Catalytic activity of HKUST-1 in the oxidation of trans-ferulic acid to vanillin

Article abstract of DOI:10.1039/c5nj00247h

HKUST-1 was used as a catalyst in the conversion of trans-ferulic acid to vanillin. The generation of unsaturated metal sites within HKUST-1 is the fundamental step in the catalytic process. When activated under vacuum, the catalyst gives complete conversion in only 1 h with a significant average reaction yield of 95%.

Full text of DOI:10.1039/c5nj00247h

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DOI: 10.1039/C5NJ00247H
LETTER
Catalytic activity of HKUST-1 in the oxidation of
trans-ferulic acid to vanillin
Cite this: DOI: 10.1039/c3nj00000x
Rebeca Yepez,a Salvador Garcta,a Persi Schachat,b Manuel SinchezꢀSinchez,c
Received 00th XXXXX 2013,
Accepted 00th XXXXX 2013
Juan H. GonzilezꢀEstefan,d Eduardo GonzilezꢀZamora,*,e Ilich A. Ibarra,*,d and
Julia AguilarꢀPliego*,a
DOI: 10.1039/c3nj00000x
www.rsc.org/njc
HKUST-1 was used as a catalyst in the conversion of trans
-
hydrogen,7 significant efforts have been also made in catalysis.8
ferulic acid to vanillin. The generation of unsaturated metal In addition to the accessibility of uncoordinated metal sites, a
sites within HKUST-1 is the fundamental step in the catalytic coordination polymerꢀbased catalyst needs to exhibit relatively
process. When activated under vacuum, the catalyst gives high thermal stability, retention of the crystallinity and
completed conversion in only 1 h with a significant average chemical resistance in order to grant a sufficient stability under
reaction yield of 95%.
the applied reaction conditions. Thus, many reports on
coordination polymerꢀbased catalysts have focused on the use
of wellꢀstudied frameworks where these parameters have been
thoroughly investigated.
A relatively new application that has been proposed for porous
coordination polymers materials relates to catalysis.1 These
materials exhibit remarkable properties such as high
crystallinity, large surface area, high pore volume and metal
content, which can provide more versatile heterogeneous
catalysts than the traditional zeolites and porous solids.2
Several research efforts have focused on the generation of
different routes for the preparation of catalytically active
coordination polymers.3 Interestingly, this catalytic activity is
provided by active (i.e. unsaturated) metals which constitute the
coordination polymer.
The
porous
coordination
polymer
HKUSTꢀ1
(Cu3(BTC)2(H2O)3, BTC = 1,3,5ꢀbenzeneꢀtricarboxylate)3a is a
wellꢀinvestigated coordination polymer that possesses
framework Cu(II) cations, from which water can be removed to
leave square planar coordinated Cu(II), (see Scheme S1, ESI†).
By combining XRD, UVꢀVis, EXAFS, XANES, and Raman
spectroscopies Bordiga et al.9 demonstrated that by removing
the coordinated water molecule, chemically bound to the Cu(II)
sites, the oxidation state of copper remains unchanged and the
crystalline nature of the material is preserved. HKUSTꢀ1 has
been widely studied as a catalyst,10 e.g. cyanosilylation of
aldehydes and ketones,10c hydrosilylation of ketones,11
However, the nature of coordination polymers synthesis
tends to result in coordinative saturation of all metal species.
