Nanoscale Pd-based catalysts for selective oxidation of glycerol with molecular oxygen: Structure–activity correlations

Article abstract of DOI:10.1016/j.poly.2016.07.017

This work investigates selective oxidation of glycerol with molecular oxygen, one of the viable routes to obtain value-added products from bio-glycerol, using nanosized Pd-based catalysts. For this, three types of 1 wt.% Pd catalysts supported on activated carbon (Ac), hydrotalcite (HTc), and activated carbon–hydrotalcite composite (Ac–HTc) were prepared. The physicochemical properties of the catalysts are characterized using various techniques, such as XRD, BET, XRF, H₂-TPR, CO₂-TPD, and TEM. The TEM images reveal the formation of Pd nanoparticles with an average diameter of 9.01, 9.10, and 11.16 nm on the surface of HTc, Ac, and Ac–HTc supports, respectively. The CO₂-TPD results show that the Pd@HTc catalyst exhibits higher concentration of basic sites compared with that of Pd@HTc-Ac and Pd@Ac catalysts. The H₂-TPR profiles show that the reducibility of Pd species is highly dependent on the nature of the supports. Catalytic activity results reveal that the conversion of glycerol over Pd catalysts increased in the following order: Pd/Ac < Pd@HTc-Ac < Pd@HTc, while selectivity of the glyceric acid increased in the following order: Pd@HTc-Ac < Pd@Ac < Pd@HTc. The presence of more number of basic sites and high dispersion of Pd nanoparticles are found to be key factors for excellent catalytic performance of Pd@HTc catalyst in the oxidation of glycerol with molecular oxygen.

Full text of DOI:10.1016/j.poly.2016.07.017

Polyhedron xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Nanoscale Pd-based catalysts for selective oxidation of glycerol
with molecular oxygen: Structure–activity correlations
a,
⇑
a
a
b
Sharifah Bee Abd Hamid , Norfatehah Basiron , Wageeh A. Yehye , Putla Sudarsanam ,
Suresh K. Bhargava
a
b
Nanotechnology & Catalysis Research Centre (NANOCAT), University of Malaya, 50603 Kuala Lumpur, Malaysia
Centre for Advanced Materials and Industrial Chemistry (CAMIC), School of Science, RMIT University, Melbourne, VIC 3001, Australia
b
a r t i c l e i n f o
a b s t r a c t
Article history:
This work investigates selective oxidation of glycerol with molecular oxygen, one of the viable routes to
obtain value-added products from bio-glycerol, using nanosized Pd-based catalysts. For this, three types
of 1 wt.% Pd catalysts supported on activated carbon (Ac), hydrotalcite (HTc), and activated carbon–hy-
drotalcite composite (Ac–HTc) were prepared. The physicochemical properties of the catalysts are char-
Received 13 May 2016
Accepted 19 July 2016
Available online xxxx
acterized using various techniques, such as XRD, BET, XRF, H
reveal the formation of Pd nanoparticles with an average diameter of 9.01, 9.10, and 11.16 nm on the sur-
face of HTc, Ac, and Ac–HTc supports, respectively. The CO -TPD results show that the Pd@HTc catalyst
exhibits higher concentration of basic sites compared with that of Pd@HTc-Ac and Pd@Ac catalysts.
The H -TPR profiles show that the reducibility of Pd species is highly dependent on the nature of the sup-
ports. Catalytic activity results reveal that the conversion of glycerol over Pd catalysts increased in the
following order: Pd/Ac < Pd@HTc-Ac < Pd@HTc, while selectivity of the glyceric acid increased in the fol-
lowing order: Pd@HTc-Ac < Pd@Ac < Pd@HTc. The presence of more number of basic sites and high dis-
persion of Pd nanoparticles are found to be key factors for excellent catalytic performance of Pd@HTc
catalyst in the oxidation of glycerol with molecular oxygen.
