Metal-containing zeolites as efficient catalysts for the transformation of highly valuable chiral biomass-derived products

Article abstract of DOI:10.1039/c3gc40267c

Metal-containing zeolites, especially Sn-Beta, perform as very efficient heterogeneous catalysts in the selective oxidation of levoglucosenone, which is considered as a platform chemical for the production of highly-valuable chemicals, towards the synthesis of the optically pure γ-lactone (S)-γ-hydroxymethyl-α,β-butenolide (HBO) using H ₂O₂ as an oxidizing agent. Using Sn-Beta as a catalyst, yields up to 75% of (S)-γ-hydroxymethyl-α,β-butenolide are achieved in a one-pot cascade reaction. When Sn-Beta is combined with an acid resin, such as Amberlyst-15, the two-step process allows yields up to 90%.

Full text of DOI:10.1039/c3gc40267c

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Metal-containing zeolites as efficient catalysts for the
transformation of highly valuable chiral
biomass-derived products
Cite this: DOI: 10.1039/c3gc40267c
Cecilia Paris, Manuel Moliner* and Avelino Corma*
Metal-containing zeolites, especially Sn-Beta, perform as very efficient heterogeneous catalysts in the
selective oxidation of levoglucosenone, which is considered as a platform chemical for the production of
highly-valuable chemicals, towards the synthesis of the optically pure γ-lactone (S)-γ-hydroxymethyl-
α,β-butenolide (HBO) using H2O2 as an oxidizing agent. Using Sn-Beta as a catalyst, yields up to 75% of
(S)-γ-hydroxymethyl-α,β-butenolide are achieved in a “one-pot” cascade reaction. When Sn-Beta is com-
bined with an acid resin, such as Amberlyst-15, the “two-step” process allows yields up to 90%.
Received 5th February 2013,
Accepted 22nd May 2013
DOI: 10.1039/c3gc40267c
www.rsc.org/greenchem
40% of the produced oxygenated products, while increasing at
the same time the production of other highly-valuable oxyge-
1. Introduction
In order to reduce the current dependence on fossil fuels, nated products, such as furfural.5 In this sense, the ability to
there is growing interest in renewable sources. Indeed, a great increase the selectivity towards target products would make
deal of efforts have been made in the last few decades to feasible the production of those key products from pyrolysis
develop efficient methods to produce valuable chemicals from oils. Interestingly, a new and feasible procedure has been
biomass, offering alternative routes besides traditional syn- recently reported for recovering LGO from pyrolytic liquids by
thetic pathways.1 In this context, lignocellulosic biomass has distillation.6
become one of the most sustainable carbon supplies for pro-
LGO and related products, such as levoglucosan, are con-
ducing valuable chemicals, and particularly, catalytic fast sidered the platform chemicals for the production of several
pyrolysis (CFP) of this lignocellulosic biomass has been deeply valuable products (see Fig. 1).7 From the different possible
studied in order to maximize the yield towards target pro- reaction pathways using LGO as a precursor, its oxidation
ducts.2 The pyrolysis oil product composition highly depends towards the unsaturated chiral γ-lactone (S)-γ-hydroxymethyl-
on the biomass source, the catalyst and the reaction con- α,β-butenolide (HBO in Fig. 1) is especially interesting
ditions used.2
since HBO is used in the synthesis of many drugs (such as
Levoglucosenone (LGO) is one of the most interesting valu- Burseran or Isostegane),8 aromatisers9 and antiviral agents
able products that can be obtained from CFP. LGO is a de- against the human immunodeficiency virus (HIV) or hepatitis B
hydrated sugar presenting two chiral and six different virus.10
functional carbon atoms, and it has been intensively used in
The most efficient method described for the synthesis of
the organic synthesis of several chemical compounds, such as HBO was reported by Koseki et al.11 Butenolide HBO is
natural products, nucleosides, anticancer drugs, and building obtained from LGO by following a two-step procedure. First,
blocks.3 It has been reported that the acid-catalyzed pyrolysis LGO is oxidized using organic peracids (such as peracetic acid
process allows increasing the selectivity towards LGO (up to or m-chloroperbenzoic acid) in acetic acid during 48 hours,
30%), using glucose, cellulose or birch and pine wood as obtaining the intermediate formate lactone (FBO, see Fig. 2).
