Synthesis and catalytic evaluation of acidic carbons in the etherification of glycerol obtained from biodiesel production

Article abstract of DOI:10.1016/j.cattod.2020.10.016

In this paper, the catalytic behaviour of carbonaceous system (Ccs) functionalized with –SO₃H groups were studied in the etherification of refined (Gly) and crude glycerol (GlyC), with benzyl alcohol (BA). This Ccs was obtained by a synthetic method with low energetic cost in only 24 h. Its catalytic activity and selectivity were studied varying the catalyst percentage (2.5, 5 and 10 wt.%), the initial reactant molar ratio and temperature (between 80 and 120 °C). A very good catalytic performance was achieved (97 % conversion after 360 min of reaction), at 120 °C, Gly:BA = 3:1 and 10 wt.% of Ccs. The high activity can be attributed to high acid site density (6.4 mmol H⁺/g), that also allowed us to working at lower reaction temperature (100 °C) and with less catalyst concentration (2.5 wt.%), without observing significant loss in BA conversion. Monoether (ME₁) was the major product of the reaction with 72 % selectivity. The material can be reused and still gives a notable conversion of BA (about 43 %) after three successive reuses. Finally, the Ccs was active and selective to the desired products in the etherification of crude glycerol (GlyC) derived of biodiesel industry. An important BA conversion (45 %) was obtained only reducing the water content of GlyC and without carrying out any other purification and/or neutralization treatment.

Full text of DOI:10.1016/j.cattod.2020.10.016

Journal Pre-proof
Synthesis and catalytic evaluation of acidic carbons in the etherification
of glycerol obtained from biodiesel production
Mar ´ı a E. Chiosso, M o´ nica L. Casella, Andrea B. Merlo
PII:
S0920-5861(20)30713-6
DOI:
https://doi.org/10.1016/j.cattod.2020.10.016
CATTOD 13181
Reference:
To appear in:
Catalysis Today
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23 April 2020
18 July 2020
12 October 2020
Please cite this article as: Chiosso ME, Casella ML, Merlo AB, Synthesis and catalytic
evaluation of acidic carbons in the etherification of glycerol obtained from biodiesel
production, Catalysis Today (2020), doi: https://doi.org/10.1016/j.cattod.2020.10.016
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SYNTHESIS AND CATALYTIC EVALUATION OF ACIDIC CARBONS IN THE
ETHERIFICATION OF GLYCEROL OBTAINED FROM BIODIESEL
PRODUCTION
María E. Chiosso1,2,3, Mónica L. Casella , Andrea B. Merlo *
1,3
3,
1
Departamento de Ciencias Básicas y Experimentales – Universidad Nacional del
Noroeste de la Provincia de Buenos Aires. Roque Sáenz Peña 456, (6000) Junín -
Argentina
2
Centro de Investigaciones y Transferencia del Noroeste de la Provincia de Buenos
Aires (CITNOBA) - UNNOBA-UNSAdA-CONICET. Monteagudo 2772, (2700)
Pergamino - Argentina
3
Centro de Investigación y Desarrollo en Ciencias Aplicadas “Dr. Jorge J. Ronco”
CINDECA) - CCT-CONICET La Plata- Universidad Nacional de La Plata. Calle 47 Nº
57 (1900) La Plata – Argentina*andreamerlo@quimica.unlp.edu.ar
(
2
Graphical abstrcat
Highlights


Etherification of glycerol obtained from biodiesel industry.
Obtaining acidic carbon enriched with surface sulfonic groups.



Good catalytic behavior in refined and crude glycerol etherification.
High catalytic selectivity: mono and diethers as the main reaction products.
KEYWORDS: ETHERIFICATION, CRUDE GLYCEROL, ACIDIC CARBON,
BENZYL ALCOHOL.
ABSTRACT
In this paper, the catalytic behaviour of carbonaceous system (Ccs) functionalized with
–
SO
3
H groups were studied in the etherification of refined (Gly) and crude glycerol
(GlyC), with benzyl alcohol (BA). This Ccs was obtained by a synthetic method with
low energetic cost in only 24 h. Its catalytic activity and selectivity were studied varying
the catalyst percentage (2.5, 5 and 10 wt.%), the initial reactant molar ratio and
temperature (between 80 and 120 ºC). A very good catalytic performance was achieved
(97% conversion after 360 min of reaction), at 120 ºC, Gly:BA= 3:1 and 10 wt.% of
+
Ccs. The high activity can be attributed to high acid site density (6.4 mmol H /g), that
also allowed us to working at lower reaction temperature (100 °C) and with less catalyst
concentration (2.5 wt.%), without observing significant loss in BA conversion.
Monoether (ME ) was the major product of the reaction with 72% selectivity. The
1
material can be reused and still gives a notable conversion of BA (about 43%) after
three successive reuses. Finally, the Ccs was active and selective to the desired products
in the etherification of crude glycerol (GlyC) derived of biodiesel industry. An
important BA conversion (45%) was obtained only reducing the water content of GlyC
and without carrying out any other purification and/or neutralization treatment.
1
. INTRODUCTION

