may
2
not be detectable by XRD. Hence, the catalysts were examined using
X-ray absorption spectroscopy.
of copper-niobium alloy (CuxNb1 ) [38,39], as shown in Fig. 1B.
−x
This is suggested by the shift of the main CuO phase peaks towards
higher 2␪ values.
Fig. 4 shows the in situ Cu K-edge XANES spectra of the catalysts.
Spectra of CuO, Cu O and Cu foil are also shown in Fig. 4 as a refer-
2
Chemical composition, BET specific surface area, micropore
area, micropore volume and specific metallic area of the catalysts
are presented in Tables 1 and 2. It can be seen that the real individual
element contents are in close agreement with the nominal values.
Moreover, all standard deviations were less than 1, thus presenting
a next real percentage and uniform dispersion. The impregnation of
ence. In addition, the quantification of oxides and metallic species
is reported in Table 3. According to Sun et al. [44], Cu2+ species in
CuO display a weak pre-edge absorption at approximately 8984 eV
assigned to a dipole-forbidden transition of 1 s to 3d orbital. On the
other hand, Cu+ compounds such as Cu O show an intense peak
2
around 8982 eV attributable to dipole-allowed transition of 1 s to
4p orbital. The fitted data of CuX/V-Al PILC samples in pre-edge
2
−1
3
−1
V-Al PILC (SB.E.T = 245 m g and V = 122 cm g ) [30] with Cu, Ce
Please cite this article in press as: F.C.F. Marcos, et al., Catalytic hydrogenation of CO into methanol and dimethyl ether over Cu-X/V-Al
2
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◦
◦
Fig. 1. XRD patterns of samples: (A) aligned XRD patterns of oxides and (B) enlargement of patterns at Bragg angles between 34 and 40 .
Fig. 2. (A) TPR profiles of CuX/V-Al PILC catalysts and V-Al PILC support; (B) enlargement of TPR profiles between CuNb/V-Al PILC and 5%Nb/V-Al PILC.
Fig. 3. In situ XRD patterns collected over reduction process of (A) CuCe/V-Al PILC, (B) CuNb/V-Al PILC and (C) Cu/V-Al PILC.
absorption is slightly different from that of the CuO phase, demon-
The linear least-square fitting results of spectra measured under
ambient atmosphere at 25 C are presented on Fig. S1 and Table S1
(see ESI). The PCA of the data for all catalysts proves that the XANES
spectra can be decomposed on the basis of one component (CuO).
2
+
◦
strating that Cu ions present in these catalysts have a chemical
environment different from that in CuO. This suggests that Cu2+
ions are in a distorted octahedral coordination environment [44].
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2
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Fig. 4. Cu K-edge in situ XANES spectra of (A) CuCe/V-Al PILC, (B) CuNb/V-Al PILC and (C) Cu/V-Al PILC. (a) CuO standard (b) samples at room (c) Cu2O standard (d) sample
reduced and (e) Cu0 standard.
Table 4
Quantitative analysis of NH3-TPD and CO2-TPD profiles of reduced catalysts.
Density of acid sites (mol g 1
−
)
−1
Density of basic sites (mol g )
Catalysts
CuCe/V-Al PILC 2.08 × 10−
3
8.49 × 10
−4
−
3
3
−4
CuNb/V-Al PILC 2.07 × 10
4.16 × 10
2.37 × 10−
8.19 × 10
−4
Cu/V-Al PILC
The fitting quality factor values of CuCe/V-Al PILC and CuNb/V-Al
PILC catalysts are higher than Cu/V-Al PIL catalyst (Table S1). This
may be due to a higher disordering in the local structure around Cu
atoms by the Ce and Nb introduction.
Cu foil shows an absorption edge at 8979 eV and two peaks at
8
993 and 9003 eV. The edge absorption at 8979 eV represents the
1
s to 4p electronic transitions [19,44]. After reduction with H was
2
expected XANES spectrum similar to Cu foil for all catalysts. The
fitted data of reduced catalysts are similar to that of Cu foil but not
equal to the standard. The least-square linear combination fits of
catalysts after reduction are showed in Fig. S2 (see ESI). The PCA
of data for all catalysts demonstrated that the XANES spectra can
Fig. 5. NH3-TPD profiles of reduced CuX/V-Al PILC catalysts.
be decomposed based on two or three components (Cu foil, Cu O
2
and CuO) by Target Transformation. They fitted reasonably, but not
perfectly, showing that the local structure around Cu in the cata-
lysts is similar but not identical to the standards. This confirms that
the CuX/V-Al PILC reduced samples are mainly composed of CuO
as A, B and C, which are characteristic of pyridine adsorbed in acid
−
1
sites, were observed for all samples. The first band (A) at 1437 cm
is characteristic of physisorbed or hydrogen-bonded pyridine on
−
1
and small amount of Cu2+ and Cu+1 ions, which were not totally
reduced.
