Development of carbon molecular-sieve membranes with tunable properties: modification of the pore size and surface affinity

UnrestrictedCarbon molecular-sieve (CMS) membranes have been studied in the past few years as an alternative to both inorganic and polymeric membranes. They are known to have considerable resistance to high temperatures and pressures for gas separation applications, such as those involving mixtures that contain H2, CO2, and CH4.; In the previous studies as well the preliminary studies in this thesis, the CMS membranes have been prepared by the carbonization of a polyetherimide precursor placed on the top of a tubular substrate. The substrate consists of a ceramic support with a thin layer of γ-alumina on the top. The membranes were prepared by dip-coating the tubular substrates in polyetherimide solution with the appropriate concentration, followed by controlled carbonization in an inert atmosphere. The single gas transport properties and separation characteristics of the resulting CMS membranes were studied, using various gases. In addition, the thermal stability of the membranes was studied at 120°C. A CO2/CH4 separation factor of 25 was obtained at room temperature. The permeance of CO2 decreased and that of CH4 increased as the temperature was raised. As a result, the corresponding separation factor also diminished.; Developing the ability to tune the membrane’s structure in order to improve its properties is desirable. So far, tuning of the CMS membrane structure has primarily been attempted through the selection of the polymeric precursors, and of the appropriate pyrolysis conditions. However, tuning based on such factors does not provide a generic solution for the membrane development for a wide range of industrial applications. The focus of the present work is to develop an alternative technique based on a post-treatment step, in order to adjust the pore size distribution of the CMS membrane, which is accomplished by activating the carbon surface using steam and depositing carbon using methane. In this study, we investigated the influence of the various activation parameters, including the temperature and duration of the treatment on the properties of the resulting CMS membranes. Steam activation is a known technique used to prepare activated carbons with various pore structures. Application of this technique to the CMS membrane resulted in the gases permeances increasing significantly after 12 h steam activation at 600 °C, accompanied with a small enhancement of the H2/CH4 separation selectivity. The resulting CMS membrane was also evaluated using BET, a structural characterization technique, in order to relate the transport properties to its structure. In addition, methane activation at 700 °C decreases the gases permeances of the resulting CMS membranes. It was shown that, relative to hydrogen, the methane treatment of the membrane impacts the CO2 permeance, resulting in a H2/CO2 separation of 10. However, the molecular sieving behavior was shown to diminish after the methane treatment.; We also studied the modification of the surface affinity of such membranes through the incorporation of metal and solid oxide nanoparticles within the structure. Three metal precursors were chosen for the preparation of metal-impregnated CMS membranes, namely, nickel-formate and nickel-acetylacetonate (as Ni precursors), and palladium acetylacetonate (as a Pd precursor). The choice of the metals was based on their strong affinity towards H2, while the choice of the metal precursors was due to the fact that they are known to self-decompose to nickel/palladium nanoparticles at elevated temperatures. Several techniques were utilized to incorporate the metal particles within the CMS membranes. The transport and separation properties of the resulting CMS membranes were characterized in terms of their permeability and selectivity using single gases, such as H2, CO2, and CH4. Compared with the base-CMS membrane, the Ni-CMS membrane, prepared by a depositing layer (using a 6% PEI solution), and 2 layers using 0.1%Ni/2%PEI solution, had higher gases permeances without showing inferior separation characteristics