Synthesis of hydrotalcites from waste steel slag with [Bmim]OH intercalated for the transesterification of glycerol carbonate

Article abstract of DOI:10.3390/molecules25194355

Ca-Mg-Al hydrotalcites were prepared by coprecipitation from Type S95 steel slag of Shanghai Baosteel Group as supports of ionic liquid in this paper. Five basic ionic liquids [Bmim][CH₃COO], [Bmim][HCOO], [Bmim]OH, [Bmim]Br and ChOH were prepared and their catalytic performance on the synthesis of glycerol carbonate by transesterification between dimethyl carbonate and glycerol was investigated. The characterization results indicated that [Bmim]OH is the best ionic liquid (IL) for the transesterification reaction of glycerol carbonate. The hydrotalcites before and after intercalation by ionic liquid were characterized by XRD, FTIR, SEM, EDS and the IL were characterized by FT-IR, ¹³C-NMR and basicity determination via the Hammett method. The analysis results implied that the dispersion of [Bmim]OH in hydrotalcites reduced the alkali density appropriately and facilitated the generation of glycerol carbonate. The yield of glycerol carbonate and the conversion rate of glycerol reached 95.0% and 96.1%, respectively, when the molar ratio of dimethyl carbonate and glycerol was 3:1, the catalyst dosage was 3 wt%, the reaction temperature was 75 ^?C and the reaction time was 120 min. The layered structure of hydrotalcites increased the stability of ionic liquid intercalated in carriers, thus the glycerol conversion and the GC yield still remained 91.9% and 90.5% in the fifth reaction cycle.

Full text of DOI:10.3390/molecules25194355

molecules
Article
Synthesis of Hydrotalcites from Waste Steel Slag with
[Bmim]OH Intercalated for the Transesterification of
Glycerol Carbonate
Guanhao Liu *, Jingyi Yang and Xinru Xu
International Joint Research Center of Green Energy Chemical Engineering, East China University of Science
and Technology, Meilong Road 130, Shanghai 200237, China; jyyang@ecust.edu.cn (J.Y.);
xrxu86@ecust.edu.cn (X.X.)
* Correspondence: 10110567@mail.ecust.edu.cn; Tel./Fax: +86-021-64252160
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Received: 19 August 2020; Accepted: 18 September 2020; Published: 23 September 2020
Abstract: Ca-Mg-Al hydrotalcites were prepared by coprecipitation from Type S95 steel slag
of Shanghai Baosteel Group as supports of ionic liquid in this paper. Five basic ionic liquids
[Bmim][CH3COO], [Bmim][HCOO], [Bmim]OH, [Bmim]Br and ChOH were prepared and their
catalytic performance on the synthesis of glycerol carbonate by transesterification between dimethyl
carbonate and glycerol was investigated. The characterization results indicated that [Bmim]OH is
the best ionic liquid (IL) for the transesterification reaction of glycerol carbonate. The hydrotalcites
before and after intercalation by ionic liquid were characterized by XRD, FTIR, SEM, EDS and the
IL were characterized by FT-IR, 13C-NMR and basicity determination via the Hammett method.
The analysis results implied that the dispersion of [Bmim]OH in hydrotalcites reduced the alkali
density appropriately and facilitated the generation of glycerol carbonate. The yield of glycerol
carbonate and the conversion rate of glycerol reached 95.0% and 96.1%, respectively, when the
molar ratio of dimethyl carbonate and glycerol was 3:1, the catalyst dosage was 3 wt%, the reaction
temperature was 75 ◦C and the reaction time was 120 min. The layered structure of hydrotalcites
increased the stability of ionic liquid intercalated in carriers, thus the glycerol conversion and the GC
yield still remained 91.9% and 90.5% in the fifth reaction cycle.
