J. Phys. Chem. 1995, 99, 8777-8781
8777
Coadsorption of Methanol and Isobutene on HY Zeolite A. Kogelbauer and J. G. Goodwin, Jr.* Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261
J. A. Lercher Department of Chemical Technology, University of Twente, P. 0. Box 21 7, 7500 AE Enschede, The Netherlands Received: December 5, 1994; In Final Form: March 20, 1995@
In order to develop a better understanding of methyl tert-butyl ether (MTBE) synthesis on zeolites, the coadsorption of methanol and isobutene on HY zeolite was investigated using IR spectroscopy. Initial adsorption of isobutene alone at 35 "C led to rapid oligomerization yielding strongly bound oligomers. The subsequent coadsorption of methanol did not induce any changes in the zeolite-adsorbate complexes. TPD following the coadsorption showed that the Bransted acid sites could be restored by temperature treatment above approximately 300 "C. When methanol was adsorbed first and isobutene was subsequently coadsorbed, MTBE was formed even at 35 "C on the catalyst surface. MTBE desorbed easily at a temperature of 70 "C, restoring a major fraction of the Brasted acid sites. Methanol was concluded to decrease the probability of oligomerization by effectively competing for the acid sites.
Introduction Since the addition of oxygenates to gasoline was mandated by some legislation in the U.S.,' there has been a steadily growing interest in methyl tert-butyl ether (MTBE) as gasoline additive. One of the major thrusts of research in this area has been the search for alternative catalysts, due to restrictions in the applicability of sulfonated ion-exchange resins, the catalyst currently in use.2 Zeolites have been suggested to be suitable alternatives because of their acid functionality, stability at elevated temperatures, ease of regeneration, and high selectivity even at unfavorable reactant feed ratio^.^ However, most reports in the open literature had the nature of screening studies rather than detailed investigations of the impact of the various zeolite properties, such as acidity, acid strength, composition, and morphology. In particular, the surface chemistry on such catalysts was not addressed. In a recent communication we reported on the reactant adsorption on zeolites and its effect on activities and selectivities during MTBE ~ynthesis.~ The adsorption of methanol prior to reaction was shown to dramatically improve the activity of zeolites in MTBE synthesis. It was suggested that the high methanol concentration in the zeolite pores may be responsible for the excellent MTBE selectivity at lower alcohollolefin ratios of zeolites compared to that of the commercially used resin catalysts. The current paper extends this work by applying infrared spectroscopy to investigate the surface chemistry during MTBE formation. The coadsorption of methanol and isobutene at low partial pressures and ambient temperature was chosen as a model for the synthesis of MTBE. As reported earlier,5s6 coadsorption experiments can provide valuable information about potential reaction precursors and intermediates, thus contributing to the understanding of reaction pathways on a molecular level.
as determined by pyridine TPD. All IR spectra were recorded in-situ on a Bruker IFS88 FTIR spectrometer using the transmission-absorption technique. Typically, 150 scans were coadded for one spectrum at a resolution of 4 cm-I, and all spectra were base line corrected in the range between 1200 and 3800 cm-I. The IR cell was equipped with a heatable sample holder enabling in-situ temperature treatment of the catalyst and was connected to a high-vacuum system achieving pressures of mbar. A detailed description of the IR system used is given in ref 7. The zeolite was pressed into a self-supporting wafer and then mbar) at a rate activated by heating under vacuum (p < of 10 "Clmin to 400 "C and maintaining that temperature for 1 h. Typically, the adsorption experiments were carried out at 35 "C; however, for one experiment the temperature was raised to 70 "C. The adsorbates (methanol, isobutene, and MTBE) were introduced by leaking the vapor from a gas manifold through a gas-dosing valve into the vacuum system and applying differential pumping in order to keep the partial pressure constant. Equilibration of the zeolite at the desired partial pressures was monitored by time-resolved IR spectroscopy. The system was then evacuated at 70 "C below low5mbar prior to temperature-programmed desorption (TPD), which was carried out using a heating rate of 10 " C h i n . For the coadsorption experiments the activated zeolite wafer was first equilibrated with mbar of one adsorbate. Subsequently, keeping the partial pressure of the first adsorbate constant, mbar of the second adsorbate was introduced, thus reaching a total pressure of 2 x mbar in the adsorption system. After equilibration, which was monitored by timeresolved IR spectroscopy, vacuum TPD was carried out using a heating rate of 10 "Clmin. In another experiment, the sequence of coadsorption was reversed to check for the existence of local adsorptioddesorption equilibrium.
