Zirconium-containing metal organic frameworks as solid acid catalysts for the esterification of free fatty acids: Synthesis of biodiesel and other compounds of interest

Article abstract of DOI:10.1016/j.cattod.2014.08.015

Zr-containing metal organic frameworks (MOFs) formed by terephthalate (UiO-66) and 2-aminoterephthalate ligands (UiO-66-NH₂) are active and stable catalysts for the acid catalyzed esterification of various saturated and unsaturated fatty acids with MeOH and EtOH, with activities comparable (in some cases superior) to other solid acid catalysts previously reported in literature. Besides the formation of the corresponding fatty acid alkyl esters as biodiesel compounds (FAMEs and FAEEs), esterification of biomass-derived fatty acids with other alcohols catalyzed by the Zr-MOFs allows preparing other compounds of interest, such as oleyl oleate or isopropyl palmitate, with good yields under mild conditions.

Full text of DOI:10.1016/j.cattod.2014.08.015

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Contents lists available at ScienceDirect
Catalysis Today
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Zirconium-containing metal organic frameworks as solid acid
catalysts for the esterification of free fatty acids: Synthesis of
biodiesel and other compounds of interest
∗
∗∗
F.G. Cirujano, A. Corma , F.X. Llabrés i Xamena
Instituto de Tecnología Química UPV-CSIC, Universidad Politécnica de Valencia, Consejo Superior de Investigaciones Científicas, Avda. de los Naranjos, s/n,
6022 Valencia, Spain
4
a r t i c l e i n f o
a b s t r a c t
Article history:
Zr-containing metal organic frameworks (MOFs) formed by terephthalate (UiO-66) and 2-
aminoterephthalate ligands (UiO-66-NH2) are active and stable catalysts for the acid catalyzed
esterification of various saturated and unsaturated fatty acids with MeOH and EtOH, with activities com-
parable (in some cases superior) to other solid acid catalysts previously reported in literature. Besides
the formation of the corresponding fatty acid alkyl esters as biodiesel compounds (FAMEs and FAEEs),
esterification of biomass-derived fatty acids with other alcohols catalyzed by the Zr-MOFs allows prepar-
ing other compounds of interest, such as oleyl oleate or isopropyl palmitate, with good yields under mild
conditions.
Received 13 May 2014
Received in revised form 17 July 2014
Accepted 18 August 2014
Available online xxx
Keywords:
Metal organic framework catalysts
Free fatty acid esterification
Solid Lewis acid catalysts
Wax esters
©
2014 Published by Elsevier B.V.
1
. Introduction
An alternative route to produce biodiesel compatible with raw
materials rich in FFA (i.e. wastes and by-products of industrial
biomass processing or from food processing plants [3]) is the direct
esterification of FFA with low molecular weight alcohols (methanol
or ethanol), as shown in Scheme 1.
Biodiesel [1,2] consists of a mixture of alkyl (methyl or ethyl)
esters of long chain fatty acids derived from oils or fats (triglyceri-
des) having properties similar to diesel obtained from oil (gasoil).
As compared with diesel derived from petroleum, biodiesel has
the main advantage of being biodegradable and its combustion
in diesel engines emits less SOx, CO, solid particles and organic
compounds. Therefore, biodiesel is considered to be in general less
toxic than petroleum diesel. The industrial process for the prepa-
ration of biodiesel involves the transesterification of triglycerides
with methanol or ethanol using stoichiometric amounts of strong
Brønsted bases, such as sodium methoxide or sodium/potassium
hydroxides [1,2]. However, this process requires raw materials hav-
ing low water contents to limit catalyst dilution or neutralization
with water, along with a low content of free fatty acids (FFA) to
avoid the formation of soaps by saponification side reactions. The
formation of these byproducts consumes the stoichiometric base,
decreases the biodiesel yield and complicates the separation of the
esters due to the formation of emulsions.
Esterification of carboxylic acids with alcohols can be accom-
plished by using an acid catalyst that rapidly drives the above
equilibrium to the products. Common acid catalysts employed in
the biodiesel production include sulfuric, phosphoric, hydrochloric,
and organic sulfonic acids [4]. However, the use of mineral acids has
serious handling, corrosion and environmental problems. In this
sense, active and stable solid acid catalysts appear as an attractive
alternative to mineral acids, allowing the easy isolation of the fatty
acid alkyl ester products and avoiding corrosion of the equipment.
In recent years, metal organic frameworks (MOFs) have
attracted much interest for their potential as heterogeneous cata-
lysts [5–9]. MOFs are crystalline porous solids formed by the linkage
of metal ions or metal–oxo clusters and polydentate organic linkers,
forming three dimensional networks defining strictly regular nano-
metric pore channels and cavities similar to those found in zeolites.
