The Role of the Hydrogen Source on the Selective Production of γ-Valerolactone and 2-Methyltetrahydrofuran from Levulinic Acid

Article abstract of DOI:10.1002/cssc.201600751

A mechanistic study of the hydrogenation reaction of levulinic acid (LA) to 2-methyltetrahydrofuyran (MTHF) was performed using three different solvents under reactive H₂ and inert N₂ atmospheres. Under the applied reaction conditions, catalytic transfer hydrogenation and hydrogenation with molecular H₂ were effective at producing high yields of γ-valerolactone. However, the conversion of this stable intermediate to MTHF required the combination of both hydrogen sources (the solvent and the H₂ atmosphere) to achieve good yields. The reaction system with 2-propanol as solvent and Ni–Cu/Al₂O₃ as catalyst allowed full conversion of LA and a MTHF yield of 80 % after 20 h reaction time at 250 °C and 40 bar of H₂ (at room temperature). The system showed the same catalytic activity at LA feed concentrations of 5 and up to 30 wt%, and also when high acetone concentration at the beginning of the reaction were added, which confirmed the potential industrial applications of this solvent/catalyst system.

Full text of DOI:10.1002/cssc.201600751

DOI: 10.1002/cssc.201600751
Full Papers
The Role of the Hydrogen Source on the Selective
Production of g-Valerolactone and
2
-Methyltetrahydrofuran from Levulinic Acid
[a, b]
[a]
[b]
[b]
Iker Obregꢀn,
Pedro L. Arias,* and Regina Palkovits
Inaki Gandarias, Mohammad G. Al-Shaal, Christian Mevissen,
[a]
[b]
A mechanistic study of the hydrogenation reaction of levulinic
acid (LA) to 2-methyltetrahydrofuyran (MTHF) was performed
using three different solvents under reactive H and inert N at-
tion system with 2-propanol as solvent and Ni–Cu/Al O as cat-
2
3
alyst allowed full conversion of LA and a MTHF yield of 80%
after 20 h reaction time at 2508C and 40 bar of H (at room
2
2
2
mospheres. Under the applied reaction conditions, catalytic
transfer hydrogenation and hydrogenation with molecular H2
were effective at producing high yields of g-valerolactone.
However, the conversion of this stable intermediate to MTHF
required the combination of both hydrogen sources (the sol-
temperature). The system showed the same catalytic activity at
LA feed concentrations of 5 and up to 30 wt%, and also when
high acetone concentration at the beginning of the reaction
were added, which confirmed the potential industrial applica-
tions of this solvent/catalyst system.
vent and the H atmosphere) to achieve good yields. The reac-
2
[
7]
Concerns about fossil fuel shortage and the environment have
triggered intensive research to find environmentally benign al-
ternatives for the non-renewable energy resources that are
currently used. Although other renewable energy sources,
such as solar or wind power, are expected to fulfill the future
tivity, improving heat and mass transfer, and facilitating prod-
[7]
uct separation.
The US Department of Energy published a list of the “Top
[8]
10” building blocks for use in future biorefineries. Levulinic
acid (LA) was included in this list according to criteria, such as
i) direct substitution of existing petrochemicals, ii) strong po-
tential as a platform chemical, and iii) underway state of the
scale-up of the production to pilot, demo, or full scale. 2-Meth-
yltetrahydrofuran (MTHF), which can be derived from LA or its
intermediate g-valerolactone (GVL) (Figure 1), is considered as
an ideal biofuel or fuel additive for currently used gasoline for-
[
1]
electricity demand, biomass-derived liquid fuels have arisen
as the only sustainable short-term alternative for the transpor-
[
2]
tation sector.
To achieve this goal, the biorefinery concept has been pro-
posed as an integrated industrial complex in which biomass-
derived feedstocks are converted into fuels and value-added
[
1]
[9]
chemicals. In the petrochemical industry, the addition of
oxygen to unfunctionalized feedstocks is typically performed
mulations and for new oil-free fuel formulations. Interesting
features such as hydrophobicity, suitable octane number, and
a similar energy density as gasoline make the use of this mole-
[
3]
in the gas phase. In contrast, most biorefinery processes in-
volve the removal of oxygen from highly oxygenated raw ma-
cule straightforward in conventional engines without any me-
[2,4]
[10–13]
terials for the production of higher added-value chemicals.
chanical modifications.
The production of MTHF from LA is
Economic considerations and thermal limitations of biomass-
derived building blocks have resulted in the need for liquid
phase processes in future biorefineries. The use of solvents for
summarized in Figure 1, along with the main side reactions.
