Isomerization and dimerization reactions of methyloxirane over various types of zeolite and zeotype

Article abstract of DOI:10.1006/jcat.2001.3186

A study on the ring-opening reactions of methyloxirane on different zeolites and zeotypes, i.e., HZSM-5, CuZSM-5, HY, AlMCM-41, SiMCM-41, and BMCM-41 at 363 K either in a pulse reactor or in a circulation system, in the presence of hydrogen or nitrogen showed that methyloxirane underwent single (isomerization) and double (deoxygenation) ring-opening, as well as dimerization reactions over different types of acidic zeolites and zeotypes. The acidic molecular sieves were active in isomerization (the products were propionaldehyde and acetone) and dimerization (the products were dioxolane and dioxane derivatives) reactions. The transformations proceeded on Bronsted or relatively strong Lewis acid sites, or their combination. The major products were dimers, i.e., dioxolane and dioxane derivatives. A confined environment was advantageous for dimerization to occur. However, this reaction route was also viable when the pore size was large and possibly on nonporous but acidic substances as well.

Full text of DOI:10.1006/jcat.2001.3186

Journal of Catalysis 200, 340–344 (2001)
doi:10.1006/jcat.2001.3186, available online at http://www.idealibrary.com on
Isomerization and Dimerization Reactions of Methyloxirane
over Various Types of Zeolite and Zeotype
,1
´
Andra´s Fa´si, Agnes Go¨mo¨ry, Istva´n Pa´linko´,† and Imre Kiricsi‡
Chemical Research Center, Hungarian Academy of Sciences, P.O. Box 17, Budapest, H-1525 Hungary; †Department of Organic Chemistry,
University of Szeged, Do´m te´r 8, Szeged, H-6720 Hungary; and ‡Department of Applied and Environmental Chemistry,
University of Szeged, Rerrich B. te´r 1, Szeged, H-6720 Hungary
Received November 20, 2000; revised January 15, 2001; accepted January 15, 2001; published online May 15, 2001
a few works (6, 7). Beside ring opening via the rupture of
one C–O bond, rearrangement was also observed (12) but
dimerization reactions were not reported until our recent
studies concerning the transformations of methyloxirane
and cis- and trans-2,3-dimethyloxiranes over HZSM-5 and
CuZSM-5 zeolites (14, 15). We have found that the isomer-
ization method of ring opening was always accompanied by
dimerization, providing dioxane and dioxolane derivatives.
Generally, this latter pathway proved to be more important
than isomerization through a simple cleavage of one C–O
bond. Since, to our knowledge, dimer formation was not re-
ported earlier over “flat” acidic contacts, we assumed that
a constrained environment was necessary for dimerization
to occur. In order to place this assumption under scrutiny,
the scope of acidic catalysts was widened in such a way that
the pore size was altered significantly. The results of this
investigation are summarized in this contribution.
The ring-opening reactions of methyloxirane on various zeolites
and zeotypes (HZSM-5, CuZSM-5, HY, AlMCM-41, SiMCM-41,
and BMCM-41) were investigated at 363 K either in a pulse re-
actor or in a circulation system, in the presence of hydrogen or
nitrogen. The acidic molecular sieves were found to be active in
isomerization (the products are propionaldehyde and acetone) and
dimerization (the products are dioxolane and dioxane derivatives)
reactions. Deoxygenation was of minor importance in hydrogen or
nitrogen atmosphere. The major product formation pathway was
dimerization on every catalytically active material. This transfor-
c
mation route was favored by the confined environment.
2001
Academic Press
Key Words: HY; HZSM-5; CuZSM-5; AlMCM-41; SiMCM-41;
BMCM-41; propylene oxide; ring-opening reactions; dimerization.
INTRODUCTION
It is widely known that epoxides are important build-
ing blocks in the syntheses of complex organic molecules,
because their preparation is relatively easy and they of-
fer a variety of ways to carry out further transformations
during or following ring opening. The methods of their
preparation are well explored both in industrial and lab-
oratory scales (for a recent review, see (1)). The laws which
govern acid-catalyzed ring-opening reaction under homo-
geneous conditions (conducted in solution) are also well
known and are even taught in fundamental organic chem-
istry courses (2). Curiously, significantly less knowledge has
been accumulated about the reaction promoted by hetero-
geneous acidic catalysts. Among the catalysts used there
were mainly oxides (Al2O3, Al2O3–SiO2, ZnO, WO3, ZrO2,
CaO, BeO, Nb2O5, etc. (3, 4)) or Nafion-H (5). The num-
ber of publications in which zeolites were chosen to catalyze
the ring-opening reaction is relatively small (6–13). In most
cases the reaction was used for synthetic purposes (9–13).
