Selective Adsorption and Separation of Xylene Isomers and Benzene

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Applications of Polymer, Composite, and Coating Materials

Selective Adsorption and Separation of Xylene Isomers and Benzene/Cyclohexane with Microporous Organic Polymers POP-1 Huiling Tan, Qibin Chen, Tingting Chen, and Honglai Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11657 • Publication Date (Web): 30 Aug 2018 Downloaded from http://pubs.acs.org on August 30, 2018

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ACS Applied Materials & Interfaces

Selective Adsorption and Separation of Xylene Isomers and Benzene/Cyclohexane with Microporous Organic Polymers POP-1 Huiling Tan, Qibin Chen*, Tingting Chen, Honglai Liu* State Key Laboratory of Chemical Engineering and School of Chemistry & Molecular Engineering, East China University of Science and Technology, Shanghai, 200237, P.R. China KEYWORDS: microporous organic polymers, xylene isomers, benzene/cyclohexane systems, separation, weak interaction

ABSTRACT: Currently, the separation of chemical substances with analogous chemical structures and physical properties is still a great challenge. In this work, triptycene-like microporous organic polymers (MOPs), POP-1, was synthesized via choosing 1,4-dimethoxybenzene (DMB) and triptycene as external crosslinkers and building blocks, respectively, and POP-1 was employed to separate xylene isomers and benzene(Bz)/cyclohexane(Cy). Results show that POP-1 has a higher uptake for m-Xylene (0.29 g/g) and Bz (1.02 g/g); more intriguingly, their complete separation can be realized within 0.6 minutes using a column packed with POP-1. The interaction between POP1 networks and adsorbates was also investigated using theoretical (density functional theory, DFT, together with non-covalent interaction analysis, NCI) and experimental (inverse gas chromatography, IGC) approaches. Especially, both results present a good agreement, that is, weak

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interactions, such as CH/π interactions, play a dominant role in defining the separation performance of POP-1 for xylene isomers and Bz/Cy mixtures. Our findings suggest that MOPs may open up a new route for separating the chemicals that are similar in structures and size.

INTRODUCTION C8 alkyl-aromatic compounds are one of the most important chemicals, each of which can be further processed into valuable products. For instance, para-xylene (pX), the precursor for terephthalic acid, is most desirable for producing polymers, such as polyethylene terephthalate and polyester.1 Currently, the efficient separation of xylene isomers is, therefore, a critical issue in chemical industry, owing to the value of the individual isomers. Actually, the separation of hydrocarbons (including aliphatic and aromatic compounds), which have similar physical properties and chemical structures, and comparable molecular size as well, is still a great challenge for the industrial applications.2 Up to date, various methods have been used to separate xylene isomers.3 In this regard, regular distillation was not an ideal option for the separation of xylene isomers, since they displayed the close boiling points (i.e., 138.4, 139.1 and 144.4℃ for pX, metaxylene (mX) and ortho-xylene (oX), respectively) and kinetic diameters (5.8, 6.5 and 6.4 Å for pX, oX and mX, respectively). Also, the crystallization,4 based on the difference in freezing points (286.4, 248.0 and 222.5℃ for pX, oX and mX, respectively), presents a poor separation efficiency, and meanwhile, the membrane separation5 has not as yet been applied into large-scale industrial separation processes due to the exquisite preparation, though it is low energy-consuming.3 Among different separation techniques, adsorption shows a great potential to be an alternative to separate hydrocarbons,6 because of its higher separation efficiency and lower energy consumption.


