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Catalysts 2015, 5, 1911-1927; doi:10.3390/catal5041911
catalysts
ISSN 2073-4344
www.mdpi.com/journal/catalysts
Article
Zr-SBA-15 Lewis Acid Catalyst: Activity in Meerwein Ponndorf
Verley Reduction
Jose Iglesias *, Juan Antonio Melero, Gabriel Morales, Jovita Moreno, Yolanda Segura,
Marta Paniagua, Alberto Cambra and Blanca Hernández
ESCET, Rey Juan Carlos University, C/Tulipán, s/n. Móstoles, E28933 Madrid, Spain;
E-Mails: juan.melero@urjc.es (J.A.M.); gabriel.morales@urjc.es (G.M.);
jovita.moreno@urjc.es (J.M.); yolanda.segura@urjc.es (Y.S.); marta.paniagua@urjc.es (M.P.);
alberto.cambra@upm.es (A.C.); blanca.hernandez@urjc.es (B.H.)
* Author to whom correspondence should be addressed; E-Mail: jose.iglesias@urjc.es;
Tel.: +34-91-488-8565; Fax: +34-91-488-7068.
Academic Editor: Rafael Luque
Received: 6 October 2015 / Accepted: 4 November 2015 / Published: 12 November 2015
Abstract: Zr-SBA-15 Lewis acid catalyst has demonstrated an outstanding catalytic activity
in the reduction of several carbonyl compounds by means of Meerwein Ponndorf Verley
(MPV) reaction, using several secondary alcohols, and showing a very high selectivity
towards the desired products. Special focus was addressed in the catalytic activity of
Zr-SBA-15 material in the production of furfuryl alcohol from furfural, which is an
important reaction for the lignocellulosic biomass valorization. In this transformation, both
the reaction temperature and the i-PrOH:Furfural molar ratio exert a positive influence on
the rate of the MPV transformation, with the influence of the former being much higher.
i-propyl-furfuryl ether, a by-product resulting from the etherification of the target product
with the sacrificing alcohol, is also found together with the main product. The production of
this side-product is highly influenced by the reaction temperature, so that low temperatures
and high sacrificing alcohol to substrate molar ratios have to be applied to keep its production
at low levels.
Keywords: mesoporous materials; heterogeneous catalysis; Lewis acids; hydrogen
transfer; zirconium
OPEN ACCESS
Catalysts 2015, 5 1912
1. Introduction
The Meerwein Schmidt Ponndorf Verley Oppenauer reaction (MPV) is a conventional procedure,
known for more than 80 years [1–3], conventionally used for reducing carbonyl groups into carbinols
by H-transfer using a sacrificing alcohol as hydrogen source. In this reaction, the sacrificing alcohol,
usually a secondary alcohol, is transformed into a ketone, whereas the starting carbonyl group, either a
terminal (aldehyde) or secondary (ketone) group, is reduced into an alcohol functionality, because of the
net transfer of a hydrogen molecule. This type of reduction is well known, but scarcely used in organic
synthetic chemistry, mainly because of the existence of highly efficient reagents able to drive the
controlled reduction of carbonyl compounds. However, the simplicity of the MPV process, its low
requirements for reactant purity, the great chemoselectivity of the transformation other insaturations
such as double and triple bonds remain unaffected, or the mild reaction conditions required to achieve
good product yields, are important advantages that makes the MPV a low-cost industrially-scalable
reduction process.
Typical MPV-active catalysts are based on transition metal complexes, including aluminum [4,5],
boron [6], magnesium [7] and zirconium [8] complexes. Despite the several advantages of these
homogeneous catalysts, their heterogeneous counterparts are preferred because of practicality, due to
their easier separation from the reaction media or their higher resistance against moisture. In this way,
important efforts have been applied to the development of heterogeneous catalysts for MPV reactions,
finding heterogeneous catalysts equivalent to those reported under homogeneous conditions. These
include a large variety of different compounds, but the use of metal oxides and grafted alkoxides seems
to be the most promising alternative. Among these materials, those based on zirconium as the active
species [9] have received much attention because of the extraordinary activity showed by this metal in
MPV hydrogen-transfer reactions, for its high resistance against water deactivation, and for the easiness
in which this heteroatom can be heterogenized onto solid supports [10,11]. In this way,
zirconium-functionalized heterogeneous catalysts used in MPV transformations comprise a large variety
of solids, including hydrous zirconia [9,12–14] both supported onto other supports like silica and as bulk
material, zirconium-functionalized silicas [12–14] and zirconium-containing zeolites [15–19], most of
them based on the BEA structure. From this whole collection of materials, hydrous zirconia displays the
lowest intrinsic catalytic activity, though it is cheap and easy to prepare [20]. On the contrary, isolated
zirconium species, such as those present in zeolites, display an outstanding intrinsic catalytic activity,
although their preparation is quite more complicated [15] and it requires of expensive reagents.
