Selective Ether/Ester C-O Cleavage of an Acetylated Lignin Model via Tandem Catalysis

Article abstract of DOI:10.1021/acscatal.5b01972

Selective ether/ester C-O bond hydrogenolysis of an acetylated lignin model is achieved using a thermodynamically leveraged tandem catalytic strategy. Acetylation serves to (1) solubilize both lignin and lignin models and to (2) modify the reactivity of pendant hydroxy groups to promote more selective C-O cleavage.

Full text of DOI:10.1021/acscatal.5b01972

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Letter
Selective Ether/Ester C-O Cleavage of an
Acetylated Lignin Model via Tandem Catalysis
Tracy L. Lohr, Zhi Li, and Tobin J. Marks
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b01972 • Publication Date (Web): 20 Oct 2015
Downloaded from http://pubs.acs.org on October 26, 2015
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Selective Ether/Ester C-O Cleavage of an Acetylated Lignin Model
via Tandem Catalysis
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Tracy L. Lohr, † Zhi Li, †,‡ and Tobin J. Marks*,†
†Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, USA.
Supporting Information Placeholder
the reaction solvent, since strongly Lewis basic, or donor
ABSTRACT: Selective ether/ester C-O bond hydrogenoly-
solvents depress the activity of the Lewis acid triflate cata-
sis of acetylated lignin models is achieved using a ther-
modynamically leveraged tandem catalytic strategy. Acet-
employed here is lignin acetylation, rendering the sub-
ylation serves to: 1) solubilize both lignin and lignin mod-
els and 2) modify the reactivity of pendant hydroxy
without compromising triflate catalytic activity (Scheme
groups to promote more selective C-O cleavage.
lysts,11a-d but usefully solubilize the lignin. The strategy
strates soluble in polar, weakly coordinating solvents
1). Aromatic hydrocarbon solvents are avoided to prevent
their competing hydrogenation by the Pd catalyst.
KEYWORDS: Lignin, Lignin models, Tandem catalysis,
Ether hydrogenolysis, Ester hydrogenolysis , biomass.
Lignin, a heterogeneous phenolic polymer which consti-
tutes roughly 15 to 20 wt% of lignocellulosic biomass (cel-
lulose, hemicellose, and lignin), represents one of the few
renewable sources of aromatic monomers.1 Current lignin
depolymerization
methodologies,
including
base-
catalyzed,2 acid-catalyzed,3 metal-catalyzed,4 ionic liquid
(IL)-assisted,5 and supercritical fluid-assisted2b,
ap-
6
proaches, typically afford low yields (~ 10-20 % or less) of
low molecular weight aromatics under relatively harsh
reaction conditions (> 300 °C).7 Recent advances include
using oxidized lignin and lignin models,8 where oxidation
of the Cα alcohol facilitates depolymerizaton, with aro-
matic monomer yields reaching up to 52% for aspen
“hardwood” lignin.9 The most common structural lignin
motifs contain a β-O-4 aryl-ether linkage,10 a primary al-
cohol in the γ skeletal position, and a secondary alcohol in
the α position (Scheme 1). This Laboratory has previously
demonstrated an effective strategy for thermodynamically
leveraged etheric and esteric C-O bond hydrogenolysis
using a tandem metal triflate + supported palladium cata-
lytic system.11 A homogeneous M(OTf)n catalyst mediates
endothermic ether or near thermoneutral ester C-O bond
scission (the reverse of hydroelementation), which is cou-
pled to exothermic Pd-catalyzed hydrogenation of the
resulting C=C unsaturation, driving the overall process
downhill. We next asked whether this tandem system
might be applicable to cleaving the β-O-4 aryl-ether bond
in lignin and lignin models. The promising results of that
investigation are communicated here.
Scheme 1. Strategy for cleaving guaiacyl-based lignins
Chlorinated solvents are used in this initial screen since
they are polar and aprotic, and can be recycled to mini-
mize environmental release. Experiments (Table 1) first
focused on the acetylated version (1b) of the model di-
meric substrate 1a, which features the prevalent β-O-4
aryl-ether linkage and is a credible model for acetylated-
lignin. From the accrued understanding of etheric and
A major consideration in lignin depolymerization using
the tandem metal triflate + palladium catalyst system is
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esteric cleavage using this tandem catalyst system,10 it is
anticipated that ester cleavage at the benzylic α position
will precede other scissions since 2° esters are far more
To suppress secondary polymerization by minimizing the
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3
4
5
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5b steady-state concentration, it was hypothesized that
slowing the ester C-O cleavage rate, i, relative to that of
olefin hydrogenation, ii (Scheme 2) might capture 5b be-
fore significant thermal/radical polymerization occurred.
