Ordered mesoporous carbon encapsulated linear poly(ionic liquid)s enabling synergy effect of surface groups and ionic moieties for CO2 fixation under mild conditions

Carbon dioxide (CO2) fixation into value-added chemicals has attracted growing attention and one promising atom-efficient pathway is via the cycloaddition with three member-ring compounds like epoxides. Herein, we demonstrated that encapsulation of linear poly(ionic liquid)s (PILs) on ordered mesoporous carbon materials provides a facile and feasible approach towards environmental-friendly heterogeneous catalysts with high performance in CO2 cycloaddition with epoxides under mild conditions. A series of novel linear phenolic hydroxyl group functional imidazolium-based PILs synthesized from hydroxymethylation reaction between 4-(imidazol-1-yl)phenol-1-butyl-imidazolium iodide and formaldehyde was loaded on ordered mesoporous carbon FDU-15–600 derived from mesoporous phenolic resin. By virtue of controlling the initial polymerization temperature, the molecular weight of PILs was facilely modulated, reaching strong host–guest interaction during the PIL immobilization. Highly stable immobilized PIL species with spatial satisfaction of ionic moieties and surface groups were thus realized to enable a synergic CO2 conversion via cycloaddition with epoxides. The optimal catalyst exhibited high yield and stable recyclability by using atmospheric CO2 under metal-additive-solvent-free conditions and the activity surprisingly exceeded the corresponding homogeneous parent IL and PIL. Excellent substrate compatibility was found by extending the transformation of more than ten epoxides including the inert ones such as disubstituted cyclohexene oxide. The significantly enhanced activity is attributed to the synergistic effect of the surface hydrogen groups and ionic moieties to accelerate the rate-determining ring-opening process.

Cycloaddition of CO 2 with epoxides usually undergoes ring opening, CO 2 insertion, and ring closure [37][38][39]. The first step is normally recognized as the rate-determining step and is triggered by an attack by a nucleophilic agent. The epoxide activation can be greatly promoted by Lewis acids (metal sites) [24-26, 28, 29] or hydrogen bond donors (HBDs) such as the hydroxyl group [5]. The anions of ILs were the efficient nucleophilic agents and the ILs' structure associating with the corresponding catalytic function can be facilely modulated at a molecular level by integrating versatile functional groups to promote CO 2 affinity and epoxide activation. Therefore, ILs and their derivatives have been extensively investigated as a large family of environmental-friendly metal-free catalysts for CO 2 cycloaddition [21,40]. For example, phenolic hydroxyl group functional IL showed excellent activity in the CO 2 cycloaddition and achieved a high yield of 96% in the conversion of epichlorohydrin (ECH) at room temperature and atmospheric conditions [33]. To facilitate catalyst recycling, great efforts have been made to develop IL-related heterogeneous catalysts mainly comprising two following strategies [41][42][43]. One is the synthesis of ionic moieties containing polymers through the polymerization of IL monomer or with other linkers through various polymerization pathways such as free radical polymerization, hypercrosslinkage, and condensation [44][45][46][47]. For example, free radial self-polymerization of 2-(dimethylamino) ethyl methacrylate derived IL monomers afforded a series of PILs that can effectively catalyze the CO 2 cycloaddition with multiple epoxides with the yields of 70-98% at 110 °C and 20 bar [46]. Bifunctional ionic polymers were directly synthesized through co-polymerization of tris (4-vinylphenyl) phosphine and functional dibromides and the resulting carboxyl-containing one exhibited the yield above 99% at 140 °C in the transformation of atmospheric CO 2 via cycloaddition with ECH [47]. Alternatively, anchoring ILs and PILs on the porous materials provided a promising approach toward IL-derived solid materials that can inherit the abundant porosity of parent supports [8,45]. More importantly, the combination of surface groups in the supports and immobilized moieties favors reaching