Effect of Water on the CO2 Adsorption Capacity ofAmine-Functionalized Carbon SorbentsPeter Psarras,† Jiajun He,‡ and Jennifer Wilcox*,†

†Department of Chemical and Biological Engineering, Colorado School of Mines, Golden, Colorado 80401, United States‡Department of Energy Resources Engineering, Stanford University, Stanford, California 94305, United States

ABSTRACT: Molecular simulation is used as a tool to improve understandingof CO2 adsorption in nitrogen-functionalized carbon sorbents in the presenceof water vapor, which is crucial to the advancement of adsorption approaches toCO2 separation from exhaust streams of coal- and natural gas-fired power plants.Molecular simulations were carried out for binary mixtures of CO2 and H2O overfour N-functionalized surfaces and three variations of the quaternary group withincreasing wt % N. The quaternary group was found to be most stable with a13% loss in CO2 capacity observed, followed by the pyrrolic and pyridonic groups,which lost 25 and 28% CO2 loading capacity, respectively. The oxidized pyridinicgroup demonstrated a dramatic loss in capacity, i.e., 58% when compared to idealloading. The quaternary group was the only functionality to display loading inexcess of 2.0 mmol CO2 g

−1 sorbent under ambient temperature and 1% humidity(2.40 mmol CO2 g

−1 sorbent). Further, the two functional groups without oxygenwere shown to be more resistant to competitive H2O adsorption at low humidity.In general, increasing nitrogen content appears to buffer the CO2 capacity loss under low humidity, yet such systems appear to beincompatible with CO2 separation at ambient temperature and 10% humidity. Further, the results of this work suggest thatmaterials modified with pyrrolic and pyridonic groups and pore size weighted in the supermicroporous region are most resistantto compromised CO2 loading under 10% humidity.

■ INTRODUCTION

Over the past several years, carbon dioxide adsorption tech-nologies have been investigated as an alternative CO2 mitigationstrategy driven in part by concerns over the water and energyintensity associated with more mature and conventional aminesolvent-based processes. Solid sorbents can offer several benefitsover their solvent-based counterparts. First, elimination ofsolvent is critical to lowering the heat of regeneration, which isinfluenced greatly by the heat capacity of solution (for example,3.496 J g−1 K−1 for 30 wt %monoethanolamine (MEA) solutionsat 298.15 K, 0.40 mol CO2/mol MEA loading).1 It has beenestimated that 80% of the energy required to release CO2 goestoward heating of the solution.2 Naturally, solid materials haveheat capacities lower than those of liquids (compare, 0.7 J g−1K−1

for graphite at 298.15 K);3 further, the gas−solid interactioncan be weaker depending upon the physical or chemical natureof the adsorption process (i.e., qads < 40 kJ mol−1).4 Moreover,solid sorbents offer tunability over pore geometries and poredimensions as well as flexibility for heteroatom doping or surfacefunctionalization, which are essential for optimized gas diffusivityand/or sorbent−CO2 interactions.5,6 Solid sorbents possessadditional advantages over amine solvents such as a relativelywide range of operating temperatures and less waste producedfor disposal.7 However, many of the factors that lead to alower theoretical energy of separation can also compromiseloading. For instance, Gray et al. estimate 3−4 mmol CO2 g

−1

sorbent (298 K, 1 atm) as the minimum CO2 working capacity

to be considered cost competitive with conventional aminescrubbing.8

Solid sorbents may also be advantageous when paired withintermittent renewable energy sources such as solar and wind.Renewable energy is increasingly identified as a necessary com-ponent in climate change mitigation with the potential toachieve carbon neutrality and negativity when paired with carboncapture utilization and sequestration (CCUS). Unfortunately,several technical barriers exist that stall significant deploymentof renewable energy development and integration, namely landmanagement and energy intermittency. In traditional solvent-based separation processes, much of the energy penalty asso-ciated with solvent heating is circumvented through heatexchange via the reboiler.9,10 This exchange is contingent onthe uninterrupted processing of flue gas, which is in turncontingent on a nonintermittent power supply; thus, pairing ofsolvent-based processes with renewable energy, even if thesesolvents show a higher working capacity for CO2, may beenergetically wasteful and unpractical. Conversely, solid sorbentsoperating under pressure swing adsorption (PSA) may integratewell with intermittent sources. Unlike temperature swing adsorp-tion (TSA), which typically experiences longer loading cycles,

