Preparation of valeric acid and valerate esters from biomass-derived levulinic acid using metal triflates + Pd/C

Article abstract of DOI:10.1039/c8gc01606b

Recently, great attention has been paid to the study of the conversion of biomass-derived compounds to value-Added chemicals. Levulinic acid (LA) is a versatile and valuable chemical and its various applications have been described. The selective conversion of biomass-derived LA to produce valeric acid and valerate esters was successfully performed in the presence of H₂, in which metal triflates and Pd/C were used as the catalysts. Under optimal conditions, a 99% conversion of LA and a 92% selectivity of valeric acid was obtained with Hf(OTf)₄ and Pd/C as the catalysts at a relatively low temperature of 150 °C. Moreover, the metal center of the catalyst, the solvent, the reaction temperature and other reaction conditions were studied. In addition, the results of the recycling experiment exhibited that the catalysts continued to have a satisfactory activity after being reused 5 times.

Full text of DOI:10.1039/c8gc01606b

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Preparation of valeric acid and valerate esters from biomass-
derived levulinic acid using metal triflates + Pd/C
Received 00th January 20xx,
Accepted 00th January 20xx
a, b
a
a
a
Jian Zhou, Rui Zhu, Jin Deng* and Yao Fu*
DOI: 10.1039/x0xx00000x
Currently, great attention has been paid on the study of the conversion of biomass-derived compounds to value-added
chemical. Levulinic acid (LA) is a versatile valuable chemical and its various applications have been described. The selective
conversion of biomass-derived LA to produce valeric acid and valerate esters has been successfully performed in the
www.rsc.org/
2
presence of H , in which metal triflates and Pd/C were used as the catalysts. Under the optimal conditions, 99% conversion
4
of LA and 92% selectivity of valeratic acid was obtained with Hf(OTf) and Pd/C as the catalysts at a relatively low
o
temperature of 150 C. Moreover, the metal center of the actalyst, the solvent, the reacrion temperature and other
reaction conditions were studied. In addition, the results of the recycling experiment exhibited that the catalysts remain a
satisfactory activity after reused for 5 times.
than current and alternative candidate biofuels (ethanol, nꢀ
butanol, EL, GVL, and 2ꢀMeTHF). Lange's method is based on
gasꢀphase catalytic reaction consisting of three steps performed
1
. Introduction
In current society, energy problems and the greenhouse effect
have already become one of studies of global change focus on.
in a flow reactor: (1) LA hydrogenation to GVL by Pt/TiO2,
followed by (2) GVL hydrogenation to VA by a Pt/HMFI
zeolite, and (3) VA esterification to valerate esters by an acidic
resin catalyst. The efficiency of this multiꢀstep process can be
improved by process integrations. Lange et al. reported the
singleꢀstep conversion of GVL to valeric esters over Pt or
The release of CO
that it absorbs when it grows, so these biomass resources will
go a long way toward reducing CO emissions and ending our
2 2
from biomass is equal to the amount of CO
2
dependence on fossil fuel. Biomass resources are renewable
and luxuriant. Through the physical, chemical, biological and
other methods, biomass resources can be transformed into a
2
Pd/TiO catalysts at 275–300°C under 10 bar, which resulted in
1
ꢀ4
20–50% selectivity of valeric esters. Since then, much works
have been carried out around LA to produce valerate esters.
variety of chemical products.
