Synthesis of glycerol carbonate over a 2D coordination polymer built with Nd3+ ions and organic ligands

Article abstract of DOI:10.1039/c8dt01803k

In this work, a two-dimensional coordination polymer was synthesized and the structure was determined by single-crystal X-ray diffraction. The crystal structure belongs to the space group Pna2₁ and was characterized by Raman and FT-infrared spectroscopy, powder X-ray diffraction and Brunauer-Emmett-Teller surface area analysis. Catalyst activities were evaluated through the synthesis of glycerol carbonate from glycerol and urea using a batch reactor. After the optimization of both reaction and reaction conditions, the activity results showed that the coordination polymer used as a heterogeneous catalyst has good values of conversions and selectivity for the manufacturing of glycerol carbonate in a fine-chemical process. The analysis of powder X-ray diffraction and spectroscopy for the coordination polymer employed, before and after the reaction, shows that some changes have taken place in the crystal structure during the process, in spite of a recovery at the end of the reaction. The advantages and limitations of the coordination polymer were discussed and compared with those of the previous heterogeneous catalysts in the literature.

Full text of DOI:10.1039/c8dt01803k

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2018, DOI: 10.1039/C8DT01803K.
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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
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PAPER
Synthesis of glycerol carbonate over 2D coordination polymer
building with Nd3+ ions and organic ligand†
Chris H. J. Francoa, Poliana Chagasb, Gabriela S. Caldeira c, Luiz C. A. Oliveira b, Patterson P.de
Souza c, Alexandre A. Leitão a, Giovanni W. Amarantea, Renata Diniz*a,b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
In this work, a two-dimensional coordination polymer was synthesized and the structure was determined by single-crystal
X-ray diffraction. The crystal structure belongs to space group Pna21 and was characterized by Raman and FT-infrared
spectroscopy, powder X-ray diffraction and Brunauer-Emmett-Teller surface area analysis. Catalyst activities were
evaluated though the synthesis of glycerol carbonate from glycerol and urea using a batch reactor. After the optimization
of both, reaction and reactor conditions, the activity results showed that the coordination polymer used as a
heterogeneous catalyst has good values of conversions and selectivity to the manufacturing of glycerol carbonate in a fine-
chemical process. The analysis of powder X-ray diffraction and spectroscopic for the coordination polymer employed,
before and after reaction show that some changes have taken place in the crystal structure during the process, in spite of
recovering at the end of the reaction. The advantages and limitations of coordination polymer were discussed and
compared with previous heterogeneous catalysts in the literature.
as Pd, Au, Ru, Cu, Pt, Ni and Ag17, 22-24. On the other hand, the
low thermal stability when compared to oxides represents the
major disadvantage and limitation in industrial processes using
1 Introduction
In the last years, a rapid growth has been observed in the
building of uni- (1D), bi- (2D) or three-dimensional (3D)
coordination polymers1, 2, not only due to their structural
properties, which support both, academic and industrial
such coordination polymers25, 26
.
A large number of different coordination polymers referred
to in the literature as metal–organic frameworks (MOFs) are
known and can be used in a wide-ranging for a variety of
heterogeneous catalytic reactions27-29. For example, the MIL-
101 can be applicate in heterogeneous catalysis due to a
pseudo-octahedral coordination sphere around chromium ions
in the crystalline structure constructed by oxygen atoms from
benzene-1,4-dicarboxylate and water molecules or a hydroxyl
group in some cases. The coordinated water molecules can be
easily removed with increasing temperature and chromium
ions may be available for use as catalytic sites in organic
reactions such as cyanosilylation of aldehydes30, phenol
hydrogenation31, hydrogenation of CO2 to formic acid24 and
hydrogenation of furfural to tetrahydrofurfuryl alcohol32.
Many processes for obtaining and synthesizing new
materials in the chemical industry are based on catalytic
studies, but also to a great interest in its potential applications
5
in several areas, such as, absorption3 and gas storage4,
,
separation processes6,
, , magnetic
nonlinear optics8-10
7
properties11-13 and catalysts14-16. In this context, different
coordination polymers have been widely used for the
production of materials with pores that have different shapes
and sizes17-19. The possibility of the incorporation of the active
metals or distinct functional groups inside the pores allows the
applications in heterogeneous catalysis20. So, the metal sites
available in these coordination polymers can be used as Lewis
acid to catalyze organic reactions16, 21. The catalytic activities
may still be provided by functional groups added by post-
synthetic modification of the organic ligands20 or may be
introduced by the incorporation of metal nanoparticles, such
processes derived from Zeolites33-36
research for new materials with catalytic properties is
important and represents promising field for further
; consequently, the
a
investigation. However, the development of an efficient
catalyst with an easy synthesis process, easy recovery and
reuse without loss of the catalytic activities has been major
practical challenge in the industry; most of the recent
successful developments are basically restricted to the
employment of supported oxides. Several coordination
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polymers described in the literature have potentials for
This work shows the synthesis, characterization and
catalytic activities in an extensive variety of organic evaluation of the catalytic properties of a coordination
reactions37-39; for example, for the conversion of benzyl alcohol polymer derived from a neodymium ion and sulphocarboxylic
to benzaldehyde with hydrogen peroxide in aqueous ligand, in addition to describing the structural properties and
42
solution40, 41, isomerization of terpene derivatives33,
,
shows a relationship with the potential of application of this
44
benzaldehyde cyanosilylation43,
and among others material in conversion reactions of commercial glycerol in
41,
reactions16,
45. The improvement of catalysts and new higher value-added products when compared with other
materials for applications in chemical reactions is very positive; catalyst materials existent in the literature.
currently, 90% of all processes in the chemical industry have
employed at least one catalytic process to obtain the
manufactured product46, 47. Various catalytic processes have
emerged in recent years due to a need, in most cases
economic, such as the use of many compounds in the
petroleum cracking48 and gasoline desulphurization
processes49. Thus, the economic need of a given process
and/or product drives research in different areas and
materials. In recent decades, the rise in the price of oil barrel
in conjecture with a demand for an alternative to less polluting
fuels has strongly improved the biofuels industry worldwide.
