Bifunctional Ni catalysts for the one-pot conversion of Organosolv lignin into cycloalkanes

Article abstract of DOI:10.1016/j.cattod.2015.11.047

In this report, Ni/ZrO₂, Ni/Al₂O₃, Ni/Al₂O₃-KF, Ni/SBA-15, and Ni/Al-SBA-15 were examined as catalysts for the hydrodeoxygenation of diphenyl ether. Adopting the degree of deoxygenation and yield of monocyclic products as the criteria for the catalyst selection, Ni/Al-SBA-15 was identified as the best catalyst for HDO of diphenyl ether. In fact, in the presence of Ni/Al-SBA-15, full conversion of the model compound into cyclohexane was achieved with high selectivity (98%). Most strikingly, Ni/Al-SBA-15 is capable of hydrodeoxygenating Organosolv lignin with selectivity to cycloalkanes higher than 99%. Owing to the similarities to hydrocarbons derived from petroleum, the lignin-derived alkanes could well be refined into drop-in fuels by conventional oil refinery processes. Moreover, in a broader perspective, the current results also highlight the importance of Al-SBA-15, as an acidic support alternative to zeolites or other acidic materials, for the HDO of phenolic streams.

Full text of DOI:10.1016/j.cattod.2015.11.047

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Bifunctional Ni catalysts for the one-pot conversion of Organosolv
lignin into cycloalkanes
Xingyu Wanga, Roberto Rinaldia,b,∗
a Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mülheim an der Ruhr, Germany
b Department of Chemical Engineering, Imperial College London, South Kensington, Campus London, SW7 2AZ, United Kingdom
a r t i c l e i n f o
a b s t r a c t
Article history:
In this report, Ni/ZrO2, Ni/Al2O3, Ni/Al2O3-KF, Ni/SBA-15, and Ni/Al-SBA-15 were examined as catalysts for
the hydrodeoxygenation of diphenyl ether. Adopting the degree of deoxygenation and yield of monocyclic
products as the criteria for the catalyst selection, Ni/Al-SBA-15 was identified as the best catalyst for
HDO of diphenyl ether. In fact, in the presence of Ni/Al-SBA-15, full conversion of the model compound
into cyclohexane was achieved with high selectivity (98%). Most strikingly, Ni/Al-SBA-15 is capable of
hydrodeoxygenating Organosolv lignin with selectivity to cycloalkanes higher than 99%. Owing to the
similarities to hydrocarbons derived from petroleum, the lignin-derived alkanes could well be refined
into drop-in fuels by conventional oil refinery processes. Moreover, in a broader perspective, the current
results also highlight the importance of Al-SBA-15, as an acidic support alternative to zeolites or other
acidic materials, for the HDO of phenolic streams.
Received 28 July 2015
Received in revised form
19 November 2015
Accepted 30 November 2015
Available online xxx
Keywords:
Lignin
Catalysis
Biofuels
Bifunctional catalysts
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
the hydrodeoxygenation (HDO) of lignin streams [3,4,10]. The first
is the in-situ depolymerisation of lignin, which generates soluble
Lignocellulosic biomass holds great potential as an alternative
raw material for the production of liquid transportation fuels [1–4].
In fact, one of the most promising routes in today’s biorefinery is
the conversion of lignocellulose into fermentable sugars for the
production of ethanol [5]. In this process, lignin is obtained as
an unavoidable residue [6,7]. Considering current productivities of
ethanol and the lignin content of lignocellulose, the production of
cellulosic ethanol may release between 1 and 3 kg lignin per kg
ethanol produced by fermentation of glucose [8]. Therefore, it is
mandatory to find outlets for lignin utilisation. In this context, the
catalytic conversion of technical lignins often generates a complex
and diluted mixture of products. Hence, the isolated yield of indi-
vidual products may not suffice to give a suitable return in today’s
chemical industry. In contrast, a lignin product mixture compris-
ing branched aliphatics could already find uses in the fuel industry.
