Highly Selective Photocatalytic Valorization of Lignin Model Compounds Using Ultrathin Metal/CdS

Article abstract of DOI:10.1021/acscatal.9b02842

Depolymerization of recalcitrant lignin is a crucial step in realizing the full potential of transforming biomass to value-added small organic molecules. Herein, we report a photocatalytic system consisting of ultrathin CdS nanosheets decorated with first-row transition metals (M/CdS) for the direct photocleavage of lignin model compounds to small aromatic products. In the meantime, the reducing power of M/CdS is utilized to simultaneously generate another valuable product, H₂, hence eliminating the necessity of sacrificial reagents and maximizing the energy conversion efficiency. We further demonstrate that, by judiciously modulating the photocatalytic conditions, it is feasible to yield different products with very high selectivity using the same catalyst of Ni/CdS. A series of control experiments were performed to investigate the mechanistic steps of each reaction and highlight the important roles played by both solvent and base in the photocleavage of the β-O-4 bond in lignin valorization.

Full text of DOI:10.1021/acscatal.9b02842

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Article
Highly selective photocatalytic valorization of
lignin model compounds using ultrathin metal/CdS
Guanqun Han, Tao Yan, Wei Zhang, Yi C Zhang, David Y. Lee, Zhi Cao, and Yujie Sun
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b02842 • Publication Date (Web): 01 Nov 2019
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Highly Selective Photocatalytic Valorization of
Lignin Model Compounds Using Ultrathin
Metal/CdS
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Guanqun Han[a], Tao Yan[b], Wei Zhang[c],[d], Yi C. Zhang[e], David Y. Lee[e], Zhi Cao[b],[c]*, and
Yujie Sun[a]*
[a] Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45040, United States
[b] Syncat@Beijing, Synfuels China Co., Ltd, Beijing 101400, China
[c] State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy
of Sciences, Taiyuan 030001, China
[d] University of Chinese Academy of Sciences, Beijing, 100049, China
[e] Department of Chemistry and Materials Science & Engineering Program, Washington State
University, Pullman WA 99163, United States
ABSTRACT. Depolymerization of recalcitrant lignin is a crucial step in realizing the full potential
of transforming biomass to value-added small organic molecules. Herein, we report a
photocatalytic system consisting of ultrathin CdS nanosheets decorated with 1st-row transition
metals (M/CdS) for the direct photocleavage of lignin model compounds to small aromatic
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products. In the meantime, the reducing power of M/CdS is utilized to simultaneously generate
another valuable product H2, hence eliminating the necessity of sacrificial reagents and
maximizing the energy conversion efficiency. We further demonstrate that by judiciously
modulating the photocatalytic conditions, it is feasible to yield different products with very high
selectivity using the same catalyst Ni/CdS. A series of control experiments were performed to
investigate the mechanistic steps of each reaction and highlight the important roles played by both
solvent and base in the photocleavage of the β-O-4 bond in lignin valorization.
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Keywords: lignin valorization • photocatalysis • selective photocleavage • ultrathin CdS • nickel
co-catalyst
Introduction
Upgrading biomass to produce value-added fuels and commodity chemicals has been considered
as a sustainable and green approach in utilizing renewable carbon resources, complementary to
the conventional chemical industry which heavily relies on fossil fuels, in that biomass consists
of contemporary carbon whose utilization will not alter the CO2 concentration in the
atmosphere.1-4 In this context, lignocellulosic biomass, including cellulose, hemicellulose, and
lignin, has attracted increasing interest in biorefinery.5-10 In particular, lignin is a highly
functionalized biopolymer that can be transformed to small aromatics, however its recalcitrant
structure renders it challenging to be depolymerized.11-13 The primary hurdle of lignin
valorization is the selective and effective cleavage of those complex linkages connecting
aromatic units in the lignin structure. One of the prevalent linkages is the β-O-4 bond which is
estimated to constitute 43-65% of all lignin linkages.14, 15
