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Highlights from the article: About half of these mini-reviews deal with current advances in adsorption in chemical engineering fields: Adsorption in Membrane Science, Coupling adsorption/mechanics; Adsorption coadsorption processes at the industrial scale; Adsorption effects in electrochemistry; Adsorption-driven heat pumps/chillers. The other half of these mini-reviews focus on advanced techniques to probe adsorption: Spectroscopic tools for adsorption; Imaging techniques to probe adsorption; Probing adsorption using Nuclear Magnetic Resonance; Kinetic approaches for molecular modeling of adsorption; Free energy techniques for simulation of adsorption. The paper of Vandamme introduces the phenomenon of adsorption-induced deformation - appearance of stresses and strains due to adsorption of fluids in porous materials [[6]].

When vitreous silica is submittedto high pressures under a heliumatmosphere, the change in volume observed is much smaller than expectedfrom its elastic properties. It results from helium penetration intothe interstitial free volume of the glass network. We present herethe results of concurrent spectroscopic experiments using either heliumor neon and molecular simulations relating the amount of gas adsorbedto the strain of the network. We show that a generalized poromechanicalapproach, describing the elastic properties of microporous materialsupon adsorption, can be applied successfully to silica glass in whichthe free volume exists only at the subnanometer scale. In that picture,the adsorption-induced deformation accounts for the small apparentcompressibility of silica observed in experiments. [ABSTRACT FROM AUTHOR]

This review article presents the fundamental and practical aspects of water adsorption in Metal-Organic Frameworks (MOFs). The state of the art of MOF stability in water, a crucial issue to many applications in which MOFs are promising candidates, is discussed here. Stability in both gaseous (such as humid gases) and aqueous media is considered. By considering a non-exhaustive yet representative set of MOFs, the different mechanisms of water adsorption in this class of materials are presented: reversible and continuous pore filling, irreversible and discontinuous pore filling through capillary condensation, and irreversibility arising from the flexibility and possible structural modifications of the host material. Water adsorption properties of more than 60 MOF samples are reported. The applications of MOFs as materials for heat-pumps and adsorbent-based chillers and proton conductors are also reviewed. Some directions for future work are suggested as concluding remarks. [ABSTRACT FROM AUTHOR]

The effect of temperature on the adsorption of a simple mixture (Ar/Kr) in disordered porous materials is investigated by means of molecular simulation. In the larger mesopores of porous silica glasses, capillary condensation occurs upon decreasing the temperature. At temperatures above the capillary condensation temperature, Kr is preferentially adsorbed at the pore surface and Ar adsorption occurs in regions of low Kr density. For temperatures below the capillary condensation temperature, Ar density surprisingly increases as temperature increases, the behaviour that is consistent with an over-solubility effect. In contrast, in the disordered sub-nanoporous carbon, filling of the pores occurs in a reversible and continuous way upon decreasing the temperature, owing to the small size and amorphous shape of the pores. These results show that the crossover between capillary condensation and continuous reversible filling observed for pure fluids in pores also exists for mixtures. We also show that the Kr selectivity exhibits a minimum in the disordered porous silica that is located at the capillary condensation temperature. In contrast, in the disordered porous carbon where no capillary condensation occurs, the selectivity decreases monotonically with increasing the temperature. These results shed light on low-temperature adsorption of mixtures confined in porous materials and provide a guide to design efficient phase separation processes. [ABSTRACT FROM PUBLISHER]

