Showing papers by "Benoit Coasne published in 2018"

Role of hydrogen bonding in hysteresis observed in sorption-induced swelling of soft nanoporous polymers.

Abstract: Hysteresis is observed in sorption-induced swelling in various soft nanoporous polymers. The associated coupling mechanism responsible for the observed sorption-induced swelling and associated hysteresis needs to be unraveled. Here we report a microscopic scenario for the molecular mechanism responsible for hysteresis in sorption-induced swelling in natural polymers such as cellulose using atom-scale simulation; moisture content and swelling exhibit hysteresis upon ad- and desorption but not swelling versus moisture content. Different hydrogen bond networks are examined; cellulose swells to form water-cellulose bonds upon adsorption but these bonds do not break upon desorption at the same chemical potential. These findings, which are supported by mechanical testing and cellulose textural assessment upon sorption, shed light on experimental observations for wood and other related materials.

Specific Surface Area Determination for Microporous/Mesoporous Materials: The Case of Mesoporous FAU-Y Zeolites

Abstract: A methodology for determining the micropore, mesopore, and external surface areas of hierarchical microporous/mesoporous materials from N2 adsorption isotherms at 77 K is described. For FAU-Y zeolites, the microporous surface area calculated using the Rouquerol criterion and the Brunauer–Emmett–Teller (BET) equation is in accord with the geometrical surface determined by the chord length distribution method. Therefore, BET surface area (SBET) is the well representative of micropore surface areas of microporous materials and of total surface area of microporous/mesoporous materials. Mechanical mixtures of mesoporous MCM-41 and microporous FAU-Y powders of known surface areas were used to calculate the respective surface areas by weighted linear combination and the results were compared to the values obtained by the t-plot method. The first slope of the t-plot determined the mesopore and external surface areas (Smes+ext). The linear fit of the first slope is in general in the range 0.01 < p/p0 < 0.17 and co...

Cavitation of water in hardened cement paste under short-term desorption measurements

Abstract: Water vapor sorption isotherm measurement is one of the promising techniques to understand the microstructure of hardened cement paste, because it always gives a higher surface area than sorption isotherm measurements performed with other adsorbents such as nitrogen and argon, which implies that water molecules can probe the widest range of the microstructure of hardened cement pastes. When, at 20 °C, the water sorption measurement is conducted such as to last for a few days, a characteristic behavior—a sudden drop in adsorbed amount around a relative humidity of 0.35—is always observed on the desorption branch. Here, we prove that this sudden drop is caused by water cavitation, based on an analysis of experimental sorption isotherms acquired at various temperatures, scanning isotherms, and length-change isotherms. Cavitation in hardened cement paste is likely to occur in the C–S–H gel pores constricted by the C–S–H interlayer space.

Revelation on the complex nature of mesoporous hierarchical FAU-Y zeolites

Abstract: The texture of mesoporous FAU-Y (FAUmes) prepared by surfactant-templating in basic media is a subject of debate. It is proposed that mesoporous FAU-Y consists of: (1) ordered mesoporous zeolite networks formed by a surfactant-assisted zeolite rearrangement process involving local dissolution and reconstruction of the crystalline framework, and (2) ordered mesoporous amorphous phases as Al-MCM-41, which coexist with zeolite nanodomains obtained by a dissolution-reassembly process. By the present systematic study, performed with FAU-Y (Si/Al = 15) in the presence of octadecyltrimethylammonium bromide and 0 < NaOH/Si ratio < 0.25 at 115 °C for 20 h, we demonstrate that mesoporous FAU zeolites consist, in fact, of a complex family of materials with textural features strongly impacted by the experimental conditions. Two main families have been disclosed: (1) for 0.0625 < NaOH/Si < 0.10, FAUmes are ordered mesoporous materials with zeolite walls, which coexist with zeolite nanodomains (100–200 nm) and (2) for ...

Saturation of the Siliceous Zeolite TON with Neon at High Pressure

Abstract: The insertion of neon and argon in the 1-D pore system of the zeolite TON was studied at high pressure by X-ray diffraction and by Monte Carlo (MC) molecular modeling. Rietveld refinements of the crystal structure of TON and the MC results indicate that 12 Ne atoms enter the unit cell of TON, completely filling the pores. This is much greater than the degree of filling observed for argon, which due to size considerations lies in a vertical plane in the pores. A phase transition from the Cmc21 to a Pbn21 structure occurs at 0.6 GPa with cell doubling. The compressibility and structural distortions, such as pore ellipticity, are considerably reduced as compared to the argon-filled or the empty-pore material. In addition, the crystalline form persists to pressures of the order of 20 GPa, and the Pbn21 phase is recovered after decompression. The results show the very strong and different effects of pore filling by noble gases on the structural stability and mechanical properties of this prototypical 1-D zeoli...

The structure and dipolar properties of CO2 adsorbed in a porous glassy chalcogel: Insights from first-principles molecular dynamics

Abstract: A thorough understanding of the interactions of CO2 with the hosting porous network is crucial for the design of new adsorbent materials with enhanced gas adsorption capacity and selectivity. In this paper, first-principles molecular dynamics simulations are performed to assess the interactions of CO2 adsorbed in a nanoporous glassy chalcogenide (i.e. chalcogel) under relevant laboratory conditions. The structure and local organization of the confined CO2 molecules are analyzed in terms of atomic density, orientational order parameter and pair correlation functions. Maximally localized Wannier functions are used to unravel the electronic structure, the local molecular dipole and the nature of the chemical bonding at the interface between CO2 and the glassy surface. Our results provide a useful insight on the complex interplay between the chemical interactions competing at the buried interface made by the CO2 molecules adsorbed in g-GeS2 chalcogel.

Adsorption on alumina nanopores with conical shape.

Abstract: Adsorption on porous solids depends on the morphology of the pores, the cylindrical one being the most studied in the literature. In this work, we present the first experimental investigation of adsorption and evaporation on conical nanopores produced by anodization of aluminium oxide. The pores are about 50 μm long, with the wide ends having a diameter of ∼79 nm and the narrow ones of ∼30 nm. Three different pores configurations are considered: open at both ends, open only at the narrow end and open only at the wide end. Despite the very small value of the conical angle α, estimated to be ∼0.06°, just barely above α = 0° corresponding to a cylindrical pore, the adsorption isotherms look strikingly different from those measured on cylindrical pores of similar size. First of all, the hysteresis loops of the conical pores with two open ends and with open wide ends practically coincide. Furthermore, they are narrower and the adsorption and evaporation branches are broader than those of the cylindrical pores with similar size. Finally, conical pores with open narrow ends exhibit a large hysteresis indicative of pore blocking. To unravel the mechanisms underlying adsorption and evaporation in such conical pores, we also report complementary results obtained using on-lattice grand canonical Monte Carlo simulations.