γ-Al2O3 catalyst

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A PT PA LE IY DSS CA L I A: GENERAL

ELSEVIER

Applied Catalysis A: General 158 (1997) LI-L6

Letter

Para-selectivity in the alkylation of toluene with isopropanol on a MCM-41/3,-A1203 catalyst J. Medina-Valtierra a'*, M i g u e l A. Sfinchez a, J.A. M o n t o y a b, Juan Navarrete b, J.A. de los Reyes c aFacultad de Ciencias Qu[micas, Universidad Autdnoma de Zacatecas, Apdo. Postal 461, Zacatecas 98000 Zac., Mixico blnstituto Mexicano del Petrdleo, Eje Ldzaro C6rdenas No. 152, DF 07730, Mixico CUniversidad Aut6noma Metropolitana-I, Av. Michoac6n y La, Purisima s/nDF 09340, Mixico

Received 25 July 1996; received in revised form 2 December 1996; accepted 17 December 1996

Abstract Alkylation of toluene with isopropanol to produce isopropyltoluenes was investigated using a MCM-41 aluminosilicate as catalyst. The MCM-41/?-A1203 catalyst presented mainly strong Lewis sites so that this material was activated by means of a liquid extraction method. It was observed that in the isopropylation of toluene, the isopropyltoluenes fraction contained more para isomer. This is due to the fact that MCM-41 materials contain relatively a few acid sites. In addition, the primary products of alkylation easily diffuse into the broad channels of this mesoporous framework. Keywords: Mesoporous molecular sieve; MCM-41 aluminosilicate; MCM-4 lkT-A1203 catalyst; Isopropylation;

Selectivity

Shape-selective conversions of benzene and toluene alkylation over zeolite catalysts have been extensively studied [1-3]. Medium pore zeolites (pore diameter 3.5-5.6/k) e.g., ZSM-5 and ZSM-35 have already been studied in a number of these processes due to the advantages that they present [4,5]. Nevertheless, there are a few studies concerning the selective conversions on MCM-41 material [6], a novel-mesoporous molecular sieve. The MCM-41 structure posses a hexagonal array of uniform mesopores in the range of 20-100,~. This material can incorporate A1 species into the siliceous framework and therefore can develop acidity and perform acid-catalyzed reactions. In this paper, we report * Corresponding author. Tel.: +91 492 31006; fax: +91 492 30137. 0926-860X/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. Pll S0926- 860X(97)00007-0

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the application of actived MCM-41 material for the catalytic alkylation of toluene as a good probe reaction for selectivity studies. MCM-41 was synthesized according to the method derived from the literature [7] and it was prepared using Catapal alumina as the source of aluminum. The molar composition in the hydrogel was: 30SiO2-A1203-4.2(TMA)209.4HDTMABr-298H20. Crystallization of the gels was carried out in a stainless steel 300 ml autoclave at 150°C for 24 h. After the autoclave was cooled the product was filtered, washed with deionized water, and dried at room temperature overnight (sample TAME3/L). To obtain the active form of this aluminosilicate, the as-synthesized sample was calcined in a static muffle at 300°C for 3 h. Then it was washed twice with an ethanol/IN NHaNO3 solution (1 g/10 ml, 25% vol. of nitrate) at 80°C for 1 h. Finally the dry solid was calcined again at 550°C for 3 h to burn off the remnant organic template (sample TAME3/E). Previous results suggest that a limited number of Al-species interact strongly with the template, therefore the loss of template removes or modifies these Al-species [8]. This effect could decrease the acidity of the final catalytic material. The MCM-41 aluminosilicate was mixed with appropriate quantities of boehmite, nitric acid and distilled water and this for giving a proportion MCM-41: ,y-alumina of 40 : 60 in the final extrudate. The extrudates (1/16 in.) were calcined in muffle with static atmosphere at 500°C for 3 h. This catalytic material is called here CATZ-4. All samples were characterized by X-ray diffraction, XRD (Siemens D500, CuKc0, IR spectroscopy with adsorbed pyridine (Nicolet FTIR 710) and ammonia temperature-programmed desorption, tpd-NH3 (Altamira Analyzer). Texture parameters of TAME3/E and CATZ-4 were determined from the adsorption isotherms of nitrogen at 75 K. The adsorption isotherms were measured with an Accusorb 2100 E instrument. 6 g of the active catalyst were loaded in the reactor, a fixed bed, placed in a stainless steel tube. Evaluations were performed at close to atmospheric pressure. The mixture of toluene and isopropanol was fed by a syringe pump, vaporized in the preheater, and passed through the reactor with the catalyst maintained at the reaction temperature. The products were trapped in a condenser at the reactor outlet and analyzed by means of FID gas chromatography (SRI instruments 9610) using a 0.25 × 60 mm DB-5 capillary column. In some of the runs at high temperatures the gaseous products were collected and identified. The XRD patterns for the actived sample and final catalyst in the 20= 1-9 region (Fig. 1) were similar to those given for MCM-41 [9]. For (a) and (b) samples four reflections the [1 0 0], [1 1 0], [2 0 0] and [2 1 0] can be indexed according to a hexagonal symmetry. In the composite catalyst the last three peaks overlap giving a singole broad line. The XRD pattern of TAME3/E reveals a dlo0 value of about 49 A, which for a hexagonal symmetry gives a=55 ,~ and a wall tickness of 2 × 6 ,~. The pore diameter for this sample was calculated to be about 43 A. The position and width of the [ 1 0 0] reflection is modified for the actived sample with a 3 A contraction with respect to the TAME3/L sample. This means that the cristallinity of the original material could be affected upon calcination as it

