Metal-exchanged magnetic β-zeolites: Valorization of lignocellulosic biomass-derived compounds to platform chemicals

Article abstract of DOI:10.1039/c7gc01178d

An array of magnetically recoverable β-zeolites exchanged with late transition metals (Pd, Fe and Ir) have been synthesized and fully characterized and had their catalytic activity evaluated on the conversion of several bio-derived compounds to other value-added platform chemicals. The nature of the transition metals exchanged in the zeolite matrix and the significant effect of reaction conditions such as temperature, time and solvent usage on both the conversion and selectivity of the process are delineated.

Full text of DOI:10.1039/c7gc01178d

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Lima, C. Lima, L. Marchini, W. N. Castelblanco, D. G. Rivera, E. A. Urquieta-González, R. S. Varma and M.
W. Paixao, Green Chem., 2017, DOI: 10.1039/C7GC01178D.
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Heterogeneous catalytic oxidation for lignin valorization into valuable
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Scheme 1. Conversion of lignocellulosic biomass to platform chemicals. Glucose,
mannose, galactose, and other sugars present in small amounts are not shown.
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their tunable acidity, micropore sizes and the possibility of cationic polymer followed by calcination and ion exchange
introducing a number of additional metals, either into their steps (Scheme 2), which were conducted to convert the
framework or for balancing the framework’s negative net catalyst from the sodium (γ-Fe2O3-β-Na+) to the acidic form (γ-
charge.4
Fe2O3-β-H+) and exchanged with the metals Pd, Ir and Fe (γ-
In this context, the zeolites ZSM-5 and β are among the most
commonly used zeolitic materials in catalytic biomass
upgrading.5 The β zeolite, for instance, has been extensively
used in fast pyrolysis of lignocellulosic biomass, mostly in its
acidic form.6 Other model reactions for the production of
platform chemicals have also been reported, with the use of
framework-modified zeolites being considerably more
common than the ion-exchanged ones. For example, Sn-
containing β zeolite was used in the isomerization of glucose7
and in the production of 5-(hydroxymethyl)furfural (5-HMF)
from carbohydrates,8 while Zr-β zeolite was employed in the
preparation of γ-valerolactone (GVL) from furfural9 and
levulinic acid,10 as well as in the conversion of glucose to
methyl levulinate.11 The use of two metals in the same zeolite
framework has also been reported, wherein the conversion of
1,3-dihydroxyacetone (DHA) into ethyl lactate (ELA) was
achieved with Sn (Lewis acid site) and Al (Brønsted acid site)
incorporated in the same framework.12 Although the use of
ion-exchanged β zeolites has been rather limited, some
interesting applications have been registered, e. g. the
synthesis of biodiesel from soybean oil and methanol catalyzed
by a La-exchanged β zeolite13 and the gas-phase dehydration
of lactic acid to produce acrylic acid with alkali-ion exchanged
β zeolites as catalysts.14
Scheme 2. Preparation of the magnetically recoverable β zeolite catalysts.
Fe2O3-β-Pd, γ-Fe2O3-β-Ir and γ-Fe2O3-β-Fe, respectively). It is
important to highlight that the magnetic support was prepared
via a solvothermal method in the form of magnetite (Fe3O4)
microspheres. Then, after an oxidative thermal treatment
(temperature up to 550
zeolite in the sodium form and in the ion exchange - Fe3O4 is
converted to maghemite -Fe2O3). This transformation,
however, was not an impediment, since -Fe2O3 also presents
permanent magnetization. In order to confirm these qualms,
we have performed thorough characterization of the
°C) - in both the immobilization of the
(
γ
γ
a
magnetic behavior of the newly developed Pd-exchanged
catalyst using both SQUID (Superconducting Quantum
Interference Device) and Mössbauer spectroscopy.
Additionally, the series of as-synthesized magnetically
recoverable catalysts was fully characterized using
complementary techniques, e.g. X-ray diffraction (XRD),
transmission electron microscopy (TEM), chemical mapping, x-
ray photoelectron spectroscopy (XPS), inductive coupled
plasma - optical emission spectroscopy (ICP-OES), temperature
programmed desorption of ammonia (TPD-NH3) and Fourier
transform infrared spectroscopy of adsorbed pyridine (FTIR-
Pyr).
Even though zeolites represent a great advance in the field of
solid acid catalysts, their removal from the reaction medium
for further reuse still entails filtration or centrifugation steps.
In this regard, catalysts supported on magnetic materials have
been gaining prominence during the last few years as a
breakthrough towards the development of highly reusable
catalysts, as they can be easily separated from the reaction
The crystalline structure of the support and the catalysts was
inferred using the XRD technique. The x-ray diffractogram of
the Fe3O4 microspheres (Figure 1) showed the characteristic
peaks for the spinel type cubic structure (JCPDS card 19-0629).
The one of the catalyst in the acid form (γ-Fe2O3-β-H+)
displayed the expected characteristic peaks of polymorph B of
β zeolite,21 denoted by B in the diffractogram, and additional
medium by using an external magnetic field.15 As
a
consequence, the use of magnetically recoverable catalysts for
the conversion of bio-derived compounds has been
increasingly reported.16
In light of the aforementioned factors and our own recent
endeavors in this context,17 we envisioned the preparation of a
series of new magnetically recoverable catalysts comprising β
zeolites in their acid form and exchanged with different
transition metals (Fe, Ir and Pd). Our interest in the use of
these specific metals arises from previous reports on their
application in the catalytic upgrading of bio-derived
compounds.18-20 Therefore, these catalysts were prepared and
evaluated in the valorization of lignocellulosic biomass-derived
compounds; the use of metal-exchanged magnetic β zeolites
as catalysts in the conversion of bio-based entities is
unprecedented, to the best of our knowledge.