The large number of available metals and organic molecules that
can be used to prepare a MOF leads to a virtually infinite num-
ber of possible combinations. This high variability, along with the
possibility to introduce new functionalities in a pre-formed MOF
by post-synthesis modification [10–12], allow to finely tune the
chemical compositions, chemical environment and pore structures
of the materials and thus, their reactivity. The Lewis acid catalytic
∗
Corresponding author. Tel.: +34 96 387 7800; fax: +34 963877809.
Corresponding author. Fax: +34 963877809.
E-mail addresses: acorma@itq.upv.es (A. Corma), fllabres@itq.upv.es
∗
∗
(
F.X. Llabrés i Xamena).
http://dx.doi.org/10.1016/j.cattod.2014.08.015
920-5861/© 2014 Published by Elsevier B.V.
0
Please cite this article in press as: F.G. Cirujano, et al., Zirconium-containing metal organic frameworks as solid acid cata-
lysts for the esterification of free fatty acids: Synthesis of biodiesel and other compounds of interest, Catal. Today (2014),
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Scheme 1. General reaction scheme for esterification of fatty acids with alochols.
◦
properties of MOFs have already been demonstrated for many reac-
tions, including cyanosilylation of carbonyl compounds [13–16],
epoxide methanolysis [17], isomerizations of ␣-pinene oxide and
citronellal [18,19], Friedländer condensation [20], alkene cyclo-
propanation [21], etc. In many cases, the Lewis acid character of
the MOF is induced by the creation of a coordination vacancy upon
at 150 C for 12 h. Likewise, Zr content in both samples was deter-
mined by ICP-OES. For both, UiO-66 and UiO-66-NH , we found
2
higher Zr contents than those expected for the ideal stoichiometric
material. These results indicate that linker defects are present in
both compounds, as already concluded by others in previous stud-
ies [26]. A detailed analysis of the linker deficiency is given in the
Supporting Information.
thermal removal of a solvent molecule (H O) initially bound to the
2
metallic nodes. Recently, Cavka et al. [22] have reported the syn-
thesis of a robust Zr terephthalate MOF, referred to as UiO-66. This
material contains Zr hexameric building blocks connected by 12
dicarboxylate ligands, forming a face-centered cubic close packing
framework. Several isoreticular materials of the family have been
obtained so far by replacing terephthalate ligand by other linear
Lewis acidity of the UiO-66 type compounds was evaluated by
means of FTIR spectroscopy of adsorbed cyclohexanone. IR mea-
surements were performed on a Nikolet i510 FTIR spectrometer
−
1
at a resolution of 4 cm . The solids were prepared in the form of
−
2
◦
thin self-supported wafers of ca. 10 mg cm and degassed at 150 C
−
3
overnight under vacuum (10 mbar). Then, cyclohexanone vapor
was admitted to the cell and, after equilibration, the samples were
degassed for 1 h at increasing temperatures (50, 100 and 150 C),
dicarboxylate molecules (such as amino- or nitroterephthalate, or
ꢀ
◦
4
,4 -biphenyl dicarboxylate), which allows tuning the dimensions
of the pores and the chemical composition of the solid. In spite
of lacking coordination vacancies in their ideal crystalline struc-
ture, Zr-containing UiO-66 type MOFs [22] also display the typical
reactivity expected for a Lewis acid catalyst [23–25]. According to
recent characterization studies [26], the catalytic activity of these
Zr-MOFs would arise from both the occurrence of a reversible (ther-
and IR spectra were recorded after each step.
2.2. General procedure for esterification reactions
Esterification reactions were performed as follows: 1 mmol of
fatty acid, and the desired amount of alcohol were contacted with
the MOF (0.07 mmol Zr) in a batch reactor at the specified tem-
perature (see footnotes in the corresponding tables for specific
conditions). The reaction was followed by GC-MS (Varian 3900)
with a 30 m long and 0.25 mm i.d. capillary column HP-5 (5%
phenylmethylpolysiloxane), using dodecane as external standard.
Retention times were compared with those of commercial stan-
dards. Turnover frequencies (TOFs) of the Zr-MOFs were calculated
as moles of product formed per moles of zirconium present in
the reaction (considering that all the zirconium atoms of the MOF
are participating in the reaction) and per hour. Data were taken
at short reaction times corresponding to low levels of conversion
12+
12+
mal) dehydroxylation of the [Zr O (OH) ]
clusters to [Zr O ]
6
4
4
6 6
exposing ␮ -vacancies, together with the systematic formation of
3
crystalline defects associated to linker deficiencies. Very recently,
de Vos and co-workers have demonstrated that the amount of miss-
ing linkers in the framework (and thus the catalytic activity of the
resulting material) can be controlled to some extend by using a
modulated synthesis approach with trifluoroacetic acid and HCl fol-
lowed by thermal activation [27]. Therefore, given the well known
Lewis acid character of UiO-66 type materials and their noticeable
thermal, chemical and mechanical stability [28], we have antici-
pated that these materials could make good candidates as solid acid
catalysts for FFA esterification for the production of biodiesel and
other compounds of interest. In this work, various alcohols have
been used along with both, saturated and unsaturated FFA of dif-
ferent chain lengths. Comparison of the results obtained with the
MOFs and with other acid catalysts reported in literature is also
provided.
