A green process for glycerol valorization to glycerol carbonateover heterogeneous hydrotalcite catalyst

Article abstract of DOI:10.1016/j.cattod.2014.01.043

Biodiesel production is accompanied by 10% w/w glycerol which needs to be valorized to into bulk andspecialty chemicals in order to make the biodiesel economics favorable. Glycerol carbonate is one suchproduct from glycerol which has many potential applications. A green process using calcined hydrotalcitesupported on hexagonal silica (CHT-HMS) catalyst was developed for the conversion of glycerol to glycerolcarbonate using dimethyl carbonate. Effects of various Al:Mg composition and loading on hexagonalmesoporous silica (HMS) were studied. Al:Mg composition of 1:2 with 15% w/w loading on HMS wasthe best catalyst. . CHT-HMS catalyst was fully characterized by various techniques such as FT-IR, EDAX, SEM, TPD, XRD, etc. The effects of various parameters such as speed of agitation, catalyst concentration,mole ratio, and temperature were studied. The catalyst is robust and recyclable. The reaction followsLangmuir-Hinshelwood-Hougen-Watson (LHHW) mechanism with weak adsoption of all species. Thus,a second order rate equation for the reaction was developed and the activation energy estimated.

Full text of DOI:10.1016/j.cattod.2014.01.043

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Contents lists available at ScienceDirect
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A green process for glycerol valorization to glycerol carbonate
over heterogeneous hydrotalcite catalyst
∗
Ganapati D. Yadav , Payal A. Chandan
Department of Chemical Engineering, Institute of Chemical Technology, Nathalal Parekh Marg, Matunga, Mumbai 400019, Maharashtra, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Biodiesel production is accompanied by 10% w/w glycerol which needs to be valorized to into bulk and
specialty chemicals in order to make the biodiesel economics favorable. Glycerol carbonate is one such
product from glycerol which has many potential applications. A green process using calcined hydrotalcite
supported on hexagonal silica (CHT-HMS) catalyst was developed for the conversion of glycerol to glycerol
carbonate using dimethyl carbonate. Effects of various Al:Mg composition and loading on hexagonal
mesoporous silica (HMS) were studied. Al:Mg composition of 1:2 with 15% w/w loading on HMS was
the best catalyst. . CHT-HMS catalyst was fully characterized by various techniques such as FT-IR, EDAX,
SEM, TPD, XRD, etc. The effects of various parameters such as speed of agitation, catalyst concentration,
mole ratio, and temperature were studied. The catalyst is robust and recyclable. The reaction follows
Langmuir-Hinshelwood-Hougen-Watson (LHHW) mechanism with weak adsoption of all species. Thus,
a second order rate equation for the reaction was developed and the activation energy estimated.
Received 15 November 2013
Received in revised form 13 January 2014
Accepted 27 January 2014
Available online xxx
Keywords:
Bioglycerol
Calcined hydrotalcite
Green chemistry
Glycerol carbonate
Kinetics
©
2014 Elsevier B.V. All rights reserved.
1
. Introduction
Glycerol carbonate is an attractive starting material due to
the presence of two competing sites of reactivity in it. Glycerol
monoesters, glycerol carbonate esters, glycerol monourethanes,
glycerol diurethanes, are obtained from glycidol or glycerol car-
bonate which are non-ionic glycerilic oxygenated surfactants, and
non-ionic glycerilic oxygenated and nitrogenated surfactants [10].
Glycerol carbonate is an intermediate in the alkylene carbonate
family with many potential areas of application such as reactive
protic solvent and solvent for battery electrolyte, filming lubricants,
agrosynthons, filming plasticizers, CO2 separation from gaseous
mixtures, phenolic resin preparation, synthesis of new function-
alized polymers like polycarbonates and polyurethanes, medicinal
and sanitary field, wetting agent ingredient for cosmetics, polymer
bases for emulsions; as a surfactant component in coating and paint
industry; and a substitute for ethylene carbonate and propylene
carbonate, etc. [11]. The production of organic carbonates is cur-
rently 80,000 TPA and carbonate based polymers 1.1 MTPA, which
is expanding continuously. The production will continue to grow,
especially if their use as additives for fuels expands [7].
Enzymatic synthesis of glycerol carbonate from glycerol and
dimethyl carbonate was reported with 74% conversion of glycerol
and 80.3% selectivity towards glycerol carbonate [12]. Sulfur
assisted carbonylation of glycerol with carbon monoxide has also
been studied [13]. A few reports deal with synthesis of glycerol
carbonate from glycerol and urea with liberation of ammonia gas
over gold [14], lanthanum oxide [15] and hydrotalcite catalyst [16].
