Textural, structural and catalytic behavior of low specific area silica-supported copper catalysts: effect of preparation method

Article abstract of DOI:10.1007/s11164-018-3508-8

Supported copper catalysts on low surface area silica were prepared by several methods and characterized by AAS, XRD, N₂ adsorption, SEM, H₂-TPR, N₂O titration, TGA-DTA, UV–Vis techniques. Their hydrogenating properties were examined in the gas-phase hydrogenation of benzaldehyde. The analysis of characterization results revealed that the choice of preparation method affected the texture, composition, and structure of the calcined and reduced Cu/SiO₂ catalysts. The dispersion and size distribution of copper species was present in different forms in the catalysts that exhibited low specific surface areas. In gas-phase hydrogenation of benzaldehyde to benzyl alcohol, the catalysts tested at the reaction temperatures of 160 and 200?°C were stable and conducted to a good catalytic activity and benzyl alcohol selectivity ranging between 5 and 39?μmol?min^?1?g^?1 and 0–95%, respectively. The activity of the catalysts in gas-phase hydrogenation also depended on the particle size and the nature of copper species formed on low surface area silica.

Full text of DOI:10.1007/s11164-018-3508-8

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-018-3508-8
Textural, structural and catalytic behavior of low specific
area silica-supported copper catalysts: effect of preparation
method
Ibtissem Lounas1 Hanane Zazoua1,2 Adel Saadi1
Zahia Mesbah Benyoucef3
•
•
•
Received: 2 March 2018 / Accepted: 12 June 2018
Ó Springer Nature B.V. 2018
Abstract
Supported copper catalysts on low surface area silica were prepared by several
methods and characterized by AAS, XRD, N2 adsorption, SEM, H2-TPR, N2O
titration, TGA-DTA, UV–Vis techniques. Their hydrogenating properties were
examined in the gas-phase hydrogenation of benzaldehyde. The analysis of char-
acterization results revealed that the choice of preparation method affected the
texture, composition, and structure of the calcined and reduced Cu/SiO2 catalysts.
The dispersion and size distribution of copper species was present in different forms
in the catalysts that exhibited low specific surface areas. In gas-phase hydrogenation
of benzaldehyde to benzyl alcohol, the catalysts tested at the reaction temperatures
of 160 and 200 °C were stable and conducted to a good catalytic activity and benzyl
alcohol selectivity ranging between 5 and 39 lmol min-1 g-1 and 0–95%,
respectively. The activity of the catalysts in gas-phase hydrogenation also depended
on the particle size and the nature of copper species formed on low surface area
silica.
Keywords Cu/SiO2 Á Benzaldehyde Á Benzyl alcohol Á Hydrogenation Á
Hydrogenolysis
&
Adel Saadi
asaadi@usthb.dz
1
2
3
Laboratory of Natural Gas Chemistry, Faculty of Chemistry, USTHB University, PO Box 32,
16111 El-Alia, Bab-ezzouar, Algiers, Algeria
Research Center in Analytical Chemistry and Physics (CRAPC), PO Box 248, Bou-Ismail,
Tipaza, Algeria
Department of Chemistry, Montmorency College, 475 Boulevard de l’Avenir, Laval,
QC H7N 5H9, Canada
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I. Lounas et al.
Introduction
Benzyl alcohol is an important product used in the chemical industry, notably in fine
chemicals. It is found in several fields, such as cosmetics, as a solvent and
preservative [1], but also in the food and pharmaceutical industries [2]. In industry,
benzyl alcohol is obtained by refluxing benzyl chloride, a long and, therefore, costly
process, which yields benzyl ether in large quantities [3] or by chlorination of
toluene followed by hydrolysis, except that this process is very toxic to the
environment [4]. For this reason, many researchers are interested in a cheaper and
especially cleaner way of producing benzyl alcohol using homogeneous or
heterogeneous catalysts. Conscious of the difficulties to use homogeneous catalysts,
especially in the catalyst–product separation, heterogeneous catalysts have been
investigated in the hydrogenation of benzaldehyde. This reaction has been the
subject of many works because of its simplicity to be implemented and which help
us to understand the mechanism of the latter [5]. In the last few years, considerable
works have been devoted to the reduction of benzaldehyde over supported precious
metal catalysts [6, 7], supported noble metal catalysts [8–10] or metal oxide
catalysts [11, 12], in liquid or gaseous phases [13, 14]. Recently, Nagaraja et al. [15]
published a review article describing the successful use of several catalysts for
catalytic hydrogenation of benzaldehyde for the selective synthesis of benzyl
alcohol. Various factors may be involved in the reduction of benzaldehyde and
favor benzyl alcohol formation, such as active metals [16, 17], acidic or basic
properties of support [18], effect of interaction oxygen support-hydrogen flow [19],
effect of electronic and geometric properties of the active phase influenced by the
support and catalysts preparation method [20–22]. Depending on the catalyst and
operating conditions, benzaldehyde can be reduced to toluene with benzyl alcohol
as intermediate or to benzene as end-product of hydrogenation or hydrogenolysis
reaction, respectively [23].
