Understanding the Mechanisms of CO2 Adsorption Enhancement in

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Understanding the Mechanisms of CO Adsorption Enhancement in Pure Silica Zeolites Under Humid Conditions WooSeok Jeong, and Jihan Kim J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b06571 • Publication Date (Web): 26 Sep 2016 Downloaded from http://pubs.acs.org on October 1, 2016

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The Journal of Physical Chemistry

Understanding the Mechanisms of CO2 Adsorption Enhancement in Pure Silica Zeolites under Humid Conditions WooSeok Jeong and Jihan Kim* Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, 291, Daehak-ro, Yuseong-gu, Daejeon, 34141, Republic of Korea.

ABSTRACT Using grand canonical Monte Carlo simulations, computational screening of hundreds of pure silica zeolites were conducted to identify materials that show enhanced CO2 uptake under humid conditions. Herein, we show that CO2 adsorption performance can be either enhanced or degraded depending on the CO2/H2O binding site separations and characteristics of CO2-H2O interaction energies. As expected, CO2 adsorption capacity is significantly degraded when its binding sites overlap with the H2O sites. On the other hand, CO2 adsorption performance is enhanced when CO2/H2O binding sites are clearly separated as shown from the molecular simulations. However, we show that there are zeolite structures where CO2 enhancement is observed despite the close distance between the CO2 and H2O binding sites. It is demonstrated that favorable long-range Coulomb interaction between CO2 and H2O molecules is responsible for enhanced CO2 adsorption performance in these materials.

ACS Paragon Plus Environment

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1. INTRODUCTION Carbon capture and sequestration (CCS) is one of the most viable technologies to reduce the amount of greenhouse gas emissions into the atmosphere.1 One practical approach to CCS is to retrofit a carbon capture facility on existing large-scale sources of greenhouse gas such as fossilfuel power plants, cement plants, and petro-chemical plants for post-combustion carbon capture.2 Among various post-combustion carbon capture methods, a wet scrubber using amine solution has been evaluated as the most mature technology.3 However, the commercial deployment of the conventional amine scrubbing for large-scale carbon capture has been delayed due to various liquid-solvent related operational issues (e.g. high regeneration cost, corrosion of facility, solvent degradation, and a loss of solvent).4 To avoid or mitigate operational problems associated with a liquid solvent, solid adsorbents have been investigated as alternatives to the traditional amine scrubber.5-6 Among these adsorbents, porous materials such as zeolites and metal-organic frameworks (MOFs) are widely viewed as promising materials for CO2 capture.7-8 In particular, zeolites are microporous crystalline aluminosilicates, which have been widely used for catalysis,9-10 ion exchange,11 and gas separation12-13 due to relatively cheap production cost, high thermal and hydrolytic stability appropriate for various harsh industrial conditions.14 Given that the number of possible materials that can be used to capture CO2 is exceedingly large, researchers have begun to use computational simulations to quickly screen through a large number of structures to identify the best materials.15-21 In particular, Lin et al. integrated the molecular simulation data with a simple process model to identify a set of puresilica zeolites that can potentially reduce the capture cost by 30-40% compared to the current


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amine technology, illustrating the power of computational screening. Unfortunately, most experimental/computational studies on CO2 capture in porous materials have focused on pure CO2 or binary gas mixtures of CO2/N27-8, 15-19, 22 whereas post-combustion flue gas contains other components (most notably 5~7% water vapor6), which can degrade CO2 adsorption capacity significantly in promising porous materials (e.g. zeolite 5A,23 zeolite 13X,23 zeolite 10X,24 MgMOF-74,25 and Ni-MOF-7426). While H2O is seen as a deterrent for most CO2 capture materials, there are few instances in which H2O has shown beneficial effect when it comes to increasing the CO2 capture performance. For example, Yazaydin et al. showed in a joint experimental/computational study that in a metal-organic framework structure called Cu-BTC, there is a slight increase in the CO2 uptake in presence of small amounts of water.26-27 Recently, there have been reports that several mesoporous silicas (e.g. MCM-41) and MOFs (e.g. Mg-MOF-74, and Ni-MOF-74) functionalized with alkylamines showed relatively large CO2 uptakes even under the humid conditions.28 Although these results are promising for carbon capture under realistic flue gas condition, from a material design’s point of view, it remains unclear on which types of materials will lead to enhancement of CO2 in presence of water. In this work, computational screening using classical molecular simulations is performed on pure-silica zeolite structures to obtain CO2 adsorption uptake under humid conditions. We focused on pure-silica zeolites as (1) large number of zeolite structures is available both in the International Zeolite Association (IZA) database of experimentally identified zeolite frameworks29-30 and a hypothetical zeolite structure database created by Deem et al.,31-32 and (2) reliable force fields have been derived by Garcia-Perez et al.33 such that we can obtain an accurate data that can help us arrive at some general conclusions about CO2 and H2O adsorption


