zeolite hybrid materials: toluene disproportionation and transalkylation with 1,2,4-trimethylbenzene

Applied Catalysis A: General 256 (2003) 173–182

Catalytic properties of heteropoly acid/zeolite hybrid materials: toluene disproportionation and transalkylation with 1,2,4-trimethylbenzene B. Sulikowski∗ , R. Rachwalik Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Niezapominajek 8, PL 30-239 Kraków, Poland Received 4 October 2002; received in revised form 6 December 2002; accepted 11 December 2002

Abstract A series of hybrid heteropoly acid (HPA)/zeolite catalysts has been prepared by supporting 12-tungstophosphoric acid (H3 PW12 O40 ) on a faujasitic matrix. It has been shown that the structure of a dealuminated zeolite Y is retained upon contact with a very strong solid acid. The catalytic properties of the hybrid catalysts were examined in the two gas-phase reactions: disproportionation of toluene and transalkylation of toluene and 1,2,4-trimethylbenzene (1,2,4-TMB). All the hybrid catalysts exhibit enhanced catalytic activity in both processes. Already the small amounts of heteropoly acid exert significant influence on the activity in both reactions studied. A pronounced shape-selectivity, expressed as p-xylene/o-xylene ratio, is evident both in toluene disproportionation and its transalkylation with 1,2,4-TMB at low temperatures, while equilibrium distribution of xylene isomers is typical at higher temperatures. As evidenced by 31 P NMR, two types of the Keggin unit are present in the catalysts. They are located in the mesopores of zeolite and on the zeolite crystals, respectively. The Keggin units interacting strongly with the support, characterised by chemical shift at −13.3 ppm and located in mesopores are probably the more active ones in title reactions. © 2003 Elsevier B.V. All rights reserved. Keywords: Dodecatungstophosphoric acid; Dealuminated faujasite; Disproportionation; Transalkylation; Toluene; Xylenes; 1,2,4-Trimethylbenzene

1. Introduction Alkylaromatic hydrocarbons undergo isomerization, disproportionation and transalkylation reactions over numerous catalysts exhibiting Brønsted and Lewis acidity. Originally, some of these reactions were performed over Friedel-Crafts type catalysts in the liquid phase (AlCl3 , GaBr3 , etc.) [1]. These processes were however abandoned due to techno∗ Corresponding author. Tel.: +48-12-6395127; fax: +48-12-4251923. E-mail address: [email protected] (B. Sulikowski).

logical difficulties and environment considerations. Some of the processes which are now implemented commercially on a large industrial scale use aluminosilicate catalysts and provide benzene, toluene, xylenes, trimethylbenzenes and higher alkylaromatic hydrocarbons [2]. A number of possibilities exists and one can, for example, produce p- and o-xylene from a C8 fraction containing xylenes and ethylbenzene (of petrochemical or coal origin), or produce benzene and xylenes using surplus toluene as a feed. The processes available are flexible, and their economics depends on the current market trends in petrochemical industry.

0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0926-860X(03)00397-1

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Transformations of alkylaromatics are also an important tool for fundamental studies. Some of these processes, like isomerization and disproportionation of toluene, are used widely for characterisation of amorphous and crystalline aluminosilicates [3,4], isomorphously substituted zeolites [5,6] and a number of related solids [7]. Heteropoly acids (HPA) with the Keggin type structure belong to the strongest known solid acids. In HPA the lower charge density is present on the surface of the nearly spherical Keggin units. Because of almost no charge localisation, the protons are very mobile, thus giving raise to extremely high Brønsted acidity [8]. 12-Tungstophosphoric acid (H3 PW12 O40 ) is the strongest one in comparison with other heteropoly oxometalates (H4 SiW12 O40 , H3 PMo12 O40 ) [9]. Pure heteropoly acids exhibit low thermal stability, hence immobilization of heteropoly acids on a support is of considerably interest. Silica, alumina, and active carbon were the most investigated systems [10–12]. Adsorption of heteropoly acids on other supports includes, inter alia, TiO2 , zeolites, and mesoporous molecular sieves. As we have demonstrated previously, heteropoly acids can be either encapsulated in the supercages of zeolite Y [13] or supported in their secondary pore system [14]. The aim of this paper is to examine catalytic activity and selectivity of a series of hybrid catalysts, containing 2–38% of 12-tungstophosphoric acid supported on the specially prepared faujasite matrix. We explored their properties in two reactions: (i) disproportionation of toluene and (ii) transalkylation of toluene and 1,2,4-trimethylbenzene (1,2,4-TMB). Behaviour of a pure dealuminated zeolite Y is reported for comparison purposes. 2. Experimental

