Synthesis, characterization and activity of homogeneous and heterogeneous (SiO2, NaY, MCM-41) iron(III) catalysts on cyclohexane and cyclohexene oxidation

Article abstract of DOI:10.1016/j.apcata.2015.09.035

This work deals with the synthesis and characterization of two homogeneous and three heterogeneous iron(III) catalysts, which were evaluated in the oxidation of cyclohexane and cyclohexene, using H₂O₂ as oxidant. The homogeneous catalyst [Fe₂(BPA)₂(μ-OCH₃)₂(Cl)₂], (1), where BPA is the deprotonated form of N-(2-hydroxybenzyl)-N-(pyridin-2-ylmethyl) amine, is a dinuclear iron(III) compound, as determined by X-ray diffraction studies. Compound [Fe^III(HBPCNOL)(Cl)₂], (2), where HBPCNOL is N-(2-hydroxybenzyl)-N-(2-pyridylmethyl)(3-chloro)(2-hydroxy) propylamine, is a mononuclear iron(III) compound. The reaction of the ligand HBPA with the 3-glycidoxypropyltrimethoxysilane molecule followed by the reaction with different inorganic matrices (silica-gel, NaY zeolite, MCM-41) resulted in the organo functionalized solids SiL, NaYL and MCM-41L, respectively. These materials were reacted with iron(III) salt, affording the heterogeneous catalysts SiLFe, NaYLFe and MCM-41LFe, respectively. They were characterized by elemental analyses, HR-CS AAS, solid state MAS NMR (²⁹Si, ¹³C), IR, UV-vis, TGA, Mossbauer, and textural analyses. The major product formed in all the oxidation reactions was the hydroperoxide derivative. When cyclohexane was the substrate, the homogeneous catalysts were more efficient than the heterogeneous ones. In contrast, the heterogeneous systems showed better results with cyclohexene than with cyclohexane, reaching about 30% in the presence of NaYLFe.

Full text of DOI:10.1016/j.apcata.2015.09.035

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Applied Catalysis A: General 507 (2015) 119–129
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Synthesis, characterization and activity of homogeneous and
heterogeneous (SiO2, NaY, MCM-41) iron(III) catalysts on cyclohexane
and cyclohexene oxidation
Paula M.A. Machadoa, Leonardo M. Lubea,b, Marcione D.E. Tiradentesa,
Christiane Fernandesa, Clícia A. Gomesa, Alexandre M. Stumboa, Rosane A.S. San Gilc,
Lorenzo C. Visentind, Dalber R. Sancheze, Vera L.A. Frescuraf, Jessee S.A. Silvaf,
Adolfo Horn Jr.a,∗
a Universidade Estadual do Norte Fluminense Darcy Ribeiro, Laboratório de Ciências Químicas, Campos dos Goytacazes, RJ 28013-602, Brazil
b Instituto Federal Fluminense, Campus Centro, Campos dos Goytacazes, RJ 28013-602, Brazil
c Universidade Federal do Rio de Janeiro, Instituto de Química, Laboratório de RMN de Sólidos, Rio de Janeiro, RJ 21941-901, Brazil
d R&D NanoBusiness e-Diffraction Pharma, Rio de Janeiro, RJ 22451-900, Brazil
e Universidade Federal Fluminense, Instituto de Física, Campus da Praia Vermelha, Niterói, RJ 24210-346, Brazil
f
Universidade Federal de Santa Catarina, Departamento de Química, Florianópolis, SC 88040-900, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 June 2015
Received in revised form 31 August 2015
Accepted 23 September 2015
Available online 30 September 2015
This work deals with the synthesis and characterization of two homogeneous and three heteroge-
neous iron(III) catalysts, which were evaluated in the oxidation of cyclohexane and cyclohexene, using
H2O2 as oxidant. The homogeneous catalyst [Fe2(BPA)2(␮-OCH3)2(Cl)2], (1), where BPA is the deproto-
nated form of N-(2-hydroxybenzyl)-N-(pyridin-2-ylmethyl) amine, is a dinuclear iron(III) compound, as
determined by X-ray diffraction studies. Compound [FeIII(HBPCꢀNOL)(Cl)2], (2), where HBPCꢀNOL is N-
(2-hydroxybenzyl)-N-(2-pyridylmethyl)(3-chloro)(2-hydroxy) propylamine, is a mononuclear iron(III)
compound. The reaction of the ligand HBPA with the 3-glycidoxypropyltrimethoxysilane molecule fol-
lowed by the reaction with different inorganic matrices (silica-gel, NaY zeolite, MCM-41) resulted in
the organo functionalized solids SiL, NaYL and MCM-41L, respectively. These materials were reacted
with iron(III) salt, affording the heterogeneous catalysts SiLFe, NaYLFe and MCM-41LFe, respectively.
