D Exchange of Ethanol Catalyzed by Acidic OH

Oct 1, 2014 - Zeolites are crystalline microporous silicates and are well- known as a ... dehydration on acidic OH groups of zeolites.5,6 Alcohols int...
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Intramolecular H/D Exchange of Ethanol Catalyzed by Acidic OH Groups on H‑ZSM‑5 Zeolite Hiroshi Yamazaki, Toshiyuki Yokoi, Takashi Tatsumi, and Junko N. Kondo* Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan S Supporting Information *

ABSTRACT: IR observation of ethanol adsorption clarified the presence of the apparent intramolecular isotope exchange from CD3CH2OH to CHD2CH2OD on acidic OH groups of H-ZSM-5 zeolite. This reaction did not proceed with CD3OH nor CH3CD2OH, implying that the β-hydrogen of alcohol had interaction with the lattice oxygen adjacent to Al and that the reaction was mediated by isotope exchange of CD3 groups of ethanol and OH groups on zeolite.

SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis

Z

Scheme 1. Schematic Adsorption Structures of CD3CH2OH and CH3CD2OH Ethanol on an Acidic OH Group of HZSM-5 Zeolite

eolites are crystalline microporous silicates and are wellknown as a representative family of solid acid catalysts with the substitution of some Si sites by Al. Active sites of zeolite catalysts originate from Brønsted acidic OH groups bridging to Si and Al, and zeolites are of practical importance especially in the field of petrochemistry.1 Acidic OH groups on zeolites are directly observable by infrared (IR) spectroscopy2 and nuclear magnetic resonance (NMR).3,4 So far, various types of interactions with hydrocarbons, oxygenate compounds, and so on have been investigated as well as their behavior alone. Among a series of IR studies, we have examined adsorption of alcohol and formation of alkoxy species accompanied by dehydration on acidic OH groups of zeolites.5,6 Alcohols interact with zeolite OH groups by forming hydrogen bonding, as illustrated in Scheme 1. In the case of ethanol, it first adsorbs molecularly (Scheme 1), and dehydration in vacuum at temperatures approximately above 453 K generates an ethoxy group.5 Ethoxy species finally decompose to ethene, leaving one of the hydrogen atoms to regenerate surface hydroxyl groups.7 As a whole, the dehydration of ethanol to ethene is catalyzed by acidic OH groups. During our effort for the clarification of the detailed mechanism of dehydration of ethoxy species to ethene using isotope-labeled ethanol, the occurrence of an intramolecular H/D isotope exchange reaction from CD3CH2OH to CHD2CH2OD on acidic OH groups on zeolites was found. Ethanol molecules are adsorbed on acidic OH groups on zeolites by forming a hydrogen bond at two adjacent sites, as illustrated in Scheme 1 (CD3CH2OH and CH3CD2OH),5,7 where OH groups of ethanol and zeolite are regarded as equivalent and exchangeable at room temperature.8 Adsorption of both CD3CH2OH and CH3CD2OH will be accompanied by dehydration (H2O) at temperatures higher than 453 K in © XXXX American Chemical Society

evacuation to form CD3CH2O and CH3CD2O ethoxy species, respectively (Scheme 1). The time course of spectral change of CD3CH2OH adsorbed on H-ZSM-5 (JRC-Z5-90H, Catalysis Society of Japan, Si/Al = 45) zeolite at 323 K is shown in Figure 1. These spectra were measured under evacuation after CD3CH2OH was supplied. The adsorption of ethanol is irreversible at this temperature, and readsorption does not occur due to evacuation. Because a background spectrum measured before ethanol adsorption is subtracted, a negative band appeared at around 3600 cm−1 due to the decrease of Received: September 3, 2014 Accepted: October 1, 2014

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Figure 2. Time course of IR spectra of CH3CD2OH ethanol adsorption at 323 K in evacuation from 5 to 60 min. The top spectrum was obtained by subtraction of the spectrum measured at 5 min from that at 60 min.

