The influence of alumina phases on the performance of Pd/Al2O3 catalyst in selective hydrogenation of benzonitrile to benzylamine

Article abstract of DOI:10.1016/j.apcata.2017.07.032

Various phase alumina supports were prepared by heating commercial pseudoboehmite at 400?°C and 1200?°C to obtain gamma (γ) and alpha (α) Al₂O₃, and by calcinations of synthetic bayerite at 400?°C and 1000?°C to obtain Eta (η) and theta (θ) Al₂O₃. These supports were then impregnated and calcined to prepare 0.5?wt% Pd catalysts. The prepared catalysts were characterized by XRD, BET, NH₃–TPD, CO–TPD, H₂–TPR, etc., and their catalytic performances were evaluated for selective hydrogenation of benzonitrile. In the order of Pd/γ-Al₂O₃, Pd/η-Al₂O₃, Pd/θ-Al₂O₃ and Pd/α-Al₂O₃, the surface areas decreased from 265 to 3.8?m²/g, the number of acidic sites was decreased, especially for strong acid sites, and meanwhile, the Pd size increased from 3.0 to 36.9?nm. The benzonitrile conversion showed limited relationship with the nature of catalysts and assumed to rely on the H₂ transmission rate. The dibenzylamine formation was considered to be a result of competitive reactions and performed as a function of the metal dispersion. Among the four Pd/Al₂O₃ catalysts, the Pd/γ-Al₂O₃ and Pd/η-Al₂O₃ both exhibited excellent stability and catalytic performance with a high benzylamine yield of ca. 90%.

Full text of DOI:10.1016/j.apcata.2017.07.032

Accepted Manuscript
Title: The influence of alumina phases on the performance of
Pd/Al2O3 catalyst in selective hydrogenation of benzonitrile to
benzylamine
Authors: Chengyong Dai, Yonggang Li, Chunli Ning,
Wenxiang Zhang, Xueguang Wang, Chunlei Zhang
PII:
S0926-860X(17)30342-3
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2017.07.032
APCATA 16337
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
4-5-2017
15-7-2017
21-7-2017
Please cite this article as: Chengyong Dai, Yonggang Li, Chunli Ning, Wenxiang Zhang,
Xueguang Wang, Chunlei Zhang, The influence of alumina phases on the performance
ofPd/Al2O3catalystinselectivehydrogenationofbenzonitriletobenzylamine, Applied
Catalysis A, Generalhttp://dx.doi.org/10.1016/j.apcata.2017.07.032
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Highlights
 Performance of various Pd/Al2O3 are compared for benzonitrile hydrogenation
 Catalytic activity of Pd/Al2O3 relies on H2 transmission rate
 Both Pd/γ-Al2O3 and Pd/η-Al2O3 achieve a high benzylamine yield of ca. 90%
 Formation of dibenzylamine has a close relation with Pd dispersion
1

The influence of alumina phases on the performance of Pd/Al2O3 catalyst
in selective hydrogenation of benzonitrile to benzylamine
Graphical abstract
3

The influence of alumina phases on the performance of
Pd/Al2O3 catalyst in selective hydrogenation of benzo-
nitrile to benzylamine
Chengyong Daia, b, Yonggang Lib, Chunli Ningb, Wenxiang Zhangb, Xueguang Wang
a,
, Chunlei Zhangb,*
a State Key Laboratory of Advanced Special Steel, School of Materials Science and
Engineering, Shanghai University, Shanghai 200072, China
b Technology Research Institute of Shanghai Huayi Group, Shanghai 200241, China
Keywords: Hydrogenation, Benzonitrile, Benzylamine, Pd/Al2O3, Alumina phases
 Corresponding author:
E-mail: wxg228@shu.edu.cn (X. G. Wang); zhangchunlei@shhuayi.com (C. L.
Zhang)
4