the highly effective synergistic conversion of CO 2 via cycloaddition with epoxide. Particularly, encapsulation of PILs on the ordered porous materials benefits to balance the stability and activity thanks to the confinement of the ordered porous channels and the spatial satisfaction of different reactive sites [48,49]. This strategy is feasible and versatile for IL solidification because it does not rely on the designation of polymerizable IL monomer for the pore formation during the polymerization process. Many ordered porous materials with the large surface area such as zeolites, MOFs, COFs, and mesoporous silica have been explored to immobilize ILs and PILs for the preparation of heterogeneous IL-derived catalysts [16,27,28,50]. Carbon materials with the features such as large surface area, tunable porosity, and abundant surface functional groups have been explored as the supports and catalysts in the CO 2 cycloadditions [51][52][53][54][55][56][57]. For instance, as a metal-free heterogeneous catalyst, graphene oxide (GOs) achieved 90.3% conversion and 98.6% selectivity in converting styrene oxide to styrene cyclic carbonate at 140 °C and 1 bar by using N,N-Dimethylformamide (DMF) as the solvent [55]. A PIL/graphene composite was prepared via an in-situ surface construction strategy and gave a yield of 99% in the coupling of 15 bar CO 2 with propylene oxide at 100 °C [56]. Carbon supported single Zn atom catalyst was constructed from the straightforward carbonization of carbon supported phenanthroline-ligated Zn(OAc) 2 complex and exhibited the yields of 93-98% in the conversion of seven epoxides via coupling with 5 bar CO 2 at 100 °C in the presence of tetrabutyl ammonium bromide (TBAB) as an additive [57]. The abundant surface oxygen groups such as hydrogen groups on the carbon materials can serve as effective HBD to promote the epoxide activation to accelerate the corresponding transformation into the target cyclic carbonates [3,41]. In addition to the disordered carbon materials mentioned above, ordered porous carbon materials such as CMK, FDU, and CGM-Cs (carbon-based Cornell Graded Materials) series [58][59][60]. Nonetheless, the encapsulation of PILs into the ordered porous carbon materials is still to be explored for the preparation of metal-free heterogeneous catalysts for CO 2 -epoxide coupling.
Herein, we triggered the application of ordered mesoporous carbon materials in the encapsulation of linear PILs and constructed a family of facilely adjustable PIL-carbon hybrid heterogeneous catalysts for CO 2 fixation via cycloaddition with epoxide. The novel linear PILs were synthesized via hydroxymethylation reaction between phenolic hydroxyl functional imidazoliumbased IL with formaldehyde solution under alkaline conditions [61], with molecular weight modulation via controlling the polymerization temperature. The target ordered mesoporous carbon material FDU-15-600 was prepared through the carbonization of mesoporous phenolic resin that was synthesized in a soft-template route [62]. The linear PILs were confined in the mesochannels of FDU-15-600, reaching strong host-guest interaction that depended on the initial molecular weight of PILs. Herein, an effective synergy between the surface hydrogen groups and the ionic moieties with the nucleophilic agent was achieved and thus resulted in high efficiency in the cycloaddition of atmospheric CO 2 with epoxides under the mild condition without any additive and solvent. High yield, stable recycling performance and broad scope were observed in the catalysis evaluation. The activity of the champion heterogeneous PIL of the present work even outperformed its parent homogeneous IL and PIL, showing the great potential of this strategy.

Materials
The commercial chemicals and reagents were used as received without extra purification unless otherwise stated. 4-(Imidazol-1-yl) phenol (97%) was provided by Meyer (Shanghai) Chemical Technology Co. LTD. Iodobutane was provided by Xilong Science Co. LTD. Triblock copolymer (PEO-PPO-PEO, pluronic F127) was purchased from Sigma-Aldrich Co. LTD. Phenol was purchased from Shanghai Lingfeng Chemical Reagent Co. LTD. Activated carbon (AC) was purchased from Shanghai Macklin Biochemical Co. LTD.

