Synthesis of Butyl Levulinate Based on Α-Angelica Lactone in the Presence of Easily Separable Heteropoly Acid Catalysts

Article abstract of DOI:10.1002/cssc.201601882

A series of choline (Ch)-exchanged heteropoly acids (HOCH₂CH₂N(CH₃)₃)_xH_(6?x)P₂W₁₈O₆₂ [abbreviated as Ch_xH_(6?x)P₂W₁₈O₆₂, x=1–6] was synthesized and used as catalysts for the reaction of α-angelica lactone (alpha-AL) with n-butanol to form butyl levulinate (BL). The solubility of Ch_xH_(6?x)P₂W₁₈O₆₂ in the reaction mixture was temperature dependent: The catalysts were soluble under the reaction conditions and precipitated upon cooling of the reaction mixture. This facilitated recovery of the catalysts from the liquid phase. Importantly, an increase of the Ch content caused a decrease of the catalyst solubility. Catalytic activity of Ch_xH_(6?x)P₂W₁₈O₆₂ for the reaction with n-butanol appeared to be in good agreement with the concentration of Br?nsted-acidic sites. The results suggest that the reaction proceeded through formation of pseudo-butyl levulinate as intermediate. Ch₂H₄P₂W₁₈O₆₂ exhibited the best balance between catalytic activity and temperature-dependent solubility. The yield of BL reached 79.4 % at full conversion of alpha-AL at a moderate temperature of 75 °C in an open system. Ch_xH_(6?x)P₂W₁₈O₆₂ could be successfully reused five times without significant loss of activity.

Full text of DOI:10.1002/cssc.201601882

Accepted Article
Title: Synthesis of butyl levulinate based on α-Angelica Lactone in the
presence of easily separable heteropoly acid catalysts
Authors: Xiaohu Yi, Ghith Al Shaal Mohammad, Ciptonugroho
Wirawan, Irina Delidovich, Xiaohong Wang, and Regina
Palkovits
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10.1002/cssc.201601882
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Synthesis of butyl levulinate based on α-Angelica Lactone in the
presence of easily separable heteropoly acid catalysts
Xiaohu Yi,a,b Mohammad G. Al-Shaal,b Wirawan Ciptonugroho,b Irina Delidovich,b Xiaohong Wang*a
and Regina Palkovits*b
Abstract:
A
series of choline exchanged heteropoly acids
[abbreviated as ChxH(6-
Though plants synthesize a great amount of carbohydrates from
CO2 and H2O utilizing sunlight as an energy source, only ca. 4%
of saccharides are currently used in industry.[2] Levulinic acid
esters (LAEs) are organic compounds able to improve the cold
flow properties of biodiesel and to serve as oxygenate additives
for diesel fuels and gasoline. Their good performance is
attributed to high lubricity, low toxicity and moderate flow
properties at low temperature. [3] LAEs can be industrially utilized
as polymer plasticizers,[4] green solvents,[5] and fragrances[6].
Consequently, the development of novel technologies for the
production of LAEs based on lignocellulose has lately attracted
great attention.[3a, 7] The production of LAEs by direct catalytic
transformation of polysaccharides is feasible, but requires harsh
reaction conditions,[8] such as high temperature (175oC) and
long reaction time (20h).[8b] LAEs can also be manufactured from
mono- and disaccharides. A reaction sequence facilitating the
efficient transformation of monosaccharides into LAEs covers (1)
dehydration of the monosaccharide into 5-hydroxymethylfurfural
(5-HMF), (2) conversion of the latter into levulinic acid (LA), and
(3) LA esterification. This cascade is especially efficient for
fructose as feedstock, though lignocellulose clearly gives rise to
glucose. Consequently, efficient glucose-fructose isomerization
processes are indispensible.[9] Several solid catalysts have been
tested for the transformation of saccharides into LAEs, such as
sulfonated materials,[10] acidic ionic liquids,[11] H-USY zeolite,[12]
and heteropoly acids (abbreviated as HPAs).[13] Additionally,
several studies focused on the production of LAEs from furfuryl
alcohol as a starting material. Again acidic catalysts, for example
ionic liquids,[14] ion exchange resin,[15] sulfonated materials,[16]
organosilica HPAs hybrid nanotubes,[17] proved to be efficient for
this transformation.
