C.P. Jiménez-Gómez et al.
MolecularCatalysis455(2018)121–131
since increases the octane value (103), the synthesis of heterocycles and
solvent in the synthesis of antimalarial drugs or methylfurfural [4,32].
Among metal active phases, copper is the most inexpensive active,
and it has been reported in the literature that basic or amphoteric
supports, such as MgO [13], ZnO [11] or CeO2 [12], or even slight
acidic, such as clay minerals or silica, improve the catalytic behavior
[8–10,15]. In fact, the presence of small Cu particles supported on SiO2
allows to attain a high FUR conversion, obtaining MF as main product,
since the hydrogenolysis of FOL is favored [16,33]. However, supports
with stronger acidity do not favor the FUR hydrogenation in gas phase,
but FUR polymerizes.
In order to develop sustainable catalysts on a larger scale, in-
expensive supports have emerged as alternative. Thus, raw especial
clays, such as bentonite and sepiolite, have been proposed as catalytic
support due to their high specific surface area and low acidity [15,34].
The present research aims at the preparation of copper supported on a
kerolitic clay, a randomly mixed layer formed by disordered turbos-
tratic talc (kerolite) and Mg-smectite (stevensite) [35,36]. Previous
research has established that kerolite can be associated to other Mg-rich
clay minerals, such as stevensite or sepiolite [35,36]. Considering that
Cu/MgO systems have shown to be active in the FUR hydrogenation
[13] and the particular characteristics of kerolitic clay, this could be a
suitable support for highly dispersed Cu nanoparticles.
incorporated, using the nitrate precursor, together with the Cu-species
considering a Cu/Ce or Cu/Zn molar ratio equal to 1. The catalysts were
labeled as K-xCu, K-xCuZn or K-xCuCe, where x is the wt.% of copper
loading.
2.3. Characterization of catalysts
The textural properties were evaluated from the N2 adsorption-
desorption isotherms at −196 °C, as determined by an automatic ASAP
2020 system from Micromeritics. Prior the measurements, the samples
were outgassed overnight at 200 °C and 10−4 mbar. The surface areas
were determined with the Brunauer, Emmett and Teller (BET) equation
[39], considering a N2 cross-section of 16.2 Å2. The total pore volume
was calculated from the adsorption isotherm at P/P0 = 0.996, and the
average pore size was determined by applying the Barrett–Joyner–Ha-
lenda (BJH) method to the desorption branch [40]. The Density Func-
tional Theory (DFT) method was employed to determine the pore-size
H2-temperature programmed reduction (H2-TPR) experiments were
performed with the catalyst precursor (0.080 g), previously treated with
a He flow (35 mL min−1) at 100 °C, for 45 min. After the sample cooled
to room temperature, the H2 consumption was monitored between 50
and 500 °C in an Ar/H2 flow (48 mL min−1, 10 vol% of H2), at a heating
rate of 10 °C min−1. The water formed in the reduction reaction was
trapped by passing the exit flow through a cold finger immersed in a
liquid N2/isopropyl alcohol bath (−80 °C). The H2 quantification was
registered on-line with a thermal conductivity detector (TCD).
XRD patterns of catalysts were obtained with a PANanalytical X’Pert
This work has also evaluated the influence of the addition of ZnO or
CeO2, as promoters, on the catalytic performance, because the elec-
tronic density of Cu nanoparticles can be modified by the presence of
2. Experimental section
Pro automated diffractometer. The patterns were recorded in a
Bragg–Brentano reflection configuration, by using a Ge (111) primary
monochromator (CuKα1) and an X’Celerator detector with a step size of
0.017° (2θ), between 2θ = 10 and 70°, with an equivalent counting
time of 712 s per step. The crystallite size (D) was calculated by using
the Williamson–Hall equation, B cos (θ) = (Kλ/D) + (2 ε sin(θ)), in
which θ is the Bragg angle, B is the full width at half-maximum
(FWHM) of the XRD peak, K is the Scherrer constant, λ is the X-ray
wavelength, and ε is the lattice strain [42]. The analysis (major ele-
ments) of untreated kerolite/Mg-smectite was carried out by means of
the MagiX X-ray fluorescence spectrometer of PANlytical. A Varian 220-
FS QU-106, atomic absorption spectrometer was used for the determi-
nation of sodium. Loss ignition was determined at 950 ºC.
