Journal of Materials Chemistry A View Article Online COMMUNICATION View Journal | View Issue A one building block approach for defect- enhanced conjugated microporous polymers: Cite this: J. Mater. Chem. A,2018,6, 15553 defect utilization for recyclable and catalytic † Received 30th June 2018 biomass conversion Accepted 24th July 2018 Kyoungil Cho,a Sang Moon Lee,b Hae Jin Kim,b Yoon-Joo Koc and Seung Uk Son *a DOI: 10.1039/c8ta06273k rsc.li/materials-a A one building block approach was studied for preparation of defect- alkynes and can be free from the network-induced suppression enhanced and terminal alkyne-rich hollow conjugated microporous of chemical reactivity. It is noteworthy that defect utilization has polymers (H-TA-CMPs). The enriched terminal alkynes were utilized in been realized in microporous inorganic materials or metal– post-synthetic modification of H-TA-CMPs with aliphatic sulfonic organic frameworks.7 Thus, we have devised a synthetic strategy acids (H-TA-CMP-ASO3H). The obtained H-TA-CMP-ASO3Hshowed to enhance defects in CMPs. excellent reactivity and recyclability in biomass conversion to Infrared absorption (IR) spectroscopy is useful for charac- 5-hydroxymethylfurfural, compared with CMPs with aromatic sulfonic terizing terminal alkynes in CMP materials.8 According to IR acids (H-control-SO3H). analysis, conventional CMPs showed trace vibration peaks of terminal alkyne groups.8 To induce more defects in CMPs, we have devised a one building block approach using dieth- The representative properties of conjugated microporous poly- ynyldihalo arene (refer to the possible chemical structure of 1 mers (CMPs) are their high surface areas and microporosity. CMPs in Fig. 1). Enriched terminal alkyne groups through Based on these properties, CMPs have been applied as adsor- enhanced defects in CMPs can be utilized to introduce bents for small molecules such as gases and water pollutants.2 To introduce additional functionalities, post-synthetic modi- cation of CMPs has been studied.3 Generally, CMPs have been prepared by the Sonogashira Published on 25 July 2018. Downloaded by Sungkyunkwan University 8/23/2018 8:41:01 AM. coupling of multiethynyl arenes and multihalo arenes.1 Thus, conventional CMPs are rich in alkyne groups. Alkynes are very versatile moieties for chemical reactions including the thiol-yne click reaction.4 Post-synthetic modication of CMPs based on the thiol-yne click reaction has been reported.3a,d,f However, in our studies, functional moieties introduced by this method were not sufficient in quantity.5 We speculate that this might be due to two factors. First, diaryl internal alkynes in CMPs may have relatively low reactivity toward the click-based thiol addi- tion. Second, the reactivity of internal alkynes can be sup- pressed in the network due to the so-called network effect.6 We considered that the possible terminal alkyne groups in the defects of the network may have higher reactivity than internal aDepartment of Chemistry, Sungkyunkwan University, Suwon 16419, Korea. E-mail: [email protected] bKorea Basic Science Institute, Daejeon 34133, Korea cLaboratory of Nuclear Magnetic Resonance, The National Center for Inter-University Research Facilities (NCIRF), Seoul National University, Seoul 08826, Korea † Electronic supplementary information (ESI) available: Experimental procedures, Fig. 1 Synthetic schemes for hollow terminal alkyne-rich CMPs and additional characterization data of CMP materials, recycled CMP materials, (H-TA-CMPs) based on a one building block approach (Synthesis A) and H-control-SO3H. See DOI: 10.1039/c8ta06273k and CMPs based on a two building block approach (Synthesis B). This journal is © The Royal Society of Chemistry 2018 J. Mater. Chem. A,2018,6, 15553–15557 | 15553 View Article Online Journal of Materials Chemistry A Communication additional functional groups. For example, aliphatic sulfonic acid can be introduced by the thiol-yne click reaction of CMPs with aliphatic sulfonate thiol. Solid sulfonic acids have been used as Brønsted acid catalysts for carbohydrate-based biomass conversion to 5-hydrox- ymethylfurfural (HMF) through water abstraction.9 Our research group has studied sulfonated microporous organic polymers10 and their application as catalysts for carbohydrate conversion to HMF. However, we found that although the sulfonated CMPs showed good reactivity in the rst run, they gradually lost their catalytic reactivity in the successive cycles. As well-documented,11 aryl sulfonic acids are not thermally stable in the presence of water to bring about facile cleavage of C–S bonds, leading to poor recyclability of aryl sulfonic acid catalysts. In contrast, it has been known that aliphatic sulfonic acids are thermally stable and show good recyclability in the dehydration reaction of sugar-based biomass