(HOCH2CH2N(CH3)3)XH(6-X)P2W18O62
X)P2W18O62 (x=1-6), Choline cation abbreviated as Ch+] was
synthesized and used as catalysts for the reaction of α-angelica
lactone (abbreviated as alpha-AL) with n-butanol forming butyl
levulinate. The solubility of ChxH(6-X)P2W18O62 in the reaction mixture
is temperature-dependent: The catalysts are soluble under the
reaction conditions and precipitate upon cooling the reaction mixture.
This facilitates recovery of the catalysts out of the liquid phase.
Importantly, an increase of the Ch content causes a decrease of the
catalysts’ solubility. Catalytic activity of ChxH(6-X)P2W18O62 for the
reaction with n-butanol appeared to be in good agreement with the
concentration of Brönsted acidic sites. The results suggest that the
reaction takes place via formation of pseudo-butyl levulinate as
intermediate. Ch2H4P2W18O62 exhibited the best balance between
catalytic activity and temperature-dependent solubility. The yield of
BL reached 79.4% at full conversion of alpha-AL at a moderate
temperature of 75°C in an open system. ChxH(6-X)P2W18O62 could be
successfully reused 5 times without significant loss of activity.
Introduction
Due to the excess exploitation and predictable depletion of
fossil feedstocks, renewable energy sources find increasingly
attention. Carbohydrate-containing biomass presents a suitable
candidate for synthesis of renewable fuels and fuel additives.[1]
[a]
[b]
X. Yi, Prof. Dr. X. Wang
Key Lab of Polyoxometalate Science of Ministry of Education
Northeast Normal University, Changchun 130024, P. R. China
E-mail: wangxh665@nenu.edu.cn
X. Yi, M.G.Al-Shaal, W. Ciptonogroho, Dr. I. Delidovich, Prof. Dr. R.
Palkovits
The direct esterification of LA with an alcohol remains the
most frequently utilized approach for synthesis of LAEs, e.g.
catalyzed by solid acids such as HPAs-based heterogeneous
catalysts[18] as well as sulfonated materials.[19] However, the
yield of LAEs is limited by the thermodynamic equilibrium. As a
result, esterification is conducted in the presence of a great
Chair of Heterogeneous Catalysis and Chemical Technology
RWTH Aachen University, Worringerweg 2, 52074 Aachen,
Germany
E-mail: Palkovits@itmc.rwth-aachen.de
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
1
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10.1002/cssc.201601882
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excess of alcohol in order to shift the equilibrium towards
product formation.[2c, 18b] Consequently, LAEs are formed in low
concentration in alcohols as substrate and solvent causing
addition costs for product separation. In this regard, synthesis of
LAEs by an equimolar addition reaction of an alcohol with alpha-
AL minimizes extensive separation procedures. Alpha-AL is
readily available by dehydration of LA.[20] Manzer reported the
production of LAEs based on alpha-AL at elevated temperatures
of 100°C to 150°C over solid acids.[21] Recently, Al-Shaal et al.
demonstrated high catalytic activity of Amberlyst-36 for the
conversion of alpha-AL into butyl levulinate (BL) with 98.4%
yield.[22] They also uncovered pseudo-LAEs (p-LAEs) as
intermediates, which can be further transformed to LAEs over
acidic sites of sufficient strength. However, all the reported
protocols were implemented under anhydrous conditions, e.g.
under inert gas atmosphere, as a special precaution to prevent
rehydration of alpha-AL to LA.[21, 22]
(inter bridge oxygen between corner-sharing WO6 octahedra,
W
895 cm−1), and VW-Oe-W (intra bridge oxygen between edge-
sharing WO6 octahedra, 716 cm−1), though somewhat shifted
owing to exchange of protons with Ch.[24] Additionally, the bands
at 3037 and 1468 cm−1 were attributed to C-H and C-N,
respectively.
