A selective synthesis of glycerol carbonate from glycerol and urea over Sn(OH)2: A solid and recyclable: In situ generated catalyst

Article abstract of DOI:10.1039/c8nj05635h

In this work, we report a selective and straightforward process to synthesize glycerol carbonate from urea and glycerol using a simple but commercially unavailable catalyst (Sn(OH)₂). This catalyst was generated in situ from the reaction of Sn(ii) halides and urea during the glycerol carbonatation process. Effects of main reaction parameters (i.e., temperature, molar ratio of urea to glycerol, catalyst concentration) were investigated. Different tin halides were assessed as catalytic precursors, with SnCl₂ being the most efficient. We found that Sn(OH)₂-catalyzed glycerol carbonation reactions with urea achieved high conversion and selectivity (ca. 87 and 85%, respectively). The samples of Sn(OH)₂ generated in situ or previously synthesized were equally active and selective catalysts toward glycerol carbonate were successfully reutilized without loss activity. This is a very attractive route based on two inexpensive and readily available feedstocks in a chemical cycle that, overall, results in the chemical fixation of carbon dioxide and, concomitantly, adds value to glycerol, a biodiesel byproduct.

Full text of DOI:10.1039/c8nj05635h

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PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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DOI: 10.1039/C8NJ05635H
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A selective synthesis of glycerol carbonate from glycerol and urea
over Sn(OH)2: a solid and recyclable in situ generate catalyst
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Received 00th January 20xx,
Accepted 00th January 20xx
Diego M. Chavesa and Márcio J. Da Silvaa,*
DOI: 10.1039/x0xx00000x
In this work, we report a selective and straight process to synthesize glycerol carbonate from urea and glycerol using a simple
and not commercially available catalyst (i.e. Sn(OH)2). This catalyst was generated in situ from the reaction of Sn(II) halides
and urea along the glycerol carbonatation process. Effects of main reaction parameters (i.e., temperature, molar ratio urea
to glycerol, catalyst concentration) were investigated. Different tin halides were assessed as catalytic precursors, being the
SnCl2 the most efficient. We have found that Sn(OH)2-catalyzed glycerol carbonatation reactions with urea achieved high
conversions and selectivity (ca. 87 and 85 % , respectively). The samples of Sn(OH)2 generated in situ or previously
synthesized were equally active and selective catalysts toward glycerol carbonate have been successfully reutilized without
loss activity. This is a very attractive route based on two inexpensive and readily available feedstock in a chemical cycle that
overall, results in the chemical fixation of carbon dioxide and concomitantly, add value to the glycerol, a biodiesel byproduct.
www.rsc.org/
dimethyl or diethyl carbonate has been a route very employed
to produce glycerol carbonate.8-10 Several alkaline catalysts (i.e.,
based on the mixture of Mg and Ca oxides) have been
successfully used, giving 100 % glycerol carbonate after 1. 5 h
reaction at 343 K.11 Nevertheless, the alkyl carbonates utilized
in the transesterification are typically generated via phosgene
reactions, an environmentally unfriendly reactant.12
Introduction
In recent years, the biodiesel industry has generated glycerol in
large surplus, a fact that has motivated the search by processes
that can aggregate value to this inexpensive, abundant and
renewable feedstock.1 The utilization of glycerol as a biorefinery
platform molecule constitute an opportunity to obtain higher
value-added products from an available and inexpensive raw
material. Among the several glycerol derivative products, the
glycerol carbonate has gained prominence in the last two
decades due to its wide use spectrum.2
Glycerol carbonate is a green solvent with high a boiling point,
low toxicity, great biodegradability, being used as the solvent in
the field of medicine, paints, cosmetics, electrolyte in the
lithium batteries, in addition to be an ingredient in the
surfactant synthesis, polymers, drugs, and agrochemicals.2-3
Therefore, the synthesis of glycerol carbonate representing a
promising pathway for the valorization of bio glycerol
generated as biodiesel waste.4
The carbonylation of glycerol using CO or CO2 is very promising
from the environmental viewpoint.13 Copper salts have been
extensively used in oxidative carbonylation of glycerol.14-16
.