Mechanistic types of investigations are described in only
EXPERIMENTAL
Materials
The catalysts used were HZSM-5 (Si/Al = 13.8), HY
(Si/Al = 30), SiMCM-41, AlMCM-41, and BMCM-41 ze-
olites and zeotypes.
The H-zeolites were prepared by wet ion exchange start-
ing from the Na forms. Four-fold ion exchange was per-
formed in 1 mol/dm3 aqueous NH4NO3 solution. The du-
ration of one exchange was 12 h and it was carried out
at ambient temperature. The degree of ion exchange af-
ter the fourth repetition was nearly 100% . The resulting
ammonium forms were then deammonized by calcination
in a vacuum for 6 h at 873 K.
The MCM-41 derivatives were synthesized in our labo-
ratory according to recipes published in the literature (16–
19). Sodium silicate was always the silica source, aluminum
isopropoxide was used for the preparation of AlMCM-41,
boric acid was applied in the synthesis of BMCM-41, and
hexadecyltrimethylammonium bromide was the surfactant
for each material. NaOH and sulfuric acid solutions were
1 To whom correspondence should be addressed. Fax: +36 62 544 619.
E-mail: palinko@chem.u-szeged.hu.
340
0021-9517/01 $35.00
c
Copyright
2001 by Academic Press
All rights of reproduction in any form reserved.

TRANSFORMATIONS OF METHYLOXIRANE ON VARIOUS ZEOLITES
341
BET measurements were performed in a conventional
volumetric adsorption apparatus cooled to the tempera-
ture of liquid nitrogen (77.4 K). Prior to measurements the
samples were pretreated in a vacuum at 573 K for 1 h.
The acid–base properties were studied by pyridine ad-
sorption followed by IR spectroscopy (Mattson Genesis I
FT-IR spectrometer, 128 scans for one spectrum, WIN-IR
software package). Self-supported wafers (10 mg/cm2 thick-
ness) were pressed from the catalyst samples. They were
pretreated in the optical cell at 573 K for 2 h in vacuum.
Next, 1.33 kPa of pyridine was adsorbed on the activated
samplesat 363K for 1h followed bycoolingto room temper-
ature under continuous evacuation. Bands at 1540 cm 1 and
1
1450 cm were used for calculating Brønsted and Lewis
acidities, respectively.
Characteristic data of the zeolites and zeotypes collected
by the different methods are displayed in Table 1.
Reactions and Analytical Method
The reactions of methyloxirane were studied in a pulse
reactor system applying either hydrogen or nitrogen as car-
rier (45 cm3/min gas flow) . The reaction temperature was
363 K. The size of the pulse was 1 L and 20 mg of catalyst
was used. For comparison, the third and fourth pulses were
used.
SCHEME˜1. Transformation pathways of methyloxirane on acidic
zeolites/zeotypes.
The ZSM-5 samples were also tested in a static closed
recirculation reactor. A mixture of 1.33 kPa of methyloxi-
rane and 20 kPa of H2 was prepared and allowed to react on
20 mg of the dehydrated zeolite (1 h evacuation at 573 K).
The reaction temperature was 363 K. Samples were with-
drawn at 5 and 15 min and the mixture was analyzed.
Analysis of the product mixture of both reactors
was done by using a GC–MS system (Hewlett Packard
(HP) 5890 gas chromatograph equipped with a HP 5970
quadrupole mass-selective detector). Good separation
was achieved on a 50-m long CPWAX 52CB-coated
CHROMPACK WCOT fused silica capillary column by ap-
plying a temperature program (303 K for 15 min, 323 K
for 20 min, and 473 K for 10 min). Product identification
used for adjusting the pH. The resulting gels were crystal-
lized for 6 days under autogenous conditions at 373 K. They
were then filtered and washed by distilled water, dried at
373 K, and, finally, calcined at 773 K for 10 h to burn the
template off.
The CuZSM-5 catalyst was prepared by the solid-state
ion-exchange method as described in the literature (20). A
certain amount (5 mol% ) of CuCl2 was intimately mixed
with well-powdered HZSM-5 in an agate mortar. The me-
chanical mixture was heat treated at 873 K for 8 h in air.
