To the question of oxidation on the surface of oxides: temperature- programmed oxidation of cyclohexanol
Temperature-programmed reaction (TPR) method with mass spectrometric control of the products was used to study of cyclohexanol oxidation into cyclohexanone on individual and mixed oxides supported by γ-Al2O3 and silica gel. In the TPR profiles the temperature of a maximum r...
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V.P. Kukhar Institute of Bioorganic Chemistry and Petrochemistry of the National Academy of Sciences of Ukraine
2022
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Catalysis and petrochemistry| _version_ | 1872009035717279744 |
|---|---|
| author | Brei, Volodymyr V. Levytska, Svitlana I. Prudius, Svitlana V. |
| author_facet | Brei, Volodymyr V. Levytska, Svitlana I. Prudius, Svitlana V. |
| author_institution_txt_mv | [
{
"author": "Volodymyr V. Brei",
"institution": "Institute for Sorption and Problems of Endoecology of the National Academy of Sciences of Ukraine, 13 General Naumov Str., Kyiv 03164, Ukraine"
},
{
"author": "Svitlana I. Levytska",
"institution": "Institute for Sorption and Problems of Endoecology of the National Academy of Sciences of Ukraine, 13 General Naumov Str., Kyiv 03164, Ukraine"
},
{
"author": "Svitlana V. Prudius",
"institution": "Institute for Sorption and Problems of Endoecology of the National Academy of Sciences of Ukraine, 13 General Naumov Str., Kyiv 03164, Ukraine"
}
] |
| author_sort | Brei, Volodymyr V. |
| baseUrl_str | https://kataliz.org.ua/index.php/journal/oai |
| collection | OJS |
| datestamp_date | 2023-01-20T09:37:29Z |
| description | Temperature-programmed reaction (TPR) method with mass spectrometric control of the products was used to study of cyclohexanol oxidation into cyclohexanone on individual and mixed oxides supported by γ-Al2O3 and silica gel. In the TPR profiles the temperature of a maximum rate of cyclohexanone formation varies from 125°C for MoO3/Al2O3 to 235°C for less active CuO/Al2O3. The catalytic activity of individual oxides decreases in the order MoO3/Al2O3> V2O5/SiO2 > Fe2O3/Al2O3 > Bi2O5/Al2O3 > TiO2/SiO2 ≈ СeO2/Al2O3 > TiO2/Al2O3 > SnO2/Al2O3. As "reactive" oxygen in our TPR experiment was supplied only from oxide lattice, oxide activity is determined by different energy of the surface Me – O bonds. The approach to search for mixed active oxides based on decreasing coordination number of O2- ions is proposed, that confirmed by the example of CuO-WO3/Al2O3 catalyst. The mixed supported oxides, especially CuOCrO3/Al2O3, CuO-MoO3/Al2O3, MoO3-SnO2/Al2O3 and Bi2O3–SnO2/Al2O3, are more active in С6Н12О + 1/2О2 → С6Н10О + Н2О oxidation. The synthesized CuO-CrO3/Al2O3 catalyst provides cyclohexanone formation without side   cyclohexanol dehydration and can be used for the oxidation of ethylene glycol – methanol mixture into methyl glycolate. CuO-Cr2O3/Al2O3 with a spinel structure of CuCr2O4 ([CuO4] 6− tetrahedra, Cu2+ sp3-hybridization) is more active in cyclohexanol oxidation than CuO/Al2O3 with flat [CuO4] 6−squares, Cu2+ dsp2-hybridization. This is explained by the lower energy of Cu-O bonds at sp3-hybridization of Cu2+ ions. |
| doi_str_mv | 10.15407/kataliz2022.33.001 |
| first_indexed | 2026-03-12T15:50:14Z |
| format | Article |
| fulltext |
Каталіз та нафтохімія, 2022, №33 1
ISSN 2707-5796. Catalysis and Petrochemistry, 2022, 33
УДК 544.47
https://doi.org/10.15407/kataliz2022.33.001
To the question of oxidation on the surface of oxides: temperature-
programmed oxidation of cyclohexanol
Volodymyr V. Brei, Svitlana I. Levytska, Svitlana V. Prudius
Institute for Sorption and Problems of Endoecology of the National Academy of Sciences of Ukraine, 13 General Naumov Str.,
Kyiv 03164, Ukraine; brei@ukr.net
Temperature-programmed reaction (TPR) method with mass spectrometric control of the products was used to
study of cyclohexanol oxidation into cyclohexanone on individual and mixed oxides supported by γ-Al2O3 and silica
gel. In the TPR profiles the temperature of a maximum rate of cyclohexanone formation varies from 125°C for
MoO3/Al2O3 to 235°C for less active CuO/Al2O3. The catalytic activity of individual oxides decreases in the order
MoO3/Al2O3> V2O5/SiO2 > Fe2O3/Al2O3 > Bi2O5/Al2O3 > TiO2/SiO2 ≈ СeO2/Al2O3 > TiO2/Al2O3 > SnO2/Al2O3. As
"reactive" oxygen in our TPR experiment was supplied only from oxide lattice, oxide activity is determined by
different energy of the surface Me – O bonds. The approach to search for mixed active oxides based on decreasing
coordination number of O2- ions is proposed, that confirmed by the example of CuO-WO3/Al2O3 catalyst. The mixed
supported oxides, especially CuO-CrO3/Al2O3, CuO-MoO3/Al2O3, MoO3-SnO2/Al2O3 and Bi2O3–SnO2/Al2O3, are more
active in С6Н12О + 1/2О2 → С6Н10О + Н2О oxidation. The synthesized CuO-CrO3/Al2O3 catalyst provides
cyclohexanone formation without side cyclohexanol dehydration and can be used for the oxidation of ethylene glycol
– methanol mixture into methyl glycolate. CuO-Cr2O3/Al2O3 with a spinel structure of CuCr2O4 ([CuO4]
6− tetrahedra,
Cu2+ sp3-hybridization) is more active in cyclohexanol oxidation than CuO/Al2O3 with flat [CuO4]
6−squares, Cu2+
dsp2-hybridization. This is explained by the lower energy of Cu-O bonds at sp3-hybridization of Cu2+ ions.
