Gold-catalyzed conversion of lignin to low molecular weight aromatics

Article abstract of DOI:10.1039/c8sc03208d

A heterogeneous catalyst system, employing Au nanoparticles (NPs) and Li-Al (1:2) layered double hydroxide (LDH) as support, showed excellent activity in aerobic oxidation of the benzylic alcohol group in β-O-4 linked lignin model dimers to the corresponding carbonyl products using molecular oxygen under atmospheric pressure. The synergistic effect between Au NPs and the basic Li-Al LDH support induces further reaction of the oxidized model compounds, facilitating facile cleavage of the β-O-4 linkage. Extension to oxidation of γ-valerolactone (GVL) extracted lignin and kraft lignin using Au/Li-Al LDH under similar conditions produced a range of aromatic monomers in high yield. Hydrolysis of the Au/Li-Al LDH oxidized lignin was found to increase the degree of lignin depolymerization, with monomer yields reaching 40% for GVL extracted lignin. Based on these results, the Au/Li-Al LDH + O₂ catalyst system shows potential to be an environmentally friendly means of depolymerizing lignin to low molecular weight aromatics under mild conditions.

Full text of DOI:10.1039/c8sc03208d

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Gold-catalyzed conversion of lignin to low
molecular weight aromatics†
Cite this: DOI: 10.1039/c8sc03208d
ab
e
Justin K. Mobley, ‡c Ali Hussain Motagamwala,cd Mark Isaacs,§{
ab
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
Yang Song,
James A. Dumesic,cd John Ralph,
and Mark Crocker
Adam F. Lee,
Karen Wilsong
cf
g
*
A heterogeneous catalyst system, employing Au nanoparticles (NPs) and Li–Al (1 : 2) layered double
hydroxide (LDH) as support, showed excellent activity in aerobic oxidation of the benzylic alcohol group
in b-O-4 linked lignin model dimers to the corresponding carbonyl products using molecular oxygen
under atmospheric pressure. The synergistic effect between Au NPs and the basic Li–Al LDH support
induces further reaction of the oxidized model compounds, facilitating facile cleavage of the b-O-4
linkage. Extension to oxidation of g-valerolactone (GVL) extracted lignin and kraft lignin using Au/Li–Al
LDH under similar conditions produced a range of aromatic monomers in high yield. Hydrolysis of the
Au/Li–Al LDH oxidized lignin was found to increase the degree of lignin depolymerization, with
monomer yields reaching 40% for GVL extracted lignin. Based on these results, the Au/Li–Al LDH + O2
catalyst system shows potential to be an environmentally friendly means of depolymerizing lignin to low
molecular weight aromatics under mild conditions.
Received 19th July 2018
Accepted 24th August 2018
DOI: 10.1039/c8sc03208d
rsc.li/chemical-science
grain ethanol,3 the utilization of lignin is limited,2 despite the
fact that the cellulosic ethanol production cost is directly
correlated to the valorization of lignin.4,5 Enzyme-induced
Introduction
Due to rising political, economic, and environmental chal-
lenges associated with the extraction and use of fossil fuels,
lignocellulosic biomass has come to prominence as a logical
renewable alternative to petroleum-based resources for
commercial carbon-based products.1,2 Although ethanol
production from the cellulosic component of lignocellulosic
biomass is being researched at a pilot scale as a replacement for
combinatorial radical polymerization of monolignols leaves
lignin with a complex, amorphous chemical structure.6–8 In view
of its structural complexity, harnessing this resource requires
chemical transformations that can depolymerize this recalci-
trant material to useable monomers with high efficiency and
selectivity.9 Although lignin possesses a heterogeneous struc-
ture, all lignins contain the alkyl aryl ether unit (i.e., contain the
b-O-4 linkage) as the most abundant structural unit (up to 60%
of all linkages)8 followed by b-5, b-b, and other minor units
(Fig. 1).
