Role of Cations in CO2 Adsorption, Dynamics, and Hydration in

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Role of Cations in CO Adsorption, Dynamics, and Hydration in Smectite Clays Under in Situ Supercritical CO Conditions 2

Geoffrey M Bowers, Herbert Todd Schaef, John S. Loring, David W. Hoyt, Sarah D Burton, Eric D. Walter, and R. James Kirkpatrick J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11542 • Publication Date (Web): 19 Dec 2016 Downloaded from http://pubs.acs.org on December 22, 2016

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The Journal of Physical Chemistry

Role of Cations in CO2 Adsorption, Dynamics, and Hydration in Smectite Clays Under in situ Supercritical CO2 Conditions Geoffrey M. Bowers1*, H. Todd Schaef 2, John S. Loring2, David W. Hoyt3, Sarah D. Burton3, Eric D. Walter3, R. James Kirkpatrick4 1

Department of Chemistry and Biochemistry, St. Mary’s College of Maryland, St. Mary’s City,

MD 20686 2

Pacific Northwest National Laboratory, Richland, WA, 99352

3

William R. Wiley Environmental and Molecular Sciences Laboratory, Pacific Northwest

National Laboratory, Richland, WA, 99352 4

College of Natural Science, Michigan State University, East Lansing, MI, 48824

*Corresponding author email: [email protected]

ABSTRACT

This paper explores the molecular-scale interactions between CO2 and the representative smectite mineral hectorite under supercritical conditions (90 bar, 50°C) using novel in situ X-ray

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diffraction (XRD), infrared (IR) spectroscopy, and magic angle spinning (MAS) nuclear magnetic resonance (NMR) techniques. Particular emphasis is placed on understanding the roles of the smectite charge balancing cation (CBC) and H2O in these interactions. The data show that supercritical CO2 (scCO2) can be adsorbed on external surfaces and in the confined interlayer spaces of hectorite at 50°C and 90 bar, with the uptake of CO2 into the interlayer favored at low H2O content and when the basal spacing is similar to a monolayer hydrate of hectorite (1WL, ~12.5 Å). These results are in agreement with published spectroscopic and molecular modeling data for the related smectite Na-montmorillonite. Charge balancing cations with small radii, large hydration energies, and low polarizabilities tend to scavenge H2O from humid scCO2 or retain the H2O they held before scCO2 exposure, swelling spontaneously to a bilayer hydrate (2WL) dominated state that largely prevents CO2-ion interactions and influences the extent of CO2 intercalation into the interlayer. In contrast, ions with large radii, low hydration energies, and large polarizabilities more readily form close associations with CO2 with the energetics enabling coexistence of CO2 and H2O in the interlayer over a wide range of scCO2 humidities. Integrating our results with those from molecular dynamics simulations of wet CO2-bearing montmorillonites suggest that adsorbed CO2 in 1WL-type interlayers is oriented with its long axis parallel to the clay sheets and experiences dynamics dominated by anisotropic rotation about the axis perpendicular to the CO2 long axis at rates of at least ~105 Hz. If appreciable CO2 is adsorbed in 2WL-type interlayers, it must experience a mean orientation and dynamic averaging affects that mimic the 1WL-type adsorption environment. External surface adsorbed CO2 is dynamically similar to the 1WL case, but the CO2 long axis samples a larger range of orientations with respect to the smectite surface and adopts a different mean angle between the long axis and the smectite surface. Our data also suggest that equilibrating hectorite with a large