The accelerated decrease in fossil resources and the growing environmental concern,
has led to the urgent need to massively develop the use of renewable energies: solar,
wind, hydraulic, biomass, etc. In the agricultural and transport sectors, the use of
biomass to produce biodiesel appears as a clear alternative.
Biodiesel consists of mono-alkyl esters of long chain fatty acids, derived from
renewable raw materials such as vegetable oils and animal fats. The advantages of using
biodiesel as an alternative energy source include: the reduction of air pollution due to
lowering the levels of CO emissions, that is an efficient resource for the reuse of waste
2
and that use of fossil fuels is decreased [1].
During biodiesel production through the transesterification process, a glyceric phase is
obtained as a co-product. This phase, known as crude glycerol (GlyC), contains many
other substances such as water, salts, matter organic non-glycerol (MONG), remaining
catalyst, soaps, free fatty acids, unreacted mono, di or triglycerides or their salts, excess
of alcohol used in the process of transesterification, etc. Therefore, GlyC require some
purification and stabilization treatment to have commercial value [2]. The main
objective of the crude glycerol purification processes is to improve its quality to adapt it
to the different applications it can have and, consequently, its insertion into the market.
For the purification of crude glycerol different techniques are used, such as distillation,
ion exchange and sequential physicochemical treatments, among others. The
purification process involves the removal of unwanted impurities according to the
different standards proposed for this compound. There are basically three types of
glycerin on the market depending on the purity and potential applicability of each one:
technical grade glycerin, USP refined glycerin (United States Pharmacopeia) and
refined glycerin FCC/Kosher (FCC Food Chemical Codex) in addition to the possible
applications of crude glycerol [3].

In the last years, the interest in developing applications for crude glycerol has increased.
There are two important reasons: the expansion of the biodiesel industry and the high
costs of the purification process [3, 4]. Glycerol excess adversely affects the biofuel
market. Consequently, several processes to produce value-added chemicals from
glycerol are becoming in promising alternatives [5- 7].
The processes for glycerol transformation include catalytic reactions of hydrogenolysis,
reduction, acetalization, dehydration, pyrolysis, oxidation, steam reforming,
etherification, among others application processes that produce value-added chemicals
compounds and which could find applications in several industrial fields
(pharmaceutical, food, polymers, fuels, fertilizers, etc) [8- 10]. The possibility of using
crude glycerol in different processes as a reagent has also been reported.
The catalytic etherification represents an important upgrading strategy for glycerol
(Gly). Depending on the reagent used in the reaction, it will be the application of the
obtained products for example, fuel additives and solvents [11, 12]. Usually, olefins as
isobutene and alcohols as tert-butyl alcohol have been used for the etherification of
glycerol to obtain di- and tri-tert-butyl ethers of glycerol in the presence of acid
catalysts, which constitute excellent additives with a great potential. These additives
could reduce the density and viscosity of fuels, leading to a better engine performance,
cleaning of engine parts and reducing volatile organic compounds emissions [13]. On
the other hand, benzyl alcohol has been used as solvent for hair dyes, however, it may
cause dermal irritation for people with a sensitive skin condition A more suitable
alternative would be the derivatives of benzyl alcohol with glycerol, specifically the
monosubstituted products [14].
On the other hand, Aimaretti et al. proposed the alkyl ethers obtention achieved via the
reaction between GlyC and olefins such as isobutylene or other olefins from FCC (Fluid