The large development of the biodiesel industry from biomass
leads to the formation of glycerol as a low-value by-product of
the reaction. The increasing abundance of glycerol in the
market has led to a drastic reduction in prices since 2016, and
in 2014, the price of the tonne of crude glycerol was estimated
at US$ 24050 and when it is imported from China can reach
2 Experimental
2.1. Syntheses of compound Nd-5sis:
A solution of 10 mmol (2.682 g) of 5-sulfoisophthalic acid
monosodium salt and 10 mmol (3.214 mg) of hydrated neodymium
acetate was prepared in 80 mL of water and transferred to a Teflon
vessel where 80 mL of ethanol was added. The solution was placed
in an autoclave under hydrothermal conditions at 160 °C for 20
hours accompanied by a cooling rate of 1.5 °C h-1 until room
temperature. At the end, it was observed the formation of pink
single-crystals suitable for analysis by single-crystal X-ray
diffraction. Yield 85% relative to the weight of all reagents. CHN
Anal. Calcd. for NdC8H11O11S (%): C, 20.91; H, 2.41. Found: C, 20.96;
H, 2.63.
2.2. Characterization
values from 50 to 70 US$/Ton51, 52
.
Single-crystal data was collected using an Oxford GEMINI A-
Ultra diffractometer with MoKα (λ = 0.71073 Å) radiation at room
temperature (298 K). The data collection, cell refinements and data
reduction were performed using the CRYSALISPRO software62 and
the structure was resolved by direct methods and refined by
SHELXL-2014 program63. All non-hydrogen atoms were refined with
anisotropic thermal parameters. H atoms connected to carbon were
placed in idealized positions and treated by rigid model, with
Uiso(H) = 1.2Ueq(C) and H atoms from water molecules were
refined with O—H = 0.85 Å and Uiso(H) = 1.5Ueq(O). The figures
In this context, the conversion of glycerol to higher value-
added products has attracted the attention of different
researchers who are trying to explore the possibilities of
conversion to different products, such as, 1,3-propanediol,
acrolein and gasoline additives53. However, the product which
has attracted most attention in recent years is glycerol
carbonate; it can be used in fine chemicals industry to find
reaction intermediates, it can also be used as bio-lubricants,
plastic, semiconductor, polymer, petrochemical and cosmetic
industries54, 55. This great versatility transforms glycerol
carbonate into a high value-added product, with a market
price of up to $ 8141/ton. Due to the high cost of production,
until now, despite the different applications, the use is quite
limited to a production of few Kilotons per year 55. Different
routes using commercial glycerol have been used for the
production of glycerol carbonate in the last years, among them
are reactions with phosgene, dimethyl carbonate, ethylene
carbonate, carbon dioxide55, 56 and reactions with urea57. All
reactions generally have routes involving both, homogeneous
and heterogeneous catalysts derived from acids, bases,
carbonates, ionic liquids, enzymes in addition to metal oxides
and hydrotalcites doped with transition metals 55, 57-59. The use
of polymeric materials is very limited and is restricted only to
the use of some resins60 and a use of transition metal polymers
for condensation of glycerol with acetone can still be seen61.
The compact literature in the field of coordination polymers
for application in the route of glycerol carbonate production
makes evident, at first, a limitation of the use of these
materials, therefore, represents a great challenge in both
academic and industrial areas.
were drawn using ORTEP-3 for Windows64
,
Mercury65 and
Topos4.066 programs. X-ray diffraction patterns of the compound
were recorded on an Advance DaVinci D8 diffractometer with
Bragg-Brentano θ-θ geometry using CuKα (1.54056 Å) at 40 kV and
40 mA and LynxEye linear detector. The measurements were
collected in the 2θ range from 5° to 60°, in steps of 0.02° and
accumulation time ranging from 0.5s to 1s by steps with a
divergence slit of 0.6 mm and primary and secondary Soller slits of
2.5°. X-ray diffraction patterns performed with temperature
variation were collected under the same conditions, using the MRI
temperature module coupled to the diffractometer with
temperature ranging from 25 °C to 700 °C and a heating rate of 10 °
min-1.
Elemental analysis for C, H and N were carried out on Perkin–
Elmer 2400 CHN analyzer. Fourier-transform Raman spectroscopy
was performed using a Bruker RFS 100 spectrometer with Nd3+/YAG
laser operating at 1064 nm excitation line in the near infrared
region using an N2 liquid and CCD detector with a spectral
resolution of 4 cm-1.
The infrared spectra were recorded on an FT-IR Perkin-Elmer
FRONTIER spectrometer using KBr pellets in the range 4000–400
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cm-1 with an average of 128 scans and 4 cm-1 of spectral resolution.
In order to compare the activities of catalyst, reactions were
The thermogravimetric data were recorded in a Shimadzu DTG60 carried out in the absence of a catalyst and using ZnSO4 calcined at
thermobalance with a heating rate of 10 °C min-1 to a temperature 300 °C for 3 hours. The products were analyzed using a gas
of 700 °C in a dynamic atmosphere of air and atmosphere of N2 chromatograph-mass spectrometer Agilent 7190-5975 system. The
with a flow of 100 mL min-1.
samples were diluted in methanol 1:250 and a 0,1 µL was injected
at 250 °C of temperature and 10:1 split ratio. The temperature
program was 80 °C for 5 min and then 3 °C min-1 until 110 °C, with a
capillary column HP-5MS. The area of glycerol was integrated with
the T.I.C. signal and a 5 steps calibration curve was used to
determine the glycerol conversion. The Linalool and cyclohexanol
were used as internal standard for quantitative conversion results.
Selectivity results were obtained with the relative area of products
formed. The 13C spectra were collected on an Avance DRX400 NMR
spectrometer at 300 K, 400 MHz as specified. The Nd-5sis leaching
study was performed on a Ray Ny EDX-800 Shimatzu X-Ray
Fluorescence equipment.
The specific apparent area data were estimated by the B.E.T
method (Brunauer, Emmett and Teller)67, 68 and were acquired from
a Quantachrome Autosorb1 equipment. The measurements were
performed using nitrogen gas at 78 K, where the compound Nd-5sis
was degassed at 130 °C for 24 hours before analysis.
The surface proprieties were estimated by a model of reaction
which involves the reaction of the water molecules on the surface
and hydrogen peroxide in a solution of acetonitrile. These studies
were performed using a system with a fixed temperature (T=25.7
°C), a pressure of approximately 0.899 atm, 1.0 mL of hydrogen
peroxide 30% (v/v) and a fixed volume of 15.0 mL for the system.
The use of acetonitrile (V = 14.0 mL) as a solvent allows contact
over the entire surface of Nd-5sis, thus avoiding limitations related
to mass diffusion effects.
