Rigid supramolecular structures based on flexible covalent bonds: A fabrication mechanism of porous organic polymers and their CO2 capture properties

Journal Pre-proofs Rigid supramolecular structures based on flexible covalent bonds: A fabrication mechanism of porous organic polymers and their CO2 capture properties Shi-Chao Qi, Guo-Xing Yu, Ding-Ming Xue, Xin Liu, Xiao-Qin Liu, Lin-Bing Sun PII: DOI: Reference:

S1385-8947(19)33393-5 https://doi.org/10.1016/j.cej.2019.123978 CEJ 123978

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Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

8 November 2019 25 December 2019 27 December 2019

Please cite this article as: S-C. Qi, G-X. Yu, D-M. Xue, X. Liu, X-Q. Liu, L-B. Sun, Rigid supramolecular structures based on flexible covalent bonds: A fabrication mechanism of porous organic polymers and their CO2 capture properties, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.123978

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Rigid supramolecular structures based on flexible covalent bonds: A fabrication mechanism of porous organic polymers and their CO2 capture properties Shi-Chao Qi, Guo-Xing Yu, Ding-Ming Xue, Xin Liu, Xiao-Qin Liu*, Lin-Bing Sun* State Key Laboratory of Materials-Oriented Chemical Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), College of Chemical Engineering, Nanjing Tech University, Nanjing 211816, China * Corresponding authors. E-mail addresses: [email protected] (X.-Q. Liu), [email protected] (L.-B. Sun).

ABSTRACT: Porous organic polymers (POPs), with dramatic textural properties and versatile uses, were traditionally fabricated by means of rigid covalent bonds or linkages on the basis of judiciously chosen monomers or reaction types. In this study, a new fabrication mechanism of POPs is proposed. For the first time, supramolecular interaction is discovered to be robust to maintain the rigid structures of POPs, even with some flexible or rotatable chemical bonds/linkages, on which a series of successfully fabricated POPs code-named PoBCs are based. Both the first-principle calculations and experimental facts confirm that, owing to π-π stacking and van der Waals’ force, a multi-helix structure can be self-assembled in the course of PoBCs polymer chain growth, which effectively inhibits bending and twisting of the polymer chains and the collapse of intrinsic pore structures. Therefore, the as-synthesized PoBC, especially the PoBC-16, with rigid supramolecular structures possesses satisfactory BET specific surface area (1170 m2 g−1) and fully developed porosity (1.08 cm3 g−1), efficiently contributing to both the selective CO2 capture, of which capacity reaches 67.2 cm3 g−1 at 0 oC

and 1 bar, and good reusability, even competitive with many representative benchmark POPs that

are based on rigid covalent linkages, for instance, that code-named APOP (BET specific surface area, 490 m2 g−1; CO2 adsorption capacity, 43.6 cm3 g−1 at 0 oC and 1 bar).

Keywords: Porous organic polymer; Supramolecule; Porosity; First principle; Carbon capture

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1. Introduction As versatile materials, porous organic polymers (POPs) with high specific surface areas, welldefined porosities, tunable architectures and multifunctionality have drawn much attention because of the broad application potentials such as gas separation and storage, catalysis, electrode materials, sensors, sustained drug release, and biomaterials [1−6]. There have been numerous reports on the fabrication and employments of POPs up to recent decade, but the successful fabrication of POPs with satisfactory textural properties is still a great challenge, owing to the spontaneous constriction the polymer chains and the inevitable collapse of the pore structures caused by the ultrahigh surface energies and large capillary pressures exerted on the newly formed pores in the course of POPs growth [7−10]. Such challenges can be overcome to some extent with carefully exploring reaction conditions to ensure the kinetical reversibility, such as self-repairing of the covalent organic frameworks under specific solvothermal conditions, but in fact, a universal principle of POPs fabrication was elaborately selecting the coupling reaction types that can effectively assemble the organic building blocks with the formation of rigid covalent bonds/linkages, for holding the highly rigid and contorted polymer chains to maintain the intrinsic porosities [11−14]. Thus, there have been only a few universal reaction types developed for the construction of POPs, such as Suzuki coupling, Yamamoto coupling, oxidative coupling, Schiff-base reaction, and nitrile cyclotrimerization [15−18], which means that some particular functional groups must be grafted onto the monomers or precursors of POPs by means of intricate and costly synthetic procedures, and thus the fabrication of POPs is largely confined. As for the reaction types or monomers forming pliable or rotatable linkages, the synthesis of POPs has to borrow ideas from the methods of sacrificial components [19,20]. For example, it was reported that POPs can be prepared from a polymeric bicontinuous microemulsion precursor, with removing one domain of the homopolymers by solvent dissolution [21,22]. However, the sacrificial components must result in a low utilization efficiency of the starting materials, and in fact, the pore structures of the resultant POPs are undeveloped in comparison with those of direct synthesis protocols, while it is well known that the presence of developed pore structures is crucial to some important applications, for instance, the gas capture and storage [23−25]. With breaking through the above-mentioned limitations of the rigid covalent bonds/linkages, a new mechanism of direct synthesis is discovered for the fabrication of POPs in this study. A common and easily-operated Friedel-Crafts (F-C) reaction between octaphenylsilsesquioxane (octaphenyl-POSS) 2