LA can be transformed into GVL by a sequence of dehydro-
genation–hydrogenation or vice versa, depending on the reac-
[
5]
[14]
these reactions is common and may serve different purposes
tion conditions. GVL can then be hydrogenated into differ-
[
6]
such as enhancing the catalytic activity, switching the selec-
ent molecules such as 1,4-pentanediol (PDO), valeric acid (VA),
or 2-butanol (2-BuOH) via 4-hydroxypentanal. Intramolecular
etherification of PDO yields MTHF whereas hydrogenolysis of
PDO can produce either 1-pentanol (1-PeOH) or 2-pentanol (2-
PeOH). The first step of the reaction, LA to GVL, can be ach-
ieved under a wide range of conditions. Although the solvent
[
a] I. Obregꢀn, Dr. I. Gandarias, Prof. Dr. P. L. Arias
Chemical and Environmental Engineering Department
University of the Basque Country (UPV/EHU)
Alameda Urquijo s/n, 48013 Bilbao (Spain)
E-mail: pedroluis.arias@ehu.eus
[15]
may interfere to a certain extent, it does not seem to play
a determining role on this step of the reaction because the re-
action proceeds at high yields under hydrogen atmosphere in
[
b] I. Obregꢀn, M. G. Al-Shaal, C. Mevissen, Prof. Dr. R. Palkovits
Chair for Heterogeneous Catalysis and Chemical Technology, Institut fꢁr
Technische und Makromolekulare Chemie, RWTH
Aachen University, Worringerweg 1
[16,17]
[15,18]
different media such as water,
1,4-dioxane,
or even under solvent-free condi-
This reaction is also reported to occur by catalytic
tetrahydro-
[19]
[20,21]
furan,
alcohols,
D-52074 Aachen (Germany)
[
15,22]
tions.
Supporting Information and the ORCID identification number(s) for the
author(s) of this article can be found under http://dx.doi.org/10.1002/
cssc.201600751.
transfer hydrogenation (CTH) over metal oxides or supported
ChemSusChem 2016, 9, 1 – 9
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[
26,28,33]
Figure 1. Synthesis of MTHF from LA.
[
23]
[24,25]
metal catalysts using alcohols or formic acid
drogen donor molecules.
(FA) as hy-
lected according to their interest and reported use in the liter-
ature; either for hydrogenation using molecular hydrogen or
for CTH reactions. 1,4-Dioxane, as an aprotic solvent, shows no
ability to serve as a hydrogen donor in the absence of degra-
dation reactions. Nonetheless, the highest reported MTHF
yields starting from LA were achieved in this solvent in the gas
The second step of the reaction, GVL to MTHF, is more chal-
[14]
lenging owing to the high thermodynamic stability of GVL.
Hence, this step requires harsher reaction conditions. Previous
investigations have shown the paramount importance of the
[31]
H pressure on the GVL conversion. Hydrogen pressures below
phase. 1-BuOH is an interesting solvent for this reaction as
recent reports have shown a simple and effective method to
separate LA from the aqueous phases through the formation
of butyl levulinate in the presence of butene or butanol and
2
8
0 bar (at room temperature, then heated up to 1908C) were
insufficient for the solvent-free hydrogenation of GVL over Ru/
[
26]
C, whereas 50–100 bar of H were required at 150–2008C to
2
produce MTHF starting from LA using homogenous Ru cata-
the use of an acid catalyst to cause spontaneous separation of
[
27–29]
[25,39]
lysts with several additives.
Additionally, in the scarce liter-
the aqueous/organic phases.
These LA esters have shown
[20,25]
ature reports of this reaction, the important role of the solvent
is evident. The most used solvent for this step is 1,4-dioxane,
similar reactivity towards GVL as neat LA.
2-PrOH was se-
lected as the third solvent owing to its high capacity for hydro-
gen donation. 2-PrOH was also reported to enable high MTHF
[
30]
[31]
under 100 bar H2 or in the vapor phase. The transforma-
[26,32]
[33]
tion of GVL is reported to be strongly inhibited by water,
yields using Ni–Cu based catalysts.
and the selectivity towards MTHF appears to be limited by
a mechanism that is still unclear. The reports of aqueous phase
The commercial Ru(5%)/C catalyst and our previously report-
ed Ni(23%)–Cu(12%)/Al O3 and Ni(35%)/Al O3 catalysts were
2
2
[
33]
[34]
GVL dehydrogenation into a-angelica lactone and LA sug-
gest that the reversibility of these reactions is a factor in the
inhibition by water. The MTHF yield significantly improves if al-
cohols are used as solvents instead of water.
selected based on their reported utilization in these reactions
[26,33]
and the significant MTHF yields achieved previously.
