The multiple equilibrium analysis model and its application to the study of adsorption.

摘要

Porous solids are used extensively in many adsorbent and separation applications, as well as serving as catalyst supports for many heterogeneously catalyzed reactions. Of the many solids available, most specialty adsorbents are carbon or silica based. Many of the carbonaceous adsorbents are derived from coal and wood chars, as well as being synthetically produced from macrorecticular resins. The siliceous based adsorbents include silica gel and zeolites. Silica, like carbon, is amorphous and provides a distribution of pore sizes capable of adsorbing probe molecules of varying size and shape. Zeolites are crystalline solids, some of which occur naturally while others are synthetically produced. Crystallinity in a solid defines the limitation and application of an adsorbent to effectively adsorb certain molecules while excluding others. Understanding those properties of a solid which favor the adsorption of probe molecules is vital in selecting an adsorbent for the appropriate application.; Currently there are several adsorption models in use, and many under development, which characterize the physical properties of solid adsorbents, namely surface area and pore size. One of the main challenges facing current adsorption models is the ability to predict adsorption isotherms for new adsorbates based on the known properties of the adsorbate and the adsorbent. Another challenge is the ability of these adsorption models to be applicable to all adsorptives and all adsorbents.; Two commercial carbonaceous adsorbents and two silica based adsorbents are examined for their ability to favorably adsorb selected probe molecules. Examination of the adsorption data by the Multiple Equilibria Analysis (MEA) model has provided great detail about the adsorption process. In an MEA interpretation, multiple adsorption processes are found to contribute to the total adsorption isotherm. These multiple adsorption processes exhibit different adsorption affinities which correspond to the adsorption of probe molecules into pores of different dimensions. In conjunction with equilibrium affinity, capacities for adsorption of a probe molecule can be transformed into accessible surface areas and corresponding pore volumes. Enthalpies of adsorption for the individual processes are calculated from the temperature dependent equilibrium constants. Correlation of the enthalpies and equilibrium constants for the selected probe molecules with the van der Waals (a) parameter is linear for those adsorbates undergoing nonspecific, dispersion interactions. This affords the opportunity of the MEA model to predict enthalpies and equilibrium constants, and in turn the adsorption isotherm, for probes not yet examined.; Extension of the MEA model to liquid-solid adsorption equilibria is described for selected adsorbates with a carbonaceous adsorbent and silica gel. Multiple equilibrium constants are found indicating the presence of more than one adsorption process. For the carbonaceous adsorbent, nonspecific interactions dominate, whereas silica undergoes specific donor-acceptor interactions.; Preliminary results for the synthesis of a silica based acid catalyst and its potential application for the production of methyl tertiary butyl ether (MTBE) are described.