J. Medina-Valtierra et al./Applied Catalysis A: General 158 (1997) L1-L6

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was previously reported [10]. The surface area SBET, calculated from the desorption branch of the hysteresis loop was 500 m 2 g i. Based on the application of the Kelvin equation a 48 ,~ diameter as the effective mean pore diameter was obtained. The acidity of "/-alumina binder and CATZ-4 as measured by tpd-NH3 are 0.302 and 0.288 mmol g - l , respectively. The quantity of acid sites proportionated by 7-alumina in CATZ-4 was 0.181 mmol/g. Fig. 2 shows the infrared spectra of pyridine in the region 1700-1400 cm -~ following its adsorption on the samples 7-A1203, TAME3/E and CATZ-4 and subsequent thermal treatment at 200-400°C. The 7-A1203 material contains only Lewis acid sites (L band at 1 4 5 0 cm -~) most of which are lost after evacuation at 300°C. TAME3/E contains a notable amount of Lewis acid sites and a smaller amount of BrOnsted acid sites (bands B at 1545 and 1640 cm-1). One can note that most of the acid sites on this sample are strong so that they still appear at 400°C. From the IR spectra of absorbed pyridine on sample CATZ-4, it is observed that most of the acid sites present are strong Lewis sites. The unique peak (L1) in the 1600-1650 cm - l range reveals the existence of homogeneity of Lewis acid sites on the composite catalyst. This Lewis acidity can be related to the presence of extra-framework A1 formed during template removing [8,11]. This has been confirmed by means of 27A1 MAS NMR analysis (data not presented here) where octahedral A1 species were observed in the washed sample [12]. It is interesting to note 7-A1203 and TAME3/E give the band due to hydrogen-bonded pyridine (H about 1585 cm-1). This observation is consistent with reports on the acidity of several pure aluminas [13]. This indicates that removal of some aluminium from the

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J. Medina-Valtierra et al./Applied Catalysis A: General 158 (1997) L1-L6

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MCM-41 framework in the course of calcination, is accompanied by the formation of non-structural A1. Our acidity results confirm that ordened mesoporous materials are not as acidic as microporous zeolites due to the relatively lower acidity of MCM-41 [14]. Possible alkylation products from toluene and isopropanol are mainly isopropyltoluenes (p, m, o) and diisopropyltoluenes. Regarding para-isopropyltoluene (p-IPT) it has a smaller kinetic diameter. Thus, this isomer is expected to be selectively formed into the channels of some shape-selective zeolites, as ZSM-5. As seen in Table 1, the CATZ-4 catalyst was active and selective for alkylation to produce p-IPT. This is of importance given there is little evidence of a selective nature of these mesoporous materials in aromatic alkylations [6,15]. Interestingly, a high selectivity (>70%) is maintained after 4 h of run. However, catalyst deactivation can be faster at higher temperatures. The catalytic activity and selectivity of CATZ-4 at different space velocities are summarized in Table 2, based on data taken after 4 h of run. As expected, an increase in WHSV from 0.25

