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J. Phys. Chem. B 1999, 103, 8538-8543
Structure of Dimerized Alkoxy Species of 2-Methylpropene on Zeolites and Silica-Alumina Studied by FT-IR Junko N. Kondo,† Hiroyuki Ishikawa,† Eisuke Yoda,† Fumitaka Wakabayashi,‡ and Kazunari Domen*,† Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan, and Department of Science and Engineering, National Science Museum, 3-23-1 Hyakunin-cho, Shinjuku-ku, Tokyo 169-0073, Japan ReceiVed: April 28, 1999; In Final Form: July 15, 1999
Adsorption and reaction of 2-methylpropene (isobutene) on zeolites (ZSM-5 and faujasite (Y zeolite)), a silica-alumina, and a silica were investigated below room temperature by infrared (IR) spectroscopy. On silica, isobutene molecules that adsorbed on silanol groups simply desorbed below 230 K in vacuum, while isobutene π-bonded to the acidic OH groups (π-complex) on acid catalysts reacted to dimer alkoxy (2,4,4trimethylpentoxy) groups. The structure of the reaction product, dimer alkoxy groups, was found to be restricted by the pore size of zeolites; 2,4,4-trimethyl-2-pentoxy species were identified on silica-alumina and Y zeolite similarly to those on mordenite, while the reaction of isobutene resulted in 2,4,4-trimethyl-1-pentoxy species on the acidic OH groups in the pores of ZSM-5. Therefore, the space restriction of the pore size of zeolites on the reaction product was directly observed by the low-temperature IR study.
1. Introduction The OH groups bridging Si and Al atoms on zeolites are wellknown to show Brønsted acidity, and zeolites are utilized as solid acid catalysts in many industrial processes because of their specific pore sizes.1 Being different from the solvated protons, which freely move in acidic solutions, protons on solid acids are stabilized at the gas-solid interface. Therefore, the interaction of the acidic OH groups on zeolites with reactant molecules is expected to be dissimilar to that of protons in solutions. We have observed the interaction of acidic OH groups on zeolites with olefin molecules by infrared (IR) spectroscopy below room temperature. In a series of our studies, several adsorption structures of olefin molecules on zeolites were identified:2 adsorption on the wall of the zeolite, adsorption on the acidic OH groups with alkyl groups of olefins, and the most stable hydrogen-bonding between acidic OH groups and π-electrons of CdC bond of olefins. The π-hydrogen-bonded structure is known to be the most stable below the temperature where protonation from OH groups to olefin molecules occurs.2-5 When protonation from acidic OH groups to ethene, propene,6,7 or normal (n-) butene4 molecules occurred, the observed species could not be assigned to the monomeric species but were identified as oligomeric5-7 or dimeric4,5 groups, although monomeric alkoxy groups were suggested to be stable by quantum chemical calculations.8-10 This was suspected to be due to the insufficient stability of the structure of formed alkoxy groups of the studied olefins (ethene, propene, 1-butene, cisand trans-2-butenes); only primary or secondary alkoxy groups are possibly produced.5 Recently, we attempted to observed the more stable tertiary (tert-) alkoxy groups by IR, which could be formed from the * To whom correspondence should be addressed. † Tokyo Institute of Technology. ‡ National Science Museum.
