# Sulfonic Resins as Catalysts for the Oxidation of Alcohols with H\({}_{2}\)O\({}_{2}\)/KBr
## 2 Results and Discussion  
The substrate for this study was a mixture of methyl 9-hydroxy- and 10-hydroxy-stearate (**1a + 1b**, represented as methyl 9(10)-hydroxystearate), obtained from the hydrogenolysis of methyl 9,10-epoxystearate as previously described [7]. First of all, the homogeneous method was tested using two sulfonic acids of different strengths, namely _p_-toluenesulffonic acid (_p_-TosOH, pK\({}_{\text{a}}\approx-2.8\)[25]) and trifici (trifluoromenthanesulfonic) acid (TOH, pK\({}_{\text{a}}=-5.9\)[25]), under conditions analogous to those described in the literature [24]: room temperature, 24 h, CH\({}_{2}\)Cl\({}_{2}\)/H\({}_{2}\)O (9:1) as solvent, 1.5 eq 60% H\({}_{2}\)O\({}_{2}\), 20 mol% KBr, and 20 mol% (_p_-TosOH) or 10 mol% (_T_OH) of acid. The results of the two reactions were very similar (conversion = 97-98%, Table 1 entries 8-9), with methyl 9(10)-oxostearate (**2a + 2b**) being the only product detected (Scheme 1). \begin{table}
\begin{tabular}{c c c c c c} \hline
**Entry** & **Sulfonic Solid** & **mol\%** & **mg** & **t (h)** & **Conversion (\%) 2** \\ \hline
1 & Aquivion PW79S & 20 & 39.4 & 24 & 66 \\  & & & & 48 & 81 \\
2 & Nafion NR50 & 20 & 112.4 & 24 & 56 \\  & & & & 48 & 72 \\
3 & Aquivion P98 & 20 & 102.0 & 6 & 54 \\  & & & & 24 & 90 \\
4 & Amberlyst 15 & 20 & 21.7 & 6 & 52 \\  & & & & 24 & \textgreater{}99 \\
5 & Dowex 50W\(\times\)8 & 20 & 22.2 & 6 & 75 \\  & & & & 24 & \textgreater{}99 \\
6 & Dowex 50W\(\times\)2 & 20 & 22.2 & 24 & 96 \\
7 & Dowex 50W\(\times\)2 & 10 & 11.1 & 24 & 49 \\
8 & Triflic acid & 10 & 7.5 & 24 & 97 \\
9 & _p_-Toluenesulffonic acid & 20 & 17.2 & 24 & 98 \\
10 & Dowex 50W\(\times\)8 3  & 20 & 22.2 & 24 & 26 \\  & & & & 48 & 44 \\
11 & Dowex 50W\(\times\)8 4  & 20 & 22.2 & 48 & 16 \\ \hline \end{tabular}
\end{table} 
Table 1: Oxidation of methyl 9(10)-hydroxystearate (**1a + 1b**) with KBr/H\({}_{2}\)O\({}_{2}\)/sulfonic solid 1. In view of these results, this procedure could possibly be improved via the substitution of the homogeneous sulfonic acid with a sulfonic resin, acting as a recoverable heterogeneous acid. Three perfluoroalkylsulfonic resins were used: Aquivion PW79S, Aquivion P98 and Nafion NR50. Although the chemical nature of these three resins is similar, Nafion possesses longer side perfluorinated chains ([PERSON], 2016). Aquivion PW97S is commercialized in powder form, and we observed some recovery problems due to the formation of slurries that were difficult to filter ([PERSON], 2016). These problems were previously solved by using Aquivion P98, which is commercialized in the form of pellets with -SO\({}_{2}\)F groups and must be activated by treatment with KOH/DMSO and then H\({}_{2}\)SO\({}_{4}\)([PERSON], 2016); this was adapted from the method involving the hydrolysis of the Nafion precursor ([PERSON], 2016). In the same way, three poly(styrene-divinylbenzene)sulfonic resins were tested: the macroreticular Amberlyst-15 (with 20% DVB crosslinker), and the gel-type Dowex 50W\(\times\)8 and 50W\(\times\)2, with an 8 and 2% DBV crosslinker. The functionalization with sulfonic groups was determined by titration, and in general, the results were in good agreement with the data provided by the suppliers (see Section 3). Thus, the weight of catalyst added to each oxidation reaction was calculated using this functionalization to obtain 20 mol% (0.1 mmol of sulfonic groups in the reaction scale of 0.5 mmol of substrate, unless otherwise is indicated in the tables). The oxidation results are shown in Table 1.  
While the reaction was not complete after 48 h with Aquivion PW79S (81% conversion, entry 1) and Nafion NR50 (72%, entry 2), Aquivion P98 led to 90% conversion after only 24 h (entry 3), showing the importance of the structural and textural properties of the perfluoroalkylsulfonic resin. Despite their weaker acidity, the results were similar or slightly better than those obtained for arylsulfonic resins, with total or nearly total (96%) conversion after 24 h, and a faster reaction in the case of Dowex 50W\(\times\)8 (entry 5).