Synthesis of oxygenated fuel additives via the solventless etherification of glycerol
Muhammad Ayoub, M.S. Khayoon⇑, Ahmad Zuhairi Abdullah
School of Chemical Engineering, Universiti Sains Malaysia, Engineering Campus, 14300 Nibong Tebal, Penang, Malaysia
a r t i c l e i n f o
Received 9 January 2012
Received in revised form 20 February 2012
Accepted 21 February 2012
Available online 3 March 2012
a b s t r a c t
The synthesis of oxygenated fuel additives via solvent freebase-catalyzed etherification of glycerol is
reported. The products of glycerol etherification arediglycerol (DG) and triglycerol (TG) with DG being
the favorable one. The catalytic activity of different homogeneous alkali catalysts (LiOH, NaOH, KOH
and Na2CO3) was investigated during the glycerol etherification process. LiOH exhibited an excellent cat-
alytic activity during this reaction, indicated by the complete glycerol conversion with a corresponding
selectivity of 33% toward DG. The best reaction conditions were a reaction temperature of 240 ?C, a cat-
alyst/glycerol mass ratio of 0.02 and a reaction time of 6 h. The influences of various reaction variables
such as nature of the catalyst, catalyst loading, reaction time and reaction temperature on glycerol ether-
ification were elucidated. Industrially, the findings attained in this study might contribute towards pro-
moting the biodiesel industry through utilization of its by-products.
? 2012 Elsevier Ltd. All rights reserved.
The forecasted decline in the production of petroleum fuels and
the growing concern about atmospheric greenhouse gas concen-
trations have necessitated the search for clean, renewable (sustain-
able), efficient and affordable alternative fuel. Biodiesel is
becoming a key fuel in motor engines if blended in certain portions
with petrodiesel (Hasheminejad et al., 2011). It can be readily pro-
duced via the transesterification of vegetable oils (edible, non-edi-
ble or reused) with low alcohols (methanol or ethanol). Indeed, the
inevitable formation of glycerol that accompanies the biodiesel
production process is affecting the process economy (Olutoye
and Hameed, 2011; Yuan et al., 2010). Moreover, the growth of
the biodiesel industry will result in overproduction of glycerol
and create a superfluity of this impure product as its production
is equivalent to 10% of the total biodiesel produced (Cardona
et al., 2007; Khayoon and Hameed, 2011).
Glycerol is an abundant carbon-neutral renewable resource for
the production of biomaterials as well as source for a variety of
chemical intermediates (Rahmat et al., 2010; García-Sancho
et al., 2011). Unfortunately, biodiesel-derived glycerol is not bio-
compatible due to its contamination with toxic alcohol (methanol
or ethanol). Therefore, global research is focused on the effective
conversion of glycerol to valuable chemicals to ameliorate the
economy of the whole biodiesel production process. Recently,
many studies have been dedicated to the transformation of this
renewable polyol by various catalytic processes (Rahmat et al.,
2010; Melero et al., 2012). This encompasses oxidation process to
obtain dihydroxyacetone, glyceraldehyde, glyceric acid, glycolic
acid and hydroxyl pyruvic acid (Liebminger et al., 2009; Augugliaro
et al., 2010); fermentation process towards 1,3-propanediolpro-
duction (Tokumoto and Tanaka, 2011); acetylation process with
acetic acid to obtain polyglycerol esters (Gonçalves et al., 2008;
Balaraju et al., 2010; Dosuna-Rodríguez et al., 2011; Khayoon and
Hameed, 2011) and acetalisation process with ketones to obtain
oxygenated acetals and ketals (Umbarkar et al., 2009; Vicente
et al., 2010; da Silva and Mota, 2011; Silva et al., 2010).
Glycerol is also an efficient platform for the synthesis of oxy-
genated components such as polyglycerols and ployglycerol ethers
by means of etherification process (Melero et al., 2010, 2012; Yuan
et al., 2011). Glycerol ethers (polyglycerols) are produced from cat-
alytic etherification of glycerol with the use of different solvents.
Particularly, these components have found colossal potential appli-
cations as fuel additives (Rahmat et al., 2010; Martin and Richter,
2011). Polyglycerols, especially diglycerol and triglycerol (called
herein later as DG and TG, respectively) are the main products of
glycerol etherification. The use of solvent could create some prob-
lems in the production process leading to a more complex overall
process. In this respect, solventless etherification process could
promise several advantages but limited information is currently
available on this mode of glycerol etherification process. Venturing
into the possibility of such process is a worthwhile research effort.
