Selective gas-phase hydrogenation of benzaldehyde in the presence of Ni-, Co- and Fe-doped BaCuO2 delafossites: effect of metal substituent on the production of benzyl alcohol

Research on Chemical Intermediates

https://doi.org/10.1007/s11164-019-03792-6

Selective gas‑phase hydrogenation of benzaldehyde

in the presence of Ni‑, Co‑ and Fe‑doped BaCuO₂

delafossites: eꢀect of metal substituent on the production

of benzyl alcohol

Hanane Zazoua¹· Fouzia Touahra₁· Moussa Sehailia · Redouane Chebout ·

,2

1,2

1

2

Djahida Lerari · Khaldoun Bachari · Adel Saadi

Received: 9 November 2018 / Accepted: 25 February 2019

©

Springer Nature B.V. 2019

Abstract

Delafossite mixed metal oxides of the form BaCuO and BaCu M O (M=Ni,

2

1

1−x

2

Fe and Co) were successfully synthesized following a sol–gel citrate method. The

products, obtained after heat treatment under N at 800 °C, were characterized

2

using several techniques such as: thermogravimetric analysis (TG), inductively cou-

pled plasma atomic emission spectroscopy (ICP), X-ray diꢀraction (XRD) spec-

troscopy, Brunauer–Emmett–Teller (BET) surface area analysis, Fourier transform

infrared (FTIR) spectroscopy and temperature-programmed reduction (TPR). The

formation of the delafossite oxide structure for all solid powders was conꢁrmed by

2

−1

XRD. The BET surface area of all catalysts was lower than 10 m g . The catalytic

performance of the synthesized catalysts was evaluated in the vapor-phase hydro-

genation of benzaldehyde. Benzyl alcohol and toluene were obtained through con-

secutive hydrogenation/hydrogenolysis reactions, while benzene was formed via

a concomitant hydrogenolysis reaction. BaCu Ni O catalyst showed better

O.47 0.51

2

activity at 250 °C with 98% benzaldehyde conversion. Under the same conditions,

BaCu Fe O catalyst showed the highest selectivity for benzyl alcohol (84% at

O.52 0.48

2

0% of conversion).

Keywords Mixed oxide · Hydrogenation · Benzaldehyde · Benzyl alcohol

Electronic supplementary material The online version of this article (https://doi.org/10.1007/s1116

4

-019-03792-6) contains supplementary material, which is available to authorized users.

*

Hanane Zazoua

hanene_zazoua@yahoo.fr

1

2

Centre de Recherche Scientiꢁque et Technique en Analyses Physico-chimiques (CRAPC), BP

3

84, 42004 Bou-Ismail, Tipaza, Algeria

Laboratoire de Chimie du Gaz Naturel, Faculté de Chimie, USTHB, BP 32, El-Alia,

6111 Bab-Ezzouar, Algiers, Algeria

1

Vol.:(012

1

3456789)

3

H. Zazoua et al.

Introduction

The hydrogenation of aromatic aldehydes to aromatic alcohols is an important

process in the synthesis of pesticides, ꢂavoring, cosmetics and pharmaceuticals

products; the process of catalytic hydrogenation can be carried out either in the

liquid or gas phase. However, gas-phase hydrogenation reactions are more indus-

trially favorable due to facile implementation and ease of product separation

when compared with liquid-phase reactions [1].

In recent times, a considerable amount of work has been devoted to the gas-

phase catalytic hydrogenation of benzaldehyde to benzyl alcohol in the pres-

ence of heterogeneous catalysts. The selectivity of the hydrogenation reaction of

benzaldehyde remains complex, as a range of intermediates and by-products can

yield hexahydro-benzaldehyde [2], toluene [3, 4], benzyl alcohol [5, 6], methyl-

cyclohexane [7], benzene [3] and cyclohexyl methanol [8]. In the gas-phase reac-

tion of benzaldehyde, the activity is mainly related to the strength of reactant

interaction with the catalyst surface, while selectivity is usually governed by the

competitive adsorption of benzaldehyde and benzyl alcohol on the catalyst’s sur-

face [9]. In the case where benzaldehyde is strongly adsorbed on the catalyst’s

surface, e.g., in the presence of strongly basic sites, benzene formation is pre-

ferred [10], although the use of reducible oxides favors interaction via the surface

hydroxyl oxygen, thus leading to the generation of toluene [11]. For this reason,

the choice of active metal is an important factor for the orientation of selectivity

in reaction products from the hydrogenation of benzaldehyde.

Literature precedent showed that catalysts based on copper [12], nickel [7] and

cobalt metals [13] have been widely studied for the hydrogenation of benzalde-

hyde. Although these metals were never used as bulk catalysts, the addition of

promoters is often required to improve the selectivity towards unsaturated alco-

hols. The selective hydrogenation of benzaldehyde can be explained on the basis

of diꢀerent factors such as the metal electron density which can inꢂuence the for-

mation of bimetallic compounds or promoting strong interaction with the support.

Other geometric eꢀects and C=O bond polarization can determine the interaction

with cationic sites on the catalyst’s surface, hence aꢀecting the selectivity of the

resulting reduced species [14].

Previously, scientists found that the conversion of benzaldehyde was maximum

for Cu/SiO and Cu/Al O ; this was due to the presence of acid–base properties

2

3

on the respective supports [11]. Higher conversion and selectivity towards the

alcohol in the presence of Cu/SiO catalyst was mainly due to the unfavorable

2

adsorption of benzyl alcohol formed during the hydrogenation of benzaldehyde

[

11]. Copper supported on titania, zirconia and silica was inactive towards hydro-

genation reactions at high temperature (350 °C). The conversion of benzaldehyde

was maximum for CuAl catalyst prepared at pH 12 as compared to CuAl prepared

at pH 8 [15]. The catalytic activity of nickel for the hydrogenation reaction of

benzaldehyde depends upon the crystallite size and the amount of nickel incor-

porated onto the support [7, 16]. Published works showed that Ni/SiO was the

2

most active nickel-based catalyst amongst Ni/CeO , Ni/TiO and Ni/Al O . The

2

3

1

3

Selective gas-phase hydrogenation of benzaldehyde in the…

higher activity of Ni/SiO compared to other catalyst is due to the formation of

2

larger nickel oxide particles (22 nm) compared to 14.4, 12 and 10.1 nm for Ni/

CeO , Ni/α-Al O and Ni/TiO , respectively. The larger particle size in Ni/SiO

2

3

2

is mainly due to the weak nickel–silica interaction which led to less dispersion

of nickel on the silica support [7]. With cobalt, the best catalytic activity was

observed for cobalt supported on γAl O . The use of Co supported on γ-Al O led

2

3

2

3

to higher reduction of benzaldehyde. The surface area of the reduced cobalt, i.e.,

2

+

0

Co to Co , also decreased, which in turn increased the activity of the catalysts

during the hydrogenation of benzaldehyde. In addition, alkaline earth metal such

as CaO, BaO and SrO were all found to favor the production of benzyl alcohol.