They were characterized by elemental analyses, HR-CS AAS, solid state MAS NMR (29Si, 13C), IR, UV–vis,
TGA, Mossbauer, and textural analyses. The major product formed in all the oxidation reactions was the
hydroperoxide derivative. When cyclohexane was the substrate, the homogeneous catalysts were more
efficient than the heterogeneous ones. In contrast, the heterogeneous systems showed better results with
cyclohexene than with cyclohexane, reaching about 30% in the presence of NaYLFe.
Keywords:
Cyclohexane oxidation
Cyclohexene oxidation
Grafting
Heterogeneous catalysis
Homogeneous catalysis
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
of cyclohexanone/cyclohexanol, phenol and methanol from cyclo-
hexane, benzene and methane, respectively, are among the top 10
Selective oxidation of unsaturated hydrocarbons is a challeng-
ing reaction due to the similarity in the electronegativity of the
carbon and hydrogen atoms [1]. In this context, the development
of catalysts that may operate at low temperature and pressure
for the transformation of relatively cheap and available substrates
into valuable functionalized products has attracted the attention
from both academy and industry [2]. For example, the production
challenges of the modern chemistry [3].
Concerning alkanes, cyclohexane oxidation is particularly inter-
esting in nylon 6 and nylon 6,6 polymers manufacturing, since
the oxidation products, cyclohexanol and cyclohexanone (KA oil),
are the precursors of adipic acid and caprolactam, obtained by
subsequent oxidation using nitric acid [4]. Furthermore, cyclohex-
anol and cyclohexanone are applied in the production of detergent
emulsions, plasticizers, herbicides, insecticides and pharmaceuti-
cals [5].
Currently, the industrial oxidation of cyclohexane employs
homogeneous cobalt catalyst and molecular oxygen at a tempera-
∗
Corresponding author.
E-mail address: adolfo@uenf.br (A. Horn Jr.).
http://dx.doi.org/10.1016/j.apcata.2015.09.035
0926-860X/© 2015 Elsevier B.V. All rights reserved.

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
120
P.M.A. Machado et al. / Applied Catalysis A: General 507 (2015) 119–129
H
CH3
Cl
Fe
N
O
O
N
O
Fe
Cl
O
N
N
H3C
H
FeCl3.6H2O
LiOH
(1)
Route 1
MeOH
H
N
HBPA =
Route 2
N
OH
Route 3
O
1. GPTMS, MeOH, 72h, 323 K
2. SiO2 or NaY or MCM-41, toluene, 72h, 383 K
Cl
MeOH, 2 h, 298 K
O
O
O
OH
N
OH
OH
N
Si
O
N
= H2BPClNOL
Cl
N
OH
(SiL or NaYL or MCM-41L)
FeCl3.6H2O
i-PrOH
FeCl3.6H2O
MeOH
O
O
O
N
Cl
N
Si
O
N
N
OH
Cl
Fe
O
OH
X
Fe
O
Cl
X
X = Cl or O
(2)
(SiLFe or NaYLFe or MCM-41LFe)
Fig. 1. Scheme of syntheses for the catalysts (1) (Route 1), (2) (Route 2) and SiLFe, NaYLFe or MCM-41LFe (Route 3).
ture between 423 and 433 K and pressure about 0.9 MPa [6]. Despite
these drastic conditions, the process obtains about 4% yield and
selectivity around 78% for cyclohexanol [1]. This reveals that there
is room for the development of more efficient and low energy cat-
alytic processes.
Concerning alkenes, their oxidation reaction is also of relevance
because it yields a lot of applicable products like alcohols, ketones,
aldehydes, acids and epoxides, which are largely used in food,
chemical and pharmaceutical industries [7].
