Catalytic performance of Pt/AlPO4 catalysts for selective hydrogenolysis of glycerol to 1,3-propanediol in the vapour phase

Article abstract of DOI:10.1039/c4ra09357g

Hydrogenolysis of glycerol to 1,3-propanediol was investigated in the vapour phase over a series of Pt/AlPO₄ catalysts with platinum loadings ranging from 0.5 to 3 wt%. The catalysts were prepared by a wet impregnation method and characterized by various techniques such as X-Ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FT-IR), BET surface area, Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and CO-chemisorption methods. Ex situ pyridine adsorbed FTIR analysis and temperature programmed desorption (TPD) of NH₃ were employed to investigate the acidic properties of the catalysts. Further, the effect of reaction temperature, hydrogen flow rate, glycerol concentration and various contents of platinum (0.5 to 3 wt%) have been investigated to find the optimum reaction conditions. Superior performance with almost 100% conversion of glycerol and above 35% selectivity to 1,3-propanediol was obtained over 2 wt% Pt/AlPO₄ at 260 °C and atmospheric pressure. The influence of acidity of the catalyst and its correlation to the catalytic performance (selectivity and conversion) has been studied. The high strength of weak acidic sites and Bronsted acidity of the catalyst measured by NH₃-TPD and Pyr-FTIR were concluded to play a key role in selective formation of 1,3-propanediol. XRD, TEM and CO-chemisorption studies revealed that platinum was well dispersed on AlPO₄ which further contributed to higher catalytic activity for glycerol hydrogenolysis. This journal is

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PAPER
Catalytic performance of Pt/AlPO4 catalysts for
selective hydrogenolysis of glycerol to 1,3-
propanediol in the vapour phase
Cite this: RSC Adv., 2014, 4, 51893
Samudrala Shanthi Priya,a Vanama Pavan Kumar,a Mannepalli Lakshmi Kantam,a
b
a
*
Suresh K. Bhargava and Komandur V. R. Chary
Hydrogenolysis of glycerol to 1,3-propanediol was investigated in the vapour phase over a series of Pt/
AlPO4 catalysts with platinum loadings ranging from 0.5 to 3 wt%. The catalysts were prepared by a wet
impregnation method and characterized by various techniques such as X-Ray Diffraction (XRD), Fourier
Transform Infrared Spectroscopy (FT-IR), BET surface area, Scanning Electron Microscopy (SEM),
Transmission Electron Microscopy (TEM) and CO-chemisorption methods. Ex situ pyridine adsorbed FTIR
analysis and temperature programmed desorption (TPD) of NH3 were employed to investigate the acidic
properties of the catalysts. Further, the effect of reaction temperature, hydrogen flow rate, glycerol
concentration and various contents of platinum (0.5 to 3 wt%) have been investigated to find the
optimum reaction conditions. Superior performance with almost 100% conversion of glycerol and above
35% selectivity to 1,3-propanediol was obtained over 2 wt% Pt/AlPO4 at 260 ꢀC and atmospheric
pressure. The influence of acidity of the catalyst and its correlation to the catalytic performance
(selectivity and conversion) has been studied. The high strength of weak acidic sites and Brønsted acidity
of the catalyst measured by NH3-TPD and Pyr-FTIR were concluded to play a key role in selective
formation of 1,3-propanediol. XRD, TEM and CO-chemisorption studies revealed that platinum was well
dispersed on AlPO4 which further contributed to higher catalytic activity for glycerol hydrogenolysis.
Received 27th August 2014
Accepted 7th October 2014
DOI: 10.1039/c4ra09357g
www.rsc.org/advances
1,2-propanediol (1,2-PDO) and 1,3-propanediol (1,3-PDO) where
as ethylene glycol (EG) is a degradative product. Over hydro-
1. Introduction
genolysis reactions produce 1-propanol (1-PrOH), 2-propanol
(2-PrOH) and propane. 1,2-Propanediol (1,2-PD) is a medium-
value commodity chemical used for polyester resins, liquid
detergents, pharmaceuticals, cosmetics, antifreeze, etc. 1,3-PDO
has received a great deal of attention than 1,2 PDO since it is a
high-value specialty chemical used primarily in polyester bres,
lms and coatings. Also an important chemical intermediate
used mostly in the manufacture of highly valuable polymer,
polytrimethylene terephthalate (PTT) and in the synthesis of
polyurethanes and cyclic compounds.6–8 Such polymers based
on 1,3-PDO exhibit many special properties such as good light
stability, biodegradability, improved elasticity, extremely stain-
resistant with high strength and stiffness. Therefore, selective
conversion of glycerol to 1,3-PDO is still regarded as a chal-
lenging process.
