NiAl and CoAl materials derived from takovite-like LDHs and related structures as efficient chemoselective hydrogenation catalysts

Article abstract of DOI:10.1039/c3cy00611e

The catalytic performance of metallic catalysts derived from layered double hydroxide (LDH) precursors with nickel or cobalt incorporated in the brucite-like layers besides aluminium (i.e., NiAl takovite and related CoAl) was investigated for the first time in the chemoselective hydrogenation of cinnamaldehyde. The precursors in the as-synthesized and calcined forms were thoroughly analysed by ICP, XRD, nitrogen physisorption, DR UV-vis spectroscopy, TPR and in situ XRD after temperature-programmed reduction. According to the XRD results, both as-synthesized samples contained purely LDH phases with degrees of crystallization depending on the nature of incorporated metal cations (i.e., the CoAl sample presents larger crystallites than the NiAl takovite-like sample). For the calcined NiAl and CoAl samples, well-crystallized oxide phases of NiO and Co₃O₄, respectively, besides amorphous alumina and corresponding spinels were evidenced by XRD. The TPR and in situ XRD results for the calcined samples indicated strong metal-alumina interactions, which resulted in depressed sintering of evolved metal nanoparticles in the high-temperature range of 600-800°C, in well agreement with TEM analysis. Materials derived from the studied LDH systems are found to be efficient catalysts for the hydrogenation of cinnamaldehyde, showing enhanced control over the activity or selectivity, depending on the nature of active metals and the thermal regime of the catalyst activation.

Full text of DOI:10.1039/c3cy00611e

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DOI: 10.1039/C3CY00611E
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ARTICLE TYPE
NiAl and CoAl Materials Derived from Takovite-like LDHs and Related
Structures as Efficient Chemoselective Hydrogenation Catalysts
Constantin Rudolf,a Brindusa Dragoi,*a Adrian Ungureanu,a Alexandru Chirieac, a Sébastien Royer, b
Alfonso Nastro, c and Emil Dumitriu a
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
The catalytic performance of metallic catalysts derived from layered double hydroxides (LDHs)
precursors with nickel or cobalt incorporated in the bruciteꢀlike layers besides aluminium (i.e., NiAl
takoviteꢀlike materials and related CoAl) was investigated for the first time in the chemoselective
10 hydrogenation of cinnamaldehyde. The precursors in the asꢀsynthesized and calcined forms were
thoroughly analysed by ICP, XRD, nitrogen physisorption, DR UVꢀvis spectroscopy, TPR and inꢀsitu
XRD after temperatureꢀprogrammed reduction. According to the XRD results, both asꢀsynthesized
samples contained purely LDH phases with degrees of crystallization depending on the nature of
incorporated metal cations (i.e., the CoAl sample presents larger crystallites than NiAl takoviteꢀlike
15 sample). For the calcined NiAl and CoAl samples, wellꢀcrystallized oxide phases of NiO and Co3O4,
respectively, besides amorphous alumina and corresponding spinels were evidenced by XRD. The TPR
and in situ XRD results for the calcined samples indicated strong metalꢀalumina interactions, which
resulted in depressed sintering of evolved metal nanoparticles in the highꢀtemperature range of 600ꢀ800
°C, in well agreement with TEM analysis. Materials derived from the studied HDL systems are found to
20 be efficient catalysts for the hydrogenation of cinnamaldehyde, showing enhanced control over the
activity or selectivity, depending on the nature of active metals and the thermal regime of the catalyst
activation.
presence of such catalysts high selectivities towards HCNA or
CNOL were obtained (e.g., 100 % unsaturated aldehyde at 79 %
conversion of CNA on Rh(III)ꢀhexamolybdate/γꢀAl2O3 [9]; ~ 88
% HCNA at total conversion of CNA on Pd/polyketone [8]; 50 %
50 unsaturated alcohol at 50 % conversion of CNA on Pt supported
on carbon aerogel [11]; 48 % unsaturated alcohol at 60 %
conversion of CNA on Ru/carbon nanofiber [18], but the catalytic
phases based on noble metals are very expensive and in limited
amounts as offer.
Introduction
The hydrogenation of cinnamaldehyde (CNA) is one of the most
25 studied catalytic reductive processes. Though CNA molecule has
high degree of unsaturation (i.e., aromatic ring, C=O and C=C
double bonds), its partial hydrogenation preferentially occurs in
the side aliphatic chain forming hydrocinnamaldehyde (HCNA)
and cinnamyl alcohol (CNOL), respectively [1; 2; 3]. Both
30 hydrogenated products are of industrial interest and therefore
there is challenging to develop high performance catalysts which
are selective either to HCNA or CNOL. HCNA is used as
inhibitor of the potato tubers sprouting [4] or for the preparation
of pharmaceutical products such as HIV protease inhibitors [5],
35 while CNOL is largely used in perfumery and flavour industries
[3]. For instance, it is frequently used in blossom compositions
such as lilac, jasmine and lily of the valley, hyacinth and
gardenia, to impart balsamic and oriental notes to the fragrances.