Keywords: heterogeneous catalysis, oxidation on oxides, supported catalysts, mixed oxides
____________________________________________________________________________________________________
Introduction
Even though many monographs and reviews
have been written on the catalytic oxidation of various
organic compounds on oxides [for example, [1-5]], the
search for a suitable catalyst remains largely luck of a
researcher. We have studied the reaction of
cyclohexanol oxidation into cyclohexanone С6Н12О +
1/2О2→ С6Н10О + Н2О on typical catalytic oxides at
linear increasing temperature with mass-spectrometry
product analysis. In particular, this TPR method has
been applied for finding correlations between the
ability of alcohols to be oxidized on CeO2/AlO3 and
their chemical shifts δ (R13СОH) [6]. Usually,
desorption mass-spectrometry is used to study
monomolecular surface reactions such as the
destruction of 1-butin-2-methyl-2ol (MBOH) test
molecule or alcohol dehydrogenation on the Cu-
catalyst [7]. However, the release of oxygen from the
oxide lattice of a catalyst allows to study bimolecular
reaction of alcohol oxidation. Obviously, it’s possible
to discuss Mars-Crevelen's mechanism only. In this
communication, the TPR spectra of cyclohexanol
oxidation on individual and mixed oxides supported by
alumina and silica are presented, and the reaction
schemes are discussed also.
Experiment
Numerous individual and mixed oxides
supported by γ-Al2O3 and SiO2 (30 samples) were used
for the cyclohexanol (chromatography purity) TPR
oxidation (Table 1, 2). The samples containing 2÷45
wt.% of supported oxides were prepared by usual
incipient wetness impregnation of commercial γ-Al2O3
and silica gel with calculated aqueous solutions of
correspondent metal salts. The supported oxides
precursors were calcined at 400÷700°C for 1÷6 h. The
samples were denoted as хМеO/Al2O3(SiO2), where х
is the MeO content in wt.%. The supported samples
lose not more than 50% of initial high surface area of
the γ-Al2O3 (260 m2/g) and silica gel (380 m2/g).
The TPR profiles of cyclohexanol products
oxidation were registered on the modernized monopole
mass-spectrometer MX-7304 (Ukraine). Previously
oxide sample (1-2 mg) in a quartz cuvette was
evacuated at 60-80°C, cooled to room temperature, and
adsorption of alcohol vapor was provided. Then a
sample was vacuumed at 30°C, and the TPR spectrum
in the range, as a rule, of m/e = 39 -72 at sweep 2
a.u.m./s and temperature raising of 15°C/min was
recorded.
Results and discussion
Individual supported oxides
In Fig. 1 some typical TPR profiles of
cyclohexanol oxidation on individual supported oxides
are presented. Two main experimental facts should be
noted:
mailto:s@ukr.net
2 Каталіз та нафтохімія, 2022, №33
ISSN 2707-5796. Catalysis and Petrochemistry, 2022, 33
- the maximum rate of cyclohexanone formation is
observed at various temperatures for several
oxides in the range from 125 to 235°C (Table 1);
- in the TPR spectra three main products are fixed,
namely, cyclohexanone (55, 42, 69 a.u.m.),
cyclohexene (54, 67, 39 a.u.m.), and CO2 (44
a.u.m.).
Cyclohexene forms at dehydration of cyclohexanol.
This is the main side reaction chartered for alcohols,
that proceeds on all studied oxides excluding CuO
(Fig. 1). Interestingly, the peaks of cyclohexene and
cyclohexanone formation often are observed at the
practically same temperatures (Fig. 1, 2), i.e.
exothermic oxidation of cyclohexanol stimulates its
endothermic dehydration. Suppression of the
dehydration of alcohols during their oxidation on solid
catalysts is an important practical question.
0 30 60 90 120 150 180 210 240 270 300 330 360 390
0
20
40
60
I,
a
.u
.
T,
o
C
44
54
55
57
5% TiO
2
/Al
2
O
3
0 30 60 90 120 150 180 210 240 270 300 330 360
0
10
20
30
40
I,
a
.u
.
T,
o
C
44
54
55
57
2%V
2
O
5
/Al
2
O
3
0 30 60 90 120 150 180 210 240 270 300 330 360 390
0
4
8
12
16
I,
a
.u
.
T,
o
C
44
54
55
57
45%CuO/Al
2
O
3
0 30 60 90 120 150 180 210 240 270 300 330
0
10
20
30I,
a
.u
.
T,
o
C
44
54
55
57
30% MoO
3
/Al
2
O
3
0 30 60 90 120 150 180 210 240
20
40
60
I,
a
.u
.
T,
o
C
44
54
55
57
CeO2/Al2O
3
0 40 80 120 160 200 240 280 320
0
20
40
60
80
I,
a
.u
.
T,
o
C
44
54
55
57
10%WO
3
/Al
2
O
3
Каталіз та нафтохімія, 2022, №33 3
ISSN 2707-5796. Catalysis and Petrochemistry, 2022, 33
0 30 60 90 120 150 180 210 240 270 300
0
20
40
60
80
I,
a
.u
.
T,
o
C
44
54
55
57
15%Bi
2
O
5
/Al
2
O
3
0 30 60 90 120 150 180 210 240 270 300 330 360 390
0
20
40
60
80
15%Bi
2
O
5
/SiO
2
I,
a
.u
.
T,
o
C
44
54
55
57
Fig.1. TPR profiles of cyclohexanol oxidation on individual supported oxides: 44 a.u.m. – CO2;
54 a.u.m. – cyclohexene; 55 a.u.m. – cyclohexanone; 57 a.u.m. - cyclohexanol.
Target cyclohexanone forms at 125°C on the most
active MoO3/Al2O3 and at 235°C for less active
CuO/Al2O3 that corresponds to change activation
energy of cyclohexanol oxidation reaction Ea ≈ 25 RTm
[6], (where Tm in K), from 19.9 kсal/mol to 25.4
kсal/mol (Table 1). Obviously that this Ea difference
could be associated with different energies of Cu-O
and Mo-O bonds on the surface of oxides.
