The Oxidation of Carbon Monoxide as an Integrated Part of the Coupled Alkane Oxidation Process: Gas-Phase Oxidation over Supported Metal-Complex Catalysts

Article abstract of DOI:10.1134/S0023158418020039

Heterogeneous rhodium–copper chloride catalysts for gas-phase oxidation processes were prepared via the cold impregnation of γ-Al₂O₃ with aqueous RhCl₃ and CuCl₂ solutions. Heptafluorobutyric or pentafluorobenzo

Full text of DOI:10.1134/S0023158418020039

ISSN 0023-1584, Kinetics and Catalysis, 2018, Vol. 59, No. 2, pp. 150–159. © Pleiades Publishing, Ltd., 2018.
Original Russian Text © E.G. Chepaikin, A.P. Bezruchenko, G.N. Menchikova, O.P. Tkachenko, L.M. Kustov, A.V. Kulikov, 2018, published in Kinetika i Kataliz, 2018, Vol. 59,
No. 2, pp. 177–187.
The Oxidation of Carbon Monoxide as an Integrated Part
of the Coupled Alkane Oxidation Process: Gas-Phase Oxidation
over Supported Metal-Complex Catalysts
a,
a
a
b
E. G. Chepaikin *, A. P. Bezruchenko , G. N. Menchikova , O. P. Tkachenko ,
b, c
d
L. M. Kustov , and A. V. Kulikov
a
Merzhanov Institute of Structural Macrokinetics and Materials Science, Russian Academy of Sciences,
Chernogolovka, Moscow oblast, 142432 Russia
b
Zelinskii Institute of Organic Chemistry, Russian Academy of Sciences, Moscow, 119991 Russia
c
Department of Chemistry, Moscow State University, Moscow, 119991 Russia
Institute of Problems of Chemical Physics, Russian Academy of Sciences,
d
Chernogolovka, Moscow oblast 142432 Russia
*
e-mail: echep@ism.ac.ru
Received February 20, 2017
Abstract⎯ Heterogeneous rhodium–copper chloride catalysts for gas-phase oxidation processes were pre-
pared via the cold impregnation of γ-Al O with aqueous RhCl and CuCl solutions. Heptafluorobutyric or
2
3
3
2
pentafluorobenzoic acids were additionally deposited onto these catalysts to simulate the action of homoge-
neous rhodium–copper chloride catalytic systems in the coupled alkane–carbon monoxide oxidation reac-
tion. The catalysts were studied in the reactions of carbon monoxide oxidation and coupled propane–CO
oxidation with dioxygen by diffuse reflectance IR Fourier transform spectroscopy (DRIFTS) and electron
paramagnetic resonance (EPR). The obtained data indicate the probable transfer of electrons between rho-
dium and copper compounds.
Keywords: carbon monoxide, catalytic oxidation, rhodium chloride, copper chloride, γ-aluminum oxide,
DRIFTS, EPR
DOI: 10.1134/S0023158418020039
Carbon monoxide oxidation is of great importance Pd(0) and Pd(I) by copper(II) occurs due to contact
from both the theoretical viewpoint (the study of gen- between them, most likely as a result of the formation
eral problems on the mechanism of homogeneous cat- of chloride bridges. The formation of such contacts in
alytic reactions) and the practical viewpoint (detoxifi- solutions is easy due to the high mobility of particles.
cation of gas emissions, respiratory protection, and Water is a component of the catalytic system. It is
purification of hydrogen from CO). Homogeneous interesting that the presence of water is also necessary for
and heterogeneous palladium-containing catalysts are the oxidation of CO on the heterogeneous Pd/γ-Al O
2
3
the most active in this reaction. The homogeneous catalyst. Surface hydroxyl groups of γ-Al O are incor-
2
3
catalysts [1–3] have been actively studied, usually as
palladium metal complexes that are active in the pres-
ence of cocatalysts (reoxidants), such as salts of cop-
per, iron, heteropoly acids, and quinones. CO oxida-
tion in the presence of homogeneous systems can be
simplistically schematized as follows:
porated into the catalytically active intermediate and
further regenerated during the interaction of this cata-
lyst with dioxygen [4].
The oxidation of carbon monoxide over the
PdCl –CuCl catalytic system on different supports
2
2
has been studied to create respiratory protection
PdCl + CO + H O → Pd(0) + CO + 2HCl, (I) devices [5–10]. The reaction was conducted at low CO
2
2
2
concentrations (<5 vol %) and room temperature, and
the initial gas was saturated with water vapor. Since the
Pd(0) + 2CuCl → PdCl + 2CuCl,
(II)
2
2
2
CuCl + ½O + 2HCl → 2CuCl + H O.
(III) adsorbability of γ-Al O with respect to water is high,
2 3
2
2
2
it is possible that the catalytic reaction occurs in the
Several other slightly modified schemes of this pro-
cess have also been proposed. The intermediates that
participate in this process are Pd(II) and, probably,
Pd(I) carbonyls. It is implied that the reoxidation of CuCl
water film on the γ-Al O surface [10].