In the literature, it has been shown that copper-based materials appear as
excellent catalysts because of their high activity for vapor-phase hydrogenation
reactions [24–27], it has been suggested that metallic copper sites are active for the
selective hydrogenation of C - O bonds and are relatively inactive for the
hydrogenolysis of C - C bonds [28]. Our previous work and the literature on Cu
catalysts supported on silica indicated that high dispersion of copper species and
strong metal-support interactions are important factors to explain the high activity
and stability in hydrogenation of benzaldehyde [15]. In addition, the copper is never
used as a bulk catalyst it always comes with an additive and/or in the supported
form in order to have a better dispersion of the copper species [29]. Furthermore the
higher activity of Cu/SiO2 compared to other catalysts is related to the better
reducibility of the copper due to the large surface area and to the participation of
acid–base properties of the silica support. Under hydrogen atmosphere a high
benzaldehyde conversion and benzyl alcohol selectivity are obtained only with
copper supported over acidic silica, which prevents adsorption of benzyl alcohol
formed during the hydrogenation of benzaldehyde. The mechanism may be due to
active hydrogen species at the surface of catalysts generated by homolytic or
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Textural, structural and catalytic behavior of low…
heterolytic cleavage of hydrogen molecule, which directly reacts with benzaldehyde
to give toluene and benzyl alcohol [30].
The high activity of a copper-based supported catalyst often requires a highly
active surface area and, therefore, for small particles, i.e., a high dispersion of the
active phase; because small metal particles can sinter even at relatively low
temperatures, these generally are used in a support material, which itself is
thermally stable [31]. With an inert support such as silica, other important physical
characteristics such as texture (specific surface area, pore size distribution, and
porous volume), density, and mechanical strength can be established [32]. Munnik
et al. [33] recently published a review that summarizes the influence of catalysts
preparation method, beyond the nature of the metal and its charge. In fact, they
highlight that the effect of conventional and unconventional methods of preparation
could have a direct influence on the dispersion of a metal on the support, and
consequently on the catalytic properties of the materials. However, some methods of
preparation often make it difficult to control the morphology of the final material
[34, 35], hence the need to search for new alternative methods capable of obtaining
supported catalysts with well-defined metal particles is the preparation of supported
catalysts [36].
Amorphous silica is usually used as a support because of its important physical
properties, thermally stable and maintains a high specific surface area up to high
temperatures, it is also known that it does not promote the formation of mixed
oxides and it provides a better approach of the particle size effect on the behavior of
supported catalysts. In addition, copper-based catalysts have generally been
supported using high surface area materials and higher than 5 wt%. In our work,
we have found it also interesting to study the crystalline silica material with low
surface area as a support for a low copper weight because no studies have been done
and also to give rise to an important contribution to the corpus of literature.
The present study reports the results obtained over 3.0 wt% of copper supported
on crystalline silica of low area prepared by several methods. The stability and
surface properties of the copper particles were examined by different characteri-
zation techniques and correlated to their catalytic activity in benzaldehyde
hydrogenation.
Experimental
Catalysts preparation
The Cu/SiO2 samples were prepared from SiO2 AldrichÒ and Cu(NO3)2Á3H2O
(Panreac 99%) precursors with a content of 3.0 wt% by several methods:
1. Deposition–precipitation (DP) synthesis: catalyst DP is prepared by precipitat-
ing a solution of copper nitrate (0.5 M) by sodium carbonate at pH * 7 in
temperature average of 85–90 °C in the presence of SiO2 support. After
precipitation, the solid is washed and dried at 100 °C overnight.
2. Ionic-exchange (IE) method: catalyst IE is obtained by ionic exchange. An
aqueous solution of copper nitrate (0.5 M) is brought to a pH * 13 by the
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I. Lounas et al.
addition of concentrated ammonia. The SiO2 support is previously added to an
ammonia solution at the same pH and stirred for 6 h at room temperature. Then
the [Cu(NH3)4]2? complex is added and the mixture is stirred and evaporated.
The solid is washed with an ammonia solution and dried overnight at 100 °C.
3. Sol–gel (SG) method: the catalyst SG is prepared by sol–gel method from SiO2
as support mixed with ethanol and an aqueous solution of copper nitrate
(0.5 M). The molar ratio SiO2/C2H5OH/H2O (1/6/8) is added to tetra-ammonia
as complex agent. The mixture is placed under stirring and the obtained gel is
dried at 100 °C overnight.
4. Hydrolysis (HY) method: the catalyst HY is prepared first of all by adding an
aqueous copper nitrate solution (0.5 M) to a concentrated ammonia 25 wt%;
this solution mix is placed under stirring at a pH equal to 9 instead to form the
[Cu(NH3)4]2? complex. Then the SiO2 support is added to the mixture and
stirred for 30 min; the dough obtained is then placed in an ice bath at 0 °C, and
500 mL of distilled water is added dropwise to the suspension to hydrolyze the
copper complex. Finally, the solution is filtered, washed three times and dried at
100 °C overnight.
After the drying step, all samples are calcined in air flow at 350 °C for 3 h with a
heating rate of 5 °C/min, characterized by different methods and tested in the
hydrogenation reaction.
Characterization techniques
The contents of copper were determined by atomic absorption spectroscopy (AAS)
using a Perkin Elmer Analyst 700 apparatus. The samples were dissolved in a 3:1
mixture of HCl/HNO3 solution. Powder X-ray diffraction (XRD) patterns were
recorded on
a Philips X’pert Pro diffractiometer with Cu-Ka radiation
(k = 0.154 nm). Diffractograms were collected at room temperature from 10° to
80° range with a scanning step size of 0.02° and a counting time of 1 s.