monomeric reaction of 2-methylpropene (isobutene) with the acidic OH groups on mordenite zeolite.11 However, only the already dimerized C8 tert-alkoxy species were observed. The adsorbed isobutene on the acidic OH groups existed below 170 K, and the dimerization was completed below 210 K. This reaction was observed at an isobutene coverage of less than 10% of the acidic OH groups under vacuum. Although the stable products of the reaction of isobutene with the acidic OH groups of zeolites are proposed to be tert-butoxy groups,8,9 the rapid reaction to the dimer species proceeded after protonation from acidic OH groups, even in the absence of any supply of isobutene molecules from the gas phase. This indicates that the migration of the adsorbed isobutene on the acidic OH groups of mordenite was rapid at around 200 K.11 The structure of the C8 reaction product of isobutene on mordenite was identified as 2,2,4-trimethyl-2-pentoxy groups by comparison of the IR spectra of 2,4,4-trimethyl-1- and 2-pentenes on mordenite and SiO2 together with that of 2,4,4-trimethyl-pentoxy groups on ZrO2.11
In this study, we examined the effects of the acid strength of acidic OH groups and the pore size of zeolites on the reaction
10.1021/jp991395f CCC: $18.00 © 1999 American Chemical Society Published on Web 09/16/1999
Alkoxy Species on Zeolites and Silica-Alumina
Figure 1. IR spectra of deuterated (a) silica, (b) silica-alumina, (c) ZSM-5, and (d) Y zeolite. Spectra were measured under vacuum at around 200 K after pretreatment.
of isobutene and the structure of the reaction product by comparing those on a faujasite (Y zeolite), a ZSM-5, and a silica-alumina. 2. Experimental Section Y zeolite, JRC-Z-HY5.6 (Si/Al ) 2.8), was provided by the Catalysis Society of Japan. H-ZSM-5 (Si/Al ) 50), silicaalumina (Si/Al ) 1.9), and silica were supplied by Sumitomo Chemical Co. Ltd., Catalysts & Chemicals Ind. Co. Ltd., and Aerosil Nippon, respectively. About 40 mg of each sample was pressed into a self-supporting IR disk (20 mm diameter) and was placed in an IR cell that was attached to a conventional closed-circulation glass system. The catalyst was pretreated by circulating O2 (100 Torr, 1 Torr ) 133.322 Pa) with a liquid nitrogen trap at 773 K for 1.5 h and evacuated (10-3 Torr) at the same temperature for 15 min in order to remove residual contaminants. Then the catalyst was further treated with circulating H2 (100 Torr) at 673 K for 1 h followed by evacuation during the cooling procedure to ca. 180 K. In the case of deuteration, the catalyst disk was treated in a manner similar to that with D2. Deuteration of the sample was performed in order to clearly observe the CH stretching bands of adsorbed olefins as well as to confirm the assignment of the OH bands by the isotopic shift to OD bands. Typical IR spectra of all the deuterated samples measured in vacuum at around 200 K are shown in Figure 1. The extent of H/D isotope exchange reaction of the samples was found to be considerably different even under the same experimental conditions. The bands at ca. 3750 cm-1 (2765 cm-1), which are common to all the catalysts, are assigned to the OH (OD) stretching of silanol groups. The acidic OH (OD) groups bridging the Si and Al atoms appear at ca. 3610 cm-1 (2670 cm-1) on zeolites (Figure 1c,d). In the case of the Y zeolite, an additional OD stretching band at 2622 cm-1 is
J. Phys. Chem. B, Vol. 103, No. 40, 1999 8539 attributed to the bridging OD groups in the sodalite cage and that at 2688 cm-1 to the super cage.12 Although zeolites are well-known to have Brønsted and Lewis acid sites, the pretreatment condition was chosen to prevent production of any Lewis acid sites, and the absence of them was confirmed by using CO molecules as a probe.12,13 Isobutene (Takachiho Trading Co., Ltd., 99.0% purity) was purified by vacuum distillation and freeze-pump-thaw cycles. D2 (Takachiho Trading Co., Ltd., 99.99% purity) was purified before deuteration of the mordenite zeolite by repeatedly passing it through a liquid nitrogen trap. 2,4,4-Trimethyl-1-pentene (97% purity), 2,4,4-trimethyl-2-pentene (99% purity), and 2,4,4trimethyl-1-pentanol (98% purity) were purchased from Aldrich Chemical Co., Inc. and also purified by vacuum distillation and freeze-pump-thaw cycles before use. The thermal change of the IR spectra of the species after adsorption at the lowest temperatures (
