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Activity and selectivity of zeolites MCM-22 and MCM-58 in the alkylation of toluene with propylene
Microporous and Mesoporous Materials 53 (2002) 121–133 www.elsevier.com/locate/micromeso
Activity and selectivity of zeolites MCM-22 and MCM-58 in the alkylation of toluene with propylene ejka Jirı C
a,*
ilkov , Andrea Krejcı a, Nadezda Z a a, Josef Kotrla a, Stefan Ernst b, Astrid Weber b
a
b
J. Heyrovsk y Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejskova 3, CZ-182 23 Prague 8, Czech Republic Department of Chemistry, Chemical Technology, University of Kaiserslautern, Erwin Schr€odinger Strasse 54, D-67663 Kaiserslautern, Germany Received 16 March 2001; received in revised form 20 November 2001; accepted 11 February 2002
Abstract The influence of the pore architecture of zeolites MCM-22 and MCM-58 on their catalytic activity and selectivity in the gas-phase alkylation of toluene with propylene has been investigated. The results obtained with MCM-22 as catalyst revealed that, despite the expectations due to the presence of 10-membered-ring pores in the structure of this zeolite, no enhanced selectivity to p-cymene is observed. Moreover, also n-propyltoluenes, which are usually formed over threedimensional 10-membered-ring zeolites (e.g., ZSM-5 and ZSM-11) via bimolecular transalkylation/isomerization reactions of cymenes with toluene, are only formed to a negligible extent. This has been rationalized by the assumption that most of the reactions occurring in this system take place on acid sites at or close to the external surface. This is supported by the FTIR-spectroscopic observation that p-cymene can reach only about one half of the bridging OH groups, while the smaller d3 -acetonitrile reaches/covers virtually all remaining Brønsted and Lewis acid sites. Over zeolite MCM-58, relatively high selectivities to n-propyltoluenes were observed, which has been ascribed to the peculiar undulating 12-membered-ring channel system of this zeolite. It is proposed that the 1,2-p-ditolylpropane-type transition state required for the formation of n-propyltoluenes is preferentially located in positions similar to those which are occupied by the N-benzyl-1,4-diazabicyclo[2.2.2]octane cations used as templates for the synthesis of zeolite MCM58. Ó 2002 Elsevier Science Inc. All rights reserved. Keywords: Toluene alkylation; Zeolites; MCM-22; MCM-58; Acid sites
1. Introduction
*
Corresponding author. Tel.: +42-2-6605-3795; fax: +42-28658-2307. ejka). E-mail address: [email protected] (J. C
The alkylation of aromatic hydrocarbons with olefins is successfully employed on a large scale in the chemical industry [1–3]. Products from such processes, in particular ethylbenzene, isopropylbenzene (cumene), p-di-iso-propylbenzene and isopropyltoluenes (cymenes) are well known as
1387-1811/02/$ - see front matter Ó 2002 Elsevier Science Inc. All rights reserved. PII: S 1 3 8 7 - 1 8 1 1 ( 0 2 ) 0 0 3 3 2 - 3
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important chemical intermediates. The classical catalysts for aromatics alkylation, viz. HF, aluminum trichloride with HCl or solid phosphoric acid exhibit many problems with respect to their handling, safety, corrosion and waste disposal. To overcome these drawbacks, zeolite-based solid acids have been successfully introduced as catalysts in the alkylation of benzene with ethylene (ethylbenzene synthesis). Moreover, several new liquidphase processes for cumene production have been announced so far or are even already commercialized by Dow Chemical, Mobil, CDTECH, UOP and Enichem in processes which are based on different acid zeolites as catalysts (viz., dealuminated mordenite, MCM-22, zeolite Y and Beta) [4,5]. The present study focuses on the alkylation of toluene with propylene to cymenes. As catalysts for this reaction, two relatively new zeolites with peculiar pore architectures, viz. MCM-22 and MCM58, were explored. In this context, the mechanism for n-propyltoluene formation over these two zeolite catalysts and the influence of their pore structures was of particular interest. The structure of zeolite MCM-22 (IZA structure code MWW) consists of two independent pore systems. One of them is defined by two-dimensional sinusoidal 10membered-ring channels (0:40 0:59 nm). The second pore system consists of large supercages with a free inner diameter of 0.71 nm which is circumscribed by 12-membered rings. The height of the large supercages amounts to 1.82 nm. These huge intracrystalline voids are accessible through 10-membered-ring apertures (0:40 0:54 nm) only [6]. The structure of MCM-58 (isostructural with SSZ-42 and ITQ-4; IZA structure code IFR [7]) is characterized by undulating one-dimensional 12membered-ring channels [8]. The pore diameter at the narrowest point is 0.64 nm, and the cage at the widest point measures 1.0 nm. Moreover, the channels exhibit side pockets similar to those observed in zeolite Beta [9]. The reaction mechanism for the alkylation of benzene or toluene with propylene (or, alternatively, with isopropyl alcohol) over ZSM-5-type catalysts has been investigated by several groups [10–15]. It follows from these investigations that the formation of n-propyltoluene (from the
reaction of toluene with propylene) proceeds essentially in two steps, viz. the formation of isopropyltoluene followed by its consecutive conversion to n-propyltoluene. Although the occurrence of the first reaction step has been widely accepted, the mechanism of n-propyltoluene formation from the primary alkylation product is still a matter of debate. Originally, it has been proposed for the analogous case of benzene alkylation with propylene that n-propylbenzene is formed from cumene via a monomolecular isomerization step [10]. However, further investigations using kinetic experiments [11] and 13 C NMR studies with labeled benzene or cumene [13,14] revealed that the formation of n-propylbenzene (or n-propyltoluenes, respectively) proceeds via a bimolecular transalkylation/isomerization reaction. Moreover, experimental evidence has been presented which suggests that there is also a selectivity-directing effect of the particular zeolite structure [15]. In order to obtain further evidence for the reaction pathway described above, the validity of the proposed bimolecular mechanism was also explored by theoretical methods using ab initio quantum chemical calculations [16,17]. These calculations were performed at the Hartree–Fock level of theory for full relaxation of all geometric parameters and imposing selected geometric constraints which could simulate the situation occurring at the channel intersections in ZSM-5- and ZSM-11-type zeolites. The results from the calculations using the constrained model suggest the existence of a reaction pathway via an anti-Markovnikov-type of proton addition for the formation of n-propylbenzene. This is in principal agreement with the experimental findings published so far. Both experimental and theoretical results suggest that, for the formation of bimolecular intermediates leading to n-propylbenzene or n-propyltoluenes, respectively, a pore architecture with a three-dimensionally interconnected channel system and pore diameters of about 0.55 nm are required, in which large intracrystalline voids are absent. These structural features seem to be prerequisites for allowing translational and rotational movements of both reactants only. In view of this, benzene or toluene alkylation with propylene or isopropyl alcohol, beside its industrial significance, could
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also be exploited to characterize the reaction space available in the intracrystalline voids of microporous molecular sieves.
2. Experimental Section Zeolite MCM-22 was synthesized according to a method published by Corma et al. [18] using hexamethyleneimine as organic template. In a typical procedure, a given amount of sodium aluminate (0.6–1.0 g, Riedel-de Ha€en, 55% of Al2 O3 , 45% of Na2 O) is added to a solution of 0.6 g of sodium hydroxide diluted in 125 g of distilled water and stirred until dissolved. Then, 8.6 g of hexamethyleneimine (Fluka) are added, and the resulting mixture is thoroughly mixed for at least 15 min. Finally, 9.2 g of Cab-O-Sil M5 (Cabot) are added, and the resulting mixture is homogenized for another 60 min. The crystallization was conducted in Teflon-lined stainless-steel autoclaves under autogenous pressure at 155 °C for 8 days under agitation. After the synthesis, the autoclaves were cooled down to room temperature by quenching in water. Then, the solid product was recovered by filtration, thoroughly washed with deionized water and finally dried at 100 °C overnight. The as-synthesized samples were calcined in air at 580 °C for 6 h. Two MCM-22 samples with different molar ratios nSi =nAl (MCM-22/1: 13.6 and MCM-22/2: 18.7) were investigated. Zeolite MCM-58 (nSi =nAl ¼ 19) was synthesized and calcined using procedures described recently [19]. The ammonium forms of both zeolites were prepared by repeated ion exchange in 0.5 M aqueous solutions of ammonium nitrate at ambient temperature. All as-synthesized, calcined and ion-exchanged zeolites were checked for their crystallinity and phase purity by X-ray powder diffraction on a Siemens D5005 diffractometer equipped with graphite monochromator and scintillation counter and using CuKa radiation in a Bragg–Brentano geometry. The concentrations of Brønsted and Lewis sites in MCM-22 and MCM-58 were determined after the adsorption of d3 -acetonitrile followed by FTIR spectroscopy using a Nicolet FTIR Protege 460
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spectrometer. For this purpose, the zeolite powders were pressed binder-free into self-supporting wafers with a density of 4.0–11.0 mg/cm2 . d3 Acetonitrile was degassed by repeated freezing and thawing cycles before its use. Prior to the adsorption of d3 -acetonitrile, the zeolites were activated in situ by overnight evacuation at 400 °C. All measured spectra were recalculated to a normalized wafer thickness of 10 mg/cm2 . For a quantitative characterization of the Brønsted acid sites (B), the CBN-B vibration at 2296 cm1 was used with an extinction coefficient of eðBÞ ¼ 2:05 0:1 cm lmol1 . For a quantitative evaluation of the Lewis acid sites (L), the CBN-L vibration at 2323 cm1 was used with an extinction coefficient of eðLÞ ¼ 3:6 0:1 cm lmol1 [20]. A similar procedure was employed to determine the accessibility of bridging OH groups of zeolites MCM-22 and MCM-58 for p-cymene molecules. Prior to its use, p-cymene was degassed by repeated freezing and thawing cycles. The adsorption of p-cymene was performed for up to 240 min at 100 °C with a partial pressure of p-cymene of 100 Pa. Afterwards, the zeolites were evacuated at 100 °C for 30 min, where upon d3 -acetonitrile was admitted to study the accessibility of the remaining free bridging OH groups. The measured spectra were again normalized to a wafer thickness of 10 mg/cm2 . The alkylation of toluene with propylene was performed under atmospheric pressure in a downflow glass microreactor (inner diameter: 10 mm, weight of catalyst: 0.1–1.5 g, WHSV ¼ 10–200 h1 ) in the temperature range from 200 to 300 °C. The particle size of the binder-free catalyst was in the range from 0.3 to 0.6 mm. The pellets were prepared by pressing the zeolite powders followed by crushing and sieving. All catalysts were activated at 450 °C in a stream of pure hydrogen for 1 h. For the catalytic experiments, a hydrogen stream containing toluene with a partial pressure of 20.8 kPa was mixed with a stream of pure propylene to adjust a molar ratio ntoluene =npropylene of 9.6. The reaction products were analyzed using an on-line gas chromatograph (Hewlett–Packard 5890 Series II) equipped with a Supelcowax 10 capillary column (30 m length, inner diameter of 0.2 mm, thickness of stationary phase of 0.2 lm) and coupled to a
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flame ionization and mass-spectrometric detector (Hewlett–Packard 5971A), respectively. 3. Results and discussion 3.1. Characterization of the zeolites used The X-ray powder patterns of as-synthesized and calcined samples of zeolites MCM-22 and MCM-58 are depicted in Figs. 1 and 2, respectively. From a comparison with the literature data [21] it can be concluded that both samples have a good crystallinity and are phase pure. The concentration of Brønsted and Lewis acid sites was determined from the adsorption of d3 -
Fig. 1. X-ray powder diffraction patterns of (A) as-synthesized and (B) calcined MCM-22/1.
