Alexander V. Neimark

Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report)

Abstract Gas adsorption is an important tool for the characterisation of porous solids and fine powders. Major advances in recent years have made it necessary to update the 1985 IUPAC manual on Reporting Physisorption Data for Gas/Solid Systems. The aims of the present document are to clarify and standardise the presentation, nomenclature and methodology associated with the application of physisorption for surface area assessment and pore size analysis and to draw attention to remaining problems in the interpretation of physisorption data.

Characterization of nanoporous materials from adsorption and desorption isotherms

Abstract We present a consistent method for calculation of pore size distributions in nanoporous materials from adsorption and desorption isotherms, which form the hysteresis loop H1 by the IUPAC classification. The method is based on the nonlocal density functional theory (NLDFT) of capillary condensation hysteresis in cylindrical pores. It is implemented for the nitrogen and argon sorption at their boiling temperatures. Using examples of MCM-41 type and SBA-15 siliceous materials, it is shown that the method gives the consonant pore size distributions calculated independently from the adsorption and desorption branches of the sorption isotherm. The pore size distributions, pore volumes and specific surface areas calculated from nitrogen and argon data are consistent. In the case of SBA-15 materials, the method evaluates also the amount of microporosity. The results of the NLDFT method are in agreement with independent estimates of pore sizes in regular nanoporous materials.

Capillary condensation in MMS and pore structure characterization

Abstract Phenomena of capillary condensation and desorption in siliceous mesoporous molecular sieves (MMS) with cylindrical channels are studied by means of the non-local density functional theory (NLDFT). The results are compared with macroscopic thermodynamic approaches based on Kelvin–Cohan (KC) and Derjaguin–Broekhoff–de Boer (DBdB) equations. We show that: The KC equations, which constitute the basis of the traditional BJH method for the pore size distribution analysis, are in error even in pores as large as 20 nm. The DBdB equations with consistently determined thickness of the adsorbed layer (disjoining pressure isotherm) can be justified for pores wider than ≈7 nm in diameter. As the pore size decreases, the macroscopic arguments become less accurate, and the NLDFT and DBdB results differ significantly in pores smaller than ≈4 nm. The adsorption–desorption isotherms predicted by NLDFT are found to be in quantitative agreement with the experimental nitrogen (77 K) and argon (87 K) isotherms on MCM-41 type materials with pores larger than 5 nm. Therewith, the experimental desorption branch corresponds to the equilibrium capillary condensation/evaporation transition. The experimental adsorption branch corresponds to the spontaneous spinodal condensation, which occurs at the limit of stability of adsorption films. The NLDFT method has been developed for the calculation of pore size distributions from both the adsorption and desorption isotherms.

Density functional theory model for calculating pore size distributions: pore structure of nanoporous catalysts

Using the example of nanoporous catalysts, we discuss the non-local density functional . theory NLDFT model applied to physical adsorption of nitrogen and argon. The model has been used for predicting adsorptionrdesorption isotherms in nanopores of different geome- . tries over a wide range of pore sizes 0.5)100 nm , and for calculating pore size distributions from adsorption isotherms based on given intermolecular fluid)fluid and fluid)solid poten- tials. The development of new nanoporous catalysts requires reliable characterization methods. We critically analyze different methods which are currently used for pore structure characterization in the range of nanometers. Calculations of the pore size distributions from nitrogen and argon adsorption isotherms are presented. Our primary method is based on the NLDFT model of adsorption on MCM-41, developed earlier. The results obtained with the NLDFT model are compared with other methods. It is shown, that the pore structure of nanoporous catalysts can be quite complex, and that Ar and N isotherms contain compli- 2 mentary information. The NLDFT model is recommended for evaluation of pore size distributions in nanoporous catalysts and other MCM-41 based materials. Q 1998 Elsevier Science B.V. All rights reserved.

Bridging scales from molecular simulations to classical thermodynamics: density functional theory of capillary condensation in nanopores

With the example of the capillary condensation of Lennard-Jones fluid in nanopores ranging from 1 to 10 nm, we show that the non-local density functional theory (NLDFT) with properly chosen parameters of intermolecular interactions bridges the scale gap from molecular simulations to macroscopic thermodynamics. On the one hand, NLDFT correctly approximates the results of Monte Carlo simulations (shift of vapour–liquid equilibrium, spinodals, density profiles, adsorption isotherms) for pores wider than about 2 nm. On the other hand, NLDFT smoothly merges (above 7–10 nm) with the Derjaguin–Broekhoff–de Boer equations which represent augmented Laplace–Kelvin equations of capillary condensation and desorption.

