Development of a method for the synthesis of basic catalysts with high number of active species

Article abstract of DOI:10.1016/j.cattod.2019.09.013

Mesoporous silica MCF and metalosilicates Nb/MCF-14 and Ce/MCF-14 were modified with anchoring of melamine followed by (3-chloropropyl)trimethoxysilane functionalization. The texture/structure parameters, chemical composition and surface properties of synthesized samples were investigated by XRD, N₂ adsorption/desorption, XPS, UV–vis, TG, elemental analysis and 2-propanol dehydration and dehydrogenation, whereas the activity of obtained catalysts was tested in the Knoevenagel condensation between benzaldehyde and malononitrile. The Knoevenagel reaction required the presence of basic active sites or a pair of acid-base active centers. The novelty of the presented work was the use of mesoporous cellular foam (MCF) also modified with niobium (Nb/MCF-14) or cerium species (Ce/MCF-14) as supports for melamine anchoring. The use of the mentioned supports allowed the investigation of the role of open structure of MCF and metal dopant on melamine loading, its stability and reactivity in the Knoevenagel condensation. The obtained results clearly demonstrate that cerium and niobium species loaded into MCF increase significantly the efficiency of melamine loading as well as its stability. The anchoring of melamine to silica and metalosilicates increase the activity of this modifier as a result of formation of secondary amine species. The presence of defected ceria in Mel/Ce/MCF-14 raises the basicity of anchored melamine due to the electron donating effect of ceria. Consequently, the amine species in melamine are more active in the Knoevenagel condensation than those in Mel/Nb/MCF-14 because niobium species do not change oxidation state as easily as defected ceria.

Full text of DOI:10.1016/j.cattod.2019.09.013

CatalysisTodayxxx(xxxx)xxx–xxx
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Catalysis Today
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Development of a method for the synthesis of basic catalysts with high
number of active species
Katarzyna Stawicka
Adam Mickiewicz University, Poznań, Faculty of Chemistry, Umultowska 89b, 61-614 Poznań, Poland
A R T I C L E I N F O
A B S T R A C T
Keywords:
MCF
Niobium
Ceria
Melamine
Knoevenagel condensation
Mesoporous silica MCF and metalosilicates Nb/MCF-14 and Ce/MCF-14 were modified with anchoring of
melamine followed by (3-chloropropyl)trimethoxysilane functionalization. The texture/structure parameters,
chemical composition and surface properties of synthesized samples were investigated by XRD, N2 adsorption/
desorption, XPS, UV–vis, TG, elemental analysis and 2-propanol dehydration and dehydrogenation, whereas the
activity of obtained catalysts was tested in the Knoevenagel condensation between benzaldehyde and mal-
ononitrile. The Knoevenagel reaction required the presence of basic active sites or a pair of acid-base active
centers. The novelty of the presented work was the use of mesoporous cellular foam (MCF) also modified with
niobium (Nb/MCF-14) or cerium species (Ce/MCF-14) as supports for melamine anchoring. The use of the
mentioned supports allowed the investigation of the role of open structure of MCF and metal dopant on mel-
amine loading, its stability and reactivity in the Knoevenagel condensation. The obtained results clearly de-
monstrate that cerium and niobium species loaded into MCF increase significantly the efficiency of melamine
loading as well as its stability. The anchoring of melamine to silica and metalosilicates increase the activity of
this modifier as a result of formation of secondary amine species. The presence of defected ceria in Mel/Ce/MCF-
14 raises the basicity of anchored melamine due to the electron donating effect of ceria. Consequently, the amine
species in melamine are more active in the Knoevenagel condensation than those in Mel/Nb/MCF-14 because
niobium species do not change oxidation state as easily as defected ceria.
1. Introduction
atom of aldehyde. The obtained intermediate product - β-hydroxyl
compound - gives then α,β-unsaturated compound after the elimination
The subject of this paper fits the global trend of synthesis and
modification of mesoporous silica, in order to obtain effective hetero-
geneous catalysts in reactions requiring the presence of available basic
active species, such as the Knoevenagel condensation. One of the pos-
sibilities of generation of basic active centers in mesoporous silica is the
immobilization of specific organic compounds containing amine or
imine species [1–5]. The basicity of such species is related to the pre-
sence of a lone pair of electrons in the nitrogen atom, which can ab-
stract a proton from the reagent molecules. Thus, amine or imine spe-
cies work as Brønsted basic sites [6].
The mechanism of the Knoevenagel condensation carried out over
amine modified materials has not been fully explained yet. Two me-
chanisms of this reaction are postulated, in which the ion-pair [7] or
imine intermediates [8] are formed. In the ion-pair mechanism the
amine species trap a proton from the methylene group of the active
methylene compound, in which the carbon atom is strongly acidic due
to the neighborhood of an electron-withdrawing group. As a result of
this reaction an anion is formed which can attack the carbonyl carbon
of a water molecule [7]. In the second mechanism the amine species are
involved in the formation of imine intermediate with aldehyde com-
pound. The obtained imine exhibits high basicity and thus it can de-
protonate the active methylene compound. As a consequence, α,β-un-
saturated compound is formed after the elimination of a water molecule
from β-hydroxyl compound [8].