Moreover, bulk substrates cannot be converted because of the limited access to the catalytic sites located
at the microporous zeolitic structure. Within this context, the incorporation of zirconium species onto
mesoporous supports, most of them silicas, has revealed to be an excellent alternative to both hydrous
zirconia and zirconium-functionalized zeolites. Thus, zirconium species have been incorporated onto the
surface of several mesoporous silicas like MCM-41 [20,21], SBA 15 [22] and TUD-1 [23,24]. These
materials display an open porous structure with pore sizes in the range of mesopores, which facilitates
the diffusion of bulk molecules inside the material, minimizing mass transfer hindrances. As for the
functionalization methods used to incorporate the active phase onto the solid support, grafting
procedures are preferred over direct syntheses in which the active phase is added together with the rest
of the precursors during the synthesis of the solid support, since the former option leads to more
Catalysts 2015, 5 1913
accessible metal sites. On the contrary, direct synthesis procedures ensure a better dispersion of the active
phases, and thus the isolation of heteroatoms is usually higher, so that a higher intrinsic catalytic activity
per catalytic site is expected in these materials.
Our previous investigations on the synthesis and use of Zr-SBA-15 have revealed a highly active
catalyst in several acid-driven reactions [25–28]. The high activity of this material has been attributed
not to the acid strength of the supported active species weak Lewis acid sites, but to the high accessibility
of the same, which occupy accessible locations on the surface of the mesopores. This feature is due to
the synthesis method and the used metal precursor, which facilitates the interaction between the
hydrophobic core of the structure directing agent micelles and the zirconium precursor, zirconocene
dichloride [27]. In this way, upon surfactant removal by calcination, the metal sites are settled on the
surface of the mesostructured support occupying accessible locations.
Within the present investigation we have taken a step forward in the investigation of the catalytic
activity of this material, in the catalytic reduction of carbonyl-containing compounds. The influence of
different operating parameters, such as the catalyst loading, reaction temperature, type of sacrificing
alcohol and secondary alcohol to substrate molar ratio, were first investigated in the reduction of
different ketones and aldehydes. Finally, the reduction of furfural, a highly versatile biomass-derived
chemical compound, has been investigated.
2. Results and Discussion
Figure 1. Characterization results for Zr-SBA-15 catalyst: (A) N2 adsorption-desorption
isotherm and pore sizes distribution (inset); and (B) X-Ray Diffraction patterns at high and
low (inset) angle range.
Zr-SBA-15 material displays type IV N2 adsorption-desorption isotherm, featured with a steep H1
hysteresis loop, typical from samples with narrow pore sizes distributions (Figure 1A). Metal loading
found in this sample is very high (8.3 wt. %) in comparison with the rather low acid loading
Catalysts 2015, 5 1914
(<0.38 meq·g−1) calculated from NH3 TPD analysis. This fact could be attributed to the formation of
zircon (ZrSiO4) and zirconium dioxide (ZrO2) particles, whose presence onto the Zr-SBA-15 material is
evident in XRD (Figure 1B) [27]. In this way, although quite a high proportion of the zirconium species
added to the synthesis media are successfully supported onto the Zr-SBA-15 material, a fraction of the
same would not be active in MPV reactions.
2.1. MPV Reduction of Cyclohexanone
The activity of Zr-SBA-15 materials in MPV reactions was first evaluated through the assessment of
their catalytic activity in the reduction of cyclohexanone to cyclohexanol. In order to get a deeper
understanding of the performance of this catalyst, the influence of the catalyst loading and the reaction
temperature were studied.