Saadi et al. successfully conꢁrmed the elevated selectivity towards benzyl alcohol

when the latter materials were utilized. [17].

Recently, the majority of published research work has concentrated on the

preparation of selective catalysts for the hydrogenation of carbonyl-containing

compounds. Potential catalytic materials can also include multi-metallic and

non-stoichiometric mixed oxides [12, 18]; these solids tend to contain highly

dispersed metal particles in the form of super-stoichiometric oxides, which are

usually in close interaction with surface cations. The formation of surface metal

cation sites may selectively promote C=O bond hydrogenation in α,β-unsaturated

aldehydes [14]. For example, gold present along with palladium was found to

improve the catalytic activity by modifying the structural and electronic proper-

ties of palladium as well as making the latter more resistant to sintering [19, 20].

For this reason, we found it interesting to test a combination of active met-

als integrated within an oxide structure, in our case, copper-based delafossite, in

the hydrogenation reaction of benzaldehyde. In addition to the originality of our

approach, this work will also give rise to a signiꢁcant contribution to the body of

literature. The utilization of delafossite-type structures in heterogeneous cataly-

sis is very scarce, with reported applications being limited to methanol synthe-

sis, N O decomposition, methanol steam reforming, photochemical reduction of

2

CO [21], photocatalytic H evolution and NO removal [22–24]. Previously, it

2

3

has been shown that cuprous delafossite oxides based on CuBaO can present an

2

interesting framework which can enable chemists to easily ꢁne-tune the chemical

properties of the resulting materials; the large interatomic copper distances allow

for better oxygen intercalation which can give rise to the formula CuBaO , thus

2

.66

enabling the presence of high-valence copper sites [25, 26].

The synthesis of BaCuO delafossite phase has been previously reported by

2

several researchers [27, 28]. The ꢁrst study of the latter oxide was performed by

Kipka et al. in 1977 [29]. The structure of BaCuO delafossite was successfully

2

determined using X-ray crystal structure analysis. The authors reported the pres-

ence of a complex polyhedral space group Im-3 m with four diꢀerent equivalent

positions for barium, four for copper and six for oxygen [30, 31].

In this study, we examine BaCuO delafossite phase, prepared by sol–gel

2

method, for the selective hydrogenation of benzaldehyde to benzyl alcohol. In

addition, we also report the inꢂuence of partial substitution of Cu in BaCuO by

2

other metals (i.e., Ni, Co and Fe) on the selective hydrogenation of benzaldehyde.

1

3

H. Zazoua et al.

Experimental

Catalyst preparation and characterization

The chemicals used in this study are citric acid (purity≥99.5%, Sigma Aldrich),

Cu(NO ) , 4H O (Biochem Chemopharm,≥99.5%), Fe(NO ) ·9H O (99%, Alfa

3

2

3 3

2

3 2

Aesar), Co(NO ) ·4H O (Biochem Chemopharm,≥98%), Ni(NO ) ·6H O (Bio-

3

2

chem Chemopharm, 98%≥purity) and Ba(NO ) ·4H O (99%, Sigma Aldrich). All

3

2

reagents were used without further puriꢁcation.

A series of BaCuO and BaCu M O (where M=Ni, Fe and Co) mixed oxides

2

1

1−x

2

were prepared according to the sol–gel method. A stock solution containing citric

acid was prepared by dissolving citric acid in de-ionized water. A stoichiometric

amount of metal nitrate was then added. The molar ratio of citric acid to the total

metal cations was 1.5; the solution was kept under vigorous agitation for 2 h, and

slowly evaporated under vacuum in a rotary evaporator, at a ꢁxed temperature of

5

5 °C. The obtained gel was then dried overnight at 100 °C in a drying oven. Finally,

the resulting powder was calcined under N at 800 °C for 15 h with increments of

2

−1

3

°C min , leading to the formation of the desired mixed oxides.

Characterizations

The chemical composition was established using inductively coupled plasma atomic

emission spectroscopy (ICP) in the presence of a multichannel Thermo Jarrel Ash

ICAP 957 spectrometer. Powder X-ray diꢀraction (XRD) patterns were obtained

using D500 diꢀractometer, with a Cu Kα radiation source (λ=1.5418 Å) at scan-

−1

ning speed of 2° min in the 10°–90° 2θ range. FTIR spectra were obtained using

an Alpha Bruker (single reꢂection diamond ATR) spectrometer in the region of

−

1

4

000–400 cm .The speciꢁc surface area of all catalysts was measured via the

adsorption isotherms of nitrogen at 77 K using the BET method (Micromerit-

ics TriStar 3000). Prior to N adsorption, the samples were out-gassed at 300 °C

2

−1

(

5 °C min ) for 5 h. Temperature-programmed reduction (TPR) was conducted

using a TriStar 3000 V6.01 instrument, equipped with a thermal conductivity detec-

tor (TCD). Prior to reduction, the sample (20 mg) was ꢂushed with argon in a quartz

reactor tube, heated at 120 °C for 2 h under argon and cooled afterwards to room

temperature. Then, the samples were ꢂushed with 5% H /Ar at 900 °C at a rate of

2

−

1

0 °C min .

