Aldol Condensation of Cyclopentanone with Valeraldehyde Over Metal Oxides

Article abstract of DOI:10.1007/s10562-019-02701-1

Kinetics of the cross aldol condensation of valeraldehyde with cyclopentanone was investigated in a batch reactor under atmospheric pressure at 130?°C using heterogeneous metal modified oxides, such as CeO₂–MgO, FeO–MgO, FeO–CaO as well as pristine CaO as catalysts. The catalysts were prepared either by evaporation impregnation or deposition precipitation methods and characterized by XRD, TEM, SEM, nitrogen adsorption, ammonia and CO₂ TPD. The results revealed that an optimum amount of strong basic sites gives the highest ratio between cross condensation and self-condensation products of valeraldehyde. The highest yield of the desired product 2-pentylidenecyclopentanone (66%) was obtained with FeO–MgO prepared by the deposition precipitation methods. Graphical Abstract: Cross-condensation of valeraldehyde with cyclopentanone was investigated over heterogeneous Fe–CaO, CeO–MgO, FeO–CaO and CaO catalysts at 130?°C using cyclopentanone as a solvent and reactant. The highest yield of the desired product, 2-pentylidene-cyclopentanone, finding applications as fragrances, flavours and pharmaceuticals, was 66% obtained over FeO–MgO catalyst exhibiting both acid and basic sites.[Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-019-02701-1

Catalysis Letters
https://doi.org/10.1007/s10562-019-02701-1
Aldol Condensation of Cyclopentanone with Valeraldehyde Over Metal
Oxides
Päivi Mäki‑Arvela1 · Nataliya Shcherban1,2 · Chloé Lozachmeur1 · Kari Eränen1 · Atte Aho1 · Annika Smeds1 ·
Narendra Kumar1 · Janne Peltonen3 · Markus Peurla4 · Vincenzo Russo5 · Konstantin P. Volcho6,7 · Dmitry Yu. Murzin1
Received: 20 December 2018 / Accepted: 30 January 2019
© The Author(s) 2019
Abstract
Kinetics of the cross aldol condensation of valeraldehyde with cyclopentanone was investigated in a batch reactor under
atmospheric pressure at 130 °C using heterogeneous metal modified oxides, such as CeO2–MgO, FeO–MgO, FeO–CaO as
well as pristine CaO as catalysts. The catalysts were prepared either by evaporation impregnation or deposition precipitation
methods and characterized by XRD, TEM, SEM, nitrogen adsorption, ammonia and CO2 TPD. The results revealed that an
optimum amount of strong basic sites gives the highest ratio between cross condensation and self-condensation products of
valeraldehyde. The highest yield of the desired product 2-pentylidenecyclopentanone (66%) was obtained with FeO–MgO
prepared by the deposition precipitation methods.
Graphical Abstract
Cross-condensation of valeraldehyde with cyclopentanone was investigated over heterogeneous Fe–CaO, CeO–MgO, FeO–
CaO and CaO catalysts at 130 °C using cyclopentanone as a solvent and reactant. The highest yield of the desired product,
2-pentylidene-cyclopentanone, finding applications as fragrances, flavours and pharmaceuticals, was 66% obtained over
FeO–MgO catalyst exhibiting both acid and basic sites.
Keywords Basic catalyst · Aldol condensation · Valeraldehyde
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10562-019-02701-1) contains
supplementary material, which is available to authorized users.
4
* Dmitry Yu. Murzin
Laboratory of Electron Microscopy, University of Turku,
Turku, Finland
dmurzin@abo.fi
5
6
7
Università di Napoli Federico II, via Cintia, 4, 80126 Napoli,
Italy
1
Johan Gadolin Process Chemistry Centre, Åbo Akademi
University, Turku, Finland
N. N. Vorozhtsov Institute of Organic Chemistry, Russian
Academy of Sciences, Novosibirsk 630090, Russia
2
L.V. Pisarzhevsky Institute of Physical Chemistry, National
Academy of Sciences of Ukraine, 31 pr. Nauky, Kiev 03028,
Ukraine
Novosibirsk State University, Novosibirsk 630090, Russia
3
Laboratory of Industrial Physics, University of Turku, Turku,
Finland
Vol.:(0123456789)
1 3

P. Mäki-Arvela et al.
List of symbols
basic and specialty chemicals to fragrances, flavours and
pharmaceuticals [1–5]. Conventionally aldol condensation
is performed using homogeneous catalysts [6, 7], such as
sodium or potassium hydroxide or Ti(O-t-Bu)4 as catalysts
[7]. Aldol condensation of isophorone was recently demon-
strated with furanic aldehydes using NaOH as a catalyst for
production of jet fuels [4]. Heterogeneous catalysts could
facilitate a more environmentally benign way to produce
chemicals, as for example in alkaline earth metal-transition
metal oxide in biodiesel production [5], or in aldol conden-
sation due to their easy separation and reuse.
K0
Equilibrium constant at standard conditions for
j
reaction j
n
Moles, mol
P
Pressure, bar
P0
R
Standard pressure, bar
Ideal gas constant, J/K/mol
Absolute temperature, K
Absolute standard temperature, K
T
T0
Greek symbols
ΔG0
Gibbs free energy of formation at standard condi-
f
Very often heterogeneous base catalysts are also quite
cheap, such as aluminophosphate [1] and hydrotalcites [2],
active in synthesis of 2-pentylidenecyclopentanone. In this
work cross-condensation of valeraldehyde with cyclopen-
tanone was investigated over several heterogeneous catalysts
of basic nature, such as Ce–MgO, CaO and Fe-modified
CaO and MgO. The aim was to study the influence of the
type of base supports and methods of catalyst synthesis on
the physico-chemical and catalytic properties in cross-con-
densation of valeraldehyde with cyclopentanone.
tions, J/mol
ΔG0
Gibbs free energy of reaction at standard condi-
tions, J/mol
r
ΔGΦ
Gibbs free energy of reaction at 1 bar and a cho-
sen temperature, J/mol
r,j
ΔH0
Enthalpy of formation at standard conditions, J/
mol
f
ΔH0
Enthalpy of reaction at standard conditions, J/mol
Stoichiometric matrix composed by i components
and j reactions
r
νi,j
Besides cross-condensation per se several reactions
(Fig. 1) can also occur, i.e. self-condensation of both valer-
aldehyde and cyclopentanone (Fig. S1) [4, 5]. In cross aldol
condensation the first step is formation of hydroxylated
1-hydroxy-2-pentylcyclopentanone 3, which in the consec-
utive step undergoes dehydration forming an unsaturated
ketone, 2-pentylidenecyclopentanone 4. This ketone being
the desired product, can further react with another aldehyde
forming 2,5-dipentylidenecyclopentanone 7.
