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Alkylation and disproportionation of aromatic hydrocarbons over mesoporous molecular sieves
Microporous and Mesoporous Materials 44±45 (2001) 499±507
www.elsevier.nl/locate/micromeso
Alkylation and disproportionation of aromatic hydrocarbons over mesoporous molecular sieves a,* Jirõ Cejka , Andrea Krejcõ a, Nadezda Zilkov a a, Jirõ Dedecek a, Jirõ Hanika b a
J. Heyrovsk y Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejskova 3, CZ-182 23 Prague 8, Czech Republic b Department of Organic Technology, Institute of Chemical Technology, Technick a 5, CZ-166 28 Prague 6, Czech Republic Received 15 February 2000; accepted 24 August 2000
Abstract Toluene alkylation with propylene and trimethylbenzene (TMB) disproportionation were investigated over mesoporous molecular sieves of MCM-41 type of dierent pore dimensions, possessing various Si/Al ratios or modi®ed with heteropoly acids (HPA) of Keggin type. It was shown that the acid strength of (Al)MCM-41 was sucient to activate propylene in the alkylation of toluene providing cymene selectivity over 96% while the rate of consecutive reactions was negligible. In contrast, TMB disproportionation proceeded at a signi®cantly lower extent than over zeolites. Siliceous MCM-41 modi®ed by HPA (HPA-MCM-41) exhibited a lower rate of cymene isomerization compared to (Al)MCM-41. Moreover, m-cymene was mainly formed at the expense of o-cymene. In TMB disproportionation over HPA-MCM-41 HPA H6 PV3 Mo9 O12 signi®cantly higher concentrations of xylenes were reached in comparison with (Al)MCM-41. m- and o-xylenes as primary products of TMBs disproportionation were preferentially formed. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Mesoporous MCM-41; Heteropoly acids; Alkylation; Disproportionation; Aromatic hydrocarbons
1. Introduction The catalytic alkylations, isomerizations and disproportionations of aromatic hydrocarbons are of signi®cant fundamental and practical importance and have been very intensively investigated in recent years [1±5]. Much of the current eort is
* Corresponding author. Tel.: +420-2-6605-3795; fax: +4202-858-2307. E-mail address: [email protected] (J. Cejka).
devoted to the understanding of various mechanistic aspects of these reactions, the role of catalyst structure and to active site characterization. These reactions can also be used as test reactions to characterize the catalytic properties of molecular sieves. Their transformations over zeolites represent a very profoundly understood area of zeolite catalysis and several reactions have been already applied at an industrial scale. The reaction pathways of practically all these reactions over microporous catalysts are strongly in¯uenced or directly controlled by the diusion phenomena changing signi®cantly the overall activities and selectivities compared to common catalysts. It has been
1387-1811/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 7 - 1 8 1 1 ( 0 1 ) 0 0 2 2 6 - 8
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reported that these transformations are controlled not only by the intrinsic activity of individual reactant molecules but also by the geometry and architecture of inner zeolite channel volume. Although product selectivities usually re¯ect the acidity of the zeolites employed, it has been reported that the diusion of reactants (xylene isomerization [3]) or desorption/transport of products (toluene alkylation with isopropyl alcohol [6]) can be the rate-determining step of the overall process. To remove the controlling role of transport phenomena mesoporous materials possessing different pore diameters have been investigated. The acid sites in MCM-41 can be formed via introduction of aluminium during the synthesis procedure or siliceous analog can be used as a support for catalytically active materials. Indeed, supported heteropoly acids (HPA) have been reported to catalyze such reactions as the Diels±Alder reaction [7], phenol alkylation [8,9], and alkylation of isobutane with 2-butene [10]. In addition several interesting review papers covering various aspects of catalytic applications of HPA have been recently issued [11±14]. In contrast to microporous molecular sieves, which have been investigated in various transformations of aromatic hydrocarbons, mesoporous aluminosilicates of MCM-41 structure are used very rarely. Only recently m-xylene isomerization [15], benzene alkylation with isopropyl alcohol [16,17], and toluene alkylation with propylene [18] were reported. In this contribution mesoporous molecular sieves of MCM-41 type are used as catalysts and catalyst support for gas-phase toluene alkylation with propylene to cymenes and trimethylbenzene (TMB) disproportionation to xylenes and tetramethylbenzenes (TeMBs). The comparison of the activity, selectivity and time-on-stream stability in these two reactions is discussed. To investigate the eect of acidity (both number and strength) (Al)MCM-41 with dierent concentrations of aluminum in silicate matrix and siliceous MCM-41 modi®ed by HPA of Keggin type were used. The catalysts are characterized by powder X-ray diffraction (XRD), sorption of nitrogen, FTIR spectroscopy of adsorbed d3 -acetonitrile, UV±
VIS diuse re¯ectance spectroscopy and catalytic tests. 2. Experimental 2.1. Catalyst preparation Three types of MCM-41 were synthesized and used in this study: (i) siliceous MCM-41 with pore dimensions of about 3.3 nm (MCM-41/1), (ii) siliceous MCM-41 with pore dimensions of about 4.5 nm (MCM-41/2), and (iii) (Al)MCM-41 with pore dimensions 3.3 nm. MCM-41/1 was prepared using sodium silicate, hexadecyltrimethylammonium bromide, ethyl acetate and aluminium hydroxide modifying the procedure described in Ref. [19]. MCM-41/2 was synthesized without swelling agents according to the recipe given in Ref. [20] using Cab-O-Sil M5 (Cabot), tetramethylammonium hydroxide and hexadecyltrimethylammonium bromide. The synthesis was carried out at 408 K for 5 days. In the case of (Al)MCM-41 cetyltrimethylammonium chloride, sodium aluminate (Riedel-de-Haen), Cab-O-Sil M5, sodium silicate and tetramethylammonium hydroxide (Aldrich) were used. The molar composition of reaction mixture was 1.0SiO2 :0.012±0.039Al2 O3 : 0.114±0.145Na2 O:0.117(CTMA)2 O:0.05(TMA)2 O: 64H2 O. The synthesis was performed without agitation at 398 K for 24 h. Modi®cation of mesoporous MCM-41 was performed with phosphomolybdenic (H3 PMo12 O40 , Fluka) and phosphovanadomolybdenic (H6 PV3 Mo12 O40 , synthesized in our laboratory) HPA of Keggin structure by shaking MCM-41 (1.0 g) overnight with this acid (0.3±1.5 g) in aqueous solution at ambient temperature. Synthesis of H6 PV3 Mo12 O40 was carried out according to the method of Tsigdinos and Hallada [21] by acidifying a mixture of solutions of Na2 HPO4 , Na2 MoO4 and NaVO3 with concentrated H2 SO4 . The HPA was extracted with diethyl ether and the residue after evaporation of the solvent was recrystallized from aqueous solution. The composition was about H5:8 PV2:8 Mo9:2 O40 which is close to the data reported by Fricke et al. [22] and Bielanski et al. [23] who used the same synthesis procedure. The
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Table 1 Characteristics of (Al)MCM-41 and siliceous MCM-41 modi®ed with HPA Catalyst
Si/Al
Pore dimensions (nm)
Heteropolyacid
MCM-41:HPA (weight)
(Al)MCM-41a (Al)MCM-41b (Al)MCM-41c P-HPA-MCM-41/1a P-HPA-MCM-41/1b P-HPA-MCM-41/1c PV3 -HPA-MCM-41/1d PV3 -HPA-MCM-41/2a
14.6 15.0 35.0 1 1 1 1 1
3.3 3.3 3.3 3.3 3.3 3.3 3.3 4.5
± ± ± H3 PMo12 O40 H3 PMo12 O40 H3 PMo12 O40 H6 PV3 Mo9 O40 H6 PV3 Mo9 O40
± ± ± 1.0:0.3 1.0:1.0 1.0:1.5 1.0:1.5 1.0:1.5
description and characteristics of these catalysts are given in Table 1. 2.2. Catalyst characterization XRD of as-synthesized, calcined and modi®ed MCM-41 was performed using an X-ray powder diractometer Siemens D5005 in the Bragg± Brentano geometry arrangement with CuKa radiation with a graphite monochromator and a scintillation detector in the region 1±10° of 2H for MCM-41 and 5±50° of 2H for HPA. The Si/Al ratio of (Al)MCM-41 was determined by an atomic absorption spectroscopy after complete dissolution of the aluminosilicate sample (Table 1). Adsorption isotherms of nitrogen were recorded at 77 K with an Accusorb 2100E instrument (Micromeritics). The samples were activated at 623 K for about 20 h at the pressure 10 4 Pa before the measurement. UV±VIS±NIR diuse re¯ectance (DR) spectra were measured using a Perkin Elmer UV±VIS± NIR spectrometer Lambda 19 equipped with a DR attachment with an integrating sphere coated by BaSO4 . Spectra of sample in 2 mm thick silica cell were recorded in a dierential mode with the parent MCM-41 as a reference. The absorption intensity was calculated from the Schuster±Kubelka±Munk equation F R1 1 R1 2 =2R1 , where R1 is the DR of a semi-in®nite layer and F R1 is proportional to the absorption coecient.