Urea has been used as a reagent to make glycerol carbonate with
lanthanum oxide as solid base catalyst [17] and zinc containing
homogeneous catalysts [18]. However, the use of urea suffers
Biodiesel production is accompanied by 10% w/w glycerol as a
co-product which needs to be utilized in order to make the eco-
nomics favorable [1–6]. The use of bio-diesel is considered as the
green alternative to reduce carbon emissions. Additionally, the
needs of new economic synthetic routes for chemicals starting
from non-petrochemical sources are desirable and therefore glyc-
erol would be acceptable as a raw material. In 2011, 66.2% of the
total glycerol was produced from bio-diesel industry. US consump-
tion of biodiesel rose from 263 million gallons in 2010 to 878 million
gallons in 2011 to 1 billion gallon in 2012, demand for biodiesel fell
and so did prices. The EPA has mandated the use of 1.28 billion gal-
lons of biodiesel in 2013. Production came from 112 biodiesel plants
with capacity of 2.2 billion gallons per year. Increasing biodiesel
demand will continue to supply glycerol abundantly and it needs
to be valorized [10].
Several bulk and specialty chemicals could be produced with
glycerol as platform [7–9]. Conversion of glycerol to glycerol car-
bonate is one of the attractive options to valorize glycerol. Glycerol
carbonate is a fairly new chemical in the market, which possibly
will present some new commercial applications [10].
∗ Corresponding author. Tel.: +91 3361 1001/1111/2222;
fax: +91 3361 1002/1020.
E-mail addresses: gd.yadav@ictmumbai.edu.in, gdyadav@yahoo.com
(
G.D. Yadav).
0
920-5861/$ – see front matter © 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cattod.2014.01.043
Please cite this article in press as: G.D. Yadav, P.A. Chandan, A green process for glycerol valorization to glycerol carbonate over
heterogeneous hydrotalcite catalyst, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.01.043

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2
using calcinated Al–Mg hydrotalcite supported on hexagonal meso-
Nomenclature
porous silica (CHT-HMS) as a heterogeneous and reusable solid base
catalyst. The results are novel and also include kinetic modeling
which will be useful for reactor design and scale-up.
solid–liquid interfacial area (cm /cm3 of liquid
2
ap
phase)
A
AS
B
BS
CA, CB
reactant species A—glycerol
chemisorbed A
reactant species B—DMC
chemisorbed B
2
. Materials and methods
2.1. Chemicals
3
concentration of A and B (mol/cm )
3
The following chemicals were procured from reputed firms
CA0, CB0 initial concentration of A and B (mol/cm )
CAS, CBS concentration of A and B at solid (catalyst) surface
and used without further purification: dimethyl carbonate (DMC),
methanol, n-undecane, ethanol, aluminum nitrate, magnesium
nitrate, sodium carbonate, sodium hydroxide (all form S.D. Fine
chem. Ltd, Mumbai, India); glycerol, tetraethylorthosilicate (Merck,
Germany), hexadecylamine (Spectrochem Ltd., Mumbai, India).
(
mol/g.cat)
CPS, CWS concentration of P and W at solid (catalyst) surface
mol/g.cat)
CS
Ct
dP
G
(
concentration of vacant sites (mol/g/cat)
total concentration of the sites (mol/g/cat)
diameter of catalyst particle (cm)
glycerol carbonate (product)
2.2. Catalyst synthesis
5
g Dodecyl amine was added to 41.8 g ethanol and 29.6 g
GS
M
chemisorbed product
distilled water to make a homogeneous solution. Tetraethy-
lorthosilicate (20.8 g) was added to the above solution under
stirring and it was aged for 18 h at room temperature to form a
white precipitate. The clear liquid above it was decanted to get
hexagonal mesoporous silica (HMS) which was then dried on a glass
methanol
rate of disappearance of A (mol.cm-3.min-1
−
rA
)
k
SLA, kSLB solid–liquid mass transfer coefficients (cm/s)
KA
equilibrium constant for adsorption of A on catalyst
3
surface (cm /mol)
◦
plate. It was calcined at 550 C in air for 3 h to remove the template
[26].
KB
equilibrium constant for adsorption of B on catalyst
3
surface(cm /mol)
5
g HMS was added to 30 cm3 an aqueous solution of 1.28 g
KSR
KG
equilibrium constant for surface reaction of
chemisorbed A and B
equilibrium constant for adsorption of G on catalyst
surface
(5.0 mmol) magnesium nitrate and 3.75 g (10.0 mmol) aluminum
◦
nitrate. The slurry was stirred at 60 C for 30 min. The solution
was then co-precipitated by adding it slowly to a solution of 1.40 g
3
(35.0 mmol) NaOH and Na CO3 1 g (9.43 mmol) in 10 cm distilled
KM(cm
3
equilibrium constant for adsorption of M on cat-
2
/mol)
◦
3
water with vigorous stirring at 30 C for 2 h. During the addition,
the pH of the solution was maintained at 9–10. The slurry was
then agitated at 30 C for 4 h, filtered, washed with distilled water
repeatedly to attain pH 7 and dried in oven at 100 C for 24 h. The
catalyst was finally calcined in a muffle furnace at 450 C for 12 h
alyst surface (cm /mol)
S
Sh
t
w
XA
vacant site
Sherwood number
◦
◦
time (min)
◦
catalyst concentration (g/cm3 of liquid phase)
to obtain 15 wt% calcined hydrotalcite on hexagonal mesoporous
silica (CHT-HMS) catalyst with Al:Mg ratio 1:2.
