Catalytic glycerol hydrogenolysis to 1,3-propanediol in a gas-solid fluidized bed

Article abstract of DOI:10.1039/c6ra27248g

Glycerol is a potential feedstock to produce 1,3-propanediol (1,3-PDO), which is a valuable commercial polyester monomer. Here, we report the gas-phase glycerol hydrogenolysis to 1,3-propanediol over Pt/WO₃/Al₂O₃ in a fluidized bed operating above 240 °C and at ambient pressure. Fluidized beds are ideal contactors for this reaction because the heat transfer rates are sufficiently high to vaporize glycerol thereby minimizing its combustion and thermal degradation. The yield of 1,3-PDO approached 14% after 2 h at 260 °C. The major co-products were 1,2-PDO (18%), 1-propanol (28%) and 2-propanol (15%). In the first step, glycerol may dehydrate to acrolein, followed by rehydration to 3-hydroxypropanal and then hydrogenation to 1,3-PDO. The concentrations of the by-products including acrolein, ethylene glycol, propane, and acetone increased with increasing temperature.

Full text of DOI:10.1039/c6ra27248g

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Catalytic glycerol hydrogenolysis to 1,3-
propanediol in a gas–solid fluidized bed
Cite this: RSC Adv., 2017, 7, 3853
a
Mahesh Edake, Marjan Dalil,a Mohammad Jaber Darabi Mahboub,a
*
Jean-Luc Duboisb and Gregory S. Patiencea
Glycerol is a potential feedstock to produce 1,3-propanediol (1,3-PDO), which is a valuable commercial
polyester monomer. Here, we report the gas-phase glycerol hydrogenolysis to 1,3-propanediol over Pt/
WO3/Al2O3 in a fluidized bed operating above 240 ꢀC and at ambient pressure. Fluidized beds are ideal
contactors for this reaction because the heat transfer rates are sufficiently high to vaporize glycerol
thereby minimizing its combustion and thermal degradation. The yield of 1,3-PDO approached 14% after
2 h at 260 ꢀC. The major co-products were 1,2-PDO (18%), 1-propanol (28%) and 2-propanol (15%). In
the first step, glycerol may dehydrate to acrolein, followed by rehydration to 3-hydroxypropanal and
then hydrogenation to 1,3-PDO. The concentrations of the by-products including acrolein, ethylene
glycol, propane, and acetone increased with increasing temperature.
Received 23rd November 2016
Accepted 26th November 2016
DOI: 10.1039/c6ra27248g
www.rsc.org/advances
thermoplastic polyesters. DuPont produces Sorona® brand
polymers from PDO and terephthalic acid and they also part-
1 Introduction
nered with Tate and Lyle to produce PDO from corn (via
a fermentation process) and captured 89% of the total market in
2014.9 Asia Pacic will be the fastest growing market, as many
Chinese manufacturers will begin commercial production of
1,3-PDO in the coming years.
Substituting fossil resources with renewable biomass to
produce biofuels, chemicals, and bioplastics remains a priority
for many countries.1,2 Glycerol is a by-product of the trans-
esterication of vegetable oils and animal fats to biodiesel.3 As
a consequence of government mandates to blend biodiesel with
petro-diesel, glycerol has ooded traditional markets. These
markets are incapable of absorbing the increased availability, so
the price of glycerol has dropped.4 The lower price makes it an
attractive feedstock for other chemicals such as 1,2-propanediol
(1,2-PDO) and 1,3-propanediol (1,3-PDO).4,5
1,3-PDO is a monomer for polytrimethylene terephthalate
(PTT) together with terephthalic acid, that has unique proper-
ties for a wide range of end-uses; it has superior stretching and
elastic recovery compared to nylon and that is difficult to ach-
ieve with other glycol based polyesters. The global value of the
1,3-PDO market was USD 310 million in 2014 and it is likely to
reach USD 620 million by 2021 with a compound annual growth
rate of 10.4% between 2014 and 2021.6 Growing demand for PTT
for end-uses in polyurethane, cosmetics, personal care and
cleaning products7 and increasing consumer preference for bio-
based chemicals is expected to drive 1,3-PDO growth.8 It can
also be used as a constituent of engine coolants, food and
beverages, de-icing uids, heat-transfer uids, and unsaturated
polyester resins.