Received: December 30, 2016Revised: April 6, 2017Accepted: May 2, 2017Published: May 2, 2017

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and where thermal integration with power plants is critical toefficient performance,11 PSA features relatively fast cycles with theabsence of thermal integration, and thus, it is capable of beingoperated in an intermittent way powered by solar or wind. Insummary, the advances of sorbent technologies create opportunitiesfor the deployment of renewable energy-assisted carbon capture.Several classes of sorbents for CO2 capture have been char-

acterized in the literature.12−14 Generally, zeolites are associatedwith moderate surface area (typically under 2000 m2/g) andmoderate production costs ($0.20/mol CO2 captured vs $0.25and $0.544/mol CO2 captured for activated carbon and MEA,respectively, assuming 2 mol CO2 loading per kg activatedcarbon, 5 mol CO2 loading per kg zeolite)

15 and typically showsensitivity to humid operating conditions. More recently, zeoliticimidazole frameworks (ZIF) exhibit enhanced chemical andthermal stability.16 Further, hydrophobic ZIFs exhibit essen-tially no loss in CO2 capacity when cofed with wet N2 (80%humidity);17 however, they remain plagued by the lack ofpore tunability. Metal organic frameworks (MOFs) are known toexhibit ultrahigh surface areas (in excess of 6000 m2/g BETspecific surface area)18 and excellent high-pressure CO2 capac-ities (as high as 54.5 mmol g−1 at 50 bar);19 yet, typical MOFssuffer from competitive water adsorption or chemical instabilityto moisture.20 For example, one family of MOF materials withsuperbCO2 capacity (upward of 23.6wt%) shows a 67% reductionin CO2 capacity in the presence of 9% humidity.21 Recently,McDonald et al. reported a chemisorption strategy to improveCO2 adsorption in the presence of moisture.22 Nevertheless, thematerial cost can be high, taking into account the scalability, timefor preparation, and postsynthesis functionalization cost.Carbonaceous sorbents are earth-abundant materials that

are generally low cost alternatives to MOFs and display reason-able chemical stability and cyclability. Additionally, the nonpolarnature of carbon materials lends to excellent stability underhumid conditions. Unfortunately, the unmodified carbon sur-face has relatively low working capacity for CO2 (<2 mmol g−1

sorbent),23 making it unsuitable for deployment at scale. It hasbeen shown that chemical surface modification of carbonsurfaces can lead to enhanced CO2 uptake at low CO2 partialpressure and, in some cases, enhanced CO2/N2 selectivity.

24,25,28

Modifications via oxygen-based functionalities26 and nitrogen-based functionalities27 have been explored via molecular simula-tions, allowing for a direct examination of the connectionbetween individual functionalities and pore size on CO2performance, i.e., working capacity and CO2/N2 selectivity.It is important to consider that a typical flue gas derived

from fossil-fuel fired power plants will contain 8−10% H2O.28

The presence of water vapor has the potential to compromisethe sorbent CO2 adsorption capacity by virtue of competitivedispersion−repulsion interactions with the surface and, moreimportantly, competing electrostatic interactions. The former isdefined by the Lennard−Jones (LJ) potential:

ε σ σ= −⎜ ⎟ ⎜ ⎟⎡⎣⎢⎛⎝

⎞⎠

⎛⎝

⎞⎠

⎤⎦⎥V r

r r( ) 4

12 6

(1)

where ε denotes the potential well depth and σ representsthe finite distance at which the interparticle potential reacheszero. Electrostatic interactions are described by a combination ofenergy contributions:2

ϕ α μ= − + − + ∂∂

E E QEr

12

12elec

2(2)

where terms on the right-hand side correspond to (from left toright) polarization, field-dipole, and the field-gradient quadru-pole. An additional contribution from sorbate−sorbate inter-actions on the surface may be included at high coverage.Competitive adsorption arises from the fact that the speciespresent in a flue gas mixture have different LJ and electrostaticparameters, as shown in Table 1.