B
Z+B
Z
Z
B
Z
Z
B
B
B
B
B
I
B
γ-Fe2O3-β-Ir
B
B
B
B
B
B
F
F
I
B
Z
F
F
B
B
Z+B
Z
Z
P
B
B
B
Z
γ-Fe2O3-β-Pd
P
B
P
Z
P
Z+B
Z
Z
B
B
B
B
B
B
B
F
F
B
B
γ-Fe2O3-β-Fe
Z
B
F
F
B
B
B
Z+B
Z
Z
B
B
Z
B
B
B
Z
B
γ-Fe2O3-β-H+
Results and discussion
Catalysts characterization
Fe3O4
The magnetic β zeolites were prepared by the immobilization
of the zeolite grains onto the magnetite microspheres using a
10
20
30
40
50
60
70
80
2θ (degrees)
Figure 1. XRD patterns of Fe3O4, the magnetically recoverable catalysts in the acid
+
2 | J. Name., 2012, 00, 1-3
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form (γ-Fe2O3-β-H ) and exchanged with Ir (γ-Fe2O3-β-Ir), Pd (γ-Fe2O3-β-Pd) and Fe (γ-
Fe2O3-β-Fe).
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a)
b)
c)
d)
Figure 2. TEM images of a) and b) the Fe3O4 microspheres and c) and d) the magnetically recoverable catalyst in the acid form (γ-Fe2O3-β-H+). Scale bar: a) 1µm , b) 50 nm, c) 100
nm and d) 200 nm.
peaks related to the structure of ZSM-12 zeolite,22 denoted by different crystalline phases present in this novel material, a
Z in the diffractogram, a common occurrence that was also sample of the magnetic sodium zeolite, the one used to
observed by others.21,22 The diffractogram of the metal- produce the array of catalysts, was submitted to a Rietveld
exchanged zeolite catalysts also showed the peaks from refinement (for details, see the ESI); the amount of polymorph
polymorph B of β zeolite and ZSM-12, as well as the peaks for B of
β zeolite was found to be 60.5%, ZSM-12 34% and
the metal oxides, PdO (JCPDS card 75-584) in the Pd- maghemite 5.5%. In spite of the fact that ZSM-12 was present
exchanged catalyst and IrO2 in the Ir-exchanged catalyst in reasonable amounts, it is likely that its co-existence would
(JCPDS card 15-870). The low intensity peaks related to the not affect the catalytic activity of the material, since these
maghemite phase (JCPDS card 39-1346) in the diffractograms zeolites present similar characteristics, e. g. micropore size.23,24
of all catalysts is a consequence of the larger amounts of the In order to confirm this premise, the synthesis, full
zeolitic material in comparison to the magnetic support. It is characterization and evaluation of the catalytic activity of the
important to highlight that the transformation of magnetite to catalysts containing the pure zeolites separately were
maghemite occurs during the calcination steps, which were conducted (For a full description on the preparation and
conducted under air atmosphere.17 Aiming to quantify the characterization of these catalysts, see the ESI).
a)
c)
e)
f)
d)
b)
Figure 3. TEM images of the magnetically recoverable catalyst exchanged with a) and b) Pd (γ-Fe2O3-β-Pd), c) and d) Ir (γ-Fe2O3-β-Ir), e) and f) Fe (γ-Fe2O3-β-Fe). Scale bar: a, c, f
= 200 nm, b and d = 100 nm and e) 500 nm.
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a)
γ-Fe2O3-β-H+
γ-Fe2O3-β-Fe
γ-Fe2O3-β-Pd
γ-Fe2O3-β-Ir
b)
γ
-Fe2O3-β-Ir
γ
-Fe2O3-
β-Pd
γ-Fe2O3-β-Fe
γ
-Fe2O3-β
-H+
150 200 250 300 350 400 450 500
Temperature (°C)
1450 1500 1550 1600 1650 1700 1750
-1
Wavenumber (cm )
Figure 4. a) Temperature programmed desorption of ammonia curves and b) Fourier transform infrared spectroscopy of adsorbed pyridine spectra of the magnetically
recoverable catalysts.
The morphological characterization of the support and With the aim of studying the acidic properties of the catalysts,
catalysts was discerned using TEM; the support (Figures 2a and temperature programmed desorption of ammonia analyses
b) showed that the material is comprised of microspheres with were undertaken. The NH3-TPD desorption curves of γ-Fe2O3-
sizes ranging from 50 to 350 nm, with an average size of β-H+, γ-Fe2O3-β-Ir and γ-Fe2O3-β-Fe (Figure 4a) revealed the
approximately 175 nm (For a detailed particle count, see the occurrence of two different types of acid sites, one with a
ESI). As for the TEM images of the catalyst in the acid form desorption peak around 207 °C, ascribed to the weak acid
(Figure 2c and d), the presence of the support particles sites, and another in 372 °C, associated to moderate to strong
surrounded by grains of the zeolite is noticeable; in some acid sites. The γ-Fe2O3-β-Pd catalyst showed an additional peak
regions, the support particles appear to be uncoated, besides those of the other catalysts, at 318 °C and related to
however, the elemental map analysis showed that even those moderate acid sites. Although the Fe-exchanged catalyst
portions present a small layer of aluminosilicates (See the ESI displayed higher total acidity, a careful analysis of Figure 4a
for additional information). The TEM images of the Pd- showed that the majority of these acid sites were weak in
exchanged catalyst (Figures 3a and b) showed, as in the case of nature. Interestingly, the Pd-exchanged catalyst, which was
the catalyst in the acid form, the presence of the support considerably more acidic than the remaining catalysts,
surrounded by zeolite grains. However, in this case it is revealed a noticeably higher amount of moderate to strong
possible to detect the presence of possible Pd nanoparticles on acid sites when compared to the other catalysts (For more
the surface of the zeolite grains (as confirmed by EDX analyses) information, see the ESI). The total amount of metals in the
with sizes varying from 2 to 12 nm with an average size around catalysts was determined by ICP-OES analysis, and the values
6 nm (See the ESI for a detailed particle count). From a were found to be 1.1% for Pd in the Pd-exchanged catalyst,
catalytic viewpoint, another readily accessible source of 0.5% of Ir in the Ir-exchanged catalyst and 1.8% for Fe in the
palladium is now available in the structure of the zeolite in the Fe-exchanged catalyst (for the determination of Fe in the Fe-
form of nanoparticles in its surface, besides the exchanged exchanged catalyst, the amount of Fe in the catalyst in the
cations into the structure. Further, the elemental chemical sodium form was determined in triplicate and subtracted from
mapping confirmed that palladium is well-dispersed that of the Fe-exchanged catalyst).
