Selective catalytic tailoring of the H unit in herbaceous lignin for methyl: P -hydroxycinnamate production over metal-based ionic liquids

Article abstract of DOI:10.1039/c8gc01252k

Selective valorization of lignin to achieve high value and commodity chemicals is attracting increasing attention. In this work, an efficient and reusable metal-based ionic liquid (MBIL) was developed for the selective tailoring of p-coumaric acid ester (pCA), a typical p-hydroxyphenyl (H) unit, into methyl p-hydroxycinnamate (MPC). Under optimized conditions and in the presence of catalyst [Bmim][FeCl₄], a volatile aromatic product of 10.5 wt% was obtained, of which, 70.5% separated as pure MPC with an isolated yield of 71.1 mg g^-1. FT-IR, ¹³C NMR, ANO and 2D HSQC demonstrated that the H unit was preferentially tailored from lignin, of which, 86.0 wt% of the H structure unit is cut off from lignin, with 70.6% being selectively converted to MPC. Further investigation demonstrated that MBIL prefers to tailor ester bonds compared to ether bonds using model compounds, and the superior catalytic ester bond cleavage performance exhibited by [Bmim][FeCl₄] can be ascribed to the relatively narrow energy gap between the lignin ester bond and [FeCl₄]^- anion and to the comparatively low absolute binding energy between the cation and anion through DFT calculations.

Full text of DOI:10.1039/c8gc01252k

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DOI: 10.1039/C8GC01252K.
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G. Chatel et al.
Heterogeneous catalytic oxidation for lignin valorization into valuable
chemicals: what results? What limitations? What trends?
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Selective catalytic tailoring of the H unit in herbaceous lignin for
methyl p-hydroxycinnamate production over metal-based ionic
liquids
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
a
a
a
DOI: 10.1039/x0xx00000x
Zhangmin Li, Zhenping Cai, Qiang Zeng, Tian Zhang, Liam John France, Changhua Song, Yaqin
b
b
c
a
a
Zhang, Hongyan He,* Lilong Jiang, Jinxing Long,* and Xuehui Li*
www.rsc.org/
Selectively valorize lignin to high value and commodity chemicals is attracting increasing attention. In this work, an
efficient and reusable metal-based ionic liquid (MBIL) was developed for the selective tailoring of p-coumaric acid ester
(pCA), a typical p-hydroxyphenyl (H) unit, into methyl p-hydroxycinnamate (MPC). Under optimized conditions and in the
presence of catalyst [Bmim][FeCl ], a volatile aromatic product of 10.5 wt% was obtained, of which, 70.5% separated as
4
-1
13
pure MPC with an isolated yield of 71.1 mg g . FT-IR, C NMR, ANO and 2D HSQC demonstrated that the H unit was
preferentially tailored from lignin, of which, 86.0 wt% of H structure unit is cutting off from lignin, with 70.6% being
selectively converted to MPC. Further investigation demonstrated that MBIL prefers to tailor ester bond compared to
ether bond by using model compounds, and superior catalytic ester bond cleavage performance exhibited by
-
[Bmim][FeCl
4
] can be ascribed to the relatively narrow energy gap between the lignin ester bond and [FeCl
4
] anion, and
the comparatively low absolute binding energy between cation and anion through DFT calculations.
of these bonds to obtain low molecular weight compounds
such as phenolic monomers, substituted benzenes and liquid
Introduction
2
,3,5-8
fuels.
Normally, these techniques are robust and efficient
The continual increase in fossil resources consumption and the
resulting negative environmental impact associated with such
activities have provoked a substantial shift within the research
community towards the generation of carbon neutral/negative
technologies. One promising candidate is the utilization of
biomass, which with its diverse range of sources and types
means that it can offer a whole range of products. Lignin has
been proven to be a particularly interesting source due to it
being abundant in most biomass. However, the major
drawback comes from the highly complex structure, which is
composed of three monolignol precursors, viz. p-coumaryl
alcohol (H), coniferyl alcohol (G), and sinapyl alcohol (S)
present through a variety of C-O-C (ester, β-O-4′, α-O-4′, 4-O-
for lignin depolymerization with good to excellent volatile
product yield. However, such products are composed of a
large distribution of chemical compounds. Usually, hundreds
of products can be generated during lignin oxidation
procedures. A total of 94 types of small molecules were
9
detected in the lignin-ethanolysis system, about 30 different
volatile products can be observed through catalytic
8
hydrogenolysis of lignin in ionic liquid (IL), while more than 40
kinds of products can be found in the ethanolysis of lignin over
1
0
nano-SiC catalyst. On the other hand, the product mixture
from current lignin depolymerization has a significantly higher
viscosity, thermosensitivity, high oxygen content and low pH
1
1,12
1
-5
value.
