Neopentane and Solid Acids: Direct Hydron Exchange before

The transition state allowing hydron exchange is most likely a carbonium species (pentacoordinated carbon) as in the case of the H/D exchange between ...
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J. Phys. Chem. B 2006, 110, 18368-18373

Neopentane and Solid Acids: Direct Hydron Exchange before Cracking Ste´ phane Walspurger,* Yinyong Sun, Abdelkarim Sani Souna Sido, and Jean Sommer Laboratoire de Physicochimie des Hydrocarbures, UMR 7513, Faculte´ de Chimie, UniVersite´ Louis Pasteur, 4 rue Blaise Pascal, 67000 Strasbourg, France ReceiVed: April 6, 2006; In Final Form: June 21, 2006

The hydrogen/deuterium exchange reaction of 2,2-dimethylpropane (neopentane) over D2O-exchanged zeolites (MOR, FAU, BEA, MFI) using a batch recirculation reactor was studied by means of gas chromatography coupled with mass spectrometer. In the temperature range 473-573 K, H/D exchange proceeds without side reaction such as cracking at short contact times. Indeed the C-H bond has appeared favorably involved in the activation of neopentane compared to the less accessible C-C bond. The transition state allowing hydron exchange is most likely a carbonium species (pentacoordinated carbon) as in the case of the H/D exchange between methane and solid acid. The activation energies of the H/D exchange between neopentane and zeolites are the same for all zeolites indicating a common carbonium ion type transition state. On the basis of previous results in the case of the exchange between methane and liquid superacids, the deuterium exchange rates in neopentane were tentatively related to the acidity of the solids. However the order of activity MOR > MFI > BEA > FAU seems to be related to the size of the pores, which may suggest the involvement of a confinement effect in the zeolites cavities. Moreover we found that H/D exchange takes also place between neopentane and deuterated sulfated zirconia (SZ) emphasizing its strong acidity.

1. Introduction Solid acids such as zeolites, sulfated zirconia, or heteropolyacids can activate C-C and C-H bonds of alkanes and are of important use particularly in industrial key reactions including cracking, isomerization, dehydrogenation, oligomerization, and disproportionation.1 The first interaction of hydrocarbons with the acidic protons on the surface of the catalyst consists of a fast hydron exchange that can be monitored using isotopic labeling.2 This technique has been applied with various small alkanes as probes to understand the activation mechanism. For instance in the case of isobutane3-5 a regiospecific exchange on the methyl groups was observed in the H/D exchange reaction with solid acids being in agreement with a general mechanism based on intermediate carbenium ions (classical trivalent carbocation). Once the carbocation is generated it is in equilibrium with its corresponding olefin on the surface by a fast deprotonation/reprotonation step. Similarly, we6,7 and others8,9 have studied more recently both the H/D exchange and the skeletal rearrangement in the reaction between propane and sulfated zirconia as well as zeolite using deuterium and carbon-13 as labels. With the help of in situ solid-state MAS NMR the intraand intermolecular rearrangement in propane were fully characterized providing strong evidence for a mechanism based on carbenium ions. Again the fast deprotonation/reprotonation process involving the equilibrium between the sec-propyl carbenium and propene explains the Markovnikov-type incorporation of deuterium in the methyl group. In the aim to correlate better the acidity to the proton transfer it is preferable to study alkanes that cannot give an olefin by deprotonation such as methane and its isolobal analogues, ethane, neopentane, etc. Despite that methyl cation or ethyl cation is not generated under * To whom correspondence should be addressed. Present address: Loker Hydrocarbon Institute, University of Southern California, Los Angeles, CA 90089-1661. Tel: +1-213-740-5979. Fax: +1-213-740-6270. E-mail: [email protected].