biomass sources, and phosphoric acid as a catalyst.4 In Second, the reaction mixture is concentrated in vacuo, and
addition, the use of zeolites as acid catalysts in the CFP methanol and HCl were added to convert formate lactone FBO
process has permitted to enhance the LGO selectivity up to into the butenolide HBO. The reaction occurs through a
Baeyer–Villiger oxidation, as Shafizadeh et al. demonstrated
using different spectroscopic data.12
Corma et al. first synthesized Sn-Beta, as a high silica
Instituto de Tecnología Química (UPV-CSIC), Universidad Politécnica de Valencia,
zeolite with Beta structure containing tetrahedrally co-
ordinated isolated tin centers.13 This zeolitic material
Consejo Superior de Investigaciones Científicas, Valencia, 46022, Spain.
E-mail: acorma@itq.upv.es, mmoliner@itq.upv.es
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Fig. 1 Different reaction pathways of levoglucosenone.
Fig. 2 The described reaction pathway for the oxidation of levoglucosenone to unsaturated γ-lactone (S)-γ-hydroxymethyl-α,β-butenolide (HBO).
performs as a heterogenous green oxidation catalyst using
hydrogen peroxide, and shows excellent product selectivities in
2. Experimental
2.1. Zeolite syntheses
the Baeyer–Villiger or Meerwein–Ponndorf–Verley (MPV) reac-
tions.14 It has been demonstrated by theoretical calculations
2.1.1. Sn-Beta. Sn-Beta zeolite was prepared as follows:
that the isolated tin centers in the zeolite framework act as 21.05 g of a tetraethylammonium hydroxide solution (Sigma-
Lewis acid sites, showing strong interactions with the carbo- Aldrich, 35 wt% in water) was mixed with 20.89 g of tetraethyl-
nyl/hydroxyl moieties of reactants.15
orthosilicate (TEOS, Sigma-Aldrich, 98 wt%) and 0.282 g of
Recently, Sn-Beta has been used for biomass transfor- tin(IV) chloride pentahydrate (Sigma-Aldrich, 98 wt%). The
mations, since many biomass routes present molecules contain- mixture was stirred until the ethanol formed upon hydrolysis
ing carbonyl groups, requiring selective catalysts capable to of TEOS was evaporated. Finally, 2.0 g of an HF solution
activate those carbonyl functionalities to direct different reac- (Sigma-Aldrich, 48 wt% in water) was added, resulting in a
tion pathways.16 Indeed, Sn-Beta has been applied as a very thick gel. The resulting gel was allowed to reach the desired
efficient heterogeneous catalyst for the isomerization of concentration by controlling the water evaporation. The final
sugars,17 conversion of sugars to furfural-based heterocycles,18 gel composition was SiO2/0.008 Sn/0.54 TEAOH/0.54 HF/3H2O.
conversion of sugars to lactic acid derivatives,19 and epimeriza- This gel was transferred to a Teflon-lined stainless steel auto-
tion of sugars for the synthesis of rare monosaccharides.20
clave and heated at 140 °C for 21 days in a rotary oven. The
Herein, we will use Sn-Beta, and related metal-containing solid was recovered by filtration, extensively washed with
zeolites, as active and selective green heterogeneous catalysts water, and dried at 100 °C overnight. The material was cal-
for the efficient oxidation of the biomass-derived levoglucose- cined at 580 °C for 6 hours to remove the organic content
none towards the unsaturated chiral γ-lactone (S)-γ-hydroxy- located within the crystalline material.
methyl-α,β-butenolide (HBO). This simple heterogeneous route
2.1.2. Zr-Beta. Zr-Beta zeolite was prepared as follows:
to synthesize optically pure lactones derived from biomass 21.00 g of a tetraethylammonium hydroxide solution (Sigma-
would open up new attractive uses of metal-containing zeolites Aldrich, 35 wt% in water) was mixed with 20.88 g of tetraethyl-
for biomass transformations.
orthosilicate (TEOS, Sigma-Aldrich, 98 wt%) and 0.263 g of
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zirconyl chloride octahydrate (Sigma-Aldrich, 98 wt%). The periods of heating at 423, 523 and 623 K, each of them fol-
mixture was stirred until the ethanol formed upon hydrolysis lowed by the IR measurement at room temperature. All the
of TEOS was evaporated. Finally, 2.0 g of an HF solution spectra were scaled according to the sample weight.
