Mesoporous niobiosilicate NbMCF modified with alkali metals in the synthesis of chromene derivatives

Article abstract of DOI:10.1016/j.cattod.2016.02.042

We report the synthesis, characterization and activity of new series of catalysts based on Niobium Mesoporous Cellular Foams (NbMCF), modified with alkaline metal elements. These materials exhibit very different acid-base properties attributed to the Nb source for the preparation of the catalysts and the presence and loading of different alkaline metals. The Me/NbMCF materials (where Me are alkaline metals) were tested in the reaction of 2-hydroxybenzaldehyde and ethyl cyanoacetate, under solvent-free conditions, at room temperature, leading to mixtures of the corresponding chromenes 4 and 5, as mixtures of diastereomers erythro/threo, in a 2:1 ratio, respectively, with good to excellent yields. Our experimental results indicate that the metal loading on the catalysts and the acid-base character, and the texture parameters are probably determining factors in the reactivity and the observed diastereoselectivity. The computational study suggests that the presence of alkaline oxides on Me/NbMCF catalyst, exhibiting strong basicity, activates the formation of the nucleophile species, the corresponding enolate, and initiates the reaction. However, the cation size has a deep impact on the stability of the reactant complex so that the effective aldolization could be obstructed by the presence of the bulkier centers.

Full text of DOI:10.1016/j.cattod.2016.02.042

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Mesoporous niobiosilicate NbMCF modified with alkali metals in the
synthesis of chromene derivatives
Agata Smuszkiewicza, Jesús Lopez-Sanza,b, Izabela Sobczakb,∗∗, Maria Ziolekb,
Rosa M. Martín-Arandaa, Elena Sorianoc,∗ ∗ ∗, Elena Pérez-Mayorala,∗
a Departamento de Química Inorgánica y Química Técnica, Universidad Nacional de Educación a Distancia, UNED, Paseo Senda del Rey 9, E-28040 Madrid,
Spain
b Adam Mickiewicz University, Faculty of Chemistry, Umultowska 89b, 61-614 Poznan, Poland
c Instituto de Química Orgánica General (CSIC), C/ Juan de la Cierva 3, E-28006 Madrid, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 July 2015
Received in revised form 11 February 2016
Accepted 19 February 2016
Available online xxx
We report the synthesis, characterization and activity of new series of catalysts based on Niobium Meso-
porous Cellular Foams (NbMCF), modified with alkaline metal elements. These materials exhibit very
different acid-base properties attributed to the Nb source for the preparation of the catalysts and the
presence and loading of different alkaline metals.
The Me/NbMCF materials (where Me are alkaline metals) were tested in the reaction of 2-
hydroxybenzaldehyde and ethyl cyanoacetate, under solvent-free conditions, at room temperature,
leading to mixtures of the corresponding chromenes 4 and 5, as mixtures of diastereomers erythro/threo,
in a 2:1 ratio, respectively, with good to excellent yields.
Keywords:
Heterogeneous catalysis
Niobium Mesoporous Cellular Foams
Heterocyclic ring systems
Theoretical calculations
Our experimental results indicate that the metal loading on the catalysts and the acid-base charac-
ter, and the texture parameters are probably determining factors in the reactivity and the observed
diastereoselectivity.
The computational study suggests that the presence of alkaline oxides on Me/NbMCF catalyst, exhibit-
ing strong basicity, activates the formation of the nucleophile species, the corresponding enolate, and
initiates the reaction. However, the cation size has a deep impact on the stability of the reactant complex
so that the effective aldolization could be obstructed by the presence of the bulkier centers.
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
[7], all of them modified with amine groups, have been reported as
efficient adsorbents of CO2. Besides, MCM-48 and SBA-15 found
Since the discovery of the mesoporous silica MCM-41 by Mobile
Corporation in 1992 [1] the interest and investigation concerning
these materials have been growing and a huge number of new
materials, with different chemical composition, structure and novel
pore geometry, have been reported [2,3]. The large surface areas
and pore size, periodical structure, the possibility of modification
of their silica walls, biocompatibility and biodegradability are the
most relevant properties of mesoporous silicas making them inter-
esting materials with application in different research areas [4].
This type of silicas has been extensively studied in adsorption and
separation of gases. Particularly, MCM-41 [5], SBA-15 [6] and MCF
their application as adsorbents of heavy metals. In other sense,
metallosilicates such as Al-SBA-15 [8] and Pt/SBA-15 [9] have been
applied as adsorbents of volatile organic compounds (VOCs).
Furthermore, mesoporous silicas have been extensively used
in catalysis. In this context, our research group possesses a wide
experience in this field, applying mesoporous silicas as heteroge-
neous catalysts in the synthesis of heterocyclic compounds with
biological properties [10]. In particular, we have recently reported
novel series of mesoporous silicas with different acid-base proper-
ties in the synthesis of quinolones and quinolines, via Friedlander
reaction, the synthesis of coumarins through Knoevenagel conden-
sation and the N-alkylation of pyrazole, all of these transformations
catalyzed by amino grafted MCM-41 and SBA-15-based materials.
We also reported the isomerization of olefins, especially Eugenol
and Safrole, catalyzed by amino-grafted metallosilicates. More
recently, we reported an experimental and theoretical study con-
cerning the catalytic behavior and of amino-grafted NbMCF in the
∗
Corresponding author.
Corresponding author.
Corresponding author.
