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n- andpara-selectivity in the alkylation of toluene with isopropanol on molecular sieves
applied catalysis A ELSEV I ER
Applied Catalysis A: General, 108 (1994) 187-204
Factors controlling iso-/n- and para-selectivity in the alkylation of toluene with isopropanol on molecular sieves Jiff Cejka, Gennadij A. Kapustin ~, Blanka Wichterlov~i* J. Heyrovsk3;Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejgkova 3, 182 23 Prague 8, Czech Republic (Received 7 July 1993, revised manuscript received 22 October 1993 )
Abstract Toluene alkylation with isopropanol was investigated over molecular sieves possessing different acidity (AI- and Fe-silicates) and structural type (Y, mordenite and MFI structure) to elucidate factors playing a decisive role in iso-/n- and para-selectivity in propyltoluenes. The desorption/ transport of bulky propyltoluenes from the zeolite channel systems was found to be the reaction rate controlling step. Of the propyltoluenes only cymenes were found with H-mordenite, and H-Y yielded n-propyltoluenes only at temperatures above 550 K, while up to 520 K cymenes were exclusively produced. On the other hand, with isomorphously substituted molecular sieves with MFI structure, having different acidities, a substantial concentration of n-propyltoluenes besides cymenes was found over the whole temperature range investigated (470~620 K). The data indicate that the dominating thctor for formation of n-propyltoluenes ( via a bimolecular mechanism involving the reaction between isopropyltoluene and toluene molecule) is the zeolite structural type. Further, the n-propyltoluene formation is enhanced by the acidic activity of the molecular sieves, controlled by the number and strength of the acid sites and by the reaction temperature, as well as by longer contact time. Thus, both the MFI structure and high acidity of the molecular sieves tend to produce n-propyltoluene. However, simultaneously this structure prefers an over-equilibrium concentration of para-alkyltoluenes. As a compromise between these factors, the highest yield of the desired product, p-cymene, can be found with molecular sieves having a MFI structure possessing a low number of bridging OH groups of a lower acid strength, (H- (Fe) ZSM-5 ), and by employing short contact times and a reaction temperature below 570 K.
Key words."cymenes; MFI; molecular sieves; mordenite; selectivity (para-) ; steric constraints; toluene alkylation; zeolites
*Corresponding author. /On leave from N.D. Zelinsky Institute of Organic Chemistry, Moscow, Russia.
SSDIO926-860X(93) E0220-7
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1. Introduction Selective synthesis of para-dialkylbenzenes, like xylene, ethyltoluene, diethylbenzene, via alkylation reactions catalyzed by zeolites have been thoroughly investigated over the last two decades [ 1-8]. It was concluded that molecular sieves with MFI structure and especially those with their surface modified by silica or basic oxides such as e.g. P205 and MgO can exhibit high para-selectivity yielding para-dialkylbenzenes with a purity higher than 95%, as is required for further processing [9-12]. The molecular sieve's para-selectivity has been ascribed mainly to substantial differences in transport rates of individual (p-, m-, o-) dialkylbenzene isomers through the MFI molecular sieve channel system, which possesses a diameter very close to the kinetic diameters of dialkylbenzene molecules [ 13-15]. Moreover, the distribution of strong acid sites between the inner channels and the crystal surface, where no geometrical constraints exist, can affect the resulting product isomer distribution [ 16]. Recently, the attention was focused on the alkylation of toluene with isopropanol over molecular sieves [ 16,17 ]. These studies were stimulated by the need for p-cymene, which is a source material for production of fungicides, pesticides, flavours as well as heat media. It has been shown that with large pore 12-membered ring H-Y and H-mordenite zeolites, a high selectivity to isopropyltoluenes (cymenes) can be obtained, in contrast to zeolites with MFI structure, which yield besides cymenes also n-propyltoluenes [ 16]. The isomorphous substitution of aluminium, gallium, iron in silicates with MFI structure resulted in a slight difference in iso-/n- and in para-selectivity. Higher selectivity to isopropyltoluenes was found with ferrisilicates, i.e. with molecular sieves of a lower acid strength compared to aluminium analogues [ 17 ]. The aim of this study was to determine and clear up the factors controlling iso-/n-alkyl and para-selectivity of molecular sieves in isopropylation of toluene. Molecular sieves with different geometry of the inner reaction volume (i.e. structural type) and number and strength of acid sites (tailored by isomorphous substitution of aluminium and iron for silicon) were employed. Simultaneously, the effect of feed composition (different tolueneto-propanol molar ratios), reaction temperature and contact time on the toluene conversion, product composition and time-on-stream behaviour of the molecular sieve catalysts was followed in detail.
