GCEP Final Report Advanced CO 2 / H 2 Separation Materials Incorporating Active Functional Agents

Polymeric membranes for preferential CO2 separation over H2 have been investigated in this project, which can be applicable, for example, in an integrated gasification combined cycle (IGCC) plant with CO2 capture and storage (CCS). Poly(amidoamine) (PAMAM) dendrimers have been employed as an effective agent for preferential CO2 separation in this project. Although PAMAM dendrimer containing membranes express excellent CO2 separation properties under humidified conditions, the separation performance drops sharply with decreasing relative humidity (less than 60 %RH), which indicates CO2 passes through the membrane most likely in the form of bicarbonate ion. In this work, selective CO2 permeation with a different mechanism has been developed by a novel supercritical CO2 (sc-CO2) directing method for use under less humid conditions. Sc-CO2 treatment would work to make a route for CO2 by a molecular imprinting manner. PAMAM dendrimer was first used for a CO2 specific compound. Membrane fabrication was tried by photopolymerization of poly(ethylene glycol) dimentacrylate (PEGDMA) in the presence of PAMAM dendrimer under sc-CO2. However, due to limited solubility of the dendrimer in sc-CO2 resulted in inhomogeneous membrane formulation. Then PAMAM dendrimer was first immobilized in a crosslinked PEGDMA, and then incubated in sc-CO2. In comparison to untreated membrane, the resulting membrane showed higher CO2 permeance (QCO2) and higher separation factor (αCO2/H2). When a polymeric membrane containing amino group is kept under sc-CO2, mechanical properties of the resulting membrane are drastically enhanced. Both of the Young’s modulus and elongation-to-break are increased by sc-CO2 treatment. This can be due to the formation of carbamate linkage, R-NHCOO ··· NH3-R’, between two amino groups and one CO2. In this case, CO2 travel through the membrane in carbamate form. In addition, carbamate acts as a quasi-crosslinking point, and the crosslinking is reversible and rearrangeable, which results in the increase of elongation-to-break. Carbamate formation after sc-CO2 treatment is confirmed by FT-IR. Especially, when the polymeric membrane is fabricated from 2-aminoethyl methacrylate and N-(3aminopropyl) methacrylamide, increase in a peak at around 1,250 cm is found, which can be assigned to the stretching vibration peak of N-C bonding of carbamate ion (νNCOO). In the sc-CO2 condition, water content of the polymeric membrane was negligible, so that CO2 permeated through the membrane in the form of carbamate by hoping between primary amines of PAMAM dendrimers. The detailed structure of the polymeric membrane was determined by laser scanning confocal microscope. Formation of bicontinuous phase structure was found upon macrophase separation, and the average PAMAM domain size was 2-4 microns depended on the dendrimer concentration and PEG length. On the other hand, glass transition temperature (Tg) of the polymer matrix was -60 °C, and the dendrimer flew at ambient condition. Rearrangement of polymer chains and phase structure can be expected, which would result in loosing CO2 penetrating channel generated by sc-CO2 treatment over time. PAMAM dendrimer was then immobilized in cellulose acetate. Tg of the polymeric matrix was above 120 °C, which suppressed rearrangement of polymers. However, the resulting polymeric membrane was inhomogeneous, and after sc-CO2 treatment, it did not show higher CO2 separation performance as expected. This would be also due to flow nature of the dendrimer. Even CO2-implinting by carbamate formation was conducted, dendrimer could move under ambient conditions. As a result, the polymeric membrane did not hold CO2 permeation pathway formed under sc-CO2, and CO2 separation properties did not change much in comparison to the untreated membrane. Primary amine is essential for preferential CO2 permeation, however, intrinsic flow nature of PAMAM dendrimer has been not suitable to fabricate CO2 penetrating pathway by sc-CO2 treatment. Then, poly(ethylene imine) (PEI) was used, which was a branching polymer having a number of primary amines. CO2 separation properties of sc-CO2 treated PEI membrane was depended on the treatment conditions, and the sc-CO2 treated membrane for more than 3 hours exhibited higher separation properties than pristine membrane, which indicated CO2 penetration and formation of carbamate in the membrane required for a couple of hours. Increase in CO2/He selectivity from 17.7 to 24.8 was accounted for the decrease in He permeance from 1.12 × 10 to 5.97 × 10 m(STP)/(m2 s Pa) with sc-CO2 treated time. Tg of PEI was -40 °C and