2 2
-TPR, CO -TPD, and TEM. The TEM images
Keywords:
Palladium catalysts
Hydrotalcite
Activated carbon
Glycerol oxidation
Glyceric acid
2
2
Ó 2016 Elsevier Ltd. All rights reserved.
1
. Introduction
The negative impacts of fossil fuels on climate change, along
attention because of its high abundance and excellent functionality
due to the presence of three hydroxyl molecules [2]. Glycerol can
be largely obtained (ꢀ10 wt.%) during biodiesel synthesis process
[1,2]. A number of catalytic processes, such as oxidation, hydro-
genation, hydrogenolysis, dehydration, esterification, and transes-
terification have been reported to efficiently convert glycerol into
valuable chemicals [4–6]. The liquid phase oxidation of glycerol
is one of the most promising routes to produce high-value chemi-
cals. As shown in Fig. 1, a number of products, such as dihydroxy-
acetone (DHA), glyceric acid (GLYAC), glyceraldehyde (GLYALD),
hydroxypyruvic acid (HYPAC), and glycolic acid (GLYCAC) are pos-
sible in glycerol oxidation [7–10]. Among all these products, glyc-
eric acid is an important chemical with potential applications
[8,11]. For example, glyceric acid can be used for treatment of skin
disorders, its ester for biodegradable fabric softener, and deriva-
tives as building block compounds for bio-based polyesters [12].
The glycerol conversion and distribution of the products in the
oxidation of glycerol are dependent on the nature of the catalyst,
reaction conditions, and oxidant source [14] The liquid phase oxi-
dation of glycerol has been investigated using few monometallic
and bimetallic catalysts based on Au, Pt, and Pd in basic medium
[4,6,15–20]. Hutchings and his research group [21,22] found that
with concerns on local economic dependencies and social sustain-
ability have motived the scientific community to search for alter-
native renewable energy sources [1,2]. In this context, biomass,
one of the important renewable resources, can be used to produce
valuable chemicals and fuels that are currently being produced
from fossil fuels. However, only 3.5% of the existing biomass is pre-
sently being used for human needs, which includes food (around
6
2%), energy use, paper and construction needs (33%), and clothing,
detergents and chemicals (5%). The remaining 96.5% of the biomass
is used in the planetary ecosystem [3]. Therefore, the transforma-
tion of biomass to useful products still represents an emerging field
that has potential to overcome the concerns of fossil fuels, however
there is an urgent need to develop substantial economical routes
for efficient biomass valorisation.
Bio-glycerol, one of the top 10 bio-platform chemicals
estimated by the US Department of Energy, has attracted much
⇑
Corresponding author.
http://dx.doi.org/10.1016/j.poly.2016.07.017
277-5387/Ó 2016 Elsevier Ltd. All rights reserved.
0
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2
S.B.A. Hamid et al. / Polyhedron xxx (2016) xxx–xxx
[
5]
O
[6]
O
[7]
O
[8]
O
O2
O2
O2
HO
OH
HO
OH
HO
HO
OH
OH
Dihydroxyacetone
O
Glycolic acid
O
Hydroxypyrunic acid
Oxalic acid
O2
O2
OH
O
HO
OH
HO
OH
Glycerol
O2
O
O
Mesooxalic acid
OH
OH
HO
O2
O2
[4]
O2
HO
O
HO
OH
HO
OH
Glyceraldehyde
O
O
O
[
1]
Glyceric acid
Tartronic acid
[
2]
[3]
Fig. 1. General reaction pathways in liquid phase oxidation of glycerol [13].
the activity of the gold catalysts is strongly dependent on the
basicity of the reaction medium. Base plays a key role in the oxida-
tion of glycerol by abstracting hydrogen from the primary OH
groups of glycerol, which is the rate limiting step in this reaction
X-ray fluorescence (XRF), H
2
-temperature-programmed reduction
-TPD),
(H -TPR), CO -temperature programmed desorption (CO
2
2
2
and transmission electron microscopy (TEM) techniques. The effi-
ciency of the catalysts was tested for liquid-phase oxidation of
glycerol with molecular oxygen. Much attention has been paid to
understand the structure–activity properties of Pd catalysts in liq-
uid phase oxidation of glycerol.
