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Article

Photocatalytic Conversion of β-O-4 Lignin Model Dimers: The Effect of Benzylic Ketones on Reaction Pathway

by
Gary. N. Sheldrake
1,
Nathan Skillen
2,
Peter. K. J. Robertson
1 and
Christopher W. J. Murnaghan
1,3,*
1
School of Chemistry and Chemical Engineering, Queen’s University Belfast, Belfast BT7 1NN, UK
2
International Centre for Brewing & Distilling, School of Engineering and Physical Sciences, Heriot Watt University, Edinburgh EH14 4AS, UK
3
Department of Biology, Edge Hill University, Ormskirk L39 4QP, UK
*
Author to whom correspondence should be addressed.
Catalysts 2025, 15(6), 525; https://doi.org/10.3390/catal15060525 (registering DOI)
Submission received: 28 March 2025 / Revised: 19 May 2025 / Accepted: 21 May 2025 / Published: 26 May 2025
(This article belongs to the Section Catalysis for Sustainable Energy)
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Versions Notes

Abstract

:
The conversion of biomass towards value-added and platform chemicals has become the focus of extensive research these past two decades. One of the methods that has been increasingly studied is the use of semiconductor-mediated photocatalysis for biomass conversion. Titanium dioxide has previously been demonstrated to be an effective commercial catalyst for the cleavage of bonds within lignin and also cellulose and hemicellulose. Described herein is the deployment of TiO2 for the cleavage of bonds within two β-O-4 lignin model compounds, one bearing a ketone in the α-position and the other an alcohol. The presence of a ketone in the benzylic position in one of the models had a pronounced effect under photolytic conditions, e.g., in the absence of a photocatalyst but with irradiation present. The subsequent reduction of the benzylic ketone resulted in observed sensitivity towards the irradiation and solely photocatalytic conversion was achieved. In addition, reaction products are proposed, which demonstrate a feasible method for β-O-4 cleavage in native lignin extracts.