Abstract:
Various phase alumina supports were prepared by heating commercial pseu-
doboehmite at 400 °C and 1200 °C to obtain gamma (γ) and alpha (α) Al2O3, and by
calcinations of synthetic bayerite at 400 °C and 1000 °C to obtain Eta (η) and theta (θ)
Al2O3. These supports were then impregnated and calcined to prepare 0.5 wt% Pd
catalysts. The prepared catalysts were characterized by XRD, BET, NH3–TPD,
CO–TPD，H2–TPR，etc., and their catalytic performances were evaluated for selective
hydrogenation of benzonitrile. In the order of Pd/γ-Al2O3, Pd/η-Al2O3, Pd/θ-Al2O3 and
Pd/α-Al2O3, the surface areas decreased from 265 to 3.8 m2/g, the number of acidic
sites was decreased, especially for strong acid sites, and meanwhile, the Pd size in-
creased from 3.0 to 36.9 nm. The benzonitrile conversion showed limited relationship
with the nature of catalysts and assumed to rely on the H2 transmission rate. The
dibenzylamine formation was considered to be a result of competitive reactions and
performed as a function of the metal dispersion. Among the four Pd/Al2O3 catalysts,
the Pd/γ-Al2O3 and Pd/η-Al2O3 both exhibited excellent stability and catalytic per-
formance with a high benzylamine yield of ca. 90%.
5

1. Introduction
Benzylamine (BA) is an important chemical compound that has been found wide-
spread applications in textile, chemical, military and agrochemical industries [1-8].
The heterogeneous catalytic hydrogenation of benzonitrile (BN) to benzylamine, re-
garded as having greater atom efficiency, is challenging due to undesired by-products
including dibenzylamine (DBA), toluene (TOL), etc., as shown in Scheme 1 [9-11]. It
is noteworthy that the byproduct DBA influences the product quality seriously, and it
is recognized as being a priority to inhibit [12]. There has been much focus on the
skeletal Ni [13] or Co [14] catalysts for their high activities to primary amines; how-
ever, considerable care is required in handling such pyrophoric catalysts on scale-up,
and moreover, reactions often need to be carried out with vast amount of extra addi-
tives. Supported metal catalysts like Ir [15], Cu [16], Ru [17], Ni [18, 19] and Pd [11,
12, 20, 21], have been investigated for the hydrogenation of benzonitrile, and the
Pd/Al2O3 catalysts exhibited the highest activity and selectivity [12]. Recently, we
have investigated the Pd/Al2O3 catalyst systematically as well and found it superior in
the trickle-bed hydrogenation of BN [22]. Over it, a long-term stability test was per-
formed and deactivation study of corresponding Pd/Al2O3 catalyst indicated a trans-
formation of the alumina phase during the running [23].
Support has been investigated in the hydrogenation of nitriles based on some tradi-
tional oxides, such as Al2O3, TiO2, MgO and SiO2 [24, 25]. These authors concen-
trated on the acid-base properties of these supports and considered that it was related
to the selectivity of amines. However, in addition to the acid-base properties, the
chemical properties of the supports might have a more pronounced effect on the cata-
lytic performance. Hence, the usage of different crystalline alumina as supports will
6

be more convincing to understand the nature of supports on the catalytic performance,
because they have the same chemical composition.
The crystal structure of support has been proved to play an interesting effect on the
catalytic performance [26-28]. Hence, it is a desire to investigate the influence of
alumina phases on the performance of Pd/Al2O3 catalyst in the hydrogenation of ben-
zonitrile.
Scheme 1 Probable reaction pathways in the hydrogenation of benzonitrile to ben-
zylamine.
2. Experimental
2.1 Catalysts preparation
Pseudoboehmite (AlO(OH), 413 m2/g) and sodium aluminate (NaAlO2) were ob-
tained from CHALCO Shandong Co., Ltd. Pd(NO3)2 aqueous solution was purchased
from Heraeus Materials Technology Shanghai Ltd. Other chemicals were graded ana-
lytically, purchased from Sinopharm Chemical Reagent Co., Ltd. and used without
further purification.
7

