Doping Metal Organic Frameworks with Ionic Liquids for

Adsorption of Tetracyclines from Water Samples

Yohannes Alula, Jie Tang and Shun Yao

College of Chemical Engineering, Sichuan University, Chengdu,610065, China

Keywords: Metal Organic Frameworks, Ionic Liquids, Tetracyclines, Adsorption.

Abstract: Ionic liquids (ILs) were supported on metal organic frameworks (MOFs) for adsorption of tetracyclines

(TCNs) from water samples. Benzothiazolium haxafluorophosphate IL when combined with Zeolitic

imidazolate framework-8 MOF ([HBth][PF

] @ZIF-8) demonstrated successful adsorption performance.

1 INTRODUCTION

The tetracyclines (TCs), including tetracycline,

chlorotetracycline and oxytetracycline, are a

chemically related group of bacteriostatic

antibiotics. They exhibit a broad spectrum of

antimicrobial activity, being effective against gram

positive and gram-negative aerobic and anaerobic

bacteria. Despite their introduction more than 50

years ago, tetracyclines are still widely popular for

human medicines (Katrin et al., 2016). Although

exposure to tetracyclines is well tolerated, they do

produce several adverse effects including

gastrointestinal irritation, phototoxicity and teeth

staining. They can also be introduced in surface

waters due to their veterinary use for livestock,

which have been ubiquitously available in the

aquatic environment via manure dispersion and

animal excretion. Their presence in natural waters

inevitably yields the spread of antibiotic resistance

in microorganisms. Therefore, there is a need to

bring effective method for the decontamination of

these drugs from water bodies.

The aim of this study is to enhance the efficiency

of potential adsorbents such as Metal Organic

Frameworks (MOFs), which have attracted

considerable attention for adsorption technology

owing to their high porosity, in removing TCs from

aquatic system. Metal–organic frameworks are a

class of crystalline porous hybrid materials

constructed after infinite connectivity of metal ions

or nodes and organic molecules called linkers via

self-assembly mechanism. MOFs have superior

adsorption properties compared to other porous

materials due to their large surface areas as well

tunable pore sizes varied from microporous to

mesoporous scale. Hence, the reported MOFs

(Rocío-Bautista et al., 2015) with various structural

topologies and unique functionalities is increasing

rapidly. Recently, MOFs have been studied for

adsorption of TCs from environmental waters. Wang

and coworkers (Wang et al., 2018) employed Fe-

based MOFs in removal of TCs by adsorption and

photocatalytic degradation simultaneously.

Moreover, some active species like metal

nanoparticles were also incorporated into the pores

of MOFs for better adsorption of TCs. Yang group

(Yang et al., 2019) reported an increase in

adsorption capacity of Cu and Co nanoparticles co-

doped MIL-101 (Cr) (Material Institute Lavoisier)

for TCs compared with the pristine MOF.

MOFs could be further customized by combining

with ionic liquids (denoted as IL@MOFs) which

could be able to overcome the drawbacks of parent

MOFs related to their poor dispersion in water. Ionic

liquids (ILs) are salts consist of positive and

negative charges bound together by electrostatic

interactions with melting points below 100

C. The

chemical and structural versatility of ILs make them

suitable guest molecule candidates for the facile

post-modification of MOFs as a host materials

(Kitagawa et al., 2015). As a consequence of the

coalition with MOFs, the ILs lose their liquid nature

but they still maintain their thermal stability and

chemical properties. Despite its great potential,

limited studies have been conducted on IL@ MOF

composite for adsorption applications. Jhung and

coworkers (Jhung et al., 2014) incorporated 1-butyl-

3-methylimidazolium chloride into the pores of

Alula, Y., Tang, J. and Yao, S.

Doping Metal Organic Frameworks with Ionic Liquids for Adsorption of Tetracyclines from Water Samples.