(Sigma-Aldrich, 48 wt% in water) was added, resulting in a
Enantioselectivity evaluation of catalytic (S)-γ-hydroxy-
thick gel. The resulting gel was allowed to reach the desired methyl-α,β-butenolide (HBO) was performed firstly by chiral-
concentration by controlling the water evaporation. The final thin layer chromatography (Fluka, RP-18 Chiral Modified TLC),
gel composition was SiO2/0.008 Zr/0.54 TEAOH/0.54 HF/3H2O. Cu2+ impregnated, with a fluorescent indicator (254 nm), and
This gel was transferred to a Teflon-lined stainless steel auto- secondly, by measuring the optical rotation for the target com-
clave and heated at 140 °C for 21 days in a rotary oven. The pound on a Jasco P-1030 polarimeter using the sodium yellow
solid was recovered by filtration, extensively washed with line (D) at 589 nm. Specific rotation values [α]2D0 were calculated
water, and dried at 100 °C overnight. The material was cal- using the following equation:
cined at 580 °C for 6 hours to remove the organic content
located within the crystalline material.
T
α ¼ ½αꢀD ꢁ c ꢁ l
[α]DT : specific rotation (dm−1 g−1 cm3), at 20 °C; c: concen-
2.1.3. Al-Beta(OH) and Al-Beta(F). The commercially avail-
able Al-Beta(OH) CP811 provided by P.Q. Industries was used
tration is reported in g cm−3; l: path length (dm).
as a Beta sample prepared in OH media. An Al-Beta sample
with a much lower amount of defects, Al-Beta(F), was syn-
thesized in fluoride media as follows: 21.04 g of a tetraethyl-
2.3. Catalytic tests
ammonium hydroxide solution (Sigma-Aldrich, 35 wt% in Levoglucosenone (95%) was purchased from Carbosynth. (S)-
water) was mixed with 20.84 g of tetraethylorthosilicate (TEOS, γ-Hydroxymethyl-α,β-butenolide (98%) was purchased from
Sigma-Aldrich, 98 wt%) and 0.363 g of aluminum isopropoxide Alfa-Aesar. Hydrogen peroxide (35% w/w) was purchased from
(IPA, Sigma-Aldrich, 98 wt%). The mixture was stirred until the Sigma-Aldrich. Acetonitrile, acetone and 1,4-dioxane were of
ethanol and isopropanol formed upon hydrolysis of TEOS and HPLC quality grade and were acquired from Alfa-Aesar. Ultra-
IPA were evaporated. Finally, 2.08 g of an HF solution (Sigma- pure laboratory grade water (MilliQ, 18.2 megaOhms, 25 °C)
Aldrich, 48 wt% in water) was added, resulting in a thick gel. was employed for HPLC analysis. All the reactants were used
The resulting gel was allowed to reach the desired concen- without further purification. Amberlyst-15 was acquired from
tration by controlling the water evaporation. The final gel com- Sigma-Aldrich.
position was SiO2/0.033 Al/0.5 TEAOH/0.5 HF/3H2O. This gel
The reaction was carried out in sealed reactors, under mag-
was transferred to a Teflon-lined stainless steel autoclave and netic stirring and heating the solution at the desired tempera-
heated at 140 °C for 14 days in a rotary oven. The solid was ture in an oil bath. In a typical reaction, 0.33 mmol (42 mg) of
recovered by filtration, extensively washed with water, and levoglucosenone was dissolved in 1 ml of 1,4-dioxane, and
dried at 100 °C overnight. The material was calcined at 580 °C 0.5 mmol (48 mg) of H2O2 (35% w/w in water) was added to
for 6 hours to remove the organic content located within the the solution. The amount of catalyst was varied depending on
crystalline material.