These negative properties make the product
5′) and C-C (β-β′, β-5′, β-1′, 5-5′) linkages. Thus, current lignin
unsuitable for the direct use as either industrial feedstock or
fuel, resulting in energy and labour-intensive post-
depolymerization technologies including oxidation, alcoholysis,
pyrolysis, and hydrogenolysis are always focus on the cleavage
1
3-15
treatments.
Hence, highly efficient strategies for the
selective depolymerization of lignin have attracted
considerable attention in recent years.
Current approaches primarily utilize the selective cleavage
of C-O and/or C-C bonds in lignin to improve depolymerization
efficiency.
A pertinant example of the above is the
organocatalytic approach for chemoselective aerobic oxidation
6
1
of aspen lignin. Depolymerization of the same biomass
source in formic acid can achieve more than 60 wt% yield of
1
7
low-molecular-mass aromatic mixture.
Formaldehyde
derived lignin can be converted to G and S monomers at near
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is converted, generating negligible volatile products (Table 1,
entry 1). IL [Bmim]Cl can promote this depolymerization,
yielding a lignin conversion of 32.0%, due to its good hydrogen
2
9
bond and C-O bond weakening capability, however, the
volatile product yield is still minor (Table 1, entry 2). In the
presence of a Lewis acid catalyst, for example, iron salts, lignin
depolymerization is substantially enhanced with a notable
increase in volatile products (Table 1, entries 3-6). This is found
to be most pertinent when they contain halogen anions,
-1
yielding values between 22.5 and 37.8 mg g of MPC (Table 1,
entries 3-5). Inspired by our previous observations that
imidazolium cations facilitate C-O bond weakening and
3
0,31
carbocation capping,
structural characterization presented in Fig. S1 and S2, ESI
are further examined. The application of these system results
theoretical yields (47-78%) through hydrogenolysis at 200 °C in substantial enhancements in lignin conversion, volatile
a series of iron-containing MBILs
Scheme 1 Current high-valued compounds derived from MPC.
(
†)
1
.
8
under 20 bar of H
2
Modification of the protecting group on product yield, MPC yield and selectivity (Table 1, entries 7-10).
the α,γ-diol-protected lignin results in the selective formation Among which, [Bmim][FeCl ] possesses the most promising
4
19
-1
of two different types of major products. However, these activity, where, a MPC yield of 77.7 mg g is achieved (Table 1,
currently developed lignin depolymerization strategies are entry 8). Analysis of the volatile fraction reveals that
mainly focused on acquiring G and S unit products from wood approximately 16 different products are generated (Fig. S3 and
lignin via C-O or C-C bond cleavage. In contrast, the H unit is Table S1, ESI
widespread in grass, for example, 20 wt% in maize lignin.
†
). Of particular note, is the relatively high
selectivity towards MPC, which approaches 70.5%. Simple
Therefore, selective conversion of the H unit from herbaceous chromatographic separation of the product mixture allows for
2
0
-1
lignin to high value chemicals is likely to be another interesting the isolation of 71.1 mg g MPC (7.1 wt%). Detailed structural
approach for lignin valorization.
and purity characterization data of this compound can be
Methyl p-hydroxycinnamate (MPC) is an excellent starting- found in Fig. S4-S8 (ESI
). This result is higher than that found
material for a variety of fine chemicals, such as valuable for previous work, where a pure major compound of 5 wt%
†
2
1-23
medicinal intermediates (Scheme 1, compounds 1-3)
and yield was obtained via catalytic C-O bond cleavage of birch
3
2
polymeric precursor monomers (Scheme 1, polymers 4 and lignin.
2
)
4,25
-
5
. Current MPC production technology utilizes relatively
It has been suggested previously that [FeCl ] , generated
4
high-cost petrochemical feedstock, noble metal or corrosive from the combination of [Bmim]Cl and FeCl acted as the
3
2
6-28
acid catalysts and complex separation procedures.