[
23]. Peroxide could also be formed and contributed to the C–C
bond cleavage of the glycerol during glycerol oxidation over the
gold catalyst [19]. The activity of the Au catalysts on different sup-
2 2 3
ports, such as TiO , Al O , and carbon has been investigated for
glycerol oxidation [8,23–26]. Among the above used supports, car-
bon was proven to be a better support for gold catalysts in glycerol
oxidation [17,25], which is due to its high surface area and larger
porosity. However, sodium glycerate is formed in alkaline solution,
thus to get free glyceric acid, neutralization and acidification
should be further needed [24]. As well, the high price and limited
availability of gold is liming the applications of Au-based catalysts
in chemical industry. Therefore, the development of novel, efficient
heterogeneous catalysts having appropriate metal active phases
and supports is needed for sustainable biomass valorisation [27].
Relatively, Pd is cheap and abundant compared to Au, and it is
widely used as heterogeneous and homogeneous catalytic compo-
nent in many industrial applications, such as vinyl synthesis, acet-
ylene hydrogenation, and automobile exhaust purification [28].
Blackburn et al. reported the first successful example of Pd cat-
alyzed aerobic oxidation of secondary alcohols to produce ketones
2
. Experimental
2.1. Materials
Na
2 4 w
PdCl (99.99%) and polyvinyl alcohol (PVA, M = 146,000–
1
86,000, 87–89% hydrolysed) were purchased from Sigma–Aldrich.
Stock activated carbon was purchased from United Chem (surface
2
ꢁ1
area = 1100–1250 m g and pH ꢀ 9–10). The other reagents used
in this work are Mg(NO O (Merck, 99%), Al(NO) ꢂ9H O (Mer-
ꢂ6H
ck, 98.5%), NaOH (Merck, 99%), Na CO (Sigma–Aldrich, 98%), and
HNO (Sigma–Aldrich, 70%). The oxygen was purchased from
MOX-Linde (99.99%).
3
)
2
2
3
2
2
3
3
2
2
.2. Catalyst preparation
.2.1. Preparation of activated carbon (Ac) support
[
29]. The successful oxidation reactions were also carried out with
3
The crude activated carbon is treated with 5 M HNO under vig-
Pd nanoparticles catalyst [26–28]. Selective oxidation of glycerol to
glyceric acid was performed using nanoscale Pd catalysts under
base-free conditions [13]. Recently, hydrotalcites are used as effi-
cient supports for dispersing the noble metals and their catalytic
efficiency was investigated for liquid-phase oxidation of primary
alcohols [30,31]. Hydrotalicites exhibit numerous catalytically
favourable properties, such as high basicity, structural stability,
homogeneous dispersion of active metals, and better resistance
to sintering [31].
orous magnetic stirring at room temperature for 24 h. The treat-
ment was performed at 500 mL/10 g ratio of HNO to carbon
sample. After the treatment, the sample was filtered off and thor-
oughly washed with distilled water until the pH of the solution
reach to ꢀ7 [32]. Afterwards, the carbon material was over-dried
at 333 K.
3
2.2.2. Preparation of hydrotalcite (HTc)
Therefore, in this work we synthesized various Pd-based cata-
lysts supported on hydrotalcite, activated carbon, and composite
of activated carbon–hydrotalicte using an immobilization method.
