The role of impurities in the La2O3 catalysed carboxylation of crude glycerol

Article abstract of DOI:10.1007/s10562-019-02679-w

The direct carboxylation of crude glycerol, obtained as a by-product of bio-diesel synthesis, with CO₂ has been investigated over lanthanum oxide as a heterogeneous catalyst for the first time. Adiponitrile is employed as a dehydrating agent in order to shift the reaction equilibrium to the product side. The selectivity of the reaction towards glycerol carbonate when using crude glycerol is significantly reduced as compared to employing refined glycerol: 2.3% cf. 17% respectively. Glycerol conversion, however remains approximately constant: 54% cf. 58%. In order to understand the role of the impurities present in crude glycerol, model systems consisting of refined glycerol and one or more of water, methanol, methyl palmitate (as a model fatty acid methyl ester), and sodium methoxide have been prepared and used as reaction media to systematically evaluate their effect. All of these impurities are seen to reduce the selectivity towards glycerol carbonate, instead favouring the formation of 4-(hydroxymethyl)oxazolidin-2-one, with the exception of methanol where no detrimental effect is observed and the measured selectivity increases slightly to ca. 22%. This effect is ascribed, in part, to improved mass transfer as a consequence of an increased solubility of carbon dioxide in the liquid media when methanol is present. Additionally, adiponitrile is observed to play a crucial role in the reaction mechanism beyond its simple role as a dehydrating agent. These results provide insights into the required purification steps for crude glycerol, and suggest the possibility of employing crude glycerol directly, and its use as a chemical feedstock; in both cases by minimising costly separation and purification steps. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-019-02679-w

Catalysis Letters
https://doi.org/10.1007/s10562-019-02679-w
The role of impurities in the La2O3 catalysed carboxylation of crude
glycerol
N. A. Razali1 · M. Conte2 · J. McGregor1
© The Author(s) 2019
Abstract
The direct carboxylation of crude glycerol, obtained as a by-product of bio-diesel synthesis, with CO2 has been investigated
over lanthanum oxide as a heterogeneous catalyst for the first time. Adiponitrile is employed as a dehydrating agent in order
to shift the reaction equilibrium to the product side. The selectivity of the reaction towards glycerol carbonate when using
crude glycerol is significantly reduced as compared to employing refined glycerol: 2.3% cf. 17% respectively. Glycerol
conversion, however remains approximately constant: 54% cf. 58%. In order to understand the role of the impurities present
in crude glycerol, model systems consisting of refined glycerol and one or more of water, methanol, methyl palmitate (as
a model fatty acid methyl ester), and sodium methoxide have been prepared and used as reaction media to systematically
evaluate their effect. All of these impurities are seen to reduce the selectivity towards glycerol carbonate, instead favouring
the formation of 4-(hydroxymethyl)oxazolidin-2-one, with the exception of methanol where no detrimental effect is observed
and the measured selectivity increases slightly to ca. 22%. This effect is ascribed, in part, to improved mass transfer as a
consequence of an increased solubility of carbon dioxide in the liquid media when methanol is present. Additionally, adi-
ponitrile is observed to play a crucial role in the reaction mechanism beyond its simple role as a dehydrating agent. These
results provide insights into the required purification steps for crude glycerol, and suggest the possibility of employing crude
glycerol directly, and its use as a chemical feedstock; in both cases by minimising costly separation and purification steps.
Graphical Abstract
No detrimental
impact
MeOH
Catalyst
H2O
deacꢀvaꢀon
Promotes
NaOMe by-product
formaꢀon
Reacts with
FAME
glycerol
Keywords Glycerol · Carbon dioxide utilisation · Glycerol carbonate · Lanthanum oxide · Glycerin · Carboxylation of
crude glycerol · Heterogeneous catalysis · Adiponitrile
Extended author information available on the last page of the article
Vol.:(0123456789)
1 3

N. A. Razali et al.
has the advantages that urea is relatively inexpensive and
the reaction can be carried out in the absence of a sol-
vent, however this reaction produces ammonia as a waste
product [22, 23]. The ammonia produced may promote
the formation of isocyanic acid, therefore it is necessary
to employ a vacuum system in order to remove NH3 [22,
24]. A promising alternative is the direct carboxylation of
glycerol with CO2 [25]. The use of CO2 also has advan-
tages in making use of a highly available, low cost, waste
material which contributes directly to climate change; CO2
concentration in the atmosphere having reached 411 ppm
in May 2018 [26]. The direct carboxylation of glycerol
can therefore be considered as a green process, but it is
thermodynamically limited [27] having a small chemical
equilibrium constant: 1.3×10−3 at 160 °C and 5 MPa [28].
Therefore, the reaction must be carried out at high pres-
sure, > 5 MPa, and elevated temperature.
1 Introduction
Glycerol is the major by-product arising from the produc-
tion of biodiesel, with 1 kg of glycerol produced for every
10 kg of biodiesel [1]. It is estimated that approximately
67% of global glycerol production is derived from bio-
diesel synthesis [2]. However, one of the major challenges
resulting from the increasing commercialisation of bio-
diesel is the disposal of waste glycerol, a costly process
which can create environmental concern [3]. This has the
consequence of lowering the market value of both refined
and crude glycerol [4, 5]. Therefore, methods able to uti-
lise glycerol through conversion to value-added products
such as glycerol carbonate present an excellent opportunity
to offset both economic and environmental concerns. Fur-
ther, there are clear benefits to minimising the number of
pre-treatment steps required in order to utilise crude glyc-
erol, that is glycerol directly obtained from the synthesis
of biodiesel and without further purification.