Table 1. Temperatures of peak formation of cyclohexanone (Tm
55) and cyclohexene (Tm
54) from
cyclohexanol adsorbed on studied oxides
Sample
Tm
55, °C
Ea=25RTm
55,
kcal/mol * Tm
54, °C
T44, °C **
5wt%TiO2/Al2O3 195 23.4 200 >300
5wt%TiO2/SiO2 190 23.2 190 >270
2 %V2O5/Al2O3 155 21.4 155 >330
2%V2O5/SiO2 135 20.4 135 >350
2%MnO2/ Al2O3 - - 205 >180
2%MnO2/ SiO2 - - - >160
7%Fe2O3/Al2O3 145 20.9 145 >270
5%Fe2O3/SiO2 - - 135 >270
45wt%СuO/Al2O3 235 25.4 240 >235
20wt%СuO/SiO2 - - - >170
30wt%MoO3/Al2O3 125 19.9 125 >280
30wt%MoO3/SiO2 - - 110 >290
10wt% SnO2/Al2O3 205 23.9 205 >295
10wt%СeO2/Al2O3 195 23.4 195 -
10wt%СeO2/SiO2 - - 130 -
10wt%WO3/Al2O3 145 20.9 145 -
10wt%WO3/SiO2 - - 115 -
15%Bi2O5/Al2O3 160 21.7 160 >240
15%Bi2O5/SiO2 - - - >270
*) Ea –activation energy of cyclohexanol oxidation
**) temperature of CO2 formation
In TPR spectra for practically all studied samples
the CO2 formation, as a result of full cyclohexanol
oxidation, is observed at temperatures higher 250°C as
rule (Fig. 1, 2; Table 1, 3). More active towards CO2
formation are MnO2/Al2O3(SiO2) samples, unsuitable
for selective alcohol oxidation. For the most part, the
oxidation TPR spectra for alumina and silica as
carriers are strikingly different, for instance, for
supported Bi2O3 (Fig. 1). On the whole, oxides
supported by alumina are more active (Table 1). A
4 Каталіз та нафтохімія, 2022, №33
ISSN 2707-5796. Catalysis and Petrochemistry, 2022, 33
sample can be considered active if in its TPR spectrum
the intensity of the fragment ion (55 a.u.m.) of
cyclohexanone exceeds one of splinter cyclohexanol
ion 57 a.u.m. at peaks. The influence of a carrier on the
activity of supported catalytic oxide is a separate
important question that is not discussed here.
The supported Cr2O3/Al2O3(SiO2) samples were
not active in the oxidation of alcohol, but unexpectedly
for us supported SnO2 and Bi2O3 oxides showed not
bad activity towards cyclohexanone formation (Table
1). Thus, among the studied samples, some individual
oxides are suitable for the oxidation of cyclohexanol
into cyclohexanone. Their activity decreases in such
order: MoO3/Al2O3 > V2O5/SiO2 > Fe2O3/Al2O3 >
Bi2O5/Al2O3 > TiO2/SiO2 ≈ СeO2/Al2O3 > TiO2/Al2O3 >
SnO2/Al2O3.
Reaction proceeding
In our experiments at low pressure (~ 10-3-5 Tor in
cuvette), oxygen for the reaction can be generated
from oxide lattice only according to the equation
2CuO = Cu2O + 1/2O2, for instance. A possible
oxidation scheme could be written as
-IVCu2+ -IIIO− + C6H11OH → -IVCu+□ + C6H10O + H2O (1)
Active surface oxygen ion returns an electron to
Cu2+ ion and formed O atom attacks the adsorbed
cyclohexanol molecule. Note, the formed oxygen atom
is easier attached to alcoholic -CH2OH group with a
reduced electron density (or a large chemical shift) on
the carbon atom [6]. As result, molecules of
cyclohexanone and water, and a virtual oxygen atom
vacancy □ or a hole are formed. Obviously, that
activation reaction energy is determined by the energy
of broken surface Me-O bonds. Studied samples
changed their color after the experiment, for instance,
light yellow СeO2/Al2O3 turned brown that chartered
for Ce2O3.
How is it possible to estimate the energy of Me-O
bond in the surface-active site? There are many data on
the heat of oxygen chemisorption QO2 on oxides, and
the known correlation between QO2 and activation
energy Ea = E0 – 0.5 QO2 or lgW = a - bQO2, where W
–reaction rate, has been determined [8]. We have tried
to obtain TPD profiles of O2 chemisorbed on studied
oxides, as Edes = QO2 max, but reliable, repeatable results
did not obtain, even after preliminary calcining
samples and adsorption of pure oxygen. Boreskov with
co-workers [8] have measured the O2 pressure (10-1-10-
4 Tor) above several oxides heated at various
temperatures (150-600°C) and calculated using the
Clapeiron equation the qo values named “binding
energies of oxygen”. These qo values are useful for
estimating the ability of oxides to lose oxygen (Table
2).
The estimation of energy of surface Me-O bond
could be based on a standard enthalpy formation of
oxide but necessary to consider the coordination
number (CNo) of oxygen ions in lattice and the
circumstance that for O2-ion on the surface its
coordination number decreases by 1. For instance, for
IVCuIVO with [OCu4] tetrahedrons (O sp3-
hybridization) and flat [CuO4]6− squares (Cu dsp2), the
binding energy of surface Cu-O could be calculated as
ECu-O = ΔH0
f (CNo-1)/CNo
2 = 37* 3/16 = 7 kcal/mol,
and for VIMnIIIO2 with rutile’s structure EMn-O =
125*2/9 = 28 kcal/mol. Such calculated EMe-O values
for studied oxides are presented in Table 2. These
values are varied from 51 kcal/mol for WO3 to
7 kcal/mol for CuO (Table 2).
