Solvent free base catalysis and transesterification over basic functionalised Metal-Organic Frameworks

Article abstract of DOI:10.1039/b915291c

Metal-Organic Frameworks (post-)functionalised with nitrogen containing moieties undergo solvent free aza-Michael condensation and transesterification, surpassing molecular and functionalised MCM-type analogues. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b915291c

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www.rsc.org/greenchem | Green Chemistry
Solvent free base catalysis and transesterification over basic functionalised
Metal-Organic Frameworks†
Marie Savonnet,a,b Sonia Aguado,a Ugo Ravon,a Delphine Bazer-Bachi,b Vincent Lecocq,b Nicolas Bats,b
Catherine Pinela and David Farrusseng*a
Received 7th June 2009, Accepted 31st July 2009
First published as an Advance Article on the web 18th August 2009
DOI: 10.1039/b915291c
Metal-Organic Frameworks (post-)functionalised with ni-
trogen containing moieties undergo solvent free aza-
Michael condensation and transesterification, surpassing
molecular and functionalised MCM-type analogues.
the porous framework. Amino-derived MOFs have been shown
to be excellent platforms for the grafting of various synthons
such as isocyanates and anhydride acids.7,8,9,10
The objective of this work is to explore functionalised MOF
materials in an aim to perform transesterification of FAMEs
and base catalysis in line with the principles of green catalysis.
Zinc carboxylate (IsoReticular Metal-Organic Framework
IRMOF-3, 1a) and triazolate (ZnF(Am2TAZ) (Am = amino,
TAZ = triazolate), 2a) were synthesised from aminoterephthalic
acid using procedures described in ref. 11 and 12 and then tested
in catalytic reactions. They both possess amino groups pointing
Despite intensive efforts to develop new efficient, selective
and recyclable solid base catalysts such as Layered Double
Hydroxides (LDH),1–3 hydrotalcite, KF and amino supported
compounds, the development of green processes involving base
catalysts remains a challenge.4
Thanks to their versatility, Metal-Organic Frameworks
(MOF), which are at the frontier between zeolites and surface
organometallic compounds, open new perspectives in hetero-
geneous catalysis.4–6 In the context of base catalysis and the
valorisation of large molecules, they possess two main assets: (i)
a porous network which is usually large enough to accommodate
molecules such as Fatty Acid Methyl Esters (FAMEs), and
(ii) a degree of basicity and a hydrophilicity/hydrophobicity
balance which can be tuned by (post)-functionalisation to
adjust reactivities and adsorption properties, respectively. Direct
functionalisation consists in selecting an already functional
linker which is directly used in the synthesis through self-
assembly, whereas post-functionalisation modifies the organic
part of the solid by a chemical reaction which takes place within
˚
to the channels which have pore openings of 7.4 and 4.6 A for 1a
and 2a, respectively (Fig. 1). Structural (X-ray diffraction, IR)
and porous characterisation (N2 physisorption at 77 K) results
match with data reported in the literature (ESI†). Physisorption
of CO2 performed at 303 K, 1 atm reveals a threefold uptake (27.1
and 8.5 ml g-1) for 1a with respect to 2a, which also corresponds
to a much higher surface area for 1a (SBET = 623 m2 g-1).
In addition, both 1a and 2a were post-functionalised with
pyridine groups in order to increase the hydrophobicity while
maintaining a similar level of basicity (pKa ~ 5 for aniline and
pyridine). The grafting on solids 1a and 2a was performed
by acylation with nicotinoyl chloride, yielding 1b and 2b,
respectively (Fig. 1).
In a typical postsynthetic modification procedure, a suspen-
sion of 1a (or 2a) in DMF is treated with nicotinoyl chloride
for 5 days at 100 ◦C, then filtered and washed three times with
DMF to provide 1b (or 2b). Infrared analyses of 1a and 1b reveal
a significant decrease in the two signals centred at 3500 and
3390 cm-1 corresponding to the stretching vibrational modes of –
NH2 species. The integration of these bands allows an estimation
of ~50% functionalisation for IRMOF-3. The peak at 3070 cm-1,
aUniversite´ Lyon 1, CNRS, UMR 5256, IRCELYON, Institut de
recherches sur la catalyse et l’environnement de Lyon, 2 avenue Albert
Einstein, F-69626, Villeurbanne, France.
E-mail: david.farrusseng@ircelyon.univ-lyon1.fr
bIFP-Lyon, BP 3, 69360, Solaize, France
† Electronic supplementary information (ESI) available: Supplementary
figures. See DOI: 10.1039/b915291c
Fig. 1 Diagram of IRMOF-3 (left), functionalised IRMOF-3 (middle) and ZnF(Am2TAZ) (right) frameworks. The yellow balls indicate the size of
the pore openings.