Among these chemical
products, levulinic acid (LA), as one of the important biomass
platform molecules, has attracted much attention because of its
9
Weckhuysen et al. reported the use of Ru/HZSMꢀ5 as a
5
, 6
heterogeneous catalyst to directly catalyze the conversion of
LA to VA and valerate esters, a yield of 45.8% of VA was
versatility and wide availability. It has been found that some
of its derivatives (such as γꢀvalerolactone (GVL), 2ꢀmethyl
tetrahydrofuran (2ꢀMeTHF), etc.) have the potential as fuels or
fuel additives. However, due to the expansion coefficient, the
miscibility with traditional fuel and other issues, these materials
1
0
achieved in dioxane as solvent. In the work of the Li group,
they used Co/HZSMꢀ5 as a catalyst and reacted at 240°C for 3
hours in a kettle reactor to give 97% yields of VA and valerate
1
1
7
esters. The Laura Prati group
mesoporous carbon (OMCs) and introduced acidic functional
groups containing Pꢀ and Sꢀ. Under the mild conditions (70 C,
introduced 1% Ru on
are difficult to apply to the combustion engine directly.
More promising routes to upgrade the use of LA are the
production of valeric acid (VA) and valeric acid esters, which
°
8
7 bar H ), the catalyst exhibited high selectivity (> 95%) in the
2
have been first proposed by Lange et al. Studies on the fuel
formation of GVL and showed a high stability after 5 cycles,
properties of the valeric biofuels showed that ethyl valerate
o
(
200 C, 40 bar H ), Ru/OMCꢀP can promote GVL ring opening
2
(EV) and methyl valerate (MV) have better fuel performance
and continuous hydrogenation to produce VA. In the previous
1
2
work of our group, it was proposed to use Ru/SBAꢀSO
3
H as a
a.
Hefei National Laboratory for Physical Sciences at the Microscale, iChEM, CAS Key
Laboratory of Urban Pollutant Conversion, Anhui Province Key Laboratory of
Biomass Clean Energy, Department of Chemistry, University of Science and
Technology of China, Hefei 230026, China. E-mail: dengjin@ustc.edu.cn;
fuyao@ustc.edu.cn.
Nano Science and Technology Institute University of Science and Technology of
China Suzhou 215123 (P. R. China)
Footnotes relating to the title and/or authors should appear here.
catalyst to react for 6 h in a kettle reactor at 250 , and finally
℃
the yield of VA and valerate esters was reached to 90%. The
valerate esters were prepared by one step and the problem that
the catalyst activity of the conventional Ru catalyst was not
high due to the lack of acidity in the reaction system. Shimizu
b.
†
13
et al. reported that the supported Pt/HMFI catalyst was used
Electronic Supplementary
DOI: 10.1039/x0xx00000x
Information
(ESI)
available.
See
in the preparation of VA from LA, and the VA was obtained by
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Scheme 1. Preparation of VA from LA as reported herein.
hydrogenation of LA with Ptꢀsupported HMFI molecular sieve
catalyst under solventꢀfree conditions. In the reaction system,
different alcohols were added to control the different valerate
2
. Experimental
The hydrogenation of LA was studied using a reactor equipped
esters obtained. In the study of reaction mechanism, they found with a 50 ml autoclave. 5 mmol of LA was added to the
that Brønsted acid carrier had a stronger effect on the structure autoclave, followed by the addition of a hydrogenation catalyst
of GVL than Lewis acid. And thus, the acid catalyzed site of and M(OTf)x, then the addition of a quantity of solvent. After
the reaction is the Brønsted acid site rather than the Lewis acid adding the stirring magnet, the reactor was installed. Filling
site. In summary, the current preparation of VA has made a hydrogen and replacing it three times. The reaction solution
great progress, but an efficient and mild preparation method is was heated to keep the temperature for several hours. After
still required.
Triflate acid is the most commonly known organic acid, and
completion of the reaction, 5 times the equivalent amount of
alcohol was added to the reaction kettle without removing the
nꢀoctane, reacted at 120°C for 2 hours, then cooled to room
temperature after the reaction. Gas chromatography (Shimadzu
GCꢀ2014, DM ꢀWAX column) and GCꢀMS (Thermal Trace GC
Ulter) for qualitative and quantitative detection.
the metal salts of triflate acid have been widely used in organic
synthesis methods such as electrophilic substitution reaction of
aromatic compounds, nucleophilic substitution reaction of
alkyne compounds, the nucleophilic addition reaction of
1
4ꢀ16
alcohol and other reactions.