The calculated enthalpies (ΔHR) of the reactions
of decomposition of higher oxides into lower ones with
the release of oxygen are changed within 90-20
kcal/mol (Table 2). Electron affinity (EA) of higher
oxides is undoubtedly a useful parameter because it
reflects their ability to change a charge of Me-cation
(Men+1 + e → Men+), but such data are available for
some oxides only (Table 2). Standard electrode
potentials also are useful. For example, comparison of
E0=0.77 v for Fe3+ +e =Fe2+ and E0 = 0.15 v for
Cu2+ + e = Cu+ allows to say that Fe2O3 is a stronger
oxidizer than CuO.
However, the comparison of experimental Tm
55, Ea
results (Table 1) and EMe-O, ΔHR values, presented in
Table 2, does not show any correlations.
Consider a possible way for the oxidation of
adsorbed alcohol molecule with the participation of
surface oxygen ion of oxide, for instance, CuO. A
surface oxygen ion with an unpaired electron can exist
either as
-CuOH group or as a strained -Cu-O*-Cu- bridge.
Such sites “sleep” until they are attacked by adsorbed
alcohol molecules. In the case of bridging structure,
the sum reaction is written as
-Cu2+-O*-Cu2+- + C6H11OH = 2 –Cu+□ + C6H10O + H2O.
Каталіз та нафтохімія, 2022, №33 5
ISSN 2707-5796. Catalysis and Petrochemistry, 2022, 33
In the case of –Cu-OH group, split-off OH•
radical could attack the hydrogen atom of β-CH2 group
of cyclohexanol, forming water and 3-cyclohexene-1-
ol (m/e=54). Of course, hydroxyl groups and strained
bridges may affect oxidation only at the beginning of a
reaction that proceeds with the participation of
molecular O2 in the gas phase.
So as the result of the first catalytic acts, surface
-Cu+□ ions are formed that are capable to chemisorb
O2 molecules, forming –Cu2+-O-O•, and then 2 –Cu2+-
O• sites. As the energy of single Cu-O and -O-O• bonds
is approximately two times less than in O2 molecule
(118 kcal/mol), obviously that these surface sites are a
source of active oxygen for oxidation. At forming
active sites by reaction –Men+□ +O2→ –Men+1-O-O•,
one O-O bond in O2 (~54 kcal/mol) is brooked and –
Men+1-O one is formed. If EMe-O< EO-O, the
chemisorption could be endothermic, but the 2–Men+□
+O2 → 2–Men+1-O• oxygen chemisorption is always
exothermic. The regeneration of active sites is a
complicated question that is not discussed here.
Table 2. Several physical-chemical parameters of individual oxides.
Oxide - ΔHf
0s,*
kcal/mol
EMe-O,**
kcal/mol
qo[8],
kcal/mol
Oxide – O
ΔHR,
kcal/mol
EA, eV***
VITiIIIO2
226 50 59
2TiO2= Ti2O3 +O
Ti4+[Ar] →Ti3+[Ar]3d1
Ti4+ +e =Ti3+, E0=-0.04v
87 1.59
VV2
IIO5
378 92 43
V2O5 = V2O3 +O2
V5+[Ar] →V3+[Ar]3d2
80 -
VIMnIIIO2
125 28 20
2MnO2=Mn2O3 +O
Mn4+[Ar]3d3→Mn3+[Ar]3d4
20 2.06
VIFe2
IVO3
197
38
33
Fe2 O3=2FeO + O
Fe3+[Ar]3d5→Fe2+[Ar]3d6
Fe3+ +e =Fe2+, E0=0.77v
68 3.06
IVCuIVO
37 7 19
2CuO = Cu2O + O
([Cu3+-O]+=[Cu2+-O])
Cu2+[Ar]3d9→Cu+[Ar]3d10
Cu2+ +e =Cu+, E0=0.15v
34 1.78
IVMoIIO3
181 45 53
2MoO3 =Mo2O5 + O
Mo6+[Kr]→Mo5+[Kr]4d1 - 3.17
VISnIIIO2
140 31 -
SnO2 = SnO + O
Sn4+[Kr]4d10→Sn2+[Kr]4d105p2
Sn4+ +2e = Sn2+, E0=0.15v
72 -
VIIICeIVO2
261 49 -
2CeO2 = Ce2O3 + O
Ce4+[Xe] →Ce3+[Xe]4f1
Ce4+ +e = Ce3+, E0=1.61v1
90 -
VIWIIO3
202 51 55
WO3 = WO2 + O
W6+[Xe]4f14 →W4+[Xe]4f145d2
60
3.30
VIBi2
IVO3
BiO2
[BiO][BiO3]
140 26 -
[Bi3+O][Bi5+O3] = VIBi2
IVO3
Bi5+[Xe]4f145d10→
Bi3+[Xe]4f145d106p2
60
3.30
* from: Turova N. Ya. Reference tables on inorganic chemistry. “Khimiya”, 1977;
https://webbook.nist.gov/chemistry/.
** EMe-O = ΔH0
f (CNo-1)/CNo
2
***https://webbook.nist.gov/chemistry/
It should be noted such an interesting option.
With an excess of adsorbed alcohol, its chemisorption
on the –Cu+□ sites with dehydrogenation to aldehyde
(or ketone) is not excluded. The mechanism may be
similar to that described in [7]: an oxygen atom of
alcoholic -OH group exchanges the hydrogen atom for
an electron from -Cu+ ion, forming -Cu2+-O-CH2-R
species; formed H• atom attacks the C-H bond forming
H2 and aldehyde with the return of the “borrowed”
electron to Cu2+ ion. The hydrogen will be oxidized in
the presence of oxygen to water so that the total
reaction (1) does not change. Known data on the
oxidation of methanol to formaldehyde confirm this
pathway.