and 4,4′-bis(chloromethyl)-1,1′-biphenyl (BCMB) is carried out to fabricate the POPs, code-named PoBC. The F-C reactions have been repeatedly tested in the fabrication of POPs but often failed. Herein, although the F-C reactions generate, in theory, the rotatable and flexible covalent bonds among monomer moieties as well [26,27], as far as we know, it is discovered for the first time in the field of POPs that the supramolecular interactions, i.e., π-π stacking and van der Waals’ force, among the biphenyl moieties anchoring onto the octaphenyl-POSS moieties may accompany the PoBCs growth, and be robust enough to maintain the rigid polymer structures and the intrinsic porosities. With gradually altering the reaction molar ratio between octaphenyl-POSS and BCMB, the evolution process of the supramolecular polymer chains is elucidated by both first-principle simulations and experimental facts. In contrast to many representative POPs reported, the as-synthesized PoBC with rigid supramolecular structures possesses large BET specific surface area (1170 m2 g−1) and ideal porosity (1.08 cm3 g−1). Then the as-synthesized PoBCs were employed for the CO2 capture. CO2 capture has drawn much attention because that the continuous emission of artificial CO2 has caused a series of severe greenhouse problems [28−31]. In contrast to the traditional chemisorption of CO2 with toxic and corrosive amine liquids, CO2 capture with porous materials has been proved to be effective and economical [32,33]. There were a variety of porous materials employed for the CO2 capture, such as porous carbons, zeolites, porous silica, and metal-organic frameworks [34−37], while many of the adsorbents were grafted extra functional groups such as amino acting as the CO2-affiliative sites for improving the CO2 capture capacities and selectivities [38,39]. As far as we know, the PoBCs fabricated in this study do not possess any typical CO2 chemisorptive sites reported, such as amines or azacyclo-species, but it is inspiring that the PoBC exhibited high CO2 capture capacity (67.2 cm3 g−1 at 0 oC and 1 bar) and good CO2 adsorption selectivity towards N2. The CO2 capture capacity of the PoBC can even be competitive with many representative benchmark POPs that are based on rigid covalent linkages.

2. Material and methods 2.1. Materials synthesis All the chemicals were commercially purchased and used as received. Octaphenyl-POSS (0.10 mmol) was dissolved into 1,2-dichloroethane (80 mL), followed by the addition of BCMB with the BCMB/octaphenyl-POSS molar ratio of x (x = 1, 2, 4, 6, 12, and 16, respectively). With the catalyst 3