Experimental Section The zeolite used was an HY zeolite (LZ210-12, UOP) with a SUA1 ratio of 6 and an acid site concentration of 1.67 mmoVg
Results
* To whom correspondence @
should be addressed.
Abstract published in Advance ACS Absrracts, May 1, 1995.
Figure 1 shows the IR spectra of HY zeolite after activation and after adsorption of mbar methanol (MeOH), isobutene (IB), and MTBE, respectively. In the spectrum of activated
0022-365419512099-8777$09.00/0 0 1995 American Chemical Society
Kogelbauer et al.
8778 J. Phys. Chem., Vol. 99, No. 21, 1995
I
' i
I
t
t An (a)
3600
3000
2500
2000
1500
,
t
,
I
3000
2500
2000
1500
1000
Wavenumbers (em-')
Wavenumbers (cm-')
HY (Figure la) bands at 3634 and 3550 cm-' are characteristic for Si-OH-A1 hydroxyl groups located in the supercages and in the sodalite cages, r e s p e ~ t i v e l y .The ~ ~ ~band at 3734 cm-' has been attributed to SiOH groups terminating the zeolite 1attice.l0 After adsorption of lo-* mbar methanol (Figure lb), the band at 3634 cm-' completely disappeared, indicating full coverage of the Si-OH-A1 groups in the supercages with methanol. The band at 3550 cm-' was reduced considerably in intensity, indicating that methanol also partly affects Bransted acid sites located in positions difficult to access. A slight decrease in the intensity of the band at 3734 cm-', which was paralleled by the appearance of a broad band of low intensity around 3300 cm-' , is attributed to methanol hydrogen-bonded to terminal silanol Broad bands at ca. 2900, 2440, and 1687 cm-' have been taken as an indication for protonation of methanol on the Bronsted acid sites of zeolites." However, recently, such bands have also been attributed to pseudobands originating from Fermi resonance between the shifted OH stretching vibration and overtones of the out-of-plane OH deformation vibration, in analogy to strongly hydrogen-bonded complexes in liquids or gases.I4 As will be seen later, the differences in these interpretations, Le., strongly hydrogenbonded methanol with or without proton transfer, do not impact upon our conclusions. The CH stretching vibrations of adsorbed methanol were observed at 2959 and 2853 cm-' and the CH deformation vibrations at 1472 and 1445 cm-'. In the spectrum of HY after isobutene adsorption (Figure IC), the band at 3634 cm-' also disappeared, indicating complete coverage of Bransted acid sites in the supercages by isobutene. The band at 3550 cm-l, however, seemed fairly unaffected by the adsorption process. The small decrease in the intensity of the band at 3734 cm-' is attributed to weak interactions of isobutene with terminal silanol groups in analogy with Figure lb. Bands due to CH stretching vibrations were observed at 2961, 2937, and 2870 cm-', and those due to CH deformation vibrations were observed at 1473 (broad) and 1368 cm-' with a shoulder at 1392 cm-l. The absence of the band at 3634 cm-' after adsorption of lo-* mbar MTBE (Figure Id) indicates complete coverage of these sites with MTBE. As was the case with methanol, a
I
3500
1000
Figure 1. IR spectra of HY at 35 "C (a) after activation and after adsorption of mbar of (b) methanol, (c) isobutene, or (d) MTBE.
