Preparation and catalytic characterisation of Al-grafted MCM-48 materials

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

755

Preparation and catalytic characterisation o f Al-grafted M C M - 4 8 materials M. Rozwadowski, *aM. Lezanska, a J. W l o c h , a K. Erdmarln, a and J. Komatowskib aFaculty of Chemistry, Nicholas Copernicus University, Gagarina 7, 87-100 Torun, Poland bLehrstuhl II fttr Technische Chemie, Technische UniversiN't Mttnchen, Lichtenbergstr. 4, 85747 Garching bei Mttnchen, Germany

Samples of A1-MCM-48 were prepared by grafting A1 onto the pure siliceous material and used as catalysts for cumene cracking and conversion of 2-propanol. The former reaction yielded mainly benzene and propene, which indicated the presence of strong Bronsted acid sites in the catalysts. The conversion of 2-propanol resulted mainly in dehydration of the substrate, yielding propene and diisopropyl ether. The catalytic activity of A1-MCM-48 grew with both the A1 content and reaction temperature. The concentrations of the Bronsted and Lewis acid sites increased with the A1 content of the material as well.

1. INTRODUCTION Mesoporous molecular sieves of the M41S family [ 1,2] have extensively been studied with respect to their unique properties [3-6]. Many efforts have been focused on silica- and alumina-based materials as potential catalysts for the reactions involving large organic molecules [7-10]. Purely siliceous M41S does not show significant catalytic activity because of its electrically neutral skeleton with no ion-exchange capability. However, substitution of silicon with various metals generates acidity in these materials and modifies their surface properties. This is a promising way to synthesise materials applicable in catalysis [11,12]; for example, introduction of boron [7,13], titanium [14-16], vanadium [17], and gallium [8] has been reported. Incorporation of aluminium is also interesting in relation to catalytic applications and has been discussed in numerous papers, especially in the case of the MCM-41 materials. Reports, although not so many, on the introduction of A1 into MCM-48, another member of the M41S family, have been published as well [18-20]. Such a modification of the M41S structure seems to be of particular importance as it can give rise to the Bronsted acid sites. These centres should primarily be responsible for the catalytic activity of the mentioned materials. Generally, the Al-containing molecular sieves can be obtained by a hydrothermal (i.e., direct) synthesis or by post-synthesis methods of impregnation or grafting. Jun and Ryoo [21 ] investigated the catalytic activity of mesoporous molecular sieves of different channel systems (MCM-41, MCM-48, and KIT-I; Si/A1 = 19 and 38) in the Friedel-Crafts alkylation reaction. They demonstrated that the materials prepared with the post-synthesis procedures were superior to those synthesised directly with respect to the structural order and

756 accessibility of the A1 centres to reactants. The authors suggest that the latter is caused by the fact that, in the case of the hydrothermal synthesis, a part of A1 becomes located inside the pore walls, especially when the A1 content is relatively low. Cheng et al. [22] showed that A1grafted MCM-41 exhibited a considerably higher acidity as compared to that of A1-MCM-41 obtained hydrothermally (both materials with Si/A1 = 20). This was reflected in the results of cumene cracking. However, when Si/A1 was in the range of 1-6 [23], the materials synthesised directly exhibited a higher acidity but their structure was not typical of MCM-41. On the other hand, the Al-grafted samples retained the MCM-41 structure. Corma et al. [24] found that the acid strength of A1-MCM-41 synthesised hydrothermally was lower than that of zeolite USY and higher than that of amorphous aluminosilicates. The aim of this work was to study the catalytic reactions of cumene cracking and conversion of 2-propanol over the Al-grafted MCM-48 samples. It was expected that the content and/or distribution of A1 might affect the strength of the Bronsted acid centres similarly as in zeolites. Therefore, we attempted to correlate the postulated reaction mechanisms with the acidic strength of these sites.

2. EXPERIMENTAL

2.1. Samples The MCM-48 material was synthesised from a mixture containing suspension of SiO2 (Ultrasil, Degussa) in water and both tetramethylammonium hydroxide and cetyltrimethylammonium chloride as templates [25]. Four different A1-MCM-48 samples were prepared by grafting aluminium onto the purely siliceous MCM-48 parent material. Aluminium isopropoxide dissolved in n-hexane was chosen as the source of aluminium for the grafting process. The resulting materials were calcined at 803 K under air for 4 h. The samples are referred to as A1-MCM-48(n) where n denotes the Si/A1 molar ratios in the reaction mixtures, equal to 32, 15, 5, and 2. The Si/A1 ratios of the calcined A1-MCM-48 samples were determined with the atomic absorption spectroscopy (AAS) (see Table 1). More details on the sample preparation can be found elsewhere [26]. 2.2. Catalysis The catalytic tests were carried out with a pulsed method using a vertical flow microreactor connected to a Shimadzu GC-14B gas chromatograph equipped with a flame ionization detector. The catalyst samples (5 mg) were placed in the reactor and treated thermally at 723 K under helium for 1 h. Cumene was injected at 25-min intervals (eight injections, 1-~tl portions) and the reaction was run at 623,673, and 723 K. The chromatographic column was packed with Carbowax 4000 and the carrier gas (helium) was flowing at a rate of 30 ml/min. In the case of 2-propanol, four injections (1-~tl portions) were applied in 15-min intervals, the reaction temperatures were 523 and 573 K, and the column was packed with Porapak N.