Thus, the incorporation of unsaturated (potentially catalytic)
metal into coordination polymers is not a trivial synthetic
oxidation of hydroquinone to p
ꢀbenzoquinone,12 oxidation of
toluene, benzene and other aromatic substrates by H2O2.13
Vanilla is one of the most widely employed flavouring agents in
the world and is extracted from the orchid Vanilla planifolia. People
think of vanilla as a modern flavouring agent, however it was the
preꢀColumbian people in Mexico (the Mayan and Aztec
challenge and so far the most successful methods comprise: (i)
generation of vacant coordination sites on metal ions that are
already occupied within the polymer structure by removal of
labile ligands, e.g. solvent;3 (ii) synthesis of heteroꢀbimetallic
coordination polymers using preꢀformed organometallic species
as building bloks;3b,4
(iii) postꢀsynthetic modification of civilisations) who first realised the potential of this flavouring by
coordination polymers by secondary coordinative or covalent mixing it with chilli peppers and honey to improve the chocolate
bond formation, to attach metallic5 or organic6 moieties within flavour.14 The vanilla plant, Vanilla planifolia, originates in
the pores. Indeed, the first method is the most commonly subtropical forests in Mexico and parts of Central America. The
Spaniards brought vanilla to Western Europe in the early 16th
employed and relies on the structural composition of a given
century and once it was discovered that vanilla orchids could be
fertilised manually, production of vanilla spread around the world.14b
The vanilla scent is due to 200 different odorant molecules, but
coordination polymer. Although, the generation of
uncoordinated (vacant) metal sites has been typically employed
for increasing the affinity of coordination polymers towards
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013
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vanillin (4ꢀhydorxyꢀ3ꢀmethoxybenzaldehyde) is the main odorant. metal sites. In order to corroborate this hypothesis, HKUSTꢀ1 was
Vanillin was first isolated in 185815 and the chemical structure activated under vacuum (10ꢀ5 bar) and 100 C for 1 h (see Fig. S2,
o
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determined in 1874.16 Nowadays, only 1% of vanilla produced is ESI†) without physical manipulation or exposure to air. Next, after
DOI: 10.1039/C5NJ00247H
obtained by the natural extraction and this represents a very long and the sample was activated, the catalytic experiment was performed as
costly process. 17 The 99% of the remaining production is chemically previously described (see Experimental). After confirming the
synthesised17 and this largeꢀscale chemical synthesis of vanillin has product composition (purified by column chromatography, see
been achieved from materials such as guaiacol, eugenol, and safrole. Experimental) by GC analysis, 1H and 13C NMR spectroscopies (see
Vanillin can be also synthesised by the oxidation of lignin present in Fig. S7 and Fig. S8, respectively, ESI†), the conversion of transꢀ
pulp wastes.18a However, these chemical processes are ferulic acid to vanillin was determined, giving a reaction yield of
environmentally unfriendly (strong oxidising agents and toxic 98%. Once the reaction was completed, the catalyst was recovered
solvents) and thus, biotechnological production of vanillin was and washed with some ethanol and characterised by PXRD (Figure
investigated.18 Biotechnologyꢀbased approaches are indeed, 1, catalyst after reaction), corroborating the retention of the overall
considerably less harmful to the environment, but they show some framework crystallinity after the catalytic reaction was finished.
disadvantages, such as many purification steps and waiting time for Additionally, in order to investigate the recyclability of the catalyst
biotransformation.18 We report herein the use of HKUSTꢀ1 as a in the reaction we ran ten catalytic experiments (with an average
catalyst on the oxidation of transꢀferulic acid to vanillin under mild reaction yield of ~95%, see Table S1, ESI†). Thus, after each
oxidation conditions (see Scheme 1). Interestingly, ferulic acid catalytic reaction was completed, the catalyst was recovered (see
(transꢀferulic acid or (E)-3ꢀ(4ꢀhydroxyꢀ3ꢀmethoxyphenyl)ꢀacrylic Table S1, ESI†) and washed with some ethanol and characterised by
acid) is a derivate of cinnamic acid which is very abundant on nature PXRD (see Fig. S3, ESI†). These PXRD experiments confirmed the
and it can be extracted from the corn bran.14c
retention of the overall framework crystallinity after 10 cycles and it
also suggested that no copper species leached into the reaction
solution which was corroborated by inductively coupled plasma
mass spectrometry (ICPꢀMS) on the hot filtrates (10 filtrates from
the catalytic reactions), see ESI†.
COOH
O
H
Activated HKUST-1
Ethanol/H2O2
OCH3
OCH3
OH
OH
Scheme 1. Oxidation of transꢀferulic acid to vanillin using HKUSTꢀ1 as a
catalyst.
Crystalline samples of HKUSTꢀ1 (Aldrich) were characterised
by PXRD (Fig. 1, catalyst before reaction), and thermogravimetric
analysis (TGA, see Fig. S1, ESI†) which confirm the purity of the
samples. In order to obtain internal standards for comparison, we ran
1
GC analyses (see ESI†) and H NMR experiments on pure transꢀ Fig. 1. PXRD patterns of the catalyst (HKUSTꢀ1) before and after the
catalytic experiment.
ferulic acid (Aldrich) and vanillin (Aldrich), respectively. Then, a
sample of HKUSTꢀ1 was activated in a conventional oven (exposed
To achieve a better understanding on the role of HKUSTꢀ1 in the
to air) at 100 oC for 1 h and the catalytic experiment (see
oxidation of transꢀferulic acid to vanillin, we performed a set of
reactions: (i) replacing HKUSTꢀ1 by Cu(CH3CO2)2, (ii) substituting
HKUSTꢀ1 for Cu(NO3)2, (iii) replacing HKUSTꢀ1 for Cu(OH)2, (iv)
in the absence of HKUSTꢀ1 in the reaction and (v) in the absence of
H2O2 in the reaction. In the cases (i), (ii) and (iii) we aimed to use
different sources of Cu(II) that could carry on the oxidation.20 In the
case (iv), we investigated the possibility of oxidising transꢀferulic
acid by only using H2O2 and finally, in the case (v) explored the
Experimental) was carried out. The product composition was
analysed by GC analysis and showed a partial conversion of transꢀ
1
ferulic acid to vanillin. Additionally, H NMR experiments on the
product composition (see Fig. S5 and Fig. S6 ESI†) confirmed this
partial conversion.
