Exceptionally High CO2 Capture in an Amorphous Polymer with

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Exceptionally High CO Capture in Amorphous Polymer with Ultramicropores Studied by Positron Annihilation Junjie Liu, Ning Qi, Bo Zhou, and Zhiquan Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b07015 • Publication Date (Web): 31 Jul 2019 Downloaded from pubs.acs.org on July 31, 2019

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ACS Applied Materials & Interfaces

Exceptionally High CO2 Capture in Amorphous Polymer with Ultramicropores Studied by Positron Annihilation Junjie Liu, Ning Qi, Bo Zhou, and Zhiquan Chen∗ Hubei Nuclear Solid Physics Key Laboratory, Department of Physics, Wuhan University, Wuhan 430072, People’s Republic of China E-mail: [email protected]

Abstract A series of amorphous melamine-based polymer networks synthesized by Schiff base chemistry (SNW) were successfully prepared by varying the strut length. The pore structure was analyzed by gas adsorption and positron annihilation methods. Positron lifetime measurements indicate the existence of ultramicropores and also larger mesopores in the SNW materials. The sizes of micropores and mesopores are almost the same in these samples, which are about 0.7 nm and 16.5 nm, respectively. The relative number of micropores increases in the order of SNW-1 < SNW-2 < SNW-3, while the number of mesopores increases in the reverse order. N2 adsorption/desorption measurements also reveal micropores and mesopores in these materials. However it gives an underestimation of the micropore volume. Benefiting from the abundant nitrogencontent and high microporosity, the SNW materials exhibit exceptionally high CO2 capture ability, which reaches a maximum value of 18.3 wt% in SNW-3 at 273 K and 1 bar, followed by SNW-2 and SNW-1. This order is exactly the same as the order of micropore volume revealed by positron annihilation measurement, suggesting that

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micropores play a crucial role in the CO2 uptake. Our results show that positron can provide more precise information about the structure of micropores, and thus can offer an accurate prediction for the adsorption capacity of complex porous materials.

Keywords amorphous polymer networks, microporous structure, gas capture, gas physisorption, positron annihilation

1

Introduction

Microporous organic polymers (MOPs) which have pores with size below 2 nm have attracted considerable attentions owing to their fascinating properties such as high porosity, large specific surface areas, high physicochemical stability and light density. 1,2 Generally, MOPs can be amorphous or crystalline depending on their topological structure. Crystalline covalent organic frameworks (COFs) with well-defined pore architecture have become a new promising material for versatile applications in various fields such as energy storage, 3 catalysis, 4 gas storage and separation. 5–7 Nevertheless, it remains a great challenge to synthesize COFs with a reasonable design at the molecular level using a simple processes. The absence of crystallinity or well-ordered structures within MOPs is actually not a drawback for gas capture. 8 Hence, the chemical diversity, cost-effectiveness and high modularity make the larger family of amorphous microporous networks more competitive. One major application of MOPs is the capture of CO2 . The CO2 adsorption capacity of MOPs has close connection with surface chemistry and pore structure. 9 Chemical functionalization with polar CO2 -philic groups such as nitrogen-rich groups 10 and oxygen-rich groups 11 can enhance the binding affinity between CO2 molecules and adsorbent resulting in the enhancement of gas capture. However, pore structure (specific surface area, pore size and pore volume) plays a more important role in the gas adsorption capacity. It has been 2


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reported that there is a good correlation between CO2 capture and the specific surface area at 50 bar. 12 Larger specific surface area can provide more adsorption sites for adsorbent. For example, the porous polymer networks PPN-4 was reported to have the highest specific surface area of 6461 m2 g−1 among all the MOPs. Its CO2 capture capacity can be as high as 48.2 mmol g−1 (212 wt%) at 50 bar and 295 K. For the porous aromatic frameworks PAF-1, it has a high specific surface area of 5640 m2 g−1 , and it also has a high CO2 uptake of 130 wt% at 298 K and 40 bar. 13,14 However this relationship is valid only for the gas adsorption under high pressure. Cooper’s group investigated the relationship between CO2 capture and specific surface area of a series of MOPs. 15 They found that the CO2 capture ability of MOPs is not solely dominated by the specific surface area at 273 K and 1 bar. A typical example is PAF-1. Although it has a ultrahigh specific surface area, the CO2 capture was as low as 2.05 mmol g−1 (9.0 wt%), which was smaller than that of PAF-3 (3.48 mmol g−1 , 15.3 wt%) in the same series but with much lower specific surface area of 2932 m2 g−1 . Another example was the crystalline covalent organic frameworks COF-6. It has a specific surface area of only 750 m2 g−1 , however its CO2 capture ability was as high as 3.84 mmol g−1 (16.9 wt%). Thus it can be seen that the specific surface area is not the principal factor for post-combustion carbon capture (at low pressure and high temperature). According to the textual research, CO2 capture is sensitive to the pore size. The pore size should be close to the diameter of CO2 , 16 so that the interaction between adsorbed molecule and walls of pore will be stronger, which leads to a higher CO2 capture ability at low pressure with given pore volume. It was reported that at pressure of 1 bar, the CO2 capture ability has a linear relationship with the pore volume when the pore sizes are smaller than 0.8 nm, suggesting that the appropriate pore size should be smaller than 0.8 nm for CO2 adsorption. 17 Attempt to characterize the pore size and pore-size distribution (PSD) resulted in development of various methods that are united by the term ‘probe methods’. A typical method for determining the pore size is gas physisorption using industrial gas with different kinetic diameters as a molecules probe, including N2 (3.64 Å), Ar (3.40 Å), CO2 (3.30 Å). 18,19 In 3