of anhydrous FeCl3 (0.20 g), the reactant solution was heated to reflux at 85 oC under vigorous magnetic stirring and N2 atmosphere for 8 h. The precipitation was filtered and eluted with ethanol repeatedly until the filtrate was colorless, followed by the treatment under vacuum at 80 oC for 12 h. The resultant product was code-named PoBC-x, of which x represents above-mentioned reaction molar ratio. 2.2. Characterization Liquid chromatography-mass spectrometry (LC-MS) was performed over Agilent 1290/6495 with SB-C18 column. A Nicolet Nexus 470 spectrometer with KBr wafer was used to record the Fourier transform infrared spectroscopy (FTIR) spectra. The 13C nuclear magnetic resonance (13C-NMR) tests were carried out on an Agilent-NMR-vnmrs600 spectrometer. Thermogravimetry (TG) and differential TG (DTG) analyses were performed with a TG209F1 apparatus. X-ray powder diffraction (XRPD) patterns were recorded using a Bruker D8 Advance diffractometer with Cu Kα at 40 kV and 40 mA. Images of high resolution transmission electron microscopy (HRTEM) were gotten in a JEM-2010 UHR electron microscope operated at 200 kV. The scanning electron microscope (SEM) images were observed on a Hitachi S-4800 and FEI Tecnai G2 T20 electron microscope. After degassing the samples at 150 °C for 4 h, N2 adsorption-desorption isotherms at 77 K were measured with a Micromeritics ASAP 2020 analyzer. The specific surface areas were calculated with the BrunauerEmmett-Teller (BET) model at the P/P0 range of 0.05–0.15. The total pore volumes were derived from the uptake at a relative pressure of 0.95. The pore size distributions were calculated based on the adsorption isotherms with the non-local density functional theory (NLDFT). 2.3. Adsorption tests Static adsorption experiments of CO2 (99.999%) and N2 (99.999%) over the adsorbents at 0 °C and 25 °C were measured by ASAP 2020 analyzer, respectively. The free space was determined using He (99.999%), with the assumption that He was not adsorbed at the temperatures investigated. Based on the static adsorption experiments of CO2 and N2, Dual-Langmuir model was employed to fit the adsorption isotherms for evaluating the adsorption selectivity of CO2 over the PoBCs. The ideal adsorption solution theory (IAST) was then used for evaluating the adsorption selectivity of binary gas separation. In order to study the nature of the interaction between CO2 and the PoBCs, the isosteric heat of adsorption (Qst) was calculated with virial equation. More mathematical details and equations for Dual-Langmuir model, IAST, and Qst are given in the Supplementary Information, respectively. 4

The tests of cyclic stability were conducted on the Micromeritics ASAP 2020 analyzer. The adsorbents were evacuated at 80 oC for 100 min to be regenerated, and then the adsorbents were saturated with CO2 up to 1 bar at 0 °C.

2.4. Computational methods In view of the sophisticated potential energy surfaces of big molecules, quasi-global minimum was probed with iterative annealing based on consistent-valence force field implemented with the LAMMPS code before first-principle calculation. With the canonical ensemble and a timestep of 1.0 fs, the molecule was subjected to 100 round trips between 300 K and 900 K by 60,000 timesteps for each cycle. The first-principle calculation was performed by Becke’s 88 exchange and Lee-Yang-Parr correlation functionals, which had been implemented in the Gaussian16 package. Pople’s 6-31G(d) basis sets were applied to all the atoms. Fully relaxed geometry optimizations were performed with Grimme’s dispersion correction of D3BJ version. Self-consistent field procedures of full accuracy were performed with tight convergence, without any orbital symmetry constraints.

3. Results and discussion It is well known that the F-C reaction between –CH2Cl groups and aromatic rings to fabricate the polymer chains is reliable [40−42], and the reaction mechanism in this study is give in Scheme 1 (a), which must form a rotatable and flexible −CH2− between two aromatic rings. The crowded configurations of octa-phenyl groups attached on the silsesquioxane cores, the big BCMB molecules, and the reactive activities of the phenyl groups codetermine that each BCMB molecule will primarily and solely link onto the para-position of the phenyl groups. In Fig. S1, the theoretical configuration of an octaphenyl-POSS reacting with numbers of BCMB molecules on the same side of the POSS core is shown on the basis of the first-principle simulation results. Intramolecular force plays a crucial role in the turnover direction of the biphenyl moieties, which must draw close to the octaphenyl-POSS axis. Besides, the biphenyl moieties stack on top of each other due to π-π stacking, with increasing the number of substituents. The confined biphenyl moieties gather to another octaphenyl-POSS fulcrum and link therein. Thus, the fabrication of a long polymer chain with supramolecular interaction is feasible. As seen in Scheme 1 (b), two octaphenyl-POSS fulcrums can theoretically be linked through 5