Further, differences in physical properties for CO2 andH2O lend to different contributions to the electrostatic poten-tial (eq 2). A comparison of these properties is presented inTable 2.

Several studies have successfully modeled the effects of wateradsorption using the parameters outlined in Table 1.34−36 Thisstudy examines the effect of water vapor presence in low (1%)and typical (10%) amounts on the loading enhancement ofseveral nitrogen-modified carbon surfaces. Though it has beendemonstrated that N-functionalization can improve CO2 uptakethrough the introduction of surface charge and interactionof the local field gradient with the CO2 quadrupole moment,the dipole interaction of water is notably stronger and isexpected to compromise CO2 loading capacity through thepreferential occupation of reactive surface sites. This effect wassimulated through the investigation of CO2/H2O binary mix-tures within pores of various widths functionalized by fourunique surfaces: pyrrolic nitrogen (N5), oxidized pyridinic nitro-gen (NO), pyridonic nitrogen (NP), and quaternary nitrogen(NQ) (Figure 1). These functional groups have been confirmedto exist within N-doped carbons and have been frequentlyreported in the literature.23,37−39 Further, previous experimentalanalysis on similar functionalities revealed no hysteresis ondesorption40 and excellent cyclic stability,41 making thesematerials suitable candidates for use in PSA. Additionally, thepresence of water vapor was tested against four surfaces ofincreasing wt %N (i.e., 3.6−28.0%), henceforth abbreviated NQ,Q2, Q4, and Q8. These results provide further knowledge ofthe relative stability of N-functionalities toward CO2 capacity invarying humidity.

Table 1. Potential Parameters Used in Calculation of ForceField

εff/kB (K) σff (Å) q(e)a ref

C (CO2) 27.0 2.8 0.70 29

O (CO2) 79.0 3.05 −0.35 29

H (H2O) 0.417 30

O (H2O) 76.58 3.15 −0.834 30

C (surface) 28.0 3.4 calcd 31

N (surface) 60.39 3.296 calcd 32, 33

O (surface) 79.0 3.1 calcd 31

H (surface) 30.0 1.31 calcd 31aBader charge assignments for surface atoms are calculated separatelyfor each system.

Table 2. Comparison of Select Physical Properties of CO2 andH2O Adapted from Ref 2

kineticdiameter(Å)

dipolemoment(Debye)

quadrupole moment(10−40 C m2)

polarizability(10−24 cm3)

CO2 0.330 0 −13.71, −10.0 2.64, 2.91, 3.02H2O 0.280 1.85 6.67 1.45, 1.48

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2. METHOD

2.1. Density Functional Theory (DFT): Model Structureand Optimization. Hierarchical, carbon-based pore structureswere approximated as a collection of independent, functionalizedgraphitic slit-pores. This approximation has been used withsuccess in several studies.26,42−44 All structures were modeled as3-layered graphitic slabs in 4 × 4 carbon ring unit cells (96 atomsfor pure graphene). Functional groups were substituted into thetop layer at a rate of one nitrogen atom per unit cell. In somecases, it was necessary to remove additional top layer carbonatoms to simulate a graphene edge; thus, nitrogen coveragevaried slightly according to functionalization. All cells weresubject to geometric optimization within the Vienna ab initiosimulation package (VASP)45 with a van der Waals correction46

