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Influence of isobutane on the formation of alkenyl carbenium ions from 1-butene over La-beta zeolite
Applied Catalysis A: General 126 ( 1995) 401-410
ELSEVIER
Influence of isobutane on the formation of alkenyl carbenium ions from 1-butene over La-beta zeolite Imre Kiricsi L* , Cristina Flego, Giuseppe Bellussi Eniricerche S.p.A., San Donate Milanese, Via Maritano 26, I-20097 Milan, Italy
Received 28 November 1994; revised 27 February 1995; accepted 27 February 1995
Abstract The influence of added isobutane on the formation of unsaturated carbenium ions (considered to be precursors of carbonaceous deposits) from I-butene was studied. The unequivocal results obtained by IR and UV-VIS spectroscopy show that the formation of alkenyl ions is strongly suppressed in the presence of isobutane. The role of isobutane is explained by its enhanced hydride ion donor character. Keywords:
Alkenyl carbenium
ions; Butene; Isobutane;
Lanthanum;
Zeolites
1. Introduction As reviewed recently by Corma and Martinez, a number of factors, such as the reaction temperature, the acidity of the solid catalyst, the space velocity of the reactants (WHSV) and the reactant ratio, affect the yield of alkylates and the operating time of the catalyst in the alkylation of isobutane with 1-butene [ 1] . The influence of process variables on the alkylation reaction has also been reported by that group [ 21. We have found, and recently reported in this journal [ 31, that the amount of alkenyl carbenium ions present in the zeolite catalyst correlates with the concentration of carbonaceous deposits formed on the catalyst operating under various experimental conditions. Alkenyl carbenium ions, as a group of unsaturated carbenium ions, described by the general formula R,C-CR-
* Corresponding
[CR-CR]
,-C +
R,
author.
I Present address: Applied Chemistry Department, Jozsef Attila University, 6720 Szeged, Rerrich B. t.1, Hungary 0926-860X/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSD10926-860X(95)00051-8
402
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(where R = alkyl group or hydrogen; while m = 0, 1, 2, etc. indicates a mono-, di-, tri-, etc. enylic carbenium ion) are generally assumed to be formed in hydrocarbon transformations over zeolite catalysts. Such carbenium ions are regarded as precursors of the carbonaceous deposits which lead to deactivation of the catalyst in hydrocarbon reactions. Formation of alkenyl carbenium ions from small alkenes, such as ethene [ 4-61, propene [ 7,8], butene [ 91 and hexene [ 10,111, has been studied on zeolites by UV-VIS, IR and NMR spectroscopy. They are identified via the UV-VIS absorptions at around 300,370 and 460 nm, the IR bands in the range 1500-1540 cm-’ and the 13C-NMR resonance at around 250 ppm. As concerns the general characteristics of alkenyl carbenium ions formed on zeolites, the accumulated results can be summarized as follows: (i) the higher the acidity of the zeolite and/or the longer the carbon chain of the alkene, the easier is the generation of alkenyl carbenium ions; (ii) both the Bronsted and the Lewis acidity influence their transformations; (iii) with increasing temperature, the transformation accelerates; (iv) alkenyl carbenium ions generated in zeolites are considered to be coke precursors, and suppression of their formation may increase the life-time of the catalyst. Few reports describe the influence of a hydrocarbon admixed with an alkene on the formation of an unsaturated carbenium ion [ 12,131. In order to widen this field, we have now studied the effect of isobutane on the formation of unsaturated ions from a 1-butene/isobutane mixture on lanthanum-exchanged beta-zeolite (La-P). This is a feature to be considered as regards the interpretation of the reactions taking place in the alkylation of isobutane with alkenes in the presence of solid acid catalysts.
2. Experimental La-p was prepared from H-P (synthesized according to the US patent 3 308 060; with a unit cell composition H 15,93Nao.o,A116Sil,603~~) by conventional ion exchange, resulting in material of composition H12.2Lao,3A112,5Sil,~,~O~~~. The BET area of the sample was 665 m’/g. Sample crystallinity was controlled by X-ray powder diffraction measurements. Spectroscopic investigations were carried out in transmission mode in the IR and in reflectance mode in the UV-VIS regions, using Perkin-Elmer FT-IR 1730 and Lambda 9 UV-VIS spectrometers, respectively. Self-supporting wafers (thickness 15 mg/cm’) or zeolite powder were placed in the optical cells, possessing either KBr or quartz windows in the IR or UV-VIS spectral ranges. Samples were degassed at 773 K in vacuum, followed by cooling to the desired temperature at which adsorbate was loaded. 1-Butene, isobutane and 1-butene/isobutane (molar ratio = 1:9) were used as adsorbates. Three different types of experiments were performed: (i) stepwise loading with the adsorbate at 295 and 353 K for a contact
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403
Wavenum ber (cm- 1) Fig. 1. Acidity of zeolite tested by pyridine adsorption. Background spectrum of La-P (a); after treatment in pyridine at 473 K, followed by evacuation at 473 K (b), 573 K (c), 673 K (d) and 773 K (e).
time of 1 min; (ii) loading with 100 kPa of adsorbate at 295 K, followed by stepwise temperature increase, for 1 h; (iii) desorption of the generated surface species at increasing temperature under evacuation. The acidity of the zeolite was determined by pyridine adsorption, using IR spectroscopic detection. The desorption of pyridine was monitored in the temperature range 473-773 K. To determine the concentration of acid sites, integral extinction coefficients of pyridine bound by Bronsted and Lewis acid sites were taken from the literature [ 141.