Characterization
1 H nuclear magnetic resonance (NMR) spectra were collected on a Bruker DPX 500 spectrometer. C, H, and N combustion chemical analyses were performed on the Vario EL cube. Brunauer-Emmett-Teller (BET) surface areas and pore volumes were measured at the temperature of liquid nitrogen (-196 °C) by using a BELSORP-MINI analyzer. Prior to the measurement, all the samples were degassed at 150 °C for 3 h to a vacuum of 10 −3 Torr. Scanning electron microscopy (SEM) images were acquired with a Hitachi S-4800 scanning electron microscope (10 kV). Transmission electron microscope (TEM) images were recorded on a JEOL JEM-2010 (200 kV) instrument. Small-angle X-ray diffraction (XRD) patterns were collected by using a Smart Lab X-ray diffractometer from Rigaku. Solid-state 13 C cross-polarization (CP)/ magic-angle-spinning (MAS) NMR ( 13 C MAS NMR) analysis was carried out on a Bruker AVANCE-III spectrometer. X-ray photoelectron spectroscopy (XPS) spectra were collected on a PHI 5000 Versa Probe X-ray photoelectron spectrometer equipped with Al Kα radiation (1486.6 eV). Thermal gravimetric analysis (TGA) experiments were performed on an STA409 instrument under N 2 flow. The molecular weight of linear PILs was analyzed by gel permeation chromatography (GPC, 1515, Waters, USA). Raman spectra were collected on a Horiba HR 800 spectrometer with a Spectra-Physics 2018 Argon/Krypton Ion Laser system (excitation line: 514 nm).

Synthesis of soluble phenolic resol
Soluble phenolic resol was prepared under basic conditions according to a reported procedure [61]. Phenol (4.0 g, 42.5 mmol) was melted at 50 °C and then mixed with a sodium hydroxide solution (20 wt%, 0.85 g, 4.25 mmol) under stirring, followed by dropwise addition of formalin (37 wt%, 6.9 g, 85 mmol). The mixture was stirred at 75 °C for 1 h and then cooled to room temperature. The pH value of the solution above was adjusted to be 7.0 by using an aqueous hydrochloric acid solution with a concentration of 0.6 M. Subsequently, water was removed by rotary evaporation under vacuum and the resulting soluble phenolic resol was re-dissolved in ethanol for further use.

Synthesis of FDU-15-600
Triblock copolymer (PEO-PPO-PEO, pluronic F127; 0.6 g) was dissolved in ethanol (10.0 g) at 55 °C and then mixed with 3.0 g of ethanol solution containing soluble phenolic resol precursors (20 wt%). The mixture was stirred for 30 min to give a homogeneous solution, which was transferred to a 150 mm culture dish. Ethanol was evaporated at room temperature for 5-8 h. The resulting transparent membrane was then heated in an oven at 100 °C for 24 h. The obtained solids were calcined at 350 °C under nitrogen for 2 h with a heating rate of 1 °C min −1 and then calcined at 600 °C for 3 h to afford FDU-15-600.

Synthesis of linear PILs
In a typical synthesis, [p-ArOH-IM] I (0.162 g) and NaOH solution (20 wt%, 10 μL) were mixed in a tube, followed by the addition of formaldehyde solution (37 wt%, 100 μL). The mixture was sonicated for 30 min and then heated at 90 °C for 60 min. After that, the solution was neutralized by an aqueous HCl solution (0.6 M) and washed subsequently with ethanol and ethyl acetate to remove the residue [p-ArOH-IM]I and NaCl. The gel-like product was dried at 90 °C under vacuum to offer a linear PIL PIL-90. Varying the polymerization temperature to room temperature (RT), 55 °C, and 110 °C afforded PIL-RT, PIL-55, and PIL-110, respectively.

Preparation of FDU-15-600 supported linear PILs
Linear PIL was immobilized on FDU-15-600 by a wetimpregnation method. Typically, PIL-90 (0.089 g) was dissolved in ethanol (5 mL) to give a homogeneous solution, which was mixed with FDU-15-600 (0.504 g, preactivated at 120 °C under vacuum for 3 h) in a 100 mL beaker. The mixture was stirred at room temperature for 12 h, with the auto-evaporation of ethanol, and then dried in a vacuum oven at 100 °C for 12 h to give the product 15%PIL-90@FDU-15-600. By varying the PIL precursor and the loading amount, various other hybrids were prepared.

15%S-PIL-90@FDU-15-600
Pre-activated FDU-15-600 (0.504 g), [p-ArOH-IM]I (0.086 g), and ethanol (5 mL) were mixed in a 50 mL flask and sonicated for 30 min to give the mixture A. An aqueous NaOH solution (20 wt%, 5 μL) was diluted with formalin (37 wt%, 50 μL) in a centrifuge tube and sonicated for 30 min to give the mixture B. The mixtures A and B were mixed in a 50 mL flask and stirred at 90 °C for 60 min under a nitrogen atmosphere. The resulting solid was dried in a vacuum oven at 100 °C for 12 h to give 15%S-PIL-90@ FDU-15-600.

