April 10-14, 2016
The global consequences of increased anthropogenic carbon dioxide emissions are well documented. To avoid the catastrophic long-term damages associated with climate change, much attention has been devoted to the development of CO2 emission mitigation strategies. A present challenge in the mitigation of anthropogenic CO2 emissions involves the design of less energy- and water-intensive capture technologies. Traditional capture methods, e.g. MEA solvent scrubbing, have prohibitive regeneration costs, and recent concerns in drought-ridden states like California have brought these water-intensive processes under increased scrutiny. Sorbent-based capture represents a promising solution to the aforementioned hurdles, as they do not incur the heavy energy penalties associated with solvent regeneration. However, to be considered competitive with their more mature, solvent-based counter-parts, these sorbents must exhibit i) high CO2 loading capacity and ii) high CO2/N2 selectivity. Ultra-microporous character and surface nitrogen functionality have been reported to be of great importance to the enhancement of CO2 capacity and CO2/N2 ideal selectivity. However, the role of pore size in combination with surface N-functionalities in the enhancement of these properties remains unclear. To investigate these effects, grand canonical Monte Carlo (GCMC) simulations were carried out on pure and functionalized 3-layer graphitic slit pore models. Theoretical isotherms were constructed as the PSD-weighted sum of individual slit-pore models of width varying from 3.5 to 200Å. In addition to pure and monovacancy-graphite, the following functional groups were isolated for testing at ~3% coverage: pyridinic nitrogen, pyrrolic nitrogen, quaternary nitrogen, and oxidized-pyridinic nitrogen. Our results reveal that nitrogen surface-functionalization can enhance CO2 loading by upwards of 90 percent over pure graphite. Increasing surface-coverage of quaternary-nitrogen resulted in enhanced loading, though higher coverage loadings converged at approximately 4 mmol CO2 g-1 sorbent. Charge analysis revealed that enhanced loading was strongly correlated with the oxidation state of surface-bound oxygen and, to a lesser extent, nitrogen at low pressures, with a decreasing effect at higher pressures. Functionally bound hydrogen contributed at higher (ambient) pressures suggesting a secondary mechanism of adsorbate stabilization. Ideal CO2/N2 selectivities were calculated for pure and N-doped models using both HL (low pressure) and IAST (working pressure) methods. Our results show that N-doping can enhance HL but not IAST selectivity. This is attributed to the introduction of a few highly active surface sites through surface N-modification. Additionally, selectivity was found to be greatly enhanced in the ultra-microporous volumes (3.5 - 7Å). These results illustrate that N-functionalization can influence CO2 and N2 adsorption behavior, particularly in narrower pores where opposing wall potentials overlap – an important consideration in the rational design of future carbon-based sorbents.
Peter Psarras and Jennifer Wilcox, "Nitrogen-functionalized porous carbons for enhanced CO2 capture" in "CO2 Summit II: Technologies and Opportunities", Holly Krutka, Tri-State Generation & Transmission Association Inc. Frank Zhu, UOP/Honeywell Eds, ECI Symposium Series, (2016). http://dc.engconfintl.org/co2_summit2/39