The increase in basicity of heterogeneous catalysts with basic ni-
trogen atoms may be obtained by using modifiers with a high number of
basic centers. This solution has been used in [4,9], where 3-[2-(2-
aminoethylamino)ethylamino]propyl-tri-methoxysilane and [3-(2-ami-
noethylamino)propyl]trimethoxysilane were applied as the basic
modifiers of mesoporous silica. The catalysts with a greater number of
basic species may be also obtained via a two-step modification of silica,
in which (3-chloropropyl)trimethoxysilane (ClPTMS) previously an-
chored on silica reacts with nitrogen containing compound possessing
at least one amine group. The available amine group in the modifier has
been used for the reaction with ClPTMS, which leads to formation of the
C–N bond between the anchored chlorine precursor and the basic
E-mail address: kstawick@amu.edu.pl.
https://doi.org/10.1016/j.cattod.2019.09.013
Received 27 February 2019; Received in revised form 18 April 2019; Accepted 11 September 2019
0920-5861/©2019ElsevierB.V.Allrightsreserved.
Pleasecitethisarticleas:KatarzynaStawicka,CatalysisToday,https://doi.org/10.1016/j.cattod.2019.09.013

K. Stawicka
CatalysisTodayxxx(xxxx)xxx–xxx
Scheme 1. The way of melamine anchoring into supports followed by ClPTMS modification.
modifier. This reaction was used for immobilization of modifiers with a
greater number of amine species as melamine [1], piperazine [10] or
biguanide [11] and was also used for the synthesis of catalysts modified
with imine species, using imidazole and 1H-1,2,3-triazole [3] as the
basic compounds.
After 1 h of gel stirring at the same temperature, tetraethoxysilane
(34.108 g, Aldrich) was introduced. The mixture was then stirred at
35–40 °C for 20 h and heated in an oven at 100 °C under static condi-
tions for 24 h. The solid product was filtered, washed with distilled
water (1200 cm3) and dried at RT (room temperature). The template
was removed by calcination at 550 °C for 8 h under static conditions
The functionalization of mesoporous material with specific func-
tional groups possessing basic nitrogen species together with compo-
nents affecting their basicity strength, could lead to changes in the
chemical behavior of the catalyst and thus its activity and selectivity.
This concept was applied in the synthesis of mesoporous silica SBA-15
modified with amine together with Zr, Mo or Nb [9], niobiosilicates and
cerium modified silicas based on SBA-15 or MCF structure modified
with imidazole, triazole and aminotriazole active species [3], or in
amino-grafted MCM-41 modified with Nb or Al together with [3-(2-
aminoethylamino)propyl] trimethoxysilane and 3-[2-(2-aminoethyla-
mino)ethylamino]propyl-trimethoxysilane [12]. The results presented
in the above mentioned papers showed, that the close neighborhood of
metal species and basic nitrogen compounds has an impact on the ac-
tivity of catalysts in the Knoevenagel condensation. Moreover, the
presence of niobium, zirconium and molybdenum in silica skeleton
increased the efficiency of modifiers loading, which is attractive for
increasing catalytic activity.
with a temperature ramp of 1 °C min−1
.
2.2. Silica modification with niobium or cerium species
The MCF support was modified with niobium or cerium species by
using impregnation method as in [15]. At first, niobium(V) ethoxide
(Aldrich) or cerium(III) nitrate hexahydrate (Aldrich) was dissolved in
dry methanol. The amount of metal precursor was chosen so that to
achieve the Si/Nb or Ce molar ratio 14. The dissolved metal sources
were then added to the dried silica (drying for 24 h at 100 °C), which
was evaporated for 1 h at 80 °C before admission of metal salt solution.
The obtained mixture was then stirred in a vacuum evaporator at 60 °C
until the methanol was removed from the solid. Then the metalosili-
cates were dried at 110 °C for 18 h and calcined at 500 °C for 4 h with a
temperature ramp of 3 °C min−1. The obtained samples are denoted as
Nb/MCF-14 and Ce/MCF-14 (the number 14 is the Si/Nb or Ce molar
ratio).
There is no doubt that the appropriate selection of the modifiers is a
key point for the design of effective catalysts. Thus, the choice of or-
ganic compounds with a high number of basic nitrogen species and a
suitable metal as an additional component, which would increase the
efficiency of basic modifier loading, lead to obtain appropriate basic
properties addressed to the Knoevenagel condensation. Thus, in this
work melamine was the basic modifier and niobium or cerium were the
additional components used for mesoporous cellular foam (MCF)
modification. The synergistic effect between metal species and aromatic
heterocycles such as imidazole, triazole and aminotriazole have been
reported in our recent paper [3]. It has been found that niobium Lewis
acidity and redox properties of cerium played an important role in the
basicity of synthesized samples. However, the specific nature of mela-
mine due to a high number of heteroatoms in its heterocyclic ring and
three available amine species [1] suggests that melamine can interact
with metal species in a different manner than the above-described
modifiers. It is well known that the basicity of aromatic amines can be
determined by the presence of electron-donating or electron-with-
drawing groups connected to the aromatic ring. The former ones in-
crease the basicity of amine species, while the latter ones lead to the
opposite phenomenon [13]. Thus, the presence of niobium and cerium
in MCF close to the anchored melamine can affect the basicity strength
of this basic modifier as a result of the transfer of electrons between
melamine and metal species.