1 Introduction
Several reactions are catalyzed by homogeneous and hetero-
geneous bases, including aldol condensation [1–4] and trans-
esterification for product of biodiesel [5]. Aldol condensa-
tion is used for synthesis of various products ranging from
Fig. 1 Reaction scheme for valeraldehyde condensation with cyclopentanone
1 3

Aldol Condensation of Cyclopentanone with Valeraldehyde Over Metal Oxides
Cross-aldol condensation of valeraldehyde with cyclo-
pentanone has been very scarcely studied over heterogene-
ous catalysts [1, 2]. The desired product 4, 2-pentylidene-
cyclopentanone has been synthesized over heterogeneous
hydrotalcites as catalysts giving maximally 90% yield using
valeraldehyde to cyclopentanone molar ratio of 1:5 at 80 °C
after 11 h via slowly adding valeraldehyde into the reac-
tor containing cyclopentanone [2]. In another work also
an excess of cyclopentanone was used in comparison with
valeraldehyde in m-xylene as a solvent at 130 °C under nitro-
gen [1]. The highest selectivity of 2-pentylidenecyclopen-
tanone, 91% at 31% conversion, was obtained at 130 °C after
2 h using amorphous aluminosilicate as a catalyst.
because of amphoteric nature of ceria and Lewis acidity of
iron oxide. Therefore, not surprisingly, iron oxides have been
used as catalysts in cyclohexanone aldol condensation [16],
while cesium was utilized in aldol condensation of heptanal
with benzaldehyde [17]. The aim in this work was to reveal
the role of basic and acid active sites, metal functions in
aldol condensation of valeraldehyde with cyclopentanone
and investigate reaction kinetics and thermodynamics. The
acidity and basicity of the catalysts were quantified by NH3
and CO2 temperature programmed desorption techniques.
According to our knowledge this is the first time when kinet-
ics and thermodynamics of valeraldehyde–cyclopentanone
aldol condensation are reported.
Other reactions relevant to aldol condensation, namely
self-condensation of cyclopentanone and valeraldehyde,
have been also investigated. Self-condensation of cyclo-
pentanone gives 2-cyclopentylidene-cyclopentanone,
which finds applications as fuel or in hydrogenated form
as fragrances and flavors [8, 9]. This reaction occurs using
potassium hydroxide as a catalyst, for example at 100 °C
[8], followed by extraction with diethylether, washing and
drying finally giving 73% yield. More recently also het-
erogeneous catalysts have been used in the synthesis of
2-cyclopentylidene-cyclopentanone by self-condensation
of cyclopentanone [10]. Mesoporous MgO–ZrO2 gave two
main products, namely 2-cyclopentylidene-cyclopentanone
composed of 10 carbon atoms and 2,5-dicyclopentylidene-
cyclopentanone [10]. A remarkable excess of the former
product with 100% selectivity was obtained after 4.5 h at
8% conversion using a lower reaction temperature 110 °C.
On the contrary at 130 °C selectivity was 92% at 85% con-
version. The same reaction was also investigated with MOF
encapsulated phosphotungstic acid as a catalyst at 130 °C
under nitrogen [11]. The yield of 2-cyclopentylidene-cyclo-
pentanone was only ca. 12% after 2 h because of its further
transformation to trimeric compounds and after removal of
hydrogen to trindane.
2 Experimental Section
2.1 Catalyst Synthesis
Fe-modified MgO and CaO and Ce-modified MgO catalysts
were prepared using deposition precipitation (DP) and evap-
oration impregnation (EIM) methods. The CeO2–MgO–DP
catalyst was prepared via dissolving cerium nitrate in dis-
tilled water adding thereafter MgO into the cerium nitrate
aqueous solution. In the deposition–precipitation procedure,
pH was first increased to 10 with ammonium hydroxide
solution (25%) to precipitate cerium followed by stirring
the slurry at 150 rpm for 24 h. After that, the catalyst was
filtered and washed with distilled water. CeO2–MgO–DP
catalyst was dried in an oven at 100 °C for 7 h. Decompo-
sition of cerium nitrate was carried out by thermal treat-
ment of CeO2–MgO–DP catalyst in a muffle oven using
a step calcination procedure. The catalyst designated as
CeO2–MgO–DP.
FeO–MgO–DP catalyst was prepared using aqueous solu-
tion of ferric nitrate by a deposition precipitation method.
The pH was increased to 10 using ammonium hydroxide
solution (25%) and the synthesis carried out for 24 h, which
was followed by drying at 100 °C for 7 h and calcination
in a muffle oven. The catalyst designated as Fe–MgO–DP.
The aqueous solution of the ferric nitrate precursor was
mixed with CaO and kept in a rotary evaporator for 24 h
at the stirring speed of 50 rpm. In the subsequent step, the
water phase was evaporated. The catalyst was removed from
the rotatory flask and dried in an oven at 100 °C for 7 h.
Decomposition of cerium nitrate was carried out in a muffle
oven at 450 °C for 4 h using a step calcination procedure.
The catalyst was designated as Fe–CaO–EIM.
Self-condensation of valeraldehyde gives 2-propylhept-
2-enal, which has found applications as an intermediate for
production of parfumes [12], and plasticizers [13, 14]. Aque-
ous NaOH is typically applied as a catalyst in industrial scale
for this reaction. It was pointed out [13], that this process
causes corrosion and exhibits poor product selectivity. TiO2
was confirmed to be an efficient heterogeneous catalyst for
synthesis of 2-propylhept-2-enal giving very high selec-
tivities to the desired product at 190 °C [13]. Zhao et al.