2.3. Apparatus, procedure and analysis of catalytic tests The alkylation of toluene with propylene was performed in a down-¯ow glass microreactor (inner diameter of 10 mm, weight of catalyst 0.3± 1.5 g, WHSV 10 h 1 ) at reaction temperature of 523 K. For the catalytic experiments, a hydrogen stream was saturated with an equilibrium concentration of toluene (18.5 mol%) at 336 K and mixed with a stream of pure propylene to reach a molar ratio ntoluene =npropylene either 4.8 or 9.6. The disproportionation of 1,2,4-trimethylbenzene (1,2,4-TMB) was carried out in the same apparatus under atmospheric pressure at a reaction temperature of 673 K with WHSV 5:0 h 1 and TMB concentration of 5.2 mol% in hydrogen. For both reactions all catalysts were activated at 723 K in a stream of pure hydrogen for 1 h. The reaction products were analyzed using an ``online'' gas chromatograph (Hewlett-Packard 5890 Series II) equipped with a Supelcowax 10 capillary column and a ¯ame ionization and mass-spectrometric detectors (Hewlett-Packard 5971A), respectively. 3. Results and discussion 3.1. Characterization of the molecular sieves used The XRD patterns of calcined MCM-41 samples (Fig. 1) possess four to ®ve clearly discernible re¯ections in the range 2h < 10°. Similar XRD
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Fig. 1. X-ray powder diraction patterns calcined (A) (Al)MCM-41, (B) MCM-41/1 and (C) MCM-41/2.
patterns were obtained for siliceous MCM-41/1 and (Al)MCM-41a while a signi®cant shift of XRD peaks to lower values of 2h was found for MCM-41/2, which indicates a higher unit cell constant. To determine the pore dimensions of dierent MCM-41 samples, nitrogen isotherms at 77 K were recorded (Fig. 2). All three isotherms evidence typical well-ordered structure of MCM-41 molecular sieves. Nitrogen isotherms of MCM-41/ 1 and (Al)MCM-41a exhibit similar shape being characterized by a steep increase at the relative pressure of P =P0 0:3. For MCM-41/2 this steep
Fig. 2. Nitrogen adsorption isotherms of (A) (Al)MCM-41, (B) MCM-41/1 and (C) MCM-41/2.
increase was found at a higher partial pressure P =P0 0:4 which indicates larger pore dimensions which is in a good agreement with XRD results. This evidences the typical well-ordered structure of MCM-41 possessing surface areas of mesopores in the range of 1000±1150 m2 /g and their volumes of about 0.830±0.890 cm3 /g. The pore dimensions for MCM-41/1 and (Al)MCM41a were estimated to be about 3.3 nm while for MCM-41/2 about 4.5 nm based on XRD and nitrogen isotherms data. XRD and UV±VIS DR spectroscopy were used to characterize the dispersity of HPA supported on MCM-41. XRD data indicate a ®ne dispersion of HPA on MCM-41 as no HPA crystallinity was observed at dierent HPA loadings. This agrees with data reported by Kozhevnikov et al. [9]. UV± VIS diuse re¯ectance spectra of bulk HPA and MCM-41 containing HPA after various treatments are given in Fig. 3. An intensive absorption edge at 18 700 (H6 PV3 Mo9 O40 ) and 22 000 cm 1 (H3 PMo12 O40 ) corresponds to bulky HPA, respectively. After incorporation of HPA (H6 PV3 Mo9 O40 ) into MCM-41, the absorption edge became complex and is shifted to 19 700 and 25 000 cm 1 , respectively. Moreover, two new intensive absorption bands at 32 000 and 42 000 cm 1 appeared. The above described spectrum is characteristic also for HPA-MCM-41/2a after alkylation reaction (Fig. 3). Unstructured absorption in the whole visible region of this spectrum corresponds to coke formation during reaction. On the other hand, PHPA-MCM-41/1b exhibits the same absorption edge as bulky HPA (H3 PMo12 O40 ). The spectrum of P-HPA-MCM-41/1b after alkylation reaction, similar to that one after disproportionation, is signi®cantly aected by the coke absorption in the UV±VIS region and identi®cation of absorption edge or absorption bands of HPA is impossible. However, after calcination of P-HPA-MCM41/1b under an oxygen stream the absorption edge is shifted to 27 000 cm 1 and new absorption bands at 33 000 and 43 000 cm 1 appeared in the spectrum. The absorption edge is characteristic for bulky materials described in the frame of solid state theory. The transformation of bulky material into
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Fig. 3. UV±VIS DR spectra of MCM-41 modi®ed with HPA. (A) As prepared PV3 -HPA-MCM-41/2a (Ð), PV3 -HPA-MCM-41/2a after reaction run (- - -), (B) bulk H6 PV3 Mo9 O40 (Ð), 0:5 10 5 M of H6 PV3 Mo9 O40 in MeOH (- - -), (C) as prepared P-HPA-MCM41/1b (Ð), P-HPA-MCM-41/1b after reaction run (- - -), P-HPA-MCM-41/1b after reaction run and calcination (± ±), (D) bulk H3 PMo12 O40 (Ð), 5 10 5 M of H3 PMo12 O40 in MeOH (- - -).
clusters is followed by the shift of the absorption edge, the loss of solid state ``nature'', i.e. formation of small clusters, di(oligo)mers or isolated molecules, is followed by the disappearance of the absorption edge in the spectrum and the presence of absorption bands corresponding to electronic transitions between individual energy levels, described in the frame of molecular orbital theory. Thus, the PV3 -HPA-MCM-41/2a is well dispersed after preparation, as follows from a signi®cant shift of the absorption edge and the presence of a discrete band structure with the 32 000 cm 1 band in the spectrum, which is similar to that of H6 PV3 Mo9 O40 in methanolic solution. Because the spectrum of new HPA species in MCM-41 was not changed during reaction con-
ditions (note that continuous absorption from VIS to UV region corresponds to the coke formed during reaction run), this well dispersed HPA is responsible for catalytic activity in alkylation or disproportionation. The nature of HPA (H3 PMo12 O40 ) did not signi®cantly change by incorporation into MCM-41, as follows from the presence of the HPA absorption edge in the spectrum of P-HPA-MCM-41/1b. However, the signi®cant shift of the HPA absorption edge after calcination followed by the presence of discrete band structure with absorption bands at 33 000 and 43 000 cm 1 similar to those obtained in methanolic solution indicate, that also HPA (H3 PMo12 O40 ) is well dispersed in MCM-41.