contrast to COFs, amorphous microporous networks have ill-defined pore structures with PSDs over a broader range and are difficult to describe. The assessment of pore volume is not a simple procedure for disordered pore structure as a result of the topological complexity, since it is likely to depend on the nature of molecule probe. 20,21 N2 adsorption isotherms measured at 77 K have been generally accepted as a standard tool for the analysis of pore structures. 22,23 PSD is usually obtained by fitting the experimental adsorption isotherm with non-linear density functional theory (NLDFT), or another modified approach called quenched-solid density functional theory (QSDFT) which takes into account the heterogeneity of adsorbate. All the same, owing to the relatively larger molecule diameter, the entrances of narrow micropores may be blocked by N2 molecules, and the micropore volume fails to be fully filled. So N2 is not an entirely satisfactory adsorptive probe for characterizing the narrow micropores if the pore width is below 0.7 nm (ultramicropore). 24 CO2 adsorption at 273 K and atmospheric pressure is often employed as a supplementation for more detailed microporosity characterization owing to the slightly smaller kinetic diameter and the higher adsorption temperature. Research suggests that the analysis of N2 adsorption underestimates the micropore volume dramatically compared to CO2 adsorption. 9,25 Similar observations have also been reported in the amorphous conjugated microporous polymers (CMPs) for smaller H2 capture. 26 A large micropore volume of small pores (1 nm or below) with narrow PSD is the key to high H2 capture. 27 But no simple correlation was observed between the micropore structure detected by N2 (3.64 Å) and H2 (2.89 Å) capture. For amorphous micropore materials, detailed insights into the pore architecture is mandatory for the optimization of gas adsorption capacity. Consequently, a more objective alternative method must be adopted for the microstructure characterization. Positron annihilation lifetime spectroscopy (PALS) has proven to be an ideal porosimetry technique to characterize the nanoporous materials. 28–30 When positrons are injected into porous materials, after thermalization in a few ps, they can either annihilate directly with electrons (from the free bulk state or vacancy trapped state), or form a metastable bound 4


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state with an electron from the material, called positronium (Ps), which is a hydrogen-like atom having the dimension of about 1.06 Å. 31 According to the different spin combinations of positron and electron in the Ps, the ground state of Ps has two states, singlet state parapositronium (p-Ps) with anti-parallel spins (↑↓) and triplet state ortho-positronium (o-Ps) with parallel spins (↑↑). In vacuum, the intrinsic annihilation lifetime of o-Ps is 142 ns, and it annihilates via three γ photons. In porous materials o-Ps will get trapped in the pores, and the annihilation lifetime of o-Ps will be significantly reduced to a few nanoseconds via picking off one surrounding electron with opposite spin from the pore surface. This is called “pick-off” annihilation and two γ photons are emitted instead of three. The pickoff annihilation lifetime of o-Ps depends on the size of pores: the larger the pores, the longer are the lifetime, and vice versa. On the other hand, the formation probability of Ps (reflected by the o-Ps intensity) contains the information of the relative number of pores. Therefore, the information of the pore structure can be accurately provided by lifetime and relative intensity of the pore-sensitive o-Ps. In contrast to the N2 adsorption measurements, positronium probe can evaluate precisely not only the ultramicropores, but also closed pores which gas molecules cannot enter. 32,33 In this study, a series of amorphous SNW materials were synthesized with different strut length. The parameters of pore structure were analyzed in depth by various probing methods, such as gas physisorption and positron annihilation lifetime spectroscopy, as illustrated in Scheme 1. The CO2 and H2 capture of these SNW materials were studied, and carefully investigated the relationship between gas capture and pore architecture.

2 2.1

Experiment section Materials

Melamine (99%), terephthalaldehyde (98%), 4,4’-biphenyldicarboxaldehyde (98%), isophthalaldehyde (98%), anhydrous dimethyl sulfoxide (DMSO, 99.9%) and tetrahydrofuran (THF, 5



e+

N2

CO2

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e-

H2

o-Ps

NH2

OHC N H2N

N N

OHC

CHO

OHC

CHO

CHO

NH2

Scheme 1: Illustration of the synthesis process of the SNW materials and the various probing methods of pore structure. 99.0%) were purchased from Aladdin Chemical Reagents Co., Ltd. (Shanghai, China). Acetone (99.0%) and dichloromethane (CH2 Cl2 , 99.5%) were purchased from Sinopharm Chemical Reagents Co., Ltd. (Shanghai, China). All reagents and solvent were used without further purification.