Keywords: steel slag; hydrotalcites; ionic liquid; transesterification
1. Introduction
Steel slag is the solid waste composed of Ca, Si, Fe, Mg, Al, Mn, P, O and so on in the steel-making
procedure [
due to the massive emissions [
1
]. The simple utilization of steel slag usually causes the loss of valuable metal resources
]. The new door to the comprehensive utilization of steel slag will
2
be opened up if the various metal resources can be used for the preparation of new multifunctional
materials with high value [3,4].
Layered double hydroxides (LDHs), also known as hydrotalcite-like compounds, are anionic
layered compounds with similar crystal structures to brucites [5]. The host layers and guest anions in
LDHs are connected by noncovalent bonds. LDHs have broad application prospects in adsorption,
catalysis, biomedicine and other fields due to their unique properties such as pore size tunability,
interlayer anion exchangeability [6] and interlayer cation compatibility [7]. Specially, LDHs are often
used as carriers in the catalysis field due to their layered structures [8].
Carbonate glycerol (GC) is widely used in food, medicine, plastics, military, new materials and
new energy as a special bio-based chemical with high boiling point, low freezing point, low volatility,
low flammability and strong polarity [9]. The transesterification of glycerol with dimethyl carbonate
Molecules 2020, 25, 4355; doi:10.3390/molecules25194355
www.mdpi.com/journal/molecules

Molecules 2020, 25, 4355
2 of 10
(DMC) over basic catalysts is considered to be an effective method for GC synthesis under mild
conditions [10]. The only reaction byproduct methanol is easily separated from the product mixture.
Meanwhile, toxic substances, relatively high reaction pressure and other harsh reaction conditions
can be avoided in the process [11]. The preparation of a catalyst with high activity and stability is the
necessary requirement for large-scale production of GC [12].
Ionic liquid has been of wide concern in the field of catalysis in recent years due to its mild
reaction conditions, high-efficiency selectivity and simple operation process [13]. However, it is difficult
to separate ionic liquid from the homogeneous system after reaction [14]. The immobilization of
ionic liquid can not only retain remarkable catalytic performance, but also overcome the separation
difficulties [15]. Meanwhile, new properties and characteristics of catalysts will be derived via the
synergism between ionic liquid and carriers [16].
In this paper, alkaline ionic liquids with different structures were designed and synthesized for
the transesterification of dimethyl carbonate and glycerol to produce glycerol carbonate. The effect of
different ions on the catalytic ability was investigated. Ca-Mg-Al hydrotalcites were prepared from
waste steel slag and used as carriers for the intercalation of ionic liquid. This process recycled the waste
steel slag, reduced the synthesis cost of hydrotalcites and solved the separation problem of ionic liquid.
2. Results and Discussion
2.1. Characterization of Ionic Liquid
2.1.1. 13C-NMR analysis of Ionic Liquid
Figure 1 showed the 13C-NMR spectra of [Bmim][CH3COO], [Bmim][HCOO], [Bmim]OH,
[Bmim]Br and ChOH. The peaks at 67.68 ppm and 58.86 ppm were the resonance signals of methylene
on quaternary ammonium cations, and the chemical shift of methyl on quaternary ammonium cations
was 54.66 ppm in the 13C-NMR spectra of ChOH. In the other four spectra, the characteristic peaks at
138.80 ppm, 124.94 ppm and 120.24 ppm corresponded to the chemical shifts of carbon on the imidazole
ring. The resonance signals of carbon on the methylene linked to the imidazole ring were 48.66 ppm,
31.10 ppm and 20.50 ppm. The peaks appeared where
to the methyl carbon on imidazole ring branch. The carbon chemical shifts of acetate were 177.14 ppm
and 21.83 ppm in the NMR spectrum of [Bmim][CH3COO]. The peak where was 166.35 ppm can
δ was 36.88 ppm, and 13.87 ppm can be attributed
δ
be ascribed to the resonance signals of formate in the NMR spectrum of [Bmim][HCOO]. The four
synthesized products had similar structure of 1-butyl-3-methyl imidazole cation while the anions were
bromine, hydroxyl, acetate and formate, respectively, according to the above NMR analysis results.