2. Experimental H-ZSM-5 zeolites with Si/A1 molar ratio 13.6 (H-ZSM-5A 1.08 mmol O H / g ) and 22.5 (H-ZSM-5B 0.69 mmol O H / g ) and crystal size in the range from 0.5 to 2.5 /zm, H-Y zeolite with Si/A1 of 2.5 (4.3 mmol O H / g ) and H-mordenite with Si/A1 7.3 ( 1.48 mmol OH/g) were supplied by the Research Institute for Oil and Hydrocarbon Gases, Slovak Republic. The details of synthesis and characterization of H- (Fe) ZSM-5 (Si/Fe = 27.5, 0.46 mmol O H / g ) have been given elsewhere [ 18]. The number and character of the strong acid sites were determined from the high-
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temperature peak of the temperature-programmed desorption of ammonia (for details see refs. [6,12] ) and IR spectra of OH groups recorded on a FT-IR Nicolet MX-1E spectrometer, respectively. The molecular sieve crystallinity was checked by X-ray diffraction and IR spectra of skeletal vibrations. Toluene alkylation with isopropanol was performed in a vapour phase continuous glass down-flow microreactor at atmospheric pressure and a weight hourly space velocity (WHSV) in the range 2.5-10.0 h ~based on toluene. Nitrogen used as a carrier gas was saturated in separate streams with toluene at 335 K to a level of 18.5 vol.-% and with isopropanol at 308 K, giving a toluene-to-isopropanol molar ratio of 9.6 (for comparison a ratio of 5.3 was also used). Zeolite catalyst (0.40 g) in a granulated form (0.3-0.7 mm) was placed in a microreactor bed (inner diameter, 10 mm) and pretreated in an oxygen stream at 770 K (H-ZSM-5A,B), at 720 K (H-mordenite) and at 670 K (H-Y, H-(Fe)ZSM-5) for 1 h. Then the reactor was cooled down to a preset reaction temperature (420-620 K). The reaction products were analyzed by means of an "on-line" high-resolution capillary gas chromatograph (Hewlett-Packard 5890 II) equipped with flame ionization and mass spectrometric detectors (Hewlett-Packard 5971A). The first analysis was done after 15 min time-on-stream, followed by further sampling of the reaction product at 40 min intervals.
3. Results
The product composition is a result of several competing and subsequent reactions. The major reaction is toluene alkylation with isopropanol and propene, formed by isopropanol dehydration, leading to a mixture of (p-, m-, o- and iso-, n-) propyltoluenes. Simultaneously, propene oligomerization and oligomer cracking, alkylation of aromatics, toluene disproportionation, propyltoluene isomerization, dealkylation and transalkylation can be expected to proceed. As will be shown further on, the actual composition of gaseous products is substantially affected, besides the mentioned reactions, also by transport/desorption processes of bulkier propyltoluenes from the molecular sieve channel systems. The light gas fractions of the products consist exclusively of propene and a small amount of butenes (less than 10 vol.-% in the light gas fraction), while no aliphatics (including methane) were found. Higher aromatics (C~ ~+ ) consist mainly of di-methylpropylbenzenes and butyltoluenes, but contain also a small amount of dipropylmethylbenzenes. This means that propene oligomers are cracked to lower alkenes, which immediately alkylate toluene and to a lower extent alkylate the products of toluene alkylation with propanol. 3.1. Molecular sieves having a M F I structure
