Cyclohexane transformations over metal oxide catalysts 2. Selective cyclohexane ring opening to form n-hexane over mono- and bimetallic rhodium catalysts

Article abstract of DOI:10.1023/A:1015495324581

The activity of monometallic Rh and Pt catalysts and bimetallic Pt-Rh catalysts on oxide supports in cyclohexane ring opening to form n-hexane was studied. The Rh-containing catalysts are highly active and selective in this reaction. Cyclohexane dehydroge

Full text of DOI:10.1023/A:1015495324581

Russian Chemical Bulletin, International Edition, Vol. 51, No. 2, pp. 255—258, February, 2002
255
Cyclohexane transformations over metal oxide catalysts
2.* Selective cyclohexane ring opening to form nꢀhexane
over monoꢀ and bimetallic rhodium catalysts
b
T. V. Vasina,а O. V. Masloboishchikova,а E. G. KhelkovskayaꢀSergeeva,а L. M. Kustov,а and P. Zeuthen

aN. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
47 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: +7 (095) 135 5328. Eꢀmail: lmk@ioc.ac.ru
bHaldor Topsoe А/S, DKꢀ2800 Lyngby, Denmark
The activity of monometallic Rh and Pt catalysts and bimetallic Pt—Rh catalysts on oxide
supports in cyclohexane ring opening to form nꢀhexane was studied. The Rhꢀcontaining cataꢀ
lysts are highly active and selective in this reaction. Cyclohexane dehydrogenation predomiꢀ
nates in the case of the Pt catalysts. The use of the bimetallic aluminaꢀsupported Pt—Rh
catalysts allows one to minimize the contribution of cyclohexane cracking and to enhance the
selectivity for nꢀhexane with the yield of the latter slightly depending on the metal ratio in the
bimetallic system under the experimental conditions.
Key words: cyclohexane, nꢀhexane, ring opening, hydrogenolysis, rhodium, platinum,
bimetallic catalysts.
As shown earlier,1—6 depending on the metal nature,
acidꢀbasic properties of a support, and reaction condiꢀ
tions, cyclohexane can undergo dehydrogenation to benꢀ
zene, isomerization to methylcyclopentane (MCP), or
hydrogenolysis (hydrocracking) to form nꢀhexane and alꢀ
kanes С1—С5 over metal oxide catalysts. Cyclohexane ring
opening to form nꢀhexane occurs mainly over Rhꢀ and
Ptꢀcontaining oxide catalysts under specific condiꢀ
tions,1—3 and the nꢀhexane yield over the Rh catalysts is
significantly higher than that over the Pt catalysts.
Bimetallic systems are often used as selective and stable
catalysts, which are more resistant to deactivation than
monometallic catalysts based on VIII group metals. The
properties of the bimetallic catalysts substantially depend
on the mode of metal introduction, component ratio,
support nature, and activation conditions.7—10. For exꢀ
ample, addition of Ag to the Rh/TiO2 system enhances
selectivity of MCP hydrogenolysis to nꢀhexane likely due
to a change in the mechanism of cyclopentane ring
opening.7
Experimental
The Al2O3 and SiO2 supports with the specific surface areas
(Ssp) of 250 and 300 m2 g–1 were used. The metals (Pt, Rh, and
Pt—Rh) were introduced from dilute aqueous solutions of
Н2PtCl6, RhCl3, and mixed Н2PtCl6 + RhCl3 solutions, respecꢀ
tively. The supports were impregnated at 20 °C, evaporated on a
rotary evaporator, and dried at 130 °C. Before operation, the
catalysts were reduced for 3 h in an H2 flow at 450 °C аnd
sometimes at 500—600 °C. Some samples were treated only with
air at 500 °C or initially with air at 500 °C and then with H2 at
450 °C. The Pt and Rh concentrations in the monometallic
catalysts were 1.0—2.0 wt.% and 1 wt.% in the bimetallic sysꢀ
tems (total metal content) at the ratios Pt : Rh = 1 : 3, 1 : 1,
and 3 : 1.
The reaction was carried out in a flow setup at temperatures
of 210—400 °C and pressures of 1.0—5.0 MPa, a volume of the
catalyst was 3 cm3, cyclohexane space feed rate was 2 h–1, and
molar ratio was Н2 : С6Н12 = 10 : 1. Analysis of the reaction
products was described previously.1
Results and Discussion
Cyclohexane ring opening to form nꢀhexane over the
monometallic Rhꢀ and Ptꢀcontaining catalysts and bimeꢀ
tallic Pt—Rh catalysts on oxide supports as well as the
effect of the Pt : Rh ratio and reaction conditions (temꢀ
perature and pressure) on the yield of nꢀhexane and selecꢀ
tivity were studied in this work.
The cyclohexane transformations were studied over
two groups of catalysts, monometallic (Pt/Al2O3,
Rh/Al2O3, Rh/SiO2) and bimetallic (Pt—Rh/Al2O3) with
various metal ratios.
Monometallic catalysts. The data of Table 1 show that
cyclohexane transformations over the 1% Pt/Al2O3 cataꢀ
* For Part 1, see Ref. 1.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 242—245, February, 2002.
1066ꢀ5285/02/5102ꢀ255 $27.00 © 2002 Plenum Publishing Corporation

