Perovskites and metal nitrides as catalysts in the base-catalysed aldol addition of isobutyraldehyde to formaldehyde

Article abstract of DOI:10.1039/c5cy01479d

Hydroxypivaldehyde represents an important intermediate in the production of neopentyl glycol, which in turn is widely applied in the production of e.g. polyesters, plasticizers, synthetic resin paints and lubricants. It is industrially produced via aldol addition of isobutyraldehyde to formaldehyde catalysed by homogeneous bases. However, the major disadvantages include the difficult separation of the base from the product stream, the formation of salts upon neutralisation and side reactions as well as the catalyst's hazard potential due to corrosion, toxicity and inflammability. A set of perovskites and metal nitrides were investigated as solid catalysts for aldol reactions establishing structure-performance correlations. For perovskites, a correlation of the catalytic activity and the degree of distortion as well as the presence of other phases is found but the most promising catalyst, BaZrO₃, shows deactivation in recycling experiments. Amongst the metal nitrides, AlN exhibits high activity and stability in batch recycling experiments. Transferring the system into a continuous fixed bed reactor reveals high stability over 150 h time on stream at 423 K. XRD and SEM/EDX confirm the formation of boehmite caused by the hydrolysis of AlN in an aqueous reaction medium. This transformation is associated with a significant increase in the basicity and acidity of the material, which is proposed to account for its high catalytic activity.

Full text of DOI:10.1039/c5cy01479d

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Perovskites and metal nitrides as catalysts in the
base-catalysed aldol addition of isobutyraldehyde
to formaldehyde†
Cite this: DOI: 10.1039/c5cy01479d
a
bc
b
b
Henrike Kleineberg, Matthias Eisenacher, Horst Lange, Heinz Strutz
a
and Regina Palkovits*
Hydroxypivaldehyde represents an important intermediate in the production of neopentyl glycol, which in
turn is widely applied in the production of e.g. polyesters, plasticizers, synthetic resin paints and lubricants.
It is industrially produced via aldol addition of isobutyraldehyde to formaldehyde catalysed by homoge-
neous bases. However, the major disadvantages include the difficult separation of the base from the prod-
uct stream, the formation of salts upon neutralisation and side reactions as well as the catalyst's hazard po-
tential due to corrosion, toxicity and inflammability. A set of perovskites and metal nitrides were
investigated as solid catalysts for aldol reactions establishing structure–performance correlations. For pe-
rovskites, a correlation of the catalytic activity and the degree of distortion as well as the presence of other
3
phases is found but the most promising catalyst, BaZrO , shows deactivation in recycling experiments.
Received 4th September 2015,
Accepted 24th April 2016
Amongst the metal nitrides, AlN exhibits high activity and stability in batch recycling experiments. Transfer-
ring the system into a continuous fixed bed reactor reveals high stability over 150 h time on stream at 423
K. XRD and SEM/EDX confirm the formation of boehmite caused by the hydrolysis of AlN in an aqueous re-
action medium. This transformation is associated with a significant increase in the basicity and acidity of
the material, which is proposed to account for its high catalytic activity.
DOI: 10.1039/c5cy01479d
www.rsc.org/catalysis
5
Therewith, yields of HPA of up to 87% are achieved. How-
ever, these catalysts exhibit a number of disadvantages,
Introduction
Heterogeneous catalysis plays an important role in a great
number of chemical production processes with major focus
on solid acids due to their great importance in petro-
which are related to the elaborate separation from the prod-
uct stream, the formation of salts upon neutralisation and
side reactions as well as the catalyst's hazard potential due to
1
9–16
chemistry. Solid base catalysts have attracted less attention,
corrosion, toxicity and inflammability.
It is therefore de-
although their application in many base-catalysed industrial
processes offers opportunities to facilitate separation, achieve
a higher purity of the product and avoid corrosion problems
and waste generation.
A base-catalysed reaction of great industrial importance is
the aldol addition of isobutyraldehyde (iBa) to formaldehyde
sirable to establish a solid base catalyst for the heteroge-
neously catalysed aldol addition of iBa to FA. To this point,
only a limited number of solid catalysts have been investi-
gated in the described aldol addition. Amongst them are ba-
2
1
6,17
18
sic ion exchange resins,
hydrated magnesium oxide as
1
3
well as base-modified clays.
(
FA) producing hydroxypivaldehyde (HPA) (Scheme 1). HPA is an
Two material classes, which have not received much atten-
tion as solid base catalysts, are perovskites and metal ni-
intermediate in the production of neopentyl glycol (NPG), which
in turn is widely applied in the production of e.g. polyesters,
plasticizers, synthetic resin paints and lubricants. Traditionally,
trides. Perovskites are described by the general formula
ABO , with cation A being of larger size than cation B. In
3
3
19
the aldol addition of iBa to FA is catalysed by homogeneous ba-
ses such as alkaline metal hydroxides or trialkylamines.
the perovskite structure, the A cation is located in the centre
of the cubic unit cell and coordinated to 12 oxygen anions.