2.3. MCF, Nb/MCF-14 and Ce/MCF-14 modification with melamine
followed by (3-chloropropyl)trimethoxysilane (ClPTMS) anchoring
The modification of the supports with (3-chloropropyl)trimethox-
ysilane (ClPTMS), as shown in Scheme 1, was made as follows. A por-
tion of 1.3 g of silica or metalosilicate (dried overnight at 100 °C) was
put into a glass flask. Then 40 cm3 of dried toluene and 1.3 cm3 of
ClPTMS (Aldrich) were introduced. The obtained mixture was then
stirred for 15 min at room temperature and then heated at 110 °C for
24 h. The solid product was then filtered, washed by 45 cm3 of dried
toluene and dried at 100 °C for 24 h.
The anchoring of melamine to the supports modified with ClPTMS
was performed as described below and illustrated in Scheme 1. A por-
tion of 1.81 g of melamine (Aldrich) and 2 cm3 of triethylamine (added
for neutralization of HCl formed during the solid modification) was
introduced to the mixture containing 1.8 g of silica or metalosilicate
and 40 cm3 of dried toluene. The obtained mixture was then heated at
110 °C for 48 h. The solid was then filtered, washed with 33 cm3 of
dichloromethane and 33 cm3 of methanol and finally dried at 100 °C for
24 h. The obtained samples are denoted as Mel/MCF, Mel/Nb/MCF-14
and Mel/Ce/MCF-14.
2.4. Samples characterization
2. Experimental
The obtained catalysts were characterized by nitrogen adsorption/
desorption, XRD, elemental analysis, UV–vis, XPS and thermogravi-
metric analysis.
2.1. Synthesis of silica support - MCF
The synthesis of MCF was performed according to the procedure
described in [14]. The surfactant Pluronic P123 (16 g, Aldrich) was
dissolved in 600 cm3 of 0.7 M solution of HCl (35–37 %, POCH) upon
vigorous stirring at 35–40 °C. Then, NH4F (0.1868 g, Aldrich) and 1,3,5-
trimethylbenzene (16 g, Aldrich) were added together to the mixture.
N2 adsorption/desorption isotherms were measured using an ASAP
2020 instrument. The catalysts before the analysis were outgassed
under vacuum at 120 °C for 10 h. The surface area was calculated by the
BET method, while the pore volume and diameter were estimated ac-
cording to the Broekhoff-de Boer method with the Frenkel-Halsey-Hills
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approximation [16].
interaction with anchored basic modifier - melamine, which was used
XRD patterns were obtained on a Bruker AXS D8 Advance apparatus
using CuKα radiation (γ =0.154 nm), with a step of 0.05° in the wide-
angle range.
because of its high number of basic nitrogen atoms.
3.1. Characterization of the materials
Elemental analysis was carried out with Elemental Analyzer Vario
EL III.
3.1.1. Structure/texture properties of the supports
The UV–vis spectra in the range between 800–190 nm were re-
corded using a Varian-Cary 300 Scan UV–vis Spectrophotometer and
Spectralon as a reference sample. Before measurements the solids were
dried overnight at 100 °C.
In this work three different supports were used for melamine
loading, i.e. silica (MCF) and metalosilicates with niobium (Nb/MCF-
14) or cerium species (Ce/MCF-14). The incorporation of metal species
did not disrupt the structure of mesoporous cellular foam as shown in
Fig. 1. The nitrogen adsorption/desorption isotherms of all supports can
be described as type IV according to IPUAC classification [17]. All
isotherms exhibit one hysteresis loop of type H1 at a relatively high
pressure (p/po), which is typical of mesoporous cellular foam [18]. The
nitrogen adsorption allowed also the estimation of texture parameters
of synthesized supports presented in Table 1. The surface area and pore
volume decreased after niobium or cerium species incorporation into
MCF, which confirmed the presence of both metals in MCF.
X-ray photoelectron measurements (XPS) were performed on an
Ultra High Vacuum (UHV) System (Specs, Germany) equipped with a
monochromatic microfocused Al Kα X-ray source (1486.6 eV). Binding
energies were referenced to the Si 2p peak at 103.4 eV.
TGA measurements were made in air atmosphere using SETARAM
SETSYS-12 apparatus in the temperature range 20–1000 °C with a
temperature ramp of 5 °C min−1
.
2.5. Test reactions
The XRD patterns shown in Fig. 2 suggest that niobium species did
not form a crystalline niobium oxide on Nb/MCF-14 surface [3]. Most
probably the formed Nb2O5 was amorphous and thus it gave no signal
in the XRD pattern of Nb/MCF-14. The presence of Nb2O5 on the sur-
face of Nb/MCF-14 was verified by the UV–vis and XP spectra shown in
Figs. 3 and 4, respectively. The UV–vis spectrum of Nb/MCF-14 (Fig. 3)
exhibited three bands, observed after spectrum deconvolution, at
217 nm, 253 nm and 310 nm. The first and the second bands are typical
of tetra- and penta-coordinated niobium species, respectively [19,20].
The presence of both bands confirms the inclusion of niobium species
into silica skeleton. This conclusion was verified by the XP spectrum of
niobiosilicate, which displayed the BE of Nb 3d5/2 at 207.5 eV (Fig. 4).
