Because of the photon contribution to the reach of the reaction activation energy, photocatalytic reactions can be easily carried out in mild conditions at ambient temperature, typically using water as a solvent and without resorting to toxic oxidizing or reducing agents. Photocatalytic processes are thus considered a very interesting option in the context of green chemistry. Moreover, photocatalytic reactions can be potentially carried out directly exploiting natural solar radiation as a photon source, further reducing the environmental impact of the implemented process
[57][105]. Several applications of photocatalysis have been studied, including water splitting for hydrogen production
[58][106], carbon dioxide reduction
[59][107] and implementations such as advanced oxidation processes (AOPs) for air
[60][108] or water
[61][109] depollution and sustainable chemical process development
[62][63][110,111].
3.5.1. Photocatalysis of Pulping Black Liquor
Prado et al.
[64][120] reported an example of photocatalytic lignin depolymerization directly performed on the pulping black liquor (i.e., without lignin separation) for the recovery of derived compounds. The authors used two different black liquors deriving respectively from organosolv pulping (with 60% ethanol at 180 °C for 90 min, solid/liquid ratio 1:4) and from ionic liquid (IL) pulping (with [Bmim][MeSO
4] under microwave, 200 °C for 30 min, solid/liquid ratio 1:10). The photocatalytic oxidation was carried out adding the photocatalyst (sol–gel synthesized TiO
2) to the pulping black liquor (2 g/L for organosolv-derived black liquor and 4 g/L for the IL-derived one). After photocatalysis, lignin was separated via acid precipitation and centrifugation while the liquid fraction was extracted with ethyl acetate to obtain the derived oil. The analysis of the precipitated lignin demonstrated a better degradation from the photocatalysis of organosolv black liquor in comparison to the photocatalyzed IL black liquor. This was attributed to the different pH values of the two reaction media (4.8 for organosolv vs. 7.0 for IL). In the extracted oil, the authors observed lignin-derived compounds and also furfural and other sugar degradation products. The yields of lignin-derived phenolics were definitely higher for the organosolv pulping medium. One of the main recovered compounds in the extracted oil was syringaldehyde, which accounted for up to 14.2% (
w/
w) of the oil fraction for organosolv black liquor and 1.2% for IL black liquor (both obtained after 0.5 h photocatalysis). The maximum recovered vanillin was 0.9% (organosolv) and 0.1% (IL), always as the oil fraction extracted after 0.5 h of photocatalytic treatment. Longer photocatalysis reaction times resulted in lower yields. The remarkably lower yields from IL black liquor photocatalysis were attributed to the formation of nitrogen-containing compounds derived from the photocatalytic degradation of the ionic liquid components. Although this is interesting from the point of view of process economy (no lignin separation must be performed), the reported results indicate that the direct photocatalytic treatment of pulping black liquors could be critical in most aspects, including the sub-optimal condition dictated by the pulping medium itself (e.g., pH) and the presence of critical amounts of extraneous compounds (e.g., sugars and pulping reagents) that can interfere with the photocatalytic process, both lowering the yields and generating undesirable byproducts.
3.5.2. Photocatalysis of Separated Lignins
The problems encountered in the direct photocatalysis of black liquor can be overcome by operating on lignins purified from the pulping medium. The application of photocatalysis to the synthesis of vanillin from sodium lignosulfonate (SLS) has been recently reported by Qiu et al.
[65][121]. The authors used a mesoporous, high specific surface area titanium dioxide photocatalyst obtained from the calcination in air of MIL-125, a Ti-based metal–organic framework. The photocatalytic conversion experiments were carried out with Xenon lamp irradiation (6 h) under air at room temperature. Of the three different catalysts prepared, calcinated at 400, 500 and 600 °C, respectively, the one calcinated at 400 °C demonstrated the highest specific surface area (174 m
2/g) and the highest vanillin yields (2.1 mg per g of SLS).
Ahmad et al.
[66][122] described the formation of vanillin and 4-hydroxybenzaldehyde in the photocatalytic degradation of lignin obtained via the delignification of rice straw residues. The lignin extraction was performed in 1 M NaOH solution at 150 °C for 1 h. The photocatalytic alkaline lignin extraction was performed in a stirred reactor using TiO
2 or ZnO-suspended photocatalyst particles. While the lignin underwent a steady degradation during the photocatalytic process, the concentration of both vanillin and 4-hydroxybenzaldehyde demonstrated, after an initial induction phase where none of the two products were detected, a rise and a following decrease after they reached the maximum value. This was attributed to the activation of the concomitant photocatalytic degradation of both products, confirmed by the authors who measured the degradation kinetics of pure vanillin and 4-hydroxybenzaldehyde in separate experiments using the same photocatalytic conditions. The maximum lignin degradation rate was found for ZnO with 2 g/L catalyst loading and, in these conditions, a higher vanillin concentration was also recorded (51.2 mg/L at 8 h process time). The 4-hydroxybenzaldehyde was formed in lower amounts, with the maximum concentration (20.4 mg/L) recorded using the TiO
2 photocatalyst at 1.5 g/L catalyst loading at 10 h process time.