What is a typical number of active sites on the
oxide surface? According to the results on
chemisorption of methanol on numerous individual
oxides [9], the typical concentration of active sites is ~
7 1014 site/cm2 [10]. In work [11] the reasonable
concentrations of active oxygen [O*]~[Cusurf] = 1.2-
6 Каталіз та нафтохімія, 2022, №33
ISSN 2707-5796. Catalysis and Petrochemistry, 2022, 33
2.6 site/nm2 (1.2 - 2.6 1014 site/cm2) for CuO-CeO2
mixed oxides have been determined. The best
CuCeOx–CP catalyst was capable to oxidize of CO at
60°C with rate 1.2∙10−8 mol CO/m2/s,
7∙10−7 mol CO/gcat/h, TOF = 10−3−10−2 s−1 [11] that
corresponds to 7.2∙1011 molecule/cm2/s (7.2∙10-3
molecule/ nm2/s, TOF = 3∙10−3 s-1) or to space yield
time SYT= 5mmol/gcat/h. Note, that for an industry
catalyst productivity, as rule, must be higher than this
SYT value. The turnover frequency TOF = 3∙10−3 s-1
means that during 1 sec the 300 sites are capable to
oxidize only one molecule of CO as follows from the
calculation of 2.1 10 14 (site/cm2) / 7.2∙1011
(molecule/cm2/s) = 300 site s/molecule.
It’s possible to calculate the collisions number of
CO with the active sites in the experiment performed
in [11]: 333 K, 1 vol% CO at normal pressure (partial
PCO = 10-1 bar [N/cm2]), using Hertz-Knudsen formula
F = P/(2πmkT)1/2 = P NA/(2πMRT10-3)1/2
[molecules/cm2/s], where NA – 6.02 1023 Avogadro
number [molecules/g-mol)], P -partial pressure
[N/cm2], M=mNA molecular weight [g/mol], R= 8.3
[J/(mol K]. Calculated collisions FCO-Cu= 2.7∙1022
PCO/[Cusurf] = 2.7∙1021/2∙1014 ~ 107 [CO
collisions/site/s]. This calculation demonstrates the
possibility of proceeding TPR reaction of absorbed
cyclohexanol with the Me-O• sites even under 10-3 Tor
vacuum (P ~ 10-6 bar) when the collision number
exceeds the [Me-O•] sites by 2-3 orders.
The observed in [11] CO oxidation rate was
7.2∙1011 molecules/cm2/s, i.e. the proportion of active,
reacting CO molecules is
N*/N = 7.2∙1011/3∙1021 ~ 2∙10−10. By Boltzmann
distribution, N*/N = e−E/RT=2∙10-10, and the active CO
molecules must have the energies
E ≥ RT ln2∙1010 ~ 11 kcal/mol, significantly exceeding
average thermal energy of molecules
ε = 3/2∙RT ~ 1 kcal/mol at 333 K.
Mixed oxides
As known, slow desorption of reaction product is
capable to brake oxidizing reaction, and for a selective
catalyst must observe “consistency between the rates
of formation and desorption of oxidized product” [5].
In the case of gas-phase alcohol oxidation, reaction
temperatures > 150°C are suitable for the quick
desorption of formed aldehydes (ketones). Therefore, a
question on the synthesis of oxide systems capable of
effectively releasing active oxygen at 150-200°C is
greatly actual. We have used the way for search of
active mixed oxides based on a decrease in the
coordination number of O2− ions (CNO) in the
framework of mixed oxide in comparison with
individual active oxide. At that, the bond Men+-O• in
the surface species could be weakened, and the
oxidation reaction accelerated. An O2- ion has different
coordination numbers in oxide lattices equal to 4 (sp3-
hybridization, CuO), 3 (sp2, TiO2), or 2 (sp, WO3).
Decreasing CNO formally leads to increasing electron
density on O2- ion, that should weaken Me-O bond. For
instance, the formation of copper tungstate by formal
reaction IVCuIVO + VIWIIO3→IVCuVIW8/3O4 decreases
formal CNf
O from 4 to 8/3 in comparison with CuO,
but increases one from 2 to 8/3 according to WO3. The
formal CNO value is calculated from an electrical
neutrality of the lattice: (2+/4) + (6+/6)+ 2 (2
-
/CNf
O) =
0, whence CNf
O =8/3. Obviously, the O2- ions are
characterized by the integer CNO= 2 or 3. The
proportion of IIO2-ions (X) with CNO=2 is calculated
from the equality 2X + 3(1-X) = 8/3, whence X =1/3,
and the proportion of IIIO2-ions is 2/3. Then, a
probability of the IVCu-IIO –VIW (bond) species
formation in CuWO4 lattice is equal W=1/2 1/3 =1/6,
i.e. [IVCu- IIO –VIW] ≤ 1/6 [CuΣ
2+]. With existence of
such species on the surface, the –Cus-IIO• sites could be
more active in comparison with pure CuO.
In work [12] the crystalline dark gray CuWO4
samples from copper nitrate and sodium tungstate have
been obtained as effective photocatalyst (Eg ~ 2.2 eV).
In calculated lattice, the double [CuO4] squares and
[WO6] octahedrons are present [12], i.e. Cu2+ and W6+
ions keep their coordination numbers in the structure
of mixed oxide. Also, the proportion of IVCu-IIO – VIW
structures in the lattice is equal to 1/3 [12].
In Fig. 2 and Table 3 the results of testing mixed
oxides supported by alumina are presented. All
prepared mixed oxides, including CuO-WO3/Al2O3,
are more active in comparison with individual
supported oxides (Table 1, 3). Quite unexpected for us
were the successful combinations of Bi2O3 and MoO3
with SnO2, and WO3 and MoO3 with CuO (Fig. 2,
Table 3). We also managed to synthesize a quite good
composition of CuO-CrO3 on Al2O3 that oxidizes
cyclohexanol into cyclohexanone at 110°C without its
dehydration in fact (Fig. 2). Also, this sample catalyzes
the oxidation of ethylene glycol – methanol vapour
mixture into methyl glycolate at 200°C with high
selectivity [14].
Каталіз та нафтохімія, 2022, №33 7
ISSN 2707-5796. Catalysis and Petrochemistry, 2022, 33
0 40 80 120 160 200 240 280 320 360 400
0
20
40
60
80
100
I,
a
.u
.
