What is limiting MOF adsorption capacity in devices: Thermodynamics, kinetics or heat transport?

SSCI-VIDE+ING+DFA:CDA; International audience; Metal-Organic Frameworks have been investigated as adsorbents for Heat-Pump applications, especially for chilling applications using water as working fluid. Today, Silica Gel bead is the current adsorbent technology which is used in adsorption chillers. While its water adsorption capacity is relatively limited, its very low manufacture cost made Silica Gel beads very competitive. Nevertheless, the performances of silica gel are still limited to address important application challenges. Technico-economical studies show that new very performing adsorbents shall be investigated for the conversion of low temperature heat produced by data center.Thanks to their very high adsorption capacity, type V isotherm and ease for regeneration thanks to their low heat of adsorption, MOFs are in principle ideal candidates for cycling adsorption and desorption of water. As consequence MOF could outperform silica gel based on technical criteria. Especially, Al-fumarate is regarded with promises as it is stable in hydrothermal conditions and can be produced without organic solvents. In addition, due to its framework flexibility upon adsorption Al-fumarate shows adsorption isotherm displacement as function of the temperature which shall enable a facilitated regeneration .However to date, technico-economical studies of Al-fumarate as adsorbent for adsorption chillers have not been published yet. To date, the promises rely mostly on water uptake at equilibrium conditions. In the frame of PRODIA, an European program, Al-fumarate beads have been developed to meet ambitious technical targets while being cost compatible.While the paper will not discuss the development of manufacture process, it is worth mentioning that the synthesis processes and shaping technics which have been developed meet industrialization criteria. The continuous mechano-chemical synthesis on Al-fumarate without solvent enables upscaling at high production rates whilst achieving high yields, adsorbent quality and higher operational safety. In addition, an original scalable shaping process was developed enabling continuous production of robust and size-controlled spheres from 0.3 to 2mm made of Al-fumarate.In the course of the development, we found that the cycling capacity on Al-fumarate beads were lower than the pristine powder at thermodynamic equilibrium. Three phenomena which could penalized adsorption capacities in were identified and investigated.1/ does the guest induced transformation of Al-fumarate occur in the time frame of the adsorption-desorption processes ?2/ are the uptake kinetics limited by a diffusion regime in the Al-fumarate porous beads ? due to the binders ? or due to the bead size ?3/ are the uptake kinetics limited by mass transport limitation in or between Al-fumarate beads ?In order to unravel the origin of kinetic limitations on Al-fumarate beads, series of beads of different diameter (0.4, 1.2, 1.6mm) and binder contents were synthesized. Mass ad heat transport were measured in a special device in adsorption and desorption modes. Kinetic results are compared with silica-gel beads. We will show that beads of 0,4 mm of diameter show fast uptake kinetics.Small spheres of 0.4 mm of Al- and Zr-MOF were produced at multi-kg scale. The performances of MOFs were evaluated in 3 liter adsorber demonstrator. We will show that MOFs outperform commercial Silica Gel under standard cycling conditions. Results in extremely low regeneration temperature will be presented and discussed in the frame application market requirements.