Direct production of naphthenes and paraffins from lignin

Article abstract of DOI:10.1039/c5cc06828b

The utilization of lignin as a fuel precursor has attracted attention, and a novel and facile process has been developed for one-pot conversion of lignin into cycloalkanes and alkanes with Ni catalysts under moderate conditions. This cascade hydrodeoxygenation approach may open the route to a new promising technique for direct liquefaction of lignin to hydrocarbons.

Full text of DOI:10.1039/c5cc06828b

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Direct Production of Naphthenes and Paraffins from Lignin
Jiechen Kong,a Mingyuan He,a Johannes A. Lercher*b and Chen Zhao*a
Received 00th xx 20xx,
Accepted 00th xx 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Utilization of lignin as a fuel precursor attracts attention, and a of high efficiency and high selectivity. In principle, it would be a
novel and facile process is developed for one-pot conversion of better strategy that hydrocarbons can be directly produced from
lignin to cycloalkanes and alkanes with Ni catalysts at moderate lignin.
condition. This cascade hydrodeoxygenation approach may open a
new promising technique for direct liquefaction of lignin to of lignin to hydrocarbons (with nearly 50% yield and 100%
hydrocarbons. selectivity) using non-precious Ni catalyst supported on
In this work, we establish a new process for one-pot converting
a
amorphous silica alumina at moderate conditions. The developed Ni
based catalysts exert multi-function, including cleaving of the
exposed external C-O-C linkages of lignin, sequentially breaking
down the Caromatic-O and Caliphatic-O bonds in the produced phenolic
oligomers, secondary and primary phenolic compounds, as well as
removing of oxygen in Ph–OCH3 and Ph–OH groups.
Selective and efficient conversion of lignocellulose to fuels and
chemicals is considered to be the greatest challenge in modern
catalysis.1-4 Lignin, the second abundant lignocellulose resource, is
rich in aromatic rings and energy density that make it attractive as a
promising renewable bio-resource to generate fuels and chemicals.5
However, the complexity of its three-dimensional cross-linking
polymeric structure leads to its low solubility in conventional
organic solvents, as well as small collision possibility of C-O-C
linkage with active catalytic sites. These specific properties of lignin
make it difficult to be converted.6
(a)
20
18
16
14
12
10
8
80
70
60
50
40
30
20
10
0
Alcohols+Ketones
Phenols
Aromatics+Olefins
Cycloalkanes+Alkanes
Conversion
The C-O cleavage of lignin derived model compounds, such as
phenolic monomers7-10 and dimers11-16, are wildly investigated by
sulfided and reduced metal catalysts. However, these catalytic
systems working well with model compounds are tested to be not
propitious or proper for real polymeric lignin deconstruction. The
efforts for lignin depolymerization, especially for breaking down the
much weaker C-O bonds in lignin, are mainly aimed at routes of
6
4
2
0
Ni/MgO Ni/SiO2
Ni/ASA
Ni/Al2O3
None Ni/ZrO2 Ni/ZnO Ni/TiO2
(b)
20
18
16
14
12
10
8
hydrolysis17 ethanolysis18 oxidation19, and reduction20-24
, , . It is
Alcohols+Ketones
Phenols
Aromatics+Olefins
Cycloalkanes+Alkanes
notable that recently Stahl and co-workers develop a two-stage
process for depolymerization of an oxidized lignin in aqueous formic
acid to give a high yield of monomeric phenolics (61 wt%) under
mild conditions17
.
However, the current techniques for
depolymerization of crude lignin are still far from the requirement
6
4
2
0
EG Dio Ben Tol Nap Eth THF n-Dod
Figure 1. a). Conversion of lignin with supported Ni catalysts. b). Conversion of
lignin in different solvents. EG: ethylene glycol, Dio: dioxane, Ben: benzene, Tol:
toluene, Nap: naphthalene, Eth: ethanol, THF: tetrahydrofuran, n-Dod:
dodecane. Reaction conditions: lignin (2.0 g), Ni/ASA catalyst (30.1 wt.%, 1.0 g),
solvent (80 mL), 250 °C, 4 MPa H2, 160 min., stirring at 700 rpm.