mild condition, due to its high energy, methane and ethane do undergo H/D exchange on solid acid without secondary reaction.10,11 This exchange can only be possible via a concerted mechanism in which the transition state resembles that of a carbonium ion presenting a 2 electron-3 center bond as in protonated methane. This species can be considered as the conjugated acid of methane. Nonclassical cations were earlier suggested as a transition state or strongly solvated intermediates in the H/D exchange reaction between methane and superacids DF/SbF512 or DSO3F/SbF5.13 Whereas the reactivity of hydrocarbons in liquid superacids is clearly explained by the σ-basicity of their C-C and C-H bonds,14,15 the mechanistic pathway should obviously involve the surface of solid acids and the participation of nucleophilic oxygen lone pairs compensating the lack of superacidity and stabilizing charges.16-18 2,2-Dimethylpropane (neopentane) is a “limiting” case on the σ-basicity scale for alkanes19 because the protolysis can only take place either on a primary C-H bond or on a C-C bond. Its behavior in superacids has been studied in the late 1960s by Olah et al. in HSO3F/SbF520,21 and Hogeveen et al. in HF/SbF5.22 Depending on the acidity and steric factors, the protolytic cleavage takes place either on the C-H bond or on a C-C bond, demonstrating the close basic character of both σ-bonds. Neopentane cracking has been previously studied over solid acids such as silica-aluminas, protonic zeolites,23-25 metalexchanged zeolites,26,27 and promoted and unpromoted sulfated zirconia.28 The analysis of products distribution has revealed a protolytic cleavage of the C-C bond yielding methane and the adsorbed tert-butyl cation as primary products. This study deals with the C-H bond activation characterized by H/D exchange with solid acids. The activation parameters and the rates of exchange allow ranking solid acids by their activity and acidity. Moreover neopentane being a bulky probe29 should be sensible to steric hindrance and the confinement effect.30

10.1021/jp0621676 CCC: $33.50 © 2006 American Chemical Society Published on Web 08/30/2006

Neopentane and Solid Acids

J. Phys. Chem. B, Vol. 110, No. 37, 2006 18369

TABLE 1: Main Characteristics of the Catalysts Employed in This Study chem composn

source

pore diameters (Å)

exchangeable hydrons (mmol·g-1)

mordenite (MOR)

Si/Al ) 10.2

Zeolyst International (CBV20A)

1.6

faujasite (FAU) beta (BEA)

Si/Al ) 2.8 Si/Al ) 25

Zeolyst International (CBV500) Zeolyst International (CP814Q)

ZSM-5 (MFI)

Si/Al ) 25

Zeolyst International (CBV5020)

sulfated zirconia (SZ)

ZrO2/SO42-

6.5 × 7.0 3.4 × 4.8 2.6 × 5.7 7.4 × 7.4 6.6 × 6.7 5.6 × 5.6 5.1 × 5.5 5.3 × 5.6

synthesized

solid acid

2. Experimental Section 2.1. Catalyst and Materials. Zeolites MOR (Si/Al ) 10.2; CBV20A), FAU (USY, Si/Al ) 2.8; CBV500), MFI (Si/Al ) 25; CBV5020), and BEA (Si/Al ) 25, CP814Q) were obtained from Zeolyst under the NH4+ form. Sulfated zirconia was obtained as follows: 19.335 g of ZrOCl2‚8H2O was dissolved in 150 mL of water. A 25 mL volume of 25% ammonia solution was added dropwise into the above solution at a rate of 0.5 mL/min. The obtained solution with precipitate was closed in a vessel and stirred for 1 h at room temperature. The solution with precipitate was filtered after aging at room temperature for 24 h. The obtained precipitate was washed with distilled water until the disappearance of chloride ions (AgNO3 test), dried at 383 K for 24 h, and powdered to below 50 mesh. The sulfation procedure was carried out by the impregnation method with 0.5 M H2SO4 solution (15 mL/g) under continuous stirring at room temperature for 1 h. The sulfated Zr(OH)4 was filtered without washing, dried again at 383 K, and calcined at 873 K for 4 h. Table 1 summarizes the main characteristics of the catalysts used during this study. The number of exchangeable hydrons has been determined using a method based on H/D exchange between D2O and catalysts,31 and the pore diameters are taken from ref 32. 2,2-Dimethylpropane (Linde Gas) was used as purchased without further purification. 2.2. General Experimental Procedures. Catalytic reactions were performed in a batch recirculation reactor at atmospheric pressure. About 500 mg of catalyst in case of SZ and 250 mg in case of zeolites were pretreated in situ with dry air (40 mL‚min-1) at 723 K (10 K‚min-1) for SZ and 823 K (5 K‚min-1) for ZSM-5 for 2 h. Deuteration of the catalyst was carried out with 3 mol % D2O in N2 at 473 K for 1 h, followed by a N2 purge of 30 min. The reactor was then brought to reaction temperature, and the alkane (ca. 2 mL) was introduced in the recirculation loop. The amount of protons provided by the hydrocarbon was in excess with respect to the acid deuterons of the solid. The reaction temperature ranged from 473 to 573 K. The gas mixture (hydrocarbon and nitrogen) was circulated by a membrane pump at speed of 5 mL‚min-1. 2.3. Kinetics of H/D Exchange of Neopentane. Samples were taken during the reaction, and analysis of deuterium content was performed by GC-MS. The reaction mixture including carrier gas was separated by a GC-8000 gas chromatograph (Carlo Erba Instruments) equipped with a 30-m capillary column DB-624 (JSW Scientific). The oven temperature was constant at 323 K. MS analysis was performed using a QMD-1000 spectrometer (Carlo Erba Instruments) with a ionization voltage of 70 eV. Neopentane was completely ionized, and its isotopic composition was calculated from the resulting tert-butyl ion’s deuterium content. To relate the intensity of the m/z signals at 57-67 of the tert-butyl ion to the actual concentration of the different isotopomers, the m/z values were corrected for the