T,
o
C
44
54
55
57
CuO-TiO
2
/Al
2
O
3
40 80 120 160 200 240 280
0
10
20
30
40 CuO-Cr
2
O
3
/Al
2
O
3I,
a
.u
.
T,
o
C
44
54
55
57
0 40 80 120 160 200 240 280 320
0
10
20
30
40
50
I,
a
.u
.
T,
o
C
44
54
55
57
CuO-CrO
3
/Al
2
O
4
0 40 80 120 160 200 240
0
5
10
15
20
25
I,
a
.u
.
T,
o
C
44
54
55
57
CuO-WO
3
/Al
2
O
3
Fig.2. TPR profiles of cyclohexanol oxidation on mixed supported oxides: 44 a.u.m. – CO2; 54 a.u.m. –
cyclohexene; 55 a.u.m. – cyclohexanone; 57 a.u.m. - cyclohexanol.
Table 3. Temperatures of peak formation of cyclohexanone (Tm
55) and cyclohexene
(Tm
55) from cyclohexanol adsorbed on mixed supported oxides
Sample Tm
55, °C Tm
54, °C T44, °C *
CuO-TiO2/Al2O3 160 160 >240
CuO-Cr2O3/Al2O3 155 155 >260
CuO-CrO3/Al2O3 110 210 >260
CuO-Fe2O3/Al2O3 140 140 >250
CuO-MoO3/Al2O3 200 200 >240
CuO-WO3/Al2O3 150 150 >200
MoO3-SnO2/Al2O3 150 150 >270
Bi2O3-SnO2/Al2O3 165 165 >250
*) temperature of CO2 formation
The TPR experiments show that CuO-
Cr2O3/Al2O3 sample with spinel structure of supported
CuCr2O4 ([CuO4]6− tetrahedra) is more active in
cyclohexanol oxidation (Tm
55 =155°C) than CuO/Al2O3
with flat [CuO4] 6− squares (Tm
55 = 235°C) (Table 1, 3).
Ion Cu2+ has 9 electrons at 3d level, and for forming 4
σ-bonds in square of [CuO4]6- are suitable 3dx2-y2, 4s
and 4px, 4py orbitals [13]. For forming [CuO4]6−
tetrahedron the 4s and 4px, 4py, 4pz orbitals of Cu2+ are
suitable only [13]. An O2− ion with 8 electrons at 2s,
2p levels forms 4 equal σ-bonds at sp3-hybridization.
The electrons density is shifted to more electronegative
oxygen atoms. As energy of 4p level is higher than 3d
one [13], the Cusq –O bond is stronger than Cutetr–O
according to the experiment (Table 3).
8 Каталіз та нафтохімія, 2022, №33
ISSN 2707-5796. Catalysis and Petrochemistry, 2022, 33
Conclusions
TPR method with mass-spectrometric control of
reaction products is useful for the study of oxidizing
reactions on oxides. Some individual oxides supported
on alumina and silica oxidize cyclohexanol into
cyclohexanone at 130 – 240°C. Their activity
decreases in the following order: MoO3/Al2O3 >
V2O5/SiO2 > Fe2O3/Al2O3 > Bi2O5/Al2O3 > TiO2/SiO2 ≈
СeO2/Al2O3 > TiO2/Al2O3 > SnO2/Al2O3.
Some mixed oxides, especially CuO-CrO3/Al2O3,
CuO-MoO3/Al2O3, MoO3-SnO2/Al2O3, Bi2O3–SnO2
/Al2O3, are more active in the cyclohexanol oxidation
and could be applied for selective oxidation of other
alcohols including glycerol and ethylene glycol. In
search of a suitable catalyst, a coordination number of
O2− as well as Men+ ions in oxide framework could be
taken into account.
References
1. Wolkenstein Th. Electronic processes on the
surface of semiconductors during chemisorption.
Nauka. Moskow. 1987. 432. [in Russian].
2. Centi G., Cavani F., Trifiro F. Selective oxidation
by heterogeneous catalysis. Kluwer, Academic
Plemiun publishers. New York. 2001.
3. Haber J. 14.11.1 Fundamentals of Hydrocarbon
Oxidation. Handbook of Heterogeneous Catalysis.
Wiley-VCH Verlag GmbH & Co. KGaA. 2008.
4. Lee E.L., Wachs I.E. Use of Oxide Ligands in
Designing Catalytic Active Sites. in Design of
Heterogeneous Catalysts Ed. Ozkan U.S. WILEY-
VCH Verlag GmbH & Co. KGaA. 2009. p. 1-24
5. Panov G.I., Starokon E.V., Ivanov D.P.,
Pirutko L.V., Kharitonov A.S. Active and super
active oxygen on metals in comparison with metal
oxides. Catalysis Rev. 2021. 63(4) 597-638.
6. Brei V.V., Mylin A.M. Oxidation of alcohols over
cerium-oxide catalyst: correlation between the
activation energy of the reaction and the chemical
shift δ (R13 COH). Ukrainian chem. J. 2019.85(8).
66-72.
7. Brei V.V., Mylin A.M. Dehydrogenation of
alcohols on copper catalyst: correlation between
activation energy of the reaction and the chemical
shift δ (R17OH). Ukrainian chem. J. 2017. 83(8).
105-110.
8. Popovsky V.V., Boreskov G.K., Muzykantov
V.S., Sazonov V.A., Shubnikov S.G. Oxygen
binding energy and catalytic activity of some
oxides. Kinetics and catalysis. 1969. 10(4). 787-
795. [in Russian].
9. Badlani M., Wachs I.E. Methanol: a ‘‘smart”
chemical probe molecule. Catal. Letters.2001. 75.
137–149.
10. Wachs I.E. Number of surface sites and turnover
frequencies for oxide catalysts. J. Catal. 2022.
405. 462–472.
11. Polster C.S., Nair H., Baertsch C.D. Study of
active sites and mechanism responsible for
highly selective CO oxidation in H2 rich
atmospheres on a mixed Cu and Ce oxide
catalyst. J. Catal. 2009. 266. 308–319.