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volume of scCO2 at 50°C and 90 bar leads to interlayer dehydration, with the extent of dehydration correlating with the hydrophilicity of the CBC. INTRODUCTION Two-dimensional nano-confinement of fluids in layer-structure materials is well known to cause significant changes in the structural and dynamical behavior of those fluids relative to the bulk phase. The oxide-based (alumino)-silicate clay minerals are among the most important layer-structure materials because of their abundance in the natural environment and their broad range of industrial applications. Not surprisingly, the behavior of H2O in the interlayer galleries of swelling clays (smectites) has been a topic of interdisciplinary interest for many years. More recently, there has been increased attention given to the incorporation of CO2 in clay interlayers due to its possible effects on the behavior of geological C-sequestration systems.1-16 CO2 occurs in the supercritical state (scCO2; Tc ~ 31oC, Pc ~ 73 bars) in such situations, where it often interacts with shales and other rocks that contain clay minerals and variable amounts of H2O (see references 1-16 for reviews). Understanding the molecular scale CO2/H2O/smectite interactions and the chemical factors that influence them at relevant temperatures and pressures is thus critical to a more complete understanding of the physics and chemistry of subsurface sequestration strategies. Recent advances in X-ray diffraction (XRD), infrared (IR) spectroscopy, quartz crystal microbalance (QCM) and nuclear magnetic resonance (NMR) methods that allow for in situ examination of samples at elevated temperature and pressure have been useful in this regard.7, 9, 12-14, 17

Studies of clay minerals using these techniques have shown that CO2 can be incorporated

into smectite interlayers and that the extent of incorporation is greatest when there is a small amount of interlayer water present, ideally enough H2O to form a monolayer hydrate (1WL) with


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a basal spacing of ~12.5 Å. The results also show that the mole fraction of CO2 sorbed by the clay decreases with increasing H2O activity in the vapor phase and that the presence of CO2 affects the structural environments and dynamical behavior of the interlayer CBCs required to stabilize electrostatic repulsions between the clay layers. For instance, recent studies by Loring and colleagues using a novel high-pressure IR titration system7,

9, 12-13

have shown that

intercalation of CO2 in Na-montmorillonite interlayers depends critically on the amount of interlayer H2O available, with dry montmorillonite and dry scCO2 leading to no intercalation, some interlayer H2O (the equivalent of 1WL of H2O or less in the interlayer) leading to significant intercalation of CO2, and larger amounts of interlayer H2O (equivalent to a bilayer of H2O molecules or more) reducing the mole fraction of interlayer CO2. In general, the studies of Loring and colleagues as well as other authors18 do not find evidence of carbonate-forming reactions in montmorillonite at 50°C and 90 bar CO2, in contrast with some earlier reports.12, 19-20 In situ X-ray diffraction studies of montmorillonites in the presence of scCO2 by Schaef and colleagues8-9, 21 and Giesting and colleagues15-16 have shown that scCO2 can lead to interlayer expansion or contraction (contraction presumably via dehydration) depending on the conditions and overall system H2O content, in agreement with the high pressure IR results, although the exact H2O/CO2 ratio in the interlayer galleries is difficult to determine by XRD alone. Several of these papers also involve novel in situ high-pressure magic-angle spinning (MAS) NMR results that generally agree with the findings from IR and XRD.13,

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For example, Bowers and

colleagues14 have used 13C and 23Na MAS NMR spectra at 90 bar CO2 and 50°C to show that the presence of interlayer CO2 affects the structural environments and dynamical behavior of interlayer Na+ in smectite clays and also smectite-fulvic acid composites.


