for the three materials derived from alginic acid, starch, and pectin [9–11].
What can be seen from a comparison of the three types of materials is that, overall, the pore volumes remain fairly constant over a wide temperature range. However, in the case of alginic‐acid‐derived materials, there is an increase at lower temperatures followed by a drop and then relative constancy. For room temperature pectin, there is evidence of increasing porosity. The total volumes are broadly constant over all three material types. The most variation is in the difference between total and mesopore volumes, indicating the extent of microporosity. Alginic‐acid‐derived materials have virtually no microporosity at any temperature and pectin a modest amount. In contrast, starch‐derived materials display very little microporosity at low pyrolysis temperatures, but from 300 °C onwards, the materials develop a considerable amount (up to c. 30%).
Critical to maintaining the porosity of the materials is that the pyrolysis and cross‐linking reactions occur before the aerogel melts or softens considerably. For alginic acid and pectin, the more reactive nature of the polysaccharides’ structure – acid and ester groups – means that this is not an issue, while for starch, a strong Brønsted acid (typically p‐toluene sulphonic acid) must be added.
3.2.2 Derivatisation
Since the development of Starbon as a high‐surface‐area mesoporous carbon, multiple reports have been published on its use as solid catalyst support. Starbon itself has a very modest catalytic activity – as will be seen next, there is little more than some weakly acidic groups on the surface. Therefore, the addition of more powerfully active groups (e.g. sulphonic acid sites or basic amines) is likely to provide a much more active material for catalytic applications, while N functionalised materials may also improve adsorbency of metals and function as ligands for catalytically active surfaces.
3.2.2.1 Sulphonation
3.2.2.1.1 Method of Sulphonation
The first published method of Starbon sulphonation was described by Budarin et al. [17]. In this synthesis, sulphonation was carried out on Starbon having been pre‐carbonised at 400 °C (Starbon‐400) as this was considered to have the best ratio of hydrophilic and hydrophobic functional groups. The Starbon‐400 was heated for four hours at 80 °C in a suspension of H2SO4 (99.999% purity) at a ratio of 10 ml acid to 1 g Starbon, and the sulphonated material was subsequently washed with distilled water until no further acid was leached. This was followed by conditioning in toluene for four hours at 150 °C and then in water for three hours at 100 °C. The resultant solid acid was dried overnight in an oven at 100 °C. Subsequent publications from the same authors noted a reduction in the aqueous conditioning temperature to 80 °C [18, 19].
Figure 3.3 Comparison of porosity of the three major Starbon types.
Source: Original data adapted from Budarin et al. [9], White et al. [10], White et al. [11].
A more recent publication expanded on this synthesis by variation of the sulphonating agent and suspension times [20]. In this report, Starbon‐300 was chosen as the base material. The first synthesis was similar to that previously reported, except that a ratio of 0.2 g Starbon to 10 ml H2SO4 (96%) was used, and a suspension time of 15 hours. Further syntheses were reported using a mixture of ClSO3H/H2SO4 as the sulphonating agent, in ratios of 2 : 10 and 3 : 10 by volume, respectively. In these cases, sulphonation was carried out at reflux for five hours.
3.2.2.1.2 Characterisation of Sulphonated Material
Sulphonated Starbon has been analysed thoroughly in terms of porosity, elemental composition, and SO3H loading: all characteristics affecting the validity of these materials as solid acid catalysts. In the original publication, the SO3H loading on Starbon‐400 was reported as 0.5 mmol g−1 as determined by thermogravimetry coupled to infrared spectroscopy (thermogravimetry‐infra red [TGIR]) [17]. Further investigation showed that the loading remained in this region for preparation temperatures of 600 °C and below. Starbon‐650 and above resulted in a lower concentration of active acid sites (0.3–0.4 mmol g−1) [18, 21]. This was further corroborated by pyridine absorption experiments showing that the acidity of sulphonated Starbon did not change considerably with preparation temperatures of 600 °C or less. However, stronger Lewis acidity was observed at lower temperatures [18, 19]. Later, Aldana‐Pérez et al. reported a –SO3H content of 1.2, 1.8, and 2.3 mmol g−1, and a total number of acid sites as 8.0, 8.2, and 10 mmol g−1, respectively, for Starbon‐300 sulphonated by H2SO4 for 15 hours, a mixture of 2 : 10 ClSO3H/H2SO4 for five hours, and a mixture of 3 : 10 ClSO3H/H2SO4 for five hours [20]. In this case, acidity was measured by potentiometric titration.
Transmission electron microscopy (s) and scanning electron microscopy (SEM) imaging of Starbon have shown that sulphonation did not considerably change the particle size or morphology of the material, which is amorphous after functionalisation, but the structure was prone to cracking at higher carbonisation temperatures as is true for the unsulphonated Starbon [18]. Starbon‐400 was shown to remain predominantly mesoporous (pore size 5–15 nm) after sulphonation, although the mean pore diameter and surface area decreased. As with the original Starbon, microporosity increased with preparation temperatures above 500 °C [19, 21]. A homogeneous distribution of elements was also observed [20]. Additional N2 adsorption studies showed that the average pore diameter of sulphonated Starbon was 8–12 nm [21]. Sulphonated Starbon‐400 had a Brunauer‐Emmett‐Teller (BET) surface area of 386 m2 g−1 and pore volume of 0.62 cm3 g−1. Aldana‐Pérez et al. reported a decrease in BET surface area on sulphonation from 163 m2 g−1 for Starbon‐300 to 66, 75, and 77 m2 g−1, respectively, for Starbon‐300 sulphonated by H2SO4 for 15 hours, a mixture of 2 : 10 ClSO3H/H2SO4 for five hours, and a mixture of 3 : 10 ClSO3H/H2SO4 for five hours [20].
Diffuse reflectance infra red (DRIFT IR) analysis of the original sulphonated Starbon‐400 showed little change in surface structures before and after sulphonation [18, 21]. Peaks in the region of 1300–600 cm−1 were attributed to –SO3H groups confirming the incorporation of sulphur into the Starbon structure, although differentiation between O–SO3H and C–SO3H was not possible. Aldana‐Pérez et al. performed fourier Transform infra red (FTIRP) photoacoustic spectra – an infrared technique particularly well suited to study dark samples – of the sulphonated and presulphonation Starbon, as shown in Table 3.1 [20]. In particular, this confirmed the presence of –SO3 groups in the sulphonated Starbon by the appearance of the S=O band at approximately 1040 cm−1. Additionally, there is a reduction in the CH2/CH3 signal at approximately 3000 cm−1, which was attributed to oxidation of aliphatic carbons to carboxylic acids, although oxidation to ketones should also not be excluded in Starbon‐300, where significant carbohydrate character (in particular –CH(OH)– groups) should remain.
Table 3.1 Summarised IR data of a range of sulphonated and unsulphonated Starbon.