throughout the whole catalyst, not only as nanoparticles, but Subsequently, the characterization of the nature of these acid
also probably exchanged into the zeolite (See the ESI for more sites was conducted by FTIR spectroscopy analyses of
information). On the other hand, the images of the Ir- adsorbed pyridine, which allowed us to determine qualitatively
exchanged catalyst (Figures 3c and d) showed the presence of the presence of Lewis and Brønsted acid sites in the materials
the zeolite grains - additionally, the grains of the microspheres (Figure 4b). The band in 1455 cm-1 is ascribed to pyridine
have completely collapsed into their smaller constituent strongly bounded to Lewis acid sites, and the one in 1497 cm-1
nanoparticles; the presence of needles of iridium oxide corresponds to both Lewis and Brønsted acid sites; the ones in
associated with the iron oxide nanoparticles are perceptible. 1553 and 1656 cm-1 are related to the pyridinium ion, which
Finally, the images of the iron-exchanged catalyst (Figures 3e implies the presence of Brønsted acid sites, while the band in
and f) showed its close morphological resemblance to that of 1603 cm-1 can be attributed to H-bonded pyridine.25
the catalyst in the acidic form (see Figure 2c); however, the The XPS analysis of the Pd-exchanged catalyst showed that Pd
elemental chemical mapping inferred that the iron is is present mainly in the form of PdO2 and PdO, as confirmed by
distributed uniformly along the whole extension of the catalyst a closer look at the Pd3d core level (See the ESI for further
(See the ESI for more information). Importantly, in none of the details on the XPS analyses of all catalysts). However, the
catalysts it was possible to discern different morphologies presence of Pd2+ ions exchanged into the zeolite was also
pertaining to the two zeolites present in the material.
detected in a considerable amount. Intermediate Pd species
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like Pd+ and Pd3+ were also present, but in a very low state (i.e., S = 5/2) in both tetrahedral and octahedral
concentration.26 As for the Ir-exchanged catalyst, it was coordination. The presence of this component with such a high
possible to observe the presence of IrO2, and of Ir3+ and Ir4+ relative area corroborate the SQUID results and confirmed our
exchanged into the zeolite, as confirmed by the analysis of the initial belief that iron oxides are present in the catalyst
Ir4f core level.27 Finally, the further study of the Fe2p core predominantly as maghemite.30 As for the other component, it
level of the Fe-exchanged catalyst showed the presence of a consisted of a doublet with a wide linewidth which could be
small amount of Fe2+ attributed to the magnetite structure, assigned to ionic Fe3+ and/or small nanoparticles within the
thus inferring that the conversion of Fe3O4 to γ-Fe2O3 was not superparamagnetic domain (characteristic of particles with
complete,17 Fe3+ from the γ-Fe2O3 structure, and Fe3+ sizes below 15 nm).
exchanged into the zeolite.28
a)
b)
c)
d)
Figure 5. SQUID magnetization curves of a) the magnetite microspheres used as support and b) the Pd-exchanged catalyst, and images depicting c) the catalyst in the reaction
right after stirring and d) the separation of the catalyst with a magnet after 10 s.
With respect to the specific surface area of the magnetic
zeolite in the sodium form and the final catalysts, it was
possible to determine by nitrogen adsorption isotherm
analysis that the exchange with metals did not markedly
affected the surface area or the micropore volume of the
materials (for details, see the ESI).
The magnetic behavior of the catalysts and support was next
examined at room temperature via SQUID measurements of
the magnetite microspheres used as support and the Pd-
exchanged catalyst (Figure 5a and b, respectively). As
expected, the maghemite microspheres presented a high
saturation magnetization (64.2 emu g-1). The corresponding
value for the Pd-exchanged catalyst was 6 emu g-1, and this
relatively low value is a consequence of the large amount of
zeolite, a diamagnetic material, when compared to the
Figure 6. 57Mössbauer spectrum of the Pd-exchanged catalyst measured at room
temperature.
Table 1. Data extracted from the least-squares fitting of the room temperature
57Mössbauer spectrum of the Pd-exchanged catalyst.
δ = isomer shift, ꢀEQ = quadrupole
splitting, Γ = linewidth, Bhf = hyperfine magnetic field, RA = relative area of the
individual spectral components. T-sites refers to the tetrahedral cation sites in the
inverse spinel structure of γ-Fe2O3 while O-sites, to the octahedral ones.
magnetic support. The amount of γ-Fe2O3 in the Pd-exchanged
catalyst, determined using the iron concentration as discerned
by ICP-OES, was found to be 2.3%. Interestingly, though, when
only the saturation magnetization of γ-Fe2O3 in the catalyst
Bhf
(T)
49.7
-
RA
(%)
89
δ
ꢀEQ
Γ
Component
(mm s-1)
0.33
0.13
(mm s-1)
-
(mm s-1)
0.61
2.61
was considered, it did increase with respect to the support,
what may be termed magnetostriction effects;29 an occurrence
that is currently being scrutinized and will be disclosed in due
time. Notably, the separation of the catalyst from the reaction
medium can be easily accomplished with the aid of a magnet,
as showed in Figure 5b.