256
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 2, February, 2002
Vasina et al.
Table 1. Effects of pressure (p) and temperature (T) on cyclohexane conversion over the Ptꢀ and Rhꢀcontaining oxide catalysts
Catalyst
р
Т
/°С
X
(%)
S
(%)
Yield of products (wt.%)
С5Н12 iꢀС6Н14 nꢀС6Н14
/MPa
C1—C4
MCP
С6Н6
1% Pt/Al2O3
1% Rh/Al2O3
2
2
3
3
5
3
3
5
3
5
370
400
400
280
280
300
280
280
320
320
14.7
49.6
41.4
39.1
55.9
91.0
89.0
83.0
80.8
92.7
35.4
25.0
42.3
90.0
87.7
65.2
78.0
84.9
35.4
56.0
—
0.2
0.3
2.1
4.9
21.2
8.8
6.9
19.6
18.6
—
0.4
0.9
1.8
2.0
9.9
10.2
5.3
32.6
22.2
0.4
1.6
3.3
—
5.2
12.4
17.5
35.2
49.0
59.3
69.4
70.5
28.6
51.9
1.4
2.3
3.7
—
—
—
—
—
—
—
7.7
32.7
15.7
—
—
—
—
—
—
—
—
0.6
0.6
0.3
—
2% Rh/Al2O3
1% Rh/SiO2
—
Note. The following designations are used here and in Tables 2, 3: X is the cyclohexane conversion, S is selectivity for nꢀhexane.
lyst occur at relatively high temperatures (370—400 °C).
Cyclohexane is mainly dehydrogenated to benzene deꢀ
spite the elevated H2 pressure in the system (1—3 MPa).
The cyclohexane conversion increases from 14.7 to 49.6%
with an increase in temperature from 370 to 400 °C. The
yield of nꢀhexane reaches 17.5 wt.% at the selectivity of
42.3% (400 °C, 3 MPa). The number of reaction products
is increased with an increase in temperature because of
the formation of MCP and isohexanes but the yield of
С1—С5 alkanes is low.
The Rh/Al2O3 and Rh/SiO2 catalysts are more active
and selective in cyclohexane ring opening. The reaction
occurs at 280—320 °C, i.e., at significantly lower temꢀ
peratures compared to the platinumꢀalumina catalyst. The
maximal yield of nꢀhexane (1% Rh/Al2O3, 300 °C, 3 MPa)
was 59.3 wt.% at selectivity of 65.2%. Benzene and MCP
were not found in the reaction products, and the yield of
isohexanes did not exceed 1 wt.%. Cyclohexane hydroꢀ
genolysis to form С1—С5 alkanes was the only side
reaction.
yield of nꢀhexane is halved. Hence, the optimal combinaꢀ
tion of the selectivity and nꢀhexane yield can be achieved
by the proper choice of temperature and pressure.
When the Rh content in the catalyst increases from
1 to 2 wt.%, the cyclohexane conversion substantially inꢀ
creases but selectivity to nꢀhexane changes insignificantly
(see Table 1). The highest yield of nꢀhexane over the
Y (wt.%)
S (%)
100
a
4
100
80
3
80
60
40
20
60
40
1
20
2
240 260 280 300 320 340 T/°C
The main channel of cyclohexane transformation over
the Rh catalysts strongly depends on temperature and
pressure (Fig. 1). The yield of nꢀhexane was 7—14 wt.% at
a pressure of 1 MPa and 240—260 °C (at selectivity up
to 90%) and increased to 32—34 wt.% with an increase in
temperature to 300—320 °C. A further increase in temꢀ
perature up to 350 °C results in a drop of the nꢀhexane
yield, and hydrogenolysis to form С1—С5 hydrocarbons
becomes the major reaction path (see Fig. 1, a).
Y (wt.%)
S (%)
100
b
100
80
1´
2´
80
60
40
60
2
40
1
20
20
The yield of nꢀhexane gradually increases with an inꢀ
crease in the pressure from 1 to 5 MPa at a constant
temperature (280 °C) and reaches 49 wt.% (see Fig. 1, b),
whereas selectivity for nꢀhexane initially increases reachꢀ
ing a 88% value and then remains unchanged. The plots
of the nꢀhexane yield and selectivity of its formation at
300 °C versus pressure pass through a maximum at 3 MPa.
When pressure increases to 5 MPa, cyclohexane hydroꢀ
genolysis to С1—С5 alkanes becomes prevailing and the
1
2
3
4
5
p/MPa
Fig. 1. a. Temperature (T ) dependence of the yield (Y ) of
nꢀhexane (1), overall yield of isohexanes (2) and hydrocarbons
С1—С5 (3), as well as selectivity (S) to nꢀhexane (4) over the
1% Rh/Al2O3 catalyst at a pressure of 1 MPa. b. Pressure depenꢀ
dence of the yield of nꢀhexane (1, 2) and selectivity to nꢀhexane
(1´, 2´) over the 1% Rh/Al2O3 catalyst at 280 (1, 1´) and
300 °C (2, 2´).