numbers of biphenyl moieties from single to quartet. The first-principle calculated configurations and formation energies are given in Table S1, in which all the formation energies are negative, indicating the thermodynamic feasibility to fabricate such configurations. The two octaphenyl-POSS fulcrums linked with single biphenyl group are impossible to form a rigid structure because of the rotatable and flexible Ph−CH2−Ph linkages generated by F-C reaction, which cannot resist the strong intramolecular forces. With the flexible linkage, the two octaphenyl-POSS fulcrums draw close to each other spontaneously, and the interspace or potential porosity thus disappears completely. In fact, the simulation result mimics and explains some failed cases in other reports, in which some flexible linkages were constructed, but the authors expected to get porosities of POPs. With double biphenyl groups anchored either adjacently or diagonally, the structure spontaneously shrinking is still inevitable. Rigid structure comes into being until triplet biphenyl groups anchoring onto the two octaphenyl-POSS fulcrums, during which the multi-helix structures of the biphenyl groups can be selfassembled due to π-π stacking and van der Waals’ force. Further first-principle simulation proves that such supramolecular interaction is robust enough, so the polymer chains can keep rigid in the course of chain growth (Fig. S2). Therefore, compared with the polymer product based on the flexible and contractive linkages, a better porosity and larger specific surface area can be expected for that based on rigid chains with multi-helix structures when the rigid polymer structures interlaced. In order to investigate the evolution process of the supramolecular polymer chains, with stepwise change of the reactant molar ratio between octaphenyl-POSS and BCMB, a group of PoBC-x were fabricated. In view of the practical reversible equilibrium, the x values were discretely selected from 1 until 16 of which the corresponding conversion of octaphenyl-POSS was little increased (Table S2). The conversion of octaphenyl-POSS can be calculated by measuring the residual concentrations of reactants by LC-MS. Moreover, in this study, PoBC-1, -2, -4, -6, -12, and -16 were chosen to exhibit the macroscopic effects of the evolution process of the rigid supramolecular structures. As shown in Fig. 1 (a), the successful fabrication of the polymer products is proved by the FTIR patterns. The strongest bands located at 1138 cm−1 are caused by the vibration of Si−O−Si bonds in the octaphenylPOSS moieties. The small peaks located at 2970 and 2850 cm−1 can be attributed to the antisymmetric and symmetrical stretching vibrations of −CH2− linked between the phenyl of octaphenyl-POSS and biphenyl group of BCMB, correspondingly. These peaks are gradually enhanced with the risen x values, indicating that an increasing number of BCMB has reacted with octaphenyl-POSS. The deepened 6

degree of reaction with the risen x values is also proved at the wavenumber range from 1400 to 1700 cm−1, at which the gradually enhanced FTIR bands are due to the overtone absorption caused by the C−H and C=C bending vibration of the substituted aromatic rings, and at the fingerprint region of 810 cm−1 indicating the para-substitution reaction between the two monomers. Note that the peak attributed to the vibration of Ar−H moves from 3050 to 3020 cm−1 with the increased x values, which might be caused by the enhanced steric effect owing to the stacked and screwed biphenyl groups linked with the octaphenyl-POSS fulcrums, supporting the above-mentioned theoretical predictions. In addition, the adsorption bands of O−H stretching vibration located at 3430 cm−1 can be observed, and gradually increase with the x values. This may be ascribed to the minimal residual ethanol in the course of elution for materials synthesis, and the removal of ethanol is increasingly difficult with the risen x values, implying the incremental adsorbability. The successful fabrication of the polymer products, as well as the presence of supramolecular interaction of the polymer chains, is further verified by the 13C-NMR profiles. As seen in Fig. S3, there are not obvious characteristic NMR peaks that indicate the 4-C atoms of phenyl or biphenyl groups bonded with −CH2− emerging on the profiles of PoBC-1 and PoBC-2, while the characteristic peaks gradually turn to be stronger with the x values increased, and the characteristic peaks for PoBC-16 are strong enough, located at 139.3 and 200.5 ppm, respectively. Meanwhile, the characteristic NMR peak (ca. 38 ppm) indicating the C atom of −CH2− bonded with octaphenyl-POSS becomes apparent with the deepened polymerization as well. These facts confirm the successful polymerization between octaphenyl-POSS and BCMB as expected, which is well consistent with the results of FTIR. It is well known that some chemical environmental factors can markedly shift the chemical displacements, such as conjugative, steric, and γ-gauche effects [43,44]. With x values increased or polymerization deepened, the 13C-NMR peaks indicating the C atoms of phenyl or biphenyl groups uniformly shift to low field apparently, for example, the peaks located at 136.6 and 183.4 ppm for PoBC-4 move to 139.3 and 190.5 ppm for PoBC-16, correspondingly, owing to the electronic density dispersed by the interlaced π orbitals of the stacked and screwed biphenyl groups, which is the direct experimental evidence for the supramolecular structures predicted in the first-principle simulation. In Fig. S4, the XRPD patterns demonstrate that all the polymer samples are amorphous. This is because although a short-range ordered supramolecular structure can self-assemble with the molar ratio of BCMB risen, the molecular weight distribution of the PoBC eventually generated should be 7