Sextet
Doublet
0.53
11
Catalytic evaluation
Finally, in order to confirm the identity of the iron phases
present in the sample, room temperature Mössbauer
spectroscopy experiments were performed (Figure 6 and Table
1); 57Fe Mössbauer spectrum showed the presence of two
spectral components: one sextet (line S1 in Figure 6) and one
doublet (line S2 in Figure 6). The sextet component was the
major one (89% relative area), and its isomer shift value is
consistent with that expected for Fe3+ ions with a high-spin
Catalyst screening: evaluation of the ion-exchanged zeolites
on the valorization of hemicellulosic biomass-derived
compounds
The conversion of furfural to other value-added products is a
complex cascade process that involves a series of sequential
catalytic steps - at times promoted by Lewis acids and
sometimes by Brønsted acids (Scheme 3). Furfural can be
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Scheme 3. Conversion of hemicellulosic biomass-derived compounds to other value-added chemicals using catalysts with Lewis and Brønsted acid sites in the presence of
secondary alcohols. FUR = furfural, FA = furfuryl alcohol, DPMF = 2-(diisopropoxymethyl)furan, PMF = 2-(isopropoxymethyl)furan, α-AL = α-angelica lactone, β-AL = β-angelica
lactone, PL = isopropyl levulinate, 4-HP = isopropyl 4-hydroxypentanoate, HVA = γ-hydroxyvaleric acid, GVL = γ-valerolactone.
reduced to furfuryl alcohol using Lewis acid catalysts in the evidence of the efficiency of the developed catalysts, which
presence of a secondary alcohol (e.g. isopropyl alcohol), which could promote two reaction steps – the Lewis acid-promoted
acts as both solvent and hydrogen donor in a process known as hydrogenation/alcoholysis sequence – with at least a 60%
MPV (Meerwein–Ponndorf–Verley) reduction;31 the presence conversion. On the other hand, only the Pd-exchanged catalyst
of Brønsted acid catalysts in the reaction medium may then (γ-Fe2O3-β-Pd) was able to produce a full conversion of
lead to the formation of alkyl levulinates via alcoholysis. Since furfural, with a remarkable complete selectivity to the ester
the latter described step proceeds with the elimination of a product, a comparable or superior result when compared to
water molecule, the formed esters are in equilibrium with other known catalysts.32 The superior performance of the Pd-
their acid counterparts. Levulinic acid may also undergo water exchanged catalyst may be ascribed to the presence of highly
elimination, forming unsaturated lactones, which can be active Pd2+ species exchanged into the zeolite allied to the PdO
reduced to γ-valerolactone. An alternative pathway involves and PdO2 nanoparticles on its surface, which may have
the hydrogenation of both levulinic acid and its ester to the assisted on the promotion of the reduction steps. Additionally,
correspondent 4-hydroxypentanoates with further cyclization this catalyst presents a higher amount of moderate to strong
to γ-valerolactone.
acid sites when compared to the others by NH3-TPD analyses.
Aiming to understand how the different metals exchanged into All other catalysts (γ-Fe2O3-β-H+, γ-Fe2O3-β-Fe, γ-Fe2O3-β-Ir)
the zeolite core could influence the reaction outcomes for the afforded respectable conversions around 60%, with the
valorization hemicellulosic biomass-derived compounds, we catalyst in the acidic form (γ-Fe2O3-β-H+) affording the higher
initiated our studies by reacting platform chemicals, namely selectivity to the ester product (~ 88%) and the other catalysts
furfural, furfuryl alcohol and ethyl levulinate, with isopropyl giving moderate selectivities (76 and 67% for the Fe and Ir-
alcohol in the presence of the ensuing magnetic catalysts. In exchanged catalysts, respectively) and generating a more
order to accomplish that, a series of reactions were set up complex mixture of products. The lower conversions in the
using 0.3 mmol of each starting material, 750 µL of isopropyl presence of these catalysts may be due to the lower amount of
alcohol, and 75 mg of each catalyst at 130 °C for 24 h.
strong acid sites; higher catalyst loadings would therefore be
Interestingly, when furfural was used as starting material required in reactions with these materials.
(Figure 7a), isopropyl levulinate was the major product of the To our delight, when the reactions were set up with furfuryl
reaction in the presence of all catalysts, an observational alcohol (Figure 7b), all four catalysts produced complete
100
80
100
80
60
40
20
0
100
80
(a)
(b)
FA conversion (%)
PL selectivity (%)
α-AL selectivity (%)
(c)
EL conversion (%)
PL selectivity (%)
GVL selectivity (%)
β-AL selectivity (%)
4-HP selectivity (%)
60
60
FUR conversion (%)
PL selectivity (%)
40
40
α-AL selectivity (%)
PMF selectivity (%)
Others selectivity (%)
6 | J. Name., 2012, 00, 1-3
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20
20
0
0
Pd2+
Pd2+
H+
Fe3+
Ir3+
H+
Fe3+
Ir3+
H+
Pd2+
Fe3+
Ir3+
Figure 7. Magnetically recoverable β zeolite-promoted valorization of a) furfural, b) furfuryl alcohol and c) ethyl levulinate. FUR = furfural, FA = furfuryl alcohol, EL = ethyl
Please do noEtxacdhajunsgtedmioanrgins
Exchanged ion
Exchanged ion
levulinate PMF = 2-(isopropoxymethyl)furan, α-AL = α-angelica lactone, β-AL = β-angelica lactone, PL = isopropyl levulinate, 4-HP = isopropyl 4-hydroxypentanoate, GVL = γ-

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conversion with high selectivities to isopropyl levulinate investigations were initiated by evaluating the effect of the
(>90%), with that of the Pd-exchanged catalyst being the temperature in the reaction conversion and selectivity (Figure
highest again. Moreover, in all cases the only observed by- 8a). The reactions were set up using 0.3 mmol of furfural, 750
product was α-angelica lactone. This result may be rationalized µL of isopropyl alcohol, and 75 mg of the γ-Fe2O3-β-Pd catalyst
on the basis that the conversion of furfuryl alcohol to isopropyl and placed for 24 h in an oil bath pre-heated at different
levulinate can be promoted by Brønsted acid catalysts; FTIR- temperatures. Interestingly, a very low conversion of the
Pyr characterization ascertained that those are abundant in all starting material was observed in the reactions at 90 and 110
catalysts.