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To have an impact on real applications, porous materials, as adsorbents, must be scalable and satisfy multiple functional criteria, such as long-term stability, selectivity, adsorption kinetics and processability, all within a viable cost envelope.7 To date, metal-organic frameworks (MOFs) have been extensively examined for separating xylene isomers.8, 9 By comparison, microporous organic polymers (MOPs), as another class of promising adsorbents, have been proved to hold a notable potential due to their diversity, high physicochemical stability and excellent adsorption capacity10, 11

and to offer a number of benefits for hydrocarbon storage and separation.12 In this context,

triptycene13-15 combined with formaldehyde dimethyl acetal was extensively investigated, in which the paddle wheel orientation of the arene rings is responsible for the inefficient packing of these building blocks, thereby producing numerous microporous structures in the polymeric network.16 However, to the best of our knowledge, the feasibility of the use of MOPs, especially triptycene derivatives, to separate C8 alkyl-aromatic compounds has not previously been demonstrated. In brief, xylene isomers consist of a benzene ring and two methyl groups, in which the major difference in the molecular structures is the relative position of substituted methyl groups, which could influence the stacking way of xylene isomers in adsorbents. If only the molecular structures of xylene and triptycene-like MOPs are taken into consideration, various weak interactions, such as π-type interactions, should be formed between guest molecules and the wall of host frameworks, favoring the separation of xylene isomers.17 Recently, the feasibility of using MOFs to separate xylene isomers motivated us to explore the possibility that MOPs can be applied in the separation of xylene isomers. In principle, more conjugated groups in adsorbents could lead to stronger π-type interactions between guest and host molecules, likely providing the higher affinity and uptake capacity for guests. In addition, since incomplete catalytic hydrogenation of benzene (Bz) generally leads to the coexistence of cyclohexane (Cy) with unreacted Bz, it is of


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great importance to separate Bz and Cy in the petrochemical industry. Accordingly, in the present work, our primary goal was 2-fold: (i) to test the feasibility of the use of triptycene-like MOPs to separate chemicals that are similar in structures and size, especially xylene isomer and Bz/Cy systems, via choosing 1,4-dimethoxybenzene (DMB) as an external crosslinker, and (ii) to determine the resulting separation efficiency. To address these issues, we synthesized a triptycene-like MOPs, POP-1, via choosing DMB and triptycene as external crosslinkers and building blocks, respectively. Synthetic routes and chemical structures are shown in Scheme 1. Also, the separation performances of POP-1 for xylene isomer and Bz/Cy systems were characterized using the isothermal adsorption and gas chromatography (GC) in this work. Additionally, it should be noted that since POP-1 was insoluble in dilute NaOH and/or HCl solution and common organic solvents as well (dichloromethane, acetone and methanol), −NH2 functional group was introduced to increase the hydrophilicity of POP-1, thereby being able to expand its application scope, e.g., the removal of industrial pollutants (like Benzene and toluene, MB) from water. Results showed that POP-1 had a high selectivity for mX/oX, mX/pX and Bz/Cy, and could efficiently eliminate Bz and MB from water.18, 19 Our findings suggest that MOPs, derived from a subtle molecular design and an appropriate selection for building blocks and/or crosslinkers, could be a good candidate for the xylene isomer and Bz/Cy separation. Scheme 1. Synthesis of POP-1.


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EXPERIMENT SECTION Materials. Reagents and chemicals: All reagents and solvents were purchased from Adamas, General-reagent and Shanghai Titan Scientific Co., Ltd. The reagents used were of reagent grade, and the solvents used were of analytical grade unless otherwise stated. 1H Nuclear Magnetic Resonance (NMR) and Fourier Transform Infrared (FT-IR) spectroscopies were used to confirm the molecular structure. Synthesis of POP-1.The synthesizing method that was used to prepare POP-1 was similar to those previously reported.20 In brief, 1,4-Dimethoxybenzene (DMB, 1.035g, 7.5mmol) and anhydrous FeCl3 (3.65g, 22.5mmol) were added to 2-aminotriptycene (0.27g, 1mmol) in 20 mL of anhydrous nitrobenzene. The mixture was heated to 80 ℃ under N2 for 5h to form the pristine framework, then heated to 120 ℃ for another 24 h to obtain the crude product. After cooling to room temperature, such crude product was collected by filtration and washed with Methanol, HCl and dichloromethane until the filtrate was nearly colorless. The product was further purified by Soxhlet extraction in Methanol for 24 h. Finally, the product was dried at 80 ℃ under vacuum to give a black solid POP-1. Characterization. 1H NMR spectra were recorded on an Avance III 400MHz NMR spectrometer (Bruker, Germany).

