Cu:Al nano catalyst for selective hydrogenolysis of glycerol to 1,2-propanediol

Article abstract of DOI:10.1007/s10562-010-0276-5

Non-chromium Cu:Al nano catalyst prepared by simultaneous co-precipitation and digestion method without any template or stabilizer, showed three times higher activity than the bulk Cu-Cr catalyst for hydrogenolysis of glycerol in both isopropanol and water solvents, with the selectivity to 1,2-Propanediol (1,2-PDO) as high as 91% at 493 K and H₂ pressure of 7 MPa in 5 h. XRD pattern showed the presence of Cu⁺ species in the activated Cu:Al nano catalyst. Although Cu⁺ is catalytically inactive in glycerol hydrogenolysis reaction, the presence of Cu⁺ helps to stabilize the particle size in a narrow range of 7-11 nm by inhibiting the sintering of copper particles under reaction conditions. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-010-0276-5

Catal Lett (2010) 135:141–147
DOI 10.1007/s10562-010-0276-5
Cu:Al Nano Catalyst for Selective Hydrogenolysis of Glycerol
to 1,2-Propanediol
•
Rasika B. Mane Amol M. Hengne Ajay A. Ghalwadkar
•
•
•
Subramanian Vijayanand Pravin H. Mohite
•
•
Hari S. Potdar Chandrashekhar V. Rode
Received: 1 December 2009 / Accepted: 11 January 2010 / Published online: 29 January 2010
Ó Springer Science+Business Media, LLC 2010
Abstract Non-chromium Cu:Al nano catalyst prepared
by simultaneous co-precipitation and digestion method
without any template or stabilizer, showed three times
higher activity than the bulk Cu–Cr catalyst for hydrog-
enolysis of glycerol in both isopropanol and water solvents,
with the selectivity to 1,2-Propanediol (1,2-PDO) as high
as 91% at 493 K and H2 pressure of 7 MPa in 5 h. XRD
pattern showed the presence of Cu? species in the activated
Cu:Al nano catalyst. Although Cu? is catalytically inactive
in glycerol hydrogenolysis reaction, the presence of Cu?
helps to stabilize the particle size in a narrow range of 7–
11 nm by inhibiting the sintering of copper particles under
reaction conditions.
molecule glycerol can be converted into value added
chemicals by various transformations and this will also
significantly improve the biodisel production economics.
Selective catalytic hydrogenolysis of glycerol represents a
low cost and green route for 1,2-propanediol which is a
major commodity chemical used in the production of
antifreeze functional fluids, paints, humectants, and poly-
ester resins [2–4]. The conventional route to produce 1,2-
PDO involves the hydration of propylene oxide derived
from propylene by chlorohydrin process or the hydro per-
oxide process [5–7]. Therefore, catalytic hydrogenolysis of
glycerol to 1,2-PDO is a sustainable process based on
renewable bio-feedstock.
Hydrogenolysis of glycerol is a two step process
involving first step of either dehydration to acetol or
dehydrogenation to glyceraldehyde depending on the acid
or base catalysis respectively and subsequent hydrogena-
tion to 1,2-PDO [8]. However, hydrogenolysis of glycerol
via dehydration is a preferred (Scheme 1) route since [9–
11] acetol is also an important intermediate in hydrogen
production by catalytic steam reforming [12], pyruvalde-
hyde synthesis through oxidation [13] and as a starting
material in various organic transformations [14, 15]. Cat-
alyst systems involving various compositions of copper
chromite as well as supported noble metals have been
reported for the conversion of glycerol to 1,2-PDO in
presence of a solvent under high pressure and temperature
conditions [9, 16–18]. Among noble metals, Pd, Pt, Ru and
Rh have been used on various supports for the hydrogen-
olysis of glycerol [19, 20]. Castle and Gomez first descri-
bed glycerol hydrogenolysis using copper and zinc catalyst
as well as sulfided Ru catalyst under very high pressure and
temperature conditions (15 MPa; 513–545 K) with 75–
85% selectivity to 1,2-PDO while [25% were cracked
products such as EG, ethanol, methanol and lactic acid [21,
Keywords Cu:Al nano catalyst ꢀ Glycerol ꢀ
Selective hydrogenolysis ꢀ 1,2-Propanediol
1 Introduction
Conversion of biomass to industrially important chemicals
offers an attractive alternative to the consumption of
petroleum based resources [1]. Large amount of glycerol is
formed as a byproduct due to the rapid development of
biodisel industries. Surplus amount of glycerol is currently
being incinerated however, being a highly functionalized
R. B. Mane ꢀ A. M. Hengne ꢀ A. A. Ghalwadkar ꢀ
P. H. Mohite ꢀ C. V. Rode (&)
Chemical Engineering and Process Development Division,
National Chemical Laboratory, Pune 411008, India
e-mail: cv.rode@ncl.res.in
S. Vijayanand ꢀ H. S. Potdar
Physical and Materials Chemistry Division, National Chemical
Laboratory, Pune 411008, India
123