It could be noted that hydrocinnamyl alcohol (HCNOL), the
40 product of total hydrogenation, has practical applications, as well.
Therefore, it is a component in perfumes [6], while in adequate
chemicals mixture has bactericidal properties [7].
55
The use of monoꢀ and bimetallic catalysts based on nonꢀnoble
metals such as Cu, Ni and Co for selective hydrogenation
reactions, including the hydrogenation of α,βꢀunsaturated
aldehydes such as cinnamaldehyde, was also reported in several
papers, either in liquid or gas phase [19-25]. Such materials
60 presented interesting catalytic performances which depend on a
wide variety of factors such as the nature of active sites,
interaction with the support, reaction and catalyst activation
conditions, nature of the second metal and so on. For instance, a
CNOL selectivity of ~ 45 % was reported on CuꢀCo/SiO2
65 bimetallic catalyst [20]. It was shown that the rate of CNOL
formation is essentially increased on spinel Cu–Ni(Co)–Zn–Al
catalysts compared to Cu/SiO2 or binary Cu–Al samples due to
the formation of surface Cuo–M2+ dual sites (M2+ = Co, Ni, Zn).
Ni0/TiO2 also manifested high activity in the hydrogenation of
70 CNA with very high selectivity to HCNA (98 % at CNA
The most part of the catalysts used for the hydrogenation of
CNA to HCNA and CNOL are based on noble metals such as Rh
45 [8-9], Pd [10-12], Pt [13, 14], Au [15-17], Ru [18], etc. In the
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conversion of 66.2 % in isoꢀpropanol and under 2 MPa hydrogen
pressure [22] while Ni0/MCMꢀ41 converted 94.6 % CNA with a
selectivity to HCNA of 44.2 % [25].
constant temperature (30 °C) and pH (9.0).
The precipitates were recovered by filtration and then washed
60 with deionized water to remove Na+ ions. After washing, the
resulting solid was dried at 40 °C up to constant weight. Before
use, the samples were calcined at 450 °C for 5 h under static air
(heating ramp of 1.5 °C min–1) and then reduced for 10 h at 500
oC under H2 flow (1 L hꢀ1), with a heating ramp of 1.5 °C minꢀ1.
In this contribution we report an original process to obtain
highly active and chemoselective catalysts based on nonꢀnoble
metals, those process is easy to perform and not expensive.
Usually, the mixed oxides of reducible cations obtained via
5
layered double hydroxides (LDHs) are effective catalytic 65 CoAl sample was also reduced at 700 oC.
precursors because the LDHs are obtained as wellꢀorganized and
Catalyst characterization
10 compositional versatile crystalline materials. Indeed, interesting
catalytic performances were previously reported in the
hydrogenation of CNA over materials derived from asꢀ
synthesized hydrotalciteꢀlike materials with Ni, Cu or Co
incorporated in MgAl or ZnAl matrixes (e.g., 60 % HCNA at
15 total conversion of CNA on MgCuAl or 90 % CNOL at 9 %
conversion of CNA on ZnNiCoAl (Ni:Co = 8 : 2)) [26].
However, it was demonstrated that the acidꢀbasic sites associated
with the magnesium or zinc cations promote the secondary
reactions of aldol condensation of aldehydes, which negatively
20 influences the yield to the hydrogenation products [27]. To
circumvent these drawbacks, in the present investigation we
report for the first time the use of Mg(Zn)ꢀfree Niꢀ and Coꢀbased
catalysts derived from binary takovite (NiAl) and related CoAlꢀ
layered double hydroxides (M/Al ratio of 2/1) in the
25 chemoselective hydrogenation of cinnamaldehyde. As it is
known, takoviteꢀbased catalysts are able to activate various
reactions such as methanation of CO, FischerꢀTropsch reactions
and steam reforming of methane [28]. Interestingly, the first
patent about the application of layered double hydroxides for the
30 catalytic hydrogenations mainly claimed the use of reduced oxide
derived from takovite and related hydrotalciteꢀlike structures
[29]. In addition, these bicomponent catalysts allow the
preparation of a large amount of metals highly dispersed on
alumina, which is difficult to achieve for metal supported
35 catalysts obtained by conventional impregnation. Materials
derived from takovite and related materials are found to be
efficient hydrogenation catalysts, showing enhanced control over
the activity or selectivity, depending on the nature of active
metals and the thermal regime of the catalyst activation.
Chemical composition (Co, Ni and Al) of the samples was
performed on a Perkin sequential scanning inductively coupled
plasma optical emission spectrometer (Optima 2000 DV). Before
70 analysis, a known amount of sample was digested in a diluted
HCl solution and then heated under microwave until complete
dissolution. XRD measurements were performed on a Bruker
AXS D8 apparatus with monochromatic CuKα radiation (λ =
0.15406 nm wavelength) at room temperature. The patterns were
75 recorded from 5 to 70 ° 2θ, with a step of 0.02 °. The crystallite
sizes were calculated using the Scherrer equation: dhkl = λ/βcosθ,
where λ = incident ray wavelength (0.15406 nm); β = peak width
at half height (rad) after Warren’s correction for instrumental
broadening; and θ = Bragg angle. Nitrogen physisorption was
80 carried out at ꢀ196 °C on an Autosorb MPꢀ1 instrument from
Quantachrome. Surface areas and pore volumes were calculated
from the corresponding isotherms using conventional algorithms.
