Production of phenolics via photocatalysis of ball milled lignin–TiO₂ mixtures in aqueous suspension

Abstract

In this study, photocatalytic conversion of lignin to valuable phenolics and aromatic hydrocarbons is demonstrated by subjecting ball milled mixtures of lignin and TiO₂ to ultraviolet (UV) radiation. Unlike a majority of the existing studies that reported photocatalytic degradation of lignin that is solubilized in alkaline medium, this study evaluates the decomposition of lignin under natural conditions in aqueous medium. In order to facilitate better contact between lignin and nano-TiO₂, the two materials were ball milled in the presence of different media, viz. without solvent, hexane, acetone and water. The ball milled mixtures were characterized using powder XRD, FT-IR and photoluminescence spectroscopy, and scanning electron microscopy. Intimate contact between lignin and TiO₂ was achieved using water and acetone as the solvents in wet milling. Photocatalysis experiments were conducted in a batch photoreactor. The aqueous phase products were analyzed using UV-visible spectroscopy, MALDI-TOF and GC mass spectrometry, while the molecular weight of solid lignin was analyzed using GPC. Ball milling resulted in the formation of phenolic compounds even during dark mixing of the mixtures prior to photocatalysis. Ball milled mixtures obtained using acetone and water resulted in a high yield of phenolic compounds after 3–4 hours of UV exposure. At long UV exposure periods, the phenolics production got saturated, possibly due to the deactivation of TiO₂ active sites by the intermediates. The main organic compounds produced during photocatalysis include ethyl benzene, acetovanillone, syringaldehyde, acetosyringone, vanillin, 2,6-dimethoxy benzoquinone and diisobutyl phthalate. Free radical depolymerization reactions of lignin mediated by active hydroxyl and superoxide radicals are responsible for the observed products.

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1. Introduction

Current research in the fields of energy generation and environmental conservation is focused on utilizing lignocellulosic biomass for obtaining valuable chemicals, materials and fuels. It is believed that the increase in dependence on a renewables-based economy would help in preserving the depleting fossil fuel-based industries.^1,2 Cellulose, hemicellulose and lignin are the major components of lignocellulosic biomass. Lignin, which constitutes 10–25 wt% of lignocellulosic biomass, is the second most abundant natural polymer.³ Cellulosic ethanol bio-refineries and paper industries utilize cellulose and hemicellulose, and reject lignin as the major byproduct. The waste lignin is primarily utilized for power generation via combustion, which is a high volume, yet low value application. Importantly, not more than 2% of the total 70 million tons of lignin produced is used in the production of phenolic resins, polyurethane foams and bio-dispersants.^3,4 Lignin is the only naturally synthesized aromatic biopolymer made up of three phenyl propane units like p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol. These monomers are connected by ether and carbon–carbon bonds such as β-aryl ether (β-O-4), α-aryl ether (α-O-4), diphenyl ether (4-O-5) and biphenyl (5–5).^3,5 Therefore, it can be regarded as a rich source of phenols, aromatic and aliphatic compounds. However, it is difficult to deconstruct lignin to various value added products because of its complex crosslinked structure.

The major lignin depolymerization techniques include biochemical, catalytic oxidation, thermochemical, electrooxidation, ionic liquid treatment and photooxidation.^1,3,5–9 Thermochemical conversion of lignin via catalytic fast pyrolysis is a well-studied process that yields simple phenols, guaiacols and syringols as the key products.^5,10 Nevertheless, lignin tars contain a complex mixture of lignin oligomers, which are not easily recoverable. Photocatalysis is an advanced oxidation process that involves the generation of highly reactive hydroxyl radicals, which mediate a number of organic oxidation reactions.¹¹ TiO₂ is a promising photocatalyst and has particular potential for lignin degradation. Owing to its non-toxic nature, physicochemical stability and strong oxidizing potential, TiO₂ is a photocatalyst of choice for a wide variety of reactions including destruction of organic pollutants like dyes, phenolics, volatile organics, pesticides and pharmaceutical compounds.^12,13

The presence of di- and tri-substituted benzene conjugated to aromatic carbonyl, α,β-unsaturated carbonyl, quinone and catechol moieties in lignin lead to ultraviolet (UV) and visible light absorption.¹⁴ As a result, lignin can be effectively depolymerized via photooxidation in presence of oxygen. Earlier studies on delignification of unbleached kraft pulps from paper industry using UV light demonstrated ca. 85% removal of lignin, which was measured by the kappa number.¹⁴ The presence of oxygen and acidic or alkaline medium were found to be detrimental in the removal of lignin. Contrastingly, irradiation under inert N₂ atmosphere led to condensation of lignin ring fragments, and hence, an effective increase in molecular weight. Studies that utilized lignin model compounds along with singlet oxygen quenching molecules unraveled the mechanism of photooxidation, which involves the cleavage of β-O-4 aryl ether bond and hydrogen abstraction reactions to form hydroperoxides, phenol, guaiacol, acetoveratrone, stilbene, syringol, vanillin, phenyl coumarone, dibenzodioxocin and their derivatives.^14,15 Very few studies are available on heterogeneous photocatalytic decomposition of lignin, and a summary of the existing studies and their salient features are listed in Table 1.^9,16–23 Importantly, as lignin is insoluble in aqueous medium in the absence of externally added alkali, the existing studies were performed at basic pH. From the mass spectrum of lignin dissolved in alkaline medium shown in Fig. S1 (in ESI data†), it is evident that a number of low molecular weight fragments (<300 Da) are produced via lignin depolymerization reactions induced by the high concentration of NaOH. Therefore, photocatalysis of this mixture results in the degradation of these small molecules more than the degradation of the polymeric structure of lignin itself. On an application viewpoint, this also results in the usage of large quantities of alkali for the dissolution of lignin. Furthermore, the mechanism of lignin/lignin oligomer decomposition will be significantly influenced by the presence of alkali. This work is an attempt to understand the mechanism of photocatalytic decomposition of lignin in the absence of externally added reagents. To the best of our knowledge, this work is the first of its kind in the following aspects: (i) utilization of lignin in solid form in aqueous medium in photocatalysis experiments, and (ii) pretreatment of lignin along with the photocatalyst, viz. nano-TiO₂, via ball milling to induce better contact between lignin and TiO₂.

