Competitive Adsorption of MethanolâAcetone on Surface

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Functional Nanostructured Materials (including low-D carbon)

Competitive Adsorption of Methanol-Acetone on Surface Functionalization (-COOH, -OH, -NH2 and -SO3H)#GCMC and DFT Simulations Yang Guo, Zheng Zeng, Liqing Li, Changqing Su, Ruofei Chen, Chunhao Wang, Ke Zhou, Xiang Xu, and Hailong Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b10804 • Publication Date (Web): 28 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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Competitive

ACS Applied Materials & Interfaces

Adsorption

of

Methanol−Acetone

on

Surface

Functionalization (-COOH, -OH, -NH2 and -SO3H)：GCMC and DFT Simulations Yang Guo, Zheng Zeng*, Liqing Li*, Changqing Su, Ruofei Chen, Chunhao Wang, Ke Zhou, Xiang Xu, Hailong Li School of Energy Science and Engineering, Central South University, Changsha 410083, Hunan, China

ABSTRACT: The capture and separation properties of surface functionalized activated carbons (AC-Rs, R= COOH, -OH, -NH2 and -SO3H) for the methanol-acetone mixture were investigated for the first time by grand canonical Monte Carlo (GCMC) simulation and density functional theory (DFT). The effects of surface functional groups and structural characteristics of AC-Rs on the adsorption and separation behaviors of methanol and acetone had been clarified. The surface functional group with strong electron-donating or electron-accepting capacity (i.e., -NH2, -OH, and -SO3H) was a crucial factor for the methanol-acetone capture and separation performance at the lower pressure range, and the accessible surface area was found to be another determinative factor. AC-NH2 with the relatively large accessible surface area (4497 m2/g) exhibited an efficient capture performance for the single component (15.7 mol/kg for methanol and 6.7 mol/kg for acetone) and the highest methanol/acetone selectivity (~23) at 0.02 kPa. While at high pressures, the surface functionalization and available pore volume of AC-Rs played pivotal roles in the adsorptive separation process. This study provided mechanistic insights on how the surface functional groups affected the capture and separation properties of ACs, which would further provide a rational alternative strategy in the preparation and synthesis of ACs for the effective gas mixture separation. KEYWORDS: activated carbons, functional groups, methanol/acetone adsorption, separation, GCMC, DFT 1

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1. INTRODUCTION Methanol-acetone mixture, used in both preparative chemistry and chemical industry, is one of the most widely used organic solvents.1 The mixture of these two compounds exhibits easy diffusion as volatile organic compounds (VOCs) into the atmosphere, which may cause a negative effect on the human health and the atmosphere.2-5 Hence, establishing effective strategies in separation and capture of the methanol-acetone mixture will be significant to the recycling and concentration of the solvent. Adsorption has been considered as one of the most favorable capture and separation technologies for its low energetic consumption and high efficiency.6-8 Many investigations on different materials, i.e., activated carbon (AC),9-11 metal oxides, 12-14 and metalorganic frameworks (MOFs),15-17 for VOCs capture have been conducted. Among these adsorbents, AC materials have been regarded as the competitive candidates because of their high specific surface area, variable surface functional groups, eco-friendly property and relatively low-cost.18,19 The VOCs adsorption capacity of AC is determined by its structural characteristics and surface modification.20 Various functional groups have been added to carbon materials to enhance their VOCs capture performance.21-27 However, since it’s difficult to calculate the selectivity of VOCs mixtures by ideal adsorbed solution theory (IAST) for the large nonideality in the adsorbed phase,

28,29

the separation performances of ACs for VOCs were rarely

reported. The use of molecular simulation allows us to have an intuitive understanding of the gas-framework interaction and the adsorptive separation process at a molecular level. Molecular simulations have been widely used to study the adsorption behavior of MOFs,30-32 covalent organic frameworks (COFs),33,34 ACs,35,36 and