fractional conversion of A
Greek letters
density of catalyst particle (g/cm3)
2.3. Catalyst characterization
ꢀ
p
2.3.1. FT-IR
FT-IR spectrum was obtained by Bruker IFS-66 single channel
from reduced pressure requirements in order to separate gaseous
ammonia and to avoid the formation of undesirable side products
such as isocyanic acid and biuret. DMC is preferred since the
severity of the process is less and the co-product methanol can
be easily separated. A process for preparation of cyclic carbonates
uses alkyl ammonium and tertiary amine catalysts and reports 92%
Fourier transform spectrophotometer. A thin pellet was prepared
by mixing the catalyst with spectroscopic grade KBr. The pellet was
subjected to a number of scans to record the spectra (Fig. 1). It
−
1
shows absorption at 1083 cm due to asymmetric stretching of
−
1
−1
Si–O–Si whereas peaks at ∼466 cm and 789 cm are attributed
to the internal bands of SiO tetrahedra. A broad absorption band at
4
conversion of glycerol with 92% selectivity to glycerol carbonate at
◦
1
20 C using tetra-butyl ammonium bromide (TBAB) as a catalyst
[
19]. Transesterification reaction of glycerol with dimethyl car-
26.0
24
bonate is also reported [20]. It has been reported that hydrotalcite
and mixed oxides efficiently catalyze the process [20–24]. The
synthesis of glycerol carobnate from glycerol and dimethyl car-
bonate (DMC) using several basic ionic liquids as both solvents and
catalysts is reported [25]. Using N-methyl-N-butylmorpholinium
dicyanamide, 95% conversion of glycerol into glycerol carbonate
22
20
18
16
14
1384.41
%T
12
10
8
◦
789.66
is achieved after four catalyst recycles at 120 C in 13 h using a
DMC/glycerol molar ratio of 3. However, the main drawbacks of
this process are its long reaction time and column chromatography
to separate catalyst and product.
6
4
1638.01
2
466.45
1
083.60
3
467.14
A few of the aforesaid processes utilize carbonates and bicarbon-
ates catalysts which are not reusable and in fact they create a lot of
waste. In the current work, the synthesis of glycerol carbonate was
achieved through reaction of glycerol and dimethyl carbonate by
0
.0
4
000.0
3000
2000
1500
1000
400.0
-1
cm
Fig. 1. FT-IR spectrum of the catalyst.
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Fig. 2. SEM images of CHT-HMS catalyst with Al:Mg 1:2.
−
1
3
467 cm was due to stretching of H bonded OH group and water
molecules; and thus the corresponding deformation mode are at
−
1
1
638 cm . FT-IR spectrum indicated the presence of functional
group which indicated the mesoporous silica structure is intact
after modifying the catalyst.
2.3.2. Surface area analysis
Surface area and pore size distribution measurements were
obtained by Micromeritics ASAP 2010 instrument by N adsorption
2
◦
◦
at −197 C, after subjecting the sample to high vacuum at 450 C
for 4 h. The adsorption desorption isotherms for the CHT-HMS cat-
alyst with Al:Mg 1:2 showed characteristic of mesoporous solids.
Characteristic nitrogen BET surface area for pure HMS was found
to be 820 m /g which decreased to 250 m /g for CHT-HMS with
Al:Mg 1:2. Pore volume for HMS and CHT-HMS were measured as
2
2
3
3
0
.39 cm /g and 0.29 cm /g, respectively. This shows the accumula-
tion of hydrotalcite on the HMS surface. The BJH adsorption pore
size distribution shows that a major portion of surface area is due to
pores with 4.6 nm diameter, suggesting pore sizes of the CHT-HMS
in the mesoporous region (figure not shown for brevity).
Fig. 3. Temperature programmed desorption of CO2 for the catalyst.
2
.3.3. SEM
2
.3.5. XRD
Surface morphology of the catalyst was captured with SEM
The X-ray scattering measurements were performed by using a
(
Camera SU 30 microscope, JEOL, Japan). The dried samples were
˚
Philips PW 1729 powder diffractometer with Cu-K␣ (1.54 A). The
scattered intensities were collected from 10 to 60 (2ꢁ) by scan-
ning at 0.025 (2ꢁ) steps with a counting time of 1 s at each step.
The catalyst exhibited a strong reflection at 2ꢁ of about 2.8 (fig-
ure not shown). In CHT-HMS catalyst with Al:Mg 1:2, the particles
which were formed on the surface and/or in the pore of HMS, were
very small to show an XRD pattern and pores of HMS blocked by the
formed Al–Mg mixed oxide, or by the walls of HMS being partially
destroyed. Hence, no peak was observed in XRD pattern of calcined
mounted on specimen studs and sputter coated with thin film
of gold to prevent charring. Topographical images in SEM were
formed from back scattered primary or low-energy secondary elec-
trons (Fig. 2). A smoother surface layer is clearly seen from the
images for CHT loaded HMS with particle size in the range of 0.1 ␮m
to 2 ␮m. The particles were uniform with small agglomerates.
