Biodiesel Glycerin Valorization into Oxygenated Fuel Additives

Article abstract of DOI:10.1007/s10562-021-03646-0

Current industrial methods of biodiesel production lead to an excess of crude glycerin which requires costly purification before commercialization. Production of oxygenated fuel additives is a potential route for glycerin valorization. Glycerin acetylation was carried out over heterogeneous acid catalysts (15%, glycerol weight basis) using glacial acetic acid (molar ratio = 9). The catalysts, containing different amounts of phosphate species (P/Si from 10 to 20 atomic ratio), were prepared by wet impregnation of commercial silica with aqueous solutions of diammonium phosphate and ortho-phosphoric acid. X-ray diffraction patterns of calcined solids presented amorphous patterns like raw silica. The prepared catalysts presented, at 120?°C, glycerol conversion higher than 89.5% after 1?h of reaction, been diacetin the major product, with triacetin selectivities lower than 26.1%. Diacetin selectivity was found to be almost invariant with catalyst acidity thus underlining the relevance of catalyst porosity due to the large acetins molecules sizes. The slow rate of triacetin diffusion in narrow pores of catalyst might be responsible for the relatively low yield obtained. Surface phosphate species showed a slow rate of leaching in the reaction medium showing high catalyst stability. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-021-03646-0

Catalysis Letters
https://doi.org/10.1007/s10562-021-03646-0
Biodiesel Glycerin Valorization into Oxygenated Fuel Additives
Ana Paula Soares Dias1 · Frederico Gomes Fonseca2 · Mónica Catarino1 · João Gomes1,3
Received: 13 January 2021 / Accepted: 2 May 2021
© The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2021
Abstract
Current industrial methods of biodiesel production lead to an excess of crude glycerin which requires costly purifcation
before commercialization. Production of oxygenated fuel additives is a potential route for glycerin valorization. Glycerin
acetylation was carried out over heterogeneous acid catalysts (15%, glycerol weight basis) using glacial acetic acid (molar
ratio=9). The catalysts, containing diferent amounts of phosphate species (P/Si from 10 to 20 atomic ratio), were prepared
by wet impregnation of commercial silica with aqueous solutions of diammonium phosphate and ortho-phosphoric acid.
X-ray difraction patterns of calcined solids presented amorphous patterns like raw silica. The prepared catalysts presented,
at 120 °C, glycerol conversion higher than 89.5% after 1 h of reaction, been diacetin the major product, with triacetin selec-
tivities lower than 26.1%. Diacetin selectivity was found to be almost invariant with catalyst acidity thus underlining the
relevance of catalyst porosity due to the large acetins molecules sizes. The slow rate of triacetin difusion in narrow pores of
catalyst might be responsible for the relatively low yield obtained. Surface phosphate species showed a slow rate of leaching
in the reaction medium showing high catalyst stability.
Graphical Abstract
Keywords Biodiesel · Catalysis · Valorization · Fuel additives · Acetylation · Acetins
Extended author information available on the last page of the article
Vol.:(0123456789)
1 3

A. P. Soares Dias et al.
1 Introduction
The increasing environmental awareness and the depletion
of fossil fuel reserves have prompted the growth of the
renewable biofuels market, with a greater impact on bio-
diesel and bioethanol, substitutes for conventional diesel
and gasoline, respectively [1].
Biodiesel is typically produced by a homogeneous base-
catalyzed transesterifcation, from vegetable oil or animal
fat [2], and bioethanol is obtained by the fermentation of
sugar [3]. Both processes co-produce glycerin, 10 wt% [4,
5] of the fnal mixture for biodiesel whereas the fermenta-
tion yields about 3 to 15 wt% (ethanol basis) [6], depend-
ing on the used feedstock.
Glycerin is mainly used in pharmaceutical, food, and
cosmetic industries but these traditional uses cannot face
the increasing biofuels-related production of glycerin, cre-
ating a surplus that not only afects its market value but
also results in a potential environmental issue that must
be addressed [4]. Simultaneously, the valorization of the
co-produced glycerin would help to improve the sustain-
ability and economic feasibility of the biodiesel produc-
tion process [7].
