Amorphous Cr/SiO2 Materials Hydrothermally Treated: Liquid Phase Cyclohexanol Oxidation

Article abstract of DOI:10.1007/s10562-018-2422-4

Abstract: Amorphous Cr–SiO₂ materials were synthesized by the sol–gel method and hydrothermally treated at temperatures between 150 and 220?°C. These materials were used as catalysts for cyclohexanol oxidation with H₂O₂ as oxidant and CH₃CN as a solvent. They were responsible for the decomposition of H₂O₂, which triggers, by a free?radical mechanism in the homogeneous phase, the oxidation or degradation of the hydrocarbon chain. Metal leaching causes a drop in catalytic activity when the material is recycled. Studies on the hydrothermal treatment effect on the leaching process have demonstrated that the higher the hydrothermal treatment temperature, the higher the metal/support interaction, leading to a diminution of the leaching process. Under mild reaction conditions, and using TBHP as oxidant, leaching was reduced, and improvements were obtained on the selectivity towards the formation of cyclohexanone. The use of these catalysts in the oxidation of verbenol, an allylic alcohol, showed a significant increase in the substrate conversion and in the selectivity to carbonyl derivative formation. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-018-2422-4

Catalysis Letters
https://doi.org/10.1007/s10562-018-2422-4
Amorphous Cr/SiO2 Materials Hydrothermally Treated: Liquid Phase
Cyclohexanol Oxidation
José Feliciano Miranda1 · Pablo M. Cuesta Zapata1 · Elio E. Gonzo2 · Mónica L. Parentis2 · Lilian E. Davies1 ·
Norberto A. Bonini1
Received: 1 February 2018 / Accepted: 13 May 2018
© Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract
Amorphous Cr–SiO2 materials were synthesized by the sol–gel method and hydrothermally treated at temperatures between
150 and 220 °C. These materials were used as catalysts for cyclohexanol oxidation with H2O2 as oxidant and CH3CN as a
solvent. They were responsible for the decomposition of H2O2, which triggers, by a free radical mechanism in the homogene-
ous phase, the oxidation or degradation of the hydrocarbon chain. Metal leaching causes a drop in catalytic activity when the
material is recycled. Studies on the hydrothermal treatment effect on the leaching process have demonstrated that the higher
the hydrothermal treatment temperature, the higher the metal/support interaction, leading to a diminution of the leaching
process. Under mild reaction conditions, and using TBHP as oxidant, leaching was reduced, and improvements were obtained
on the selectivity towards the formation of cyclohexanone. The use of these catalysts in the oxidation of verbenol, an allylic
alcohol, showed a significant increase in the substrate conversion and in the selectivity to carbonyl derivative formation.
Graphical Abstract
Keywords Cr/SiO2 · Cyclohexanol · H2O2 · TBHP · Cyclohexanone · Hydrothermal treatment
1 Introduction
Cyclohexanone production is a process of great impor-
tance in the chemical industry. Annually, millions of tons
of cyclohexanone are produced mostly for the synthesis of
adipic acid and caprolactam, main precursors for the manu-
facture of Nylon-6 and Nylon-66 [1–5]. Cyclohexanone also
* Norberto A. Bonini
bonini@exa.unsa.edu.ar
Extended author information available on the last page of the article
Vol.:(0123456789)
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J. F. Miranda et al.
finds application as additive and solvent in a wide variety
of products, from insecticides to herbicides and pharma-
ceuticals products, paints, dyes, lacquers, varnishes and as
homogenizer and stabilizer for soaps, among other minor
applications and as a precursor in organic synthesis [6, 7].
In the synthesis process of adipic acid, cyclohexanone is
produced by oxidation of cyclohexane with air, in the pres-
ence of cobalt naphthenate as a homogeneous catalyst and
boric acid to stabilize the oxidation mixture [8, 9]. The final
product is a mixture of cyclohexane (C–H), cyclohexanol
(C–OL), cyclohexanone (C-ONE) and cyclohexyl hydroper-
oxide (C-HP), been the last the intermediate in the oxidation
process. The unreacted C–H is distilling off and recycling
while the low-volatile liquids, mainly C-OL and C-ONE, are
purified by distillation leading to a mixture known as KA
(ketone+alcohol) oil. The whole process is extremely costly
as a consequence of the high temperatures (145–165 °C) and
high O2 pressures (1–2 Mpa) required. In this process, the
C–H conversion is kept low (<10%) to avoid over-oxidation
of cyclohexanone and excessive waste production [6, 8, 10,
11].
oxidation of organic compounds is one of the main chal-
lenges for the environmentally “green” catalysis.
The sol–gel method for preparing catalysts is suitable for
incorporating metal ions homogeneously dispersed into the
silica structure. However, the stability of these metal ions
is not enough to avoid leaching into the liquid phase [6,
16, 20]. Recently, our research group has reported the syn-
thesis of Cr–SiO2 catalysts by sol–gel technique followed
by hydrothermal treatment [25–27] up to 220 °C. It was
observed a progressive incorporation of Cr3+ ions in the
silica network as the hydrothermal temperature increases
from 120 to 220 °C. The nature of Cr–SiO2 interaction was
studied by techniques such as XPS, UV–Visible DRS, and
adsorption of probe molecules followed by FTIR analysis.