142
R. B. Mane et al.
22]. Tomishige and co-workers developed Ru—solid acid
bifunctional catalyst system for the hydrogenolysis of
glycerol, however the highest selectivity to 1,2-PDO
achieved was 82% with 40% conversion of glycerol [23–
26]. The effect of sulphur addition to Ru/C catalyst was
also investigated by Lahr et al. [27] in which the selectivity
to 1,2-PDO linearly increased with sulphur loading nev-
ertheless, maximum selectivity to 1,2-PDO obtained was
up to 45%. It is clear from the literature that although Pt
and Ru exhibited higher activity among the noble metal
catalysts for the hydrogenolysis of glycerol, but Ru leads to
C–C bond cleavage forming undesirable byproducts such
as ethylene glycol and methane while Pt gives higher
selectivity to lactate [8]. 1,2-PDO formation via glycerol
hydrogenolysis involves selective cleavage of C–O bond
without breaking C–C bond, for which Cu based catalysts
are desirable. Dasari et al. [9] have reported copper chro-
mite catalysts with the highest selectivity of 85% to 1,2-
PDO with 55% glycerol conversion after 24 h at 473 K and
1.4 MPa H2 pressure. Since the use of Cu–Cr catalysts is
not favourable because of the toxicity associated with
chromium, several researchers have focused their work on
Cu–ZnO catalysts. Chaminand et al. [28] reported glycerol
conversion of 20% with [90% selectivity to 1,2-PDO,
using Cu–ZnO in presence of tungstic acid in 92 h. Wang
and Liu proposed bifunctional Cu–ZnO catalysts and
obtained 84% selectivity to 1,2-PDO with 23% glycerol
conversion in 12 h [29]. Recently, highest selectivity of
93% to 1,2-PDO was achieved with 34% glycerol con-
version for Cu–ZnO catalyst in 16 h reaction time [30].
Although, Cu–ZnO catalyst system could be acceptable
due to elimination of chromium, but longer reaction times
make it unpractical from process point view. Therefore, we
aimed at developing highly efficient non-chromium nano
(Cu:Al) catalysts by the reduction of cubic spinel-type
phases, which contain excess copper ions that will exhibit
good catalytic performance and do not require any pro-
moter for the glycerol hydrogenolysis. As per the expec-
tation, our non chromium Cu:Al nano catalyst (7–11 nm)
exhibited greater than two fold activity as compared to the
bulk Cu–Cr catalyst ([25 nm) with the highest selectivity
of 91% to 1,2-PDO for aqueous phase hydrogenolysis of
glycerol in a very short reaction time of 5 h. The catalyst
was characterized for its physico-chemical properties based
on which the observed activity results have been discussed.
Effect of various reaction parameters on conversion of
glycerol and selectivity to 1,2-PDO has also been reported
for the nanostructured Cu:Al catalyst.
2 Experimental
2.1 Materials
Glycerol (99%), ethylene glycol were purchased from
Merck Specialities, Mumbai, India while acetol, 1,2-Pro-
panediol, and iso-propanol were purchased from Sigma–
Aldrich, Bangalore, India. Copper nitrate, aluminium
nitrate were purchased from Loba Chemie, Mumbai, India.
Hydrogen and nitrogen of high purity ([99.99%) were
obtained from Inox-I.