3. Results 3.1. Acidity of the sample
Spectra of pyridine adsorbed on La-p are reported in Fig. 1. The major bands at 1545 cm-’ are attributed to the Bronsted site-bound pyridine, while the bands at 1445 and 1456 cm- ’are attributed to Lewis site-bound pyridine. These latter bands are assigned to pyridine on La3 + and A13+ -containing Lewis sites, respectively. 3.2. Adsorption of I-butene After La-p was loaded with 0.34 molecules 1-butene/unit cell at room temperature, the IR band of bridged OH groups (at 3605 cm- ‘) broadened and shifted to
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3400
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126 (1995) 401dlO
3000 (cm- 1)
Fig. 2. Influence of 1-butene adsorption on the hydroxyl groups of La-p. Background spectrum of zeolite evacuated at 773 K for 2 h in vacuum (a); after loading to 0.34 (b) or 2.7 (c) molecules of I-butene/unit cell at room temperature, (d) c after treatment at 353 K, (e) d after evacuation at 353 K.
lower wavenumbers. At higher loading (2.7 molecules/unit cell), only the position of terminal OH groups (at 3745 cm- ’) remained unchanged (see spectrum c in Fig. 2). Absorptions characteristic of -CH3 and -CH,- groups developed in the range 2800-3 100 cm- ‘, increased with increasing loading, and also increased after heat treatment of the sample at 353 K. Evacuation at 353 K resulted in a partial regeneration of OH bands in the spectrum. Adsorption of I-butene on La-/? at room temperature led to the appearance of two bands, at 1504 and 1533 cm-‘, assigned to alkenyl carbenium ions, and to other absorptions at 1368, 1382 and 1460 cm-’ characteristic of the presence of oligomers [ 151 (see Fig. 3). The intensities of the 1533 and 1504 cm- ’ bands passed through a maximum (at 433 K) with increasing desorption temperature. At 573 K, only broad bands centred at 1490 and 1380 cm-’ were observed. After evacuation at 673 K, practically the background spectrum of the zeolite was registered. UV-VIS spectra of surface species formed upon adsorption of 1-butene on La-p are depicted in Fig. 4. Two main characteristics can be seen immediately. Bands of very low intensities were obtained in the spectra of zeolites pretreated in the presence of 1-butene below 423 K. An intense band at 300 nm and a weaker one at 380 nm appeared in spectra registered after contact with 1-butene at 423 K (spectra d and e). In agreement with the former view [ 151, we assigned these bands to mono- and dienylic carbenium ions, respectively.
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L
i0
Wavenumber (cmV11 Fig. 3. Difference IR spectra of surface species formed from I-butene on La-p. Sample loaded with 1.33 kPa of I-butene, followed by evacuation at room temperature (a), 353 K ( b) ,393 K (c) ,433 K (d) ,473 K (e) and 573 (f).
220
300
400
480
Wavelength (nm) Fig. 4. UV-VIS spectra of surface species generated from I-butene on La-p. Immediately after admission of lbutene at room temperature (a), after 1 h (b), after treatment at 353 K (c), 423 K (d) after 1 h in the presence of alkene and evacuation at 423 K for 1 h (e)
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16 Wavenumber hi’
)
Fig. 5. Difference IR spectra of surface species formed during adsorption of I-butene/isobutane mixture on Lap. After adsorption of 0.0133 kPa (a), 0.133 kPa (b), 2.3 kPa (c), 10 kPa (d) and 15.7 kPa (e) of mixture at 353 K, followed by evacuation at room temperature.
3.3. Adsorption of isobutane After adsorption of isobutane on La-p at 353 at 2800-3 100 and 1300-1600 cm- ’ or the detected, even on prolonged contact time. This was not followed by any further transformation
K, no change in either the IR ranges 600-200 nm UV-VIS range was confirms that isobutane adsorption of isobutane.