Nitrogen-physisorption measurements were performed at - 196 °C using an
ASAP 2020 Micromeritics instrument. Before each measurement the samples were
first outgassed at 700 °C for 12 h and then at room temperature for 2 h. The BET
method was used to determine the specific surface areas whereas pore volumes/di-
ameters were calculated by the BJH theory [37].
H2-Temperature-programmed reduction (H2-TPR) of samples were carried out
on a Micromeritics II-Autochem 2920 instrument, using H2/Ar (5% v/v) as the
reducing gas mixture. The sample of 100 mg mass was heated from room
temperature to 850 °C at a heating rate of 7 °C/min after pretreatment at 350 °C in
argon gas flow. The H2 (total flow 25 mL/min) consumed during the experiment
was measured with a thermal conductivity detector (TCD). The water formed during
the reduction of the catalyst was condensed and trapped by a cold mixture
(isoprapanol ? liquid nitrogen, T = - 60 °C) and copper loading was estimated
from the H2 consumption by assuming the following reduction stoechiometry:
CuO ? H2 ? Cu ? H2O. The copper surface areas and copper dispersions were
determined according to N2O titration using the same equipment. Before
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Textural, structural and catalytic behavior of low…
measurements the calcined sample underwent a H2-TPR process in H2/Ar (5% v/v)
as reducing gas mixture from 50 to 500 °C for 2 h. After cooling down, the sample
was exposed to N2O/Ar (5% v/v) and oxidized at a flow rate of 25 ml/min at 60 °C
for 1 h, followed by Ar purging and cooling the sample bed down to room
temperature. Finally, H2-TPR was passed again with a gas mixture of H2/Ar to
500 °C. Copper dispersion was calculated by dividing the amount of surface copper
sites by the total number of Cu atoms; copper surface area was calculated by
assuming spherical shape of the Cu metal particles and a surface concentration of
1.47 9 1019 Cu atoms/m2.
Thermal gravimetry analysis (TGA) was performed by an SDT Q600 TA
Instruments heating rate 10 °C/min (nitrogen flow of 100 mL/min) and the
materials were placed in an alumina crucible. Diffuse reflectance UV–Vis
spectroscopic measurements were performed by SPECORD Plus spectrometer.
The spectra were collected at 200–1100 nm referenced to BaSO4. The scanning
electronic microscopy (SEM) images were acquired with a Fei-Quanta 250
microscope.
Catalytic experiments
The catalytic performances of the samples were evaluated in the gas phase
hydrogenation of benzaldehyde at reaction temperatures of 160 and 200 °C using a
fixed-bed glass tubular reactor with 200 mg samples at atmospheric pressure and a
total flow rate of 50 cm3 min-1. Before the catalytic test, each catalyst was reduced
in situ for 2 h at 250 °C in a current of H2 with a flow rate of 20 cm3 min-1
.
Gaseous benzaldehyde was obtained by bubbling of nitrogen gas in liquid
benzaldehyde (Aldrich, 99.98%) maintained at constant temperature (50 °C) in a
suitable saturator. After the pre-reduction step, the reactant gas feed consisted of
4.8 torr of benzaldehyde and 250 torr of H2 diluted in N2 (total flow rate of
50 cm3 min-1).
The gaseous reactant and products were analyzed on line using a Delsi IGC-121
ML chromatograph equipped with a FID detector and 10% CP-SIL 8 CB/
Chromosorb W column. Each reaction temperature was maintained constant until
the corresponding steady-state was reached as indicated by the gas chromatograph
analysis of the exit gases samples. Finally, no mass transfer was observed with
200 mg of the sample weight.
Results and discussion
Catalysts characterization
Copper loading and BET surface data of SiO2, DP, EI, SG and HY catalysts are
summarized in Table 1.
The elemental composition of catalysts was determined by atomic absorption
spectroscopy, and the results show that the samples contain only the copper phase
on silica support. The Cu contents are very close to the nominal value of 3% weight
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I. Lounas et al.
Table 1 Textural and structural properties of SiO2 and Cu/SiO2 catalysts
Catalyst
SiO2
Cu/SiO2-DP Cu/SiO2-IE Cu/SiO2-SG Cu/SiO2-HY
Cu loading (wt%)a
–
3.1
2.3
3.2
3.3
SBET (m2 g-1
Average pore volume (cm3 g-1 b
Average pore diameter (nm)b
)
7.0
0.0035
36.0
–
8.4
8.3
8.3
9.1
)
0.0079
5.2
0.0080
5.4
0.0192
0.0228
13.0
15.4
Theoretical H2 consumption
(mmol)c
48.8
36.2
50.4
51.9
Experimental H2 consumption
(mmol)c
–
45.6
34.5
46.8
44.6
% of total H2 consumptionc
–
–
–
93.4
1.3
95.3
3.1
92.9
2.4
85.9
3.5
Cu0 surface area (m2 g-1 d
)
Copper dispersion degree (%)
38.4
29.3
30.9
25.5
aDetermined by AAS method
bDetermined using BJH method
cCalculated assuming total reduction of Cu/SiO2
dDetermined by N2O titration
except on Cu/SiO2-IE catalysts, which exhibit a Cu content at around about
2.3 wt%, indicating an incomplete exchange with the Cu species.
