Liquid-phase catalytic transfer hydrogenation and cyclization of levulinic acid and its esters to γ-valerolactone over metal oxide catalysts

Article abstract of DOI:10.1039/c1cc14748j

Levulinic acid and its esters are converted to γ-valerolactone over metal oxide catalysts by catalytic transfer hydrogenation via the Meerwein-Ponndorf-Verley reaction. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1cc14748j

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COMMUNICATION
Liquid-phase catalytic transfer hydrogenation and cyclization of levulinic
acid and its esters to c-valerolactone over metal oxide catalystsw
Mei Chia and James A. Dumesic*
Received 1st August 2011, Accepted 6th October 2011
DOI: 10.1039/c1cc14748j
Levulinic acid and its esters are converted to c-valerolactone
over metal oxide catalysts by catalytic transfer hydrogenation
via the Meerwein–Ponndorf–Verley reaction.
reaction conditions (i.e., hydrothermal conditions in the presence
of salts12,13) for both the generation of H2 from FA and the
subsequent hydrogenation of LA to GVL.
In contrast to previous work, we note that CTH through the
Meerwein–Ponndorf–Verley (MPV) reaction remains an
unexplored alternative chemistry for this reduction step, offering
important advantages for its application. For example, the MPV
reaction possesses exceptional chemo-selectivity for the reduction
of carbonyl groups under mild reaction conditions and in the
presence of other functional groups (such as CQC double
bonds). Significantly, the MPV reaction can take place over
non-precious metal heterogeneous catalysts.14 Moreover, the
hydrogen donor in the MPV reaction, usually a secondary
alcohol, can be recycled after hydrogenation over base metal
catalysts such as nickel15–17 or copper,18,19 or even sold as a
commodity chemical in its oxidized form (i.e., ketones, Fig. 1, 4).
We demonstrate in this communication that CTH via the
MPV reaction is a viable means for the hydrogenation of LA and
its esters over inexpensive, heterogeneous catalysts that are easily
regenerable, with attainment of close to quantitative yields of
GVL under appropriate reaction conditions. In this respect, the
MPV reaction is uniquely able to meet the above-mentioned
techno-economic demands for the production of GVL from LA
and its esters. Importantly, we demonstrate here for the first time
that the MPV reaction can be exploited for the formation of
GVL from LA and its esters over heterogeneous catalysts, which
has hitherto been reported in only one instance to occur in the
presence of homogenous catalysts.20
The hydrogenation of biomass-derivable levulinic acid (LA) and
its esters1–5 to g-valerolactone (GVL) (Fig. 1) is a key reaction in
the development of economically viable and carbon-efficient
biorenewable routes to chemicals6,7 and liquid transportation
fuels.8 Although Group VIII metals, notably ruthenium,7 have
been shown to facilitate this hydrogenation step using molecular
H2, it has recently been suggested9 that the use of precious metal
catalysts is detrimental to the overall process economics for the
production of high volume and relatively low value liquid
transportation fuels. Thus, we report in this communication a
new route for the conversion of LA to GVL using inexpensive
heterogeneous catalysts, such as zirconium oxide.
As an attractive alternative to the reduction of LA to GVL
using H2 over metal catalysts, we have explored the reduction of
LA by catalytic transfer hydrogenation (CTH), whereby a
hydrogen source other than molecular H2 is used. Recent
literature on the CTH of LA has focused on using formic acid
(FA) as the hydrogen donor, since FA is a by-product of LA
production from C6 sugars. However, the use of FA entails
several unresolved disadvantages, such as the need for precious
metals (e.g., Pd, Rh), homogeneous catalysts,10,11 and/or harsh
In initial reaction kinetic studies using batch reactors, it was
found that a variety of metal oxides are active materials for the
transfer hydrogenation of butyl levulinate (BL) to GVL
(Table 1, Entries 1–5). Of the catalysts examined, ZrO2 was
the most active material for CTH. We have further observed
that the CTH reaction occurs at reaction temperatures >100 1C,
and the rate of formation of GVL increases with increasing
reaction temperature (Table 1, Entries 9–11), with no observable
loss in GVL selectivity at temperatures of up to 220 1C when
secondary alcohols (e.g., isopropanol (IPA), 2BuOH) are
employed as the hydrogen donor. The only by-products
detected in the product mixtures were those formed through
the transesterification of the levulinate ester with the secondary
alcohol solvent (e.g., isobutyl levulinate as the product).
In contrast to using secondary alcohols as hydrogen donors,
we found that although primary alcohols such as 1-butanol
Fig. 1 Catalytic transfer hydrogenation of levulinic acid (1, R1 = H)
and its esters (1, R1 = CxH2x+1) to g-valerolactone (5) using a
secondary alcohol as the hydrogen donor (2, R2 = CyH2y+1).
