CuFe and CuCo supported on pillared clay as catalysts for
CO2 hydrogenation into value-added products in one-step
Francielle C. F. Marcos1, José M. Assaf1, and Elisabete M. Assaf2,*
1Federal
University of São Carlos, São Carlos-SP,13565-905, Brazil
University of SãoPaulo, São Carlos-SP, 13560-970 Brazil
*[email protected]
2
Keywords: pillared clay, bifunctional catalyst, CO hydrogenation, methanol and DME.
1. Introduction
The capture and utilization of carbon dioxide (CO2)
in the production of high value-added products has
gained increasing importance due to the necessity of
reducing the emissions of this greenhouse gas in the
atmosphere and the dependence of fossil fuels [1].
CO2 hydrogenation studies have focused on catalysts
to methanol and Fischer-Tropsch syntheses, such as
Fe, Co and Cu [2]. Bimetallic combinations of Cu
with small amounts of Fe or Co supported on acidbasic solid can be the key to development of a new
bifunctional catalyst (containing both metallic and
acid sites) for one-step synthesis reactions. Pillared
clays (PILC´s) can be used as support due to their low
cost and acid-basic properties. The introduction of Al
polycations in the clay leads to a significant increase
of specific area and the formation of new acid sites
[3]. The aim of this study is evaluates the effect of the
different metallic and acid-basic sites on the CO2
hydrogenation for production of value added
compounds in a one-step reaction.
2. Experimental
The synthesis of the aluminum-pillared clay (V-Al
PILC) was carried out from a pillaring agent prepared
from the hydrolysis of an AlCl3.6H2O 1M solution
with NaOH 0.4 M (OH−/Al3+ = 2.0), as reported in
details elsewhere [4]. Cu-Fe and Cu-Co catalysts
supported on V-Al PILC were prepared by the
impregnation method using 10 wt% of Cu and 5 wt%
of Fe or Co. Additionally, one catalyst containing
only Cu on pillared clay was synthesized as a
reference; more details about this catalyst has been
described elsewhere [5]. The catalysts were labeled
as CuFe/V-Al PILC, CuCo/V- Al PILC and Cu/V- Al
PILC. These materials were analyzed by X-ray
diffraction (XRD), energy dispersive spectroscopy
(EDS), N2 adsorption/desorption isotherms and
temperature programmed reduction (TPR). The
acidity of catalysts was characterized by means of
Fourier transform infrared spectroscopy after
pyridine adsorption (Py-FTIR) and temperature
programmed desorption of ammonia (NH3-TPD).
The basic sites was evaluated by Temperature
programmed desorption of CO2 (CO2-TPD).
Catalytic activity was carried out in high-pressure
reactor containing 0.2 g of catalyst diluted with 0.2 g
of SiC. Before the reactions, the catalysts were
reduced with hydrogen (30 mL min-1) at 300 °C by 1
h. Experiments were performed at temperatures
ranging from 250 to 300 °C, pressure of 40 bar and
mixture of H2:CO2 = 3:1 (molar ratio). The gaseous
reagents and products were analyzed in a gas
chromatograph (TCD and FID).
3. Results and Discussion
The chemical composition determined by EDS is in
close agreement with the nominal values.
Table 1 presents the BET specific surface area,
micropore area, and specific metallic area of the
catalysts. The Cu-Co/V-Al PILC catalyst showed the
largest surface area and copper metallic area in
comparison with others catalysts. All catalysts
showed pore size distribution centered at 1.9 nm.
Table 1. Specific surface area (SB.E.T ), micropore area
(Stplot) and specific copper metallic area (SCu) of catalysts
Catalyst
SB.E.T
St-plot
SCu
(m2.g-1) (m2.g-1)
(m2.g-1)
CuFe/V-Al PILC
107
47
31
CuCo/V-Al PILC
165
91
55
Cu/V-Al PILC
52
21
17
Diffraction peaks corresponding to the monoclinic
CuO phase were clearly visible in all patterns (JCPDS
89-2530). In addition, other peaks characteristic of
montmorillonite and quartz phases also were
observed. The XRD pattern of CuFe/V-Al PILC
catalyst showed an additional peak of Fe3O4 phase
(JCPDS 03-0863) in the same position of monoclinic
CuO. The XRD pattern of CuCo/V-Al PILC catalyst
exhibited all peaks characteristic of CuO,
montmorillonite and quartz phases as ascribed above.