l
Figure 2. Difference between the IR spectra of HY (a) before and after adsorption of isobutene on the activated zeolite, (b) before and after coadsorption of isobutene and methanol on the activated zeolite, and (c) after coadsorption of isobutene and methanol following = H mbar, PIE= adsorption of isobutene (Tads= 35 "C, P M ~ O
mbar). significant reduction in the intensity of the band of the Bronsted acid sites located in the sodalite cages (3550 cm-') was observed. The diminished intensity of the band at 3734 cm-', as compared to the activated zeolite, indicates also weak interaction of MTBE with terminal SiOH groups. Broad bands at approximately 2400 and 1700 cm-', very similar to those present after methanol adsorption, were attributed to OH stretching and deformation vibrations, most likely originating from methanol formed by dissociation of MTBE. Altematively, this could also indicate the presence of hydrogen-bonded MTBE for which the perturbed OH band has a Fermi resonance with the out-of-plane bending vibration of the Si-OH-A1 group.I4 The intense band at 2979 cm-' is typical for the CH stretching vibration of tert-butyl groups in the vicinity of oxygen.15 Additional bands due to CH stretching vibrations were less clearly resolved at 2960, 2921, 2870, and 2850 cm-I. Bands at 1469, 1431, 1397, and 1374 cm-' were attributed to CH deformation vibration^.'^ Figure 2 shows IR spectra obtained after adsorption of lo-* mbar isobutene and after coadsorption of isobutene and methanol on HY. The spectra are represented as difference spectra in order to better visualize the changes occurring on the zeolite surface as a consequence of the coadsorption process. In such an illustration, bands pointing downward are characteristic of species whose surface concentration decreases upon adsorption whereas bands pointing upward indicate species that are generated on the catalyst surface upon adsorption. Figure 2b shows the difference in the IR spectrum after coadsorption of isobutene and methanol compared with that of the activated zeolite. Negative bands at 3734, 3634, and 3550 cm-' indicate that all types of hydroxyl groups interact during coadsorption. Bands due to CH vibrations were observed at 2965 and 2870 cm-' (stretching modes) as well as 1472 (broad) and 1368 cm-' with a shoulder at 1392 cm-'. Except for the negative band at 3550 cm-I, the spectrum after coadsorption of isobutene and methanol (Figure 2b) completely resembled the one obtained after adsorption of isobutene only (Figure 2a). Figure 2c shows the changes that appear as a consequence of the coadsorption
Coadsorption of Methanol and Isobutene on HY Zeolite I
I
t
J. Phys. Chem., Vol. 99, No. 21, 1995 8779
- 1
h
3500
3000
2500
2000
1500
1000
3500
after adsorption of methanol on the activated zeolite, (b) before and after coadsorption of methanol and isobutene on the activated zeolite, and (c) after coadsorption of methanol and isobutene following adsorption of methanol (Tads= 35 "C, PM~OH = lo-' mbar, PIB= lo-' mbar).