Catalytic activity studies

The gas-phase hydrogenation of benzaldehyde was studied in a ꢁxed-bed glass tubu-

lar reactor with 100-mg samples at atmospheric pressure; the total ꢂow rate was

3

−1

5

0 cm min . The reacting gas feed consisted of 4.8 Torr of benzaldehyde and

50 Torr of H diluted in N . Gaseous benzaldehyde was obtained by bubbling N

2

in liquid benzaldehyde at constant temperature (50 °C) in a suitable saturator. Prior

1

3

Selective gas-phase hydrogenation of benzaldehyde in the…

to testing, the catalysts were reduced in situ for 1 h at 450 °C under H at a ꢂow rate

2

3

−1

of 20 cm min . The gaseous reactant and products were heated upstream and out-

stream in order to avoid condensation; the resulting products were analyzed online

using an gas chromatograph (Delsi IGC 121 ML) equipped with a ꢂame ionization

detector (FID) and a carbosieve column. Each reaction temperature was maintained

constant until the corresponding steady state was reached as indicated by the gas

chromatograph analysis of the exiting gas samples. For each catalyst, no mass trans-

fer was observed with 100 mg of the sample weight, and the reaction temperature

was changed in a crossing order: 250 °C, 80 °C, 150 °C, 50 °C and 110 °C.

Results and discussion

Catalysts characterization

Catalyst composition (ICP‑AES)

The chemical composition of BaCuO and BaCu M O (M=Ni, Fe and Co) sol-

2

1

1−x

2

ids, after calcination, was determined using ICP; the results are presented in Table 1.

The data of elemental analyses indicated that the concentration of Ba, Cu, Fe and Ni

for all samples was close to the intended value, indicating a good preparation of the

precursor oxides.

X‑ray diꢀraction analysis

Figure 1 shows XRD patterns for BaCuO and BaCu M O (M = Ni, Fe and

2

1

1−x

2

Co) mixed oxides. XRD analysis of BaCuO (Fig. 1A) shows diꢀraction peaks

2

that resemble the characteristics of delafossite-type structure (JPCDS card no.

3

8-1402). The peaks observed at 2θ = 35.46° and 38.71° are due to the presence

of CuO (JPCDS card no. 05-0661). However, partial substitution of Cu with Ni

forming BaCu Ni O ) shows a shift in the principle peak from 2θ = 29.28°

(

0

.47

0.51

2

to 2θ = 30.20°. The diꢀraction peak is observed between that of BaCuO

2

(

2θ = 29.28°) and BaNiO (2θ = 33.15°; JPCDS card nos. 38-1402 and 04-0510,

2

respectively). The diꢀraction peaks at 2θ = 34.67° and 38.71° are attributed to

Table 1 Elemental chemical composition (at.%) and surface area of various mixed oxides

Catalysts

Elemental chemical composition

at.%)

Experimental

formula

Surface

Pore size (nm)

(

area (S

)

BET

2

−1

(

m g

)

Ba

Cu

Fe

Co

Ni

BaCuO₂

29.20 28.98

–

BaCuO₂

BaCu₀_.₅₂Fe₀_.₄₈O₂

BaCu₀_.₅₁Co₀_.₄₉O₂

3.51

5.25

6.23

8.62

25.52

26.54

37.58

40.91

BaCuFeO₂29.51 15.22 13.27

BaCuCoO₂28.89 14.31

BaCuNiO₂30.21 13.88

–

13.80

–

14.90 BaCu₀_.₄₇Ni₀_.₅₁O₂

1

3

H. Zazoua et al.

Fig. 1 XRD of BaCuO (A), BaCu Ni O (B) and BaCu Co O (C) and (D) after partial sub-

2

0.47 0.51

2

0.51

0.49

2

stitution of Cu by Fe. Black diamond BaCuO ; white circle CuO; black square BaCu Ni O ; white

2

0.47 0.51

2

with black square BaCu₀Co O ; black circle CuFeO and times in white box BaO

.51

0.49

2

presence of CuO where a weak increase in intensity was observed (Fig. 1B).

The partial substitution of Cu with Co (BaCu Co O ) prompted the peaks to

0

.51

0.49

2

shift between those of BaCuO and BaCoO (JPCDS card no. 40-0722; Fig. 1C).

2

On the other hand, XRD diꢀractograms for the partially Fe-substituted com-

pound (Fig. 1D) showed the presence of a mixture oxides, CuFeO (JPCDS card

2

no. 39-0246), BaCuO (JPCDS card no. 38-1402) and BaO (JPCDS card no.

2

2-1056).

FTIR spectroscopic measurements

−

1

Infrared spectra recorded in the range 400–4000 cm

for BaCuO and

2

BaCu M O (M = Ni, Fe and Co) mixed oxides are depicted in Fig. 2. The

1

1−x

2

−

1

bands in the region 420–750 cm are assigned to the stretching vibrations

modes of M–O resulting from the interaction between oxygen and metal cations.

−

1

The absorption band around 3700 cm can be attributed to the hydroxyl (O–H)

−

1

stretching vibrations, while the absorption peak at 1558 cm is assigned to the

bending vibrations of water, δ(H O), which is usually caused by the presence of

2

−

1

water molecules adsorbed on the surface of the material. The band at 1442 cm

−

2

is a characteristic of O–C–O stretching vibration in CO . The data obtained are

3

in agreement with results observed by other scientists [32–34].

1

3

Selective gas-phase hydrogenation of benzaldehyde in the…

BaCuO

2

BaCu

Fe

.52 0.48

O

0

2

Ni

O

0

.47 0.51

2

BaCu

Co

O

0

.51 0.49

2

5

63

443

3745

1558

1442

4

000

3500

3000

2500

2000

1500

1000

500

-

1

(cm )

Fig. 2 FTIR spectroscopy of BaCuO and BaCu M O (M=Ni, Fe and Co)

2

1

x−1

2

BaCu

Ni

O

0

.47 0.51

2

BaCu

Fe

O

2

0

.52 0.48

BaCu

Co

O

2

0

.51 0.49

CuBaO

2

0

.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (p/p )

0

Fig. 3 N adsorption–desorption isotherms of BaCuO and BaCu M O (M=Ni, Co and Fe)

2

1

x−1

2

N₂adsorption–desorption analysis

The adsorption–desorption isotherms of the synthesized catalysts are presented in

Fig. 3. According to the International Union of Pure and Applied Chemistry (IUPAC

classiꢁcation), all samples have type IV adsorption isotherms with a hysteresis loop

(

at the relative pressure P/Po>0.5), which is a characteristic of mesoporous materi-

als. The BET speciꢁc surface areas and the pore diameter calculated by the Bar-

rett–Joyner–Halenda (BJH) theory of all catalysts are listed in Table 1.

