et al. [20].
| Wavenumber (cm−1) | Vibration | Appearance |
|---|---|---|
| 1040 | S=O symmetric stretch | Sulphonated |
| 1610 | Aromatic C=C stretch | Both |
| 1719 | C=O stretch | Both |
| 3000 | CH2/CH3 | Unsulphonated |
| 3440 | C(O)OH/PhOH stretch | Both |
Solid and liquid 13C NMR data were also collected for the sulphonated catalyst, Starbons‐300‐2–H2SO4–ClSO3H–TEPO, using triethylphosphine oxide (TEPO) as a probe molecule to find the relative acid strength [20]. This was done by dissolving 0.05 g of material in 5 ml of 1 M TEPO in hexane, stirring for one hour, followed by drying at 40 °C under vacuum. The resulting solid was dissolved in deuterated chloroform. Having managed to get better resolution of signals in the liquid state, bands at 128.94, 131.05, and 132.54 ppm were assigned to aromatic carbons while band 167.94 ppm was assigned to carboxylic carbons. The 31P MAS NMR (magic angle spinning nuclear magnetic resonance) of the same material showed the shift of a signal from 53.89 ppm in the unsulphonated Starbon to 85.30 ppm in the sulphonated material, which was attributed to the change from hydroxyl to sulphonic groups, whose interaction with TEPO confirmed strong Brønsted acid character.
Elemental analysis of the original sulphonated Starbon gave a sulphur content of 1.9% (Starbon‐400‐SO3H), 1.4% (Starbon‐650‐SO3H), and 1.3% (Starbon‐750‐SO3H) [21], whereas Aldana‐Pérez et al. reported a higher sulphur content of 3.30% by X‐ray photoelectron spectroscopy (XPS), possibly due to the much longer duration of sulphonation in their method [20].
3.2.2.2 N‐Starbons
3.2.2.2.1 Methods of N Incorporation
Attard et al. first reported a method for the N‐doping of Starbon in 2018 [12] by using chitosan (a natural N‐containing polysaccharide) or ammonia as nitrogen sources. Three methods were described. Route A involved the gelation of chitosan and alginic acid together in the formation of Starbon. Routes B and C involved injecting ammonia into the Starbon at the aerogel stage and into the final product, respectively, where ammonia was adsorbed onto the surface of the material. These N‐doped materials are collectively referred to as N‐Starbon.
Another study used the impregnation of Starbon with monoethanolamine [22]. This involved mixing Starbon derived from both corn and potato starch with a known amount of amine in a solution of ethanol and water. The product was subsequently dried at 105 °C.
3.2.2.2.2 Characterisation
In the account by Attard et al. [12], N‐doped Starbon produced by either the inclusion of chitosan or ammonia displayed similar characteristics. A significant amount of nitrile was observed in these materials, something that was known to be unusual in other forms of N‐doped carbons [12]. The TGIR results showed that chitosan released ammonia upon heating to low temperatures, whereas N‐Starbon did not. It was concluded that the amine groups in chitosan may become trapped in the form of nitriles, the presence of which was confirmed by a DRIFT absorbance band at 2210 cm−1. The nitrile was assumed chemically bonded to the Starbon as it remained present after generous washing with ethanol and water.
The presence of nitriles was further explored by preparing a range of materials and it was found that nitriles only form at carbonisation temperatures above 300 °C. It was also shown that the combination of chitosan and polysaccharide precursor to Starbon was necessary to retain the mesoporous structure of the N‐doped materials, as chitosan alone collapsed to a microporous structure on carbonisation.
Porosimetry was carried out on N‐Starbon from each synthetic route, showing that N‐doping did not negatively influence mesoporosity. Route A, using a chitosan/polysaccharide combination, resulted in larger pore volumes and surface areas than the ammonia adsorption method. A summary of BET surface area studies and pore volume is shown in Table 3.2.
A large amount of nitrogen was detected in all samples, as confirmed by elemental analysis and XPS, and displayed in Table 3.3.
In a study to produce N‐Starbon for the application of carbon capture, Sreedhar et al. found that higher carbonisation temperatures of 750 °C for a duration of six hours were required to achieve the high surface areas and pore volumes needed [22]. Different weight loadings of monoethanolamine (10, 20, and 30%) were characterised by FTIR, with amine signals being present at 1650–1550 cm−1. Further studies by X ray diffraction (XRD) confirmed that N‐Starbons from both corn and potato starch were amorphous materials with very little difference between them.
Table 3.2 Textural properties of N‐doped Starbon compared with N‐free analogues.
Source: Data from Attard et al. [12].
| Carbonisation temp. (°C) | Material | Mesoporosity (%) | Total pore volume (cm3 g−1) | BET surface area (m2 g−1) |
|---|---|---|---|---|
| 300 | Starbon N‐Starbon‐A N‐Starbon‐B | 88.3 86.7 98.7 | 0.627 0.663 0.645 | 174.2 412.6 240.5 |
| 450 | Starbon N‐Starbon‐A | 84.5 81.9 | 0.515 0.615 | 339.5 448.3 |
| 600 | Starbon N‐Starbon‐A N‐Starbon‐C | 79.8 83.5 92.8 | 0.686 0.750 0.324 | 519.5 519.2 249.5 |
Table 3.3 Nitrogen content of N‐doped Starbons.
Source: Data from Attard et al. [12].
| Carbonisation temp. (°C) | Nitrogen content (wt%) | C/N ratio | ||
|---|---|---|---|---|
| CHN | XPS | CHN | XPS | |
| 300‐A | 6.4 | 4.9 | 9.2 | 15.3 |
| 300‐B | 6.8 |