Fig. 1 Acetylation reaction of glycerin with acetic acid
which are toxic, corrosive, difcult to remove from the
fnal reaction mixture, and generate wastes that may pose
risks for human health and the environment [12]. Moreo-
ver, the main challenge regarding glycerin acetylation is
the low selectivity towards TAG, which can be enhanced
by increasing the catalyst acidity. However, when homo-
geneous catalysts are used, an increase in catalyst acidity
implies higher equipment costs [11].
One of the main applications of crude glycerin (non-
purifed glycerin co-produced with biodiesel) is an alterna-
tive fuel however, its physical and chemical properties are
limiting factors in combustion [8]. Moreover, the uncon-
trolled combustion of glycerin leads to the formation of
acrolein, a substance highly toxic for human health and
ecosystems [9, 10]. On the other hand, traditional indus-
trial uses require high glycerin purity, which would penal-
ize even further the biodiesel production process [11].
In this context, new approaches have been studied and
glycerin acetylation, to produce mono-, di-, and triacetins
(MAG; DAG, and TAG), have stood out as a potential
option since the three products have a wide range of appli-
cations [12], particularly TAG that can be used as an oxy-
genated fuel bio-additive to improve fuel combustion [13].
Additionally, DAG can be used in biodegradable polyester
production, helping to mitigate the problem related to the
pollution of marine and freshwater environments [7, 14].
Glycerin acetylation (Fig. 1) can be carried out using
either acetic anhydride or acetic acid as an acetylation
agent [15]. However, despite the former being thermody-
namically favorable, achieving higher TAG selectivity, the
latter is more practical since it is more stable during stor-
age periods and has a lower price [7, 13]. Additionally, the
reaction with acetic anhydride is more violent, demanding
more resistant, and therefore expensive, equipment [7].
The acetylation is an acid-catalyzed reaction that can be
performed in a homogeneous or heterogeneous medium.
Usually, mineral acids are used as homogeneous catalysts,
The use of heterogeneous catalysts in glycerin acetylation
not only avoids all the mentioned problems but also ofers
the possibility of reuse the catalyst several times, becoming
an interesting option from an economic and environmental
point of view [11]. Therefore, several solid catalysts have
been studied, for diferent reaction conditions (Table 1).
For most of the tested catalysts in Table 1, the selectivity
towards TAG, the most valuable acetin, remains low. Data
reported by Abida et al. [16] show that SO42− studded over
silica coated Fe2O3, magnetic and nanostructured catalysts,
permits to obtain 100% of TAG selectivity after 80 min of
reaction. According to the depicted reaction mechanism, the
active phase seems to be related to Si–SO4H species on the
catalyst surface. The best catalyst lost about 50% of TAG
selectivity after 6 consecutive reaction batches, possibly by
leaching the –SO4H species into the reaction medium. The
leaching active species into the reaction medium can have a
non-negligible homogeneous catalytic contribution.
Recently data from Ramalingam et al. [17] seems to show
a major role of the catalyst mesoporous on the triacetin for-
mation. The authors reported a selectivity 50.4% of TAG
selectivity obtained with mesoporous silica modifed with
copper and ruthenium. Both, Lewis and Brønsted acid cata-
lysts have been used in glycerin acetylation. Brønsted acety-
lation mechanism (Fisher esterifcation) is believed to start
with acetic acid protonation as schematized in Fig. 2 [18].