3 Results and Discussion
3.1. Structural description
2.3. Catalytic testing
The crystallographic data for compound Nd-5sis are shown in
Table 1. The compound Nd-5sis was crystallized in the
orthorhombic crystalline system and a non-centrosymmetric space
group Pna21 and presents a deprotonated 5sis ligand and an Nd3+
ion in the crystal structure, Fig 2. The Flack parameter is at 0.18(5),
A schematic illustration of the experimental apparatus is shown
in Fig. 1. Catalytic tests were performed in a batch reactor 1 (150
mL) which has a controlled rotation system 2 (130 rpm) for better
distribution of heat between reagents and catalyst inside reactor, a
tank 3 (500 mL) with temperature control and containing a heating
oil bath, a thermometer 4, a trap system 5 with hydrochloric acid (1
mol L-1) to capture ammonia produced in the reaction and a
vacuum pump 6 for removing ammonia with a pressure value at 10
kPa. Compound Nd-5sis was used as a heterogeneous catalyst and
commercial glycerol, urea and solvents were purchased from
Aldrich. In a typical experiment glycerol (12.61 g), urea (8.225 g)
and compound Nd-5sis (0.250 g) were placed in a batch reactor and
heated in bath oil at 140 °C with constant stirring (130 rpm) under a
reduced pressure. After 90 minutes of reaction, methanol was
added and the resulting solution was centrifuged at 3500 rpm for
15 minutes. Then, the compound Nd-5sis (catalyst) was separated
by filtration. The method used was reproduced; however, a heat
pre-treatment was performed for the compound Nd-5sis. 250 mg of
Nd-5sis was left in a muffle with a heating rate of 10 ° min-1 to a
temperature of 160 °C for 2 hours. After this time, the dehydrated
compound, as denoted Nd-5sis-desh, was added to the reaction
mixture.
showing that the absolute structure is correct for this compound69
.
It is observed that Nd3+ ion is coordinated to nine oxygen atoms,
four of which are derived from water molecules (Nd1-O8, Nd1-O9,
Nd1-O10 and Nd1-O11) with average distance Nd-O of 2.498(11) Å
Table 1 Crystal data and refinement parameters for compound Nd-
5sis.
Compound
Formula
Formula weight (g mol-1)
Crystal system
Space group
Nd-5sis
C8H11NdO11
459.47
S
Orthorhombic
Pna21
a(Å)
7.3216(5)
16.6789(9)
10.4430(5)
90
b(Å)
c(Å)
α = β = γ(°)
V(Å3)
1275.26(13)
4
Z
Crystal size (mm)
µ (Mo-Kα) (mm-1)
Flack parameter
Refl. measured/independt
Observed refl.[F2obs > 2σ(F2obs)]
No.of parameters refined
R(F0), WR (F02), S
0.20 x 0.18 x 0.11
0.71073
0.18(2)
9010 / 3089
2622
137
0.035, 0.140, 1.14
1.72, -0.72
Fig. 1 Schematic illustration experimental system.
Δρmax, Δρmin (e Å−3
)
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By the map, it is possible to see that Nd3+ ion presents a much
greater contribution when compared with the carbon, oxygen and
sulfur atoms present in the structure, data coherent with the
number of electrons of the atoms. In this sense, the intensity of the
reflections in the crystal is attenuated by the absorption effect of
Nd3+ ions in the structure. If the absorption correction is not
applied, it is possible observed high levels of anisotropic
displacement or negative (physically impossible) to different atoms
and with a high residual electron density near the Nd3+ ions.
Furthermore, even if the atomic positions of the heavy atoms (Nd3+)
are not affected during the refinement, it is noted that the position
of the light atoms such as carbon, oxygen and sulfur is
compromised due to the formation of several scattered electron
density peaks in the electron density map around these atoms. Fig.
S3† shows the residual electronic density map of Nd-5sis after
refinement process. The correction of empirical absorption using
spherical harmonics71 and considering strong absorption centers
were fundamental for the determination of the crystalline structure
in the compound Nd-5sis. Although it is still possible to verify a
marked residual value of untreated electron density around the
Nd3+ atom (1.72 eÅ-3), thus indicating that, even with the absorption
treatment in the reflections, the model is not yet the most
appropriate for the structure. The compound Nd-5sis is
isostructural to a series of compounds which were synthesized in
the literature by Liu and co-workers with Ho, Dy, Tb, Sm, Pr ions and
5sis ligand72. By crystallographic data of the compounds, it can be
seen that the crystalline structures, in spite of presenting statistical
data acceptable to the International Union of Crystallography
(IUCr), show some limitations, mainly in the anisotropic parameters
of some carbon and oxygen atoms that have been refined
isotropically due to negative values of atomic displacement, or even
the various restrictions applied in order to minimize such problems
in the refinement. All structures have high values of the absorption
coefficient and, mainly, high values of residual electronic density,
except for Ho (0.615 eÅ-3) and Pr (0.754 eÅ-3), indicating that the
effect of absorption is very evident in these type of compounds
involving lanthanide atoms. However, the electron density
accumulation enveloped by these ions in different materials
represents a potential for the application of these compounds in
the catalysis area and may be exploited in some organic reactions,
Fig. 2 The crystal structure for Nd-5sis. Symmetry code: (i) x, y, z–1
e (ii) -1/2–x, 1/2 +y, z–1/2. Some hydrogen atoms were omitted for
more clarity and the ellipsoids were drawn with 50% probability.
Fig. 3 Packing for Nd-5sis. (a) Extension of the 2D polymer along of
bc-plane. Colour code: C: gray, O: red, S: yellow, Nd: light pink. (b)
Formation of the 3D supramolecular arrangement of polymer along
of a-axis. Some atoms have been omitted for better visualization.
75
such as Diels-Alder reactions73 and π-allyl bond formation74,
and other atoms from the carboxylate (Nd1-O1ii, Nd1-O2ii, Nd1-O3i,
Nd1-O4i) and sulfonate groups present in the organic ligand (Nd1-
O6). The crystalline structure of Nd-5sis leads to the building of a
two-dimensional polymer (2D) extending along the crystallographic
bc-plane, are as shown in Fig 3(a). Thus, the structure of Nd-5sis is
defined as a uni-nodal (3)-connected and can be described as a
point symbol 63 and is deposited in RCSR as a net of type hcb, with
where the products of reactions depend on the kind of added rare
earth element76-81
.