aDepartment of Chemistry, University of Kentucky, 505 Rose Street, Lexington, KY
40506, USA
bCenter for Applied Energy Research (CAER), University of Kentucky, 2540 Research
Park Drive, Lexington, KY 40511, USA. E-mail: mark.crocker@uky.edu
cGreat Lakes Bioenergy Research Center, The Wisconsin Energy Institute, University of
Wisconsin-Madison, 1552 University Avenue, Madison, WI 53726, USA
dDepartment of Chemical and Biological Engineering, University of Wisconsin-
Madison, 1415 Engineering Drive, Madison, WI 53726, USA
eEuropean Bioenergy Research Institute, Aston University, Aston Triangle, Birmingham,
B4 7ET, UK
Although much effort has been invested in hydrogenolytic
approaches to lignin depolymerization, the chemistry can be
non-selective;10 moreover, most phenolics produced via reduc-
tive pathways would face competition from the low cost
fDepartment of Biochemistry, University of Wisconsin–Madison, 433 Babcock Drive,
Madison, WI 53706, USA
gSchool of Science, RMIT University, 124 La Trobe Street, Melbourne, VIC 3000,
Australia
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8sc03208d
‡ Present affiliation: Department of Chemistry, University of Kentucky, 505 Rose
Street, Lexington, KY 40506, USA.
§ Present affiliation: Harwell XPS, Research Complex at Harwell (RCaH)
Rutherford Appleton Laboratory, Harwell Oxford, Didcot, OX11 0FA (UK).
{ Present affiliation: Department of Chemistry, University College London, 20
Gordon St, Kings Cross, London WC1H 0AJ (UK).
Fig.
1
Structural representation of
a
lignin fragment illustrating
selected linkages.
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phenolics produced from petroleum.11 Compared to hydro-
genolytic methods, catalytic oxidative lignin depolymerization
is desirable as the products exhibit increased functionalization
and complexity,11–13 otherwise not readily available from petro-
leum resources.6 Many recent studies utilize homogenous
catalytic systems for oxidative lignin depolymerization, and
while promising results have been achieved for lignin model
compounds, many systems suffer from a lack of selectivity,10,14
catalyst decomposition,12 and the requirement for harsh reac-
tion conditions.15 In addition, problems inherent in the recy-
cling of homogenous catalysts make the industrial application
of these systems difficult. Heterogeneous catalyst systems are
generally better suited for industrial applications,16 and recent
reports have shown moderate to high yields in heterogeneously
catalyzed oxidation of lignin model compounds and mono-
aromatic substrates,17–23 albeit with few successful examples of
lignin depolymerization having been reported. Aside from
catalyst type, the oxidant is another key factor in oxidative lignin
depolymerization processes, molecular oxygen being favored as
the terminal oxidant for any large scale oxidative conversion
process.17,24 In the past 20 years, Au nanoparticles (NPs) sup-
ported on metal oxide and layered double hydroxide (LDH)
supports (Au/TiO2, Au/CeO2, Au/Mg–Al LDH, Au/Ni–Al LDH,
etc.) have been shown to selectively catalyze aerobic oxidations
of alcohols to the corresponding carbonyl compounds.25–29
Herein, we introduce a heterogeneous catalyst system for the
aerobic oxidation and depolymerization of lignin by targeting
the lignin b-aryl ether fragment, utilizing Au NPs supported on
a basic Li–Al LDH30 under molecular oxygen at atmospheric
pressure. We highlight the high oxidation activity of Au/Li–Al
LDH with a variety of lignin model compounds, and its appli-
cability towards lignin depolymerization.
Table 2 Aerobic oxidation of simple benzylic alcohols using Au/Li–Al
LDHa
Entry
Substrate
t [h]
Conv. [%]
Product
Sel. [%]
1
2
3
4
80
78
2
98
98
0.5
>99
>99
4
2
4
2
67
>99
>99
>99
5
80
6b
>99
7
0.5
>99
>99
a
Substrate (1 mmol), Au/Li–Al LDH (50 mg, 1 wt% Au), diphenyl ether
(DPE, 10 mL), 80 ꢁC, p ¼ 1 atm. O2 (10 mL minꢀ1). Conversion and
selectivity were determined by GC-MS using dodecane as internal
b
standard. Substrate (1 mmol), Au/Li–Al LDH (50 mg, 1 wt% Au),
diphenyl ether (DPE, 10 mL), 100 ꢁC, p ¼ 1 atm. O2 (10 mL minꢀ1).
