Biobased catalyst in biorefinery processes: Sulphonated hydrothermal carbon for glycerol esterification

Article abstract of DOI:10.1039/c5cy00059a

Sulphonated hydrothermal carbon (SHTC), obtained from d-glucose by mild hydrothermal carbonisation and subsequent sulphonation with sulphuric acid, is able to catalyse the esterification of glycerol with different carboxylic acids, namely, acetic, butyric and caprylic acids. Product selectivity can be tuned by simply controlling the reaction conditions. On the one hand, SHTC provides one of the best selectivity towards monoacetins described up to now without the need for an excess of glycerol. On the other hand, excellent selectivity towards triacylglycerides (TAG) can be obtained, beyond those described with other solid catalysts, including well-known sulphonic resins. Recovery of the catalyst showed partial deactivation of the solid. The formation of sulphonate esters on the surface, confirmed by solid state NMR, was the cause of this behaviour. Acid treatment of the used catalyst, with subsequent hydrolysis of the surface sulphonate esters, allows SHTC to recover its activity. The higher selectivity towards mono- and triesters and its renewable origin makes SHTC an attractive catalyst in biorefinery processes.

Full text of DOI:10.1039/c5cy00059a

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Biobased catalyst in biorefinery processes:
sulphonated hydrothermal carbon for
Cite this: DOI: 10.1039/c5cy00059a
glycerol esterification†
c
Carlos de la Calle,a José M. Fraile,ab Enrique García-Bordejé, Elísabet Pires ab and
*
Laura Roldánc
Sulphonated hydrothermal carbon (SHTC), obtained from D-glucose by mild hydrothermal carbonisation
and subsequent sulphonation with sulphuric acid, is able to catalyse the esterification of glycerol with dif-
ferent carboxylic acids, namely, acetic, butyric and caprylic acids. Product selectivity can be tuned by sim-
ply controlling the reaction conditions. On the one hand, SHTC provides one of the best selectivity towards
monoacetins described up to now without the need for an excess of glycerol. On the other hand, excellent
selectivity towards triacylglycerides (TAG) can be obtained, beyond those described with other solid cata-
lysts, including well-known sulphonic resins. Recovery of the catalyst showed partial deactivation of the
solid. The formation of sulphonate esters on the surface, confirmed by solid state NMR, was the cause of
this behaviour. Acid treatment of the used catalyst, with subsequent hydrolysis of the surface sulphonate
esters, allows SHTC to recover its activity. The higher selectivity towards mono- and triesters and its
renewable origin makes SHTC an attractive catalyst in biorefinery processes.
Received 12th January 2015,
Accepted 18th March 2015
DOI: 10.1039/c5cy00059a
www.rsc.org/catalysis
used as solvents in three characteristic organic reactions,
namely, nucleophilic substitution, Suzuki cross-coupling
Introduction
Even though biodiesel production has decreased in the past
years, the use of glycerol itself and glycerol derivatives has
been the focus of the interest of many researchers, probably
due to their availability, renewable origin and usefulness as
building blocks1 or as solvents2 in industrial applications.
Among glycerol derivatives, esters have been profusely used in
industry, above all triacylglycerols. For instance, ca. twenty
new applications for triacetin appear monthly. As some exam-
ples, triacetin has been used as a solvent for caffeine extrac-
tion from coffee or tea,3 as well as in ceramic preparation via
enzymatic catalysis acting as a pH regulator when hydrolyzed
in reaction media.4,5 On the other hand, monoacylglycerols
with ester moieties with 1 to 7 carbon atoms have been used
as solvents in antibacterial or anti-mould formulations,6 as
well as cellulose plasticizers.7 Finally, acylglycerols have also
been described as fuel additives.8
reaction and enzymatic asymmetric reduction.9 Triacetin
plays simultaneously the roles of a solvent and an acyl donor
in the synthesis of isoamyl acetate10,11 catalyzed by lipase
CALB or Amberlyst 36 and in the one-pot synthesis of
cinnamyl acetate from cinnamaldehyde,12 and only that of a
reaction medium in the production of paclitaxel,13 an anti-
cancer drug.