Figure 1. Time course of IR spectra of CD3CH2OH ethanol adsorption at 323 K in evacuation from 5 to 60 min. The top spectrum was obtained by subtraction of the spectrum measured at 5 min from that at 60 min.

above conclusion (S-Figure 1, Supporting Information). Further confirmation was made upon adsorption of propanol isotopes. Only CH3CD2CH2OH led to intramolecular isotope exchange, while nothing happened on the adsorption of the others (CD3CH2CH2OH and CH3CH2CD2OH), similar to cases of CH3CD2OH and CD3OH. (S-Figure 2, Supporting Information) Acidic OH groups bridging to Si and Al are essential for this reaction because it did not proceed over silicalite-1 nor a TS-1 titanosilicate, which have the same zeolite topology. Therefore, the intramolecular isotope exchange reaction can be mediated by the acidic OH groups of H-ZSM-5 zeolite. Therefore, this reaction should be apparent intramolecular isotope exchange. It should be reminded that hydroxyl groups of alcohols and zeolites are exchangeable in structures of alcohol adsorption (Scheme 1). Thus, OH groups on H-ZSM-5 should be also exchanged to OD in an equivalent amount to that of ethanol (Scheme 2). For the confirmation, H-ZSM-5 and the isotope-

isolated acidic OH groups of H-ZSM-5. Alternatively, an extremely broad band ranging from 3200 to 1400 cm−1 and below was observed, which was attributed to the hydrogenbonded OH groups of the zeolite.5,7,9 The other OH band at 3600−3400 cm−1 was assigned to OH groups of adsorbed ethanol. Other bands attributed to CD3 and CH2 groups of adsorbed CD3CH2OH molecules were also observed at 2400− 2200 and 3000−2800 cm−1, respectively. The occupancy of acidic OH groups of H-ZSM-5 was estimated as about 90% by the decrease of the isolated acidic OH band in integrated intensity. At the beginning, the absorption region of the OD stretching band (2800−2400 cm−1) was silent, but a new band appeared and increased in intensity in the time course (dotted line in Figure 1). The top spectrum was obtained by subtraction of a spectrum measured at 5 min from that at 60 min after CD3CH2OH adsorption, which emphasizes the spectral change from 5 to 60 min. The increase of CH and OD stretching bands in intensity is recognized as well as the decrease of CD and OH stretching bands, indicating the conversion of CD3 and OH groups to CHD2 and OD groups. The frequency of the generated OD band is a match of the isotope-shifted OH band at 3600−3400 cm−1 of ethanol molecules. Therefore, intramolecular isotope exchange from CD3CH2OH to CHD2CH2OD was found by IR observation. As a comparative assessment of the intramolecular isotope exchange of ethanol, adsorption of CH3CD2OH was conducted. CH3CD2OH was adsorbed at 323 K followed by evacuation at the same temperature. Similarly to spectra in Figure 1, background-subtracted spectra are arrayed in the time course in Figure 2. A broad band due to the hydrogen-bonded OH groups of H-ZSM-5 was observed at 3200−1400 cm−1 and below, in addition to CH3, CD2, and OH stretching and CH3 deformation bands of CH3CD2OH adsorbed on OH groups of H-ZSM-5. These bands resulted from the adsorption structure in Scheme 1 (bottom scheme). No spectral changes were evident during the time course of 60 min, as clearly confirmed by the subtracted spectrum of that at 5 min from that at 60 min (top spectrum in Figure 2). Accordingly, only β-hydrogen was found to lead to the intramolecular isotope exchange on OH groups of the adsorbed ethanol. The absence of H/D isotope exchange during adsorption of CD3OH on H-ZSM-5 and successive methoxy (CD3O) formation7 on zeolites support the

Scheme 2. Equilibrium of Adsorbed Alcohol (R−OD) on the Acidic OH Group of Zeolitea

a

R−OD on OH and R−OH on OD.

