Molecular Simulation of Adsorption and Transport in
Hierarchical Porous Materials
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Abstract
Adsorption and transport in hierarchical
porous solids with micro-
(∼1 nm) and mesoporosities (>2 nm) are investigated by molecular
simulation. Two models of hierarchical solids are considered: microporous
materials in which mesopores are carved out (model A) and mesoporous
materials in which microporous nanoparticles are inserted (model B).
Adsorption isotherms for model A can be described as a linear combination
of the adsorption isotherms for pure mesoporous and microporous solids.
In contrast, adsorption in model B departs from adsorption in pure
microporous and mesoporous solids; the inserted microporous particles
act as defects, which help nucleate the liquid phase within the mesopore
and shift capillary condensation toward lower pressures. As far as
transport under a pressure gradient is concerned, the flux in hierarchical
materials consisting of microporous solids in which mesopores are
carved out obeys the Navier–Stokes equation so that Darcy’s
law is verified within the mesopore. Moreover, the flow in such materials
is larger than in a single mesopore, due to the transfer between micropores
and mesopores. This nonzero velocity at the mesopore surface implies
that transport in such hierarchical materials involves slippage at
the mesopore surface, although the adsorbate has a strong affinity
for the surface. In contrast to model A, flux in model B is smaller
than in a single mesopore, as the nanoparticles act as constrictions
that hinder transport. By a subtle effect arising from fast transport
in the mesopores, the presence of mesopores increases the number of
molecules in the microporosity in hierarchical materials and, hence,
decreases the flow in the micropores (due to mass conservation). As
a result, we do not observe faster diffusion in the micropores of
hierarchical materials upon flow but slower diffusion, which increases
the contact time between the adsorbate and the surface of the microporosity