The physicochemical properties of the catalysts were investigated
The hydrotalcite (Mg/Al ratio = 4) was prepared using a co-pre-
cipitation method [33]. In brief, the requisite amounts of
2.40 mol Mg(NO ) ꢂ6H O and 0.6 mol Al(NO ) ꢂ9H O were dis-
3
2
2
3 3
2
solved in 227.5 mL distilled water (solution A). The solution A
was then added drop wise to another solution B, which contained
2
using powder X-ray diffraction (XRD), N adsorption–desorption,
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S.B.A. Hamid et al. / Polyhedron xxx (2016) xxx–xxx
3
2
.0
.6
(a)
(b)
3
2
2
1
1
00
50
00
50
00
1
1.2
0
.8
.4
0
50
0
0.0
0.0
0
.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Relative Pressure (P/P0)
Relative Pressure (P/P0)
(c)
250
200
(d)
250
200
150
100
1
1
50
00
5
0
0
50
0
0
.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Relative Pressure (P/P0)
Relative Pressure (P/P )
0
2
1
1
00
60
20
200
160
120
80
(
e)
(f)
8
0
0
0
4
40
0
0
.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Relative Pressure (P/P0)
Relative Pressure (P/P0)
Fig. 2. N
2
adsorption/desorption isotherms of (a) Ac, (b) HTc, (c) HTc-Ac (d) Pd@Ac, (e) Pd@HTc and (f) Pd@HTc-Ac: desorption
adsorption.
Table 1
Textural properties of the supports and Pd catalysts.
2 3
250.9 mL aq. solution of Na CO (2 mol) and NaOH (6 mol) under
vigorous stirring at room temperature. The suspension was left
for 12 h at 300 K and the pH of the sol–gel solution was around
13. The sample was then filtered off and washed with distilled
water until the pH of the solution reach to ꢀ7. The resultant gel
Catalyst
BET surface area Pore volume
Pore
diameter
Pd particle
size (nm)
2
ꢁ1
3
ꢁ1
(m
g
)
(cm
g
)
(
nm)
XRD
TEM
was dried at 333 K overnight and calcined at 723 K in N
to obtain its oxide form.
2
for 5 h
Ac
HTc
917.9
126.7
648.1
917.1
125.6
0.4664
0.0291
0.1444
0.0294
0.2754
0.0516
3.590
17.379
8.644
4.450
7.853
8.113
–
–
–
–
–
–
HTc-Ac
Pd@Ac
Pd@HTc
Pd@HTc- 632.2
Ac
11.00 9.10
8.90 9.01
22.01 11.16
2.2.3. Preparation of activated carbon–hydrotalcite composite
The activated carbon and hydrotalcite were mixed by taking a
:1 weight ratio to obtain the composite of activated carbon–hy-
1
aDetermined by XRF.
drotalcite (Ac–HTc).
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4
S.B.A. Hamid et al. / Polyhedron xxx (2016) xxx–xxx
2
.2.4. Synthesis of supported Pd catalysts
The supported Pd catalysts with 1% Pd loading were prepared
using an immobilization method with polyvinyl alcohol. The Pd
precursor was pre-reduced with NaBH with a 2:1 M ratio of
NaBH :Pd (formation of dark brown sol). Within a few minutes
of sol generation, the colloid was immobilized by adding support
activated carbon, hydrotalcite, and composite of hydrotalcite
2.3. Catalyst characterization
The specific surface area, pore volume, and average pore size of
the catalysts were measured by N adsorption–desorption tech-
2
4
4
nique using Micromeritics Tristar ASAP 3020 at 77 K. The Pd load-
ings on the surface of the catalysts were determined by XRF
(S4EXPLORER, Bruker AXS). The H
using Thermofinnigan TPDRO 1100 in a fixed-bed reactor equipped
with a thermal conductivity detector using 5% H balance in N
(
2
-TPR studies were carried out
and activate carbon) under vigorous stirring. The slurry was stirred
until all the palladium was deposited on the support. Afterwards,
the sample was filtered off and dried at 333 K overnight. The cata-
lyst was then calcined at 673 K under N
reduced in a stream of N
for 4 h at 723 K.