Due to the increasing interest in glycerol-derived esters, in
the past few years great attention has been paid to glycerol
acetylation with acetic acid (Scheme 1) and several studies
have been published dealing with the use of solid acid cata-
lysts, such as Amberlyst 15 or other sulphonic resins,14–17
mesoporous materials with anchored sulphonic groups,16,18,19
K10 montmorillonite,17 SnCl2,20 hydroxylated magnesium fluo-
ride,21 and heteropolyacids.22,23 In general sulphonic solids,
either resins or mesoporous materials, and K10 have shown
much better performance than zeolites and other solid acids,
such as niobic acid and sulphated zirconia.14,16,17
Acylglycerols have been used as reaction media but to a
lesser extent. Diacetin, triacetin and tributyrin have been
a Departamento de Química Orgánica, Facultad de Ciencias, Universidad de
Zaragoza, E-50009 Zaragoza, Spain. E-mail: epires@unizar.es
b Instituto de Síntesis Química y Catálisis Homogénea (ISQCH), Facultad de
Ciencias, CSIC – Universidad de Zaragoza, E-50009 Zaragoza, Spain
c Instituto de Carboquímica (ICB-CSIC), Miguel Luesma Castán 4, E-50018
Zaragoza, Spain
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5cy00059a
Scheme 1 Acylation of glycerol with carboxylic acids.
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Table 1 Composition, textural properties and acidity of hydrothermal
carbons
Some of these studies showed that glycerol conversion
and reaction selectivity towards mono-, di- or triacetins
strongly depend on the nature (hydrophobicity–hydrophilic-
ity)14 of the catalyst surface and the density and strength of
the acid catalytic sites.16,18,19
Sample
HTC
SHTC Used SHTC
Compositiona
H/C
O/C
0.796 0.514
0.304 0.546
0.000 0.009
0.616
0.578
0.007
n.d.
In the past years, sulphonated carbons24 have emerged as
an interesting alternative to both organic and inorganic
acidic materials, with the additional advantage of being pre-
pared from renewables.25 The use of sulphonated carbons in
glycerol esterification has been recently described.26,27
Sulphonated pyrolised (400 °C) sucrose provided excellent
glycerol conversion and a maximum of 50% selectivity
towards TAG at 180 °C.26 Sulphonated activated carbon led to
mixtures of 38% MAG, 28% DAG and 34% TAG at 120–135 °C
with glycerol conversions over 90%.27
S/C
N2 adsorption
SA (m2 g−1
)
7.0
5.9
VP (cm3 g−1
)
)
0.014 0.009
142.5 224.3
0.057 0.090
n.d.
CO2 adsorption
Acidity (mmol g−1
SA (m2 g−1
)
256.0
0.100
4.61
0.32
4.29
VP (cm3 g−1
Totalb
)
3.42
0.00
3.42
5.43
0.40
5.03
Sulphonicc
Non-sulphonicd
a
Molar ratio determined by CHS combustion analysis and O analysis
by pyrolysis. b Determined by back-titration. Determined by sulphur
c
analysis. d Calculated by difference.
Carbons prepared under mild conditions, such as hydro-
thermal carbon (HTC), are very attractive materials from a
sustainability point of view. Sulphonated hydrothermal car-
bon (SHTC) has been used as a solid acid catalyst in esterifi-
cation reactions of fatty acids,28–31 showing remarkable per-
formance despite the apparently very low surface area. In this
manuscript, we present a deep study of the acetylation of
glycerol catalysed by sulphonated hydrothermal carbon
(SHTC). Several reaction parameters have been studied, such
as reaction temperature, excess of acid or catalyst loading, as
well as the possibility of catalyst recovery. The activity of
SHTC has been compared with that of some commercial
sulphonic solids, and the optimised reaction conditions have
been applied to the esterification of glycerol with butyric and
caprylic (octanoic) acids, in order to broaden the scope of
application of this catalyst.
