Ultrasound irradiation as an effective tool in synthesis of the slag-based catalysts for carboxymethylation

Article abstract of DOI:10.1016/j.ultsonch.2021.105503

Waste minimization strategy was applied in the current work for synthesis of the catalysts from industrial solid waste, namely desulfurization slag. The starting slag material comprising CaCO₃, Ca(OH)₂, SiO₂, Al₂O₃, Fe₂O₃, and TiO₂ was processed by various treating agents systematically varying the synthesis parameters. A novel efficient technique – ultrasound irradiation, was applied as an additional synthesis step for intensification of the slag dissolution and crystallization of the new phases. Physico-chemical properties of the starting materials and synthesized catalysts were evaluated by several analytical techniques. Treatment of the industrial slag possessing initially poor crystal morphology and a low surface area (6 m²/g) resulted in formation of highly-crystalline catalysts with well-developed structural properties. Surface area was increased up to 49 m²/g. High basicity of the neat slag as well as materials synthesized on its basis makes possible application of these materials in the reactions requiring basic active sites. Catalytic performance of the synthesized catalysts was elucidated in the synthesis of carbonate esters by carboxymethylation of cinnamyl alcohol with dimethyl carbonate carried out at 150 °C in a batch mode. Ultrasonication of the slag had a positive effect on the catalytic activity. Synthesized catalysts while exhibiting similar selectivity to the desired product (ca. 84%), demonstrated a trend of activity increase for materials prepared using ultrasonication pretreatment. The choice of the treating agent also played an important role in the catalytic performance. The highest selectivity to the desired cinnamyl methyl carbonate (88%) together with the highest activity (TOF₃₅ = 3.89*10^?7 (mol/g*s)) was achieved over the material synthesized using 0.6 M NaOH solution as the treating agent with the ultrasound pre-treatment at 80 W for 4 h.

Full text of DOI:10.1016/j.ultsonch.2021.105503

Ultrasonics Sonochemistry 73 (2021) 105503
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry
journal homepage: www.elsevier.com/locate/ultson
Ultrasound irradiation as an effective tool in synthesis of the slag-based
catalysts for carboxymethylation
a
a
a
b
b
¨
Ekaterina Kholkina , Narendra Kumar , Kari Eranen , Markus Peurla , Heikki Palonen ,
Jarno Salonenb, Juha Lehtonenc, Dmitry Yu. Murzina
,
*
a Process Chemistry Centre, Åbo Akademi University, FI-20500 Turku/Åbo, Finland
b University of Turku, FI-20014 Turku, Finland
c VTT Technical Research Centre of Finland Ltd, FI-02150 Espoo, Finland
A R T I C L E I N F O
A B S T R A C T
Keywords:
Waste minimization strategy was applied in the current work for synthesis of the catalysts from industrial solid
waste, namely desulfurization slag. The starting slag material comprising CaCO3, Ca(OH)2, SiO2, Al2O3, Fe2O3,
and TiO2 was processed by various treating agents systematically varying the synthesis parameters. A novel
efficient technique – ultrasound irradiation, was applied as an additional synthesis step for intensification of the
slag dissolution and crystallization of the new phases. Physico-chemical properties of the starting materials and
synthesized catalysts were evaluated by several analytical techniques. Treatment of the industrial slag possessing
initially poor crystal morphology and a low surface area (6 m2/g) resulted in formation of highly-crystalline
catalysts with well-developed structural properties. Surface area was increased up to 49 m2/g. High basicity
of the neat slag as well as materials synthesized on its basis makes possible application of these materials in the
reactions requiring basic active sites. Catalytic performance of the synthesized catalysts was elucidated in the
synthesis of carbonate esters by carboxymethylation of cinnamyl alcohol with dimethyl carbonate carried out at
150 ◦C in a batch mode. Ultrasonication of the slag had a positive effect on the catalytic activity. Synthesized
catalysts while exhibiting similar selectivity to the desired product (ca. 84%), demonstrated a trend of activity
increase for materials prepared using ultrasonication pretreatment. The choice of the treating agent also played
an important role in the catalytic performance. The highest selectivity to the desired cinnamyl methyl carbonate
(88%) together with the highest activity (TOF35 = 3.89*10ꢀ 7 (mol/g*s)) was achieved over the material syn-
thesized using 0.6 M NaOH solution as the treating agent with the ultrasound pre-treatment at 80 W for 4 h.
Desulfurization slag
Slag catalysts
Ultrasound irradiation
Physico-chemical characterization
Carboxymethylation
1. Introduction
in removal of such impurities as sulfur and alkali metals, which in turn
generates high basicity of the formed slag [1,2]. Desulfurization steel
Generation of the industrial wastes grows along with the continuous
development of the metal industry including steel manufacturing. Most
of metallurgical technologies contaminate the environment by incom-
plete recycling of the generated by-products. Promotion of the total
utilization of the industrial by-products is a concept of the closed-loop
system supposed to have not only a positive environmental impact
through a decrease of waste from the landfill, but also a resource-
conservation effect.
slag is a part of the big group of materials – ferrous slags, with a complex
mineral composition represented by a mixture of oxides (CaO, SiO2,
FeO, MnO, MgO and Al2O3). Elemental content and properties of the
solid wastes depends on the particular production process and used
feedstock (iron ore and limestone) [2].
The current slag application fields settle mainly in the construction
areas, namely cement production, road construction, utilization as a
fertilizer, wastewater treatment and others disposing of ca. 70% of
produced waste with landfilling constituting the rest [3,4]. An alterna-
tive way of slag utilization is related to the application in catalysis
benefitting from its unique properties. In particular, slag elemental
composition comprising a range of basic oxides along with some
Desulfurization steel slag used in the current work is a by-product of
steel manufacturing formed at the stage prior to converting a hot metal
to crude steel. Desulfurization of the hot metal occurs after the blast
furnace and consists in addition of lime to the melted iron ore resulting
* Corresponding author.
E-mail address: dmurzin@abo.fi (D.Yu. Murzin).
https://doi.org/10.1016/j.ultsonch.2021.105503
Received 13 January 2021; Received in revised form 18 February 2021; Accepted 21 February 2021
Available online 24 February 2021
1350-4177/© 2021 The Authors.
Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).

E. Kholkina et al.
Ultrasonics Sonochemistry 73 (2021) 105503
amounts of oxides exhibiting acidic properties allows synthesis of effi-
cient, low-cost and environmentally friendly catalytic materials
contributing to the waste minimization. Thus, utilization of the steel slag
in the synthesis of zeolites A and ZSM-5 [5], hydroxyapatite–zeolite
composites [6] was successfully performed by several research groups.
Other studies are dedicated to slag application as a catalytic support [7]
or a catalyst itself [8]. In our previous works [9,10], the treated steel slag
was successfully applied as a low-cost catalyst for the pyrolysis of woody
biomass. Synthesis of catalytic materials comprised alkaline and acid
treatments with sodium hydroxide and ethylenediaminetetraacetic acid
(EDTA), respectively. It was demonstrated that activity and product
distribution depend on the catalyst synthesis conditions as well as the
type of the treating agent.
2. Experimental
2.1. Materials and reagents
Industrial desulfurization slag used as a starting material for the
catalyst synthesis was provided by SSAB, Finland. The slag was crushed
by ball milling and sieved to the fraction below 90 μm.
