Transformation of vinyloxirane on Pt-SiO2 and Pd-SiO2

Article abstract of DOI:10.1006/jcat.1998.1969

The transformation of vinyloxirane and ethyloxirane were studied on Pt-SiO₂ and Pd-SiO₂ catalysts at 273 and 301 K in a recirculation reactor, in the presence of hydrogen or deuterium. There are significant differences, not only in reaction rates, but also in reaction routes, depending upon the nature of the metal. The main reaction routes are: deoxygenation and hydrogen or deuterium addition on Pt-SiO₂; isomerization and hydrogenolysis on Pd-SiO₂. The reaction routes leading to the formation of the individual products are interpreted on the basis of the experimental results. In the case of the hydrogenolysis of oxiranes, Pd was shown to exhibit a special reactivity, unlike earlier observations described in the literature.

Full text of DOI:10.1006/jcat.1998.1969

JOURNAL OF CATALYSIS 175, 40–47 (1998)
ARTICLE NO. CA981969
Transformation of Vinyloxirane on Pt–SiO and Pd–SiO
2
2
Mih a´ ly Bart o´ k, Andr a´ s F a´ si, and Ferenc Notheisz
Department of Organic Chemistry and Organic Catalysis Research Group of the Hungarian Academy of Sciences,
J o´ zsef Attila University, D o´ m t e´ r 8, H-6720 Szeged, Hungary
Received August 5, 1997; revised November 7, 1997; accepted November 7, 1997
vinyloxirane in the chemical industry, because nowadays
The transformation of vinyloxirane and ethyloxirane were stud- vinyloxirane can be prepared easily by the selective epoxi-
ied on Pt–SiO2 and Pd–SiO2 catalysts at 273 and 301 K in a recircu- dation of butadiene (9). The isomerization and hydrogenol-
lation reactor, in the presence of hydrogen or deuterium. There are
significant differences, not only in reaction rates, but also in reaction
ysis of the oxirane ring are catalyzed by Rh-complexes
(
10, 11). In the case of Pd- and Ir-complexes, selective hy-
routes, depending upon the nature of the metal. The main reaction
routes are: deoxygenation and hydrogen or deuterium addition on
Pt–SiO2; isomerization and hydrogenolysis on Pd–SiO2. The reac-
tion routes leading to the formation of the individual products are
interpreted on the basis of the experimental results. In the case of
the hydrogenolysis of oxiranes, Pd was shown to exhibit a special
reactivity, unlike earlier observations described in the literature.
drogenation of the C == C bond has been observed (12). The
utilization of metal catalysts (Rh, Pd, Pd–Au, Pd–Re) has
been published in patents (13–15) reporting the selective
hydrogenation of the C == C bond. Hydrogenolysis of the
oxirane ring was observed during hydrogenation of steroid
vinyloxiranes catalyzed by Raney and Ni (16).
�
c 1998 Academic Press
According to earlier studies (17), the main products of
Key Words: vinyl- and ethyloxirane; Pt–SiO2 and Pd–SiO2 cata- the transformation of ethyloxirane on Pt and Pd catalysts at
lysts; effects of hydrogen and deuterium; mechanism of ring open- 300–400 K are 2-butanol and 2-butanone. In the course of
ing; deoxygenation; hydrogenolysis; isomerization; hydrogenation; the hydrogenative transformation of monosubstituted oxi-
deuteration; isotope effect.
–
ranes the cleavage of the C O bond near the substituent
yields primary alcohol and aldehyde while the cleavage of
the other C–O bond results in the formation of secondary
INTRODUCTION
alcohol and ketone. Regioselectivity depends primarily on
the quality of the metal catalyst. While in the case of Pt
and Pd, bond cleavage occurs from the sterically less hin-
dered direction, on Ni and Cu it proceeds from the other,
sterically more hindered direction (17). These reactions are
also accompanied by deoxygenation, depending upon the
catalyst.