of methanol onto the zeolite preequilibrated with isobutene. A negative band at 3550 cm-' was paralleled by bands at 2967, 1466, and 1443 cm-' as well as a broad band at approximately 1450 cm-'. Additionally, broad bands of very weak intensity were observed around 2550 cm-' and between 1800 and 1650 cm-'. The results of the reversed coadsorption sequence, adsorption of methanol first followed by coadsorption of methanol and isobutene, are depicted in Figure 3. Figure 3b was obtained by subtraction of the IR spectrum of the activated zeolite from the IR spectrum of the zeolite after equilibration with methanol and isobutene. The negative bands at 3734, 3634, and 3550 cm-I again indicate that all zeolite hydroxyl groups were involved in the interaction with adsorbate molecules. Broad bands due to OH vibrations very similar to the ones of adsorbed methanol were observed in the region between 3500 and 3200 cm-' and at 2440 and 1690 cm-I. Bands resulting from CH stretching vibrations were observed at 2980, 2957, 2920, and 285 1 cm-I. The corresponding deformation vibrations gave bands at 1495,1470,1434 (weak), 1398, and 1375 cm-I. Figure 3c, representing the changes induced by coadsorption of isobutene onto the zeolite preequilibrated with methanol, showed a broad negative band around 3560 cm-' accompanied by a negative band in the region of deformation vibrations at 1470 and 1446 cm-I. Additionally, bands characteristic of MTBE were observed at 2980, 2870, 1850 (broad), 1464, 1430, 1398, and 1375 cm-I. Figure 4 shows the difference between the IR spectrum of the zeolite after coadsorption of methanol and isobutene at 35 "C and the IR spectrum obtained after subsequent heating of the system to 70 "C at constant partial pressures. Bands at 3634 and at 3550 cm-' (more intense) indicate the desorption of adsorbates from both types of Bransted acid sites. The negative bands at 2980, 1398, and 1375 cm-' indicate that it was mainly MTBE which disappeared from the zeolite surface. At the same time, bands at 2960,2940,2905, and 2871 cm-' (CH stretching vibrations) and at 1471, 1449, 1387, and 1364 cm-' (CH
2500
2000
1000
1500
Wavenumbers (cm-')
Wavenumbers (cm-')
Figure 3. Difference between the IR spectra of HY (a) before and
3000
Figure 4. Difference between the IR spectra of HY after coadsorption of methanol and isobutene at 70 and 35 "C (PM~OH = lo-* mbar, PIB = mbar).
\ 3800
. . .
.
3200
.
,
2600
,
.
,
2000
.
1
. I 1400
Wavenumbers (cm")
Figure 5. IR spectra recorded during TPD from HY after adsorption of isobutene and subsequent coadsorption of isobutene and methanol (T& = 35 "c, P M ~ O=H lo-* mbar, PIB= lo-' mbar).
deformation vibrations) gained intensity, indicating oligomerization of isobutene. Figure 5 shows the IR spectra recorded during TPD from the zeolite sample after adsorption of isobutene and subsequent coadsorption of methanol. The IR spectra obtained during TPD after adsorbing methanol first followed by coadsorption of isobutene are depicted in Figure 6. In Figure 5 only the band of the Bransted acid sites located in sodalite cages was observed after evacuation of the zeolite prior to TPD (spectrum at 40 "C). With increasing temperature to 360 "C, however, the intensity of the zeolite band at 3634 cm-' was almost completely restored, and the bands arising from CH vibrations had essentially disappeared from the spectrum. The maximum in the rate of desorption was found at 310 "C by differentiating the integrated areas of the OH band at 3634 cm-' with temperature. As was the case with the zeolite that had been exposed to isobutene first, the intensity of the band at 3634 cm-' was completely restored during TPD at the expense of that of all CH vibrations when methanol was adsorbed first (Figure 6). However, the band at 3634 cm-' was readily observed after evacuation of the zeolite at 70 "C prior to TPD, suggesting a significantly lower stability of the zeolite-adsorbate complex when methanol was adsorbed first.
8780 J. Phys. Chem., Vol. 99, No. 21, 1995
\
3800
.
,
.
,
3200
,
,
2600
,
Kogelbauer et al.
.
1
2000
,
.
, I
1400
Wavenumben (cm")
Figure 6. IR spectra recorded during TPD from HY after adsorption of methanol and subsequent coadsorption of methanol and isobutene (Tads= 35 "C, PM~OH = lo-* mbar, PIB = lo-* mbar).