Clearly, active species and exhibited a good catalytic performance for
a greener and more cost-effective route for the production of the cleavage of ester bond in poly(ethylene terephthalate).
MPC from renewable resources would be attractive, matching Meanwhile, [Bmim] cations can build complexes with
well with the requirements of sustainable development. metalates, which can further promote the activity of [FeCl ] .
3
3
+
3
4
-
4
Hence, unlike previous C-O or C-C bonds cleavage For herbaceous lignin, the pCA structural unit is normally
3
5-37
technologies, we propose a methodology for selective catalytic connected with other units through ester bond.
Hence, it is
-
tailoring of the ester linkages of H units from herbaceous lignin plausible that the catalytic activity of [FeCl ] towards ester
4
under mild conditions for the production of MPC, as it has a bond breakage can couple with the promotional effect of
+
similar structure to that of p-coumaric acid ester, a typical H [Bmim] , yielding
a
synergetic influence on catalytic
structural unit in herbaceous lignin. A series of control performance. Such an explanation would help to rationalise
experiments, characterization methods, model compound the elevated performance of the combination, [Bmim][FeCl4],
experiments and density functional theory (DFT) calculations compared to the individual [Bmim]Cl and FeCl components. It
3
are carried out to analyze the relationship between the is apparent that this effect can be further extended to
structure of MBILs and their special catalytic activities for MPC [Bmim] [FeCl ] and [Bmim][FeClBr ] MBILs, which exhibit the
2
4
3
production. We believe that the findings in the study will be common [Bmim][FeCl ] structure. (Table 1, entries 7-9). Table
helpful in developing effective methods for lignin valorization.
x
2 4 3
1 shows that Fe (SO ) exhibits no MPC production (Table 1,
entry 6), whereas, when [Bmim]Cl is added in conjunction with
it, a sharp increase in lignin conversion from 24.5 to 56.7% is
-1
found, generating 56.9 mg g MPC (Table 1, entry 10). This
Results and discussion
Lignin conversion over various catalysts
increase can be tentatively ascribed to the formation of
[
Bmim][FeCl
x
]
species generated via ligand substitution
Table 1 summarizes the lignin depolymerization performance
over various catalysts. Without catalysts, less than 19.0% lignin
processes.
2
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a
Table 1 Lignin conversion over different iron salts and MBIL catalysts
-
1
Y
VP (mg g )
Entry
Catalyst
C
L
(wt%)
S
MPC (%)
Y
MPC
Y
Others
Total
1
2
3
4
5
6
7
8
9
-
19.0
32.0
31.3
43.1
42.0
24.5
44.0
47.6
43.8
56.7
37.3
27.6
32.0
30.0
24.4
31.9
24.8
45.4
43.2
55.6
-
-
0.6
0.6
1.2
-
[Bmim]Cl
1.2
-
FeCl
FeCl
2
37.8
27.0
22.5
Trace
69.0
77.7
31.6
56.9
11.3
6.5
29.4
45.1
20.4
38.7
41.9
32.5
27.4
21.7
18.7
12.3
9.9
67.2
72.1
42.9
38.7
110.9
110.2
59.0
78.2
30.0
18.8
19.7
69.3
34.1
25.0
9.8
56.3
37.4
52.4
-
3
FeBr
Fe (SO
[Bmim] [FeCl
[Bmim][FeCl
[Bmim][FeClBr
[Bmim]Cl/Fe (SO
[Bmim][CrCl
[Bmim] [CrCl
[Bmim] [MnCl
[CoCl
3
2
4 3
)
2
4
]
62.2
70.5
53.6
72.8
36.7
34.6
49.7
30.9
17.5
70.9
24.5
74.9
45.8
17.8
4
]
3
]
1
1
1
1
1
1
1
1
1
1
2
0
1
2
3
4
5
6
7
8
9
0
2
4 3
)
4
]
2
4
]
2
4
]
9.8
[Bmim]
2
4
]
21.4
6.0
47.9
28.1
7.0
[Bmim]
2
[NiCl
4
]
[Bmim]
2
[CuCl
4
]
18.0
2.4
[Bmim]
2
[ZnCl
4
]
7.4
[Bmim][FeCl
3
Br]
64.4
31.5
20.7
21.6
37.3
95.6
86.0
68.7
116.3
[Bmim][FeBr
4
]
b
HCl
a
b
Reaction conditions: 0.25 g lignin, 1.0 mmol catalyst, 10.0 mL CH
anhydrous
3
OH, 420 K, 6.0 h. HCl (1.0 mmol per 10.0 mL methanol) was prepared by dissolve dry HCl gas into
methanol.
