Facile Synthesis of a Pentiptycene-Based Highly Microporous Organic

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Applications of Polymer, Composite, and Coating Materials

Facile Synthesis of a Pentiptycene-Based Highly Microporous Organic Polymer for Gas Storage and Water Treatment Shuangjiang Luo, Qinnan Zhang, Yizhou Zhang, Kevin P Weaver, William A. Phillip, and Ruilan Guo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02566 • Publication Date (Web): 16 Apr 2018 Downloaded from http://pubs.acs.org on April 16, 2018

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ACS Applied Materials & Interfaces

Facile Synthesis of a Pentiptycene-Based Highly Microporous Organic Polymer for Gas Storage and Water Treatment Shuangjiang Luo, Qinnan Zhang, Yizhou Zhang, Kevin P. Weaver, William A. Phillip, Ruilan Guo* Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, IN 46556, United States * Corresponding author: +1-574-631-3453 (tel), +1-574-631-8366 (fax), [email protected] ABSTRACT: Rigid H-shaped pentiptycene units, with an intrinsic hierarchical structure, were employed to fabricate a highly microporous organic polymer sorbent via Friedel-Crafts reaction/polymerization. The obtained microporous polymer exhibits good thermal stability, a high BET surface area of 1604 m2 g-1, outstanding CO2, H2 and CH4 storage capacities, as well as good adsorption selectivities for the separation of CO2/N2 and CO2/CH4 gas pairs. The CO2 uptake reached values as high as 5.00 mmol g-1 (1.0 bar and 273 K), which, along with high adsorption selectivity values (e.g., 47.1 for CO2/N2), make the pentiptycene-based microporous organic polymer (PMOP) a promising sorbent material for carbon capture from flue gas and natural gas purification. Moreover, the PMOP material displayed superior absorption capacities for organic solvents and dyes. For example, the maximum adsorption capacities for Methylene Blue and Congo Red were 394 and 932 mg g-1, respectively, promoting the potential of the PMOP as an excellent sorbent for environmental remediation and water treatment.

Keywords: pentiptycene, microporous polymers, sorbent, gas storage, water treatment

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1. Introduction Worldwide attention has been focused on efficient CO2 capture because the gradually increasing CO2 level has induced a variety of environmental problems such as climate change and ocean acidification.1 Another serious environmental issue involves water pollution by organic solvents and dyes discharged from the textiles, tannery, and printing industries, which is a grievous issue threatening the human health and the natural environment.2 Among the various methods for CO2 capture and water treatment, adsorption of target molecules on solid sorbents has been proven as an economic and versatile approach.3 A variety of sorbent materials including activated carbon,4 zeolites5, and natural fibres6 have been reported. It should be noted that most reported sorbents are in general designed for specific applications. Recently, microporous organic polymers (MOPs) have attracted much attention as sorbent materials due to their high surface area, good stability, diverse functionality, and low regeneration energy.7-10 Numerous MOPs including polymers of intrinsic microporosity (PIMs),11 hyper-crosslinked polymers,12 conjugated microporous polymers,13 and covalent organic frameworks (COFs)14 have been designed and fabricated for use as sorbent materials. However, the practical application of MOPs is limited by either low adsorption capacities or complicated material fabrication procedures. As such, the development of facile and cost-effective synthesis methods for MOPs that incorporate hierarchical building blocks and targeted functional groups is critical to producing high-performance sorbents for gas storage and separation, environmental remediation, and water treatment applications. Recently, triptycene and its derivatives have been demonstrated as useful building blocks in the preparation of MOPs10, 15-20 and separation membrane materials,21-25 due to their hierarchical, rigid paddlewheel-like structures that generate high microporosity in the corresponding polymers.26 Additionally, the open space between the benzene blades of triptycene moieties, known as “internal free volume”,27 introduces significant amount of permanent microcavities that are desirable for the efficient adsorption of target molecules. Similarly, pentiptycene, another simple member of the iptycene family featuring five benzene rings fused into rigid H-shaped scaffold (Scheme 1), has an even bulkier configuration and larger internal free volume than triptycene.27-28 Specifically, the total 2 ACS Paragon Plus Environment