Yields of 1,2-PDO from glycerol hydrogenolysis are highest
with bifunctional catalysts including a hydrogenation metal
and an acid co-catalyst.10–12 Producing 1,3-PDO is more chal-
lenging. The ratio of 1,2-PDO/1,3-PDO can be controlled by
selectively cleaving the C–O bond of the primary or secondary
hydroxyl groups of glycerol, which mostly depend on the type of
catalyst. Chaminand13 improved the 1,3-PDO yield at 200 ꢀC and
80 bar adding H2WO4 to Rh/SiO2 in a sulfolane medium. A Pt/
WO3/ZrO2 catalyst achieved a 24% yield of 1,3-PDO in 1,3-
dimethyl-2-imidazolidinone (DMI) as a solvent at 170 ꢀC and 80
bar.14 A Cu–H4SiW12O40/SiO2 catalyst reached a 27% yield of 1,3-
PDO in the vapor phase and aqueous media at 5 bar and 210 ꢀC.
The 1,3-PDO yield over Pt deposited on a super-acid sulfated
ZrO2 support was 56% at 170 C and 73 bar.15 Environmental
ꢀ
and economic concerns limit the use of organic solvents for
a commercially viable process. Water is the ideal solvent for the
process as glycerol is obtained in the aqueous phase aer the
transesterication reaction.16
Heterogeneous noble metals (Ir, Rh, or Pt) combined with
oxophilic metals such as Mo, Re, and W catalyse glycerol
hydrogenolysis in the aqueous phase.17 Tomishige’s group
In 1990, Shell developed catalytic technology to convert
ethylene oxide to PDO, which led to the Corterra® brand of
18
19
studied Rh–ReOx/SiO2
and Ir/SiO2
catalytic systems for
glycerol hydrogenolysis. H2SO4 modied the structure of the
catalyst. The authors reported 38% 1,3-PDO selectivity at 120 ꢀC
and 80 bar over Ir/SiO2. However, Re leached. More robust and
a
´
´
Department of Chemical Engineering, Polytechnique Montreal, Succ. CV Montreal,
´
H3C 3A7 Quebec, Canada. E-mail: maheshedake1@gmail.com; Tel: +1 514 243 4062
bARKEMA, 420 Rue d’Estienne d’Orves, 92705 Colombes, France
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stable Pt–WO3 based catalytic systems are options to avoid the
leaching problems associated with Re. Recently, Priya et al.
studied the hydrogenolysis of glycerol to 1,3-PDO over bime-
tallic catalysts supported on mordenite in the liquid phase. The
maximum 1,3-PDO selectivity was 58.5%.20,21 Selectivity of 1,3-
PDO reached 32% with Pt/WO3/ZrO2 at 130 ꢀC and 40 bar 16 but
the highest selectivity to date was 66% (Table 1) with a Pt/WO3/
AlOOH catalyst.22 Recent studies show that acrolein in an
aqueous solution reacts to form 3-hydroxypropanal in a yield of
20%. Subsequently, hydrogen reacts with this intermediate to
form 1,3-PDO.23,24
Homogeneous catalytic processes suffered from a catalyst
separation problem and liquid phase heterogeneous catalytic
processes need high pressure. We report the gas phase glycerol
hydrꢀogenolysis over a Pt/WO3/Al2O3 catalyst in a uidized bed at
260 C and at atmospheric pressure, producing 14% 1,3-PDO.
Glycerol conversion and PDOs’ selectivity over the Pt/WO3/Al2O3
catalyst are related to the physicochemical properties of the
catalyst.