Two different atmospheres were chosen for the experiments,
namely, N and H . In the activity tests performed under N
2
2
2
The results in ethanol were similar to those in water (<1%
MTHF yield). However, 10% MTHF yield was obtained in 1-bu-
tanol (1-BuOH), and nearly complete GVL conversion and 46%
pressure, the CTH was the principal source of hydrogen for the
reaction. In the reactions performed under H pressure, the re-
2
quired hydrogen could come from both molecular hydrogen
and CTH.
[
33]
MTHF yield were obtained in 2-propanol (2-PrOH). Converse-
[
35,36]
[37]
ly, high PDO
and MTHF yields were reported in aqueous
reactions using noble metal based catalysts, emphasizing that
further studies are required to fully understand this complex
reaction. The production of MTHF utilizing FA as hydrogen
source has also been reported. An MTHF yield of 72% was ob-
tained under microwave conditions when LA was fed with
Results and Discussion
The catalytic activities of the three catalysts in the solvents and
reaction atmospheres are summarized in Figure 2. For clarity,
the results of the experiments under N2 will be discussed
[
38]
a 170% excess of FA in the presence of Pd(5%)/C.
It is evident that the presence of a hydrogen donor in the
reaction medium, either an alcohol or FA, enhances the yield
of MTHF. To investigate the effects of the two possible hydro-
gen sources (molecular hydrogen and CTH) on the hydrogena-
tion of LA, this work presents a systematic study of the reac-
tion with a set of three solvents (1,4-dioxane, 1-BuOH, and
based on GVL yields, whereas the results under H atmosphere
2
will be discussed based on MTHF yields. The reason for this
consideration is that under H atmosphere, the reaction of LA
2
to GVL is very fast and reaches full conversion in all cases,
making activity comparisons impossible. Under N atmosphere,
2
as previously discussed, CTH reactions are the principal source
of hydrogen for the transformation of LA into GVL. However,
this in situ-generated hydrogen alone is insufficient to convert
2
-PrOH) and three catalysts (Ru(5%)/C, Ni(35%)/Al O , and
2 3
Ni(23%)–Cu (12%)/Al O ). These catalysts and solvents were se-
2
3
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Figure 2. LA conversion and product distribution for different catalysts, solvents, and reaction atmospheres. Reaction conditions: 5 wt% LA in solvent, LA/Cat.
ꢁ
1
1
0 gg , 2508C, 40 bar initial pressure and 5 h reaction time. The intermediates considered in this graph are AL, PDO, and LA esters. Quantified degradation
products are VA, 2-BuOH, 1-PeOH, and 2-PeOH. Other detected but not quantified degradation products: pentane, butane, methanol, and methane. The cor-
responding carbon balances are shown in Table S2.
the highly stable GVL into MTHF. These results are discussed in
detail in the following sections.
The performance of all the three catalysts improved with
2-PrOH as the solvent, which is a well-known hydrogen
[40]
donor. Complete conversion of LA was achieved and only
trace amounts of 2-propyl levulinate were detected in all
cases. The most noticeable change was observed for the Ru/C
catalyst, which achieved a 71% yield of GVL. The production of
GVL with the non-noble metal catalysts also increased, deliver-
ing 74–82% GVL yields and VA as the main by-product (3%).
The VA yield is expressed as the sum of free VA and VA esters.
Notably, none of the experiments that were performed
Activity tests under a N atmosphere
2
Low hydrogen availability provided by a poor hydrogen donor
(
1,4-dioxane) and a N atmosphere resulted in relatively low LA
2
conversions for the three tested catalysts. The Ru-based cata-
lyst allowed up to 20% LA conversion with GVL as the main
product. Interestingly, significantly higher LA conversions and
GVL yields (up to 53%) were achieved with Ni and Ni–Cu
based catalysts. The hydrogen required for the transformation
of LA to GVL was most probably provided by solvent degrada-
tion. Indeed, 1,4-dioxane derivatives (e.g., ethanol, 2-ethoxy-
ethanol, and ethanol 2-ethoxy methoxy) were detected among
the liquid reaction products using GC–MS. Additionally, neat
under N atmosphere produced significant amounts of MTHF
2
(<2%). To confirm these results, additional experiments using
GVL as substrate were performed in 2-PrOH under N atmos-
2
phere with the Ru/C and Ni–Cu/Al O3 catalysts. The results
2
were consistent and showed low MTHF yields. As shown in
Table 3, very low (ꢀ10%) GVL conversions were achieved
along with low MTHF yields (ꢀ2%) for both catalysts. The
main difference was the higher VA yield obtained by using Ni–
Cu/Al O and the higher 2-BuOH yield achieved by using Ru/C,
1,4-dioxane degradation experiments under N2 atmosphere
showed the presence of molecular hydrogen in the gas phase,
confirming the production of hydrogen by degradation of the
solvent or solvent derivatives. According to these data, it is
reasonable to argue that the low catalytic activity of Ru/C
under the reaction conditions can be related to its lower activi-
ty for 1,4-dioxane degradation and the use of the in situ-gener-
ated hydrogen in the conversion of LA to GVL.
