The Isomerization of Cyclopropane over Reduced Molybdena-Alumina Catalysts

Article abstract of DOI:10.1006/jcat.1997.1830

The isomerization and metathesis of cyclopropane were carried out over a series of reduced molybdena-alumina catalysts (1.7-13 wt%Mo). The distribution of Mo oxidation states was determined by XPS. The average Mo oxidation state estimated from XPS agreed with that obtained by measuring the O₂ consumption on reoxidation for a given catalyst. A good correlation was found between the catalytic activity for the isomerization reaction and the relative intensity of the XPS peak attributed to paired double-bonded Mo (IV) species (as in MoO₂). The results indicate a direct relationship between these Mo (IV) species and the coordinative unsaturated sites involved in the formation of metallocyclobutane. The latter is reportedly the intermediate required for the isomerization reaction to take place.

Full text of DOI:10.1006/jcat.1997.1830

JOURNAL OF CATALYSIS 171, 485–489 (1997)
ARTICLE NO. CA971830
The Isomerization of Cyclopropane over Reduced
Molybdena–Alumina Catalysts
�
�
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,1
I. Oliveros, M. J. P e´ rez Zurita, C. Scott, M. R. Goldwasser, J. Goldwasser, S. Rond o´ n,†
M. Houalla,† and D. M. Hercules†
�
Centro de Cat a´ lisis, Petr o´ leo y Petroqu ´ı mica, Escuela de Qu ´ı mica, Facultad de Ciencias, Universidad Central de Venezuela, Apartado 47102, Los
Chaguaramos, Caracas 1020-A, Venezuela; and †Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235
Received March 17, 1997; revised July 1, 1997; accepted July 14, 1997
on the isomerization of cyclopropane being acid-catalyzed
The isomerization and metathesis of cyclopropane were car- over reduced molybdena–alumina catalysts.
ried out over a series of reduced molybdena–alumina catalysts
More recent work (11) suggested that the ring opening
1.7–13 wt% Mo). The distribution of Mo oxidation states was deter-
(
was carried out through metallocyclobutane intermediates
MCB) bydirect insertion ofthe coordinativelyunsaturated
transition metal ion into the cyclopropane ring, in agree-
ment with models presented in the literature (12). Other
data supported the necessity for the formation of the MCB
intermediate for the isomerization reaction to take place
mined by XPS. The average Mo oxidation state estimated from XPS
agreed with that obtained by measuring the O2 consumption on re-
oxidation for a given catalyst. A good correlation was found between
the catalytic activity for the isomerization reaction and the relative
intensity of the XPS peak attributed to paired double-bonded Mo
(
(
IV) species (as in MoO2). The results indicate a direct relationship
(
13, 14).
between these Mo (IV) species and the coordinative unsaturated
sites involved in the formation of metallocyclobutane. The latter is
The preceding results highlight the importance of MCB
reportedly the intermediate required for the isomerization reaction as a key intermediate in the isomerization of cyclopropane
to take place.
�c 1997 Academic Press
over reduced molybdena–alumina catalysts. Interestingly,
coordinatively unsaturated sites (CUS) are essential for
the formation of this intermediate. Different authors (8, 9,
INTRODUCTION
1
5–20) have suggested Mo (IV) species as having adequate
unsaturation to promote the direct insertion reaction, and,
thus, the formation of the MCB intermediate, crucial for
the ring opening of cyclopropane.
From the previous discussion it follows that while a cor-
relation between the catalytic activity for the isomerization
reaction and the extent of reduction has been established
The isomerization of cyclopropane has been the subject
of detailed research for over 50 years (1–7). The homo-
geneous gas phase reaction (1–3) has been shown to be
unimolecular and extremely slow at temperatures below
�
4
25 C. Acids have long been known to catalyze the open-
ing of the cyclopropane ring (4–7), the reaction being quite
facile. For example, the isomerization can be carried out
over solid Brønsted acids, such as silica–alumina at near
room temperature (5).
Over molybdena–alumina catalysts the reaction has been
thoroughly investigated by W. K. Hall and co-workers
(
8), no attempts have been made to investigate a possible
quantitative relationship between catalytic activity and the
abundance of discrete Mo oxidation states. These studies
are important since they may shed light on the nature of
the active species responsible for the isomerization reac-
tion. The lack of these studies was the driving force for the
present paper. The main purpose was to study the possi-
ble correlations between the activity for the isomerization
of cyclopropane and the relative abundances of different
oxidation states of Mo species. The latter were obtained
from the analysis of X-ray photoelectron spectra (XPS) of
a series of reduced molybdena–alumina catalysts.
(
8–11). Early work showed that the activity for the isomer-
ization reaction increased with the extent of reduction of
the catalyst (8 wt% Mo/Al2O3) (8). The reaction was poi-
soned by the adsorption of water, and the activity was re-
�
stored by evacuation of the catalyst at 500 C. These results
were contrary to the findings over acid catalysts, e.g., silica–
alumina (5). Further work (10) showed clear evidence for
the presence of Brønsted acidity for the oxidic catalyst but
not for the reduced system. These resultscast seriousdoubts
EXPERIMENTAL
2
1
� -Alumina (Ketjen CK-300), surface area 210 m /g,
To whom all correspondence should be addressed. E-mail: jgoldwas@
strix.ciens.ucv.ve.
was used as the support. The catalysts were prepared by
485
0021-9517/97 $25.00
Copyright �c 1997 by Academic Press
All rights of reproduction in any form reserved.