3.4. Adsorption of isobutane/l -butene mixture When the I-butene/isobutane mixture was adsorbed on La-p at 353 K, several bands due to adsorbed hydrocarbons were observed. Absorptions in the 2800-3 100 and 1300-1500 cm-’ regions (characteristic of methyl and methylene groups) were attributed to oligomers and/or alkylates. At 353 K a very weak band reflecting alkenyl carbenium ions was found (see spectra u-d in Fig. 5). However, after 100 kRa of mixture was introduced into the optical cell, followed by heat treatment at 353 K for 1 h, an additional very weak band was observed at 1533 cm-’ (spectrum e). The bands characteristic of oligomers were also clearly seen in these cases. Fig. 6 depicts two spectra taken after identical pretreatment conditions (353 K, partial pressure of 1-butene = 10 kPa, 1 min contact time). Spectrum a refers to the surface species formed from 1-butene, while spectrum b is due to the surface intermediates formed from the 1-butenejisobutane mixture. No band due to alkenyl carbenium
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w Wavenumber (cti’ ) Fig. 6. IR spectra of La-/3 after loading of 1.33 kE’a of 1-butene (a) and I-butene/isobutane K for 1 min, followed by evacuation at room temperature.
mixture (b) at 353
ions is seen in the latter case, but absorptions attributed to oligomeric species are detected in both cases. UV-VIS-active surface species were not formed in amounts detectable by UVVIS from the isobutane/ 1-butene mixture on the zeolite treated similarly as in IR spectroscopic investigations.
4. Discussion Acidity measurements showed that, with increasing desorption temperature (from 473 to 773 K), the concentrations of pyridine bound to both Bronsted and Lewis centres decreased, although the ratio of the two adsorbed species did not change much at 473,573 and 673 K (0.3 1,0.32 and 0.30, respectively), but did so after evacuation at 773 K (0.1) . This feature differs from what was observed for H-/3-zeolite, where this ratio decreased continuously on elevation of the desorption temperature from 473 to 773 K (1.46, 1.07, 0.93 and 0.55, respectively) [ 161. Comparison of the concentrations of acid sites for La-p and H-P revealed that (i) the concentration of Bronsted acid sites was much higher for H-P than for La-p, and (ii) the concentration of Lewis sites was 10 times higher than that of the Bronsted centres measured after pretreatment at 773 K (this was generally the activation temperature in the spectroscopic experiments) for La-p.
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As alkylation reactions are generally catalyzed by Bronsted acid sites [ 11, a lower activity was expected for La-p than for the H-P catalyst, containing many more Bronsted sites. Indeed, the conversion, the octane yield and the selectivity in the reactions of the 1-butene/isobutane (9/ 1) mixture over the H-P and La-p catalysts (WHSV = 0.5 h- ‘, P = 35 atm, T= 353 K) differed [ 171. La-p proved to be less active and selective. As no structural modification of the La-/3 catalyst was observed, the lower catalytic activity and selectivity are attributed to the higher number of Lewis acid sites in this sample. A similar explanation was earlier given for the role of La ions in beta-zeolite [ 181. This catalytic behaviour is in accordance with the acidity measurement results and reveals that the Bronsted rather than the Lewis acidity is important in the alkylation reaction. In the following, the carbenium ions generated on acidic sites are regarded as secondary Lewis acid centres since they are capable of hydride ion abstraction from neutral hydrocarbon molecules. 1-Butene undergoes two main types of reactions: isomerization to 2-butenes and oligomerization, depending on the acidity of the catalyst [ 19-251. Over zeolites containing no Bronsted acidity, isomerization takes place exclusively [ 26-281. On acidic zeolites, both reactions occur, even at room temperature. The zeolite studied in this work possessed both Bronsted and Lewis acid centres. Although both sites are active in the transformations of 1-butene, the predominant role was attributed by Datka [29] to Bransted acid sites, especially in the oligomerization reaction. Forster and Kiricsi [ 91 found evidence that Lewis acid sites play an important role in the formation of alkenyl carbenium ions from 1-butene in zeolites. These observations allow the roles of Bronsted and Lewis acid sites to be separated at least partly. The first alkyl (set-butyl) and alkenyl (methyl-allyl) carbenium ions can be formed from I-butene on Bronsted and Lewis acid sites via proton addition and hydride ion abstraction, respectively: +H+
-+
CH3-CH2-CH-CH3
(1)
CH,;H-CH-CH,
(2)
Bransted
CH*=CH-CH2-CH3 --H-
+ Lewis
_-_--
