Adsorptive Separation of MethanolâAcetone on Isostructural Series of

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Adsorptive separation of methanol-acetone on isostructural series of metalorganic frameworks M-BTC (M = Ti, Fe, Cu, Co, Ru, Mo): A computational study of adsorption mechanisms and metal-substitution impacts Ying Wu, Huiyong Chen, Jing Xiao, Defei Liu, Zewei Liu, Yu Qian, and Hongxia Xi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b07665 • Publication Date (Web): 18 Nov 2015 Downloaded from http://pubs.acs.org on November 24, 2015

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ACS Applied Materials & Interfaces

Adsorptive separation of methanol-acetone on isostructural series of metal-organic frameworks M-BTC (M = Ti, Fe, Cu, Co, Ru, Mo): A computational study of adsorption mechanisms and metal-substitution impacts Ying Wu a, Huiyong Chen b, Jing Xiao a, Defei Liu a, Zewei Liu a, Yu Qian a, Hongxia Xi a,* a- The School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, Guangdong, People’s Republic of China 510641 b- School of Chemical Engineering, Northwest University, Xi’an, Shanxi, People’s Republic of China 710069 * Corresponding author: Hongxia Xi *, Tel.: 86-13825124468 / Fax: 86-020-87113735. E-mail: [email protected] (H. Xi).

ABSTRACT The adsorptive separation properties of M-BTC isostructural series (M = Ti, Fe, Cu, Co, Ru, Mo) for methanol-acetone mixtures were investigated by using various computational procedures of grand canonical Monte Carlo simulations (GCMC), density functional theory (DFT) and ideal adsorbed solution theory (IAST), following with comprehensive understanding of adsorbate-metal interactions on the adsorptive separation behaviors. The obtained results showed that the single component adsorptions were driven by adsorbate-framework interactions at low pressures and by framework structures at high pressures, among which the mass effects, electrostatics and geometric accessibility of the metal sites also played roles. In the case of methanol-acetone separation, the selectivity of methanol on M-BTCs decreased with rising pressures due to the pressure-dependent separation mechanisms: the cooperative effects between methanol and acetone hindered the separation at low

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pressures, whereas the competitive effects of acetone further resulted in the lower selectivity at high pressures. Among these M-BTCs, Ti and Fe analogues exhibited the highest thermodynamic methanol/acetone selectivity, making them promising for adsorptive methanol/acetone separation processes. The investigation provides mechanistic insights on how the nature of metal centers affects the adsorption properties of MOFs, and will further promote the rational design of new MOF materials for effective gas mixture separation. KEYWORDS: Metal-organic frameworks, Adsorptive separation, Methanol/acetone, DFT, GCMC, IAST.

1. INTRODUCTION Methanol-acetone mixtures are well known as one of the most versatile solvents with widely applications in both chemical industry1 and biology-relevant processes.2 Apart from integrating the merits of the apolar CH3 groups and polar C=O groups in acetone, as well as the H-donor OH groups in methanol, the solvation properties (e.g. density and dielectric constant) of the mixture can also be tuned by mixing both in varied proportion1. However, the binary mixture exhibits full miscibility to be a maximum pressure azeotrope3 and shows ease diffusion as volatile organic compounds (VOCs) to the air, making it a challenge in efficient concentration and separation that is necessary for recycling of the solvent.4 Thus, pursuing effective strategies for capture and separation of the methanol-acetone mixtures are highly urged by the widespread use of the solvent in the industrial processes. 2


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Among various separation technologies,5-6 adsorption is of practical interest in organic gas capture with the advantages of ease operation, low energetic consumption and less financial investment7. Many investigations have been deployed using different porous adsorbents for VOCs adsorption, e.g. activated carbon,8 silica,9 nevertheless, their adsorption properties related to varied molecular structure of VOCs are still unsatisfactory. In this respect, metal-organic frameworks (MOFs) exhibit significant potential to be VOC adsorbents, owing to their highly tunable structures and functionalities, as well as the dense CUS (coordinatively unsaturated metal sites)10 for the superior adsorptive separation of various organic molecules.11-12 It has been widely accepted that CUS is thermodynamically favored by small organic molecules, allowing adsorption uptakes at relatively low pressures due to its strong interaction affinity with adsorbates.13-14 However, few studies have been carried out to investigate the impacts of different CUS on the adsorption of organic molecules, which are of significance for screening suitable MOF materials for gas adsorption and separation. Such investigation can be performed by creating isostructural MOF series using metal substitution, since it eliminates other variables that may impose MOF adsorptions, e.g. pore shape and structural topologies, and thereby offering direct insight

into

the

nature

of

adsorbate-metal

interactions.15

Meanwhile,

metal-substitution is also a valuable strategy to tune the adsorption properties of MOFs.16-17 One representative case is the extensive family of M-MOF-74, alternatively known as M2(DOBDC) (DOBDC = 2,5-dioxy-1,4-benzenedicarboxylate, M = Mg, 3



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Mn, Fe, Co, Ni, Cu, or Zn).18-20 Glover et al.21 experimentally reported a remarkable performance of M-MOF-74 (M = Zn, Co, Ni, Mg) series for the adsorption of ammonia and octane, with exception of cyanogen chloride that was dramatically interfered by the presence of water. Geier et al.22 demonstrated that MOF-74 series were capable of offering remarkable selectivities for ethylene-ethane and porylene-propane mixtures. Apart from MOF-74, Cu-BTC23 (or HKUST-1, BTC = 1,3,5-benzentricarboxylate) is another emblematic example of CUS-containing prototype MOF with abundant metal substitutions, including Cr, Ni, Zn, Ru and Mo-substituted analogues.24-28 The crystalline structures of M-BTCs consist of the well-known paddle-wheel (PW) unit29 of metal connectors bridged by the BTC linkers are illustrated in Fig. 1. Surrounded by the apolar small T1 pockets, the large cages are categorized into the less polar L2 and the polar L3 types by the varied orientation of the open metal sites. Cu-BTC and its analogues (M-BTCs) show great promises in numerous applications, involving the removal of VOCs.30-31 Mo-BTC28 exhibits a great ability in gas adsorption (mainly in O2, N2 and H216), profited from its quadruply bonded dimetal units and the relative large BET surface area. Ru3(BTC)2(Cl)x(OH)1.5-x27 possesses well-ordered porosity, which is also suitable for gas adsorption. However, the adsorption and separation of methanol and acetone on M-BTCs have not been reported. In the last decades, both theoretical and experimental studies have been widely applied to investigate the adsorption on isostructural series of MOFs. Commonly, the structural analysis of the as-synthesized MOFs and the corresponding binding 4


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geometry is achieved by the spectroscopy measurements, including the neutron powder diffraction (NPD)32 and in situ diffraction33. While the adsorption and breakthrough21 experiments perform through isosteric method, in order to acquire the adsorption capacity21 as well as the isosteric heat of adsorption32. Furthermore, the experimental isotherms of single components can be fitted from various adsorption models using ideal adsorbed solution theory (IAST) to predict the selectivity of the mixtures on MOFs22. Nevertheless, restricted by the high cost and time-consumption in the synthesis of isostructural materials with varied metal centers, it is quite difficult for screening of MOFs with suitable CUS which can be used for selective adsorption. In this respect, the employment of molecular simulation to predict the adsorption properties of isostructural series of M-BTCs is highly desirable. For instance, the first principle calculations can be used to assess and screen the most possible isostructural MOFs among various metal-substituted variants

34

, and further clarify the relations

between the adsorption properties and the electrostatic structures of MOFs34-35. Moreover, molecular simulation can provide a complementary method with better understanding of the adsorbate-metal interactions and the adsorption mechanisms in atomic level36, which is unattainable by experimental methods.

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Fig. 1 Schematic of M-BTCs (M= Ti, Fe, Cu, Co, Ru, Mo) in (a) top view and (b) perspective view, with (c) the cluster model taken from unit cell. The spheres in green, purple and red represent the guest-free space inside the T1, L2 and L3 pores, respectively. Color code of atoms: M (metal) = orange, C= gray, H= white, O= red.