Powder X-ray diffraction patterns were used for further
investigating the structure of ChxH(6-X)P2W18O62 and the parent
HPA (Fig. S2). The latter demonstrates reflections typical of a
Dawson-structured HPAs, e.g. resulting in an intensive reflection
at 2θ ca. 25 °. ChxH(6-X)P2W18O62 also exhibit crystalline structure,
though the reflections possess lower intensity indicating reduced
crystallinity upon introduction of choline cations. Additionally, a
shift of the reflections characteristic of the Dawson anion (2θ 20-
40°) towards lower angles takes place, whereas a strong
reflection at 2θ of ca. 8° appears. The latter can be explained by
formation of planes with large interplanar spacing upon
exchange of protons with bulky choline cations. [25]
Herein, we report choline-exchanged Dawson-type heteropoly
acids (ChxH(6-X)P2W18O62, where Ch means the choline cation
The catalysts were studied by 31P MAS NMR (Fig. S3). The
+
[24]
HOCH2CH2N(Me)3 ) as recyclable catalysts for the reaction of
parent HPAs results in a resonance at -12.9 ppm.
Upon
butanol with alpha-AL. These catalysts exhibit a temperature-
dependent solubility, being soluble at elevated temperature and
hardly soluble at room temperature.[23] As a result, the dissolved
HPAs efficiently catalyze the reaction of butanol with alpha-AL
by molecular Brönsted acidic species avoiding potential
diffusional hindrances often encountered in heterogeneous
catalysis. The HPAs precipitate upon cooling the reaction
mixture to room temperature, facilitating catalyst separation and
recycling. These ChxH(6-X)P2W18O62 catalysts demonstrate
excellent catalytic activity and selectivity for BL production
already at very mild reaction temperatures of only 75°C even
working in air, instead of an inert atmosphere.
introduction of the choline cation, a shift of the signal to -14.43
ppm occurred due to a change of the chemical environment of
the P2W18O626- anion.
Ch2H4P2W18O62 was characterized by high resolution TEM
and energy-dispersive X-ray techniques. According to TEM data,
Ch2H4P2W18O62 forms relatively irregular micellar particles with
sizes ranging from 10 nm to 20 nm (Fig S4(a)). Moreover, in Fig
S4(b), EDX analysis confirms that the material contains
phosphorous, tungsten and oxygen of the P2W18O626- heteropoly
anion, as well as carbon and nitrogen from the choline cation.
The critical micelle concentration (CMC) of ChXH(6-X)P2W18O62
was determined (Fig. S5). Two straight lines exhibiting inflection
points at ca. 0.89 mM are present at a conductivity graph plotted
versus concentration of the catalysts. This points at the
assemblance of ChXH(6-X)P2W18O62 amphiphilic molecules into
micelles.[23b]
Results and Discussion
Characterization of catalysts
The Brönsted acid strength (Hammett acidity) of ChxH(6-
x)P2W18O62 was measured by using a UV-visible indicator 4-
nitroaniline possessing a Hammett constant of 0.99.[26] The
obtained spectra are shown in Figure S6, and the calculated Ho
values are listed in Table S2. In general, the lower the Ho of an
acid is, the stronger is the Brönsted acidity of the catalyst. As
expected, the Ho values increase along with increasing choline
content. The Brönsted acid strength decreases in the order of
A series of catalysts, ChxH(6-x)P2W18O62 with x=1-6, was
prepared by partial exchange of protons of the HPAs into the
choline cation. The elemental compositions of these catalysts
are listed in Table S1. The contents of C, N, P, W found by
elemental analysis of the catalysts are in good agreement with
the calculated values.
FTIR spectra of ChxH(6-X)P2W18O62 are shown in Fig S1. They
exhibit four characteristic signals for ChxH(6-X)P2W18O62 (x=1-6) at
1084, 954, 901, and 735 cm−1, respectively. These bands
correspond to the heteropoly anion, e.g. P-O stretching
vibrations (1081 cm−1), VW-Od (terminal oxygen, 958 cm−1), VW-Oc-
H6P2W18O62
>
Ch1H5P2W18O62
Ch4H2P2W18O62
>
Ch2H4P2W18O62
> Ch5H1P2W18O62
>
>
Ch3H3P2W18O62
Ch6P2W18O62.