4
–8
20
a
Institut für Technische und Makromolekulare Chemie, RWTH Aachen University,
Worringerweg 2, 52074 Aachen, Germany. E-mail: palkovits@itmc.rwth-aachen.de
b
OXEA GmbH, Otto-Roelen-Strasse 3, 46147 Oberhausen, Germany
c
TH Köln University of Applied Sciences, CHEMPARK Leverkusen, Kaiser-Wilhelm-
Allee, 51368 Leverkusen, Germany
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
Scheme 1 Base-catalysed aldol addition of iBa to FA followed by
c5cy01479d
hydrogenation to produce NPG.
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The B cation is found at the corners of the cubic unit cell
and is 6-fold coordinated with oxygen anions. An ideal perov-
skite structure is formed if the ionic radii of the involved cat-
ions and anions yield a tolerance factor t of 1 according to
range of 3–90° with an angular step of 0.02°. X-ray fluores-
cence (XRF) spectroscopy was used to determine the chemical
composition of the perovskite samples. Spectra were recorded
under vacuum on an Eagle II spectrometer (Röntgenanalytik
Systeme). Temperature-programmed desorption (TPD) of CO2
1
9
the equation:
.
With deviating toler-
3
and NH , respectively, were measured on a Quantachrome In-
ance factor, the structure becomes increasingly distorted and
struments ChemBET Pulsar TPD/TPR apparatus with a ther-
mal conductivity detector. Prior to the measurement, the
sample was pretreated at 723 K in a helium flow (24 mL
2
0,21
at t < 0.8, the perovskite structure is no longer formed.
To the best of our knowledge, the only application of perov-
skites as catalysts for aldol reactions to this point was
−1
−1
min in CO -TPD and 30 mL min in NH -TPD, respectively)
2
3
2
2
reported by Torres-Martínez et al. They catalysed the gas
for 30 min. The adsorption of the probe molecules occurred
phase aldol condensation of acetone by alkali tantalates
−1
at 298 K for 30 min in a flow of CO
2
−1
(99.995%, 24 mL min )
(
ATaO
3
, with A = Li, Na and K). With respect to metal ni-
or NH (50% in argon, 30 mL min ), respectively. The weakly
3
2
3
trides, Bej et al. reported the application of molybdenum
adsorbed probe gas was removed by flushing the measure-
ment cell with helium for another 30 min. The TPD measure-
nitride as a catalyst in the gas phase aldol condensation of
2
4,25
acetone. Hasni et al.
discussed the oxynitride-catalysed al-
−1
ment was carried out by ramping in helium with 20 K min
dol condensation of cyclopentanone with valeraldehyde and
heptanal with benzaldehyde, respectively.
to 973 K. Scanning electron microscopy with energy disper-
sive X-ray spectroscopy (SEM/EDX) was performed by the Cen-
tral Facility for Electron Microscopy (GFE) of RWTH Aachen
University. The sample powder was fixed in liquid conductive
silver and polished by a cross section polisher. Diffuse reflec-
tance infrared Fourier transform spectra (DRIFTS) were mea-
sured on a Bruker VERTEX 70 spectrometer, equipped with a
Praying Mantis Diffuse Reflection Accessory and a high tem-
perature reaction chamber (Harrick). Before the measure-
ment, the sample was pretreated at 723 K for 30 min in a ni-
Herein, we present the potential of perovskites and metal
nitrides as solid bases, evaluating their catalytic performance
in aldol addition of iBa to FA. The investigated perovskites in-
clude the material series A = Ca, Sr and Ba, and B = Ti, Zr and
Ce. Amongst the metal nitrides, aluminium nitride, silicon ni-
tride, titanium nitride, titanium carbon nitride as well as tan-
talum nitride were applied. The materials were characterised,
tested in aldol addition and recycled in batch experiments.
Additionally, the most promising catalyst was studied in a
continuous fixed bed reactor to examine the catalyst stability.
trogen stream. The spectra were recorded at 303 K in a
−1
nitrogen stream (24 mL min after CO adsorption and 55
2
−
1
mL min after pyridine adsorption, respectively), collecting
12 scans with a resolution of 4 cm in the wavenumber
range of 4000–850 cm . The spectrum of the pretreated sam-
ple before probe gas adsorption was used as the respective
background spectrum. Thermogravimetric analyses (TGA)
were conducted on a Netzsch STA 409 device.
−
1
Experimental
Catalyst preparation
5
−
1
The synthesis of perovskites containing titanium as the B cation
26
was performed according to the method described by Li et al.
In a typical synthesis, 0.87 g (0.011 mol) of titanium oxide
TiO
, technical, Sigma-Aldrich, ≥99%) were intensively mixed
with an equimolar amount of calcium nitrate (CaIJNO ·xH O,
Aldrich, 99.997%), strontium nitrate (Sr(NO ) , Sigma-Aldrich,
(
2
Catalytic experiments
)
3 2
2
Autoclave experiments. Batch experiments of the aldol ad-
dition of iBa to FA were carried out in a Berghof DAB-2 pres-
sure autoclave with a 50 mL PTFE inlet. 12 mL (0.13 mol) of
iBa and 9 mL (0.12 mol) of FA solution (aqueous, 37%) were
reacted with 5 wt% catalyst for 90 min at 423 K under argon
atmosphere and rigorous stirring. After cooling down the re-
action mixture to room temperature, the catalyst was sepa-
rated by centrifugation and the biphasic product solution
was homogenised by addition of isopropanol. The resulting
solution was analysed by gas chromatography (GC), using a
flame ionisation detector (FID) and a 30 m Agilent DBWax
column.