There are different approaches in the literature devoted to the
development of catalysts aiming hydrocarbon oxidation, includ-
ing both homogeneous and heterogeneous processes. Some of
them are inspired in biological systems, like cytochrome P-450 and
methanemonoxygenase enzymes [8–11]. These iron metalloen-
zymes are oxygenases, able to catalyze the stereospecific oxidation
of hydrocarbons under mild conditions, using electron transfer
reactions and oxygen transport [12]. These oxygenases generally
use molecular oxygen as oxidant to introduce one or two oxy-
gen atoms into their substrates [13]. Concerning cytochrome P450,
it is a family of heme protein that can catalyze oxidation reac-
tions like hydroxylation and epoxidation of numerous alkane and
alkene substrates [14]. On the other hand, iron methanemonoxyge-
nase is a non heme enzyme which converts methane to methanol,
into methanotrofic bacteria metabolism [15,16]. Other oxygenases
also can oxidize alkenes like toluene ortho-monooxygenase, that
convert substrates like toluene, naphthalene and trichloroethy-
lene [13], and Rieske dioxygenases, a class of enzymes able to
cis-hydroxylation of aromatic hydrocarbons [17].
The biotransformation, under mild conditions, by these
enzymes is selective and efficient. This has called attention to the
development of bioinspired catalysts that try to reproduce the
active center of these enzymes and solve the oxidation mech-
anisms, as well [18]. However these enzymes have a limited
application in industries, since their isolation and purification are
very expensive, there are difficulties in stabilizing the biocatalyst in
the reaction medium, problems with product separation, recovery
and recycling [19].
Trying to overcome this drawback, iron complexes containing
N, O, S donor ligands have been used as bio-inspired catalysts for
hydrocarbon oxidation, since similar donor groups are present in
the active site of oxidases [20–26]. These systems are characterized
by working at room temperature and in the presence of oxidant
agents such as peroxides or iodosilbenzene.
Due to the satisfactory results obtained previously with the
bioinspired homogeneous catalyst [FeIII(HBPCꢀNOL)(Cl)2] (Fig. 1,
compound (2)) in the cyclohexane oxidation [27], we decided
to evaluate if the similar, but simpler, compound [Fe2(BPA)2(␮-
OCH3)2(Cl)2] would be more effective in the oxidation of
cyclohexane and cyclohexene than the former one. Furthermore,
the ligand H2BPCꢀNOL was immobilized in silica, NaY-zeolite and
MCM-41 matrices, resulting, after reacting with iron(III), in the
heterogeneized versions of the catalyst [FeIII(HBPCꢀNOL)(Cl)2],

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
P.M.A. Machado et al. / Applied Catalysis A: General 507 (2015) 119–129
121
allowing a direct comparison between the efficiency of homoge-
neous and heterogeneous systems.
ble sub-boiling distillation in a poly(tetrafluoroethylene) system
(Kürner Analysentechnik). Supra pure hydrogen peroxide 30–32%
(Vetec) was used as source of oxygen in promoting the digestion.
To accomplish the dissolution of the insoluble fluorides, boric acid
saturated solution (Vetec) 5.0% m/v was used, followed by a second
treatment in the microwave oven. Fe standard solution (SpecSol,
Jacarei, São Paulo) was used in the calibration curve, which range
was from 10.0 to 50 ␮g dm−3. An aliquot of about 20.0 mg of the
samples were weighed directly into the microwave Teflon vessels,
3.0 cm3 of nitric acid, 1.0 cm3 of hydrochloric acid and 1.0 cm3 of
hydrogen peroxide were added. The mixture was kept standing for
1 h to avoid strong reactions inside the instrument. After this time
0.5 cm3 of HF was added, the vessels were closed and submitted to
the microwave assisted program (Table S2). In this procedure the
power was controlled instead of the temperature. At the end of the
digestion and cooling steps, the vessels were opened and 4.0 cm3
of boric acid saturated solution (5.0% m/v) was added. The vessels
were closed and submitted to the same microwave assisted pro-
gram again. After the digestion procedure, the vessels were allowed
to achieve a temperature around 70 ◦C. They were then opened and
kept standing in the fume hood at room temperature. The samples
were transferred to 50.0 cm3 polyethylene tubes and filled up to a
final volume of 50 cm3 with high purity water. Before the quantifi-
cation, the samples were diluted 1000 times.