Sodium hydroxide forslag treatment was providedby Baker Analyzed™
A.C.S. Reagent (NaOH, >97%), J.T.Baker™. EDTA (ethylenediaminetetra-
acetic acid disodium salt dehydrate, Titriplex®III for analysis, ACS, ISO,
Reag. Ph Eur, CAS-No.:6381-92-6) andHCl(hydrochloric acid, foranalysis)
were supplied by Merck and TEAH (tetraethylammonium hydroxide, 40%
in water, reagent grade) was purchased from Fluka.
In the current study, desulfurization steel slag as well as materials
synthesized on its basis were employed as catalysts in the base-catalyzed
reaction. Carboxymethylation of cinnamyl alcohol with dimethyl car-
bonate was chosen as a model reaction for catalytic evaluation (Fig. 1).
The current process produces cinnamyl methyl carbonate (product I)
as the main and desired compound with some amounts of the side
products. The desired product I finds its application in organic synthesis
[11] and can be used in the synthesis of biologically active compounds
2.2. Catalyst preparation
2.2.1. Synthesis without ultrasound irradiation
Catalytic materials synthesized without the US step were obtained
upon variation of the treating agent type (distilled water, 0.6 M NaOH,
0.6 M HCl solution, 0.6 M EDTA–0.6 M NaOH mixture, 0.1 M TEAH–0.6
M NaOH mixture), synthesis temperature and time. A detailed descrip-
tion of the synthesis using alkaline, EDTA and structure-directing agent
solutions as well as the preparation of the treating solutions (0.6 M
NaOH solution, 0.6 M EDTA–0.6 M NaOH and 0.1 M TEAH–0.6 M NaOH
mixtures) was reported previously [9,10]. Here the most pertinent de-
tails are given. The choice of the solution concentrations was also based
on the previous experience. Synthesis of the water-treated and acid-
treated (0.6 M HCl) materials was done by stirring at the ambient con-
ditions for 48 h. EDTA treatment without US was performed for 15 h.
Hydrothermal synthesis of the slag catalysts using distilled water and a
sodium hydroxide solution were done in autoclaves operated in the
rotation mode for 48 h at 150 ◦C.
[12–14]. Numerous solid basic catalysts (K2CO3, CsF/αAl2O3, NaOH,
MgO, hydrotalcite, NaAlO2) were utilized as catalysts for carbox-
ymethylation of various alcohols with dimethyl carbonate under
different temperatures (90–180 ◦C) and reaction time (up to 96 h)
[15–17]. High values of alcohol conversion (>90%) and selectivity to
cinnamyl methyl carbonate (>80%) were achieved over the catalysts
exhibiting basic properties.
Desulfurization steel slag is potentially applicable as a catalyst in the
cinnamyl alcohol carboxymethylation due to the its basicity. However,
the presence of sulfur in the slag composition, which is catalytic poison,
as well as poor structural properties of the solid waste require a certain
slag upgrading prior to the application in catalysis.
Ultrasound irradiation (US) can be considered as a modern and
perspective tool in the processing of industrial solid wastes. Ultra-
sonication has been proved as an effective technique in the synthesis of
nanostructured materials [18–20]. Effect of the US on crystallization
suggests formation of the localized hot spots resulting in the high-energy
chemical and physical effects [18,21]. Utilization of US improves crys-
tallization kinetics by intensification of the solute transfer and nucle-
ation rates [22].
2.2.2. Synthesis with ultrasonication step
Synthesis of the slag-based materials with US treatment was done in
two main steps, one of which was pre-treatment with US and another
one – post-treatment at the ambient or hydrothermal conditions. At the
first step, treating solution was mixed with the milled slag in the round
bottom flask and placed into the water bath connected to a high-
intensity transducer system. Ultrasonic equipment was constructed in-
house and already used for other reaction intensification studies [23].
For the current research the sonication set-up was operated in a time
cycle mode (on/off) with the power variation (50–100 W). All treat-
ments with an US step were performed at the lowest sonication fre-
quency (20 kHz) due to a physical impact on the treated materials, while
high frequencies can impose chemical effects complicating analysis of
results [24].
The current work describes synthesis of the low-cost catalytic ma-
terials starting from the desulfurization steel slag by different treatment
techniques including ultrasonication as an additional synthesis step. For
this purpose, such synthesis parameters as the treating agent type, US
time and power, time and temperature of the post-treatment were varied
to optimize structural and textural properties of the synthesized mate-
rials. Efficiency of ultrasonication in the synthesis of the catalytic ma-
terials was verified by comparison of the physico-chemical and catalytic
properties of the slag-based materials synthesized in the absence and
presence of the US pre-treatment at the same post-synthesis conditions.
Solvent temperature at US step was kept ambient due to the negative
impact of a high solvent temperature on stable sonication [24]. Addi-
tional measurements were performed to investigate the temperature
changes during ultrasonication. At the lowest power (50 W) the tem-
perature increase was just 6 ◦C during 4 h of measurement, namely from
21 to 27 ◦C, while at the highest power (100 W) the difference was ca.
Fig. 1. Carboxymethylation of cinnamyl alcohol with dimethyl carbonate.
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E. Kholkina et al.
Ultrasonics Sonochemistry 73 (2021) 105503
20 ◦C.
periodicity of pores and metal oxides particle size was performed by
transmission electron microscopy using JEM-1400 Plus with 120 kV
acceleration voltage and resolution of 0.38 nm equipped with OSIS
Quemesa 11 Mpix bottom mounted digital camera.
The post-treatment consisted of stirring at ambient conditions for 48
◦
h or hydrothermal synthesis in the rotation mode for 48 h at 150 C.
Post-treatment parameters (concentrations of the treating agents, syn-
thesis time and temperature at hydrothermal conditions) were selected
based on the previous studies [9,10], where the influence of the syn-
thesis parameters on physico-chemical and catalytic properties of slag-
based materials in biomass pyrolysis was systematically explored.
The catalysts were prepared upon variation of such synthesis con-
ditions as the treating agent type (distilled water, 0.6 M NaOH and 0.6 M
HCl solutions, 0.6 M EDTA–0.6 M NaOH or 0.1 M TEAH–0.6 M NaOH
mixtures), sonication power (50, 80 or 100 W) and time (4 or 8 h), post-
synthesis temperature (25 or 150 ◦C). Time of the post-treatment was 48
h for all types of synthesis except EDTA (15 or 48 h).
2.3.4. Concentrations of basic sites
Concentrations of basic sites, namely their presence, type, amount
and strength, were determined by temperature programmed desorption
of CO2 (TPD-CO2). Measurements were performed on Micromeritics
AutoChem 2910 instrument equipped with a thermal conductivity de-
tector (TCD). Basicity of the materials was calculated by integration of
the TPD-CO2 profiles.
2.3.5. Crystallinity and phase purity
Influence of US on the properties of the catalyst, which was syn-
thesized by two-step synthesis and showed the highest activity in fast
pyrolysis of wood biomass [10], was also investigated. The initial cat-
alytic material was prepared by treatment with 0.1 M TEAH–0.6 M
NaOH mixture (stirring at 25 ◦C for 4 h) of the already EDTA-treated slag
XRD analysis was performed using Philips X’Pert Pro MPD with
monochromatized Cu-Kα radiation and voltage of 40 kV. A 7.5 mm anti-
scatter slit was used in the diffracted beam side prior to the proportional
counter. The samples were ground before the measurements to minimize
the sample texture (preferred crystal orientation). Copper sample
holders were used.
◦
(synthesis with 0.6 M EDTA-NaOH mixture for 15 h at 25 C). Three
materials were produced using US treatment for the different synthesis
steps such as EDTA pre-treatment (TEAH EDTA-US), TEAH-NaOH post-
treatment (TEAH-US EDTA) and both synthesis steps (TEAH-US EDTA-
US).