(<15–20%).
3. Results and discussion
3.1. Acid properties of UiO-66 and UiO-66-NH2
As commented above, the acid character of UiO-66 type mate-
2
. Experimental
rials has been indirectly established for several acid catalyzed
reactions [23–25], as well as directly determined experimentally by
means of CO adsorption FTIR and microcalorimetry [30], FTIR spec-
troscopy of adsorbed 5-nonanone [31], temperature programmed
desorption (TPD) of NH3 [23], and has been theoretically evalu-
ated by DFT [25]. Furthermore, experimental evidences by TPD of
CO2 [23] and FTIR of adsorbed CDCl3 [31] also revealed the pres-
2.1. Synthesis of the MOFs
UiO-66 and UiO-66-NH2 solids were prepared according to the
reported procedure [29]. Briefly, 750 mg of ZrCl and either 740 mg
of terephthalic acid (UiO-66) or 800 mg of 2-aminoterephthalic
4
acid (UiO-66-NH ) were dissolved in 90 mL of DMF (Zr:ligand:DMF
ence of basic sites of medium strength in UiO-66-NH , which are
2
2
molar ratio of 1:1:220) and the solution was keep in a round bottom
absent in UiO-66. In the present work, we have used FTIR spec-
troscopy of adsorbed cyclohexanone to evaluate comparatively the
acid properties of the UiO-66 and UiO-66-NH2 samples that will
be later used in the catalytic studies. Cyclohexanone is a conve-
nient probe molecule to characterize Lewis acidity of solid acids
[32,33]. Adsorption of cyclohexanone on Lewis acid sites produces
a bathochromic shift of the C O stretching mode with respect to the
value of the free molecule, and the magnitude of the shift depends
on the strength of the acid site. Unfortunately, both UiO-66 and
UiO-66-NH2 have strong absorption bands in the region expected
◦
◦
flask without stirring at 80 C for 12 h and at 100 C for another 24 h
in an oil heating bath. The resulting material was recovered by fil-
tration and washed thoroughly with fresh DMF. Then the solid was
washed three times by putting it in contact with dichloromethane
for 3 h. Finally, the solid was recovered by filtration and dried under
vacuum for another 1 h. X-ray diffraction (Phillips X’Pert, Cu K␣
radiation) was used to confirm the expected crystalline structure
of the materials.
Elemental analysis (C, H and N) of UiO-66 and UiO-66-NH2
was performed on the solids previously degassed under vacuum
−
1
for the ␯(C O) mode of cyclohexanone (ca. 1700–1670 cm ) due to
Please cite this article in press as: F.G. Cirujano, et al., Zirconium-containing metal organic frameworks as solid acid cata-
lysts for the esterification of free fatty acids: Synthesis of biodiesel and other compounds of interest, Catal. Today (2014),
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a)
b)
c)
1
0
0
0
0
0
,0
,8
,6
,4
,2
,0
3000
2900
2800
3000
2900
2800
50
100
150
-1
-1
wavenumber, cm
wavenumber, cm
Outgassing temperature, ºC
Fig. 1. FTIR spectroscopy of cyclohexanone adsorbed on UiO-66 (part a) and UiO-66-NH2 (part b) normalized to the Zr amount in each sample. Cyclohexanone was adsorbed
◦
at room temperature and desorbed at increasing temperature, at (from top to bottom) 50, 100 and 150 C. Part c shows the amount of cyclohexanone that remain adsorbed
(
as normalized intensity of the C H peaks) as a function of the outgassing temperature on UiO-66 (-ꢀ-) and UiO-66-NH2 (-ꢁ-).
the terephthalate linkers, thus limiting the use of cyclohexanone as
probe molecule for this type of compounds. Nevertheless, the Lewis
acid strength of the solids can still be inferred from the correspond-
ing C H stretching modes of cyclohexanone, by evaluating the
amount of ketone that remains adsorbed after degassing at a certain
temperature. Thus, cyclohexanone was adsorbed at room tem-
the higher nucleophilic character of MeOH and the corresponding
lower activation energy of the addition step. Blank experiments
revealed that these reactions are auto-catalyzed to some extend by
the acid reagent, C12, yielding under the reaction condition used
up to 56% of methyl laurate and 12% ethyl laurate after 24 h of
reaction, respectively (see Table 1, entry 1, and Table 2, entry 1).
In comparison, the MOF catalyzed reaction is considerably faster
than the auto-catalyzed process (compare entry 1 with entries 2
and 3 in Tables 1 and 2). Additional control experiments in the
presence of either terephthalic or aminoterephthalic acid indicates
that the contribution from eventual traces of free ligands that might
be present in the MOFs is negligible (conversions attained in both
cases were practically identical than in the blank experiment).