Conversion and selectivity were determined by GC-MS using dodecane
as internal standard. Partial results shown, for full list of benzylic
alcohols tested refer to ESI (see ESI Table S2).
used as solvent for comparison with literature data, Au/Li–Al
LDH exhibited the highest turnover frequency (TOF) compared
to other supported Au NP catalyst systems that effectively
oxidize 1-phenylethanol such as Au/Mg–Al LDH (3213 hꢀ1),31
Au/Ni–Al LDH (5310 hꢀ1),32 and Au/Al2O3 (825 hꢀ1).27
The high oxidation activity of Au/Li–Al LDH can be explained
by the degree of charge transfer from the basic support to the Au
NPs.30,35 According to CO2 pulse chemisorption measurements,
the basic site concentration follows the order Mg–Al LDH
(44.8 mmol gcatꢀ1) < Ni–Al LDH (80.5 mmol gcatꢀ1) < Li–Al LDH
(102.7 mmol gcatꢀ1), while evidence of charge transfer to the
supported Au NPs was provided by X-ray photoelectron spec-
troscopy (XPS): as the basicity of the support increases, the
binding energy of the Au 4f7/2 peak is shied towards lower
energy (see Fig. S1 and S2 in ESI†).
Results and discussion
Our initial studies focused on the oxidation of simple benzylic
alcohols (Tables 1 and 2). Encouragingly, we found that by
employing a strongly basic Li–Al (1 : 2) LDH30 as support, the
activity for benzylic alcohol oxidation increased signicantly
compared to less basic Au/LDH catalysts. When toluene was
Table 1 Oxidation of 1-phenylethanol to acetophenone using Au/
LDHsa
Besides the basic nature of the Li–Al LDH, the Au nano-
particle size also plays an important role. It has been well
established that reactivity towards alcohol oxidation using Au
NPs decreases signicantly when the Au particle size surpasses
5 nm;36–39 we observe via transmission electron microscopy
(TEM) that most of the Au NPs in our catalyst are <5 nm, the
median particle size being 2.1 nm (Fig. S3†), which should be
optimal for alcohol oxidation. The Li–Al LDH used in this
catalyst system possesses a crystalline structure (Fig. S4†) and
exhibits a characteristic sand rose morphology (Fig. S5†) with
a relatively high surface area of 85 m2 gꢀ1 (Table S1†), which
favors adsorption of the alcohol and facilitates the formation of
small Au NPs on the catalyst surface.
Catalyst
t [h]
Conv.b [%]
Sel.b [%]
TOFc [hꢀ1
]
Au/Mg–Al LDH
Au/Ni–Al LDH
Au/Li–Al LDH
1
1
1
75
34
98
>99
>99
>99
5926
1317
11 061 (29 708)d
a
1-Phenylethanol (1 mmol), catalyst (0.1 g, 1 wt% Au), diphenyl ether
(10 mL), 80 ꢁC, p ¼ 1 atm. O2 (10 mL minꢀ1). Conversion and
b
selectivity were determined by GC-MS using dodecane as internal
c
standard. TOF values are based on the number of surface Au atoms
calculated for the mean Au particle size found via TEM analysis (see
d
ESI Fig. S3).33,34 TOF in parentheses was determined using toluene
as solvent. Note: a control experiment conducted without catalyst
showed no conversion.