Fig. 2. X-ray powder diffraction patterns of (A) as-synthesized and (B) calcined MCM-58.
acetonitrile onto the activated zeolites followed by IR spectroscopy. Figs. 3 and 4 present the IR spectra of the hydroxyl group region and the region of CBN vibrations of d3 -acetonitrile for MCM-22/1 and MCM-58 before and after acetonitrile adsorption. In the case of MCM-22 four infrared bands were found in the hydroxyl region. The absorption band at 3742 cm1 belongs to the vibrations of terminal Si–OH groups, while the band at 3730 cm1 can probably be assigned to perturbed Si–OH groups interacting with other hydroxyl groups. The most intensive band at 3620 cm1 represents vibrations of acid bridging
Fig. 3. FTIR spectra of MCM-22 before (1) and after (2) adsorption of d3 -acetonitrile: (A) hydroxyl group region and (B) region of nitrile vibrations of d3 -acetonitrile.
Fig. 4. FTIR spectra of MCM-58 before (1) and (2) after adsorption of d3 -acetonitrile: (A) hydroxyl group region and (B) region of nitrile vibrations of d3 -acetonitrile.
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Si–OH–Al groups, and a small band at 3670 cm1 is usually assigned to Al–OH groups which can be present in some zeolites possessing extra-framework aluminum species. In contrast to the results of Corma et al. [22] we did not find a broad band at 3500 cm1 assigned to vibrations of internal silanol groups. The infrared spectrum of adsorbed d3 -acetonitrile consists of two typical bands due to the acetonitrile adsorption on acid Si–OH–Al groups at 2296 cm1 and on Al-Lewis sites at 2323 cm1 (Fig. 3). The concentrations of Brønsted and Lewis sites determined from d3 -acetonitrile adsorption using the extinction coefficients reported in [20] were 0.34 mmol/g for the Brønsted sites and 0.42 mmol/g for the Lewis sites (MCM-22/1, Si/ Al ¼ 13.6) and 0.27 mmol/g of Brønsted sites and 0.33 mmol/g of Lewis sites (MCM-22/2, Si/ Al ¼ 18.7), respectively. The hydroxyl region of MCM-58 contained an absorption band of terminal Si–OH groups at 3745 cm1 , two bands of bridging OH groups at 3630 and 3485 cm1 (their assignment is not yet clear) and a band at 3690 cm1 which probably belongs again to Al–OH groups. In addition, a new band appeared at 3780 cm1 . This band has already been observed for zeolite Beta and ascribed to OH groups bound to framework Al with a defective structure [23]. In contrast to two absorption bands of adsorbed acetonitrile over MCM-22, in the case of MCM-58 an additional band at 2276 cm1 was found. This band is usually assigned to d3 -acetonitrile in interaction with silanol Si–OH groups, which is in agreement with the decrease in the intensity of this band after acetonitrile adsorption (Fig. 4). While this band remained almost unchanged even after evacuation at 100 °C, it is inferred that this acetonitrile is caught in the cages of the undulating channel system of MCM-58 zeolite. The concentration of Brønsted sites of zeolite MCM-58 (Si/Al ¼ 19) was 0.20 mmol/g while the concentration of Lewis sites was 0.19 mmol/g. 3.2. Alkylation of toluene with propylene Tables 1 and 2 summarize the conversions and the product distributions after different timeson-stream (T-O-S) over H-MCM-22/1 and HMCM-58 respectively, in the alkylation of toluene
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Table 1 Conversion and product distribution in toluene alkylation with propylene over MCM-22/1 T-O-S (min) 15
55
135
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Toluene conversion (%) Propylene conversion (%) Selectivity (vol%) Benzene Ethylbenzene p-Xylene m-Xylene o-Xylene Cumene n-Propylbenzene p-Ethyltoluene m-Ethyltoluene o-Ethyltoluene m-Cymene p-Cymene o-Cymene 1,3,5-Trimethylbenzene 1,2,4-Trimethylbenzene 1,2,3-Trimethylbenzene m-n-Propyltoluene p-n-Propyltoluene o-n-Propyltoluene Higher aromatics
8.0 81.8
8.6 87.7
9.8 88.8
10.2 88.5
6.5 0.4 5.3 1.2 0.4 0.5 0.3 1.2 1.1 0.0 51.0 21.5 3.6 0.0 1.9 1.9 0.0 1.0 0.5 1.7
2.6 0.0 1.3 0.0 0.0 0.3 0.0 0.5 0.0 0.0 59.3 24.2 4.5 0.0 1.9 1.5 0.0 0.5 0.5 2.9
1.6 0.0 0.7 0.0 0.0 0.2 0.0 0.3 0.0 0.0 60.0 25.7 4.6 0.0 1.5 1.4 0.0 0.4 0.5 3.1
1.4 0.0 0.6 0.0 0.0 0.1 0.0 0.2 0.0 0.0 59.9 26.4 4.7 0.0 1.7 1.3 0.0 0.4 0.5 2.8
RCymenesa p-Cymene m-Cymene o-Cymene
76.1 28.3 67.0 4.7
88.0 27.5 67.4 5.1
90.3 28.4 66.4 5.1
91.0 29.1 65.8 5.1
iso-/n-Propyltoluenes
50.7
98.0
100.0
101.0
T ¼ 250 °C, WHSV ¼ 10.0 h1 , ntoluene =npropylene ¼ 9:6. a Thermodynamic composition at 225 °C (para: 28.2%, meta: 56.5%, ortho: 15.3%).
with propylene at 250 °C. It can be seen that alkylation is the major reaction, leading to a mixture of isopropyltoluenes (cymenes). Moreover, this major reaction pathway is accompanied by various competitive and consecutive reactions which yield benzene, xylenes, ethyltoluenes, propyltoluenes, trimethylbenzenes and, in particular, n-propyltoluenes. It has been reported that the actual composition of the reaction products is strongly influenced not only by the reaction conditions applied, but also by the reaction volume and the pore architecture of the particular zeolite structure [12].
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Table 2 Conversion and product distribution in toluene alkylation with propylene over MCM-58 T-O-S (min) 15
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Toluene conversion (%) Propylene conversion (%) Selectivity (vol.%) Benzene Ethylbenzene p-Xylene m-Xylene o-Xylene Cumene n-Propylbenzene p-Ethyltoluene m-Ethyltoluene o-Ethyltoluene m-Cymene p-Cymene o-Cymene 1,3,5-Trimethylbenzene 1,2,4-Trimethylbenzene 1,2,3-Trimethylbenzene m-n-Propyltoluene p-n-Propyltoluene o-n-Propyltoluene Higher aromatics
10.8 73.2
10.4 78.7
10.3 81.9
10.1 82.6
15.6 0.3 6.1 7.2 4.3 2.2 0.7 1.9 2.8 0.4 31.9 13.9 2.3 0.0. 0.8 1.4 4.3 2.0 0.1 1.7
9.8 0.2 4.1 4.5 2.8 1.9 0.4 1.6 1.8 0.2 41.6 17.6 3.2 0.0 0.7 1.1 2.7 1.4 0.1 4.1
6.3 0.1 2.7 2.8 1.9 1.6 0.3 1.4 1.3 0.2 48.2 20.2 3.8 0.0 0.5 1.0 1.7 1.0 0.1 4.9
4.8 0.0 2.0 2.1 1.5 1.4 0.2 1.4 1.1 0.1 51.8 21.6 4.2 0.0 0.4 0.9 1.3 0.8 0.1 4.2
RCymenesa p-Cymene m-Cymene o-Cymene
48.1 28.8 66.4 4.9
62.4 28.2 66.6 5.2
72.3 28.0 66.7 5.3
77.6 27.9 66.8 5.4
7.5
20.0
24.9
33.7
iso-/n-Propyltoluenes
T ¼ 250 °C, WHSV ¼ 10.0 h1 , ntoluene =npropylene ¼ 9:6. a Thermodynamic composition at 225 °C (para: 28.2%, meta: 56.5%, ortho: 15.3%).
