Tunable LiCl@UiO-66 composites for water sorption-based heat transformation applications

Abstract

Porous composite materials are potential candidates for water-based adsorptive heat transformation (AHT) applications. Here, a solid adsorbent LiCl@UiO-66 as a 'composite salt inside porous matrix’ (CSPM) has been prepared by incorporating hygroscopic lithium chloride into a microporous metal–organic framework (MOF) UiO-66 as a host matrix through the wet impregnation method. In our wet impregnation we did not let the excess salt solution dry to prevent salt precipitation on the matrix surface. This yielded a true salt@MOF composite with no deliquescence of LiCl and strongly enhanced the water adsorption capacity of UiO-66 through the salt content. At p/p₀ = 0.1 the water vapor sorption isotherms show a hydration state of LiCl inside the MOF of LiCl·2–4H₂O which is much higher than for neat LiCl with 0.5H₂O, due to the dispersion of a small particle size inside the matrix. LiCl@UiO-66 with a 30 wt% LiCl content (LiCl@UiO-66_30) has a 3 to 8 times higher water uptake over neat UiO-66 (depending on relative pressure) and could reach a volumetric and gravimetric water uptake of over 2.15 g g⁻¹ at p/p₀ = 0.9, which outperforms the so far known UiO-66-based composites. Cycling tests confirmed the hydrothermal stability of the LiCl@UiO-66 composites. Kinetic evaluation of the gravimetric water uptake (at 90% relative humidity) over time yielded rate coefficients up to 2.0(1) × 10⁻⁴ s⁻¹ which is slower than that in neat UiO-66 (6.7(6) × 10⁻⁴ s⁻¹) but faster than that for salt@silica gel composites. The coefficient of performance for the heat pumping mode (at T_des/T_ads/T_evap set to 90/40/10 °C) of 1.64 for LiCl@UiO-66_30 exceeds those of other MOFs, salt@MOF or salt@silica gel composites. For thermal battery applications the heat storage capacity (C_HS) for LiCl@UiO-66_30 is 900 kJ kg⁻¹ (=0.25 kW h kg⁻¹), which can reach the Department of Energy (DOE) value of 2.5 kW h/35 kg with just 10 kg of material and outperforms CaCl₂@UiO-66_38 with a C_HS value of 367 kJ kg⁻¹.