The precursor, synthetic bayerite, was prepared by the following method similar to
reported literature [29-31]. Initially, 250 mL of 2.4 mol/L NaAlO2 aqueous solution
were heated in a stainless steel batch reactor at 40 °C by a water bath. Then carbon
dioxide was introduced and the pressure was increased to 1 MPa. With mechanical
stirring, precipitate produced during the carbonation process was aged in the mother
solution for 48 h. The resulting white precipitate was filtered and washed with dis-
tilled water as free as possible from alkali, then dried at 120 °C and was identified as
Al(OH)3 with crystal structure of bayerite, by powder X–ray diffraction (XRD),
TG–DSC and FT–IR [31]. Eta (η) and theta (θ) phase alumina were obtained through
calcinations of obtained bayerite at 400 °C and 1000 °C for 6 h, respectively. Simi-
larly, gamma (γ) and alpha (α) phase alumina were obtained through calcinations of
pseudoboehmite at 400 °C and 1200 °C for 6 h, respectively [32-35]. Various phase
alumina synthesized above are characterized and identified as detailed below.
Various phase alumina supported Pd catalysts containing 0.5 wt% Pd were pre-
pared through the incipient wetness impregnation of metal nitrates with above Al2O3
supports. As an example, 10 g of Pd(NO3)2 aqueous solution (0.047 mol/L, adjusted to
pH of 1.5 by nitric acid) was dropped slowly into 10 g of γ-Al2O3 support (40–60
mesh) with vigorous mixing. The composites were aged at room temperature for 12 h,
dried at 100 °C overnight and then calcined at 400 °C in air in a muffle furnace for 4 h.
Then the fresh catalyst labeled Pd/γ-Al2O3 was obtained. The final Pd loadings of all
supported catalysts were determined to be within the range of 0.47  0.02 wt% by in-
ductively coupled plasma optical emission spectrometry (ICP-OES).
2.2 Catalyst evaluation and analysis
8

Hydrogenation of benzonitrile was performed in a batch reactor at 90 °C under 1.5
MPa. The temperature was monitored by a thermocouple inserted in a thermowell
within the solution. After filling the reactor with 1.25 g catalyst and 40 g ethanol, the
system was purged with H2 and pressurized to 1.5 MPa. Then, the solvent/catalyst
mixtures were heated to 90 °C and kept for 2 h in order to reduce the catalyst under
stirring at 1000 rpm [36]. 10 g BN was added to the batch reactor via a pressurized
filling tank, in this way, the exact starting time of reaction could be assured. The eth-
anol serves as an inert solvent under reaction conditions. Samples were withdrawn
periodically from the reaction mixture via an outlet valve and analyzed by Agilent
7890A equipped with a FID detector, employing a DB-5 capillary column. Benzo-
nitrile conversion (X) and product selectivity (Si) were calculated with the equation
(Eqs. 1, 2), shown as follows:
[BN ]in [BN ]out
(1)
(2)
X(%) 
Si (%) 
100
100
[BN ]in
[Product]i  Ni
[BN ]in [BN ]out
Where [BN]in and [BN]out are the molar amounts of BN in the inlet and outlet
streams, respectively. [Product]i is the molar amounts of product i in the outlet mix-
ture and Ni is the stoichiometric coefficient for product i with respect to BN, for ex-
ample, the stoichiometric coefficient is 2 for DBA, and 1 for BA and TOL.
Turnover frequency (TOF) values are calculated by the formula as given below：
Reaction rate
TOF =
Number of Pd sites
Where reaction rate (mol/s) is calculated by the BN conversion within the initial
reaction time of 15 min, and number of Pd sites (mol) is determined by CO chemi-
sorption method.
9

2.3 Catalyst characterization
The X-ray diffraction (XRD) patterns were obtained at room temperature using
Bruker D8-advance X-ray diffractometer. The powdered samples were scanned using
Cu Kα radiation at a power of 40 kV and a current of 40 mA in the 2θ range of 10 –
80° with a scan rate of 2° per minute.
Thermogravimetry and differential scanning calorimeter measurements (TG –
DSC) were performed with a simultaneous thermal analyzer model STA449F3 of
NETZSCH Instruments. Non-isothermal experiments were carried out under dynamic
conditions from room temperature to 1100 °C at a heating rate of 10 °C/min under air
atmospheres. The average sample weight was about 30 mg, and the gas flow rate was
50 mL/min.
Fourier transform infrared (FT–IR) was carried on a Nicolet NEXUS 670 spec-
trometer with a resolution of 2 cm−1 using the KBr method.
Transmission electron microscopy (TEM) analyses were performed on a JEOL
JEM-2100F microscope operating at 200 kV. The sample was prepared by placing a
drop of ethanol solution of a well-ground catalyst powder onto a carbon-coated cop-
per grid, followed by evaporation of ethanol.
N2 adsorption and desorption isotherms were measured using a Quantachrome Au-
tosorb–iQ analyzer at liquid nitrogen temperature (−196 oC). Prior to analysis, the
sample was processed by vacuum degassing treatment for 16 h at 250 °C. The specific
surface area (SBET) was evaluated using the Brunauer–Emmett–Teller (BET) method
in the P/P0 range from 0.05 to 0.25. Pore size distribution curves were calculated us-
ing the desorption branch of the isotherms and the Barrett–Joyner–Halenda (BJH)
method. Pore sizes were calculated by the BJH method and pore volumes were calcu-
lated from the adsorbed amount at P/P0 = 0.990. The pore volume of Pd/α-Al2O3 was
10

