Capture of Carbon Dioxide from Air and Flue Gas in the Alkylamine-Appended Metal−Organic Framework mmen-Mg2(dobpdc)

Journal of the American Chemical Society, 2012, 134, 7056−7065
Capture of Carbon Dioxide from Air and Flue Gas in the Alkylamine-Appended Metal−Organic Framework mmen-Mg2(dobpdc)
Thomas M. McDonald,† Woo Ram Lee,§ Jarad A. Mason,† Brian M. Wiers,† Chang Seop Hong,*,§
and Jeffrey R. Long*,†
†Department of Chemistry, University of California, Berkeley, California 94720, United States
§Department of Chemistry, Research Institute for Natural Sciences, Korea University, Seoul 136-713, Republic of Korea
ABSTRACT:
Two new metal−organic frameworks, M2(dobpdc) (M = Zn (1), Mg (2); dobpdc4− = 4,4′-dioxido-3,3′-biphenyldicarboxylate), adopting an expanded MOF-74 structure type, were synthesized via solvothermal and microwave methods. Coordinatively unsaturated Mg2+ cations lining the 18.4-Å-diameter channels of 2 were functionalized with N,N′-dimethylethylenediamine (mmen) to afford Mg2(dobpdc)- (mmen)1.6(H2O)0.4 (mmen-Mg2(dobpdc)). This compound displays an exceptional capacity for CO2 adsorption at low pressures, taking up 2.0 mmol/g (8.1 wt %) at 0.39 mbar and 25 °C, conditions relevant to removal of CO2 from air, and 3.14 mmol/g (12.1 wt %) at 0.15 bar and 40 °C, conditions relevant to CO2 capture from flue gas. Dynamic gas adsorption/desorption cycling experiments demonstrate that mmen-Mg2(dobpdc) can be regenerated upon repeated exposures to simulated air and flue gas mixtures, with cycling capacities of 1.05 mmol/g (4.4 wt %) after 1 h of exposure to flowing 390 ppm CO2 in simulated air at 25 °C and 2.52 mmol/g (9.9 wt %) after 15 min of exposure to flowing 15% CO2 in N2 at 40 °C. The purity of the CO2 removed from dry air and flue gas in these processes was estimated to be 96% and 98%, respectively. As a flue gas adsorbent, the regeneration energy was estimated through differential scanning calorimetry experiments to be 2.34 MJ/kg CO2 adsorbed. Overall, the performance characteristics of mmen-Mg2(dobpdc) indicate it to be an exceptional new adsorbent for CO2 capture, comparing favorably with both amine-grafted silicas and aqueous amine solutions.
INTRODUCTION
The concentration of CO2 in the Earth’s atmosphere is presently 390 ppm,1 an increase of approximately 110 ppm since the start of the Industrial Revolution.2 The combustion of fossil fuels is largely responsible for this increase,3 yet fossil fuels will continue to be heavily utilized for energy production during the 21st century. Currently, there is significant interest in the development and implementation of technologies that slow CO2 emissions and thus forestall the most severe consequences of global warming. For limiting future CO2 emissions from large, stationary sources like coal-fired power plants, carbon capture and sequestration (CCS) has been proposed.4 The CCS process involves the selective removal of CO2 from gas mixtures, the compression of pure CO2 to a supercritical fluid, transportation to an injection site, and finally permanent subterranean or submarine storage.5 For the retrofit of existing power plants, post-combustion CO2 capture is a likely configuration. In this design, fuel is burned in air and CO2 is removed from the effluent. For coal-fired power plants, the largest flue gas components by volume are N2 (70−75%), CO2 (15−16%), H2O (5−7%), and O2 (3−4%), with total pressures near 1 bar and temperatures between 40 and 60 °C.
Aqueous amine solutions are currently the most viable absorbents for carbon capture under the aforementioned conditions, and they are presently used for the removal of CO2 from industrial commodities like natural gas.7 While a variety of advanced amines are available, 30% monoethanolamine (MEA) in water is the benchmark solvent against which competing technologies are generally compared. The low solvent cost and proven effectiveness make MEA an attractive absorbent for many applications. However, if MEA were to be utilized for CCS, electricity prices are projected to increase by 86%.8 The U.S. Department of Energy has targeted a maximum 35% increase for the cost of electricity produced from a coal power plant that captures 90% of the CO2 it generates. The diversion of steam from the electricity generation cycle to the solvent regeneration cycle sharply reduces the net electricity output of the plant, drastically increasing electricity costs. Previous work has demonstrated that plant efficiency is highly dependent on the solvent regeneration energy.
Presently, there is significant interest in the development of solid adsorbents that selectively adsorb CO2 at partial pressures applicable to CCS.10 Solid adsorbents are promising candidates because the significantly smaller heat capacities of solids may reduce the sensible heat required for regeneration. In addition, solvent loss and corrosion issues resulting from the use of aqueous amines would be minimized if solids adsorbents were instead utilized.
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