12. Xie X., Liub M., Wang C., Chen L., Xu J., Cheng
Y., Dong H., Lu F., Wang W.-H., Liu H., Wang
W. Efficient photo-degradation of dyes using
CuWO4 nanoparticles with electron sacrificial
agents: A combination of experimental and
theoretical exploration. RSC Advances, 2016. 6.
953–959.
13. Gray H.B. Electrons and chemical bonding.
W.A. Beniamin, Inc., New York, Amsterdam,
1965.
14. Varvarin A.M., Levytska S.I., Mylin A.M.,
Zinchenko O.Yu., Brei V.V. Vapor-phase
oxidation of ethylene glycol methanolic solution
into methyl glycolate over Cu-containing
catalysts. Catalysis and Petrochemistry. 2022. N 33.
59-65.
Надійшла до редакції 17.08.2022 р
Каталіз та нафтохімія, 2022, №33 9
ISSN 2707-5796. Catalysis and Petrochemistry, 2022, 33
Окиснення на поверхні оксидів: термопрограмоване окиснення циклогексанолу
Володимир В. Брей, Світлана І. Левицька, Світлана В. Прудіус
Інститут сорбції та проблем ендоекології НАН України, вул. Генерала Наумова, 13; Київ, 03164, Україна
brei@ukr.net
Метод термопрограмованої реакції (ТПР) з мас-спектрометричним контролем продуктів застосовано для
дослідження реакції окиснення циклогексанолу до циклогексанону на індивідуальних та змішаних оксидах,
нанесених на γ-Al2O3 та силікагель. В зареєстрованих ТПР профілях окиснення циклогексанолу температура
макси-мальної швидкості утворення циклогексанону (55 а.о.м.) змінюється від 125°C для MoO3/Al2O3 до
235°C для менш активного CuO/Al2O3, що відповідає зміні енергії активації реакції від 19.9 до 25.4 ккал/моль.
В ТПР мас-спектрах спостерігаються також піки утворення циклогексену (54, 67 а.о.м.), як продукту
дегідратації циклогесанолу, та СО2 (44 а.о.м.), як продукту його повного окиснення за температур > 200°С.
Каталітична активність нанесених індивідуальних оксидів щодо утворення циклогексанону зменшується в
ряду MoO3/Al2O3 > V2O5/SiO2 > Fe2O3/Al2O3 > Bi2O5/Al2O3 > TiO2/SiO2 ≈ СeO2/Al2O3 > TiO2/Al2O3 > SnO2/Al2O3.
Оскільки «реакційний» кисень в умовах нашого ТПР експерименту може постачатись тільки граткою оксиду,
їх активність обумовлюється різною енергією поверхневих Ме – О зв’язків. Запропоновано підхід до пошуку
змішаних активних оксидів, що базується на зменшенні координаційного числа іонів О2-, який підтверджено
на прикладі CuO-WO3/Al2O3 каталізатора. Встановлено, що змішані оксиди, нанесені на γ-Al2O3, особливо,
CuO-CrO3/Al2O3, CuO-MoO3 /Al2O3, MoO3-SnO2/Al2O3 та Bi2O3–SnO2/Al2O3, є більш активними в С6Н12О +
1/2О2 → С6Н10О + Н2О окисненні. Слід зазначити, що синтезований CuO-CrO3/Al2O3 каталізатор забезпечує
утворення циклогексанону практично без побічної дегідратації циклогексанолу і може застосовуватись для
окиснення етиленгліколю з метанолом до метилгліколату. ТПР експеримент показує, що CuO-Cr2O3/Al2O3 з
шпінельною структурою нанесеного CuCr2O4 ([CuO4]6− тетраедри, Cu2+ 4s4p3-гібридизація) є більш активним в
окисненні циклогексанолу (Tm
55 =155°C), ніж CuO/Al2O3 з пласкими [CuO4]6- квадратами, Cu2+ 3d4s4p2-
гібридизація (Tm
55 =235°C). Це пояснюється меншою енергією Cu-O зв’язків при 4s4p3-гібридизації іонів Cu2+.
Ключові слова: гетерогенний каталіз, окиснення на оксидах, нанесені каталізатори, змішані оксиди
mailto:s@ukr.net
|
| id | oai:katalizorgua:article-76 |
| institution | Catalysis and petrochemistry |
| keywords_txt_mv | keywords |
| language | English |
| last_indexed | 2026-03-12T15:50:14Z |
| publishDate | 2022 |
| publisher | V.P. Kukhar Institute of Bioorganic Chemistry and Petrochemistry of the National Academy of Sciences of Ukraine |
| record_format | ojs |
| resource_txt_mv | katalizorgua/37/ed93a7c28fc62e202e2b683f180b4137.pdf |
| spelling | oai:katalizorgua:article-762023-01-20T09:37:29Z To the question of oxidation on the surface of oxides: temperature- programmed oxidation of cyclohexanol To the question of oxidation on the surface of oxides: temperature- programmed oxidation of cyclohexanol Brei, Volodymyr V. Levytska, Svitlana I. Prudius, Svitlana V. heterogeneous catalysis, oxidation on oxides, supported catalysts, mixed oxides гетерогенний каталіз, окиснення на оксидах, нанесені каталізатори, змішані оксиди Temperature-programmed reaction (TPR) method with mass spectrometric control of the products was used to study of cyclohexanol&nbsp;oxidation into cyclohexanone on individual and mixed oxides supported by γ-Al2O3&nbsp;and silica gel. In the TPR profiles the temperature of a maximum rate of cyclohexanone formation varies from 125°C for MoO3/Al2O3&nbsp;to 235°C for less active CuO/Al2O3. The catalytic activity of individual oxides decreases in the order MoO3/Al2O3&gt; V2O5/SiO2&nbsp;&gt; Fe2O3/Al2O3&nbsp;&gt; Bi2O5/Al2O3&nbsp;&gt; TiO2/SiO2&nbsp;≈ СeO2/Al2O3&nbsp;&gt; TiO2/Al2O3&nbsp;&gt; SnO2/Al2O3. As "reactive" oxygen in our TPR experiment was supplied only from oxide lattice, oxide activity is determined by different energy of the surface Me – O bonds. The approach to search for mixed active oxides