2
2
(30 mL/min). The amount of catalyst was 50 mg and the tempera-
ꢁ
1
2
flow (50 mL min ) and
ture was increased from room temperature to 800 K at a heating
ꢁ1
ꢁ1
2
and H
2
(90:10) at 150 mL min flow rate
rate 1 K min . The CO
2
-TPD studies were conducted using Ther-
mofinnigan TPDRO 1100. The catalyst (50 mg) was placed in a
7
10 K
1
wt% Pd@Htc-Ac
wt% Pd@HTc 519 K
wt% Pd@Ac
720 K
1
1
Ac
HTc
400
500
600
Temperature (K)
700
800
2
Fig. 3. H -TPR profiles of the supports and corresponding Pd catalysts.
1
wt% Pd@HTc-Ac
1
wt% Pd@HTc
1
wt% Pd@Ac
HTc
Ac
4
00
500
600
700
800
Temperature (K)
2
Fig. 4. CO -TPD profiles of the supports and corresponding Pd catalysts.
Table 2
Liquid phase oxidation of glycerol using supports and Pd catalysts.
TOF (h 1
ꢁ
)
Catalyst
CO
2
uptake amount (mmol/g)
Conversion (%)
Selectivity (%)
DHA
GLYALD
GLYAC
TARAC
Ac
–
<1
n.d
n.d
n.d
n.d
n.d
HTc
2.357
0.321
1.936
1.421
<1
n.d
n.d
n.d
n.d
n.d
Pd@Ac
Pd@HTc
Pd@HTc-Ac
51.45
70.35
63.78
11.21
n.d.
n.d
13.20
0.84
1.46
57.66
80.37
35.18
17.93
18.79
17.56
125.30
165.58
147.47
Reaction conditions: 0.3 M glycerol solution, glycerol/Pd = 3500 mol/mol, 363 K, 1000 rpm and mole ratio of NaOH/glycerol = 2, O₂ pressure = 8 bar, and reaction
time = 180 min, n.d: not determined.
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S.B.A. Hamid et al. / Polyhedron xxx (2016) xxx–xxx
5
tubular quartz sample tube and the sample was pre-treated at
a stirring speed of 1000 rpm. The concentration of the reactant and
products were analysed by high-performance liquid chromatogra-
phy (Shimadzu HPLC) equipped with refractive index (RI) detector.
ꢁ1
ambient condition under N
sample was exposed to CO
2
flow of 10 mL min for 30 min. The
at room temperature to the 393 K
2
ꢁ1
(
10 K min ). Desorption was carried out from room temperature
An Agilent Hi-Plex H, 7.7 ꢃ 300 mm, 8
l
m column was employed
ꢁ
1
to 873 K at a heating rate of 10 K min . The X-ray diffraction
XRD) patterns were recorded using X-ray diffractometer (Bruker)
with Cu K (k = 0.154 nm) radiation. The particle size of the cata-
lysts were measured by transmission electron microscope (TEM)
JEM 2100F:JEOL).
for product separation at 338 K with 0.0085 M H
2
SO solution as
4
ꢁ1
(
themobile phaseflowing at 0.55 mL min . Products were identified
by comparison of HPLC standard compounds.
a
(
3
. Results and discussion
2.4. Glycerol oxidation
3.1. Catalyst characterization
A batch stainless steel autoclave reactor (200 mL of Top Indus-
3.1.1. N
2
adsorption–desorption and elemental analyses
adsorption–desorption isotherms of the catalysts are
tries) was used for the liquid phase oxidation of glycerol. In a typical
experiment, 0.3 M of glycerol with glycerol/Pd = 3500 and 2 mol
ratio of NaOH/glycerol were charged into the reactor. The autoclave
The N
2
shown in Fig. 2. The BET surface area, pore size, and pore volume
of the catalysts are presented in Table 1. The isotherm of activated
was purged with O
2
and then pressurized to 8 bar at room temper-
carbon can be classified as type I with H
4
type hysteresis,
ature. The reaction mixture was heated to 363 K for 180 min under
indicating the presence of both micropores and mesopores. The
Fig. 5. Powder XRD patterns of the supports and Pd-based catalysts.