DOI: 10.5220/0008185700690072

In The Second International Conference on Materials Chemistry and Environmental Protection (MEEP 2018), pages 69-72

ISBN: 978-989-758-360-5

MIL-101 by means of simple impregnation for the

adsorptive removal of benzothiophene from liquid

fuel. Later on, the same research group synthesized

the IL inside inside MIL-101 porous cavities via a

ship-in-bottle (SIB) technique for similar objectives

(Jhung et al., 2016). In this work, we explored

several ILs and MOFs when combined produces

synergetic effect for successful adsorptive

elimination of TCs from water samples. To the best

of our knowledge, no other published work

attempted to investigate IL@MOF for the same

application.

2 EXPERIMENTAL

2.1 Chemicals

Terephthalic acid (TPA, 98%), Chromium chloride

(CrCl

.6H

O, 96%), Zinc nitrate hexahydrate

(Zn(NO

)

.6H

O, 99%), 2-Methylimidazole (H-

MIM, 99%), Benzene- 1,3,5-tricarboxylicacid

btc, 95%), Cu(NO

)

.2.5H

O, Chlorotetracycline,

Oxytetracycline and Tetracycline were supplied by

Kelong Chemicals (Chengdu, China). Ultrapure

water was obtained by a water purification system

A10 MilliPore (Bedford, MA). The ten ionic liquids

were chosen for this study, which included 1-butyl-

3-methylimidazolium chloride ([C

MIM][Cl

]), 1-

hexyl-3-methylimidazolium chloride ([C

MIM][Cl

]), 1-butyl-3-methylimidazolium tetrafluoroborate

([C

MIM][BF

]), 1-hexyl-3-methylimidazolium

tetrafluoroborate ([C

MIM][BF

]), 1-butyl-3-

methylimidazolium hexafluorophosphate

([C

MIM][PF

]), 1-hexyl-3-methylimidazolium

hexafluorophosphate ([C

MIM][PF

]),

benzothiazolium tetrafluoroborate ([HBth][BF

]),

benzothiazolium hexafluorophosphate

([HBth][PF

]), 8-hydroxyquinolinephosphate

([HOQu][H

]), 8-hydroxyquinolinesulfate

([HOQu][HSO

]) and were synthesized in our lab

according to reported methods.

2.2 Synthesis of MOFs

2.2.1 Synthesis of ZIF-8

ZIF-8 (Zeolitic Imidazolate Framework) was

synthesized according to the procedure of Kitagawa

group (Kitagawa et al., 2015). A methanol solution

(75 mL) of Zn(NO

)2•6H

O (1.10 g, 3.70 mmol)

was added to a methanol solution (75 mL) of

H(MeIM) (1.22 g, 14.8 mmol) and triethylamine

(3.00 g, 29.7 mmol) at ambient temperature. The

mixture was stirred vigorously for a period of 1 h at

room temperature. The resultant white-colored

precipitate was collected using centrifugation, and

washed five times by methanol followed by drying

at 150

2.2.2 Synthesis of MIL-101

MIL-101(Cr) was synthesized according to Jhung

(Jhung et al., 2016) group. In a typical synthesis,

MIL-101 was prepared from CrCl

•6H

O, TPA and

deionized water with reactants composition 1.0

CrCl

•6H

O : 1.0 TPA : 300 H

O. The precursor of

30 g was loaded in a Teflon-lined autoclave and put

in a preheated electric oven at 210

C for 8 hours.

After the reaction, the autoclave was cooled to room

temperature and solid green-colored products were

recovered by filtration. The MOF obtained was

purified in three steps. In the first step, 1.0 g MIL-

101 was added to 300 mL water and stirred

magnetically for 5 hours at 70

C. Then the MOF

was filtered and dried overnight. In the second step,

the dried MOF was added to 250 mL ethanol; stirred

magnetically at 60

C for 3 hours and then filtered.