3.2. Thermal stability studies
The results of the thermogravimetric analysis for compound Nd-
5sis are displayed in Fig. S4† and S5†. When the compound Nd-5sis
was heated in air at a rate of 10 °C min-1, two events of weight loss
are presented. The first event occurs in the temperature range from
110 to 160 °C accompanied by an endothermic event at 150 °C. The
weight decrease (15%) observed is in good agreement with the
removal of four mols of water with a relative deviation of
approximately 5%, and it is in agreement with CHN elemental
analysis. The compound Nd-5sis shows thermal stability between
160-550 °C; however, an oxidative process is observed which origins
a slight weight increase in this temperature range. The second
hexagonal
symmetry
P6mm70
.
Three-dimensional
(3D)
supramolecular arrangement in the compound is formed mainly by
moderate hydrogen bonds between the 2D sheets along of the
crystallographic a-axis with an average distance of 2.915(13) Å, Fig
3(b). In Fig. S1† and S2† (supplementary material), it is possible to
observe the electron density map (eÅ-3) neighboring of Nd3+ ion and
the plane of the aromatic ring of 5sis ligand.
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weight loss event may be attributed to a thermodecomposition that depending on the crystalline phase. These results are in agreement
causes the collapse of the crystal structure. The DTA curve shows an with thermal analysis data in an air atmosphere, where the
exothermic event at 604 °C, where the calculation of weight loss of oxidation processes are observed in the region of 160 °C to 500 °C.
40% indicates that the residue is related to oxidation of the However, a heating and thermodecomposition of the sample lead
products with the formation of 1/2 mol of neodymium oxysulphate, to the overlap of different diffraction peaks. In order to check the
Nd2O(SO4)2, with a relative deviation approximately of 6%. In Fig overlaps, a CHN elemental analysis of the sample was made during
S5†, it is observed that the thermodecomposition curve in nitrogen the formation of the different crystalline phases. The chemical
atmosphere has two events of weight loss. The first event of 15% in composition of the products formed during sample, heating
the temperature range from 110 °C to 180 °C accompanied by an together with the experimental observations of X-ray and thermal
endothermic event at 155 °C may be attributed to the loss of four analysis, suggests coherence in relation to crystalline phases
mols of water molecules (relative deviation of 5%). No weight formed, results are shown in Table 2.
change was observed in the range from 180 °C to 550 °C. The
second stage of weight loss of 35% with a slightly exothermic event
3.3. Structure and surface effects
at 622 °C refers to the formation of 1/2 mol of neodymium
oxysulphate, Nd2O(SO4)2, (relative deviation 2%). Comparative
One of the complications in working with the heterogeneous
results of the thermogravimetric analysis in air and nitrogen catalyst is a need to increase and stabilize the surface acidity, so a
atmosphere show that the thermodecomposition occurs in a similar study of the quantification of the sites that cover the internal
way in which the compound Nd-5sis is thermally stable at and/or external surface of the solid is usually necessary85, 86. Several
approximately 550 °C. But, it is noted that the two methods are used to characterize and quantify the acid-base sites,
thermodecomposition patterns are quite different in the some of these used for site density determination and others for
temperature range of 180 °C to 550 °C due to an oxidation process site nature characterization, such as programmed thermal
88
which results in a slight weight increase. Oxidation process in air desorption87, and spectroscopy in the infrared region89-91. Both
atmosphere is related to the formation of carbonates and techniques are well established for the characterization of inorganic
oxycarbonates of neodymium with dehydration of the crystal materials and oxides used as catalysts92-94
. However, for
structure.
coordination polymers, these techniques generally show a lot of
limitations, since the use of high temperatures are required or the
formation of numerous adsorption bands occurs which overlaps the
marker bands of the probe molecules used. In this sense, a model of
reaction might be used to determine and quantify more exactly the
superficial properties of coordination polymers such as Nd-5sis.
According to the X-ray analysis, the thermodecomposition of
the compound Nd-5sis can also be separated into three steps which
are related to the formation of the different intermediate
crystalline phases of carbonates and oxycarbonates of neodymium
after the loss of the water molecules. Polycrystalline diffraction
patterns of Nd-5sis at different temperatures (heating rate of 10 ° C
min-1) are shown in Fig 4 and indicates that the compound exhibits
a stable crystalline phase at 130 °C. The increase in temperature
allows the removal of the water molecules and a reduction of the
crystallinity of the compound with the formation of intermediate
crystalline phases as shown in Fig 5. Phases of Neodymium
carbonate intermediates, Nd2(CO3)3 (orthorhombic, Pccn) and
Nd2O2CO3 (hexagonal, P63/mmc)82, with different initial ratios
83
appear in 160 °C. The thermal treatment of the compound up to
300 °C allows a gradual reduction of neodymium carbonate phase,
which can be accompanied by a reduction of the intensity of
diffraction peak at 2θ = 19.42° (CuKα) besides a slight increase of
the neodymium oxycarbonate phase (2θ = 11.09°, 2θ = 23.41° and
2θ = 26.80°, CuKα). It should be noted that the formation of
oxycarbonate phase from Nd-5sis occurs much more softly (160 °C-
200 °C) when compared to previous reports found in the literature
(450 °C) for thermodecomposition of carbonate phases of rare
earth82. At 350 °C, the formation of a neodymium oxysulphate
phase is observed (2θ = 17,65°, 2θ = 34,50° and 2θ = 36,20°, CuKα)
and it is possible to observe that the crystal structure of Nd-5sis
remains partially crystalline up to 400 °C (peak at 2θ = 13.07°) and
above 450 ° C, only the neodymium oxysulfate phase is observed.
The decomposition of neodymium oxysulphate to neodymium
oxide was not detected during the experiments and it is consistent
with data found in the literature84, in which the formation of Nd2O3
is observed at temperatures between 900 °C and 1100 °C
Fig. 4 Diffraction patterns for the compound Nd-5sis with a
temperature range of 30 to 650 °C.
Fig. 5 Scheme of formation of different crystalline phases with the
heating of the compound Nd-5sis.
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Table 2 Chemical analysis of the products formed with the thermodecomposition of Nd-5sis
.