Metal triflates are relatively
stable in physical and chemical properties, and more
importantly, they have higher reactivity and can catalyze the 3. Results and discussion
reaction under more moderate conditions, which has recently
3
.1 The effect of the catalyst type
attracted the attention of chemists. What we can see in Tobin J.
1
7
18
Marks
and our previous work
are that since the metal We mainly studied the process of catalytic conversion of LA to
triflates have a strong Lewis acidity, the metal ions can bind to VA by the metal triflates and metal supported hydrogenation
the electrons on the oxygen atoms of the ethers and promote the
break of CꢀO bond. Unlike the Brønsted acid catalyzed pathway
catalyst. We use the precious metal load on the activated carbon
carrier. Firstly, we investigated the effect of hydrogenation on
the catalytic hydrogenation of LA with different noble metals
on carbon. We mainly screened Ru, Pt, Pd (Table 1 ,Entries 1ꢀ
1
3
reported by Shimizu, we propose the use of metal triflates as
a Lewis acid to catalyze the conversion of LA and to make the
reaction more moderately effective. In this paper, we have
further developed this catalytic system with the use of
hydrogenation catalyst and metal triflates, achieved in a more
moderate conditions using one pot two steps to LA into VA and
valerate esters method, and can obtain 92% VA and valerate
esters yield. In addition, we also explored the effects of various
reaction conditions, such as the metal center of the catalyst, the
solvent, the reaction temperature, the reaction time, the
hydrogen pressure and so on.
3
). It can be seen that the hydrogenation effect of Pd/C is the
best among the three metalꢀsupported catalysts, and a yield of
2% of VA is obtained. Ru/C was less effective, obtaining a
9
19
yield of 81% VA. From the work of M. Albert Vannice we
know that the metal itself has an important effect on the
reaction kinetics. In this reaction system, the performance of
Pd/C shows the best.
2
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Then we further investigated the effect of the metal triflates
Lastly, we conducted controlled trials (Table 1, Entries 14ꢀ
with different metal centers. This group of experiments using 16). Comparing entry 3 and entry 14, it can be seen that the
Pd/C as a hydrogenation catalyst, nꢀoctane as a solvent, heated catalytic effect of HfCl4 was not good and VA was not
o
to 150 C after 6 hours. The product distribution is shown in obtained. These experiments illustrated that in addition to
ꢀ
Table 1. As can be seen from the table (Table 1, Entries 7ꢀ12) , cations, the anion (i.e., TfO ) in the metal triflate also played an
1
8, 19
the catalytic activities of Ce(OTf)
3
, Sm(OTf)
3
, Fe(OTf)
3
,
important role. According to our previous work
,
ꢀ
Ga(OTf) , La(OTf) , Er(OTf) were not high, the yields of VA nucleophilic TfO attacked the C=O bond activated by metal
3
3
3
4
were below 30%, while the catalytic activities of Hf(OTf) , and cations and formed active oxonium triflate. This reactive
Zr(OTf) were relatively high (Table 1, Entries 3,5), the yields intermediate (i.e. oxonium triflate) reduced the energy barrier
4
of VA reached 92% and 85% respectively. From the of the subsequent reaction and made the reaction easier. When
predecessors' work, we found that the catalytic activities of the only Pd/C was added (Entry 15), GVL was detected in the
metal triflates have a great relationship with the effective product and no VA was generated. When only Hf(OTf)
4
was
1
7ꢀ20
3+
4+
charge density of the metal center
. From Er to Hf , the added (Entry 16), neither GVL nor VA was detected.
effective charge density is gradually increased. The increase in Therefore, we finally chose Hf(OTf)
the effective charge density enhances the acidity of the Lewis optimized reaction catalyst.