4.4 0.64 0.64 1.8

contribution of fragment peaks by following the method proposed by Iglesia et al.33 Without any side reaction, the H/D exchange between neopentane and deuterated solid acid can be considered as a first-order equilibration reaction between a hydrogen pool (alkane) and deuterium pool (solid acid). The former can be represented as 12 × [C5H12]in, the initial amount of hydrogen atoms on neopentane, and the latter is represented as [DZ]in, the initial amount of deuterium atoms on the solid acid. At equilibrium, the “degree of deuteration”, Φeq, is equal on the solid acid and on neopentane, while the degree of deuteration of neopentane (Φ) during the reaction can be measured by mass spectrometer analysis: 12

∑1 x[i-C5H12-xDx]

Φ)

12

12 ×

∑0 [i-C5H12-xDx]

Φeq can be determined after the titration of the number of exchangeable hydrons in the solid acid ([DZ]in) by applying the method developed by Louis et al.:29

Φeq )

[DZ]in 12 × [i-C5H12]in + [DZ]in

The exchange rate k, i.e. the number of exchange reactions per hydroxyl per minute, can be calculated from the slope of the straight line that is obtained when ln(Φeq - Φ) is plotted versus time according to

ln(Φeq - Φ) )

-k t + ln Φeq 12 × [i-C5H12]in‚Φeq

3. Results and Discussion 3.1. H/D Exchange between Neopentane and D2OExchanged Solid Acid and Cracking of Neopentane. Several groups have investigated the neopentane conversion on solid acids and proposed a mechanistic pathway for the initial activation in full agreement with Haag and Dessau hypothesis.34 The first step consists of the protolysis of a C-C bond leading to the carbonium species (I) as a discrete intermediate or a transition state. In the next step methane and the adsorbed tertbutyl cation are simultaneously formed and further deprotonation of the latter leads to various oligomerization/cracking reactions (Scheme 1). Intermediate II is generallly discarded in the literature, mainly because it would generate the primary highly energetic neopentyl cation. Whereas this second pathway was highlighted in superacidic media by the formation of tert-amyl cation resulting from the 1,2 methyl shift, several authors described the C-C

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SCHEME 1: Protolysis and H/D Exchange in the Activation of Neopentane on Solid Acids