CO 2 cycloaddition with epoxide
The cycloaddition of atmospheric CO 2 with epoxides was carried out in a Schleck tube reactor (25 mL) equipped with a CO 2 balloon. Epoxide (5 mmol) and 15%PIL-90@FDU-15-600 (50 mg, 0.4 mol%) were stirred at the desired temperature for the preset time. After the reaction, the solid was separated by centrifugation and the internal standard n-dodecane (0.5 g) was added into the liquid phase, which was further diluted with ethyl acetate. The resulting solution was qualitatively analyzed by a gas chromatography-mass spectrometer (Bruker Scion 436 GC-MS) and quantitatively analyzed by a gas chromatograph (Agilent 7890B) equipped with a flame ionization detector (FID) and a capillary column (HP-5, 30 m × 0.25 mm × 0.25 μm). The cycloaddition of highpressure CO 2 with epoxide was conducted in a stainlesssteel autoclave (25 mL) and the product was analyzed by GC and GC-MS as mentioned above.
The catalyst was isolated after the cycloaddition reaction, washed with ethyl acetate several times and dried at 90 °C for 12 h in a vacuum. Afterward, the catalyst was reused for the next catalytic run under the same reaction conditions. This procedure was repeated five times to examine the recyclability of the catalyst PIL-90@ FDU-15-600.

Structure of FDU-15-600 supported linear PIL
Scheme 1 depicts the preparation procedure of ordered mesoporous carbon confined linear PILs for CO 2 cycloaddition with epoxide. The ordered mesoporous carbon FDU-15-600 was synthesized by using F127 as the soft template and calcined at 350 °C to remove the template, with further carbonization at 600 °C. The linear PILs were synthesized via hydroxymethylation reaction, where [p-ArOH-IM]I bearing phenolic hydroxyl group and formaldehyde respectively served as the α-H resource component and a cross-linking agent [63,64]. The linear PILs were immobilized on FDU-15-600 via the wet-impregnation process. By varying the polymerization temperature from room temperature (RT) to 110 °C, a series of linear PILs were synthesized and named PIL-T, where T = RT, 55, 90, and 110, represent the polymerization temperature. As shown in Fig. S2, the color of the PIL-T series gradually become shallower with increasing polymerization temperature. CHN elemental analyses indicated that the PIL-T series had similar chemical composition to the IL precursor [p-ArOH-IM] I while slightly lower H content (Table S1 entries1-5), attributable to the α-H elimination during the polymerization [40]. The polymerization degree of PIL-T was analyzed by gel permeation chromatography (GPC) (Fig. S3). The molecular weights of PIL-T were listed in Table 1. Polymerization at RT caused a low molecular weight that was about three times that of IL precursor (  [3][4][5]. This result implies that the polymerization degree of PIL-T was facilely modulated by controlling the polymerization temperature. A family of carbon encapsulated linear PIL samples, termed X%PIL-T@FDU-15-600 (X% index the loading amount), were prepared by using PIL-T with different loading amounts, including 15%PIL-T@FDU-15-600, 10%PIL-90-FDU-15-600, and 20%PIL-90@FDU-15-600. There was no N element in FDU-15-600 and the detectable N species on X%PIL-T@FDU-15-600 samples reflected the immobilization of PIL-T (Table S1, entries 6-12). The PIL content linearly increased with the loading amount, while close to each other by fixing the loading amount. The small-angle XRD patterns of the parent FDU-15-600 and X%PIL-T@FDU-15-600 were displayed in Fig. 1a and Fig. S4a. For FDU-15-600, a strong diffraction peak was observed at the 2θ of 0.86, corresponding to the 100 plane [61]. Two weak peaks were found at the 2θ of 1.36 and 1.67, corresponding to the 110 and 200 planes [62]. The observation of these diffraction peaks revealed the formation of a well-ordered 2D hexagonal structure with a space group of p6mm [65]. The remaining of these peaks in the XRD patterns of X%PIL-T@FDU-15-600 indicated the preservation of the parent highly ordered hexagonal mesostructure after PIL-T loading. The weakening of the peak intensity came from the occupation of the guest PIL-T inside the meso-channels. The solid-state 13 C MAS NMR spectra of FDU-15-600 and the typical PIL loaded sample  Fig. 1b. Both of them exhibited similar and distinct peaks at ~ 28.1, ~ 127, and 153.4 ppm, which are respectively corresponded to the C atom of the methylene group linking the two benzene rings, the C atoms