wide, and the particles stacking was random. However, a morphologic saltation of the PoBCs can be seen in the SEM images (Fig. S5). The fragments of polymers from PoBC-1 to PoBC-12 look like being shaly and dense, while gritty and loose structures are dominant on PoBC-16. Note that the sizes of the PoBCs fragments shown in the SEM images are generally smaller than 5 μm, which might be too small to be employed in fixed bed operations, and should be difficult to be fluidized because of cohesive forces. Therefore, pelletization for the adsorbents is needed in the industrial fixed bed at the expense of the specific surface areas/porosities and the diffusion rate of gas molecules in the particles, and externally-assisted techniques can be considered in the fluidized bed operations, such as mechanical vibration [45] and electric field [46]. The HRTEM images provide more details about the morphologic differences (Fig. S6). Compared with the dense structures of the PoBC counterparts, PoBC-16 presents a spidery appearance (Fig. 2), indicating a larger specific surface area and better porosity. Wormlike microstructures can be observed in all HRTEM snapshots under the visual field of 10 nm, which is a typical character of amorphous polymers, but the density of bright spot of PoBC-16 is much higher than those of others, which indicates that there are more micropores of PoBC-16 than other PoBC counterparts. However, it is interesting that the loose or porous structure of PoBC-16 possesses better thermostability than any other PoBC counterparts. As Figs. 1 (b) and S7 exhibit, the TG and DTG profiles show that the rapid weight losses of PoBC-12 and PoBC-16 take place only at the temperature range from 550 oC to 600 oC,

and their residual weight percentages are higher than 77%. In contrast, rapid weight losses come

with the entire temperature rise period for other PoBCs, and the residual weight percentages are closely related to the x values. The thermal stability is an important advantage allowing for the CO2 separation at high temperature, for example, at the circumstance of flue gases. Besides, the technology of temperature swing adsorption (TSA) requires the adsorbents that can be regenerated at elevated temperature. The PoBCs can remain stable before 300 oC, which is enough to bear the operating conditions of majority of TSA technologies. N2 adsorption-desorption isotherms confirm the porosities of PoBCs, and the calculated textural parameters are given in Table S2. As shown in Fig. 1 (c), all PoBCs are mainly featured type-I isotherms with rapid N2 adsorption at the low P/P0 range, indicating the presence of micropores, and exhibit hysteresis loops due to the existence of mesoporous structures caused by particle accumulation, except that of PoBC-1. The BET specific surface areas of PoBCs rise with the x values, and that of 8

PoBC-16 reaches the highest BET specific surface area of 1170 m2 g−1, in contrast to other PoBCs (82, 704, 843, 1000, and 1089 m2 g−1 for -1, -2, -4, -6, and -12, respectively). Besides the specific surface area, the porosity is also an important criterion in many application fields, such as the gas storage and separation. It is obvious that the pore structures of PoBCs continue to develop with the x values, and especially PoBC-16, which exhibits a sudden promotion of the micropore volume (Fig. 1, d), which reaches 0.46 cm3 g−1, in contrast to other PoBC counterparts (0.02, 0.22, 0.30, 0.32, and 0.34 cm3 g−1 for -1, -2, -4, -6, and -12, respectively). These experimental facts support the first-principle predictions very well, and prove that it is effective to get differentiated textural characteristics of the polymers through stepwise changing the reactant molar ratio, and some important properties, such as the thermostability and microporosity, must develop with the relative content of the multi-helix supramolecular structures in the polymer chains, which fully formed only at high x values. The developed specific surface areas and porosities endow the PoBCs with great potential of gas storage and separation, for example, the CO2 capture with adsorbents. In addition, the experiments to check the degree of the BCMB self-polymerization were performed, while it was found that under the same polymerization conditions of PoBCs, the conversion (< 8%) of the BCMB, as well as the BET specific surface area (32 m2 g−1) and the pore volume (0.02 cm3 g−1) of the precipitate, was ultra-low in contrast to those of PoBCs. Besides, both the yield and the textural property of PoBC did not change much anymore when the x value was larger than 16, which means that the octaphenyl-POSS can be fully substituted by BCMB in accordance with the theoretical maximal molar ratio (1:4) only after the practical rate of charge reaches 1:16, in view of the reaction equilibrium. These facts confirm that the effects of the BCMB self-polymerization ought to be negligible herein. In Fig. 3, the CO2 adsorption isotherms of the PoBCs show the stepwise increased CO2 capture capacities owing to the stepwise developed porosities as well as the BET specific surface areas, which must be crucial for the CO2 adsorption because, as far as we know, there are not any theoretical CO2 chemisorptive sites on them. PoBC-16 exhibits better CO2 capture capacity of 67.2 cm3 g−1 at 0 oC and 1 bar (31.4 cm3 g−1 at 25 oC and 1 bar) than any other PoBC counterparts, because it possesses the large enough specific surface area and fully developed pore structures to which the presence of micropores effectively contribute. The large specific surface area provides sufficient sites to adsorb CO2 molecules, while the fully developed porosity makes it possible to stably capture and then restrict CO2 molecules therein. As the small molecules, the presence of micropores, especially the ultra9

