A new photocatalyst for the degradation of tnt by metal organic framework NH2-MIL-88B (Fe)

Research 
Journal of Military Science and Technology, Special Issue, No.51A, 11 - 2017 71 
A NEW PHOTOCATALYST FOR THE DEGRADATION OF TNT BY 
METAL ORGANIC FRAMEWORK NH2-MIL-88B(Fe) 
Tran Dinh Tuan1*, Le Viet Ha2, Nguyen Thi Hoai Phuong1, Ninh Duc Ha1 
Abstract: NH2-MIL-88B (Fe), (MIL = Materials of Institute Lavoisier) a 
mesoporous metal-organic framework (MOF) with high surface area was 
synthesized. The structural characteristics of NH2-MIL-88B(Fe) have exhibited an 
advantageous application potentialities in gas storage, separation and 
heterogeneous catalysis. This MOF is usually obtained by the hydrothermal 
synthesis in a Teflon-lined autoclave at high temperature (>150oC) under static 
conditions. However, this method exhibits several disadvantages such as high 
temperature, high pressure, long reaction time, and relatively low MOF yield. In 
this study, NH2-MIL-88B(Fe) has been successfully synthesized by using reflux 
method at low-temperature. Structure and properties of materials have been 
characterized by using: XRD, IR, SEM, TGA. The study has showed that NH2-
MIL-88B(Fe) is as catalysts and the solution of trinitrotoluene (TNT) 50 ppm was 
decolorized after an hour under simulated sun-light radiation. 
Keywords: Metal-organic frameworks (MOFs), NH2-MIL-88B(Fe), Reflux method, Photocatalyst. 
I. INTRODUCTION 
Metal-organic frameworks (MOFs) are crystalline porous materials that consist 
of metal-carboxylate units, metal ions and organic linkers. These metal ions are 
coordinated to form the rigid organic molecules with one, two or three-dimensional 
structures that can make the very porous framework [1-3]. The special interest in 
MOF materials is due to the easy tunability of their pore size and shape from a 
micro to a mesoscale by changing the connectivity of the inorganic moiety and the 
nature of organic linkers. MOFs are well known for their various applications in 
adsorption and gas store [4], separation [5], catalysis [6], drug delivery [7], and for 
photocatalysis [8-10]. MOFs exhibit comparative advantages over to traditional 
catalysts because of their desirable topology and high surface area that allow for 
accommodation of guest molecules [1]. Additionally, the HOMO-LUMO gap can 
easily be tuned through modification of the inorganic or organic units of the 
molecule during its synthesis [3]. 
2,4,6-trinitrotoluene (TNT) is one of the most common toxic pollutant present 
in wastewater generated from ammunitions plants. Due to its potential 
carcinogenic characteristics, TNT present in water bodies represents a risk for 
human health and aquatic life [11]. Wastewater containing TNT can be treated by 
adsorption on activated carbon or advanced oxidation processes (AOPs) [12]. 
Besides, these photo induced techniques, semiconductor photocatalyst has been 
proven to be more effective in degradation of nitroaromatic compounds. Among 
various semiconductors, nano TiO2 was a widely used as photocatalyst, due to its 
chemical inertness, photostability and non-toxicity. However, these processes have 
limitations associated with the regeneration and ultimate disposal of the 
contaminated catalysts. Besides, TiO2 is only photoactive under UV light 
irradiation due to its wide band gap energy (3.2 eV: anatase). 
Chemistry & Environment 
 T. D. Tuan, , N. D. Ha, “A new photocatalyst for  NH2-MIL-88B(Fe).” 72 
In this study, a new method for synthesis of photocatalytic material based on 
NH2-MIL-88B(Fe) at low temperature (80
oC) and atmospheric pressure by reaction 
of iron(III) chloride hexahydrate (FeCl3.6H2O) and 2-aminoterephtalic acid (H2N-
BDC) under reflux condition like in [13] has been presented. Due to the synthesis 
conducted with agitation, the obtained NH2-MIL-88B(Fe) has exhibited high yield 
and large specific surface area, accompanied with low synthesis temperature and 
short synthesis time. This method could be applied for MOF synthesis in large 
scale. Its photocatalytic activity was also investigated over the degradation of TNT 
using visible light irradiation. This is the first time, TNT has been treated by this 
MOF photocatalyst. 
II. EXPERIMENTS 
2.1. Materials 
All chemicals used such as iron(III) chloride hexahydrate (FeCl3.6H2O), 2-
aminoterephtalic acid (H2N-BDC), dimethylformamide (DMF), ethanol (C2H5OH) 
are chemicals with analytical purity grade prepared in de-ionized water. 