Graphical Abstract

1. Introduction

Lignin, a complex aromatic biopolymer, represents one of the most abundant renewable resources on Earth, making up approximately 15–30% of lignocellulosic biomass [1]. The valorization of lignin (a representative portion is shown in Figure 1) into high-value aromatic compounds has garnered significant attention in recent years due to its potential to replace petroleum-based chemicals and contribute to a more sustainable bioeconomy [2,3,4,5,6,7]. Among the various approaches to lignin conversion, Advanced Oxidation Processes (AOPs) such as photocatalysis have emerged as a promising method due to its mild reaction conditions, low-carbon footprint, photoredox-neutral nature, and good functional group tolerance [8]. Photocatalytic conversion of lignin involves the use of light-activated species to ultimately facilitate the cleavage of C-O and C-C bonds between the aromatic units of lignin as a result of the generation of reactive oxygen species (ROS). ROS such as hydroxyl radicals [9] and superoxide radicals [10,11] are produced in situ by reactions at the interface of the catalyst surface and bulk solution. These species are capable of cleaving bonds due to relatively high oxidation potentials of 2.8 eV and 1.3 eV for hydroxyl and superoxide radicals, respectively [12]. The ROS produced from semiconductor photocatalysts have frequently been utilized for the remediation of contaminated water [13,14,15,16,17] and air primarily as a result of their nonselective nature, which subsequently facilitates the attack of bonds present in pollutants and often the cleavage of the weakest within the structure.
Recent advancements in photocatalytic systems have demonstrated the potential for efficient lignin valorization [18,19,20,21]. For instance, Zhang et al. developed a hybrid photocatalytic redox system [22] combining an Ir-based molecular photocatalyst with Pd(OAc)2 to effectively drive the preoxidation of lignin model compounds. Additional innovative approaches include the use of ZnIn2S4 photocatalysts for self-hydrogen transfer hydrogenolysis of lignin models [23] and the development of π-conjugated carbazolic copolymers, which was found to efficiently cleave C-O bonds in lignin models [24]. Studies have also demonstrated the employment of visible light irradiation for the conversion of lignin models with heterogeneous catalysis [18,19,20,21]. In addition, our previous publications have demonstrated increased advancement and understanding of photocatalytic lignin valorization by focusing on more representative lignin models such as β-5 [25] and an oligomeric hexamer compound [26], which are both shown in Figure 2. The work conducted with β-5 was the first to demonstrate photocatalytic degradation of the key lignin linkage, with two studies that showed conversion over commercial TiO2, and, subsequently, over visible-light-activated material C3N4 [27]. An additional study focused on the valorization of a hexamer model lignin compound with multiple functionalities [8]. The presence of 5-5′, β-5, and β-O-4 linkages within the compound provided a more representative model substrate for use in a photocatalytic system. The study provided key insights into the fundamental processes occurring during model compound conversion in relation to bond cleavage of β-5 and β-O-4 and the subsequent formation of reaction intermediates. Interestingly, it also demonstrated that the 5-5′ biphenyl linkage remained intact within the LC-MS identified reaction intermediates, suggesting it was resistant to photocatalytic cleavage. This was further proven by monitoring the impact of photocatalysis on a 5-5′ biphenyl model dimer, which confirmed the C-C bond remained intact due to the calculated bond enthalpy (115 to 118 KJ mol−1).
This report builds on our previous publications to further enhance and support the utilization of photocatalysis for lignin valorization by focusing on the mechanism of selective bond cleavage. The work presented here directly builds upon our previous findings in relation to the hexamer lignin model compound, which was subjected to TiO2-mediated photodegradation. During that study, it was observed that there was significant cleavage in the side chains of the model and the central 5-5′ biphenyl linkage remained intact throughout the process. The majority of the cleavages in that model were focused in the β-O-4 region, which resulted in further cleavages within the molecule. A subsequent question that arose following that work was to what extent the oxidation of the α-position in the β-O-4 linkage impacted the conversion under photolytic and photocatalytic conditions. The data presented here address this point by monitoring the photo-conversion of nonphenolic β-O-4 compounds, which include guaiacyl (G/β-O-4) and a reduced guaiacyl analogue (RG/β-O-4); Figure 2. The full experimental details are available in the Supplementary Materials; however, in brief, TiO2 (0.5 g L−1 P25) was deployed under irradiation from a low-power (~5 W) 370 nm UV-LED in a reaction solution containing a model substrate in a suspension of 50% acetonitrile (CH3CN) in H2O. The degradation of the substrates and formation of intermediates and products was monitored using UV-Vis spectroscopy, HPLC, and LC-MS.