[13] stated that acidity is also required for production of
2-propylhept-2-enal.
In the current work metal oxides have been applied
as catalysts in aldol condensation of valeraldehyde with
cyclopentanone. Since it is known that aldol condensation
requires acids or bases as catalysts [15], basic supports such
as CaO and MgO were modified with ceria and iron oxide,
2.2 Catalyst Characterization Methods
The Philips X’Pert Pro MPD X-ray powder diffractometer
was used in the XRD measurements. The diffractometer
1 3

P. Mäki-Arvela et al.
was operated in Bragg–Brentano diffraction mode, and the
monochromatized Cu–Kα radiation (λ= 1.541874 Å) was
generated with a voltage of 40 kV and a current of 45 mA.
The measured diffractograms were analyzed with Philips
X’Pert HighScore and MAUD [18] programs. The Powder
Diffraction File 2 (PDF-2) database, Inorganic Crystal Struc-
ture Database (ICSD), and Crystallography Open Database
(COD) were used as references [19–21].
with an excess of cyclopentanone (Sigma Aldrich, >99%),
which was also applied as a solvent (60 ml), if not other-
wise stated. The initial valeraldehyde concentration was
0.50 mol/l if not specifically mentioned. The experiments
were performed by adding valeraldehyde, cyclopentanone
and the catalyst into the cold reactor, which was then heated
to 130 °C. The reactor system was equipped with a con-
denser, which exhibited temperature close to zero. Valeral-
dehyde and a part of cyclohexanone were thus condensed
back to the liquid phase. The experiments were carried out
under argon atmosphere. The weight ratio of valeraldehyde
to cyclopentanone in the kinetic experiments was 0.05, while
the amount of catalyst was 1.6 g. The mass ratio of valer-
aldehyde to catalyst was 1.7 being the same as was used in
[1]. Small catalyst particles (below 63 µm) were used under
vigorous stirring (600 rpm) to avoid internal and external
mass transfer limitations. In some experiments mesitylene
(Sigma Aldrich, 97%) was used as a solvent. The products
were analyzed by a GC equipped with HP-5 column (30 m,
di=320 µm, film thickness 0.5 µm) using the following tem-
perature programme: 60 °C (2 min)–8 °C/min–124 °C–4 °C/
min–200 °C–12 °C/min–280 °C (10 min). The reaction
products were identified by GC–MS (Agilent Technologies,
6870N) equipped with DB-1 column (30 m, 250 µm, film
thickness 0.50 µm).
The scanning electron microscopy coupled with an
energy dispersive X-ray analyzer was utilized to study mor-
phology of the fresh catalysts using Zeiss Leo Gemini 1530
microscope combined with secondary electron and back-
scattered electron detectors. Acceleration voltage of 15 kV
was used for X-ray analyzer. In order to perform analysis,
the catalyst was placed as a thin layer on top of the carbon
coating to enhance the conductivity and allow high quality
of magnified images.
The transmission electron microscopy was utilized to
study the structural properties, porosity, metal particle size
and distribution using JEM-1400Plus (by JEOL ltd. Japan)
of 120 kV maximal acceleration voltage. Interpretation of
TEM images and determination of the particles sizes of the
fresh and spent catalyst were done using ImageJ program.
The specific surface area was determined with Sorptom-
eter 1900 (Carlo Erba Instrument) using nitrogen physisorp-
tion. The catalysts were outgassed at 150 °C for 3 h prior to
the measurement. The BET equation was used to calculate
the specific surface area.
CO2 TPD measurements were performed to determine
the concentration of basic sites using Autochem 2010,
Micromeritics instrument. The catalyst, 200 mg, was first
dried at 150 °C for 30 min under helium (AGA, 99.996%)
after which it was cooled to 100 °C. Thereafter CO2 (AGA)
was adsorbed on the catalyst surface during 30 min. The
physisorbed CO2 was flushed from the catalyst surface at
100 °C for 30 min after which the temperature was increased
by 10 °C/min up to 700 °C.
3 Results and Discussion
3.1 Catalyst Characterization Results
XRD results revealed that FeO–CaO–EIM catalyst contained
iron only as Ca2Fe2O5 phase (Table 1; Fig. 2). Ca2Fe2O5 in
[22] was prepared by coprecipitation of Ca and Fe nitrates
and calcined at 500 °C. The size of Ca2Fe2O5 is about 60 nm
based on XRD [23]. On the other hand, metal oxide Fe2O3
particles are threefold smaller than Ca2Fe2O5 (Table 1).
XRD results of CeO2–MgO showed the presence of cubic
CeO2 [24] and MgO [28] (Table 2). XRD of FeO-MgO-
DP displayed only the presence of cubic MgO. This result
indicates that iron either is in an amorphous form or it well
dispersed on the support with the particle size below 3 nm,
not being detectable by XRD.
Ammonia TPD was performed with Micromeritics (Auto-
Chem 2910) using helium as a carrier gas. The sample was
dried prior to the measurement at 250 °C for 30 min after
which ammonia (5 vol% in helium, AGA) was adsorbed on
the catalyst for 60 min at 25 °C. Then the gas supply was
stopped and these conditions were hold for 30 min. In the
next step the temperature was increased by 10oC/min to
100 °C and hold at 100 °C for 30 min. The ammonia TPD
was measured using the following temperature programme:
100 °C–10 °C/min–600 °C.
SEM images of different catalysts are depicted in Fig. 3.