2


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zeolites.37,38 In terms of AC, the slit pore model has been most commonly adopted to study the influence of functional groups and pore size distribution (PSD) on its adsorption capacity. 39-41 However, this simplified model is not indicative of presenting the complex nature of real carbon materials. Recently, a random structure AC model42, consisting of several coronene-shaped graphitic basic structural unit (BSU), was reported to have a good match with the pore topology and morphology of the real carbon. This random structure model has been successfully and widely used to predict the adsorption performance of CO2/CH4 in ACs, and the calculated adsorption amounts are well consistent with the experimental results.43 More importantly, functional groups can be connected to the graphitic BSU to constitute the modified AC frameworks. For example, Lu et al.44 predicted the adsorption and separation performance of a binary CO2/CH4 mixture in edge functionalized ACs, and found that adding appropriate surface functional groups to the ACs could improve the capture and separation performance of the CO2/CH4 mixture. In this work, the random structure AC model was constructed to simulate internal structure of real carbons. The graphitic BSU was intercepted from a sheet of graphene that consists of 24 carbon atoms, and four functional groups (-COOH, -OH, -NH2 and -SO3H) were connected to the BSU to constitute the modified frameworks. Grand canonical Monte Carlo (GCMC)45,46 simulations were used to predict the thermodynamic equilibrium properties of single- and mixture-component of methanol-acetone in ACs. Density functional theory (DFT) was performed to assess the most stable structure of the graphitic BSUs, and to establish the optimal adsorption position of molecules on the surface of ACs. The intrinsic enhancement mechanism in adsorption capacity was elucidated by analyzing the pore topology and electrostatic interaction contribution. The calculated results of potential energies of adsorbate-framework and adsorbate-adsorbate interactions (electrostatic and van der Waals

3



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(vdW)) as well as the optimal adsorption position clarified the different adsorption behaviors of methanol and acetone in ACs at both low and high pressures. The adsorption of a binary methanol-acetone mixture in ACs was preformed to investigate the separation performance of the chosen functional groups. This study provided the detailed information on the capture, competitive adsorption and separation of the binary methanol-acetone mixture on the surface functionalized ACs, which paves a new way for designing ACs for the effective gas mixture separation.

2. SIMULATION METHODOLOGY To study the capture and separation properties of surface functionalized activated carbons (AC-Rs, R= COOH, -OH, -NH2 and -SO3H) for methanol-acetone mixtures, functional groups were connected to the graphitic BSU to constitute the modified AC frameworks (see Figure 1).

Figure 1. The structure of graphitic BSU and AC frameworks. All the functionalized BSUs were optimized with the DFT geometry optimization in Gaussian 09 package.47 Mulliken charge was used as the atomic partial charge and the B3LYP functional combined with the 6-31+G(d,p) basis set was used to do the spin-unrestricted all-electron quantum-chemistry calculation.48,49 The optimized BSU 4


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structure and the atom charge were presented in Figure S1 and Table S1, and the calculated atom charge was further compared with literatures which proved to be reasonable as shown in Table S2. The AC models were then constructed from a collection of geometry optimized BSUs at a mass density of 0.542 g/cm3.42 The visualization and detailed modeling process of the unit cell of the five AC-Rs (R=None, COOH, OH, NH2, SO3H) models were shown in Figure S2-S4. The minimum energy configurations of methanol/acetone molecule adsorbed on the BSUs were calculated by density functional dispersion correction