This value is higher than that observed for Nb2O5 (207.1 eV [21]),
which further confirmed the niobium incorporation into silica matrix,
as concluded on the basis of the UV–vis measurement. The UV–vis
spectrum of Nb/MCF-14, among the bands typical of niobium species
included into silica skeleton, showed furthermore the third, less intense,
band at 310 nm assigned to extra-framework niobium [19]. To check if
some niobium oxide is present in Nb/MCF-14, the XP spectra of oxygen
were analyzed (Fig. 4). The deconvoluted O 1s spectrum, apart from the
intensive band at 532.8 eV assigned to the oxygen in silanol species of
MCF silica support [22], revealed an additional less intense component
at 530.0 eV typical of the lattice oxygen in metal oxides [23].
In the case of XRD pattern of Ce/MCF-14 (Fig. 2) the reflexes typical
of cerium oxide (structure of fluorite) were detected (JCPDS ICDD PDF
Card – 00-043-1002). The CeO2 crystals were of the size of 14 nm. The
UV–vis spectrum of Ce/MCF-14 (Fig. 3), after its deconvolution, shows
the two components at 247 nm and 302 nm, which are assigned to the
electron charge transfer between oxygen and cerium species at +3 and
+4 oxidation state, respectively [15,24]. The presence of cerium spe-
cies in different oxidation states in Ce/MCF-14 was furthermore con-
firmed by the XPS Ce 3d spectrum (Fig. 4). The deconvoluted spectrum
shows several components at 884.6 eV and 901.6 eV, which evidence
the presence of Ce3+ - O2−, and the bands at 881.7 eV, 887.6 eV,
898.7 eV, 905.5 eV and 917.1 eV typical of Ce4+ - O2− [25,26]. The
surface Ce3+/(Ce3++Ce4+) ratio for Ce/MCF-14 was 0.41. The pre-
sence of cerium species at both oxidation states reaffirm the defected
nature of cerium oxide loaded on Ce/MCF-14 surface. This observation
2.5.1. 2-propanol dehydration and dehydrogenation
The 2-propanol decomposition was performed using
a micro-
catalytic pulse reactor (Ø 6 mm) containing the catalyst bed (0.02 g).
The reactor was connected to the column (5% Carbowax20 m) of an SRI
310 chromatograph. The granulated (0.5 < ø < 1 mm) catalyst placed
in the reactor was first activated at 350 °C for 2 h under nitrogen flow
(40 cm3 min−1). Then, the 3 μl pulses of 2-propanol (POCH) after in-
troduction into the reactor were vaporized and passed through the
catalyst bed under nitrogen flow, 65 cm3 min−1. The reaction was
performed in the temperature range between 150–300 °C. The products
of the reaction were identified by GC equipped with a FID detector
under nitrogen as a carrier gas.
2.5.2. Knoevenagel condensation
All catalysts were tested in the Knoevenagel condensation carried
out without any solvent (Scheme 2). A mixture of benzaldehyde
(20 mmol, 2.12 g) (Aldrich) and malononitrile (20 mmol, 1.32 g) (Al-
drich) together with 0.5 wt.% of the catalyst (preliminary dried over-
night at 100 °C) was placed in a quartz reactor in an EasyMax system
and heated at 60 °C upon vigorous stirring (200 rpm). The reactions
were performed for 5, 15 and 30 min. Products were analyzed by a gas
chromatograph (Thermo Scientific) equipped with 60 m VF-5 ms ca-
pillary column and a FID detector.
3. Results and discussion
The idea of this work was the synthesis of strong basic catalysts with
open structure active in the Knoevenagel condensation under mild re-
action conditions. For this purpose, the mesoporous cellular foams were
synthesized, modified with metal species (niobium or cerium) and
melamine as the precursor of basic nitrogen species. MCF was used as
the support because of its attractive structure, made of cells of size ca
35 nm connected by windows of size ca 20 nm. Such a construction of
silica avoids diffusional problems during the reactions performed in
liquid phase such as the Knoevenagel condensation. The choice of metal
species for MCF modification was dictated by the possibility of their
Scheme 2. The Knoevenagel condensation between benzal-
dehyde and malononitrile.
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Fig. 1. The nitrogen adsorption/desorption isotherms of silica MCF and metalosilicate supports before and after melamine loading.
Table 1
Texture/structure parameters of synthesized samples.
Catalyst
Surface area, m2/g
Pore volumea, cm3/g
Window diameterb, nm
Cell diameterc, nm
MCF
708 +/− 1.2
576 +/− 0.9
573 +/− 0.9
177 +/− 0.5
150 +/− 0.4
141 +/− 0.3
2.61
2.15
2.27
0.81
0.64
0.61
19.8
19.7
20.1
17.9
16.5
17.8
35.1
35.4
32.5
32.6
32.2
33.5
Ce/MCF-14
Nb/MCF-14
Mel/MCF
Mel/Ce/MCF-14
Mel/Nb/MCF-14
a,c – determined from adsorption branches of N2 isotherms (BdB–FHH method).
b
determined from desorption branches of N2 isotherms (BdB–FHH method).
Fig. 2. The wide-angle XRD patterns of modified samples and melamine.
was moreover verified by the deconvoluted O 1s XP spectrum of Ce/
MCF-14, which revealed the three components at 529.7 eV, 532.7 eV
and 534.0 eV. The most intense component at 532.7 eV assigned to
hydroxyl groups from silanol species in MCF, and the one at 529.7 eV
typical of lattice oxygen in metal oxides were also observed for Nb/
MCF-14, whereas the band at 534.0 eV was detected only in the spec-
trum of Ce/MCF-14. The latter band is attributed to the oxygen in de-
fected metal oxides [27] and its presence confirmed that Ce/MCF-14
contained cerium species at +3 oxidation state. The number of surface
oxygen vacancies in Ce/MCF-14 can be evaluated from the ratio of OV/
OV+OL, where OV and OL refer to O2− in oxygen vacancies (V) or in the
lattice (L). The surface ratio of OV/OV+OL for Ce/MCF-14 was 0.80.