While both UiO-type materials are active catalysts for C12 ester-
ification with MeOH and EtOH, UiO-66-NH2 was found to be more
active than UiO-66, providing higher turnover frequencies (TOF)
and final yields of the ester. In principle, one could expect differ-
ences in the acid character of the two materials as a consequence
of the electronic effects introduced by the amino groups of the
linkers. In this sense, the groups of Ahn [23], de Vos [24,25] and
Timofeeva [31] have recently reported the comparative catalytic
properties of isoreticular UiOs bearing various functional groups in
the terephthalate linkers. According to these authors, the catalytic
properties of the UiOs are highly dependent on the acid character of
the materials, and this acidity can be modulated by the presence of
functional groups in the MOF structure. Thus for instance, accord-
ing to the kinetic data obtained for citronellal cyclization [25],
introduction of electron withdrawing groups in the terephtaha-
late linkers (such as nitro groups) increases the Lewis acid strength
of the Zr clusters due to an electronic induction effect, resulting
in a higher catalytic activity (with respect to non-functionalized
UiO-66). Conversely, the presence of electron donating groups
◦
perature on previously outgassed samples (150 C under vacuum
overnight), and then desorbed successively at increasing tempera-
◦
tures (50, 100 and 150 C) for 1 h. The results obtained are shown
in Fig. 1, for UiO-66 (part a) and UiO-66-NH (part b), together with
2
the normalized integrated intensity of the C H stretching bands of
cyclohexanone as a function of the outgassing temperature (part
c). Note that the spectra in parts a and b have been previously nor-
malized to the zirconium content of the wafer to allow a direct
comparison. As it can be seen in Fig. 1, both materials can adsorb
cyclohexanone, thus revealing a certain Lewis acidity, though the
amount of cyclohexanone adsorbed on UiO-66 is about two times
higher than on UiO-66-NH2 (see Fig. 1c). Furthermore, cyclohex-
anone is completely removed from UiO-66-NH after outgassing at
2
◦
1
50 C for 1 h, while it still remains a significant fraction of cyclo-
hexanone adsorbed on UiO-66 after the same thermal treatment
◦
(
ca. 23% of the amount adsorbed at 50 C). Both results indicate
that UiO-66 contains twice as much Lewis acid centers than UiO-
6-NH , and their acid strength is higher, in agreement with the
6
2
conclusions drawn in previous studies [23,30,31].
3.2. Esterification of lauric acid (C12) with methanol or ethanol
In order to test the catalytic activity of Zr-containing UiO-type
MOFs for the esterification of free fatty acids, we first considered
lauric acid (dodecanoic acid, hereafter C12 for short) as model com-
pound, and methanol and ethanol as alcohols. The results obtained
for UiO-66, UiO-66-NH2 (i.e., an isoreticular UiO-66 material built
up by aminoterephthalate linkers) and other reference catalysts are
shown in Fig. 2 and Tables 1 and 2. In all cases, esterification with
MeOH is considerable faster than with EtOH, as expected due to
(such as amino groups) produces the opposite effect and, hence,
a drop of the catalytic activity. A similar reactivity trend was
also observed for benzaldehyde acetalization [31]. However, this
is not the reactivity trend that we have observed in the present
Please cite this article in press as: F.G. Cirujano, et al., Zirconium-containing metal organic frameworks as solid acid cata-
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100
4
b) 100
a)
3
3
1
4
1
2
5
5
0
50
5
2
6
2,0
0
0
0
,0
0,5
1,0
1,5
0
2
4
6
8
time (h)
time (h)
◦
Fig. 2. (a) Esterification of lauric acid (C12) with methanol at 60 C over: UiO-66-NH2 (curve 1), UiO-66 (2), ZrCp2Cl2 (3), H2SO4 (4), Zr(acac)4 (5) and tetragonal ZrO2 (6). In
+
◦
all cases, 8 mol% Zr (or 8 mol% H in the case of H2SO4) and a C12:MeOH: molar ratio of 1:26 was used. (b) Esterification of lauric acid with ethanol at 78 C over: UiO-66-NH2
(
curve 1), UiO-66 (2), ZrCp2Cl2 (3), H2SO4 (4) and Zr(acac)4 (5). In all cases, 8 mol% Zr (or 8 mol% H+ in the case of H2SO4) and a C12:EtOH: molar ratio of 1:18 was used.