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In parallel with the experimental studies, computational molecular modeling using molecular dynamics (MD) and Monte Carlo (MC) methods10-11, 23-25 is providing molecular scale insight into the structural and dynamical behavior of CO2 in smectite interlayers. The MC calculations of Botan et al.10 show that the CO2/(CO2+H2O) ratio in the interlayer (interlayer mole fraction CO2) of Na-montmorillonite in equilibrium with both a CO2-rich and H2O-rich external fluid phase at 75°C increases with increasing pressure and decreases with increasing interlayer expansion. For 1WL and 2WL hydrate structures, the interlayer mole fraction of CO2 is in the range of ~3 – 12%, is greater than in the coexisting H2O rich phase, and is larger for the 1WL structure. The results show that the CO2 molecules are at the center of the interlayer and that the orientation of their O-C-O axis is parallel to the basal clay surface for 1WL interlayers. For the 2WL hydrate, the CO2 molecules are displaced from the center of the interlayer towards the basal surfaces, are not oriented parallel to the basal surface, but are also not oriented perpendicular to it despite the fact that such an orientation is geometrically possible. The MD results of Sena et al.11 for the 1WL hydrate of Na-montmorillonite with an interlayer CO2 mole fraction of 0.25 also show that the O-C-O axis of the CO2 is parallel to the basal clay surface. They find that the CO2 forms clusters or chains composed of a few molecules, with CO2-CO2 interactions playing an important role in forming the clusters. The CO2 molecules are mostly oriented in the so-called slipped parallel orientation with respect to each other, although some have a distorted Torientation. These clusters and chains provide percolation paths for diffusion of the CO2 molecules parallel to the basal surface. The clusters are also dynamic, breaking and reforming many times during the 20 ns duration of the simulation. In addition, these results show that the H2O molecules cluster around the Na+ ions and that few of the Na+ are coordinated by OCO2. The MD results of Krishnan et al.25 for a 1WL-type Na-montmorillonite interlayer (1WL-type


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referring to an interlayer with a ~12.5 Å basal spacing but with variable H2O from zero up to the level required to form a 1WL hydrate) containing only interlayer CO2 (interlayer mole fraction = 1) shows that the OCO2 are centered at the mid-plane of the interlayer, that the average position of the OCO2 is also at this position, but that most of the time the O-C-O axis of the CO2 molecules is not in this orientation but tipped relative to the basal surface. The angular distribution has a fullwidth at half-height of ±40o from parallel, but the CO2 molecules are almost never oriented perpendicular to the basal surfaces, in agreement with the MC simulations of Botan et al.10 It is well known that the hydration energy and polarizability of the interlayer cation greatly affects the structure, dynamics and expansion behavior of smectite clays.26-32 Thus, it is likely that CO2-clay interactions will be cation dependent given the importance of the interlayer hydration state demonstrated in the Na-montmorillonite studies summarized above. The results of Schaef and colleagues9 for montmorillonite-CO2-H2O systems with interlayer Na+, Ca2+, and Mg2+ using in situ scCO2 XRD, IR, and QCM experiments, in combination with those of Loring and colleagues,12 confirm that the conclusions drawn from the Na-montmorillonite systems apply to montmorillonite with any of these cations with high affinities for H2O (reflected in their relatively high hydration energies). With these cations, there is little interlayer incorporation of CO2 for anhydrous clays; CO2 incorporation in the interlayer is a maximum for 1WL-type interlayers; and adsorbed CO2 decreases at higher hydration states. The QCM results show that Ca-montmorillonite incorporates more CO2 than Na-montmorillonite at a given PCO2, as expected since fewer Ca2+ ions are required to balance the negative structural charge of the clay, thereby providing more interlayer sorption sites for CO2. The difference between the interlayer CO2 uptake with different cations decreases with increasing PCO2. The results also show that CO2 incorporation is a maximum near 90 bars for both Ca- and Na-montmorillonite and that there is


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significant CO2 adsorption on external surfaces that increases with increasing PCO2. These results are consistent with the in situ XRD results of Giesting et al.15-16 for Na- and Ca-montmorillonite. Little is known, however, about the behavior of smectites in contact with scCO2 when they contain cations with low hydration energies, such as Cs+ and K+. To the best of our knowledge, the XRD work of Giesting et al. on K-montmorillonite are the only published results for such materials.16 Overall, the expansion behavior of K-montmorillonite with high pressure CO2 and variable H2O is similar to that of Ca-montmorillonite, although the kinetics of expansion in the K-bearing system is faster. For K- and Ca-montmorillonite, the basal spacing decreases after exposure to CO2, indicating removal of H2O from the interlayers due to equilibration with bulk scCO2. This paper examines the smectite mineral hectorite containing CBCs with both high hydration energies (Na+, Ca2+) and low hydration energies (Cs+) at 50°C and 90 bar CO2 using a combination of in situ NMR, XRD, and IR. Many of the NMR results presented here were acquired using a newly developed CO2-charging system that largely prevents H2O loss from the NMR rotor during equilibration with scCO2. These experiments build on our previous results for Na-hectorite that show scCO2 influences the structural environments and relaxation behavior of the charge balancing Na+.14