2. Experimental
2.1. Materials and methods
Reagents and solvents that were used in the syntheses were of
the high purity grade and were obtained from Sigma–Aldrich and
Synth. Cyclohexane and cyclohexene, used in the oxidation assays,
were distilled and stored under argon atmosphere. Hydrogen per-
oxide 30% v/v was obtained from Vetec and its concentration was
previously determined by titration. The solid matrices employed
in this study were silicagel-60 (35–70 mesh ASTM), NaY zeolite
(Fábrica Carioca de Catalisadores) and MCM-41 [28]. NaY zeolite
and silica were heated at a rate of 3 ◦C/min from room tempera-
ture to 350 ◦C, for 4 h, and were kept in a desiccator under reduced
pressure.
FTIR analyses were carried out with samples supported
in KBr pellets using a Shimadzu IRAffinity-1 spectrometer in
4000–400 cm−1 range. Full scan mass spectra (MS mode) were
obtained on
a MicroTOF LC Bruker Daltonics spectrometer
equipped with an electrospray source operating in positive and
negative ion mode. Sample was dissolved in a acetonitrile and
injected in the apparatus by direct infusion. Cyclic voltammetry was
carried out with an Autolab PGSTAT 10 potentiostat/galvanostat
in acetonitrile containing 0.1 mol dm−3 tetrabutylammonium per-
chlorate (TBAClO4) as the supporting electrolyte under argon
atmosphere at room temperature. The electrochemical cell
employed was a standard three-electrode configuration: a glassy
carbon working electrode, a platinum-wire auxiliary electrode
and a commercial Ag/AgCl electrode immersed in a salt bridge
containing 0.1 mol dm−3 TBAClO4. The formal potential of the
ferrocenium/ferrocene couple was 0.426 V vs the reference elec-
trode Ag/AgCl, being established as 0.400 V vs NHE. The redox
potential is given vs Fc/Fc+. UV–vis spectra were recorded in a Shi-
madzu spectrometer, model UV-1800, in the range from 200 to
1100 nm. The compound [Fe2(BPA)2(␮-OCH3)2(Cl)2] was analyzed
in CH3CN solution while the solid catalysts were analyzed quali-
tatively in mineral oil suspension. Elemental analyses (CHN) were
carried out by a CHNSO Analyzer model Flash 2000, from Thermo
Scientific. Termograms of heterogeneous catalysts and inorganic
supports were recorded by a SII TG/DTA 63000, model Exstar, from
Seiko. The analyses were performed under synthetic air flux of
100 cm3 min−1. The heating was started at 25–700 ◦C (10 ◦C/min)
and finished with range from 700 ◦C to 1000 ◦C (15 ◦C/min). Deter-
mination of Fe in the heterogeneous catalysts were carried out in
a high resolution continuum source atomic absorption spectrome-
ter (HR-CS AAS), model ContrAA 700 (Analityk Jena, Jena, Germany)
equipped with a graphite furnace atomizer with transverse heating.
Pyrolytically treated graphite tubes with pyrolytically integrated
platforms (Analytik Jena Part No. 407-A81.025) were used in all
measurements and the wavelength was 248.3270 nm. The oper-
ating conditions of the graphite furnace are shown in Table SI1,
as Supplementary information.The samples were treated in a
microwave oven model Ethos Plus, (Milestone, Sorisole, Italy).
Microbalance (Mettler, Toledo, Switzerland) was used on the sam-
ples mass measurements. Reagents used in this work were at
least of analytical grade purity. Water purified in a Milli-Q sys-
tem (Millipore, Bradford, MA, USA), to a resistivity of 18.2 Mꢁ cm
was used to prepare the samples and the solutions. Nitric acid
65% v/v (Carlo Erba Reagenti, Milan, Italy) and hydrochloric acid
37% v/v (Vetec Rio de Janeiro, Brazil), were purified by double
sub-boiling distillation in a quartz still (Kürner Analysentechnik,
Rosenheim, Germany). Hydrofluoric acid was also purified by dou-
Solution 1H NMR data were obtained from a JEOL eclipse 400+
spectrometer. Solid state 13C and 29Si MAS NMR were recorded
from a Bruker WB Avance III 400 spectrometer (9.4T) operating at
Larmor frequencies of 100.65 and 79.51 MHz, respectively. Exper-
imental conditions for 13C: a 3.2 ZrO2 triple channel probe, with
3.2 mm rotors (Vespel caps) spinning at 10 kHz; cross polarization
pulse sequence (CPMAS ramp) with 2 ms contact time, 4 s repeti-
tion time and number of scans between 8 K and 20 K. Experimental
conditions for 29Si: a 7.0 mm two channel broadband probe with
7.0 mm rotors (Kel-F caps) spinning at 5 kHz; cross polarization
pulse sequence (CPMAS) with 4 ms contact time and 4 s repeti-
tion time [29,30]. For NaY precursor HPDEC pulse sequence with
repetition time of 60 s was used, due to low amount of hydrogen
atoms present in the sample. Adamantane (higher frequency signal
at 37.85 ppm) and kaolinite (at −91.5 ppm) were used as references
for the chemical shifts of 13C and 29Si, respectively.
