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Molecular simulation of preferential adsorption of CO2 over CH4 in Na-montmorillonite clay material
Accepted Manuscript Title: Molecular Simulation of Preferential Adsorption of CO2 Over CH4 in Na-montmorillonite Clay Material Author: Nannan Yang Shuyan Liu Xiaoning Yang PII: DOI: Reference:
S0169-4332(15)01914-5 http://dx.doi.org/doi:10.1016/j.apsusc.2015.08.101 APSUSC 31048
To appear in:
APSUSC
Received date: Revised date: Accepted date:
6-5-2015 24-7-2015 12-8-2015
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Molecular Simulation of Preferential Adsorption of CO2 Over CH4 in Na-montmorillonite Clay Material Nannan Yang1, Shuyan Liu2, Xiaoning Yang1,* College of Chemistry and Chemical Engineering, 2: School of Mechanical and Power Engineering
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Nanjing Tech University,Nanjing 210009, China
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1: State Key Laboratory of Materials-Oriented Chemical Engineering,
ABSTRACT: Grand canonical Monte Carlo simulations have been conducted to study the adsorption of carbon dioxide and methane, as well as their binary mixtures
the
clay
structure
for
the
two
an
on Na-montmorillonite clay material. It was found that the adsorption behavior near species
is
distinctively
different.
The
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Na-montmorillonite clay shows obviously high adsorption capacity for CO2, as compared with CH4. The adsorption behavior and mechanism have been
d
characterized by the interlayer interfacial structures and isosteric heats of adsorption.
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Meanwhile, the mixture adsorption demonstrates that CO2 molecules with enhanced adsorption strength are able to competitively replace CH4 molecules within the clay
Ac ce p
structure. The high separation selectivity of CO2 over CH4 implies the possibility of separating CO2 from natural gas mixtures using the clay minerals. The interlayer
sodium cations and negatively-charged clay surface can provide enhanced interaction with CO2 molecules that have high quadrupole moment, which is
responsible for the higher adsorption loading of CO2. Keywords: montmorillonite clay, CO2, CH4, adsorption, separation, molecular simulations.
*Corresponding author, E-mail address: [email protected] 1
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1. Introduction The effect of increasing atmospheric CO2 concentration on global warming is considered as important environmental concern for society [1]. At present, various
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strategies and technologies for CO2 capture and sequestering have been proposed to
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reduce CO2 burden on the atmosphere. In this aspect, adsorption is the potential
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technique that is feasible and efficient in CO2 capture [2]. In general, development of adsorbents with high adsorption capacity and selectivity is essential. Recently, various
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types of porous adsorbents, such as activated carbons, zeolites [3-5], and metal organic frameworks (MOFs) [6-9], have been claimed to offer promising applications
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for CO2 adsorption and separation. As an readily available cheap material, naturally occurring clay minerals might provide an attractive alternative to more complex
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functional synthesized materials as adsorbents [10]. Furthermore, as the major impurity in natural gas, the presence of CO2 can reduce the conversion rate and
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energy content of natural gas. As a result, the removal of CO2 from natural gas
mixture is very critical in many industrial fields. There is an increasing need to develop new-typed porous adsorbents to remove CO2 from natural gas with low-cost
and high-energy efficiency.
From another perspective, long-term storage of CO2 in different geological sites
[11], including gas-rich shales, active or depleted oil and gas fields, deep saline aquifers, unminable coal seams, has been suggested as an important approach for CO2 sequestration. Among these geological sequestration options, that, which allows the production of additional byproduct, such as methane gas, is the most attractive
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approach [12]. This includes sequestration of CO2 in geological formations with the simultaneous recovery of CH4. For example, the BP-Amoco Corporation has demonstrated that CH4 could be effectively removed and recovered from deep,
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unmineable coals using CO2 injection. Shale gas reservoir has been considered as an
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important energy resource in recent years [13]. Shale is mainly composed of organic
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kerogen materials and inorganic clay minerals. It has been suggested that high pressure injection of CO2 into the shale rocks could benefit the methane removal. The
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mechanism of the process is CO2 has stronger adsorption affinity with clay materials, which can efficiently replace CH4 already adsorbed within the shale microspores.
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Understanding the CO2-clay and CH4-clay adsorption interactions is an important fundamental aspect for the above applications. In particular, it is highly required to
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clarify the competitive adsorption behavior between CO2 and CH4 in the clay confined pores. Reliable prediction of the gas-retention adsorption capacity for
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CO2/CH4 mixtures is necessary for the validity evaluation of the simultaneous CO2 capture and CH4 removal in geological formations. Meanwhile, predictions
concerning the process stability require the knowledge of how CO2/CH4 molecules
are held within the clay pores.
In general, clay structures are comprised of layered aluminosilicates with
negatively charged due to tetrahedral substitutions of Si by Al or octahedral substitutions Al by Mg. Cations in the interlayer spaces compensate the negative charge of mineral layers and hold the clay sheets together. The typical montmorillonite interlayer cations are Na+, K+, Ca2+, and Mg2+. The layer spacing
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between neighboring montmorillonite layers is generally variable. Clay minerals have found extensive application in several areas such as catalytic activity, filtration of fluids, and waste disposal process [14, 15].
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Experimental and computational studies always focused on the structure and
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dynamics of interlayer water and counterions in clay pores [16-24]. In addition, there
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are quite a few experiments on the adsorption of heavy metals on clays [25-27], demonstrating that clay mineral could be an excellent adsorbent. Currently, the
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adsorption capacity of CO2 on clay materials still remains unresolved. Very limited experimental studies have shown that adsorption ability of CO2 in hydrated clays is
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relatively lower [28, 29]. Experiment [30] shows that the gas adsorption capacity on clay minerals under moisture-equilibrated condition is significantly reduced as
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compared with that under dry condition. The interlayer water molecules might suppress the intercalation of CO2 molecules into the clay pores [31]. Ross [32]
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experimentally confirmed that the gas sorption capacity of shales under dry conditions is substantially larger than under moisture equilibrated condition. In the situation without interlayer water molecules, the nanoscale clay pore is expected to provide the maximum nanochannel for CO2 intercalation, and thus maximizing the adsorption ability of CO2 molecules. In recent years, there are several molecular simulation
reports on the interfacial structural and dynamics properties for CO2 intercalated within clay pores [33-36]. However, these studies mainly focus on the CO2 formation in the clay layer structures and did not provide microscopic interaction mechanism, in particular, the effect of interlayer counterions is not clear for CO2 adsorption.
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Meanwhile, the competitive adsorption and interaction mechanism for CO2/CH4 mixtures on clay structures remain largely unexplored. With the above in mind, in this work, grand ensemble Monte Carlo (GCMC)
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simulation was employed to investigate the adsorption of pure CO2 and CH4, as well
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as CO2/CH4 mixtures on the common Na-montmorillonite. For this purposes,
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adsorption isotherms from low pressure to 20 MPa have been simulated. Meanwhile, the interfacial structural properties and the isosteric heat of adsorption were used to
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explore the adsorption mechanism. In the previous work [33], the mechanics and thermodynamics stability of clay materials shows that there exist multiple stable
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spacings of clays in the presence of supercritical CO2. Therefore, in this work, two types of pore widths with the basal spacing 14.0 Å and the spacing 20.6 Å were
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investigated. This paper is organized as follows: the intermolecular potentials and simulations methodology will be given in the following section. Then we present the
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results and discussion, and paper concludes with a brief summary and conclusion of our research in the last section.
2. Models and Simulation Method The clay model used in this simulation is the Na-montmorillonite whose molecular
formula is Na0.75 [Si8] (Al3.25Mg0.75) O20 (OH) 4[37, 38]. The negative charge caused
by the substitution of Mg2+ for Al3+ in the octahedral sheet is balanced by the interlayer Na+ cations. The simulation cell is a patch with two half layers of the montmorillonite sheet, and with the interlayer space in between. To model the montmorillonite structure, three-dimensional periodic boundary condition was applied
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to a simulation box composed of 32 (8×4×1) unit cells, so making up a clay patch of 42.24 Å(x-dimension) by 36.56 Å (y-dimension), and 6.56 Å in the z dimension. We selected two stable basal spacings: a smaller spacing 14.0 Å and a larger spacing
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20.6Å. The two clay spacings were considered based on the previous study [33].
