Unprecedented selectivities in aldol condensation over Mg-Al hydrotalcite in a fixed bed reactor setup

Article abstract of DOI:10.1016/j.catcom.2014.09.002

Aldol condensation of furfural with acetone (molar ratio 1:10) was carried out in a flow fixed bed setup at 50 °C using calcined hydrotalcite with Mg/Al of 3 as a catalyst. Complete conversion of furfural and stable catalyst performance was obtained during the initial 50 h on stream. This period was followed by a rapid catalyst deactivation. In contrast to previous reports, higher molecular weight products that were identified as products of successive aldol condensation of acetone with furfural were observed. Their concentration was time-dependent with a maximum formation after 10 to 30 h on stream.

Full text of DOI:10.1016/j.catcom.2014.09.002

Catalysis Communications 58 (2015) 89–92
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Unprecedented selectivities in aldol condensation over Mg–Al
hydrotalcite in a fixed bed reactor setup
⁎
Oleg Kikhtyanin, Lukáš Hora, David Kubička
Research Institute of Inorganic Chemistry, UniCRE-RENTECH, Areál Chempark Záluží, 436 70 Litvínov, Czech Republic
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 May 2014
Received in revised form 15 August 2014
Accepted 1 September 2014
Available online 16 September 2014
Aldol condensation of furfural with acetone (molar ratio 1:10) was carried out in a flow fixed bed setup at 50 °C
using calcined hydrotalcite with Mg/Al of 3 as a catalyst. Complete conversion of furfural and stable catalyst
performance was obtained during the initial 50 h on stream. This period was followed by a rapid catalyst deacti-
vation. In contrast to previous reports, higher molecular weight products that were identified as products of
successive aldol condensation of acetone with furfural were observed. Their concentration was time-dependent
with a maximum formation after 10 to 30 h on stream.
Keywords:
© 2014 Elsevier B.V. All rights reserved.
Aldol condensation
Hydrotalcites
Furfural
Acetone
1. Introduction
flow reaction systems which are more favorable from a practical point
of view. Moreover, only the application of a flow reactor system
Biomass-derived short-chain aldehydes and ketones originating
from pyrolysis of lignocellulosic biomass have rather limited direct
applicability. Aldol condensation has been proposed to valorize these
primary products by combining them into more complex reaction prod-
ucts with higher molecular weight. The reaction between furfural and
acetone, which are suitable model compounds, results in the formation
of ketones with 8 or 13 carbon atoms [1] (Scheme 1). Moreover, acetone
molecule can interact not only with aldehydes (e.g. furfural) but also
with ketones, hence also with another acetone molecule. This acetone
self-condensation produces compounds with 6 or 9 carbon atoms [2].
Both acetone self-condensation and aldol condensation of furfural and
acetone can be competitive reactions and proceed over the same cata-
lyst at the same reaction conditions [1]. Industrially, homogenous base
catalysts, such as sodium and calcium hydroxide, are used for aldol con-
densation. However, due to ecological and economic considerations the
introduction of heterogeneous basic catalysts becomes very relevant [3].
Among these catalysts, layered double oxides, for example hydrotalcites
(HTC), seem to be the most promising [4]. Calcined Mg–Al hydrotalcites
possess high activity in the aldol condensation of furfural and acetone
even at reaction temperatures below 100 °C [1]. Aldol condensation
over heterogeneous basic catalysts, similar to a homogeneous process,
is usually investigated in a batch reactor (a glass reactor or an autoclave)
equipped with a stirring system. Nevertheless, to our best knowledge no
information has been provided to date on the behavior of HTC as a
catalyst for the aldol condensation between furfural and acetone in
makes it possible to investigate the long-term stability (durability) of
HTC materials in aldol condensation, which is essential for evaluating
their prospects in industrial applications. In the present work we have
studied aldol condensation of furfural and acetone using a flow reactor
system with a fixed bed of a catalyst, a Mg–Al hydrotalcite with a
molar ratio of Mg:Al = 3:1 calcined at 450 °C.
2. Experimental
Mg–Al layered double HTC with a Mg:Al molar ratio of 3:1 was pre-
pared according to a procedure described in [1]. The crystallographic
structure of the dried HTC catalyst was determined by X-ray powder dif-
fraction using a Philips MPD 1880 instrument. Aldol condensation of
furfural and acetone was carried out in a fixed-bed continuous-flow
reactor with a catalyst loading of 10 g. The catalyst with a particle size
of 0.125–0.150 mm was placed in the isothermal zone of the reactor,
activated at 450 °C for 16 h under nitrogen flow, and then cooled
down to 50 °C. The liquid feed, consisting of furfural (Acros Organics,
99%) and acetone (Lach:Ner, p.a.) in the molar ratio of 1:10 was fed
into the reactor by a micro-metering pump at weight hourly space
velocity (WHSV) of the total reaction feed equal to 2 h−1. The reaction
conditions were T = 50 °C and P = 0.5 MPa. During the experiment,
liquid reaction products were withdrawn from a cooler-condenser
(T = 0 °C) every 2 h, weighed and analyzed by an Agilent 7890A GC
equipped with a flame ionization detector and an HP-5 capillary column
(30 m/0.32 mm ID/0.25 μm). The yield of liquid products during the ex-
periment was 98% which can be explained by losses due to acetone
evaporation. Finally, the carbon balance in GC analyses was calculated
⁎
Corresponding author. Tel.: +420 476163735; fax: +420 475 212 079.
E-mail address: david.kubicka@vuanch.cz (D. Kubička).
http://dx.doi.org/10.1016/j.catcom.2014.09.002
1566-7367/© 2014 Elsevier B.V. All rights reserved.