It was shown that chromium species are incorporated into
the silica framework by forming chromasiloxane rings with
hydrothermal treatment [28–30]. It was also showed that
even small amounts of Cr3+ coordinative unsaturated sites
are just enough to produce the dehydration of cyclohexanol
in gas phase diminishing the selectivity to cyclohexanone
[28–30]. Therefore, the sol–gel technique combined with
hydrothermal treatments seem to be an interesting method
for preparing materials with Cr3+/Cr6+ centers, highly dis-
persed, and stable from leaching into the liquid phase.
In the present work, the catalytic activity of the chromium
catalysts, prepared by the sol–gel technique and hydrother-
mally treated, was evaluated. They were tested against the
oxidation of different substrates, mainly cyclohexanol, to
cyclohexanone in the liquid phase, using H2O2 as oxidizer
and acetonitrile as solvent. The effect of hydrotreatment tem-
perature on catalytic activity was studied. For comparison,
the behavior of the system when TBHP (70% in water) is
used as an alternative oxidizing agent was analyzed. Finally,
the behavior of verbenol, a cyclic allylic alcohol, was com-
pared with that of cyclohexanol using H2O2 as the oxidizing
agent.
Another industrial process for the production of cyclohex-
anone is the gas phase dehydrogenation of cyclohexanol in
the presence of a heterogeneous copper catalyst [7, 12–15].
In this process, with the aim of having conversions close
to 60 and 99% selectivity to cyclohexanone, a temperature
of 260 °C [12, 14, 15] is used which causes loss of activity
over time due to sintering. As a result, there is an increasing
interest in developing new liquid phase oxidation processes
exhibiting operative advantages such as mild conditions
of reaction and the use of “soft” oxidants such as molecu-
lar oxygen, hydrogen peroxide or ter-butyl hydroperoxide
(TBHP). However, these homogeneous liquid phase oxi-
dations also need the presence of soluble transition metal
complexes which cause damage to the environment; compel-
ling their replacement by heterogeneous catalysts, which are
easier to separate, recover and recycle [7, 16].
Between heterogeneous catalysts suitable for this reac-
tion and obtained from supported transition metals [17, 18],
chromium is particularly attractive due to its wide variety
of oxidation states. In fact, chromium/silica catalysts were
probed for cycloalkane liquid phase oxidations [6, 10, 16,
18–22], where the activity was ascribed to the presence of
the redox pair Cr3+/Cr6+ [19]. Although Adam et al. [10] and
Sakthivel et al. [6] have synthesized Cr/SiO2 catalysts with
very high catalytic activity, these authors also have reported
the leaching of chromium species into the reaction mixture,
becoming the reaction carried out partially into the homo-
geneous phase. Chromium leaching seems to be a recurring
problem with this kind of materials [16, 20, 23], especially
when using TBHP or H2O2 in the presence of polar solvents
such as water or acids [24]. Thus, the development of hetero-
geneous, highly stable, and active catalysts for the selective
2 Experimental
2.1 Catalysts Preparation
Tetraethyl orthosilicate (TEOS, Merck) was used as the sil-
ica source in all syntheses. It was mixed with ethanol (EtOH)
(Merck 99.9%), stirred at 60 °C, and the pH adjusted to 9.5
with NH4OH (Merck). Then, a solution of Cr(NO3)3.9H2O
(ANEDRA > 98%) dissolved in an EtOH/H2O (1:1 v/v)
mixture was dropped, under stirring. The amount of chro-
mium was the required to obtain a Cr/TEOS molar ratio
of 0.06 and adequate to produce a 5% w/w of chromium
on silica catalyst (5SG). The final mixture, (molar ratio of
1.0TEOS:225.0H2O:30.3EtOH), was vigorously stirred for
one hour and allowed to gel during 40 h. Portions of the gel
1 3

Amorphous Cr/SiO2 Materials Hydrothermally Treated: Liquid Phase Cyclohexanol Oxidation
were autoclaved in a Teflon vessel at different temperatures
(120–220 °C) for 24 h. After hydrothermal treatment, the
solid product was filtered and washed; first with water and
then with an EtOH/acetone (1/1) mixture to avoid deposition
of soluble chromium salts during drying. Finally, samples
were dried in air up to 110 °C. Materials were assigned as
5SGxxxyyy where xxx is the hydrotreatment temperature and
yyy the highest treatment temperature in air. Non-hydro-
Difference spectra were obtained by subtracting the spec-
tra of the pure sample (baseline spectrum) from those of the
sample interacting with the adsorbed molecule after vacuum
treatment. For quantitative determinations, the spectra were
normalized using, as an internal standard, the overtones and
combination bands in 2100–1800 cm−1 region.
2.3 Catalytic Activity
treated samples were denoted as 5SGnHT
.
Catalytic activity tests were performed in an 18 mL flat bot-
tom vial (Perkin Elmer) with a plastic cap and PTFE/silicone
septum. A mixture of 10 mg of catalyst, 5 mL of solvent
(acetonitrile or chlorobenzene), 5 mmol of cyclohexanol and
5 mmol of oxidant was introduced into the reactor. The reac-
tion was carried out at 70 °C under stirring. The addition
of the oxidizing agent (H2O2 30% or TBHP 70% in H2O)
was performed on aliquots at scheduled times (t=0 and 2 h,
2 mmol, and t = 3, 4 and 6.5 h, 1 mmol). Aliquots of the
reaction mixture (0.1 mL) were taken at different reaction
times, filtered through a membrane filter (PTFE-0.45 µm),
dissolved in acetonitrile and cooled at 0 °C (to ensure
quickly quenching of the reaction) until chromatographic
analysis. The concentration of reactants and products was
determined by gas chromatography (GC) with a Hewlett
Packard-5890 Series II gas chromatograph equipped with
an HP-20 capillary column (L=25 m, DI=0.32 mm) and
a flame ionization detector (FID) using chlorobenzene as
internal standard (SI). Products and by-products of the reac-
tion were identified by gas chromatography (Perkin Elmer
Clarus 680) coupled to a mass detector (MS) (Perkin Elmer
Clarus 600, Elite-5MS column, L = 30 m, DI = 0.32 mm)
and their mass spectra compared with those in NIST 08 and
Wiley 2010 libraries.