Figure 2 depicts the results achieved in the MPV reduction of cyclohexanone with i-PrOH. Selectivity
of the transformation was, in every case, close to 100%, since no by-products were detected under the
tested reaction conditions. As for the catalytic behavior of Zr-SBA-15, this evidenced a positive net
influence in the conversion of the ketone towards cyclohexanol, as compared to a blank reaction test,
performed in absence of catalyst, where negligible substrate conversion was detected. The analysis of
the kinetic data (Figure 2A) suggests that the acceleration driven by the Zr-SBA-15 material is high
enough to ensure an almost complete conversion of the substrate into the desired product in just 6 h. The
high substrate conversion obtained in presence of these materials seems, however, to be a consequence
of the use of a quite high amount of catalyst in the reaction media (catalyst to substrate mass ratio = 1.0).
Product yield is largely influenced by the amount of catalyst, as it is evident from the results achieved
when using different substrate to catalyst mass ratios (4.0–1.0; catalyst loadings of 0.05–0.20 g;
Figure 2B). In this way, reducing the catalyst loading in the reaction media to 0.05 g (substrate:catalyst
mass ratio of 4.0) led to a significant decreasing in product yield, though 37% of total amount of substrate
was transformed into cyclohexanol. This behavior could be linked to the high amount of sacrificing
alcohol in the reaction media. The interaction of the secondary alcohol with the catalytically active
zirconium sites could lead to the saturation of the metal coordination sphere, reducing its ability to bind
the ketone substrate, thus decreasing the intrinsic catalytic activity [22,29,30]. This influence is much
more pronounced as the catalyst loading decreases because of the higher proportion of sacrificing alcohol
to catalyst.
As for the influence of the reaction temperature, decreasing this parameter (Figure 2C) yields, as
expected, lower amounts of the reaction product. Nevertheless, for the considered reaction time 6 h,
100 °C is suitable to provide 98% of cyclohexanol, so that, higher temperature conditions are not
required to drive this transformation further.
For comparison purposes, two reference catalytic tests, carried out in presence of zircon and
zirconium dioxide, were accomplished. Zircon and hydrous zirconia were selected because of their
presence in the Zr-SBA-15 material, as stated from XRD, to ensure their catalytic capability in MPV
reactions and assess their influence in that shown by Zr-SBA-15. Both materials produced negligible
substrate conversion or product yields, under the tested reaction conditions (T = 110 °C, t = 6 h; substrate
to catalyst mass ratio = 1.0), indicating that the catalytic activity of Zr-SBA-15 comes not from the
presence of these crystalline dense phases of zirconium silicate and hydrous zirconia, but from other
Catalysts 2015, 5 1915
types of zirconium species, placed at accessible locations onto the surface of the mesostructured material.
The catalytic sites can be present either as isolated zirconium sites or as zirconium-oxygen-zirconium
species [28], which have been proved, both of them, to be catalytically active in Lewis-acid
driven reactions [31].
Figure 2. Reaction results from the MPV reduction of cyclohexanone with i-PrOH in
presence of Zr-SBA-15. (A) Influence of the reaction time. Temperature: 110 °C; Catalyst
loading: 0.2 g; i-PrOH to cyclohexanone molar ratio: 50; Cyclohexanone to catalyst mass
ratio: 1.0. (B) Influence of the catalyst loading. Reaction time: 6 h. (C) Influence of the
reaction temperature. Reaction time: 6 h; Catalyst loading: 0.2 g.
Table 1 lists the results obtained in the reduction of different ketones with a variety of secondary
alcohols, looking for the influence of the different substituents in both reactants on the catalytic behavior
of Zr-SBA-15. Thus, in a first set of experiments, different secondary alcohols have been assayed in the
reduction of cyclohexanone (entries 1–4). Increasing the size of the secondary alcohol leads to lower
conversion rates, as it is inferred from the lower product yield obtained in the reduction of
cyclohexanone insofar as the alkyl chain in the sacrificing alcohol increases. However, when using
2-methyl-cyclohexanol as secondary alcohol (entry 4), a higher product yield is obtained (71%),
probably as a consequence of the more accessible conformation of the carbonyl group in the cyclic
ketone. As for the influence of the structure of the ketone substrate (entries 5–7), higher reaction rates
are detected for the smaller molecules (2-butanone compared to penta- and hexanone), being the
presence of steric hindrances around the carbonyl group, and the subsequent difficulty in the
coordination of the same to the zirconium catalytically active sites, the most probable causes of lower
activities. In this way, 2-butanone is almost quantitatively transformed into its corresponding alcohol,
whereas 2-methyl-cyclohexanone is scarcely reduced (8%).