2. Results

2.1. Photo-Degradation of G/β-O-4 and RG/β-O-4

The β-O-4 bonding pattern is abundant in naturally formed lignin and, as a result, has frequently been used as a model compound [28,29,30]. Alongside this, the presence of a carbonyl at the α-position in the β-O-4 unit has also been reported [31]; therefore, the model compounds reported here represent a realistic approach towards consideration for raw lignin conversion. Two synthesized β-O-4 models (G/β-O-4 and RG/β-O-4) were used in this study to monitor the impact structural differences had on degradation. In doing so, this work has further expanded our existing portfolio of lignin model photocatalytic systems and enhanced our understanding of the fundamental mechanism taking place.
The impact of photocatalytic, photolytic, and dark control conditions on the degradation of G/β-O-4 and RG/β-O-4 is shown in Figure 3. A photocatalytic and photolytic response was noted for G/β-O-4 (Figure 3a), while, for RG/β-O-4 (Figure 3b), photocatalysis appeared to be primarily responsible for the observed degradation. It was noted through the implementation of control reactions that the presence of the carbonyl at the α-position in G/β-O-4 showed an increased sensitivity under photolytic conditions, i.e., in the absence of a catalyst but presence of irradiation.
Regarding G/β-O-4, the data shown in Figure 3a are also supported by observed changes in the UV-Vis spectra (shown in Supplementary Materials Figure S15), which demonstrated a significant increase in absorption in the 250 nm range under both photocatalytic (a) and photolytic conditions (b), owing to the potential formation of reaction intermediates. These observations suggested different processes were occurring under photocatalytic and photolytic conditions and the reactions were not proceeding through the same pathway. Within the literature, it has been reported that a Norrish reaction and transfer hydrogenation reaction is responsible for the degradation under UV-only conditions for similar functionalities [32,33,34]. This would also suggest that, under photocatalytic conditions, the mechanism was a combination of both photocatalysis ROS oxidation and UV photolysis. Upon consultation with the HPLC results, this observation was confirmed as repeatable (reactions were performed in duplicate with error bars calculated as standard deviation from mean).
Consideration of the HPLC plots, however, demonstrated that there was an increased rate of consumption of the substrate molecule under photolytic conditions in comparison to the photocatalyzed reaction. This confirms the previous assertion that the two processes through which the substrate molecule was consumed were not the same and may even be a synergy between the two. Interestingly, the consumption of G/β-O-4 by the photolytic reaction was to a greater extent than the photocatalyzed reaction also.
While photocatalytic and photolytic responses were noted for G/β-O-4 degradation, RG/β-O-4 degradation was primarily a result of photocatalysis. Considering the proposed high reactivity pattern of G/β-O-4 as a result of the ketone being present, the removal of the ketone should remove any sensitivity towards the UV irradiation in the absence of the photocatalyst. The reduction of G/β-O-4 with NaBH4 provided the benzylic alcohol RG/β-O-4 with a yield of 92%. Subjecting the reduced dimer to the same experimental conditions as G/β-O-4 confirmed the presence of the ketone increased the susceptibility of the compound to photodegradation. Figure 3b (and Figure S16) demonstrates minimal removal of RG/β-O-4 was recorded under dark (catalyst present with no irradiation) and photolytic (catalyst absent with irradiation) conditions. The data also confirm minimal adsorption of the substrate onto the surface of the catalyst, which suggests the reactions and subsequent breakdown under photocatalysis occurred through a reaction between the substrate molecules and ROS in the bulk solution. A comparison of the rate of consumption for the photocatalytic degradation of both G/β-O-4 and RG/β-O-4 is shown in Table 1, along with photolytic conditions for G/β-O-4
The lack of ketone in the benzylic position of RG/β-O-4 demonstrated that, under photolytic conditions (e.g., UV irradiation only), the reaction was insufficient for bond cleavage within the molecule. HPLC analysis confirmed that no reaction was observed (i.e., no intermediates detected) when the solution was under irradiation but TiO2 was absent. Considering the proportion of lignin that is categorized as containing carbonyl functionalities [35], it is important to note that there will be increased sensitivity towards similar UV irradiating conditions when raw lignin is under investigation. The data presented in Figure 3 and supporting figures in Supplementary Materials demonstrated that photodegradation of both model lignin compounds occurred via multiple processes and reactions. Therefore, using the same analysis profile, LC-MS was undertaken to enable the elucidation of proposed products being formed in the reaction solution.

