Selective C-C Bond Cleavage of Methylene-Linked Lignin Models and Kraft Lignin

Article abstract of DOI:10.1021/acscatal.8b00200

Biorefinery and paper pulping lignins, referred hereto as technical lignins, contain condensed C-C interunit linkages. These robust C-C linkages with higher bond dissociation energies are difficult to disrupt under hydrogenolysis conditions, which are gen

Full text of DOI:10.1021/acscatal.8b00200

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Article
Selective C-C Bond Cleavage of Methylene-
linked Lignin Models and Kraft Lignin
Li Shuai, Jake Sitison, Sunitha Sadula, Junhuan Ding, Mark Charles Thies, and Basudeb Saha
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00200 • Publication Date (Web): 05 Jun 2018
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Selective C−C Bond Cleavage of Methylene-linked Lignin Models and Kraft Lignin
Li Shuai¥, Jake Sitison ¥, Sunitha Sadula ¥, Junhuan Ding§, Mark C. Thies§, Basudeb Saha¥*
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¥Catalysis Center for Energy Innovation, University of Delaware, 221 Academy Street, Newark,
DE 19716, USA
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§Department of Chemical and Biomolecular Engineering. Clemson University
221 Earle Hall, Clemson, SC 29634, USA
Corresponding author’s email: bsaha@udel.edu
ABSTRACT
Biorefinery and paper pulping lignins, referred hereto as technical lignins, contain condensed
C−C interunit linkages. These robust C−C linkages of higher bond dissociation energies are
difficult to disrupt under hydrogenolysis condition, which is generally used for cleaving C−O
bonds of native lignin in biomass or model C−O linked compounds. Thus, selective C−C bonds
cleavage to release aromatic monomers for high value applications is a challenge. We report an
effective catalytic system to cleave such C−C bonds selectively under mild conditions. A
representative methyleneꢀlinked CꢀC model dimer achieves 88% yield of mainly two aromatic
monomers within 1.5 h at reasonably low temperature (250 ºC) using a commercial CoS2
catalyst. Aromatic monomers convert to nonꢀaromatic products upon reaction for a prolonged
time. C−C bond of the dimer becomes unreactive to cleavage upon dehydroxylation of aromatic
rings while the methoxyl group has a little effect to the cleavage. βꢀ1 and 5ꢀ5 CꢀC linked model
dimers formed demethoxylated hydroxyl dimers as the major products. Depolymerization of a
purified kraft lignin fraction yields five aromatic monomers and lower molecular weight soluble
products. This study opens up the possibility of valorization of technical lignin using inexpensive
catalysts.
Keywords: Technical lignin, kraft lignin, CꢀC cleavage, model lignin, biorefinery lignin,
selective catalysis.
INTRODUCTION
Technical lignins, generated by biorefinery and paper pulping industries, are considered as
low value waste streams, and their valorization have intrigued researchers for decades.
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Researchers have made numerous efforts to disrupt technical lignins into low molecular weight
(Mw) fragments to upgrade to valueꢀadded aromatic products.1, 2 Recent efforts on lignin
valorization have been mainly focused on selective cleavage of β−O−4 (β−ether) linkages, which
represent the major interunit linkages (50ꢀ60%) in native lignin of lignocellulosic biomass,3ꢀ6
with the balance being α−O−4 and various C−C linkages, such as β−5, β−1 and 5−5.3, 7
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Scheme 1. Representative lignin structure featuring interunit C−O (e.g. β−O−4, highlighted in
blue) and C−C (e.g. β−β, β−5 and 5−5, highlighted in red) linkages and current ‘ligninꢀfirst’
strategy to produce monomer in which CꢀC bonds are uncleavable.
Hydrolysis,8 hydrogenolysis,9ꢀ14 and oxidation15, 16 pathways have been widely investigated for
lignin depolymerization (Scheme 1). Hydrogenolysis is a selective method to depolymerize
lignin to monomers via β−O−4 bonds cleavage;4, 9ꢀ12 however, total monomer yield is limited by
robust C−C interunit linkages.