higher values of surface areas were obtained (up to 224
m2 g−1) by CO2 adsorption, demonstrating the presence of
ultramicropores in the solid. Back titration with a solution of
0.01 M NaOH allowed the determination of the total acidity.
In the case of SHTC, the total acidity was 5.43 mmol g−1
,
whereas the number of sulphonic groups, calculated from
the S content, is only 0.4 mmol g−1. The rest of the acidic
sites (5.03 mmol g−1) corresponds to carboxylic and hydroxyl
sites. This high functionalization forms a dense hydrogen-
bond network responsible for the microporosity detected by
CO2 adsorption, which is not permanent as shown by the full
access to the acid sites in the presence of a polar protic
solvent.35
Screening of reaction conditions
Results and discussion
The activity of SHTC was first evaluated in the acetylation of
glycerol with acetic acid. Screening of two reaction parame-
ters was made, namely, the temperature (bath temperature
40, 80 and 115 °C) and the acetic acid/glycerol molar ratio
(3 : 1, 6 : 1, and 9 : 1). Conversion of glycerol and selectivities
towards mono-, di- and triacetins were chosen as response
factors. A 10% w/w catalyst was used, that is 0.4% molar ratio
of SO3H sites with respect to glycerol. The reaction time was
fixed at 10 h. The reaction was also carried out in the absence
of the catalyst at 40 °C and 115 °C. The results are gathered
in Table 2.
A big influence of the reaction temperature on glycerol
conversion and reaction selectivity was observed. Thus at 40
°C, glycerol conversion reached 70% at 10 h of reaction time
and monoacetins were the major products (up to 89% selec-
tivity) regardless of the amount of acetic acid. To the best of
our knowledge, this overall result is among the best ones
described in the literature, comparable with the 84% MAG
selectivity at 82% conversion with sulphonic SBA15 (ref. 19)
or the 90% MAG selectivity at 55% conversion with K10
montmorillonite,17 without the need for an excess of glyc-
erol.36 At 40 °C, the reaction nearly did not take place in the
absence of the catalyst, even with an excess of acetic acid.
Preparation and characterization of the solids
Hydrothermal carbon (HTC) was prepared by introducing an
aqueous solution (1 M) of glucose in a Teflon lined autoclave
and keeping it at 195 °C for 19 h. HTC was obtained in the
form of nanospheres (300–400 nm diameter), slightly smaller
than those reported for analogous materials.31 Sulphonation
of HTC was carried out by treatment with concentrated
sulphuric acid at 150 °C for 15 h. Apparently, the nanosphere
morphology was not modified by the sulphonation process.32
The main properties of both solids are collected in Table 1.
HTC is a carbonaceous material with relatively high oxy-
gen and hydrogen contents IJO/C ratio = 0.304; H/C ratio =
0.796). This composition corresponds to a structure based
mainly on furan moieties directly bonded or linked through
alkyl (methylene) or ketone bridges.33 Sulphonation reduces
the hydrogen content and increases the oxygen content, in
agreement with an oxidation process, occurring in parallel
with the sulphonation reaction. The surface area, determined
by nitrogen adsorption isotherm measurement, was always
low (6–7 m2 g−1), but slightly higher than the values reported
in the literature,31,34 in agreement with the smaller particle
size. The pore volume is also low (9–14 μl g−1). However,
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Table 2 Glycerol conversion and product selectivities in the acetylation
of glycerol with acetic acid catalysed by SHTCa
Temp.
AcOH/gly
Conversion
%MAG
%DAG
%TAG
40 °C
3 : 1
6 : 1
9 : 1
9 : 1
3 : 1
6 : 1
9 : 1
3 : 1
6 : 1
9 : 1
9 : 1
32
70
70
7
90
93
97
99
98
98
95
85
88
89
100
46
33
28
9
15
12
11
0
50
57
62
51
49
38
57
0
0
0
0
Blank
80 °C
4
10
10
40
45
57
11
115 °C
6
5
32
Blank
a
Results determined by GC. Reaction time 10 h, 10% w/w catalyst.