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Because the homolytic bond dissociation enthalpy of Cβ-O has been theoretically predicted to
decrease from 69.2 to 55.9 kcal/mol once the benzylic Cα-OH group is oxidized to ketone,16 a
two-step β-O-4 cleavage pathway has been widely adopted for lignin depolymerization, wherein
the oxidation of Cα-OH to ketone is followed by the break of the Cβ-O bond.17-19 Consequently,
oxidative cleavage of lignin and its model compounds has been the subject of intensive
investigation.15 Nevertheless, the majority of reported methods require harsh conditions and
expensive/toxic chemical oxidants with unsatisfactory yield and/or selectivity.20 Along the theme
of green chemistry, O2 is a desirable oxidant owning to its low cost, large abundance, and benign
nature. For instance, Stahl et al reported an AcNH-TEMPO-mediated (AcNH-TEMPO = 4-
acetamido-2,2,6,6-tetramethylpiperidine-N-oxyl) catalytic system for the cleavage of lignin
models utilizing O2 as the external oxidant for the first step, Cα-OH oxidation to ketone
(Figure1a).21 Taking advantage of electricity and light, together with powerful hydrogen atom
transfer mediators like N-hydroxyphthalimide (NHPI) and expensive Ir photocatalyst (i.e.,
[Ir(ppy)2(dtbbpy)](PF6)), Stephenson’s group reported a one-pot two-step strategy for the
cleavage of lignin model compounds, electrocatalytic oxidation of benzylic alcohol followed by
photocatalytic β-O-4 cleavage (Figure 1b).22-24 However, the homogeneous nature of the above
two catalytic systems makes the product separation and purification challenging. In addition, the
cost and long-term robustness of redox mediators (i.e., AcNH-TEMPO and NHPI) and Ir-based
photocatalysts also question their applicability on a large scale. Compared to the aforementioned
homogeneous catalytic systems, solid-state catalysts have been recognized with unique
advantages like easy separation and potential recyclability. A particularly attractive option is
photocatalytic valorization on semiconductors.25 As shown in Figure 1c, Wang et al developed a
transfer hydrogenolysis protocol for the photocatalytic valorization of lignin models on
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ZnIn2S4.26 Although the ketone intermediate could be obtained, its yield is always quite low
(32% in 1 atm O2 atmosphere while in most cases less than 10%). In the meantime, the direct
cleaved products of acetophenone and phenol could be detected with good to poor yields.
Overall, this system lacks high selectivity for desirable products.
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Figure 1. Reported oxidation and cleavage strategies of lignin model compounds and our
proposed strategy of photocleavage with two possible pathways. (a) Chemical oxidation. (b)
Electrocatalytic/photocatalytic tandem cleavage. (c) Photocatalytic cleavage. (d) Our strategy.
On the other hand, solar-driven H2 production via semiconductor/co-catalyst systems is generally
regarded as a promising approach in transforming and storing intermittent solar energy in H2, in
that H2 is not only a clean fuel but also an important commodity chemical.27, 28 This process only
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utilizes excited electrons generated in semiconductors upon light irradiation. However, the
excited holes will compete to recombine with excited electrons and a common strategy to
mitigate charge recombination is to use sacrificial electron donors to quench those excited holes,
which eventually wastes the oxidizing power of excited holes. A more desirable strategy is to
make the use of both excited electrons and holes for two valuable reactions. Within this context,
it has become an increasing interest in coupling oxidative biomass valorization with concomitant
H2 production, which will not only eliminate sacrificial reagents, but also produce two types of
value-added products with maximized photo energy utilization. Our group has initiated a
program focusing on this integrated photocatalysis strategy and recently reported the
photocatalytic upgrading of biomass-derived HMF (5-hydroxymethyl furfural) to more valuable
DFF (2,5-diformylfuran) with the co-production of H2 under ambient conditions.29 Herein, we
explore a library of co-catalysts decorated on two-dimensional (2D) ultrathin CdS nanosheets
(M/CdS, M = Mn, Fe, Co, Ni, Cu) and achieve different products from the oxidative
photocleavage of lignin model compounds in highly selective manners (Figure 1d).
