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

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. Atp/p(0)= 0.1 the water vapor sorption isotherms show a hydration state of LiCl inside the MOF of LiCl center dot 2-4H(2)O which is much higher than for neat LiCl with 0.5H(2)O, 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)atp/p(0)= 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) x 10(-4)s(-1)which is slower than that in neat UiO-66 (6.7(6) x 10(-4)s(-1)) but faster than that for salt@silica gel composites. The coefficient of performance for the heat pumping mode (atT(des)/T-ads/T(evap)set to 90/40/10 degrees 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 aC(HS)value of 367 kJ kg(-1).