Y. Grillet

Effect of pore size on adsorbate condensation and hysteresis within a potential model adsorbent: M41S

Abstract The use of M41S (MCM-41) materials, which have cylindrical-like mesopores, has highlighted the effect of pore diameter on the position, presence and size of the hysteresis loop observed in association with argon and nitrogen adsorbate-adsorbate co-operative condensation. A distinct step in the adsorption isotherm was observed for each of three materials with varying pore diameters (2.5, 4.0 and 4.5 nm). An increase in the pore size leads to a shift in this step to higher relative pressures and to the appearance of a significant hysteresis loop. This may indicate the transition from secondary micropores to mesopores with respect to the nitrogen adsorbate.

Two-dimensional freezing of nitrogen or argon on differently graphitized carbons

Abstract On a set of differently graphitized carbon blacks (Sterling heated between 1500 and 2700°C), chosen to reproduce, in some respects, the set used in 1954 by Beebe and Young (J. Phys. Chem. 58, 93) in their famous experiments, adsorption microcalorimetry at 77 K gives experimental evidence of a two-dimensional fluid → solid phase change at the completion of the argon or nitrogen monolayer. That phenomenon (which is confirmed by a slight substep of the isotherms) develops earlier (i.e., on a less graphitized surface) in the case of argon than in the case of nitrogen: This is explained by the epitaxial crystallization of nitrogen which makes it very sensitive to the size of the graphitic sheets.

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 comparison of the adsorption of various gases on two microporous adsorbents: a model aluminophosphate and a natural clay

Abstract A comparison was made of the physisorption mechanisms of various adsorptives (Ar, CH 4 , N 2 and CO) on two microporous adsorbents: an aluminophosphate, AlPO 4 -5 with an electrically neutral framework and cylindrical pores of 0.73 nm diameter, and sepiolite, a fibrous natural clay with micropores of rectangular cross-section (1.34×0.67 nm 2 ). Isothermal microcalorimetry at 77 K of probe molecules, either with or without a permanent moment (dipolar or quadrupolar), allowed the determination of the successive steps of micropore filling, an evaluation of their relative importance and, finally, an indication of apparent changes in the phase adsorbed by the aluminophosphate. In the case of AlPO 4 -5, the adsorption of the various gases occurred by site in the micropores in only one stage. However, for sepiolite, the adsorption occurred in two steps, initially by site in the intrafibrous structural micropores and then by volume in the interfibrous micropores.

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.

On the physisorption isotherm of the MFI-type zeolites: The high-pressure hysteresis

Abstract Characterization of model (Al)MFI-type samples, carried out initially by scanning electron microscopy and 27 Al nuclear magnetic resonance spectroscopy, aided the elucidation of the mechanisms involved in producing the high-pressure H4-type hysteresis observed in argon, oxygen, nitrogen, and carbon monoxide isotherms. This hysteresis seems to be the resultant of two different adsorbate related phenomena: swelling of microporous defect regions, accessible via larger fissures (created during calcination), occurs in conjunction with swelling between parallel crystal “slabs”. In both cases the adsorbate needs to be above its triple point ( i.e., a three-dimensional liquid).

Application of CRTA to the study of microporosity by thermodesorption of pre-adsorbed water

Thermodesorption is here considered for its possibility of giving access to the microporosity of adsorbents. The requirements of this application (good separation of successive desorption steps, good control of the desorption pressure and temperature throughout the sample, possibility of a safe kinetic analysis of each step) are here fulfilled by carrying out the thermodesorption in the Controlled transformation Rate Thermal Analysis (CRTA) mode. The method is applied to 4 zeolites (3A, 4A, 5A and 13X) and a well characterized charcoal, from −25 to 325°C, after pre-adsorption of water.ZusammenfassungThermodesorption wird in Erwägung gezogen, um einen möglichen Zugang zur Mikroporösität von Adsorbenten zu erhalten. Die Anforderungen an diese Anwendung (gute Trennung aufeinanderfolgender Desorptionsschritte, gute Kontrollmöglichkeit von Desorptionsdruck und-temperatur innerhalb der gesamten Probe, Möglichkeit einer sicheren kinetischen Analyse jedes Einzelschrittes) werden hier durch die Ausführung der Thermodesorption mittels der CRTA-Methode erfüllt. Diese Methode wurde nach Präadsorption von Wasser zwischen −25 und 325°C an 4 Zeolithen (3A, 4A, 5A und 13X) und einer sehr gut definierten künstlichen Kohle angewendet.