9
8
7
6
5
4
3
2
0
2
4
6
8
10
12
14
16
18
Diameter (nm)
Fig. 6a. TEM overview image of Pd@HTc and palladium particle distribution.
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6
S.B.A. Hamid et al. / Polyhedron xxx (2016) xxx–xxx
estimated average pore diameter of Ac support is 3.59 nm (Table 1).
In contrast, the HTc support exhibits type IV isotherm with H type
hysteresis loop and the obtained average pore diameter is 17.3 nm,
confirming that HTc has mesoporous structure [34–36]. The iso-
therm of HTc-Ac is almost similar to Ac isotherm, i.e. type I with
the addition of Ac. Due to the relatively low Pd loadings used in
this study, there was no significant variation in the specific surface
area of Pd@Ac and Pd@HTc catalysts with respect to supports.
However, the BET surface area of the Ac–HTc is considerably
1
2
ꢁ1
decreased from 648.1 to 632.2 m g
after the addition of Pd.
H
4
hysteresis type. The estimated average pore diameter of HTc-
The XRF measurements showed that the employed method pro-
vides better Pd retention efficiencies and the determined Pd con-
tents in Pd@Ac Pd@HTc and Pd@HTc-Ac were 1.02, 1.12, and
1.20%, respectively (Table 1).
Ac is ꢀ8.644 nm. There was no effect of Pd addition on the iso-
therm of the Ac and Ac–HTc supports: the estimated average pore
diameters are 4.450 and 8.113 nm, respectively. In contrast, the
addition of Pd to HTc resulted in a significant variation in shape
of isotherm and the obtained pore diameter of Pd@HTc catalyst
is 7.853 nm. The Pd@HTc-Ac catalyst shows type I with H type
4
hysteresis, which indicates the presence of both micropores and
mesopores.
3
.1.2. H
The H
2
-TPR studies
-TPR profiles of the Pd catalysts, including the supports
2
are shown in Fig. 3. As can be noted from Fig. 3, no reduction peaks
were found in the case of supports. On the other hand, the Pd@Ac
catalyst shows a major peak at ꢀ525 K, which can be assigned to
It can be noted from Table 1 that the Ac support shows high BET
2
ꢁ1
surface area of 917.9 m g [32]. The HTc support has low BET sur-
face area (126.7 m g ), which is improved to 648.1 m g after
2+
the reduction of Pd species to metallic Pd [37]. The position of
2
ꢁ1
2
ꢁ1
this peak is shifted to higher temperatures (ꢀ715 K) for the
7
6
5
4
3
2
1
0
2
4
6
8
10
12
14
16
18
Particles size(nm)
Fig. 6b. TEM overview image of Pd@Ac and palladium particle size distribution.
6
5
4
3
2
1
0
0
2
4
6
8
10
12
14
16
18
Particle size (nm)
Fig. 6c. TEM overview image of Pd@HTc-Ac and palladium particle size distribution.
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S.B.A. Hamid et al. / Polyhedron xxx (2016) xxx–xxx
7
Pd@HTc and Pd@HTc-Ac catalysts. The presence of reduction peak
at higher temperatures may be due to the strong interaction of Pd
particles with support or existence of highly coordinated types,
causing more resistance to reduce Pd2 to Pd [37,38].