based on decreasing coordination number of O2-&nbsp;ions is proposed, that confirmed by the example of CuO-WO3/Al2O3&nbsp;catalyst. The mixed supported oxides, especially CuOCrO3/Al2O3, CuO-MoO3/Al2O3, MoO3-SnO2/Al2O3&nbsp;and Bi2O3–SnO2/Al2O3, are more active in С6Н12О + 1/2О2&nbsp;→ С6Н10О + Н2О oxidation. The synthesized CuO-CrO3/Al2O3&nbsp;catalyst provides cyclohexanone formation without side &nbsp;&nbsp;cyclohexanol dehydration and can be used for the oxidation of ethylene glycol – methanol mixture into methyl glycolate. CuO-Cr2O3/Al2O3&nbsp;with a spinel structure of CuCr2O4&nbsp;([CuO4]&nbsp;6−&nbsp;tetrahedra, Cu2+&nbsp;sp3-hybridization) is more active in cyclohexanol oxidation than CuO/Al2O3&nbsp;with flat [CuO4]&nbsp;6−squares, Cu2+&nbsp;dsp2-hybridization. This is explained by the lower energy of Cu-O bonds at sp3-hybridization of Cu2+&nbsp;ions. Temperature-programmed reaction (TPR) method with mass spectrometric control of the products was used to study of cyclohexanol&nbsp;oxidation into cyclohexanone on individual and mixed oxides supported by γ-Al2O3&nbsp;and silica gel. In the TPR profiles the temperature of a maximum rate of cyclohexanone formation varies from 125°C for MoO3/Al2O3&nbsp;to 235°C for less active CuO/Al2O3. The catalytic activity of individual oxides decreases in the order MoO3/Al2O3&gt; V2O5/SiO2&nbsp;&gt; Fe2O3/Al2O3&nbsp;&gt; Bi2O5/Al2O3&nbsp;&gt; TiO2/SiO2&nbsp;≈ СeO2/Al2O3&nbsp;&gt; TiO2/Al2O3&nbsp;&gt; SnO2/Al2O3. As "reactive" oxygen in our TPR experiment was supplied only from oxide lattice, oxide activity is determined by different energy of the surface Me – O bonds. The approach to search for mixed active oxides based on decreasing coordination number of O2-&nbsp;ions is proposed, that confirmed by the example of CuO-WO3/Al2O3&nbsp;catalyst. The mixed supported oxides, especially CuOCrO3/Al2O3, CuO-MoO3/Al2O3, MoO3-SnO2/Al2O3&nbsp;and Bi2O3–SnO2/Al2O3, are more active in С6Н12О + 1/2О2&nbsp;→ С6Н10О + Н2О oxidation. The synthesized CuO-CrO3/Al2O3&nbsp;catalyst provides cyclohexanone formation without side &nbsp;&nbsp;cyclohexanol dehydration and can be used for the oxidation of ethylene glycol – methanol mixture into methyl glycolate. CuO-Cr2O3/Al2O3&nbsp;with a spinel structure of CuCr2O4&nbsp;([CuO4]&nbsp;6−&nbsp;tetrahedra, Cu2+&nbsp;sp3-hybridization) is more active in cyclohexanol oxidation than CuO/Al2O3&nbsp;with flat [CuO4]&nbsp;6−squares, Cu2+&nbsp;dsp2-hybridization. This is explained by the lower energy of Cu-O bonds at sp3-hybridization of Cu2+&nbsp;ions. V.P. Kukhar Institute of Bioorganic Chemistry and Petrochemistry of the National Academy of Sciences of Ukraine 2022-10-26 Article Article application/pdf https://kataliz.org.ua/index.php/journal/article/view/76 10.15407/kataliz2022.33.001 Catalysis and petrochemistry; No. 33 (2022): Catalysis and petrochemistry; 1-9 Каталіз та нафтохімія; № 33 (2022): Каталіз та нафтохімія; 1-9 2707-5796 2412-4176 10.15407/kataliz2022.33 en https://kataliz.org.ua/index.php/journal/article/view/76/77 Copyright (c) 2022 Catalysis and petrochemistry |
| spellingShingle | гетерогенний каталіз окиснення на оксидах нанесені каталізатори змішані оксиди Brei, Volodymyr V. Levytska, Svitlana I. Prudius, Svitlana V. To the question of oxidation on the surface of oxides: temperature- programmed oxidation of cyclohexanol |
| title | To the question of oxidation on the surface of oxides: temperature- programmed oxidation of cyclohexanol |
| title_alt | To the question of oxidation on the surface of oxides: temperature- programmed oxidation of cyclohexanol |
| title_full | To the question of oxidation on the surface of oxides: temperature- programmed oxidation of cyclohexanol |
| title_fullStr | To the question of oxidation on the surface of oxides: temperature- programmed oxidation of cyclohexanol |
| title_full_unstemmed | To the question of oxidation on the surface of oxides: temperature- programmed oxidation of cyclohexanol |
| title_short | To the question of oxidation on the surface of oxides: temperature- programmed oxidation of cyclohexanol |
| title_sort | to the question of oxidation on the surface of oxides: temperature- programmed oxidation of cyclohexanol |
| topic | гетерогенний каталіз окиснення на оксидах нанесені каталізатори змішані оксиди |
| topic_facet | heterogeneous catalysis oxidation on oxides supported catalysts mixed oxides гетерогенний каталіз окиснення на оксидах нанесені каталізатори змішані оксиди |
| url | https://kataliz.org.ua/index.php/journal/article/view/76 |
| work_keys_str_mv | AT breivolodymyrv tothequestionofoxidationonthesurfaceofoxidestemperatureprogrammedoxidationofcyclohexanol AT levytskasvitlanai tothequestionofoxidationonthesurfaceofoxidestemperatureprogrammedoxidationofcyclohexanol AT prudiussvitlanav tothequestionofoxidationonthesurfaceofoxidestemperatureprogrammedoxidationofcyclohexanol |