∗∗
∗ ∗ ∗
E-mail address: eperez@ccia.uned.es (E. Pérez-Mayoral).
http://dx.doi.org/10.1016/j.cattod.2016.02.042
0920-5861/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: A. Smuszkiewicz, et al., Mesoporous niobiosilicate NbMCF modified with alkali metals in the synthesis
of chromene derivatives, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.02.042

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2.1.2. NbMCF materials
2
For the preparation of NbMCF materials, the synthesis procedure
was modified in order to introduce niobium. Two kinds of niobium-
containing materials were prepared: NbMCF(ox) and NbMCF(Et),
depending on the metal source. In the NbMCF(ox) synthesis ammo-
nium niobate(V) oxalate hydrate (C4H4NNbO9) was added to the
gel containing Si source while keeping the TEOS/Nb molar ratio = 64
constant. The source of Nb was added to the mixture 10 min after
TEOS admission.
Scheme 1. Synthesis of chromenes 1 from 2-hydroxybenzaldehydes 2 and nitriles
substituted at aposition 3.
The preparation of NbMCF(Et) material followed the procedure
described above, but TMB/PL ratio = 0.75 was applied (8 g, 1.4 mmol
of Pluronic and 6 g, 49.92 mmol of TMB). Niobium ethoxide was
used as a source of niobium (Si/Nb molar ratio = 64 as assumed).
In order to establish niobium content in NbMCF, XRF technique
was applied.
synthesis of nitrogen heterocycles also through the Friedländer
reaction [11].
Mesoporous Cellular Foams (MCF) are a relatively new class of
silica materials, easy to be modified, with a cell size between 24
and 42 nm interconnected by window of size between 9 and 22 nm,
exhibiting very high surface area and porosity [12]. As other meso-
porous silicas, their properties can be changed by incorporation of
metals within the framework. In this sense, this type of mesoporous
silicas has been reported by some of us in the production of biofuels
additives [13], esterification reactions with glycerol [14], and also
for the selective oxidation of methanol to formaldehyde [15].
The goal of this paper is the development of a new envi-
ronmental friendly methodology for the efficient synthesis of
2-amino-4H-chromenes by using NbMCF materials as catalytic
agents. 4H-Chromene derivatives are six-membered oxygen het-
erocycles widely distributed in nature [16] and synthetic drugs,
with relevant biological properties [17]. One of the simplest syn-
thetic approaches for the preparation of 2-amino-4H-chromenes
1 is the reaction between 2-hydroxybenzaldehydes 2 and nitriles
substituted at ␣ position 3 (Scheme 1). However, only a few exam-
ples of heterogeneous catalysts for this transformation have been
reported. Thus, Amberlyst A-21 [18], tin(IV) oxide doped hydrotal-
cites [19], molecular sieves and Al2O3 [20] have been reported as
catalysts for the synthesis of 4H-chromenes 1 with goods yields
although during prolonged reaction times or in the presence of
solvents.
2.2. Modification of NbMCF materials with alkali metals
NbMCF supports were impregnated by using the incipient wet-
ness method with aqueous solution of MeCH3COO (Me = Li, Na, K,
Rb, Cs). The outgassed NbMCF(ox) and NbMCF(Et) (after treatment
at 373 K for 3 h in the oven and 1 h at 373 K in an evaporator flask)
were filled in with the appropriate amount of an aqueous solution of
alkali metal acetate (CH3COOLi, CH3COONa, CH3COOK, CH3COORb
or CH3COOCs, a volume of the solution was a little bit higher than
the pore volume of the support) and rotated and heated in an evapo-
rator flask at 373 K. The impregnated powder was dried at 373 K for
18 h and then calcined at 723 K for 10 h in the oven.Two series of cat-
alysts were obtained: Me/NbMCF(ox) and Me/NbMCF(Et), where
Me = alkali metal.
The amount of alkali metal acetate used for the impregnation
was calculated for the loading of metal equal to 0.5 mmol per 1 g of
NbMCF support. Such value gives rise to the following weight per-
cent of metals: Li—0.35%; Na—1.15%; K—1.96%; Rb—4.27; Cs—6.65%
Rb-4.27%. The surface area of the modified materials differs from
that of the parent one and therefore the mole number of alkali metal
cations calculated per m2g−1 of the supports was calculated and
shown in Table 1.
In continuation with our studies, we report herein the investi-
gation of the catalytic behavior of novel series of NbMCF materials,
prepared from two different Nb sources and modified with alka-
line metals, in the synthesis of 4H-chromene derivatives (1),
through multicomponent reactions (MCRs), between substituted
2-hydroxybenzaldehydes (2) and ethyl cyanoacetate (3), under
solvent-free and mild conditions. Our aim also is to understand
the reaction mechanism and the influence of basic character of the
MCF materials on the reaction.
2.3. Characterization
The materials prepared were characterized using XRF, N2
adsorption/desorption, UV–vis and test reactions.
X-ray fluorescence (XRF) was applied using MiniPal-Philips. The
measurements were done using calibration curves prepared from
mixtures of silica and Nb2O5 (Si/Nb from 5 to 300).
The N2 adsorption/desorption isotherms at 77 K on materials
under study were recorded using a static volumetric appara-
tus ASAP 2020 (Micromeritics). In order to attain a sufficient
accuracy in the accumulation of the adsorption data, this instru-
ment is equipped with pressure transducers covering the 133 Pa
and133 kPa ranges. Before each sorption measurement the samples
were outgassed at 383–393 K overnight until the residual pres-
sure was lower than 0.7 Pa. The surface area was calculated by the
BET method, whereas the volume and the diameter of mesopores
were estimated according to Broekhoff-de Boer method with the
Frenkel–Halsey–Hill equation (BdB FHH).