CeO2–DP exhibits both spherical and plate-like character
(Fig. 3a). FeO–MgO–DP contains plates (Fig. 3b) whereas
FeO–CaO–EIM exhibited large spherical particles between
105 and 148 nm (Fig. 3c). Particles in FeO–CaO–EIM were
both of irregular and spherical shapes in the range of 50
to 600 nm (Fig. 3d). The particle sizes in CaO varied in a
broader range of 50 to 400 nm. These changes can be related
to the presence of different phases in these materials. CaO is
2.3 Kinetic Experiments
Catalytic experiments were typically performed under flow-
ing helium at 130 °C in a glass reactor equipped with a stir-
rer. Typically valeraldehyde (Aldrich, 97%) was reacting
1 3

Aldol Condensation of Cyclopentanone with Valeraldehyde Over Metal Oxides
Table 1 Phase composition and
particle size of CaO and FeO–
CaO–EIM determined by XRD
[24, 25]
Phase
Framework
hkl at 2Θ (o)
CaO
wt%
FeO–CaO–EIM
D (nm)
38 25
wt%
D (nm)
CaCO3
Ca(OH)2
Rhombohedral
Hexagonal
104 (29.40), 018 (47.38)
16
83
6
5
19 17
92 55
001 (17.98), 104 (29.40),
26
6
52
6
24
0
7
101(34,12), 102 (47.38)
Fe2O3
Ca2Fe2O5
Rhombohedral
Orthorhombic
104 (33.18), 024 (49.52)
0
0
0
0
0
020 (11.99), 002, (32.00),
22
8
60 12
200 (33.06), 141 (33.49)
CaO
Cubic
200 (37.38), 220 (53.90)
1
1
n.d
7
3
65 30
25000
+
59% of the initial value for pristine CaO due to a relatively
high Fe loading, 10%. In addition the concentration of strong
basic sites decreased in this catalyst by iron modification
being only 21% of that of CaO. Nevertheless, basicity of
FeO–CaO–EIM was quite high, partially due to high basic-
ity of Ca2Fe2O5 (Fig. 5a) [5], which was comparative to
the parent CaO. The composite material CeO2–MgO–DP
with 24 wt% Ce exhibited a total basicity higher than the
pristine MgO. The amount of strong basic sites in both
CeO2–MgO–DP was, however, quite low compared to the
parent MgO, which according to the literature exhibits more
strong basic sites than CeO2 [30].
+
20000
o
^
+
+
o
+
^
+
CaO
^
+
^
^
15000
^
10000
+
^
5000
0
+
*
o
o
+
+
*
*
^
o
+
^
Fe-CaO EIM
*
*
+
+
*
^
*
Acidity, determined by ammonia TPD, follows the order
(Table 3; Fig. 5 b): 21 wt% CeO2–MgO–DP > 10 wt%
FeO–CaO–EIM>7 wt% FeO–MgO–DP. The lowest acid-
ity was measured for FeO–MgO–DP, containing the low-
est metal amount. In fact for this material no Fe2O3 was
observed according to XRD (Table 2) and most probable
iron is present in the form of finely dispersed iron oxide
particles. Results in Table 3 showing some acidity for
FeO–MgO–DP can be interpreted by taking into account that
Fe2O3 exhibits both Brønsted and Lewis acidity [31]. The
highest acidity of CeO2–MgO–DP is explained by its high
CeO2 loading. It is known that CeO2 is amphoteric which
influences its acidic and basic properties. Acidity of 10 wt%
FeO–CaO–EIM is related to presence of iron+3 possessing
Lewis acidity [32] in the form of Ca2Fe2O5.
20
40
60
80
100
2 theta (o)
Fig. 2 XRD patterns from CaO, and FeO–CaO–EIM. Notation:
Ca(OH)2+, CaCO3^, CaO o, Ca2Fe2O5*, Fe2O3#
Table 2 Phase composition and particle size of FeO-MgO-DP and
CeO-MgO catalysts determined by XRD [24, 26–28]
Catalyst
Phase
Framework
wt%
D (nm)
FeO–MgO–DP
CeO2–MgO–DP
MgO
CeO2
MgO
Cubic
Cubic
Cubic
100
20 2.0
9.0 2.0
31 19
69 19
17
3
3.2 Reaction Network and Thermodynamic Analysis
rich in hexagonal Ca(OH)2, however, FeO–CaO–EIM con-
tained mainly CaCO3.
The reaction network for aldol condensation of valeral-
dehyde with cyclopentanone is shown in Fig. 1. It should
be pointed out here, that as mentioned in the Introduction
self-condensation of cyclopentanone forming 2-cyclopentyl-
idene-cyclopentanone, requires presence of strong basicity,
thus its contribution to valeraldehyde self- and cross-con-
densation reactions was considered to be less significant.
According to the literature [2] valeraldehyde condensa-
tion proceeds via formation of an intermediate 1-hydroxy-
2-pentylcyclopentanone 3, which is dehydrated forming the
desired product 2-pentylidenecyclopentanone 4. In addition
The TEM images showed also large differences in the
particle sizes and shapes (Fig. 4). FeO–CaO–EIM contains
spherical particles (Fig. 4a), whereas the parent CaO exhib-
ited 10 to 50 nm particles which were not easy to separate
from each other.
Basicity determined by CO2 temperature programmed
desorption showed that pure CaO exhibited the highest
basicity in accordance with the literature (Table 3; Fig. 5a)
[23, 29]. CaO contained large amounts of medium and
strong acid sites. When CaO was modified by Fe using the
evaporation impregnation method its basicity decreased to
1 3

P. Mäki-Arvela et al.
Fig. 3 SEM images of a CeO2–MgO–DP, b FeO–MgO–DP, c FeO–CaO–EIM, and d CaO
Fig. 4 TEM images of a FeO–
CaO–EIM, b CaO
to formation of the desired cross-condensation product, the
latter can also be isomerized to 2-pentyl-2-cyclopenten-
1-one 5, especially at higher temperature, when this isomer
is thermodynamically more stable than the desired product 4
[2]. The desired product can also react further with another
valeraldehyde molecule giving 2-(1-hydropentyl)-5-pentyl-
idene cyclopentanone 6 and its corresponding dehydrated
product, 2,5-dipentylidene cyclopentanone 7. Furthermore,
other reactions can also occur, namely self-condensation of
valeraldehyde giving 2-propyl-3-hydroxyheptanal, 1 which
1 3

Aldol Condensation of Cyclopentanone with Valeraldehyde Over Metal Oxides
Table 3 Metal content, basicity and acidity of the catalysts
Catalyst
Metal content
by EDXA
(wt%)
Amount of basic sites
Weak (desorbed
Amount of NH3
desorbed (µmol/
T
max (oC)
for ammonia
desorption
Medium (desorbed Strong (desorbed
between 277– above 477 °C)
477 °C) µmol/gcat µmol/gcat
Sum of
basic
sites
gcat)
below 277oC)
µmol/gcat
CaO
0
22
13
99
222
76
905
962
731
354
967
1179
259
168
118
43
2105
1234
989
695
1087
n.d
n.d
430
n.d
306
380
FeO–CaO–EIM 10
MgO
FeO–MgO–DP 7.0
CeO2–MgO DP 24
305
n.d
45
0
152
valerate 8 could be seen, which in general can be formed via
the Tischenko reaction.