exchanged D-ZSM-5 were exposed to CH3CH2OH under the same conditions (323 K), and the time courses of the integrated intensity between 3600 and 3400 cm−1, which corresponds to the hydrogen-bonded OH stretching band of ethanol, were compared (S-Figure 3, Supporting Information). After 20 min, when adsorption of ethanol reached equilibrium, the integrated intensity of the OH band of ethanol on D-ZSM5 was about half of that on H-ZSM-5. Therefore, the exchange reaction of hydroxyl groups of the zeolite and those of alcohol (Scheme 2) was rapid at 323 K. A careful look at the top spectrum in Figure 1 notifies the decrease of the broad band at 3529

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Figure 3. Time course of the integrated intensity of the OD band of ethanol on H-ZSM-5 (A) and an Arrhenius plot for the apparent intramolecular isotope exchange reaction (B).

around 1600 cm−1 in intensity, which is attributed to the hydrogen-bonded OH groups of H-ZSM-5. The resultant hydrogen-bonded OD band is expected to appear at 2300− 1000 cm−1 with stronger intensity at the lower-frequency region than the isolated ones. Such a band overlaps with IR absorption of zeolite (SiO2) and is not evident, although the slight upward band (at around 1400 cm−1) in the subtracted spectrum in Figure 1 may reflect the presence of an OD band of D-ZSM-5. For quantitative analysis, the integrated intensities of the OD stretching band at 2720−2520 cm−1, which is attributed to the OD stretching band of adsorbed ethanol (dotted lines in Figures 1 and 2), are plotted in Figure 3A, where the difference of CD3CH2OH and CH3CD2OH in reactivity is apparent. Additionally, adsorption of CH3CH2OD was observed under the same condition. In this case, rapid isotope exchange between OH on the zeolite and OD of ethanol reached instantaneous equilibrium (Scheme 2), and thus, the amount of OD groups in ethanol on H-ZSM-5 is regarded as about half of that on D-ZSM-5 (the reverse reaction of that in S-Figure 3, Supporting Information). The integrated intensity of the OD band of adsorbed CH3CH2OD decreased in time course in the reverse manner as that of CD3CH2OH; a similar intramolecular interaction of CH3CH2OD and CH2DCH2OH occurred. The activation energy for the apparent intramolecular isotope exchange reaction was estimated at 313−343 K as 72 ± 4 kJ· mol−1 (Figure 3B). The reaction proceeds under evacuation at temperatures below 350 K, where desorption of ethanol and the formation of ethoxy groups do not occur.7 It is also reminded that the activation energy for the apparent intramolecular isotope exchange is much lower than that of ethene production from ethoxy groups (181 ± 2 kJ·mol−1 on H-ZSM5).5 While a detailed reaction mechanism is not yet clear, the reaction proceeds most probably via the concerted mechanism, as depicted in Scheme 3. One of the β-D atoms has the interaction with a lattice oxygen next to Al, accompanied by a

simultaneous interaction of β-carbon with the acidic H. Then, isotope exchange of CD3 and OH of the acid site first occurs to CHD2 and OD, followed by the equilibrium of the adsorption of CHD2CH2OH on OD and CHD2CH2OD on OH (two structures in Scheme 3). The adsorbed ethanol molecule involved in the first and second steps should not be necessarily the same because of the free migration of adsorbed molecules. Thus, the isotope exchange of silanol groups from OH to OD in Figure 1 can be explained by the presence of the second step reaction over silanol groups similarly to that over zeolite (Scheme 2). It should be mentioned that the isotope exchange of CD3CH2OH to CHD2CH2OD via intramolecular dehydration and rehydration of ethene is one of the candidate mechanisms. CD3CH 2OH → CD2 CH 2 + HDO → CHD2 CH 2OD