2.2

Synthesis of the SNW materials

The amorphous melamine-based polymer networks were synthesized according to the method invented by Schwab et al. from the one-pot polycondensation reaction of a equimolar ratio of amine and aldehyde groups at 453 K for 72 h in anhydrous dimethyl sulfoxide. 34 The aminal network’s name is Schiff base network. Typically, melamine (313 mg, 2.485 mmol), terephthalaldehyde (500 mg, 3.728 mmol) and dimethyl sulfoxide (15.5 ml) were added into a flame-dried Schlenk flask, which was fitted with a condenser and a magnetic stirring bar. Then the mixture was heated to 453 K for 72 h under an inert atmosphere. After cooling down to room temperature, the precipitated powders were isolated by filtration over a Büchner funnel and were washed three times (once each) with excess acetone, tetrahydrofuran and dichloromethane. The purified off-white powders were dried in a vacuum oven at 373 K for 12 h to obtain the final SNW-1. SNW-2 and SNW-3 were synthesized in a way similar to the preparation of SNW-1, with different strut length by changing the composition of

6


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aromatic aldehydes (Figure S1).

2.3

Characterization

Fourier transform-infrared (FT-IR) spectroscopy was collected on a NICOLET 5700 FTIR Spectrometer. Elemental analysis (C, H, N) was measured on RARIO EL III. Thermogravimetric analysis (TGA) was performed with a TA thermal analyzer (Q50) under N2 (flowing rate of 30 mL min−1 ) at a heating rate of 10 K min−1 . The surface morphology of microporous polymer networks were observed by a Zeiss SIGMA field emission scanning electron microscope. N2 adsorption/desorption measurements was taken using a Micromeritics ASAP 2020 instrument. Low-pressure CO2 sorption were measured separately at 273 K and 298 K, and the isosteric heats of adsorption (Q st ) of the SNW materials were calculated from the Clausius-Clapeyron equation. 35 H2 adsorption isotherm was measured at 77 K and 1 bar using Quantachrome Instruments. Prior to gas sorption analysis, samples were degassed at 393 K for 12–16 h under vacuum. In order to reach the absorption equilibrium, the equilibration time was extended to 180 sec (in the time interval, the consecutive pressure value agrees within 1.3 × 10−4 bar). Specific surface areas was calculated by applying the theory of Brunauer, Emmett, and Teller (BET) from the N2 adsorption isotherm. The pressure range between 0.02 and 0.13 P/P0 was selected which yields a high linear region (R2 > 0.999). PSDs were calculated using the QSDFT for N2 adsorption and the NLDFT for CO2 adsorption using the slit-shaped pore carbon model. By contrast, the smallest probe o-Ps was also used to characterize the microstructure of these materials by measuring the positron lifetime spectrum, which was performed under vacuum (1 × 10−5 Torr) at ambient temperature using an automated EG&G Ortec fast-fast coincidence spectrometer (Figure S5). A

22

Na positron source (∼10 µCi) deposited in an envelope of commercial DuPont™

Kapton films (7.5 µm thick) was sandwiched between two identical sample pieces, and was sealed in vacuum chamber. The integrated counts of each spectrum are approximately 2 million, and two spectra were collected for each sample to verify the stability of the instru7



ment. Positron lifetime spectrum shows a collection of the annihilation statistical events of the injected positrons, and it is a sum of several exponentially decaying components. 36 Accurate discrimination of each annihilation lifetime component and relative intensity from spectrum is a key step. The obtained positron lifetime spectra were modeled as the sum of four decaying exponentials (lifetimes τ 1 , τ 2 , τ 3 and τ 4 ) with their intensities (I 1 , I 2 , I 3 , and I 4 ) using the PATFIT program after source correction needed for the sealing films, 37 or analyzed as a continuous lifetime distribution using the Laplace inversion algorithm by measuring the positron lifetime spectrum of a reference material (Si) at the same experimental condition. 38 The advantages of this algorithm lie in that: 1) determination of the instrumental resolution function was avoided by measuring the lifetime spectrum of a reference material with a known single lifetime component under the same experimental condition; 2) the number of lifetime components is not needed to be specified before the analysis.

3

Results and discussion

3.1

Structure and porosity of SNW networks

The chemical nature of synthesized SNW materials were investigated by FT-IR spectra and elemental analysis. As shown in Figure 1, the characteristic peaks of quadrant (1552 cm−1 ) and semicircle stretching (1479 cm−1 ) of the triazine ring appear in the spectra, indicating that the melamine nodes have successfully incorporated in these networks. Furthermore, the band at 1690 cm−1 corresponding to carbonyl group (C=O) stretching are absent. The broad peak at around 3411 cm−1 might be from the free N-H groups. Elemental analysis show that the nitrogen content of these materials is nearly 40 wt% (Table S1). All these results demonstrated that the extended aminal network of SNW polymers were successfully synthesized. The dominant pore of a fragment in SNW-1 was shown in Figure S3, the pore diameter was evaluated to be around 0.6 nm. The surface morphology and size of the SNW particles were studied by field-emission 8