Lennard-Jones (L-J) and Coulomb terms:
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(1)
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Etotal
⎧ ⎡⎛ σ ⎞12 ⎛ σ ⎞6 ⎤ q q ⎫ ⎪ ⎪ = ∑∑ ⎨4ε ij ⎢⎜ ij ⎟ − ⎜ ij ⎟ ⎥ + i j ⎬ ⎜ ⎟ ⎜ ⎟ ⎢ r i j >i ⎪ ⎝ rij ⎠ ⎥⎦ rij ⎪⎭ ⎩ ⎣⎝ ij ⎠
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The total potential energy (Etotal) of the simulated system involves the
where ε and σ are the energetic and size parameters, respectively, in the L-J potential.
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qi is the charge of particle i. The long-range Coulomb interaction was handled using the Ewald technique. Carbon dioxide was treated with the three-site EPM2 model [39,
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40]. Methane is represented by a single L-J sphere [41] in the TraPPE force field, which was developed to reproduce the properties of alkanes. The interactions
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involving the clay and adsorbate follow the methods described by Boek et al[42]. The parameter for clay is derived directly from the models introduced by Skipper et al [19,
37, 43]. All the interaction parameters for CO2, CH4, and clay were listed in Table S1. In this work, MUSIC simulation code [44] was used for the GCMC simulation,
wherein the temperature T (318.15 K), volume V, and chemical potential μ were fixed. A series of bulk phase pressures (up to 20 MPa) were chosen and the corresponding chemical potentials of bulk phase were calculated using the Peng-Robinson equation of state (P-REOS) [45]. In general, four types of trial moves were applied for the gas molecules in the GCMC simulation: attempts to translate a molecule; attempts to
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insert a molecule, attempts to delete an existing molecule, and attempts to rotate a molecule. For the sodium counterions confined within the clay pore, only the translation move was adopted. The energy biasing technique [46, 47] was applied for
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the molecule insertion. Rigid model was used for the clay structure throughout this
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work. For each simulation run, the GCMC simulation consisted of 1×107 steps, the
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first half of these steps was used to guarantee equilibration, and the remaining steps were taken for calculating the ensemble averages. To ensure fast GCMC equilibrium,
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the final configuration of the last run was used as the initial configuration of the next
3. Results and Discussion
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Single component adsorption
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run.
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Figures 1(a) and 1(b) depict the absolute adsorption isotherms of pure gases (CO2 and CH4) on the Na-Wyoming montmorillonite, as a function of pressure on
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the basal spacing of 14.0 Å and 20.6 Å, respectively. It is noticed that the CO2
isotherms exhibit a steep rise at low pressure. The curve shape can be described as type-Ι (Langmuir type), which is characteristic of microporous adsorbent with pore sizes of molecular dimensions [48]. Comparatively, the adsorption isotherm of CH4 demonstrates a gradual rise with pressure. The adsorption amount of CO2 is
obviously larger than that of CH4, which could be attributed to CO2 molecules that have higher interaction with Na-montmorillonite Clay. In particular, the existing quadrupole of CO2 can induce stronger electrostatic interaction with the charged clay structures. It is observed that the statured adsorption amount is ~5.1 mmol/g for CO2 7
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in the smaller spacing (12.6 Å) and ~13.0 mmol/g in the larger spacing (20.6 Å). The adsorption amount is comparable to the experimental data [49] for CO2 on carbon materials. Although this CO2 adsorption on the clay is less than the value on the
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synthesized IRMOF-1 adsorbent [50], the economical feature of natural clay sources
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still demonstrates that it is attractive porous adsorbent for CO2 capture.
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A careful inspection of Figure 1(a) (see insert) shows that, at low-pressure range (below 0.1 MPa), the adsorption amount of CO2 in the small pore (14 Å) is larger
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than that in the larger pore (20.6 Å). This phenomenon is due to the relative stronger surface interaction in the 14.0 Å spacing that has two clay planes. At lower pressure
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condition, owing to weak adsorption, the adsorbate-adsorbent interaction is the
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controlling factor determining the adsorption performance. With the pressure
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increasing, more adsorbate molecules present in the confined clay pore and the effect of adsorbate-adsorbate interaction becomes pronounced, inducing an enhanced
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adsorption. As a result, under higher pressure, the larger clay pore (20.6 Å) with more adsorption space available is able to produce the larger adsorption, as compared with the smaller pore (14.0 Å). Figures 1(c) and 1(d) present the corresponding excess adsorption isotherms. The
excess adsorption amount, nex, is related to the absolute adsorption amount, nab, from
the following equation: n ex = n ab − V g ρ g
(2)
where Vg is the pore volume of clay, ρg is the molar density of the bulk fluid phase,
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which was estimated from the P-R EOS [45]. Each excess adsorption isotherm shows a maximum in the medium pressure range, which is typical of high pressure adsorption. This phenomenon is usually caused by a significant change in the bulk
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phase fluid density near its critical point. The simulated isotherms of CO2 are
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obviously higher than the experimental ones in the hydrolyzed Na-montmorillonite
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clay with a relative humidity of 40-60% [28]. This behavior is due to the fact that the interlayer water impedes CO2 adsorption.
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The local density distributions for pure CO2 and CH4 molecules adsorbed in the interlayer region of clay were presented in Figure S1 of supplementary material. In
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the smaller clay pore, usually one density peak appears on the middle of pores.
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However, at higher pressure (5 MPa); two symmetrical separated peaks are formed in
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the density profile. In the large pore (20.6 Å), the density distribution of CO2 exhibit two significant peaks close to the clay surfaces, and two shoulder peaks develop on
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the middle of clay pore at 5 MPa. With the pressure increasing, the primary peaks become more close to the clay surface. Comparatively, the density peak of CH4
molecules is obviously low. This is consistent with the lower adsorption amount of CH4.
The corresponding configuration snapshots for the gas adsorption in the clay pores
were shown in Figures S2 and S3. It was observed that Na+ cations are generally located closer to the surface than gas molecules. At lower pressure, CO2 molecules preferentially occupy the position near Na+ counterions. With pressure increasing, CO2 and CH4 molecules can occupy the positions near the clay surface, and the 9
Page 9 of 32
adsorption in the midplane is due to the enhanced intermolecular interactions. The snapshots also show CO2 molecules have an orientation preference in parallel with the
confined within aluminum silicate layers [51] and clay pore [33].
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clay sheets, which is in good agreement with the previous results regarding CO2
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In order to quantitatively evaluate the interaction strengths of CO2 and CH4 with
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the clay, we also computed the isosteric heats of adsorption (Qst). Qst is defined as the difference between the partial molar enthalpy of adsorbates in the bulk phase and the
(
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partial molar internal energy in the adsorbed phase [52]:
)
(
a
∂n a
)
T
(3)
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Qst = − hb ,* − hb + RT − ∂U
where -(hb,*-hb) stands for the departure function of the partial molar enthalpy of bulk
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fluid and it was calculated by the PR-EOS of fluid. Ua is the average internal energy
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of adsorbate in the adsorbed phase, obtained from the GCMC simulation. n is the
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adsorbed number of CO2. The partial derivative term in Equation (3) was generally evaluated in two different methods: numerical differentiation and ensemble fluctuation [47,52]. In this work, we used the numerical differentiation method. The Qst is mainly determined by the two components: one is the molar enthalpy -(hb,*-hb)
(Hb for short) for the bulk phase, and the other is the partial molar internal energy of
(
adsorbate - ∂U
a
)
∂n a T (Ua for short). The Ua can be further decomposed into the
a contributions from the adsorbate-adsorbate interaction ( U gg for short), and the
adsorbate-adsorbent interaction ( U gsa for short). Figure 2 shows the isosteric heats of adsorption of CO2 and CH4 as a function of
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the adsorbed amount in the two clay spacings. The adsorption heat ( Qst0 ) in the limit of zero loading can be used to characterize the strength of surface interaction. For CO2, The Qst0 is ~ 55.4 kJ/mol in the small pore and ~ 40.5 kJ/mol in the large pore.
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However, this value is comparable to that in NaX [54].