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C NMR measurements were performed on a 9.4 T Avance

NMR spectrometer (Bruker, Germany). Attenuated total reflectance (ATR) FT-IR spectra were obtained on a Nicolet

6700 ATR

FTIR

spectrometer (ThermoFisher, USA). N2

adsorption/desorption data were acquired on ASAP2020 physisorption analyser (Micromeritics, USA). Thermogravimetric analysis (TGA) were conducted using a thermal analysis system (NETZSCH STA 449 F3, Germany). Powder X-ray diffraction (PXRD) patterns were collected in the range of 10-80° (2θ) using a Rigaku D/MAX 2550 VB/PC powder diffractometer (Cu K


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radiation, λ=1.5406Å). The morphology of samples was characterized using a Nova NanoS 450 field emission scanning electron microscope (FE-SEM). The adsorption isotherms of POP-1 for xylene isomers, Bz and Cy were analyzed with IGA-100b (Hidden, UK). UV-visible (UV-vis) spectra of the adsorbent were recorded using a CARY 500 spectrophotometer. All measurements were acquired under ambient laboratory conditions. Fitting the adsorption isotherm. The adsorption isotherms for xylene isomers, Bz and Cy were evaluated by the single-site Langmuir model.21 𝑞=

𝑞sat 𝑏𝑝 1 + 𝑏𝑝

(1)

where q (g/g) is the adsorbed amount at relative pressure p, qsat (g/g) is the saturation adsorbed amount, and b is the Langmuir parameter. These parameters were determined by fitting adsorption isotherms at 298 K (Table S1). The q (g/g) and p (-) are obtained from the experiment. Therefore, the parameters qsat (g/g) and b could be determined by nonlinear regression where the Gauss-newton algorithm is adopted. The adsorbent selectivity (α)22 was estimated according to Eq (2): 𝛼1/2 =

𝑏1 × 𝑞sat,1 𝑏2 × 𝑞sat,2

(2)

where qsat,1 and qsat,2 are the saturation amount adsorbed in g adsorbate 1 and adsorbate 2 per g POP-1; b1 and b2 are the Langmuir parameter for adsorbate 1 and adsorbate 2. According to this model, if α >1, it is sufficient to separate such two materials. Separating xylene isomers and Bz/Cy. The separation of xylene isomers and Bz/Cy was also studied via a common GC method on Inverse Gas Chromatography-Surface Energy Analyzer (IGC-SEA, London, United Kingdom) equipped with a thermal conductivity detector (TCD) and a flame-ionization detector (FID). The chromatographic profiles were recorded at 298 K, whereas


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the detector and injector were heated at 453 K. The flow rate of the carrier gas, He, is 10 cm3/min. Alkylated glass columns (30 cm long and 3 mm inner diameter) were previously cleaned and weighed, and then packed with POP-1. The packed columns were degassed overnight at 393 K under a helium flow. The mixture of xylene isomers and Bz/Cy with equivalent moles were injected at a surface coverage of 0.01. The resolution (RS)23 for analytes 1 and 2 on the column packed with POP-1 was calculated from chromatogram results according to the following equation: 𝑅S =

2 × (𝑡1 − 𝑡2 ) 𝑊1 + 𝑊2

(3)

where t1 and t2 are the retention time of analytes 1 and 2, respectively. W1 and W2 are the peak width of analytes 1 and 2, respectively. Removal of Bz and MB. POP-1 was also used to remove the pollutants in water, e.g., Bz and MB. In brief, 4 mg of POP-1 was added to 4 mL of Bz and MB aqueous solution (concentration: 0.5 g/L); the suspension was stirred at 298K for 6h and then separated by centrifugation at 8000 rmp for 15 min and filtration through 0.22-µm filter; the concentration of Bz and MB in the filtrate was determined on an UV-vis spectrophotometer. Surface Energy Analysis. The distribution of surface energy for POP-1 was carried out on IGC-SEA. The operation condition was the same as that used in the separation of xylene isomer and Bz/Cy systems. In this test, n-hexane, n-heptane and n-octane were served as non-polar solvents, ethyl acetate and dichloromethane were treated as polar solvents; Peak Max method was used to manage the experimental peaks; Schultz24 and Della Vople methods were adopted to analyze the surface energy of POP-1. The details on how to operate IGC and calculate the surface energy were reported by Jin et. al,25 which were adopted in this work.