In this work, six metal-substituted M-BTCs, including the experimentally existed analogues (Fe, Cu, Ru, Mo) and the theoretically constructed ones (Ti, Co), were selected as adsorbents for the adsorptive separation of methanol-acetone, and a combination of grand canonical Monte Carlo simulations (GCMC), density functional theory (DFT) and IAST was employed to study and predict the adsorption and separation behaviors of M-BTCs. Interaction energy calculations coupled with detailed textural analysis of M-BTC series were undertaken to clarify the single-component (methanol/acetone) adsorption mechanisms in varied pressure ranges. Binary adsorption isotherms and selectivity were predicted to examine the separation performance of the selected M-BTCs. Multilayer binding energies were calculated to illustrate the cooperative and competitive effects during the separation process. The obtained results will provide the detailed information of the metal center

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and the local interactions, further realize the screening of suitable adsorbent for specific VOCs molecules on purpose. Such investigation is of great scientific interest in both theory and practice, which is also with ingratiation for the emergence of VOCs capture and organic solvents recycling with novel porous materials.

2. SIMULATION METHODOLOGY 2.1 Model constructions Based on the experimental lattice information obtained from Cambridge Crystallographic Data Centre (CCDC), the crystal structures of Fe-BTC,37 Cu-BTC,23 Ru-BTC27 and Mo-BTC28 were fabricated using the Visualizer module in Materials Studio package (Version 5.0).38 The unit cells of these isostructural analogues have a similar face-centered cubic structure with 3 symmetry. Additionally, we also theoretically constructed Ti and Co analogues based on the initial model of Cu-BTC. The structures and the unit cell dimensions of Ti- and Co-BTCs were determined via geometry optimization in Forcite code. The framework atoms depicted by universal force field (UFF) were allowed to fully relax, with the smart algorithm parameters set as 1×10-5 kcal·mol-1, 0.0005 kcal mol-1 Å-1, and 5.0×10-6 Å for energy, forces and displacement, respectively. The reliability of these two isostructural analogues have been confirmed by Koh et al.34 using revPBE-vdW functional, as Ti- and Co-BTC preserved the similar paddle-wheel geometry to their parent Cu-BTC. To imitate the activated state of the frameworks for adsorption, the terminal and/or disordered water molecules on M-BTCs were eliminated to construct the dehydrated frameworks with 7



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bare metal sites.

2.2 Interatomic potentials To accurately describe the adsorption performance of M-BTCs, both electrostatic and van der Waals (vdW) interactions were taken into account. The electrostatic potential (ESP) charges were used as the atomic partial charges, which were calculated from DFT with Gaussian 09 package.39 Although the experimental synthesized Fe- and Ru- analogues possessed mixed valence CUS (MII,III), in contrast to other M-BTCs with common MII,II units, the usage of the average charge for the metal sites was still effective to predict the adsorption affinity with the adsorbates, as evidenced by Koh et al.’s work,34 in which the average charge of CUS (including Fe-BTC) was used as a descriptor to identify the MOFs with targeted performance instead of the adsorption isotherms. The calculation was performed using the fragmented cluster model with the dangling bonds saturated by methyl groups, as shown in supporting Fig. S1. The ChelpG method was employed with unrestricted B3LYP functional, where the LANL2DZ was used for metal atoms, and 6-31+G* was deployed for the rest of the framework atoms. The vdW radii of the metal atoms was adopted from Cordero et al.’s work,40 as the radii were missed in the basic set used. Such charge calculation approach has been successfully applied to estimate the ESP charges as well as the binding energies in many MOFs including Cu-BTC.13, 41-42 In addition, the vdW interactions was described by 12-6 Lennard-Jones (LJ),43 with the LJ parameters for the framework nonmetal atoms obtained from Yang et al.’s 8


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work,44 while the all-atom universal force field (UFF)45 was adopted for the metal atoms as they were not treated by Yang. This force field has been justified to offer well-reproduction of the experimental adsorption isotherms for CO, CO2 and CH4 on Cu-BTC.44 The LJ parameters and the calculated charges were shown in supporting

Table S1. In terms of methanol and acetone adsorbates, both LJ parameters and the atomic partial charges were obtained from the TraPPE-UA (united atom) force field.46-47 The force field represents the CH3 groups with pseudo united-atoms located at the position of carbon atoms, which has been proved to perform well in describing the adsorption over MOFs contained open CUS.13, 48 The LJ parameters and atomic charges of the adsorbate molecules were listed in supporting Table S2.

2.3 Computational details GCMC simulations were carried out to model the optimal adsorption isotherms for methanol and acetone over M-BTC series at 298 K using MUSIC code.49 The long-range electrostatic interactions were managed by Ewald summation technique, while the vdW interactions were adopted to a cutoff radius of 13.0 Å, in which the cross LJ parameters in adsorbate-MOF interactions were determined by the Lorentz-Berthelot (LB) mixing rules.50 The single unit cells of M-BTCs were performed in the simulations, with the periodic boundary conditions (PBC) taken into account in all three dimensions. Leaving the adsorbate molecules relaxed, the framework atoms were held fixed to replicate the bulk behavior of M-BTCs. It has 9



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been previously justified that the framework of Cu-BTC can be treated as rigid when simulating the equilibrium adsorption of small gases,31, 51 as the framework flexibility have negligible effects on the adsorption.52 During the GCMC simulations, four types of random moves (translation, insertion, delection and rotation) were used for the adsorbate molecules. For each state point, the calculation contained 5 × 106 trials to ensure the equilibration, and the following 5 × 106 trials were used to sample the average production. To verify the atomic charge and the LJ parameters used in this work, the simulated absolute adsorption uptakes (Nabs) were converted into excess adsorption uptakes (Nex) using eq. 1, in order to make a comparison with experiments.

= −

(1)

Where Vg represents the guest-free pore volume of MOFs that is detected by helium as nonadsorbing gas,53 while ρg represents the density of the adsorbate estimated by the Peng-Robinson equation of state.54 In addition, we undertook a simple Monte Carlo program55 to calculate the accessible surface areas (Sacc) of M-BTCs. DFT calculations by the Gaussian 0939 were performed to obtain the energy-minimum configurations and the corresponding binding energies (BEs) between adsorbates and frameworks based on the dispersion-corrected ωB97X-D functional. The DFT approach has been successfully employed to estimate the interaction energies between organic molecules.56-57 The frozen cluster model shown in Fig. 1 was used in the calculation, while the adsorbate molecules were left fully 10


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relaxed. For each adsorbate-framework complex, 6-31G(d, p) was used to search for the minimum energy configurations, while a higher level of 6-311++G(d, p) was adopted for the energetic calculations, with the basis set superposition error (BSSE) considered by the counterpoise corrections.58 Previous work59 has clarified that such mixed basis set method was capable of providing comparative results with the case that all the calculations were based on high-level theory. BE was calculated by eq. 2 as follows.

BE = Ecomplex – (EMOF + Eadsorbate)

(2)

Where the Ecomplex is the total energy of the optimized adsorption complex, while

EMOF and Eadsorbate is the single-point energy of M-BTC and adsorbate molecule, respectively. To investigate the adsorptive separation properties of binary mixtures on M-BTCs, the adsorption isotherms of equimolar mixtures were calculated using GCMC simulations, with 1.5 × 107 trials employed for equilibration, and the subsequent 1.5 × 107 trials for production in each state point. On the basis of the binary mixture isotherms, the adsorptive selectivity (S) was computed by eq. 3, with

and representing the mole fraction of components j (j=1 or 2) in the adsorbed and bulk phases, respectively.

S = ×

(3)

In addition, IAST-predicted selectivities,60 fitting from the GCMC-simulated single component isotherms, were carried out to compare with the direct GCMC-calculated results, since IAST procedure was justified to offer more 11



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thermodynamically stable results than GCMC calculations.41 After testing several adsorption models, the dual-site Langmuir-Freundlich (DSLF) was proved to be the best model in describing the isotherms on M-BTCs (with the correlation coefficient

R2 as high as 0.999), as shown in eq. 4.

N = ×

( )

( )

+N ×

( )

( )

(4)

Where N is the adsorbed uptakes on M-BTCs, Ni (i=1 or 2) represents the saturated capacities of the two adsorption sites on the framework, p is the total bulk pressure, b and n are the affinity coefficients and the deviations from ideal homogeneous surface, respectively. The fitting parameters were listed in supporting

Table S3.