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The infinite adsorption heat is smaller than those in MOF-177 [6] and in silicalite [53].
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It is observed in Figure 2(a) and (b), that the Qst of CO2 decreases at first with the adsorption loading, revealing the heterogeneous nature of the clay surface. This
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changing behavior of Qst is not unusual. Similar behavior has been observed for gas adsorptions on other porous materials [50, 55-57]. A decomposition of Qst shows that
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the decrease at the adsorption beginning is due to the contribution ( U gsa ) of dominated
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adsorbate-adsorbent interaction. This demonstrates that CO2 molecules first occupy
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the more energetically favorable sites in the confined clay pores, and then adsorb at the less favorable sites, corresponding to a gradually decreasing contribution ( U gsa ). A
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further split of U gsa can reveal that its loading dependence is corporately determined by the CO2-Na+ interaction and the CO2-surface interaction (see Figure S4). At the
initial adsorption stage, the large adsorption heat of CO2, along with the rapid adsorption in the adsorption isotherm (Figure 1), is mainly controlled by the strong attraction of the CO2 quadrupole with the positive charged sodium ions. As discussed in the preceding section, the adsobate-adsorbent interaction provides important contribution to the CO2 adsorption heat on clay. In order to further understand the molecular origin of the gas-solid interaction, the average normalized interaction energies for one CO2 molecule in the adsorption phase with the 11
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Na-montmorillonite Clay was decomposed into the electrostatic component and the L-J component. As shown in Figure S6, the electrostatic interaction between CO2 and sodium ions provides the main contribution to the CO2-clay interaction energy,
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especially at the lower adsorption loading. This confirms again the conclusion that the
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electrostatic interaction from charged Na+ is responsible for the higher adsorption heat
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of CO2. Comparatively, the electrostatic interaction from the clay layer is minor.
a ) increases monotonically in the two The adsorbate-adsorbate contribution ( U gg
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spacings clays and it has certain effect on the adsorption heat at high adsorption loading. This can result in an increase in the adsorption heat (Qst) in the larger pore. It
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is noticed that the isosteric heat of adsorption shows a final rapid decrease at very
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higher loading. This behavior is ascribed to the corresponding contribution from the
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fluid density.
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bulk enthalpy -(hb,*-hb), which changes the shape of the isosteric heat curves at high
Comparatively, as shown in Figure 2(c) and 2(d), the adsorption heat of CH4 is
obviously smaller than that for CO2, which agrees with the lower adsorption amount of CH4. This signifies the weak interaction strength of CH4 with the clay. The Qst0 for
CH4 is just 18.9 kJ/mol in the small pore (also see Figure S5), in agreement with the experimental value (16.0 kJ/mol) on the pillared clays [58]. It is noted that the adsorption heat of CH4 firstly increases and then decreases with the adsorption loading. The dependence of Qst on the adsorption loading is consistent with the observation for the CH4 adsorption on other microporous materials [50, 52, 57, 59]. This behavior can be attributed to the fairly homogeneous adsorption feature for 12
Page 12 of 32
nonpolar CH4 in the clay pores, wherein the methane-clay interaction remains less change during the adsorption process.
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Adsorption of CO2/CH4 mixtures In this work, GCMC simulations were further performed on the competitive
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adsorption of CO2/CH4 mixtures in the clay. Figure 3 displays the adsorption
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isotherms for the binary mixtures as a function of bulk phase compositions at the pressures of 0.1 MPa and 1.0 MPa. With the mole function of CO2 increasing, the
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adsorption amount of CO2 rises steadily, while the adsorption of CH4 decreases. This
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result represents the competitive adsorption behavior between the two species. It is found that CO2 is more preferentially adsorbed than CH4 within the clay structures
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due to the stronger interaction between CO2 and clay. This is consistent with the
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different adsorption amounts of pure CO2 and pure CH4, as demonstrated in the
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preceding section. In the 14.0 Å pore, the interlayer space is fully filled with CO2 molecules and the adsorption loading of CH4 is almost negligible when the mole fraction of CO2 in bulk phase exceeds 0.5. As expected, at high pressure condition, the adsorption amounts of both CO2 and CH4 become higher. Our simulation provides a
theoretical support that CO2 molecules could competitively replace CH4 molecules in
Na-montmorillonite clay.
To further evaluate the adsorption separation performance of the clay material for CO2/CH4 mixtures, we calculated the adsorption selectivity (S) of carbon dioxide with respect to methane as follows,
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S =
xCO2 xCH 4
(4)
yCO2 y CH 4
where x, y are the mole fractions of the two species in the adsorbed and bulk phases,
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respectively. The selectivity in the clay structures with two kinds of spacings is shown
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in Figure 4. For the two pore widths, the selectivity is much greater than unity, confirming that the clay material has high adsorption separation performance for
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CO2/CH4 mixture. The adsorption selectivity in the small pore is larger than that in the
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large pore. This can be attributed to the relatively low adsorption amount of CH4 in the small clay pore. For instance, in the small pore size (14.0 Å), the adsorption
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amounts of CH4 almost approaches to zero with yCO2 increasing (see Figure 9(a)). According to our simulation results, for the clay structure with the 14.0 Å
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spacing, the separation factor can be achieved to be 110.7 at 0.1 MPa with the bulk phase yCO2=0.2, which represents a CH4-rich content in natural gas, as seen from
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Figure 4. For the larger pore (20.6 Å), the selectivity also reaches the range of 26-70, depending on various pressures. This simulated selectivity of clays is obviously large than the simulated values for other materials. For example, the selectivity was reported to be ~10 in the C60 intercalated graphite at 0.1 MPa and 298 K [57], 6.2 in
Cu-BTC, and 1.8 in MOF-5 [60]. At yCO2=0.5, the separation factor is generally in the range of 25.0-46.8 in the larger clay pore. The separation factor of equimolar CO2/CH4 mixture has also been reported in other porous materials. For example, the selectivity is only 6 in Cu-BTC and 2 in MOF-5 [60]. In short, the CO2/CH4
separation performance of clay structure shows superiority over some current porous
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materials. It is demonstrated that Na-montmorillonite could be considered as promising porous adsorbent to separate CO2 from the natural gas mixtures. Moreover, this result also suggests that the clay-based geological formations could be applied for
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concurrently capturing CO2 and removing CH4.
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For the two sizes of clay pores, the adsorption selectivity behaves very differently.
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It is observed that the adsorption selectivity shows a decreasing behavior with yCO2 increasing in the larger pore (20.6 Å). This can be explained as the increase in the
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CH4 adsorption due to sufficiently large pore space. However, the selectivity shows fluctuation in the 14.0 Å spacing clay. This behavior reflects the complicated packing
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features of the mixture in the small confined pore. From Figure 4, it can be found that
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the selectivity at 0.1 MPa is significantly larger than that at 1 MPa. The pressure
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influence on the selectivity is similar with the observation of C2H6/CH4 in the pillared clay [58]. From our simulation result, the clay materials with larger pores generally
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show satisfactory separation performance from both adsorption capacity and adsorption selectivity.
For an improved understanding of the mixture adsorption behavior in the clay pores,
we also investigated the interlayer structure properties for the mixture. As shown in Figure 5, in the smaller pore, the density distributions of CO2 and CH4 molecules
present relatively broad peaks at the middle region of pore. This is identical with the confined configurations of pure species in the clay pore (Figure S1). The density distribution of CO2 molecules shows a transition from one to two layers with an increase in the bulk concentration. From the snapshots in the small pore, we can see 15
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the CO2 molecules are homogeneously distributed in the center region and obviously fewer CH4 molecules also appear in the midplane. Comparatively, the sodium cations
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are always located close to the clay plane. In the larger spacing, the density distributions of CO2 and CH4 show two separated
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symmetrical peaks. As shown in Figure 6, the density peak of CO2 is close to the
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plane, as compared with CH4. The corresponding snapshots also show that CO2/CH4 molecules can coexist near the clay plane. This molecule packing arrangement is
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consistent with the previous result [33] for the intercalation of supercritical CO2 molecules within the clay pores. Under higher pressure (1.0 MPa), similar behavior
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can be observed, as shown in Figures 7 and 8. In the larger pore size (20.6 Å), the
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CH4 molecules are repelled from the clay plane as the CO2 bulk composition
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two species.