applied for proper optimization of graphitic layer spacing.A plane-wave basis set was employed and truncated at 750 eVto achieve a balance of computational efficiency within thedesired force threshold of 1 meV/Å. Projector augmented wave(PAW)47 potentials were used to describe core electronicbehavior. The exchange-correlation functional of Perdew, Burke,and Ernzerhof (PBE)48 was employed for nonlocal corrections,and a 6 × 6 × 1 Monkhorst−Pack49 grid was used to sample thefirst Brillouin zone. All optimizations were carried out using theconjugate-gradient algorithm. Charge partitioning was achievedthrough the Bader method,50 and core-level charge was includedin the integrations. The fine-grid was adjusted until the totalsystem charge was preserved to within ±0.001 e.The molecular simulations result in pore density profiles;

however, if a target material pore size distribution (PSD) ismade available, adsorption isotherms may be predicted. Here, weuse a PSD based on the work of He et al.,41 which served as thebasis for adsorption isotherm predictions in greater detail inprevious work and is demonstrated in Figure 2 as an example.27

The material (named SU-MAC 500, for Stanford Universitymesoporous activated carbon, with an activation temperature of500 °C) demonstrated excellent CO2 working capacity, CO2/N2

selectivity, and reasonable cyclic stability under humid condi-tions. The detailed methodology for the experimental PSD isprovided in our previous study.41

The experimentally determined PSDwas partitioned such thatthe full structure could be approximated as a weighted sum ofpores with the following internal diameters: 3.5, 4, 5, 6, 7, 8, 9, 10,12, 14, 18, 22, 28, 34, 40, 50, 60, 70, 80, 90, 100, 120, 140, 160,180, and 200 Å, where the pore width was assumed as thedistance between carbon atoms at opposing pore walls less thecollision diameter of a surface carbon atom (3.35 Å). Pores werecreated by first transposing the functional group from the centerof the cell (where optimized) to off-center and then mirroringthe VASP-optimized cells about a point of inversion at the porecenter. This method prevented unnatural overlap of opposingwall functionalities in the smaller pore sizes.

2.2. Molecular Simulations. Grand canonical Monte Carlo(GCMC) simulations51 were carried out to describe CO2 andH2O adsorption on the idealized functional pores previouslydiscussed. Carbon dioxide was described according to TraPPEparameters29 which have been shown to quantitatively reproduceCO2 vapor−liquid equilibria. This three-site rigid model treatsthe intrinsic quadrupole moment of CO2 by assignment of partialcharge at each site (qC = 0.70 e and qO = −0.35 e). The CObond lengths were fixed at 1.16 Å, and the OCObond anglewas fixed at 180.0°. Water was modeled using TIP3P param-eters,30 which effectively captures the intrinsic dipole moment.This three-site rigid model assigns partial charges of +0.417 e totwo hydrogen sites and −0.834 e to the oxygen site with H−Obond lengths of 0.957 Å and an H−O−H bond angle of 104.52°.

Figure 1. Surface view of the seven unique three-layered modified graphitic models examined in this study. Model abbreviations are included inparentheses.

Figure 2. Pore size distribution used as the basis for adsorption isothermpredictions. Modeled after the material SU-MAC 500 from ref 41.

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Potential parameters for surface carbon, oxygen, and hydrogenwere modeled after the studies of Steele52 and Tenney andLastoskie,31 respectively, while the parameters for surfacenitrogen were modeled after the work of Gubbins et al.33 Allpotentials associated with different LJ sites were calculated usingstandard Lorentz−Berthelot mixing rules.53 The surface chargewas assigned according to those calculated in the Bader methoddescribed previously.Simulations were carried out in the μVT ensemble at 298 and

333 K and for a pressure range of 0.001 to 1 bar. The Peng−Robinson equation of state was used to calculate fugacity and torelate bulk experimental pressure to chemical potential. Acceptedmove types included energy-biased insertions, deletions,rotations and translations. A total of 10 million GCMC moveswere attempted during each simulation to ensure adequatesystem equilibration. To reduce system load, a rigid frameworkwas assumed, and sorbate−surface interactions were interpolatedto a pretabulated potential map with grid spacing of 0.1 Å(sorbate−sorbate interactions were calculated on-the-fly).