2.2 Catalyst Preparation
The nanostructured Cu (Cu:Al) catalyst was prepared by
coprecipitaion method. 0.05 M aqueous solution of each
Cu (NO3)2ꢀ3H2O and Al (NO3)3ꢀ9H2O were taken and
precipitated using 0.2 M aqueous potassium carbonate at
room temperature. The precipitate was aged further for 6 h
at room temperature. Then the precipitate was separated by
filtration and washed with deionized water to remove the
traces of potassium. The precipitate thus obtained was
dried in static air oven at 373 K for 8 h and calcined at
673 K for 4 h.
Bulk copper chromite catalyst (Cu–Cr A) was prepared
by a co precipitation method. First, aqueous ammonium
chromate solution was prepared by drop wise addition of
19 mL of 30% aqueous ammonia to aqueous solution of
ammonium dichromate. This solution was then added to an
aqueous copper nitrate solution to give a reddish brown
precipitate of copper chromate. This precipitate was then
filtered, dried and calcined at 673 K for 5 h. Another bulk
copper chromite catalyst (Cu–Cr B) was prepared follow-
ing the patented procedure in which barium was used as an
additive [31]. Copper content of Cu–Cr A and Cu–Cr B
catalysts was 67 and 60%, respectively.
2.3 Characterisation
H
H
X-ray powder diffraction patterns were recorded on a
Rigaku, D-Max III VC model, using nickel filtered CuKa
radiation. The samples were scanned in the 2h range of
1.5–80°. TPD measurements were carried out on a
Quantachrome CHEMBET 3000 instrument. In order to
evaluate acidity of the catalysts, ammonia TPD measure-
ments were carried out by: (1) pre-treating the samples
from room temperature to 473 K under nitrogen flow rate
OH
O
C
H
O
OH
O
C
OH OH
Dehydration
-H2O
Hydrogenation
H2
H2C
CH2
H2C
CH3
H2C
C
H
CH3
Glycerol
Acetol
1, 2-Propanediol
Scheme 1 Hydrogenolysis of glycerol to 1,2-Propanediol
123

Selective Hydrogenolysis of Glycerol to 1,2-Propanediol
143
of 65 mL/min. (2) adsorption of ammonia at room tem-
perature (3) desorption of adsorbed ammonia with a heat-
ing rate of 10 °C min-1 starting from the adsorption
temperature to 973 K.
2.4 Catalyst Activity
Glycerol hydrogenolysis reactions were carried out in a
300 mL capacity autoclave (Parr Instruments Co., USA) at
a stirring speed of 1,000 rpm. The typical hydrogenolysis
conditions were: temperature, 493 K; glycerol concentra-
tion, 20 wt%; catalyst loading, 0.8 g; and hydrogen pres-
sure 7–8 MPa. The catalysts were pre-reduced under H2 at
1.4 MPa and 473 K for 12 h. Liquid samples were ana-
lyzed by GC (Varian 3600) equipped with a flame ioni-
zation detector and a capillary column (HP-FFAP 30 m,
0.53 mm, 1 lm). Conversion of glycerol was calculated as
follows,
moles of glycerol reacted
Conversionð%Þ ¼
ꢁ 100
ð1Þ
initial moles of glycerol
The mass balance was also calculated in each experiment
which was always[95%. Selectivity of various products was
calculated as follows,
moles of a product formed
P
Selectivityð%Þ ¼
ꢁ 100
ð2Þ
moles of all the product
Catalyst activity has been also expressed in terms of turn
over frequency (TOF, h-1) as given by eq. (3).
Concn: of glycerol consumed
TOFðhꢂ1Þ ¼
ð3Þ
½Concn: of catalyst ðactive metalÞꢃðhÞ
3 Results and Discussion
Fig. 1 a TEM image of nano catalyst b area of diffraction from the
planes (110), (111), (311) and (204)
3.1 Catalyst Characterisation
correspond to metallic Cu (B2 in Fig. 3b) [33]. While
reduced nano catalyst shows broader peaks at 36.54° and
43.36° which could be assigned to Cu? and metallic Cu
respectively, indicating the inadequate reduction of Cu in
nano catalyst. This also confirms that the Cu2? in nano
Cu:Al catalyst undergoes the sequential reduction as CuO
first reduced to Cu2O cubic phase as a stable intermediate
and then to Cu° [32]. Using Scherrer–Warren equation, the
crystallite sizes of fresh and used nano catalysts were found
to be 7 and 11 nm while those of bulk catalysts were 24
and 42 nm, respectively. This shows that the extent of
aggregation for nano Cu:Al catalyst was much less than
that observed for the bulk catalyst under reaction condi-
tions which could be due to the presence of Cu? species in
the nano catalyst.