thus rearrangement of polymer chain would take place even after crosslinking in membrane preparation. The high CO2 separation performance might not last for a period of service time. Finally, we chose methacrylamide or methacrylate monomers bearing primary amines for membrane preparation. Especially, the corresponding polymers obtained by photopolymerization show higher Tg above 100 °C, and thermal rearrangement of polymer chains at the operating condition would be suppressed considerably. After scCO2 treatment, the resulting methacrylamide-base membrane displayed higher CO2 separation properties over certain period. In addition, under pressurized condition (0.56 MPa of CO2 partial pressure), CO2/He selectivity increased with decreasing in He permeance. This could result from “molecular-gate” effect that crosslinks by carbamate inhibited sorption of other gas molecules, leading to high CO2 separation performance. Through these investigations, the effect of sc-CO2 directing method was confirmed for preferential CO2 permeation, and this technology would provide insights in designing and developing novel CO2 separation materials. Introduction CO2 capture and storage (CCS) is an important option for mitigating CO2 emission so as to suppress global warming. However, in terms of present-day technology, CCS consumes a large amount of energy and is costly, especially in CO2 capturing. Various CO2 separations have been studied in this research group, including solvent absorption, membrane, membrane-absorption hybrid, and adsorption technologies. Among those separation methods, solvent absorption has gained current acceptance and can be commercialized soon. On the other hand, because membrane separation requires the least energy, it would be the most promising and expected to follow the solvent absorption as a next generation technology. Polymeric membranes for CO2 separation over H2 have been investigated extensively for the use in an IGCC plant in this project. Lately, He has been used as a substituent of H2 as a safety reason. We have developed advanced composite materials having a functional agent by nanoarchitecture controlling technologies in polymeric and inorganic materials. For example, selective extraction of CO2 from CO2 and H2 gas mixture was enabled by introducing amine, which specifically interacted to CO2. The materials consisted of the active functional agents in the nanopores of a porous substrate and polymeric matrix. We control the morphology, surface atoms/molecules and the compositions of the pore/matrix and functional compounds to create the desired molecular interaction. Our research is going to provide insights for development of effective and promising CO2 separation materials. Basic concept of CO2 molecular gate membrane Figure 1 shows the basic outline of the CO2 “molecular gate” membrane. Free volume of the membrane is occupied by CO2, which acts as a gate to block the sorption of other gases by forming carbamate with primary amines. As a consequence, permeation of H2 is significantly suppressed, and high concentrations of CO2 can be obtained in the permeate. The molecular gate membrane realizes CO2 separation over smaller H2 in reverse molecular sieving mechanism. Figure 1 (b) explains details of the preferential CO2 separation with amine compounds, such as PAMAM dendrimers. A carbamate ion pair is formed with one CO2 molecule and two amine moieties of the dendrimer. The resulting carbamate ion pair works as a quasi cross-linkage that inhibits H2 permeation through the membrane. On the other hand, because formation of carbamate ion pairs are in equilibrium with free CO2 and amine moieties, CO2 in carbamate form can diffuse into the membrane from feeding side to permeate side to form another carbamate ion pair with free amine moieties next to the carbamate. As a result, CO2 molecules can permeate the membrane by hoping mechanism. (a) Smaller distance of CO2 affinity molecule CO2 H2 etc. (b) Larger distance of CO2 affinity molecule CO2 H2 etc. Lower permeability Lower selectivity CO2 affinity molecule Figure 1: Concept of the CO2 molecular gate membrane. The CO2 molecular gate requires precise alignment of amine moieties. In Figure 1, the distance between two amine moieties should be controlled to express the CO2 molecular gate function. If the distance is closer than an appropriate distance, the hydrogen bonding formation will be suppressed. As a result, the membrane will not have sufficient CO2 permeability (Figure 2(a)). On the other hand, if the distance is too far, a quasi cross-linkage to prevent H2 permeation will not be achieved. Precise control of the molecular alignments will be thus required as represented in Figure 2(b).