2
3
According to our preliminary data, the PdCl –
2
/γ-Al O catalysts sustain self-heating (70–
2 3
2
150

THE OXIDATION OF CARBON MON
151
8
0°C) at a CO concentration above 5 vol % due to the complexes [12–14, 19]. The homogeneous oxidation
low efficiency of withdrawal of heat and lose activity of propane generally occurs according to the scheme
due to the reduction of palladium to the metal state.
i-C H OAcf
3
7
CO oxidation plays an essential part in such
important reactions as the oxidation and oxidative car-
bonylation of alkanes. Carbon monoxide is a reducing
agent (coreducer) of dioxygen. The direct partial oxi-
dation of alkanes from natural and associated petro-
leum gases is considered as a promising method for the
synthesis of key petrochemical products. The oxida- acids is also observed.
tion of alkanes over the traditional heterogeneous cat-
alysts is characterized by a low selectivity due to the included the preparation of heterogenized homoge-
side deep oxidation process [11]. A much higher selec- neous catalysts for the gas-phase process to prevent
tivity is demonstrated by the metal complex catalysts the contact of these media with equipment. Such cat-
O2, CO
AcfOH
C3H8
n-C H OAcf
3
7
CH3 C(O) CH3
The formation of small amounts of methyl- and ethyl-
trifluoroacertates and formic, acetic, and propionic
Since acidic media are aggressive, our work also
alysts can be prepared via the deposition of homoge-
neous catalytic system solutions onto porous supports
in solvents with high boiling temperatures (presum-
ably, in the form of thin films). Similar catalysts in the
form of an ionic liquid solution of a metal complex,
which was immobilized on a support [23] or intro-
duced into its pores [24], were studied earlier in hydro-
formylation reactions. The application prospects of
[
12–14]. As a rule, they are more efficient in acidic
media, in particular, in the presence of aqueous per-
fluorocarboxylic acids [12–20]. As shown by the stud-
ies of different oxygenases for the activation of dioxy-
gen under mild conditions, it is necessary to comple-
ment the system with reducing agents [21], among
which hydrogen and carbon monoxide are the most
acceptable ones. Preference is given to CO, as CO2 catalysts of this type were considered in [25].
formed after its oxidation can easily be removed from
As can be judged from the results of studies on
a reactor. One of the most active catalysts for the cou- homogeneous catalytic systems [12–20], the coupled
pled oxidation of alkanes and carbon monoxide is our oxidation of CO and alkanes should be expected to
occur at high CO concentrations (~10 vol %) and tem-
rhodium–copper chloride system formed upon the
peratures of no less than 80–90°C. The reduction of
dissolution of RhCl ⋅ (H O) and copper compounds
3
2
3
RhCl with carbon monoxide into carbonyl complexes
3
(
CuO, Cu O, and Cu(OAc ) in an aqueous trifluoro-
2 f 2
and the oxidation of carbon monoxide in the presence
acetic acid (Ac OH) solution [18]:
f
of rhodium carbonyls with participation of FeCl and
3
benzoquinones as stoichiometric oxidants have been
R−H + CO + O + Ac OH
2
f
studied in aqueous solutions [1].
(
IV)
→
R−O−Ac + CO + H O.
f 2 2
The RhCl –CuCl catalytic system has almost not
3
2
been studied in the considered reaction and the sup-
ported RhCl –CuCl composition has not been syn-
The role of CO consists of the conversion of dioxy-
gen, which is inert under mild conditions, into active
two-electron oxidants, such as hydrogen peroxide or
its analogues over the catalytic system:
3
2
thesized and studied at all by anyone until now. Nev-
ertheless, it is much more stable at 80–90°C, but less
active than the Pd-containing systems. Since the cou-
pled oxidation of alkanes and carbon monoxide occurs
at temperatures of no less than above-mentioned ones,
V) the RhCl –CuCl /γ-Al O catalytic systems should be
2
+
2
Rh"
Cu + CO + H O
2
(
2+
+
3
2
2
3
"
⎯
⎯ ⎯ → Cu + 2H + CO ,
2
2
considered to be more promising than the Pd-con-
taining catalysts. The gas-phase catalytic oxidation of
2
+
2+
2+
carbon monoxide was studied on Rh/TiO and
Cu + O → Cu −O−O−Cu
2
2
2
(VI)
+
Rh/Fe(OH) catalysts and, for comparison, on
2+
x
2
H
⎯
⎯ ⎯ → 2Cu + H O .
2 2
Rh/Al O catalyst in [26, 27]. The activation of oxygen
2
3
on Rh/Al O occurs at an increased temperature,
2
3
Some data [22] indicate that the reaction between
while Rh/TiO and Rh/Fe(OH) are efficient at low
1+
2
x
Cu and dioxygen under certain conditions may lead
temperatures. In the authors’ opinion, this is due to
the formation of heteronuclear structures such as Rh–
O–O–Ti and Rh–O–O–Fe, which provide the possi-
to the accumulation of Н О2 in an appreciable
2
amount. We also considered some probable mecha-
nisms of the action of this rhodium–copper chloride bility of successive competition between oxygen and
catalytic system with the participation of bridging het- carbon monoxide during their activation on a catalyst.
eronuclear intermediates that incorporate rhodium
The objective of this work was to develop and study
and copper and the activation of alkanes on peroxo supported rhodium–copper chloride catalytic sys-
KINETICS AND CATALYSIS Vol. 59 No. 2 2018

1
52
CHEPAIKIN et al.
Table 1. Prepared catalysts and reference samples*
Copper salt
Catalyst
Acidic component
content, wt %
Cu content, wt % Acidic component
CuCl ⋅ 2H O
Cu(OAc ) , wt %
f 2
2
2
CT-1
+
–
3.5
7.0
6.54
3.5
3.5
7.0
5.25
5.25
3.5
_
–
–
–
–
–
CT-2
+
+
+
+
+
+
+
+
–
_
–
3.04
–
CT-3
–
CT-4
Pr COOH
5.7
5.0
5.0
–
f
CT-5
–
Ph COOH
f
CT-6
–
Pr COOH
f
CT-7
–
–
CT-8
–
Pr COOH
8.4
–
f
CT-9
–
–
–
_
CT-10
CT-11А
CT-12А
CT-13А
–
–
_
_
–
–
+
_
–
Pr COOH
7.5
–
f
−
3.5
–
*
Reference samples are CT-9 (CuCl /γ-Al O ), CT-10 (RhCl /γ-Al O ), CT-11A (RhCl /γ-Al O ), CT-12A (RhCl –Pr COOH/γ-Al O
2 2 3 3 2 3 3 2 3 3 f 2 3
(7.5 wt % Pr COOH)), and CT-13A (RhCl –CuCl /γ-Al O ). The copper content in the copper-containing samples is 3.5 wt %.
