Hollow and Microporous Organic Polymers Bearing Sulfonic Acids:Antifouling Seed Materials for Polyketone SynthesisNojin Park,†,# Yu Na Lim,‡,# Shin Young Kang,† Sang Moon Lee,§ Hae Jin Kim,§ Yoon-Joo Ko,∥

Bun Yeoul Lee,⊥ Hye-Young Jang,*,‡ and Seung Uk Son*,†

†Department of Chemistry and Department of Energy Science, Sungkyunkwan University, Suwon 16419, Korea‡Division of Energy Systems Research, Ajou University, Suwon 16499, Korea§Korea Basic Science Institute, Daejeon 350-333, Korea∥Laboratory of Nuclear Magnetic Resonance, National Center for Inter-University Research Facilities (NCIRF), Seoul NationalUniversity, Seoul 08826, Korea⊥Department of Molecular Science and Technology, Ajou University, Wonchon-dong, Suwon 16499, Korea

*S Supporting Information

ABSTRACT: The reactor fouling is a notorious obstacle inmanaging continuous synthetic processes of polyketones. Thefouling raises synthetic costs and reduces the bulk density ofpolymers. Thus, efficient seed materials are required for antifoulingperformance. Hollow microporous organic polymers (HMOPs)were prepared using ZIF-8 nanoparticles as templating materials.Sulfonic groups were incorporated into HMOP via a post syntheticapproach. The resultant HMOP−SO3H (20−90 μg/mL inmethanol) showed successful catalyst activation and antifouling performance in polyketone synthesis.

Chemical fixation and conversion of C1 gases such as CO,CO2, and CH4 are important issues in managing global

warming.1 CO can be used to synthesize useful chemicals.2 Forexample, thermoplastic polyketones have been produced inmethanol using CO, ethylene, and Pd catalysts (Scheme 1a), in

which methanolysis is conducted by Pd catalysts.3 Thegenerated methoxy or hydride ligand initiates polymerizationthrough migratory insertion. The original anionic ligands in Pdcatalyst precursors are critical for methanolysis.4 For example,(P−P)Pd(OAc)2 which has poor reactivity in methanolysis canbe activated by sulfonic acids to form (P−P)Pd(OSO2R)2 withless coordinating sulfonate ligands (Scheme 1b).4

One of the technical problems in the synthesis ofpolyketones is the notorious fouling of reactors which is anobstacle in managing continuous synthetic processes.5 More-

over, the fouling raises synthetic costs and reduces the bulkdensity of polymers. Thus, antifouling technologies are criticalfor the commercialization of polyketones.6 As an antifoulingmethod, seed materials have been added to the reactionmedia.5,7 However, further exploration is required to reduce theamount of seed materials. Considering the catalyst activation of(P−P)Pd(OAc)2 by sulfonic acids, we expect that solidmaterials having sulfonic acids can be used as seed materialsin polyketone synthesis.Recently, various microporous organic polymers (MOPs)

have been prepared by the coupling of organic buildingblocks.8,9 Our research group has shown that hollow MOPs canbe engineered using templates.10 These hollow MOPs arehighly dispersible in organic solvents including methanol.11 Inaddition, functional groups can be easily introduced to hollowMOPs through postsynthetic approaches11 due to the shortdiffusion pathways of reagents. Thus, we believe that antifoulingseed materials for polyketone synthesis can be developed basedon MOPs. However, as far as we are aware, there have been noreports investigating MOP-based antifouling seed materials inpolymer synthesis. In this work, we report nonfoulingpolyketone synthesis using hollow MOPs with sulfonic acids(HMOP-SO3H) as seed materials.Figure 1 shows the synthetic route for HMOP-SO3H.