In this context, several studies have been reported on the
conversion of glycerol into 1,3-PDO through homogeneous or
heterogeneous processes. Homogeneous processes reported9,10
so far suffered from the common problem of catalyst separation
and thus direct hydrogenolysis of glycerol over heterogeneous
catalysts is a preferred route. Chaminand and co-workers11
employed the use of tungstic acid to Ru/C catalytic system and
reported 1,3-PDO selectivity of 12% in sulfolane. A Pt/WO3/ZrO2
Glycerol, a promising renewable resource, produced as a major
by-product in different processes such as soap manufacture,
fatty acid production, microbial fermentation and also during
the production of biodiesel by the transesterication of
triglycerides, is readily available at low cost.1,2 For that reason,
the optimum exploitation of glycerol as a raw material should
be encouraged for its transformation to value-added products to
ensure minimum environmental impact and maximum
economic benet. Until now, a great deal of effort has been put
towards the utilization of glycerol, a highly functionalised
platform chemical into value-added chemicals by various reac-
tions.3–5 One of the attractive outlets of glycerol is to produce
propanediols, by selective hydrogenolysis of glycerol. This
process provides a clean and economically competitive route for
the production of commercially valuable propanediols from
renewable glycerol instead of from non renewable petroleum.
The primary products of glycerol hydrogenolysis are
aCatalysis Division, CSIR-Indian Institute of Chemical Technology, Hyderabad-500
007, India. E-mail: kvrchary@iict.res.in; Fax: +91-40-27160921; Tel: +91-40-
27193162
bSchool of Applied Sciences, RMIT University, GPO Box 2476V, Melbourne, VIC 3001,
Australia
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catalyst was used to catalyze the hydrogenolysis of glycerol to surface area, XRD, FTIR, NH3-TPD, Pyr-FTIR, SEM, TEM and
1,3-PDO with an yield of 24% at 170 ꢀC and 8 MPa H2.12 CO-chemisorption methods. The aim of this work is to estimate
Bifunctional Cu–H4SiW12O40/SiO2 catalyst presented the selec- the dispersion of platinum on AlPO4, to study the acidic prop-
tivity of 1,3-PDO of 32.1% in vapour phase.13 The yield of 1,3- erties of the catalyst and its contribution to vapour phase
PDO reached 38% over Ir–ReOx/SiO2 catalyst with H2SO4 as an hydrogenolysis of glycerol to 1,3-PDO. In addition, the reaction
additive in a batch reactor.14 Pt-sulfated zirconia with 1,3- is optimized under various reaction parameters such as effect of
dimethyl-2-imidazolidinone was investigated for glycerol metal loading, effect of temperature, effect of H2 ow rate and
hydrogenolysis and the selectivity of 1,3-PDO as 55.6% at 170 ꢀC time-on-stream studies. The research work reported in the
for 24 h with an initial H2 pressure of 7.3 MPa was reported.15 paper is the rst of its kind in successful use of AlPO4 as a
The selective hydrogenolysis of glycerol to 1,3-PDO was studied catalyst support for 1,3-PDO production from glycerol.
over zirconia supported catalysts containing Pt and hetero-
polyacids where 1,3-PDO and 1,2-PDO were produced with 48.1
and 16.5% selectivity, respectively.16
2. Experimental section
Feng et al.17 demonstrated gas phase glycerol hydrogenolysis
over Cu/ZnO/MOx (MOx ¼ Al2O3, TiO2, and ZrO2) catalysts and
found that weak acidic sites of the support favoured 1,3-pro-
panediol formation, however the selectivity of 1,3-PDO was
found to be 10%. Pt/Al2O3 with tungsten additive for the
selective formation of 1,3-PDO in a batch process was investi-
gated recently and the selectivity of 1,3-PDO was found to be
28%.18 A reusable Pt-AlOx/WO3 catalyst was employed for the
selective hydrogenolysis of glycerol to 1,3-PDO in water; through
the process 1,3-PDO was produced with 66% yield at 180 ꢀC and
5 MPa H2.19 The major disadvantages of existing heterogeneous
processes arise from use of organic solvents and high reaction
pressure which will greatly reduce the environmental and
economic viability.
Despite several reports, there is a need for eco-friendly and
catalytically efficient practical alternative for this important
transformation which might work under mild and cheaper
conditions. Hence the most promising alternative is to perform
hydrogenolysis reaction in vapour phase at moderate tempera-
ture and atmospheric pressure to achieve fairly good conver-
sions and selectivities. Vapour phase transformation is a
continuous process and a lower reaction time is necessary for a
given conversion hence vapour phase reaction is economically
viable and eco-friendly process.
2.1 Catalyst preparation
2.1.1 Synthesis of AlPO4 support. Amorphous aluminum
phosphate with a P/Al ratio of 0.9 was prepared from Al(NO)3-
$6H2O and (NH4)2HPO4 by the following reported proce-
dures.27–29 The starting materials were dissolved in deionized
water (400 mL of 0.5 M of Al nitrate solution and 350 mL of
0.5 M of (NH4)2HPO4 solution) and acidied with nitric acid. A
hydrogel was then formed by adding 700 mL of 10% solution of
ammonia to the acidied solutions of Al and P precursors until
a pH of 8.0 was achieved. Aer 1 h, the contents were ltered
and the hydrogel was washed with twice its volume of distilled
water. The hydrogel was dried at 110 ꢀC for 16 h and calcined at
ꢀ
500 C in air for 0.5 h.
2.1.2 Synthesis of Pt/AlPO4 catalyst. A series of platinum
catalysts with Pt loadings varying from 0.5 to 3 wt% were
prepared by impregnation with requisite amount of Pt(NH3)4-
Cl2$xH2O (Aldrich, 98%) on AlPO4 support. The catalysts were
dried at 110 ꢀC for 16 h and subsequently calcined at 450 ꢀC for
3 h in air at a heating rate of 10 ꢀC minꢁ1. The same sets of
catalysts were used for all characterization and evaluation
studies.