2.2. Synthesis method 
NH2-MIL-88B(Fe) was prepared at low-temperature in atmospheric pressure. 
Taken FeCl3.6H2O (0.72 g), H2N-BDC (0.24 g) and an additional amounts of DMF 
(28 mL) were mixed and charged into a three-neck round bottom flask equipped 
with a magnetic stirrer with 300 rpm for 8 hrs under reflux condenser at 80oC. The 
reaction product was purified three times with a extraction solvent including de-
ionized water (350 mL) and ethanol (350 mL) at 70oC for 24 hrs, finally dried in a 
vacuum desiccator at 150oC for 10 hrs. 
 2.3. Characterization 
- X-ray diffraction (XRD) measurements were conducted using standard 
powder diffraction analysis procedures. 
- SEM images were performed to identify the morphologies and particle size 
distribution of the crystals. 
- Functional groups were determined by FT-IR spectra. 
-The thermal stability of NH2-MIL-88B(Fe) was investigated by thermal 
gravimetric analysis (TGA). 
2.4. Photocatalytic reaction 
The photocatalytic activity of the NH2-MIL-88B(Fe) was tested by degradation 
of TNT. 50 mg of catalyst was added in 100 mL TNT solution (concentration of 50 
ppm) with 0.5 mL of H2O2 solution 30%, pH = 7, stirring for 60 min in the dark to 
reach adsorption-desorption balance stage.The photocatalytic degradation started 
under the irradiating with simulated sun-light (sunlight simulation lamp, 15 W, 4-
6% UV, 340-315 nm wavelength), for each 15 min, taking 2 mL solution, 
centrifuging to get TNT left. The TNT concentrations left were analysized by 
HPLC method [10]. The other two series of experiments such as the samples 
involving only TNT or TNT plus H2O2 were carried out to comprising. 
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Journal of Military Science and Technology, Special Issue, No.51A, 11 - 2017 73 
III. RESULTS AND DISCUSSION 
3.1. Structure and components of NH2-MIL-88B(Fe) 
Morphologies of MOF that have been identified by SEM micrographs. NH2-
MIL-88B (Fe) microsized crystals produced from an aqueous reaction mixture of 
FeCl3.6H2O and H2N-BDC resulted in the morphology of bipyramidal hexagonal 
prism, and size of 10 µm in length and 2 µm in width. 
Composition and structure of the material were analyzed by X-ray diffraction 
(XRD), SEM and infrared spectroscopy,the results are shown in Fig 1 and Fig 2. 
Samples showed good crystallinity. Fig 1 showed that the peak was very sharp. 
Characteristic peaks of the NH2-MIL-88B(Fe) are 2θ = 8,9
o; 10o; 17,8o, 27,72o 
respectively. 
Fig 1. SEM images and XRD pattern of NH2-MIL-88B(Fe). 
The FT-IR spectrum of NH2-MIL-88B(Fe) (left fig 2) is similar to the previous 
results in [8]. The two bands at 1578 cm-1 and 1422 cm-1 are corresponding to the 
symmetric and asymmetric C-O stretching vibrations of carboxylates, thus 
indicates the presence of 2-aminoterephtalic acid anions. Band at 3334 cm-1 
corresponds to the asymmetrical and symmetrical stretching of the amine moieties 
in 2-aminoterephthalic acid. Band at 1336 cm-1 indicates the C-N stretching 
absorption distinctive of aromatic amines (2-aminoterephthalic acid). 
Fig 2. IR pattern and thermogravimetric analyses of NH2-MIL-88B(Fe). 
3.2. Thermogravimetric analysis of NH2-MIL-88B(Fe) 
The TGA curve of NH2-MIL-88B(Fe) shows two steps of weight loss between 
25 and 600oC (right fig 2). The first one (≈ 9.5 wt.%) in a temperature range from 
Chemistry & Environment 
 T. D. Tuan, , N. D. Ha, “A new photocatalyst for  NH2-MIL-88B(Fe).” 74 
25 to 120oC, is attributed to the desorption of the free water molecules inside the 
pores. The second weight loss (≈ 13 wt.%) in the temperature range from 120 to 
320oC is related to the decomposition of NH2-BDC to produce iron oxide. This 
clearly indicates that the overall thermal stability of the current NH2-MIL-88B(Fe) 
sample is similar to that of the samples synthesized by other methods in [8]. 
3.3. Adsorption and photocatalytic activity of NH2-MIL-88B(Fe) 
3.3.1. Efficiency adsorption of NH2-MIL-88B(Fe) 
The experiment was carried out in the dark. The TNT concentration left are 
shown in Table 1. The results showed that TNT concentrations decreased by 
increasing adsorption time, for 60 min, can decrease from 50 to 8.54 ppm. This 
proves that TNT has been adsorbed from solution onto Fe-MOF material. 