The decomposition of Cu/SiO2 precursors was studied by TGA technique under
nitrogen flow in the temperature range 50–650 °C (Fig. 1). The obtained curves
show essentially three main and well-defined mass losses: (1) the first step
appearing below 200 °C is attributed to the desorption of physisorbed water [38],
(2) the second step situated between 200 and 350 °C corresponded to the
decomposition of nitrate and ammonium ions giving CuO phase [39], (3) the last
step appearing above 350 °C is probably due to the decomposition of carbonate
species and copper complex [40]. The analyzed results show that setting the
calcination temperature at 350 °C does not make it possible to eliminate all the
impurities that resisted washing during the preparation except for the ‘‘SG’’ and
‘‘IE’’ precursors.
The successive weight losses (%), the corresponding temperatures and the nature
of evolved gases observed in the TGA curves show that the decomposition of ‘‘IE’’
and ‘‘SG’’ precursors conducted to two mass losses, whereas ‘‘HY’’ and ‘‘DP’’
precursors show three mass losses. The first weight loss common to all precursors at
100 °C to ca. 200 °C can be attributed to the corresponding to the removal of
physisorbed water with a mass loss of 0.8–4.5%. The second decomposition step,
which starts at 200 °C and ends at 450 °C, may be caused by the departure of nitrate
and ammonium ions (weight loss: 1.1–18%). A continuous weight loss occurs above
450 °C, appears only on ‘‘HY’’ and ‘‘DP’’ precursors and is attributed to the
decomposition of carbonate and complex [Cu(NH3)4]2? species.
Figure 2 shows the XRD patterns of SiO2 (a), Cu/SiO2-IE (b), Cu/SiO2-SG (c),
Cu/SiO2-HY (d), Cu/SiO2-DP (e) catalysts calcined at 350 °C, and the results only
123

Textural, structural and catalytic behavior of low…
100.0
99.5
99.0
98.5
98.0
97.5
97.0
96.5
96.0
-0.005
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
100.0
99.5
99.0
98.5
98.0
97.5
97.0
96.5
IE
DP
-0.002
0.000
0.002
0.004
0.006
0.008
0.010
0.012
0.014
183
289
220
TGA
DTA
TGA
DTA
450
205
100
200
300
400
500
600
100
200
300
400
500
600
Temperature °C
Temperature °C
100
99
98
97
96
95
0.000
-0.05
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
100
95
90
85
80
75
70
HY
SG
0.005
0.010
0.015
0.020
225
530
155
TGA
DTA
TGA
DTA
268
306
100
200
300
400
500
600
100
200
300
400
500
600
Temperature °C
Temprature °C
Fig. 1 TGA-DTA curves of Cu/SiO2-IE, Cu/SiO2-DP, Cu/SiO2-HY, Cu/SiO2-SG. Measurements done
under nitrogen flow 100 mL/min, from 50 to 650 °C with 10 °C/min rate
exhibit the characteristic diffractions of a highly crystalline SiO2-like phase
(Fig. 2a). The wide angle powder XRD of calcined CuO/SiO2 catalysts display
typical images of quartz SiO2 species positioned at 21°, 26°, 50° corresponding to
(100), (101) and (112) lattice planes (JCPDS-ICDD card: 46-1045). However, no
reflection plan due to crystalline copper species (CuO, JCPDS-ICDD card: 05-0661)
and (Cu2O, JCPDS-ICDD card: 05-0667), copper hydroxide (Cu(OH)2, JCPDS-
ICDD card: 35-0505), or either metallic copper (JCPDS-ICDD card: 04-0836)
appear for any of the catalysts, which can be explained by the presence of
amorphous or very small copper species highly dispersed within the matrix [41, 42].
The textural properties and the N2 adsorption isotherms of calcined Cu/SiO2
catalysts are given in Table 1 and Fig. 3. The results show that SiO2 support and
Cu/SiO2 catalysts exhibit low and similar specific surface area with a value does not
exceed 10 m2 g-1. The BET surface area of the catalysts increases weakly
compared to the surface of the silica support and this increase is the same order for
each catalyst independently of the preparation method. The gap of the specific
123

I. Lounas et al.
♦
♦
Quartz, syn
(101)
♦
(100)
♦
(112)
♦
♦
♦ ♦
♦
♦
e
d
c
b
a
20
30
40
50
60
70
80
2 Theta (degress)
Fig. 2 X-ray diffraction patterns of calcined: a SiO2, b Cu/SiO2-IE, c Cu/SiO2-SG, d Cu/SiO2-HY, e Cu/
SiO2-DP catalysts. Filled diamond SiO2, Quartz. Scanning step of 0.02° under Cu-Ka radiation
(k = 0.154 nm)
Fig. 3 Nitrogen adsorption isotherms of samples prepared by several methods
surface area (20–30%) can be attributed to the addition of copper and calcination
effect.
According to IUPAC classification of adsorption isotherms, Fig. 3 shows that the
isotherms are typical of type II and the mean pore diameters calculated by BJH
theory indicate that all catalysts are in the mesoporous range (Table 1), but probably
contain also some macropores and/or non-porous grain aggregates. The comparison
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Textural, structural and catalytic behavior of low…
of mean pore volume and diameter reveals that ‘‘DP’’ and ‘‘IE’’ catalysts are similar
with a pore volume (pore diameter) of around 0.008 cm3 g-1 (5.3–5.4 nm). The
same result is obtained for ‘‘SG’’ and ‘‘HY’’ catalysts with pore volume (pore
diameter) of around 0.02 cm3 g-1 (13–15.4 nm). The decrease of mean pore
volume and diameter on ‘‘DP’’ and ‘‘IE’’ catalysts can be related to the placement of
copper nanoparticles around and inside the pores.
