Journal ofMaterials Chemistry A

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A one building b

aDepartment of Chemistry, Sungkyunkwan

sson@skku.edubKorea Basic Science Institute, Daejeon 3413cLaboratory of Nuclear Magnetic Resonance

Research Facilities (NCIRF), Seoul National

† Electronic supplementary information (Eand additional characterization data of Cand H-control-SO3H. See DOI: 10.1039/c8t

Cite this: J. Mater. Chem. A, 2018, 6,15553

Received 30th June 2018Accepted 24th July 2018

DOI: 10.1039/c8ta06273k

rsc.li/materials-a

This journal is © The Royal Society of C

lock approach for defect-enhanced conjugated microporous polymers:defect utilization for recyclable and catalyticbiomass conversion†

Kyoungil Cho,a Sang Moon Lee,b Hae Jin Kim,b Yoon-Joo Koc and Seung Uk Son *a

A one building block approach was studied for preparation of defect-

enhanced and terminal alkyne-rich hollow conjugated microporous

polymers (H-TA-CMPs). The enriched terminal alkynes were utilized in

post-synthetic modification of H-TA-CMPs with aliphatic sulfonic

acids (H-TA-CMP-ASO3H). The obtained H-TA-CMP-ASO3H showed

excellent reactivity and recyclability in biomass conversion to

5-hydroxymethylfurfural, compared with CMPs with aromatic sulfonic

acids (H-control-SO3H).

The representative properties of conjugated microporous poly-mers (CMPs) are their high surface areas and microporosity.1

Based on these properties, CMPs have been applied as adsor-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 Sonogashiracoupling of multiethynyl arenes and multihalo arenes.1 Thus,conventional CMPs are rich in alkyne groups. Alkynes are veryversatile moieties for chemical reactions including the thiol-yneclick reaction.4 Post-synthetic modication of CMPs based onthe thiol-yne click reaction has been reported.3a,d,f However, inour studies, functional moieties introduced by this methodwere not sufficient in quantity.5 We speculate that this might bedue to two factors. First, diaryl internal alkynes in CMPs mayhave 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 Weconsidered that the possible terminal alkyne groups in thedefects of the network may have higher reactivity than internal

University, Suwon 16419, Korea. E-mail:

3, Korea

, The National Center for Inter-University

University, Seoul 08826, Korea

SI) available: Experimental procedures,MP materials, recycled CMP materials,a06273k

hemistry 2018

alkynes and can be free from the network-induced suppressionof chemical reactivity. It is noteworthy that defect utilization hasbeen realized in microporous inorganic materials or metal–organic frameworks.7 Thus, we have devised a synthetic strategyto enhance defects in CMPs.

Infrared absorption (IR) spectroscopy is useful for charac-terizing terminal alkynes in CMP materials.8 According to IRanalysis, conventional CMPs showed trace vibration peaks ofterminal alkyne groups.8 To induce more defects in CMPs, wehave devised a one building block approach using dieth-ynyldihalo arene (refer to the possible chemical structure ofCMPs in Fig. 1). Enriched terminal alkyne groups throughenhanced defects in CMPs can be utilized to introduce

Fig. 1 Synthetic schemes for hollow terminal alkyne-rich CMPs(H-TA-CMPs) based on a one building block approach (Synthesis A)and CMPs based on a two building block approach (Synthesis B).

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Fig. 2 (a) SEM images of H-TA-CMP and CMP materials prepared for2, 4, 6, and 12 h by the one building block approach (Synthesis A) andtwo building block approach (Synthesis B), respectively. IR spectra of(b) H-TA-CMP and (c) CMP materials prepared by Syntheses A and Bfor 4, 6, and 12 h, respectively.

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additional functional groups. For example, aliphatic sulfonicacid can be introduced by the thiol-yne click reaction of CMPswith aliphatic sulfonate thiol.

Solid sulfonic acids have been used as Brønsted acid catalystsfor carbohydrate-based biomass conversion to 5-hydrox-ymethylfurfural (HMF) through water abstraction.9 Our researchgroup has studied sulfonated microporous organic polymers10

and their application as catalysts for carbohydrate conversion toHMF. However, we found that although the sulfonated CMPsshowed good reactivity in the rst run, they gradually lost theircatalytic reactivity in the successive cycles. As well-documented,11

aryl sulfonic acids are not thermally stable in the presence ofwater to bring about facile cleavage of C–S bonds, leading to poorrecyclability of aryl sulfonic acid catalysts. In contrast, it has beenknown that aliphatic sulfonic acids are thermally stable and showgood recyclability in the dehydration reaction of sugar-basedbiomass to furan derivatives.12 Thus, CMPs with aliphaticsulfonic acids are promising solid catalysts for biomass conver-sion to HMF. However, as far as we are aware, CMPs withaliphatic sulfonic acids have not been reported yet.