The influence of the reaction temperature (at WHSV ¼ 10 h1 and ntoluene =npropylene ¼ 9:6) on the T-O-S dependence of the conversion of toluene and the selectivities to cymenes are plotted in Fig. 5 for the three zeolite catalysts explored in the present study. Moreover, the influence of the reaction temperature on the selectivity to n-propyltoluenes and on the iso-/n-propyl toluene ratios are depicted in Fig. 6. At a reaction temperature of 200 °C, no significant differences in the catalytic activities could be observed between toluene conversions over H-MCM-22/1, H-MCM-22/2 and HMCM-58, despite their differences in the pore
Fig. 5. T-O-S dependence of toluene conversion and cymenes selectivity over MCM-22/1 (M), MCM-22/2 () and MCM-58 (); ntoluene =npropylene ¼ 9:6, WHSV ¼ 10 h1 , reaction temperature ¼ 200 °C (A,D), 250 °C (B,E) and 300 °C (C,F).
structures and in the concentration of active sites. Moreover, toluene conversion over all three catalysts was quite stable with T-O-S, and only a slight increase in cymene selectivities for both MCM-22 zeolites directly after the onset of the experiments was observed. This increase in cymene selectivities was accompanied by a decrease in the selectivities to benzene and xylenes, viz. the products from toluene conversion. With increasing reaction temperature, H-MCM-58 turned out to be more active than either of the two MCM-22 catalysts. It is anticipated here that toluene conversion over H-MCM-58 at low reaction temperature (i.e. at 200 °C) is strongly influenced by desorption and/or the mass transport effects due to the relatively bulky product molecules, viz. the propyltoluene species. This influence decreases with increasing reaction temperature. With both MCM-22-type catalysts, practically the same toluene conversions are observed, although these catalysts differ in the
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Fig. 6. T-O-S dependence of n-propyltoluenes selectivity and iso-/n-propyltoluene ratio over MCM-22/1 (M), MCM-22/2 () and MCM-58 (); ntoluene =npropylene ¼ 9:6, WHSV ¼ 10 h1 , reaction temperature ¼ 200 °C (A,D), 250 °C (B,E) and 300 °C (C,F).
concentration of bridging OH groups. These results are in a good agreement with those obtained for the ZSM-5-type zeolites possessing different nSi =nAl ratios [11]. In both cases, a variation of WHSV did not result in significant changes of conversion, which has been explained in terms of desorption and transport of products as rate controlling steps. The catalytic data obtained with H-MCM-22 obtained in the present study led us to the conclusion that desorption of cymenes plays a rate controlling role in the overall reaction scheme of toluene alkylation with propylene. At a reaction temperature of 250 °C, toluene conversions in the quasi-stationary state are again very similar, however, for short values of T-O-S, H-MCM-22/1, the sample with a higher concentration of Brønsted-acid OH groups as compared to H-MCM-22/2, yields a lower conversion which, however, increases during the first 3 h on-stream. This increase has also been reported for H-ZSM-5-
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type catalysts [12]. The selectivities to cymenes increase with increasing T-O-S and reach more than 90% for both H-MCM-22 and 80% for the large-pore H-MCM-58 sample. The lower selectivity to cymenes over H-MCM-58, in particular with increasing reaction temperature, is probably due to the relatively large intracrystalline volume in which competitive and consecutive reactions may proceed. The trends observed already at 250 °C are even more significant at a reaction temperature of 300 °C where both H-MCM-22 samples exhibit practically the same catalytic activity (toluene conversion of about 5%) while toluene conversion over H-MCM-58 is significantly higher (17% after 15 min T-O-S). The high conversion of toluene over H-MCM-58 is probably mainly due to a substantial increase in the rate of reactions, which are in competition with toluene alkylation, in particular toluene disproportionation to benzene and xylenes (cf. Table 2). Moreover, at 300 °C there is already a small but significant difference in the selectivities to cymenes for H-MCM-22/1 and H-MCM-22/2 (cf. Fig. 5E). In a manner similar to H-ZSM-5-type zeolites [11] with different nSi =nAl ratios, also for MCM-22 the selectivity to cymenes increases with decreasing concentration of active sites (i.e. bridging OH groups). Over H-MCM-58, the selectivity to cymenes is initially strongly suppressed due to competitive reactions, however, it increases with decreasing toluene conversions and longer T-O-S values (Fig. 5C): The selectivity to cymenes increases with T-O-S from only a few percent after 15 min up to almost 60% after 215 min (see Fig. 5F). This can be explained by an increasing deactivation of the catalyst and the resulting decrease in toluene conversion. The formation of n-propyltoluenes via a bimolecular transalkylation/isomerization step between cymenes and toluene [11,13,17] is practically negligible at 200 °C and a WHSV of 10 h1 with n-propyltoluene selectivities below 1%. Upon increasing the reaction temperature to 250 °C, npropyltoluenes are still formed in low amounts over both MCM-22 samples (1–2%), while for the large-pore MCM-58 zeolite, the selectivity to npropyltoluenes amounts to more than 6% after 15 min of T-O-S. A completely different situation is
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observed at 300 °C, where the product distribution is influenced to a considerable extent by competitive reactions and is most probably governed by thermodynamics. In order to obtain more detailed information on the role of the zeolites used in toluene alkylation with propylene towards the formation of n-propylbenzene a series of catalytic runs at 250 °C was performed with a variation of WHSV in the range from 10 to 200 h1 (cf. Fig. 7). Although the residence time (1/WHSV) was varied by more than one order of magnitude, the observed changes in toluene conversion were very low, and the lowest toluene conversion obtained was 8% (Fig. 7A). As both zeolites investigated in the present study
Fig. 7. WHSV dependence of (A) toluene conversion, (B) cymenes selectivity and (C) iso-/n-propyltoluene ratio over MCM-22/2 (M) and MCM-58 (); ntoluene =npropylene ¼ 9:6, reaction temperature ¼ 250 °C, T-O-S ¼ 55 min.
differ significantly in their structures it is likely that desorption and transport play a significant role in the control of the rate of the alkylation reaction studied here. It was pointed out by Corma et al. for MCM-22 [24] and Holtermann and Innes for the isostructural SSZ-25 [25] for the case of benzene alkylation with ethylene or propylene, that the alkylation reaction proceeds predominantly on the ‘‘external’’ surface of these zeolites, thus, only those sites located on the external surface contribute to the alkylation reaction. On the other hand, it is assumed that the acid sites located in the 10-membered-ring channels contribute to propylene oligomerization [25]. For all three zeolites, the initial selectivities for the formation of cymenes decrease with increasing reaction temperature. Moreover, differences in the ‘‘steady-state’’ concentrations among individual catalysts were observed mainly at higher reaction temperatures. The selectivity to cymenes decreases when either a more open zeolite structure is used and/or a higher number of bridging OH groups is present in the same zeolite topology. Both factors, in principle, can promote an increasing rate of competitive and consecutive reactions. The observed selectivities to p-cymene formed under all reaction conditions applied in the present study were very close to the thermodynamic equilibrium values, while the selectivity to m-cymene was 10% higher at the expense of the ortho-isomer. This observation probably indicates that diffusion does not significantly influence the composition of the cymene isomers over these zeolite catalysts. With increasing WHSV, the overall selectivity to cymenes for MCM-22 increases up to 97% with selectivities to the para-isomers reaching 38%. This value is significantly lower as compared to the paraselectivity in cymenes formation over ZSM-5 or even over ZSM-12 [15]. In the whole WHSV range studied here, the selectivity to cymenes for MCM58 was lower than that obtained over MCM-22 (Fig. 7B), which indicates that also other reactions beside alkylation proceed inside the channels of this zeolite. As expected, with increasing WHSV the selectivities to cymenes increase with WHSV and reach about 80% at a WHSV around 200 h1 . The conversion to xylenes over H-MCM-58 at 200 °C, with comparable selectivities to p- and
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m-xylene, are far from the thermodynamic values. This could indicate that the formation of xylenes is influenced by transition state selectivity in toluene disproportionation, favoring the formation of pand m-xylenes at the expense of o-xylene. As the diffusion coefficient of o-xylene in MCM-22 is relatively low as compared to that of benzene and toluene [26] it seems that the formation of xylenes could preferentially occur at the external surface or close to the external surface of the MCM-22-type catalysts. The investigation of the role of the zeolite structure on the selectivity to individual cymene isomers revealed that by decreasing the diameter of the channels or the windows in the sequence zeolite Y > mordenite > ZSM-12 > ðAl; FeÞZSM-5 led to a significant increase in the selectivity towards p-cymene (from 28.1% to 86.8%, calculated as pcymene selectivity in cymenes) [15]. However, a further decrease in the channel diameters in using MCM-22 did not lead to a higher selectivity to pcymene, rather the selectivity obtained with this zeolite is quite comparable to that obtained with large-pore zeolite catalysts. Thus, based on the results presented here, it can be assumed that toluene alkylation with propylene to cymenes does not proceed inside the 10-membered-ring channels of MCM-22, since a pronounced shape selectivity is lacking. Recently, moreover, kinetic experiments on benzene alkylation with propylene to cumene carried out by Corma et al. gave additional evidence that this type of alkylation reaction proceeds mainly at or close to the external surface of MCM22 zeolites. These authors proposed that the alkylation reaction proceeds via an Eley–Rideal mechanism and that the evenly structured external surface of the MWW-type catalysts, consisting of 0:7 0:7 nm pockets, probably provides an optimum surrounding for the alkylation reaction. The cited authors even assumed that, the higher the concentration of these pockets, the higher will be the activity of the catalyst [24].