study for C12 esterification over UiO-66 and UiO-66-NH . Thus, an
or basic sites are no longer needed to attain high activities and
selectivities, but rather the reaction is better performed with mild
acid–base pairs in close proximity [35–38], such as those present
2
alternative origin is probably behind the differences in reactivity
of the two materials. One possible explanation is that, as pointed
out by Morris et al. [34], about one third of the amino groups
in the as-synthesized zirconium terephthalate are in the form of
in UiO-66-NH . Thus, our proposal to explain the higher activity of
2
UiO-66-NH2 with respect to UiO-66 is that the Zr Lewis acid sites
(which are present in both UiO-66 and UiO-66-NH2 as revealed by
FTIR spectroscopy of adsorbed cyclohexanone) can adsorb the fatty
acid, thus increasing the electrophilic character of the carboxylic
protonated NH3+Cl salts, formed by the hydrolysis of the ZrCl4
precursor and HCl released during the MOF synthesis. These highly
electronegative NH3+ groups could act as electron withdrawing
groups, thus enhancing the Lewis acidity of the zirconium oxoclus-
ters in the same way as NO2 groups do. In any case, this situation
would imply the presence of two coexisting and opposite elec-
tronic induction effects acting simultaneously, NH3+ increasing
and NH2 decreasing the Lewis acidity of Zr centers, that would
neutralize each other to some extend. Another possible explanation
for the higher catalytic activity of UiO-66-NH2 for C12 esterifica-
tion could involve the direct participation of the amino groups in
the activation of the reaction substrate, by assisting in the activa-
tion of the nucleophilic character of the alcohol and elimination
of the water molecule. Thus, UiO-66-NH2 would be acting as a
bifunctional acid–base catalyst for the esterification reaction, as
shown in Scheme 2. In this dual activation, strong individual acid
−
carbon atom. Meanwhile, even if the NH2 groups in UiO-66-NH2
−
are not basic enough to deprotonate CH OH to yield CH O , they
3
3
can still form hydrogen bonded adducts, which will increase the
nucleophilic character of the O atom of the alcohol, thus favor-
ing the condensation with the activated carboxylic carbon of the
fatty acid. In spite of UiO-66-NH having less and weaker acid sites
2
than UiO-66, its higher catalytic activity can be explained by the
occurrence of a dual activation mechanism, which make not nec-
essary the presence of strong acid sites. On the contrary, UiO-66
lacking these amino groups, cannot benefit from a dual activation
mechanism as must rely only on the Lewis acid sites to catalyze
the reaction. The same reactivity trend observed for fatty acid
esterification (i.e., UiO-66-NH > UiO-66) was also observed for CO2
2
Table 1
Esterification of lauric acid (C12) with methanol to obtain methyl laurate (2).
.
◦
a
b
−1
Catalyst
C12:MeOH molar ratio
t (h)
2(24)
2
2
2
2
2
2
10
2
2
2
2
6
2
15
2
Temp. ( C)
Catalyst (mol%)
Yield of 2 (mol%)
TOF /Product (h )
Reference
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
Blank
UiO-66
UiO-66-NH2
H2SO4
ZrO2(tet)
1:26
1:26
1:26
1:26
1:26
1:26
1:26
10:1
1:6
60
60
60
60
60
60
60
65
100
140
100
140
65
160
120
140
–
8
8
8
8
8
8
5
9 (56)
94
>99
>99
<1
20
>99
72
39.4
95.7
19
79
99
–
This work
This work
This work
This work
This work
This work
This work
[47]
[48]
[48]
[49]
[49]
16/6
25/6
30/6
Nil
Zr(acac)4
1/1
Zr(Cp)2Cl2
Zr Oxophospate
Zn5(OH)8(NO3)2
Zn5(OH)8(NO3)2
Mn(Laurate)2
Mn(Laurate)2
SiO2-SO3H
Halloysite
MoO3/SiO2
Bi2O3
36/6
c
1.5 /1
3.2
9.6
4
–/6
–/5
–/2
0
1
2
3
4
5
6
1:6
1:14
1:14
1:35
1:12
1:12
1:20
4
–/10
–/12
–/3
–/16
–/16
1.4
16.3
0.4
2
[50]
[51]
[52]
[53]
95
97
65
a
b
c
Turnover frequencies (TOF) cannot be calculated for most of the reactions taken from literature since no conversions at short reaction times were provided.
Productivity of the catalyst, as moles of product formed per mol of catalyst and per hour, calculated at the end of the reaction.
TOF has been estimated from the data shown in Fig. 7 of reference [47].
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Table 2
Esterification of lauric acid (C12) with ethanol to obtain ethyl laurate (3).
.
