Polyamine-tethered porous polymer networks for carbon dioxide capture from flue gas.

One of the most pressing environmental concerns of our age is the escalating level of atmospheric CO2, which is largely correlated to the combustion of fossil fuels. For the foreseeable future, however, it seems that the ever-growing energy demand will most likely necessitate the consumption of these indispensable sources of energy. Carbon capture and sequestration (CCS), a process to separate CO2 from the flue gas of coal-fired power plants and then store it underground, has been proposed to reduce the anthropogenic CO2 emissions. Current CO2 capture processes employed in power plants worldwide are post-combustion “wet scrubbing” methods involving the chemical adsorption of CO2 by amine solutions such as monoethanolamine (MEA). The formation of carbamate from two MEA molecules and one CO2 molecule endows the scrubber with a high capacity and selectivity for CO2. However, this process suffers from a series of inherent problems, such as high regeneration costs that arise from heating the solution (ca. 30% of the power produced by the plant), fouling of the equipment, and solvent boil-off. To sidestep the huge energy demand, corrosion problem, and other limitations of traditional wet scrubbers, intensive efforts have been made to investigate the use of solid adsorbents as an alternative approach. Compared to wet scrubbing, in which a large amount of water (70% w/w) must be heated and cooled during the regeneration of the dissolved amines, the solid adsorbent approach has the tremendous advantage of improving the energy efficiency of the regeneration process by eliminating the need to heat water. Porous materials, such as MOF-210, NU-100, and PPN-4, have been deemed to be viable storage alternatives because of their high porosity and, therefore, significantly increased accessible contact area with gas molecules. This could be advantageous because separation and regeneration could be performed under relatively mild conditions compared to amine wet scrubbing systems. Unfortunately, the record high storage capacities do not translate to high selectivities and only moderate CO2-uptake capacities were observed under carbon capture conditions. The polarizability and large quadrupole moment of CO2 can be taken advantage of by introducing CO2-philic moieties that create strong interactions between the material surface and the CO2. This will improve the loading capacities and selectivity of CO2 over other gases. Indeed, this approach has already been proven to be very successful in enhancing the enthalpy of CO2 adsorption, [6] which can be calculated from CO2 sorption isotherms at different temperatures and used to quantify the interaction between the material and CO2. It is worth pointing out that the porosity of the material will be compromised by the introduction of functional groups. CO2 loading capacities at ambient conditions are dependent on the adsorption enthalpy and porosity (both surface area and pore volume), which must be balanced to achieve high loading. Besides the loading capacity, another important factor when quantifying how well a material will perform is CO2 selectivity over N2. Taking amine scrubbing as the model, aminated porous materials that have been synthesized usually exhibit very large adsorption enthalpies for CO2 and high CO2/N2 selectivities. Recently, Long and co-workers demonstrated the incorporation of N,N-dimethylethylenediamine (mmen) at exposed metal centers of CuBTTri (H3BTTri= 1,3,5-tri(1H-1,2,3-triazol-4-yl)benzene), with the new compound mmen-CuBTTri showing drastic enhancement of CO2 uptake at low pressure, as well as CO2/N2 selectivity. It has the highest heat of adsorption for CO2 (96 kJmol ) and one of the highest CO2/N2 adsorption selectivities (SIAST= 327 at 25 8C) reported to date. Despite its large isosteric heat of CO2 adsorption, mmen-CuBTTri could easily be regenerated at 60 8C. It is always important to keep physicochemical stability in mind for practical applications. Purely organic porous polymers are a class of adsorbents that exhibit surface areas comparable to those of metal–organic frameworks (MOFs), but have much higher physicochemical stability because of the entirely covalently bonded network. The key to obtaining porous polymers with high amine loading is to judiciously select efficient reactions and starting materials with ultrahigh surface areas. PPN-6 (PPN stands for porous polymer networks), which has an exceptionally high surface area and an extremely robust all-carbon scaffold [*] Dr. W. Lu, J. P. Sculley, Z. Wei, Prof. Dr. H.-C. Zhou Department of Chemistry, Texas A&M University College Station, TX 77842 (USA) E-mail: [email protected] Homepage: http://www.chem.tamu.edu/rgroup/zhou/