Catalytic Cracking) [15]. Nouredduni et al. studied the effect of impurities on glycerol
etherification catalysed with Amberlyst 15 (5% weight). The GlyC conversion obtained
was poor compared to the refined glycerol, at the same reaction conditions [16].
Ruppert et al. carried out the crude glycerol etherification with octane and acid zeolites
(H-Beta) as catalyst. They reported low conversions, and the alkaline nature of the
substrate demonstrated to be responsible for this low activity [17]. Pico et al. compared
the results obtained in the etherification with refined and non-purified glycerol. In the
catalytic tests with Amberlyst 15, the conversion notably decreases (from 52% to about
6
%), which was attributed to both the deactivation of the catalyst with -OH groups and
the proton exchange with metallic cations (K and Na) present in the non-purified
glycerol [18].
The Gly etherification reaction can either be carried out with homogeneous or
heterogeneous catalysts. It has been studied with several heterogeneous acid catalysts,
because these systems offer the greatest possibilities for reusing and allow avoiding
environmental and operational issues. Carbonaceous systems are very good alternative
and show high promise as potential catalytic materials [19, 20]. Currently, they are
interesting solids to be used in heterogeneous catalysis area, either as supports for
actives phases or as catalysts themselves. The performance will depend on their textural
characteristics, such as pore sizes and high surface area [21- 23].
A way to obtain a carbonaceous material is using direct pyrolysis of sugars, but solids
with low porosity and surface area are generally generated. However, the textural
characteristics can be improved using different kinds of “templates” during the
synthesis process [24]. Among these materials, those functionalized with sulfonic acid
groups have been investigated as potential catalysts in glycerol etherification. Acid
functional groups can be incorporated into the solid using a two-step procedure or

during the synthesis method. The increase in catalytic activity has directly been
associated with the sulfonic groups in the surface of the acidic carbons [20, 25, 26].
Based on these antecedents, in this work it is proposed as objectives to obtain a
functionalized carbonaceous system (enriched with sulfonic groups), and evaluating its
catalytic performance (activity, selectivity and stability) towards the glycerol
etherification with benzyl alcohol (BA). The reactions were performed in a solventless
system in liquid phase and both refined (Gly) and crude glycerol (GlyC: coming from
the biodiesel production) were used as substrates. The reactant molar ratio, as well as
the effects of temperature and catalyst concentration on the product distribution was
also studied.
2
. MATERIAL AND METHODS
2
.1 Synthesis of the acidic carbon
The acidic carbon material that was used as catalyst was synthesized adapting the
method reported by Zhao et al. [27]. In a typical procedure, 10 g of dextrose
(anhydrous, Anedra Pro-analysis) and 20 g 4-hydroxybenzenesulfonic acid (p-HBSA
Aldrich, 65% aqueous solution) were dissolved in 30 mL distilled water. The solution
was heated at 85 ºC under constant stirring for about 2 h. The paste was kept at 30 ºC
for 1 h to evaporate part of water and allow forming a black gel (resin). The resin was
dried at 105 ºC overnight and then heated in concentrated H
2
SO (98%), using a ratio of
4
1
5 mL of acid per gram of solid, at 150 ºC for 4 h. Finally, it was washed with distilled
water for several times until no sulfate ions were detected. The acidic solid thus
prepared was named as Ccs and it was ground to a particle size of 60 to 100 mesh,
before performing the catalytic tests.

For comparison, others acidic carbons (denoted as Cst-SO
3
H and Cst-PhSO H) were
3
also used. These catalysts were prepared according to the method reported by Chiosso,
et al [19].
The catalyst (Cst) was obtained using a template-assisted carbonization process [24].
The anhydrous dextrose was used as carbon precursor (15 g) and the commercial
sodium silicate solution (20 mL) as source of SiO to develop the template.
2
Carbonization of the solid was performed under a nitrogen atmosphere in two steps: one
at 200 ºC and one at 450 ºC, both for a period of 5 h. The resulting material was stirred
-1
in NaOH (3 mol L ) for 5 h to remove the silica "template". Then, the carbon was
recovered by filtration and dried.
Two methods for the sulfonation of the carbon material were used. In the first, the solid
was contacted with concentrated H
of solid, at 150 ºC for 15 h. Then, the sulfonated material was washed with distilled
water, filtered and dried at 105 ºC (Cst-SO H). The second method consisted of the
reaction between 4-benzene-diazonium sulfonate and the carbon material, using
hypophosphorous acid (H PO ) as the reducing agent [25]. The Cst-PhSO H obtained
was filtered and washed with distilled water.
.2 Acidic Carbon characterization
2
SO (98%), using a ratio of 15 mL of acid per gram
4
3
3
2
3
2
The density of surface acid sites of the synthesized material was measured by a back-
titration method. In a typical analysis, 0.1 g of dried solid were added to 50 mL of
-1
-1
NaOH (0.01 mol L ) or NaHCO
be determined (total or -SO H + -COOH). The reacting mixture was kept under stirring
for 30 min at room temperature. Then, the remaining of basic solution was titrated by a
3
(0.05 mol L ), depending on the type of acid sites to
3
-1
standard solution of HCl (0.01 mol L ).