Glycerol etherification has been extensively investigated with
or without the use of organic solvents. For both cases, different
homogeneous alkali catalysts like carbonates and hydroxides or
heterogeneous catalysts like zeolite, mesporous silica and metal
oxides have been applied (Clacens et al., 2002; Jerome et al.,
2008; Martin and Richter, 2011). DG and TG are produced from
the consecutive condensation of two or three glycerol molecules,
0960-8524/$ - see front matter ? 2012 Elsevier Ltd. All rights reserved.
⇑Corresponding author. Fax: +60 4 594 1013.
E-mail address: email@example.com (M.S. Khayoon).
Bioresource Technology 112 (2012) 308–312
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journal homepage: www.elsevier.com/locate/biortech
respectively. They can be synthesized as linear, branched, or cyclic
isomers based on the condensation process which may take place
between primary and secondary hydroxyls or even through an intra-
molecular condensation (Corma et al., 2007). Heterogeneous catalysts
show some advantages such as ease of their separation from reaction
mixture and the potential for reusability. However, they generally
show lower catalytic activity as compared to homogeneous catalysts.
Some of them cannot even be applied industrially. Usually, heteroge-
neouscatalystsrequire relativelyhighertemperatureandlonger reac-
tion time than those needed in the case of homogeneous catalysts. In
addition, the high cost, difficulty to functionalize, solubility in polar
media, leaching of metal clusters from their surfaces, low surface
area (in some cases) and their thermal stability are the main disad-
vantages of some heterogeneous catalysts.
Previously, several bases have been examined as homogeneous
Carbonates and metal oxides are attractive catalysts for glycerol
due to the stronger base character of the former. Contrary, oxides
such as ZnO, MgO and CaO are less active in the aforementioned
reaction due to the solubility issue. Nevertheless, these reactions
are usually not sufficiently fast (in terms of glycerol conversion) or
do not selectively produce DG apart from difficulties in filtration,
to eliminate the solvents from the homogeneous catalysts is a very
challenging task (Clacens et al., 1998). Charles et al. (2003) reported
that 96% glycerol conversion with a corresponding selectivity to DG
of 24% were achieved using 2% Na2CO3catalyst at 260 ?C and 24 h.
Alkaline metals impregnated into mesoporous catalysts have been
reported to achieve 80% conversion of glycerol with not less than
40% selectivity to DG at 260 ?C and a long reaction period of 24 h
(Clacens et al., 2002). In 2008, the use of zeolitic catalyst for glycerol
etherification was attempted and 80% of glycerol conversion and
less than 20% selectivity to DG at 260 ?C were achieved (Krisnandi
et al., 2008). Recently, glycerol etherification was performed at a
lower reaction temperature of 220 ?C using Mg Al mixed oxide cat-
alyst. The maximum conversion of 50% was recorded for this mixed
oxide catalyst while a high selectivity to DG of ?90% was recorded
ent capability to produce the desired product.
Glycerol etherification has been proposed as an equilibrium-
limited reaction and the presence of water molecules might en-
hance the backward reaction. Therefore, the water content in the
reaction medium should be diminished either by reactive distilla-
tion to displace the co produced water or by employing anhydrous
feedstock. On the other hand, the biodiesel derived glycerol cannot
be utilized directly in industrial applications due to its being im-
pure and the high water content. Thus, the current work was car-
ried out using anhydrous glycerol to eliminate the undesired
In this work, the use of LiOH as an efficient and selective homo-
geneous catalyst for the solvent free etherification of glycerol has
been attempted. This study has been performed to find an applica-
ble, feasible, eco-friendly and cost effective approach to synthesize
an oxygenated fuel additive i.e. DG via a controllable homogeneous
batch process. The catalytic behaviors of LiOH have been evaluated
and compared against those of NaOH, KOH and Na2CO3catalysts.
Anhydrous glycerol of high purity (>99%) was purchased from
R&M Chemicals, Ltd., Malaysia while lithium hydroxide (99.9%)
was supplied by BDH, UK. Potassium hydroxide (85%), sodium car-
bonate (>99%) and sodium hydroxide (>98%) were obtained from
Sigma–Aldrich, Malaysia. Meanwhile, glycerol (99%) from Sigma–
Aldrich, diglycerol (>90%) from Solvay Chemicals and triglycerol
(>90%) from Sigma–Aldrich were used as GC standards. All these
chemicals were used without any further treatment.
