High-Performance Materials from Bio-based Feedstocks. Группа авторов

Starbon surface, the results of which will be discussed in more detail in the catalysis section. The first approach, from Matharu et al. [23], utilises the surface hydroxyls of the uncarbonised material and of a Starbon‐350 material to attach a complex ligand to the Starbon surface via succinimidyl carbonate functionalisation. This approach resulted in significant loadings (a degree of substitution of 0.33 was achieved for the uncarbonised material), representing an approximately 10% functionalisation of the total hydroxyl population (and therefore significantly more in terms of accessible surface groups).

      In an alternative approach, Carneiro et al. [24] used the unsaturation present in a Starbon‐700 to first brominate and then displace the bromine atoms to build a bis‐oxazolidine structure onto the surface. The lability of the bromines suggests that alkene functionality is common on such materials, as the alternative bromination of (electron‐rich) aromatics, which are also present, might occur even under the very mild conditions used, but this would lead to less active sites for functionalisation. A 10% drop in the C content of the brominated material suggests a substantial amount of bromination took place, and loadings of the catalyst (0.5 wt% Cu) are significant.

      3.2.3 Applications

       3.2.3.1 Catalysis

      A range of catalytic applications of Starbon materials has been investigated, mainly with sulphonated Starbons, where strong acid functionality has been included via reaction with sulphuric acid.

       3.2.3.1.1 Sulphonated Starbon in Esterifications

      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].

       3.2.3.1.2 Sulphonated Starbon in Dehydrations

Millán et al. [26] have demonstrated the use of Starbon‐450‐SO₃H in the dehydration of xylose to furfural in biphasic conditions. They found that, in a single aqueous phase, conversion of xylose was readily achieved, but furfural was further degraded, meaning selectivity and yield of furfural was low (38% yield and 65% selectivity at best). The addition of cyclopentyl methyl ether (CPME) as a water‐immiscible co‐solvent was found to aid the reaction considerably by rapidly removing furfural from the aqueous solution, leading to optimal performance of 96% conversion and 72.5% selectivity after one hour at 200 °C (Figure 3.6). Interestingly, given the rather challenging conditions, reusability (measured at 175 °C for 18 hours) was excellent, with no change in performance over 3 runs.

       3.2.3.1.3 Sulphonated Starbon in Amide Synthesis

      Starbon‐400‐SO₃H has been used to form amides in excellent yields from a range of anilines and acetic acid under microwave irradiation [27].

      After 15 minutes, at a maximum temperature of 130 °C, yields approaching quantitative were achieved for a range of aliphatic amines and anilines. Interestingly, the simplest primary amines (C₅, C₆, and C₁₀) tended to give slightly poorer yields, despite them being typically more active than aromatic systems. As expected in amide formation, selectivity was excellent. Other aliphatic acids were also screened with very good results, with secondary acids being slightly less reactive on steric grounds. Other strong solid acids were investigated with significantly lower conversions.

The same catalyst was utilised by Mesquita et al. in the acid‐catalysed Ritter reaction of steroids, again forming amide products [28]. Starting from epoxy steroids and acetonitrile, the authors found that Starbon‐400‐SO₃H catalysed the ring opening, and trapping of the resultant carbocation via hydration gave the resultant product as shown in Figure 3.7.