Tables 1 and 2 present the typical dependence of toluene conversion and product distribution on time-on-stream (TOS) for H-ZSM-5A and B. Besides toluene conversion the values of isopropanol conversion to alkylaromatics and to propene are given in all tables. In no case isopropanol was completely transformed into alkylaromatics. Over both types of H-ZSM-5 the toluene conversion with TOS increases to the "quasi steady-state" value of approx. 7% (at longer TOS the conversion values start to decline). The concentration of isopropyltoluenes (IPTs) (except for the o-isomer) increases substantially with TOS at the
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Table 1 TOS dependence of toluene conversion and product distribution in toluene alkylation with isopropanol over HZSM-5A (temperature 520 K, WHSV 10.0 h ~, toluene-to-isopropanol molar ratio 9.6) Time (min) Toluene conversion ( % ) Propanol conversion ( % )
15 5.8 100.0 17.2 82.8
55 6.4 100.0 12.5 87.5
95 6.8 100.0 10.5 89.5
Selectivity (vol.-%) Benzene Ethylbenzene p-xylene m-xylene Cumene o-xylene Propylbenzene p-ethyltoluene m-ethyltoluene o-ethyltoluene p-cymene m-cymene o-cymene p-n-propyltoluene m-n-propyltoluene o-n-propyltoluene Higher aromatics
2.7 0.9 0.7 0.9 0.8 1.4 9.0 22.0 12.5 29.0 4.8 15.2
1.2 0.5 0.2 0.4 0.5 0.5 13.4 29.7 12.4 24.9 3.8 12.5
0.7 0.3 0. I 0.2 0.4 0.3 17.6 32.0 11.6 20.8 3.3 12.6
Selectivity (voL-%) ,~ cymenes n-propyltoluenes ~' propyltoluenes
31.0 46.3 77.3
43.1 41.1 74.2
49.6 35.7 85.3
0.8
1.2
1.5
Selectivity (%) p-cymene m-cymene o-cymene
29.1 70.9 -
3 I. 1 68.9 -
35.5 64.5
p-n-propyltoluene m-n-propyltoluene o-n-propyltoluene
27.0 62.6 10.4
30.2 60.6 9.2
32.5 58.3 9.2
C3-C 4 (vol-,%) a
Aromatics (vol-.%)"
iso-/n- ratio
"Percent isopropanol transformed into C3 C4 or aromatics. Thermodynamic equilibrium composition at 498 K: isopropyltoluenes: para 28.2, meta 56.4, ortho 15.4; n-propyltoluenes: para 28.3, meta 56.6, ortho 15.1 [ 17]. The equilibrium ratio of IPTs/NPTs was estimated from the cumene/n-propylbenzene ratio to be 0.6 at 500 K [20].
e x p e n s e o f n - p r o p y l t o l u e n e s ( N P T s ) ; t h e s e l e c t i v i t y v a l u e s w i t h r e s p e c t to I P T s a n d N P T s are i n v e r s e l y p r o p o r t i o n a l e a c h to o t h e r ( T a b l e s 1 a n d 2 ) . T h e r e f o r e , t h e i s o - / n - p r o p y l t o l u e n e ratio i n c r e a s e s s i g n i f i c a n t l y w i t h t i m e - o n - s t r e a m , r e a c h i n g a " s t e a d y s t a t e " v a l u e at
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191
Table 2 TOS dependence of toluene conversion and product distribution in toluene alkylation with isopropanol over HZSM-5B (temperature 520 K, WHSV 10.0 h ~, toluene-to-isopropanol molar ratio 9.6) Time (min) Toluene conversion (%) Propanol conversion (%) C3-C4 ( vol.-% )" Aromatics ( vol.-% )"
15 4.4 100.0 17.7 82.3
55 6.2 100.0 14.8 85.2
95 6.6 100.0 17.9 82.1
175 6.7 100.0 17.4 82.6
-
-
+ -
-
-
-
Selectivity ( vol.- %) Benzene Ethylbenzene p-xylene m-xylene Cumene o-xylene Propylbenzene p-ethyltoluene m-ethyholuene o-ethyltoluene p-cymene m-cymene o-cymene p-n-propyltoluene m-n-propyholuene o-n-propyltoluene Higher aromatics
2.4 . 1.5 . . 1.1 1.8 0.3 . 26.7 8.3
0.4 .
.
.
+ . .
. .
. .
+ 0.2 .
.