◦
◦
◦
◦
2.3.4. TPD
◦
◦
HT on HMS from 10 to 70 .
Temperature programmed desorption (TPD) was performed by
using a Micromeritics AutoChem 2910 (USA) instrument. The basic-
2
.3.6. EDAX
ity of CHT-HMS catalyst samples was studied by CO -TPD. A sample
2
◦
Elemental analysis of the catalyst was carried out which showed
was loaded and activated at 450 C, in a quartz cell; under continu-
ous helium flow (20 cm /min) for a period of 2 h. Pre-saturation was
accomplished by passing 10% v/v CO2 in He for 1 h at 30 C. Then
the sample was flushed with He for 1 h at 50 C to remove excess
3
exact % composition of each component in the catalyst. During the
synthesis, 1:2 ratio of Al:Mg was used, which was almost matching
with the values obtained from EDAX results (Table 1). Around 15%
loading of (MgO + Al O ) was observed.
◦
◦
2
3
CO . The adsorbed CO was desorbed in He flow (30 ml/min) with
2
2
◦
a heating rate of 10 C/min and the temperature was raised from
◦
◦
5
0 C to 450 C. The desorbed CO was detected by using a ther-
Table 1
Elemental analysis of CHT-HMS catalyst.
2
mal conductivity detector (TCD). A broad peak with a maximum
◦
at 114 C corresponds to desorption of adsorbed CO (Fig. 3). The
2
Composition, w/w
CHT-HMS
O (%)
51.70
Mg (%)
5.38
Al (%)
2.93
Si (%)
39.99
amount of CO2 adsorbed was 9.7 ml/g of the catalyst at STP. This
suggests the presence of basic sites in the catalyst.
Please cite this article in press as: G.D. Yadav, P.A. Chandan, A green process for glycerol valorization to glycerol carbonate over
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4
OH
O
O
O
O
OH H C
CH3 CHT-HMS
+
3
+
2 CH OH
3
O
O
HO
Dimethyl carbonate
Glycerol
HO
Methanol
Glycerol carbonate
Scheme 1. Synthesis of glycerol carbonate from glycerol and DMC over CHT-HMS.
2
.4. Reaction procedure
glycerol was studied with glycerol as a limiting reactant. Concen-
tration profiles of all reactants and products were measured as
function of time. Over 80% conversion was obtained for 1:3 mol
ratio of glycerol to DMC in standard experiment in 90 min.
The experiments were carried out in a 100 cm3 stainless auto-
clave (Amar equipment, Mumbai, India) with stirrer with pitched
turbine impeller. For the standard reaction, the reactor was charged
with 0.109 mol glycerol, 0.326 mol dimethyl carbonate (DMC),
3.1. Screening of various catalysts
3
3
0
.002 g/cm CHT-HMS catalyst with Al:Mg 1:2, 1.0 cm n-undecane
as an internal standard and methanol as a solvent to make the total
Varying ratios of Al–Mg containing CHT-HMS catalysts were
3
◦
volume of 55 cm at 170 C for 3 h. The autogenous pressure of
the reaction mixture was about 6.5 atm to 7.0 atm. As the reaction
proceeded, methanol and CO2 were generated in the reaction and
therefore the pressure reached to 17.0 atm to 18.0 atm at the end of
the reaction. At the end of the reaction, the autoclave was cooled to
room temperature, fewer atmospheric pressure and then opened
to enable the reaction liquid to be filtered.
prepared by a standard aqueous preparation and heat crystalliza-
tion process. Selection of the catalyst was based on the conversion
of glycerol and selectivity towards glycerol carbonate. This catalyst
contains two types of sites, acidic as well as basic. Acidic sites were
fewer as compared to the basic sites. The role of both the sites is
demonstrated in the reaction mechanism. HMS is slightly acidic in
nature due to presence of surface –OH groups and thus the pres-
ence of intact HMS structure shown from XRD and FT-IR indicates
2.5. Analysis
the presence of acidic sites in the catalyst. CO TPD shows the pres-
2
ence of basic site due to the presence of alkaline earth metal Mg
in the catalyst [25,26]. The presence of silica support increases the
surface area and gives uniform pore size and therefore was more
advantageous to sacrifice the small activity of the catalyst due to
acid neutralization of this solid base catalyst in the presence of sil-
ica. High thermal stability, larger surface area, uniform-sized pores
and adsorption capacity for aromatic organic molecules turn these
materials into interesting catalysts. The catalyst was found to have
excellent activity towards transesterification reaction of glycerol.
CHT-HMS catalysts with different Al and Mg ratios were screened
for the reaction under similar experimental conditions (Table 2).
The reaction was also studied with pure HMS as a catalyst to verify
the role of active sites in the catalyst. 0.11 g catalyst was added to
the reaction mixture. No conversion was obtained when pure HMS
was used since it is an inert support. Catalyst with Al:Mg ratio of
The samples were withdrawn periodically from the autoclave,
after stopping the agitation temporarily, by using a sampling valve.
The samples were centrifuged to allow the catalyst particle to set-
tle down. The upper clear liquid was taken further for the analysis.