◦
a
b
−1
Catalyst
C12:EtOH molar ratio
t (h)
8(24)
8(20)
8
Temp. ( C)
Catalyst (mol%)
Yield of 3 (mol%)
TOF /Product (h )
Reference
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
Blank
UiO-66
1:18
1:18
1:18
1:18
1:5
1:18
1:18
1:18
1:18
1:18
1:6
78
78
78
78
78
78
78
78
78
–
8
8
8
8
8
8
8
8
4(12)
64(80)
99
60
42(55)
94
7
72
99
34
23.1
77.2
75
99
87.1
95
–
This work
This work
This work
This work
This work
This work
This work
This work
This work
This work
[48]
[48]
[49]
[50]
[51]
[52]
[54]
[54]
2.8/1
7.4/1.5
2.1/0.9
1.5/0.5
17.5/6
Nil
3.4/1
21.7/6
<1
UiO-66-NH2
UiO-66-NH2
UiO-66-NH2
H2SO4
ZrO2(tet)
Zr(acac)4
Zr(Cp)2Cl2
Zr-Beta zeolite
Zn5(OH)8(NO3)2
Zn5(OH)8(NO3)2
Mn(Laurate)2
SiO2-SO3H
c
8
8(20)
2
20
8
2
20
2
2
2
7
2
0
1
2
3
4
5
6
7
8
78
2
100
140
140
80
160
120
78
3.2
3.2
4
1.4
16.3
0.4
0.1
0.1
–/4
1:6
–/12
–/10
–/12
–/3
1:14
1:35
1:12
1:12
1:3
Halloysite
MoO3/SiO2
HPA/Ta2O5
HPA/Ta2O5
d
15
3
3
21 /16
65.6
99
–/219
e
1:9
78
800 /330
a
b
c
Turnover frequencies (TOF) cannot be calculated for most of the reactions taken from literature since no conversions at short reaction times were provided.
Productivity of the catalyst, as moles of product formed per mol of catalyst and per hour, calculated at the end of the reaction.
A H2O:C12 molar ratio of 15:1 was added at the beginning of the reaction.
TOF has been estimated from the data shown in Fig. 6 of reference [52].
TOF has been estimated from the data shown in Fig. 8 of reference [54].
d
e
cycloaddition to styrene oxide [23] and cross-aldol condensation
analysis of the liquid filtrate after the reaction presented no traces
of Zr, evidencing the lack of appreciable leaching from the solid
during the reaction. As an example of the recyclability of the UiO
materials, Fig. S1 in Supporting Information shows the XRD of fresh
and used UiO-66 and UiO-66-NH2 catalysts, while the correspond-
ing kinetic curves obtained for the esterification of C12 with EtOH
over UiO-66 is shown in Fig. S2.
In order to put into perspective our results obtained with the
two MOFs, we have studied the esterification of C12 using other
acid catalysts (both homogeneous and heterogeneous) under the
same reaction conditions. The most representative results obtained
[
24], for which similar acid–base bifunctional mechanisms were
proposed.
The stability of the UiO-66 and UiO-66-NH2 catalysts under
the reaction conditions was checked in all cases by comparing
the XRD patterns of the fresh materials with those of the solids
recovered after the reaction. In all the cases, the crystallinity of the
materials was preserved and the intact solids recovered after the
reaction can be re-used without significant differences for at least
two additional catalytic cycles. Elemental analysis of the catalysts
before and after the reaction, revealed the same Zr content, while
Scheme 2. Plausible reaction mechanism for the alcohol esterification reaction over (a) a monofunctional acid catalyst (UiO-66); or (b) a bifunctional acid-base catalyst
(
UiO-66-NH2).
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are included in Tables 1 and 2. In our hands, the performances of
the Zr-MOFs with MeOH and EtOH were superior to other zirco-
nium compounds and zeolites tested, such as those reported in
0,6
0
0
0
0
0
0
,5
,4
,3
,2
,1
,0
Tables 1 and 2, and were only surpassed by zircocene, Zr(Cp) Cl ,
2
2
and H SO .
2
4
On the other hand, the comparison with catalytic data from lit-
erature is not straightforward, since in many cases the reaction
conditions used (temperature, C12:alcohol ratio and amount of
catalyst) are very different. Nevertheless, Tables 1 and 2 also con-
tain data extracted from previous studies using various types of
solid acid catalysts. In general, these studies do not provide enough
kinetic data to calculate the corresponding turnover frequencies
(
TOFs), for which conversion at short reaction times is required.
Thus, for the sake of comparison, the productivity of the catalyst,
calculated at the end of the reaction as moles of product formed per
mol of catalyst used and per hour, is provided in Tables 1 and 2. The
same calculation has also been extended to the catalysts measured
in the present work. In general, the activities of the Zr-MOFs for
C12 esterification with MeOH and EtOH are comparable (in some
cases superior) to mild acid catalysts and clearly lower than strong
acid materials, such as supported heteropolyacids. Note that most
of the studies considered in Tables 1 and 2 were carried out at
temperatures well above those used in the present study. Hence,
given the endothermic character of the esterification reaction, this
can explain, in part, the better productivities obtained with these
catalysts with respect to the Zr-MOFs.
C12
C16
C18
C18:1
C18:2
C18:3
Fatty acid
Fig. 3. Pseudo-first order reaction rate constant (k) of esterification of vari-
ous saturated and unsaturated fatty acids with EtOH over UiO-66-NH . (Fatty
acid:EtOH = 1:18, 8 mol% Zr with respect to fatty acid, 78 C).
2
◦
for C12 esterification commented above). As it can be observed, the
reaction rate decreases as the chain length and the degree of unsa-
turation of the fatty acid increases. This is probably due to higher
adsorption of the unsaturated fatty acid (or fatty ester) on the sur-
face of the solid, which causes the progressive deactivativation of
the catalyst. However, it is worth mentioning that this deactivation
due to product adsorption is fully reversible, and the activity of the
catalysts is completely recovered by simply washing with EtOH.