the levoglucosenone/active site ratio. 10 μl samples were col-
lected along the reaction (by cooling down the reactor with ice)
and diluted with 1.5 ml of acetonitrile. Separation of the cata-
2.2. Characterization
Powder X-ray diffraction (PXRD) measurements were per- lyst was done by filtration (nylon 0.2 μm syringe filters).
formed in multisample Philips X’Pert diffractometer The experiment performed following a two-step procedure
equipped with a graphite monochromator, operating at 45 kV (entry 11 of Table 2) was carried out as follows: 0.33 mmol
and 40 mA, and using Cu Kα radiation (λ = 0.1542 nm). (42 mg) of levoglucosenone was dissolved in 1 ml of 1,4-
a
The chemical analyses were carried out in a Varian 715-ES dioxane, and 0.5 mmol (48 mg) of H2O2 (35% w/w in water)
ICP-Optical Emission spectrometer, after solid dissolution in was added to the solution. This feed was contacted with Sn-
an HNO3–HCl–HF aqueous solution.
Beta catalyst (the LGO/Sn ratio was fixed at 20), and the reac-
The morphology of the samples was studied by scanning tion was kept at 100 °C for 4 hours. Then, the reaction solution
electron microscopy (SEM) using a JEOL JSM-6300 microscope. was separated from the catalyst by filtration, and contacted
UV–Vis spectra were obtained with a PerkinElmer (Lambda with 140 mg of the acid resin Amberlyst-15 at room tempera-
19) spectrometer equipped with an integrating sphere with ture for six hours.
BaSO4 as the reference.
The evolution of reactions was followed by HPLC, injecting
Infrared spectra were measured with a Nicolet 710 FT IR 10 μl of samples, and monitoring product formation at
spectrometer. Pyridine adsorption–desorption experiments 220 nm. Samples from the catalytic test were analyzed by
were performed on self-supported wafers (10 mg cm−1) of ori- HPLC (Varian ProStar 330, equipped with a Quaternary Pump,
ginal samples previously activated at 673 K and 10−2 Pa for a Rheodyne injector and a UV-Vis PDA detector), using a C18
2 hours. After wafer activation, the base spectrum was recorded chromatographic column (Mediterranean Sea C18, 5 μm, 0.46
and pyridine vapour (6.5 × 102 Pa) was admitted in the vacuum × 250 mm). A gradient elution system (0.8 ml min−1) was de-
IR cell and adsorbed onto the zeolite. Desorption of pyridine veloped for complete separation of the reaction crude (from
was performed in vacuum over three consecutive one-hour t = 0 to t = 5 min: H2O–acetonitrile 85% : 15%; from t = 5 to
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t = 10 min: H2O–acetonitrile 90% : 10%; from t = 10 to
t = 15 min: H2O–acetonitrile 90% : 10%; from t = 15 to t = 20 min:
H2O–acetonitrile 85% : 15%).
Table 1 Chemical analyses and crystal size of the different metal-containing
zeolites
Crystal BET surface Micropore
sizeb
areac
volumec
(cm3 g−1
)
Sample
Heteroatom Si/Het.a (μm)
(m2 g−1
)
Al-Beta(OH) Al
13
33
114
127
0.1–0.2 594
0.18
0.22
0.21
0.20
3. Results and discussion
3.1. Zeolite synthesis and characterization
Al-Beta(F)
Sn-Beta
Zr-Beta
Al
Sn
Zr
0.8–2
1–2
505
490
474
1–3
Different tetrahedrally-coordinated isolated metals in zeolite
frameworks perform as excellent Lewis acid active sites,14,16
and isolated tin and zirconium atoms in the framework of
Beta zeolite have been described as green and highly stable
Lewis acid catalysts, even in aqueous media where Lewis
acidity is often suppressed.1,17,21 Beta zeolite is a very open tri-
directional large pore molecular sieve with channel openings
between 6.5 and 7 Å.22 In this sense, Sn-Beta and Zr-Beta could
be adequate candidates for their use as catalysts in the oxi-
dation of levoglucosenone to γ-lactone (S)-γ-hydroxymethyl-
α,β-butenolide (HBO), because, on the one hand, metal centers
would introduce the Lewis acidity required in the reaction, and
on the other hand, their pore size would allow the diffusion of
the reactants and products involved in the reaction.
a Measured by ICP. b Obtained from SEM. c Acquired from N2
adsorption measurements.