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O. Kikhtyanin et al. / Catalysis Communications 58 (2015) 89–92
O
OH
O
O
O
O
O
+
O
_
H2O
O
F
Ac
FAc
FAc-OH
_
O
O
O
O
O
+
O
H2O
F2Ac
Scheme 1. Reaction pathways of aldol condensation of furfural and acetone showing only the main products.
as the number of carbon atoms analyzed in each GC sample (C3, C5, C8,
C13, etc.) divided by the number of carbon atoms (C3 and C5) in the
feedstock.
scheme. The primary reaction product, FAc-OH, is completely absent
during the initial 45 h of TOS indicating good dehydrating properties
of fresh HTC catalyst. FAc is the main reaction product of aldol conden-
sation of furfural and acetone, and selectivity to this compound exhibits
an intricate dependence on TOS. At the beginning of the experiment the
selectivity to FAc is 62–63%; it decreases to 54% at TOS = 20 h but
reaches a maximum value of 73–74% at TOS = 42–44 h, i.e. at the
very end of the range of total furfural conversion. Further, as the conver-
sion of furfural rapidly drops, FAc selectivity tends to decrease to
66–67%. Fig. 1(A) shows that this decrease is accompanied by an
increase in selectivity to primary reaction product, FAc-OH, proving a
weakening of the dehydrating properties of the catalyst and, according-
ly, a reduction of its total activity. A consecutive interaction of FAc with
furfural results in F2Ac formation. Fig. 1(A) shows that the selectivity to
this product is equal to 33% at the start of experiment, but decreases to
5.6% after 20 h of TOS. As TOS further grows, the selectivity to F2Ac also
increases followed by a stabilization at 16–18%.
The peculiar selectivity dependence can be attributed to the forma-
tion of heavier condensation products. Fig. 1(B) shows that the group
of heavier condensation products, i.e. F2Ac2-OH, F2Ac2, F2Ac3 and
F3Ac2, is almost absent in the reaction mixture at low TOS. Their selec-
tivity increases with the increasing TOS and reaches a maximum value
at TOS = 10–30 h followed by a continuous decrease. Finally, these
compounds totally disappear in reaction products at TOS N 60 h. Fig. 1
shows that the selectivities to FAc and F2Ac, on the one hand, and to
F2Ac2-OH, F2Ac2, F2Ac3 and F3Ac2, on the other hand, can be described
as “inverse volcano” and “volcano” curves, respectively. Apparently,
the increase in F2Ac2-OH, F2Ac2, F2Ac3 and F3Ac2 formation takes place
at the expense of FAc and F2Ac formation in the range of TOS = 10–30 h.
The observed variation in the composition of the reaction products
during the experiment clearly has to reflect a change in the properties
of the catalyst. At the initial phase (0–4 h), the pre-calcined HTC sample
promotes the formation of FAc and F2Ac in accordance with the com-
monly accepted reaction route of the aldol condensation of furfural
3. Results and discussion
In accordance with the generally accepted reaction route, aldol con-
densation of furfural and acetone over basic catalysts results in the suc-
cessive formation of the following reaction products: FAc-OH, FAc and
F2Ac (Scheme 1). GC analyses of mixtures obtained under flow reaction
conditions revealed, however, the presence of additional reaction prod-
ucts. Their combined selectivity was as high as 35%. Using GC-MS anal-
ysis the following molecular weights (MWs) of the unknown products
were established: 176, 218, 254, 272, 312 and 350. By combining the
MWs of acetone and furfural while taking into account that furfural
and acetone can interact not only with each other, but also with the
products of acetone self-condensation and the main aldol condensation
products, i.e. FAc-OH, FAc and F2Ac, it was possible to identify plausible
until now unknown reaction products. The classical products of the
aldol condensation of furfural and acetone (FAc-OH, FAc and F2Ac) can
participate in the successive condensation reactions either with furfural
or with acetone owing to the presence of α-H atoms in their structure.
Based on the GC-MS results, the formation of the following products
can be deduced: FAc2 (MW 176), FAc3 (MW 218), F2Ac2-OH (MW
272), F2Ac2 (MW 254), F2Ac3 (MW 312) and F3Ac2 (MW350). More-
over, the involvement of acetone self-condensation products in aldol
condensation with furfural is probable as well as these by-products
were found in the reaction products too. The reaction between furfural
and Ac2 or Ac3 would yield FAc2 (MW 176) and FAc3 (MW 218), respec-
tively. The verification of both the exact molecular structure of
these products and the proposed reaction pathways leading to their
formation is an objective of our forthcoming study. There is no doubt,
however, that aldol condensation of acetone and furfural is more com-
plex than the reaction route presented in Scheme 1, resulting in a
large number of products having high molecular weight that have not
yet been reported and that could be attractive as automotive fuel com-
ponents or their precursors. A tentative simplified reaction network
leading to the formation of these products is depicted in Scheme 2.
As it will be shown further, these products might affect the long-term
stability of the catalytic activity of HTC catalysts.
Fig. 1 shows that complete furfural conversion was obtained during
the initial 45 h of TOS (Fig. 1(A)). The observed high activity of the HTC
sample during a long period proves that aldol condensation can be
successfully carried out using a flow reactor with a fixed catalyst bed.
Nevertheless, after TOS = 45 h furfural conversion has rapidly de-
creased and reached ca. 50% after 64 h. This indicates a rapid HTC cata-
lyst deactivation. Fig. 1 also shows changes in selectivity to the reaction
products with the increasing TOS. These changes reflect the alteration of
the catalyst performance with TOS. At the very beginning, the composi-
tion of reaction products is limited to two compounds, FAc and F2Ac
(Fig. 1), which is in accordance with the commonly accepted reaction
Scheme 2. Tentative reaction network leading to the formation of heavier condensation
products of furfural (F) and acetone (Ac). The shaded areas correspond to acetone self-
condensation (blue) and the main reactions of furfural and acetone condensation (pink).
For the sake of clarity, only dehydrated products are shown.