of the benzene ring adjacent the phenolic hydroxyl groups, and the other C atoms of the benzene ring [61,63]. Comparing the spectrum of 15%PIL-90@FDU-15-600 with FDU-15-600, it was found that there existed an extra shoulder band at 138 ppm, attributable to the C2 atom of the imidazole ring [63]. This phenomenon is in line with the encapsulation of imidazolium based PIL in the FDU-15-600. Notably, a slight shift of the strong peak at 126.6 ppm to 127.4 ppm, which may be caused by the coincidence of the imaged C4 and C5 carbon atoms with the benzene ring carbon atoms, resulting in the change of the chemical shift of the characteristic peak of the aromatic carbon and imidazolium ring (C4 and C5 atoms) not obvious there [63]. Nitrogen sorption isotherms of FDU-15-600 and X%PIL-T@FDU-15-600 were investigated to provide the pore information (Fig. 1c). All of them displayed the typical IV type isotherm with an apparent H1-type hysteresis loop, characteristic of classical 2D-hexagonal mesopores [61,64]. The pore size distribution curves calculated by using Barret-Joyner-Halenda (BJH) method further visualized the existence of mesoporous structure (Fig. 1d). The Brunauer-Emmett-Teller (BET) surface areas and pore volumes were summarized in Table 1. FDU-15-600 had a high surface area of 798 m 2 g −1 and a large pore volume of 0.62 cm 3 g −1 (Table 1, entry 6). Decreased surface area and pore volume were observed after loading of PIL-T, and the higher loading, the less porosity (Table 1, entries 7-12). For example, the typical sample 15%PIL-90@FDU-15-600 has a surface area of 283 m 2 g −1 and a pore volume of 0.29 cm 3 g −1 (Table 1, entry 9). These results indicate that PIL-T occupies inside the mesopores.
X-ray photoelectron spectroscopy (XPS) analyses were performed to discern the interactions between the guests' PIL-X and the supports in 15%PIL-X@FDU-15-600 (X = RT, 55, 90, and 110). For comparison, FDU-15-600, [p-ArOH-IM]I, and 15%IL@FDU-15-600 were measured in parallel. The survey scan XPS spectrum of FDU-15-600 showed the signals at 284 and 532 eV for C1 and O1s species, respectively (Fig. S5a) [66]. Additional signals at 402 eV and 618 eV respectively for N1s and I3d species (Fig. S5a) were observed in the IL or PIL containing samples, reflecting the loading of these guests [67,68]. The high-resolution N1s and I3d XPS (Figs. 2a and b) spectra indicated that [p-ArOH-IM]I exhibited one signal at 402.5 eV for N1s (N I species) and a set of double peaks at Scheme 1 Schematical illustration of the preparation of ordered mesoporous carbon (FDU-15-600) encapsulated linear poly(ionic liquid) for CO 2 cycloaddition with epoxides 630.8 eV for I3d(3/2) and 619.3 eV for I3d(5/2) (I I species) [33,69]. Loading [p-ArOH-IM]I on FDU-15-600 caused the emergence of one small N1s signal at 400.0 eV and a set of double I3d signals at 632.7/620.6 eV in the N1s and I3d XPS spectra of 15%IL@FDU-15-600, respectively. These singles came from the formation of less positively charged N species (N II ) and less negatively charged I species (I II ), attributable to the strong host-guest interaction that varied the cation-anion interaction in the immobilized [p-ArOH-IM]I on FDU-15-600. A similar phenomenon was observed in the N1s and I3d XPS spectra of 15%PIL-X@FDU-15-600. The proportion of different N and I species were calculated from their peak area and summarized in Table S2 and Table S3. More N II and I II species formed in 15%PIL-X@FDU-15-600 relative to 15%IL@FDU-15-600. With increasing polymerization temperature in the preparation of PIL precursor, the content of I II species varied slightly, while the content of N II species firstly increased and then decreased, reaching the maximum value at T = 90 for 15%PIL-90@FDU-15-600. This result indicated that modulating the polymerization temperature can facilely adjust the interaction between PIL and FDU-15-600, reaching a strong interaction at moderate polymerization temperature. The reason can be assigned that linear PIL with moderate molecular weight was synthesized under this condition, providing more binding sites to strengthen the host-guest interaction while avoiding the potential agglomeration by using the PIL precursor with excessive molecular weight. Besides, the O1s XPS spectra of all these samples were deconvoluted into two signals at 532 eV and 533 eV, respectively for C = O and C-OH species, proving the existence of hydroxyl species (Fig. S5b).
