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Materials and Interfaces

Fabrication of microporous aminal-linked polymers with tunable porosity toward highly efficient adsorption of CO, H, organic vapors and volatile iodine 2

2

Meng Rong, Liangrong Yang, Li Wang, Huifang Xing, Jiemiao Yu, Hongnan Qu, and Huizhou Liu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b03126 • Publication Date (Web): 22 Aug 2019 Downloaded from pubs.acs.org on August 27, 2019

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Industrial & Engineering Chemistry Research

Manuscript submitted to Industrial & Engineering Chemistry Research

Fabrication of microporous aminal-linked polymers with tuneable porosity toward highly efficient adsorption of CO2, H2， organic vapor and volatile iodine Meng Rong,a,b Liangrong Yang,*a Li Wang,a Huifang Xing,a Jiemiao Yu,a Hongnan Qu,a and Huizhou Liu*a,b a

CAS Key Laboratory of Green Process and Engineering, Institute of Process

Engineering, Chinese Academy of Sciences, Beijing 100190, China b

School of Chemical Engineering, University of the Chinese Academy of Sciences,

Beijing 100049, China Corresponding Authors: Prof. Huizhou Liu and Dr. Liangrong Yang Institute of Process Engineering, Chinese Academy of Sciences, China P.O. 353, Beijing 100190, China Tel: +86-10-82544912; Fax: +86-10-62554264 Email address: [email protected]; [email protected]

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ABSTRACT In this work, a series of microporous aminal-linked polymers (MALPs) are designed and prepared through the polycondensation of 1,4-bis-(2,4-diamino-1,3,5-triazine)benzene and four rigid tetra-aldehydes such as tetra(4-formylphenyl)methane, tetra(4formylphenyl)silane, 1,3,5,7-tetra(4-formyphenyl)adamantane and 9,9'-spirobi[9Hfluorene]-2,2',7,7'-tetracarboxaldehyde. Benefiting from highly contorted structural units introduced into the polymer network, the nitrogen-rich MALPs exhibit large surface areas (1093-1179 m2/g) and narrow pore size distributions (0.52, 0.93 nm), which endow them with superior small molecules adsorption performances such as CO2, toxic organic vapors and volatile iodine. It is found that higher amounts of micropores greatly improve the small molecules adsorption performance. For example, among the prepared four MALPs, MALP-2 showed a largest adsorbed amount of 18.6 wt % CO2 (273 K, 1 bar), and high adsorption selectivity of CO2/N2 (22.5) and CO2/CH4 (6.3). Notably, MALP-2 could uptake 35.4 wt % benzene, 30.7 wt % cyclohexane, 35.7 wt % toluene even at 298 K and a very low-pressure of P/P0 =0.1, surpassing many other porous organic polymers. MALP-2 also displays an excellent iodine vapor adsorption capacity (218.5 wt %) and remarkable solution iodine adsorption ability. The stable physicochemical properties and excellent adsorption performances towards CO2, toxic organic vapor and iodine demonstrate that MALPs are promising adsorbents for environmental remediation.


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INTRODUCTION Over the past decades, intensive efforts have been devoted to exploring novel solid sorbents and their applications in gas adsorption, organic vapor removal, and radioactive iodine capture with respect to the increasing public concern about climate change and environmental pollution issues.1-2 Up to now, numerous solid inorganic porous adsorbents including porous carbons,3 silver-based zeolites4 and metal organic frameworks (MOFs)5 have been reported. Nevertheless, the organic vapor and iodine adsorption capacities of porous carbons and silver-based zeolites are small owing to their limited available surface area or weak interaction. Besides, most of MOFs are moisture-sensitive, which would furtherly impede their practical application such as humid flue gas and iodine waste separation. In contrast, covalently bonded microporous organic polymers (MOPs) has attracted intensive interest for their potential application in gas capture and separation,6-7 heterogeneous catalysis8 and chemo sensing by virtue of their large surface area, tunable porosity, available functionality and stable physicochemical properties.



Scheme 1 The synthetic route used to prepare microporous aminal-linked polymers, NALPs.