2.2. Reaction procedure
The catalytic activity of the homogeneous catalysts was studied
based on the solvent free etherification of glycerol at atmospheric
pressure in temperature range of 180?260 ?C using 2 wt% of cata-
lyst (based on glycerol weight). A three-neck glass reactor vessel
(250 mL) that was used to carry out glycerol etherification was
placed on astirring-heating mantle equipped with a PID tempera-
ture controller. To avoid glycerol oxidation, the reaction was per-
formed under inert environment using continues flow of nitrogen
gas. In this set up, the forward reaction was promoted by displac-
ing and collecting the formed water using a Dean–Stark apparatus
attached to the reactor vessel. In a typical experimental run, the
reactor was charged with 50 g of anhydrous glycerol and 1.0 g of
catalyst was then added to it and then heated up to the desired
2.3. Analysis of reaction products
The reaction samples were collected at 2 h intervals (0–8 h) and
qualitatively analyzed using a GC/MS Perkin Elmer system (Clarus
600 gas chromatograph attached to a Clarus 600T mass spectrom-
eter) equipped with a DB-5 column. Prior to GC quantitative anal-
ysis, asilylation pretreatment according to the method reported by
Sweeley and coworkers (1963) was performed on the samples. A
known mass of reaction sample (50 mg) was first mixed with care-
fully dried pyridine (1.5 mL) in a screw-capped septum vial (4 mL).
After dissolution, 0.2 mL of hexamethyldisilazane (HMDS) and
0.1 mL of trimethylchlorosilane (TMCS) were gently added and
the resulting mixture was heated up to 70 ?C for 1 h. An aliquot
of the solution (0.05 mL) was then diluted in dried toluene
(2 mL). Finally, 1 lL of the liquid was injected into a capillary polar
GC column DB-HT5 (15 m ? 0.32 mm ? 0.10 lm) mounted in a gas
chromatograph GCD 7820A system (Agilent Technologies, USA).
The analysis was performed in a temperature-programmed mode
from 60 to 250 ?C with a ramping rate of 10 ?C/min. The typical
retention times (tR) of the silylated components were to 3.2, 8.6
and 13.1 min for glycerol, DG and TG, respectively. The calibration
curves of glycerol, DG and TG were obtained by injecting their
respective standard reagents. Glycerol conversion (%), DG yield
(%) and DG selectivity (%) were calculated using the following
Glycerol conversionð%Þ ¼moles of glycerol reacted
moles of glycerol taken
DG yieldð%Þ ¼moles of DG produced
moles of glycerol taken? 100%
DG selectivityð%Þ ¼Moles of DG produced
Total moles of products? 100%
3. Results and discussion
3.1. Etherification reaction
Selective etherification of glycerol to polyglycerol using either
homogenous or heterogeneous catalysts is an attractive pathway
M. Ayoub et al./Bioresource Technology 112 (2012) 308–312
for organic chemists as it directly gives access to value-added
chemicals. The products of glycerol etherification are DG, TG and
tetraglycerol. Among these products, DG is the most favorable
one due to its characters as a moisturizer in cosmetics and pharma-
ceutical industries and more recently as a biodiesel additive. The
reaction involves the conversion of glycerol to DG by removing
one water molecule, and subsequent removal leads to the forma-
tion of TG and tetragycerol.
The catalytic activity of the investigated catalysts was measured
based on glycerol conversion and corresponding selectivity to DG
during the solventless etherification of glycerol. The reaction was
carried out at 240 ?C for up to 8 h. In some cases, leaching of the
active phase into the reaction medium and the formation of acro-
lein at high reaction temperature are the two main drawbacks of
heterogeneous catalysts. On the other hand, the use of organic sol-
vents for glycerol etherification in the presence of homogeneous
catalysts might complicate their separation after the reaction to
consequently affect the process economy and products purity.
Therefore, the current study addressed a solvent free process in
the presence of alkali metals. The effects of different reaction vari-
ables were particularly investigated. The reproducibility of the ob-
tained results were checked by repeating some experiments and
no more than 7% error was observed which might be attributable
to experimental aspects or analytical dissimilarity.
3.2. Performance of different catalysts
Fig. 1 presents the performance of 2 wt% of four different cata-
lysts used for selective glycerol conversion to DG via etherification
at 240 ?C. The highest conversion of glycerol (?100%) was achieved
after 6 h in the presence LiOH and NaOH catalysts. However, the
application of 2 wt% of NaOH resulted in slightly lower glycerol
conversion and DG selectivity than those obtained using 2 wt%
LiOH catalyst. This difference became significant after 2 h of reac-
tion, explaining the key role of Li ions in promoting glycerol con-
version and enhancing DG formation. The catalytic activities of
NaOH, KOH and Na2CO3were also examined, and the results re-
vealed that significant glycerol conversion with palpable DG selec-
tivity were achieved. However, their catalytic activities were found
to be lower than that of LiOH. It was also observed that Na2CO3,
which was used as reference catalyst, presented poor glycerol con-
version and DG selectivity compared to those attained using LiOH.
