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 ReceiVed: February 28, 1997; In Final Form: April 24, 1997X
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 characteristic 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 generated by elimination of BAS from zeolites. The skeletal1 and the open structures2 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. +
CHRdCHCH2R′ + H+ a CH2R-CHCH2R′ a CH2R-CHdCHR′ + H+ The protonated species, namely carbenium ions, are regarded as important intermediates for many acid-catalyzed rections.3,4 The importance of the protonated intermediates and the mechanism is also emphasized for catalysis of solid acids.5,6 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 homogeneous chemistry have been accepted.7 Recently, direct observation of the BAS and their interaction with adsorbed molecules have become possible by IR and NMR methods.8-10 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 recently developed.9,11 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 †
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
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-1 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 SiOD groups.
sample disk was treated under the condition that no LAS were produced.12 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 © 1997 American Chemical Society
5478 J. Phys. Chem. B, Vol. 101, No. 28, 1997
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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 the characteristic bands as follows: the band at 1627 cm-1 for 1-butene, total intensity of the bands at 1644 and 1658 cm-1 for 2-butene, the broad band from 2600 to 2100 cm-1 for OD groups, and the broad band from 3550 to 3100 cm-1 for OH groups. For 2-butene, intensity ratio of the two bands obtained from adsorption of each 2-butene was used for calibration.
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 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.
occupied by adsorbate, and the intermolecular interaction was avoided. The IR cell was then gradually warmed (ca. 6 K‚min-1) while evacuating. If the reactant remains in gas phase at higher temperatures, protonation and reaction of the protonated 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. 1-Butene adsorbed on D-ZSM-5 gives an IR spectrum as 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 band at 3618 cm-1 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 1-butene appeared at 2302 cm-1. The IR spectrum of adsorbed 1-butene is characterized by bands of olefinic CH stretching at 3078 cm-1, CdC stretching at 1627 cm-1, and skeletal vibrations between 1500 and 1300 cm-1. The other CdC stretching at 1642 cm-1 is due to weakly adsorbed species observed only at a very low temperature range, which is not discussed here. 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
increased in intensity: an olefinic CH stretching band at 3021 cm-1, two bands in the CdC stretching region at 1658 and 1644 cm-1, 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 hydrogen-bonded OD stretching at 2302 cm-1 observed at 204 K shifted to 2258 cm-1 at 230 K accompanied by the reaction 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. 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 to 1-butene (2302 cm-1) transformed to those to 2-butene (2358 cm-1) 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. CH2 CHCHCH3 D Al
O
CH3CH CHCH3
νOD = 2302 cm–1 Si
Al
D O
νOD = 2258 cm–1 Si
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
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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 exchange reaction of BAS is generally conceived as mediated by either the alkoxyl or the carbenium species.5
CH3CHD–CHCH3 CH3CH CHCH3
Al
D O
Al
O
Si
CH3CD CHCH3
alkoxyl species Si
+
CH3CHD–CHCH3 –O Al Si
Al
D O
Si
carbenium species
Simultaneously, bands due to 2-butenes decreased in intensity and the spectral feature changed, indicating that the reaction of 2-butene starts above 230 K (Figure 3) accompanied by the isotope exchange reaction of BAS. It is again shown in Figure 3 that the DBM of 1-butene and the isotope exchange reaction 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,
therefore, the beginning of dimerization upon proton transfer 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. 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. References and Notes (1) Kokotailo, G. T.; Chu, P.; Lawton, S. L.; Meier, W. M. Nature 1978, 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 & 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. (5) Corma, A. Chem. ReV. 1995, 95, 559. (6) Blaszkowski, S. R.; Jansen, A. P.; Nascimento, M. A. C.; van Santen, R. A. J. Phys. Chem. 1994, 98, 12938. (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. (11) van Santen, R. A.; Kramer, G. J. Chem. ReV. 1995, 95, 637. (12) Wakabayashi,F.; Kondo, J. N.; Domen, K.; Hirose, C. J. Phys. Chem. 1995, 99, 10573.
