Exploring the ruthenium catalysed synthesis of γ-valerolactone in alcohols and utilisation of mild solvent-free reaction conditions

Article abstract of DOI:10.1039/c2gc16631c

Levulinic acid and alkyl-levulinates have been hydrogenated using a range of supported catalysts. The different reaction outcomes obtained in alternate solvents have been rationalized and the influence of varying catalyst supports examined. A range of solvent free conditions have been investigated with complete LA conversion obtained at temperatures as low as 25 °C.

Full text of DOI:10.1039/c2gc16631c

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COMMUNICATION
Exploring the ruthenium catalysed synthesis of γ-valerolactone in alcohols
and utilisation of mild solvent-free reaction conditions†
Mohammad G. Al-Shaal, William R. H. Wright and Regina Palkovits*
Received 20th December 2011, Accepted 16th February 2012
DOI: 10.1039/c2gc16631c
1
3
Levulinic acid and alkyl-levulinates have been hydrogenated
using a range of supported catalysts. The different reaction
outcomes obtained in alternate solvents have been rational-
ized and the influence of varying catalyst supports examined.
A range of solvent free conditions have been investigated
with complete LA conversion obtained at temperatures as
low as 25 °C.
acid (γVA) as reaction intermediates (Scheme 1). Notably, the
relative favorability of each reaction pathway has not yet been
determined, although it has long been established that both
pseudo-LA and Anjelica Lactone can readily convert back to
14
LA. Furthermore, when alcohols are employed as reaction sol-
15
vents, both LA and γVA can undergo esterification. However,
the overall impact of ester formation on the efficiency of γVl pro-
duction has not yet been fully explored. This is of interest as the
utilisation of small alcohols as a medium for LA hydrogenation
could prove to be advantageous, as such solvents arguably have
a relatively low net environmental impact and can be derived
Secondly, conversion of lignocellulosic
biomass to levulinic esters instead of LA could enable higher
yields and easier product separation, suggesting that future γVl
Despite numerous difficulties being associated with exploitation
of fossil reserves, these resources still form the bases of world-
1
,2
wide fuel and chemical production. Ultimately, these finite
reserves will be depleted, inevitably forcing a switch to more
sustainable feedstocks, such as those obtained from renewable
9,16
from biomass.
3
17
biomass. However, before widespread biomass utilisation can
be realised, new processing methodologies must be developed,
that are undemanding in energy and close to carbon neutral.
production could be based on hydrogenation of levulinic
esters.
4
18
This represents a considerable challenge as biomass components
With the goal of assessing the impact of ester formation upon
LA hydrogenation, we have examined the ability of Ru–C to cat-
alyse LA hydrogenation in methanol, ethanol and 1-butanol at
130 °C (Table 1, entries 1–3). As efficient LA hydrogenation has
already been demonstrated in 1,4-dioxane, for comparison this
solvent was also utilised (Table 1, entry 4). Of the alcohols
screened, methanol facilitated the highest γVl yield and selectiv-
ity, allowing a similarly high catalyst activity to that obtained
using 1,4-dioxane. The conversion of LA in ethanol was notably
lower, and significantly worse in 1-butanol, although high selec-
tivities were obtained in all three alcohols (81–85%). To evaluate
the extent of ester formation in methanol and 1-butanol, mixtures
comprising each alcohol and LA were prepared and heated under
(
e.g. sugars) are typically rich in functionalities, with energy
expenditure often required for their conversion to less functiona-
5
lised more usable compounds. Despite this difficulty, the US
19
DOE has identified key target molecules that can be derived
6
from biomass, including γ-valerolactone (γVl) which may find
7
8
use as a renewable solvent, fuel additive, or precursor to
9
alkene based transportation fuels. The production of γVl entails
the hydrogenation of levulinic acid (LA), a versatile and viable
platform chemical that has already been produced efficiently
1
0,11
from lignocellulosic biomass on a pilot plant scale.
Overall,
the maximum economic and environmental benefits associated
with utilisation of γVl will only be realised if minimal energy is
employed during its production. With this in mind, we have re-
examined the production of γVl, with the aim of developing less
energy demanding ‘greener’ protocols with lower environmental
impact and higher sustainability.
The capacity of a range of bases and precious metals to cata-
12
lyse LA hydrogenation has already been extensively explored,
with the highest γVl yields invariably obtained from Ru-sup-
12,13
ported catalysts.