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3 4 1 1

T r a n s m is s io n ( % )

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3 8 5 0

1 5 5 2

S N W -1

1 4 7 9

S N W -2 S N W -3

3 3 0 0

2 7 5 0 2 2 0 0 1 6 5 0 W a v e n u m b e r ( c m -1 )

1 1 0 0

5 5 0

Figure 1: FT-IR spectra of the SNW materials. scanning electron microscopy as shown in Figure 2. No significant change in the size of the particles was observed with the systematic variation of the monomer strut length, and the spherical shape with diameter in the range of 20–40 nm can be observed for the prepared nanoparticles. Besides, the SNW materials exhibit high thermal stability as evidenced by TGA analysis (Figure S4). It was obvious that these materials show a small weight loss of ∼10% at about 609 K. (b)

(a)

(c)

100 nm

100 nm

100 nm

Figure 2: SEM images of (a) SNW-1, (b) SNW-2 and (c) SNW-3. The pore structure was investigated by two different methods, i. e. N2 adsorption/desorption and positron annihilation lifetime measurements. Figure 3 shows the N2 adsorption/desorption isotherms measured at 77 K for the SNW materials. The adsorption isotherms show a steep uptake at very low relative pressures (P/P0 < 0.001) owing to enhanced adsorbent-adsorptive interactions in the micropores of molecular dimensions, and then followed by a virtually horizontal adsorption in the intermediate pressure region. All networks give rise to the reversible 9



type I isotherms according to IUPAC classifications, indicating that the SNW materials are microporous polymer in nature. 1 A sharp increase in N2 adsorption capacity at high relative pressures (P/P0 > 0.8) suggests the existence of larger mesopores, which may result from the interparticle voids of the nanoparticle powder. The inset in Figure 3 shows the pore size distributions in the micropore range. The PSD curves in the whole range including micropores and mesopores can be found in the Figure S7. All these networks show a narrow dominant pore width centered at around 0.7 nm and a shoulder peak between 1 and 2 nm with different intensities. The dominant PSD peak shifts slightly rightward in the order of SNW-3, SNW-1 and SNW-2 with the increase of strut length, while the shoulder peak has obvious rightward shift in the order of SNW-1, SNW-2 and SNW-3. 0 .0 8

Å

-1

1 0 0 0

S N W -1

0 .0 6

S N W -2 S N W -3

3

g

-1

8 0 0

d V (w ) / c m

3

g

-1

)

1 2 0 0

Q u a n t it y A d s o r b e d ( c m

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

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6 0 0

0 .0 4 0 .0 2 0 .0 0

4 0 0

0 .3

2 0 0 0

0 .0

0 .2

0 .7

1 .1 1 .5 1 .9 P o r e W id t h ( n m )

2 .3

0 .4 0 .6 R e la t iv e P r e s s u r e ( P / P 0 )

0 .8

1 .0

Figure 3: N2 adsorption/desorption isotherms of the SNW materials at 77 K and PSDs calculated by QSDFT method (inset). As shown in Table 1, the specific Brunauer-Emmett-Teller surface area (SBET ) is in the order of SNW-1 > SNW-3 > SNW-2, varying between 665 and 796 m2 g−1 . The micropore surface area (Smicro ) in SNW-1, SNW-2 and SNW-3 are 554 m2 g−1 , 446 m2 g−1 and 641 m2 g−1 , respectively, and the micropore volume (Vmicro ) are 0.23 cm3 g−1 , 0.19 cm3 g−1 and 0.23 cm3 g−1 , respectively. Apparently, despite that the SNW materials are highly irregular, the pore structure such as pore size and specific surface area could be fine-tailored by varying the building block length, like the crystalline COFs and metal organic frameworks (MOFs). 39–41 10


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In general, the specific surface area decreases with the increase of strut length between the nodes, which has been reported in HCPs and CMPs. 26,42,43 For SNW-2, the lowest specific surface area as well as the lower micropore volume of 0.19 cm3 g−1 than SNW-1 can be attributed to the longer strut segment between the melamine nodes, which has much higher conformational freedom of three-dimensional structure due to the bending out of the plane at the junction of benzene rings in biphenyl linker. In terms of SNW-3, it has the shortest strut length in SNW system, however the specific surface area is lower than that of SNW-1. This might be due to the effect of the dissymmetrical structure of isophthalaldehyde on the geometry of strut, which leads to more disordered network structure than SNW-1. As a result, SNW-3 has a slightly lower specific surface area compared to SNW-1. There is yet another possible reason for the lower specific area in SNW-3. It can be seen from Table 1 that the micropores contribute most of the specific area, while for N2 adsorption measurements, the characterization of micropores especially ultramicropores (d < 0.7 nm) may have some uncertainty, which will report unreal micropore volume and specific surface area. Table 1: Summary of porosity for the SNW Networks Strut

