A Multiple-Process Equilibrium Analysis of Silica Gel and HZSM-5

Abstract

Adsorption isotherms are reported for HZSM-5 and silica gel using a series of gas adsorptives at several temperatures above their critical temperature. The data are analyzed with a new multiple equilibria adsorption model producing equilibrium constants (K i ), capacities (n i ), and enthalpies (−ΔH i ) for each of the processes. Unlike the amorphous adsorbents studied earlier, which contain a distribution of pore sizes, zeolitic materials have uniform pore dimensions. This uniformity provides a test of our interpretation of the number of processes required in the isotherm fits of the multiple equilibrium analysis, MEA. As expected from the two pores in the structure of HZSM-5, most adsorbates require two processes (K 1,ads and K 2,ads) to fit the adsorption isotherms. HZSM-5 is compared to amorphous carbonaceous adsorbents, revealing fundamental differences in their behavior. Small cylindrical channels in HZSM-5 lead to an unfavorable entropic contribution from restrictions imposed by adsorptive packing and interactions with the channel walls. The pores of the carbons studied (Drago, R. S.; Kassel, W. S.; Burns, D. S.; McGilvray, J. M.; Lafrenz, T. J.; Showalter, S. K. J. Phys. Chem. B 1997, 101, 7548−7555) are slit-shaped, leading to less restriction and larger equilibrium constants. For several adsorbates, the greater enthalpic interactions in the small HZSM-5 pores are accompanied by lower equilibrium constants than in the larger pores because of unfavorable entropic contributions. Finally, the larger total micropore volumes of the carbons studied for these adsorptives (Drago, R. S.; Kassel, W. S.; Burns, D. S.; McGilvray, J. M.; Lafrenz, T. J.; Showalter, S. K. J. Phys. Chem. B 1997, 101, 7548−7555) result in increased capacity compared to HZSM-5. The process capacities from MEA (mol g-1) are converted to pore volumes using the molar volume. Surface areas are calculated from molecular areas of the adsorbates. Pore volumes and surface areas calculated from the process capacities are compared to those from conventional N2 porosimetry and are shown to provide a more detailed and more accurate assessment of areas and volumes. These results show that MEA has the potential of becoming a standard characterization method for microporous solids that will lead to an increased understanding of their behavior in gas adsorption and catalysis.

Diffusion in tight confinement: A lattice-gas cellular automaton approach. I. Structural equilibrium properties

“…Since each equilibrium can be represented by a single Langmuir isotherm, the adsorption isotherm for n simultaneous equilibria is well reproduced by an n-site Langmuir isotherm. 10 This result has been confirmed by molecular dynamics and grandcanonical Monte Carlo simulations, 12 and has also been used in KMC models 13 to take into account the loading dependence of the self-diffusion coefficient in the study of diffusion in zeolites. In our model we differentiate the adsorption energies of the sites in order to mimic the real situation in which different adsorption locations in the cavity influence the ways a particle have access to a window ͑to reach an exit site, in our model͒ and then migrate into the adjacent cavity.…”

“…The choice to differentiate two types of site into each cell arises from the experimental evidence of two different types of adsorption locations in various zeolites, 11 or more generally, of n types of sites differing in their ability to bind a guest species. 10 The Langmuir model 22 is often used to describe equilibrium adsorption in zeolites. The particles do not interact except by excluding each other from the adsorption sites and one assumes that all sites are capable of holding at most one molecule each.…”

“…In a cell, the exit sites differ from the inner sites ͑which are not connected to neighboring cells͒ in binding energy. A differentiation among several types of adsorption sites, mutually differing in adsorption energy, has been shown to be a feature of most nanoporous materials, [10][11][12][13] giving rise to a strong dependence of both equilibrium and transport properties on loading and temperature. 7,[14][15][16] For example, experimental evidences 17 have shown that in NaY zeolite benzene molecules appear to occupy two types of sites, cationic sites on the walls of the supercage and less favorable window sites in apertures between adjacent supercages, a behavior consistent with results from molecular dynamics simulations.…”

Exceptional Affinity of Nanostructured Organic–Inorganic Hybrid Materials towards Dioxygen: Confinement Effect of Copper Complexes

“…The O 2 adsorption isotherms were analyzed by using a multiple-site adsorption process involving a two Langmuirtype adsorption model [71][72][73] as has been described for cobalt-corrole xerogels, [40] and [CoA C H T U N G T R E N N U N G (cyclam)] 2 + grafted onto silica. [52] The calculated isotherms are given in Figure 1 and Table 1 along with experimental isotherms for CO and N 2 adsorption.…”

“…The first Langmuir-type isotherms reflect the chemisorption of dioxygen with a high energetic interaction, while the second illustrates a very low energetic interaction through a nonselective adsorption process, resulting from dissolution and diffusion of the gas into the solid material. [71][72][73][74] Furthermore, the use of only two Langmuir adsorption components to fit the experimental isotherms shows that all the sites are accessible. [40,74] Conversely, since N 2 is adsorbed onto the solid through a non-selective physisorption process and no coordination to the metal ions occurs, a single Langmuir adsorption model was used for this gas.…”