Cyclohexane ring opening to nꢀhexane
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 2, February, 2002
257
X, S (%)
Table 2. Effect of pretreatment conditions on activity of the
1% Rh/Al2O3 catalyst in cyclohexane ring opening at 300 °С
2
a
80
Pretreatment conditions
р
X
(%)
S
Yield of
1´
2´
/MPa
(%) nꢀhexane
Medium
T/°С
60
40
(wt.%)
H2
H2
450
520
2
3
2
3
2
2
3
2
3
85.5 63.3
91.0 65.2
87.1 63.6
97.1 51.6
89.5 62.7
67.7 64.5
100
48.1 73.2
60.4 71.5
54.1
59.3
55.4
50.1
56.1
43.7
—
1
20
H2
Air
600
500
1
2
3
4
5
p/MPa
X, S (%)
—
4
100
b
Air
Air + H2
500
500 a, 450 b
35.2
43.2
3´
80
60
40
a Air.
b H2.
3
genolysis of the С—С bond in cycloalkanes. An electron
deficiency arises on the metal clusters because of strong
interaction of Pt and Rh with the support, thus favoring
the formation of particles with various charges, disperꢀ
sion, etc., thereby affecting11 the catalytic properties of
the whole system.
4´
1
2
3
4
5
p/MPa
Fig. 2. Pressure dependence of the cyclohexane conversion (X )
(1, 2, 3, 4) and selectivity (S) to nꢀhexane (1´, 2´, 3´, 4´) over the
1% Rh/SiO2 (a) and 1% Rh/Al2O3 (b) catalysts at temperatures
280 (3, 3´), 300 (1, 1´, 4, 4´), and 320 °C (2, 2´).
Hence, the activation conditions are important for the
formation of the active catalyst surface because they afꢀ
fect the acidꢀbasic properties of the system, the electron
state of the metal, the dispersion, and the morphology of
metal particles. Table 2 presents the data on the influence
of the pretreatment conditions on cyclohexane transforꢀ
mations on the 1% Rh/Al2O3 catalyst. Reduction in an
H2 atmosphere produces the catalytic system that is the
most active and selective in cyclohexane ring opening.
Variation of the temperature of catalyst treatment (within
the range of 450—600 °C) does not affect the activity in
fact. The system formed during catalyst activation in an
oxidative medium exhibits the enhanced activity in
cyclohexane hydrocracking to form С1—С5 alkanes. The
successive treatment of the catalyst with air and H2 is
more efficient than the treatment only with air but signifiꢀ
cantly less efficient than activation in the H2 flow. The
larger the Rh particles or RhОх phase are likely formed in
the oxidative medium, facilitating cyclohexane hydroꢀ
genolysis to form light alkanes (see Table 2). The electron
state of the metal also depends on the activation conꢀ
ditions.
2% Rh/Al2O3 catalyst at 280 °C and a pressure of 5 MPa
is 70.5 wt.% at selectivity of 84.9%.
Hence, depending on the process conditions, selecꢀ
tive cyclohexane ring opening to form nꢀhexane or
hydrogenolysis to С1—С5 hydrocarbons are the main paths
of cyclohexane transformation over Rh/Al2O3 (see Fig. 1,
Table 1).
The temperature and pressure at which the
1% Rh/SiO2 catalyst shows the highest activity in cycloꢀ
hexane ring opening to form nꢀhexane are higher than
those for the aluminaꢀbased system.1,3 The temperature
of 320 °C is an optimal temperature for the reaction over
Rh/SiO2 (it is 40 °C higher than over Rh/Al2O3) (see
Table 1) as well as a pressure of 5 MPa (Fig. 2). The
maximal yields of nꢀhexane over the 1% Rh/SiO2 and
1% Rh/Al2O3 catalysts are comparable. However, selecꢀ
tivity of cyclohexane ring opening is higher over the latter,
since С1—С5 hydrocarbons are formed over 1% Rh/SiO2
in the amount of up to 50 wt.%.
In contrast to Pt/Al2O3 catalysts, cyclohexane deꢀ
hydrogenation to benzene and isomerization to MCP do
not occur over Rh/Al2O3 and Rh/SiO2 catalysts under the
experimental conditions used (240—350 °C, 1—5 MPa).
A possible reason for this is that hydrogenatingꢀdehydroꢀ
genating properties are typical of the Pt catalysts, while
the Rhꢀcontaining systems are characterized by strong
cracking activity, which manifests itself in the hydroꢀ
Bimetallic catalysts. The introduction of the second
metal into the system is often used to enhance selectivity
and stability of the catalyst. We studied the effect of Pt on
activity and selectivity of the Pt—Rh bimetallic systems in
cyclohexane ring opening (Fig. 3, Table 3). As can be
seen in Fig. 3, the addition of Pt to Rh/Al2O3 decreases
the contribution of hydrogenolysis to С1—С5 alkanes and
increases selectivity of cyclohexane transformation for

