Experimental investigation for water adsorption characteristics on functionalized MIL-125 (Ti) MOFs: enhanced water transfer and kinetics for heat transformation systems

Abstract

Employing adsorption technology, metal organic frameworks (MOFs) are found promising to generate cooling, heating, and desalinated water with zero or negligible carbon footprint. In this article, we present various functional groups namely hydroxyl (-OH), amino (-NH2), nitro (-NO2), bromo (-Br) and pyridine (-C5H5N) assisted MIL-125 (Ti) MOFs to control water adsorption/ desorption capacity under the working domain of adsorption heat transformation (A-HT) process. The MOFs are at first synthesized, and later these are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), thermo-gravimetric analyzer (TGA) and nitrogen (N2) adsorption techniques. The water adsorption isotherms and kinetics are measured by gravimetric methods. Using isotherms data in pressure-temperature coordinate system, the isosteric adsorption-heat (Qst) is evaluated. The experimental results show that the functional group not only influences the hydrophobic and hydrophilic behaviors of MIL-125 (Ti) MOFs but also controls the water transfer per adsorption-desorption cycle with variable uptake/offtake rates (kinetics). Employing isotherms and kinetics data of functionalized (MOFs + water) systems plus the thermodynamic modeling of A-HT cycle, the performance parameters such as cooling, heating and desalination capacities are estimated for various cycle times and heat inputs. The mass and heat transfer processes involved in each component of A-HT system are presented. It is found that OH-MIL-125 (Ti) provides 3 times faster kinetics as compared with the parent MIL-125 (Ti) MOFs. Hence the cooling/heating and water production of A-HT are improved significantly with the addition of functional group, and NH2-MIL-125 (Ti) shows the best cooling and water production performances.