Theoretically, Lewis and Brønsted mechanisms are similar
1 3

Biodiesel Glycerin Valorization into Oxygenated Fuel Additives
Table 1 Triacetin (TAG) selectivity in glycerin acetylation from the literature
Catalyst
Parameters (reaction time, temperature, HAc: glycerin molar
% TAG selectivity
Reference
ratio; catalyst loading)
Acid-exchange resins
4 h; 120 °C; 9:1; 4 wt% (glycerin basis)
4.5 h; 110 °C; 9:1; 82.96 mmol H+/L
8 h; 80 °C; 6:1; 5 wt% (glycerin basis)
3 h; 105 °C; 3:1; 5 wt% (glycerin basis)
24 h; 80 °C; 6:1; 9 wt% (glycerin basis)
0.5 h; Refux temp.; 3:1; 2 mmol of acid sites
1 h; 120 °C; 3:1; 1 g
4 h; 105 °C; 9:1; 7 wt% (glycerin basis)
3 h; 120 °C; 8:1; 0.8 g
5 h; 110 °C; 3:1; 2 wt% (glycerin basis)
1.5 h; 100 °C; 8:1; 5 wt% (glycerin basis) – ultrasound-assisted 100
24 h; 80 °C; 6:1; 9 wt% (glycerin basis)
6 h; 80 °C; 3:1; 75 mg
8 h; 80 °C; 6:1; 5 wt% (glycerin basis)
3 h; 105 °C; 3:1; 5 wt% (glycerin basis)
2 h; 50 °C; 6:1; 0.2 g
4.5 h; 120 °C; 6:1; 4 wt% (glycerin basis)
5 h; 120°;10:1;wt% undefned
8 h; 80 °C; 6:1; 5 wt% (glycerin basis)
3 h; 105 °C; 3:1; 5 wt% (glycerin basis)
3 h; 120 °C; 6:1; 5 wt% (glycerin basis)
1 h; 120 °C; 6:1; 0.1 g
8 h; 80 °C; 6:1; 5 wt% (glycerin basis)
6 h; 80 °C; 3:1; 75 mg
24 h; 105 °C; 6:1; 5 wt% (glycerin basis)
20 h; 105 °C; 12:1; 10 wt% (glycerin basis)
4 h; 120 °C; 5:1; 0.2 g
≤34
≤44.5
15.1
12
26
5
≤56
≤41.0
34
[34]
[35]
[36]
[37]
[12]
[32]
[38]
[39]
[40]
[41]
[11]
[12]
[42]
[36]
[37]
[43]
[44]
[17]
[36]
[37]
[45]
[46]
[36]
[42]
[47]
[48]
[49]
[50]
[16]
K-10 montmorillonite
Activated carbons
≤45
≤23
≤18
≤27.0
≤49
≤4.62
35
≤50.4
0.5
≤5
≤7.52
≤16.5
≤0.9
≤30
≤71.4
30
Mesoporous and modifed silica
Modifed zirconia
Heteropolyacids loaded solids
20
≤14
100%
3 h; 120 °C; 16:1; 0.2 g
45 min; 80 °C; 6:1; 7%wt% (glycerin basis)
Magnetic nanostructured
SO42−/(SiO2–Fe2O3)
50% for 6th reaction batch
Phosphotungstic, silicotungstic,
phosphomolybdic acids/polymeric
material
6 h; 110 °C; 1:20; 7%wt% (glycerin basis)
34%
[24]
Sulphated siliceous zirconia
40 min; 80 °C; 9:1; 3%wt% (glycerin basis)
93%
[22]
[18] and some researchers claim a synergy efect between
both acid sites during glycerin acetylation over Keggin het-
eropolyacid catalysts [19].
[23] are also highly active for the glycerin acetylation show-
ing a lower leaching rate, but with lower TAG selectivity
than that reported by Abida and Ali.
Since silica can be obtained from natural sources and
industrial wastes, the use of this kind of catalyst would help
to further increase the sustainability of all biodiesel pro-
duction processes [20]. According to published data strong
Brønsted acid, sites can be created by calcination of phos-
phoric acid impregnated silica [21]. Abida and Ali [22] stud-
ied the catalytic behavior of sulfated siliceous zirconia cata-
lysts. The authors reported strong Brønsted acidity, which
is responsible for the exceptional TAG selectivity (Table 1).
The catalyst was deactivated by leaching sulfate species into
the reaction medium. Sulfate species over carbon materials
Magar et al. [24] reported the catalytic behavior of phos-
photungstic, silicotungstic, and phosphotungstic acids sup-
ported over polymeric materials during glycerin acetylation.
The authors underlined the fact that polymeric support mate-
rial adsorbed the water formed during reaction thus shift-
ing the equilibrium towards the reaction products. The best
catalyst allowed 34% of TAG selectivity (Table 1).
A clever synthesis route, to avoid the glycerin co-produc-
tion with biodiesel, was recently proposed by Dahwan et al.
[25] The authors did not assess whether the acetin content
of the methyl esters produced meets the biodiesel standards.