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increases. This behavior again demonstrates the different surface interactions for the
The presence of interlayer counter cations is expected to have significant effect on
the adsorption performances of CO2 and CH4. Previous experimental studies [61]
have indicated that the enhanced adsorption of CO2 on NaA and NaX zeolites can be
ascribed to the strong interaction between the high quadrupole moment of CO2 and the
extra-framework cations. In order to explore the different interactions between the
sodium counterion and the CO2/CH4 mixture species in the clay pores, we show the radial distribution functions (RDFs) between the Na+ cations and the mixture components in the two clay pores in Figure 9. The position of the first peak in the Na-CO2 RDF is at ~3.8 Å, which is coincident with the coordination of pure CO2 near 16
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the sodium cations in the clay pores [33]. Similar result has been reported previously in the simulation [62] for the supercritical CO2 adsorption in the NaA zeolites.
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The main peak of Na-CH4 is relatively low and broad, and occurs at a distance of ~5.5 Å. From the RDFs of the CO2/CH4 mixtures, the separation position of first
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peaks of the Na-CO2 RDFs is obviously shorter than those of the Na-CH4 RDFs. This
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indicates that there is a preferential attractive accumulation of CO2 molecules near the Na+ cations, meaning the existence of strong interaction between the CO2 molecules
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with quadrupole moment and the positively charged Na+. This interaction could lead
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to a greater selectivity of CO2 over CH4 within the clay pores.
4. Conclusions
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The adsorption of pure CO2 and pure CH4, as well as their binary mixture on the
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Na-montmorillonite clay were studied using the GCMC simulations. The adsorption
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isotherms, the isosteric adsorption heats, and the interfacial properties have been simulated. The adsorption amount of CO2 on the clay material is obviously larger than
that of CH4. The simulation configurations show that the CO2 molecules arrange
themselves near the clay plane in a parallel arrangement with the respect to the clay sheet. The adsorption heats of CO2 and CH4 on the clay pores have been applied to characterize the adsorption interaction strengths for the two species. The enhanced interaction acting on the CO2 molecules from the negatively charged clay surfaces and the positive sodium counterions is responsible for the higher adsorption amount of CO2. Meanwhile, the CO2/CH4 mixture adsorption behavior has also been simulated for various pressures, bulk compositions, and pore sizes. A higher CO2 selectivity can 17
Page 17 of 32
be achieved with the clay. This simulation result demonstrates that CO2 molecules competitively replace CH4 molecules within the clay confined pores, suggesting that the clay materials can be used to separate CO2 from the gas mixture. A further
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configuration analysis of the mixture indicates that CO2 molecules are preferentially
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adsorbed at the position close to the clay surface, as compared with the CH4
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molecules. In conclusion, our simulation shows that the clay material with economical feature is an attractive porous adsorbent for CO2 capture and separation. In particular,
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the simulation result provides the theoretical support that geological storage of CO2 possibly allows the production of additional methane byproduct, already adsorbed
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within the gas-rich shales.
Conflict of Interests
Acknowledgement
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The authors declare no competing financial interests.
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This work was supported by the Natural Science Foundation of China under Grant
21176114,
21376116,
Research
funding
from
State
Key
Laboratory
of
Materials-Oriented Chemical Engineering (ZK201404), and A PAPD Project of Jiangsu Higher Education Institution.
Appendix A. Supplementary data: Supplementary data associated with this article can be found in the online version.
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and nitrogen on zeolite 13X at high pressures, J. Chem. Eng. Data 19 (2004) 1095-1101. [4] P.J.E. Harlick, F.H. Tezel, An experimental adsorbent screening study for CO2 removal from N2, Microporous Mesoporous Mater. 76 (2004) 71-79. [5] D. Aaron, C. Tsouris, Separation of CO2 from flue gas: A review, Separation Science and
ip t
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Carlo and Molecular Dynamics Simulations, J. Phys. Chem. B 107 (2003) 2281-2290. [25] S.H. Lin, R.S. Juang, Heavy metal removal from water by sorption using surfactant-modified
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carbon Dioxide, J. Phys. Chem. A 118 (2014) 7454-7468.
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Page 21 of 32
Density Functional Theory Study, IEC Res 43 (2004) 6577-6586. [46] S. Chempath, R.Q. Snurr, J.J. Low, Molecular Modeling of Binary Liquid-Phase Adsorption of Aromatics in Silicalite, AIChE J. 50 (2004) 463-469. [47] X.P. Yue, X.N. Yang, Molecular Simulation Study of Adsorption and Diffusion on Silicalite for a
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Benzene/CO2 Mixture, Langmuir, 22 (2006) 3138-3147.
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[55] J.W. Jiang, S.I. Sandler, Separation of CO2 and N2 by Adsorption in C168Schwarzite: A Combination of Quantum Mechanics and Molecular Simulation Study, J. Am. Chem. Soc. 127 (2005) 11989-11997. [56] A. Khelifa, L. Benchehida, Z. Derriche, Adsorption of carbon dioxide by X zeolites exchanged with Ni2+ and Cr3+: isotherms and isosteric heat, Journal of Colloid and Interface Sci. 278 (2004) 9-17. [57] X. Peng, D.P. Cao, W.C. Wang, Computational Study on Purification of CO2 from Natural Gas by C60 Intercalated Graphite, Ind. Eng. Chem. Res. 49 (2010) 8787-8796. 22
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[58] P.R. Pereira, J. Pires, M.B. de Carvalho, Adsorption of methane and ethane in zirconium oxide pillared clays, Separation and Purification Technology 21 (2001) 237-246. [59] T. Vuong, P.A. Monson, Monte Carlo Simulation Studies of Heats of Adsorption in Heterogeneous Solids, Langmuir 12 (1996) 5425-5432.
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[60] Q.Y. Yang, C.L. Zhong, Molecular Simulation of Carbon Dioxide/Methane/Hydrogen Mixture Adsorption in Metal-Organic Frameworks, J. Phys. Chem. B 110 (2006) 17776-17783.
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[61] R.V. Siriwardane, M.S. Shen, E.P. Fisher, J. Losch, Adsorption of CO2 on zeolites at moderate temperatures, Energy Fuels 19 (2005) 1153-1159.
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[62] S.S. Liu, X.N. Yang, Gibbs ensemble Monte Carlo simulation of supercritical CO2 adsorption on
an
NaA and NaX zeolites, J. Chem. Phys. 124 (2006) 244705.
Highlights The
adsorption
of
CO2,
M
•
CH4,
and
their
mixtures
on
Na-montmorillonite clay materials was simulated; It was demonstrated that the clay material exhibits high adsorption
d
•
selectivity for CO2 over CH4;
The different adsorption mechanisms for the two species near the
te
•
Ac ce p
clay surfaces have been revealed
•
The
interlayer
sodium
counterions
can
provide
enhanced
electrostatic interaction with CO2.
23
Page 23 of 32
ed
M
an
us
cr
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Figure
Ac
ce pt
Figure 1.Absolute adsorption isotherms of (a) CO2 and (b) CH4; Excess adsorption isotherms of (c) CO2 and (d) CH4；The basal spacing: 14.0 Å (black solid line) and 20.6 Å (red dash line).
Page 24 of 32
ip t cr us an M
ed
Figure 2.Isosteric adsorption heats of pure species and the relevant decomposition terms as a function of adsorption coverage on the basal spacings at 14.0 Å (left) and 20.6Å (right) at 318K, (a) and (b): CO2; (c) and (d): CH4. The decomposition terms of Qst : the molar enthalpy (Hb) a
ce pt
(magenta line), average internal energy term of adsorbate-adsorbate ( U gg ) (red line), average a
Ac
internal energy term of adsorbate-adsorbent ( U gs ) (blue line).
Page 25 of 32
ip t cr us an M
ed
Figure 3 Adsorption isotherms of CO2/CH4 mixtures as a function of the mole fraction of CO2 on the basal spacings of (a) 14.0 Å and (b) 20.6 Å at 0.1 MPa, and (c) 14.0 Å and (d) 20.6 Å at 1
Ac
ce pt
MPa.
Page 26 of 32
ip t cr us an
Ac
ce pt
ed
basal spacings (a) 14.0 Å and (b) 20.6 Å.
