MOFs as multifunctional catalysts: One-pot synthesis of menthol from citronellal over a bifunctional MIL-101 catalyst

Article abstract of DOI:10.1039/c2dt12480g

A bifunctional MOF catalyst containing coordinatively unsaturated Cr ³⁺ sites and palladium nanoparticles (Pd@MIL-101) has been used for the cyclization of citronellal to isopulegol and for the one-pot tandem isomerization/hydrogenation of citronellal to menthol. The MOF was found to be stable under the reaction conditions used, and the results obtained indicate that the performance of this bifunctional solid catalyst is comparable with other state-of-the-art materials for the tandem reaction: Full citronellal conversion was attained over Pd@MIL-101 in 18 h, with 86% selectivity to menthols and a diastereoselectivity of 81% to the desired (-)-menthol, while up to 30 h were necessary for attaining similar values over Ir/H-beta under analogous reaction conditions. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2dt12480g

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PAPER
MOFs as multifunctional catalysts: One-pot synthesis of menthol from
citronellal over a bifunctional MIL-101 catalyst†
F. G. Cirujano, F. X. Llabrés i Xamena* and A. Corma*
Received 23rd December 2011, Accepted 27th January 2012
DOI: 10.1039/c2dt12480g
A bifunctional MOF catalyst containing coordinatively unsaturated Cr3+ sites and palladium nanoparticles
(Pd@MIL-101) has been used for the cyclization of citronellal to isopulegol and for the one-pot tandem
isomerization/hydrogenation of citronellal to menthol. The MOF was found to be stable under the
reaction conditions used, and the results obtained indicate that the performance of this bifunctional solid
catalyst is comparable with other state-of-the-art materials for the tandem reaction: Full citronellal
conversion was attained over Pd@MIL-101 in 18 h, with 86% selectivity to menthols and a
diastereoselectivity of 81% to the desired (−)-menthol, while up to 30 h were necessary for attaining
similar values over Ir/H-beta under analogous reaction conditions.
of bifunctional acid–base MOFs, namely UiO-66(NH2), which
Introduction
contained basic amino groups close to coordinatively unsaturated
Zr4+ acid sites generated in situ by controlled dehydroxylation.
In recent years, metal–organic framework compounds (MOFs)
have attracted much interest for their potential as heterogeneous
catalysts.1–3 Besides their high surface areas and pore volumes,
which facilitate diffusion and the interfacial contact between the
active site and the reagents, much of the potential of MOFs as
heterogeneous catalysts relies on the possibility to fine-tune their
chemical composition, pore dimensions and chemical environ-
ment inside the pores, by either selecting the appropriate build-
ing units or by post-synthesis modifications.4 The high flexibility
in the design of MOFs facilitates the engineering of the catalytic
sites of the material, since a number of different strategies can be
used to introduce active centers. On one hand, the inorganic
component can bear an active site, such as coordinatively unsatu-
rated metal ions showing Lewis acidity or redox properties, or
they can be used as anchoring points for introducing additional
functionalities.5 On the other hand, the organic ligands can also
hold functional groups,6,7 or they can be designed as coordi-
nation points to introduce metal active centers.8,9 Finally, the
available pore space inside the MOFs can be efficiently used to
introduce a variety of encapsulated species, including metal or
metal oxide nanoparticles,10,11 polymers or discrete molecules,
which can bring about new catalytic functions. However, in spite
of this high tunability, the simultaneous introduction in MOFs of
two or more active sites is an elusive subject, being the number
of existing reports on multifunctional MOF catalysts very scarce
to date.12–14 Vermoortele et al.12 have described the preparation
The authors used this material for the cross aldol-condensation
of benzaldehyde and heptanal to form jasminaldehyde. For this
reaction, the acid–base pairs work in a concerted way to activate
simultaneously the two reactants, as was previously shown with
other inorganic and organic catalysts with acid–base pairs.15–18
Similarly, Wu et al.13 prepared a lanthanide MOF containing
Lewis acid Tb3+ and Lewis base triphenylamine sites, and used
it for Knoevenagel an cyanosilylation reactions. Pan et al.14
reported on the use of Pd@MIL-101 as a bifunctional Lewis
acid/hydrogenation catalyst, which allowed the one-pot synthesis
of methyl isobutyl ketone from acetone and H2, through conden-
sation, dehydration and hydrogenation. In the present work we
have used a bifunctional hydrogenation/acid catalyst based on
the Cr3+-containing MIL-101 MOF19 that shows excellent
activity and selectivity for the one-pot two-steps synthesis of
(−)-menthol from citronellal.