3. RESULTS AND DISCUSSION3.1. Effect of Functionality on CO2 Loading Capacity.

Ideal CO2 adsorption isotherms are presented here againstbinary CO2:H2O mixtures for humidity conditions of 1% H2Oand 10% H2O (Figure 3). Adsorption isotherms presented onthe left represent CO2 loading over NQ, N5, NO, and NP inmild humidity (1% H2O). These results are presented againstideal adsorption isotherms for the same materials obtained inan earlier study27 to help directly visualize the loss in loading

capacity as a function of water vapor presence. The quaternarygroup (NQ) was found to be most stable with a 13% loss in CO2capacity observed, followed by the pyrrolic and pyridonic groupswhich lost 25 and 28% CO2 loading capacity, respectively. Theoxidized pyridinic group saw a dramatic loss in capacity (58%)when compared to ideal loading; however, due to exceptionalideal loading performance, the relative capacity for CO2 uptake at1 bar (1.81 mmol CO2 g

−1 sorbent) is similar to the N5 and NPgroups (1.76 and 1.95 mmol CO2 g

−1 sorbent, respectively). Thequaternary group was the only functionality to display loadingin excess of 2.0 mmol CO2 g

−1 sorbent under these conditions(2.40 mmol CO2 g

−1 sorbent). Further, the two functional groupswithout oxygen were shown to be more resistant to competitiveH2O adsorption at low humidity.Generally, CO2 loading capacity was greatly compromised

for all modified sorbents at 10% humidity, with the greatestpercentage loss in loading corresponding to the quaternary(98%) and oxidized pyridinic groups (97%). As indicated inthe adsorption isotherm (Figure 3, top right), CO2 loading isdramatically reduced (even at low pressures), and marginalloading is observed for p/p° 0.1. Interestingly, the pyrrolic groupis considerably more stable over 0 < p/p° < 0.25, with a sharpdecline in loading observed at p/p° = 0.3. A comparison of thezoomed-in low-pressure isotherms reveals that CO2 remainscompetitive with H2O for high-energy surface sites (functionalsubstitutions) at low pressures, as indicated by the relativeisotherm slopes for the low and moderate humidity conditions.An exception to this trend is observed for the pyrrolic group,whereby initial competition for pyrrolic sites appears to be

Figure 3.Comparison of CO2 loading on 4 uniqueN-functionalized surfaces at 298 K. Ideal loading presented as a reference and reproduced from ref 27with permission from the PCCP Owner Societies. Binary CO2 loading indicated by hollow symbols. The lower plots are zoomed-in regions of the fullisotherm, representing low pressure (i.e., 0−10mbar) loading performance. Isotherms on the right-hand side show greatly compromised CO2 loading intypical flue conditions (10% H2O).

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dominated byH2O. Considering that the pyrrolic group is indeedthe most stable under the moderate humidity of the four groupsstudied here, it is worth a moment to comment on the pyrrolicsurface chemistry.Aside from the absence of oxygen, the pyrrolic nitrogen has a

Bader partial charge smaller than those of the quaternary andpyridonic groups, a consequence of bonding to the less elec-tronegative hydrogen (Figure 4). Additionally, the strained five-membered pyrrolic ring creates more vacancy space within theimmediate vicinity of the functional group. The combination ofthese latter two effects may leave the pyrrolic nitrogen steric-ally unhindered in spite of early (low pressure) H2O bonding,allowing the pyrrolic nitrogen to continue to participate in CO2

loading at higher partial pressures.These simulations were repeated at 333.15 K. Resulting