(a)
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
Aldol condensation can occur both on basic and acid sites
according to the literature suggestions [2]. Cross aldol con-
densation over basic sites can be described by a mechanism,
[2] when the base accepts a proton from the ketone form-
ing in the current case negatively charged O− in cyclopen-
tenone which attacks the aldehyde via a nucleophile form-
ing hydroxypentylcyclopentanone. Dehydration occurs via
formation of a carbocation which after deprotonation gives
2-pentylidenecyclopentanone.
FeO-CaO-EIM
CeO2-MgO-DP
FeO-MgO-DP
0.4
0.2
0.0
In acid catalyzed aldol condensation the reaction has been
proposed to start with enolization of the aldehyde over Lewis
acid sites. Subsequently the carbocation formed from the alde-
hyde on Brønsted acid site reacts with the enol leading to for-
mation of a cross condensation product [33].
0
200
400
600
Temperature (oC)
While the main focus in this work was on kinetics of the
cross condensation of valeraldehyde with cyclopentanone, in
order to understand if thermodynamics governs the overall
reaction, first thermodynamic analysis was performed. Accord-
ing to our knowledge thermodynamics for the cross condensa-
tion and relevant reactions has not been reported. Evaluation
(b)
FeO-CaO-EIM
4
2
0
of thermodynamics was done by calculating enthalpy (ΔH0 )
r
and Gibbs free energy (ΔG0 ) at standard conditions using the
approach from [34] and starrting from the standard enthalpy
(ΔH0 ) and Gibbs free energy (ΔG0 ) of formation from the
CeO2-MgO-DP
f
f
elements estimated with the Joback approach [35–37]:
ꢀ
ΔHr0,j
ΔG0r,j
=
ꢀi,j ⋅ ΔHf0,i
FeO-MgO-DP
(1)
(2)
j
ꢀ
ꢀi,j ⋅ ΔG0f,i
0
100
200
300
400
500
=
j
Temperature (oC)
The equilibrium constant of each reaction was calculated
from its definition
Fig. 5 a CO2 TPD of FeO–CaO–EIM, FeO–MgO–DP and CeO2–
MgO–DP, b NH3 TPD of FeO–CaO–EIM and CeO2–MgO–DP
(
)
ΔG0r,i
Kj0 = exp
−
(3)
RT
can be dehydrated to the corresponding unsaturated alde-
hyde, 2-propylhept-2-enal 2. In the current case no pentyl
1 3

P. Mäki-Arvela et al.
Table 4 Enthalpy and Gibbs free energy formation for each component (i) (given in bold) and stoichiometric matrix for component i for reaction
j
Component
ΔH0f (kJ/mol)
ΔG0f (kJ/mol)
i/j
1
2
3
4
5
6
7
8
Valeraldehyde
2-Propyl-3-hydroxyheptanal
2-Propylhept-2-enal
Cyclopentanone
1-Hydroxy-2-pentylcyclopentanone
Water
−232
−498
−228
−195
−484
−242
−231
−260
−512
−108
−20.8
5.47
−87.1
−192
−228
0.45
1
−2
1
0
0
0
0
0
0
0
0
−1
1
0
0
1
0
0
0
−1
0
0
−1
1
0
0
0
0
0
0
0
0
−1
1
1
0
0
0
0
0
0
0
0
−1
1
0
−1
0
0
0
0
0
−1
0
0
0
0
0
0
1
0
0
−2
0
0
0
0
0
0
0
0
2
3
4
5
6
7
8
9
2-Pentylidenecyclopentanone
2-Pentyl-2-cyclopenten-1-one
−24.7
−104
(Z)-2-(1-hydroxypentyl)-5-2-pentyl-
1
−1
idenecyclopentanone
2,5-Dipentylidene cyclopentanone
Pentyl-valerate
−258
−495
88.0
−201
10
11
0
0
0
0
0
0
0
0
0
0
0
0
1
0
0
1
r1
r2
r3
r4
r5
r6
r7
r8
40
30
Table 5 Enthalpy and Gibbs free energy for each reaction (j) at stand-
ard conditions, equilibrium constants at standard conditions (K0 )
j
20
j
ΔH0r,j (kJ/mol)
ΔG0r,j (kJ/mol)
K0
j
10
1
2
3
4
5
6
7
8
−33.9
28.4
−57.6
12.1
−29.7
−48.9
12.1
0.87
−15.1
0.34
−36.0
−25.1
0.34
2.99×10−2
4.37×102
2.51×10−1
2.04×106
2.53×104
2.51×10−1
2.04×106
1.57×10−3
0
-10
-20
-30
-40
-50
-60
−36.0
16.0
−30.3
350 360 370 380 390 400 410 420 430
T [K]
Fig. 6 Gibbs free energy for each reaction (j) as a function of tem-
The dependence of the reaction free Gibbs energy on tem-
perature
perature was included by implementing the Gibbs–Helmholtz
equation valid at P=1 bar (ΔGΦ
)
r,j
2-hydroxyl-2-pentylcyclopentanone to the desired prod-
uct and formation of 2,5-dipentylidene cyclopentanone
(Table 5; Fig. 6). It can also be seen from Fig. 6 that both
of these reactions become even more thermodynamically
feasible at higher temperatures. Interestingly the ∆G for
formation of 2-pentyl-2-cyclopentyl-1-one, isomer of the
desired product, was independent on temperature.