Such a mixture could well serve both as performance-enhancer
of synthetic fuels produced by the Fischer–Tropsch process and
drop-in fuel in its own right [9,10].
lignin fragments for the catalytic conversion [4]. For some Organo-
solv lignins (obtained from a pulping process without added acid
catalysts), a considerable quantity of relatively weak ether linkages
(e.g., ␤-O-4) may still be present in the isolated polymer. In these
cases, the depolymerisation may easily take place through non-
catalytic thermolysis or solvolysis at temperatures between 200
and 300 ◦C [11–13]. The second step consists of the catalytic con-
version of (low molecular weight) fragments of lignin. Regarding
the production of biofuels from lignin, the main aim of the catalyzed
reactions is to remove O-functionalities selectively [10]. By reduc-
tion of ketone and aldehyde intermediates, lignin fragments are
prevented from recondensation [14–17]. The prevention of recon-
densation is essential to mitigating problems associated with the
formation of biochars, and thus improving the yield of liquid prod-
ucts [18–20]. Furthermore, catalytic reactions should proceed to
target a reduction in O/C ratio and an increase in H/C ratio to achieve
optimum fuel properties and energy density in the product [8,10].
Importantly, the cracking of the carbon backbone of lignin should
target hydrocarbon products from C9 to C18 , as an eventual exten-
sive cracking of the carbon backbone decreases the energy density
of the prospective fuel candidates [21].
In the current approaches for conversion of lignin into hydrocar-
bons, at least two steps are essential to achieving high efficiency in
Undoubtedly, noble metals combined with acid catalysts are
promising systems for the use of lignin as feedstock for the produc-
tion of biofuels via HDO [22]. Nonetheless, the high price of noble
∗
Corresponding author at: Department of Chemical Engineering, Imperial College
London, South Kensington, Campus London, SW7 2AZ, United Kingdom.
E-mail address: rrinaldi@imperial.ac.uk (R. Rinaldi).
http://dx.doi.org/10.1016/j.cattod.2015.11.047
0920-5861/© 2016 Elsevier B.V. All rights reserved.
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increased by 1 ◦C min−1 from 25 to 550 ◦C. In sequence, they were
reduced under H2/Ar (20 v/v% H2, 250 cm3 min−1) at 550 ◦C for 5 h.
The activated Ni catalysts were left to cool down to room temper-
ature under H2/Ar, and stored under argon in a glove box.
metals may pose problems for such processes on a large-scale. To
circumvent this limitation, the use of less expensive base metal
catalysts for HDO of lignin-derived streams has been extensively
explored [10,23]. In spite of the large number of reports and reviews
on the upgrade of biophenols (model compounds and pyrolysis oil)
available [10,24–31], to the best of our knowledge, there is no report
on the use of bifunctional Ni catalysts in the HDO of Organosolv
lignin rendering cycloalkanes as a distinct class of products.
In this report, aiming at improved HDO of Organosolv lignin, we
first explored Ni supported on acidic or basic supports (ZrO2, Al2O3,
Al2O3/KF, SBA-15 and Al-SBA-15) for the HDO of diphenyl ether,
as a model compound representing strong ether linkages (4-O-5ꢀ)
in the lignin fragments. In these experiments, we identified Ni/Al-
SBA–15 as the best catalyst for HDO of diphenyl ether, as it fully
converts the substrate into cyclohexane with high selectivity (98%).
Most strikingly, for lignin valorisation, we found that Ni/Al-SBA-
15 is capable of hydrodeoxygenating Organosolv lignin, rendering
cyclic alkanes at selectivity higher than 99%.
2.3. Catalyst characterisation
Transmission electron microscopy (TEM) images were taken on
an HF-2000 FE transmission electron microscope at a voltage of
200 kV. Energy-dispersive X-ray spectroscopy (EDX) analysis was
performed by using a Noran System Six EDX with a Si (Li) detector.