In contrast, technical lignin contains mostly C−C bonds, which are formed during biomass
pretreatment or pulping processing. Higher bond dissociation energies (BDEs) of C−C bonds
(75–118 Kcal/mol) than that of C−O linkages (70 Kcal/mol) pose challenges to depolymerize
technical lignins selectively under mild conditions (Scheme 1).17, 18 For example, hydrogenolysis
depolymerzation of organosolv lignin achieved only 7% monomer yield,11 while
depolymerization of native lignin or uncondensed lignin yielded up to 48% monomer upon
cleaving β−O−4 bonds.3, 19, 20 Similarly, hydrogenolysis depolymerization of kraft lignin with
Pt/Al2O3 at 250−300 °C formed about 7% monomer yield.21, 22 Catalytic pyrolysis of alkali kraft
lignins with strong solid acids or solid acidꢀsupported catalysts at elevated temperatures (up to
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650 °C) reported <10% lignin oil, with majority being undesired coke (up to 40%), and
deactivation of the catalysts.23, 24
Another common issue of current processes is very low selectivity of each product,
rendering any separation approach energyꢀintensive and expensive. For example, kraft lignin
depolymerization in ethanol under pyrolysis condition (380 °C) produced over three dozen
monomers, out of which total aromatic phenols was only 10%.25 Efforts have been made to
study the cleavage of model compounds featuring 5−5, β−5 and β−1 linkages, but the saturation
or disruption of aromatic rings and C−O cleavage were observed, instead of C−C cleavage, under
hydrogenolytic and oxidative conditions.26, 27 A CoMo/Al2O3 catalyst was found to be ineffective
to cleave 5ꢀ5′ linkage of a model dimer in a prior study28 while a flower like hierarchical MoS2
composite29 offered unselective bioꢀoil yield at higher temperature (310 °C), likely via thermoꢀ
pyrolysis. Because of rigidity of C−C linkages and the structural complexity, majority of 80
million tons of technical lignins annually generated in pulping and biorefinery processes is
considered as a lowꢀvalue waste and burned only for heat recovery.30
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Efficient conversion technologies to depolymerize technical lignins into highꢀvalue fuels and
aromatic products are necessary to maximize waste utilization and carbon efficiency in bioꢀ
products, and to create additional revenue for pulping and biorefineries. Prior reports suggest that
methylene linkages could be formed in technical lignins during selfꢀcondensation or inꢀsitu
generated formaldehydeꢀinduced condensation of depolymerized lignin fragments containing
aromatic rings with αꢀhydroxyl side chain functionality in acidic and alkaline media.25, 31ꢀ33
Herein, we report an effective approach to improve selective depolymerization of technical
lignin using model lignin dimers (dimethylguaiacylmethane DMGM; Figure 1A and its
dehydroxylated analogues) and purified kraft lignin as representative examples. First, we
evaluate the activity of a few types of readily available metal sulfides to disrupt methylene
linkage of DMGM. The synthesis of DMGM featuring a methylene linkage is shown in the
supporting information (Figures S1ꢀS3). The reasons to choose this type of catalysts are: (1) kraft
lignin generally contains 1.5−3% of sulfur which comes from Na2S, a chemical reagent
promoting lignin removal during biomass pretreatment. Metal sulfides are expected to be
resistant to sulfur poisoning, and (2) metal sulfides, such as FeS2, CoS2 and MoS2, exist naturally
in highly pure forms, therefore these inexpensive materials are economically suitable for
upgrading waste lignin liquor.