ΔGꢀr,j(T) ΔG0r,j
=
(
)
1
1
T0
+ ΔHr0,j
−
(4)
T0
T
T
The calculated enthalpy and Gibbs free energy for-
mation for each component (i) are reported in Table 4.
Numbering of reactions is from Fig. 1.
Starting from these values, the enthalpy and Gibbs free
energy for each reaction (j) at standard conditions, equi-
3.3 Catalytic Results
librium constants at standard conditions (K0 ), enthalpy
j
In the preliminary work valeraldehyde condensation with
cyclopentanone was investigated using mesitylene as a
solvent, since in [1] successful aldol condensation in this
reaction was reported to occur in another substituted aro-
matic compound, o-xylene, as a solvent. In the current work
and Gibbs free energy at different temperatures and
pressure were calculated. A temperature range between
Tmin=353.15 K and Tmax=423.15 K was investigated. It
can be seen that thermodynamically most favorable reac-
tions are 4 and 7, namely dehydration of the intermediate
1 3

Aldol Condensation of Cyclopentanone with Valeraldehyde Over Metal Oxides
aldol condensation of valeraldehyde with cyclopentanone
was performed analogously to the work of Hasni et al. [1]
using 1:5 molar ratio of the reactants, however, instead of
m-xylene in mesitylene as a solvent over CeO2–MgO–DP.
The results showed, however, that with a low initial valeral-
dehyde concentration (0.09 mol/l) in mesitylene as a solvent,
mainly self-condensation products of valeraldehyde (98%)
were formed in 3 h at 56% conversion at 130 °C, when 1:5
molar ratio of valeraldehyde to cyclopentanone was applied.
In addition, minor oxidation of mesitylene occurred result-
ing in formation of 3,5-dimethylbenzaldehyde, confirmed by
GC–MS, which might be possible by lattice oxygen originat-
ing from CeO2. After the initial experiments in mesitylene
as a solvent, valeraldehyde condensation was subsequently
performed using cyclopentanone both as a solvent and a
reactant.
100
90
80
70
60
50
40
30
20
10
0
FeO-CaO-EIM
FeO-MgO-DP
CaO
thermal
CeO2-MgO-DP
0
200
400
600
800
1000
1200
Amount of strong basic sites (µmol/gcat
)
Both the thermal reaction between valeraldehyde and
cyclohexanone in the absence of any catalyst as well as
catalytic experiments were carried out using an excess of
cyclopentanone. The concentration profiles for valeralde-
hyde and its self- and cross-condensation products are shown
in Figs. 7, 8, 9, 10, 11, 12. In thermal cross condensation of
valeraldehyde at the initial concentration of 0.53 mol/l 95%
conversion of valeraldehyde was obtained. The main product
was 2-propylhept-2-enal. In addition relative large amounts
of both 2-propyl-3-hydroxyheptanal, identified based on the
confirmation of the m/z+peak at 173 in GC–MS analysis
reported in [38] and 1-hydroxy-2-pentylcyclopentanone and
Fig. 8 Conversion of valeraldehyde after 1 h. The initial valeralde-
hyde concentration was 0.53 mol/l in cyclohexane. The experiments
were performed at 130 °C under argon flow
an isomer of the desired 2-pentyl-2-cyclopent-1-one were
formed. This result clearly shows that valeraldehyde self-
condensation does not require any catalyst under the stud-
ied conditions. In addition the isomerization of the desired
product, 2-pentylidenecyclopentanone can occur, especially
at relatively high temperatures due to the fact that 2-cyclo-
pent-1-one 5 is thermodynamically more stable than 4 due
0.5
0.4
0.3
0.2
0.1
0.0
0.5
0.4
0.3
0.2
0.1
0.0
0
60
120
180
240
0
60
120
180
240
300
Time (min)
Time (min)
Fig. 7 Concentration of valeraldehyde and its self- and cross-conden-
sation products as a function of time in valeraldehyde cross-conden-
sation with cyclopentanone. The initial valeraldehyde concentration
was 0.53 mol/l in cyclohexane. The experiments were performed at
130 °C under argon flow. Symbols: valeraldehyde (white square),
2-pentylidenecyclopentanone 4 (black circle), 2-pentyl-2-cyclopent-
1-one 5 (white circle), 2-propyl-3-hydroxyheptanal (+) 1, 2-propyl-
hept-2-enal (black up-pointing triangle) 2 and 1-hydroxy-2-pentylcy-
clopentanone (black square) 3
Fig. 9 Concentration of valeraldehyde and its self- and cross-con-
densation products as a function of time in valeraldehyde cross-
condensation with cyclopentanone over FeO–CaO–EIM. The initial
valeraldehyde concentration was 0.53 mol/l in cyclohexane. The
experiments were performed at 130 °C under argon flow. Symbols:
valeraldehyde (white square), 2-pentylidenecyclopentanone 4 (black
circle), 2-pentyl-2-cyclopent-1-one 5 (white circle), 2-propyl-3-hy-
droxyheptanal (+) 1, 2-propylhept-2-enal (black up-pointing triangle)
2 and 1-hydroxy-2-pentylcyclopentanone (black square) 3
1 3