Highly crystalline zeolite NaY with overall Si/Al = 2.47 and Na/Al = 1.09 (Inowrocław, M˛atwy, Poland), as determined by atomic absorption spectroscopy (AAS), was dealuminated using the Kerr method [16,17]. Thus, ethylene-diaminotetraacetic acid (H4 EDTA) was added stepwise to a suspension of the zeolite in water. The suspension was continuously stirred at 366 ± 2 K, and the rate of H4 EDTA addition, which is essential to minimise amorphization of the zeolite, was 0.01 g of H4 EDTA per 1 g of zeolite per 1 h. The suspension was heated and stirred further for 4 h after all the acid had been added. The zeolitic product was washed with hot distilled water, dried, and ion-exchanged with 10 wt.% ammonium chloride. Preliminary impregnations were carried out using water solutions of the heteropoly acid. All the samples studied here were prepared using ethyl ether as a solvent. Thus, the dealuminated zeolite matrix was dried at 120 ◦ C and immersed immediately in the ethyl ether solution containing known amounts of PW12 . The mixture was stirred in a closed vessel at ambient temperature for 1.5 h. Then the solid was filtrated, washed with ethyl ether and left at room temperature until the solvent was evaporated. The samples were labelled as z% PW/Y , where z% denotes the concentration of PW12 (wt.%). They were characterised by XRD, BET and NMR. 2.2. X-ray diffraction Powder X-ray diffraction patterns were acquired on a Siemens D5005 automatic diffractometer using Cu K␣ radiation (55 kV, 30 mA) selected by a graphite monochromator in the diffracted beam. 2.3. BET A volumetric sorption unit of standard design was used for sorption of argon. The samples were outgassed at 623 K.

2.1. Samples preparation 2.4. NMR spectroscopy Dodecatungstophosphoric acid (H3 PW12 O40 , labelled onward as PW12 ) has been synthesised from sodium phosphate and sodium tungstate. The product was purified according to a known procedure [15]. Its purity has been verified by FT-IR and NMR spectroscopies.

The NMR spectra were obtained at room temperature on a home-made 300 MHz pulse NMR spectrometer. A high speed, high-power Bruker HP-WB MAS probe equipped with 4 mm rotors spun at 6 kHz were used for acquiring 29 Si and 27 Al spectra, and at 2 kHz

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for 31 P spectra. The 2 ␮s rf excitation pulse corresponded to 90◦ flipping angle for 29 Si and 31 P, and to 18◦ for 27 Al. The repetition rates were equal to 120, 60, and 1s for 29 Si, 31 P, and 27 Al, respectively. One hundred twenty-eight scans were accumulated for silicon and aluminium spectra, and 64–1320 for phosphorous spectra. An exponential line broadening of 20 and 100 Hz was applied to 29 Si and 27 Al spectra. 2.5. Catalytic tests The disproportionation and transalkylation reactions were performed in a tubular down-flow stainless steel reactor (length 10 cm, 1.0 cm i.d.). The tests were performed in the pulse mode, in order to avoid excess coke formation. Deposition of coke can obscure the intrinsic activity and selectivity of the catalysts. Catalysts were pressed into binder-free wafers, crushed and sieved to 0.200–0.315 mm fraction. The catalyst particles (0.05 g) were mixed with the equal volume of SiC chips and loaded into the microreactor. Additional 2 cm3 of SiC chips (>0.40 mm) were placed onto the top of the catalyst bed. Before the use SiC was cleaned with concentrated nitric acid and intensively washed with distilled water. A catalyst was preheated in the dry helium flow (grade 99.999%, 30 cm3 min−1 ) at 723 K for 1 h. Then the temperature was decreased to 623 K and 10 injections of a 10 ␮l of a substrate were applied in a 1 min sequence to stabilise the performance of a catalyst. The temperature was decreased further to 573 K and catalytic tests were performed at 573–723 K temperature range under the atmospheric pressure (carrier gas flow F = 30 cm3 min−1 ). Typically, three injections of the reactants (1 ␮l) were applied at each temperature. The substrate and products were analysed by a Hewlett-Packard 6890 gas chromatograph connected on-line to the microreactor system, using a packed column (3 m, 2.0 mm i.d.) with 8% Bentone-34, 6% didecyl phthalate and 1% silicon oil A on Chromosorb W (60–80 mesh) and a TCD detector. The response factors of the detector were established using as standards the aromatic hydrocarbons (reagent grade) and mixtures thereof. All reaction parameters were kept constant during the tests to maintain maximum reproducibility. Carbon and mass balances were kept below ±5%. Conversion and selectivity were calculated as shown below.