Sn-Beta and Zr-Beta were prepared with the same Si to
metal ratio (Si/M = 125, see the Experimental section for
details). Both materials show the characteristic powder X-ray
diffractogram (PXRD) of Beta zeolite (see Fig. 3), and chemical
analyses indicate that the real Si/M ratios in the resulting
solids are similar to the gel ratios (Si/Sn = 114, and Si/Zr = 127,
for Sn-Beta and Zr-Beta, respectively, see Table 1). These two Fig. 4 Scanning electron microscopy (SEM) images of: (A) Al-Beta(OH), (B)
Al-Beta(F), (C) Sn-Beta, and (D) Zr-Beta.
metallozeolites show comparable crystal sizes between 1 and
3 μm (see SEM images C and D in Fig. 4). Sn-Beta and Zr-Beta
were also studied by UV-Vis spectroscopy to identify if Sn and
Zr species have been inserted in framework positions. As
observed in Fig. 5, Sn-Beta presents a band centered at
200 nm, and Zr-Beta shows a band centered at 215 nm, which
have been previously assigned to the characteristic UV radi-
ation absorption of these metals in zeolitic framework
positions.23
As a general description, the introduction of aluminum
atoms in the framework of zeolites creates negative charges in
Fig. 5 UV-Vis spectra of Sn-Beta and Zr-Beta.
the structure, which are primarily balanced by protons,
making Al-containing zeolites extraordinary Brönsted acid
materials.24 In this sense, two different Al-Beta materials
showing Brönsted acidity were selected for comparing their
activity in the oxidation of LGO with the Lewis acid hetero-
geneous catalysts. On the one hand, the commercially
Fig. 3 Powder X-ray diffractograms (PXRD) of the different metal-containing
Beta zeolites in their calcined form.
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available Beta CP811 provided by P.Q. Industries [named here as furan-derivatives in the presence of Al-Beta(F). Then, in
Al-Beta(OH)], which presents a Si/Al ratio of 13 and a very order to see if HBO remains stable in the presence of Al-
small crystal size (∼100–150 nm, see Fig. 4A), and, on the Beta(F), a blank experiment was performed, and the result
other hand, an Al-Beta zeolite synthesized in fluoride media, given in entry 7, Table 2, clearly reveals that 25% of HBO is
Al-Beta(F), with a crystal size comparable to those of Sn- and degraded after 20 hours of reaction.
Zr-Beta, were prepared and tested as catalysts (see the Experi-
mental section for details, Fig. 4 and Table 1).
At this point, it is important to comment on the relevant
catalytic differences observed between Al-Beta(OH) and Al-
Beta(F) (entries 3 and 4 in Table 2). Although both samples present
Brönsted acidity, Al-Beta(F) shows much higher activity and
selectivity to FBO and HBO. Those two samples were character-
3.2. Catalytic tests: “one-pot” selective oxidation of
levoglucosenone
The levoglucosenone oxidation experiments were performed in ized by in situ infrared study of pyridine adsorption/desorption
2 ml reactors at 100 °C using H2O2 as an oxidant and dioxane in order to see the acidity of the zeolites. Al-Beta(OH) and
as a solvent (see the Experimental section for details). The Al-Beta(F) show an IR spectrum band at 1545 cm−1, which has
amount of each catalyst introduced in the reaction was varied been assigned to protonated pyridine and, therefore, to
keeping the levoglucosenone/metal active site molar ratio Brönsted acid sites in the zeolites (see Fig. 6). It can be seen
(LGO/M) constant at 50. The reaction was followed by HPLC. that the 1545 cm−1 IR band of the pyridine adsorbed after eva-
From the preliminary screening results summarized in cuation at 150 °C is more intense for Al-Beta(OH) than Al-Beta(F),
Table 2, it can be observed that Al-Beta(F), Sn-Beta and Zr-Beta in clear agreement with the lower Si/Al ratio in the former. On
are very active catalysts for LGO oxidation, with conversions the other hand, Al-Beta(OH) zeolite also shows much more
higher than 90% after 4 hours of reaction. In all cases, the intense IR spectra bands at 1452 and 1622 cm−1 than Al-Beta(F).