Network SNW-1 SNW-2 SNW-3

OHC

CHO

OHC

CHO

OHC

CHO

Lstrut a SBET b Vtotal c Smicro d Vmicro d 2 −1 3 −1 [Å] [m g ] [cm g ] [m2 g−1 ] [cm3 g−1 ] 796 0.57 554 0.23 5.77 665 0.48 446 0.19 10.08 703 0.43 641 0.23 5.01

CO2 uptake [cm3 g−1 ] 72 (54) 75 (61) 93 (74)

e

H2 uptake [cm3 g−1 ] 99 113 120

f

Monomer strut length (Figure S2). Specific surface area calculated from N2 adsorption isotherms using the BET equation (Figure S6). c Total pore volume calculated from N2 adsorption at P/P0 =0.8. d The micropore surface area and micropore volume calculated from N2 adsorption isotherms by QSDFT. e Data collected at 273 K and 1 bar. The data in parentheses is the adsorbance collected at 298 K and 1 bar. f Data collected at 77 K and 1 bar.

a

b

In order to obtain an accurate and comprehensive picture of the pore structures in the amorphous SNW materials, positron annihilation spectroscopy was applied in this paper. Figure 4 shows the peak-normalized positron lifetime spectra measured for SNW-1, SNW2 and SNW-3. The data of the lifetimes and their relative intensities are listed in Table 2. These data are crucial in interpretation of the microstructure. 44 The shortest lifetime 11



τ1 (about 188 ps) is ascribed to the self-annihilation of p-Ps and free-annihilation of the positron. The intermediate lifetime τ2 (about 0.44–0.45 ns) is due to the annihilation of positron localized at open volume defects such as free volume holes or pores. 45 The longlived lifetimes τ3 (∼2.8 ns) and τ4 (∼110 ns) stem from the annihilation of o-Ps in micropore and mesopores in the SNW materials, respectively. The intensity of o-Ps lifetime reflects the relative number of the corresponding pores.

C o u n ts

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1 0 5

1 0 4

1 0 3

1 0 2

S N W -1 S N W -2 S N W -3

0

1 0 0 0 2 0 0 0 3 0 0 0 C h a n n e ls ( t im e / c h = 5 0 .3 p s )

4 0 0 0

Figure 4: Peak-normalized positron lifetime spectrum measured for the SNW materials. A semi-empirical equation (Eq. 1) has been proposed to correlate the o-Ps lifetime with the average radius (R) of the spherical pores by Tao and Eldrup (Tao-Eldrup model) as given below. 46 −1 τo−P s

1 R R + sin 2π =2 1− R + ∆R 2π R + ∆R

fV = C(

4π 3 R )I 3

(1) (2)

where the parameter ∆R (0.1656 nm) is the electron layer thickness, which is a constant obtained by empirical calibration. The pore density can be calculated using Eq. 2, where C is a proportionality constant depending on materials. fV can be used to characterize the relative pore volume. When the pore is too large, some of the o-Ps fail to pick electron from the wall, so the self annihilation of o-Ps cannot be neglected, which leads to ultra long o-Ps 12


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lifetime component. In this case, the above Tao-Eldrup model is no longer applicable. Ito et al. modified the above model by considering the intrinsic three γ annihilations in large pores, resulting in the following equation: −1 τo−P s

" b # Ra 1 2πRa R − Ra 1 =2 1− + sin( ) × 1− + Ra + ∆R 2π Ra + ∆R R + ∆R 142

(3)

where Ra (0.88 nm) and b (0.55) are fitted parameters. 47 It was argued that Ito’s model gives overestimation of the pore size, since the calibration of this model used o-Ps lifetime data measured in air instead of vacuum. Wada et al. also presented a simple model for larger spherical pores: 48 λR o−P s =

3 vth P + λT (R ≥ 0.96 nm or 1/λo−P s ≥ 21.1 ns) 4 R − r0

(4)

where R is the radius of the spherical pore, vth P = 0.021 ± 0.002 nm/ns and 2r0 =1.14 nm. λT is the self annihilation rate of o-Ps in vacuum (1/142 ns−1 ). By using Eq. 1, the average sizes of micropores in SNW-1, SNW-2 and SNW-3 are estimated to be 0.70 nm, 0.71 nm, and 0.71 nm, respectively. These values are consistent with the dominant micropore width obtained by N2 adsorption. It is noteworthy that the relative intensity I3 increases in the order of SNW-1, SNW-2 and SNW-3 despite that the average pore width changes a little, as shown in Table 2. The relative micropores density calculated using Eq. 2 gradually increases from 3.04%, 3.51% to 3.80%. It indicates that SNW-3 has the largest micropore volume with similar average pore width at around 0.7 nm, followed by SNW-2 and SNW-1. Significantly, there is also a ultralong lifetime component τ4 (110–116 ns) with intensity between 7.7%–16.8%, which corresponds to the mesopores in the SNW materials. By using Eq. 3, the average mesopore size is between 32–43 nm, while using Wada’s spherical model (Eq. 4), it is around 16.5–21.3 nm. The whole PSD curves calculated from N2 adsorption (Figure S7) have unequivocally confirmed the existence of mesopores owing to amorphous structure or the interparticle space. 13