2. Materials and methods
2.1. Synthesis of MCF and NbMCF materials
2.1.1. MCM material
MCF was prepared by one-pot synthesis method described
in Ref. [21]. First, Pluronic 123 (PL—poly(ethylene glycol)-block-
poly(propylene glycol)-block-poly(ethylene glycol)-block) (8 g,
1.4 mmol) was dissolved in 300 g of 0.7 M HCl solution at
308–313 K. Next, 1,3,5trimethylbenzene (TMB) (12 g, 99.84 mmol)
(TMB/PL = 1.5) and NH4F (0.0934 g, 2.52 mmol) were added under
vigorous stirring. Following 2 h of stirring, TEOS (17.054 g,
81.99 mmol) was added. The final mixture was stirred at 308–313 K
for 24 h and then transferred into a polypropylene bottle and heated
at 373 K under static conditions for 24 h. The solid product was
recovered by filtration, washed with distilled water and dried at
room temperature. The template was removed from the as synthe-
sized material by calcination at 773 K for 8 h under static conditions.
Table 1 lists some characterization data of the catalysts under
study.
UV–vis spectra were recorded using a Varian-Cary 300 Scan
UV–visible Spectrophotometer. Sample powders were placed into
the cell equipped with a quartz window. The spectra were recorded
in the range from 800 to 190 nm. Spectral on (fluoropolymer stan-
dard provided with Varian Spectrometer) was used as a reference
material.
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Table 1
Alkali metal (Me) loading and textural parameters of NbMCF catalysts.
Sample
Me weightloading [%]
Me loading(mol/m2 g−1)*10−6
Surface area, BET[m2/g]
Mean pore volume [cm3/g]
BdB FHH
adsorption
NbMCF(ox)
–
–
672
361
296
248
226
231
717
331
341
303
250
234
2.7
1.6
1.3
1.2
1.2
1.1
2.8
1.6
1.5
1.7
1.3
1.4
Li/NbMCF(ox)
Na/NbMCF(ox)
K/NbMCF(ox)
Rb/NbMCF(ox)
Cs/NbMCF(ox)
NbMCF(Et)
Li/NbMCF(Et)
Na/NbMCF(Et)
K/NbMCF(Et)
Rb/NbMCF(Et)
Cs/NbMCF(Et)
0.35
1.15
1.96
4.27
6.67
–
0.35
1.15
1.96
4.27
6.67
1.39
1.69
2.02
2.21
1.99
–
1.51
1.47
1.65
2.00
2.14
Two test reactions were used to determine acid-base properties
of catalysts. They were 2.5-hexanedione cyclisation and dehydra-
tion and 2-propanol decomposition.
A tubular, down-flow reactor was used in 2,5-hexanedione
cyclisation reaction that was carried out at atmospheric pressure,
using nitrogen as the carrier gas. In a typical experiment, the cata-
lyst bed (0.05 g) was first activated for 2 h at 673 K under nitrogen
flow (40 cm3 min−1). Subsequently, 0.5 cm3 of 2.5-hexanedione
was passed continuously over the catalyst at 623 K. The substrate
was delivered with a pump system and vaporized before being
passed through the catalyst with the flow of nitrogen carrier gas
(40 cm3 min−1). The reaction products were collected for 30 min
downstream of the reactor in the cold trap (mixture of 2-propanol
and liquid nitrogen) and analysed by gas chromatography (CHROM-
5, Silicone SE-30/Chromosorb G column).
The 2-propanol conversion (dehydration and dehydrogena-
tion) was performed, using a microcatalytic pulse reactor inserted
between the sample inlet and the column of a CHROM-5 chromato-
graph. In a typical experiment, the catalyst bed (0.02 g) was first
activated at 673 K for 2 h under helium flow (40 cm3 min−1). The 2-
propanol conversion was studied at 423, 473, 523 and 573 K using
3 mL pulses of alcohol under helium flow (40 cm3 min−1). The reac-
tant and the reaction products: propene, 2-propanone (acetone)
and diisopropyl ether were analysed using CHROM-5 gas chro-
matograph on line with microreactor. The reaction mixture was
separated on 2 m column filled with Carbowax 400 (80–100 mesh)
at 338 K in helium flow (40 cm3 min−1) and detected by TCD.
Chart 1. Structures selected as theoretical models for calculations.
300(300 MHz for 1H). 1H chemical shifts (␦) in CDCl3 are given from
internal tetramethylsilane.
All reagents and solvents were purchased from Aldrich, Fluka
and Alfa-Aesar.
2.5. Theoretical calculations
The calculations were carried out with the Gaussian 09 suite of
programs [22]. Geometry optimizations were performed using the
B3LYP functional using the standard double-z quality plus polariza-
tion functions 6–31G(d,p) for all atoms but Nb and Cs, which were
treated with the Los Alamos ECP [23]. Reactants and intermediates
were characterized by frequency calculations, and have positive
definite Hessian matrices. Transition-state structures (TS’s) show
only one negative eigenvalue in their diagonalized force-constant
matrices, and their associated eigenvectors were confirmed to
correspond to the motion along the reaction coordinate under con-
sideration using the intrinsic reaction coordinate (IRC) method
[24]. Vibrational frequencies were also performed to extract vibra-
tional zero-point and thermal corrections from the thermodynamic
results. Single-point calculations on the optimized geometries were
performed with the dispersion-corrected M06 functional [25] with
the expanded 6–311G(2d,p) basis sets to estimate more rigorous
values.
2.4. Catalytic performance
The reactions were performed in a liquid phase under atmo-
spheric pressure in a multi-experiment work station StarFish
(Radley’s Discovery Technologies UK).