bond protolysis as the major pathway on solid acids rather than the C-H bond protolysis.23-28 We show here that species II should not be neglected as a direct exchange between primary protons and the solid acid was observed. 3.1.1. H/D Exchange with Zeolites. Neopentane was recirculated on deuterated mordenite (D-MOR) at temperatures between 473 and 548 K. H/D exchange took place without any side reaction at the beginning of the reaction, and then small amounts of methane could be detected by GC after a time of recirculation which is more or less short depending on the temperature. This is due to the competition between C-H and C-C protolysis which can even be noticed from the deuteration rate of the neopentane. Indeed the primary product of C-C bond cleavage is the tert-butyl cation which is in equilibrium with isobutene by a fast protonation/deprotonation. This process is known to be a fast reaction on zeolite35-37 and consumes a lot of deuterium thus decreasing the rate of deuteration of neopentane. This statement is demonstrated when the isotopic analysis of the C-C bond cleavage products is performed. Indeed, among those propane is the main component in accordance with literature23,24 and is found to be extensively deuterated. As a consequence the first-order rate of deuteration of neopentane itself can only be determined at short contact times. It should be pointed out that the isotopomers of neopentane are formed in a consecutive way: D1 f D2 f D3 f D4 .... This corresponds to an H/D exchange reaction where one hydron is exchanged at a time. Figure 1 shows the first-order rates of the H/D exchange between neopentane and D2O-exchanged mordenite. At higher temperatures, the competitive C-C bond cleavage takes place in a non-negligeable rate. For this reason we limit H/D exchange of neopentane as follows: on D-USY between 523 and 573 K, D-BEA between 513 and 548 K, and D-ZSM5 between 513 and 543 K following the same experimental procedure. 3.1.2. H/D Exchange with Sulfated Zirconia. Sulfated zirconia is known to activate hydrocarbons at low temperature.38 In the

case of the well-studied n-butane isomerization but also for propane rearrangement the first step is most probably an oxidative dehydrogenation.39,40 The synthesis of the material constitutes also a crucial point for its activity toward alkanes.41,42 The catalyst we used in this study readily exchanged its deuterons with the hydrons of neopentane in the temperature range 483-513 K. At higher temperature C-C protolysis competes significantly with H/D exchange. As can be seen on Figure 2, only a few points follow first-order kinetic at 513 K. However, these results highlight the ability of acidic sites of sulfated zirconia to generate a carbonium ion (species II, Scheme 1) at such moderate temperature. Sulfated zirconia apart from its well-known oxidative properties exhibits sufficiently strong acid sites allowing the generation of a pentacoordinated carbon similarly to carbonium-type ions in liquid superacids. 3.2. H/D Exchange and Acidity. 3.2.1. ActiVation Parameters on Solids. The analysis of kinetic data of the H/D exchange allowed us to determinate the activation energies for each solid acid. The Eyring equation separates the free enthalpy from the entropic term for a better comparison and comprehension of the transition state involved in the proton-transfer process:

k)

kB ‚T‚e-∆H*/RT‚e-∆S*/R h

From Figure 3, it appears clearly that the free enthalpy of activation ∆Hq of the H/D exchange between neopentane and zeolites is very close for all zeolites tested, taking into account the precision on rates. The similitude between these values is consistent with a common transition state in the H/D exchange step on zeolites. Further analysis of the entropic term (-222, -226, -228, and -236 J‚mol-1‚K-1 for MOR, BEA, MFI, and USY, respectively) reveals an important participation of the surface in the transition state. The significant loss of entropy gives evidence for a strongly concerted mechanism at the surface. Species II (Scheme 1) should rather be represented as an activated complex in which the surface of the solid acid

Neopentane and Solid Acids

J. Phys. Chem. B, Vol. 110, No. 37, 2006 18371

Figure 1. Rates of H/D exchange between neopentane and D2O-exchanged mordenite (D-MOR).

Figure 2. Rates of H/D exchange between neopentane and D2O-exchanged sulfated zirconia (D-SZ).

delocalizes extensively the charge (Chart 1), as it has been proposed.16-18 Accordingly, the value ∆Hq calculated for sulfated zirconia differs widely from those of zeolites, most probably due to the difference in chemical composition of the surface. This result constitutes a further proof of the strong involvement of the surface in the transition state. This situation is nevertheless very similar to what happens in liquid superacids for the activation of methane. Recently we have studied the H/D exchange between methane and DSO3F/SbF5 varying the SbF5 concentration and we have demonstrated not only a unique transition state for all systems but also a strong participation of the anionic part in the transition state.13 An analogy between these superacidic media and zeolites can then be proposed on the basis of kinetical analysis, and the rate of exchange at a given temperature should correlate directly with the acidity of the system. 3.2.2. Rates of H/D Exchange, Acidity and Confinement Effect. Table 2 shows the temperature range in which H/D exchange rates were determined and gives also the rate of H/D exchange between neopentane and each solid acid at 523 K. For an easier comparison the relative rates are also given.