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2.6. Disproportionation of toluene


 100 − ct × 100 100   (cp + cm + co ) × 2 SD = × 100 100 − ct

Xt =

where Xt is the conversion of toluene (mol%); SD the selectivity of toluene disproportionation (%); and ct , cp , cm , co the concentration (mol%) of toluene, p-, mand o-xylene in products, respectively. The apparent activation energy of toluene disproportionation was calculated using the Bassett–Habgood equation [18] for the effective rate constant of toluene conversion
 F0 1  k = kK = ln 273RW 1−x where k is the effective rate constant of toluene conversion (mol m s kg−2 ); k the rate constant of surface reaction (s−1 ); K the Henry’s constant (mol m s2 kg−2 ); F0 the normalised carrier gas flow (m3 s−1 ); R is 8.314 (J K−1 mol−1 ); W the weight of catalyst (kg); and x the conversion of toluene (molar fraction). F0 =

273 pA × ×F Tf pA + p M

where pA is the atmospheric pressure (hPa); pM the pressure of the carrier gas at the microreactor inlet (hPa); and F the carrier gas flow (m3 s−1 ) at the outlet of the GC column in temperature Tf (K). 2.7. Transalkylation of toluene and 1,2,4-trimethylbenzene X1,2,4-TMB = ST =

SI =

48.79 − c1,2,4-TMB × 100 48.79

(cp + cm + co )/2 × 100 48.79 − c1,2,4-TMB

(c1,3,5-TMB − 0.04) + (c1,2,3-TMB − 1.17) 48.79 − c1,2,4-TMB × 100

where X1,2,4-TMB is the conversion of 1,2,4-trimethylbenzene (mol%); ST the selectivity of 1,2,4-TMB

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transalkylation (%); SI the selectivity of 1,2,4-TMB isomerization (%); and c1,2,4-TMB the concentration of 1,2,4,-TMB in products (mol%). Concentrations of 1,2,4,-TMB, 1,3,5-TMB and 1,2,3-TMB in feed were 48.79, 0.04, and 1.17 mol%, respectively. 3. Results and discussion 3.1. The support and hybrid catalysts The support for the heteropoly acid should be stable chemically and thermally. It should contain a pore system large enough to accommodate H3 PW12 O40 molecules. Moreover, the stability of the zeolite upon contact with the very strong solid acid should be considered. The primary structure of the 12-tungstophosphoric acid consists of the Keggin heteropolyanions with diameter of about 12 Å, so it is evident that unmodified zeolite Y with the entrance to the pore system of about 7.4 Å is too small for PW12 molecules to enter it. In this case the adsorption would take place exclusively on the external surface of zeolite crystals. Taking into account these facts we have decided: (i) to increase the stability of zeolite by decreasing the aluminium content and (ii) to choose a method of dealumination yielding simultaneously a secondary pore system in zeolite matrix, thus allowing deposition of the large H3 PW12 O40 molecules. Aluminium can be extracted from zeolite framework by mineral or organic acids (tartaric, H4 EDTA), COCl2 , SO2 Cl2 and SiCl4 , to mention a few reagents [17]. We have chosen dealumination by H4 EDTA acid, because extraction of framework aluminium to give more siliceous solid with the enhanced Si/Al framework ratio is accompanied by formation of a secondary pore system with the diameter of 15 Å [14,19]. Chemical analysis of the dealuminated sample Y performed by AAS yielded overall Si/Al = 4.40. 29 Si NMR spectra (not shown) of the samples were used for calculation the Si/Al framework ratio, equal to 4.24. It is clear that the extracted aluminium was also removed completely from the zeolite pore system. This was confirmed independently by 27 Al NMR spectra [14]. After dealumination the tetrahedral signal was dominating the spectrum (98%), and the contribution from

