Toward understanding of the role of Lewis acidity in aldol condensation of acetone and furfural using MOF and zeolite catalysts

Article abstract of DOI:10.1016/j.cattod.2014.08.016

The aldol condensation of furfural and acetone allow producing higher value products from simple organic compounds resulting from biomass processing. In this paper, a number of metal organic framework (MOF) materials possessing Lewis acidity were investigated as catalysts in this reaction. Experiments were carried out under batch reaction conditions in Parr stirred autoclave at 100 °C during 0-4 h. The catalytic results indicated that the aldol condensation over MOFs took place with the participation of acidic rather than basic centers. The catalytic performance of these materials exhibited could not be explained by their reported Lewis acidity. Experiments with heat-activated and re-hydrated samples showed that Br?nsted acid sites could also be present in MOFs as a consequence of interaction of metal cation with surrounding water molecules. As a consequence, the catalysts having such generated Br?nsted acidity exhibited enhanced activity in the reaction.

Full text of DOI:10.1016/j.cattod.2014.08.016

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Toward understanding of the role of Lewis acidity in aldol
condensation of acetone and furfural using MOF and zeolite catalysts
a
b,c
Oleg Kikhtyanina,∗, David Kubicˇka , Jirˇí Cejka
ˇ
a Research Institute of Inorganic Chemistry – UniCRE-RENTECH, Chempark Litvínov, 436 70 Litvínov, Czech Republic
b J. Heyrovsky´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, v.v.i., Dolejsˇkova 3, CZ-182 23 Prague 8, Czech Republic
c Center of Research Excellence in Petroleum Refining and Petrochemicals, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia
a r t i c l e i n f o
a b s t r a c t
Article history:
The aldol condensation of furfural and acetone allow producing higher value products from simple organic
compounds resulting from biomass processing. In this paper, a number of metal organic framework (MOF)
materials possessing Lewis acidity were investigated as catalysts in this reaction. Experiments were
carried out under batch reaction conditions in Parr stirred autoclave at 100 ◦C during 0–4 h. The catalytic
results indicated that the aldol condensation over MOFs took place with the participation of acidic rather
than basic centers. The catalytic performance of these materials exhibited could not be explained by
their reported Lewis acidity. Experiments with heat-activated and re-hydrated samples showed that
Brønsted acid sites could also be present in MOFs as a consequence of interaction of metal cation with
surrounding water molecules. As a consequence, the catalysts having such generated Brønsted acidity
exhibited enhanced activity in the reaction.
Received 16 June 2014
Received in revised form 28 July 2014
Accepted 18 August 2014
Available online xxx
Keywords:
Metal organic framework
Aldol condensation
Acidic catalysis
Lewis acidity
Brønsted acidity
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
Solid catalysts with acid–base character are considered as a
promising alternative to homogeneous catalysts for aldol conden-
Geopolitical and environmental concerns faced by our society
are reflected in the extensive research activities focused on produc-
tion of automotive fuels and different valuable products using other
resources than fossil ones. Among them, lignocellulosic biomass is
considered as one of the most promising bio-based feedstocks for
the production of fuels and other chemical products not competing
with food crops, fodder, and natural habitat [1]. Furanic compounds
(furfural, 5-hydroxymethlyfurfural) and acetone are readily avail-
able chemicals, which can be produced by biomass hydrolysis [2].