The surface area and textural properties of the catalyst were analysed by the adsorption/
desorption of nitrogen at -196 °C using a Micromeritics Accusorb 2100E. Surface area
was determined by the BET method and pore volumes were evaluated by single point
adsorption at P/P ≥ 0.99. The morphology of the carbon was studied by scanning
0
electron microscopy (SEM) in a Philips SEM 505 microscope. Energy dispersive X-ray
spectroscopic (EDX) analysis was performed to confirm the presence of sulphur in the
systems.
Thermogravimetric analysis (TGA) was performed using a Shimadzu TGA-50 thermal
analyser. Sample mass was recorded during heating at a constant temperature rate (10
-1
-1
ºC min ) from 25 to 700 ºC, under a N
2
flow of 100 mL min . FTIR spectra were
recorded on Jasco 4200 equipment. The sample was pelletized with KBr (1:100) and the
-1
spectrum was obtained between 400- 4000 cm .
2
.3 Catalytic test
The catalytic performance of the acidic carbon was studied in the etherification of
commercial glycerol (Gly, Biopack 99.5%) and crude glycerol (GlyC) with benzyl
alcohol (BA, Anedra analytic reagent). The reaction was carried out in a batch glass
reactor (100 mL) without solvent, under continuous stirring and N atmosphere. In the
2
catalytic tests, the influence of different operational variables was studied: the Gly:BA
molar ratio (3:1, 1:1 and 1:3), the mass of catalyst in relation to the mass of glycerol
employed (2.5, 5 and 10 wt.%) and different temperatures (80, 100 and 120 ºC). Stirring
was fixed for all experiments at 1200 rpm to avoid external diffusion limitations.
After 360 min of reaction, a sample was taken, filtered and analysed by gas
chromatography. The conversion was calculated from chromatographic data obtained
using a Varian CG-3800 gas chromatograph equipped with a capillary column CP
WAX 52 CB (30 m; 0.53 mm d.i.; 1.0 m d.f.) and an FID detector. Butyl alcohol was

used as internal standard for the calibration curve and ethanol as solvent. Commercial
standards of (±) 3-benzyloxy-1,2-propanediol and 1,3-dibenzyloxy-2-propanol were
utilized to obtain the corresponding response factors. GC/MS was employed for the
identification of the products using a Shimadzu QP2010 Ultra gas chromatograph with
an SH-Rtx-5Sil MS (30 m; 0.25 mm d.i.; 0.25 m d.f) column.
The stability of the Ccs carbonaceous material was evaluated through its reuse in
successive reaction. To do so, after finishing the reaction, the catalyst was separated by
filtration, washed with water at 60 ºC for 3 h and then dried in an oven at 105 ºC for 24
h. The solid material was reused under the following reaction conditions: 120 ºC, 10%
of catalyst and Gly:BA molar ratio 3:1.
Finally, the etherification of crude glycerol (GlyC) supplied by Biobin S.A. was carried
out following an identical procedure to that above mentioned. The GlyC
0
was heated at
1
00 ºC to remove some water without an important change in the sample composition.
Four samples were obtained as can be seen in Table 1. For the determination of the
quality parameters and the water content of crude glycerol, ASTM D 6304-07 was used.
The reactions were performed under these conditions: 120 ºC, 10 % of catalyst and 3:1
Gly:BA molar ratio.
3
. RESULT AND DISCUSSION
3
.1 Catalyst characterizations
The acid site density of synthesized carbon (Ccs) determined by the acid–base back-
titration method are shown in Table 2. This material has higher density of acid sites
compared to the others solids previously obtained in our laboratory (Cst-SO
3
H= 3.7
H= 2.2 mmol H g ) [19]. These results indicated the
polymerization of dextrosa and p-HBSA for the formation of solid containing sulfonic
+
-1
+
-1
mmol H g and Cst-PhSO
3