formil-ester product (FBO) is first formed, and later trans- These two bands are associated with pyridine coordinated to
formed to the unsaturated chiral γ-lactone (S)-γ-hydroxy- Lewis acid sites that can be related to the presence of extra-
methyl-α,β-butenolide (HBO). The selectivity towards desired framework aluminum. Indeed, it is believed that Lewis acidity
HBO is higher for Al-Beta(F) than for Sn-Beta and Zr-Beta, after in Al-containing zeolitic materials is generated by extra-frame-
4 and 20 hours (see entries 4, 5, and 6 in Table 2), but the work octahedral aluminum species. Therefore, if we assume
overall yield to both products (FBO + HBO) is much lower for that the above described reaction must occur through a Lewis
the former (i.e. 62% for Al-Beta(F) versus 87% and 89% for acid mechanism, the lower activity of Al-Beta(OH) when com-
Zr-Beta and Sn-Beta, respectively after 20 h). In this sense, the pared to Al-Beta(F) cannot be explained. However, it has been
Brönsted acidity of Al-Beta(F) would favor the hydrolysis of the described in the literature that some Al-Beta materials are
formil ester FBO towards HBO but, at the same time, this active for Lewis acid based reactions, such as Meerwein–Ponn-
chiral sugar could also be converted to other by-products, such dorf–Verley (MPV).14a,25 Van Bekkum et al. concluded that
Table 2 Results of the oxidation of levoglucosenone using different metal-containing zeolites. Reaction conditions: 0.33 mmol LGO, 0.5 mmol H2O2, 1 ml of
solvent at given temperatures and LGO/metal ratios
Entry
Catalyst
LGO/metal
Solvent
T (°C)
t (h)
Conversion
SFBO
SHBO
STOTAL
1
2
3
—
—
—
50
Dioxane
Dioxane
Dioxane
100
100
100
100
100
100
100
100
100
100
100
100
80
4
4
4
20
4
20
4
20
4
20
4
20
20
48
4
20
4
3
9
0
0
39
8
45
5
67
40
72
41
—
—
45
2
61
18
45
12
52
2
0
0
0
0
Si-Beta
Al-Beta(OH)
63
89
97
100
92
98
91
100
11
25
97
99
95
100
98
100
97
99
62
95
12
33
30
57
23
47
24
48
—
—
44
69
26
59
50
75
44
89
7
51
41
75
62
90
87
96
89
—
—
89
71
87
77
95
87
96
91
64
55
4
Al-Beta(F)
Zr-Beta
50
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
5
50
6
Sn-Beta
50
7
Al-Beta(F)
Al-Beta(F)
Sn-Beta + Al-Beta(F)
Sn-Beta
50 (HBO)a
8
50
80
9
50 (Sn)–200 (Al)b
100
100
100
100
100
R.T.
100
100
10
11
20
20
4
6
4
4
Sn-Beta
20
—
50
50
Amberlyst-15c
Sn-Beta
12
13
Acetonitrile
Methanol
57
0
Sn-Beta
55
a HBO/Al ratio. b LGO/Sn and LGO/Al ratios of 50 and 200, respectively. c 140 mg of the commercial acid resin Amberlyst-15.
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Fig. 6 Transmission FTIR spectra in the stretching C–C region of the different metal-containing zeolites after adsorbing pyridine followed by desorption at 150,
250, and 350 °C.
extra-framework aluminum species are inactive in the MPV Brönsted acid sites, the HBO yield improves up to 59%, but at
reaction, and alternatively proposed that the active sites are the same time, the total yield to FBO plus HBO drops when
octahedrally coordinated framework aluminums, connected compared to Sn-Beta (see STOTAL in entries 6 and 9 of Table 2).
with four bonds to the framework, one water molecule and a That means that although the Brönsted acidity introduced in
hydronium ion.25 Unfortunately, this specific aluminum site the reaction mixture is very low, HBO is still being degraded to
was not unambiguously observed by Van Bekkum et al.,25 but other by-products on the Brönsted acid sites.