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Results
Synthesis and characterization of photocatalysts M/CdS
Our ultrathin CdS nanosheets were obtained from the reaction between CdCl2 and S in
diethylenetriamine at 80 °C for 30 min under microwave irradiation. Subsequently, the resulting
CdS nanosheets were mixed with common metal chloride salts and NaBH4 to obtain the final
M/CdS photocatalysts (M = Mn, Fe, Co, Ni, or Cu). Such a two-step microwave-assisted
synthetic strategy enabled us to readily synthesize five different photocatalysts for a systematic
study. Figure 2a compiles the X-ray diffraction (XRD) patterns of M/CdS as well as the pristine
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CdS. Compared to the standard XRD pattern (JCPDS 41-1049) of CdS, it is apparent that the
decoration of 1st-row transition metals did not alter the crystal structure of each catalyst which
still maintained the wurtzite CdS structure. The relatively poor crystallinity of these samples was
most likely due to their ultrathin thickness as discussed later. ICP-OES data were collecred for
the all M/CdS samples and the Ni weight percentage in Ni/CdS was ~5.0% (Table S1). The
scanning electron microscopy (SEM) images of Mn/CdS (Figure S1), Fe/CdS (Figure S2),
Co/CdS (Figure S3), Ni/CdS (Figure 2b and Figure S4), and Cu/CdS (Figure S5) were collected
to confirm their nanosheet morphology. No apparent nanoparticles or aggregates were observed,
implying the very small size of those decorated transition metals on CdS. Elemental mapping
analysis also confirmed the identity and homogeneous distribution of Cd, S, and Ni in Ni/CdS
(Figure 2c and Figure S6). Similar results could be obtained for the other four samples (Figure
S7 – S10). In line with the aforementioned characterization results, X-ray photoelectron
spectroscopy (XPS) further verified the composition of anticipated elements in these
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photocatalysts, including Cd, S, and the corresponding transition metal (Figure S11). Figure 2d
presents the high-resolution Cd 3d XPS spectra of all the samples including the bare CdS,
wherein the two distinctive peaks at 411.4 and 404.6 eV can be attributed to the Cd 3d5/2 and
3d3/2 features, respectively.30 For the S 2p spectra shown in Figure 2e, those two peaks at 162.8
and 161.7 eV are corresponding to S 2p3/2 and 2p1/2, respectively.31 Again, the nearly identical
Cd and S XPS spectra of M/CdS and CdS demonstrate that the decoration of transition metals on
CdS does not substantially alter the valence states of both Cd and S. As an example, the high-
resolution Ni 2p spectrum of Ni/CdS was plotted and fitted in Figure 2f. The three sub-peaks at
853.0, 855.9, and 861.5 eV can be assigned to the metallic Ni 2p3/2, Ni2+ 2p3/2, and NiOx 2p3/2
satellite peaks, respectively.32,33 The other subset of peaks at 870.3, 873.7, and 880 eV is
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attributed to the Ni 2p1/2 features. The presence of oxidized nickel species is probably resulting
from exposure to air during synthesis and post treatment. The high-resolution metal 2p XPS
spectra of other samples are included in Figure S12. Atomic force microscopy (AFM)
measurement of Ni/CdS was conducted to confirm its ultrathin thickness. As shown in Figure 2g
and Figure S13, the thickness of Ni/CdS was only ~1.2 nm with no apparent thickness change
after nickel decoration. In order to determine the size of nickel species in Ni/CdS, scanning
tunneling microscopy (STM) was performed. Figure 2h presents the STM image of Ni/CdS
wherein those white spots with a diameter of a few nanometers can be attributed to the supported
nickel. The height profile in Figure 2i demonstrates the average thickness and size of these nickel
nanoparticles in Ni/CdS are ca 0.3 and 3.5 nm, respectively. These observations are drastically
different from that of the pristine CdS nanosheets which possess a very smooth surface (Figure
S14).
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Figure 2. Characterizations of M/CdS. (a) XRD patterns of CdS and M/CdS together with the
standard CdS JCPDS 41-1049. (b) SEM image of Ni/CdS. (c) Elemental mapping images of
Ni/CdS. (d, e) High-resolution XPS spectra of Cd 3d (d) and S 2p (e) of M/CdS and CdS. (f)
High-resolution XPS spectrum of Ni 2p of Ni/CdS. (g) AFM image of Ni/CdS with height
profiles in selected areas. (h) STM image of Ni/CdS. (i) Height vs. displacement cross-sections
of all the nickel species in Ni/CdS in Figure 2h.