3.3. Isopropyltoluene transformation into n-propyltoluene The initial proposal of a bimolecular reaction mechanism for the formation of n-propylbenzene
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or n-propyltoluenes from cumene or cymenes, respectively, was based on 13 C NMR investigations [13] using labeled benzene or cumene and kinetic experiments [11]. In addition, the results of ab initio quantum chemical calculations [16,17] supported this mechanism. It has been proposed that the formation of n-propylbenzene (n-propyltoluenes) predominantly proceeds in the threedimensional channel system of the medium-pore channel systems of zeolites of ZSM-5 and ZSM-11, while only a low concentration of n-propyltoluene isomers was obtained over large-pore zeolite Beta with its two sets of channel systems (pore diameter of 0:76 0:64 nm and 0:55 0:55 nm, respectively) which are interconnected via common intersections. In the case of zeolite Beta, a significantly larger reaction space at the channel intersections is available which decreases the probability of the formation of the proper bimolecular complex. The geometry and the architecture of the intersections in zeolite ZSM-5 was found to be a tentative optimum for its formation, and when the bimolecular transalkylation/isomerization reaction proceeds with catalysts exhibiting larger free reaction space, energetically more favored pathways are preferred which do not result in n-propyltoluene formation [16,17]. A completely different behavior was found in the case of H-MCM-22 and H-MCM-58 as catalysts. While with zeolite MCM-22 containing intersecting channels of 0:40 0:59 nm only negligible concentrations of n-propyltoluenes were observed (1.5% of n-propyltoluenes over H-MCM-22/1 at 250 °C, with iso-/n-propyltoluenes molar ratio of 50 after 15 min of T-O-S, see Table 1), significant yields of n-propyltoluenes are obtained under the same reaction conditions over the large-pore zeolite MCM-58 (viz. 6.4% of n-propyltoluenes with an iso-/n-propyltoluenes molar ratio of 7.5 for the same T-O-S value, see Table 2). This indicates that the structure of both zeolites investigated in the present study used has a significant influence on the course of the bimolecular transalkylation/isomerization reaction. With increasing WHSV, the iso-/n-propyltoluene ratio significantly increased. For MCM-22 at WHSV values above 100 h1 , the concentration of n-propyltoluenes was negligible. Moreover, the increase in the iso-/n-propyl ratio
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for MCM-58 was not as dramatic as for MCM-22 (Fig. 7C). Lawton et al. [6] reported that MCM-22 crystallizes as very thin plates, and this was confirmed for the samples prepared in the present study. From its known structure, a high concentration of pockets on the external surface with 12-membered-ring openings having an approximate depth of 0.71 nm should be present. Thus, especially these pockets are easily accessible for reactant molecules, and if a portion of the active sites is located in these pockets, the alkylation and particularly transalkylation/ isomerization can proceed inside. Due to the spacious nature of these pockets, it is hard to conceive that they supply the surrounding which is believed to be necessary for n-propyltoluene formation, as argued earlier (viz. Refs. [11,16]). No suitable steric conditions are present in these pockets to align the reacting molecules in a proper way to facilitate the energetically less favored mechanism. Recent molecular dynamics simulations performed by Sastre et al. [27] and studying the diffusion of a mixture of benzene and propylene in siliceous MCM-22 during cumene synthesis suggested that the formation of cumene probably occurs at the external surface or close to the external surface of the MCM-22 crystallites. These data are also consistent with diffusion studies done with xylene isomers over MCM-22 [24]. Our experimental results on toluene alkylation with propylene led us to a related conclusion, viz. that cymenes are probably not formed inside the 10membered-ring channels of this zeolite catalyst. From our data and from experimental data reported in the literature together with results from molecular dynamics simulations it can be inferred that toluene alkylation with propylene proceeds to a major extent at the external surface of MCM-22 zeolite sheet-like crystallites, perhaps in the large pockets located at the external surface. Thus, only a limited amount of n-propyltoluenes is formed, and no increased formation of para-substituted isomers is achieved. As for H-MCM-58 at a reaction temperature of 250 °C, the concentration of n-propyltoluenes formed is close to the values obtained with zeolite ZSM-5 under the same reaction conditions, although these two zeolite structures differ signifi-
Fig. 8. Location of the N-benzyl-1,4-diazabicyclo[2.2.2]octane cation in the undulating channel of zeolite MCM-58 (according to Ref. [8]) (A) and proposed location of 1,2-p-ditolylpropane transition state in the same zeolite (B).
cantly from each other and there do not exist any channel intersections in MCM-58. A hypothetical mechanism for n-propyltoluenes formation over zeolite MCM-58 can be inferred from the template used for the synthesis of this structure, viz. Nbenzyl-1,4-diazabicyclo[2,2,2]octane [8]. The location of this organic cation in the undulating channel is depicted in Fig. 8A. It is evident that the shape and the size of this cation are quite similar to the shape and the size of the bimolecular intermediate required for n-propyltoluenes formation. Fig. 8B presents the tentative location of a 1,2ditolylmethane-type of transition state located in the undulating channel structure of MCM-58, underlining the possibility of a preferred formation of n-propyltoluenes in toluene alkylation with propylene. This pictorial view indicates that a significant concentration of n-propyltoluenes can also be formed in one-dimensional large-pore channels when their structure enables the accommodation of the required transition state. 3.4. Adsorption of p-cymene on zeolites MCM-22 and MCM-58 To evaluate the accessibility of active sites for pcymene, as one of the major product molecules,
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the adsorption of p-cymene on MCM-22/1 and MCM-58 was followed by FTIR spectroscopy. After overnight evacuation of the zeolite, p-cymene was admitted to the high-vacuum cell at a partial pressure of 100 Pa and a temperature of 100 °C. The spectra were recorded every 30 min for a total period of 240 min at the same temperature. After that, d3 -acetonitrile was admitted to the IR cell. The results obtained with H-MCM-22/ 1 are depicted in Fig. 9 for the OH vibration region (Fig. 9A) and for the region of p-cymene vibrations (Fig. 9B). It is clearly seen that p-cymene interacts with bridging OH groups of MCM-22/1 which results in a decrease in the intensity of the absorption band at 3618 cm1 . About 50% of these OH groups interact with p-cymene even after an adsorption time of 240 min. Hence, approximately the same concentration of OH groups is, under the conditions applied here, not accessible for p-cymene molecules. It should be stressed that all remaining free OH groups are still accessible for d3 -acetonitrile. This could indicate that the sites actively involved in the toluene alkylation are mainly located on the external surface in the 12membered-ring pockets or close to the external surface. Experiments in which of di-tert-butylpyridine was adsorbed on MCM-22 led others to the conclusion that about 10% of acid sites are accessible for this bulky probe molecule [28]. For MCM-22/1 this concentration was found to be
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Fig. 10. FTIR spectra of p-cymene adsorption over MCM-58 (parent, 1; after 30 min, 2; after 240 min, 3), at 100 °C, 100 Pa of p-cymene.
even lower than 10% [29]. It is without any doubt that the accessibility of acid sites for the smaller pcymene should be higher than these 10%. We assume that also acid sites close to the entrances of the 10-membered-ring channels can be accessed by p-cymene but not by di-tert-butylpyridine. In the case of H-MCM-58, two absorption bands of bridging OH groups were found with maxima at 3625 and 3490 cm1 (Fig. 10). While the OH groups at 3490 cm1 interact only weakly with p-cymene, the intensity of the absorption band at 3625 cm1 is reduced to a minimum. Thus, in contrast to H-MCM-22/1, all bridging OH groups of H-MCM-58 represented by the absorption maximum at 3625 cm1 in the IR spectra are occupied by adsorbed p-cymene.
4. Conclusions
Fig. 9. FTIR spectra of p-cymene adsorption over MCM-22/1 (parent, 1; after 30 min, 2; after 240 min, 3), at 100 °C, 100 Pa of p-cymene.
The alkylation of toluene with propylene was investigated over acidic forms of the aluminosilicates MCM-22 and MCM-58. It has been found that the structure of these zeolite catalysts governs to a certain extent the reaction pathway and in particular controls the course of the bimolecular transalkylation/isomerization reaction leading to n-propyltoluenes as products. In the case of zeolite MCM-22, the alkylation reaction seems to preferentially proceed at or close
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to the external surface of the zeolite crystallites. Hence, the composition of the reaction products is not influenced to a considerable extent by product shape selectivity effects. Only negligible concentrations of n-propyltoluenes are observed over MCM-22, which can be attributed to the absence of the proper steric environment required for the formation of the corresponding bimolecular transition state. In contrast to zeolite MCM-22, n-propyltoluenes are formed in significant concentrations in the one-dimensional 12-membered-ring channels of zeolite MCM-58 at 250 °C. This is probably due to their peculiar undulating shape. The formation of 1,2-p-ditolylpropane seems to be supported by the pore architecture of zeolite MCM-58, since this intermediate has a similar molecular shape as the N-benzyl-1,4-diazabicyclo[2.2.2]octane template which is used for the synthesis of the IFR topology. With both zeolite structures used in the present investigation, no enhanced p-selectivity among the cymene isomers was observed. Moreover, the particular pore geometry of zeolite MCM-58 seems to promote 1,2-ditolylpropane-type transition states which are prerequisites for the formation of npropyltoluenes, while with MCM-22 as catalyst, no n-propyltoluenes are formed. The results obtained with FTIR spectroscopy using adsorbed p-cymene on MCM-22/1 suggest that a significant part of the acidic OH groups is not accessible for the relatively large p-cymene molecules, while d3 -acetonitrile molecules do have access to all remaining Brønsted-acid sites. It is suggested that this observation also indicates that the formation of cymenes should preferentially occur over active sites located at or close to the external surface of the MCM-22 crystallites.
Acknowledgements Financial support of the Grant Agency of the Academy of Sciences of the Czech Republic (A4040001) and Volkswagen-Stiftung (I/75 886) is highly acknowledged. The work of J.K. was supported by a postdoctoral grant from the Grant
Agency of the Czech Republic (203/00/P033). S.E. and A.W. gratefully acknowledge financial support by Deutsche Forschungsgemeinschaft and Fonds der Chemischen Industrie.
References [1] H.G. Franck, J.W. Stadelhofer, Industrial Aromatic Chemistry, Springer, Berlin, 1988, 488 pp. [2] K. Tanabe, W.F. H€ olderich, Appl. Catal. A: General 181 (1999) 399. ejka, B. Wichterlova, Catal. Rev. -Sci. Engng., in press. [3] J.C [4] G.R. Meima, Cattech 3 (1998) 5. [5] C. Perego, S. Amarilli, A. Carati, C. Flego, G. Pazzuconi, C. Rizzo, G. Bellussi, Micropor. Mesopor. Mater. 27 (1999) 345. [6] S.L. Lawton, M.E. Leonowicz, R.D. Patridge, P. Chu, M.K. Rubin, Micropor. Mesopor. Mater. 23 (1998) 109. [7] M.A. Camblor, A. Corma, L.A. Villaescusa, Chem. Commun. (1997) 749. [8] C.Y. Chen, L.W. Finger, R.C. Medrud, C.L. Kibby, P.A. Crozier, I.Y. Chan, T.V. Harris, L.W. Beck, S.I. Zones, Chem. Eur. J. 4 (1998) 1312. [9] J.M. Newsam, M.M.J. Tracy, W.T. Koetsier, C.B. de Gruyter, Proc. Roy. Soc. London A 420 (1988) 375. [10] D. Fraenkel, M. Levy, J. Catal. 118 (1989) 10. ejka, J. Catal. 136 (1994) 523. [11] B. Wichterlova, J. C ejka, G.I. Kapustin, B. Wichterlova, Appl. Catal. A: [12] J. C General 108 (1994) 187. [13] I.I. Ivanova, D. Brunel, J.B. Nagy, G. Daelen, E.G. Derouane, in: M. Guisnet, J. Barbier, J. Barrault, C. Bouchoule, D. Duprez, G. Perot, C. Montassier (Eds.), Heterogeneous Catalysis and Fine Chemicals III, Studies in Surface Science and Catalysis, vol. 78, Elsevier, Amsterdam, 1993, p. 587. [14] I.I. Ivanova, D. Brunel, J.B. Nagy, E.G. Derouane, J. Mol. Catal. A Chemical 95 (1995) 243. ejka, N. Z ilkova, Micropor. Mater. 6 [15] B. Wichterlova, J. C (1996) 405. poner, J. S poner, J. C ejka, B. Wichterlova, J. Phys. [16] J. S Chem. B 102 (1998) 7169. ejka, N. Z poner, B. Wichterlova, Collect. ilkova, J.E. S [17] J. C Czech. Chem. Commun. 63 (1998) 1769. [18] A. Corma, C. Corell, J. Perez-Pariente, Zeolites 15 (1995) 2. [19] S. Ernst, M. Hunger, J. Weitkamp, Chem. -Ing. -Tech. 6 (1997) 77. [20] B. Wichterlova, Z. Tvar uzkova, Z. Sobalık, P. Sarv, Micropor. Mesopor. Mater. 24 (1998) 223. [21] S. Ernst, in: H.G. Karge, J. Weitkamp (Eds.), Molecular Sieves––Science and Technology, vol. 1, Springer, Berlin, 1998, p. 65. [22] A. Corma, C. Corell, V. Fornes, W. Kolodziejski, J. PerezPariente, Zeolites 15 (1995) 576.