In a typical procedure, 2-hydroxybenzaldehyde (2) (2 mmol), and
ethyl cyanoacetate (3) (4 mmol) were added to the 3-necked vessel
of 10 mL of capacity, equipped with condenser and thermometer
and magnetic stirred (0.8 cm). Subsequently, the catalyst (50 mg)
was added and the reaction mixture was stirred (250 rpm) at room
temperature, 298 K, during the appropriated reaction time shown
in figures or tables. The samples were taken at different reac-
tion times by diluting a small amount of the reaction mixture in
dichloromethane (0.5 mL). Subsequently, the catalyst was filtered
off and the solvent evaporated in vacuo.
In accordance with the basis of our accumulated observations
on these materials, the selected models for the calculations are
depicted in Chart 1.
3. Results and discussion
Activation of the catalyst samples was carried out at 333 K
overnight.
3.1. Nb loading and structure/texture characterization of MCF
materials
The reactions were followed by TLC chromatography performed
on DC-Aulofolien/Kieselgel 60 F245 (Merck) using mixtures of
CH2Cl2/ EtOH in 98:2 as eluent.
The procedure of MCF synthesis originally proposed by Stucky
et al. [21a] was modified in this work by the addition of nio-
bium source at the end of the synthesis route [21c]. Two various
TMB/PL ratios were applied (TMB/PL = 1.5 and 0.75) depending on
The characterization of the reaction products was carried out by
1H NMR. NMR spectra were recorded with a Bruker AVANCE DPX-
Please cite this article in press as: A. Smuszkiewicz, et al., Mesoporous niobiosilicate NbMCF modified with alkali metals in the synthesis
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223
256
4
NbMCF(ox)
Li/NbMCF(ox)
Na/NbMCF(ox)
K/NbMCF(ox)
Rb/NbMCF(ox)
Cs/NbMCF(ox)
226
3
2
1
0
254
200
300
400
500
600
Wavelength, nm
3
2
1
0
268
261
NbMCF(Et)
Li/NbMCF(Et)
Na/NbMCF(Et)
K/NbMCF(Et)
Rb/NbMCF(Et)
Cs/NbMCF(Et)
227
Fig. 1. Nitrogen adsorption isotherms of NbMCF supports.
the source of Nb used (ammonium niobate(V) oxalate hydrate or
niobium ethoxide). In the case of NbMCF(Et), niobium ethoxide
produces additional ethanol during the synthesis (this compound
is also released during hydrolysis of TEOS). The higher concentra-
tion of ethanol enhances the transformation of cylindrical SBA-15
towards the spherical MCF phase. The lower TMB/PL ratio was used
in this synthesis because the higher value of this ratio does not allow
obtaining the proper material.
200
300
400
500
600
Wavelength, nm
Fig. 2. UV–vis spectra of NbMCF catalysts before and after the modification with
alkali metals.
The loading of niobium in NbMCF materials was estimated on
the basis of XRF data and it is lower than assumed (real Si/Nb ratio
is 260 for NbMCF(ox) and 169 for NbMCF(Et)). The weight percent
of Nb in NbMCF(ox) is significantly lower (0.5 0.04 wt.%) than
in NbMCF(Et) (0.8 0.03 wt.%). It has been reported in the litera-
ture, that the introduction of niobium from Nb(V) oxalate under
acid conditions in the synthesis typically gives materials with low
niobium concentration [26].
surface area of 250 m2/g. Textural parameters calculated from N2
adsorption/desorption isotherms are shown in Table 1.
The morphology of NbMCF supports is illustrated by SEM images
(Fig. S1—Supplementary data). All materials show a coral-type mor-
phology characteristic of MCF materials.
The N2 adsorption/desorption isotherms of MCF and NbMCF
supports shown in Fig. 1 are typical of mesostructured cellular
foams of MCF materials [28a] exhibiting a H1 type hysteresis loop
at a relatively high pressure (p/p0) with parallel adsorption and
desorption branches for MCF and NbMCF(ox).
The nitrogen adsorption/desorption isotherm of NbMCF(Et) is
slightly deformed in the desorption branch at the high p/p0 ratio.
The shape of isotherms does not change significantly for sam-
ples after modification with alkali metals with the exception of
Cs/NbMCF(ox).
The texture parameters of NbMCF supports significantly change
after their modification by impregnation with alkali metals
(Table 1). The samples containing alkali metals exhibit much
lower surface area than the unmodified ones. BET surface area is
strongly dependent on the metal loading (expressed in mol/m2g−1
in Table 1) and it decreases with the growth of the alkali metal
incorporated. Pore volume is changed in the same order. The nature
of the alkali metal and the method of NbMCF preparation (ox
or Et niobium source) are not crucial for the BET surface area
value. As example for the role of metal loading, K/NbMCF(ox)
(Me loading 2.02 mol/m2g−1) shows BET surface area 248 m2/g
whereas Rb/NbMCF(Et) (Me loading 2.00 mol/m2g−1) exhibiting
3.2. Niobium state in MCF materials
The location of niobium introduced during the synthesis of MCF
material (NbMCF) was explained by metal coordination studied
using UV–vis technique. UV–vis spectra of both series of Nb-
containing catalysts (Fig. 2A and B) show two UV bands at 223
and 256 nm for NbMCF(ox) and 227 and 268 nm for NbMCF(Et)
from low coordinated (tetra and penta) niobium species localized in
the framework of MCF [21c,27]. These two bands can be assigned
to the electron charge transfer typical of niobium in the frame-
work positions in different surroundings [21c]. This diversity of
surroundings comes from the structure of MCF (windows and cells)
and the possible non-uniform distribution of niobium. However, it
is important to note, that the relative intensity of the UV–vis bands
varies depending on the NbMCF series. The lack of the UV–vis bands
at ∼330 nm characteristic of octahedrally coordinated Nb proves
the absence of extra framework niobium species on the surface of
all samples studied. It is important to stress that the small variation
in the position of the bands in UV–vis spectra of NbMCF materials
after modification indicates the interaction of alkali metal species
with niobium located in the supports.