Additional characterizations were performed by taking the typical sample 15%PIL-90@FDU-15-600 as an example and the parent FDU-15-600 was tested in parallel to demonstrate the variation before and after immobilization of linear PIL. The morphology of FDU-15-600 and 15%PIL-90@FDU-15-600 were monitored by scanning electron microscopy (SEM) images ( Fig. 3a and b). FDU-15-600 was composed of irregular particles on the micrometer scale, while 15%PIL-90@FDU-15-600 shows the highly ordered mesoporous structure (Fig. 3c) [62,63]. The corresponding elemental mapping images displayed the relatively homogeneous dispersion of C, N, O, and I elements, representative of the highly dispersive ionic moieties (Fig. 3d-g).

CO 2 fixation through cycloaddition with epoxide
The catalytic performance of the above [p-ArOH-IM]I, PIL-T, and X%PIL-T@FDU-15-600 samples were evaluated in the CO 2 cycloaddition with epoxides to produce cyclic carbonates ( Table 2). The investigation started with CO 2 coupling with styrene oxide (SO) under a relatively mild condition of 110 °C and atmospheric pressure without any additive or solvent. For comparison, various control catalysts were tested in parallel, including the parent support FDU-15-600, IL precursor [p-ArOH-IM] I, and PIL-T series. FDU-15-600 was inactive in the reaction, giving a low yield of 2.6% (Table 2, entry 1). Feeding 0.4 mol% [p-ArOH-IM]I relative to SO led to a moderate yield of 80.0% and a turnover number (TON) of 200 ( Table 2, entry 2). In parallel, the catalytic performance of PIL-T series and other solid catalysts below unless otherwise mentioned were tested by using the same dosage of ionic moieties to [p-ArOH-IM]I. The yield was 61.7% over PIL-RT and continuously increased along with the elevated polymerization temperature ( Table 2, entry 3). Notably, the yield of PIL-90 (71.7%) was close to that of PIL-110 (72.6%), suggesting that excessive polymerization temperature caused only slight activity variation. Despite this, loading PIL-T on FDU-15-600 presented a different picture, with a maximum yield of 94.5% over 15%PIL-90@FDU-15-600. The influence of the reaction temperature and time was presented in Fig. S9, demonstrating a rapid increase in the yield with the temperature or time. Compared with previous encapsulated PILs catalysts (Table 3) [3,27,71], it is the first time to reach effective CO 2 cycloaddition with epoxides under atmospheric conditions over an encapsulated PIL catalyst. Considering the substrate difference, Table 3 listed the activity involving various substrates. The comparison indicated that previous carbon supported IL or PIL catalysts can only catalyze the transformation of robust and moderate active substrates under high pressure [27,42,[72][73][74]. By contrast, the scope surveying indicated the great substrate tolerance of 15%PIL-90@FDU-15-600, offering high yields even for those inert ones such as long-alkyl chain bearing terminal epoxide and disubstituted cyclohexene oxide under atmospheric condition, further demonstrating the superiority of this catalyst.