First, we rationalised the reaction time was not long enough and
therefore, by carrying out the reaction for longer than 1 h, the
conversion should be better. Thus, the catalysts HKUSTꢀ1 was
possibility of oxidising ferulic acid only with HKUSTꢀ1. After all of
activated as described above and a catalytic experiment was
these reactions were completed, the product composition, analysed
performed (see Experimental), extending the reaction time to 4 and
by GC, showed no conversion of transꢀferulic acid to vanillin.
12 h. Interestingly, the conversion of transꢀferulic acid to vanillin
Thus, it became clear that the responsible for the oxidation of
was not improved, suggesting that the reaction time was not the
transꢀferulic acid to vanillin is the heterogeneous catalyst HKUSTꢀ1
limiting experimental parameter.
in combination with the oxidant H2O2. Once we managed to identify
It is very well known that the generation of coordinativelyꢀ
the catalytic conditions, we investigated the formation of
unsaturated metal sites in porous coordination polymers is a
coordinatively unsaturated Cu(II) metal sites within the catalyst.
fundamental step that entirely relies on the activation conditions.19
Then, FTIR spectra were recorded at 25 oC over three different
Thus, it is reasonable to expect that the activity of the catalyst
samples. First, the FTIR spectrum of HKUSTꢀ1 at room
(HKUSTꢀ1) is determined by the accessibility to the vacant Cu(II)
temperature, nonꢀactivated, (Figure 2, see Fig. S4, ESI†) shows a
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broad absorption band at 3400 cmꢀ1, confirming the presence of the FTIR experiments (see above) and the fact that H2O2 is
uncoordinated water in the material, and a characteristic sharp peak remarkably more reactive in nucleophilic displacements than H2O,22
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at 3680 cmꢀ1 that is assigned to metalꢀcoordinated water.21
an anionic pathway similar to an electrophilic addition to double
DOI: 10.1039/C5NJ00247H
Then, a sample of HKUSTꢀ1 was activated at 100 oC during 1 h bond could take place. In this context a possible reaction mechanism
in a conventional oven (exposed to air). After the sample was back accounts for the formation of vanillin 10 is depicted in Scheme S2
o
to room temperature, (25 C) the FTIR spectrum of it (HKUSTꢀ1 (ESI†). Initial activation of hydrogen peroxide by HKUSTꢀ1 (1)
activated, exposed to air) showed a reduction in the intensity of the gives the copper ion 2,13 which by transfer of OH+ to the vinyl
broad absorption band at 3400 cmꢀ1 but with retention of the sharp system of transꢀferulic acid 3 gives the carbocation intermediate 5,
peak at 3680 cmꢀ1 (Figure 2, see Fig. S4, ESI†). This indicated the which should be favoured due to stabilisation of the carbocation in
loss of some free water (uncoordinated water) in the framework and benzylic position. Nucleophilic attack of water on to the electrophilic
that the coordinated water is not removed from the framework under carbon atom gives, after release of a proton, the dihydroxy
these conditions and remains bound to Cu(II). Finally, a sample of compound 6, which by loss of water and subsequent oxidation by a
o
HKUSTꢀ1 was activated under vacuum (10ꢀ5 bar) at 100 C for 1h second molecule of hydrogen peroxides provide vanillin 10 as the
and after cooling down to room temperature, the FTIR spectrum main product (98%). Oxalic acid and traces of acetic acid were
exhibited a significant reduction in the intensity of the broad detected as byꢀproducts. The presence of vanillin, oxalic acid, acetic
absorption band (3400 cmꢀ1) and the sharp peak at 3680 cmꢀ1 is also acid reported in the scheme were found in the bulk of the solution
lost from the spectra (Figure 2, see Fig. S4, ESI†). Therefore, when and it was validated by HPLC and H1ꢀNMR. 4ꢀvinylguaiacol,
the catalyst is activated under these last conditions all water vanillic acid or protocatechuic acid were not detected in the bulk of
molecules (uncoordinated and coordinated) are removed from the solution, as we also previously observed in another work.23
material generating coordinativelyꢀunsaturated Cu(II) sites (see
Scheme S1, ESI†).