+
-1
groups. The total acid site density reached 6.4 mmol H g while that of the sum of (-
+
-1
COOH + -SO
in the catalyst. These acidic sites might be hydroxyl groups on polymerized p-HBSA
27].
Table 2 shows the surface area of Ccs. As it can be seen, the surface area obtained is
3
H) is 4.2 mmol H g , indicating the presence of other sites than –SO H
3
[
2
-1
very low (SBET <10 m g ), similarly to what is reported for materials synthesized with
analogous methods. The low surface area and high acid density of the solid indicate that
part of the sulfonic groups were in the bulk of the catalysts. It is very likely that during
the “swelling” of the Ccs catalyst in the reaction mixture, more acid sites would be
exposed, thus accounting for the high catalytic activity showed. In the same sense as the
results presented here, Mo et al. studied a number of acid carbonaceous materials of low
2
-1
surface area (<2 m g ) and high acid density and explained their results taking into
2
account that if all the -SO
3
H groups were on the outermost surface, then 1 Å of the
carbon would have to accommodate four sulphur atoms, which is not possible [26].
The morphology of the synthesized carbon is shown in the SEM micrograph, Figure 1.
The Ccs system presents rather small carbon aggregates with flat surfaces, according to
the low surface area measured. The EDX spectrum confirms the presence of sulphur,
carbon and oxygen coming from the carbon precursor (dextrose) and p-HBSA. It is
observed that the S is evenly distributed on the surface of catalyst. Thus, it can be said
that the prepared material has a homogeneous composition [23].
Aiming at evaluating its thermal stability, the Ccs catalyst was subjected to a
thermogravimetric analysis. Figure 2a shows the mass loss as a function of temperature
(TGA curve) and its first derivative (DrTGA curve). A first mass loss, representing less
than 15%, starts at a temperature ca. 50 ºC and ends at 160 ºC approximately. This
process can be attributed to loss of water. The second mass loss process between 170

and 325 ºC can be assigned to sulfonic and carboxylic groups decomposition.
According to what is reported in the literature, in this temperature range SO and SO
are detached from the solid, this result clearly indicated the presence of –SO H in the
catalyst, and the –COOH group is lost in the form of gaseous molecules such as CO and
CO [27, 29]. Total mass loss was approximately 50%, but given that the working
2
3
3
2
temperature used will be 120 °C maximum, it can be stated that the system will not
undergo thermal decomposition during the etherification of glycerol.
The presence of –SO H group in the studied material was also confirmed by FTIR
3
analysis (Figure 2b). Furthermore, this carbonaceous material is rich in functional
groups such as phenolic hydroxyl group, carbonyls, and carboxylic, all of them
originated during the synthesis and acidic treatment applied.
−
1
The important band at 3440 cm is referred to the stretching vibrations of –OH
-1
attributed at phenolic hydroxyl group. The peak at 1710 cm is assigned to the
stretching vibration of the C=O bond of carboxylic acids and the band located at 1600
-1
cm corresponds to the stretching modes of the aromatic C-C bonds from the polycyclic
-1
aromatic compounds of the carbon material [30, 31]. A band of around 1175 cm can
also be seen, which can be attributed to the symmetric stretching of the -SO H groups
3
incorporated in the material synthesis. This is accompanied by a band at 1040 cm-1
corresponding to the asymmetric stretch of the S=O bond. Therefore, this confirms the
presence of sulfonic groups in the synthesized catalytic system. Additionally, the
-1
material has a band at 800 cm , which is attributed to the S-O bonds and a band at 575
-1
cm , assigned to the C–S stretch mode [29]. Therefore, all these features confirm the
presence of sulfonic groups in the synthesized Ccs catalyst.
3
.2 Catalytic results

The glycerol (Gly) etherification with benzyl alcohol involves a set of consecutive
equilibrium reactions catalyzed by acids, and it is well-known that its effectivity
depends on the amount and nature of the acidic sites, and the structural characteristics of
catalytic system. The sulphonic acid groups have been reported as responsible for the
improvement at conversion of glycerol to its ethers [27].
Glycerol etherification with benzyl alcohol (BA) gives rise to a mixture of products.
Two monoethers can be formed, 3-benzyloxy-1,2-propanediol (ME
,3-propanediol (ME ); two diethers: 1,2-dibenzyloxy-3-propanol (DE
dibenzyloxy-2-propanol (DE
1
) and 2-benzyloxy-
1
2
1
) and 1,3-
2
) and a triether: 1,2,3-tribenzyloxypropane (TE). The
+
reaction mechanism previously reported in the literature, proposes that the H of -SO
3
H
group are available for the protonation of BA (pKa = 15.40), which will then lose a
water molecule to became a primary carbocation, which in turn that will be attacked by
either Gly or another BA molecule. This allows us to explain the formation of dibenzyl
ether (BE). The dimerization of benzyl alcohol is an independent side reaction that
influences the main reactions of etherification through the undesired consumption of
BA (Figure 3) [19, 32]. If water is continuously removed, this system can become an
irreversible process, otherwise the equilibrium shifts towards the reactants, not
achieving the formation of the triether (TE). In other words, the presence of water could
be decrease the amounts of DE and TE formed, due to their own hydrolysis. [33].
Under the reaction conditions selected in this work, it was observed that the Ccs catalyst
was selective towards ME
has been observed.
1
, ME
2
and DE . In addition, the formation of water and BE
1
The BA conversion was defined according to Eq. (1), being C
i
and C the benzyl alcohol
t
-1
concentration at zero and t times, respectively (mol Kg ).
ꢀ
푖−ꢀ푡
퐶표푛푣. %퐵퐴 = (
) ∗ 100 Eq. (1)
푖
ꢀ