2
3
in good agreement with the well-known decarbonylation activ-
[26]
ity of Ru catalysts.
The conversion of LA and the GVL yields were higher for
each catalyst when a better hydrogen donor solvent was used,
and for each solvent when a more CTH-active catalyst was
used. These results clearly indicate that under the applied reac-
tion conditions in situ-generated hydrogen alone is insufficient
to convert the highly stable GVL into MTHF even if a high
excess of active hydrogen donor molecules is used. In contrast
with the presented results, significant MTHF yields were report-
ed using FA as a hydrogen donor molecule both under micro-
A similar GVL yield (up to 10%) was obtained using Ru/C
[40]
and 1-BuOH, which is a moderate hydrogen donor molecule,
whereas the use of Ni–Cu/Al O resulted in a GVL yield of 65%.
2
3
In this solvent, full conversion of LA was achieved over the
three tested catalysts but with 27–90% yield of butyl levuli-
nate, a by-product caused by esterification.
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wave irradiation and fixed bed reactor conditions, however,
2-PrOH as solvent. This different behavior is indicative of more
complex interactions and reaction mechanisms in the presence
[
38]
the catalyst lost its activity after a short reaction time.
of a high H pressure. Nevertheless, for all the catalysts, the
2
highest MTHF yields were obtained when the best hydrogen
donor, 2-PrOH, was used as a solvent.
Activity tests under a H atmosphere
2
Full LA conversion was achieved by using Ru/C in 1,4-dioxane,
with a GVL yield of 85.3% and a MTHF yield of 3%. Similar GVL
and MTHF yields were observed using 1-BuOH as solvent. The
use of 2-PrOH as solvent triggered the production of MTHF
with a yield of 29%. However, up to 36% yield of degradation
products (mainly 2-BuOH and VA) were obtained using this re-
action system, which showed a lack of selectivity towards the
desired product. The large differences in the MTHF yield ob-
In terms of the carbon balances of these experiments, those
under N
atmosphere showed a higher deviation (79.6–104.3)
atmosphere (88.6–99.3), as
2
than those obtained under a H
2
shown in Table S2 (Supporting Information). This difference
can be associated with sufficient availability of hydrogen for
the reduction of the secondary reactions.
tained with each solvent under H atmosphere are in good
Hydrogenation mechanism
2
agreement with the results under N atmosphere. In addition,
2
In view of the important role of the CTH in the reaction under
N2 and H2 atmospheres, a direct correlation between the
amount of dehydrogenation products (acetone or butanal) re-
sulting from the solvent and the yield of the desired product
was expected. Although such a trend could not be found in
Figure 3 (left) some remarks are worth noting. As expected,
acetone and butanal were present at lower concentrations
under H2 atmosphere than under N2 atmosphere (Table 1).
Under H2 atmosphere, all the catalysts showed very similar
concentrations of the dehydrogenated donor near the thermo-
dynamic equilibrium conditions, which indicates that the reac-
tion SolventꢁH QH +Solvent reached equilibrium. Conversely,
significant concentrations of solvent dehydrogenation prod-
ucts (acetone and butanal) were detected in the experiments
under 100 bar H (Table 1). Similar to the experiments under
2
N , solvent dehydrogenation produced no measurable increase
2
in the total reaction pressure, thus, keeping the dissolved H2
concentration constant for each solvent.
Analogous conclusions were drawn from the results using
non-noble metal catalysts. Similar to the results under N at-
2
mosphere, the general trend was as follows: for each catalyst,
the better the hydrogen donating capacity of the solvent, the
higher the MTHF yield; and, for each solvent, the higher the
CTH activity of the catalyst, the higher the obtained MTHF
2
2
underN atmosphere, different concentrations of dehydrogena-
2
yield. However, two exceptions were found under H atmos-
2
tion products were measured depending on the catalysts
used, which, considering the reaction atmosphere, suggests
possible kinetic control in the solvent dehydrogenation reac-
tion. Nevertheless, considerable differences in the product
yields can be observed with similar solvent dehydrogenation
product concentrations. This implies that it is important for the
catalyst to effectively use the in situ produced hydrogen atoms
for the hydrogenation reactions, and to reduce their combina-
tion and desorption as hydrogen molecules.
phere. First, a higher MTHF yield was obtained using Ni–Cu/
Al O in 1,4-dioxane than in 1-BuOH. Second, a higher MTHF
2
3
yield was observed using Ru/C in comparison to Ni/Al O in
2
3
Table 1. Butanal and acetone concentrations for the different catalysts
and reaction atmospheres.