°C (23.4 and 30%, respectively), with a mixture of products
We then turned out our attention to evaluate ethyl levulinate as present in the reaction medium by the end of the reaction
the starting material (Figure 7c); in all cases isopropyl time. Remarkably, at 130 °C a full conversion of furfural was
levulinate was observed as a major product, a result of the observed with complete selectivity to PL, and the increase of
transesterification reaction of ethyl levulinate with isopropyl the reaction temperature to 150 °C did not lead to any losses
alcohol. Interestingly, the γ-Fe2O3-β-H+, γ-Fe2O3-β-Pd, γ-Fe2O3- in neither the conversion nor the selectivity.
β-Ir catalysts afforded conversions below 50%, while the Fe- Having defined 130 °C as the optimal operational temperature,
exchanged catalyst achieved a full conversion of the starting the influence of the catalyst loading in this protocol was then
material. Unfortunately, though, a mixture of products was evaluated (Figure 8b). The reactions were set up using 0.3
obtained, with isopropyl 4-hydroxypentanoate, a product of mmol of furfural, 750 µL of isopropyl alcohol, and different
the reduction of isopropyl levulinate, being the major one.
amounts of the γ-Fe2O3-β-Pd catalyst at 130 °C for 24 h. This
We further tried to combine the Fe-exchanged catalyst with factor apparently exerted a very significant effect in both the
the one in the acidic form (γ-Fe2O3-β-H+) aiming to induce the conversion and selectivity; when a very small amount of
cyclization of the 4-HP intermediate to GVL – however, even catalyst was employed (5 mg), a conversion around 6.5% was
after extensive studies, we were not able to achieve the observed, and only α-AL and the ether DPMF were present as
optimal conditions for this transformation via a cooperative products. When the catalyst loading was increased to 15 mg,
catalysis mediated by the designed catalysts (See the ESI for the conversion was slightly higher (7.6%), but traces of the
additional information). Consequently, the low conversions ester product started to emerge among the products, and with
and selectivities observed in the processes using this starting 25 mg the conversion was still low, but PL was then the major
material discouraged us to pursue this path.
reaction product. A further increase of the catalyst loading to
Since the Pd-catalyst showed remarkable activity in the 50 mg resulted in a large increase in both the conversion (76%)
conversion of both furfural and furfuryl alcohol to isopropyl and selectivity to PL (around 91%). Finally, with 75 mg of
levulinate, we next devoted our efforts to study how factors catalyst, the conversion of furfural achieved completion with
such as temperature, catalyst loading and concentration of the total selectivity to PL.
reaction medium could influence the outcome of these The concentration of the reaction medium was next examined
transformations.
(Figure 8c); the reactions were conducted with 0.3 mmol of
furfural, variable amounts of isopropyl alcohol, and 75 mg of
Conversion of furfural to isopropyl levulinate using the
100
80
60
40
20
0
100
80
60
40
20
0
100
80
60
40
20
0
(b)
(c)
(a)
FUR conversion (%)
PL selectivity (%)
α-AL selectivity (%)
PMF selectivity (%)
DPMF selectivity (%)
FUR conversion (%)
FUR conversion (%)
PL selectivity (%)
Others selectivity (%)
PL selectivity (%)
α−AL selectivity (%)
DPMF selectivity (%)
5 mg 15 mg 25 mg 50 mg 75 mg
Catalyst loading
1:10
1:20
1:30
1:40
90 °C
110 °C
130 °C
150 °C
Molar ratio FUR:iPrOH
Temperature
Figure 8. Study of the reaction parameters in the valorization of furfural using a magnetically recoverable Pd-exchanged β zeolite catalyst. FUR = furfural, DPMF = 2-
(diisopropoxymethyl)furan, PMF = 2-(isopropoxymethyl)furan, α-AL = α-angelica lactone, PL = isopropyl levulinate.
magnetically recoverable Pd-exchanged
the reaction parameters
β
zeolite: study of
the γ-Fe2O3-β-Pd catalyst at 130 °C for 24 h. The use of a
relatively concentrated reaction medium (molar ratio
FUR:iPrOH 1:10, when using 230 µL of isopropyl alcohol) led to
a full conversion of furfural, but also to the formation of non-
identified by-products, and consequently, to a diminished
selectivity to PL (approximately 89%). A decrease in the molar
ratio FUR:iPrOH to 1:20 and 1:30 (with the corresponding
isopropyl alcohol amounts of 460 and 750µL, respectively)
As aforementioned, we believe that the Pd-exchanged catalyst
showed superior catalytic activity when compared to the
others due the cooperative effect between the Pd2+ ions
exchanged along the extension of the zeolite with the PdO and
PdO2 nanoparticles on its surface. On that account, the
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caused the conversion to remain the same, but a full selectivity attain a reaction condition wherein a 76% conversion to
to PL was obtained in both cases. A further decrease in the exclusively furfuryl alcohol was achieved from furfural (Table
molar ratio led to a significant drop in the conversion to 2, entry 2).
approximately 62%, although the selectivity was unaltered.
Table 2. Evaluation of the hydrogenation of furfural using the Pd-exchanged catalyst
With the aim of comprehending how the reaction conversion
and sodium formate as hydrogen source.
and composition changed over time, we next conducted
experiments under the identified optimal conditions by varying
the reaction time (Figure 9a). At the first analyzed reaction
time (3h), a very low conversion of furfural was observed
Molar ratio
FUR:HCOONa
1:40
Solvent amount
(mL)
Entrya
Conversionb
(approximately 13%), with the formation of PL with a 75%
selectivity and α-angelica lactone being the only other reaction
product. The conversion of furfural increases progressively
with the increase of the reaction time, with the rise of the
selectivity to the ester product being less pronounced over
time. The conversion attains its maximum with 18h of reaction
time, but only at 24h the selectivity to PL reaches its maximum
(100%). Considering the great improvements that microwave-
induced reactions have represented in the valorization of bio-
derived compounds in recent years,33 we decided to evaluate
1
2
1 mL
41.6%
76.2%
61.9%
29.8%
0%
1:20
1 mL
3
1:20
0.5 mL
0.5 mL
1 mL
4
1:10
5c
6d
1:20
1:20
1 mL
77.6%
aUnless stated otherwise, the reactions were set up in a sealed vial using 0.15
mmol of furfural, 3 mmol of sodium formate, 1mL of water and 32.5 mg of
catalyst for 24h . bThe conversion of furfural was determined by GCMS. cReaction
this alternative heating source in addition to the conventional performed with 18.75 mg of catalyst. dReaction performed under microwave
irradiation for 1h.
one (Figure 9b). Remarkably, we could achieve
a full
conversion of furfural with 96% selectivity to PL in only 45
minutes and a complete selectivity within 1h of reaction.