Analysis of the reaction mixture was performed by GC (Chemito,
model 1000) equipped with FID and a BPX-50 capillary column
0.25 mm diameter and 30 m length). The calculations were based
on the limiting reactant glycerol. The product glycerol carbonate
and all the other impurities were confirmed by GC–MS instrument
(
(
(
Perkin Elmer Clarus 500) equipped with BP-1 capillary column
30 m length, 0.025 mm i.d.).
3
. Results and discussion
1
:2 was found to be the best for the reaction and therefore it was
When DMC was used as a solvent, two immiscible phases
taken for the further reaction studies.
were formed which made the kinetic study of the reaction dif-
ficult (Scheme 1). Methanol was found to be highly miscible
with the reaction mixture, hence it was taken as solvent. In
methanol as a solvent, high reaction temperature was required
to shift the equilibrium towards forward direction as it was
also a co-product in the reaction. The other side products were
3.2. Effect of speed of agitation
The effect of speed of agitation was studied in the range of
3
600–1000 rpm (Fig. 4) with 0.002 g/cm catalyst concentration at
170 C. The mole ratio of glycerol to DMC was kept as 1:3. No signifi-
◦
4
-(methoxymethyl)-1,3-dioxolan-2-one, 3-methoxy propane-1,2-
diol and bis[(2-oxo-1,3-dioxolan-4yl)methyl] carbonate in small
quantity. The effect of various parameters on the conversion of
cant change in the rate and conversion pattern was observed, which
indicated the absence of any resistance to external mass transfer of
Table 2
a
Catalyst screening for the synthesis of glycerol carbonate .
Entry
Catalysts with different
Al:Mg ratio
Reaction
time (h)
Conversion of
glycerol (%)
Selectivity to glycerol
carbonate (%)
1
2
3
4
5
Al:Mg (1:3)
Al:Mg (1:2)
Al:Mg (2:1)
Al:Mg (3:1)
HMS
2.5
2.5
2.5
2.5
2.5
83.1
84.7
73.5
72.9
0
86.6
84.3
88.3
86.5
–
a
Reaction conditions: 0.326 mol dimethyl carbonate, 0.109 mol glycerol, 0.002 g/cm CHT-HMS catalyst, 1.0 cm3 n-undecane as internal standard, 20 cm3 methanol,
3
◦
5
5.0 cm3 total reaction volume, speed of agitation 1000 rpm, temperature 170 C.
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100
5
100
80
60
40
20
0
80
60
40
20
0
0
30
60
90
120
150
180
210
0
30
60
90
120
150
180
Time (min)
800 rpm
Time (min)
600 rpm
1000 rpm
0
0
.001 g/cm3
.003 g/cm3
0.002 g/cm3
0.004 g/cm3
Fig. 4. Effect of speed of agitation on conversion of glycerol. Reaction conditions:
3
0
1
.326 mol dimethyl carbonate, 0.109 mol glycerol, 0.002 g/cm CHT-HMS catalyst,
Fig. 5. Effect of catalyst loading on conversion of glycerol. Reaction conditions:
.326 mol dimethyl carbonate, 0.109 mol glycerol, 1.0 cm n-undecane as inter-
nal standard, 20 cm methanol, speed of agitation, 55 cm total reaction volume,
000 rpm, temperature 170 C.
.0 cm3 n-undecane as internal standard, 20 cm3 methanol, 55.0 cm total reaction
3
3
0
◦
volume, speed of agitation 1000 rpm, temperature 170 C.
3
3
◦
1
reactants to the external surface of the catalyst. However, all fur-
ther reactions were carried out at a speed of 1000 rpm. Theoretical
calculations were also done to prove the absence of external mass
transfer resistance as per some of our earlier work [27–30].
was required to shift the reaction equilibrium towards the prod-
uct and 80% conversion was obtained. Under such conditions no
reaction of dimethyl carbonate with the third OH group of glycerol
was seen. Due to the proximity of the carbonate group, the reactiv-
ity of the third OH group was lower than that of the first one. The
intramolecular reaction with the second OH group proceeds much
faster than the intermolecular one with the next dimethyl carbon-
ate molecule and thermodynamically stable five-membered cyclic
carbonate is formed exclusively. Mole ratio effect was studied in
the range of 1:1 to 1:4 and 1:3 was found as the best (Fig. 6).
3.2.1. Effect of solid-liquid mass transfer resistance
The rate of mass transfer was calculated from the mass trans-
fer coefficients for both the reactants, which were obtained from
their bulk liquid phase diffusivities. The solid-liquid mass trans-
fer resistance was calculated using the Wilke–Chang equation and
Sherwood number. The values of solid to liquid phase mass tran-
fer co-efficients kSLA and k
were calculated from Sherwood
SLB
3
−
−3
number (Sh) as 7.45 × 10 cm/s and 6.93 × 10 cm/s, respec-
tively. To be on the safer side, the limiting value of the Sherwood
number was taken as 2. The actual value was far greater than
3.5. Effect of temperature
◦
◦
The effect of temperature was varied from 150 C to 180 C
Fig. 7). The rate of reaction increased with an increase in tem-
2
due to intense agitation. The effect of mass transfer resis-
(
tance due A and B was given by 1/(kSLA × ap) and 1/(kSLB × ap)
perature. However, selectivity towards glycerol carbonate was
decreased at higher temperature due to the formation of other
impurities. Hence 170 C was taken as optimum temperature for
where, ap represents the external surface area per unit liquid vol-
ꢀ
ꢀ
ꢁ
ꢁ
4
−1
ume ap = 6w/ꢀpdp = 2.94 × 10 cm
and the values obtained
◦
−3
−4
were 4.761 × 10 s and 4.9 × 10 s, respectively. The mass trans-
the reaction.