1

3

H. Zazoua et al.

The results showed that the speciꢁc surface area of all samples is lower than

2

−1

2

−1

1

0 m g . The speciꢁc surface of particles in BaCuO is 3.51 m g . However, all

2

particles saw an increase in their surface areas following partial substitution of Cu

2

−1

2 −1

with Co and Fe (5.25 m g and 6.23 m g . respectively). Interestingly, the high-

2

−1

est surface area was obtained after partial substitution of Cu with Ni (8.62 m g ).

The results of pore diameter reveal that BaCu Ni O and BaCu Co O cata-

0

.47 0.51

2

0.51

0.49

2

lysts are more important with pore diameters of 37 and 40 nm, respectively. Graphs

showing the pore size distribution of the synthesized catalysts are shown in Fig. S 1.

Temperature‑programmed reduction (H ‑TPR)

2

The reducibility of BaCuO , BaCu Ni O , BaCu Co

O

2

and

2

0.47 0.51

2

0.51

0.49

BaCu Fe O was investigated using H -TPR analysis; reduction proꢁles are

0

.5 0.48

2

shown in Fig. 4. It is hard to deꢁne a clear boundary between each of the reduction

steps. For this reason, we used XRD of the reduced catalysts at diꢀerent tempera-

tures (350, 450 and 650 °C).

XRD patterns of the composites following initial reduction (prior to reaction) at

diꢀerent temperatures (350, 450 and 650 °C) for 1 h are depicted in Fig. 5. Fol-

lowing reduction of BaCuO at 350 °C, predominant phases corresponding to

2

0

the presence of BaCuO and metallic copper (Cu ), located at 2θ=43.29° and

2

θ=50.43° (JPCDS card no. 04-0836) were observed. Ba(OH) peaks also appeared

2

at 2θ=24.6°, 26° and 35.2° (JPCDS card no. 44-0585). Intensities of peaks for

0

Cu and Ba(OH) increased as the reduction temperature increased (450 °C and

2

6

50 °C); this was accompanied with the disappearance of the characteristic peaks

for BaCuO . BaCu Ni O , reduced at 350 °C, showed the presence of two dis-

2

0.47 0.51

2

0

tinct peaks of BaCu Ni O and Cu . This result indicated that nickel and copper

0

.47 0.51

2

in BaCu Ni O have not been reduced at this temperature. At 450 °C, the inten-

0

.47 0.51

2

sity of the peaks corresponding to BaCu Ni O decreased, while the intensity

0

.47 0.51

2

1

.25

.20

.15

.10

.05

.00

3

10 °C

6

80 °C

^B^a^C^u^C^o^O2

3

15 °C

6

05 °C

^B^a^C^u^F^e^O2

BaCuNiO₂

BaCuO₂

4

30 °C 522 °C

320 °C

4

86 °C

1

00 200 300 400 500 600 700 800 900

Temperature (°C)

Fig. 4 TPR of: BaCuO , BaCu Ni O , BaCu₀_.₅₁Co₀_.₄₉O₂and BaCu₀_.₅₂Fe₀_.₄₈O₂(5%H /Ar ꢂow 1.2

2

0.47 0.53

2

)

2

−1

L/h, at a temperature ramp of 10 °C min

1

3

Selective gas-phase hydrogenation of benzaldehyde in the…

(

A)

(C)

(

B)

(

D)

Fig. 5 XRD of BaCuO (a), BaCu Ni O (b), and BaCu Co O (c) and (d) after partial sub-

2

0.47 0.51

2

0.51

0.49

2

stitution of Cu by Fe reduced at various temperatures (5%H /Ar ꢂow 1.2 L/h, at a temperature ramp of

2

−1

1

0 °C min ). Black diamond BaCuO ; black square BaCu Ni O ; white inverted triangle Ba(OH) ;

2

0.47 0.5

2

0

down arrow NiO; black inverted triangle Cu ; Ni ; Co ; star BaCoO ; times in white square BaO;

2

0

black diamond in white diamond Fe O ; white square Fe O ; dot in white circle Fe

2

3

4

of metallic Cu peaks increased. The other peaks appearing at 2θ=37°, 43° and

6

2.8° (JPCDS card no. 47-1049) and 2θ=44.5°, 51.8° and 76.3° (JPCDS card no.

0

4-0850) both correspond to the presence of NiO and Ni metal, respectively. At

higher temperature, i.e., 650 °C, no peaks corresponding to BaCu Ni O were

0

.47 0.51

2

0

observed, indicating a complete reduction of the material to Cu , Ni and Ba(OH) .

2

BaCu Co O composite, reduced at 350 °C, was totally converted to BaCoO

0

.51

0.49

2

0

phase (JPCDS card no. 40-0722), BaCuO , Ba(OH) , Cu and Co (JPCDS card no.

2

1

5-0806). Following reduction at higher temperature (450 and 650 °C), the XRD

0

diꢀractogram patterns showed peaks only for Ba(OH) , Cu and Co . With regards

2

to the sample obtained after partial substitution of Cu with Fe, XRD patterns fol-

lowing reduction at 350 °C showed the appearance of new peaks corresponding to

0

Fe O and Cu ; this is explained by the reduction of CuFeO species to form Fe O

2

3

2

3

0

and Cu . XRD patterns obtained after reduction at 450 and 650 °C showed the

0

presence of diꢀraction lines resulting from the formation of Ba(OH) , Cu , Fe O

2

3

4

0

(

JPCDS card no. 26-1136) and Fe .

1

3

H. Zazoua et al.

The reduction of BaCuO (Fig. 4) proceeds in two steps, at 320 and 486 °C. Both

2

0

+

peaks are attributed to the reduction of Cu , whereby the ꢁrst peak corresponds

2

+

2+

to a reduction of Cu in CuO (Cu →Cu ), while the second peak is assigned

2

+

to the reduction temperature of Cu to Cu in BaCuO [35, 36]. TPR proꢁles of

2

BaCu Fe O sample exhibited two maximum reduction temperatures located at

0

.52 0.48

2

3

15 and 605 °C. The peak appearing in the temperature range of 200–400 °C cor-

responds to the reduction of CuFeO to metallic Cu and Fe O (Eq. 1). At tempera-

2

3

tures higher than 400 °C, Fe O oxides continued to be reduced to Fe O (Eq. 2) and

2

3

4

0

subsequent reduction to the metallic Fe (Eqs. 3, 4). Those results are in agreement

with results obtained from the XRD diꢀractogram in Fig. 2.

