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The final stage of pyrolysis involves a slow temperature ramp from room temperature to the desired temperature. This drives off water, a complex bio‐oil (typically formed between 250 and 400 °C) and smaller gases (CO₂, CO, etc.) which are the dominant species at higher temperatures. With increasing temperature, the nature of the surface changes from polysaccharide‐like, gradually increasing the C : O ratio from 1 : 1 to around 30 : 1 at 800 °C. The increasingly carbon‐like nature of the material underlies profound changes in chemistry. While these changes are complex and not fully understood, globally it can be said that, for starch at least, initial dehydrations occur which lead to unsaturation (C=C and C=O functionality) leading to an enhanced reactivity. These groups react further, cross‐linking and releasing small organic molecules into the gas phase to form a broadly aromatic matrix [9]. This is somewhat of an oversimplification as significant aliphatic functionality is always seen by ¹³C NMR. This may be due, in part at least, to the early dehydration of sugars, which can lead initially to –CH₂–C(=O)– units, although the reactivity of such groups suggests that more structural features of greater stability are initially formed. After the initial substantial mass loss, further heating causes much more gradual mass loss, predominantly in the form of small molecules such as water, CO, and CO₂.

Figure 3.2 Evolution of porosity as a function of water : butanol composition. Eutectic points are at 25 wt% t‐BuOH and at 95 wt% t‐BuOH.

Source: Original data from Borisova et al. [16].

Thus, the chemical functionality of the surface of the Starbon materials represents a continuum of changing functionality ranging from hydroxylic to highly functional (hydroxylic, unsaturated) to a more aromatic, low oxygen structure.

The mesoporosity of the materials compared to the total (meso + micro) porosity are shown in Figure 3.3 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.