Ind. Eng. Chem. Res. 1990,29, 1579-1582
1579
KINETICS AND CATALYSIS
Infrared Investigations of the Alkylation of Toluene with Methanol by Alkali-Modified Zeolites Elzbieta Mielczarski and Mark E. Davis* Department of Chemical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061
The infrared spectra of zeolites NaX, ion-exchanged CsNaX, and cesium acetate impregnated CsNaY (CsAce/CsNaY) exposed to methanol and toluene a t batch and continuous flow conditions over the temperature range 200-420 "C have been recorded in situ in order to investigate the types of adsorbed species that may exist on these catalysts during side-chain alkylation of toluene with methanol to form styrene. The results from all three materials indicate that methanol and toluene adsorb a t different sites within the zeolite. Zeolites with no acidity (CsAce/CsNaY) do not show the presence of formaldehyde. These data and those from our previous catalytic experiments on side-chain alkylation are used to speculate on new catalyst designs necessary for further rate/selectivity enhancements over existing technology.
Introduction The side-chain alkylation of toluene with methanol to form styrene may have commercial advantages to existing processes if reaction rates and selectivities can be appropriatly fixed. To date, no results are available to suggest that the reaction rates are sufficient for commercialization (Unland and Barker, 1978; Unland and Barker, 1981; Garces et al., 1985; Engelhardt et al., 1987; Zheng et al., 1988). The alkylation of toluene with methanol has been studied extensively. However, there is no consensus concerning the mechanism of the reaction. It is generally accepted that zeolites with some basicity, e.g., CsNaX, RbNaX, and KNaX, catalyze side-chain alkylation to styrene which is subsequently hydrogenated to ethylbenzene while those which do not contain base sites accomplish only ring alkylation to form xylenes. Although methanol is the starting reactant, it most probably dehydrogenates to formaldehyde, and the formaldehyde serves as the alkylating agent (Yashima et al., 1972). CsNaX zeolite (Garces et al., 1985) or CsNaX zeolites impregnated with boric acid (Unland and Barker; 1981) and/or copper nitrate (Lacroix et al., 1984) appear to be the most effective catalysts for side-chain alkylation. Several factors have been suggested to play a role in side-chain alkylation on CsNaX zeolites: (i) active base sites, (ii) spatial constraints found within the zeolite pores, and (iii) stabilization of the formaldehyde. Although it is known that strong acid sites promote ring alkylation, a certain degree of acidity is thought to enhance the stability of the formaldehyde (Unland and Barker, 1981; Itoh et al., 1980) and thus suppress its decomposition to CO and H,. In fact, Itoh et al. (1980) speculate that both acid and base sites are required for side-chain alkylation to occur. Due to the potential importance of styrene production from toluene and methanol, we have investigated this reaction further. Hathaway and Davis (1989a,b) showed that zeolite CsNaY impregnated with cesium acetate
* To whom correspondence
should be addressed.
0888-5885/90/2629-1579$02.50/0
(CsAce/CsNaY) possessed (i) significantly higher activity for the formation of acetone from 2-propanol (normally attributed to base sites) and (ii) virtually no Bransted acid sites. Also, CsAce/CsNaY was shown to contain a greater basicity than CsNaX, and NaX was essentially void of base sites (lack of acetone production from 2-propanol). Thus, this series of materials is ideal for investigating further the alkylation of toluene with methanol due to the variability in acid/base characteristics. Very recently, we reported on the catalytic activity of toluene alkylation with methanol using CsAce/CsNaY and CsNaX catalysts (Hathaway and Davis, 19894. These data will be used in conjunction with the results presented here to provide an overall picture of the alkylation of toluene with methanol. The objective of this work is to investigate the reaction pathways involved in the alkylation of toluene with methanol by in situ infrared monitoring of reaction processes on NaX, CsNaX, and CsAce/CsNaY catalysts.