Additionally, acetone is available as a by-product in phenol pro-
duction. Aldehydes and ketones can react by aldol condensation
and afford higher added value products from simple and cheap
ones. Aldol condensation proceeds in the presence of either basic
or acidic catalysts [3,4]. The most effective industrially available
methods rely on using liquid bases (NaOH or KOH) or mineral acid
(H2SO4) as catalysts [3]. However, these catalysts pose a major envi-
ronmental threat due to waste water production and equipment
corrosion. To address this issue, new industrially viable catalysts
must be developed.
sation of furfural and acetone [5–13]. In accordance with generally
accepted reaction scheme, 4-(2-furyl)-4-hydroxybutan-2-on (alco-
hol FAc-OH; Scheme 1) is the primary intermediate product of
the interaction between furfural and acetone. In the presence
of catalysts having strong acidic or basic centers, this inter-
mediate is easily dehydrated to 4-(2-furyl)-3-buten-2-on (FAc;
Scheme 1), the primary product of aldol condensation. Further,
F2Ac (1,4 pentadiene-3-on-1,5-di-2-furanyl) formation takes place
as a result of a consecutive reaction of FAc with another furfural
molecule. When acidic zeolites are used as catalysts for the reac-
tion, the formation of an additional product, (FAc)2, occurs as a
result of FAc dimerization over Brønsted acid sites (Scheme 1) [11].
Among solid catalysts, Al–Mg hydrotalcites and different metal
oxides or double mixed oxides with basic character are consid-
ered as the most promising catalysts for this reaction [5–13]. The
essential disadvantages of such basic catalysts are their high sen-
sitivity to ambient CO2, which transforms them into a catalytically
inactive form and the lack of reliable methods for recovering their
catalytic properties after regeneration [14–16]. Recently, zeolites
were reported as catalysts for aldol condensation of acetone and
furfural [11]. It was shown that by using zeolites the disadvantages
of basic catalysts i.e. susceptibility to CO2 and poor regenera-
bility could be avoided. On the other hand, their activity in the
aldol condensation of acetone and furfural is lower than that of
∗
Corresponding author.
E-mail address: oleg.kichtanin@vuanch.cz (O. Kikhtyanin).
http://dx.doi.org/10.1016/j.cattod.2014.08.016
0920-5861/© 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: O. Kikhtyanin, et al., Toward understanding of the role of Lewis acidity in aldol condensation of acetone
and furfural using MOF and zeolite catalysts, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.08.016

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Scheme 1. Aldol condensation between furfural and acetone over acidic or basic catalysts and the formation of dimer product (FAc)2 from FAc compound over acidic catalysts.
hydrotalcites. Moreover, they are rapidly deactivated due to coke
formation during the reaction [11]. Obviously, the optimization
of the properties of solid acidic catalysts in the aldol condensa-
tion of acetone and furfural is not possible without understanding
the importance of different physico-chemical characteristics, espe-
cially the role of Brønsted (BAS) and Lewis acid sites (LAS), for the
reaction. Unfortunately, zeolites contain generally both types of
acidic centers, so an unambiguous conclusion about the degree of
participation of BAS and LAS in the aldol condensation of furfural
and acetone is virtually impossible. This problem can be overcome
by using other solid acidic catalysts which possess only one type of
acid sites.
Fe-BTC has also been reported to be a highly active catalyst
for some reactions requiring Lewis acidity, such as methylation
of amines [32], Claisen–Schmidt condensation between acetophe-
none and benzaldehyde [33], aldehyde acetalization and epoxide
ring-opening reactions by methanol [34–36], the isomerization of
␣-pinene oxide [29]. To the best of our knowledge, no information
yet is available about the use of MOFs as catalysts for aldol conden-
sation of furfural and acetone. The performance of both Cu-BTC and
Fe-BTC in this reaction is of a special interest since these materials
with well-expressed Lewis acidity could provide information about
the importance of such sites for the reaction between acetone and
furfural.
MOFs are heterogeneous porous crystalline solids, which are
characterized by reasonable thermal stability [17] and outstand-
ing textural properties [18]. Unlike zeolites containing both Lewis
and Brønsted acid sites [19–21], MOFs are assumed to possess
mild Lewis acidity [22,23]. It makes them active in a num-
ber of organic reactions [24–27]. The presence of Lewis acidity
in two commercially available MOF materials, Cu-BTC (copper
benzene-1,3,5-tricarboxylate) and Fe-BTC (iron benzene-1,3,5-
tricarboxylate), is well-documented [28,29]. Due to the presence
of Lewis sites, Cu-BTC exhibits good catalytic properties in var-
ious reactions, such as the rearrangement of ␣-pinene oxide to
campholenic aldehyde and the cyclization of citronellal to isopule-
gol [30], the Friedländer reaction between 2-aminobenzophenones
and acetylacetone [23], coumarin synthesis by the Pechmann reac-
tion [31], and Beckmann rearrangement [28].