The carbenium ions from reactions ( 1) and (2) may react with further I-butene molecules, resulting in the formation of analogous carbenium ions with longer chains. Besides these oligomerization reactions, hydride ion abstraction from lbutene by alkyl and alkenyl carbenium ions may also occur. This leads to the formation of mono- and dienyl carbenium ions on the surface, while alkane and alkene molecules are released, respectively. In our experiments with 1-butene on La-p, alkenyl (particularly mono- and dienylic) carbenium ions were detected (identified via the IR absorptions at 1533
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and 1504 cm-‘, and the absorptions at 300 and 380 nm in the UV-VIS spectra). The other IR bands, at 1460,1382 and 1368 cm- ‘, were due to paraffinic oligomers, confirming that such reactions likewise occurred. Since no IR band due to cyclic surface species was observed at 353 K, the detected carbenium ions should be aliphatic. Assuming identical activity for Bronsted and Lewis sites, simultaneous formation of alkyl and alkenyl carbenium ions is to be expected, although that of alkyl carbenium ions is favoured. On a zeolite with predominantly Lewis acid sites, and taking into account the presence of secondary Lewis sites (carbenium ions), extensive alkenyl carbenium ion formation is expected. The generation of alkenyl carbenium ions should be enhanced by increase of the temperature and contact time. This was exactly what we observed in the reactions of 1-butene over La-p. Isobutane takes part in the chain-grow reaction with 1-butene, producing isooctane. When a mixture of an alkene ( 1-butene) and an alkane containing tertiary hydrogen (isobutane) reacts over an acidic zeolite, the initial step can be either formation of the pentacoordinated carbonium ion from isobutane and the Bronsted acidic proton, or generation of a carbenium ion upon interaction of 1-butene with protons. The latter process is energetically much more favoured. Supposing the formation of the pentacoordinated carbonium ion to be the initial step, no alkenyl carbenium ion generation should be observed during adsorption at around 370 K. Literature data indicate that formation of carbonium ions from alkanes over acidic zeolites takes place only at much higher temperature than that required for alkylation [ 301. Therefore, we can discard the direct reaction of isobutane with Bronsted acidic protons under these conditions. We assume that the role of isobutane is to supply hydride ions to the carbenium ions and thus convert them to paraffinic and olefinic compounds, while isobutane is transformed to isobutyl carbenium ion and remains bonded to Bronsted acid sites. As a result of these reactions, the acid sites are covered with isobutyl carbenium ions, while the amounts of alkenyl and longchain alkyl ions on the surface should be diminished. This assumption was verified experimentally (see spectra in Fig. 6). At higher temperatures, independently of the reactant composition, alkenyl carbenium ions were detected. These unsaturated carbenium ions can be the precursors of carbonaceous deposits which result in catalyst deactivation. Carbonaceous deposits formed in the alkylation of isobutane with butenes on LaHY zeolite at 353 K exhibited an H/C atomic ratio of 1.8 [ 311. This value is very similar to that expected for alkenyl carbenium ions.
5. Conclusions Formation of alkenyl carbenium ions from I-butene on La-p was proved by IR and UV-VIS spectroscopy. No generation of unsaturated carbenium ion was observed from isobutane under the conditions used in the alkylation of isobutane
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with I-butene. Formation of alkenyl carbenium ions was almost completely suppressed when isobutane was admixed with the 1-butene, due to the enhanced hydride ion donor activity of isobutane, which may contribute to a lowering of the rate of catalyst deactivation.
Acknowledgements The authors wish to thank Eniricerche mission to publish these results.
S.p.A. and Agip Petroli S.p.A. for per-
References A. Corma and A. Martinez, Catal. Rev. Sci. Eng., 35 ( 1993) 483. A. Corma, A. Martinez and C. Martinez. J. Catal., 149 ( 1994) 52. C. Flego, I. Kiricsi, W.O. Parker, Jr. and M.G. Clerici, Appl. Catal. A, 124 ( 1995) 107. J.C. Vedrine, P. Dejaifve, E.D. Garbowski and E.D. Derouane, Stud. Surf. Sci. Catal., 5 ( 1980) 29. M. Laniecki and H.G. Karge, Proc. Vth Intern. Symp. Heterog. Catal., Sofia, Bulgaria, Vol. 2, 1987, p. 129. J. Melsheimer and D. Ziegler, J. Chem. Sot.. Faraday Trans. 1,88 ( 1992) 2101. E.D. Garbowski and H. Praliaud, J. Chim. Phys., Phys. Chim. Biol., 76 (1979) 687 [ 81H. Forster, S. Franke and J. Seebode, J. Chem. Sot., Faraday Trans. I, 79 ( 1983) 373. [9] H. Forster and I. Kiricsi, Zeolites, 7 (1987) 508. [ IO] I. Kiricsi, H. Forster and Gy. Tasi, Stud. Surf. Sci. Catal., 46 ( 1989) 355 [ 111 S. Maixner. C.Y. Chen, P.J. Grobet, P.A. Jacobs and J. Weitkamp, Stud. Surf. Sci. Catal., 28 ( 1986) 693. [ 121 V.N. Andreev and V.B. Kazansky, Izv. Akad. Nauk SSSR, Ser. Khim., ( 1974) 1887. [ 131 H. Forster, Gy. Tasi. I. Kiricsi and I. Hannus, J. Mol. Strut., 296 ( 1993) 61. 1141 J. Take, T. Yamaguchi, K. Miyamoto, H. Okayama and M. Misono, Stud. Surf. Sci. Catal., 28 ( 1986) 495. [ 151 I. Kiricsi and H. Forater, J. Chem. Sot., Faraday Trans. 