Discussion From NMR results of methanol adsorption on HZSMS it was concluded that methanol was protonated to a significant amount." On the basis of the similarity of the IR bands for CH and OH vibrations with those found on HZSM5," we conclude that methanol was adsorbed in a similar structure on the strong Bronsted OH groups of HY that give rise to the HF band. Although the Bronsted acid sites in the supercages were completely covered, the sites in the double six-rings were hardly accessible. Because these sites are only accessible through sixmembered rings with an average free diameter of 0.26 nmI6 and the kinetic diameter for methanol is on the order of 0.39 nm, the sites should be in principle not accessible. Two possibilities exist to explain the availability of few of these OH groups for interaction. (i) The zeolite lattice was partially dealuminated, leading to a mesoporous structure, in which the SI site in double six-rings is partly accessible. (ii) Protons may be mobile and hence separated from their original sites'7.'8 by interacting with methanol. The extent of that interaction would depend upon the base strength of the adsorbate, the acid strength of the proton, and the minimum distance between both. For MTBE two types of bands were observed. One type is clearly attributed to MTBE adsorbed on the zeolite and includes the CH stretching vibration at 2979 cm-' characteristic of a tert-butyl group next to an oxygen atomI5 and the doublet of the CH deformation vibration of the same group at 1397 and 1374 cm-I (with the 1374 cm-' band being more intense). The second set of bands resembles those for adsorbed methanol and is dominated by the broad bands resulting from distorted OH vibrations. From steric requirements it is concluded that MTBE was not able to interact with Bronsted acid sites in hardly accessible locations. However, it should completely cover all accessible sites since the corresponding OH band at 3634 cm-I was missing in the IR spectrum. The fact that the band at 3550 cm-' also decreased to some extent in intensity upon MTBE adsorption is tentatively explained by the interaction of these sites with methanol formed as a consequence of MTBE decomposition following adsorption. This is supported by the similarity of the OH bands and the identical band positions in the spectra of adsorbed MTBE and adsorbed methanol. Also, thermodynamics predicts some extent of MTBE disso~iation.'~ Olefins have traditionally been considered to be adsorbed as carbenium ions on zeolites. Isobutene is expected to undergo proton transfer, because the resulting tertiary carbenium ion is relatively stable.20 It has been suggested, however, that the
adsorbate complex is better to be seen as covalently bound alkoxy groups (alkyl silyl ether).21,22On the basis of I3C MASNMR studies at low temperatures, it has been recently suggested that isobutene is hydrogen-bonded to the zeolite and forms tertbutyl carbenium ions upon thermal a ~ t i v a t i o n .However, ~~ all these species follow typical carbocation chemistry, e.g., oligomerize at ambient temperature following an ionic m e ~ h a n i s m . ~ ~ - ~ ~ In our case, hydrogen bonding of zeolite hydroxyls with isobutene was excluded because the intense, strongly shifted band due to distorted hydroxyl groups, that Liengme and Hall2? observed upon adsorption of ethylene on HY, was not observed. The lack of bands from olefinic CH and C=C stretching vibrations in the IR spectrum of adsorbed isobutene suggests that the olefinic character of isobutene has been lost as a consequence of the adsorption process, indicating oligomerization or sorption as isobutoxy group. However, a broad and relatively weak band was observed after isobutene adsorption at 3500 cm-' (see Figure 2a). A similar effect was observed by Datka for the oligomerization of butenes on NaHY28 and was attributed to hydrogen-bonding interactions between the Brginsted acid sites in the supercages and butene oligomers. Similar positions of perturbed OH bands have been observed also after adsorption of paraffins on protonic zeolite^.^^^^^ The bands of CH vibrations observed in the IR spectrum after isobutene adsorption are characteristic for an aliphatic, branched hydrocarbon of longer chain length. Similar observations have been reported for pr~pene:~.