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RESULTS and DISCUSSION
FeS2 has been used as a catalyst for hydrocracking of heavy oils containing aromatic units at
300−450 °C. These studies have found that FeS2 can cleave interunit C−C bridge linkages
between aromatic rings.34 Therefore, we first tested FeS2 as the catalyst to disrupt the methylene
linkage of DMGM at 250 °C and 50 bar H2. Monomeric and a small amount of dimeric products
are formed (Figures 1A and S4). Some of the resultant monomers having one or two isomers are
grouped for simplification and their retention times are shown in Figure S4. Without FeS2,
DMGM depolymerization at 250 °C and 50 bar H2 for 15 h achieves only 15% conversion
(Figure S5) likely via thermal pyrolysis. Pyrolysis of lignin to monomers without catalysts and
H2 input has been reported previously.35 In the presence of FeS2, monomer yield of 64% with
97% selectivity is achieved under comparable reaction conditions. This temperature (250 °C) has
been widely used in hydrogenolysis cleavage of β−O−4 linkages of native lignin or model lignin
compounds with Ni/C, Ru/C and Pt/C catalysts,1, 4, 5, 11, 19, 36 which allows a fair comparison of
the catalytic activity of the two types of catalysts for cleavage of the respective CꢀC and CꢀO
bonds (vide infra). The reaction is ineffective at 200 °C, giving no conversion of DMGM in the
presence of FeS2. While the results with FeS2 at 250 °C, giving 64% total monomer yield
containing five phenolic products, is more selective than prior arts demonstrating formation of
heterogeneous slates of molecules under pyrolysis conditions, the separation of desired monomer
from the five products is still a challenge. In addition, 30% of demethoxylated and
dehydroxylated dimeric products with partial ringꢀhydrogenation (Figures S4 and S5) are
formed, which are proven to be uncleavable (vide infra). Therefore, we tested two other metal
sulfides, MoS2 and CoS2.
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The MoS2ꢀcatalyzed reactions under comparable conditions as FeS2 formed a little amount
of monomer (8%) and a significant amount of demethoxylated and dehydroxylated dimers with
or without partial ring hydrogenation (Figures S4 and S6), which remained uncleaved. In
contrast, initial CoS2ꢀcatalyzed reaction at 250 °C and 50 bar H2 for 15 h produced only three
monomers (B, D and E in Figure 1A and Figure S6). The clean dimer region in the GC
chromatogram (retention time of 11.7−16.5 min in Figure S6) indicates the methylene linkage of
DMGM was selectively cleaved without formation of uncleavable dimeric products. However,
the initial reaction for 15 h gives only low yield of aromatic monomers. We hypothesize the low
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yield could be due to the further conversion of aromatic monomers to other products during a
long reaction time of 15 h. Therefore, a timeꢀcourse experiment was conducted to study the
reaction profile of the CoS2ꢀcatalyzed disruption of DMGM.
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The timeꢀcourse results show selective formation of two aromatic monomers B and C
(Figure 1B) in the beginning of the reaction and total monomer yield reached to 88% within 1.5
h (Figures 1A and S7, Table S1). B and C convert to other two aromatic monomers, D and E
(Figure 1A) through possible dehydroxylation and demethoxylation, when the reaction continued
for 4 h. Complete and partial ring hydrogenated nonꢀaromatics products, methyl cyclohexene and
cyclohexane (Figures 1A and S7, Table S1) are formed upon continuing the reaction for 15 h. A
small amount demthoxylated and dehydroxylated dimeric products are also formed. We
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B
Figure 1. (A) Proposed pathways for metal sulfideꢀcatalyzed methylene linkage disruption of
dimethylguaiacylmethane (DMGM); (B) The timeꢀcourse reaction profile for CoS2ꢀcatalyzed
DMGM disruption, showing selective formation of aromatic monomer in short reaction time.
Reaction conditions for the timeꢀcourse experiments: 20 mg DMGM, 10 mg CoS2, 50 bar H2,
250 °C, 20 mL heptane.
hypothesize that the dimers become uncleavable after losing the methoxyl and/or hydroxyl
groups of DMGM. To validate the hypothesis, we investigate C−C cleavage of other model
dimers containing a methylene linkage, but without methoxyl and/or hydroxyl groups (Figure
S8). The results show the methylene linkage of a model dimer with a phenolic hydroxyl on both
aromatic rings but without methoxyl can still be cleaved with 77% conversion in 1.5 h,
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suggesting that the methoxyl group has a little effect on the cleavage of C−C bonds. Another
model dimer with one phenolic hydroxyl group on one ring and no phenolic hydroxyl on the
second ring results in low conversion (20%) under comparable reaction conditions. This
indicates the cleavage of the methylene linkage depends on the chemical environments of lignin
molecules, especially the vicinal phenolic groups which likely interact with Co2+ sites of cobaltꢀ
sulfide and strong electronꢀdonating effect of the phenolic groups weaken the methylene linkage.