To elucidate the role of Lewis and Brønsted acid sites in aldol
condensation of acetone and furfural, the performance of sev-
eral metal organic framework (MOF) catalysts with well-expressed
Lewis acidity is compared with the performance of several zeolites
possessing both Brønsted and Lewis acid sites.
2. Experimental
The following MOFs were purchased from Sigma–Aldrich Cu-
BTC (Basolite C300), Fe-BTC (Basolite F300), Mg-formate (Basosiv
M050), and ZIF-8 (Basolite Z1200). The zeolites used as reference
catalysts included MFI (Si/Al = 11.5) and BEA (Si/Al = 19) obtained
from Zeolyst. Their catalytic properties were investigated in aldol
condensation of furfural and acetone. The catalytic experiments
were carried out in a 200 ml stirred batch reactor (Parr autoclave) at
Please cite this article in press as: O. Kikhtyanin, et al., Toward understanding of the role of Lewis acidity in aldol condensation of acetone
and furfural using MOF and zeolite catalysts, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.08.016

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Table 1
Physico-chemical characteristics of MOFs (taken from [28]) and reference zeolite catalysts (taken from [11]).
Sample
BET, m2/g
Dmicropore, nm
Vtot, cm3/g
Vmic, cm3/g
CLAS (mmol/g)
CBAS (mmol/g)
Cu-BTC
Fe-BTC
MFI
1500
1060
378
0.90, 0.50, 0.35
0.86
0.64
0.33
0.126
0.218
2.30
2.56
0.22
0.179
–
–
0.51 × 0.55, 0.53 × 0.56
0.210
0.334
0.353
0.138
BEA
617
0.66 × 0.67, 0.56 × 0.56
100 ◦C. Before the start of the catalytic runs, 1.0 g (unless indicated
otherwise) of the as-obtained catalyst was mixed together with a
mixture of 19.75 g of acetone and 3.25 g of furfural (i.e. acetone-
to-furfural molar ratio of 10-to-1) and loaded into the autoclave.
After initiation of the heating the desired temperature, 100 ◦C, was
achieved in ∼60 min, and the autoclave was kept at T = 100 ◦C for
an additional 2 or 4 h.
than Cu-BTC (conversion 13.7%), but lower than the fresh Fe-BTC
catalyst (Fig. 1A). Elemental analysis of the organic reaction prod-
ucts proved that no leaching of Cu or Fe cations into the liquid phase
took place. So, catalytic action of dissolved Fe3+ and Cu2+cations
could be neglected and only physico-chemical characteristics of Fe-
BTC and Cu-BTC should be taken into account for the discussion of
catalytic results.
The Fe-BTC catalyst was also reactivated after the catalytic
experiment by extraction of the soluble deposits using ethanol as a
solvent as the conventional regeneration technique based on oxi-
dation of the deposits is not suitable in the case of MOFs due to
their low thermal stability.
Analysis of the reaction products was performed using an Agi-
lent 7890A GC unit equipped with a flame ionization detector using
a HP-5 capillary column (30 m/0.32 ID/0.25 ␮m). The obtained
products were identified based on the standard reference com-
pounds and GC–MS analysis of representative product samples.
The catalytic activity is expressed in terms of conversion, which is
defined as the fraction of furfural which was converted. The selec-
tivity was calculated by dividing the percentage of furfural moles
presented in the products by the total moles of converted furfural.
