LITMIR.BIZ

Figure 3.11 Selective acylation of a diol by a supported Cu‐bis‐oxalodinone/Starbon catalyst.

Part of the rationale for choosing Starbon as a support for the titania catalyst is that the TiO₂ particles need to be nanoscale in order to have sufficient surface area to function efficiently. This raises two issues. First, separation of the nanoparticles from water is a major challenge, and second, agglomeration of the nanoparticles is a significant issue, which reduces the activity and efficiency of the catalytic system.

      In their work, Colmenares et al. [32] utilised ultrasound‐assisted formation and deposition of titania onto thermally pre‐treated Starbon, avoiding the use of surfactants, and adding to the green credentials of the process. The materials were then dried and re‐calcined to produce a hybrid material containing 25 wt% of TiO₂. The authors suggest that carboxylic groups on the surface act as nucleation sites for the titania particles, which is further discussed in a second paper [33]. Interestingly, only the anatase form of titania was formed on Starbon, whereas similar amounts of anatase and rutile were produced on Norit, a standard activated carbon. This is important because anatase is the catalytically active form. The crystallinity was also found to be excellent on Starbon, meaning few defects are present – defects reduce the efficiency of the photocatalytic process by facilitating electron‐hole recombination. Graphene oxide seemed to produce almost no discernible crystalline material. Titania particle sizes for Starbon‐ and Norit‐derived materials were largely similar and in the range 20–30 nm. Loading of the inorganic phase resulted in a reduction in surface area by a factor of 1.6 for Starbon, and 1.3 for graphene oxide, but the microporous Norit suffered a sevenfold decrease in surface area.

Photochemical mineralisation of phenol was carried out using 50 ppm solutions of phenol in water, a Hg lamp (365 nm) and without bubbling of air through the solution (i.e. passive diffusion of oxygen into the solution). The Starbon materials had substantially greater activity than either Norit or graphene oxide‐derived materials, with the mineralisation rate constant being approximately 3 times higher for Starbon‐titania than for either of the other two systems. This was ascribed mainly to the pure, highly crystalline anatase phase formed on Starbon, as well as the mesoporosity of the system that allowed much better transport of phenol. Runs utilising Starbon itself also indicated that Starbon’s ability to adsorb phenol from the aqueous solutions was very good, meaning that it can pick up phenol from water and deliver it to the catalytic sites efficiently. Physical mixtures of Starbon and commercial anatase (Evonik P25) likewise showed excellent results, having approximately 50% higher rate constant than the ultrasound synthesised materials. Rate curves for the various systems indicated that Starbon‐based systems reduced phenol levels to 0.3% of the initial (i.e. to 0.3 ppm, well below the 1 ppm target) compared to 0.5 ppm for Norit and 0.7 ppm for graphene oxide. Not only were the final levels lower, but the rate of removal was faster as well.

      3.2.4 Adsorption Processes

      One of the classic uses of high‐surface‐area materials is the adsorption of dissolved or gas‐phase species, either with a view to concentrating and winning high‐value products, or to decontaminating waste streams, polluted air or environments. Starbon has been used widely in all these application areas, showing considerable promise in several instances, which are discussed next.

       3.2.4.1 Adsorption of Gases

      Milescu et al. [34] have published data on the adsorption of three toxic gases on starch, alginic acid, and pectin‐based materials and found some very interesting behaviour. The authors studied the room temperature adsorption of the gases from 5000 ppm in air mixtures, and looked at ammonia, hydrogen sulphide, and sulphur dioxide (separately). They found excellent adsorption in many cases, with materials outperforming standard activated carbons such as Norit, and also giving impressive results compared with a range of other adsorbents discussed in the literature.

      For ammonia, adsorption was best with low‐temperature‐activated alginic acid material (in particular, the uncarbonised A000) with performance dropping to very low levels with the higher temperature materials. This correlates very well with the availability of acidic functionality (via the alginic acid‐repeating units) which is highest at carbonisation temperatures <200 °C. This functionality is lost above this temperature, and with it, the ability to adsorb ammonia drops dramatically. For both the starch and the pectin‐derived materials, there is no inherent acidity and the best performance for these materials is seen at 300 °C activation. This activity likely correlates well with a high level of functionality, including aldehydic/ketonic groups formed by dehydration of vicinal diols, and also possibly by ring scission. Such groups will readily form imines, trapping the nitrogen. Evidence for nitrile formation is also strong, possibly via dehydration of primary amides, from acidic functionality that can also develop during carbonisation. Further carbonisation leads to the gradual loss of this rich functionality, leading to a reduced ability to adsorb ammonia.

For both the sulphur‐containing gases, adsorption ability was dominated by the highest temperature materials (in this case, 800 °C was the maximum used). Indeed, for hydrogen sulphide, the pectin‐derived materials were exceptionally active, adsorbing 20 times as 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

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