In order to overcome this limitation, acetonitrile [29],
2-cyanopyridine [25] and benzonitrile [30] have previ-
ously been employed as dehydrating agents in this reac-
tion. Dehydrating agents act as a chemical water trap, thus
shifting the equilibrium to the product side; additionally
2-cyanopyridine has also been implicated in CO2 activa-
tion [31]. A range of heterogeneous catalysts have pre-
viously been employed and tested for the conversion of
glycerol to glycerol carbonate including: zeolites, basic
ion-exchange resins as well as Al2O3 and Nb2O5 supported
on CeO2 [32–34]. Employing CeO2 directly as a catalyst
resulted in an 19% yield to glycerol carbonate [31]. The
use of La2O2CO3-ZnO [29], Cu/La2O3 [35] and Cu based
catalysts supported on: (i) acidic supports; (ii) basic sup-
ports; (iii) acidic and basic bifunctional supports; and (iv)
supports with neither acidic nor basic sites for the car-
boxylation of glycerol in the presence of acetonitrile have
also been studied [36, 37]. Transition metals have demon-
strated potential to activate CO2; however the performance
of these catalysts is highly dependent upon metal particle
size and the degree of metal dispersion [35, 36]. Else-
where, hydrotalcite catalysts such as Zn/Al/La/M (M= Li,
Mg and Zr) and Zn/Al/La/X (X = F, Cl and Br) have been
employed resulting in yields of glycerol carbonate of up
to 14% [29, 30, 38].
Crude glycerol consists of: glycerol, methanol, residual
catalyst from biodiesel synthesis, fatty acid methyl esters
(FAMEs) and water [2]. The presence of these impurities
can limit the direct conversion of crude glycerol and lead
to products or processes with a low yield, e.g., as a conse-
quence of catalyst poisoning. This can make utilisation of
crude glycerol unfavourable in many circumstances. Distil-
lation, filtration, chemical treatment, adsorption using acti-
vated carbon, ion-exchange, extraction and crystallization
are the typical methods employed to purify crude glycerol
[6], and in turn at significant processing cost; the costs of
separation and purification often dominate the overall cost
of a chemical process [7]. This has therefore stimulated
recent research on the direct utilisation of crude glycerol
into valuable chemicals, e.g. glycerol carbonate [8, 9],
polyester [10], 1,3-propanediol [11], and n-butanol [12].
Glycerol carbonate is of particular interest due to its varied
potential applications as an electrolyte in lithium ion bat-
teries, an intermediate in polymer synthesis and wide use
in the cosmetic and pharmaceutical industries [13, 14].
Additionally, glycerol carbonate has been proposed as a
solvent for the pre-treatment of sugarcane bagasse [15]
suggesting a role for this versatile chemical in low and
middle income agricultural economies where small-scale,
local, biodiesel production may play an important role in
sustainable energy generation, but where complex separa-
tion processes are not available.
Considering the use of crude, unrefined glycerol as a
feedstock, Okoye et al. [39] have studied the influence of
water in the synthesis of glycerol carbonate synthesis from
dimethyl carbonate and they observed a strong decrease in
carbonate yield from 75 to 50% upon the addition of 5 wt.%
of water. Elsewhere, Teng et al. [9] studied the microwave-
assisted transesterification of crude glycerol over a CaO cat-
alyst. In this case, a yield of glycerol carbonate of 94% was
achieved if in the presence of sodium methoxide (NaOMe);
in contrast a yield of only 8% was achieved if using pure
glycerol under the same reaction conditions.
Glycerol carbonate can be produced from glycerol
through reaction with phosgene or via transesterifica-
tion with dimethyl or diethyl carbonate [16, 17], however
these acyclic organic carbonates are themselves commonly
manufactured from phosgene [18]. Alternatives to employ-
ing phosgene include the use of urea [19–21]. This route
1 3

The role of impurities in the La2O3 catalysed carboxylation of crude glycerol
Further attempts at synthesising glycerol carbonate
from crude glycerol have been made via the glycerolysis
of urea. Indran and co-workers reported that the presence
of water significantly reduced the conversion of glycerol
from 94 to 50% and the yield of glycerol carbonate from
85 to 35%; an effect ascribed to the instability of glyc-
erol carbonate in water [8]. This study also revealed that
methanol has a negative effect on the glycerolysis of urea,
as it promotes the decarboxylation of glycerol carbonate
to glycidol.
2.3 Characterisation Techniques
The morphology of the catalyst was studied by scanning
electron microscopy (SEM) (JEOL JSM-6010LA). The
solid catalyst was mounted on carbon and gold coated
prior to study, and particle size analysis was conducted
using Image J software. Brunauer Emmett Teller (BET)
measurements (Micromeritics 3-Flex) identified the total
surface area: 0.5 g catalyst was loaded into the sample
tube and degassed at 250 °C for 3 h prior to analysis
using Vac Prep 061. Surface functionalities were identi-
fied by Fourier transform infrared spectroscopy (FTIR)
(Shimadzu IRAffinity-IS). Samples were placed in Specac
Quest ATR cell and spectra were collected over the range
400 to 4000 cm− 1 with a resolution of 4 cm− 1 X-ray dif-
fraction (Bruker D2 Phaser) employing CuKα1 radiation
(λ=1.5406 Å, monochromator tube voltage 30 kV, current
10 mA) was used to determine the crystallographic phases
present.
