Synthesis of Polymeric Sorbents with Magnetic Properties

OLEG MANAENKOV, OLGA KISLITSA

Department of biotechnology, chemistry and standardization

Tver State Technical University

Tver, Af. Nikitina, 22

RUSSIA

Abstract: In this work, a procedure for the synthesis of polymeric sorbents based on hypercrosslinked

polystyrene (HPS) with magnetic properties was developed. The technique is based on the reduction of iron

salts with polyhydric alcohols at high temperature in an inert atmosphere. The resulting sorbents retain their

original characteristics: an extended specific surface area, micro-mesoporous structure, acquiring magnetic

properties, which makes it possible to use them as magnetically separable sorbents, supports for the active

phase of heterogeneous catalytic systems etc.

Key-Words: hypercrosslinked polystyrene, sorbent, magnetite, materials with magnetic properties.

Received: May 23, 2022. Revised: October 17, 2022. Accepted: November 23, 2022. Published: December 31, 2022.

1 Introduction

Hypercrosslinked polystyrene (HPS) is a porous

structure spontaneously formed during synthesis.

This polymer is characterized by the presence of

rigid cavities with a diameter of about 2–3 nm, and

has a high degree of crosslinking (more than 100 %)

[1]. The micro-mesoporous structure of the HPS

polymer matrix, and an extended specific surface

area (usually about 1000 m2/g) determine the

excellent sorption properties of the material.

Sorbents based on HPS simultaneously combine

high adsorptive capacity, the ability to swell in

solvents of various nature, high molecular-size

selectivity with easy regeneration, good kinetic

characteristics, stability in aggressive medium, and

the absence of volumetric changes varying ionic

strength, pH, or type of solvent [2, 3].

Other advantages of HPS include its mechanical

strength, chemical and thermal (up to 400 °C)

stability. It makes the HPS an excellent alternative

to traditional supports (C, Al2O3, SiO2, etc.) used for

the synthesis of heterogeneous catalytic systems [2-

4]. Structural features of HPS provide wide

opportunities for its use as a matrix for

nanocomposites. The movement and agglomeration

of particles embedded in HPS cavities is difficult

due to the small size of the pores and channels of

the polymeric network. Since the network

parameters are determined by the synthesis

conditions, the pore size differs insignificantly. This

fact is of the great importance when developing

different materials, in particular, catalysts, since the

enlargement of particles decreases the inner surface

of the particles, and, consequently, the activity of

the catalyst [5-7].

In this regard, the modification of HPS in order

to impart magnetic properties is of high interest.

Sorbents with magnetic properties can be very

useful in various industries due to its ability to be

quickly and easily separated by a magnetic field [8].

There are two methods for obtaining magnetic

polymeric sorbents: the synthesis of magnetic

nanoparticles (MNPs) in pores of the matrix and the

synthesis of a polymer in the presence of MNPs.

The first method seems to be the simplest. It

includes the chemical precipitation of magnetite

(Fe3O4) from iron (II) and (III) salts [9]. For

example, Pastukhov and colleagues [10] developed

a magnetic nanocomposite material based on

MN200 HPS. To introduce iron hydroxide

nanoparticles, the above neutral polymers were

impregnated with a solution of Fe (II) and Fe (III)

salts and then treated with an aqueous solution of

ammonia. It was shown that the impregnated

magnetite nanoparticles were found to have a

particle size up to 6 nm in microporous composites,

while for the biporous HPS (with micro-mesoporous

structure), magnetite nanoparticles were about 16

nm. According to the results of sorption studies, the

developed nanocomposite magnetic sorbents have

high capacity with respect to some physiologically

active compounds along with the original HPS.

Another approach was implemented in [11].

Preliminarily synthesized magnetite nanoparticles

were introduced into the polymer matrix at the stage

of hypercrosslinking. The resulting magnetic

sorbent showed high sorption capacity to antibiotics.

International Journal of Chemical Engineering and Materials

DOI: 10.37394/232031.2022.1.5

Oleg Manaenkov, Olga Kislitsa

E-ISSN: 2945-0519

Volume 1, 2022

The approach based on the in-situ synthesis of

magnetic nanoparticles showed a better distribution

of nanoparticles in the porous structure of resins,

while no agglomeration phenomena were detected

[2]. On the contrary, the ex-situ method revealed in

some cases of magnetite nanoparticles

agglomeration on the surface of resin supports [12].

In the present work, we propose a new method

for the synthesis of magnetically recoverable

composites with high saturation of magnetization

based on MN270 HPS (non-functionalized), which

can be used directly as sorbents or as supports for

heterogeneous catalytic systems.

2 Experimental

Fe3O4/HPS MN270 composites were synthesized

according to the following procedure. FeCl3∙H2O

was dissolved in 95 wt.% ethanol. As received HPS

MN270 was added to the resulting solution, mixed

thoroughly and left for 15 min for impregnation.