All the 15%PIL-T@FDU-15-600 catalysts showed a higher yield than the corresponding PIL-T sample and the activity of 15%PIL-90@FDU-15-600 is even superior to that of [p-ArOH-IM]I ( Table 2, entries 2-10), revealing a synergistically catalytic process by combining the PIL-T and carbon support. Immobilization of PIL-110 on FDU-15-600 caused slight activity enhancement, assignable to the limited dispersion of the bulky linear polymer in a confined nano-space. In addition, the loading amount of PIL-90 on FDU-15-600 was explored by preparing two control samples with either lower (10%) or higher (20%) loading. The nitrogen sorption result indicated a negative relationship between the loading amount and the porosity, i.e. the higher loading, the lower surface area and pore volume ( Fig. S10 and Table 1, entries 9, 11, and 12). Varying the loading amount always provided a higher yield than the parent PIL-90 and moderate loading of 15% gave the highest activity when charging the same dosage of ionic moieties (Table 2, entries 9, 11, and 12), further reflecting the cooperation of PIL-90 and FDU-15-600. This is further supported by the result that 15%PIL-90@FDU-15-600 was more active than 15%PIL-90 + FDU-15-600, the physical mixture of FDU-15-600 and PIL-90 (yield: 94.5% vs 78.9%) ( Table 2, entries 9 and 13).
For additional comparison, PIL-90 was impregnated on commercial activated carbon (AC) and a disordered carbon (LOMC) (Table S1, entries [13][14][15][16]. AC is an amorphous carbon with a large surface area of 1526 m 2 g −1 and a pore volume of 0.52 cm 3 g −1 (Fig. S11 and Table 1, entry 13). After loading with 15%PIL-90, the resulting sample 15%PIL-90@AC had a surface area of 942 m 2 g −1 and a pore volume of 0.40 cm 3 g −1 (   14), suggesting the lack of a synergistic effect between PIL-90 and AC. LOMC was derived from the carbonization of a disordered phenolic resin that was synthesized with the same procedure as that of FDU-15-600 except for the involvement of excessive soft-template F127. Small-angle XRD patterns of LOMC revealed a disordered structure, which was also observable on the sample 15%PIL-90@LOMC after loading with PIL-90 (Fig. S4b). Nitrogen sorption isotherm and pore size distribution curve revealed the mesoporous structure of LOMC, with a similar surface area and pore volume to FDU-15-600 ( Fig. S11 and Table 1, entry 15). Loaded sample 15%PIL-90@LOMC had a surface area of 227 m 2 g −1 and a pore volume of 0.19 cm 3 g −1 (Table 1, entry 16). Under the identical reaction conditions, 15%PIL-90@LOMC exhibited a yield of 80.4% (Table 2, entry 15), close to that over [p-ArOH-IM]I but lower than the one over 15%PIL-90@ FDU-15-600. These comparisons further reflect that During the CO 2 cycloaddition with epoxide, the ringopening is usually the rate-determining step and the epoxide activation can be promoted by the HBD through H-bond interaction [8,21,[81][82][83]. For the supported ILs Scheme 2 Proposed reaction route for CO2 cycloaddition with epoxide catalyzed by 15%PIL-90@FDU-  or PILs, the HBD species can come from the functional groups in ionic moieties and the support, but requiring spatial satisfactory [3,14,73,84]. Previously, we have carefully investigated the role of phenolic hydroxyl group of imidazolium-based ILs in the cycloaddition with epoxide and afforded a highly active IL [p-ArOH-IM]I [33]. Herein, the heterogenization of this homogeneous IL was reached by first polymerization and successive loading on ordered mesoporous carbon FDU-15-600. The delocalized I − anions of ionic moieties have well leaving ability, allowing superior spatial facilitation to complete the ring-opening with the assistance of HBD groups in the ionic moieties and surface groups. Based on the result above and previous mechanism investigation [19,64,85], the most probable reaction pathway is proposed for 15%PIL-90@ FDU-15-600 catalyzed CO 2 cycloaddition with epoxide (Scheme 2), in which the Ianions served as the nucleophilic reagent [40], while the abundant phenolic hydroxyl groups in the ionic moieties and inside the wall of mesochannels of FDU-15-600 acted as efficient HBD [5,63,86]. In the beginning, epoxide was adsorbed into the mesochannels of 15%PIL-90@FDU-15-600 and activated by the hydroxyl groups with the polarization of C-O bonds of epoxide through hydrogen bonding interaction [33,40].