The present work therefore extends these previous stud-
ies by investigating the direct carboxylation of crude glyc-
erol with CO2 in the presence of adiponitrile as a dehy-
drating agent over lanthanum based catalysts. This has the
advantages of overcoming the thermodynamic limitations
of the reaction through the use of a dehydrating agent,
and of improving the overall economic and environmen-
tal sustainability of the process by avoiding the use of
phosgene and bypassing the separation and purification of
crude glycerol. La2O3 is widely used in the carboxylation
of pure glycerol to glycerol carbonate directly as a single
component catalyst, mixed metal oxide or a support and
hence its use allows direct comparison with previous stud-
ies of this reaction [40–43].
In addition, ATR-FTIR was also employed to characterise
liquid samples of the reaction mixture. The same spectrom-
eter, accessory and acquisition conditions were used as for
the characterisation of the solid catalyst.
2.4 Catalytic Testing
2 Materials and Experimental Method
Carboxylation of glycerol was carried out over 18 h in a
45 ml stainless steel autoclave (Parr instrument, Model
4714) equipped with a magnetic stirrer. La2O3 (6 wt.% with
regard to the initial reaction mixture), 22.5 mmol of glyc-
erol (2.1 g) and 45 mmol (5 ml) of adiponitrile were loaded
into the reactor. The reactor was purged and cycled with
CO2 (2.0 MPa). The reactor was then pressurised at 3.4 MPa
CO2 at room temperature resulting in an initial pressure at
reaction temperature of 4.5 MPa. The reactor was placed in
an oil bath pre-heated at 160 °C. Once the temperature had
stabilised at 160 °C 2 °C, the magnetic stirrer was turned
on. The reaction was stopped after 18 h and quenched in
ice water. The pressure inside the reactor before and after
cooling was recorded and then the reactor was depressurised
slowly.
2.1 Characterisation of Crude Glycerol
Crude glycerol was obtained as the by-product of biodiesel
synthesis from sunflower oil and methanol, catalysed by
sodium methoxide. The reaction was carried out on a
Golden Ray biodiesel processer. The composition of all
organic species present in the crude glycerol was deter-
mined using GC-MS (Shimadzu GC-MS 2010) through
comparison with the NIST library. The water content was
determined using the Karl Fisher titration method; carried
out by Quality Context Limited. The quantity of sodium
salt remaining in the crude glycerol was analysed by ICP-
MS; while the pH of the crude glycerol was estimated
using pH paper.
Model crude glycerol mixtures were prepared by mixing
glycerol and methanol in an 80:20 mol ratio (22.5 mmol in
total). 1 wt.% sodium methoxide, 10 wt.% methyl palmitate
(97%) and/or 10 wt.% water were added to the glycerol and
methanol mixture. 5 ml of adiponitrile and 6 wt.% of La2O3
catalyst were also loaded into the reactor. 2.1 g of crude
glycerol was loaded into the reactor along with 5 ml of adi-
ponitrile and 6 wt.% of catalyst. 5 wt.% of methanol and 1
wt.% of sodium methoxide were also added. Catalytic testing
was then conducted following the same methodology as for
pure glycerol.
2.2 Catalyst and Reagents
Commercial La2O3 (99.9%, Sigma–Aldrich) was employed
as the catalyst. Prior to use the catalysts were calcined at
400 °C in static air with a heating ramp from room tem-
perature of 40 °C min−1. Additionally, glycerol (99%,
Sigma–Aldrich) and adiponitrile (99%, Sigma–Aldrich)
were utilised as reagents.
1 3

N. A. Razali et al.
can therefore be considered predominately mesoporous in
nature. For comparison, the kinetic diameter of glycerol is
0.61 nm [44] hence mass transfer of the reactant within the
catalyst pores is expected to be relatively facile; it therefore
more likely that mass transfer between the gas-phase (CO2)
and the catalyst surface in the liquid (glycerol) phase is the
dominant transport-limitation effect.
2.5 Analysis of Liquid Products
ATR-FTIR analysis of the liquid samples was conducted
employing a Shimadzu IRAffinity-1S. Spectra were col-
lected over the range 400–4000 cm−1 with a resolution of
4 cm−1. Gas chromatography-mass spectrometry (Shimadzu
GC-MSQP2012SE) was carried out using a 30 m HP-Inno-
wax capillary column in order to determine the composition
of the liquid products collected from the reaction. Ethanol
was used as a solvent to dilute samples of the reaction mix-
ture for GC-MS analysis. The conversion of glycerol and
selectivity to glycerol carbonate were calculated as follows:
XRD (Fig. 2) was used in order to confirm the crystal-
line phase of the La2O3 catalyst. It is possible to observe
the dominant phase is in a hexagonal crystal phase (JCPDS
83-1344); characterised by peaks at 27.5°, 28.1°, 29.3°,
30.1°, 39.6°, 46.2° 48.9° and 52° 2θ. The presence of
La(OH)3 was inferred from the peak at 15.9° [35]. The for-
mation of La(OH)3 is a result of the interaction of La2O3
with moisture in atmosphere [35, 45].
(
)
moles of glycerol consumed
moles of glycerol introduced
% conversion = 100 ×
Furthermore, due to the basic properties of La2O3 this
can further react with atmospheric CO2 and water [45].
These react on the La2O3 surface forming the chemisorbed
surface carbonate and bicarbonate species, which although
not detected via XRD, were confirmed by infrared spectros-
copy with characteristic peaks identified at 3608 cm−1 and
634 cm−1 (Fig. 3) [46, 47].
(
)
moles of glycerol carbonate formed
moles of glycerol consumed
% selectivity = 100 ×
The yield of glycerol carbonate from glycerol is the prod-
uct of conversion and selectivity.