This argument becomes more pronounced when an analogous diphenyl model compound
without any phenolic hydroxyl group substituent showed no C−C cleavage. Likewise, βꢀ1 and 5ꢀ
5 linked model dimers formed their corresponding demethoxylated hydroxyl dimers as the major
products (Figure S8).
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To elucidate the transformation of monomers B and C to monomers D and E, B and C were
reacted individually with the CoS2 catalyst under comparable reaction conditions used for the
disruption of model dimers. B converts to E through dehydroxylation, but demethoxylation of C
formed B and E. A reaction with a mixture of B and C also gives E as the sole product. Based on
the aforementioned discussion (Figures S7ꢀS9), a proposed reaction pathway for the CoS2ꢀ
catalyzed DMGM disruption is shown in Figure 1, although the exact pathway of formation of D
is inconclusive.
Next, we test the role of H2 for cleavage of the methylene linkage of DMGM. At 10 bar H2,
DMGM conversion achieved only 21% as opposed to nearly 90% at 50 bar H2 (Table S2).
Another experiment at 10 bar H2 but total pressure of 50 bar using N2 as an inert gas improved
monomer yield slightly (27%) due to better H2 solubility in the solvent. Without H2, DMGM is
unreactive, indicating the CꢀC cleavage is likely H2ꢀassisted as reported for CꢀO cleavage of
lignin compounds.37
We used Ru/C and Pt/C catalysts to compare their CꢀC cleavage performance with CoS2.
These noble metal catalysts have been used to selectively cleave β−O−4 linkages of model and
native lignins via hydrogenolysis, but C−C cleavage of lignin by these catalysts is rare.26, 27, 38
The results in Table 1 show that CoS2 exhibits much higher activity to cleave CꢀC linkage to two
monomers (B and C) with 81% yield out of total molar yield of 88% (entry 1 in Table 1). In
contrast, the Ru/C and Pt/C catalyzed reactions yield very little monomer (total yields of B and C
are 9.9 % and 4.1%, respectively; entries 2 and 3 in Table 1), which likely result from small
amount of thermoꢀpyrolysis at 250 °C as observed in the blank experiment without the catalyst
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(discussed above). The results reveal that CoS2 is very effective and selective towards aromatic
monomers with high yield at high conversion of DMGM in heptane.
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Table 1. Comparison of CoS2ꢀ and noble metal catalyzed DMGM disruption
and kraft lignin depolymerisation.[a]
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Entry
Feedstock
Solvents
Catalysts
Catalyst
loading (mg) monomer
yield (%)
Aromatic
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2
3
4
5
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7
DMGM
DMGM
DMGM
Lignine
Lignine
Lignine
Lignine
Heptane
Heptane
Heptane
Heptane
Dioxane
Dioxane
THF
CoS2
Ru/C
Pt/C
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88[b]
9.9[c]
4.1[d]
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CoS2
CoS2
CoS2
CoS2
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50
11.7
13.2
12.6
100
50
[a] Other reaction conditions: 50 bar H2, 250 °C, 1.5 h for dimer or 15 h for
kraft lignin, 50 mg kraft lignin or 20 mg DMGM. [b] dimer conversion 100%;
[c] dimer conversion 15.3%; [d] dimer conversion 5%; [e] kraft lignin (alkali)
fraction purified by ALPHA process.39 Comparison of breakdown monomer
yield from CoS2, Ru/C and Pt/C catalysis of DMGM is in Table S3. Ru/C
catalyzed kraft lignin depolymerized monomer yields in heptane and dioxane
are shown in Table S4.