(−)-Menthol is a chemical compound widely used in flavour-
ing and pharmaceutical applications. Its synthesis is usually
carried out by multistep processes, such as the Haarmann and
Reimer process starting from thymol,20 and the Takasago
process starting from myrcene.21 However, another alternative
method consists in the one-pot sequential transformation of citro-
nellal (1) into isopulegol (2),22 followed by hydrogenation to
menthol, as shown in Scheme 1. Besides isopulegol, citronellal
cyclization can lead to three other stereoisomers (3, 4 and 5),
which after hydrogenation lead to four menthol stereoisomers, of
which only that coming from isopulegol (2) is the desired
product. Thus, it is important to determine the diastereoselectiv-
ity ratio to evaluate the performance of a catalyst for this reac-
tion. To afford citronellal transformation into menthol in a one-
pot, a bifunctional catalyst bearing acid and metal sites can be
used. Jacobs et al.23,24 have reported that a 3% Ir-impregnated
Instituto de Tecnología Química UPV-CSIC, Universidad Politécnica de
Valencia, Consejo Superior de Investigaciones Científicas, Avda. de los
Naranjos, s/n, 46022 Valencia, Spain. E-mail: acorma@itq.upv.es,
fllabres@itq.upv.es; Fax: +34 963877809
†Electronic supplementary information (ESI) available. See DOI:
10.1039/c2dt12480g
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 4249–4254 | 4249

Scheme 1
Fig. 1 (a) Time-conversion plots of citronellal (1) and yields to isopulegol (2) and other isopulegols (3–5) obtained using Cr3+–MIL-101 as catalyst.
(b) Comparison of the citronellal conversion obtained using Cr3+–MIL-101 or Pd@MIL-101 as catalyst. (c) Time-conversion plots of citronellal (1)
and yields to isopulegol (2) and other isopulegols (3–5) obtained using Pd@MIL-101 as catalyst. (d) Time-evolution of products during hydrogenation
of isopulegols to menthols.
H-BEA zeolite catalyzes the one-pot synthesis of menthol from
citronellal in two steps. First, the cyclization step was carried out
under N2 atmosphere, and then H2 was added to carry out the
hydrogenation. Complete citronellal conversion was achieved
after 30 h, with 95% overall selectivity to the menthol isomers,
of which 75% was the desired (−)-menthol. When this tandem
reaction was performed in one-step (introducing H2 from the
beginning), reaction times were shorter (16 h), but this resulted
in lower selectivities, since citronellol and 3,7-dimethyl-octanol
were also formed.23,24 Later reports described the use of Pt/H-
beta zeolites25 and Pd-heteropolyacids26 for the same reaction of
menthol synthesis from citronellal, or even starting from citral,
which requires an additional hydrogenation step.27,28 These are
excellent examples of catalysts based on supported metal nano-
particles in which the support not only affords a medium for dis-
persing the metal nanoparticles, but it does also participate
actively in the catalytic process. In that sense, the high surface
areas and pore volumes characteristic of some MOFs, make
them excellent candidates for attaining high dispersion of metal
nanoparticles.10 This, together with the possibility to introduce a
second functionality, such as the presence of Lewis acid centers
at the inorganic nodes, give MOFs the possibility to act as multi-
functional catalysts.
