Novel route of double-bond migration of an olefin without protonated species on ZSM-5 zeolite

Article abstract of DOI:10.1021/jp9707504

A novel reaction of the double-bond migration (DBM) of 1-butene on Bronsted acid sites (BAS) of zeolites in the absence of proton transfer from BAS to the adsorbed olefin was found. The widely accepted protonated intermediate should result in the conversion of the acidic OD groups to OH upon the DBM. Nevertheless, the OD groups forming a 1:1 complex with the adsorbed 1-butene were unchanged even after the reaction. The isotope exchange of OD groups occurred at a higher temperature than that of the DBM. Thus, the existence of a new type of reaction on BAS is demonstrated, which takes place more easily than the proton transfer.

Full text of DOI:10.1021/jp9707504

J. Phys. Chem. B 1997, 101, 5477-5479
5477
A Novel Route of Double-Bond Migration of an Olefin without Protonated Species on
ZSM-5 Zeolite
Junko N. Kondo and Kazunari Domen*,†
Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta,
Midori-ku, Yokohama 226, Japan
Fumitaka Wakabayashi‡
Department of Science and Engineering, National Science Museum, 3-23-1 Hyakunin-cho,
Shinjuku-ku, Tokyo 169, Japan
X
ReceiVed: February 28, 1997; In Final Form: April 24, 1997
A novel reaction of the double-bond migration (DBM) of 1-butene on Brønsted acid sites (BAS) of zeolites
in the absence of proton transfer from BAS to the adsorbed olefin was found. The widely accepted protonated
intermediate should result in the conversion of the acidic OD groups to OH upon the DBM. Nevertheless,
the OD groups forming a 1:1 complex with the adsorbed 1-butene were unchanged even after the reaction.
The isotope exchange of OD groups occurred at a higher temperature than that of the DBM. Thus, the
existence of a new type of reaction on BAS is demonstrated, which takes place more easily than the proton
transfer.
Solid-catalyzed reactions are utilized in syntheses of many
compounds and are now indispensable for industrial chemistry.
Zeolites, which are a family of aluminosilicates with charac-
teristic porous structures, are one of the most attractive materials
for catalysts. The acidity of zeolites is regarded as the origin
of various acid-catalyzed reactions on zeolites and classified
into Brønsted and Lewis acid sites (BAS and LAS). BAS exist
as bridging OH groups to Al and Si atoms, and LAS are
1
generated by elimination of BAS from zeolites. The skeletal
2
and the open structures of H-ZSM-5, one of the typical zeolites,
are depicted in Figure 1 with model structures of the Brønsted
acidic OH and OD groups and their IR spectra.
The double-bond migration (DBM) of olefins are widely
known to proceed catalyzed by Brønsted acids.
+
+
CHRdCHCH R′ + H a CH R-CHCH R′ a
2
2
2
+
CH R-CHdCHR′ + H
2
The protonated species, namely carbenium ions, are regarded
3
,4
as important intermediates for many acid-catalyzed rections.
The importance of the protonated intermediates and the mech-
5
,6
anism is also emphasized for catalysis of solid acids.
The reactivity of acidic OH groups of zeolites and the reaction
mechanisms have been studied by indirect methods for a long
time, and reaction mechanisms similar to those established in
Figure 1. Skeletal diagram (A) and the open apertures (B) of ZSM-5
and IR spectra of H- and D-ZSM-5 (C). (A) In the skeletal structure,
each straight line consists of either Si or Al atoms at the both ends and
an O atom or an OH group in the middle. Brønsted acidity appears
when an OH group is bonded to Al and Si atoms. (B) The small and
large circles represent O atoms and either Si or Al atoms. (C) The IR
bands at 3618 and 2671 cm are attributed to the Brønsted acidic OH
and OD groups oscillating free from any interactions. The other bands
observed at high frequencies are due to the neutral Si-OH and Si-
OD groups.
7
homogeneous chemistry have been accepted. Recently, direct
observation of the BAS and their interaction with adsorbed
8
-10
molecules have become possible by IR and NMR methods.
On the other hand, the assessment of the proposed models and
activated complexes (the intermediates) of the reactions greatly
relies on the quantum chemical calculations that also have been
-
1
9,11
recently developed.
A thin disk of a H-ZSM-5 zeolite (Si/Al ) 50) was placed
in an IR cell connected to a closed gas-circulation system. The
sample disk was treated under the condition that no LAS were
1
2
†
produced. Deuteration of H-ZSM-5 to form D-ZSM-5 was
conducted by exposing the H-ZSM-5 disk to D2 at 573 K. A
small amount of butenes (0.2 Torr) was introduced below 150
K and was immediately evacuated. About 80% of BAS are
Phone, +81-45-924-5238; Fax, +81-45-924-5176; e-mail, kdomen@
res.titech.ac.jp.
‡
Phone, +81-3-3364-2311; Fax, +81-3-3364-7104; e-mail, f-waka@
kahaku.go.jp.
X
Abstract published in AdVance ACS Abstracts, June 15, 1997.
S1089-5647(97)00750-5 CCC: $14.00 © 1997 American Chemical Society