For porous polymer sorbent, pore structure (surface area, pore width) and polar groups play a complementary role in determining the CO2 adsorption performances.1 The introduction of ultramicropore (less than 0.7 nm) and heteroatoms (N, O, F) into POPs could significantly facilitate the increase of CO2 affinity and selectivity through pore-size-exclusion effect and strong dipole-quadrupole interaction or hydrogen boding between CO2 molecule and polar sites.9-10 Very recently, a series of conjugated MOPs have been also found to be effective in capture and removal of organic vapors and volatile iodine, which mainly depend on the conjugated  electron structure, surface area and polymer skeleton polarity.2, 11-13 Therefore, rational monomer structure design and optimization is a key to construct highly porous functional MOPs toward large CO2 adsorption capacity and selectivity as well as excellent capture and recovery of organic vapors and radioactive iodine. Polyaminals are firstly reported by Klaus Müllen through the amination reaction of melamine and aromatic aldehydes, their large surface area and nitrogen-rich merit have attracted significant attention in gas adsorption and heterogeneous catalysis.14 A series


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of melamine or diaminotriazine-based polyaminals have been reported with excellent CO2 adsorption capacity and selectivity.15-17 Nevertheless, these polyaminals are mainly constructed from monoaldehyde, dialdehyde or ternary aldehyde with planar molecule structure, whereas highly contorted three-dimensional aromatic aldehydes are rarely used. Three-dimensional contorted building units such as triphenylamine, triptycene, spirobifluorene, tetrahedral tetraphenyl unit with varied cores (carbon atom, silicon atom, germanium and adamantane) are widely used to construct highly porous organic polymers with large surface area and narrow pore size distributions, e.g. porous aromatic frameworks,18 conjugated microporous polymers,19 covalent organic frameworks,20 and many other functional MOPs.21-23 The introduction of contorted structure into polyaminal networks may sufficiently prevent tight space packing during the period of the formation of polymer networks, furtherly to retain large porosity. In addition, previous studies on polyaminals have focused on the adsorption performances of CO2, CH4, and H2, whereas studies on the capture and recovery of volatile organic vapors and iodine remain are rare. Considering all the above-stated reasons, herein, three-dimensional aromatic tetraaldehyde

such

as

tetra(4-formylphenyl)methane

(TFPM),

tetra(4-

formyltetraphenyl)silane (TFPSi), 1,3,5,7-tetra(4-formylphenyl)adamantane (TFPAd), and

9,9'-spirobi[9H-fluorene]-2,2',7,7'-tetracarbaldehyde (TFPSp) are designed and

successfully synthesized. Subsequently, a series of nitrogen-rich MALPs are prepared through the polycondensation from 1,4-bis-(2,4-diamino-1,3,5-triazine)-benzene (SL1) and tetra-aldehydes. The former three aldehydes have the same tetrahedral geometry,



but with varied center cores. Spirobifluorene-based aldehyde possesses a 90o kink and cross-like shape. These contorted monomer structures could prevent efficient space packing and obtain high, available free volume during the formation of polymer network.24 In addition, the selected building blocks with varied cores and structures are expected to tune porosity parameters and chemical compositions. The capture and adsorption performances of small molecules (CO2, H2, CH4, N2, benzene, cyclohexane, toluene, methanol and iodine) are studied in terms of the polymer chemical compositions and their porous properties in detail.

EXPERIMENTAL SECTION Material N, N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), dichloromethane, tetrahydrofuran (THF) and methanol were purchased from Beijing Chemical Works. DMSO was dehydrated with CaH2 and furtherly distilled under reduced pressure before use. SL-1,25 TFPM,26 TFPSi,27 TFPAd,28 and TFPSp,29 were synthesized in according or slightly modified with the known procedures (detailed synthetic procedures, Supporting Information). Instrumentation Fourier transform infrared spectra (FTIR) of the starting monomers and prepared polymers were recorded using BRUKER TENSOR 27 in the region of 400-4000 cm-1. Solid-state

13C

CP/MAS spectra were measured on a BRUKER AVANCE HD 500

spectrometer at 125.78 MHz at MAS rate of 12.0 kHz using zirconis rotors (diameter: 4 mm), a contact time of 1500 μs and relaxation delay of 5.0 s. Elemental analysis were