Results and discussion
We started our study by determining the performance of the
Cr3+–MIL-101 support for the cyclization of citronellal. The
reaction was carried out at 80 °C under N2 atmosphere, using
cyclohexane as solvent (see Experimental), and the results are
shown in Fig. 1a. As it can be seen there, full citronellal conver-
sion was attained with Cr3+–MIL-101 after 18 h. No products
other than the isopulegols (2–5) were detected, while the selec-
tivity for the isopulegol diastereomer (2) with respect to the
other three diastereomers (3–5) was 74 : 26. These results
4250 | Dalton Trans., 2012, 41, 4249–4254
This journal is © The Royal Society of Chemistry 2012

demonstrate that the Cr3+ sites present in the MOF can catalyze
this reaction. Indeed, it is known that MIL-101 features Cr3+
ions coordinated to a labile water molecule that can be easily
removed to create a coordination vacancy on the metal ion. The
resulting coordinatively unsaturated Cr3+ sites endow MIL-101
with Lewis acid character, in a similar way as in the case of
[Cu3(BTC)2] (BTC = 1,3,5,benzenetricarboxylate) which has
unsaturated Cu2+ ions and can catalyze the above isomerization
reaction.29
effectively used for the one-pot two-steps transformation of citro-
nellal into menthol, attaining full citronellal conversion and an
overall yield of the desired menthol of 74% after 18 h.
The Pd@MIL-101 was found to be stable under the reaction
conditions used, according to the XRD pattern and the ICP
analysis of the solid recovered after the reaction (see Fig. S1 in
ESI†). According to the TEM analysis, some aggregation of the
Pd nanoparticles occur upon catalytic use, resulting in an
average particle diameter of ca. 5 nm in the used catalyst (as
compared to 3 nm in the fresh material), with a fraction of larger
particles (see Fig. S2 in ESI†). Nevertheless the material still
maintained a considerable fraction of specific surface area: SBET
= 1620 m2 g−1 (see Table S1 in the ESI†). No leached Pd was
detected in the solution by ICP analysis. Nevertheless, the contri-
bution of homogeneous catalysis by eventually leached species
after hot filtration was studied and the results in Fig. S3 in ESI†
show negligible activity in the filtrate. In order to evaluate the
recyclability of the catalyst, the solid recovered after the reaction
was washed with cyclohexane and acetone and used in a second
catalytic cycle. Similar results were obtained in this case, with
only a small decrease of the activity during the cyclization step:
18 h were needed to attain full citronellal conversion, while only
12 h were necessary with the fresh catalyst. No appreciable
decrease of activity was observed for the hydrogenation step.
The selectivities to isopulegol in the first step and to menthol in
the second step were practically the same with the fresh and with
the used Pd@MIL-101.
Alternatively, the tandem transformation of citronellal into
menthol can also be carried out in just one step: i.e., by perform-
ing the reaction under H2 atmosphere from the beginning.
However, in this case the hydrogenation of the CvO and CvC
bonds of citronellal can compete with the cyclization, leading to
citronellol (6), 3,7-dimethyloctanal (7) and 3,7-dimethyloctanol
(8), as shown in Scheme 2. The occurrence of these side reac-
tions produces a decrease in the selectivity of citronellal to iso-
pulegols and, hence, to the final desired menthols. When
Pd@MIL-101 was used as catalyst (see Experimental), we
obtained full citronellal conversion after 20 h. Unfortunately, the
main product observed (60% yield) under the reaction conditions
used was 3,7-dimethyloctanol (8), which indicates that reduction
of the CvO and/or CvC bonds is preferred over the cyclization.