O. Kikhtyanin et al. / Catalysis Communications 58 (2015) 89–92
91
[9]. This process results in the enhancement of the catalytic activity of
base-catalyzed reactions, for which OH− sites act as active Brønsted
base sites [4,6,8,10]. Our results show that these Brønsted basic sites
in the reconstructed material have a pronounced ability to activate
α-H atoms in ketones thus promoting an additional formation of
bulky compounds and changing the product selectivity. However,
these strong basic sites have been recently suggested to be responsible
for catalyst deactivation in this kind of reactions [2]. The formation of
bulkier compounds than F3Ac2 and F2Ac3 determined by GC may be
also expected due to the further interaction of reaction products.
These bulkier compounds would be less prone to desorption from the
HTC surface resulting in the gradual deactivation of the catalyst as the
one observed here, i.e. due to restricting/preventing the access of reac-
tants to active sites. Nonetheless, further investigations of the formation
of the heavier compounds and particularly of their interaction with the
catalyst surface are needed.
Important supporting information for the conclusions presented
above can be obtained from detailed analysis of acetone transformation.
Acetone conversion and selectivity to the products of acetone self-
condensation observed during the long-term run are shown in Fig. 3.
The initial conversion of acetone is ≈15% with its slight increase to
17% after 12–14 h of the experiment. This increase in catalyst activity
can be again explained by HTC reconstruction due to its re-hydration
and the formation of Brønsted basic sites, which appear to have an
increased ability to the abstract the α-proton in acetone molecule.
Acetone conversion remains constant during the next ca. 30 h followed
by its sharp decrease, which is in line with the trend observed for furfu-
ral conversion. Moreover, as the acetone conversion is stable for about
40–45 h at around 15–17% it can be unequivocally concluded that
under the selected reaction conditions the HTC catalyst exhibits stable
activity followed by a sharp deterioration of the catalyst performance.
This conclusion is supported by the comparison of the reported thermo-
dynamic equilibrium of acetone self-condensation to mesityl oxide,
which is ca. 20% at 120 °C [11], and the conversion to mesityl oxide
and diacetone alcohol reported here, which is about 5% (Fig. 3). As a
result, it is concluded that the acetone conversion is not limited by the
thermodynamical equilibrium.
Ac2-OH is the main product of acetone self-condensation with a
selectivity of 23–27%, and this compound gives Ac2 upon dehydration
(the remaining converted acetone yields condensation products with
furfural described above). Fig. 3 depicts the complex behavior of Ac2 se-
lectivity during the experiment. At the beginning of the run Ac2 selectiv-
ity exceeds 5%, followed by a sharp decrease to b1% at TOS = 8–10 h.
This drop in selectivity may be explained by the increased affinity of
the freshly calcined HTC catalyst to interaction with water resulting
in the formation of the reconstructed HTC material as proved by
XRD (Fig. 2). As a consequence, at the initial time the equilibrium
Fig. 1. A—Furfural conversion and selectivity to main reaction -FAc, FAc-OH and F2Ac.
B—Selectivity to heavier reaction products.
and acetone (Scheme 1). In the course of the reaction water is formed as
one of the products of dehydration of a reaction intermediate, FAc-OH.
According to [5], сalcined hydrotalcites can be reconstructed back to
a layered structure if put in contact with water and appropriate anions.
A comparison of the XRD patterns of as-synthesized HTC, the material
calcined at 450 °C and the catalyst after long-term run (Fig. 2), proves
that during the reaction the reconstruction of MgO phase which is char-
acteristic of the calcined material to the layered HTC structure really
takes place. Also, rehydration enhances the catalytic activity of recon-
structed HTC materials in base-catalyzed reactions, including aldol con-
densation [1,6–8]. Thus, water formed due to FAc-OH dehydration could
induce the reconstruction of HTC and cause a Lewis-to-Brønsted site
transformation resulting from the reaction of O2− basic sites with H2O
Fig. 3. Acetone conversion and selectivity to the reaction products of acetone self-
condensation (the rest of selectivity is due to the formation of compounds of F + Ac
interaction).
Fig. 2. XRD patterns of the initial as-synthesized hydrotalcite (A), calcined material (B) and
catalyst after long-run test (C). ★—HTC phase, —MgO phase.

