(788f) 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

1. Introduction

Dendrimers are diverse
families of highly branched macromolecules with well-defined molecular
architecture and have been developed with various chemistries.^[1]
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.^[4]
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.^[5] 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.^[6]
Recently, an effective incorporation of PAMAM dendrimer in a cross-linked
poly(vinyl alcohol) (PVA) was developed.^[7] 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.^[8] 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.^[9] The CO₂
separation properties are readily tunable by altering the dendrimer
concentration and generation.

2. Experimental

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.

1. The CO₂ separation properties are increased with increase in PAMAM dendrimer content in the polymeri membrane.
2. 0^th generation PAMAM dendrimer exhibits the highest CO₂ separation properties when the dendrimer content is kept to 50 wt%.
3. The polymeric membrane is opaque, which indicates macrophase separation between the dendrimer and cross-linked PEG.
4. The phase separation takes place in a couple of microns scale and the phase-separated structure is related to CO₂ separation properties.
5. PEG length, corresponded to the distance between cross-linking points, determines the CO₂ separation properties.
6. 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.

4. Acknowledgements

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.

5. References

(1) Newkome,
G. R.; Moorefield, C. N.; V?gtle, F. Dendritic
Molecules: Concepts, Syntheses, Perspectives; Wiley-VCH; Weinheim, 1996.

(2) Rosi, N. L.; Mirkin, C. A. Chem. Rev. 2005, 105, 1547-1562.

(3) Lee, C. C.;
MacKay, J. A.; Fr?chet, 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.