isotherms are presented together for 1 and 10% humidity, asshown in Figure 5. For the surfaces NQ, N5, and NP at 1%humidity, CO2 loading declined by 38, 19, and 19%, respectively,relative to the 1% humidity capacity at 298.15 K. From the kinetictheory of gases, it is intuitive to anticipate a drop in single-component working capacity at elevated temperature. For gasmixtures, a balance exists between the increasing kinetic energyof each gas-phase component, the relative heats of adsorption,and relative partial pressures; thus, an increase in loading for onecomponent in a binary mixture may be observed if, for instance, acompetitive sorbate is more susceptible to temperature change.This effect is manifested here, albeit slightly, by a 1% increase inCO2 loading over the NO surface, which also demonstrated thestrongest CO2 capacity of the 4 surfaces (1.71 mmol CO2 g

−1

sorbent). The relative CO2 capacity at 333.15 K and 1% humiditydecreased in the order of NO > NP > NQ > N5, which does notalign with the trend observed at 298.15 K (NQ > NP > NO >N5), indicating that specific functional performance is nonlinearwith temperature.At 10% humidity, CO2 capacity increased dramatically for

NQ, N5, and NO (with a concurrent decline in capacity ofca. 10% for NP) relative to the 298.15 K 10% humidity results.

Adsorption isotherms for NQ and NP pass through a maximumat low pressure, and then loading steadily declines to 1 bar.Because H2O is likely adsorbedmore strongly than CO2 (see eq 2and Table 2), CO2 is gradually displaced at higher partialpressures of H2O. Interestingly, this trend is absent from the N5isotherm, further supporting how low incorporation of pyrrolicgroups yields a surface with moderate hydrophobicity. Theobserved relative increase in CO2 loading can be attributed tothe effect of temperature on the interaction between H2O andthe surface. This is discussed further in Section 3.3.

3.2. Effect of Increased N wt % on CO2 LoadingCapacity. The models examined in Section 3.1 had similarnitrogen content, ca. 3.6 wt %. Here, we examine the effect ofincreased nitrogen content via systematic substitution of nitro-gen for graphitic surface carbon to obtain models of 7.22, 14.4,and 28.0 wt % N (Q2, Q4, and Q8, respectively, from Figure 1).Results are presented for 298.15 and 333.15 K in Figure 6.

Figure 4. Bader partial charge assignments for select surface atoms. Bonds and select atoms omitted for clarity. Smaller magnitude charges (|q|≤ 0.10 e)not shown.

Figure 5. Comparison of CO2 loading at 1 and 10% humidity at333.15 K. Carbon dioxide is shown to be more competitive with H2Oover N-modified surfaces at elevated temperatures (when compared toresults at 298.15 K, Figure 3). Loading for the pyrrolic group is relativelyunaffected when the humidity is increased from 1 to 10%, an extensionof the trend observed at 298.15 K.

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Loading at ambient temperature was found to be similar toloading at elevated temperature for all models with variationsmeasured at ±1 to 3%. Likewise, the loss in 1 bar loading relativeto ideal CO2 capacity is reported here as a decrease in 52, 52,

and 45% for the Q2, Q4, and Q8 systems, respectively. Only theheavily modified Q8 system demonstrated a working capacityin excess of 2.0 mmol CO2 g

−1 sorbent. At ambient temperature,all systems suffered great losses in CO2 loading capacity at 10%

Figure 6.Carbon dioxide adsorption isotherms for three surfaces with increasing wt %N at 298.15 K (left) and 333.15 K (right) in 1% (circles) and 10%humidity. Only the Q8 model (28.0 wt % N) demonstrated +2.0 mmol CO2 g

−1 sorbent loading at both temperatures. Increase in temperature had anegligible effect on loading on all models at 1% humidity, while higher temperatures appear to stabilize CO2 on theN-modified surface at 10% humidity.