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3.2. (Al)MCM-41 in toluene alkylation and disproportionation Toluene alkylation with propylene represents an acid catalyzed electrophilic substitution reaction which proceeds, as commonly accepted, via carbenium type mechanism [24] and can be catalyzed by acid zeolites of dierent acid strength [18,25]. In a series of isomorphously substituted zeolites of ZSM-5 structure with Al, Fe and In it has been evidenced that despite the signi®cant dierences in the acid strength of bridging OH groups (acid strength order Al > Fe > In) all three zeolites are able to activate isopropyl alcohol for toluene alkylation to similar conversion level [26]. On the other hand in toluene disproportionation or its alkylation with ethylene, toluene conversion over (In)ZSM-5 was signi®cantly lower compared to (Al) and (Fe)ZSM-5. To characterize the acid strength in (Al)MCM-41 toluene alkylation with propylene and TMB disproportionation, which requires substantially higher acid strength, were employed. The adsorption of d3 -acetonitrile followed by infrared spectroscopy on (Al)MCM-41 (not presented in this paper) revealed only its interaction with Lewis acid sites, absorption band at
2324 cm 1 , with the absence of interaction with bridging Si±OH±Al sites. Over (Al)MCM-41, toluene alkylation with propylene ntoluene =npropylene 9:6 leads exclusively to the formation of alkylation products (cymenes, di-isopropyltoluenes, cf. Table 2). This is in agreement with the results of Perego et al. [18], while for MCM-41 modi®ed with alumina formation of ethylbenzene and xylenes was also reported [16]. The overall selectivity to cymenes despite the toluene conversion level was higher than 96% (Fig. 4). From Table 2 it is evident that cymene isomerization proceeds very easily via 1,2isopropyl shift over benzene ring which leads to the formation of all three cymene isomers. The higher the toluene conversion, the higher the extent of cymene isomerization (toluene conversion 9.0±10.0%±m-cymene selectivity 46.0±51.0% vs. toluene conversion 1.5±2.0%±m-cymene 30.0%). These results indicate that the ®rst alkylation step proceeds to the ortho- and para-positions in agreement with 13 C NMR results of Corma and Ivanova [5] for toluene alkylation with methanol. The absence of steric constraints in the internal space of MCM-41 results in the absence of npropyltoluenes among the reaction products.
Table 2 Toluene alkylation with propylene over (Al)MCM-41 and HPA-MCM-41 (reaction temperature 520 K, WHSV 10 h 1 , 15 min of time-on-stream) (Al)MCM-41b
(Al)MCM-41b
P-MCM-41/1c
PV3 -MCM-41/1d
PV3 -MCM-41/2a
T/P ratioa Conversion (%) Toluene Propylene
9.6
4.8
9.6
9.6
9.6
9.7 81.9
16.3 73.5
6.4 58.5
1.3 10.9
1.0 10.1
Selectivity (vol.%) C8-aromatics p-Cymene m-Cymene o-Cymene Di-isopropyltoluenes
0.0 36.7 51.2 9.0 3.0
0.4 33.2 46.7 10.2 9.8
0.0 45.6 38.6 12.9 2.5
0.0 49.9 26.5 23.7 0.0
0.0 49.3 30.0 20.7 0.0
Cymene selectivityb p-Cymene m-Cymene o-Cymene
37.9 52.8 9.3
36.9 51.8 11.3
47.0 39.8 13.2
49.9 26.5 23.7
49.3 30.0 20.7
a b
T/P ratio is the toluene/propylene molar ratio. Selectivities to cymene isomers.
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zeolites the reactant molecules are not strongly held by the electrostatic ®eld in the channels which leads to the decrease in the rate of disproportionation. These results con®rm that the acid strength of active sites associated to aluminum in MCM-41 is signi®cantly lower compared to the aluminosilicate zeolites. For toluene alkylation with propylene substantially lower acid strength is required which was already shown for series of isomorphously substituted ZSM-5 zeolites (Al, Fe, In) [26] and, thus, comparable selectivities to cymenes are obtained for (Al)MCM-41. On the other hand, the acid sites of (Al)MCM-41 are not able to activate TMB molecules, which leads to a low conversion compared to zeolites. 3.3. Heteropoly MCM-41
Fig. 4. Time-on-stream dependence of toluene conversion (A, C) and cymenes selectivity (B, D) in toluene alkylation with propylene over (A,B) (Al)MCM-41a and (C,D) (Al)MCM-41c for ntoluene =npropylene molar ratio 9.6 (d) and 4.8 ( ).
The decrease in ntoluene =npropylene molar ratio from 9.6 to 4.8 results in an increase in toluene conversion for lower Si/Al ratio (Fig. 4). In contrast to microporous zeolites [6] the decrease in this ratio did not lead to a signi®cant rate of deactivation due to an oligomerization of propylene. The concentration of by-products formed (cumene, ethyltoluenes, TMBs) was lower than 0.5% with simultaneous increase in the concentration of di-isopropyltoluenes (Table 2). A completely dierent behavior of (Al)MCM41 compared to zeolite catalysts was observed in TMB disproportionation. Relatively low conversion of 1,2,4-TMB was found over all (Al)MCM41 catalysts (Table 3, Fig. 5) which is three to four times lower compared to zeolite Beta or mordenite possessing Si/Al ratios of 12.5 or 10, respectively [27]. In addition, 1,2,4-TMB isomerization dominates over disproportionation providing only 8± 13% of xylenes at dierent conversion levels. It could be inferred that in contrast to microporous
acid
supported
on
siliceous
It has been previously reported that HPA supported on mesoporous MCM-41 exhibit signi®cantly higher activity in alkylation of tertbutylphenol compared to bulk HPA due to a large number of surface active sites [9]. In the case of toluene alkylation with propylene over P-HPAMCM-41 with dierent HPA loadings the toluene conversion increases with increasing concentration of HPA (P-HPA-MCM-41/1a±c in Fig. 5). Table 2 shows that there is practically no formation of byproducts and cymenes are exclusively formed. The selectivity to cymenes is, thus, higher than over (Al)MCM-41. Again, no formation of n-propyltoluenes was observed. Although HPA are known as strongly acidic catalysts [28], it seems that (Al)MCM-41 isomerizes p- and o-cymenes to m-cymene much faster than P-HPA-MCM-41. Moreover, surprisingly the formation of m-cymene over P-HPA-MCM-41 proceeds mainly at the expense of o-cymene. As for PV3 -HPA-MCM-41 with dierent pore dimensions (3.3 and 4.5 nm, respectively, cf. Table 1) and the same HPA loading no signi®cant differences were found in toluene alkylation which could indicate that at these pore dimensions, there are no steric or transport eects on the reaction which could cause some dierences in toluene conversion (Table 2).