3. RESULTS AND DISCUSSION 3.1 Pure component adsorption on M-BTCs Prior to comprehensively investigation of the adsorption properties on M-BTCs, the calculated adsorption isotherms of methanol and acetone on Cu-BTC were compared with experimental results (see Fig. 2), in order to validate the atomic charge distribution and the force field used in the simulation. The experimental data were obtained from gravimetric measurements, and the details were performed in the supporting information. The adsorption capacity of acetone climbed up quickly at the onset of adsorption and subsequently stabilized at about 9 mmol/g, whereas the isotherm of methanol exceeded that of acetone as the pressures reached over 0.5 kPa

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and the adsorption capacity approached to saturation at around 19 mmol/g. Such obtained results suggested the different interaction mechanics of the two adsorbates with the frameworks, which might involve the varied preferential adsorption sites as well as the steric effects at different pressure ranges, and could be further understood in section 3.2. Additionally, the simulated isotherms for both methanol and acetone matched reasonably well with the referred experimental results, although slight discrepancies were observed among the entire pressure range, similar as CO2 adsorption on Cu-TDPAT, which contained similar open Cu-sites as Cu-BTC.61 The underestimation of simulation at low pressures probably resulted from the strong interactions between adsorbate molecules and open metal sites, which were difficult to model by simple force field.62 In contrast, the calculated isotherms transcended the experimental data at higher pressures, ascribing to the idealized framework model used in this work was capable of accommodating more molecules compared to the real material with inaccessible pores. It has been reported that the simulated results exhibited a good accordance with experiments if the inaccessibility of the framework was considered to imitate the real sample.63 Therefore, we believed that the combination of interatomic potentials could provide rational description for the adsorption properties of Cu-BTC, and further extended to the subsequent calculations in other analogues.

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Fig. 2 Simulated and experimental adsorption isotherms of methanol (Me) and acetone (Ace) on Cu-BTC at 298 K.

In addition, it should be mentioned that the synthetized Fe-, Ru- and Mo-BTC contained additional counterions (usually the Cl- or OH- ions) to maintain the electron-neutrality of the framework. We therefore examined the influence of the counterions within Fe- and Ru-BTC on the adsorption, as illustrated in supporting Fig. S5 and Fig. S6. It could be expected that the adsorption property was restricted in the presence of the counterions, ascribing to the Cl- ions that might block the open metal sites and enhance the weight of the framework. Such counterions were probably disordered inside the pores (Fe-BTC37) or coordinated with the metal centers (Ru-BTC36), making the system more complex and difficult to compared with other common M-BTCs. Thus, it was more worthwhile to study a less complex scenario with the counterions eliminated, in order to exclusively investigate the electrostatic

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influence of different CUS in this study.

Fig. 3 Simulated single-component adsorption isotherms of (a) methanol and (b) acetone on different M-BTCs (M= Ti, Fe, Cu, Co, Ru, Mo) at 298K.

Fig. 3 illustrated the simulated adsorption isotherms of single methanol and acetone on different M-BTCs at the pressure range from 0 to 10 kPa. The isotherms of two adsorbates nearly reached their initial saturation at 10 kPa and maintained steady as the pressures rose up to 100 kPa (see supporting Fig. S7), we therefore majorly focused on the investigation of their adsorption properties at the low pressure range. The capacities of both methanol and acetone on these M-BTCs followed the order of Ti-BTC > Fe-BTC > Cu-BTC > Co-BTC > Mo-BTC > Ru-BTC. The isotherms on Tiand Fe-BTC started at a high uptake (9 mmol/g and 6 mmol/g for methanol and acetone, respectively) at the low pressure of 0.02 kPa, and remained steady at a higher level compared to the other M-BTCs in the entire pressure range. In terms of Cu-, Co-, Ru- and Mo-BTC, the isotherms could be divided into three regions, (1) slow grew at

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onset of adsorption, (2) a rapid rise in a narrow pressure range (step region), (3) another slow increase at higher pressures, showing the type-IV isotherm category according to the International Union of Pure and Applied Chemistry (IUPAC) classification.64 As the adsorption performance of MOFs commonly varied with pressures,31, 34, 51 it was more rational to promote the investigation in different pressure ranges. The BEs between adsorbate and adsorbent were commonly used to measure the adsorption mechanics at low pressures, since BEs were more sensitive to describe guest-host interactions than isotherms at low concentrations of adsorbates.13,

65

We therefore

calculated the BEs between adsorbate molecules and the open metal sites of M-BTCs. It should be mentioned that only the configurations of oxygen atoms of adsorbate molecules oriented to the CUS of M-BTCs were taken into account, as it had been claimed that the adsorption dominantly occurred around the regions of CUS at low pressures, while the interactions in the other adsorption sites (e.g. organic linkers) or orientations were less significant at low pressures.13-14, 65 The obtained results were shown in Fig. 4, and the corresponding optimized complexes were illustrated in supporting Fig. S8 and Fig. S9.

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Fig.4 Binding energies (BEs) of methanol and acetone with oxygen atoms oriented to the CUS of different M-BTCs (M= Ti, Fe, Cu, Co, Ru, Mo).

Fig. 4 showed that the BEs for both methanol and acetone on M-BTCs followed the order of Fe-BTC > Ti-BTC > Cu-BTC > Co-BTC > Ru-BTC > Mo-BTC, which generally agreed with the order of the adsorption uptakes at low pressures (see Fig. 3), except those of Fe-BTC and Ru-BTC. Although previous studies31, 51, 65 indicated that the adsorption on MOFs at low pressures was dominated by adsorbate-adsorbent interactions (involving the electrostatic and dispersive interaction), the obtained results in this work suggested that the interaction energy might not be the single factor controlling the adsorption process at low pressures. As listed in Table 1, Fe-BTC (Fe= 55.0 g/mol) and Ru-BTC (Ru= 101.07 g/mol) possessed higher weight compared to Ti-BTC (Ti= 47.8 g/mol) and Mo-BTC (Mo= 95.9 g/mol), respectively, indicated that the capacity reduction on Fe-BTC and Ru-BTC was likely attributed to the heavy mass of the frameworks. Additionally, although the open metal ions with smaller ionic radius were expected to significantly evoke the polarization of guest molecules,20 17



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leading to stronger interaction strength, an allowance for the “burrowing” effects in the four-fold-coordination environment of the PW metal nodes34 should also be made. Such textural effects arisen from smaller ionic radius of CUS led to a “burrowing” of the open metal sites within the SBU, which made CUS possess poor accessibility to the adsorbed molecules. Thus, Ti- and Mo-BTC with relatively larger metal radius were able to adsorb more molecules compared to Fe- and Ru-BTC at a given pressure. In addition, methanol adsorbed on Fe-BTC possessed the greatest BE (about -70 kJ/mol) among the M-BTCs. Theoretically, such strong interaction affinity of methanol seemed to hinder the desorption process by large energetic consumption. However, Koh et al.34 implied that the adsorption enthalpies for CO2 capture ranking between -40 to -75 kJ/mol might be the most desirable under the consideration of regeneration efficiency, while the actual adsorption energy of CO2 on Fe-BTC only reached -30 kJ/mol, which was low compared to methanol (-70 kJ/mol). In this respect, the optimal energy range for methanol adsorption should be at least higher than that of CO2. Therefore, we believed that the interaction strength between methanol and Fe-BTC (-70 kJ/mol) might not significantly inhibit the regeneration of the framework. Furthermore, we also concerned about the effects of electrostatic structures of the M-BTCs on the interaction energies with the adsorbates. Rana et al.35 reported that the higher charge on the CUS of M-DOBDC family was capable of enhancing the interaction between MOF and CH4, resulting in the higher BEs of the adsorption. However, we found that the effective charge of the CUS followed the order: Ti-BTC > 18