M
Figure 4. Adsorption selectivity of CO2 over CH4 as a function of the mole fraction of CO2 on the
Page 27 of 32
ip t cr us an M ed ce pt
Ac
Figure 5. Equilibrium configuration snapshots (left) and density distributions of CO2 and CH4 mixtures along the z-direction (right) on the 14.0 Å basal spacing at 0.1 MPa with three bulk phase compositions: (a) yco2=0.1; (b) yco2=0.5; (c) yco2=0.9. Sticks, clay framework; violet spheres, Na; red spheres, carbon dioxide O; cyan spheres, carbon dioxide C; magenta spheres, methane.
Page 28 of 32
ip t cr us an M ed ce pt
Ac
Figure 6. Same as Figure 5 but on the clay with 20.6 Å basal spacing.
Page 29 of 32
ip t cr us an M ed
Ac
ce pt
Figure 7. Same as Figure 5 but at 1.0 MPa.
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ip t cr us an M ed ce pt
Ac
Figure 8. Same as Figure 6 but at 1 MPa.
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ip t cr us an M Ac
ce pt
ed
Figure 9 The radial distribution functions of the CO2 (solid line) and CH4 (dash line) mixtures on the spacings at (a) 14.0 Å and (b) 20.6 Å at 0.1 MPa, (c) 14.0 Å and (d) 20.6 Å at 1 MPa.
Page 32 of 32
S0169-4332(15)01914-5 http://dx.doi.org/doi:10.1016/j.apsusc.2015.08.101 APSUSC 31048
To appear in:
APSUSC
Received date: Revised date: Accepted date:
6-5-2015 24-7-2015 12-8-2015
Please cite this article as:
Molecular Simulation of Preferential Adsorption of CO2 Over CH4 in Na-montmorillonite Clay Material Nannan Yang1, Shuyan Liu2, Xiaoning Yang1,* College of Chemistry and Chemical Engineering, 2: School of Mechanical and Power Engineering
us
cr
Nanjing Tech University,Nanjing 210009, China
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1: State Key Laboratory of Materials-Oriented Chemical Engineering,
ABSTRACT: Grand canonical Monte Carlo simulations have been conducted to study the adsorption of carbon dioxide and methane, as well as their binary mixtures
the
clay
structure
for
the
two
an
on Na-montmorillonite clay material. It was found that the adsorption behavior near species
is
distinctively
different.
The
M
Na-montmorillonite clay shows obviously high adsorption capacity for CO2, as compared with CH4. The adsorption behavior and mechanism have been
d
characterized by the interlayer interfacial structures and isosteric heats of adsorption.
te
Meanwhile, the mixture adsorption demonstrates that CO2 molecules with enhanced adsorption strength are able to competitively replace CH4 molecules within the clay
Ac ce p
structure. The high separation selectivity of CO2 over CH4 implies the possibility of separating CO2 from natural gas mixtures using the clay minerals. The interlayer
sodium cations and negatively-charged clay surface can provide enhanced interaction with CO2 molecules that have high quadrupole moment, which is
responsible for the higher adsorption loading of CO2. Keywords: montmorillonite clay, CO2, CH4, adsorption, separation, molecular simulations.
*Corresponding author, E-mail address: [email protected] 1
Page 1 of 32
1. Introduction The effect of increasing atmospheric CO2 concentration on global warming is considered as important environmental concern for society [1]. At present, various
ip t
strategies and technologies for CO2 capture and sequestering have been proposed to
cr
reduce CO2 burden on the atmosphere. In this aspect, adsorption is the potential
us
technique that is feasible and efficient in CO2 capture [2]. In general, development of adsorbents with high adsorption capacity and selectivity is essential. Recently, various
an
types of porous adsorbents, such as activated carbons, zeolites [3-5], and metal organic frameworks (MOFs) [6-9], have been claimed to offer promising applications
M
for CO2 adsorption and separation. As an readily available cheap material, naturally occurring clay minerals might provide an attractive alternative to more complex
te
d
functional synthesized materials as adsorbents [10]. Furthermore, as the major impurity in natural gas, the presence of CO2 can reduce the conversion rate and
Ac ce p
energy content of natural gas. As a result, the removal of CO2 from natural gas
mixture is very critical in many industrial fields. There is an increasing need to develop new-typed porous adsorbents to remove CO2 from natural gas with low-cost
and high-energy efficiency.
From another perspective, long-term storage of CO2 in different geological sites
[11], including gas-rich shales, active or depleted oil and gas fields, deep saline aquifers, unminable coal seams, has been suggested as an important approach for CO2 sequestration. Among these geological sequestration options, that, which allows the production of additional byproduct, such as methane gas, is the most attractive
2
Page 2 of 32
approach [12]. This includes sequestration of CO2 in geological formations with the simultaneous recovery of CH4. For example, the BP-Amoco Corporation has demonstrated that CH4 could be effectively removed and recovered from deep,
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unmineable coals using CO2 injection. Shale gas reservoir has been considered as an
cr
important energy resource in recent years [13]. Shale is mainly composed of organic
us
kerogen materials and inorganic clay minerals. It has been suggested that high pressure injection of CO2 into the shale rocks could benefit the methane removal. The
an
mechanism of the process is CO2 has stronger adsorption affinity with clay materials, which can efficiently replace CH4 already adsorbed within the shale microspores.
M
Understanding the CO2-clay and CH4-clay adsorption interactions is an important fundamental aspect for the above applications. In particular, it is highly required to
te
d
clarify the competitive adsorption behavior between CO2 and CH4 in the clay confined pores. Reliable prediction of the gas-retention adsorption capacity for
Ac ce p
CO2/CH4 mixtures is necessary for the validity evaluation of the simultaneous CO2 capture and CH4 removal in geological formations. Meanwhile, predictions
concerning the process stability require the knowledge of how CO2/CH4 molecules
are held within the clay pores.
In general, clay structures are comprised of layered aluminosilicates with
negatively charged due to tetrahedral substitutions of Si by Al or octahedral substitutions Al by Mg. Cations in the interlayer spaces compensate the negative charge of mineral layers and hold the clay sheets together. The typical montmorillonite interlayer cations are Na+, K+, Ca2+, and Mg2+. The layer spacing
3
Page 3 of 32
between neighboring montmorillonite layers is generally variable. Clay minerals have found extensive application in several areas such as catalytic activity, filtration of fluids, and waste disposal process [14, 15].
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Experimental and computational studies always focused on the structure and
cr
dynamics of interlayer water and counterions in clay pores [16-24]. In addition, there
us
are quite a few experiments on the adsorption of heavy metals on clays [25-27], demonstrating that clay mineral could be an excellent adsorbent. Currently, the
an
adsorption capacity of CO2 on clay materials still remains unresolved. Very limited experimental studies have shown that adsorption ability of CO2 in hydrated clays is
M
relatively lower [28, 29]. Experiment [30] shows that the gas adsorption capacity on clay minerals under moisture-equilibrated condition is significantly reduced as
te
d
compared with that under dry condition. The interlayer water molecules might suppress the intercalation of CO2 molecules into the clay pores [31]. Ross [32]
Ac ce p
experimentally confirmed that the gas sorption capacity of shales under dry conditions is substantially larger than under moisture equilibrated condition. In the situation without interlayer water molecules, the nanoscale clay pore is expected to provide the maximum nanochannel for CO2 intercalation, and thus maximizing the adsorption ability of CO2 molecules. In recent years, there are several molecular simulation
reports on the interfacial structural and dynamics properties for CO2 intercalated within clay pores [33-36]. However, these studies mainly focus on the CO2 formation in the clay layer structures and did not provide microscopic interaction mechanism, in particular, the effect of interlayer counterions is not clear for CO2 adsorption.