Experimental Section The zeolites NaX, CsNaX, and CsAce/CsNaY are the same samples as reported previously (Hathaway and Davis, 1989a-c). The powdered zeolites were used without binder. Infrared measurements were performed on an IBM IR/32 FTIR spectrometer equipped with a Spectra Tech drifts controlled environment chamber which was attached to a gas-handling system. Initially, we used KBr windows. However, the specular reflected component of the light masks the diffuse scattering below 1300 cm-' for neat samples. Thus, we decided to use CaFz windows since they are not attacked by water and are transparent above 1300 cm-'. Neat samples are studied in the region 4000-1300 cm-', and an average of 1000 scans are collected per spectrum. Samples NaX, CsNaX, and CsAce/CsNaY were heated under vacuum to 150 "C for 6-8 h and then to 500 "C for 4 h in the IR cell prior to cooling and exposure to reactant gases. For zeolite CsAce/CsNaY, a calcination in flowing O2at 500 "C for 4 h was performed in a calcination furnace to decompose the acetate (Hathaway and Davis, 1989a,b) 0 1990 American Chemical Society
1580 Ind. Eng. Chem. Res., Vol. 29, No. 8, 1990 2949
3635
T
2945
1603
I 2832
C
3738
400
4000
I
3500
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um: (a) NaX; (b) CsNaX; (c) CsAce/CsNaY.
prior to the in situ heating treatment given above. Samples were exposed to 40 mmHg toluene or 100 mmHg methanol in batch experiments. Toluene and methanol were also introduced into the IR cell in a flowing N2 stream by bubbling the N2 carrier gas through either methanol or toluene at room temperature. For the experiments where toluene and methanol were simultaneously passed over the catalyst, the outlets of the toluene and methanol bubblers were connected prior to the IR cell. Infrared spectra were collected at temperatures of 200 and 420 "C. Background spectra were the samples just prior to contact with the reactants. That is, each sample spectrum is ratioed against the spectrum of zeolite recorded at the same temperature. This procedure reduces enhancement due to infrared emission which becomes more visible at higher temperatures.
Results and Discussion Zeolites. Figure 1 shows the infrared spectra for the hydroxyl group stretching frequency region of NaX, CsNaX, and CsAce/CsNaY. For all spectra illustrated in Figure 1, no absorptions are observed at -1640 cm-' (bending vibration of H20). Thus, each spectrum is not influenced by the presence of occluded water. Treatment of all samples with D20 vapor at room temperature before heating gave spectra which replicate the shape and relative intensities of the bands shown in Figure 1. However, the frequencies were all shifted to a lower wavenumber by a factor of 1.355 as expected for D substitution of H. Thus, all of the bands observed in Figure 1 are from hydroxyl
3000
2000
cm'
Figure 2. Infrared spectra of zeolites after exposure to methanol. Background spectra are the zeolites just prior to methanol contact: (a) NaX a t 200 "C; (b) NaX a t 420 "C; (c) CsNaX a t 200 OC; (d) CsAce/CsNaY a t 200 OC.
groups. Four bands are observed on NaX. The band at 3735 cm-' is from the O-H stretching mode of terminal silanol groups (Si-O-H). The band at 3684 cm-' is assigned to the OH stretching mode of Al-O-H (Kiselev and Lygin, 1975). The peaks at 3635 and 3572 cm-' are from bridging hydroxyl groups (Si-OH-AI) located in the supercages and sodalite cages, respectively (Jacobs, 1982). Thus, NaX possesses acid sites. CsNaX shows evidence of acid sites as well since bands at 3570 and 3640 cm-' are present. (Typically, the silanol groups do not contribute to catalytic activity of the type we are discussing here.) Notice that the intensity of the 3570-cm-' band is much greater than that of the 3640-cm-' band for CsNaX. Unless ion movement between the sodalite and supercages is possible at reaction conditions, CsNaX should be much less acidic than NaX because reactant molecules cannot penetrate the sodalite cages. Finally, CsAce/CsNaY shows essentially no bridging OH bands, indicating that it should have no acidity. We have shown previously that NaX >> CsNaX > CsAce/CsNaY in converting 2-propanol to propene (acid catalyzed) and that CsAce/CsNaY most probably contains no acidity (Hathaway and Davis, 1989a,b). Thus, the infrared results shown in Figure 1are consistent with measurements of 2-propanol dehydration activity. Adsorption of Methanol. Figure 2 illustrates the infrared spectra of NaX, CsNaX, and CsAce/CsNaY upon adsorption of methanol at 200 and 420 "C. The adsorption of methanol on NaX at 200 "C (Figure Sa) eliminates bridging hydroxyl groups (negative peak at 3684 cm-'). The broad absorption band centered at 3400 cm-' is due to hydrogen-bonded OH groups from methanol and water
