Fernando Rojas

Capillary condensation in heterogeneous mesoporous networks consisting of variable connectivity and pore-size correlation

Heterogeneous three-dimensional mesoporous networks (A. J. Ramirez-Cuesta, S. Cordero, F. Rojas, R. J. Faccio and J. L. Riccardo, J. Porous Mater., 2001, 8, 61, ) constructed under the premises of the dual site–bond model have been used as probe substrates to study the effects of variable connectivity and pore-size correlation on the aspects of both hysteresis loops and primary sorption scanning curves. The shapes of the hysteresis loops obtained from sorption simulation in networks of diverse morphologies are compared and discussed. It is found that the precursor structural parameters of the Monte Carlo simulated networks together with the sorption algorithm used in this work, can lead to IUPAC types H1, H2 and H3-like hysteresis loops, depending on the values chosen for the pore-size distribution parameters and mean connectivity. Network morphology also influences greatly the mechanisms of sorption processes in poorly or highly size correlated porous substrates. Sorption results on these 3-D porous specimens help to visualize the extents of pore blocking (vapour percolation) and delayed adsorption (liquid percolation) phenomena and also to foresee the most appropriate methods to ascertain the structure of porous materials.

On comparing BJH and NLDFT pore-size distributions determined from N2 sorption on SBA-15 substrata

SBA-15 silica materials consisting of a collection of non-intersecting cylindrical pores of varying diameters have been utilized to try to reconcile the pore-size distribution results proceeding from the classical Barrett–Joyner–Halenda (BJH) and modern non-local density functional theory (NLDFT) approaches. To assess such pretended concordance, it is necessary to perform BJH pore-size estimates on the basis of a modified Kelvin equation that makes allowance for the adsorption potential field emanating from the solid walls of the adsorbent towards the adsorbate molecules. Under this context, critical conditions for capillary condensation and evaporation to happen in cylindrical pores have been specifically calculated via a treatment previously developed by Broekhoff and de Boer (BdB). In this way, BJH-BdB pore-size distribution results, obtained from the analyses of both ascending and descending boundary curves of N2 sorption isotherms at 76 K on a series of model SBA-15 substrata that have been synthesized in this work, are compared with homologous curves proceeding from a NLDFT treatment performed on the descending boundary curve and very reasonable agreement has been found.

Nitrogen-Sorption Characterization of the Microporous Structure of Clinoptilolite-Type Zeolites

The micropore-filling characteristics of a series of natural and modified microporous clinoptilolite-type zeolites with N2 at 76 K are measured and analysed. The adsorption behaviour of these substrata is examined in the range of relative pressures between 10−5–1. Several methods such as: Sings αs-plots, de Boers t-plots, Lee and Newnhams direct comparison plots, Dubinins classical methods and a difference isotherm procedure proposed here, are used to assess the microporosity of the samples. Natural samples are used as reference materials to perform these sorption analyses of the modified samples. The effect of narrow micropore constrictions on the adsorption behaviour of clinoptilolites is explored experimentally. The occurrence of a low-pressure hysteresis loop along the sorption isotherm of a modified sample is frequently found and may be due to the strong adsorption of adsorbate molecules at the entrance of necked micropores that interfere with the diffusion of adsorbate molecules inside the porous structure of these zeolites.

Mechanistic study of surface processes on adsorbents: I. Statistical description of adsorptive surfaces

Abstract A dual description based on a network of “sites” and “bonds” is developed for the characterization of the adsorptive energy of a heterogeneous surface. This description is more complete than previous ones based on only one of those elements. The joint site-bond energy distribution is determined through a correlation function in such a way that the maximum degree of randomness is attained in the network. The degree of randomness is limited by the “Construction Principle”: according to this, the adsorptive energy at a site must be deeper than that of any bond connected to that site. This correlation function contains valuable information about the topology of the energy surface, which plays an important role in adsorption equilibrium and dynamics.

Pore network interactions in ascending processes relative to capillary condensation

It is currently accepted that domain interdependence in adsorption hysteresis (i.e. pore-blocking effects due to hindered liquid–vapour transitions in which the state of any domain depends on those adopted by its neighbours), occurs during the descending (desorption) processes associated with capillary evaporation. In contrast to this behaviour, network effects are thought to be absent during the ascending processes inherent in capillary condensation. However, we have considered the possibility of strong vapour–liquid transitions of an assisted kind taking place during capillary condensation. This situation seems to be the rule, rather than the exception, in a wide variety of porous materials. The interactive effect arises as a consequence of menisci coalescence at the meeting point of capillaries, and it becomes more important as the extent to which the network is filled with capillary condensate increases. When a critical proportion (which depends on the connectivity and geometry of the porous media) of filled elements in the network has been reached, the whole condensation occurs suddenly. This implies that the usual analysis of the ascending boundary curve does not lead to the true pore-size distribution. However, the ascending curve can be predicted from the size distribution and connectivity of the porous network.