Department of Chemical and Biological Engineering,
University of Wisconsin-Madison, 1415 Engineering Drive, Madison,
WI 53706, USA. E-mail: dumesic@engr.wisc.edu;
Fax: 01-608-262-5434; Tel: 01-608-262-109
w Electronic supplementary information (ESI) available: Table S1 and
experimental details. See DOI: 10.1039/c1cc14748j
c
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Chem. Commun., 2011, 47, 12233–12235 12233

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Table 1 Catalytic transfer hydrogenation of levulinate esters using various alcohols as the hydrogen donora
Time (h) Mass ratio catalyst : ester Solvent Conv (%) GVL yield (%) GVL formation rate (mmol gÀ1 minÀ1
Entry Catalyst
)
1
2
3
4
5
6
7
8
MgO/Al2O3 16
1 : 2.4
1 : 2.4
1 : 2.4
1 : 2.4
1 : 2.4
1 : 4.8
1 : 1.2
1 : 1.2
1 : 4.8
1 : 4.8
1 : 4.8
1 : 4.8
1 : 1.2
2BuOH
2BuOH
2BuOH
2BuOH
17.0
12.6
19.7
37.0
14.6
8.0
1.9
1.1
2.0
3.9
—
65.6
4.0
MgO/ZrO2
CeZrOx
g-Al2O3
ZrO2
ZrO2
ZrO2
16
16
16
16
4
16
16
4
15.8
29.6
84.7
55.6
42.8
49.2
12.2
55.0
80.5
62.4
1.5
2BuOH >99.9
2BuOH
1BuOH
EtOH
IPA
IPA
IPA
70.1
60.5
98.2
36.6
77.2
93.2
70.8
5.8
ZrO2
ZrO2
—
9b
10
11c
12d
13e
14.2
70.8
108.4
86.0
5.2
ZrO2
ZrO2
4
4
ZrO2
ZrO2
4
16
IPA
THP
a
b
c
d
e
Batch reactions, 5 wt% BL in respective solvents as feed, 150 1C, 300 psig He. 120 1C. 180 1C. 5 wt% EL as feed. 5 wt% IL as feed. Entries
1–6 and 9–12: the only by-products detected were isobutyl or isopropyl levulinate, formed through the transesterification of BL or EL with the
solvent.
(Table 1, Entry 7, 1BuOH) and ethanol (Table 1, Entry 8, EtOH)
are able to facilitate CTH of levulinate esters leading to the
formation of GVL, various by-products are formed causing a
decrease in GVL selectivity. These by-products were identified
using GC-MS as forming through the self-condensation of the
aldehyde formed after dehydrogenation of the hydrogen donor,
or by forming through reaction of the aldehyde with levulinate
esters. Significant differences are not observed in the rate of GVL
formation when using different levulinate esters, e.g., BL and
ethyl levulinate (EL) (Table 1, Entries 10 and 12). Additionally,
4-hydroxypentanoic acid (Fig. 1, 3) was not detected in any
product mixtures throughout this study, indicating that rapid
ring-closing to form GVL occurs under the reaction conditions
employed. To investigate whether CTH of levulinate esters could
be occurring through direct intramolecular hydrogen transfer
within the ester, a reaction mixture of isopropyl levulinate (IL) in
tetrahydropyran (THP) was used (Table 1, Entry 13). The
formation of small amounts of GVL suggests that intramolecular
CTH does appear to take place, but at a much lower rate (e.g., by
a factor of approximately 20) than through the MPV reaction
with a hydrogen donor.
