# Advances in Organic and Organic-Inorganic Hybrid Polymeric Supports for Catalytic Applications
## 2 Organocatalysed Non-Asymmetric Reactions
### Bronsted Acid Catalysed Reactions
#### 2.1.1. Esterification Reactions  
Recently, [PERSON] et al. reported the synthesis of poly-ionic liquid (PIL) acid **3** and its use as a catalyst in esterification reactions (Scheme 1) [3]. The acid **3** was obtained by polymerization of ionic liquid monomer **2** with styrene and ethyleneglycol dimethacrylate in the presence of azobisiso-butyronitrile (AIBN). Ionic liquid monomer **2** was prepared in two synthetic steps: (a) reaction between 4-vinylpyridine and 1,4-butylsulfone; (b) treatment of intermediate **1** with trifluoromethane-sulfonic acid. Under optimized conditions, the poly-ionic liquid acid **3** was then used in the synthesis of various esters at appropriate reaction temperatures and times, and quantitative yields were achieved. The activity and stability of the catalyst were maintained after five recycles. Furthermore, the continuous formation of methyl methacrylate was investigated using a fixed bed reactor and the methyl methacrylate product was obtained in high yield (>96%). In 2013, [PERSON] et al. reported the use of the macroporous phenolsulfonic acid-formaldehyde resin **4** as a heterogeneous polymeric acid catalyst in the direct dehydrative esterification of alcohols and carboxylic acids. The corresponding esters were obtained in 91%-95% yield without removal of water (Scheme 2) [4]. In 2016, the resin **4** was employed with excellent results (yields of 91%-95%) in the esterification of oleic acid with various alcohols and its recyclability was evaluated [5]. The intrinsic physical properties of resin **4** allowed increasing the yield by shifting the equilibrium toward esterification without the need for a water-removal procedure. In fact, the slightly hydrophilic meso-/macroporous structure of the resins could absorb fairly hydrophilic substrates and transform them into more hydrophobic esters, which in turn could readily be excluded from the catalyst surface. After esterification, the resin was filtered, washed with ethanol and treated with concentrated sulfuric acid in order to regenerate the active sulfonic acid sites. The catalyst was recyclable up to at least 30 times without any significant loss in its activity. Thanks to its surface properties and porous structure, resin **4** exhibited higher catalytic activity than a variety of commercially available heterogeneous  
Figure 1: Synthesis of poly ionic liquid acid **3** and its catalytic activity in esterification reactions. solid-acid catalysts tested in the esterification of oleic acid and methanol (Montmorillonite K-10, Amberlyst 15, Amberlite IRA 400, sulfonated ZrO\({}_{2}\), H-ZSM-5, PTSA). The same year, poly(styrene sulphonic acid) was anchored onto polyhedral oligomeric octavinyl-silsesquioxanes (o-POSS) [6], thus providing novel hydrophobic POSS-derived solid acid catalysts for esterification of glycerol, an unavoidable side product of the biodiesel production process. These POSS-derived solid acid catalysts, denoted as **5a-d** (Scheme 3), were prepared through two synthetic steps: (a) free radical copolymerization of octavinylsilsesquioxanes [7; 8] with sodium \(p\)-styrene sulfonate varying their molar ratio and (b) treatment of the obtained copolymers with H\({}_{2}\)SO\({}_{4}\). POSS-derived solid acid catalysts **5a**, **5b**, **5c**, **5d** were formed with molar ratios of POSS to Ph-SO\({}_{3}\)H of approximately 1:1, 1:2, 1:3, and 1:4, respectively. All synthesized catalysts presented high conversions (above 95%) with remarkable combined selectivity (above 99%) towards glyceryl diacetate (DAG) and glyceryl triacetate (TAG) in the esterification of glycerol with acetic acid at 120 \({}^{\circ}\)C. After each cycle, the catalyst was separated from the reaction system by filtration and reused for five-run tests without any decrease of catalytic activity. The application of **5b** was extended to the esterification of various substrates (Scheme 3), obtaining high conversions (81%-99%). **Scheme 3. Synthesis of POSS-based poly(styrenesulphonic acid) catalysts **5a-c** and their use in esterification.