Adsorption and transport in hierarchicalporous solids with micro-(∼1 nm) and mesoporosities (>2 nm) are investigated by molecularsimulation. Two models of hierarchical solids are considered: microporousmaterials in which mesopores are carved out (model A) and mesoporousmaterials in which microporous nanoparticles are inserted (model B).Adsorption isotherms for model A can be described as a linear combinationof the adsorption isotherms for pure mesoporous and microporous solids.In contrast, adsorption in model B departs from adsorption in puremicroporous and mesoporous solids; the inserted microporous particlesact as defects, which help nucleate the liquid phase within the mesoporeand shift capillary condensation toward lower pressures. As far astransport under a pressure gradient is concerned, the flux in hierarchicalmaterials consisting of microporous solids in which mesopores arecarved out obeys the Navier–Stokes equation so that Darcy’slaw is verified within the mesopore. Moreover, the flow in such materialsis larger than in a single mesopore, due to the transfer between microporesand mesopores. This nonzero velocity at the mesopore surface impliesthat transport in such hierarchical materials involves slippage atthe mesopore surface, although the adsorbate has a strong affinityfor the surface. In contrast to model A, flux in model B is smallerthan in a single mesopore, as the nanoparticles act as constrictionsthat hinder transport. By a subtle effect arising from fast transportin the mesopores, the presence of mesopores increases the number ofmolecules in the microporosity in hierarchical materials and, hence,decreases the flow in the micropores (due to mass conservation). Asa result, we do not observe faster diffusion in the micropores ofhierarchical materials upon flow but slower diffusion, which increasesthe contact time between the adsorbate and the surface of the microporosity. [ABSTRACT FROM AUTHOR]

The structural, morphological, and adsorption propertiesof MCM-41porous silicas are investigated using a realistic numerical modelobtained by means of ab initiocalculations [Ugliengo,P.; et al. Adv. Mater.2008, 20, 1]. Simulated X-ray diffraction, small angle neutronscattering, and electronic microscopy for the atomistic model arein good agreement with experimental data. The morphological featuresare also assessed from chord length distributions and porous volumeand specific geometrical surface calculations, etc. The N2, CO2, and H2O adsorption isotherms in theatomistic model of MCM-41 are also in reasonable agreement with theirexperimental counterpart. An important finding of the present workis that water forms a film adsorbed on specific hydrophilic regionsof the surface while the rest of the surface is depleted in watermolecules. This result suggests that the surface of MCM-41 materialsis heterogeneous, as it is made up of both hydrophilic and hydrophobicpatches. While adsorption and irreversible capillary condensationcan be described using the thermodynamical approach by Derjaguin (alsoknown as the Derjaguin–Broekhoff–De Boer model), theFreundlich equation fits nicely the data for reversible and continuousfilling in small pores. [ABSTRACT FROM AUTHOR]

We report the development of a realistic molecular model for mesoporous silica SBA-15, which includes both the large cylindrical mesopores and the smaller micropores in the pore walls. The methodology for modeling the SBA-15 structure involves molecular and mesoscale simulations combined with geometrical interpolation techniques. First, a mesoscale model is prepared by mimicking the synthesis process using lattice Monte Carlo simulations. The main physical features of this mesoscale pore model are then carved out of an atomistic silica block; both the mesopores and the micropores are incorporated from the mimetic simulations. The calculated pore size distribution, surface area, and simulated TEM images of the model structure are in good agreement with those obtained from experimental samples of SBA-15. We then investigate the adsorption of argon in this structure using Grand Canonical Monte Carlo (GCMC) simulations. The adsorption results for our SBA-15 model are compared with those for a similar model that does not include the micropores; we also compare with results obtained in a regular cylindrical pore. The simulated adsorption isotherm for the SBA-15 model shows semiquantitative agreement with the experimental isotherm for a SBA-15 sample having a similar pore size. We observe that the presence of the micropores leads to increased adsorption at low pressure compared to the case of a model without micropores in the pore walls. At higher pressures, for all models, the filling proceeds via the monolayer−multilayer adsorption on the mesopore surface followed by capillary condensation, which is mainly controlled by the mesopore diameter and is not influenced by the presence of the micropores. [ABSTRACT FROM AUTHOR]