Stress-Based Model for the Breathing of Metal−Organic Frameworks

Gas adsorption in pores of flexible metal-organic frameworks (MOF) induces elastic deformation and structural transitions associated with stepwise expansion and contraction of the material, known as breathing transitions between large pore (lp) and narrow pore (np) phases. We present here a simple yet instructive model for the physical mechanism of this enigmatic phenomenon considering the adsorption-induced stress exerted on the material as a stimulus that triggers breathing transitions. The proposed model implies that the structural transitions in MOFs occur when the stress reaches a certain critical threshold. We showcase this model by drawing on the example of Xe adsorption in MIL-53 (Al) at 220 K, which exhibits two consecutive hysteretic breathing transitions between lp and np phases. We also propose an explanation for the experimentally observed coexistence of np and lp phases in MIL-53 materials.

Plugged hexagonal templated silica: a unique micro- and mesoporous composite material with internal silica nanocapsulesElectronic supplementary information (ESI) available: Fig. S1: X-ray diffractogram of a PHTS material. Fig. S2: TEM images of SBA-15 and PHTS-2. Fig. S3: hydrothermal stabilities. See http://www.rsc.org/suppdata/cc/b2/b201424f/

We describe in this paper the development of plugged hexagonal templated silicas (PHTS) which are hexagonally ordered materials, with internal microporous silica nanocapsules; they have a combined micro- and mesoporosity and a tuneable amount of both open and encapsulated mesopores and are much more stable than other tested micellar templated structures.

Cavitation in Metastable Liquid Nitrogen Confined to Nanoscale Pores

We studied cavitation in metastable fluids drawing on the example of liquid nitrogen confined to spheroidal pores of specially prepared well-characterized mesoporous silica materials with mean pore diameters ranging from approximately 6 to approximately 35 nm. Cavitation was monitored in the process of evaporation/desorption from fully saturated samples with gradually decreasing vapor pressure at the isothermal conditions. The onset of cavitation was displayed by a sharp step on the desorption isotherm. We found that the vapor pressure at the onset of cavitation depended on the pore size for the samples with pores smaller than approximately 11 nm and remained practically unchanged for the samples with larger pores. We suggest that the observed independence of the cavitation pressure on the size of confinement indicates that the conditions of bubble nucleation in pores larger than approximately 11 nm approach the nucleation conditions in the bulk metastable liquid. To test this hypothesis and to evaluate the nucleation barriers, we performed grand canonical and gauge cell Monte Carlo simulations of nitrogen adsorption and desorption in spherical silica pores ranging from 5.5 to 10 nm in diameter. Simulated and experimental adsorption isotherms were in good agreement. Exploiting the correlation between the experimental cavitation pressure and the simulated nucleation barrier, we found that the nucleation barrier increased almost linearly from approximately 40 to approximately 70 k(B)T in the range of pores from approximately 6 to approximately 11 nm, and varied in diapason of 70-75 k(B)T in larger pores, up to 35 nm. We constructed the dependence of the nucleation barrier on the vapor pressure, which asymptotically approaches the predictions of the classical nucleation theory for the metastable bulk liquid at larger relative pressures (>0.6). Our findings suggest that there is a limit to the influence of the confinement on the onset of cavitation, and thus, cavitation of nanoconfined fluids may be employed to explore cavitation in macroscopic systems.

The Behavior of Flexible MIL-53(Al) upon CH4 and CO2 Adsorption

The use of the osmotic thermodynamic model, combined with a series of methane and carbon dioxide gas adsorption experiments at various temperatures, has allowed shedding some new light on the fascinating phase behavior of flexible MIL-53(Al) metal−organic frameworks. A generic temperature-loading phase diagram has been derived; it is shown that the breathing effect in MIL-53 is a very general phenomenon, which should be observed in a limited temperature range regardless of the guest molecule. In addition, the previously proposed stress model for the structural transitions of MIL-53 is shown to be transferable from xenon to methane adsorption. The stress model also provides a theoretical framework for understanding the existence of lp/np phase mixtures at pressures close to the breathing transition pressure, without having to invoke an inhomogeneous distribution of the adsorbate in the porous sample.

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).