2.2 Sample Characterization
2.2.1 Textural Characterization
Adsorption–desorption isotherms were carried out in a
Micromeritics ASAP-2020 sorptometer, employing N2 as
the adsorbent. Specific surface area (SBET) was calculated
using the standard BET method. The pore size distribu-
tions were determined from the adsorption branch of the
isotherms using the BJH method.
2.2.2 Atomic Absorption Spectroscopy
Chromium loading was determined by atomic absorption
(AA) spectroscopy, with a GBC 904 AA spectrometer, after
dissolving the samples with HF.
Leaching tests on the liquid supernatant were performed,
at 70 °C, after 7 h of contacting either the catalyst with sub-
strate and solvent or the catalyst with the substrate, solvent,
and oxidant. The same amount of liquid sample was used for
leaching tests as well as for catalytic activity assays.
2.2.3 UV–Vis Diffuse Reflectance Spectroscopy (DRS)
The H2O2 decomposition was determined by iodometric
titration of aliquots taken off at different periods.
A GBC-918 equipment, with a BaSO4 diffuse reflectance
sphere as a reference, was used in the 190–900 nm range for
the UV–Vis diffuse reflectance analysis. The spectra were
determined over finely grounded materials gently compacted
into the sample compartment of the spectrophotometer.
3 Results and Discussion
3.1 Characterization
3.1.1 N2 Adsorption
2.2.4 FTIR‑2,6‑lutidine Adsorption
The FTIR spectra were recorded on a Spectrum GX-FTIR
Perkin-Elmer Spectrophotometer. FTIR measurements in
vacuum and 2,6-lutidine (Lu) adsorption were carried out
over a self-supporting wafer (20 mg approx.) in a cell with
KRS-5 windows that allows vacuum (10−5 mmHg) and heat-
ing treatments “in-situ”. Each sample was evacuated at dif-
ferent temperatures and the spectrum recorded (baseline).
Gaseous Lu was introduced and left to equilibrate at its cor-
responding vapor pressure at room temperature (RT). After
Lu adsorption, the cell was evacuated at different evacuation
temperatures and the spectra recorded.
Table 1 shows textural properties for 5SG samples deter-
mined from the N2 adsorption–desorption isotherms: BET
specific surface area (SBET), mean pore diameter (DP), and
pore volume (VTP).
Both the specific surface area (SBET) and the pore vol-
ume (VTP) decreased when the hydrotreatment temperature
increased (Table 1). The pore diameter distribution (Dp) is
narrow and centered around 43 Å for the sample without
hydrotreatment (5SGnHT). Hydrothermally treated sam-
ples developed a bimodal profile with a broad distribution
1 3

J. F. Miranda et al.
Table 1 5% Chromium content and textural properties of SG materi-
als
Sample SBET (m2 g−1
)
VTP (cm3g−1
)
DP (Å) % Cr (w/w) (*)
5SGnHT 608
0.735
0.358
0.154
43
18–34
19–25
4.1 0.1
4.5 0.4
4.9 0.4
5SG150
5SG220
240
136
(*) from AA analysis
characteristic of amorphous siliceous materials. Moreo-
ver, the chromium loading (Table 1), in all materials, was
lower than the theoretical amount added because the non-
anchored chromium was released by washing before drying
(see “Experimental” section). On the other hand, the amount
of chromium was lesser into the 5MEnHT material than in
the hydrotreated samples. This difference is attributed to the
lower amount of water and surface hydroxyl groups pre-
sent on hydrotreated materials as result of the SBET decrease
(Table 1). However, it must also consider that, under hydro-
thermal conditions, a partial solubility of the Si-oligomers
occurs, leading to a consequent increase in the final chro-
mium content.
Fig. 1 DRUV–vis spectra obtained from samples prepared under
different hydrothermal conditions: a 5SGnHT110, b 5SG150110, c
5SG220110. The shoulder between 680 and 710 nm corresponds to a
spin-forbidden transition
3.1.2 UV–Vis Diffuse Reflectance Spectroscopy (DRS)
Cr3+ is a d3 ion that presents three intense absorption bands,
in the 200–900 nm spectral region, corresponding to three
allowed electronic transitions. The [Cr(H2O)6]3+ ion shows
these bands centered at 580, 412 and 260 nm, and also a
weak band as a shoulder at 680 nm corresponding to a spin-
these materials produce the partial oxidation of the Cr3+ with
the development of mono and polychromate Cr6+ species.
Such a behavior was confirmed by the partial disappearance
4
4
of the (4T2g − A2g) and (4T1g − A2g) bands and a strong
increase of the Cr6+ metal to ligand charge transfer band into
the 230–350 nm spectral region [27].