Figure 1. 13C-NMR spectra of [Bmim]OH, [Bmim][CH3COO], [Bmim][HCOO], [Bmim]Br and ChOH.

Molecules 2020, 25, 4355
3 of 10
2.1.2. FT-IR Analysis of Ionic Liquid
The FT-IR spectra of different ionic liquids were displayed in Figure 2. The related characteristic
absorption peaks of imidazole were shown in the spectra of [Bmim][CH3COO], [Bmim][HCOO],
[Bmim]OH and [Bmim]Br. The stretching vibration peaks of unsaturated C-H on imidazole rings
appeared at 3123 cm−1 and 3082 cm−1. The absorption peaks at 2965 cm−1, 2954 cm−1 and 2866 cm−1
were corresponded to the stretching vibration of saturated C-H bond on branch chain. The characteristic
peaks at 1645 cm−1 and 1163 cm−1 were attributed to the stretching vibration of C-N and C=C on
imidazole ring. Duan [17] reported similar results about the imidazolium cation. The absorption
peak at about 3500 cm−1 was caused by the stretching vibration of hydroxyl. In the infrared spectra
of [Bmim][CH3COO] and [Bmim][HCOO], the absorption peak at 1375 cm−1 was ascribed to the
asymmetric stretching vibration of C-O in carboxylate. The absorption peak at 1656 cm−1 was
corresponded to N-C stretching vibration of quaternary ammonium cations in the FT-IR spectra of
ChOH. The stretching vibration peaks of methylene and methyl on the branched chain of quaternary
ammonium cation appeared at 2989 cm−1 and 2699 cm−1. All the above indicated that the ionic liquid
had been synthesized successfully.
Figure 2. FT-IR spectra of ChOH (
[Bmim]OH (e).
a), [Bmim]Br (b), [Bmim][HCOO] (c), [Bmim][CH3COO] (d) and
2.1.3. Results of Basicity of Ionic Liquids
The catalyst basicity was tested, and the influence on the reaction activity was studied in this
section. The basic amount of [Bmim]Br, [Bmim][HCOO], [Bmim][CH3COO], ChOH and [Bmim]OH
was 0.69 mmol/g, 0.87 mmol/g, 0.92 mmol/g, 1.09 mmol/g and 1.17 mmol/g, respectively, in Table 1.
The glycerol conversion and the GC yield mounted up with the increase of basicity while the reaction
selectivity decreased because glycerol carbonate was further decarboxylated to generate glycidol in
the strong alkaline environment. The intercalation of ionic liquid in hydrotalcites reduced the alkali
density appropriately, which not only helped to improve the selectivity, but also solved the separation
problem of ionic liquid.

Molecules 2020, 25, 4355
4 of 10
Table 1. Effect of ionic liquid basicity on glycerol conversation and carbonate glycerol (GC) yield.
Glycerol Conv.
(%)
GC Yield
(%)
Basic Amount
(mmol/g)
Basic Strength
(H_)
Catalyst
[Bmim]HCOO
[Bmim][CH3COO]
[Bmim]OH
81.6
90.5
97.5
75.2
96.5
81.1
90.3
92.1
75.0
91.4
0.87
0.92
1.17
0.69
1.09
H_ < 9.8
9.8 < H_ < 15.0
18.4 < H_ < 26.5
H_ < 9.8
[Bmim]Br
ChOH
18.4 < H_ < 26.5
2.2. Characterization of IL-CaMgAl
2.2.1. XRD Analysis
Figure 3 was the XRD patterns of pure hydrotalcites and ionic liquid intercalated hydrotalcites.
