SO3H-functionalized porous organic polymer with amphiphilic and swelling properties: A highly efficient solid acid catalyst for organic transformations in water
Graphical abstract
Introduction
Performing organic reactions in water has received considerable attention due to the intrinsic advantages of using water as an environmentally benign solvent [[1], [2], [3]]. However, performing organic reactions in water with hydrophobic organic substrates often encounters low reaction efficiency because of the high mass transfer resistance [4,5]. On the other hand, heterogeneous catalysis is preferred over homogeneous catalysis for industrial application due to its obvious advantages, such as high productivity, less waste, and easy process control [[6], [7], [8], [9]]. However, the development of highly active and water-compatible heterogeneous catalysts that allow the nice contact with both the hydrophobic and hydrophilic reactants is still a challenging task [[10], [11], [12], [13]]. To address this challenging topic, many attempts have been made, while successful examples are limited to building emulsion systems [14], and utilizing amphiphilic catalyst support [[10], [11], [12],15].
Porous organic polymers (POPs), which feature high framework stability, designable chemical functionalities, larger surface area, and tunable surface wettability, have attracted great interest because of their potential to combine the advantages of both heterogeneous and homogeneous catalysis [7,9,16,17]. Recently, a series of porous organic polymer-based catalysts have been developed via a solvothermal and free radical polymerization method [[18], [19], [20], [21]]. Previous studies demonstrated that porous catalysts synthesized by such polymerization method featured excellent swelling properties [9,[18], [19], [20], [21]], which allow the easy access of catalytic sites and high enrichment of the reactants, thus endowing the solid catalysts with high catalytic activities that could be comparable to, or even higher than their homogeneous analogues [[18], [19], [20]].
The vast majority of POPs were constituted by hydrophobic aromatic networks, endowing the POPs with hydrophobic surface property [[18], [19], [20],22,23]. However, hydrophobic POPs show poor dispersion in organic-aqueous biphasic liquid mixture, and they are easy to adsorb organic substrate and stay in organic phase [24], thus restricting their catalytic applications in water. To solve this problem, the polymers can be designed to be amphiphilic [15]. Porous polymers with amphiphilic properties benefit the miscibility of both the hydrophilic and hydrophobic reactants [21]. However, with traditional post-grafting method, it is not easy to introduce a surfactant-like moiety into the framework of porous organic polymers. Fortunately, the recent emergence of poly (ionic liquid)s [21,[25], [26], [27]] gave us a hint that it is possible to introduce an ionic-liquid moiety into the framework of porous organic polymers, endowing, to some extent, the porous polymers with phase-transfer ability.
On the other hand, Brønsted acids have been widely used as efficient catalysts for various water-mediated organic reactions, including many hydrolysis and hydration reactions [2,[28], [29], [30], [31], [32]]. However, performing aqueous organic reactions over traditional Brønsted acids (e.g., H2SO4, HCl and sulfonic acid) have some drawbacks, such as strong corrosivity and large volumes of wastes [33,34]. To tackle these problems, various solid acids have been developed to replace the liquid acids [35,36]. Nevertheless, typical solid acids, such as zeolites, ion-exchange resins, and sulfonated porous carbons [11,33,34], usually exhibited inferior catalytic activities in water due to their poor surface amphiphilicity. Therefore, it is valuable to develop water‐compatible solid acids, which show high catalytic activity for water-mediated organic reactions.
To develop highly efficient solid acids for aqueous organic reactions, and as part of our interest in preparation of amphiphilic catalysts [23], herein, an amphiphilic and swelling SO3H-functionalized porous organic polymer was successfully prepared. Interestingly, the prepared solid acid displayed excellent catalytic performances in water for the hydrolysis of cyclohexyl acetate and hydration of phenyl acetylene.
Section snippets
Materials
All materials were of analytical grade and used as received. 4-Bromostyrene, phosphorus trichloride, 4-vinylbenzyl chloride (90%), sodium p-styrene sulfonate (SBS), sodium bisulfate, 4-dodecylbenzenesulfonic acid (DBSA), 2,2′-azobis (2-methylpropionitrile) (AIBN), N,N-dimethylformamide, cyclohexyl acetate, phenyl acetylene, benzenesulfonic acid were purchased from Energy Chemical (Shanghai) Co. Ltd. Amberlyst-15 was got from Sigma-Aldrich Company. Concentrated sulfuric acid was available from
Characterization
P (QP-BSA)-n catalysts, where n stand for the molar ratio of SBS with QP, were successfully prepared in quantitative yields via the free radical copolymerization of SBS and QP, followed by ion-exchange of P (QP-SBS)-n with 1 M sulfuric acid (Scheme 1). The obtained solid acids were characterized by a series of physicochemical methods including FT-IR, solid-state 13C NMR, XPS, elemental analysis, N2 adsorption-desorption, SEM, TEM, TGA, and contact angle measurement.
Fig. 1 shows the FT-IR
Conclusion
In summary, amphiphilic and swelling SO3H-functionalized porous organic polymers were successfully prepared by a solvothermal and free radical copolymerization of vinyl-functionalized surfactant-like monomer and vinyl benzenesulfonate monomer. The resultant materials featured high adsorption capacities towards organic substrates in water, acting as remarkable solid acid catalysts with good reusability for aqueous organic reactions. The remarkable catalytic performances could be attributed to
CRediT authorship contribution statement
Yizhu Lei: Conceptualization, Methodology, Writing - review & editing. Maomin Zhang: Investigation, Formal analysis, Writing - original draft. Guojun Leng: Investigation, Formal analysis. Chao Ding: Validation, Investigation. Youming Ni: Supervision.
Declaration of competing interest
The authors claim no conflicts of interest.
Acknowledgements
We acknowledge the financial support by the National Natural Science Foundation of China (21763017); the National Natural Science Foundation of Guizhou Province (qian ke he ji chu [2018]1414); the Scientific and Technological Innovation Platform of Liupanshui (52020-2018-03-02 and 52020-2017-02-02); the Academician Workstation of Liupanshui Normal University (qiankehepingtairencai [2019]5604 hao); and the Fund of Liupanshui Normal University (LPSSYZDXK201602).
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