3
2
≥
99,0%) or barium nitrate (Ba(NO
3
)
2
, Sigma-Aldrich, ≥99%),
respectively. The mixture was calcined at 600 °C for 10 h
−1
(heating rate: 4 °C min ).
The perovskites containing zirconium or cerium as the B cat-
ion with A = Ca, Sr or Ba were synthesised by the thermal de-
composition of the respective metal nitrate salts. Typically, the
particular nitrate salt was thoroughly mixed in an equimolar ra-
tio to produce 2 g of the desired perovskite. The nitrate mixture
−1
was calcined at 650 °C for 10 h (heating rate: 2 °C min ).
The metal nitrides AlN (Grade C and K), Si (Grade M9
95.7% purity) and M11 (90.8% purity)), TiN (Grade C), Ti(C,
3 4
N
(
Catalyst recycling. In order to investigate the recyclability
of the chosen catalysts, the aldol addition was first carried
out under standard conditions. After the reaction was fin-
ished, the catalyst was separated from the product solution,
washed with 100 mL of distilled water and 75 mL of iso-
propanol and subsequently dried at 100 °C for 20 h. The cata-
lyst was then re-calcined under the conditions applied during
synthesis. The regenerated catalyst was applied again in the
N) (Grade C, 50/50) and TaN used in this work were commer-
cial samples obtained from H.C. Starck and were applied
without any further pretreatment.
Catalyst characterisation
Powder X-ray diffraction (XRD) patterns were acquired with a
D500 Siemens diffractometer using Cu Kα radiation in the 2θ
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aldol addition, adjusting the substrate amounts when needed
so as to retain the original catalyst to substrate ratio.
termined by XRF. The perovskite structure requires an A to B
cation ratio of 1 : 1. As shown in Table 1, the majority of the
determined values for the amount of A and B cations within
the respective compounds do not comply with this ratio.
However, in most cases it is shifted towards the B cation,
meaning that the A cation has only partially been incorpo-
rated into the structure or an additional phase of the B cation
has been formed.
Continuous reaction. The continuous aldol reaction of iBa
with FA was conducted in a fixed bed reactor setup. The sub-
strate solution was pumped into the reactor by an HPLC
−
1
pump, applying a constant flow rate of 0.025 mL min . The
fixed bed reactor was positioned in an oven to adjust the re-
action temperature. The pressure within the setup was con-
trolled by a pressure regulation unit, allowing a constant
pressure of 30 bar with consistent flow rate. The substrate so-
lution used contained iBa and FA in a ratio of 1 : 1 and 16
vol% isopropanol with respect to the volume of substrates to
obtain a single-phase solution. The catalyst bed consisted of
A special case among the considered perovskites is the Ca
and Ce containing material. In contrast to the other XRD pat-
terns, that of the named compound does not show any reflec-
tions of the perovskite phase. Instead, the reflections of the
coexisting CaO and CeO2 phases are observed. A glance at
the calculated tolerance factor for this compound (Table 2)
explains this fact, as tolerance factors of t < 0.8 do not per-
2
.85 g of the 63–90 μm particle size fraction. The contact
−
1
time at a flow rate of 0.025 mL min averaged 108 min. Sam-
ples of the product solution were taken at defined time inter-
vals, diluted in isopropanol and analysed by GC according to
the description above.
2
1
mit the formation of the perovskite phase.
As the applied metal nitrides were commercial samples
and used without further purification, the phase purity of the
samples was validated by XRD measurements. The obtained
XRD patterns shown in Fig. 2 confirm the presence of the re-
spective metal nitride phases in all cases. The XRD patterns
of Si N and TaN additionally exhibit some small reflections
Results and discussion
3
.1. Catalyst characterisation
3
4
The synthesised perovskite catalysts as well as the commer-
cial metal nitrides all together exhibited specific surface areas
that were too low to be determined with sufficient accuracy
by nitrogen sorption measurements.
besides the nitride phase which can be attributed to impuri-
ties and TaC, respectively.
Table 1 Composition of the synthesised perovskite-type catalysts, deter-
mined by XRF
The XRD patterns of the synthesised perovskite-type cata-
lysts generally display the typical reflections of the desired
compounds (Fig. 1). Besides the typical perovskite reflections,
most XRD patterns also show additional reflections of the B
oxides, i.e. TiO , ZrO or CeO . This observation can be attrib-
Perovskite ABO
3
n(cation A) [mol%]
n(cation B) [mol%]
CaTiO
SrTiO3
3
43.2
57.2
28.9
34.0
50.0
43.0
—
56.8
42.8
71.1
66.0
50.0
57.0
—
2
2
2
BaTiO
CaZrO
3
uted to non-stoichiometry within the perovskite structure
which is commonly known for this class of materials and has
3
SrZrO
BaZrO
3
1
9
been described elsewhere. The B cations form a stable
framework which allows A cation vacancies to occur, e.g. due
to volatilisation at elevated temperatures, without impairing
the perovskite structure. The defective perovskite structures
can also be observed in the composition of the materials de-
3
a
CaCeO
3
SrCeO
BaCeO
3
41.8
38.9
58.2
61.1
3
a
No sufficient material available for analysis.