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The used cellulolytic enzyme lignin originates from corncob coke was observed. Catalyzed by the 30 wt.% Ni/oxides catalysts
after enzymatically hydrolyzing of cellulose and hemicellulose, (prepared by deposition precipitation method) at identical
attaining a Klason lignin content of 80 wt.% (Tables S1a and S1b). conditions, more than 90 kinds of products were detected, mainly
The soluble molecular weight of lignin attained an Mw/Mn value of including oxygenates, cycloalkanes, and aromatics (Table S2). The Ni
3061 (Mn = 1.04), as determined from the measurement of gel catalysts, that is, supported on oxides of SiO2, Al2O3, or ZrO2, ZnO,
permeation chromatography (GPC). The elementary analysis TiO2, and MgO, yielded a maximum liquid of 11 wt.%, leading to
suggested that the structure unit of lignin was C10H10.3O3.8 with a high concentrations of oxygenates formation. For example, Ni/Al2O3
molecular weight of 191 g∙mol-1, and accordingly, lignin contained reached 84% selectivity of oxygenates (Fig. 1a).
1
at least 19 structural units. Measurements of infrared and H NMR
In comparison, Ni supported on amorphous-silica-alumina
spectroscopy manifested that lignin was rich in phenyl, guaiacyl, (Ni/ASA) led to the highest lignin conversion at 78 wt.% with an 18
and syringyl unit structures (Fig. S1). The metallic Ni center showed wt.% liquid yield (Fig. 1a), attaining a lignin conversion rate of 0.58
-
high activities in conversion of phenolic monomers and dimers in glig∙gcat-1∙h-1 and a turnover frequency (TOF) of 17.4 mollig∙molsurf
.
Ni
water, but these developed catalyst systems were tested to be not 1∙h-1 (Ni dispersion: 3.5%, Mw lignin unit = 191 g∙mol-1). The much lower
suitable for real lignin conversion due to severe coke or tar liquid yield (18%) compared to the lignin conversion value (78%)
formation.9,11 In the blank test at 250 °C in presence of 4.0 MPa maybe attributed to the undetected high molecular-weight
hydrogen, only 2 wt.% liquid fragments were formed with 100% phenolic oligomers by GC-GC×MS, whereas these heavier
substituted phenols in dodecane after 160 min.. In addition, serious compounds were identified by HPLC-MS measurement (Fig. S3). It
should be addressed that majorities of liquid products over Ni/ASA
(a)
50
45
40
35
30
25
20
15
10
80
70
60
50
40
30
20
10
0
were alkanes and aromatic hydrocarbons (74% selectivity, Fig. 1a),
with a small fraction of 26% oxygenates formation (phenols,
ketones, and alcohol, Table S3). This indicated that the acidity
especially the specific Brönsted acid sites on Ni/ASA (0.107 mmol∙g-1,
Table S2) facilitate the oxygen removal from phenolic compounds
and downstream oxygenate fragments, leading to the final
hydrocarbons production. The relatively lower liquid yield (18 wt.%)
compared to the high lignin conversion (78 wt.%) implied that
transformation of the phenolic oligomers intermediate is the
slowest step in the selected system.
Conversion
Liquid Yield
Alkanes Yield
Phenols+Alcohols
+Ketones Yield
5
4
3
2
1
0
240 250 260 270 280 290 300 310 320 330
Temp. (°C)
The impact of solvents (Fig. 1b) revealed that Ni/ASA performed
much better in apolar solvents (benzene, toluene, naphthane, and
tetrahydrofuran) than in some polar solvents (ethylene glycol,
ethanol, and dioxane). In fact dioxane and ethanol can well dissolve
lignin, but Ni nano-clusters exert poorly due to their oxidation or
instability in these polar solvents.25 The highest liquid yield (18.0
wt.%) was achieved in dodecane (Fig. 1b). In addition, variation of
temperatures from 250 °C to 320 °C in dodecane led to a liquid yield
increase from 18.0 wt.% (conv: 52.9 wt.%) to 43.9 wt.% (conv: 79.0
wt.%) over Ni/ASA (Fig. 2a), proportional to a cycloalkane mixture
yield from 13.0% to 42.0%. In parallel, an increase in H2 pressure
from 3.0 to 6.0 MPa enhanced the liquid yield from 30.1 wt.% (conv:
73.5 wt.%) to 46. 2 wt.% (conv: 78.0 wt.%) at 300 °C (Fig. 2b). The
liquid yields remarkably increase accompanied with nearly stable
lignin conversions, suggesting that the enhanced temperatures and
hydrogen pressures were beneficial for cleaving of lignin-derived
phenolic oligomers to much smaller fragments.
(b)
48
80
70
60
50
40
30
20
46
44
42
40
38
36
34
32
30
28
Conversion
Liquid Yield
(100% Selectivity to Alkanes)
3.0
3.5
4.0
4.5
5.0
5.5
6.0
H2 Pressure (MPa)
(c)
50
40
30
20
10
0
100
80
60
40
20
0
(d)
CH4 C2H6 C3H8 n-C4H10 CO2
The high activity of Ni nanoclusters was probably attributed to
the high surface area of ASA (SBET: 292 m2∙g-1, Fig. 3a and Table S2)
supported Ni nanoclusters with good dispersed (Fig. 3b) and with
small sizes (d = 6.5 ± 2.1 nm as determined by TEM image, see Fig.