An increase in temperature to 80 °C provided reaction
mixtures in which diacetins are the major products, with
more than 60% selectivity at a high acetic acid/glycerol molar
ratio. Finally, good selectivity towards triacetin was obtained
(57%) at higher temperature by using a larger excess of acetic
acid. Interestingly, this high selectivity towards TAG is
obtained with only small amounts of MAG in the reaction
mixture. The triacetin selectivity can be improved up to 61%
at 24 h of reaction time. This result is also remarkable, as in
the literature14,16–19,23,23,26,27 the described triacetin yields are
normally lower than the yields of diacetins, regardless of the
temperature and the excess of acetic acid, whereas in our
case triacetin is the major product. Just very recently,
Khayoon et al.37 described a similar triacetin selectivity, using
Y-SBA as a catalyst, but in that case MAG selectivity was
higher, up to 19%. When the reaction was carried out with-
out the catalyst, good glycerol conversions are achieved but
diacetin is in this case the major product, with very low
triacetin selectivity.
As mentioned in the introduction, triacetin is an interest-
ing product due to its industrial applications. Above this,
triacetin is also the most challenging product and selectivity
towards TAG can be used as an indicator of the catalytic
activity. In view of this, the optimized conditions to obtain
TAG, that is an acetic acid/glycerol molar ratio of 9 : 1 and
115 °C, were used to continue the study. First, a test to deter-
mine the influence of reducing the amount of catalyst from
10% w/w to 5% w/w and 1% w/w was carried out. Reactions
were monitored with time and the results are presented in
Fig. 1.
Fig. 1 Influence of the amount of catalyst in glycerol conversion (a)
and yields of monoacetins (b), diacetins (c), and triacetin (d) in glycerol
acetylation with acetic acid catalysed by SHTC (reaction temperature
115 °C, acetic/glycerol ratio 9 : 1; results determined by GC).
TAG/DAG ratio decreased to 1.15. When the 1% w/w catalyst
was used, the results appeared to be identical to the ones in
the blank reaction, that is 95.3% glycerol conversion in 4.5 h
of reaction time with 41% yield of monoacetins, 51% yield of
diacetins and only 7% yield of triacetin. These results can be
considered as normal, given that the 1% w/w catalyst repre-
sents only 0.04 mol% of SO3H with respect to glycerol.
Comparison of several solid acid catalysts
When we tried to compare the results of glycerol esterifica-
tion catalysed by SHTC with those of other sulphonic solids,
previously published, we found that the reaction conditions
were different in many cases and conclusions about their rel-
ative activity were difficult to extract.
Two arylsulphonic resins (Amberlyst 15, Dowex 50W ×2)
and one alkylsulphonic resin (Deloxan) were tested under the
optimal reaction conditions (115 °C and 9 : 1 AcOH : glycerol
molar ratio) to obtain triacetin. The weight of the catalyst
was varied in order to maintain in all cases a constant ratio
of the number of sulphonic sites per mol of glycerol, that is
0.4% molar ratio. The results are gathered in Table 3.
The TON value (productivity) with respect to the glycerol
conversion is highly dependent on the initial glycerol/acid
ratio, as the conversion is in general close to quantitative due
to the easy first esterification reaction to produce mono-
acetins. Similar TON values are achieved with all the solids.
However, the synthesis of diacetins and triacetin represent
two and three esterification reactions, respectively, and thus
the TON calculated would be more significant with respect to
esterification reactions, that is the number of acetate groups
formed per acid site.