As in our previous NMR studies of interlayer structure and

dynamics, we use hectorite rather than montmorillonite, because the San Bernardino hectorite has a very low Fe content and yields NMR spectra with much higher sensitivity and resolution than with most montmorillonites. Interpretation of the experimental observations draws heavily on recently published molecular modeling results for smectite clays containing H2O and CO210-11 and provides significant new spectroscopic insight into the molecular scale structure and dynamics of CO2 in smectite clay interlayers. We find that a fraction of CO2 molecules


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associates with the external surfaces of smectite at low RH in the scCO2 fluid; that some CO2 is incorporated into interlayers with ~12.5 Å basal spacing; that the total CO2 uptake, mole fraction of CO2 in the interlayer fluid, and the ratio of surface-to-interlayer CO2 depends on the CBC and available H2O; and that Cs-hectorite adsorbs a larger fraction of CO2 molecules at all conditions than Ca- or Na-hectorite. The

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C NMR spectra show that adsorbed CO2 exhibits rotational

dynamics about an axis perpendicular to the molecular long axis, with the long axis remaining parallel to the clay surface in 1WL-type interlayers (basal spacing of ~12.5 Å) and at a different mean orientation with respect to the basal surface when the molecule is adsorbed at the external surfaces. Either CO2 is excluded from 2WL-type interlayers (smectites with basal spacing ~15 Å) or exhibits a similar mean orientation and dynamic averaging effects to the 1WL-type interlayer case. Small amounts of bicarbonate and/or carbonate are observed in samples where the overall H2O/CO2 ratio is 1-1.5, but despite our best efforts to remove carbonate contamination from the parent hectorite, it appears that this material was present before exposure to scCO2.

MATERIALS AND EXPERIMENTAL METHODS Materials and Sample Preparation The smectite used in our studies is the natural San Bernardino hectorite available from the Source Clays Repository of the Clay Mineral Society, SHCa-1. We used the < 1 µm fraction isolated via differential centrifugation, which was shown previously to have the structural formula (Na0.19Mg0.07Sr0.01K0.01)+0.36[(Mg2.65Li0.35)-0.35(Si3.99Al0.01)O10(F1.1OH0.9)].28 Note that the Ca-hectorite was prepared with

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Ca-enriched material to facilitate rapid acquisition of NMR

data for this otherwise challenging NMR nucleus. Ion exchange procedures to generate saturated


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smectites followed those used previously by our group.31-32 IR and TGA analysis show that the < 1 µm fraction isolated via differential centrifugation does contain a small amount of carbonate or bicarbonate amounting to < 1% on average (see Supporting Information). For the NMR experiments, different initial hydration states of the hectorite samples were prepared by equilibrating the clays in appropriate relative humidity (RH) buffers identified from controlled RH X-ray diffraction experiments performed and published previously.31-32 Hectorite samples equilibrated under conditions that produce a system dominated by interlayers containing enough H2O molecules to form two water layers (a bilayer hydrate) are denoted as 2WL in the remainder of the paper. Systems with variable H2O/CO2 ratios but that exhibit a basal spacing similar to the 2WL state (~15 Å) are called 2WL-type interlayers. Samples equilibrated under conditions that produce a system dominated by interlayers containing one monolayer of H2O are denoted as 1WL, with systems containing variable H2O/CO2 ratios that exhibit a similar ~12.5 Å basal spacing denoted as 1WL-type interlayers.

The nominally dry NMR samples were

produced by equilibrating initially freeze-dried samples over P2O5, however, they were exposed to the lab atmosphere briefly during rotor packing and shipping to PNNL. They most likely to contain small and variable amounts of H2O and are denoted as

Role of Cations in CO2 Adsorption, Dynamics, and Hydration in

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