In this study, the catalytic activities of MBILs with a variety similarly found to influence the extent and selectivity of the
of different transition metal ions were further investigated reaction. It is notable that only product yields follow a volcano
(
Table 1, entries 11-17). Unfortunately, most of them exhibit trend (maximizing at 420 K) while a continuous increase and
very weak ability for the production of MPC. What’s more, decrease is found for conversion and selectivity, respectively.
FeCl , a material that is known to possess less Lewis acidity It is quite clear from these trends that higher temperatures
than FeCl
Table 1, entry 3 vs 4). The above demonstrates that Lewis more favorable, which is further evidenced by the continuous
2
3
8
3
, is found to be more efficient for MPC production make the conversion of higher molecular weight G and S units
(
acidity is not a decisive factor for most active catalyst decline in Re-lignin molecular mass (Fig. 1d). Reaction time
combinations in this process. Moreover, HCl, a typical (Fig. 1e) is found to exhibit a similar trend for yields and
Brønsted acid, can promote lignin conversion, but results in selectivity to temperature variation (optimized time is found at
low MPC yield and selectivity (Table 1, entry 20). In actuality, 6 hours), however, longer reaction times result in reduced
the bulk of the volatile products in this case is generally conversion, which is attributable to an increased
3
1,39
derived from lignin guaiacol (G) and syringol (S) units.
repolymerization degree (Fig. 1f). Fig. 1g further demonstrates
Hence, considering the remarkably different product that MPC yield is improved with increasing methanol dosage
distribution, it is believed that selective lignin conversion in the between 5.0 to 10.0 mL, beyond the latter dosage level
4
presence of [Bmim][FeCl ] is more likely to be mediated by reduced catalytic efficiency results due to lower catalyst
coordination rather than acidity.
concentration. This result coincides with observations made by
gel permeation chromatography (GPC) for the Re-lignin, as
show in Fig. 1h. In general, optimized conditions were found to
Effect of reaction conditions
This process is substantially influenced by the catalyst dosage, be 1.0 mmol catalyst, 10.0 mL methanol, 420 K and 6.0 h for
reaction temperature, time and solvent volume. Fig. 1a 0.25 g of bagasse lignin. Further investigation shows that
demonstrates that catalyst dosage has significant influence on process efficiency of selective lignin depolymerization
lignin conversion and MPC production. In general, a clear increases with the decrease of carbon number or branching
volcano-type plot is observed for the change in catalyst loading degree of the alcohol (Fig. S9, ESI
vs MPC selectivity and yield, however, conversion appears to via steric hindrance effects.
maximize at 1.0 mmol, with only a minor decline above this
†), which is readily explained
Comparative characterization of the original lignin and Re-lignin
value. Considering the increase in the molar mass of Re-lignin
between 1.0 and 1.5 mmol (Fig. 1b), it becomes apparent that In order to obtain a clearer understanding on the
this trend is indicative of repolymerization of phenolic depolymerization performance order of lignin structural units,
oligomers, suggesting that those related to H units may be 2D HSQC NMR spectroscopy was applied to analyze the
preferentially consumed. Reaction temperature (Fig. 1c) is chemical linkages of the original lignin and regenerated lignin
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Fig. 1 The effect of reaction conditions on lignin depolymerization (a, c, e, and g) and molecular weight distribution of Re-lignin (b, d, f and h).
(
Re-lignin), where all determined cross peaks are designated been selectively converted, while the remaining absorbance
-1 31,43
with reference to a typical herbaceous lignin in previous features consistent with G (1269, 1217 and 1030 cm )
1
and
6,32
-1 43
As shown in Fig. 2, the cross- S units (1325 and 1120 cm ) can be observed clearly.
studies (Table S2, ESI
†).