2.4. Catalytic performance
Carboxymethylation of cinnamyl alcohol (CA, Sigma Aldrich, 98%)
with dimethyl carbonate (DMC, ReagentPlus®, 99%) used both as a
reactant and a solvent was carried out in the autoclave (300 mL, Parr
Instruments) operating in a batch mode with mechanical agitating (540
rpm). The reactor was equipped with an electrical heater and water
cooling system to keep the desired temperature and prevent over-
heating. The initial concentration of CA was 0.59 mol/L. The catalysts
were dried into an oven at 110 ◦C for moisture removal one day prior to
the experiment. The reactor filled with CA, 100 mL of the solvent and the
dried catalyst (1.18 g), was sealed and flushed with argon (AGA,
99.999%) for 10 min for air removal. Thereafter, the autoclave was
pressurized to 10 bar with argon and kept for 5 min for the leakage test.
The temperature was increased to 150 ◦C with the ramping rate 5 ◦C/
min. Stirring was initiated after reaching the desired temperature. The
samples were periodically withdrawn from the reactor and analysed by
GC equipped with a DB-1 column (30 m, 250 µm, 0.50 µm). The peaks
were identified via GC–MS (Agilent Technologies 5973 GC/MSD)
equipped with a DB-1 column (30 m, 250 µm, 0.50 µm) and compared
with the corresponding data for the neat compounds.
After synthesis, all samples were filtered, washed with distilled
water, dried at 100 ◦C for 7 h and then calcined at 400 ◦C using a step
calcination procedure with holding at 200 ◦C for 40 min.
The synthesized catalytic materials were designated according to the
treating agent, presence of an US step of a certain power and time, post-
synthesis mode of stirring (rotation (rot) vs stirring (st)), its time and
temperature (only in the case of hydrothermal synthesis). For example,
the catalyst denoted as NaOH US (50 W, 4 h) rot 48 h, 150 ◦C was
synthesized using 0.6 M NaOH as a treating agent with application of
ultrasound irradiation for 4 h at power 50 W with the further hydro-
thermal synthesis in the rotation mode for 48 h at 150 ◦C.
2.3. Catalyst characterization
The characterization of the physico-chemical properties of the syn-
thesized catalysts was carried out using nitrogen physisorption, scan-
ning electron microscopy (SEM), transmission electron microscopy
(TEM), energy dispersive X-ray analysis (EDXA), temperature pro-
grammed desorption of carbon dioxide (TPD-CO2), X-ray powder
diffraction (XRD).
Activity of the catalysts (TOF) was calculated as the number of
converted moles of cinnamyl alcohol per mass of catalyst per unit time:
nCA
gcat*t
2.3.1. Surface area and pore volume
TOF =
(1)
Measurements of the surface area and pore volume were performed
by nitrogen physisorption using MicroActive 3Flex™ 3500 (Micro-
meritics®). BET (Brunauer-Emmett-Teller) method was applied for
determination of the specific surface area. Calculations of the external
surface area were performed using the t-plot method. The pore volume
was obtained using the BJH (Barrett-Joyner-Halenda) method. Catalysts
were pretreated under vacuum (0.05 mbar) and heating to 150 ◦C for at
least 7 h for moisture removal prior to measurements preformed at 77 K.
where nca is the number of converted moles of cinnamyl alcohol, gcat is
the mass of the catalyst (g) and t is time (s).
Leaching of the slag components from the some of synthesized cat-
alysts into the reaction media was analysed after 24 h experiments by
inductively coupled plasma optical emission spectrometry (ICP-OES).
After filtration of the spent catalysts the liquid samples were taken for
the measurements, which were carried out using Optima 5X00™ DV
ICP-OES spectrometer (PerkinElmer Inc.).
2.3.2. Crystal morphology and elemental composition
Crystal morphology, namely shape, size and distribution of crystals,
were studied by scanning electron microscopy. Energy dispersive X-ray
microanalysis was used for determination of the elemental composition.
SEM micrographs were obtained with a LEO Gemini 1530 Scanning
Electron Microscope with a Thermo Scientific UltraDry Silicon Drift
Detector (SDD). For some of the slag catalytic materials spot elemental
analysis was performed.
3. Results and discussion
3.1. Crystal morphology
Textural properties such as crystal size, shape and distributions of
crystals were investigated by SEM. Micrographs of the neat slag and
catalysts synthesized on its basis are illustrated in Figs. 2–7, respectively.
It should be noted that some of these Figures are presented at different
magnification. Fig. 2 depicts electron micrograph of the initial
2.3.3. Internal structure
Analysis of the internal structure, textural properties, porosity,
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E. Kholkina et al.
Ultrasonics Sonochemistry 73 (2021) 105503
synthesis of steel slag [9,10]. A survey of literature shows that the
assignment of this type of crystals is controversial, as they can be
attributed to different phases – TiO2 (anatase [31,32] or rutile [32]),
Fe2O3 [33,34] or CaCO3 [30,35]. Moreover, presence of the numerous
impurities complicates the process of phase identifications even further,
as illustrated for Fe2O3 doped with inorganic cations [33]. Performed
spot elemental analysis of the round shape crystals (Fig. A2; Table A2)
showed a high Ca content, which indicates calcium crystallization under
the hydrothermal conditions. At the same time, the content of other
phases may be too small to recognize their presence. Utilization of the
ultrasound irradiation showed a significant influence on the crystal
structure in the case of water treatment (Fig. 4).
In particular, ultrasonication applied as a method additional to the
treatment in water showed a difference in comparison to synthesis
without US. Formation of the needle shape crystals at the ambient
conditions was accompanied with the creation of round shape particles,
previously observed only at hydrothermal synthesis. An exception was a
material synthesized at the highest US time – 8 h (Fig. 4b), exhibiting
presence of fibers. In the case of hydrothermal synthesis, materials
synthesized at different US power and the same hydrothermal condi-
tions showed a difference in their crystal morphology. The lowest power
(50 W) resulted in crystallization of only calcium particles, whereas
formation of a mixed texture containing needle and round shape crystals
with some amounts of agglomerates (Fig. 4e and f) was observed upon
elevation of the US power to 100 W. Alkaline-treated materials syn-
thesized without an additional US step (Fig. 5) and with US (Fig. 6)
displayed a significant difference in crystal morphology in comparison
with the water-treated catalysts.
Fig. 2. SEM micrograph of desulfurization industrial slag.
desulfurization slag.
Crystal morphology of the untreated desulfurization slag is repre-
sented as a mixture of needle shape crystals formed on the platelets. This
complex slag morphology is attributed to the complexity of the
elemental composition, namely presence of several oxides. According to
the literature, large agglomerate species can be assigned to calcium-
containing compounds (calcium hydroxide, calcium carbonate) [25],
while prismatic needle shape crystals can represent an ettringite phase
(hydrous calcium aluminum sulfate mineral) [26,27] – a product of the
clinker phases hydration present in the desulfurization slag [28,29].
Micrographs of the materials synthesized from the desulfurization steel
slag using different treatment methods are shown in Figs. 3–7.