The active phase of the catalyst for the carboxylation
reaction is postulated to be La2O3. La2(CO3)3, La(HCO3)3
both contain CO2 in the form of carbonate CO32−, which
is inert for this reaction, and while La(OH)3 can adsorb
CO2 (a pre-requisite to CO2 activation), this adsorption is
likely to be very strong and result in reactant poisoning of
this phase through the formation of unreactive La2CO3. By
elimination it is therefore suggested that the active phase is
La2O3. Although La2O3 as a bulk metal oxide is basic [48],
the surface contains both Lewis acid sites (uncoordinated
La3+ centres) and Lewis basic sites (uncoordinated O22− or
3 Results and discussion
3.1 Catalyst Characterisation
The solid La2O3 catalyst after calcination at 400 °C was
characterised via SEM (Fig. 1) to determine its morphology
and estimate its grain size. The catalyst appears to have a
relatively homogenous morphology, with a typical particle
size of ~1 µm.
BET measurements yield a surface area of 13.6 m2g−1,
and a pore volume, determined from the desorption branch
of the isotherm via the BJH method, of 0.015 cm3g−1
,
with an average pore diameter of 3.1 nm. The catalyst
20
∆
Ο
Ο
Ο
15
10
5
Ο
Ο
Ο
Ο
Ο
0
10
20
30
40
50
60
70
80
2θ (°)
Fig. 2 XRD pattern of La2O3 catalyst, where (Δ) indicates La(OH)3
Fig. 1 SEM micrograph of La2O3. Magnification is 250× collected
and (Ο) indicates hexagonal La2O3
with an acceleration voltage of 20 kV
1 3

The role of impurities in the La2O3 catalysed carboxylation of crude glycerol
105
advanced operando techniques. Additionally, studies on the
recyclability of the catalyst would further shed light on the
influence of structural and morphological changes on cata-
lyst activity.
100
La2O3 (calcined)
95
La2O3
90
3.2 Characterisation of Crude Glycerol
85
3608 cm-1
The organic species present in crude glycerol were charac-
terised by GC-MS, while the water content was tested using
the Karl Fisher titration method. Furthermore, the concen-
tration of sodium, residual from biodiesel synthesis where
sodium methoxide forms in situ as the active catalyst, was
determined by ICP-MS. The impurities identified in both
crude and pure glycerol are summarised in Table 1.
In addition to these impurities, the crude glycerol also
contained fatty acid methyl esters (FAMEs), which were
analysed via GC-MS, with compound identification through
comparison with the NIST database. The chain-length of
the detected FAMEs ranged from C17 (methyl palmitate) to
C20 (ethyl linoleate). Such compounds are well established
constituents of biodiesel synthesised from oil feedstocks [50,
51].
80
75
634 cm-1
1000
70
65
4000
3000
2000
Wavenumber (cm-1)
Fig. 3 FTIR spectra of La2O3 (red) and La2O3 calcined at 400 °C
(blue). The peaks at 3608 cm−1 and 634 cm−1 are characteristic of
surface carbonate and bicarbonate species
O22− bridges) [49]. CO2 TPD adsorption studies over La2O3
surfaces [47] have previously shown that O22− can bind CO2
with different levels of strength without necessarily evolv-
ing to carbonate species. In view of these data and consid-
3.3 Direct carboxylation of Glycerol: Comparison
of Pure and Crude Glycerol
2−
erations, it is proposed that it is such O2 bridges on the
La2O3 surface that are responsible for the catalytic activity
of La2O3 towards the formation of glycerol carbonate by
CO2. This could be confirmed through the application of
By using GC-MS a glycerol conversion of 58% and a
selectivity to glycerol carbonate of 17% were determined
when pure glycerol was used as the feedstock. These values
Table 1 Impurities contained in
Properties
Crude glycerol (purity 74%)
Pure glycerol (purity 99%)
crude and 99% purity glycerol.
“n.d.” indicates that the
presence of a component was
not detected
Appearance
Brown viscous liquid
5%
74%
<1%
8
Colourless viscous liquid
Moisture/ Water (wt/wt%)
Glycerol purity (wt/wt%)
Methanol (wt/wt%)
pH
1%
99%
n.d.
7
Salt (g/l)
6.85
n.d.
Table 2 Impact of individual
impurities on the carboxylation
of glycerol. Reaction conditions:
P=4.5 MPa, 6 wt.% La2O3,
22.5 mmol glycerol, 18 h,
45 mmol adiponitrile and
reaction temperature 160°C
Feedstock composition
Conversion of
glycerol (%)
Selectivity to glycerol Yield of glyc-
carbonate (%)
erol carbonate
(%)
Glycerol
Crude glycerol
Glycerol: Methanol (80:20)
Glycerol+1 wt.% sodium methoxide
aGlycerol+1 wt.% sodium methoxide
Glycerol+10 wt.% of FAME
Glycerol+10 wt.% of water
58
54
61
51
43
76
19
17
2.3
22
9.9
1.2
13
11
5.6
2.9
0.8
0.5
6.8
1.0
2.6
aReaction carried out in the absence of La2O3
1 3

N. A. Razali et al.
decreased slightly for the conversion to 54% and rather sig-
nificantly to ca. 2.3% for the selectivity to glycerol carbon-
ate, when crude glycerol is employed (Table 2), indicating
that the impurities present have a significant impact on the
selectivity of the reaction and that alternative reaction path-
ways may be operating in the presence of these impurities.