The product was cooled to ambient temperature, washed
free of chloride, and then dried at 373 K.
Methyloxirane (1; for the structure, please see Scheme 1)
was a commercially (Fluka) available compound (racemic
mixture was purchased) and was used as received. Hydro-
gen was produced in a Matheson generator. The high-purity
nitrogen contained less than 0.0001% contamination.
TABLE 1
Characterization Data for the Zeolites and Zeotypes
a
d001
BET surface areas
Brønsted/Lewis
sitesb
1
Zeolites
(nm)
(m2 g
)
Characterization
HZSM-5
CuZSM-5c
HY
SiMCM-41
BMCM-41
AlMCM-41
—
—
—
3.994
4.028
4.059
336
318
604
1169
872
932
0.88
0.09
8.67
0.03
0.05
0.62
The samples were characterized by powder X-ray diffrac-
tometry (DRON 3 diffractometer) and BET surface area
measurements, and the metal content of CuZSM-5 was de-
termined by X-ray fluorescence spectroscopy.
X-ray diffractograms were registered on well-powdered
samples with a DRON 3 diffractometer in order to check
crystallinity and to determine d100 distances for the MCM-
41 samples.
a Calcined samples.
b Calculated on the basis of IR spectra of adsorbed pyridine.
c Prepared by solid-state ion exchange.

´
FASI ET AL.
342
was based on the NBS/NIH /EPA /MSDC database, clean
It is to be seen that dimerization was predominant on
samples and chemical evidences detailed in Ref. (15), and the ZSM-5 zeolites, while HY and AlMCM-41 preferred
molecular modeling (AM1 method included in the Hy- single ring opening. Compared to dimerization the ZSM-5
perChem 6.02 package (21)). The experimental, literature, catalysts performed poorly in single ring opening with some
and computational methods indicate that the most proba- preference for the rupture of the sterically more hindered
ble dimer products have trans configuration (trans-2-ethyl- C–O bond on HZSM-5. Nearly equal isomerization activity
4-methyl-1,3-dioxolane (5), trans-2,5-dimethyl-1,4-dioxane was found in both directions on CuZSM-5. Deoxygenation
(7), and trans-2,6-dimethyl-1,4-dioxane (8)).
was the most important pathway on HY in the first two
pulses (100% and 80 % selectivities in the first and second
pulses, respectively). From the third pulse isomerization
and dimerization took over and deoxygenation became of
minor importance. The formation of the single ring-opening
products was overwhelming on HY and AlMCM-41 and,
clearly, propionaldehyde (3) was always more significant
than acetone (2). The proportion of dimerization decreases
in the order CuZSM-5 HZSM-5 > HY > AlMCM-41.
Among the dimers 2-ethyl-4-methyl-1,3-dioxolane (5) is
the predominant product. Dioxolane formation seems to
be preferred on every zeolite except CuZSM-5 where the
quantity of dioxanes surpasses that of dioxolanes. Interest-
ingly, a significant dimerization product, 2,6-dimethyl-1,4-
dioxane (8) that is, was not formed on HY and AlMCM-41
at all.
RESULTS
Transformation Pathways of Methyloxirane
The various acidic zeolite or zeotype catalysts used in this
study behaved similarly as far as the main reaction chan-
nels are concerned. Cleavage of one C–O bond (single ring-
opening pathway) provided acetone (2) and propionalde-
hyde (3). Rupture of both C–O bonds (double ring-opening
pathway) gave propylene (9). Dimerization resulted in vari-
ous 1,3-dioxolane (4, 5) and 1,4-dioxane (6, 7, 8) derivatives.
The transformation pathways are depicted in Scheme 1.
When hydrogen was the carrier, the double ring-opening
route was suppressed on every catalyst but HY. Applying
hydrogen as carrier resulted in an induction period, and
products were first observed after the third pulse.
Details are different, however, as far as specific prod-
ucts and product distribution are concerned. Before giving
a more detailed overview, let us mention that the nonacidic
SiMCM-41 was not active at all, and the weakly Lewis acidic
BMCM-41 displayed only very little activity (conversion
was less than 1% ). Here, only dimerization occurred; how-
ever, even that reaction was very slow.
Finally, let us mention that changing the hydrogen carrier
to nitrogen does not result in major alterations in product
formation activities and selectivities.