Photocatalytic oxidation of benzylic alcohol and its derivatives
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Prior to the photocatalytic oxidation of lignin model compounds, we utilized benzylic alcohol
oxidation as a test reaction to screen the activity of all the synthesized M/CdS samples. As
shown in Figure 3, upon visible light irradiation (8 W LED, 440 – 460 nm) for 2 h in deaerated
water, the pristine CdS nanosheets exhibited negligible activity with less than 5% conversion of
benzylic alcohol and a small amount of H2 was generated. With the decoration of 1st-row
transition metal co-catalyts, the overall performance including benzylic alcohol conversion,
benzaldehyde yield, and H2 production, was substantially increased. Among these M/CdS (M =
Mn, Fe, Co, Ni, and Cu) photocatalysts, it is apparent that Ni/CdS demonstrated the best activity,
achieving 93.4% conversion of benzylic alcohol (Figure 3 and S15). The selectivity of
benzaldehyde was nearly 100% with a reaction yield of 93.4%. In the meantime, nearly similar
yield of H2 was obtained from Ni/CdS (Figure S16 and S17), which was also the highest yield
among those obtained from all the M/CdS photocatalysts (M = Mn, Fe, Co, Ni, and Cu). It
should be noted that all the generated H2 volumes were consistent with the amounts of consumed
benzyl alcohol, implying that H2 was produced from benzyl alcohol instead of H2O. In fact,
similar volume of H2 could be obtained if the photolysis was conducted in CH3CN as shown in
Figure S18. In addition, full conversion from benzyl alcohol to benzaldehyde could be obtained
by increasing the irradiation time to 2.5 h (Figure S19). It is worth mentioning that both metallic
Ni and Ni2+ species in Ni/CdS (Figure 2f) are beneficial for the excellent photocatalytic
property. It has been acknowledged that metallic Ni is an excellent H2 evolution catalyst while
oxidized Ni species can promote organic oxidation reactions.34 Therefore, the co-presence of
metallic Ni and oxidized Ni is anticipated to be beneficial to the overall photocatalytic activity of
H2 evolution and benzyl alcohol oxidation. In order to further demonstrate the significant role of
Ni species, a control experiment was conducted by using parent CdS which was treated with
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NaBH4 without Ni precursor. The corresponding 1H NMR spectra were included in Figure S20
and only a small amount of hydrobenzoin was obtained while no benzaldehyde was detected.
This control experiment unambiguously proved the critical role of Ni species in the
photocatalytic oxidation of benzyl alcohol to benzaldehyde on Ni/CdS. Besides the excellent
activity, our Ni/CdS also exhibited robust stability. Three consecutive cycles using the same
Ni/CdS photocatalyst indicated nearly no degradation during the whole process (Figure S21).
Inspired by the above promising data of Ni/CdS, the following photocatalytic reactions were
performed using Ni/CdS as the catalyst unless noted otherwise.
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Figure 3. Activity comparison of M/CdS photocatalysts for the oxidation of benzyl alcohol,
showing the conversion of benzyl alcohol together with the yields of benzaldehyde yield and H2
using different photocatalysts. Conditions: 10 mM benzyl alcohol and 10 mg photocatalyst in 10
mL deaerated H2O under N2 at room temperature, illumination for 2 h.
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Next, we investigated the influence of electronic effect on the photocatalytic oxidation of
benzylic alcohol on Ni/CdS. A series of benzylic alcohol derivatives with either an electron-
donating (-CH3, 1b) or an electron-withdrawing (-F, 1c or –CF3, 1d) group at the para-position
were employed as the organic substrates. Since Ni/CdS was able to achieve complete benzyl
alcohol oxidation within 2.5 h (Figure S19), we purposely conducted a short period (0.5 h) of
photocatalytic oxidation of these benzyl alcohol derivatives on Ni/CdS. As listed in Table 1,
entries 1 – 4 compared the yields of aldehyde and H2 using each benzyl alcohol derivative. It was
found that 1b with the electron-donating methyl group exhibited the highest yield of the 4-
methylbenzaldehyde (2b, 54%), followed by the yield of benzaldehyde (2a, 46%) from the
oxidation of the parent benzyl alcohol (1a). The installation of electron-withdrawing groups like
–F (1c) and –CF3 (1d) apparently suppressed the reaction rate, resulting in the yields of 4-
fluorobenzaldehyde (2c) and 4-trifluoromethylbenzaldehyde (2d) as 41% and 23%, respectively.
