An integrated strategy for the conversion of cellulosic biomass into γ-valerolactone

Article abstract of DOI:10.1039/c4cy00717d

An integrated method for the production of γ-valerolactone from cellulosic biomass is presented here. A combination of acidic water hydrolysis of the biomass followed by extraction with methyltetrahydrofuran is used to generate a levulinic acid feed, which is further hydrogenated into the platform chemical γ-valerolactone using RANEYnickel as a catalyst. This journal is

Full text of DOI:10.1039/c4cy00717d

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An integrated strategy for the conversion of
cellulosic biomass into γ-valerolactone†
Cite this: Catal. Sci. Technol., 2014,
4, 3626
*
Valerio Molinari, Markus Antonietti and Davide Esposito
Received 2nd June 2014,
Accepted 9th August 2014
DOI: 10.1039/c4cy00717d
www.rsc.org/catalysis
An integrated method for the production of γ-valerolactone from
cellulosic biomass is presented here. A combination of acidic
water hydrolysis of the biomass followed by extraction with
methyltetrahydrofuran is used to generate a levulinic acid feed,
which is further hydrogenated into the platform chemical
γ-valerolactone using RANEY® nickel as a catalyst.
extraction has been used to convert LA into different aliphatic
esters,17,18 with the drawback of adding a synthetic step to
the process. Alternatively, extraction with alkylphenol solvents
has been proposed to selectively separate LA from the water
phase,19 although in this case, the use of an environmentally
questionable solvent was required. Ideally, the use of a sus-
tainable and green solvent to replace the alkylphenol could
tremendously improve such an approach. In addition, in
order to favor the industrial application of these methodolo-
gies, the exploitation of rare and expensive transition metals
for the preparation of the hydrogenation catalysts should be
avoided, while the development of catalytic schemes based on
cheap and widely available metals should be encouraged.
In this work we propose an efficient integrated strategy
to convert biomass into GVL, which is based on the use
of the biomass-derived solvent 2-methyltetrahydrofuran
(2-MeTHF) to extract LA from aqueous feeds obtained by
acidic hydrolysis of lignocellulosic materials. The so-obtained
organic feed is thus subjected to a continuous-flow hydrogena-
tion catalyzed by the commercial and relatively economical20
RANEY® nickel.
The suggested integrated scheme for the conversion of
lignocellulosic carbohydrates into GVL is depicted in Fig. 1.
Initially, following a previously published route,21 biomass
is conveniently pretreated by ball milling to obtain water-
soluble partially depolymerized lignocellulosic fragments.
Then, a batch reactor is employed to perform carbohydrate
hydrolysis using sulfuric acid as the catalyst. The water
phase can be introduced into an extractor after separation
of the by-products derived from the decomposition of carbo-
hydrates. Extraction of LA using 2-MeTHF regenerates the
water phase, which can be recycled for iterative hydrolysis
steps, while producing a LA-containing organic feed ready to be
introduced in the continuous reactor R2. Here, hydrogenolysis
with RANEY® Ni yields GVL that can be conveniently
separated from 2-MeTHF by distillation. 2-MeTHF is distilled
as a stable azeotrope with water,22 which is still suitable for
further reactions.
Fuels and chemicals produced in a sustainable fashion from
non-edible lignocellulosic biomass are currently considered
as a promising alternative to fossil feedstocks.1 A variety of
different molecules have been identified as possible high
value products of the catalytic upgrade of biomass. One of the
illustrative examples in this direction is represented by
γ-valerolactone (GVL),2 a cyclic ester with optimal properties
as a fuel additive3 and solvent4 and as a precursor for differ-
ent platform chemicals and fuels.3,5 GVL can be produced by
hydrogenation of levulinic acid (LA), obtained in turn by
hydrolysis of the cellulosic portion of raw biomass.6 Several
hydrogenation schemes relying on the use of different hetero-
geneous catalysts have been proposed for the conversion of
LA into GVL.7–11 In general, Ru-based catalysts have shown
the highest efficiency, followed by other transition metal-
based catalysts containing Pd, Pt, Re or RANEY® nickel.2,12–16
Since LA is usually produced by acidic hydrolysis of biomass
with mineral acids (sulfuric acid in particular), novel prepara-
tion of catalysts with increased tolerance towards acids and
poisoning agents like sulfur has been attempted5 in order to
enable the direct processing of biomass-derived LA. Despite
the success, the stability of such catalysts still requires
additional improvements to become compatible with high-
throughput conversion schemes. Alternative solutions have
been proposed to bypass this problem; for instance, reactive
Max-Planck-Institute of Colloids and Interfaces, Department of Colloid Chemistry,
14424 Potsdam, Germany. E-mail: davide.esposito@mpikg.mpg.de;
Tel: (+49) 0331 567 9538
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4cy00717d
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Fig. 1 Schematic representation of the integrated strategy for the multistep synthesis of GVL from lignocellulosic biomass.