4
Page 4 of 32
Meanwhile, the competitive adsorption and interaction mechanism for CO2/CH4 mixtures on clay structures remain largely unexplored. With the above in mind, in this work, grand ensemble Monte Carlo (GCMC)
ip t
simulation was employed to investigate the adsorption of pure CO2 and CH4, as well
cr
as CO2/CH4 mixtures on the common Na-montmorillonite. For this purposes,
us
adsorption isotherms from low pressure to 20 MPa have been simulated. Meanwhile, the interfacial structural properties and the isosteric heat of adsorption were used to
an
explore the adsorption mechanism. In the previous work [33], the mechanics and thermodynamics stability of clay materials shows that there exist multiple stable
M
spacings of clays in the presence of supercritical CO2. Therefore, in this work, two types of pore widths with the basal spacing 14.0 Å and the spacing 20.6 Å were
te
d
investigated. This paper is organized as follows: the intermolecular potentials and simulations methodology will be given in the following section. Then we present the
Ac ce p
results and discussion, and paper concludes with a brief summary and conclusion of our research in the last section.
2. Models and Simulation Method The clay model used in this simulation is the Na-montmorillonite whose molecular
formula is Na0.75 [Si8] (Al3.25Mg0.75) O20 (OH) 4[37, 38]. The negative charge caused
by the substitution of Mg2+ for Al3+ in the octahedral sheet is balanced by the interlayer Na+ cations. The simulation cell is a patch with two half layers of the montmorillonite sheet, and with the interlayer space in between. To model the montmorillonite structure, three-dimensional periodic boundary condition was applied
5
Page 5 of 32
to a simulation box composed of 32 (8×4×1) unit cells, so making up a clay patch of 42.24 Å(x-dimension) by 36.56 Å (y-dimension), and 6.56 Å in the z dimension. We selected two stable basal spacings: a smaller spacing 14.0 Å and a larger spacing
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20.6Å. The two clay spacings were considered based on the previous study [33].
Lennard-Jones (L-J) and Coulomb terms:
us
(1)
an
Etotal
⎧ ⎡⎛ σ ⎞12 ⎛ σ ⎞6 ⎤ q q ⎫ ⎪ ⎪ = ∑∑ ⎨4ε ij ⎢⎜ ij ⎟ − ⎜ ij ⎟ ⎥ + i j ⎬ ⎜ ⎟ ⎜ ⎟ ⎢ r i j >i ⎪ ⎝ rij ⎠ ⎥⎦ rij ⎪⎭ ⎩ ⎣⎝ ij ⎠
cr
The total potential energy (Etotal) of the simulated system involves the
where ε and σ are the energetic and size parameters, respectively, in the L-J potential.
M
qi is the charge of particle i. The long-range Coulomb interaction was handled using the Ewald technique. Carbon dioxide was treated with the three-site EPM2 model [39,
te
d
40]. Methane is represented by a single L-J sphere [41] in the TraPPE force field, which was developed to reproduce the properties of alkanes. The interactions
Ac ce p
involving the clay and adsorbate follow the methods described by Boek et al[42]. The parameter for clay is derived directly from the models introduced by Skipper et al [19,
37, 43]. All the interaction parameters for CO2, CH4, and clay were listed in Table S1. In this work, MUSIC simulation code [44] was used for the GCMC simulation,
wherein the temperature T (318.15 K), volume V, and chemical potential μ were fixed. A series of bulk phase pressures (up to 20 MPa) were chosen and the corresponding chemical potentials of bulk phase were calculated using the Peng-Robinson equation of state (P-REOS) [45]. In general, four types of trial moves were applied for the gas molecules in the GCMC simulation: attempts to translate a molecule; attempts to
6
Page 6 of 32
insert a molecule, attempts to delete an existing molecule, and attempts to rotate a molecule. For the sodium counterions confined within the clay pore, only the translation move was adopted. The energy biasing technique [46, 47] was applied for
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the molecule insertion. Rigid model was used for the clay structure throughout this
cr
work. For each simulation run, the GCMC simulation consisted of 1×107 steps, the
us
first half of these steps was used to guarantee equilibration, and the remaining steps were taken for calculating the ensemble averages. To ensure fast GCMC equilibrium,
an
the final configuration of the last run was used as the initial configuration of the next
3. Results and Discussion
d
Single component adsorption
M
run.
te
Figures 1(a) and 1(b) depict the absolute adsorption isotherms of pure gases (CO2 and CH4) on the Na-Wyoming montmorillonite, as a function of pressure on
Ac ce p
the basal spacing of 14.0 Å and 20.6 Å, respectively. It is noticed that the CO2
isotherms exhibit a steep rise at low pressure. The curve shape can be described as type-Ι (Langmuir type), which is characteristic of microporous adsorbent with pore sizes of molecular dimensions [48]. Comparatively, the adsorption isotherm of CH4 demonstrates a gradual rise with pressure. The adsorption amount of CO2 is
obviously larger than that of CH4, which could be attributed to CO2 molecules that have higher interaction with Na-montmorillonite Clay. In particular, the existing quadrupole of CO2 can induce stronger electrostatic interaction with the charged clay structures. It is observed that the statured adsorption amount is ~5.1 mmol/g for CO2 7
Page 7 of 32
in the smaller spacing (12.6 Å) and ~13.0 mmol/g in the larger spacing (20.6 Å). The adsorption amount is comparable to the experimental data [49] for CO2 on carbon materials. Although this CO2 adsorption on the clay is less than the value on the
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synthesized IRMOF-1 adsorbent [50], the economical feature of natural clay sources
cr
still demonstrates that it is attractive porous adsorbent for CO2 capture.
us
A careful inspection of Figure 1(a) (see insert) shows that, at low-pressure range (below 0.1 MPa), the adsorption amount of CO2 in the small pore (14 Å) is larger
an
than that in the larger pore (20.6 Å). This phenomenon is due to the relative stronger surface interaction in the 14.0 Å spacing that has two clay planes. At lower pressure
M
condition, owing to weak adsorption, the adsorbate-adsorbent interaction is the
d
controlling factor determining the adsorption performance. With the pressure
te
increasing, more adsorbate molecules present in the confined clay pore and the effect of adsorbate-adsorbate interaction becomes pronounced, inducing an enhanced
Ac ce p
adsorption. As a result, under higher pressure, the larger clay pore (20.6 Å) with more adsorption space available is able to produce the larger adsorption, as compared with the smaller pore (14.0 Å). Figures 1(c) and 1(d) present the corresponding excess adsorption isotherms. The
excess adsorption amount, nex, is related to the absolute adsorption amount, nab, from
the following equation: n ex = n ab − V g ρ g
(2)
where Vg is the pore volume of clay, ρg is the molar density of the bulk fluid phase,
8
Page 8 of 32
which was estimated from the P-R EOS [45]. Each excess adsorption isotherm shows a maximum in the medium pressure range, which is typical of high pressure adsorption. This phenomenon is usually caused by a significant change in the bulk
ip t
phase fluid density near its critical point. The simulated isotherms of CO2 are
cr
obviously higher than the experimental ones in the hydrolyzed Na-montmorillonite
us
clay with a relative humidity of 40-60% [28]. This behavior is due to the fact that the interlayer water impedes CO2 adsorption.
an
The local density distributions for pure CO2 and CH4 molecules adsorbed in the interlayer region of clay were presented in Figure S1 of supplementary material. In
M
the smaller clay pore, usually one density peak appears on the middle of pores.
d
However, at higher pressure (5 MPa); two symmetrical separated peaks are formed in
te
the density profile. In the large pore (20.6 Å), the density distribution of CO2 exhibit two significant peaks close to the clay surfaces, and two shoulder peaks develop on
Ac ce p
the middle of clay pore at 5 MPa. With the pressure increasing, the primary peaks become more close to the clay surface. Comparatively, the density peak of CH4
molecules is obviously low. This is consistent with the lower adsorption amount of CH4.