Ind. Eng. Chem. Res., Vol. 29, No. 8, 1990 1581 (bending band at 1640 cm-' indicates the presence of H20 which is most probably formed from the interaction of methanol and the zeolite). The bands at 2949 and 2843 cm-' are from C-H stretching modes from CH3 groups. Heating the sample to 420 "C causes the spectrum to change significantly. New bands arise at 3017,2810,2720, and 1618 cm-', and the bands at 3400,1640, and 2843 cm-' disappeared. The bands at 2810 and 2720 cm-' are C-H stretching modes, and the former is most likely from an aldehyde group. Based upon the work of King and Garces (1987),the bands at 2720 and 1618 cm-' are from bidentate formate. Water is desorbed from the zeolite by heating to 420 "C, and methane is formed (peak at 3017 cm-'). No methane was observed in our continuous flow, catalytic experiments (Hathaway and Davis, 1989~).However, methane has been observed by others during side-chain alkylation (Moon et al., 1987). Thus, here the extended contact of methanol with NaX in a closed reaction vessel must allow for methane formation. Methane is the only gas-phase species detectable. When methanol or toluene vapors were introduced to the IR chamber without catalyst (diamond powder used as solid), observed IR bands have about 4% of the intensity of the bands observed after adsorption on the zeolite surface at the same conditions. Thus, all other infrared bands are from sorbed species (compare to continuous flow experiments, vide infra). The results in Figure 2a and 2b show that methanol is converted to methane, formaldehyde, and some bidentate formate. King and Garces (1987) showed that no bidentate formate was formed on NaX at temperatures similar to those used here. It is not clear what the differences between samples are at this time; however, our samples are exposed to a vacuum at 420 "C while those of King and Garces were not. Figure 2 shows also spectra from CsNaX contacted with methanol at 200 "C. Figure 2c appears qualitatively similar to Figure 2a. However, the band at 2843 cm-' in NaX is shifted to 2832 cm-' on CsNaX, and new bands at 1603, 2617, and 2730 cm-' appear on CsNaX. As with NaX at 420 "C, the bands at 2730 and 1603 cm-' are most probably due to bidentate formate on CsNaX at 200 "C as is the band at 2617 cm-' (King and Garces, 1983). (The 2617cm-' band is most probably too weak to observe on NaX.) The explanation of the vibrational modes of the bidentate formate is given elsewhere (King and Garces, 1987). When CsNaX was heated to 420 "C (not shown), the bands at 2730,2617, and 1603 cm-' disappeared while the bands at 2945 and 2832 cm-' remained. Also, a peak at 3017 cm-' from methane is observed. Thus, bidentate formate is removed from CsNaX by heating under vacuum to 420 "C. King and Garces (1987) obtained data on CsNaX similar to the data shown here. Figure 2d shows the infrared spectrum of CsAce/CsNaY when exposed to methanol at 200 "C. As previously described, the bands at 1606, 2611, and 2745 cm-' are assigned to bidentate formate. The peak at 3017 cm-' again indicates the presence of methane. The intensity near 3040 cm-' is most probably due to the formation of carbonaceous residue which contains some olefinic character. It is interesting that we observe the formation of residue only on the most basic zeolite, namely, CsAce/CsNaY. At 420 "C, the spectrum of CsAce/CsNaY remains relatively unchanged from that observed at 200 "C. Thus, the bidentate formate is more stable on CsAce/CsNaY than on CsNaX. Adsorption of Toluene. The infrared spectra for the C-H stretching frequency region obtained from adsorbing toluene on NaX, CsNaX, and CsAce/CsNaY are shown
2922
3q40
A
2920
h2870
-
\
2754
3030 , 2920
,"c. 2745
3200
2800
cm-
Figure 3. Infrared spectra of zeolites after exposure to toluene. Background spectra are the zeolites just prior to toluene contact: (a) NaX at 420 OC; (b) CsNaX at 420 "C; (c) CsAce/CsNaY at 420 O C .