2.2 Catalyst characterization
In our efforts to design a highly active and selective catalyst X-ray diffraction patterns were obtained on Rigaku miniex
for hydrogenolysis of glycerol to 1,3-PDO, we have focused our diffractometer using graphite ltered Cu Ka (K ¼ 0.15406 nm)
attention on aluminium phosphate as support material and radiation. Measurements were recorded in steps of 0.045ꢀ with a
then deposited Pt on it. Aluminium phosphates have found count time of 0.5 s in the 2q range of 2–65ꢀ. Identication of the
increasing interest as catalysts or catalyst supports for a variety phase was made with the help of the Joint Committee on
of catalytic reactions in the last three decades.20 Amorphous Powder Diffraction Standards (JCPDS) les. Morphology of the
aluminium phosphate is built of tetrahedral units of AlO4 and catalyst samples were investigated by scanning electron
PO4 and is structurally similar to silica. Aluminium phosphate microscopy (SEM) by mounting the sample on an aluminum
has been studied extensively due to its high surface area, large support using a double adhesive tape coated with gold and
average pore size, thermal stability and surface acid–base observed in Hitachi S-520 SEM instrument.
properties.21–25 The acid–base properties of AlPO4 play an
The surface area of the calcined catalysts were analysed
important role in catalytic reactions. The close relationship using N2 adsorption at ꢁ196 ꢀC by the multipoint BET method
between silica and aluminium phosphate, which are isoelec- taking 0.0162 nm2 as its cross-sectional area using Autosorb
tronic and isostructural, has prompted many others to examine 1 (Quantachrome instruments, USA).
the use of AlPO4 as either a catalytic material or as a support for
numerous catalytic applications.26,27
Transmission electron microscopy (TEM) images were taken
on a JEOL model of 1010 microscope operated at 100 kV.
In the present investigation, we developed a catalytic strategy Samples for TEM analyses were prepared by adding 1 mg of
for vapour phase hydrogenolysis of glycerol to 1,3-propanediol reduced sample to 5 mL of methanol followed by sonication for
over a series of platinum catalysts (0.5–3 wt%) supported on 10 min. A few drops of suspension were placed on a hollow
aluminium phosphate. The catalysts were characterized by BET copper grid coated with a carbon lm made in the laboratory.
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TPD experiments were also conducted on AutoChem 2910 ionization detector (FID). The conversion of glycerol and
(Micromeritics, USA) instrument. In a typical experiment for selectivity of products were calculated as follows.
TPD studies, 100 mg of oven dried sample was taken in a U
shaped quartz sample tube. The catalyst was mounted on a
quartz wool plug. Prior to TPD studies, the sample was pre-
Conversion ð%Þ ¼
moles of glycerol ðinÞ ꢁ moles of glycerol ðoutÞ
ꢂ 100
moles of glycerol ðinÞ
treated by passage of high purity (99.995%) helium (50 mL
minꢁ1) at 200 ꢀC for 1 h. Aer pretreatment, the sample was
saturated with highly pure anhydrous ammonia (50 mL minꢁ1
)
moles of one product
moles of all products
with a mixture of 10% NH3–He at 80 ꢀC for 1 h and subsequently
ushed with He ow (50 mL minꢁ1) at 80 ꢀC for 30 min to
remove physisorbed ammonia. TPD analysis was carried out
Selectivity ð%Þ ¼
ꢂ 100
ꢀ
ꢀ
The carbon mass balance is found to be >98% unless
otherwise stated.
from ambient temperature to 800 C at a heating rate of 10 C
minꢁ1. The amount of NH3 desorbed was calculated using
GRAMS/32 soware.
The ex-situ experiments of FTIR spectra of pyridine adsorbed
samples were carried out to nd the Brønsted and Lewis acid
sites. Pyridine was adsorbed on the activated catalysts at 120 ꢀC
until saturation. Prior to adsorption experiments the catalysts
3. Results and discussion
3.1 Characterization techniques
3.1.1 X-ray diffraction (XRD). X-ray diffraction patterns of
pure aluminium phosphate support and various Pt/AlPO4
catalysts with Pt loadings ranging from 0.5 to 3 wt% are shown
in Fig. 1. XRD results suggest that the synthesized aluminium
phosphate is found to be X-ray amorphous. A broad peak in the
range of 2q between 15 and 30ꢀ which is centred at 26ꢀ is due to
amorphous AlPO4. This nding is in good agreement with XRD
results of previous literature.29–31 At higher Pt loadings (3 wt%),
less intense peaks due to crystalline Pt phase are observed at 2q
¼ 39.7ꢀ and 46.1ꢀ coinciding with the (111), (200) lattice planes
of crystalline Pt. However, at lower Pt loadings the absence of
crystalline Pt peaks indicates that the active phase is present in
highly dispersed form or the crystallite size might be less than
4 nm in size, which is beyond the detection capacity of the XRD
technique.
