12 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
Continuous Partial Hydrogenation Reactions by Pd@unconventional Bimodal Porous Titania Monolith Catalysts
Pd nanoparticles are immobilized by a green procedure
onto unconventional
dual porosity titania monoliths. The material is used in catalytic
continuous-flow hydrogenation reactions showing excellent efficiency,
selectivity, and durability
Experiment and Theory of Low-Pressure Nitrogen Adsorption in Organic Layers Supported or Grafted on Inorganic Adsorbents: Toward a Tool To Characterize Surfaces of Hybrid Organic/Inorganic Systems
We report experimental nitrogen adsorption isotherms
of organics-coated
silicas, which exhibit a low-pressure desorption branch that does
not meet the adsorption branch upon emptying of the pores. To address
the physical origin of such a hysteresis loop, we propose an equilibrium
thermodynamic model that enables one to explain this phenomenon. The
present model assumes that, upon adsorption, a small amount of nitrogen
molecules penetrate within the organic layer and reach adsorption
sites that are located on the inorganic surface, between the grafted
or adsorbed organic molecules. The number of accessible adsorption
sites thus varies with the increasing gas pressure, and then we assume
that it stays constant upon desorption. Comparison with experimental
data shows that our model captures the features of nitrogen adsorption
on such hybrid organic/inorganic materials. In particular, in addition
to predicting the shape of the adsorption isotherm, the model is able
to estimate, with a reasonable number of adjustable parameters, the
height of the low-pressure hysteresis loop and to assess in a qualitative
fashion the local density of the organic chains at the surface of
the material
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
Diffusion Properties of Hexane in Pseudomorphic MCM-41 Mesoporous Silicas Explored by Pulsed Field Gradient NMR
Pulsed field gradient (PFG) NMR is a powerful tool to
examine diffusion
of adsorbates in porous systems. The use of mesoporous silicas with
uniform particle sizes allowed us to demonstrate the possibilities
of this technique. In particular, we confirmed that, in the Mitra
mathematical approach of diffusion, the surface-to-volume ratio is
related to the geometry of the whole particle and not of a single
pore. Hexane diffusion measured by PFG-NMR was efficient to study
innovative materials like pseudomorphic MCM-41 mesoporous silicas
presenting different pore topologies. The thorough analysis of the
diffusion data allows monitoring the extension of the restricted diffusion
domain. This method gives quantitative information on diffusion processes
in bimodal pore systems and permits to gain insight into the internal
structure of the pseudomorphic materials at different synthesis times.
For a simple pore geometry, it is observed that the diffusion coefficient
increases with the pore size. However, when materials possess a bimodal
pore system (as for the intermediate materials of the pseudomorphic
transformation), the diffusion can either decrease or increase depending
on the connectivity of the secondary large mesopores with the main
mesoporous channels. By PFG-NMR it was possible to detect the rearrangement
of the mesoporous network of MCM-41 with synthesis time and to confirm
the time necessary for the ordered mesoporous channels of MCM-41 to
run through the whole particle. This type of measurement can nicely
complement usual characterization techniques (N<sub>2</sub> adsorption,
SEM, TEM, etc.) in order to give a better picture of diverse porous
materials
Apparent Anomalous Temperature Dependence of Self-Diffusion Studied by Pulsed-Field Gradient Nuclear Magnetic Resonance and Thermodynamic Modeling
The self-diffusivity of cyclohexane and n-octane
adsorbed in hierarchical zeolite monoliths has been investigated by
using PFG-NMR. In these samples, the intrinsic FAU-X zeolite microporosity
combines with a complex macroporous network composed of aggregated
zeolite nanocrystals. As temperature is increased, cyclohexane self-diffusivity
apparently decreases, reaches a minimum, and then starts increasing
upon further increasing the temperature. Such striking, i.e., non-Arrhenius,
temperature dependence is not observed for n-octane
in the same samples and for cyclohexane adsorbed in purely microporous
FAU-X. Through thermodynamic modeling, we show that this anomalous
behavior can be rationalized by considering the evolution in the adsorbate
populations when changing the temperature. In more detail, we show
that the slow and fast diffusing species present in the microporosity
and secondary porosity arising from the packing of zeolite nanocrystals
vary significantly with a strong impact on the effective diffusivity.
