Photocatalytic Based Degradation Processes of Lignin Derivatives

Table 3

Parameters, analytical methods, and results from different work groups.

Reference	Parameter studied	Analytics	Result
MacHado et al. [42]	Role of hydroxyl radicals, irradiation of lignin in the absence and presence of photocatalyst TiO2 and H2O2	Ultraviolet-visible (UV-Vis) spectroscopy, ionization absorption spectroscopy (IAS), size exclusion chromatography (SEC)	A sharp decrease in the phenolic content observed for reactions involving direct photolysis; SEC: a reduction of almost 50% in the average molecular weight of lignin equal to 1.4 kD after 90 min of irradiation
Ksibi et al. [47]	Irradiation of lignin in the absence and presence of photocatalyst TiO2-P25	UV-Vis spectroscopy, ^13C-nuclear magnetic resonance (NMR) solid state, total ion gas chromatography (TIC), induction coupling plasma (ICP), chemical oxygen demand (COD)	56% degradation rate with TiO2 catalyst after 420 min, reaction time; ethyl acetate-extractable products showed vanillin, vanillic acid, palmitic acid, biphenyl structures, and 3,4,5-trimethoxy benzaldehyde; presence of magnesium and calcium ions; COD removal is higher for the initially low concentrations of lignin solution
Kansal et al. [48]	Catalyst dose, pH, oxidant concentration, initial substrate concentration, ZnO catalyst in slurry and immobilized mode	UV-Vis spectroscopy, COD	Optimum catalyst dose is 1 g/L; optimum oxidant concentration:  M; gradual decrease of absorption peak indicating decomposition of organics; COD removal is higher for the initially low concentrations of lignin solution
Dahm and Lucia [49]	Catalyst dose, illumination intensity	UV-Vis spectroscopy, total organic carbon (TOC), capillary ion electrophoresis analysis (CIA)	Gradual decrease in absorption peak indicating decomposition of organics; optimal catalyst dose of 10 mg/m; higher illumination intensities correlated well with higher initial degradation rate; 74% disappearance of TOC
Portjanskaja and Preis [50]	Influence of ferrous ions, N-doped catalyst effect,; sprayed catalyst on support and submersed catalyst	COD, UV-Vis spectroscopy, biochemical oxygen demand (BOD), colorimetric measurement at 570 nm	Addition of Fe^2+, up to 2.8 mg/L leads to 25% increase in photocatalytic efficiency; sprayed catalyst exhibited 1.5 times higher efficiency than the one attached by submersion; negligible effect of N-doped catalyst; increase of aldehyde concentration over reaction time; neutral media was most beneficial for biodegradability; 80% of free phenols removed under neutral conditions
Tanaka et al. [51]	Catalyst loading, lignin concentration, illumination time	UV-Vis spectroscopy, TOC, gel permeation chromatography (GPC), ^1HNMR, Fourier transformation infrared (FTIR) spectroscopy	FTIR measurement revealed a fast transformation of aromatic moiety present in lignin; characteristic bands of aromatic rings, methoxy, and aliphatic side chains; decrease in TOC values over time; decrease in degradation rate with increase catalyst dosage. But after catalyst threshold value is attained, catalyst increase causes a decrease in degradation rates.; FTIR peaks are shifted towards lower molecular weight region after photocatalysis
Tonucci et al. [33]	Test of catalytic systems to obtain fractions with reduced degrees of polymerization,; comparison of thermal and photochemical reactions	^1HNMR, gas chromatography-mass spectroscopy (GC-MS)	POMs are less selective when used as photocatalysts and no appreciable bleaching of the solution was seen when POM was used as thermal catalyst; derived chemicals from experiment: vanillin, hydroxyl methoxy-acetophenone, coniferyl alcohol, coniferyl aldehyde, methanol, formic acid, acetic acid, and sometimes small amounts of C-2 and C-3 alcohols
Miyata et al. [24]	Examination of cell wall structure of lignin (wood flour) before and after photocatalysis	GC-MS, scanning electron microscopy (SEM), ^1HNMR	High delignification activity, delignification confined to the surface of lignin; derived chemicals experiment: vanillin
Shende et al. [17]	Test of combined action of bio- and photocatalytic systems	UV-Vis spectroscopy, GC-MS, X-ray dispersive energy spectroscopy and X-ray diffraction analysis (EDX, XRD), SEM	Detection of following chemicals: acetyl guaiacol, 4-ethoxymethyl-2-methoxy phenol, methoxyphenyl oxime, guaiacol, succinic acid, acetyl guaiacol, vanillic acid, and vanillin
Tian et al. [52] and Pan et al. [53]	Test of combined action of electro-and photocatalytic systems	UV-Vis spectroscopy, FTIR, SEM, X-ray (EDX), high performance liquid chromatography (HPLC), COD	Detection of the following chemicals: carbonyl functionality, vanillin, and vanillic acid
Awungacha Lekelefac et al. [54], [55]	Comparison of degradation rates by different catalyst	HPLC, fluorescence and UV-Vis spectroscopy, SEM, TOC	UV-Vis results reveal faster degradation of the aliphatic moiety compared to the aromatic moiety of ligninsulfonate obtained from paper waste water. Peaks observed during HPLC analysis. Some of the peaks produced after photocatalysis had fluorescence signals. This suggests the production of new substances and fluorophores. Coatings produced through Sol-Gel procedures are stable and can be used many times.