The 1H NMR and GC spectra of these reactions after 0.5 h photocatalysis were presented in
Figures S22-S26. Figure S27 presents the yield evolution of the corresponding aldehyde of each
reaction during 3 h photocatalysis, wherein 1a, 1b, and 1c were also completely transformed to
their respective aldehydes while 1d only reached a conversion of ca. 60%. Such a drastic
difference in reaction rate is in good agreement with their oxidation potentials. The cyclic
voltammograms of 1a-d were measured in CH3CN and compiled in Figure S28. An irreversible
oxidation potential was observed for all the four compounds whose oxidation potentials follow
this order: 1b < 1a < 1c < 1d. This oxidation potential trend is consistent with the
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aforementioned photocatalysis reaction rate. Hence, the much slower reaction rate of 4-
trifluomethylbenzylic alcohol (1d) can be rationalized that the overall reaction was most likely
driven by an outer-sphere electron transfer mechanism, similar to alcohol oxidation via redox
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mediators (e.g., TEMPO) while different from those reactions requiring the deprotonation of the
alcohol group to form a metal alkoxide species as the rate-limiting step.35 In order to further
support this mechanism, a Hammett plot was conducted as shown in Figure S29. During the
initial 30 min, an excellent linear relationship between benzaldehyde yield and time could be
obtained (Figure S29a). From the slopes of those fitting curves, 1b exhibited the largest rate
constant of 1.780 min-1 and the smallest was obtained by 1d, 0.779 min-1. Overall, the reaction
rates were in agreement with the Hammett’s relationship,36 which has a basic equation of log
(k/k0) = σρ, where k is the rate constant of the oxidation of each substituted benzyl alcohol, k0 is
the rate constant of the oxidation of parent benzyl alcohol, σ is the substituent constant and it
only depends on the substituent, and ρ is the reaction constant, which only depends on the type
of the reaction but not on the substituent used. By plotting log k vs. σ, a straight line was
obtained (Figure S29b). Such a remarkable linear relationship strongly supports the electronic
effect exerted by those para substituents on the oxidation of benzyl alcohols to benzaldehydes.37
Table 1. Comparison of photocatalytic oxidation of benzylic alcohol and its derivatives.
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Entry
1a
Substrate
Product
Organic Yield (%)
H Yield (%)
2
OH
O
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46
54
38
17
0
1a
2a
OH
O
2a
3a
4a
5b
6b
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0
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2b
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O
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F
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2c
OH
O
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C
F3C
1d
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2d
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O
OH
O
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OH
O
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3c
4c
a10 mM organic substrate, 10 mg Ni/CdS in10 mL deaerated H2O under N2 at room temperature,
illumination for 0.5 h; b10 mM organic substrate, 10 mg Ni/CdS in 10 mL CH3CN/H2O (v/v = 9/1)
under N2 at room temperature, illumination for 24 h.
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In addition to benzylic alcohol groups, lignin is primarily composed of three cinnamyl alcohol
units, including coumaryl, coniferyl, and sinapyl alcohols. As the oxidation of the alcohol group
at the Cα position (Cα-OH) is a necessity for the cleavage of the β-O-4 bond, it is highly
important that a catalyst of lignin valorization expresses high selectivity for the oxidation of Cα-
OH versus Cβ-OH and Cγ-OH. In order to determine whether Ni/CdS’s oxidation capability is
selective to Cα-OH, we utilized 3-phenyl-1-propanol (3a) and 2-phenylethyl alcohol (3b) as
organic substrates which bear Cγ-OH and Cβ-OH groups, respectively. After 24 h photocatalysis,
no corresponding aldehydes, 3-phenylpropionaldehyde (4a) or phenylacetaldehyde (4b), was
obtained (Table 1 and Figures S30-33) for the oxidation of 3a and 3b, suggesting that Ni/CdS
was able to tolerate both Cβ-OH and Cγ-OH groups. Encouraged by these results, we further
expanded the alcohol scope to cinnamyl alcohol (3c). In addition to the terminal hydroxymethyl
group, vinyl group in 3c is also susceptible to oxidation. The concentration evolution of
cinnamyl alcohol and its oxidation products during a 24 h photocatalysis was plotted in Figure
S34. The only products obtained were cinnamyl aldehyde (yield: ~90%) and 3-
phenylpropionaldehyde (yield: ~10%) whose 1H NMR spectra were shown in Figure S35. No
vinyl oxidation products were obtained.
