Degradation of Lignin
ciently cleaving the bꢀOꢀ4 linkage could be an optional strat-
egy for the degradation of lignin while preserving the aromatic
character of the fragments.
Acid and base-catalyzed routes to lignin depolymerization
are known; these fundamental processes involve strong acids,
caustic alkali, sulfocompounds and volatile toxic solvents that
can have negative effects on the environment. Kraft pulping, a
common process for the depolymerization of lignin, mainly
employs NaOH and NaSH (or anthraquinone) to cleave the b-
ether bonds in lignin.[9,11] Acid-catalyzed hydrolysis is another
method for cleaving the b-ether bonds of lignin.[12] Studies on
the hydrolysis of b-ether bonds in phenylpropane dimer model
compounds have been carried out with hydrochloric acid or
AlCl3 as the catalyst in dioxane–water or ethanol–water sol-
vents.[13,14]
Scheme 1. The cleavage of bꢀOꢀ4 bond of lignin model compounds.
and it is needed for the hydrolysis reaction, so a controlled
amount of water was added at a level that led to approximate-
ly 2 wt% H2O. The Cꢀ2 protons of 1,3-disubstituted imidazoli-
um cations are acidic,[26,27] therefore, two ILs, 1-butyl-3-methyl-
imidazolium chloride ([BMIM]Cl) and 1-butyl-2,3-dimethylimida-
zolium chloride ([BdMIM]Cl, a Cꢀ2 substituted IL) were em-
ployed to test the effect of the Cꢀ2 proton. After GG (8 mg)
and H2O (2.25 mL) were heated at 1308C for 120 min in 100 mg
of [BMIM]Cl or [BdMIM]Cl, less than 5% GG conversion was ob-
served. No guaiacol was detected by HPLC; a trace amount of
EE was detected by HPLC and verified by NMR spectroscopy.
Although a controlled amount of H2O was added into our
system, the results are consistent with the low EE yield that
was reported in [BMIM]Cl after 180 min at 1208C,[22] which indi-
cates the Cꢀ2 proton has no effect on the GG conversion and
hydrolysis of the bꢀOꢀ4 bond.
Ionic liquids (ILs) have attracted much attention as a
medium for biomass conversion, primarily in the conversion of
carbohydrates.[15–17] However, few studies report on the reactiv-
ity of lignin in ILs. Recently, ILs have been found to be a direct
wood pulp solvent capable of solubilizing lignocelluloses.[18–21]
Guaiacylglycerol-b-guaiacyl ether (GG) is commonly employed
as a model compound for the phenolic bꢀOꢀ4 ether linkages
in lignin. Recently, Kubo et al. found that an enol ether (EE), 3-
(4-hydroxy-3-methoxyphenyl)-2-(2-methoxy-phenoxy)-2-prope-
nol, was the primary decomposition product from GG in dialky-
limidazolium chloride and acetate ILs.[22] However, EE essential-
ly is a dehydration product from GG, which implies these ILs
did not cleave the bꢀOꢀ4 bond. Binder et al. reported the
dealkylation of alkyl substituted 2-methoxyphenols, which
served as lignin model compounds, in a variety of ILs and real-
ized up to 11.6% yield of the dealkylation product.[23] However,
the high concentration of CꢀO bonds in lignin suggests cleav-
ing these bonds, especially the bꢀOꢀ4 bonds, is a more viable
degradation strategy. Due to the complex chemical structure
of lignin, one has to recognize the limitations of extrapolating
results with simple model compounds featuring the bꢀOꢀ4
bond to lignin. Nonetheless, model compounds, such as em-
ployed herein, facilitate understanding lignin chemistry.
Moreau et al. reported 1-H-3-methylimidazolium chloride
([HMIM]Cl) acted as both solvent and catalyst for the dehydra-
tion of sugars.[24] Since [HMIM]Cl is an easily synthesized and
low-cost acidic IL from the BASIL technology,[25] and a non-vol-
atile IL, we explored its potential as the acid catalyst for the hy-
drolytic cleavage of bꢀOꢀ4 linkages common in lignin. Herein,
we report initial results on the cleavage of bꢀOꢀ4 bonds in
both phenolic and non-phenolic lignin model compounds in
[HMIM]Cl.
Figure 2a presents the results of reacting GG in [HMIM]Cl at
various temperatures, in which GG (8 mg) and H2O (2.25 mL)
were added into [HMIM]Cl (100 mg) for each experimental run.
GG was effectively cleaved at temperatures as low as 1108C,
producing guaiacol in [HMIM]Cl. The guaiacol yield increased
with reaction temperature, reaching 71.5% at 1508C after
60 min. At the higher temperatures, the guaiacol yield curves
displayed a maximum, possibly because guaiacol underwent
subsequent reactions.[28] The GG conversion was essentially
100% in all the experimental runs (Figure 2a) except after
15 min at 1108C, for which the GG conversion was 68.4%, and
6% yield of EE was detected. EE was not detected at longer
times or higher temperatures.
In general, lignin consists of more etherified phenylpropane
units, so we used veratrylglycerol-b-guaiacyl ether (VG) as a
non-phenolic lignin model compound. The phenolic lignin
model compound, GG, is considered to be more reactive. As il-
lustrated in Figure 2b, the bꢀOꢀ4 bond of VG was cleaved as
steadily as GG at 1508C. The guaiacol yield was similar to that
from GG and decreased a little after 120 min. The VG conver-
sion also exhibited the same pattern as GG with essentially
100% conversion in all the experimental runs. Both GG and VG
were converted rapidly into intermediate products, some of
which reacted to guaiacol (see below).
Results and Discussion
Cleavage of bꢀOꢀ4 bonds in lignin model compounds
GG, a common dimeric lignin model compound, is employed
for phenolic lignin units featuring the bꢀOꢀ4 bond. Because
guaiacol is liberated after the bꢀOꢀ4 bond of GG is hydrolyzed
(Scheme 1), guaiacol yield was monitored to track bꢀOꢀ4
bond cleavage. Water is ubiquitous in hydroscopic systems
Acid-catalyzed hydrolysis of GG and VG should also produce
Hibbert’s ketones.[12,29] Figure 3 shows the FTIR spectra of GG,
VG, and guaiacol. The absorbance around 3400–3500 cmꢀ1 is
associated with the hydroxyl group vibrational stretching
modes for the three compounds; symmetric and asymmetric
ChemSusChem 2010, 3, 1078 – 1084
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