1, 84 (1988) 491. [ 161 I. Kiricsi, C. Flego, G. Pazzuconi, W.O. Parker Jr., R. Millini, C. Perego and G. Bellussi, J. Phys. Chem., 97 ( 1994) 6460. [ 171 A. de Angelis, private communication. [ 181K.S.N. Reddy, M.J. Eapen. H.S. Soni and V.P. Shiralkar, J. Phys. Chem., 96 (1992) 7923. 1191 J. Haber, J. Komorek-Hlodzik and T. Romotowski, Zeolites. 2 ( 1982) 179 [20] S. Ceckievicz. A. Branski and J. Galuszka, J. Chem. Sot., Faraday Trans. 1.74 ( 1987) 2027 [ 2 1] J. Datka, J. Chem. Sot., Faraday Trans. I, 76 ( 1980) 2437. [ 221 J. Datka, J. Chem. Sot., Faraday Trans. I, 77 ( 1981) 391. 1231 J. Datka. J. Chem. Sot., Faraday Trans. I, 77 (1981) 1309. [24] J. Datka, J. Chem. Sot., Faraday Trans. I, 77 ( 1981) 2633. [25] D. Michael, W. Meiler, H. Pfeifer, H.J. Rauscher and H. Segel, J. Mol. Catal., 5 ( 1979) 263. [ 261 H. Forster and R. Seelemann, J. Chem. Sot., Faraday Trans. 1, 74 ( 1978) 1435. 1271 H. FSrster and R. Seelemann, J. Chem. SIX., Faraday Trans. 1,75 ( 1979) 2744. [ 281 H. Fijrster and R. Seelemann, J. Chem. Sot., Faraday Trans. I, 77 ( 198 I ) 1359. [29] _I.Datka, Zeolites, 1 (1981) 113. [30] G.A. Olah, Chem. Brit., 8 (1972) 281. [ 311 J. Weitkamp and S. Maixner, Zeolites, 7 ( 1987) 6. [l] [2] 131 [4] [5] [6] [7]
ELSEVIER
Influence of isobutane on the formation of alkenyl carbenium ions from 1-butene over La-beta zeolite Imre Kiricsi L* , Cristina Flego, Giuseppe Bellussi Eniricerche S.p.A., San Donate Milanese, Via Maritano 26, I-20097 Milan, Italy
Received 28 November 1994; revised 27 February 1995; accepted 27 February 1995
Abstract The influence of added isobutane on the formation of unsaturated carbenium ions (considered to be precursors of carbonaceous deposits) from I-butene was studied. The unequivocal results obtained by IR and UV-VIS spectroscopy show that the formation of alkenyl ions is strongly suppressed in the presence of isobutane. The role of isobutane is explained by its enhanced hydride ion donor character. Keywords:
Alkenyl carbenium
ions; Butene; Isobutane;
Lanthanum;
Zeolites
1. Introduction As reviewed recently by Corma and Martinez, a number of factors, such as the reaction temperature, the acidity of the solid catalyst, the space velocity of the reactants (WHSV) and the reactant ratio, affect the yield of alkylates and the operating time of the catalyst in the alkylation of isobutane with 1-butene [ 1] . The influence of process variables on the alkylation reaction has also been reported by that group [ 21. We have found, and recently reported in this journal [ 31, that the amount of alkenyl carbenium ions present in the zeolite catalyst correlates with the concentration of carbonaceous deposits formed on the catalyst operating under various experimental conditions. Alkenyl carbenium ions, as a group of unsaturated carbenium ions, described by the general formula R,C-CR-
* Corresponding
[CR-CR]
,-C +
R,
author.
I Present address: Applied Chemistry Department, Jozsef Attila University, 6720 Szeged, Rerrich B. t.1, Hungary 0926-860X/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSD10926-860X(95)00051-8
402
I. Kiricsi et al. /Applied
Catalysis A: General 126 (1995) 401410
(where R = alkyl group or hydrogen; while m = 0, 1, 2, etc. indicates a mono-, di-, tri-, etc. enylic carbenium ion) are generally assumed to be formed in hydrocarbon transformations over zeolite catalysts. Such carbenium ions are regarded as precursors of the carbonaceous deposits which lead to deactivation of the catalyst in hydrocarbon reactions. Formation of alkenyl carbenium ions from small alkenes, such as ethene [ 4-61, propene [ 7,8], butene [ 91 and hexene [ 10,111, has been studied on zeolites by UV-VIS, IR and NMR spectroscopy. They are identified via the UV-VIS absorptions at around 300,370 and 460 nm, the IR bands in the range 1500-1540 cm-’ and the 13C-NMR resonance at around 250 ppm. As concerns the general characteristics of alkenyl carbenium ions formed on zeolites, the accumulated results can be summarized as follows: (i) the higher the acidity of the zeolite and/or the longer the carbon chain of the alkene, the easier is the generation of alkenyl carbenium ions; (ii) both the Bronsted and the Lewis acidity influence their transformations; (iii) with increasing temperature, the transformation accelerates; (iv) alkenyl carbenium ions generated in zeolites are considered to be coke precursors, and suppression of their formation may increase the life-time of the catalyst. Few reports describe the influence of a hydrocarbon admixed with an alkene on the formation of an unsaturated carbenium ion [ 12,131. In order to widen this field, we have now studied the effect of isobutane on the formation of unsaturated ions from a 1-butene/isobutane mixture on lanthanum-exchanged beta-zeolite (La-P). This is a feature to be considered as regards the interpretation of the reactions taking place in the alkylation of isobutane with alkenes in the presence of solid acid catalysts.