Water-treated slag catalysts exhibit various crystal morphology
depending on the treatment procedure. Thus, synthesis without ultra-
sonication (Fig. 3) resulted in the formation of needle shape crystals
predominantly at both ambient and hydrothermal conditions. A detailed
elemental analysis of the formed crystals in the water-treated material
The ordinary alkaline treatment at ambient conditions resulted in
creation of oblong crystals attributed to the phase similar to Mg-calcite
[35]. In the case of hydrothermal synthesis calcium compounds under-
went a partial transformation to the tobermorite phase [36] present as
platelets with round shape crystals formed in between. Application of
ultrasound irradiation during alkaline treatment led to crystal formation
different from synthesis without US as was discovered before for water
treatment (Fig. 6). Synthesis at ambient conditions resulted in crystal-
lization rhombohedral calcite crystals [30] regardless of the applied US
power, while round shape together with oblong crystals were formed for
the hydrothermal treatment after the US step. The size of the round
shape crystals obtained by SEM and TEM analysis as well as diameters of
pores and internal channels from TEM micrographs are given in
Table A3. It should be noted that smaller particle sizes were achieved
using US step in comparison to its absence. The same phenomenon was
also observed for synthesis of zeolites [37,38]. In the case of slag ma-
terials, such effect was achieved due to interconnected phenomena.
Ultrasound irradiation, known as a technique enhancing the mass
transfer and reaction rates [39,40], accelerated dissolution of calcium
from the raw material. Crystallization of the new phases in the gel
◦
synthesized without an US step for 48 h at 25 C is presented in Ap-
pendix A (Fig. A1, A2; Table A1, A2). A spot elemental analysis was
performed in two spots attributed to areas filled with needle shape
crystals and shapeless agglomerates. Obtained results showed that
needle shape crystals (spot 1) correspond to crystals of Fe-containing
compounds, while agglomerates (spot 2) have a high Ca and Si con-
tent. Massive, platy crystals (agglomerates) present in the slag are
attributed to calcium-containing compounds coming from the lime used
in slag manufacturing [25,26,30]. Application of the high synthesis
temperature in the case of water treatment leads to formation not only of
needle shape, but also round shape crystals with an average size 128 ±
42 nm (Table A3) typically observed in the case of hydrothermal
Fig. 3. SEM micrographs of slag catalytic materials synthesized via water treatment without an US step: a) st 48 h; b) rot 48 h, 150 ◦C.
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Ultrasonics Sonochemistry 73 (2021) 105503
Fig. 4. SEM micrographs of slag-based materials synthesized via water treatment with US step: a) US (50 W, 4 h) st 48 h; b) US (50 W, 8 h) st 48 h; c) US (80 W, 4 h)
st 48 h; d) US (100 W, 4 h) st 48 h; e) US (50 W, 4 h) rot 48 h, 150 ◦C; f) US (100 W, 4 h) rot 48 h, 150 ◦C.
Fig. 5. SEM micrographs of NaOH-treated industrial slag without US step: a) st 48 h; b) rot 48 h, 150 ◦C.
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Ultrasonics Sonochemistry 73 (2021) 105503
Fig. 6. SEM micrographs of slag catalysts treated by alkaline solution US step: a) US (50 W, 4 h) st 48 h; b) US (80 W, 4 h) st 48 h; c) US (100 W, 4 h) st 48 h; d) US
(50 W, 4 h) rot 48 h, 150 ◦C.
Fig. 7. SEM micrographs of slag catalysts treated by other leaching agents: a) 0.6 M HCl st 48 h; b) EDTA st 15 h; c) TEAH (EDTA-pretreated slag) st 4 h.
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Ultrasonics Sonochemistry 73 (2021) 105503
formed after US occurred from the new-formed nuclei while crystalli-
zation in the treatment without US occurred by growing of the new
phases on partially dissolved particles.
interesting result was achieved at a relatively high US power (80 W),
where formation of internal channels with the width of 2 nm was
observed (Fig. 9b).
Utilization of different treating agents such as water (Fig. 3), an
alkaline solution (Fig. 5), different acids and a surfactant (Fig. 7) illus-
trated the influence of pH on the textural properties. The reaction me-
dium has a strong impact on phase transformations, especially for calcite
[41]. If in the case of the water and alkaline treatments the starting
material underwent dissolution and further transformations into new
highly crystalline phases, then in the case of the strong leaching agents
such as HCl (Fig. 7a) or EDTA (Fig. 7b and c) the synthesized catalysts
are amorphous materials with a certain amount of the crystalline phases.
Application of the ultrasound irradiation in HCl, EDTA and surfac-
tant treatment did not lead to significant changes in the crystal
morphology (Fig. A3). For all cases the treatment resulted in formation
of amorphous phases with some amounts of the needle shape crystals
and platelets, which shows a significant difference from the alkaline and
water treatment. Formation of the amorphous phases during HCl and
EDTA treatment can be attributed to the acidic pH of the solution con-
taining the slag material, thereby inhibiting crystallization. Variation of
the numerous factors – synthesis conditions (temperature, treating
agent), pretreatment parameters (time and power of US), led to catalysts
very different in their morphology and distribution of crystal phases
which is worth exploring in various catalytic applications.
Alkaline treatment under ordinary conditions and with the presence
of US illustrated certain trends in formation of pores in the crystals
attributed to the leaching effect of sodium hydroxide under the ambient
conditions (Fig. 10a and c) [43]. The same trend as for treatment with
water, namely formation of round shape crystals under hydrothermal
conditions was preserved for materials synthesized using an alkaline
solution. A significant difference of the alkaline treating agent from
other procedures consists in formation of the fibers (Figs. 10c and A5)
typical for tricalcium silicate (C3S, alite) present in the slag materials
[44,45].
Variation of the treating agents showed diverse crystal structures
presented in Figs. 11 and A6, A7. The application of TEAH in the slag
treatment led to creation of uniform internal channels (Fig. 11b). Tet-
raethylammonium hydroxide is typically used as a structure directing
organic compound in the synthesis of microporous zeolites. Formation
of the uniform porosity and internal channels can be attributed to the
presence of TEAH during the slag treatment.
Intensification of the catalyst synthesis by employment of the ul-
trasound irradiation was clearly seen by TEM analysis. Formation of the
round shape crystals, internal pores and channels was observed for the
US-treated materials.
Formation of crystals with a different morphology in the catalysts
synthesized from the same starting material varying the treatment
conditions led to the following conclusion. Neat slag phases undergo
dissolution with further surface reactions resulting in formation of a gel
with subsequent recrystallization and precipitation of the new phases.
Application of ultrasonication as an additional intensification tool led to
certain changes in the crystallinity of the synthesized materials
depending on the treating agent and synthesis conditions. Thus, water
and alkaline treatment with the presence of US step had a strong impact
on the phase transformations and led to formation of new phases.
3.3. Surface area and pore volume
One of the most significant disadvantages of industrial slags is their
poor structural properties, namely surface area and pore volume,
resulting in a low active surface barely suitable for catalysis. An increase
of the surface area and porosity was performed by selective calcium
leaching and recrystallization upon different treating techniques. Re-
sults of the surface area measurements of the starting materials and
synthesized catalysts performed by nitrogen physisorption are shown in
Table 1.