The differences in chemical behavior of niobium and cerium species
loaded on MCF were investigated in 2-propanol dehydration and de-
hydrogenation (Table 2). The products formed in this reaction are the
indicators of acidity or basicity of tested samples. 2-Propanol is trans-
formed into propene on acidic active centers, whereas acetone is
formed on basic active centers [28]. Diisopropyl ether appears if pairs
of acid-base centers are present on the catalyst surface. Both, niobium
and cerium species loaded into MCF catalyzed the 2-propanol
conversion, however Nb/MCF-14 showed higher activity than Ce/MCF-
14. The niobium modified silica catalyzed the 2-propanol conversion
only to propene, while Ce/MCF-14 exhibited the selectivity also to
other products such as acetone and diisopropyl ether. The obtained
results proved that the surface of both metalosilicates shows different
properties. The surface of Nb/MCF-14 is acidic, while that of Ce/MCF-
14 is more basic.
3.1.2. The impact of melamine loaded on the supports
The anchoring of melamine caused significant changes in the shape
of nitrogen adsorption isotherms, as shown in Fig. 1. The volume of
adsorbed nitrogen decreased significantly for the melamine modified
samples, compared to its volume adsorbed on the corresponding sup-
ports. The hysteresis loop of the samples modified with melamine is still
of type H1. The calculated textural parameters of the materials mod-
ified with melamine species (Table 1) show a significant reduction of
surface area and pore volume in comparison to the supports without
modification. This confirms the anchoring of a modifier to the surfaces
of MCF, Nb/MCF-14 and Ce/MCF-14. Moreover, the anchoring of
melamine caused a decrease in the windows and cells diameter in the
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Fig. 3. The UV–vis spectra of modified MCF materials and melamine.
obtained samples with exception of Mel/Nb/MCF-14. In this sample the
cells size increased after melamine loading.
spectra of Mel/Nb/MCF-14 and Mel/Ce/MCF-14 might be covered by
the band assigned to niobium in pentahedral coordination (238 nm) for
Mel/Nb/MCF-14 or by the band characteristic of the electron charge
transfer between oxygen and cerium in +3 (242 nm) in the case of Mel/
Ce/MCF-14. The formation of the bond between chlorine precursor and
melamine might be verified by N 1s XP spectra shown in Fig. 5. The
spectra after deconvolution showed the two components. The first of
them at ca 399.4 eV is typical of nitrogen connected to carbon (C]N) in
heterocyclic ring, whereas the second band at 397.0 eV corresponds to
the nitrogen in amine species [29]. These components in the N 1s XP
spectrum of melamine show similar intensities because of the sym-
metric structure of this substance [29]. In the XP spectra of the samples
modified with melamine the intensity of the first band was lower,
especially in the case of modified metalosilicates. The probable reason
is that one of the amine groups in melamine formed bond with ClPTMS
anchored to the supports. As a result, the electrons from the C]N bond
in melamine were delocalized within the whole modifier, i.e. melamine
and propyl precursor from previously anchored ClPTMS (Scheme 1).
Moreover, the band assigned to C]N was shifted towards higher BE in
the XP spectra of Mel/Nb/MCF-14 and Mel/Ce/MCF-14, which implies
the interaction between niobium or cerium species with anchored
melamine.
The XRD patterns of melamine modified samples (Fig. 2) confirm
the modifier inclusion into the supports by the presence of the reflexes
characteristic of melamine. The XRD pattern of Mel/Nb/MCF-14 did
not show the reflexes characteristic of niobium oxide. In the XRD pat-
tern of Mel/Ce/MCF-14 the reflexes typical of CeO2 were still visible.
Nevertheless, they were overlapped by the reflexes characteristic of
melamine, thus it was impossible to calculate the crystals size of cerium
oxide in this material.
The anchoring of melamine was also proven by the UV–vis (Fig. 3)
and XPS measurements (Fig. 5). The UV–vis spectrum of melamine
showed a band at 203 nm. This band was well resolved in the spectra of
Mel/MCF (211 nm) and Mel/Ce/MCF-14 (214 nm), while for Mel/Nb/
MCF-14 it could be overlapped by the band characteristic of niobium in
tetrahedral coordination (210 nm). The presence of the band typical of
free melamine implies that part of this substance used for the supports
modification was not reacted with the previously anchored ClPTMS, as
shown in Scheme 1, but it is hydrogen bonded to the surface of sup-
ports. That could explain the appearance of the reflexes characteristic of
melamine in the XRD patterns of the samples modified with this sub-
stance. Interestingly, in the UV–vis spectrum of Mel/MCF, an additional
band at 237 nm was detected (Fig. 3). The presence of this band could
be assigned to the anchoring of melamine by chlorine precursor
(ClPTMS) to MCF support as presented in Scheme 1. This band in the
As mentioned above, the UV–vis spectrum of Mel/Nb/MCF-14 ex-
hibited the bands typical of metal included into silica skeleton, i.e.
tetra- (210 nm) and penta-coordinated (238 nm) niobium (Fig. 3). The
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Fig. 4. The Nb 3d, Ce 3d and O 1s XP spectra of metalosilicates.