Next we evaluate depolymerization of a low Mw fraction of a kraft lignin of average Mw of
1600 Da and the highest Mw species ranged up to 10000 Da (Figure 2) with CoS2. The kraft
lignin fraction was obtained from purification and fractionation of an alkali kraft lignin via the
Aqueous Lignin Purification with Hot Acids (ALPHA) process,39 followed by a single water
wash to remove the acetic acid solvent, reduce the total metals content to <200 ppm, and then
drying to remove excess water. The ALPHA process had no effect on the original sulfur content
of the kraft lignin. Depolymerization with Ru/C was also performed for comparison. Initial
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results show poor depolymerization of the kraft lignin fraction in heptane (entry 4 in Table 1),
which we ascribed to its low solubility in heptane. The solvent screening results suggest that the
kraft lignin fraction has higher solubility in dioxane and tetrahydrofuran (THF). In dioxane, CoS2
catalysis achieves maximum 13% monomer yield (entries 5ꢀ6 in Table 1). A reaction in THF
yields similar amount of monomers (entry 7). In contrast, Ru/Cꢀcatalyzed reactions in heptane
and dioxane at two different catalyst loadings (20 mg and 100 mg) yield maximum 5% aromatic
monomers (Figure S10; Table S4), which are likely formed via cleavage of a small amount of
β−O−4 linkages (5ꢀ7%21) present in the kraft lignin fraction. However, the structural complexity
of the condensed kraft lignin makes it difficult to explain of which linkages (methylene versus
other C−C bonds) were disrupted during the reaction. In addition, Mw species that are outside
the detectable range of GC/GCꢀMS could be formed. Therefore, gel permeation chromatography
(GPC) was used to qualitatively characterize the change in Mw of lignin species before and after
depolymerization.
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Figure 2 shows the highest Mw species in the kraft lignin fraction before depolymerization
is ranged up to 10000 Da with an average Mw of about 1600 Da. In contrast, the average Mw of
depolymerized lignin is decreased by about 3ꢀ5 folds and the Mw distribution range is
significantly narrowed. The shape of the peak, Mw distribution range and average Mw of
depolymerized lignin at two different Ru/C loadings are similar, indicating that increasing Ru/C
loading could not further cleave interunit C−O or C−C linkages, consistent with similar
monomer yields obtained from the two reactions, (Table S4). In case of CoS2, Mw distribution
range of depolymerized lignin is narrower than that of the Ru/C catalyzed depolymerization,
indicating a greater degree of depolymerization of kraft lignin occurred. Furthermore, average
Mw and the range of the highest Mw of the depolymerized species decreased with an increase in
CoS2 loadings, indicating C−C cleavage occurred by the CoS2 catalyst, in addition to any CꢀO
cleave of small amount of CꢀO linkages that are present in kraft lignin. This observation is also
consistent with the results of DMGM cleavage in which monomer yield increased with CoS2
loadings.
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Ru_20
Ru_100
Co_50
Co_100
Co_400
Kraft lignin
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100
1000
10000
Molecular Weight
Figure 2. GPC of a kraft lignin fraction before and after depolymerisation with CoS2 and Ru/C.
CoS2 catalyzed reaction also generated H2S (Figure S11), hypothesizing CoS2 could be reduced
to CoS by H2 during the reaction. The XRD pattern of the recovered catalyst validates the
hypothesis, which showed the characteristic peaks for CoS (Figure S12). In addition, CoS2 was
treated in heptane and dioxane individually in the absence and presence of H2 without DMGM or
lignin, and the recovered catalyst was characterized by the XRD. The recovered catalyst retained
characteristic CoS2 peaks in the absence of H2, however it changed to CoS in the presence H2
(Figure S13) in both the solvents. Thus, CoS2 may not be the active phase for the CꢀC cleavage.
A CoS, synthesized by the reported procedure,40 yield 7.7% and 11.1% monomer from the kraft
lignin fraction at the catalyst loading of 50 mg and 100 mg, respectively, which are slightly
lower than that from the CoS2ꢀcatalyzed reactions (entries 5 and 6 in Table1).