1
signals of methoxyls and β-O-4′ substructures are the most This observation is further verified via H NMR spectroscopy.
prominent in the side chain region of both original lignin and Comparative examination of the original lignin (Fig. S10a, ESI
Re-lignin (Fig. 2a and b), suggesting the preservation of the and Re-lignin (Fig. S10b, ESI ) demonstrated that the intensity
core structure” after depolymerization. However, the 2D of the signal attributed to the H unit (7.49 ppm) is notably
HSQC spectrum of Re-lignin shows a dramatically declined A reduced. However, those associated with G and S units (6.78
CA) signal, which is ascribed to the pCA structure linked to and 6.70 ppm, respectively) are still present in the Re-lignin
the γ-OH of syringyl moieties in herbaceous lignin by an ester spectrum. In addition, a strong signal at 3.7 ppm, consistent
†)
†
4
0
“
γ
2
9
(γ−p
4
0
43,44
bond. On the other hand, in the aromatic region of Re-lignin, with methoxyl protons (-OCH
3
),
can also be observed. GC-
the signals of the H structure unit (here it is mainly consisted MS results demonstrate that MPC, generated by the abscission
of pCA structure) decreased sharply, in comparison with those and followed methanol esterification of the H unit, is
of S and G lignin. Therefore, results obtained from 2D HSQC predominant in the volatile products (Table S1 and Fig. S3, ESI
†
spectroscopy suggest that the H structural unit is the most ), while methyl ferulate, generated by the abscission and
flexible, whereas G and S units are more recalcitrant in this subsequent methanol esterification of the G unit, is negligible.
1
Therefore, results obtained from FT-IR, H NMR spectroscopy
lignin depolymerization process.
Fig. 3 shows that raw bagasse lignin has conventional FT-IR and GC-MS further confirm those obtained from the above 2D
bands consistent with those of herbaceous lignin, exhibiting HSQC spectroscopy, viz., the H structure unit is more readily
characteristic absorption bands associated with H, G and S converted than either G or S units.
1
3
structures, detailed FT-IR spectroscopy assignments are shown
in Table S3 (ESI ). After treatment with [Bmim][FeCl ], Re- and alkaline nitrobenzene oxidation (ANO) methods
lignin exhibits a similar FT-IR spectrum as that of the original demonstrate that the ratio of H: G: S changed from 15:45:40
Further quantitative analysis using C NMR (Fig. S11, ESI†)
†
4
-1
lignin. However the absorption band at 1169 cm , consistent (original lignin) to 4:45:51 (Re-lignin). Detailed results (Table
4
1,42
with carbonyl stretching in H-lignin units (Fig. 3a and c),
S4) and discussion are described in the ESI.
†
These
exhibits a significantly reduced intensity, suggesting that it has observations also verify the fact that H is preferentially
4
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Fig. 2 2D HSQC NMR spectrum relates to the lignin interunit distribution. (Aliphatic regions (a, b) and aromatic regions (c, d) of 2D C-
1
6
H correlation (HSQC) spectroscopy of original lignin (a, c) and Re-lignin (b, d) in DMSO-d .)
converted (86 wt% of H unit is converted according to confirming the preservation of the main basic lignin structural
equation (2)). Meanwhile, most of the G and S units are
retained in Re-lignin without significant structure change.
Therefore, we consider that this effective MPC production
process from renewable lignin offers highly selective molecular
tailoring, where H unit (pCA structure) is dissociated through
selective cleavage of the ester bond, leaving G and S units with
insignificant structural change in Re-lignin.
GPC demonstrates an obvious difference between
molecular weight distribution and average molecular weight of
the original lignin and Re-lignin. As shown in Fig. S12 and Table
S5 (ESI†), after reaction, a significant decrease of average
-1
molecular weight (from 1860 to 1299 g mol ) is found with a
broadening of the molecular weight distribution and increased
polydispersity. Elemental analysis (Table S6, ESI
the carbon content of the Re-lignin increases from 60.88 to
4.37%, due to the dissociation of relatively low carbon
products as listed in Table S1 (ESI ). However, it should be
noted that the main elemental compositions of Re-lignin are
†) shows that
6
†
Fig. 3 FT-IR of raw lignin (a), Re-lignin without catalyst (b) and with [Bmim][FeCl4] (c)
3
1,45
under the optimized conditions.
still in the range of conventional organosolv lignin,
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units. This result accords well with the above discussion the MBIL. Further discussion can be referred to ESI.