258
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 2, February, 2002
Vasina et al.
X (%), Y (wt.%)
The yield of nꢀhexane over the bimetallic catalysts
(3—5 MPa, 300—320 °C) reaches 53 wt.% at selectivity
of up to 83% (see Table 3). The optimal temperature of
cyclohexane ring opening over the Pt—Rh bimetallic cataꢀ
lysts is 20—30 °C higher than that over 1% Rh/Al2O3 but
essentially lower than that over 1% Pt/Al2O3. A weak deꢀ
pendence of the nꢀhexane yield on the Pt : Rh ratio over
the bimetallic aluminaꢀbased catalysts is likely due to the
formation of the Pt—Rh alloy phase enriched in Rh,10
which decreases the concentration of free electrons. This
can lead to weakening the substrate—metal bond and, as
a sequence, to a lesser probability of the rupture of the
С—С bonds in the substrate molecule.12
100
1
2
3
80
60
40
20
Hence, the Rhꢀcontaining alumina catalysts are the
most active and selective in cyclohexane ring opening to
form nꢀhexane.
1 Pt 0.75 Pt—0.25 Rh 0.5 Pt—0.5 Rh 0.25 Pt—0.75 Rh 1 Rh
Fig. 3. Effect of the Pt and Rh contents (wt.%) in the bimetallic
Pt—Rh/Al2O3 catalysts on the cyclohexane conversion (X ) (1)
References
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14
(YC —C ) (3) in cyclohexane 6hydrogenolysis (320 °C, 2 MPa).
1
5
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47.9
39.1
55.9
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nꢀhexane (320 °C, 2 MPa). The cyclohexane conversion
on the monometallic catalyst 1% Pt/Al2O3 under these
conditions is >1%, whereas cyclohexane completely transꢀ
forms to the С1—С5 gaseous cracking products over the
1% Rh/Al2O3 catalyst. The yield of nꢀhexane over the
bimetallic catalysts slightly depends on the Pt : Rh ratio,
but the selectivities for nꢀhexane over these catalysts sigꢀ
nificantly differ. The (0.5 wt.% Rh—0.5 wt.% Pt)/Al2O3
system is the most selective, and the yield of the С1—С5
cracking products is 2—3 times lower than that over the
Pt—Rh catalysts with a different composition.
Received July 26, 2001;
in revised form October 11, 2001