1 3

A. P. Soares Dias et al.
Fig. 2 Brønsted acid mechanism of glycerin acetylation (adapted from [18])
Given the value of TAG as an oxygenated additive for
diesel, the low TAG selectivity reported in the literature, for
Table 2 Evonik Sipernat 22 characteristics
Density (g/L)
Pore volume
BJH (cm3/g)
Average pore size
BJH (nm)
d50 Average par-
ticle diameter
laser difraction
(μm)
most of the tested catalysts, justifes further investigation in
the heterogenous catalysts for glycerin acetylation reaction.
The focus of this work lays in the employment of phosphate
modifed silica (commercial fumed silica), a non-expensive
and easy to prepare catalysts while changing some reaction
parameters to promote the esterifcation of glycerin using
acetic acid.
220
120.7
1.15
32.5
precursor can be either o-phosphoric acid (H3PO4, Fisher
Chemical, reagent grade) or diammonium phosphate
[(NH4)2HPO4, Merck, pro analysis]. Diammonium phos-
phate is expected to allow a better anchorage of P species
on the silica surface thus contributing to better dispersion
of the acidic species and improved catalytic behavior.
The slurries were slowly evaporated (≈ 80 °C) under
vigorous stirring until dryness. The obtained samples were
dried overnight at 120 °C and calcined for 5 h at 475 °C
in a mufe.
2 Materials and Methods
2.1 Preparation and Characterization
of the Catalysts
Phosphated silica catalysts were prepared by contact-
ing commercial silica powder (Sipernat 22, character-
istics in Table 2) with aqueous solutions containing the
desired amount of the phosphate precursor. Samples with
atomic ratios P/Si in the range 10–20 were obtained and
are referred to using the mnemonic nXpercursor (e.g.:
10XH3PO4), in which n is the P/Si atomic ratio and
The crystallinity of fresh catalysts was investigated by
X-ray difraction (XRD) using a Burke D8 Advance X-ray
difractometer with Cu kα radiation at 40 kV and 40 mA.
Each difractogram was acquired in the range of 5–70°
1 3

Biodiesel Glycerin Valorization into Oxygenated Fuel Additives
at 0.02° s−1 and then compared with standard JCPDS
patterns.
Samples were prepared by mixing some drops of the prod-
uct with Isopropyl Alcohol (IPA) (GC grade) as an internal
standard. About 2 μL of the sample was then injected into
the column. The catalyst stability was evaluated using the
same sample for consecutive reaction batches without any
intermediate reactivation procedure. The GC analysis was
unable to distinguish MAG and DAG isomers, most likely
due to their high boiling points, hence the products are sim-
ply referred to as “MAG and DAG”.
To identify the surface species involved in the catalytic
process, which remain adsorbed, and thus infer about the
stability of the catalysts, both fresh and post-reaction solid
catalysts were characterized by HATR-FTIR with a Piker
Elmer Spectrum Two IR spectrometer, using a 4 cm−1
resolution. HATR-FTIR spectra were corrected using the
Kubelka–Munk function.
The acidity of catalysts was also evaluated using the
standard reaction of skeletal isomerization of 1-butene. The
catalytic tests were carried out at 222 °C in a conventional
apparatus for continuous catalytic tests. The reaction was
carried out using 520 mg of catalyst with Y01 - but = 12.8% in
an inert atmosphere of N2 and a total fow of 4.3 L h−1. The
reaction efuent was quantifed by gas chromatography (GC)
using an FID detector.
3 Results and Discussion
The surface acidity of raw silica and prepared catalysts was
evaluated by skeletal isomerization of 1-butene. Both the
reaction speed (r1-but) and the trans/cis selectivity ratio of
the skeletal isomerization of 1-butene (Table 4) are directly
related to the surface acidity of catalysts (zeolites) [26].
Acidic catalysts produce larger amounts of trans-isomer [26]
thus having trans/cis-butene-2 ratios higher than 1. For the
tested catalysts, the trans/cis butene-2 isomers ratios were
always higher than 1, evidencing the acidic character of the
catalysts. Also, the 1-butene conversion was usually greater
for the catalysts prepared with diammonium phosphate solu-
tions and tends to increase with the P/Si ratio, which indi-
cates greater surface acidity and therefore greater potential
for glycerin acetylation performance.