In this work, we report a one building block approach forhollow terminal alkyne rich CMPs (H-TA-CMPs), chemicalmanagement of enhanced defects to prepare hollow CMPs withaliphatic sulfonic acids (H-TA-CMP-ASO3H), and their catalyticperformance in biomass conversion to HMF.

Fig. 1 shows the synthetic scheme for H-TA-CMPs. First, weprepared 1,4-dibromo-2,5-diethynylbenzene13 for use asa building block for the one building block approach. To use astemplates, we prepared silica spheres with an average diameterof 254 nm.14 CMP layers with a thickness of 20–25 nm (videinfra) were formed on the silica spheres by the Sonogashiracoupling of 1,4-dibromo-2,5-diethynylbenzene. Etching of silicatemplates resulted in H-TA-CMPs (Synthesis A shown in Fig. 1).For comparison, we tried to prepare comparatively hollow CMPmaterials using 1,3,5-triethynylbenzene and 1,4-dibromo-benzene (Synthesis B shown in Fig. 1).

Formation processes of H-TA-CMPs were investigated byscanning electron microscopy (SEM), IR spectroscopy, and theanalysis of N2 sorption isotherm curves. As shown in Fig. 2a, wefound that in the case of the one building block approach(Synthesis A), CMP materials were quickly formed on silicaspheres. Aer 4 h, complete hollow CMPs were obtained aersilica etching. In contrast, in the two building block approach(Synthesis B) with 1,3,5-triethynylbenzene and 1,4-dibromo-benzene, the formation of CMPs was relatively slow and goodquality hollow CMPs could not be obtained eventually. Instead,conventional nonhollow CMPs were obtained aer 12 h(Fig. 2a). We speculate that the Sonogashira coupling of1,4-dibromo-2,5-diethynylbenzene might be more facile thanthat between 1,3,5-triethynylbenzene and 1,4-dibromobenzene.In the one building block approach, isolated yields of H-TA-CMPs obtained aer 2, 4, 6, and 12 h were 127, 179, 173, and182%, respectively.15 In contrast, in the two building blockapproach, isolated yields of CMPs obtained aer 2, 4, 6, and12 h were 31, 70, 86, and 83%, respectively.

IR spectra of H-TA-CMPs obtained by Synthesis A showedstrong vibration peaks of terminal alkynes at 3295 cm�1

15554 | J. Mater. Chem. A, 2018, 6, 15553–15557

(Fig. 2b). Even aer reaction for 12 h, the H-TA-CMPs showeda signicant vibration peak of terminal alkynes, indicating theenhanced defects and the terminal alkyne-rich nature of H-TA-CMPs. In contrast, in the case of Synthesis B, vibration peaks ofterminal alkynes in CMPs gradually decreased with theincreasing reaction time.16 (Fig. 2c) While surface areas of H-TA-CMPs obtained by Synthesis A for 4, 6, and 12 h were measuredto be 677, 661, and 590 m2 g�1, respectively, those of CMPsobtained by Synthesis B for 4, 6, and 12 h were measured to be698, 660, and 589 m2 g�1, respectively (Fig. S1 in the ESI†).

Considering the terminal alkyne-rich nature of H-TA-CMPmaterials, we introduced aliphatic sulfonic acid by the thiol-yne click reaction to form H-TA-CMP-ASO3H (Fig. 3a). Asdescribed in the introduction part, solid sulfonic acid is animportant catalyst for sugar-based biomass conversion toHMF.9 It has been known that while aryl sulfonic acids arethermally unstable,11 aliphatic sulfonic acids are quite stable.12

According to transmission electron microscopy (TEM), aerincorporation of aliphatic sulfonic acids into H-TA-CMP, theoriginal hollow structure was completely retained (Fig. 3band c). Elemental mapping based on energy dispersive X-rayspectroscopy (EDS) showed homogeneous distributions of Sand O in H-TA-CMP-ASO3H, supporting the successful incor-poration of aliphatic sulfonic acid into H-TA-CMPs (Fig. 3d).