Diffuse reflectance UV–visible (DR UV–vis) spectra were
recorded on a Shimadzu UVꢀ2450 spectrometer equipped with an
85 integrating sphere unit (ISRꢀ2200). The spectra were collected in
the range between 190 – 800 nm by using BaSO4 as the
reflectance standard. In situ powder XRD patterns were recorded
on a Bruker D8 ADVANCE Xꢀray diffractometer equipped with
a VANTECꢀ1 detector, using a CuKα radiation (λ = 1.54184 Å)
90 as Xꢀray source. The powder samples were placed on a kanthal
filament (FeCrAl) cavity. The data were collected in the 2θ range
from 10 to 80° with a step of 0.05° (step time of 2s). Phase
identification was made by comparison with ICDD database. The
diffractograms were recorded after in situ reduction from 30 to
95 800 °C under a flow of 3 vol.% H2 in He (30 mL minꢀ1). The data
were collected 3 times at each temperature. TEM images were
obtained using a on a JEOL 2100 instrument (operated at 200 kV
with a LaB6 source and equipped with a Gatan Ultra scan
camera). Temperature programmed reduction experiments were
100 performed on an Autochem chemisorption analyser from
Micromeritics, equipped with a TCD and MS (Omnistar, Pfeiffer)
detectors. About 100 mg of asꢀsynthesized LDH were inserted in
a Uꢀshape microreactor. Before each TPR run, the catalyst was
activated at 120 °C for 4 h under a flow of simulated air (30 mL
105 ꢁminꢀ1). After cooling to 50 °C, the H2 containing flow was
stabilized (30 mL minꢀ1, 3 vol.% H2 in Ar) and the TPR was
performed from 50 to 800 °C with a temperature ramp of 2 °C
minꢀ1.
40 Experimental
Catalyst preparation
All chemicals were used as received: aluminium nitrate
(Al(NO3)3ꢁ9H2O, 98%, SigmaꢀAldrich), nickel nitrate
45 (Ni(NO3)2ꢁ6H2O, 98%, SigmaꢀAldrich) and cobalt nitrate
(Co(NO3)2ꢁ6H2O, 98%, SigmaꢀAldrich) as metal precursors,
sodium hydroxide (NaOH ) and sodium carbonate (Na2CO3
99.8%, Merck) as precipitating agents. For catalytic runs, the
chemicals were also used as purchased: transꢀcinnamaldehyde
50 (C6H5CH=CHCHO, 98%, Merck) as reagent and propylene
carbonate (C4H6O3, 99%, SigmaꢀAldrich) as solvent.
Two samples of layered double hydroxides, CoAl and NiAl
with a molar ratio 2:1, were prepared by coꢀprecipitation at low
supraꢀsaturation method. An aqueous solution containing the
55 corresponding metal sources and a solution containing Na2CO3
(0.8 M solution) and NaOH (1.6 M solution) were simultaneously
added at a flow rate of 0.7 ml.min–1 under vigorous stirring and
Hydrogenation of trans-cinnamaldehyde
110 The catalytic runs were carried out in a roundꢀbottom threeꢀneck
glass reactor equipped with a bubbler, magnetic stirrer and a
reflux condenser. The reaction was performed at atmospheric
pressure under the following conditions: 0.265 g of catalyst, 1.0
ml of transꢀcinnamaldehyde, 25.0 ml of propylene carbonate as
2
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solvent, hydrogen flow of 1.0 L hꢀ1, reaction temperature of 150
oC, and agitation speed of 900 rpm. Samples of reaction mixture
were periodically withdrawn and analyzed with
The crystalline structure of LDH precursors was investigated
by powder Xꢀray diffraction. As shown in Fig. 1, the diffraction
gasꢀ 35 patterns for the asꢀsynthesized LDH samples display the
a
chromatograph (HP 5890 equipped with a DBꢀ5 capillary column
(25 m × 0.20 mm × 0.33 ꢀm) and FID detector). The
identification of the products from the reaction mixture was
accomplished from the retention time of the pure compounds and
characteristic diffraction peaks of crystalline hydrotalciteꢀlike
LDHs, which are indexed in a hexagonal lattice with Rꢀ3m
rhombohedral symmetry [1]. It should be noted that for CoAl
sample the diffraction peaks are sharper and narrower suggesting
5
verified by GCꢀMS (Agilent 6890N system equipped with an 40 improved crystallinity (larger particle size) than for NiAl sample.