2.2 Glycerin Acetylation Catalytic Tests
The glycerin (Glyc, 99% from Fluka) acetylation with acetic
acid (HAc, glacial from Fluka) was studied in a 500 mL
round bottom Pyrex fask equipped with a condenser and
a magnetic stirrer. The reaction temperature was kept at
120 °C using a nest shape electrical heating jacket with
temperature control, whereas the temperature sensor is
immersed in the reaction medium. The catalytic tests were
carried out in conditions selected from the literature [42]
using 500 mg of catalyst (15 wt%, glycerin basis), HAc/
Glyc = 9 molar ratio. After the reaction, the catalyst was
removed by fltration and the liquid efuent was analyzed
by GC using a Varian CP 3800 chromatograph equipped
with a fame ionization detector. The pure MAG, DAG,
TAG, glycerin, and acetic acid were injected separately to
determine the retention time of each component for a fused
silica capillary column (0.32 mm of internal diameter, 15 m
length with a 0.1 μm of flm thickness of Methyl 5% Phenyl
Silicone). Chromatograms were acquired in the conditions
listed in Table 3.
It appears that the diammonium precursor allowed a bet-
ter dispersion of P species on the silica surface thus improv-
ing surface acidity. Additionally, the volatilization of ammo-
nium species during calcination can contribute to creating a
higher porosity surface layer over silica material.
The difractograms of fresh catalysts (Fig. 3) showed
patterns ascribable to amorphous materials like raw silica
(amorphous according to the supplier). As reported in the
literature [27], the broad feature around 2θ=22.5° belongs
Table 4 1-butene isomerization. Average rate of conversion (r1-but
)
and trans/cis selectivity ratios. Y01−but = 12.8% in N2; Wcat = 0.52g;
total fow of 4.3 L/h and 222 °C
Table 3 Gas chromatograph experimental conditions
Catalyst
1-butene con-
version (%)
r1-but
trans/cis
2-butene
selectivity
ratio
(μmol·gcat−1·s−1
)
Conditions
Carrier gas
Helium
Initial temperature
Ramp
Injector temperature
FID temperature
Detector gases (mL/min)
110 °C
10 °C/min to 150 °C
280 °C; split ratio 10
300 °C
30
300
30
6.7
Silica
3.7
12.3
16.7
12.3
11.7
11.6
21.7
29.1
0.45
1.50
2.04
1.50
1.43
1.41
2.64
3.56
1.1
1.5
1.4
1.5
1.4
1.4
1.5
1.6
10XH3PO4
10XNH4PO4
15XH3PO4
15XNH4PO4
15XNH4PO4
20XH3PO4
20XNH4PO4
H2
Air
He
Total pressure (psi)
1 3

A. P. Soares Dias et al.
Fig. 3 XRD patterns of fresh
catalysts 15XH3PO4 and
15XNH4PO3
to amorphous silica. Difraction lines of silica phosphate are
absent which seems to confrm the good dispersion of P spe-
cies with the probable formation of small (nanostructured)
aggregates of silica phosphates.
which can be correlated with catalytic activity and deacti-
vation. The main spectral features for fresh and post-reaction
catalysts are presented in Fig. 4.
The raw silica shows absorption bands at 1080, 963,
and 802 cm−1 respectively attributable to antisymmetric
Si–O–Si, in-plane Si–O, and symmetric Si–O stretching
The fresh and post-reaction catalysts were characterized
by HATR-FTIR spectroscopy to identify surface species
post reaction
PR1hsiNH4pfosf10X
PR1hsiNH4pfosf15X
PR1hsiNH4pfosf20X
PR1hsipfosf10X
sipernat
sipfosf10X
sipfosf15X
sipfosf20X
sipNH4fosf10X
sipNH4fosf15X
sipNH4fosf20X
PR1hsipfosf15X
PR1hsipfosf20X
PR2hsiNH4pfosf15X
PR2hsipfosf15X
1400 1300 1200 1100 1000
900
800
700
600
1400 1300 1200 1100 1000
900
800
700
600
Wave number (cm-1)
Wave number (cm-1)
Fig. 4 HATR-FTIR spectra of fresh and post-reaction catalysts
1 3

Biodiesel Glycerin Valorization into Oxygenated Fuel Additives
vibrations [28]. The small bands at 947 cm−1 and 802 cm−1
are assigned to the fexion modes of SiO–H and Si–O–,
respectively [29]. Fresh catalysts display analogous IR
features but the most intense band became wider due to
the overlay of O–P–O and O=P–O vibrations at 1140 and
1230 cm−1 [30]. For the post-reaction samples, the most
intense band results from the overlapping of features belong-
ing to SiO2 and PO43− species; for samples prepared using
diammonium phosphate this efect was more noticeable.