Then, a calculated portion of sodium acetate was

added dropwise to the solution. The mixture was

dried at a temperature of 70 °C until complete

removal of ethanol. The powder was wetted with

ethylene glycol and placed in a quartz tube purged

with an argon. The tube was heated in an electric

furnace up to 300 °C and kept for 5 h in an argon

flow. The synthesized Fe3O4/HPS MN270 powder

was washed several times with water, then with

ethanol, and dried to constant weight in an oven at a

temperature of 70 °С .

The thermal stability of the initial HPS MN270

sample was studied using a TG 209 IRIS

thermogravimetric analyzer (NETZSCH, Germany).

The values of the specific surface area of the

samples were determined by the method of low-

temperature nitrogen physisorption using a

Beckman CoulterSA 3100 surface analyzer (USA).

Transmission electron microscopy (TEM) was

performed using a JEM1010 microscope (USA).

The magnetic properties of the samples were

determined using a VIBRACh vibrating

magnetometer (TVGU, Russia). X-ray fluorescence

analysis was performed on a Zeiss Jena VRA-30

spectrometer (Germany).

3 Results and discussion

To estimate the thermal stability of the HPS

polymer matrix, a thermogravimetric (TG) study of

the prepared and crushed polymer (particle size less

than 60 μm) was carried out. The results of

thermogravimetric analysis of the initial HPS

MN270 sample are shown in Figure 1.

An intense, multi-stage decomposition of the

polymer probably associated with the destruction of

methylene crosslinks seems to begin at a

temperature of about 450 °C. This temperature

corresponds to the maximum rate of polymer weight

loss, 10 %/min. The resulting weight loss was

approximately 55 %. These facts confirmed the

possibility of synthesizing magnetite particles in the

pores of the HPS matrix by thermal reduction of

iron salts at temperatures up to 400 °C.

Fig. 1 Results of thermogravimetric analysis of

HPS MN270.

In the study, the method for synthesizing

magnetite particles in HPS pores was optimized.

The use of iron (III) nitrate as a precursor was found

to be unacceptable due to the formation of a

significant amount of oxygen during the nitrate

thermal decomposition, as it was shown in the

previous study [13].

4Fe(NO3)3 → 2Fe2O3 + 12NO2 + 3O2,

The high amount of oxygen formed can lead to the

destruction of the porous structure HPS due to its

oxidation (Table 1).

Table 1. Specific surface area of samples.

Sample

SBET,

m2/g

SL,

m2/g

St,

m2/g

HPS MN270

1075

1191

2651;

8072

Fe3O4/HPS MN270

450

480

1601;

2892

Fe3O4/HPS MN270

301;

SBET is the specific surface area (BET model); SL is the

specific surface area (Langmuir model); St is the

specific surface area (t-plot); 1specific surface area of

micropores; 2specific surface area according to a t-plot

model.

It can be seen from the data in the table that the

specific surface area of a sample obtained using iron

International Journal of Chemical Engineering and Materials

DOI: 10.37394/232031.2022.1.5

Oleg Manaenkov, Olga Kislitsa

E-ISSN: 2945-0519

Volume 1, 2022

(III) nitrate (Entry 3) is by 45-50 times less than that

for the sample synthesized using FeCl3 (Entry 2) as

a precursor.

Thus, for the synthesis of Fe3O4/HPS MN270

composites, iron (III) chloride was used as a

precursor. Then the reactions occurring during the

synthesis according to the above method can be

represented by the equation below. As a result of the

exchange reaction, Fe(CH3COO)3 is formed in the

pores of the polymer:

FeCl3 + 3CH3COONa → Fe(CH3COO)3 +

3NaCl.

In this case, it is important to use 95 % ethanol as

a solvent to prevent the hydrolysis of the resulting

iron acetate. In the case when hydrolysis does not

occur, the subsequent thermal decomposition of

acetate at 300 °C proceeds according to the

following mechanism [14]:

6Fe(CH3COO)3 → 2Fe3O4 + 9CH3COCH3 +

9CO2 + ½O2,

4Fe(CH3COO)3 → 2Fe2O3 + 6CH3COCH3 +

6CO2,

2Fe3O4 + ½O2 → 3Fe2O3.

It should be noted that in these reactions, a much

smaller amount of oxygen is formed. It can be

quickly removed from the reaction zone by constant

purging with an inert gas (argon). Thus, the oxygen

destructive effect on the polymer matrix is

significantly reduced.

The synthesized samples of Fe3O4/HPS MN270

composites were characterized by physicochemical

methods. Table 2 presents the results of elemental

analysis, which shows that the developed synthesis

procedure makes it possible to obtain samples with

desired iron content in the magnetite composition.

