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CONTROLLING THE ACTIVITY AND CHEMOSELECTIVITY IN THE

CINNAMALDEHYDE HYDROGENATION BY INSERTION OF NON-

NOBLE METALS IN THE MATRIX OF HYDROALCITE-LIKE

MATERIALS

Alexandru Chirieac, Brindusa Dragoi, Adrian Ungureanu, Alina Moscu Corcodel, Constantin

Rudolf, Alexandra Sasu, Emil Dumitriu1

Department of Organic and Biochemical Engineering, Faculty of Chemical Engineering and

Environmental Protection, “Gheorghe Asachi” Technical University of Iasi,

73 Prof.dr.docent Dimitrie Mangeron Street,700050-Iasi, Romania,

Abstract

The purpose of this study was to prepare new nanocomposite catalysts of Metal/LDHs type

by mild reduction of transition metals Co2+ and Cu2+ cations substituted in the matrix of ZnAl-LDH

at 150°C and atmospheric pressure, under hydrogen flow. To this end, a series of ZnCuCoAl

layered double hydroxides (LDH) precursors were synthesized by co-precipitation method. M2+/M3+

(i.e., Zn2+/Al3+) molar ratios were kept constant while the Cu2+/Co2+ molar ratios were varied in

order to investigate their influence on the structural and textural properties of the resulted catalysts.

The LDH precursors were systematically characterized by different techniques such as powder

XRD, nitrogen physisorption, FT-IR and DR UV-Vis spectroscopy as well as temperature

programmed reduction (TPR). The characterization results indicate that all the precursors present

well-crystallized layered structures and a high degree of substitution of cobalt and copper cations in

the ZnAl matrix. Upon the mild reduction treatment the layered structure of precursors was

maintained, whereas a certain part of transition cations was reduced to metallic centers acting as

highly dispersed catalytic active sites. The effect of chemical composition of the ZnCuCoAl LDHs

on the catalytic properties in the selective hydrogenation of cinnamaldehyde was studied in propylene carbonate at 150°C and atmospheric pressure. It was observed that catalytic activity

gradually increases with the Cu content of LDH, whereas the selectivity to unsaturated alcohol does

not depend on the chemical composition. A substantial high chemoselectivity of LDHs can be

however attained by replacing Cu with Ni in ZnCoAl LDH precursors at the expense of decreased

activity..

Keywords: cinnamaldehyde hydrogenation, layered double hydroxides, Co-based catalysts, Cu-

based catalysts1 Author to whom all correspondence should be addressed: E-mail: [email protected]; Phone: +40 232278683; Fax:+40 232271311

mailto:[email protected]






1. Introduction

The intensive demand for new environmentally friendly materials and technologies led to

the development of the catalysis field which now provides sustainable routes to convert efficientlyand economically the raw materials into valuable chemicals, fuels and other products. The synthesis

of fine chemicals is an area of great interest where catalysis should be more deeply involved since

this industry is under great pressure to develop new efficient processes with low energy and raw

materials consumption and minimal wastes. Nonexpensive catalysts based on transitional metals are

intensively studied nowadays to meet these requirements.

The selective hydrogenation of α, β-unsaturated aldehydes such as cinnamaldehyde is of

great importance in fine chemicals industry especially in flavoring, perfume and pharmaceutical(Castelijins et al. 1998, Tessonnier et al., 2005; Gallezot et al., 1998; Coq et al., 1998; Ponec, 1997)

because it leads to the formation of useful products (i.e., cinnamylalcohol (CNOL) and

hydrocinnamaldehyde (HCNA)). However, the hydrogenation of C=C bonds is favored from

thermodynamic and kinetic considerations. Therefore the hydrogenation of C=O bond to obtain

CNOL is a continuous and challenging scientific task since it requires chemoselective catalysts.

High selectivity to CNOL was obtained with Ir- and Os-based catalysts (Mäki-Arvela et al., 2005),

while the noble metals from Pt group display selectivity to CNOL when promoters are used.

However, the high cost and low availability of these catalysts cause a serious problem, and

therefore enhanced research efforts are made nowadays to develop active, chemoselective and

inexpensive catalysts which are based on non-noble metals.

In this regard, the synthesis of nanocomposite materials affords new opportunities on the

design of more active and selective catalysts. This study is focused on the synthesis of some

Me/LDH nanocomposites (Me = Co, Cu) obtained from layered double hydroxide (LDH)

precursors with applications in the liquid phase hydrogenation of the cinnamaldehyde. The general

formula for the LDHs is [M1− x2+M x

3+(OH)2] x+(An−) x/n·mH2O, where M2+ and M3+ are di- and

trivalent cations respectively, including Mg2+, Zn2+, Ni2+, Co2+, or Cu2+ and Al3+, Ga3+, Cr 3+ or Fe3+,

respectively; x is equal to the molar ratio M2+/(M2++M3+), and An− can be simple or complex anions.