Figure 7. (a) Pore densities for pores of width 5, 10, and 22 Å at 1 bar. Notable is the relative stability of the pyrrolic and pyridonic groups at ambienttemperature and 10% humidity and likewise the absence of CO2 loading for the 5 Å pore at the same conditions. (b) Probability density functiondistributions for CO2 and H2O loading taken along the z cell axis at ambient temperature and low humidity. The lack of CO2 loading observed in the 5 Åpore may be a result of complete pore filling by H2O at higher humidity, as indicated by the preference of H2O near the pore wall. Note changes in they-axis scaling.

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humidity, with decreases ranging from 97 to 99% of the idealCO2 capacity. Naturally, such a loss in CO2 loading is accom-panied by increased water loading. For example, at 1 bar and333.15 K, the H2O loading jumps from the marginal 0.02 and0.01 mmol H2O g−1 sorbent for Q4 and Q8, respectively, to8.58 and 7.76 mmol H2O g−1 sorbent, respectively, when thehumidity increases from 1 to 10%. At a lower temperature of298.15 K and 10% humidity, loadings of 13.7 and 18.1 mmolH2O g−1 sorbent were achieved for the same sorbents at 1 bar.In summary, increasing nitrogen content appears to buffer theCO2 capacity loss under low humidity, yet such systems appearto be incompatible with CO2 separation at ambient temperatureand 10% humidity.3.3. Pore Densities at Select Pore Widths. To gain a

better understanding of the influence of material design onH2O stability, pore densities are reported for select pore widthswithin the ultramicroporous (d = 5.0 Å), supermicroporous(d = 10.0 Å), and near mesoporous (d = 22.0 Å) regions. Resultsare presented in Figure 7.Generally, loading decreased with pore widening at low (1%)

humidity and likewise with an increase in temperature. Carbondioxide loading in the 5 Å pore was similar at both temperaturesin the oxidized pyridinic and pyridonic models with greater lossin capacity observed in the wider pores. This effect is commonlyobserved in narrow pores, whereby opposing pore wall poten-tials can overlap to enhance sorbate uptake. It has been demon-strated elsewhere that loading enhancement induced by surfacefunctionalization becomes marginal for supermicropores andconverges with the unmodified surface for mesopores.27 Thistrend has inspired materials with large ultramicroporous volumessuch as the SU-MAC 500, which are designed to maximizebenefits in loading and selective CO2 uptake. However, poredensity results in the presence of 10% humidity, which indicatesthat very narrow pores are in fact nonideal for CO2 uptake(Figure 7a). Further, these results suggest that materials modifiedwith pyrrolic and pyridonic groups and pore size weighted in thesupermicroporous region are most resistant to compromisedCO2 loading under 10% humidity.An examination of the probability density functions (PDFs)

for CO2 and H2O in 5 Å pore widths reveal that H2O pre-ferentially adsorbs at the pore wall (represented as the left andright vertical axes in Figure 7b), whereas CO2 exists primarily inthe pore center; thus, as the humidity is increased to 10%, ultra-micropores become saturated with H2O and are consequentlyinaccessible to CO2. Here, results for the NQ and NO groups arepresented, where multilayering appears to be slightly enhancedfor the NO model. This observation is reflected in higher poredensities, as reported in Figure 7a.Two distinct trends can be observed when considering the

effect of temperature on loading capacity. The first involvesloadings at 1% humidity in the 5 Å pore, where loadings for NPand NO are marginally reduced, and N5 and NQ undergo largereductions. Here, the NP andNO pores are clearly saturated withCO2 in both cases, indicating that the strength of interactionbetween these functionalities and CO2 outweighs the decrease insurface residence time onset by elevated temperature. This isnot the case, however, for the N5 and NQ functionalized pores.As these latter functionalities can be considered less polar thanNO and NP (i.e., they lack oxygen), it is unlikely that H2O ispreferentially adsorbing at elevated temperature, an effect notseen for NP and NO by virtue of unchanged loading at 333.15 K;rather, the decrease in loading is likely a function of lowerisosteric hearts of adsorption for N5 and NQ and decreased