The starting material – desulfurization slag, as well as the granulated
slag exhibit low surface areas, which is typical for industrial slags
[6,46,47]. All applied treatments led to development of the surface area
and pore volume in comparison with the neat slag. The highest surface
area was achieved for the material synthesized by the multi-step syn-
thesis with EDTA and TEAH (Table 1, entry 19). Relatively high surface
area (49 m2/g) was achieved due to formation of internal channels
observed by TEM. Most likely formation of the channel system was
initiated during the first step of EDTA treatment, where prominent
leaching took place. Further processing with a surfactant continued
development of the structural properties with increasing channel width
from 4 to 5 nm (Table A3) and surface area (Table 1). It should be noted
that application of ultrasound irradiation in this synthesis procedure
influences negatively the surface area of EDTA- and TEAH-EDTA treated
materials. The plausible explanation of the surface area decrease
(Table 1) for some of the slag catalysts treated with ultrasound step can
be due to distortions of pores and internal structure. The eye-catching
difference of the TEAH-US EDTA-US material (entry 22) expressed in
two other samples can be elucidated by the presence of internal channels
(Fig. A7). In the case of the treatment with water and ultrasound an
opposite result was achieved. Application of US in combination with
synthesis at ambient conditions resulted in the surface area higher than
in conventional synthesis. The alkaline treatment showed the same
trend for the material synthesized at the lowest US power (entry 12). An
increase of the power led to lower surface area and porosity of the
synthesized materials. These results can be explained by the phase and
crystal transformations taking place during synthesis. A previously
observed trend of the surface area and porosity decrease for catalysts
synthesized at high temperature [9,10] was also seen in the current
work. An exception is the water-treated catalyst (entry 3), which
structural properties were developed at hydrothermal conditions.
3.2. Internal structure
TEM analysis was performed for investigation of the changes of the
internal structure after the treatment, determination of the particle size
and its distribution, as well as parameters of the channel system. TEM
micrographs of the starting material and catalysts synthesized on its
basis are given in Figs. 8–11 and A4–A7.
The internal structure of the untreated desulfurization slag repre-
sents a crystallized ettringite phase over agglomerates [42]. Heteroge-
neous morphology of the starting material underwent significant
transformations under the treatment. Utilization of distilled water as a
treating agent resulted in the formation of clearly defined metal particles
under hydrothermal conditions (Figs. 9a, d and A4e). As mentioned
before, application of ultrasound irradiation let to crystallization of the
particles with a smaller size than in the absence of US (Table A3). An
Fig. 8. TEM micrograph of the initial desulfurization steel slag.
7

E. Kholkina et al.
Ultrasonics Sonochemistry 73 (2021) 105503
Fig. 9. TEM micrographs of water-treated slag catalysts synthesized without US step at 150 ◦C for 48 h (a) and with US step: b) US (80 W, 4 h) st 48 h; c) US (100 W,
4 h) st 48 h; d) US (50 W, 4 h) rot 48 h, 150 ◦C.
Fig. 10. TEM micrographs of the alkaline-treated slag without US: a) st 48 h; b) rot 48 h, 150 ◦C; and with US: c) US (50 W, 4 h) st 48 h; d) US (50 W, 4 h) rot 48
h, 150 ◦C.
Water and alkaline treatment in the presence of US step resulted in a
higher surface area for some synthesis conditions (50 W of US for 4 h).
An increase of US power from 50 W led to a decrease of the surface area
due to changes in crystallinity of the slag catalysts. Intensification of the
EDTA- and surfactant-treatment by ultrasonication had a negative effect
on the development of the surface area due to high leaching of the slag
components into the solution and their poor further recrystallization.
8

E. Kholkina et al.
Ultrasonics Sonochemistry 73 (2021) 105503
Fig. 11. TEM micrographs of slag catalysts treated by different leaching agents: a) 0.6 M HCl st 48 h; b) TEAH EDTA-US; c) TEAH-US EDTA.
3.4. Elemental composition
significant results were achieved for the materials treated with a well-
known leaching agent EDTA [10,49,50]. The calcium content was
decreased at least two-fold. Such profound leaching of calcium and
sulfur led to an obvious increase of the relative concentration of other
compounds. Moreover, some other elements (Cr, K) were detected being
hidden previously under the calcium layer.
Changes in the morphological and structural properties of the syn-
thesized materials are attributed to modifications in the elemental
composition and phase transformations taking place during synthesis.
Complexity of the elemental composition was determined by SEM-EDXA
and presented in Tables 2 and A4 for the neat slags and the synthesized
catalysts, respectively.
Ultrasonication had a significant effect on the elemental composition
of the synthesized catalysts in comparison with the neat slag and ma-
terials synthesized without the US step. Treatment using distilled water
resulted in a high degree of recrystallization of the dissolved compo-
nents into the new phases (Table A4). At the same time leaching of
sulfur, which is a well known poison for catalysts, was high. Utilization
of the alkaline solution in the slag processing had a higher leaching ef-
fect, especially for the calcium-containing compound and sulfur. A
positive effect on the sulfur leaching was achieved in the cases of EDTA-
and surfactant-treatment. However, a high leaching effect of these
chemical agents resulted in a low calcium amount in the synthesized
catalysts influencing their basicity.
Elemental content of the raw material – desulfurization slag
(Table 2) is dominated by calcium (48 wt%) resulting in high basicity.
However, such high calcium content is responsible for pore blockage
decreasing the active surface of the catalysis and preventing also crys-
tallization of other phases. At the same time, the starting material con-
tains a high amount of sulfur (3 wt%), which presence can influence
negatively the catalytic behavior including activity, stability and even
product distribution. Based on these premises the slag treatment should
be focused on two aims. The first is related to selective leaching of
calcium aiming at a higher surface area and a larger pore volume, while
the goal of the second one is an efficient removal of sulfur, present in the
starting material in the sulfide form [48].
3.5. Concentration of basic sites
For all treatment techniques lower amounts of sulfur were achieved
than in the starting slag (Table A4). The lowest sulfur content (trace
amounts) was reached in the material produced by alkaline treatment at
150 ◦C for 48 h (Table A4, entry 11). Equally successful results (ca. 0.30
wt%) were shown for catalysts prepared by a multi-step synthesis pro-
cedure using 0.1 M TEAH, 0.6 M EDTA and 0.6 M NaOH as treating
agents both with and without US. The same effect was achieved for the
material denoted as NaOH US (50 W. 4 h) st 48 h (entry 12 in Table S4).
For the rest of the materials the sulfur amount was varied in the range of
0.4–1.6 wt%. It should be noted that the application of US resulted in a
lower sulfur content in comparison with the materials synthesized using
the same treating agents without ultrasound.
Basicity of the starting material and the catalysts synthesized on its
basis was evaluated by temperature programmed desorption of CO2
(TPD-CO2.). Neat slag as well as the catalysts synthesized on its basis
exhibit pronounced basic properties due to the presence of high amounts
of the basic compounds as CaCO3/Ca(OH)2, MgO and Na2O. Quantita-
tive characterization of the basic sites amount (μmol/g) and basic
strength distribution are presented in Table 3. The basic sites were
divided into three groups – weak (<750 K), medium (750 K–1050 K) and
strong (>1050 K) based on the ability to desorb the probe molecule
(CO2) from the surface of active sites.
The starting material containing a mixture of basic oxides has high
basicity. Further changes in the amount and strength of the active sites
The calcium content was decreased for most of treatments. The most
9

E. Kholkina et al.
Ultrasonics Sonochemistry 73 (2021) 105503
Table 1
Table 3
Specific and external surface areas, and pore volumes of the neat industrial slags
and synthesized catalysts.
Measurement of amounts of basic sites (weak, medium, strong and total) in the
synthesized slag catalytic materials.
Entry
Catalyst
Specific
External
Pore
Entry
Catalyst
Concentration of basic sites.