N2 sorption analyses were carried out on an ASAP 2000 surface
area analyser (Micromeritics). The specific surface was determined
by using the BET method. The pore volume and pore size distribu-
tion were calculated from the desorption profiles of the isotherms
by using the BJH method.
2.4. Extraction of lignin from Poplar wood through Organosolv
process
2. Experimental
Poplar wood (500 g, 2 mm chips, J. Rettenmaier & Söhne) was
suspended in a 3-L solution of ethanol:water (1:1, v/v) in a 5-L reac-
tor equipped with an overhead mechanical stirrer. The suspension
was heated from 25 to 178 ◦C at 1.5 ◦C min−1 under mechanical
stirring. The suspension was processed at 178 ◦C for 3.3 h. At this
temperature, the autogenous pressure was 1.5 MPa. In sequence,
the mixture was left to cool down to room temperature. After
the wood fibers had been filtered off, a reddish–brown solution
was obtained. By using a rotoevaporator, the solvent was removed
under reduced pressure at 40 ◦C. A solid reddish–brown residue
(116 g, Organosolv lignin) was obtained. Elemental analysis: C,
59.0%; H, 6.2%; N, 0.1%, O, 34.7%. GPC characterisation (values given
by comparison with PS standards in THF): Mw 960 Da; Mn 330 Da.
2.1. Preparation of the catalytic supports
Al2O3-KF was prepared by suspending fumed aluminum oxide
(9.0 g, Aeroxide® Alu C, Evonik) in a 0.4 mol L−1 KF solution (40 mL).
The suspension was stirred at 70 ◦C for 2 h. The solid was filtered
under reduced pressure and washed with distilled water at 50 ◦C.
In sequence, the solid was dried at 120 ◦C under reduced pressure.
The material was calcined at 450 ◦C for 3 h. The temperature was
increased by 5 ◦C min−1 from 25 to 450 ◦C.
SBA-15 was prepared as described elsewhere [32]. Typically,
to a solution of Pluronic P123 (4.0 g, Aldrich) dissolved in Milli-
Q water (30 mL), a 2 mol L−1 HCl solution (120 mL) was added. In
sequence, tetraethoxysilane (8.4 g, Aldrich, 99%) was added drop-
wise to the solution under vigorous stirring. The mixture was first
aged under stirring at 38 ◦C for 20 h, and then statically at 100 ◦C for
an additional 24 h. The white solid product was separated by filtra-
tion and dried in open air at room temperature. Finally, to remove
the organic template, the material was calcined at 550 ◦C for 5 h.
The temperature was increased by 5 ◦C min−1 from 25 to 550 ◦C.
Al-SBA-15 was prepared as described in Refs. [33–35].
Tetraethoxysilane (8.4 g), aluminum 2-propoxide (0.5 g, Aldrich,
97%) and HCl aqueous solution (10 mL, pH = 1.5) were mixed and
stirred at room temperature for 3.5 h (Solution I). P123 (4 g),
NH4F (0.05 g, Aldrich, 99.99%) and HCl aqueous solution (150 mL,
pH = 1.5) were mixed and stirred at 45 ◦C for 1 h (Solution II). Solu-
tion I was slowly added (within 10 min) to Solution II under stirring
at 45 ◦C. The mixture was aged under stirring at 45 ◦C for 20 h. The
solution was then transferred into a Teflon-lined autoclave, and
aged under static conditions at 100 ◦C for 24 h. A white precipitate
was formed. The white solid was collected by filtration and washed
with water until pH 7. This material was dried overnight at 50 ◦C.
Finally, the material was calcined at 550 ◦C for 4 h. The temperature
was increased by 0.4 ◦C min−1 from 25 to 550 ◦C.