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inside porous materials. Large-scale screening on hundreds of structures was conducted to observe common, emerging patterns that lead to either enhancement or degradation of CO2 adsorption in presence of water. In this context, the focus of this work has been made on the interplay between the CO2 and the H2O adsorbed inside the zeolites as well as their respective binding sites. Thereby the insights gained from this study can be applied to other porous materials such as metal-organic frameworks and polymers. This paper is organized as follows. In the Computational Methodology section, the detailed procedures used to conduct the computational simulations are described. To compare CO2 adsorption performances of various zeolites structures, the water content is fixed to be equal for all structures, thus enabling us to isolate just the effects from the zeolite geometry and topology. For analysis, the degree of overlap between CO2 and H2O binding sites are quantified to evaluate its impact on CO2 adsorption. In the Results and Discussion section, optimal zeolite frameworks are identified through the screening process. A few representative zeolite materials are selected for further investigation from the analysis using the ratio of the Henry coefficients. The underlying mechanisms for the enhancement/degradation of CO2 adsorption under humid conditions are discussed using distance histogram and energy contours. Finally, in Conclusions, some of the important findings in our study are summarized.

2. COMPUTATIONAL METHODOLOGY 391 zeolite structures are selected from the IZA database of 217 zeolite frameworks29-30 and from the 174 diverse hypothetical zeolite frameworks34 that are selected with consideration of maximal dissimilarity from predicted zeolite structures (i.e., PCOD) generated by Deem and coworkers.31-32 In order to compute the Henry coefficients (KH) and the adsorption isotherms, the


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Widom particle insertion method and grand canonical Monte Carlo (GCMC) simulations are utilized, respectively.35-36 All of the interatomic interactions are comprised of Lennard-Jones and electrostatic interactions. A force field of 12-6 Lennard-Jones potential model for pure silica zeolites and CO2 developed by E. Garcia-Perez et al. and Lorentz-Berthelot mixing rule were adopted.33 The simple point charge (SPC) model37 is used as a basic water model for molecular simulations, and the Tip4p38 and Tip5p39 water models are adopted for comparison and validation of several zeolites. For the electrostatic interactions, the Ewald summation method is used to expedite long-range interaction calculations. To accelerate the simulations, an energy grid with a grid size of 0.15 Å are generated and used with a cut-off distance of 12 Å. To ensure fair comparisons amongst all of the zeolite materials, humid conditions are set to be the same for each of the different zeolite materials. Specifically, two different volumetric water contents (100% and 300%) were used where 100% (300%) refers to having one (three) water molecule(s) inside a volume of 12 Å × 12 Å × 12 Å zeolite framework. In terms of gravimetric water content, 100% (300%) correspond to an average weight percentage of 1.04wt% (3.14wt%), respectively (where the values were computed via averaging all the zeolite materials examined in this work). The temperature is set to be 300 K in all of our molecular simulations. The adsorption isotherms are evaluated at 0.1 bar, which is around the partial pressure of CO2 within the flue gas as well as at 1 bar. In order to obtain accurate statistics, millions of GCMC moves (for pure CO2, production cycles: 1,000,000 ~ 8,000,000; for CO2/H2O

mixtures, initialization steps: 50,000, production cycles: 500,000 ~ 10,000,000, for each

Monte Carlo cycle N moves with a minimum limit of 20 are conducted, where N is the number of molecules in a simulation box to fully equilibrate the system consisting of large molecules)


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and Widom insertion moves (350,000,000 insertions) are conducted, leading to standard error of less than 3% for most structures. As the main criterion for the screening, the CO2 uptake enhancement ratio, where the CO2 uptakes are computed using GCMC simulations, is defined as follows: Enhancement ratio =

(1)

For a better understanding of behaviors of CO2 and H2O molecules at a molecular level, the CO2 KH is computed with and without the H2O molecules at their lowest energy positions, and the ratio between the two is labeled “KH ratio” (defined below). Accordingly, the dynamics of CO2 and H2O in the GCMC simulations is simplified as the snapshots of frozen H2O molecules located at their minimum energy configurations. ! ratio

=

"# $ ! %$ & & '% "# $ '%&

(2)