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Table 3 TMB disproportionation over (Al)MCM-41 and HPA-MCM-41 (reaction temperature 670 K, WHSV 5 h 1 , 15 min of time-onstream) (Al)MCM-41c Conversion of 1,2,4-TMB (%)
(Al)MCM-41a
P-MCM-41/1a
P-MCM-41/1c
PV3 -MCM-41/1d
PV3 -MCM-41/2a
3.7
18.6
4.5
23.4
11.2
13.7
Selectivity (vol.%) Low ole®ns Benzene Toluene p-Xylene m-Xylene o-Xylene 1,3,5-TMBa 1,2,3-TMB 1,2,4,5TeMBb 1,2,3,5TeMB 1,2,3,4TeMB PeMBc
0.0 0.0 4.4 2.5 6.3 4.2 32.0 39.5 6.8 3.6 1.2 0.0
0.0 0.0 2.8 1.4 3.7 3.4 45.6 34.0 3.3 3.9 1.0 0.8
0.0 0.0 5.1 1.8 3.4 4.5 37.3 38.8 4.7 2.4 1.0 1.0
1.6 0.4 1.9 3.1 7.6 7.6 36.2 25.4 6.6 7.4 2.0 0.2
2.5 0.0 1.4 5.3 14.4 14.1 14.8 16.2 17.9 10.7 2.7 0.0
2.8 0.0 1.3 5.3 14.0 15.0 18.4 18.0 12.9 9.5 2.8 0.0
P Xylenes P TeMBs
13.0 11.6
8.5 8.2
9.7 8.1
18.3 16.0
33.7 31.3
34.3 25.2
a
TMB ± trimethylbenzene. TeMB ± tetramethylbenzene. c PeMB ± pentamethylbenzene. b
Fig. 5. Time-on-stream dependence of toluene conversion (A) and cymenes selectivity (B) in toluene alkylation with propylene over (d) P-HPA-MCM-41/1a, (m) P-HPA-MCM-41/1b and ( ) P-HPA-MCM-41/1c.
Signi®cant dierences in disproportionation of 1,2,4-TMB over P- and PV3 -HPA-MCM-41 were observed for dierent HPA (cf. Table 3 and Fig. 6). More than 30% of xylenes are formed over PV3 -HPA-MCM-41/1d and 2a. However, these
Fig. 6. Time-on-stream dependence of TMB conversion (A) and xylenes and TeMBs selectivity (B) in 1,2,4-TMB disproportionation over (d) P-HPA-MCM-41/1a and ( ) P-HPAMCM-41/1c, (s, h) xylenes and (d, ) TeMBs, respectively.
selectivities are still considerably low compared to zeolites operating under the same reaction conditions [27]. Concentrations of xylene isomers are far from equilibrium values, particularly the amount
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of p-xylene is rather low (Table 3). This could be caused by the fact that mainly o- and p-xylenes are primary products of TMB disproportionation (1,3,5-TMB ! m-xylene, 1,2,4-TMB ! mainly mor o-xylene, 1,2,3-TMB ! o-xylene) and monomolecular xylene isomerization is rather limited.
4. Conclusions The acid strength of (Al)MCM-41 was found to be sucient for the activation of propylene in toluene alkylation leading to cymene selectivities over 96% while the rate of consecutive reactions was negligible. In contrast, TMB disproportionation proceeded at a signi®cantly lower extent than over zeolites. Moreover, 1,2,4-TMB isomerization dominates over disproportionation on (Al)MCM41. It could be inferred that in contrast to microporous zeolites the reactant molecules are not strongly held by electrostatic ®eld in the channels which leads to a decrease in the rate of disproportionation. Over siliceous MCM-41 modi®ed by phosphoromolybdenic and phosphorovanadomolybdenic HPA only cymenes and di-isopropyltoluenes were found with a lower rate of cymene isomerization compared to (Al)MCM-41. In addition, m-cymene was mainly formed at the expense of o-cymene. 1,2,4-TMB disproportionation over PV3 -HPA-MCM-41 (HPA H6 PV3 Mo9 O12 ) led to signi®cantly higher concentrations of xylenes in comparison with (Al)MCM-41 with preferential formation of o- and m-xylenes.
Acknowledgements Financial support of the Grant Agency of the Academy of Sciences of the Czech Republic (A4040001) and Grant Agency of the Czech Republic (104/99/0840) is highly acknowledged.
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References [1] D.H. Olson, W.O. Haag, ACS Symp. Series 248 (1984) 275. [2] A. Corma, E. Sastre, J. Chem. Commun., Chem. Soc. (1991) 594. [3] G. Mirth, J. Cejka, J.A. Lercher, J. Catal. 139 (1993) 24. [4] I.I. Ivanova, D. Brunel, G. Daelen, J.B. Nagy, E.G. Derouane, Stud. Surf. Sci. Catal. 78 (1993) 587. [5] I.I. Ivanova, A. Corma, J. Phys. Chem. B 101 (1997) 547. [6] J. Cejka, G.I. Kapustin, B. Wichterlova, Appl. Catal. A 108 (1994) 187. [7] M. Misono, Catal. Rev.-Sci. Eng. 29 (1987) 269. [8] G.J. Meuzelaar, L. Maat, R.A. Sheldon, I.V. Khozevnikov, Catal. Lett. 45 (1997) 269. [9] I.V. Khozevnikov, A. Sinnema, R.J.J. Jansen, K. Pamin, H. van Bekkum, Catal. Lett. 30 (1995) 241. [10] T. Blasco, A. Corma, A. Martinez, P. Martinez-Escolano, J. Catal. 177 (1998) 306. [11] A. Corma, Chem. Rev. 95 (1995) 559. [12] T. Okuhara, N. Mizuno, M. Misono, Adv. Catal. 41 (1996) 113. [13] A. Corma, Chem. Rev. 97 (1997) 2373. [14] I.V. Khozevnikov, Chem. Rev. 98 (1998) 171. [15] S. Morin, P. Ayrault, S.El. Mouahid, N.S. Gnep, M. Guisnet, Appl. Catal. A 159 (1999) 317. [16] J. Medina-Valtierra, O. Zaldivar, M.A. Sanchez, J.A. Montoya, J. Navarrete, J.A. de los Reyes, Appl. Catal. A 166 (1998) 387. [17] J. Cejka, N. Zilkov a, J.E. Sponer, B. Wichterlova, Collect. Czech. Chem. Commun. 63 (1998) 1769. [18] C. Perego, S. Amarilli, A. Carati, C. Flego, G. Pazzuconi, C. Rizzo, G. Bellussi, Micropor. Mesopor. Mater. 27 (1999) 345. [19] G. Schulz-Eklo, J. Rathousky, A. Zukal, J. Inorg. Mater. 1 (1999) 1462. [20] A. Corma, Q. Kan, M.T. Navarro, J. Perez-Pariente, F. Rey, Chem. Mater. 9 (1997) 2123. [21] G.A. Tsigdinos, C.J. Hallada, Inorg. Chem. 7 (1968) 437. } [22] R. Fricke, H.G. Jerschkewitz, G. Ohlmann, J. Chem. Soc., Faraday Trans. I 82 (1986) 3479. [23] A. Bielanski, A. Malecka, L. Kubelkova, J. Chem. Soc., Faraday Trans. I 85 (1989) 2847. [24] P.B. Venuto, L.A. Hamilton, P.S. Landis, J. Catal. 5 (1966) 484. [25] B. Wichterlova, J. Cejka, J. Catal. 146 (1994) 523. [26] J. Cejka, A. Vondrova, B. Wichterlova, G. Vorbeck, R. Fricke, Zeolites 14 (1994) 147. [27] J. Cejka, A. Krejcõ, J. Hanika, Stud. Surf. Sci. Catal. 125 (1999) 351. [28] M. Misono, T. Okuhara, Chemtech 11 (1993) 23.
