3 research outputs found
Adsorption and Dynamics in Hierarchical Metal–Organic Frameworks
Adsorption and dynamics in hierarchical
metal–organic frameworks
are investigated by means of molecular simulation. The models of hierarchical
porous solids are obtained by carving mesopores of different diameters
out of a crystal of Cu-BTC (model A) or by inserting a microporous
particle of Cu-BTC in an amorphous silica mesopore (model B). We show
that the nitrogen adsorption isotherms at 77 K for the solids corresponding
to model A can be described as a linear combination of reference adsorption
isotherms for pure microporous and mesoporous solids. In contrast,
the adsorption isotherms for model B cannot be described accurately
as a sum of reference microporous and mesoporous adsorption isotherms.
The inserted particle acts as a constriction which helps nucleate
the liquid phase within the mesopore so that no capillary condensation
hysteresis is observed. The dynamics of nitrogen adsorbed at 77 K
inside the porosity of the hierarchical solids is also investigated.
The Fickian regime is reached at long times which are not attainable
with molecular dynamics simulations. At higher temperature, the faster
self-diffusion makes it possible to obtain the diffusivity of the
adsorbate. Nitrogen adsorbed in the microporosity of the hierarchical
porous solids has a self-diffusion coefficient close to that of nitrogen
adsorbed in pure Cu-BTC. In contrast, diffusion in the mesoporosity
is faster than in the microporosity so that the overall diffusivity
is faster than in pure Cu-BTC
Molecular Simulation of Adsorption and Transport in Hierarchical Porous Materials
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
Validity of the <i>t‑plot</i> Method to Assess Microporosity in Hierarchical Micro/Mesoporous Materials
The <i>t-plot</i> method is a well-known technique which
allows determining the micro- and/or mesoporous volumes and the specific
surface area of a sample by comparison with a reference adsorption
isotherm of a nonporous material having the same surface chemistry.
In this paper, the validity of the <i>t-plot</i> method
is discussed in the case of hierarchical porous materials exhibiting
both micro- and mesoporosities. Different hierarchical zeolites with
MCM-41 type ordered mesoporosity are prepared using pseudomorphic
transformation. For comparison, we also consider simple mechanical
mixtures of microporous and mesoporous materials. We first show an
intrinsic failure of the <i>t-plot</i> method; this method
does not describe the fact that, for a given surface chemistry and
pressure, the thickness of the film adsorbed in micropores or small
mesopores (< 10σ, σ being the diameter of the adsorbate)
increases with decreasing the pore size (curvature effect). We further
show that such an effect, which arises from the fact that the surface
area and, hence, the free energy of the curved gas/liquid interface
decreases with increasing the film thickness, is captured using the
simple thermodynamical model by Derjaguin. The effect of such a drawback
on the ability of the <i>t-plot</i> method to estimate the
micro- and mesoporous volumes of hierarchical samples is then discussed,
and an abacus is given to correct the underestimated microporous volume
by the <i>t-plot</i> method