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internal free volume of a pentiptycene unit is more than three times that of a triptycene unit (i.e., 326 Å3 vs. 93 Å3, or 0.44 cm3 g-1 vs. 0.21 cm3 g-1).29 It has been shown that gas separation membranes containing pentiptycene building blocks have significantly increased fractional free volume and gas permeability compared with the ones containing triptycene.29-33 In this regard, pentiptycene-based building blocks have great potential to improve the microporosity and enhance the molecular adsorption capabilities of MOPs. Herein, we report for the first time the synthesis of a pentiptycene-based MOP (PMOP) using Friedel-Crafts alkylation polymerization,34 and the obtained PMOP displays a high versatility for both CO2 capture and water treatment. Specifically, a pentiptycene-based building block was “knitted” into a three-dimensional, highly porous structure using dimethoxymethane (DMM) as a crosslinker. Moreover, heteroatoms (i.e., hydroxyl groups) were introduced on pentiptycene moieties to enhance the interaction between the sorbates and the PMOP surface.35-37 The synthesized PMOP was characterized thoroughly in terms of chemical structure, thermal property, surface area, and microporous structure. The microporous material was subjected to gas adsorption tests involving H2, CO2, N2, and CH4 to assess its gas storage capacity and applicability for carbon capture and natural gas upgrading. Furthermore, the PMOP was challenged in absorption tests with eight different organic solvents as well as in dye removal tests to evaluate its potential as a material for environmental remediation and water treatment applications.

2. Experimental Materials Anthracene (Alfa Aesar, 97%), p-benzoquinone (Sigma-Aldrich, ≥98%), p-chloranil (Alfa Aesar, 97%), sodium hydrosulfite (Sigma-Aldrich, 85%), dimethoxymethane (DMM, TCI, >98%), and anhydrous iron (III) chloride (Alfa Aesar, 98%) were used as received. All other chemicals and reagents were obtained from commercial suppliers and used as received. Synthesis of PMOP Pentiptycene diol29 (2.33 g, 5 mmol), dimethoxymethane (DMM, 3.07 g, 40 mmol), and 1,2-dichloroethane (60 mL) were mixed in a flame-dried two-necked flask equipped with a magnetic stirring, a nitrogen inlet, and a condenser. After purging with N2 for 10 min, anhydrous iron (III) chloride (6.54 g, 40 mmol) was added. The reaction mixture was then stirred and heated at 3 ACS Paragon Plus Environment


80 oC for 24 h. After cooling to room temperature, the insoluble solid was collected by filtration and washed extensively with methanol, 1 M HCl solution, water, and methanol again until the filtrate turned clear. The polymer was further purified by Soxhlet extraction with methanol and chloroform for 24 h, respectively, to remove any unreacted monomers, crosslinker or iron residues. Finally, the product was dried under vacuum at 120 oC for 24 h to obtain a light brown powder. Characterizations Solid state 13C cross polarization magic angle spinning nuclear magnetic resonance (13C CP/MAS NMR) spectrum was recorded on an ECX 300 MHz JEOL spectrometer with 7200 scans, contact time of 2 ms, and pulse delay of 3 s. Fourier transform infrared (ATR-FTIR) spectra of the solid were obtained on a Jasco FT/IR-6300 spectrometer in attenuated total reflection (ATR) mode. Thermal gravimetric analysis (TGA) was performed under nitrogen purge on a TGA Q500 instrument (TA Instruments) by heating to 800 oC at a heating rate of 10 oC min-1. The morphology of the PMOP was examined using field emission scanning electron microscope (FE-SEM, FEI-MAGELLAN 400). Wide-angle X-ray diffraction (WAXD) patterns were recorded on a Bruker D8 Advance Davinci diffractometer by depositing powder on glass substrate with 1.54 Å wavelength of Cu Ka radiation source in the reflection mode at room temperature. The scan speed and step size were 5 s per step and 0.02o per step, respectively, and the d-spacing values were calculated from Bragg’s equation in the 2θ range of 5-45o. Surface area and microcavity size distribution of the PMOP were tested using nitrogen adsorption and desorption at 77 K on a Micromeritics ASAP2020 volumetric adsorption analyzer. Sample was degassed at 150 oC for 12 h prior to analysis. The surface area of the polymer was calculated using the Brunauer-Emmett-Teller (BET) method in the relative pressure range of 0.05-0.2, and the total volumes was obtained at a relative pressure of P/P0 = 0.995. The cavity size distribution was derived from N2 adsorption isotherms using the non-local density functional theory (NLDFT). The H2 adsorption isotherm was measured at 77 K; CO2, N2, and CH4 adsorption isotherms were monitored at 273 and 298 K, respectively, with the same degassing procedure. The absorption of organic solvents was carried out following an established procedure.38 Eight different organic solvents (i.e., chloroform, dimethyl sulfoxide, toluene, ethanol, acetonitrile, acetone, ethyl acetate, hexanes), as well as water were tested in this study. A certain amount of the PMOP was weighted and loaded in a small glass tube with both ends blocked with adsorbent cotton, 4 ACS Paragon Plus Environment