0

CuFeO + H → 2Cu + Fe O + H O

2

(1)

(2)

(3)

(4)

2

3

2

3

∕2Fe O + 1∕2H → Fe O + 1∕2H O

2 3 2 3 4 2

Fe O + H → 3FeO + H O

2

3

4

2

0

FeO + H → Fe + H O

2

TPR proꢁles for BaCu Ni O catalyst exhibited three hydrogen consumption

0

.47 0.51

2

+

0

peaks. The ꢁrst peak at 320 °C is assigned to the reduction of Cu to Cu in CuO.

2

+

0

The peak at 430 °C is assigned to the reduction of Ni to Ni . Another reduction

peak at 522 °C, which shifted~36 °C to higher temperature compared to BaCuO

2

due to trench interaction between copper and nickel, is attributed to the reduction

2

+

0

of Cu to Cu . In addition, BaCu Co O material (Fig. 4) shows two peaks

0

.51

0.49

2

+

0

2+

at 310 and 680 °C, which is attributed to the reduction of Cu to Cu and Co to

0

Co , respectively. To further explain the catalytic properties of our materials, the

pre-reduction of our catalysts up to 450 °C before reaction leads to the formation of

0

Cu species on the substituted catalysts.

Catalytic activity

Benzaldehyde hydrogenation

The adsorption of benzaldehyde on copper favors hydrogenation of the C=O group

in the benzaldehyde molecule, forming mainly the unsaturated alcohol. Reaction

selectivity is challenging in benzaldehyde hydrogenation as the formation of a wide

range of intermediates and by-products are possible. Generally, the main products

resulting from the reaction are benzyl alcohol, toluene and benzene. Most of our

catalysts were active and selective in the gas-phase hydrogenation of benzaldehyde

at atmospheric pressure and temperature range 50–250 °C. Thus, we envisage the

presence of a synergistic eꢀect between Cu/Ni, Cu/Fe and Cu/Co for the purpose of

selective hydrogenation of benzaldehyde.

The inꢂuence of temperature on the hydrogenation reaction was studied.

Table 2 indicates the presence of a non-regular trend in terms of conversion yields

and selectivity for catalysts substituted by Co, Ni and Fe in the temperature range

8

0–250 °C. Our results reveal that benzaldehyde is mainly transformed to benzyl

1

3

Selective gas-phase hydrogenation of benzaldehyde in the…

Table 2 Catalytic results for gas-phase hydrogenation of benzaldehyde

Catalysts

T_r_e_a_c_t_i_o_n(°C)

Conv. (%)^a

Selectivity (%)^a

Benzyl alcohol

Toluene

Benzene

BaCuO₂

50

0.94

1.24

2.38

12.08

89.86

13.2

6.3

11.0

72.6

98.3

4.5

9.8

3.1

3.0

20.1

3.1

100

60

50

0

–

40

50

100

–

7.4

45.3

96.5

–

7.6

–

8

0

110

150

250

BaCu₀_.₄₇Ni₀_.₅₁O₂

BaCu₀_.₅₁Fe₀_.₄₉O₂

BaCu₀_.₅Co₀_.₄₈O₂

50

100

90.3

32.1

2.4

100

84.4

100

73.8

5.9

8

0

–

110

150

250

2.3

22.6

1.1

–

8.0

–

50

8

0

110

150

250

50

8

0

5.9

3.2

42.7

91.0

110

150

250

17.5

4.7

8.7

89.4

^aAfter 90 min of reaction

alcohol at lower temperatures (50–110 °C); this is in spite of the low conversion

values of our starting material. At elevated temperatures (> 110 °C), however,

selectivity toward benzyl alcohol tends to decrease with the best optimum results

obtained for BaCu Co O at 150 °C, followed by BaCu Fe O at 250 °C.

0

.5

0.48

2

0.51 0.49

2

The latter results indicate the presence of suitable surface characteristics which

allows for the facile desorption of the resulting benzyl alcohol from the surface

of catalysts. On the other hand, excellent selectivity for toluene was best achieved

with BaCuO delafossite at 250 °C, primarily owing to the unique adsorption of

2

benzyl alcohol intermediate on the surface of the catalyst. As a result, further

hydrodeoxygenation of benzyl alcohol is achieved to furnish toluene as an end

product. Interestingly, better selectivity for the direct formation of benzene was

observed at 250 °C when BaCu Ni O and BaCu Co O were used. At

0

.47 0.51

2

0.5

0.48

2

this temperature, both of these catalysts tend to favor direct decarbonylation of

benzaldehyde to form benzene rather than eꢀecting the hydrogenation process.

These results are in agreement with previously reported works by Vannice et al.

[

4]. as well as our group.

The maximum conversion is obtained at 250 °C on all catalysts studied, which

may be due to the fact that more active sites are promoted at high temperature.

1

3

H. Zazoua et al.

1

00

BaCuO

BaCu

2

Ni

O

0

.47 0.51

2

8

6

4

2

0

BaCu Fe

O

0.5 0.48 2

BaCu Co

O

0.5 0.49 2

5

0

100

150

200

250

Temperature (°C)

Fig. 6 Benzaldehyde conversion under H ꢂow as a function of the reaction temperature for: BaCuO ,

2

BaCu Ni O , BaCu Fe O and BaCu Co O . Pretreatment for 1 h at 450 °C in a current of

0

.47 0.51

2

0.5 0.48

2

0.5

0.49

2

3

−1

dihydrogen (H ) with a ꢂow rate of 20 cm min

2

This result signiꢁcantly shows that the activity of our catalysts is dependent on

both the reaction temperature and nature of the metal. The remarkable activity of

CuBaO catalyst (89% conversion at 250 °C) is due to the good reducibility of the

2

catalyst.