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Fe-BTC > Co-BTC > Cu-BTC > Mo-BTC > Ru-BTC (see Table 1), which did not match well with the order of BEs. The obtained result was likely ascribed to the distance from CUS to the neighboring oxygen atoms in the framework. Such oxygen atoms with strong electronegativity could repulse the electron-donating oxygen atoms in adsorbates, and further restricted the electrostatic interactions between adsorbates and CUS. Therefore, Ti-, Co- and Mo-BTC possessed lower BEs although they had higher CUS charge compared to Fe-, Cu- and Ru-BTC, respectively, since the framework oxygen atoms packed closer to Ti-, Co- and Mo- sites than to Fe-, Cu- and Ru- sites (see Table 1). Fig. 4 also demonstrated that the BEs for methanol adsorption in selected M-BTCs were always more exothermic than those for acetone adsorption. It can be ascribed to the TraPPE force field which defines a stronger interaction between oxygen atom of methanol molecule and the framework of M-BTCs, as listed in supporting Table S2. The obtained results indicate that apart from the adsorbate-adsorbent

interactions

(energetic

effects),

the

framework

mass,

electrostatics and the metal radius of CUS also have impacts on the low pressure adsorption properties of M-BTCs. On the other hand, the adsorption isotherms were essentially linear at low pressure range (below 0.1 kPa), indicating that the adsorption of methanol and acetone throughout the pressure range obeyed the Henry’s law behavior. Therefore, apart from the BEs, we also computed the Henry’s constants (kH) to describe the adsorption affinity at low pressures, as listed in Table 2, which were fitted from the 19



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slope of linear with the first 4 adsorption data points, corresponding to the pressures less than 0.1 kPa. The values of kH for both methanol and acetone followed the order Ti-BTC > Fe-BTC > Cu-BTC > Co-BTC > Mo-BTC > Ru-BTC, which agreed well with the order of adsorption uptakes, but have exception of Fe- and Ru-BTC when compared with the order of BEs. A plausible explanation lied in the BEs calculation that only focused on the open CUS sites, while the kH fitted from isotherms was able to describe the adsorption combined with various factors, involving the higher framework weight and the smaller CUS radius of Fe- and Ru-BTC that might inhibit the adsorption, as have mentioned earlier. Additionally, it was interesting that acetone have a larger value of kH than methanol, in contrast to the order of BEs, suggesting that more acetone molecules were preferentially adsorbed in other portions of the framework (e.g. organic linkers or small cages) rather than the CUS, which was further discussed in section 3.2.

Table 1 Physical and textural properties of M-BTCs (M= Ti, Fe, Cu, Co, Ru, Mo). Properties

Ti-BTC

Fe-BTC

Cu-BTC

Co-BTC

Mo-BTC

Ru-BTC

Metal radius (Å)

1.45

1.26

1.28

1.25

1.39

1.34

Metal relative mass (g/mol)

47.8

55.8

63.5

58.9

95.9

101.1

Metal charge (eV)

1.386

1.289

1.089

1.269

1.195

0.821

M-O (Å)

1.994

2.047

1.952

1.813

1.977

2.001

Accessible surface (m2/g)

2484.2

2362.5

2167.0

2113.9

2095.1

2142.1

Unit cell lattice (Å)

26.64

26.63

26.34

26.00

27.13

26.55

20


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Table 2 Henry’s constants (cm3g-1kPa-1) at 298 K. Ti-BTC

Fe-BTC

Cu-BTC

Co-BTC

Mo-BTC

Ru-BTC

Methanol

384.16

188.46

154.94

63.06

32.15

16.80

Acetone

493.74

413.73

319.07

143.51

140.04

91.60

In high pressure range, the textural properties of framework turned to play more significant roles rather than CUS, as most metal sites have been terminated by adsorbed molecules.31, 50 Table 1 showed that the Sacc of M-BTCs followed the order of Ti-BTC > Fe-BTC > Cu-BTC > Ru-BTC > Co-BTC > Mo-BTC, generally correlating with the order of adsorption capacity for both methanol and acetone at high pressures (as illustrated in Fig. 3), excluding the case of Ru analogue. Ru-BTC possessed relatively higher Sacc (2142.1 m2/g) but the adsorption uptakes fell to the end of the scale, only offering about 15 mmol/g for methanol and 8 mmol/g for acetone. Similar as the case in low pressure adsorption, the capacity reduction of Ru-BTC could be associated with negative mass effects, as Ru-BTC possessed the heaviest mass (Ru= 101.1 g/mol) among the M-BTCs studied. The results suggested that the high pressure adsorption on M-BTCs was not only governed by the textural properties of the frameworks, the other factors, e.g. the framework mass or density should also be taken into account.

3.2 Influences of electrostatic and dispersive interactions The additional adsorption isotherms of methanol and acetone on Ti-BTC with the adsorbate-framework electrostatic interactions (EIs) switching off, were further 21



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calculated to fundamentally investigate the interaction mechanisms between adsorbates and adsorbents. Ti-BTC, which had the greatest charge value (1.386 eV, as listed in Table 1) of CUS among the M-BTCs, was expected to offer more distinct information about the influence of EIs in the adsorption.34 The isotherms without EIs were compared with their counterparts with EIs, as illustrated in Fig. 5, where the percentage contributions of EIs were also laid out to quantitatively indicate the impacts of EIs on the adsorption. The EI contribution to the adsorption capacity was defined as eq. 5.

!" $%&'()*+')%& =

|-./01 2-./01340 | -./01

× 100%

(5)

Where Nwith and Nwithout represented the absolute adsorption uptakes with and without EI between adsorbate and framework, respectively.

22


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Fig. 5 Adsorption isotherms of (a) methanol and (b) acetone on Ti-BTC at 298 K, with and without the consideration of the adsorbate-framework electrostatic interactions (EI); the percentage of EI contributions is also performed.

Without EIs, the adsorption uptakes of methanol dramatically plummeted and remained at an extremely low level during 0~3 kPa, while the uptakes of acetone rapidly increased after a slight decrease in a narrow pressure range (< 1 kPa). As the adsorption at low pressures dominantly occurred around CUS,13 we proposed that the regions surrounding CUS were capable of affording larger concentrations of methanol than acetone. It was likely associated with the stronger EIs affinity of methanol, as 23



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evidence by the greater EI contribution of methanol (95%) compared to acetone (85%), consistent with the BE results illustrated in Fig. 4. After CUS reached saturation, the adsorbate molecules began to rank in multilayers and further filled the cages of the framework,13 and the EIs between adsorbates and CUS turned to be less significant to the adsorption, with the contributions dramatically dropped to 5.7% and 6.9% for methanol and acetone, respectively. The obtained results revealed a sequential adsorption behavior in the framework. Methanol exhibited a pronounced stepwise adsorption process as the stronger EIs affinity allowed the CUS-interaction stage lasted for a relatively wide pressure range, whereas such EI stage in the entire adsorption process of acetone showed less effects.

Fig. 6 Potential energies of site-site interactions for (a) methanol and (b) acetone on Ti-BTC in four cases: EIs between adsorbate-adsorbate, DIs between adsorbate-adsorbate, EIs between adsorbate-framework, DIs between adsorbate-framework.

In addition, the potential energies of four interaction cases, i.e. (1) EIs in 24


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adsorbate-adsorbate interaction, (2) EIs in adsorbate-framework interactions, (3) dispersive interactions (DIs) in adsorbate-adsorbate interaction, and (4) DIs in adsorbate-framework interactions, were further investigated to thermodynamically clarify the underlying adsorption mechanisms of methanol and acetone on Ti-BTC, as presented in Fig. 6. The EIs between methanol and Ti-BTC stood at a high energetic level of -36 kJ/mol at the onset of the adsorption, but subsequently dropped and maintained at the level of -16 kJ/mol. Meanwhile, the energies of DIs between methanol and Ti-BTC rose with the decreasing EIs, but remained at a lower level (-10 kJ/mol) compared to EIs. The observation reflected the process that the CUS were gradually blocked by methanol molecules, corresponding to the decrease stage when switching off EIs in Fig. 5(a). On the other hand, the EIs between methanol-methanol interactions which reached a remarkably high level of -24 kJ/mol when the pressures rose up to 3 kPa, suggesting

that

the

multilayer

adsorption

of

methanol

and

the

further

molecule-clustering were prone to occur through the intermolecular EIs and further contributed to the adsorption, coincided with the information provided by the density distribution contours of methanol on Cu-BTC.13 Such results could give a rational explanation to the insignificant role of EIs between methanol and Ti-BTC after the CUS reaching saturation (see Fig. 5(a)), where the DIs between methanol and Ti-BTC as well as the EIs between methanol molecules dominantly control the adsorption at higher pressures. With respect to acetone (see Fig. 6(b)), significant variations were found 25