Reactions in the Circulation Reactor on HZSM-5
and CuZSM-5 and Comparison of the Observed
Characteristics to Those of the Pulse System
HZSM-5 and CuZSM-5 were picked for a more detailed
investigation in a more sophisticated reactor system, since
they provided the unexpected dimers in the highest propor-
tion. In this reactor, similarly to the pulse system, methy-
loxirane was transformed through single C–O scission as
well as dimerization (15). Product distribution data are dis-
played in Table 3.
Reactions in the Pulse System
Product distributions in the pulse reactor on various ze-
olites and zeotypes are summarized in Table 2.
TABLE 2
First, let us describe features similar in both reactor types
when hydrogen was added to methyloxirane. The products
were essentially the same, except that 2,2,3-trimethyl-1,3-
dioxolane (4) was not formed on HZSM-5 in the circulation
reactor. However, one should note that the proportion of
this compound was also very low in the pulse reactor. The
CuZSM-5 zeolite was more active than the HZSM-5 zeo-
lite. The dimerization pathway was more important than
the single ring-opening route on both zeolites. 2-Ethyl-4-
methyl-1,3-dioxolane (5) was the most important product,
even taking into account the overall transformation. And,
finally, deoxygenation (double C–O scission) did not occur.
Besides the above-listed similarities, considerable dif-
ferences could be observed. They are as follows. Among
the single C–O scission pathways the opening of the steri-
cally more hindered C–O bond was clearly overwhelming
in the circulation reactor. Although 2-ethyl-4-methyl-1,3-
dioxolane (5) remained the most important product in the
Product Distribution in theZeolite/Zeotype-Catalyzed Reactions
ofMethyloxirane(1)in thePulseReactor UsingHydrogen/Nitrogen
as the Carrier—Data Correspond to the Third Pulse (20 mg of
Catalyst, 45cm3/min GasFlow, 1 LPulseofMethyloxirane, 363K)
Composition/mol% a
HZSM-5
CuZSM-5b
HY
AlMCM-41
2
3
4
5
6
7
8
9
4.5/4.9
7.1/9.8
0.9/1.5
51.9/47.3
8.6/8.3
14.7/14.4
12.3/12.9
0/0.9
3.8
3.6
2.7
35.1
5.8
20.1
28.9
0
21.0/18.2
41.9/47.8
0/0
17.6/12.4
6.9/8.0
5.6/2.6
0/0
10.0/10.5
65.5/62.4
0/0
15.1/15.9
0/0
9.4/10.7
0/0
0/0.5
7.0/11.0
a Conversion of methyloxirane: HZSM-5, 67.8% /67.1% ; CuZSM-5,
87.4% ; HY, 90.4% /93.1% ; AlMCM-41, 93.6% /98.7% .
b Carrier: hydrogen.

TRANSFORMATIONS OF METHYLOXIRANE ON VARIOUS ZEOLITES
343
TABLE 3
As far as dimerization is concerned, it is clear that a more
constrained environment (smaller pore size) is advanta-
geous for dimerization to occur. The decreasing proportion
of dimerization correlates with the increasing pore diame-
Product Distribution in the Zeolite-Catalyzed Reactions of
Methyloxirane (1) under Reductive Atmosphere in the Circula-
tion Reactor (20 mg of Catalyst, 1.33 kPa Methyloxirane, 20 kPa
Hydrogen, 363 K)
˚
˚ ˚ ˚
5.6 A; Y, 7.4 A; AlMCM-41, 40.6 A).
ters (ZSM-5, 5.3 A
It is important to note, however, that dimerization remains
an important reaction pathway even on AlMCM-41. In the
following the suggested mechanism of one five-membered-
ring product is detailed as an example, since the formation
of five-membered-ring dimerization products is preferred
on every zeolite/zeotype applied. Moreover, the 2-ethyl-4-
methyl-1,3- dioxolane (5) is always the major product. The
first step of its formation is proposed to be the formation
of propanal precursors via the opening of the more hin-
dered C–O bond oftwo methyloxirane molecules. These ad-
sorbed species are then combined by establishing new C–O
bonds. During ring formation a 1,2-hydride shift should also
occur.