Catalysts 2015, 5 1916
Ethyl levulinate and furfural have also been tested as substrates in these preliminary MPV tests
(Table 1, entries 8 and 9). In the first case, the reduction of the carbonyl group at ethyl levulinate leads
to the production of 4-hydroxy-ethyl valerate, which evolves through an intramolecular
transesterification pathway towards γ-valerolactone (GVL), decreasing the selectivity of the process to
the desired alcohol (42%). On the other hand, the low reaction rate observed for this substrate could be
attributed to the presence of the carboxylate group in the ester moiety, which can lead to different
molecular adsorption modes on the zirconium sites [32] either through the carbonyl or through the
carboxylic acid group. In this way, the former is favorable to MPV conversion, whereas the latter is not,
yielding only 24% of the desired alcohol product.
As for the reduction of furfural, in contrast with the results provided by other authors for the reduction
of aldehydes in presence of similar catalytic systems to those applied hereby [22,33], furfural is
converted in a lower extension, as compared to the other tested molecules, into its corresponding alcohol,
probably as a consequence of two different reasons: the lower reactivity of the carbonyl group due to the
electron donating nature of the furan ring, and the planar adsorption mode of the same onto the support
surface silanol groups [34] thus leading to a competition with the adsorption on zirconium active sites
and reducing the MPV reaction rate. However, the high interest for furfuryl alcohol, which serves as
starting material in numerous organic syntheses [35] and the simplicity of the catalytic system, has
prompted us to investigate the influence of several operation parameters looking for the best reaction
conditions to enhance this transformation.
Table 1. Results from MPV catalytic tests in presence of Zr-SBA-15.
Entries Sacrificing alcohol Substrate Product yield (mol %)
1 2-Propanol Cyclohexanone 99
2 2-Butanol Cyclohexanone 97
3 Cyclopentanol Cyclohexanone 24
4 2-methyl cyclohexanol Cyclohexanone 71
5 2-Propanol 2-Butanone 85
6 2-Propanol Cyclopentanone 21
7 2-Propanol 2-methyl cyclohexanone 8
8 2-Propanol Ethyl levulinate 42 a
9 2-Propanol Furfural 24
Reaction conditions: Sacrificing alcohol 4 g; Catalyst loading: 0.2 g; Reaction temperature 110 °C; Sacrificing
alcohol to substrate molar ratio of 30. a Includes γ-valerolactone (GVL), coming from the
intra-transesterification of the MPV product.
2.2. MPV Reduction of Furfural
The study of the catalytic performance of Zr-SBA-15 catalyst in the MPV reduction of furfural has
been investigated by using i-PrOH as sacrificing alcohol to produce furfuryl alcohol. For this purpose,
different reaction parameters have been assayed: the reaction time, the reaction temperature, and the
sacrificing alcohol to substrate molar ratio. Figure 3 depicts the influence of the reaction temperature, as
a function of time, on the conversion of furfural and the yield towards the detected products present in
the reaction media. First thing to emphasize is that furfural is converted in high extension only if the
applied reaction temperature is raised from 90 up to 130 °C. Under these conditions (130 °C), furfural
Catalysts 2015, 5 1917
is completely converted after 4 h of reaction. However, the existence of side reactions leads to the
formation of undesired by-products such as that coming from the etherification between furfuryl alcohol
and the sacrificing alcohol (i-propyl-furfuryl ether), which is also present in the reaction media. Alcohol
etherification is usually accomplished by the reaction in the presence of mild acid catalysts [36,37], and
bearing in mind that a blank reaction performed by heating a solution of furfuryl alcohol in i-PrOH not
shown, does not lead to the formation of the ether product, Zr-SBA-15 material necessarily plays a
crucial role in the assembly of this chemical from furfuryl alcohol. However, the influence of the catalyst
seems to be, though necessary, also limited, since the assays performed under different temperature
conditions lead to very different yields towards furfuryl alcohol and i-propyl-furfuryl ether. Thus, the
former appears as the main product only at low temperatures (90 °C, Figure 3A), reaching a product
yield of 41%, for a substrate conversion of 54% after 6 h of reaction. Under these conditions, only 15%
of the starting furfural is converted into the ether by-product. Increasing the reaction temperature up to
110 °C does not improve the production of the target product, obtaining a furfuryl alcohol yield (29%)
lower than that achieved at 90 °C, and being the amount of produced i-propyl-furfuryl ether much higher
(38%). In this way, insofar as the reaction temperature is increased, the yield to i-propyl-furfuryl ether
is also enhanced, becoming the main reaction product at 130 °C (85%), and providing a poor 9% yield
towards furfuryl alcohol.