110
forbidden transition [31]. The 5SGnHT material shows
a shift of the 580 nm band toward higher wavelengths at
604 nm. This red shift is due to the presence of ammonia
as ligand and the incorporation of Si–O groups in the outer
coordination sphere of Cr3+ ions [27]. A shift of the two
bands in the visible region to higher wavelengths, due to
the effect produced by the hydrothermal-treatment, was
observed (Fig. 1; Table 2). Thus, a gradual increase of the
hydrotreatment temperature during the synthesis produces
The resistance of Cr3+ to Cr6+ oxidation depends on the
hydrotreatment. So, while after calcination at 450 °C in the
5SGnHT materials almost all the Cr3+ signals disappear, these
signals are evident on the 5SG220450 material [27].
3.1.3 FTIR Study of 2,6‑lutidine Adsorption
The presence of CU Cr3+ ions anchored to the silica sur-
face produces Lewis acid centers, and their oxidation in air
promotes the development of weak Brønsted sites [27]. The
evolution of these acid centers was studied by FTIR through
4
4
a gradual shift in the T2g(F) − A2g(F) absorption band,
from 604 nm (5SG110) to 622 nm (5SG220). This behavior
must be justified by the formation of Si–O–Cr bonds. The
shift was accompanied by a strong change in the relative
intensities of the absorption bands. As a result, the ratio R
= (4T2g − 4A2g)/(4T1g − 4A2g) goes from 1, for 5SGnHT110, to
2 for 5SG220110 (Table 2; Fig. 1).
Table 2 Electronic transitions of Cr3+ ions observed by DRUV–Vis
spectroscopy
Sample
4T2g(F) →
4T1g(F) →
R=I(620 nm)/I(425 nm)
These changes reflect the distortion of the octahedral field
produced by the incorporation of Cr3+ ions in the tetrahedral
structure of silica. This distortion is related to the formation
of strained chromium siloxane rings reported in the literature
[28–30]. These Cr3+ ions constitute coordinative unsaturated
(CU) centers. The calcination in air from 300 up to 450 °C of
4A2g(F) (nm)
4A2g(F) (nm)
110
5SGnHT
5SG150
5SG220
604
616
622
397
439
441
0.82
1.65
2.00
110
110
1 3

Amorphous Cr/SiO2 Materials Hydrothermally Treated: Liquid Phase Cyclohexanol Oxidation
Fig. 3 BLu species developed over different 5SG materials calcined
Fig. 2 Lu adsorbed on 5SGnHT material calcined at: a 150 °C, b
350 °C, c 450 ºC
at 450 °C. a 5SGnHT450, b 5SG220450. The spectra were determined
after evacuation for 30 min at room temperature
110
the adsorption of 2,6-lutidine (Lu) molecules. On 5SGnHT
oxidation of Cr3+ ions on hydrothermally treated samples as
it was observed by DRUV–Vis experiences [27].
materials previously evacuated at 250 °C, the Lu adsorption
leads to the development of IR bands at 1602, 1583, 1475,
1461, 1408 and 1382 cm−1 (Fig. 2a). These bands are assign-
able to hydrogen-bonded lutidine (HBLu) and/or Lu inter-
acting with Lewis acid sites (LLu). Discriminating between
them is only possible by vacuum treatment. Therefore, while
HBLu completely disappears in vacuum at 50 °C, removing
LLu requires vacuum treatment higher than 100 °C.
To verify whether Brønsted centers correlates to the Cr6+
presence and their leachability, a 5SG220450 sample, which
after calcination in air developed a light yellow color, was
washed with acetone and ethanol. After washing and drying
in air at room temperature, a self-supported wafer of this
material was made; it was evacuated at 150 °C into the FTIR
cell and put in contact whit Lu vapors. Figure 4 shows that
only LLu/HBLu bands formed on this material after interac-
tion with Lu. A new heating of the washed sample in air at
450 °C (Fig. 4b) shows the development of new Brønsted
acid centers at the expense of the Lewis ones. These results
let us conclude that the development of Brønsted centers is
directly related to the Cr6+ species formed during calcina-
tion in air.
While in samples heated in N2 or in vacuum up to 450 °C
before the adsorption of Lu the nature of the adsorbed spe-
cies don´t change, the Lu adsorption on materials calcined
in air leads to the development of a new band at 1644 cm−1
with a shoulder at 1629 cm−1 (Fig. 2b, c). These bands cor-
respond to the appearance of a protonated 2,6-lutidinium
ion, formed by interaction of Lu with Brønsted acid sites
(BLu). The amount of BLu depends on both the previous
hydrothermal treatment and the temperature of in-air calci-
nation. The rise of the calcination temperature increases the
amount of BLu at expense of Lewis species. Therefore, on
5SGnht calcined at 450 °C the most intense bands observed
correspond to BLu (Fig. 2c). These BLu species resist the
vacuum treatment up to 100 °C. From these results, we con-
clude that LLu is related to Cr3+ ions, while BLu appears
when Cr6+ forms by calcination.
Conversely, the intensity of the BLu bands decreased
when the sample, before the adsorption of Lu, was treated
in vacuum at increasingly higher temperatures (Fig. 4c, d).
The color of the sample turns progressively to dark blue-
green instead of the yellow color observed on the oxidized
one. So, in vacuum at 450 °C, almost all Cr6+ transforms to
Cr3+. This behavior shows that the Cr3+–Cr6+ redox process
on the Cr/SiO2 system is reversible. The amount of both ions
depends on the temperature and the oxidative character of
the atmosphere used during calcination, as well as on the
presence of an oxidizing agent (H2O2 or TBHP) in the liquid
phase as will be discussed later.