The combined selectivity to menthols amounted to 34%, with a
diastereoselectivity of 79% to the desired (−)-menthol (see
Table 1B.2). Similar results were found for Ir/H-beta when the
reaction was performed at temperatures or H2 pressures lower
than the optimum.23
As mentioned above, the most likely active sites in Cr3+–
MIL-101 for the citronellal isomerization are coordinatively
unsaturated Cr3+ sites, created upon removal of the apical H2O
ligand. Nevertheless, some studies30–32 have revealed
the presence of Bronsted type acidity in this class of solids,
which could also contribute to the observed catalytic activity.
Since Cr3+–MIL-101 was active for the first step of the global
process for converting citronellal into menthol, we could proceed
to introduce the second catalytic function required. Then, the
bifunctional Pd@MIL-101 material, containing coordinatively
unsaturated Cr3+ Lewis acid sites and encapsulated Pd nanoparti-
cles, was prepared (see Experimental) and used as catalyst for
the one-pot two step transformation of citronellal into menthol.
The first cyclization step was carried out under N2 atmosphere,
at the same conditions as those used previously with Cr3+–
MIL-101. The results obtained for the citronellal conversion and
the yield to isopulegol (2) and other isopulegols (3–5) in the first
reaction step using Pd@MIL-101 as catalyst are shown in
Fig. 1c. As it can be observed in Fig. 1c, almost full conversion
(97 mol%) of citronellal can be achieved over Pd@MIL-101
after 12 h, yielding isopulegols as the only detectable products,
with a 73 : 27 diastereoselectivity ratio. One could expect that
the presence of Pd nanoparticles could have a detrimental effect
on the acid-catalyzed cyclization, taking into account the
decrease of surface area and pore volume that occurs when Pd
was incorporated inside the pores of MIL-101 (see Table S1 in
ESI†). However, Pd@MIL-101 shows a slightly higher activity
for the citronellal cyclization as compared with MIL-101 (see
Fig. 1b). This difference can indicate the presence in
Pd@MIL-101 of a fraction of Pd2+, not completely reduced
during the hydrogenation of the palladium precursor, which can
contribute to the acid-catalyzed citronellal cyclization. A similar
observation was also described by Iosif et al.23 using iridium-
impregnated H-beta zeolite and an analogous interpretation was
also given.
The second step of the tandem reaction to convert citronellal
to menthol involves the hydrogenation of the isopulegol formed
in the first step. This reaction is catalyzed by the encapsulated Pd
nanoparticles. Thus, once the cyclization step was completed,
the N2 atmosphere inside the reactor was replaced by H2 (0.8
MPa) while keeping the temperature unchanged. As can be
observed in Fig. 1d, hydrogenation of the cyclization products,
isopulegol (2) and other pulegols (3–5), can be accomplished in
about 6 h, yielding selectively the corresponding menthols: 86%
menthols (70 mol% of the desired menthol coming from isopule-
gol (2) and 16 mol% of other menthols), together with some
small amounts of remaining non-hydrogenated isopulegols. This
70 : 16 ratio represents a diastereoselectivity of 81% for the
(−)-menthol with respect to other menthols. From the results
obtained it can be concluded that Pd@MIL-101 can be
Comparison of Pd@MIL-101 and Cr3+–MIL-101 with other
relevant catalysts
At this point, and in order to put the activity and selectivity of
the MOF catalyst into perspective, we have compared
Pd@MIL-101 and the Cr3+–MIL-101 support with other related
catalysts; i.e., Sn-beta, Ir/H-beta catalyst and the MOF
[Cu3(BTC)2]. A summary of the most relevant information is
shown in Table 1. We have compared the performance of the cat-
alysts for both cyclization of citrolellal to isopulegols
(Table 1A), and for the tandem cyclization/hydrogenation of
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 4249–4254 | 4251

Scheme 2
Table 1 Summary of the catalytic results obtained for the isomerization of citronellal to isopulegol and tandem isomerization/hydrogenation of
citronellal to menthol obtained with different catalysts
(A) Citronellal to isopulegol
Catalyst
Temp. (°C)
t (h)
Solvent
Conv. (%)
Select.a (%)
Diastereoselect.b (%)
Ref.
