3.2.2.3 Derivatisation via Bromination and Activation of Hydroxyl Functionality
According to the functionality of the Starbon materials, particularly (but not exclusively) the lower temperature materials should have a significant number of –OH functionalities as well as several alkene and similar sites. Such materials clearly have the potential for functionalisation too, although less work has been published in utilising these functional groups as anchor points for groups (e.g. epoxides which could be further elaborated). Sreedhar’s approach is likely to bind the amines to the surface via aldehydes and other active groups on the surface, reactions that should be readily achieved under mild conditions [22]. Two other articles indicate the potential for further functionalisation of the 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