>
2
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Scheme 1. Plausible reaction network of BLformation based on alpha-AL and BuOH in the presence of heteropoly acids
DRIFT pyridine adsorption enabled a further characterization
of the catalysts’ surface acidity (Fig S7). Pyridine molecules
interact with protons of HPAs. Accordingly, a signal at 1540 cm-1
emerges corresponding to Brönsted acidic sites and confirms
this interaction.[27] The presence of a band at 1490 cm-1
correlates to both Brönsted and Lewis acidity, though HPAs
typically exhibit Brönsted acid sites only. Additionally, titration
with n-butylamine allowed determining the total acid content
(Table 1)
was identified in the reaction mixtures, especially in the
presence of catalysts with low proton content, such as
Ch5H1P2W18O62 or Ch6P2W18O62. This confirms reaction pathway
1 as the main pathway of BL formation over the studied HPAs-
based catalysts. Additionally, LA formed with a moderate yield in
the range of 3-10% (Fig. 1). The formation of LA can take place
either via reaction pathway 2 or owing to hydrolysis of BL by
traces of water present in the reaction mixture.
Catalytic activity of ChxH(6-X)P2W18O62
We tested the reaction between n-butanol and alpha-AL over
pure HPAs, the series of ChxH(6-x)P2W18O62 (x=1,2,3,4,5 and 6)
as well as pure ChCl. A blank reaction without any catalyst was
also carried out (Figure 1). The screening was carried out using
equimolar ratios of alpha-AL and n-butanol at 75 °C for 1 h.
HPAs and all ChxH(6-x)P2W18O62 catalysts enabled full conversion
of alpha-AL in a short time. Under the same conditions, the
alpha-AL conversion was very low over ChCl (13%) as well as in
the absence of any catalyst (5%), respectively. Figure
1
demonstrates the results of the catalytic tests indicating that the
yield of BL correlates well with Brönsted acidity of the catalysts.
Analysis of the reaction mixture showed the presence of p-BL
along with LA. In line, Scheme 1 summarizes the two possible
reaction pathways of BL formation as recently reported by Al-
Shaal et al. in the presence of Amberlyst 36.[22] P-BL presents
an intermediate of reaction pathway 1. Subsequent ring opening
of p-BL to BL could be identified as rate-determining step and
requires the presence of a catalyst of sufficient strength of the
acidic sites. An alternative reaction pathway 2 comprehends an
addition of water to a molecule of alpha-AL forming pseudo-
levulinic acid followed by ring opening. A great amount of p-BL
Fig 1. Synthesis of BL by reaction between BuOH and alpha-AL in the
presence of pure H6P2W18O62, different ChxH(6-x)P2W18O62, and pure choline.
No catalyst was added to the reaction mixture for the blank reaction.
Conditions: alpha-AL (0.625 g, 6.375 mmol), BuOH (0.473 g, 6.375 mmol),
catalyst (20 wt% = 125.12 mg), 75°C, 1h
Optimizing the catalyst composition for the proposed reaction
requires a compromise between catalytic activity and solubility.
For example, molecular HPAs facilitates a superior yield of BL,
though its separation from the reaction mixture poses a major
challenge. The exchange of protons by choline cations leads to
3
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Table 1 Determination of acidic properties for ChXH(6-X)P2W18O62 and yield of BL a
Catalyst
Total acidity content b (mmol.g-1)
Ho c
Yield of BL d (%)
H6P2W18O62
3.94
2.54
2.32
1.23
0.86
0.98
1.6
95.1
84.5
79.4
38.2
21.5
Ch1H5P2W18O62
Ch2H4P2W18O62
Ch3H3P2W18O62
Ch4H2P2W18O62
1.61
1.86
1.92
Ch5H1P2W18O62
Ch6P2W18O62
0.56
0.48
1.96
2.04
9.9
9.2
a
Reaction conditions: alpha-AL (0.625 g, 6.375 mmol), BuOH (0.437 g, 6.375 mmol), catalyst (20 wt%=125.12 mg),
75 , 1h.
℃
b 0.05 g of a solid was suspended in 45 mL of acetonitrile and stirred for 3 h under 60oC. Then the suspension was
titrated with a solution of n-butylamine in acetonitrile (0.05 M) using the indicator anthraquinone (pKa=−8.2 ). Total
acidity is represented only by Brönsted acidity.
c
A sample of the catalyst (100 mg) and 100 mg Hammett indicator (p-nitroaniline, pK(I)aq= 0.99) were dissolved in
20ml of twice distilled H2O and stirred for 12h. Hammett acidity functions were determined by UV-visible
spectroscopy at 380 nm.
d The yield is determined by GC.
decreasing solubility of the HPAs at room temperature, with the
lowest solubility for the highest degree of cation exchange,
though an exchange of protons with two choline cations already
leads to very low solubility. At the same time, the presence of
the protons is of utmost importance for the catalysis (Fig. 1). In
this regard, Ch2H4P2W18O62 demonstrated high catalytic activity
in a row: methanol>ethanol>butanol>iso-propanol, pointing at
the impact of steric hindrances. In all cases the corresponding
levulinate was obtained in high yield in the range of 72-90% in
only 1 h reaction at 75 °C.
towards BL formation,
a very low solubility at ambient
temperature as well as high solubility at elevated temperature.