[
a]
ꢁ1
Entry
Product
Atmosphere
Concentration [mgg ]
Ru/C
Ni/Al
2
O
3
2 3
Ni–Cu/Al O
A linear trend can be observed between the hydrogenation
performance (plotted as MTHF yield) under H2 atmosphere
versus CTH performance of the system (plotted as GVL yield)
1
2
3
4
butanal
butanal
acetone
acetone
N
H
N
H
2
2
2
2
15.6
1.7
179.5
34.9
29.5
1.1
135.3
35.0
30.4
1.6
128.1
37.3
under N atmosphere, as shown in Figure 3 (right). This correla-
2
tion suggests that a catalyst that shows a high CTH per-
formance combined with a good hydrogen donor solvent pro-
motes the production of MTHF.
[
a] Equilibrium concentrations (for pure alcohol dehydrogenation reac-
tions) were calculated using Aspen Plus software (see the Supporting In-
formation for details).
ꢁ
1
Figure 3. Correlation between the solvent dehydrogenation product concentration (mgg of reaction media) and desired product yields (left) and correlation
between GVL yields under a N
2
atmosphere and MTHF yields under a H
2
atmosphere (right).
&
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In this study, the maximum MTHF yield was obtained with
the combination of Ni–Cu/Al O catalyst and 2-PrOH as solvent.
Considering that the reaction is composed of hydrogenation
and dehydrogenation steps, both hydrogenating and acidic
functionalities are expected to play a role. It is well known that
the CTH mechanism (the metal-hydride route) requires close
2
3
Nonetheless, in addition to the mentioned CTH active catalyst/
solvent system, a H atmosphere is necessary to convert the
2
[40]
stable GVL into MTHF. Based on this evidence, we suggest that
both sources of hydrogen, molecular hydrogen and the hydro-
gen donor, play a significant and synergetic role in the reac-
tion. The dynamic hydrogenation–dehydrogenation chemical
equilibrium of the donor increases the amount of adsorbed hy-
drogen atoms on the catalyst surface. Meanwhile, a high mo-
proximity of both functionalities. Therefore, the concentra-
tions of the metal and acid sites on the catalyst surface were
determined.
It appears likely that a higher concentration of active metal
sites in the catalyst could be the reason for the improved cata-
lytic performance of Ni–Cu/Al O . Therefore, CO chemisorption
2
3
lecular hydrogen pressure increases the H dissolved in the re-
measurements of the three fresh catalysts were performed
2
action medium, which in turn promotes the adsorption of hy-
drogen atoms and reduces their desorption rate. This im-
proved hydrogen availability seems necessary for the slow and
(Table 2). Ni/Al O3 contained the largest amount of active
2
metal sites, followed by Ru/C; surprisingly Ni–Cu/Al O showed
2
3
the lowest amount of sites, two times less than Ni/Al O . These
2
3
high-hydrogen-demanding reaction (2 mol H per mol GVL) of
results showed significant activity differences related to the
active metal sites of each catalyst. Normalized productivities
using 2-PrOH as a solvent under N and H atmospheres were
2
GVL to PDO and MTHF (Figure 1). The two possible hydrogen
sources are illustrated in Figure 4, which shows the LA to GVL
and GVL to MTHF reactions under a N and H atmosphere, re-
2
2
used for comparison purposes. Under a N atmosphere, the Ru
2
2
2
spectively.
metal sites were 1.4 times more active than the Ni sites, where-
as the active sites in the Ni–Cu catalyst were 2.6 times more
active. These differences increased under a H2 atmosphere,
Characterization of the catalyst was conducted to determine
the origin of the observed differences in the performance.
Figure 4. Proposed reaction mechanism of hydrogen adsorption, 2-PrOH dehydrogenation, and LA to GVL reaction under N
under H (right) atmosphere.
2
(left) and GVL to MTHF reaction
2
Table 2. Surface characterization of the fresh catalyst and normalized productivities.
[
a]
Cat.ꢁ1
[c]
ꢁ1
Entry
Catalyst
Amt. CO
Acidity [mmolNH3
Weak
g
]
Normalized productivity [h ]
Cat.ꢁ
1
2
2
MTHF (H )
[mmolCO
g
]
Medium
Strong
GVL (N
)
[
b]
[b]
1
2
3
Ru/C
Ni/Al
Ni–Cu/Al
0.0614
0.0895
0.0375
0.147 (3108C)
0.380 (3258C)
0.297 (3648C)
–
–
201
142
378
82
45
189
2
O
3
0.242 (5648C)
0.245 (5378C)
0.179 (7858C)
0.275 (7428C)
2
O
3
[a] The strength of the acidity was assessed by peak fitting. The maximum of the peaks is expressed in brackets. [b] Ru/C catalyst is decomposed during
the TPD analysis for temperatures above 4008C, hence, its acidity cannot be determined above this temperature [c] Normalized productivities are calculat-
ꢁ
1
ed by dividing the produced GVL or MTHF mol by the amount of metallic active sites (g Cat. xCO molg Cat.) and reaction time (5 h).