Although glycerol conversions were excellent in all cases at
short reaction times (nearly quantitative in less than 1 h with
5% w/w), the product selectivity was strongly dependent on
the amount of catalyst and reaction time. As can be seen, a
reduction of the amount of catalyst from 10% to 5% w/w did
not produce any significant change in glycerol conversion or
in the yield of monoacetins, but had given rise to different
TAG/DAG distributions. Thus, at 24 h of reaction time with
the 10% w/w catalyst, triacetin was the major product, with a
TAG/DAG ratio of 1.79, while with the 5% w/w catalyst the
With this parameter, differences are observed when calcu-
lating TON with respect to the number of acetate groups
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Table 4 Glycerol conversion and product selectivities in SHTCa cata-
lyzed esterification of glycerol with butyric and caprylic acidsb
Table 3 Comparison of the productivity of SHTC and other sulphonic
solids in the esterification of glycerol with acetic acid under the same
reaction conditions
Acid
Time (h)
Conversion
%MAG
%DAG
%TAG
Reaction time 1 h
Reaction time 10 h
Butyric
1
95
99
99
99
76
98
100
93
43
1
1
11
61
2
52
24
18
62
37
42
19
67
5
TON
TON
TON
TON
10
24
24
1
10
24
24
75
81
27
2
56
80
28
Catalyst
(conv)a
(acet)b
(conv)a
(acet)b
Blank
Caprylic
SHTC
187
180
181
181
393(138)
324(73)
294(21)
307(36)
189
189
188
187
477 (324)
453 (231)
436 (234)
436 (236)
Dowex 50W ×2
Amberlyst 15
Deloxan
1
5
Blank
Reaction conditions: 0.4 mol% catalyst, acetic acid/glycerol ratio of
a
a
Another batch of SHTC was used, with a functionalization of
9 : 1, 115 °C. Mol of glycerol converted per mol of sulphonic group.
b
b
0.59 mmol of S per g. Results determined by GC. Reaction con-
ditions: 10% w/w catalyst, 115 °C, acid/glycerol molar ratio of 9 : 1.
Mol of acetic acid converted per mol of sulphonic group or acetate
group formed per sulphonic acid site. In parentheses, the TON only
of the third acetate in triacetin (mol of triacetin formed per mol of
sulphonic group).
providing high selectivities towards tributyrin and tricaprylin
at longer reaction times.
formed, especially triacetin. Thus at 1 hour of reaction time,
SHTC and Dowex showed higher values than Ambelyst and
Deloxan that exhibited similar activity. But SHTC provided
twice the value of TON, with respect to triacetin, of Dowex. At
10 h of reaction time, acetate TON of the solids become simi-
lar but again SHTC exhibited higher values of TON in
triacetin, demonstrating a higher activity of this catalyst
under the same reaction conditions. This higher activity was
also previously observed in the esterification of palmitic acid
with methanol.35 This behaviour was attributed to the coop-
erative effect of acid sites, due to their high density in the
solid. The same cooperative catalysis by adjacent Brønsted
acid sites has been also recently observed in fructose dehy-
dration catalysed by HZSM5.38 The authors reported that the
multiple nearby sites can interact simultaneously with one
reactant molecule to favour its activation. In our case, the
favourable interaction of the catalyst surface with the poly-
functional diacetins could explain the higher reactivity to pro-
mote the esterification of the third hydroxyl group of glycerol
with the nearby activated acetic acid and hence provoking an
increased triacetin selectivity.
Study of the recoverability of SHTC
The reuse of the catalyst was also studied and in all the cases
SHTC was significantly deactivated upon recovery, as it hap-
pened in the esterification of palmitic acid.28 At 10 h of reac-
tion time, the fresh catalyst exhibited a TON of 188 for glyc-
erol conversion while upon recovery, the TON slightly
decreased to 152. However, the product distribution strongly
varied, and with the reused catalyst MAG and DAG were the
major products. For the sake of comparison, a carboxylic
acrylic resin Dowex CCR-2 was tested in the acetylation of
glycerol under the same conditions as SHTC and reused
SHTC. The results gathered in Fig. 2 show that upon reuse,
SHTC exhibited the same results as the carboxylic resin and
as the blank reaction, behaving as if all the sulphonic groups
were deactivated.
Analysis of the catalyst after the reaction (Table 1) revealed
that 80% of the sulphur content remained in the solid and
the surface area and pore volume did not decrease, thus
deactivation could not be attributed to the leaching of
Reactions of glycerol with butyric and caprylic acids
Once the optimization of the reaction conditions and the
screening of the catalysts were performed, SHTC was also
tested in the esterification of glycerol with carboxylic acids
having a longer hydrocarbon chain, such as butyric and
caprylic acids. The idea of using butyric acid came from the
interest in mono-, di- and tributyrins at an industrial
level.39,40 The use of caprylic acid would demonstrate the
general applicability of this solid even with fatty acids.