Agilent 5973 MSD detector and a 30ꢀm DBꢀ5ꢀms column). The
On the basis of the difractogramms, the structural parameters
(i.e., basal distances –d003, a and c parameters and the crystallite
sizes) were calculated and are provided in Table 1. As it can be
observed, d003 displays values of 7.82 and 7.75 Å for CoAl and
10 quantitative analyses were performed by taking into consideration
the FID response factor for each compound. The total conversion
of cinnamaldehyde and the selectivity towards different
hydrogenation products were calculated as previously reported 45 NiAl, respectively, which are higher than that of the MgAl
[30]. For the recyclability tests, the catalysts were recovered by
15 centrifugation from the end reaction mixture, washed with fresh
propylene carbonate and ethanol, and then dried under vacuum
before the reactivation.
hydrotalcite (7.65 Å) [32]. However, it should be taken into
account that the thickness of the brucite layer of hydrotalcite is
4.8 Å [32], the amount of water (data not shown here) is almost
the same for both samples while the counteranion is carbonate in
50 both cases. Hence, the increase of the d003 value for NiAl and
CoAl can be explained by the increase in the thickness of bruciteꢀ
like sheets due to the substitution of Al3+ by Ni2+ and Co2+ [31].
These structural results confirm the incorporation of the metallic
cations into the bruciteꢀlike layers of the samples.
Results and discussion
Catalysts characterization
20 The chemical compositions of LDH precursors were evaluated by
ICP and are presented in Table 1.
55 Table 1 Physical properties of the asꢀsynthesized LDHs
M/Ala
(molar ratio)
Ionic
radius
for M2+
(pm)b
74.5
c
f
g
d003
ad
ce
D(003) D(110)
Sample
(Å)
(Å) (Å) (nm) (nm)
Synthesis ICP
7.82
7.75
CoAl
NiAl
2
2
2.18
2.05
3.06 23.46 17.41 32.64
69.0
2.96 23.25 4.33
ec = 3d003
9.73
aM = Co, Ni; b[33];
;
da = 2d110
;
;
fD(003)
=
c
crystallite size in c direction; gD(110) = crystallite size in a direction.
Fig. 1 Powder X-ray diffraction for the as-synthesized LDHs.
25
As noted, the Co/Al and Ni/Al ratios in the final solids are
slightly different from those of the synthesis mixture (2.18 and
2.05 for Co/Al and Ni/Al, respectively vs 2.0). These small
variations of the final M2+/Al ratios are usual and are due to the
preferential precipitation of one cation to another [31]. According
30 to these values, the complete incorporation of aluminium at pH =
9 did not occur during the synthesis, especially for the Co/Al
sample.
Fig. 2 DR UV-Vis spectra for the as-synthesized LDHs
Information on the oxidation state as well as the coordination
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of the cations into the bruciteꢀlike sheets was provided by DR
600 nm is observed and it may be attributed to d–d transitions in
UVꢀvis spectrometry. The recorded spectra are displayed in Fig. 15 low spin of octahedrally coordinated Co3+ cations [39].
2. Because Al3+ has d0 configuration, it does not show absorption
bands in the UVꢀvis spectra. Therefore, the spectra show the
bands mainly associated to the d – d transitions of Ni2+ (d8) and
Co2+ (d7) cations. As illustrated in Fig. 2, a characteristic band
Nevertheless, this band is much less intense in comparison to the
band at ~ 520 nm, indicating that a minor amount of cobalt exists
as Co3+.
5
For NiAl sample, two double important bands are observed.
around 205 and 250 nm for NiAl and CoAl, respectively due to 20 The first one is located at ~ 375 and 420 nm and the second one
3
the O2ꢀ → Mn+ charge transfer transitions was identified in each
corresponding spectrum [34-37].
The incorporation of cobalt in CoAl sample resulted in the
formation of a three component absorption band positioned at ~
at ~ 630 and 730 nm. Two spinꢀallowed transitions A2g(F) →
3T1g(F) and A2g(F) → T1g(P) were assigned to the bands at 375
and 730 nm, respectively, while the bands at 420 and 630 nm
were assigned to spin forbidden transitions 3A2g(F) → 1Eg(D) and
3
3
10
450, 490 and 520 nm which was ascribed to Co2+ in octahedral 25 A2g(F) → T2g(D) [28]. Both bands correspond to Ni2+ in
3
1
coordination [38]. Additionally, an absorption band centered at ~
octahedral coordination.
Fig. 3 N2 adsorption-desorption isotherms for the as-synthesized (A) and calcined-mixed oxides (B) samples
CoAlꢀMO) samples the isotherms illustrated in Fig. 3B are of
Table 2 Textural properties of asꢀsynthesized and calcined LDHs
45 type IV with larger hysteresis loops, especially for NiAl sample.
This indicates improved mesoporosity, which is a positive feature
for catalysis as the diffusional limitations are restricted. As a
result, the surface areas of MOꢀmaterials are almost three times
the surface areas calculated for the asꢀsynthesized materials.
50 These increases might also indicate that the formation of highly
stable spinels was avoided, which typically occurs for LDHs
containing transition metals cations in the layer and carbonate as
counteranion [46].