Analysis of hydrogenation reactions using
GC-MS has implicated both pseudo-LA and γ-hydroxyvaleric
Institut für Technische und Makromolekulare Chemie, RWTH,
Aachen University, Worringerweg 1, D-52074 Aachen, Germany.
E-mail: palkovits@itmc.rwth-aachen.de
†
Electronic supplementary information (ESI) available: Full experimen-
tal details alongside supporting charts and tables. See DOI: 10.1039/
c2gc16631c
Scheme 1 Reaction pathways for the hydrogenation of LA.
1260 | Green Chem., 2012, 14, 1260–1263
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Table 1 LA hydrogenation using supported ruthenium catalysts
a
b
c
d
e
f
Catalyst
Sub.
Solvent
% LA conv.
% γVl select.
% γVl yield
Moles γVl/gRu
1
2
3
4
5
6
7
8
9
1
Ru–C
Ru–C
Ru–C
Ru–C
Ru–C
Ru–C
LA
LA
LA
LA
Methanol
Ethanol
99.0
75.5
48.6
98.8
97.8
91.1
96.4
100
96.2
99.2
98.8
99.5
0.0
85.3
81.0
81.7
97.7
89.4
82.3
81.4
98.3
87.7
89.8
76.0
86.6
—
84.4
61.1
39.7
95.9
87.4
75.1
78.4
98.3
84.5
89.1
75.1
86.2
—
2.9
2.1
1.3
3.3
3.0
2.5
2.6
3.3
2.9
3.0
2.5
2.9
—
1-Butanol
1,4-Dioxane
Methanol
Methanol
Methanol
Butanol
Methanol–H
Ethanol–H
Butanol–H O
H O
2
Ethanol
Ethanol–H
Ethanol
Ethanol–H
Ethanol
Ethanol–H
Ethanol
g
ML
h
BL
i
Ru–C
Ru–C
LA
LA
LA
LA
LA
LA
LA
LA
LA
LA
LA
LA
LA
LA
i
Ru–C
Ru–C
Ru–C
Ru–C
Ru–TiO
Ru–TiO
Ru–TiO
Ru–TiO
Ru–Al
Ru–Al
Ru–SiO
Ru–SiO
2
2
O
O
0
1
1
2
1
1
1
1
1
1
1
1
2
2
3
4
5
6
7
8
9
0
2
2
2
2
(T)
(T)
(D)
(D)
2
2
2
2
O
O
O
O
0.0
—
—
—
67.7
81.2
37.7
94.7
82.9
98.0
92.2
87.8
85.8
80.4
92.8
76.5
62.4
71.2
32.3
76.2
77.0
74.9
2.1
2.4
1.1
2.6
2.6
2.5
2
O
O
3
2
3
2
2
Ethanol–H
a
General conditions: LA (500 mg, 4.31 mmol); Ru (5%) support (25 mg, 0.012 mmols of Ru); Solvent (10 mL); 130 °C; H
2
(12 bar); 160 min;
b
1
30 °C. Alcohols of LC-MS grade were purchased from Sigma Aldrich and used as received; with alcohol–water mixtures being generated by
c
d
mixing 9 mL of an alcohol with 1 mL of water. (Total moles of all products)/(moles LA) × 100. (Moles γVl)/(total moles of all products) × 100.
e
f
g
h
i
(
2
Moles of γVl)/(moles LA) × 100. (Moles of γVl)/(mass of Ru (g)). ML (560 mg, 4.31 mmol). BL (681 mg, 4.31 mmol). H pressure of 20 bar
utilized.
a pressure of H (12 bar, 130 °C, 160 min) in the absence of
Ru–C. Analysis of both mixtures at room temperature using H
NMR spectroscopy demonstrated that the extent of esterification
in the methanol and 1-butanol mixtures was 0.5 and 4.0%
respectively. To further explore the influence of ester formation
on LA hydrogenation, discrete samples of methyl-levulinate
10). More noticeably, mixing water and 1-butanol resulted in a
substantial increase in LA conversion, without considerable
modification of γVl selectivity (Table 1, entry 3 vs. 11). Overall,
the presence of water can enhance γVl production, which is for-
tunate as generation of LA via hydrolysis of lignocellulosic
biomass will result in LA product streams that invariably contain
water. To further evaluate the suitability of water as a reaction
solvent, LA hydrogenation in pure water was assessed (Table 1,
entry 12). This showed that water enables efficient γVl pro-
duction with notably higher yields obtained than for ethanol or
2
1
11
(ML) and butyl-levulinate (BL) were both hydrogenated in
methanol, utilizing identical conditions to those employed for
LA (Table 1, entries 5–6). This resulted in similar conversions
and yields to those obtained from LA, establishing that esters of
LA can be readily hydrogenated to γVl, and that alkyl-levulinate
formation in alcohol solvents does not significantly inhibit γVl
production. Thus, while a marginally higher extent of ester for-
mation occurs in 1-butanol, another factor results in the lower
γVl yields obtained from this system. Notably, H2 is more
1-butanol, again indicating that the high k value of water can
h
enable higher reaction yields.