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[27] G. Sastre, C.R.A. Catlow, A. Corma, J. Phys. Chem. B 103 (1999) 5187. [28] A. Corma, V. Fornes, L. Forni, F. Marquez, J. MartınezTriquero, D. Mascotti, J. Catal. 179 (1998) 451. ejka, J. Dedecek, J. Kotrla, M. Tudor, N. Z ilkova, S. [29] J. C Ernst, in: A. Galarneau, F. di Renzo, F. Fajula, J. Vedrine (Eds.), Zeolites and Mesoporous Materials at the Dawn of the 21st Century, Studies in Surface Science and Catalysis, vol. 135, Elsevier, Amsterdam, 2001, p. 14-P-21.
Activity and selectivity of zeolites MCM-22 and MCM-58 in the alkylation of toluene with propylene ejka Jirı C
a,*
ilkov , Andrea Krejcı a, Nadezda Z a a, Josef Kotrla a, Stefan Ernst b, Astrid Weber b
a
b
J. Heyrovsk y Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejskova 3, CZ-182 23 Prague 8, Czech Republic Department of Chemistry, Chemical Technology, University of Kaiserslautern, Erwin Schr€odinger Strasse 54, D-67663 Kaiserslautern, Germany Received 16 March 2001; received in revised form 20 November 2001; accepted 11 February 2002
Abstract The influence of the pore architecture of zeolites MCM-22 and MCM-58 on their catalytic activity and selectivity in the gas-phase alkylation of toluene with propylene has been investigated. The results obtained with MCM-22 as catalyst revealed that, despite the expectations due to the presence of 10-membered-ring pores in the structure of this zeolite, no enhanced selectivity to p-cymene is observed. Moreover, also n-propyltoluenes, which are usually formed over threedimensional 10-membered-ring zeolites (e.g., ZSM-5 and ZSM-11) via bimolecular transalkylation/isomerization reactions of cymenes with toluene, are only formed to a negligible extent. This has been rationalized by the assumption that most of the reactions occurring in this system take place on acid sites at or close to the external surface. This is supported by the FTIR-spectroscopic observation that p-cymene can reach only about one half of the bridging OH groups, while the smaller d3 -acetonitrile reaches/covers virtually all remaining Brønsted and Lewis acid sites. Over zeolite MCM-58, relatively high selectivities to n-propyltoluenes were observed, which has been ascribed to the peculiar undulating 12-membered-ring channel system of this zeolite. It is proposed that the 1,2-p-ditolylpropane-type transition state required for the formation of n-propyltoluenes is preferentially located in positions similar to those which are occupied by the N-benzyl-1,4-diazabicyclo[2.2.2]octane cations used as templates for the synthesis of zeolite MCM58. Ó 2002 Elsevier Science Inc. All rights reserved. Keywords: Toluene alkylation; Zeolites; MCM-22; MCM-58; Acid sites
1. Introduction
*
Corresponding author. Tel.: +42-2-6605-3795; fax: +42-28658-2307. ejka). E-mail address: [email protected] (J. C
The alkylation of aromatic hydrocarbons with olefins is successfully employed on a large scale in the chemical industry [1–3]. Products from such processes, in particular ethylbenzene, isopropylbenzene (cumene), p-di-iso-propylbenzene and isopropyltoluenes (cymenes) are well known as
1387-1811/02/$ - see front matter Ó 2002 Elsevier Science Inc. All rights reserved. PII: S 1 3 8 7 - 1 8 1 1 ( 0 2 ) 0 0 3 3 2 - 3
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important chemical intermediates. The classical catalysts for aromatics alkylation, viz. HF, aluminum trichloride with HCl or solid phosphoric acid exhibit many problems with respect to their handling, safety, corrosion and waste disposal. To overcome these drawbacks, zeolite-based solid acids have been successfully introduced as catalysts in the alkylation of benzene with ethylene (ethylbenzene synthesis). Moreover, several new liquidphase processes for cumene production have been announced so far or are even already commercialized by Dow Chemical, Mobil, CDTECH, UOP and Enichem in processes which are based on different acid zeolites as catalysts (viz., dealuminated mordenite, MCM-22, zeolite Y and Beta) [4,5]. The present study focuses on the alkylation of toluene with propylene to cymenes. As catalysts for this reaction, two relatively new zeolites with peculiar pore architectures, viz. MCM-22 and MCM58, were explored. In this context, the mechanism for n-propyltoluene formation over these two zeolite catalysts and the influence of their pore structures was of particular interest. The structure of zeolite MCM-22 (IZA structure code MWW) consists of two independent pore systems. One of them is defined by two-dimensional sinusoidal 10membered-ring channels (0:40 0:59 nm). The second pore system consists of large supercages with a free inner diameter of 0.71 nm which is circumscribed by 12-membered rings. The height of the large supercages amounts to 1.82 nm. These huge intracrystalline voids are accessible through 10-membered-ring apertures (0:40 0:54 nm) only [6]. The structure of MCM-58 (isostructural with SSZ-42 and ITQ-4; IZA structure code IFR [7]) is characterized by undulating one-dimensional 12membered-ring channels [8]. The pore diameter at the narrowest point is 0.64 nm, and the cage at the widest point measures 1.0 nm. Moreover, the channels exhibit side pockets similar to those observed in zeolite Beta [9]. The reaction mechanism for the alkylation of benzene or toluene with propylene (or, alternatively, with isopropyl alcohol) over ZSM-5-type catalysts has been investigated by several groups [10–15]. It follows from these investigations that the formation of n-propyltoluene (from the
reaction of toluene with propylene) proceeds essentially in two steps, viz. the formation of isopropyltoluene followed by its consecutive conversion to n-propyltoluene. Although the occurrence of the first reaction step has been widely accepted, the mechanism of n-propyltoluene formation from the primary alkylation product is still a matter of debate. Originally, it has been proposed for the analogous case of benzene alkylation with propylene that n-propylbenzene is formed from cumene via a monomolecular isomerization step [10]. However, further investigations using kinetic experiments [11] and 13 C NMR studies with labeled benzene or cumene [13,14] revealed that the formation of n-propylbenzene (or n-propyltoluenes, respectively) proceeds via a bimolecular transalkylation/isomerization reaction. Moreover, experimental evidence has been presented which suggests that there is also a selectivity-directing effect of the particular zeolite structure [15]. In order to obtain further evidence for the reaction pathway described above, the validity of the proposed bimolecular mechanism was also explored by theoretical methods using ab initio quantum chemical calculations [16,17]. These calculations were performed at the Hartree–Fock level of theory for full relaxation of all geometric parameters and imposing selected geometric constraints which could simulate the situation occurring at the channel intersections in ZSM-5- and ZSM-11-type zeolites. The results from the calculations using the constrained model suggest the existence of a reaction pathway via an anti-Markovnikov-type of proton addition for the formation of n-propylbenzene. This is in principal agreement with the experimental findings published so far. Both experimental and theoretical results suggest that, for the formation of bimolecular intermediates leading to n-propylbenzene or n-propyltoluenes, respectively, a pore architecture with a three-dimensionally interconnected channel system and pore diameters of about 0.55 nm are required, in which large intracrystalline voids are absent. These structural features seem to be prerequisites for allowing translational and rotational movements of both reactants only. In view of this, benzene or toluene alkylation with propylene or isopropyl alcohol, beside its industrial significance, could
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also be exploited to characterize the reaction space available in the intracrystalline voids of microporous molecular sieves.