The Selectivity (Eq. 2) to monoethers (SME
1
, SME
2
), diether (SDE ) and benzyl ether
1
(SBE), has been defined as:
ꢀꢁꢂ
%
푆푋푖 = (∑
) ∗ 100 Eq. (2)
ꢀ푀퐸+ꢀ퐷퐸+ꢀ퐵퐸
-1
Where CXi= CME
The catalytic performance of the Ccs was evaluated in the etherification of pure Gly
with BA at 120 ºC, and it was compared with Cst-SO H and Cst-PhSO H catalysts,
1
, CME
2
, CDE
1
, or CBE expressed as mol Kg .
3
3
previously studied in our research group [19]. As it can be seen in Figure 4, with the
three systems an almost complete conversion of BA (more than 95%) was obtained after
3
60 min of reaction. In addition, the three catalysts render the preferential formation of
ME , since the etherification of glycerol is sterically preferred in the primary –OH
1
groups, because the BA is voluminous molecule [23, 29-32, 34]. On the other hand, it is
noteworthy that the Ccs catalyst led to much less formation of the self-condensation
product of BA (SBE= only 1%).
Consequently, the carbon sulfonation process giving rise to the Ccs catalyst
demonstrated to be an efficient alternative to produce an acid catalyst, with a promising
behaviour in etherification reactions. On the other hand, it can be said that the
incorporation of the sulphonic groups does not limit the accessibility of the reagents to
the active sites, which results in a very good catalytic performance even with the small
2
-1
surface area of the synthesized carbon (2.6 m g ).
.3 Influence of the experimental variables
3
Thinking about improving the process, tending to obtain the desired products through a
more efficient transformation, the influence of some experimental variables (mass of
catalyst, temperature, etc.) was evaluated. At first, the study on the effects of
concentration of catalyst (between 2.5 and 10 wt.%) on the BA conversion and product
distribution, was carried out using Ccs at 120 °C.

In Figure 5, it can be seen that conversion of BA was practically not affected as the
amount of catalyst decreased from 10 to 2.5 wt.%. The high acidity of the prepared
+
-1
material (6.4 mmol H g ) allowed obtaining a very significant catalytic performance
even when working with a catalyst concentration four times lower. Furthermore, the
product distribution presented only a slight variation. Figure 5 shows that the selectivity
to ME increased as the amount of catalyst decreased. Based on the results obtained, the
2
Gly (refined) etherification could be done with less mass of Ccs catalyst.
The effect on initial concentration of the reactants was also studied at 120 ºC, using
three Gly:BA molar ratios: 1:3, 1:1 and 3:1. The results depicted in Figure 6 show that
the conversion of BA did not have a significantly change with different molar ratios
employed. Moreover, a clear effect on the product distribution is observed. The main
products were ME when working under conditions of excess glycerol (3:1), while for
the 1:1 and 1:3 molar ratios, DE became more important. Therefore, by decreasing the
1
Gly:BA initial molar ratio, the selectivity to undesirable product (BE) increases
considerably, in agreement with previously works [19, 35]. Beside, when excess or
stoichiometric amount of BA was used, the other products formation was also favoured.
These by-products were identified as 2-benzyl-benzyl alcohol and 1, 2-diphenylethane
(Figure 7).
Finally, the effect of reaction temperature on the Ccs catalytic activity was analysed. In
Figure 8 it can be seen the BA conversion obtained when a temperature range between
1
20 and 80 ºC was employed. As expected, the trend towards higher conversion by
increasing the temperature was confirmed. ME is the major product in all three
1
temperatures. At 80 ºC, the conversion was the lowest (about 25%) and a significant
selectivity to BE, about 25% after 360 min of reaction, was obtained. At 100 ºC, the
selectivity to ME showed a slight decline, as result of the formation of DE through a
1