IL-CaMgAl still had the characteristic peaks of hydrotalcite lattice planes (003), (006), (009), (015),
(018), (110) and (113) [6]. The position of each characteristic diffraction peak in IL-CaMgAl was
slightly shifted but can be clearly distinguished compared with the XRD spectrum of pure hydrotalcites,
which indicated that the intercalation of [Bmim]OH did not change the crystal structure of hydrotalcites.
Figure 3. XRD patterns of pure hydrotalcites (a) and IL-CaMgAl (b).
2.2.2. FT-IR Analysis
FT-IR spectra of CaMgAl hydrotalcites before and after intercalation by ionic liquid were shown
in Figure 4. The absorption peak of pure hydrotalcites at 3470 cm−1 was related to hydroxyl stretching
vibration linked to metal cations. The absorption vibration peak of carbonyl in carbonates was observed
at 1430 cm−1. The characteristic peaks of imidazole ionic liquid appeared, obviously, on the infrared
spectrum of hydrotalcites after intercalation by [Bmim]OH. The absorption peaks at 3250 cm−1 and
3090 cm−1 corresponded to the unsaturated C-H on imidazole rings. The stretching vibration peaks at
2960 cm−1, 2930 cm−1 and 2880 cm−1 were attributed to the saturated carbon-hydrogen bond on the
branch chain of imidazole rings. The absorption peaks at 1567 cm−1 and 1164 cm−1 were ascribed to
C=N and C-C on imidazole rings, respectively. All indicated that [Bmim]OH had been intercalated
into hydrotalcites successfully.

Molecules 2020, 25, 4355
5 of 10
Figure 4. FT-IR spectra of pure hydrotalcites (a) and IL-CaMgAl (b).
2.2.3. Element Analysis
Table 2 was the main element analysis of pure hydrotalcites and IL-CaMgAl. The contents
of oxygen, calcium, aluminum, carbon and magnesium in the pure hydrotalcites were 45.49 wt%,
28.74 wt%, 9.99 wt%, 6.11 wt% and 4.04 wt%, respectively, while the contents of oxygen, calcium,
carbon, aluminum, nitrogen and magnesium in the IL-CaMgAl were 41.93 wt%, 27.53 wt%, 12.30 wt%,
7.98 wt%, 5.25 wt% and 3.66 wt%, respectively. The occurrence of nitrogen and the increase of carbon
indicated that the ionic liquid had immobilized in the hydrotalcites since [Bmim]OH contained a large
amount of carbon and nitrogen elements.
Table 2. Surface element content of CaMgAl and IL-CaMgAl.
Element (wt.%)
C
N
O
Mg
Al
Ca
Others
CaMgAl
IL-CaMgAl
6.11
12.30
NA *
5.25
45.49
41.93
4.04
3.66
9.99
7.98
28.74
27.53
5.63
1.35
* NA means the element content can not be detected by the instrument.
2.2.4. SEM Analysis
The petaloid layered structures was the characteristic morphology of hydrotalcites [
7].
The polygonal or circular lamellas can be seen clearly in the SEM images of pure hydrotalcites
in Figure 5. The layered morphology was still maintained after intercalation of [Bmim]OH in the SEM
images of IL-CaMgAl, but the lamellas were distorted to some extent.
Figure 5. SEM images of pure hydrotalcites (A) and IL-CaMgAl (B).

Molecules 2020, 25, 4355
6 of 10
2.3. Transesterification of Glycerol and Dimethyl Carbonate
The intercalation of [Bmim]OH with strong alkalinity changed the catalytic activity of hydrotalcites,
obviously. The esterification of dimethyl carbonate and glycerol was carried out catalyzed by IL-CaMgAl
and pure hydrotalcites respectively under the condition that the catalyst dosage was 3 wt%, the molar
ratio of DMC and glycerol was 3:1, the reaction time was 120 min and the reaction temperature was
75 ◦C in order to investigate the influence of ionic liquid intercalation on esterification. The reaction
results were presented in Figure 6.