Single crystal X-ray studies were performed with the compound
[Fe2(BPA)2(␮-OCH3)2(Cl)2] (1). The X-ray data were collected from
a Bruker KAPPA CCD diffractometer [31], at 295 K and MoK˛
monochromatic-graphite radiation. The cell parameters for (1)
were obtained using the PHICHI and DIRAX programs [32,33]. The
average data were reduced using the EvalCCD program and the
absorption correction was performed with the SADABS programs
[34,35]. The structure was solved by direct methods via SHELXS97
and refined via SHELXL97 by a full-matrix least-squares treatment
with anisotropic temperature parameters for all non H atoms [36].
H atoms of the carbon were positioned geometrically (C H = 0.93 Å
for Csp2 atoms, and C H = 0.96 and 0.97 Å for Csp3 atoms) and
treated as riding on their respective C atoms, with Uiso(H) values set
at 1.2 Ueq Csp2 and 1.5 Ueq Csp3 . The H atom bond in N1 was posi-
tioned free and located in Fourier map. The crystal data are listed
in Table SI3.
Textural analyses were carried out by nitrogen physisorption
at 77 K with an Autosorb 1C equipment (Quantachrome Instru-
ments). The samples were degassed under vacuum for 2.5 h at 453 K
before the tests. Surface areas were calculated by the BET method
and total pore volumes were determined using the BJH model and
the desorption branch of the isotherms obtained for silica gel and
MCM-41 compounds. The t-plot method was used for NaY zeolite
compounds.
The Mössbauer spectroscopy of 57Fe was performed at room
temperature in transmission geometry. The spectra were taken

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
122
P.M.A. Machado et al. / Applied Catalysis A: General 507 (2015) 119–129
Fig. 2. A perspective view of molecular structure of (1) with atomic labeling. The ellipsoids are shown with a 50% probability level. Hydrogen atoms were omitted for clarity.
Main bond lengths (Å): Fe(1) Cꢀ(2), 2.337(8); Fe(1) O(2), 1.887(2); Fe(1) O(3), 1.957(2); Fe(1) O(3)#, 2.019(1); Fe(1) N(1), 2.185(2); Fe(1) N(2), 2.180(2); Fe(1) Fe(1),
3.115(1).
with the Co-57 in Rh-matrix source and the powdered sample
maintained at the same temperature, moving in a sinusoidal mode.
The isomer shift (␦) values are in relation to metallic iron. All the
spectra were fitted with only one paramagnetic component (dou-
blet) and the quadrupole splitting ꢂEQ, isomer shift ␦, the linewidth
ꢃ and the absorption area were the free fitting parameters.
and the solid was washed with methanol by Soxhlet extraction for
48 h and dried at 363 K for 24 h, to give a brown solid named SiL,
NaYL or MCM-41L.
2.2.3.2. Complexation with iron(III) salt. A solution of iron(III) chlo-
ride, FeCl3·6H2O (0.44 g, 1.63 mmol), was added to a suspension of
5 g of organo functionalized matrices (SiL, NaYL or MCM-41L), both
in methanol (25 cm3 each), and the reaction mixture was stirred
for 24 h (Fig. 1, Route 3). The resulting solid was filtered, washed
exhaustively with ethanol in a Soxhlet extractor for 72 h and dried
at 363 K for 24 h, resulting in blue solids, named SiLFe, NaYLFe or
MCM-41LFe.