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Table 3. PDO, GVL, and MTHF conversion and product distribution for different catalysts and reaction atmospheres.
[
a]
Entry
Solvent
Gas
Catalyst
Conv. [%]
PDO
Yields [%]
MTHF
CB
[%]
VA
0.0
2-BuOH
0.0
1-PeOH
2-PeOH
0.0
Others
1
2
2-PrOH
2-PrOH
H
H
2
2
-
2.3
99.4
1.5
99.0
traces
0.4
0.0
0.8
99.2
100.8
Ni–Cu/Al
2
O
3
0.0
0.0
0.0
GVL
12.6
5.4
97.4
44.1
MTHF
1.9
VA
2-BuOH
1-PeOH
0.0
0.1
0.0
1.1
2-PeOH
Others
3
4
5
6
2-PrOH
2-PrOH
2-PrOH
2-PrOH
N
N
H
H
2
2
2
2
Ru/C
Ni–Cu/Al
Ru/C
3.3
4.6
1.8
8.5
3.0
0.5
37.3
2.5
0.4
0.0
18.9
0.5
0.7
0.0
9.2
0.5
96.7
102.3
84.7
2
O
O
3
3
2.5
14.9
30.3
Ni–Cu/Al
2
99.3
MTHF
98.5
10.3
–
–
–
VA
0.0
0.0
2-BuOH
1-PeOH
0.0
0.1
2-PeOH
Others
7
8
2-PrOH
2-PrOH
H
H
2
2
Ru/C
Ni–Cu/Al
5.4
1.0
51.8
3.8
15.8
0.3
74.5
94.9
2
O
3
[a] Carbon balances over 100% are probably caused by experimental or analytical errors.
whereby the Ru sites were 2 times more active and the Ni–Cu
catalyst were 4.2 times more active than the Ni sites.
a high activity with almost full conversion of GVL in 2-PrOH,
with MTHF, 2-BuOH, and 2-PeOH as the main products in
yields of 15, 37, and 19%, respectively. Similarly, MTHF was
fully converted over Ru/C in 2-PrOH with 2-PeOH as the main
product and small quantities of 2-BuOH. These experiments
(Table 3, Entries 5 and 7) clearly differentiate the origin of the
degradation products if Ru/C is used: The high yields of
2-BuOH observed in the LA hydrogenation experiments were
mainly produced from the conversion of GVL rather than the
degradation of MTHF, as MTHF degradation produced mainly
2-PeOH. Conversely, using Ni–Cu/Al O in 2-PrOH, 44% conver-
The superior performance of the active sites on Ni–Cu/Al O ,
2
3
as compared to Ni/Al O , is indicative of a significant promo-
2
3
tion effect by the addition of Cu, considering that Cu was
[33]
found to be a less active metal than Ni for this reaction. The
lower concentration of metal sites on the surface of Ni–Cu/
Al O reveals that Cu promotion effect was not caused by
2
3
metal dispersion enhancement, but rather by a bimetallic
effect produced by an especially active Ni–Cu mixed phase.
This bimetallic effect has previously been detected in this cata-
lyst, which showed incorporation of Cu in the Ni crystal struc-
2
3
sion of GVL was achieved with a MTHF yield of 30% and VA as
the main by-product (8%). Under the same conditions, in the
presence of this catalyst, MTHF conversion only reached 10%
with 2-PeOH as the main product. This catalyst, which showed
a significant activity for the challenging GVL conversion, also
showed high selectivity towards MTHF. Its low activity for the
degradation of MTHF makes it a suitable catalyst for selective
production of MTHF. Meanwhile, Ru/C is also a very active cata-
lyst for the conversion of GVL in the presence of 2-PrOH. How-
ever, the selectivity towards MTHF is considerably lower owing
to the formation of 2-BuOH from GVL and the high activity of
this catalyst for the degradation of MTHF (mainly to 2-PeOH).
Taking into consideration the presented activity results and
the features of the studied catalysts and solvents, the most
active and selective system was chosen for a more in-depth
analysis. Ni–Cu/Al O was found to produce the highest MTHF
[
33]
ture. The presented evidence is consistent with the literature
data, which shows improved CTH activity of Ni–Cu bimetallic
[41,42]
catalysts compared to Ni or Cu monometallic catalysts.