f 3 2 2 3
The rhodium content in all the samples except CT-9 is 1.5 wt %. The symbol A in the name of a sample means that it was treated with
CO or a reaction vapor–gas mixture under catalytic reaction conditions.
tems, which would be predeterminedly active in First, the hot solution was filtered; Cu(OAc ) was
f 2
homogeneous coupled alkane–CO oxidation reac- then separated after cooling, dried in a vacuum, and
tions. The presence of copper salts promotes the acti- stored in a exiccator.
vation of O . Their activity in the model carbon mon-
2
СО (99.9%), C H (99.8%), O (99.9%), helium of
3
8
2
oxide oxidation reaction was selected as a simple and
convenient catalyst efficiency criterion. This criterion
was used to determine the optimal conditions for the
preparation of stable and active catalysts and to study the
state of rhodium and copper in a catalyst and the possi-
bility of their interaction on the surface of a support.
grade А, electrolytic hydrogen, and purified atmo-
spheric air were used as gaseous reagents.
Preparation of Catalysts
To prepare CT-1–CT-3 catalysts (Table 1), a cal-
culated amount of RhCl and CuCl was dissolved in
3
2
MATERIALS AND METHODS
distilled water (12 mL); γ-Al O (5 g) was then added
2 3
and the mixture was allowed to stand for 12 h. Water
was further distilled out on a rotary evaporator and the
residue was dried at room temperature in air.
Materials
RhCl ⋅ (H O) (34.5% Rh), CuCl ⋅ 2H O (chem-
3
2
n
2
2
ically pure for analysis grade), γ-Al O (GOST (State
2
3
CT-4–CT-6 Catalysts were synthesized via the
Standard) 8136-85) in the form of 0.5–1-mm grains
deposition of a calculated amount of Pr COOH or
f
2
with a BET specific surface area of 205 m /g, distilled
Ph COOH onto CT-1 from their solution in acetone.
f
water, and rectified acetone, methanol, propanol, iso-
propanol, and n-butanol (all of chemically pure grade)
were used in the experiments. Trifluoroacetic acid
The latter was further distilled out on a rotary evapo-
rator and the residue was dried in a vacuum at room
temperature and allowed to stand in air. Reference
catalysts CT-9 and CT-10 were synthesized by the
same method as for CT-1.
(
Ac OH) (Aldrich, 99%, extra-pure grade), hepraflu-
f
orobutyric acid (Pr COOH) (Aldrich, 99%, extra pure
f
grade), and pentafluorobenzoic acid (Ph COOH)
f
(
Aldrich, 99%, extra pure grade) were used without
purification.
Copper trifluoroacetate Cu(OAc ) was synthe-
Catalytic Experiments
f 2
The experiments were performed on a flow-type
sized by dissolving CuO in Ac OH under boiling. setup (Fig. 1) at atmospheric pressure. The initial gas
f
KINETICS AND CATALYSIS
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THE OXIDATION OF CARBON MON
153
2
V-2
Reaction gas V-4
V-1
Helium
4
Loop
5
To chromatograph
7
Reaction gas
Reaction gas
1
V-5
Helium
Loop
6
3
To chromatograph
Reaction gas V-3
8
Fig. 1. Experimental setup: (1) bottle with a mixture of initial gases, (2) pressure gauge, (3) rheometer, (4) saturator for the satu-
ration of gas with water vapor, (5) reactor with a temperature-controlled jacket, (6) condensing cooler, (7), (8) dosing valves for
the introduction of samples into chromatograph columns; V-1 is the fine-control valve, V-2–V-5 are shutoff valves.
was prepared in steel bottle 1 by mixing the compo-
The chromatographic data were used as a basis to
nents, which were dosed with a reference pressure determine the CO conversion and the catalyst activity
gauge and diluted to prevent the formation of explo- (А) as
sive mixtures. In the coupled propane–CO oxidation
experiments, the mixture was diluted with propane.
The flow rate of the initial gas mixture was determined
0
WVC αk
CO
A =
,
4
10 × 22.4m
–1
with precalibrated rheometer 3. The gas at the inlet where W is the gas hourly space velocity (HSV) (h ),
into the reactor was saturated with water vapor, whose
partial pressure was adjusted by changing the tempera-
ture in the jacket of steam bubbler 4. The steam bub-
bler height was 130 mm; the end of its inlet tube was
equipped with a porous glass filtering plate to promote
the formation of small bubbles and their saturation
with water vapor. It turned out to be impossible to
measure the moisture content with an IVTM-7
hygrometer due to flow pulsations despite that they
0
СО
3
V is the catalyst volume (cm ), С is the CO content
in the initial gas (%), α is the CO conversion (%), k is
the coefficient for the reduction of gas to normal con-
ditions, and m is the amount of RhCl in a catalyst
3
(
mmol).
Analysis of Reaction Products
The gas and liquid phases were analyzed on a Crys-
were small. The pipeline from the bubbler to the reac- tallLux-4000M chromatograph (Meta-chrom, Rus-
tor was carefully thermoinsulated to prevent the con- sia) using NetChrom V2.1 software. The gas phase was
analyzed at a column temperature of 55°C using
densation of water. The amount of water entrapped in
the condenser for 2–3 h was 0.75–0.85 of the calcu-
lated amount of the passed water vapor. Temperature
in the jacket of reactor 5 (inner diameter, 9.5 m;
height, 130 mm) was kept using a water thermostat.