The H2-TPR measurement was carried out to study the redox properties of
samples previously calcined at 350 °C and the interaction between the copper
species and the low surface area silica support. It is especially suitable for studying
highly dispersed systems with low loading [43]. The obtained profiles of reduced
copper supported catalysts are represented in Fig. 4, and the reduction of copper
species is accomplished at different temperatures, which starts at 230 °C and ends at
450 °C. Despite the low copper content (3% wt) on silica support, the TPR profiles
show that each catalyst has a particular profile; therefore, it can be assumed that
each method of preparation provides different characteristics to each catalyst.
Indeed, the catalyst prepared by hydrolysis ‘‘HY’’ was reduced in one single step
(258 °C), the ‘‘DP’’ catalyst in two steps (396–412 °C), the ‘‘IE’’ catalyst in three
steps (238–305–356 °C) and finally the ‘‘SG’’ catalyst in four steps (292–350–
362–414 °C). The appearance of several reduction peaks indicates the presence of
copper nanoparticles with of different sizes in more or less strong interaction with
silica support. It is well known that CuO phase can be reduced in the temperature
range 230–500 °C [43] and may be related to the copper content, method of
preparation and the nature of the support.
The TPR study made by Dow et al. [44, 45] showed that two peaks were
obtained. The first peak appearing in the low temperature region is attributed to the
reduction of a highly dispersed copper species, including isolated Cu2? ions that
258°C
1,5
382°C
305°C
350°C
414°C
412°C
396°C
356°C
238°C
292°C
Cu/SiO2-HY
Cu/SiO2-SG
Cu/SiO2-IE
Cu/SiO2-DP
0,0
50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800
Temperature (C)
Fig. 4 H2-TPR profiles of Cu/SiO2-IE, Cu/SiO2-DP, Cu/SiO2-HY, Cu/SiO2-SG catalysts. Sample
mass = 100 mg, from 25 to 850 °C under H2/Ar flow (5% v/v) with a total flow of H2 25 mL/min
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I. Lounas et al.
strongly interact with the support, the isolated Cu2? that weakly interact with the
support (the cupric ions have close contact with each other), and small two-
dimensional clusters or three-dimensional ones. The second peak observed at higher
temperature region is ascribed to the reduction of bulk CuO.
In our case, the XRD results of the samples calcined at 350 °C revealed that no
larger CuO particles could be observed (no appeared peaks) but only amorphous or
highly dispersed Cu2? species and small clusters that may form. In addition, the
TGA study has shown that carbonate and ammonium ions present on ‘‘DP’’ and
‘‘HY’’ catalysts, respectively, resist thermal treatment and they are eliminated above
350 °C, which can make the reduction of copper species more difficult.
For the ‘‘DP’’ catalyst, the TPR profile analysis shows that the peak centered at
ca. 396–412 °C is attributed to the reduction of encapsulated CuO species with low
surface area silica support or trapped inside the pores because of the presence of
carbonate species. The small shoulder observed at 396 °C could account for the
two-step reduction (Cu2? ? Cu? ? Cu). The same behavior could be observed on
the ‘‘SG’’ catalyst. In addition to the peak obtained at 414 °C, two other peaks
occurring successively at about 350 and 382 °C are related to the presence of two
population types of small Cu2? sizes in weakly interaction with support. In the case
of Cu/SiO2-IE, the peak centered at 305 °C with a shoulder at 356 °C, is related to
the reduction of isolated Cu2? that weakly interact with the support (the cupric ions
have close contact with each other), and small two-dimensional or three-
dimensional clusters. The small broad peaks and shoulder observed at a low
temperature region (before 300 °C) are assigned to the reduction of isolated highly
dispersed Cu2? species.
The total H2 consumption ratio, reported in Table 1, shows that is superior to
90% in all cases except Cu/SiO2-HY catalyst, thus suggesting a good accessibility
of copper to the reduction step (from Cu2? to Cu? to Cu0). The Cu/SiO2-HY
catalyst shows the lowest total H2 consumption ratio, suggesting a more difficult
reduction, which can be ascribed to a diffusion of copper to the matrix due to the
strong metal-support interaction. As shown in Table 1 and Fig. 4, the copper
dispersion measured by N2O titration increases monotonically from 25.5 to 38.4%
with the increase of reduction temperature. The catalysts prepared by hydrolysis
exhibits low copper dispersion, whereas the catalyst prepared by deposition leads
high copper dispersion. As a result, the introduction of a low amount of copper on
low specific area silica results in the good metal dispersion. Meanwhile, Cu surface
areas exhibit a decreasing trend with the increase of the copper dispersion degree,
indicative of the decrease in the surface metallic copper sites. However, the DP
sample has the smallest Cu surface area (1.23 m2 g-1), which is ascribed to the
agglomeration of Cu particles or to encapsulated CuO species. To explain catalytic
properties, the TPR profiles show that the pre-reduction of catalysts before catalytic
reaction up to 250 °C leads mostly to the presence of Cu2? species on low surface
area silica support except on the catalysts IE and SG.