4
86
OLIVEROS ET AL.
impregnating with (NH4)6Mo7O24 � 4H2O (BDH, reagent
grade) to incipient wetness. The solids were dried for 2 h at
�
�
1
00 C and calcined for 5 h at 500 C. The Mo loading was
determined by atomic absorption spectroscopy.
�
Reactions were carried out at 25 C in an all-glass recircu-
lation system (15). The initial amount of cyclopropane used
was 50 cc (NTP). The reaction products were analyzed by
on-line gas chromatography. The analytical procedures are
fully described elsewhere (9, 11). The initial slopes of the
plots of conversion (percentage of each hydrocarbon) vs
time provided the way of calculating initial rates of appear-
ance ofthe different reaction products. Prior to the reaction,
3
�
the catalyst was treated with pure O2 (60 cm /min) at 500 C
for 8 h to condition it. The catalysts were reduced with pure
�
H2 at 500 C for 2 h and then evacuated at this tempera-
ture (absence of HR) or at 25 C (presence of HR) for 1 h.
�
The average Mo oxidation state (AOS) was determined
�
by measuring the O2 consumption (at 500 C) required for
complete reoxidation, after the reduction procedure. Vol-
umetric techniques (21–23) were used.
XPS spectra of the reduced catalysts were obtained with
a modified AEI ES200 spectrometer equipped with an alu-
minum anode (Al K� = 1486.6 eV) at a power of 240 W
(
12 KV and 20 mA). The catalysts were pressed as pellets
2
at 2000 kg/cm and mounted on a sealable probe which per-
mitted transfer of the pretreatred catalyst from an external
reaction chamber to the spectrometer without exposure to
air. The spectrometer was interfaced to an IBM PC com-
patible for data collection. The subsequent data analysis
was also carried out in the PC environment under DOS, us-
ing programs (GOOGLY software) written in-house (24).
A damped nonlinear least-squares fitting (NLLSF) rou-
tine was used to curve fit the Mo 3d envelopes. The bind-
ing energy (BE) values were referenced to the Al 2p line
(
74.5 eV). The methodology used for curve fitting the Mo 3d
envelope and the assumptions made for quantitative analy-
sis of Mo oxidation states, in molybdena–alumina catalysts,
is similar to those used in different papers (25, 26) by D. M.
Hercules and co-workers.
FIG. 1. Mo 3d curve-fitted spectra for reduced 1.7% Mo and 8.8% Mo
catalysts. The Mo 3d5/2–3d3/2 doublets are designated by their assigned
oxidation states. Mo (V) (Mo 3d5/2 BE 231.5 � 0.1 eV) refers to Mo (V) and
ionic Mo (IV), whereas Mo (IV) (Mo 3d5/2, BE 229.7 � 0.1 eV) designates
paired double-bonded Mo (IV). See text for details.
RESULTS AND DISCUSSION
Figure 1 shows typical Mo 3d curve-fitted spectra for the
reduced 1.7% Mo and 8.8% Mo catalysts. As shown, only
three sets of Mo 3d doublets were necessary to satisfacto- the curve fit procedure. The Mo 3d binding energies for Mo
rily curve fit the data. The doublets are identified by their (VI), Mo (V), and Mo (IV), for the 8.8% Mo catalyst, agree
oxidation state assignment (Table 1).
within 0.2 eV with earlier results obtained on the same in-
Table 1 shows the XPS parameters, BEs, and full widths strument, for an 8 wt% molybdena–alumina catalyst (25),
�
at half maximum (fwhm’s), obtained for the series of re- reduced at 500 C. It should be noted however that two bind-
duced molybdena–catalysts. The BE and fwhm values for ing energy values were reported for Mo (IV) (29–32): a high
the Mo (VI) 3d doublet were obtained from the XPSspectra binding energy value (Mo 3d5/2 231.5–232 eV) attributed to
of the oxidic catalyst. A good agreement with other values ionic “isolated” Mo (IV) and a low binding energy value
reported in the literature is observed (25–29). The XPS pa- (Mo 3d5/2 ca. 230 eV) typically associated with Mo (IV) as
rameters found for the Mo (V) and Mo (IV) were a result of in MoO2 and ascribed to paired double-bonded Mo (IV).