However, there are limitations, such as the high toxicity of CO,
the high pressures required. In addition, poor yields have been
obtained in the reactions with CO2 due to the thermodynamic
limitations, which compromise these processes.17
Alternatively, the use of urea as a carbonylic source to produce
glycerol carbonate have shown an attractive phosgene-free
route.18 Urea is an inexpensive and commercially available
reactant. The reactions of carbonatation of glycerol with urea
have been efficiently catalyzed by Lewis acid such as salts and
oxides of transition metals.19-22 In addition to use a high load of
catalyst metal, some of these processes undergo with a low
intrinsic activity and poor selectivity.
Solid-supported Keggin heteropolyacids (i.e. H4SiW12O40/ MCM-
41 or H4SiW12O40/ H-zeolite) are efficient catalysts on the
carbonatation of glycerol with urea.23,24 Similarly, solid-
supported transition metal catalysts have been also successfully
utilized in urea glycerolize reactions.25
The main difference between the routes to produce glycerol
carbonate consists in the choice of which will be the reactant
used to incorporate the carbonyl group to the glycerol.5-7 On
this sense, the alkaline transesterification of glycerol with
a.Chemistry department, Federal University of Viçosa, Avenida Peter Henry Rolfs,
s/n, Viçosa, Minas Gerais, MG, Brazil, 36570-900. E-mail: silvamj2003@ufv.br;
Fax: +55-31-3899-3065; Tel: +55-31-3899-3210
Tin based catalysts are also utilized in synthesis of glycerol
carbonate. Srikanth et al. synthesized Cs-exchanged Keggin
heteropolyacid catalysts impregnated with Sn(II) different loads
(i.e., Sn-Cs3PW12O40), and evaluated their activity in the
synthesis of glycerol carbonate from urea.26 Those authors
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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found that at 413 K temperature and under vacuum, the is a solvent free reaction, the samples were diluted in methyl alcohol
reactions achieved high conversion and selectivity toward before GC analyses.
glycerol carbonate (ca. 91 and 81 %, respectively). Lingaiah et
al. assessed the urea glycerolize over tin-tungsten mixed oxide Purification and product identification
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DOI: 10.1039/C8NJ05635H
catalysts prepared by precipitation method and verified that
Glycerol carbonate was purified trough liquid-liquid extraction
using a 2:1 molar ratio of Sn to W, ca. 52 % of glycerol was
procedures (Fig SM1). The mass spectrum of the GC was obtained on
converted with a high carbonate selectivity (ca. 95 %).27
a Shimadzu MS-QP 2010 Ultra mass spectrometer instrument,
Although all the catalysts described by us herein until now
coupled to Shimadzu 2010 GC (Tokyo, Japan) with He as the carrier
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were synthesized in the laboratory, available commercially
gas (ca. 1.18 mL min-1). Column and chromatographic conditions
Lewis acid catalysts are also an attractive option in reactions of
were the same of the GC analyses. The injector and MS ion source
urea alcoholysis. In special, tin(II) halides demonstrated to be
temperatures were 250 and 200 °C, respectively. The MS detector
efficient catalysts in reactions of urea with terpenic
operated in the EI mode at 70 eV, with a scanning range of m/z 0-
alcohols.28,29 These tin(II) halides are easily handling solids, have
400. The 1H and 13C NMR spectra was recorded on the Mercury-300
high water tolerance and low cost, have been successfully used
Varian Spectrometer at 300 and 75 MHz respectively, in CDCl3
solution. FT-IR/ ATR spectroscopy analysis was recorded in Varian
in several reactions to valorize glycerol.23, 30-32
Inspired by these findings, in this work we investigate the
660 FT-IR Spectrometer. The spectroscopic data of the glycerol
catalytic activity of tin(II) halides in reactions of urea glycerolize
carbonate are shown as follow (Fig 1).
to synthesize glycerol carbonate. Although our initial intention
has been to assess the activity of tin(II) halides in carbonatation
of glycerol with urea, we have found that at reaction conditions
studied, those compounds were converted to Sn(OH)2.