4
and Pd/C as the
1
7
acid. Prior to this, Tobin J. Marks et al. reported that metal
triflates can effectively promote the ether compounds in the Cꢀ
O bond break. It was found that the higher the effective charge
density of the metal ions in the metal triflates, the more
advantageous breakpoint of the CꢀO bond. This rule is also
3
.2 The effect of the solvents
After the research about the catalysts, we further investigated
the effects of the solvents on the reaction. First of all, we tried a
solventꢀfree system at 150 C, 5MPa of H
o
2
. We first get αꢀ
consistent with the above experimental conclusion. However, angelica lactone and βꢀangelica lactone, and we found that the
from the experimental data in the above table Table 1, Entry 5 two lactones were polymerized and no VA was obtained under
（
6
+
）
we find that the effective charge density of W in W(OTf)6 solventꢀfree conditions by GCꢀMS detection. Therefore, we
followed the solvent screening. Fig. 1 shows the product
distribution in five different solvents of acetic acid (AcOH), nꢀ
octane, tetrahydrofuran (THF), dioxane and DMF. In these
typical solvents, LA was almost completely converted, the
product is mainly GVL and VA. The following reactions were
is also quite large, but its catalytic efficiency is not ideal, in
which the yield of VA is only 55 % and the yield of byproduct
GVL was 43%. This may due to the production of water in the
reaction since the W(OTf) is very sensitive to water, and thus
6
the catalytic effect is poor.
carried out at 150°C for 6h with Pd/C and Hf(OTf)
4
as catalyst.
1
8, 23
Based on our recent work
, we first selected AcOH as
Table 1. Product distributions for LA hydrogenation with various catalysts.
the reaction solvent. The experimental results showed that LA
was completely converted in the AcOH, but a mixture
containing GVL as a main product was obtained and the VA
yield was only 24%. Considering that the solubility to hydrogen
of AcOH is poor and the reaction rate is relatively slow, we
extended the reaction time from 6h to 12h, and the VA yield
can be further increased to 36%. Due to the strong hydrogen
bonding between the target product VA and the solvent AcOH,
subsequent separation may be difficult, so we did not further
optimize the reaction conditions to the AcOH solvent.
Considering the weak molecular interaction between the alkane
solvent and the carboxylic acid product, the greater solubility to
x
Reaction conditions: 5mmol of LA, 0.5mol% of Pd/C, 2mol% of M(OTf) ,
o
1
0 mL of nꢀoctane, 6 h, 150 C and 5 MPa H
2
Selectivity (%)
GVL
Entry
Catalyst
VA
90
81
92
55
85
52
28
24
26
22
18
18
--
1
2
3
4
5
6
7
8
9
Pt/C-Hf(OTf)
4
6
Ru/C-Hf(OTf)4
14
7
Pd/C-Hf(OTf)
Pd/C-W(OTf)
4
2
1
hydrogen and better heat and mass transfer, we selected nꢀ
octane as the solvent consequently. The experimental results
showed that LA was also completely converted in nꢀoctane and
VA was obtained in a yield of 92%. For reasons of such high
yields, we speculate that it may be due to the fact that the metal
triflate is only complexed with the oxygenꢀcontaining substrate
and is not affected by the nonꢀfunctionalized alkanes. Other
solvents, such as THF, dioxane and DMF, will cause a
complexation effect with Lewis acid and can form donor ligand
with the metal center. In addition, GCꢀMS showed that the
6
35
10
36
55
66
67
66
71
74
96
62
-
Pd/C-Zr(OTf)
Pd/C-Al(OTf)3
Pd/C-Fe(OTf)
Pd/C-Sm(OTf)3
Pd/C-Ce(OTf)
Pd/C-Ga(OTf)
Pd/C-La(OTf)
Pd/C-Er(OTf)
4
3
3
10
12
13
14
15
16
3
3
3
2
1
metal triflate promoted ringꢀopening polymerization of THF .
Therefore, these solvents containing polar functional groups
were not suitable for this reaction. Thus, nꢀoctane is selected as
the solvent for the reaction.