~'b ~ t e n e , and ~ ~ ,h ~e ~~ e n epo~~ lymerization on NaHY. The doublet of the symmetric CH deformation vibrations (1392 and 1368 cm-') which indicates the presence of tert-butyl groups was significantly less intense than the band due to asymmetric CH deformation vibrations at 1473 cm-I. This indicates that at least some contribution of deformation vibrations from CH:! groups was present in the band at 1473 cm-'. The fact that the CH stretching region is dominated by CH stretching vibrations stemming from CH3 groups (2961 and 2870 cm-I) on the other hand suggests that relatively short chains were formed (dimers or trimers). This agrees well with the fact that treatment of HY after isobutene adsorption at elevated temperatures is sufficient to desorb oligomerization products (viz. TPD) and to restore catalytic activity for MTBE ~ynthesis.~ The hydroxyl groups characterized by the band at 3550 cm-', which interacted partly with methanol adsorbed from the gas phase or formed by dissociation of MTBE on the zeolite surface, were not able to interact with isobutene since its kinetic diameter of about 0.49 nm is too large to allow access to those sites through the six-membered rings. The coadsorption of methanol onto the zeolite preequilibrated with isobutene leads only to the coverage of (still) vacant sites by methanol. Isobutene, or more precisely its oligomers, was concluded to be unreactive because changes in the spectral features pertaining to the olefin adsorption were not observed (see Figure 2c). This can be considered indirect evidence for the absence of isolated terr-butylcarbenium ions which should be highly reactive toward methanol. It is interesting to note that the CH stretching vibration of adsorbed methanol, found at 2967 cm-' in Figure 2c, has been attributed to the formation of surface methoxy groups.34 Such groups are considered reactive for alkylation reactions.34 Another interesting point lies in the fact that, although isobutene oligomers were present in the zeolite pores, methanol could still reach sites which were inaccessible for isobutene. This indicates that the oligomers do not block the whole zeolite channel system but rather poison individual sites due to strong adsorption. On the contrary, when methanol was preadsorbed, the coadsorption of isobutene led to the formation of MTBE even
J. Phys. Chem., Vol. 99, No. 21, 1995 8781
Coadsorption of Methanol and Isobutene on HY Zeolite at ambient temperature. The fact that oligomerization was not observed in this case suggests that isobutene does not replace methanol from the surface and that adsorbed methanol does not induce oligomerization by acting as a protonating agent. The formation of MTBE suggests that isobutene reacts directly with adsorbed methanol to form MTBE. This is supported by the fact that once these sites are regenerated (Le., through desorption of MTBE at 70 "C) the oligomerization of isobutene can take place immediately. From these results it appears that adsorbed methanol decreases the probability of oligomerization by competing effectively for the acid sites. This explanation agrees well with reaction results obtained after reactant preadsorption where a dramatically increased MTBE synthesis activity was found after methanol pread~orption.~ Combining the results of the current study with those of steady-state reaction results: the following reaction routes for methanol and isobutene on HY zeolite can be proposed (I and 11). Z
IB
Z-IB
Z-
MeOH
-. Z-DIB IB
-no
Z-MeOH
IB Z-MTBE
MeOH
reaction
(I)
Conclusions Coadsorption of methanol and isobutene on HY zeolite revealed that isobutene, once adsorbed, quickly oligomerizes. The products of this oligomerization reaction were determined to be strongly adsorbed branched olefins which are most likely responsible for the deactivation observed during fixed bed reaction. Heating under vacuum or in an inert flow was sufficient to restore the catalyst surface to its original state. Methanol adsorption before isobutene adsorption yielded MTBE even at ambient temperature. Although some details of MTBE formation on zeolites are still not well understood and require further study, the present work demonstrates that methanol effectively inhibits the adsorption and subsequent oligomerization of isobutene on the acid sites of the zeolite.
Acknowledgment. Financial support from the Materials Research Center of the University of Pittsburgh through AFOSR Grant 91-0441 is gratefully acknowledged. References and Notes (1) Peeples, J. E. Fuel Reformulation 1991, 1 ( l ) , 27. (2) Takesono, T.; Fujiwara, Y. U.S. Patent 4 182 913, 1980.