At 523 K the rates of exchange were found to be in the same order of magnitude for all zeolites tested. In agreement with the previous studies on neopentane cracking, mordenite showed the highest activity among zeolites. Again, in full analogy with our previous results in superacids,13 since the transition state is unique for all zeolites as well as in the case of the exchange between CH4 and DSO3F/SbF5, we suggest that the activity in H/D exchange can be related to the acidity of zeolites. The order of activity appears to be the following: MOR > MFI > BEA > FAU. Generally the Si/Al ratio is considered as a major parameter to characterize the acidity of the zeolites. Nevertheless if one takes into account our experimental data for BEA and MFI (both having a Si/Al ratio of 25 here), the Si/Al ratio is not sufficient to explain the difference observed for the activity in H/D exchange. In fact the activity of zeolites seems to fit more with the size of the pores (Table 2). Considering the most confined space or the smallest pore of zeolite allows finding the order of activity of zeolites in H/D exchange with neopentane. Recent data in the literature confirm the essential role of the cage effect in acid catalysis. The closer the molecular size of the probe fits with the pores, the more basic the probe

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Figure 3. Free enthalpy of activation ∆Hq for the H/D exchange between neopentane and D2O-exchanged FAU (b), BEA (2), MFI (1), MOR (9), and SZ ([).

CHART 1: Carbonium-like Ion Involved in the H/D Exchange between Neopentane and Zeolitesa

a This illustration emphasizes the important role of the surface in such reactions.

TABLE 2: Temperature Range and Activity of Catalysts in the H/D Exchange Reaction with Neopentane temp range for solid acid H/D exchange (K)

rate of H/D exchange (exchange/OH·s-1) (rel rates) at 523 K pore sizes (Å)

FAU BEA MFI MOR SZ

1.12 × 10-9 (1.0) 3.70 × 10-9 (3.3) 5.01 × 10-9 (4.5) 9.44 × 10-9 (8.4) 29.0 × 10-9 a (26)

a

523-573 513-548 513-543 473-548 483-513

7.4 × 7.4 5.6 × 5.6 5.1 × 5.5 3.4 × 4.8

Extrapolated value.

becomes, thus facilitating the acid catalysis.43 Busca et al. and Thibault-Starzyck et al. have published very recently their results concerning the adsorption of nitriles and pyridines in zeolite.44-47 It is very interesting to note that acetonitrile can enter such small pores as the “side pockets” of mordenite (3.4 × 4.8 Å) at low temperature. In contrast these authors demonstrated by IR spectroscopy that pivalonitrile could not enter these small pores and remains in the main channel of mordenite. Neopentane is an isolobal analogue of pivalonitrile having a diameter of 6.2 Å which prevents the entry in the “side pockets”. However computational studies showed clearly that a methyl group having

a critical diameter of 4 Å enters the “side pockets” of mordenite when temperature is increased.48 Indeed the increase of temperature implies a greater flexibility of the framework and allows the distortion of the molecular probe.49,50 On the same basis, Nascimento has also shown that neopentane can enter the MFI channel (5.3 × 5.6 Å).51 If we take into account these results, it seems that the order of activity of the zeolites we present here emphasizes the confinement effect. Mordenite is the most active catalyst for the H/D exchange of neopentane as well as for its cracking, and H/D exchange takes place at lower temperature than with the other zeolites. Considering the “side pockets” as ideal sites where the methyl group of neopentane can be optimally protonated can explain these experimental data. Therefore, the order of activity found for H/D exchange between zeolites and neopentane seems to be strongly related to the size of the pores (Tables 1 and 2). The rate of exchange on sulfated zirconia is much higher than on zeolites. Free enthalpy of activation is however higher on SZ than on zeolites. We have noticed a similar phenomenon during the H/D exchange between methane and superacids. Indeed DF/SbF5 exchanges much faster than DSO3F/SbF5 whereas the free enthalpy of activation is lower in the latter system.13 A generalized relation between the rates of H/D exchange and the acidity of these systems appears rather risky because the transition states are of different nature mostly due to the participation of the anion (in the case of superacids) or the surface (zeolites and sulfated zirconia). 4. Conclusion H/D exchange between solid acids and neopentane is only possible through a carbonium ion (species with pentacoordinated carbon) since no primary product from oxidation can be stabilized by fast deprotonation/reprotonation process. Protonic zeolites mordenite, faujasite, beta, and MFI, as well as sulfated zirconia, are suitable solids to perform this reaction. Their acidic character is sufficient to generate the highly electrophilic carbonium ion, and we have to emphasize that we demonstrate here the significant acidity of the sulfated zirconia. The

