Читать книгу High-Performance Materials from Bio-based Feedstocks - Группа авторов - Страница 67

Only a very few examples of supported metal complexes and their catalytic activity have been reported, perhaps surprising, given the large numbers that exist with silica as support. Interestingly, little has been done to extend the well‐established organo‐silica chemistry beyond the initial studies by Doi et al. [30] who used expanded starch (i.e. non‐pyrolysed Starbon) as a support. Two approaches that successfully attach complex catalytic species to the surface of Starbon are available, and are discussed next.

Matharu et al. [23] published details of an Fe‐NHC catalytic system (NHC = N‐heterocyclic carbene), which they tethered to the surface of a starch‐based Starbon‐350 and also to the surface of the precursor‐expanded starch (Figure 3.9). They utilised a multistage ligand synthesis, anchored the ligand to the surface, and then finally attached the metal species [23].

The synthesis of the heterocyclic ligand containing amine functionality as an anchor was carried out. Separately, Starbon surface was functionalised with a succinimyl carbonate group, following an adapted literature procedure, where the toxic dimethyl formamide (DMF) solvent was replaced with propylene carbonate, a safer alternative to dipolar aprotics [31]. Finally, the ligand was bound to the functionalised Starbon surface. Anchoring of the ligand system was achieved by reaction of the functionalised Starbon with the amine pendant on the ligand moiety. The degree of substitution achieved for the succinimidyl carbonate grafting was 0.33, approximating to 1 in 10 hydroxyls being substituted (in the case of the expanded starch, this is likely to be somewhat higher for the Starbon‐350, although the complexity of the structure is much greater with a wider range of functionalities). Given the extensive H‐bonding and steric hindrance pertinent to the majority of the hydroxyls in such polysaccharides, this is a reasonably significant degree of substitution, and led to metal centre loadings of 0.26 and 0.3 mmol g⁻¹, well within the range of loadings achieved for highly porous silicas.

Catalytic activity was very promising in the dehydration of fructose to 5‐hydroxymethyl‐2‐furaldehyde (HMF), with the expanded starch catalyst slightly outperforming the Starbon‐350 material (86% vs. 81% yield after 0.5 hour at 100 °C). Reuse was also very good, with consistent performance over 5 runs, and no discernible leaching of iron. Given the simpler route to the expanded starch material, it is clear that this is the catalyst of choice here.

While Matharu’s approach utilised the abundant hydroxyl groups on the surface of the material, Silva and co‐workers have described the synthesis of a chiral bis‐oxazoline catalyst on the surface of a Starbon‐700 material utilising the abundant unsaturation present in the higher temperature materials [24]. Given the low O content of these higher temperature materials, the typical functionalisation routes involving surface hydroxyls (i.e. reaction with silane esters such as (RO)₃SiR′ [30] and the succinimidyl carbonate route described earlier) are unsuitable. To circumvent this problem, Carneiro et al. [24] utilised a bromine functionalisation, reacting bromine with double bonds on the surface, to give a 1,2‐dibromo‐functionalised surface (Figure 3.10). As the ligand to be attached is a diol, the formation of pairs of anchor sites close together is particularly attractive.

Figure 3.9 Synthesis of an N‐heterocyclic carbine‐based catalyst on the Starbon surface.

The resultant materials were used in the kinetic resolution of 1,2‐diphenylethane‐1,2‐diol. While activity was high, the enantiomeric excess of the process was significantly lower than that obtained by a solution‐phase process (Figure 3.11). This was attributed to a low concentration of catalytic groups on the surface of the catalyst, and the high conversion suggests non‐chiral‐active sites may also be present.