2.2. Catalysts preparation
2.2.1. Syntheses of the ligands
The ligand HBPA (N-(2-hydroxybenzyl)-N-(pyridin-2-ylmethyl)
amine) (Fig. 1) was synthesized as described previously [38],
while the ligand H2BPCꢀNOL, N-(2-hydroxybenzyl)-N-(2-
pyridylmethyl)[(3-chloro)(2-hydroxy) propylamine] (Fig. 1),
was obtained in the reaction between HBPA and epichlorohydrin
[39].
2.3. Catalytic tests
The oxidation of cyclohexane or cyclohexene was performed
using hydrogen peroxide as the oxidant and acetonitrile as sol-
vent in the presence of the catalysts. The molar ratios tested for
catalyst:substrate:oxidant were 1:1000:1000 and 1:1000:100 in a
total reaction volume of 5 cm3. Quantitative analyses of the oxi-
dation products of cyclohexane and cyclohexene were executed
in a Varian GC-430-FID equipped with a 30 m × 0.25 mm (0.25 ␮m
i.d.) CP-WAX 58 column. The typical procedure of oxidation reac-
tions was: to a stirred solution of catalyst (3.5 ␮mol, 2 cm3 of
acetonitrile), at 298 K and under argon atmosphere, it was added
the substrate (1000 eq., 3.5 mmol) and acetonitrile to bring the
total volume to 2.5 cm3. Subsequently, 2.5 cm3 of oxidant solu-
tion (1000, 3.5 mmol or 100 eq., 0.35 mmol of 30% aqueous H2O2
in acetonitrile) was added dropwise over 5 min, resulting in 5 cm3
of reaction volume of the catalyst:substrate:oxidant in two ratios,
1:1000:1000 or 1:1000:100. After 24 h, 50 mm3 of the reaction
were added to 450 mm3 of the solution containing 2-ethylhexanol
(70 mmol dm−3), which is used as internal standard. The resulting
solution was analyzed in duplicate in GC-FID before and after the
addition of triphenylphosphine, which was used to quantify the
presence of alkylhydroperoxides [37]. Total yield is to amount of
product detected by GC-FID, considering that a 100% yield means
that all the oxidant present in the reaction was converted to prod-
ucts and generates one equivalent of oxidized product. Selectivity
is the percentage of each product quantified by GC-FID in relation
to the total yield. Only the products shown in Tables 4 and 5 were
observed in the GC-FID analyses.
2.2.2. Syntheses of the homogenous catalysts
The complex [Fe2III(BPA)2(␮-CH3O)2Cꢀ2] (1) (Fig. 1, Route 1)
was obtained by the reaction of HBPA (1 mmol, 0.24 g), LiOH·H2O
(0.058 g; 1 mmol) and [Fe(H2O)6]Cꢀ3 (1 mmol, 0.27 g) in methanol,
resulting in a dark blue solution. After 30 min, a deep blue solid was
formed, which was filtered off and washed with cold propan-2-ol.
Crystals were isolated from the mother solution, which allowed
the determination of the molecular structure by X-ray diffraction.
Yield: 0.145 g (67%). Anal. found (%): C = 49.86; H = 4.67; N = 8.14.
Calcd. (%) for Fe2C28H32N4Cꢀ2O4: C = 50.11; H = 4.81; N = 8.45; ꢄM
(CH3CN): 22 ꢁ−1 cm2 mol−1, neutral compound [40]. IR: 3420,
3213, 1605, 1597, 1570, 1477 1591, 1479, 1449, 1272, 1034, 767,
759 cm−1. The complex [Fe(BPCꢀNOL)Cꢀ2] (2) (Fig. 1, Route 2) was
synthesized as described previously [27].
2.2.3. Syntheses of the heterogeneous catalysts
2.2.3.1. Organofuncionalization of inorganic matrices. The incorpo-
ration of the chelating unit HBPA on the surface of inorganic
solids (silica gel, NaY or MCM-41) was performed in two
steps. The first step was based in the reaction between 3-
glycidoxypropyltrimethoxysilane (GPTMS) (23 mmol, 5.0 cm3) and
HBPA (23 mmol, 4.9 g), in methanol (100 cm3). The mixture was
stirred at 323 K in an inert atmosphere for 72 h. Then, a sample
of 5.0 g of the inorganic matrix was suspended in 100 cm3 of dry
toluene and was added to this solution, at 383 K (Fig. 1, Route 3).
After 72 h, the reaction was cooled to room temperature, filtered