Pd/C-HfCl
Pd/C
4
-
Hf(OTf)
4
-
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tends to be stable. In view of the above, considering the
reaction conditions and the reaction yield and other aspects of
the impact, 150°C was chosen as the reaction temperature.
3
.4 The effect of hydrogen pressure
We continued to investigate the effect of the initial hydrogen
pressure on the reaction. The reaction was carried out with
Pd/C and Hf(OTf) as the reaction catalysts. The reaction was
4
carried out at 150°C for 6h. As shown in Fig.3, when the
hydrogen pressure of the reaction was 1 MPa, the yield of VA
is 56%; when the hydrogen pressure of the reaction was 2 MPa,
the yield of VA is 77%, so we speculate that the effect of
hydrogen pressure is still critical; then we continue to increase
the hydrogen pressure of the reaction and the yield of GVL
decreases to 11% when the hydrogen pressure of the reaction
increases to 3 MPa. When the rate of reaction increased to
Fig 1. Effects of solvents on the product selectivities for the hydrogenation of
4
MPa and 5MPa, LA was completely transformed.
LA at full conversion. Reaction conditions: 5mmol of LA, 0.5mol% of Pd/C
Therefore, we hypothesize that hydrogen pressure plays a
o
，
2mol% of Hf(OTf)
4
, 10 mL of solvent, 6 h, 150 C and 5 MPa H
2
. (a).
crucial role in the conversion of LA. When the hydrogen
pressure reaches 3MPa, the increasing of the yield of VA was
not obvious. Finally, 3 MPa hydrogen pressure was finalized as
the reaction condition for subsequent studies.
5
mmol of LA, 0.5mol% of Pd/C，2mol% of Hf(OTf)
4
, 10 mL of solvent, 12
o
2
h, 150 C and 5 MPa H .
3
.3 The effect of the reaction temperature
100
90
80
70
60
50
40
30
20
10
0
1
00
%
%
(GVL)
(VA)
80
60
40
20
0
Con
Yield of VA
1
2
3
4
5
Hydrogen pressure (MPa)
130
140
150
160
170
180
Temperature (℃)
Fig 3. Effects of initial hydrogen pressure on the product selectivities for the
hydrogenation of LA at full conversion. Reaction conditions: 5mmol of LA,
Fig 2. Effects of temperature on the product selectivities for the
hydrogenation of LA at full conversion. Reaction conditions: 5mmol of LA,
o
0
.5mol% of Pd/C, 2mol% of Hf(OTf)
4
, 150 C, 10 mL of nꢀoctane, and 6h.
0
4 2
.5mol% of Pd/C, 2mol% of Hf(OTf) , 10 mL of nꢀoctane, 6 h, and 5 MPa H
3
.5 The effect of Substrate concentration
After studying the effect of solvents on the reaction, we
investigated the effect of reaction temperature on the reaction
In the next, we explored the effect of substrate concentration on
the reaction results. It can be seen from the data in Fig.4 that
the selectivity of VA increased followed by a gradual decrease
when the concentration of LA increased. The selectivity of
GVL was significantly decreased. By GCꢀMS, it was found that
the polymer of the macromolecule appeared during the
reaction, and it was deduced that the intermediate product had
been polymerized. Therefore, as the substrate concentration
increases, the reaction was more prone to polymerization,
resulting in slower reaction. Thus, in conjunction with the
above data we can see that in this reaction system, as the
concentration of LA decreases, the efficiency of the reaction
increases accordingly.
