much as any other material tested, and approximately 4 times as much as a pectin‐derived P550 material. Interestingly, XPS studies on the loaded samples showed that, while most of the sulphur loaded was in the original oxidation state, there were significant quantities of oxidised sulphur species present (S(IV) and S(VI)). Small amounts of reduced sulphur species were noted on SO2 adsorption, along with more prevalent oxidised species. The presence of high oxidation state N species in very low quantities on the surface of the materials was suggested as a potential catalytic route to oxidation of the S species. The significant amounts of inorganics in the materials may also play a role, especially in the high‐temperature materials, where the inorganics are concentrated via loss of organics – S800 has only 2.6% inorganics, A800 a significant 8.8%, and P800 (from pectin) a huge 28.6%, much of it potassium.
Dura et al. published results relating to the impressive ability of Starbon to adsorb carbon dioxide, which is an important topic for control of CO2 levels and also to concentrate CO2 prior to its conversion into valuable chemicals such as cyclic carbonates [35]. The authors compared the predominantly mesoporous Starbon (68–92% of the total pore volume was in the mesopore range) with Norit‐activated carbon which is c. 75% microporous. They utilised both starch‐based and alginic‐acid‐derived materials in their study, and these were prepared over a wide temperature range from 300 to 1200 °C.
Pressure swing adsorption data were collected over 5 cycles at 10 bar pressure. This demonstrated that, in both the starch‐ and alginic‐acid‐derived materials, CO2 adsorption increased from low levels for the S300 and A300, reaching a maximum at the S800 and A800 materials. Beyond the 800 materials, there was a slight drop in the case of the starch materials, but a substantial reduction in adsorption in the alginic acid series. The optimal adsorption capacity was 40% (S800) and 50% greater (A800) than for Norit. Adsorption kinetics were the same for all three adsorbents, with saturation after 30 minutes at 5 bar pressure or after 10 minutes for 10 bar pressure. Desorption under atmospheric pressure took 20 minutes in all cases.
Simultaneous thermal analysis was used to measure adsorption/desorption under flowing conditions alternating between CO2 and nitrogen. This indicated rapid and complete reversibility of adsorption over many cycles, as well as allowing estimation of the enthalpy of adsorption. The values (between −14 and −18 kJ mol−1) indicate that physisorption is the predominant mechanism. Regression analysis was used to develop an empirical equation to correlate the adsorption to textural properties and led to the equation:
In other words, the adsorption behaviour is strongly linked both to micropore adsorption and to a combination of micropores and mesopores, suggesting that the combination of mesopores and micropores allows excellent transport to the adsorption sites as well as some mesopore adsorption.
Very importantly, CO2 to nitrogen selectivity was measured for Norit, S800 and A800 using mixed gas streams at 298 and 323 K. What was striking here was that selectivity to CO2 was much higher for the Starbon materials (14.0–20.3) than for Norit (5.4 at 298 K and 4.0 at 323 K) meaning that adsorption in real situations would benefit from the use of the Starbon materials.
3.2.4.2 Adsorption or Organics from Solution
Parker et al. [36] demonstrated that Starbon materials have a very impressive ability to adsorb a range of small phenolic molecules from water. They chose the relatively hydrophobic and low oxygen content Starbon‐800 materials as their adsorbents, and compared starch‐derived systems with alginic‐acid‐derived systems. Both materials exhibited equally good adsorbency behaviour over a range of phenols in an aqueous solution (Table 3.4). The starch‐derived S800 had around twice the surface area of the alginate A800, but much of the ‘extra’ surface area was present as micropores and ultramicropores, which likely contribute less to the adsorption behaviour. It is tempting to think that the process is therefore as simple as being directly related to a similar mesopore surface area; however, a more detailed analysis of the relevant adsorption isotherms indicated that somewhat more complexity is involved.
Table 3.4 Collected data for the adsorption of phenols on alginic‐acid‐derived Starbon (A) and on starch‐derived Starbon (S).
Source: Data from Parker et al. [36].
| Phenol | ΔG (kJ mol−1) | ΔH (kJ mol−1) | ΔS (J mol−1K−1) | Qo (mg g−1) | n | |||||
|---|---|---|---|---|---|---|---|---|---|---|
| A | S | A | S | A | S | A | S | A | S | |
| H‐ | −0.13 | −0.91 | 1.7 | −3.8 | 6.0 | −9.7 | 40.3 | 100 | 0.03 | 5.7 |
| 2‐Me | −2.8 | −1.6 | −16 | 4.2 | −4.3 | 20 | 0.08 | 94.6 | 3.6 | 5.8 |
| 2‐F | −2.0 | −2.5 | −11 | −4.1 | −32 | −5.4 | 206 | 131.8 | 0.01 | 6 |
| 3‐NH2 | −0.42 | −0.57 | 8.9 | −1.8 | 32 | −4.0 | 241 | 101.2 | 0.01 | 6.6 |
| 4‐OMe | −1.2 | −2.8 | 37 | −4.9 | 128 | −6.9 | 146 | 118.6 | 3.8 | 14.1 |
First, using Langmuir and Freundlich isotherms, it was postulated that the surfaces have significant heterogeneity,