octahedral Al was at the same level as in the parent sample (2%). The dealuminated zeolite was impregnated with water solutions of 12-tungstophosphoric acid. However, despite of the PW12 concentration and the conditions applied, we observed by XRD a partial or total destruction of the zeolite matrix for all the samples prepared from water solution. Consequently, the next series of samples was prepared using ethyl ether as a solvent. XRD patterns of the H3 PW12 O40 ·26H2 O, zeolite matrix and representative catalysts are visualised in Fig. 1. As-prepared PW12 acid is a mixture of triclinic and cubic forms. As seen, the structure of the zeolite does not collapse upon loading with the strong heteropoly acid. Apparent decrease of the lines intensity is due to the lowered amount of zeolite in the samples upon loading with PW12 . For the highly loaded samples the second phase can be seen in the X-ray patterns. These broad signals were assigned to the cubic form of H3 PW12 O40 ·26H2 O (a = 23.28 Å). Finally, BET data show decrease of the sorption capacity with the PW12 loading (Table 1). In 31 P MAS NMR spectra two signals at −15 and −13.3 ppm can be discerned [14]. It is therefore evident that heteropoly acid is present in the samples in two different forms. The first type of Keggin unit interacts strongly with the support, gives rise to NMR signal at −13.3 ppm and is located in the mesopores of zeolite matrix. The second one is characterised by essentially unperturbed 31 P NMR spectra with

Fig. 1. X-ray diffraction patterns of (a) dealuminated zeolite matrix (H,Na)-Y ; (b) H3 PW12 O40 crystals; (c) 2% PW/Y ; (d) 16% PW/Y ; and (e) 38% PW/Y .

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Table 1 Characteristics of the dealuminated zeolite (H,Na)-Y and supported samples containing 12-dodecatungstophosphoric acid Sample

PW12 in dry zeolite (H,Na)-Ya (wt.%)

BET (m2 g−1 )

(H,Na)-Y

0

669

2% PW/Y

1.8

664

5% PW/Y

5.3

676

16% PW/Y

15.8

560

27% PW/Y

27.3

523

38% PW/Y

38.2

423

31 P

MAS NMR signals

Chemical shift (ppm)

Ratio of −15/−13.3 signals

– – −13.3 −14.9 −13.4 −15.0 −13.2 −15.0 −13.3 −15.0 −13.3 −15.0

– 1.2 2.6 1.7 3.7 4.1

Calculated as wt.% of anhydrous heteropoly acid in the dehydrated zeolite (27.7 wt.% loss of zeolite (H,Na)-Y 1 on heating from 293 to 1003 K). a

the chemical shift at ca. −15 ppm. This species is therefore interacting weakly with the matrix, behaves similar to the bulk PW12 and is located at the external surface of zeolite crystals. The ratio between the two types of Keggin units depends on the matrix loading. At low coverage PW12 is deposited both in mesopores and on crystals, while at high loading the non-interacting Keggin units prevail (cf. Table 1, last column). 3.2. Disproportionation of toluene

bution shows (Tables 2 and 3) that benzene and xylene isomers are the main toluene conversion products (traces of TMB were only found at 723 K). If the heteropoly acid is supported on the matrix, the overall activity of the catalysts increases correspondingly. The more heteropoly acid is loaded, the higher conversion of toluene is observed, reaching finally 28% for the sample 38% PW/YY . Simultaneously, as seen in Fig. 2, the reaction commences at lower temperature (∼570 K). Product distribution shows clearly that in addition to benzene and xylenes, all three isomers