However, experimental facts exclude the possibility. First, ethanol adsorbs on zeolites only molecularly at temperatures below 373 K.5,9 Further heating at above 453 K results in dehydration between ethanol molecules and acidic OH groups to form surface alkoxy groups.5,10 Then, ethene is evolved at slightly higher temperatures than dehydration.5 Therefore, the first step of the above reaction scheme (intramolecular dehydration) does not occur under the present experimental conditions. Furthermore, ethene adsorption at 323 K results in oligomerization11 even in the copresence of water (not shown for simplicity). Accordingly, the intramolecular dehydration and rehydration mechanism for the present isotope exchange reaction from CD3CH2OH to CHD2CH2OD on H-ZSM-5 is disqualified. In addition, because such a reaction did not take place over amorphous silica aluminas, the zeolite structure was found to play an important role. The adsorption and the behavior of CD3CH2OH were observed on Al-containing H-form zeolites with various topologies, FER (10), *BEA (12.5), MOR (45), FAU (2.8), and CHA (46), where numbers in paretheses indicate Si/Al ratios. The order of acid strength is regared as follows based on the heats of adsorption of ammmonia measured by temperature-programmed desorption (TPD):12,13

Scheme 3. Proposed Reaction Mechanism for the Apparent Intramolecular Isotope Exchange of Ethanol on H-ZSM-5

CHA > MOR > FER > MFI > *BEA > FAU

Among the zeolites tested, only the *BEA structure allowed the apparent intramolecular isotope exchange reaction from CD3CH2OH to CHD2CH2OD on acidic OH groups in addition to MFI (H-ZSM-5). Thus, the strong acidity cannot be the origin of the reaction. The topology as well as the local structure of zeolites, the location of the acidic OH group, and 3530

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(7) Kondo, J. N.; Nishioka, D.; Kubota, J.; Domen, K. Activation Energies for the Reaction of Ethoxy Species to Ethene over Zeolites. J. Phys. Chem. C 2010, 114, 20107−20113. (8) Haase, F.; Sauer, J. Interaction of Methanol with Brønsted Acid Sites of Zeolite Catalysts: An Ab Initio Study. J. Am. Chem. Soc. 1995, 117, 3780−3789. (9) Pazé, C.; Bordiga, S.; Lamberti, C.; Salvalaggio, M.; Zecchina, A.; Bellussi, G. Acidic Properties of H−β Zeolite As Probed by Bases with Proton Affinity in the 118−204 kcal mol−1 Range: A FTIR Investigation. J. Phys. Chem. B 1997, 101, 4740−4751. (10) Wang, W.; Jiao, J.; Jiang, Y.; Ray, S. S.; Hunger, M. Formation and Decomposition of Surface Ethoxy Species on Acidic Zeolite Y. ChemPhysChem 2005, 6, 1467−1469. (11) Spoto, G.; Bordiga, S.; Ricchiardi, G.; Scarano, D.; Zecchina, A.; Borello, E. IR Study of Ethene and Propene Oligomerization on HZSM-5: Hydrogen-Bonded Precursor Formation, Initiation and Propagation Mechanisms and Structure of the Entrapped Oligomers. J. Chem. Soc., Faraday Trans. 1994, 90, 2827−2835. (12) Suzuki, K.; Noda, T.; Katada, N.; Niwa, M. IRMS-TPD of Ammonia: Direct and Individual Measurement of Brønsted Acidity in Zeolites and Its Relationship with the Catalytic Cracking. J. Catal. 2007, 250, 151−160. (13) Katada, N.; Nouno, K.; Lee, J. K.; Shin, J.; Hong, S. B.; Niwa, M. Acidic Properties of Cage-Based, Small-Pore Zeolites with Different Framework Topologies and Their Silicoaluminophosphate Analogues. J. Phys. Chem. C 2011, 115, 22505−22513. (14) Zones, S. I. Nitrogen Containing Cation Derived from 1Adamantane, 3-Quinuclidinol or 2-Exo-Aminnorbornane As Template. U.S. Patent 4,544,538, Oct. 1, 1985. (15) Watanabe, R.; Yokoi, T.; Tatsumi, T. Synthesis and Application of Colloidal Nanocrystals of the MFI-Type Zeolites. J. Colloid Interface Sci. 2011, 356, 434−441.

the acid density could be the driving forces for the apparent isotope exchange reaction. Many issues should be clarified and disscussed, and details are still under investigation.