After calcination, on hydrothermally treated samples
(5SG220450) the relative amount of Brønsted to Lewis acid
sites is markedly lower than in non-hydrothermally treated
materials (5SGnHT450) (Fig. 3). The lower amount of Brøn-
sted acid centers coincides with a greater resistance toward
From the above results and those reported in previous
papers [25–27], we can conclude that Cr3+ species were the
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J. F. Miranda et al.
Fig. 5 Oxidation of different substrates in the presence of H2O2 on
Cr/SiO2 catalyst. Temperature: 70 °C. Solvent: CH3CN (5 mL). Cata-
110
Fig. 4 Difference spectra of Lu adsorbed on 5SG220450 materials after
lyst: 5SG150 (10 mg). Substrate: 5 mmol. Oxidizing agent: H2O2
450
(30%) added at programmed stages to 10 mmol (molar ratio R=sub-
strate/H2O2 =0.5) except for cyclohexanol for which R=1
different treatments: a 5SG220 washed with acetone/EtOH, b sam-
ple a calcined in air up to 450 °C; (c–e) Lu adsorption on sample b
after treatment in vacuum (1 h – 10−5 mmHg) at 250, 350 and 450 °C
successively
cyclohexane, conversion exceeded 90% after 6 h with selec-
tivity to cyclohexanol and/or cyclohexanone lower than 10%.
The main by-products were short chain carboxylic acids and
CO2. This behavior agrees with the results obtained by Adam
et al. [10] on Cr/SiO2 catalyst prepared by the impregnation
method, in which the major products were cyclohexanol,
cyclohexanone, adipic acid and decomposition products of the
hydrocarbon chain, such as valeric, acetic and butyric acids,
among others.
only one into the catalyst samples dried at 110 °C, whereas
in samples treated at temperatures higher than 110 °C, both
Cr3+ and Cr6+ species were present. Similar results were
obtained over Cr-MCM-41 materials on which the relative
amount of both species was quantified by XPS [25, 26].
All the above characterization results let to conclude that
hydrothermal treatment enhances the incorporation of Cr3+
into the silica structure, through the Si–O–Cr bonds forma-
tion, avoiding the leaching and also the formation of either
dichromate or polychromate polymeric species [25–27].
The X-ray diffraction profiles of 5SG materials (not shown)
established the absence of crystalline species, even after
calcination in air up to 450 °C. The last because of the low
loading and high chromium dispersion. The superficial chro-
mium species formed, studied by FTIR of chemisorbed pyri-
dine (Py) [25, 27] and 2,6-lutidine (Lu), allowed establishing
the presence of highly unsaturated Cr3+ centers that partially
develop, after calcination in air at 450 °C, Cr6+ centers asso-
ciate to the presence of Brønsted acid sites.
When the molar ratio R, between the substrate and oxygen
peroxide, was equal to 2, the process resembles the organic
substrates decomposition with H2O2 in an acid medium in the
presence of Fe2+ salt (Fenton process). In our case, the high
tendency to chain degradation and low selectivity to carbonyl
derivatives is attributed to the strong surface acidity of 5SG
catalyst and the chromium ability to generate HO• radicals
from H2O2 (Scheme 1).
Like cyclohexane, cyclohexene also suffers rapid oxidation
in the presence of H2O2. Thus, after 3 h of reaction (Fig. 5),
more than 60% of the substrate was converted to degrada-
tion products. The oxidation of cyclohexene with H2O2 using
polar solvents such as acetonitrile or H2O is one of the ways
commonly chosen for the direct preparation of adipic acid.
The reaction mechanism involves the formation of interme-
diates species such as cyclohexene oxide, 1,2-cyclohexan-
ediol, 2-hydroxycyclohexanone and adipic anhydride [16, 32,
3.2 Catalytic Activity in Liquid Phase
3.2.1 Oxidation of Different Substrates
The oxidation reaction of cyclohexane, cyclohexene, and
cyclohexanol to cyclohexanone was studied to investigate
the catalytic activity of the 5SG150110 materials. Results are
shown in Fig. 5.
After 5 h of reaction, conversions near to 70% were
observed for the cyclohexane and cyclohexene substrates. For
Scheme 1 H2O2 decomposition on Cr/SiO2 catalysts
1 3

Amorphous Cr/SiO2 Materials Hydrothermally Treated: Liquid Phase Cyclohexanol Oxidation
110
33]. Among the oxidation products obtained with 5SG150
catalyst, small amounts of cyclohexene oxide, 1,2-cyclohex-
anediol, and 2-hydroxycyclohexanone but not anhydride or
adipic acid were detected by gas chromatography and mass
spectrometry.
than at 50 °C. At 50 °C and after 6 h of reaction, the CC−OH
was near 10% with null selectivity to cyclohexanone (Fig. 6,
right). Meanwhile, at 70 °C, the C-OH conversion reaches
approximately 45% and selectivity towards cyclohexanone
25%. Figure 6, shows that, at the beginning and until two
hours of reaction at 70 °C, when the concentration of hydro-
gen peroxide is high, the degradation of the hydrocarbon
chain is important with SC−ONE near 10%. This behavior sug-
gests that degradation and oxidation reactions run through
independent paths not being cyclohexanone an intermediate
in the chain degradation reaction.