(DFT-D) in Dmol3 program package.50 Generalized gradient

approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE)51 functional was chosen to predict the adsorbatesurface interaction for its accuracy in describing the exchange–correlation interaction.52,53 The DFT semi-core pseudopotential (DSPP)52 processing and the double numerical including polarization (DNP) basis set were chosen for the atoms. GCMC simulation was carried out to estimate the adsorption isotherms for single or mixed gas components. All the GCMC calculations were performed in the simulation software RASPA.54,55 The GCMC simulations were began with an equilibration run of 1.0×105 cycles, followed by a production run of 5.0×105 cycles. The potential energy of the host-sorbate as well as the sorbate-sorbate system was calculated by the sum of the electrostatic and the vdW energy. The calculation formulas of the vdW and the electrostatic potential energy were presented in the SI. In addition, the Lennard Jones (12-6) model56 with a cutoff radius of 18 Å was established to describe the vdW energy, while the electrostatic energy was calculated by the Ewald summation. The L-J potential parameters, for the AC-Rs, were taken from Dreiding force field57. The L-J potential parameters are provided in Table S3. The L-J potential parameters and partial charges of the methanol and acetone adsorbates were obtained from

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TraPPE-UA (united atom) force field.58,59 The L-J potential parameters and atomic partial charges of the gas molecule are shown in Table S4.In this force field, the methyl groups in the methanol and acetone molecules were considered to be a united-atom located on a carbon atom, which has been widely adopted in predicting the adsorption behavior in AC23 and MOFs31.

3. RESULTS AND DISCUSSION 3.1. Structural Characteristics of AC-Rs. To investigate how the functionalization affects the pore topology and morphology of AC, the physical characteristics of the five AC models are calculated (Table 1). The structural features of the AC-Rs in Table 1 were calculated in RASPA. And the relevant computational methods and definitions are elaborated in the SI. Compared to AC-None (1.339 cm3/g), the presence of -SO3H (1.457 cm3/g), -COOH (1.396 cm3/g) or -OH (1.379 cm3/g) functional groups leads to the enlargement of the total available pore volume (Vtot). The variation of the maximum pore diameter (Dm) is in accordance with that of Vtot, that is, the AC-SO3H exhibits the largest expansion in Dm values (19.13 Å) compared with AC-None (14.13 Å), then followed by AC-COOH (15.92 Å) and AC-OH (14.66 Å). The above results indicate that all the selected functional groups enlarge the Vtot and Dm of AC except AC-NH2, which has a Vtot of 1.302 cm3/g and a maximum pore diameter of 13.77 Å. Table 1. Structural characteristics of the AC-Rs. -None

-COOH

-OH

-NH2

-SO3H

40

20

40

30

20

Dimensions of the model (Å )

32.81

37.03

39.22

35.35

42.62

Porosity, Фc (%)

72.49

75.66

74.67

70.55

78.97

AC-Rs Number of BSUs 3

6


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Total available pore volume, Vtot (cm3/g)

1.339

1.396

1.379

1.302

1.457

Maximum pore diameter, Dm(Å)

14.13

15.92

14.66

13.77

19.13

Accessible surface area (m2/g)

4390

4043

4443

4497

3684

The accessible surface area of the framework is another key factor to obtain a higher adsorption capacity, since a larger accessible surface area means more physical adsorption sites.60 The AC-NH2 (4497 m2/g ) and ACOH (4443 m2/g) has the relatively large accessible surface area among the functionalized ACs, which is larger than that of carbon nanotubes (CNTs) (153-1315 m2/g)61 and mesoporous high-surface-area activated carbon (931-2191 m2/g)62. However, the accessible surface areas of other surface functionalized AC models are smaller than that of the AC-None. Hence, the -NH2 functional group has a positive effect on the accessible surface area but a negative effect on the Vtot, to which the -SO3H, -COOH and -OH functional groups are adverse. This is further verified by analyzing the pore size distributions (PSDs) shown in Figure 2. And the visualization of the pore shape and connectivity is presented in Figure S5. The introducing of -NH2 functional group moves the pore sizes of AC-None (3.80-14.13 Å) to a smaller range (3.38-13.77 Å), whereas the other three functional groups lead to an expansion in the PSDs, especially the -SO3H (4.00-19.13 Å). More detailly, the pore size of AC-NH2, with two peaks at ∼7.1 and ∼9.1 Å, is dominantly distributed under 10 Å, which means large proportion of smaller pores. Obviously, the smaller pores have a higher contribution to the accessible surface area but a lower contribution to the Vtot. Instead, when it comes to AC-SO3H, whose pores are dominantly distributed in 10.0018.0 Å, it exhibits the largest Vtot but the smallest accessible surface area among all the ACs. All the PSDs are within a pore size range of 3.38-19.13Å, dominated by supermicropores (within 7.00-20.0 Å) with less proportion of ultramicropores ( AC-SO3H > AC-OH >