Additionally, formic acid was evaluated as a hydrogen source,
but no furfural conversion was observed. Once again, very
similar results in terms of conversion and selectivity could be
attained using microwave irradiation in a considerably shorter
time (1h, Table 2, entry 6).
With the optimized conditions in hand, we decided to exploit
another interesting feature of our newly developed Pd catalyst
– the presence of well-dispersed PdO nanoparticles on its
surface.34 We envisioned the use of alternative hydrogen
sources for the Lewis acid-promoted reduction steps and the
possibility of conducting the reaction in water. It is well-known
that water is a deactivation agent for zeolites,4a however, we
expected that the presence of the PdO nanoparticles in the
surface of the catalyst would prevent its complete
deactivation. In order to confirm this hypothesis, we set up a
reaction protocol by using furfural as the starting material in
the presence of the Pd-exchanged catalyst in water at 60 °C
using sodium formate as a hydrogen source. This salt was
chosen as hydrogen source because it is readily available,
Conversion of furfuryl alcohol to isopropyl levulinate using the
magnetically recoverable Pd-exchanged β zeolite: study of the
reaction parameters
As described earlier for furfural, we started our investigations
on the valorization of furfuryl alcohol by assessing the effect of
the temperature over the conversion and selectivity of this
process (Figure 10a). Following the aforementioned path, the
reactions were initially set up using 0.3 mmol of furfuryl
alcohol, 750 µL of isopropyl alcohol, and 75 mg of the γ-Fe2O3-
β-Pd catalyst and placed for 24 h in an oil bath pre-heated at
varying temperatures.
reacts neatly in water and at relatively low temperatures.35
a)
Fortunately, after an evaluation of the sodium formate
amount and the reaction concentration (Table 2), we could
b)
100
80
100
80
FUR conversion (%)
PL selectivity (%)
α-AL selectivity (%)
FUR conversion (%)
PL selectivity (%)
α-AL selectivity (%)
60
60
40
20
0
40
20
0
DPMF selectivity (%)
15 min
30 min
45 min
60 min
3 h
6 h
9 h
12 h
15 h
18 h
21 h
24 h
Reaction Time
Reaction Time
Figure 9. Evaluation of the conversion and selectivity in the valorization of furfural over time using a magnetically recoverable Pd-exchanged β zeolite catalyst under a)
conventional heating and b) microwave irradiation. The reactions
FUR = furfural,
DPMF = 2-(diisopropoxymethyl)furan, α-AL = α-angelica lactone, PL = isopropyl levulinate.
8 | J. Name., 2012, 00, 1-3
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100
80
60
40
20
0
100
100
80
60
40
20
0
a)
80
60
40
20
0
FA conversion (%)
PL selectivity (%)
α-AL selectivity (%)
PMF selectivity (%)
Others (%)
FA conversion (%)
PL selectivity (%)
AF conversion (%)
PL selectivity (%)
α-AL selectivity (%)
α-AL selectivity (%)
5 mg 15 mg 25 mg 50 mg 75 mg
Catalyst loading
1:10
1:20
1:30
1:40
90 °C
110 °C
130 °C
Temperature
150 °C
Molar ratio AF:iPrOH
Figure 10. Evaluation of the reaction parameters in the valorization of furfuryl alcohol using a magnetically recoverable Pd-exchanged β zeolite catalyst. FA = Furfuryl Alcohol, PMF
= 2-(isopropoxymethyl)furan, α-AL = α-angelica lactone, PL = isopropyl levulinate.
In this case, even at lower temperatures full conversions of selectivity were achieved in this approach in 30 minutes
furfuryl alcohol were obtained, with moderate selectivities (> (Figure 11b).
80%) to isopropyl levulinate. The increase of the temperature
b)
c)
Recyclability of the Pd-exchanged catalyst in the upgrading of
furfural and furfuryl alcohol to isopropyl levulinate
to 130 °C caused the selectivity to achieve its maximum, and
the further heating to 150°C did not afford any dramatic
changes in yields or selectivity
Next, we set out to address the possible changes in the
Concerning the catalyst loading, it is discerned that it does not catalyst’s morphological structure after the reactions for FUR
exert a dramatic effect on the conversion, but that it and FA upgrading. The catalysts were characterized after the
completely dictates the selectivity (Figure 10b). That reactions using TEM to discern possible morphological
occurrence is explained on the basis that the etherification changes, and ICP-OES, to rule out eventual metal leaching
product, PMF, can be readily formed even when small issues. Gratifyingly, for both reactions no significant changes in
amounts of catalyst are present in the reaction medium, but the morphology of the materials were observed in the TEM
its full conversion to isopropyl levulinate requires a minimum images, and the amount of Pd remained virtually unchanged
catalyst loading, in this case, 75 mg. The evaluation of the (see the ESI for additional information).
molar ratio AF:iPrOH showed that for values in the range of Finally, we evaluated the recyclability of the Pd-exchanged
100
80
60
40
20
0
100
80
60
40
20
0
FA conversion (%)
PL selectivity (%)
α-AL selectivity (%)
FPE selectivity (%)
FA conversion (%)
PL selectivity (%)
α-AL selectivity (%)
PMF selectivity (%)
15 min 30 min 45 min 1 h
2 h
3 h
6 h
9 h
12 h 15 h 18 h 21 h 24 h
5 min
15 min
Reaction Time
30 min
Reaction Time
Figure 11. Evaluation of the conversion and selectivity in the valorization of furfuryl alcohol over time using a magnetically recoverable Pd-exchanged β zeolite catalyst under a)
conventional heating and b) microwave irradiation. The reactions
FA = Furfuryl
Alcohol, PMF = 2-(isopropoxymethyl)furan, α-AL = α-angelica lactone, PL = isopropyl levulinate.