The conversion of benzaldehyde was higher in the presence of Ni and Co cat-

alysts than Fe and CuBaO catalysts (Fig. 6). However, the maximum conversion

2

was attained with BaCu Ni O (98%) and BaCu Co O (91%) catalysts at

0

.47 0.51

2

0.51

0.49

2

50 °C. This result can be explained by the presence of several active metals in the

hydrogenation reaction of benzaldehyde on the same catalyst, such as Cu/Ni and

Cu/Co which can provide more catalytically active sites for hydrogen dissociation

and thus higher conversion. Also, to further correlate the catalytic activity with our

material characterization results, the higher values obtained are mainly associated

with the high speciꢁc surface area in conjunction with the larger pore sizes found

over BaCu Ni O and BaCu Co O catalysts (40 and 37 nm, respectively).

0

.47 0.51

2

0.51

0.49

2

Recently, Lounas et al. [37] explained that the large size and volume of the pores

ensure greater accessibility and dissociation of hydrogen on the diꢀerent species

of copper formed on the surface of the catalysts, while the presence of low pore

volume and size can lead to decreased adsorption and lower interaction of the car-

bonyl group with the active sites. In addition, and based on TPR results, the pres-

0

ence of Cu , NiO and CoO species on these catalysts signiꢁcantly aꢀects the cata-

lytic activity. However, the nature of interaction existing between Cu–Ni and Cu–Co

in BaCu Ni O and BaCu Co O catalysts have a strong inꢂuence on the

0

.47 0.51

2

0.51

0.49

2

adsorption and desorption of the reactants and products during the reaction [38,

3

9] (Scheme 1). Indeed, the higher activity obtained with BaCu Ni O catalyst

0.47 0.51 2

at 250 °C is probably due to the presence of small particles of nickel, which are

1

3

Selective gas-phase hydrogenation of benzaldehyde in the…

Scheme 1 Proposed mechanism of benzaldehyde activation on the surface of the mixed oxide delafossite

obtained in high dispersion, thus leading to the obtention of high metallic surface

area in BaCu Ni O catalysts.

0

.47 0.51

2

It is also noted that BaCu Fe O is less active (20% at 250 °C) when com-

0

.5 0.48

2

pared to other catalysts; sintering might be reason for the decrease of the catalyst

activity, which leads to a decline in benzaldehyde conversion. On the other hand,

Xiao et al. found that the reduced activity of Cu-Fe bimetallic nanoparticles dur-

ing CO hydrogenation was mainly due to the occurrence of two distinct structural

changes during the reaction, i.e., sintering and phase separation of Cu and Fe spe-

cies [40].

Catalyst selectivity

The hydrogenation products resulting from benzaldehyde reduction under the stated

reaction conditions are: benzyl alcohol, toluene and benzene. In fact, the eꢀect of

1

00

8

6

4

2

0

BaCuO

2

BaCu

Ni O

0

.47 0.51 2

BaCu Fe

O

2

0

.5 0.48

BaCu Co

O

0

.5 0.49

2

5

0

100

150

200

250

Temperature (°C)

Fig. 7 Selectivity to benzyl alcohol under H ꢂow as a function of the reaction temperature for: BaCuO ,

2

BaCu Ni O , BaCu Fe O and BaCu Co O . Pretreatment for 1 h at 450 °C in a current of

0

.47 0.51

2

0.5 0.48

2

0.5

0.49

2

3

−1

dihydrogen (H ) with a ꢂow rate of 20 cm min

2

1

3

H. Zazoua et al.

temperature is well pronounced when it comes to product selectivity. Our results

clearly demonstrate that selectivity to obtain benzyl alcohol from benzaldehyde

hydrogenation was dependent on temperature. Analysis of the results (Fig. 7)

revealed that high selectivity towards benzyl alcohol occurred at low reaction tem-

peratures (≤110 °C), whereas benzaldehyde conversions were lower. According to

previous results given in literature, the hydrogenation of benzaldehyde at low tem-

perature in the presence of nickel supported on various other metal oxides tends to

favor the formation of benzyl alcohol; however, at higher temperatures, benzene

and toluene are formed [16, 41]. Previously, our group [7, 11] reported preferen-

tial production of benzyl alcohol over supported Cu at a low reaction temperature

(

<100 °C). It is generally believed that the active sites in copper-catalyzed hydro-

+

genation reactions are associated with Cu in an octahedral environment with

hydride ion [11, 42–44]. These sites would be ionic in nature. The existence of these

two sites on BaCu Ni O catalyst would explain their high reactivity toward

0

.47 0.51

2

the highly polarized C=O double bond to yield benzyl alcohol [42–45]. Also, the

presence of nickel in BaCu Ni O is found to be active towards the hydrode-

0

.47 0.51

2

oxygenation of the C–OH single bond present in benzyl alcohol (Scheme 2) [4, 42].

Toluene and benzene were the other products formed as a result of benzaldehyde

hydrogenation [16]. Benzyl alcohol was produced on the BaCu Co O catalyst

0

.51

0.49

2

(

74% selectivity at 150 °C and 43% conversion). This result proves that the presence

of Co increased the selectivity towards benzyl alcohol. These results are consistent

with previously reported results by Konga et al. [46]. The authors found that high

Scheme. 2 Proposed mechanism for the formation of benzyl alcohol

1

3

Selective gas-phase hydrogenation of benzaldehyde in the…

selectivity towards benzyl alcohol was exhibited by Co/Al O –SiO which could be

2

3

2

due to the low acidity of the catalyst surface.

At high reaction temperature (150–250 °C), the most important and remarkable

result is the high selectivity towards benzyl alcohol (>80%) coupled with a weak

benzaldehyde conversion (up to 20%) observed in the case of BaCu Fe O cata-

0

.52 0.48

2

3

+

lyst (Fig. 7). This behavior may be explained by the presence of Fe ions which can

lead to a facile adsorption/desorption of the reactants and products. Similar explana-

tions were proposed on the role of Fe-doped catalysts on hydrogenation reactions of

unsaturated aldehydes. Schroder et al. [47] examined the hydrogenation of unsatu-

rated aldehydes in the presence and absence of FeCl . The authors suggested that Fe

2

deposited on the catalyst surface can create more adsorption sites for the carbonyl

group, thus lowering the activation energy associated with the hydrogenation reac-

tion. On the basis of previous work by Fleischer et al. [48], as well as our own ꢁnd-

ings, we have concluded that Fe can be used to aꢀect the eꢃcient hydrogenation of

carbonyl compounds to alcohol.