SBET
Vpore
Sample
(m2.gꢀ1)
(cm3.gꢀ1)
CoAl
NiAl
CoAlꢀMOa
NiAlꢀMOa
68
69
0.44
0.49
0.93
1.27
219
229
30 a MO ꢀ mixed oxides
The textural properties of the samples were evaluated from
the nitrogen adsorptionꢀdesorption isotherms (Fig. 3A). The
isotherms are of type IIb [45, 46] and originate from the
35 adsorption of nitrogen in the void spaces between the plateꢀlike
particles. The shape of the hysteresis loops (type H3) indicates a
large distribution of the slitꢀshaped mesopores for both samples.
The textural properties, i.e. the specific surface areas and pore
volumes are summarized in Table 2. The obtained values are in
40 agreement with those already reported for these kinds of
materials [31, 32]. As expected, the calcination of these solids at
450 °C gave rise to changes in the texture of solids. Indeed, for
the calcined CoAl (denoted as CoAlꢀMO) and NiAl (denoted as
The reducibility and extent of metalꢀalumina interactions for
55 the calcined LDHs precursors were investigated by TPR (Fig. 4).
The TPR profile for CoAlꢀMO sample displays two main maxima
of reduction. Based on inꢀsitu XRD results (see below), the first
peak, centered at ~ 350 °C, was assigned to the reduction of
Co3O4 spinel to CoO while the second one, centered at ~ 700 °C,
60 was attributed to the reduction of CoO to Co0. Nevertheless, the
reduction of Co2+ in amorphous CoAl2O4 cannot be totally ruled
out at 700 °C [40, 41], although the characteristic peaks of the
spinel were not detected by in situ XRD (see below). The TPR
profile of NiAlꢀMO sample shows one minor and broad peak
4
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between 200 and 400 °C and a major peak at 400 ꢀ 800 °C with 25 49] phases, respectively. The Co3O4 spinel is detected in the XRD
maximum at ~ 630 °C and a shoulder at ~ 560 °C. The first peak
is attributed to the reduction of Ni2+ in a minor NiO phase with no
interaction with the support [42]. The reduction temperatures at ~
560 °C and ~ 630 °C are assigned to the reduction of Ni2+ in NiO
patterns of the CoAlꢀMO up to 300 °C. After reduction at 400 °C,
the typical diffraction lines of the Coꢀspinel disappeared while
new diffraction peaks appeared at 2θ = 36.47, 42.37 and 61.47°.
These peaks are unambiguously assigned to the diffraction from
5
in strong interaction with alumina and/or NiAl2O4 [41, 43, 44], 30 the (111), (200) and (220) planes of CoO crystal lattice (ICDD
also noticing that the characteristic peaks of the spinel were not
detected by in situ XRD (see below). Because the TCD signal
baseline is recovered after the reduction of nickel and cobalt
48ꢀ1719) [50] (Fig. 5d). As observed from Fig. 5A, the
intermediate CoO phases are not reducible up to a reduction
temperature of 700 °C, providing clear evidence that the peak at
o
10 cations, it can be affirmed that (i) there is good accessibility of H2
700 C in the TPR profile for CoAlꢀMO (Fig. 4) is due to the
to completely reduce nickel and cobalt cations and (ii) metallic 35 reduction of CoO to metallic cobalt. Therefore, it is obvious that
Ni0 and Co0 are in some extent supported on alumina and
accessible to gas phase.
the calcination of CoAl generated mainly Co3O4 whose reduction
under H2 takes place in two steps. However, this high reduction
temperature (i.e., 700 °C) suggests a strong interaction between
cobalt and alumina. Additionally, XRD showed that the alumina
40 generated by the calcination of the CoAl sample (and NiAl
sample) is amorphous [51]. As noted, the crystalline CoAl2O4
spinel is still not observed in the diffractograms recorded at 700
°C reinforcing the idea of the amorphous state of alumina in the
sample. The strong interaction between cobalt and alumina can
45 also explain the absence of the typical diffraction peaks of the
metallic Co0 in the diffractograms after further reduction at 800
°C, which indicates that the generated nanoparticles are stable
against sintering. For NiAlꢀMO sample, traces of NiO can be still
observed in the XRD patterns after reduction at 600 °C (Fig. 5d),
50 which is in accord with TPR showing that the complete reduction
of NiO takes place at ~ 750 °C. After reduction at 500 °C, new
small diffraction lines attributed to metallic Ni0 phase (ICDD 04–
0850) [52] appeared in the diffractograms. . Thus, the most
intense peak (2θ = 44.5 °) is attributed to (111) plane of Ni0
55 overlapping with the diffraction peak of the kanthal sample
holder. The other two broad peaks observed at ~ 52 and ~ 76 °
and associated to the (200) and (220) planes [52], respectively,
are well defined but they are too small to accurately calculate the
average crystallite size of Ni0 nanoparticles. The reduction at 600
60 °C generated new Ni0 phases and the diffraction peaks are more
intense. The calculated average crystallite size of the resulting Ni0
nanoparticles is 3.6 nm according to Scherrer’s equation. Further
increase in the reduction temperature to 700 and 800 °C results in
the formation of slightly larger Ni0 nanoparticles with average
65 crystallite sizes are of 5.5 and 5.8 nm, respectively. In similarity
with the CoAl sample, the results obtained for NiAl suggest a
limited sintering of nickel nanoparticles in the highꢀtemperature
range of 600ꢀ800 °C because of the strong interaction between
nickel and alumina [52].
