Fractal Structure in Silica and Composites Aerogels.

Silica aerogels are known to be materials with exceptional characteristics, such as ultra-low density, high surface area, high porosity, high adsorption, and low-thermal conductivity. In addition, these unique properties are mainly related to their specific processing. Depending on the aerogel synthesis procedure, the aerogels texture can be tailored with meso and/or macroporosity. Fractal geometry has been observed and used to describe silica aerogels at nanoscales in certain conditions. In this review paper, we describe the fractal structure of silica aerogels that can develop depending on the synthesis conditions. X-ray and neutron scattering measurements allow to show that silica aerogels can exhibit a fractal structure over one or even more than two orders of magnitude in length. The fractal dimension does not depend directly on the material density but can vary with the synthesis conditions. It ranges typically between 1.6 and 2.4. The effect of the introduction of silica particles or of further thermal treatment or compression of the silica aerogels on their microstructure and their fractal characteristics is also resumed.

Highly Selective Removal of Perfluorinated Contaminants by Adsorption on All-Silica Zeolite Beta.

Perfluorinated alkylated substances (PFASs) are widely used in industrial and commercial applications, leading to a widespread occurrence of these persistent and harmful contaminants in our environment. Removal of these compounds from surface and waste waters is being mandated by European and U.S. governments. Currently, there are no treatment techniques available that lower the concentrations of these compounds for large water bodies in a cost- and energy-efficient way. We hereby propose a hydrophobic, all-silica zeolite Beta material that is a highly selective and high-capacity adsorbent for PFASs, even in the presence of organic competitors. Advanced characterization data demonstrate that the adsorption process is driven by a very negative adsorption enthalpy and favorable steric factors.

Microcalorimetry Study of the Adsorption of Asphaltenes and Asphaltene Model Compounds at the Liquid-Solid Surface.

The adsorption of an acidic polyaromatic asphaltene model compound (C5PeC11) and indigenous C6-asphaltenes onto the liquid-solid surface is studied. Model compound C5PeC11 exhibits a similar type of adsorption with a plateau adsorbed amount as C6-asphaltenes onto three surfaces (silica, calcite, and stainless steel). Model compound BisAC11, with aliphatic end groups and no acidic functionality, does not adsorb at the liquid-silica surface, indicating the importance of polar interactions on adsorption. The values of the adsorption enthalpy characterized by the ΔHz parameter (the enthalpy at zero coverage) indicate that the type of adsorption and the driving force depend on the surface, a key feature when discussing asphaltene deposition. The adsorption of C5PeC11 onto silica is shown to be driven primarily by H bonding (ΔHz = -34.9 kJ/mol), unlike adsorption onto calcite where polar van der Waals and acidic/basic interactions are thought to be predominant (ΔHz = -23.5 kJ/mol). Interactions between C5PeC11 and stainless steel are found to be weak (ΔHz = -7.7 kJ/mol). Comparing C6-asphaltenes and their esterified counterpart shows that adsorption at the liquid-solid surface is not influenced by the formation of H bonds. This was evidenced by the similar adsorbed amounts obtained. Finally, C5PeC11 captures, to a certain extent, the adsorption interactions of asphaltenes present at the calcite-oil and stainless steel-oil surfaces.

Mechanical energy storage performance of an aluminum fumarate metal-organic framework.

The aluminum fumarate MOF A520 or MIL-53-FA is revealed to be a promising material for mechanical energy-related applications with performances in terms of work and heat energies which surpass those of any porous solids reported so far. Complementary experimental and computational tools are deployed to finely characterize and understand the pressure-induced structural transition at the origin of these unprecedented levels of performance.

Adsorptive desulfurization with CPO-27/MOF-74: an experimental and computational investigation.

By combining experimental adsorption isotherms, microcalorimetric data, infrared spectroscopy and quantum chemical calculations the adsorption behaviour of the CPO-27/MOF-74 series (Ni, Co, Mg, Cu, and Zn) in the desulfurization of fuels is evaluated. The results show a clear influence of the metal ion on the adsorption capacity and affinity for S-heterocyclic compounds, with CPO-27(Ni) being the best performing material both in terms of capacity and affinity. The microcalorimetric data and infrared spectroscopy confirm the high affinity of CPO-27(Ni) for thiophene and similar compounds, while the computational data reveal that the origin of this outstanding adsorption performance is the strong sulfur-metal interaction.

The direct heat measurement of mechanical energy storage metal-organic frameworks.

In any process, the heat exchanged is an essential property required in its development. Whilst the work related to structural transitions of some flexible metal-organic frameworks (MOFs) has been quantified and linked with potential applications such as molecular springs or shock absorbers, the heat related to such transitions has never been directly measured. This has now been carried out with MIL-53(Al) using specifically devised calorimetry experiments. We project the importance of these heats in devices such as molecular springs or dampers.

N/S-heterocyclic contaminant removal from fuels by the mesoporous metal-organic framework MIL-100: the role of the metal ion.