2.5. Catalytic experiments
2.5.1. Hydrodeoxygenation of diphenyl ether
All the procedures involving the activated catalysts were per-
formed in a glove–box under Argon. In a glass inlet, a catalyst
(100 mg) was suspended in a solution containing diphenyl ether
(500 mg, Aldrich, 99.9%), n-dibutyl ether (100 mg, internal stan-
dard, Aldrich, 99.3%) and solvent (15 mL, methylcyclohexane,
Aldrich 99.9%). The glass inlet was then placed into a stainless steel
reactor (30 mL). After removing the autoclave from the glove–box,
the reactor vessel was flushed with H2 and then loaded with an H2
pressure of 5 MPa (measured at 25 ◦C). The experiments were per-
formed under overhead mechanical stirring (400 rpm) at 200 ◦C for
40 min. The reaction medium was analysed by GC–FID and GC–MS.
2.5.2. Hydrodeoxygenation of Organosolv lignin
In a glass inlet, Organosolv lignin (500 mg) and Ni/Al-SBA-15
(150 mg) were suspended in methylcyclohexane (15 mL) under
argon (glove–box). The glass inlet was then placed into a stain-
less steel reactor (30 mL). After removing the autoclave from the
glove–box, the reactor vessel was flushed with H2 and then loaded
with an initial H2 pressure of 7 MPa (measured at 25 ◦C). The
experiments were performed under overhead mechanical stirring
(400–rpm) at 300 ◦C for 8 h. After the reaction, n-dibutyl ether
was added to the reaction mixture (external standard for the
GC × GC–FID analysis). The reaction mixture was filtered. The solid
residue (containing the catalyst and lignin residues) was washed
with methylcyclohexane, dried at 60 ◦C under vacuum, and then
weighed. The consumption of lignin was determined by weight
difference.
2.2. Catalyst preparation
Precursors of Ni/Al2O3, Ni/Al2O3-KF and Ni/ZrO2 were prepared
by incipient impregnation of Al2O3 (Aeroxide® Alu C, Evonik),
Al2O3-KF, Zr(OH)2 (Aldrich, 97%), respectively, with a volume of
an 1.4 mol L−1 Ni(NO3)2 solution enough to achieve a nominal
Ni loading of 10 wt%. In turn, through impregnation of SBA-15
and Al-SBA-15 with the same Ni(II) source, 5 wt% Ni/SBA-15 and
Ni/Al-SBA-15 precursors were synthesised. Typically, the dried
precursors were calcined at 550 ◦C for 5 h. The temperature was
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2.6. Quantification of the products
2.6.1. GC analysis
The samples were analysed on a gas chromatograph (Shimadzu
QP2010 Plus) equipped with a Rxi-1 ms column (30 m). The tem-
perature program started with an isothermal step at 40 ◦C for 6 min.
Next, the temperature was increased by 30 ◦C min−1 to another
isothermal step at 160 ◦C for 1 min. Again, the temperature was
raised to 200 ◦C by 30 ◦C min−1. From this point on, the tempera-
ture was increased at 10 ◦C min−1 up to the last isothermal step at
240 ◦C for 1 min. The components were identified by using mass
spectra (MS) libraries NIST 08, NIST 08s and Wiley 9. The quan-
tification was performed using the calibration curves based on the
response of the flame ionisation detector (FID) for each component
of the mixture.
2.6.2. GC × GC Analysis
The reaction mixtures were analysed by using 2D GC × GC–MS
(1st column: Rxi-1 ms 30 m, 0.25 mm ID, df 0.25 ␮m; 2nd column:
BPX50, 1 m, 0.15 mm ID, df 0.15 ␮m) in a gas chromatograph (Shi-
madzu QP2010 Plus) equipped with a ZX1 thermal modulation
system (Zoex). The temperature program started with an isother-
mal step at 40 ◦C for 5 min. Next, the temperature was increased
from 30 ◦C to 300 ◦C by 5.2 ◦C min−1. The program finished with an
isothermal step at 300 ◦C for 5 min. The modulation applied to the
comprehensive GC × GC analysis was a hot-jet pulse (400 ms) every
9000 ms. 2D chromatograms were processed by using GC Image
software (Zoex). The products were identified by using MS libraries
NIST 08, NIST 08s, and Wiley 9. The semiquantification of the prod-
ucts was performed using the GC × GC–FID images and estimated
response factors.