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THEORETICAL CALCULATIONS DFT calculations and NCI analysis. The interaction energy of the complexes was obtained by DFT calculations by means of the Gaussian 09 package.26 Therefore, the geometry optimizations at a low level of M06-2x27/6-31G (d)28 was used to find the energy minimum configurations, while a higher level of M06-2x/6311+G (d, p) was employed for the single-point energy calculations. To simplify the calculation process, a general ligand was utilized to symbolize the POP-1 networks.29 In the interaction energy calculations, the counterpoise correction30 applied to the optimized structures was taken into account for basis set superposition error (BSSE). The value of interaction energy was calculated by subtracting the sum of the single-point energies of adsorbate (Eadsorbate) and the ligand cluster (Eligand) from the energy of the optimized adsorbate-ligand complex (Eligand-adsorbate), as shown in Eq (4). A more negative ΔE value indicates a stronger adsorption. The interaction energy is defined as follows: ∆𝐸 = 𝐸ligand−adsorbate − 𝐸ligand − 𝐸adsorbate

(4)

Meanwhile, the NCI analysis31 was performed by Multiwfn program 3.3.732 to visualize the interaction regions as well as discriminate weak interaction types. The corresponding results were processed by Visual Molecular Dynamics (VMD)33 software to obtain a final isosurfaces graph. In this graph, the value of sign (λ2(r)) ρ(r) is mapped to the Reduced Density Gradient (RDG) isosurfaces in different colors. In general, the blue, green and red colors indicate the strong attraction, van der Waals interaction and the repulsion force, respectively.


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RESULTS AND DISCUSSION Characterization of POP-1. The chemical structure of POP-1 was studied by FT-IR spectroscopy10,34,35 (Figure 1a) and 13C NMR14,20,36 (Figure 1b). In Figure 1a, peaks at 1626, 1494 and 1459 cm-1 correspond to skeleton vibrations of Bz rings and the signal at around 3420 cm-1 is assigned to the stretching vibration of the −NH2 moieties. As expected, such spectra present C−H asymmetrical stretching vibrations and symmetrical stretching vibrations at 2932 and 2847 cm-1. Additionally, the peak at 1043 cm-1 can be attributed to the methoxy group, proving the existence of unreacted p-dimethoxybenzene. In Figure 1b, the resonance peaks at 124 and 115, 103 ppm can be assigned to the substituted aromatic carbon and non-substituted aromatic carbon, respectively. Moreover, the signals at 145 ppm correspond to the N-substituted aromatic carbons, indicating the existence of −NH2 functioned triptycene. The uncompleted reaction is confirmed by the peaks that appear at 55 and 153 ppm, which are related to carbons in and connected to unreacted methoxy groups, respectively.


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Figure 1. a) FT-IR spectrum of POP-1 b) Solid 13C NMR spectrum of POP-1. The X-ray diffraction (XRD) patterns of POP-1 are shown in Figure S3. A broad peak with low-intensity in XRD pattern suggested amorphous nature of POP-1. The amorphous characteristics of POP-1 (Figure 2) were confirmed by FE-SEM, agreeing well with the XRD analysis. The thermal stability was investigated by the thermogravimetric analysis (TGA). TGA curves exhibited a slight decrease in the temperature range from 25 to 265 ℃, attributed to the removal of the adsorbed water and the solvent molecules that existed in the channels of POP-1. The char yield at 800 ℃ was 21.2 wt% (Figure S4). In addition, the mass loss rate curve was also given, helpful for determining the thermal decomposition temperature due to the existence of a visible breakpoint.