μ
mol/g
surface area
surface area (t-
volume,
Weak
48.1
Medium
23.7
Strong
3.9
Total
(BET), m2/g
plot), m2/g
cm3/g
1
Desulfurization slag
75.6
1
Desulfurization slag
6
4
0.05
Water-treated slag catalysts
Water-treated slag catalysts
2
3
4
5
6
7
8
H2O st 48 h
10.7
14.1
14.2
12.5
12.1
11.0
11.9
51.5
67.6
122.3
95.8
94.3
81.4
68.7
3.5
3.8
0
65.7
2
3
4
H2O st 48 h
16
19
27
14
16
21
0.08
0.13
0.08
H2O rot 48 h, 150 ◦
C
85.4
H2O rot 48 h, 150 ◦
C
H2O US (50 W, 4 h) st 48 h
H2O US (80 W, 4 h) st 48 h
H2O US (100 W, 4 h) st 48 h
H2O US (50 W, 8 h) st 48 h
H2O US (50 W, 4 h) rot 48 h,
136.4
109.7
108.4
94.9
H2O US (50 W, 4 h) st
48 h
1.4
2.1
2.5
3.9
5
6
7
8
9
H2O US (80 W, 4 h) st
48 h
22
20
22
9
19
17
16
5
0.06
0.06
0.07
0.05
0.10
84.5
H2O US (100 W, 4 h)
st 48 h
150 ◦
C
9
H2O US (100 W, 4 h) rot 48 h,
13.8
76.3
3.6
93.8
H2O US (50 W, 8 h) st
48 h
150 ◦
C
H2O US (50 W, 4 h)
Alkaline-treated slag catalysts
rot 48 h, 150 ◦
C
10
11
12
13
14
15
NaOH st 48 h
12.8
9.9
106.2
33.2
79.7
41.2
54.3
66.2
0
119.1
44.9
88.7
51.7
64.7
77.3
NaOH rot 48 h, 150 ◦
C
1.8
0
H2O US (100 W, 4 h)
19
16
rot 48 h, 150 ◦
C
NaOH US (50 W, 4 h) st 48 h
NaOH US (80 W, 4 h) st 48 h
NaOH US (100 W, 4 h) st 48 h
NaOH US (50 W, 4 h) rot 48 h,
9.0
7.9
2.6
2.1
0
Alkaline-treated slag catalysts
8.3
10
11
NaOH st 48 h
21
16
17
12
0.07
0.06
11.0
NaOH rot 48 h,
150 ◦
C
150 ◦
C
12
13
14
15
NaOH US (50 W, 4 h)
st 48 h
27
15
15
7
21
13
13
4
0.11
0.09
0.08
0.02
Slag-based catalysts treated with EDTA-NaOH mixture
16
17
18
EDTA st 15 h
8.6
9.4
5.8
9.3
4.8
3.5
4.8
22.9
27.7
22.8
NaOH US (80 W, 4 h)
st 48 h
EDTA US (50 W, 4 h) st 15 h
EDTA US (50 W, 4 h) st 48 h
18.4
8.7
NaOH US (100 W, 4
h) st 48 h
Slag-based catalysts treated with TEAH-NaOH mixture of EDTA-pretreated slag
19
TEAH-NaOH (EDTA-pretreated
slag)
25
15
5
45
NaOH US (50 W, 4 h)
rot 48 h, 150 ◦
C
20
21
22
TEAH EDTA-US
TEAH-US EDTA
TEAH-US EDTA-US
19.6
16.2
13.8
9.9
3.1
3.1
2.7
32.6
31.0
33.9
Slag-based catalysts treated with EDTA-NaOH mixture
11.7
17.4
16
17
EDTA st 15 h
EDTA US (50 W, 4 h)
st 15 h
31
23
27
21
0.11
0.12
HCl-treated synthesized without US step
23 HCl st 48 h
16.7
66.6
3.8
87.1
18
EDTA US (50 W, 4 h)
st 48 h
20
15
0.07
Slag-based catalysts treated with TEAH-NaOH mixture of EDTA-pretreated slag
for the synthesis at ambient conditions (entries 4–6). The leaching of
large amounts of Ca, Mg, Na in the aqueous phase due to enhanced
sonication can attribute to the decrease in amounts of basic sites.
However, carrying out synthesis at hydrothermal conditions did not
have a significant influence on basic site formation (entry 3 vs. 8).
Interestingly that an increase of US power up to 100 W with a further
hydrothermal synthesis resulted in higher basicity of the final material
(entry 9). For the water treatment predominance of medium basic sites
was observed.
19
TEAH-NaOH (EDTA-
pretreated slag)
49
47
0.17
20
21
22
TEAH EDTA-US
25
32
39
22
28
36
0.13
0.12
0.14
TEAH-US EDTA
TEAH-US EDTA-US
HCl-treated synthesized without US step
23 HCl st 48 h 43
41
0.14
are dependent on the leaching ability of the treating agent and synthesis
conditions resulting in crystallization of the certain phases. Thereby,
treatment with EDTA (entries 16–18) showing shapeless morphology
and a low calcium content after synthesis (Table A4), resulted in low
basicity of the final materials. Further processing of EDTA-pretreated
samples with a surfactant showed diverse results. Application of US in
this multi-step treatment has a negative influence not only on the
morphological and structural properties, but also on the formation of
active sites (entries 20–22). At the same time synthesis without US
(entry 19) resulted in moderate amount of basic sites. In the surfactant-
treated catalysts the predominance of weak and medium basic sites is
noticeable.
Opposite to water media, alkaline conditions combined with the US
step and post-treatment at room temperature led to a lower amount of
the basic sites in comparison with the conventional treatment (entries
10, 12–14). An increase of the US power has a negative effect on the
active site formation. Most likely this phenomenon is the result of high
calcium leaching at US conditions (Table A4). The reverse effect was
observed for the hydrothermal post-treatment elevating basicity (entry
11 vs 15). As in the case of the water treatment, alkaline synthesis
resulted in the formation of active sites mainly of medium strength.
It was suggested in the literature that the concentration of the active
sites has a trend similar to changes in the surface area values [51].
However, as reported earlier [9,10] and confirmed in the current work
the synthesized slag catalysts do not follow this trend, which can be
explained by complexity of the elemental composition and diverse in-
fluence of the synthesis parameters.
In the case of using water as a treating agent, US played an important
role in the creation of basic sites increasing two fold their amount. An
enhancement of the sonication power resulted in a decrease of basicity
Table 2
Elemental composition of steel slag (wt%).
O
Na
Mg
Al
Si
S
Ca
Ti
Fe
Mn
Desulfurization slag
34.33
2.49
0.90
1.47
5.80
2.90
48.21
0.50
2.50
0.90
10

E. Kholkina et al.
Ultrasonics Sonochemistry 73 (2021) 105503
Basicity of the synthesized catalysts has a direct relationship to the
predominant phase, the alkaline treatment led to prevalence not only
CaCO3 but also SiO2 (2θ = 20.7◦ and 26.6◦) and TiO2 (2θ = 27.3◦ and
28.1◦). This difference can be clearly seen in the material synthesized
upon hydrothermal conditions. Carrying out synthesis at a high tem-
perature showed its significant influence on TiO2 crystallization.
Observed elevation of the peak intensity indicates dynamical crystalli-
zation of TiO2. According to SEM analysis it was difficult to conclude,
which phase is attributed to formation of the round shape crystals.
However, a requirement of hydrothermal conditions for titania crystal-
lization [32,55] is an argument in favour of crystalline titania presence
among round shape crystals along with silica.
elemental composition, which in turn is related to the employed treating
agent and the synthesis conditions. Thus, water- and alkaline-treatment
demonstrated a positive effect on basic sites formation after ultra-
sonication. Utilization of the chemical agent resulting in a high calcium
leaching (EDTA, TEAH and HCl) led to moderate or even low basicity of
the final materials (Table 3) decreasing with application of US.