Figure 6. Transesterification of dimethyl carbonate and glycerol catalyzed by [Bmim]OH, layered double
hydroxides (LDHs) and IL-CaMgAl.
The conversion rate of glycerol and the yield of glycerol carbonate catalyzed by IL-CaMgAl were
much higher than that by pure hydrotalcites in the same reaction time. The glycerol conversion rate
and the GC yield catalyzed by pure hydrotalcites were both at a low level all along. The conversion rate
of glycerol catalyzed by ionic liquid intercalated hydrotalcites reached 60.1% without by-product when
the reaction lasted for 40 min. The GC yield was 95.0% and the glycerol conversion was 96.1% when the
reaction time was 120 min. However, the yield of GC decreased slightly although the conversion rate
of glycerol increased when the reaction time continued to mount up since the further decarboxylation
of glycerol carbonate generated glycidol in strong alkaline environment. The optimal reaction time
was 120 min under the research conditions based on the above experimental results. The catalytic
performance comparison of different catalysts was listed in Table 3.
Table 3. Comparison of IL-CaMgAl with reported catalysts in transesterification of dimethyl carbonate
(DMC) and glycerol.
Catalyst
Dosage
(wt.%)
Reaction
Temperature
(°C)
Molar Ratio
of DMC to
Glycerol
GC
Yield
(%)
Reaction Time
(min)
Catalyst
Ref.
CaO
Na2SiO3-200
3
5
75
75
2:1
4:1
30
150
90.2
95.5
[18]
[19]
Calcined
Dolomite
6
75
3:1
90
94.0
[20]
NaY zeolite
IL-CaMgAl
10
3
70
75
3:1
3:1
240
90
80.0
96.2
[21]
This work
2.4. Reusability Test of IL-CaMgAl
Continuous industrial reaction required high stability of catalyst materials to resist rapid
deactivation and leaching of active sites. The cycle experiment was carried out when the molar

Molecules 2020, 25, 4355
7 of 10
ratio of DMC and glycerol was 3:1, the catalyst dosage was 3 wt%, the reaction temperature was 75 ◦C
and the reaction time was 120 min. IL-CaMgAl was washed with methanol after each reaction cycle.
Figure 7 displayed the XRD patterns and FT-IR spectra of fresh catalyst and five times reused catalyst.
The crystal morphology of catalyst after five cycles was almost identical to the fresh catalyst. However,
the adsorption peak of [Bmim]OH became weak and the stretching vibration peak of ester group at
1710 cm−1 appeared in the FT-IR spectrum of five times reused catalysts, which indicated the loss of
active components and the adhesion of organics.
Figure 7. XRD patterns (A) and FT-IR spectra (B) of five times reused catalyst (a) and fresh catalyst (b).
Figure 8 showed the change of catalytic activity of IL-CaMgAl in the five cycles. The catalyst
remained 91.9% glycerol conversion rate and 90.5% GC yield in the fifth reaction cycle, which meant
IL-CaMgAl had remarkable reusability in the continuous reaction. The hydrotalcites with layered
structures can immobilized the ionic liquid and, thus, reduce the loss of active components in
the catalyst.
Figure 8. Reusability of IL-CaMgAl.
3. Experimental
3.1. Preparation of Ionic Liquid
First, 1-methylimidazole (0.05 mol) was added into the four-neck flask with the condenser tube
and heated slowly to 70 ◦C. Some n-butylbromide, in the constant pressure funnel, was added dropwise
at 1:1.1 molar ratio of 1-methylimidazole to n-butylbromide. The reaction was carried out at 70 ◦C

Molecules 2020, 25, 4355
8 of 10
under the atmosphere of nitrogen. The yellowish intermediate [Bmim]Br was obtained after washing
with ethyl acetate and drying in a vacuum oven at 70 ◦C for 24 h.