2. Experimental Section Zeolite MCM-22 was synthesized according to a method published by Corma et al. [18] using hexamethyleneimine as organic template. In a typical procedure, a given amount of sodium aluminate (0.6–1.0 g, Riedel-de Ha€en, 55% of Al2 O3 , 45% of Na2 O) is added to a solution of 0.6 g of sodium hydroxide diluted in 125 g of distilled water and stirred until dissolved. Then, 8.6 g of hexamethyleneimine (Fluka) are added, and the resulting mixture is thoroughly mixed for at least 15 min. Finally, 9.2 g of Cab-O-Sil M5 (Cabot) are added, and the resulting mixture is homogenized for another 60 min. The crystallization was conducted in Teflon-lined stainless-steel autoclaves under autogenous pressure at 155 °C for 8 days under agitation. After the synthesis, the autoclaves were cooled down to room temperature by quenching in water. Then, the solid product was recovered by filtration, thoroughly washed with deionized water and finally dried at 100 °C overnight. The as-synthesized samples were calcined in air at 580 °C for 6 h. Two MCM-22 samples with different molar ratios nSi =nAl (MCM-22/1: 13.6 and MCM-22/2: 18.7) were investigated. Zeolite MCM-58 (nSi =nAl ¼ 19) was synthesized and calcined using procedures described recently [19]. The ammonium forms of both zeolites were prepared by repeated ion exchange in 0.5 M aqueous solutions of ammonium nitrate at ambient temperature. All as-synthesized, calcined and ion-exchanged zeolites were checked for their crystallinity and phase purity by X-ray powder diffraction on a Siemens D5005 diffractometer equipped with graphite monochromator and scintillation counter and using CuKa radiation in a Bragg–Brentano geometry. The concentrations of Brønsted and Lewis sites in MCM-22 and MCM-58 were determined after the adsorption of d3 -acetonitrile followed by FTIR spectroscopy using a Nicolet FTIR Protege 460
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spectrometer. For this purpose, the zeolite powders were pressed binder-free into self-supporting wafers with a density of 4.0–11.0 mg/cm2 . d3 Acetonitrile was degassed by repeated freezing and thawing cycles before its use. Prior to the adsorption of d3 -acetonitrile, the zeolites were activated in situ by overnight evacuation at 400 °C. All measured spectra were recalculated to a normalized wafer thickness of 10 mg/cm2 . For a quantitative characterization of the Brønsted acid sites (B), the CBN-B vibration at 2296 cm1 was used with an extinction coefficient of eðBÞ ¼ 2:05 0:1 cm lmol1 . For a quantitative evaluation of the Lewis acid sites (L), the CBN-L vibration at 2323 cm1 was used with an extinction coefficient of eðLÞ ¼ 3:6 0:1 cm lmol1 [20]. A similar procedure was employed to determine the accessibility of bridging OH groups of zeolites MCM-22 and MCM-58 for p-cymene molecules. Prior to its use, p-cymene was degassed by repeated freezing and thawing cycles. The adsorption of p-cymene was performed for up to 240 min at 100 °C with a partial pressure of p-cymene of 100 Pa. Afterwards, the zeolites were evacuated at 100 °C for 30 min, where upon d3 -acetonitrile was admitted to study the accessibility of the remaining free bridging OH groups. The measured spectra were again normalized to a wafer thickness of 10 mg/cm2 . The alkylation of toluene with propylene was performed under atmospheric pressure in a downflow glass microreactor (inner diameter: 10 mm, weight of catalyst: 0.1–1.5 g, WHSV ¼ 10–200 h1 ) in the temperature range from 200 to 300 °C. The particle size of the binder-free catalyst was in the range from 0.3 to 0.6 mm. The pellets were prepared by pressing the zeolite powders followed by crushing and sieving. All catalysts were activated at 450 °C in a stream of pure hydrogen for 1 h. For the catalytic experiments, a hydrogen stream containing toluene with a partial pressure of 20.8 kPa was mixed with a stream of pure propylene to adjust a molar ratio ntoluene =npropylene of 9.6. The reaction products were analyzed using an on-line gas chromatograph (Hewlett–Packard 5890 Series II) equipped with a Supelcowax 10 capillary column (30 m length, inner diameter of 0.2 mm, thickness of stationary phase of 0.2 lm) and coupled to a
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flame ionization and mass-spectrometric detector (Hewlett–Packard 5971A), respectively. 3. Results and discussion 3.1. Characterization of the zeolites used The X-ray powder patterns of as-synthesized and calcined samples of zeolites MCM-22 and MCM-58 are depicted in Figs. 1 and 2, respectively. From a comparison with the literature data [21] it can be concluded that both samples have a good crystallinity and are phase pure. The concentration of Brønsted and Lewis acid sites was determined from the adsorption of d3 -
Fig. 1. X-ray powder diffraction patterns of (A) as-synthesized and (B) calcined MCM-22/1.
Fig. 2. X-ray powder diffraction patterns of (A) as-synthesized and (B) calcined MCM-58.
acetonitrile onto the activated zeolites followed by IR spectroscopy. Figs. 3 and 4 present the IR spectra of the hydroxyl group region and the region of CBN vibrations of d3 -acetonitrile for MCM-22/1 and MCM-58 before and after acetonitrile adsorption. In the case of MCM-22 four infrared bands were found in the hydroxyl region. The absorption band at 3742 cm1 belongs to the vibrations of terminal Si–OH groups, while the band at 3730 cm1 can probably be assigned to perturbed Si–OH groups interacting with other hydroxyl groups. The most intensive band at 3620 cm1 represents vibrations of acid bridging
Fig. 3. FTIR spectra of MCM-22 before (1) and after (2) adsorption of d3 -acetonitrile: (A) hydroxyl group region and (B) region of nitrile vibrations of d3 -acetonitrile.
Fig. 4. FTIR spectra of MCM-58 before (1) and (2) after adsorption of d3 -acetonitrile: (A) hydroxyl group region and (B) region of nitrile vibrations of d3 -acetonitrile.
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Si–OH–Al groups, and a small band at 3670 cm1 is usually assigned to Al–OH groups which can be present in some zeolites possessing extra-framework aluminum species. In contrast to the results of Corma et al. [22] we did not find a broad band at 3500 cm1 assigned to vibrations of internal silanol groups. The infrared spectrum of adsorbed d3 -acetonitrile consists of two typical bands due to the acetonitrile adsorption on acid Si–OH–Al groups at 2296 cm1 and on Al-Lewis sites at 2323 cm1 (Fig. 3). The concentrations of Brønsted and Lewis sites determined from d3 -acetonitrile adsorption using the extinction coefficients reported in [20] were 0.34 mmol/g for the Brønsted sites and 0.42 mmol/g for the Lewis sites (MCM-22/1, Si/ Al ¼ 13.6) and 0.27 mmol/g of Brønsted sites and 0.33 mmol/g of Lewis sites (MCM-22/2, Si/ Al ¼ 18.7), respectively. The hydroxyl region of MCM-58 contained an absorption band of terminal Si–OH groups at 3745 cm1 , two bands of bridging OH groups at 3630 and 3485 cm1 (their assignment is not yet clear) and a band at 3690 cm1 which probably belongs again to Al–OH groups. In addition, a new band appeared at 3780 cm1 . This band has already been observed for zeolite Beta and ascribed to OH groups bound to framework Al with a defective structure [23]. In contrast to two absorption bands of adsorbed acetonitrile over MCM-22, in the case of MCM-58 an additional band at 2276 cm1 was found. This band is usually assigned to d3 -acetonitrile in interaction with silanol Si–OH groups, which is in agreement with the decrease in the intensity of this band after acetonitrile adsorption (Fig. 4). While this band remained almost unchanged even after evacuation at 100 °C, it is inferred that this acetonitrile is caught in the cages of the undulating channel system of MCM-58 zeolite. The concentration of Brønsted sites of zeolite MCM-58 (Si/Al ¼ 19) was 0.20 mmol/g while the concentration of Lewis sites was 0.19 mmol/g. 3.2. Alkylation of toluene with propylene Tables 1 and 2 summarize the conversions and the product distributions after different timeson-stream (T-O-S) over H-MCM-22/1 and HMCM-58 respectively, in the alkylation of toluene
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Table 1 Conversion and product distribution in toluene alkylation with propylene over MCM-22/1 T-O-S (min) 15
55
135
215
Toluene conversion (%) Propylene conversion (%) Selectivity (vol%) Benzene Ethylbenzene p-Xylene m-Xylene o-Xylene Cumene n-Propylbenzene p-Ethyltoluene m-Ethyltoluene o-Ethyltoluene m-Cymene p-Cymene o-Cymene 1,3,5-Trimethylbenzene 1,2,4-Trimethylbenzene 1,2,3-Trimethylbenzene m-n-Propyltoluene p-n-Propyltoluene o-n-Propyltoluene Higher aromatics
8.0 81.8
8.6 87.7
9.8 88.8
10.2 88.5
6.5 0.4 5.3 1.2 0.4 0.5 0.3 1.2 1.1 0.0 51.0 21.5 3.6 0.0 1.9 1.9 0.0 1.0 0.5 1.7
2.6 0.0 1.3 0.0 0.0 0.3 0.0 0.5 0.0 0.0 59.3 24.2 4.5 0.0 1.9 1.5 0.0 0.5 0.5 2.9
1.6 0.0 0.7 0.0 0.0 0.2 0.0 0.3 0.0 0.0 60.0 25.7 4.6 0.0 1.5 1.4 0.0 0.4 0.5 3.1
1.4 0.0 0.6 0.0 0.0 0.1 0.0 0.2 0.0 0.0 59.9 26.4 4.7 0.0 1.7 1.3 0.0 0.4 0.5 2.8
RCymenesa p-Cymene m-Cymene o-Cymene
76.1 28.3 67.0 4.7
88.0 27.5 67.4 5.1
90.3 28.4 66.4 5.1
91.0 29.1 65.8 5.1
iso-/n-Propyltoluenes
50.7
98.0
100.0
101.0
T ¼ 250 °C, WHSV ¼ 10.0 h1 , ntoluene =npropylene ¼ 9:6. a Thermodynamic composition at 225 °C (para: 28.2%, meta: 56.5%, ortho: 15.3%).
with propylene at 250 °C. It can be seen that alkylation is the major reaction, leading to a mixture of isopropyltoluenes (cymenes). Moreover, this major reaction pathway is accompanied by various competitive and consecutive reactions which yield benzene, xylenes, ethyltoluenes, propyltoluenes, trimethylbenzenes and, in particular, n-propyltoluenes. It has been reported that the actual composition of the reaction products is strongly influenced not only by the reaction conditions applied, but also by the reaction volume and the pore architecture of the particular zeolite structure [12].
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Table 2 Conversion and product distribution in toluene alkylation with propylene over MCM-58 T-O-S (min) 15
55
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Toluene conversion (%) Propylene conversion (%) Selectivity (vol.%) Benzene Ethylbenzene p-Xylene m-Xylene o-Xylene Cumene n-Propylbenzene p-Ethyltoluene m-Ethyltoluene o-Ethyltoluene m-Cymene p-Cymene o-Cymene 1,3,5-Trimethylbenzene 1,2,4-Trimethylbenzene 1,2,3-Trimethylbenzene m-n-Propyltoluene p-n-Propyltoluene o-n-Propyltoluene Higher aromatics
10.8 73.2
10.4 78.7
10.3 81.9
10.1 82.6
15.6 0.3 6.1 7.2 4.3 2.2 0.7 1.9 2.8 0.4 31.9 13.9 2.3 0.0. 0.8 1.4 4.3 2.0 0.1 1.7
9.8 0.2 4.1 4.5 2.8 1.9 0.4 1.6 1.8 0.2 41.6 17.6 3.2 0.0 0.7 1.1 2.7 1.4 0.1 4.1
6.3 0.1 2.7 2.8 1.9 1.6 0.3 1.4 1.3 0.2 48.2 20.2 3.8 0.0 0.5 1.0 1.7 1.0 0.1 4.9
4.8 0.0 2.0 2.1 1.5 1.4 0.2 1.4 1.1 0.1 51.8 21.6 4.2 0.0 0.4 0.9 1.3 0.8 0.1 4.2
RCymenesa p-Cymene m-Cymene o-Cymene
48.1 28.8 66.4 4.9
62.4 28.2 66.6 5.2
72.3 28.0 66.7 5.3
77.6 27.9 66.8 5.4
7.5
20.0
24.9
33.7
iso-/n-Propyltoluenes
T ¼ 250 °C, WHSV ¼ 10.0 h1 , ntoluene =npropylene ¼ 9:6. a Thermodynamic composition at 225 °C (para: 28.2%, meta: 56.5%, ortho: 15.3%).
