5.3 RESULTS AND DISCUSSIONS
5.3.2 Structure Description
The guest molecules encapsulated within the pores of the as-synthesized Al-MIL-101-X- CE materials were removed in a two-step activation procedure similar to that reported for the previously reported26 Al-MIL-101-NH2 material. In the first step, the guest molecules were exchanged with more volatile and thus easily removable methanol molecules by heating the as- synthesized compounds in methanol. In the second step, the methanol molecules were removed from the pores by heating the filtered materials at 130 °C under dynamic vacuum for 24 h in order to obtain the thermally activated materials. The thermally activated Al-MIL-101-X-CE materials were not sensitive towards moisture from air. However, the crystallinity (and hence structural integrity) of the samples deteriorated significantly when treated with water, as verified by the XRPD experiments (Figure 5.2). It is worthy to note that MIL-101 analogues of Fe(III)54 and Al(III)44 were formerly found to be unstable in water, whereas those of Cr(III)27 possess high hydrothermal stability. Furthermore, the MIL-53 analogues of Fe(III),55 Al(III)25 and Cr(III)56 are stable in water. The higher hydrothermal stability of MIL-53 analogues compared to the MIL- 101 variants might be ascribed57 to the sterically more shielded [MO6] inorganic building units (IBUs) of MIL-53 as compared to the open ones of MIL-101. Hence, except for Cr-MIL-101, the IBUs of Fe(III) and Al(III) analogues of MIL-101 are prone to attack by water. The hydrothermal stability of Cr-MIL-101 is typically attributed58 to the considerably low exchange rates of the ligands with water because of the gain in energy by ligand field stabilization energy.
(ST). Further connection of the ST with each other in three-dimension leads to the augmented zeolite Mobil Thirty-Nine (MTN) type of framework.
Figure 5.3 Calculated XRPD pattern of Cr-MIL-101 (pink) and experimental XRPD patterns of the thermally activated 1-CH3 (black), 2-NO2 (blue), 3-OCH3 (green), 4-C6H4 (red), 5-F2 (magenta), 6- (CH3)2 (orange), 7-(OCH3)2 (cyan) and Al-MIL-101-NH2 (violet) synthesized by solvothermal method.
Figure 5.4 Calculated XRPD pattern of Cr-MIL-101 (pink) and experimental XRPD patterns of the as- synthesized 1-CH3 (black), 3-OCH3 (green), 4-C6H4 (red) and 7-(OCH3)2 (cyan) synthesized by
Figure 5.5 (a) Framework structure of Cr-MIL-101 having MTN topology and containing smaller (green) and larger (red) mesoporous cages. The structure is constructed from (b) supertetrahedra (ST), which consist of trimeric oxido-centered [Cr3(μ3-O)(F)(H2O)2]6+ building blocks at the vertices interconnected the by terephthalate ligands. The smaller and larger cages bear only pentagonal (c) or a combination of pentagonal (c) and hexagonal (d) windows, respectively. Color codes: Cr, green octahedra; C, gray; O, red. The MTN framework (a) and the portions of Cr-MIL-101 network (b-d) have been drawn by utilizing the atomic coordinates given in “Database of Zeolite Structures”60 and ref. 27, respectively.
It is noteworthy that the labile coordination sites of the [AlO6] octahedra in the as- synthesized Al-MIL-101-X materials are filled with the O-donor atoms from DEF molecules or chlorine atoms, rather than fluorine atoms or water molecules as in the case of Cr-MIL-101. The existence of such coordinated DEF molecules in the structures of the as-synthesized Al-MIL- 101-X materials has been verified by IR spectroscopy and TG analyses (cf. Infrared Spectroscopy and Thermal Stability sections). The presence of coordinated chloride anions has been corroborated by energy dispersive X-ray (EDX) experiments (Figures 5.6 to 5.13). The frameworks comprise two types of mesoporous cages. The smaller cage is built up of 12 pentagonal rings possessing a free diameter of ca. 12 Å, and the accessible diameter of the cage
is ca. 29 Å. The larger cage consists of 12 pentagonal and 4 hexagonal rings with a free diameter of the hexagonal rings of ca. 16 Å and an accessible cage diameter of ca. 34 Å. The smaller and larger cages are composed of 20 and 28 ST, respectively. They are present in a ratio of 2:1.
It is worthy to mention that the molecular formula (i.e., [Al3OCl(DEF)2(BDC-X)3]) of the as-synthesized Al-MIL-101-X materials have been assigned based on the fact that the existence of chloride anions and DEF molecules in 1:2 ratios will make the structures electrostatically neutral. The assignment of the molecular formulae of the as-synthesized materials by spectroscopic techniques (TG, EDX and IR) is tentative. This is due to the fact that the chloride anions or DEF molecules ascribed as coordinated ligands might also correspond to the guest species inside the pores. For example, the investigation of the structure of Al-MIL-100 by solid- state NMR spectroscopy disclosed that the pores also encapsulate different species such as nitrate, unreacted trimesic acid and water molecules.61 It is noteworthy that the DMF molecules were found to serve as the coordinated ligands in the previously reported Al-MIL-101-NH2
material.26
Figure 5.6 EDX spectrum of thermally activated 1-CH3.
Figure 5.7 EDX spectrum of thermally activated 2-NO2.
Figure 5.8 EDX spectrum of thermally activated 3-OCH3.
Figure 5.9 EDX spectrum of thermally activated 4-C6H4.
Figure 5.10 EDX spectrum of thermally activated 5-F.
Figure 5.11 EDX spectrum of thermally activated 6-(CH3)2.
Figure 5.13 EDX spectrum of thermally activated Al-MIL-101-NH2.
The TG curves and FT-IR spectra of the thermally activated materials indicate that the coordinated DEF molecules are absent in the structures. Thus, we hypothesize the following: during the activation process, the coordinated DEF molecules have been exchanged with methanol molecules, which have been subsequently removed by heating. The thermally activated materials absorb water from air after cooling to room temperature. In this way, the activated Al-MIL-101-X materials incorporate coordinated H2O molecules in the structures and they bear tentative molecular formula of [Al3OCl(H2O)2(BDC-X)3.Lx(H2BD- X)] (Table 5.4).