Figure 1 shows TEM image of the activated Cu:Al nano
catalyst as an aggregation of the metal particles the size of
which was estimated to be *10 nm (Fig. 1a). Figure 1b
shows the diffraction planes (110), (111), (311), (204)
which correspond to acicular CuO nano particles [32].
TEM of bulk Cu–Cr (Fig. 2) clearly shows the higher
extent of agglomeration of metal particles from which the
particle size was found to be in the range of 25–30 nm.
Figure 3a, b show the XRD patterns of Cu:Al nano and
the bulk Cu–Cr A catalysts respectively at various stages of
preparation. The XRD patterns after calcination for both
Cu-nano (A1 in Fig. 3a) as well as bulk Cu–Cr A catalysts
(B1 in Fig. 3b) showed dominant peaks at 2 theta values of
35.46° and 38.7° corresponding to Cu2? [29]. After
reduction of Cu–Cr A in hydrogen, narrow well defined
peaks appeared at 43.36°, 50.96°, and 74.5° which
Since, the first step of glycerol hydrogenolysis involves
its dehydration to acetol; the strength and nature of acid
123

144
R. B. Mane et al.
(a)
A3
A2
A1
0
10
20
30
40
50
60
70
80
90
2 Theta
(b)
Fig. 2 TEM image of bulk Cu–Cr catalyst
sites of our catalysts were determined by NH3-TPD.
Table 1 shows the value of ammonia adsorbed and
distribution of acidic sites of nano Cu:Al, Cu–Cr A and
Cu–Cr B catalyst samples. The nano Cu:Al catalyst shows
the highest total concentration of acidic sites as
1.567 mmol g-1 among all the three catalysts indicating
that hydrogenolysis of glycerol proceeds through acetol via
dehydration (Scheme 1) without needing a separate acidic
catalyst.
B3
B2
B1
The results of liquid phase glycerol hydrogenolysis in
isopropanol at 493 K over different copper catalysts are
presented in Table 2. Cu:Al nano catalyst showed the highest
activity (TOF, 2.96 h-1) with 88% selectivity to 1,2-PDO
and 7% to acetol, while Cu–Cr A catalyst showed the lowest
activity (TOF, 0.92 h-1) with 74% selectivity to 1,2-PDO
and 24% to acetol. Bulk catalysts (Cu–Cr A and Cu–Cr B)
showed [20% selectivity to acetol due to lower rate of
hydrogenation of acetol. The activity of Cu:Al nano catalyst
was almost five fold higher than the bulk Cu–Cr catalysts
reported in the literature [9, 29]. The novelty of our
catalysts was that the formation of degradation product viz.
ethylene glycol was much lower (B5%) than that compared
to bulk copper catalysts reported in the literature for which
ethylene glycol along with methanol, propanols and gaseous
products to the extent of[50% were formed [9, 29]. Also the
major side product formed was only the acetol which even-
tually undergoes hydrogenation to give 1,2-PDO.
20
30
40
50
60
70
80
2 Theta
Fig. 3 a XRD pattern of nano catalyst (A1) calcined, (A2) reduced,
(A3) after reaction. b XRD pattern of Cu–Cr A catalyst (B1) calcined,
(B2) reduced, (B3) after reaction
Table 1 Ammonia TPD results of nano Cu-nano, Cu–Cr A and
Cu–Cr B31
Catalysts
NH3 adsorbed
(mmol g-1
Distribution of acidic sites
)
(85–200 °C)
(200–450 °C)
Table 3 shows the activity results of various catalysts
for the hydrogenolysis of glycerol using water as a solvent.
Nano Cu:Al
Cu–Cr A
1.567
0.482
1.184
0.145
0.0241
0.145
0.910
0.1713
0.910
Cu:Al nano catalyst showed the highest (TOF, 2.63 h-1
)
Cu–Cr B
activity and selectivity (91%) to 1,2-PDO. Activity as well
123

Selective Hydrogenolysis of Glycerol to 1,2-Propanediol
145
(Fig. 3b) consists of pure metallic copper phase i.e. Cu°,
the Cu? species in Cu:Al nano catalyst formed due to
alumina is responsible for the higher catalytic activity due
to the stabilization of the catalyst by inhibiting the sintering
of active species.
