A recyclable post-synthetically modified Al(III) based metal-organic framework for fast and
5.3 Results and discussion
5.3.1 Characterization of material
The post-synthetically modified metal-organic framework (4-NH2@THB) was successfully characterized by employing a number of analytical techniques such as FT-IR spectroscopy, ESI-MS, NMR spectroscopy, XRPD, TG analysis, N2
sorption analysis and FE-SEM measurement.
A drastic colour change (yellow to orange) of the material after post- synthetic modification (Figure 5.1) is an indirect proof for the formation of newly functionalized compound. Moreover, in the FT-IR spectrum of 4-NH2 (Figure 5.2), the two teeth-like sharp bands at 3487 and 3388 cm-1 are due to the asymmetrical and symmetrical stretching vibrations of the N−H bond, respectively.51, 52 These two bands were vanished and only one broad band was observed in that region of the FT-IR spectrum of 4-NH2@THB, which suggests the richness of hydroxyl group after post-synthetic modification.32 The strong absorption band near 1633 cm-1 signifies the formation of imine bond between amine moiety of MIL-53-NH2 and aldehyde functionality of 2,3,4-trihydroxy benzaldehyde, which was not observed in the FT-IR spectrum of 4-NH2.53
Figure 5.1 Digital images of (a) 4-NH2 and (b) 4-NH2@THB in solid state.
Figure 5.2 FT-IR spectra of 4-NH2 (black) and 4-NH2@THB (red).
The ESI-MS spectrum of digested 4-NH2@THB material (Figure 5.3) exhibits two intense peaks at m/z = 182.0519 and 318.0697 (measured in positive ion mode), which can be ascribed to the (M+H)+ ion of H2BDC-NH2 ligand and imine-functionalized ligand (i.e. product of condensation reaction between H2BDC- NH2 ligand and 2,3,4-trihydroxy benzaldehyde). Hence, it becomes evident that during post-synthetic modification reaction, partial conversion of amine functionality to imine functionality was successfully achieved.
Figure 5.3 ESI-MS spectrum of the digested framework of 4-NH2@THB showing m/z (positive ion mode) peaks at 182.0519 and 318.0697, which correspond to (M+H)+ ion (M
= mass of ligands) of H2BDC-NH2 ligand and the imine-functionalized ligand, respectively.
To determine the percentage conversion of the amine moiety to imine functionality, 1H NMR spectrum of digested 4-NH2 and 4-NH2@THB material was compared (Figure 5.4). The new signals at δ = 8.11, 7.97.7.75, 7.11 and 6.50 ppm are due to the aromatic protons from imine-functionalized ligand, whereas the new signal at δ = 9.71 ppm is ascribed to the proton of the imine bond. On the basis of peak area ratio of aromatic protons of H2BDC-NH2 ligand and the imine- functionalized ligand of digested 4-NH2@THB, the calculated percentage conversion from amine to imine functionality is ∼51%.
Figure 5.4 1H NMR spectra of (a) 4-NH2 and (b) 4-NH2@THB after framework digestion in K3PO4/D2O. The assignment of the NMR peaks for 4-NH2@THB was interpreted according to the presence of the new peaks observed for the phenyl and imine moiety. To calculate the percent of conversion, the aromatic proton peaks corresponding to H2BDC- NH2 ligand were set to an integration of 1 and all new peaks were integrated accordingly Digestion protocol of the MOF sample for recording NMR spectra: 10 mg of each MOF sample was added to 400 µL of DMSO-d6. To this solution, 200 µL of saturated K3PO4 in D2O was added. After shaking for 5 min, the MOF sample was totally dissolved and the organic phase was analyzed by 1H NMR spectroscopy immediately.
Compounds 4-NH2 and 4-NH2@THB show very similar X-ray diffraction patterns (Figure 5.5). The similar XRPD patterns of 4-NH2 and 4-NH2@THB
clearly indicate that during post-synthetic modification, no loss in structural integrity was occurred.
Figure 5.5 XRPD patterns of Al-MIL-53 (calculated) (a), 4-NH2 (b) and 4- NH2@THB (c).
The TG traces of 4-NH2 and 4-NH2@THB (Figure 5.6) show that the parent and post-synthetically modified materials have almost similar thermal stability up to 440 °C. Hence, the post-synthetically modified compound was achieved without harming the thermal stability of the material. During post-synthetic modification, the amine functionality grafted with the parent material is converted to imine functionality but the framework structure remains same. Therefore, the material after post-synthetic modification shows almost similar thermal stability as the unmodified compound.54 In the TG trace of 4-NH2, the first weight loss of 5.0 wt.%
in the temperature range 0-125 °C shows good agreement with the removal of 0.7 guest water molecule per formula unit (calcd.: 5.3 wt.%). After 440 °C, sudden break in the TG curve suggests the framework decomposition. Hence, the formula of 4-NH2 is [Al(OH)(C8H5NO4)]∙0.7H2O. On the other hand, in the TG trace of 4- NH2@THB, the first weight loss of 4.5 wt.% in the temperature range 0-125 °C is ascribed to the removal of 0.8 guest water molecule per formula unit (calcd.: 4.7 wt.%). The sudden weight loss near about 430 °C signifies the collapse of the structure of the material. Based on the above calculation and 1H NMR study, the formula of the material is [Al(OH)(C15H9NO7)0.51(C8H5NO4)0.49]∙0.8H2O.
Figure 5.6 TG curves of 4-NH2 and 4-NH2@THB recorded in the temperature range of 25-800 °C with a heating rate of 10 °C min-1.
N2 sorption isotherms of 4-NH2 and 4-NH2@THB were measured at –196
°C to investigate the porosity and surface area of the materials (Figure 5.7). As derived from the sorption profiles of 4-NH2, the specific BET surface area is 1088 m2 g-1 and the micropore volume is 0.72 cm3 g-1 (calculated at p/p0 = 0.5). On the other side, after post-synthetic modification, the specific surface area and micropore volume of the material become 52 m2 g-1 and 0.05 cm3 g-1, respectively. Such drastic reduction of both surface area and micropore volume indicates the introduction of 2,3,4-trihydroxy benzaldehyde molecule in the framework. The incorporation of this bulky molecule blocks the pores of the material. Consequently, such lower surface area is obtained after post-synthetic modification.
Figure 5.7 N2 adsorption and desorption isotherms of (a) 4-NH2 and (b) 4-NH2@THB recorded at –196 °C.
FE-SEM images were collected to ensure the retention in morphology after post-synthetic modification of 4-NH2. Figure 5.8 displayed that both 4-NH2 and 4- NH2@THB crystals have similar size and shape. Hence, it can be concluded that during PSM process, no alteration in the morphology of the parent material occurred.
Figure 5.8 FE-SEM images of 4-NH2 (a, b) and 4-NH2@THB (c, d).