Hydrogenative ring opening of oxiranes has been stud-
ied extensively (see (1–3)). The reaction has both theoret-
ical and practical significance, also proven by the reports
published lately (e.g., (4–8)). Additionally, oxiranes show
exclusively ring opening under very mild conditions (low
temperature and hydrogen pressure) without any side re-
action. Methyloxirane is probably one of the most popular
reactants in catalytic chemistry. Its hydrogenative transfor-
mation is a frequently used model reaction for characteri-
zation of metal catalysts.
EXPERIMENTAL
Materials
Studieson oxiranes, containinga C == C bond are ofspecial
Vinyloxirane and ethyloxirane were purchased from
importance, as the presence of two functional groups within Fluka and Aldrich. Prior to use, they were purified by sev-
a single molecule makes possible the investigation of the se- eral cycles of freezing–degassing–thawing.
lectivity of catalytic hydrogenolysis or hydrogenation which
Three percent Pd–SiO2 (D = 15% ) and Pt–SiO2 (D =
is important from the point of view of synthesis, on the one 14% ) were prepared by impregnation (18, 19). The catalysts
hand, and for studies on the mechanism of metal catalytic were reduced in flowing hydrogen at 623 K. The dispersion
reactions, on the other. For these two purposes a more de- of the catalysts was determined by H2 and CO chemisorp-
tailed elucidation of the hydrogenation and hydrogenolysis tion and were checked bytransmission electron microscopy.
of vinyloxirane was set as the aim of the present work.
In the course of the pretreatment the catalyst was heated
Relatively little information has been published on the to 423 K at 26.6 kPa hydrogen or deuterium pressure, kept
catalytic transformation of vinyloxirane in the presence of at this temperature for 1 h and cooled to the reaction tem-
hydrogen. This is despite the huge interest for the use of perature under continuous vacuum.
40
0021-9517/98 $25.00
Copyright �c 1998 by Academic Press
All rights of reproduction in any form reserved.

Pt- AND Pd-CATALYZED RING OPENING
41
Methods
Measurements were carried out in a circulation reactor
3
(
20). The volume of the reactor was 69 cm and that of
3
the entire system was 170 cm . The reactor was heated by
an air thermostat. Analysis was carried out in a gas chro-
matograph (HP 5890) with a quadrupole mass selective de-
tector (HP 5970 MSD) attached to the system. Separation
was done in a Carbowax 20M capillary column (HP-20M).
Data were processed by CSW and HP 5970 chromatogram
analysis programs.
Hydrogen used for the measurements was generated in
a Matheson type 8326 electrolysis apparatus equipped with
a Pd diffusion cell. Deuterium was obtained by a General
Electric 15 EHG generator (the purity of D2O was 99.8% ).
Deuterium was deoxygenated by a Model 1000 Oxygen
Trap (CRS, USA).
FIG. 1. Transformation of ethyloxirane and vinyloxirane on Pt–
SiO2 and Pd–SiO2 catalysts (catalyst = 10 mg, 273 K, poxirane = 1.33 kPa,
pdeuterium or hydrogen = 20 kPa).
Procedure
Fresh catalyst (10 mg) and 1.33 kPa of vinyloxirane or
ethyloxirane were added for each measurement. For com-
petition studies a 1 : 1 mixture of the two oxiraneswasadded
to a total pressure of 1.33 kPa. The pressure of hydrogen
or deuterium was 20 kPa in all cases; reaction temperatures
were 273 and 301 K in the case of both molecules.
Samples taken at various times were used for the calcula-
tion of molar percentage values which served for the basis
of comparison. Deuterium distribution was calculated from
the data obtained in measurements with deuterium.
experimental observation that the conversion of vinyloxi-
rane on Pd is nearly identical in hydrogen and in deuterium.
This means that at 301 K, under the given conditions the
�
-hydride phase no more plays a determinative role.
Since on well-hydrogenating catalysts (Pt, Pd) vinyloxi-
rane also gives rise to ethyloxirane, hydrogenolysis and iso-
merization of ethyloxirane were first examined.