15
Fig.4 TPR profiles of the calcined LDHs.
In order to elucidate the nature of intermediate crystalline
phases formed during the reduction process, the calcined CoAlꢀ
MO and NiAlꢀMO samples were reduced under hydrogen flow at
different temperatures in the range of 30 – 800 °C and analyzed
20 in situ by XRD. The recorded diffractograms are displayed in Fig.
5a (for CoAlꢀMO) and Fig. 5b (for NiAlꢀMO). The XRD patterns
registered at 30 °C for NiAlꢀMO and CoAlꢀMO (enlarged in Fig.
5c) show diffraction lines that fit the characteristic patterns for
NiO (ICDD 047ꢀ1049) [47] and Co3O4 (ICDD 42ꢀ1467) [43, 48,
70
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Fig.5 In situ XRD after reduction at different temperatures for the calcined samples: A) CoAl-MO; B) NiAl-MO; C) XRD patterns registered for both
samples after thermal treatment at 30 °C; D) XRD patterns registered for both samples after thermal treatment at 600 °C.
5
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The morphostructural properties of the NiAl and CoAl catalysts
after reduction under hydrogen were investigated by TEM
analysis. Representative images are displayed in Fig. 6.
double bonds in the side chain that makes the hydrogenation
reaction to follow two possible ways: (i) the hydrogenation of
C=C bond with the formation of hydrocinnamaldehyde (HCNA)
and (ii) the hydrogenation of C=O bond when cinnamyl alcohol
45 (CNOL) is formed (Scheme 1). The chemoselective
hydrogenation towards the unsaturated alcohol is difficult to
achieve since the thermodynamics favors the hydrogenation of
the C=C over the C=O by ~35 kJ/mol and for kinetic reasons the
reactivity of the C=C group is higher than C=O group in the
50 hydrogenation reactions [54].
Depending on the catalyst composition, different catalytic
behaviors can be noted (Fig. 7). Therefore, NiAl sample exhibited
very high catalytic activity while CoAlꢀMO sample was
moderately active. These results clearly show that a reduction
55 temperature of 500 °C was sufficient to generate very active
metallic sites in the case of NiAl catalyst (complete conversion of
CNA after 90 min of reaction), albeit the TPR results suggested
that the nickel cations are not fully reduced to metallic nickel
under these conditions. However, since the catalyst used in the
60 catalytic runs were kept much longer under hydrogen at this
reduction temperature, the degree of reduction is likely to be
more elevated. For CoAl sample, the generated metallic sites
converted ~ 50 % of CNA after 300 min of reaction. Moreover, it
can be observed that the main product is HCNA on NiAl sample,
65 while CNOL is the main reaction product on CoAl sample (Fig
7). Also, the catalytic curves displayed in Fig. 7 show that in the
case of NiAl sample, a selectivity of ~ 95 % HCNA can be
achieved. The distribution of products is more equilibrated on
CoAl sample, with the highest selectivity obtained for CNOL at ~
70 60 % whatever the CNA conversion.
5
Fig. 6 TEM images for NiAl (a) and CoAl (b) after reduction at 800 °C.
As a first remark, traces of typical morphology with
hexagonal plateꢀshaped particles are observed for NiAl sample,
while for CoAl the bruciteꢀlike sheets are no longer maintained.
This difference suggests higher stability of the bruciteꢀlike sheets
10 when they contain nickel besides aluminium. Likewise, a
monodisperse distribution of the metal particles with a size of 5ꢀ6
nm is observed for NiAl sample (Fig.6A). For CoAl sample, very
small metal particles are observed in Fig. 6B, those sizes are
difficult to evaluate. These results are in good agreement with
15 XRD data which indicated nickel crystallite size of ~ 6 nm for
NiAl sample and highly dispersed cobalt metal particles in the
case of CoAl sample.
Hydrogenation of the cinnamaldehyde
The catalytic properties of calcined NiAl and CoAl samples after
20 reduction at 500 °C were evaluated for the liquid phase
It is well documented that the catalytic performance of a solid
in the hydrogenation of CNA is influenced by several factors,
between them the particle sizes and the electronic structure of the
metallic active sites [1], both being considered to explain the
75 catalytic results reported herein. As it was shown by inꢀsitu XRD,
larger Ni0 nanoparticles are supposed to form in comparison with
Co0 nanoparticles at similar reduction temperature. Although the
calculation of the particles sizes after reduction at 500 °C was not
feasible, the difference in size was suggested by the presence of
80 the weak diffraction peaks of Ni0 and absence of those for Co0.