The gas at the outlet from the reactor was cooled in
condenser 6 to separate water vapor. In the coupled
propane–CO oxidation experiments, the trap was
cooled to –40°C to provide the entrapment of pro-
pane oxidation products. The cooled gas was passed
to gas dosing valves 7, through which it was fed into a
column with molecular sieves or valves 8 (into a
Poparak Q column). Before the measurements, a
vapor–gas mixture was passed through the reactor
helium as a carrier gas and a thermal conductivity
detector, the analysis for O , N , CH , CO was per-
2
2
4
formed in a column, which was packed with 5-Å
molecular sieves and had a length l = 3 m and a diam-
eter d = 3 mm, at a grain size of 0.2–0.3 mm, and a
carrier gas flow rate of 30 mL/min; the analysis for
CO and propane was performed using Poparak Q as a
2
packing with a grain size of 0.115–0.200 mm at l = 2 m,
d = 2.5 mm, and a carrier gas flow rate of 20 mL/min.
The liquid phase was analyzed using a flame ioniza-
tion detector and an Agilent CP-Sil 5CB capillary col-
umn with l = 25 m and d = 0.15 mm at a temperature
programmed from 40 to 150°C at a rate of 5 K/min, a
He flow rate of 20 mL/min, an inlet column pressure
for 1 h to attain a steady-state regime. The composi- of 1.3 atm, splitting the flow at a ratio of 1 : 70 at its rate
tion of the outlet gas (CO, CO , O ) was then mea- of 0.287 mL/min.
2
2
sured three times for 30–40 min together with its
flow rate (with a foam rheometer).
The catalysts and reference samples were studied
by diffuse reflectance IR Fourier transform spectros-
KINETICS AND CATALYSIS Vol. 59 No. 2 2018

1
54
CHEPAIKIN et al.
Table 2. Results of testing in the CO oxidation reaction for catalysts CT-1, CT-3, and CT-7*
Water vapor
content, vol %
А,
W, h–1
Experiment no.
Catalyst
Т, °C
α, %
molСО/(molRh h)
1
2
3
4
5
6
7
8
9
50
60
70
80
251
249
251
280
351
780
250
553
627
277
270
1023
69.0
80.0
83.7
92.4
88
9.7
11.2
11.9
14.8
17.4
38.6
9.6
2
.8
CT-1
2.8
19.8
2.8
9
0
87.6
68.1
69.7
78.0
23.6
1.44
89.8
50
80
90
2.8
21.5
27.2
3.3
CT-2
7.4
10
CT-3
80
2.8
7.4
11
CT-3 in 2 h
CT-7
0.20
49.5
12
90
3
*
1
Catalyst weight, 2.1 g (V = 4.2 cm ); m = 0.3 mmol. Gas composition (vol %): О , 5.12; % СО, 9.67, helium, balance. Experiments
Rh 2
–7 were performed on the same catalyst.
copy (DRIFT) on a NICOLET Protege 460 spec- ising support (at least, for Pd-catalysts) is γ-Al O [4–
2
3
trometer (France) equipped with a diffuse reflectance 8], which we also used. The composition of the cata-
attachment [28] developed in the Zelinskii Institute of lysts and reference samples we prepared is given in
Organic Chemistry of the Russian Academy of Sci- Table 1. CT-1–CT-3 catalysts were compared with
ences at room temperature in the region from 6000 to CT-4–CT-6 and CT-8 containing perfluorocarbox-
–
1
–1
4
00 cm at a step of 4 cm . To attain a satisfactory ylic acids. In this case, we made the assumption that
signal/noise ratio, 500 spectra were accumulated. γ-Al O has the rather high acidity required for the
CaF was used as a standard. Before CO was fed into the oxidation of alkanes.
ampoule with a catalyst it was evacuated to 8 × 10 Torr
at room temperature for 5 min.
2
3
2
–2
Catalyst testing results are given in Tables 2 and 3.
From Table 2 it follows that CT-1 and CT-2 catalysts
EPR spectra were recorded on a Radiopan SE/X exhibit high activity (А) and stability within a tempera-
544 spectrometer (Poland) at room temperature in ture range of 70–90°C, where (by analogy with the
2
ampoules with valves for the evacuation and feeding of action of homogeneous catalysts) the coupled oxida-
gases. The second EPR spectrum integral (I) normal- tion of propane and carbon monoxide can occur. The
ized to three instrumental parameters (amplification CT-3 catalyst containing copper partially in the form
coefficient, microwave power, and magnetic field of Cu(OAc ) (see Table 1) proved to have low activity
f 2
modulation) was proportional to the number of para- and was unstable (Table 2, experiments 10 and 11).
magnetic sites in a catalyst. Carbon monoxide or air The activity grows with an increase in the partial
was fed into preevacuated ampoules until the atmo- water-vapor pressure (experiments 5 and 6) and the
spheric pressure was attained.
copper content (experiments 4, 8 and 6, 9). Experi-
ments 1 and 7 indicate the stability of the catalysts.
The CT-7 catalyst was slightly more active than CT-2,
probably due to its larger specific surface area. The use
RESULTS AND DISCUSSION
of copper in the form of CuCl at a content of ~5% is
2
As shown earlier, homogeneous rhodium–copper
chloride catalytic systems are efficient in an aqueous
optimal.
trifluoroacetic acid (Ac OH) medium [12–14]. How-
From Table 3 it follows that the introduction of
f
ever, Ac OH is not a suitable component for a heterog- perfluorocarboxylic acids slightly decreases the activ-
f
enized system due to its low boiling temperature ity of catalysts. Nevertheless, the CT-4 basic catalyst
(
(
72.2°C). For this reason, heptafluorobutyric acid still remains rather active (Table 3, experiments 1–4).
melting temperature, 17.5°C; boiling temperature, It is noteworthy that CT-5, which contain the high
1
20°C) and pentafluorobenzoic acid (melting tem- boiling temperature Ph COOH acid (boiling tempera-
f
perature, 100–102°C; boiling temperature, 220°C) ture 220°C) is also active. An increase in the copper
were selected as acidic components. The most prom- content from 3.5 to 7.0 wt % improves the activity
KINETICS AND CATALYSIS
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THE OXIDATION OF CARBON MON
155
Table 3. The results of testing in the CO oxidation reaction for catalysts CT-4, CT-5, and CT-6*
Water vapor
content, vol %
A,
W, h–1
Experiment no.
