Gas adsorption and separation properties of porous material

Abstract

The development of new porous materials for use in applications such as gas storage and separation processes, catalysis, catalysts supports and the removal of environmentally unfriendly species has increased rapidly over the past decade. Research into the development of these new materials has been dominated by metal organic frameworks, covalent organic frameworks, nanoporous polymers and, most recently, porous organic cage molecules. This thesis describes adsorption studies of a metal organic framework, Zn (TBAPy) and a porous tetrahedral organic cage molecule of ~ 1 nm diameter formed by the condensation reaction of 1,3,5- triformylbenzene with 1,2-ethylenediamine. The development of metal organic frameworks has traditionally involved the formation of rigid network structures, analogous to that of zeolites. More recently the focus has shifted to those of dynamic, flexible framework materials, and the response of these materials to adsorption of gases and vapours. The metal organic framework Zn (TBAPy) is based on a zinc metal centre functionalised with benzoate fragments. The initial two-dimensional structure undergoes rearrangement of the paddlewheel units to form a 3D framework, Zn (TBAPy)' upon desolvation. The ability of this 3D network to separate p-xylene and m-xylene was investigated. It was found that these isomers produced different effects on the framework, with p-xylene producing a typical Type I isotherm, whereas m-xylene induced a structural change within the material, with a much slower rate of m-xylene adsorption at higher pressures. This could potentially lead to the equilibrium separation of these two isomers by the metal organic framework Zn (TBAPy)'. The 1 nm diameter tetrahedral cage molecules formed by the condensation reaction of 1,3,5-triformylbenzene with 1,2-ethylenediamine can exist in a number of stable polymorphs, Cage 1α, Cage 1β and Cage 1γ. These polymorphs can be interconverted by exposure to certain organic vapours/solvents. The conversion of Cage 1β to Cage 1α by adsorption of probe molecules ethyl acetate, 2-butanone, diethyl ether, pentane and methanol was studied. Adsorption of ethyl acetate, 2- butanone and diethyl ether produced unusual adsorption isotherms, which included desorption of adsorbed vapour with increasing pressure during the adsorption isotherms. This desorption is attributed to the structural change from Cage 1β to Cage 1α. The unusual desorption step is not observed for methanol or pentane adsorption. The adsorption of methyl acetate was studied over a wide temperature range in order to assess the thermodynamic and kinetic characteristics of the unusual desorption step. The adsorption of dichloromethane showed the reverse transformation of Cage 1α to Cage 1β, showing that the inter conversion produces stable polymorphs. The kinetics of the structural transformation followed an Avrami model and the mechanism is an activated process. Cage 1α has voids between the cages, which are connected by very narrow constrictions that allow the kinetic molecular sieving of oxygen, carbon dioxide and nitrogen. It was found that oxygen adsorbs approximately ten times faster than nitrogen on Cage 1α, with selectivity and rate constants similar to those observed for carbon molecular sieves. The thermodynamics and kinetic results are discussed in terms of structural characteristics and diffusion into molecular cage materials. The kinetic molecular sieving is not present in the polymorph Cage 1β, which has wider pores.