To identify the lowest energy H2O configurations, an energy grid for H2O-framework interaction is generated first and a H2O molecule is randomly rotated 100 times with respect to an oxygen atom in each of the grid points to get the average Boltzmann weighted energies. From this process, the minimum average Boltzmann weighted energy over all the grid points is obtained and labelled, Emin. Then the entire H2O energy grid is scanned and H2O energy values that are below a certain energy threshold (herein, we choose Emin + 3kBT for narrow blocking and Emin + 11kBT for broad blocking) are all blocked to high energy, thereby making these spatial points inaccessible to all molecules. To ensure that the vicinities of these strong H2O binding sites are all blocked out, all of the grid points within a radius of 1.25 Å (slightly larger than the O-H bond of SPC water model, 1.0 Å) with respect to the identified minimum point are selected and set to very high energy value. For the simulations, two kinds of molecular simulation codes are used because these two codes have different capability: CPU Monte Carlo


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code40 and GPU (graphics processing unit) code41-42. The CPU code is used to calculate CO2 uptake with/without H2O (moving H2O method; translation and rotational moves of H2O molecules are allowed during MC simulations) and to record CO2/H2O snapshots during Monte Carlo simulations. The GPU code is utilized to compute CO2 KH ratio and CO2 enhancement ratio with fixed H2O molecules (fixed H2O method), and to generate CO2-framework interaction energy contour.

3. RESULTS AND DISCUSSION 3.1. Computational Screening. The CO2 enhancement ratios with 100% and 300% H2O contents are plotted as a function of the CO2 uptake at 300 K and 0.1 bar (Figure 1a) and 1 bar (Figure 1b) for IZA and diverse set of hypothetical zeolite frameworks. The screening results indicate that just a small portion of the 391 zeolite materials exhibit more than 10% increase in CO2 adsorption with CO2 uptake of more than 0.1 mol/kg: 5.12% and 7.42% of all the zeolite structures at 0.1 bar, and 2.56% and 5.37% of all the zeolite structures at 1 bar for water contents of 100% and 300%, respectively. In general, there is an inverse relationship between the CO2 uptake and enhancement, suggesting that structures with strong CO2 binding sites do not perform well in presence of water, which seems reasonable since CO2 and H2O most likely compete for the same binding sites. In addition, the structures with large enhancement tend to display higher enhancement ratio at 0.1 bar compared to 1 bar as the proportion of CO2 molecules involved in favorable CO2-H2O interactions (which lead to the enhancement) becomes greater at lower pressures. By comparison, at higher water content (300%) the spread in the enhancement ratio (amongst all of the zeolite structures) becomes larger as higher water content can lead to more extreme adsorption properties (both favorable and adverse). For 300% water content at 0.1 bar,


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PCOD8208795 structure exhibits the largest enhancement ratio of 1.84. The maximum CO2 enhancement ratio amongst the IZA structures is found for OWE at 1.68. On the other hand, PCOD8323306 and AWW zeolite structures show significant decrease in the CO2 adsorption capacity (93% and 84% loss of CO2 capacity, respectively) compared to the dry conditions.

Figure 1. Computational screening results for IZA and diverse sets with respect to CO2 uptake enhancement ratio at 300 K (left: water 100%, right: water 300%). Three best and worst frameworks for each IZA and diverse set are indicated individually. (a) P = 0.1 bar, (b) P = 1 bar.


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3.2. Classification of CO2/H2O Binding Site Separations. As the first step of analysis, we test our hypothesis that the extent of overlap between CO2 and H2O binding sites heavily influences the CO2 adsorption performance under humid conditions by using the CO2 KH ratio (defined from Equation (2)). CO2 KH under humid condition is significantly reduced compared to that of dry condition when the H2O and the CO2 binding sites overlap with one another. In the extreme case where the overlapping adsorption sites also serve as dominant sites in the overall CO2 adsorption properties, the CO2 KH ratio becomes close to zero. The CO2 uptake enhancement ratios at 0.1 bar are plotted as a function of CO2 KH ratio in Figure 2. The CO2 KH ratios with/without blocked H2O low energy regions are computed for total 258 frameworks from IZA and diverse sets. To obtain the maximum possible enhancement under a wide range of humidity conditions for each of the materials, the CO2 uptake enhancement ratios for each framework are computed from 100% to 1500% in intervals of 200%. In our analysis, all of the zeolite structures start to show decrease in the CO2 uptake enhancement ratio beyond the point of 1500% H2O content (15 H2O molecules per 12 Å × 12 Å × 12 Å), thus the reason for this upper limit. For majority of zeolites, there is CO2 uptake degradation for all humid conditions, and in order to meaningfully display this in Figure 2, the smaller CO2 uptake enhancement ratio (which are all below 1) from 100% and 300% water contents was plotted.