P. Mäki-Arvela et al.
0.5
0.4
0.3
0.2
0.1
0.0
0.5
0.4
0.3
0.2
0.1
0.0
0
60
120
180
240
300
0
60
120
180
240
Time (min)
Time (min)
Fig. 10 Concentration of valeraldehyde and its self- and cross-
condensation products as a function of time in valeraldehyde cross-
condensation with cyclopentanone over FeO–MgO–DP. The initial
valeraldehyde concentration was 0.53 mol/l in cyclohexane. The
experiments were performed at 130 °C under argon flow. Symbols:
valeraldehyde (white square), 2-pentylidenecyclopentanone 4 (black
circle), 2-pentyl-2-cyclopent-1-one 5 (white circle), 2-propyl-3-hy-
droxyheptanal (+) 1, 2-propylhept-2-enal (black up-pointing triangle)
2 and 1-hydroxy-2-pentylcyclopentanone (black square) 3
Fig. 12 Concentration of valeraldehyde and its self- and cross-
condensation products as a function of time in valeraldehyde cross-
condensation with cyclopentanone over CaO. The initial valeralde-
hyde concentration was 0.53 mol/l in cyclohexane. The experiments
were performed at 130 °C under argon flow. Symbols: valeralde-
hyde (white square), 2-pentylidenecyclopentanone 4 (black circle),
2-pentyl-2-cyclopent-1-one 5 (white circle), 2-propyl-3-hydroxyhep-
tanal (+) 1, 2-propylhept-2-enal (black up-pointing triangle) 2 and
1-hydroxy-2-pentylcyclopentanone (black square) 3
0.5
0.4
0.3
0.2
0.1
0.0
concentrations was close to 100%. Since thermal aldol con-
densation of valeraldehyde–cyclopentanone showed prom-
ising results, it was worth to investigate the performance
of catalysts at the same conditions. Valeraldehyde self-con-
densation is a thermodynamically feasible reaction even if
approximate calculations of the Gibbs free energy for forma-
tion of the intermediate 2-propyl-3-hydroxyheptanal, gave
a positive value.
Catalytic cross-condensation of valeraldehyde
with cylopentanone was investigated over two dif-
ferent FeO catalysts supported on MgO and CaO as
well as CaO per se and CeO2–MgO–DP (Table 6;
Figs. 7, 8, 9, 10, 11, 12). The initial rates for valeralde-
hyde transformation decreased in the following order:
FeO–MgO–DP>CeO2–MgO–DP>FeO–CaO–EIM>CaO
(Table 6). This order shows that the catalysts with the low-
est basicity exhibited also the lowest initial valeraldehyde
transformation rate, apart from a highly basic CaO. This
could be explained by initial poisoning of CaO by traces
of water and CO2 despite drying over night at 100 °C in an
oven. Based on the literature [39] dehydration of the basic
catalysts is required to remove adsorbed water and CO2. It
is believed that this type of poisoning could easily occur
for very basic CaO even in few minutes when loading the
catalyst into the reactor. In addition when comparing the
converted amounts of valeraldehyde after 120 min obtained
with CaO (Fig. 12) and FeO–CaO–EIM (Fig. 11), it can be
0
60
120
180
240
Time (min)
Fig. 11 Concentration of valeraldehyde and its self- and cross-
condensation products as a function of time in valeraldehyde cross-
condensation with cyclopentanone over CeO2–MgO–DP. The initial
valeraldehyde concentration was 0.53 mol/l in cyclohexane. The
experiments were performed at 130 °C under argon flow. Symbols:
valeraldehyde (white square), 2-pentylidenecyclopentanone 4 (black
circle), 2-pentyl-2-cyclopent-1-one 5 (white circle), 2-propyl-3-hy-
droxyheptanal (+) 1, 2-propylhept-2-enal (black up-pointing triangle)
2 and 1-hydroxy-2-pentylcyclopentanone (black square) 3
to the presence of a conjugated double bonds, i.e. a car-
bonyl bond and a cyclic olefinic bond and high temperatures
favor isomerization [2]. The sum of the reactant and product
1 3

Aldol Condensation of Cyclopentanone with Valeraldehyde Over Metal Oxides
seen that for neat CaO the conversion was only 83% com-
pared with 99% obtained by FeO–CaO–EIM showing also
that addition of just 10 wt% FeO increased activity of CaO.
Comparison of conversion levels over different cata-
lysts shows that CaO, FeO–CaO–EIM and CeO2–MgO–DP
with strong basicity efficiently converted valeraldehyde.
The conversion levels increased also with the increasing
amount of strong basic sites (Fig. 8). The sums of the liquid
phase products determined by GC (GCLPA) were high with
FeO–MgO–DP and FeO–CaO–EIM, whereas CaO despite
its high basicity exhibited a lower sum of reactant and prod-
uct masses determined by GC analysis of the liquid phase.
These results may be related to formation of oligomers, in
the latter case not visible in GC.
A comparison of the total concentration, i.e. the sum of
the reactant and product concentrations from valeraldehyde
self- and cross-condensation in the liquid phase, which are
visible in GC analysis, as a function of conversion is shown
in Fig. 13. The initial concentration of valeraldehyde was
0.53 mol/l and as it can be seen the highest total concentra-
tion is obtained in the absence of catalyst being after 3 h. In
addition the total concentration increased with increasing
conversion for those catalysts which were relatively good
for catalyzing the formation of the desired product 4, i.e.
FeO–CaO–EIM, FeO–MgO–DP and CaO. It can also be
seen from Fig. 13 that the lowest total concentrations were
obtained with CeO–MgO–DP and FeO–CaO–DP. The lat-
ter catalyst exhibited a very high concentration of acid sites
most probably promoting oligomerisation.
0.6
0.5
FeO-MgO-DP
Selectivity in the cross-condensation of valeraldehyde
with cyclopentanone in comparison with valeraldehyde self-
condensation can be evaluated by considering the ratio of the
cross-condensation to valeraldehyde self-condensation prod-
ucts for thermal and catalytic experiments (Table 6) showing
that this ratio was very low for the thermal reaction and rea-
sonably high for catalytic experiments with basic catalysts.