3. Results and discussion
3.1. Characterisation of the supported Ni catalysts
Shown in Fig. 1 are TEM images of 10 wt% Ni/ZrO2, Ni/Al2O3 and
Ni/Al2O3-KF catalysts. Clearly, the supports consist of packed par-
ticles. The particle packing generates a non-structural pore system
in the materials. Moreover, the metal nanoparticles can be dis-
tinguished (dark spots) from the support. The average size of Ni
particles depends on the carriers. Ni particles on ZnO2 and Al2O3
were formed with an average size of 5–10 nm, while on Al2O3-KF
with 10–20 nm.
Displayed in Fig. 2 are TEM images of 5 wt% Ni/SBA-15 and
Ni/Al-SBA-15. For both materials, the highly ordered hexagonal
mesostructure was preserved throughout the procedure of cata-
lyst preparation. Small Ni particles (∼6 nm) are detected inside
the channels (Fig. 2b and d), but a significant amount of large
Ni particles (30–50 nm) are located on the external support sur-
face (Fig. 2a–c). The presence of Ni particles on the outer surface
of Al-SBA-15 most likely constitutes an advantageous feature for
the hydrogenolysis and hydrogenation of large fragments of lignin,
which could face problems to access the internal surface of Al-SBA-
15 for catalytic conversion, accounting for the outstanding results
obtained from the HDO of lignin using this material as a catalyst
(vide infra).
Listed in Table 1 are the textural properties of Ni catalysts as
determined by N2 physisorption. As expected, Ni/ZrO2, Ni/Al2O3
and Ni/Al2O3-KF have low BET surface area (76–110 m2 g−1) and
pore volume (0.2–0.5 cm3 g−1). The loading of Ni slightly decreased
the surface area of Al2O3 (from 110 to 99 m2 g−1), but considerably
increased the pore volume (from 0.19 to 0.45 cm3 g−1, entries 1
and 2). It is clear that the incipient impregnation led to aggrega-
tion of individual Al2O3 particles, which reduces the surface area,
Fig. 1. TEM images of (a) Ni/ZrO2, (b) Ni/Al2O3, (c) Ni/Al2O3-KF.
Table 1
Textural properties of supports and Ni catalysts.
Entry Material
BET surface
Pore volume
Average pore size
(nm)
(m2 g−1
)
(cm3 g−1
)
1
2
3
4
5
6
7
8
Ni/ZrO2
Al2O3
77
110
99
0.20
0.19
0.45
0.47
1.23
1.32
0.90
0.93
11.3
11.6
27.1
33.4
7.2
8.2
6.7
7.7
Ni/Al2O3
Ni/Al2O3-KF
SBA-15
Al-SBA-15
Ni/SBA-15
99
837
821
645
Ni/Al-SBA-15 607
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Fig. 2. TEM images of Ni catalysts supported on a, b): SBA-15 and c, d): Al-SBA-15.
but creates a pore structure, and therefore, increases the pore vol-
ume. KF treatment did not significantly affect the pore structure of
Ni/Al2O3-KF, as revealed by the similar textural properties found for
Ni/Al2O3 and Ni/Al2O3-KF (Table 1, entries 3 and 4). In turn, SBA-
15 and Al-SBA-15 show high surface area (> 800 m2 g−1) and high
pore volume (> 1.2 cm3 g−1) with an average pore size between 7
and 8 nm. As expected, the loading of Ni decreased the surface area,
pore volume, and average pore size (Table 1, entries 5–8). The drop
in surface area is another evidence of a partial deposition of Ni par-
ticles inside the channels of SBA-15 and Al-SBA-15, as also seen in
TEM images of these materials (Fig. 2).
