Exploiting the photocatalytic activity of TiO2towards the depolymerization of Kraft lignin

Article abstract of DOI:10.1039/d1nj03043d

Lignin is a promising renewable source of aromatic chemicals. Described herein is a new photocatalytic methodology for depolymerization of oxidized Kraft lignin to industrially relevant aromatic platform chemicals. The photooxidation route begins with oxidation of alcohol moieties using a gold nanoparticle/hydrotalcite composite. Next, the oxidized lignin is subjected to UVA irradiation in the presence of TiO₂, leading to a 3-fold decrease of its molecular weight and to the formation of various monolignols. The products obtained in both steps were characterized using FTIR, SEC, GC-MS, and 2D NMR spectroscopy, which confirm the depolymerization of lignin to smaller molecular weight products. This method offers both energetic and chemical advantages over thermochemical processing for lignin valorisation.

Full text of DOI:10.1039/d1nj03043d

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Exploiting the photocatalytic activity of TiO2
towards the depolymerization of Kraft lignin†
Cite this: DOI: 10.1039/d1nj03043d
Charles-Oneil L. Crites, *a Fernanda Gomes de Mendonça,b
b
e
Jose Carlos Netto-Ferreira,cd R. Tom Baker,
´
Geniece L. Hallett-Tapley
and
Luc Tremblayf
Lignin is a promising renewable source of aromatic chemicals. Described herein is a new photocatalytic
methodology for depolymerization of oxidized Kraft lignin to industrially relevant aromatic platform
chemicals. The photooxidation route begins with oxidation of alcohol moieties using a gold nanoparticle/
hydrotalcite composite. Next, the oxidized lignin is subjected to UVA irradiation in the presence of TiO2,
leading to a 3-fold decrease of its molecular weight and to the formation of various monolignols. The
products obtained in both steps were characterized using FTIR, SEC, GC-MS, and 2D NMR spectroscopy,
which confirm the depolymerization of lignin to smaller molecular weight products. This method offers both
energetic and chemical advantages over thermochemical processing for lignin valorisation.
Received 21st June 2021,
Accepted 29th July 2021
DOI: 10.1039/d1nj03043d
rsc.li/njc
One approach to transforming lignin into value-added che-
micals, involves breaking it into smaller molecules. Previous
Introduction
Lignin is the third main constituent of wood, after cellulose studies have reported both complete3 and partial pyrolysis2 of
and hemicellulose.1 During the milling process, cellulose is lignin as an alternative towards its valorisation, although most
isolated and used in paper manufacture, while lignin is most of the oxygenated functional groups are lost through these
commonly discarded and considered as waste. Indeed, lignin is processes. Moreover, the large number of compounds formed
often burned to produce heat for paper mill operations, con- make such thermochemical processes unsuitable for fine
tributing to a large excess of greenhouse gas emissions at the chemical production. A more targeted approach employs a
expense of the destruction of an abundant, natural source of catalyst to break specific bonds in lignin while retaining some
aromatic compounds. The valorization of lignin presents an
ecological option for sourcing aromatic molecules for indus-
trial applications. The current focus of this valorization process
has been three specific linkages in the lignin macromolecular
backbone: b-O-4, b-50, and 4-O-50 (Fig. 1).2 These linkages are
most common in softwood lignin, but the percentage of each is
source dependent.
a
´
Secteur Science, Universite de Moncton, Edmundston, New Brunswick, E3V 2S8,
Canada. E-mail: charles-oneil.crites@umoncton.ca
b Department of Chemistry and Biomolecular Sciences and Centre for Catalysis
Research and Innovation, University of Ottawa, Ottawa, Ontario K1N 6N5,
Canada
c Department of Organic Chemistry, Universidade Federal Rural do Rio de Janeiro,
´
Seropedica, Rio de Janeiro, 23890-000, Brazil
d Department of Organic Chemistry, Federal University of Rio de Janeiro, CT,
´
Block A, Cidade Universitaria, 21941-909, Rio de Janeiro, RJ, Brazil
e Department of Chemistry, St. Francis Xavier University, P. O. Box 5000,
Antigonish, NS, B2G 2W5, Canada
f
´
Department of Chemistry and Biochemistry, Universite de Moncton, Moncton,
New Brunswick, E1A 3E9, Canada
† Electronic supplementary information (ESI) available: 1H and GC-MS of cata-
Fig. 1 Representation of the substructure of lignin highlighting the
b-O-4, b-50 and 4-O-50 linkages.