group in SBP completely unconverted. This high selectivity for
conversion of LA versus reduction of SBP has practical implica-
tions, because we have shown elsewhere that alkylphenol solvents
can be used to extract levulinic acid from aqueous solutions of
sulfuric acid used to achieve the deconstruction of cellulose.5
Although the use of equimolar concentrations of hydrogen
donor (i.e., IPA) and ester result in relatively low reaction rates
(Table S1, ESI, Entry 1), it was observed that modest increases in
the amount of IPA were effective in raising the yield of GVL. For
instance, an increase in molar ratio of IPA : EL from 1 : 1 to 4 : 1
(Table S1, ESI, Entries 1–2) increased the GVL yields from 18%
to 73%. At a molar ratio of IPA : EL of 7 : 1 (Table S1, ESI,
Entry 3), it was found that the rate of GVL formation was
comparable to that when pure IPA is used as the solvent
(Table 1, Entry 10), and close to quantitative yields of GVL
were attained (Table S1, ESI, Entry 4). We note that the high
chemo-selectivity inherent in the MPV reaction, required in
production processes for conversion of LA and its esters in the
presence of highly functionalized extraction solvents, such as
those recently reported by Alonso et al.,5 is not easily attainable
over conventional heterogeneous metal catalysts using molecular
H2. In particular, the hydrogenation of CQC over CQO bonds
is thermodynamically favoured, and typically requires the use of
precious metals (e.g., Ru, Pt) modified with a base metal
(e.g., Sn) at the expense of lower catalytic activity.5,21
A key advantage of the MPV reaction is that the high chemo-
selectivity for hydrogenation of carbonyl groups allows the CTH
of levulinate esters to GVL to occur in the presence of other
highly functionalized molecules. For example, we have observed
that the reduction of levulinate esters occurs effectively in the
presence of an aromatic diluent such as sec-butyl phenol
(Table S1, ESI, SBP), whilst leaving the aromatic functional
We have observed that LA undergoes CTH less readily than
its esters, with GVL formation rates of 4 mmol gÀ1 minÀ1
(Table 2, Entry 3). In comparison, the rate of GVL formation
Table 2 Catalytic transfer hydrogenation of levulinic acid and levulinate esters using 2-butanol as the solvent and hydrogen donora
Mass ratio
catalyst : LA
GVL formation rate
Time (h) GVL yield (%) Ester & LA conv (%) (mmol gÀ1 minÀ1
Entry LA (wt%) BL (wt%)
Catalyst
)
1
2
3
0.5
1
5
5
5
1
1
1
1
5
5
0
0
0
0
0
0
0
—
—
ZrO2
ZrO2
ZrO2
ZrO2
ZrO2
ZrO2
MgO/ZrO2 16
g-Al2O3
CeZrOx
4
4
16
16
16
16
30
13
22
71
39
92
54
56
11
30.1
26.0
52.0
>99.9
>99.9
>99.9
>99.9
79.9
42.1
14.9
3.9
—
16.7
—
—
2.0
0.4
1 : 2
1 : 2
1 : 5
2 : 1
2 : 1
2 : 1
2 : 1
4b
5c
6
7
8
9
16
16
43.4
a
b
c
Batch reactions, 150 1C, 300 psig He. 220 1C. MgO added to reaction mixture; mass ratio of MgO:ZrO2 = 1 : 1.
c
12234 Chem. Commun., 2011, 47, 12233–12235
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Importantly, the initial activity of the catalyst is fully regained
after calcination in air at 450 1C, and the catalyst was then found
to display the same deactivation profile as the fresh catalyst after
each regeneration cycle. After the second regeneration cycle and
500 h time-on-stream, it was observed that the catalyst stabilized
to a similar activity level as the fresh catalyst (i.e., at 100 h). This
ease of repeated catalyst regeneration with no loss in catalyst
performance, and the stabilization in catalytic activity with time-
on-stream, are significant advantages for the use of this catalyst
system in industrial applications.
In conclusion, we have found that reduction of LA and its
esters to GVL can be accomplished by CTH through the MPV
reaction over various metal oxide catalysts using secondary
alcohols as the hydrogen donor. ZrO2 was demonstrated to be
a highly active material for CTH, in both batch and continuous
flow reactor studies. While the activity of this catalyst decreased
and then stabilized during operation for 100 h of time-on-stream,
the initial activity of the catalyst was repeatedly regenerable by
calcination in air, with no observable loss in catalytic activity.
This material is based upon work supported by the National
Science Foundation under Award No. EEC-0813570.
Fig. 2 Plot of g-valerolactone (GVL) formation rate (’) and yield of
GVL (J) as a function of time-on-stream for the catalytic transfer
hydrogenation of butyl levulinate (BL) to GVL over ZrO2 in a
continuous flow reaction system. Refer to ESI for experimental details.
Catalyst was regenerated at 150 and 300 h (dotted lines).
from BL was approximately 70 mmol gÀ1 minÀ1. For all runs with
LA (Table 2), the sole by-product detected was isobutyl
levulinate, formed through the esterification of LA with the
solvent over the catalyst. Entries 1–2 in Table 2 demonstrate that
the addition of LA to BL and 2BuOH results in a decrease in the
rate of GVL formation over ZrO2 from 42 to 15 mmol gÀ1 minÀ1
as the concentration of LA is increased from 0.5 to 1 wt%.
We suggest that the observed inhibiting effect of LA on CTH is
due to the strong binding of the acid functional group in LA to
basic sites on ZrO2, which is known to be amphoteric;22,23 the
number of acidic and basic sites for the ZrO2 material used here
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c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 12233–12235 12235