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Figure 2. SEM images of POP-1 at low (a) and high (b) magnification. Scale bar: 5 µm (a) and 1 µm (b). Porosity measurements. The porosity of POP-1 was evaluated by the nitrogen adsorption/desorption isotherms at 77 K. As shown in Figure 3a, the initial sharp increase of uptake at low pressure (P/P0 < 0.1) is peculiar of type I isotherms. For POP-1, the BET specific surface area was 486 m2/g, while the Langmuir surface area was 718 m2/g. A slightly increasing uptake of gas up to a relative pressure above 0.9 suggested the occurrence of macropores and interparticle voids.37 More importantly, the presence of a notable hysteresis in the N2 desorption curve of POP1 suggested a mesoporous character in the whole range of relative pressures10. The ratio of the micropore surface area to the total BET surface area is calculated, listed in Table 1. These results indicate that POP-1 mainly consists of micropores, further confirmed by the pore size distribution (PSD). Herein, PSD curves (Figure 3b) based on the non-local density functional theory indicate that most of the pores are smaller than 2.0 nm in POP-1. Table 1. The properties of porosity for POP-1

a

Polymer

SABETa (m2/g)

SALangb (m2/g)

Vtotalc (cm3/g)

SMicro/SBET

POP-1

486

718

0.34

0.55

Surface area is calculated from the nitrogen adsorption branch according to the BET model.


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b

Surface area is calculated from the nitrogen adsorption branch based on the Langmuir model.

c

The total pore volume is calculated from single point nitrogen uptake at P/P0 = 0.90.

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Figure 3. a) N2 adsorption/desorption isotherms, b) Pore size distributions (PSD) of POP-1 at 77K. Separating performance of POP-1 for xylene isomers. In this work, we aimed at understanding the role that weak interactions, such as CH/π interactions, influenced the purification of chemical substances, especially when molecules were close in size and structures. Thus, the adsorption capacity for xylene isomers was recorded at 298 K up to P/P0 = 1 on IGA100b analyzer, shown in Figure 4a. Prior to the measurement, the samples were degassed under a vacuum at 120 ℃ for 12 h. POP-1 had a saturation uptake of 0.29, 0.25 and 0.23 g/g for mX, pX


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and oX, respectively. As is apparent, these uptake capacities of POP-1 for xylene isomers are higher than those of MOFs reported recently, e.g., MIL-125(Ti)-NH238 (0.15 g/g for pX, and 0.1g for mX and oX) and Cu(CDC)39 (0.12 g/g for pX, 0.06g/g for mX and 0.01 g/g for oX). Despite this, with regard to xylene isomers, a modest deviation in the adsorbed amount obtained in this work is a likely consequence of the different host-guest interactions, mainly resulting from the structural difference in xylene isomers, i.e., the unidentical methyl substituted sites, which will be further confirmed by the following DFT calculations. In this work, the adsorption selectivity for two different xylene isomers was calculated by Eq (2), compiled in Table 2. Herein, αmX/oX, αmX/pX and αpX/oX were higher than 1, which was higher than that for MIL-4740 (0.90 for αmX/pX and 0.60 for αpX/oX) and comparable to that for HKUST-16 (2.40 for αmX/oX, 1.10 for αmX/pX and 1.43 for αpX/oX) and ZIF-841 (0.87 for αmX/pX and 1.50 for αpX/oX). These results suggested that POP-1 had a promising potential for the separation of xylene isomers. However, αpX/oX (1.12) was much lower than αmX/pX (2.04) and αmX/oX (2.28). It might result from closer interaction energies between pX (oX) and host ligand. In order to further investigate the separation performance and mechanism of POP-1 for xylene isomers, we also conducted IGC analysis (vide post). Additionally, the reproducibility of POP-1 for mX adsorption was investigated via multiple recycle experiments. In such experiments, adsorption was performed at 298 K under P/P0 = 1, and desorption was carried out at 298 K under vacuum for 6h. After 5 cycles, the re-adsorption capacity for mX was comparable to the initial adsorption capacity, confirming the stability of POP-1(Figure S6). Table 2. The calculated selectivity of different adsorbate

