mesoporous composites in toluene alkylation with propylene

Applied Catalysis A: General 281 (2005) 85–91 www.elsevier.com/locate/apcata

Catalytic activity of micro/mesoporous composites in toluene alkylation with propylene Pavla Prokesˇova´a,b, Nadeˇzˇda Zˇilkova´a, Svetlana Mintovab, Thomas Beinb, Jirˇ´ı Cˇejkaa,* a

J. Heyrovsky´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejsˇkova 3, CZ-182 23, Prague 8, Czech Republic b Department of Chemistry, University of Munich (LMU), Butenandtstr. 11 (E), 81377 Munich, Germany Received 20 September 2004; received in revised form 4 November 2004; accepted 9 November 2004 Available online 16 December 2004

Abstract The gas phase alkylation of toluene with propylene was used to investigate the catalytic activity of pure zeolite Beta, pure mesoporous molecular sieve Al-MCM-41 and micro/mesoporous composites. The influence of the structural features and the concentration of acid sites of these catalysts on the toluene conversion and the selectivity to isopropyltoluenes (cymenes) was studied. Various characterization techniques, i.e., DLS, SEM, XRD, FT-IR, NMR, and nitrogen sorption measurements were applied for complete characterization of the catalysts. In addition, the concentrations of Bro¨nsted and Lewis acid sites were determined based on the amount of adsorbed d3-acetonitrile using FT-IR spectroscopy. The catalytic activity in toluene alkylation with propylene was increased in the order: Al-MCM-41 < micro/mesoporous composites < commercial zeolites Beta < nanosized zeolite Beta. This sequence clearly shows the role of strong Bro¨nsted acid sites in the reaction together with the decrease in the transport limitations with decreasing size of zeolite crystals. The enhancement of toluene conversion over micro/mesoporous composites compared to Al-MCM-41 is due to the presence of zeolite particles in composite catalyst increasing the acidity of this material. # 2004 Elsevier B.V. All rights reserved. Keywords: Micro/mesoporous composite; Nanocrystalline zeolite Beta; Toluene alkylation

1. Introduction Zeolites represent the most frequently used heterogeneous catalysts in the chemical technology. Refineries and petrochemical processes are mainly based on these materials [1–6], however, there is still a great area for the improvement of their catalytic properties. This is especially due to the fact that zeolite pores, restricted to the maximum pore size of about 1 nm, limit the access of larger molecules and many processes are controlled by the transport phenomena and desorption of bulky products [2]. From these reasons, the possibilities of the preparation of new types of heterogeneous catalysts combining acidic properties of zeolites with a higher accessibility of active site for large organic molecules are intensively investigated [7]. * Corresponding author. Tel.: +420 26605 3795; fax: +420 28658 2307. E-mail address: [email protected] (J. Cˇejka). 0926-860X/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.11.016

Zeolites with extra-large pores [8,9] and delaminated materials [10] were prepared but still the limiting dimensions of the micropores are around 1 nm. Instead of increasing the pore size, another possibility is to decrease the crystal size of the zeolite [11–13]. Nanosized zeolites have received much attention in catalytic applications including fluid catalytic cracking, hydrocracking of gas oil, hydroxylation of phenol, and the hydration of cyclohexene to cyclohexanol. For nanosized Beta and ZSM-5 zeolites, a higher catalytic activity, low rate of catalyst deactivation and higher product quality compared to conventional type of microcrystalline catalyst were reported for toluene acylation with acetic anhydride and hydroisomerization of n-heptane, respectively [14,15]. On the other hand, it was shown that the selectivity on nanosized zeolites could be low due to the high amount of acid sites on external surfaces [16]. In the case of mesoporous molecular sieves the main concern is still devoted to a low acidity and hydrothermal