measured by the water displacement method, and meanwhile, the average pore diam-
eter was calculated from the pore volume and BET surface area data by assuming all
pores have cylindrical shapes [37, 38]. In addition, the mesoporous structure of
Pd/α-Al2O3 was also measured by a Quantachrome Poremaster-60 GT mercury po-
rosimetry analyzer which provided the data of pore diameter and pore volume.
The temperature–programmed desorption of ammonia (NH3–TPD) were conducted
on a Micromeritics AutoChem II 2920 apparatus. Typically, about 50 mg sample was
loaded in a quartz reactor and treated at 200 °C in He flow (30 mL/min) for 1 h, and
then free cooled to 50 °C. Afterwards, the sample was pretreated with a 10 vol%
NH3/He mixed gas flow (30 mL/min) for 30 min and then purged in He flow (30
mL/min) for 60 min. Then the sample was heated to 800 °C at a rate of 10 °C/min
with the flow of helium gas. Desorption of ammonia was monitored with a thermal
conductivity detector (TCD).
Dispersion of metal Pd was determined by CO chemisorption on the same instru-
ment as NH3–TPD. About 50 mg sample was reduced in a 10 vol% H2/He flow (30
mL/min) at 300 °C for 1 h, and then cooled to 50 °C and kept in a He flow (30
mL/min) for 30 min. After this, the sample was treated with a 10 vol% CO/He flow
(30 mL/min) for 30 min, and then purged in flowing He (30 mL/min) for 60 min. The
sample was heated to 600 °C at a rate of 10 °C/min in the He flow. The increase of
the TCD signal area indicated the CO uptake chemisorbed. The metal dispersion (D)
was calculated from the CO chemisorption data, where CO is known to adsorb on Pd
in both an end-on configuration (CO/Pd = 1) and a bridged configuration (CO/Pd =
0.5) [39–42]. The configuration status are identified by the CO–TPD peak position
and detailed in the text. The average particle size for reduced Pd0 (dPd) was calculated
by the equation of dPd = 1.12/D (nm) [43, 44].
11

Temperature programmed reduction in hydrogen (H2–TPR) was carried out on the
same instrument as NH3–TPD. About 50 mg sample was initially treated at 200 °C in
He flow (30 mL/min) for 1 h and then cooled to 50 °C. Then a 10 vol% H2/He mix-
ture (30 mL/min) was admitted to the reactor and the sample was heated to 120 oC at a
rate of 10 °C/min. H2 consumption and evolution were measured with a TCD detector
as a function of the temperature.
3. Result and discussion
3.1 Pseudoboehmite and bayerite
Pseudoboehmite used as the precursor to synthesize the γ- and α- phase Al2O3, was
verified by XRD (see Fig. 1) [32]. The synthetic bayerite, applied for the synthesis of
η- and θ- phase Al2O3, was identified by XRD, TG–DSC and FT–IR method, as
shown in Fig. 2. The XRD result (Fig. 2a) showed the compounds were identified to
Al(OH)3 with primary crystal structure of bayerite according to JCPDS no. 74-1119.
Fig. 2b showed the typical TG–DSC curves for the thermal decomposition of synthet-
o
ic compound from room temperature to 1100 C. The curves showed a sharp decom-
position with a noticeable endothermic peak at ca. 255 oC, where the mass loss was ca.
30 wt%. These results were consistent with the theoretical value for the thermal de-
composition of aluminum hydroxide to aluminum oxide, and were in good agreement
with the data of bayerite in the reported literature [30, 31]. FT–IR spectra (Fig. 2c)
verified the existence of crystalline bayerite. Five distinguished peaks due to O–H
stretching vibration during 3400–3700 cm-1 were ascribed to the bayerite [30, 45]. In
addition, O–H bending bands at 1024 and 974 cm-1 and a broad band centered at
around 770 cm-1, due to Al–OH group, were clearly observed for the composition.
The IR spectrum of the crystallized compound was in good agreement with absorption
12