Received: November 8, 2016Accepted: November 11, 2016Published: November 15, 2016

Scheme 1. (a) Polyketone Synthesis from CO and Ethyleneby Pd Catalyst and (b) Catalyst Activation by Sulfonic Acids

Letter

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© 2016 American Chemical Society 1322 DOI: 10.1021/acsmacrolett.6b00846ACS Macro Lett. 2016, 5, 1322−1326

We used zeolitic imidazolate framework (ZIF-813) particleswith a diameter of 473 ± 21 nm as templates to engineerHMOP-SO3H (Figures 2a,b). It has been known that ZIF-8 is agood adsorbent and can be disassembled by acetic acid.13b The(PPh3)2PdCl2 and CuI catalysts were adsorbed in ZIF-8. Usingtetrakis(4-ethynylphenyl)methane14 and 1,4-diiodobenzene asbuilding blocks, MOPs were formed on the surface of ZIF-8particles through the Sonogashira coupling (Figure S1 in theSupporting Information, SI). After treating the ZIF-8@MOPwith acetic acid for 1 h, the hollow MOP (HMOP) wasobtained. As shown in Figure 2c, scanning electron microscopy(SEM) indicated the hollow nature of HMOP.The sulfonation of HMOP with ClSO3H was conducted

following the procedure in the literature.15a The SEM analysisof HMOP-SO3H showed hollow polyhedrons with a diameterof 521 ± 21 nm (Figures 2d,e). Transmission electronmicroscopy (TEM) showed a wall thickness of 25 ± 2 nm(Figure 2f). Analyzing the N2 sorption isotherm curves ofHMOP based on the Brunauer, Emmett, and Teller theoryshowed a surface area of 623 m2/g and microporosity (pore size< 2 nm) (Figure 2g). The incorporation of sulfonic acids intoHMOP resulted in a decrease of the surface area to 232 m2/g,matching well with observations in the literature.11,13a PowderX-ray diffraction (PXRD) studies showed the conventionalamorphous characteristics of HMOP and HMOP-SO3H(Figure S2 in the SI).8−12,15 The infrared (IR) absorptionspectroscopy of HMOP-SO3H revealed vibration peaks ofsulfonic acids at 3475, 1218, and 1047 cm−1 (Figure 2h).15 Thesolid-phase 13C nuclear magnetic resonance (NMR) spectros-copy of HMOP and HMOP-SO3H showed 13C peaks of benzylcarbon and phenyl rings at 64 and 110−150 ppm, respectively(Figure 2i). The aromatic 13C peaks changed significantly bythe incorporation of sulfonic acid groups to phenyl rings. Thesimilar change of 13C NMR spectra was reported in theliterature13a on the sulfonated MOPs by ClSO3H. The

13Cpeaks at 85−95 ppm disappeared, indicating the acid additionto internal alkynes (Figure 2i). The addition of halosulfonic

acid and HCl16 to internal alkynes was well reported in theliterature.17 The amount of sulfonic groups in HMOP-SO3H(2.16 mmol SO3H/g) was analyzed by elemental analysis (6.91wt % sulfur)18 and acid−base titration.19 HMOP-SO3H washighly dispersible in methanol to form a colloidal solution(Figures 1 and S3 in the SI). Considering its excellentdispersion in methanol, we studied the HMOP-SO3H aspotential antifouling seed materials for polyketone synthesis.Table 1 summarizes the results.[1,3-Bis(di-o-methoxyphenylphosphino)propane]Pd(OTs)2,

i.e., (domppp)Pd(OTs)2, has been developed as an efficientindustrial catalyst for polyketone synthesis.20 In comparison,(domppp)Pd(OAc)2 showed very poor activity for polyketonesynthesis (entry 1 in Table 1). The soluble p-toluenesulfonic

Figure 1. Synthetic scheme and the suggested structure for hollowmicroporous organic polymers bearing sulfonic acids (HMOP-SO3H)and a photograph of colloidal HMOP-SO3H in methanol.

Figure 2. SEM images of (a,b) ZIF-8 particles, (c) HMOP, and (d,e)HMOP-SO3H. (f) TEM image of HMOP-SO3H. (g) N2 adsorption−desorption isotherm curves at 77 K, pore size distribution diagrambased on the DFT method (inset), (h) IR absorption spectra, and (i)solid-phase 13C NMR spectra of HMOP and HMOP-SO3H.