Therefore, we selected Ch2H4P2W18O62 for further investigations
and optimization of the reaction conditions.
(a)
(b)
(c)
Fig. 2 demonstrates the temperature-dependent solubility of
Ch2H4P2W18O62 catalyst. The successful precipitation can be
visually noticed. Moreover, ICP-OES analysis of the reaction
solution after removal of the catalyst by filtration emphasized
recyclability with P and W amounts under detection limit.
Nevertheless, it is noticeable that the reaction solution turned
red after the reaction. The origin of this coloration is not fully
clear yet, but could be related to side reactions. Furthermore,
the Ch2H4P2W18O62 appeared to be applicable for the
transformation of alpha-AL with other alcohols, not only BuOH.
The results of reactions with MeOH, EtOH, and i-PrOH are
provided in Table S3. In all cases the corresponding pseudo-
levulinate intermediate formed. The reaction rate decreases
Fig 2. Photographs of Ch2H4P2W18O2 in the presence of reaction mixture (a)
before reaction at room temperature (b) during reaction at 75°C (c) after
reaction and cooling to room temperature.
Substrate adsorption on Ch2H4P2W18O62 was studied.
According to TEM analysis (Fig. S4a), self-assembly of
Ch2H4P2W18O62 enables formation of micellar spheres (Scheme
S1). These spheres interact with hydrophobic organic substrates
and products as evidenced by FTIR spectroscopy of
Ch2H4P2W18O62 before and after catalytic reaction (Fig. S8).
Indeed, the IR spectrum of the used catalyst exhibits
characteristic vibration bands of heteropoly anions with Dawson
6-
structure P2W18O62 at 1090 (VP-Oa), 961(VW-Od), 912(VW-Ob-W),
775(VW-Oc-W) cm-1. Moreover, several additional vibration bands
4
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Fig 3. Conversion of alpha-AL (black) and yields for BL (red), Pseudo-BL (blue), LA (pink) observed under the following reaction conditions: (a) alpha-AL (0.625 g,
6.375 mmol), BuOH (0.437 g, 6.375 mmol), catalyst (20 wt%=125.12 mg), 1h. (b) alpha-AL (0.625 g, 6.375 mmol), BuOH (0.437 g, 6.375 mmol), 75 °C, 1h
appeared including characteristic VC-C signals at 1370 cm-1 and
1540 cm-1. These were assigned to the alkyl chain of BL or
BuOH. The characteristic VC=O bands at 1750 cm-1 to 1735cm-1
were assigned to the C=O vibration of the ester group, which
may be attributed to p-BL as well as BL, respectively.
of BL are equal 95.1%, 84.5%, 79.4% over HPAs,
Ch1H5P2W18O62 and Ch2H4P2W18O62, respectively. A noticeable
deviation in BL yield was observed, though the same amount of
produced BL in the presence of all the catalysts would be
expected. This can be explained by deviating contents of water
in the catalysts. Due to high polarity of the heteropoly anion,
water sorption upon exposure to air cannot be fully excluded.
The presence of water causes hydrolysis of BL, therefore,
somewhat lower yield of BL in the presence of higher moisture-
content could occur. Importantly, BL demonstrated excellent
stability in the presence of HPAs as well as the choline-
containing salts under reaction conditions for at least 3 hours.
On the contrary, the yield of LA reached its maximum of 7-10%
in 10 minutes, thereafter a slow but steady decrease of the LA
concentration occurred, apparently due to decomposition of LA.