Na2CO3might possess higher catalytic activity, than that recorded
in this study if applied at reaction temperature higher than 240 ?C
or longer reaction period than 12 h (Clacens et al., 2002). However,
such extreme conditions were not used in this work to avoid signif-
icant degradation reactions on the reactants, intermediates and
Fig. 2 provides the measured original pH values of glycerol and
the mixtures of different alkali catalysts with glycerol. It is obvious
that different catalysts resulted in different pH value after mixing
with glycerol. The pH value increased in the order of: LiOH +
glycerol > NaOH + glycerol > KOH + glycerol > Na2CO3+ glycerol.
This observation suggested that a mixture with higher pH value
performed better during the solvent free glycerol etherification.
LiOH was the most active catalyst during the selective glycerol
conversion to DG and it might be attributable to its highest alkalin-
ity in the reaction mixture. Li has smaller ionicsize and higher
atomic electronegativity than other metals used in this study.
Higher nuclear charge enabled it to make stronger attraction for
electrons or protons during the etherification reaction. This might
interpret the relationship between the atomic characteristics of the
metals with their catalytic performance in glycerol etherification
reaction. After this preliminary screening test, LiOH was selected
for further study on the effects of several important reaction
3.3. Effect of catalyst loading
Effect of LiOH catalyst loading was studied using the same
reaction conditions as those mentioned in Section 3.2. The catalyst
concentration in the reaction medium was varied in the range of
10.4–83.5 mmole while the other reaction variables were kept con-
stant. It was observed that different catalyst loadings resulted
indifferent reaction profiles (expressed by glycerol conversion
and DG yield) with nonlinear relationship as shown in Fig. 3(a)
and (b). The conversion of glycerol attained its maximum level of
100% after 6 has the amount of LiOH was increased from 2 to
4 wt%. The highest DG selectivity of about 30% was achieved after
4 h with 2 wt% of LiOH. The conversion of glycerol and DG selectiv-
ity were found to gradually increase as the LiOH amount was in-
creased from 0.5 to 2 wt%. However, further increase beyond
2 wt% resulted in decreasing trends observed in both glycerol con-
version and DG selectivity. The same trend of DG yield was ob-
served and the maximum DG yield achieved was 29%. It was a
level at which the highest reaction rate was achieved. Further in-
crease did not bring about the desired effect as the reaction could
have been limited by the mass transfer during the reaction. Based
on Fig. 3(a) and (b), the selectivity and yield of DG at 4 wt.% of LiOH
loading suddenly decreased after 2 h of reaction and came close to
zero value after 6 h. Bearing in mind nearly complete glycerol con-
version was obtained at the same point, low selectivity and yield
for DG indicated the subsequent etherification reaction beyond
DG to form TG and other oligomers.
Fig. 1. Performance of different catalystsin the etherification process measured in
terms of DG yield% (Reaction temperature:240 ?C, catalyst loading: 2 wt.%).
Glycerol 2% LiOH+
Fig. 2. Measured pH value of glycerol in the presence of different catalysts prior to
glycerol etherification reactions.
M. Ayoub et al./Bioresource Technology 112 (2012) 308–312
Expectedly, increasing LiOH amount from 0.5 to 2 wt% resulted
in improving conversion of glycerol molecules to polymerize into
DG molecule by dehydration. Besides promoting over-polymeriza-
tion reaction, increasing LiOH amount further to 4 wt.% under
these reaction conditions could also resulted in back-scission of
DG to glycerol. Martin and Richter (2011) reported that the inter-
action between the B–OH base with glycerol could weaken one
of the glycerol OH bonds and enhanced the nucleophilic character
of the hydroxyl oxygen. Attack on this polarized glycerol molecule
by the hydroxyl group of a second glycerol molecule would simul-
taneous split off of water molecule resulting in DG formation. As
2 wt% of LiOH catalyst showed the best DG yield during the reac-
tions, this loading was selected for the subsequent research work.
3.4. Effect of reaction time
The influence of reaction time on reaction profiles (glycerol con-
version, DG selectivity and DG yield) was evaluated using 2 wt%
LiOH catalyst at 240 ?C and the results are also shown in Fig. 3(a)
and (b). It was observed that after 2 h of reaction, the selectivity to-
ward DG attained its highest value of ?30%, but decreased as the
reaction was further prolonged to 8 h. Whilst, glycerol conversion
was found to linearly increase with reaction time to 8 h, the DG
yield was 29% after 4 h and behaved like the selectivity of DG. Nev-
ertheless, increasing reaction time further resulted in a decrease in
DG yield for all the catalysts used, except for 4 wt% LiOH which
showed decreasing DG yield after 2 h.