Fig. 1 Schematic of the experimental set-up.
at 120 ꢀC overnight and tꢁh1en calcined in air at 550 ꢀC at
a heating rate of 2 ꢀC min for 4 h. In the second step, we
loaded a mass fraction of 2% platinum on a supported tungsten
oxide catalyst. The precursor was chloroplatinic acid hydrate,
H2PtCl6$xH2O (Sigma Aldrich 99.99%). The catalyst was dried
and calcined under the same conditions as during the rst step.
2 Experimental
2.1 Catalyst preparation
2.2 Catalyst characterization
We prepared the Pt/WO3/Al2O3 catalyst by an incipient wetness
impregnation method. The support was Al2O3 with a particle
size ranging from 100 mm to 150 mm, which is suitable to ach-
ieve uniform uidization. Al2O3 (10 g) was impregnated with 20
wt% ammonium metatungstate ((NH4)6(H2W12O40)$nH2O) dis-
solved in 120 mL deionized water at 25 ꢀC. The paste was dried
An Autosorb-1 system (Quantachrome Instruments) recorded
the N2 adsorption/desorption isotherms to give textural prop-
erties (surface area, pore volume, and pore size distributions).
The samples were degassed at T ¼ 200 ꢀC under vacuum for 3 h.
We calculated the specic surface area (SSA) following the
multipoint Brunauer–Emmett–Teller (BET) procedure. A Philips
Xpert diffractometer measured the X-ray diffraction pattern and
crystallinity at room temperature (Cu anode (K ¼ 0.15406 nm) at
50 kV voltage and 40 mA current). It scanned 2q with a 0.020
step size. We identied the catalyst phases with JCPDS les. A
JEOL JSM-7600TFE instrument scanned the surface of the
catalysts and produced eld emission scanning electron
micrographs. We mounted the samples on an aluminum
support covered with double-sided adhesive tape. A laser scat-
tering PSD analyzer (LA-950, Horiba) measured the particle size
distribution of the catalyst.
A TA-Q50 instrument recorded the thermogravimetric curves
for the used catalysts. 20 mg samples of the catalysts were
loaded onto a 10 mm aluminium crucible. The resolution of the
balance was 0.1 mg. Prior to each experiment, we purged the
samples with a 40 mL minꢁ1 stream of nitrogen to remove
volatiles adsorbed on the catalyst. Simultaneously, the furnace
ramped the temperature at 10 ꢀC minꢁ1 up to 300 ꢀC and
remained constant for 15 min. We substituted nitrogen with
Table 1 State of the art catalysts for the selective hydrogenolysis of
glycerol to 1,3-propanediol
P
(bar)
Conv. gly
(%)
Yield
(%)
Catalyst
T (ꢀC)
(Batch mode)
15
Pt/sulfated-ZrO2
73
80
80
55
40
80
55
80
80
40
40
50
55
170
120
170
170
170
120
180
180
120
170
200
180
180
67
50
86
46
45
79
15
32
14
45
49
100
18
56
25
24
13
13
11
7
4
1
29
28
66
39
19
Ir/ReOx–SiO2
Pt/WO3/ZrO2
14
25
Pt/WO3/ZrO2
Pt–Re/C26
10
Rh–ReOx/SiO2
27
Pt/WO3/ZrO2/SiO2
Rh/C13
28
Rh/SiO2
Pt–Re/C29
30
Pt/Al2O3
22
Pt/WO3/AlOx
Pt/m-WO3
31
(Fixed bed-continuous)
29
Pt/WO3/ZrO2
Cu/STA/SiO2
Pt–STA/ZrO2
Pt/WO3/Zro2
40
50
50
40
50
1
1
1
1
130
210
180
130
180
260
280
210
225
70
83
24
70
54
100
62
90
32
27
48
46
52
35
20
58.5
48.6
Table 2 Textural properties of the catalysts
16
32
SA, m2 gꢁ1
PV, cm3 gꢁ1
Pd, A
˚
Catalyst
16
8
Pt/WOx/ZrO2
Al2O3
Pt/Al2O3
Pt/WO3/Al2O3 (fresh)
Pt/WO3/Al2O3 (used)
131
130
120
108
0.27
0.25
0.22
0.20
56
56
48
48
33
Pt/AlPO4
Ru/MCM34
Pt/Cu/Mor20
Pt/H-mordenite21
94.9
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Fig. 4 Glycerol hydrogenolysis product distribution.
oxygen while heating to 550 ꢀC with the same ramp conditions.