Results obtained from experiments performed at different temperatures have been used to perform a
kinetic study of the identified reactions by formulating a pseudo-homogeneous first-order kinetic model
(Equations (1)–(3), Supplementary Information) for both chemical transformations, the reduction of
furfural and the etherification of furfuryl alcohol with the hydrogen-donor solvent 2-propanol. This
model was fitted to the experimental data by means of non-linear regression, applying the simplex
optimization method, to find the optimal values for the kinetic constants, minimizing the sum of squares
error, which has been used as objective function (Equation (4), Supplementary Information). In this
function, the error was defined as the difference between the experimental value of concentration for
each reaction time and the predicted concentration value for the different products involved in the
proposed reaction scheme.
The kinetic model fitted quite fine to the experimental concentration values obtained for each
compound during the overall reaction temperatures (Figure S1, Supporting Information). Thus, the
obtained kinetic constants for the different assayed reaction temperature values were fitted to an
Arrehnius model, providing the pre-exponential and apparent activation energy values for each reaction
(Table S1, Supplementary Information). The results reveal that the fastest reaction is that corresponding
to the etherification of the MPV reaction product, which is, in addition, more sensitive to the reaction
temperature as compared to the MPV transformation of furfural, which is congruent with the results
previously described as well as with the higher reactivity reported for furfuryl alcohol, as compared
to furfural.
Catalysts 2015, 5 1918
Figure 3. MPV reduction of furfural with i-PrOH in presence of Zr-SBA-15—Influence of
the reaction temperature. Reaction Conditions: Catalyst loading: 0.2 g; i-PrOH to furfural
molar ratio: 50; furfural to catalyst mass ratio: 1.0; X = Substrate conversion;
Y = Product Yield.
As for the influence of the sacrificing alcohol to substrate molar ratio on the catalytic performance of
Zr-SBA-15 material in the MPV reduction of furfural, this has been assessed in the range
25:1–200:1. Furfural conversion and yields (left axes) and selectivities (right axes) towards the different
products have been depicted in Figure 4. As in the previously described assays, i-propyl-furfuryl ether
is produced together with furfuryl alcohol under all the tested experimental conditions, as a consequence
of the etherification of the same with the sacrificing alcohol. In terms of product yield, the influence of
the i-PrOH:Furfural molar ratio is positive for the production of furfuryl alcohol.
Catalysts 2015, 5 1919
Figure 4. MPV reduction of furfural with i-PrOH in presence of Zr-SBA-15—influence of
the sacrificing alcohol to substrate molar ratio. Reaction Conditions: Catalyst loading: 0.2 g;
reaction temperature: 90 °C; furfural to catalyst mass ratio: 1.0; i-PrOH to furfural molar
ratio: (A) 25:1, (B) 50:1; (C) 100:1; (D) 200:1. (X = Substrate conversion; Y = Product Yield;
S = Selectivity towards the product).
Thus, a poor conversion of furfural is achieved (24%) for the lower amount of i-PrOH present in the
reaction media (i-PrOH:Furfural = 25:1, Figure 4A), yielding 18% of furfuryl alcohol with a low
production of i-propyl-furfuryl ether. Increasing the amount of sacrificing alcohol in the reaction media,
Catalysts 2015, 5 1920
as previously observed for cyclohexanone, progressively enhances the production of the target product,
up to yield 45% of the starting furfural as furfuryl alcohol (Figure 4D), when using a sacrificing alcohol
to substrate molar ratio of 200. The production of i-propyl-furfuryl ether follows a similar trend to that
observed for furfuryl alcohol with the amount of i-PrOH in the reaction media.
Thus, a higher production of this by-product is observed as the i-PrOH:Furfural molar ratio increases,
which is not only due to a higher concentration of furfuryl alcohol in the reaction media, but also because
a higher quantity of i-PrOH drives faster the etherification of the target product, which is inferred from
the steeper slopes in the selectivity curves (Figure 4, right axes) . However, the influence of i-PrOH:Furfural
molar ratio seems to be limited as compared to that observed for the reaction temperature. Thus, under
the most favorable conditions (i-PrOH:Furfural molar ratio of 200), i-propyl-furfuryl ether production
reaches maximum yields around 14%, which involves a maximum selectivity towards this product of
27%. These values are quite low in comparison to those obtained when increasing the reaction
temperature i-propyl furfuryl ether becomes the major product at 110 °C, supporting the fact that the
influence of the reaction temperature is, in this case, much higher than the proportion of the different
reactants involved in the MPV transformation.