SEM images of calcined samples are displayed in Fig. 5. It can be seen that the
silica framework is intrinsically different, but well crystallized. The morphology
comparison of Cu/SiO2 catalysts shows that the ‘‘SG’’ and ‘‘DP’’ samples exhibited
spherical grains of uniform distribution with diameters lower than1 lm and 3 lm,
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Textural, structural and catalytic behavior of low…
Sol-Gel ‘SG’
Deposition-Precipitation ‘DP’
Hydrolysis ‘HY’
Ion-Exchange ‘IE’
Fig. 5 SEM images of Cu/SiO2 catalysts prepared per sol–gel (SG), deposition–precipitation (DP),
hydrolysis (HY) and ionic exchange (IE)
respectively, whereas more grain aggregation with irregular morphology is noticed
in the case of samples prepared by ion exchange and hydrolysis methods.
The UV–Vis spectra of the calcined samples prepared by different methods are
shown in Fig. 6. The results indicate that copper incorporated samples exhibit a
significant absorption in the visible light region due to the presence of Cu2? species.
The spectral analysis shows bands for the catalysts ‘‘DP’’ and ‘‘HY’’ in an interval
of 600–800 nm in d-d transitions of Cu2? ions, bands among 400–440 nm, which
correspond to the transfer of O–Cu–O and Cu–O–Cu that appears also in ‘‘EI’’ and
‘‘DP’’ catalysts [46].
Catalytic activity
The catalytic behavior of copper-based catalysts supported on low surface area
silica prepared by four methods has been examined in the gas phase reduction of
benzaldehyde under hydrogen flow at 160 and 200 °C reaction temperature after a
pre-reduction step at 250 °C. Figure 7a, b shows the catalytic activities of the
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I. Lounas et al.
564
SG
DP
689
607
IE
HY
573
400
500
600
700
800
Wavelength (nm)
Fig. 6 UV–Vis spectra of Cu/SiO2-IE, Cu/SiO2-SG, Cu/SiO2-HY and Cu/SiO2-DP catalysts
benzaldehyde reduction with time on stream at reaction temperature 160 and
200 °C, respectively. The low surface area silica used as support is totally inactive,
whereas copper-based supported catalysts are active and selective in the gas phase
reduction of benzaldehyde, and the reaction products are benzyl alcohol, toluene,
and benzene. The activities obtained in the presence of copper prove that the latter is
the main component of the active site.
At 160 °C reaction temperature (Fig. 7a), the copper-based catalysts prepared by
hydrolysis (HY) and deposition (DP) methods exhibited initial activation before the
steady-state is established after 4 h of reaction and the degree of activation
depended on the preparation method. It can be noted that Cu/SiO2 prepared by sol–
gel method (SG) is the less active whereas the same catalyst prepared by hydrolysis
method (HY) is the most active that exhibits higher activation with time on stream
(an increase from 18 to 32 lmol min-1 g-1). On Cu/SiO2-DP catalyst, an increase
of activity from 7 to 19 lmol min-1 g-1 is observed after about 4 h. Conversely,
Cu/SiO2-IE catalyst is the most stable since the beginning of the reaction. At 200 °C
reaction temperature, the inverse phenomenon is observed on all catalysts except
Cu/SiO2 prepared by sol–gel method (SG), whose initial activity rate was
maintained with time on stream and no deactivation was observed (Fig. 7b). The
remainder of the catalysts show initial deactivation before steady-state. On Cu/SiO2
catalysts prepared by deposition (DP) and hydrolysis (HY), a drop of activity from
49–21 to 53–39 lmol min-1 g-1, respectively, is observed after about 4 h working.
The decreased activity is the result of poisoning or blocking of active sites by
strongly adsorbed oxygen molecules [30].
The steady-state activity rate for all catalysts (Table 2) showed that the level and
the order of activity depended on both the preparation method and reaction
123

Textural, structural and catalytic behavior of low…
40
(a)
36
Reaction temperature = 160°C
HY
IE
DP
SG
32
28
24
20
16
12
8
4
0
0
50
100
150
200
250
300
350
400
Time (min)
72
68
64
60
56
52
48
44
40
36
32
28
24
20
16
12
8
(b)
HY
IE
DP
SG
Reaction temperature = 200°C
4
0
0
50
100
150
200
250
300
350
400
Time (min)
Fig. 7 Catalytic activity per lmol min-1 g-1 versus time on stream under H2 flow. Pretreatment
conditions: H2/250 °C. reaction temperatures: a 160 °C, b 200 °C
temperature. At 160 and 200 °C, the catalyst prepared by hydrolysis is by far the
most active, and the catalytic activity order obtained at 200 °C is a follows:
Cu=SiO2-HY ꢀ Cu=SiO2-SG [ Cu=SiO2-DP ꢀ Cu=SiO2-IE
To correlate the catalytic activity with the characterization results, the decreasing
order of activity according to the mode of preparation appears to be related to the
pore volume and diameter (see Table 1). Although there is insufficient published
data to arrive at any reliable trends regarding pores volume and size effects in
–CH=O hydrogenation, we can note the reported decrease in catalytic activity over
Cu/SiO2 prepared by deposition (DP) and ion-exchange (IE) methods
(\ 0.01 cm3 g-1 and 6 nm) and increase with increasing of pores volume and size
over Cu/SiO2-SG and Cu/SiO2-HY ([ 0.01 cm3 g-1 and 10 nm). The size and
volume of the larger pores ensure greater accessibility and dissociation of hydrogen
123