5478 J. Phys. Chem. B, Vol. 101, No. 28, 1997
Letters
Figure 3. Temperature dependence of the relative amount of the
adsorbed species during thermal reaction of 1-butene on BAS of
D-ZSM-5: b, 1-butene; O, 2-butene; 9, OD groups, and 0, OH groups.
The amount of each species was calculated by integrated intensity of
-1
the characteristic bands as follows: the band at 1627 cm for 1-butene,
-
1
total intensity of the bands at 1644 and 1658 cm for 2-butene, the
-
1
broad band from 2600 to 2100 cm for OD groups, and the broad
-
1
band from 3550 to 3100 cm for OH groups. For 2-butene, intensity
ratio of the two bands obtained from adsorption of each 2-butene was
used for calibration.
increased in intensity: an olefinic CH stretching band at 3021
-1
Figure 2. IR spectra of 1-butene adsorbed on D-ZSM-5 at 204 K (a)
and its thermal change at 212 K (b) and 230 K (c) and reference spectra
of trans-2-butene on D-ZSM-5 (d) and cis-2-butene on D-ZSM-5 (e).
The simultaneous increase of the reverse band of free OD groups and
the broad band of hydrogen-bonded OD groups from 204 to 212 K
results from the increase of the amount of adsorbed 1-butene. This is
because when the temperature was gradually increased, initially trapped
cm , two bands in the CdC stretching region at 1658 and 1644
-1
cm , and those in the skeletal vibration region. The great
decrease of the bands of 1-butene indicates the disappearance
of most of the 1-butene, and appearance of the alternative bands
and their increase means the generation of new adsorbed species
from 1-butene. The reference spectra of trans- and cis-2-butenes
adsorbed on D-ZSM-5 are shown in Figure 2d,e, and thus the
produced species (Figure 2c) are reasonably assigned to the
mixture of cis- and trans-2-butenes. It must be noted that the
1
-butene inside the cell at low temperatures desorbed, and a part of
the desorbed 1-butene readsorbed on the D-ZSM-5 disk even under
evacuation.
-
1
hydrogen-bonded OD stretching at 2302 cm observed at 204
occupied by adsorbate, and the intermolecular interaction was
-1
K shifted to 2258 cm at 230 K accompanied by the reaction
avoided. The IR cell was then gradually warmed (ca. 6
of 1-butene, which confirms the production of 2-butene. Great
attention must be paid to the fact that the hydrogen-bonded OD
groups did not convert to OH ones below 230 K even after the
conversion of the adsorbed 1-butene to 2-butene.
-1
K‚min ) while evacuating. If the reactant remains in gas phase
at higher temperatures, protonation and reaction of the proto-
nated species with molecules in gas phase occur simultaneously,
and interpretation suffers from the complexity of the observation.
On the other hand, this method enables extraction of the initial
interaction of BAS and adsorbed olefins followed by stepwise
thermal reactions.
From Figure 2 it was found that the thermal reaction of
1
-butene formed both cis- and trans-2-butenes by the DBM on
D-ZSM-5. From the facts that the hydrogen-bonding OD groups
-1
to 1-butene (2302 cm ) transformed to those to 2-butene (2358
-
1
1-Butene adsorbed on D-ZSM-5 gives an IR spectrum as
cm ) and that the reaction proceeds on D-ZSM-5 where no
LAS exist, it is concluded that adsorbed 1-butene was converted
to either cis- or trans-2-butene on Brønsted acidic OD groups
below 230 K in the absence of proton transfer from D-ZSM-5.
shown in Figure 2a. Since the spectrum of D-ZSM-5 measured
in the absence of adsorbates was subtracted, the decreased band
of free OD groups is shown as a reverse peak. (The reverse
-
1
band at 3618 cm is attributed to the OH groups of BAS that
remained even after the D2 treatment.) Alternatively, a broad
band due to the hydrogen-bonded OD groups with adsorbed
CH3CH CHCH3
D
CH2 CHCHCH3
D
νOD = 2302 cm–1
νOD = 2258 cm–1
-
1
1
1
3
-butene appeared at 2302 cm . The IR spectrum of adsorbed
O
O
Al
Si
Al
Si
-butene is characterized by bands of olefinic CH stretching at
078 cm , CdC stretching at 1627 cm , and skeletal
vibrations between 1500 and 1300 cm . The other CdC
stretching at 1642 cm is due to weakly adsorbed species
observed only at a very low temperature range, which is not
discussed here.
-
1
-1
The absence of the protonated intermediate was evidenced by
a fact that the hydrogen-bonded OD band remained even after
the DBM. The result of quantitative analysis by using the
integrated intensity of characteristic bands of each species is
shown in Figure 3. Almost complete decrease of 1-butene and
concurrent production of 2-butene are clearly shown below 230
K where the amount of OD groups stays constant.
The isotope exchange reaction of OD groups of D-ZSM-5
was observed following the DBM above 230 K after 1-butene
was almost completely altered to 2-butene. The band due to
-
1
-
1
At 212 K, the IR spectrum started to change from that
measured at 204 K, and further increase in temperature to 230
K caused complete change in IR spectra, where all the
characteristic bands of the adsorbed 1-butene (Figure 1a) almost
disappeared. On the other hand, new bands appeared and