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and then the glass tube was immersed in organic solvents until all the polymers were infiltrated with solvents by the capillary effect. Weight measurements were performed immediately after the glass tube was taken out of the solvents to avoid evaporation of absorbed solvents. The absorption capacity values (in mL/g) were calculated by the ratio of the volume absorbed to the PMOP dry mass. Dye removal tests were carried out using two organic dyes, i.e., Methylene Blue (MEB) and Congo Red (CR). The adsorption kinetics of the dyes were performed by the addition of 10 mg of PMOP into 10 mL of aqueous dye solution (100 mg L-1), the mixture was stirred at the rate of 500 rpm for different periods of time. PMOP was then removed from the mixture and the resulting solutions were analyzed by a Cary 60 UV-Vis spectrophotometer (Agilent Technologies). The adsorption isotherm was obtained by the addition of 10 mg of PMOP into 10 mL aqueous dye solutions of different initial concentrations from 200 to 1400 mg L-1, and the suspensions were stirred overnight to reach adsorption equilibrium before the PMOP was removed and the solution was analyzed. The dye adsorption isotherm was analyzed using both the linearized Langmuir model, Ce / Qe = 1/( K L Qm ) + Ce / Qm , and the linearized Freundlich model, ln Qe = ln K F + 1/ n ln Ce , where Qe (mg g-1) and Qm (mg g-1) are the equilibrium adsorption uptake and the maximum adsorption capacity, respectively, Ce (mg L-1) is the equilibrium dye concentration in solution, KL is the Langmuir constant, and KF and n are the Freundlich constants.

3. Results and discussion The PMOP precursor, i.e., pentiptycene diol, was prepared from a one-pot Diels-Alder addition between p-benzoquinone and anthracene, followed by a reduction reaction (Scheme S1 in Supporting Information).29 The reactions produce highly pure product with a high total yield of greater than 90% with no needs of purification processes, which is facile and straightforward when compared to the synthesis of monomers/precursors used in the preparation of recently reported microporous polymers such as polycarbazoles39 and nitrogen-rich triptycene-based porous polymers.16 The synthesis of PMOP (Scheme 1) was carried out via FeCl3-catalyzed Friedel-Crafts cross-linking reaction using 1,2-dichloroethane as the solvent to produce a light-brown powder with a yield of 98%. The resulting PMOP was insoluble in any common organic solvents.



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Scheme 1 Synthesis of pentiptycene-based MOP. The chemical structure of the PMOP was characterized using FT-IR and solid-state

13

C

cross-polarization magic-angle spinning (CP/MAS) NMR. As shown in FT-IR (Figure S1), the broad peak located around 3400 cm-1 is ascribed to the hydroxyl groups within the microporous polymer networks. The bands at 2840 and 2933 cm-1 are attributed to the stretching vibration of methylene. As expected, three types of carbon signals are observed in the

13

C CP/MAS NMR

spectrum (Figure S2): methylene carbon (a) with a chemical shift of 31 ppm, pentiptycene bridgehead carbon (b) at 44 ppm, and aromatic carbons (c-h) in the range of 120-140 ppm. The powder wide angle X-ray diffraction (WAXD) pattern (Figure S3) of the PMOP displays a broad peak, suggesting amorphous chain packing in the polymer networks; the calculated d-spacing value of 9.0 Å indicates general microporous structure within the PMOP. The PMOP is thermally stable up to 350 oC with a high char yield of 67% at 800 oC under a nitrogen atmosphere as revealed by thermal gravimetric analysis (TGA, Figure S4). The small weight loss below 200 oC is possibly due to the trapped moisture. The PMOP material appeared as irregular granules (average particle size ~100 µm) when imaged by field-emission scanning electron microscope (FE-SEM) (Figure S5), which is possibly a result from strong hydrogen bonding within the polymer network. Additionally, the surface of the particles appears to be rather rough and highly porous indicating possibly high porosity in the bulk material. The porosity of the PMOP was investigated by nitrogen adsorption/desorption isotherm measurements at 77 K. As shown in Figure 1a, a Type I adsorption profile is observed for the PMOP, where the sharp increase of nitrogen uptake at low relative pressure (P/P0 < 0.01) evidences the microporous nature of the PMOP. Previous study has demonstrated that the microporosity of a 6 ACS Paragon Plus Environment