The particle morphology was studied by Transmission Electron
Microscopy (TEM), using FEI Talos F200X equipment (Thermo Fisher
Scientific). This equipment combines outstanding high-resolution S/
TEM and TEM imaging with industry-leading energy dispersive X-ray
spectroscopy (EDS) signal detection, and 3D chemical characterization
with compositional mapping. The samples were suspended in isopropyl
alcohol and dropped onto a perforated carbon film grid.
Metal surface area and dispersion were evaluated by N2O titration
[11,12]. This method is based on the formation of a monolayer of Cu2O
by oxidation of superficial Cu0 with a N2O flow, according to the re-
action: 2Cu0 + N2O → Cu2O + N2. Before analysis, the CuO phase is
reduced under a 10 vol.% H2/Ar flow (48 mL min−1) and a rate of
5 °C min−1, until 300 °C during 1 h, for all catalysts except in the case of
K15CuCe, which was reduced at 230 °C. Then, samples were purged
under a He flow and cooled down to 60 °C. The oxidation of Cu0 to Cu+
is carried out by chemisorption of N2O (5 vol.% N2O/He) at 60 °C
during 1 h. Later, the catalyst was again purged with an Ar flow and
cooled to room temperature. After this, the reduction of Cu2O to me-
tallic Cu was accomplished by raising the temperature up to the
aforementioned values for all catalysts (300 °C), except for K-15CuCe
catalyst (230 °C).
2.1. Reagents
The synthesis of the Cu-based catalysts was carried out using copper
nitrate trihydrate, Cu(NO3)2·3H2O (Sigma-Aldrich, 99%), zinc nitrate
tetrahydrate, Zn(NO3)2 4H2O (Merck, 99%), cerium nitrate tihydrate,
Ce(NO3)4 3H2O (Aldrich, 99%), ethanol, CH3CH2OH (Prolabo, 95%
vol.), ethylene glycol, C2H6O2, (Sigma-Aldrich, 99%), sodium carbo-
nate, Na2CO3(Sigma-Aldrich, 99%).
Kerolitic clay was collected from the Esquivias deposits (Madrid
Basin, Spain). Previous research has established that these deposits are
composed mostly of mixed layered kerolite-stevensite [35]. More re-
cently, it has also been reported the presence of sepiolite and saponite
Chemicals employed in furfural hydrogenation were furfural
(Sigma-Aldrich, 99%), cyclopentyl methyl ether (Sigma-Aldrich,
99.9%) as solvent, and o-xylene (Sigma-Aldrich, 99.9%) as internal
standard. The gases employed were He (Air Liquide 99.99%), H2 (Air
Liquide 99.999%), N2 (Air Liquide 99.9999%) N2O/He (5 vol.% in
N2O) and H2/Ar (10 vol.% in H2, Air Liquide 99.99%).
2.2. Synthesis of catalysts
Copper-based catalysts were synthesized by a co-precipitation-de-
position method, according to previous research reported in the lit-
erature [15]. Briefly, 1 g of kerolitic clay was dispersed under stirring in
75 mL of water for 30 min. Later, a solution composed by 10 mL H2O,
20 mL ethylene glycol, 50 mL ethanol and the corresponding percentage
of copper precursor, for achieving a final copper loading ranging be-
tween 5 and 30 wt.%, was added to the clay suspension. In order to
precipitate the Cu-species, pH was increased until pH = 11 by dropwise
addition of a Na2CO3 (0.5 M) aqueous solution, at 80 °C. After the ad-
dition of the precipitant solution, the resulting solution was cooled and
aged at room temperature for 24 h, without stirring. The solid was fil-
tered and dried at 90 °C for 12 h, and calcined at 400 °C for 1 h.
In the case of the catalysts with ZnO or CeO2, the synthetic proce-
dure was similar to that previously indicated. The Ce or Zn-species was
XPS spectra were obtained with a Physical Electronics PHI 5700
spectrometer with non-monochromatic MgKα radiation (300 W, 15 kV,
1253.6 eV) with a multichannel detector. The spectra were recorded in
the constant-pass energy mode at 29.35 eV with a 0.72 mm diameter
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