Indeed, as the etherification reaction was prolonged, more
glycerol molecules underwent dehydration or any other form of
reaction which resulted in an increase in the conversion of glycerol.
Unfortunately, during this conversion, the cleavage of glycerol
molecules might not exactly result in polyglycerol form, but in
some other forms of by-products such as acrolein due to the double
dehydration of glycerol. These by-products are not desirable in the
polymerization etherification reaction and might lead to unfavor-
able products (Clacens et al., 2002). Subsequently, the DG mole-
cules that formed might be converted into higher glycerol ethers
under uncontrolled reaction conditions. This may also happen, in
some cases due to the weak formation of two polymerized glycerol
molecules (DG isomers) which might not be highly stable under gi-
ven reaction conditions. Therefore, the selectivity and yield of DG
gradually started to decrease after 2 h of reaction time.
3.5. Effect of reaction temperature
It has been well established that chemical reaction rate is
strongly influenced by reaction temperature. Therefore, the influ-
ence of reaction temperature in the range of 180–260 ?C on glyc-
erol etherification was investigated. As shown in Fig. 4, the
maximum glycerol conversion after 6 h was 100% and it was
achieved in the presence of 2 wt% LiOH at 240 ?C. However, the
selectivity to DG at this point was not high (<20%) and was
decreasing with increasing temperature and reaction time. Mean-
while, etherification reaction rate was lower at a lower reaction
temperature of 220 ?C, achieving a glycerol conversion of 23%
(after 8 h) and a DG selectivity of lower than 5%.
Fig. 4 shows that glycerol conversion increased uniformly with
increasing reaction temperature from 240 to 260 ?C. On the con-
trary, DG selectivity decreased with increasing reaction tempera-
ture beyond 240 ?C. The observation suggested that reaction
temperatures higher than 240 ?C might have speeded up the con-
version of the remaining glycerol and enhanced the consecutive
etherification reaction of DG to higher glycerol oligomers to conse-
quently result in decreasing DG yield. These observations were in
good agreement with earlier reported results (Clacens et al.,
1998; Charles et al., 2003).
Overall, Table 1 presents a detailed comparison with those re-
ported in previous studies implemented using different homoge-
neous catalysts. This study investigated the catalytic glycerol
etherification using alkaline metals catalysts (LiOH, NaOH, KOH
and Na2CO3) at comparable reaction temperature of 240 ?C and
very short reaction time that is maximum of 8 h. More specifically,
this work focused on exploring the catalytic potential of different
alkaline metals as an example of eco-friendly catalysts.
Fig. 3. Influence of LiOH catalyst loading on (a) glycerol conversion and corre-
sponding DG selectivity and, (b) DG yield% (Reaction temperature: 240 ?C).
Fig. 4. Effect of reaction temperature on glycerol conversion and corresponding DG
selectivity. (Catalyst loading:2 wt.% of LiOH).
M. Ayoub et al./Bioresource Technology 112 (2012) 308–312
4. Conclusions Download full-text
In this work, solventless etherification of glycerol to DG using
various alkaline metal precursors was successfully investigated
and the obtained results were compared with those obtained with
Na2CO3as a reference catalyst. LiOH catalyst showed unique activ-
ity for glycerol etherification, achieving complete glycerol conver-
sion with a corresponding 33% selectivity toward DG as the desired
product. The best reaction conditions were 240 ?C of reaction tem-
perature, 2 wt% of catalyst loading and 6 h. Glycerol conversion
and DG selectivity were found to be influenced by the alkalinity
of the metal catalyst as well as the reaction conditions.
The authors sincerely acknowledge the financial support pro-
vided by UniversitiSains Malaysiaunder Research University (RU)
grant (Project No: 814126).
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Comparison of the catalytic activity (expressed by glycerol conversion and products selectivity) of different homogenous catalysts in the glycerol etherification process.
Selectivity (%) after 8 hRef.
Cottin et al. (1998)
Charles et al. (2003)
Barrault et al. (2004)
Calatayud et al. (2009)
Ruppert et al. (2008)
Cottin et al. (1998)
Krisnandi et al. (2008)
Richter et al. (2008)
Richter et al. (2008)
Richter et al. (2008)
*Xg: Glycerol conversion (%).
M. Ayoub et al./Bioresource Technology 112 (2012) 308–312