To gain more detail on the pore structure of the materials, N2
adsorption–desorption isotherms were examined (Fig. 3 and Fig. 4).
N2 adsorption and desorption profile found for Ni/ZrO2 shows a
type IV isotherm with H1 + H3 hysteresis loops (Fig. 3a), which
characterise a mesoporous structure with undefined pore shape
[36,37]. Again, the sharp step at P/P0 range of 0.9–1.0 suggests a
contribution of non-structural pores formed by particle packing.
Furthermore, isotherms of type II were observed for Al2O3, Ni/Al2O3
and Ni/Al2O3-KF (Fig. 3b–d), indicating that these materials are
non-porous solids. Again, type H3 hysteresis loop indicates the
presence of non-structural mesopores formed by particle packing
[38].
N2 adsorption–desorption isotherms found for SBA-15 and Al-
SBA-15 is of type IV with an H1 hysteresis loop (Fig. 4a and b),
which describes a mesoporous material with cylindrical pores with
a narrow size distribution [39]. For Ni/SBA-15 and Ni/Al-SBA-15, a
second step in isotherms is detected at P/P0 ∼ 0.8 (Fig. 4c and d).
The stepwise adsorption isotherm (Type IV) observed for Ni/SBA-
15 and Ni/Al-SBA-15 indicates the presence of a multi-model pore
distribution, which is expected from the non-uniform loading of
the pore system by Ni nanoparticles, as shown by TEM images
(Fig. 4). Despite this, the mesoporous structure was preserved after
the loading of the Ni phase, as shown by HR-TEM (Fig. 2).
3.2. Catalytic conversion of diphenyl ether
Hydrogenolysis of diphenyl ether was investigated as a model
reaction for the upgrade of lignin. He et al. ]40–42] demonstrated
that the product distribution obtained from the Ni-catalyzed con-
version of diphenyl ether is determined by two parallel reaction
channels, namely, hydrogenation of aromatic ring and hydrogenol-
ysis of C O ether bond (Scheme 1) [40–42]. Notably, upon the
stepwise saturation of diphenyl ether (i.e. 1 → 2 → 3), the
reactivity of the ether bond toward hydrogenolysis dramatically
decreases. In fact, monofunctional Ni catalysts are inactive for the
hydrogenolysis of Csp3 Csp3 linkages [4,40–42]. Accordingly,
O
the full saturation of diphenyl ether leads to a dead end in the
HDO process if no acidic functionality is present in the catalyst [4].
However, provided that acid sites of adequate strength are present
on the catalyst surface, the dialkyl ether intermediate may then
undergo hydrolysis, consuming the water formed by the dehydra-
tion of cyclohexanol intermediate through other reaction channels.
As a result, cyclohexane is obtained from the reaction channel:
cyclohexanol → cyclohexene → cyclohexane, as represented in
Scheme 1.
To examine the catalytic performance of the supported Ni
materials, the reactions were carried out in methylcyclohexane
(solvent) under 5 MPa H2 (r.t.) at 200 ◦C for 40 min. Summarised
in Table 2 are the results of the catalytic conversion of diphenyl
ether, selectivity to products, and the ratio of monocyclic prod-
ucts to bicyclic products [ꢀ(4–7)/ꢀ(2,3)] in addition to the ratio
of oxygenated monocyclic products to deoxygenated monocyclic
products [ꢀ(4,6)/ꢀ(5,7)].
In the presence of Raney Ni, Ni/SBA-15 or Ni/Al-SBA-15, full
conversion of diphenyl ether was achieved after 40 min (entries
1, 5 and 6). From these experiments, only saturated products were
obtained. However, with Ni/ZrO2, Ni/Al2O3 or Ni/Al2O3-KF, high
conversion (96–99%) was achieved, but unsaturated compounds
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Fig. 3. Nitrogen adsorption–desorption isotherms of (a) Ni/ZrO2, (b) Al2O3, (c) Ni/Al2O3 and (d) Ni/Al2O3-KF.