From the previous results it can be concluded that low reaction temperatures (≤90 °C) as well as high
i-PrOH:Furfural molar ratios (≥200:1) are required to maximize the production of furfuryl alcohol by
MPV reduction of furfural in presence of the Zr-SBA-15 material. However, the formation of
i-propyl-furfuryl ether as a by-product seems to be inevitable when furfuryl alcohol is present in a
practical concentration in the reaction media, leading to the decrease of the achieved yield towards the
desired product. However, this by-product finds interesting applications as biofuel additive, so that its
production could also provide benefits.
Nevertheless, regarding the catalytic activity of Zr-SBA-15 in the studied transformation, it must be
noted that, although either increasing the reaction temperature, or the i-PrOH:Furfural molar ratio, leads
to the consumption of furfuryl alcohol in favor of i-propyl-furfuryl ether, there are important features
that support the good performance of Zr-SBA-15 as catalyst for the MPV reduction of furfural. Thus,
the existence of i-propyl-furfuryl ether is only a consequence of the prior formation of furfuryl alcohol
in a very fast manner—much faster as the reaction temperature increases, which obviously involves the
MPV reduction of furfural. In this way, if furfuryl alcohol and i-propyl-furfuryl ether are considered
together as products coming from the MPV reduction of the substrate, an outstanding combined yield of
94% (94% selectivity) towards furfural-reduction-derived products is obtained when operating at 130 °C.
This involves a second feature that evidences the good catalytic behavior in MPV reactions, which is the
absence of secondary by-products—obtained from side-reactions such as the acetalization of
furfural [38], which is produced by reaction of furfural with alcohols in presence of weak acid catalysts.
Remarkably, these a priori plausible by-products are absent in the reaction media even when operating
with the maximum quantity of sacrificing alcohol, supporting the good selectivity of the Zr-SBA-15
catalysts towards H-transfer reactions.
A comparison between Zr-SBA-15 and hydrous zirconia, a typical catalyst used in MPV
transformations, has been established. This comparison was performed in order to determine whether
the mesostructured catalyst involves, or not, catalytic advantages over the conventional catalyst to
convert furfural into furfuryl alcohol through a MPV reduction.
Catalysts 2015, 5 1921
Figure 5 shows the results in terms of product yield at 3 different temperatures: 90, 110 and 130 °C.
Unlike Zr-SBA-15, hydrous ZrO2 provided increasing furfuryl alcohol yields with temperature, although
the achieved values were very poor, even at 130 °C, yielding ~11% of furfuryl alcohol, and evidencing
a much lower catalytic activity than Zr-SBA-15 material.
Figure 5. MPV reduction of furfural with i-PrOH—catalyst screening. Reaction Conditions:
Catalyst loading: 0.2 g; reaction temperature: 90–110–130 °C; furfural to catalyst mass ratio:
1.0; i-PrOH to furfural molar ratio: 50; reaction time: 6 h.
The low catalytic activity shown by the hydrous zirconia could be ascribed to the low textural
properties of this material. Despite the zirconium content is higher in hydrous ZrO2 than in Zr-SBA-15,
its lower surface area (~10 m2/g) limits the extent of the reaction interface, so that a low conversion of
the substrate into furfuryl alcohol is achieved. However, the most interesting difference between
Zr-SBA-15 and hydrous zirconia catalysts can be the absence of the by-product furfuryl-isopropyl
ether—when using hydrous ZrO2. This difference between both materials could be ascribed to lower
acidity of hydrous ZrO2, as compared with Zr-SBA-15. The high dispersion achieved for zirconium
species in Zr-SBA-15 material leads to a great extension of the reaction interface, leading to a higher
catalytic activity, but also to the presence of isolated zirconium species, which are associated to the
occurrence of acid sites. These sites are the responsible for the transformation of furfuryl alcohol into
furfuryl-isopropyl ether. Since these are absent in hydrous zirconia, this material is inactive in the
etherification reaction, even if high temperature values are applied.