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determined by a Vario MACRO cube. Field-emission scanning electron microscopy (FE-SEM) were observed on a JSM-7800(Prime) with 30 kV accelerating voltage. Highly resolved transmission electron microscopy (HR-TEM) images were obtained on a JEM-2100F. Powder X-ray diffractions (PXRD) from 5o to 90o were performed on X'Pert PRO MPD with a scanning rate of 5o/min. Thermogravimetric analysis curves (TGA) were measured on a TG-DTA6300 thermal analyzer by heating the samples (∼10 mg) from 25 up to 700 °C at a heating rate of 10 °C/min under nitrogen atmosphere. UV-vis spectra were recorded on a U-4100 (Hitachi). Gas sorption measurements for all the gas and organic solvent vapor were conducted on an AutosorbIQ analyzer. The samples were degassed at 120 oC under high vacuum for 12 hours before the sorption measurement. 77 K N2 adsorption-desorption isotherms were collected to calculate the BET specific surface area (Brunauer–Emmett–Teller model, P/P0 ranging from 0.01 to 0.1) and pore size distribution (slit/cylinder pore, QSDFT adsorption mode). The CO2, N2, and CH4 adsorption-desorption isotherms were collected at 273, 298 K with a feed up to 1 bar. The H2 adsorption-desorption isotherms were measured at 77, 87 K up to 1 bar. The benzene, cyclohexane, toluene, water and methanol vapor adsorption isotherms at 298 K were measured up to 0.9 P/P0. Synthesis of Tetraphenylmethane-based Microporous Polyaminal (MALP-1) Microporous aminal-linked polymers MALPs were prepared in accordance to the procedures of reported polycondensation method.14 Only the preparation process of MALP-1 is described here as a representative. SL-1 (1 mmol, 0.2975 g), TFPM (0.5 mmol, 0.2167g) and 10 ml freshly distilled DMSO were added into a dry 50 ml Schlenk



tube equipped with a magnetic stir bar and a condenser under N2 flow. The mixture was heated slowly to become completely dissolved. Subsequently, the temperature was increased to 180 oC and the system was kept stirred for 72 h. Finally, the system was cooled to room temperature and the resulting insoluble solid was carefully transferred to plastic centrifuge tube. The insoluble solid was washed and centrifuged successively with DMF, dichloromethane, THF and methanol until the filtrate became clear. The resulting product was furtherly Soxhlet extracted with THF for 24 h and then dried at 120 oC under vacuum for 24 h to get white fluffy powder. The product yield is 76.6%. Elemental analysis (wt%) calculated for (C41N10H28)n: C, 74.54; N, 21.21; H, 4.24. Found: C, 51.73; N, 30.49; H, 3.95. Synthesis of Tetraphenylsilane-based Microporous Polyaminal (MALP-2) The preparation procedure of MALP-2 is the same as that of MALP-1 except that the same amount of aldehyde monomer added into the system was TFPSi instead of TFPM. The product yield is 74.3 %. Elemental analysis (wt%) calculated for (C41N10H28Si)n: C, 71.01; N, 20.71; H, 4.14. Found: C, 51.36; N, 29.13; H, 4.40. Synthesis of Tetraphenyladmantane-based Microporous Polyaminal (MALP-3) The preparation procedure of MALP-3 is the same as that of MALP-1 except that the same amount of aldehyde monomer added into the system was TFPAd instead of TFPM. The product yield is 73.3%. Elemental analysis (wt%) calculated for (C60N10H40)n: C, 80.00; N, 15.56; H, 4.44. Found: C, 52.88; N, 30.94; H, 4.62. Synthesis of Spirobifluorene-based Microporous Polyaminal (MALP-4) The preparation procedure of MALP-4 is the same as that of MALP-1 except that the


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same amount of aldehyde monomer added into the system was TFPSp instead of TFPM. The product yield is 79.7%. Elemental analysis (wt%) calculated for (C41N10H24)n: C, 75.00; N, 21.34; H, 3.66. Found: C, 51.58; N, 30.30; H, 4.40. Iodine Capture and Adsorption The iodine vapor capture experiments were investigated by weighing method. 40 mg activated MALPs solid powder was loaded into a small beaker, which was placed into a sealed glass container with excessive iodine powder. The system was heated at 77 oC under ambient pressure. After certain contact time, it was cooled down to room temperature and weighed. The iodine uptakes for MALPs were calculated by weight gain:  = (m2 - m1) / m1 × 100wt%, where  is the iodine adsorption capacity, m1 and m2 are the weight of samples before and after the adsorption of solid iodine. Iodine adsorption in solution for MALPs were measured by soaking 20 mg powder in an iodine-hexane solution (4 ml, 2 mg/ml). The UV-vis spectra of supernatant were measured after some contact time interval. The solution iodine uptake efficiency was calculated by subtracting the remaining iodine in solution from the initial amount of iodine.