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EXPERIMENTAL METHODS H-ZSM-5 (JRC-Z5-90H, Si/Al = 45), Beta (JRC-Z-HB25, Si/ Al = 12.5), mordenite (JRC-Z-HM90, Si/Al = 45), HY (JRC-ZHY5.6(2), Si/Al = 2.8), and TS-1 (ARC-TS1CL, Si/Ti = 35) were provided by the Catalysis Society of Japan. Ferrierite (CP914, Si/Al = 10) purchased from Zeolyst International. H-SSZ13 and Silicalite-1 were synthesized according to the literature.14,15 Ethanol (CH3CH2OH, 99.8%, Wako Pure Chemical Industries, Ltd.), CD3CH2OH (99%, Sigma-Aldrich Co. LLC.), CH3CD2OH (98%, Sigma-Aldrich Co. LLC.), and CH3CH2OD (99.5%, Sigma-Aldrich Co. LLC.) were used. The self-supporting disk of the zeolites was place in a quartz cell, attached to a conventional closed-gas circulation system. The disk was heated up to 773 K under evacuation and was maintained at its temperature for 1 h. FT-IR spectra were recorded using a Jasco 4100 FT/IR spectrometer equipped with a mercury cadmium telluride (MCT) detector at a resolution of 4 cm−1 and a typical average of 64 scans. FT-IR spectra of the pretreated disk recorded at several temperatures were used as background spectra. Background-subtracted IR spectra showing adsorbed species are presented throughout this work. The amount of ethanol-OD can be estimated from the integrated intensity of the band of the OD stretching mode, which determines the rate (k) of formation of ethanol-OD.

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ASSOCIATED CONTENT

S Supporting Information *

Figures of the time course of IR spectra of CD3OH adsorption at 323 K, that of CH3CD2CH2OH, CD3CH2CH2OH, and CH3CH2CD2OH adsorption at 323 K, and that of the integrated intensity of the OH band of adsorbed CH3CH2OH on H- and D-ZSM-5 at 323 K. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Vermeiren, W.; Gilson, J.-P. Impact of Zeolites on the Petroleum and Petrochemical Industry. Top. Catal. 2009, 52, 1131−1161. (2) Busca, G. Acid Catalysts in Industrial Hydrocarbon Chemistry. Chem. Rev. 2007, 107, 5366−5410. (3) Wang, W.; Buchholz, A.; Seiler, M.; Hunger, M. Evidence for an Initiation of the Methanol-to-Olefin Process by Reactive Surface Methoxy Groups on Acidic Zeolite Catalysts. J. Am. Chem. Soc. 2003, 125, 15260−15267. (4) Hio, H.; Peng, L.; Gan, Z.; Grey, C. P. Solid-State MAS NMR Studies of Brønsted Acid Sites in Zeolite H-Mordenite. J. Am. Chem. Soc. 2012, 134, 9708−9720. (5) Kondo, J. N.; Ito, K.; Yoda, E.; Wakabayashi, F.; Domen, K. An Ethoxy Intermediate in Ethanol Dehydration on Brønsted Acid Sites in Zeolite. J. Phys. Chem. B 2005, 109, 10969−10972. (6) Yamazaki, H.; Shima, H.; Imai, H.; Yokoi, T.; Tatsumi, T.; Kondo, J. N. Evidence for a “Carbene-Like” Intermediate during the Reaction of Methoxy Species with Light Alkenes on H-ZSM-5. Angew. Chem., Int. Ed. 2011, 50, 1853−1856. 3531

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