3
3
fer resistances were 2.94 s/cm /mol and 0.82 s/cm /mol for transfer
of A and B, respectively. Whereas the initial rate (r ) for the
A
−
6
7
3
standard reaction was found to be 6.7 × 10 mol/cm /s. Thus, the
100
3
resistance due to reaction (1/r ) 1.49 × 10 s/cm /mol was very
A
3
much greater than mass transfer resistances of 2.94 s/cm /mol and
3
0
.82 s/cm /mol. This confirmed that mass transfer rates are much
80
higher than rates of reaction and hence speed of agitation had no
influence on reaction rate.
6
0
0
3.3. Effect of catalyst concentration
4
It was observed that the rate of reaction increased with increase
in catalyst quantity in the range of 0.001 g/cm to 0.004 g/cm3
3
20
0
(
Fig. 5). This is due to the proportional increase in active sites.
3
Beyond catalyst concentration of 0.002 g/cm the effect was small
to increase conversion which suggested that more number of active
sites were available than required the available reactant molecules.
0
30
60
90
120
150
180
Time (min)
Gly:DMC = 1:1
Gly:DMC = 1:3
Gly:DMC = 1:2
Gly:DMC = 1:4
3.4. Effect of mole ratio
When the mole ratio of glycerol to dimethyl carbonate was less
than 1:3 during the reaction, it was observed that both the conver-
sion and selectivity decreased. Glycerol to DMC mole ratio of 1:3
3
Fig. 6. Effect of mole ratio of glycerol: DMC. Reaction conditions: 0.002 g/cm CHT-
3
3
3
HMS catalyst, 1.0 cm n-undecane as internal standard, 20 cm methanol, 55 cm
◦
total reaction volume, speed of agitation 1000 rpm, temperature 170 C.
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100
The Langmuir–Hinshelwood–Hougen–Watson (LHHW) model
was used. The reaction involves two reactants namely glycerol (A)
and DMC (B) to give the desired product glycerol carbonate (G) and
methanol (M).
80
60
40
20
0
Adsorption of glycerol (A) on vacant site (S) was given by
K
A
A + S←→ AS
Adsorption of DMC (B) on vacant site was given by
(1)
KB
B + S←→ BS
(2)
Surface reaction of AS and BS leading to the formation of Glycerol
carbonate (GS) on the site
0
30
60
90
Time (min)
50 °C 160 °C 170 °C 180 °C
120
150
180
K
SR
AS + BS←→ GS + MS
(3)
Desorption of glycerol carbonate (PS) and methanol (MS) was
given by
1
Fig. 7. Effect of temperature on conversion of glycerol. Reaction conditions:
1
/K
G
3
0
1
5
.326 mol dimethyl carbonate, 0.109 mol glycerol, 0.002 g/cm CHT-HMS catalyst,
GS←→ G + S
(4)
(5)
.0 cm n-undecane as internal standard, 20 cm3 methanol, total reaction volume
3
1
/KM
3
5 cm , speed of agitation 1000 rpm.
MS←→ M + S
The total concentration of the sites, Ct expressed as
3
.6. Development of kinetic model
Ct = C + C + CBS + CGS + CMS
(6)
(7)
S
AS
The above results were used to build a kinetic model. From the
or
Ct = C + K C C + KBCBC + K C C + KMCMC
calculated values of mass transfer rates of A and B and the initial
rate, it was evident that the reaction was devoid of inter and intra-
particle diffusion resistances. Thus the reaction may be controlled
by any of these 3 steps namely adsorption, surface reaction and
desorption.
The catalytic mechanism is given in Scheme 2. First glycerol and
dimethyl carbonate get adsorbed on the surface of the catalyst. The
adsorbed species reacts to give intermediate which is adsorbed on
the catalytic surface, this intermediate rearranges to form methanol
and glycerol carbonate. The catalyst gets regenerated at the end of
the reaction.
