277565 Preferential CO2 Separation Over H2 with Poly(amidoamine) Dendrimer Immobilized in a Poly(ethylene glycol) Network
Preferential CO2 Separation over H2 with Poly(amidoamine) Dendrimer Immobilized in A Poly(ethylene glycol) Network
Ikuo Taniguchi,* Teruhiko Kai, Shuhong Duan, Shingo Kazama,
Chemical Research Group, Research Institute of Innovative Technology for the Earth,
9-2 Kidzugawadai, Kizugawa, Kyoto 619-0292, Japan
Tel: +81-774-75-2305, Fax: +81-774-75-2318
Dendrimers are diverse families of highly branched macromolecules with well-defined molecular architecture and have been developed with various chemistries. The dendritic molecules have gained particular interest in the area of nanotechnology, macromolecular science and engineering.[2,3] Among a wide variety of dendrimers, poly(amidoamine) (PAMAM) dendrimers are the first successfully developed and one of the most investigated dendritic structures. Potential applications of the dendrimers have been investigated, including biomedical and pharmaceutical uses. Besides the versatile potentials, another fascinating feature of PAMAM dendrimer was found by Sirker et al. which was that the dendrimer exhibited preferential permeation of CO2 over a gas mixture with a liquid immobilized membrane system. The sorption of CO2 was facilitated by the specific interaction between CO2 and the primary amines of the branching end of PAMAM dendrimer. PAMAM dendrimer would be an essential agent in the selective CO2 separation.
In this research group, further extension of CO2 separation with PAMAM dendrimer has been investigated. For example, PAMAM dendrimers with lower generations flow at ambient condition, and thus the liquid dendrimer should be stably immobilized in a matrix especially for use under a pressurized condition. Recently, an effective incorporation of PAMAM dendrimer in a cross-linked poly(vinyl alcohol) (PVA) was developed. PVA network was formed with a cross-linker in the presence of PAMAM dendrimer in aqueous media, so that the dendrimer content in the membrane was readily controlled. The polymeric membrane showed a preferential CO2 permeation over even smaller H2 with preferable mechanical properties, and the CO2/H2 selectivity exceeded 30 at 0.56 MPa of CO2 partial pressure (total pressure: 0.70 MPa). The dendrimer containing polymeric membranes hold potential for CO2 separation, for example in an integrated gasification combined cycle (IGCC) plant with CO2 capture and storage (CCS). In the coal-fired plant, the syngas consisted of H2 (60 vol%) and CO2 (40 vol%) after the water-gas shift reaction with 2.5 MPa in total pressure. Selective separation of CO2 has been examined in the IGCC plant, and a solvent absorption with Selexol has gained current acceptance. Based on our simulation, a CO2 separation with a membrane could be competitive or even more preferable and cost effective in comparison to the solvent absorption method. However, in the membrane formulation of PAMAM dendrimer in a cross-linked PVA network, the time-consuming drying process required further improvement and optimization in terms of large-scale production and processability.
As for the selective CO2 separation over H2, an alternate immobilization of PAMAM dendrimer has been exploited, which is photocross-linking of poly(ethylene glycol) dimethacrylate (PEGDMA) in the presence of the dendrimer.[9,10] The cross-linking reaction takes place very rapidly under mild conditions, and the polymeric membrane is obtained in a couple of minutes. Thus unfavorable side reactions would be suppressed, such as Michael addition reaction between methacrylate and primary amine. The obtained polymeric membrane demonstrated an excellent CO2 selectivity over H2. The CO2 separation properties are readily tunable by altering the dendrimer concentration and generation.
Fabrication of polymeric membranes
PAMAM dendrimer (2.0 g, 3.9 mmol), and PEGDMA (2.0 g, 2.7 mmol) were dissolved in ethanol (2.0 g) containing HCPK (9.1 mg, 44 mmol) under shading. The reaction mixture (2.5 g) was poured into a Teflon dish (ID: 60 mm) and exposed to the UV light for 90 s. The thickness of the obtained membrane was ca. 500 mm after dried.