Table 1 List of the existing studies on photocatalytic degradation of lignin reported in the literature^a

Sl. No.	Type of lignin	Photocatalyst	Reaction conditions	Products	Reference
a HPML – high pressure mercury lamp, MPML – medium pressure mercury lamp.
1	Kraft lignin	TiO2	25 g L^−1 rutile TiO2; 0.02 wt% of lignin in 20 mL of solution; 500 W HPML; pH > 8	Methanol, ethanol, formaldehyde, formic acid, oxalic acid and traces of methane and ethane	Kobayakawa et al. (189)^16
2	Coniferous wood lignin	TiO2	0.7 g L^−1 catalyst; 0.01% of lignin in 300 mL reaction solution; 100 W HPML	Carboxylate, aldehyde	Tanaka et al. (1999)^17
3	Peroxy formic acid lignin	TiO2	5 mg L^−1 catalyst; lignin dissolved in 1 : 4 v/v ethanol–water mixture at 250 mg L^−1 in 25 mL of reactant solution; 400 W MPML	Complete mineralization	Machado et al. (2000)^18
4	Lignin precipitate from black liquor	TiO2	1 g L^−1 catalyst; 330 mg L^−1 lignin in 1 L of solution; 9 pH; 125 W HPML	Vanillin, coniferylic alchol, vanillic acid, p-coumanic acid, syringaldehyde	Ksibi et al. (2003)^19
5	Wheat straw lignin	TiO2–ZnO	1 g L^−1 catalyst; 11 pH; 30 W UV tubes	Complete mineralization	Kansal et al. (2008)^20
6	Synthetic lignin	Pt–TiO2	1 g L^−1 catalyst; 251 mg L^−1 of reactant mixture; 11 pH; 35 W UV tubes	Mineralization monitored using dissolved organic carbon content	Ma et al. (2008)^21
7	Kraft lignin	TiO2–laccase	3 g L^−1 of photocatalyst; 5.55 g L^−1 H2O2; laccase in 1 g L^−1 lignin in 10 mL of reactor solution; 5 pH; 24 W UV tubes	Succinic acid, malonic acid, acetic acid, vanillin	Kamwilaisak and Wright (2012)^22
8	Organosolv lignin black liquor	TiO2	2 g L^−1 catalyst; was added to 50 mL of lignin; 300 W UV lamp	Syringol, pyrocatechol, vanillin, syringaldehde, sinapyladehyde	Prado et al. (2013)^9
9	Alkali lignin	Ag–AgCl–ZnO	0.5 g L^−1 catalyst; 50 mg L^−1 lignin solution; 11 pH; solar light	Methane	Li et al. (2015)^23
10	Industrial lignin	Aeroxide TiO2	0.4 g of 1 : 1 w/w lignin–TiO2 ball milled mixtures suspended in 200 mL aq. medium; natural pH; 125 W HPML	Ethyl benzene, vanillin, acetovanillone, acetosyrigone, syringaldehyde	This work

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Pretreatment of lignin is essential for improving the yield of phenolic compounds in any conversion process. Many physical and chemical pretreatment methods like ball milling, dilute acid, steam, hot water, ionic liquid and organic solvent treatments have been successfully employed to overcome the recalcitrance of biomass and lignin, and improve the extraction of products.²⁴ Among these pretreatment techniques, ball milling is a promising pretreatment process in terms of polydispersity reduction and reactivity improvement. Yamashita et al.²⁵ revealed that a combination of ball milling and phosphoric acid is an effective pretreatment method for the production of ethanol from paper sludge. In addition, other investigations indicated that wet ball milling (WM) is better than dry ball milling (DM).²⁶ The use of planetary mill-based pretreatment is an efficient and environment friendly method as it imparts artificial gravity to the grinding medium via a centrifugal force field. This causes a non-uniform field of centripetal acceleration. As a result, the balls in planetary mill have notably higher impact energies.²⁷ Importantly, milling is shown to produce relatively lesser amount of soluble phenolics compared to alkali treatment.²⁷

The objectives of this work are four fold: (i) preparation of lignin–TiO₂ mixtures in a ball mill via dry milling and wet milling (using different solvents like water, hexane and acetone), (ii) characterization of the mixtures using various techniques such as Fourier transform infrared spectroscopy, photoluminescence spectroscopy, powder X-ray diffraction, and scanning electron microscopy, (iii) photo degradation of aqueous lignin–TiO₂ suspensions and evaluation of concentration of total phenolics by UV-visible spectrophotometer, and (iv) identification of various phenolic compounds using mass spectrometry and molecular weight of solid phase lignin by gel permeation chromatography.

2. Experimental section

2.1. Materials

Commercial lignin was procured from Asian Lignin Manufacturing (ALM), India. This lignin is extracted from non-woody biomasses like wheat straw and sarkanda grass by soda pulping process using aq. NaOH.²⁸ The number average and weight average molecular weights of ALM lignin are reported to be 1000 Da and 2500–3400 Da, respectively.²⁸ Commercial TiO₂ (Aeroxide® P25) was obtained from Sigma-Aldrich. Water, n-hexane (Merck) and acetone (Sisco Research Laboratories Pvt. Ltd. India) were used as solvents in the wet ball milling process. Double deionized water was used as the reaction medium for photocatalysis experiments. The solvents, dichloromethane, tetrahydrofuran (THF), and dimethyl formamide (DMF), were procured from Merck, India. o-Nitrobenzaldehyde (Avra Synthesis Pvt. Ltd., India) was used to determine the intensity of the ultraviolet (UV) lamp via actinometry.²⁹

2.2. Preparation of lignin–TiO₂ mixtures

Lignin and TiO₂ were thoroughly mixed in a planetary ball mill (Fritsch Pulverisette) equipped with zirconia milling jar of 250 mL capacity. The jar containing 100 steel balls of 10 mm diameter was loaded with 5 g of lignin–TiO₂ mixture (1 : 1 w/w), and milled at 120 rpm for 6 hours. The sample thus obtained is called as dry milled mixture (DM). For wet milling, solvents such as water, hexane or acetone were also added to the lignin–TiO₂ mixture at 1 : 2 w/w of mixture : solvent ratio before the grinding process. The choice of these solvents is based on their widely different polarities (water-highly polar, acetone-medium polar, hexane-non-polar). The mixtures obtained after wet milling are henceforth denoted as WMW, WMH and WMA, corresponding to water, hexane and acetone, respectively. The mixtures thus obtained were filtered and dried at 40 °C. The dried mixtures were washed with water and then vacuum filtered. The residue was dried in hot air oven at 100 °C overnight. The recovery of the solids was calculated as the ratio of final dried mass of ball milled samples to the initial mass of lignin–TiO₂ mixture. Lignin, without TiO₂, was also ball milled in the presence and absence of the above solvents in order to perform control experiments.