Additionally, longer reaction times (2 h instead of 1 h) faded
this efect, which possibly indicates that phosphorus surface
species are leached during the reaction.
Kinetics data, obtained using raw silica catalyst (Fig. 5),
confrmed the sequential mechanism of the acetylation reac-
tion [31]. The total conversion of glycerin was achieved in a
short time, frst by MAG formation, whose maximum selec-
tivity was reached at the beginning of the acetylation. For
longer reaction times MAG concentration decreased, being
replaced by DAG. On the other hand, DAG was slowly con-
verted in TAG, at a reaction rate constant and independent
of DAG concentration. A decrease in DAG concentration is
only observable for low concentrations of MAG. The low
rate of triacetin formation can be explained by the porosity
of the catalyst since the TAG molecule is larger than the
glycerin one and, therefore, more susceptible to difusional
limitations. The kinetics test was replicated 3 times, allow-
ing to compute relative errors for conversion and selectivi-
ties (3% for conversion and 7% for selectivities).
100
90
80
70
60
50
40
30
20
10
0
The tests performed (Table 5) allowed us to analyze the
efect of the catalyst acidity and the reaction time on the
total conversion and, especially, on the selectivity towards
triacetin production.
The efect of higher catalyst acidity is readily notice-
able as the selectivity of TAG reaction increases steadily,
especially when compared to the unmodifed silica (Fig. 6).
The efect of the reaction temperature is observable in the
10XNH4PO4 tests and seems to impact the overall glycerin
conversion more than the products’ selectivity.
0
20
40
60
80
100
120
Time (min)
The efect of test duration is noticeable through higher
selectivity towards heavier products (TAG) but lower over-
all conversion. This trend should lead to greater consump-
tion of MAG and DAG over higher reaction times, and an
conv
sel MAG
sel DAG
sel TAG
Fig. 5 Glycerin conversion and acetins (MAG, DAG and TAG) selec-
tivities versus reaction time (120 °C; HAC/Glyc = 9 molar ratio;
15 wt.% glycerin basis; raw silica catalyst)
Table 5 Catalytic performances of the prepared catalysts during the acetylation of glycerin with acetic acid
HAc/Glyc
Catalyst load- T (°C)
ing
Reaction
time (h)
Conversion, selectivities (%)
Molar ratio
Conv
Acetol
MAG
DAG
TAG
(wt%, glyc-
erin basis)
Sipernat
(NH4)2HPO4
9.0
8.6
9.0
8.8
8.7
8.8
8.6
8.3
8.8
8.7
9.0
9.0
8.7
8.6
8.9
15.0
15.1
15.9
15.6
15.4
15.5
15.3
15.6
15.6
15.3
16.2
14.9
15.4
15.2
15.7
120
120
130
120
112
120
120
120
120
120
120
120
120
120
120
1
2
2
1
2
1
2
2
1
2
2
1
1
2
1
89.5
92.6
91.1
91.3
94.0
92.9
95.1
94.4
93.0
94.8
93.3
91.3
90.3
92.7
91.0
–
8.7
–
–
5.7
–
–
–
–
–
–
–
–
–
41.7
18.6
17.5
21.1
19.7
21.4
19.5
19.1
20.8
20.6
16.4
18.7
19.1
17.2
14.2
50.3
60.2
67.0
65.5
61.2
65.2
64.8
64.8
65.4
64.3
66.0
66.4
66.4
65.5
59.7
8.0
10X
10X
10X
10X
15X
15X
15X, Batch#2
20X
20X
10X
10X
15X
20X
12.5
15.5
13.5
13.4
13.4
15.7
16.2
13.8
15.1
17.6
14.9
14.5
17.3
26.1
H3PO4
20X
–
1 3

A. P. Soares Dias et al.
20
16
12
8
The results in Table 5 showed a catalytic behavior for
P/Silica catalysts superior to most catalysts displayed in
Table 4. P/SiO2 catalysts allowed similar TAG selectivity
for shorter reaction times.