Fig. 1 XRD patterns of the synthesised perovskite-type catalysts.
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Table 2 Ionic radii and tolerance factors of the synthesised perovskite-
type catalysts
a,b
Ionic radius [Å]
Perovskite
ABO
Tolerance
factor
c
3
Cation A
Cation B
2
+
4+
CaTiO
3
Ca (VIII)
1.12
1.44
1.61
1.12
1.44
1.61
1.12
1.44
1.61
Ti (VI)
0.605
0.605
0.605
0.72
0.72
0.72
0.87
0.87
0.87
0.89
1.00
1.06
0.84
0.95
1.00
0.78
0.88
0.94
2
+
4+
SrTiO
BaTiO
3
Sr (XII)
Ti (VI)
2
+
4+
3
Ba (XII)
Ti (VI)
2
+
4+
CaZrO
3
Ca (VIII)
Zr (VI)
2
+
4+
Fig. 3 Catalytic results of the perovskite-type materials in the aldol
addition of iBa to FA at 423 K for 90 min using 5 wt% of catalyst.
SrZrO
BaZrO
3
Sr (XII)
Zr (VI)
2
+
4+
3
Ba (XII)
Zr (VI)
2
+
4+
CaCeO
3
Ca (VIII)
Ce (VI)
2
+
4+
SrCeO
BaCeO
3
Sr (XII)
Ce (VI)
2
+
4+
3
Ba (XII)
Ce (VI)
ever, considering their low amount in the samples, a major
contribution to the catalytic activity appears unlikely.
a
b
Data from ref. 27. Numbers in parentheses describe the coordination
c
2−
of the ion within the perovskite. Ionic radius of O (VI): 1.40 Å.
The commercially obtained metal nitride samples were
likewise applied in the catalytic aldol addition of iBa to FA.
The catalytic results gained from these experiments are
depicted in Fig. 4. The catalytic activities of the investigated
metal nitrides differ quite strongly from each other. Interest-
ingly, silicon nitride with a higher purity (grade M9) shows a
lower conversion of iBa than the material with greater impu-
rities (grade M11). This leads to the assumption that not
Si N but the impurities represent the active species, or the
3
4
impurities create some sort of defects in the nitride structure,
therefore enhancing the catalytic activity. Comparing the cat-
alytic results of TiN and Ti(C,N), Ti(C,N) exhibits consider-
ably higher conversion of iBa and selectivity towards HPA.
Thus, the presence of titanium carbide within the compound
has a positive influence on the catalytic activity. The lowest
catalytic conversion was obtained using TaN, while AlN
showed the highest activity with a conversion of 88% and an
HPA yield of 52%.
Fig. 2 XRD patterns of metal nitride catalysts.
3
.2. Catalyst screening
3.3 Catalyst recycling
Regardless of the actual crystal phase observed by XRD, all
synthesised perovskite-type materials were applied in the
batch aldol addition of iBa to FA. The obtained catalytic re-
sults are presented in Fig. 3. The highest yields of the desired
Because of its promising catalytic activity, BaZrO was found
3
exemplary as a perovskite to investigate its stability and recy-
clability in the aldol addition of iBa to FA. The gained results
for this recycling experiment over three runs are shown in
Fig. 5. The conversion of iBa experiences a slight decrease
over three reaction runs, while the selectivity towards HPA re-
mains constant. Upon homogenisation with isopropanol, col-
orless needle-shaped crystals precipitated from the product
solution. These were separated, dried and investigated by
XRD measurement. Although we were not able to attribute
3 3
product are obtained using SrZrO or BaZrO as a catalyst,
producing 53% HPA at an iBa conversion of 91%. Within the
group of compounds containing Ti as the B cation, the con-
version of iBa appears to correlate with the tolerance factor
of the perovskites (Table 2). As the tolerance factor is a mea-
sure for the ideality or distortion of the cubic perovskite
structure, the activity of the perovskites can be related to the
degree of their distortion. This observation was also made by
2
2
Torres-Martínez et al. who applied alkali tantalates in the
gas phase aldol reaction of acetone. They found that the most
distorted LiTaO showed the highest basicity and activity in
3
the reaction. In contrast, the opposite trend occurs for Zr and
Ce as B cations exhibiting increasing catalytic activity for a
tolerance factor of approximately 1. In line with this, a possi-
ble explanation could be related to the potential contribution
of the other phases existing in the material besides the perov-
skite phase. Indeed, reference experiments with ZrO2 and
Fig. 4 Catalytic results of the metal nitrides in the aldol addition of iBa
CeO revealed 44 and 37% iBa conversion, respectively. How-
to FA at 423 K for 90 min using 5 wt% of catalyst.
2
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3
Fig. 5 Recyclability of BaZrO in the aldol addition of iBa to FA at 423
K for 90 min using 5 wt% of catalyst.
Fig. 7 XRD patterns of (a) fresh AlN and regenerated catalysts after
the (b) first, (c) second and (d) third run.
the obtained reflections to a specific crystal phase, we sug-
gest that the precipitate is some sort of organic compound of
barium or zirconium.