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compared to those of methanol. DIs between acetone and Ti-BTC majorly governed the adsorption in the entire pressure range with the potential energies stood at -22 kJ/mol, while the EIs experienced a rapid decrease to -15 kJ/mol and kept steady to the end of the pressure range. It should be ascribed to the two methyl groups of acetone that provided stronger DIs affinity compared to the single methyl group of methanol, further led to the adsorption occur near the organic linkers, which consistent with CO2 adsorption on ZIF-68.66 In contrast, the oxygen atom of acetone in electrostatics to the CUS was less favorable than methanol, made EIs only account for relatively low occupancy in acetone-Ti-BTC interactions, which further consolidated the results observed in Fig. 5(b). On the other hand, the strong affinity of DIs (BE = -12 kJ/mol) between acetone-acetone suggested that the formation of acetone multilayers occurred despite the connection between acetone molecules was weaker than that between methanol molecules with EIs (BE = -24 kJ/mol). When comparing with Fig. 6(a) and (b), the varied interaction preferences of methanol and acetone also made better understanding of the different isotherm trends of the two adsorbates (see Fig. 2). Since the DIs with organic linkers governed the adsorption of acetone and the organic linkers were highly dense in the framework, the acetone molecules were capable of freely adsorbing throughout the entire framework at low pressures. In contrast, the low-pressure adsorption of methanol mainly focused on the CUS which were less accessible than organic linkers, resulting in the lower adsorption uptakes of methanol than acetone. As the pressures increased, the uptakes of methanol gradually exceeded that of acetone, which resulted from the multilayer 26


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adsorption occurred in the pores by the greater intermolecular EIs affinity of methanol molecules. Meanwhile, the negative steric effects due to the bulky size of acetone molecules also hindered the pore-filling process at high pressures. Therefore, the two complementary factors gave rise to the lower adsorption capacity of acetone compared to methanol in higher pressure range.

3.3 Binary separation of methanol/acetone mixtures Fig. 7 presented the methanol selectivities over methanol/acetone equimolar mixtures, calculated from GCMC simulations of the binary mixture adsorption isotherms at 298 K. The methanol selectivities on all M-BTCs decreased with rising pressures, giving the maximum value of selectivity at the onset of the adsorption processes, similar as other common organic gases separation systems.67 Fe- and Ti-BTC (about 1.2) exhibited at least 4 times higher selectivities in comparison to other M-BTCs, which was probably associated with the advantage of the stronger interaction affinity with methanol (as evidenced by Fig. 4), as well as the relatively low weight of the frameworks, as mentioned earlier. Followed by Fe- and Ti-BTC, the selectivity on Mo-BTC started from a relatively low level (about 0.3), while the cases of Cu-, Co- and Ru-BTC fell to the lowest end of the scale (< 0.15). Additionally, undulation of the selectivities was found at the low pressure range, which was likely resulted from the insufficient equilibration of the binary GCMC simulations,50 as the two adsorbates might have different interaction mechanisms with the M-BTCs at low pressures. 27



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Apart from the GCMC simulations, IAST calculations based on DSLF model were also performed to predict the methanol selectivity, as illustrated in supporting Fig. S10. Different from the direct GCMC calculations, the IAST-predicted selectivities on M-BTCs increased with the rising pressures, and tended to generate overestimated data. The discrepancy between IAST and GCMC simulations revealed a large non-ideality in the adsorbed phase,8 which should be attributed to the high polarity of the two adsorbates that could provide strong interaction affinity in the electrostatic environment of the CUS-contained MOFs. Therefore, the assumption of IAST was less applicability to methanol-acetone separation system, whereas the GCMC simulation, which was capable of considering the non-ideality of the system, was more reliable to further investigate the separation process.

Fig. 7 GCMC-calculated selectivities for equimolar mixtures of methanol/acetone on M-BTCs (M= Ti, Fe, Cu, Co, Ru, Mo), as a function of total bulk pressures at 298 K.

28


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Fig. 8 Density distribution contours for equimolar mixtures of (a, b, c) methanol and (d, e, f) acetone in the (001) plane of Ti-BTC at 0.2 kPa (left), 2 kPa (center), 10 kPa (right), respectively. The category of cages (T1, L2 and L3) was denoted in (a) for clarity.

To clarify the separation mechanics of methanol/acetone mixtures on M-BTCs, Ti-BTC was selected as representative to illustrate the preferential adsorption regions for the equimolar methanol/acetone mixtures. Fig. 8 showed the density distribution contours for the center of mass (COM) of methanol and acetone at different pressures, where 5000 GCMC-equilibrium configurations of adsorbate locations were accumulated within each graphic. At the onset of the adsorption, both adsorbate molecules were far below adsorptive equilibration and allowed to independently

29



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interact with the framework, which generated a certain degree of methanol selectivity owing to the higher interaction affinity of methanol than acetone with the CUS, as confirmed by the energetic results in Fig. 4 and Fig. 6. When the pressures grew to 0.2 kPa (see Fig. 8(a)), methanol molecules preferentially occupied the CUS as well as the windows connecting T1 cages and the neighboring L3 cages, while acetone molecules were prone to adsorb into the T1 pockets (see Fig. 8(d)). Theoretically, acetone molecules with less EIs affinity were predicted to adsorb in the apolar regions (e.g. near the organic linkers), however, the obtained results indicated that a certain amount of acetone molecules were capable of adsorbing in the polar regions similar as methanol.

Fig. 9 Binding energies for methanol (navy) and acetone (red) on the CUS of Ti-BTC. The arrows represent the process of adsorbates interacting with the framework or another adsorbates.

To analyze such a specific behavior of acetone, Fig. 9 was performed to depict

30


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the interaction strength (BEs) in the mixture adsorption process, and the corresponding optimized complexes were illustrated in supporting Fig. S11. The energy of guest-free Ti-BTC was set as the reference (0 kJ/mol), and the navy and red arrows referred to the adsorption processes of methanol and acetone, respectively. As have mentioned in Fig. 4, the BEs of adsorbing one methanol molecule (-62.15 kJ/mol) was more exothermic than that of acetone (-46.18 kJ/mol). Additionally, acetone molecules distributed more widely throughout the framework (including the T1 pockets, as illustrated in Fig. 8), indicating the stronger entropic effects compared to methanol. In this case, according to the definition of Gibbs energy ∆G = ∆H −

T∆S (∆H = enthalpy, ∆S = entropy), the obtained resulted suggested that methanol with greater ∆G was more thermodynamically favored by Ti-BTC compared to acetone. After preferentially adsorbing the first methanol molecule, the strong lateral interactions between methanol and acetone were capable of diminishing the total energy of the adsorption system to -118.47 kJ/mol, which was even more exothermic than adding a second incoming methanol molecule to the adsorbed methanol (-80.76 kJ/mol), suggesting that the presence of the adsorbed methanol helped to intensify the additional adsorption of acetone. In the case of acetone, the BE was low but somewhat close to the liquefaction enthalpy of acetone adsorption (about -32.1 kJ/mol68), suggesting that the capillary condensation might occur during the adsorption of acetone, as evidenced by the rapid increase step in the single acetone adsorption isotherm (see Fig. 2). Thus, acetone could also adsorbed on the framework and promoted methanol adsorption. Such enhancement in the mixture adsorption 31



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process was called the cooperative adsorption effects,69 similar as the behaviors of ethane/ethylene separation in ZIFs.50 Apart from adsorbing in the polar regions similar as methanol, acetone molecules were able to encapsulate into the confined space of T1 pockets (see Fig. 8(d)) due to the relatively stronger DI affinity of acetone towards the organic linkers, differed from the adsorption of methanol, reflecting that the entropic effects70 of the small (T1) cages can also provide benefits when DIs control the adsorption. Therefore, these two counterbalancing factors gave rise to the increased adsorption uptakes of acetone, eventually resulted in the decreased methanol selectivity on Ti-BTC at the pressure of 0.2 kPa (see Fig. 7). After the limited regions surrounding the open CUS in the polar L3 cages were blocked by the adsorbed molecules (2 kPa), the multilayer adsorption and further molecule-clustering processes would occur, as denoted in Fig. 8(b) and (e). In this stage, acetone molecules further migrated into the T1 pockets as well as the L2 cages, and gradually displaced the previously adsorbed methanol molecules near the open windows of the T1 cages. Whereas methanol molecules were forced to clustered together inwards the L2 cages, in which their amount was slightly higher than that of acetone, ascribing to the stronger intermolecular EIs strength between methanol molecules than the weak acetone-acetone DIs, as mentioned in Fig. 6. However, the total adsorbed amount of acetone was still higher than that of methanol, and thus led to the continuously decreasing methanol selectivity with rising pressures. When the pressures reached 10 kPa, as shown in Fig. 8(c) and (f), both methanol and acetone were forced to fill the void regions in the framework. In this respect, the 32