Catalytic properties of the dealuminated zeolite Y impregnated with different amounts of the 12-tungstophosphoric acid were tested in the disproportionation of toluene and transalkylation of toluene and 1,2,4-trimethylbenzene. The catalytic activity of a pure zeolitic matrix was also assessed for comparison purposes. It is recognised that both processes are reversible and being catalysed by acid centres [2]. Disproportionation of the two toluene molecules yields an equimolar mixture of xylene isomers and benzene. Depending on the reaction conditions, it may be accompanied by a parallel dealkylation of toluene to benzene, and also by a secondary transformations of xylenes. The dealuminated zeolite is active in toluene disproportionation above 600 K. The activity increases slowly to about 10% at 723 K (Fig. 2). Product distri-

Fig. 2. Conversion of toluene (mol%) on zeolite matrix and PW12 / zeolite hybrid catalysts. Conditions: catalyst weight W = 50 mg; helium flow F = 30 cm3 min−1 .

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Table 2 Product distribution and selectivity of toluene disproportionation on the parent zeolite (H,Na)-Y and supported samples 16% PW/Y and 38% PW/Ya Product

Benzene Toluene p-Xylene m-Xylene o-Xylene SD

613 K

653 K

(H,Na)-Y

PW/Y

16%

– 100.00 – – – –

1.94 97.53 0.11 0.38 0.04 47.34

38%

PW/Y

2.86 96.40 0.29 0.38 0.06 51.00

(H,Na)-Y

16% PW/Y

38% PW/Y

1.30 97.87 0.22 0.47 0.14 77.80

4.93 92.18 0.77 1.55 0.57 78.68

4.96 90.70 1.18 2.35 0.82 82.20

Normalised distribution of xylene isomers Equilibrium (620 K)b

Experimental p-Xylene m-Xylene o-Xylene p-Xylene/o-xylene a b

– – – –

28.4 62.3 9.3 3.1

36.6 53.7 9.7 3.8

23.72 53.00 23.28 1.02

Experimental 26.5 56.5 17.0 1.6

26.5 53.8 19.8 1.3

28.4 54.5 17.3 1.6

Product distribution is given in mol%. Equilibrium at 620 K [20].

of trimethylbenzene are formed at higher temperature (>690 K). The origin of trimethylbenzenes can be rationalised as being due to the secondary disproportionation of xy-

lene isomers towards toluene and trimethylbenzenes. As shown previously [21], no disproportionation was observed if pure m-xylene was used as a feed at 623 K on a dealuminated matrix, while a pronounced one was

Table 3 Product distribution and selectivity of toluene disproportionation for the transformation of toluene on the parent zeolite (H,Na)-Y and supported samples 16% PW/Y and 38% PW/Ya Product

Benzene Toluene p-Xylene m-Xylene o-Xylene 1,3,5-TMB 1,2,4-TMB 1,2,3-TMB SD

693 K

723 K

(H,Na)-Y

PW/Y

16%

3.55 93.65 0.69 1.45 0.66 – – – 88.15

8.22 85.50 1.48 3.27 1.34 0.03 0.15 – 83.37

38%

PW/Y

11.94 80.11 1.83 4.09 1.73 0.04 0.23 0.03 78.09

(H,Na)-Y

16% PW/Y

38% PW/Y

5.87 88.96 1.29 2.75 1.00 0.02 0.11 – 89.12

14.24 75.30 2.37 5.22 2.27 0.14 0.41 0.06 79.63

17.64 71.67 2.52 5.66 2.00 0.11 0.34 0.05 74.28

Normalised distribution of xylene isomers Equilibrium (700 K)b

Experimental p-Xylene 24.7 m-Xylene 52.3 o-Xylene 23.0 p-Xylene/o-xylene 1.1 a b

24.7 53.5 21.8 1.1

Product distribution is given in mol%. Equilibrium at 700 K [20].