Lewis acid sites. The band (B) at 1490 cm
is related to both
−
1
Brønsted and Lewis acids sites. The band (C) at 1543 cm is associ-
ated with pyridinium ion (Py H ), which is produced by reaction of
+
Fig. 5 displays the NH -TPD profiles of reduced catalysts and the
3
pyridine with Brønsted acid sites [45,46]. The CuCe/V-Al PILC cat-
alyst showed the highest intensity of Lewis band in comparison to
the other catalysts.
total density of acid sites is summarized in Table 4. The total den-
sity of acid sites was similar for all the samples. Considering these
◦
profiles (Fig. 5), the catalysts exhibited weak (100–300 C), medium
◦
◦
Fig. 7 shows the CO -TPD profiles obtained from reduced cata-
(
300–450 C) and strong acid sites (450–900 C) with different dis-
2
lysts and the density of basic sites is summarized in Table 4. The
tributions and densities. All catalysts presented more intense peaks
at higher temperatures indicating that strong acid sites are preva-
lent.
To evaluate the type of sites present in the catalysts, FTIR experi-
ments using pyridine as probe molecule were done. Py-FTIR spectra
of the solid-acid catalysts are shown in Fig. 6. Three bands labeled
catalysts showed tree peaks corresponding to CO desorbed at dif-
2
ferent temperatures. The first peak at low temperature is related
to a weak basic site while the two others are associated to strong
basic sites. The catalysts showed the following order of total basic
sites density: CuCe/V-Al PILC > Cu/V-Al PILC > CuNb/V-Al PILC.
Please cite this article in press as: F.C.F. Marcos, et al., Catalytic hydrogenation of CO into methanol and dimethyl ether over Cu-X/V-Al
2
PILC (X = Ce and Nb) catalysts, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.08.007

G Model
CATTOD-10345; No. of Pages8
ARTICLE IN PRESS
6
F.C.F. Marcos et al. / Catalysis Today xxx (2016) xxx–xxx
Fig. 6. Py-FTIR spectra of (A) CuCe/V-Al PILC, (B) CuNb/V-Al PILC and (C) Cu/V-Al PILC.
performance for all catalysts on the CO conversion. The interaction
2
between Fe and Cu facilitates a strong CO and H adsorption. These
2
2
data are in accordance with Liu et al. [50].
The products distribution through the catalytic tests was
affected by temperature. MeOH, DME, CH4 and CO were the main
◦
products formed under all temperatures from 200 to 300 C. At
higher reaction temperature, smaller amounts of C and C hydro-
2
3
carbons were formed. There was no carbon formation for any
experimental condition tested in the catalytic CO2 hydrogenation
(see ESI Figs. S3 and S4).
Two competing reactions can occur during CO2 hydrogenation
over metallic sites, MS and RWGS, Eqs. (1) and (2), respec-
tively [1,10,14]. Methanation also occurs and is thermodynamically
−
1
favored (ꢀG298K = −130.8 kJ mol ), Eq. (9) [11].
CO + 4H → CH + 2H O ꢀH298K = −252.9 kJ mol−
1
(9)
2
2
4
2
◦
At 200 C, it was observed that MS occurs predominantly over
RWGS (Fig. 8). In parallel, the dehydration of MeOH into DME (Eq.
Fig. 7. CO2-TPD profiles of reduced CuX/V-Al PILC catalysts.
(3)) and methanation (Eq. (9)) also occurred but in a smaller extent.