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Figure 2. Potential CO2 uptake enhancement ratio at P = 0.1 bar as function of CO2 KH ratio. Several high/low performance zeolite frameworks are labeled. H2O low energy regions are blocked (left: energy threshold is Emin + 3 kBT for narrow blocking, right: Emin + 11 kBT for broad blocking) in computing the CO2 KH ratio. For the H2O energy region blocking case with narrow blocking energy threshold of Emin + 3kBT (Figure 2a), all of the zeolite structures that possess CO2 KH ratio near zero (e.g. AWW, and PCOD8306594) shows degradation under humid conditions. This is expected as all of the strong CO2 sites overlap with the strong H2O sites. For CO2 KH ratio > 0.5, zeolite structures with CO2 enhancement start to appear. However, even in cases where the CO2 KH ratio is close to 1 (implying zero overlap between strong CO2 and strong H2O binding sites), there is a considerable number of zeolite structures that show severe degradation of CO2 uptake in the presence of water. This suggests that some of the strong CO2 sites might overlap with the relatively weak H2O sites, that are not blocked with the narrow energy threshold of Emin + 3kBT, and due to entropic effects, water occupies these sites and they are not constrained to their


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stronger binding sites. To verify this hypothesis, the energy threshold for defining the strong H2O low region is increased to Emin + 11kBT (Figure 2b). In other words, H2O binding sites with strong to intermediate strength are all set to be inaccessible as well. Accordingly, the data points collectively shift towards CO2 KH ratio = 0 as more regions are blocked off due to higher threshold. However, even under this renewed criterion, few structures still possess large CO2 KH ratio (e.g. PCOD8197781 and PCOD8194213) and show degradation (Figure 2b) despite seemingly clear separation between CO2 and strong/intermediate H2O sites. These interesting structures will be investigated further in the later section. On the contrary, there are a few structures that still demonstrate enhancement (e.g. PCOD8319944 and PCOD8208795), near CO2 KH ratio = 1 region, indicating that these structures most likely possess separated CO2 and H2O binding sites. Finally, zeolites APC and OWE exhibit very large enhancement despite showing considerable amount of overlap in the CO2 and the H2O binding regions. Given the diverse structural properties as shown in Figure 2, several structures (both IZA and hypothetical) with large enhancement/degradation properties (AWW, SAS, APC, OWE, PCOD8306594, PCOD8208795, PCOD8197781 and PCOD8194213) are selected as representative zeolite structures for further investigation. We have classified the zeolite structures into three distinct cases that lead to differing CO2 enhancement/degradation properties as shown by the snapshots of CO2 and H2O molecules taken from the Monte Carlo simulations of (Figure 3 and Figure S1): (1) zeolite materials where binding sites for CO2 and H2O are clearly separated (Figure 3a). In this case, CO2 and H2O molecules adsorb at different channels and/or cages such that the distance separating the two sites is larger than 3 Å. These zeolite materials with separated binding sites (e.g. SAS and PCOD8319944) show enhancement of CO2 adsorption under humid conditions. (2) zeolite materials where CO2 and H2O binding sites


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overlap in the same channels and/or cages (Figure 3b). Here, CO2 and H2O binding sites are congregated together and separated by a very short distance (less than 1 Å). The zeolite materials that fall into this category (e.g. such AWW and PCOD8306594) possess overlapping binding sites leading to significant decrease in CO2 adsorption in presence of moisture. (3) zeolite materials where CO2 and H2O binding sites are close but do not completely overlap with one another in the same channels and/or cages (Figure 3c and 3d). In these structures, water molecules tend to adsorb near the wall of channels while CO2 molecules prefer to adsorb away from the wall toward the center of the channels and/or cages. Accordingly, the CO2 and the H2O binding sites are separated energetically but located closely in physical space. Interestingly enough in this case, there are structures in which enhancement is observed (e.g. OWE and APC) as well as degradation (e.g. PCOD8197781 and PCOD8194213).