ꢀ
were activated in N2 ow at 300 C for 1 h to remove moisture
from the samples. Aer such activation the samples were cooled
to room temperature. The IR spectra were recorded using a IR
(model: GC-FT-IR Nicolet 670) spectrometer by KBr disc method
under ambient conditions.
CO chemisorption measurements were carried out on
AutoChem 2910 (Micromeritics, USA) instrument. Prior to
adsorption measurements, ca. 100 mg of the sample was
reduced in a ow of hydrogen (50 mL minꢁ1) at 300 C for 3 h
ꢀ
and ushed out subsequently in a pure helium gas ow for an
hour at 300 ꢀC. The sample was subsequently cooled to ambient
temperature in the same He stream. CO uptake was determined
by injecting pulses of 9.96% CO balanced helium from a cali-
brated on-line sampling valve into the helium stream passing
over the reduced samples at 300 ꢀC. Metal surface area,
percentage dispersion and Pt average particle size were calcu-
lated assuming the stoichiometric factor (CO/Pt) as 1. Adsorp-
tion was deemed to be complete aer three successive runs
showed similar peak areas.
3.1.2 BET surface area. Nitrogen adsorption desorption
measurements have been carried out to measure the BET surface
area and the results are presented in Table 1. As can be seen from
the results of Table 1, platinum loadings have shown a clear
impact on the surface area of the aluminium phosphate support.
The surface area of the pure aluminium phosphate support was
2.3 Catalyst testing
Hydrogenolysis of glycerol (>99% MERCK Chemicals) was
carried out over the catalysts in a vertical down-ow glass
reactor with an inner diameter of 9 mm operating under normal
atmospheric pressure. In the typical reaction ca. 500 mg of the
catalyst, diluted with double the amount of quartz grains was
packed between the layers of quartz wool. The upper portion of
the reactor was lled with glass beads, which served as pre-
heater for the reactants. Prior to the reaction, the catalyst was
reduced in a ow of hydrogen (100 mL minꢁ1) at 350 ꢀC for 2 h.
Aer cooling down to the reaction temperature (260 ꢀC),
hydrogen (140 mL minꢁ1) and an aqueous solution of 10 wt%
glycerol were introduced into the reactor through a heated
evaporator. The liquid products were collected in a condenser in
order to be analysed every 60 min by GC fed. The reaction
products were analyzed by Shimadzu-GC 2014 gas chromato-
graph equipped with a DB-wax capillary column with a ame- Fig. 1 XRD patterns of pure AlPO4 and various Pt/AlPO4 catalysts.
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Table 1 Results of temperature-programmed desorption and BET
surface area of various Pt/AlPO4 catalysts
TPD
NH3 uptake
Catalyst
(mmol gꢁ1 STP)
Tmax (ꢀC)
BET S.A. (m2 gꢁ1
)
Pure AlPO4
0.5 Pt/AlPO4
1 Pt/AlPO4
2 Pt/AlPO4
3 Pt/AlPO4
354
892
1061
1453
905
184.7
198.2
210.0
216.4
187.5
251
196
178
165
139
Fig. 3 SEM images of pure AlPO4 and various Pt/AlPO4 catalysts.
are shown in Fig. 2. A broad band centred at ꢃ3500 cmꢁ1 is
attributed to the isolated OH stretching vibration and the
vibration band at ꢃ1640 cmꢁ1 is due to the H2O molecule
(HOH). The band at around 1118 and 492 cmꢁ1 could be
attributed to the triply degenerate P–O stretching vibration
mode and to the triply degenerate O–P–O bending vibration
mode of tetrahedral (PO4)3ꢁ, respectively.32–34
Fig. 2 FT-IR spectra of pure AlPO4 and various Pt/AlPO4 catalysts.
3.1.4 Scanning electron microscopy. The surface
morphology of the samples was examined by SEM and Fig. 3
shows the representative SEM micrographs of pure aluminium
phosphate, supported platinum catalysts with different load-
ings (0.5–3 wt%). SEM analysis of AlPO4 material (Fig. 3a)
showed micron scale rounder agglomerates of much smaller
primary particles. The granular type particles were observed in
all Pt/AlPO4 catalysts.
found to be 251 m2 gꢁ1and decreases as a function of platinum
content (Table 1). The decrease in surface area with increasing Pt
loading can be attributed to blocking of the pores of the support
by crystallites of platinum as evidenced from XRD.
3.1.3 Fourier transform infrared spectroscopy. The FT-IR
analysis for the calcined Pt/AlPO4 catalysts was carried out to
conrm the state of hydroxyl groups on AlPO4 and the results
Table 2 Results of CO uptake, dispersion, metal area and average particle size of various Pt/AlPO4 catalysts
Metal surface
Pt (wt%)
Dispersion (%)
CO uptake (mmol gꢁ1
)
area (m2 gꢁ1
)
cat
Particle sizea (nm)
Particle sizeb (nm)
0.5
1
2
74.1
54.4
38.9
28.9
18.9
27.8
39.9
44.6
0.74
1.09
1.56
1.74
1.88
2.56
3.59
4.82
2.76
3.65
5.02
6.55
3
a
Determined from CO uptake values. b Determined from TEM analysis.