Acidity measurements revealed that Ni/Al O and Ni–Cu/Al O
2
3
2
3
3
ꢁ
1
had a similar total acidity (ꢀ0.8 mmolg ). Conversely, the NH
temperature programmed desorption (TPD) profiles showed
that Ni had a higher amount of weak acid sites whereas the
Ni–Cu catalyst had more strong acid sites (Table 2). The TPD
profile of Ru/C was difficult to analyze as further contributions
(including decomposition) could not be excluded. The profile
showed some weak acidity on this catalyst but, for tempera-
tures above 4008C, carbon is expected to decompose and,
hence, the measurements allowed no direct interpretation. The
low acidity of the Ru/C catalysts was emphasized in CTH stud-
ies in which partial oxidation of Ru was necessary to provide
the required acidity for the reaction mechanism. This catalyst
2
3
yields (35–40%) after 5 h reaction time in both 2-PrOH and
[
43]
was partially deactivated by in situ reduction, showing both
1,4-dioxane under a H atmosphere. As discussed above, these
2
[40]
metallic and acid sites to be necessary for CTH reactions.
Hence, the better performance of the Ni–Cu/Al O catalyst as
superior results are derived from a high efficiency of the cata-
lyst for CTH alongside high selectivity for the conversion of
GVL to MTHF and a very low activity for the degradation of
MTHF. 2-PrOH was selected as a green solvent alternative to
2
3
compared to Ru/C under the applied reaction conditions can
be ascribed to a balance amount of adjacent acidic and metal
active sites.
[44]
1,4-dioxane, which was not stable under the applied reac-
tion conditions.
Additionally, the reactivity of PDO, GVL, and MTHF in 2-PrOH
was analyzed to gain a deeper insight into the reaction mecha-
nism. PDO was found to be stable in the absence of a catalyst.
However, when using a catalyst, full conversion to MTHF was
achieved in less than 90 min with trace amounts of 2-BuOH,
The time evolution profile in Figure 5 showed complete con-
version of LA in less than one hour in 2-PrOH using Ni–Cu/
Al O under H atmosphere. The MTHF yield continuously in-
2
3
2
creased from 16% after the first hour to 80% after a 20 h reac-
tion time. GVL was the main product after short reaction times
1-PeOH, 2-PeOH, and GVL. As shown in Table 3, Ru/C showed
&
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Full Papers
Conclusions
The two possible sources of hydrogen (catalytic transfer hydro-
genation (CTH) and molecular hydrogen) for the conversion of
LA to MTHF were studied using three different catalysts. Al-
though hydrogenation through CTH or molecular H alone was
2
able to produce GVL yields of up to 82–93%, only trace
amounts of MTHF (<3%) were detected under these condi-
tions. The combination of both sources of hydrogen was indis-
pensable to achieve significant yields. Furthermore, the linear
relationship found between the results under N and H atmos-
2
2
pheres indicates the important role of CTH in the hydrogena-
tion of LA to MTHF even under a high H pressure. Hydrogen
2
pressure is considered to contribute to the reaction by increas-
ing the hydrogen dissolved in the reaction medium and, as
a consequence, by reducing hydrogen desorption from the
catalyst surface. This enhanced hydrogen availability allows an
efficient conversion of GVL and results in a high yield of MTHF.
Provided that the solvent was a good hydrogen donor, Ru/C
also proved to be a very active catalyst for the conversion of
LA and GVL. However, its selectivity towards MTHF from GVL is
low, and it is further hampered owing to the high activity for
the degradation of MTHF. In contrast, Ni–Cu/Al O showed the
Figure 5. Time evolution profile of the reaction in 2-PrOH with Ni–Cu/Al
Reaction conditions: 5 wt% LA in 2-PrOH, LA/Cat. 10 gg , 2508C, 40 bar H
initial pressure. The term “others” includes VA (up to 9.9% yield), 2-BuOH
2
O
3
.
ꢁ
1
2
(up to 2.3%), and 1-PeOH (up to 2.9%).
and steadily decreased from 77% after 1 h to 6% after 20 h.
The main by-product detected was VA, which reached 10%
yield after 20 h. Minor amounts of 2-BuOH and 1-PeOH were
also detected, reaching 2% and 3%, respectively, at the end of
the experiment.