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material can be estimated by the ratio of micropore volume (determined by the t-plot method) to the total pore volume at P/P0 = 0.995 (i.e., Vmicro/Vtotal).10

For the PMOP, a Vmicro/Vtotal value of 0.65

was obtained based on the micropore volume and the total pore volume of 0.575 and 0.885 cm3 g-1, respectively, suggesting a high degree of microporosity in the PMOP. This could be attributed largely to the internal free volume of pentiptycene units. The nitrogen adsorption isotherm displays a minor hysteresis in the whole range of relative pressure. This is possibly due to the segmental flexibility of the polymer networks. The PMOP exhibited a very high BET surface area of 1604 m2 g-1 (Langmuir surface area of 1871 m2 g-1), which is much higher than that of the triptycene-based MOPs.10, 15-19 Again, this is attributed to the much larger internal free volume of pentiptycene (326 Å3) than that of triptycene (93 Å3), suggesting the great potential of pentiptycene moieties in fabricating highly microporous materials. Figure 1b shows the pore size distribution derived from non-local density functional theory (NLDFT), which suggests a dominant pore width of ~1.3 nm and several small peaks with pore width of less than 2 nm. This observation confirms the predominantly microporous nature of the synthesized PMOP and no macropores (i.e., pore diameter greater than 50 nm) were observed in the PMOP.



Figure 1 (a) Nitrogen adsorption/desorption isotherm measured at 77 K, (b) pore size distribution calculated by NLDFT method, (c) hydrogen adsorption/desorption isotherm at 77 K, and (d) CH4 adsorption/desorption isotherms at 273 K and 298 K. The adsorption and desorption isothermals are labelled with filled and empty symbols, respectively. To evaluate the gas storage capacities of the PMOP, hydrogen adsorption tests of the PMOP were conducted at 77 K (Figure 1c). Due to the high internal free volume and high surface area, the PMOP exhibits a ultra-high hydrogen uptake of 1.86 wt % at 1 bar, which is markedly higher than those of recently reported triptycene-based MOPs that showed H2 uptake capacities in the range of 1.09−1.71 wt%.15-16, 40-41 Similarly, the CH4 uptakes at 273 K and 298 K up to 1.13 bar were evaluated. As shown in Figure 1d, reversible adsorption/desorption isotherms were observed at both temperatures. The CH4 uptakes are 2.41 and 1.50 wt % at 273 and 298 K, respectively, which are comparable to many current microporous polymers for methane storage under similar conditions.19, 42

The macromolecular design of the PMOP purposefully features two important characteristics that allow for high capacity CO2 uptake, i.e., high microporosity induced by internal free volume of the pentiptycene and the Friedel-Crafts cross-linking reaction as well as the CO2-philic sites on the PMOP network. As shown in Figure 2a, the CO2 isotherms of PMOP display a steep rise at low pressures and reversible CO2 uptakes with negligible hysteresis; no saturation of CO2 uptake is observed in the studied pressure and temperature range. CO2 uptakes (1 bar) are 5.00 mmol g-1 (22.0 wt %) and 3.17 mmol g-1 (13.9 wt %) at 273 and 298 K, respectively. The CO2 uptake of PMOP is about 2.4 times that of the commercial BPL carbon (9.2 wt % at 1 bar and 273 K).43 Besides, this value is much higher than that of the reported highly porous aromatic framework, PAF-1 (9.1 wt %), which has a ultra-high BET surface area of 5460 m2 g-1.44 Moreover, the PMOP displays higher CO2 adsorption capacities than most of the aromatic hyper-crosslinked polymers7, 34-36

and triptycene-based MOPs10, 15-18. For instance, the CO2 uptake of the PMOP is ~40% higher

than that of a triptycene-based hyper-crosslinked polymer sponge at 1 bar and 273 K.15 A comprehensive comparison of CO2 capture performance between the PMOP in this work and a variety of other reported MOPs is illuminated in Figure S6 by plotting the CO2 uptake as a function of BET surface area. As shown, the PMOP displays superior CO2 adsorption capacity that outperforms most of previously reported MOPs with comparable BET surface area. It should also be noted that the preparation of current high-performance MOPs generally involves rather 8 ACS Paragon Plus Environment