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adsorption space was dominantly occupied by acetone molecules, while only limited pore volume was available for methanol adsorption. The observation suggested that acetone possessed stronger adsorptive competition than methanol, which was likely resulted from the lower liquefaction enthalpy of acetone (-32.1 kJ/mol) than that of methanol (-38.4 kJ/mol), whereas the steric effect of acetone molecules seemed to play less significant roles in the separation processes. Thereby, the obtained results revealed that the factors determining the separation performance of M-BTCs vary with pressures. The cooperative effects at low pressures reduced the methanol selectivity by inducing the additional adsorption of acetone molecules, whereas the competitive effects of acetone turned to be more significant at high pressures and further diminished the methanol selectivity.

4. CONCLUSIONS The foregoing results demonstrated the potential utility of the isostructural series of metal-substituted M-BTCs (M= Ti, Fe, Cu, Co, Ru, Mo) for methanol-acetone adsorptive separation. The single component adsorption isotherms for methanol and acetone on these M-BTCs obeyed a similar trend, and their adsorption capacities followed the order of Ti-BTC > Fe-BTC > Cu-BTC > Co-BTC > Mo-BTC > Ru-BTC. By examining the binding energetics and the textural properties of the frameworks, we found that the factors affecting the adsorption varied with pressures: the adsorbate-framework interactions governed the adsorption at low pressures, while the framework structures turned to be more significant at high pressures. On the other 33



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hand, the strong electrostatic interaction potentials in methanol-CUS and methanol-methanol interactions gave rise to the centralization of methanol molecules around CUS and the subsequent multilayer adsorption, while the acetone adsorption was dominantly governed by dispersive interactions between acetone and framework. With respect to the separation of equimolar methanol/acetone mixtures, the selectivities of methanol decreased with rising pressures, attributing to the combined effects of the cooperation and competition between methanol and acetone at different pressure ranges. According to the present study, the electrostatics comprised a significant portion of adsorbate-adsorbent interactions, suggesting that the M-BTCs containing high electropositive CUS with less compact neighboring oxygen atoms were capable of offering strong interaction affinity with the adsorbates. Additionally, the promising adsorption and separation performances should take the advantage of lower weight of the framework, as well as the appropriate ionic radius of CUS that allowed the steric accessibility of the CUS for the adsorbates. Yet, changing one of these properties might worsen the others and caused drawbacks to the results. Fe- and Ti-BTC showed highest performances in both adsorption and separation processes, as they balanced the electrostatic and structural properties mentioned above. Mechanic insights on M-BTC property- separation performance relationship provided in this work will be of value in guiding synthesis efforts towards promising MOFs for methanol-acetone mixture separation.

34


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ASSOCIATED CONTENT Supporting Information Cluster model of M-BTCs, LJ potential parameters and partial charges for adsorbates and M-BTCs, the fitting parameters of DSLF model, detailed experimental synthesis, XRD, SEM and nitrogen adsorption on Cu-BTC, optimized interaction configurations of adsorbates on M-BTCs. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Hongxia Xi Phone: 86-13825124468. Fax: 86-020-87113735. E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS We gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 21436005 and 21176084), the National High Technology Research and Development Program of China (No. 2013AA065005), Specialized Research Fund for the Doctoral Program (No. 20130172110012) and Guangdong Natural Science Foundation (S2011030001366).

35



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REFERENCES (1) Idrissi, A.; Hantal, G.; Jedlovszky, P. Properties of the Liquid-Vapor Interface of Acetone-Methanol Mixtures, as Seen from Computer Simulation and Itim Surface Analysis. Phys. Chem. Chem. Phys. 2015, 17, 8913-8926. (2) Lijian, L.; Xingzhong, Y.; Huajun, H.; Jianguang, S.; Hou, W.; Xiaohong, C.; Guangming, Z. Bio-Char Derived from Sewage Sludge by Liquefaction: Characterization and Application for Dye Adsorption. Appl. Surf. Sci. 2015, 346, 223-31. (3) Idrissi, A.; Polok, K.; Barj, M.; Marekha, B.; Kiselev, M.; Jedlovszky, P. Free Energy of Mixing of Acetone and Methanol: A Computer Simulation Investigation. J.

Phys. Chem. B 2013, 117, 16157-16164. (4) Gil, I. D.; Botía, D. C.; Ortiz, P.; Sánchez, O. F. Extractive Distillation of Acetone/Methanol Mixture Using Water as Entrainer. Ind. Eng. Chem. Res. 2009, 48, 4858-4865. (5) Demeestere, K.; Dewulf, J.; De Witte, B.; Van Langenhove, H. Sample Preparation for the Analysis of Volatile Organic Compounds in Air and Water Matrices. J. Chromatogr. A 2007, 1153, 130-144. (6) Verriele, M.; Plaisance, H.; Depelchin, L.; Benchabane, S.; Locoge, N.; Meunier, G. Determination of 14 Amines in Air Samples Using Midget Impingers Sampling Followed by Analysis with Ion Chromatography in Tandem with Mass Spectrometry.

J. Environ. Monit. 2012, 14, 402-408. (7) Zhang, W.; Qu, Z.; Li, X.; Wang, Y.; Ma, D.; Wu, J. Comparison of Dynamic Adsorption/Desorption Characteristics of Toluene on Different Porous Materials. J.

Environ. Sci. 2012, 24, 520-528. (8) Ushiki, I.; Ota, M.; Sato, Y.; Inomata, H. Prediction of VOCs Adsorption Equilibria on Activated Carbon in Supercritical Carbon Dioxide over a Wide Range of Temperature and Pressure by Using Pure Component Adsorption Data: Combined Approach of the Dubinin-Astakhov Equation and the Non-Ideal Adsorbed Solution Theory (NIAST). Fluid Phase Equilib. 2014, 375, 293-305. (9) Kaloni, T. P.; Schreckenbach, G.; Freund, M. S. Large Enhancement and Tunable Band Gap in Silicene by Small Organic Molecule Adsorption. J. Phys. Chem. C 2014,

118, 23361-23367. 36


Page 36 of 44

Page 37 of 44

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


(10) Huang, C. Y.; Song, M.; Gu, Z. Y.; Wang, H. F.; Yan, X. P. Probing the Adsorption Characteristic of Metal-Organic Framework Mil-101 for Volatile Organic Compounds by Quartz Crystal Microbalance. Environ. Sci. Technol. 2011, 45, 4490-4496. (11) Ellern, I.; Venkatasubramanian, A.; Jin-Hwan, L.; Hesketh, P.; Stavila, V.; Robinson, A.; Allendorf, M. HKUST-1 Coated Piezoresistive Microcantilever Array for Volatile Organic Compound Sensing. Micro Nano Lett. 2013, 8, 766-9. (12) Gutierrez, I.; Diaz, E.; Vega, A.; Ordonez, S. Consequences of Cavity Size and Chemical Environment on the Adsorption Properties of Isoreticular Metal-Organic Frameworks: An Inverse Gas Chromatography Study. J. Chromatogr. A 2013, 1274, 173-180. (13) Wu, Y.; Liu, D.; Chen, H.; Qian, Y.; Xi, H.; Xia, Q. Enhancement Effect of Lithium-Doping Functionalization on Methanol Adsorption in Copper-Based Metal-Organic Framework. Chem. Eng. Sci. 2015, 123, 1-10. (14) Wu, Y.; Xiao, J.; Wu, L.; Chen, M.; Xi, H.; Li, Z.; Wang, H. Adsorptive Denitrogenation of Fuel over Metal Organic Frameworks: Effect of N-Types and Adsorption Mechanisms. J. Phys. Chem. C 2014, 118, 22533-22543. (15) Bondi, A. Van Der Waals Volumes and Radii. J. Phys. Chem. 1964, 68, 441-451. (16) Wade, C. R.; Dinca, M. Investigation of the Synthesis, Activation, and Isosteric Heats of CO2 Adsorption of the Isostructural Series of Metal-Organic Frameworks M3(BTC)2 (M = Cr, Fe, Ni, Cu, Mo, Ru). Dalton Trans. 2012, 41, 7931-7938. (17) Schneemann, A.; Henke, S.; Schwedler, I.; Fischer, R. A. Targeted Manipulation of Metal-Organic Frameworks to Direct Sorption Properties. ChemPhysChem 2014,