I. Lounas et al.
Table 2 Summary of catalytic activity and selectivity for the gaseous phase hydrogenation over Cu/SiO2
catalysts
Catalysts
TReaction (°C)
Activitya lmol min-1 g-1
Products distribution (% selectivity)a
Benzyl alcohol
Toluene
Benzene
Cu/SiO2-DP
Cu/SiO2-IE
Cu/SiO2-SG
Cu/SiO2-HY
160
200
160
200
160
200
160
200
19
21
10
16
5
80
45
95
76
97
20
44
–
5
15
48
–
7
05
24
–
–
03
05
06
09
35
32
39
75
50
91
aResults obtained at steady-state after 4 h of reaction time. Reaction conditions: carrier gas = H2 and
pressure = 1 atm, pre-reduction temperature = 250 °C, reaction temperature = 160 and 200 °C
on the different species of copper formed and its reaction with the reagent. With
agreement to the surface structure, the presence of low pore volume and size can
lead to decreased adsorption and reaction of carbonyl group with the active sites.
The presence of different sizes of copper nano-crystallites interacting with the silica
support must lead to a more or less homogeneous distribution of the copper species
in the silica support, which makes it difficult to adsorb and dissociate hydrogen
[47, 48].
Based on metallic copper surface area and dispersion degree, one important key
to high catalytic performance is a large, accessible Cu surface area because larger
copper surface area can provide more catalytically active sites for hydrogen
dissociation and thus higher conversion. Indeed, the catalyst prepared by hydrolysis
with the largest active Cu surface area and smallest dispersion degree exhibits
superior catalytic performance to other methods, but favor toluene and benzene as
by-products. It can be deduced that the Cu species should be the effective
catalytically active centers for dissociating hydrogen in the consecutive hydro-
genation of benzaldehyde to toluene. For the rest of the preparation methods, the
obtaining of small Cu surface areas is synonymous of the predominance of Cu2?
species on the catalysts surface, which leads to a low conversion of benzaldehyde.
The higher copper dispersion degree confirms the formation of weak metal-support
interactions, making Cu2? reduction more difficult.
On the other hand, the ‘‘IE’’ catalyst with the lowest copper content (2.3 wt%) is
the least efficient, whereas the higher copper content obtained on the HY catalyst
(3.3 wt%) led to higher activity. In addition and based on TGA results, the presence
of heat-resistant carbonate species (Fig. 1) on the ‘‘DP’’ catalyst significantly affects
also the catalytic activity. On the contrary, the existence of the copper ammonium
complex on the ‘‘HY’’ catalyst conducting to the formation of a single population of
readily reducible copper particles (see TPR profiles) could be related to the high
activity.
123

Textural, structural and catalytic behavior of low…
On the basis of the results given in the literature, a strong adsorption of the
oxygenated species (as the reagent) on the support surface could cause a blocking of
the active sites. Indeed, the adsorption studies of benzaldehyde over several metal
oxides confirm the contribution of acid–base properties of few supports such as
silica in the reduction of –C=O function. The FTIR study [48] revealed that the
adsorption of benzaldehyde on silica involves hydrogen bonding between the
oxygen atom of the C=O group and the surface silanol groups, producing surface
benzoate species [49, 50].
Selectivity
The gas phase hydrogenation of benzaldehyde is used in an attempt to correlate the
benzylacohol selectivity as the main product with the physicochemical properties of
the catalysts. As seen in Table 2, over Cu/SiO2 prepared by several methods,
benzaldehyde is converted mainly to benzyl alcohol, toluene and benzene, which are
the expected products from successive hydrogenation/hydrogenolysis and decar-
bonylation, respectively [30, 51]. Under the operating conditions fixed to this
reaction, no aromatic ring hydrogenation or its opening was observed. The aromatic
ring is unreactive while the –C=O group undergoes a transformation suggesting that
benzaldehyde reacts primarily with the catalyst via carbonyl function. In this case,
both the aromatic ring and the carbonyl group lie parallel to the active surface [52]
and the adsorbed hydrogen preferably attacks the carbonyl group because of the
energy barriers [52, 53]. According to the results given in the literature, reaction
selectivity is challenging in benzaldehyde hydrogenation due to the number of
intermediates and by-products formed, which depends on several parameters such as
the nature of metal [6], the nature of phase reaction (liquid or gas) [18, 54], the
nature of gas pretreatment and reaction when carried out in gas phase [18], the
nature of the solvent when the reaction is carried out in the liquid phase [13].
Whatever the preparation method, benzyl alcohol was obtained over all catalysts
at 160 and 200 °C. The observed increasing of benzyl alcohol selectivity with
reaction time suggests that it is mostly formed as the primary product of direct
hydrogenation of benzaldehyde (Fig. 8). Between 160 and 200 °C, the results
summarized in Table 2 reveal that the high benzyl alcohol selectivity appeared on
the least active catalysts prepared by deposition (80% at 160 °C), sol–gel (97% et
160 °C) and ion-exchange methods (95% at 160 °C), whereas the high activities
encouraged the apparition of toluene as the principal product with high selectivity
particularly at 200 °C on Cu/SiO2 catalysts prepared by sol–gel and hydrolysis
methods. For all catalysts, when the reaction temperature increases, the benzyl
alcohol selectivity decreases and benzaldehyde conversion increases. From the
characterization of catalysts, we can conclude that the differences of selectivity and
conversion would not be related to the specific surface area since all catalysts have
the similar surface data. The state and distribution of copper species in the catalysts
may be the key factors to the reaction.