1:10 – 1:30, the conversion and selectivity were not affected to catalyst in the conversion of furfural and furfuryl alcohol to
a great extent, but the further dilution of the reaction medium isopropyl levulinate under the optimized conditions (Figure
(molar ratio 1:40, when using 920 µL of isopropyl alcohol) 12a and b). Therefore, the reactions were set up using 0.6
caused a marked decrease in both the conversion and mmol of furfural (or FA), 150 mg of catalyst, and 920 µL of
selectivity to isopropyl levulinate (Figure 10c).
isopropyl alcohol (460 µL for the reaction with FA) at 130 °C for
The study of the conversion and selectivity throughout the 24 h (21 h for FA). After each reaction run, the catalyst was
reaction showed that at the initial evaluated time (15 min) the removed from the reaction medium with the aid of an external
conversion was already complete, with a selectivity around magnet, dried for at least 12 h at 100 °C and reused in a new
70% to isopropyl levulinate; the selectivity then slowly reaction cycle.
increases from this point until it reaches its maximum after When furfural was used as starting material, the catalyst could
21h of reaction (Figure 11a). Again, the use of microwave be recycled by two additional reaction cycles maintaining
irradiation rather than conventional heating proved to be conversions above 60% and selectivities over 80% to isopropyl
highly advantageous, since the same levels of conversion and levulinate; at the 4th reaction cycle, however, the conversion
was significantly reduced. In the case of furfuryl alcohol, no
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J. Name., 2013, 00, 1-3 | 9
a)
b)
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100
100
80
60
40
20
0
b)
a)
FA conversion (%)
PL selectivity (%)
α−AL selectivity (%)
PMF selectivity (%)
FUR conversion (%)
80
PL selectivity (%)
α−AL selectivity (%)
DPMF selectivity (%)
60
40
20
0
1
2
3
4
1'
1
2
3
4
1'
Reaction Run
Reaction Run
Figure 12. Evaluation of the recyclability of the Pd-exchanged catalyst in the upgrading of a) furfural and b) furfuryl alcohol to isopropyl levulinates. The reactions
FA = Furfuryl
Alcohol, DPMF = 2-(diisopropoxymethyl)furan, PMF = 2-(isopropoxymethyl)furan, α-AL = α-angelica lactone, PL = isopropyl levulinate.
substantial drops were noticed in the conversion, but the reaction run, when starting from furfuryl alcohol; however, the
selectivity to isopropyl alcohol was noticeably affected, being catalyst could be re-activated by a simple calcination step
40 % in the 3rd reaction cycle. In the case of the reaction with which bodes well for its application.
furfural, the conversion decreased after each reaction cycle
probably due to the formation of humins in the medium,
Experimental
which possibly blocked the Lewis acid sites needed to promote
the first step of the reaction, the reduction of furfural to
Synthesis of the Fe3O4 microspheres
furfuryl alcohol. In the case of FA, however, the conversion
The magnetic Fe3O4 microspheres used as support were
remains high even after a series of reaction cycles. These
prepared using a modified solvothermal method.36 FeCl3·6H2O
results can be rationalized on the basis that even a small
(3.9 g), trisodium citrate (1.2 g), and sodium acetate (NaOAc,
amount of Brønsted acids present could promote the
7.2 g) were dispersed in ethylene glycol (60 mL) under
conversion of the alcohol to the ether intermediate (PMF); the
magnetic stirring and the resulting suspension was transferred
evaluation of the composition of reaction medium over time
to a 200 mL Teflon-lined stainless-steel autoclave and placed in
had already shown that the conversion of FA to PMF occurred
an oven at 200 °C for 10 h. After cooling to room temperature,
very quickly (see Figure 11a). However, when the selectivity to
the obtained black product was isolated with the aid of a
PL was examined in more detail, similar trends were observed
magnet and washed several times with deionized water and
for the reactions with FUR and FA.
ethanol. The magnetite microspheres were dispersed and
With the aim to confirm that the humins formed in the surface
stored in distilled water for further use.
and pores of the catalyst were indeed responsible for its
deactivation, we performed recalcination after the fourth
Functionalization of the Fe3O4 microspheres with PDDA
reaction cycle (at 500 °C for 5h), and reexamined the
The functionalization of the magnetite microspheres was
upgrading of FUR and FA; pleasantly, the catalytic activity of
conducted using poly(dimethyldiallylammonium) chloride
this material was fully restored in both processes (reaction run
(PDDA) using the procedure described by Lv and co-workers.37
denoted by 1’ in Figures 12a and b), with conversions and
In this approach, 1.0 g of wet magnetite nanospheres were
selectivity to PL similar to the ones achieved when the fresh
added to an aqueous ammonia solution (2.0 wt%, 30.0 mL),
catalyst was used.
magnetically stirred at room temperature for 30 min, followed
by the addition of 5 mL of PDDA solution (20 wt% in H2O). The
mixture was then stirred at room temperature for 24 h.
Conclusions
Subsequently, the ensuing PDDA-functionalized magnetite
In summary, we have designed, synthetized and fully
microspheres were magnetically separated from the reaction
characterized a series of new catalysts comprising magnetically
medium and kept in water for further functionalization with
recoverable β zeolites exchanged with transition metals that
the zeolite.
displayed an immense potential in the catalytic valorization of
hemicellulosic biomass-derived compounds. The Pd-exchanged
Synthesis of the β zeolite in the sodium form (β-Na+)
catalyst, especially, have shown remarkable efficiency in the
The β-zeolite in the sodium form was synthetized using the
procedure described by Bhat et al.38 In a typical procedure, 7.2
conversion of both furfural and furfuryl alcohol to isopropyl
levulinate,
a platform chemical with broad potential
g of silica gel were suspended in 44 mL of distilled water under
magnetic stirring followed by the addition of 246 mg of sodium
hydroxide. Then, 7.35 g of tetraethylammonium hydroxide
were added to the mixture, which was stirred for 30 minutes.