RESULTS AND DISCUSSION Synthesis and Structure Characterization In order to investigate the influence of the tetra-aldehyde structure on the texture properties for the prepared polymers, the polymerization reaction conditions including the monomer dosage, concentration, solvent and temperature were set to be the same. The resulting products are insoluble in any common organic solvents and exhibit a



characteristic hyper-crosslinked structure. The chemical structures of MALPs were evaluated by FT-IR spectra, solid-state

13C

NMR spectra and elemental analysis. As

shown in Fig. 1a, the intensive absorption band of aldehydic (-CHO) stretching vibration at around 1700 cm-1 disappeared (Fig. S1), meanwhile the occurrence of a broad absorption band attached to the stretching vibration of secondary amine

Fig. 1 FTIR spectra of MALPs and SL-1(A); 13C CP-MAS NMR spectra of MALPs (B) and * represents the spinning sidebands.

(3414-3418 cm-1) and typical triazine ring quadrant stretching (1541, 1370 cm-1) confirm the successful formation of aminal (-C-NH-) linkage.30-31 Solid-state 13C NMR spectra (Fig. 1b) also show the carbon resonances signals at 164 ppm (carbon atoms in triazine ring),32 137 ppm (carbon atoms adjacent to triazine ring),32 126 ppm (phenyl


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Fig. 2 FE-SEM images of MALP-1, MALP-2, MALP-3 and MALP-4.

ring),16 52-55 ppm (carbon atom in aminal linkage),16 65 ppm (quaternary carbon atoms in TFPM and TFPSp),33-34 and 38 ppm (secondary carbon atom in symmetric adamantane core).33 Obviously, experimental elemental analysis data show lower carbon and higher nitrogen content relative to theoretical values. On the one hand, the decomposed byproducts (formaldehyde) of DMSO at a high temperature of 180 oC could furtherly react with SL-1 to form aminal linkage.35 On the other hand, the adsorbed gas and water molecules could also affect the elemental analysis result. This phenomenon is common for previously reported polyaminals such as SNWs14 and TMPs.36 Wide angle X-ray diffraction spectra (Figure S2) show no obvious crystal diffraction peak, implying that MALPs are amorphous in nature. FE-SEM (Fig. 2) and HR-TEM images (Fig. S3) exhibit that MALPs are composed of loosely aggregated super tiny irregular nanoparticles. The thermogravimetric analysis (TGA) analysis (Fig. S4) show that MALPs are thermal stable within the temperature of 350 oC under



nitrogen atmosphere. Porosity parameter analysis The porous properties of four polyaminal polymers are analyzed with 77 K nitrogen adsorption branch isotherms and the obtained results are summarized in Table 1. As shown in Fig. 3A, all samples show a rapid N2 uptake rise at a very low relative pressure region (P/P0 < 0.01), which is a characteristic indicative of the presence of substantial micropores in the polymer skeleton. The nearly reversible adsorption-desorption isotherms imply that the network architectures are rigid enough, whereas desorption hysteresis effects are frequently seen in various MOPs due to the elastic deformation of polymer skeleton.37 Interestingly, a steep N2 uptake increase occurred at high relative

Fig. 3 77 K adsorption (filled) and desorption (empty) isotherms of N2 for MALP-1, MALP-2 (+15), MALP-3 (+30), and MALP-4 (+45) (A). Pore size distributions for MALPs (B).

pressure (P/P0 > 0.9) indicates the existence of some macropores caused by the interparticulate voids of loosely packed small particles, which could be also seen in the SEM images.


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The summarized porosity data in Table 1 show that all MALPs possess large surface areas in a range from 1093 to 1179 m2/g and the values surpass most of melamine-based polyaminals (452-907 m2/g)15, 36, 38 and diaminotriazine-based polyaminals (458-995 m2/g).16-17, 31 Fig. 3B shows that the predominant pores are located at 0.524-0.59 nm (ultramicropore), 0.926-0.966 nm, combined with some mesopores. The pore size distribution results indicate that MALPs belong to microporous polymers and the formed ultramicropores favor in the adsorption of smaller CO2 molecule than N2 and CH4 molecules (CH4 (0.38 nm) > N2 (0.364 nm) > CO2 (0.33 nm)).39 The above pore structure parameters show that the incorporation of highly contorted building blocks could sufficiently open up the segments of polymer network and prevent them from efficiently packing.40 Thus, the resulting polymer networks could form adequate small free voids and large surface areas. As for the influence of tetra-aldehyde structure on the porosity parameters of the resulting polymers, MALPs exhibit varying textural properties. Spirobifluorene possesses smaller internal molecular free volume and less stereo-contorted structure relative to that of tetraphenylmethane, tetraphenylsilane and tetraphenyladamantane, which leads to the smallest SBET, Smicro, and Vmicro for MALP-4, among the four polymers.18, 41 MALP-1, MALP-2 and MALP-3 show close BET surface areas due to their similar tetrahedral monomer structure. However, MALP-2 shows the highest Smicro (739 m2/g), Vmicro (0.314 cm3/g), and highest ratio of micropores, which could also be seen in the 77 K N2 isotherms and pore size distributions.18 Notably, MALPs show similar pore size distributions but with varying ratio of micropore of different sizes,