Reactions were carried out under the best conditions
determined for acetylglycerol derivatives, that is 115 °C, 9 : 1
acid/glycerol molar ratio and 10% w/w catalyst. The results
gathered in Table 4 show that, as in the case of using acetic
acid, high glycerol conversions were achieved in only 1 hour
and SHTC appeared to be also active in these reactions
Fig. 2 Yields (determined by GC) of MAG, DAG and TAG in glycerol
acetylation catalysed by SHTC, reused SHTC, and Dowex CCR2, at
115 °C and AcOH/glycerol ratio of 9 : 1.
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sulphur, given that the reaction also takes place with half the
amount of catalyst as shown in Fig. 1, nor to the blocking of
the pores, as the textural properties (Table 1) are similar to
those of the fresh catalyst.
In the case of the reaction of palmitic acid with MeOH,
the esterification of the sulphonic sites was shown to be the
main deactivation mechanism,28,35 and a similar effect may
be also driven by glycerol. In order to confirm this hypothe-
sis, the 13C-CP-MAS-NMR spectrum of the used catalyst was
recorded and compared with that of the fresh SHTC (Fig. 3).
The 13C-CP-MAS-NMR spectrum of the catalyst used in the
reaction at 115 °C (Fig. 3) shows the presence of new bands,
a small one at 20 ppm, compatible with –OOC–CH3 groups,
and the most prominent one at 69 ppm, compatible with
glycerol species that may have reacted with the acidic groups
on the SHTC surface, as it happened in the case of esterifica-
tion with methanol.28,35 To prove the nature of this band,
SHTC was treated with glycerol under different conditions
and the spectra are collected in Fig. 4.
First of all, glycerol was adsorbed on SHTC at room tem-
perature, to prevent any reaction with the acidic groups. As
can be seen (Fig. 4b), two signals at 73 and 64 ppm are
obtained, typical for the carbons of the secondary and pri-
mary alcohols of glycerol, respectively. When SHTC is treated
with glycerol at 115 °C and thoroughly washed with metha-
nol, a broad signal at 69 ppm is obtained with a higher inten-
sity for increasing treatment times (Fig. 4c and d). It is also
significant that the signals of physisorbed glycerol are much
thinner than those of the solids treated at 115 °C, in agree-
ment with the higher mobility of the physisorbed molecules
in comparison with that of those covalently bonded on the
surface. In Fig. 4 the two possible types of surface species
formed are also presented, bonded either with only one or
with two covalent bonds. The question about the formation
of surface sulphonates or only carboxylates was studied by
Fig. 4 13C-CP-MAS-NMR spectra: a) SHTC, b) SHTC with glycerol
physisorbed at room temperature, c) SHTC treated with glycerol at
115 °C for 4 h and washed with methanol, d) SHTC treated with
glycerol at 115 °C for 24 h and washed with methanol.
adsorption of triethylphosphine oxide (TEPO) as a probe mol-
ecule. The bonding of triethylphosphine oxide (TEPO) to the
acid site of a surface produces a systematic change in the 31P
isotropic chemical shift that is proportional to the acid
strength of the adsorption site.41 It has been also described
that alkylsulphonic groups give the 31P chemical shift of
TEPO in the range of 71–77 ppm, whereas the value for
arylsulphonic groups in Amberlyst 15 is 86 ppm.35,42
The intensity of the 31P-MAS-NMR band at 87 ppm, corre-
sponding to TEPO adsorbed on sulphonic sites, is signifi-
cantly reduced in the SHTC catalyst used in one reaction at
60 °C (Fig. 5), whereas it completely disappears in the solid
treated with glycerol at 115 °C. Only the band at 62 ppm, cor-
responding to TEPO adsorbed on carboxylic sites, remains
visible. This result demonstrates that sulphonic sites are
involved in the formation of surface esters, the main mecha-
nism for deactivation of SHTC. As the deactivation of the cat-
alyst was due to the esterification of the sulphonic sites by
glycerol, an attempt to recover the catalyst was done by acid
treatment of the solid after the reaction. The catalyst was
treated at 150 °C for 15 h with H2SO4 (98%) or H2SO4 (66%).