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stability of these materials [7]. The strategies that have been investigated to improve hydrothermal stability and acidity of the mesoporous molecular sieves include (i) decreasing the content of silanol groups by silylation of the surface –OH groups in order to make the surface more hydrophobic and thereby improve the stability in water, (ii) thickening the walls of MCM-41 by post-treatment of MCM-41 to improve hydrothermal stability with subsequent grafting of Al centers into the framework walls, (iii) synthesis of mesoporous structures with thick walls using triblock copolymer and neutral Gemini amine surfactants, (iv) adding salts to the synthesis gels to facilitate the condensation of silanol groups during the formation of the framework and thereby improving framework cross-linking, (v) increasing the acidity by supporting of heteropolyacids (HPAs) and SO42
/ZrO2 in the mesopores or by their functionalization with sulfonic acid groups, and finally (vi) generating microporous–mesoporous composite materials [7]. Basically three methodologies exist for the preparation of micro/mesoporous composite materials combining crystalline zeolites and mesoporous molecular sieves. One approach is to prepare mesoporous molecular sieves and then to recrystallize the originally amorphous walls into crystalline zeolitic walls using proper structure directing agents [17–23]. The second strategy is based on the use of protozeolitic nanoclusters as assembly precursors. Structures, analogous to MCM-41, SBA-15 or disordered wormlike systems, characterized by enhanced catalytic activity have been obtained based on the usage of zeolites Y, ZSM-5, Beta and L [24–35]. Such composite aluminosilicates exhibit high hydrothermal stability. Recently, an improved catalytic activity of prepared materials in comparison with conventional Al-MCM-41 and Al-SBA15 was demonstrated in cumene cracking reaction [24]. Mesopores can been introduced into zeolite crystals also by occluding small nanosized carbon particles (12 or 18 nm) or macromolecules during the synthesis, followed by particle removal through calcination [36–38]. These mesoporous zeolite single crystals with the MFI structure exhibit improved catalytic activities and selectivities as compared to the conventional zeolite catalysts, in the alkylation of benzene with ethylene. Previously, Holland et al. [39] have published the synthesis of macroporous zeolites prepared by filling the voids between polymer sphere arrays with synthesis gel, while Antonietti et al. [40] have formed meso/macroporous solids by combining polymer sphere templating with gels for MCM-41 materials. In this paper we report on the comparison of the catalytic properties of materials related to zeolite with BEA type structure, namely colloidal zeolite Beta, commercial zeolite Beta and micro/mesoporous composite materials. The composites have been prepared based on nanosized zeolite Beta with different concentrations of active sites. For comparison, pure mesoporous Al-MCM-41 material was utilized. Toluene alkylation with propylene was used as a test reaction as this reaction offers to correlate several

catalytic parameters (toluene and propylene conversions, selectivity to cymenes, para-selectivity and formation of npropyltoluenes) with the structure and acidity of catalysts under investigation.

2. Experimental 2.1. Catalyst preparation The catalysts used are industrial zeolites Beta purchased from Zeolyst (USA) possessing crystal size between 0.1 and 0.2 mm, nanosized (colloidal) zeolites Beta, pure mesoporous Al-MCM-41 and the micro/mesoporous composite materials. Nanosized crystals of zeolite Beta were synthesized from a colloidal precursor solution having the following chemical composition [41]—solution B: 0.35 Na2O:9 TEAOH:0.125– 0.25 Al2O3:25 SiO2:295 H2O. Crystallization was performed at 100 8C for 68–264 h. The precursor solution used for the synthesis of pure AlMCM-41 mesophase has the following chemical composition [42]—solution M: 0.0089 Al2O3:1 SiO2:19.4 NH3:103 EtOH:0.28 CTAB:826 H2O. Al-MCM-41 was prepared via stirring of the precursor solution at room temperature for 24 h. The micro/mesoporous composites were prepared via two different synthetic procedures. The first one includes hydrothermal treatment (HT) of solution B containing zeolite Beta seeds and freshly prepared precursor solution M. For the preparation of micro/mesoporous composite materials always pure-siliceous mesoporous precursor was used without aluminum. The final mixtures have the following chemical composition—solution BM: 1 Al2O3:103.4 SiO2:31.8 TEAOH:0.8 CTAB. The second procedure is based on the application of solution B but not freshly prepared; it has been treated at 100 8C for 27 h. After cooling to the room temperature the solution was mixed with a freshly prepared precursor solution M. Further, crystallization of these materials was carried out at 100 8C for 0–40 h. For sample BM4 the crystallization time of precursor solution B was even 68 h. After the first crystallization step the solution B was mixed with solution M, and the crystal growth of micro/ mesoporous composite material was completed after 40 h of heating at 100 8C. Samples prepared via the second procedure are abbreviated as BM1–3. Zeolite Beta samples were calcined in a stream of air at 500 8C for 7 h, pure Al-MCM-41 at 550 8C for 1 h in nitrogen, followed by 6 h in air, and the composite materials at 470 8C for 1 h in nitrogen, followed by 6 h in air. All materials were converted into acidic form via ion-exchange using 0.5 M solution of NH4NO3. 2.2. Catalyst characterization The nanosized zeolites Beta and micro/mesoporous composite materials were characterized using dynamic