Generally, the structure of LDHs consists of positively charged layers which are stacked on top of

each other (Fig. 1.a). These positive charges are compensated by anions which lie in the interlayer

region between two sheets. In the space between layers are also accommodated molecules of

crystallization water.



2.1. Preparation of LDHs

Layered double hydroxides synthesis containing Zn, Cu, Co and Al were prepared using the

low supersaturation co-precipitation method at room temperature. Two solutions, first containing

metal nitrates ([Zn2+

] + [Cu2+

] + [Co2+

] + [Al3+

] = 1.0 M) and a second alkaline solution (NaOH + Na2CO3 = 1.1 M) were simultaneously added in a three-necked round-bottom flask fitted with a pH

electrode and containing a small amount of deionized water. The suspension was vigorously stirred

and maintained at the desired pH (~ 8) by adjusting the relative flow rates of the two solutions. The

final suspension was aged under stirring at room temperature for 12 h. The precipitate was filtered

off, and then washed two times with carbonate solution in order to remove nitrate ions from the

interlayers, and finally repeatedly washed with sufficient deionized water to remove residual

sodium. The resulting precipitates were dried at 40 °C for 24 h.The solids are labeled according to the ratio between cooper and cobalt as ZCuCoA XY in

which X and Y represents the molar ratio of the substituted elements (Cu and Co) in the ZnAl

matrix.

2.2. Characterization of the catalysts

Powder X-ray diffraction (XRD) patterns were recorded on a SEIFERT diffractometer using

monochromatized Cu Kα radiation (λ = 1.542 Å). Usually, the diffraction data were collected using

continuous scan mode with a scan speed of 2 °·min-1 over the scan range 2θ = 2 – 70 °.

N2 adsorption and desorption isotherms were obtained at 77 K on Autosorb 1-MP

(Quantachrome) instrument. Prior to the measurements, samples (ca. 80 – 100 mg) were outgassed

under high vacuum at 150 °C for 3h. The volume of adsorbed N 2 was normalized to standard

temperature and pressure.

FT-IR spectra were recorded on a Perkin-Elmer FT1730 instrument, using KBr pellets; 100

spectra (recorded with a nominal resolution of 4 cm−1) were averaged to improve the signal-to-noise

ratio.

Diffuse reflectance UV – visible (DR UV – vis) spectra were recorded on a Shimadzu UV-

2450 spectrometer equipped with integrating sphere unit (ISR-2200). The spectra were collected at

190 – 800 nm using BaSO4 as reference.

Temperature-programmed reduction (TPR) measurements were carried with a TPDRO 1100

instrument. The reducing agent was H2/Ar (5% vol). The gas flow (50 ml·min−1 ), sample weight

(15 – 20 mg) and heating rate (10 °C·min−1) were chosen according to preliminary tests aiming to



optimize resolution of the curves. A 13X molecular sieve trap was used to remove water.

Calibration of the instrument was carried out with CuO (Merck).

2.3. Catalytic activity

For each test, the non-calcined catalyst was first reduced at 150 °C for 2 h (heating rate of 6

°C·min – 1) under hydrogen flow (1 L.h – 1) and then transferred to the reaction system. The catalytic

hydrogenation of trans-cinnamaldehyde test was carried out at atmospheric pressure in a three neck

glass reactor equipped with reflux condenser and magnetic stirrer (900 rpm) under the following

conditions: 1 ml of reagent (7.94 × 10 – 3 mol), 25 ml of propylene carbonate as solvent, and 0.265 g

of catalyst, hydrogen flow 1 L.h – 1, and constant reaction temperature of 150 °C.

Samples of the reaction mixture were withdrawn periodically and the identification andquantification of reaction products were performed with GC (HP 5890 gas chromatograph, FID

detector, DB-5 column) and were based on the retention times and suitable response factors,

respectively. In order to confirm the identity of the products, GC-MS analyses were performed with

an Agilent 6890N system equipped with an Agilent 5973 MSD detector and a DB-5-ms column.

3. Results and Discussion

3.1. Chemical and physical properties

There are few reports about synthesis and properties of multicomponent LDHs which

contain Co in their structure; it could be mentioned some ternay LDHs: CoZnAl (Benito et al,.