Table 1 Effect of flow and temperature on the hydrogenation of LA into
We commenced our studies by evaluating the possible
hydrogenation of LA using commercial RANEY® nickel. Inter-
estingly, the successful use of RANEY® nickel for the hydroge-
nation of ethyl levulinate to GVL has been recently reported
on simulated feeds.14 In contrast, impurities and poisoning
agents contained in biomass-derived LA were not tolerated. It
is worth noting that the use of RANEY® Ni for the direct
hydrogenation of LA in batch has also been proposed,12,13
although the authors reported very low efficiency under
moderate conditions, whereas adequate preparative condi-
tions were attained at high temperatures (175–200 °C) and
pressure (35 bar).
GVL in different solventsa
Entry
Solvent
T (°C)
Conversion (%)
1
2
3
4
5
EtOH
EtOH
EtOH
EtOH/H2O
2-MeTHF
115
125
135
125
125
57
>99
>99
>99
>99
a
Reaction conditions: 0.9 g RaNi (wet), 12 bar, 0.3 mL min−1
,
50 mM LA in EtOH, EtOH/water and 2-MeTHF.
We believe that such findings could be due to a non-
optimized reaction setup, while milder reaction conditions
could be obtained by careful engineering of the reaction
parameters in a continuous reactor. In this direction, a recent
work proposed the continuous synthesis of γ-valerolactone
exploiting RANEY® nickel as a catalyst and water as solvent,
obtaining interesting results.23 We therefore investigated the
use of a continuous system but focused on the use of organic
solvents, which integrate better in our strategy. In particular,
we screened the effect of temperature and solvent on the
reactions using a fixed-bed reactor equipped with a packed
catalyst column (0.9 g of wet RANEY® Ni) and supplied with
LA and H2 feeds (for details see ESI†). The results are
summarized in Table 1.
LA was already fully converted into GVL at moderate con-
ditions (entries 1–4). We continued to explore the possible
use of other solvents characterized by a low environmental
footprint and we focused our attention on 2-MeTHF, a
biomass-derived solvent,22 which can be in fact obtained
by reduction of GVL.24,25 Interestingly, during hydrogenation
of LA in ethanol and ethanol–water, we did not observe
the formation of 2-MeTHF, which probably requires more
severe conditions. The use of 2-MeTHF as the reaction
solvent (entry 5) yielded GVL with full conversion and
selectivity under the same conditions applied in the case of
ethanol and ethanol–water. Having demonstrated the viability
of reducing LA in flow using RANEY® Ni under moderate con-
ditions, we proceeded to evaluate a possible strategy to extend
this approach to LA feeds extracted from an acidic hydrolysate
of the lignocellulosic material. As mentioned above, one of the
major problems associated with the hydrogenation of LA in
biomass-derived feeds is the presence of mineral acids and
impurities which result in the deactivation of the catalyst.
The results obtained with 2-MeTHF (Table 1, entry 5)
therefore appeared particularly attractive since this solvent
could be used in principle to extract LA from acidic aqueous
solutions. For instance, exploiting a continuous extraction
system would enable accumulation of LA in the organic
phase and potentially lead to the recycling of the aqueous
sulfuric acid for iterative production of LA. Interestingly,
2-MeTHF has been recently exploited for biomass processing
in water-containing biphasic mixtures to extract apolar lignin
fragments from the aqueous phase.26
To assess the feasibility of such an approach, we per-
formed experiments to determine the partition ratio (Kd)
for LA in the system 2-MeTHF/H2SO4 (0.5 M) (Fig. S4†). The
estimated Kd resulted in a value of ca. 1.57, anticipating the
possible extraction of LA using 2-MeTHF.
We performed the reduction of LA using a simulated feed
obtained by partitioning a water solution of LA in 0.5 M
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Table 2 Hydrothermal synthesis of LA using different biomass precursors and effect of flow and temperature on the continuous hydrogenation of the
corresponding LA feeds in 2-MeTHF using RANEY® nickel
Hydrolysis
Hydrogenation
Entry
1
2
3
4
5
6
7
8
Substrate
Simulated feeda
Simulated feed
Simulated feed
Glucose
Glucose
Glucose
Glucose
Glucose
LA yield (w/w %)c
Feed conc. (mM)
Flow (mL min−1
)
T (°C)
125
135
150
125
125
125
125
135
135
135
135
125
125
125
125
125
125
135
135
Conversion (%)e
>99
98
>99
>99
87
64
54
>99
98
85
64
>99
66
36
93
81
49
>99
88
52d
52
52
52d
52
52
52
52
52
52
52
36d
36
36
40d
40
40
40
40
0.3
0.5
1.0
0.3
0.4
0.5
1.0
0.3
0.4
0.5
1.0
0.5
0.7
1.0
0.5
0.7
1.0
0.5
1.0
32c
—
—
—
—
—
—
—
23c
—
—
13c
—
—
—
—
9
Glucose
Glucose
10
11
12
13
14
15
16
17
18
19
Glucose
Celluloseb
Cellulose
Cellulose
Beechwoodb
Beechwood
Beechwood
Beechwood
Beechwood
a
Hydrolysis conditions: substrate in 0.5 M H2SO4, 220 °C, 8 h. Hydrogenation conditions: LA in 2-MeTHF, RANEY® nickel, H2, 12 bar. Obtained
b
c
by partitioning a LA solution in 0.5 M H2SO4 with 2-MeTHF. Pretreated via ball milling with catalytic H2SO4. Determined by HPLC analysis;
d
reactions were carried out multiple times and yields were averaged. Determined by HPLC analysis measuring the difference in the concentra-
tion of LA in the water phase before and after extraction with 2-MeTHF. Determined by GC-MS analysis.