-
. 72.7 I 1. I
25.8 16.4 3.4 12.3
63.1 1 1.0 _ 12.6 4.6 0.9 7.1
7.4 3.4 0.5 4.9
73.7 1I. 1 _ 7.3 3.4 0.3 4.2
35.0 45.6 80.6
74.1 18.1 92.2
83.8 11.3 95.1
84.8 11.0 95.8
0.8
4.3
7.4
7.7
p-cymene m-cymene o-cymene
76.3 23.7 -
85. I 14.9
86.8 13.2 -
86.9 13.1 -
p-n-propyltoluene m-n-propyltoluene o-n-propyltoluene
56.6 36.0 7.4
69.6 25.4 5.0
65.5 30.0 4.5
66.4 30.9 2.7
Selectivity (vol.-%) X cymenes n-propyltoluenes X propyltoluenes iso-/n- ratio
Selectivity (%)
"Percent isopropanol transformed into Ct-Ca or aromatics. + traces of product. a t i m e w h e n the t o l u e n e c o n v e r s i o n a l s o a p p r o a c h e s a c o n s t a n t v a l u e . S i m i l a r t r e n d s in toluene conversion and product distribution were observed by Fraenkel and Levy [16]. S i m u l t a n e o u s l y , d u r i n g the w h o l e r e a c t i o n r u n the o v e r a l l s e l e c t i v i t y to p r o p y l t o l u e n e s i n c r e a s e s a n d , o n the o t h e r h a n d , the s e l e c t i v i t y to b y - p r o d u c t s ( C 6 - C 9 a n d C ~~+ a r o m a t i c s ) s u b s t a n t i a l l y d e c r e a s e s . T h i s c l e a r l y i n d i c a t e s that a l t h o u g h the t o l u e n e c o n v e r s i o n i n c r e a s e s d u r i n g the first s t a g e s o f the r e a c t i o n , the z e o l i t e d e a c t i v a t e s w i t h T O S . T h e h i g h e r the
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192
Table 3 TOS dependence of toluene conversion and product distribution in toluene alkylation with isopropanol over HZSM-5B (temperature 520 K, WHSV 5.0 h ~, toluene-to-isopropanol molar ratio 9.6) Time (min) Toluene conversion ( % ) Propanol conversion ( % ) C~-C4 ( vol.-% )" Aromatics ( vol.-% )"
15 4.0 100.0 19.4 80.6
55 5.8 100.0 14.6 85.4
95 6.6 100.0 13.3 86.7
Seleetivity ( vol. - % ) Benzene Ethylbenzene p-xylene m-xylene Cumene o-xylene Propylbenzene p-ethyltoluene m-ethyltoluene o-ethyltoluene p-cymene m-cymene o-cymene p-n-propyltoluene m-n-propyltoluene o-n-propyltoluene Higher aromatics
4.9 0.4 2.8 0.3 + + 1.8 2.5 1.2 . 19.3 9.3 20.8 22.3 3.9 10.6
1.4 + 0.8 + 0.6 0.9 +
0.6 0.4 -
Selectivity ( vol.- % ) cymenes -Yn-propyltoluenes X propyltoluenes
.
0.2 0.4 .
175 6.8 100.0 13.5 86.5
-
.
41.0 13.3 18.7 10.6 2. I 10.6
57.8 14.5 I 1.4 6.2 1. I 7.5
69.8 12.6 0.7 6.7 3.5 0.8 5.9
28.6 47.0 75.6
54.3 31.4 85.7
72.3 18.7 91.0
82.4 I 1.0 93.4
0.6
1.7
3.9
7.5
Seleetivity (%) p-cymene m-cymene o-cymene
67.5 32.5 -
75.4 24.6 -
80.0 20.0 -
84.1 15.1 0.8
p-n-propyltoluene m-n-propyltoluene o-n-propyltoluene
44.2 47.5 8.3
59.6 33.8 6.6
60.7 33.3 6.0
60.9 31.8 7.3
iso-/n-ratio
-
"Percent isopropanol translbrmed into C -Ca or aromatics. + traces of product. n u m b e r o f b r i d g i n g O H g r o u p s o f t h e z e o l i t e is, t h e l o w e r is the s e l e c t i v i t y to p r o p y l t o l u e n e s , to p - c y m e n e ( p - I P T ) a n d p - n - p r o p y l t o l u e n e ( p - N P T )