A Platinel II thermocouple placed 2 mm above the sample pan
monitored the temperature.
Fig. 2 XRD pattern for fresh and used Pt/WO3/Al2O3.
We evaluated the catalyst surface acidity by Fourier trans-
form infrared spectroscopy with pyridine as the probe molecule.
We placed samples in an infrared quartz cell, with CaF2
windows and evacuated the chamber. Thereaer, we ramped
the temperature in air to 150 ꢀC. We degassed the cell at the
same temperature for 1 h then introduced pyridine into the cell
aer returning to ambient temperature. All reported spectra
were obtained by subtracting the spectrum of the activated
catalyst (aer pretreatment but before pyridine adsorption)
from those aer pyridine adsorption. We characterized the used
catalyst aer 2 h reaction time to evaluate the effect of time on
stream. In this case, we took out the used catalyst from the
reactor without regeneration.
2.3 Experimental set-up
An ideal uidized bed operates isothermally and its radial
concentration gradients are smaller than in packed beds. Mix-
ing in micro-uidized beds is inferior to larger reactors, since
the bubbles are smaller, but we consider that the temperature
Fig. 3 FE-SEM micrographs of (a) fresh catalyst at ꢂ150, (b) used
catalyst after reaction ꢂ150 (c) fresh catalyst at ꢂ500, (d) used catalyst
after reaction at ꢂ500.
Table 3 Catalytic activity study of Pt/WO3/Al2O3 during vapor phase glycerol hydrogenolysis
Selectivity%
Expt. no.
T, ꢀC
H2/gly ratio
WHSV, hꢁ1
Conversion%
1,3-PD
1,2-PD
1-PrOH
2-PrOH
Othera
1
2
3
4
5
6
7
8
240
240
240
260
260
260
260
280
280
280
280
300
300
300
300
18
28
14
9
14
18
28
14
18
28
14
18
9
0.09
0.14
0.14
0.09
0.14
0.09
0.14
0.12
0.09
0.14
0.14
0.09
0.09
0.14
0.14
76
78
78
98
98
99
99+
99+
99+
99+
99+
99+
99+
99+
99+
6
7
7
12
13
13
14
6
4
4
3
4
5
6
5
13
11
12
9
5
7
4
4
16
18
19
23
25
26
28
36
34
37
36
36
35
37
38
14
13
13
15
16
15
17
13
15
13
15
15
14
13
13
59
56
56
37
35
34
32
40
40
42
41
42
44
43
40
9
10
11
12
13
14
15
3
4
3
4
3
4
5
14
28
a
Other: CO, CO2, propane, acetone, methanol, propanal, acrolein, ethylene glycol, hydroxyacetone etc.
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Fig. 7 Pyridine FTIR analysis of the fresh and used Pt/WO3/Al2O3
catalyst.
Table 4 Mass loss of the Pt/WO3/Al2O3 in a TGA fed with oxygen
Temp (ꢀC)
H2/gly
WHSV, hꢁ1
Wt. loss (%)
260
260
260
28
14
9
0.14
0.14
0.09
0.45
0.46
0.6
Fig. 5 Selectivity vs. temperature for (a) 1,3-PDO and (b) 1,2-PDO at
WHSV ¼ 0.09 to 0.14 hꢁ1, residence time ¼ 0.43 s and 0.65 s and H2/
glycerol ratio ¼ 9, 14, 18 and 28.
and concentration gradients of these beds are smaller than in
comparable xed bed reactors. Whilst the smaller bubbles in
micro-uidized beds reduce the overall mixing, they also reduce
bypassing and for this reason conversion approaches 100%
even in shallow beds of 10 mm. Atomizing gases and liquids
into uidized beds increases the heat transfer rates by orders of
magnitude in comparison with xed beds to derive the
kinetics.35,36
We loaded 2.8 g of the catalyst into a 30 mm long quartz tube
with a 7 mm inner diameter housed in a stainless steel tube to
operate at high pressure (Fig. 1). A ceramic distributer in the
middle of the tube distributed the gas uniformly across the
diameter. A furnace heated the reactor to the set point (mostly
260 ꢀC for the initial catalyst reduction with H2). A type-K
thermocouple inside the bed monitored its temperature.