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The higher shift of the band position is noted for the series of
catalysts based on NbMCF(Et). In this series the intensity of UV–vis
band at 268 nm corresponding to the charge transfer in pentacoor-
dinated oxo-niobium species is much higher than that at ca 227 nm
coming from charge transfer in tetracoordinated niobium. It is not
a case of NbMCF(ox) series where both UV–vis bands show almost
the same intensity. It is a significant difference in the both series of
catalysts. Recently calculations [28] have indicated that this pen-
tacoordinated niobium species, being one of the most stable, is
tetra-anchored to the silica structure and the fifth coordination is
with hydroxyl group – potentially catalytic active centers. Anchor-
ing of Nb-species via ethoxy groups present in Nb-source (Et) is
much easier by interaction with silanol groups than via oxalate
ligands in Nb-source(ox). Therefore, in the case of NbMCF(Et) tetra-
anchored pentacoordinated niobium species dominate.
O
DMF
H+
O
O
O
OH-
MCP
Scheme 2. 2,5-Hexanedione transformation in acid and in basic medium.
A
2000
NbMCF(ox)
NbMCF(Et)
1500
3.3. Acid-base properties of catalysts—test reactions
The presence of Nb in mesoporous structure of MCF influences
the basic properties of the alkali modified samples. Two types of
reactions have been tested in this work to study the acidity/basicity
of the surface of the catalysts prepared: 2-propanol decomposition
and 2,5-hexanedione cyclisation and dehydration.
1000
500
0
3.3.1. Propanol (2-PrOH) decomposition
-
Na
Rb
K
Cs
Li
The 2-propanol decomposition is a test reaction for characteri-
sation of acidic (Brønsted or Lewis) and/or basic properties of solids
[29]. Dehydration of alcohol to propene and/or diisopropyl ether
requires acidic centers (Lewis or Brønsted), whereas the dehydro-
genation to acetone occurs on Lewis basic sites.
B
100
80
60
40
20
0
As shown in Table 2, over both Nb-containing matrices
(NbMCF(ox), NbMCF(Et)) the main reaction product of 2-propanol
decomposition, at 523 and 573 K, is propene, so these results indi-
cate the acidic character of these supports. The activity of MCF is
below 1% of 2-propanol conversion. The activity of MCF increases
when Nb is introduced to the MCF framework (Table 2) and depends
on the preparation procedure (ox or Et used as Nb source). It is
clear that more niobium species (Lewis acid sites) incorporated
from ethoxide give rise to the higher acidic activity in 2-propanol
reaction. Moreover, one can suppose that hydroxyls present in
pentacoordinated niobium species are also responsible for this
activity. Interestingly, NbMCF(ox) shows not only dehydration but
also dehydrogenation activity indicating that this support contains
also Lewis basic sites responsible for transformation of 2-propanol
to acetone.
The impregnation of acidic NbMCF supports with alkali metal
acetates decreases the activity of the catalyst towards 2-PrOH dehy-
dration/dehydrogenation below 1% of conversion. As a result of the
poisoning of the supports acidic centers by alkali metals, the cata-
lysts based on niobiosilicates are not active. Therefore the results
of 2-propanol decomposition on alkali modified materials are not
considered in the discussion.
NbMCF(ox)
NbMCF(Et)
-
Na
Rb
K
Cs
Li
Fig. 3. (A) Dependence of the MCP/DMF selectivity ratio on the nature alkali
metals located on the surface of NbMCF(ox) and NbMCF(Et); (B) conversion of 2,5-
hexanedione.
The conversion of 2,5-hexanedione and selectivity ratio
MCP/DMF are shown in Fig. 3. The conversion of 2,5-hexanedione
depends strongly on the composition of the matrix. The activity is
higher for NbMCF(ox) than NbMCF(Et). The sequence of conver-
sion for NbMCF supports differs from that of 2-propanol reaction.
However, one should remember that the mechanisms of both pro-
cesses are different. In 2-propanol conversion a large variety of
reaction pathways is possible involving both types of acid (Brønsted
and Lewis) and Lewis base centers. In 2,5-hexanedione cyclisation
Brønsted acid and/or base centers are required.
3.3.2. 2,5-Hexanedione cyclisation and dehydration
The cyclisation and dehydration of 2,5-hexanedione has been
used as a test reaction for Brønsted basicity/acidity properties
of the catalysts [30]. The formation of 2,5-dimethylfuran (DMF)
occurs on acidic centers, whereas in the production of 3-methyl-
2-cyclopentenone (MCP) basic centers take part (Scheme 2). On
the basis of the ratio of selectivity to MCP/selectivity to DMF, the
sequence of the basicity of the prepared catalysts can be estimated.
According to the literature [30,31] the basicity of the catalyst is
stated if MCP/DMF » 1. When MCP/DMF « 1 the catalyst exhibits
acidic properties, while for MCP/DMF ∼ 1 the acid–base character
of catalysts is postulated.
NbMCF(Et) and NbMCF(ox) materials exhibit acidic properties
(MCP/DMF « 1).
The effect of the alkali metal on the activity and selectivity of cat-
alysts is clearly visible. The decrease of activity of NbMCF supported
materials is observed. It indicates that impregnation procedure
blocks most of the Brønsted acid centers present on the surfaces of
NbMCF(ox) and NbMCF(Et). The introduction of alkali metals gen-
erates Brønsted basic centers—the selectivity to MCF significantly
increases for all modified catalysts studied. It can be observed a
volcanic dependence of MCP/DMF selectivity ratio vs the nature of
alkali metal element for NbMCF(ox) supported catalysts (Fig. 3A),
Please cite this article in press as: A. Smuszkiewicz, et al., Mesoporous niobiosilicate NbMCF modified with alkali metals in the synthesis
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Table 2
The results of 2-propanol decomposition.