2.3. Acid sites For the analysis of the Bronsted and Lewis acid sites present in the studied A1-MCM-48 materials, the IR spectra were recorded with a Bruker 48 PC spectrometer equipped with a MCT detector. The samples in the form of wafers were activated in situ in the IR cell at 633 K for 1 h. Then, pyridine (POCh, Poland, dried over KOH) taken in excess of the amount necessary to neutralise all the acid sites was adsorbed at 430 K. Subsequently, the physisorbed

757 pyridine was removed under 30-min evacuation at the same temperature and then the IR spectra were recorded. Concentrations of both the Bronsted and Lewis acid sites were calculated from intensities of the IR bands assigned to pyridinium ions (HPy +) and to pyridine molecules bonded to Lewis sites (PyL) at 1545 and 1455 cm-1, respectively. The extinction coefficients used for the calculations were determined for pyridine adsorbed on both the zeolite HY containing only the Bronsted acid sites and the dehydroxylated zeolite HY containing only the Lewis acid sites. They were equal to 0.070 and 0.100 cm 2 gmo1-1 for HPy + and PyL, respectively.

3. RESULTS AND DISCUSSION

The low-angle XRD powder patterns of the studied samples demonstrated a set of peaks (including the 211 and 220 reflections), indicating a typical system of uniform cubic pores [26]. These pores are considered as the primary mesopores while void space between adjoining crystallites and large mesopores in the particles that do not form any ordered structures are referred to as the secondary mesopores [26]. The combined volume of both the primary and secondary mesopores is defined as a total pore volume. Table 1 shows some structural parameters of the studied samples. Although the values of SBET and Vt somewhat decreased with the increase in the content of A1, they were relatively high. This suggested that the materials might exhibit noteworthy catalytic properties. The reaction of the catalytic cracking of cumene results in a series of compounds with different numbers of carbon atoms in a molecule, propene and benzene being the main products [27,28]. Comparison of the level of the cumene conversion performed over different samples at a given temperature allows one to arrange these samples with respect to their acidity [29]. Here, it was found that the cumene conversion increased with the content of aluminium in the A1-MCM-48 materials (Fig. 1). For a given sample, the cumene conversion Table 1 Structural parameters of the studied MCM-48 materials [26] Parameter

Sample parent A1-MCMA1-MCMA1-MCMA1-MCMMCM-48 48(32) 48(15) 48(5) 48(2) Si/A1 (AAS) n.a. 34.5 12.7 3.8 3.5 d211 [nm] 3.71 3.40 n.d. 3.32 n.d. ao [nm] 9.10 8.33 n.d. 8.14 n.d. SBET [m2 g-~] 1315 1245 1188 1051 1030 2 -1 St [m g ] 1294 1213 1164 1010 995 Next [m2 g-l] 284 259 136 154 209 Sp [m2 g-l] 1010 954 1028 856 786 Vp [cm3g-l] 0.718 0.662 0.673 0.538 0.513 Vt [cm 3 g-l] 0.958 0.884 0.794 0.713 0.711 d211 is the (211) interplanar spacing, a 0 - unit cell parameter, aBET- the BET specific surface area, St - total surface area, Sr - external surface area, Sp - surface area of primary mesopores, Vp - volume of primary mesopores, Vt - total pore volume, n.a. - not applicable, and n.d. - not determined.

758 increased also with the reaction temperature, as seen for A1-MCM-48(5) (Fig. 2). A similar picture was observed for A1-MCM-48(2)while for A1-MCM-48(15)and A1-MCM-48(32)the increase in the conversion with the temperature was clearly lower. In general, the conversion decreased slightly with the number of injections (Figs. 1 and 2). The rate of this decrease was more pronounced for the samples with higher contents of A1 and practically independent of the reaction temperature. These observations suggest a more efficient coking of the catalysts with the higher A1 contents. 50

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Figure 1. Cumene conversion at 723 K over A1-MCM-48 with different Si/A1 ratios: 3.5 (+), 3.8 (O), 12.7 (A), and 34.5 ( 9

Figure 2. Cumene conversion at 623 ( 9 673 (A), and 723 K (F]) over A1-MCM-48 with Si/A1 = 3.8.

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Figure 4. Selectivity of cumene conversion at temperatures indicated over A1-MCM-48 with Si/A1 = 3.8; b, benzene, p, propene, m, a-methylstyrene; 1 and 4 denote selectivities after the injections no. 1 and 4, respectively.