lytic reactions. See DOI: 10.1039/d1nj03043d
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critical structural characteristics.2 Metal-based catalysts and, in wavenumbers between 4000–600 cmꢁ1. Size exclusion chroma-
particular, photocatalysts, may allow for specific bond cleavage tography (SEC) was carried out using an Agilent 1200 HPLC
in lignin while preserving its functional groups, given previous system fitted with a diode-array detector (ldetection = 254 nm).
successes on smaller molecules with similar substituents.4–8
Components were separated on a SUPREMA analytical Linear S
To date, much of the work on lignin valorisation has been column (8 ꢂ 300 mm; 5 mm) set to 40 1C and using a 20/80 CH3OH
performed on small, model molecules, which are not fully (HPLC grade, BDH) and 10 mM ammonium bicarbonate(aq.)
representative of the lignin structure. In industry, most lignin (Sigma-Aldrich) eluent (flow rate = 0.3 mL minꢁ1; injec-
waste is isolated from the Kraft process, which leads to the tion volume = 40 mL). SEC column calibration was performed
incorporation of sulfur functionalities in its molecular using polystyrene sulfonate sodium salts, salicylic acid and
structure.9 Sulfur’s role in heterogeneous catalyst deactiva- acetone. 2D NMR spectra were acquired at room temperature
tion has likely hampered many past lignin transformation using a Bruker AVANCE 500 MHz spectrometer, based on the
processes.10 Nonetheless, the investigation of processes using method reported by Stahl et al.20 In short, an aliquot of the
bulk lignin is paramount towards improving the industrial reaction mixture was evaporated under reduced pressure and
relevance of valorisation pathways.
dissolved in 1 mL of DMSO-d6. The structural units of lignin
The valorisation of lignin through several photochemical were assigned by comparison with previously reported litera-
approaches has been reported.7,11–15 For instance, Chen et al. ture values.21 GC-MS analyses were performed on an Agilent
demonstrated the photo-depolymerization of a lignin model Technologies 6890N GC system equipped with an Agilent
using In2S3 nanoparticles.7 In this case, aromatic monomers Technologies 5973 Network mass spectrometer. The chroma-
including acetovanillone and vanillin were obtained from tographic conditions were as follows:
a Rtx-5 column
guaiacylglycerol-b-guaiacyl ether after 12 h under visible light (105 m ꢂ 0.32 mm ꢂ 1 mm) from Restek was employed, and
irradiation. Nevertheless, the application of this process to a the injector temperature was set to 250 1C in split mode with
real lignin fraction was not investigated. Additional examples an injection volume of 2 mL. Helium was the carrier gas at
rely on the use of the well-known photooxidant TiO2 towards 1.6 mL minꢁ1. The transfer line and source temperatures were
lignin depolymerization.16–19 Wang et al. described a tandem set to 280 1C. The oven temperature was initially at 55 1C for
method for cleaving C–O bonds of b-O-4 lignin models using 4 min, then increased at a rate of 35 1C minꢁ1 until 150 1C and
Pd/ZnIn2S4 and TiO2–NaOAc through photooxidation/hydroge- held for 5 min. A second increase in temperature was then
nolysis.18 In this one-pot strategy, selective oxidation of a-C–OH applied at 30 1C minꢁ1 until 250 1C, with a hold time equal to
takes place over Pd/ZnIn2S4 under a 455 nm light, while the 5 min. The mass spectral database NIST/EPA/NIH EI (NIST05)
TiO2–NaOAc system cleaves C–O bonds through hydrogenolysis was used for identification purposes.
by switching to a 365 nm light. Once again, lignin models were
the sole targets of this study.
Synthesis of gold nanoparticles supported on hydrotalcite
(AuNP/HT)
Herein, we present the use of UV-light activated TiO2 in real
lignin depolymerization. More specifically, Kraft lignin is oxi- The synthesis of AuNP/HT was adapted from Gupta et al.22 In a
dized and then photodepolymerized to afford monolignol-like round-bottomed flask, 41.2 mg (0.11 mmol) of HAuCl4ꢀ3H2O
products. This approach results in value-added products from was dissolved in 40 mL of ultrapure water. The pH was adjusted
lignin through an economical, energy-efficient process as com- to 10 using an aqueous solution of NH3 (28% v/v in H2O). The
pared to traditional pyrolysis pathways.
mixture was stirred for 6 h at room temperature and then
heated to 100 1C for 30 min. The yellow solid (as a result of
Au(OH)3 precipitation and deposition) was filtered, washed
with ultrapure water and dried in an oven at 200 1C for 4 h to
yield 1.0 g a purple solid. The catalyst tended to lose activity a
few weeks after synthesis, as was evidenced by no discernible
variations in the lignin oxidation IR spectrum.