3.6. Phase purity
Determination of the phase purity of the industrial slag and catalysts
synthesized on its basis was performed by XRD. XRD patterns of the
water- and alkaline-treated materials are demonstrated in Figs. 12, A8
and 13, respectively. XRD patterns of the EDTA- and surfactant-treated
catalysts are given in Figs. A9 and A10. All types of treatment led to
changes in XRD patterns in comparison with the initial material. Highly
crystalline phases containing CaCO3 (rhombohedral, calcite), Ca(OH)2
(hexagonal, portlandite), SiO2 (hexagonal), Al2O3 (rhombohedral),
Fe2O3 (rhombohedral), TiO2 (tetragonal, rutile) were observed in the
treated slag materials with a clear difference in peak intensities of the
particular phases upon various treatment types [52,53]. Fig. 12 illus-
trates the diffractogram of the fresh desulfurization slag and the cata-
lysts synthesized by treatment in water. The diffractogram, typical for a
slag material [54], confirms a high calcium content in line with the
elemental analysis.
XRD patterns of the EDTA- and surfactant-treated catalysts synthe-
sized in a presence and absence of US step are presented in Figs. A9 and
A10. Opposite to the water- and alkaline-treated materials exhibiting
Ca-containing peaks of a high intensity, EDTA- and TEAH-treated slag
catalysts synthesized with US step demonstrated a highly crystalline
phase of silica.
Synthesized slag catalysts exhibit highly crystalline phases for the
most types of treatment. In the case of synthesis with water or alkaline
solution, the effect of the ultrasound irradiation on the phase distribu-
tion was not visible. Formation of the new phases was not observed.
Treating agents with a high leaching effect illustrated a significant dif-
ference between materials synthesized with and without US step. Thus,
employment of ultrasonication resulting in a high calcium leaching for
EDTA- and TEAH-treatment led to visible changes in XRD patterns
namely presence of silica peaks with a high intensity.
Treatment of the slag with chemical agents demonstrated a signifi-
cant increase of crystalline CaCO3 (2θ = 29.4◦, 39.4◦, 47.5◦, and 48.5◦)
while the latter was almost absent in the starting material. The main
phase present in the desulfurization slag is Ca(OH)2 (peaks at 2θ = 18.1,
31.1 and 50.8◦). It should be noted that the peaks attributed to the
particular phases did not shift under changes of the synthesis parame-
ters. The narrow character of the peaks indicates a growth of the crys-
tallite size during synthesis, which was confirmed by TEM and SEM
analysis. Application of the ultrasonication as well as its power variation
does not affect the formation of new phases (Fig. A8). However, an in-
crease of the peak intensity with the power elevation was observed.
Fig. 13 depicts the diffractograms of the starting material and the
catalysts synthesized by alkaline treatment with 0.6 M NaOH solution.
Contrary to synthesis in the water medium, where CaCO3 was a
3.7. Catalytic performance
Synthesized slag catalysts were evaluated in carboxymethylation of
CA with DMC at 150 ◦C. Activity of the slag materials at 35% conversion
(TON35) as well as selectivity to desired product – cinnamyl methyl
carbonate, are given in Table 4.
Selectivity to desired product over synthesized slag-based catalyst
was varied in the range 80–89% at 35% conversion. Untreated slag
possessed high selectivity to cinnamyl methyl carbonate, however, its
activity was the lowest among the presented slag materials. Utilization
of the chemical agents and different treatment methods improved cat-
alytic activity in comparison with initial material. Water-treated mate-
rials synthesized without US step illustrated low activity not allowing to
Fig. 12. XRD patterns of the raw material and samples obtained via treatment
in water. Notation: initial slag (black); H2O st 48 h (red); H2O US (50 W, 4 h) st
48 h (light blue); H2O US (80 W, 4 h) st 48 h (pink); H2O US (100 W, 4 h) st 48 h
(green); H2O US (50 W, 4 h) rot 48 h, 150 ◦C (dark blue). Symbols: SiO2 (●);
Fig. 13. XRD patterns of the raw material and materials obtained by alkaline
treatment with 0.6 M NaOH. Notation: initial slag (black); NaOH US (50 W, 4 h)
st 48 h (red); NaOH US (80 W, 4 h) st 48 h (dark blue); NaOH US (50 W, 4 h) rot
48 h, 150 ◦C (pink). Symbols: SiO2 (●); Al2O3 (◆); Fe2O3 (▾); TiO2 ( ); Ca
○
○
Al2O3 (◆); Fe2O3 (▾); TiO2 ( ); Ca(OH)2 (▴); CaCO3 (■). (For interpretation of
the references to colour in this figure legend, the reader is referred to the web
version of this article.)
(OH)2 (▴);CaCO3 (■). (For interpretation of the references to colour in this
figure legend, the reader is referred to the web version of this article.)
11

E. Kholkina et al.
Table 4
Ultrasonics Sonochemistry 73 (2021) 105503
Selectivity to desired product and catalytic activity (TOF35) at 35% CA conversion (XCA = 35%) over slag catalysts evaluated at 150 ◦C.
Entry
Catalyst
Selectivity to cinnamyl methyl carbonate at XCA = 35%, (%)
TOF35, 10ꢀ 7 (mol/g*s)
1
Desulfurization slag
89 (X = 33%)
1.19 (X = 33%)
Water-treated slag catalysts
2
3
4
5
6
7
8
9
H2O st 48 h
84 (X = 34%)
1.68 (X = 34%)
H2O rot 48 h, 150 ◦
C
n/a
83
n/a
H2O US (50 W, 4 h) st 48 h
H2O US (80 W, 4 h) st 48 h
H2O US (100 W, 4 h) st 48 h
H2O US (50 W, 8 h) st 48 h
H2O US (50 W, 4 h) rot 48 h, 150 ◦
1.96
1.90
2.71
n/a
83
85
n/a
n/a
84
C
n/a
H2O US (100 W, 4 h) rot 48 h, 150 ◦
C
2.30
Alkaline-treated slag catalysts
10
11
12
13
14
15
NaOH st 48 h
84 (X = 33%)
2.03 (X = 33%)
NaOH rot 48 h, 150 ◦
C
n/a
85
88
89
86
n/a
NaOH US (50 W, 4 h) st 48 h
NaOH US (80 W, 4 h) st 48 h
NaOH US (100 W, 4 h) st 48 h
2.10
3.89
2.95
1.99
NaOH US (50 W, 4 h) rot 48 h, 150 ◦
C
Slag-based catalysts treated with EDTA-NaOH mixture
16
17
18
EDTA st 15 h
83
2.21
n/a
n/a
EDTA US (50 W, 4 h) st 15 h
EDTA US (50 W, 4 h) st 48 h
n/a
n/a
Slag-based catalysts treated with TEAH-NaOH mixture of EDTA-pretreated slag
19
20
21
22
TEAH-NaOH (EDTA-pretreated slag)
TEAH EDTA-US
84
84
85
84
3.17
1.83
2.39
2.79
TEAH-US EDTA
TEAH-US EDTA-US
HCl-treated synthesized without US step
23 HCl st 48 h
80
2.17
*n/a – not available.
reach even 35% CA conversion. Application of ultrasound irradiation for
4 h with power 50 W (Table 4, entry 4) slightly increased activity of the
catalyst. However, prolonged US time (entry 7) as well as high tem-
perature of the post-synthesis (entry 8) resulted in inactive materials.