In conclusion, the above experiments demonstrates that both
Zr-containing UiOs can efficiently catalyze the esterification of var-
ious fatty acids with MeOH and EtOH, being less active as the alkyl
chain length and degree of unsaturation of the acid increases. It is
also worth mentioning that in all the reactions tested, the Zr-MOFs
were found to be stable and reusable without significant loss of
activity, as we have previously demonstrated for the esterification
of C12 with EtOH over UiO-66-NH2.
The esterification of carboxylic acids with alcohol is an equi-
librium governed reaction that determines the maximum yield of
ester that can be obtained at a given temperature (the presence of
the catalyst only lowers the time needed to attain this equilibrium).
Therefore, since H O is a byproduct of the esterification reaction,
2
reasonably the presence of H O in the reaction medium will dis-
2
place the equilibrium to the left, and less ester will be produced.
Thus for instance, when a large excess of H O (15 equivalents with
2
respect to C12) was intentionally added, the final yield of ethyl lau-
rate produced after 8 h passed from 99 to 60% (compare entries
3
and 4 in Table 2), resulting in a decrease of the corresponding
−1
TOF from 7.4 to 2.1 h . Meanwhile, if the excess of EtOH used
is lowered, the equilibrium of the esterification reaction will be
less displaced toward product formation. Thus, when the C12:EtOH
molar ratio was lowered from 1:18 to 1:5, the final amount of ester
formed also decreased, passing from 99% after 8 h to a maximum
3.4. FFA esterification for the production of other compounds of
interest
−
1
yield of 55% after 20 h, while the observed TOF was 1.5 h (compare
entries 3 and 5 in Table 2).
Thus far, we have demonstrated that Zr-containing UiO-type
MOFs can be used as stable and recyclable heterogeneous catalysts
for the production of fatty acid methyl and ethyl esters (FAMEs and
FAEEs) from biomass derived free fatty acids. These compounds
form the so-called biodiesel. However, besides the preparation of
biodiesel molecules, FFA esterification has interest for the prepara-
tion of other compounds that find application as food and cosmetic
emulsifiers, lubricants, solvents, surfactants, detergent additives,
etc. Herein we will show that UiOs can also be used for the synthe-
sis of valuable chemicals based on esterification of readily available
FFAs with various alcohols.
3.3. Esterification of other saturated (C16, C18) and unsaturated
(
C18:1, C18:2, C18:3) fatty acids
In view of the good catalytic activity and recyclability of UiO-
type MOFs for the esterification of lauric acid with MeOH and EtOH,
we wanted to investigate the applicability of the MOFs to other
biomass derived free fatty acids with longer chain lengths, both
saturated and unsaturated. Thus, we extended our study to the
esterification with MeOH and EtOH of palmitic (hexadecanoic acid,
C16), Stearic (octadecanoic acid, C18), Oleic (cis-9-octadecenoic
acid C18:1), linoleic (cis,cis-9,12-octadecadienoic acid, C18:2) and
Esterification of fatty acids with long chain alcohols gives high
molecular weight esters, known as wax esters. Among them, oleyl
oleate, a synthetic analog of jojoba oil, finds application as lubri-
cant for high-speed machinery [39] and is used as base material in
cosmetics, pharmaceuticals, paints, wood coatings and perfumery
products [40–42]. The results of the esterification of oleic acid with
oleyl alcohol to obtain oleyl oleate in the presence of UiO-66-NH2
are shown in Fig. 4. Analogous results were obtained with UiO-66 as
catalyst. As it can be seen in Fig. 4, the presence of the Zr-MOF cat-
alyst clearly improves the esterification reaction and increases the
yield obtained with respect to the autocatalyzed reaction. However,
␣
-linolenic acids (cis,cis,cis-9,12,15-octadecatrienoic acid, C18:3).
For the sake of brevity, the complete catalytic data obtained for each
fatty acid and the comparison with other acid catalysts from the
literature is provided as Supporting Information (Tables S1–S10).
In order to illustrate the dependence of the chain length and
unsaturation degree of the fatty acid on reaction rate, Fig. 3 shows
the calculated pseudo-first order reaction rate constants, k, of ester-
ification of various fatty acids with ethanol over UiO-66-NH . The
same tendency was also observed for UiO-66, although this mate-
rial was in general less active than UiO-66-NH (as already observed
2
◦
when the reaction was performed at 80 C the time-conversion
2
Please cite this article in press as: F.G. Cirujano, et al., Zirconium-containing metal organic frameworks as solid acid cata-
lysts for the esterification of free fatty acids: Synthesis of biodiesel and other compounds of interest, Catal. Today (2014),
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F.G. Cirujano et al. / Catalysis Today xxx (2014) xxx–xxx
7
a) 100
b)100
80
60
40
20
0
80
60
40
20
0
0
5
10
15
20
0
5
10
15
20
time (h)
time (h)
◦
◦
Fig. 4. Esterification of oleic acid with oleyl alcohol at: (a) 80 C and (b) 110 C, over UiO-66-NH2 (—ꢀ—) (10 mol% Zr) and in the absence of catalyst (—ꢁ—).