Figure 3. Representative frameworks for different types of CO2/H2O binding site separation: (a) CO2/H2O binding sites are clearly separated [enhanced framework], (b) CO2/H2O binding sites are overlapped [degraded framework], (c) CO2/H2O binding sites are close [enhanced framework] and (d) CO2/H2O binding sites are close [degraded framework]. CO2 and H2O snapshots from total 1,000 snapshots (1,000,000 Initialization steps and 10,000 number of


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cycles) are plotted as point (framework, green; C of CO2, gray; O of CO2, red; H of H2O, cyan; O of H2O, blue). To obtain a comprehensive picture of the enhancement/degradation behaviors in these representative structures, the entire CO2 adsorption isotherms were computed at 300% water content (Figure 4). To check for possible sensitivity issues that might arise with the selection in the water force fields, three different water models (SPC, Tip4p, and Tip5p) were used. The adsorption isotherm data indicates that CO2 adsorption is insensitive to the water force field parameters and that enhancement (as well as degradation) can be observed through a wide range of pressures for these representative structures. In terms of pure CO2 uptake (as opposed to enhancement), structures with the overlapped CO2/H2O binding sites show the largest amounts of CO2 adsorbed at dry conditions (1.459 mol/kg for AWW and 1.161 mol/kg for PCOD8306594 at 0.1 bar). In contrast, zeolite structures of clearly separated CO2/H2O binding sites possess the least CO2 uptakes (0.0917 mol/kg for SAS and 0.133 mol/kg for PCOD8319944 at 0.1 bar).


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Figure 4. Adsorption isotherms for representative zeolite frameworks with/without co-adsorbed water molecules (300%) using various water models at 300 K: (a) CO2/H2O binding sites are clearly separated [enhanced framework], (b) CO2/H2O binding sites are overlapped [degraded framework], (c) CO2/H2O binding sites are close [enhanced framework] and (d) CO2/H2O binding sites are close [degraded framework]. Next, histograms compiled from the distances between the CO2 and H2O molecules in these representative zeolite structures are shown in Figure 5. To obtain CO2 and H2O coordinates


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separately, the CO2 GCMC simulations at 300 K and 0.1 bar, and fixed number of H2O Monte Carlo equivalent to 300% water contents at 300 K were performed. From the molecular positions, all the closest distances between the three atoms in the CO2 molecule and the three atoms in the H2O molecule of every CO2-H2O pairs that are within 10 Å of one another are taken into account and counted to each histogram bin with bin size of 0.01 Å. The generated distance frequency is normalized to the total number of distance data. The distance histogram for zeolite SAS contains very few distance values below 3 Å (Figure 5a), demonstrating clearly separated CO2/H2O binding sites. On the other hand, for zeolite AWW, which has overlapping CO2/H2O binding sites (Figure 5b), relatively large peak exists in the distance range from 0 to 1 Å. Given that the C-O bond length of 1.16 Å in the CO2 model used in this study,33 the presence of the large peak less than 1 Å clearly shows that there is considerable overlap between CO2 and H2O binding sites. In the case of zeolites OWE and PCOD8197781, where the binding sites are close but not overlapped (Figure 5c and 5d), the histogram distributions at small separations (i.e. less than 3 Angstroms) show a small but nonnegligible distributions, making these structures different from the two aforementioned cases. However, in these intermediate cases, zeolites can show significant differences amongst one another in terms of CO2 enhancement, (OWE: enhanced CO2 uptake, PCOD8197781: degraded CO2 adsorption) under humid conditions.


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Figure 5. Distance histograms for representative zeolite frameworks: (a) CO2/H2O binding sites are clearly separated [enhanced framework], (b) CO2/H2O binding sites are overlapped [degraded framework], (c) CO2/H2O binding sites are close [enhanced framework] and (d) CO2/H2O binding sites are close [degraded framework]. All closest atom(CO2) - atom(H2O) distances between every CO2-H2O pair are calculated and normalized to generate the distance histograms. CO2 and H2O coordinates are obtained separately from total 1,000 snapshots (1,000,000 Initialization steps and 10,000 number of cycles).


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Next, various geometric features of the representative zeolite materials were investigated using the Zeo++ software43. As shown in Table 1, spherical probe is used to compute the largest included sphere diameter (i.e. largest pore size), largest free sphere diameter (i.e. minimum channel window size on a diffusion path), number of channel, dimensionality of channel system, accessible pore volume (i.e. AV pore V), and non-accessible pore volume (i.e. NAV pore V). In particular, features of channel and pore volume were calculated using two different probe sizes (2.65 Å and 3.3 Å), each corresponding to the CO2 and the H2O kinetic diameters. For zeolite APC, CO2-accessible pore volume is zero since the minimum channel window size (i.e. 2.69 Å) is smaller than the kinetic diameter of CO2, preventing CO2 diffusion into the pore. Accordingly, although APC has a specific pore shape to enhance CO2 adsorption in presence of water, its usage as a CO2 capture material is limited due to the narrow pore entrance window. Unfortunately, there is no clear topological distinction among three categories of CO2/H2O binding site separation, which makes it difficult to develop simple geometry-based indicators that can differentiate the best materials for CO2 adsorption under humid conditions.