Bronkhorst mass ow controllers (from 200 mL minꢁ1 to 1000
mL minꢁ1) maintained the gas ow rates. A syringe pump
injected the aqueous glycerol solution (from 10% to 50%) via
a 1/1600 tube from the bottom of the reactor. The injector passed
through the gas distributor and entered the catalyst bed at
a height of 10 mm. Prior to the activity test, pure H2 reduced the
catalyst with a ow of 50 mL minꢁ1 at 350 C for 1 h.
ꢀ
80 mL minꢁ1 of H2 uidized the Pt/WO3/Al2O3 catalyst (2.8 g
for each test), while argon atomized the feed solution into the
catalyst bed. All experiments lasted for 2 h. 30% oxygen in argon
ꢀ
regenerated the catalyst at 550 C.
Atomizing the liquid into uniform drops is important for
operating the uidized bed: large droplets cause the catalyst to
agglomerate and block the injector and/or accumulate on top of
the grid.35,37 We examined several gas and liquid ow rates and
glycerol concentrations to identify the optimum conditions.
Fig. 6 Selectivity vs. temperature for (a) 1-PrOH and (b) 2-PrOH at
WHSV ¼ 0.09 to 0.14 hꢁ1, residence time ¼ 0.43 s and 0.65 s and H2/
glycerol ratio ¼ 9, 14, 18 and 28.
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Fig. 8 Proposed reaction mechanism for glycerol hydrogenolysis to 1,3-PDO in a catalytic fluidized bed reactor at atmospheric pressure.
The total ow rate (QAr + QH + Qglycerol) changed from 140 mL mass spectrometer was calibrated before and aer the exper-
2
minꢁ1 to 210 mL minꢁ1, which corresponds to a velocity 3 or 5 iment with a mix of Ar, CO, CO2, H2O, CH4 and H2 at two
times that of the minimum uidization velocity (Umf). To concentrations close to the concentration of the reactor
change the feed composition, we manipulated the feed rates of effluent.
glycerol, hydrogen and Ar (the inert gas).
A Bruker GC (equipped with Hyesep Q, Molsieve 5A with
Therefore to reach the desired H2/glycerol ratio, the mole FFAP columns) and a Varian HPLC (Metacarb 87H column)
fractions of the gases were modied according to the following: analysed the glycerol, 1,3-PDO, 1,2-PDO, 1-PrOH, 2-PrOH as well
– Ar/H2/glycerol/water (%) ¼ 3.2/81/9/1.8 to reach H2/gly ¼ 9. as other degradation products.
– Ar/H2/glycerol/water (%) ¼ 3.8/84/6/1.2 to reach H2/gly ¼
The conversion of glycerol (Xgly.) as well as selectivities
toward products (Sp) were calculated as follows;
14.
– Ar/H2/glycerol/water (%) ¼ 4/90/5/1 to reach H2/gly ¼ 18.
– Ar/H2/glycerol/water (%) ¼ 3.6/92.4/3.3/0.7 to reach H2/
in
gly:
out
gly:
n
ꢁ n
Xgly: ðmol%Þ ¼
ꢂ 100
(1)
(2)
in
gly:
n
gly ¼ 28.
A quench trapped all the condensables and the mist coming
np
zp
Sp ðmol%Þ ¼
ꢂ
gly:
ꢂ 100
from the effluent gas and these were analyzed off-line using GC,
GC-MS and HPLC. The gas products were monitored on-line
using MS.
in
n
out
zgly:
ꢁ n
gly:
in
gly.
out
gly.