their description would explain the results we have obtained
for the two different Al-Beta materials in the levoglucosenone Sn-Beta (LGO/Sn = 50, see Fig. 7A) reveals that Lewis acid sites
oxidation. introduced by isolated tin centers in framework positions are
The oxidation catalytic profile of levoglucosenone using
As has been introduced above, Al-Beta(F) zeolite is active in able to perform the hydrolysis of the formil ester FBO product
the LGO oxidation when the reaction is performed at 100 °C, to HBO. This hydrolysis rate is slower when compared to
but unfortunately undesired degradation reactions also occur Brönsted acid Al-Beta(F), but at the same time, the presence of
affecting the total selectivity of FBO and HBO. In attempts to undesired competitive degradation reactions catalyzed by
reduce the undesired pathways, a new experiment using Al- Brönsted acid sites are prevented with Sn-Beta. To ensure the
Beta(F) at 80 °C is carried out. After 20 hours, levoglucosenone absence of Brönsted acid sites in Sn-Beta, this material was
is almost fully converted (see entry 8 of Table 2) and, interest- characterized by in situ infrared study of pyridine adsorption/
ingly, competitive degradation reaction is highly prevented desorption, and as seen in Fig. 6, the unique bands present in
(STOTAL = 89%) but the hydrolysis step of the formil ester FBO the spectrum after desorption at 150 °C are those associated
to HBO is very slow. After 48 hours of reaction, the selectivity with Lewis acid sites (1452 and 1622 cm−1). Then, it could be
to HBO is almost 70%.
predicted that if the amount of Sn-Beta is increased in the
Sn-Beta performs better than the other metallozeolites in reaction mixture, the hydrolysis rate of FBO to HBO will also
terms of total yield to FBO and HBO at 100 °C with very high be increased, while the presence of undesired competitive
selectivities to these products (see entry 6 of Table 2). However, reactions should mostly be prevented. Having that in mind, a
after 20 hours of reaction, the selectivity to the desired HBO is new experiment using Sn-Beta as a catalyst is performed fixing
still low (see Fig. 7A). In order to increase the hydrolysis of the the LGO/Sn ratio at 20. As seen in Fig. 7B and Table 2 entry 10,
formil ester to HBO when Sn-Beta (LGO/Sn = 50) is used as a the hydrolysis rate to HBO is notoriously increased, achieving
catalyst, we decided to combine Sn-Beta with Al-Beta(F), keeping a HBO yield of 75% after 20 hours. Interestingly, the total yield
the LGO/Sn ratio at 50 and the LGO/Al ratio at 200. Using this to the main products (FBO + HBO) is fixed after increasing the
combination of Lewis acid sites with a small amount of catalytic active sites in the reaction (compare entries 6 and 10
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Fig. 7 Levoglucosenone (LGO) oxidation reaction profiles and product distributions as a function of time when Sn-Beta is used as a catalyst with a LGO/Sn ratio of
50 (A) and 20 (B). Reaction conditions: 0.33 mmol LGO, 0.5 mmol H2O2, 1 ml dioxane at 100 °C.
in Table 2). This result is interesting since the previous yield crude obtained after reacting levoglucosenone with Sn-Beta for
described in homogeneous catalysis using organic peroxides 20 hours in dioxane (entry 10 in Table 2), HBO product was
was 65%.11 Furthermore, the efficiency of H2O2 was measured, isolated and purified in a column. On the one hand, a special
obtaining values higher than 90% after 1 and 4 hours of thin-layer chromatography (TLC) plate for enantiomeric chiral
reaction.
analysis (see the Experimental section) was used to evaluate
In addition to dioxane, other protic and aprotic solvents the optical purity of HBO using acetonitrile–CH2Cl2 (10 : 90
with similar polarities were studied using Sn-Beta as a catalyst. v/v) as the mobile phase. Only one compound was observed in
As seen in Table 2, the oxidation of LGO in acetonitrile shows the TLC plate, with the same Rf = 0.19 as the commercial enan-
lower activity and very low selectivity to HBO (entry 12), while tiomeric pure sample of HBO. On the other hand, the optical
in methanol the activity is similar to dioxane but the total purity of the isolated HBO product after reaction was also
selectivity to FBO and HBO is low (entry 13).
determined by measuring the specific rotation [α]TD (see the
Experimental section for details) for the HBO-catalytic
product. The achieved value [α]TD = −114.56 for the isolated
3.3. Catalytic tests: “two-step” selective oxidation of
levoglucosenone
HBO was in agreement with the value obtained ([α]DT
=
−117.01) for the commercially available γ-lactone (S)-γ-hydroxy-
methyl-α,β-butenolide, measured under the same experimental
conditions. From both the experimental results, it could be
said that S-enantiomer of HBO is formed in more than 95%,
by reacting LGO in the presence of Sn-Beta catalyst.