2. Experimental La-p was prepared from H-P (synthesized according to the US patent 3 308 060; with a unit cell composition H 15,93Nao.o,A116Sil,603~~) by conventional ion exchange, resulting in material of composition H12.2Lao,3A112,5Sil,~,~O~~~. The BET area of the sample was 665 m’/g. Sample crystallinity was controlled by X-ray powder diffraction measurements. Spectroscopic investigations were carried out in transmission mode in the IR and in reflectance mode in the UV-VIS regions, using Perkin-Elmer FT-IR 1730 and Lambda 9 UV-VIS spectrometers, respectively. Self-supporting wafers (thickness 15 mg/cm’) or zeolite powder were placed in the optical cells, possessing either KBr or quartz windows in the IR or UV-VIS spectral ranges. Samples were degassed at 773 K in vacuum, followed by cooling to the desired temperature at which adsorbate was loaded. 1-Butene, isobutane and 1-butene/isobutane (molar ratio = 1:9) were used as adsorbates. Three different types of experiments were performed: (i) stepwise loading with the adsorbate at 295 and 353 K for a contact
I. Kiricsi et al. /Applied
Catalysis A: General 126 (1995) 401410
403
Wavenum ber (cm- 1) Fig. 1. Acidity of zeolite tested by pyridine adsorption. Background spectrum of La-P (a); after treatment in pyridine at 473 K, followed by evacuation at 473 K (b), 573 K (c), 673 K (d) and 773 K (e).
time of 1 min; (ii) loading with 100 kPa of adsorbate at 295 K, followed by stepwise temperature increase, for 1 h; (iii) desorption of the generated surface species at increasing temperature under evacuation. The acidity of the zeolite was determined by pyridine adsorption, using IR spectroscopic detection. The desorption of pyridine was monitored in the temperature range 473-773 K. To determine the concentration of acid sites, integral extinction coefficients of pyridine bound by Bronsted and Lewis acid sites were taken from the literature [ 141.
3. Results 3.1. Acidity of the sample
Spectra of pyridine adsorbed on La-p are reported in Fig. 1. The major bands at 1545 cm-’ are attributed to the Bronsted site-bound pyridine, while the bands at 1445 and 1456 cm- ’are attributed to Lewis site-bound pyridine. These latter bands are assigned to pyridine on La3 + and A13+ -containing Lewis sites, respectively. 3.2. Adsorption of I-butene After La-p was loaded with 0.34 molecules 1-butene/unit cell at room temperature, the IR band of bridged OH groups (at 3605 cm- ‘) broadened and shifted to
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I. Kiricsi
et al. /Applied
3700
Catalysis A: General
3400
Wavenumber
126 (1995) 401dlO
3000 (cm- 1)
Fig. 2. Influence of 1-butene adsorption on the hydroxyl groups of La-p. Background spectrum of zeolite evacuated at 773 K for 2 h in vacuum (a); after loading to 0.34 (b) or 2.7 (c) molecules of I-butene/unit cell at room temperature, (d) c after treatment at 353 K, (e) d after evacuation at 353 K.
lower wavenumbers. At higher loading (2.7 molecules/unit cell), only the position of terminal OH groups (at 3745 cm- ’) remained unchanged (see spectrum c in Fig. 2). Absorptions characteristic of -CH3 and -CH,- groups developed in the range 2800-3 100 cm- ‘, increased with increasing loading, and also increased after heat treatment of the sample at 353 K. Evacuation at 353 K resulted in a partial regeneration of OH bands in the spectrum. Adsorption of I-butene on La-/? at room temperature led to the appearance of two bands, at 1504 and 1533 cm-‘, assigned to alkenyl carbenium ions, and to other absorptions at 1368, 1382 and 1460 cm-’ characteristic of the presence of oligomers [ 151 (see Fig. 3). The intensities of the 1533 and 1504 cm- ’ bands passed through a maximum (at 433 K) with increasing desorption temperature. At 573 K, only broad bands centred at 1490 and 1380 cm-’ were observed. After evacuation at 673 K, practically the background spectrum of the zeolite was registered. UV-VIS spectra of surface species formed upon adsorption of 1-butene on La-p are depicted in Fig. 4. Two main characteristics can be seen immediately. Bands of very low intensities were obtained in the spectra of zeolites pretreated in the presence of 1-butene below 423 K. An intense band at 300 nm and a weaker one at 380 nm appeared in spectra registered after contact with 1-butene at 423 K (spectra d and e). In agreement with the former view [ 151, we assigned these bands to mono- and dienylic carbenium ions, respectively.