due to bayerite [29, 45]. The specific surface area of the sample was 9.1 m2/g and the
results are in good consistent with other studies [30, 46]. All in all, the synthetic
compound was verified to be a relatively high purity bayerite.
Pseudoboehmite
+
+
+
20
15
10
5
+
+
+
+
+
0
10
20
30
40
50
60
70
80
2 θ (o)
Fig. 1 XRD patterns of pseudoboehmite.
13

Bayerite
*
*
20
15
10
5
a
*
*
*
*
*
*
*
*
0
10
20
30
40
50
60
70
80
2 θ (o)
0
-10
-20
-30
-40
b
1.0
Endo.
400
200
600
800
1000
Temperature (oC)
80
80
60
40
20
0
c
60
40
20
0
3657
3617
3550
3524
3462
770
800
1024 974
1000
600
3800 3600 3400 3200
Wavenumber (cm-1)
Fig. 2 Identification of synthetic bayerite. (a) XRD, (b) TG–DSC and (c) FT–IR.
3.2 Catalysts
14

3.2.1 XRD analysis
The XRD patterns of various phase alumina supported Pd catalysts were illustrated
in Fig. 3. No diffraction peaks associated with Pd were observed in all cases due to
the low loading and high dispersion of metal Pd.
Among the four Pd/Al2O3 catalysts, the θ-Al2O3 and α-Al2O3 supported catalysts
exhibited characteristic XRD patterns which can be identified clearly according to
JCPDS file no. 35–0121 for θ-Al2O3 and JCPDS file no. 10–0173 for α-Al2O3, re-
spectively. The XRD patterns of the γ-Al2O3 and η-Al2O3 supported catalysts were
very diffuse and exhibited extensive reflection overlapping. According to the method
mentioned in other literatures, the γ and η alumina phases were distinguished through
the comparison including the full width at half maximum (FWHM) of individual re-
flections with their hkl's and relative intensities [47, 48]. As a result, these samples
were all proved to be well crystallized single-phase alumina and complied with the
results in the literatures [28, 35, 47, 49].
15

Pd/θ-Al2O3
+
θ-Al2O3
+
+
+
2000
+
+
+
+
+
+
+
+
+
+
+
+
Pd/α-Al2O3
α-Al2O3
*
*
*
15000
*
*
*
*
*
*
*
*
10
20
30
40
50
60
70
80
2 θ (o)
+
+
γ-Al2O3
Pd/γ-Al2O3
+
+
1000
+
+
+
*
η-Al2O3
*
Pd/η-Al2O3
*
*
*
1000
*
*
*
20
30
40
2 θ (o)
50
60
70
80
Fig. 3 XRD patterns of Pd/θ-Al2O3, Pd/α-Al2O3, Pd/γ-Al2O3 and Pd/η-Al2O3.
3.2.2 TEM analysis
As shown in Fig. 4a, b and c for γ-, η- and θ-Al2O3 supported catalysts, Pd particles
could not be identified in the low magnification due to the low Pd loading and the in-
terference of alumina supports. However, the presence of spherical Pd particles could
be clearly evidenced by HR-TEM. The distance of 0.227 nm between the adjacent
lattice fringes agreed well with the (1 1 1) lattice spacing of cubic metallic palladium
nanoparticles [50, 51]. The sizes of Pd particles were located in the range of 3–8 nm,
and increased gradually by the order of Pd/γ-Al2O3, Pd/η-Al2O3 and Pd/θ-Al2O3. In
16

