J.P. Coulomb

Thermodynamic and structural properties of physisorbed phases within the model mesoporous adsorbent M41S (pore diameter 2.5 nm)

An M41S sample with a pore diameter of 2.5 nm has been characterised by the physisorption of various probe molecules (N2, CO, D2, CH4, CD4, Ar and Kr). This gives rise to a distinct step in the adsorption isotherm as if a capillary condensation mechanism occurs for these adsorbates. However, not all of the isotherms present a hysteresis in the desorption branch of the isotherm as would be expected for a capillary condensation-type mechanism. Thermodynamic (isothermal microcalorimetry) studies reveal an output of differential enthalpy around |1–2.5 kJ mol−1| above the enthalpy of liquefaction during this step. Furthermore, in the case of krypton, a “solidification” is suspected. A structural (neutron diffraction) study however, indicates that the adsorption of deuterium and methane is characterised by short range order, even at 3 K.

A microcalorimetric study of the different states of argon and nitrogen adsorbed AT 77 K on silicalite-I and ZSM-5

The adsorption of argon and nitrogen on a series of MFI-type zeolites (silicalite-I (Si/Al>1000) and HZSM-5 (16<Si/Al<120)) was studied by isothermal microcalorimetry, volumetry and neutron diffraction.The adsorption of argon and nitrogen present a ‘liquid-like’ to ‘solid-like’ adsorbate phase change. The ‘solid-like’ structures of both adsorbates are similar and imposed by the zeolite channel system. Increasing the aluminium content produces an overall increase in the enhanced adsorption sites for nitrogen whereas the behaviour of argon is unmodified. On HZSM-5, the phase changes of both adsorbates still occur, but, particularly for nitrogen, in a less distinct manner with increasing aluminium content.ZusammenfassungMittels Mikrokalorimetrie, Volumetrie und Neutronendiffraktion wurde die Adsorption von Argon und Stickstoff an einer Reihe von MFI-Zeolithen ((Si/Al1000) und HZSM-5 (16<Si/Al<120)).Die Adsorption von Argon und Stickstoff stellt einen Phasenübergang von einem “flüssigkeitsartigen” zu einem “feststoffartigen” Adsorbat dar. Die “feststoffartigen” Strukturen beider Adsorbate sind einander ähnlich und durch das Zeolith-Tunnelsystem geprägt. Die Erhöhung des Aluminiumgehaltes verursacht bei Stickstoff eine allgemeine Zunahme der belegten Adsorptionsstellen, während das Verhalten von Argon unverändert bleibt. An HZSM-5 vollzieht sich zwar die Phasenumwandlung bei beiden Adsorbaten, aber — insbesondere bei Stickstoff — durch Erhöhung des Aluminiumgehaltes in einer weniger ausgeprägten Weise.

Sorption of argon and nitrogen on network types of zeolites and aluminophosphates

Abstract Synthetic zeolites and aluminophosphates comprising 10- and 12- membered ring openings, unidimensional and network type of pore systems (MFI, MEL, ERI, LTA, AEL, AFI and FAU) were used as model adsorbents to examine the impact of micropore structure on the sorption properties. Argon and nitrogen were employed as adsorptives. Adsorption measurements were carried out on gravimetric and volumetric sorption devices and also monitored by microcalorimetry. From the low coverage regime of the isotherm Henrys constants and isosteric heats of adsorption were derived. Both quantities allowed the discrimination between 10- and 12- membered ring systems. Unidimensional 10- and 12- membered ring zeolites and aluminophosphates gave Type I isotherms for argon and nitrogen. Stepped isotherms were observed for argon and nitrogen on network types of molecular sieves. On MFI type zeolites with nitrogen a distinct hysteresis was observed between p/p o = 0.1 and 0.15, as reported earlier. In-situ measurements of the system Silicalite I / nitrogen at 77 K by neutron diffraction experiments indicated discontinous changes in the diffraction pattern of both MFI and nitrogen upon increasing adsorbate coverage.

Structural property of methane (CD4) and hydrogen (D2) sorbed phases on MCM-41 (Ø=25 Å).

A structural study by neutron diffraction of methane (CD4) and hydrogen (D2) physisorbed at T=77.5 K and T=16.4 K respectively on MCM-41 (O=25 ) has pointed out that the two sorbed phases are characterized by a molecular short range order (20≤Lcoher.≤30 ) both in the film formation regime and in the bulk mesopore filling regime. When decreasing the temperature down to T=3 K no crystallization is boserved. Such short range order is the characteristic of fluid phases or amorphous solid phases.

Structural analysis by neutron diffraction of simples gases(H2, Ar, CH4and CF4) sorbed phases in AIPO4-5.

Abstract The inner surface of the one-dimensional micropore network of AIPO 4 -5 is particularly simple(composed mainly of hexagonal adsorption sites). Such aluminophosphate is well suited for the modelling of the sorbate phase properties. Our neutron diffraction study of D 2 , 36 Ar, CD 4 and CF 4 sorbed phases in AIPO 4 -5 has pointed out that the diffraction spectrum of the bare zeolite is strongly modified during the gas sorption(even when no adsorbate phase transition is observed). Only the diffraction peak intensities vary(not the peak positions) indicating that the AIPO 4 -5 structure does not change upon gas adsorption. Neutron diffraction spectra give direct information about the range of the molecular order in the sorbate phases. This molecular organization depends greatly on the sorbate characteristics. For small molecules such as D 2 and Ar, we have observed a large influence of the AIPO 4 -5 adsorption sites in the medium and high loading regime, the sorbate phase looks like lattice fluid . At very high loading the argon sorbate phase solidifies in a kind of vitreous solid characterized by a short range order(L coh. ≃ 7 A). For CF 4 , which is a relatively large molecule, the sorbate phase is completely disordered( fluid like ). Only CH 4 undergoes a phase transition when adsorbing on the AIPO 4 -5 inner surface. From our neutron diffraction analysis we have deduced the nature of the phase change lattice fluid ⇔ commensurate solid phase .