These differences are particularly apparent when comparing
the yield of glycerol carbonate for both feedstocks where
a ~ 10-fold reduction is observed on moving from pure to
crude glycerol. This is despite the similarity in overall con-
version between the two reactions, which is ascribed to a
compensation effect where the decrease in selectivity to, and
yield of, glycerol carbonate is compensated by an increase
in by-product formation.
H2CO3 and in turn CO32−, species which are inert towards
the reaction of glycerol to glycerol carbonate. This would
have an overall effect to decrease the selectivity to glycerol
carbonate when crude glycerol is used, in concert with the
other effects described in this work.
In addition to glycerol carbonate, 4-(hydroxymethyl)
oxazolidin-2-one (C4H7NO3, 4-HMO) is formed as a co-
product. In this context parallel reaction pathways leading
to glycerol carbonate and 4-HMO are proposed (Fig. 4).
It is suggested that 4-HMO is produced from the reaction
of glycerol, CO2 and NH3 derived from the decomposition
of adipamide. Specifically, CO2 attacks the carbon on the
primary and secondary alcohol groups of glycerol to form
2,3-dihydroxypropyl hydrogen carbonate and [2-hydroxy-
1-(hydroxymethyl)ethyl] hydrogen carbonate. The reac-
tion of these species with NH3 then results in the formation
of (2-amino-3-hydroxy-propyl) hydrogen carbonate and
[1-(aminomethyl)-2-hydroxy-ethyl] hydrogen carbonate
respectively. Cyclisation of these intermediates then leads to
the formation of 4-HMO isomers (B1 and B2, Fig. 4). Previ-
ous studies on the synthesis of glycerol carbonate employing
In this context though, and for completeness, whereas
impurities in crude glycerol play a role for both conversion
and selectivity another parameter that may influence the
reaction is pH. Crude glycerol has a higher and slightly basic
pH~8, as compared to pure glycerol (pH~7). A higher pH
combined to the presence of water would increase the solu-
bility of CO2 in the reaction mixture, but to the formation of
O
A
O
O
OH
H
2O
NH2
OH
OH
NH3
CO2
HO
O
H
O
HO
O
O
HO
OH
H
2O
CO2
OH
2O
OH
O
O
O
O
OH
OH
B1
HO
HO
NH
O
NH3
OH
H
2O
O
O
OH
NH
O
B2
NH2
HO
H
2O
Fig. 4 Parallel reaction pathways of glycerol and carbon dioxide to glycerol carbonate (A) and 4-(hydroxymethyl)oxazolidin-2-one (B1 and B2)
1 3

The role of impurities in the La2O3 catalysed carboxylation of crude glycerol
105
100
95
nitrogen-containing dehydrating agents have identified by-
products such as acetamide but the formation of 4-HMO has
not been previously demonstrated [29]. In the present study,
the presence of 4-HMO was confirmed by GC-MS analysis
using (S)-4-HMO (Sigma–Aldrich) as a standard. In the case
of crude glycerol, sodium salts of carboxylic acids (‘soap’)
are an alternative theoretical product. No evidence of soap
formation is however observed. This is most likely due to
the need for a stoichiometric quantity of sodium methoxide
to FAME in the transesterification reaction.
reaction sample
model mixture
90
85
1662 cm-1
80
75
Alongside GC-MS analysis, the liquid products produced
have been characterised by FTIR. FTIR spectra for liquid
samples arising from the carboxylation of pure and crude
70
glycerol (Fig. 5) showed peaks at 1730 and 1790 cm−1
,
65
1900 1850 1800 1750 1700 1650 1600 1550 1500
Wavenumber (cm-1)
which are indicative of the presence of a carbonyl C=O
group within a cyclic five-membered ring. In particular
the band at 1790 cm−1 corresponds to the C=O of glyc-
erol carbonate, and the band at 1730 cm−1 to the C=O of
4-HMO [52]. Notably, this peak at 1730 cm−1 is absent for
the reaction employing crude glycerol, and this is consistent
with GC-MS analysis which shows only trace amounts of
4-HMO. Note however that glycerol conversion is similar
for both pure and crude glycerol, hence these data, as men-
tioned above, are indicative of the formation of alternative
products. These are probably obtained through the rapid
transformation of carbonates to an amide product, as indi-
cated by a strong band present in the crude glycerol reaction
sample at 1622 cm−1. This is representative of C=O of an
Fig. 6 FTIR spectra of reaction mixtures (liquid samples) obtained
from the direct carboxylation of pure glycerol with CO2 in the pres-
ence of adiponitrile (raction sample, blue), and the reaction of glyc-
erol carbonate and adiponitrile in present of CO2 (model mixture,
red). Reaction conditions: P=4.5 MPa, 6 wt.% of La2O3, 2.1 g of
glycerol or glycerol carbonate, 45 mmol of adiponitrile. The reaction
was carried out at 160 °C for 18 h. The peak at 1662 cm−1 is indica-
tive of the presence of an amide product
amidic group (N–C=O) and hence this suggests a secondary
by-product from glycerol carbonate and adiponitrile [19]. In
order to test this hypothesis, glycerol carbonate and adiponi-
trile were reacted directly in both the presence and absence
of CO2. FTIR analysis of this reaction mixture (Fig. 6),
clearly shows the presence of a peak at 1662 cm−1, thus
supporting this assumption. The formation of 4-HMO is a
consequence of the reaction of ammonia, resulting from adi-
ponitrile decomposition, and 2,3-dihydroxypropyl hydrogen
carbonate. As adiponitrile is used for both crude and pure
glycerol reactions it should not be surprising this species is
detected in both these mixtures, although in lower amount
when pure glycerol is used. The same rationale applies for
the interpretation of FT-IR data from reaction mixtures con-
taining methanol where 4-HMO is also detected (Sect. 3.5).