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complicated multi-step chemical synthesis to introduce the CO2-philic N-containing heterocyclic rings in the building block structures to realize high CO2 adsorption. In this regard, the simplicity of PMOP synthesis reported in this work represents a significant leap forward in developing novel MOP

materials

for

high-capacity

gas

storage/separation

applications.

Moreover,

specifically-designed chemical modifications on the reported pentiptycene-based MOP, such as incorporating CO2-philic functional groups (e.g., amine), hold great promise to push the CO2 adsorption capacity of MOP materials even further.

Figure 2 (a) CO2 adsorption/desorption isotherms measured at 273 and 298 K, (b) isosteric heat of adsorption as a function of CO2 loading, (c) four continuous CO2 adsorption/desorption cycles of the PMOP at 273 K with degas regeneration at 298 K for 1.5 h on the same analysis port (after the initial cycle, represented in black, an offset of 200 mmHg was applied to the isotherm curves measured during each subsequent cycle), and (d) Henry selectivities of CO2/N2 and CO2/CH4 gas pairs. (The adsorption and desorption isothermals are labeled with filled and empty symbols, respectively.) To provide a better understanding of the fundamental gas adsorption properties, the isosteric heat of adsorption (Qst) for PMOP was calculated from the CO2 adsorption isotherms at 273 and 298 K using the Clausius-Clapeyron equation, and the results are shown in Figure 2b. The PMOP exhibits an adsorption of heat in the range of 24.2–32.2 kJ mol-1. The onset adsorption heat of 32.2 kJ mol-1 is higher than the reported values of many other MOPs,45 and comparable to some of the 9 ACS Paragon Plus Environment


CO2-selective MOPs9,

46-47

featuring amine or acid functionalized pore walls. The high CO2

adsorption and binding energy of the PMOP can be ascribed to the favorable interactions between the polarizable CO2 molecules and the polar –OH functionalized pore walls in the PMOP through dipole-quadrupole interaction and/or hydrogen bonding. On the other hand, the value of the adsorption heat is well below the energy of chemical bonds, implying moderate CO2 interactions with the PMOP networks that allow for a facile sorbent regeneration without applying extensive heat. This is verified by the completely reversible adsorption/desorption isotherms with no hysteresis (Figure 2a), as well as almost the same adsorption/desorption behavior in four continuous cycles (Figure 2c). A simple degas regeneration at room temperature for 1.5 h was used after each cycle. To further evaluate the potential of using the PMOP for carbon capture from flue gas (CO2/N2) and natural gas purification (CO2/CH4), N2 and CH4 adsorption tests were also performed up to 1.13 bar. As shown in Figure S7, N2 uptake is the lowest among the three gases, and the relatively higher CH4 uptake may be attributed to the H-π interactions between CH4 and pentiptycene moieties.48 A sharp rise of gas uptake at low pressures is observed for CO2 compared to N2 and CH4, indicating the high affinity of CO2 toward the polar hydroxyl groups of pentiptycene moieties. For instance, the CO2 uptake is 1.36 mmol g-1 at 273 K and 0.1 bar, and the corresponding values are 0.042 and 0.26 mmol g-1 for N2 and CH4, respectively, implying high separation capabilities of CO2 over N2 and CH4. Quantitatively, adsorption selectivities of CO2/N2 and CO2/CH4 were estimated based on the ratios of the onset slopes of gas adsorption isotherms (Figure S8 and Figure S9), and the results are summarized in Figure 2d. The calculated CO2/N2 adsorption selectivity for PMOP is 47.1 at 273 K and 28.7 at 298 K, which are higher than many of the reported MOPs such as porous polymer frameworks (PPFs) (e.g., 14.5−20.4 at 273 K),49 hydroxyl-functionalized hyper-cross-linked polymers (16−26 at 298 K),35 triptycene-based MOPs (18−45 at 273 K),10, 15-16 and carbazolic porous organic frameworks (Cz-POFs) (19−37 at 273 K).39, 50 While several MOPs were reported to have very high CO2/N2 selectivity in the range of 38–140 due to the incorporation of CO2-philic amine or acid functionality,45 such high CO2/N2 selectivity is typically accompanied by moderate CO2 uptake capacities, which is much lower than that of PMOP in this study under the same conditions. For instance, a pyrrole-based aromatic heterocyclic microporous polymer (Py-1)7 10 ACS Paragon Plus Environment