CO2 separation experiment
CO2 separation properties over H2 of the polymeric membrane were measured at 298 K under atmospheric pressure. A CO2/H2 gas with 5 vol% of CO2 was humidified at 298 K and fed at a flow rate of 100 mL/min. Helium was supplied to the permeate side of the cell as a sweep gas at a flow rate of 10 mL/min. Compositions of the permeate and the feed sides were analyzed on a gas chromatograph.
3. Results and Discussion
For selective CO2 separation, facile fabrication of polymeric membranes, in which PAMAM dendrimer is stably entrapped, has been investigated in this research group. Photopolymerization of PEGDMA in the presence of the dendrimer is one of the successful examples, which is motivated by the several considerations: (1) the reaction takes place rapidly and efficiently under mild conditions, minimizing unfavorable side reactions and providing macroscopically homogenous membranes. (2) PAMAM dendrimer was immobilized with a desired mixture ratio without remarkable leakage of the dendrimer. Thus, (3) the CO2 separation properties can be readily controlled. (4) The thickness of the membrane is also tunable, which is associated with permeance of gases. (5) The compatible solvent can be eliminated after membrane formulation.
With PAMAM dendrimer immobilized PEG network, following results are obtained.
- The CO2 separation properties are increased with increase in PAMAM dendrimer content in the polymeri membrane.
- 0th generation PAMAM dendrimer exhibits the highest CO2 separation properties when the dendrimer content is kept to 50 wt%.
- The polymeric membrane is opaque, which indicates macrophase separation between the dendrimer and cross-linked PEG.
- The phase separation takes place in a couple of microns scale and the phase-separated structure is related to CO2 separation properties.
- PEG length, corresponded to the distance between cross-linking points, determines the CO2 separation properties.
- Humidity is also a key to characterize the CO2 separation properties, and higher separation properties results from operations under higher humidity.
For example, the separation factor increases with PAMAM dendrimer concentration and reaches about 500 when the dendrimer content is 50 wt% at 298 K and 5 kPa of CO2 partial pressure under 80 % of relative humidity. The resulting polymeric mixture would be promising as a novel CO2 separation material.
In this presentation, structural analysis of the polymeric membrane and interplay with CO2 separation performance will be discussed.
This work was financially supported by the Japanese Ministry of Economy, Trade and Industry. The authors are also thankful for the assistance of Ms. Hiromi Urai and Rie Sugimoto throughout the membrane fabrication processes and gas chromatography measurements.
(1) Newkome, G. R.; Moorefield, C. N.; V; Wiley-VCH; Weinheim, 1996.
(2) Rosi, N. L.; Mirkin, C. A. Chem. Rev. 2005, 105, 1547-1562.
(3) Lee, C. C.; MacKay, J. A.; Frchet, J. M. J.; Szoka, F. C. Nature Biotech. 2005, 23, 1517-1526.
(4) Tomalia, D. A.; Baker, H.; Dewald, J.; Hall, M.; Kallos, G.; Martin, S.; Roeck, J.; Ryder, J.; Smith, P. Polym. J. 1985, 17, 117-132.
(5) Kowali, A. S.; Chen H.; Sirkar, K. K. J. Am. Chem. Soc. 2000, 122, 7594-7595.
(6) Kouketsu, T.; Duan, S.; Kai, T.; Kazama S.; Yamada, K. J. Membr. Sci. 2007, 287, 51-59.
(7) Duan, S.; Kai, T.; Taniguchi, I; Kazama S., Y. J. Membr. Sci. submitted to J. Membr. Sci.
(8) Nagumo, R.; Kazama, S.; Fujioka, Y. Energy Procedia, 2009, 1, 4089-4093.
(9) Taniguchi, I.; Duan, S.; Kazama, S.; Fujioka, Y. J. Membr. Sci. 2008, 322, 227-280.
(10) Taniguchi, I.; Kazama, S.; Jinnai, H. J. Polym. Sci. Part B, Polym. Phys. in press.