In addition, there were peaks corresponding to the
spinel Co3O4 phase (JCPDS 42-1467).
Fig. 1 (A) displays the TPR profiles of catalysts. Both
CuCo/V-Al PILC and Cu/V-Al PILC catalysts
presented similar onset reduction temperatures for
CuO reduction to Cu0. In addition, at 300 °C the
CuCe/V-Al PILC catalyst presented the Co3O4
superficial reduction to Co0. Beyond that, this catalyst
showed at temperatures higher than 300 °C others
peaks that can be attributed to Co oxide and support
reductions. CuFe/V-Al-PILC catalyst showed two
peaks at 220 °C and 280 °C that may be ascribed to
both reductions: Cu2+ to Cu0 and Fe3+ to Fe2+. Besides
these, at temperatures higher than 300 °C it presented
peaks that can be attributed to Fe oxide and support
reductions.
Py-FTIR spectra of catalysts are shown in Fig. 1 (B).
Two bands labeled as A and B characteristic of
pyridine adsorbed in acid sites were observed for all
samples. The first band (A) at 1437 cm−1 is
characteristic of physisorbed or hydrogen-bonded
pyridine on Lewis acid sites. The band (B) at 1490
cm−1 is related to both Bronsted and Lewis acids sites.
The CuCo/V-Al PILC catalyst showed the highest
intensity of Lewis band in comparison to the other
catalysts. The NH3-TPD profiles of reduced catalysts
are displayed in Fig. 1 (C).
Different strengths of acidity were observed in the
catalysts: weak (100–300 °C), medium (300–450 °C)
and strong (450–900 °C). At higher temperatures, all
catalysts presented more intense peaks suggesting
that strong acid sites are predominant. The following
order of total acid sites density was determined:
CuFe/V-Al PILC>CuCo/V-Al PILC >Cu/V-Al PILC.
Fig. 1 (D) shows the CO2-TPD profiles obtained from
reduced catalysts. Different profiles of CO2 desorbed
were observed: at low temperature (> 300 °C) the
peak is related to a weak basic site while at higher
temperatures is associated to medium and strong
basic sites. The catalysts showed the following order
of total basic sites density: CuFe/V-Al PILC > Cu/VAl PILC > CuCo/V-Al PILC.
Fig. 2 displays the results of catalytic CO2
hydrogenation in the temperature range of 250 °C and
300 °C. The CO2 conversion into value-added
products was dependent on the temperature reaction.
At low temperature (250 °C), the reaction was more
selective to methanol and DME synthesis, while at
higher temperature the selectivity to methane was
increased.
Figure 2. CO2 conversion (X(CO2)) and selectivity of
products (Si(%)) on catalysts.
Our results have demonstrated that the interaction
between Cu-Fe and Cu-Co improved the catalytic
performance to methanol and DME production due to
the increase of specific metallic area.
The methanol synthesis is influenced by metallic and
basic sites, while DME production is correlated to
acid sites. The CuCo/V-Al PILC catalyst showed the
higher CO2 conversion in agreement with the higher
metallic surface, probably because of the lower total
density of basic sites, as observed in Fig. 1D. This
catalyst was more active for methane synthesis. The
CuFe/V-Al PILC showed the best performance for
methanol and DME synthesis. This performance is
related to the highest acid-basic sites.
4. Conclusions
The present study showed that bimetallic
combinations of Cu with Fe or Co favored the
increase of specific surface area, specific metallic
area and CO2 conversion. The products observed are
directly correlated with metallic and acid-basic sites.
The best performance in methanol and DME
production was obtained with CuFe/V-Al PILC
catalyst.
Acknowledgments
The authors thank to FAPESP (processes number 2012/17957-3
and 15/06246-7) for financial support.
References
Figure 1. (A) TPR profiles (B) Py-FTIR spectra of
catalysts (C) NH3-TPD profiles and (D) CO2-TPD profiles
of reduced catalysts.
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