TY: THES T1 - Synthesis and characterization of small pore membranes and their application to the separation of hydrogen containing mixtures A1 - Cardoso, Simão Pedro Pereira N2 - Among various methods for the separation of hydrogen containing mixtures, membrane technology has received special attention due to its simplicity of operation, moderate energy consumption, cost effectiveness, it is environmentally friendly and can be easily coupled with other separation methods. In particular, inorganic membranes exhibit relevant advantages such as high thermal, chemical and mechanical stabilities, minor plasticization and better control of pore size distribution. They can be divided into five main families: metallic and ceramic proton conducting membranes (dense phases), and silica, zeolite and carbon molecular sieve membranes (porous materials). With more than 200 framework types, zeolites possess uniform molecular pore dimensions and unique properties for catalytic, ion exchange, adsorption and membrane applications. This thesis is essentially devoted to zeolite (titanosilicate) membranes for hydrogen and light gases separation. Titanosilicates offer important benefits over the remaining zeolites: they are usually synthesized without organic templates, can be prepared under moderate pH conditions, exhibit novel possibilities of isomorphous framework substitution, and generally offer strong alkalinity. The main objectives of this dissertation were: (i) synthesis and characterization of titanosilicate membranes. The materials structure and morphology were examined by X-ray diffraction (XRD) and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDS); the dynamic characterization of membranes was accomplished by permeation assays with pure gases; (ii) deep analysis of experimental results, with the objective to disclose the mass transport mechanisms that prevail in the membrane; (iii) accurate modeling of permeation data using theoretically sound approaches like the generalized Maxwell-Stefan (MS) formalism; (iv) development of new MS thermodynamic factors to represent the surface diffusion of pure and multicomponent gases through microporous membranes. Here, several important (though less studied) isotherms were tested, the influence of the solid loading upon surface diffusivity was deeply evaluated, and the predictive ability of the whole model was validated. The hydrothermal synthesis and characterization of AM-2 and AM-3 powder and supported membranes have been performed. Altogether, ca. thirty and more than one hundred syntheses of AM-2 and AM-3 powder, respectively, have been accomplished, and eight AM-2 and twenty AM-3 membranes have been prepared on tubular -alumina and stainless-steel supports by seeding and secondary growth. All powders were characterized by XRD and/or SEM, and three AM-2 and seven AM-3 membranes were dynamically tested. The permeation of pure gases at programmed temperature and fixed transmembrane pressure drop ( P) was carried out in order to evaluate the membranes quality and identify the governing transport mechanisms (e.g. viscous flow, Knudsen regime, surface diffusion, activated gaseous diffusion). Additionally, more than one hundred permeance measurements were conducted at various temperatures and pressures using different gases (H2, He, N2, O2 and CO2). The best AM-3 membrane was studied in detail at P=0.5, 1.0 and 1.5 bar, at fixed and programmed temperatures between 304 and 394 K, using H2, He, N2, O2 and CO2. The results provided selectivities towards hydrogen up to 3.5, and disclosed in all cases an activated behavior typical of surface diffusion, a small contribution of viscous flow due to macro-defects, and a minor contribution of Knudsen mechanism due to meso-defects. The gas permeation was accurately modeled using Maxwell-Stefan theory, achieving an average deviation of only 3.42 % for all molecules. The rigorous MS thermodynamic factors of various relevant isotherms ? Toth, Dubinin-Astakhov (DA) and Dubinin-Radushkevich (DR) ? were derived in this work for the first time, with the objective to model the permeation of pure and mixed gases through zeolite membranes. The DA and DR isotherms received special attention, taking into account they are often adopted to represent the adsorption equilibrium of most industrial adsorbents that possess a welldeveloped porous structure, especially of carbonaceous origin. The MS factors for DA and DR isotherms were validated using equilibrium and flux data for methane/silicalite-1, ethane/silicalite-1 and (methane,ethane)/ silicalite-1 systems according to the following procedure: (i) the unary isotherms were fitted to the corresponding data, while their binary expressions were predicted from the previous ones; (ii) the influence of surface loading upon methane and ethane diffusivities was carefully assessed; (iii) the MS diffusion parameters of each gas were obtained from unary permeation data; (iv) the MS counter-sorption diffusivities were then predicted using the Vignes correlation; (v) the separation of methane/ethane mixtures using the silicalite-1 membrane was totally predicted using the new MS thermodynamic factors combined with the binary isotherms. The results demonstrated the validity of this approach, particularly if DA isotherm is selected. Besides the main focus of this PhD on membranes, ion exchange experiments and modeling were also included. The removal of ionic contaminants (Cd2+ and Hg2+) from aqueous solutions was studied using synthesized stannosilicate AV- 6 and titanosilicate ETS-4. In the case of the Cd2+/K+/AV-6 system, the kinetic removal was modeled with Maxwell-Stefan (MS) and Nernst-Plank (NP) equations, while equilibrium was modeled with mass action law and activity coefficients in both solution (Debye-Hu ckel and Pitzer models) and exchanger (Wilson model) phases. The experimental data proved the ion exchange ability of AV-6 to sorb cadmium(II), and excellent results were achieved for the equilibrium isotherm (deviations around 6.6 %) and kinetic modeling (MS, 3.74 %; NP, 3.71 %). It was shown that the solid phase is clearly non-ideal. The investigation of the ternary system (Cd2+,Hg2+)/Na+/ETS-4 confirmed the large capacity of ETS-4 (5.44 meq g-1) and its selectivity towards both toxic metals, particularly Cd2+. However, the trends of the ion exchange kinetics were very distinct, because the internal diffusivity of cadmium(II) is higher than that of mercury(II). These results are in accordance with the effective ionic radius of both species, namely, 102 pm for Hg2+ and only 95 for Cd2+. UR - https://ria.ua.pt/handle/10773/21173 Y1 - 2016 PB - Universidade de Aveiro