3.3.2. Photocatalytic efficiency of NH2-MIL-88B(Fe) 
The experiments were carried out as suggested in previous item (2.4). The 
experimental data showed that in the sample involving MOFs, TNT, the 
photodegradation efficiency of TNT is higher than others. This photodegradation 
of TNT might be explained as following. 
The main proposed pathways of TNT photodegradation by MOFs under visible 
light irradiation are shown, presented in [9, 10]. It was considered that the initial 
process of photocatalysis was the generation of electron-hole pairs in the MOFs. 
After absorption of energy equals to or greater than the band gap of the MOFs, the 
electrons (e-) were excited from the valence band (VB) and entered into the 
conduction band (CB), leaving the holes (h+) in the VB. The electrons and holes 
migrated to the surface of the MOFs, then electrons reducing the oxygen (O2) to 
form oxygen radicals (*O2
-), holes oxidizing the hydroxyl (H2O) to hydroxyl 
radicals (*OH). Hydroxyl radicals (*OH) and oxygen radicals (*O2
-) were capable 
of decomposing TNT effectively. 
h+(MOFs) + H2O 
*OH + H+ + MOFs 
e-(MOFs) + O2 MOFs + 
*O2
- 
The results of TNT concentration remaining after experiment are shown in 
table 1. 
The results in table 1 show that photocatalytic reaction is fast. After an hour of 
treatment, TNT is metabolized 99%. With the second loop, the reaction is similar. 
Table 1. TNT concentration left after treament by 
 NH2-MIL-88B(Fe) (ppm) versus time. 
Time (min) 0 15 30 45 60 
TNT adsorption 50 20.18 10.95 9.62 8.54 
TNT Photochemical 50 49.50 49.11 48.75 48.18 
TNT Photochemical with 
H2O2 
50 48.98 47.84 46.69 45.22 
TNT Photocatalysis with 
MOFs 
50 13.53 2.52 1.08 0.52 
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Journal of Military Science and Technology, Special Issue, No.51A, 11 - 2017 75 
IV. CONCLUSIONS 
NH2-MIL-88B(Fe) material was synthesized at low temperature (80
oC) and 
atmospheric pressure for 8 h. The material has the particle size of 10 µm in length 
and 2 µm in width and is crystalline in structure. It has high potentiality for 
applications in adsorption and photocatalysis to treat of TNT from solution. 
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TÓM TẮT 
CHẤT XÚC TÁC QUANG ĐỂ XỬ LÝ TNT TRÊN CƠ SỞ 
VẬT LIỆU NH2-MIL-88B(Fe) 
Vật liệu khung cơ kim (MOFs) trên cơ sở NH2-MIL-88B(Fe) có diện tích 
bề mặt cao. Các đặc tính cấu trúc của NH2-MIL-88B(Fe) đã thể hiện cho 
thấy vật liệu có nhiều tiềm năng ứng dụng trong hấp phụ, tách và lưu trữ khí, 
xúc tác dị thể... Thông thường, NH2-MIL-88B(Fe) được tổng hợp bằng 
phương pháp thủy nhiệt ở nhiệt độ cao trong điều kiện tĩnh. Tuy nhiên, 
phương pháp này có những nhược điểm như: nhiệt độ và áp suất cao, thời 
gian phản ứng dài, năng suất thấp... Trong nghiên cứu này, vật liệu NH2-
MIL-88B(Fe) đã được tổng hợp thành công bằng phương pháp hồi lưu ở 
nhiệt độ thấp. Đặc tính, cấu trúc của vật liệu đã được xác định bằng các 
phương pháp: XRD, IR, SEM, TGA. Nghiên cứu phản ứng quang xúc tác khi 
sử dụng vật liệu NH2-MIL-88B(Fe) cho thấy, dung dịch trinitrotoluen (TNT) 
50 ppm bị xử lý sau một giờ dưới ánh sáng mô phỏng. 
Từ khóa: Vật liệu khung cơ kim (MOFs), NH2-MIL-88B(Fe), Phương pháp hồi lưu, Chất quang xúc tác. 
Received date, 22th Sep., 2017 
Revised manuscript, 07th Oct., 2017 
Published, 01st Nov., 2017 
Author affiliations: 
1 Institute of Chemistry and Materials/Academy of Military Science and Technology; 
2 Brigade No.5/Army of Commando; 
*Corresponding author: dinhtuanmta39@gmail.com