Product distribution/mol% a
5 min
15 min
CuZSM-5(s)
HZSM-5
CuZSM-5
HZSM-5
2
3
4
5
6
7
8
4.4
22.7
0
35.3
27.1
2.4
5.4
16.1
2.4
7.7
26.4
0
6.1
18.3
2.4
31.8
17.9
10.3
13.2
32.3
18.9
10.6
14.3
30.9
23.4
2.5
8.1
9.1
a Conversion of methyloxirane. HZSM-5: 5 min, 29.5% ; 15 min, 36.4% .
CuZSM-5: 5 min, 76.4% ; 15 min, 94.4% .
The above proposal is summarized in Scheme 3.
It is thought that C–O bond scission, C–O bond forma-
tion, and the hydride shift are probably sequential reac-
tions. The first and second pulses, that are largely adsorbed
in/on the materials, react with the following pulses and the
combined products are observed as dimers. (In the static
circulation reactor there is plenty of time for reactions like
these to occur.) This suggestion is supported by the product
distribution in the ring-opening reactions of methyloxirane
circulation system too, the sum of dioxane derivatives ex-
ceeded that of the dioxolanes.
DISCUSSION
The Reactions
Dimerization is a kind of ring opening; thus, ring open- performed in the liquid phase (but otherwise under simi-
ing may be considered to be the sole transformation path- lar conditions as described in this work) on HZSM-5 (22).
way for methyloxirane under the conditions applied. Three Here, dimerization is suppressed relative to what was ex-
types of this reaction may be distinguished on these sub- perienced in the gas phase. Solvation certainly hinders the
stances: (i) single C–O bond cleavage leading to aldehyde interaction of the reactants; nonetheless, dimerization was
and ketone; (ii) dimerization involving simultaneous bond not completely ceased. This observation indicates that a
making breaking; and (iii) olefin formation meaning dou- dimerization pathway with concomitant C–O scission and
ble C–O bond scission. Single ring opening is a typical acid- formation of new C–O bonds may also be operable.
catalyzed reaction proceeding on Brønsted as well as Lewis
acid sites. It is known to proceed via carbenium ions. Since
the secondary carbenium ion is more stable than the pri-
The Molecular Sieves
mary, the ring-openingroute usingthe former cation (propi-
The complete lack of reactivity on the all-Si zeotype
onaldehyde is forming) is preferred (Scheme 2). However,
strongly indicates the necessity of acid sites for the reaction
steric factors favor the cleavage of the sterically less hin-
to occur. Moreover, relatively strong acid sites are needed,
dered C–O bond (acetone is forming). The two effects com-
since the activity of BMCM-41 possessing only weak acid
pete; nevertheless, electronic factors tend to overwhelm
sites is negligibly low. The transformations should proceed
steric factors.
on Brønsted or Lewis centers—the activity of CuZSM-5
containing mostly Lewis sites was comparable to that of
HZSM-5 containing mostly Brønsted sites—or their combi-
nation. Although the initial CuZSM-5 zeolite has predom-
inantly Lewis sites, the working zeolite possibly contains a
steady state composition of Cu(II), Cu(I), and Cu(0) clus-
ters together with extra protonic sites, due to the interplay
of autoreduction, the reducing environment (hydrogen or
nitrogen as added gas), trace amounts of water, and the
oxidizing effect of methyloxirane.
SCHEME 2. Schematic representation of the suggested mechanism for
As has been mentioned already, the nature of added
gas (hydrogen/nitrogen) does not influence product
single ring opening of methyloxirane leading to aldehyde formation on a
Lewis acid site.

´
FASI ET AL.
344
SCHEME 3. Schematic representation of the suggested mechanism for the dimerization of methyloxirane leading to dioxolane formation on Brønsted
acid sites.
distribution to a significant extent; however, added hydro-
gen suppresses or even stops deoxygenation.
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CONCLUSIONS
Methyloxirane undergoes single (isomerization) and
double (deoxygenation) ring-opening as well as dimeriza-
tion reactions over various types of acidic zeolites and
zeotypes. The transformations proceed on Brønsted or rel-
atively strong Lewis acid sites or their combination. The
major products are dimers: dioxolane and dioxane deriva-
tives. It was found that a confined environment is advan-
tageous for dimerization to occur; however, this reaction
route also is viable when the pore size is large and possibly
on nonporous but acidic substances as well.
8. Takeuchi, H., Kitajima, K., Yamamato, Y., and Mizuno, K., J. Chem.
Soc., Perkin Trans. 2 199 (1993).
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ACKNOWLEDGMENT
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This research was supported by an OTKA Grant (T034184).
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