Journal of Materials Chemistry A PAPER View Article Online View Journal | View Issue A porous Brønsted superacid as an efficient and durable solid catalyst† Cite this: J. Mater. Chem. A,2018,6, 18712 Qi Sun,‡*a Kewei Hu,‡a Kunyue Leng,b Xianfeng Yi,c Briana Aguila, d Yinyong Sun, c Anmin Zheng, c Xiangju Meng,a Shengqian Mad and Feng-Shou Xiao ‡*a The development of catalysts able to assist industrial chemical transformations is a topic of high importance. In view of the versatile catalytic capabilities of acid catalysts, extensive research efforts are being made to develop porous superacid materials with a high density of accessible active sites to replace molecular acid catalysts. Herein, we report the rational development of a porous Brønsted superacid by combining important elements that target high strength acidity into one material, as demonstrated by grafting the sulfonic acid group onto a highly fluorinated porous framework, where the acid strength and stability are greatly enhanced by an electron-withdrawing environment provided by the polymer backbone, reminiscent of that seen in Nafion® resin. In addition, the densely arranged acid groups that are confined in the three-dimensional nanospace facilitate the transfer of hydrons, thereby further increasing the acidity. By virtue of the pore structure and strong acidity, this system exhibits Received 7th July 2018 excellent performance for a wide range of reactions, far outperforming commercial acid resins under Accepted 12th September 2018 repeated batch and flow reaction conditions. Our findings demonstrate how this synthetic approach may DOI: 10.1039/c8ta06516k instruct the future design of heterogeneous acid catalysts with advantageous reaction capabilities and rsc.li/materials-a stability. Introduction superacidity.17 In addition, the lack of suitable porous materials with unique properties to compensate such an inevitable loss The development of porous materials offers great improve- aer immobilization leads to the acidity of the resultant solids ments in catalyst design for efficient large-scale chemical being much weaker than their liquid structural analogues. processes.1–6 In particular, solid acid catalysts,7–12 as represented Therefore, the development of nanostructured strongly acidic Published on 13 September 2018. Downloaded by University of South Florida 10/15/2018 4:25:05 PM. by zeolites, have played an indispensable role in the petroleum catalysts, particularly those with superacidity (acids stronger renery industry for continuous throughput and efficient than 100% sulfuric acid), is still in the portfolio of researchers separation.13,14 Yet, due to the lack of high performance solid and such materials will pave the way toward alleviating the acid catalysts, a signicant number of acid-catalyzed reactions various existing burdens imposed by current synthetic tech- still mainly rely on molecular acids.15,16 Transferring the high nologies pertaining to the environment and the overall cost.18,19 activity of these catalysts into their solid states remains chal- To tackle the challenges of designing high performance acid lenging because the low graing degree of acid groups and the materials and especially those with the superacidic character, rigidity of solid supports prevent them from cooperating in the we were inspired by peruororesinsulfonic acids, such as transfer of protons, an essential feature in the manifestation of Naon®,20 well-known superacid catalysts, and numerous developed elegant systems on the journey to achieve efficient 21–28 aKey Lab of Applied Chemistry of Zhejiang Province, Department of Chemistry, porous acid catalysts. From the established knowledge that Zhejiang University, Hangzhou 310028, China. E-mail: [email protected]; the strength of acidity is primarily determined by the nature of [email protected] the acid group (e.g. –SO3H>–COOH), the electronic properties bSchool of Chemical Engineering and Technology, Harbin Institute of Technology, of the backbone and the density of acid groups in the materials, Harbin 150001, China wherein reducing the distance strengthens the acidity by cState Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, enhancing their cooperation, it is believable that combining all Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China these essential elements into one material may give rise to dDepartment of Chemistry, University of South Florida, 4202 E. Fowler Avenue, Tampa, a superior catalyst. We thus postulate that if the sulfonic acid Florida 33620, USA groups can be graed onto a highly uorinated porous frame- † Electronic supplementary information (ESI) available: Experimental methods work with a preset position for modication, a catalyst with and supporting gures and tables. See DOI: 10.1039/c8ta06516k a high density yet readily accessible acid sites that are arranged ‡ These authors contributed equally to this work. 18712 | J. Mater. Chem. A,2018,6,18712–18719 This journal is © The Royal Society of Chemistry 2018 View Article Online Paper Journal of Materials Chemistry