8


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AC-COOH> AC-None at the low pressures. Among the five AC-Rs models, AC-NH2 has the highest uptake (15.7 and 6.7 mol/kg for methanol and acetone, respectively) at 0.02 kPa, while AC-COOH shows a relatively weak adsorption capacity of two adsorbates. Adsorption isotherms also demonstrate that the adsorption capacity for methanol on selected AC-Rs at low pressures were more exothermic than those for acetone except AC-None.

(a)

(b)

Figure 3. Absolute adsorption isotherms of methanol (a) and acetone (b) on AC-Rs. Moreover, the adsorption energies (Eads) between the adsorbent and the adsorbate at 0.02 kPa are shown in Figure 4. The Eads of the two adsorbates on ACs follows the order of AC-SO3H>AC-NH2>AC-OH>ACCOOH>AC-None. The introduction of appropriate functional groups has a positive effect on the interaction between the adsorbates and ACs at ultralow pressures. Note that the selected functional groups enhance the absolute value of Eads from ∼23 to ∼73 kJ/mol for methanol and from ∼29 to ∼63 kJ/mol for acetone. AC-SO3H exhibits the strongest interaction with the two adsorbates (-73.2 kJ/mol for methanol and -63.4 kJ/mol for acetone), but a weaker adsorption capacity than AC-NH2 at 0.02 kPa, since the accessible surface area for providing physical adsorption sites of AC-NH2 (4497 m2/g) is larger than that of AC-SO3H (3684 m2/g). The largest uptake of AC-NH2 at low pressures is a consequence of a high Eads combined with the largest accessible surface area.

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Figure 4. Adsorption energies for methanol and acetone on AC-Rs. The isotherms of methanol and acetone nearly reach their saturation at 12 and 18 kPa, respectively. Moreover, the largest enhancement of methanol and acetone uptake on AC-SO3H at high pressures can be attributed to its largest total available pore volume (1.457 cm3/g) and its highest Eads. Hence, the adsorption capacity of AC-Rs under high pressures is related to its total pore volume and functionalization property. In terms of isotherm type, the absolute adsorption isotherms of methanol and acetone exhibit type-I Langmuir adsorption behavior,63 except the adsorption isotherm of methanol on the AC-None. Apparently, the adsorption of methanol on the AC-None shows a low adsorption capacity at low pressures but a rapid rise at higher pressures, which is similar to the adsorption behavior of water on the graphite surface and exhibits the type-Ⅲ according to the IUPAC. 3.3. Influences of Electrostatic and Van Der Waals Interactions. Consider that the introduction of surface

10


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functional groups can simultaneously enhance the adsorption capacity and Eads of methanol and acetone on ACs, the electrostatic interaction (EI) contribution to the adsorption capacity of the two adsorbates is investigated. The EI contribution is defined as Eq. 1. EI contribution =

|𝑄𝑤𝑖𝑡ℎ ― 𝑄𝑤𝑖𝑡ℎ𝑜𝑢𝑡| 𝑄𝑤𝑖𝑡ℎ

(1)