ISOMERIZATION OF CYCLOPROPANE
487
TABLE 1
Mo 3d5/2, 231.5 � 0.1 eV; Mo 3d3/2, 234.6 � 0.1 eV). These
results are consistent with the implied assumption in the
present study that the distribution of Mo oxidation states
in the sample prior to the catalytic run is similar to that of
the catalyst subjected to the same pretreatment conditions
and analyzed by XPS. As the Mo loading is increased the
average oxidation state of the Mo phase decreases from 5.4
XPS Parameters Derived for the Series of Reduced
Molybdena–Alumina Catalysts
Binding energies, eVa
fwhm, eVc
Mo (VI) Mo (V) Mo (IV) Mo (VI) Mo (V) Mo (IV)
b
Catalysts
3d5/2
3d5/2
3d5/2
3d5/2
3d5/2
3d5/2
(
1.7% Mo) to 4.2 (13.0% Mo). This can be understood in
1
3
5
8
.7% Mo
.8% Mo
.5% Mo
.8% Mo
232.9
232.9
232.9
232.8
232.9
231.6
231.5
231.5
231.6
231.6
229.7
229.6
229.7
229.7
229.7
2.5
2.4
2.3
2.3
2.2
2.3
2.2
2.2
2.1
2.2
2.3
2.2
2.1
2.2
2.0
terms of the higher reducibility of the high loading Mo cata-
lysts compared to the low loading solids. The latter report-
edly contain predominantly difficult to reduce monomeric
Mo species, while a large proportion of easier to reduce
polymolybdate species are found in the high-loading Mo
catalysts (22).
1
3.0% Mo
a
The difference between the binding energy values of Mo 3d3/2 and Mo
d5/2 was kept constant (3.1 eV) for all oxidation states. For more details
3
Figure 2 shows the distribution of Mo oxidation states vs
see Ref. (25).
b
Mo (V) (Mo 3d5/2, BE 231.5 � 0.1 eV) refers to Mo (V) and ionic “iso- metalloading. The relative abundance ofeach Mo oxidation
lated” Mo (IV), whereas Mo (IV) (Mo 3d5/2, BE 229.7 � 0.1 eV) designates state was calculated by dividing the Mo 3d area for a given
paired double-bonded Mo (IV). For more details see text.
oxidation state by the total area of the Mo 3d envelope (25).
It can be seen that the percentage of Mo (VI) decreases with
increasing metal loading. The relative abundance of Mo (V)
shows an increase, with Mo loading, up to the 3.8% Mo
c
Identical Mo 3d3/2 and Mo 3d5/2 fwhm values were used. For more
details see Ref. (25).
catalyst. For higher loading, the variation is small, although
showingsome random behavior. The percentage ofMo (IV)
shows a steady increase with increasing Mo loading up to
Since the high binding energy value is comparable to that
typically used for Mo (V), one cannot rule out that the
Mo 3d doublet attributed to Mo (V) is in fact a convolution
of the contributions from Mo (V) and ionic Mo (IV). Thus,
in this manuscript, for the sake of simplicity, Mo (V) (Mo
5
.5% Mo. For higher Mo loading, the percentages of the
Mo phase present as Mo (IV) are very similar.
3d5/2, BE 231.5–231.6 eV) will refer to Mo (V) and ionic
Mo (IV) whereas Mo (IV) (Mo 3d5/2, BE 229.7 eV) will
designate paired double-bonded Mo (IV).
No significant shift in the binding energy of the different
Mo oxidation states was observed as the Mo loading was
increased. The fwhm’s for the different oxidation states, for
any particular catalyst, were within 0.2 eV.
The variation of the extent of reduction of the Mo phase
as a function of Mo loading is shown in Table 2. A reason-
able agreement is found between the values obtained from
reoxidation experiments and those estimated from the XPS
data. The latter were calculated based on the assumption
that only Mo (V) contributes to the Mo 3d doublet (BE,
TABLE 2
Average Mo Oxidation State (AOS) in Mo Catalysts Estimated
a
from O2 Consumption and XPS Experiments
Catalyst
(AOS)O2
(AOS)XPS
1
3
5
8
.7% Mo
.8% Mo
.5% Mo
.8% Mo
5.4
5.0
4.8
4.4
4.2
5.4
5.1
4.9
4.7
4.6
FIG. 2. Distribution of Mo oxidation states vs weight percentage of
Mo. The percent of each Mo oxidation state was calculated by dividing
the Mo 3d area for a given oxidation state by the total area of the Mo 3d
envelope. Mo (V) (Mo 3d5/2, BE 231.5 � 0.1 eV) refers to Mo (V) and
ionic Mo (IV), whereas Mo (IV) (Mo 3d5/2, BE 229.7 � 0.1 eV) designates
paired double-bonded Mo (IV). See text for details.
1
3.0% Mo
a
The average Mo oxidation state (AOS) was determined by measur-
�
ing the O2 consumption (at 500 C) required for complete reoxidation,
after the reduction procedure.