Table 2
Nb 3d XP spectrum of Mel/Nb/MCF-14, the detected BE of Nb 3d5/2
was 207.3 eV. This value is lower than that observed for Nb/MCF-14
(207.5 eV) and is closer to that observed for bulk Nb2O5 (207.1 eV) in
which niobium is in extra-framework position. The presence of niobium
oxide in Mel/Nb/MCF-14 was evidenced by the O 1s XP spectrum,
which showed the component at 530.2 eV typical of lattice oxygen. The
presence of melamine in niobiosilicate caused also a shift of BE of O 1s
towards higher value (530.2 eV in Mel/Nb/MCF-14 from 530.0 eV in
Nb/MCF-14), which might be caused by the transfer of electrons be-
tween melamine and niobium oxide localized on the surface of Mel/Nb/
MCF-14.
The influence of melamine on metal species was also observed for
Mel/Ce/MCF-14. The UV–vis spectrum of this sample, besides the band
typical of melamine, showed the bands assigned to the electron charge
transfer between oxygen and cerium in +3 (242 nm) and +4 oxidation
state (300 nm) (Fig. 3). The positions of both bands were blue shifted in
comparison to the spectrum of Ce/MCF-14. Interestingly, the band ty-
pical of Ce3+ - O2− was better resolved in the spectrum of Mel/Ce/
MCF-14 than in the Ce/MCF-14. However, this band might be over-
lapped by the band characteristic of melamine anchored to the support
by ClPTMS precursor. Nevertheless, the visible differences in the posi-
tion and intensity of the bands characteristic of both cerium species
indicated the interaction between melamine and cerium cations, simi-
larly as was observed for niobium modified sample.
The results of 2-propanol dehydration and dehydrogenation performed at
300 °C.
Sample
Conversion
2-PrOH, %
Selectivity, %
Acetone
Propene
Ether
MCF
Ce/MCF-14
Nb/MCF-14
Traces
4
68
0
15
0
100
84
100
0
1
0
intensities of these bands were different than those observed in the
spectrum of Nb/MCF-14. The intensity of the first band was lower,
while that of the second one was higher. It implies that both bands
might be overlapped by the bands typical of melamine, as suggested
above. Apart from the differences in intensity of the bands at 210 nm
and 238 nm, they occupied different positions. Both bands were shifted
to the blue region of UV–vis in comparison to those in the spectrum of
Nb/MCF-14. Additionally, the UV–vis spectrum of Mel/Nb/MCF-14
showed the band with a maximum at 280 nm and with a long tail up to
330 nm typical of niobium in extra-framework position. This band was
more intense than that observed in the spectrum of Nb/MCF-14. It
seems that Nb/MCF-14 modification with melamine followed by
ClPTMS anchoring caused the extraction of a part of niobium from the
silica skeleton. As a result, greater amount of Nb2O5 was formed on the
surface of Mel/Nb/MCF-14. This phenomenon could cause the increase
in the cells diameter in Mel/Nb/MCF-14 (Table 1). The differences in
the intensity and position of both bands as well as detection of the new
band characteristic of octahedral coordinated niobium at UV–vis
spectrum of Mel/Nb/MCF-14 proved a strong interaction between
melamine and niobium species in the niobiosilicate support.
The differences appeared on the surface of Ce/MCF-14 after mela-
mine anchoring were also certified by XPS measurement. The Ce 3d XP
spectrum of Mel/Ce/MCF-14 (Fig. 6) exhibited the components as-
signed to Ce3+ (883.8 eV and 901.5 eV) and Ce4+ (882.3 eV, 886.7 eV,
898.6 eV, 906.7 eV and 917.4 eV) bonded with O2− as for Ce/MCF-14.
The positions and intensities of the listed components were changed
with respect to those in the XP spectrum of Ce/MCF-14 (Fig. 4). A
significant difference in the band’s area was observed for the compo-
nents at 883.8 eV and 917.4 eV characteristic of the electron charge
The impact of melamine on niobium species in Nb/MCF-14 is fur-
thermore manifested in the XP spectra shown in Fig. 6. As concerns the
6

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CatalysisTodayxxx(xxxx)xxx–xxx
Fig. 5. The N 1s XP spectra of melamine modified samples.
Fig. 6. The XP spectra of metalosilicate supports after melamine loading.
7

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transfer between O2− and Ce3+ or Ce4+, respectively. The intensity of
the first band decreased, while the second one increased in comparison
to those in the spectrum of Ce/MCF-14. As a consequence, the surface
ratio of Ce3+/Ce3++Ce4+ decreased by 47% for Mel/Ce/MCF-14
(from 0.41 to 0.22). Consequently, the ratio of oxygen vacancies on the
surface of Mel/Ce/MCF-14 also decreased from 0.80 to 0.50. Further-
more, the position of the bands typical of oxygen in oxygen vacancies
(534.2 eV) and lattice oxygen in metal oxides (530.2 eV) was shifted to
higher BE (from 534.0 eV and 529.7 eV for Ce/MCF-14, respectively)
after melamine loading into Ce/MCF-14. A similar phenomenon was
observed for Mel/Nb/MCF-14. The differences which appeared for Mel/
Ce/MCF-14 indicated that melamine played a role of an electron-
withdrawing modifier, which caused the change in oxidation state of
surface cerium species from +3 to +4 (XPS analyses only the surface of
the materials). It is well known that cerium cations in defected cerium
oxide can exist in two forms, in a more stable tetravalent state and
trivalent state having the deficiency of positive charge compensated by
oxygen vacancies [30]. Experimental studies have shown that the
highly mobile lattice oxygen available at the surface of ceria together
with the oxygen vacancies are responsible for the transfer of electrons
in the fluorite structure of ceria [31]. Ce3+ has one electron in the 4f
orbital and this electron is mobile [32]. The close neighborhood of
melamine with resonate structure of heterocyclic ring bonded with
three amine species to cerium oxide in Mel/Ce/MCF-14 caused the
migration of mobile electrons from the surface of ceria to melamine. As
a result the very unstable Ce3+ cations in Mel/Ce/MCF-14 were im-
mediately transformed into Ce4+ state. At the same time the number of
oxygen vacancies and lattice oxygen on the surface of Mel/Ce/MCF-14
was changed.