www.elsevier.nl/locate/micromeso
Alkylation and disproportionation of aromatic hydrocarbons over mesoporous molecular sieves a,* Jirõ Cejka , Andrea Krejcõ a, Nadezda Zilkov a a, Jirõ Dedecek a, Jirõ Hanika b a
J. Heyrovsk y Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejskova 3, CZ-182 23 Prague 8, Czech Republic b Department of Organic Technology, Institute of Chemical Technology, Technick a 5, CZ-166 28 Prague 6, Czech Republic Received 15 February 2000; accepted 24 August 2000
Abstract Toluene alkylation with propylene and trimethylbenzene (TMB) disproportionation were investigated over mesoporous molecular sieves of MCM-41 type of dierent pore dimensions, possessing various Si/Al ratios or modi®ed with heteropoly acids (HPA) of Keggin type. It was shown that the acid strength of (Al)MCM-41 was sucient to activate propylene in the alkylation of toluene providing cymene selectivity over 96% while the rate of consecutive reactions was negligible. In contrast, TMB disproportionation proceeded at a signi®cantly lower extent than over zeolites. Siliceous MCM-41 modi®ed by HPA (HPA-MCM-41) exhibited a lower rate of cymene isomerization compared to (Al)MCM-41. Moreover, m-cymene was mainly formed at the expense of o-cymene. In TMB disproportionation over HPA-MCM-41 HPA H6 PV3 Mo9 O12 signi®cantly higher concentrations of xylenes were reached in comparison with (Al)MCM-41. m- and o-xylenes as primary products of TMBs disproportionation were preferentially formed. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Mesoporous MCM-41; Heteropoly acids; Alkylation; Disproportionation; Aromatic hydrocarbons
1. Introduction The catalytic alkylations, isomerizations and disproportionations of aromatic hydrocarbons are of signi®cant fundamental and practical importance and have been very intensively investigated in recent years [1±5]. Much of the current eort is
* Corresponding author. Tel.: +420-2-6605-3795; fax: +4202-858-2307. E-mail address: [email protected] (J. Cejka).
devoted to the understanding of various mechanistic aspects of these reactions, the role of catalyst structure and to active site characterization. These reactions can also be used as test reactions to characterize the catalytic properties of molecular sieves. Their transformations over zeolites represent a very profoundly understood area of zeolite catalysis and several reactions have been already applied at an industrial scale. The reaction pathways of practically all these reactions over microporous catalysts are strongly in¯uenced or directly controlled by the diusion phenomena changing signi®cantly the overall activities and selectivities compared to common catalysts. It has been
1387-1811/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 7 - 1 8 1 1 ( 0 1 ) 0 0 2 2 6 - 8
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reported that these transformations are controlled not only by the intrinsic activity of individual reactant molecules but also by the geometry and architecture of inner zeolite channel volume. Although product selectivities usually re¯ect the acidity of the zeolites employed, it has been reported that the diusion of reactants (xylene isomerization [3]) or desorption/transport of products (toluene alkylation with isopropyl alcohol [6]) can be the rate-determining step of the overall process. To remove the controlling role of transport phenomena mesoporous materials possessing different pore diameters have been investigated. The acid sites in MCM-41 can be formed via introduction of aluminium during the synthesis procedure or siliceous analog can be used as a support for catalytically active materials. Indeed, supported heteropoly acids (HPA) have been reported to catalyze such reactions as the Diels±Alder reaction [7], phenol alkylation [8,9], and alkylation of isobutane with 2-butene [10]. In addition several interesting review papers covering various aspects of catalytic applications of HPA have been recently issued [11±14]. In contrast to microporous molecular sieves, which have been investigated in various transformations of aromatic hydrocarbons, mesoporous aluminosilicates of MCM-41 structure are used very rarely. Only recently m-xylene isomerization [15], benzene alkylation with isopropyl alcohol [16,17], and toluene alkylation with propylene [18] were reported. In this contribution mesoporous molecular sieves of MCM-41 type are used as catalysts and catalyst support for gas-phase toluene alkylation with propylene to cymenes and trimethylbenzene (TMB) disproportionation to xylenes and tetramethylbenzenes (TeMBs). The comparison of the activity, selectivity and time-on-stream stability in these two reactions is discussed. To investigate the eect of acidity (both number and strength) (Al)MCM-41 with dierent concentrations of aluminum in silicate matrix and siliceous MCM-41 modi®ed by HPA of Keggin type were used. The catalysts are characterized by powder X-ray diffraction (XRD), sorption of nitrogen, FTIR spectroscopy of adsorbed d3 -acetonitrile, UV±
VIS diuse re¯ectance spectroscopy and catalytic tests. 2. Experimental 2.1. Catalyst preparation Three types of MCM-41 were synthesized and used in this study: (i) siliceous MCM-41 with pore dimensions of about 3.3 nm (MCM-41/1), (ii) siliceous MCM-41 with pore dimensions of about 4.5 nm (MCM-41/2), and (iii) (Al)MCM-41 with pore dimensions 3.3 nm. MCM-41/1 was prepared using sodium silicate, hexadecyltrimethylammonium bromide, ethyl acetate and aluminium hydroxide modifying the procedure described in Ref. [19]. MCM-41/2 was synthesized without swelling agents according to the recipe given in Ref. [20] using Cab-O-Sil M5 (Cabot), tetramethylammonium hydroxide and hexadecyltrimethylammonium bromide. The synthesis was carried out at 408 K for 5 days. In the case of (Al)MCM-41 cetyltrimethylammonium chloride, sodium aluminate (Riedel-de-Haen), Cab-O-Sil M5, sodium silicate and tetramethylammonium hydroxide (Aldrich) were used. The molar composition of reaction mixture was 1.0SiO2 :0.012±0.039Al2 O3 : 0.114±0.145Na2 O:0.117(CTMA)2 O:0.05(TMA)2 O: 64H2 O. The synthesis was performed without agitation at 398 K for 24 h. Modi®cation of mesoporous MCM-41 was performed with phosphomolybdenic (H3 PMo12 O40 , Fluka) and phosphovanadomolybdenic (H6 PV3 Mo12 O40 , synthesized in our laboratory) HPA of Keggin structure by shaking MCM-41 (1.0 g) overnight with this acid (0.3±1.5 g) in aqueous solution at ambient temperature. Synthesis of H6 PV3 Mo12 O40 was carried out according to the method of Tsigdinos and Hallada [21] by acidifying a mixture of solutions of Na2 HPO4 , Na2 MoO4 and NaVO3 with concentrated H2 SO4 . The HPA was extracted with diethyl ether and the residue after evaporation of the solvent was recrystallized from aqueous solution. The composition was about H5:8 PV2:8 Mo9:2 O40 which is close to the data reported by Fricke et al. [22] and Bielanski et al. [23] who used the same synthesis procedure. The
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Table 1 Characteristics of (Al)MCM-41 and siliceous MCM-41 modi®ed with HPA Catalyst
Si/Al
Pore dimensions (nm)
Heteropolyacid
MCM-41:HPA (weight)
(Al)MCM-41a (Al)MCM-41b (Al)MCM-41c P-HPA-MCM-41/1a P-HPA-MCM-41/1b P-HPA-MCM-41/1c PV3 -HPA-MCM-41/1d PV3 -HPA-MCM-41/2a
14.6 15.0 35.0 1 1 1 1 1
3.3 3.3 3.3 3.3 3.3 3.3 3.3 4.5
± ± ± H3 PMo12 O40 H3 PMo12 O40 H3 PMo12 O40 H6 PV3 Mo9 O40 H6 PV3 Mo9 O40
± ± ± 1.0:0.3 1.0:1.0 1.0:1.5 1.0:1.5 1.0:1.5
description and characteristics of these catalysts are given in Table 1. 2.2. Catalyst characterization XRD of as-synthesized, calcined and modi®ed MCM-41 was performed using an X-ray powder diractometer Siemens D5005 in the Bragg± Brentano geometry arrangement with CuKa radiation with a graphite monochromator and a scintillation detector in the region 1±10° of 2H for MCM-41 and 5±50° of 2H for HPA. The Si/Al ratio of (Al)MCM-41 was determined by an atomic absorption spectroscopy after complete dissolution of the aluminosilicate sample (Table 1). Adsorption isotherms of nitrogen were recorded at 77 K with an Accusorb 2100E instrument (Micromeritics). The samples were activated at 623 K for about 20 h at the pressure 10 4 Pa before the measurement. UV±VIS±NIR diuse re¯ectance (DR) spectra were measured using a Perkin Elmer UV±VIS± NIR spectrometer Lambda 19 equipped with a DR attachment with an integrating sphere coated by BaSO4 . Spectra of sample in 2 mm thick silica cell were recorded in a dierential mode with the parent MCM-41 as a reference. The absorption intensity was calculated from the Schuster±Kubelka±Munk equation F R1 1 R1 2 =2R1 , where R1 is the DR of a semi-in®nite layer and F R1 is proportional to the absorption coecient.