Cr3+–MIL-101
Pd@MIL-101
Sn-beta
H-beta
Ir/H-beta
80
80
80
80
80
110
18
12
1
8
8
Cyclohexane
Cyclohexane
Acetonitrile
Cyclohexane
Cyclohexane
Toluene
>99
>99
97
∼92
∼96
>99
>99
>99
>99
>99
>99
>99
74
73
85
75
75
65–69
This work
This work
22
24
24
29
[Cu3(BTC)2]
∼80
(B) One-pot citronellal to menthol
Catalyst p(H2) (MPa)
(B.1) Two-steps reaction: first step in N2 atmosphere, second step in H2 atmosphere
t (h)
Conv. (%)
Select.c (%)
Diastereoselect.d (%)
Ref.
Pd@MIL-101
Ir/H-beta
0.8
0.8
18
30
>99
>99
86
95
81
75
This work
23
(B.2) Reaction in one-step: H2 supplied from the beginning
Pd@MIL-101
Ir/H-beta
0.8
0.8
20
16
>99
>99
34
93
79
75
This work
23
a Selectivity for the four diastereomeric isopulegols with respect to other products. b Selectivity for the isopulegol diastereomer (2) with respect to the
other three diastereomers (3–5). c Overall selectivity to menthols. d Selectivity to (−)-menthol coming from isopulegol (2) with respect to other
menthols coming from isopulegols (3–5).
citronellal to menthol (Table 1B), either in one-step or in two-
steps. Note that the reaction conditions that we have chosen for
this study (see Experimental) are the same as those reported as
the best for Ir/H-beta: temperature, solvent, H2 pressure, and a
citronellal to metal molar ratio of ∼710. Therefore, comparison
between Pd@MIL-101 and Ir/H-beta is straightforward.
As we have already commented above, Pd@MIL-101 is
slightly more active for the citronellal isomerization to isopule-
gol that Cr3+–MIL-101 (see Fig. 1b), while both catalysts yield
similar values of diastereoselectivities to isopulegol (2), of
73–74%. From a simple inspection of Table 1A, it is evident that
the activities and selectivities of both MIL-101 catalysts are
4252 | Dalton Trans., 2012, 41, 4249–4254
This journal is © The Royal Society of Chemistry 2012

much lower than those of Sn-beta,21 which allows full citronellal
conversion in just 1 h with a significantly higher diastereoselec-
tivity to isopulegol (2) of 85%. The comparison of the MIL-101
catalysts and Sn-beta cannot be made directly, since the reactions
are not done in exactly the same conditions, but they are rather
similar: Same temperature and comparable amounts of catalyst,
though different solvents were used in both cases. Nevertheless,
the differences between Sn-beta and the MIL-101 solids are so
large in favor of the Sn-containing material that they cannot be
attributed to the slightly different experimental conditions used,
but to a clearly better performance of the tin catalyst. On the
other hand, both Cr3+–MIL-101 and Pd@MIL-101 are better cat-
alysts for citronellal cyclization than [Cu3(BTC)2], even when a
significantly higher temperature was used for the reaction with
copper catalyst. Less time was required to achieve full conver-
sion over the MIL-101 catalysts, while yielding higher selectiv-
ities to the desired isopulegol product (2): e.g., 74% selectivity
for Cr3+–MIL-101 after 18 h, as compared to 65–69% for
[Cu3(BTC)2] after (at least) 48 h, depending upon the preparation
procedure.29
Experimental
Catalysts preparation
MIL-101 was prepared according to the reported procedures,19
while Pd@MIL-101 was obtained by impregnation of MIL-101
with Pd(NO3)2·2H2O and reduction in a H2 flow. Details on the
synthesis procedures can be found in the ESI.† Both materials
were thoroughly characterized by XRD, N2 adsorption iso-
therms, TGA, TEM and ICP analysis (see ESI†). Comparison of
the XRD patterns of MIL-101 and Pd@MIL-101 were found to
be practically identical, and both corresponded to the expected
structure for the Cr-containing MOF (see Fig. S1 in ESI†).