24.6 52.9 22.1 1.1

24.4 52.2 23.4 1.04

Experimental 25.5 53.7 20.8 1.2

24.2 52.9 22.8 1.1

24.7 54.2 21.1 1.2

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exhibited upon PW12 /zeolite hybrid catalysts. The results of toluene disproportionation are therefore consistent with previous measurements. The plot of toluene conversion versus heteropoly acid content at 693 K shows clearly that first portions of 12-tungstophosphoric acid (up to ca. 10%) give rise to the most pronounced effect of the catalyst activity. Moreover, the relationship in this region is essentially linear (Fig. 3). Further loading with the PW12 increases the conversion from 15 to 20%. The distribution of xylene isomers is also given in each table. The data shows that distribution is very close to thermodynamic equilibrium. This can be rationalised as follows. Isomerization of dialkylbenzenes is a typical reversible reaction. If starting from any of the three xylene isomers, the secondary isomerization occurs and the system proceeds towards the thermodynamic equilibrium (for example, at 700 K the equilibrium data for xylene isomers are: 24.4% para, 52.2% meta and 23.4% ortho [20]; the p-xylene to o-xylene ratio is 1.04. Shape-selectivity of the reaction, expressed in terms of the p-xylene/o-xylene ratio, is visualised in Fig. 4 at two temperatures. As seen, at 613 K a preponderance of p-xylene is observed, and the para/ortho ratio is increasing with PW12 content in the catalyst (from about 2 to 3.5). If the temperature of the reaction is increased to 653 K, the secondary isomerization between the xylene molecules shifts the para/ortho ratios to values which are very close to the thermodynamic equilibrium. We also note an insignif-

179

Fig. 4. p-Xylene/o-xylene ratio on the zeolite matrix (H,Na)-Y and PW12 /zeolite hybrid catalysts at 613 K (a) and 653 K (b) as a function of the heteropoly acid content (wt.%).

icant effect of the PW12 content in the sample, manifesting itself in rising the para/ortho ratio to 1.2 for the catalyst containing 38% PW12 (Fig. 4, line b). It is also interesting that the selectivity of disproportionation is lower than 100%. This is due to the secondary reactions proceeding on the catalysts: a secondary transalkylation occurring between toluene and the xylene molecules, yielding trimethylbenzenes and benzene, and disproportionation of xylene isomers, as discussed earlier. These secondary reactions were observed clearly during studies of m-xylene transformations [22]. It was also of interest to estimate the apparent activation energy of toluene transformation over the catalyst studied. By using the Bassett–Habgood approach [18], the effective rate constant of overall toluene conversion and apparent activation energies were calculated. The results are listed in Table 4. As seen, the Table 4 Apparent activation energy of toluene transformation on the dealuminated zeolite and PW12 /zeolite hybrid catalysts

Fig. 3. Overall conversion of toluene on the zeolite matrix (H,Na)-Y and PW12 /zeolite hybrid catalysts as a function of the heteropoly acid content (wt.%) at 693 K. Conditions as in Fig. 2.

Sample

Apparent activation energy (kJ mol−1 )

(H,Na)-Y 2% PW/Y 5% PW/Y 9% PW/Y 16% PW/Y 27% PW/Y 38% PW/Y

87.4 87.0 73.9 71.6 70.3 70.1 61.2

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highest activation energy was found for the pure zeolite and a catalyst with a very low PW12 content. Upon increasing the PW12 content, we observed an abrupt decrease of E to about 70 kJ mol−1 . This value remained constant for the catalysts containing 5–28% PW12 and decreased further to 61.2 kJ mol−1 for the 38% PW/Y sample. 3.3. Transalkylation of toluene and 1,2,4-trimethylbenzene Transalkylation was studied using equimolar mixture of the two hydrocarbons. Transalkylation of toluene and 1,2,4-trimethylbenzene gives xylene isomers as primary products. Secondary reactions involve here dealkylation and isomerization as well. As found for disproportionation of toluene, loading of the matrix with 9 and 38% of PW12 led to a drastic increase of the overall conversion in comparison with the pure zeolite. This is seen in Fig. 5 in the temperature range of 530–650 K. Shape-selective effects were evident only at the lowest temperature 573 K (Fig. 6). Both increasing the temperature and the matrix loading with PW12 give rise to the equilibration between the xylene molecules, thus giving para/ortho value of 1 (see a normalised xylene distribution in Table 5). Two other processes are accompanying title transalkylation: isomerization of 1,2,4-TMB and its