This is furthermore supported by GC-MS analysis where
two unidentified product peaks were detected.
crude glycerol
1790 cm-1
1730 cm-1
1662 cm-1
pure glycerol
3.4 Carboxylation of Model Crude Glycerol
4000
3000
2000
1000
Wavenumber (cm-1)
In order to understand and determine the impact of the impu-
rities present in crude glycerol on the reaction, model crude
glycerol feedstocks have been prepared comprising glyc-
erol and the known impurities [2, 9]. In addition, glycerol
feedstock standards have been prepared with each impurity
present individually, in order to ascertain their effect in the
absence of mutual interactions. Additionally, feedstocks with
two impurities present simultaneously were also prepared
Fig. 5 FTIR spectra of reaction mixtures (liquid sample) of crude
(blue) and pure (red) glycerol. Spectra are offset for clarity. Reac-
tion conditions: P=4.5 MPa, 6 wt.% of La2O3, 2.1 g of pure or
crude glycerol, 45 mmol of adiponitrile, reaction T=160 °C, reac-
tion time=18 h. The bands at 1790 cm−1, 1730 cm−1 and 1662 cm−1
are ascribed to C=O of glycerol carbonate, C=O of 4-HMO and an
aminde funtionality respectively
1 3

N. A. Razali et al.
to allow an analysis of the interplay between these effects.
The impurities added are: methanol, methyl palmitate (as a
model FAME), water and sodium methoxide. The quantities
of each impurity have been selected in order for their impact
to be clearly evident. These individual effects can then be
correlated with the differences observed between crude and
pure glycerol discussed in Sect. 3.3.
of the carbonyl group) which comprises the formation of a
transition state with a large charge separation: [–NH2(+)-
CO(−)-OH]‡. Sodium methoxide could stabilise this transi-
tion state, and in turn decrease the activation energy of this
step, or directly promote the removal of a H+ species from
this transition state and in turn promote product formation.
The addition of 10 wt.% FAME to the glycerol reactant
resulted in a strong increase of conversion up to 76%; how-
ever, selectivity to glycerol carbonate drops to only 1%.
Due to the high conversion and low selectivity to glycerol
carbonate, it is proposed that the glycerolysis of fatty acid
methyl esters to monoglycerides occurs during the reaction
[55, 56]. FTIR analysis (Fig. 7) was conducted in order to
investigate the origin of this reduced selectivity to glycerol
carbonate. In comparison to the reaction with pure glycerol
the peak at 1790 cm−1 (ascribed to C=O of glycerol car-
bonate) is no longer present. This coincides with a reduced
intensity of the peak at 1730 cm−1 (C=O of 4-HMO) and
with the appearance of new peaks at 1715 cm−1, 1360 cm−1
and 1220 cm−1. The band at 1715 cm−1 is typical of a car-
bonyl group of a ketone (R2C=O) stretch, while the peaks
at 1360 and 1220 cm−1 are characteristic of nitro- (RNO2)
and phenolic species respectively. The absence of a band
at 1790 cm−1 is consistent with the low selectivity towards
glycerol carbonate and hence the role that FAMEs play in
hindering glycerol carbonate formation.
Table 2 shows the impact of each impurity consid-
ered individually on catalytic performance. It is possible
to observe that impurities of methyl palmitate, water and
sodium methoxide, all decrease the selectivity of the reac-
tion towards glycerol carbonate. In contrast the addition of
methanol does not have a detrimental effect on either meas-
ured selectivity, which increases from 17 to 22%, or on con-
version, which increases from 58 to 61%. It is a noteworthy
that there is likely to be improved mass transfer in the pres-
ence of methanol as a consequence of a higher miscibility
of glycerol and CO2. In fact, CO2 is only slightly miscible
in glycerol [53], however the equilibrium of CO2, metha-
nol and glycerol is improved in favour of dissolved CO2
upon increasing methanol concentration [54]. Additionally,
methanol may also inhibit secondary reactions of glycerol
carbonate thereby increasing the reaction selectivity. We
anticipate a more detailed study on the effect of methanol
in order to determine its precise role. In the present study,
further evidence of enhanced selectivity to glycerol carbon-
ate is discussed in Sect. 3.6.
The influence of water on the reaction has also been
investigated. The addition of 10 wt.% of water in the reaction
mixture resulted in a significant decrease in glycerol conver-
sion to 19% and glycerol carbonate selectivity to 2.6%. This
The impact of 1 wt.% sodium methoxide on the car-
boxylation reaction is to reduce both conversion (51%) and
selectivity to glycerol carbonate (11%) instead. The reduced
selectivity to glycerol carbonate is reflected in an eightfold
increase in the quantity of 4-HMO. As sodium methoxide
acts as a homogeneous catalyst in biodiesel synthesis, the
efficacy of this species as a catalyst for the direct conver-
sion of glycerol was tested by repeating the reaction in the
absence of La2O3 (Table 2). A 43% conversion of glycerol
was observed - a decrease of less than 10% as compared to
the reaction using La2O3, in the absence of sodium meth-
oxide. However, the selectivity to glycerol carbonate was
further reduced to 6.8%. Sodium methoxide therefore acts as
an active homogeneous catalyst for glycerol conversion, but
by altering the selectivity compared with La2O3 in favour of
4-HMO. This effect may arise through two possible mecha-
nisms: firstly, NaOCH3 can react with glycerol carbonate to
induce ring opening reactions, in turn reducing the selectiv-
ity towards glycerol carbonate and thus indirectly increasing
the selectivity towards 4-HMO. An alternative parallel path-
way is an acid-base reaction between 2,3-dihydroxypropyl
hydrogen carbonate and the methoxide, with the formation
of a carboxylic salt which would not be able to proceed to
glycerol carbonate formation. The second possible mecha-
nism to explain the effect of sodium methoxide in the for-
mation of 4-HMO is via a cyclization step (–NH2 attack
(a)
1220 cm-1
1715 cm-1
(b)
1360 cm-1
(c)
1730 cm-1
1790 cm-1
1900
1800
1700
1600
1500
1400
1300
1200
Wavenumber (cm-1)
Fig. 7 FTIR spectra of reaction mixtures (liquid samples) from the
reaction of glycerol with impurities and CO2 over La2O3 catalyst.