†
regarding selective tailoring of the pCA structure from original
Model compounds study
lignin preferentially and the retention of G and S units in Re-
a
lignin.
Table 2 Conversion of model compounds over different MBIL catalysts
O
O
OH
[
Bmim][MCl
x
]
Conversion of different herbaceous lignin
O
Methanol,
O
6
h, 420 K
a
b
1
The conversion of a series of different herbaceous lignin types
was carried out to extend the feedstock applicability of the
O
[Bmim][MCl
x
]
O
OH
Methanol,
6 h, 420 K
[
Bmim][FeCl
4
] catalytic system. As shown in Fig. 4 and Table S7
2
c
d
(
ESI†), all lignin forms demonstrated appreciable catalytic
Conv. (%)
Yield (%)
Entry
Catalyst
-
depolymerization performance with MPC as the major product
in most cases. However, lignin depolymerization and MPC
production are conspicuously variable depending upon the
choice of lignin feed, due to different H unit content. But, it is
found that a linear correlation exists between MPC yield and H
unit content of the lignin. For example, Corncob has the
highest H unit content of 17.1%, achieving the highest MPC
1
2
a
b
c
-
d
1
2
3
4
5
6
7
8
9
7.4
2.0
7.1
7.2
1.1
FeCl
4 3
Fe (SO )
3
98.3 52.4 89.9 92.1
89.3
12.9
2
99.8 95.2 81.9 68.4 174.8 12.0
[Bmim][CrCl
[Bmim] [MnCl
[Bmim][FeCl
[Bmim] [FeCl
[Bmim] [CoCl
[Bmim]
4
]
59.1
62.9
98.4
98.6
97.1
18.1
42.5
61.4
98.6
5.9
1.9
4.0
1.3
1.7
2.8
4.0
1.5
2.1
56.3 60.0
62.9 62.2
96.6 96.7
99.4 96.5
98.2 95.0
17.3 17.6
41.6 41.8
60.7 60.7
98.7 96.2
4.8
0.1
3.0
0.5
0.1
1.4
2.3
0.1
1.3
44.3
4.7
1.46
3.6
2
4
]
4
]
-1
-1
2
4
]
1.5
yield of 90.9 mg g (Table S7, entry 1). While, only 4.7 mg g
2
4
]
1.6
MPC yield is found for haulm lignin, as it contains no more
than 5.0% H unit (Table S7, entry 7). Consequently, no MPC
was produced when lignin from the softwood of pine was used
2
[NiCl
4
]
2.3
1
1
1
1
0
1
2
3
[Bmim]
[Bmim]
2
[CuCl
4
]
3.9
2
[ZnCl
4
]
1.1
(Table S7, entry 8), as there is no H unit in this non-herbaceous
[Bmim]Cl/FeSO
4
4
2.1
lignin. Similarly, FT-IR spectroscopy shows that all these Re-
lignins preserve the core structure of the original lignin expect
for the observed decrease of the absorption peak intensity of
[Bmim]Cl/Fe (SO
2
)
3
98.1 28.5 93.3 91.6
11.8
a
Reaction conditions: 2.0 mmol phenethyl phenylacetate (0.4806 g) or benzyl
-1
ether (0.3965 g), 5.0 mol% catalyst, 5.0 mL methanol, 420 K, 6.0 h.
H structural unit at 1169 cm (Fig. S13, ESI
†). Thus, this result
further confirms the aforementioned conclusion that MBIL
Herbaceous lignin is generally composed of C-C and C-O
35,37
[Bmim][FeCl ] is highly selective for MPC production through
4
linkages including ester (e.g. pCA and ferulate)
35,36
and ether
the molecular tailoring of the H structural unit from
bond (such as β-O-4 and 4-O-5).
Amongst which, the C-C
herbaceous lignin.
bond is much more rigid than those of C-O (ether and ester)
The reusability of the MBIL catalyst was also examined. Fig.
bonds, hence, many current technical methods look towards
1,6,32
S14 (ESI
selectivity of 42.4 and 66.1%, respectively, can be obtained
after five consecutive reaction cycles. Detailed
†
) shows that a satisfactory lignin conversion and MPC
cleaving the C-O linkage.
