Vapor-phase adsorption of alkylaromatics on aluminum-trimesate MIL-96: An unusual increase of adsorption capacity with temperature

Porous aluminum-trimesate (MIL-96), one of the metal–organic frameworks (MOFs), has been synthesized hydrothermally under autogeneous pressure in water as the solvent. The MIL-96 adsorbs nitrogen readily at liquid nitrogen temperature to show permanent porosity (SBET = 532 m2/g, SLangmuir = 700 m2/g). Moreover, the desorption of adsorbed nitrogen does not to show any hysteresis between the adsorption and desorption isotherms. A vapor-phase adsorption study (temperature: 30–110 °C) shows that MIL-96 can uptake aromatic compounds such as p-xylene, m-xylene and 1,3,5-TMB (trimethylbenzene). However, a larger molecule such as 1,3,5-TiPB (triisopropylbenzene) is only negligibly adsorbed on MIL-96. The adsorption rate of 1,3,5-TMB (even at a high temperature of 70 °C) is very slow compared with that of p-xylene (at 30 °C). Very interestingly, the adsorption capacity of 1,3,5-TMB increases with increasing adsorption temperature (up to 90 °C), which is very rare because an adsorption is usually an exothermic process. All of these results may be explained by the fact that the MIL-96 has a pore size similar to a kinetic diameter of 1,3,5-TMB. Moreover, the results may suggest that the framework of MIL-96 is flexible and the lattice vibrates more widely at high temperature to increase the effective pore size. The flexible structure of MIL-96 demonstrated in this study may enlarge the applications of MOFs for the storage of chemicals and controlled release such as drug delivery.