2.2. Proposed Reaction Pathways for G/β-O-4 and RG/β-O-4

Examining the rate and overall consumption data between the two substrates (Table 1) demonstrated that there was an increased reaction rate and consumption for the photocatalytic reaction of RG/β-O-4 (0.0948 mg mL−1) compared to G/β-O-4 (0.0832 mg mL−1). For the G/β-O-4 compound, this suggested that multiple processes were occurring within the reaction solution and alludes to potential competition for the substrate between the photolytic process and any potential ROS in the bulk solution. This can be explained by the presence of TiO2 (as a suspended particle within the reaction solution) absorbing UV photons and, in doing so, lowering the concentration available to photolytically cleave bonds. The absorption of UV photons by TiO2, however, subsequently resulted in ROS generation, which led to photodegradation, albeit at a slower rate than pure photolysis. The comparison of the results for the initial rate of consumption (Table 1) between the photolytic reaction of G/β-O-4 (2.70 × 10−3 mg mL−1 min−1) and the photocatalytic reaction of RG/β-O-4 (1.92 × 10−3 mg mL−1 min−1) was significant. This was assumed to be due to the time required for photo-excitation to occur through promotion of the electron from the valence band to the conduction band, followed by subsequent formation of the ROS. In addition, it may also have been due to the rate of reaction for direct hole oxidation of the substrate at the catalyst surface or ROS oxidation in the bulk solution. If both photocatalytic profiles are considered in Figure 3, it can be seen that, during the equilibration time (30 min dark period prior to illumination) for both substrates, there is an increased amount of RG/β-O-4 adsorbed (4.34%) onto the surface of the catalyst. This observation may point towards direct hole oxidation being responsible for consumption of the substrate during the initial period of irradiation. In contrast, there was minimal G/β-O-4 adsorbed (X%) onto the surface of the TiO2, which conversely points towards reaction of the substrate with the ROS in the bulk reaction medium. To support the rationalization of the reaction progression and proposed mechanism, LC-MS was employed to identify key reaction intermediates.
All of the reactions undertaken in this study were analyzed using LC-MS with the proposed products formed from G/β-O-4 and RG/β-O-4 shown in Scheme 1 and Scheme 2, respectively. Comparison of the LC-MS analysis with the previous results showed there were several different products formed during the two processes. A key observation, however, was that there appeared to be a common product between the two sets of reactions for G/β-O-4. In the photocatalytic reaction (i.e., in the presence of TiO2 and irradiation), two main products were identified from the HPLC and LC-MS results. The first product identified as 1 is proposed to be because of a cleavage of the C-O bond between the guaiacyl B-ring and the oxygen it was bonded to. Interestingly, in this reaction, the co-product (guaiacol 2) was also identified by matching retention time on the HPLC (LC-MS had 150 Da cut-off). The remainder of the products, which appeared in the chromatogram stack of the photocatalytic reaction, are the focus of ongoing work and, as such, have not been fully characterized in this study.
With regards to the photolytic reaction of G/β-O-4, there were three products identified in the reaction solution, with one being common to the photocatalytic reaction products (1). Considering the nature of the photolytic reaction, this was unexpected, as the main area where the reaction is expected to occur in such a molecule would be at the benzylic ketone. This would proceed through an excitation mechanism and result in a benzylic radical species, which, although stable, could subsequently undergo a range of reactions, including Norrish cleavages and also Norrish–Yang reactions. It is proposed that, in this reaction, there is a Norrish–Yang-type reaction to furnish proposed product 4, where an intramolecular cyclization reaction occurs. Under such conditions, the excitation of the benzylic ketone will result in an excited state and the formation of a benzylic radical. Following this, the γ-abstraction of the proton in the CH2 will result in a small ring being formed in the molecule, which also matches what would be expected for the retention time where the more polar diol is formed from the ketone.
Proposed product 3 appears to be the result of cleavage between the α and β carbon atoms in the skeleton of the linkage. This cleavage is proposed to have resulted in the formation of an aldehyde species with the aldehyde at the benzylic position of the A-ring. It is noteworthy that a previously cited article demonstrated the use of UV irradiation for the cleavage of bonds within a lignin model bearing a ketone in the α-position [36]. In that study, however, the presence of only two products was observed because of cleavage between the β-carbon and the oxygen of the B-ring. In contrast, the findings presented here demonstrate that this is not the case and that there are different products formed in the reaction solution. This is likely due to the other substituents which may be present on the A- and B-rings.
In relation to the degradation of RG/β-O-4, Figure 4 shows that the structure of G/β-O-4 was observed as a proposed product in the reaction solution. This was expected as, under photocatalytic conditions, the highly oxidative reaction solution would oxidize alcohols [37], especially so that the secondary alcohol will form the stable ketone that is no longer susceptible to direct oxidation unless cleavage of C-C bonds is the preceding event. It was also plausible that the generation of G/β-O-4 in the reaction solution would open a pathway for the photolytic degradation of this formed product, making it, therefore, an intermediate that is consumed as a result of the irradiation. Proposed product 5 (in Scheme 2) appears to be due to an elimination of a CH2OH unit from the hydroxymethyl on the β-carbon with the formation of an alkene in its place. This would subsequently tautomerize to form the benzylic ketone. The formation of product 6 in the reaction solution appears to be a result of the oxidation of the α-alcohol to the benzylic ketone. It has been previously reported and shown that the oxidation of the benzylic alcohol to the corresponding ketone will, in turn, weaken the β-C-O bond [38,39] by as much as 15.9 kcal mol−1. The oxidation of the benzylic alcohol to the ketone is proposed to be the preceding event to the cleavage of the β-C-O bond to provide product 6. The cleavage of the ethyl ether unit to provide the corresponding phenol in the A-ring of RG/β-O-4, which provides product 7, is similar to other work that has employed a TiO2 catalytic system [40]. Finally, product 8 is proposed to be the result of the primary alcohol in the substrate being fully oxidized to the corresponding carboxylic acid. This is a rational pathway for the substrate to be consumed, as the initial oxidation to the aldehyde would be a quick process. This would subsequently produce an aldehydic intermediate, which is extremely vulnerable under the highly oxidizing conditions and would be quickly consumed. The oxidation of this primary alcohol in this fashion is similar to work which has been performed previously in our own lab, where a hexameric lignin model compound was subjected to photocatalytic conversion [8].