After thorough washing with water and drying overnight,
the regenerated catalysts were reused in the acetylation of
glycerol. Although a slight influence of the acid concentration
is observed on the product distribution (Fig. 6), the solids
treated with sulphuric acid behave as the fresh SHTC.
Sulphur analysis of SHTC before and after acid treatment
did not show any increase in sulphur content, precluding
any additional sulphonation of HTC. On the contrary, the
13C-CP-MAS-NMR spectrum (Fig. 3c) showed that the bands
assigned to bonded glycerol and acetate completely dis-
appeared. Thus the recovery of the activity was due to
Fig.
3
13C-CP-MAS-NMR spectra: a) SHTC, b) SHTC used in the
reaction of glycerol with acetic acid at 115 °C, c) SHTC used and
regenerated with H2SO4 (66%).
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80 °C or 115 °C). The reaction was monitored by gas chroma-
tography. At the end of the reaction, methanol was added to
the reaction medium; the catalyst was filtered off, thoroughly
washed with methanol and dried overnight at 115 °C prior to
reuse. In the case of using sulphonic resins, the same loading
of sulphonic groups (0.4 mol%) was used in each case. Reac-
tions of glycerol with butyric and octanoic acid were carried
out in the same way as acetylation reactions.
Regeneration of the catalyst by acid treatment
The catalyst recovered from the reaction medium as indi-
cated in the prior section was treated with 96% or 65%
sulphuric acid (15 ml g−1) at 150 °C for 15 h and thoroughly
washed with hot water (90 °C). Then the solid was dried over-
night at 115 °C and reused under the same reaction condi-
tions previously described.
Conclusions
Fig.
5
31P-MAS-NMR spectra of TEPO adsorbed from methanol
SHTC showed a high activity in the esterification of glycerol
with acetic, butyric and caprylic acids. Product selectivity
could be tuned by adjusting the reaction temperature. Thus
at 40 °C high selectivity toward monoacetylglycerol was
obtained without the need for an excess of glycerol. When
high reaction temperatures were used, triesters were mainly
obtained. High reaction temperatures and long reaction
times also favoured the deactivation of the catalyst by esterifi-
cation of the sulphonic groups with glycerol. The superiority
of the catalyst is attributed to cooperative effects. The acid
treatment of deactivated SHTC made the recovery of the activ-
ity of the catalyst possible by hydrolysis of the surface
sulphonic esters.
solutions on: a) SHTC, b) SHTC used in the reaction of glycerol with
acetic acid at 60 °C for 4 h, and c) SHTC treated with glycerol at
115 °C for 24 h.
Acknowledgements
This work was made possible by the generous financial sup-
port from the Spanish “Ministerio de Economía
y
Competitividad” (project CTQ2011-28124), the “Diputación
General de Aragón” (E11 Group co-financed by the European
Regional Development Funds) and the European Commission
(FP7, FREECATS project grant agreement no. 280658).
Fig. 6 Yields of products in acetylation of glycerol catalysed by fresh,
used and regenerated SHTC (acetic acid/glycerol ratio of 9 : 1, 115 °C,
time reaction 24 h, determined by GC).
References
hydrolysis of the sulphonated esters and hence confirmed
that the esterification of the sulphonic groups was the rea-
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Experimental
Esterification of glycerol
Glycerol (0.3 g, 3.25 mmol), acetic acid (0.6, 1.2 or 1.8 ml;
9.75, 19.5 or 29.25 mmol), SHTC (0.03 g, 0.012 mmol SO3H),
and 1-methylnaphthalene (0.03 g, 0.21 mmol) as an internal
standard were stirred (≈1000 rpm) in
immersed in a silicone bath at different temperatures (40 °C,
a round flask
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