VIETNAM JOURNAL OE CHEMISTRY VOL. 51(5) 611-615 OCTOBER 2013
THE EFFECT OF SYNTHESIZED CONDITION ON THE FORMATION OF MESOPOROUS STRUCTURE IN MIL-101 METAL-ORGANIC
FRAMEWORK MATERIALS
Vo Thi Thanh Chau^*, Tran Thai Hoa\ Dinh Quang Khieu^
'Department of Technology, Quang Ngai Campus, Industrial University of Ho Chi Minh City and the College of Sciences, Hue University
^The College of Sciences, Hue University Received 15 August 2013
Abstract
Mesostmctured MIL-101 metal-orgamc framework materials were successfully synthesized by solvothermal process thought controlling molar ratio of chromium (III) nitrate (Cr) to 1,4-benzene dicarboxylic acid (HiBDC). The obtained sarrqiles were characterized by powder X-ray diffraction, scanning electron microscope (SEM), transmission electron microscope (TEM), thermal analysis (TG-DTA), fourier transform infrared spectroscopy (FT-IR) and nitrogen adsorption/desorption isotherms at 77K, The results showedthat in an approximate synthetised condition, the MIL-101 with a mesoporous structure and highly crystallized regular octahedron with a perfect cubic symmetry is obtained.
Keywords: MIL-101, mesoprous structure, metal organic frameworks.
1. INTRODUCTION
Metal-organic frameworks (MOFs), a new class of hybrid porous solids, are potentially a type of prominent porous adsorption [1] and catalysis [2] because of the exfremely large pore volume and surface area (usually > 2000 m /g) [3]. MOFs are crystalline hybrid porous sohds with ordered three dimensional network frameworks via sfrong metal-ligand bonds between metal cations and organic linkers. Many kinds of MOFs including MIL-47, MOE-5, MIL- 53,...have been developed so far. First time m 2005, Ferey and co-workers [4] reported the synthesis and characterization of the metal- organic framework MIL-101 (MIL stands for Matenal of Institut Lavoisier) with the formular of [(Cr3X(H20)20(BDC)3; X=F, OH;
BDC=benzene-l,4-dicarboxylate)] has a cubic stmcture and huge pore volume [5], The pore sizes of MIL-101 area round 2.9-3.4 nm [5]. This material exhibits an inherent mesoporous behavior owing to mesoporous cages. However, the feature of mesoporous structure was not reported in many works [6-8]. Most of reported data only shows the characteristic diffraction at small angles of 29 range of 2.5-3.5°. Huang et al [9] improved the
mesotructure of MIL-101 through synthesis conditions by adding supramolecular template of cetylfrimethylammonium bromide. The characteristic diffraction of mesoporous structure at 29 of 1,4° was appeared but very broad and weak. In this work, we present result of study on the formation of mesoporous structure by confrolled synthesized conditions in which the effect of molar ratio of chromium nifrate to terephthalic acid was focused.
2. EXPERIMENTAL
Chromium (III) nifrate nonahydrates (99%, Merck), 1,4-benzene dicarboxylic acid (H^BDC, Merck) (99%) and fluorhydnc acid (HF, Merck) (40%) were used in this work.
MIL-101 was synthesized based on reference [7].
In the present work, we focus on investigating into the effect of molar ratio of chromium nifrate to terephthalic acid on the formation of mesostmcture of MIL-101. In a typical procedure, a mixture of terephthalic acid (HjBDC) (1.66 g, 10 mmol), Cr(N03)3'9H20 (x mmol) with x = 5, 7,5, 8.5, 10, 12.5, 17,5, HF (40%, 0.11 mL, 2.5 mmol) and H3O (47.70 mL) was heated in Teflon-lmed stainless steel autoclave at 200°C for 8 hours. The resulting green solid matenals were filtered. The green raw product
VJCVol. 51(5), 2013
was washed with hot water (70°C, 5h, 1 time) and with hot ethanol (100°C, 3h, 1 time) and an aqueous solution of 30mM NH4F (60°C, lOh, 1 time), the precipitates were filtered and washed three times with distilled water and dried overnight in an oven at 100°C. The obtained MILlOl were denoted as MIL-101 (0.5:1), MIL- 101 (0,75:1), MIL-101 (0.85:1) MIL-101(1:1), MIL-101 (1.25:1), MIL-101 (0.175:1) corresponding MIL-101 with molar ratio of chromium (IH) nifrate to acid 0.5, 0.75, 0.85, 1, 1.25, 1.75, respectively.