This assumption is based on the finding in the same reac-
tion using Zn/Al and Co/Al hydrotalcites as catalysts. Therein,
the precipitated crystals could be identified as the formate
salts of zinc and cobalt, respectively. Furthermore, comparing
solely the reflections of the AlN phase, the XRD patterns of
the three regenerated samples show additional reflections of
boehmite (AlOOH). The boehmite phase is generated in the
hydrolysis of AlN with water from the aqueous reaction solu-
tion. According to the corresponding reaction pathway pub-
lished by Bowen et al. AlN reacts at room temperature with
water and under elimination of ammonia to produce amor-
phous AlOOH. In a basic aqueous reaction medium, the
3
the XRD spectra of fresh BaZrO and the regenerated catalyst
2
8
after the third reaction run (Fig. 6), the reflections assigned
to ZrO2 are much more pronounced in the latter. This im-
plies that barium leached from the perovskite structure, leav-
ing a greater amount of ZrO
2
to coexist with the perovskite
3
amorphous AlOOH then recrystallizes to give AlIJOH) .
phase. Thus, the leaching of barium species could be an ex-
planation for the decrease in catalytic activity.
As for the perovskites, AlN was selected for a recycling ex-
periment because of its outstanding catalytic performance
among the metal nitrides. The results obtained for three runs
are depicted in Fig. 7. As the conversion of iBa and the selec-
tivity towards HPA remain constant in all three reactions, no
deactivation of the catalyst could be determined. In order to
observe a possible change in the catalyst's structure during
recycling, fresh AlN as well as the catalysts after reaction and
regeneration were investigated using XRD. Hereby, the reg-
enerated samples are described as AlN-Rx, with x being the
regeneration step after the xth reaction run. The recorded
XRD patterns are shown in Fig. 8. It has to be noted before-
hand that only a small sample amount was available for the
measurement of AlN-R2, thus the reflections in this XRD pat-
tern are of low intensity. While the XRD of fresh AlN shows
This model was extended for elevated temperatures by Kocjan
2
9
et al. They suggested that some of the initially formed
amorphous AlOOH is dissolved and reprecipitated to form
crystalline AlOOH, while some transforms to crystalline
AlIJOH)
posed that Al(OH)
via a dissolution/recrystallization process. It is pro-
is formed at low temperatures, high pH
3
3
values and long hydrolysis times. However, at higher temper-
atures and moderate pH values boehmite is thermodynami-
cally more stable.
Fig. 6 XRD patterns of fresh BaZrO
after the third run (BaZrO -R3).
3
and the regenerated catalyst
Fig. 8 Recyclability of AlN in the aldol addition of iBa to FA at 423 K
for 90 min using 5 wt% of catalyst.
3
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The aldol addition investigated in this work was carried out
at 150 °C for 90 min in a relatively large volume of aqueous
solution, in which pH variation is expected to be rather low.
Regarding the reaction conditions applied, it is obvious why
no reflections of AlIJOH) are observable in the XRD patterns
of the regenerated catalyst samples.
3
2 3
Fig. 10 (A) TPD-CO and (B) TPD-NH spectra of (a) TaN, (b) fresh AlN
and (c) regenerated AlN catalyst after the first run (AlN-R1).
The formation of an oxygen-containing phase in alumi-
num nitride during the reaction was furthermore observed
using SEM/EDX measurements. Fig. 9 shows the SEM images
of fresh AlN and the regenerated AlN-R3 sample. The streaky
appearance of the particles of both materials is due to the
polishing procedure during sample preparation. Concerning
fresh AlN, only aluminum and nitrogen are detected both in
the middle and the marginal area of the particle shown.
These observations indicate the presence of pure aluminum
nitride. For the AlN-R3 sample, however, additional oxygen,
and at the edge of the particle only aluminum and oxygen
are detected. This leads to the assumption that the hydrolysis
of AlN does not only take place at the surface, but advances
inside the particle. The silver signal that appears in the spec-
trum of AlN-R3 does not actually present part of the sample
but results from the conductive silver, which was used for
sample fixture.
nitride itself does not possess any basic or acidic sites. The
spectra of fresh AlN show only very weak signals that,
according to the desorption temperature at which they occur,
can be attributed to mainly weak basic and acidic sites. The
origin of these sites is not fully clarified yet but probably re-
lated to partial surface hydrolysis. The spectra of AlN-R1,
however, reveal significantly stronger signals both in the TPD
2 3
spectra of CO and NH . Corresponding to the strong desorp-
tion signal in the TPD-CO spectra at around 378 K, a major
2
part of the basicity arises from the weak basic sites. In the
TPD-NH spectra, the most obvious signal is found at 383 K,
3
which can be assigned to weak acid sites. Additionally, spec-
tra deconvolution suggests further contributions at around
4
43 and 684 K for medium to strong acid sites (Fig. S1, ESI†).
The signal at about 903 K which appears both in the TPD-
CO and TPD-NH spectra is presumed to be a result of water
In order to explain the high catalytic activity of aluminum
nitride in the aldol addition of iBa to FA, the basic and acidic
properties of the fresh AlN and the already applied catalyst
material were investigated using temperature-programmed
2
3
that is formed during the decomposition of the boehmite
phase. Integration of the TPD signals allowed the estimation
of the concentration of basic and acidic sites of the respective
catalysts. The data are presented in Table 3. The total basicity
of fresh AlN increased almost by a factor of 20 in AlN-R1 due
to the hydrolysis of AlN and thus the formation of boehmite.