consecutive reaction. At 120 ºC, the highest conversion and ME selectivity were
obtained after 360 min of reaction.
It is possible that at 80 ºC both catalytic activity and selectivity are being affected by the
presence of water in the reaction medium. Water is the most polar component in this
system, competing with BA and Gly for the active sites, which affects the reaction
equilibrium and prevents the formation of the desired products. The influence of water
in the etherification reaction has been thoroughly studied in the literature and it is
generally accepted that the presence of water favours the backward reaction [33, 36].
3
.4 Catalyst reusing
For evaluation of the operational stability of the synthesized system (Ccs), the catalyst
was recovered and recycled for further reaction. The catalyst was reused in three
catalytic cycles at 120 ºC, Gly:BA molar ratio= 3:1 and 10 wt.% of catalyst. After each
use, the solid was recovered by filtration, washed and dried in oven at 105 °C during 24
h. It is important to note that no reactivation treatment was performed after each
reaction cycle.
As can be seen in Figure 9, the conversion of BA almost remained unchanged for the
first two uses. For the 3rd. reaction (Reuse 2), the catalyst still provides a notable
conversion of BA about 43%, as shown in this figure. According to studies reported in
the literature, the decreasing of catalytic activity can be associated with two different
factors: the loss of active sites in the reaction medium by leaching or blocking of acidic
sites [21, 26, 37]. In the case of the present study, the gradual deactivation of the
catalyst as from Reuse 2 could be originated by the adsorption of reactants or products
that cannot be removed by the simple washing treatment applied between each reuse.
Besides, to confirm if leaching could be another reason for the loss of catalytic activity,
the acidic site density was determined on the recovered Ccs after Reuse 3. The result

showed that several superficial groups were solubilized in the reaction medium, since
+
-1
acidic sites density decreased to 1.8 mmol H g after Reuse 3. In agreement with
previously reported works by other authors, the leaching of polycyclic aromatic
hydrocarbons containing -SO H groups of this catalysts could take place during the
3
reaction [26]. Another explanation might be the unavoidable loss of catalyst mass
during the recycling step.
On the other hand, the selectivity to ME+DE was rather constant up to Reuse 2, but for
Reuse 3 the selectivity to the undesired by-product BE increased to reach a value ca.
3
8%. This fact might be due to the decrease of acidic sites that favours of self-
condensation product. The findings attained with Ccs overcome the previously systems
studied in our working group [19], because this catalyst maintains the activity and
selectivity towards the products of interest, at least on two successive uses.
3
.5 Etherification of crude glycerol with benzyl alcohol
There is a great interest in producing glycerol ethers using crude glycerol. Thus, some
experiments were carried out with crude glycerol (GlyC) obtained from biodiesel
production, supplied by Biobin S.A. GlyC was obtained after biodiesel production
0
process from fatty acids transesterification with methanol and its characteristics are
reported in Table 1.
As already mentioned, the etherification reaction takes place through a series of
reversible equilibrium steps, in each one of which water is generated. The presence of
water negatively influences the formation of the desired products. In the case of crude
glycerol, the concentration of water of each sample is given by the % moisture value
reported in Table 1. In order to analyse the effect of water on the catalytic results, a
series of crude glycerol samples with different moisture contents were prepared. These
samples were obtained by heating the GlyC to 100 °C to favour water evaporation and
0

were taking samples at different times. The samples correspond to those indicated as
GlyC , GlyC and GlyC
1
2
3
.
The catalytic tests were carried out with a Gly:BA molar ratio= 3:1, taking into account
the percentage of water in each sample was. In each test, a Ccs catalyst load of 10% by
weight of the glycerol in the sample was employed, at 120 °C and with constant stirring
(1200 rpm.) for 360 min. In Table 3, the results obtained with each sample are
presented.
First, the etherification reaction was carried out evaluating the activity of the Ccs
catalyst with the GlyC
Despite the fact that under these conditions the activity is low, the formation of the
products of interest predominates (selectivity to ME +ME +DE = 73%).
0
sample. In this case, a BA conversion of 20% was obtained.
1
2
1
In the following tests, GlyC samples with decreasing water content were evaluated.
From the results shown in Table 3, it can be seen that when removing some of the
water, the results obtained in terms of BA conversion are clearly improved, being 45%
for the GlyC
3
sample (2.36% moisture). Regarding the distribution of products, ME is
1
always the main product.
4
. CONCLUSION
The catalytic behavior of a synthesized carbon material, Ccs, was studied in the
etherification of pure Gly with BA. This system was highly active (about 97%) and
selective, at 120 ºC. The high activity can be attributed to high acid site density (6.4
+
-1
mmol H g ). The main reaction product was ME (72%).
1
Given the acidic characteristics of the material, it is possible to work at lower reaction
temperature (100 °C) and with less catalyst concentration (2.5 wt.%), without observing
significant loss in BA conversion.