In summary, we demonstrated the application of the porous
coordination polymer HKUSTꢀ1 as a heterogeneous catalyst in the
conversion of transꢀferulic acid to vanillin. This work represents the
first example of the use of HKUSTꢀ1 in the synthesis, via oxidation,
of vanillin. The generation of vacant coordination sites within
HKUSTꢀ1 (corroborated by FTIR) is the key step in the catalytic
process and this strongly depends on the activation conditions. If the
catalyst is activated in air, only a partial conversion of transꢀferulic
acid to vanillin was observed. However, when HKUSTꢀ1 was
activated under vacuum, the catalytic conversion took place in 1 h
with a remarkable conversion yield of 98%. In addition, after the
reaction was completed the catalyst was entirely recovered and the
recyclability of it was confirmed to up to ten catalytic cycles
(average reaction yield of ~ 95%) and by PXRD the retention of the
structure crystallinity was verified. Additionally, no copper species
leached into the reaction solution were corroborated by ICPꢀMS.
Indeed, from an economical and ecological point of view, this
efficient route to highly precious vanillin could be extraordinarily
attractive. We are currently investigating the oxidation of other α,βꢀ
unsaturated carboxylic acid under the oxidation conditions presented
in here.
Fig. 2. FTIR spectra of the catalyst (HKUSTꢀ1) at 25 oC. Nonꢀactivated
(black line), activated in a conventional oven (red line) and activated
under vacuum (blue line).
On the basis of the above experimental results, the removal of
coordinated water from HKUSTꢀ1 is the key to create an active
catalyst and as we previously described, vide infra, this activated
catalyst in combination with H2O2 are necessarily for the oxidation
of transꢀferulic acid to vanillin. Interestingly, in the catalytic
reaction (see Experimental) there is also H2O (since H2O2 is added at
30%v aqueous solution) that could reꢀcoordinated these Cu(II) sites.
Thus, the nucleophilicity of the reacting species towards the Cu(II)
sites is what determines the path of the catalytic reaction. In fact,
H2O2 is an excellent nucleophile which has a considerably higher
reactivity than H2O22 and therefore it coordinates first to
uncoordinated Cu(II) sites even when H2O is present.
In order to gain some insight about the oxidation mechanism
using HKUSTꢀ1 in the presence of H2O2, Baiker et al.13 used
hydroquinone as a scavenger to stop a hydroxylation process, but
hydroquinone did not stop the reaction, indicating a nonꢀradical
reaction mechanism. The authors proposed an electrophilic aromatic
substitution for the hydroxylation process, based on the reaction
between aromatic compounds and electrophilic species like OH+.
Taking the mechanistic considerations aforementioned into account,
Experimental
Activated catalyst (HKUSTꢀ1), 0.005g (3.2 mol%), suspended in ethanol (20
mL) was mixed with 0.5 mL of H2O2 (30%v aqueous), 0.025mL of
acetonitrile and 0.050g (2.57 X 10ꢀ4 mol) of ferulic acid. The reaction mixture
was heated to reflux for 1 h. After the reaction was finished, the catalyst was
recovered by filtration; the filtrate was extracted with ethyl acetate and
washed with a saturated solution of NH4Cl. The combined organic phases
were dried with anhydrous Na2SO4, filtrated and concentrated under vacuum.
Finally, the residue was purified by column chromatography (AcOEtꢀhexane
5:95).
Notes and references
a UAM-Azcapotzalco, San Pablo 180, Col. Reynosa-Tamaulipas,
Azcapotzalco, C.P. 02200, México, D.F., Mexico.
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bIMP | Instituto Mexicano del Petroleo, Eje Central Lázaro Cárdenas Norte
152 Col. San Bartolo Atepehuacan, C.P. 07730, México D.F., Mexico.
cInstituto de Catálisis y Petroleoquímica, ICP-CSIC, C/ Marie Curie, 2
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dInstituto de Investigaciones en Materiales, Universidad Nacional Autónoma
de México, Circuito Exterior s/n, CU, Del. Coyoacán, 04510, México D. F.,
Mexico. E-mail: argel@unam.mx
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Iztapalapa, San Rafael Atlixco 186, Col. Vicentina, Iztapalapa, C. P. 09340,
México D. F., Mexico.
Acknowledgements
The authors thank Dr. A. TejedaꢀCruz (Xꢀray; IIMꢀUNAM). R.Y. thanks
CINVESTAV, Mexico for technical support. M.S.S acknowledges the
financial support by Spanish Government, MINECO (MAT2012ꢀ31127).
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