About 75% of the basicity can be attributed to the weak basic
sites. These occur in the form of Brønsted hydroxyl groups,
which in turn are known to be the active species in catalytic
desorption of CO2 and NH , respectively. Additionally, TaN
3
was examined as its performance in the aldol addition had
been proven to be rather low. The obtained TPD spectra of
all three materials are depicted in Fig. 10. It shows that for
TaN no measurable signals of desorbed CO or NH , respec-
2
3
tively, were detected which leads to the conclusion that this
3
0–34
aldol reactions.
Further, the concentration of acidic sites
in AlN-R1 is more than an order of magnitude higher than
that in fresh AlN. Accordingly, the observed catalytic activity
of AlN in the first catalytic run is likely caused by fast surface
reconstruction of the material into amorphous AlOOH
followed by recrystallization providing an enhanced concen-
tration of the surface basic and acidic sites responsible for
the catalytic activity.
Table 3 Basic and acidic characteristics of the metal nitrides
c(basic sites)
c(acidic sites)
−
1
−1
[
mmol g
]
[mmol g
]
Catalyst
Total
Weak
Total
Weak
TaN
AlN
AlN-R1
—
0.005
0.093
—
0.002
0.068
—
0.079
0.987
—
0.012
0.208
Fig. 9 SEM/EDX results of (A) fresh AlN and (B) the regenerated
catalyst after the third run.
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To determine the nature of the basic and acidic sites on
ammonia, a leaching experiment was carried out. Therefore,
the aldol addition was first carried out under standard condi-
tions. The catalyst was removed from the product solution
and the reaction continued under equal reaction conditions
without any catalyst. The gained results in Fig. 12 show that
there is no further increase in conversion observable. Thus, it
can be assumed that the hydroxide ions resulting from hy-
drolysis do not represent the active species in the catalytic al-
dol addition. Meanwhile, the slight decrease in selectivity to-
wards HPA may be attributed to consecutive reactions of HPA
in the second reaction run. An additional reference experi-
ment using a commercially available boehmite (Pural TH
100, Sasol) emphasised its catalytic activity for this reaction,
delivering an iBa conversion of 73% compare to 81% for AlN
(Fig. 7) under these reaction conditions.
AlN-R1, DRIFT spectroscopy after adsorption of CO or pyri-
2
dine, respectively, was realized. The recorded spectra are
shown in Fig. 11. The spectra after adsorption of CO reveal
2
the distinct signals of bicarbonate species at 1650, 1443, and
−
1
−1 35
1
219 cm , and unidentate carbonate at 1558–1505 cm .
These surface species of adsorbed CO2 are related to the
2
−
Brønsted basic hydroxyl groups and Lewis basic O anions,
respectively, which is in accordance with the thermal stability
of the carbonates regarding the disappearance of the corre-
sponding signals at elevated temperatures. Two further sig-
−
1
nals appear at 1593 and 1392 cm , which are thought to be
related to the weakly bound species due to their disappear-
ance at 373–473 K. The spectra after pyridine adsorption ex-
−
1
hibit strong signals at 1591 and 1442 cm resulting from hy-
3
6,37
drogen bridged pyridine.
This type of adsorbed species is
found on Brønsted acidic hydroxyl groups. The signal at 1442
3.4. Continuous reaction
−
1
−1
cm masks the signal at about 1445 cm , becoming visible
only at elevated temperatures due to the desorption of hydro-
gen bridged pyridine, and thus the disappearance of the cor-
responding signal. Because of the position and thermal sta-
bility of the revealed signal, it is suggested to be associated
To estimate the durability of a catalyst in aldol addition of
iBa to FA, continuous reaction experiments in a fixed bed re-
actor setup were conducted. The reaction was run for 150 h
at reaction temperatures of 423 and 393 K, respectively.
Fig. 13 presents the catalytic results obtained for both reac-
tion runs. The initial course of the conversion in both cases
shows that a few hours at the beginning of the reaction are
required until steady state conditions are reached. The rea-
son for that is the mean residence time of over five hours
within the reaction setup is related to the small volumetric
flow rate. At 423 K, 81% of iBa are converted to HPA with a
selectivity of 70% once the steady state is attained. However,
within the first 32 h, both the conversion of iBa and the se-
lectivity to HPA are subject to a slight decrease. At 32 h, a
constant selectivity of 64% is observed, and the conversion
remains constant at 73% from 120 h, producing an HPA yield
of 47%. At a lower reaction temperature of 393 K, the conver-
sion of iBa decreases immediately after the beginning of the
reaction, leading to HPA yields of mostly below 30% after a
time-on-stream of 24 h. However, in comparison, a much
higher selectivity to HPA of 85% is recorded throughout the
entire reaction run, caused by a lower degree of consecutive
reactions of HPA at lower temperatures.
3
+
with pyridine coordinated on strong Lewis acidic Al cations.
−
1
At 1490 and 1479 cm , two additional signals attributed to
the presence of Brønsted and Lewis acidic sites are observed.
The deviation of the results obtained from DRIFT spectro-
scopy and TPD is found to be due to different thermal treat-
ment procedures, and especially in the characterization of
acidic properties, the use of two different probe molecules.