This goal was achieved by employing less Lewis acidic yet
labile La(OTf)3, increasing the H2 concentration/pressure,
and increasing the Pd catalyst loading relative to that of
La(OTf)3. Using a tandem catalyst with La(OTf)3:Pd = 5:1
Table 1. Catalytic Hf(OTf)4 + Pd/C/H2 mediated acety-
lated dimer cleavage dataa
1
9
affords guaiacol (3)in 39% yield by H NMR and GC-MS
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assay (Table 2, entry 1), with negligible dimethoxyphenyl
(DM) products. Doubling both catalyst loadings (Table 2,
entry 2) yields cleavage products 3 and 4a-c, and monoac-
tetate 2b, however, the mass balance of recovered mono-
mers is only 21% for guaiacol-based monomers and 13%
for dimethoxyphenyl based monomers. No starting 1b is
recovered, indicating that mass loss likely results from
styrenic 5b polymerization. Increasing the Pd catalyst
loading to La:Pd = 5:3 (Table 2 entries 3 and 4) increases
the recovered monomer yields to 66% and 58% for guaia-
col and dimethoxyphenyl based monomers, respectively.
Finally, decreasing the La:Pd ratio to 1:1 results in 97% and
96%, recovery of the guaiacol and dimethoxyphenyl mon-
omers, respectively (Table 2, entry 5), indicating an opti-
mal catalyst ratio for reducing the steady-state 5b con-
centration and suppressing secondary polymerization.
Concentrating (Table 2, entry 6) the reaction solution
results in slightly lower monomer unit recovery, albeit
with similar conversions for β-O-4 aryl-ether cleavage
(28% remaining 2b versus 27% 2b for entry 5). Diluting
the reaction solution (Table 2, entry 7) at similar catalyst
ratios yields higher dimethoxyphenyl monomer recovery
(96%) but lower β-O-4 aryl-ether cleavage (52 % 2b).
Mass
Temp Yields (%)
Balance
%
Entry
ᵒ
C
1b
2b
3
ꢀ
4a 4b 4c G : DM
b
1
70
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
0 : 0
100
c
2
25
100
ꢀ
ꢀ
c
3
100
ꢀ
0 : 0
a
Reaction conditions: 1 mL of 0.1 M solution of 1b in CDCl3,
The results in entries 5-7 indicate that conducting the
reaction in Cl2C2H4 at 0.025 M substrate and 5 mol% Pd
and La is optimum for 1b hydrogenolysis. Interestingly,
changing the solvent to higher boiling Cl4C2H2 acceler-
ates β-O-4 aryl-ether cleavage with only 14% 2b recov-
ered, but with lower monomer yields -- 73 and 61 % for
guaiacol and dimethoxyphenyl units, respectively. Analy-
sis of the reaction mixture by GC-MS reveals the presence
of dimeric tetrachlorodienes (Cl4C4H2), likely resulting
from Pd-catalyzed Cl4C2H2 dehydrohalogenation, fol-
lowed by radical coupling of the resulting dichloroal-
kenes.12 This may depress monomer yields by either “poi-
soning” the Pd catalyst, in turn increasing the concentra-
tion of styrenic unit 5b, leading to increased polymeriza-
tion, and/or via HCl release inducing monomer degrada-
tion and cationic styrenic polymerization.
0.5 mol% Hf(OTf)4, 0.2 mol% Pd/C (10 wt%) for 16 h. Upon
completion, reactor cooled to 25 °C, and contents filtered to re-
move catalyst. Solvent removed in vacuo to yield beige oily sol-
ids. G = guaiacol unit recovered (yields of 2b & 3 versus starting
1b). DM = dimethoxyphenyl unit recovered (yields of 2b, 4a, 4b,
4c vs starting 1b. “-“ = none detected. b 1 psi H2 c 600 psi H2.
reactive than 1° esters and 2° ethers. Initial attempts to
use highly Lewis acidic Hf(OTf)4 at 1 bar H2/70 °C, or at
600 psi H2/100 °C, did not effect selective cleavage of 1b to
the constituent monomer units (Table 1). The starting
dimer 1 is not recovered, and broad signals in the 1H NMR
spectrum of the waxy solid product indicate that Hf(OTf)4
+ Pd/C most likely induces polymerization of 1b. Analysis
of this product by GC-MS reveals only traces of styrenic
intermediate 5b, the result of secondary ester cleavage at
the α position, without subsequent hydrogenation (see
Scheme 2 below). Observation of 5b by GC-MS suggests
that this intermediate is formed in non-negligible
amounts, and is most likely responsible for the dimer
polymerization since styrenic polymerization should be
This tandem lignin hydrogenolysis was also investigated
in the room temperature ionic liquid [EMIM][OTf]
(EMIM = ethylmethyl-imadizolium) as an alternative sol-
vent to 1,2-dichloroethane.13 Since commercially available
Pd/C is not stable in in [EMIM][OTf], atomic layer depos-
ited Pd@ALD12d was utilized. In this case, no monomeric
cleavage products are obtained up to 185 ⁰C, most likely
facile under these reaction conditions.12 Running the re-
action at room temperature under 600 psi H2 yielded no
reaction (Table 1 entry 2) under these conditions.