Normally, the ill-defined geometry of amorphous microporous networks makes the PSD to have only statistical significance, i. e. the pore size should have a distribution. To further characterize the porosity, the continuous positron lifetime analysis was performed by CONTIN program using Laplace inversion algorithm. The PSDs of the SNW materials can be then represented by Gaussian peaks from the continuous o-Ps lifetime using Eq. 1. The analytic results from the PATFIT and CONTIN programs are cross-checked against each other. After spectrum analysis by CONTIN program, five positron annihilation lifetime (reciprocal of annihilation rate, λ−1 ) distribution peaks are observed clearly, with two shorter lifetime −1 −1 −1 −1 components (λ−1 1 , λ2 ) and three longer lifetime components (λ3 , λ4 , λ5 ), as shown in

Figure 5(a). The longest lifetime component λ−1 5 in the CONTIN analysis result is in good agreement with results of τ4 in PATFIT analysis, which corresponds to the o-Ps annihilation in mesopores in the SNW materials. The result of the PSDs in the micropore region is shown in Figure 5(b), and the average lifetime and intensity are listed in Table 2 for comparison. It is clear that the average pore width centered at around 0.7 nm estimated from τ3 resolved by PATFIT program may consist of two components: a narrow dominant peak centered at around 0.6 nm and a shoulder peak with center varying between 0.9 nm and 1.2 nm. Notable is, the dominant pore size estimated by positron annihilation spectroscopy agrees fairly closely with the theoretical calculation. The overall intensity I λ3 +λ4 is basically consistent with I 3 in the PATFIT result. This is in agreement with the PSD calculated by the N2 adsorption. More importantly, the shoulder peak position shifts towards the ultramicroporous region in the order of SNW-1, SNW-2 and SNW-3, and the overall intensity (I λ3 +λ4 ) increases gradually. It reaffirms that SNW-3 has the largest micropore volume with pore diameter below 1 nm, followed by SNW-2 and SNW-1. The possibility of the influence of chemical structure on positronium formation and the validity of positron data are discussed in the Supporting Information.

14


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(a ) P r o b a b ilit y D e n s it y F u n c t io n

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0 .1

1

2 3

S N W -1

4

(b ) 5

S N W -2

S N W -3

1 1 0 P o s it r o n lif e t im e ( n s )

3

P r o b a b ilit y D e n s it y F u n c t io n

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1 0 0

0 .4

4

S N W -1

S N W -2

S N W -3

0 .7 1 .0 P o r e d ia m e t e r ( n m )

1 .3

Figure 5: (a) Positron lifetime distribution and (b) micropore size distribution in the SNW materials. Table 2: PALS results of the SNW materials Network SNW-1 SNW-2 SNW-3

3.2

PATFIT τ1 [ps] τ2 [ps] τ3 [ns] τ4 [ns] I3 [%] I4 [%] [ns] 178.4 ± 4.3 437.1 ± 6.2 2.79 ± 0.03 110.05 ± 0.86 9.54 ± 0.09 16.80 ± 0.06 2.26 188.5 ± 4.1 451.2 ± 6.9 2.89 ± 0.02 110.44 ± 1.00 10.39 ± 0.09 14.29 ± 0.06 2.18 188.7 ± 3.6 442.4 ± 7.6 2.86 ± 0.02 116.27 ± 1.83 11.45 ± 0.09 7.66 ± 0.06 2.16

CONTIN [ns] 8.43 8.42 5.11 8.41 4.65 8.41

I λ3 +λ4 [%] 9.92 10.49 11.44

Gas adsorption

Due to ultrahigh nitrogen content of up to 40 wt%, the SNW materials show enhanced CO2 capture ability as the reported nitrogen-rich microporous polymers networks. 10 For these reasons, SNW-1 was often chose as a potential candidate for mixed-matrix membranes (MMMs) fabrication. 49–51 Nevertheless, despite of these advantages which are significant for gas storage, the adsorption property of other SNW materials (SNW-2 and SNW-3) synthesized from similar approach are barely reported. Therefore the gas uptake performance was investigated for all the SNW materials. Figure 6(a) and (b) shows the volumetric CO2 adsorption/desorption curves of SNW materials up to 1 bar at 273 K and 298 K respectively, and Figure 6(c) shows H2 adsorption curves measured at 77 K and 1 bar. It can be seen that the adsorption capacity of SNW-1 is in accordance with previously reported results. 49 Remarkably, SNW-3 exhibits exceptionally outstanding CO2 capture ability (18.3 wt%, 703 m2 g−1 ) at 273 K and 1 bar, which is ∼38% higher as compared to SNW-1. As shown in Figure 7, the CO2 capture ability in SNW-3 is much higher than that of many other MOPs under the same conditions such as PAF-1 (9.0 wt%, 5640 m2 g−1 ), 13 PAF-3 (15.3 wt%, 2932 15