The CuCe/V-Al PILC and CuNb/V-Al PILC catalysts led to higher
MeOH yields than the Cu/V-Al PILC catalyst. This is possibly due
to the larger quantity of Cu and CuO species [10] and higher cop-
3.2. Catalytic activity
+
Fig. 8 displays the results of catalytic CO hydrogenation in the
temperature range of 200–300 C. First, the effect of temperature on
per metallic area (Table 3 and 2). The Cu/V-Al PILC catalyst showed
2
◦
the lowest CO conversion in all the range of temperatures studied,
2
the CO conversion (XC O2) was investigated (Fig. 8A). The CuCe/V-Al
the lowest DME production and the highest methane production.
This can be due to the lower metallic surface area (Table 2) and to
the presence of Cu2+ species, as observed by in situ XANES (Table 3),
which may has not favored the MS reaction.
2
PILC and CuNb/V-Al PILC catalysts showed the highest CO conver-
2
sion to methanol over all temperatures in comparison with Cu/V-Al
PILC catalyst (Fig. 8B–D). This behavior can be due to the higher
1+
◦
Cu /CuO ratio in the CuNb/V-Al PILC and CuCe/V-Al PILC catalysts,
At 250 C, it was observed that the methanation and dehydra-
+
as shown in Table 3. According to literature, Cu and CuO species
tion of MeOH to DME reactions were improved. The DME selectivity
is influenced by MS reaction, acid sites and other parameters, such
as temperature, gas hour space velocity (GHSV) and pressure [14].
Moreover, during the DME synthesis from MeOH dehydration, the
+
are important for methanol production and the Cu /CuO ratio is
determining on the specific activity of the reaction [26,47,48]. The
+
formation of Cu species in CuCe/V-Al PILC catalyst may be due to
electron transfer between the CeO and the CuO particles, leading
to formation of Cu species, as shown in Eq. (4) [25,49]. This is in
H O molecules compete with the MeOH molecules for the adsorp-
2
2
+
tive Lewis acid sites. Therefore, both Brønsted and Lewis acid-base
pair sites are the active sites for MeOH dehydration into DME
[51,52]. The catalysts did not lead to complete MeOH conversion
agreement with the copper surface area (SCu- Table 2), where the
CuCe/V-Al PILC and CuNb/V-Al PILC catalysts presented higher val-
ues of area than Cu/V-Al PILC catalyst, which means more actives
sites avaliable for methanol formation.
Furthermore, the presence of Fe in the V-Al PILC support, as
identify by TPR analysis (Fig. 2), may also have contributed to the
into DME. This may be due to the H O adsorption on Lewis acid sites
2
and also the high gas hour space velocity (GHSV). As reported by
Bonura et al. [14], there is a significant decrease in DME selectivity
when the GHSV is increased.
Please cite this article in press as: F.C.F. Marcos, et al., Catalytic hydrogenation of CO into methanol and dimethyl ether over Cu-X/V-Al
2
PILC (X = Ce and Nb) catalysts, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.08.007

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CATTOD-10345; No. of Pages8
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F.C.F. Marcos et al. / Catalysis Today xxx (2016) xxx–xxx
7
Fig. 8. CO2 hydrogenation reaction: (A) CO2 conversion; Selectivity on (B) CuCe/V-Al PILC; (C) CuNb/V-Al PILC and (D) Cu/V-Al PILC.
The largest DME selectivity was found for the catalytic CO
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◦
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CuX/V-Al PILC catalysts were active on the CO2 conversion to
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+
were directly related to the Cu and CuO species and with acid and
basic sites available on catalyst surface. The best performance of the
CuCe/V-Al PILC catalyst in the production of DME may be due to the
good relationship between metallic, basic and acid sites available
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The authors are grateful for the financial support provided by
São Paulo Research Foundation −FAPESP (grant Proc. 2012/17957-
[
3
), Prof. Dr. Fergus Gessner at University of São Paulo for supplying
the commercial clay, National Laboratory of Synchrotron Light
LNLS) in Campinas, Brazil, for the in situ XRD and XANES facilities,
(
[
[
[
[
13] S.S. Akarmazyan, P. Panagiotopoulou, A. Kambolis, C. Papadopoulou, D.I.
Kondarides, Methanol dehydration to dimethylether over Al2O3 catalysts,
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and the sponsorship FAPESP and BG Brasil through the Research
Centre for Gas Innovation, FAPESP grant Proc. 2014/50279-4.
Appendix A. Supplementary data
1016/j.cattod.2013.11.017.
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.cattod.2016.08.
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