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acids (p-TsOH) activated the (domppp)Pd(OAc)2 (entries 2−4). The use of p-TsOH as an activator in the range of 1 to 2equiv with respect to (domppp)Pd(OAc)2 showed goodactivities in polyketone synthesis (entries 3−4). However, asexpected, the severe fouling of reactor was observed (entries 3−4 and Figure 3). When HMOP-SO3H was used as a catalystactivator for polyketone synthesis, the reaction showed goodactivity without reactor fouling. Through screening thesynthetic conditions, it was found that there is an optimalrange of seed concentrations (entries 5−10). When 0.2−0.9 mgof HMOP-SO3H was used in 10 mL of methanol, the systemsshowed excellent antifouling performances (entries 5−7).When the seed concentration exceeded 180 μg/mL, thecatalytic activities gradually decreased (entries 8−10). It isnoteworthy that a very small amount of HMOP-SO3H (20−90μg/mL in methanol) showed successful antifouling perform-ance during polyketone synthesis. In the literature, a relativelylarge amount (2−6 wt %) of solid seed materials withoutcatalyst activating groups (SO3H) have been used to solve thefouling problem.5,21

According to the stoichiometric consideration, the(domppp)Pd(OAc)2 used in entries 5−7 in Table 1 (2.0μmol) is in excess to the amount of SO3H groups (0.43−1.9μmol) in the HMOP-SO3H, implying that some Pdcompounds did not participate in the polyketone synthesis.Even when we gradually reduced the amount of Pd catalystsfrom 1.5 to 0.32 mg (0.43 μmol) with 0.40 mg (0.86 μmolSO3H) of HMOP-SO3H, the catalytic activities weresignificantly retained (entries 6 and 11−13). The system of(domppp)Pd(OAc)2 and HMOP-SO3H in this work showedactivities of up to 46 kg polyketones/g of Pd (entry 13). Thebulk densities of polyketones were measured up to 0.29 g/mL(entries 5−7). When we used HMOP without sulfonic acids(instead of HMOP-SO3H), the catalytic system showed verypoor catalytic activity, indicating that the Pd catalyst wasactivated by the sulfonic acids in HMOP-SO3H (entry 14).The NMR spectroscopy confirmed the chemical structure of

polyketones with major 13C peaks of carbonyl and ethylene at

212 and 35 ppm, respectively (Figures S4−5 in the SI). Theinfrared (IR) absorption spectroscopy showed the vibrationpeaks at 1693 (CO), 1407 (C−H), and 1334 (C−H) cm−1

(Figure S6 in the SI). Elemental analysis of polyketones showed63.85−64.11% of carbon, 7.01−7.17% of hydrogen, and 28.11−28.38% of oxygen (theoretical values of (C3H4O)n: 64.27%, H:7.19%, O: 28.54%). The endothermic peaks by the melting ofpolyketones were observed in a range of 252−260 °C indifferential scanning calorimetry (DSC) curves (Figure S6 inthe SI). These characterization data match well with those ofpolyketones in the literature.3,4

To understand the difference in the reaction processesbetween the homogeneous catalyst system using p-TsOH

Table 1. Synthesis of Polyketones by Pd Catalystsa

catalystb seedc yield activity densityd

entry (mg) (mg) (g) (kg/g-Pd) (g/mL) fouling

1 1.5e 0 0.1 0.48 - -2f 1.5 0 1.3 6.3 - -3g 1.5 0 4.4 21 - yes4h 1.5 0 5.0 24 - yes5 1.5 0.2 3.1 15 0.24 no6 1.5 0.4 4.0 19 0.29 no7 1.5 0.9 3.8 18 0.27 no8 1.5 1.8 2.7 13 - some9 1.5 3.6 2.2 10 - some10 1.5 9.3 0.36 1.7 - -11 0.90 0.4 4.1 32 0.24 no12 0.65 0.4 3.6 39 0.22 no13 0.32 0.4 2.1 46 0.13 no14 1.5 0.4i 0.16 0.77 - -

aReaction conditions: ethylene (25 bar), CO (35 bar), MeOH (10mL), 15 h, and 90 °C. b[1,3-Bis(di-o-methoxyphenylphosphino)-propane]Pd(OAc)2.

cHMOP-SO3H (2.16 mmol SO3H/g).dBulk

density. e2.0 μmol. f0.36 equiv of p-TsOH was used. g1.0 equiv of p-TsOH was used. h2.0 equiv of p-TsOH was used. iHMOP was usedinstead of HMOP-SO3H.