e
sulfuric acid with 2-MeTHF (Table 2, entries 1–3). The effi-
ciency of the resulting reaction was unaffected after the
extraction (>99% conversion, corresponding to a GVL yield
of 96%, based on GC analysis) independent of the tempera-
ture, indicating that no significant amounts of sulfuric acid
were carried into the organic feed. At this point, the robust-
ness of the reduction step was challenged by processing real
levulinic acid feeds obtained by acidic hydrolysis of biomass
models and lignocellulosic biomass in order to take into
account the presence of possible additional poisoning agents
that originated during sugar depolymerization. While glucose
was used initially as a model substrate, we later focused our
attention on real biomass. The direct use of biomass is never-
theless complicated by the relative hydrolytic stability and low
solubility of cellulose and lignin. To overcome this problem,
several techniques have been recently applied to process bio-
mass into water-soluble partially depolymerized cellulosic frag-
ments. Among others, the mechano-catalytic depolymerization
of lignocellulose has been successfully exploited as an entry
process for the preparation of oligosaccharides, alcohols and
furfurals.21,27,28 The main benefits of this technique reside in
the efficient comminution of the substrate, which makes it
compatible with various reaction setups, and the simultaneous
partial depolymerization of polysaccharides and lignin, which
facilitates the successive hydrolytic treatments. We performed
our experiments using water-soluble oligosaccharides prepared
by mechano-catalytic processing of cellulose and beechwood. LA
was synthesized following known procedures6,29 treating the
desired precursor in 0.5 M aqueous sulfuric acid at 220 °C in
autoclaves for 8 hours. Reaction time was not optimized further,
while a range of concentrations between 30 and 50 mg mL−1
was explored without observing significant changes in the
yield. The obtained water phase was thus filtered to remove
the humins and the insoluble products generated during the
hydrolysis.30 LA was thus extracted with 2-MeTHF, and the
resulting concentration, depending on the initial concentration
of biomass used for the hydrolysis, was measured by chromato-
graphic methods. Formic acid is a known co-product of the
hydrolysis of sugars into levulinic acid.25 Although the
formic acid potentially extracted in the organic phase can
undergo transfer hydrogenation processes,31,32 its presence
was neglected due to the excess of hydrogen applied during
hydrogenation. Three different standardized feeds obtained
from glucose, milled cellulose, and milled wood were supplied
to the continuous reactor, and the effect of temperature and
flow rate on the conversion was analyzed.
Hydrolysis of glucose afforded LA in 32% weight yield,
ca. 50% on a molar basis (Table 2, entry 4). Hydrogenation of
a 2-MeTHF extract using RANEY® nickel proceeded smoothly,
already yielding to full conversion at 125 °C (Table 2, entry 4).
Increasing the flow rate (entries 5, 6 and 7) resulted in a
progressive decrease in the conversion. This effect could be
compensated for by increasing the reaction temperature
(entries 8–11). Interestingly, these experiments showed that
the use of a feed obtained from real biomass does not affect
the efficiency of the conversion of LA into GVL.
Performing hydrolysis of water-soluble oligosaccharides
(WSO) obtained by mechano-catalytic depolymerization
of cellulose with sulfuric acid33 (entry 12) produced LA
with yields that are similar to the ones obtained from
glucose, confirming the efficiency of ball milling for biomass
pretreatment.