Water of
hydration in the
sample
Heating
temperature / °C
Composition
C%
H%
Crystalline
phase* (%)
30
Nd(5sis).4H2O
Experimental
Calculated
Relative error / %
Experimental
Calculated
Relative error / %
Experimental
Calculated
20.96
20.91
2
21.24
21.41
1
23.19
21.72
6
2.63
2.41
8
2.63
2.03
22
1.51
1.16
23
100
160
200
Nd(5sis)
Nd2(CO3)3
98
2
3
1
Nd(5sis)
Nd2(CO3)3
Nd2O2CO3
90
5
5
Relative error / %
*Crystalline phase estimated by the analysis of the chemical composition of the products
The quantification of Nd3+ sites acting as Lewis acid on the
Based on crystallographic data together with the elemental
surface of Nd-5sis can be determined by considering a chemical analysis data, it is possible to calculate the amount of Nd3+ sites
reaction between water molecules on the surface of Nd-5sis and noting that there is a proportion of 4 molecules of water per Nd3+
-
H2O2 species, which allows the formation of OOH and H3O+ ions, as site in the crystal structure and consequently 4 molecules of H3O+
shown in Fig 6. Based on empirical observations, a model was formed in the system per Nd3+. As a consequence of the use of
proposed and a mathematical equation was obtained that describes this model of reaction, when Nd-5sis weight is equal to zero,
the number of active sites in the relation to the amount of Nd-5sis, the pH assumes pH value of the solution, pH = 3.35, and this
considering some limitations. A plot of pH versus the amount of Nd- initial value must be considered to obtain the mathematical
5sis was constructed and it is shown in Fig 7(a). For a constant value equation 1 (eq.1).
of the H2O2 solution (1 mL), it is observed that, when the amount of
Nd-5sis increases, pH of the solution is reduced, and after 50 mg of
ꢌꢍ
ꢀꢁꢂꢃ
ꢈ 5 ꢊ 4 . 10 ꢎ 40 ꢊ 2 . 10ꢌꢏ. ꢐ
ꢉeq. 1ꢋ
ꢉ
ꢋ
ꢉ
ꢋ
ꢄꢅꢆꢇ
Nd-5sis, the pH remains practically constant and suffers small
oscillations. It is known that hydrogen peroxide solution employed
in the experiment is commercially acidified (2 ≤ pH ≤ 4) so that
there is a chemical equilibrium, Fig 7(b), in order to maintain a
constant concentration of H2O2 species in the system.
Where ꢀꢁꢂꢃ represents the number of Nd3+ sites on
the surface of Nd-5sis in mols and
ꢄꢅꢆꢇ
w the amount of compound
in grams. It is also perceived that at the limit where the weight
tends to zero, the comportment of the reaction assumes
values close to real values and there was not restoring effect
of the H2O2 species. As a result, considering the equation
obtained by fit linear analyses of the graph in Fig 8, the
compound Nd-5sis has a number of Nd3+ sites in the range of
38.01 to 42.09 μmol per one gram.
Thus, it is experimentally observed that increasing the
concentration of Nd3+ sites (weight of Nd-5sis) on the surface
leads to a higher consumption of H2O2 species and a formation
of H3O+ ions. At the moment that an excess of H3O+ ions occurs
in the system, the chemical equilibrium in solution is displaced
in order to restore the concentration of H2O2 species in the
system, attenuating variation of pH of the solution, Fig 7(b).
Thus, after addition of 50 mg of Nd-5sis, it is possible to see
that the equilibrium works as a buffer effect in the system
avoiding pH variations in the solution. So, a linear function of
the amount of Nd-5sis and number of Nd3+ sites present on
the surface can be obtained considering the number of H3O+
ions formed in the system in the range of 5 to 35 mg of Nd-
5sis, as a shown in Fig 8.
Fig. 7 (a) Graph of the variation of pH in relation to the amount
of Nd-5sis and (b) chemical equilibrium in peroxide solution
30% (v/v).
Fig.
molecules and hydrogen peroxide on the surface of compound
Nd-5sis
6 Reaction model for the reaction between water
.
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To support a chemical reaction with a catalyst, the desired
requirement is that the step controlling the reaction is the
chemical kinetics. However, during the process several other
physical and chemical phenomena may occur, in addition to
phenomena of diffusion and mass transfer from bulk phase to
active site. A great difficulty in determining of kinetic data in
heterogeneous catalysis is the external effect of heat and mass
transfer, especially into the pores of catalysts with high specific
area. At first, it is important that initially, to define the
conversion values of glycerol, it is necessary to estimate these
catalytic phenomena involved in the process and consequently
specify how much these terms are significant or not to
estimate the real conversion values. As previously mentioned,
Fig. 8 Graph of the amount of Nd-5sis and number of Nd3+ reaction temperature is an important factor affecting the
sites on the surface in the range of 5 to 35 mg.
conversion values and reaction rate by increasing the
equilibrium constant in addition to contributing to the
Therefore, using this data, it is possible to determine the alteration of the viscosity of the mixture, decreasing the
rate of reaction per active site (TOF - Turnover frequencies) diffusion process and increasing the contact between the
from consumption values of glycerol in the catalytic tests reactants and the catalyst particles. An increase in
performed.
temperature for such a reaction above 140 °C may result in a
loss of thermal stability of the Nd-5sis catalyst.
3.4. Catalytic activity
However, in this case, a number of kinetic analysis
experiments were performed in the range of 80-280 rpm to
find suitable speed of agitation on percentage of conversion
values. The results are show in Fig S6†. Based on the kinetic
analysis results, it is possible to confirm that the agitation
speed does not directly influence the conversion values, on the
other hand, plays an important role in controlling the process
of diffusion and mass transfer combined with a temperature at
140°C, thus securing the chemical kinetics of the reaction. For
experimental facilities the stirring speed of 130 rpm was
selected for the reaction.
The catalyst tests were made with commercial glycerol,
urea and Nd-5sis as a catalyst, Fig 9. Incorporation of a weak
Lewis acid, such as Nd3+ ions, can favor the reaction, in
addition to decreasing significantly the interaction of reactants
and products formed avoiding surface contamination of the
solid by the slight adsorption of all species in the system. It has
been different from what occurs in the literature that basically
uses Lewis acids derived from transition metals60. The
conversion and selectivity of glycerol to glycerol carbonate are
affected by different parameters of the reaction conditions,
such as temperature and pressure. Then, for a reduced
pressure of 20 Pa and a temperature of 25 °C, the equilibrium
constant has a value of 0.516 x 102 while for a temperature of
140 °C, it has a value of 3.658 x 107 95. Indeed, the increase in
temperature favors the formation of glycerol carbonate,
however, a degradation effect of the product was considered
with a drastic increase in temperature, as well as excessive
energy costs. In order to favor the thermodynamic conditions
of the reaction, the temperature of 140 °C was established as
well as a reduced pressure, based also on what it has been
done in literature57, 96. The experimental results for the
catalytic tests obtained in the absence of catalyst, ZnSO4 and
with Nd-5sis were compared with the conversion and
selectivity data of a series of compounds which have been
prepared and employed as a heterogeneous catalyst for
The effect of catalyst amount of the reaction was
investigated in a range from 0.25% to 2.4% of the catalyst
weight percentage and at 90 minutes for reaction time (Fig
S7†). The increase of Nd-5sis catalyst amount promotes an
increase of glycerol conversion; on the other hand, further
increasing on the catalyst amount to above 1.2% w/w cannot
improve the conversion values. Fig 10 shows glycerol
conversion and selectivity values in the absence of catalyst of
37% and 73%, respectively. It is emphasized that it is not
possible to compare real conversion values in a quantitative
and rigorous way for different catalysts under similar
conditions. However, Fig 10 can be useful for understanding
the maximum potential of the catalysts under their optimized
conditions current in the literature. Catalyst derived from
Zeolites and hydrotalcites doped with Mg2+, Zn2+, Zn2+/Li+ and
59
Fe2+/Mg2+
assume values ranging from 70% to 93% for
synthesis of glycerol carbonate57, 59, 96
.