Figure 3. (a) Total pressure changes of reaction systems duringpolyketone synthesis using HMOP-SO3H (entry 6 in Table 1) and p-TsOH (entry 4 in Table 1) as catalyst activators. Photographs of (b)the HMOP-SO3H seed materials induced polymerization (entry 6 inTable 1) and (c) the homogeneous polymerization with reactorfouling (entry 4 in Table 1). (d) Illustration of the polymerizationinduced by the HMOP-SO3H seeds and the homogeneous polymer-ization induced by p-TsOH. (e) PXRD patterns of polyketonesprepared using HMOP-SO3H and p-TsOH as a catalyst activator(refer to Figure S8 in the SI for detailed analysis). SEM images ofHMOP-SO3H (f) before and (g) after reaction (see additional SEMimages in Figure S9 in the SI).

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(entry 4 in Table 1) and the heterogeneous HMOP-SO3H seedsystem (entry 6 in Table 1), we investigated the pressurechanges during the reactions. As shown in Figure 3a, thehomogeneous system with (domppp)Pd(OAc)2 and p-TsOHshowed a sharp decrease in the total pressure during the earlystage of reaction. In comparison, the HMOP-SO3H seedsystem showed a gradual decrease in the total pressure,indicating the controlled formation of polymers around theseed materials, as illustrated in Figure 3d. While thephotographs in Figure 3b show the nonfouling synthesis ofpolyketones, those in Figure 3c show the typical reactor foulingduring polymer synthesis by the homogeneous catalyst system(Figure S7 in the SI).It has been reported that the relatively slow crystallization of

polyketones induces α packing22a which is denser and morestable than β packing.22b Wide-angle XRD study showed adominant α phase of polyketone obtained using HMOP-SO3Hand a β phase of polyketone obtained using p-TsOH (Figures3e and S8 in the SI), indicating the slower and closer packing inseed-mediated polymerization.22c Through centrifugation ofpolyketone solution in hexafluoroisopropanol (HFIP), wecould isolate the minute HMOP-SO3H materials. The SEMstudies showed the homogeneous decoration of HMOP-SO3Hmaterials with polymers, inducing an increase of diameter andwall thickness from 521 and 25 to 625 and 120 nm, respectively(Figures 3f,g and S9 in the SI). This observation supports theaction of HMOP-SO3H as seed materials.In conclusion, this work shows that MOP chemistry can be

applied to develop the polymerization processes of polyketonesby creating efficient antifouling seed materials. The seed-mediated polymerization by HMOP-SO3H showed a moregradual formation of polymers, hence controlling the packingstructure of polyketones to a denser α packing, compared withthe β packing formed through homogeneous polymerization byp-TsOH. We believe that the strategy in this work can beapplied for industrial polyketone processes.23 In addition,shape-controlled MOP materials can be further designed anddeveloped as highly efficient seed materials for more diversetypes of antifouling polymerization.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsmacro-lett.6b00846.

Synthetic procedure, TEM images of ZIF-8@MOP,PXRD patterns, characterization data of polyketones,and SEM images of HMOP-SO3H before and afterreaction (PDF)

■ AUTHOR INFORMATION

Corresponding Authors*E-mail: sson@skku.edu.*E-mail: hyjang2@ajou.ac.kr.

Author Contributions#These authors contributed equally

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This work was supported by C1-gas Refinary Research program(No. 2015M3D3A1A01065436) through NRF of Korea fundedby the Ministry of Science, ICT & Future Planning.

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