2.3. Characterization of lignin–TiO₂ mixtures

Powder X-ray diffractograms (XRD) of lignin, TiO₂ and the ball milled mixtures were collected in D8 Discover (Bruker) diffractometer using Cu-K_α radiation. Fourier transform infrared (FT-IR) spectra were obtained in an Agilent Cary 660 FT-IR spectrometer in the wavenumber range of 400–4000 cm⁻¹ in transmittance mode at a resolution of 2 cm⁻¹. The fine powders were cast in the form of pellets using KBr. The surface morphology and energy dispersive X-ray analyses (EDS) of the mixtures were performed using a Hitachi S-4800 high resolution scanning electron microscope (SEM). The UV absorption spectra of the mixtures were collected in a UV-visible photodiode array (PDA) spectrophotometer (Agilent Cary 8454) by suspending the mixtures in water. Photoluminescence emission (PL) spectra of TiO₂, WMW, WMA, WMH and DM mixtures were recorded in an Agilent Cary Eclipse fluorescence spectrometer at an excitation wavelength of 257 nm. 70 mg of the samples were dry pressed and analyzed to compare the intensities of the samples. Thermogravimetric analyses (TGA) of the mixtures were performed in a SDT Q600 TG analyzer (T.A. Instruments) at 10 °C min⁻¹ under continuous flow of N₂ gas at 100 mL min⁻¹. Typically, 5 ± 0.3 mg of the samples were pyrolyzed and mass loss of the sample with temperature was monitored. Differential mass loss and onset degradation temperature were evaluated.

2.4. Photocatalytic treatment of lignin–TiO₂ mixtures

Photocatalytic degradation of only lignin, physical mixtures of both untreated and ball milled lignin and TiO₂, and the various mixtures, viz. DM, WMW, WMH and WMA, were carried out in aqueous medium in an annular type photoreactor. It is important to note that the lignin–TiO₂ mixtures were well suspended in water without the dissolution of lignin in alkaline medium. The photoreactor consisted of a jacketed quartz tube containing a high pressure mercury lamp (Philips 125 W) that emits UV radiation at a wavelength of 365 nm. The intensity and photon flux of the UV lamp measured by o-nitrobenzaldehyde actinometry were 1.11 × 10⁻⁷ einstein L⁻¹ s⁻¹ and 4.82 W m⁻², respectively. The temperature of the reaction mixture was maintained at 25 ± 3 °C by circulating chilled water through the jacket of the quartz tube. The quartz tube was immersed in the reaction vessel for uniform illumination of the reaction mixture. The details of the photocatalytic reactor can be found elsewhere.³⁰ Unless otherwise specified, the concentration of lignin–TiO₂ mixture taken for experiments was 2 g L⁻¹ in aqueous medium. Prior to illumination, the mixtures were stirred well in the dark to ensure that low molecular weight fragments such as monomers and dimers of lignin are dissolved in the aqueous medium. Typically, dark mixing was carried out until the concentration of phenolics in the aqueous medium reached a constant value. After this period, UV lamp was turned on, and the samples were collected at regular time intervals. The total reaction time was 360 min. Prior to analyses, the aliquots were centrifuged to separate the unreacted solids.

2.5. Characterization of aqueous phase products

The concentration of total phenolics produced was determined using UV-visible PDA spectrophotometer (Agilent Cary 8454). A characteristic peak at 280 nm signified the phenolic compounds. Absorbance vs. concentration calibration graph was constructed to quantify the total phenolic compounds using Beer–Lambert's law. For the calibration of total phenolics, lignin was dissolved in alkaline medium. The experiments were repeated three times and the uncertainty in the concentration of total phenolics was within 7%.

In order to identify the molecular weight of the compounds produced, the aqueous samples were subjected to Matrix Assisted Laser Desorption-Ionization Time-of-Flight Mass Spectrometry (MALDI-TOF). MALDI-TOF analyses were carried out in a Voyager-DE STR Biospectrometry Workstation (Applied Biosystems). 2,5-Dihydroxybenzoic acid (DHB) was used as the matrix, and the analyses were carried out in negative mode. In order to exactly identify the structure of the compounds, gas chromatography-mass spectrometric (GC/MS) technique was adopted. The lignin degradation products in aqueous medium were extracted in dichloromethane (DCM) solvent, and the DCM extract was injected in Shimadzu QP2010 Plus GC/MS equipped with a capillary column RTi-5 MS column (30 m × 0.25 mm; 0.25 μm film thickness, Restek, USA). Ultra high pure helium gas (5.5 grade) was used as the carrier gas at a total flow rate of 12.5 mL min⁻¹. 1 μL of the sample was injected at a split ratio of 5 : 1. The column oven was initially held at 40 °C for 4 min, followed by heating at a rate of 5 °C min⁻¹ to 280 °C, and finally held at 280 °C for 10 min. The injector, interface and ion source temperatures were 250 °C, 280 °C, and 250 °C, respectively. The mass spectra of the products were acquired in the m/z range of 40–400 Da. The mass spectra of the unknown peaks were compared with NIST mass spectral database to identify the organic compounds. A minimum cut-off of 85% was set as the search criteria in the NIST database. The salient products were reconfirmed by matching the retention time using pure standards, and quantified by constructing calibration graphs.

2.6. Characterization of solid phase lignin

The molecular weight of the degraded lignin in the solid phase was determined using a gel permeation chromatograph (GPC) (Agilent GPC 1260 Infinity series). Lignin was dissolved in DMF-0.1% LiBr solution at a concentration of 3 g L⁻¹. The GPC system consisted of a PLgel 5 μm MiniMIX-C column (250 mm length × 4.6 mm i.d.), Rheodyne injector, 50 μL sample loop and Agilent differential refractive index detector. THF was used as the mobile phase at 0.3 mL min⁻¹. The column was calibrated using twelve poly(methyl methacrylate) (PMMA) standards ranging in molecular weight from 550 to 2 136 000 g mol⁻¹. The calibration curve was fitted to a 5^th order polynomial with a regression coefficient of 0.99. The calibration plot and equation are available in Fig. S2 (in ESI data†).