samples, indicating that these samples do not show any basic prop-
erties. In contrast, the HTc, Pd@HTc, and Pd@HTc-Ac samples exhi-
bit a major peak in the range of 600–700 K. This peak indicates the
presence of medium basic sites in the synthesized catalysts. The
medium basic sites are related to the metallic oxygen bridge like
+
2
+
2ꢁ
3+
2ꢁ
3
.1.3. CO
The basic properties of the catalysts were investigated using
-TPD technique (Fig. 4). No peaks were found in Ac and Pd@Ac
2
-TPD studies
Mg -O and Al -O pairs. As shown in Table 2, the Pd@HTc cat-
alyst shows a strong ability towards CO uptake compared to
Pd@HTc-Ac and Pd@Ac catalysts, indicating presence of more
2
CO
2
1
00
100
100
80
60
40
20
0
100
(
a)
(b)
Glycerol conversion
Glycerol conversion
Sglyceraldehyde
90
Sglyceraldehyde
Sglycericacid
Sdihydroxyacetone
Sglyceric acid
80
80
60
40
20
0
80
Sdihydroxyacetone
70
6
4
2
0
0
0
0
60
50
40
30
20
1
0
0
0
50
100
150
0
50
100
150
Time (min)
Time (min)
1
00
100
Glyerol conversion
Sglyceraldehyde
Sglyceric acid
(c)
8
6
4
2
0
0
0
0
0
80
60
40
20
0
Sdihydroxyacetone
0
50
100
150
Time (min)
Fig. 7. Time course of glycerol oxidation over (a) Pd@Ac (b) Pd@HTc and (c) Pd@HTc-Ac catalysts.
OH
OH
OH
OH
Pd supported
catalyst
O2
O2
HO
OH
HO
H
HO
OH
HO
OH
3
3
1
bar O2 ,
33 K,
80 mins
O
O
O
O
Glyceraldehyde
GLYALD)
Glyceric acid
(GLYAC)
Tartronic acid
TARAC)
0.3M glycerol
(
(
O2
O
HO
OH
Dihydroxyacetone
DHA)
(
Fig. 8. Reaction pathway for liquid-phase oxidation of glycerol over Pd catalysts.
Please cite this article in press as: S.B.A. Hamid et al., Polyhedron (2016), http://dx.doi.org/10.1016/j.poly.2016.07.017

8
S.B.A. Hamid et al. / Polyhedron xxx (2016) xxx–xxx
number of basic sites in Pd@HTc catalyst, which is highly beneficial
for glycerol oxidation, as discussed in the activity part [38].
3.1.6. Catalytic activity studies
The liquid phase oxidation of glycerol with molecular oxygen
was studied using the Pd catalysts in a 200 mL autoclave vessel
[
42,43]. The reaction conditions used for the glycerol oxidation
3
.1.4. Powder XRD studies
The XRD patterns of the supports and corresponding Pd cata-
are 8 bar O , 363 K and 1000 rpm. The obtained results as a func-
2
tion of time are presented in Fig. 7. For comparison purpose, the
catalytic efficiency of the supports was also tested for glycerol oxi-
dation at 180 min and the results are resented in Table 2. It can be
noted from Table 2 that HTc and Ac show a negligible performance
in glycerol oxidation, indicating that HTc and Ac could only play a
support role for dispersing the Pd catalysts. As shown in Fig. 7, the
conversion of glycerol increases with the increase of time for all the
Pd catalysts. Among the catalysts tested, the Pd@HTc catalyst exhi-
bits the highest glycerol conversion of 70.35% followed by Pd@HTc-
Ac (63.78%) and Pd@Ac (51.45%) (Table 2). Interestingly, the selec-
tivity of the glyceric acid follows different trends, depending up on
the catalysts. As shown in Fig. 7, the selectivity of glyceric acid is
very low at initial times for Pd@Ac catalyst, which is significantly
increased to 27 and 57.66% for 60 and 180 min, respectively. How-
ever, there was no much improvement in the selectivity of the
glyceric acid with time in the case of Pd@HTc-Ac catalyst: the
observed selectivity of the glyceric acid was 35.18% for 180 min.
