Читать книгу High-Performance Materials from Bio-based Feedstocks - Группа авторов - Страница 63

As discussed earlier, reaction of Starbon with concentrated sulphuric acid has been achieved [18, 19] and good loadings of sulphonic acid/sulphuric acid esters have been obtained. These materials have proved themselves to be very promising solid acids in a range of reactions as described next.

Esterification of succinic acid was shown to be possible, even in water‐rich environments. Succinic acid, an important platform molecule, can be produced in high concentration (up to c. 12 wt%) by fermentation of waste polysaccharides [25]. Clark and Budarin focused on valorising such fermentation broths containing succinic acid, as recovery of the acid itself from such complex media is extremely difficult and generates considerable waste. Conversion to di‐ethyl succinate, a liquid with low miscibility with the broths, is an elegant solution, but esterifications in water are theoretically likely to be unsuccessful due to reversibility. Interestingly, Clark et al. showed that sulphonated Starbon was a very effective esterification catalyst, giving the diester in excellent yields (95% after eight hours’ reaction), and under conditions where a range of other acid catalysts failed to give more than modest quantities of diester, even after considerably longer periods of reaction [18, 19].

As can be seen from Figure 3.4, there is a significant rate dependence on the Starbon carbonisation temperature, and the optimum activity is not a simple function of acid site density. Textural properties are relatively similar throughout, and therefore there may be a significant role being played by the nature of the surface beyond the active sites themselves. One potential factor is hydrophobicity – pores that are prone to exclude water (even partially) will allow the esterification equilibrium to shift toward the ester product, whereas hydrophilic pores will do the opposite. Low acidity in the bulk liquid will ensure that related esters will hydrolyse only very slowly.

Further bio‐derived acids (itaconic, fumaric, and levulinic) were also successfully converted to their esters/diesters under similar conditions [17]. In the same paper, a range of benzylic alcohols were also esterified with acetic acid under microwave irradiation within one minute, with phenol also reacting, albeit 10 times more slowly.

Oleic acid was successfully esterified to give ethyl oleate using sulphonated Starbon‐300 [20]. The blank reaction and Starbon‐300 gave essentially no conversion, but Starbon‐300 sulphonated with sulphuric acid gave a 45% conversion to the ester after 24 hours. Starbon‐300 sulphonated with a mixture of ClSO₃H and sulphuric acid shows considerably better activity and gave conversions of 90% after 10 hours. The rationale for the enhanced activity was that the latter catalysts had a significantly higher loading of acidic groups, which lead to approximately 20‐fold greater turnover frequencies. However, the comparisons shown in Figure 3.5 suggest that, as mentioned earlier, there are other factors at play. The nonaqueous reaction medium (in contrast to the succinic acid system as mentioned earlier) means that hydrophobicity/hydrophilicity is unlikely to be as relevant in this case. It may well be that the strongly acidic nature of the sulphonation medium alters the surface by, e.g. dehydration or cross‐linking as well as via sulphonation, and that this also plays a role in the activity of the materials.

Figure 3.4 Diesterification of succinic acid in aqueous ethanol as a function of Starbon preparation temperature.

Source: Data from Clark et al. [19].

Figure 3.5 Esterification of oleic acid with sulphonated Starbon materials.

Source: Data from Aldana‐Pérez et al. [20].