The yields of the desired product 4 and self-condensation
product of valeraldehyde 2 over different catalysts after 4 h
reaction illustrate that the highest yield of the desired prod-
uct was obtained over FeO–MgO–DP followed by CaO and
FeO–CaO–EIM (Table 6; Figs. 9, 10, 12). Repeatability in
the cross condensation reactions was adequate showing for
FeO–CaO–EIM catalyst minor deviations in the yield of
the desired product 4, 42 3%. All these catalysts exhib-
ited strong basicity (Table 3). Intensive 2-propylhept-2-enal
formation occurred only in the absence of heterogeneous
catalysts. This result is in accordance with [1] where it was
thermal
0.4
CaO
0.3
FeO-CaO-EIM
CeO-MgO-DP
0.2
0.1
0.0
0.0
0.2
0.4
0.6
0.8
1.0
Conversion (%)
Fig. 13 A comparison of the total concentration, i.e. the sum of reac-
tant and product concentrations from valeraldehyde self- and cross-
condensation in the liquid phase, which are visible in GC analysis,
as a function of conversion. The initial valeraldehyde concentration
was 0.53 mol/l in cyclohexane. The experiments were performed at
130 °C under argon flow
Table 6 Results from valeraldehyde cyclopentanone cross condensation in thermal and catalytic experiments using cyclopentanone both as a
solvent and reactant at 130 °C under argon flow
∑
c0,
VA
Entry
Catalyst
GCLPA (mol- Init. rate
Conversion
(%) after 4 h
Yield of 4 Yield of 2 Yield of 5 Ratio
∑
b
c0,
VA−CP
%) after 4 h
(mmol/min
(mol-%)
(mol-%)
(mol-%)
between 4/2
gcat
)
after 4 h
1
2
3
4
5
Thermal
CeO2–MgO–DP
CaO
FeO–CaO–EIM
FeO–MgO–DP
40a
60
77
100
92
n.d
95a
99
100
100
95
2.3
1
26
53
45
66
28
<1
2
3
2
11
11
19
25
19
0.03
47
27
15
33
0.17
0.08
0.11
0.27
0.54
0.08
0.01
0.07
2-pentylidenecyclopentanone 4, 2-propylhept-2-enal 2, isomer of 2-pentylidenecyclopentanone 5. GCLPA denotes the sum of the concentrations
of valeraldehyde and its self- and cross condensation products visible in GC analysis. The initial valeraldehyde concentration was 0.53 mol/l,
cyclopentanone was used both as a reactant and solvent
a3∑h
c0,
VA
b
∑
denotes the initial ratio of the sum of the concentration VA self-condensation products/sum of the concentration of VA-CP cross con-
c0,
VA−CP
densation products
1 3

P. Mäki-Arvela et al.
70
60
50
40
30
20
10
0
line with the thermodynamic calculations, showing that such
reaction is unfeasible in the studied temperature range.
Comparison of the yield of 2-pentylidenecyclopentanone
2 obtained by all catalysts at 4 h reaction time (Fig. 14)
shows that an optimum amount of the largest total amount
of basic sites is required for production of cross conden-
sation products with the highest yield of 2 generated over
FeO–MgO–DP. In [1] the conclusion was reached that either
strongly acidic (e.g. Al2O3–SiO2) or base catalysts, (e.g.
MgO) are required, rather than bifunctional catalysts.
FeO-MgO-DP
CaO
FeO-CaO-EIM
CeO2-MgO-DP
4 Conclusions
thermal
0
200
400
600
800
1000
Ce-, Fe-modified CaO, CeO2 and MgO base catalysts were
successfully synthesized using deposition–precipitation (DP)
and evaporation impregnation methods (EIM). The physico-
chemical characterization of the catalysts using TPD–CO2
showed the presence of the largest total amount of basic
sites after CaO in FeO–CaO–EIM (1234 µmol/g) followed
by CeO–MgO–DP (1087 µmol/g) catalyst. FeO–CaO–EIM
catalyst resulted in the highest conversion (100%) of valer-
aldehyde during cyclopentanone cross-condensation with
valeraldehyde. FeO–MgO–DP catalyst synthesized using the
deposition precipitation method afforded the highest yield
of the desired product 2-pentylidenecyclopentanone (66%),
clearly indicating that the method of catalyst synthesis has
a tremendous influence on conversion and selectivity to the
desired products. Correlation of valeraldehyde conversion
with the amount of medium strong basic sites revealed that
these sites affect the aldol condensation reaction.
Amount of medium basic sites (µmol/gcat
)
Fig. 14 The yield of 2-pentylidenecyclopentanone 4 h as a function
of medium strong basic sites in the catalyst
concluded that formation of 2-propylhept-2-enal was the
lowest with MgO and Al2O3–SiO2, having acid-base site
pairs.
The isomer of the desired product (e.g. compound 5)
was formed mainly over FeO-MgO-DP as well as over CaO
and FeO–CaO–EIM showing that double bond migration
occurs not only on acidic, but also on basic sites. According
to ammonia TPD FeO–CaO–EIM, in which the iron contain-
ing phase is Ca2Fe2O5, is also acidic promoting formation of
the desired product isomer (Table 3; Fig. 9). When acidity
of CeO2–MgO–DP was lower, subsequently lower amounts
of 2-hydroxy-2-pentylcyclopentanone were formed (Table 6,
entry 2) compared to FeO–CaO–EIM (Table 6, entry 4).
Noteworthy is also that valeraldehyde self-condensation
to form 2-propylhept-2-enal was concluded to be more acid
catalyzed, when it was investigated [11] over a more acidic
TiO2 catalyst. In this work, however, self-condensation with
a relatively highly acidic FeO–CaO–EIM was not able to
produce high amounts of 2-propylhept-2-enal. In addition,
activity of FeO–CaO–EIM catalyst was decreased after
60 min since dehydration of 2-hydroxy-2-pentylcyclopen-
tanone to the desired product was inhibited (Fig. 9).
Only traces of 2,5-dipentylidene cyclopentanone 7 were
formed despite its negative Gibbs free energy. This result
indicates that consecutive condensation was retarded. Fur-
thermore, all valeraldehyde has reacted after 120 min over
FeO–MgO–DP, when the concentration of 2-pentylidenecy-
clopentanone was high indicating that valeraldehyde reacted
already to some heavy compounds before that, since the
GCLPA was 92%.
Acknowledgments Open access funding provided by Abo Akademi
University (ABO).
Open Access This article is distributed under the terms of the Crea-
tive Commons Attribution 4.0 International License (http://creativeco
mmons.org/licenses/by/4.0/), which permits unrestricted use, distribu-
tion, and reproduction in any medium, provided you give appropriate
credit to the original author(s) and the source, provide a link to the
Creative Commons license, and indicate if changes were made.
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1 3