The electronic structure of the metal is taken into consideration
because it defines the interaction between the metal surface and
the reactant molecule. Previous studies stated that nickel has
similar behavior as Pd and Rh meaning that it favors the
85 hydrogenation of C=C bond while cobalt is included in the same
group with Pt and Ru having a moderate selectivity to CNOL
[55].
OH
cinnamylacohol
(CNOL)
O
OH
H
O
hydrocinnamylalcohol
H
cinnamaldehyde
(CNA)
(CNOL)
hydrocinnamaldehyde
(HCNA)
hydrogenation of cinnamaldehyde (CNA).
Scheme Reaction pathways for hydrogenation of
cinnamaldehyde
1
25
It is worth remembering that (i) these two catalysts contain
alumina that bring some acidity in the reaction medium and (ii)
depending on the nature of the active sites (redox and/or acidꢀ
base sites), the reaction scheme of the conversion of
cinnamaldehyde can be very complex [53]. Despite the presence
From theoretical considerations [56] and studies of the
reaction mechanism [53], the formation of CNOL should be
90 favored by enhancing the electron density of the metal that will
determine the repulsive four electron interaction of the metal with
the C=C bond to decrease. The electron density can be correlated
with the metal dꢀband width. Indeed, cobalt with larger dꢀband
width than nickel (4.0 vs 3.0 eV) [57] is more selective toward
95 the unsaturated alcohol. However, this factor cannot be
independently discussed because the electron density depends
also on the particle size. Thus, the electron density of the dꢀ
orbitals declines with the decrease of the particle size with direct
30 of alumina, the reaction products identified by GCꢀMS are mainly
the result of the hydrogenation reaction. More precisely, in the
case of NiAlꢀMO, condensation products below 2 mol % were
identified whereas no traces of condensation or any other
additional products were observed in the case of CoAlꢀMO
35 sample. Consequently, the results of the hydrogenation reaction
are exclusively discussed herein. The evolution of CNA
conversion with the reaction time as well as the selectivities to
the hydrogenation products are comparatively shown for both
catalysts in Fig. 7.
40
As mentioned in the introduction, CNA molecule has two
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consequence on the selectivity to CNOL. Nitta et al. [58] found
for supported Co nanoparticles prepared from cobalt chrysotile
and cobalt precipitated on silica, that the selectivity to CNOL
increases with the particle size, up to ~ 95 mol % for cobalt
5
nanoparticle of ~ 24 nm in size. Similar sizeꢀdependent effects
were observed in the case of platinum supported on carbon and
graphite [55].
Fig. 7 CNA conversion vs time and selectivity to reactions products for NiAl (A) and CoAl (B) samples (Test conditions: Treduction = 500 oC; Treaction = 150 °C,
10
0.265 g catalyst; 1 mL of CNA, 25 mL propylene carbonate as solvent, H2 flow = 1 L h-1, stirring rate = 900 rpm).
same effect on the catalytic behavior as silica. Thus, copper
For the catalytic results presented herein, the size of the
cobalt nanoparticles lower than the detection limit of the XRD (<
3 nm) should be taken into consideration to explain the moderate
supported on alumina and silica by impregnation showed similar
selectivity to CNOL (51ꢀ73 and 48ꢀ75 % for Cu/Al2O3 and
Cu/SiO2, respectively at 15ꢀ30 % CNA conversion; the reaction
15 selectivity to CNOL (~ 60%) and the low selectivity to HCNA (~ 45 was performed at 1 MPa and isoꢀpropanol as solvent). Lashdaf et
30 %). It can be assumed that that the adsorption of both double
bonds is allowed on the small particles of cobalt but the
adsorption of C=O bond is predominant. Indeed, Fig. 7 right
shows that hydrogenations of C=C and C=O bonds are parallel
al. [61] showed that platinum catalysts supported on silica by gasꢀ
phase deposition and impregnation are more active in the
hydrogenation of CNA that those on alumina, prepared by the
same routes (the reaction was performed at 10 bar and isoꢀ
20 reactions, even when the total hydrogenation to HCNOL becomes 50 propanol as solvent). In a study on aluminaꢀsupported platinum
obvious (i.e., at isoꢀconversion of ~ 30 %).