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which may be attributed to their similar contorted three-dimensional monomer structure. MALPs are amorphous in nature and the influence of difference of tetra-aldehyde monomer structure on the porosity may be weakened by the loss of ordered structure. In general, the porous properties of MALPs can be efficiently tuned by the change of tetra-aldehyde monomer structures. Table 1 The porosity parameters of MALPs calculated from N2 adsorption isotherms at 77 K. sample

SBETa (m2/g)

Smicrob (m2/g)

Vmicroc (m3/g)

Vtotald (m3/g)

Pore size (nm)

MALP-1

1179

617

0.268 (30.7)

0.874

0.59, 0.926

MALP-2

1126

739

0.314 (41.6)

0.754

0.524, 0.926

MALP-3

1141

635

0.276 (32.8)

0.841

0.524, 0.966

MALP-4

1093

550

0.244 (29.0)

0.841

0.567, 0.966

aCalculated cThe

using BET method. bThe micropore specific surface area calculated using t-plot method.

micropore volume calculated using t-plot method (the values in parenthesis are the percentage

of micropore volume relative to total pore volume). dThe total pore volume at P/P0 = 0.90.

Low Pressure Gas Sorption and Selectivity Studies Motivated by the excellent porosity parameters and rich nitrogen dopants of MALPs, the gas storage/adsorption performances at low pressure are investigated, especially for CO2. Firstly, the single CO2 adsorption-desorption isotherms are measured at 273 K and 298 K. As shown in Fig. 4A, the CO2 uptake increases with the pressure and has


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Fig. 4 CO2 uptake isotherms (A), H2 uptake isotherms (B), CH4 uptake isotherms of MALPs and isosteric heat of adsorption of CO2 (D), H2 (E) and CH4 (F).

not reached saturation, implying that it could continue to increase at higher pressure. The reversible adsorption-desorption isotherms mean that the polymer adsorbents could be easily regenerated under vacuum or pressure reduction, which is extremely desirable to the actual CO2 capture and separation applications. Notably, the data in Table 2 exhibit that all MALPs possess high CO2 uptakes at 1 bar. And MALP-2 exhibits a highest CO2 sorption capacity of 186 mg/g (4.22 mmol/g) at 273 K and 1 bar, which exceed or can compete with that of all melamine-based polyaminals (100-179 mg/g),15, 42

and most of diaminotriazine-based polyaminals (66-187 mg/g).16 Besides, the CO2

uptakes are also higher than numerous functional microporous organic polymers, such Table 2. Gas uptake and selectivity (CO2/N2, CO2/CH4) for MALPs. H2 at 1 bara

CO2 at 1 bara

CH4 at 1 bara

N2 at 1 bara

Selectivityb

sample

77 K

87 K

Q0

273 K

298 K

Q0

273 K

298 K

Q0

273 K

298 K

CO2/N2

CO2/CH4

MALP-1

15.5

12.3

6.97

179

100

28.5

22.6

10.5

23.9

15.0

6.0

18.3(18.6)

5.2(5.6)

MALP-2

16.4

12.1

7.24

186

102

32.0

22.1

12.0

20.8

14.0

6.2

22.4(22.5)

6.1(6.3)

MALP-3

14.8

12.2

6.68

173

97

29.7

22.3

10.3

29.7

15.3

5.7

18.5(18.2)

5.4(5.6)