Table 1. CO2 uptake and enhancement ratios (water 300 %) at 300 K for representative zeolite frameworks

Category

CO2/H2O binding sites are clearly separated

CO2/H2O binding sites are overlapped

CO2 adsorption variation under humid conditions

Enhanced

Degraded

CO2/H2O binding sites are close

Enhanced

Degraded

Framework

SAS

PCOD 8319944

AWW

PCOD 8306594

OWE

APC

PCOD 8197781

PCOD 8194213

Largest included sphere diameter [Å]

8.548

5.612

6.894

8.357

5.215

3.825

6.468

6.873

Largest free sphere diameter [Å]

3.819

4.748

3.766

3.688

3.385

2.690

5.381

5.408


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Probe dia. = 3.3 Å (CO2 kinetic diameter)

Probe dia. = 2.65 Å (H2O kinetic diameter)

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# of channel

2

4

2

1

1

0

1

1

Channel dimensionality

11

1111

11

1

1

0

1

1

AV pore V [cm3/g]

0.0661

0.00882

0.0298

0.0367

0.0150

0

0.0127

0.0143

NAV pore V [cm3/g]

0

0.000193

0

0.00217

0

0.00111

0.00181

0.000562

# of channel

2

6

2

2

1

4

2

2

Channel dimensionality

11

111 111

11

11

2

1111

11

11

AV pore V [cm3/g]

0.0947

0.0164

0.0506

0.0593

0.0392

0.0105

0.0256

0.0240

NAV pore V [cm3/g]

0

0.00122

0

0.0003

0

0

0.000153

0.000869

Finally, CO2 adsorption properties for aluminosilicate zeolites under humid conditions is analyzed for the representative zeolite structures that showed enhancement of CO2 uptake in presence of moisture (i.e., APC, OWE, SAS, PCOD8208795 and PCOD8319944). The aluminosilicate structures with Si/Al ratios of 1, 2, 3, 5 and 10, were created by randomly replacing Si atoms with Al atoms, obeying the Lӧwenstein’s rule.16,

44

with the appropriate

number of sodium cations added to ensure charge neutrality in the system. The force field derived by Lennard Joos et al.45 was used in our analysis as their work successfully reproduced the CO2 experimental data under humid conditions for aluminosilicate zeolite FAU. In all zeolites except for APC, pure CO2 uptake amounts under dry conditions increased from the pure silica values upon converting the material to aluminosilicates (Table S3). However, under humid conditions, the CO2 uptake decreased significantly (Figure S5) demonstrating that enhancement properties do not translate well upon adding cations, which is reasonable given that presence of cations can highly disturb the delicate conditions that led to enhancement in the first place for the pure silica structures.


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3.3. Mechanisms for Enhancement/Degradation. To explain the mechanisms behind CO2 enhancement, potential energy contours for CO2-zeolite interactions were generated with and without water (where water is assumed to be part of the “framework”) (Figure 6 and S4). The H2O snapshots from the final recorded MC cycle were inserted into the zeolite framework supercell, which was expanded beyond 2 times the cutoff radius in all three spatial directions. To differentiate the impacts between Coulomb and Lennard-Jones interactions between CO2 and H2O, three different cases (one with just Lennard-Jones, one with only Coulomb, and one with both Lennard-Jones and Coulomb interactions) were examined. In the case of clearly separated binding sites of zeolite SAS (Figure 6a), the overall locations of the relatively low energy regions remain relatively unchanged. However, the minimum CO2 interaction energy is decreased as listed in Table 2 as the presence of water in SAS leads to favorable Lennard-Jones/Coulomb interactions with the CO2 binding sites, and enhances the strength of CO2 binding energy. For PCOD8319944 (Figures S1, S3 and S4), which also has clearly separated binding sites, similar behavior can be observed. However, due to the larger distance (4 Å) that separates the CO2 and H2O binding sites compared to that of SAS (3 Å), contribution for the favorable interactions largely comes from Coulomb interactions as shown in Table 2. This favorable interaction across pore wall is similar to the reported longrange adsorbate-adsorbate interactions in IRMOF-74 by H. S. Cho et al.46


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Figure 6. CO2 energy contours for representative frameworks. (Top left: CO2-zeolite framework interaction energy contour, top right: CO2 E contour after the addition of 1 snapshot of H2O, bottom left: CO2 E contour with CO2-H2O Coulomb interaction, bottom right: CO2 E contour with CO2-H2O Lennard-Jones interaction: (a) CO2/H2O binding sites are clearly separated [enhanced framework], (b) CO2/H2O binding sites are overlapped [degraded framework], (c) CO2/H2O binding sites are close [enhanced framework] and (d) CO2/H2O binding sites are close [degraded framework]. The energy contours in the left column differ from the right column by a view angle. Table 2. Variation of the minimum CO2-framework interaction energy (in kJ/mol) at 300 K due to the addition of water molecules