Finally, γ-alumina—a common Lewis-acid catalyst used in MPV transformations—was also used as
reference catalysts. In this case, unlike hydrous zirconia, γ-alumina provided furfuryl alcohol yield of
52% after 6 h at 90 °C (Catalyst loading: 0.2 g; furfural to catalyst mass ratio: 1.0; i-PrOH to furfural
molar ratio: 50), a comparable result to that shown by Zr-SBA-15. Nevertheless, if considering the
completely different metal loading shown by g-alumina and Zr-SBA-15 material, the results, as in the
case of hydrous zirconia, confirm a superior intrinsic catalytic activity in the mesostructured material,
probably as consequence of the better catalytic performance of isolated zirconium species as compared
to bulk zirconia or bulk aluminum oxide-based materials.
Catalysts 2015, 5 1922
In order to check the reusability of Zr-SBA-15 material, recycling tests were performed for four
consecutive reaction tests, involving intermediate washing with solvents between recycling tests 1 and
2, and reactivation by calcination for the rest of the recycling tests (Figure S2; Supporting Information).
Results evidence a strong catalytic activity decay from the first to the second consecutive reaction runs,
probably as consequence of deposition of products and product intermediates onto the catalytic sites,
thus preventing the access of new reactant molecules. Intermediate washing was applied between the
reaction tests accomplished in presence of the fresh catalyst and the first recycling test. In this way, some
of the organic compounds deposited onto the catalytic centers could remain adsorbed even after the
washing treatment, thus leading to catalytic activity decay. On the contrary, intermediate calcination,
which was applied for the rest of the study on the reusability of the Zr-SBA-15 material, allowed, though
not fully, a partial recovering of the initial catalytic activity. However, this seemed to be stabilized
between the second and the third recycling tests. These results suggest that the starting
Zr-SBA-15 material contain some unstable catalytic sites, which are loss during the first assay, and
afterwards, the material shows quite a stable behavior, though calcination has to be applied between
consecutive reaction runs in order to completely remove organic deposits from the catalysts.
3. Experimental Section
3.1. Materials and Methods
Zirconocene dichloride (ZrCl2Cp2, Aldrich, San Luis, MO, USA) and tetraethyl orthosilicate (TEOS,
Aldrich, San Luis, MO, USA) were used as metal and silicon precursors, together with Pluronic P123
(Aldrich, San Luis, MO, USA) as structure directing agent, in the synthesis of Zr-SBA-15. 2-propanol
(i-PrOH, Scharlab, Barcelona, Spain) and 2-butanol (2-BuOH, Scharlab, Barcelona, Spain) have been
used, without previous purification, as solvents and sacrificing alcohols in MPV reaction tests.
Cyclohexanone, furfural and ethyl levulinate (Aldrich, San Luis, MO, USA) were used as substrates in
MPV reaction tests without previous purification. Zirconium silicate (Zircon, Aldrich, San Luis, MO,
USA), and hydrous zirconia [39] were used as reference catalytic materials for comparison purposes.
3.2. Synthesis of Catalysts
Zr-SBA-15 catalyst was prepared according to the method previously described in literature [27]. In
a typical synthesis, ZrCl2Cp2 is dissolved in a P123-containing hydrochloric acid aqueous solution at
40 °C. After two hours, an appropriate amount of TEOS is added into the dark-red solution and the
resultant suspension is stirred for 20 h at the same temperature. The resultant solution is then transferred
into a Teflon-lined stainless-steel autoclave to be hydrothermally aged at 130 °C for 24 h. The resultant
material is recovered by filtration and air-dried overnight. Surfactant removal is then accomplished by
calcination in air at 450 °C for 5 h to provide the final material as a white powder.
3.3. Catalyst Characterization
Physicochemical properties of Zr-SBA-15 material have been determined by means of a wide variety
of analytical techniques. Textural properties—surface area, pore size and volume—were determined
from nitrogen adsorption-desorption isotherms recorded at 77 K on a Micromeritics TriStar-3000 unit.
Catalysts 2015, 5 1923
Surface area was calculated from BET equation, whereas mean pore size was obtained as the maximum
of the pore sizes distribution applying the BJH method with the KJS correction. Total pore volume was
assumed to be that recorded at p/p0 = 0.985. Mesoscopic ordering and the presence of crystalline phases
were assessed by means of XRD recorded on a Philips X’pert unit, using the Cu Kα line and recording
the diffraction patterns in the 2θ range from 0.5 to 5° for long range ordering and from 5 to 50° for
atomic ordering. Metal content was determined by means of IPC-OES using a Vista Pro IPC
spectrophotometer (Varian Inc., Palo Alto, CA, USA), previously calibrated with Zr-containing standard
stock solutions. Acid loading and strength was evaluated by means of ammonia thermal programmed
desorption experiments performed on a Autochem 2900 unit (Micromeritics, Norcross, GA, USA).