J. Medina-Valtierra et al./Applied Catalysis A: General 158 (1997) L1-L6

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Table 1 The catalytic activity of CATZ-4 for the alkylation of toluene with isopropanol Reaction time (min) Toluene conversion (%)

20 20.08

60 19.53

110 18.70

140 17.63

180 18.02

220 17.47

61.44 15.41 21.25 1.90

76.54 21.29 0.68 1.49

75.06 20.78 0.54 3.62

70.87 21.98 0.47 6.68

71.40 19.18 0.40 9.02

71.30 22.49 0.36 5.85

Selectivity (wt%) p-Isopropyltoluene m-Isopropyltoluene o-Isopropyltoluene Di-isopropyltoluenes

Temperature: 200°C; WHSV=0.85 h - l ; toluene : isopropanol-3 : l(mol).

Table 2 Effect of space velocity in the alkylation of toluene with isopropanol on CATZ-4 WHSV (h 1) Toluene conversion (%)

Selectivity (wt%) Benzene p-Xylene m-Xylene p-Isopropyltoluene m-Isopropyltoluene o-Isopropyltoluene Di-isopropyltoluenes

1.49 13.55

0.85 18.32

0.47 19.59

0.25 22.44

0.0 0.0 0.0 71.70 27.80 0.50 0.0

0.0 0.0 0.0 71.25 24.61 0.0 4.13

5.53 0.66 0.0 66.75 24.64 0.0 2.40

10.63 2.24 1.50 64.72 20.05 0.34 1.00

Temperature: 200°C; toluene : isopropanol=3 : l(mol).

to 1.5 h 1 resulted in a decrease of activity. However, it was observed that the high selectivity for p-IPT decreases generally when toluene conversion is increased. The yield of meta-isopropyltoluene (m-IPT) was significant, especially with the low conversions of toluene. Table 3 shows the results of the alkylation reaction in the temperature range of 150-400°C. The level of toluene conversion increased and p-IPT selectivity was lowered when the reaction temperature increased. Side reactions such as toluene disproportionation and cracking took place at high temperatures. These side reactions are confirmed by the presence of benzene and xylenes at 250°C, and by the presence of notable amounts of benzene and C1-C2 products at higher temperatures. As the MCM-41 structure has broad channels (>40 ,~), it is expected that spatial constrains do not occur in the formation of products. From the data presented here it does not appear that selectivity is very much affected by the change in space velocity. It seems that enhanced para-selectivity is not related so much to diffusional effects but more to the fact that the MCM-41 material contains relatively a few acid sites and this when compared with microporous zeolites. Hence, p-IPT is formed as the primary product of isopropylation, which do not easily isomerize to the m-IPT due to the lower probability of reacting. Therefore, it

J. Medina-Valtierra et al./Applied Catalysis A: General 158 (1997) L1-L6

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Table 3 Effect of reaction temperature in the alkylation of toluene with isopropanol on CATZ-4 Temperature (°C) Toluene conversion (%)

150 4.77

200 14.55

250 18.70

300 22.00

400 26.50

Toluene conversion to gaseous products" (wt%) Cb C2 -C3 --

---

1.1 --

4.18 0.92

6.10 1.20

Selectivi~ of liquid products (wt%) Benzene 0.0 Xylenes 0.0 p-Isopropyltoluene 61.61 m-lsopropyltoluene 32.39 o-lsopropyltoluene 0.00 Di-isopropyltoluenes 0.0

0.0 0.0 71.00 22.13 6.87 0.0

14.20 2.22 53.87 10.0 0.75 6.04

29.70 10.86 50.60 6.80 1.50 1.10

38.80 42.24 16.27 1.87 0.80 0.0

WHSV=0.85 h -1, toluene : isopropanol=3 : l(mol).

was of interest to find that with MCM-41 catalyst the isopropyltoluenes fraction contained more para isomer. In this case the primary products of alkylation diffuse into the broad channels without undergoing further transformation. Due to that larger molecules as alkylnaphthalenes too easily diffuse in the mesopore channel of MCM-41 [16], we conclude that the main out-of-pore products are those favored kinetically on active sites. The results of this work - - while still preliminary - demonstrate the potential of MCM-41 material as a selective catalyst for some alkylation reactions.

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