surface residence time of CO2. Second, we consider the sharpincrease in loading for N5 (22 Å) at 333.15 K and 10% humidity.It should be pointed out that all functionalities observed anincrease in CO2 loading within the 22 Å pore at elevated tem-peratures. This observation is counterintuitive from a kineticview, leaving only H2O presence as responsible for the enhanceduptake in larger pores, possibly through new acid−baseinteractions of the coadsorbed species. Interestingly, the greatestenhancement was observed for N5 and NQ, the two function-alities most comprised in 5 Å loading. Collectively, these resultsunderlie how the mechanism of loading as a function of tem-perature is extremely sensitive to both surface chemistry and poresize, where subtle changes to either property can yield largechanges in loading behavior. This phenomenon has beenobserved elsewhere.54

■ CONCLUSIONS

Molecular simulations were carried out for CO2 and H2O binarymixtures over fourN-functionalized surfaces and three variationsof the quaternary group at increasing wt % N. Generally, thesematerials were found to be largely unsuitable for CO2 captureunder 10% humidity at ambient temperature with severe loss toCO2 loading observed and a decrease of up to 99% of the idealcapacity. Reductions in loading were partially offset by increasedtemperature (333 K); however, loading results for individualfunctionalities were found to be nonlinear with increasing tem-perature. Of the functionalities tested, the pyrrolic group wasfound to bemost stable under 1 and 10% humidity, a result that islikely a consequence of additional surface vacancy space createdby the strained five-membered pyrrolic ring and the relative“softness” of the nitrogen partial charge due to bonding with theless-electronegative hydrogen. The latter condition creates a lesspolar environment and, due to vacancy spacing, an environmentthat is likely less sterically hindered to accommodate CO2adsorption in spite of water presence.Increasing nitrogen content upward of 28.0 wt % yielded ade-

quate CO2 loading (2.4 mmol g−1 sorbent) despite low humidity.

This could be one tool to offset anticipated reductions to CO2capacity in practice. However, the aforementioned reductions inCO2 loading capacity at 10% humidity further underscore theneed for moderate stream dehydration. Further studies will focuson optimizing the material−humidity level balance. Additionally,ultramicroporous volumes were shown to be especially suscep-tible to water uptake. This challenges the viewpoint that materialsshould be designed with high ultramicroporous volumes tomaximize pore wall-induced benefits in CO2 uptake and selec-tivity. Instead, materials weighted toward the supermicroporousvolumes may represent a suitable compromise between func-tionality enhancement and stability to moisture.Future work will focus on the effects of other common flue

constituents. For example, coal-derived flue gas contains fly ashparticles, oxygen, moisture, carbon monoxide, and many acidgases. A typical untreated flue gas derived from the combustionof a United States low sulfur Eastern bituminous coal cancontain: 5−7% H2O, 3−4% O2, 15−16% CO2, 1 ppb total Hg,20 ppmCO, 10 ppm hydrocarbons, 100 ppmHCl, 800 ppm SO2,10 ppm SO3, 500 ppm NOx, and balance N2.

55−57 There can bepotential impacts of acid gases such as SO2, SO3, NO, NO2, andHCl upon the CO2 capture through competing acid−basesurface chemistries. The possibility of oxidative degradation ofthe sorbent by oxygen and these same acid gas species also meritsfuture study.58

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■ AUTHOR INFORMATIONCorresponding Author*E-mail: wilcox@mines.edu.

ORCIDJennifer Wilcox: 0000-0001-8241-727XFundingThis work was partially funded by the Global Climate and EnergyProject (GCEP) and Precourt Institute for Energy.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis paper was identified by Session Chair William J. Koros(Georgia Institute of Technology, United States) as the BestPresentation from the session “Novel Material for GasSeparation, Storage & Utilization” from the 2016 ACS FallMeeting in Philadelphia, PA.

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