The influence of the metal ion in the mesoporous metal trimesate MIL-100(Al(3+), Cr(3+), Fe(3+), V(3+)) on the adsorptive removal of N/S-heterocyclic molecules from fuels has been investigated by combining isotherms for adsorption from a model fuel solution with microcalorimetric and IR spectroscopic characterizations. The results show a clear influence of the different metals (Al, Fe, Cr, V) on the affinity for the heterocyclic compounds, on the integral adsorption enthalpies, and on the uptake capacities. Among several factors, the availability of coordinatively unsaturated sites and the presence of basic sites next to the coordinative vacancies are important factors contributing to the observed affinity differences for N-heterocyclic compounds. These trends were deduced from IR spectroscopic observation of adsorbed indole molecules, which can be chemisorbed coordinatively or by formation of hydrogen bonded species. On the basis of our results we are able to propose an optimized adsorbent for the deep and selective removal of nitrogen contaminants out of fuel feeds, namely MIL-100(V).

Inverse suspension polymerization as a new tool for the synthesis of ion-imprinted polymers.

Ion-imprinted polymer beads are prepared for the first time by inverse suspension polymerization in mineral oil using nickel(II) as the template ion. As water is not used as the continuous phase, this new route of synthesis avoids the risk that the ion template leaves the suspension for the aqueous phase. The leaching of nickel from the resin beads is very good due to the large porosity of the polymer beads. The ratio between the ligand and the crosslinker has been increased leading to higher adsorption capacities. Comparing these values with those of the non-imprinted polymers and studying the effect of some interfering ions proves that an optimum can be found for the ratio ligand/crosslinker.

p-Xylene-selective metal-organic frameworks: a case of topology-directed selectivity.

Para-disubstituted alkylaromatics such as p-xylene are preferentially adsorbed from an isomer mixture on three isostructural metal-organic frameworks: MIL-125(Ti) ([Ti(8)O(8)(OH)(4)(BDC)(6)]), MIL-125(Ti)-NH(2) ([Ti(8)O(8)(OH)(4)(BDC-NH(2))(6)]), and CAU-1(Al)-NH(2) ([Al(8)(OH)(4)(OCH(3))(8)(BDC-NH(2))(6)]) (BDC = 1,4-benzenedicarboxylate). Their unique structure contains octahedral cages, which can separate molecules on the basis of differences in packing and interaction with the pore walls, as well as smaller tetrahedral cages, which are capable of separating molecules by molecular sieving. These experimental data are in line with predictions by molecular simulations. Additional adsorption and microcalorimetric experiments provide insight in the complementary role of the two cage types in providing the para selectivity.

Structural transitions in MIL-53 (Cr): view from outside and inside.

We present a unified thermodynamic description of the breathing transitions between large pore (lp) and narrow pore (np) phases of MIL-53 (Cr) observed during the adsorption of guest molecules and the mechanical compression in the process of mercury porosimetry. By revisiting recent experimental data on mercury intrusion and in situ XRD during CO(2) adsorption, we demonstrate that the magnitude of the adsorption stress exerted inside the pores by guest molecules, which is required for inducing the breathing transition, corresponds to the magnitude of the external pressure applied from the outside that causes the respective transformation between lp and np phases. We show that, when a stimulus is applied to breathing MOFs of MIL-53 type, these materials exhibit small reversible elastic deformations of lp and np phases of the order of 2-4%, while the breathing transition is associated with irreversible plastic deformation that leads to up to ∼40% change of the sample volume and a pronounced hysteresis. These results shed light on the specifics of the structural transformations in MIL-53 (Cr) and other soft porous crystals (SPC).

Hydration sequence of swelling clays: evolutions of specific surface area and hydration energy.

In order to identify the key steps and the driving force for the hydration process of swelling clays, the water adsorption isotherms and enthalpies were measured on monoionic montmorillonite samples saturated with alkali or calcium ions, and on bi-ionic samples saturated with a sodium-calcium mixture. The specific surface area evolution along the hydration process was determined using a recent interpretation of the experimental adsorption isotherms of swelling solids. Results are interpreted in structural terms. Compared with additional data from sample-controlled thermal analysis (SCTA), the results confirm experimentally that the hydration of Li- and Na-montmorillonite is mainly a cation-controlled process, in contrast with the hydration of Cs samples in which the cation contribution to hydration is negligible, as we have already demonstrated using electrostatic calculations or conductivity measurements.

Detailed in situ XRD and calorimetric study of the formation of silicate/mixed surfactant mesophases under alkaline conditions. Influence of surfactant chain length and synthesis temperature.

The formation of mesoscopically ordered silica/surfactant composites under alkaline synthesis conditions has been studied by time-resolved in situ small-angle X-ray diffraction with synchrotron radiation. Alkyltrimethylammoniumbromide surfactants, C(n)()TAB, of different chain lengths (n = 14, 16, and 18) as well as mixtures thereof were used as structure directing agents and the measurements were carried out at two different temperatures. A linear relationship between the mean surfactant chain length and the d spacing of the hexagonal phase was observed, suggesting an ideal mixing of the surfactants in the supramolecular surfactant aggregates. It is shown that the formation of the hexagonal phase is kinetically controlled mainly by the rate of silicate condensation, while the effect of changes in the surfactant chain length on the kinetics is small under the studied conditions. Two concominant, albeit partly interlinked, processes, suggested being intra- and intermicellar condensation, followed by aggregate-aggregate condensation, govern the nucleation and growth of the hexagonal phase. The two-step mechanism is confirmed by a microcalorimetric study where the heat evolved during the hydrolysis-condensation reactions is followed as a function of time.