Catalyst
Т, °C
α, %
molСО/(molRh h)
1
2
3
4
5
6
7
8
70
70
80
90
80
90
80
90
3.0
231
360
490
542
280
280
425
434
70.4
49.0
56.8
47.4
50.5
68.5
60.0
76.3
8.5
10
CT-4
7.4
14.0
17.5
7.0
CT-5
CT-6
7.4
9.4
10.0
11.5
14.4
21.2**
15.7
100
9
CT-8***
90
9.6
633
19.5
3
*
Catalyst weight, 2.1 g (V = 4 cm ); m = 0.3 mmol. Gas composition in experiments 1–6 and 9 (vol %): О , 5.20; СО, 9.10;
Rh 2
helium, balance; gas composition in experiments 7 and 8 (vol %): О , 7.40; СО, 8.00; helium, balance.
2
** The catalyst changed its color from beige to light yellow by 20 min after the beginning of experiment and its activity increased.
*** CT-8 weight, 1.06 g.
(
experiments 3, 7 and 4, 8), and an increase in the
f
(2) RhCl –CuCl –Pr COOH/γ-Al O (1.5 wt %
3 2 f 2 3
Pr COOH content from 5.0 to 8.4%, on the contrary, Rh, 3.5 wt % Cu, 5.7 wt % Pr COOH) (CT-4);
f
slightly decreases it (experiments 4 and 9). Hence,
perfluorocarboxylic acids introduced at 5–8 wt %
slightly decrease the activity of catalysts, but increase their
acidity as required for the coupled oxidation of alkanes and
carbon monoxide. In experiment 8 (Table 3), this unex-
pectedly increased to А = 21.2 mol /(mol h) in
(
3) CT-4A: catalyst CT-4 that was pretreated with
CO and stored in a CO atmosphere;
(
4) CuCl /γ-Al O (3.5 wt % Cu) (CT-9);
2 2 3
(5) RhCl /γ-Al O (1.5 wt % Rh) (CT-10);
(
3
2
3
СО
Rh
6) RhCl /γ-Al O (1.5 wt % Rh) (CT-11A);
3 2 3
2
0 min after a steady-state regime with an activity of
(
7) RhCl –Pr COOH/γ-Al O (1.5 wt % Rh, 7.5 wt %
1
4.4 mol /(mol h) was attained and the catalyst
3 f 2 3
СО
Rh
Pr COOH) (CT-12A); and
changed its color from beige to light-yellow. It seems
that the CT-6 catalyst may exist in at least two redox
forms with different activities.
f
(
8) RhCl –CuCl /γ-Al O (1.5 wt % Rh, 3.5 wt %
3 2 2 3
Cu) (CT-13A).
Attempts were made to perform the coupled oxida-
The assignment of spectral bands was accom-
tion of propane and carbon monoxide under the con- plished with consideration for the literature data [29–
o
ditions specified in Tables 2 and 3 at 80 and 90 C 32]. The panoramic spectra of catalysts and reference
(
helium was replaced by propane); only traces of samples are shown in Fig. 2. As can be seen, the spec-
organic oxygenates were revealed among the reaction tra of freshly prepared catalysts and reference samples do
products. We also tried to use a catalyst containing not differ from the spectrum of γ-Al O except CT-4A.
2
3
1
.5 wt % Rh, 5 wt % Cu, and 8.5 wt % Pr COOH on The initial spectrum recorded for this catalyst in an air
f
γ-Al O pretreated with a 0.5 M sulfuric acid solution; atmosphere contains three bands at 2079, 2023, and
2
3
–1
however, the attempt was unsuccessful as well. The 1804 cm . The first two of these belong to the anti-
inactivity of these catalysts in the propane oxidation symmetric and symmetric vibrations of dicarbonyl
+
–1
reaction may be explained by the fact that we did not Rh (CO) . The band at 1804 cm lies in the region of
2
manage to completely simulate the conditions for the
homogeneous variant of this reaction (see, e.g., [20]).
the absorption bands of bridging carbonyl groups [32]
and could be assigned to the vibrations of such a group
in the rhodium complex or the heteronuclear Rh–Cu
compound. To determine the origin of this band, CT-11A
reference samples (without Cu), CT-12A (without Cu,
Diffuse Reflectance IR Fourier Transform Spectroscopy
but with Pr COOH) and CT-13A were prepared. Their
f
The eight different catalysts and reference samples
that were studied were
spectra do not contain this band (Fig. 2). It follows
–1
that the band at 1804 cm is not associated with the
(
1) RhCl –CuCl /γ-Al O (1.5 wt % Rh, 3.5 wt % formation of the rhodium complex with a bridging
3
2
2
3
Cu) (CT-1);
carbonyl group or its heteronuclear analogue. This
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1
56
CHEPAIKIN et al.
1
804
070
091
2114
2
2
5
0.1
CT-12А
CT-13А
+
air, 100 h
+
air, 160 min
+
air, 70 min
CT-11А
CT-4А
2163
2167
+air, 5 min
CT-4A
CT-4
CT-1
CO, 41 h
CO, 17 h
CO, 1 h
CT-9
2 3
Al O
CT-10
2
500 2400 2300 2200
2100
2000
6000
5000
4000
3000
2000
1000
Wavenumber, cm–1
Wavenumber, cm–1
Fig. 3. DRIFT spectra of carbon monoxide and a CO + air
mixture on the CT-9 catalyst.