however it undergoes sublimation at temperatures below its melting
point [33]. The DTG curves presented the maxima of the first mass loss
at 324 °C for Mel/MCF, 329 °C for Mel/Ce/MCF-14 and 335 °C for Mel/
Nb/MCF-14. These temperatures are close to the melting point of
melamine, thus they could be correlated with the sublimation of un-
reacted form of this modifier. The second less intense mass loss ob-
served in the DTG curves has its maximum at 476 °C for Mel/Ce/MCF-
14 and at 556 °C for Mel/Nb/MCF-14, while for Mel/MCF a broad
maximum corresponding to mass loss was detected. This mass loss
might be linked to the decomposition of melamine anchored via a
propyl chain of ClPTMS to the supports (Scheme 1). The formed che-
mical bonding between these two compounds could increase the mel-
amine stability and thus could be responsible for the shift of the tem-
perature of its decomposition to the higher value.
The temperature of the maximum that corresponds to the decom-
position of melamine is an indicator of the modifier stability. It is visible
that melamine was more stable on the sample containing niobium than
on that with cerium species. The temperature of melamine decom-
position was higher by 6 °C for the first mass loss (329 °C in comparison
to 335 °C) and by 80 °C for the second mass loss (476 °C compared to
556 °C). The visible difference between Mel/Ce/MCF-14 and Mel/Nb/
MCF-14 might be caused by the change in the oxidation state of cerium
species in Mel/Ce/MCF-14 after melamine anchoring. A similar phe-
nomenon of decreasing stability of anchored basic modifier in cerium
modified silicas in comparison to niobiosilicates was observed for the
samples modified with imidazole species [3].
3.2. Results of the Knoevenagel condensation
It has been already reported that the Knoevenagel condensation
between aldehydes and active methylenes proceeds efficiently in the
presence of catalysts which exhibit base or acid-base properties [9].
Thus, the synthesis of catalysts containingmelamine, as a basic modi-
fier, and different supports to which it was anchored, such as MCF, Nb/
MCF-14 and Ce/MCF-14, seems to be the appropriate. Melamine con-
tains a high number of basic nitrogen species, which could play a role of
active sites in the Knoevenagel condensation, while the used supports
exhibit different properties (acid or basic one) and thus they can affect
the efficiency of melamine loading and its basicity strength. The per-
formed reactions between benzaldehyde and malononitrile, illustrated
in Scheme 2, allowed to support some conclusions drawn from the
structural/textural characterization and chemical composition of the
synthesized samples. Moreover, the above reactions provided the in-
formation that helped explain the differences in the strength of basicity
of melamine anchored to the silica and metalosilicates.
The results of the Knoevenagel condensation between benzaldehyde
and malononitrile in the presence of the synthesized samples are given
in Fig. 8. It is important to note that the selectivity of the reaction was
100% to the main product, benzylidenemalononitrile. It evidenced that
anchoring of melamine to MCF, Nb/MCF-14 and Ce/MCF-14 caused a
significant increase in the activity of benzaldehyde conversion in
comparison to the activity of silica and metalosilicate supports. It is
worth noting that melamine was not detected in the reaction mixtures
of Mel/MCF, Mel/Nb/MCF-14 and Mel/Ce/MCF-14 by GC–MS. It im-
plies that melamine did not leach from these catalysts during the
Knoevenagel condensation. In order to check if the reaction was not
limited by external or internal diffusion, the additional reactions were
performed in which a mixture of benzaldehyde and malononitrile with
Mel/Ce/MCF-14 was stirred at 100 rpm, 200 rpm and 400 rpm. The
obtained results clearly demonstrated that the reactions performed over
the modified metalosilicate were not controlled by external or internal
diffusion because for each reaction a similar benzaldehyde conversion
was obtained, ca 87%. Interestingly, the reaction performed over free
melamine (2 mg) was not so efficient as for samples modified with this
substance. Thus, the role of the support in increasing the basicity
strength of melamine was evidenced. This occurrence might be
3.1.3. The efficiency of melamine loading and its stability
The efficiency of melamine loading can be estimated from the ni-
trogen content. The weight percentage of nitrogen and moles of an-
chored melamine calculated from the elemental analysis data are given
in Table 3. It is evidenced that the effectiveness of melamine anchoring
depends on the composition of the support used. The presence of nio-
bium or cerium in MCF enhanced (more than twice) the modifier
loading. It can be caused by the strong interaction between metal
species and melamine inferred from the UV–vis and XP spectra
(Figs. 3–6). The increase in modifier loading is higher for Nb/MCF-14
than Ce/MCF-14 support. Similar results were obtained in our earlier
paper in which the chemical interaction between niobium species and
imidazole, triazole or aminopropyl species was proved [3]. A similar
interaction can occur with melamine species and thus the visible in-
crease in efficiency of melamine loading in Nb/MCF-14 support was
obtained.