Neopentane and Solid Acids comparison of the activation parameters of the H/D exchange is in agreement a common transition state for all zeolites having a free enthalpy of activation of about 95 kJ‚mol-1. We have also to point out the strong involvement of the surface of solid acids in the transition state (solvated carbonium ion) confirmed by an important loss of entropy. Finally the comparison of H/D exchange rates for 523 K allows stressing and highlighting the crucial role of the zeolite structure in the activation of alkanes. Combining the recent literature data devoted to this effect with our experimental results, we found a direct correlation among the confinement effect, the acidity, and the activation of alkanes (H/D exchange as well as protolysis). The confinement effect afforded by zeolites cages should also explain the possibility of generation of superelectrophiles in zeolites.52 Acknowledgment. Financial support of the Loker Hydrocarbon Institute, USC, Los Angeles, CA, is gratefully acknowledged. We thank Cle´ment Richert for the mass spectrometry analysis and Delphine Boulic and Laetitia Fouillen for their work and ideas in this study. Dr Franc¸ ois Fajula is gratefully acknowledged for fruitful discussion and clever advice. References and Notes (1) Olah, G. A.; Molnar, A. Hydrocarbon Chemistry; John Wiley: New York, 1995. (2) Narbeshuber, T. F.; Stockenhuber, M.; Brait, A.; Seshan, K.; Lercher, J. A. J. Catal. 1996, 160, 183. (3) Sommer, J.; Habermacher, D.; Jost, R.; Sassi, A.; Stepanov, A. G.; Luzgin, M. V.; Freude, D.; Ernst, H.; Martens, J. J. Catal. 1999, 181, 265. (4) Hua, W.; Sassi, A.; Goeppert, A.; Taulelle, F.; Lorentz, C.; Sommer, J. J. Catal. 2001, 204, 460. (5) Schoofs, B.; Schuermans, J.; Schoonheydt, R. A. Microporous Mesoporous Mater. 2000, 35-36, 99. (6) Haouas, M.; Walspurger, S.; Sommer, J. J. Catal. 2003, 215, 122. (7) Haouas, M.; Walspurger, S.; Taulelle, F.; Sommer, J. J. Am. Chem. Soc. 2004, 126, 599. (8) Stepanov, A. G.; Arzumanov, S. S.; Luzgin M. V.; Ernst, H.; Freude, D.; Parmon, V. N. J. Catal. 2005, 235, 221. (9) Arzumanov, S. S.; Reshetnikov, S. I.; Stepanov, A. G.; Parmon, V. N.; Freude, D. J. Phys. Chem. B 2005, 109, 19748. (10) Hua, W.; Goeppert, A.; Sommer, J. J. Catal. 2001, 197, 406. (11) Schoofs, B.; Martens, J. A.; Jacobs, P. A.; Schoonheydt, R. A. J. Catal. 1999, 183, 355. (12) Ahlberg, P.; Karlsson, A.; Goeppert, A.; Lill, S. O. N.; Diner, P.; Sommer, J. Chem.sEur. J. 2001, 7, 1936. (13) Walspurger, S.; Goeppert, A.; Haouas, M.; Sommer, J. New. J. Chem. 2004, 28, 266. (14) Olah, G. A. Angew. Chem., Int. Ed. Engl. 1973, 12, 173. (15) Goeppert, A.; Sommer, J. New J. Chem. 2002, 26, 1335. (16) Vollmer, J. M.; Truong, T. N. J. Phys. Chem. B 2000, 104, 6308. (17) Kramer, G. J.; Van Santen, R. A.; Emeis, C. A.; Nowak, A. K. Nature 1993, 363, 529. (18) Mota, C. J. A. J. Phys. Chem. B 1999, 103, 10417. (19) Olah, G. A.; Halpern, Y.; Shen, J.; Mo, Y. K. J. Am. Chem. Soc. 1973, 95, 4960.

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