According to analysis of N2 sorption isotherm curves basedon the Brunauer–Emmett–Teller theory, surface areas andmicropore volumes decreased from 677m2 g�1 and 0.20 cm3 g�1

(H-TA-CMP) to 387m2 g�1 and 0.11 cm3 g�1 (H-TA-CMP-ASO3H),respectively, through incorporation of aliphatic sulfonic groups,matching well with the observed trends in post-synthetic

This journal is © The Royal Society of Chemistry 2018

Fig. 3 (a) Synthetic scheme for H-TA-CMP-ASO3H by post-syntheticmodification based on the thiol-yne click reaction. TEM images of (b)H-TA-CMPs and (c) H-TA-CMP-ASO3H. (d) SEM and TEM-EDSelemental mapping images of H-TA-CMP-ASO3H. (e) N2 adsorption–desorption isotherm curves obtained at 77 K, inset: pore size distri-bution diagrams (based on the DFT method), (f) IR absorption spectra,and (g) solid state 13C NMR spectra of H-TA-CMPs and H-TA-CMP-ASO3H.

Fig. 4 (a) Scheme of catalytic fructose conversion to HMF by H-TA-CMP-ASO3H. (b) Temperature dependent catalytic conversion offructose to HMF by H-TA-CMP-ASO3H (2 mol% SO3H to fructose).Biphenyl was used as an internal standard. (c) Recyclability tests of H-TA-CMP-ASO3H and H-control-SO3H (temperature: 100 �C, time: 5 hfor H-TA-CMP-ASO3H and 2 h for H-control-SO3H, 2 mol% SO3H tofructose). H-Control-SO3H with aromatic SO3H was prepared bydirect sulfonation of H-TA-CMPs with ClSO3H (refer to the Experi-mental section in the ESI†). (d) TGA curves of H-TA-CMP-ASO3H andH-control-SO3H. (e) SEM images of H-TA-CMP-ASO3H retrievedbefore and after five successive reactions.

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modication of CMP materials in the literature.3 (Fig. 3e) WhileH-TA-CMPs showed major vibration peaks at 3295, 1460, and886 cm�1, corresponding to terminal alkyne and aromatic C]Cand C–H vibrations, respectively, H-TA-CMP-ASO3H showed newstrong vibration peaks at 3442, 1633, and 1203 cm�1, corre-sponding to vibrations of O–H (SO3H), C]C (alkene neigh-boring sulde), and S]O (SO3H), respectively8 (Fig. 3f).

Solid state 13C nuclear magnetic resonance spectroscopy(NMR) of H-TA-CMPs showed aromatic 13C peaks at 136 and125 ppm (Fig. 3g). In addition, two kinds of alkyne 13C peakswere clearly observed at 93 and 80 ppm, corresponding tointernal and terminal alkyne moieties, respectively, indicatingthe terminal alkyne rich-nature of H-TA-CMPs.

Interestingly, in the 13C NMR spectrum of H-TA-CMP-ASO3H,while the 13C peak of internal alkynes at 93 ppm was mostlyretained (indicated by a black-dotted arrow in Fig. 3g), that of

This journal is © The Royal Society of Chemistry 2018

terminal alkynes at 80 ppm signicantly disappeared (indicatedby a green-dotted arrow in Fig. 3g), indicating higher reactivityof terminal alkynes than internal alkynes. New aliphatic 13Cpeaks of H-TA-CMP-ASO3H were observed at 50 and 25–31 ppm,indicating successful incorporation of aliphatic sulfonicgroups. According to elemental analysis, the content of sulfonicacids in H-TA-CMP-ASO3H was measured to be 0.713 mmol g�1

(S: 4.56 wt%).17 According to powder X-ray diffraction studies,both H-TA-CMP and H-TA-CMP-ASO3H were amorphous,matching well with the conventional properties of CMP mate-rials in the literature1 (Fig. S2 in the ESI†).