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Fig. 4 TEM images of pure AlPO4 and various Pt/AlPO4 catalysts.
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Fig. 5 TPD of ammonia profiles of pure AlPO4 and various Pt/AlPO4
catalysts.
3.1.5 CO-chemisorption. The Pt dispersion, metal surface
area, and average particle size were measured from the irre-
versible CO-chemisorption on various aluminium phosphate
supported platinum catalysts and the results are illustrated in
Table 2. The results reveal that the CO uptake value increases
with increase in Pt loading on AlPO4 support. From Table 2 it is
evident that the particle size of Pt increases with loading on
AlPO4, due to agglomeration of Pt particles on the support. The
platinum dispersion was found to decrease from 74.1% to
28.9% with increase in platinum loading from 0.5 to 3 wt% on
AlPO4. This is because as the platinum loading increases, the
deposition of excess platinum on the external surface of AlPO4
leads to decrease in the distance between the metal particles
and thereby promotes the agglomeration. These results are in
good agreement with the results obtained from the TEM and
Fig. 6 Ex-situ pyridine adsorbed FTIR of pure AlPO4 and various Pt/
AlPO4 catalysts.
XRD studies. Previous studies also reported a systematic study
on the dispersion of platinum by CO-chemisorption
method.16,35
3.1.6 Transmission electron microscopy (TEM). The size
and morphology of various AlPO4 supported Pt catalysts are
determined by TEM. The TEM images of 0.5, 1, 2 and 3 wt% Pt/
AlPO4 catalysts are presented in Fig. 4 and corresponding
particle sizes are given in Table 2. The TEM images show the
spherical particles of Pt/AlPO4 catalysts. The average particle
size of Pt in 0.5 wt% and 1 wt% catalysts is around 2.76 nm and
Table 3 Effect of metal loading of various Pt/AlPO4 catalysts on conversion or selectivity of glycerol hydrogenolysisa
Selectivity (%)
Pt wt%
Conversion (%)
1,3-PDO
1,2-PDO
1-PrOH
2-PrOH
Acetone
Methanol
Others
Cb (wt%)
0.5
1
2
100
100
100
75
19.7
27.8
35.4
15.7
—
—
—
—
21.7
23.4
13.5
15.9
14.2
12
21.3
18.7
24
16
4.4
8.8
4.8
9.7
1.64
1.51
1.49
2.03
15.1
14.8
25.4
12.9
10.2
14.6
3
a
Reaction conditions: catalyst: Pt/AlPO4 (0.5 g); reaction temperature ¼ 260 ꢀC; H2 ow rate: 140 mL minꢁ1; WHSV ¼ 1.02 hꢁ1. 1,2-PDO ¼ 1,2-
propanediol; 1,3-PDO ¼ 1,3-propanediol; 1-PrOH ¼ 1-propanol; 2-PrOH ¼ 2-propanol; others include ethanol, acetaldehyde and propane.
b
Carbon estimated aer the 10 h continuous operation on the used catalysts.
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3.65 nm respectively. 2Pt/AlPO4 catalyst particle size is 5.02 nm
and for 3 wt% it is around 6.55 nm. As shown in Fig. 4, the TEM
images of Pt/AlPO4 samples reveal the highly dispersed Pt
particles conned to pores of AlPO4. The small size of Pt
particles at lower loadings is due to the well-dispersed state of Pt
metal particles. The large particles for 3Pt/AlPO4 catalysts are
due to agglomeration of platinum particles i.e., decrease in the
dispersion of platinum. The average particle size of platinum
particles estimated from TEM results is in good agreement with
that estimated from CO-chemisorption results.
3.1.7 Temperature-programmed desorption of ammonia
(NH3-TPD). NH3-TPD measurements were performed to deter-
mine the acid strength and amounts of acid sites on catalyst
surface, using ammonia as an adsorbate. Ammonia is used
frequently as a probe molecule because of its small molecular
size, stability and strong basic strength. Desorption peaks with
maxima in the range 180–250 ꢀC, 260–330 ꢀC, 340–500 ꢀC in the
NH3-TPD pattern are commonly attributed to NH3 that has been
chemisorbed on weak, moderate and strong acid sites, respec-
tively.36,37 If there is more than one binding site for a molecule
on a surface, then this will result in multiple peaks in the TPD
spectrum. The NH3-TPD spectra of pure AlPO4 and different
wt% loadings of Pt/AlPO4 catalysts are presented in Fig. 5. This
gure clearly demonstrated the effect of acidic properties of
different wt% of platinum loadings on aluminium phosphate
support. All the adsorbed NH3 on AlPO4 desorbed below 200 ꢀC,
indicating the absence of strong acid sites on the surface. The
temperatures of desorption maxima (Tmax) and the volume of
NH3 desorbed of the catalysts are summarized in Table 1. The
pure aluminium phosphate supporꢀt exhibits one broad peak in
the temperature range of 180–250 C. The NH3-TPD spectra of
all catalyst samples (Fig. 5) show a broad peak at low tempera-
ture, which correspond to weak acid sites. It is clearly observed
that the area under the desorption peak for samples 0.5Pt/
AlPO4, 1Pt/AlPO4 and 3Pt/AlPO4 is much smaller than that in
2Pt/AlPO4 sample; therefore, the total amount of acid sites of
sample 2Pt/AlPO4 is much larger than that of the other catalysts.