But, such processes lack
selectivity as most of the bonds are broken simultaneously,
yielding a mixture comprised of up to hundreds of compounds.
In this study, the pCA structure could be selectively tailored via
the cleavage of the ester linkages, without significant cleavage
of the other C-O linkages associated with G and S units (Fig. 2
characterizations of the fresh and spent catalysts using
elemental analysis (Table S8), FT-IR (Fig. S15), Raman (Fig.
S16), TGA (Fig. S17) and ESI-MS (Fig. S2A, B, I and J)
demonstrate that the slight catalytic activity loss could be
-
attributed to gradual leaching of the [FeCl ] anion during
&
3). In order to provide a more in-depth understanding of the
4
catalytic activity exhibited by these MBIL catalysts, their
cleavage performance towards ester and ether bonds were
investigated, using phenethyl phenylacetate and benzyl ether
as ester and ether model compounds respectively. It was
found that 98.4% of phenethyl phenylacetate is converted to
methyl phenylacetate and phenethyl alcohol by selective
operation and the presence of lignin remnants complexed with
cleavage of the ester bond when [Bmim][FeCl
4
] was used
(Table 2, entry 6). Other iron-containing MBILs, which showed
very good catalytic activity towards lignin depolymerization
and MPC production (Table 1, entries 7, 8 and 10) are also
active, resulting in more than 97.1% conversion (Table 2,
entries 6, 7 and 13). In contrast, less than 4.0% of the ether
bond in benzyl ether is broken for most MBIL catalysts used in
this study under the same conditions (Table 2, entries 4-11).
4
This result further verifies that [Bmim][FeCl ] shows the best
catalyst performance for MPC production, through selective
Fig. 4 The conversion of different herbaceous lignin for MPC production. (Reaction tailoring of the ester bond in H lignin (Table 1, entry 8).
4 3
conditions: 0.25 g lignin, 1.0 mmol [Bmim][FeCl ], 10.0 mL CH OH, 420 K, 6.0 h.)
Quantum chemical calculation
6
| J. Name., 2012, 00, 1-3
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S19, ESI†) correctly predict the nature of the resulting shifts
and suggest that they are caused by interactions between the
cationic and anionic species of the catalyst.
4
Fig. 6 illustrates that the [Bmim]-[FeCl ] ion pair has a
relatively low absolute binding energy between cation and
anion than any other MBILs or [Bmim]Cl, indicating weaker
+
-
4
attractive forces between [Bmim] and [FeCl ] . That means C-
H bonds are shorter and that vibrations appear at higher
frequencies. What’s more, the shorter length and higher
energy of the Fe-Cl bond (Table S10, ESI
the strong coordination ability of Fe and Cl atoms leads to
weak hydrogen bonding between C-H and Cl in [Bmim][FeCl ].
†) also indicates that
4
Fig. 5 Energy diagram of anions and ester model compound (phenethyl phenylacetate).
Therefore, the reduction of these interactions improves
-
efficient dissociation of [FeCl ] (the main catalytic active
4
+
centre) and [Bmim] (the carbocation capping species). This
In order to elucidate the difference in catalytic activity, the
HOMO-LUMO energy gap and optimized cation-anion
structures of the MBIL catalysts were investigated with DFT
calculation as well (Fig. 5). It is found that the [FeCl4]-
phenethyl phenylacetate combination has the smallest energy
difference between the LUMO of the anionic complex and the
HOMO of the model compound, detailed data refer to Table S9
substantial interaction of Fe-Cl and relative weak
intramolecular effect of the [Bmim][FeCl ] (ion pair effect, Fig.
4
6
) are considered to be responsible for its high catalytic activity
-
on the lignin selective tailoring. Systematic substitution of Cl
-
with Br leads to a gradual decline in activity and selectivity,
(Table 1, entries 8, 9, 18 and 19), which can be attributed to a
lower interaction between Fe-Br when compared to Fe-Cl and
an increase in the ion pair effect due to lower electronegativity
(
ESI
†). The HOMO-LUMO gap is usually used to predict the
reactivity of catalysts, for example, the smallest LUMO-HOMO
4
-
48
6,47
gap can relate to the highest catalytic activity of a complex.