m2 g−1 ), 13 CMP-1 (9.5 wt%, 837 m2 g−1 ), 11 and COF-6 (16.9 wt%, 750 m2 g−1 ), 52 even though they have a larger specific surface area. The enthalpy of CO2 adsorption is another important factor for evaluating the CO2 capture performance of materials, which can reflect the intermolecular force between adsorbate and adsorbent. As shown in Figure 6(d), the calculation of initial Q st yields very high values of 38.2, 29.4 and 31.4 kJ mol−1 for SNW-1, SNW-2 and SNW-3, respectively, indicating a strong interaction between CO2 molecules and the SNW materials. 53 This may be attributed to the abundance of nitrogen atoms, and the multiwall interactions from the micro- and ultramicropores. It is clear that the Q st value for SNW-1 has a rapid decline trend with the increasing CO2 capture. This shows a stronger interaction between SNW-3 and CO2 as compared to SNW-1. But eventually, the CO2 capture has no direct connection with isosteric heat for these materials. Strangely enough, the CO2 capture ability follows the sequence of SNW-3 > SNW-2 > SNW-1 at either 273 K or 298 K, bearing little relation with the information of microstructure obtained from the N2 adsorption/desorption measurements, as shown in Table 1. Similarly, SNW-3 also exhibits the largest H2 capture, followed by SNW-2 and SNW-1, which agrees with that of CO2 capture. This suggests that there might be other factors which play more important role on CO2 capture. According to the reported results, CO2 capture was sensitive to the specific surface area and pore dimensions in MOPs. 17,18 Specific surface area is definitely not the primary reason for CO2 capture at low pressure (1 bar) for the SNW materials as shown in Figure 6. CO2 molecules prefer to be adsorbed in micropores with pore width below 1 nm, because if the pore width is close to the kinetic diameter of molecules, the binding energy between adsorbent and the gas molecules will be stronger. Therefore the micropore volume with pore size in this range will govern the CO2 capture. 18 According to the PSDs obtained from N2 adsorption/desorption, the SNW materials all exhibit a narrow dominant pore width in the micropore region, however there is no correlation between the CO2 capture or H2 capture ability with the micropore volume calculated from N2 probe. A reasonable explanation for 16


Page 16 of 28

Page 17 of 28

7 5

a d so rb e d (c m

3

g

-1

8 0 6 0

S N W -1

2 0

S N W -2

0 .0

1 4 0

0 .2

0 .4 0 .6 P re ssu re (b a r)

S N W -3

0 .8

-1

)

Q s t ( k J m o l-1 )

8 0 6 0 4 0 0 .0

S N W -1

1 5

S N W -2

0 .0

0 .2

0 .4 0 .6 P re ssu re (b a r)

(d )

0 .8

S N W -3

1 .0

3 0

g 3

a d so rb e d (c m

3 0

3 6

1 0 0

2

4 5

0

1 .0

(c )

1 2 0

2 0

6 0

2

4 0

0

(b )

)

(a )

V o lu m e C O

V o lu m e C O

2

a d so rb e d (c m

3

g

-1

)

1 0 0

V o lu m e H

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


0 .2

0 .4 0 .6 P re ssu re (b a r)

0 .8

S N W -1 S N W -2 S N W -3

2 4 1 8 1 2 6 0

1 .0

0 .0

0 .4

0 .8

C O

1 .2 2

1 .6 2 .0 2 .4 u p t a k e ( m m o l g -1 )

S N W -1 S N W -2 S N W -3

2 .8

3 .2

Figure 6: Volumetric CO2 adsorption isotherms at 273 K (a) and 298 K (b), Volumetric H2 adsorption isotherms at 77 K (c), isosteric heats of CO2 adsorption for the SNW materials (d). this result might be the inaccurate estimation of micropore volume by N2 gas molecule, since N2 molecule may be kinetically restricted in the ultramicropores. As can be seen from Figure 6(a) and Figure 6(b), all the desorption branches of the isotherms show an apparent delay due to the lack of equilibrium, known as low-pressure hysteresis phenomena. 54 This phenomenon indicates that the micropores in these materials certainly contain narrow necks where CO2 molecule have problems of accessibility. 1,25 Therefore, we may conjecture that these narrow necks are more inaccessible for N2 . The relative dimensions of molecule probes and cavity where they can touch determine the probe-accessible area. To further check the accessibility of N2 and CO2 molecules to micropores, the cumulative micropore volume was calculated from N2 sorption and CO2 sorption curves, and they are compared in Figure 8. As shown in Figure S9, the dominant pore size derived from the

17


4 .2

S N W -3

3 .5

S N W -2

C O F -6

S N W -1

2 .8 2 .1 1 .4

C O F -1

H C P -4 H C P -2

0

Page 18 of 28

P A F -3

H C P -3 H C P -1 P A F -4

C M P -1 C M P -1 -(O H )2 C M P -1 -C H 3 C O F -8 C O F -5 C M P -1 -N H 2 C O F -1 0

C M P -1 -C O O H

-1

(m m o l g 2

C O

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

)


P A F -1

C O F -1 0 3 C O F -1 0 2

1 5 0 0 3 0 0 0 4 5 0 0 S p e c if ic s u r f a c e a r e a ( m 2 g

-1

)

