(144a) Pei/MCM-48 Composite Membranes for Carbon Dioxide Separation

Capturing and sequestration of CO2 emissions from flue gas mixtures are receiving significant attention due to concerns over global warming and clean fuel issues. Among several technologies for separating CO2 from gas mixtures, membrane separation is of particular interest due to potentially enhanced transport and separation properties, continuous high production rate compared to batch processes and reduced operating costs. Ordered porous membranes with nano-sized pores arranged in a periodic fashion have received considerable attention owing to their favorable high surface area and uniform pore structural properties. MCM-41 and MCM-48 are examples of this class of ordered mesoporous materials that has been successfully fabricated in the form of membranes. Gas transport in these membranes is mainly governed by Knudsen-type diffusion which exhibit poor CO2 selectivity over other gases including N2 and O2. Functionalization of internal pores with proper groups possessing high affinity toward CO2 could potentially enhance CO2 separation properties of these novel membranes.

In this work, mesoporous MCM-48 membranes with cubic pore structure were prepared by solution growth method and characterized by XRD, SEM/EDS and unsteady state gas permeation. The membranes were fabricated on symmetric á-alumina supports under hydrothermal conditions using cetyltrimethylamonium bromide (CTAB) as a surfactant. The surfactant was removed by calcination and Soxhlet extraction at 100 0C using ethanol/HCl mixture. The X-ray diffraction results showed that these membranes possessed a cubic MCM-48 structure. N2 adsorption studies indicated high pore volumes and pore size distribution in mesoporous regime with an average pore diameter of ~ 2.6 nm. Single gas permeation experiments for N2, CO2 on these membranes showed that best quality membranes were synthesized when the surfactant was removed by Soxhlet extraction. For these membranes the N2 and CO2 permeation was found to be governed by Knudsen diffusion. The permeance was independent of feed pressure, indicating an absence of viscous flow contribution to gas transport.

The internal pores of MCM-48 were functionalized with basic polyethyleneimine (PEI) which has high affinity towards acidic CO2. The PEI attachment to MCM-48 powders was characterized by IR spectroscopy, N2 adsorption-desorption isotherms, CHN/Si elemental analysis and TGA. The membrane quality and gas transport properties were investigated after PEI attachment by CO2 and N2 single gas permeation. The TGA, CHN/Si analysis and IR spectra of MCM-48 powders confirmed that polyethyleneimine was successfully attached to the pore surface. Gas sorption studies on amine modified MCM-48 powders displayed high equilibrium uptake capacity of CO2 (44 mg CO2/g polyethyleneimine-modified MCM48) as compared to N2, which was almost negligible. This suggested that PEI-modified MCM-48 is expected to be a superior adsorbent material for CO2/N2 separation. The CO2/N2 binary gas permeation behavior of these composite membranes was investigated at 20-100 0C, 1.5-5 atm and 0-20% water vapor, which indicated that these novel composite membranes are highly promising for CO2 separation from low purity sources. These membranes exhibited N2/CO2 selectivity (~ 30) in the presence of water vapor at room temperature, while lower selectivities were observed in the absence of water vapor and at higher temperatures.