to furan derivatives.12 Thus, CMPs with aliphatic sulfonic acids are promising solid catalysts for biomass conver- sion to HMF. However, as far as we are aware, CMPs with aliphatic sulfonic acids have not been reported yet. In this work, we report a one building block approach for Fig. 2 (a) SEM images of H-TA-CMP and CMP materials prepared for hollow terminal alkyne rich CMPs (H-TA-CMPs), chemical 2, 4, 6, and 12 h by the one building block approach (Synthesis A) and management of enhanced defects to prepare hollow CMPs with two building block approach (Synthesis B), respectively. IR spectra of (b) H-TA-CMP and (c) CMP materials prepared by Syntheses A and B aliphatic sulfonic acids (H-TA-CMP-ASO3H), and their catalytic performance in biomass conversion to HMF. for 4, 6, and 12 h, respectively. Fig. 1 shows the synthetic scheme for H-TA-CMPs. First, we prepared 1,4-dibromo-2,5-diethynylbenzene13 for use as a building block for the one building block approach. To use as (Fig. 2b). Even aer reaction for 12 h, the H-TA-CMPs showed templates, we prepared silica spheres with an average diameter a signicant vibration peak of terminal alkynes, indicating the of 254 nm.14 CMP layers with a thickness of 20–25 nm (vide enhanced defects and the terminal alkyne-rich nature of H-TA- infra) were formed on the silica spheres by the Sonogashira CMPs. In contrast, in the case of Synthesis B, vibration peaks of coupling of 1,4-dibromo-2,5-diethynylbenzene. Etching of silica terminal alkynes in CMPs gradually decreased with the templates resulted in H-TA-CMPs (Synthesis A shown in Fig. 1). increasing reaction time.16 (Fig. 2c) While surface areas of H-TA- For comparison, we tried to prepare comparatively hollow CMP CMPs obtained by Synthesis A for 4, 6, and 12 h were measured À materials using 1,3,5-triethynylbenzene and 1,4-dibromo- to be 677, 661, and 590 m2 g 1, respectively, those of CMPs Published on 25 July 2018. Downloaded by Sungkyunkwan University 8/23/2018 8:41:01 AM. benzene (Synthesis B shown in Fig. 1). obtained by Synthesis B for 4, 6, and 12 h were measured to be À Formation processes of H-TA-CMPs were investigated by 698, 660, and 589 m2 g 1, respectively (Fig. S1 in the ESI†). scanning electron microscopy (SEM), IR spectroscopy, and the Considering the terminal alkyne-rich nature of H-TA-CMP analysis of N2 sorption isotherm curves. As shown in Fig. 2a, we materials, we introduced aliphatic sulfonic acid by the thiol- found that in the case of the one building block approach yne click reaction to form H-TA-CMP-ASO3H (Fig. 3a). As (Synthesis A), CMP materials were quickly formed on silica described in the introduction part, solid sulfonic acid is an spheres. Aer 4 h, complete hollow CMPs were obtained aer important catalyst for sugar-based biomass conversion to silica etching. In contrast, in the two building block approach HMF.9 It has been known that while aryl sulfonic acids are (Synthesis B) with 1,3,5-triethynylbenzene and 1,4-dibromo- thermally unstable,11 aliphatic sulfonic acids are quite stable.12 benzene, the formation of CMPs was relatively slow and good According to transmission electron microscopy (TEM), aer quality hollow CMPs could not be obtained eventually. Instead, incorporation of aliphatic sulfonic acids into H-TA-CMP, the conventional nonhollow CMPs were obtained aer 12 h original hollow structure was completely retained (Fig. 3b (Fig. 2a). We speculate that the Sonogashira coupling of and c). Elemental mapping based on energy dispersive X-ray 1,4-dibromo-2,5-diethynylbenzene might be more facile than spectroscopy (EDS) showed homogeneous distributions of S that between 1,3,5-triethynylbenzene and 1,4-dibromobenzene. and O in H-TA-CMP-ASO3H, supporting the successful incor- In the one building block approach, isolated yields of H-TA- poration of aliphatic sulfonic acid into H-TA-CMPs (Fig. 3d). CMPs obtained a er 2, 4, 6, and 12 h were 127, 179, 173, and According to analysis of N2 sorption isotherm curves based 182%, respectively.15 In contrast, in the two building block on the Brunauer–Emmett–Teller theory, surface areas and À À approach, isolated yields of CMPs obtained aer 2, 4, 6, and micropore volumes decreased from 677 m2 g 1 and 0.20 cm3 g 1 2 À1 3 À1 12 h were 31, 70, 86, and 83%, respectively. (H-TA-CMP) to 387 m g and 0.11 cm g (H-TA-CMP-ASO3H), IR spectra of H-TA-CMPs obtained by Synthesis A showed respectively, through incorporation of aliphatic sulfonic groups, À strong vibration peaks of terminal alkynes at 3295 cm 1 matching well with the observed trends in post-synthetic 15554 | J. Mater. Chem. A,2018,6, 15553–15557 This journal is © The Royal Society of Chemistry 2018 View Article Online Communication Journal of Materials Chemistry A terminal alkynes at 80 ppm signicantly disappeared (indicated by a green-dotted arrow in Fig.