3.1.8 Pyridine adsorbed Fourier transform infrared spec-
troscopy (Pyr-FTIR). FT-IR aer pyridine adsorption is a useful
tool to determine the nature and amount of acid sites. Fig. 6
illustrates the FTIR spectra aer pyridine adsorption on pure
aluminium phosphate and supported platinum catalysts.
Fig. 7 Effect of metal loading of various Pt/AlPO4 catalysts on conver-
sion of glycerol hydrogenolysis reaction. Reaction conditions: reaction
temperature ¼ 260 ꢀC; H2 flow rate ¼ 140 mL minꢁ1, WHSV – 1.02 hꢁ1
.
Fig. 8 Effect of temperature on hydrogenolysis of glycerol to propane
diols. Reaction conditions: reaction temperature ¼ 150–280 ꢀC; H2
flow rate ¼ 140 mL minꢁ1; WHSV – 1.02 hꢁ1
.
Table 4 Effect of temperature on conversion or selectivity of glycerol hydrogenolysisa
Selectivity (%)
Reaction temp. (ꢀC)
Conversion (%)
1,3-PDO
1,2-PDO
1-PrOH
2-PrOH
HA
Acetone
Methanol
Others
150
180
230
250
260
280
—
37.5
61
100
100
100
—
—
16.7
26.4
35.4
—
—
—
—
—
—
—
—
—
—
18.6
13.5
—
—
—
—
28.7
21.3
100
—
—
32
11.8
12.8
14.8
—
—
28
8.2
7.2
10.2
—
—
12.5
54
—
—
—
27.5
9.3
6.3
4.8
—
a
Reaction conditions: catalyst: 2 wt% Pt/AlPO4 (0.5 g), H2 ow rate: 140 mL minꢁ1, WHSV ¼ 1.02 hꢁ1; 1,2-PDO ¼ 1,2-propanediol; 1,3-PDO ¼
1,3-propanediol; 1-PrOH ¼ 1-propanol; 2-PrOH ¼ 2-propanol; HA ¼ hydroxyacetone; others include acetaldehyde, ethanol and propane.
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Pyridine adsorption at Brønsted (B) acid sites and Lewis (L) acid
sites exhibited typical bands centering at 1540–1548 cmꢁ1 and
1445–1460 cmꢁ1, respectively.38 Furthermore, the bands corre-
sponding to combination of both Brønsted and Lewis (B + L)
acid sites appear at 1490–1500 cmꢁ1. All the catalysts have
shown bands at 1454 cmꢁ1 corresponding to Lewis acid sites
and the other band appeared at 1551 cmꢁ1 is attributed to
Brønsted acid sites (Fig. 6). Although the surface acidity is low,
Bronsted acid sites are signicantly observed in AlPO4 catalysts.
One possible explanation would be that pyridine adsorption on
AlPO4 catalysts leads to the formation of protonated (bands at
1551 and 1493 cmꢁ1) and coordinated (bands at 1493 and 1454
cmꢁ1) species. Weaker acidity has been associated with
aluminium atoms while P–OH sites have been associated to
stronger acidity. P–OH groups are most probably responsible
for Bronsted acidity on AlPO4 surfaces and are quite stable.39,40
In addition, P–OH acidity may be further enhanced by hydrogen
bonding to Al–OH groups, as stated by Moffat et al.41 However,
the acid properties of AlPO4 can be modied by introducing
elements different from Al and P in the framework at different
loading. Thus, it is evident from Fig. 6 that the incorporation of
Pt to AlPO4 results in increase in the number of both Bronsted
and Lewis acid sites with increase in loading (0.5 to 3 wt%).
Scheme 1 Reaction scheme.
3.2 Catalytic activity studies
3.2.1 Effect of platinum loading on glycerol hydro-
genolysis. Table 3 summarizes the results of effect of platinum
loading on the catalytic performance of glycerol hydrogenolysis
Fig. 9 Effect of Glycerol concentration on hydrogenolysis of glycerol
to propanediols. Reaction conditions: reaction temperature ¼ 260 ꢀC;
ꢀ
H2 flow rate ¼ 140 mL minꢁ1, WHSV – 1.02 hꢁ1; WHSV – 1.01 hꢁ1
&
over Pt/AlPO4 catalysts at 260 C and atmospheric pressure. In
WHSV – 1.01 hꢁ1
.
general, 1,2-PDO and 1,3-PDO are the primary products of
glycerol hydrogenolysis whereas EG, 1-PrOH and 2-PrOH are the
minor products. It is well known that the dispersion of active
metal and acidity of catalyst play a key role in the bi-functional
hydrogenolysis mechanism of glycerol. The catalytic activity
exhibited by various Pt/AlPO4 catalysts (0.5–3 wt%) showed
superior performance in vapour phase hydrogenolysis reaction
of glycerol at 260 ꢀC and low hydrogen ow rates [140 mL
minꢁ1] under atmospheric pressure. Interestingly, 1,3-PDO was
exclusively produced as a major product over Pt/AlPO4 catalyst
eliminating the formation of 1,2-PDO.