759 In accordance with the literature findings, benzene and propene were the main products of the cumene cracking over A1-MCM-48 (Figs. 3 and 4). a-Methylstyrene was another product found in significant amounts. Some not determined compounds were also observed though they were present in trace amounts only. These products were neglected while calculating selectivity. At 723 K, the relative yield of benzene and propene slightly increased and that of a-methylstyrene slightly decreased with the growth of the aluminium content of the catalysts (Fig. 3). On the other hand, the relative yield of benzene and a-methylstyrene slightly decreased whereas that of propene slightly increased with the rising reaction temperature, as observed for A1-MCM-48(5) (Fig. 4). The selectivities of all the products did not change much with the injection number (Fig. 4). As known [27,29], benzene and propene are formed on strong Bronsted acid sites while a-methylstyrene forms at electron-acceptor centres. Thus, the presented observations (Fig. 3) suggest that the number of the electron-acceptor centres decreased while that of the Bronsted acid sites slightly increased with the A1 content of the catalysts. According to stoichiometry of the reaction, the cracking of cumene should yield equal amounts of benzene and propene. The observed lower amounts of propene (Figs. 3 and 4) result most probably from the fact that propene undergoes to a greater extent the conversion to carbonaceous deposits, especially at lower temperatures. In the case of conversion of 2-propanol, two reactions were assumed to occur: (i) dehydration, which leads to formation of propene and diisopropyl ether and (ii) dehydrogenation, which yields acetone [30,31 ]. As found here, the conversion at 523 K increased from c a . 50 to 100% with the Si/A1 of the A1-MCM-48 samples decreasing from 34.5 to 3.5 (Fig. 5). Thus, the catalytic activity of the studied materials grew clearly with the A1 content of the catalysts. At 573 K, the conversion over all the catalysts was approximately 100%. Interestingly, the conversion did not depend on the number of injections. Propene was the main

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Figure 5. 2-Propanol conversion at 523 K over A1-MCM-48 with different Si/A1 ratios: 3.5 (+), 3.8 ([]), 12.7 (zx), and 34.5 (9

760 product of this reaction. At 523 K, selectivity toward propene was c a . 98.5% for A1-MCM48(32) and it increased up to c a . 100% with the content of A1 (Fig. 6). Diisopropyl ether was the other important product while acetone was detected in trace amounts only. The selectivity toward propene decreased slightly with the number of injections of 2-propanol. At 573 K, however, the contribution of propene for all the samples was practically 100% and did not decrease with the injection number. The relation between the level of conversion of the examined compounds and the A1 content was confirmed by the IR analysis of the acid centres. As found, the studied A1-MCM48 catalysts differ in the concentrations of the Bronsted and Lewis acid centres that determine the course of the conversion of cumene and 2-propanol. The calculated concentrations of the sites in the parent MCM-48 material and selected A1-MCM-48 samples are listed in Table 2. Some amount of the Lewis sites detected in the parent material is presumably due to traces of A1 present in the reagents used for the synthesis. As seen from the table, the Bronsted acidity of A1-MCM-48(5) is only c a . 3.5 times higher than that of A1-MCM-48(32) although the A1 content is ca. 10 times higher. This implies that the Al-rich sample contains likely a relatively high amount of aluminium that is not incorporated into the structure of the material and does not give rise to the Bronsted acidity. Another reason for the observed catalytic behaviour of the studied samples may be connected with a different acid strength of the catalyst centres. The acid strength can decrease with the rising concentration of the centres that control the examined reactions. Such a dependence, although not very clear, has been found by us for the MCM-41 materials [ 10]. The analysis of the acid strength of the centres of the A1-MCM-48 samples is in progress. These results and comparison of the catalytic activity between Al-grafted MCM-48 and other molecular sieves (zeolites, amorphous aluminosilicates) are planned to be included in a next paper. 100

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761 Table 2 Concentrations of Bronsted and Lewis acid sites Acid sites [~tmol g-l] Bronsted Lewis

MCM-48 0 32

Sample A1-MCM-48(32) 32 150

A1-MCM-48(5) 110

430

4. CONCLUSIONS The examined A1-MCM-48 materials differ in the concentrations of the Bronsted and Lewis acid sites, which increase with the A1 content. High conversion of the reaction of cumene cracking over A1-MCM-48 indicates the presence of strongly acidic Bronsted sites. The conversions of cumene and 2-propanol grow with both the A1 content and reaction temperature. Benzene and propene are the main products of the cumene cracking, a-methylstyrene being another product found in considerable amounts. In the case of the 2-propanol conversion, dehydration is the principal reaction. It leads to formation of propene in predominating amounts and of diisopropyl ether. The concurrent reaction of dehydrogenation yields acetone in trace amounts only. In spite of large differences in the A1 contents of the catalyst, the results of the catalytic reactions do not indicate significant differences in their Bronsted acidity. An increase in the concentration of the Bronsted sites may cause some decrease in their acidic strength. Further investigations are in progress.

ACKNOWLEDGEMENT Thanks are due to Prof. J. Datka (Krakow, Poland) for the IR analysis of acid centres. The work was supported in part by the State Committee for Scientific Research (KBN).

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