Temperature screening revealed BL, p-BL, and LA as main
reaction products in the range of 25-100 °C (Fig. 3a). The
maximum yield of BL appeared to be nearly the same at 75 and
100 °C indicating a weak dependence of the equilibrium on
temperature (Fig. 3a). For a catalyst loading higher than 20 wt.%,
the maximum yield is reached within 1 h. (Fig. 3b)
Comparing the results on catalytic activity of HPAs with the
literature data is of interest. Only scares data on the interaction
of alpha-AL with alcohols is currently available. The catalytic
activity of acids was found to be superior to bases. According to
Manzer, Dabco® as base catalyst gave rise to 58% yield of BL
only [21a]. Amberlyst® 15 and Amberlyst® 36 as acid catalysts
enabled the highest reported yield of BL of 98% [21b, 22]. Catalytic
activity of the Amberlyst® resins exceeds that of other ion-
[21b]
exchange resins,
HBEA zeolites, sulfonated ZrO2, and
WO3/ZrO2 [22]. Reported here yield of BL of ca. 80% is somewhat
lower, though the reaction rate over Ch2H4P2W18O62 surpasses
the previously published results. In the presence of the HPAs,
the maximum is reached in 1.5 h only at 75°C, whereas 4 h are
required in the presence of Amberlyst®.
Optimization of reaction conditions
The reaction conditions were optimized by screening of main
reaction parameters. Decreasing the concentration of Brönsted
catalytic sites leads to a decrease of the catalyst weight based
reaction rate. Thus, it takes 20, 50, and 90 minutes to reach the
maximum conversion in the presence of HPAs, ChH5P2W18O62,
and Ch2H4P2W18O62 at 75℃, respectively. In order to compare
the activity of the catalysts under the same reaction conditions,
we selected 1 h as a standard reaction time for the reaction
between alpha-AL and BuOH. Under these conditions, the yields
Plotting concentrations of the substrates and products against
time (Figure S9a) further supports the previously deduced
reaction sequence in line with reaction pathway 1 (Scheme 1),
e.g. through formation of p-BL as intermediate. For comparison,
the esterification of LA over Ch2H4P2W18O62 was also
investigated.(Figure S9b) In this case, high selectivity towards
BL was observed, whereas only traces of p-BL were detected in
the reaction mixture. However, the yield of BL based on LA is
restricted to ca. 60% due to the presence of water in the reaction
5
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10.1002/cssc.201601882
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mixture, whereas a BL yield close to 80% can be achieved when
starting from alpha-AL (Figure S9b).
Thereafter the mixture was cooled down, the catalyst was
removed by filtration, and alpha-AL was added and allowed to
react for another hour. Under these conditions, a low conversion
of alpha-AL of 10% occurred (Fig. S11), supporting the good
separation of the catalyst by filtration.
Our results suggest that high selectivity for BL is achievable in
the presence of the HPAs-based catalysts in an open, e.g. not
water-free, system. However, minor formation of LA points at
hydration of alpha-AL with residual water along with the reaction
with butanol. We carried out an experiment upon addition of 100
mol% of water (referred to alpha-AL) into the reaction mixture.
This caused a significant decrease of the BL yield to 60%
accompanied by an increase of the LA yield to 20%. Thus, the
concentration of water in the reaction mixture should be
minimized for an efficient BL formation (Fig. S10b)
The IR spectrum of the used Ch2H4P2W18O62 (Fig S12b) did not
significantly differ from that of fresh Ch2H4P2W18O62 (Fig S12a),
indicating that the structure of the heteropoly anion remains
intact. Characterization with 31P MAS NMR revealed the
presence of chemical shifts at -14.43 ppm and -14.51 ppm of the
fresh (Fig S3a) and the recycled Ch2H4P2W18O62 (Fig S3b),
respectively. X-Ray diffraction patterns further confirm the
stability of Ch2H4P2W18O62 under the reaction conditions. (Figure
S13) The TEM examination of used catalyst (Fig S4c) revealed
remaining nanosphere morphology, as observed for the fresh
catalyst (Fig S4a). EDX analysis indicated the presence of
phosphorous, tungsten ,oxygen, carbon, nitrogen of recycled
catalyst (Fig. S4d). These results demonstrate that the
temperature-responsive Ch2H4P2W18O62 catalyst remains intact
The reusability and stability of the catalyst Ch2H6P2W18O62.