Therefore, we demonstrate that Sn(OH)2-catalyzed reactions
were a selective and straight synthesis pathway to obtain
glycerol carbonate from urea and glycerol. The effects of main
Fig.
carbonate)
1
4-(hydroxymethyl)-1,3-dioxolan-2-one
(glycerol
reaction parameters (i.e. temperature, molar ratio of urea to
glycerol, nature of pre-catalyst, catalyst concentration) were
investigate in solvent-free processes carried out under air flux.
FT-IR/ ATR (ν (cm-1)/ attribution): 3365/ (ν OH); 2881-2935/ (ν CH2
and CH); 1774/ (ν C=O cyclic 5-membered carbonate); 1170/ ν C-C);
1043/ (ν C-O of C-OH bond).
GC-MS ((m/z)/relative intensity): 88/ 22, 87/ 31, 44/ 95, 43/ 100, 31/
77 and 29/ 34.
1H NMR (300 MHz, CDCl3) δ (ppm): 3.60-3.78 (m, 1H, H-6’*), 3.89-
4.08 (m, 1H, H-6*), 4.24-4.63 (m, 2H, H-5), 4.65-4.96 (m, 1H, H-4),
5.43-5-48 (m, OH). *signals may be exchanged.
13C NMR (75 MHz, CDCl3): δ (ppm); 61.8 (C-5); 65.9 (C-6); 76.7 (C-4);
155.5 (C-2).
Experimental
Chemicals
All chemicals and solvents purchased from commercial sources and
used as received. All tin (II) salts (SnCl2 (ca. 98 wt. %), SnF2 and SnBr2
(ca. 99 wt. %)) were acquired from Sigma-Aldrich, as well the glycerol
(ca. 99.5 wt. %). Urea was GE (99.5 wt. %). Methyl alcohol was Vetec
(ca. ≥ 98.9 wt. %).
Sn(OH)2-catalyzed urea hydrolysis reactions with glycerol
Characterization of Sn(OH)2 generated in situ in the reactions of
urea glycerolize
The catalytic runs were performed in a 25 mL three-necked glass
flask, equipped with sampling system, a reflux condenser, in
thermostatic bath with magnetic stirrer. Typically, urea (34.3 mmol)
was added to pure glycerol (34.3 mmol), which were stirred and
heated to 413 K; then, after the adding Sn(II) pre-catalyst (ca. 4.9 mol
%), the reaction was started. The air flow used was 415 cm3min-1.
The tin content on the solid recovered from the reaction was
confirmed by AAS using an Agilent Atomic Absorption Spectrometer,
model Spectra 240FS AA. Samples were digested with HCl at room
temperature. Thermal gravimetric analysis (TGA) of the solid
generated during the reaction was conducted by a Simultaneous
Thermal Analyzer (STA) 6000 Perkin Elmer (Fig.SM2). To do it, the
solid sample (ca. 3.990 mg) was heated from 30.0 to 850.0 ºC at a
rate of 10.0 ºC min-1, under N2 flux (50.0 mL min-1). FT-IR/ATR
spectroscopy analyses were carried out in Varian 660 FT-IR
Spectrometer (Fig. 2). To verify the possible presence of organic
compounds in the solid formed during the reactions, elemental
analysis was done using a Perkin-Elmer CHN analyzer.
The structural properties were measured on the Quantachrome
NOVA 1200 apparatus, using the vacuum degasser at 80 °C for 5 h.
The BET surface area was determined from MultiPoint BET. The size
and pore volume distribution were obtained from the BJH model
(Table SM1).
Reaction monitoring
The reaction progress was followed taking aliquots at regular
intervals and analyzing them via gas chromatography (Shimadzu GC-
2010 Plus, FID), fitted with RTX-Wax capillary column (0.25 m x 0.25
mm x 30 m)). The temperature program was as follows: 150° C/ 3
min, 10 ° C/min up to 230 ° C, then 5 ° C/min up to 250 ° C hold time
of 3 min. Both injector and detector were kept at 250 °C
temperature, respectively. Hydrogen was the carrier gas (ca. 1.2 mL
min-1). The conversion and reaction yields were calculated by
matching the GC peak areas of the pure compounds (i.e., glycerol and
glycerol carbonate, respectively) in the calibration curves. Because it
2 | J. Name., 2012, 00, 1-3
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Catalytic tests: a quick in situ conversion of hVoiemwoArgtieclne eOonluinse
catalyst SnCl2 to solid catalyst Sn(OH)2
Results and discussion
General aspects
DOI: 10.1039/C8NJ05635H
Tin(II) halides have been efficiently used as catalysts in several
reactions to add value to the glycerol such as esterification and
ketalization.34-36 However, as far we know, they were not
assessed in glycerol carbonatation reactions with urea. Herein,
initially we carried out the reactions in the presence SnCl2 as
catalyst (Fig. 2).