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1 2 3 MALP-4 14.8 11.8 6.58 174 98 29.1 22.0 10.3 25.6 14.8 5.6 20.1(20.2) 5.6(6.2) 4 a b 5 Gas uptake in mg/g and enthalpy of adsorption at zero coverage (Q0) in kJ/mol. Selectivity was calculated from initial slope calculation and (IAST) at 6 273 K. 7 8 as polybenzimidazoles BILP-2,5,10,14,15,16 (114-176 mg/g),43 poly(schiff-base)s 9 10 (MOPs, PINs and PSNs: 47-150 mg/g),26, 44-47 and polyimides MOPIs (128-167 11 12 13 mg/g).48 CO2 uptakes display obvious decrease with the temperature increase and this 14 15 is attributed to the physisorption characteristics. MALPs can still uptake 97-102 mg/g 16 17 18 CO2 at 298 K and 1 bar, which are among one of the highest reported CO2 uptakes of 19 20 21 MOPs at 298 K, such as ALPs (8.1-14.3 wt %)21 and BILPs (8.7-15.8 wt %).43, 49 The 22 23 detailed comparisons of CO2 sorption capacity between MALPs and other reported 24 25 26 porous solid sorbents are also summarized and listed in Table S1. To estimate the 27 28 binding affinity between CO2 and polymer skeleton, the limiting enthalpy of adsorption 29 30 31 (Q0) at zero coverage and the isosteric enthalpy of adsorption (Qst) are calculated using 32 33 34 the virial plots and Clausius-Clapeyron equation, respectively (calculation method, 35 36 Supporting Information). Fig. 4D shows that Qst value gradually drops with the CO2 37 38 39 loading increase and the trend is becoming slower and slower, indicating stronger 40 41 affinity between CO2 molecules and polymer pore wall than that between CO2 42 43 44 molecules. Such phenomenon can also be seen in the Qst figures of CH4 and H2 (Fig. 45 46 47 4E and Fig. 4F). At smaller adsorbed amounts area, the Qst value is mainly determined 48 49 by the interaction between adsorbate gas molecules and polymer framework. As the 50 51 52 pore surface is gradually covered adsorbate gas molecules and the adsorbed amounts 53 54 increase, the dominant influencing factor of Qst becomes the interaction between 55 56 57 adsorbate gas molecules. Relative to other three MALPs, MALP-2 displays the largest 58 59 60 Q0 of 32.0 kJ/mol, consistent with the ranking order of Qst and CO2 sorption capacity.


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The obtained Qst and Q0 values are less than 40 kJ/mol, which also confirms that the physical adsorption characteristics for CO2 adsorption in MALPs.50 Similarly, the recyclability of CO2 adsorption experiments shows no apparent loss in CO2 uptake after 10 consecutive cycles (Fig. 5).

Fig. 5 Ten cycles of CO2 uptake for MALP-2 up to 1 bar at 298 K.

In addition to CO2, the H2 and CH4 storage performances are also investigated by the measured gas sorption isotherms. As illustrated in Fig. 3B and Table 2, at 77 K and 1 bar, MALP-2 exhibits a relatively higher H2 uptake of 1.64 wt% and other MALPs can uptake 1.48-1.55 wt% H2. It is noteworthy that the higher amount of micropore lead to more H2 uptake for MALP-2. The calculated Q0 values of MALPs are in the range of 6.58-7.24 kJ/mol, which is superior to non-functional MOPs and comparable to hetero atom-doped MOPs such as ALPs (7.9-8.5 kJ/mol),21 PAN-T (7.41 kJ/mol),42 and COF



Fig. 6 CO2, CH4 and N2 uptake isotherms of MALPs at 273 K, and their singlesite Langmuir-Freundlich fitting curves (solid line).

(6.0-7. 0 kJ/mol).20 Fig. 3D shows that all CH4 adsorption-desorption isotherms are reversible. And the CH4 uptakes are found to be in the range of 22.0-22.6 mg/g at 273 K and 1 bar, exceeding or competing with many other MOPs like triptycene-based microporous polymers (TMPs, 14.7-19.2 mg/g),51 ALPs (14.3-26.0 mg/g),21 and BILPs (14-27 mg/g).43 Similarly, the calculated Q0 values of MALPs toward CH4 are in the range of 20.8-29.7 kJ/mol and higher than most of functional MOPs, such as ALPs (20.8-21.2 kJ/mol)21 and NPOF-4-NH2 (20.7 kJ/mol).52 The adsorption selectivities of CO2 over N2 and CH4 are investigated by the measured adsorption isotherms of CO2, CH4, and N2. As shown in Fig. 6, all MALPs show much higher CO2 uptakes than CH4 and N2 in the whole measured pressure region. For each experimental adsorption isotherm, the corresponding single-site Langmuir-Freundlich fitting curve fits well. Two methods including initial slope and ideal adsorbed solution