a No impurities present; b with methyl palmitate impurity; c with
methyl palmitate and methanol impurities. The peaks at 1790 cm−1
,
1730 cm−1, 1715 cm−1, 1360 cm−1 and 1220 cm−1 are ascribed to
C=O of glycerol carbonate, C=O of 4-HMO, R2C=O, RNO2, and
phenolic species respectively
1 3

The role of impurities in the La2O3 catalysed carboxylation of crude glycerol
is likely attributable to the selective strong adsorption of
water molecules on the active sites of La2O3 [39] leading to
catalyst deactivation.
moieties are not observed by FTIR again confirming the
impact of these impurities on the reaction.
The addition of sodium methoxide to glycerol/methanol
mixture increases the selectivity to glycerol carbonate, from
22 to 25%; while the addition of FAME results in a decrease
in the selectivity to glycerol carbonate to 3.4% (Table 3).
The most complex model feedstock; comprising of glyc-
erol, methanol, sodium methoxide and FAME yields only
4% selectivity to glycerol carbonate, again confirming that
the presence of FAME reduces glycerol carbonate selectiv-
ity. Complete separation of FAME, i.e. biodiesel is therefore
both commercially advantageous to increase biodiesel yield
and to facilitate conversion of the glycerol by-product.
Water and FAMEs have a negative impact on the car-
boxylation of glycerol and significantly reduced the selec-
tivity to glycerol carbonate from 17% to <3%. The pres-
ence of sodium methoxide also reduces glycerol carbonate
selectivity: to 11% and 6.8% in the presence and absence of
the La2O3 respectively. This decrease is correlated with an
increase in the formation of 4-HMO and other by-products.
In contrast, the addition of methanol to the reaction mixture
has no such detrimental effect and results in glycerol con-
version and glycerol carbonate selectivity of 61% and 22%
respectively.
As the impurities in crude glycerol are likely to interact
with one another, and with any subsequent products pro-
duced from the impurities, the behaviour of model feed-
stocks containing two or more impurities has also been
investigated. In this case, the presence of methanol is kept
constant across all model systems with glycerol and metha-
nol in an 80:20 molar ratio.
3.5 Influence of Additives on the Conversion
of Crude Glycerol
Given the observation that methanol does not diminish, and
may slightly increase, the efficiency of the carboxylation
reaction with respect to glycerol carbonate production when
compared with the use of pure glycerol, the influence of the
addition of methanol to crude glycerol was also investigated
(Fig. 8). The addition of 5 wt.% methanol to crude glycerol
resulted in a decrease in glycerol conversion to 38%; this is
coupled with an increase in selectivity to glycerol carbon-
ate to 10%. In this study the reaction time is kept constant
at 18 h. It is therefore necessary to be cautious when com-
paring selectivities among different reactions as the overall
conversions achieved differ significantly. The increase in
selectivity is however in agreement with studies utilising
model feedstocks, where feeds containing a binary mixture
of impurities including methanol achieved higher selectivi-
ties than those containing only sodium methoxide of FAMEs
as the impurity (Tables 2 and 3).
Using these conditions, the addition of 1 wt.% sodium
methoxide does not have the same stark reduction in glycerol
carbonate selectivity as it does in the absence of methanol.
Indeed, this system has the highest selectivity of any sys-
tem investigated in this work (25%), although this occurs
at a cost of a reduced conversion of 46% as compared to
58% for pure glycerol, and 61% for the glycerol/methanol
mixture. In contrast, the addition of FAME results in a sig-
nificantly reduced selectivity alongside high conversion, as
it was observed in the absence of methanol. This trend is
also observed if methanol, sodium methoxide and FAME are
all present; suggesting that the effect of FAME is to reduce
glycerol carbonate selectivity.
In contrast to a reaction mixture containing methyl palmi-
tate as a single impurity, the FTIR spectrum for the sample
containing methanol and the FAME (Fig. 7) does not show
significant intensity at 1715 cm−1. This suggests that the
previously observed ketone functionality is not formed in the
presence of methanol, or that it is rapidly consumed. When
the reaction mixture comprises glycerol, methanol, FAME
and CO2, the phenolic (1360 cm−1) and nitro- (1220 cm−1)
The comparison of FTIR spectra for the reaction mix-
tures post-reaction liquid samples (Fig. 9) from the reactions
involving: (a) crude glycerol mixed with methanol and (b)
pure glycerol mixed with methanol, shows that the peak at
1790 cm−1 (glycerol carbonate) is not observed in the sample
arising from crude glycerol. However, bands with strong
intensities are observed for the peaks at 1730 cm−1 (4-HMO)
and 1662 cm−1 (the peak at 1662 cm−1 has previously been
Table 3 The impact of multiple impurities on carboxylation of glycerol over La2O3. Reaction conditions: P=4.5 MPa, 6 wt.% of La2O3-C,
glycerol:methanol (22.5 mmol in total) and 45 mmol of adiponitrile and 18 h
Feedstock composition
Conversion of glycerol Selectivity to glycerol
Yield of glyc-
erol carbonate
(%)
(%)
carbonate (%)
Glycerol+methanol (80:20 ratio)+1 wt.% sodium methoxide
Glycerol+methanol (80:20 ratio)+10 wt.% FAME
46
85
73
25
3.4
4.0
12
2.9
2.9
Glycerol+methanol (80:20 ratio)+10 wt.% FAME+1 wt.% sodium
methoxide
1 3