3c) at a high loading (Ni content: 30 wt%). The SEM image (Fig. 3d)
C3-C4 C5 C6 C7 C8 C9
C15 C16 C17
C10 C11 C12 C13 C14
also manifested
a mesoporous external surface of Ni/ASA.
Meanwhile, rates for lignin conversion were shown to be sensitive
to Ni particle sizes (Fig. S4), as evidenced by the fact that the larger
Ni particles (18 nm, calcined at 750 °C) reduced 50% liquid yield
than smaller Ni particles (11 nm, calcined at 450 °C). The sensitivity
of Ni size to lignin conversion efficiency was attributed to the fact
that the in the initial stage small Ni nanoclusters attack external
Figure 2. a) Temperatures and b) hydrogen pressures impact towards
lignin conversion and product distributions. c) Alkane compositions in
liquid phase and d) gas phase product distributions from lignin conversion
at 300 °C in presence of 6 MPa H2. Reaction conditions: lignin (2.0 g),
Ni/ASA catalyst (30.1 wt.%, 1.0 g), dodecane (80 mL), 160 min., stirring at
700 rpm.
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exposed C-O-C linkages of lignin in dodecane as the first step. The
The stability of Ni/ASA on converting of lignin was tested in
Ni leaching was undetectable (< ICP detection limitation: 1 ppm) four consecutive recycling runs (Fig. 5). After each reaction, the
during the whole conversion.
used catalyst was washed by dimethyl sulfoxide (DMSO) to dissolve
the unreacted lignin feedstock, and was then sequential calcined in
air, and reduced in hydrogen. After four runs, the lignin conversion,
the liquid yield and the alkane selectivity still reached 74.3%, 40.1%
and 92.1% respectively, quite approaching the activity obtained
with the fresh catalyst. These data indicated that Ni/ASA catalyst
possessed high durability in the selected catalytic system. The spent
Ni/ASA catalyst was sequentially characterized by measurements of
XRD, SEM, TEM, and N2 adsorption (see supporting information Fig.
S8). The slightly enlarged Ni nanoparticles (dTEM = 7.6 ± 2.3 nm) and
the shrunk specific surface areas (SBET: 254 m2·g-1) of the used
Ni/ASA catalyst may attribute to the small decrease of the
hydrodeoxygenation activity during catalytic runs.
(a)
(b)
(d)
(c)
140
Conversion
Liquid Yield
Alkane Selectivity
100
120
80
60
40
20
0
Figure 3. Characterization of Ni/ASA catalyst by a) N2 adsorption and
desorption isotherms, b) SEM-mapping image, c) TEM and d) SEM images.
Under optimized conditions (300 °C and 6.0 MPa H2), the
formed transparent liquid products (yield: 46.2%, Fig. 2c and Fig. S5)
included a selectivity of 80.8% C3-C9 (gasoline range), as well as 6.5%
C10-C14 (kerosene range) and 12.7% C14-C17 (diesel range) naphtha
and paraffins (Fig. 4 and Table S4) affording nearly 100% selectivity
of hydrocarbons, while small amounts of C3-C6 light alkanes as well
as trace tetrahydropyran and 2-methyl tetrahydropyran were partly
originated from residual cellulose and hemicellulose conversion
(Table S4 and Fig. S6). Gaseous products consisted of 98% CH4 and 2%
C2-C4 light alkanes (Fig. 2d, Fig. S7).
Run 1
Run 2
Run 3
Run 4
Figure 5. Recycling tests of lignin conversion over Ni/ASA. Conditions: lignin
(2.0 g), Ni/ASA (30.1 wt.%, 1.0 g), dodecane (80 mL), 300 °C, 6 MPa H2, 160
min., stirring at 700 rpm.