Transformation of Ethyloxirane on Pt–SiO2
After deoxygenation and hydrogenation of the surface
intermediate, ethyloxirane is selectively converted into bu-
tane on Pt catalysts at temperatures as low as 273 K at the
experimental conditions outlined in Fig. 1. According to
the evidence of the measurements in deuterium, simultane-
ously with deoxygenation the C–H bonds are also cleaved
RESULTS AND DISCUSSION
Activity of Catalysts
Changes in the conversion of the two oxiranes studied
as a function of reaction time in the presence of hydro-
gen or deuterium at 273 and 301 K are shown in Figs. 1
and 2.
According to these data, under identical experimental
conditions the activity of Pt is higher than that of Pd in the
hydrogenolysis of ethyloxirane (the conversion of ethyloxi-
rane is minimal at 273 K on Pd). This is probably caused
by the presence of the Pd-�-hydride, which is inactive in
C–O hydrogenolysis. As regards the rate of conversion, the
situation is the same for vinyloxirane at 273 K because the
�
-hydride is inactive in hydrogenation of C == C bond (21).
At 301 K, however, the transformation of vinyloxirane is
faster on Pd than on Pt. This is likely explained by the in-
stability of the �-hydride in the presence of oxygen, since
trace amounts of oxygen can destroy this phase (22). The
vinyloxirane can be deoxygenated at 301 K, producing a
surface oxide layer. The formation of ionic type active sites
is also manifested in the reaction routes because the main
products on Pd–SiO2 originate from the cleavage of the
more hindered C–O bond of vinyloxirane. It is an important
FIG. 2. Transformation of ethyloxirane and vinyloxirane on Pt–
SiO2 and Pd SiO2 catalysts (catalyst = 10 mg, 301 K, poxirane = 1.33 kPa,
pdeuterium or hydrogen = 20 kPa).
–

´
´
4
2
BARTOK, FASI, AND NOTHEISZ
TABLE 1
Transformation of Ethyloxirane in Deuterium over Pt–SiO2 Catalyst
Distribution of deuterium (% )
In butane
In 2-butanol
t (min)
conv. (% )
Sbutane
S2-butanol
S2-butanonea
d2
d3
d4
d5
d6
d7
d2
d3
5
9
24
34
44
0.75
0.76
0.76
0.75
0.21
0.22
0.22
0.23
0.04
0.02
0.02
0.02
69.6
57.7
52.9
50.4
26.2
26.7
25.7
24.2
4.2
12.0
12.7
13.2
0
0
0
2.8
3.7
0
0
0
1.1
100
100
95.0
92.2
0
0
5.0
7.8
1
2
3
5
5
5
3.5
5.9
7.4
Note. Catalyst = 10 mg, 301 K, poxirane = 1.33 kPa, pdeuterium = 20 kPa.
a
Only 2-butane-d1 formed.
(
e.g., 75% butane-d2, 21% butane-d3, and 4% butane-d4 bility of readsorption in the presence of oxiranes is very
were formed at the conversion of 3% ). At 301 K the main low. It should be noted that conversions on the Pt cata-
product is still butane (Sbutane = 77% ) but, as a results of lyst recorded in the measurements of hydrogen are in good
the cleavage of the sterically less hindered bond, 2-butanol agreement with those of deuterium at both temperatures,
(
S2-butanol = 22% ) and small amounts of 2-butanone (less which means that there is no isotope effect.
than 1% ) also appear. This suggests that the first event is
the formation of PtO sites on the surface and the active sites
for isomerization are formed in the course of this process.
