Photocatalysis, belonging to the advanced oxidation processes (AOPs), is a potential new transformation technology for lignin derivatives to value added products (e.g., phenol, benzene, toluene, and xylene). Moreover, lignin represents the only viable source to produce aromatic compounds as fossil fuel alternative. This review covers recent advancement made in the photochemical transformation of industrial lignins. It starts with the photochemical reaction principle followed by results obtained by varying process parameters. In this context, influences of photocatalysts, metal ions, additives, lignin concentration, and illumination intensity and the influence of pH are presented and discussed. Furthermore, an overview is given on several used process analytical methods describing the results obtained from the degradation of lignin derivatives. Finally, a promising concept by coupling photocatalysis with a consecutive biocatalytic process was briefly reviewed.
Photocatalytic degradation experiments were done with lignin sulfonate in a circulating reactor. Catalysts (TiO 2-P25-SiO 2 + Pt, TiO 2-P25-SiO 2, TiOSO 4 _30.6 wt%, ZnO + TiO 2-P25-SiO 2), synthesized via the sol-gel method, were immobilized on porous glass support material. A comparative study was done regarding morphology of coatings, degradation rates, reaction rates, dissolved carbon (DC), formation of peaks, and fluorescence of products formed from the photocatalytic degradation of lignin sulfonate obtained from a local paper plant. Through simultaneous reaction-extraction pathways applying dialysis filtration and highly porous polystyrene divinylbenzene adsorbent resin (HR-P) for solid-phase extraction (SPE), an attempt was been made to isolate smaller molecules produced from photocatalytic degradation. Moreover, relatively high lignin sulfonate (0.5 g/L) concentrations are used in the reactions. UV-Vis spectroscopy revealed a faster reduction in the concentration values for the aliphatic moiety compared to the aromatic moiety. Peaks were observed by both fluorescence spectroscopy and high-performance liquid chromotography (HPLC), suggesting the production of new substances and fluorophores.
The synthesis of immobilized catalyst on porous glass support material via the sol-gel route is reported. TiO2-P25-SiO2+ Pt, TiO2-P25-SiO2, TiOSO4_30.6 wt%, and ZnO + TiO2-P25-SiO2catalysts were synthesized and a comparative study is done regarding morphology of coatings, degradation rates, reaction rates, dissolved carbon (DC), formation of peaks, and fluorescence of products formed from the photocatalytic degradation of lignin sulfonate obtained from a local paper plant. Through simultaneous reaction-extraction pathways applying dialysis filtration and highly porous polystyrene divinylbenzene adsorbent resin (HR-P) for solid phase extraction (SPE), an attempt has been made to isolate smaller molecules produced from photocatalytic degradation. Moreover relatively high lignin sulfonate (0.5 g/L) concentrations are used in the reactions. UV-Vis spectroscopy revealed a faster reduction in the concentration values for the aliphatic moiety compared to the aromatic moiety. Peaks were observed by both fluorescence spectroscopy and HPLC suggesting the production of new substances and fluorophores.
A comparative study between ten different photocatalytic active coatings was done. The effectiveness and photocatalytic activity of the coatings were studied by degradation experiments of methylene blue (MB) dye under UV light illumination. The reactor design consisting of sintered glass packed in a borosilicate tube placed between two planar dielectric barrier discharge lamps (Osram Planon) is reported for the first time. The coatings consisted of either titania, silica, or zinc on sintered borosilicate glass. The advantage of sol-gel in catalyst preparation was exploited to combine catalyst to act as cocatalyst. TiO2-P25 widely applied in suspension systems was effectively immobilized on sintered glass support with the aid of tetraethylorthosilicate (TEOS) solution which acted as support material. Results indicated that TiO2-P25+SiO2, TiO2-P25+SiO2+Pt, and TiOSO4_30,6wt% films showed highest degradation rates close to 100% after 90 min illumination with degradation rates exceeding 50% after 30 minutes. TTIP+Pt showed lowest degradation rate.
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