Fig. 5. Conversion of 1,2,4-trimethylbenzene (mol%) as a function of temperature for the transalkylation process between toluene and 1,2,4-TMB for (a) zeolite matrix; (b) 9% PW12 /Y ; and (c) 38% PW12 /Y .

Fig. 6. p-Xylene/o-xylene ratio on the zeolite matrix (H,Na)-Y and hybrid catalysts at 573 K (a) and 653 K (b).

dealkylation. Typical selectivities for the three reactions are shown as a function of heteropoly acid loading (Fig. 7). As seen, there is a constant rising trend for the selectivity of transalkylation at the expense of selectivity of isomerization and dealkylation. As dealkylation is known to proceed on a very strong acid centres [2], this observation can be explained by a slight decrease of the acid centres strength with the PW12 content, despite of the increase of their overall number. This is in line with a generally observed trend that isolation of the acid sites enhances their

Fig. 7. Transalkylation of 1,2,4-TMB and toluene (1:1) molar ratio at 693 K. Selectivity of 1,2,4-TMB conversion along the three pathways: (a) transalkylation; (b) isomerization; and (c) dealkylation is shown as a function of PW12 loading. Conditions as in Fig. 2.

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Table 5 Product distribution and selectivity of 1,2,4-trimethylbenzene transformation along different paths for the transalkylation reaction between toluene and 1,2,4-TMBa Product

Benzene Toluene p-Xylene m-Xylene o-Xylene 1,3,5-TMB 1,2,4-TMB 1,2,3-TMB ST b Si c SDe d

693 K

723 K

(H,Na)-Y

PW/Y

9%

2.48 36.59 9.63 22.62 9.61 5.55 13.00 3.00 58.72 20.56 20.7

2.30 35.39 9.78 22.30 9.73 6.16 14.22 2.42 60.31 21.22 18.5

38%

PW/Y

4.06 34.16 10.46 23.78 10.05 6.00 13.67 1.88 63.46 19.05 17.5

(H,Na)-Y

9% PW/Y

38% PW/Y

5.13 39.96 10.00 22.71 10.11 4.38 10.75 2.10 56.49 14.03 29.5

4.27 37.32 10.04 22.57 9.99 4.95 12.11 3.03 57.42 16.22 26.4

5.47 39.05 10.16 22.99 10.17 4.52 10.67 2.44 58.23 15.38 26.4

Normalized distribution of xylene isomers Experimental p-Xylene m-Xylene o-Xylene

23.2 54.0 22.8

Equilibrium (700 K) 23.4 53.4 23.2

23.6 53.8 22.6

Experimental 24.4 52.2 23.4

23.4 52.9 23.7

23.5 53.0 23.5

23.6 52.9 23.5

a

Product distribution is given in mol%. Selectivity of 1,2,4-TMB transalkylation. c Selectivity of 1,2,4-TMB isomerization. d Selectivity of 1,2,4-TMB dealkylation. b

strength (phenomenon found for dealuminated zeolites [23] and also in a series of dicarboxylic organic acids). How can one rationalize the catalytic data obtained? We suggest these findings can be explained taking into account the ratio between the two PW12 entities, as evidenced by NMR studies. Because the abrupt influence of PW12 content on the catalytic activity is clearly seen in reactions studied in this paper, and also in transformations of pure m-xylene, as shown earlier [21], the more catalytically active form of the Keggin unit is probably the one prevailing at low PW12 loading and being characterised by the chemical shift at −13.3 ppm. 4. Conclusions We have demonstrated that it is possible to prepare a stable hybrid catalysts based on the dealuminated faujasite matrix. The catalysts retain their structure and exhibit good sorption properties.