Next, a solution containing 465 mg of sodium aluminate in 5.6
mL of distilled water was added and the resulting suspension
applications. Additionally, the same catalyst was also very
promising in the environmentally friendly reduction of furfural
to furfuryl alcohol using sodium formate as a hydrogen source
in aqueous medium. Still, recyclability studies have showed
that the Pd-exchanged catalyst was only deactivated after in
the 4th reaction run when starting from furfural, or in the 3rd
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was stirred for 1h. The mixture was transferred to a Teflon- furfuryl alcohol under 300W power level in a CEM Discover
lined stainless steel autoclave and placed in an oven at 150 °C Microwave Reactor.
for 5 days. The formed white power was filtered, washed with After completion of the reaction, the vial was cooled to room
distilled water several times, and dried at 60 °C for 12h. Finally, temperature and the catalyst was separated with the aid of a
the material was submitted to calcination at 550 °C for 6h magnet and a small amount of the reaction liquid (50 µL) was
under air atmosphere.
removed from the vial, diluted in isopropyl alcohol and the
products were analyzed using GC-MS; the retention times of
the products were compared to those of commercial
standards.
Preparation of the magnetically recoverable catalyst in the
acid form (γ-Fe2O3-β-H+)
Catalytic evaluation – preparation of furfuryl alcohol from furfural
or using the γ-Fe2O3-β- Pd catalyst
300 mg of the PDDA-functionalized magnetite microspheres
were dispersed in 50 mL of distilled water with the subsequent
addition of 2.2 g of the zeolite β-Na+ powder and stirring for 48 In a typical catalytic test, 0.15 mmol of furfural, 1 mL of water,
h at room temperature. The resulting composite Fe3O4-PDDA- 204 mg of sodium formate and 32.5 mg of the magnetically
β-Na+ was magnetically separated from the aqueous mixture recoverable catalyst were placed in a glass vial containing a
and dried at 60 °C for 24 h under vacuum. The material was magnetic stir bar. The vial was sealed and immersed in a pre-
then calcinated under air atmosphere at 550 °C for 5 h, heated oil bath at 60 °C and magnetically stirred at 700 rpm for
affording the composite γ-Fe2O3-β-Na+. In order to convert the 24h.
sodium form of the material to its acid form (γ-Fe2O3-β-H+), 1.0 For the reactions under microwave irradiation, the reaction
g of the γ-Fe2O3-β-Na+ magnetic zeolite was dispersed in 100 mixture was stirred in a 10 mL microwave tube at 60 °C for 1h
mL of an aqueous solution of NH4NO3 (1.0 mol L-1) and the under 300W power level in a CEM Discover Microwave
mixture was vigorously stirred (1000 rpm) at 80 °C for 2 h. The Reactor.
material was separated from the media with the aid of an After completion of the reaction, the vial was cooled to room
external magnet and resubmitted to this cationic exchange temperature and the catalyst was separated with the aid of a
process twice more. The solid was then dried at 60 °C under magnet and a small amount of the reaction liquid (50 µL) was
vacuum for 24 hours and calcinated under air atmosphere at removed from the vial, diluted in isopropyl alcohol and the
500 °C for 5 h, generating an orange-brown powder.
products were analyzed using GC-MS; the retention times of
the products were compared to those of commercial
standards.
Preparation of the magnetically recoverable catalysts
exchanged with Pd, Fe and Ir (γ-Fe2O3-β- Pd, γ-Fe2O3-β- Fe, γ-
Fe2O3-β- Ir)
Acknowledgements
500 mg of the Fe3O4-PDDA- β-Na+ were dispersed in 10 mL of
distilled water followed by the addition of the metal salts
[Pd(NO3)2.H2O, IrCl3.H2O, Fe(NO3)3.6H2O for Pd, Ir and Fe
respectively] to attain a concentration of 0,01 mol L-1 for Pd
and Ir and 1 mol L-1 for Fe. The mixture was stirred at room
temperature for 6h, the solid was removed via magnetic
decantation and dried at 60 °C for 12 h. This ion exchange
procedure was repeated twice more. Finally, the obtained
powder was calcinated at 550 °C for 5h.
E. Y. C. Jorge and C. G. S. Lima acknowledge CAPES for their
PhD and Postdoc fellowships, respectively. M. W. Paixão
gratefully acknowledges financial support from CNPq and
FAPESP (14/50249-8 and 15/17141-1). The authors
acknowledge GSK, as well, for the financial support. We also
would like to thank Prof. Dra. Elizabete Moreira Assaf for the
analyses of FTIR-Pyr, Prof. Dr. Joaquim de Araújo Nóbrega and
Dr. Thiago Linhares Marques for the ICP-OES analyses, Prof. Dr.
Adilson J. A. de Oliveira for the SQUID analyses and Prof. Dr.
João Batista Marimom da Cunha for the Mössbauer analyses.
Catalytic evaluation – preparation of isopropyl levulinates
from furfural or furfuryl alcohol using the γ-Fe2O3-β- Pd
catalyst using conventional heating
Notes and references
In a typical catalytic test, 0.3 mmol of the starting material
(furfural, furfuryl alcohol or ethyl levulinate), 750 µL of
isopropyl alcohol for the reaction with furfural or ethyl
levulinate or 230µL for the reaction with furfuryl alcohol and
75 mg of the magnetically recoverable catalyst were added to
a glass vial containing a magnetic stir bar. The vial was sealed
and immersed in a pre-heated oil bath at 130 °C and
magnetically stirred at 700 rpm for 24h for the reaction with
furfural or ethyl levulinate or for 21h for the reaction with
furfuryl alcohol.
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