as w e l l as the l o w e r is t h e i s o - /
n - p r o p y l t o l u e n e ratio. W h e n v a r y i n g the W H S V v a l u e s ( r e c i p r o c a l c o n t a c t t i m e ) f o r H - Z S M - 5 B
(cf. Tables
2 - 4 ) , t h e " q u a s i s t e a d y - s t a t e " c o n v e r s i o n s (i.e. c o n s t a n t c o n v e r s i o n v a l u e s o b t a i n e d at T O S a b o v e 2 0 0 m i n ) r e a c h p r a c t i c a l l y the s a m e v a l u e s ( 6 . 2 , 6.8 a n d 6 . 7 ) at d i f f e r e n t
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193
Table 4 TOS dependence of toluene conversion and product distribution in toluene alkylation with isopropanol over HZSM-5B (temperature 520 K, W H S V 2.5 h - ~, toluene-to-isopropanol molar ratio 9.6) Time ( m i n ) Toluene conversion ( % ) Propanol conversion ( % ) C3-C4 (vol.-%)" Aromatics ( vol.-% )"
15 3.1 100.0 22.8 77.2
55 5.4 100.0 16.6 83.4
135 5.5 100.0 13.8 86.2
215 6.2 100.0 9.5 90.5
14.7 0.6 4.5 1.6 . . 2.4 2.4 2.7 . 8.2 12.5 . 1 1.0 29.8 4.0 5.6
2.9 + 1.6 + . . 1.2 1.2 0.5 . 20.5 17.1 . 18.8 20.6 3.6 12.0
1.9 1.1
0.5 0.5 -
0.8 0.9 0.2
0.3 -
27.6 16.0
51.0 18.4
20.3 15.8 3.0 12.2
10.5 7.8 1.7 9.4
Selectivity ( vol.- % ) Benzene Ethylbenzene p-xylene m-xylene Cumene o-xylene Propylbenzene p-ethyltoluene m-ethyltoluene o-ethyltoluene p-cymene m-cymene o-cymene p-n-propyltoluene m-n-propyltoluene o-n-propyltoluene Higher aromatics
. .
.
.
. .
.
.
Selectivity (vol.-%) S, cymenes n-propyltoluenes propyltoluenes
20.9 44.8 65.7
37.6 43.0 80.6
43.6 39. I 82.7
69.4 20.0 89.4
0.5
0.9
1.1
3.5
p-cymene m-cymene o-cymene
39.4 60.6 -
54.6 45.4
63.2 36.8 -
73.5 26.5 -
p-n-propyltoluene m-n-propyltoluene o-n-propyltoluene
24.6 66.5 8.9
43.8 47.9 8.3
52.0 40.4 7.6
52.4 38.9 8.7
iso-/n- ratio
Selectivi O' (%)
"Percent isopropanol transformed into C~-C4 or aromatics. + traces of the product.
W H S V s of 2.5, 5.0 and 10.0 h J, respectively, while the "initial" conversion is the lowest for the longest contact time. Simultaneously, the longest contact time ( W H S V 2.5 h ~) gives, as expected, a higher selectivity to C6-C9 and C~j+ by-products, especially at the beginning of the reaction, as the catalyst needs a longer reaction time to be deactivated to the same degree as that operated at higher W H S V values. As may be expected, with a lower toluene-to-isopropanol molar ratio in the feed (5.3), a higher conversion of toluene was
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Table 5 TOS dependence of toluene conversion and product distribution in toluene alkylation with isopropanol over HZSM-5B ( temperature 520 K, WHSV 10.0 h- ~, toluene-to-isopropanol molar ratio 5.3 ) Time (min) Toluene conversion ( % ) Propanol conversion ( % ) C3-C 4 ( vol,-% )" Aromatics (vol.-%)"