Two types of Keggin units exist in the catalysts: (i) those interacting strongly with the support, thus chemically modified, located in the mesopores of zeolite matrix and giving NMR signal at −13.3 ppm and (ii) those weakly interacting with the matrix, characterized by essentially unperturbed 31 P NMR spectra with the chemical shift at ca. −15 ppm, and present at the external surface of zeolite crystals. The ratio between the two species depends on the matrix loading. At low coverage the former species prevails, while at high loading the latter ones are dominant in the samples. Pure zeolite matrix is active in both reactions studied. The activity is however increased markedly upon loading with the heteropoly acid molecules. The effect of the loading is very clearly seen at low heteropoly acid loading (below 10 wt.% of PW12 in the catalysts). The more catalytically active form of the Keggin unit is probably the one prevailing at low PW12 loading and being characterized by the chemical shift at −13.3 ppm. Finally, a pronounced shape-selectivity of the catalysts is observed at low temperatures in both processes studied.

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Acknowledgements We thank A. Kubacka, M.Sc., for samples preparation and Dr. Z. Olejniczak for NMR spectra of the catalysts. References [1] A. Streitwieser, L. Reif, J. Am. Chem. Soc. 82 (1960) 5003. [2] N.Y. Chen, W.E. Garwood, F.G. Dwyer, Shape Selective Catalysis in Industrial Applications, Marcel Dekker, New York, 1989. [3] J. Dewing, J. Mol. Catal. 27 (1984) 25. [4] B. Sulikowski, React. Kinet. Catal. Lett. 16 (1981) 39; B. Sulikowski, React. Kinet. Catal. Lett. 31 (1986) 215. [5] B. Sulikowski, J. Klinowski, Z. Phys. Chem. (München) 177 (1992) 93. [6] B. Sulikowski, J. Klinowski, J. Phys. Chem. 96 (1992) 5030. [7] B. Sulikowski, J. Datka, B. Gil, J. Ptaszynski, J. Klinowski, J. Phys. Chem. B 101 (1997) 6929]. [8] T. Okuhara, N. Mizuno, M. Misono, Adv. Catal. 41 (1996) 113.

[9] P. Hudec, K. Prandová, Collect. Czech. Chem. Commun. 60 (1995) 443. [10] R. Fricke, G. Öhlmann, J. Chem. Soc., Faraday Trans. 1 82 (1986) 263, 273. [11] J.B. Moffat, S. Kasztelan, J. Catal. 109 (1988) 206. [12] K. Brückman, M. Che, J. Haber, J.M. Tatibouet, Catal. Lett. 25 (1994) 225. [13] B. Sulikowski, J. Haber, A. Kubacka, K. Pamin, Z. Olejniczak, J. Ptaszynski, Catal. Lett. 39 (1996) 27. [14] Z. Olejniczak, B. Sulikowski, A. Kubacka, M. Gasior, Top. Catal. 11–12 (2000) 391. [15] H.S. Booth (Ed.), Inorganic Synthesis, vol. 1, McGraw-Hill, New York, 1939, p. 132. [16] G.T. Kerr, J. Phys. Chem. 73 (1968) 2780. [17] B. Sulikowski, Heterogeneous Chem. Rev. 3 (1996) 203. [18] D.W. Bassett, J.W. Habgood, J. Phys. Chem. 64 (1960) 769. [19] B. Sulikowski, J. Phys. Chem. 97 (1993) 1420. [20] W.J. Taylor, D.W. Wagman, M.G. Williams, K.S. Pitzer, F.D. Rossini, J. Res. Natl. Bur. Stand. 37 (1946) 95. [21] K. Pamin, A. Kubacka, Z. Olejniczak, J. Haber, B. Sulikowski, Appl. Catal. A 194–195 (2000) 137. [22] B. Sulikowski, et al., to be published. [23] R. Beaumont, D. Barthomeuf, J. Catal. 26 (1972) 218; D. Barthomeuf, R. Beaumont, J. Catal. 30 (1973) 288.