Where: n and n are the molar ow rates of glycerol at
the reactor’s entrance and exit. In eqn (2), np is the molar stream
of each product. zp and zgly. represent the number of carbon
atoms of the products and glycerol.
2.4 Product analysis
A Hiden QIC-200 quadrupole mass spectrometer monitored
the partial pressure of the reaction products in real time. The
Table 5 Mechanistic pathway study by 1,3- and 1,2-PDO injection fed over Pt/WO3/Al2O3 under the same reaction conditions
Selectivity%
Expt. no.
Reactant
Conversion%
1,3-PDO
1,2-PDO
1-PrOH
2-PrOH
C3H8
Acrolein
Othera
1
2
3
4
5
6
1,3-PDO
1,2-PDO
1-PrOH
2-PrOH
Acrolein
3-HPA
11
42
32
36
49
80
—
1
0
0
9
2
—
2
2
0
3
62
54
—
3
8
14
3
14
2
—
3
19
8
23
12
11
14
1
1
2
3
—
—
8
6
8
15
20
29
20
4
a
Other: CO, CO2, propanaldehyde, ethylene glycol, acetone etc.
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glycerol–water solution (a mass fraction of 20%), the catalyst
was activated in a stream of H2 at 350 C for 2 h.
3 Results and discussion
3.1 Physicochemical properties
ꢀ
The catalyst produced the most 1,3-PDO at 260 ꢀC. Both
glycerol conversion and 1,3-PDO selectivity over the Pt/WO3/
Al2O3 catalyst depend on temperature,16,25,32 (Fig. 4). Glycerol
conversion was generally very high and exceeded 98% above
260 ꢀC whereas 1,3-PDO selectivity reached a maximum at
260 ꢀC. 1,3-PDO product selectivity doubled from 6% to oꢀver
14% when we increased the temperature from 240 to 260 C.
However, it decreased at higher temperatures presumably due
to subsequent hydrogenation to 1-PrOH and 2-PrOH. Further
increasing the temperature to 300 ꢀC reduced the 1,3-PDO
selectivity and increased by-products like 1,2-PDO, 1-PrOH and
2-PrOH. Similar results published over a bi-functional Pt/WO3/
TiO2/SiO2 catalyst in a batch reactor, explain how the increased
temperature activates the terminal hydroxyl groups of glycerol.25
1,2-PDO formed in parallel with 1,3-PDO and its selectivity
was lower at all temperatures except 280 ꢀC (Fig. 4). As with 1,3-
PDO, it reached a maximum selectivity of 13% at 260 ꢀC. Other
byproducts were 1-propanol and 2-propanol. We conrmed that
these two compounds form directly from the hydrogenolysis of
1,3-PDO and 1,2-PDO. At 260 ꢀC, 1-propanol selectivity was 25%
and it increased with temperature to reach 38% at 300 ꢀC. 2-
Propanol selectivity increased less and reached a maximum of
17% at 260 ꢀC.
The surface area of the Al2O3 support was 131 m2 gꢁ1, with an
3
˚
average pore diameter of 56 A, and total pore volume of 0.27 cm
gꢁ1. The surface area of the Al2O3 support was the same aer Pt
impregnation (Table 2). It dropped by 10% aer we added WO3
and another 10% aer the reaction. The bulk support pore
volume dropped from 0.27 cm3 gꢁ1 to 0.20 cm3 gꢁ1 due to
tungsten oxide and platinum oxide species and/or coke block-
ing the Al2O3 pores.
The X-ray diffraction patterns of fresh and used Pt/WO3/
Al2O3 suggest that the synthesized catalyst is crystalline (Fig. 2).
Both samples exhibit characteristic peaks of alumina. Peaks at
37.5ꢀ, 45.4ꢀ, 66.9ꢀ for 2q values were attributed to crystalline
Al2O3. Diffraction peaks attributable to crystalline WO3 were
absent indicating that the tungsten species are uniformly
distributed. Diffraction peaks of metallic or oxidized Pt phases
were absent, indicating that the metal was well dispersed across
the surface of the support, which agrees with the literature
studies.22 Aer the reaction, the XRD diffraction pattern was
unchanged.