X-ray diffraction (XRD) analysis was carried on D8-Advance (Brucker, Germany) X-ray Diffractometer with CuK„ (X = 1.5406 A) radiation. Thermogravimetric analysis (TGA) was performed on Labsys TG SET ARAM of France at a heating rate of 5 K min"' in ambient atmosphere. Specific surface area was measured by adsorption-desorption of N2 gas at 77 K on Micromentics Tristar 3000 (USA). The sample was degassed at 150°C for 10 hours before adsorption/desorption measurement was started, FT-IR specfra were recorded using potassium bromide pellets in the range of 400-4000 cm""' on
Vo Thi Thanh Chau, etal TENSOR37 specfrometer. TEM images were obtained by JEOL JEM-2100F. SEM/EDX analyses were carried out by SEM JMS-5300LV (Japan) and JED- 2300 JEOL.
3. RESULTS AND DISCUSSION
MS (
I _ '
•^ W I — A U . , ^ ^-A.A- .
l4.AvJk—•^-___
Cr/HjBDC 1.75:1
1.25:1
„ ^ 1.00:1
U . ^ ., . A 0.75'1 V A . ..A. K. ^ A ^-^'-^
Fig. ;.• XRD patterns of MIL-101 synthesized with different molar ratios of Cr/H2BDC Table 1: The d-spacing and hkl values of XRD powder diagram of MIL-101(1.25:1) and MIL-101 as
literature report [4]
MIL-101 26
2.81 3.29 3.97 5.16 5.88 8.44 9.06 10.34 11.26 16.53 16.92 17.27 19.56
d,A
31.420 26.795 22.217 17.103 15.022 10.473 9.755 8.551 7.855 5.359 5.237 5.131 4.535
h
0 1 0 1 1 2 1 2 0 5 4 10
8 k
2 1 0 5,3 3 2 9,3
2 8 9 4 10
8 1
2 3 4 3 5 8 5 10
8 13 16 10 16
MIL-101(1.25:1) 29
1,69 2.80 3.30 3.92 5.15 5.89 8.45 9.07 10.35 11.36 16.55 17.34 17.99 18.86
d,A 52.188 31.529 26.720 22.486 17.147 14.991 10.452 9.736 8.538 7.778 5.350 5.107 4.925 4.700
h
0 1 0 1 1 2 1 2 0 5 4 10
8 k
2 1 0 5,3
3 2 9,3
2 8 9 4 10
8 1
2 3 4 3 5 8 5 10 8 13 16 10 16
VJC, Vol. 51(5). 2013
The powder X-ray diffraction patterns for obtained samples are shown in Fig. 1. The charactenstic diffractions of HjBDC at 26 of 17, 25.5 and 28" were not observed that confirmed that acid is removed completely. At low ratio of chromium (III) to HjBDC, the XRD pattern of MIL-101 (0.5:1), MIL-101 (0.75:1), MIL-101 (1:0.85) and MIL-101 (1:1) is similar to the many reported partens for MIL-101 [ 1, 6-8] m which the diffractions at low angle m the 2 theta range of 2.5-3.5" were observed. When chromium amoimt ' increases at certain level as MIL-101 (1.25:1) or MIL-101 (1.75:1), thefr XRD patterns were the similar to the simulated pattern of MIL-101 reported [4] in which the sharp and sfrong diffraction at 29 around l" were observed clearly.
The Miller index of other reports and the present works are listed m table 1. These results confirm the presence of mesoporous stmcture.
The effect of synthesized condition ..
observed due to possible impurities from mitial chemicals. Paired samples /-test for each pafr of element analyzed point showed that composition of each pafr is not statistically different (t = 0.697, p = 0.517); (t = 0.698, p = 0.516); (t = 0.000, p = 1.000) indicating that the homogeneous elements in MIL-101 are obtained.