According to the TEM analysis, Pd@MIL-101 was found to
contain Pd nanoparticles of ca. 3 nm, homogeneously distributed
throughout the MIL-101 (see Fig. S2 in ESI†). According to the
elemental analysis, the Pd content was 0.28 wt% Pd. These
results are completely analogous to those reported for samples
prepared by similar methods.14
The performance of Pd@MIL-101 and the support Cr3+–
MIL-101 for the isomerization of citronellal is similar to that of
Ir/H-beta and the H-beta support, both in terms of activity and
selectivity to isopulegol (2), being the beta catalysts only slightly
more active than the MIL-101 MOFs (see Table 1A). In both
systems, the metal-loaded catalysts are found to be slightly more
active than the corresponding supports, which is attributed to the
presence of a fraction of non-completely reduced metal (Pd or
Ir), which contributes to the acid-catalyzed cyclization of
citronellal.
Catalytic reactions
The citronellal isomerization reaction was performed at 80 °C
under a N2 atmosphere. 60 μl of racemic citronellal and 30 mg
of MOF were placed inside a round bottom flask with 0.5 ml of
cyclohexane. The reaction was followed by GC-MS, using
dodecane as external standard. Samples were analyzed by GC
(Varian 3900), using a 30 m long and 0.25 mm i.d. capillary
column HP-5 (5% phenylmethylpolysiloxane). Retention times
were compared with those of commercial standards.
Concerning the tandem isomerization/hydrogenation reaction,
the results obtained for Pd@MIL-101 were very different when
the reaction was performed in two-steps or in one-step. When
citronellal was allowed to isomerize under a N2 atmosphere until
complete conversion before supplying H2, the tandem reaction
proceeded smoothly to the formation of the desired menthol. In
this case, the overall performance of the Pd@MIL-101 catalysts
was comparable to the best results obtained with Ir/H-beta. Con-
sidering that the reaction was performed with a citronellal to
metal ratio of 710 with both Ir/H-beta and Pd@MIL-101, and
that citronellal was quantitatively converted in both cases to iso-
pulegols and menthols, one can calculate the mols of
(−)-menthol produced per mol of noble metal and per catalytic
cycle over the two materials, obtaining very similar values: 494
and 505 for Pd@MIL-101 and Ir/H-beta, respectively. However,
longer reaction times were required for the beta catalyst (30 h)
than for the MIL-101 solid (18 h).
For the tandem isomerization/hydrogenation reaction, the N2
atmosphere was replaced by a H2 atmosphere (p(H2) = 0.8
MPa), upon completion of the isomerization reaction. Both
Cr3+–MIL-101 and Pd@MIL-101 were used in the reaction as-
prepared, without any further pre-activation.
Acknowledgements
Financial support by Ministerio de Educación y Ciencia e Inno-
vación (Project MIYCIN, CSD2009-00050; PROGRAMA CON-
SOLIDER. INGENIO 2009), Generalidad Valenciana (GV
PROMETEO/2008/130) and the CSIC (Proyectos Intramurales
Especiales 201080I020) is gratefully acknowledged.
References
However, when a H2 atmosphere was used from the beginning
of the reaction, the selectivity to menthols when Pd@MIL-101
was used as catalyst dropped significantly, with 3,7-dimethyl-
octanol (8) being the main reaction product. This result indicates
that hydrogenation of the CvC and CvO bonds is faster than
the isomerization of citronellal to isopulegol. A similar result
was also observed for the Ir/H-beta when a temperature or a H2
pressure lower than the optimum was used. Overall, Ir/H-beta
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the one-pot one-step conversion of citronellal to menthol than
Pd@MIL-101, although the performances of the two catalysts
were comparable when the reaction was performed in two-steps.
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