Table 2
Results of the conversion of diphenyl ether in the presence of a benchmark catalyst (Raney Ni) and supported Ni catalysts.
Entry
Catalyst
Conv. (%)
Selectivity (%)
2
ꢀ(4–7)
ꢀ(2,3)
ꢀ(4,6)
ꢀ(5,7)
3
4
5
6
7
1
2
3
4
Raney Ni
Ni/ZrO2
Ni/Al2O3
Ni/Al2O3-KF
Ni/SBA-15
Ni/Al-SBA-15
100
96
99
99
100
100
0
14
3
2
0
38
19
44
35
50
0
0
0
0
0
0
0
0
8
0
0
0
0
29
24
24
32
23
0
32
32
29
31
27
98
1.6
1.9
1.2
1.7
1.0
98/0
0.91
0.61
0.85
1.01
0.87
0.00
5
6b
0
aReaction conditions: diphenyl ether (2.9 mmol), dibutyl ether (internal standard, 0.78 mmol), catalyst (100 mg) and solvent (15 mL), 5 MPa (r.t.), 200 ◦C for 40 min.
b1,1ꢀ-bicyclohexane was obtained as a minor product (2%).
were still detected (entries 2–4). To compare the selectivity val-
ues, the ratios [ꢀ(4–7)/ꢀ(2,3)] and [ꢀ(4,6)/ꢀ(5,7)] were analysed.
On the one hand, the ratio ꢀ(4–7)/ꢀ(2,3) describes the selectiv-
ity to monocyclic products produced by the cleavage of C O bond.
On the other hand, the ratio ꢀ(4,6)/ꢀ(5,7) defines the selectivity
to hydrodeoxygenation after the ether bond cleavage, that is, the
ability of the catalyst to perform the reaction sequence: phenol →
cyclohexanol → cyclohexene → cyclohexane [4,20].
is slightly higher than with Raney Ni (1.6). Remarkably, subjecting
diphenyl ether to Ni/Al-SBA-15 renders a 98% yield of cyclohex-
ane. Overall, the selectivity to monocyclic products increases as
follows Ni/SBA-15 < Ni/Al2O3 < Raney Ni < Ni/Al2O3-KF < Ni/ZrO2 «<
Ni/Al-SBA-15.
Regarding the degree of hydrodeoxygenation, the ratio
ꢀ(4,6)/ꢀ(5,7) obtained from experiments with Ni/ZrO2 (0.61),
Ni/Al2O3 (0.85), Ni/SBA-15 (0.87) and Raney Ni (0.91) indicate
that hydrodeoxygenation is achieved to only a very limited
extent. In turn, Ni/Al2O3-KF shows no deoxygenation activity
(ꢀ(4,6)/ꢀ(5,7) = 1.0). Obviously, the support acidity is of great sig-
nificance in the deoxygenation. ZrO2 and Al2O3 both contain acidic
In Table 2, entries 1–5 indicate that the ꢀ(4–7)/ꢀ(2,3) ratio
obtained from experiments with Ni/Al2O3 (1.2) and Ni/SBA-15
(1.0) is lower than that found for the experiment with Raney Ni
(1.6). However, with Ni/ZrO2 (1.9) and Ni/Al2O3-KF(1.7) the ratio
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Fig. 4. Nitrogen adsorption–desorption isotherms of (a) SBA-15, (b) Al-SBA-15, (c) Ni/SBA-15 and (d) Ni/Al-SBA-15.