catalysts, Arai et al. [62] found that the catalysts, reduced with
sodium tetrahydroborate at 30 °C, display high selectivity to
cinnamyl alcohol (78 % at 50 % CNA conversion) in comparison
with the catalysts reduced with hydrogen at 400 °C, that did not
On the other side, the adsorption of CNA on NiAl sample
predominantly occurs via C=C bond, as also observed for highly
dispersed nickel nanoparticles supported on SBAꢀ15 [59]. In that
25 case, the reduction of Ni/SBAꢀ15 at 350 °C generated a catalyst 55 display selectivity to cinnamyl alcohol. Szollozi et al. [63]
that totally converted CNA in 300 min of reaction with a
selectivity to HCNA of 94 mol % on the whole range of
conversion. It can be concluded that the very high activity of
NiAl sample doubled by the outstanding selectivity to HCNA
revealed that the activity and the selectivity to cinnamyl alcohol
ꢀ1
was lower with platinum on alumina (e.g., 32 x 103 mol.gꢀ1
h
Pt
and 47 % CNOL) than platinum on silica (e.g., 72 x 103 mol.gꢀ
1
ꢀ1
h
and 79 % CNOL), when the catalysts are submitted to
Pt
30 endorse this sample as a highꢀperformance catalyst in the 60 sonication under hydrogen for 10 min previous the reaction. The
hydrogenation of C=C double bond of cinnamaldehyde.
next example is more suitable to our situation due to the similar
reaction conditions. In this case, the comparison is made between
copper supported on alumina by impregnation and CuꢀAl
mesoporous material prepared by direct synthesis [64]. The
Due to the presence of the alumina in the catalyst
composition, it is interesting to observe if there is any influence
of it on the catalytic behavior of the samples. On the basis of the
35 literature reports, the influence of the alumina on the catalytic 65 authors found that the Cuꢀimpregnated alumina exhibited
performance of NiAl and CoAl samples is in some extent
debatable because, depending on the nature and amount of the
metallic active sites, the preparation method as well as the
reaction conditions, different behaviors were noted. For instance,
40 Volpe et al. [59] reported that alumina support manifests the
selectivity to CNOL of 30 % at 35 % CNA conversion while for
CuꢀAl mesoporous catalysts, a selectivity to CNOL of 55 % at the
same isoꢀconversion was noted. On the basis of these studies, a
beneficial effect of reduction from cation incorporated into the
8
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alumina lattice to generate strong metal interaction with residual
alumina support to give improved selectivities can be pointed out.
Fig. 8 CNA conversion vs time and selectivity to reactions products for CoAl samples after reduction at 500 and 700 °C (Test conditions: Treaction = 150 °C,
5
0.265 g catalyst; 1 ml of CNA, 25 mL propylene carbonate as solvent, H2 flow = 1 L h-1, stirring rate = 900 rpm).
According to TPR and inꢀsitu XRD after temperatureꢀ
programmed reduction, the total reduction of cobalt take place at
700 °C, reason why the hydrogenation of cinnamaldehyde on
CoAl sample was also carried out after reduction at 700 °C. The
10 conversion curve and selectivity to CNOL are displayed together
with those recorded after reduction at 500 °C in Fig. 8. As
observed, the activity is twice the activity obtained after reduction
at 500 °C suggesting the generation of a higher number of active
sites by reduction at higher temperatures (i.e., 700 °C). Also, the
15 selectivity to CNOL increased with ~ 15 % while the selectivity
to HCNA decreased suggesting that the adsorption of the
carbonyl bond is further favored over the olefin bond. This
improvement in the chemoselectivity can be related to the
modification of adsorption site density with metal particle size. It
20 is assumed that upon reduction of the sample at 700 °C the
particle size slightly increased as compared to the sample reduced
o
at 500 C because of particle sintering. Indeed, previous studies
demonstrated that as compared with small particles, large cobalt
particles are more favourable to the CNA adsorption via C=O
25 bond due to a higher proportion of dense Co (111) faces [58, 65].
It is also interesting to note that the high CNOL selectivity
obtained over CoAl is maintained up to high CNA conversions
(Fig. 8, i.e., ~60% at 90% conversion), suggesting that during the
reaction the adsorption of unsaturated alcohol on the catalyst
30 surface is weaker than the adsorption of saturated or unsaturated
aldehyde.
40 Fig. 9 Recycling tests for NiAl and CoAl samples
.
As a conclusion, this study shows the interest to use binary
precursors of NiAl and CoAl LDHs to prepare metallic catalysts
with controlled and stable catalytic performances in terms of both
activity and chemoselectivity.
For a catalyst, the stability during several catalytic tests is very
important for the economy of the process. Therefore, several
catalytic runs with recycled catalysts were performed and the
35 catalytic activities were evaluated (Fig. 9). It can be observed that
the catalytic performances of both catalysts are not essentially
changed over the three catalytic runs demonstrating their good
stability and reusability.
45 Conclusions
Highly dispersed and thermostable nickel and cobalt
nanoparticles showing high performance in cinnamaldehyde
hydrogenation were prepared from NiAl takoviteꢀlike LDH
precursors and related materials. The lamellar structure of the
50 LDH precursors was confirmed by XRD while DR UVꢀvis
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30 Finally, it can be concluded that effective and low cost
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Acknowledgment
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35 This work was partially supported by a grant of the Romanian
National Authority for Scientific Research, CNCS – UEFISCDI,
project number PNꢀIIꢀRUꢀTEꢀ2012ꢀ3ꢀ0403. C.R. acknowledges
the project CUANTUMDOC “Doctoral Studies for European
Performances in Research and Innovation” ID79407 funded by
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