2.3. Apparatus, procedure and analysis of catalytic tests The alkylation of toluene with propylene was performed in a down-¯ow glass microreactor (inner diameter of 10 mm, weight of catalyst 0.3± 1.5 g, WHSV 10 h 1 ) at reaction temperature of 523 K. For the catalytic experiments, a hydrogen stream was saturated with an equilibrium concentration of toluene (18.5 mol%) at 336 K and mixed with a stream of pure propylene to reach a molar ratio ntoluene =npropylene either 4.8 or 9.6. The disproportionation of 1,2,4-trimethylbenzene (1,2,4-TMB) was carried out in the same apparatus under atmospheric pressure at a reaction temperature of 673 K with WHSV 5:0 h 1 and TMB concentration of 5.2 mol% in hydrogen. For both reactions all catalysts were activated at 723 K in a stream of pure hydrogen for 1 h. The reaction products were analyzed using an ``online'' gas chromatograph (Hewlett-Packard 5890 Series II) equipped with a Supelcowax 10 capillary column and a ¯ame ionization and mass-spectrometric detectors (Hewlett-Packard 5971A), respectively. 3. Results and discussion 3.1. Characterization of the molecular sieves used The XRD patterns of calcined MCM-41 samples (Fig. 1) possess four to ®ve clearly discernible re¯ections in the range 2h < 10°. Similar XRD
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Fig. 1. X-ray powder diraction patterns calcined (A) (Al)MCM-41, (B) MCM-41/1 and (C) MCM-41/2.
patterns were obtained for siliceous MCM-41/1 and (Al)MCM-41a while a signi®cant shift of XRD peaks to lower values of 2h was found for MCM-41/2, which indicates a higher unit cell constant. To determine the pore dimensions of dierent MCM-41 samples, nitrogen isotherms at 77 K were recorded (Fig. 2). All three isotherms evidence typical well-ordered structure of MCM-41 molecular sieves. Nitrogen isotherms of MCM-41/ 1 and (Al)MCM-41a exhibit similar shape being characterized by a steep increase at the relative pressure of P =P0 0:3. For MCM-41/2 this steep
Fig. 2. Nitrogen adsorption isotherms of (A) (Al)MCM-41, (B) MCM-41/1 and (C) MCM-41/2.
increase was found at a higher partial pressure P =P0 0:4 which indicates larger pore dimensions which is in a good agreement with XRD results. This evidences the typical well-ordered structure of MCM-41 possessing surface areas of mesopores in the range of 1000±1150 m2 /g and their volumes of about 0.830±0.890 cm3 /g. The pore dimensions for MCM-41/1 and (Al)MCM41a were estimated to be about 3.3 nm while for MCM-41/2 about 4.5 nm based on XRD and nitrogen isotherms data. XRD and UV±VIS DR spectroscopy were used to characterize the dispersity of HPA supported on MCM-41. XRD data indicate a ®ne dispersion of HPA on MCM-41 as no HPA crystallinity was observed at dierent HPA loadings. This agrees with data reported by Kozhevnikov et al. [9]. UV± VIS diuse re¯ectance spectra of bulk HPA and MCM-41 containing HPA after various treatments are given in Fig. 3. An intensive absorption edge at 18 700 (H6 PV3 Mo9 O40 ) and 22 000 cm 1 (H3 PMo12 O40 ) corresponds to bulky HPA, respectively. After incorporation of HPA (H6 PV3 Mo9 O40 ) into MCM-41, the absorption edge became complex and is shifted to 19 700 and 25 000 cm 1 , respectively. Moreover, two new intensive absorption bands at 32 000 and 42 000 cm 1 appeared. The above described spectrum is characteristic also for HPA-MCM-41/2a after alkylation reaction (Fig. 3). Unstructured absorption in the whole visible region of this spectrum corresponds to coke formation during reaction. On the other hand, PHPA-MCM-41/1b exhibits the same absorption edge as bulky HPA (H3 PMo12 O40 ). The spectrum of P-HPA-MCM-41/1b after alkylation reaction, similar to that one after disproportionation, is signi®cantly aected by the coke absorption in the UV±VIS region and identi®cation of absorption edge or absorption bands of HPA is impossible. However, after calcination of P-HPA-MCM41/1b under an oxygen stream the absorption edge is shifted to 27 000 cm 1 and new absorption bands at 33 000 and 43 000 cm 1 appeared in the spectrum. The absorption edge is characteristic for bulky materials described in the frame of solid state theory. The transformation of bulky material into
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503
Fig. 3. UV±VIS DR spectra of MCM-41 modi®ed with HPA. (A) As prepared PV3 -HPA-MCM-41/2a (Ð), PV3 -HPA-MCM-41/2a after reaction run (- - -), (B) bulk H6 PV3 Mo9 O40 (Ð), 0:5 10 5 M of H6 PV3 Mo9 O40 in MeOH (- - -), (C) as prepared P-HPA-MCM41/1b (Ð), P-HPA-MCM-41/1b after reaction run (- - -), P-HPA-MCM-41/1b after reaction run and calcination (± ±), (D) bulk H3 PMo12 O40 (Ð), 5 10 5 M of H3 PMo12 O40 in MeOH (- - -).
clusters is followed by the shift of the absorption edge, the loss of solid state ``nature'', i.e. formation of small clusters, di(oligo)mers or isolated molecules, is followed by the disappearance of the absorption edge in the spectrum and the presence of absorption bands corresponding to electronic transitions between individual energy levels, described in the frame of molecular orbital theory. Thus, the PV3 -HPA-MCM-41/2a is well dispersed after preparation, as follows from a signi®cant shift of the absorption edge and the presence of a discrete band structure with the 32 000 cm 1 band in the spectrum, which is similar to that of H6 PV3 Mo9 O40 in methanolic solution. Because the spectrum of new HPA species in MCM-41 was not changed during reaction con-
ditions (note that continuous absorption from VIS to UV region corresponds to the coke formed during reaction run), this well dispersed HPA is responsible for catalytic activity in alkylation or disproportionation. The nature of HPA (H3 PMo12 O40 ) did not signi®cantly change by incorporation into MCM-41, as follows from the presence of the HPA absorption edge in the spectrum of P-HPA-MCM-41/1b. However, the signi®cant shift of the HPA absorption edge after calcination followed by the presence of discrete band structure with absorption bands at 33 000 and 43 000 cm 1 similar to those obtained in methanolic solution indicate, that also HPA (H3 PMo12 O40 ) is well dispersed in MCM-41.
