High-Performance Materials from Bio-based Feedstocks. Группа авторов
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Название: High-Performance Materials from Bio-based Feedstocks

Автор: Группа авторов

Издательство: John Wiley & Sons Limited

Жанр: Химия

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isbn: 9781119655626

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СКАЧАТЬ catalyst on the surface of a Starbon‐700 material utilising the abundant unsaturation present in the higher temperature materials [24]. Given the low O content of these higher temperature materials, the typical functionalisation routes involving surface hydroxyls (i.e. reaction with silane esters such as (RO)3SiR′ [30] and the succinimidyl carbonate route described earlier) are unsuitable. To circumvent this problem, Carneiro et al. [24] utilised a bromine functionalisation, reacting bromine with double bonds on the surface, to give a 1,2‐dibromo‐functionalised surface (Figure 3.10). As the ligand to be attached is a diol, the formation of pairs of anchor sites close together is particularly attractive.

Schematic illustration of synthesis of an N-heterocyclic carbine-based catalyst on the Starbon surface.

       3.2.3.1.6 Photocatalytic Processes

      A further aspect of Starbon catalysis relates to the photocatalytic decomposition of water pollutants. Colmenares and colleagues have recently published work [32] that illustrates the excellent activity of Starbon‐based titanium dioxide photocatalysts for the total oxidation of phenol, one of the more toxic of water‐borne pollutants.

Schematic illustration of preparation of a chiral bis-oxalolidinone catalyst via surface bromination/displacement. Schematic illustration of selective acylation of a diol by a supported Cu-bis-oxalodinone/Starbon catalyst.

      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.

      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.