Evaluation of PET-derived metal organic frameworks (MOFs) for water adsorption and heat storage

In this work, we report on five different Metal Organic frameworks (MOFs) materials synthesized from Polyethylene terephthalate (PET) waste as a direct precursor for 1,4-benzenedicarboxylic acid (1,4-H₂BDC), a large component in BDC-based MOFs for water adsorption processes. These materials were characterized by physicochemical techniques including surface area analyser, Powder X-Ray diffraction, scanning electron microscopy and thermogravimetric analyser. The high surface areas obtained from N₂ adsorption indicated the successful synthesis and utilization of the BDC from PET waste material. The diffraction patterns of all materials were characterized by major peaks present in the materials. The morphology of the different MOFs was distinct and showed the presence of elements on the surface of all materials. Thermogravimetric analysis indicated the weight loss (%) in the samples at different stages. Adsorption isotherms were measured using dynamic vapor sorption analyzer (DVS) at 25 and 55 °C. MIL-101(Cr) superseded all other MOF materials at 55 °C with water uptake of 1.53 g_H2o/g_ads at a relative pressure of 0.99, followed by 1.15 g_H2o/g_ads at p/p0 = 0.9 for UiO-66(Zr), 0.78 g_H2o/g_ads at p/p0 = 0.9 for MIL-101(Fe) and 0.77 g_H2o/g_ads at p/p0 = 0.95 for MIL-88B(Fe). Aluminium fumarate presented the lowest water uptake (0.52 g_H2o/g_ads at a relative pressure of 0.96). MOFs were exposed to 24 successive adsorption/desorption cycles. MIL-101(Cr) exhibited better hydrothermal stability and fast adsorption kinetics compared to other four materials. The Dubinin-Astakhov and LDF were used to model the measured adsorption characteristics and the results showed that the model prediction of the experimental data with a mean relative deviation of 6%. Heat storage capacities were measured by TG-DSC and all MOFs presented an increase in capacities at higher temperature (55 °C). The results showed that higher surface area and high temperature range measurements yields high water uptake and heat storage capacities for heat storage applications.