Fig. 4d for Pd/α-Al2O3, the Pd particles exhibited obvious agglomeration. Similar re-
sults were also observed by other studies for Pd/α-Al2O3 [52]. The mean sizes of Pd
particles were listed in the following Table 2 for convenient comparison.
a
b
50 nm
c
d
Fig. 4 TEM images of the reduced Pd/Al2O3 catalysts: (a) Pd/γ-Al2O3, (b) Pd/η-Al2O3,
(c) Pd/θ-Al2O3 and (d) Pd/α-Al2O3。
3.2.3 Textural properties
The specific surface areas, pore volumes, and average pore diameters of the sup-
port precursors and the Pd/Al2O3 catalysts are given in Table 1. Due to the low Pd
loading, the textural properties of catalysts are ascribed to the nature of supports itself.
17

Compared with pseudoboehmite, Pd/γ-Al2O3 and Pd/α-Al2O3 showed an apparent de-
crease in surface area and pore volume, which could be attributed to particle sintering
and structure collapse during calcination [53]. Considering the fact that the calcination
temperature of Pd/α-Al2O3 is up to 1200 °C and the specific surface area is only 3.8
m2/g, Pd/α-Al2O3 has scarcely pore structure. On the contrary, Pd/η-Al2O3 and
Pd/θ-Al2O3 exhibited porous structures while its precursor, namely the bayerite, has
low surface area of 9.1 m2/g and small pore volume. It might be caused by the ream-
ing effects in the dehydration process of bayerite during calcination.
The pore size distributions of the Pd/Al2O3 catalysts are illustrated in Fig. 5. A sin-
gle pore structure was observed for Pd/γ-Al2O3 and Pd/η-Al2O3 except the latter
showed a much narrower pore size distribution. The Pd/θ-Al2O3 exhibited broad pore
size distribution with large pore size, indicating the poor pore structure originated
from the accumulation of particles.
Table 1 Textural properties of the precursors and Pd/Al2O3 catalysts.
Samples
Pseudoboehmite
Bayerite
SBET (m2/g)
413
Pore diameter (nm) Pore volume (cm3/g)
5.2
24.9
0.45
0.06
9.1
Pd/γ-Al2O3
Pd/η-Al2O3
Pd/θ-Al2O3
Pd/α-Al2O3
265
9.9
0.68
256
5.3
0.30
55.3
3.8
22.9
0.32
147b / 241c
0.14a / 0.16c
a Measured by the water displacement method.
b Average pore diameter = (4×pore volume) / (BET surface area), by assuming
all pores have cylindrical shapes.
18

c Measured by a mercury porosimeter.
0.05
0.01
γ
η
θ
5
10
15
20
25
30
Pore size (nm)
Fig. 5 BJH pore size distributions of the Pd/Al2O3 catalysts.
3.2.4 NH3–TPD analysis
The NH3–TPD profiles in Fig. 6 indicate the acidity of the Pd/Al2O3 catalysts de-
creased in the order of γ-Al2O3 > η-Al2O3 > θ-Al2O3 > α-Al2O3 by the supports. The
tendency was approximately consistent with the increasing calcination temperature of
supports due to the continuous dehydration at higher temperature. The γ and η phase
alumina were calcined at the same temperature and both of them showed large
amounts of weak and strong acidic positions. However, because of the different
structure or type of precursor being considered possibly, γ-Al2O3 showed a greater
numbers of the acid sites than that of η-Al2O3. The results were in good agreement
with the report by D.S. Maciver [49], who found the water content of γ-Al2O3 was
much greater than that of η-Al2O3 and showed higher acidity. As a complete dehydra-
tion product, the α-Al2O3 had rarely acidic positions.
19