Simulation of three-dimensional porous networks

Abstract Simulation of porous networks, with characteristics similar to those of real media, is essential for the study of capillary processes that take place within these substrata. The dual site-bond model (DSBM) provides a theoretical basis from which it is possible to adequately describe and simulate porous networks of diverse structural properties. Following the DSBM principles, heterogeneous 3-D cubic porous networks have been built by a Monte Carlo method. The desired topological properties of these substrata have been introduced by considering: (i) different sizes of the void entities (sites or cavities and bonds or throats); (ii) different connectivities ( C ) of the pore elements with their neighbours, i.e. the number of throats (bonds) that surround and connect a pore cavity (site) with its homologous entities is not constant throughout the network; (iii) geometrical restrictions, in the sense that the sizes of the bonds that meet into a site must be of such values as to avoid any mutual interference. The overlapping ( Ω ) between the site and bond distribution functions, the connectivity ( C ) and the geometrical restrictions ( G ), are the three fundamental factors that promote segregation effects in the substrate. For regular networks (i.e. those of constant C ) subjected to G and high Ω , it is found that big sites: (i) prefer big bonds as neighbours, and (ii) are less affected by geometrical restrictions than small ones. In turn, for irregular networks of varying C subjected to G and large Ω it is found that: (i) the smallest sites are linked to the biggest possible bonds thus acquiring a low connectivity, and (ii) the biggest sites adopt the maximum possible connectivity and allocate small and medium size bonds rather than large ones. All these particularities strongly influence the topology of a porous network and hence the repartition of fluids inside the pores during a capillary process.

Ordered SiO2?(phenolic-formaldehyde resin) in situ nanocomposites

Nanocomposite materials consisting of monodisperse SiO2 particles embedded in a polymerized resin matrix were produced by the adhesion of silica globules on the surface of a chemically modified phenolic-formaldehyde resin (MPFR) substrate that incorporates carboxylic groups in its molecules. Two routes were followed to obtain SiO2 nanoparticles?MPFR materials. The first procedure consisted of the growth of an SiO2 phase concurrently with the presence of MPFR molecules. The second procedure involved the preparation of a monodisperse SiO2 sol that was subsequently mixed with an MPFR solution. The thermal curing of the MPFR resin phase at 80??C brought about thin SiO2?MPFR flakes from samples obtained from procedure 1 whilst monolithic pieces arose from samples from procedure 2. During the curing process, silanol surface groups of the silica globules reacted with carboxylic groups of the MPFR molecules to create a reinforced SiO2?MPFR substance that displayed ester bonds across the interface. Thermal treatments of specimens prepared by procedure?2 were performed at 150, 250, 400, 600 and 800??C to monitor the integrity of the resultant hybrid substrates. To assess the characteristics of SiO2?MPFR materials, some of the main chemical, structural and textural characteristics of several specimens have been determined via FTIR, SEM and N2 adsorption studies.

Adsorption Characteristics of Natural Erionite, Clinoptilolite and Mordenite Zeolites from Mexico

Nitrogen sorption properties inherent to some natural zeolites from Mexico, such as erionites, clinoptilolites and mordenites, are determined and compared with corresponding sorption properties of homologous synthetic or acid modified forms. The mineralogies of natural zeolites are determined by X-ray analysis. N2 low-pressure hysteresis loops are displayed by some substrata while are absent in others; key factors for this phenomenon to occur are the micropore structure and the ion-exchange treatment to which the natural precursors are subjected. Argon sorption at 76 K on selected samples evidence further the strong adsorption and the pore blocking effects at pore necks in the zeolites.

Domain complexions in capillary condensation. Part 1.—The ascending boundary curve

A statement of the general principles of capillary condensation in porous networks and the ascertainment of its particularities for a given structure are difficult, since either independent or dependent vapour–liquid transitions arise at each point of the network and also because porous materials occurring in nature and in industrial processes possess extremely variable morphologies. However, the following stages enable one to achieve these objectives readily: (i) development of general expressions for the probability that the various elements fill with capillary condensate, according to their type (sites or bonds) and size, (ii) classification of all possible morphologies of porous structures into a few unambiguous types and (iii) for each of these types, simplification of the general expressions to obtain particular equations allowing a straightforward derivation of domain complexions and ascending boundary curves. It appears that, even if in one structural type, the less frequently encountered domains behave as though independent, for the other types, corresponding to most materials, an interdependence must be taken into account. As an extreme case of domain interactivity (also pertaining to structures represented fully, once a certain degree of filling is reached, a phenomenon arises in which the whole configuration of capillary condensate becomes unstable, the entire network then being filled.

Structure and texture of self-assembled nanoporous SnO2

Synthesis of nanoporous SnO2 by a surfactant self-assembling sol–gel technique is described in this work. The synthesis was performed at room temperature by employing tin(IV) tetra-tert-amyloxide, Sn (OAmt)4, as the porous SnO2 frame precursor in the presence of the micelles of a cationic surfactant solution of cetyltrimethylammonium bromide (CTAB), which, in turn, acts as the SnO2 particle nanostructure director. The pH of the Sn alkoxide–CTAB mixture was adjusted to a value of 2 with HCl in order to eventually achieve a finely dispersed SnO2 gel structure. The final annealed materials were attained after thermal treatment of the above mentioned SnO2 gel systems between 300 and 500 °C. This annealing procedure burned off different numbers of CTAB micelles and caused some sintering of the substrates depending on the final calcination temperature, thus producing truly mesoporous (cylindrical) SnO2 skeletons of pore sizes 5–9 nm and surface areas between 60 and 100 m2 g−1 as well as some other assorted structural and textural characteristics.