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Hydrogenation of 32 mM LA extracted in 2-MeTHF
afforded GVL quantitatively at 125 °C with a flow rate of
0.5 mL min−1.
reaction we observed the formation of a biphasic system
composed of water and a less dense organic layer. We attrib-
uted this to the presence of 2-MeTHF (ca. 7–8%) dissolved in
the water phase after extraction. A control experiment
performed by treating a 7% solution of 2-MeTHF in 0.5 M
H2SO4 at 220 °C for 8 hours resulted in the formation of
a similar bio-oil. GC-MS analysis revealed a complicated
mixture of different aliphatic and aromatic molecules,
probably originating through non-selective decomposition
pathways involving 2-MeTHF. In addition, gel permeation
chromatography and ESI mass analysis of a bio-oil obtained
after hydrothermal treatment of beechwood during recycling
experiments revealed the presence of molecular species with
mass in the range of 500–100 g mol−1. We surmised that such
a “bio-oil” could be responsible for the solubilization of the
solid by-products otherwise observed during the acidic hydro-
thermal treatment of the biomass, including lignin fragments
corresponding to different degrees of depolymerization.
In this case, the advantage of working with dilute LA feeds
was clearly visible, since in comparison, the use of a 48 mM
solution of LA and a volumetric flow of 0.5 mL min−1 resulted
in only 64% conversion at 125 °C (entry 6). Direct hydrolysis
of milled beechwood as an exemplary real biomass afforded
LA in 13% yield (w/w). Considering the composition of
beechwood, which contains ca. 41 wt% glucans (see the ESI†),
this corresponds to a LA yield of 44% on a molar basis.
HPLC analysis of the water phase revealed the presence of
LA, FA and acetic acid together with additional minor impuri-
ties. Insoluble products containing humins and lignin frag-
ments were isolated at the end of the reaction by filtration.
Extraction of a concentrated beechwood hydrolysate with
2-MeTHF was used to prepare a 40 mM LA feed. Interestingly,
GC analysis of this solution revealed the presence of the sole
LA in 2-MeTHF, indicating that the extraction proceeds in a
very selective manner. Hydrogenation was performed at dif-
ferent temperatures and full conversion could be achieved at
135 °C with a flow of 0.5 mL min−1 (entry 18), in line with
the trend observed for glucose- and cellulose-derived LA
feeds. It is worth noting that the efficiency of LA hydrogena-
tion proved to be independent of the nature of the precursor
exploited, whereas the feed concentration played a determin-
ing role. The stability of RANEY® Ni as a function of time was
evaluated showing that the catalyst retains its activity up
to 130 hours on stream with the values for the conversion of
LA into γ-valerolactone remaining stable (Fig. S6†). The effi-
ciency of the proposed integrated scheme relies on the possi-
bility of recycling the acidic aqueous phase for iterative
hydrolytic steps. In order to verify this condition, we recycled
the acidic water phase for the hydrolysis of glucose after
extraction with 2-MeTHF over 4 cycles without observing
a substantial decrease in the yield of LA (Fig. 2, red bars).
Similarly, recycling of the water phase after hydrolysis of
beechwood over four cycles did not compromise LA forma-
tion (green bars). It is worth noting that performing recycling
experiments after extraction with 2-MeTHF prevented the
formation of insoluble precipitates. In fact, at the end of the
Conclusions
In conclusion, we described a convenient integrated strategy
for the conversion of biomass into GVL. Ball milling is used
as an efficient entry step to generate water-soluble partially
depolymerized lignocellulosic feeds. Thus, an acidic hydro-
thermal treatment is used to produce LA. Following extrac-
tion with 2-MeTHF to prepare a LA-containing feed, a final
RANEY® nickel-catalyzed hydrogenation affords GVL. Ball
milling was performed with catalytic amounts of sulfuric acid
in line with the following sulfuric acid-catalyzed hydrolysis.
The use of 2-MeTHF resulted in a particularly convenient
strategy providing LA in acid/sulfur-free organic feeds which
do not compromise the activity of RANEY® nickel. Further-
more, 2-MeTHF proved to be an optimal solvent for hydroge-
nation reactions.
One of the main advantages of the proposed scheme is
the possibility of recycling both the water and the organic
streams for iterative processing. It is worth noting that blend-
ing of the water phase with little amounts of 2-MeTHF during
hydrolysis prevented the formation of insoluble precipitates
and in turn resulted in the formation of an enriched
“bio-oil”, which in the case of beechwood contained different
lignin fragments. This renders the proposed refining schemes
particularly attractive due to the possible simultaneous
upgrading of both the cellulosic and the lignin fractions
of the biomass. Since 2-MeTHF is considered a green/
ecofriendly solvent22,34,35 and RANEY® Ni is prepared from
an alloy of two widely available metals, Al and Ni, the
strategy disclosed in this paper represents a very sustainable
alternative to methods based on the use of rare transition
metals or non-green reaction media and could facilitate the
preparation of GVL from biomass on an industrial scale.
The possible extension of the proposed integrated strategy
also to the preparation of 2-MeTHF, which would render
the proposed approach even more sustainable, will be the
subject of future studies.
Fig. 2 Levulinic acid yield per cycle during recycling experiments of the
acidic aqueous phase for the hydrolysis of glucose and beechwood.
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