Methods and materials
Materials
Hydrotalcite (HT) was purchased from Sigma-Aldrich and used as
received. TiO2 P25 was obtained from Evonik Degussa and used as
received. Tetrachloroauric acid (HAuCl4ꢀ3H2O) was purchased
from Alfa Aesar. Ammonia (28% v/v in H2O) was purchased from
Fisher Scientific Canada. Toluene (Anachemia) and methanol
(Fisher Scientific Canada) were used for all catalytic reactions.
Ultrapure water was collected in-house (18.2 MO cm at 25 1C) and
used in the synthesis of all nanoparticle composites. Hydroxylated
Kraft lignin (FPInnovations Canada) was used in all oxidation and
depolymerization reactions.
AuNP/HT-mediated oxidation of Kraft H-lignin
In a round-bottomed flask, 100 mg of H-lignin was dissolved in
20 mL of toluene. Next, 50 mg of AuNP/HT was added. The
reaction mixture was stirred and refluxed for 4 h under a
continuous oxygen purge. Different reaction times were investi-
gated (e.g., 2, 6, and 8 h), although 4 h afforded the best results.
Control reactions (HT, no catalyst) were performed under the
same reaction conditions to confirm the need for AuNP.
Instrumental analysis
Photolysis in the presence of TiO2
ATR-FTIR analysis was performed on a Cary 630 Fourier trans- In a glass vial, 30 mg of lignin (Kraft H-lignin or oxidized Kraft
form infrared spectrometer with 8 scans per sample in H-lignin; OKHL) was dissolved in 6 mL of methanol in the
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presence of 10 mg of TiO2. The mixture was then irradiated for
6 and 18 h using two UVA LEDs centred at 365 nm (21.6 W mꢁ2
)
and the irradiance was measured by a Luzchem Spectroradi-
ometer. Control reactions were also performed in the absence
of TiO2 using the same reaction conditions.
Results and discussion
Oxidation of Kraft H-lignin
The primary step of our depolymerization approach relies on
the gold nanoparticle/hydrotalcite (AuNP/HT) catalyzed oxida-
tion of the alcohol moiety in lignin. AuNP/HT was chosen as a
catalyst based on the work of Mitsudome et al.23 that clearly
demonstrated its high activity towards alcohol oxidation reac-
tions. A subsequent TiO2-photoinduced scission of the b-O-4
linkages would then afford the desired aromatic building
blocks as represented in Scheme 1 (lignin model is shown for
clarification). The AuNP/HT-promoted alcohol oxidation was
initially assessed using ATR-FTIR (Fig. 2). No significant
changes were observed when comparing the spectrum of origi-
nal H-Kraft lignin with the spectra collected after oxidation
performed in the absence of catalysts or using pristine HT.
However, in the presence of AuNP/HT, the OH stretching band
at B3300 cmꢁ1 showed a significant decrease while the relative
intensity of the band at B1700 cmꢁ1 increased. This is the first
indication of successful OH oxidation generating carbonyl
groups conjugated with an aromatic ring.16 These preliminary
screening trials suggest that AuNP/HT catalysts are capable of
successfully oxidizing the alcohol moiety of the b-O-4 linkage
under mild conditions (toluene reflux) and a short time (4 h). This
finding is congruent with previous literature reports on the
success of supported AuNP-mediated alcohol oxidation.23–26
Size exclusion chromatography (SEC) was performed to
monitor the overall changes to lignin macrostructure through
the detailed analysis of its molecular weight. Ideally, the simple
oxidation of the b-O-4 linkage should not result in substantive
changes in the lignin molecular weight, regardless of the
reaction conditions. In all cases, the maximum MW remains
relatively constant between 1500–1800 g molꢁ1 (Fig. 3).
Fig. 2 Normalized ATR-FTIR spectra of H-Kraft native lignin before and
after oxidation under the following conditions: no catalyst, HT only and
AuNP/HT. Highlighted: bands at B3300 and B1700 cmꢁ1
.