On the other hand, a low oxidation reaction rate and
selectivity to cyclohexanone higher than 30% were observed
when cyclohexanol was used as the substrate. In this case, by
GC-MS analysis, cyclohexanone was identified as the main
product, along with 3-hydroxycyclohexanone, 4-hydroxy-
cyclohexanone, 2-hydroxycyclohexanone and 2-cyclohex-
enone as minor products. These results allow proposing that
after the oxidation to cyclohexanone, the formation of more
oxygenated by-products follow the path to ring opening. In
the absence of the catalyst, the oxidation of these substrates
showed very low conversions. So, the presence of the cata-
lyst was essential to trigger off the H2O2 decomposition.
From these results, and to have a better insight of the
process, new experiences were programmed to investigate
the effect of different variables on the activity and selectiv-
ity of 5SG catalysts over the cyclohexanol oxidation in the
presence of acetonitrile as solvent. This system is interesting
to study since the little knowledge of this reaction performed
in non-halogenated solvents [19].
3.2.2.1 The Effect of Catalyst Amount Loaded Figure 7,
shows the effect of the amount of catalyst added on CC−OH
and also in the SC−ONE. In the absence of the catalyst, the
reaction rate is very low (CC−OH = 5%). After 7 h, cyclohex-
anone was not found between the oxidation products; it
was detected after 23 h of reaction with selectivity close
to 7%. This behavior reflects the role of the catalyst in the
decomposition of the oxidizing agent. Accordingly, moni-
toring H2O2 decomposition in the absence of the substrate
shows that the 10 mmol of H2O2 fully decomposed over the
5SG150110 catalyst after 5 h of reaction at 70 °C.
Figure 7 also shows that different amounts of loaded cata-
lytic material produce almost the same CC−OL and negligible
differences in SC−ONE. These results seem to reflect that,
although the catalyst plays a leading role in the decomposi-
tion of H2O2, it induces, through a free radical mechanism in
homogeneous phase (see Scheme 1 above), the oxidation and
the subsequent degradation of the hydrocarbon chain. Other
authors [10] have also reported similar results.
3.2.2 Reaction Temperature Effect on the Cyclohexanol
Conversion
The effect of the reaction temperature on the total conversion
(CC−OL) and selectivity to cyclohexanone (SC−ONE) over the
5SG150110 material, hydrothermally prepared at 150 °C, was
determined. As expected, reaction proceeds faster at 70 °C
110
Fig. 6 Effect of reaction temperature on a cyclohexanol conversion and b cyclohexanone selectivity. Catalyst: 5SG150 (10 mg). Solvent:
CH3CN (5 mL). Substrate: 5 mmol. Oxidizing agent: H2O2 (0.7 mL, 30%). (R=1)
1 3

J. F. Miranda et al.
110
Fig. 7 Effect of the amount of catalyst loaded. a Cyclohexanol conversion. b Cyclohexanone selectivity. Temperature: 70 °C. Catalyst: 5SG150
(10 mg/ 20 mg). Solvent: CH3CN (5 mL). Substrate: cyclohexanol (5 mmol). Oxidizing agent: H2O2 (0.7 mL, 30%). (R=1)
3.2.2.2 Catalyst Lifetime and Number of Cycles Three
cycles of reuse were carried out to analyze the lifetime and
reusability of the catalyst. After each cycle, the Cr/SiO2
material was filtered, washed with hot acetonitrile and added
to a fresh reaction mixture for running a new activity test.
It was observed a loss of catalytic activity as the number
of cycles increased (Fig. 8a). However, the loss of activity
was not accompanied by a significant change to cyclohex-
anone selectivity (Fig. 8b).
removal of lixiviated metal, we conclude that hydrogen per-
oxide favors the leaching of the supported Cr ions by Cr6+
formation. These results are coincident with that observed
by Lu-adsorption FTIR (Fig. 4) were the Cr+3 to Cr6+ oxida-
tion, when catalysts are heated in air, was produced.
This comportment coincides with the violet coloration
acquired by the reaction medium after addition of hydro-
gen peroxide that disappears as the reaction proceeds.
The same but more intense coloration was observed when
a chromium(III) salt was dissolved in acetonitrile in the
presence of hydrogen peroxide and heated up to 70 °C.
This behavior is related to the formation of the soluble
Cr6+ peroxides, like the blue (CrO(O2)2H2O) and violet
The slight drop in CC−OL conversion through the succes-
sive cycles suggests that a fraction of chromium ions are lost
by the leaching process. Took into account the careful wash-
ing of the solid done after each cycle to ensure the complete
Fig. 8 Behavior as a function of the number of cycles. a Cyclohexanol conversion. b Cyclohexanone selectivity. Temperature: 70 °C. Solvent:
CH3CN (5 mL). Catalyst: 5SG150110 (10 mg). Substrate: cyclohexanol (0.5 mL). Oxidizing agent: H2O2 (0.7 mL, 30%)
1 3

Amorphous Cr/SiO2 Materials Hydrothermally Treated: Liquid Phase Cyclohexanol Oxidation
(CrO(O2)2OH−) peroxides [34]. Both complexes readily
decompose to form Cr(III) ions, in acid media.