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The difficulty of activating O2 represents one of the rate
Chemical Science
limiting factors in oxidation catalysis.40 It is contended that Au
NPs adsorbed on the surface of a basic support contain coor-
dinatively unsaturated Au atoms that are efficient in O2 disso-
ciation, and the activity of adsorbed O2 on metallic Au follows
Brønsted acid–base reaction patterns.38,40 Our proposed mech-
anism (Fig. 2) suggests that catalysis occurs at the Au–O–Li
interface. Electron rich Au NPs activate molecular oxygen via
electron donation to the LUMO (p*) of O2 (ref. 41) and simul-
taneously, the support deprotonates the benzylic hydroxyl
group to allow adsorption to the catalyst surface. The oxidation
process is then completed by a series of deprotonation,
elimination, and catalyst regeneration steps. It should also be
noted that the reaction proceeds via a two-electron pathway, as
evidenced by the selective oxidation of a-cyclopropyl-
benzenemethanol without ring opening (entry 6 in Table 2).
Compared to radical pathways, a two-electron oxidation mech-
anism preserves desirable aromaticity in most products, and
prevents repolymerization of intermediates that generate olig-
omers via irreversible bond formation.42–44
Fig. 3 Aerobic oxidation of lignin model dimer 2a using Au/Li–Al LDH.
Conditions: substrate (1 mmol), Au/Li–Al LDH (0.1 g, 1 wt% Au), DPE (10
mL), 120 ꢁC, p ¼ 1 atm. O2 (10 mL minꢀ1). All reaction rates were
obtained by oxidizing the individual intermediates using the same
conditions. For full details refer to ESI (Fig. S5–S7 in ESI†). Conversion
and yields were determined by GC-MS using dodecane as internal
standard.
The properties of Au/Li–Al LDH in aerobic oxidation were
further evaluated using a lignin model dimer, 2a, containing
the b-O-4 interunit linkage (Fig. 3). Our catalyst system showed
selective oxidation of the secondary benzylic alcohol in 2a with
a reaction rate of 10.8 mmol hꢀ1 gcatꢀ1, achieving a 37% yield of
2b in 5 h with 68% conversion of 2a (near complete conversion
of 2a was seen aer 24 h). Under these conditions 2b reacted
further to give the dehydration product 2c (rate ¼ 3.6 mmol hꢀ1 support, such reactions having been reported in the litera-
gcatꢀ1) and the retro-aldol product 2d (rate ¼ 36 mmol hꢀ1 ture.45,46 While 2c was not observed to react further (Fig. S6†), 2d
gcatꢀ1). The high reaction rate of the retro-aldol product reached a maximum yield of 40% aer 24 h, in turn producing
formation may be attributed to the basic property of the Li–Al guaiacol (2e) and p-anisic acid (2f) at a rate of 10.8 mmol hꢀ1
gcatꢀ1. Formation of products 2e and 2f via cleavage of the b-O-4
bond in 2d using molecular oxygen has also been observed in
various literature reports.47–49 Indeed, the propensity of the
oxidized (ketone) form of b-O-4 model compounds to undergo
oxidative cleavage to monomers has been attributed to the
signicantly weaker C–O bond present in the ketone form
compared to the initial benzylic alcohol compound, and
a decrease in pKa of the proton located at Cb.48,50 Schoenebeck
and coworkers49 suggested that the rst step would most likely
involve the formation of an enolate intermediate aer depro-
tonation of the Cb–H, which can be further oxidized to give
a hydroperoxide species. Thereaer, two possible routes would
give products 2e and 2f: (1) an anionic pathway involving
formation of a dioxetane intermediate, which could undergo
subsequent fragmentation to a carboxylate and a phenyl ester;
(2) a homolytic O–O scission step forms an O centred radical,
which undergoes further C–C cleavage, as suggested by Wang
and co-workers.47
In a subsequent esterication reaction, 2e and 2f reacted to
form 2g with a rate of 5.4 mmol hꢀ1 gcatꢀ1. A similar rate of
product formation was observed when only the Li–Al LDH
support was used, consistent with base catalyzed esterication.
Although esterication is an undesirable side-reaction, the ester
linkage in 2g can be cleaved by means of simple hydrolysis,
which, when translated to lignin, would result in net cleavage of
the b-aryl ether linkage.
Fig. 2 Proposed mechanism for aerobic oxidation of benzylic alco-
hols over Au/Li–Al LDH.