3. Results and discussions

3.1. Effect of ball milling on lignin–TiO₂ mixture

The recovery of solids after ball milling of various lignin–TiO₂ mixtures were 94%, 93%, 90% and 70% for DM, WMH, WMW and WMA samples, respectively. Qu et al.³¹ reported 95% solid recovery after wet milling lignin with water, which is comparable with our study. The higher weight loss in the case of WMA can be related to the higher solubility of lignin in acetone, which was also observed during the preparation. From Fig. 1, visual changes in the appearance of wet milled mixtures using polar solvents like acetone and water are evident. The change in the appearance of WMW and WMA mixtures prepared using polar solvents can be related to the intermolecular interactions of the solvent with lignin and TiO₂.^32–34 The hydroxyl groups of the phenolic molecules present in lignin are likely to form hydrogen bonds with water and acetone.³² It is found that acetone and water have a strong tendency to fill the oxygen vacant sites in TiO₂.³³ Moreover, these two solvents are reported to alter the bond length of α-O-4 linkage, and decrease the electronic levels, viz. HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital), and band gap of lignin.³⁵ The interactions are further validated by analyzing the FT-IR and photoluminescence spectra of the mixtures as discussed in the next section.

Fig. 1 Preparation methodology of various ball milled lignin–TiO₂ mixtures.

3.2. Characterization of different lignin–TiO₂ mixtures

Fig. 2 depicts the XRD patterns of pure TiO₂, lignin and different lignin–TiO₂ mixtures. All the lignin–TiO₂ mixtures exhibited peaks at 2θ = 25.3, 37.9, 48.1, 54.3, corresponding to anatase phase of TiO₂ (JCPDS, no. 84-1286), and at 2θ = 27.7, 36.2, 41.4, 55.3, corresponding to rutile phase of TiO₂ (JCPDS, no. 88-1175). The XRD pattern of pristine lignin showed no crystalline peaks confirming its amorphous nature. Only for wet milled mixtures with acetone and water, a new peak at 31.7° was visible which is attributed to the incorporation of carbon from lignin onto TiO₂. This is an evidence for strong surface attachment and interactions between TiO₂ and lignin, which is desired. Kim et al.³⁶ also observed a similar peak in the XRD of TiO₂–carbon composites using rice husk as the carbon precursor at high loading. The crystallite sizes of Aeroxide® TiO₂, and TiO₂ in DM, WMH, WMA and WMW mixtures were similar, i.e. 28 ± 5 nm.

Fig. 2 XRD patterns of TiO₂, lignin and different wet milled mixtures of the two.

The FT-IR spectra of lignin and different lignin–TiO₂ mixtures (Fig. S3 in ESI data†) were analyzed to understand the structural modifications in the mixtures caused by any interaction between lignin and TiO₂. The signature peaks of lignin are observed at 1720 cm⁻¹ (C=O stretch of the carbonyl group mostly attached to β or γ carbon of the propane unit of lignin), 1603 and 1514 cm⁻¹ (aromatic C=C stretch of the phenolic groups), 1451, 1425 and 1364 cm⁻¹ (phenolic O–H bending),³⁷ and 1326, 1268, 1220, 1121 and 1033 cm⁻¹ (condensed guaiacyl and syringyl units of lignin).^31,38 The peak at 1630 cm⁻¹ in the FT-IR spectra of TiO₂ is attributed to the bending vibration of co-ordinated H₂O as well as Ti–O–H. The FT-IR spectra of DM and WMH (hexane) lignin–TiO₂ mixtures show the presence of all the above signature peaks of lignin. However, C=O stretching (1720 cm⁻¹) and phenolic O–H bending (1364 cm⁻¹) vibrations are not observed with WMW and WMA mixtures. Carbonyl groups (aldehyde and ketone) are not directly associated with aromatic rings of lignin, but are present in α, β, and γ carbons. The absence of carbonyl vibration is an indication that these may be involved in reactions in presence of polar solvents. A marked decrease in wavenumber of aromatic C=C stretching (1603 cm⁻¹) by 10 cm⁻¹ was observed for WMA and WMW mixtures. These changes may be due to molecular level interactions between lignin and TiO₂ induced by ball milling in the presence of polar solvents. Parthasarathi et al.³⁹ showed the existence of hydrogen bonding of type C–H⋯O, O–H⋯O in phenol–water clusters via density functional theory calculations, which partly substantiates the shifts in wavenumbers. The phenoxide groups of lignin are known to be neutralized by protonation, which is expected to occur in presence of water.³⁴ Moreover, acetone and water are known to compete and adsorb onto TiO₂ (100) sites via oxygen (H–“O”–H and H₃C–C(=“O”)–CH₃).³³ Therefore, two way interactions are envisaged to occur between lignin and polar solvents, and between TiO₂ and polar solvents. These interactions might result in association of lignin with TiO₂ mediated by the solvent molecules.

SEM images of lignin and ball milled mixtures are shown in Fig. 3. It is observed that the surface of lignin (Fig. 3(a)) is non-uniform with micron sized structures. No significant change in surface morphology was observed in the dry milled mixture. However, the particle size and surface morphology of the lignin–TiO₂ mixtures display interesting changes after wet milling. For WMH mixture, the size distribution of lignin–TiO₂ clusters is uniform (Fig. 3(b)), while the formation of large sized lignin–TiO₂ aggregates are observed in WMA mixture (Fig. 3(c)). The morphology of WMW sample in Fig. 3(d) showed a smoother surface with distribution of agglomerated particles. Owing to strong hydrophobicity, aggregated islands of lignin molecules could be observed when ball milled with water.³¹ The EDS data was analyzed at different spatial locations of the samples to evaluate the percent incorporation of TiO₂ in the surface of lignin (Table S1 in ESI data†). It is evident that the incorporation of TiO₂ on the surface of the mixtures is 14–18 wt% of Ti (25–31 wt% of TiO₂). As EDS technique probes only the sample surface, the rest of TiO₂ (19–25 wt%) is obviously present within the lignin matrix. Fig. S4 (in ESI data†) depicts the UV-visible spectra of the lignin–TiO₂ mixtures. A distinct peak at 280 nm for the mixtures signifies the phenolic groups from lignin. The absorption band edges for the various mixtures were 380 nm for WMA, 420 nm for WMW, 436 nm for Aeroxide TiO₂, and 475 for DM. WMH mixture exhibited a very broad band without a specific absorption band edge.

Fig. 3 SEM images of (a) lignin (b) WMH (c) WMA, and (d) WMW lignin–TiO₂ mixtures.