In conclusion, the high catalytic activity of AlN-R1 is
suggested to be a result of the increased concentration of
both basic and acidic sites on the catalyst's surface. This ob-
servation is in good accordance with the findings of Hasni
2
4
et al. whose results reveal that amphoteric Al O generates
2
3
higher conversions in the aldol condensation of cyclopenta-
none with valeraldehyde than strongly basic MgO.
As described above, during hydrolysis of aluminum ni-
tride, ammonia is released, which may lead to an increased
concentration of hydroxide ions in an aqueous solution. In
order to estimate whether the catalytic activity in the aldol
addition of iBa to FA is actually caused by the applied cata-
lyst, or purely an effect of an increased pH due to emitted
In order to estimate the strong decrease in conversion of
iBa at 393 K, while the conversion remained relatively con-
stant at 423 K, thermogravimetric analyses of the fresh AlN
Fig. 11 DRIFT spectra of regenerated AlN after the first run (AlN-R1) after
Fig. 12 Leaching experiment of AlN in the aldol addition of iBa to FA
adsorption of (A) CO
2
and (B) pyridine, respectively, and thermal treatment.
at 423 K for 90 min using 5 wt% of catalyst.
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Fig. 13 Continuous aldol reaction of isobutyraldehyde with formaldehyde with AlN as the catalyst at (A) 423 K and (B) 393 K over 150 h.
as well as the used catalysts of both reaction runs were car-
ried out. The recorded TGA curves presented in Fig. 14 show
severe differences between the fresh material and the used
catalysts. Hence, fresh AlN is stable up to temperatures above
the mass loss amounts to 2.2%, while the catalyst applied at
423 K loses only 2.0% of its initial mass. The low catalytic ac-
tivity of AlN at the temperature of 393 K may therefore be
explained by a higher degree of organic species adsorbed on
the surface, thus blocking the active sites of the catalyst to a
greater extent. As desorption is an endothermic process, the
amount of adsorbed species is lower at a higher reaction tem-
perature, allowing for a higher catalytic activity.
1
173 K. The used catalysts, however, undergo two mass losses
at 277–523 K and 523–753 K, respectively. As mentioned
above, aluminum nitride is hydrolysed to boehmite by water
contained in the substrate solution of the aldol addition reac-
tion. Accordingly, the mass loss at 523–753 K can be attrib-
2 3
uted to the dehydration of AlOOH, forming Al O . The larger
Conclusions
mass loss of the catalyst found in the reaction at 423 K may
be explained by a higher content of boehmite, which is
formed due to the harsher reaction conditions. The mass loss
observed at 277–523 K differs slightly for the catalysts used at
Different perovskites and metal nitrides were applied as
heterogeneous base catalysts in aldol addition of iBa to FA.
The catalysts were first tested in batch experiments. Within
the group of compounds containing Ti as the B cation, the
conversion of iBa appears to correlate with the tolerance fac-
tor of the perovskites. The opposite trend becomes apparent
for Zr and Ce as B cations, which is potentially related to the
contribution of the other phases in the materials to the cata-
3
93 and 423 K, respectively. The mass loss is proposed to re-
sult from organic compounds, e.g. formic acid, which is
formed as a typical by-product of aldol addition, adsorbed on
the catalyst surface. For the material after reaction at 393 K,
lytic activity. BaZrO as an exemplary compound of its class
3
lost activity upon recycling, which could be attributed to the
leaching of barium from the perovskite structure. Across the
metal nitrides, strong deviations in their catalytic activity oc-
curred. Aluminium nitride proved to be the most active can-
didate. Recycling tests revealed a constant activity over three
runs. Investigations using XRD and SEM/EDX demonstrated
the formation of a boehmite phase due to hydrolysis of AlN
with water from the aqueous reaction solution. Results of
TPD with CO and NH showed that the basicity as well as
2
3
the acidity of the material containing boehmite was largely
increased in comparison with the fresh aluminum nitride. It
can be concluded that these properties are responsible for
the high catalytic activity of the material. The stability test of
Fig. 14 TGA of (a) fresh AlN and the applied catalyst in the continuous
aldol addition of iBa to FA at (b) 393 K and (c) 423 K, respectively.
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this catalyst in a continuous fixed bed reactor for 150 h at
18 T. Kwok and W. Watkins, Eastman Chemical Company, WO
9735825A1, 1997.
4
23 K revealed stable conversion and selectivity producing
the desired product at a yield of 46%. For a decreased reac-
tion temperature of 393 K, the selectivity increased from 70
to 85%. In contrast, the conversion decreased probably due
to a higher degree of adsorbed organic species at 393 com-
pared to that at 423 K.
19 M. A. Peña and J. L. G. Fierro, Chem. Rev., 2001, 101,
1981–2018.
20 M. Misono, Heterogeneous Catalysis of Mixed Oxides - Perov-
skite and Heteropoly Catalysts, Elsevier, Oxford, 2013.
21 J. Twu and P. K. Gallagher, Preparation of Bulk and
Supported Perovskites, In: Properties and Applications of
Perovskite-Type Oxides, ed. L. G. Tejuca and J. L. G. Fierro
Acknowledgements
(
Hrsg.), Marcel Dekker, New York, 1993.