2009), CoNiAl (Rives et al., 2003), quaternary LDHs: MgCoNiAl (Tichit et al., 2001), CoCuZnAl

(Velu et al, 2001) and even with five elements CuZnCoAlCr (Porta et al., 1995). However, to the

best of our knowledge, there are no previous reports about Me/LDHs nanocomposites obtained bymild reduction from layered double hydroxides (LDHs) precursors containing Co2+ and Cu2+

transition metal cations substituted in the matrix of ZnAl-LDH.

The characterization of the LDH solids was performed to study the obtained crystalline phases,

phase purity, texture as well as to prove the substitution of transitional metals in the brucite-like

layers. To this aim, the as-synthesized solids were first characterized from crystallographic point of

view. Figure 2 shows the XRD patterns of the LDH precursors with constant Zn/Al ratios, constant

M2+/M3+ ratios, but with variable Cu2+/Co2+ ratios.



Figure 2. XRD patterns for the as-made ZnCuCoAl LDHs obtained by co-precipitation

These diffractograms indicate that pure layered double hydroxides were synthesized for all

compositions. The shape and symmetry of XRD peaks are characteristic of well-crystallized

materials. The XRD patterns show sharp and symmetrical reflections at low 2Θ angles (planes

(003), (006), (110) and (113)) and broad and asymmetric peaks at high 2Θ angles (planes (012),

(015) and (018)). According to literature, these peaks are characteristic for crystalline phases of

hydrotalcite-like LDHs (Cavani et al., 1991). The absence of other crystalline impurity phases (e.g.,

malachite Cu2CO3(OH)2) suggests that ions of Co2+ and Cu2+ are isomorphous substituted in the

brucite-like layers. It can be also observed in Figure 2 that ZCuCoA 10 sample (the Co-free sample)

exhibits a high crystallinity, which gradually decreases with the increase in the content of Co, in

well agreement with Velu et al., 2001. From the XRD diffractograms, the lattice parameters ”a” and

“c“ as well as the crystallite sizes were calculated and their values are included Table 1. As it can be

observed, with the increase in the copper content the value of parameter ”a” increases, in agreement

with the larger ionic radius of Cu2+ with respect to Co2+ in octahedral environments, while the

value of parameter ”c” decreases. The molar fractions of Zn2+ and Al3+ are almost constant in all

samples, indicating the compliance of Vegard’s rule. It is worthy of note that upon the mild

reduction treatment, the XRD results (not show) show that the hydrotalcite-like structures of LDH

precursors were maintained.



Figure 3. Nitrogen adsorption-desorption isotherms for as-made ZnCuCoAl LDHs

obtained by co-precipitation

The textural properties such as BET surface area, the average pores diameter and pore

volume for the investigated LDH precursors are also shown in Table 1 and are in agreement withthose reported in the literature (Benito et al., 2006). ZnCuCoAl – LDH exhibit characteristic type IV

isotherms (Figure 3), which are typical for mesoporous materials, with high BET surface) and

average pore diameter higher than 300 Å for all samples. The hysteresis loop is of H3 type, which is

usually associated with the condensation-evaporation phenomena between aggregates of platelet

particles giving rise to slit-shaped pores.

Table 1. Textural and structural characteristics for the samples ZnCuCoAl series obtained by co-precipitation

Sample Molar ratio Crystalline phase a c D(003) D(110) S BET

(m2.g -1 )

V t

(cm3.g -1 )

APS

Zn Cu Co Al (Å) (Å) (nm) (nm) (Å)

ZCuCoA 10 1.0 1.0 0.0 1.0 LDH 3.076 22.59 19.9 13.8 77 0.72 373

ZCuCoA 82 1.0 0.8 0.2 1.0 LDH 3.072 22.70 17.5 14.5 96 0.83 349

ZCuCoA 55 1.0 0.5 0.5 1.0 LDH 3.072 22.68 14.5 20.3 102 0.90 353

ZCuCoA 28 1.0 0.2 0.8 1.0 LDH 3.071 22.68 15.1 22.5 102 0.79 310

ZCuCoA 01 1.0 0.0 1.0 1.0 LDH 3.065 22.86 16.1 28.6 93 0.72 311

The characterization of as-synthesized materials was further accomplished by FTIR

spectroscopy to analyze the substitution of the two cations in the ZnAl matrix. The corresponding



FTIR spectra are shown in Figure 4. Beside the three groups of absorption bands at 1630, 1480 and

1357 cm-1 discussed elsewhere (Dragoi et al., 2010a), the spectra show absorption bands below

1000 cm-1 which indicates the formation of crystalline networks containing transitional metals.