Photoluminescence (PL) technique is used to understand the surface processes involving charge carriers, and to evaluate the efficiency of charge carrier trapping, migration and transfer between composite materials.^40,41 Fig. 4 depicts the PL emission spectra of TiO₂, dry milled and wet milled lignin–TiO₂ mixtures when excited at 257 nm. The major emission peaks at 410 and 483 nm can be attributed to free exciton emission of TiO₂ and Ti⁴⁺–OH, respectively.⁴⁰ The PL intensity for the ball milled mixtures follows the order: WMH > DM > WMA ≈ WMW. While the reduction in PL intensities of the ball milled lignin–TiO₂ mixtures can be attributed to the low amount of TiO₂ in the samples and sample heterogeneity, these differences can also be attributed to the variation in electron transfer from the excited state of TiO₂ to the new levels or defects introduced by lignin. Importantly, the same trend in PL intensity was observed with samples chosen from different spatial locations of the mixtures. The interactions depend on how well TiO₂ and lignin are mixed. The low intensity or high extent of quenching of fluorescence observed with WMA and WMW mixtures is an indication that the probability of electron transfer to lignin is more, which might lead to a lower recombination of charge carriers. Using quantum chemistry calculations, it was shown that water and acetone greatly modify the electronic states (HOMO and LUMO) and band gap associated with α-O-4 and β–β bonds of lignin.³⁵ For example, the valence band and conduction band edges of TiO₂ are −7.46 eV (vs. vacuum) and −4.26 eV, respectively,⁴² and the HOMO and LUMO states of α-O-4 of lignin in water solvent are −5.076 eV and −1.714 eV, respectively.³⁵ This shows that the electron transfer can occur from conduction band of TiO₂ to HOMO of lignin during excitation. This trapped electron can initiate reactions in lignin. Importantly, the HOMO and LUMO states are said to vary in presence of different solvents. This supports the low PL intensity observed with WMA and WMW mixtures. However, when lignin–TiO₂ mixture is ball milled in the absence of solvent or in presence of a non-polar solvent like hexane, the recombination of charge carriers may be high, and this can lead to a low photocatalytic activity.

Fig. 4 Photoluminescence spectra of TiO₂, dry and wet ball milled mixtures of lignin and TiO₂.

The thermal stability of the mixtures was evaluated using TGA. Fig. 5 depicts the mass loss and differential mass loss profiles of lignin, dry milled and wet milled mixtures of lignin and TiO₂. It is evident that the sample mass remaining at the final temperature, 900 °C, is significantly more for the mixtures, which is attributed to the presence of TiO₂. The mass loss profiles of WMW and WMA mixtures are less steep in the temperature range of 200–400 °C, which signifies the slow rate of decomposition. The absence of shoulders at 200 and 250 °C in the differential mass loss profile was also evident for the WMW mixture. The onset degradation temperature (T_onset) follows the trend: WMW (235.5 °C) > WMA (213 °C) > lignin (203.2 °C) > DM, WMH (199.3 °C). This shows that the WMW and WMA mixtures are more stable than the others, which is indirect evidence that demonstrates the probable interactions between lignin and TiO₂ in mixtures prepared using polar solvents. This also stands as a supporting analysis for the claims made via XRD and PL studies.

Fig. 5 Mass loss and differential mass loss profiles of lignin and various lignin–TiO₂ mixtures.

3.3. Production of phenolics during dark mixing

Fig. 6 depicts the concentration profiles of the phenolic compounds produced during dark mixing and UV illumination under different conditions. It is observed that within 3 h of stirring the lignin–TiO₂ mixtures in the dark, a constant production of phenolics was observed. This is attributed to the dissolution of low molecular weight fragments such as monomers and dimers that are inherently present in the lignin sample or those produced by ball milling. The concentration (in mg L⁻¹) of phenolics produced at the end of 3 h of dark stirring of various mixtures in aqueous medium followed the trend: 268 (WMW) > 133 (WMH) > 124 (DM) > 117 (ball milled lignin) > 88 (physical mixture of lignin and TiO₂) ≈ 88 (only lignin) > 64 (WMA). With 1 g L⁻¹ and 4 g L⁻¹ of WMW, 220 and 484 mg L⁻¹ of phenolics were produced, respectively, which is expected. A similar trend was also observed when only lignin (in the absence of TiO₂) was ball milled in the absence and presence of different solvents (Fig. 7). This shows that ball milling results in the production of more oligomers of lignin that are easily soluble in aqueous medium. The use of acetone as the solvent resulted in lower production of phenolics during dark mixing. It is important to note that nearly 30% mass loss of lignin was observed when wet milled with acetone. This shows that the use of acetone results in the loss of a majority of the monomeric phenols during the pretreatment step. This means that the dried mixture after pretreatment predominantly contains higher oligomers and long chains of lignin attached to TiO₂. Fig. S5 (in ESI data†) depicts the particle size distribution of as received lignin and lignin ball milled for 6 h. It is evident that ball milling results in the broadening of lignin particle size distribution towards smaller size range. Moreover, a significant decrease in d₅₀ from 22.15 μm (for as received lignin) to 13.4 μm (for ball milled lignin) is also observed. This is also supported by the molecular weight distribution of lignin depicted in Fig. S6 (in ESI data†). It is evident that this variety of lignin inherently contains a large fraction of low molecular weight fragments in the range of 500–2000 g mol⁻¹, which are easily broken down to phenolics during ball milling process. Depolymerization of lignin is expected to occur during this process by the cleavage of β-aryl ether links.³¹ This substantiates the increase in production of phenolics in the initial 3 h period with ball milled lignin compared to untreated lignin. The results also demonstrate that the presence of water during ball milling is favorable as it partially weakens the linkages in lignin, besides aiding in the generation of more surface hydroxyl moieties associated with TiO₂ and lignin.

Fig. 6 Concentration profiles of phenolic compounds formed during dark mixing and photocatalysis of various lignin–TiO₂ mixtures in aqueous medium.

Fig. 7 Production of phenolics during dark mixing and UV irradiation from 2 g L⁻¹ of physical mixtures of lignin and TiO₂. Importantly, TiO₂ was not mixed with lignin during ball milling, but added only during the photocatalysis experiments.