4 Conclusions
Acetylation of glycerin with acetic acid was carried out
over solid acid catalysts. Phosphate modifed silica was
prepared using ortho-phosphoric acid or diammonium
phosphate. TAG selectivity remains almost invariant with
catalyst surface acidity, while emphasizing the importance
of pore size due to the large kinetic diameter of the heavi-
est product. It appears that slow difusion in narrow pores
might be responsible for the low selectivity of the reac-
tion to triacetin. Diammonium phosphate precursor led to
higher acidity solids. Surface phosphate species showed
a slow rate of leaching in the reaction medium showing
high catalyst stability.
4
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
r1-but (µmol·gcat-1·s-1)
Fig. 6 TAG selectivity (120 °C; 2h reaction time) versus rate of
butene-1 isomerization (a measure of the catalysts acidity)
increasingly higher concentration of TAG in the medium,
thus longer reaction times seem to favor TAG production.
Production of acetol (α-hydroxyacetone) was only
observed for experiments using the 10XNH4PO4 and
15XNH4PO4 catalysts and might be a product of dehydra-
tion of glycerin. Dehydration tends to be an endothermic
reaction, which explains the low selectivity due to the low
temperature used. However, Gonçalves et al. [32] reported
better yields of acetol for lower acidity catalysts, such as
the case, which might relate to the formation of this prod-
uct with the acidity of the catalyst. Acetol could be an
interesting intermediate in a new technological route for
the production of propylene glycol from glycerin [33].
The comparison of data in Table 5 (current work) and
Table 1 (from literature) is hard since there is a wide dis-
persion of reaction variables (catalyst loading, tempera-
ture, reaction time, and HAc/ Glyc ratio) values. Still,
from data in Table 5 and Table 1, it stands out the lower
TAG selectivities obtained in this study when compared
with data reported for mesoporous silica catalysts. The
catalyst morphology seems to have a chief role in TAG
formation because the highest selectivities were reported
for mesoporous silica catalysts. Despite the low porosity of
the used silica data in Table 5 showed a catalytic behavior
for P/Silica catalysts superior to most catalysts displayed
in Table 1. P/SiO2 catalysts allowed similar TAG selectiv-
ity for shorter reaction times.
Acknowledgements The team would like to acknowledge Raisa Paes
Santiago and André Aguilar for all the help given during glycerin
acetylation tests and acknowledge FCT (Fundacão para a Ciência e
Tecnologia, Portugal) for funding project PTDC/EMS-ENE/4865/2014.
The author F. G. Fonseca is a member of the Bioeconomy Graduate
Program BBW ForWerts.
Author Contributions APSD Conceptualization, Resources, Super-
vision, Funding acquisition, Writing-Reviewing and editing. FGF
Investigation, Formal Analysis, Writing Original Draft, Reviewing
and Editing. MC Investigation, Writing Original Draft. JG Writing,
Reviewing and Editing.
Declarations
Conflict of interest The authors declare that they have no known com-
peting fnancial interests or personal relationships that could have ap-
peared to infuence the work reported in this paper.
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Authors and Afliations
Ana Paula Soares Dias1 · Frederico Gomes Fonseca2 · Mónica Catarino1 · João Gomes1,3
3
* Ana Paula Soares Dias
ADEQ, Instituto Superior de Engenharia de Lisboa, Instituto
apsoares@tecnico.ulisboa.pt
Politécnico de Lisboa, R. Conselheiro Emídio Navarro, 1,
1959-007 Lisbon, Portugal
1
IDMEC, CERENA, Instituto Superior Técnico, Universidade
de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisbon, Portugal
2
Institute of Catalysis Research and Technology
(IKFT), Karlsruhe Institute of Technology (KIT),
76344 Eggenstein-Leopoldshaven, Germany
1 3