Deoxygenation of oxiranes under relatively mild experi-
Transformation of Ethyloxirane on Pd–SiO2
The extent of the conversion of ethyloxirane on the Pd
mentalconditionsisa well-known reaction (23). The driving catalyst at 273 K in the presence of hydrogen or deuterium
force to the reaction is the strain energy of the three- is very small, only traces are converted to 2-butanol. At
membered ring. Due to its stereospecific character, this 301 K it is mainly the sterically less hindered C–O bond
reaction is used in the synthetic organic chemistry for the that is cleaved and 2-butanol is formed (even in the 35th
stereoselective formation of C == C bonds. The process takes minute, conversion to butane, butanal, and 2-butanone is
place at room temperature on the surface of Pt and leads to only 2.9% ). Thus, under identical conditions the amount of
the formation of PtO. Under the given experimental con- butane formed on Pd is two orders of magnitude less than
ditions, PtO is readily converted back to Pt by hydrogen on Pt and the main product is 2-butanol. Consequently, on
which makes deoxygenation continuous. The latter, well- Pd the main reaction route is not deoxygenation but hy-
known reaction is used frequently for the characterization drogenolysis of the C–O bond. The repression of deoxy-
of Pt catalysts (hydrogen–oxygen titration method to mea- genation may be explained by the presence of surface PdO
sure the surface area of supported platinum catalysts which is generated at the beginning of the reaction and is
(
24, 25)).
Measurements in the presence of deuterium (Table 1) According to the new literature data (26) (XPS analysis of
showed that hydrogenolysis and isomerization give rise to palladium oxide layers and particles): “The reduction rate
-butanol-d2 and 2-butanone-d1, respectively.
of the surface oxide is � 8 times lower than the reduction
In the case of the products containing oxygen, the place rate of the bulk oxide.”
of the incorporation of deuterium may also be determined Conversion was considerably higher in the measure-
less readily reduced than PtO under the given conditions.
2
on the basis of the evaluation of mass spectra. Deuterium is ments using hydrogen than in those with deuterium (Fig. 2),
located on the methyl group and the oxygen in 2-butanol-d2 indicating the operation of the isotope effect. Our earlier
and on the methyl group in 2-butanone-d1 (Scheme 1). (It data (7) indicated that at a higher temperature (363K) there
has to be noted that the deuterium in the O–D group is ex- was no significant difference in the behaviour between the
changed for hydrogen in the capillary glass column; there- two isotopes in the case of 2,3-dimethyloxirane. No iso-
fore this D is not detected in the mass spectrum. The occur- tope effect was observed by other authors in the course
rence of the exchange on the column was verified by our of the hydrogenolysis and deuterolysis of oxiranes either
measurements with perdeuterated methanol.) The scarcity (17, 27–32). The evidence for the isotope effect in the cou-
of multiple exchange events in the case of 2-butanone and rse of the hydrogenolysis of ethyloxirane at a relatively low
2
-butanol allows us to conclude that the intermediate temperature on Pd allows a very important conclusion to
formed after ring opening spends very little time on the sur- be drawn regarding the reaction mechanism, namely, that
face; i.e., the oxygen-containing products are very rapidly hydrogen or deuterium adsorbed on the surface plays a role
desorbed from the surface of the catalyst and the proba- in the rate-limiting step of the reaction, i.e. ring-opening. At

Pt- AND Pd-CATALYZED RING OPENING
43
SCHEME 1. Mechanism of ethyloxirane transformation on Pt–SiO2 and Pd–SiO2 catalysts.
higher temperatures, where no isotope effect is observed, adsorption (27). Pd also tends to form such electrophilic
the reaction of the hydrogen atom with the oxirane ring is centers by the surface oxidation, while Pt—due to the rapid
preceded by the dissociation of the C–O bond on the sur- reduction of PtO—is less prone to this course of events.
face. At lower temperatures, however, the three-membered
ring is opened by hydrogen or deuterium chemisorbed on in unconverted ethyloxirane on either catalyst studied. This
the surface. observation again supports the earlier notion that oxirane
It is important to mention that no deuterium was found
The entry of deuterium into the oxygen-containing prod- is irreversibly bound to the surface of Pt and Pd catalysts
ucts is similar to the process described for Pt. There is no and is desorbed only as a product.
detectable level of multiple exchanges; therefore the pres-
Transformation of Vinyloxirane on Pt–SiO2
ence of 2-butanol-d2 and 2-butanone-d1 again indicates fast
desorption into the gas phase and the absence of readsorp-
Reaction routes of the transformations of vinyloxirane
tion. In the case of butanal-d1 formed via cleavage of the on Pt and Pd catalysts in the presence of hydrogen or deu-
sterically more hindered bond, the situation is similar. terium are demonstrated on Scheme 2. The extent of the
On the basis of our experimental results, the isomeriza- participation of the individual processes depends on the ex-
tion and hydrogenolysis of ethyloxirane may be interpreted perimental conditions and, naturally, on the quality of the
as shown in Scheme 1.
catalyst. In the absence of hydrogen (mixed with 67 kPa of
According to the reaction pathway outlined in Scheme 1, He), vinyloxirane is not transformed either on reduced Pt,
-butanol and 2-butanone are formed on reduced metal or on reduced Pd, at 273 or 301 K.