0.01
γ
η
θ
α
150
300
450
600
750
Temperature (oC)
Fig. 6 NH3–TPD profiles of the Pd/Al2O3 catalysts.
3.2.5 CO–TPD analysis
The CO–TPD profiles of various catalysts are illustrated in Fig. 7. The CO adsorp-
tion results, Pd dispersions and the average Pd metal particle sizes determined from
CO chemisorption are given in Table 2. The CO desorption curves indicate a diversity
of adsorption sites for CO on Pd as well as variation in the proportion and distribution
of CO species. Pd/γ-Al2O3 and Pd/η-Al2O3 catalysts showed similar behavior with
o
three peaks at ca. 88, 173 and 410 C, except for the Pd/η-Al2O3 catalyst the desorp-
tion of these peaks were slightly shifted to higher temperatures. The Pd/θ-Al2O3 cata-
lyst exhibited a single peak at ca. 135 oC, whereas Pd/α-Al2O3 catalyst showed only
tiny desorption. As indicated by Monteiro et al. [54], the two peaks (L1, L2) at low
temperatures (<300 oC) were attributed to linearly-bonded CO on Pd (1 0 0) and Pd (1
o
1 1) planes, respectively, and the peak (B) at high temperature (>300 C) was as-
signed to bridged–bonded CO on Pd (1 1 1) plane. The distribution of these species
changed as supports change (Table 2). Where, in the order of Pd/γ-Al2O3, Pd/η-Al2O3,
Pd/θ-Al2O3 and Pd/α-Al2O3, the molar ratio of bridge to linear (B/L) and L2 to L1
20

(L2/L1) decreased sharply, reveals the transformation from strongly adsorbed species
to weakly adsorbed ones.
The mean diameters of Pd particles determined by CO chemisorptions in Table 2
are about 3.0, 5.3 8.5 and 36.9 nm for Pd/γ-Al2O3, Pd/η-Al2O3, Pd/θ-Al2O3 and
Pd/α-Al2O3, respectively. These values are consistent with the sizes of Pd particles by
TEM shown in Table 2.
B
0.25
L2
γ
L1
η
θ
α
0
150
300
450
600
750
Temperature (oC)
Fig. 7 CO–TPD profiles of the Pd/Al2O3 catalysts.
Table 2 CO chemisorption results of the Pd/Al2O3 catalysts.
CO lineara
CO bridgedb
Pd particle size
(nm)
Pd dis-
persion
(%)
L2/L1
ratio
B/L
ratioc
(μmol/g)
(μmol/g)
Catalysts
dCO
dTEM
L1
L2
B
Pd/γ-Al2O3 0.11 1.8
Pd/η-Al2O3 0.31 1.2
17.0
4.0
–
7.8
4.2
2.4
0.7
4.1
2.7
1.7
37.4
21.1
13.6
3.0 3.5+1.2
5.3 4.8+1.6
8.5 8.0+3.0
Pd/θ-Al2O3 1.4
Pd/α-Al2O3
–
–
–
–
–
3.0
36.9 32+18
a L1 and L2 are the linearly bonded CO, respectively.
b B is the bridged-bonded CO.
c L = L1 + L2.
21

3.2.6 H2–TPR analysis
The TPR curves of the catalysts are shown in Fig. 8, where no consumption peaks
o
were observed in all cases due to the fact that PdO can be reduced below 20 C [55,
56]. Except for the Pd/γ-Al2O3, all the other three catalysts exhibited a negative H2
peak at around 65 oC. This peak is attributed to the H2 desorption from the decompo-
sition of β–PdH formed at room temperature (5 min) before the start of TPR run
[57-60]. The formation of β–PdH depends on the particle size of palladium; larger Pd
particles more easily form β–PdH phase, whereas smaller Pd particles may be unable
to form the Pd hydride [59, 61]. Some studies indicated the critical size of crystallite
Pd is 2–3 nm [39, 54] which are in good agreement with the catalysts behavior in this
study. Furthermore, the β–PdH decomposition peak of Pd/α-Al2O3 shifted to higher
temperatures and the area decreased 25% by compared with that of Pd/θ-Al2O3. The
reason may lie in the inadequate reaction of bulk palladium and its lag during the de-
composition of β–PdH phase.
γ
H2 release
η
0.01
θ
α
50
60
70
80
90 100 110 120
Temperature (oC)
Fig. 8 H2–TPR profiles of the Pd/Al2O3 catalysts.
22