The corresponding configuration snapshots for the gas adsorption in the clay pores
were shown in Figures S2 and S3. It was observed that Na+ cations are generally located closer to the surface than gas molecules. At lower pressure, CO2 molecules preferentially occupy the position near Na+ counterions. With pressure increasing, CO2 and CH4 molecules can occupy the positions near the clay surface, and the 9
Page 9 of 32
adsorption in the midplane is due to the enhanced intermolecular interactions. The snapshots also show CO2 molecules have an orientation preference in parallel with the
confined within aluminum silicate layers [51] and clay pore [33].
ip t
clay sheets, which is in good agreement with the previous results regarding CO2
cr
In order to quantitatively evaluate the interaction strengths of CO2 and CH4 with
us
the clay, we also computed the isosteric heats of adsorption (Qst). Qst is defined as the difference between the partial molar enthalpy of adsorbates in the bulk phase and the
(
an
partial molar internal energy in the adsorbed phase [52]:
)
(
a
∂n a
)
T
(3)
M
Qst = − hb ,* − hb + RT − ∂U
where -(hb,*-hb) stands for the departure function of the partial molar enthalpy of bulk
d
fluid and it was calculated by the PR-EOS of fluid. Ua is the average internal energy
te
of adsorbate in the adsorbed phase, obtained from the GCMC simulation. n is the
Ac ce p
adsorbed number of CO2. The partial derivative term in Equation (3) was generally evaluated in two different methods: numerical differentiation and ensemble fluctuation [47,52]. In this work, we used the numerical differentiation method. The Qst is mainly determined by the two components: one is the molar enthalpy -(hb,*-hb)
(Hb for short) for the bulk phase, and the other is the partial molar internal energy of
(
adsorbate - ∂U
a
)
∂n a T (Ua for short). The Ua can be further decomposed into the
a contributions from the adsorbate-adsorbate interaction ( U gg for short), and the
adsorbate-adsorbent interaction ( U gsa for short). Figure 2 shows the isosteric heats of adsorption of CO2 and CH4 as a function of
10
Page 10 of 32
the adsorbed amount in the two clay spacings. The adsorption heat ( Qst0 ) in the limit of zero loading can be used to characterize the strength of surface interaction. For CO2, The Qst0 is ~ 55.4 kJ/mol in the small pore and ~ 40.5 kJ/mol in the large pore.
cr
However, this value is comparable to that in NaX [54].
ip t
The infinite adsorption heat is smaller than those in MOF-177 [6] and in silicalite [53].
us
It is observed in Figure 2(a) and (b), that the Qst of CO2 decreases at first with the adsorption loading, revealing the heterogeneous nature of the clay surface. This
an
changing behavior of Qst is not unusual. Similar behavior has been observed for gas adsorptions on other porous materials [50, 55-57]. A decomposition of Qst shows that
M
the decrease at the adsorption beginning is due to the contribution ( U gsa ) of dominated
d
adsorbate-adsorbent interaction. This demonstrates that CO2 molecules first occupy
te
the more energetically favorable sites in the confined clay pores, and then adsorb at the less favorable sites, corresponding to a gradually decreasing contribution ( U gsa ). A
Ac ce p
further split of U gsa can reveal that its loading dependence is corporately determined by the CO2-Na+ interaction and the CO2-surface interaction (see Figure S4). At the
initial adsorption stage, the large adsorption heat of CO2, along with the rapid adsorption in the adsorption isotherm (Figure 1), is mainly controlled by the strong attraction of the CO2 quadrupole with the positive charged sodium ions. As discussed in the preceding section, the adsobate-adsorbent interaction provides important contribution to the CO2 adsorption heat on clay. In order to further understand the molecular origin of the gas-solid interaction, the average normalized interaction energies for one CO2 molecule in the adsorption phase with the 11
Page 11 of 32
Na-montmorillonite Clay was decomposed into the electrostatic component and the L-J component. As shown in Figure S6, the electrostatic interaction between CO2 and sodium ions provides the main contribution to the CO2-clay interaction energy,
ip t
especially at the lower adsorption loading. This confirms again the conclusion that the
cr
electrostatic interaction from charged Na+ is responsible for the higher adsorption heat
us
of CO2. Comparatively, the electrostatic interaction from the clay layer is minor.
a ) increases monotonically in the two The adsorbate-adsorbate contribution ( U gg
an
spacings clays and it has certain effect on the adsorption heat at high adsorption loading. This can result in an increase in the adsorption heat (Qst) in the larger pore. It
M
is noticed that the isosteric heat of adsorption shows a final rapid decrease at very
d
higher loading. This behavior is ascribed to the corresponding contribution from the
Ac ce p
fluid density.
te
bulk enthalpy -(hb,*-hb), which changes the shape of the isosteric heat curves at high
Comparatively, as shown in Figure 2(c) and 2(d), the adsorption heat of CH4 is
obviously smaller than that for CO2, which agrees with the lower adsorption amount of CH4. This signifies the weak interaction strength of CH4 with the clay. The Qst0 for
CH4 is just 18.9 kJ/mol in the small pore (also see Figure S5), in agreement with the experimental value (16.0 kJ/mol) on the pillared clays [58]. It is noted that the adsorption heat of CH4 firstly increases and then decreases with the adsorption loading. The dependence of Qst on the adsorption loading is consistent with the observation for the CH4 adsorption on other microporous materials [50, 52, 57, 59]. This behavior can be attributed to the fairly homogeneous adsorption feature for 12
Page 12 of 32
nonpolar CH4 in the clay pores, wherein the methane-clay interaction remains less change during the adsorption process.
ip t
Adsorption of CO2/CH4 mixtures In this work, GCMC simulations were further performed on the competitive
cr
adsorption of CO2/CH4 mixtures in the clay. Figure 3 displays the adsorption
us
isotherms for the binary mixtures as a function of bulk phase compositions at the pressures of 0.1 MPa and 1.0 MPa. With the mole function of CO2 increasing, the
an
adsorption amount of CO2 rises steadily, while the adsorption of CH4 decreases. This
M
result represents the competitive adsorption behavior between the two species. It is found that CO2 is more preferentially adsorbed than CH4 within the clay structures
d
due to the stronger interaction between CO2 and clay. This is consistent with the
te
different adsorption amounts of pure CO2 and pure CH4, as demonstrated in the
Ac ce p
preceding section. In the 14.0 Å pore, the interlayer space is fully filled with CO2 molecules and the adsorption loading of CH4 is almost negligible when the mole fraction of CO2 in bulk phase exceeds 0.5. As expected, at high pressure condition, the adsorption amounts of both CO2 and CH4 become higher. Our simulation provides a
theoretical support that CO2 molecules could competitively replace CH4 molecules in
Na-montmorillonite clay.
To further evaluate the adsorption separation performance of the clay material for CO2/CH4 mixtures, we calculated the adsorption selectivity (S) of carbon dioxide with respect to methane as follows,
13
Page 13 of 32
S =
xCO2 xCH 4
(4)
yCO2 y CH 4
where x, y are the mole fractions of the two species in the adsorbed and bulk phases,
ip t
respectively. The selectivity in the clay structures with two kinds of spacings is shown
cr
in Figure 4. For the two pore widths, the selectivity is much greater than unity, confirming that the clay material has high adsorption separation performance for
us
CO2/CH4 mixture. The adsorption selectivity in the small pore is larger than that in the
an
large pore. This can be attributed to the relatively low adsorption amount of CH4 in the small clay pore. For instance, in the small pore size (14.0 Å), the adsorption
M
amounts of CH4 almost approaches to zero with yCO2 increasing (see Figure 9(a)). According to our simulation results, for the clay structure with the 14.0 Å
te
d
spacing, the separation factor can be achieved to be 110.7 at 0.1 MPa with the bulk phase yCO2=0.2, which represents a CH4-rich content in natural gas, as seen from
Ac ce p
Figure 4. For the larger pore (20.6 Å), the selectivity also reaches the range of 26-70, depending on various pressures. This simulated selectivity of clays is obviously large than the simulated values for other materials. For example, the selectivity was reported to be ~10 in the C60 intercalated graphite at 0.1 MPa and 298 K [57], 6.2 in
Cu-BTC, and 1.8 in MOF-5 [60]. At yCO2=0.5, the separation factor is generally in the range of 25.0-46.8 in the larger clay pore. The separation factor of equimolar CO2/CH4 mixture has also been reported in other porous materials. For example, the selectivity is only 6 in Cu-BTC and 2 in MOF-5 [60]. In short, the CO2/CH4
separation performance of clay structure shows superiority over some current porous
14
Page 14 of 32
materials. It is demonstrated that Na-montmorillonite could be considered as promising porous adsorbent to separate CO2 from the natural gas mixtures. Moreover, this result also suggests that the clay-based geological formations could be applied for
ip t
concurrently capturing CO2 and removing CH4.
cr
For the two sizes of clay pores, the adsorption selectivity behaves very differently.
us
It is observed that the adsorption selectivity shows a decreasing behavior with yCO2 increasing in the larger pore (20.6 Å). This can be explained as the increase in the
an
CH4 adsorption due to sufficiently large pore space. However, the selectivity shows fluctuation in the 14.0 Å spacing clay. This behavior reflects the complicated packing
M
features of the mixture in the small confined pore. From Figure 4, it can be found that
d
the selectivity at 0.1 MPa is significantly larger than that at 1 MPa. The pressure
te
influence on the selectivity is similar with the observation of C2H6/CH4 in the pillared clay [58]. From our simulation result, the clay materials with larger pores generally
Ac ce p
show satisfactory separation performance from both adsorption capacity and adsorption selectivity.