Photocatalytic cleavage of lignin model compounds
Encouraged by the excellent activity and selectivity towards the Cα-OH oxidation of benzylic
alcohol and its derivatives on Ni/CdS, we sought to explore its photocleavage performance for a
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lignin model compound, 2-phenoxy-1-phenylethanol (5), which contains Cα-OH and an ether
linkage. As shown in Figure 4a, in pure CH3CN, complete conversion of 5 to the ketone product
2-phenoxy-1-phenylethanone (6) was obtained, achieving nearly 100% selectivity and yield after
18 h photocatalysis. The 1H NMR spectra of 5 before and after photocatalysis in CH3CN are
presented in Figure S36, confirming that no 5 existed post photocatalysis and the only product
was 6. In the meantime, H2 was detected as the only reduction product. The sole oxidation
product 6 allowed us to adopt subsequent chemical oxidation under ambient condition to produce
benzoic acid and phenol with nearly unity yields (Figure S37), in which the Cα-Cβ bond was also
cleaved. In the contrast, upon the addition of water, a much rapid conversion of 5 was observed
in CH3CN/H2O (v/v = 2/8) on Ni/CdS. Figure 4b indicates that nearly complete consumption of
5 was realized within the first 8 h of photocatalysis. In addition to the ketone product 6, directly
cleaved products acetophenone (7) and phenol (8) were also obtained. Further elongating the
illumination time to 18 h did not alter the relative ratio between 6, 7, and 8, whose final yields
were 47%, 51%, and 52%, respectively (Figure S38). The co-production of 6, 7, and 8 with the
addition of water in CH3CN prompted us to explore the complete photocleavage of 5 to produce
7 and 8 directly as the only products. Under alkaline conditions, a thin layer of Ni(OH)2 or NiOx
may form on the surface of Ni/CdS. Those oxidized nickel species are poorer H2 evolution
catalysts compared to metallic nickel. The absorbed hydrogen species will prefer to transfer to
the Cβ-O bond, leading to ether bond cleavage. Indeed, as shown in Figure 4c, in a mixture of
CH3CN and 0.1 M KOH in water (v/v = 2/8), a much faster consumption rate of 5 was observed,
showing complete conversion after 3 h photocatalysis. In the meantime, the cleaved products 7
and 8 were produced immediately and reached over 90% yield within 2 h and nearly unity yields
were achieved for both 7 and 8 after 3 h photocatalysis. The HPLC spectral evolution showing
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the consumption of 5 and the concentration rise of 7 and 8 is presented in Figure S39. The
corresponding 1H NMR spectra of 5 before and after photocatalysis in CH3CN/0.1 M KOH (v/v
= 2/8) are shown in Figure S40, further confirming the full conversion of 5 to 7 and 8. In order to
determine the beneficial role of Ni in Ni/CdS for photocatalysis under alkaline condition, a
control experiment using CdS as photocatalyst was performed. As shown in Figure S41, only
cleaved products were detected, however, the reaction rate was much slower. Even after 9 h
irradiation, conversion of lignin model was only 61%. In contrast, if Ni/CdS was used, full
conversion could be achieved within 3 h under the same condition. Post-photocatalysis
characterization of Ni/CdS, including SEM (Figure S42a), XRD (Figure S42b), and XPS (Figure
S43) measurements, was conducted for Ni/CdS. As shown in Figures S42-43, our Ni/CdS
maintained its nanosheet morphology and CdS wurtzite crystalline structure. The main
composition still contained Cd, S, and Ni. Therefore, our Ni/CdS demonstrated strong robustness
for long-term photocatalysis, in addition to its great selectivity in producing different products
under different photocatalysis conditions.
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Figure 4. Conversion of substrate and yield of products over time during photocatalysis. All the
photocatalytic reactions were performed with 5 mM 2-phenoxy-1-phenylethanol (5), 20 mg
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Ni/CdS in different deaerated solvents: (a) CH3CN, (b) CH3CN/H2O (v/v = 2/8), and (c)
CH3CN/0.1 M KOH (v/v = 2/8).
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Such a highly selective photocleavage performance of Ni/CdS towards the lignin model
compound 5 prompted us to propose the following mechanistic steps sketched in Figure 5. Upon
visible light irradiation, the excited holes generated in Ni/CdS are able to oxidize the Cα-OH
group in 5 to produce the corresponding ketone product 6. In pure CH3CN, the great solubility of
6 would allow it to dissociate from the catalyst surface and remain as the main product. In the
meantime, the extracted protons from 5 yield H2 with the aid of Ni/CdS. However, the relatively
slow conversion rate of 5 in CH3CN implies that fast charge recombination takes place for
photo-irradiated Ni/CdS in CH3CN. With the addition of water, the solubility of 6 is decreased
and hence it has a better chance to linger on the surface of Ni/CdS. It should be noted that the
excited electrons of Ni/CdS will facilitate the formation of adsorbed hydrogen species on the
surface of Ni/CdS, which prefer to transfer to the Cβ-O bond [26], leading to ether bond cleavage
and the formation of acetophenone (7) and phenol (8). Overall, the dissociation of 6 from the
surface of Ni/CdS and its further cleavage compete with each other. In the mixture solvent of
CH3CN/H2O (v/v = 2/8), a delicate balance is achieved and all the products of 6, 7, and 8 are
obtained. However, upon the addition of 0.1 M KOH, the adsorbed hydrogen species more likely
migrate to the Cβ-O bond and only the directly cleaved products 7 and 8 are detected (pathway I
in Figure 5). However, if the ketone product 6 is the sole product (i.e., in pure CH3CN), a facile
chemical oxidation using H2O2 can not only cleave the β-O-4 bond but also the Cα-Cβ bond,
which can lead to phenol and benzoic acid (9), a product not commonly observed from the direct
photocleavage of lignin model compounds (pathway II in Figure 5).