The surface morphology of the calcined Pt/WO3/Al2O3 was
examined using FE-SEM (Fig. 3). A high magnication view of
the catalyst shows that the particles were spheroidal. The
surface was rough and the particles had a uniform diameter of
approximately 80–120 mm. Catalyst assemblies were closely
packed and no cracks were apparent on the grain surfaces.
Pyridine-IR analysis compared the strengths of the Lewis and
Brønsted acid sites in PtWO3//Al2O3. Liquid pyridine was
adsorbed on the catalyst at room temperature and desorbed at
150 ꢀC in a vacuum. The FT-IR spectra of the pyridine adsorbed
The residence time varied from 0.43 s to 0.65 s (due to the
change of total ow rate). However, in this study we conducted
all experiments with a very high hydrogen ow, which is used as
reactant as well as for uidization purposes. Therefore the
results of selectivity show that the products were independent
of residence time changes.
Other by-products including propane, propanal, acrolein,
ethylene glycerol, acetone, methanol and hydroxyacetone were
also detected. The hydroxyacetone, acetone, methanol and
propane concentrations were highest at 240 ꢀC. CO and CO2
catalysts were analyzed in the region of 1700 cmꢁ1 to 1400 cmꢁ1
.
Bronsted (B) acid sites exhibited typical bands centered at 1640
cmꢁ1 and 1540 cmꢁ1, and Lewis (L) acid sites at 1460 cmꢁ1. The
fresh catalyst showed bands at 1454 cmꢁ1 corresponding to
Lewis acid sites and the other band observed at 1640 cmꢁ1 is
attributed to Brønsted acid sites. Although the total surface
acidity is low, the Brønsted acid sites were the most apparent on
Pt/WO3/Al2O3.
ꢀ
formation increased with temperature from 240 C to 300 ꢀC.
Both glycerol conversion and product selectivities are inde-
pendent of the H2/glycerol ratio over the Pt/WO3/Al2O3 (Fig. 5).
Under each of the conditions, the selectivity to the diols and
mono-alcohols were in the margin of error of the experimental
data (Fig. 6). We varied the H2/glycerol ratio from 9 to 29. We
tested a WHSV of 0.09 hꢁ1 and 0.14 hꢁ1
.
3.2 Hydrogenolysis
Glycerol hydrogenolysis is a multi-step process in which an acid
dehydrates glycerol, and a heterogeneous catalyst hydrogenates
3.3 Analysis of the used catalyst
the intermediates to 1,2-PDO and 1,3-PDO. Generally, the 1,2- The XRD patterns of the catalyst remained the same aer the
PDO yield is higher than that of 1,3-PDO. Maximizing 1,3-PDO reaction as did the morphology and the crystalline size (Fig. 2).
selectivity requires Brønsted acid sites to selectively dehydrate Crystalline Pt phases were absent aer the reaction, which
the secondary carbon hydroxyl bond. The acid plays a decisive suggests that the Pt species were well dispersed while reacting
role in eliminating a hydroxyl group, and a fast sequential (Fig. 2). The BET surface area of the catalyst decreased aer the
hydrogenation prevents further dehydration. In this study, we reaction from 120 m2 gꢁ1 to 107 m2 gꢁ1. The total pore volume
tested three catalysts while varying the feed composition and dropped from 0.22 cm3 gꢁ1 to 0.20 cm3 gꢁ1 (Table 2).