The presence of water in LA product streams could create
additional challenges with regard to catalyst stability. Conse-
quently, the identification of water tolerant catalysts would be a
major factor in facilitating closer process integration and increas-
ing efficiency, by removing the costly need to separate water
from LA feeds prior to hydrogenation. Thus, we assessed the
capacity of 5 wt% Ru supported on TiO (Tronox), Al O and
soluble in methanol,‡ with the k values for methanol, ethanol
h
and 1-butanol previously reported as being 596, 452 and 358
20
MPa respectively. Hence, the reduced catalyst activity observed
in ethanol and 1-butanol could result from lower concentrations
of H in the reaction mixture. In contrast, at increased H press-
2
2 3
SiO , to catalyse LA hydrogenation in ethanol and mixtures of
2
2
2
ures (20 bars) we have observed that 1-butanol facilitates higher
ethanol–water (Table 1, entries 13–20). For Ru–TiO (Tronox)
2
reaction yields than methanol (Table 1, entries 7–8).
no LA conversion was observed in either ethanol or ethanol–
One general means by which the concentration of H can be
water (Table 1, entries 13–14). In contrast, Ru–TiO (Degussa
2
2
increased in a given mixture is by the addition of water, which
P25) did initiate γVl production (Table 1, entries 15–16), with
the higher specific surface area of this support enhancing cataly-
sis, either by facilitating substrate absorption or enabling a
higher Ru dispersion. Overall, the different reaction outcomes
rendered by each form of TiO2 demonstrate that the catalyst
support structure can have a profound influence on reaction
yields, and further investigations are currently underway to
rationalise these effects. Ru–Al O was able to catalyze the pro-
with an associated k value of 7500 MPa has a high capacity to
h
21
dissolve H2. Thus, adding water to the reaction mixtures could
permit high H concentrations, without requiring high pressures
2
and associated engineering costs. With this in mind, mixtures
comprised of water (10% by volume) and a given alcohol (90%
by volume), were utilized as solvents for LA hydrogenation
(Table 1, entries 9–11). For methanol, water addition did not
2
3
modify γVl production (Table 1, entry 1 vs. 9), although using
ethanol–water slightly enhanced γVl yield (Table 1, entry 2 vs.
duction of γVl in ethanol, albeit with a low conversion and yield
(Table 1, entry 17). Notably, higher catalyst activity was rendered
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Table 2 Ruthenium catalysed LA hydrogenation at 25 °C
a
b
c
d
e
f
Catalyst
Reaction time (h)
% LA conver.
% γVA select.
% γVl select.
% γVl yield
Moles γVl/gRu
Ru–C
Ru–C
Ru–C
Ru–C
Ru–SiO
Ru–Al
2.6
24
40
50
50
50
10.6
83.7
86.1
100
2.1
75.7
13.1
7.2
2.4
19
24.2
86.8
92.7
97.5
81.0
75.1
2.5
0.08
2.50
2.75
3.36
0.05
0.02
72.7
79.8
97.5
1.7
2
O
2
3
8.43
24.9
6.3
a
General conditions: LA (5.0 g, 43.1 mmol); Ru(5%) support (250 mg, 0.12 mmols Ru); H
2
(12 bar); Samples of LA acid were initially heated to
b
3
5 °C to induce the melting required for catalyst mixing, although, hydrogenation was conducted exclusively at 25 °C. (Total moles of all products)/
c
d
e
(
moles LA) × 100. (Total moles of γVA)/(moles LA) × 100. (Moles γVl)/(total moles of all products) × 100. (Moles of γVl)/(moles LA) × 100.
f
(Moles of γVl)/(mass of Ru (g)).
by Ru–Al O in the ethanol–water mixture, enabling a signifi-
cantly increased γVl yield (Table 1, entry 18). Similarly, for the
catalyst productivity at the different reaction conditions (see
Chart S1†). Nevertheless, these results indicate that using low
reaction temperatures could enable longer catalyst lifetimes, a
significant finding as catalyst stability is an important consider-
ation in the development of new sustainable processes.
