Double step heating synthesis of MIL-101(Cr) composites for water harvesting applications

Energy storage is a valuable mechanism for managing fluctuations in energy demand by allowing the distribution of energy production needed during peak demand over a more extended period. Thermal energy storage is among the most valuable categories of energy storage. The optimal sorbent material must satisfy thermal, physical, chemical, and environmental requirements for designing sorption heat storage systems. MIL-101(Cr) is a Metal-Organic Framework (MOF) with remarkable properties such as a high surface area, hydrothermal stability and good water sorbent capacity that make it a potential candidate as a working sorbent. Their physicochemical properties can be modulated by functionalizing the organic linker or by modifying its structure, improving the interaction with the target sorbate, mainly environmentally friendly and non-toxic sorbates such as water. However, good water sorption capacity is not the only characteristic to be considered when selecting the sorbent material for sorption heat storage systems. Here, we propose an alternative method to obtain a sorbent material with the objective of reducing the reaction time and achieving a good balance between the physical properties of the material, such as the density and the thermal conductivity, without substantially decreasing its water uptake capacity. For this aim, we show that structural modification of MIL-101(Cr) induced by CaCl2 and graphene oxide (GO) can improve the water sorption capacity and optical properties compared to the pristine sorbent. The MIL-101(Cr)/GO/CaCl2 composite had a higher water uptake capacity of 1.08 gwater g−1composite-1, even compared with MIL-101(Cr)/CaCl2 composite with a water uptake capacity of 0.68 gwater g−1composite. Additionally, MIL-101(Cr)/GO and MIL-101(Cr)/GO/CaCl2 sorbents tend to absorb light toward the near-infrared region and increase the temperature around twice as much compared to sorbents without GO when illuminated with a flash lamp. These results demonstrate that GO added at the MIL-101(Cr) structure allows a better distribution of CaCl2 in the MIL-101(Cr) pores and it may be advantageous in the water desorption process making use of renewable infrared radiation sources such as solar energy.