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In order to examine the reusability of Au/Li–Al LDH under of lignin in DMF. Fig. 4 shows 2D HSQC NMR spectra
these reaction conditions, recycle reactions were carried out comparing KL and GVL before and aer oxidation with O2 in the
using a slightly simpler model compound, 3a (Table S3 and presence of Au/Li–Al LDH (oxidized KL and GVL are abbreviated
S4†). Aer three runs, the conversion of 3a measured at 4 h as KLox and GVLox, respectively).
showed no change (ꢂ58%). However, for recycle experiments
Analysis of the oxygenated aliphatic region of GVLox and KLox
performed at longer reaction times (16 h), a gradual decrease in revealed the absence of signals corresponding to b-aryl ether (A)
the yields of 2e and 2f, formed from 2d, was apparent. Yields of and phenylcoumaran (B) units, albeit compared to KLox, some
2e, 2f, and 2g were largely restored aer the spent catalyst was resinol (C) remains in GVLox. It is hypothesized that as for the b-
washed with water at room temperature (4th run in Table S4†), aryl ether model dimer 2a, the b-aryl ether units in lignin may
consistent with the removal of adsorbed 2e and 2d (identied in have gone through a similar oxidation-cleavage-re-coupling
the water washings). This indicates that over time 2e and 2d process forming the ester moiety, which has no sidechain
accumulate on the catalyst surface, as would be expected for C–H signals to observe via 2D HSQC NMR. Moreover, cross
acidic compounds, slowing the further conversion of 2d. peaks correlating to the guaiacyl (G) units are no longer present
Leaching of the catalytically active species into the reaction in the HSQC spectrum aer oxidation of KL and GVL (KLox and
medium is an underlying concern for heterogeneously catalyzed GVLox), and vanillate (VA) analogs are the only remaining
lignin depolymerization, as observed by Bolm and coworkers.17 signals for both. Additionally, the HSQC spectrum of GVLox
In the present work, when Au/Li–Al LDH was removed via hot displayed a signicant decrease in syringyl (S) and an increase
ltration aer 2 h of oxidation of 2a, no further conversion was in oxidized syringyl units (S0).
observed aer an additional 10 h reaction time. Consistent with
these ndings, concentrations of Au, Li, and Al in solution were lysis with NaOH (0.1 M), aer which the mixture was neutral-
determined to be <0.1 ppm by ICP-OES. ized with HCl (1 M) and the soluble material was extracted with
The oxidized lignin samples were next subjected to hydro-
Next, our focus shied to the use of process lignins, namely, ethyl acetate (EtOAc). KLox yielded 20 wt% of EtOAc soluble
Indulin AT kra lignin (sowood, from pine) and g-valer- material aer hydrolysis (Fig. 5A), which is double the amount
olactone extracted lignin51 from maple wood (hereaer referred obtained from the initial oxidized lignin sample. Notably, GVLox
to as KL and GVL, respectively), as substrate. Reaction condi- aer hydrolysis afforded 56 wt% EtOAc soluble material, versus
tions similar to those used in the lignin model compound 20 wt% without the additional hydrolysis step (Fig. 5A). The low
experiments were applied, albeit dimethylformamide (DMF) percentage of soluble organic products formed from KLox is not
was used as solvent instead of DPE due to the higher solubility surprising given that the kra pulping process is known to
Fig. 4 2D HSQC NMR spectra of KL, KLox, GVL, and GVLox lignin. All samples dissolved in 4 : 1 DMSO-d6/pyridine-d5; contours color-coded to
structures responsible; percentages are determined from volume integrals based on aC–H with the exception of A0 which uses the bC–H signal.
The assignment of peaks is based on known lignin spectra and available model compound data.
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to date for heterogeneously catalyzed oxidative lignin depoly-
merization55 and approach those reported for the most effective
homogenous catalysts.56,57 Notably, the lignins employed in the
latter case were either extracted enzymatically or via mild
acidolysis, which preserves a high fraction of b-aryl ether link-
ages.57 The fact that we observe a 40 wt% monomer yield from
a lignin that is extracted via a scalable method, utilizing
a biomass-derived solvent, renders the Au/Li–Al LDH catalyst
system particularly promising.