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theory (IAST) are utilized to calculated the adsorption selectivity, respectively. Two typical gas pairs including flue gas (15% CO2/85% N2) and landfill gas (50% CO2/50% CH4) are selected to determine the IAST selectivities of CO2/N2 and CO2/CH4. Table 2 shows that the adsorption selectivity of CO2/N2 as determined by initial slope are in the range of 18.3-22.4, which is consistent with the selectivity of 18.2-22.5 calculated by the IAST method. MALP-2 has a higher CO2/N2 selectivity of 22.5 and is comparable to APOPs (23.8-27.5),16 imine-linked porous polymer framework PPF-3 (20.5),53 and aniline/benzene co-polymers (15.9-49.2).54 CH4 molecule is much polar than N2 molecule, thus, MALPs show much lower CO2/CH4 adsorption selectivity of 5.6-6.3 (273 K and 1 bar) relative to that of CO2/N2. Adsorption of Volatile Organic Vapors The adsorption isotherms of benzene, cyclohexane, toluene, and methanol vapors at 298 K are measured and displayed in Fig. 7. Relative to the characteristic small organic vapor adsorption capacity at a low-pressure region for many porous organic polymers (type Ⅱ adsorption isotherm), MALPs exhibit a steep vapor uptake rise at the very initial pressure range (P/P0 < 0.1), indicating that there is a strong binding affinity between organic vapor molecules and polymer skeleton. For example, the comparisons in Table 3 show that MALPs could uptake 30.0-33.8 wt% benzene, 25.5-30.7 wt% cyclohexane, 30.2-35.7 wt% toluene and 18.1-20.4 wt% methanol vapors at a very low relative pressure of P/P0 = 0.1, which surpass most of MOPs under the same measurement condition. As the relative pressure increase furtherly, the adsorbed amounts of organic vapor show a steady increase. Such organic vapor adsorption



Page 20 of 40

behaviour at low- pressure region is mainly attributed to the rich micropores and the aromaticity of polymers,

Fig. 7 Adsorption isotherms of benzene (A), cyclohexane (B), toluene (C), and methanol (D) vapor of MALPs measured at 298 K. Table 3. The comparisons of uptakes of benzene, cyclohexane, toluene and methanol vapor at 298 K between MALPs and other porous organic polymers.

sample MALP-1 MALP-2 MALP-3 MALP-4

PAF-2 PSN-3 SMPI-10 PAF-5 PCN-AD sPI aOrganic

benzene (wt%)a

cyclohexane (wt%) a

toluene (wt%) a

methanol (wt%) a

P/P0

P/P0

P/P0

P/P0

0.1 33.8 35.4 31.6 30.0 8.6 14.4 25.6 25.6 27.1 39.2

0.9 58.5 54.5 57.1 55.8 13.6 80.5 133.8 85.9 98.0 176.6

0.1 29.0 30.7 26.7 25.5 0.2 11.2 11.6 NA 13.0 22.1

0.9 49.2 47.2 48.8 46.5 0.6 63.7 42.1 NA 57.4 78.1

0.1 34.0 35.7 31.6 30.2 NA NA NA NA NA 22.1

0.9

0.9 43.1 43.8 41.6 40.6

ref. this work this work this work this work

NA

55

NA

45

NA

0.1 20.4 19.8 18.5 18.1 NA NA NA

NA

56

NA

8.2

84.3

57

NA

NA

NA

58

78.1

NA

NA

59

48.0 48.9 46.4 45.1 NA NA

solvent vapors uptakes measured at P/P0 = 0.1 and 0.9. NA means not available.


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which are also evidenced by the adsorption capacity order: MALP-2 > MALP-1 > MALP-3 > MALP-4. MALPs are constructed from aromatic phenyl ring, triazine and aminal linkage, which would exhibit strong  affinity toward aromatic organic vapors like benzene and toluene. The adsorbed amounts of methanol vapor is much larger because of its smallest molecule size. Compared with MALP-1, the cycloaliphatic adamantane unit decrease the aromaticity of MALP-3 polymer to some extent and leads to the reduction of aromatic vapor adsorption amounts. The above results indicate that MALPs are advantageous in the application of capturing lowconcentration organic vapors such as indoor toxic organic vapors (TVOCs). Iodine Capture and Recovery by MALPs Iodine vapor capture studies were conducted in a closed glass vessel at 77 oC and ambient pressure. After exposing the samples to iodine vapor, the colour became darkbrown quickly (Fig. S10). Fig. 8 shows that the equilibrium iodine uptakes of MALP1, MALP-2, MALP-3 and MALP-4 are up to 208.6, 218.5, 186.7 and 203.8 wt %, respectively. The iodine loading capacity of MALP-2 is one of the highest among the reported porous adsorbents and the detailed comparisons with respect to that of other solid adsorbents are summarized in Table S4. By virtue of a larger micropore volume, MALP-2 displays a highest iodine vapor uptake. MALP-3 has the smallest iodine vapor uptake owing to the cycloaliphatic adamantane units, which significantly decrease the conjugacy of polymer network (less conjugated  electron). Meanwhile, the iodine vapor capture rates are fast and reach a plateau after 1.5 h. Such a phenomenon could be attributed to the rich micropore, high nitrogen content and conjugated  electron



structure.

