A. Yohannes, J. Li and S. Yao
Journal of Molecular Liquids 318 (2020) 114304
liquid fuel. Later on, the same research group synthesized the IL inside
MIL-101 porous cavities via a ship-in-bottle (SIB) technique for similar
objectives [22]. In this work, it aimed to explore 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 in the
same application for the three representative TCs.
method [24]. The typical procedure is as follows. (1) 0.05 mol
(6.7590 g) benzothiazole, 0.06 mol methyl bromide and 0.06 mol so-
dium hexafluorophosphate or 0.06 mol fluoroboric acid were placed
in a 50 mL round-bottom flask equipped with a magnetic stirrer;
(2) The reaction mixture was reacted for 6 h at 100 °C, and yellow
paste crude product was collected. It was mixed with proper amount
of distilled water and filtered to remove unreacted salts. The filter
cake was washed with 10 mL ether for three times, and then washed
with distilled water until there was no precipitate formation in the
washed water with the silver nitrate test. The filter cake was re-
crystallized from hot anhydrous ethanol and colorless flake crystal
was finally obtained after vacuum dryness. The yields of these
benzothiazolium ILs were in the range of 92%–97%.
2. Experimental
2.1. Materials and instruments
3 2 2
Ethanol, zinc nitrate hexahydrate (Zn(NO ) ·6H O, 99%), 2-
methylimidazole (99%), benzene-1,3,5-tricarboxylic acid (H3BTC, 95%),
terephthalic acid (98%), chromium chloride (CrCl ·6H O, 96%), copper
nitrate hemipentahydrate Cu(NO ·5H O, fluoroboric acid, 8-
hydroxyquinoline, phosphoric acid, chlorotetracycline, oxytetracycline
and tetracycline were supplied by Kelong Chemicals (Chengdu, China).
Benzothiazole, methyl bromide and sodium hexafluorophosphate were
supplied by Sigma-Aldrich Co. Ltd. (USA). Ultrapure water was attained
by a water purification system A10 MilliPore (Bedford, MA). The ten
ionic liquids with three main types of cations and four kinds of anions
were chosen for this study, including 1-butyl-3-methylimidazolium chlo-
Finally, the following imidazolium ILs were synthesized according to
previously reported procedures [25]:1-butyl-3-methylimidazolium chlo-
3
2
)
3 2
2
ride ([C
[Cl]), 1-butyl-3-methylimidazolium tetrafluoroborate ([C
hexyl-3-methylimidazolium tetrafluoroborate ([C MIM][BF
3-methylimidazolium hexafluorophosphate ([C MIM][PF ]), 1-hexyl-3-
methylimidazolium hexafluorophosphate ([C MIM][PF ]). The reaction
6
MIM][Cl]), 1-hexyl-3-methylimidazolium chloride ([C
MIM][BF
]), 1-butyl-
4
MIM]
4
]), 1-
4
6
4
4
6
6
6
process was described briefly as follows: bromobutane (0.8 mol) and N-
methylimidazole (0.1 mmol) were mixed in a round bottom flask at
82 °C under refluxing and stirring. After stirring for 24 h, the residual sol-
ride ([C
6
MIM][Cl], 1), 1-hexyl-3-methylimidazolium chloride ([C
MIM][BF
MIM][BF ], 4),
MIM][PF ], 5),
MIM][PF ], 6),
], 7), benzothiazolium
], 8), 8-hydroxyquinolinephosphate
], 9), and 8-hydroxyquinolinesulfate ([HOQu][HSO ], 10)
4
MIM]
vents were removed under vacuum at 40 °C. Then the resulting [[C
Br products were washed with appropriate solvents and were placed in a
vacuum drying oven for 24 h. Proportional mole ratio of [[C MIM]Br and
NaBF were made to react using acetone as solvent for 10 h at 40 °C under
vigorous agitation. The reaction product was separated, and dichloro-
methane was added to the residue of reaction mixture, and white solids
precipitate was recovered. In the next step, the solid product was isolated
4
MIM]
[Cl], 2), 1-butyl-3-methylimidazolium tetrafluoroborate ([C
4
4
],
3), 1-hexyl-3-methylimidazolium tetrafluoroborate ([C
1-butyl-3-methylimidazolium hexafluorophosphate ([C
1-hexyl-3-methylimidazolium hexafluorophosphate ([C
6
4
4
4
6
4
6
6
benzothiazolium tetrafluoroborate ([HBth][BF
hexafluorophosphate ([HBth][PF
[HOQu][H PO
4
6
(
2
4
4
by filtration while the IL ([C
80 °C for 2 h to avoid the residual dichloromethane. In a similar fashion,
([C MIM][Cl]) and ([C MIM][PF ]) were prepared by using HCl and KPF
instead of NaBF Moreover, ([C MIM][Cl]), ([C MIM][BF ]) and
([C MIM][PF ]) were prepared by using bromohexane instead of
4 4
MIM][BF ]), was vacuum dried in an oven at
were synthesized in our lab, which were identified by their melting points
and spectral absorbance. All the structures of TCs and ILs are shown in
Scheme 1.