2
clusters by associative and dissociative mechanisms, respec-
In the presence of hydrogen, on Pt catalyst at 273 K
tively. In this case the isomerization takes place on metal 1-butene, butane, and ethyloxirane are formed (Fig. 3).
atoms where the molecule is bound to the surface by �-allyl The hydrogenation and deoxygenation of vinyloxirane take
SCHEME 2. Reaction pathways in the transformation of vinyloxirane on Pt–SiO2 and Pd–SiO2 catalysts.

´ ´
BARTOK, FASI, AND NOTHEISZ
4
4
hindered bond that is typical, it is improbable that butanal
originates from ethyloxirane. It is more probable that bu-
tanal is formed in the course of the hydrogenation of the in-
termediate product originating from vinyloxirane. Just like
in the ethyloxirane/Pt system, there is no significant differ-
ence between the measurements carried out in hydrogen
and in deuterium (the differences are within 10% ).
Deuterium distribution in ethyloxirane formed in the
course of deuterium addition is presented in Table 3.
Besides the expected maximum in d2 and further H–D ex-
change in the ethyl group, the amounts of d0 and d1 are
relatively high, which is attributable to the fast dissociation
of the vinylic and allylic hydrogens in vinyloxirane.
The reaction route of the transformation of vinyloxi-
rane on a Pt–SiO2 catalyst, in the presence of hydrogen, at
FIG. 3. Transformation of vinyloxirane on Pt–SiO2 catalyst (cata-
lyst = 10 mg, poxirane = 1.33 kPa, phydrogen = 20 kPa).
2
73–301 K is shown in Scheme 3.
place simultaneously. As long as vinyloxirane molecules Transformation of Vinyloxirane on Pd SiO2
–
are present in the system, the methyloxirane and butene
On a Pd catalyst in the presence of hydrogen at 273 K,
hydrogenation of vinyloxirane yields mostly ethyloxirane
Fig. 4); some 3-methoxypropene is also formed via C–C
bond cleavage, but traces of 1-butene are detectable as well.
On the basis of the product composition it appears unam-
biguous that the geometry of the adsorption of vinyloxirane
on Pd is different from that on Pt.
The representation of conversion versus time (Figs. 4
and 5) reveals that products formation has an induction pe-
riod. This may be caused by the presence of the �-hydride
at this temperature which is well known to be inactive in
hydrogenation of alkenes (21). Vinyloxirane, however, de-
stroys the hydride phase by its adsorption.
molecules cannot adsorb on the surface; therefore their
transformation cannot occur. This is supported by the fact
that the 1-butene/butane ratio is more or less constant in
the course of the reaction and 1-butene is predominant all
along (Table 2).
Under the given chromatographic conditions, butane and
butene are eluted in one peak; therefore they were quan-
tified on the basis of the characteristic m/e values (56, 58).
Formation of butane mainly from ethyloxirane is also sup-
ported in an indirect way by the near identity of the amount
of hydrocarbon formed in the ethyloxirane/Pt and vinyloxi-
rane/Pt systems which, on the other hand, allows one to con-
clude that the effect of the substituent on deoxygenation is
not the determinant.
(
The results obtained in measurements with deuterium
(
Fig. 5) are slightly different from those with hydrogen as
At 301 K, the main product is ethyloxirane but the
amount of 1-butene and butane is still significant (Fig. 3);
moreover, cleavage in the sterically more hindered direc-
tion also takes place, giving rise to butanal. Since in the
ethyloxirane/Pt system it is the cleavage of the sterically less
novel products appear: trans-2-butenal, cis-2-butenal, and,
via their hydrogenation after an extended reaction time,
butanal (the combined amount of the latter two products is
below 1% ).