2.3. Role of Solvent

All of the reactions described in this report were performed in the presence of acetonitrile as a co-solvent to aid in the dissolution of the substrate. It is important to consider and discuss the role of acetonitrile within the described reactions. The use of acetonitrile for these reactions was rationalized, as the use of alternatives such as acetone has previously shown to act as a scavenger for ROS being formed in the bulk solution [40]. Acetonitrile provides good solvation of the substrate while providing good miscibility with water, while facilitating free diffusion of any ROS in the bulk solution. It has, however, been reported in the literature that, in the presence of acetonitrile, Degussa TiO2 (the catalyst used in this study) will form an acetamide species on the surface of the catalyst, therefore potentially inhibiting its activity. It was demonstrated, however, in this report that, for the RG/β-O-4 model compound, the adsorption onto the surface of the catalyst would not be fully possible if the catalyst surface was swamped with an acetamide-type species. Similarly, another report by Augugliaro et al. showed that TiO2 in the presence of acetonitrile and natural sunlight irradiation produced cyanide and formate ions, each of which could play a plausible role in the consumption of both substrates shown here. The work to fully elucidate the progress of this reaction, aside from the products being formed and the consumption, is the focus of ongoing work and will include a mechanistic study with electron trapping experiments. Although, it is proposed that, in the reactions which have been conducted in this report, the pathways of degradation for the photocatalytic reactions are not solely because of either direct hole oxidation or ROS. The case may be, in fact, that the generation of ROS or direct hole oxidation produces reactive intermediates from acetonitrile, which subsequently degrade the substrates. Figure 4 demonstrates the possible reactions that may be occurring at the surface of the catalyst and shows the possible pathways through which there may be a range of reactive intermediates being formed.