Catalyst
MCF
Temperature [K]
2-PrOH conv. [%]
Propene sel. [%]
Acetone sel. [%]
523
573
523
573
523
573
<1
<1
1
7
18
59
100
100
85
97
100
100
–
–
15
3
–
NbMCF(ox)
NbMCF(Et)
–
100
80
60
40
20
0
on which the highest MCP/DMF ratio was noted for sample con-
taining rubidium. It is not the case of NbMCF(Et) series. For this
type of catalysts the MCP/DMF ratio systematically increases from
Li to Cs modified material. Moreover, NbMCF(Et) based catalysts
are characterized by the lowest Brønsted basicity. However, it is
important to stress that for both series of catalysts the highest activ-
ity in the cyclisation and dehydration of 2,5-hexanedione is noted
for samples modified with potassium (Fig. 3B). Taking into account
the MCP/DMF ratio for potassium modified matrices, the following
order is observed: K/NbMCF(ox) > K/NbMCF(Et). The higher acidic
activity of the support the lower basic activity of potassium con-
taining catalysts is achieved.
NbMCF (ox)
NbMCF (Et)
A)
Li
Na
K
Rb
Cs
3.4. Catalytic performance
100
NbMCF (ox)
NbMCF (Et)
B)
MCF catalysts were tested in the reaction of 2-
hydroxybenzaldehyde (2a) and ethyl cyanoacetate (3), at room
temperature, under solvent-free conditions (Scheme 3). Firstly,
we carried out the reaction in the presence of MCF and both
NbMCF supports (ox and Et series). It should be noted that only
in the presence of NbMCF(ox) a mixture chromenes 4a and 5a
were obtained in almost 5% of yield after 4 h of reaction time.
These preliminary results suggest that the environment provided
by hydroxyl groups and Nb present in the investigated supports,
exhibiting very different acid-base properties, are not probably
involved in the reaction or they are barely able to activate the
first step of the reaction, that is, the aldolic condensation between
reagents. Remarkably, the blank experiment under the same
experimental conditions did not produce any reaction product
recovering the unaltered starting reagents.
80
60
40
20
0
Li
Na
K
Rb
Cs
Fig. 4. Yields of chromenes 4a and 5a from 2-hydroxybenzaldehyde (2a) and ethyl
cyanoacetate (3), at room temperature, under solvent-free conditions, catalyzed by
Me/NbMCF materials, (A) after 60 min and (B) after 120 min of reaction times.
The modification of the NbMCF supports with alkaline met-
als yielded, in all the investigated cases, diastereomeric mixtures
(Erythro/Threo isomers) of the chromenes 4a and 5a. Fig. 4 shows
the yield of chromenes 4a and 5a in the reaction catalyzed by
Me/NbMCF materials, after 60 min (Fig. 4A) and 120 min of reac-
tion times (Fig. 4B). It can be observed a different catalytic behavior
for the modified NbMCF materials as function of the used NbMCF
support, as expected. The yield of chromenes 4a and 5a in the
presence of Me/NbMCF(Et) decreases when the cation size of the
alkaline metal is increased. Similar trend was observed when using
the NbMCF(ox) series, Li/NbMCF being an exception. It is impor-
tant to note that the best yields of compounds 4a and 5a were
obtained when the reaction was catalyzed by Na or K/NbMCF(ox)
after 60 min of reaction time (Fig. 4A), although similar conversion
were observed when the reaction was carried out in the presence
of Li/NbMCF(Et) after 120 min.
These results were rationalized in terms of metal loading on the
corresponding NbMCF series but also regarding the acid-base prop-
erties of the corresponding catalysts. Concerning to the alkaline
metal content, Na or K/NbMCF(ox) showed higher metal load-
ing (expressed in mol/m2g−1 × 10−6, Table 1) as compared with
their Et series homologues. This fact is also demonstrated for the
Li/NbMCF(Et) catalyst. In other sense, it is clear the influence of the
alkaline metal in the reaction since other Me/NbMCF (ox or Et) with
higher metal loadings afforded mixtures of compounds 4a and 5a
with lower yields (Table 1 and Fig. 4).
The results seem to indicate that the Brønsted basic centers cre-
ated by the modification of NbMCF(ox) with alkaline metals could
be the responsible of the observed reactivity. Also, the reaction is
catalyzed by Lewis basic centers, although in lower extent. This
circumstance is likewise demonstrated by the observed lower cat-
alytic activity for the NbMCF(ox) support and Me/NbMCF(Et), the
Et series characterized by exhibiting lower Brønsted basicity.
The yields of compounds 4a and 5a, at different reaction times,
when the reaction is catalyzed by Me/NbMCF(ox) are shown
in Fig. 5. As can be observed, the best conversion values were
obtained in the presence of Na or K/NbMCF(ox) affording mix-
tures of compounds 4a and 5a with almost quantitative yields after
5 h of reaction time. The major difference with their Me/NbMCF
analogues is at shorter reaction times. Taken into account that
Rb/NbMCF(ox) was the most basic catalyst, it then seems that
the optimum Brønsted basicity is required to obtain the highest
conversion values to chromenes 4a and 5a and it is provided by
modification of NbMCF(ox) with Na or K acetates.