[Bmim]Br (0.03 mol) and potassium acetate were weighed at the molar ratio of 1:1 and dissolved
◦
in methanol. The reaction lasted for 8 h at 25 C, 10 mL ether was added after reaction and the
white precipitate was removed by filtration. The filtrate was evaporated for 1 h at 55 ◦C and dried
in the vacuum oven at 70 ◦C for 24 h to obtain ionic liquid [Bmim][CH3COO]. [Bmim][HCOO] and
[Bmim]OH can be prepared by changing the above-mentioned potassium acetate into potassium
formate and sodium hydroxide, respectively.
The quaternary ammonium ionic liquid ChOH was synthesized by using choline chloride and
sodium hydroxide as raw materials at the molar ratio of 1:1. Choline chloride and sodium hydroxide
were weighed and dissolved in dichloromethane solution. The dichloromethane was removed by
rotary evaporation after the reaction, and ChOH was obtained after washing with ethyl acetate,
filtering and drying.
3.2. Preparation of Ca-Mg-Al Hydrotalcites Intercalated by [Bmim]OH
Hydrotalcites were synthesized by coprecipitation method in this study. The steel slag from
Shanghai Baosteel Group was dissolved in 34 wt% nitric acid solution to prepare solution A.
Appropriately, NaOH and Na2CO3, at the molar ratio of 1:16, was dissolved in deionized water
to prepare solution B. Then, 50 mL deionized water and [Bmim]OH was add into a four-neck
flask with stirring devices. Solution A and solution B were dropped by double-titration method.
◦
The system temperature was kept at 70 C, and the pH value was kept between 10 and 11 during
titration. The obtained white slurry was aged in a constant-temperature water bath at 70 ◦C for 24 h.
The hydrotalcites intercalated by ionic liquid were synthesized after filtering, washing with deionized
water and drying in the vacuum oven at 70 ◦C for 24 h. Pure hydrotalcites can be prepared without
adding ionic liquid in the coprecipitation.
3.3. Characterization Methods
The crystallite sizes of solid catalyst were characterized by X-ray diffraction (XRD) on Rigaku
D/max2550VB/PC (Rigaku International Corporation, Tokyo, Japan) with CuK
α radiation (λ = 0.154 nm,
100 mA, 45 kV). The scanning range was 10–80◦(2 ) with a rate of 0.02◦/s. Surface elements were
θ
measured by Edax Falcon energy dispersive spectrometer (EDS) (EDAX Co., Ltd., Mahwah, NJ,
USA). The morphologies of hydrotalcites was observed by scanning electron microscope (SEM,
Hitachi S-3400N, Hitachi Co., Ltd., Tokyo, Japan). Fourier transform infrared (FT-IR) absorption spectra
of the samples were determined by Nicolet Magna-IR 550 via KBr pellet (Thermo Fisher Scientific
,
Waltham, MA, USA). Avance 500 nuclear magnetic resonance spectrometer of Bruker company in
Germany was used to determine the 13C-NMR spectrum of samples. Hammett indicators including
4-nitroaniline (H_ = 18.4), 2.4-dinitroaniline (H_ = 15.0), phenolphthalein (H_ = 9.8) and 4-chloroaniline
(H_ = 26.5) were used to measure the basic strength of ionic liquid by various color. The ionic
liquid was diluted with deionized water and titrated with hydrochloric acid standard solution using
phenolphthalein as indicator. The alkali amount can be calculated by the amount of hydrochloric
acid consumed.
3.4. Transesterification Procedure
Glycerol and dimethyl carbonate according to the molar ratio of 1:3 was added into the flask
with the stirring device and reflux device. The mixture was heated to 75 ◦C under the atmosphere of
nitrogen. Then, 3 wt% IL-CaMgAl was added to catalyze the transesterification between glycerol and
dimethyl carbonate. The product was separated from solid catalyst after 120 min by centrifugation
and analyzed by gas chromatography. The gas chromatograph (Jinghe GC-7860, Jing He Analysis
Instrument Co., Ltd., Shanghai, China) was equipped with a flame ionization detector (FID) (Agilent

Molecules 2020, 25, 4355
9 of 10
Technologies Inc., Santa Clara, CA, USA) and a capillary column (HP-PONA, 50 m by 0.200 mm by
0.50 µm). The injection temperature and detector temperature were 325 ◦C and 280 ◦C, respectively.