The metal species in MCF also have an important effect on the
stability of anchored melamine, which is particularly significant for
processes performed in liquid phase, such as the Knoevenagel con-
densation. The stability of anchored melamine was estimated by ther-
mogravimetric analysis whose results are shown in Fig. 7. The TG
curves revealed two mass loss events between 270–390 °C and
400–600 °C. The first greater mass loss could be assigned to the de-
composition of melamine that did not react with anchored ClPTMS. The
chlorine precursor decomposed from MCF and NbMCF-14 at tempera-
ture below 300 °C [14]. Melamine melts and decomposes at 347 °C,
Table 3
The melamine loading on silica and metalosilicate supports (based on elemental
analysis).
Catalyst
N, wt.%
Melamine, mmol/g
Mel/MCF
Mel/Ce/MCF-14
Mel/Nb/MCF -14
13.1
41.2
44.2
2.2
4.9
5.3
8

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CatalysisTodayxxx(xxxx)xxx–xxx
Fig. 7. The thermogravimetry analysis of melamine modified samples.
metalosilicates than to Mel/MCF as shown in Table 3. However, the
differences between the benzaldehyde conversion achieved over Mel/
Ce/MCF-14 and Mel/Nb/MCF-14 suggest that the higher activity of the
former sample might be caused by the electron donation to melamine
by defected cerium oxide in Mel/Ce/MCF-14 as deduced from the Ce 3d
and O 1s XP spectra (Fig. 6). It is well known that the electron donating
groups increase the basicity of aromatic amines due to displacement of
negative charge on the carbon connected to nitrogen in amine group.
Consequently, the lone pair of electrons placed in the nitrogen atom of
amine species is more reactive [13] as visible for Mel/Ce/MCF-14.
Moreover, the ceria in Mel/Ce/MCF-14 still exhibit defected nature as
confirmed by the XP spectra (Fig. 6), in which basic mobile oxygen
could participate in the reaction besides melamine. The basicity of Ce/
MCF-14 support was confirmed by 2-propanol dehydration and dehy-
drogenation (Table 2). In contrast, the acidic niobiosilicate (as con-
firmed by 2-propanol conversion (Table 2)) did not promote the con-
densation between benzaldehyde and malononitrile as ceria did in Mel/
Ce/MCF-14. This might be caused by different nature of niobium spe-
cies loaded into MCF support, which are at +5 oxidation state, the most
stable state of niobium [35]. Thus, niobium included into MCF and
niobium oxide on the surface of Mel/Nb/MCF-14 could not play the
role of electron donating species as it did ceria in Mel/Ce/MCF-14.
The activity of samples modified with melamine was moreover in-
vestigated at different reaction time (Fig. 9). The conversion of ben-
zaldehyde increased gradually with increasing reaction times for all
investigated catalysts. Mel/Ce/MCF-14 showed the highest activity
among the tested samples in each reaction time due to its stronger
basicity.
Fig. 8. The results of the Knoevenagel condensation between benzaldehyde and
malononitrile performed for 15 min in the presence of synthesized samples and
melamine. The reaction mixture was stirred at 200 rpm with exception of * -
100 rpm and # - 400 rpm.
correlated with the formation of secondary amine species in the mela-
mine modified materials. The non-anchored melamine has only primary
amine species and tertiary amine within its heterocyclic ring, which are
less basic than secondary amine [34]. Consequently, the presence of
secondary amine groups in melamine anchored to the supports raised
the benzaldehyde conversion in comparison to the homogeneous mel-
amine system.
From among the catalysts tested, the highest conversion of benzal-
dehyde equal to 87.0% was achieved over Mel/Ce/MCF-14. This phe-
nomenon could be due to a higher number of melamine anchored to
As the amount of anchored melamine was different, depending on
the type of support used for modifier anchoring (Table 3), a comparison
of the activities of Mel/MCF, Mel/Ce/MCF-14 and Mel/Nb/MCF-14 was
made on the basis of both benzaldehyde conversion and TOF (turnover
frequency) calculated for the reactions performed for 5 min (Fig. 9).
Melamine anchored to the metalosilicates was less active than when it
was loaded into MCF. However, if melamine was anchored to Ce/MCF-
14, its activity increased relative to that of melamine anchored to Nb/
9

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CatalysisTodayxxx(xxxx)xxx–xxx
efficiently than in Mel/Nb/MCF-14 and Mel/MCF. Niobium species in
Mel/Nb/MCF-14 did not donate amine species in anchored melamine as
ceria did in Mel/Ce/MCF-14 because of different properties of niobium,
which does not change its oxidation state as easily as defected ceria.
Acknowledgement
National Science Centre in Poland (Project No UMO-2018/29/B/
ST5/00137) is gratefully acknowledged for its financial support. The
author wishes to thank Prof. Maria Ziolek (Adam Mickiewicz
University, Poznań, Poland) for a fruitful discussion and valuable re-
marks and Ms. A. Wrublińska for assistance in experimental work.
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