Selectivity (α)

mX/pX

mX/oX

pX/oX

2.04

2.28

1.12


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In Figure 4b is shown a typical chromatographic profile recorded at 298 K for the ternary mixture of xylene isomers with a 1:1:1 molar ratio using POP-1 columns at a surface coverage of 0.01 n/nm, with a flow rate of 10 sccm. Most strikingly, the elution peaks of three isomers appeared well resolved and the calculated resolution (RS) was larger than 1.5 (2.67 for mX/pX, 3.85 for mX/oX and 1.56 for pX/oX, seen in Table S2), which was higher than that for MCF-5042(1.541.65 for mX/pX and 1.90 for mX/oX) and ZIF@PDMS43(1.45 for mX/pX and 1.23 for mX/oX), comparable to that for MOF-CJ344(1.35 for mX/pX and 3.85 for mX/oX). These results also indicated that POP-1 was capable of achieving a complete separation for the mixture of xylene isomers. Moreover, as for xylene isomers, all the retention time is within 0.6 minutes, far less than that of most reported MOFs (e.g., MAF-645: 2.7 minutes, HKUST-146: 6.5 minutes), which benefits the fast separation for xylene isomers. Taken together, both results demonstrate POP-1 is a promising candidate for the separation of xylene isomers. In addition, mX has the larger retention time, implying a stronger guest-host interaction, which is in good accordance with the adsorption capacity of POP-1.


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Figure 4. a) pX, mX and oX adsorption isotherms of POP-1 at 298K, b) Chromatographic separation of xylene isomers with POP-1 packed column at 298 K. The composition of the mixture is pX/mX/oX = 1:1:1(molar ratio). Separating performance of POP-1 for Bz and Cy mixture. Since the benzene ring was introduced in the crosslinkers and the content of arene rings in POP-1 was relatively high, the separation performance of POP-1 for Bz and Cy was also investigated, shown in Figure 5a. Results show that the Bz uptake is higher than that for Cy, with an adsorption selectivity of 1.55 for Bz/Cy. This might be attributed to the existence of the aromatic ring in the former case, resulting in a stronger host-guest interaction, so that POP-1 shows a preference for the adsorption of Bz molecules. More intriguingly, the Bz adsorption capacity at P/P0 = 1 is as high as 1.02 g/g (102


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wt%) in this study, which is higher than that of most reported MOFs (such as DAT-MOF-12:11.7 wt%, 2-bpe47: 10.8 wt% and Ce-LOF48:23.7 wt%) and MOPs (for instance, PCN-TPPC49:77.8 wt%, cyanate resins50: 34.4-58.5 wt%). Taken together, the greater adsorption selectivity and the higher uptake capacity relative to the most porous materials reported recently, highlight the contribution of π-type interactions to the separation performance of POP-1. Therefore, our results imply the πtype interaction plays a crucial role in defining the separation performance of porous materials, that is, the high content of π-conjugated systems in host frameworks could endow them with more affinity for aromatic guest molecules (e.g., Bz). In addition, recycle experiments demonstrate that POP-1 has a great reproducibility for Bz adsorption (more details seen in Figure S6). In Figure 5b, the common GC approach was used to conduct the separation of Bz/Cy using columns packed with POP-1. Herein, POP-1 was also capable of completely separating the Bz/Cy mixture, with a resolution (RS) of 2.34 (Table S2), similar to xylene isomers. The retention time less than 0.5 minutes indicates that POP-1 is of high-efficiency for the separation of Bz/Cy, less than most MOFs reported (MAF-645: 2.7 minutes, HKUST-151: 2 minutes). The fact that Bz has the larger retention time, compared with Cy, reveals a stronger guest-host interaction, which is also in agreement with the isothermal adsorption shown above.