Table 2 Hydrogenolysis of glycerol in isopropanol
Catalysts
TOF h-1 Conversion (%) Selectivity (%)
1,2-PDO Acetol EG
Nano Cu:Al 2.96
47
16
24
88
74
63
7
24
32
5
2
5
Cu–Cr A
Cu–Cr B
0.92
1.26
3.2 Parameter Studies
Reaction conditions: 23 wt% glycerol aqueous solution (100 mL),
7 MPa H2 pressure, 0.8 g catalyst, 493 K, 5 h
3.2.1 Effect of Temperature
Figure 4 shows the influence of reaction temperature on
conversion and selectivity pattern in glycerol hydrogenol-
ysis over Cu:Al nano catalyst. Glycerol conversion
increased from 29 to 76% with increase in temperature
from 473 to 513 K. Maximum conversion of 76% was
obtained at 513 K however, selectivity to 1,2-PDO
decreased marginally from 93 to 89%, correspondingly
increasing the selectivity to ethylene glycol from 6 to 10%,
indicating that the higher temperature favoured degradation
products due to C–C bond cleavage.
as selectivity to 1,2-PDO decreased drastically for bulk
Cu–Cr catalysts in comparison with Cu:Al nano catalyst.
The drop in selectivity to 1,2-PDO was mainly due to the
accumulation of an intermediate, acetol indicating slower
kinetics of acetol hydrogenation to 1,2-PDO over bulk Cu–
Cr catalyst. The activity and selectivity exhibited by Cu:Al
nano catalyst in water was almost comparable to that in
isopropanol solvent indicated the better water tolerance of
the Cu:Al nano catalyst hence, aqueous glycerol solution
can be directly used as a feed for its selective hydrogen-
olysis to 1,2-PDO.
3.2.2 Effect of Catalyst Loading
The highest activity of the Cu:Al nano catalyst for
glycerol hydrogenolysis could be due to the following two
reasons. (1) The higher acidic sites as shown by ammonia
TPD results. As the glycerol hydrogenolysis is a two step
process, the first step being dehydration of glycerol to
acetol followed by the hydrogenation to give 1,2-PDO.
Aluminium present in our nano catalyst gets converted to
alumina during calcination step which is normally
responsible for higher acidity leading to faster dehydration
to form acetol. Sato et al. [34] proposed that alumina
supported copper as well as pure copper were effective
catalysts for the dehydration of glycerol to acetol under
inert conditions, and (2) acetol thus formed undergoes
probably a very fast hydrogenation to 1,2-PDO, catalyzed
by nano size Cu°. The nano size Cu° is stabilized due to the
inhibition of sintering by Cu?, the presence of which is
evidenced by the XRD pattern (Fig. 3a). Huang et al. [35]
also have suggested that the Cu? formed during the
reduction treatment, although catalytically inactive in
glycerol reaction, helps to inhibit the sintering of copper
particles during the reaction. While Cu–Cr A catalyst
Figure 5 shows the effect of catalyst loading on glycerol
conversion and selectivity for hydrogenolysis of glycerol at
493 K. It was found that conversion of glycerol increased
almost three fold with increase in catalyst loading from 0.4
to 1.6 g. The selectivity to 1,2-PDO slightly decreased at
higher catalyst loading due to formation of degradation
products. The increase in conversion with increase in cat-
alyst loading could be due to more availability of the active
sites on the catalyst surface for the reaction which also
leads to excessive hydrogenation of 1,2-PDO to lower
alcohols.
