R. Estevez, et al.
Molecular Catalysis 488 (2020) 110921
etherification reaction of glycerol with tert-butyl alcohol. Specifically,
the amount of sulfonic acid groups in the catalyst has been directly
associated to its catalytic activity [30,31,33].
The advantages of using microwaves instead of conventional
heating in the etherification reaction have also been revealed in the
present study. Thus, a noticeable better catalytic performance (X = 84
G
The acidity values of the solids here studied obtained from ele-
% and Yh-GTBE = 22 %) was obtained on the OC(2)100Mw catalyst,
under microwave irradiation and after 15 min, than after 6 h under
+
mental analysis (mmol of SO
3
H/g) and from titration (mmol H /g) are
collected in Table 1. The acidity values related to the sulfonic acid
groups are in agreement with the amount of sulfur incorporated in the
carbons, since all this sulfur is in sulfonic form as it has been demon-
strated by XPS (Fig. 3). Thus, the OC(0.5)100 sample with the lowest
amount of sulfur also exhibited the lowest acidity value (0.4 mmol
SO H/g). The rest of carbons exhibited very similar acidity values. The
3
same tendency was observed in the acidity values calculated from acid-
base titration. However, these values of acidity were higher than those
G
conventional heating at similar reaction conditions (X = 45 % and Yh-
GTBE = 9%), Table 2. This fact could be ascribed to the higher energy
generated by the microwave in comparison to the conventional heating,
which would give rise the breaking of the hydrogen bonds formed be-
tween the free hydroxyl groups of the catalyst surface and the sulfonic
acid sites. In order to verify this, a reaction at a higher temperature
(100 °C) was carried out under conventional heating. The results ob-
G
tained (X = 82 % and Yh-GTBE = 21 %), Table 2, were very similar to
obtained by elemental analysis (mmol SO
3
H/g). This fact is logical
those obtained under microwave irradiation.
given the contribution of other acid groups existing in the catalyst,
mainly the carboxylic acid groups.
These results brings to light that the use of microwave promotes an
important reduction, not only in the sulfonation time but also in the
time and temperature employed in the etherification reaction, favoring
then a reduction of the costs and consequently, an improvement of the
economy of the global procedure.
3.3. Microwave-assisted etherification of glycerol
It has been widely reported that the reaction mechanism of the
etherification of glycerol with tert-butyl alcohol occur via a fast proto-
nation of TBA on acid sites, giving rise to a tertiary carbocation that
reacts with a glycerol molecule, generating the first MTBGs. These
MTBGs are able to react with other molecule of TBA to form the DTBGs,
which finally form the TTBG in a subsequent reaction with another TBA
molecule. Furthermore, water is also produced in every step of the re-
action [27,30,41].
In the present study, in addition to the aforementioned ethers, a
small amount of IB (< 5%), coming from the dehydration of the alcohol
was also obtained. Besides, it has to be highlighted that, at the reaction
conditions here studied, the untreated carbon (OC) did not show ac-
tivity in the etherification reaction.
The evolution of the glycerol conversion with time of reaction on all
the solids studied is shown in Fig. 5. In general, the glycerol conversion
G
(X ) increased from 3 to 15 min of reaction and then, remained prac-
tically constant. Furthermore, all the solids exhibited similar glycerol
conversion (70–80 %) values after 15 min, with the exception of the
sulfonated carbon OC(0.5)100.
3.3.1. Comparison with other reported catalysts
The catalytic performance in the etherification of glycerol with tert-
butyl alcohol of the catalysts here studied was compared with the ac-
tivity exhibited by some of the reported sulfonated carbons, obtained
from different wastes, Table 3. Thus, under conventional heating and at
similar experimental conditions (weight of catalyst and TBA/G molar
ratio), the yield to h-GTBE (22 %) attained on OC(2)100Mw after 6 h of
reaction was very similar to that obtained on some of the sulfonated
carbons reported in literature [32,33,35] but at higher reaction tem-
peratures (120 °C).
Regarding the results here obtained under microwave irradiation,
similar values of yield to h-GTBE were obtained but at softer reaction
conditions, i.e., lower reaction time and temperature than those re-
ported [32,33,35]. Therefore, we consider that an important advance in
relation to the viability of the complete process (synthesis of the car-
bons and etherification reaction) has been made.
3.3.2. Reusability and catalyst stability
Regarding to selectivity, a similar tendency as that observed for the
glycerol conversion was obtained over all the catalysts studied. In this
sense, after 15 min, the MTBGs and h-GTBE selectivity values did not
change (Fig. 6). These results are in clear concordance with those
previously reported in the microwave-assisted etherification of glycerol
over amorphous organosilica-aluminumphosphates [31].
The glycerol conversion also increased notably (from 55 % to 84 %)
with the increasing reaction temperature from 65 °C to 85 °C, as is
shown in Table 2 for OC(2)100. Similarly, the values of selectivity (and
yield) to h-GTBE increased, obtaining the maximum value of yield to h-
GTBE (22 %) at 75 °C, while the selectivity to MTBGs decreased. A
further increase of temperature to 95 °C led to a high decrease of the
The reusability of the solids was studied by consecutive etherifica-
tion reactions. It has to be highlighted that the solid did not undergo
any acid washing between the reactions, which is typically done to
reactivate the active sites.
According to the results shown in Fig. 7, after the first use, not only
the glycerol conversion values were relatively constant, but also the
yield to h-GTBE value remained practically constant. However, a very
low decrease (∼8%) was observed after the third use. At this point, an
elemental analysis of sulfur and SEM experiments were done to the
spent catalyst (OC(5)100Mw). The results obtained indicated that no
leaching of the sulfonic groups took place. Hence, we could associate
the small decrease in the activity to the presence of some molecules of
reactants and reaction products in the catalyst, which were retained in
the course of the etherification reaction, as we previously reported on
other catalytic systems [30,31].
G
activity (X = 30 % and Yh-GTBE = 2%). This behavior of the catalysts
here studied as a function of the temperature is very similar to that
exhibited by other catalysts previously studied such as Amberlite and
sulfonated silica [], as well as organosilica-aluminumphosphates [],
using microwave or conventional heating.
4. Conclusions
As can be seen, a relationship between the acidity of the solids and
their catalytic behavior exists. Thus, the OC(0.5)100 catalyst, with the
lowest value of acidity also exhibited the lowest value of conversion (31
Olive stone from olives of Andalusia, one of the most productive
regions of these fruits in the world, has been the raw material for the
synthesis of acid catalysts with sulfonic groups (sulfonated carbons).
These acid catalysts have been tested in the production of high ethers
(h-GTBE) by etherification reaction of glycerol with tert-butyl alcohol.
The h-GTBEs have a great potential as an additives for diesel and bio-
diesel.
The sulfonated carbons, OC(X)T(Z), were prepared by treatment of
carbon with sulfuric acid at different sulfonation times (X = 0.5, 2 and
5 h) and temperatures (T = 100 and 150 °C) and using either,
%) and of yield to h-GTBE (6%). The rest of the solids showed similar
conversion values (66–84 %) and yield to h-GTBE (16-22 %) as corre-
spond with the higher values of acidity that they showed. Hence, the
present results corroborate those previously obtained on sulfonated
carbons obtained from different wastes [32,33], and also over catalysts
of different nature, such as sulfonated zeolites [42], sulfonated hybrid
silicas [30] and sulfonated organosilica-aluminumphosphates [31], in
which the higher the acidity, the higher the activity of the catalysts.
7