15, 823-839. (18) Bhattacharjee, S.; Choi, J. S.; Yang, S. T.; Choi, S. B.; Kim, J.; Ahn, W. S. Solvothermal Synthesis of Fe-MOF-74 and Its Catalytic Properties in Phenol Hydroxylation. J. Nanosci. Nanotechnol. 2010, 10, 135-141. (19)Bloch, E. D.; Murray, L. J.; Queen, W. L.; Chavan, S.; Maximoff, S. N.; Bigi, J. P.; Krishna, R.; Peterson, V. K.; Grandjean, F.; Long, G. J.; Smit, B.; Bordiga, S.; Brown, C. M.; Long, J. R. Selective Binding of O2 over N2 in a Redox–Active Metal–Organic Framework with Open Iron(Ii) Coordination Sites. J. Am. Chem. Soc. 2011, 133, 14814-14822. (20) Zhou, W.; Wu, H.; Yildirim, T. Enhanced H2 Adsorption in Isostructural 37



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Page 38 of 44

Metal−Organic Frameworks with Open Metal Sites: Strong Dependence of the Binding Strength on Metal Ions. J. Am. Chem. Soc. 2008, 130, 15268-15269. (21) Grant Glover, T.; Peterson, G. W.; Schindler, B. J.; Britt, D.; Yaghi, O. MOF-74 Building Unit Has a Direct Impact on Toxic Gas Adsorption. Chem. Eng. Sci. 2011,

66, 163-170. (22) Geier, S. J.; Mason, J. A.; Bloch, E. D.; Queen, W. L.; Hudson, M. R.; Brown, C. M.; Long, J. R. Selective Adsorption of Ethylene over Ethane and Propylene over Propane in the Metal-Organic Frameworks M-2(DOBDC) (M = Mg, Mn, Fe, Co, Ni, Zn). Chem. Sci. 2013, 4, 2054-2061. (23) Chui, S. S.; Lo, S. M.; Charmant, J. P.; Orpen, A. G.; Williams, I. D. A Chemically Functionalizable Nanoporous Material. Science 1999, 283, 1148-50. (24) Murray, L. J.; Dinca, M.; Yano, J.; Chavan, S.; Bordiga, S.; Brown, C. M.; Long, J.

R.

Highly-Selective

and

Reversible

O2

Binding

in

Cr3(1,3,5-Benzenetricarboxylate)2. J. Am. Chem. Soc. 2010, 132, 7856-7857. (25) Maniam, P.; Stock, N. Investigation of Porous Ni-Based Metal–Organic Frameworks Containing Paddle-Wheel Type Inorganic Building Units Via High-Throughput Methods. Inorg. Chem. 2011, 50, 5085-5097. (26) Lu, J.; Mondal, A.; Moulton, B.; Zaworotko, M. J. Polygons and Faceted Polyhedra and Nanoporous Networks. Angew. Chem., Int. Ed. 2001, 40, 2113-2116. (27) Kozachuk, O.; Yusenko, K.; Noei, H.; Wang, Y.; Walleck, S.; Glaser, T.; Fischer, R. A. Solvothermal Growth of a Ruthenium Metal-Organic Framework Featuring HKUST-1 Structure Type as Thin Films on Oxide Surfaces. Chem. Commun. 2011, 47, 8509-8511. (28) Kramer, M.; Schwarz, U.; Kaskel, S. Synthesis and Properties of the Metal-Organic Framework Mo3(BTC)2 (TUDMOF-1). J. Mater. Chem. 2006, 16, 2245-2248. (29) Koeberl, M.; Cokoja, M.; Herrmann, W. A.; Kuehn, F. E. From Molecules to Materials: Molecular Paddle-Wheel Synthons of Macromolecules, Cage Compounds and Metal-Organic Frameworks. Dalton Trans. 2011, 40, 6834-6859. (30) Peralta, D.; Barthelet, K.; Perez-Pellitero, J.; Chizallet, C.; Chaplais, G.; Simon-Masseron, A.; Pirngruber, G. D. Adsorption and Separation of Xylene Isomers: CPO-27-Ni Vs HKUST-1 Vs Nay. J. Phys. Chem. C 2012, 116, 21844-21855. (31)Van Assche, T. R. C.; Duerinck, T.; Gutierrez Sevillano, J. J.; Calero, S.; Baron, G. 38


Page 39 of 44

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


V.; Denayer, J. F. M. High Adsorption Capacities and Two-Step Adsorption of Polar Adsorbates on Copper Benzene-1,3,5-Tricarboxylate Metal-Organic Framework. J.

Phys. Chem. C 2013, 117, 18100-18111. (32) Queen, W. L.; Hudson, M. R.; Bloch, E. D.; Mason, J. A.; Gonzalez, M. I.; Lee, J. S.; Gygi, D.; Howe, J. D.; Lee, K.; Darwish, T. A.; James, M.; Peterson, V. K.; Teat, S. J.; Smit, B.; Neaton, J. B.; Long, J. R.; Brown, C. M. Comprehensive Study of Carbon Dioxide Adsorption in the Metal-Organic Frameworks M-2(DOBDC) (M = Mg, Mn, Fe, Co, Ni, Cu, Zn). Chem. Sci. 2014, 5, 4569-4581. (33) Terencio, T.; Di Renzo, F.; Berthomieu, D.; Trens, P. Adsorption of Acetone Vapor by Cu-BTC: An Experimental and Computational Study. J. Phys. Chem. C 2013, 117, 26156-26165. (34) Koh, H. S.; Rana, M. K.; Hwang, J.; Siegel, D. J. Thermodynamic Screening of Metal-Substituted MOFs for Carbon Capture. Phys. Chem. Chem. Phys. 2013, 15, 4573-4581. (35) Rana, M. K.; Koh, H. S.; Zuberi, H.; Siegel, D. J. Methane Storage in Metal-Substituted Metal-Organic Frameworks: Thermodynamics, Usable Capacity, and the Impact of Enhanced Binding Sites. J. Phys. Chem. C 2014, 118, 2929-2942. (36) Noei, H.; Kozachuk, O.; Amirjalayer, S.; Bureekaew, S.; Kauer, M.; Schmid, R.; Marler, B.; Muhler, M.; Fischer, R. A.; Wang, Y. Co Adsorption on a Mixed-Valence Ruthenium Metal-Organic Framework Studied by Uhv-Ftir Spectroscopy and DFT Calculations. J. Phys. Chem. C 2013, 117, 5658-5666. (37) Xie, L.; Liu, S.; Gao, C.; Cao, R.; Cao, J.; Sun, C.; Su, Z. Mixed-Valence Iron(Ii, Iii) Trimesates with Open Frameworks Modulated by Solvents. Inorg. Chem. 2007, 46, 7782-7788. (38) Materials Studio 5.0. Accelrys: San Diego, CA, 2010. (39) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, M. 39



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

Page 40 of 44

J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09. Revision D. 01; Gaussian, Inc.:

Wallingford, CT, 2009. (40)Cordero, B.; Gomez, V.; Platero-Prats, A. E.; Reves, M.; Echeverria, J.; Cremades, E.; Barragan, F.; Alvarez, S. Covalent Radii Revisited. Dalton Trans. 2008, 21, 2832-2838. (41) Wu, Y.; Chen, H.; Liu, D.; Xiao, J.; Qian, Y.; Xi, H. Effective Ligand Functionalization of Zirconium-Based Metal–Organic Frameworks for the Adsorption and Separation of Benzene and Toluene: A Multiscale Computational Study. ACS

Appl. Mater. Interfaces 2015, 7, 5775-5787. (42) Yang, Q.; Guillerm, V.; Ragon, F.; Wiersum, A. D.; Llewellyn, P. L.; Zhong, C.; Devic, T.; Serre, C.; Maurin, G. CH4 Storage and CO2 Capture in Highly Porous Zirconium Oxide Based Metal-Organic Frameworks. Chem. Commun. 2012, 48, 9831-9833. (43) Martin, M. G.; Siepmann, J. I. Novel Configurational-Bias Monte Carlo Method for Branched Molecules. Transferable Potentials for Phase Equilibria. 2. United-Atom Description of Branched Alkanes. J. Phys. Chem. B 1999, 103, 4508-4517. (44) Yang,

Q.;

Zhong,

C.