The experimental data of the transformation of vinyloxi-
rane at a higher temperature (301 K) show that the induc-
tion periods observed at 273 K are absent at this temper-
ature. (As temperature increases, the �-hydride phase is
most probably transformed faster by the oxirane adsorbed;
therefore the induction period is not detected.)
TABLE 2
Distribution of 1-Butene and Butane in the Transformation
of Vinyloxirane over Pt–SiO2
The primary products are further converted: hydrogena-
tion of 3-methoxypropene yields methoxypropane; and the
hydrogenations of trans- and cis-butenal lead to butanal.
Their connection is indicated by the decrease of concen-
tration of the unsaturated molecules after a maximum and
the increase of the saturated molecules after an induction
period. 1-Butene formed by deoxygenation is gradually
reduced to butane (Table 2). These statements hold true
both in hydrogen and in deuterium. The main difference
between the two systems is the operation of the isotope
effect during hydrogenation of the C == C bond. Secondary
Distribution of 1-butene and butane (% )
Pt/SiO2
Pd/SiO2
301 K
2
73 K
301 K
�
�
�
�
�
�
t (min) m/e = 56 m/e = 58 m/e = 56 m/e = 58 m/e = 56 m/e = 58
5
5
5
5
75
75
77
77
25
25
23
23
73
76
75
73
27
24
25
27
100
88
62
0
12
38
63
1
2
3
37
Note. Catalyst = 10 mg, poxirane = 1.33 kPa, phydrogen = 20 kPa.

Pt- AND Pd-CATALYZED RING OPENING
45
TABLE 3
a
b
Transformation of Vinyloxirane in Deuterium over Pt–SiO2 and Pd–SiO2 Catalysts
Distribution of deuterium in (% )
ethyloxirane (a)
ethyloxirane (b) 3-methoxypropene (b) methoxypropane (b)
trans-2-butenal (b)
butanal (b)
t (min) d0 d1 d2 d3 d4 d0 d1 d2 d3 d0
d1
d2
d3 d1
d2
d3
d4 d0
d1
d2
d3 d0 d1 d2 d3 d4 d5 d6
5
5
5
5
27 39 29
26 36 29
24 37 29
24 37 29
6
7
8
8
0
1
2
2
46 39 15
28 42 21
33 39 19
31 41 20
0
9
9
9
59
27
10
—
39
33
39
—
3
33
52
—
0
7
0
—
—
—
—
70
51
32
—
29
34
40
—
1
13
25
—
0
2
4
23 33 27 14
16 30 27 16
3
8
0
3
4
6
0
0
0
1
a
a
a
a
1
2
3
50
9
24
24
26
35
0
33
12 25 29 20 10
23 27 22 12
—
—
9
Note. Catalysts = 10 mg, 301 K, poxirane = 1.33 kPa, pdeuterium = 20 kPa.
a
Traces.
processes are, therefore, easier to follow using deuterium. on either catalyst. The conclusions drawn from this fact
Comparison of the results obtained at 273 and 301 K re- are identical with those in the case of ethyloxirane. Deu-
veals that on increasing the temperature, cleavage in the terium distributions determined in the products of vinyloxi-
sterically more hindered direction is significantly enhanced rane (Table 3) exhibit interestinganomalies. The statements
and becomes dominant. Cleavage in the sterically more regarding the ethyloxirane formed on Pt also hold true in
hindered direction has already been observed in the case the case of Pd. The situation is similar also in the case of the
of 2,2-dimethyloxirane (28). In the present case the driv- other products (methoxypropane, butanal); the deuterium
ing force of ring opening in the sterically hindered direc- content of the molecules is lower than expected.
tion is the heterolysis of the weak allylic C–O bond which
is at the same time part of the strained three-membered genatad carbonaceous residue from the oxirane leads to
ring. a pool of surface hydrogen atoms and the d0 in oxirane.