100
90
80
70
60
50
40
30
20
10
0
80
70
60
50
40
30
20
10
0
ETG
1,2-PDO
Acetol
Conversion
Table 3 Hydrogenolysis of glycerol in water
Catalysts
TOF h-1 Conversion (%) Selectivity (%)
1,2-PDO Acetol EG
Nano Cu:Al 2.63
38
9
91
55
38
4
44
62
5
470
480
490
500
510
520
Cu–Cr A
Cu–Cr B
0.51
0.94
1
Temperature, K
18
\0.01
Reaction conditions: 20 wt% glycerol aqueous solution (100 mL),
7 MPa H2 pressure, 0.8 g catalyst, 493 K, 5 h
Fig. 4 Effect of temperature. Reaction conditions: 20 wt% glycerol
aqueous solution (100 mL), 7 MPa H2 pressure, 0.8 g catalyst, 5 h
123

146
R. B. Mane et al.
100
90
80
70
60
50
40
30
20
10
0
80
70
60
50
40
30
20
10
0
effect of glycerol concentration on the selectivity pattern.
For the highest glycerol loading of 60%, Cu:Al nano cat-
alyst showed several fold ([10) higher activity (TOF,
10.2 h-1) than the bulk copper catalysts [9, 29].
ETG
1,2-PDO
Acetol
Others
Conversion
3.2.4 Effect of Hydrogen Pressure
The results of effect of H2 pressure on glycerol hydrog-
enolysis studied at a constant temperature of 493 K are
shown in Fig. 7. Glycerol conversion increased from 23 to
38% as the hydrogen pressure increased from 3.5 to 7 MPa.
The increase in conversion was due to the higher avail-
ability of hydrogen at the catalyst surface at higher H2
pressure. It is interesting to note that the selectivity to
propanediol and acetol was not affected with change in H2
pressure.
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Catalyst Weight (g)
Fig. 5 Effect of catalyst loading. Reaction conditions: 20 wt%
glycerol aqueous solution (100 mL), 7 MPa H2 pressure, 493 K, 5 h
3.2.5 Catalyst Recycle Study
3.2.3 Effect of Glycerol Concentration
In order to establish the reusability of catalyst for glycerol
hydrogenolysis reaction the catalyst was filtered after the
first reaction and washed with a solvent. Then it was dried
in oven at 383 K and regenerated under hydrogen and used
for the subsequent hydrogenolysis of glycerol. This pro-
cedure was followed for two subsequent hydrogenolysis
reactions and the results are shown in Fig. 8. Our Cu-nano
catalyst showed significant activity even after the second
recycle in terms of TOF. The turn over frequency of Cu-
nano catalyst decreased from 2.96 to 2.27 which could be
mainly due to the handling losses of the catalyst as well as
metal sintering under reaction conditions of high temper-
ature (493 K) for long time duration (5 h).
In order to achieve maximum productivity of 1,2-PDO, the
effect of aqueous glycerol concentration on the conversion
was also studied. Figure 6 shows that conversion of glyc-
erol increased from 38 to 45% with increase in glycerol
concentration up to 60 wt% beyond which it decreased
drastically to 23%. This is because the limiting number of
catalyst active sites was available since the catalyst con-
centration was constant. Substrate inhibited kinetics at
higher glycerol concentration could be another explanation
for decrease in conversion beyond 60% glycerol concen-
tration. Similar observation was reported in case of butyne-
diol hydrogenation also [36]. There was no significant
100
80
60
40
20
0
60
50
40
30
20
10
0
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
ETG
1,2-PDO
Acetol
Conversion
ETG
1,2-PDO
Acetol
Conversion
20
30
40
50
60
70
80
3
4
5
6
7
8
Glycerol Concentration (%)
Pressure(MPa)
Fig. 6 Effect of glycerol concentration. Reaction conditions: 7 MPa
H2 pressure, 0.8 g catalyst, 493 K, 5 h
Fig. 7 Effect of hydrogen pressure. Reaction conditions: 20 wt%
glycerol aqueous solution (100 mL), 0.8 g catalyst, 493 K, 5 h
123

Selective Hydrogenolysis of Glycerol to 1,2-Propanediol
147
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while the only degradation product formed was ethylene
glycol (\5%). Presence of Cu? as evidenced by XRD, in
the Cu-nano catalyst helps to inhibit the sintering of the
active phase (Cu°) under reaction conditions leading to
higher activity of the Cu-nano catalyst. Ammonia TPD
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non-chromium catalyst which is responsible for the
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Acknowledgments We gratefully acknowledge Council of Scien-
tific and Industrial Research, New Delhi for its financial support to
this work under NMITLI program.
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