The adsorption−desorption isotherms and relaxation curves of water in chromium terephthalate metal−organic frameworks (MOFs), MIL-101, were measured by the gravimetric method at 298 K and 1 atm. The obtained isotherms were compared to those obtained by the volumetric method, which showed good agreement. The measured isotherms exhibited three-step and two-step curves during adsorption and desorption, respectively. The hysteresis between adsorption and desorption isotherms was not pronounced, and the difference in relative humidity between the adsorption and desorption isotherms was 0.05−0.10 at identical adsorbed mass. Regarding the water adsorption relaxation curves, when the relative humidity was varied stepwise from 0.40 to 0.47 and the water supply rate was small, the relaxation curves could be fitted to two straight lines, indicating that, initially, the middle cages filled with water, followed by the large cages at a different adsorption rate. When the relative humidity was changed stepwise from 0.40 to 0.90 and the water supply rate was large, the relaxation curves could be fitted to a single straight line until the equilibrium state, and the relaxation time was about 40 s. The kinetics of water transport inside MIL-101 is strongly dependent on its unique pore structure and hydrophilicity−hydrophobicity spatial distribution.
The
effect of pore length on the water filling and emptying rates
was studied using mesoporous silica (MPS) with same pore diameter
but different pore lengths. The pore diameter of the synthesized MPS
was ∼8 nm, whereas the average pore lengths were 460, 1,770,
and 4000 nm. The gravimetric method was employed to record the time
course of the adsorbed mass of water in MPS at 298 K and 1 atm. In
both the filling and emptying processes, the relaxation curves (time
course of adsorbed mass of water per unit mass of sample) were not
significantly related to the pore length. This independence of the
initial adsorption and desorption rates on the pore length suggests
that the surface of the MPS aggregates is the bottleneck in the overall
adsorption and desorption processes and that the initial mass flux
in each nanopore is inversely proportional to the pore length. Furthermore,
because the relaxation times to reach the equilibrium state were independent
of the pore length, the mass flux of water uptake, release, and transport
probably increase with an increase in the pore length during the entire
adsorption and desorption processes. A transport model to describe
these phenomena was proposed.
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