P. Prokesˇova´ et al. / Applied Catalysis A: General 281 (2005) 85–91

light scattering (DLS, ALV), scanning electron microscopy (SEM, Philips XL 40), X-ray diffraction (Scintag XDS 2000), infrared spectroscopy (FT-IR Bruker Equinox spectrometer), 29Si and 27Al MAS NMR spectroscopy (4 mm ZrO2 rotors in a commercial double resonance probe, Bruker DSX Avance 500) and nitrogen sorption (Quantachrome Corporation NOVA 4200). Chemical composition of nanosized zeolites Beta, industrial samples of Beta, Al-MCM-41 and micro/ mesoporous composites was determined by an ICP-AES (Varian, Vista). Concentrations of Lewis (cLS) and Bro¨ nsted (cBS) acid sites were determined using adsorption of d3-acetonitrile followed by FT-IR spectroscopy. FT-IR spectra of adsorbed d3-acetonitrile were recorded on a Nicolet FT-IR Prote´ ge´ 460 spectrometer with a cooled MCT-A detector in the adsorption mode with a resolution of 4 cm
1 with 64 scans. Samples were pressed into self-supported wafers with a density approximately 8–14 mg cm
2. Homogenously pressed wafers were placed in a three-position sample holder in a vacuum cell with KBr windows equipped with a programmed heating and a vacuum/gas dosing system. Prior to adsorption, samples were activated in situ at 450 8C under overnight evacuation and cooled to the adsorption temperature. d3-Acetonitrile (Aldrich) was degassed by freeze– pump–thaw cycles. Spectra of adsorbed d3-acetonitrile on zeolite samples were taken after 30 min of adsorption (ca. 900 Pa) and subsequent 10 min of desorption at room temperature. All measured spectra were recalculated to the uniform value of 10 mg cm
2. The IR bands of adsorbed d3-acetonitrile at 2297 and 2325 cm
1 corresponds to the interaction of the CBBN group with Bro¨ nsted sites and Lewis sites, respectively. The concentrations of Bro¨ nsted and Lewis sites were calculated from the intensities of the relevant bands by using extinction coefficients eB = 2.05  0.05 and eL = 3.62  0.1 cm mmol [43]. The intensities were measured after deconvolution of the IR bands of adsorbed d3-acetonitrile into the Gaussian profiles. The silicon to aluminium ratio was calculated from the concentration of Bro¨ nsted and Lewis acid sites assuming that the concentration of aluminium cAl in the zeolite is cAl = cBS + 2cLS [44] and the molar formula characteristic for zeolite Beta is HxAlxSi64
xO128 [45]. 2.3. Catalytic experiments Toluene alkylation with propylene was investigated in the gas phase at atmospheric pressure using a glass fixed bed reactor. Each catalyst was pressed into the pellets, crushed and sieved to obtain particles with a diameter in the range of 0.041–0.250 mm. Prior to the reaction, 0.15 g of the catalyst was in situ activated at 450 8C for 120 min in a stream of nitrogen (20 ml min
1), and then the activated catalyst was cooled down to the reaction temperature (250 8C). All experiments were performed using WHSV related to toluene

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equal to 20 h
1. The standard concentration of toluene in nitrogen was 21.3 mol% and the toluene to propylene molar ratio was 9.0 or 2.0. The reaction mixture was analyzed using an on-line gas chromatograph (HP 6890) equipped by a FID detector and a capillary column (DB-5, 50 m  320 mm  1 mm). The first sample was taken after 15 min of time-on-stream (T-O-S) and the other samples were taken in the interval of 60 min.