On the other hand, toluene was observed as a product at moderate reaction

temperature (110 °C) in the presence of BaCuO (40%) and BaCu Ni

O

2

0.47 0.51

(

2%) (Fig. 8); the latter compound is only formed at higher reaction temperatures

(

≥150 °C) in the presence of BaCu Fe O (8%) and BaCu Co O (17%)

0

.52 0.48

2

0.51

0.49

2

catalysts. Toluene is the product of a subsequent hydrogenation reaction of benzyl

alcohol [3, 49, 50]. It is envisaged that the presence of toluene in the product mix-

ture is associated with the presence of highly active catalytic sites, leading to the

rapid transformation of benzyl alcohol to toluene at a very early stage of the reaction

process, where the partial pressure of both reactants and products are low. Another

possible route to this could be that benzyl alcohol disproportionates to form toluene,

benzaldehyde and water [50–52]. At higher reaction temperature (250 °C), toluene

1

00

BaCuO

BaCu

2

Ni

O

0

.47 0.51

2

8

6

4

2

0

BaCu Fe

O

0

.5 0.48

2

BaCu Co

O

0

.5 0.49

2

5

0

100

150

200

250

Temperature (°C)

Fig. 8 Selectivity to toluene under H ꢂow as a function of the reaction temperature for: BaCuO ,

2

BaCu Ni O , BaCu Fe O and BaCu Co O . Pretreatment for 1 h at 450 °C in a current of

0

.47 0.51

2

0.5 0.48

2

0.5

0.49

2

3

−1

dihydrogen (H ) with a ꢂow rate of 20 cm min

2

1

3

H. Zazoua et al.

was produced as the sole product over BaCuO . Indeed, Saadi et al. showed that the

2

use the BaO in the reduction of benzaldehyde led to toluene appearing as a second-

ary product in the reaction mixture at 150 °C [17]. This result conꢁrms the role of

BaO species present in our catalysts in the formation of toluene. These results also

indicate that production of toluene from benzaldehyde is favored in the presence of

strong interaction between the carbonyl oxygen and the active sites present on the

surface of the catalyst, thus leading to a hydrogenolytic cleavage of the C=O bond

by dissociated hydrogen [9].

In the present case, benzene selectivity increased with elevated reaction tempera-

ture, while maximum selectivity was attained at 250 °C for BaCu Ni O (96%)

0

.47 0.51

2

and BaCu Co O (89%) catalysts (Table 2). In contrast, the high activities

0

.51

0.49

2

encouraged the apparition of benzene as the principal product with high selectivity,

particularly at 250 °C for catalysts substituted by cobalt and nickel. There was no

detectable benzene formation in any reaction over CuBaO . Formation of benzene

2

as a function of reaction temperature (Fig. 9) showed that the product is directly pro-

duced from the reacting molecules and not via alcohol and/or toluene intermediates.

Benzene is believed to be the product of the decarbonylation of the external C=O

bond of benzaldehyde [41, 53].

The aforementioned results show that the presence of bimetallic species on the

surface of catalysts can inꢂuence the rate of adsorption/desorption of diꢀerent oxy-

genated benzylic species. These results are in alignment with other similar studies,

such as the selective gas-phase hydrogenation of crotonaldehyde, where it was found

that bimetallic Pt–Sn/SiO catalyst was more stable than monometallic Pt/SiO2

2

catalyst; this explanation was further evidenced by in situ FTIR experiments which

had indicated the presence of CO stretching vibrations (resulting from the decar-

−

1

bonylation of crotonaldehyde) at 2047 and 1838 cm on Pt–Sn/SiO catalyst. How-

2

ever, the intensity of those peaks signiꢁcantly decreased when Pt/SiO was used.

2

1

00

BaCuO

BaCu

2

Ni

O

2

0

.47 0.51

8

6

4

2

0

BaCu Fe

O

0

.5 0.48 2

BaCu Co

O

2

0

.5 0.49

5

0

100

150

200

250

Temperature (°C)

Fig. 9 Selectivity to benzene under H ꢂow as a function of the reaction temperature for: BaCuO ,

2

BaCu Ni O , BaCu Fe O and BaCu Co O . Pretreatment for 1 h at 450 °C in a current of

0

.47 0.51

2

0.5 0.48

2

0.5

0.49

2

3

−1

dihydrogen (H ) with a ꢂow rate of 20 cm min

2

1

3

Selective gas-phase hydrogenation of benzaldehyde in the…

Furthermore, in situ FTIR also showed the presence of other non-desorbed organic

molecules on the surface of Pt/SiO during the reaction which can signiꢁcantly

2

aꢀect the activity of the catalyst [54].

Conclusion

Results from this work clearly indicate that addition of transition metals (Ni, Co and

Fe) to BaCuO delafossite can improve the catalytic activity in the reaction of ben-

2

zaldehyde hydrogenation. The order of activity was attributed to both the reaction

temperature and nature of the substituting metal. The conversion of benzaldehyde

was higher with Ni-doped catalyst than Co- and Fe-containing catalyst; these results

are attributed to the particle size of the active sites, degree of dispersion and to the

high metallic speciꢁc surface area. In this respect, benzyl alcohol was selectively

obtained (84% at 20% conversion) at 250 °C in the presence of Fe-doped catalyst,

whereas benzene was obtained on catalysts substituted by cobalt and nickel. Toluene

was also formed from the hydrogenolysis process of benzyl alcohol, while benzene

was directly formed from benzaldehyde decarbonylation.

References

1

2

3

4

5

6

7

8

9

.