ｘ100%

where 𝑄𝑤𝑖𝑡ℎ and 𝑄𝑤𝑖𝑡ℎ𝑜𝑢𝑡 represent the absolute adsorption capacity with or without the gas-adsorbent EI. The 𝑄𝑤𝑖𝑡ℎ𝑜𝑢𝑡 was calculated by setting the atomic partial charges of the adsorbent (AC-Rs) to zero. Figure 5 shows the EI contributions of gas-framework at different pressure ranges. Surface-functionalized AC-Rs exhibit a larger EI contribution than the AC-None at low pressures, indicating that the functionalization enhances the EI between the adsorbent and the adsorbate. The selected electron-donating groups (-NH2, -OH) and electron-accepting group (-SO3H, -COOH) exhibit strong electrostatic interactions for their donating/accepting electron density capacity (a)

(b)

Figure 5. The gas-framework EI contributions on the uptake of (a) methanol and (b) acetone. Figure 6 exhibits that the N atoms in BSU-NH2 show an electronegative property by gaining electrons from the direct-connected C atoms, while the H atoms in the -NH2 group and the peripheral C in the activated carbon exhibit a high electropositivity by donating electrons. Those electropositive/electronegative atoms on the 11



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activated carbon surface become polar adsorption sites for methanol and acetone molecules, which effectively strength the electrostatic interactions between the adsorbate and framework especially at low pressures. Moreover, stronger electropositive/electronegative property of the functional group leads to more effects on the atomic partial charge of activated carbon. As shown in Table S5, the N atom in -NH2 group exhibits stronger electronegativity than the O atoms in -OH and -COOH functional groups, whereas the S atom in the -SO3H functional group shows a high electropositivity.

0.296 -0.711 0.360 0.323 -0.822 -0.302 0.317

0.291 -0.666

0.311 -0.515

0.326 -0.820 0.324

-0.053

-0.508

1.024

0.619

0.061

-0.400

0.350

H N C

0.331 0.302 0.325 -0.699 0.311 -0.720 -0.668 0.335 -0.532

0.070

0.076

0.046

0.118

-0.798 -0.623

0.302

-0.097

0.807

0.030

1.338

-0.426

1.012

-0.348

-0.305

0.327

0.352

-0.248

-0.676 -0.850 0.295 0.357 0.331 0.304

0.285 -0.668 0.322

0.324 -0.672 0.290

The direct-connected C with N atom

The peripheral C

Figure 6. Partial charges for the BSU-NH2. It can also be found that EI contributions of the gas-framework on the adsorption capacity decrease with the rising pressure. The adsorbate gas molecules firstly take up the optimal adsorption sites on the surface of the ACRs because of the strong adsorbate-framework electrostatic interactions at low pressures. However, when these 12


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physical adsorption sites reach their saturation, the monolayer adsorption is changed into multilayer one64 and the EIs between the adsorbate and adsorbent become less significant to the adsorption. Moreover, EI contributions of gas-framework on the uptake of methanol remain stable at a relatively high level before 1 kPa, and then follow a rapid drop at high pressures, which is much different from that of acetone (decreasing in a stepwise manner). It can be inferred that the methanol with a higher electrostatic affinity is more sensitive to electrostatic interactions at the low pressure. It can be verified from the adsorption isotherms (see Figure 2). The surface functionalized activated carbon exhibits a lager methanol uptake than that of acetone at the low pressures. In addition, to clarify the adsorption mechanisms of the two adsorbates on ACs at both low and high pressures, we take AC and AC-SO3H as examples, and calculate the potential energies of four interaction cases by GCMC (Figure 7): (i) the adsorbate-framework electrostatic interactions (EIs), (ii) the adsorbate-framework van der Waals interactions (VIs), (iii) the adsorbate-adsorbate EIs, and (iv) the adsorbate-adsorbate VIs.