Reusability and stability of catalysts are crucial for practical
applications. Based on the above discussion, Ch2H4P2W18O62
can be recovered out of reaction mixture since it precipitates
upon cooling the reaction solution (Fig. 2c). We recycled
Ch2H4P2W18O62 for 5 times. After each catalytic run, the catalyst
was recovered by centrifugation, washed with ethanol and water, after reaction.
and then dried before reuse. After the fifth run, the amount of the
recovered catalyst was 87.4% of its initial amount. Most
Conclusion
importantly, Ch2H4P2W18O62 was successfully reused five times
without significant loss of catalytic activity and selectivity (Fig. 4).
This study demonstrated a series of choline exchanged
Thus, the yield of BL somewhat decreased from 79.4% at the 1st
run to 69.9% at the 5th run, respectively, though full conversion
was achieved in each runs.
heteropoly acids ChxH(6-X)P2W18O62 (x=1-6) as outstanding
catalysts for reaction of n-butanol with alpha-AL forming butyl
levulinate. ChxH(6-X)P2W18O62 (x=1-6) were prepared by
a
precipitation/ion exchange method, of which ChCl and
H6P2W18O62 are precursors. The activity of the catalysts
appeared to be in good correlation with the amount of Brönsted
sites. Heteropoly acidic catalysts containing Ch cation are
soluble in the reaction mixture under the reaction conditions, but
precipitate upon cooling the reaction solution to room
temperature. Such temperature behaviour facilitates
a
homogeneously catalysed reaction and simultaneously enables
easy recovery of the catalyst. The best balance between
solubility properties and catalytic activity was observed for
Ch2H4P2W18O62 providing up to 79.4% BL yield. Analysis of the
reaction network suggests a transformation of alpha-AL with n-
butanol to yield p-BL with further conversion to BL.
Ch2H4P2W18O62 was recovered and reused five times without
significant loss of activity. Therefore, Ch2H4P2W18O62 possesses
a high potential for the production of valuable chemicals such as
levulinate acid ester from biomass-based platform molecules.
Figure 4. The reusability tests of the catalyst Ch2H6P2W18O62 in model
reaction. Reaction conditions: alpha-AL (0.625 g, 6.375 mmol), BuOH (0.473 g,
6.375 mmol), Ch2H6P2W18O62 (20 wt%=125.12 mg), 75℃, 1h.
Additionally, a control experiment has been performed to
evaluate the complete separation of catalytically active species
from the reaction solution. For this purpose BuOH was first
stirred with Ch2H4P2W18O62 (20wt%= 125.12 mg) at 75℃ for 1h.
Experimental Section
6
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10.1002/cssc.201601882
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Catalyst Preparation
dioxane and analyzed by GC after adding diethylene glycol
dimethyl ether as an internal standard.
H6P2W18O62 was synthesized according to the previously
published method.[28] All other reagents were of analytical
reagent (AR) grade and used without further purification.
The (HOCH2CH2N(CH3)3)XH(6-X)P2W18O62 [abbreviated as
ChxH(6-X)P2W18O62 (x=1-6), Choline cation abbreviated as Ch+]
catalysts were prepared according to the previously reported
method with some modifications.[29] Ch2H4P2W18O62 serves as
an example: Choline chloride (referred to as ChCl in what
follows) (4 mmol) was added to a solution of H6P2W18O6 (2
mmol) in 20 mL of distilled water and stirred at room
temperature for 8 h. The obtained white precipitate was washed
with distilled water until no free HPAs or chloride (AgNO3 test)
were found. The catalysts were recrystallized twice from CH3CN
and dried at 60oC. White products of ChxH(6-X)P2W18O62 were
obtained in ca. 70% yield. Other compositions ChxH(6-X)P2W18O62
were synthesized by the same method using different molar
ratios between H6P2W18O6 and ChCl.
Sample analysis
The identity of reaction products e.g. pseudo-butyl levulinate
(abbreviated as p-BL), β-angelicalactone, γ-methylene-γ-
butyrolactone, 2,4-pentadienoic acid, and LA was confirmed by
GC-MS analysis and compared with authentic samples.
Quantitative analysis of samples of the addition reactions was
performed using a chromatograph (Agilent 6890) equipped with
an Agilent 19091J-416 capillary column and a flame ionization
detector. Qualitative analysis was performed by GC-MS on a
Trace GC chromatograph 1310 equipped with a Restek Rxi-1
MS column (60 m × 250 μm × 0.5 μm) and a Thermo Scientific
ISQ mass spectrometer (EI+, 70 eV, 250°C).