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3.2. (Al)MCM-41 in toluene alkylation and disproportionation Toluene alkylation with propylene represents an acid catalyzed electrophilic substitution reaction which proceeds, as commonly accepted, via carbenium type mechanism [24] and can be catalyzed by acid zeolites of dierent acid strength [18,25]. In a series of isomorphously substituted zeolites of ZSM-5 structure with Al, Fe and In it has been evidenced that despite the signi®cant dierences in the acid strength of bridging OH groups (acid strength order Al > Fe > In) all three zeolites are able to activate isopropyl alcohol for toluene alkylation to similar conversion level [26]. On the other hand in toluene disproportionation or its alkylation with ethylene, toluene conversion over (In)ZSM-5 was signi®cantly lower compared to (Al) and (Fe)ZSM-5. To characterize the acid strength in (Al)MCM-41 toluene alkylation with propylene and TMB disproportionation, which requires substantially higher acid strength, were employed. The adsorption of d3 -acetonitrile followed by infrared spectroscopy on (Al)MCM-41 (not presented in this paper) revealed only its interaction with Lewis acid sites, absorption band at
2324 cm 1 , with the absence of interaction with bridging Si±OH±Al sites. Over (Al)MCM-41, toluene alkylation with propylene ntoluene =npropylene 9:6 leads exclusively to the formation of alkylation products (cymenes, di-isopropyltoluenes, cf. Table 2). This is in agreement with the results of Perego et al. [18], while for MCM-41 modi®ed with alumina formation of ethylbenzene and xylenes was also reported [16]. The overall selectivity to cymenes despite the toluene conversion level was higher than 96% (Fig. 4). From Table 2 it is evident that cymene isomerization proceeds very easily via 1,2isopropyl shift over benzene ring which leads to the formation of all three cymene isomers. The higher the toluene conversion, the higher the extent of cymene isomerization (toluene conversion 9.0±10.0%±m-cymene selectivity 46.0±51.0% vs. toluene conversion 1.5±2.0%±m-cymene 30.0%). These results indicate that the ®rst alkylation step proceeds to the ortho- and para-positions in agreement with 13 C NMR results of Corma and Ivanova [5] for toluene alkylation with methanol. The absence of steric constraints in the internal space of MCM-41 results in the absence of npropyltoluenes among the reaction products.
Table 2 Toluene alkylation with propylene over (Al)MCM-41 and HPA-MCM-41 (reaction temperature 520 K, WHSV 10 h 1 , 15 min of time-on-stream) (Al)MCM-41b
(Al)MCM-41b
P-MCM-41/1c
PV3 -MCM-41/1d
PV3 -MCM-41/2a
T/P ratioa Conversion (%) Toluene Propylene
9.6
4.8
9.6
9.6
9.6
9.7 81.9
16.3 73.5
6.4 58.5
1.3 10.9
1.0 10.1
Selectivity (vol.%) C8-aromatics p-Cymene m-Cymene o-Cymene Di-isopropyltoluenes
0.0 36.7 51.2 9.0 3.0
0.4 33.2 46.7 10.2 9.8
0.0 45.6 38.6 12.9 2.5
0.0 49.9 26.5 23.7 0.0
0.0 49.3 30.0 20.7 0.0
Cymene selectivityb p-Cymene m-Cymene o-Cymene
37.9 52.8 9.3
36.9 51.8 11.3
47.0 39.8 13.2
49.9 26.5 23.7
49.3 30.0 20.7
a b
T/P ratio is the toluene/propylene molar ratio. Selectivities to cymene isomers.
J. Cejka et al. / Microporous and Mesoporous Materials 44±45 (2001) 499±507
505
zeolites the reactant molecules are not strongly held by the electrostatic ®eld in the channels which leads to the decrease in the rate of disproportionation. These results con®rm that the acid strength of active sites associated to aluminum in MCM-41 is signi®cantly lower compared to the aluminosilicate zeolites. For toluene alkylation with propylene substantially lower acid strength is required which was already shown for series of isomorphously substituted ZSM-5 zeolites (Al, Fe, In) [26] and, thus, comparable selectivities to cymenes are obtained for (Al)MCM-41. On the other hand, the acid sites of (Al)MCM-41 are not able to activate TMB molecules, which leads to a low conversion compared to zeolites. 3.3. Heteropoly MCM-41
Fig. 4. Time-on-stream dependence of toluene conversion (A, C) and cymenes selectivity (B, D) in toluene alkylation with propylene over (A,B) (Al)MCM-41a and (C,D) (Al)MCM-41c for ntoluene =npropylene molar ratio 9.6 (d) and 4.8 ( ).
The decrease in ntoluene =npropylene molar ratio from 9.6 to 4.8 results in an increase in toluene conversion for lower Si/Al ratio (Fig. 4). In contrast to microporous zeolites [6] the decrease in this ratio did not lead to a signi®cant rate of deactivation due to an oligomerization of propylene. The concentration of by-products formed (cumene, ethyltoluenes, TMBs) was lower than 0.5% with simultaneous increase in the concentration of di-isopropyltoluenes (Table 2). A completely dierent behavior of (Al)MCM41 compared to zeolite catalysts was observed in TMB disproportionation. Relatively low conversion of 1,2,4-TMB was found over all (Al)MCM41 catalysts (Table 3, Fig. 5) which is three to four times lower compared to zeolite Beta or mordenite possessing Si/Al ratios of 12.5 or 10, respectively [27]. In addition, 1,2,4-TMB isomerization dominates over disproportionation providing only 8± 13% of xylenes at dierent conversion levels. It could be inferred that in contrast to microporous
acid
supported
on
siliceous
It has been previously reported that HPA supported on mesoporous MCM-41 exhibit signi®cantly higher activity in alkylation of tertbutylphenol compared to bulk HPA due to a large number of surface active sites [9]. In the case of toluene alkylation with propylene over P-HPAMCM-41 with dierent HPA loadings the toluene conversion increases with increasing concentration of HPA (P-HPA-MCM-41/1a±c in Fig. 5). Table 2 shows that there is practically no formation of byproducts and cymenes are exclusively formed. The selectivity to cymenes is, thus, higher than over (Al)MCM-41. Again, no formation of n-propyltoluenes was observed. Although HPA are known as strongly acidic catalysts [28], it seems that (Al)MCM-41 isomerizes p- and o-cymenes to m-cymene much faster than P-HPA-MCM-41. Moreover, surprisingly the formation of m-cymene over P-HPA-MCM-41 proceeds mainly at the expense of o-cymene. As for PV3 -HPA-MCM-41 with dierent pore dimensions (3.3 and 4.5 nm, respectively, cf. Table 1) and the same HPA loading no signi®cant differences were found in toluene alkylation which could indicate that at these pore dimensions, there are no steric or transport eects on the reaction which could cause some dierences in toluene conversion (Table 2).