The HBO yield can be increased from 75% to 90% by designing
a very simple “two-step” procedure using a combination of
Sn-Beta and the acid resin Amberlyst-15 (see entry 11 of Table 2
and the Experimental section). The LGO feed is first contacted
with the Sn-Beta catalyst at 100 °C for 4 hours, and later, the
catalyst is separated from the reaction solution by filtration. At
that moment, the LGO conversion is almost complete (97%),
and the yield to the intermediate formil butenolide (FBO) is
close to 50%. We have seen above that for a longer time using
Sn-Beta as a catalyst favors the FBO transformation into HBO
(see entry 10 in Table 2), but the hydrolysis rate is low. This
hydrolysis rate can be increased by using Brönsted acid cata-
lysts, such as Al-Beta zeolites, but undesired by-products are
formed under those elevated temperatures. To avoid this degra-
dation, we propose as an alternative the use of an acid resin,
such as commercially available Amberlyst-15. This acid resin
shows less diffusion problems than Al-Beta zeolites and the
3.5. Catalyst stability
Finally, the stability of Sn-Beta was studied. The catalyst was
filtered and washed with acetone after the catalytic test for its
re-use. As seen in Table 3, the recycled catalyst shows a slightly
lower activity than fresh Sn-Beta after 1 hour of reaction, and it
also shows lower selectivities to HBO after 1 and 4 hours of
reaction. The recovered solid from the 1st catalytic cycle was
calcined in air at 550 °C for 6 h. This calcined catalyst was
tested in the levoglucosenone oxidation, and as observed in
hydrolysis step of FBO to HBO could be performed at lower Table 3 Study of the Sn-Beta stability after different catalytic cycles
temperatures avoiding undesired degradation reactions. As seen
Si/Sna
117
LGO/Sn
20
Time (h)
Conversion
SHBO
in entry 11 of Table 2, Amberlyst-15 performs the complete
hydrolysis of the remaining FBO at room temperature after
6 hours, achieving up to 90% yield of desired HBO.
Fresh
1
4
1
4
1
4
90
99
71
98
86
99
22
49
15
31
19
42
1st Cycle
2nd Cycleb
122
20
3.4. (S)-γ-Hydroxymethyl-α,β-butenolide characterization
120
20
Since the desired product is the chiral γ-lactone (S)-γ-hydroxy-
methyl-α,β-butenolide (HBO), an additional characterization
a Measured by ICP after each experiment. b Catalyst recovered from the
has been performed for studying its optical purity. From the 1st cycle was calcined in air at 550 °C for 6 h.
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Efficient heterogeneous catalysts based on metal-containing zeo-
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towards the unsaturated chiral γ-lactone (S)-γ-hydroxymethyl-
α,β-butenolide (HBO) using H2O2 as a green oxidant. This cataly-
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products obtained from biomass fast pyrolysis, and the chiral
γ-lactone HBO is a high-valuable chemical with applications in
flavors and biomedicine. Sn-Beta performs better than other
metalloaluminates, achieving HBO yields of up to 75% in a “one-
pot” system. This value is higher than that previously reported
with homogeneous catalysts using organic peracids as oxidants
(65%). The HBO yield is increased up to 90% if Sn-Beta catalyst is
combined with an acid resin in a “two-step” procedure.
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Acknowledgements
This work has been supported by the Spanish Government
MINECO through Consolider Ingenio 2010-Multicat and
MAT2012-37160, and by UPV through PAID-06-11 (n.1952).
Manuel Moliner also acknowledges “Subprograma Ramon y
Cajal” for contract RYC-2011-08972. ITQ thanks the “Program
Severo Ochoa” for financial support.
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