Several pertinent studies have examined the importance of the different oxidation
states of copper in the hydrogenation reactions, but without a general agreement on the
copper oxidation state at the origin of benzyl alcohol formation from benzaldehyde.
123

I. Lounas et al.
Reaction temperature = 160°C
(a)
100
90
80
70
60
50
40
30
20
10
0
HY
IE
DP
SG
0
50
100
150
200
250
300
350
400
Time (min)
(b)
100
Reaction temperature = 200°C
HY
IE
DP
90
80
70
60
50
40
30
20
10
0
SG
0
50
100
150
200
250
300
350
400
Time (min)
Fig. 8 Benzyl alcohol selectivity versus time on stream under H2 flow, pressure = 1 atm, reaction
temperatures: a 160 °C, b 200 °C
Before the reduction temperature 250 °C and in agreement with N2O titration results,
the TPR analysis (Fig. 4) shows that no Cu? or Cu0 species are detected in all catalysts
except on ‘‘Cu/SiO2-HY’’ and ‘‘Cu/SiO2-IE’’ (a small amount is reduced). For the rest
of the catalysts the predominant species are mainly the isolated Cu2? that weakly
interacts with the support and small copper clusters. On the catalyst prepared by
hydrolysis method, the reduction of a good amount of Cu2? species in strong
interaction with the support could be at the origin of the low benzyl alcohol selectivity.
The formation of benzyl alcohol as a primary product from benzaldehyde
hydrogenation occurs via a nucleophilic mechanism where the carbonyl group is
adsorbed and activated at the metal/support interface [30]. A co-planar form of
benzaldehyde adsorption to the catalyst surface facilitates C=O polarization, making
the oxygen sensitive to nucleophilic attack with proton transfer to generate benzyl
123

Textural, structural and catalytic behavior of low…
alcohol [21]. The possible reaction pathways proposed for hydrogenation of
benzaldehyde are shown in Fig. 9.
At the same time, high selectivity of toluene is obtained with ‘‘HY’’ catalyst. So
it is easy to conclude that the ability of hydrogenation/hydrogenolysis is related to
the CuO species reduced to Cu0 during pretreatment and reaction, which is the main
reactive site for toluene formation. The results suggest that the highly dispersed
Cu2? ions were the active phase for the obtaining of highly toluene selectivity,
whereas the weakly dispersed Cu2? species were the active site for the benzyl
alcohol formation. The obtained toluene as product from successive reaction
hydrogenation of benzaldehyde or benzyl alcohol may be favored by a strong
interaction between oxygen of carbonyl or alcohol functions and the surface with
Fig. 9 Proposed mechanism for benzaldehyde hydrogenation over Cu/SiO2 catalysts
123

I. Lounas et al.
hydrogenolytic cleavage of C=O or C–OH by dissociated hydrogen. The toluene
production from benzaldehydde could be the most probable pathway because of the
electrophilic character of C=O which renders this functionality more reactive
relative to O–H [13]. Keane et al. [6] showed that the rate of toluene production
over Ni/Al2O3 from benzaldehyde was significantly higher than that obtained from
benzyl alcohol and this pathway can be taken to be the predominant route to toluene.
In contrast to the other preparation methods, the most selective catalyst to benzyl
alcohol (Cu/SiO2-IE) does not form any benzene as a by-product. This remarkable
difference suggests that the formation of benzene requires an important hydro-
genation/hydrogenolysis activity. Indeed, benzene selectivity obtained on the ‘‘DP’’
catalyst (15–48%), suggests the participation of copper species, which are difficult
to reduce (from 350 to 450 °C in the TPR profile). This phenomenon is probably
attributed to isolated Cu(II) enveloped within the SiO2 matrix and formed small
clusters, which might find access difficult or reduced by hydrogen or reagent
molecules.
Conclusion
Cu/SiO2 catalysts have been successfully synthesized by several methods and the
surface structure of the catalysts is determined by the preparation method. High
selectivity to benzyl alcohol and low conversion of benzaldehyde were observed on
the ‘‘IE’’ catalyst, whereas high selectivity to toluene and high benzaldehyde
conversion were observed on the ‘‘HY’’ sample. The hydrogenation activities of all
catalysts were strongly dependent on their surface structures. Based on the textural
and structural results, the different distribution of Cu2? species over all catalysts
have been observed. Readily reducible copper species that may be reduced to
metallic copper are responsible for the formation of toluene, while Cu2? species
which reduce only at higher temperatures lead to the formation of benzyl alcohol
and benzene. The small CuO clusters that may not be reduced to metallic copper are
the active sites to catalyze the benzaldehyde hydrogenation reaction and to give
benzyl alcohol as the primary product. The results presented in this work provided
insight into the relationship between the surface structures of the catalysts
determined by the preparation methods and the catalytic behaviors that were
accordingly affected.
Compliance with ethical standards
Conflict of interest All authors declare that they have no conflict of interest.
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