The carbonatation of glycerol with urea is a reversible reaction
(Scheme 1), which generally requires temperatures higher than
413 K.21 Although at these temperatures, it occurs without a
catalyst, its presence is a key aspect for that a high yield of
glycerol carbonate can be obtained.
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glycerol conversion
glycerol carbonate
90
80
75
SnCl2/Sn(OH)2
60
45
30
15
0
60
40
20
0
Scheme 1. Glycerol carbonate formation via urea glycerolize
Table 1. Metal-catalyzed urea glycerolize reactions
no catalyst
Catalysts
(mol %)
aAu/
Con.
(%)
Sel.
(%)
Time
(h)
T
(K)
Removal
NH3
Ref.
17
0
1
2
3
4
blank-reaction
SnCl2/Sn(OH)2
time/ h
(a)
(b)
support/
(0.02 Au3+)
aZnSO4/ (1.0
Zn2+)
aMgO/ (4.14
Mg2+)
81
68
4
423
N2 flux or
vacuum
Fig. 2 Kinetic curves of urea glycerolizes in the presence and
absence of SnCl2 (a) and conversion/ glycerol carbonate
selectivity (b)a
aReaction conditions: glycerol (34.3 mmol); urea (34.3 mmol);
temperature (413 K); Sn(II) concentration (4.9 mol %); air flow
(415 cm3min-1).
83
59
58
37
4
4
423
423
vacuum
(14.7 KPa) 30
b(RIm)2ZnX2
/ (1.0 Zn2+)
93
75
93
77
6
8
413
423
bNa4SiW12O4
0/MCM-41
We have found that the after 2 min, the addition of the SnCl2
triggered the formation of a white solid. The solid formed was
removed from the reaction, washed with hot water, dried at
373 K, and identified by elemental analysis (i.e., AAS), and FT-IR
spectroscopy analyses as Sn(OH)2 (Scheme 2). AAS analysis of
liquid phase collected from reaction indicated the absence of
tin(II), which reinforce the total presence of the tin in the
formed solid.
N2 purge
31
(0.1
4-
SiW12O40
)
bZn2+ salts/
(2.0 Zn2+)
bLa2O3/
85
69
93
98
2
1
423
413
2.67 kPa
3 kPa
21
32
(0.37 La3+)
bPS-
(Im)2ZnX2/
(0.3 Zn2+)
bSnO2/WO3/
(6.0 Sn4+)
bZn2+ salts
(3.7)
93
93
6
413
14.7 kPa
23
Reduced
pression
52
84
87
95
97
85
4
3
4
413
413
27
33
3 kPa
Sn(OH)2/
This
work
413
Air flux
(4.9 Sn2+)
aGlycerol: urea molar proportion = 1:1.5; Glycerol: urea molar
b
proportion = 1:1
Several solid supported metal catalysts or soluble have been used in
reactions of glycerol carbonatation with urea. Table 1 summarizes
some important examples. Different approaches have been used to
remove ammonia from reaction medium (Table 1). Zinc,
molybdenum, tungsten and tin are the most used catalysts in
reactions to synthesize glycerol carbonate (Table 1). In general, the
reactions are carried out at temperatures of 413 or 423 K, under
reduced pressure or vacuum, being the ammonia removed
throughout the process (Table 1).
Scheme 2. Formation of Sn(OH)2 from reaction of SnCl2 with
urea at 413 K in net glycerol
Figure 3 shows a comparison of FT-IR/ATR spectra obtained
from two samples of Sn(OH)2; the first one formed on the urea
glycerolize reaction and second sample that was previously
synthesized.
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