To understand the individual steps in lignin depolymerization,
the kinetics curve revealed that the cycloalkanes yields were
increased up to 43% (with exceeding 80% selectivity) along with the
reaction time (Fig. 6a), while the important intermediates of
substituted phenols and cyclohexanols were increased to the
maximum yields of respective 1.0% and 2.5% at 40 min., and
subsequently decreased to zero as a function of time. These results
indicated that the crucial fragments from lignin degradation were
substituted phenols and alcohols, which were swiftly converted to
final cycloalkanes and alkanes over Ni/ASA via complex cascade
steps (Fig. 6b). To clarify the complex C-O bond cleavage process,
the separate kinetics test of typical phenolic dimer linkage (β-O-4
model compounds) conversion was performed over Ni/ASA at
identical conditions (Fig. S10a). It revealed that hydrogenolysis of
Caliphatic–O and Caromatic–O bonds appeared as the primary step, and
the sequential hydrodeoxygenation steps on phenol, 2-
phenylethanol and aromatics fragments followed (Fig. S10a). The C-
C coupling reaction (yield: 7.2%) was observed among phenolic
intermediates via the alkylation route as well. Furthermore, the
kinetics curves on guaiacol conversion over Ni/ASA (Fig. S10b)
demonstrated that the parallel steps of removing of
methoxy/hydroxy groups as well as hydrogenation of benzene-ring
appeared as the initial step. The sequential dehydration,
hydrogenolysis, and hydrogenation integrated steps on the formed
cycloketone, cycloalcohols, and cyclic ether generated the final
(a)
uV(x1,000)
Chromatogram
10.0
9.0
8.0
7.0
6.0
5.0
4.0
3.0
2.0
1.0
0.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
min
(b)
uV(x1,000)
Chromatogram
10.0
9.0
8.0
7.0
6.0
5.0
4.0
3.0
2.0
1.0
13.5
14.0
14.5
15.0
15.5
16.0
16.5
17.0
17.5
18.0
18.5
19.0
19.5
20.0
20.5
n
Figure 4. Liquid products of a) C3-C10 and b) C12-C17 hydrocarbons from one-pot
lignin conversion over Ni/ASA. Conditions: lignin (2.0 g), Ni/ASA catalyst (30.1
wt.%, 1.0 g), dodecane (80 mL), 300 °C, 6 MPa H2, 160 min., stirring at 700 rpm.
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cycloalkanes and alkanes (Fig. S10b). A variety of lignin-derived
In summary, we have developed a new and simple route for
phenolic monomers and dimers were quantitatively converted to directly producing C3-C17 hydrocarbons from cellulolytic enzyme
cyclic alkanes over Ni/ASA at selected conditions, as well (Fig. S11).
lignin with the non-precious metal Ni/ASA catalyst in aploar solvent
via cascade steps in a one-pot process. These obtained naphthene
and paraffin hydrocarbons are promising precursors to be further
manufactured to clean fuels of gasoline, kerosene, diesel, as well as
aromatics chemicals. We believe that the developed highly
integrated new and efficient lignin transformation system can
provide intrinsic insights for direct liquefaction of the abundant
cellulolytic enzyme lignin, as well as inspiring thoughts in exploiting
waste lignocellulose and sulfated/alkali lignin to produce value-
added fuels and chemicals.
(a)
We gratefully acknowledge the financial support by the
Recruitment Program of Global Young Experts in China, National
Natural Science Foundation of China (Grant No: 21573075),
and Shanghai Pujiang Program PJ1403500.
(b)
Notes and references
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Figure 6. a) Kinetics of lignin conversion with Ni/ASA in the apolar solvent.
Conditions: lignin (2.0 g), Ni/ASA catalyst (30.1 wt.%, 1.0 g), dodecane (80 mL),
300 °C, 6 MPa H2, stirring at 700 rpm. b) Reaction pathways in one-pot direct
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Combining these results allowed us to depict the overall
reaction pathway for one-step conversion of cellulolytic enzyme
lignin to cycloalkanes (Fig. 6b). Under appropriate conditions, lignin
was dissolved in aploar solvent dodecane in equilibrium (k1, not
determined), which fast shifts as lignin was consumed gradually.
Subsequently, the small and uniform Ni nanoclusters supported on
ASA cleaved the exposed external C-O-C linkages of the lignin
macromolecule to form phenolic oligomers (k2, designated as the
rate of lignin conversion). The multi-steps of C-O cleavage on
phenolic oligomers follow (k3, designated as the rate of liquid yield
formation from lignin). The sequential hydrogenolysis of phenolic
dimers and trimers produces guaiacol and syringol derivatives (k4).
Followed by multi-steps of demethoxygenation, hydrogenation, and
dehydration, guaiacol and syringol derivatives were converted to
target cycloalkanes (k5). Meanwhile, the hydrogenolysis of Ph–OCH3
group produced the dominant CH4 in the gas phase. Based on the
kinetics of lignin (k2, k3) as well as of typical phenolic monomer (k5)
and dimer conversion (k4), it could be roughly estimated that the
individual reaction rates follow the order (unit: g∙g-1∙h-1): k5 (27.8) >
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≫
k4 (20.6)
k2 (0.58) > k3 (0.13), suggesting that the rate-
determining step was the cleavage of phenolic oligomers.
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