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Figure 5. a) Bz and Cy adsorption isotherms of POP-1 at 298K, b) Chromatographic separation of Bz/Cy mixture with POP-1 packed column at 298K. The composition of the mixture is Bz/Cy=1:1(molar ratio). Separation Mechanism POP-1 for xylene isomers and Bz/Cy. The binding energy between adsorbates and ligands was determined by DFT calculations, based on their optimized configurations, and the corresponding non-covalent interaction (NCI) analysis was conducted. Herein, it should be noted that the ligand was limited to a building unit, composed of a triptycene modified with an amine group and a methoxybenzene, due to the intrinsic nature of MOPs. On the one hand, the order for the binding energy of xylene isomers apparently follows: mX (-31.6 kJ/mol) > pX (-27.1 kJ/mol) > oX (-24.7 kJ/mol), as given in Figure 6, consistent well with the


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sequence for their adsorbed amounts. Therefore, the high αmX/pX (2.04) and αmX/oX (2.28) might result from that POP-1 shows a strongest interaction for mX among xylene isomers. By contrast, the αpX/oX (1.12) might stem from the relatively closer and lower interaction energies between pX (oX) and host ligand. Especially, xylene isomers differ in their orientations when incorporated in host frameworks, that is, they exhibit a notable variation in a dihedral angle between two planes of guest benzene and the host benzene connected −NH2 from triptycene: 46.9° for mX, 56.8° for pX and 83.8° for oX (more details given in Figure S7). Despite this, all orientations are still at an intermediate position between parallel stacked and T-shaped configurations for the benzene dimer.52 Based on the surface electrostatic potentials of xylene isomers and ligand (Seen Figure S8), such geometries favor facilitating CH/π interactions between guests and ligands. It is confirmed by the distance between the hydrogen of the C-H from guest molecules and the center of the nearest aromatic ring in a range of 2.5-4.2 Å (seen Figure S9). Accordingly, the differences in the binding energy and in the molecular orientation of guest molecules are a likely consequence of the discrepancy in the sites of two substituted methyl groups, thereby generating dissimilar hostguest interactions and promoting the separation of xylene isomers. On the other hand, POP-1 was employed to separate Bz and Cy, having an analogous size. As expected, the interaction between Bz and the ligand had a larger binding energy of -26.1 kJ/mol than that of Cy (-23.0 kJ/mol, seen in Figure S10), as a result of the presence of the aromatic ring in Bz molecules (Figure S11) and the intrinsic surface electrostatic potential (Figure S12). More impressively, POP-1 revealed a high uptake capacity of 1.02 g/g for Bz, which is higher than that of most reported MOFs and MOPs, suggesting that POP-1 is capable of selectively adsorbing Bz. In addition, it is worth noting that the lower uptake of POP-1 for mX and pX relative to Bz is a likely result of the steric hindrance effect of methyl groups, although both of them have a larger interaction energy than Bz, stemming


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from the presence of methyl groups and in turn providing additional interactions and enhancing the interaction strength.53

Figure 6. Energy and NCI analysis for ligand-adsorbate (a mX, b pX, c oX) interactions (C gray, N blue, O red, H white). In order to further investigate the adsorption/separation mechanism of POP-1, IGC was used to examine its surface energy distribution. In general, the surface energy, consisting of dispersive and specific terms,54 could reveal the interaction between adsorbate and adsorbent. Herein, the


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dispersive terms are calculated by Henry Law from the infinite dilute solvent, which is related to the particle morphology, like pore structures, while the specific terms include acid-base interactions, magnetism, metallicity and hydrogen bonds.55 In this work, the acid-base interaction may dominate the specific terms, so that the use of acid-base interaction analysis could reflect the property of surface functional groups. As shown in Figure 7, the dispersive energy was markedly higher than the acid-base free energy under different solvent coverage. This result indicates that the dispersive energy is predominant in the surface of POP-1. To date, it has been perceived that dispersion plays a major role in the attractive nature of CH/π interactions have a more dispersive character.56 Therefore, we concluded CH/π interactions were major contributions to the dispersive energy, thereby dominating the host-guest interactions, while the interaction induced by functional groups played a minor role in defining the resultant adsorbate-adsorbent interactions. 500