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O. Kikhtyanin et al. / Catalysis Communications 58 (2015) 89–92
Ac2OH ↔ Ac2 + H2O is shifted to the right. As the process of HTC recon-
struction is completed, the formed material does not react with water as
easily thus resulting in the drop of Ac2 selectivity.
successive condensation products of acetone and furfural and they
could be plausibly exploited as valuable diesel fuel components, partic-
ularly after deoxygenation.
However, with the further increase of TOS Ac2 selectivity starts to
increase and attains 2% at TOS = 20 h. The observed increase in the
dehydration ability of the catalyst reflects the gradual changes in
the properties of the active sites in reconstructed HTC, possibly, due to
the increase of their strength. Obviously, a detailed investigation of the
change of the properties of calcined hydrotalcite during the reaction
under flow reaction conditions requires a separate study.
The results presented in Figs. 1 and 2 show that the behavior of Ac2
selectivity and selectivities to bulky products of acetone/furfural aldol
condensation observed at TOS = 10–50 h is very similar. It could be
concluded that the formation of these compounds takes place on the
strong sites which are responsible for both Ac2-OH dehydration and
α-proton activation in ketones. However, the same sites could be also
responsible for the formation of non-desorbed reaction products
which result in catalyst deactivation and the sharp decrease of its
activity after 50 h of TOS.
Acknowledgments
This publication is a result of project no. P106/12/G015 supported by
the Czech Science Foundation which is being carried out in the UniCRE
center (CZ.1.05/2.1.00/03.0071) whose infrastructure was supported
by the European Regional Development Fund and the state budget of
the Czech Republic.
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It has been demonstrated for the first time that aldol condensation of
furfural and acetone under flow conditions over calcined hydrotalcite
catalyst results in the formation on a much wider variety of products
than those typically reported for aldol condensation in a batch setup.
While total furfural conversion could be obtained over the initial 40 h
on stream, this period was followed by sharp decrease in activity due
to rapid catalyst deactivation. The heavier products were identified as