3.4. Production of phenolics during UV illumination

Fig. 8 depicts the time evolution of UV-visible spectra during dark mixing and photocatalysis experiments for WMW mixture at 2 g L⁻¹ concentration. It is evident that the peak corresponding to total phenolics at 280 nm increases both during dark mixing and UV illumination upto 3 hours. During UV illumination period, no appreciable increase in concentration of phenolic compounds in the aqueous phase could be observed with only lignin or a simple physical mixture of lignin with TiO₂ (Fig. 7). This shows that (i) solid lignin does not photodegrade in the absence of any photocatalyst, and (ii) the mere presence of TiO₂ in the suspension along with lignin particles without any intimate contact between the two does not result in the degradation of lignin. The slight decrease in concentration of phenolics in the aqueous phase at long time periods (>3 h) of UV exposure can be attributed to the degradation of phenolics that were already released during the dark stirring period to the aqueous phase. Photocatalysis of ball milled lignin without TiO₂ also exhibits a flat concentration profile of phenolics in the initial 3 h period of UV illumination, and then shows a drop in concentration. This shows that the presence of TiO₂ in contact with lignin particles is a must for the production of phenolics under UV illumination, and ball milling of lignin without TiO₂ only aids in the production of slightly more amount of phenolics in the dark stirring period.

Fig. 8 UV-visible spectra depicting the evolution of phenolics during dark stirring and photocatalysis of WMW lignin–TiO₂ mixture (2 g L⁻¹).

Photocatalysis of DM mixture of lignin and TiO₂ results only in a slight increase in the production of phenolics from 124 to 155 mg L⁻¹ in the initial 3 h period, which shows that significant contact between TiO₂ and lignin is not developed by dry milling (Fig. 6). However, significant production of phenolics is observed when the wet milled mixtures are subjected to UV irradiation. The increase in phenolics concentration is from 64 to 143 mg L⁻¹, and 133 to 215 mg L⁻¹ for WMA and WMH mixtures, respectively, at the end of 3 h. Thereafter, the phenolics concentration in aqueous phase decreases owing to their photodegradation. Interestingly, the phenolics concentrations are even higher with WMW mixtures. The increase in concentrations are from 220 to 352 mg L⁻¹, 268 to 464 mg L⁻¹ and 484 to 623 mg L⁻¹ for WMW mixtures of different concentrations, viz. 1 g L⁻¹, 2 g L⁻¹ and 4 g L⁻¹, respectively. The percentage contribution by photocatalysis to the overall production of phenolics is also evaluated using the following expression for different lignin–TiO₂ mixtures.

C_max corresponds to the maximum concentration of total phenolics produced, which is usually the concentration from third to fifth hour. Table 2 depicts the values of C_max and % contribution by photocatalysis for different mixtures including physical mixtures of ball milled lignin with TiO₂ and ball milled lignin + TiO₂. It is evident that the addition of TiO₂ to ball milled lignin during UV irradiation period (i.e. physical mixture) results in lower production of phenolics in aqueous phase compared to that from ball milled lignin + TiO₂ (Fig. 7). This substantiates the role played by wet milling in improving the contact between lignin and TiO₂, and hence, the electron transfer. Based on the parameter, % contribution by photocatalysis, the mixtures can be ranked as follows: WMA-2 g L⁻¹ (55%) > WMW-2 g L⁻¹ (42%) > WMH-2 g L⁻¹ (38%) ≈WMW-1 g L⁻¹ (38%) > WMW-4 g L⁻¹ (22%) ≈ DM-2 g L⁻¹ (20%). Even though the maximum concentration of phenolics produced is low with wet milled mixture prepared with acetone, equal contribution from photocatalysis and dark mixing is observed, whereas for high mass concentrations of water based mixtures, more phenolics are formed during dark mixing period compared to photocatalysis. For all the WMW mixtures, the concentration of total phenolics saturated and started to slowly decrease after 4 h of UV illumination. This is also evident from the UV-visible spectra in Fig. 8. While this can be ascribed to the mineralization of the aqueous phase phenolics due to the action of UV radiation and TiO₂,^12,30 this also shows the suppression of TiO₂ activity due to the adsorption of lignin oligomers and intermediates on the TiO₂ active sites. It is important to note that when lignin is completely dissolved in the aqueous medium by the use of alkali, the photocatalytic reaction is kinetically controlled, whereas in our experiments, the photocatalytic reaction is both reaction as well as mass transfer controlled. Lignin decomposition occurs exclusively on the catalyst surface mediated by TiO₂. Even though the lignin–TiO₂ solid mixture is continuously stirred during UV illumination, the transfer of phenolic compounds from the solid lignin to aqueous phase is influenced by mass diffusion.

Table 2 Maximum concentration of phenolics produced in the aqueous phase via both dark mixing and photocatalysis, and % contribution by photocatalysis for physical and ball milled mixtures of lignin with TiO₂

	Cmax (mg L^−1)	% contribution by photocatalysis
Untreated lignin	88	0
Physical mixture of untreated lignin with TiO2	94	6
Physical mixtures
Dry milled lignin and TiO2	127	38
Wet milled (acetone) lignin and TiO2	77	58
Wet milled (hexane) lignin and TiO2	116	24
Wet milled (water) lignin and TiO2	183	13
Ball milled mixtures
DM	155	20
WMA	143	55
WMH	215	38
WMW	464	42

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It is important to note that the mass ratio of lignin : TiO₂ (1 : 1 wt/wt) used in this study is comparable with those employed in existing reports on photocatalytic degradation of lignin (refer Table 1).^{9,16,19,21–23} Fig. S7 (in ESI data†) depicts the concentration profiles of total phenolics produced when 1 : 1 and 3 : 1 wt/wt WMW mixtures of lignin–TiO₂ are subjected to dark stirring and UV irradiation. During the initial dark mixing phase, slightly more phenolics are produced in the aqueous medium from 3 : 1 lignin–TiO₂ mixture (307 vs. 268 mg L⁻¹), which is due to the high amount of lignin in the mixture that depolymerizes to phenolics after the wet milling process. However, during photocatalysis, high concentration of phenolics is produced from equal composition mixture. The % contribution by photocatalysis is only 15% with 3 : 1 lignin–TiO₂, while it is 42% with 1 : 1 lignin–TiO₂ mixture. This substantiates that equal composition of lignin–TiO₂ is preferred.

The reduction in molecular weight of solid phase WMW mixture was probed using GPC. It is evident from Table 3 that ball milling results in a slight drop in M_w of lignin (1980–1777 g mol⁻¹). A similar magnitude of decrease in M_w is also observed during dark stirring period (1777–1590 g mol⁻¹). This shows that the monomers/oligomers that are formed during the ball milling pretreatment are dissolved in aqueous medium. Nevertheless, UV photocatalysis causes a gradual and significant reduction of M_w of lignin (1590–1040 g mol⁻¹). This is also in line with the high production of phenolics in the aqueous phase from WMW mixture during UV irradiation. From the molecular weight distribution graph in Fig. S6 (in ESI data†), it is evident that the fraction of high molecular weight lignin decreases, while that of low mass fragments (<200 g mol⁻¹) increases with UV treatment. Such a significant change in M_w was not observed with other ball milled mixtures and physical mixtures of lignin and TiO₂. For example, with DM, WMH and WMA mixtures, the decrease in M_w after 6 h of UV treatment was less than 400 g mol⁻¹. From the GPC analysis, it can be concluded that although a significant decrease in M_w of lignin is achieved via UV photocatalysis, a significant fraction in the molecular weight range of 800–3000 g mol⁻¹ is unconverted.