Surface energy (mJ·m-2)

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γd γab γt

400 300 200

Scale:Della Volpe Method:Schultz Parameter:Peak Max

100 0 0.000 0.002 0.004 0.006 0.008 0.010 0.012

Surface coverage (n/nm)

Figure 7. Surface energy distribution of POP-1(γd: is the dispersive energy; γab: is the acid-base energy; γt is the total energy). The effect of −NH2 on the Bz and MB removal of POP-1. In order to enhance the hydrophilicity of POP-1, −NH2 was introduced in the building subunit, triptycene; and then POP1 was utilized for the removal of hazardous aromatic compounds (Bz and MB) from water. As shown in Figure S5, POP-1 could completely remove Bz and MB from solution, showing that


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POP-1 has a good potential for future applications in the removal of hazardous aromatic compounds form water. Finally, although the difference in the contribution to the adsorbate-adsorbent interactions in IGC results lend support for a secondary role of functional groups, we sought additional evidence for quantitatively assessing the role of the substituted −NH2 based on DFT calculation. In order to define the effect of −NH2 on the selective separation of POP-1 for xylene isomers, DFT calculation was carried out: holding the intermolecular separation and the spatial geometries of host and guest constant, while replacing the substituted −NH2 with a hydrogen atom afforded essentially identical differences of ca. -2.0 kJ/mol in the interaction energies between with and without −NH2, as shown in Table S3. Consequently, this result suggests that the contribution of the substituted −NH2 to the adsorbate-adsorbent interactions is almost comparable.

CONCLUSION In summary, xylene isomers and Bz/Cy mixtures are successfully separated using triptycenelike MOPs, POP-1, in this work. Results indicate that: i) the molecular orientations of xylene isomers are different, thereby producing distinctive host-guest interactions, resulting from the different substituted position of two methyl groups; ii) Bz and Cy differ in the binding modes, stemming from the discrepancy in the molecular nature between Bz and Cy, despite having analogous size. Additionally, the introduction of −NH2 has a minor effect on the binding energy and in turn the separation performance, apart from increasing the hydrophilicity. Our findings demonstrate that the weak interactions, such as CH/π interactions, can be employed to tune the host-guest interaction and to achieve a specific application, e.g., separating chemical substances; in particular, a subtle molecular design and an appropriate selection for building subunits can make MOPs have a promising potential to separate xylene isomers, Bz/Cy mixtures and other chemicals


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that have similar structures and size. Meanwhile, MOPs could be applied into scaled-up manufacturing due to the commercial raw material, simple and mild reaction conditions and high yield products.

ASSOCIATED CONTENT Supporting Information One scheme, twelve figures and three tables are shown in Support information, including the synthesis scheme and 1H NMR spectra of the modified triptycene monomers, XRD pattern, TG curve, UV-Vis spectra for Bz and MB(methylbenzene) and sorption cycles for Bz and mX of POP1, and electrostatic potential maps. The theoretical results (host-guest energy and optimized configuration), and data for adsorption isotherm fitting parameters, calculated resolutions and energy differences are also presented. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] [email protected] ORCID Qibin Chen: 0000-0002-1095-1307 Honglai Liu: 0000-0002-5682-2295 Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENTS This work is supported by National Natural Science Foundation of China, the Fundamental Research Funds for the Central Universities of China and the 111 Project of Ministry of Education of China. Qibin Chen received funding from National Natural Science Foundation of China (No. 21576079) and the Fundamental Research Funds for the Central Universities of China (No. WK1213003). Honglai Liu received funding from National Natural Science Foundation of China (No. 91334203) and the 111 Project of Ministry of Education of China (No. B08021). REFERENCES (1)

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Selective Adsorption and Separation of Xylene Isomers and Benzene

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