Fig. 3 EDX spectiaim of MIL-101 (1.25:1)
Fig 2.IRspectiiimofME.-101 (1.25:1)
The characteristic function groups in MIL-101 were analyzed by the FT-IR spectrum. Its FT-IR spectrum is similar to the previous results [1]. The bands at 1018 and 748 cm"' attributed to 6(C-H) and Y(C-H) vibrations of aromatic rings, respectively. The sfrong and broad band at 3436 cm"' most likely indicate the sfretchmg OH vibrations. Sfrong bands in the region of 1642, 1510 and 1406 cm"' correspond to Vas(coo)' ^(coo), and v^c-o vibrations, implying the presence of dicarboxylate linker in the MIL framework. Weak bands in the specfral region of 700-600 cm"' are most likely due to in-plane and out-of-plane bending modes of COO-groups [1].
Furthemiore, the dispersed degree of elements was analysized by EDX as shown in Fig. 3. The main element of Cr was observed. In addition, the magical amounts of nifrogen and iron were
Fig. •^,-TG-DTA profile of MIL-101 (1.25:1)
Thermal behaviour of obtained MIL-101 (exfracted sample) was investigated. Thermal analysis for an exfracted matenal was earned under air flow as shown in Fig. 4. TG profile showed a sigmficant weight loss of = 53% at below 100°C corresponding to endothermic peak (DTA) that could atfributed to the water desorption within the pore channels of extracted sample. Next weight loss ( = 34%) together with an endothermic peak at 405°C (DTA) were found for buming/oxidizing process of organic ligands. The very sharp peak in DTA and clear decreasing step in TG indicated the single phase of MIL-101 was obtamed.
SEM image shows the overall octahedral morphology of MIL-101 (Fig. 5). The discrete octahedral nano-crystals do not aggregate and exhibit a uniform crystal size ranging from 100 to 200 nm.
Representative TEM observation of MIL-101 show the similar morphology as shown in Fig. 6 a. By pyramidal octahedrons are clearly observed using TEM images along [111], [110], [001] and [112] as shown in Fig. 6
VJC, Vol. 51(5), 2013 f'o Thi Thanh Chau, et al b, 6c, 6d. The clear images in difference views crystallized regular octahedron with a perfect cubic confirm that the synthesized MIL-101 is a highly symmetry.
/•iu .V SEM obscnalioiisofMlL-ini |1.25:l)
a. [Ill] b, [001] c. [110] d. [112]
Fig 6: TEM images observed along different direction and corresponding schematic representations:
(a) along the [111] direction, (b) along the [110] direction, (c) along the [001] direction, and (d) along the [112] direction
The textural properties of MEL-lOl were The isotherm curve exhibits two secondary inflection investigated by the N2 adsorption-desorption points near p/p" = 0.1 and p/p" = 0.2, indicating the isotherm of MEL-lOl at 77 K as shown in Fig. 7. presence of two nano porous windows in the
614
V J C V o l . 5 1 ( 5 ) , 2 0 1 3
framework [7]. T h e B E T a n d L a n g m u i r surface area o f obtained p o w d e r , calculated from N2 adsorption-desorption data at 77 K using B E T and L a n g m u i r e q u a t i o n s , w e r e found to b e 2663.84 mVg a n d 3 9 3 7 . 1 5 m^/g, respectively, with the total p o r e v o l u m e o f 1.44 cm^/g at p/p° = 0.99. These values are closed to p r e v i o u s reported values for M I L - 1 0 1 [ 1 4 ] .
1 0 0 0 - .
3 BOO-
1
& ' 0 0 -
a BOO-
> 500-
3 0 0 -
1 J
I
1
11
MIL-101(1.25:l)y
Rdatlve Pressure, P/p' Fig. 7: Adsorption/desorption isotherms of
M I L - 1 0 1 ( 1 . 2 5 : 1 ) 4. C O N C L U S I O N S
Mesostmctured M I L - 1 0 1 metal-organic framework was successfiilly synthesized under solvothermal synthesis condition b y confrolling molar ratio of c h r o m i u m (III) nifrate (Cr) to 1,4- benzene dicarboxylic acid (H2BDC). T h e M I L - lOl samples synthesized in the l o w molar ratio of Cr/HiBDC condition possess p o o r ordered mesoporous s t m c t u r e . W h e r e a s , samples synthesised in the h i g h m o l a r ratio of Cr/H2BDC condition possess well-defined m e s o p o r o u s sfructure. T h e obtained s a m p l e of M I L - 101(1.25:1) with m e s o p o r o u s s t m c t u r e exhibites high speccific surface area of 2 6 6 3 m^/g and highly crystallized regular octahedron with particle size around 150 n m .
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