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Figure 5. Proposed mechanistic steps for the two pathways of the photocatalytic oxidation of 2-
phenoxy-1-phenylethanol (5) on Ni/CdS to produce different products with high selectivity. 2-
Phenoxy-1-phenylethanone (6) is yielded as the sole product in CH3CN, which will be fully
transformed to acetophenone (7) and phenol (8) in CH3CN/0.1 M KOH (v/v = 2/8) (pathway I)
or benzoic acid (9) and phenol through a chemical oxidation (pathway II).
In conclusion, we have reported the photocatalytic oxidation and cleavage of lignin model
compounds to yield three types of products with very high selectivity on ultrathin 2D
cocatalyst/CdS nanosheets. Among a series of 1st-row transition metal cocatalysts, including Mn,
Fe, Co, Ni, and Cu, Ni/CdS exhibited the best performance for both alcohol oxidation and H2
evolution under visible light irradiation. By judiciously modulating the solvent mixture and
alkalinity, we demonstrated that it was possible to selectively achieve 2-phenoxy-1-
phenylethanone from 2-phenoxy-1-phenylethanol oxidation with nearly unity yield in pure
CH3CN on Ni/CdS. A facile chemical oxidation of 2-phenoxy-1-phenylethanone could lead to
the formation of benzoic acid and phenol. In contrast, photocatalysis performed in a mixture of
acetonitrile and water produced 2-phenoxy-1-phenylethanone together with the direct β-O-4
cleaved products, acetophenone and phenol. Increasing pH of the solvent mixture via the
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addition of 0.1 M KOH could result in the complete photocleavage of 2-phenoxy-1-
phenylethanol to acetophenone and phenol. A possible mechanism involving adsorbed hydrogen
species on Ni/CdS during photocatalysis was proposed to rationalize the observed photocatalysis
results. Overall, this work represents a rare example of controlling the photocatalytic selectivity
of lignin valorization by finely tuning the reaction conditions, which is potentially applicable to
other related organic transformations driven by light irradiation.
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Methods
Synthesis of CdS. A facile microwave-assisted synthesis was adopted for the preparation of
ultrathin CdS nanosheets. Briefly, 0.122 g CdCl2 2.5H2O was suspended in 20 mL DETA. After
ultrasonication for about 30 min, 0.107 g S was added and ultrasonicated for another 10 min. The
resultant dark green suspension was subjected to microwave irradiation at 80 °C for 30 min.
Subsequently, the obtained CdS was washed with copious amounts of methanol and water
followed by centrifugation and finally dried under vacuum at room temperature.
Synthesis of M/CdS (M = Mn, Fe, Co, Ni, and Cu). A simple chemical reduction method was
used for the synthesis of 1st-row transition metal cocatalysts decorated CdS (M/CdS). Typically,
80 mg CdS and a certain amount of metal (II) chloride (with a molar percentage of 15%) were
dispersed in 18 mL water and sonicated for 30 min, followed by the addition of a 2 mL solution
of NaBH4 (0.4 M) and NaOH (2.5 M). After stirring at room temperature for one hour, the final
M/CdS samples were collected by centrifugation, washed with copious amounts of water and
methanol, and dried under vacuum at room temperature.
Physical characterization. X-ray diffraction patterns were collected on a Philips X'Pert Pro
PW3040/00 (PAN analytical) instrument. The scan range was set from 10° to 90° (in 2θ) with a
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Cu-tube operating at 45 kV and 40 mA. Scanning electron microscopy (FEI XL30, 15 kV) and
transmission electron microscopy (FEI CM20, 300 kV) were used to characterize morphology of
each sample. Scanning/transmission electron microscope (FEI Talos F200X) equipped with
super-EDX operating at 200 kV was used to conduct element mapping. UV-vis spectra were
collected on an Agilent 8453 UV-visible spectrophotometer. An ambient McAllister scanning
tunneling microscope (STM) with an R9 controller (from RHK Technologies) was used to
characterize the catalyst surface with sub-nm resolution. Atomic force microscope measurements
were performed in contact mode using a Bioscope scan head and motorized Digital Instruments
(Bruker) backplane custom fitted to an inverted optical microscope. X-ray photoelectron
spectroscopy measurements were carried out on an X-ray photoelectron spectrometer (K-alpha,
Thermo Fisher Scientific, USA) using a monochromatic Al Kα X-ray source. C 1s peak at 284.6
eV was used as an internal standard for spectral calibration. Each sample was sputtered by 1 keV
Ar+ for 60 s to remove adventitious contaminants on the surface.