The nucleophilic attack of Ianions to the C atom of activated epoxide led to the ring-opening, generating iodinealkoxide followed by CO 2 insertion. After that, ring closure produced cyclic carbonate, with the left of Ianions to regenerate the catalyst. The ring-opening is normally recognized as the rate-determining step [21], and the synergy of nucleophilic reagent and HBD is crucial for the epoxide activation and transition state stabilization [8,33]. Modulating the molecular weight of PIL precursors can finely adjust the host-guest interaction to reach stable immobilization and favorable dispersion. Owing to this balance, the optimal sample 15%PIL-90@FDU-15-600 allowed the Ianions to near the surface hydroxyl groups, satisfying the requirable synergistic effect. By contrast, the loading of IL precursor resulted in unstable active sites, showing apparent deactivation during the recycling test ( Table 2, entry 18) due to the leaching of loaded IL molecules as demonstrated by the decline of the N content in the elemental analysis of the spent catalyst (Table S1, entry 19).

Reusability and scope
The recycling stability of a heterogeneous catalyst is of great importance for practical application. Delightedly, the separation of 15%PIL-90@FDU-15-600 from the   (Fig. 5b). The preservation of the chemical composition and porosity during the recycling cycloaddition with epoxide was evidenced by the almost same elemental analysis result (Table S1, entry 20), N 2 sorption isotherm and the pore size distribution curve (Figs. 5c and 5d) of the spent 15%PIL-90@FDU-15-600 as the fresh one. All of these are responsive to excellent recyclability and stability.
The scope 15%PIL-90@FDU-15-600 was extended to the CO 2 fixation with various epoxides (Table 4). Good to excellent yields were achieved in these terminal epoxides comprising the ones containing long alky chains, the conversion of which was more difficult than those active epoxides like epichlorohydrin (Table 4, entries 1-10) [3,41]. Notably, the effective cycloaddition of atmospheric CO 2 with inert disubstituted cyclohexene oxide over the present catalyst afforded a high yield of 91.7% (Table 4, entry 11). This is rarely reached by heterogeneous catalysts before [3,40]. The scope surveying indicated the great substrate tolerance of this catalyst. It is worth mentioning that a high TON of 2645 and turnover frequency (TOF) of 7935 (Table 4, entry 12) was observed over 15%PIL-90@FDU-15-600 by using high pressure CO 2 (3 MPa), greatly outperforming those carbon encapsulated ILs and PILs listed in Table 3. Though various effective heterogeneous catalysts with high TON and TOF have been reported before, it is still scarcely for metal-free heterogeneous catalysts under additive and solve free conditions. A comprehensive comparison with previous catalysts under similar conditions [3,45,[81][82][83] indicated that the above TON and TOF over 15%PIL-90@FDU-15-600 under harsh conditions are superior to majority of those state-of-art ones, and particularly, a record-high TOF was reached over this metal-free catalyst. All of these reveal the high performance of 15%PIL-90@FDU-15-600 in the heterogeneous CO 2 cycloaddition with epoxides.

Conclusion
A family of linear PILs with tunable molecular weight was synthesized through a hydroxy methylation reaction between a phenolic hydroxyl group functional imidazolium-based IL and formaldehyde by controlling the polymerization temperature. The resulting PILs were encapsulated on the mesoporous phenolic resin derived ordered mesoporous carbon FDU-15-600 to give the corresponding heterogeneous PIL-carbon hybrid catalysts. The interaction between PILs and carbon was strengthened by involving PIL with moderate molecular weight synthesized using moderate polymerization temperature. Not only the stability was improved, but also the suitable spatial adjacency of the ionic moieties and surface groups in the wall of mesochannels was reached, affording synergistically catalytic CO 2 cycloaddition with epoxide. The best-performing catalyst was highly active in the transformation of a series of epoxides through coupling with CO 2 under the atmospheric condition without any metal species, additive or solvent, and even more active than the parent IL and PIL precursors. The catalyst was facilely recovered and reused, and stable activity was observable during the recycling test. This work provides a facile and feasible perspective toward IL-derived heterogeneous catalysts and highlights the great potential of ordered mesoporous carbon materials in the rational designation of highly effective metal-free catalysts for CO 2 fixation.