2
3
best results, even using a poor hydrogen donor solvent such
as 1,4-dioxane. This catalyst was very active for the conversion
of the highly stable GVL into MTHF and showed very low activ-
ity for further transformations of MTHF, resulting in high MTHF
selectivity. Notably, this bimetallic catalyst, which produced the
highest MTHF yields, has the lowest active site concentration
among the tested catalysts. This confirmed the bimetallic pro-
motion effect of the catalyst, which produced less active sites
with much higher activity instead of improving the dispersion
of the metal. Overall, the use of Ni–Cu/Al O resulted in MTHF
Additional experiments were performed to show the poten-
tial applicability of this reaction system. As 2-PrOH readily de-
hydrogenates and produces acetone as part of the CTH mech-
anism, the activity of the system may decrease if acetone
builds up in the reaction mixture. This was investigated by per-
forming an activity test with 5 wt% LA in a mixture of 2-PrOH
and acetone (up to 4:1 weight ratio). The results showed that
even at this high initial acetone loading there was no decrease
in the activity of the system, which reached complete conver-
sion of LA with MTHF, GVL, and VA yields of 40, 39, and 6%, re-
spectively, after 5 h of reaction with 91% carbon balance. Fur-
thermore, the acetone concentration in the reaction mixture
2
3
yields as high as 80% using a good hydrogen donor solvent,
2-PrOH, under a H atmosphere after 20 h reaction time.
2
Experimental Section
ꢁ
1
decreased from 190 to 55 mgg at the end of the reaction,
only slightly above the concentration in the experiment with-
A commercial Ru(5%)/C catalyst was purchased from Sigma–
Aldrich and used without further modifications. The catalyst was
always preserved under a N2 atmosphere. Ni(35%)/Al O and
ꢁ
1
out acetone addition (37 mgg ). This fact, along with compa-
2
3
rable LA hydrogenation results, indicates that the 2-PrOHQace-
Ni(23%)–Cu(12%)/Al O catalysts were prepared using a simple
2 3
wet impregnation procedure reported previously. Briefly, the de-
[16]
tone+H reaction occurs much faster than the conversion of
2
sired amounts of g-Al O (Alfa-Aesar) were mixed with deionized
2
3
LA to GVL and MTHF. Consequently, the kinetics of the trans-
formation of 2-PrOH do not affect the conversion of LA. This
would be an important advantage for a possible scale-up of
the process since no ex situ acetone hydrogenation would be
required for solvent recycling in the reactor.
water in a 1:9 weight ratio and the appropriate amounts of metal
precursor salts (Ni(NO ) ·6H O, Sigma–Aldrich and
Cu(NO ) ·5/2H O, Alfa-Aesar) were added and stirred overnight at
3
2
2
3
2
2
9
0 rpm. Water was removed by heating the solution to 608C under
vacuum. The catalysts were then dried overnight at 1108C,
ꢁ
1
The use of more concentrated solutions is of paramount im-
portance for possible industrial application. Hence, an experi-
ment was performed with a 30 wt% LA solution in 2-PrOH,
keeping the LA/catalysts weight ratio constant at 10. The re-
sults were in good agreement with the results using the 5 wt%
feed and showed that full LA conversion was facilitated after
crushed, and calcined at 3008C for 2 h (2 Kmin ramp). Prior to
1
ꢁ
the activity tests, the catalysts were reduced at 4508C (10 Kmin
heating ramp) for 1 h under H2 flow and cooled down under Ar
flow.
Activity tests were performed in 50 mL Hastelloy autoclaves. The
typical reaction mixtures consisted of 5.3 g of a 5 wt% substrate in
solvent solution with a substrate/catalyst weight ratio of 10. The
autoclaves were fed with the reduced catalysts and the reaction
mixture, sealed, flushed three times with the appropriate gas,
loaded with 40 bar H or N , placed in preheated heating plates,
5
h of reaction time with MTHF, GVL, and VA yields of 47, 43,
and 6% at 98% carbon balance.
2
2
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7
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&
These are not the final page numbers! ÞÞ

Full Papers
and stirred at 500 rpm for 5 h. The autoclaves were cooled down,
and the pressure in the autoclaves was slowly released. The reac-
tion products were analyzed by GC-FID in an Agilent HP6890 series
device equipped with a CP-WAX-52-CB column (60 mꢂ0.25 mmꢂ
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ꢁ
ꢀ
ꢁ
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C
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[
[
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[
Keywords: 2-methyltetrahydrofuran · biofuel · heterogeneous
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Received: June 6, 2016
Published online on && &&, 0000
&
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8
ꢁ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

FULL PAPERS
Under H pressure! The important role
I. Obregꢀn, I. Gandarias, M. G. Al-Shaal,
C. Mevissen, P. L. Arias,* R. Palkovits
2
of both the H pressure and the catalyt-
2
ic transfer hydrogenation mechanism
was assessed in the one-pot production
of 2-methyltetrahydrofuran from levulin-
ic acid. The best yields, up to 80%, were
achieved using Ni–Cu/Al O catalysts in
&
& – &&
The Role of the Hydrogen Source on
the Selective Production of g-
Valerolactone and
2
3
2-propanol as a solvent.
2-Methyltetrahydrofuran from
Levulinic Acid
ChemSusChem 2016, 9, 1 – 9
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ꢁ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