The previously reported results of aldol condensation of furfural
with acetone over zeolites indicate that both Lewis and Brønsted
acid sites might be involved in the reaction [11,37]. References on
the study of Cu-BTC and Fe-BTC as catalysts for different organic
reactions show [29–31] that the studied MOFs have a hard Lewis
acid character. Unfortunately, our experiments with MOFs possess-
ing similar Lewis acidity (Table 1) do not unambiguously explain
the effect of LAS in aldol condensation of furfural with acetone as
Fe-BTC affords 10 times higher conversion than Cu-BTC (Fig. 1;
Table 2) under the same experimental condition despite having
only 10% higher concentration of LAS (Table 1). Hence, the role of
Lewis acidity remains unclear and further factors have to be taken
into account.
It can be seen (Fig. 1B) that Cu-BTC exhibits only weak dehydrat-
ing properties; selectivity to alcohol, the primary product of the
interaction between furfural and acetone, is as high as 46% after 2 h
and reduces to 30% after 4 h of the reaction (Fig. 1B). The absence of
the second condensation product, F2Ac (Fig. 1B), serves as an addi-
tional evidence of the poor catalytic behavior of Cu-BTC. In contrast,
the selectivity to alcohol over Fe-BTC is just 10% after 2 h of the
reaction. Additionally, F2Ac is found among the reaction products
over this catalyst with a selectivity above 15% and when increasing
the time of the experiment to 4 h. Moreover, (FAc)2, i.e. a prod-
uct ascribed to FAc dimerization [11], is observed in the products
(S = 1.4%) as well. The composition over the regenerated catalyst
has changed: selectivity to alcohol increases to 22.1% while that to
F2Ac decreases to 8%.
From the comparison of the catalytic properties of MOFs with
those of zeolites (Table 2) it can be elucidated that the acid
sites in MOFs are weaker than those in zeolites as evidenced by
their significantly lower dehydration activity of the primary alco-
hol product. In fact, FAc-OH is not observed among the products
over zeolites at comparable conversion levels, though this may be
influenced by the presence of Brønsted acid sites responsible for
dehydration [38,39].
It was also demonstrated previously [11] that the formation of
(FAc)2 can be related to the Brønsted acidity present in zeolites;
the selectivity was as high as 11.5% over zeolite BEA at conversion
around 29%, i.e. similar to that observed for Fe-BTC. Thus, the pres-
ence of (FAc)2 in the reaction products suggests that, first, the aldol
condensation of furfural and acetone over Fe-BTC is definitely acid
rather basic catalyzed reaction; second, the presence of not only LAS
but also BAS in Fe-BTC. Indeed, it was proposed in [34] that weak
Brønsted acid sites could be present in Fe-BTC due to the existence
of structural defects. If so, the catalytic activity of MOF materials in
the aldol condensation might be attributed to the presence of BAS
rather than LAS since the Lewis acidic Cu-BTC affords only neg-
ligible furfural conversion. Moreover, the very recent theoretical
calculations by P. Nachtigall et al. [40] have shown that LAS alone
are not able to decrease the activation barrier for aldolization, while
BAS decrease it.
3. Results and discussion
Several MOF catalysts, i.e. Basosiv M050 and Basolite Z1200,
exhibited only negligible activity in the aldol condensation of
furfural and acetone (furfural conversion < 1%) and are thus not dis-
cussed here further. On the other hand, Cu-basolite (Cu-BTC) and
Fe-basolite (Fe-BTC) were active in the reaction. These MOFs are
commercially available materials and the textural and acidic prop-
erties of the samples under study were determined by Opanasenko
et al. [28]. Hence, we used for our investigation the published
data. The Cu-BTC possesses higher BET surface area (1500 m2/g)
than Fe-BTC (1060 m2/g) and similar Lewis acid sites (LAS) con-
centration, 2.3 and 2.56 mmol/g, respectively (Table 1). While the
concentration of LAS in Fe-BTC is only ca. 10% higher than in Cu-
BTC, its BET area is ca. 30% lower and thus, judging from these
physico-chemical characteristics, similar catalytic activity could
be expected. Surprisingly, the furfural conversion during its aldol
condensation with acetone over Cu-BTC is significantly lower in
comparison with Fe-BTC (Fig. 1). Furfural conversion over Cu-BTC
is 2.7% after 2 h of experiment whereas it is nearly 10 times higher
over Fe-BTC, 26.2% (Table 2). It should also be noted that increasing
the reaction time up to 4 h does not further increase the furfural
conversion. Moreover, the Fe-BTC was re-used in aldol condensa-
tion after regeneration with ethanol and it showed better activity
Table 2
Catalytic properties of MOFs and reference zeolite samples in the aldol condensation
of furfural and acetone.