Table 3 shows the percent of chromium leached out based
on the total chromium content of the catalyst (Table 1). It
is observed that, regardless of hydrothermal treatment,
without the oxidizing agent metal leaching does not occur
due to the insolubility of Cr3+ in CH3CN. By contrast, in
the presence of H2O2, the amount of metal leached from
5SGnHT material was 73% after the first cycle, reaching 95%
after the second one. Note that the higher was the hydro-
thermal treatment temperature, the lower the metal leached
out. Consequently, for the hydrothermally treated material
at 220 °C (5SG220110), the leaching was only 12% and 18%
after the first and second cycle, respectively. An intermedi-
ate behavior was observed for the material hydrotreated at
150 °C; being the total chromium leached out 29 and 49%
in the first and second washings, respectively. The leaching
behavior confirms the previously reported [25–27] regard-
ing the increase of the metal-support interaction with the
rise of the hydrothermal treatment temperature. In all cases,
the leaching process was accompanied by the development
of a violet coloration whose intensity depends on the metal
3.2.2.3 Homogeneous Phase Reaction To investigate the
leaching of chromium ions into the homogeneous phase,
the catalytic activity was evaluated by removing the catalyst
after one-hour reaction [22]. Figure 9 shows a comparison
110
between the reaction performed in the presence of 5SGnHT
catalyst and the reaction when the catalyst was removed
after one hour of reaction. For comparison, and consider-
110
ing that the leaching and reusability of the 5SG150 cata-
lyst were determined previously (Fig. 8), the catalyst with
no hydrothermal treatment was chosen since it presents the
weakest metal-support interaction and consequently it is the
most susceptible to leaching [25–27].
Figure 9 shows that, after removing the catalyst, the reac-
tion proceeds in the homogeneous phase with almost identi-
cal conversion to that obtained in the presence of the cata-
lyst. On the other hand, the selectivity to carbonyl derivative
decreases when the reaction is completed in homogeneous
phase. This behavior confirms that hydrogen peroxide initi-
ates the reaction by oxidation of surface Cr3+ ions to form
soluble Cr6+ peroxides. They catalyze the homogeneous
phase reaction until the disappearance of the oxidizing agent
promoting the degradation of the cyclic chain. So, the cata-
lyst presence exerts an improving effect on the selectivity to
the carbonyl derivative.
Table 3 Total chromium loss in leaching tests
a
Sample
CH3CNa
CH3CN+H2O2
First wash (%)
First wash Second
Second
wash
(%)
(%)
wash (%)
5SG110
0
0
0
0
0
0
73
29
12
95
49
18
nHT
3.2.2.4 Leaching Tests To determine the leaching degree of
chromium ions during the progress of the reaction, we car-
ried out some experiments using CH3CN as solvent at 70 °C
for 7 h, both in the presence and absence of H2O2.
5SG110
5SG110
150
220
aWashing made after 7 h at 70 °C under stirring
110
110
Fig. 9 Comparison of the system with 5SGnHT
catalyst and the
Solvent: CH3CN (5 mL). Catalyst: 5SGnHT
(10 mg). Substrate:
same system one hour after removing the catalyst. a Cyclohex-
anol conversion. b Cyclohexanone selectivity. Temperature: 70 °C.
cyclohexanol (0.5 mL). Oxidizing agent: H2O2 (0.7 mL, 30%)
1 3

J. F. Miranda et al.
concentration in solution, given support to the presence of
Cr6+ peroxide species into the solvent.
3.2.2.5 Effect of hydrothermal treatment temperature The
cyclohexanol conversion and cyclohexanone selectivity
on hydrothermally treated materials at different tempera-
tures (nHT, 150 °C, 220 °C) are depicted in Fig. 10. Fig-
ure 10a shows that increasing the hydrotreating temperature
reduces both the catalytic activity (total cyclohexanol con-
version) and selectivity towards cyclohexanone. After 7 h
of reaction in presence of the material without hydrotreat-
ment (5SGnHT110) the CC−OH was 60% approximately and
SC−ONE around 25%. Under the same reaction conditions,
the 5SG220110 material gave a 30% conversion and cyclohex-
110
anone selectivity as low as 10%. The 5SG150 material
Fig. 11 Initial reaction rates for 5SG materials determined by the
(hydrothermal temperature of 150 °C) showed an interme-
diate behavior.
conversion at 4 h of reaction
On the other hand, the initial reaction rates values
(Fig. 11) for both total C-OH conversion (mmol of C-OH
h−1 m−2 mol Cr−1) and C-ONE formation (mmol of C-ONE
So, the higher the hydrotreatment temperature, the
stronger the Cr3+/support interaction and its resistance to
being leached out, diminishing the availability of Cr6+ spe-
cies in solution for the redox process (Fig. 10a). However,
the strong metal/support interaction increases the amount
of coordinative unsaturated Cr+3 centers and hence the
Lewis superficial acidity [27]. These centers promote the
opening of the cyclic hydrocarbon chain and its later deg-
radation. Both effects produce a maximum in the reaction
rate (Fig. 11) and therefore, reduction of leaching degree
and acidity increase, result in a lower reactivity and a lower
selectivity to carbonyl derivatives for the 5SG220110 material.
A similar effect was reported in the oxy-dehydrogenation
110
h−1 m−2 mol Cr−1) show a volcano shape, with the 5SG150
material displaying the maximum value. This maximum is
due to the competition between the homogeneous reaction
in the liquid face and the heterogeneous reaction on the solid
surface.
Besides, the difference observed between rC−OH and
rC−ONE resembling selectivity (Fig. 10b) must be inter-
preted in terms not only of the amount of Cr6+ leached off
but also as the consequence of the acidity changes (amount
and strength) occurring into the surface of 5SG materials by
the hydrotreatment.