13



(a)

(b)

(c)

(d)

Page 14 of 29

Figure 7. Potential energies of (a, c) methanol and (b, d) acetone on AC-None and AC-SO3H in four cases: the adsorbate-framework EIs, the adsorbate-framework VIs, the adsorbate-adsorbate EIs, the adsorbate-adsorbate VIs. As shown in Figure 7a, the potential energy of EI between the methanol and AC-None starts at a relatively high level at 0.02 kPa, then drops constantly with an increase of pressure (from -20.1 to -11.9 kJ/mol). When it comes to acetone (see Figure 7b), the vdW force dominates the gas-framework interaction. The VI energy between the acetone and AC-None reaches a high level (-38.6 kJ/mol) at 0.02 kPa, which is remarkably higher than that of methanol (-4.5 kJ/mol). This should be attributed to different configurations of methanol and acetone. As shown in Figure S6, the two methyl groups of acetone interact strongly with the AC-None, while the methanol with typical polar hydroxyl groups shows remarkable EI affinity (hydrogen bonding interaction).65 The varied interaction behaviors of methanol and acetone lead to their different isotherm trends (type-Ⅲ vs. type-I) on AC14


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None at low pressures. As a consequence, the AC-None with less polar adsorption sites exhibits a low methanol uptake at the low pressures. In terms of the AC-SO3H (see Figure 7c and d). The EI energy between the methanol and AC-SO3H achieves an extremely high level (-69.6 kJ/mol) at 0.02 kPa, while VI energy still remains at a low level (about -3.4 kJ/mol). Meanwhile, the dominating interaction between the acetone and framework changes from VI to EI after the introduction of -SO3H, since the strong polar adsorption sites on the AC-SO3H surface exhibit distinctly EI with the carbonyl group in acetone (see Figure S6). In addition, Figure 7 also shows that the potential energy of gas-gas interactions (both VIs and EIs) increases with the increased pressures. And the interaction between the methanol and methanol is stronger than that between the acetone and acetone. Moreover, the DFT-D calculated results also indicate that the adsorption distance between the methanol and methanol is smaller than that between the acetone and acetone (Figure S7). Obviously, smaller adsorption distance provides more electrostatic interaction contributions for polar adsorptions sites, which finally leads to a larger saturated methanol adsorption capacity on the AC-Rs. 3.4. Separation of Methanol/Acetone Mixture by AC-Rs. The adsorption isotherms of the methanol-acetone mixture (mole bulk composition 50:50) are calculated by the GCMC simulation. The selectivity (S) of methanol over acetone is computed by Eq. 2: 𝑥1

𝑦2

S = 𝑥2 × 𝑦1

(2)

where 𝑥1 and 𝑥2 represent the mole fractions of methanol and acetone in the adsorbed phase, while 𝑦1 and 𝑦2 are the corresponding mole fractions in the bulk phase. Figure 8 presents the methanol-acetone selectivity on ACs at 298 K. The selectivity of methanol over acetone follows the order of AC-NH2 >AC-OH > AC-SO3H >AC-COOH > AC-None under the entire pressure range.

15



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Introduction of -NH2, -OH, -SO3H functional groups significantly enhances the selectivity of methanol over acetone at the ultra-low pressure for the strong electron donating/accepting ability of the functional groups (see section 3.3). Surface functionalization provides polar adsorption sites for methanol molecules at the ultra-low pressure resulted from their high electrostatic affinity. Moreover, the tendency of selectivity curves (a rapid descent followed by a steady rise) of the -NH2, -OH, and -SO3H modified AC-Rs is similar to the CO2/CH4 selectivity curves of nanoporous carbons66 and PPN-1.67

Figure 8. Selectivity of methanol over acetone in AC-Rs at 298 K To clarify the separation mechanism of the mixtures, the density distribution contours of the two adsorbates on the AC-NH2 at 0.02, 0.50, 2.00 kPa are presented in Figure 9. The results also give an explanation of the initial decrease in the selectivity curve. At the adsorption onset (0.02 kPa), the methanol molecules with stronger polar hydroxyl functional groups68 quickly take up most of the polar functionality sites by the hydrogen bond interaction, while the acetone molecules only occupy a partial ultramicropore space (see Figure 9a, d). The obvious competitive effects lead to a relatively high methanol-acetone separation rate. When the pressure rises from 0.02 to 0.50 kPa (see Figure 9b, e), the acetone molecules begin to be adsorbed at benzene ring areas due to their 16