On the other hand, the Pd@HTc shows a prominent role in achiev-
ing higher selectivity of the glyceric acid: a 80.37% of glyceric acid
was found for 180 min. Turn over frequency (TOF) values were cal-
culated at highest conversion for each catalyst using Eq. (1). In
addition to glyceric acid, various by-products, such as glyceralde-
hyde and dihydroxyacetone are also found as shown in Fig. 8.
lysts are shown in Fig. 5. The Ac support shows two peaks at
6.31 and 42.03°, which can be assigned to pure carbon and carbon
oxide, respectively. This is due to the pre-treatment of HNO on the
carbon surface. Calcined HTc shows various reflections as shown in
Fig. 5, and clearly indicates that the structure of HTc collapsed dur-
ing the calcination at 773 K (PDF 89–0460). The noticed peaks at 2
theta of 29.56, 31.84, and 48.51° in HTc support indicate the pres-
2
3
ence of Al
8–0107) [39] . In addition, the supported Pd catalysts show a
minor peak at 40.03°, which corresponds to metallic Pd (PDF88-
335). No detectable diffraction peaks correspond to PdO crystal-
2 3
O , resulted from the impurities of the hydrotalcite (PDF
8
2
lites were found. This is due to low concentration of PdO and/or
its high dispersion on the support. However, it cannot be ruled
out that the PdO crystallites might be present as smaller crystal-
lites, which are unable to be detected by powder XRD [37]. The
average Pd crystallite size of the Pd catalyst was estimated using
a Scherrer equation and the data was presented in Table 1 [40].
Among the Pd catalysts, the Pd@HTc catalyst has the smallest Pd
crystallites size (8.90 nm).
3.1.5. TEM studies
The particle size and morphology of the Pd catalysts were deter-
mined by TEM analysis. The TEM images of Pd@HTc-Ac, Pd@Ac,
and Pd@HTc catalysts are shown in Fig. 6. A close observation of
all images reveals the formation of different size of palladium par-
ticles with irregular morphologies. It is a known fact that aggrega-
tion of smaller seeds of spherical particles leads to large size
nanoparticles [41]. Fitting by Log-Normal function, the particle size
of Pd species was estimated and the data was presented in Fig. 6.
The estimated average diameter of palladium particles is summa-
rized in Table 1. It can be seen from Fig. 6a that the Pd@HTc cata-
lyst has better dispersion of Pd particles with an average diameter
mmol of converted glycerol
mmol of total Pd ꢃ Reaction time ðhourÞ
TOF ¼
ð1Þ
It is a well-known fact that the basicity of the catalysts plays a
key role in the glycerol oxidation [16–18,44]. As well, a high dis-
persion of active phase species is needed for any heterogeneous
catalytic reaction, including glycerol oxidation. It can be noted
from Fig. 6 and Table 2 that the Pd@HTc catalyst has highly dis-
persed palladium species and more number of basic sites com-
pared to other synthesized Pd catalysts. These improved
properties are the main reasons for high catalytic performance of
the Pd@HTc catalyst in glycerol oxidation (Fig. 7 and Table 2).
ꢀ9.01 nm (Table 1). In contrast, different sized particles can be
found ranging from 2 to 16 and 14 to 18 nm for Pd@Ac (average
diameter is ꢀ9.10 nm) and Pd@HTc-Ac catalysts (average diameter
is ꢀ11.16 nm), respectively (Table 1).
Fig. 9. Proposed mechanism of glycerol oxidation over Pd@HTc catalyst.
Please cite this article in press as: S.B.A. Hamid et al., Polyhedron (2016), http://dx.doi.org/10.1016/j.poly.2016.07.017

S.B.A. Hamid et al. / Polyhedron xxx (2016) xxx–xxx
9
3
.1.7. Proposed reaction mechanism
Based on the results obtained in this study and previously
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(
bits more number of basic sites compared to Pd@HTc-Ac and
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