For an improved understanding of the mixture adsorption behavior in the clay pores,
we also investigated the interlayer structure properties for the mixture. As shown in Figure 5, in the smaller pore, the density distributions of CO2 and CH4 molecules
present relatively broad peaks at the middle region of pore. This is identical with the confined configurations of pure species in the clay pore (Figure S1). The density distribution of CO2 molecules shows a transition from one to two layers with an increase in the bulk concentration. From the snapshots in the small pore, we can see 15
Page 15 of 32
the CO2 molecules are homogeneously distributed in the center region and obviously fewer CH4 molecules also appear in the midplane. Comparatively, the sodium cations
ip t
are always located close to the clay plane. In the larger spacing, the density distributions of CO2 and CH4 show two separated
cr
symmetrical peaks. As shown in Figure 6, the density peak of CO2 is close to the
us
plane, as compared with CH4. The corresponding snapshots also show that CO2/CH4 molecules can coexist near the clay plane. This molecule packing arrangement is
an
consistent with the previous result [33] for the intercalation of supercritical CO2 molecules within the clay pores. Under higher pressure (1.0 MPa), similar behavior
M
can be observed, as shown in Figures 7 and 8. In the larger pore size (20.6 Å), the
d
CH4 molecules are repelled from the clay plane as the CO2 bulk composition
Ac ce p
two species.
te
increases. This behavior again demonstrates the different surface interactions for the
The presence of interlayer counter cations is expected to have significant effect on
the adsorption performances of CO2 and CH4. Previous experimental studies [61]
have indicated that the enhanced adsorption of CO2 on NaA and NaX zeolites can be
ascribed to the strong interaction between the high quadrupole moment of CO2 and the
extra-framework cations. In order to explore the different interactions between the
sodium counterion and the CO2/CH4 mixture species in the clay pores, we show the radial distribution functions (RDFs) between the Na+ cations and the mixture components in the two clay pores in Figure 9. The position of the first peak in the Na-CO2 RDF is at ~3.8 Å, which is coincident with the coordination of pure CO2 near 16
Page 16 of 32
the sodium cations in the clay pores [33]. Similar result has been reported previously in the simulation [62] for the supercritical CO2 adsorption in the NaA zeolites.
ip t
The main peak of Na-CH4 is relatively low and broad, and occurs at a distance of ~5.5 Å. From the RDFs of the CO2/CH4 mixtures, the separation position of first
cr
peaks of the Na-CO2 RDFs is obviously shorter than those of the Na-CH4 RDFs. This
us
indicates that there is a preferential attractive accumulation of CO2 molecules near the Na+ cations, meaning the existence of strong interaction between the CO2 molecules
an
with quadrupole moment and the positively charged Na+. This interaction could lead
M
to a greater selectivity of CO2 over CH4 within the clay pores.
4. Conclusions
d
The adsorption of pure CO2 and pure CH4, as well as their binary mixture on the
te
Na-montmorillonite clay were studied using the GCMC simulations. The adsorption
Ac ce p
isotherms, the isosteric adsorption heats, and the interfacial properties have been simulated. The adsorption amount of CO2 on the clay material is obviously larger than
that of CH4. The simulation configurations show that the CO2 molecules arrange
themselves near the clay plane in a parallel arrangement with the respect to the clay sheet. The adsorption heats of CO2 and CH4 on the clay pores have been applied to characterize the adsorption interaction strengths for the two species. The enhanced interaction acting on the CO2 molecules from the negatively charged clay surfaces and the positive sodium counterions is responsible for the higher adsorption amount of CO2. Meanwhile, the CO2/CH4 mixture adsorption behavior has also been simulated for various pressures, bulk compositions, and pore sizes. A higher CO2 selectivity can 17
Page 17 of 32
be achieved with the clay. This simulation result demonstrates that CO2 molecules competitively replace CH4 molecules within the clay confined pores, suggesting that the clay materials can be used to separate CO2 from the gas mixture. A further
ip t
configuration analysis of the mixture indicates that CO2 molecules are preferentially
cr
adsorbed at the position close to the clay surface, as compared with the CH4
us
molecules. In conclusion, our simulation shows that the clay material with economical feature is an attractive porous adsorbent for CO2 capture and separation. In particular,
an
the simulation result provides the theoretical support that geological storage of CO2 possibly allows the production of additional methane byproduct, already adsorbed
M
within the gas-rich shales.
Conflict of Interests
Acknowledgement
te
d
The authors declare no competing financial interests.
Ac ce p
This work was supported by the Natural Science Foundation of China under Grant
21176114,
21376116,
Research
funding
from
State
Key
Laboratory
of
Materials-Oriented Chemical Engineering (ZK201404), and A PAPD Project of Jiangsu Higher Education Institution.
Appendix A. Supplementary data: Supplementary data associated with this article can be found in the online version.
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Page 18 of 32
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ip t
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Highlights The
adsorption
of
CO2,
M
•
CH4,
and
their
mixtures
on
Na-montmorillonite clay materials was simulated; It was demonstrated that the clay material exhibits high adsorption
d
•
selectivity for CO2 over CH4;
The different adsorption mechanisms for the two species near the
te
•
Ac ce p
clay surfaces have been revealed
•
The
interlayer
sodium
counterions
can
provide
enhanced
electrostatic interaction with CO2.
23
Page 23 of 32
ed
M
an
us
cr
ip t
Figure
Ac
ce pt
Figure 1.Absolute adsorption isotherms of (a) CO2 and (b) CH4; Excess adsorption isotherms of (c) CO2 and (d) CH4；The basal spacing: 14.0 Å (black solid line) and 20.6 Å (red dash line).
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Figure 2.Isosteric adsorption heats of pure species and the relevant decomposition terms as a function of adsorption coverage on the basal spacings at 14.0 Å (left) and 20.6Å (right) at 318K, (a) and (b): CO2; (c) and (d): CH4. The decomposition terms of Qst : the molar enthalpy (Hb) a
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(magenta line), average internal energy term of adsorbate-adsorbate ( U gg ) (red line), average a
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internal energy term of adsorbate-adsorbent ( U gs ) (blue line).
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Figure 3 Adsorption isotherms of CO2/CH4 mixtures as a function of the mole fraction of CO2 on the basal spacings of (a) 14.0 Å and (b) 20.6 Å at 0.1 MPa, and (c) 14.0 Å and (d) 20.6 Å at 1
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MPa.
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basal spacings (a) 14.0 Å and (b) 20.6 Å.
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Figure 4. Adsorption selectivity of CO2 over CH4 as a function of the mole fraction of CO2 on the
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Figure 5. Equilibrium configuration snapshots (left) and density distributions of CO2 and CH4 mixtures along the z-direction (right) on the 14.0 Å basal spacing at 0.1 MPa with three bulk phase compositions: (a) yco2=0.1; (b) yco2=0.5; (c) yco2=0.9. Sticks, clay framework; violet spheres, Na; red spheres, carbon dioxide O; cyan spheres, carbon dioxide C; magenta spheres, methane.
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Figure 6. Same as Figure 5 but on the clay with 20.6 Å basal spacing.
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Figure 7. Same as Figure 5 but at 1.0 MPa.
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Figure 8. Same as Figure 6 but at 1 MPa.
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Figure 9 The radial distribution functions of the CO2 (solid line) and CH4 (dash line) mixtures on the spacings at (a) 14.0 Å and (b) 20.6 Å at 0.1 MPa, (c) 14.0 Å and (d) 20.6 Å at 1 MPa.
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