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stronger vdW interaction with the benzene rings. The acetone uptake increases rapidly at this pressure range, which results in a decrease in the methanol-acetone selectivity. With the increase of pressure from 0.50 to 2.00 kPa, methanol tends to stuff the unoccupied pore space (e.g. supermicropores space) (see Figure 9c, f), which results in a constant increase in the methanol-acetone selectivity with a rise of pressure as shown in Figure 8. (a)

(c)

(b)

(b)

Acetone adsorbed at the benzene rings area.

Methanol occupies the

Methanol

polar functionality

unoccupied pore space.

sites.

(d)

stuffs

the

(f)

(e)

Figure 9. Density distribution contours of methanol (a, b, c) and acetone (d, e, f) on AC-NH2 at 0.02 (left), 0.50 (middle), 2.00 kPa (right), respectively. Green represents a strong adsorption site, light blue represents a medium attachment site, and dark blue represents a weak adsorption site. Moreover, to analyze the binary mixture adsorption process at the high pressure range, the Eads of four stable 17



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adsorbate-adsorbate interaction models are established by DFT-D in RASPA (as shown in Figure 10). And the specific steps are presented in SI (Figure S8). The 𝐸𝑎𝑑𝑠 can be described as follows: 𝐸𝑎𝑑𝑠 = 𝐸𝑠𝑖𝑛𝑔𝑙𝑒 𝑎𝑑𝑠𝑜𝑟𝑏𝑎𝑡𝑒 𝑠𝑦𝑠𝑡𝑒𝑚 + 𝐸𝑎𝑑𝑠𝑜𝑟𝑏𝑎𝑡𝑒 ― 𝐸𝑡𝑤𝑜 𝑎𝑑𝑠𝑜𝑟𝑏𝑎𝑡𝑒𝑠 𝑠𝑦𝑠𝑡𝑒𝑚

(3)

where 𝐸𝑠𝑖𝑛𝑔𝑙𝑒 𝑎𝑑𝑠𝑜𝑟𝑏𝑎𝑡𝑒 𝑠𝑦𝑠𝑡𝑒𝑚 is the energy of the BSU with a single adsorbate molecule (methanol- acetone) , 𝐸𝑎𝑑𝑠𝑜𝑟𝑏𝑎𝑡𝑒 is the energy of the methanol or acetone molecule , and

𝐸𝑡𝑤𝑜 𝑎𝑑𝑠𝑜𝑟𝑏𝑎𝑡𝑒𝑠 𝑠𝑦𝑠𝑡𝑒𝑚 is the energy of the

BSU with two adsorbate molecules (the second gas molecule adsorbed on the previous adsorbed one). The 𝐸𝑎𝑑𝑠 for the adsorbed methanol molecule (adsorbed on the polar functionality sites) to catch another methanol and acetone are -47.81 and -21.88 kJ/mol respectively (see Figure 10a, b). And the Eads for the adsorbed acetone molecule (adsorbed on the benzene ring areas) to catch another methanol and acetone are -36.63 kJ/mol and -19.52 kJ/mol respectively (see Figure 10c, d). Obviously, the Eads of the single adsorbate system to capture another methanol molecule is larger than that to capture another acetone molecule, which also suggests the presence of a competitive adsorption effect at the high pressures. Moreover, the acetone molecules adsorbed at benzene ring areas become new polar functionality sites for the methanol molecule, which further enhances the methanol selectivity (see Figure 9c). In addition, we find that AC-SO3H exhibits a higher EI contribution (see Figure 5) but a weaker separation performance than AC-NH2 and AC-OH. The relatively weaker separation performance of AC-SO3H is a result of its smaller accessible surface area (3684 m2/g). Therefore, the effect of accessible surface area on separation is also investigated. Another two AC-SO3H (4133 and 2845 m2/g) models with different accessible surface areas are built and the related physical characteristics is shown in Table S6.