Table 3 Variation of weight average and number average molecular weights of wet milled (water) mixture (WMW) of lignin and TiO₂ during dark stirring and UV irradiation period^a

	Time (min)	Mw (g mol^−1)	Mn (g mol^−1)
a It is important to note that the above molecular weight values are based on PMMA standards, and cannot be compared with the reported molecular weight of lignin, which is 2500–3400 g mol^−1.^28b Corresponds to lignin + TiO2 after wet milling in water medium.
Before wet milling		1980	450
Dark stirring period	−180^b	1777	400
	−120	1722	300
UV irradiation period	0	1590	485
	60	1418	250
	180	1160	265
	360	1040	260

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The various reactions taking place during photocatalysis include charge carrier generation (conduction band electrons and valence band holes), generation of active hydroxyl radicals (OH˙) via (a) hole pathway involving the reaction of holes with surface OH⁻ groups and water, and (b) electron pathway involving the reaction of electrons with dissolved oxygen, superoxide radicals, and H₂O₂ in a series of steps.^12,13,43 The generation of highly reactive OH˙ radicals can result in the degradation of lignin due to the scission of α-O-4 and β-O-4 bonds. This results in the generation of alkoxy, benzyl and alkyl free radicals that take part in a variety of lignin depolymerization reactions to form low molecular weight phenolics and lignin fragments. The OH˙ radicals can also directly attack the phenyl rings of lignin to form catechol, resorcinol and hydroquinone.⁴³ Moreover, the H⁺ ions generated (as a by-product of reaction of holes with water) can also react with conduction band electrons to produce more H˙,¹¹ which take part in hydrogen abstraction reactions. Effectively, the availability of H˙ for H-abstraction reactions is greatly improved via photocatalysis. As long chain molecules/oligomers of lignin are insoluble in aqueous medium, they are either in contact with TiO₂ or stay in the suspension. The degradation products of lignin, primarily phenolics and dimers, can also block and deactivate the TiO₂ active sites, and lead to the saturation of phenolics production at long time periods. In order to probe the identity of the various products produced during dark mixing and photocatalysis, mass spectrometric characterization of the products was carried out.

3.5. Identification of products via mass spectrometry

MALDI-TOF mass spectra of the aqueous phase samples from different experiments are shown in Fig. 9. As the reported intensities in the graph are normalized with respect to the DHB peak, the various curves can be compared. It is evident that with DM and WMH mixtures, the production of phenolics in the low mass range is insignificant with very low intensities. However, with WMA and WMH mixtures, higher production of lignin monomers and dimers with molecular weights 168, 184, 196, 206, 233, 249, 288, 315, 320, 327, 340 and 358 g mol⁻¹ are observed. Importantly, it can be noticed that a number of peaks are spaced at intervals of ca. 30 mass units, which signifies the difference between a guaiacol and syringol intermediate (that varies by an –OCH₃ group). Lignin monomeric units such as phenyl propane guaiacol (168) and phenyl propane syringol (196) are also observed.⁹ Moreover, with increase in photocatalysis time, increase in intensity of the major peaks is also observed for the WMW mixture, which shows the increase in concentration of the phenolic compounds. Fig. 10 depicts the GC/MS total ion chromatograms of the DCM extract obtained after 3 h of dark mixing of the WMW mixture, and after 1 h and 3 h of UV treatment. The structure, molecular weight and the typical mass fragments observed for the major phenolic compounds and aromatic hydrocarbons are listed in Table 4. The listed compounds are identified with more than 90% confidence based on NIST mass spectral database. It is evident that styrene, vanillin, acetyl vanillin and acetosyringone are the major products obtained after dark stirring, while a number of other compounds are produced after subjecting the mixture to UV irradiation. For example, dialkyl phthalates are reported to be the major products during lignin depolymerization via hydrogen-free hydrogenolysis in presence of hydrogen donor solvents and metallic catalysts like noble metal-doped Al-SBA-15.⁴⁴ The formation of diisobutyl phthalate in aqueous phase photocatalysis shows that mild hydrogenolysis of lignin might occur in the case of WMW mixture. The molecular weight of a number of these compounds also matches with the MALDI-TOF mass spectra in Fig. 9.

Fig. 9 MALDI-TOF mass spectra of the liquid phase after subjecting various ball milled mixtures to UV irradiation for different time periods.

Fig. 10 GC/MS total ion chromatograms of DCM extract obtained from photocatalysis of WMW liginin–TiO₂ mixture at different durations. The peaks correspond to 1 – ethyl benzene, 2 – acetovanillone, 3 – 2,6-dimethoxybenzoquinone, 4 – syringaldehyde, 5 – diisobutyl phthalate, 6 – styrene, 7 – vanillin, 8 – acetyl vanillin, 9 – acetosyringone.

Table 4 Major products obtained from photocatalysis of WMW lignin–TiO₂ mixture identified by GC/MS

Compounds identified in the DCM extract via GC/MS	Molecular weight (g mol^−1)	Structure	% similarity with NIST	Major ions
Vanillin	152.15		95	51, 53, 65, 81, 93, 109, 123, 137, 151, 152
Acetovanillone	166.17		90	43, 52, 65, 73, 80, 93, 95, 108, 123, 136, 151, 166
2,6-Dimethoxy benzoquinone	168.15		85	53, 59, 69, 80, 82, 97, 109, 112, 123, 127, 138, 169, 170
Syringaldehyde	182.17		93	51, 65, 79, 93, 96, 111, 139, 153, 167, 182, 183
Acetosyringone	196.19		95	43, 65, 67, 79, 85, 93, 108, 123, 138, 153, 167, 181, 196
Acetyl vanillin	194.18		90	44, 52, 65, 79, 122, 152
Diisobutyl phthalate	278.35		91	41, 57, 65, 76, 104, 121, 149, 150, 167, 223
Ethyl benzene	106.17		98	51, 65, 74, 77, 91, 106, 107
Styrene	104.15		95	43, 50, 51, 77, 78, 84, 104