Category

CO2 adsorption variation under humid conditions

CO2/H2O binding sites are clearly separated

Enhanced

CO2/H2O binding sites are overlapped

Degraded

Change of the minimum CO2-framework interaction energy under humid conditions [kJ/mol] Framework

All interactions [Coulomb + Lennard-Jones]

Only Lennard-Jones interaction between CO2-H2O

Contribution from Coulomb interaction between CO2-H2O

SAS

-1.00 (-3.99%)

-0.42 (-1.70%)

-0.57 (-2.29%)

PCOD8319944

-3.67 (-9.31%)

-0.32 (-0.80%)

-3.35 (-8.51%)

AWW

+2.26 (+4.82%)

+0.92 (+1.87%)

+1.34 (+2.95%)

PCOD8306594

+8.89 (+18.79%)

+8.50 (+17.96%)

+0.39 (+0.83 %)

OWE

-5.75 (-22.78%)

-0.94 (-3.72%)

-4.81 (-19.06 %)

APC

-7.06 (-23.64%)

-1.55 (-5.18%)

-5.51 (-18.46 %)

PCOD8197781

+1.80 (+4.59%)

-1.69 (-4.32%)

+3.48 (+8.91 %)

PCOD8194213

+1.86 (+5.41%)

-1.60 (-4.66%)

+3.47 (+10.07 %)

Enhanced CO2/H2O binding sites are close Degraded

On the other hand, zeolites in which the locations of the CO2 and the H2O binding sites are much closer to one another exhibit more radical changes in the CO2 energy contours. For example, in zeolites AWW and PCOD8306594, which possess overlapping binding sites, (Figure 6b and Figure S4) a large portion of the strong CO2 binding sites disappears due to short-range


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Lennard-Jones repulsive interactions from the water molecules. In the case where the binding sites are close but not completely overlapped, different energy contours arise depending on the level of CO2 enhancement. In case of OWE where CO2 uptake is enhanced (Figure 6c), the proportion of the strong CO2 binding region is decreased (due to the presence of water) but its strength has increased due to the favorable CO2-H2O interactions. In contrast, for degraded structure PCOD8197781 (Figure 6d), the strong CO2 binding regions are much diminished in terms of both strength and volume. As listed in Table 2, for enhanced frameworks, there are reductions in the minimum CO2framework energy whereas degraded frameworks show increase in the minimum CO2framework energy compared with those before the addition of water molecules. This indicates that variation of strength of strong CO2 binding sites (herein, change of minimum CO2framework interaction energy) dominates CO2 adsorption performance. Particularly, long-range Coulomb interactions between CO2-H2O contributes to the increase in CO2 adsorption unless the strong CO2 binding sites are screened by short-range Lennard-Jones repulsive interactions. Next, the sensitivity of CO2 adsorption property to the orientations of the H2O molecules is examined (Figure 7). Initially, the H2O molecule coordinates are taken from a final MC cycle and fixed as part of the framework for the subsequent CO2 GCMC simulations. The results here are different from when the case of H2O molecules moving around but the trends of enhancement/degradation agrees well (Table S2). Next, the orientation of the frozen H2O molecules is adjusted by rotating H-O bonds on O atom to observe the rotational effects of the H2O molecules on the CO2 adsorption properties.


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Figure 7. Variation of CO2 uptake enhancement ratio with respect to the direction of H2O molecules adsorbed. One H2O coordinate from total 1,000 snapshots (1,000,000 Initialization steps and 10,000 number of cycles) is added and fixed to a framework. For comparison, three different snapshots (last, 101th and 201th from the last) are used. GCMC simulations (500,000 initialization steps and 10,000,000 number of cycles) are conducted using GPU code to calculate CO2 uptake enhancement ratio. Consequently,

in

frameworks

with

overlapping

binding

sites

(AWW

and

PCOD8306594), enhancement ratios remain unchanged regardless of the orientation of the H2O molecules (Figure 7a). This seems reasonable given that when CO2 and H2O binding sites are overlapped, their repulsive interactions do not vary with respect to the orientation of the H2O molecules. On the other hand, in cases where CO2 and H2O binding sites are close but not overlapped, we observe that the enhancement ratios are sensitive to the direction of H2O molecules. In particular, the performance of OWE and PCOD8197781 reverse from CO2 enhancement to degradation and from degradation to enhancement, respectively when the when