M. Okamoto, T. Hirao, T. Yamaai, J. Catal. 276, 423 (2010)

L. Červený, Z. Bĕlohlav, M.N.H. Hamed, Res. Chem. Intermed. 22, 15 (1996)

D. Haꢀad, U. Kameswari, M. Bettahar, A. Chambellan, J. Lavalley, J. Catal. 172, 85 (1997)

M.A. Vannice, D. Poondi, J. Catal. 169, 166 (1997)

D. Procházková, P. Zámostný, M. Bejblová, J. Červený, J. Čejka, Appl. Catal. A 332, 56 (2007)

M. Arai, A. Obata, Y. Nishiyama, J. Catal. 166, 115 (1997)

A. Saadi, R. Merabti, Z. Rassoul, M. Bettahar, J. Mol. Catal. A Chem. 253, 79 (2006)

P. Kluson, L. Červený, J. Mol. Catal. A Chem. 108, 107 (1996)

R. Merabti, K. Bachari, D. Halliche, Z. Rassoul, A. Saadi, React. Kinet. Mech. Catal. 101, 195

(

2010)

1

0. M.A. Peralta, T. Sooknoi, T. Danuthai, D.E. Resasco, J. Mol. Catal. A Chem. 312, 78 (2009)

1. A. Saadi, Z. Rassoul, M. Bettahar, J. Mol. Catal. A Chem. 164, 205 (2000)

2. T. Jayesh, K. Itika, G.R. Babu, K.R. Rao, R. Keri, A.H. Jadhav, B. Nagaraja, Catal. Commun. 106,

7

3 (2018)

1

3. Y. Nitta, Y. Hiramatsu, T. Imanaka, J. Catal. 126, 235 (1990)

4. A.J. Marchi, D.A. Gordo, A.F. Trasarti, C.R. Apesteguıa, Appl. Catal. A 249, 53 (2003)

5. N. Haddad, A. Saadi, A. Löfberg, R. Vannier, E. Bordes-Richard, C. Rabia, J. Mol. Catal. A Chem.

3

96, 207 (2015)

1

2

6. A. Saadi, K. Lanasri, K. Bachari, D. Halliche, C. Rabia, J. Phys. Chem. 2, 73 (2012)

7. A. Saadi, Z. Rassoul, M. Bettahar, J. Mol. Catal. A Chem. 258, 59 (2006)

8. H. Zazoua, A. Saadi, K. Bachari, D. Halliche, C. Rabia, Res. Chem. Intermed. 40, 931 (2014)

9. J. Weitkamp, G. Ertl, H. Knözinger, Handbook of heterogeneous catalysis (Wiley, New York, 1997)

0. D. Kumar, M. Chen, D. Goodman, Catal. Today 123, 77 (2007)

1. J.W. Lekse, M.K. Underwood, J.P. Lewis, C. Matranga, J. Phys. Chem. C 116, 1865 (2012)

2. J. Monnier, M. Hanrahan, G. Apai, J. Catal. 92, 119 (1985)

3. J. Christopher, C. Swamy, J. Mater. Sci. 27, 1353 (1992)

4. S. Kameoka, M. Okada, A.P. Tsai, Catal. Lett. 120, 252 (2008)

5. R. Cava, H. Zandbergen, A. Ramirez, H. Takagi, C. Chen, J. Krajewski, W. Peck Jr., J. Waszczak, G.

Meigs, R. Roth, J. Solid State Chem. 104, 437 (1993)

2

6. O. Garlea, C. Darie, C. Bougerol, O. Isnard, P. Bordet, Solid State Sci. 5, 1095 (2003)

1

3

H. Zazoua et al.

2

3

7. C.-Q. Jin, S. Adachi, X.-J. Wu, H. Yamauchi, S. Tanaka, Physica C 223, 238 (1994)

8. M. Alario-Franco, C. Chaillout, J. Capponi, J.-L. Tholence, B. Souletie, Physica C 222, 52 (1994)

9. R. Kipka, H. Müller-Buschbaum, Z. Naturforsch. B 32, 121 (1977)

0. M. Weller, D. Lines, J. Chem. Soc. Chem. Commun. 8, 484 (1989)

1. W. Gutau, H. Müller-Buschbaum, J. Less Common Metals 152, L11 (1989)

2. S. Kumar, M. Miclau, C. Martin, Chem. Mater. 25, 2083 (2013)

3. M. Stefanescu, M. Barbu, T. Vlase, P. Barvinschi, L. Barbu-Tudoran, M. Stoia, Thermochim. Acta

5

26, 130 (2011)

3

4

4. A. Athawale, P. Desai, Ceram. Int. 37, 3037 (2011)

5. H. Falcon, M. Martinez-Lope, J. Alonso, J. Fierro, Appl. Catal. B 26, 131 (2000)

6. R. Hu, R. Ding, J. Chen, J. Hu, Y. Zhang, Catal. Commun. 21, 38 (2012)

7. I. Lounas, H. Zazoua, A. Saadi, Z.M. Benyoucef, Res. Chem. Intermed. 44, 6549 (2018)

8. M. Englisch, A. Jentys, J.A. Lercher, J. Catal. 166, 25 (1997)

9. J.L. Margitfalvi, A. Tompos, I. Kolosova, J. Valyon, J. Catal. 174, 246 (1998)

0. K. Xiao, Z. Bao, X. Qi, X. Wang, L. Zhong, K. Fang, M. Lin, Y. Sun, J. Mol. Catal. A Chem. 378,

3

19 (2013)

4

5

1. M.A. Keane, J. Mol. Catal. A Chem. 118, 261 (1997)

2. L. Jalowiecki, G. Wrobel, M. Daage, J. Bonnelle, J. Catal. 107, 375 (1987)

3. S. Siegel, J. Catal. 30, 139 (1973)

4. R. Bechara, G. Wrobel, M. Daage, J. Bonnelle, Appl. Catal. 16, 15 (1985)

5. J. Jenck, J.-E. Germain, J. Catal. 65, 141 (1980)

6. X. Konga, L. Chen, Appl. Catal. 476, 34 (2014)

7. U. Schroder, L. Deverdier, J. Catal. 142, 490 (1993)

8. S. Fleischer, S. Zhou, K. Junge, M. Beller, Angew. Chem. 125, 5224 (2013)

9. E.-A.I. Heiba, P.S. Landis, J. Catal. 3, 471 (1964)

0. M. Jayamani, C. Pillai, J. Catal. 82, 485 (1983)

1. R. Sreekumar, C.N. Pillai, Catal. Lett. 19, 281 (1993)

2. M. Jayamani, N. Murugasen, C. Pillai, J. Catal. 85, 527 (1984)

3. S. King, E. Strojny, J. Catal. 76, 274 (1982)

4. F. Coloma, J. Llorca, N. Homs, P.R. Piscina, F. Rodríguez-Reinoso, A. Sepúlveda-Escribano, Phys.

Chem. Chem. Phys. 2, 3063 (2000)

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Article Doi

Selective gas-phase hydrogenation of benzaldehyde in the presence of Ni-, Co- and Fe-doped BaCuO2 delafossites: effect of metal substituent on the production of benzyl alcohol

DOI: 10.1007/s11164-019-03792-6

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