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assigned to nArC-H, reveals the presence of the nicotinoylgroup,
while the absence of a signal at 3600 cm-1 indicates that there are
no Zn-OH bonds.6 Further proofs of grafting are revealed by
1H NMR and mass spectrometry (MS) analysis of digested 1b
in DCl/D2O/DMSO-d6. The negative mode MS clearly shows
a base peak at m/z 285.1 (M-H-) corresponding to the grafted
linker. In addition, aromatic shifts of the grafted compounds
were assigned by COSY experiments (1H–1H) (d = 7.71, 7.82,
8.20, 8.36, 8.88, 9.20 and 12.16 ppm) (Fig. 2). The 1H integration
indicates a grafting rate of ~60%, which is close to that obtained
via infrared measurements. Although the washing routine with
DMF removes unreacted nicotinoyl chloride, 0.3 molecules
per unit cage still remain in the porous network (Fig. 2). The
decrease of the microporous surface from 650 m2 g-1 for 1a to
180 m2 g-1 for 1b confirms a reduction in free available space
provoked by the introduction of basic groups and the presence
of residual acid in the pore, which might be the cause of the loss
of crystallinity.
Fig. 3 Diagram of one example of the aza-Michael reaction. Reaction
of dibutylamine with cyclohexen-1-one.
For comparison purposes, alkylamino-functionalised meso-
porous ordered silica MCM-41 (Mobil Composition of Mat-
ter No.41)17,18 (3a) and corresponding post-functionalised
compounds19 with 1.5% mol based on amino groups (3b) were
prepared following the same procedure and then tested. More-
over, test reactions were carried out in homogeneous conditions
with molecular analogues, namely aniline and pyridine which
are the corresponding active centres for 1a, 2a, 3a and 1b, 2b,
3b, respectively.
Cyclic and acyclic aliphatic amines underwent 1,4-addition
with different a,b-unsaturated compounds to afford the corre-
sponding aza-Michael product without formation of byproduct
(Table 1). The molecular catalysts i.e. aniline and pyridine show
similar yields which is consistent with their very close pKa, 4.6,
and 5.2, respectively. Clearly, the functionalised MOF materials
exhibit superior yields with regards to their molecular analogues
and also with regards to MCM functionalised materials. Finally,
highest yields are always found for post-functionalised materials
1b, 2b and 3b, with respect to their parent forms 1a, 2a and
3a. These self-consistent results point out the superiority of
MOF catalysts over molecular and inorganic analogues. It has
to be pointed out that the difference in yields according to the
substrates results from the electrophilic character of the acceptor
and the relative nucleophilicity of the donor. Highest yields
are achieved for the most activated acceptor and/or the most
nucleophilic donor.
Fig. 2 1H-NMR of digested 1b (top) and functionalised linker as
reference (bottom).
In contrast, the crystallinity of the functionalised zinc
triazolate (2b) is fully preserved. Unfortunately, the NMR
quantification of the grafting rate cannot be performed on
2b due to the absence of 1H on the linker. A grafting rate
of approximatively 60% was measured by elementary analysis
based on the N and C contents. In addition, mass spectrometry
analysis performed after digestion shows a signal at m/z = 309
corresponding to the doubly functionalised linker.
For ethyldecanoate transesterification with MeOH (1:28), all
MOF compounds reach 95% conversion after 24 h stirring at
Table 1 Catalytic results of aza-Michael reactions at room temperature
based on 1.5 mol% on basic (aniline or pyridine type) groups and with a
stoichiometry donor:acceptor of 1.0:1.1. The bold numbers indicate the
highest yield obtained for each reaction.
Yield (%)
Catalytic materials were evaluated using aza-Michael reaction
and simple transesterification as model reactions. Knoevenagel
condensations are usually chosen as model reactions for evalu-
ating base catalysts. However, the condensation liberates water
which in our case could modify the structure of IRMOF-3.13–15
In contrast, aza-Michael condensations do not liberate water
and is appropriate to probe weak demanding base reactions. In
addition, the aza-Michael reaction has found practical appli-
cations. The conjugate addition of amines to a,b-unsaturated
compounds to form b-amino carbonyl compounds constitutes
a key reaction in the synthesis of various complex natural
products, antibiotics, b-amino alcohols and chiral auxillaries16
(Fig. 3).
Substrates
Aniline Pyridine 1a 1b 2a 2b 3a 3b
17
19
44 47 23 28 21 34
66
59
82
70
57
83
76 77 59 79 63 65
66 72 42 85 72 80
90 94 88 90 84 86
79
19
81
22
76 94 75 87 76 85
24 29 20 33 18 24
In an aim of probing the catalytic centres and getting rid
of possible mass transfer limitations, ethyldecanoate was used
as model compound for transesterification instead of bulkier
triglycerides.
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180 ◦C (1.5 mol% catalyst), which corresponds to equilibrium.