4
process. Pd/C and Hf(OTf) were used as catalysts , the
selectivity of two products and reaction temperature as shown
in Fig. 2. Under the respective reaction conditions, LA was
completely converted. From the data we can see that when the
reaction temperature is reduced to 130°C, the yield of VA is
only about 71%. When the reaction temperature is 140°C, the
LA in the reaction system was also completely converted and
the yield of VA was 82%. Subsequently, the reaction
temperature was continuously increased. When the reaction
temperature was raised to 150°C, the yield of VA also
increased to 92%. When further increase the reaction
temperature to 160°C, 170°C, and 180°C, the reaction system
4
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3
.7 Reusability of the catalyst
VA
GVL
100
GVL
VA
1
00
80
60
40
20
0
8
0
0
0
0
0
)
(
6
4
2
0
.0
0.2
0.4
0.6
0.8
1.0
Substrate concentration (mmol/ml)
1
2
3
4
5
6
a
Cy cle i nd ex
Fig 4. Effects of Substrate concentration on the product selectivities for the
hydrogenation of LA at full conversion. Reaction conditions: 0.5mol% of
o
Pd/C，2mol% of Hf(OTf)
4
, 10 mL of nꢀoctane, 150 C, 6 h, and 5 MPa H2.
Fig 6. The reusability of Pd/C and Hf(OTf)
Reaction conditions:5mmol of LA, 0.5mol% of Pd/C，2mol% of Hf(OTf)
10 mL of nꢀoctane, 6h,150 °C and 5 MPa H . (a). Reaction conditions:5mmol
4
at full conversion of LA.
4
,
3
.6 Dynamics research
2
of LA, 0.5mol% of Pd/C，2mol% of Hf(OTf)
C and 5 MPa H
4
, 10 mL of nꢀoctane, 12h,150
°
2
.
1
00
In order to evaluate the efficiency of the catalytic system in
practical application, we conducted a catalyst for repeatability
studies. Since the metal triflate is a homogeneous catalyst and
is easy to hydrolyze, the catalyst cycle we designed is that the
Pd/C is filtered off and the metal triflate is added again. The
results were listed in Fig 6. The yield of VA was declined
slightly after five cycles, which indicated that the catalytic
effect of Pd/C decreased. To further illustrate this issue, ICP
test (SI, Table S1) and BET test (SI, Table S2, Figure S1,
Figure S2) were conducted on fresh Pd/C and used Pd/C
respectively. The ICP test showed that after five cycles of
recycling, the Pd content in Pd/C decreased and Pd has lost.
This has become an important reason for the decline in Pd/C
catalytic activity. From the result of BET test, after five cycles
of Pd/C recycling, the specific surface area was significantly
8
6
4
2
0
0
0
0
0
%
%
%
(Con)
(GVL)
(VA)
0
2
4
6
8
10
12
14
16
Reaction time (h)
Fig 5. Formation of GVL, VA as a function of reaction time for the
hydrogenation of LA using Pd/C and Hf(OTf) . Reaction conditions: 5mmol
of LA, 0.5mol% of Pd/C, 2mol% of Hf(OTf) , 10 mL of nꢀoctane 130 °C,
and 5 MPa H
4
4
2
In order to elucidate the reaction mechanism of the reaction
system catalyzing the conversion of LA to VA, we investigated
the reaction kinetics of LA hydrogenation process. The
2
1
selectivity distribution of LA hydrogenated products with the decreased. Combined with our previous work , the polymer
reaction time is shown in Fig 5. We monitored the selectivity
produced by the reaction blocked the pores of the catalyst and
distribution of the product in the range of 1 h to 15 h. After 0.5
the catalyst also agglomerated. In addition, the operating losses
h
of reaction, LA has been substantially completely
transformed, and the product at this time is predominantly during the catalyst recovery process could also lead to a
GVL. With the increasing of reaction time, the selectivity of
VA gradually increased, and the selectivity of GVL decreased
gradually. On the basis of previous studies, combined with the
reduction of the yields. We extended the reaction time after the
catalyst was used for five times (Fig 6) and found that the yield
4
experimental results, we conclude that Pd/CꢀHf(OTf) catalyzes of VA increased. Therefore, the reduced catalytic efficiency of
the conversion of LA to VA as shown in Fig.5. First, LA is
catalyzed by rapid hydrogenation to GVL, followed by GVL
conversion under the catalysis of Lewis acid to hydrogenation
to VA.
the reused Pd/C can be compensated by means of prolonging
the reaction time.