6 0 0 0

Figure 7: CO2 capture ability at 1 bar and 273 K versus BET specific surface area for different MOPs. NLDFT model for CO2 sorption (273 K) are in good agreement with the QSDFT model for N2 sorption (77 K). That indicates that the measurement temperature hardly influences the maxima of the distribution curves. This conclusion may apply equally to PALS estimated at room temperature (298 K). The existence of a higher micropore volume of pores width at about 0.7 nm is further confirmed for SNW materials. Significantly, benefiting from the slightly smaller kinetic diameter and enhanced kinetics at higher temperature, CO2 molecules can access the narrower ultramicropores more easily. All the pore width distribution obtained from the CO2 probe starts from the smallest diameter of internal width 0.3 nm regardless of the struts conformation. As a result, the micropore volume obtained from N2 adsorption measurement may be underestimated owing to a large number of ultramicropores in the SNW materials. For this reason, the micropore volume presented here are not consistent with the results obtained from N2 adsorption in the order of SNW-3, SNW-1 and SNW-2. The accessible pore volumes of amorphous micropores networks obtained from the larger N2 molecule probe is inappropriate for describing the relationship between micropore volume and the adsorption capacity of CO2 and H2 . It should be taken into account that when a probe is used to characterize the pore structure as a predictive tool for assessing the potential of MOPs for the capture of a given adsorbate, the probe should be small enough compared 18


0 .2 4

0 .0 0

0 .3

)

(b )

-1

0 .1 2 S N W -1 S N W -2 S N W -3

0 .8 1 .3 1 .8 P o r e d ia m e t e r ( n m )

C u m u la t iv e p o r e v o lu m e ( c m

3

0 .1 8

0 .0 6

0 .3 2

g

-1

(a )

g 3

C u m u la t iv e p o r e v o lu m e ( c m

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


)

Page 19 of 28

0 .2 4 0 .1 6 0 .0 8 0 .0 0

0 .3

S N W -1 S N W -2 S N W -3

0 .6 0 .9 1 .2 1 .5 P o r e d ia m e t e r ( n m )

Figure 8: Cumulative pore volume curves calculated by (a) QSDFT for N2 sorption and (b) NLDFT for CO2 sorption. with the pore size. This is especially serious in the polymers of intrinsic microporosity (PIMs) which exhibit pseudo-microporous and “nonporous” characteristics obtained from N2 probe. 55,56 For positronium, its intrinsic diameter is as small as 1.06 Å, which is much smaller than any gas molecules, so it can give more precise information about the micropores, especially the ultramicropores with size below 0.7 nm. From Table 2, it can be seen that the intensity I 3 shows continuous increase in the order of SNW-1, SNW-2 and SNW-3, while the lifetime τ3 shows nearly no change, which means the increase of micropore volume. This result well explains the close correlation between micropore volume and CO2 (or H2 ) capture ability in SNW materials. Therefore positronium could be a more potential and sensitive probe for micropores in MOPs.

4

Conclusions

A series of amorphous melamine-based polymer networks were successfully synthesized by varying the monomer strut length, and the relationship between gas capture and pore architecture was investigated by positron annihilation lifetime spectroscopy for the first time. Due

19



to ultrahigh nitrogen-content and high microporosity, the SNW materials show a remarkable CO2 uptake at 273 K and 1 bar. Especially, SNW-3 exhibit a ultrahigh CO2 capture (18.3 wt%) among these materials, followed by SNW-2 and SNW-1. Positron annihilation lifetime spectroscopy implies that the proportion of micropore (pore width below 1 nm) volume increases in the sequence of SNW-1 < SNW-2 < SNW-3, which well explains the CO2 capture ability in SNW materials. Combining with the analysis of the N2 and CO2 adsorption isotherms, the micropores volume derived from N2 probe significantly underestimated the pore volume in the ultramicropore range, owing to the relatively large molecule diameter of N2 . Our results indicate that positronium is a powerful tool for the evaluation of pore structure in MOPs and other microporous materials.

5

Conflicts of interest

There are no conflicts to declare.

Acknowledgement This work was supported by the National Natural Science Foundation of China under Grant Nos. 11575131, 11775163 and 11875208, and the Natural Science Foundation of Hubei province under Grant No. 2016CFA080. We would like to thank the Analytical & Testing Center of Sichuan University for providing Materials studio and we would be grateful to Daichuan Ma for his help of computational simulation. Special thanks to the fruitful discussions with Prof. Yoshinori Kobayashi from National Institute of Advanced Industrial Science and Technology (AIST), Japan.

20


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Supporting Information Available Details of synthesis route, N2 adsorption isotherms, BET calculation, Thermogravimetric analysis, Elemental analysis, Schematic diagram of the fast–fast PALS system, Isosteric heats calculation, PSDs derived from CO2 adsorption at 273 K.

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Corresponding Author ∗

E-mail: [email protected]

ORCID Zhiquan Chen: 0000–0002–9518–7837

Graphical TOC Entry

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Exceptionally High CO2 Capture in an Amorphous Polymer with

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