Catalytic runs were carried out at lower temperature (130 C)
aminoterephthalic acid under stirring at room temperature for
◦
90 min. The powder was collected by repeated centrifugation
and thoroug◦h DMF washing for three times. Then, they were
dried at 130 C under static air.11
ZnF(Am2TAZ) was prepared by placing a mixture of 0.2 g
(2 mmol) of 3,5-diamino-1,2,4-triazole, 0.35 g (2 mmol) of
in order to discriminate activities between the various MOFs
(Fig. 4). Here again, 1a and 1b show the highest catalytic
activities, with a significant enhancement of the yield after
post-functionalisation with pyridine (TOF of 3 h-1). On the
other hand, for the MCM catalyst, a decrease in activity was
observed after functionalisation (Fig. 4, 3b). No further reaction
takes place after removing the catalysts from reaction mixture,
indicating the absence of leaching. After filtration, the MOF
catalysts can be re-used twice without any loss of activity.
R
ZnF2·4H2O and 10 mL of water into a 40 mL Teflonꢀ lined
autoclave. The resulting mixture was stirred for 5 min prior to
sealing the autocla◦ve. The autoclave was then placed in an oven
and heated to 160 C for 3.5 days.12
(Post-)functionalisation
All experiments were carried out under an inert atm◦osphere.
A sample of IRMOF-3 (0.85 g) activated at 250 C for 12 h
was suspended in dry, freshly distilled DMF and treated with
one equivalent of nicotinoyl chloride (0.35 g) in the presence
of an excess of distilled pyridine (5 mL) and DMAP (0.1 g)
◦
at 100 C for five days. The resulting powder was collected by
filtration, then thoroughly washed three times with DMF and
dried under vacuum at 170 ◦C for 12 h. The same procedure was
performed for the post-functionalisation of ZnF(Am2TAZ) and
MCM-41-NH2.
Fig. 4 Catalytic results of the transesterification of ethyldecanoate with
MeOH (1:28) at 130 ◦C for 24 h.
Catalytic reactions
Ethyldecanoate (Fluka, 99%), dibutylamine (Fluka, 99%),
N-methylcyclohexylamine (Aldrich, 99%), methyl acrylate
(aldrich, 99%), acrylonitrile (Aldrich, 99%), cyclohexen-1-one
(Fluka, 98%), methanol (Aldrich, 99%), toluene (Chimie-Plus,
99%) and decane (Alfa Aesar, 99%) were used as received.
Aza-Michael reaction. The reaction of the donor group
(5 mmol) with the acceptor group (5.5 mmol) in the presence of
1.5 mol% of catalyst based on the amino group was carried out
at room temperature for 24 h.3 After the reaction was completed
and the catalyst filtered off, a sample of the filtrate was diluted
in n-decane with 5% toluene as internal standard and analysed
by gas chromatography (HP 6890 N equipped with a 30 m HP5
column).
The aza-Michael reaction does not require strong basic
sites since it can proceed on pyridine polymers.20 In contrast,
stronger immobilized nitrogen bases such as guanidines are
required for low temperature transesterification (e.g. 80 ◦C).
However, weaker sites such as described herein are sufficient to
perform the reaction in current process conditions (T = 190 ◦C,
P ~ 30 bars).
We anticipate that the superiority of functionalised MOFs
(1b and 2b) against MCM analogue catalysts arises from the
activation of the substrates via strong adsorption in the organic
micropore framework whereas such confinement is not found in
inorganic mesoporous MCM materials.
We report herein, for the first time, the application of
functionalised Metal-Organic Frameworks to base catalysis. We
believe that the development of post-functionalisation routes
will make it possible to achieve tailor made materials that will
create new breakthroughs in selective catalysis.
Transesterification. Ethyldecanoate (2.5 mL) and methanol
(10 mL) along with 1.5 mol% catalyst based on basic (amino or
R
aniline) groups were made to react in a Teflonꢀ lined stainless
steel digestion bomb (TopIndustrie) for 24 h at 180 ◦C or
130 ◦C. After the reaction, the catalyst was recovered by filtration
and a sample of the filtrate was diluted in CH2Cl2 and analysed
by GC.
Experimental
Synthesis
Acknowledgements
All chemicals were used as received: N,N¢-dimethylformamide,
DMF (Aldrich, 99.8%), Zn(NO3)2·4H2O (Merck, 98.5%), 2-
aminoterephtalic acid (Alfa Aesar, 99%), nicotinoyl chlo-
ride hydrochloride (Aldrich, 97%), dichloromethane (Acros
Organics, 99.99%), triethylamine (Riedel-de Hae¨n, 99%), 4-
(dimethylamino)pyridine DMAP (Aldrich, 99%), ZnF2·4H2O
(Alfa Aesar, 98%), 3,5-diamino-1,2,4-triazole (Alfa Aesar,
98+%).
We thank Dr. A. Galarneau and M. F. Driole of the Institut
Charles Gerhardt of Montpellier (France) for having provided
us MCM-41 materials. We also thank A. Camarata and
Dr. J. Pawlesa for the technical work, along with IRCELYON
Scientific Services.
IRMOF-3 was prepared according to the procedure reported
by Huang et al.:11 4.4 mL (31.6 mmol) of triethylamine was
directly added to a DMF solution (80 mL) containing 2.4 g
(8 mmol) of Zn(NO3)2·4H2O and 0.66 g (4 mmol) of 2-
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