In the third step, the MOF from the second step was

added to 150 mL 30 mM NH

F solution and stirred

for 10 hours at 60

C. Finally, it was filtered, washed

five times and then dried at 150

2.2.3 Synthesis of HKUST-1

HKUST-1 was synthesized according to the

literature (Rocío-Bautista et al., 2015). Typically,

btc (0.630 gram, 3 mmol) was dissolved in 15 mL

of ethanol. Secondly, Cu(NO

)

•2.5H

O (1.255

gram, 5.4 mmol) was dissolved in 15 mL of

deionized water and then added dropwise to the

ethanolic solution of H

btc while stirring for 10 min.

The resultant solution mixture was transferred into a

45 mL Teflon lined stainless steel autoclave and

kept at 110

C for 24h. Then, the autoclave was

cooled down to room temperature and the crystalline

powder was isolated by filtration, washed with

ethanol several times and air dried at 50

C. Finally,

the purified MIL-101 was dehydrated at 150

overnight.

2.3 Synthesis of IL@MOF Composites

Wet impregnation method (Henni et al., 2018) was

used for synthesizing the IL/MOF composites. The

specified amount of IL was weighed out in a

scintillation vial. Then 3 mL of methanol was added

to the specified amount of IL, and the vial was

gently mixed until the IL was dissolved. The total

MEEP 2018 - The Second International Conference on Materials Chemistry and Environmental Protection

amount of ionic liquid used for the impregnation

varied depending on the maximum value of IL

desired in the resulting composite. The solution was

added dropwise to the preweighted ZIF-8 material,

and the composite was dried at 70 °C overnight to

remove any remaining solvent and stored in a

desiccator.

2.4 Characterization

The size and image of microcrystals was observed

with JSM-7001F scanning electron microscopy

(SEM) (JEOL Co., Ltd., Tokyo, Japan) and X-ray

diffraction (XRD) patterns were recorded on a D8

X-ray diffractometer equipped with accessional

analytical system (Bruke, Karlsruhe, Deutschland).

Thermogravimetric analysis (TGA) was performed

on a TG 209 Fl Iris instrument (NETZSCH-

Gerätebau GmbH, Selb, Deutschland) with a heating

rate of 10

C min

1 from 30 to 800

C under

nitrogen.

2.5 Adsorption Experiment

All the IL@MOFs were heated at 150

C for 12

hours in a vacuum oven before being used as an

adsorbent. The adsorption study was carried out as

follows: 5 mg IL@MOF mixed with 20 mL sample

solution of 10 ppm in 50 mL Erlenmeyer flask. The

mixture was shaken in a water-bath oscillator (100

rpm) for 35 min 25

C. After full adsorption, the

flask was centrifuged at 6000 rpm for 5 min, and

then the UV-visible absorbance of the supernatant

was measured at 355 nm.

Table 1: Data for adsorption efficiency of Oxytetracycline

over ZIF-8 and [HBth][PF

]@ZIF-8.

Adsorbent

E (%)

ZIF-8

10% IL@ZIF-8

20% IL@ZIF-8

25% IL@ZIF-8

30% IL@ZIF-8

Table 2: Data for adsorption efficiency of TCNs over

[HBth][PF

]@ZIF-8 in 0.30 to 1 molar ratio.

TCNs

E(%)

chlorotetracycline

82.2

oxytetracycline

89.8

tetracycline

91.5

E % = ((C

– C

) × 100%)/C

(1)

3 RESULTS AND DISCUSSION

3.1 Properties of Adsorbents

The SEM images presented in Figure 1 and 2 show

that the surface morphology of ZIF-8 changed after

incorporation of ILs which indicates the existence of

guests inside the pores of MOFs. The XRD

experiment illustrated that the crystal phases of

pristine and modified MOFs have remarkable

similarity, which is confirming successful synthesis

and stability of the MOFs even after decorated with

ionic liquids. TGA analysis revealed the synthesized

adsorbents are stable up to nearly 400

Figure 1: SEM image of ZIF-8 before incorporation of IL.