Generally, these bands are associated with the stretching and deformation modes in M-O, M-O-M’

and O-M-O specific groups in the brucite-like layers, together with other modes of the carbonateanion (Chang et al., 2005; Rives et al., 2003; Holgado et al., 2001).

Figure 4. FT-IR spectra for as-made ZnCuCoAl LDHs obtained by co-precipitation

Figure 5 presents the DR UV-Vis spectra for the layered double hydroxides of the

ZnCuCoAl series. Because Zn2+ and Al3+ have d 10 and d 0 electronic configurations, they do not

show absorption bands in the UV-Vis spectra. Therefore, the spectra from Figure 3 show bands

associated with d-d transitions of Cu2+ and Co2+ cations in octahedral coordination in the brucite-

like layer. The position of the absorption bands depends on the nature of cations while their

intensity depends on their concentrations. As consequence, the absorption band at 528 nm is more

intense as the content of substituted Co2+ increases. As cobalt is gradually replaced by copper, the

intensity of this band decreases until its disappearance along with an increase in the intensity of a

band centered at ~750 nm. For the Co-free sample (ZCuCoA 10) only a band at ~750 nm was



observed, which is attributed to the Cu2+ cations in an octahedral coordination and situated on the

edges of the brucite like-layers.

Figure 5. DR UV-Vis spectra for as-made ZnCuCoAl LDHs obtained by co-precipitation

To confirm that the band at ~750 nm is indeed characteristic of Cu2+ cations, selected

samples were reduced at 150°C under hydrogen flow and their corresponding spectra were recorded

(Figure 5). It can be observed that this band completely disappeared and a new band become visible

at ~600 nm. Moreover, the intensity of this last band increases with the Cu content. According toliterature, the band at ~600 nm is attributed to absorption of the Cu o atoms (Sojka et al., 2008). It is

interesting to observe that the adsorption bands at ~240 nm attributed to octahedral Cu2+ are still

visible after reduction at 150°C, suggesting that only copper cations placed on the edges of the

brucite layers and on the crystal defects are reducible under these conditions.



Figure 6. DR UV-Vis spectra for ZnCuCoAl LDHs after reduction at 150°C.

The reducibility of Cu2+ and Co2+ in ZnNiCoAl-LDH precursors were investigated by means

of TPR. Figure 7 illustrates the TPR curves for two selected samples with different Cu2+/Co2+ molar

ratio. Because Zn2+ and Al3+ cations are not reducible in the studied temperature range 25 – 800 °C,

the TPR curves reflect the reduction of cobalt and nickel cations to the corresponding zero valent

states. For the Cu-rich LDH (sample ZCuCoA 82), a first reduction main peak at ~261°C together

with a shoulder at ~236 °C can be observed, which may be assigned to the reduction of Cu2+ species

in two different chemical environments and/or locations in the brucite-like layers (Velu et al.,

2001). According with the DR UV Vis results (Figure 5), the low temperature peak can be

tentatively attributed to the reduction of copper cations in highly defective positions in the brucite-

like layers and may explain the formation of Cu0/LDH nanocomposites by mild reduction treatment.

Beside these peaks, a broad and low intense peak can be observed centered at ~ 521 °C, which is

associated with hardly reducible Co2+ cations (Velu et al., 2000) (S. Velu, K. Suzuki, M. P. Kapoor,

S. Tomura, F. Ohashi and T. Osaki, Chem. Mater., 2000, 12, 719.). The Co-rich LDH (sample

ZCuCoA 28) shows peaks at similar reduction temperatures as Cu-rich sample (~245 and ~512 °C)

but of different contributions at the total hydrogen consumption, in agreement with its higher Co/Cu

ratio. It is finally worthy of note that comparing with the Co- and Cu-free LDH samples, the

reduction temperatures of both Cu2+ and Co2+ are lower in bi-component LDHs (~ 325 °C for Co-



alcohol (CNOL) (route 1), hydrocinnamaldehyde (HCNA) (route 2) and hydrocinnamyl alcohol

(HCNOL) (routes 3 and 4).

Scheme 1. Reaction pathways for CNA hydrogenation

Figure 8 illustrates the catalytic activity of Me/LDH nanocomposites obtained by the mild

reduction at 150o

C of as-synthesized ZnCuCoAl LDH precursors. Under such mild conditions,according to TPR data, only Cu2+ are susceptible to be reduced to Cu0 while cobalt remains in

cation state due to its low reducibility. It is thus expected that activity mainly depends on the

content of Cu in the LDH precursors. Indeed, it can be observed in Figure 8 that after 6 h of reaction

the total conversion of CNA drastically increased from ~1 % to 98.53 % for Cu-free and Co-free

LDH, respectively. Moreover, the activity gradually increases with the increase in the Cu/Co ratio.