Fig. 2. Broad-range DRIFT spectra of the catalysts and
reference samples.
2124
2023
2103
2
2343
2092
+air, 41 h
020
1
0.5
2126
2113
+air, 55 min
2346
+
air, 75 h
air, 130 min
air, 1 h
CO, 175 min
+
+air, 5 min
CO, 155 min
CO, 95 min
CO, 20 min
+
2124
CO, 95 min
CO, 20 min
CO, 5 min
CO, 5 min
2
500 2400 2300 2200 2100 2000 1900 1800
2500 2400 2300 2200 2100 2000 1900 1800
Wavenumber, cm–1
Wavenumber, cm–1
Fig. 4. DRIFT spectra of carbon monoxide and a CO + air
mixture on the CT-10 catalyst.
Fig. 5. DRIFT spectra of carbon monoxide and a CO + air
mixture on the CT-1 catalyst.
band seems to be due to the presence of carbonate in the presence of air. In this case, the spectrum also
3+
structures. The spectrum of the CT-9 catalyst exposed contains the Rh (CO) band. The reduction of
3
+
+
to a CO atmosphere (20–30 Torr) is characterized by Rh (CO) to Rh (CO) leads to the occurrence of the
2
+
–1
–1
the occurrence of the Cu (CO) band at 2114 cm ,
which persists even 100 h after air was fed in (Fig. 3).
At the initial stage of interaction between CT-9 and
adsorbed СО band (2345 cm ).
2
Hence, air-stable carbonyl complexes of low-
valence Rh and Cu ions are formed on the surface of
CT-9 and CT-10. Both samples do not exhibit any cat-
alytic activity in the CO oxidation reaction.
2+
CO, the spectrum contains the Cu (CO) band at
–
1
2
167 cm . The band of carbonyl complexes occurs in
the spectrum of catalyst CT-10 under the same condi-
3+
–1
tions as soon as in 5 min (Rh (CO) band at 2126 cm in
The interaction between CO and catalyst CT-1 first
Fig. 4). Bands at 2092 cm–1 (antisymmetric Rh (CO)
+
3+
2
lead to the occurrence of the Rh (CO) band at
–1
+
–1
–1
vibrations) and 2023 cm (symmetric Rh (CO)
2124 cm (Fig. 5). The band at 2020 cm (symmetric
2
+
vibrations) then occur, and their intensity increases Rh (CO) vibrations) and the broadened band at
2
1
–
with time. In 175 h, the cell was filled with extra air to 2113 cm , which is a superposition of bands from
the atmospheric pressure, and the intensity of the
+
+
antisymmetric Rh (CO) vibrations and Cu (CO)
2
–1
bands at 2092 and 2023 cm had additionally
vibrations, occur later on. The intensity of the bands at
increased in 75 h after this was accomplished. Hence,
–1
2
020 and 2103 cm increased with time. These bands
+
the formation of dicarbonyl Rh (CO) continued even disappeared in 41 h after air was fed in, thereupon the
2
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THE OXIDATION OF CARBON MON
157
2
116
3+
–1
2345
Rh (CO) band at 2124 cm with a shoulder at
2135
–1
1
20 h
~
2114 cm occurred probably due to the presence of
2098 2029
+
Cu (CO). This indicates that reduced catalyst CT-1 is
efficiently oxidized with air in contrast to CT-9 and
CT-10.
1
8 h
0.5
The band assigned to a superposition of bands from
4
1
h
h
+
+
antisymmetric Rh (CO) and Cu (CO) vibrations is
2
–1
shifted from 2113 to 2103 cm with time. It is likely
that this shift is caused by the formation of heteronu-
2001
+
+
clear carbonyl Rh and Cu complexes linked with
each other by chloride, hydroxide, or oxide bridges.
No alternative explanation seems possible for the con-
figuration of the discussed spectra and the fact that the
CO oxidation reaction is catalyzed.
+air, 10 min
2124
CO, 18 h
CO, 5 min
2
500 2400 2300 2200 2100 2000 1900 1800
Wavenumber, cm–1
The behavior of the CT-4 catalyst, which contained
Fig. 6. DRIFT spectra of carbon monoxide and a CO + air
mixture on the CT-4 catalyst.
heptafluorobutyric acid in addition to RhCl and
3
CuCl , is also interesting. Pr COOH was introduced as
2
f
a high boiling temperature solvent and a strong acid,
which is a necessary component of this catalytic sys-
tem. The interaction with carbon monoxide leads to
2
027
2
096
+
the occurrence of the band of symmetric Rh (CO)
2
2
133
–
1
–1
2348
vibrations at 2029 cm , i.e., it is shifted at 6 cm in
comparison with the spectrum of the CT-1 catalyst
(Fig. 6). Moreover, a partial resolution between the
1
4
38 h
2 h
1
+
+
1
8 h
bands of antisymmetric Rh (CO) and Cu –CO
2
1
.5 h
–1
vibrations (2098 and 2116 cm , respectively) is
CO + air, 10 min
observed. The band at 2345 cm– (СО ) occurs after
1
2
CO, 180 min
air is fed in, while the other three bands persist for 4 h
2131
CO, 55 min
CO, 5 min
–1
and then disappear. Only the band at 2135 cm
3
+
(
Rh –CO) remains in the spectrum in 120 h. Let us
note that Rh (CO) is more quickly oxidized than
Cu (CO). According to the spectral data, the behavior of
2
500 2400 2300 2200 2100 2000 1900 1800
+
2
Wavenumber, cm–1
+
CT-4A almost does not differ from the behavior of CT-4.