micropores of which pore diameters are smaller than 1 nm, play important roles in the CO2 adsorption [42]. The PoBCs are abundant in ultra-micropores, especially PoBC-16, which explains the high CO2 capture capacities. In fact, PoBC-16 is also competitive with many reported similar POPs in terms of both textural properties and CO2 capture capacity. As seen in Table S3, PoBC-16 not only shows advantages in contrast to other POPs with flexible covalent bonds, for example, the POP code-named CP of which BET specific surface area was only 67 m2 g−1 and CO2 adsorption capacity was only 21.3 cm3 g−1 at 0 oC

and 1 bar [47]; but also performs better than some POPs based on rigid covalent linkages, for

instance, that code-named APOP with the BET specific surface area of 490 m2 g−1 and CO2 adsorption capacity of 43.6 cm3 g−1 at 0 oC and 1 bar [48], which further confirms the feasibility of the fabrication mechanism we proposed. As shown in Fig. S8, the flat Qst profiles indicate the highly and evenly dispersive adsorption sites on the PoBCs. The initial Qst values for all the PoBCs are lower than 40 kJ mol−1, which is the threshold to identify physisorption or chemisorption. This fact confirms that CO2 is physically and uniformly adsorbed onto the PoBCs, and there are not any special CO2-affiliative sites grafted on the PoBCs. However, the CO2 adsorption selectivities of the PoBCs towards N2 are still satisfactory, particularly that of PoBC-16, with fitting the adsorption isotherms by Dual-Langmuir model. The Dual-Langmuir model has been proved to be more suitable to accurately describe or fit the experimental adsorption isotherms than some other adsorption models, for example, the Langmuir model that roughly describes the monolayer adsorption onto the homogeneous surface, because the Dual-Langmuir model includes two types of potential adsorption sites, or in other words, involves two possible statuses of adsorption, effectively reducing the deviation of homogeneous models [49]. As Table S4 depicted, all the fitting correlation coefficients (R2) are larger than 0.999, which shows the accuracy of the Dual-Langmuir model to describe the experimental data. The Langmuir equilibrium constants (kI) of CO2 over the PoBCs are generally much larger than that of N2 at both 0 °C and 25 °C, giving good discriminability between CO2 and N2. For example, the kI value of CO2 over PoBC-16 at 0 °C is 5.01 bar−1, and that of N2 over it is only 0.24 bar−1, indicating the much stronger affinity of CO2 than that of N2 over PoBCs. Moreover, Fig. S9 clearly exhibits the good selectivity of CO2 over PoBCs at the entire range of CO2 molar fraction based on the IAST. It is well consistent with the Dual-Langmuir fitting. However, as shown in Table S3, the CO2/N2 selectivity of PoBC-16 is good, but cannot compete with the POPs that 10

involves CO2 chemisorptive sites such as amines or azacyclo-species. It can be expected that, once the CO2 chemisorptive sites are involved in the future study, the CO2/N2 selectivity of the PoBC-16 derivative must be dramatically improved. Besides, regenerability is another criterion to evaluate adsorbents, related to the stability of the adsorbents. Herein, PoBC-16 performs very well because the attenuation of CO2 capture capacity is not observed even after adsorption-desorption cycles six times (Fig. S10). In view of the fact that heating was proved to be very efficient for desorbing CO2 and the ideal thermal stability of the PoBCs, it can be expected that the TSA process might be given priority in practical application of the absorbent regeneration [50−53]. The above-mentioned high CO2 capture capacity, good CO2 selectivity and regenerability endow PoBC-16 with promising application in terms of carbon capture and separation, which can be attributed to the well-developed porosity and sufficient thermalstability due to the dominated rigid supramolecular polymer chains with multi-helix structures.