N. A. Razali et al.
60
40
20
0
Conversion
1730 cm-1
100
90
(a)
(b)
Glycerol carbonate
selectivity
80
1790 cm-1
70
1662 cm-1
1900 1850 1800 1750 1700 1650 1600 1550 1500
Wavenumber (cm-1)
Crude glycerol Crude glycerol Crude glycerol
+ 5 wt.%
methanol
+ 5 wt.%
methanol + 1
wt.% NaOMe
Fig. 9 FTIR spectra of a the reaction of crude glycerol and 5 wt.%
of methanol and CO2 and b the direct carboxylation of pure glyc-
erol: methanol to the molar ratio of 80:20. Reaction conditions:
P=4.5 MPa, 6 wt.% La2O3, 80:20 glycerol:methanol or 2.1 g of
crude glycerol and 5 wt.% of methanol, 5 ml adiponitrile; reaction
T=160 °C, reaction time=18 h
This trend may indicate that the main influence of the addi-
tives is alter the overall rate of reactions(s) rather than alter
the reaction pathways [9]. A detailed investigation of dif-
ferent times-on-stream, which is outwith the scope of this
work, would provide insights into this. The overall impact
of sodium methoxide is less pronounced than in the model
feedstock systems. Overall, additives may act to increase
the selectivity of the reaction but further investigation is
required to confirm this effect and determine its origin.
4 Conclusions
Fig. 8 Conversion of glycerol, selectivity and yield to glycerol car-
bonate for crude glycerol and crude glycerol plus methanol and
sodium methoxide. Reaction conditions: P=4.5 MPa, 6 wt.% La2O3,
2.1 g glycerol, 5 wt.% methanol, 1 wt.% of sodium methoxide, 5 ml
adiponitrile; reaction T=160 °C, reaction time=18 h
In this work, the efficacy of La2O3 as a catalyst for the syn-
thesis of glycerol carbonate via the carboxylation of crude
glycerol was demonstrated for the first time. The carboxyla-
tion of crude glycerol shows a significantly lower selectivity to
glycerol carbonate (2.3%) as compared to using pure glycerol
where a selectivity of 17% was obtained. In order to understand
the impact of impurities on glycerol conversion and product
selectivity, several model crude glycerol feedstocks containing
one or more impurities (water, methanol, sodium methoxide
and FAME) were prepared. The addition of methanol slightly
increased the measured selectivity from 17 to 22%. Water,
sodium methoxide and FAME lowered the glycerol carbonate
selectivity to <10%. Glycerol conversion increased upon the
addition of FAME to 76%, but only 1% selectivity to glycerol
carbonate was observed. FTIR analysis confirmed the presence
of additional by-products. Interestingly, sodium methoxide was
able to synthesise glycerol carbonate even in the absence of
assigned to an amide product formed from glycerol carbon-
ate) [19]. This may therefore support the suggestion that the
lower selectivity observed is indicative of the secondary con-
version of synthesised glycerol carbonate to such products
rather than an absence of formation of this product.
The addition of both 1 wt.% sodium methoxide and meth-
anol to the crude glycerol has the effect of increasing conver-
sion relative to the sample with only methanol added from
38 to 43%, while decreasing selectivity to glycerol carbonate
from 10 to 7.8%. Note that for all reactions employing crude
glycerol, conversion and selectivity follow an inverse corre-
lation with selectivity decreasing with increased conversion.
1 3

The role of impurities in the La2O3 catalysed carboxylation of crude glycerol
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Acknowledgements The SEM data presented were acquired with the
support of the Engineering and Physical Sciences Research Council
(EPSRC) “4CU” programme grant, aimed at sustainable conversion of
carbon dioxide into fuels, led by The University of Sheffield and carried
out in collaboration with University College London, the University
of Manchester, and Queen’s University Belfast. The authors acknowl-
edge the EPSRC for supporting this work financially (Grant No. EP/
K001329/1). NR gratefully acknowledges the Ministry of Education,
Malaysia for the award of a PhD scholarship.
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Open Access This article is distributed under the terms of the Crea-
tive Commons Attribution 4.0 International License (http://creativeco
mmons.org/licenses/by/4.0/), which permits unrestricted use, distribu-
tion, and reproduction in any medium, provided you give appropriate
credit to the original author(s) and the source, provide a link to the
Creative Commons license, and indicate if changes were made.
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Affiliations
N. A. Razali1 · M. Conte2 · J. McGregor1
2
* J. McGregor
Department of Chemistry, University of Sheffield, Brook
james.mcgregor@sheffield.ac.uk
Hill, Sheffield S3 7HF, UK
1
Department of Chemical and Biological Engineering,
University of Sheffield, Mappin Street, Sheffield S1 3JD, UK
1 3