Table 2. Results of elemental analysis of Fe3O4/HPS

MN270 samples.

Sample

Fe, %

Fe3O4/HPS MN270 (1:1)*

17.1

Fe3O4/HPS MN270 (1:2)

19.6

Fe3O4/HPS MN270 (1:3)

25.4

* - parentheses indicate the mass ratio of the initial HPS

and FeCl3.

The results of the study by the method of low-

temperature nitrogen adsorption showed that the

synthesized composites seem to be micro-

mesoporous. The ratio of micro- and mesopores and

their specific surface area are significantly affected

by the introduction of magnetite particles into the

polymer matrix (Table 3).

Table 3. Specific surface area and pore volume of

samples with different iron contents.

Sample

SBET,

m2/g

SL,

m2/g

St,

m2/g

cm3/g

HPS MN270

1075

1191

2651;

8072

0.37

Fe3O4/HPS

MN270 (1:1)

730

825

2241;

5062

0.22

Fe3O4/HPS

MN270 (1:2)

656

752

1581;

4982

0.23

Fe3O4/HPS

MN270 (1:3)

450

480

1601;

2892

0.13

SBET is the specific surface area (BET model); SL

is the specific surface area (Langmuir model); St is

the specific surface area (t-plot); 1specific surface

area of micropores; 2specific surface area according

to a t-plot model.; V – micropores volume.

It was assumed that the formation of magnetite

particles occurs mainly on the HPS surface and at

the mouths of the pores. The last ones can be

blocked by the magnetite nanoparticles and, as a

result, a decrease in the specific surface area and a

change in the ratio of micro-, meso-, and

macropores of the samples can be observed. This

assumption was confirmed by the TEM results. It is

shown that (Figure 2) the formation of Fe3O4

actually occurs mainly on the surface, at the mouths

of the pores of the HPS polymer matrix. The particle

size of magnetite was found to be 40 ± 5 nm.

Fig. 2 TEM images of the Fe3O4/HPS MN270

sample.

International Journal of Chemical Engineering and Materials

DOI: 10.37394/232031.2022.1.5

Oleg Manaenkov, Olga Kislitsa

E-ISSN: 2945-0519

Volume 1, 2022

The composition of the surface of

nanocomposites was determined by X-ray

photoelectron spectroscopy (XPS) (Table 4).

Table 4. Surface composition of Fe3O4/HPS MN270

based on XPS analysis results.

Element

at%

wt%

C 1s

66.8

45.5

O 1s

25.8

23.5

N 1s

0.2

Cl 2p

0.4

0.8

Fe 2p3/2

3.5

11.1

The magnetic properties of Fe3O4/HPS MN270

were also studied.

Fig. 3 Magnetization curves Fe3O4/HPS MN270

(1:3).

It has been shown that the experimental samples

have a five times higher saturation magnetization

(about 4.0 ± 0.5 emu/g, Figure 3) than the value for

magnetically separable catalysts based on

mesoporous silicon dioxide developed by us in

previous studies [13]. Such magnetization allows

the composite to be easily separated from the liquid

phase by external magnetic field.

The characteristic shape of the magnetization

curves indicates the superparamagnetic nature of the

sample and confirms the formation of magnetite

particles in the pores of the HPS polymer matrix.

The magnetite nature of the magnetic particles

was also confirmed by X-ray powder diffraction

(Figure 4). The diffraction pattern of Fe3O4/HPS

MN270 contains a set of clear Bragg reflections, the

intensity and position of which are typical for

magnetite.

Fig. 4 XRD pattern of the Fe3O4/HPS MN270

(1:3).

4 Conclusion

Based on the results of the study, the following

conclusions can be done:

- a new method for the synthesis of sorbents

based on a polymer matrix of hypercrosslinked

polystyrene with magnetic properties (saturation

magnetization of at least 4.0 ± 0.5 emu/g) was

proposed. This method is characterized by the

absence of processes of destruction of the porous

polymer structure;

- the magnetite nature of the particles formed in

the pores of HPS during the synthesis was

confirmed by the various methods;

- the synthesized composites can be used directly

as sorbents or as support for heterogeneous catalytic

systems.

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Oleg Manaenkov, Olga Kislitsa

E-ISSN: 2945-0519

Volume 1, 2022

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Oleg Manaenkov, Olga Kislitsa ‒ investigation,

data curation, original draft preparation, review and

editing.

This work was supported by the Russian Science

Foundation (22-79-10096).

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(Attribution 4.0 International, CC BY 4.0)

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Commons Attribution License 4.0

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International Journal of Chemical Engineering and Materials

DOI: 10.37394/232031.2022.1.5

Oleg Manaenkov, Olga Kislitsa

E-ISSN: 2945-0519

Volume 1, 2022