2
3
Ru–SiO catalyst, a larger γVl yield and catalyst productivity
2
was obtained in the presence of water, although the discrepancy
is less noticeable (Table 1, entries 19–20). Overall, these results
demonstrate that addition of water to a reaction mixture can
enhance the total γVl yields generated by a range of supported
Ru catalysts. To assess the relative stability of the reaction pro-
ducts, the Ru–SiO2 ethanol reaction (Table 1, entry 19) was
Of the catalysts screened at 25 °C, Ru–C provided the highest
yields (Table 2), with significantly lower activities given by Ru–
SiO , Ru–Al O . While dilution of LA with water or γVl mark-
2
2 3
edly decreased Ru–C activity, it was found that this can be miti-
gated by increasing catalyst loadings (Table S1† entries 1–2).
1
filtered, stirred at 25 °C for 24 h, and re-analysed using H NMR
spectroscopy. The composition of this reaction mixture remained
Thus, mixtures of γVl/LA and H O–LA were hydrogenated
2
unaltered with the same amount of γVA persisting in solution
using Ru–C at 25 °C to give γVl yields of 91.4% and 88.8%
respectively, with the only other reaction product being γhydrox-
(7.2%). Evidently, the dehydration of γVA requires elevated
temperatures or the presence of a catalyst.
yvaleric acid (γVA). This demonstrates that LA–H O product
2
While tolerance of water in LA streams could enable closer
process integration, further optimisation of LA hydrogenation,
may reduce overall process costs. Total process efficiency could
be increased by employing solvent-free conditions that would
streams could be hydrogenated at low temperatures and that γVl
could be added to reaction mixtures to ensure fluidity. As an
alternative to the 50 h reaction times required at 25 °C, we have
also found that heating mixtures of LA and Ru–C to 190 °C
22
facilitate the later isolation and processing of γVl derivatives.
under an atmosphere of H (12 bar) results in complete conver-
2
Thus, we have strived to develop mild solvent-free conditions
for the hydrogenation of LA, and have identified that γVl pro-
duction can be achieved at 25 °C, without the requirement of
any additional reaction solvents (Table 2). In our investigations,
stirring mixtures of LA, 5 wt% Ru–C under an atmosphere of H2
sion of LA to γVl within 40 minutes. Such a system could prove
to be highly attractive for technical implementation, with the fast
reaction times enabling very high space time yields permitting
minimal net energy expenditure. In conclusion, we have estab-
lished that for many systems solvent selection can have a signifi-
cant influence on γVl yields with water addition often enhancing
γVl production. Furthermore, solvent-free conditions have been
demonstrated, both at 25 °C and 190 °C.
(
(
12 bar), resulted in near complete conversion to γVl after 50 h
Table 2). This longer reaction time was necessary as the rate of
hydrogenation is considerably diminished at 25 °C. Indeed, the
1
H NMR spectra presented by the reaction mixture after 2.6 h
indicated that most of the LA had not been hydrogenated, with
γVA identified as the principle reaction product (Table 2). After
Acknowledgements
2
4 h, the reaction mixture consisted mainly of γVl, although sig-
We wish to thank the Higher Institute of Applied Science and
Technology of Syria for the studentship of Mohammad G. Al-
Shaal (Grant Number 1596/MT). This work was supported by
the Robert Bosch Foundation in the frame of the Robert Bosch
Fellowship for the efficient utilization of renewable resources.
nificant amounts of γVA and LA were present in solution. These
components slowly convert to γVl, which after 50 h is contami-
nated by only trace γVA (2.4%), according to H NMR spec-
1
troscopy. Only one previous example of LA hydrogenation at
2
5 °C has been reported, in which lower LA conversions (87%
2
3
after 48 h) were rendered by a Pt catalyst. As our previous
Notes and references
experiments established that γVA does not readily dehydrate at
‡
The mole fraction of H
Henry’s law equation, p(H
and c is H solution concentration.
2
in a given solution can be determined using
) = k × c(H ). Where p is H partial pressure
2
5 °C in the absence of a catalyst, in this instance it is probable
2
h
2
2
that this transformation is induced by Ru–C. After 50 h no
further modification of the reaction mixture occurred, and the
Ru–C was removed from the product mixture by filtration. The
Ru–C catalyst was found to be recyclable with marginally better
catalyst stability observed at 25 °C than at 130 °C, although
direct comparison is certainly not possible due to variations in
2
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