The influence of the reaction temperature (at WHSV ¼ 10 h1 and ntoluene =npropylene ¼ 9:6) on the T-O-S dependence of the conversion of toluene and the selectivities to cymenes are plotted in Fig. 5 for the three zeolite catalysts explored in the present study. Moreover, the influence of the reaction temperature on the selectivity to n-propyltoluenes and on the iso-/n-propyl toluene ratios are depicted in Fig. 6. At a reaction temperature of 200 °C, no significant differences in the catalytic activities could be observed between toluene conversions over H-MCM-22/1, H-MCM-22/2 and HMCM-58, despite their differences in the pore
Fig. 5. T-O-S dependence of toluene conversion and cymenes selectivity over MCM-22/1 (M), MCM-22/2 () and MCM-58 (); ntoluene =npropylene ¼ 9:6, WHSV ¼ 10 h1 , reaction temperature ¼ 200 °C (A,D), 250 °C (B,E) and 300 °C (C,F).
structures and in the concentration of active sites. Moreover, toluene conversion over all three catalysts was quite stable with T-O-S, and only a slight increase in cymene selectivities for both MCM-22 zeolites directly after the onset of the experiments was observed. This increase in cymene selectivities was accompanied by a decrease in the selectivities to benzene and xylenes, viz. the products from toluene conversion. With increasing reaction temperature, H-MCM-58 turned out to be more active than either of the two MCM-22 catalysts. It is anticipated here that toluene conversion over H-MCM-58 at low reaction temperature (i.e. at 200 °C) is strongly influenced by desorption and/or the mass transport effects due to the relatively bulky product molecules, viz. the propyltoluene species. This influence decreases with increasing reaction temperature. With both MCM-22-type catalysts, practically the same toluene conversions are observed, although these catalysts differ in the
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Fig. 6. T-O-S dependence of n-propyltoluenes selectivity and iso-/n-propyltoluene ratio over MCM-22/1 (M), MCM-22/2 () and MCM-58 (); ntoluene =npropylene ¼ 9:6, WHSV ¼ 10 h1 , reaction temperature ¼ 200 °C (A,D), 250 °C (B,E) and 300 °C (C,F).
concentration of bridging OH groups. These results are in a good agreement with those obtained for the ZSM-5-type zeolites possessing different nSi =nAl ratios [11]. In both cases, a variation of WHSV did not result in significant changes of conversion, which has been explained in terms of desorption and transport of products as rate controlling steps. The catalytic data obtained with H-MCM-22 obtained in the present study led us to the conclusion that desorption of cymenes plays a rate controlling role in the overall reaction scheme of toluene alkylation with propylene. At a reaction temperature of 250 °C, toluene conversions in the quasi-stationary state are again very similar, however, for short values of T-O-S, H-MCM-22/1, the sample with a higher concentration of Brønsted-acid OH groups as compared to H-MCM-22/2, yields a lower conversion which, however, increases during the first 3 h on-stream. This increase has also been reported for H-ZSM-5-
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type catalysts [12]. The selectivities to cymenes increase with increasing T-O-S and reach more than 90% for both H-MCM-22 and 80% for the large-pore H-MCM-58 sample. The lower selectivity to cymenes over H-MCM-58, in particular with increasing reaction temperature, is probably due to the relatively large intracrystalline volume in which competitive and consecutive reactions may proceed. The trends observed already at 250 °C are even more significant at a reaction temperature of 300 °C where both H-MCM-22 samples exhibit practically the same catalytic activity (toluene conversion of about 5%) while toluene conversion over H-MCM-58 is significantly higher (17% after 15 min T-O-S). The high conversion of toluene over H-MCM-58 is probably mainly due to a substantial increase in the rate of reactions, which are in competition with toluene alkylation, in particular toluene disproportionation to benzene and xylenes (cf. Table 2). Moreover, at 300 °C there is already a small but significant difference in the selectivities to cymenes for H-MCM-22/1 and H-MCM-22/2 (cf. Fig. 5E). In a manner similar to H-ZSM-5-type zeolites [11] with different nSi =nAl ratios, also for MCM-22 the selectivity to cymenes increases with decreasing concentration of active sites (i.e. bridging OH groups). Over H-MCM-58, the selectivity to cymenes is initially strongly suppressed due to competitive reactions, however, it increases with decreasing toluene conversions and longer T-O-S values (Fig. 5C): The selectivity to cymenes increases with T-O-S from only a few percent after 15 min up to almost 60% after 215 min (see Fig. 5F). This can be explained by an increasing deactivation of the catalyst and the resulting decrease in toluene conversion. The formation of n-propyltoluenes via a bimolecular transalkylation/isomerization step between cymenes and toluene [11,13,17] is practically negligible at 200 °C and a WHSV of 10 h1 with n-propyltoluene selectivities below 1%. Upon increasing the reaction temperature to 250 °C, npropyltoluenes are still formed in low amounts over both MCM-22 samples (1–2%), while for the large-pore MCM-58 zeolite, the selectivity to npropyltoluenes amounts to more than 6% after 15 min of T-O-S. A completely different situation is
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observed at 300 °C, where the product distribution is influenced to a considerable extent by competitive reactions and is most probably governed by thermodynamics. In order to obtain more detailed information on the role of the zeolites used in toluene alkylation with propylene towards the formation of n-propylbenzene a series of catalytic runs at 250 °C was performed with a variation of WHSV in the range from 10 to 200 h1 (cf. Fig. 7). Although the residence time (1/WHSV) was varied by more than one order of magnitude, the observed changes in toluene conversion were very low, and the lowest toluene conversion obtained was 8% (Fig. 7A). As both zeolites investigated in the present study
Fig. 7. WHSV dependence of (A) toluene conversion, (B) cymenes selectivity and (C) iso-/n-propyltoluene ratio over MCM-22/2 (M) and MCM-58 (); ntoluene =npropylene ¼ 9:6, reaction temperature ¼ 250 °C, T-O-S ¼ 55 min.
differ significantly in their structures it is likely that desorption and transport play a significant role in the control of the rate of the alkylation reaction studied here. It was pointed out by Corma et al. for MCM-22 [24] and Holtermann and Innes for the isostructural SSZ-25 [25] for the case of benzene alkylation with ethylene or propylene, that the alkylation reaction proceeds predominantly on the ‘‘external’’ surface of these zeolites, thus, only those sites located on the external surface contribute to the alkylation reaction. On the other hand, it is assumed that the acid sites located in the 10-membered-ring channels contribute to propylene oligomerization [25]. For all three zeolites, the initial selectivities for the formation of cymenes decrease with increasing reaction temperature. Moreover, differences in the ‘‘steady-state’’ concentrations among individual catalysts were observed mainly at higher reaction temperatures. The selectivity to cymenes decreases when either a more open zeolite structure is used and/or a higher number of bridging OH groups is present in the same zeolite topology. Both factors, in principle, can promote an increasing rate of competitive and consecutive reactions. The observed selectivities to p-cymene formed under all reaction conditions applied in the present study were very close to the thermodynamic equilibrium values, while the selectivity to m-cymene was 10% higher at the expense of the ortho-isomer. This observation probably indicates that diffusion does not significantly influence the composition of the cymene isomers over these zeolite catalysts. With increasing WHSV, the overall selectivity to cymenes for MCM-22 increases up to 97% with selectivities to the para-isomers reaching 38%. This value is significantly lower as compared to the paraselectivity in cymenes formation over ZSM-5 or even over ZSM-12 [15]. In the whole WHSV range studied here, the selectivity to cymenes for MCM58 was lower than that obtained over MCM-22 (Fig. 7B), which indicates that also other reactions beside alkylation proceed inside the channels of this zeolite. As expected, with increasing WHSV the selectivities to cymenes increase with WHSV and reach about 80% at a WHSV around 200 h1 . The conversion to xylenes over H-MCM-58 at 200 °C, with comparable selectivities to p- and
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m-xylene, are far from the thermodynamic values. This could indicate that the formation of xylenes is influenced by transition state selectivity in toluene disproportionation, favoring the formation of pand m-xylenes at the expense of o-xylene. As the diffusion coefficient of o-xylene in MCM-22 is relatively low as compared to that of benzene and toluene [26] it seems that the formation of xylenes could preferentially occur at the external surface or close to the external surface of the MCM-22-type catalysts. The investigation of the role of the zeolite structure on the selectivity to individual cymene isomers revealed that by decreasing the diameter of the channels or the windows in the sequence zeolite Y > mordenite > ZSM-12 > ðAl; FeÞZSM-5 led to a significant increase in the selectivity towards p-cymene (from 28.1% to 86.8%, calculated as pcymene selectivity in cymenes) [15]. However, a further decrease in the channel diameters in using MCM-22 did not lead to a higher selectivity to pcymene, rather the selectivity obtained with this zeolite is quite comparable to that obtained with large-pore zeolite catalysts. Thus, based on the results presented here, it can be assumed that toluene alkylation with propylene to cymenes does not proceed inside the 10-membered-ring channels of MCM-22, since a pronounced shape selectivity is lacking. Recently, moreover, kinetic experiments on benzene alkylation with propylene to cumene carried out by Corma et al. gave additional evidence that this type of alkylation reaction proceeds mainly at or close to the external surface of MCM22 zeolites. These authors proposed that the alkylation reaction proceeds via an Eley–Rideal mechanism and that the evenly structured external surface of the MWW-type catalysts, consisting of 0:7 0:7 nm pockets, probably provides an optimum surrounding for the alkylation reaction. The cited authors even assumed that, the higher the concentration of these pockets, the higher will be the activity of the catalyst [24].
