Adsorbate Effect on AlO₄(OH)₂Centers in the Metal−Organic Framework MIL-53 Investigated by Solid-State NMR Spectroscopy

Abstract

1H and 27Al MAS NMR spectroscopies have been applied for studying the effect of water molecules, nitrogen bases, and o-xylene on the hydroxyl protons of bridging AlOH groups and framework aluminum atoms in the metal−organic framework (MOF) MIL-53. For water molecules adsorbed on the low-temperature form MIL-53lt, two 1H MAS NMR signals were found indicating the formation of different O−H···O hydrogen bonds to neighboring oxygen atoms, such as to carboxylic oxygens. Upon adsorption of the nitrogen bases acetonitrile, ammonia, and pyridine, a linear increase of the quadrupole coupling constant, C Q, of the framework aluminum atoms in dehydrated MIL-53 from C Q = 8.5 MHz (unloaded material) to maximum 10.8 MHz (pyridine-loaded material) as a function of the proton affinity of the adsorbates was observed. Adsorption of o-xylene led to three stepwise changes of the quadrupole coupling constants, C Q, of framework aluminum atoms in dehydrated MIL-53. While the first two stepwise changes of the C Q values (C Q = 8.0 and 8.7 MHz) occur for o-xylene loadings of lower than 4 molecules per unit cell and for all AlO4(OH)2 centers, the third change of the C Q value to 9.4 MHz was observed for o-xylene loadings higher than 4 o-xylene molecules per unit cell and for maximum 50% of the framework aluminum atoms. This third adsorbate-induced change of the C Q value of framework aluminum atoms in MIL-53 is accompanied by a significant decrease of the adsorbate mobility.

The Metal–Organic Framework MIL‐53(Al) Constructed from Multiple Metal Sources: Alumina, Aluminum Hydroxide, and Boehmite

“…Theoretically, MIL-53(Al) is built up by aluminum atoms that coordinate to terephthalates as organic linkers. [17,24] In the regular hydrothermal synthesis of MIL-53(Al), aluminum is present as the Al(H 2 O) 6 3 + ion in the aqueous solution. The conversion can be described as the replacement of a water molecule from the [Al(H 2 O) 6 ] 3 + ion, thus resulting in the formation of AlO 4 (OH) 2 centers.…”

Solid-state nuclear magnetic resonance investigations of the nature, property, and activity of acid sites on solid catalysts

“…In a number of solid materials, bridging OH groups (T(OH)T) between framework metal atoms T exist, also called structural OH groups. In the microporous metal-organic framework MIL-53, e.g., Al(OH)Al groups are formed at Al-O-Al bridges between two adjacent framework aluminum atoms [53,54]. The octahedrally coordinated framework aluminum atoms of MIL-53 are coordinated to four oxygen atoms of organic linkers and two oxygen atoms of the above-mentioned bridging OH groups causing 1 H MAS NMR signals at d 1H ¼1.7-2.6 ppm in the hydrated and dehydrated state (see Table 1).…”

“…Depending on the structure type of zeolites and their cation exchange degree, the 1 H MAS NMR signals of Si(OH)Al groups can be observed at d 1H ¼3.6-4.0 and 4.8-5.2 ppm due to bridging OH groups located in large cages and small structural units, respectively (see Table 1). The larger [54], parent zeolite H,Na-Y (n Si /n Al ¼ 2.7) (b), dealuminated zeolite deH,Na-Y/ 81.5 prepared by steaming at 748 K at a water vapor pressure of 81.5 kPa and not hydrated after steaming and nitrogen purging (c) [74], and the heteropoly acid AlPW 12 O 40 dehydrated at 473 K (d) [69]. Experimental spectra (top) are compared with simulated spectra (bottom).…”

“…Another approach for investigating the strength of solid acids is the application of the weak bases acetonitrile and trichloroacetonitrile as probe molecules in connection with their 1 H MAS NMR spectroscopic observation [54,46,47,[109][110][111]. Jaenchen et al [110] used the adsorbate-induced low-field shift, Dd 1H , caused by hydrogen bondings with deuterated acetonitrile and trichloroacetonitrile to study the influence of the framework n Si /n Al ratio and structure type of zeolites on their acid strength.…”

13C NMR Study of CO2 Adsorbed in Highly Flexible Porous Metal‐Organic Frameworks

“…[30][31][32][33] Recently, 129 Xe NMR was employed to investigate the Xe molecule itself in its adsorption to MIL-53 (Al). 34 However, solid-state 13 C NMR studies of CO 2 in MIL-53 have not been reported until now, even though 13 CO 2 can act as a direct probe molecule to examine the pore environments.…”