A in electron-withdrawing three-dimensional nanospaces will be well as approaching or exceeding those observed for H2SO4 as achieved. The strong electronegativity of the uorine atoms not a catalyst. Most notably, this catalyst, with its hierarchical only makes the molecule less susceptible to chemical and porosity, is highly active and durable under xed bed condi- electrophilic attack, but also stabilizes the sulfonic acid anions tions, allowing for the elimination of solvent and easy separa- and thereby facilitates the leaving of the protons on them. This tion of the reaction products, thereby holding great promise to is anticipated to afford new acid materials with fast reaction replace the existing technologies. kinetics and high acid strength, in conjugation with great stabilities, which surmount the prime issues of low efficiency Materials and methods encountered by Naon® as a result of the low surface area and low density of acid groups,29 as well as the low acid strength of Materials many other solid catalysts.30 To fulll these tasks, porous Commercially available reagents were purchased in high purity organic polymers (POPs)31–33 showing great promise as versatile and used without purication. Solvents were puried according candidates for acid catalysis were used. Due to their amenable to standard laboratory methods. THF and DMF were distilled synthesis, high surface areas, and exceptional chemical over LiAlH4 and CaH2, respectively. stability, they have emerged as one of the prime alternatives to traditional porous materials, such as active carbon and silica Catalyst preparation gels, with great potential to address the ongoing energy and Synthesis of the uorinated porous organic polymer (FPOP). environmental challenges.34–40 In this contribution, we demon- 2,20,4,40,6,60-Hexauoro-3,30-divinyl-1,10-biphenyl (1.0 g) was strate that the sulfonic acid groups graed on a highly uori- dissolved in THF (10 mL), followed by addition of azobisiso- nated porous polymer framework act as a catalyst in a range of butyronitrile (AIBN, 25 mg). The mixture was transferred into an industrially important transformations with a combination of autoclave at 100 C for 24 h. The title material was obtained high activity and selectivity that are superior to those of all aer being washed with CH Cl and dried under vacuum. benchmark heterogeneous acid catalysts tested. 2 2 Synthesis of the sulfonated FPOP (FPOP-SO H). Fuming In developing versatile POP-based acid catalysts, we sought 3 sulfonic acid (20%, 40 mL) was added dropwise to a 100 mL to employ 2,20,4,40,6,60-hexauorobiphenyl as a building block ask containing 500 mg of FPOP. The mixture was stirred at given that it is highly uorinated yet molecularly designable. room temperature for 48 h, and then 50 C for another 48 h, The modulation property of this compound ensures its feasi- aer which the mixture was poured into ice water and washed bility to be constructed into a highly porous framework and to with an excess of water until the pH of the ltrate reached be endowed with abundant active sites. Moreover, the perfor- around 7. mance of the acid groups, in turn, could benet from the electron-withdrawing polymer backbone (Scheme 1). In view of materials synthesis for practical applications, free-radical Characterization induced polymerization of vinyl-functionalized monomers is The gas adsorption isotherms were collected on a surface area particularly attractive with these considerations: in addition to analyzer ASAP 2020. The N2 sorption isotherms were measured ff the excellent monomer tolerance and cost-e ective synthesis, at 77 K using a liquid N2 bath. Scanning electron microscopy Published on 13 September 2018. Downloaded by University of South Florida 10/15/2018 4:25:05 PM. the adapted polymerization conditions allow adjustment of the (SEM) images were collected using a Hitachi SU 1510 or Hitachi pore structure and introduce hierarchical porosity for the SU 8000. TGA was carried out on a Q50 thermogravimetric 41,42 enhancement of mass transfer. We thus surmised that analyzer under a N2 atmosphere at a heating rate of À modifying sulfonic acid moieties onto such a framework could 10 C min 1. IR spectra were recorded on a Nicolet Impact serve as a means to obtain a very efficient solid catalyst for 410 FTIR spectrometer. X-ray photoelectron spectroscopy (XPS) a wide range of acid catalyses. Indeed, in liquid phase spectra were recorded on a Thermo ESCALAB 250 with Al Ka processes, the resultant catalyst with higher intrinsic acidity irradiation at q ¼ 90 for X-ray sources, and the binding energies exhibits outstanding properties that are many times better than were calibrated using the C 1s peak at 284.9 eV. CHNS elemental those of the sulfonic acid groups graed on an analogous analyses were performed on a Perkin-Elmer series II CHNS nonuorinated porous polymer and commercial acid resins, as analyzer 2400. Diffuse reectance ultraviolet-visible (UV-vis) spectra were measured with a PE Lambda 20 spectrometer, 1 and BaSO4 was used as an internal standard sample.