Fig. 7. DRIFT spectra of carbon monoxide and a CO+ air
Using a standard method, the CT-12A and CT-13A
catalysts were also studied. The absorption bands
assigned to carbonyl groups in the spectra of these cat-
alysts are better resolved than in the spectra of CT-1,
CT-4, and CT-10 (Figs. 7 and 8). This is likely to be
mixture on the CT-12A catalyst.
due to the presence of Pr COOH. It is important to
1
2131
f
2343
1
38 h
2 h
8 h
note that the spectrum of CT-13A does not contain
+
–1
the Cu –CO band at 2114 cm .
4
According to the X-ray diffraction data [8], γ-Al O
2
3
1
supported copper exists in the form of paratacamite
Cu Cl(OH) , as was also confirmed by our X-ray diffrac-
2
3
1.5 h
tion data for the CT-1 catalyst (RhCl –CuCl /γ-Al O ).
3
2
2
3
2094
CO + air, 10 min
CO, 180 min
No rhodium has been revealed in the CT-1 and CT-10
catalysts; this indicates that it is amorphous and prob-
ably finely dispersed.
2129
2026
CO, 55 min
CO, 5 min
EPR Study of the Catalysts
2
500 2400 2300 2200 2100 2000 1900 1800
The EPR spectra of freshly prepared CT-9 catalyst
and the same catalyst treated in a CO atmosphere
coincide with each other in their intensity (I) and the
shape of the signal (Fig. 9). Hence, Cu(II) is not sub-
Wavenumber, сm–1
Fig. 8. DRIFT spectra of carbon monoxide and a CO + air
mixture on the CT-13A catalyst.
+
jected to any appreciable reduction to Cu (CO) or Cu
KINETICS AND CATALYSIS Vol. 59 No. 2 2018

1
58
CHEPAIKIN et al.
under these conditions. The spectrum of CT-2 with an
initial molar ratio CuCl : RhCl = 7.5 contains the
g ≈ 2.1
CT-9
2
3
signal with g = 2.10 and an intensity I = 15900 arb.
units (Fig. 10). The signals in the spectra of the cata-
lyst and individual CuCl ⋅ 2H O (Fig. 10) differ from
CT-9 A
2
2
each other in their shapes. This is likely to be due to the
formation of paratacamite or copper compounds,
whose structure is close to paratacamite, on the sur-
face of γ-Al O . After treatment in a CO atmosphere,
2
3
the intensity of this signal abruptly decreases to I = 480
2
+
(
3% of the initial level). Hence, at least 7 Cu ions
200
250
300
350
400
subjected to the reduction with carbon monoxide by
reaction (V) catalyzed by rhodium complexes are
Magnetic field, mT
+
available for the interaction with one Rh (CO) ion.
2
Fig. 9. EPR spectra of the CT-9 catalyst and the same catalyst
The copper compound with a paratacamite struc-
ture is unlikely to be able to migrate over the surface
due to steric hindrance and probably due to its rather
strong bonding with γ-Al O . The analysis of DRIFTS
treated with carbon monoxide (CT-9A) at room temperature.
2
3
1
2
g⊥ ≈ 2.1
I = 15900
spectra shows that rhodium in the reduced catalysts
+
generally exists in the form of Rh (CO) ; therefore,
2
the possibility of its migration exists, which provides
I = 480
2+
the interaction with Cu via the formation of chloride
bridges. From the analysis of EPR spectra it follows
that copper in the CT-9 catalyst is not reduced in a CO
3
I = 11000
+
atmosphere. Carbonyl Cu (CO) revealed in the
CuCl2 · 2H2O
DRIFTS spectra of the CT-1, CT-4, and CT-9 cata-
lysts seems to be formed in very small amounts from a
certain especially active compound of copper, which is
present in the freshly prepared samples and disappears
after the catalytic reaction (e.g., in catalyst CT-13A).
g⊥
g⎜⎜
250
300
Magnetic field, mT
350
Almost all the copper exists in the catalyst in the
form of Cu(I) after treatment in a CO atmosphere, is
not bonded to a carbonyl ligand and, in contrast to
Fig. 10. EPR spectra recorded at room temperature for
CuCl2 ⋅ 2H2O and the CT-2 catalyst (1) without treatment and
(2), (3) after treatment with (2) carbon monoxide and (3) air.
+
Cu (CO), can easily be oxidized with air oxygen to
Cu(II), which further oxidizes Rh(I) containing parti-
+
cles. Hence, Cu (CO) does not participate in the cat-
Cu(II) before their contact with CO is weak and is not
exhibited in the spectra. Another factor may be associ-
ated with the peculiarities of catalyst preparation. As
an example, the catalysts from the work [8] were evac-
alytic oxidation of carbon monoxide.
Interaction between the Catalyst Components
–8
uated to 2 × 10 Torr for 4 h for the removal of phys-
ically adsorbed water before recording their DRIFTS
spectra. At the same time, the activity of catalysts
appreciably increased after water vapor was added to
the dry gas [7, 8]. Since our catalysts were subjected
only to short-term evacuation before the addition of
One of the important issues in estimating the effect
of the catalysts we prepared and catalysts similar to
them is the interaction between rhodium (or palla-
dium) and copper compounds on the surface of
γ-Al O . This issue was also mentioned in [8], where
2
3
no signs of such an interaction could be revealed in carbon monoxide, water and heprafluorobutyric acid
freshly prepared catalysts, even using a large variety of (if used) might persist in a catalyst.
physical methods. Heteronuclear [(PdCl ) Cu-
2
2
Hence, it has been demonstrated using DRIFTS
and EPR that rhodium(I) carbonyl formed during the
Cl (DMFA) ] was separated out and characterized by
2
4 n
X-ray diffraction when performing the Wacker process interaction of catalysts with carbon monoxide reduces
in a dimethylformamide (DMFA) medium, and Cu(II) to Cu(I). Here, there are up to seven Cu(II)
[
(PdCl ) (CuO) (HMFT) ] with oxo groups serving ions available for the interaction with rhodium. This
2
6
4
4 n
as bridges between Pd and Cu was identified when argues for the possibility of the migration of catalyst
hexamethylphosphotriamide (HMFT) was used as a components or one of them (most probably, rho-
solvent [33]. It is possible to hypothesize that the dium(I) carbonyl) over the solvent film on the support
interaction between the components of freshly pre- surface. Their migration may result in the formation of
pared catalysts containing Rh(III) or Pd(II) and heteronuclear complexes with chloride, hydroxide, or
KINETICS AND CATALYSIS
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2018

THE OXIDATION OF CARBON MON
159
oxide bridges and the transfer of electrons between
Rh(I) and Cu(II).