4. Conclusions
The partial metal elements such as Ca, Mg and Al in waste steel slag can be utilized effectively by
preparing the environment-friendly catalyst in this paper. The intercalation of ionic liquid [Bmim]OH
adjusted the basicity of hydrotalcites and improved the GC yield in the transesterification of dimethyl
carbonate and glycerol. The highest yield of glycerol carbonate can reach 95.0% under the condition
that the molar ratio of DMC and glycerol was 3:1, the catalyst dosage was 3 wt%, the reaction time was
120 min and the reaction temperature was 75 ◦C. Meanwhile, the supported solid catalyst also solved
the separation difficulty and loss problem of ionic liquid. The conversion rate of glycerol and the
yield of GC still remained 91.9% and 90.5% after five cycles catalyzed by IL-CaMgAl. The significant
reusability and high catalytic activity of IL-CaMgAl made it meet the requirements of continuous
production and have extensive application perspective in industry. However, all experiments in this
paper were carried out in a lab scale, thus the pilot-scale study was necessary for the further research.
Author Contributions: Conceptualization, G.L. and X.X.; methodology, G.L.; software, J.Y.; validation, G.L., J.Y.
and X.X.; formal analysis, G.L.; investigation, G.L.; resources, J.Y.; data curation, X.X.; writing—original draft
preparation, G.L.; writing—review and editing, J.Y.; visualization, X.X.; supervision, G.L.; project administration,
G.L.; funding acquisition, J.Y. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
2.
3.
4.
5.
6.
7.
Tae, S.J.; Morita, K. Immobilization of Cr (VI) in stainless steel slag and Cd, As, and Pb in wastewater using
blast furnace slag via a hydrothermal treatment. Met. Mater. Int. 2017, 23, 576–581. [CrossRef]
˙
Yüksel, I. A review of steel slag usage in construction industry for sustainable development.
Environ. Dev. Sustain. 2017, 19, 369–384. [CrossRef]
Wang, L.; Liu, W.; Hu, J. Indirect mineral carbonation of titanium-bearing blast furnace slag coupled with
recovery of TiO2 and Al2O3. Chin. J. Chem. Eng. 2017, 26, 583–592. [CrossRef]
Purohit, A.; Satapathy, A. Dry sliding wear characteristics of epoxy composites filled with steel industry slag
and sludge particles: A comparative study. Mater. Today Proc. 2018, 5, 11906–11913. [CrossRef]
Zhang, F.; Du, N.; Li, H. Sorption of Cr(VI) on Mg-Al-Fe layered double hydroxides synthesized by a
mechanochemical method. RSC Adv. 2014, 4, 46823–46830. [CrossRef]
Yu, X.Y.; Luo, T.; Jia, Y. Three-dimensional hierarchical flower-like Mg-Al-layered double hydroxides:
Highly efficient adsorbents for As(V) and Cr(VI) removal. Nanoscale 2012, 4, 3466–3474. [CrossRef]
Hussein, M.Z.B.; Zainal, Z.; Yahaya, A.H. Controlled release of a plant growth regulator, α-naphthaleneacetate
from the lamella of Zn-Al-layered double hydroxide nanocomposite. J. Control. Release 2012, 82, 417–427.
[CrossRef]
8.
9.
Nowicki, J.; Lach, J.; Organek, M.; Sabura, E. Transesterification of rapeseed oil to biodiesel over Zr-doped
MgAl hydrotalcites. Appl. Catal. A 2016, 524, 17–24. [CrossRef]
Wan, Y.; Lei, Y.; Lan, G.; Liu, D.; Li, G.; Bai, R. Synthesis of glycerol carbonate from glycerol and
dimethyl carbonate over DABCO embedded porous organic polymer as a bifunctional and robust Catalyst.