Letters
J. Phys. Chem. B, Vol. 101, No. 28, 1997 5479
OD groups hydrogen bonded to 2-butene commenced to
decrease above 230 K accompanied by increase of the broad
band due to hydrogen-bonded OH groups. The isotope ex-
change reaction of BAS is generally conceived as mediated by
therefore, the beginning of dimerization upon proton trans-
fer evidences the occurrence of migration of the adsorbed
species.
The DBM of 1-butene was not observed on weak BAS (Y
type zeolite) nor on a Na-exchanged ZSM-5 but on zeolites with
strong acidity (ZSM-5 and mordenite). The detailed mechanism
of the reaction is continuously examined. The possibilty of the
intramolecular proton transfer from the third carbon to the first
one with such a small activation barrier is now assessed by
density functional theoretical calculation.
5
either the alkoxyl or the carbenium species.
CH3CHD–CHCH3
O
CH3CH CHCH3
Al
Si
CH3CD CHCH3
D
O
alkoxyl species
In summary, a novel reaction pathway on solid-acid catalysts
was demonstrated: the DBM of 1-butene was catalyzed by BAS
of zeolites without protonated intermediates below 230 K.
D
O
+
Al
Si
CH CHD–CHCH3
3
Al
Si
–O
Al
Si
References and Notes
carbenium species
(1) Kokotailo, G. T.; Chu, P.; Lawton, S. L.; Meier, W. M. Nature
1
978, 275, 119.
(2) Meier, W. M.; Olson, D. H.; Baerlocher, C. Zeolites 1996, 17, 1.
3) Olah, G. A.; Prakash, G. K. S.; Sommer, J. Superacids; John Wiley
(
Simultaneously, bands due to 2-butenes decreased in intensity
and the spectral feature changed, indicating that the reaction of
&
Sons: New York, 1985.
4) Olah, G. A.; Prakash, G. K. S.; Williams, R. E.; Field, J. D.; Wade,
D. Hydrocarbon Chemistry; John Wiley & Sons: New York 1987.
(
2
-butene starts above 230 K (Figure 3) accompanied by the
isotope exchange reaction of BAS. It is again shown in Figure
that the DBM of 1-butene and the isotope exchange reaction
(
5) Corma, A. Chem. ReV. 1995, 95, 559.
(
6) Blaszkowski, S. R.; Jansen, A. P.; Nascimento, M. A. C.; van
3
Santen, R. A. J. Phys. Chem. 1994, 98, 12938.
of BAS are clearly distinguished by the reaction temperature.
The latter reaction of 2-butene has been confirmed to be the
dimerization of adsorbed 2-butene molecules by our studies
conducted at higher temperatures (∼300 K). From the pore
structure (Figure 1) and the Si/Al ratio (50) of the ZSM-5, each
BAS is regarded as being separated by the structure of ZSM-5,
(7) Zamaraev; K. I.; Thomas, J. M. AdV. Catal. 1996, 41, 335.
(
8) Thomas, J. M.; Klinowski, J. AdV. Catal. 1985, 33, 199.
(
9) Sauer, J.; Ugliengo, P.; Garrone, E.; Saunders, V. R. Chem. ReV.
1994, 94, 2095.
(
(
(
10) Farneth, W. E.; Gorte, R. J. Chem. ReV. 1995, 95, 615.
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