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Photocatalytic experiments. Typically, a mixture of 10 mM organic substrate and 10 mg
M/CdS was thoroughly mixed in 10 mL solvent which was charged in a 50 mL sealed round-
bottom flask. The reaction mixture was deaerated with N2 bubbling for 30 min prior to
photocatalysis. Subsequently, the suspension was irradiated under royal blue light (λ = 440 – 460
nm) using an 8 W LED (SinkPAD-II, LuxeonStarLEDs) for a certain period.
Synthesis of 2-phenoxy-1-phenylethanone. 2-Phenoxy-1-phenylethanone was synthesized
according to a reported procedure.38 Specifically, phenol (0.520 g, 5.5 mmol) and K2CO3 (1.040
g, 7.6 mmol) were mixed in 50 mL acetone under stirring at room temperature. To the above
solution, a 50 mL solution of 2-bromoacetophenone (1.000 g, 5.0 mmol) in acetone was added
dropwisely over 30 min. The resultant mixture was refluxed for 4 h under stirring followed by
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filtering and drying under vacuum. The crude product was purified by column chromatography
with hexane/ethyl acetate = 20:1 and final yield was 90%.
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Chemical cleavage of 2-phenoxy-1-phenylethanone. 2-Phenoxy-1-phenylethanone (10.6 mg)
was mixed with 0.1 mL 30% H2O2 (0.88 mmol), 0.5 mL 2.0 M NaOH (1 mmol), 0.6 mL
CH3OH, and 0.6 mL THF. The resultant mixture was stirred at 50 °C for 10 h, followed by
acidification using 0.5 M HCl to pH 3. The final product was extracted with ethyl acetate and
dried under vacuum with an overall yield close to 100%.
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Product quantification. The evolved H2 from photocatalysis was quantified using gas
chromatography (GC, SRI 8610C) equipped with a molecular sieve 13 packed column, a
HayesSep D packed column, and a thermal conductivity detector. The oven was kept at 80 °C
and argon was used as the carrier gas. The quantity of H2 was determined from a calibration
curve using methane as an internal standard. High-performance liquid chromatography (HPLC)
on a Shimadzu Prominence LC-2030C system at room temperature was used to analyze the
organics. The HPLC was equipped with an UV-visible detector and a 4.6 mm × 150 mm Shim-
pack GWS 5 μm C 18 column. A mixture of eluting solvents (A: 5 mM ammonium acetate
aqueous solution and B: methanol) was utilized. For the separation and quantification of benzyl
alcohol and benzaldehyde, the elution mixture consisted of 40% A and 60% B for each 20 min
run time and the flow rate was 0.5 mL min-1. For the separation and quantification of cinnamyl
alcohol and cinnamaldehyde, the elution mixture consisted of 45% A and 55% B with same flow
rate. For the separation and quantification of 2-phenoxy-1-phenylethanol, 2-phenoxy-1-
phenylethanone, phenol, and acetophenone, the elution mixture consisted of 20% A and 80% B
and the flow rate was 1.0 mL min-1. The identification and quantification of organic compounds
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were determined based on their corresponding standard HPLC spectra and calibration curves. 1H
NMR spectra were collected on a Bruker Advance III HD Ascend 500 MHz NMR spectrometer.
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Corresponding Author
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Z.C. (email: caozhi@sxicc.ac.cn)
Y.S. (email: yujie.sun@uc.edu)
ASSOCIATED CONTENT
Supporting Information
The supporting information is available free of charge on the ACS Publications website at DOI:
Additional characterization data, including ICP, SEM, EDX, SEM, elemental mapping, XPS,
AFM, STM, and CV. Products analysis, including 1H NMR, GC, and HPLC results.
Acknowledgements
Y.S. acknowledges the financial support of Herman Frasch Foundation (820-HF17), National
Science Foundation (CHE-1914546), and the University of Cincinnati. Z.C. is grateful for the
financial support from the State Key Laboratory of Coal Conversion, Institute of Coal
Chemistry, and Synfuels China, Co. Ltd. Z.C. also acknowledges the Hundred-Talent Program of
Chinese Academy of Sciences and Shanxi Hundred-Talent Program (Y9SW911981).
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