plot rapidly reached a plateau and only partial conversion was
attained (34% yield after 20 h of reaction). This maximum yield
can be increased by slightly rising the temperature to 110 C (88%
100
◦
3
yield after 20 h), although in this case the difference with respect to
the autocatalyzed reaction was less pronounced. For comparison,
Sunitha et al. [43] reported the synthesis of various wax esters using
1
2
a Lewis acidic ionic liquid catalyst (choline chloride·2ZnCl ). Among
2
them, oleyl oleate was obtained in 98% yield after 12 h of reaction
◦
at 110 C, but up to 1 equivalent of catalyst was used (i.e.; 100 mol%,
50
ten times more catalyst than the amount used in the present study).
ZrOCl ·8H O has also been used as heterogeneous catalyst for the
2
2
4
preparation of wax esters, but low yields were obtained in the case
of acids and alcohols with chains longer than C14 length [44].
Another product of interest that can be obtained by esterifica-
tion of free fatty acids is isopropyl palmitate, obtained from palmitic
acid and isopropanol. This compound is a dry and soft emollient
with good adsorption characteristics used in cosmetics [45], lubri-
cants and is an excellent solvent for mineral oil, silicone and lanolin
5
0
0
4
8
12
16
20
[
46].
In order to determine the applicability of Zr-MOFs for the pro-
duction of isopropyl palmitate, palmitic acid was contacted with
time (h)
◦
◦
5
eq isopropanol in the presence of UiO-NH2 (8 mol%) at 100 C,
Fig. 5. Esterification of palmitic acid with isopropyl alcohol at 100 C in the presence
of various catalysts: (1) UiO-66-NH2 (8 mol% Zr); (2) reused UiO-66-NH2 (8 mol% Zr);
3) H2SO4 (8 mol% H ); (4) Zinc acetate supported on succinic acid-modified silica
0.2 mol% Zn); and (5) blank experiment, in the absence of catalyst.
and the kinetic data obtained is shown in Fig. 5. Under these condi-
tions, isopropyl palmitate was quantitatively obtained (99% yield)
after 20 h, while the blank, autocatalyzed reaction only afforded 9%
yield. For comparison, time-conversions attained with other homo-
geneous (H SO ) and heterogeneous (zinc acetate supported on
+
(
(
long-term stability of the material under continuous operation in
a fix bed reactor.
2
4
succinic acid-modified SiO2 as reported in [46]) catalysts are also
included in the same plot. As it can be seen, the activity of the
UiO material is comparable with that of H SO4 measured in the
4. Conclusions
2
same conditions in our lab, while it clearly outperform the results
obtained with the supported zinc acetate (although in this case
the amount of catalyst used was sensibly lower: 0.2 mol% versus
In summary, in the present work we have demonstrated that
zirconium containing MOFs, UiO-66 and UiO-66-NH , are active,
2
8
mol% in the present study). In any case, our results demonstrate
stable and reusable heterogeneous Lewis acid catalysts for the
esterification of biomass-derived free fatty acids with various alco-
hols. The superior activity of UiO-66-NH2 with respect to UiO-66
indicates the occurrence of a possible cooperative acid–base catal-
ysis, leading to a dual activation of the acid by the coordinatively
unsaturated Zr vacancies, along with an assisted deprotonation
of the alcohol or water elimination by the amino groups of the
that UiO-type MOFs can be used as effective heterogeneous cata-
lysts for the production of isopropyl palmitate. The maintenance of
the crystalline structure and the fair preservation of the catalytic
activity upon reuse indicate that the material can withstand the
mild reaction conditions used. In order to determine the full poten-
tial of these Zr-containing MOFs, we are currently investigating the
Please cite this article in press as: F.G. Cirujano, et al., Zirconium-containing metal organic frameworks as solid acid cata-
lysts for the esterification of free fatty acids: Synthesis of biodiesel and other compounds of interest, Catal. Today (2014),
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material. Various products ranging from biodiesel compounds
fatty acids ethyl and methyl esters), and other compounds of inter-
est (viz., oleyl oleate and isopropyl palmitate) can be prepared using
these Zr-MOFs, with catalytic activities and productivities compa-
rable (in some cases superior) to other solid acid catalysts.
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Please cite this article in press as: F.G. Cirujano, et al., Zirconium-containing metal organic frameworks as solid acid cata-
lysts for the esterification of free fatty acids: Synthesis of biodiesel and other compounds of interest, Catal. Today (2014),
http://dx.doi.org/10.1016/j.cattod.2014.08.015