15 8.5 100.0 18.6 81.4
55 9.0 100.0 37.3 62.7
95 9.4 100.0 37.3 62.7
52.0 15.2 1.2 13.3 5.0 1.3 9.3
74.1 17.0 2.0 2.9
75.8 15.3 1.7 2.7
1.4
1.5
0.4 2.2
0.3 2.7
68.4 19.6 88.0
93. I 4.7 97.8
92.8 4.5 97.3
3.5
20.0
20.7
p-cymene m-cymene o-cymene
75.9 22.3 1.8
79.7 18.2 2.1
81.7 16.4 1.9
p-n-propyltoluene m-n-propyltoluene o-n-propyltoluene
67.7 25.6 6.8
61.4 31.2 7.4
59.4 33.4 7.2
Selectivi~ ( vol. - % )
Benzene Ethylbenzene p-xylene m-xylene Cumene o-xylene Propylbenzene p-ethyltoluene m-ethyltoluene o-ethyltoluene p-cymene m-cymene o-cymene p-n-propyltoluene m-n-propyltoluene o-n-propyltoluene Higher aromatics
0.8 0.6
0.4 0.8
Selectivity ( vol.- % )
v cymenes n-propyltoluenes ,~ propyltoluenes iso-/n- ratio Selectivity (%)
"Percent isopropanol transformed into C3-Ca or aromatics. o b s e r v e d ( T a b l e s 2 a n d 5 ) . M o r e o v e r , a h i g h e r s e l e c t i v i t y to p r o p y l t o l u e n e s a n d a c o n s i d erably h i g h e r i s o - / n - a l k y l i s o m e r ratio w a s f o u n d i n d i c a t i n g that side r e a c t i o n s o f t o l u e n e leading to o t h e r d i a l k y l b e n z e n e s are s u p p r e s s e d . It is r a t h e r difficult to e v a l u a t e the e f f e c t o f t e m p e r a t u r e o n the zeolite activity, e s p e c i a l l y in the initial stages o f the r e a c t i o n , i.e. after 15 , i n o f T O S , w h e n n o n - s t a t i o n a r y c o n v e r s i o n and p r o d u c t c o m p o s i t i o n is o b t a i n e d . A t a low t e m p e r a t u r e o f 4 7 0 K the t o l u e n e c o n v e r s i o n
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195
Table 6 Temperature dependence of toluene conversion and product distribution in toluene alkylation with isopropanol over H-ZSM-5B after 95 min ofTOS (WHSV 10.0 h - ~, toluene-to-isopropanol molar ratio 9.6) Temperature (K) Toluene conversion ( % ) Propanol conversion (%) C3-C4 ( vol.-% )" Aromatics (vol.-%)"
470 1.6 100.0 76.4 23.6
520 6.6 100.0 17.9 82.1
570 4.7 100.0 28.1 71.9
620 5.5 100.0 41.5 58.5
. 52.8 38.8 . 7.5 0.3 0.6
+ -
5.5 1.3 3.2 0.7 0.2 0.2 0.7 5.7 3.4
22.2 5.9 5.9 5.2 0.1 1.7 0.3 10.5 15.4
8.3 6.1
2. I 2.7
7.4 3.4 0.5 4.9
18.5 25.4 2.2 18.6
3.7 9. I 0.7 14.4
Selectivi~ (vol.-%) Benzene Ethylbenzene p-xylene m-xylene Cumene o-xylene Propylbenzene p-ethyltoluene m-ethyltoluene o-ethyltoluene p-cymene m-cymene o-cymene p-n-propyltoluene m-n-propyholuene o-n-propyltoluene Higher aromatics
.
.
.
72.7 11.1 .
.
.
Selectivity (vol.-%) v cymenes n-propyltoluenes propyltoluenes
91.6 7.8 99.4
83.8 1 1.3 95.1
14.4 46.1 60.5
4.8 13.5 18.3
iso-/n- ratio
I 1.7
7.4
0.3
0.3
p-cymene m-cymene o-cymene
57.6 42.4 .
86.8 13.2
57.5 42.5
42.7 57.3
p-n-propyholuene m-n-propyltoluene o-n-propyltoluene
96. l 3.9 -
40. I 55. I 4.8
27.4 67.4 5.2
Selectivity (%)
.
. 65.5 30.0 4.5
.