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exhibits an excellent CO2/N2 adsorption selectivity of 117, however, with a CO2 uptake of only 2.7 mmol g-1 at 273 K and 1 bar while PMOP in this study has a CO2 uptake of 5.0 mmol g-1 under the same condition. Moreover, PMOP shows a high CO2/N2 selectivity of 28.7 at 298 K, a temperature that is closer to the operation condition for actual post-combustion capture. This value is also higher than some of the hydroxyl-functionalized MOPs (e.g., adsorption selectivity of 16−27),35,

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suggesting that pentiptycene can improve selective adsorption of CO2 over N2 due to the well-defined microcavity size of internal free volume.29 Additionally, the CO2 selectivity over CH4 was also calculated for PMOP based on the onset slops of adsorption isotherms and found to be 8.6 at 273 K and 6.7 at 298 K (Figure 2d). These values outperform the reported values for carbazolic POF (4.4−7.1 at 273 K),50, 52 promoting PMOP as a potential sorbent material for natural gas upgrading.

Figure 3 Absorption capacities of PMOP for various organic solvents. Considering its high surface area and hierarchical pore structures, PMOP also holds great potential to be used as a sorbent in environmental remediation and water treatment applications such as the removal of organic solvents and dyes from aqueous solutions. The removal of organic solvents was evaluated by the absorption capacities of eight different organic solvents, i.e., toluene, chloroform, dimethyl sulfoxide (DMSO), ethanol, acetonitrile, acetone, ethyl acetate, and hexanes. 11 ACS Paragon Plus Environment


As shown in Figure 3, the PMOP displays excellent efficiency in absorbing organic solvents with the uptake values in the range of 15.3-24.5 mL g-1. These values are much higher than the micropore volume of dry PMOP (0.575 mL g-1), and this is because the PMOP underwent significant swelling to form sponge-like matter upon organic absorption allowing for accommodating large volume of solvents. Specifically, the PMOP can absorb up to around 30 times as much as its weight, and this value is much higher than those recently reported active carbon (~9 times) and PFCMP-0 (~19 times).38 The PMOP exhibits the highest absorption capacity of 24.5 mL g-1 for ethanol possibly due to the hydrogen bonding between hydroxyl groups. Moreover, high uptake values of 18.5-20.9 mL g-1 were obtained for other polar organic solvent, such as DMSO, acetone, acetonitrile, and ethyl acetate; this excellent absorbing performance could be attributed to the highly porous structures as well as the polar functional hydroxyl groups on the pore wall. Due to its hydrophobicity, the PMOP displays a relatively low absorption capacity of 2.4 mL g-1 for water, which is desirable for the application of removing polar organic solvent from water. Furthermore, the PMOP with absorbed organic solvents can be easily regenerated by drying under vacuum. A negligible reduction of the absorption capacity was observed after the PMOP was recycled for four times, as shown in Figure S10.


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Figure 4 UV-vis adsorption spectra of (a) MEB and (b) CR aqueous solutions after exposure to the PMOP sorbent for different time intervals, the initial concentration is 100 mg mL-1 for both dyes. Adsorption rates of (c) MEB and (d) CR on the PMOP, the corresponding solution images are shown inset. The zero values in the figures correspond to values lower than the detection limit of the Cary 60 UV-Vis spectrophotometer. Besides organic solvents, the PMOP was also challenged with aqueous solutions containing organic dyes, such as Methylene Blue (MEB) and Congo Red (CR), which are the toxic pollutants in water resources.53 The adsorption kinetics of the dyes onto the PMOP were monitored by recording the changes in intensities at the maximum adsorption wavelengths for MEB and CR in aqueous solutions (663 and 498 nm, respectively) using UV-visible (UV-vis) adsorption spectroscopy. As observed from the adsorption spectra at different time intervals in Figure 4, the characteristic adsorption intensities of MEB and CR aqueous solutions become weaker with increasing treatment time. For instance, more than 80% of CR were adsorbed by PMOP within 30 min at room temperature, implying fast adsorption kinetics of PMOP for organic dyes. Compared with CR, MEB displays an even faster adsorption kinetics with almost 100% of dyes adsorbed within 5 min. This difference in uptake rates is possibly due to the relatively smaller molecule diameter of MEB (14.0 Å) than CR (20.0 Å).15 On the other hand, the negatively divalent nature of CR could also play a role in the slower kinetics through adsorbed CR molecules electrostatically repelling approaching CR molecules. The adsorption process of dyes can be also easily visualized by the color changes over time. As shown in the inset figures of Figure 4, the colors of the dye solutions become lighter and finally fade after 5 and 60 min for MEB and CR, respectively.