conversion and in the range from 5% to 88% in terms of
selectivity. Moreover, it is also possible to note that for
neodymium oxide, Nd2O3 96, the conversion and selectivity
results are approximately 30% and 93%, respectively, and
despite the high selectivity, conversion value is comparable to
reaction in the absence of the catalyst and it is indication of a
low catalytic activity, with a yield of 27.5%.
Fig. 9 Scheme of reaction between commercial glycerol and
urea.
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-
However, when comparing the reaction rate (mmol gcatalyst
h-1), Fig 11, the compound Nd-5sis has a reaction rate with
1
reasonable values, thus favoring its application. Wang et al
reports the use of calcined lanthanum oxide, denoted as
La2O3-600, at 600 °C to catalyze the synthesis of glycerol
carbonate from glycerol and urea with a yield of 67.6 % 96. The
experimental data suggests a reaction rate of 1149 mmol
-1
gcatalyst h-1, initially given much higher when compared to Nd-
5sis (278 mmol catalyst-1 h-1).
The reaction rate results show that the reaction with Nd-
5sis may contain several reactive molecules per active site
(μmol g-1), which are much larger than the series of catalyst
which have been tested and employed for synthesis of
carbonate of glycerol.
Fig. 10 Catalyst performance for the synthesis of glycerol
carbonate over different catalyst. Reaction conditions for 3.5. Resonance magnetic nuclear
absence of catalyst: Molar ratio Glycerol/Urea = 1, 1.5 h,
The reaction mixture of the glycerol reaction with urea,
140°C, 10 KPa. For Nd-5sis and Nd-5sis-desh: Molar ratio
Glycerol/Urea = 1, 1.2wt%, 1.5 h, 140°C, 10 KPa and for ZnSO4:
Molar ratio Glycerol/Urea = 1, 4wt%, 1.5 h, 140°C, 10 KPa. For
Nd2O3 and La2O3-600: Molar ratio Glycerol/Urea = 1, 0.4wt%, 1
h, 140°C, 4 KPa. For the other catalysts: Molar ratio
Glycerol/Urea = 1, 5wt%, 5 h, 145°C, 4 KPa.
after separation process of Nd-5sis with methanol, was cooled
and results of 13C NMR analysis for the reaction mixture with
deuterated acetone solvent are shown in supplementary
content, Fig S8†. The presence of four chemical shiꢀ signals (δ)
of 13C in 63.7; 66.4; 71.0 and 159.0 ppm confirm the reaction
of glycerol with urea with the loss of the terminal hydroxyl
group of glycerol and formation of glycerol carbonate,
chemical shifts are in agreement with data reported in the
literature for glycerol carbonate (61, 65 and 70 ppm,
deuterated methanol)53. The presence of urea (δ = 162.7 ppm)
and small traces of possible by-products in addition to glycerol
(δ = 64.0 ppm, δ =73.4 ppm) were observed.
High conversion and selectivity results are found for the
compound La2O3 96, with values of 68.9% and 98.1%,
respectively. For catalyst test using anhydrous zinc sulphate,
the experimental results of conversion were of 71 % while of
selectivity were of 77%. The process of manufacturing glycerol
carbonate by ZnSO4 and other heterogeneous Zn-derived
catalysts has already been described in the literature by Yoo et
3.6. Glycerol consumption rate and glycerol carbonate
production rate in a process
al 57
and presented an economic viability with a yield of 86%
.
glycerol carbonate. The yield results differing from the
theoretical data may indicate that pressure in the batch
reactor still need to be adjusted, since the authors use reduced
pressure to 4 kPa.
It is possible to calculate the rate of production of glycerol
carbonate per unit volume of the catalyst employed, Nd-5sis
,
with experimental data obtained discounting the limitations of
the mass and energy transport phenomena in the system
during the reaction in a batch reactor.
For Nd-5sis, the reaction showed conversion values of 76%
and selectivity of 74%, values similar to reaction with ZnSO4.
When this compound is dehydrated at 160 °C for 2 hours,
designated by Nd-5sis-desh (Fig 10), it has values of 74% and
81% for conversion and selectivity, respectively. The slight
change in the conversion values might be associated with the
dehydration process of Nd-5sis, which allows to formation and
disposal of surface active metal sites that work as a Lewis acid
favoring the reaction or, in combination with X-ray result, the
phases of neodymium oxycarbonate can be found, Nd2O2CO3
in Nd-5sis-desh catalyst. It alters the acidity and basicity of the
surface approving the selectivity results on the reaction
suggesting that these modifications play a significant role in
the activity of the catalyst. When comparing the data with
Nd2O3, it is noted that Nd-5sis has a more active catalyst and
considering the experimental data obtained for conversion and
selectivity of the catalyst tests with Nd-5sis and Nd-5sis-desh it
can be observed that these catalysts have similar values with
several systems described in the literature and presented in
this work.
-1
Fig 11 Reaction rate results (mmol gcatalyst h-1) for different
catalysts. The rates were calculated based on the mole of
glycerol converted per gram catalyst per hour of reaction.