8. Titov, D.N., Ustyugov, A.V., Tkachenko, O.P., Kus-
tov, L.M., Zubavichus, Ya.V., Veligzhanin, A.A.,
Sadovskaya, N.V., Oshanina, I.V., Bruk, L.G., and
Temkin, O.N., Kinet. Catal., 2012, vol. 53, no. 2, p. 262.
CONCLUSIONS
9. Golodov, V.A., Sokolsky, D.V., and Noskova, N.F., J.
Mol. Catal.,1977, nos. 1–3, p. 51.
In this work, heterogenized γ-Al O supported cat-
2
3
1
0. Lee, J.S. and Park, E.D., Top. Catal., 2002, vol. 18,
alysts containing rhodium and copper compounds
with and without perfluorocarboxylic acids were pre-
pared for their testing in a coupled propane–carbon
monoxide oxidation process. The catalysts were active
nos. 1–2, p. 67.
1
1. Arutyunov, V.S. and Krylov, O.V., Okislitel’nye
prevrashcheniya metana (Oxidative transformations of
methane), Moscow: Nauka, 1998.
and stable in the CO oxidation reaction at atmospheric 12. Chepaikin, E.G., Usp. Khim., 2011, vol. 80, no. 4,
p. 384.
pressure and a temperature up to 90°C; however, only
traces of organic oxygenates were revealed when 13. Chepaikin, E.G., J. Mol. Catal. A.: Chem, 2014,
vol. 385, p. 160.
attempting to perform the coupled oxidation of pro-
pane and carbon monoxide under these conditions.
1
4. Chepaikin, E.G. and Borshch, V.N., J. Organomet.
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5. Sen, A., Acc. Chem. Res., 1998, vol. 31, no. 9, p. 550.
6. Lin, M., Hogan, T., and Sen, A., J. Am. Chem. Soc.,
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Using DRIFTS, it has been shown that carbonyl
rhodium(I) and copper(I) complexes are formed in
the prepared catalysts and reference samples. They are
stable in air in the reference samples and subjected to
oxidation in the catalysts. It is most likely that cop-
per(I) carbonyl does not catalyze CO oxidation. As
follows from the analysis of the EPR spectra, cop-
1
1
1
1
7. Chepaikin, E.G., Bezruchenko, A.P., Leshcheva, A.A.,
Boyko, G.N., Kuzmenkov, I.V., Grigoryan, E.H., and
Shilov, A.E., J. Mol. Catal. A.: Chem, 2001, vol. 169,
p. 89.
per(II) in the reference sample is not reduced by car- 18. Chepaikin, E.G., Bezruchenko, A.P., and Leshcheva, A.A.,
bon monoxide to any appreciable extent. Reduction
Kinet. Catal., 2002, vol. 43, no. 4, p. 507.
takes place only in the presence of both rhodium and 19. Chepaikin, E.G., Bezruchenko, A.P., Boiko, G.N.,
Gekhman, A.E., and Moiseev, I.I., Kinet. Catal., 2006,
copper compounds. In this case, the intensity of the
vol. 47, no. 1, p. 12.
Cu(II) signal in the EPR spectrum decreases nearly to
2
0. Chepaikin, E.G., Bezruchenko, A.P., Menchikova, G.N.,
and Gekhman, A.E., J. Mol. Catal. A.: Chem., 2017,
vol. 426, p. 389.
zero even at an initial atomic ratio Rh(III) : Cu(II) =
1
: 7.5. Hence, at least seven copper(II) ions are avail-
able for the interaction with rhodium.
2
1. Shilov, A.E., Metal Complexes in Biomimetic Chemical
Reactions, Boca Raton: CRC Press, 1997.
2
2. Zuberbuhler, A., Helv. Chim. Acta, 1967, vol. 50, no. 2,
ACKNOWLEDGMENTS
p. 466.
The authors are grateful to D.Yu. Kovalev for per-
forming the X-ray diffraction analysis. E.G. Che-
paikin, A.P. Bezruchenko, and G.N. Menchikova are
grateful to the Russian Foundation for Basic Research
2
2
3. Mehnert, C.P., Chem. – Eur. J., 2005, vol. 11, p. 50.
4. Haumann, M., Dentler, K., Joni, J., Rissager, A., and
Wassersheid, P., Adv. Synth. Catal., 2007, vol. 349,
p. 425.
for financial support (project no. 17-03-00336). 25. Oliver-Bourbigou, H., Magna, L., and Morvan, D.,
Appl. Catal., A., 2010, vol. 373, p. 1.
L.M. Kustov and O.P. Tkachenko are grateful to the
Russian Scientific Foundation for financial support
2
6. Guan, H.L., Lin, J., Qiao, B.T., Yang, X.F., Li, L.,
Miao, S., Liu, J.Y., Wang, A.Q., Wang, X.D., and
Zang, T., Angew. Chem., 2016, vol. 55, p. 2820.
(grant no. 14-50-00126).
2
7. Guan, H.L., Lin, J., Li, L., Wang, X.D., and Zang, T.,
Appl. Catal., B., 2016, vol. 184, p. 299.
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Translated by E. Glushachenkova
KINETICS AND CATALYSIS Vol. 59 No. 2 2018