Having these results in mind, it seems reasonable to compare the
catalytic behavior of the most efficient catalysts, Me/NbMCF (where
Me is Na or K) with the analogues alkali metal oxides supported
over MCF pure silica. The reaction under the same experimental
conditions, in the presence of Na/MCF or K/MCF, yielded mixtures
of chromenes 4a/5a in 58 and 47%, after 1 h of reaction time, respec-
tively; these yields are considerably lower than that obtained for
Please cite this article in press as: A. Smuszkiewicz, et al., Mesoporous niobiosilicate NbMCF modified with alkali metals in the synthesis
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Scheme 3. Synthesis of 2-amino-4H-chromenes (4 and 5) from 2-hydroxybenzaldehyde (2a) and ethyl cyanoacetate (3).
Li/NbMCF
100
Na/NbMCF
K/NbMCF
Rb/NbMCF
Cs/NbMCF
Na/NbMCF(ox)
Rb/NbMCF(ox)
100
80
60
40
20
0
80
60
40
20
0
0
50
100
150
Time (min)
200
250
300
350
0
60
120
180
240
300
Time (min)
Fig. 7. Variation of selectivity to 4a vs time for the reaction between 2-
hydroxybenzaldehyde (2a) and ethyl cyanoacetate (3), at room temperature, under
solvent-free conditions, catalyzed by Me/NbMCF materials (where Me is Na or Rb).
Fig. 5. Plot of yields of mixtures of chromenes 4a and 5a vs time, for the reaction
between 2-hydroxybenzaldehyde (2a) and ethyl cyanoacetate (3), at room temper-
ature, under solvent-free conditions, catalyzed by Me/NbMCF(ox) materials.
Table 3
Synthesis of chromenes
hydroxybenzaldehydes (2) and ethyl cyanoacetate (3) catalyzed by Na/NbMCF.
NbMCF(ox)
NbMCF(Et)
100
80
60
40
20
0
4
and
5
from differently substituted 2-
R
Time (min)
Yield (%)a
Selectivity to 4
H
MeO
Br
NO2
Cl
240
300
300
240
60
93 (72)
87 (44)
72 (41)
95 (19)
82
65
64
71
75
70
a
Conversion values in parenthesis after 60 min of reaction time.
Li
Na
K
Rb
Cs
Fig. 6. Selectivity to chromene 4a, for the reaction between 2-hydroxybenzaldehyde
(2a) and ethyl cyanoacetate (3), at room temperature, under solvent-free conditions,
catalyzed by Me/NbMCF materials, after 300 min of reaction time.
5 h of reaction time) and also decreasing the estereoselectivity to
compound 4a.
The scope of the methodology was investigated by starting from
differently 5-substituted-2-hydroxybenzaldehydes (2) in the pres-
ence of one of the most active catalyst, Na/MCF(ox) (Table 3). In
this sense, cromenes 4 and 5 were efficiently synthetized with
good to excellent yields. The reactivity order of 5-substituted-2-
hydroxybenzaldehydes 2 was Cl > H > MeO, Br > NO2. Taken into
account the experimental results, it can be concluded that the sub-
stitution at position 5− has a negative effect on the yield values
obtained after 60 min of reaction times. However, the presence of
the Cl atom at position 5− produces a notable increasing of con-
version values to products 4 and 5. In general, the presence of
electro-withdrawing substituents at position 5− induces a slightly
increasing of the selectivity to the major isomer 4 due probably to
confinement effects.
Finally, the Table 4 summarizes the experimental conditions and
results as comparison of the performance of the catalyst Na/NbMCF
with respect to the previous literature. We report herein a new
methodology for the synthesis of chromenes which uses the lowest
catalyst amount, in absence of any solvent, operating at room tem-
perature, during relatively short reaction times affording mixtures
of chromenes 4 and 5 with good to excellent yields.
Me/NbMCF. Therefore, Nb in the Me/NbMCF should be probably
involved in the reaction favoring the process.
In general, chromenes 4a and 5a were obtained in approxi-
mately 2:1 ratio, the regioselectivity remaining constant along the
reaction course. However, selectivity to compound 4a, the most
stable diastereomer, was notably increased when using Na and
Rb/NbMCF(ox), 95 and 88, respectively (Fig. 6). Firstly, it could be
due to the confinement effects derived from the smallest pore vol-
ume of these materials, 1.3 and 1.2 cm3/g, respectively. In Fig. 7 it
is shown the variation of selectivity to chromene 4a when using
Na and Rb/NbMCF(ox) catalysts. As can be observed, the selectivity
remains constant at the shortest reaction times as expected. How-
ever, an increase of the selectivity to 4a is produced at the highest
reaction times. These results indicate that in the presence of these
catalysts, another competitive reaction, consisting of the epimer-
ization reaction of the isomer 5a into 4a, is probably produced.
We also performed the reaction in the presence of small amount
(25 mg) of the best catalytic material, Na/NbMCF(ox), yielding a
mixture of chromenes 4a and 5a with lower conversion (66% after
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Table 4
Heterogeneous catalysts involved in the synthesis of chromenes 4 and 5.
Catalyst
HB/ECA
Catalyst amount (g)
Solvent
T (◦C)
Time (h)
Yield (%)
.
Amberlyst-21
Substituted 2-hydroxybenzaldehydes 5/10
2-hydroxybenzaldehyde 5/10
Substituted 2-hydroxybenzaldehydes 1/2
2-Hydroxybenzaldehyde 1/2
Substituted 2-hydroxybenzaldehydes 1/2
2-Hydroxybenzaldehyde 1/2
Substituted 2-hydroxybenzaldehydes 10/22
5-Bromo-2-hydroxybenzaldehyde 10/22
2
2
EtOH
EtOH
–
–
–
r.t.
r.t.