From Table 3, it was found that the conversion of glycerol
was remarkably 100% with increase in platinum loading from
0.5 to 2 wt%. This can be explained by the high dispersion of
platinum and increase in the number of platinum active sites at
2 wt% which accelerated the reaction process, Further increase
in platinum loading (3 wt%) decreased the conversion of glyc-
erol from 100% to 75%. The selectivity of 1,3-PDO increased
from 19.7 to 35.4% with increase in platinum loading from 0.5–
2 wt% further decreased to 15.7% at 3 wt%. This may be caused
by that excess Pt generated agglomerates, which blocked the
acid sites and reduced the dispersion of Pt as evidenced from
XRD, TEM and CO-chemisorption results. Total carbon content
in used catalysts was determined by CHNS Analyzer-
ELEMENTAR Vario micro cube model (results shown in
Table 3).
Fig. 10 Effect of H2 flow rate on hydrogenolysis of glycerol to pro-
panediols. Reaction conditions: reaction temperature ¼ 260 ꢀC; H2
flow rate ¼ 60 mL minꢁ1, 100 mL minꢁ1 & 140 mL minꢁ1, WHSV –
1.02 hꢁ1
.
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performance, the following investigations were conducted on
this sample. The reaction temperature (150–280 ꢀC) depen-
dence of glycerol hydrogenolysis over 2 wt% Pt/AlPO4 is illus-
trated in Fig. 8. Increasing the reaction temperature has a
positive effect on the conversion of glycerol (Table 4). As
expected, glycerol conversion improved dramatically from
ꢀ
ꢀ
37.5% (180 C) to 61% (230 C) and then remained at 100% as
the temperature elevated (250, 260 and 280 ꢀC). However,
conversion of glycerol was found to be remarkably minuscale at
lower temperature (150 ꢀC) indicating that it was not favourable
to promote hydrogenolysis of glycerol. While, the selectivity to
1,3-PDO increased from 16.7% to 35.4% when the temperature
ꢀ
ꢀ
increased from 230 C to 260 C. Notably, 1,3-PDO formation
was not observed at lower temperatures (150 ꢀC, 180 ꢀC) and
elevated temperature (280 ꢀC) which is related to the formation
of large amounts of undesired by-products, such as over
hydrogenolysis products 1-PrOH, 2-PrOH, and the degradative
products methanol, ethanol, acetone and propane. So the
optimal reaction temperature to perform vapour phase glycerol
hydrogenolysis to 1,3-PDO selectively over 2 wt% Pt/AlPO4
ꢀ
catalyst is 260 C.
3.2.3 Effect of glycerol concentration. The effect of glycerol
concentration on glycerol hydrogenolysis was investigated in
the range of 5–20 wt%. The results presented in Fig. 9 clearly
show that a considerable decline in glycerol conversion from
100% to 84% is noticed with increase in glycerol concentration
from 5 wt% to 20 wt% in the feed. This is probably due to
adsorption of reactant molecules on the surface of catalyst
signicantly decreasing the surface area of the catalyst as a
result of blockage of the pores. Earlier studies suggest that
higher glycerol conversion is favourable at low glycerol
concentration.46,47 The selectivity of 1,3-PDO also increased to
35.4% at 10 wt% glycerol concentration and further decreased
to 18.4% at 20 wt%. For 20 wt% glycerol solution, higher
viscosity lowers the rate of reaction. The maximum conversion
of glycerol and selectivity of 1,3-PDO was obtained at 10 wt%
glycerol concentration. Miyazawa et al. also reported a high
glycerol conversion when the glycerol concentration was 10%.48
3.2.4 Effect of H2 ow rate. The role of H2-ow rate on
hydrogenolysis of glycerol was studied by carrying out the
reaction under H2 ow rates 60 mL minꢁ1, 100 mL minꢁ1 and
140 mL minꢁ1 at reaction temperature 260 ꢀC. Fig. 10 shows the
effect of H2 ow rate on conversion and selectivity during
glycerol hydrogenolysis. The glycerol conversion increased to
100% with increase in the H2 ow rates accompanied with an
increase in the selectivity towards 1,3-PDO from 22.6% to
35.4%. A similar tendency of glycerol conversion and selectivity
with hydrogen pressure has been reported over a Pt/WO3/ZrO2
catalyst.49 The high conversion and selectivity of glycerol with
increase in H2-ow rate is due to the availability of number of Pt
sites for the hydrogenolysis of glycerol during the reaction and
may be ascribed to the fact that proton and hydride transfer are
involved in the formation of 1,3-PDO from glycerol. In contrast,
considerable decline in the selectivities of over hydrogenolysis
and degradative products is also noticed with increase in H2
ow rate.
Fig. 11 TOS on hydrogenolysis of glycerol to propane diols ((A) 0.5–3
wt% Pt/AlPO4; (B) 2 wt% Pt/AlPO4 catalysts). Reaction conditions:
reaction temperature ¼ 260 ꢀC; H2 flow rate ¼ 140 mL minꢁ1, WHSV –
1.02 hꢁ1
.