4
88
OLIVEROS ET AL.
TABLE 3
1
8
Initial Rates/(molecules/gMo � min) � 10 for the Formation
a
of C3H6 (Isomerization) and C2H4 (Metathesis)
�
�
Evac. temp. = 500 C
Evac. temp. = 25 C
(TOF/10 4)b
�
C2H4
C3H6
(TOF/10 )b
� 4
%
Mo
C2H4
C3H6
1
.7
.8
.5
.8
—
2.1
6.1
32
13
28
42
50
48
(2.9)
(2.9)
(3.0)
(3.3)
(3.4)
—
—
1.1
1.5
1.9
6.1
15.0
18.2
20.6
20.4
(1.3)
(1.5)
(1.3)
(1.3)
(1.4)
3
5
8
1
3.0
29
a
�
Reaction temperature = 25 C. Initial cyclopropane pressure = 150 Torr.
Weight of catalyst = 0.3–0.6 g.
b
The numbers in parentheses are the turnover frequencies for the forma-
tion of propylene, expressed in molecules of C3H6 per Mo (IV) per second.
Table 3 shows the initial rates for the formation of propy-
lene (isomerization) and ethylene (metathesis) obtained
over the series of reduced Mo-alumina catalysts. An equi-
valent amount of 2-butenes (compared to C2H4) was also
formed in the metathesis reaction. Two sets of data, in the
presence and in the absence of HR, are presented. As shown
FIG. 4. Initial rate of formation of C H vs relative abundance of Mo
3
6
in Table 3 and Fig. 3 there is an increase in the rate of ethy- (IV). Mo (IV) (Mo 3d5/2, BE 229.7 � 0.1 eV) designates paired double-
bonded Mo (IV). See text for details.
lene and propylene formation with Mo loading. For cata-
lysts evacuated at 500 C, the activity for the formation of
�
both products was higher compared to that obtained over
as molecules of C3H6 per Mo (IV) per second, are also
catalysts evacuated at room temperature (presence of HR),
shown in Table 3. It can be seen that the TOF remain essen-
in agreement with previous results (13). The turnover fre-
tially constant for the series of catalysts, for a given evac-
quencies (TOF) for the formation of propylene, expressed
uation temperature. These results strongly suggest that a
linear correlation must exist between the initial rate of for-
mation of propylene and the abundance of Mo (IV). This is
clearly illustrated in Fig. 4. Two straight lines (correlation
coefficients of 0.99 and 0.98, with and without HR, respec-
tively) were obtained. The latter underlines the direct re-
lationship between paired double-bonded Mo (IV) species
and the rate for cyclopropane isomerization.
In order to investigate possible correlations with other
Mo species or oxidation states, an attempt was made to cor-
relate the abundance of Mo (V) species with the catalytic
activity for the isomerization reaction. The data are shown
in Fig. 5. No linear correlation was found for the series of
catalysts used in the present study. Although this result is
consistent with the idea of Mo (IV) being directly related to
the CUS involved in the formation of the MCB intermedi-
ate, it remains inconclusive because of the fact noted above
that both Mo (V) and “ionic” Mo (IV) may contribute to
the Mo 3d doublet typically attributed to Mo (V).
CONCLUSIONS
A good correlation between the relative intensity of the
XPS peak attributed to paired double-bonded Mo (IV)
FIG. 3. Initial rate of formation of C3H6 vs weight percentage of Mo.
species (as in MoO2) and the activity for cyclopropane

ISOMERIZATION OF CYCLOPROPANE
489
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ACKNOWLEDGMENT
2
2
2
3
This work was partially supported by the Consejo de Desarrollo
Cient ´ı fico y Human ´ı stico (CDCH) through Grant 03.1447.83.
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