3. Results and discussion 3.1. Characterization of catalysts X-ray powder diffraction confirmed the high crystallinity and phase purity of the nanosized zeolites Beta samples. The particles size of pure Beta nanocrystals is about 50 nm determined based on SEM and DLS measurements [42]. The formation of micro/mesoporous composite materials was performed according to the procedures described in the previous section; hydrothermally treated solution of zeolite Beta seeds and freshly prepared pure-siliceous mesoporous precursor solutions were transformed into composites within 40 h heating at 100 8C (samples BM1–3). The combination of XRD, DLS and SEM confirmed the formation of composite materials, and no physically separated two phases were observed. The composite contained particles of 90– 135 nm in radius, which were much smaller than the particles in the pure mesoporous sample Al-MCM-41 (290 nm in radius) and slightly bigger than the pure nanosized zeolite Beta crystals (50 nm in radius) [41,42]. The evaluation of the nitrogen sorption results confirmed the presence of micropores, mesopores and high textural porosity in the composite material. However, the particle size distribution of sample BM4 was very broad indicating the existence of non-homogeneous sample with particles having sizes from 50 to 300 nm. In addition, the XRD pattern contains all characteristic reflections for pure zeolite Beta. This is an indication for a presence of quite large particles with Beta type structure in the sample BM4. The use of the fully crystalline milky solution of zeolite Beta in sample BM4 instead of the zeolite Beta seeds solution led to the formation of samples with a lower specific surface area (SBET = 760 m2 g
1) in comparison to the sample BM3 (SBET = 915 m2 g
1). For more details concerning the physicochemical characterizations of all samples, see Refs. [41,42]. 3.2. Characterization of Bro¨ nsted and Lewis acid sites using FT-IR spectroscopy The concentration and the type of acid sites in all samples including industrial and nanosized pure zeolite Beta, pure mesoporous Al-MCM-41 material and the micro/mesoporous composites were determined based on the amount of adsorbed d3-acetonitrile using FT-IR spectroscopy (Table 1).

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Table 1 d3-Acetonitrile FT-IR data of nanocrystalline and industrial zeolite Beta, micro/mesoporous composites and mesoporous Al-MCM-41 Si/AlFT-IR

cBS (mmol g
1)

cLS (mmol g
1)

Crel,BS (%)

Nanosized Beta B1 33.2 B2 23.8 B3 14.8

27.2 22.0 16.8

0.17 0.21 0.20

0.21 0.26 0.37

46 44 35

Industrial Beta iB1 iB2 iB3

83.3 35.4 11.2

0.10 0.19 0.16

0.05 0.14 0.60

66 58 21

Micro/mesoporous composites BM1 33.6 41.8 BM2 33.1 40.0 BM3 33.5 40.1 BM4 31.0 28.4

0.04 0.03 0.03 0.13

0.18 0.19 0.19 0.22

18 15 15 38

Mesoporous molecular sieve Al-MCM-41 48.5 72.0

0.000

0.110

0

Samples

Si/Alc

75.0 37.5 13.4

Si/Alc, Si/Al ratio measured by ICP-AES analysis; Si/AlFT-IR, Si/Al ratio determined from FT-IR adsorption measurements; cBS and cLS, concentration of Bro¨ nsted and Lewis acid sites; Crel,BS, relative concentration of Bro¨ nsted sites (Crel,BS + Crel,LS = 100%).

A good agreement between Si/Al ratios calculated from the FT-IR measurements with those from the chemical analysis was found for the Beta nanosized and industrial samples. It also indicates a good accessibility of acid sites for the probe molecule during adsorption measurements with d3-acetonitrile. It has been found that with increasing the concentration of Al in zeolite Beta, the relative concentration of Bro¨ nsted acid sites decreases while the relative concentration of Lewis acid sites increases. In the case of commercial zeolite Beta (Table 1, samples iB1–3) the relative concentration of Bro¨ nsted acid sites varies from

66 to 21% when the ratio of Si/Al decreases from 75 to 13.4. For pure nanosized zeolite Beta (samples B1–3) the relative concentration of Bro¨ nsted acid sites varies from 46 to 35% when the ratio of Si/Al is decreasing from 33 to 14.8. In the case of pure mesoporous Al-MCM-41 sample, less than 70% of aluminum in the mesoporous structure is accessible for the probe molecule and no Bro¨ nsted acid sites were observed. This observation is in a good agreement with the data reported by Deˇ decˇ ek et al. [46] showing that some portion of aluminum, depending on the synthesis procedure, is hidden in the walls of Al-MCM-41. The concentration of aluminum in the micro/mesoporous composites detected by adsorption of d3-acetonitrile was lower in comparison with that determined from the chemical analysis (Table 1). This indicates that some aluminum atoms in the composite material were not accessible for the probe molecule. In addition, it is supported by the fact that for composite sample BM4 formed from largest Beta particles, the concentration of Al determined from the adsorption of d3-acetonitrile is comparable with data obtained from the chemical analysis. When small colloidal particles of zeolite Beta were used (samples BM1–3), a part of aluminum is hidden in the walls of the composite samples. 3.3. Toluene alkylation catalytic experiments Toluene alkylation with propylene is usually carried out in a gas phase with a high toluene to propylene molar ratios [2,5,6]. Under the reaction conditions in our experiments, the toluene conversions over all catalysts were in the range of 9–10.5% after 15 min of T-O-S (with exception of pure mesoporous Al-MCM-41), and therefore it was not possible to easily distinguish the role of structure of the catalyst and concentration of acid sites on their catalytic properties.