I. Kiricsi et al. /Applied Catalysis A: General 126 (1995) 401410
405
L
i0
Wavenumber (cmV11 Fig. 3. Difference IR spectra of surface species formed from I-butene on La-p. Sample loaded with 1.33 kPa of I-butene, followed by evacuation at room temperature (a), 353 K ( b) ,393 K (c) ,433 K (d) ,473 K (e) and 573 (f).
220
300
400
480
Wavelength (nm) Fig. 4. UV-VIS spectra of surface species generated from I-butene on La-p. Immediately after admission of lbutene at room temperature (a), after 1 h (b), after treatment at 353 K (c), 423 K (d) after 1 h in the presence of alkene and evacuation at 423 K for 1 h (e)
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16 Wavenumber hi’
)
Fig. 5. Difference IR spectra of surface species formed during adsorption of I-butene/isobutane mixture on Lap. After adsorption of 0.0133 kPa (a), 0.133 kPa (b), 2.3 kPa (c), 10 kPa (d) and 15.7 kPa (e) of mixture at 353 K, followed by evacuation at room temperature.
3.3. Adsorption of isobutane After adsorption of isobutane on La-p at 353 at 2800-3 100 and 1300-1600 cm- ’ or the detected, even on prolonged contact time. This was not followed by any further transformation
K, no change in either the IR ranges 600-200 nm UV-VIS range was confirms that isobutane adsorption of isobutane.
3.4. Adsorption of isobutane/l -butene mixture When the I-butene/isobutane mixture was adsorbed on La-p at 353 K, several bands due to adsorbed hydrocarbons were observed. Absorptions in the 2800-3 100 and 1300-1500 cm-’ regions (characteristic of methyl and methylene groups) were attributed to oligomers and/or alkylates. At 353 K a very weak band reflecting alkenyl carbenium ions was found (see spectra u-d in Fig. 5). However, after 100 kRa of mixture was introduced into the optical cell, followed by heat treatment at 353 K for 1 h, an additional very weak band was observed at 1533 cm-’ (spectrum e). The bands characteristic of oligomers were also clearly seen in these cases. Fig. 6 depicts two spectra taken after identical pretreatment conditions (353 K, partial pressure of 1-butene = 10 kPa, 1 min contact time). Spectrum a refers to the surface species formed from 1-butene, while spectrum b is due to the surface intermediates formed from the 1-butenejisobutane mixture. No band due to alkenyl carbenium
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w Wavenumber (cti’ ) Fig. 6. IR spectra of La-/3 after loading of 1.33 kE’a of 1-butene (a) and I-butene/isobutane K for 1 min, followed by evacuation at room temperature.
mixture (b) at 353
ions is seen in the latter case, but absorptions attributed to oligomeric species are detected in both cases. UV-VIS-active surface species were not formed in amounts detectable by UVVIS from the isobutane/ 1-butene mixture on the zeolite treated similarly as in IR spectroscopic investigations.
4. Discussion Acidity measurements showed that, with increasing desorption temperature (from 473 to 773 K), the concentrations of pyridine bound to both Bronsted and Lewis centres decreased, although the ratio of the two adsorbed species did not change much at 473,573 and 673 K (0.3 1,0.32 and 0.30, respectively), but did so after evacuation at 773 K (0.1) . This feature differs from what was observed for H-/3-zeolite, where this ratio decreased continuously on elevation of the desorption temperature from 473 to 773 K (1.46, 1.07, 0.93 and 0.55, respectively) [ 161. Comparison of the concentrations of acid sites for La-p and H-P revealed that (i) the concentration of Bronsted acid sites was much higher for H-P than for La-p, and (ii) the concentration of Lewis sites was 10 times higher than that of the Bronsted centres measured after pretreatment at 773 K (this was generally the activation temperature in the spectroscopic experiments) for La-p.