Sample
Furfural conversion, %
Selectivity, %
FAc-OH
FAc
F2Ac
(FAc)2
Cu-BTC
Fe-BTC
MFI
2.7
26.2
6.4a
29a
30
8
70
71
0
20
0a
0
1
0a
0a
100a
83a
0a
BEA
5.5a
11.5a
a
Taken from [11].
Please cite this article in press as: O. Kikhtyanin, et al., Toward understanding of the role of Lewis acidity in aldol condensation of acetone
and furfural using MOF and zeolite catalysts, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.08.016

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Fig. 1. Conversion of furfural over Cu-BTC and Fe-BTC (A) and the selectivity to the main aldol condensation products (B).
Fig. 2. Catalytic properties of activated Cu-BTC and Fe-BTC and rehydrated Fe-BTC samples. (A) Furfural conversion. (B) Products selectivity. 1 – As-received Cu-BTC. 2 –
Cu-BTC activated at 150 ◦C. 3 – Fe-BTC activated at 150 ◦C. 4 – Activated and rehydrated Fe-BTC. Note: only 0.25 g of regenerated catalyst was used in the experiments with
Fe-BTC.
The question remains, however, whether it is possible to
attribute the existence of BAS in MOF materials exclusively to
the presence of structural defects? To answer this question, we
have performed a thermal activation of Cu-BTC and Fe-BTC at
150 ◦C to remove water which could be present in these materi-
als. Fig. 2 shows that furfural conversion of thermally activated
Cu-BTC decreases by an order of magnitude, the only reaction
product being alcohol. The additional experiments with Fe-BTC
were conducted using 0.25 g of the regenerated catalyst. Catalytic
results obtained over the heat-treated samples have shown that
furfural conversion over activated Fe-BTC is 1.6% and the reaction
products consist of two compounds: FAc-OH with the selectiv-
ity of 58% and FAc with the selectivity of 42%. Another part of
Fe-BTC sample activated at 150 ◦C was impregnated with a few
drops of water, followed by drying in air for 24 h. The catalytic
test of such re-hydrated sample evidenced an increase in the fur-
fural conversion up to 4.2% (Fig. 2A). Also, the selectivity to alcohol
was significantly reduced (from ca. 60% to about 14%), and the
second condensation product F2Ac was obtained with the selec-
tivity of 14% (Fig. 2B).
The results demonstrate that water present in the fresh, as-
obtained Fe-BTC, introduces a promoting effect on the catalytic
properties of the MOF material in comparison with the dried cat-
alysts, plausibly due to the generation of BAS. It is also obvious
(Fig. 2B) that the active sites present in Cu-BTC are not capable of
the second aldol condensation step affording F2Ac while this prod-
uct is obtained over Fe-BTC. This suggests that the concentration of
Brønsted acid sites in Cu-BTC is lower than in Fe-BTC or they are
weaker than those in Fe-BTC. As in both catalysts a clear effect of
water (moisture) is visible, it was investigated further.
To prove the importance of water for the presence of BAS in
hydrated iron and copper MOF catalysts, additional experiments
were carried out using copper and iron hydrated nitrates as cat-
alysts for aldol condensation (Fig. 3). Furfural conversion over
Cu(NO3)2·6H2O was 1.1%, and the reaction products consisted of
alcohol with a selectivity of 52.2% and FAc with a selectivity of
Fig. 3. Comparison of catalytic properties of iron and copper nitrates in the aldol condensation of furfural and acetone.