Determination of acidic properties
Hammett acidity was measured according to Jiao’s work.[26]
A
catalyst sample (100 mg) and 100 mg Hammett indicator (p-
nitroaniline, pK(I)aq = 0.99) were dissolved in 20 mL of double-
distilled water. The mixture was stirred for 12h in a sealed vessel
at room temperature, and then the solid was separated by
filtration. The color intensity of the remaining liquid was
measured using a Cary 500 UV/Vis/NIR spectrophotometer at
380 nm.
Catalyst Characterization
Elemental analysis was carried out using a Leeman Plasma
Spec (I) ICP-ES and a P-E 2400 CHN elemental analyzer. IR
spectra (4000-500cm-1) were acquired as potassium bromide
discs on a Nicolet Magna 560IR spectrometer. The 31P MAS
NMR spectra were obtained using a Bruker AM500 spectrometer
at 202.5 MHz using 85% H3PO4 as external standard.
Transmission electron microscopy (TEM) micrographs and EDX
spectra were collected using a JEOL-2020F electron microscope
with accelerating voltage of 200kV. Powder samples of the
catalysts were dispersed in ethanol under ultra-sonication and
mounted on copper grids. X-ray diffraction (XRD) patterns of the
samples were collected on a Japan Rigaku D/max-2000 X-ray
diffractometer with Cu K radiation 40 kV/40 mA (λ=0.154178
nm). The critical micellar concentration (CMC) of ChxH(6-
X)P2W18O62 was determined by a plot of conductivity versus
concentration using a conductometer model DDS-11A. UV-Vis
spectra (200–400 nm) were recorded on a Cary 500 UV-vis-NIR
spectrophotometer.
The surface Brönsted acidity was investigated using the
irreversible adsorption of pyridine on the solid surfaces
according to the reported method.[27, 30]
Titration was used to evaluate the total acid content of the
solids. 0.05 g of solid was suspended in 45 mL of acetonitrile
and stirred for 3 h at 60oC. Then the suspension was titrated
with a solution of n-butylamine in acetonitrile (0.05 M) using the
indicator anthraquinone ( pKa = −8.2 ).[30]
2.6. Critical micelle concentration (CMC) determination
The CMC of ChxH(6-X)P2W18O62 was determined by the break
points of two nearly straight line. The inflection value from a
conductivity graph plotted versus concentration indicated that
the catalyst assembled into micelles. [23b]
General procedure for catalytic test
A 15 mL thick-wall pressure glass vessel with a Teflon cap
was charged with alpha-AL or LA (6.375 mmol), n-butanol (6.375
mmol), and catalyst (125.12 mg), respectively. The reactor was
heated using a parallel plate reactor with an aluminium block
under stirring with a magnetic stirrer at 350 rpm. Samples of the
reaction mixture (≈40 mg) were taken with a syringe and
centrifuged. The resulting clear samples were diluted with 1,4-
Acknowledgements
This work was mainly supported by the National Natural
Science Foundation of China (No. 51578119), the major projects
of Jilin Provincial Science and Technology Department
(20140204085GX). X.Y. thanks the China Scholarship Council
for financial support. M. G. Al-Shaal thanks the Higher Institute
7
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10.1002/cssc.201601882
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of Applied Science and Technology of Syria for the studentship.
W. Ciptonugroho thanks the support of Indonesian Ministry of
National Education Scholarship. Part of this work was performed
within the Cluster of Excellence “Tailor-Made Fuels from
Biomass” (DFG-EXC 236) funded by the Excellence Initiative by
the German federal and state governments to promote science
and research at German universities.
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8
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Entry for the Table of Contents (Please choose one layout)
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Xiaohu Yi,a,b Mohammad G. Al-Shaal,b
Wirawan Ciptonugroho,b Irina
Delidovich,b Xiaohong Wang*a and
Regina Palkovits*b
Page No. – Page No.
A series of compounds ChxH(6-x)P2W18O62 with temperature-dependent solubility
allowed an efficient butyl levulinate formation from α-angelica lactone and an
alcohol under mild reaction avoiding any by-products. The catalyst was easily
separable and can be reused for 5 times without significant loss of activity.
Synthesis of butyl levulinate based
on α-Angelica Lactone in the
presence
of
easily
separable
heteropoly acid catalysts
9
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