in Figure 3. The spectra are qualitatively the same and are indicative of adsorbed toluene. Thus, toluene is not strongly interacting with the zeolite framework in the sense that no bond rupturing is observed upon adsorption. The exception to this statement is CsAce/CsNaY. Initial adsorption of toluene on CsAce/CsNaY revealed the presence of methane, which is likely to result from the dealkylation of toluene. Subsequent adsorptions gave no methane, indicating that certain adsorptionfreaction sites are most probably poisoned/deactivated. Dealkylation of alkylbenzenes is known to be base mediated (Pines, 1972). It is interesting to note that dealkylation occurs only on CsAce/CsNaY, which we showed previously to be the best base catalyst for 2-propanol decompositions (Hathaway and Davis, 1989a-c). Consecutive Adsorptions. NaX, CsNaX, and CsAce/CsNaY were exposed to methanol and subsequently toluene. Also, the adsorption order was reversed. In all cases, the infrared spectra appear to be simple additions of the spectra illustrated in Figures 2 and 3; e.g., for NaX combine Figure 2b with Figure 3a. Thus, it appears that on all three samples methanol and toluene occupy different sites. Continuous Flow Experiments. In order to more closely approximate our continuous flow reactor experiments, several in situ continuous flow infrared studies were conducted. First, toluene was passed over each of the three
1582 Ind. Eng. Chem. Res., Vol. 29, No. 8, 1990
catalysts in a flowing N2 stream at 420 "C. After approximately 1 h, toluene was substituted by methanol. Similar experiments were performed also in which the order of the reactants was reversed. Second, toluene and methanol were simultaneously passed over the catalysts at 420 "C. The results of these studies reveal that the data shown in Figures 2 and 3 are indicative of those obtained with flowing reactants. The exception is that the methane formation is depressed or eliminated by flow. Again, the order of reactant contact does not affect the final spectra, and they appear qualitatively the same as those obtained from co-feeds of toluene and methanol. Summary and Recommendations. The results of this study indicate that methanol and toluene adsorb at different sites within the zeolite. Variations in the acid/base characteristics of the zeolite do not significantly affect the adsorption of toluene while they greatly influence the adsorption/reaction of methanol. The presence of some acidity has been suggested by others (Unland and Barker, 1981; Itoh et al., 1980) to stabilize formaldehyde. Results from our catalytic studies (Hathaway and Davis, 1989~) reveal that microporosity is important and thus indicates that geometric factors play an important role in side-chain alkylation. We showed also that, contrary to previous speculations, acid sites are not necessary for side-chain alkylation to occur (CsAce/CsNaY catalyst). However, this catalyst decomposed methanol to CO and C02 while CsNaX formed formaldehyde. Thus, the catalytic results are consistent with the infrared data shown here in that some acidity is necessary to observe formaldehyde. From our infrared and catalytic results, we are not able to determine whether the appearance of formaldehyde is due to its stabilization by acid sites or to the absence of strong base sites which can react it to CO and COz. If a zeolite catalyst with strong acid and base sites could be prepared, then this question could be addressed. Our data along with that reported in the literature lead one to believe that the ideal catalyst for side-chain alkylation of toluene with methanol probably will be one which possesses microporosity and "well-tuned" acid/base properties. It is most likely that existing large-porezeolites (e.g., faujasite types (X, Y), zeolite L, etc.) will not be ideal. We base this comment solely on the fact that most if not all the combinations of acid/base characteristics have been previously investigated on faujasite type zeolites. We suggest that larger pore (10-15 A) materials should be attempted as catalysts for side-chain alkylation due to the high porosity that existing data on faujasites are most likely influenced by diffusional resistances. VPI-5 (Davis et al., 1988; Davis et al., 1989) contains a pore size within the 10-15-A range. However, it is a phosphate-based molecular sieve and most probably will not be stable to base. If an aluminosilicate analogue of VPI-5 can be synthesized, then it may be appropriately modified, e.g., by alkali like Cs, to form a useful side-chain alkylation catalyst. To date, we feel that one avenue worthy of exploration that has not been tested is that of pillared hydrotalcites. Hydrotalcites are solid anion exchangers which can serve as base catalysts (Reichle, 1985). Hydrotalcites can be pillared (Carrado et al., 1988),and if properly done, micropores in the 10-15-A range may be obtainable. From "tuning" of the acid/ base properties of the pillared hydrotalcite, a solid with ideal physicochemical characteristics for side-chain alkylation of toluene with methanol may be possible.
Acknowledgment Finanacial support of this work was provided by the National Science Foundation and the Dow Chemical Co. under a Presidential Young Investigator Award to M.E.D. We thank Drs. Paul E. Hathaway and Stan T. King of the Dow Chemical Co. for their helpful discussions. Registry No. CACE, 3396-11-0; toluene, 108-88-3; methanol, 67-56-1; styrene, 100-42-5.