Conclusions
The heterogeneous catalyst Au/Li–Al LDH showed excellent
activity and selectivity in the oxidation of benzylic alcohols
using molecular oxygen as the terminal oxidant. XPS data
suggest the high activity of this catalyst is facilitated by charge
transfer from the basic Li–Al LDH support to the Au nano-
particles. As evidenced by the model b-O-4 linkage dimer
oxidation results, our catalytic system was able to oxidatively
cleave the linkage aer selective benzylic alcohol oxidation,
ultimately forming an ester moiety that can be depolymerized
Fig. 5 (A) Percent organic soluble fraction obtained from KLox and
GVLox pre- and post-hydrolysis. Oxidized lignin samples (50 mg) were
dissolved in 0.1 M NaOH (5 mL), followed by addition of 1 M HCl until
pH 2 was reached, and were then extracted with EtOAc. For additional
details refer to ESI. (B) Yields of monomer products from KLox and (C)
GVLox after hydrolysis. Corresponding yields for control experiments via simple hydrolysis. Similarly, NMR data obtained on oxidized
(conducted without catalyst) are shown in the ESI.†
kra lignin (KLox) and GVL lignin (GVLox) showed the disap-
pearance of benzylic alcohol groups, consistent with ester
formation. Aer hydrolysis was performed on oxidized lignin
produce a recalcitrant lignin that is C–C cross-linked, and hence
difficult to depolymerize.8 However, the results obtained with
GVLox are signicant given that GVL more closely resembles the
structure of native lignin than does KL.52 Gel permeation
chromatography (GPC) revealed two peaks for both KLox and
GVLox aer hydrolysis (Fig. S9†). The rst peak in both chro-
matograms represent fractions containing high molecular
weight oligomers, and the second peak corresponds to low
molecular weight components. The high molecular weight
region shows a ꢂ3500 Da decrease in molecular weight for KLox
post-hydrolysis while a decrease of ꢂ1000 Da was observed for
GVLox post-hydrolysis. The low molecular weight range shows
monomer production for both lignins, however, a signicantly
higher signal intensity was obtained from GVLox post-hydrolysis
compared to KLox post-hydrolysis, consistent with the results
previously mentioned in Fig. 5A.
Compound identication and quantication via GC-MS
revealed that S- and G-derived carboxylic acids and aldehydes
are the most prominent products (Fig. 5B and C). The yield of
GC-MS identiable monomers from GVL totaled 40 wt%,
whereas a 10 wt% yield of GC-MS identiable monomers was
obtained from KL. Among the identied monomers, vanillin,
vanillic acid, ferulic acid, and coniferyl alcohol were present
from both lignins. Yields of these monomers, except for
vanillin, were found to be higher from GVLox than from KLox. S-
derived products were only obtained from GVL, as the KL used
in this work was derived from pine, which consists of almost
exclusively G units, a small amount of H units, and no S units.53
Notably, the monomers obtained, including vanillin, vanillic
acid, and syringaldehyde, are of higher market value than those
typically derived from hydrogenative methods of lignin depo-
lymerization.54 These monomer yields are the highest reported
samples, a 40 wt% yield of aromatic monomers was obtained
from GVLox, while KLox afforded 10 wt% monomers. These
monomer yields are the highest reported for heterogeneously
catalyzed oxidative lignin depolymerization. Based on these
results, Au/Li–Al LDH is identied as a promising catalyst
system for lignin valorization to value-added low molecular
weight aromatics.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This research was funded by the National Science Foundation
under Cooperative Agreement No. 1355438 and by the Global
Bioenergy, Biofuels, and Biorening network (GB3-Net) sup-
ported by the British Council and the Department for Business,
Innovation and Skills (BIS). This work was also funded in part
by the DOE Great Lakes Bioenergy Research Center (DOE Office
of Science BER DE-FC02-07ER64494 and DE-SC0018409). The
authors thank Shelley Hopps for performing the XRD
measurements and Dr Mark Meier for helpful discussions.
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