The lowest surface area was a plausible reason of a low activity of the
material synthesized at hydrothermal conditions. In the case of
increased US time (8 h), such important parameters as basicity and
surface area did not change noticeably, while crystal morphology un-
derwent destructive transformations (Figs. 4 and A4). An increase of US
power resulted in catalytic activity going through the minimum at 80 W
and achieving the maximum activity at 100 W. Catalyst synthesized at
the highest US power exhibits moderate values of basicity and surface
area in comparison to other water-treated catalysts. The main difference
of this material consists in mixed crystal morphology detected by SEM
analysis (Fig. 3). A positive effect of ultrasound irradiation was clearly
observed for the water-treated sample synthesized at hydrothermal
conditions (Table 4, entry 9). This material exhibits higher activity in
comparison with sample synthesized without US step (entry 3), and also
catalysts synthesized at lower US power (entries 4 and 5). Comparing to
the catalyst synthesized at 100 W of US and stirring at ambient condi-
tions (entry 6), catalyst produced at the same US power and high post-
synthesis temperature not only demonstrated near values of the sur-
face area and basicity, but also illustrated similarities in crystal
morphology. More visible comparison of catalytic activity of the water-
treated slag catalysts presented as changes of the CA consumption dur-
ing the reaction is given in Fig. A11.
treated materials, US power resulted in a more significant changes in
catalyst behavior. An increase of US power allowed achieving not only
higher activity, but also higher selectivity to cinnamyl methyl carbonate
up to 89% at 35% of CA conversion (Table 4, entries 12–14). The highest
activity was achieved using alkaline-treated material synthesized at 80
W of US and post-treatment at ambient conditions.
Utilization of the treating agents with a high leaching ability such as
EDTA and HCl demonstrated a negative effect on catalytic properties.
Catalysts synthesized without ultrasonication exhibited a moderate ac-
tivity, while selectivity to cinnamyl methyl carbonate was relatively
low. Application of US in the treatment with EDTA had a negative
impact on synthesis of the effective catalysts. Such effect was achieved
by significant influence of US on the leaching of slag components into
the solution during synthesis resulting a high loss of active sites (Table 3)
and decrease of surface area as well (Table 1). Opposite to treatment
with EDTA, HCl-treated material showed a moderate parameters of
surface area and basicity. The main similarity between EDTA- and HCl-
treated catalysts plausibly influencing catalytic activity was an amor-
phous texture (Fig. 7).
Multi-step synthesis with EDTA and surfactant (TEAH) resulted in
extraordinary results (Table 4; Fig. A13). Thus, catalyst synthesized
without US exhibited high activity with a moderate selectivity to the
desired product (84%). Application of ultrasonication at the different
synthesis steps showed diversity of the catalytic behavior. It should be
noted, that all catalysts synthesized with EDTA and TEAH exhibited
similar selectivity to the desired product (ca. 84%), while the main
difference consisted in activity (TON35) of the slag materials. Catalyst
synthesized by EDTA treatment with US step illustrated not only poor
physico-chemical properties, namely low surface area and crystallinity,
but also insufficient activity in the carboxymethylation of cinnamyl
alcohol with DMC. Employment of the US step at surfactant treatment
(Table 4, entry 21) led to improvement of catalytic properties in com-
parison with the previously discussed sample (entry 20) and catalyst
synthesized by EDTA treatment without US (entry 16). A higher activity
Comparison of the alkaline-treated materials with water-treated ones
synthesized at the same conditions demonstrated significant impact of
the treating solution on the catalytic behavior of the slag materials.
Alkaline-treated catalysts demonstrated higher activity and selectivity.
An exception, as in the case of the water treatment, was material ob-
tained by hydrothermal synthesis exhibiting a poor catalytic activity
(Fig. A12; Table 4, entry 11). Application of US allowed a slight eleva-
tion of activity and selectivity to desired product. As in the case of water-
12

E. Kholkina et al.
Ultrasonics Sonochemistry 73 (2021) 105503
was observed for the catalyst obtained by multi-step synthesis with
EDTA and TEAH, and application of ultrasonication on the both steps of
synthesis. Comparison of these materials synthesized with an agent
exhibiting high leaching ability to calcium (EDTA) and a structure-
directing agent (TEAH) showed an importance of the ultrasonication
disposition in catalyst synthesis.
NaOH as a treating agent and 80 W of US as a means of resulted in an
active catalyst allowing high selectivity (88%) to the desired cinnamyl
methyl carbonate. Analysis of the reaction solution after the experi-
ments showed absence of leaching of the slag components into the re-
action media.
There are just few studies reported in literature and related to the
current reaction [15,16]. Typically employed basic catalysts – K2CO3
[15], NaAlO2 [16] and hydrotalcites [16], were evaluated at tempera-
tures different to the temperature employed in the current work (90 ◦C
[16] or 165 ◦C [15] vs 150 ◦C). Approximate comparison between
current study and catalysts evaluated at 165 ◦C [15] demonstrated much
higher activity of the potassium carbonate (91% conversion in 3 h [15])
in comparison with the slag catalysts. However, such high activity can
be afforded by a partial solubility of the catalyst components into the
reaction media, which is typical for the materials such as K2CO3 [56] or
NaAlO2 [57]. A visual inspection of the spent catalysts as well as analysis
of the reaction media after the experiment by ICP-OES demonstrated
absence of leaching of the slag components into the reaction media.
Thereby, synthesized slag-based materials can be considered as
promising heterogeneous catalysts for the based-catalyzed carbox-
ymethylation of cinnamyl alcohol with dimethyl carbonate.
CRediT authorship contribution statement
Ekaterina Kholkina: Investigation, Data curation, Writing - original
draft. Narendra Kumar: Conceptualization, Supervision, Writing - re-
¨
view & editing. Kari Eranen: Methodology. Markus Peurla: Data
curation. Heikki Palonen: Data curation. Jarno Salonen: Data cura-
tion, Methodology. Juha Lehtonen: Funding acquisition, Methodology.
Dmitry Yu. Murzin: Conceptualization, Funding acquisition, Supervi-
sion, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgements
4. Conclusions
Authors acknowledge the financial support of the Finnish Funding
Agency for Innovation (Tekes, 2016–2017) and Business Finland
(L4Value project, 2017-2020). E. Kholkina is grateful to Åbo Akademi
University for the partial financial support (Doctoral research grant
2020).
Novel low-cost catalytic materials were synthesized from the desul-
furization steel slag by treatment with different treating agents under
various synthesis conditions. Ultrasound irradiation was applied as an
additional tool for the slag processing. Properties of the synthesized
materials were determined by a wide range of analytical techniques.
Based on the obtained results ultrasound irradiation can be considered
as an effective tool influencing not only slag dissolution but also crys-
tallization of the certain phases.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.ultsonch.2021.105503.
Crystal morphology such as shape, size and distributions of the metal
oxide nanoparticles were observed to be influenced by the variation in
synthesis parameters of ultrasound irradiation such as power and
duration. Based on the observed changes in crystallinity during treat-
ment of the slag it can be concluded, that the initial waste material
undergoes dissolution with formation of a gel and its subsequent
recrystallization into new phases. Most types of synthesis resulted in
highly crystalline materials in comparison with the raw industrial slag.
However, utilization of the treating agents with a high leaching effect –
HCl and EDTA, led to a shape-less morphology of the resulting materials
due to high calcium leaching and its poor precipitation. At the same
time, high calcium dissolution resulted in formation of the internal
channels being the cause of a high surface area of the synthesized
materials.
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