Infrared Spectroscopy L1600300 (PerkinElmer, Fremont, USA) was
utilized for identification of function groups of the composite IL@ZIF
material. The size and image of microcrystals was viewed with JSM-
4
4
6
6
4
.
6
6
4
6
6
bromobutane. The yields of these imidazolium ILs were in the range of
90%–94%.
7
001F scanning electron microscopy (SEM) (JEOL Co., Ltd., Tokyo,
Japan) and X-ray diffraction (XRD) patterns were recorded on a D8
2.3. Synthesis of IL@MOF (ZIF-8)
type X-ray diffractometer fitted with accessional analytical system
(
Bruke, Karlsruhe, Deutschland). Thermogravimetric analysis (TGA)
The standard method of MOF synthesis is solvothermal method in
which reactants are mixed in high boiling polar solvents such as
water, dialkyl formamides, dimethyl sulfoxide or acetonitrile [26]. Hy-
drothermal method is a subset of solvothermal method which uses
water as a solvent. In the following part, three kinds of MOF materials
(including ZIF-8, MIL-101 and HKUST-1) were firstly synthesized in pre-
vious ways [27–29] and then combined with the former ILs.
was implemented using TG 209 Fl Iris instrument (NETZSCH-Gerätebau
−1
GmbH, Selb, Deutschland) with heating rate of 10 °C min from 30 to
00 °C under nitrogen. For evaluation of adsorbed target TCs UV–Vis
8
spectrometer TU-1810 (Purkinje General Instrument Co., Ltd., Beijing,
China) was used.
2
.2. Preparation of candidate ionic liquids
The former reported strategy for the synthesis of IL@MOF composites
was based on ionothermal method by the use of an IL as both the solvent
and template or structure-directing agent [30]. However, cations are not
able to show similar physicochemical behaviors as the bulk ILs, which is
a disadvantage for subsequent applications. The recent strategy towards
the synthesis of IL@MOF composite involves heterogenization of ILs
based on post-synthetic modification of MOFs. The typical procedure for
the synthesis of IL@ZIF-8 by wet impregnation was as follows: certain
amount of the previously synthesized IL was weighed and placed in an Er-
lenmeyer flask. Then 3 mL of methanol was added to the flask containing
IL, and the vial was gently mixed until the IL was dissolved. The total
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 compos-
ite was dried at 70 °C overnight to remove any remaining solvent and
stored in a desiccator. In another strategy called capillary action, the IL
and MOF are directly mixed without using a solvent and ILs are loaded
into the pores of MOFs through capillary action. The MOF is first dried
to remove all impurities from the pores before being mixed with the IL.
Firstly, the synthesis of quinolinum ionic liquids was made following
the procedures previously reported by our group with minor modifica-
tions [23]. Typically, 8-hydroxyquinoline (0.05 mol) was dissolved in
5
phosphoric acid (0.05 mol) dissolved in 30 mL anhydrous ethanol was
added dropwise into the aforementioned solution over a period of
3
and the crude products were washed three times with 15 mL acetone.
The obtained crude product was recrystallized from anhydrous ethanol
and further purified by recrystallization from mixed solution of ethyl ac-
etate and ethanol. At last, the crystals were filtered and pale pink
0 mL anhydrous ethanol under stirring at 5 °C in an ice bath. Then,
0 min. After 4 h, the residual ethanol was removed under vacuum
needle-like crystals were obtained. [HOQu][HSO
the same procedure except that different acid (sulfuric acid) was used.
As a result, the yield of [HOQu][HSO ] was 97%.
Secondly, the synthesis of benzothiazole tetrafluoroborate ([HBth]
]) and hexafluorophosphate ([HBth][PF ]) was carried out by refer-
ring the procedure previously developed in our lab via “one-pot”
4
] was prepared under
4
[BF
4
6
2