Fluids in large and small pores display different behaviors with a crossover described through the concept of critical capillarity. Here we report experimental and simulation data for various siliceous zeolites and adsorbates that show unexpected reminiscent capillarity for such nanoporous materials. For pore sizes D exceeding the fluid molecule size, the filling pressures p are found to follow a generic behavior kBT ln p ∼ γ/ρD where γ and ρ are the fluid surface tension and density. This result is rationalized by showing that the filling chemical potential for such ultra-small pores is the sum of an adsorption energy and a capillary energy that remains meaningful even for severe confinements. A phenomenological model, based on Derjaguin's formalism to bridge macroscopic and molecular theories for condensation in porous materials, is developed to account for the behavior of fluids confined down to the molecular scale from simple parameters. Confined fluids in porous media exhibit different behaviors in large and small pores, the crossover between the two regimes being not well understood. Here the authors show, by experiments and simulations, that capillarity is reminiscent even for very small pore diameters, providing a unified picture. [ABSTRACT FROM AUTHOR]

Review: 172 refs. [ABSTRACT FROM AUTHOR]

Hierarchical zeolites presenting different pore domains have the potential to reduce problems associated with low diffusion in zeolites, thus holding great promise in a wide range of application fields. Nevertheless, efficient transfer between the different porous networks is required for the mesopores (2–50 nm) and macropores (>50 nm) to be effective in promoting the transit of molecules towards the micropores (<2 nm). In this work, the connectivity between the various pore domains in a class of hierarchical zeolites – denominated as nanoparticle aggregates – is investigated through 2D exchange spectroscopy (EXSY) 1H nuclear magnetic resonance (NMR) using paraxylene as a probe molecule. This approach allows to track whether the probe molecule visits distinct pore environments over time, providing unique insight into pore connectivity. The analysis of scanning curves through N 2 adsorption/desorption cycles completes the characterization of pore connectivity. Furthermore, electron microscopy and tomography are used as visual techniques to confirm the arrangement of the mesoporous and macroporous networks generated by the nanoparticle aggregates. • Hierarchical zeolites are analyzed using a broad spectrum of experimental methods • 2D exchange NMR is used to gain insight into pore connectivity • Scanning N 2 adsorption is combined with electron microscopy and tomography. [ABSTRACT FROM AUTHOR]

Experiment and molecular simulation are used to investigate adsorption in heterogeneous porous media consisting of hierarchical solids (combining different porosity scales) or composite solids (such as silver nanoparticles adsorbed at the external surface of zeolite). It is shown that adsorption in such heterogeneous materials can be written as a linear combination of the adsorption isotherms in its different domains (i.e. porosity scales for the hierarchical sample and constituents for the composite sample). In the case of the composite material, we also show that the linear combination can be used with weighing parameters obtained for a different adsorbate. Such a superimposition principle , which is validated using well-characterized experimental samples, is of interest for characterization purpose as well as industrial applications as they can be used to determine accurately the amount of phases in a given sample (volume corresponding to a given porosity scale or constituent). In contrast, significant departure between the experimental adsorption isotherm and the linear combination can be used to detect coupling effects between the different domains or restrained access to a given domain type. Such a characterization strategy of complex heterogeneous media is complementary to other experiments, such as those probing capillary hysteresis shapes, scanning curves and subloops, which allow determining the distribution of domains within the framework of the independent domain theory. [ABSTRACT FROM AUTHOR]

Silica monoliths with hierarchical porosity (macro-/mesoporous), prepared by a combination of phase separation (spinodal decomposition) and sol–gel process, have demonstrated remarkable potential as supports for catalysts and adsorbents with improved efficiency and productivity of a number of applications. This is due to perfect homogeneous interconnected porous networks enabling an exceptional mass transfer and a fine control of contact times. Silica monoliths have been functionalized by an important variety of moities and techniques, such as grafting with versatile species (acidic, basic), by alumina coating, immobilization of ionic liquids, in-situ synthesis of nanoparticles of Pd, MOF, and NiMoS 2 , pseudomorphic transformation of the skeleton into MCM-41 or zeolites (SOD, LTA). These functional materials reveal great opportunities for process intensification. [ABSTRACT FROM AUTHOR]