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Fig. 11 shows the variation of concentration of the major compounds present in DCM extract after different treatment durations. It is interesting to note that styrene concentration drops to zero after the mixture is irradiated, while ethyl benzene production increases drastically. Maximum production of ethyl benzene is observed at 1 h of irradiation. Vanillin and acetyl vanillin production decreases after UV-irradiation, and an increase in production of acetovanillone is observed. Syringaldehyde and acetosyringone production continuously increases with irradiation time. The overall mass conversion of lignin to organic compounds in the total time period including dark stirring and photocatalysis is found to be in the range of 17–20 wt% for TiO₂–lignin mixtures obtained by wet milling in presence of water. In order to understand the structure of lignin that is used in the experiments, and the similarity of the products obtained via photocatalysis versus fast pyrolysis, which is a reasonably well established technique, analytical pyrolysis of the lignin sample was performed in a micropyrolyzer coupled with GC/MS (Py-GC/MS). Table S2 (in ESI data†) depicts the typical products obtained and the relative area% contributions. Based on the products obtained from Py-GC/MS, the contribution of coniferyl, sinapyl and coumaryl units to the total phenolics present in lignin is found to be 56%, 28% and 16%, respectively. This is also in agreement with a literature report.²⁸ This shows that ALM lignin contains a significant fraction of sinapyl units, which is observed as syringol derivatives after photocatalytic depolymerization. It is evident that the major products formed during photocatalysis, including acetosyringone, acetovanillone, syringaldehyde and vanillin, are also observed in fast pyrolysis. This shows that the mechanism of transformation of lignin and its oligomers to phenolic compounds in both these processes follows a free radical pathway involving the cleavage of alkyl-aryl ether (α-O-4 and β-O-4), aryl–aryl ether (4-O-5) and aryl–aryl (5–5) bonds, hydrogen abstraction and β-scission reactions. While thermal energy is the main driving force for these bond fission reactions in thermolysis, UV radiation and the ˙OH radicals initiate these reactions in photocatalysis. Importantly, hydroxyl radicals can react with benzene ring via electrophilic addition and cause the cleavage of α-O-4 or β-O-4 ether links in lignin.¹⁵ As a result, OH group substitution is achieved. The formation of dimethoxy benzoquinone was earlier proposed to occur by the action of singlet oxygen (¹O₂) or superoxide radicals (O₂˙⁻) on the phenolic ring, which results in the cleavage of the bond between aromatic and the α-carbon.¹⁵ Dimethoxy benzoquinone is also formed during ionic liquid assisted depolymerization of lignin. The decrease in concentration of acetyl vanillin on UV irradiation shows that deacetylation reaction also occurs during photocatalysis. Moreover, demethylation, dealkoxylation and hydroxylation are some other reactions mediated by hydroxyl radicals.¹⁵ The first two reactions convert methoxy substituents to hydroxy substituents. Thus, sinapyl derivatives can be effectively converted to guaiacol derivatives, and guaiacol derivatives can be converted to simple phenols. On long duration exposure to UV radiation, simple phenolics and guaiacols can be converted to ring opened fragments such as C4–C6 linear carboxylic acids.^15,45 These are finally mineralized to CO₂ and H₂O.

Fig. 11 Variation of concentration (in mg g_lignin⁻¹) of the major products obtained in the DCM extract from photocatalysis of WMW lignin–TiO₂ mixture.

While wet ball milling of lignin–TiO₂ mixtures followed by photocatalysis is demonstrated to be a promising method for the production of phenolics, the saturation of catalytic activity after 6 h of UV exposure presents a reasonable challenge to adopt this method for the treatment of lignin in a practical setting. One way to recover the catalyst is to first separate the solid phase lignin–TiO₂ mixture after photocatalysis, and then dissolve it in alkaline medium. This will solubilize lignin and adsorbed intermediates, and will free the active sites. Thus, the degradation of lignin–TiO₂ suspension can be an initial step for alkaline degradation of lignin. Importantly, this technique can be used to initially reduce the concentration of lignin, especially from paper and pulp industry reject, to a reasonable value before subjecting it to other processing techniques.

4. Conclusions

In this work, we have shown that wet milling of lignin and TiO₂ using polar solvents improves the contact between lignin and TiO₂, thus facilitating lignin depolymerization to produce valuable phenolics and aromatic hydrocarbons. For the first time, photocatalysis of lignin is carried out by suspending the lignin–TiO₂ mixtures in water, without completely dissolving lignin in alkaline medium. The change in intensity of emission lines in photoluminescence spectra, appearance of carbon peak in XRD pattern and high onset degradation temperature in TGA for WMW and WMA indicate that interactions may be involved when lignin and TiO₂ are wet milled in presence of polar solvents. Wet milling in presence of water is shown to depolymerize lignin besides causing particle size reduction and change in particle size distribution. The extent of depolymerization caused by wet milling was evident from the formation of water soluble phenolic compounds during the dark stirring period prior to photocatalysis. Based on the concentration of phenolics produced during the dark stirring period, various ball milled lignin–TiO₂ mixtures can be ranked as follows: wet milled (water) > wet milled (hexane) > dry milled > wet milled (acetone). Compared to the dry milled and physical mixtures, photocatalysis of wet milled mixtures resulted in high production of phenolics.

Based on the total phenolics produced during photocatalysis, the mixtures can be ranked as follows: wet milled (acetone) > wet milled (water) > wet milled (hexane) > dry milled > physical mixture. The major organic compounds identified via MALDI-TOF/MS and GC/MS include ethyl benzene, acetovanillone, acetosyringone, syringaldehyde, styrene, acetyl vanillin, vanillin, 2,6-dimethoxy benzoquinone and diisobutyl phthalate. Importantly, the production of the first four compounds in the above list increased with UV irradiation time. Moreover, a significant reduction in molecular weight of lignin was observed during photocatalysis of wet milled (water) mixture. The promising results of this study demonstrate that value added phenolic compounds can be extracted from lignin via photocatalysis. The key challenges are to improve (a) the yield and selectivity of phenolics by enhancing the contact between lignin and TiO₂, and (b) the life time of the catalyst by avoiding deactivation. More detailed studies are required to unravel the mechanism of photocatalytic production of these organic compounds from lignin extracted from different biomass feedstocks.

Acknowledgements

The corresponding author thanks Chevron Inc. for funding the project via alumni grant. National Center for Combustion Research and Development (NCCRD) is sponsored by Department of Science and Technology (DST), India. The authors thank the anonymous reviewers for the constructive suggestions to improve the manuscript.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra25954a

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