Table 2 Toluene alkylation with propylene: the toluene conversion, the selectivity to cymenes and the para-selectivity in cymenes at 15, 75 and 135 min of T-O-S (toluene/propylene molar ratio of 2.0, WHSV = 20 h
1, reaction temperature 250 8C) over zeolite Beta, micro/mesoporous composites and mesoporous AlMCM-41 Samples

Si/Alc

Toluene conversion, XT (%)

Selectivity to cymenes, SC (mol%)

p-Selectivity, Sp-C (mol%)

15 min

75 min

135 min

15 min

75 min

135 min

15 min

75 min

135 min

Nanosized Beta B1 B2 B3

33 24 15

32.4 30.7 27.0

20.2 24.7 14.5

15.5 17.7 13.4

87.8 85.9 88.1

87.7 91.7 90.0

90.9 91.3 91.1

30.0 30.2 32.1

38.2 38.3 39.6

40.4 40.7 42.0

Industrial Beta iB1 iB2 iB3

75 38 13

25.5 27.7 28.2

18.4 17.5 13.7

15.6 13.4 10.7

94.9 89.4 92.1

90.9 90.8 90.7

92.4 92.6 91.7

34.8 31.8 32.6

41.9 42.2 41.8

43.4 43.4 43.0

19.0 14.1 12.5 27.0

7.8 8.4 9.4 13.9

6.5 7.2 8.0 10.6

41.5 63.6 87.4 90.6

87.7 88.0 88.0 91.5

89.5 89.4 89.4 92.4

42.6 43.1 43.4 35.2

42.8 43.1 43.6 42.9

42.7 43.1 43.5 44.0

9.0

5.6

4.3

86.5

90.5

96.9

43.6

43.3

43.1

Micro/mesoporous composites BM1 34 BM2 33 BM3 34 BM4 31 Mesoporous molecular sieve Al-MCM-41 49

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Table 3 Toluene alkylation with propylene conversions and product distributions at 15 and 135 min of T-O-S for zeolite Beta, micro/mesoporous composites and mesoporous Al-MCM-41 (toluene/propylene molar ratio of 2.0, WHSV = 20 h
1, reaction temperature 250 8C) Samples iB2

B1

BM3

Al-MCM-41

15 (min)

135 (min)

15 (min)

135 (min)

15 (min)

135 (min)

15 (min)

Toluene conversion (%) Propylene conversion (%)

27.7 69.8

13.4 31.4

32.4 77.1

15.5 35.8

12.5 28.4

8.0 17.7

9.0 19.7

4.3 8.6

Product composition (mol%) Aromatics C3–C4 olefins

84.8 15.2

73.9 26.1

88.0 12.0

76.2 23.9

73.5 26.5

70.7 29.3

71.4 28.6

68.7 31.3

Selectivity (mol%) Benzene Xylene (p-, o-) Isopropylbenzene p-Ethyltoluene m-Isopropyltoluene p-Isopropyltoluene o-Isopropyltoluene n-Propyltoluene (p-, o-) Higher aromatics Isopropyltoluenes