(3) Chu, P.; Ktihl, G. H. Ind. Eng. Chem. Res. 1987, 26, 366. (4) Kogelbauer, A,; Nikolopoulos, A. A,; Goodwin, J. G., Jr.; Marcelin, G.J . Catal. 1995, 152, 122. (5) Mirth, G.;Lercher, J. A. J . Phys. Chem. 1991, 95, 3736. (6) Kogelbauer, A.; Lercher, J. A. J . Chem. Soc., Faraday Trans. 1992, 88, 2283. (7) Kogelbauer, A.; Lercher, J. A.; Steinberg, K. H.; Roessner, F.; Soellner, A.; Dmitriev, R. V. Zeolites 1989, 9, 224. (8) Ward, J. W. J . Catal. 1967, 9, 225. (9) Ward, J. W. J . Phys. Chem. 1967, 7, 3106. (10) Qin, G.; Zheng, L.; Xie, Y.; Wu, C. J . Catal. 1985, 95, 609. (1 1) Mirth, G.; Lercher, J. A,; Anderson, M. W.; Klinowski, J. J . Chem. SOC.,Faraday Trans. 1990, 86, 3039. (12) Borello, E.; Zecchina, A.; Morterra, C. J . Phys. Chem. 1967, 71, 2938. (13) Borello, E.; Zecchina, A.; Morterra, C.; Ghiotti, G. J . Phys. Chem. 1967, 71, 2945. (14) Pelmenschikov, A. G.; van Santen, R. A,; Janchen, J.; Meijer, E. J . Phys. Chem. 1993, 97, 11071. (15) Bellamy, L. J. The Infra-red Spectra of Complex Molecules; John Wiley & Sons: New York, 1964. (16) Breck, D. W. Zeolite Molecular Sieves-Structure, Chemistry and Use; John Wiley & Sons: New York, 1974. (17) Barthomeuf, D. J . Phys. Chem. 1979, 83, 249. (18) Barthomeuf, D. ACS Symp. Ser. 1977, No. 40, 453. (19) Tejero, J.; Cunill, F.; Izquierdo, J. F. Ind. Eng. Chem. Res. 1988, 27, 338. (20) Bernecker, R. R.; Long, F. A. J . Phys. Chem. 1961, 65, 1565. (21) Aronson, M. T.; Gorte, R. J.; Fameth, W. E.; White, D. J . Am. Chem. SOC. 1989, 111, 840. (22) Haw, F. J.; Richardson, B. R.; Oshiro, I. S.; Lazo, N. D.; Speed, J. A. J . Am. Chem. SOC.1989, 111, 2052. (23) Lazo, N. D.; Richardson, B. R.; Schettler, P. D.; White, J. L.; Munson, E. J.; Haw, J. F. J. Phys. Chem. 1991, 95, 9420. (24) Datka, J. J . Chem. Soc., Faraday Trans. I 1980, 76, 2437. (25) Wolthuizen, J. P.; van den Berg, J. P.; van Hooff, J. H. C. Stud. Surf. Sci. Catal. 1980, 5, 85. (26) van den Berg, J. P.; Wolthuizen, J. P.; Clague, A. D. H.; Hays, G. R.; Huis, R.; van Hooff, J. H. C. J . Catal. 1983, 80, 130. (27) Liengme, B. V.; Hall, W. K. Trans. Faraday SOC.1966,62, 3229. (28) Datka, J. Stud. Suif. Sci. Catal. 1980, 5, 121. (29) Datka, J. J . Chem. SOC.,Faraday Trans. I 1980, 76, 705. (30) Kogelbauer, A.; Lercher, J. A. J . Catal. 1990, 125, 197. (31) Kubelkova, L.; Novakova, J.; Jiru, P. React. Kinet. Catal. Lett. 1976, 4, 151. (32) Weeks, T. J.; Angell, C. L.; Ladd, I. R.; Bolton, A. B. J . Catal. 1974, 33, 256. (33) Eberly, P. E., Jr. J . Phys. Chem. 1967, 71, 1717. (34) Mirth, G.; Lercher, J. A. Natural Gus Conversion; Elsevier: Amsterdam, 1991; p 437. Jp9432298