Figure 5 Dye adsorption isothermals and percentage removal of (a) Methylene Blue and (b) Congo Red as a function of the equilibrium concentrations. The Langmuir isothermal model fitting of (c) Methylene Blue and (d) Congo Red. The adsorption behaviors of PMOP for dyes were further examined by analyzing the adsorption isotherms with both Langmuir (Figure 5) and Freundlich (Figure S11) isotherm models, and the results are summarized in Table S2. Compared with the Freundlich isotherm model, both dyes display better fitting with the Langmuir isothermal model, which are evidenced by the higher correlation coefficients (R2) in Table S2, suggesting that the adsorption of both MEB and CR can be well described by the Langmuir isotherm model. The Langmuir adsorption behavior indicates that the adsorption of organic dyes by the PMOP eventually saturates at a maximum adsorption capacity. From the linear fittings of the adsorption isotherms by the Langmuir model (Figure 5c and 5d), the maximum adsorption capacities of the PMOP for MEB and CR were calculated as 394 and 932 mg g-1, respectively. Those values are greater than those of most common commercial and state-of-the-art sorbents, such as granular active carbon (264 mg g-1 for MEB),54 carbon nanotubes (216 mg g-1 for MEB54 , 882 mg g-1 for CR55), porous boron nitride (BN) nanosheets (782 mg g-1 for CR, 313 mg g-1 for MEB),56 BN hollow spheres (117 mg g-1 for MEB),57 FeOOH hollow spheres (275 mg g-1 for CR),53 and triptycene-based hyper-cross-linked polymer sponge (330 mg g-1 for MEB)15. The fast adsorption kinetics and high uptake capacities make PMOP a highly 14 ACS Paragon Plus Environment

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competitive sorbent for the removal of both anionic dyes and cationic dyes due to their high specific surface area as well as hierarchical microporosity.

4. Conclusion In conclusion, a pentiptycene-based highly microporous organic polymer network (PMOP) with excellent thermal stability and high BET surface area (1604 m2 g-1) was fabricated from a facile one-pot method via Friedel-Crafts reaction/polymerization. The obtained PMOP exhibits excellent gas storage capabilities and high adsorption selectivities for CO2/N2 and CO2/CH4, suggesting that the PMOP is a promising sorbent material for CO2 capture from flue gas as well as natural gas upgrading. Moreover, the PMOP displays fast absorption kinetics, excellent recyclability, and high absorption capacities for organic solvents and dyes, and thus is a versatile sorbent material that can also be deployed for environmental remediation and water purification. Featured with low cost, high surface area, and hierarchical porous architecture, PMOP may lead to rapid progress in a wide range of large-scale applications including gas storage, separation, and water treatment.

ASSOCIATED CONTENT Supporting Information FT-IR,

13

C MAS NMR, WAXD, TGA, SEM, CO2, N2, and CH4 adsorption isotherms, initial gas

uptake slopes, recycling of PMOP for chloroform absorption, and the Freundlich isothermal model fittings.

AUTHOR INFORMATION Corresponding Author *Telephone: +1-574-631-3453 Fax: +1-574-631-8366 E-mail: [email protected]

ACKNOWLEDGMENTS R. Guo gratefully acknowledges the financial support from the Division of Chemical Sciences, Biosciences, and Geosciences, Office of Basic Energy Sciences of the U.S. Department of Energy (DOE) under award DE-SC0010330. We thank the ND Energy Materials Characterization Facility for the use of the Micromeritics adsorption analyzer.



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Facile Synthesis of a Pentiptycene-Based Highly Microporous Organic

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