0.0 0.7 1.5 0.5 54.7 28.5 6.2 3.6 4.3 89.4

0.0 0.0 0.0 0.0 38.9 40.2 13.5 0.0 7.4 92.6

0.6 1.4 2.0 0.8 56.2 26.3 5.3 3.7 3.8 87.8

0.0 0.0 0.0 0.0 40.9 36.7 13.3 0.0 9.1 90.9

0.0 0.0 0.0 0.0 32.7 37.9 16.8 0.0 12.6 87.4

0.0 0.0 0.0 0.0 29.8 38.9 20.7 0.0 10.6 89.4

0.0 0.0 0.0 0.0 29.0 37.8 19.8 0.0 13.5 86.5

0.0 0.0 0.0 0.0 28.5 41.8 26.6 0.0 3.1 96.9

Tables 2 and 3 summarize conversions, selectivities and product distributions after different T-O-S for the toluene/ propylene molar ratio of 2.0 over nanocrystalline and industrial zeolite Beta, as well as over the micro/mesoporous composites and pure mesoporous Al-MCM-41. The alkylation of toluene was the major reaction over all samples and a mixture of cymenes was formed. In addition, benzene, xylenes (p-, o-), isopropylbenzene, p-ethyltoluene, npropyltoluenes (p-, o-) were formed over zeolite Beta, while various higher aromatics were detected in each reaction mixture, particularly on Al-MCM-41. For all samples used, the conversion of toluene, XT (toluene/propylene molar ratio = 2.0) was decreasing with T-O-S due to the deactivation of the catalyst with simultaneous increase in the selectivity to cymenes and the para-selectivity in cymenes (see Tables 2 and 3). As can be seen from Table 2 and Fig. 1, the toluene conversion varied significantly due to different structures and concentrations of active sites of the catalysts used (XT = 9.0– 32.4% at 15 min of T-O-S, XT = 4.3–15.5% at 135 min of T-O-S). The highest initial activity (15 min of T-O-S) was obtained for nanocrystalline zeolites Beta (XT = 27.0– 32.4%) and industrial Beta samples (25.5–28.2%). However, the micro/mesoporous composites exhibited toluene conversions between 12.5 and 27.0%. The highest conversion obtained over BM4 catalyst is probably due to the highest amount of Beta crystallites of this sample in comparison with BM1–3. After 135 min of T-O-S, the nanosized and industrial Beta zeolites exhibited XT of 13.0–17.7% and 10.7–15.6%, respectively. Still lower conversion were obtained for micro/mesoporous composites (XT = 6.5– 10.6%), however, these values were much higher compared to pure mesoporous Al-MCM-41 sample (XT = 4.3%). The

135 (min)

sequence of the toluene conversions observed in the investigated samples, i.e., nanosized Beta > industrial Beta > composite > mesoporous Al-MCM-41 is probably due to the following two factors: (i) the toluene conversion increases with the increase in the concentration of strong bridging Si–OH–Al groups, and (ii) the toluene conversion increases with the decrease in the particle size of the catalysts possessing similar acidic properties. This clearly demonstrates that the incorporation of Beta seeds into mesoporous structures increases significantly the acidity of the catalyst compared to pure mesoporous Al-MCM-41. The selectivity to cymenes over all samples was at least 85% despite the initial toluene conversion (with exception of BM1 and BM2 after 15 min of T-O-S) and it was further

Fig. 1. Alkylation of toluene with propylene (toluene/propylene molar ratio of 2.0, WHSV = 20 h
1, reaction temperature 250 8C) over nanosized Beta, B1 (^); industrial Beta, iB2 (*); micro/mesoporous composite, BM3 (5); and mesoporous Al-MCM-41 (). (A) T-O-S values of the toluene conversion, XT and (B) T-O-S values of the selectivity to cymenes, SC.

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increased with the prolongation of T-O-S (89–93% for T-OS of 135 min). The highest selectivity was found for mesoporous Al-MCM-41 (96.9% after 135 min), which is due to the lowest conversion of toluene among all samples. However, the formation of by-products is also particularly affected by the structure and acidity of catalysts under investigation (see Table 3). For nanosized and industrial Beta zeolites, the disproportionation and transalkylation reactions, in some extent, are leading to the formation of benzene, xylenes, isopropylbenzene and even n-propyltoluenes. With decreasing acidity and increasing pore size of the composites and mesoporous Al-MCM-41, the alkylation of cymenes to form higher aromatics proceeds (10–14%), which is not possible over zeolites Beta because of sterical reasons. The T-O-S dependence of the selectivity to o-, m- and p-cymene in alkylation of toluene with propylene is depicted in Tables 2 and 3. It is clear that the acidity controls not only the rate of alkylation reaction but also the rate of consecutive isomerization of cymenes, which is reflected in the changes of selectivities to individual cymene products (cf. Table 3 and Fig. 2). It has been proposed that the first alkylation step proceeds to ortho or para positions [2], which is followed by isomerization. Therefore, zeolites Beta exhibiting the highest acid strength, posses the highest selectivity to mcymene at T-O-S of 15 min. With increasing T-O-S the selectivity to m-cymene is decreasing due to a partial deactivation of the catalyst. In contrast, the isomerization activity is rather limited over micro/mesoporous composite materials and mesoporous Al-MCM-41 that reflects in higher selectivities to o- and p-cymenes compared to the thermodynamic values (ortho, 15.7%; meta, 56.3%; para, 28.0%). Fig. 2 illustrates the influence of the toluene conversion on the selectivities to o-, m- and p-cymene in cymenes for all four types of materials. As can be seen, the ratio of the