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catalyst and the metal center in the Lewis acid is critical to the
reaction system.
3
.8 Preparation of various alkyl valerate esters
After exploring the optimum conditions for the preparation of
VA, we continue to explore the preparation of valerate esters
under optimal conditions. In the work of Shawn K. Collins, we
know that metal triflate can catalyze the direct esterification of
Conflicts of interest
2
4
carboxylic acids and alcohols . In our work, a oneꢀpot twoꢀstep There are no conflicts to declare.
method was used. After converting the LA to VA, different
alcohols were added into the original system and the
corresponding valerate esters can be achieved. From the above
data above, valerate esters were substantially completely
Acknowledgements
This work was supported by the NSFC (21732006, 21572212,
1402181), MOST (2017YFA0303502), CAS (YZ201563), FRFCU
converted except for the tertꢀbutanol. It is judged that this may
be because the esterification mechanism of tertꢀbutyl alcohol is
2
different from other alcohols. The esterification mechanism of (WK3530000004) and PCSIRT. The authors thank the Hefei Leaf
tertꢀbutyl alcohol is the cleavage of the alkoxy bond, which is Biotech Co., Ltd. and Anhui Kemi Machinery Technology Co.,
difficult to be carried out by the addition elimination Ltd. for free samples and equipment that benefited our ability
mechanism, and the steric hindrance is too large, so it is
difficult to carry out esterification. It can also be seen that the
yield of the corresponding valerate esters also decreases as the
chain length of the alcohol increases.
to conduct this study.
Notes and references
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Fig 7. Preparation of various alkyl valerate esters over Pd/C and Hf(OTf) at
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In summary, the effective charge density of the metal center
and the stability of the metal triflate have an important effect in
the process of catalyzing the production of VA and valerate
esters. We have developed an one pot two step method
preparation of valerate esters from biomassꢀderived LA by
using Pd/CꢀHf(OTf) . We first studied the distribution of the
4
reaction product in different metal centers. The selectivity of
VA in the product increased with the effective charge density of
1
1
1
1
4
2
3
7. Z Li, R. S. Assary, A. C. Atesin, L. A. Curtiss, T. J. Marks, J.
Am. Chem. Soc, 2013, 136, 104ꢀ107.
the metal center, and finally reached 92% when using 18. R. Zhu, J ꢀL. Jiang, X ꢀL. Li, J. Deng, Y. Fu, ACS Catalysis,
Hf(OTf) , (LA conversion>99%). Through the dynamics of
2017, 7, 7520ꢀ7528.
4
research and combined with the work of predecessors, possible 19.
Z
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ꢀY. Xie, J. Deng, Y. Fu, ChemSusChem, 2018,
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ways of reaction was put forward. First, the catalytic
2
2
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Wu, Y. Fu, ChemSusChem, 2015, 8, 4250ꢀ4255.
hydrogenation of LA was converted to GVL, then GVL was
hydrogenated to produce VA. Finally, in the original system by
adding different chain length of the fatty alcohol to get a variety
of corresponding valerate esters. In summary, the
hydrogenation activity of the appropriate hydrogenation
2
2
2. K. Zhang, X. L. Li, S. Y. Chen, H. J. Xu, J. Deng, Y. Fu,
ChemSusChem, 2018, 11, 726ꢀ734.
3. X ꢀL. Li, K. Zhang, J ꢀL. Jiang, R. Zhu, W ꢀP. Wu, J. Deng and
6
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ARTICLE
Y. Fu, Green Chem, 2018, 20, 362ꢀ368.
4. M. de Léséleuc, S. K. Collins, ACS Catalysis, 2015, 5, 1462ꢀ
467.
2
1
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