18


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(b)

(a)

𝐸𝑎𝑑𝑠= -21.88 kJ/mol


(d)

(c)



Figure 10. Stable adsorption configurations of the two adsorbate systems: (a)methanol-methanol; (b) methanolacetone; (c) acetone-acetone; (d) acetone-methanol. The selectivity of methanol from an equimolar methanol–acetone mixture on the three kinds of AC-SO3H 19



Page 20 of 29

at 298 K are shown in Figure 11. The selectivity follows the order of AC-SO3H (4133 m2/g)>AC-SO3H (3684 m2/g)>AC-SO3H (2845 m2/g) in the entire pressure range, which is well consistent with the accessible surface area order. Meanwhile, the adsorption isotherms for the methanol and acetone mixture in the three AC-SO3H models are shown in Figure S9. The isotherms of acetone nearly reach their saturation at 1 kPa, while that of methanol increase continuously with the increasing pressure. Obviously, the selectivity at the high pressure range is dominated by the saturated adsorption capacity of methanol and acetone. The AC-SO3H (4133 m2/g) with a larger Vtot (1.994 cm3/g) provides more space for methanol molecules, leading to a prominent separation performance at high pressures.

Figure 11. Selectivity of methanol over acetone in three AC-SO3H models. Thereby, the obtained results in this section reveal the competitive adsorption effects between the two adsorbates in the entire pressure range. The separation performance of AC-Rs is ruled by the polar functionality site and accessible surface area at low pressures, whereas the functionalization and total available pore volume turns to be more important at high pressures.

20


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4.


CONCLUSIONS In this study, the adsorption and separation performance of AC-Rs for methanol-acetone has been

investigated. AC-NH2 with the relatively large accessible surface area (4497 m2/g) and Eads (-73.2 kJ/mol for methanol and -63.4 kJ/mol for acetone) exhibits the highest uptake for methanol and acetone at low pressures. AC-SO3H shows the maximum saturated methanol and acetone adsorption capacity at high pressures for its extremely large available pore volume (1.457 cm3/g). Meanwhile, the different configurations between methanol and acetone provide us with a novel approach to separate a binary methanol-acetone mixture. Methanol has a typically polar hydroxyl group and a methyl group, while acetone has a carbonyl group and two methyl groups. The ACs materials with strong electron-donating or electron-accepting functional groups and relative larger accessible surface area (i.e., AC-NH2, AC-OH and AC-SO3H) exhibit a strong methanol adsorption capacity, and thus have a significant selectivity towards methanol. Overall, strong electron-donating or electron-accepting functional groups, large accessible surface area and total available pore volume are the key factors for improving the methanol-acetone separation performance of activated carbons, which provides a superior alternative strategy in the preparation and synthesis of activated carbon for the methanol-acetone mixture separation.

21



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ASSOCIATED CONTENT * Supporting Information Lennard-Jones parameters and partial charges for the adsorbents and the adsorbates; Visualization of BSUs of AC-Rs; Stable adsorption configurations of methanol and acetone on BSUs; Physical Characteristics of the three AC-SO3H models; The adsorption isotherms for the equimolar methanol/acetone mixture on the three ACSO3H models.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected], [email protected] Notes The authors declare no competing financial interest

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21878338), the Key Research and Development Project of Hunan Province, China (No. 2018SK2038).

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Table of Content graphic

Methanol

Methanol-Acetone

Selectivity (Methanol/Acetone)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


AC-Rs

Pressure (kPa)

(R= COOH, OH, NH2, SO3H) BSUs

29


Competitive Adsorption of MethanolâAcetone on Surface

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