It is highly probable that the formation of dehydro-
Since reaction routes operative on a Pd catalyst are dif- However, the tendency of deuterium incorporation to in-
ferent from those on Pt, measurements were also carried crease with the progress of reaction time is quite obvious.
out in hydrogen at 363 K (Fig. 6). The main products are The formation of trans-2-butenal is the result of a hydroi-
butanal, methoxypropane, and dibutyl ether, as secondary somerization reaction involving the incorporation of a deu-
products (Scheme 4). Dibutylether is derived from butanal terium atom. H–D exchange in the primary product natu-
by the reaction verified earlier (semiacetal formation be- rally yields d2 and d3 compounds.
tween adsorbed butanal and adsorbed butanol on the sur-
The reaction of vinyloxirane on a Pd–SiO2 catalyst, in
face, followed by hydrogenolysis (33)). The hydrocarbon the presence of hydrogen or deuterium at 273–301 K may
formed is mainly butane, but small amounts of methane and be interpreted on the basis of Scheme 4. The considerable
ethylene are also detectable. CO produced by the cleavage differences, as compared to Pt, may be attributed to the dif-
of butanal is most certainly retained on the surface of the ferent affinities of the two metals toward oxygen, since the
catalyst and poisons part of the active centers.
competition between the two reactive parts of the molecule
In the course of the experiments in deuterium, untrans- (strained oxirane ring and C == C bond) is determined by this
formed vinyloxirane was shown to contain no deuterium characteristic. The formation of 3-methoxypropene and,
SCHEME 3. Mechanism of vinyloxirane transformation on Pt–SiO2 catalyst.

´ ´
BARTOK, FASI, AND NOTHEISZ
4
6
FIG. 4. Transformation of vinyloxirane on Pd–SiO2 catalyst (cata-
lyst = 10 mg, 273 K, empty symbols, and 301 K; black symbols, poxirane =
.33 kPa, phydrogen = 20 kPa).
FIG. 5. Transformation of vinyloxirane on Pd–SiO2 catalyst (cata-
lyst = 10 mg, 273 K, empty symbols, 5, trans-2-butenal; ♦, 3-methoxy-
propene, and 301 K, black symbols, poxirane = 1.33 kPa, pdeuterium = 20 kPa).
1
dered bond (2-butenal and butanal). The difference in re-
action routes between the two catalysts may be interpreted
by the different structures of the surface species compris-
ing the reactant, on the one hand, and the catalyst surface
developing in the course of the reaction, on the other. In
furthermore, the decrease in the rate of deoxygenation
show that vinyloxirane is able to be adsorbed on Pd without
the formation of a metal–oxygen bond. It is well known, for
example, that hydrogenation of aliphatic ketones usually
does not happen on Pd (34, 35). There is also a significant
difference between the reactions of the two metals and their
oxides in the presence of oxygen and hydrogen (24, 25, 36).
CONCLUSION
Our measurementsat lowtemperaturesallowed the iden-
tification ofnewphenomena in metalcatalyzed hydrogenol-
ysis of oxiranes. Under identical experimental conditions,
ethyloxirane undergoes hydrogenolysis on Pd and deoxy-
genation on Pt.
The difference in reaction routes is even larger when the
conversion of vinyloxirane is studied in the presence of hy-
drogen and/or deuterium, on a Pt– or Pd–SiO2 catalyst. On
Pt–SiO2, in addition to the hydrogenation of vinyloxirane to
yield ethyloxirane, the dominant reaction is deoxygenation.
On a Pd–SiO2 catalyst, the main reaction route is the forma-
FIG. 6. Transformation of vinyloxirane on Pd SiO2 catalyst (cata-
–
tion ofproductsderived bythe cleavage ofthe stericallyhin- lyst = 10 mg, 363 K, poxirane = 1.33 kPa, pdeuterium = 20 kPa).

Pt- AND Pd-CATALYZED RING OPENING
47
SCHEME 4. Mechanism of vinyloxirane transformation on Pd–SiO2 catalyst.
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