8 | J. Name., 2012, 00, 1-3
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The rate of a chemical reaction using solid-state catalyst
Where ꢓꢔꢕꢖ is the density of the mixture reactional (g
depends directly on the number of active sites on the surface cm3); ꢐꢕ,ꢛꢜꢕꢒꢞꢆꢟꢜ is the initial weight of glycerol (g), ꢠ% is the
of the catalyst and the activity of the catalyst is generally conversion value, ꢙꢙꢛꢜꢝꢒꢞꢆꢟꢜ is the molar mass of the
proportional to the area of the active component in the glycerol, ꢐꢤ is the sum of the weight of reagents (g) used
catalyst per weight unit or per volume unit. Since the surface (glycerol and urea) and ꢥꢆ is the reaction time (s). The number
area per volume unit depends on the size of the particles and of active sites on the surface, crystallite size and surface area
the bulk density of the material. When it is possible to fill an of the compounds are parameters that directly influence the
entire reactor with the catalyst, the bulk density of the amount of catalyst and are embedded in equation 2 by the
material can be calculated, however, when it is not possible, conversion value, at least for the Nd-5sis and ZnSO4.
the bulk density is not considered, a situation often used in Considering the selectivity values of the reaction, it is possible
fine-chemical processes for batch reactors97. With these to determine the rate of production of glycerol carbonate
considerations, TOF can be calculated from the ratio of the (118.09 g mol-1), in mol m-3 s-1, or Ton m-3 month-1. Thus, for a
glycerol consumption rate at 140 °C, the number of active sites process of conversion of glycerol to glycerol carbonate
-
in the catalyst (10 µmol) estimated by the formation of OOH occurring over 12 hours, which a good time for the operation
on the surface of Nd-5sis for the weight of 250 mg and total of a fine-chemical industry, the production of glycerol
time of 5400s. It can be noted that the TOF for the catalyst has carbonate can be determined as 136.2 Ton per cubic meter of
a value of 1.93 s-1 and indicates that, approximately, two Nd-5sis in the period of one month, considering 74% for the
glycerol molecules are converted to glycerol carbonate per selectivity value, with a cost of 189.0 Ton m-3 month-1 of
active site per second. The specific surface area of Nd-5sis was glycerol and 123.2 Ton m-3 month-1 of urea, or a production of
estimated at 11 m2 g-1 for calculated particle size of 100 nm by 56.9 kg of glycerol carbonate with consumption of 78.9 kg of
X-ray analyses98. The nitrogen adsorption and desorption glycerol and 51.5 kg of urea per 1 kg of Nd-5sis. Experimental
59, 96
isotherms, in Fig S9†, of Nd-5sis presents a typical type IV data of different catalyst in the literature
were compared
curves with H3 hysteresis loops. The curve does not exhibit any and the production of glycerol carbonate per kilogram of the
limiting adsorption at high P/P0, suggestive of the existence of spent catalyst was obtained based on values of conversion and
non-rigid aggregates of plate-like particles or slit-shaped pores selectivity found by different authors, these results are
assemblages. This result does not provide a consistent summarized in Fig 12.
calculation of the pore size distribution.
From the data, it can be observed that the compound Nd-
The weight fraction of the catalyst used is 1.2 wt% and 5sis as well as Nd-5sis-desh have a higher glycerol carbonate
density of the mixture in the batch reactor of 1.30 g cm-3 (a production rate, compared to several catalysts used in the
good approximation since the density of the urea is 1.32 g cm-3 literature. On the other hand, it has higher values of glycerol
and commercial glycerol is 1.26 g cm-3). Considering the (X-ray) consumption, which is a limiting factor for its application.
catalyst density of 2.393 g cm-3, the volume fraction of the La2O3-600 shows a better relationship between production of
catalyst can be estimated with a value of 6.519 x 10-3 m3 m-3as glycerol carbonate and consumption of glycerol, standing out
well as the surface area in the volume fraction of the catalyst against all catalysts available in the literature to date. Different
(1.716 x 105 m2 m-3) in the reactor with a crystallite size of 100 heterogeneous catalysts have been tested in recent years;
nm. The number of active atoms available for the reaction is although they have advantages and limitations in their use. In
lower than the number of atoms on the surface, which in Nd- addition to the catalysts, which are produced by hydrotalcite
5sis is estimated as 2.20 x 1018 atoms m-2, or the number of (HTC), they have a low reaction rate and require high energy
active atoms per volume of the catalyst has a value of 3.78 x consumption for the calcination process and subsequent
1023 atoms m-3. Then, the number of mols of glycerol recovery of the sites59. Catalysts derived from oxides or
converted each second per volume of the catalyst utilized can mixtures of oxides generally have simple methods of synthesis,
be determined. Considering TOF value and the number of sites easy separation and recovery of the compounds in the
active per volume of the catalyst obtains a value of 7.29 x 1023 mixture, besides good regeneration ability, however, they are
atoms m-3 s-1, or 1.21 mol m-3 s-1.
sensitive and deactivated in the presence of water and their
In summary, the bulk density effect of compounds to sites are contaminants in the final product. Catalytic reactions
catalytic tests and the transport limitations that occur in the in the batch reactor have limitations due to low diffusion in the
batch reactor may be neglected; a reasonable consideration, mixture and, consequently, low catalytic activity54, 55, 97. For
since the system is maintained with a good distribution of calcined zinc sulfate, it is required a higher energy for
heat, constant density and good agitation. Therefore, the activation of the sites, besides this, part of the catalyst is
conversion rate of glycerol (mol m-3 s-1) is given approximated soluble in the reaction, which needs
by equation 2 (eq.2). good candidate for the
separation57. La2O3-600 is
a subsequent
a
manufacture of glycerol carbonate, has an excellent reaction
rate, easy separation and good recycling, but has high costs for
calcination of the material, since it requires high
temperatures96. The compound Nd-5sis, in addition to the
reasonable reaction rate, shows a good production of glycerol
carbonate, it requires low temperatures for the synthesis with
ꢉ
ꢌꢂꢋ
ꢓꢔꢕꢖ ꢗ ꢘꢙ ꢚ ꢐꢕ,ꢛꢜꢝꢒꢞꢆꢟꢜ ꢗ ꢚ ꢠ%
ꢉ ꢋ
ꢉ ꢋ
ꢑꢒ ꢈ
ꢚ 10ꢏ ꢉeq. 2ꢋ
ꢉ
ꢌꢣꢋ
ꢉ ꢋ
ꢦ
ꢙꢙꢛꢜꢝꢒꢞꢆꢟꢜ ꢗ ꢙꢡꢢ ꢚ ꢐꢤ ꢗ ꢚ ꢥꢆ
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a high yield and shows a facility for the separation process. interfere in the bulk of Nd-5sis, i.e., the crystalline structure of
Features which make Nd-5sis a good candidate in the the Nd-5sis, as shown in Fig S11†, indicating that this small
manufacturing process. Besides this, the use of the catalyst increase of metal ions is possibly associated with the
with Nd3+ metal site favors the reaction and have been formation of an amorphous phase in the system.
responsible for a good catalytic activity.