Table 2 lists the physicochemical properties of the Zr-SBA-15 catalyst. Textural propert ies,
calculated by means of N2-adsorption/desorption experiments, as well as XRD patterns are those
typically attributed to mesostructured samples with SBA-15 topology.
Table 2. Physicochemical properties of Zr-SBA-15 catalyst.
Sample SBET
a (m2·g−1) Vp
b (cm3·g−1)Dp
c (Å) a0 d (Å) Zr e (%w/w) Acid
f (meq·g−1)
Zr-SBA 553 1.26 123 135 8.3 0.38
a Specific Surface area calculated by the B.E.T. method. b Total pore volume recorded at p/p0 = 0.985.
c Mean pore size calculated as the maximum of the B.J.H. pore sizes distribution applying the K.J.S. correction.
d Unit cell size calculated as 2/(√3·d100), where d100 is the Bragg’s lattice parameter obtained as
(d100 + √3·d110 + √4·d200)/3. e Metal loading calculated by means of ICP-OES. f Acid loading calculated by NH3
temperature programmed desorption analysis.
3.4. MPV Catalytic Reaction Tests
The catalytic activity of Zr-SBA-15 material in the MPV reactions was evaluated under different
operating conditions using different sacrificing alcohols. Prior to its use in these tests, Zr-SBA-15 was
calcined in air. In a typical assay, 0.2 g of the calcined catalyst were mixed together with 5 mL of a
solution of the carbonyl substrate in the sacrificing alcohol (typically with a sacrificing alcohol to
substrate molar ratio of 50:1). The resultant suspension was transferred into a 10 mL ACE pressure flask,
hermetically closed and warmed up to the reaction temperature. The reaction was then allowed to
proceed for 6 h before cooling down in an ice/water bath to quench the reaction. Reaction media aliquots
were periodically withdrawn and product analyses were conducted on a gas chromatograph fitted with a
FID detector and using a Zebron ZB-WAXPLUX capillary column (L: 30m × ID: 0.53 mm × df: 1 μm,
Phenomenex, Torrance, CA, USA).
4. Conclusions
Zr-SBA-15 material has proved to be a highly active catalyst in MPV reductions of both ketones and
aldehydes. The material has displayed an outstanding catalytic activity in the reduction of cyclic ketones,
such as cyclohexanone, with several secondary alcohols, including i-propanol and 2-butanol, showing a
very high selectivity towards the desired products. It is also effective in the reduction of a variety of
linear ketones, though the structure of both the substrate and the sacrificing alcohol plays a crucial role
in the extent of the MPV transformation. The catalytic activity of Zr-SBA-15 material has been extended
Catalysts 2015, 5 1924
to the preparation of furfuryl alcohol by MPV reduction of furfural with i-propanol. The influence of
several reaction parameters, such as the reaction temperature or the i-PrOH:Furfural molar ratio, has
been investigated on the catalytic performance of Zr-SBA-15 material. Both parameters exert a positive
influence on the rate of the MPV transformation, being the influence of the reaction temperature much
higher. Together with the target product (furfuryl alcohol) the by-product coming from its etherification
with the sacrificing alcohol (i-propyl-furfuryl ether) is also found under all the tested experimental
conditions. The yield to this by-product is highly influenced by the reaction temperature, so that low
temperatures have to be applied to keep its production at low levels. However, i-propyl-furfuryl ether is
just a consequence of the previous MPV reduction of furfural, and thus, its presence in high yields in the
reaction media is just a consequence of the high activity shown by Zr-SBA-15 in the MPV reduction
of furfural.
Acknowledgments
Financial support from the Spanish Ministry of Economy and Competitiveness (Project
CTQ2014-52907-R) and from the Regional Government of Madrid (Project S2013/MAE-2882) is kindly
acknowledged. B. Hernández thanks the Spanish Ministry of Economy and Competitiveness for a FPI
grant (BES-2012-061144).
Author Contributions
M.P, A.C., & B.H. conducted the experimental part of the investigation, under the supervision of J.I.,
J.A., & G.M. Finally, J.I., J.M., Y.S., & M.P processed the results, performed their discussion, and wrote
the manuscript.
Conflicts of Interest
The authors declare no conflict of interest.
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