3. Conclusions

Described in this study is the use of commercial TiO2-mediated photocatalytic conversion of lignin model compounds G/β-O-4 and RG/β-O-4. The presence of a ketone in the benzylic position for G/β-O-4 was observed to increase sensitivity towards photolytic conditions (e.g., irradiation alone and in the absence of photocatalyst) and, subsequently, an increased rate of conversion towards products. The use of LC-MS was instrumental in the proposal of several of the products and intermediates, which were observed to form in the reaction solution of the photolytic conversion of G/β-O-4. The reduction of the ketone in the α-position of the G/β-O-4 provided the benzylic alcohol, which was observed to have no enhanced sensitivity to UV irradiation alone. The photocatalytic conversion of this model (RG/β-O-4) provided a number of products arising from oxidation and cleavage events within the molecule. The deployment of TiO2 as a commercial catalyst for this degradation process has shown that there is potential for the photocatalytic conversion of lignin. Even still, the presence of a ketone at the benzylic position in the β-O-4 linkage shows an enhanced amount of reactivity to UV irradiation alone and, therefore, provides another strategy for the cleavage of this linkage and the conversion towards value-added products. The role of the solvent in these degradation processes is one which remains to be elusive and is currently the focus of ongoing work to determine the potential pathways of degradation towards reactive intermediates arising from acetonitrile reacting with the photocatalytic conditions. This work is part of a larger mechanistic study that will be used to form the basis of a holistic review of photocatalytic processes and their feasibility within organic solvents.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal15060525/s1. Figure S1. Synthetic route towards the guaiacyl β-O-4 dimer (G/β-O-4). Figure S2. Reduction of the guaiacyl β-O-4 dimer to afford RG/β-O-4. Figure S3. UV-Vis spectra for a standard of 5 model substrate. Figure S4. UV-Vis spectra for a standard of 7 model substrate. Figure S5. Image of the experimental set up for photocatalytic reactions showing a reaction solution containing a model substrate and catalyst under irradiation from a UV LED. Figure S6. Spectral output of LED used in this study. Figure S7. Mass spectrum of proposed product 4. Figure S8. Mass spectrum of proposed product 3. Figure S9. Mass spectrum of proposed product 1. Figure S10. Mass spectrum of proposed product 5. Figure S11. Mass spectrum of proposed product 6. Figure S12. Mass spectrum of proposed product 7. Figure S13. Mass spectrum of proposed product 8. Figure S14. Mass spectrum of proposed product G/β-O-4. Figure S15. Degradation of G/β-O-4 under (a) photocatalytic and (b) photolytic conditions along with (c) a normalised time-absorbance profile for photocatalysis (∎) and photolysis (•) at 310 nm. The red arrows indicate the increase and decrease of spectra in accordance with irradiation time. Figure S16. Degradation of RG/β-O-4 under (a) photocatalytic and (b) photolytic conditions along with inserts (c) and (d) showing a magnified view of the photolysis spectra. The red arrows indicate the increase and decrease of spectra in accordance with irradiation time.

Author Contributions

Conceptualization, G.N.S.; methodology, C.W.J.M.; validation, C.W.J.M.; investigation, C.W.J.M.; writing—original draft preparation, C.W.J.M.; writing—review and editing, N.S., G.N.S. and C.W.J.M.; supervision, G.N.S. and P.K.J.R.; funding acquisition, G.N.S. All authors have read and agreed to the published version of the manuscript.

Funding

Christopher Murnaghan would like to acknowledge EPSRC for funding his studentship under project EP/N509541/1. We would like to acknowledge the EPSRC project EP/S018077/1 for funding the LC-MS instrument under Dr Peter Knipe’s kind advice.

Data Availability Statement

Data is fully available upon request and immediate data present in Supplementary Information.

Conflicts of Interest

No conflicts of interest to declare.