Please cite this article in press as: O. Kikhtyanin, et al., Toward understanding of the role of Lewis acidity in aldol condensation of acetone
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47.8%. Furfural conversion in the presence of Fe(NO3)3·9H2O was
9.8% and the reaction products were not limited to FAc-OH and FAc,
but F2Ac with a selectivity of 3.8% and (FAc)2 with a selectivity of
6.6% were also present. The last result shows that the hydrated iron
nitrate definitely exhibits Brønsted acidity. Indeed, it is known that
transition metal ions like Fe3+or Cu2+ cannot be Brønsted acids by
themselves (as they do not possess any proton to donate), but in
water solutions they are causing the hydrolysis of water molecules
being in their first coordination sphere. Thus, the existing metal
aqua complex can be considered as donor of the proton, forming
hydroxoaqua complex in the process and giving up H+ ions to the
medium. Water molecules co-ordinatively bound to one of these
metal ions are more acidic than normal water molecules.
Taking into account the results of the catalytic tests, it can be
suggested that even in organic medium Fe3+ cations in hydrated
iron nitrate may act as donors for the neighboring water molecules,
thus resulting in the appearance of H+ ions. Unfortunately, the exact
structure of Fe-BTC remains unknown due to its poor crystallinity
[34] making it difficult to describe its interaction with water. How-
ever, similar catalytic properties of iron hydrated nitrate and iron
1,3,5-benzenetricarboxylate (Fe-BTC) make us suggest that Fe-BTC
also contains bound water molecules acting, by analogy, as pro-
ton donors. As a result, Fe-BTC also possesses Brønsted acidity as
evidenced by the formation of the dimerization product, (FAc)2.
Moreover, trivalent ions pull the electrons more strongly than diva-
lent ions. That means that the hydrogen atoms in the ligand water
molecules will have a greater positive charge in a trivalent ion, i.e.
being more acidic. This could explain the string difference in the
catalytic behavior of Cu-BTC and Fe-BTC.
was supported by the European Regional Development Fund and
the state budget of the Czech Republic.
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Catalytic behavior of Fe-BTC and Cu-BTC in the aldol conden-
sation of furfural and acetone suggests that the reaction proceeds
with the participation of acidic rather than basic sites. Solid cata-
lysts having exclusively Lewis acid sites possess substantially lower
activity in the aldol condensation of acetone and furfural, as proved
by the results over Cu-BTC in this reaction, particularly when suf-
ficiently dried. Fe-BTC, which is also assumed as a material with
Lewis acidity, shows a fairly good activity in the aldol condensa-
tion of acetone and furfural. Such catalytic behavior of Fe-BTC can
be explained by the presence of (weak) Brønsted acid sites in addi-
tion to Lewis acid sites. These centers are present in Fe-BTC either
due to the existence of structural defects, or due to the activat-
ing effect of Fe3+ cation on the surrounding (coordinated) water
molecules, resulting in the appearance of H+ ions in the reaction
medium, i.e. of Brønsted acidity. The consequences of the gener-
ated Brønsted acidity in the catalyst are the increase in the activity
of MOF materials and the change in the composition of reaction
products. In particular, the formation of (FAc)2 takes place which,
in turn, may be considered as a proof for the presence of Brønsted
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This publication is a result of project no. P106/12/G015 sup-
ported by the Czech Science Foundation which is being carried out
in the UniCRE center (CZ.1.05/2.1.00/03.0071) whose infrastructure
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http://dx.doi.org/10.1002/cctc.201402411.
Please cite this article in press as: O. Kikhtyanin, et al., Toward understanding of the role of Lewis acidity in aldol condensation of acetone
and furfural using MOF and zeolite catalysts, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.08.016