Fig. 8 Iodine uptake curves for MALPs at different times at 350 K (A). Kinetic studies of iodine adsorption by MALPs in 2 mg/ml hexane solution (B).

In addition, the iodine adsorption performances of MALPs in solution are also evaluated by immersing some polymer powder into an iodine-hexane solution. UV-vis spectroscopy is used to detect the remaining iodine concentration in solution after some interval contact (Fig. S13). After adding polymer samples into iodine-hexane solution, the dark-purple solution quickly changed to deep red and finally to colourless (Fig. S12). Fig. 8B shows the kinetic of solution iodine adsorption of MALPs. All MALPs demonstrated similar solution iodine adsorption behaviour, the iodine removal efficiency increases sharply at the initial stage and then to a plateau (UV-vis spectra change, Fig. S11). After 12 h, the iodine removal efficiency reaches over 95% and the solution became completely clear. To evaluate the cycle iodine capture performances, MALP-2 was regenerated through Soxhlet extracted with anhydrous ethanol for 24 hours and dried under vacuum, then it was used for next cycle. The degree of reduction in capture efficiency is very small (Fig. S12), most of iodine remaining on surface and in the pores could be easily removed by washing with ethanol. After 5 successive cycle


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experiments, MALP-2 could still remain about 97.2% iodine capture capacity. Such excellent capture and adsorption results imply MALPs are ideal adsorbents for fast removal of iodine vapor or solution iodine.

CONCLUSIONS In summary, four nitrogen-rich microporous diaminotriazine-based polyaminals with tuneable porosity parameters are successfully prepared by incorporating threedimensional building blocks into polymer networks. In general, large surface areas and rich narrow micropores are advantageous for small molecules storage and adsorption. One of MALPs demonstrates higher gas uptakes such as 18.6 wt% CO2 (273K, 1bar) and 35.4 wt % benzene, 30.7 wt % cyclohexane, 35.7 wt % toluene (298 K, 0.1 P/P0), as well as excellent iodine vapor capture capacity of 218.5 wt % and remarkable solution iodine adsorption. Their stable physicochemical properties and excellent small molecule adsorption performances show promising application prospects in CO2 capture and storage, recovery of toxic organic vapor and radioactive iodine.

ASSOCIATED CONTENT Supporting Information Monomer materials synthesis, calculation methods for Qst, Q0, and selectivity, Figure S1-S12, and Table S1-S3.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]; *E-mail: [email protected].



ORCID Meng Rong: 0000-0002-6851-2788 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work is supported by the National Technology Research and Development Program of China (2015CB251402), the National Key Natural Science Foundation of China (U1507203), the National Natural Science Foundation of China (21676273), and the Youth Innovation Promotion Association, CAS (Grant No. 2016043).

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TOC



Scheme 1 The synthetic route used to prepare microporous aminal-linked polymers MALPs. 149x72mm (300 x 300 DPI)


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Fig. 1 FTIR spectra of MALPs and SL-1(A); 13C CP-MAS NMR spectra of MALPs (B) and * represents the spinning sidebands.



Fig. 2 FE-SEM images of MALP-1 (A), MALP-2 (B), MALP-3 (C) and MALP-4 (D).


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Fig. 3 77 K adsorption (filled) and desorption (empty) isotherms of N2 for MALP-1, MALP-2 (+15), MALP-3 (+30), and MALP-4 (+45) (A). Pore size distributions for MALPs (B).



Fig. 4 CO2 uptake isotherms (A), H2 uptake isotherms (B), CH4 uptake isotherms of MALPs and isosteric heat of adsorption of CO2 (D), H2 (E) and CH4 (F). 508x334mm (150 x 150 DPI)


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Fig. 5 Ten cycles of CO2 uptake for MALP-2 up to 1 bar at 298 K. 162x152mm (300 x 300 DPI)



Fig. 6 CO2, CH4 and N2 uptake isotherms of MALPs at 273 K, and their single-site Langmuir-Freundlich fitting curves (solid line).


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Fig. 7 Adsorption isotherms of benzene (A), cyclohexane (B), toluene (C), and methanol (D) vapor of MALPs measured at 298 K.



Fig. 8 Iodine uptake curves for MALPs at different times at 350 K (A). Kinetic studies of iodine adsorption by MALPs in 2 mg/ml hexane solution (B).


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Fabrication of microporous aminal-linked polymers ... - ACS Publications

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