Computer analysis of the nitrogen adsorption process on MIL-53(Al) metal–organic frameworks

Abstract

This paper evaluates the effect of different synthesis conditions on the structural properties of the porous metal–organic frameworks MIL-53(Al) produced by the microwave method at different reaction temperatures (120, 150, and 180 °C) and times (30, 180 min). The morphologies, chemical contents and crystal structures of the synthesized samples were determined by the SEM-EDS and XRD methods, respectively. The structural properties of MIL-53(Al)s were analysed according to isotherm data obtained from nitrogen gas adsorption by applying the Brunauer–Emmett–Teller (BET), Langmuir, Dubinin–Radushkevich (DR), t-plot, Barrett–Joyner–Halenda (BJH) and quenched solid density functional theory (QSDFT) methods, as well as the new numerical clustering-based adsorption analysis (LBET) method, which is a unique numerical method that provides advanced assessment for porous structures. The LBET analysis showed that the MIL-53(Al) sample at a reaction temperature of 180 °C for 30 min has the best adsorption properties, with the highest volume of the first adsorbed layer and the highest value of the energy parameter for higher layers. Due the LBET method, it was possible to evaluate the porous structure of the MIL-53(Al) samples with high precision and reliability, and thus to determine the precise influence of the conditions of their preparation on the formation of their structure. In turn, based on the shapes of the pore size distributions (PSD), the materials analysed have very similar PSD, with only the MIL-35(AL)S1 sample having a significantly lower PSD height. In conclusion, attention was paid to the necessity of complex evaluation of adsorbent properties, taking into account not only their adsorption properties but also their mechanical and functional properties, as well as production costs and the impact of adsorbent production and utilization on the environment.