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H2O molecules are rotated by 180 degrees from their original positions. For randomized H2O rotations, these frameworks show an enhancement ratio close to 1 (Figure 7b). For zeolite APC that share similar properties as OWE (i.e., CO2 enhancement and close CO2-H2O binding sites), however, the enhancement ratio is unaffected by the orientation of adsorbed water molecules since CO2/H2O binding sites are located alternatively at appropriate distance in space (Figure S1(c)). Finally, for zeolite structures with separated CO2-H2O binding sites (SAS and PCOD8319944), enhancement ratio is decreased for randomized rotation of the H2O molecules, but it still remains larger than 1. Accordingly, we can conclude that the enhancement effect depends not just on the location of the binding sites but also on the specific orientations of the molecules. Finally, based on the discussed mechanisms above, a mixed form of zeolites framework can be analyzed. In many structures, CO2/H2O binding sites are not just clearly separate or overlapped, but have mixtures of all these properties. As an example, PCOD8208795, which has the highest enhancement ratio (1.84) at 300% water content and 0.1 bar, has both separated and close CO2/H2O binding site characteristics as illustrated in Figure 8. The distance histogram for PCOD8208795 has narrow tail in the range of 0 to 3 Å, showing intermediate thickness of enhanced frameworks with separated binding sites (Figure 5a) and those with close binding sites (Figure 5c and 5d).


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Figure 8. Mixed type of CO2/H2O binding site separation (CO2/H2O binding sites are clearly separated and close), distance histogram and CO2 energy contour for PCOD8208795.

4. CONCLUSIONS Hundreds of pure silica zeolite frameworks have been screened for CO2 capture at humid conditions using grand canonical Monte Carlo simulations. To simplify the analysis and secure reliable data, cations are excluded and all of the structures investigated are pure-silica zeolite frameworks. Although there aren’t any reported experimental data on enhanced CO2 adsorption uptake in pure-zeolites under wet conditions, the screening results show that CO2 uptake enhancement can occur in several structures in presence of moisture. Through detailed investigation, three types of behaviors can occur when CO2 and H2O are co-adsorbed inside a zeolite material. In the first case, CO2 and H2O binding sites are clearly separated in different channels/cages and a long-range favorable adsorbate-adsorbate interaction between CO2 and


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H2O molecules beyond a pore wall leads to improved CO2 adsorption performance. In the second case, CO2 and H2O binding sites overlap with one another and the CO2 adsorption capacity is significantly reduced with the molecules competing for the same sites. In the final case, CO2 and H2O binding sites are close to one another yet not completely overlapped. In this scenario, depending on the specificity of the structure, there can be either enhancement or degradation of the CO2 uptake in presence of water. For the enhanced structure, the reduced CO2 pore volume due to presence of water is offset by the enhanced CO2-H2O interactions that lead to stronger CO2 binding sites. In the degraded framework, this enhancement does not occur. Unfortunately, aluminosilicate zeolites of the identical frameworks, which showed enhanced CO2 adsorption uptake in siliceous zeolites at humid conditions, are expected to lose its beneficial effect due to the addition of sodium cations. Although these results are confined to just the zeolites, we expect there to be similar behaviors in other classes of porous materials (e.g. metal-organic frameworks, porous polymer networks), which can be instrumental to finding the best CO2 capture materials under realistic flue gas conditions.

Supporting Information. Simulation results of CO2 adsorption performance for representative zeolite frameworks (Tables S1-S3). Additional analysis results for representative zeolite frameworks (Figures S1-S4) and simulation results of aluminosilicate zeolites with different Si/Al ratios for the enhanced frameworks (Figure S5). The supporting Information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION


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Corresponding Author *E-mail: [email protected]. Tel: +82-42-350-7311 Author Contributions W. J. conducted the simulations, performed analysis on the results, and wrote manuscript. J. K. designed the simulations, supervised the development of work, edited the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors gratefully acknowledge the financial support from the Saudi Aramco KAIST CO2 Management Center. This research used computing resources of the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the US Department of Energy.

ABBREVIATIONS AV, accessible volume; CCS, carbon capture and sequestration; MOFs, metal-organic frameworks; IZA, international zeolite association; PCOD, predicted crystallography open database; KH, Henry coefficient; GCMC, grand canonical Monte Carlo; MC, Monte Carlo; NAV, non-accessible volume; SPC, simple point charge; Emin, the minimum average Boltzmann weighted energy; GPU, graphics processing unit.


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Understanding the Mechanisms of CO2 Adsorption Enhancement in

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