3.3. Isopropyltoluene transformation into n-propyltoluene The initial proposal of a bimolecular reaction mechanism for the formation of n-propylbenzene
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or n-propyltoluenes from cumene or cymenes, respectively, was based on 13 C NMR investigations [13] using labeled benzene or cumene and kinetic experiments [11]. In addition, the results of ab initio quantum chemical calculations [16,17] supported this mechanism. It has been proposed that the formation of n-propylbenzene (n-propyltoluenes) predominantly proceeds in the threedimensional channel system of the medium-pore channel systems of zeolites of ZSM-5 and ZSM-11, while only a low concentration of n-propyltoluene isomers was obtained over large-pore zeolite Beta with its two sets of channel systems (pore diameter of 0:76 0:64 nm and 0:55 0:55 nm, respectively) which are interconnected via common intersections. In the case of zeolite Beta, a significantly larger reaction space at the channel intersections is available which decreases the probability of the formation of the proper bimolecular complex. The geometry and the architecture of the intersections in zeolite ZSM-5 was found to be a tentative optimum for its formation, and when the bimolecular transalkylation/isomerization reaction proceeds with catalysts exhibiting larger free reaction space, energetically more favored pathways are preferred which do not result in n-propyltoluene formation [16,17]. A completely different behavior was found in the case of H-MCM-22 and H-MCM-58 as catalysts. While with zeolite MCM-22 containing intersecting channels of 0:40 0:59 nm only negligible concentrations of n-propyltoluenes were observed (1.5% of n-propyltoluenes over H-MCM-22/1 at 250 °C, with iso-/n-propyltoluenes molar ratio of 50 after 15 min of T-O-S, see Table 1), significant yields of n-propyltoluenes are obtained under the same reaction conditions over the large-pore zeolite MCM-58 (viz. 6.4% of n-propyltoluenes with an iso-/n-propyltoluenes molar ratio of 7.5 for the same T-O-S value, see Table 2). This indicates that the structure of both zeolites investigated in the present study used has a significant influence on the course of the bimolecular transalkylation/isomerization reaction. With increasing WHSV, the iso-/n-propyltoluene ratio significantly increased. For MCM-22 at WHSV values above 100 h1 , the concentration of n-propyltoluenes was negligible. Moreover, the increase in the iso-/n-propyl ratio
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for MCM-58 was not as dramatic as for MCM-22 (Fig. 7C). Lawton et al. [6] reported that MCM-22 crystallizes as very thin plates, and this was confirmed for the samples prepared in the present study. From its known structure, a high concentration of pockets on the external surface with 12-membered-ring openings having an approximate depth of 0.71 nm should be present. Thus, especially these pockets are easily accessible for reactant molecules, and if a portion of the active sites is located in these pockets, the alkylation and particularly transalkylation/ isomerization can proceed inside. Due to the spacious nature of these pockets, it is hard to conceive that they supply the surrounding which is believed to be necessary for n-propyltoluene formation, as argued earlier (viz. Refs. [11,16]). No suitable steric conditions are present in these pockets to align the reacting molecules in a proper way to facilitate the energetically less favored mechanism. Recent molecular dynamics simulations performed by Sastre et al. [27] and studying the diffusion of a mixture of benzene and propylene in siliceous MCM-22 during cumene synthesis suggested that the formation of cumene probably occurs at the external surface or close to the external surface of the MCM-22 crystallites. These data are also consistent with diffusion studies done with xylene isomers over MCM-22 [24]. Our experimental results on toluene alkylation with propylene led us to a related conclusion, viz. that cymenes are probably not formed inside the 10membered-ring channels of this zeolite catalyst. From our data and from experimental data reported in the literature together with results from molecular dynamics simulations it can be inferred that toluene alkylation with propylene proceeds to a major extent at the external surface of MCM-22 zeolite sheet-like crystallites, perhaps in the large pockets located at the external surface. Thus, only a limited amount of n-propyltoluenes is formed, and no increased formation of para-substituted isomers is achieved. As for H-MCM-58 at a reaction temperature of 250 °C, the concentration of n-propyltoluenes formed is close to the values obtained with zeolite ZSM-5 under the same reaction conditions, although these two zeolite structures differ signifi-
Fig. 8. Location of the N-benzyl-1,4-diazabicyclo[2.2.2]octane cation in the undulating channel of zeolite MCM-58 (according to Ref. [8]) (A) and proposed location of 1,2-p-ditolylpropane transition state in the same zeolite (B).
cantly from each other and there do not exist any channel intersections in MCM-58. A hypothetical mechanism for n-propyltoluenes formation over zeolite MCM-58 can be inferred from the template used for the synthesis of this structure, viz. Nbenzyl-1,4-diazabicyclo[2,2,2]octane [8]. The location of this organic cation in the undulating channel is depicted in Fig. 8A. It is evident that the shape and the size of this cation are quite similar to the shape and the size of the bimolecular intermediate required for n-propyltoluenes formation. Fig. 8B presents the tentative location of a 1,2ditolylmethane-type of transition state located in the undulating channel structure of MCM-58, underlining the possibility of a preferred formation of n-propyltoluenes in toluene alkylation with propylene. This pictorial view indicates that a significant concentration of n-propyltoluenes can also be formed in one-dimensional large-pore channels when their structure enables the accommodation of the required transition state. 3.4. Adsorption of p-cymene on zeolites MCM-22 and MCM-58 To evaluate the accessibility of active sites for pcymene, as one of the major product molecules,
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the adsorption of p-cymene on MCM-22/1 and MCM-58 was followed by FTIR spectroscopy. After overnight evacuation of the zeolite, p-cymene was admitted to the high-vacuum cell at a partial pressure of 100 Pa and a temperature of 100 °C. The spectra were recorded every 30 min for a total period of 240 min at the same temperature. After that, d3 -acetonitrile was admitted to the IR cell. The results obtained with H-MCM-22/ 1 are depicted in Fig. 9 for the OH vibration region (Fig. 9A) and for the region of p-cymene vibrations (Fig. 9B). It is clearly seen that p-cymene interacts with bridging OH groups of MCM-22/1 which results in a decrease in the intensity of the absorption band at 3618 cm1 . About 50% of these OH groups interact with p-cymene even after an adsorption time of 240 min. Hence, approximately the same concentration of OH groups is, under the conditions applied here, not accessible for p-cymene molecules. It should be stressed that all remaining free OH groups are still accessible for d3 -acetonitrile. This could indicate that the sites actively involved in the toluene alkylation are mainly located on the external surface in the 12membered-ring pockets or close to the external surface. Experiments in which of di-tert-butylpyridine was adsorbed on MCM-22 led others to the conclusion that about 10% of acid sites are accessible for this bulky probe molecule [28]. For MCM-22/1 this concentration was found to be
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Fig. 10. FTIR spectra of p-cymene adsorption over MCM-58 (parent, 1; after 30 min, 2; after 240 min, 3), at 100 °C, 100 Pa of p-cymene.
even lower than 10% [29]. It is without any doubt that the accessibility of acid sites for the smaller pcymene should be higher than these 10%. We assume that also acid sites close to the entrances of the 10-membered-ring channels can be accessed by p-cymene but not by di-tert-butylpyridine. In the case of H-MCM-58, two absorption bands of bridging OH groups were found with maxima at 3625 and 3490 cm1 (Fig. 10). While the OH groups at 3490 cm1 interact only weakly with p-cymene, the intensity of the absorption band at 3625 cm1 is reduced to a minimum. Thus, in contrast to H-MCM-22/1, all bridging OH groups of H-MCM-58 represented by the absorption maximum at 3625 cm1 in the IR spectra are occupied by adsorbed p-cymene.
4. Conclusions
Fig. 9. FTIR spectra of p-cymene adsorption over MCM-22/1 (parent, 1; after 30 min, 2; after 240 min, 3), at 100 °C, 100 Pa of p-cymene.
The alkylation of toluene with propylene was investigated over acidic forms of the aluminosilicates MCM-22 and MCM-58. It has been found that the structure of these zeolite catalysts governs to a certain extent the reaction pathway and in particular controls the course of the bimolecular transalkylation/isomerization reaction leading to n-propyltoluenes as products. In the case of zeolite MCM-22, the alkylation reaction seems to preferentially proceed at or close
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to the external surface of the zeolite crystallites. Hence, the composition of the reaction products is not influenced to a considerable extent by product shape selectivity effects. Only negligible concentrations of n-propyltoluenes are observed over MCM-22, which can be attributed to the absence of the proper steric environment required for the formation of the corresponding bimolecular transition state. In contrast to zeolite MCM-22, n-propyltoluenes are formed in significant concentrations in the one-dimensional 12-membered-ring channels of zeolite MCM-58 at 250 °C. This is probably due to their peculiar undulating shape. The formation of 1,2-p-ditolylpropane seems to be supported by the pore architecture of zeolite MCM-58, since this intermediate has a similar molecular shape as the N-benzyl-1,4-diazabicyclo[2.2.2]octane template which is used for the synthesis of the IFR topology. With both zeolite structures used in the present investigation, no enhanced p-selectivity among the cymene isomers was observed. Moreover, the particular pore geometry of zeolite MCM-58 seems to promote 1,2-ditolylpropane-type transition states which are prerequisites for the formation of npropyltoluenes, while with MCM-22 as catalyst, no n-propyltoluenes are formed. The results obtained with FTIR spectroscopy using adsorbed p-cymene on MCM-22/1 suggest that a significant part of the acidic OH groups is not accessible for the relatively large p-cymene molecules, while d3 -acetonitrile molecules do have access to all remaining Brønsted-acid sites. It is suggested that this observation also indicates that the formation of cymenes should preferentially occur over active sites located at or close to the external surface of the MCM-22 crystallites.
Acknowledgements Financial support of the Grant Agency of the Academy of Sciences of the Czech Republic (A4040001) and Volkswagen-Stiftung (I/75 886) is highly acknowledged. The work of J.K. was supported by a postdoctoral grant from the Grant
Agency of the Czech Republic (203/00/P033). S.E. and A.W. gratefully acknowledge financial support by Deutsche Forschungsgemeinschaft and Fonds der Chemischen Industrie.
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