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Scheme 2. Step-wise reaction profile for 1a,b C-O hydrogenolysis using a tandem M(OTf)n + Pd/C catalyst
Table 2. Acetylated dimer hydrogenolysis with a tandem La(OTf)3 + Pd/C catalyst
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Mass Bal-
ance %
G : DM
39 : 0
21 : 13
38 : 35
66 : 58
97 : 96
78 : 64
13 : 97
73 : 61
Temp
(ºC)
H2
(psi)
Time
(h)
Yields (%)
Entry
La(OTf)3
Pd/C
1b
-
-
-
-
-
-
-
-
2b
-
8
3
4a 4b 4c
1
2
3
4
5
5%
10%
5%
10%
5%
5%
5%
5%
1%
2%
3%
6%
5%
5%
5%
5%
140
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140
600
600
600
600
600
600
600
600
2
2
39
13
24 14
23 43
-
-
-
2
6
2
2
9
1
3
9
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2
17
27 69 18 19 32
28 56 19 11
52 13 22 13 12
14 59 14 34
6b
7c
8d
6
7
aReaction conditions: 4 mL Cl2C2H4, 0.1 mmol 1b ([1b] = 0.25 mM). Upon completion, reactor was cooled to 25 °C, and
contents filtered to remove catalyst. Solvent then removed in vacuo to yield beige/brown oily solids. Internal CH3NO3 stand-
ard added. Product yields determined by 1H NMR with reference to the internal standard by comparison to known compo-
nent spectra. GC-MS analysis also performed on each sample as confirmation. G = guaiacol unit recovered (2b and 3 yields
from 1b). DM = dimethoxyphenyl unit recovered; 2b, 4a, 4b, and 4c yields from 1b. b2 mL (0.50 mM). c6 mL (0.17 mM). d.
4 mL in Cl4C2H2 (0.25 mM).
due to the reduced activity of Pd@ALD in comparison to
Pd/C.14
sus that for hydrogenation. Future efforts will investigate
the application of this cleavage reaction process to acety-
lated guaiacol-based Kraft lignins. Preliminary experi-
ments indicate that this catalyst system also yields aro-
matic products from acetylated lignins.15
In conclusion, we have successfully applied a tandem
M(OTf)n + Pd/C catalytic system to the thermodynamical-
ly leveraged hydrogenolysis of acetylated model lignins.
Selective cleavage is achieved via kinetic control of the
two principal reaction steps, the rate of C-O scission ver-
ASSOCIATED CONTENT
AUTHOR INFORMATION
Corresponding Author
Experimental details, procedures, and characterization. This
material is available free of charge via the Internet at
http://pubs.acs.org.
* t-marks@northwestern.edu
Present Addresses
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‡ (Z.L.) ShanghaiTech University, 100 Haike Road, Pudong
Wang, X. Y.; Rinaldi, R., Angew. Chem. Int. Ed., 2013, 52,
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District, Shanghai 201210, China.
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Zhang, J. J.; Yu, W. Q.; Xu, J., Energy Environ. Sci. 2013, 6,
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Yao, K. S., Angew. Chem. Int. Ed. 2013, 52, 12674-12678; (o)
Xu, X.; Li, Y.; Gong, Y. T.; Zhang, P. F.; Li, H. R.; Wang, Y., J.
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Notes
The author’s declare no competing financial interest.
ACKNOWLEDGMENT
This work was supported by the U.S. Department of Energy
under contract DE-AC0206CH11357. NSF grant CHE-1213235
supported T.L.L. and provided reactor equipment. This ma-
terial is based partially upon work supported as part of the
Institute of Atom-efficient Chemical Transformation (IACT),
an Energy Frontier Research Center funded by the U.S. De-
partment of Energy, Office of Sciences, and Office of Basic
Energy Sciences, which supported Z.L.
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Atesin, A. C.; Ray, N. A.; Stair, P. C.; Marks, T. J., J. Am. Chem.
cling.13 Therefore, using 1,2-dichloroethane (widely used in-
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Soc., 2012, 134, 14682-14685; (e) Lohr, T. L.; Marks, T. J., Nat.
Chem. 2015, 7, 477-482.
dustrially) was deemed reasonable as it is much easier to
recycle, contain, and limit overall waste generation. Fur-
thermore, 1,2-dichloroethane has very low solubility in H2O
(0.84g/100g), which would enable easier cleanup of potential
waste water streams.
15 La(OTf)3 + Pd/C can also produce aromatic monomers
from phenolic lignin, however, unlike the methoxy-
substituted aromatic groups described in the present contri-
bution, significant amounts of the phenolic aromatic groups
are over-hydrogenated to cycloalkanes.
12.
Hui, A. W.; Hamielec, A. E., J. Appl. Polym. Sci. 1972,
16, 749-&.
13.
Stärk, K.; Taccardi, N.; Bösmann, A.; Wasserscheid,
P., ChemSusChem 2010, 3, 719-723.
14. The dimer and monomer products were extracted from
the ionic liquid (after dilution with water) with both toluene
and dichloromethane, which both increases the amount of
waste generated, but reduces the ease of ionic liquid recy-
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Insert Table of Contents artwork
Tandem Catalysis for C-O cleavage
OMe
O
OMe
OMe
O
O
OMe
R
OH
OMe
O
OMe
OMe
O
O
MeO
MeO
O
M(OTf)n Pd/C, H2
ꢀAcOH
O
R = H, C(O)CH3, OH
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