Fig. 10 Influence of hydrothermal treatment temperature. a Cyclohexanol conversion. b Cyclohexanone selectivity. Temperature: 70 °C. Sol-
vent: CH3CN (5 mL). Catalyst: 10 mg. Substrate: cyclohexanol (0.5 mL). Oxidizing agent: H2O2 (0.7 mL, 30%)
1 3

Amorphous Cr/SiO2 Materials Hydrothermally Treated: Liquid Phase Cyclohexanol Oxidation
of alcohols (2-propanol and C–OH) in the gas phase [27];
where materials without hydrothermal treatment showed
greater selectivity towards the carbonyl derivatives than
those materials prepared by hydrothermal treatment at
220 °C. So, the latter ones showed, in the gas phase at
280 °C, almost exclusively dehydrating activity over 2-pro-
panol and cyclohexanol; whereas materials without hydro-
thermal treatment, also showed dehydrogenating activity.
All these results indicate that the main reaction, when
CH3CN is employed as the solvent, develops into the homo-
geneous phase. The role of the catalyst surface is to modify
the selectivity to cyclohexanone as the acidity changes by
the hydrotreatment.
Cr3+ to Cr6+ who starts the reaction into the homogeneous
phase promoting the degradation of the hydrocarbon chain.
So, in spite of TBHP is less reactive than H2O2 it is more
selective to the carbonyl derivative (55%). This behavior
probably relates to that the TBHP (70% in H2O) intro-
duces a smaller amount of water to the reaction medium
and explains previous results reported in the bibliography
[19] wherein, using a non-aqueous solvent (Cl-benzene)
and anhydrous TBHP as the oxidizing agent, selectivity
remains close to 90% under mild oxidation conditions.
3.2.3 Verbenol to Verbenone Oxidation
3.2.2.6 Oxidizing Agent Effect Figure 12 shows the effect
of two different oxidizing agents on the cyclohexanol con-
version and cyclohexanone selectivity. There, the oxidizing
behaviors of H2O2 (30%) and TBHP (70% weight) in water
are compared. The amount of the oxidizing agent, in both
cases, was the necessary to keep the molar ratio (R) of the
substrate to oxidizing agent equal to one. All the other reac-
tion conditions were that described in the “Experimental”
section.
To complement the previous results, we investigate the
oxidation of verbenol to verbenone. Like any allylic alco-
hol, we expected a straightforward oxidation towards the
carbonyl derivative. The reaction was carried out, using
H2O2 (30%) as an oxidizing agent, over the catalyst hydro-
thermally treated at 150 °C (5SG150110), as it is the one
with an intermediate activity.
It can be seen (Fig. 13) that verbenol conversion reached
94% after 6 h of reaction with a conversion to verbenone of
53% (selectivity 56%). This selectivity was superior to that
obtained with the other cyclic substrates (cyclohexanol,
cyclohexene, and cyclohexanone) under the same condi-
tions of reaction. The difference is mainly attributed to that
verbenol oxidation to α-β carbonyl derivative is thermody-
namically favored, while in the other cyclic compounds,
the ring opening is privileged, so the reaction is largely
directed towards degradation of the ring.
After 7 h of reaction, although hydrogen peroxide showed
greater activity than TBHP (CC−OH = 60 vs. 25%, respec-
tively), the last one showed a higher selectivity to the car-
bonyl derivative (up to 55%); the highest from the previously
described experiment.
As it was discussed above, H2O2 quickly decomposes
in the presence of the chromium catalyst, oxidizing the
Fig. 12 Oxidizing agent effect on a cyclohexanol conversion and b
cyclohexanone selectivity. Oxidant agents: H2O2 (0.7 mL, 30%) and
TBHP (0.9 mL, 70%), R (molar ratio substrate/oxidant)=1. Reaction
conditions: Temperature: 70 °C. Solvent: CH3CN (5 mL). Catalyst:
5SGnHT110 (10 mg). Substrate: cyclohexanol (0.5 mL)
1 3

J. F. Miranda et al.
Funding Funding was provided by Consejo de Investigación, Uni-
versidad Nacional de Salta and Consejo Nacional de Investigaciones
Científicas y Técnicas.
Compliance with Ethical Standards
Conflict of interest Authors declare they have not any ethical con-
flict of interest related to the publication and its content.
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Cr/SiO2 in acetonitrile constitutes a catalytic system
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Acknowledgements This work was performed under the auspices and
the financial support of INIQUI-CONICET and CIUNSa.
1 3

Amorphous Cr/SiO2 Materials Hydrothermally Treated: Liquid Phase Cyclohexanol Oxidation
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Affiliations
José Feliciano Miranda1 · Pablo M. Cuesta Zapata1 · Elio E. Gonzo2 · Mónica L. Parentis2 · Lilian E. Davies1 ·
Norberto A. Bonini1
José Feliciano Miranda
josefm.86@gmail.com
Lilian E. Davies
lilian@exa.unsa.edu.ar
Pablo M. Cuesta Zapata
pcuesta@unsa.edu.ar
1
2
Facultad de Ciencias Exactas, INIQUI – CONICET,
Universidad Nacional de Salta, Avda. Bolivia 5150,
4400 Salta, Argentina
Elio E. Gonzo
gonzo@unsa.edu.ar
Facultad de Ingeniería, INIQUI – CONICET, Universidad
Nacional de Salta, Avda. Bolivia 5150, 4400 Salta, Argentina
Mónica L. Parentis
mparentis@unsa.edu.ar
1 3