"Percent isopropanol transtbrmed into C C4 or aromatics. + traces of product. d e c r e a s e s s t e a d i l y w i t h T O S o w i n g to a v e r y s l o w d e s o r p t i o n o f t h e p r o d u c t s ( 3 . 9 % a f t e r 15 m i n , 1.6% a f t e r 95 m i n ) . T h i s is in a c c o r d w i t h ( i ) the o b s e r v e d so c a l l e d " l o w temperature coke"
f o r m e d at l o w t e m p e r a t u r e s b y a l k e n e o l i g o m e r s , d e s c r i b e d first b y
K a r g e et al. [ 1 9 ] , a n d ( i i ) s l o w d e s o r p t i o n rate o f d i e t h y l b e n z e n e s in the e t h y l b e n z e n e a l k y l a t i o n w i t h e t h y l e n e [ 4 ] . T h e r e f o r e , the " q u a s i s t e a d y s t a t e " o f the r e a c t i o n ( T O S a b o v e 95 m i n ) w a s c h o s e n to c o m p a r e the p r o d u c t c o m p o s i t i o n o v e r H - Z S M - 5 B at v a r i o u s
196
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Table 7 TOS dependence of toluene conversion and product distribution in toluene alkylation with isopropanol over H(Fe)ZSM-5 (WHSV 10.0 h - ~, temperature 520 K, toluene-to-isopropanol molar ratio 9.6) Time (min) Toluene conversion ( % ) Propanol conversion (%) C3-C4 (vol.-%)" Aromatics ( vol.-% )"
15 6.5 100.0 21.6 78.4
55 7.0 100.0 21.0 79.0
95 7.1 100.0 20.6 79,4
135 7.1 100.0 21.6 78.4
-
-
74.7 14.1
75.1 13.7
2,9 3.1 0.8 4.4
2.8 3.0 0.7 4.7
Selectivi~ ( vol.- % ) Benzene Ethylbenzene p-xylene m-xylene Cumene o-xylene Propylbenzene p-ethyltoluene m-ethyltoluene o-ethyltoluene p-cymene m-cymene o-cymene p-n-propyholuene m-n-propyltoluene o-n-propyltoluene Higher aromatics
. . . . . . 70.3 16.1 . 2.7 3.3 1.5 6.1
Selectivi~ (vol.- % ) cymenes n-propyltoluenes propyltoluenes
86.4 7.5 93.9
87.2 7.6 94.8
88.8 6.8 95.6
88.8 6.5 95.3
iso-/n- ratio
I 1.6
11.7
13.0
13.5
Selectivi~' (%) p-cymene m-cymene o-cymene
81.3 18.7 -
83.6 16.4 -
84.2 15.8
84.6 15.4 -
p-n-propyltoluene m-n-propyltoluene o-n-propyltoluene
35.8 44.8 19.4
40.9 46.5 12.6
42.4 45.5 12.1
43. l 46.2 10.7
. . .
. . .
. . .
. . .
. . . 72,9 14.3 . 3, I 3.5 1.0 5.2
.
. . .
.
"Percent isopropanol transformed into C3-C4 or aromatics. t e m p e r a t u r e s ( T a b l e 6 ) . W h i l e at 4 7 0 a n d 5 2 0 K a h i g h s e l e c t i v i t y to p r o p y l t o l u e n e s
is
r e a c h e d ( 9 9 a n d 9 5 % , r e s p e c t i v e l y ) , at 5 7 0 a n d 6 2 0 K a c o n s i d e r a b l e a m o u n t o f b y - p r o d u c t s (C6-C9 and C~+
aromatics)
was observed
d e c r e a s i n g t h i s s e l e c t i v i t y to 6 0 a n d 1 8 % ,
r e s p e c t i v e l y . S i m u l t a n e o u s l y , t h e N P T c o n c e n t r a t i o n is h i g h e r t h a n t h a t o f I P T . T h e r e f o r e , t h e o v e r a l l s e l e c t i v i t y to P T s d e c r e a s e s s h a r p l y w i t h i n c r e a s i n g t e m p e r a t u r e a n d t h e i s o - / n - r a t i o a l s o d e c r e a s e s , a p p r o a c h i n g a v a l u e c l o s e to t h a t o f a n e q u i l i b r i u m ( T a b l e 1 ) [ 1 7 , 2 0 ].