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As alkylation reactions are generally catalyzed by Bronsted acid sites [ 11, a lower activity was expected for La-p than for the H-P catalyst, containing many more Bronsted sites. Indeed, the conversion, the octane yield and the selectivity in the reactions of the 1-butene/isobutane (9/ 1) mixture over the H-P and La-p catalysts (WHSV = 0.5 h- ‘, P = 35 atm, T= 353 K) differed [ 171. La-p proved to be less active and selective. As no structural modification of the La-/3 catalyst was observed, the lower catalytic activity and selectivity are attributed to the higher number of Lewis acid sites in this sample. A similar explanation was earlier given for the role of La ions in beta-zeolite [ 181. This catalytic behaviour is in accordance with the acidity measurement results and reveals that the Bronsted rather than the Lewis acidity is important in the alkylation reaction. In the following, the carbenium ions generated on acidic sites are regarded as secondary Lewis acid centres since they are capable of hydride ion abstraction from neutral hydrocarbon molecules. 1-Butene undergoes two main types of reactions: isomerization to 2-butenes and oligomerization, depending on the acidity of the catalyst [ 19-251. Over zeolites containing no Bronsted acidity, isomerization takes place exclusively [ 26-281. On acidic zeolites, both reactions occur, even at room temperature. The zeolite studied in this work possessed both Bronsted and Lewis acid centres. Although both sites are active in the transformations of 1-butene, the predominant role was attributed by Datka [29] to Bransted acid sites, especially in the oligomerization reaction. Forster and Kiricsi [ 91 found evidence that Lewis acid sites play an important role in the formation of alkenyl carbenium ions from 1-butene in zeolites. These observations allow the roles of Bronsted and Lewis acid sites to be separated at least partly. The first alkyl (set-butyl) and alkenyl (methyl-allyl) carbenium ions can be formed from I-butene on Bronsted and Lewis acid sites via proton addition and hydride ion abstraction, respectively: +H+
-+
CH3-CH2-CH-CH3
(1)
CH,;H-CH-CH,
(2)
Bransted
CH*=CH-CH2-CH3 --H-
+ Lewis
_-_--
The carbenium ions from reactions ( 1) and (2) may react with further I-butene molecules, resulting in the formation of analogous carbenium ions with longer chains. Besides these oligomerization reactions, hydride ion abstraction from lbutene by alkyl and alkenyl carbenium ions may also occur. This leads to the formation of mono- and dienyl carbenium ions on the surface, while alkane and alkene molecules are released, respectively. In our experiments with 1-butene on La-p, alkenyl (particularly mono- and dienylic) carbenium ions were detected (identified via the IR absorptions at 1533
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and 1504 cm-‘, and the absorptions at 300 and 380 nm in the UV-VIS spectra). The other IR bands, at 1460,1382 and 1368 cm- ‘, were due to paraffinic oligomers, confirming that such reactions likewise occurred. Since no IR band due to cyclic surface species was observed at 353 K, the detected carbenium ions should be aliphatic. Assuming identical activity for Bronsted and Lewis sites, simultaneous formation of alkyl and alkenyl carbenium ions is to be expected, although that of alkyl carbenium ions is favoured. On a zeolite with predominantly Lewis acid sites, and taking into account the presence of secondary Lewis sites (carbenium ions), extensive alkenyl carbenium ion formation is expected. The generation of alkenyl carbenium ions should be enhanced by increase of the temperature and contact time. This was exactly what we observed in the reactions of 1-butene over La-p. Isobutane takes part in the chain-grow reaction with 1-butene, producing isooctane. When a mixture of an alkene ( 1-butene) and an alkane containing tertiary hydrogen (isobutane) reacts over an acidic zeolite, the initial step can be either formation of the pentacoordinated carbonium ion from isobutane and the Bronsted acidic proton, or generation of a carbenium ion upon interaction of 1-butene with protons. The latter process is energetically much more favoured. Supposing the formation of the pentacoordinated carbonium ion to be the initial step, no alkenyl carbenium ion generation should be observed during adsorption at around 370 K. Literature data indicate that formation of carbonium ions from alkanes over acidic zeolites takes place only at much higher temperature than that required for alkylation [ 301. Therefore, we can discard the direct reaction of isobutane with Bronsted acidic protons under these conditions. We assume that the role of isobutane is to supply hydride ions to the carbenium ions and thus convert them to paraffinic and olefinic compounds, while isobutane is transformed to isobutyl carbenium ion and remains bonded to Bronsted acid sites. As a result of these reactions, the acid sites are covered with isobutyl carbenium ions, while the amounts of alkenyl and longchain alkyl ions on the surface should be diminished. This assumption was verified experimentally (see spectra in Fig. 6). At higher temperatures, independently of the reactant composition, alkenyl carbenium ions were detected. These unsaturated carbenium ions can be the precursors of carbonaceous deposits which result in catalyst deactivation. Carbonaceous deposits formed in the alkylation of isobutane with butenes on LaHY zeolite at 353 K exhibited an H/C atomic ratio of 1.8 [ 311. This value is very similar to that expected for alkenyl carbenium ions.
5. Conclusions Formation of alkenyl carbenium ions from I-butene on La-p was proved by IR and UV-VIS spectroscopy. No generation of unsaturated carbenium ion was observed from isobutane under the conditions used in the alkylation of isobutane
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with I-butene. Formation of alkenyl carbenium ions was almost completely suppressed when isobutane was admixed with the 1-butene, due to the enhanced hydride ion donor activity of isobutane, which may contribute to a lowering of the rate of catalyst deactivation.
Acknowledgements The authors wish to thank Eniricerche mission to publish these results.
S.p.A. and Agip Petroli S.p.A. for per-
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