Two-dimensional phase changes of argon adsorbed on boron nitride at 77°K

Abstract Adsorption volumetry and microcalorimetry both show a two-dimensional phase change of the argon monolayer adsorbed on boron nitride at 77°K. A second transition takes place near the completion of the second layer. The former phase change is explained as a two-dimensional crystallization in the light of previous work on the argon/graphite and nitrogen/graphite systems which lend themselves to complementary neutron diffraction experiments. Both transitions had remained undetected in previously employed experimental approaches.

Crystalline structure analysis by neutron diffraction of argon sorbed phases observed in the high loading regime of silicalite I and ZSM-5 (Si/Al=23) zeolites.

Abstract Recently for the first time, phase transitions have been observed both by calorimetric and adsorption isotherm measurements during the sorption of simple gases (argon, nitrogen) on the zeolite Silicalite I [1]. Such phenomena appear in the high loading and low temperature sorption regime. Silicalite I belongs to the MFI micropore structural network, it is the pure silica form of ZSM-5 zeolite which is of great importance in petroleum catalytic cracking. The Silicalite I pore network is rather complex. It is composed of straight channels and sinusoidal channels (their respective elliptical diameters are 5.4 × 5.6 A and 5.1 × 5.4 A). Our neutron diffraction studies, concerning the sorbed phases of both 36-argon and n-argon, have pointed out that the phase transition is associated with the appearance of a crystalline order (long range order) in the sorbed argon species. Moreover the Silicalite I structure modification seems to be very small during the gas sorption. These observations result from the large neutron coherent scattering length b difference between 36-argon and n-argon (b 36Ar =2.43 and b Ar =0.20). We can conclude that the adsorption isotherm substep observed during the argon loading of the Silicalite I zeolite is undoubtedly the signature of a phase transition concerning the sorbed species and not a consequence of a Silicalite I structural modification. The two sorbed phases are a disordered phase characterized by a short range atomic order (presumably a fluid phase) and a crystalline solid phase characterized by a periodic long range atomic order. The periodic organization of the argon atoms is induced by the Silicalite I adsorption sites. The phase transition corresponds to a change in the adsorption nature: from a mobile adsorption to a localized adsorption. Our neutron diffraction investigation of the argon/ZSM-5 (Si/Al=23) system has shown that the same phase transition is observed. The inner surface of the Silicalite I zeolite, which is accessible to gas molecules of lower diameter than 5.6 A, is quite complex at the atomic level. This surface complexity is due to the two different familly of micropores (straight micropores, sinusoidal micropores and their intersections) and to the different local symmetry of the adsorption sites (square, pentagonal and hexagonal symmetry). The laboratory of Prof. K.K. Unger at Mainz was the first laboratory which succeded to prepare Silicalite I samples composed of large crystallites with high level of crystallinity. As a consequence sorption studies by isothermal volumetry and microcalorimetric measurements, of simple gases on these Silicalite I samples have revealed new and interesting phenomena. For the first time, the “signature” of a phase transition (as adsorption isotherm substep or exothermic heat peak of adsorption) was observed during the sorption of argon and nitrogen [1-2]. Recently several new examples of isotherm substeps have been observed during the sorption studies of Kr, CO, O 2 and D 2 on Silicalite I, Figure 1 [3-4].

Investigation on the Adsorption of N2 Ar, CO and CH4 on Aluminophosphates

Abstract Adsorption isotherms at different temperatures and isothermal micro-calorimetric curves were measured for N 2 Ar, CO, and CH 4 on various molecular sieves as VPI-5, MCM-9, AlPO 4 -8, AlPO 4 -H, AlPO 4 -5, SAPO-5, AlPO 4 -39, AlPO 4 -C. It was shown that the isosteric heats of adsorption correlated with the pore-sizes of the studied molecular sieves.