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J. Cejka et al. / Microporous and Mesoporous Materials 44±45 (2001) 499±507
Table 3 TMB disproportionation over (Al)MCM-41 and HPA-MCM-41 (reaction temperature 670 K, WHSV 5 h 1 , 15 min of time-onstream) (Al)MCM-41c Conversion of 1,2,4-TMB (%)
(Al)MCM-41a
P-MCM-41/1a
P-MCM-41/1c
PV3 -MCM-41/1d
PV3 -MCM-41/2a
3.7
18.6
4.5
23.4
11.2
13.7
Selectivity (vol.%) Low ole®ns Benzene Toluene p-Xylene m-Xylene o-Xylene 1,3,5-TMBa 1,2,3-TMB 1,2,4,5TeMBb 1,2,3,5TeMB 1,2,3,4TeMB PeMBc
0.0 0.0 4.4 2.5 6.3 4.2 32.0 39.5 6.8 3.6 1.2 0.0
0.0 0.0 2.8 1.4 3.7 3.4 45.6 34.0 3.3 3.9 1.0 0.8
0.0 0.0 5.1 1.8 3.4 4.5 37.3 38.8 4.7 2.4 1.0 1.0
1.6 0.4 1.9 3.1 7.6 7.6 36.2 25.4 6.6 7.4 2.0 0.2
2.5 0.0 1.4 5.3 14.4 14.1 14.8 16.2 17.9 10.7 2.7 0.0
2.8 0.0 1.3 5.3 14.0 15.0 18.4 18.0 12.9 9.5 2.8 0.0
P Xylenes P TeMBs
13.0 11.6
8.5 8.2
9.7 8.1
18.3 16.0
33.7 31.3
34.3 25.2
a
TMB ± trimethylbenzene. TeMB ± tetramethylbenzene. c PeMB ± pentamethylbenzene. b
Fig. 5. Time-on-stream dependence of toluene conversion (A) and cymenes selectivity (B) in toluene alkylation with propylene over (d) P-HPA-MCM-41/1a, (m) P-HPA-MCM-41/1b and ( ) P-HPA-MCM-41/1c.
Signi®cant dierences in disproportionation of 1,2,4-TMB over P- and PV3 -HPA-MCM-41 were observed for dierent HPA (cf. Table 3 and Fig. 6). More than 30% of xylenes are formed over PV3 -HPA-MCM-41/1d and 2a. However, these
Fig. 6. Time-on-stream dependence of TMB conversion (A) and xylenes and TeMBs selectivity (B) in 1,2,4-TMB disproportionation over (d) P-HPA-MCM-41/1a and ( ) P-HPAMCM-41/1c, (s, h) xylenes and (d, ) TeMBs, respectively.
selectivities are still considerably low compared to zeolites operating under the same reaction conditions [27]. Concentrations of xylene isomers are far from equilibrium values, particularly the amount
J. Cejka et al. / Microporous and Mesoporous Materials 44±45 (2001) 499±507
of p-xylene is rather low (Table 3). This could be caused by the fact that mainly o- and p-xylenes are primary products of TMB disproportionation (1,3,5-TMB ! m-xylene, 1,2,4-TMB ! mainly mor o-xylene, 1,2,3-TMB ! o-xylene) and monomolecular xylene isomerization is rather limited.
4. Conclusions The acid strength of (Al)MCM-41 was found to be sucient for the activation of propylene in toluene alkylation leading to cymene selectivities over 96% while the rate of consecutive reactions was negligible. In contrast, TMB disproportionation proceeded at a signi®cantly lower extent than over zeolites. Moreover, 1,2,4-TMB isomerization dominates over disproportionation on (Al)MCM41. It could be inferred that in contrast to microporous zeolites the reactant molecules are not strongly held by electrostatic ®eld in the channels which leads to a decrease in the rate of disproportionation. Over siliceous MCM-41 modi®ed by phosphoromolybdenic and phosphorovanadomolybdenic HPA only cymenes and di-isopropyltoluenes were found with a lower rate of cymene isomerization compared to (Al)MCM-41. In addition, m-cymene was mainly formed at the expense of o-cymene. 1,2,4-TMB disproportionation over PV3 -HPA-MCM-41 (HPA H6 PV3 Mo9 O12 ) led to signi®cantly higher concentrations of xylenes in comparison with (Al)MCM-41 with preferential formation of o- and m-xylenes.
Acknowledgements Financial support of the Grant Agency of the Academy of Sciences of the Czech Republic (A4040001) and Grant Agency of the Czech Republic (104/99/0840) is highly acknowledged.
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References [1] D.H. Olson, W.O. Haag, ACS Symp. Series 248 (1984) 275. [2] A. Corma, E. Sastre, J. Chem. Commun., Chem. Soc. (1991) 594. [3] G. Mirth, J. Cejka, J.A. Lercher, J. Catal. 139 (1993) 24. [4] I.I. Ivanova, D. Brunel, G. Daelen, J.B. Nagy, E.G. Derouane, Stud. Surf. Sci. Catal. 78 (1993) 587. [5] I.I. Ivanova, A. Corma, J. Phys. Chem. B 101 (1997) 547. [6] J. Cejka, G.I. Kapustin, B. Wichterlova, Appl. Catal. A 108 (1994) 187. [7] M. Misono, Catal. Rev.-Sci. Eng. 29 (1987) 269. [8] G.J. Meuzelaar, L. Maat, R.A. Sheldon, I.V. Khozevnikov, Catal. Lett. 45 (1997) 269. [9] I.V. Khozevnikov, A. Sinnema, R.J.J. Jansen, K. Pamin, H. van Bekkum, Catal. Lett. 30 (1995) 241. [10] T. Blasco, A. Corma, A. Martinez, P. Martinez-Escolano, J. Catal. 177 (1998) 306. [11] A. Corma, Chem. Rev. 95 (1995) 559. [12] T. Okuhara, N. Mizuno, M. Misono, Adv. Catal. 41 (1996) 113. [13] A. Corma, Chem. Rev. 97 (1997) 2373. [14] I.V. Khozevnikov, Chem. Rev. 98 (1998) 171. [15] S. Morin, P. Ayrault, S.El. Mouahid, N.S. Gnep, M. Guisnet, Appl. Catal. A 159 (1999) 317. [16] J. Medina-Valtierra, O. Zaldivar, M.A. Sanchez, J.A. Montoya, J. Navarrete, J.A. de los Reyes, Appl. Catal. A 166 (1998) 387. [17] J. Cejka, N. Zilkov a, J.E. Sponer, B. Wichterlova, Collect. Czech. Chem. Commun. 63 (1998) 1769. [18] C. Perego, S. Amarilli, A. Carati, C. Flego, G. Pazzuconi, C. Rizzo, G. Bellussi, Micropor. Mesopor. Mater. 27 (1999) 345. [19] G. Schulz-Eklo, J. Rathousky, A. Zukal, J. Inorg. Mater. 1 (1999) 1462. [20] A. Corma, Q. Kan, M.T. Navarro, J. Perez-Pariente, F. Rey, Chem. Mater. 9 (1997) 2123. [21] G.A. Tsigdinos, C.J. Hallada, Inorg. Chem. 7 (1968) 437. } [22] R. Fricke, H.G. Jerschkewitz, G. Ohlmann, J. Chem. Soc., Faraday Trans. I 82 (1986) 3479. [23] A. Bielanski, A. Malecka, L. Kubelkova, J. Chem. Soc., Faraday Trans. I 85 (1989) 2847. [24] P.B. Venuto, L.A. Hamilton, P.S. Landis, J. Catal. 5 (1966) 484. [25] B. Wichterlova, J. Cejka, J. Catal. 146 (1994) 523. [26] J. Cejka, A. Vondrova, B. Wichterlova, G. Vorbeck, R. Fricke, Zeolites 14 (1994) 147. [27] J. Cejka, A. Krejcõ, J. Hanika, Stud. Surf. Sci. Catal. 125 (1999) 351. [28] M. Misono, T. Okuhara, Chemtech 11 (1993) 23.