Molecular

Simulation

of

Carbon

Dioxide/Methane/Hydrogen Mixture Adsorption in Metal−Organic Frameworks. J.

Phys. Chem. B 2006, 110, 17776-17783. (45) Rappe, A. K.; Casewit, C. J.; Colwell, K. S.; Goddard, W. A.; Skiff, W. M. Uff, a Full Periodic Table Force Field for Molecular Mechanics and Molecular Dynamics Simulations. J. Am. Chem. Soc. 1992, 114, 10024-10035. (46) Maerzke, K. A.; Schultz, N. E.; Ross, R. B.; Siepmann, J. I. Trappe-Ua Force Field for Acrylates and Monte Carlo Simulations for Their Mixtures with Alkanes and Alcohols. J. Phys. Chem. B 2009, 113, 6415-6425. (47) Chen, B.; Potoff, J. J.; Siepmann, J. I. Monte Carlo Calculations for Alcohols and Their Mixtures with Alkanes. Transferable Potentials for Phase Equilibria. 5. United-Atom Description of Primary, Secondary, and Tertiary Alcohols. J. Phys.

Chem. B 2001, 105, 3093-3104. 40


Page 41 of 44

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


(48) De Lange, M. F.; Gutierrez-Sevillano, J. J.; Hamad, S.; Vlugt, T. J. H.; Calero, S.; Gascon, J.; Kapteijn, F. Understanding Adsorption of Highly Polar Vapors on Mesoporous Mil-100(Cr) and Mil-101 (Cr): Experiments and Molecular Simulations.

J. Phys. Chem. C 2013, 117, 7613-7622. (49) Chempath, S.; Düren, T.; Sarkisov, L.; Snurr, R. Q. Experiences with the Publicly Available Multipurpose Simulation Code, Music. Mol. Simul. 2013, 39, 1223-1232. (50) Wu, Y.; Chen, H.; Liu, D.; Qian, Y.; Xi, H. Adsorption and Separation of Ethane/Ethylene on ZIFs with Various Topologies: Combining Gcmc Simulation with the Ideal Adsorbed Solution Theory (Iast). Chem. Eng. Sci. 2015, 124, 144-153. (51) Jose Gutierrez-Sevillano, J.; Manuel Vicent-Luna, J.; Dubbeldam, D.; Calero, S. Molecular Mechanisms for Adsorption in Cu-BTC Metal Organic Framework. J. Phys.

Chem. C 2013, 117, 11357-11366. (52) Skoulidas, A. I. Molecular Dynamics Simulations of Gas Diffusion in Metal−Organic Frameworks: Argon in CuBTC. J. Am. Chem. Soc. 2004, 126, 1356-1357. (53) Talu, O.; Myers, A. L. Molecular Simulation of Adsorption: Gibbs Dividing Surface and Comparison with Experiment. AIChE J. 2001, 47, 1160-1168. (54) Cho, J.; Rho, S.; Park, S.; Lee, J.; Kim, H. A Comparison between Anderko's Model with Composition-Dependent Physical Interaction Parameters and Srk-Based Hexamer Model for HFC and HF Systems. Fluid Phase Equilib. 1998, 144, 69-75. (55) Düren, T.; Snurr, R. Q. Assessment of Isoreticular Metal−Organic Frameworks for Adsorption Separations: A Molecular Simulation Study of Methane/N-Butane Mixtures. J. Phys. Chem. B 2004, 108, 15703-15708. (56) Wang, W.; Zhang, Y.; Wang, Y. B. Noncovalent Π⋅⋅⋅Π Interaction between Graphene and Aromatic Molecule: Structure, Energy, and Nature. J. Chem. Phys. 2014, 140, 094302, 1-6. (57) Xiao, J.; Sitamraju, S.; Janik, M. J. Co2 Adsorption Thermodynamics over N-Substituted/Grafted Graphanes: A DFT Study. Langmuir 2014, 30, 1837-1844. (58) Boys, S. F.; Bernardi, F. The Calculation of Small Molecular Interactions by the Differences of Separate Total Energies. Some Procedures with Reduced Errors. Mol.

Phys. 1970, 19, 553-566. (59) Kasuriya, S.; Namuangruk, S.; Treesukol, P.; Tirtowidjojo, M.; Limtrakul, J. Adsorption of Ethylene, Benzene, and Ethylbenzene over Faujasite Zeolites 41



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

Investigated by the Oniom Method. J. Catal. 2003, 219, 320-328. (60) Myers, A. L.; Prausnitz, J. M. Thermodynamics of Mixed-Gas Adsorption.

AIChE J. 1965, 11, 121-127. (61) Zhang, Z.; Li, Z.; Li, J. Computational Study of Adsorption and Separation of CO2, CH4, and N2 by an Rht-Type Metal-Organic Framework. Langmuir 2012, 28, 12122-33. (62) Bae, Y. S.; Lee, C. Y.; Kim, K. C.; Farha, O. K.; Nickias, P.; Hupp, J. T.; Nguyen, S. T.; Snurr, R. Q. High Propene/Propane Selectivity in Isostructural Metal–Organic Frameworks with High Densities of Open Metal Sites. Angew. Chem., Int. Ed. 2012,

51, 1857-1860. (63) García-Pérez, E.; Gascón, J.; Morales-Flórez, V.; Castillo, J. M.; Kapteijn, F.; Calero, S. Identification of Adsorption Sites in Cu-BTC by Experimentation and Molecular Simulation. Langmuir 2009, 25, 1725-1731. (64) Sing, K.; Everett, D.; Haul, R.; Moscou, L.; Pierotti, R.; Rouquerol, J.; Siemieniewska, T. Reporting Physisorption Data for Gas/Solid Systems, with Special Reference to the Determination of Surface Area and Porosity (Recommendations 1984). Pure Appl. Chem. 1985, 57, 603-619. (65) Gutierrez-Sevillano, J. J.; Dubbeldam, D.; Bellarosa, L.; Lopez, N.; Liu, X.; Vlugt, T. J. H.; Calero, S. Strategies to Simultaneously Enhance the Hydrostability and the Alcohol-Water Separation Behavior of Cu-BTC. J. Phys. Chem. C 2013, 117, 20706-20714. (66) Liu, Y.; Liu, J.; Chang, M.; Zheng, C. G. Effect of Functionalized Linker on CO2 Binding in Zeolitic Imidazolate Frameworks: Density Functional Theory Study. J.

Phys. Chem. C 2012, 116, 16985-16991. (67) First, E. L.; Gounaris, C. E.; Floudas, C. A. Predictive Framework for Shape-Selective Separations in Three-Dimensional Zeolites and Metal-Organic Frameworks. Langmuir 2013, 29, 5599-5608. (68) Soni, M.; Ramjugernath, D.; Raal, J. D. Vapor–Liquid Equilibrium for Binary Systems of 2,3-Pentanedione with Diacetyl and Acetone. J. Chem. Eng. Data 2008,

53, 745-749. (69) Do, D. D.; Do, H. D. Cooperative and Competitive Adsorption of Ethylene, Ethane, Nitrogen and Argon on Graphitized Carbon Black and in Slit Pores.

Adsorption 2005, 11, 35-50. 42


Page 42 of 44

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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


(70) Zeng, Y.; Zhu, X.; Yuan, Y.; Zhang, X.; Ju, S. Molecular Simulations for Adsorption and Separation of Thiophene and Benzene in Cu-BTC and IRMOF-1 Metal–Organic Frameworks. Sep. Purif. Technol. 2012, 95, 149-156.

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