Recycling study of CoS2 shows (Figure S14) the recovered catalyst is slightly less active and
total monomer yield in the 3rd cycle is 14% lower than that in the 1st cycle. Compositional
analyses of CoS2 and the recovered catalyst by XRF (for Co and S) and elemental analyzer (for
S) indicate the atomic ratio of Co to S in the catalyst before the reaction is 1: 1.7 while the ratio
decreased to 1:1 in the recovered catalyst after the reaction. The compositional analysis data is
consistent with the XRD pattern discussed above and suggests that the catalyst was reduced to
CoS by H2 during the reaction and. XPS was employed to examine the surface composition and
oxidation states of Co and S of the catalysts before and after CꢀC cleavage of DMGM (Figure
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S15). Deconvoluted Co 2p3/2 band shows peaks for CoS2, CoS, Co(OH)2 and CoO at binding
energies (BE) of ~777.2ꢁeV, 781.5ꢁeV, 784.0ꢁeV and 786.8ꢁeV respectively, according to prior
assignment.41ꢀ43 The recovered catalyst shows an increased intensity ratio of CoS and a decreased
ratio of CoS2, which is consistent with the changing trend of S in the deconvoluted S 2p band.
The S 2p band of the catalyst before reaction shows BE peaks for S2− (monosulfide), S2
2−
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2−
(disulfide), Sn (nꢁ>ꢁ2, polysulfides) and S0 (elemental S) at ~161.5ꢁeV, 162.5ꢁeV, 163.7ꢁeV and
164.4ꢁeV, respectively, according to prior assignment.44 After reaction, the ratio of S2ꢀ increased
2ꢀ
and the ratio of S2 decreased, indicating the structural change of the catalyst to CoS occurred
during the reaction. A controlled experiment using a commercial cobalt oxide at comparable
reaction conditions as Coꢀsulfide formed only 2.4% monomer, indicating cobalt oxide is not an
active species. The recycling experiment and the catalyst characterization results suggest that
CoS is formed during the reaction and is active for CꢀC cleavage, however the role of CoS2
cannot be precluded for the reason that the reaction starting with CoS2 gives a slightly higher
amounts of monomers.
CONCLUSIONS
Selective cleavage of interunit C−C linkages of condensed technical lignins has been a longꢀ
standing challenge. This study demonstrates that such cleavage of C−C bonds in lignin is
possible selectively using an inexpensive cobalt sulfide catalyst. C−C cleavage of a
representative CꢀC linked lignin motif, dimethylguaiacylmethane (DGMG), gives two aromatic
monomers with high yield and selectivity at mild conditions and in short reaction time. The
reaction for a prolonged time forms nonꢀaromatic products. Methoxy substituent has a little
effect to the C−C cleavage, while the dimer becomes unreactive to the C−C cleavage upon
dehydroxylation. Depolymerization of a kraft lignin fraction gives lower molecular weight (Mw)
soluble lignin species including up to 13% monomer, containing 5 detected functional aromatic
monomers.
ASSOCIATED CONTENT
Supporting Information. The Supporting Information is available free of charge on the ACS
Publications website at DOI: XXXXX. Experimental procedure for DMGM synthesis and
catalysis of DMGM and kraft lignin; analytical methodology for analysis of liquid products;
quantification of liquid products; characterization of catalysts; H, 13C and HSQC NMR of
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DMGM; GC and GCꢀMS chromatogram and spectra of products; XRD patterns and
deconvoluted XPS spectra; controlled experimental data; structures of monomers and dimers
from DMGM and kraft lignins; conversion and yield data.
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AUTHOR INFORMATION
Corresponding Author
Eꢀmail: bsaha@udel.edu
ORCID
Basudeb Saha: 0000ꢀ0002ꢀ3591ꢀ3227
Notes
The authors declare no competing financial interest.
ACKNOWLEDGEMENTS
This work was conducted with financial support as part of the Catalysis Center for Energy
Innovation, an Energy Frontier Research Center funded by the U.S. Department of Energy,
Office of Science, Office of Basic Energy Sciences under Award Number DEꢀSC0001004. MCT
acknowledges the National Science Foundation for supporting kraft lignin fractionation and
purification work via ALPHA process (Grant No. CBETꢀ1403873).
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