3.5.3. Photocatalysis with Combined Processes
A possible strategy for enhancing the overall performance of photocatalytic oxidation is the combination with other complementary approaches developing an integrated process with optimized performances. Few works involving the specific production of vanillin are available in the literature on this topic. An example targeted at lignin oxidation was proposed by Tian et al.
[67][128] which described a combination of photocatalytic and electrochemical processes for the degradation of Kraft lignin. The authors studied various combinations of a Ti/TiO
2 nanotube electrode and a Ti/TaO
5-IrO
2 electrode in electrochemical and/or photocatalytic operation modes. The counter electrode was a Pt foil, while an Ag/AgCl electrode was used as a reference electrode. The authors studied the kinetics of the lignin degradation process measuring the lignin concentration with UV spectrophotometry (295 nm). All studied processes resulted in pseudo-first-order kinetics. The highest rate constant (0.021 min
−1) was demonstrated by the combined photocatalytic/electrochemical process obtained and by connecting both electrodes at +600 mV (vs. the Ag/AgCl reference electrode) under UV irradiation. The separate electrochemical oxidation, performed with the Ti/TaO
5-IrO
2 electrode held at +600 mV without UV irradiation resulted in less than half the degradation rate constant (0.009 min
−1).
The combined photocatalytic and biocatalytic degradation of Kraft lignin was studied by Kamwilaisak et al.
[68][116] using TiO
2 and laccase. The authors studied both a combined dual-step process (photocatalysis followed by biocatalysis) and a combined single-step process (photocatalysis and biocatalysis running concurrently in the same batch). Separate photocatalysis and biocatalysis processes were performed as references. All batches were carried out at 50 °C and pH 5 for 24 h using 1 g/L lignin and 3 g/L TiO
2 (for photocatalytic processes) and 2.5 units/mL laccase (for biocatalytic processes). The photocatalytic processes were carried out under UV-A light by fluorescent tubes at 5.3 µE cm
−2 s
−1 (~1.7 W/cm
2 at 370 nm). The authors studied also the effect of the addition of H
2O
2 (5.55 g/L) in all experiments. The lignin degradation yield of the different systems was biocatalysis << photocatalysis ≈ combined single-step < combined dual-step. The addition of H
2O
2 increased the degradation yield of all the processes with the sole exception of the combined single-step. Several degradation products were identified, mainly, organic acids and carbohydrates. Vanillin was identified but not quantified. This study indicated the potentialities of combined strategies but also the possible problems due to the interference of different processes running simultaneously in the same reactor.
3.5.4. Perspectives for Vanillin Production by Controlled Photocatalytic Lignin Degradation
Although currently still at the experimental laboratory stage, the production of vanillin by the controlled photocatalytic oxidation of lignin appears to be a potentially very interesting goal, particularly considering the increasing focus on the sustainability of industrial chemical processes. However, several problems must still be resolved, and more research is needed for an effective deployment of the technology. Despite the overall mild operating conditions, the typically high amount of hydroxyl radicals generated in the photocatalytic process can establish a quite reactive environment that possibly causes a significant degradation of the generated vanillin. Catalytic systems with enhanced selectivity are needed for an effective production of the target molecule, reducing competitive degradation pathways of the original lignin and of the product itself. In this regard, it is interesting to note that the mild operating conditions potentially enable the use of tailored on-process separating systems such as membranes or adsorbing resins in order to separate the neo-formed vanillin from the reaction medium, thus avoiding possible product degradation. Original approaches on the reactor construction may therefore be interesting research topics
[69][130].
4. Conclusions
Vanillin has been known, for a long time, for its properties as a flavoring agent for food, beverages and pharmaceuticals. Moreover, its belonging to the large class of phenolic compounds has pushed the research more recently on its antioxidant properties and its use as a natural building block for many biobased polymers. In this framework, vanillin obtained from a biobased waste residue such as lignin offers various significant environmental and economic benefits including waste reduction and natural resource recycling. The development of new technologies and processes for the production of vanillin from lignin thus represents a promising approach to valorize an underutilized renewable resource and also address the growing global demand for natural vanillin. Chemical, enzymatic, microbial and photochemical conversion methods have been described here and have shown great potentialities in providing a more sustainable and cost-effective source of vanillin. However, it appears that highly significant challenges remain in terms of vanillin yield, selectivity, and process efficiency.