temperature to identify the optimum selectivity. We analyzed
Based on FT-IR pyridine analysis both the concentration of
the product stream aer a reaction time of 2 h. Here, we react the Lewis and Brønsted acid sites decreased aer the reaction
glycerol in the vapour phase at 240 ꢀC to 300 ꢀC, at a hydrogen to (Fig. 7). The ratio of Brønsted sites to Lewis acid cites was 1.18
glycerol ratio of 9 to 28, and WHSV between 0.09 and 0.14 hꢁ1 aer the reaction. Future work will examine more closely the
and at ambient pressure (Table 3). Before atomizing the change in acidity with time on-stream as well as during the
3858 | RSC Adv., 2017, 7, 3853–3860
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reoxidation process. Dalil et al.38 have already demonstrated by experiments with glycerol by GC and GC-MS: 1-PrOH, propane
FT-IR pyridine analysis on a WO3/TiO2 catalyst that the strong and CO, CO2, propanal, ethylene glycol and acetone.
acid sites were changing for up to 14 h during for the dehy-
dration of glycerol to acrolein. During this time the selectivity to
acrolein increased while the by-products decreased and the
4 Conclusions
medium acidic and basic sites decreased.
We demonstrate
a
catalyst system—Pt/WO3/Al2O3—that
Air regenerated the used catalyst in a TGA from which we
derived the mass loss due to carbon that accumulated during
the reaction (Table 4). It was relatively insensitive to H2/gly: the
ratio changed by a factor of 3 but only 25% more carbon evolved
from the catalyst. The catalyst has a small proportion of basic
sites that became coked with time but the catalyst was stable
over all of the conditions. The pore volume and surface area
changed slightly, which conrms that the catalyst is stable and
carbon builds up on a small fraction of the catalyst.
converts glycerol to 1,3-PDO and 1,2-PDO in the gas phase at
ambient pressure and elevated temperatures in a uidized bed
reactor. The maximum yield of 1,3-PDO, the desired product,
was 14%. It decreased with increasing temperature and
produced more 1-PrOH. The other by-products were 2-PrOH,
propanal, methanol, ethylene glycol, acetone, CO and CO2. The
consecutive reaction rate of 1,3-PDO to 1-PrOH is low; the most
important factor for increasing selectivity is to reduce the
parallel reaction to 1,2-PDO. A proposed mechanism pathway
shows that the reaction occurs via a double dehydration of
glycerol – rehydration of acrolein – followed by metal supported
hydrogenation to give 1,3-PDO.
3.4 Reaction mechanism
The two step dehydration–hydrogenation pathway to convert
glycerol to 1,3-PDO is widely accepted.22 In the rst step, glycerol
dehydrates to 3-hydroxypropanal (3-HPA) and then hydroge- Acknowledgements
nates in a second step to 1,3-PDO (Fig. 8, Path 2). Over acidic
The authors would like to acknowledge CRIBIQ and MITACS
catalysts, the double dehydration of glycerol gives acrolein,
which could be the precursor for 3-hydroxypropanaldehyde (3-
HPA) aer rehydration (Fig. 8, Path 1). The second dehydration
to acrolein and the rehydration would be an equilibrium, which
depends on water partial pressure and temperature. This
equilibrium stage is important: PDO selectivity increases with
higher H2O partial pressure, and lower temperature (to limit the
second dehydration) but this will also affect the rst dehydra-
tion. A detailed mechanism of acrolein formation with 3-HPA as
an intermediate has been stated in our recently published work
on glycerol dehydration over a WO3–TiO2 catalyst.37
The reaction mechanism includes 3-steps: (1) double dehy-
dration of glycerol to acrolein, (2) rehydration of acrolein to 3-
HPA, (3) hydrogenation of 3-HPA to 1,3-PDO (Fig. 8, Path 1).
We validated this mechanism by feeding intermediate
products—1,3-PDO, 1,2-PDO, 1-PrOH, 2-PrOH, and acrolein—to
the Pt/WO3/Al2O3 catalyst under the same reaction conditions
(Table 5). 1,3-PDO was less active than 1,2-PDO, the propanols,
and glycerol but its selectivity to 1-PrOH was 62% versus 54%.
This result accounts for the yield of 1,3-PDO being higher versus
1,2-PDO during glycerol hydrogenolysis. The selectivity of 1-
PrOH from 1,2-PDO was higher than for 2-PrOH. During glyc-
(Canada) for their partial nancial support of this study.
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