During the reaction dimethylcarbonate is consumed and two
moles of methanol is formed and hence there is change in the total
reaction volume. Initial total volume of the reaction was 55 cm ,
whereas after 84.7% conversion in 2.5 h the total volume of the
reaction mixture was calculated as 54.3 cm . As this change in the
S
A
A
S
S
G
G
S
S
The concentration of vacant sites,
Ct
CS = 1
(8)
+ K CA + KBCB + K C + KMCM
A
G G
If the surface reaction (C) controls the rate of reaction, then the
rate of reaction of A was given by,
dCA
−rA = − dt = kSRCASCBS − kSꢀ RCGSCMS
(9)
ꢂ
ꢀ
ꢁꢃ
kSR K K C C − K K C CM/KSR
B
B
G G
M
Ct2
dCA
dt
A
A
−
=
(10)
2
(
1 + KACA + KBCB + KGCG + KMCM)
3
When the reaction was far away from equilibrium,
3
2
dCA
dt
k
K KBC CBCt
SR
A
A
volume was very less, it would not affect the rate and the conversion
pattern based on catalyst concentration.
−
=
= ꢀ
ꢄ
ꢁ
(11)
(12)
2
1
+
KiC
i
krwC CB
A
ꢀ
ꢄ
ꢁ
Glycerol
Dimethyl carbonate
2
1
+
KiCi
O
O
HO
O H
where krw = k KAKBCt; w is catalyst concentration. Analysis of ini-
SR
O
H
O
tial rate data showed that all species were weakly adsorbed and the
Langmuir–Hinshelwood–Hougen–Watson (LHHW) model reduced
to the power law model. It would mean reactants DMC and glycerol
were weekly adsorbed on the catalyst.
B - A - B - A - B
Glycerol carbonate
HO
B - A - B - A - B
O
H
Then the above equation reduces to,
O
O
dCA
O
−
= k C C w
(13)
r
A
B
dt
O
O
HO
Let CB0/CA0 = M, the molar ratio of glycerol to DMC at time t = 0.
Then, Eq. (13) can be written in terms of fractional conversion as,
HO
O
O -
+
H
O
H
O
O
dXA
2
-
O
O
=
dt
krC w(1 − X )(M − X )
(14)
B - A - B - A - B
A0
A
A
+
H
O
H
O
O
This upon integration leads to:
H
O
O
B - A - B - A - B
ꢅ
ꢆ
M − XA
O
H
Ln
= k CA0(M − 1)t
(15)
1
M(1 − X )
A
B - A - B - A - B
Scheme 2. Catalytic reaction mechanism.
A plot of ln[(M − X )/M × (1 − X )] against time gave an excel-
A
A
lent fit, thereby supporting the model (Fig. 8). The initial reaction
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1.40
1.20
1.00
0.80
0.60
0.40
0.20
0.00
thereafter a steady activity was observed for the next cycles with
maintained high selectivity.
y = 0.0176x
R² = 0.9979
y = 0.0148x
R² = 0.9957
4. Conclusions
This work dealt with conversion of glycerol and dimethyl car-
bonate into glycerol carbonate using CHT-HMS as the solid base
catalyst. A new catalytic method was developed. A reusable CHT-
HMS (Al:Mg:: 1:2; 15% w/w on HMS), calcined hydrotalcite catalyst
for the conversion of glycerol to glycerol carbonate was found to the
best. It was fully characterized to understand its superior activity.
All the reaction parameters were optimized to get high conversion
of glycerol. The reaction was free from any external mass transfer
resistance and intra-particle diffusion resistance and was intrinsi-
cally kinetically controlled. A kinetic model for the reaction was
developed and validated. The reaction was found to be overall sec-
ond order with activation energy of 12.56 kcal/mol. The catalyst
activity was tested upto 5 cycles with intact activity after the first
run.
y = 0.0115x
R² = 0.9817
y = 0.007x
R² = 0.9933
0
20
40
Time (min)
170 C 160 C
60
80
100
1
80 C
150 C
Fig. 8. Plot of Ln[(M − XA)/M(1 − XA)] against time.
0
Acknowledgment
-1
This research work was supported by CSIR-NMITLI project on
“
Bioglycerol based chemicals”. PAC received JRF under this project.
-
2
GDY acknowledges support from the R. T. Mody Distinguished Pro-
fessor Endowment and J. C. Bose National Fellowship (DST-GOI).
-3
y = -6141.3x + 9.4566
R² = 0.9413
References
-4
[
[
1] http://www.eia.gov/biofuels/biodiesel/production/ (accessed 12.01.14).
2] R. Medina, L.E. Cerdan, A. Gimenez, B.C. Paez, J. Biotechnol. 70 (1999)
-5
379–391.
[
[
3] J.M. Clacens, Y. Pouilloux, J. Barrault, Appl. Catal., A 227 (2002) 181–190.
4] M. Roice, K.P. Subhashchandran, A.V. Gean, J. Franklin, V.N. Rajasekharan Pillai,
Polymer 44 (2003) 911–922.
-
6
0
.00215
0.0022
0.00225 0.0023
0.00235 0.0024
-
1
[5] G. Olga, R. Juana, B. Jorge, R. Jose, C. Tomas, Recent Patents Chem. Eng. 2 (2009)
1–21.
1
/T (°K )
1
[
[
6] F. Xiaohu, B. Rachel, Z. Yongchang, Open Fuels Energy Sci. J. 3 (2010) 17–22.
7] G.D. Yadav, P.A. Chandan, N. Gopalaswami, Clean Technol. Environ. Policy 14
Fig. 9. Arrhenius plot.