The reuse of Ccs catalyst was investigated at four successive reactions. It was observed
that the final conversion (after 360 min) and selectivity to the desired products were
kept between the first and second use. In the second reuse, the material still gives a
notable conversion of BA (about 43%), but leaching was confirmed as one of the causes
of loss of activity. The studies showed that some superficial groups were solubilized in
the reaction medium.
Finally, the Ccs was active and selective to the desired products for the etherification of
crude glycerol (GlyC) derived from biodiesel industry. A high BA conversion was
obtained with only reducing the water content. Therefore, this catalyst is an inexpensive
material, due to it was made by a synthetic method whit less energetic cost and only in
2
4 hours, and a promising alternative for crude glycerol transformation in value-added
chemicals.
ACKNOWLEDGEMENTS
This work was supported by the Universidad Nacional de La Plata (Projects X757 and
X802), ANPCyT (PICT 2016-0148), UNNOBA (project SIB 2019, Exp. 0596/2019),
and CICPBA (project (PIT-AP-BA, Res. Nº428/2016) from Argentina. The authors are
grateful to P. Fetsis for BET analysis, Dr. A. Sosa for FT-IR analysis and Dr. I.D. Lick
for TGA analysis.
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FIGURE CAPTIONS
Figure 1. Scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy
(
EDX) for Ccs.
Figure 2. TGA and DrTGA curves associated to mass loss of Ccs (a) and FTIR spectrum of Ccs
b).
(
Figure 3. Schematic network of the etherification reaction of glycerol (Gly) with benzyl alcohol
BA) [19, 32].
(

Figure 4. Conversion of BA and selectivity obtained with different carbon catalysts: Ccs, Cst-
SO H and Cst-PhSO H. Reaction conditions: Gly:BA molar ratio 3:1, 10 wt.% of catalyst, 120
3
3
ºC and 360 min reaction time. (For interpretation of colour, the reader is referred to the web
version of this article).
Figure 5. Etherification of Gly with BA, using a molar ratio 3:1, 120 ºC and different
percentage of Ccs catalyst. (For interpretation of colour, the reader is referred to the web
version of this article).

Figure 6. Influence of the reactants ratio on the BA conversion and products distribution. Gly:
BA molar ratio between 3:1 to 1:3. Catalyst: Ccs (10 wt.%). Reaction temperature: 120 ºC. (For
interpretation of colour, the reader is referred to the web version of this article).
Figure 7. Other products of BA condensation. T= 120 ºC, 10 wt.% of catalyst and Gly:BA= 1:1
or 1:3.

Figure 8. BA conversion and products distribution employing 10 wt.% of Ccs, Gly:BA molar
ratio 3:1 and reaction temperature between 120 ºC to 80 ºC. (For interpretation of colour, the
reader is referred to the web version of this article).
Figure 9. Ccs system reuse in the Gly etherification with BA. Reaction conditions: Gly:BA
molar ratio 3:1, 10 wt.% of catalyst, 120 ºC and 360 min. (For interpretation of colour, the
reader is referred to the web version of this article).
Table 1. Characterization of crude glycerol.
Sample % moisture* % ash % glycerol %MeOH %MONG** pH
GlyC
0
1
12.36
8.17
4.78
4.86
81.95
85.5
0.06
0.04
0.9
4.2
GlyC
1.47
4.96

GlyC
2
3
5.78
2.36
5.14
5.41
88.3
0.02
0.01
0.78
1.94
5.02
5.12
GlyC
90.29
*
Obtained by heating at 100 ºC for different times
*MONG: matter organic non-glycerol.
*
Table 2. Acid site density, surface area and pore volume of Ccs synthesized in this
work.
Characterizations
Total*
6.4
4.2
2.6
+
-1
Acid site (mmol H g )
(
-COOH -SO H)**
3
2
-1
S
BET (m g )
*
*
titration with NaOH (pKa= 15.74) [28].
3
* titration with NaHCO (pKa= 6.37) [28].
Tabla 3. Results of etherification of GlyC with BA. T= 120 ºC, ratio Gly: BA= 3:1 and
0 wt.% of catalyst.
1
Selectividad** (%)
Gly
Conv**. BA (%)
ME
72
64
62
57
53
1
ME
14
8
2
DE
10
1
1
BE
Gly* 97
1
GlyC
GlyC
GlyC
GlyC
0
1
2
3
20
23
33
45
27
10
9
1
27
1
33
10
1
36
*
Glicerol refined. GlyC
GlyC : 2.36 % moisture.
* After 360 min of reaction
0
: 12.36 % moisture, GlyC
1
: 8.17 % moisture, GlyC
2
: 5.78 % moisture and
3
*