Fig. 3 SEC analysis of reaction mixtures following oxidation of Kraft lignin
(black) oxidation (O2, 110 1C, 4 h) using the following catalytic conditions:
no catalyst (yellow), HT only (green) and AuNP/HT (blue). Size exclusion
chromatography (SEC) was performed to monitor the overall changes to
lignin macrostructure through the detailed analysis of its molecular weight.
Ideally, the simple oxidation of the b-O-4 linkage should not result in
substantive.
Depolymerization of lignin H-Kraft using UV light-activated
TiO2
The successful oxidation of the OH groups in the b-O-4 linkage is
crucial for the effective depolymerization of lignin. To support this
claim, a series of control reactions were performed on non-oxidized
H-Kraft lignin with and without TiO2 under an inert atmosphere.
ATR-FTIR spectra collected after these experiments indicate some
alcohol oxidation (Fig. 4). Here, a decrease in the relative intensity of
the OH signal located between 3600–3000 cmꢁ1 was observed when
TiO2 was employed as the catalyst under sample irradiation, sug-
gesting that alcohol oxidation was the sole outcome of this experi-
ment. SEC experiments were not performed due to lack of solubility
in methanol, again indicating that depolymerization did not occur.
When the reaction was carried out in the presence of oxygen, lignin
was completely mineralized, as indicated by the presence of only few
milligrams of starting material at the end of the reaction.27
Photodepolymerization of oxidized Kraft H-lignin (OKHL)
The photodepolymerization of OKHL was performed using
UV-activated TiO2 over 6 and 18 h, with longer reaction times
Scheme 1 Two-step reaction of depolymerization of b-O-4 lignin model
2-phenyl-1-phenylethan-1-ol.
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with longer irradiation period (i.e., 6 vs. 18 h). Second, the
depolymerization is highly dependent on the presence of the
TiO2 photocatalyst. In the absence of TiO2, UV exposure of
the OKHL mixture afforded relatively low amounts of smaller
MW products, given by the low signal intensity, suggesting that
the products are likely oligomeric in structure. However, a
higher depolymerization degree was reached using TiO2 as
the photocatalyst after 18 h of UV irradiation, affording a
high-intensity signal at considerably lower MW values than
the OKHL starting material.
Using 2D NMR (1H–13C HSQC) we were able to closely
examine the success of TiO2-mediated OKHL depolymerization
and to give more insight into possible changes in the chemical
structure of lignin.21 Specifically, previous reports have sug-
gested that the disappearance of certain resonances in the
aliphatic region of the NMR spectra provide critical informa-
tion regarding changes in the nature of specific linkages.29 For
instance, if alcohol oxidation occurs, we would observe the
complete or partial disappearance of Ha (Scheme 2A). The NMR
resonances related to Ha appear at around 75 ppm/6.1 ppm in
the 13C/1H NMR spectra.30 In our case, no signals related to Ha
were observed in the HSQC spectrum of OKHL (Fig. 7A),
indicating that significant alcohol oxidation took place in the
first step (i.e., the oxidation of H-Kraft lignin with AuNP/HT).
2D NMR spectra also point to significant structural changes
in OKHL following photodepolymerization under UV irradia-
tion with and without TiO2 (Fig. 7B and C, respectively). Since
lignin is a polymer, its depolymerization will afford small
oligomers or monomers with lower molecular weights
(Scheme 2B), therefore reducing the chemical environment
diversity and leading to smaller correlation peaks in HSQC or
even the absence of these peaks. In fact, the OMe-related
resonances are present in all spectra but are less predominant
after UV irradiation (Fig. 7B and C), with new chemical shift
correlations in the OMe region pointing to structural changes
in OKHL.
Fig. 4 Normalized ATR-FTIR spectra of native Kraft H-lignin following UV
irradiation with and without TiO2 (reaction conditions: N2 atmosphere;
18 h).
giving rise to more effective valorization. The ATR-FTIR spectra
(Fig. 5) highlight significant changes to the oxidized lignin
following UV irradiation. First, OH stretching bands emerged
at approximately 3600 cmꢁ1 following UV light treatment of
OKHL, presenting a higher intensity in the presence of TiO2
photocatalyst. This trend can be attributed to the generation of
phenolic compounds,28 one of the major products of the lignin
depolymerization process. Though initial assessments of reac-
tion mixtures strongly suggest depolymerization is taking place,
a more in-depth analysis using SEC and NMR spectroscopy was
required.
