Metal–organic frameworks represent
a class of microporous
adsorbents with high application potential for adsorption heat transformation.
Here, we present a functional, full-scale heat exchanger coated with
the microporous aluminum fumarate MOF Basolite A520 using a polysiloxane-based
binding agent. The function of the heat exchanger was evaluated resulting
in a gross cooling power of 2900 W (at the beginning of the adsorption
cycle) or, respectively an average cooling power of 690 W (up to a limit of 90% equilibrium
loading in 7 min) under the working conditions of a realistic adsorption
chiller of 90 °C – 30 °C – 18 °C
(temperature level of heat source, heat rejection/condenser, and evaporator).
Measured, via pulsed field gradient (PFG) NMR, and computed molecular dynamics (MD) were utilized for the study of the phase equilibrium and kinetics of water sorbed in a bed of MIL-100(Al) crystallites. The computations rely on our recent methodology for modeling water equilibria and dynamics in the Fe-homologue MIL-100 crystal; in that sense, the particular NMR technique serves also as a validation tool of the previous simulation work which is adapted to the current system. In addition, a computational scheme for assigning partial charges on the host framework atoms was devised; it involves density functional theory (DFT) combined with electronegativity equalization method (EEM) calculations. The derived this way electronegativity, hardness, and gamma parameters for the specific MIL-100(Al) atoms can be used in EEM calculations of other aluminum metalorganic frameworks (MOF) bearing similar atom types. The thermodynamics predictions obtained via MD, comprising equilibria, enthalpies, adsorbate probability densities, and host's terminal species effects, were compared with data from the real system's phase equilibria measured in this work. The intracrystalline self-diffusivity of the sorbed water was extracted by means of the spin echo curves obtained by PFG NMR for various guest loadings as a function of observation time and a theoretical short-time expansion of the diffusion coefficient of random walkers, assuming spherical particles under reflecting boundary conditions following Mitra et al. The experimental activation energies for diffusion confirmed previous, in MIL-100(Fe), and current modeling results, with respect to the adsorbed water dynamics and singlet probability density distribution.
Metal–organic
frameworks (MOFs) currently receive high interest
for cycling water adsorption applications like adsorption heat transformation
for air-conditioning purposes. For practical use in adsorption heat
pumps (AHPs), the microcrystalline powders must be formulated such
that their high porosity and pore accessibility are retained. In this
work, the preparation of millimeter-scaled pellets of MIL-160(Al),
Al-fumarate (Basolite A520), UiO-66(Zr), and Zr-fumarate (MOF-801)
is reported by applying the freeze granulation method. The use of
poly(vinyl alcohol) (PVA) as a binder reproducibly resulted in highly
stable, uniformly shaped PVA/MOF pellets with 80 wt % MOF loading,
with essentially unchanged MOF porosity properties after shaping.
The shaped pellets were analyzed for the application in AHPs by water
adsorption isotherms, over 1000 water adsorption/desorption cycles,
and thermal and mechanical stability tests. Furthermore, the Al-fum
pellets were applied in a fixed-bed, full-scale heat exchanger, yielding
specific cooling powers from 349 up to 431 W/kg (adsorbent), which
outperforms the current commercially used silica gel grains in AHPs
under comparable operating conditions.
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