4. Conclusions In summary, the supramolecular interaction is discovered to be effective to maintain the rigid polymer chains in this study, in addition to the general idea limited to rigid covalent bonds. Porous organic polymers are successfully fabricated with the monomers of octaphenylsilsesquioxane (octaphenyl-POSS) and 4,4′-bis(chloromethyl)-1,1′-biphenyl (BCMB), of which Friedel-Crafts reaction only generates rotatable and flexible Ph−CH2−Ph bonds that cannot theoretically resist the shrinkage of the polymer chains caused by intramolecular forces. However, both first-principle predictions and experimental facts indicate that the multi-helix supramolecular structures of the biphenyl moieties of BCMB self-assembled due to π-π stacking and van der Waals’ force are robust enough to maintain the rigid polymer chains, provided that octaphenyl-POSS can fully react with BCMB. The resultant properties of the polymer products, such as BET specific surface area, porosity, and thermalstability, can be largely improved. Even without being doped any CO2 chemisorptive sites, the polymers with large BET specific surface area and fully developed porosity perform on CO2 capture in terms of capture capacity, selectivity, and regenerability very well. This study may open up a route for the fabrication of porous polymers by means of supramolecular interactions, and extend the list of feasible reaction types for the fabrication of organic porous materials.

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Acknowledgements We acknowledge financial support of this work by the National Natural Science Foundation of China (21808105, 21676138, 21722606, and 21576137) and the Natural Science Foundation of Jiangsu Province (BK20180709). We are also grateful to the High Performance Computing Center of Nanjing Tech University for supporting the computational resources.

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Scheme 1. The polycondensation scheme between octaphenyl-POSS and BCMB. (a) The reaction mechanism, in which the four linkages are in parallel; (b) First-principle calculated patterns of polycondensation between octaphenyl-POSS and different number of BCMB, in which [A], [B1], [B2], [C], and [D] represent two octaphenyl-POSS fulcrums with one BCMB, with two diagonal BCMBs, with two adjacent BCMBs, with three BCMBs, and with four BCMBs, respectively.

17

1700 1400 1138

3020

(a)

810

100

(b)

PoBC-16 Transmittance (a.u.)

90 PoBC-12 PoBC-6

80

PoBC-16 PoBC-12 PoBC-6 PoBC-4 PoBC-2 PoBC-1

PoBC-4 PoBC-2 PoBC-1

70 60

Weight percentage (%)

3430

2970 2850 3050

50 0 100 200 300 400 500 600 700 800 Temperature (oC) (d)

PoBC-16 PoBC-6 PoBC-2

PoBC-12 PoBC-4 PoBC-1 dVp (a.u.)

Vads (cm3/g, STP)

4000 3500 3000 2500 2000 1500 1000 Wavenumber (cm-1) 750 PoBC-16 PoBC-12 (c) PoBC-6 PoBC-4 PoBC-2 PoBC-1 600 450 300 150 0

0

0.2

0.4

0.6 P/P0

0.8

1.0 5.0 Pore diameter (nm)

1.0

10

Fig. 1. (a) FTIR patterns, (b) TG profiles, (c) N2 adsorption-desorption isotherms, and (d) pore size distributions of all PoBCs.

18

Fig. 2. HRTEM images of PoBC-16.

19

CO2@0oC

80

CO2@25oC

N2@0oC

PoBC-16

N2@25oC

PoBC-12

60 60 40

40

20

20

Adsorbance (cm3/g)

0 60

0 50

PoBC-6

PoBC-4

40 40

30 20

20

10 0

0 PoBC-2

40 30

30

20

20

10

10

0

0

0.2

0.4 0.6 0.8 Pressure (bar)

PoBC-1

40

1.0

0

0

0.2

0.4 0.6 0.8 Pressure (bar)

Fig. 3. CO2 and N2 adsorption isotherms of all PoBCs at 0 oC and 25 oC.

20

1.0

21

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

22

GRAPHICAL ABSTRACT

23

HIGHLIGHTS 

Constraints of rigid covalent bonds for the POPs fabrication are broken through.



Rigid polymer structures are fabricated by means of supramolecular interaction.



The supramolecular structures of POPs are based on flexible covalent linkages.



POPs are endowed with competitive textural properties and CO2 capture abilities.

24