We gratefully acknowledge the cooperation with OXEA GmbH
as our project partner. Moreover, we would like to thank H.C.
Starck for the supply of a range of different metal nitride sam-
ples. We thank B. Hermanns for XRF measurements and K.-J.
Vaeßen for XRD and TG analyses.
22 L. M. Torres-Martínez, M. E. Meza-de la Rosa, L. L. Garza-
Tovar, I. Juárez-Ramírez, F. Tzompantzi, G. Del Angel, J. M.
Padilla and R. Gómez, Adv. Mater. Res., 2010, 132, 61–67.
23 S. K. Bej and L. T. Thompson, Appl. Catal., A, 2004, 264,
141–150.
2
4 M. Hasni, G. Prado, J. Rouchaud, P. Grange, M. Devillers
and S. Delsarte, J. Mol. Catal. A: Chem., 2006, 247, 116–123.
5 M. Hasni, J. Rouchaud, P. Grande, M. Devillers and S.
Delsarte, Mater. Sci. Forum, 2007, 554, 37–42.
Notes and references
2
1
2
3
H. Hattori, Chem. Rev., 1995, 95, 537–558.
Y. Ono, J. Catal., 2003, 216, 406–415.
26 J. Li, Y. C. Zhang, T. X. Wang and M. Zhang, Mater. Lett.,
2011, 65, 1556–1558.
27 R. D. Shannon, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr.,
Theor. Gen. Crystallogr., 1976, 32, 751–767.
28 P. Bowen, J. G. Highfield, A. Mocellin and T. A. Ring, J. Am.
Ceram. Soc., 1990, 73, 724–728.
H.-J. Arpe, Industrial Organic Chemistry, 5th Edition, Wiley-
VCH, Weinheim, 2010.
H. Müler, Riedel-de Haën Aktiengesellschaft, DE 1020614, 1956.
G. Jacobsen, H. Fernholz and D. Freudenberger, Farbwerke
Hoechst Ag, DE 1768274A1, 1968.
4
5
6
7
P. Lappe, H. Springer and J. Weber, Chem.-Ztg., 1989, 113, 293–304.
F. Merger, W. Fuchs, S. Winderl and E. Haarer, BASF AG, DE
29 A. Kocjan, K. Krnel and T. Kosmač, J. Eur. Ceram. Soc.,
2008, 28, 1003–1008.
1
957591A1, 1969.
30 V. K. Díez, C. A. Ferretti, P. A. Torresi, C. R. Apesteguía and
J. I. Di Cosimo, Catal. Today, 2011, 173, 21–27.
31 S. Abelló, F. Medina, D. Tichit, J. Pérez-Ramírez, J. C. Groen,
J. E. Sueiras, P. Salagre and Y. Cesteros, Chem. – Eur. J.,
2005, 11, 728–739.
8
9
N. Breitkopf, W. Höfs, H. Kalbfell, F. Thönnessen, P. Lappe
and H. Springer, Ruhrchemie AG, DE 3644675A1, 1986.
R. Fischer, R. Rakoczy, F. Rößner, O. Meyer, S. Grieshammer
and B. J. Berger, Süd-Chemie AG, DE 102010055399A1, 2010.
1
1
0 G. Dämbkes, P. Lappe, F. Thönnessen and H. Springer,
Hoechst Aktiengesellschaft, US 5072058A, 1990.
1 D. Kratz, H. Rust, R. Krokoszinski and V. Helf, BASF AG, DE
32 M. J. Climent, A. Corma, S. Iborra and A. Velty, Catal. Lett.,
2002, 79, 157–163.
33 K. K. Rao, M. Gravelle, J. Sanchez Valente and F. Figueras,
J. Catal., 1998, 173, 115–121.
1
9948112A1, 1999.
1
1
2 B. Cornils, Hydrocarbon Process., 1980, 59, 93–102.
3 R. Culp and W. Beavers, Eastman Chemical Company, WO
34 D. Tichit, M. Naciri Bennani, F. Figueras, R. Tessier and J.
Kervennal, Appl. Clay Sci., 1998, 13, 401–415.
35 J. I. Cosimo, V. K. Díez, M. Xu, E. Iglesia and C. R.
Apesteguía, J. Catal., 1998, 178, 499–510.
36 H. Knözinger, Specific Poisoning and Characterisation of
Catalytically Active Oxide Surfaces, In: Advances in Catalysis,
ed. D. D. Eley, H. Pines and B. W. Paul, Academic Press,
1976, vol. 25.
0
000456A1, 1999.
1
1
4 D. Pankaj and B. G. Sampat, Chem. Wkly., 2008, 201–204.
5 M. Steiniger, M. Guixa Guardia and K. Stehmeier, BASF SE,
WO 2011141470A1, 2011.
1
6 E. Paatero, E. Nummi, L. P. Lindfors, H. Nousiainen, J. Hietala,
L. Lahtinen and R. Haakana, Neste Oy, WO 9829374A1, 1997.
7 M. A. Tike, A. M. Gharde and V. V. Mahajani, Asia-Pac. J.
Chem. Eng., 2008, 3, 333–346.
1
37 G. Larsen, S. Raghavan, M. Márquez and E. Lotero, Catal.
Lett., 1996, 37, 57–62.
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