Water sorption in hydrophobic porous materials : isotherm shapes and their meanings for the mesoporous MCM-41 and the microporous AIPO4-5

We report on extensive neutron diffraction and incoherent quasi-elastic neutron scattering analyses for the water sorption in two hydrophobic porous materials: the mesoporous material MCM-41 and the microporous zeolite AIPO 4 -5. Water sorption isotherms have, in the both porous materials, the characteristics of type V isotherms: vertical step at p/p 0 > 0.3 and Hl hysteresis loop. Whatever the pore diameter (either mesoporous 20 A < 0 < 40 A or microporous 0 =7.3 A), whatever the pore wall structure (either amorphous SiO 2 , or crystalline AIPO 4 ), water sorption phenomenon looks like the so-called capillary condensation phase transition. Our neutron scattering results clearly validate such an expected behaviour in the mesoporous confinement range (20 A < O MCM-41 < 40 A). Concerning water confinement in the microporous range (O AIPO4-5 = 7.3 A), our results are more surprising. Type V sorption isotherm is the signature of a crystallization phenomenon at room temperature (T = 300 K). The confined water crystallizes in two helices that are commensurate with the AIPO 4 -5 micropore structure. The confined ice has a density of 1.2 g.cm -3 .

Adsorption and neutron scattering studies: a reliable way to characterize both the mesoporous MCM-41 and the filling mode of the adsorbed species

Numerous studies concern the MCM-41 material, and yet the MCM-41 porosity description and especially the MCM-41 pore diameter (O) characterization is still open for discussion. Here, we report on adsorption and extensive neutron diffraction analyses for the hydrogen sorption in the mesoporous material MCM-41. The type IV isotherm for hydrogen sorption in MCM-41 is similar to many species sorption isotherms such as N 2 , Ar, Kr. The hydrogen isotherm particularity is that the two distinctive parts of the isotherm (first vertical uptake at the low relative pressure p/p 0 ≤ 0.1 and second uptake at the high relative pressure p/p 0 > 0.1) are equally developed. Thus, the neutron scattering experiments realized during the hydrogen adsorption gave accurate data for each adsorption mechanism associated to these two main uptakes, and consequently for MCM-41 porosity structural characterisation. The main findings are the low density of the MCM-41 silica walls (20% voids) and the fine description of D 2 adsorption mechanism: at the low relative adsorption pressure D 2 fills the wall voids and forms a layer on the rough wall surface. Then at the relative pressure of the second uptake, D 2 fills the whole free MCM-41 mesopores as expected (capillary condensation phenomenon). Even the solid capillary phase does not grow layer by layer on the inner pore walls but is growing up along the pore axis.

Structural and dynamic properties of confined water in model porous materials (AlPO4-5, MCM-41)

Confined water presents unusual properties in comparison with other sorbate species. First of all, the sorption isotherm is of type III, even in the microporous confinement range (O<20 ). Whatever the pore diameter, water sorption phenomenon looks like the so-called capillary condensation phase transition. Our results clearly valid such an expected behaviour in the mesoporous confinement range (20 <O<40 ). The water confined phase is a liquid phase characterized by a short range order and a high translational molecular mobility. The confinement induces a strong displacement towards the low temperature of the water confined liquid solidification Tsol. (for instance, Tsol.=230 K for D2O confined liquid in MCM-41 (O=24 ). We have determined the structure of the water confined solid phase observed below Tsol.. It looks like those of the cubic ice structure affected by strong quasi-isotropic finite size effects induced by the confinement. Such a quasi-(1d) solid appears as a polycrystalline column rather than a single crystalline nano fiber. Concerning water confinement in the microporous range (as for example, AlPO4-5 zeolite (O=7.3 )), our results are more surprising. Type III sorption isotherm is the signature of a crystallization phenomenon at room temperature (T=300 K); A commensurate ice chain characterized by a double helix morphology and a high density (d=1.5) is observed.

Phase transition types observed during the sorption of van der Waals gases on model zeolites: Silicalite I and AlPO4-5

Abstract Physiorption of van der Waals (vdW) gases on a uniform flat substrate, usually gives rise to a two-dimensional (2d) film which presents three types of phases and phase transitions ((2d)-gas, (2d)-liquid and (2d)-crystalline solid). On the other hand, it is well known that no phase transition exists for a one-dimensional vdW molecular assembly. Van der Waals sorbed phases on microporous crystalline materials present an intermediate behaviour. In general no phase transition is observed during the sorption on microporous zeolites. The sorbed phase is characterized by a large entropic factor which prevents any phase trnasition in the usual investigated temperature range 77–300 K. But in some specical cases, the characteristics of the zeolite inner surface favour the crystallization of the sorbed phase and induce a phase transition between a disordered fluid phase and a crytalline solid phase.

Measure of the Translational Mobility Dt of Methane Molecules Sorbed in the One-Dimensional Micropore Network of the AlPO4-5 Zeolite

We have measured the translational mobility Dt of methane molecules sorbed in the AIPO4-5 micropores, in the low and medium loading regime. Dt greatly depends both on the temperature and on the sorbed quantity. At T = 96.5 K, Dt decreases from 1.0 10-5 cm2/s to 0.25 10-5 cm2/s when increasing the loading from 0.2 to 0.6. For a CH4 loading equal to 0.2, Dt increases by one order of magnitude in the temperature range 57–97 K. The methane sorbed phase is a fluid-like phase, its translational mobility is of the Brownian type.