3.3 Catalytic performance
105
α
θ
40
30
20
η
α
90
75
60
γ
θ
γ
6
4
η
45
12
η
γ
η
γ
90
80
70
60
50
10
8
6
θ
α
θ
α
4
2
0
0
3
6
9
12 15 18
0
3
6
9
12 15 18
Time (h)
Fig. 9 Catalytic performance of Pd/Al2O3 catalysts as a function of reaction time.
Table 3 Catalytic reaction rate and TOF values of the Pd/ Al2O3 catalysts.
TOF (s-1)
Samples
Reaction ratea (10-3 g/s gcatalyst
)
Pd/γ-Al2O3
Pd/η-Al2O3
Pd/θ-Al2O3
Pd/α-Al2O3
1.4
1.3
1.1
1.3
0.8
1.4
2.0
9.7
a BN consumption rate based on the initial 15 min results.
For the comparison of these catalysts supported on different phase alumina, con-
trast tests are carried out in a batch reactor under the same conditions. Comparative
results including the BN conversion and product selectivity (DBA, TOL and BA) are
demonstrated in Fig. 9. The curves of BN conversions for the four catalysts are simi-
lar and they all reached total conversion after 6 h.
23

Table 3 listed the initial catalytic activities and corresponding TOF values. The ini-
tial BN conversion rate are all in the range of 1.1-1.4 ×10-3 g/s gcatalyst and the
Pd/α-Al2O3 with the minimum surface area and the maximum Pd particle size exhib-
ited close performance with other catalysts. However, the TOF values showed a sig-
nificant difference over these catalysts, which varied from 0.8 to 9.7 s-1. Considering
the similar Pd loading and identical preparation methods of these catalysts, these re-
sults all implied the BN conversion has no direct relationship with the surface area,
supports acidity, or the metal dispersion. The reason can be explained as that in our
previous work [22], that the terrible hydrogenation transmission is regarded as the
limiting step in batch reactor and plays a key role on the BN conversion. As a result,
the conversion of BN relied on the H2 transmission rate rather than the nature of cata-
lysts itself.
Concerning the condensation by-product DBA formation, the plots of the DBA se-
lectivity showed that the weaker acidity of the catalyst, the higher DBA selectivity
was achieved. It is opposite to conventional belief that DBA formation could be ac-
celerated by acidic sites. The results inferred that the condensation reactions proceed
with no participation of the support and in agreement with other reports [62, 63]. The
behavior of the DBA selectivity for the four catalysts was considered to be a result of
competitive reactions. As shown in Scheme 1, DBA is formed via a condensation re-
action which relies on the amounts of semi-hydrogenated intermediate of BN. Larger
particle size, or lower metal dispersion, can reduce the activity of the catalyst. It pro-
moted the formation of intermediate products instead of the hydrogenolysis to toluene,
and thus enhanced the DBA formation [22]. Hence, the DBA selectivity performed as
a function of the catalyst activity which mainly related to the metal dispersion. Fur-
thermore, the DBA selectivity for Pd/γ-Al2O3 and Pd/η-Al2O3 decreases gradually
24

with the reaction time after reaching a peak value at 3 h. In consideration of other
products, a further hydrogenolysis and deamination of DBA and BA are carried out
and generate TOL over them. The behavior did not appear on the Pd/θ-Al2O3 and
Pd/α-Al2O3 catalysts. Their DBA selectivity climbed up and remained stable as a
function of reaction time, indicating the hydrogenation of BN over these catalysts was
weakened, and most likely, the catalysts were deactivated.
Overall, among the four Pd/Al2O3 catalysts, the Pd/γ-Al2O3 and Pd/η-Al2O3 cata-
lysts both exhibited excellent selectivity of BA at high activity with the extremely
lower DBA selectivity, and simultaneously, a high BA yield up to 90% was achieved
over them, which was superior to that in other literatures [21, 39].
4. Conclusions
Different transition alumina including γ, η, θ and α phase alumina, were prepared
by calcinations of commercial pseudoboehmite and synthetic bayerite. These supports
were then impregnated and calcined to prepare 0.5 wt% Pd catalysts. The prepared
catalysts were characterized by XRD, BET, NH3–TPD, CO–TPD，H2–TPR，etc., and
showed different active sites, dispersion and adsorption strength. Their catalytic per-
formances were evaluated for selective hydrogenation of benzonitrile. The BN con-
version showed no direct relationship with the nature of catalysts and assumed to be
relied to the H2 transmission rate. The DBA formation was considered to be a result of
competitive reactions and performed as a function of the metal dispersion. Among the
four Pd/Al2O3 catalysts, the Pd/γ-Al2O3 and Pd/η-Al2O3 both exhibited excellent sta-
bility and catalytic performance with high BA yield of ca. 90%.
25

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