Fig. 8. Hydrogenation of trans-cinnamaldehyde over Me/LDHs nanocomposites derived from

ZnCuCoAl LDH precursors .

(Reaction conditions: T = 150 °C, 0.265 g catalyst; CNA = 1 ml, solvent = propylene carbonate (25

mL), rate stirring = 900 rpm, H2 flow = 1L.h-1).



The high activity of Me/LDH nanocomposites in hydrogenation can be associated without

any doubt with the concentration of copper catalytic active species, in particular the types present

on the surface of nanocomposites. It is considered that the reduction of copper cations placed in the

most favorable positions such as edges and vertices of LDH crystals, crystal defects, etc leads to

highly active catalytic active centers. As discussed in our previous study (Dragoi et al., 2010b),copper cations are statistically distributed in the LDH framework and thus, when some of the Cu-O-

M bonds (M = Co, Mg, Al) are broken by mild reduction (see Figure 1), Cu0 will be highly

dispersed in the LDH component of the nanocomposite. Consequently, our data confirm that

metallic copper is catalytically active in hydrogenation only when it is in a highly dispersion state

(i.e., high electron densities) (Maki-Arvela et al., 2005). As concerns the influence of Co2+, it seems

that its main role is to improve the reducibility of Cu2+ cations, since the selectivity to CNOL does

not depend at all on the Co content of nanocomposites (maximum of ~ 45 % at a isoconversion of CNA of ~9 % for all compositions).



Fig. 9. Selectivity vs CNA conversion over Me/LDH nanocomposites obtained from ZNiCoA 82

(A) and ZCuCoA 82 (B) precursors. (Reaction conditions: T = 150 °C, 0.265 g catalyst; CNA = 1

ml, solvent = propylene carbonate (25 mL), rate stirring = 900 rpm, H2 flow = 1L.h-1).

However, when Cu is replaced with Ni in the ZnAl matrix, the role played by cobalt is

totally changed. Figure 9 shows a comparison between the selectivity of catalysts derived from

ZNiCoA 82 and ZCuCo 82 LDH precursors, which illustrates that the activity and chemoselectivityof Me/LDH nanocomposites can be finely controlled by an adequate choice of the transition non-noble

metal. Thus, as compared with the ZnCuCoAl system, substitution of Co and Ni in the ZnAl matrix results in

a lower catalytic activity (9 vs 55 % CNA conversion after 6 h of reaction) but in an enhanced selectivity to

unsaturated alcohol (85 vs 45 % at an isoconversion of CNA of 9 %) (Figure 9A and B).

Conclusions

Experimental data described in the present work provide evidence of the possibility of using

non-calcined layered double hydroxide substituted in brucite-like layers with cations of non-noble

transition metals (Ni2+, Co2+, Cu2+) as precursors for the composites containing nanosized metal

particles in LDH matrix. Their layered structure and the substitution of Cu2+ and Co2+ in the brucite-

like layers were proved by various methods (XRD, FT-IR, DR UV-Vis, TPR, etc.). Reduction of

some transition metal cations located in favorable positions and their encapsulation in the matrix do

not lead to a drastic loss in the layered structure when the activation process is carried out at 150°C,

under hydrogen flow. At the same time, the resulted nanocomposite materials of metal/LDH typeexhibited high catalytic activity in the liquid-phase hydrogenation of cinnamaldehyde. On the one



hand, Cu and Co substitution offered a better catalytic activity in terms of CNA conversion. On the

other hand, the catalyst derived from CoNi-containing LDH was found to be more promising for the

selective production of CNOL. Certainly, the catalytic properties could be tailored by modifying the

chemical composition of LDH precursors. Thus, nanocomposites of this kind could attract much

attention for their potential practical applications as catalytic materials, which are easily synthesized, andusefully for carrying out hydrogenation processes under gentle reaction conditions.

Acknowledgement: The authors (A.C., A.U., B.D., E.D.) acknowledge the financial support from

CNCSIS-UEFISCSU (Romania) under PNII-Idei Project No. 485/2009. C. Rudolf acknowledges the

financial support of POSDRU CUANTUMDOC “Doctoral Studies for European Performances in

Research and Inovation” ID 79407 project funded by the European Social Found and Romanian

Government. A. Sasu acknowledges the financial support of EURODOC “Doctoral Scholarships for

research performance at European level" ID 59410 project, finance by the European Social Found

and Romanian Government.

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