2–5
4
1–24
18 (1)
20–24
24
14
14
14
14
68–94
85
83–94
98 (94)
83–98
88
46–86
56
50
[18]
[18]
[19]
[19]
[19]
[19]
[20]
[20]
[20]
[20]
This work
This work
SnMgAl-1
MgAl
0.05
0.05
0.05
0.05
3
3
3
3
0.05
0.05
40 ◦
40 ◦
40 ◦
40 ◦
r.t.
C
C
C
C
–
Molecular sieve 3A
Molecular sieve 4A
Molecular sieve 5A
Al2O3
EtOH
EtOH
EtOH
EtOH
–
r.t.
r.t.
r.t.
r.t.
63
72–95
70
Na/NbMCF
Substituted 2-hydroxybenzaldehydes 2/4
2-Hydroxybenzaldehyde 2/4
1–4
1
–
r.t.
Fig. 8. Optimized structures for the aldolization step catalyzed by Na/NbMCF.
HB/ECA: 2-hydroxybenzaldehydes (mmol)/ethyl cyanoacetate
(mmol).
could promote the approaching of the reactants by interaction with
the cyanocetate.
Our calculations reveal that the activation of the ethyl cyanoc-
etate 3 by the alkaline oxide is a highly exothermic step and
proceeds without activation barrier, as expected from such strong
bases. The computed free energies of this activation are −39.5,
−82.5 and −94.3 kcal/mol for Li, Na and Cs, respectively.
Once the nucleophile has been activated, the interaction with
the Nb supported structure leads to the reactant complex R (Fig. 8),
where the hydroxylic substituent forms a strong hydrogen-bond
´
3.5. Computational study
Initially, we considered the model A in our calculations (Chart 1).
Thus, it could be envisaged that the alkaline center could activate
the ethyl cyanocetate 3 to form the C C bond with salicylaldehyde
2a. However, the postulated reaction mode did not lead to a pro-
ductive reaction, in part due to the lack of a proper stabilization of
the negative charge developed in the aldolic intermediate.
On other hand, if we assume the model B, we can propose that
the basicity of the alkaline oxide is strong enough to easily activate
the ethyl cyanocetate 3, by abstraction of the methylenic proton, to
form the reactive enolate. Under these conditions, the hydroxylic
moiety of the catalyst can act as a Brønsted acid site, thus stabilizing
the forming aldol transition structure. In addition, the Nb center
˚
with the aldehyde carbonyl (1.661, 1.675 and 1.723 A, for Li, Na
and Cs). Moreover, the alkaline center interacts with two methoxy
moieties of the silica support, and the Nb interacts with the cyano
´
˚
group (2.275, 2.276 and 2.286 A, for Li, Na and Cs). These inter-
actions promote the reactants approaching to generate the aldol
intermediate. It should be noted that this reactant complex, R, is
more stable than the isolated reactants for the reaction involving Li
Please cite this article in press as: A. Smuszkiewicz, et al., Mesoporous niobiosilicate NbMCF modified with alkali metals in the synthesis
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9
and Na (−16.9 and −7.8 kcal/mol, respectively) while the presence
of Cs leads to a less stable complex (5.2 kcal/mol above the isolated
reactants), likely due to its bulkiness.
Acknowledgements
This work has been supported in part by MICINN (projects
and CTQ2011-27935 and CTM2014-56668-R) and National Sci-
ence Centre in Poland (Project No. 2014/15/B/ST5/00167). We are
grateful to the Centro de Supercomputación de Galicia (CESGA) for
generous allocation of computing resources.
The formation of the C C bond in the transition state, TS, is more
advanced in the presence of Cs, as suggest the forming C C (Li:
´
˚
2.172, Na: 2.179, Cs: 2.138 A) whereas the O H bond distance fol-
´
˚
lows the opposite trend (Li: 1.085, Na: 1.112, Cs: 1.171 A) which
indicates a more asynchronous structure for the smaller alkaline
metals. The activation barrier to reach the transition structure fol-
lows the order Cs (10.5) < Na (12.2) < Li (15.6 kcal/mol).
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.cattod.2016.02.
042.
TS leads to the aldol intermediate, I, displaying the full formation
of the C C and O H bonds. This structure shows notable differences
depending of the alkaline center. Thus, the interaction between the
Li and the methoxy moieties, on one hand, and the aldolic hydroxyl
with the new sp2 oxygen of the support, on the other hand, drives to
a less stable complex than the reactant species R (by 3.4 kcal/mol).
Conversely, for the aldolization involving Na and Cs centers, the
aldolic hydroxyl moiety is displaced by the alkaline which thus
interacts with the support deprotonated oxygen. In these cases, the
reaction step is exothermic, being I more stable than the reactant
complex R (−5.4 and −11.3 kcal/mol, for Na and Cs, respectively).
In summary, the strong basicity of the alkaline oxides activates
the formation of the nucleophile. These results agree with the
experimental observations since they revealed that the reaction
is probably promoted by Brønsted basic sites and the reactivity
is increased in the presence of the alkaline oxides. However, the
bulkiness of the alkaline center has a deep impact on the forma-
tion and stability of the reactant complex, which is obstructed in
the presence of the bulkier centers thus restricting the effective
aldolization, in agreement with the reactivity found in the presence
of the most basic materials.
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The computational study suggests that the strong basicity of
alkaline oxides initiates the reaction by activation of the reactants.
However, the formation of the reactant complex and effective reac-
tion requires a compromise between this basicity and the alkaline
center size.
Please cite this article in press as: A. Smuszkiewicz, et al., Mesoporous niobiosilicate NbMCF modified with alkali metals in the synthesis
of chromene derivatives, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.02.042

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Please cite this article in press as: A. Smuszkiewicz, et al., Mesoporous niobiosilicate NbMCF modified with alkali metals in the synthesis
of chromene derivatives, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.02.042