From the experimental data it is possible to verify that the
reaction of urea with glycerol at 140 ° C under the surface of
the Nd-5sis catalyst occurs basically in two steps. In the first
stage of the reaction the adsorption of glycerol and urea
occurs on the surface of the catalyst, followed by the
conversion of the glycerol to the glycerol carbamate
(intermediate) and the loss of an ammonia molecule at the
surface active sites. Subsequently, a process of cyclization of
the structure of the intermediate leads to the desorption of
glycerol carbonate and abstraction of another molecule of
ammonia. The second stage has a lower kinetic velocity than
the first one, due the low conversion value (30%) for reaction
in the absence of the catalyst, being, therefore, a determining
step of the reaction. The use of the catalyst with the Nd3+
metal site favors the reaction, being responsible for the good
catalytic activity. Mechanism similar to that occurring with
transition metals present on the surface of hydrotalcites59 and
shown in Fig 14.
3.7. Recyclability of Nd-5sis
The chemical and structural stability of the Nd-5sis catalyst
was investigated after the first reaction cycle, the
spectroscopic and X-ray data, are presented in Fig S9† and
S10†, respectively. At the end of the reaction, the catalyst was
centrifuged and vacuum filtered, then, it was washed with a
mixture of water and methanol before the analyses. It is
possible to see in Fig S10†, in comparison to fresh Nd-5sis
catalyst, that the absorption bands at 2944 cm-1 and 2891 cm-1
which are attributed to the stretching modes of CH groups
from methyl groups, in conjunction with a band at 1715 cm-1,
which are related to stretching of C = O group, were associated
with the formation of glycerol carbamate, an intermediate of
the reaction, adsorbed on the surface of Nd-5sis. By Raman
scattering spectrum, the bands at 2946 cm-1 and 2886 cm-1 in
addition to a shoulder at 1719 cm-1 are correlated to the
formation of the intermediate in the reaction.
At the end of the reaction, Nd-5sis exhibited a loss of
crystallinity, mainly due to the adsorption of reactants,
products and intermediates on its surface, as shown in Fig
S11†. The Nd-5sis recovered was recrystallized using a mixture
of water and ethanol at 160 °C for 12 hours and at the end of
the process, the crystalline structure is preserved. Recycling of
catalyst was investigated until the fourth reaction cycle, as
shown in Fig 13. No significant variations in glycerol conversion
values were observed even after four consecutive cycles. It is
observed that both the catalytic activity and the crystalline
structure of the compound are preserved during the recycle
tests. The process of recrystallization of the catalyst leads to
the removal of impurities adsorbed on the surface of the
catalyst and was considered to contribute to a good reuse of
the material. It is still verified that after each cycle of reaction
a greater time in the process of recrystallization, reaching up
to six days at 160 °C to the fourth cycle. The catalyst will
maintain catalytic activity after four subsequent cycles, it is
Fig. 12 Production data of glycerol carbonate and consumption
of glycerol. Results calculated by equation 2. *Values obtained
from experimental data in this work.
according to present in literature for the others catalysts57, 99
.
One of the possible deactivation mechanisms of Nd-5sis in
system was investigated by leaching study of Nd3+ ions. The
analysis of Nd3+ before and after first cycle of reaction was
carried out by X-ray fluorescence semi-quantitative analysis.
The results of XRF suggest that after the first reaction cycle,
the Nd-5sis has an increase of 5% over the initial amount of
Nd3+ ions present in the fresh Nd-5sis, ensuring the initial
catalytic activity for repeated batch reactions. Results are
consistent with the catalyst recycling tests up to the fourth
reaction cycle, as can be seen in Fig 13. This slight alteration in
the amount of Nd3+ ions may probably be related with the
leaching process of organic ligand, which may have occurred
during the washing, filtration and recrystallization process of
Nd-5sis. However, this ligand leaching process does not
Fig. 13. Effects of recycling times on the conversion reaction of
glycerol.
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Acknowledgements
The authors would like thank the Crystallography
Laboratory (LabCri-UFMG) for providing facilities for the
measurement of X-ray diffraction and the Coordination of
Improvement of Higher Education Personnel (CAPES), the
Foundation for Research Support of Minas Gerais (FAPEMIG)
and the National Council for Scientific and Technological
Development (CNPq) for recourses used in this work.
Fig. 14. Mechanism for the reaction of urea with glycerol.
Conclusions
Notes and references
Two-dimensional polymer (Nd-5sis) was synthesized by
‡ CCDC 1428527 has the supplementary crystallographic data for
this work and it can be obtained free of charge from The
means of a solvothermal reaction, with relatively simple
solvents, which guaranteed
a better control of the
Cambridge
Crystallographic
http://www.ccdc.cam.ac.uk/conts/retrieving.html.
Data
Centre
at
experimental parameters and, therefore, their great
reproducibility. The study of thermal stability, when associated
with thermodynamic parameters of the conversion reaction of
glycerol to glycerol carbonate, allowed establishing a suitable
working temperature range for the 2D coordination polymer,
denoted as Nd-5sis. With this work it was possible to propose
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a
good methodology for the characterization and
quantification of active superficial sites from the study of
structure and surface by means of a suitable model reaction
using a hydrogen peroxide solution where the structural
determination of Nd-5sis was fundamental to the proposal.
In addition, a simple mathematical equation was proposed
to compare some results in the literature with those calculated
in this work, allowing estimating the consumption and
formation of glycerol and glycerol carbonate in a process. It
noted that Nd-5sis has coordinated water molecules in the
metal sites. Based on X-ray diffraction and thermogravimetric
experimental data, it is possible verify that Nd-5sis has ability
to generate free open-metal sites into structure if the
abstraction of coordinated water molecules occurs. This is an
7.
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17.
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18.
The Nd-5sis exhibits a satisfactory catalytic performance in
the conversion process when compared to different catalysts
19.
in the literature; it can still be easily recovered without
significant losses in relation to initial weight, chemical and
structural modification. The present work has shown that the
coordination polymers can be promising in the conversion
process of glycerol, a low-value industrial waste and despite
the limitations in the process; the study can help in the
competitiveness of biodiesel companies in the market in the
near future. But, for the application of these kinds of catalysts
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Conflicts of interest
There are no conflicts to declare.
This journal is © The Royal Society of Chemistry 20xx
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Table of Contents:
Production of glycerol carbonate from glycerol and urea using a 2D coordination
polymer as the heterogeneous catalyst.