Figure 2: SEM image of [HBth][PF

]@ZIF-8 composite.

3.2 Adsorption Efficiency of MOFs

An initial investigation was carried out to identify

the appropriate MOF as an adsorbent based on

calculation of adsorption efficiency (E %) for each

MOF according to equation 1, where, C

and C

(mgL

-1

) are Concentrations of TCs before and after

adsorption respectively. Considering that the

adsorption is performed in aqueous solution, the

potential adsorbents needs to be water stable. Three

Metal Organic frame works specifically, MIL-101,

HKUST-1 and ZIF-8 were chosen and examined for

adsorptive removal of one type of TCs (named

oxytetracycline, OTC) from aqueous solution.

Parameters that affect the adsorption condition such

as, initial concentration of adsorbent and adsorption

time were optimized for each of the three MOFs

before the study of their adsorptive performances.

Doping Metal Organic Frameworks with Ionic Liquids for Adsorption of Tetracyclines from Water Samples

The results reveal that E (%) is 58.2%, 55.6%, 49.3

% for MIL-101, HKUST-1 and ZIF-8, respectively.

This trend could be attributed to the complex

formation reaction between TCNs and

coordinatively unsaturated (CUS) sites in MIL-101

and HKUST-1. The absence of open metal site may

have an impact in the case of ZIF-8 for its relatively

lowered adsorption performance.

3.3 Effect of Ionic Liquids

One of the alternatives considered to upgrade the

adsorption properties of those MOFs was to decorate

with ILs following a post-synthetic modification. It

was expected to lead to a higher performance for

adsorption than the pristine MOFs. Ten ionic liquids

were tested as potential guests, introduced into the

pores of virgin MOFs targeted for TCs adsorption.

Tetracyclines are complex hydrophilic drugs having

high solubility in water and can exist both in acidic

and basic form. The highest adsorption efficiency of

77.4% were obtained for the three TCs during the

combination of [HBth][PF6] with ZIF-8. The best

result was specifically obtained when [HBth][PF6]

to ZIF-8 ratio was increased to 25 % as depicted on

Table 1. The high adsorption efficiency of the

IL@MOFs could be due to the capability of ILs to

create complex interaction of columbic forces and π-

π interaction with TCs. Generally the ILs improves

the adsorption efficiency of all the studied MOFs to

a greater extent.

3.4 Doping by Capillary Action

Once [HBth][PF6] @ZIF-8 was selected as the best

adsorbent, improving the stability of the hybrid

composite, in order to keep the ILs inside the pores

of MOF was considered. The effect of capillary

action was studied as an alternative synthesis

strategy, since ZIF-8 has no open metal sites. The

[HBth][PF

] was mixed with activated ZIF-8 powder

using mortar and pestle in a molar ratio of 0.20:1,

0.25:1, 0.30:1 and 0.35:1. The mixture was heated

and stored overnight to enhance the diffusion of the

IL through pores of ZIF-8. As shown in Table 2, the

adsorbed quantities of TCNs increased to a

significant extent until the IL to MOF ratio became

0.30 to 1. The capillary action improves the

confinement of IL inside ZIF-8 pores and hence, the

stability of the resulting composite, which brought

great potential in the way of increasing the

adsorption efficiency of TCNs.

4 CONCLUSIONS

In summary, imidazolium, hydroxyquinolium and

benzothiazolium ionic liquids were impregnated on

water stable metal organic frameworks and

investigated for adsorption of TCNs. ZIF-8 when

combined with one type of benthothiazolium IL

showed highest adsorption efficiency, especially

after doping by capillary action. The result proves

that the adsorption properties of MOFs could be well

improved by using appropriate ILs.

ACKNOWLEDGEMENTS

The authors thank the National Natural Science

Foundation of China (No. 81373284, 81673316).

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MEEP 2018 - The Second International Conference on Materials Chemistry and Environmental Protection