Applying the temperature evolution of their relative fractions to
a simple two-phase diffusion model helps obtain insights into the
physicochemical factors responsible for the complex behavior of effective
self-diffusivity in hierarchical zeolites
Positronium Production in Engineered Porous Silica
Positronium
(Ps) has been the subject of several experimental and
theoretical investigations due to its many scientific applications.
In this work high positronium yield was found in engineered porous
silica. The studied materials were pellets of swollen MCM-41 and of
commercial Davicat 1700, obtained by different compression pressures,
with mesopores characterized by different structural and chemical
features. The measurements were performed with a variable energy positron
beam at room temperature. An estimation of the Ps mean diffusion length
was obtained by measuring capped samples. A selected swollen MCM-41
sample (0.39 g/cm<sup>3</sup>) was measured also at cryogenic temperature
(8 K). In this material both the Ps yield and the Ps diffusion length
are found to be independent of temperature. The pore surface of the
swollen MCM-41 samples is very interesting in comparison to commercial
silica as it possesses hydrophobic patches to avoid ice formation
at low temperature. Positron lifetime measurements show a high Ps
survival time inside the mesoporous materials (∼110 ns), which
promotes a high Ps mobility during cooling inside the pores favoring
diffusion lengths up to 1 μm for swollen MCM-41 materials. Besides,
it was possible to estimate the total Ps yield coming up outside the
sample at high implantation energies and the time between the implantation
of positrons and the Ps release
Apparent Anomalous Temperature Dependence of Self-Diffusion Studied by Pulsed-Field Gradient Nuclear Magnetic Resonance and Thermodynamic Modeling
The self-diffusivity of cyclohexane and n-octane
adsorbed in hierarchical zeolite monoliths has been investigated by
using PFG-NMR. In these samples, the intrinsic FAU-X zeolite microporosity
combines with a complex macroporous network composed of aggregated
zeolite nanocrystals. As temperature is increased, cyclohexane self-diffusivity
apparently decreases, reaches a minimum, and then starts increasing
upon further increasing the temperature. Such striking, i.e., non-Arrhenius,
temperature dependence is not observed for n-octane
in the same samples and for cyclohexane adsorbed in purely microporous
FAU-X. Through thermodynamic modeling, we show that this anomalous
behavior can be rationalized by considering the evolution in the adsorbate
populations when changing the temperature. In more detail, we show
that the slow and fast diffusing species present in the microporosity
and secondary porosity arising from the packing of zeolite nanocrystals
vary significantly with a strong impact on the effective diffusivity.
Applying the temperature evolution of their relative fractions to
a simple two-phase diffusion model helps obtain insights into the
physicochemical factors responsible for the complex behavior of effective
self-diffusivity in hierarchical zeolites
Ionic Liquid Mediated Sol-Gel Synthesis in the Presence of Water or Formic Acid: Which Synthesis for Which Material?
Sol-gel syntheses involving either neutral water or formic
acid
as a reactant have been investigated (1) to determine the best conditions
to confine a maximum of ionic liquid (IL) inside silica-based matrixes
and (2) to reach the highest porosity after removing the IL from the
ion gels (washed gels). Several sets of ionogels were prepared from
various 1-butyl-3-methylimidazolium ILs and various silica or organosilica
sources. The study evidenced a critical effect of the anion on the
morphology (monolith, powder) and texture of the resulting washed
gels. Particularly, tetrafluoroborate anion led to monolith ionogels
by a simple hydrolytic method, affording highly condensed mesoporous
silicas with some fluorinated surface sites. Such sites have never
been reported before and were evidenced by <sup>19</sup>F NMR. On
the other hand, formic acid solvolysis turned out to be the only method
to get non-exuding, crack-free, and transparent monoliths from ILs
containing bis(trifluoromethylsulfonyl)imide [NTf<sub>2</sub>] anion,
with promising applications in photochemistry or photosensing. With
bulky imidazolium and pyridinium cations, removal of the IL led to
highly porous silicas with pore diameters and pore volumes as high
as 10–15 nm and 3 cm<sup>3</sup> g<sup>–1</sup>, respectively.
These silicas could find applications as supports for immobilizing
bulky molecules