sites of moderate acid strength [43]. which can catalyze the deoxy-
confirm that the hydrogenation ability of Ni phase together with
high surface area (607 m2 g−1) of the highly acidic support (Al-SBA-
15) [35] hold the key to providing a highly active material for the
conversion of diphenyl ether into cyclohexane.
genation via dehydration of cyclic alcohol [44,45]. More important,
however, are the results obtained from the experiment in the pres-
ence of Ni/Al-SBA-15. In this case, the hydrogenolysis of diphenyl
ether proceeded to full conversion yielding cyclohexane (98%) and
1,1ꢀ-bicyclohexane (2%) (Table 2, Entry 6). Conversely, the reaction
in the presence of Ni/SBA-15 rendered dicyclohexyl ether (50%),
cyclohexanol (23%) and cyclohexane (27%). Clearly, the absence
of strong acid sites in SBA-15 causes the accumulation of dicyclo-
hexyl ether and cyclohexanol in the product mixture. These results
3.3. Conversion of Organosolv lignin
To assess the performance of Ni/Al-SBA-15 in the conversion
of Organosolv lignin the substrate was subjected to Ni/Al-SBA-15
suspended in methylcyclohexane under an H2 pressure of 7.0 MPa
Fig. 5. GC × GC image of the products obtained from the HDO of organosolv lignin. Reaction conditions: reaction conditions: Organosolv lignin (0.5 g), methylcyclohexane
(15 mL), Ni/Al-SBA-15 (200 mg), 7 MPa H2 (r.t.), 8 h.
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Scheme 1. Possible reaction channels involved in the conversion of diphenyl ether into cyclohexane. SA stands for solid acid.
H2 (measured at room temperature) at 300 ◦C for 8 h. Under these
4. Conclusion
conditions, an 84% conversion of Organosolv lignin was achieved,
as determined by weight difference. To gain in-depth insight into
the composition of the product mixture, the product mixture was
analysed by using GC × GC–MS/FID.
In this report, we demonstrated that Ni/Al-SBA-15 is a highly
active catalyst for the hydrodeoxygenation of aromatic streams.
Remarkably, full conversion of diphenyl ether into cyclohexane, in
the presence of Ni/Al-SBA-15, was achieved. Organosolv lignin was
also successfully converted into a mixture of cycloalkanes. Clearly,
such a mixture of products is highly interesting for the produc-
tion of drop-in transportation fuel because of the ease of product
fractionation by conventional refining processes [48]. In a broader
perspective, the current results also point to the importance of Al-
SBA-15, as an acidic support alternative to zeolites or other acidic
materials, for the HDO of phenolic streams.
Shown in Fig. 5 is GC × GC–MS image obtained from the product
mixture. The amount of volatile compounds visible to the chro-
matographic analysis was estimated at approximately 45% (with an
injector temperature of 300 ◦C). Remarkably, Organosolv lignin was
almost fully converted into saturated hydrocarbons. Semiquantifi-
cation by GC × GC–FID shows that more than 99% detected products
are cycloalkanes, with only less than 1% cyclic alcohols remained
as byproducts (retention time shorter than 9 min and not shown in
Fig. 5). This result demonstrates that Ni/Al-SBA-15 can successfully
catalyze the conversion of lignin into saturated alkanes through a
one-pot process. Moreover, the current results starkly contrast to
those reported by Toledano et al. [46,47] for conversion of lignin
under microwave irradiation with hydrogen-donating solvents in
the presence of Ni/Al-SBA-15. Under those conditions, the satura-
tion of the phenolic ring could not be achieved by the H-transfer
process, and therefore, extensive deoxygenation of the stream, via
concurrent dehydration and hydrogenation, could not be accom-
plished. As a result, that process produced a mixture of phenols via
the depolymerisation of lignin.
Acknowledgements
The funds for this research were provided by the Alexander von
Humboldt Foundation (Sofja Kovalevskaja Award 2010). This work
was performed as part of the Cluster of Excellence “Tailor-Made
Fuels from Biomass” (DFG).
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