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Figure 1. Representative structure of lignin with key bonding patterns highlighted.
Figure 1. Representative structure of lignin with key bonding patterns highlighted.
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Figure 2. An overview of the lignin model compound structures utilized in (a) our previous studies and (b) the present study. All synthetic data are available in ESI.
Figure 2. An overview of the lignin model compound structures utilized in (a) our previous studies and (b) the present study. All synthetic data are available in ESI.
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Figure 3. Concentration vs. time plot of lignin model compound G/β-O-4 (a) and RG/β-O-4 (b) under photocatalytic (•), photolytic (•) (light only, no catalyst present), and dark control (•) (catalyst only, no light present) reaction conditions. Irradiation was started at Time 0, with each reaction given 30 min in the dark to reach equilibrium based on control data (shown in Supplementary Materials). The starting concentration is depicted at Time −30 min (•). All concentrations were determined based on a calibration graph of standards with known concentrations and utilizing the equation shown in the Supplementary Materials. HPLC analysis confirmed there were no non-photocatalytic reactions occurring in the absence of light or in the absence of a catalyst.
Figure 3. Concentration vs. time plot of lignin model compound G/β-O-4 (a) and RG/β-O-4 (b) under photocatalytic (•), photolytic (•) (light only, no catalyst present), and dark control (•) (catalyst only, no light present) reaction conditions. Irradiation was started at Time 0, with each reaction given 30 min in the dark to reach equilibrium based on control data (shown in Supplementary Materials). The starting concentration is depicted at Time −30 min (•). All concentrations were determined based on a calibration graph of standards with known concentrations and utilizing the equation shown in the Supplementary Materials. HPLC analysis confirmed there were no non-photocatalytic reactions occurring in the absence of light or in the absence of a catalyst.
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Scheme 1. Proposed products arising from both the photolytic and photocatalytic reactions of the G/β-O-4 lignin model dimer.
Scheme 1. Proposed products arising from both the photolytic and photocatalytic reactions of the G/β-O-4 lignin model dimer.
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Figure 4. Overview [25] of the potential photocatalytic degradation of lignin models, which shows (a) the general photocatalysis mechanism as applied to TiO2, (b) oxidation reactions at the valence band, including (1) OH• formation, (2) direct hole oxidation, and (3) OH• attack on lignin models and MeCN, and (c) the formation of O2•− at the conduction band and OH• generation via the fission of H2O2.
Figure 4. Overview [25] of the potential photocatalytic degradation of lignin models, which shows (a) the general photocatalysis mechanism as applied to TiO2, (b) oxidation reactions at the valence band, including (1) OH• formation, (2) direct hole oxidation, and (3) OH• attack on lignin models and MeCN, and (c) the formation of O2•− at the conduction band and OH• generation via the fission of H2O2.
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Scheme 2. Proposed products arising from the photocatalytic conversion of lignin model dimer RG/β-O-4.
Scheme 2. Proposed products arising from the photocatalytic conversion of lignin model dimer RG/β-O-4.
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Table 1. Table of values for the photocatalytic and photolytic reactions for G/β-O-4 and RG/β-O-4.
Table 1. Table of values for the photocatalytic and photolytic reactions for G/β-O-4 and RG/β-O-4.
Reaction of G/β-O-4Rate of Consumption (First 15 min)Overall Consumption 1
Photocatalytic1.22 × 10−3 mg mL−1 min−10.0832 mg mL−1
Photolytic2.70 × 10−3 mg mL−1 min−10.0905 mg mL−1
Reaction of RG/β-O-4Rate of Consumption (First 15 min)Overall Consumption 1
Photocatalytic1.92 × 10−3 mg mL−1 min−10.0948 mg mL−1
1 Calculated to LOD on HPLC, details in Supplementary Materials.
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MDPI and ACS Style

Sheldrake, G.N.; Skillen, N.; Robertson, P.K.J.; Murnaghan, C.W.J. Photocatalytic Conversion of β-O-4 Lignin Model Dimers: The Effect of Benzylic Ketones on Reaction Pathway. Catalysts 2025, 15, 525. https://doi.org/10.3390/catal15060525

AMA Style

Sheldrake GN, Skillen N, Robertson PKJ, Murnaghan CWJ. Photocatalytic Conversion of β-O-4 Lignin Model Dimers: The Effect of Benzylic Ketones on Reaction Pathway. Catalysts. 2025; 15(6):525. https://doi.org/10.3390/catal15060525

Chicago/Turabian Style

Sheldrake, Gary. N., Nathan Skillen, Peter. K. J. Robertson, and Christopher W. J. Murnaghan. 2025. "Photocatalytic Conversion of β-O-4 Lignin Model Dimers: The Effect of Benzylic Ketones on Reaction Pathway" Catalysts 15, no. 6: 525. https://doi.org/10.3390/catal15060525

APA Style

Sheldrake, G. N., Skillen, N., Robertson, P. K. J., & Murnaghan, C. W. J. (2025). Photocatalytic Conversion of β-O-4 Lignin Model Dimers: The Effect of Benzylic Ketones on Reaction Pathway. Catalysts, 15(6), 525. https://doi.org/10.3390/catal15060525

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