Considering aliphatic sulfonic groups in H-TA-CMP-ASO3H,we studied its catalytic activity in the model biomass conversionof fructose to HMF (Fig. 4a). Fig. 4 summarizes the results. Inthe literature, the conversion of fructose to HMF was mostlystudied at a high temperature of�150 �C.9 To lower the reactiontemperature, we studied the temperature dependent catalyticactivity of H-TA-CMP-ASO3H (2 mol% SO3H to fructose). Asshown in Fig. 4b, while the H-TA-CMP-ASO3H showed goodconversion of fructose to HMF (91–92% yields aer 5 h) at 120

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and 140 �C, it maintained good catalytic activity (85% yield ofHMF aer 5 h) at 100 �C. It is noteworthy that microporousorganic polymers with sulfonic groups have been recentlydeveloped as solid catalysts for fructose conversion to HMF,showing catalytic turnover numbers (TONs) of 5.83 and 6.11 at120 and 140 �C.18 In comparison, the H-TA-CMP-ASO3H showedsuperior TONs of 42.5 and 45.5 at 100 and 120 �C, respectively.We think that the excellent performance of H-TA-CMP-ASO3H isattributable to its microporosity and thin hollow structure.19

Importantly, the H-TA-CMP-ASO3H showed excellent recycla-bility in the ve successive catalytic reactions with 85, 84, 85, 83,and 82% yields of HMF for the rst, second, third, fourth, andh run, respectively (Fig. 4c). According to thermogravimetricanalysis (TGA), the H-TA-CMPs and H-TA-CMP-ASO3H werestable up to 247 and 205 �C, respectively (Fig. 4d and S3 in theESI†). SEM and IR analysis showed that the H-TA-CMP-ASO3Hrecovered aer the h run maintained its original hollowstructure and aliphatic sulfonic acids (Fig. 4e and S4 in theESI†).

In comparison, we prepared hollow aromatic sulfonic acids(H-control-SO3H) as control materials by reaction of H-TA-CMPswith ClSO3H (Experimental section and Fig. S5 in the ESI†). Asshown in Fig. 4c and d, the H-control-SO3H with aromatic SO3Hshowed poor recyclability,20 poor thermal stability, anda gradual desulfonation in IR analysis (Fig. S6 in the ESI†),matching with the thermal instability of aromatic sulfonic acidsin the literature.12

In conclusion, this work shows that defects can be enhancedand utilized for functionalization of CMP materials. Versatiledefective terminal alkynes can be incorporated into CMPmaterials by the Sonogashira coupling of 1,4-dibromo-2,5-diethynybenzene. Aliphatic sulfonic acids can be easily intro-duced into H-TA-CMPs by the thiol-yne click reaction ofterminal alkyne groups. The resultant H-TA-CMP-ASO3Hshowed recyclable catalytic performance in fructose conversionto HMF. We believe that more diverse functional groups can beincorporated by the reaction of terminal alkynes of H-TA-CMPswith tailored functional reactants.

Conflicts of interest

There are no conicts to declare.

Acknowledgements

This work was supported by “Next Generation Carbon UpcyclingProject” (Project No. 2017M1A2A2043146) through the NationalResearch Foundation (NRF) funded by the Ministry of Scienceand ICT, Republic of Korea.

Notes and references

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12 M. Balakrishnan, E. R. Sacia and A. T. Bell, ChemSusChem,2014, 7, 1078–1085.

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15 The quantitative yield (100%) was dened assumingcomplete and nondefective networking.

16 For H-TA-CMPs, intensity ratios of the terminal alkyne peaksat 3295 cm�1 to vibration peaks at 1468 cm�1 were 0.76 (4 h),0.63 (6 h), and 0.55 (12 h). In comparison, for CMPmaterials,intensity ratios of the terminal alkyne peaks at 3296 cm�1 tovibration peaks at 1484 cm�1 were 0.52 (4 h), 0.36 (6 h), and0.27 (12 h).

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17 Acid-base titration of H-TA-CMP-ASO3H with 0.10 M NaOHconrmed the same amount of sulfonic acids in thematerials.

18 S. Mondal, J. Modal and A. Bhaumik, ChemCatChem, 2015, 7,3570–3578.

19 K. Cho, J. Yoo, H.-W. Noh, S. M. Lee, H. J. Kim, Y.-J. Ko,H.-Y. Jang and S. U. Son, J. Mater. Chem. A, 2017, 5, 8922–8926.

20 When we tested the hollow microporous organic polymerwith aryl sulfonic acids in ref. 10 as a catalyst for fructoseconversion to HMF, it also showed poor recyclability dueto desulfonation.12.

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