Fig. 2. The toluene conversion dependence of the selectivity to o-cymene (^); m-, o- and p-cymene (5) in the alkylation of toluene with propylene over nanosized Beta, B1; industrial Beta, iB2; micro/mesoporous composite, BM3; and mesoporous Al-MCM-41 (toluene/propylene molar ratio of 2.0, WHSV = 20 h
1, reaction temperature 250 8C); comparison with the equilibrium composition.

individual isomers of cymenes was influenced mainly by toluene conversion, which strongly depends on the overall acidity of zeolite-based catalysts used. The selectivities to o, m- and p-cymenes on zeolite Beta are very similar to those on micro/mesoporous composites (the toluene conversions are XT = 12–15%). In the conversion range between 4 and 10%, the selectivities to o- and p-cymenes are much higher compared to the thermodynamic values, which is due to the high ratio of alkylation to isomerization rates. With increasing toluene conversion, the isomerization activity is increasing and the selectivities are much closer to the thermodynamic values. It seems that this explains the high selectivities to p-cymene, and not the preferential transport of the smallest cymene molecule in catalysts derived from the zeolite Beta crystals. No straightforward relationship between the concentration of acid sites and toluene conversion and selectivity to cymenes in the toluene alkylation with propylene in nanosized Beta (samples B1–3, Si/Al = 33–15) and industrial zeolite Beta (samples iB1–iB3, Si/Al = 75–13) was found. It seems that for catalysts exhibiting the highest conversion at 15 min of T-O-S, the deactivation with increasing T-O-S was faster than for those providing a lower toluene conversion at 15 min of T-O-S. Thus, at T-O-S for 15 min, the sequence of toluene conversions for the nanosized zeolites Beta was B1 > B2 > B3, while for the industrial samples was iB3 > iB2 > iB1 (Table 2); while the T-O-S at 135 min was B2 > B1 > B3 and iB1 > iB2 > iB3. Beta catalysts showing higher initial conversion (samples B1 and iB3) suffered from faster deactivation in comparison with the catalyst giving lower initial conversion (samples B2 and iB1).

4. Conclusions The gas phase alkylation of toluene with propylene was investigated over nanosized (colloidal) and industrial zeolite Beta with various Si/Al ratios, mesoporous Al-MCM-41, and micro/mesoporous composites prepared from Beta zeolite seeds and fully crystalline nanozeolite. The increase in the concentration of Bro¨ nsted acid sites in the micro/mesoporous composites in comparison with the mesoporous Al-MCM-41 was proved by FT-IR d3-acetonitrile adsorption study. Toluene alkylation with propylene over nanosized and commercial zeolite Beta samples, micro/mesoporous composites and mesoporous Al-MCM-41 clearly showed that the toluene conversion depends on the following two parameters: (i) size of the zeolite crystals and (ii) acidity of the used catalysts. Toluene conversion was found to increase with decreasing size of the crystals, due to the restricted role of transport of product molecules, i.e. nanosized Beta versus commercial Beta, as well as with increasing acidity of the catalysts, i.e. micro/meso composites versus pure mesoporous Al-MCM-41.

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No clear influence of the concentration of acid sites in zeolite Beta on the toluene conversion and selectivity to cymenes in the toluene alkylation with propylene was found, which is probably due to the significant role of desorption of products.

Acknowledgements This investigation was supported by the Marie Curie training program and DFG-CNRS. The work of PP was also supported by the Grant Agency of the Czech Republic (203/03/H140), and JC thanks for the grant of the Academy of Sciences of the Czech Republic (B4040402). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

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