SEC chromatograms presented in Fig. 6 show that UV light
exposure promotes a substantial decrease in the molecular
weight of the products, strongly supporting initial conclusions
from IR spectra (Fig. 5). Two main conclusions can be drawn
from the SEC data: first, OKHL depolymerization increased
Fig. 5 Normalized ATR-FTIR spectra of OKHL starting material, UV irra-
diated OKHL for 6 and 18 h, and UV irradiated TiO2/OKHL for 6 and 18 h. All
reactions were performed under N2.
Fig. 6 SEC chromatograms of OKHL starting material (blue), UV irradiated
OKHL for 6 (black) and 18 h (gray) and UV irradiated TiO2/OKHL for
6 (green) and 18 h (purple). All reactions were performed under N2.
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Scheme 2 (A) Alcohol oxidation and (B) depolymerization of lignin via C–
O bond cleavage of the b-O-4 linkage.
In the case of OKHL after UV irradiation in the presence of
1
TiO2, a strong resonance at 8.0 ppm in the H NMR spectrum
could point to the presence of the monolignol, indicating lignin
depolymerization21 (see ESI† for 1H NMR spectra of all samples,
Fig. S1). The new correlation at 50 ppm/3.1 ppm observed after
UV treatment can be related to a CH3 group from methanol.
Alternatively, it could also be assigned to the methoxy groups
present in the lignin fragment generated following depolymeri-
zation.
Previous work has demonstrated the ability of AuNP sup-
ported on custom-made hydrotalcite-related materials to facil-
itate the transformation of lignin to monolignol-type
products.31 However, in our case significant formation of a
monolignol-type product did not take place in the oxidation
step in the presence of AuNP/HT, as previously discussed.
Therefore, it is reasonable to assume that AuNP/HT itself is
not able to depolymerise the H-Kraft lignin, which takes place
upon UV irradiation in the presence of TiO2.
GC-MS analysis of the UV-irradiated OKHL reaction mixture
in the presence of TiO2 likewise indicated lignin depolymeriza-
tion and the generation of products which are consistent with
monolignols (Fig. S4, ESI†). On the other hand, both OKHL and
UV irradiated OKHL (in the absence of TiO2) did not present
any peaks related to monolignol-like products, except a small
peak related to vanillin. In these cases, the peaks observed in
the chromatogram could be attributed to products with mole-
cular weights that are not compatible with monolignols (Fig. S2
and S3, ESI†).
In agreement with previous reports by Scaiano and co-
workers,32 UV irradiation of OKHL by itself induces effective
cleavage of the b-O-4 linkage through a photogenerated triplet
state of the carbonyl moiety. However, the current work shows
that addition of TiO2 to the reaction media under UV irradia-
tion favours the production of monolignol-like products. There-
fore, considering that the end goal of lignin depolymerization is
to obtain monolignol-like products, the use of TiO2 is a definite
Fig. 7 HSQC 2D NMR spectrum of (A) OKHL, (B) OKHL following 18 h of
UV irradiation. And (C) OKHL following 18 h of UV irradiation in the
presence of TiO2.
advantage. In terms of the mechanism involved in this process,
33
¨
Reckenthaler et al. demonstrated that electron transfer from
catalyzed by TiO2 and the direct reaction using UV). Given this,
traditional radical scavenging experiments to identify free
radicals generated throughout the course of the mechanism
would lack precision.
Moreover, since monolignol products are more present
when TiO2 was used rather than in its absence, further electron
transfer events from TiO2 to lignin cannot be ruled out.
Electron transfer to other bonds in the lignin chemical
TiO2 to acetophenone can afford the ketyl radical. Thenceforth,
a similar electron transfer event may also play a key role here,
through interaction with OKHL to form a ketyl radical and
undergo a similar b-O-4 cleavage as suggested by Netto-Ferreira
and co-workers.34 The product formation in the presence of
TiO2 is summarized in Scheme 3 and is the basis for this
proposed mechanism. In the current study, two free radical
reactions are likely operating in parallel (i.e., the reaction
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Acknowledgements
´
COLC would like to acknowledge l’Universite de Moncton for a
research grant. GHT thanks the Natural Sciences and Engineer-
ing Research Council of Canada Discovery Grant Program
(NSERC-DG), the Canada Foundation for Innovation (CFI) and
ResearchNS for gracious financial support of her research
program. RTB and FGM thank the Canada Research Chairs
Program for operating support.
Notes and references
Scheme 3 Proposed mechanism for photocatalyzed depolymerization
by TiO2 (activated by UV light) of a representative molecule present in
lignin containing the b-O-4 bond.
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Conflicts of interest
The authors declare no conflicts of interest.
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