Based on the literature, it is believed that the formation of
1,3-propanediol involves two steps: dehydration of glycerol to
3-hydroxypropionaldehyde on acid sites and subsequently rapid
hydrogenation of 3-hydroxypropionaldehyde to 1,3-propanediol
over metal catalyst.42–45 Feng et al.17 suggested that weak acid
sites favoured the formation of 1,3-PDO whereas strong acid
sites lead to the formation of 1,2-PDO through hydroxyacetone.
Interestingly, weak acid sites dominantly presented in Pt/AlPO4
catalyst, evidenced from NH3-TPD results, favoured the dehy-
dration of glycerol to 3-hydroxypropionaldehyde, and further
hydrogenation to 1,3-propanediol. However signicant
amounts of degradative products such as ethanol, methanol,
acetone, acetaldehyde and propane were observed. Evidently
these results have demonstrated that 2Pt/AlPO4 can be a good
catalyst for the selective hydrogenolysis of glycerol to 1,3-PDO
(Fig. 7) which offers a promising alternative route (Scheme 1).
3.2.2 Effect of reaction temperature. Since the Pt/AlPO4
catalyst with 2 wt% Pt loading exhibited superior catalytic
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Fig. 12 XRD and SEM images of spent catalyst 2 wt% Pt/AlPO4.
3.2.5 Effect of time on stream. The time on stream exper-
3.2.6 Structural aspects of spent catalysts. The spent cata-
iments for glycerol hydrogenolysis were carried out over 0.5–3 lyst of 2 wt% Pt/AlPO4 was characterized by XRD, SEM, BET
wt% Pt/AlPO4 catalysts to understand the stability of catalysts surface area and the results were compared with those of the
and the results are presented in Fig. 11. These results show that fresh catalyst. As shown in Fig. 12a, the spent and fresh catalysts
2Pt/AlPO4 catalyst exhibit higher conversion (100%) and showed similar XRD patterns in which no crystalline phase
showed stability compared to other catalysts. The catalysts related to the Pt species was observed in the spent catalyst. This
prepared by impregnation method exhibited better conversions can be ascribed to the fact that the Pt species were homoge-
and good selectivities towards 1,3-propanediol. The results neously dispersed on the support surface and did not agglom-
suggest that 3Pt/AlPO4 show slightly lower conversion and erate during the reaction. The BET surface area result also
selectivity than 2Pt/AlPO4, due to their increased crystallite conrmed that the surface area of catalyst had no remarkable
sizes. Although the initial activity is better for 3Pt/AlPO4 cata- change during the reaction. The SEM (Fig. 12b) images of the
lysts, the activity abruptly dropped from 72% to 63% within 10 fresh and spent samples showed similar morphologies, which
hours of operation. The catalysts 0.5Pt/AlPO4 and 1Pt/AlPO4 implies that the structure of this catalyst was rather stable. The
exhibited higher conversions 92% and 100% but their activity conversion of glycerol and selectivity of 1,3-PDO was also
decreased with time compared to 2Pt/AlPO4 catalyst, suggesting studied over the spent catalyst of 2 wt% Pt/AlPO4 while main-
that 2Pt/AlPO4 was a best catalyst for the glycerol hydrogenolysis taining the same reaction conditions (260 ꢀC, 0.1 MPa). The
of our present investigation. The catalytic activity is correlated results are presented in Table 5. The used catalyst achieved
with the particle size of Pt on different catalysts. The 3Pt/AlPO4 100% conversion same as that of fresh catalyst however the
catalyst is attributed to increase in the particle size of platinum selectivity of 1,3-PDO slightly dropped to 34.2%. These results
on AlPO4 due to agglomeration as evident from XRD, TEM, suggest that the structural features of spent catalyst did not
BET-SA and CO-chemisorption results.
change appreciably and the efficiency of the catalyst remained
during glycerol hydrogenolysis reaction.
Table 5 Studies of the spent catalyst 2 wt% Pt/AlPO4
4. Conclusion
Conversion
(%)
Selectivity of
1,3-PDO (%)
BET surface area
Catalyst
(m2 gꢁ1
)
Aluminium phosphate supported platinum catalysts prepared
by impregnation method were identied as the most efficient
catalysts for the selective hydrogenolysis of glycerol to 1,3-pro-
panediol in vapour phase. Under the reaction condition of
Fresh
Spent
100
100
35.4
34.2
165
154
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260 ꢀC and atmospheric pressure (0.1 MPa) 100% conversion of 25 B. Rebenstorf, T. Lindblad and S. L. T. Andersson, J. Catal.,
glycerol with 35.4% selectivity to 1,3-PDO were achieved over 1991, 128, 293.
2 wt% Pt/AlPO4 catalyst. This was attributed to the appropriate 26 S. A. El-Hakam, M. R. Mostafa and A. M. Youssef, Adsorpt.
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chemisorption, promoted the selective production of 1,3-pro-
panediol from glycerol over Pt/AlPO4 catalyst.
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Acknowledgements
SSP thanks CSIR-IICT & RMIT for the award of Junior Research
Fellowship. The authors gratefully acknowledge Dr Selvakannan
Periasamy, RMIT University for supporting the catalyst TEM
characterization work.
31 C. Srilakshmi, P. S. Sai Prasad, S. K. Rajusth and
A. K. Shukla, JEST-M., 2013, 2, 25.
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