On the other hand, [Bmim][FeCl ] shows an outstandingly
and charge density of Br . In addition, as a paramagnetic
MBIL, [Bmim][FeCl ] has many unique properties, such as good
4
4
4
9
-1
solvating ability and moisture-stability, which facilitate its
contact with the solvent and the lignin molecules, making a
contribution for the high catalytic activity as well.
strong interaction (-30.38 kJ mol ) when it combines with
ester in the presence of methanol (Fig. S18, ESI ). This
†
indicates that [Bmim][FeCl ] preferentially interacts with ester
4
bond in model lignin compounds, which may partly help to
explain the activity and selectivity enhancements found for
this complex with real lignin (Table 1, entry 8).
Conclusions
FT-IR analysis of these MBILs (Fig. S1, ESI
†
) shows that in
We have demonstrated a selective herbaceous lignin
comparison with conventional IL ([Bmim]Cl), MBILs containing tailoring process to produce MPC, a versatile chemical
Fe and Cr exhibit an obvious change in absorption bands. For currently from the petrochemical industry, catalyzed by MBIL
-1
example, the C-H stretching vibrational absorption bands of catalyst [Bmim][FeCl ]. Under mild conditions, 71.1 mg g of
4
the imidazolium ring show blue shift from 3143 for [Bmim)Cl pure isolated MPC can be achieved from bagasse lignin, along
-1
to 3155 for [Bmim][FeCl
S1, ESI ). While in the fingerprint region, in/out-of-plane investigation of the volatile product fraction, original lignin and
bending modes associated with the IL imidazolium ring shift Re-lignin demonstrates that this effective MPC production
4 4
] and 3167 cm for [Bmim][CrCl ] (Fig. with a total volatile aromatic fraction of 10.5 wt%. Extensive
†
-1
4 4
from 868 to 826 ([Bmim][FeCl ]) and 853 cm ([Bmim][CrCl ]). process can be classified as a highly selective molecular
In this study, simulation results based on DFT calculations (Fig. tailoring approach in which H unit (pCA structure) is
dissociated through the selective cleavage of the ester bond.
Simulation results reveal that the excellent catalytic
4
performance exhibited by [Bmim][FeCl ] can be ascribed to the
relatively narrow energy gap between the lignin ester bond
-
and [FeCl
amongst catalyst, lignin and methanol. Furthermore, the
catalyst ([Bmim][FeCl ]) is chemically stable, can be easily
4
] anions, in addition to intensive interactions
4
separated and is reusable for several consecutive runs.
Consequently, this selective catalytic tailoring strategy
demonstrates a highly efficient and selective approach for the
generation of fundamental chemicals from renewable lignin.
Experimental
Fig. 6 Optimized structures and binding energies between cation and anion of MBILs.
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All reagents in this study are of analytical grade and used
directly without further purification. The MBIL catalysts were
Acknowledgements
5
0
The authors gratefully acknowledge financial support from the
Natural Science Foundation of China (Grant No. 21736003,
synthesized according to previous methods under dry
nitrogen, and characterized by FT-IR (Fig. S1) and ESI-MS (Fig.
2
1336002, 21690083, 21676108 and 21406230), the Natural
Science Foundation of Guangdong Province, China (No.
015A030311048), and the Science and Technology Program
S2), the purity (≈98%) and water content (
measured by elemental analysis and Karl Fisher titration,
respectively (Table S11, ESI ). Raman (Fig. S16), TGA (Fig. S17)
and elemental analysis (Table S8) were carried out for fresh
and recycled [Bmim][FeCl ] (ESI ). Herbaceous lignin was
separated from biomass according to our previous work, and
the separated weights are shown in Table S12 (ESI ). Detailed
IL synthesis, lignin separation and lignin characterization
﹤0.11 wt%) were
2
†
of Guangzhou, China (201804020014), as well as Dr. Qiuping
Mao for his assistance in planning and conducting 2D HSQC
NMR experiments.
4
†
5
1
†
Notes and references
1
13
methods (GPC, H NMR, C NMR, 2D HSQC.) are presented in
the ESI.
1
2
3
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Typical process for lignin conversion
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respectively, using dimethyl phthalate as the internal standard
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Graphical Abstract
Selective catalytic tailoring of herbaceous lignin for production of methyl
p-hydroxycinnamate (MPC).