Literature Cited Carrado, K. A.; Kostapapas, A.; Suib, S. L. Layered Double Hydroxides (LDHs). Solid State Ionics 1988,26,77-86. Davis, M. E.; Saldarriaga, C.; Montes, C.; Garces, J.; Crowder, C. Nature 1988,331, 698-699. Davis, M. E.; Montes, C.; Hathaway, P. E.; Arhancent, J. P.; Hasha, D. L.; Garces, J. M. J . A m . Chem. SOC.1989,111, 3919-3924. Engelhardt, J.; Szanyi, J.; Valyon, J. Alkylation of Toluene with Methanol on Commercial X Zeolite in Different Alkali Cation Forms. J . Catal. 1987,107,296-306. Garces, J. M.; Vrieland, G . E.; Bates, S. I.; Scheidt, F. M. Basic Molecular Sieve Catalysts/Side-Chain Alkylation of Toluene by Methanol. In Catalysis by Acids and Bases; Imelik, B., Naccache, C., Coudurier. G., Taarit, Y. Ben, Vedrine, J. C., Eds.; Elsevier: Amsterdam, 1985. Hathaway, P. E.; Davis, M. E. Base Catalysis by Alkali-Modified Zeolites. I. Catalvtic Activity. J. Catal. 19898. 116. 263-278. Hathaway, P. E.; Daiis, M. E. Base Catalysis by Alkali-Modified Zeolites. 11. Nature of the Active Site. J . Catal. 1989b,116, 279-284. Hathaway, P. E.; Davis, M. E. Base Catalysis by Alkali-Modified Zeolites. 111. Alkylation with Methanol. J . Catal. 1989c,119, 497-507. Itoh, H.; Miyamoto, A,; Murakami, Y. Mechanism of the Side-Chain Alkylation of Toluene with Methanol. J. Catal. 1980,64,284-294. Jacobs, P. A. Acid Zeolites: An Attempt to Develop Unifying Concepts. Catal. Rev.-Sci. Eng. 1982,24,415-440. King, S. T.; Garces, J. M. In-Situ Infrared Study of Alkylation of Toluene with Methanol on Alkali Cation Exchanged Zeolites. J. Catal. 1987,104, 59-70. Kiselev, A. V.; Lygin, V. J. Infrared Spectra of Surface Compounds; Wiley: New York, 1975; p 305. Lacroix, C.; Deluzarche, A.; Kiennemann, A.; Boyer, A. Promotion Role of Some Metals (Cu, Ag) in the Side Chain Alkylation of Toluene by Methanol. Zeolites 1984,4,109-111. Moon, S. K.; Kim, H. J.; Seo, K. T.; Pack, S. W. Influence of Methanol-Decomposition in the Side-Chain Alkylation of Toluene with Methanol. J . Korean Inst. Chem. Eng. 1987,25,601-606. Pines, H. Four Decades of Research in Catalysis of Hydrocarbon Reactions and Related Studies. Intra-Sci. Chem. Rept. 1972,6 , 1-29. Reichle, W. T. Catalytic Reactions by Thermally Activated, Synthetic, Anionic Clay Minerals. J . Catal. 1985,94, 547-557. Unland, M. L.; Barker, G. E. US.Patent 4,115,424, 1978. Unland, M. L., Barker, G. E. Catalysis of the Toluene-Methanol Reaction to Form Styrene and Ethylbenzene. In Catalysis of Organic Reactions; Moser, W. R., Ed.; Marcel Dekker: New York, 1981. Yashima, T.; Sato, K.; Hayasaka, T.; Hara, N. Alkylation on Synthetic Zeolites. 111. Alkylation of Toluene with Methanol and Formaldehyde on Alkali Cation Exchanged Zeolites. J . Catal. 1972,26,303-312. Zheng, S. A.; Cai, J. J.; Liu, D. C. The Effect of Pore Structure and Acid-Base Properties on the Selective Alkylation of Modified Zeolite Catalysts. In Catalysis: Theory to Practice: Proceedings of the 9th International Congress on Catalysis, Calgary, Canada; The chemical Institute of Canada: Ottawa, 1988.
Received for review December 15, 1989 Revised manuscript receiued March 30, 1990 Accepted April 10, 1990