(
2012) 85–95.
8] G.D. Yadav, P.A. Chandan, D.P. Tekale, Ind. Eng. Chem. Res. 51 (2012)
549–1562.
[9] G.D. Yadav, R.V. Sharma, S.O. Katole, Ind. Eng. Chem. Res. 52 (2013)
0133–10144.
10] M.O. Sonnati, S. Amigoni, E.P. Taffin de Givenchy, T. Darmanin, O. Choulet, F.
Guittard, Green Chem. 15 (2013) 283–306.
11] M. Tudorache, L. Protesescu, S. Coman, V.I. Parvulescu, Green Chem. 14 (2012)
[
Table 3
Catalyst reusability.
1
1
Sr. no. Catalyst
reusability
Reaction
time (h)
Conversion of
glycerol (%)
Selectivity to glycerol
carbonate (%)
[
[
1
2
3
4
5
6
Fresh
2.5
2.5
2.5
2.5
2.5
2.5
84.7
77.8
76.6
76.2
75.7
75.6
84.3
94.6
92.9
93.4
92.8
92.3
478–482.
1st Reuse
2nd Reuse
3rd Reuse
4th Reuse
5th Reuse
[
[
12] T. Mizuno, T. Nakai, M. Mihara, Heteroat. Chem. 21 (2010) 99–102.
13] C. Hammond, J.A. Lopez-Sanchez, M.H. Ab Rahim, N. Dimitratos, R.L. Jenkins,
A.F. Carley, Q. He, C.J. Kiely, D.W. Knight, G.J. Hutchings, Dalton Trans. 40 (2011)
3
927–3937.
14] W. Liguo, M. Yubo, W. Ying, L. Shimin, D. Youquan, Catal. Commun. 12 (2011)
458–1462.
[
[
1
15] M.J. Climent, A. Corma, P. De Frutos, S. Iborra, M. Noy, A. Velty, P. Concepcion,
J. Catal. 269 (2010) 140–149.
rates were calculated and the Arrhenius plot gave activation energy
2.56 kcal/mol (Fig. 9) which indicated an intrinsically kinetically
controlled reaction with overall second order.
[16] L. Wang, Y. Ma, Y. Wang, S. Liu, Y. Deng, Catal. Commun. 12 (2011) 1458–1462.
[
[
[
17] S. Fujita, Y. Yamanishi, M. Arai, J. Catal. 297 (2013) 137–141.
18] R.A. Grey, US5091543, Arco Chemical Tech. Inc., USA, 1992.
19] J.R. Ochoa-Gomez, O.G. Jimenez-Aberasturi, B.M. Madurga, A.P. Rodrıguez, C.R.
Lopez, L.L. Ibarreta, J.T. Soria, M.C. Villara Velasco, Appl. Catal., A 366 (2009)
1
3
15–324.
3.7. Catalyst reusability
[20] M.G. Alvareza, A.M. Segarra, S. Contreras, J.E. Sueiras, F. Medina, F. Figueras,
Chem. Eng. J. 161 (2010) 340–345.
[
[
21] C. Chiappe, S. Rajamani, Pure Appl. Chem. 84 (2012) 755–762.
22] G. Rokicki, P. Rakoczy, P. Parzuchowski, M. Sobiecki, Green Chem. 7 (2005)
The reaction mixture was filtered under vacuum; the cata-
lyst was washed with methanol and dried. The dried catalyst was
refluxed in methanol to remove the adsorbed species, again filtered
529–539.
[
[
23] A. Kumar, K. Iwatani, S. Nishimura, A. Takagaki, K. Ebitani, Catal. Today 185
◦
(2012) 241–246.
and dried in oven at 120 C. The residual catalyst was reused for
24] A. Takagaki, K. Iwatani, S. Nishimura, K. Ebitani, Green Chem. 12 (2010)
578–581.
same the reaction up to 5 cycles (Table 3). From the results it was
clearly seen that, the catalyst was reusable with minimum loss in its
activity and stability. The conversion of glycerol and the selectivity
towards desired glycerol carbonate remained high. The initial drop
in the catalyst activity could be due to loosely bound active compo-
nent which could have been leached during the first run; however,
[25] G.D. Yadav, P. Aduri, J. Mol. Catal. A: Chem. 355 (2012) 142–154.
[
[
[
26] G.D. Yadav, G.P. Fernandes, Catal. Today 207 (2013) 162–169.
27] G.D. Yadav, M.S. Krishnan, Org. Process Res. Dev. 2 (1998) 86–95.
28] G.D. Yadav, S. Sengupta, Org. Process Res. Dev. 6 (2002) 256–262.
[29] G.D. Yadav, M.S. Krishnan, Chem. Eng. Sci. 54 (1999) 4189–4197.
30] G.D. Yadav, P.H. Mehta, Ind. Eng. Chem. Res. 33 (1994) 2198–2208.
[
Please cite this article in press as: G.D. Yadav, P.A. Chandan, A green process for glycerol valorization to glycerol carbonate over
heterogeneous hydrotalcite catalyst, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.01.043