Appl. Catal. A 2018, 562, 267–275. [CrossRef]
10. Wang, S.; Wan, J.; Sun, P.; Xu, L.; Okoye, P.U.; Li, S. Disposable baby diapers waste derived catalyst for
synthesizing glycerol carbonate by the transesterification of glycerol with dimethyl carbonate. J. Cleaner Prod.
2019, 211, 330–341. [CrossRef]
11. Liu, J.; He, D. Transformation of CO2 with glycerol to glycerol carbonate by a novel ZnWO4-ZnO catalyst.
J. CO2 Util. 2018, 26, 370–379. [CrossRef]
12. Chaves, D.M.; Márcio, J.; Da, S. A selective synthesis of glycerol carbonate from glycerol and urea over
Sn(OH)2: A solid and recyclable in situ generated catalyst. New J. Chem. 2019, 43, 3698–3706. [CrossRef]

Molecules 2020, 25, 4355
10 of 10
13. Olkiewica, M.; Plechkova, N.V.; Earle, M.J.; Fabregat, A.; Stüber, F. Biodiesel production from sewage sludge
lipids catalysed by Brønsted acidic ionic liquids. Appl. Catal. B 2016, 181, 738–746. [CrossRef]
14. Zhang, P.; Liu, Y.; Fan, M.; Jiang, P. Catalytic performance of a novel amphiphilic alkaline ionic liquid for
biodiesel production: Influence of basicity and conductivity. Renew. Energ. 2016, 86, 99–105. [CrossRef]
15. Shirani, M.; Semnani, A.; Habibollahi, S. Synthesis and application of magnetic NaY zeolite composite
immobilized with ionic liquid for adsorption desulfurization of fuel using response surface methodology.
J. Porous Mat. 2016, 23, 701–712. [CrossRef]
16. Yao, H.; Wang, G.; Zuo, C. Deep hydrodenitrification of pyridine by solid catalyst coupling with ionic liquids
under mild conditions. Green Chem. 2017, 19, 1692–1700. [CrossRef]
17. Duan, J.; Sun, Y.; Shi, L. Three different types of heterocycle of nitrogen-containing alkaline ionic liquids
treatment of acid oil to remove naphthenic acids. Catal Today. 2013, 212, 180–185. [CrossRef]
18. Simanjuntak, F.S.H.; Kim, T.K.; Lee, S.D.; Ahn, B.S. CaO-Catalyzed synthesis of glycerol carbonate from
glycerol and dimethyl carbonate: Isolation and characterization of an active Ca species. Appl. Catal. A 2011
401, 220–225. [CrossRef]
,
19. Wang, S.; Hao, P.; Li, S.; Zhang, A.; Guan, Y.; Zhang, L. Synthesis of glycerol carbonate from glycerol and
dimethyl carbonate catalyzed by calcined silicates. Appl. Catal. A 2017, 542, 174–181. [CrossRef]
20. Algoufi, Y.T.; Kabir, G.; Hameed, B.H. Synthesis of glycerol carbonate from biodiesel by-product glycerol
over calcined Dolomite. J. Taiwan Inst. Chem. Eng. 2017, 70, 179–187. [CrossRef]
21. Pan, S.; Zheng, L.; Nie, R.; Xia, S.; Chen, P.; Hou, Z. Transesterification of glycerol with dimethyl carbonate to
glycerol carbonate over Na–Based Zeolites. Chin. J. Catal. 2012, 33, 1772–1777. [CrossRef]
Sample Availability: Samples of the compounds Ca-Mg-Al hydrotalcites intercalated by [Bmim]OH and pure
hydrotalcites are available from the authors.
©
2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).