J. ~ejka et al. / Applied Catalysis A." General 108 (1994) 18 ~ 204
197
Table 8 Temperature dependence of toluene conversion and product distribution in toluene alkylation with isopropanol over H-Y zeolite after 15 and 95 min ofTOS (WHSV 10.0 h J, toluene-to-isopropanol molar ratio 9.6) Temperature (K) Time (rain) Toluene conversion ( % ) Propanol conversion ( % ) C3-C~ ( vol.-% )" Aromatics ( vol.-% )"
520 15 9.6 100.0 12.6 87.4
520 95 8.7 100.0 9.9 90. I
620 15 8.1 100.0 45.0 55.0
620 95 7.8 100.0 50.9 49.1
2.0 0.2 2.0 2.7 2.3 1.6 2.3 2.4 0.6 23.5 54.0 4.6 -
1.7 0.1 2.4 1.2 2.0 1.7 1.7 1.6 0.4 24.8 56.0 4.8 1.6
26.3 1.6 14.4 17.9 0.4 8.1 2.5 2.6 5.5 1.3 2.5 5.5 + 2.5 6.2 2.6 -
23.1 1.0 15.7 16.5 0.5 7.5 1.9 2.3 4.7 1.0 3.4 7.6 + 2.7 6.7 2.0 3.4
8.0 I 1.3 19.3
I 1.0 I 1.4 22.4
Selectivio, ( vol.- % ) Benzene Ethylbenzene p-xylene m-xylene Cumene o-xylene Propylbenzene p-ethyltoluene m-ethyltoluene o-ethyltoluene p-cymene m-cymene o-cymene p-n-propyltoluene m-n-propyltoluene o-n-propyltoluene Higher aromatics
1.7
Selectivity (wd.-%) Y, cymenes n-propyltoluenes ~' propyltoluenes
82.1 82. I
85.6
iso-/n- ratio
~
~c
28.6 65.8 5.6
29.0 65.4 5.6
3 I. I 68.9 -
31.1 68.9
-
-
22. I 54.9 23.0
23.7 58.8 17.5
85.6
0.71
0.96
Selectivio, (%) p-cymene m-cymene o-cymene p-n-propyltoluene m-n-propyltoluene o-n-propyltoluene
"Percent isopropanol transformed into C 3 - C 4 o r aromatics. + traces of product. F o r the H - ( F e ) Z S M - 5
c o n t a i n i n g b r i d g i n g O H g r o u p s o f l o w e r a c i d i t y c o m p a r e d to
aluminium analogues, nearly only PTs -products (no C6-C9 aromatics)
mainly IPTs --
are f o u n d a m o n g the r e a c t i o n
( T a b l e s 2 a n d 7 ) . H o w e v e r , C ~ t + a r o m a t i c s a p p e a r in
c o n c e n t r a t i o n s c o m p a r a b l e to t h o s e o b t a i n e d w i t h a l u m i n i u m a n a l o g u e s . A t a t e m p e r a t u r e o f 5 2 0 K an i n c r e a s e in t o l u e n e c o n v e r s i o n , a n d in I P T s a n d p - I P T c o n c e n t r a t i o n s w i t h T O S
198
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Table 9 Temperature dependence of toluene conversion and product distribution in toluene alkylation with isopropanol over H-Mordenite after 15 and 95 min ofTOS (WHSV 10.0 h - ~, toluene-to-isopropanol molar ratio 9.6) Temperature (K) Time (min) Toluene conversion (%) Propanol conversion (%) C3-C4 (vol.-%)" Aromatics ( vol.-%)"
520 15 7.7 100.0 24.5 75.5
520 95 2.4 100.0 62.1 37.9
620 15 10.1 100.0 51.4 48.6
620 95 1.8 100.0 77.2 22.8
51.3 34.7 13.1
46.9 0.8 10.9 ! 9.4 0.2 7.7 0.3 0.5 1.0 0.8 2.2 5.1 0.4
33.4 59.0 7.6
3.8
-
7.7
100.0
89.1
7.7
100.0
~
o~
Selectivi~ (vol.-%) Benzene Ethylbenzene p-xylene m-xylene Cumene o-xylene Propylbenzene p-ethyltoluene m-ethyltoluene o-ethyltoluene p-cymene m-cymene o-cymene p-n-propyltoluene m-n-propyltoluene o-n-propyltoluene Higher aromatics
8.7 0.2 5.9 3.5 1.9 1.5 0.4 0.5 0.6 23.4 47.3 5.5 . . . 0.6
. . .
. . .
. . .
0.9
Selectivity (vol.- %) cymenes ,~ n-propyltoluenes ~' propyltoluenes
76.2 . 76.2
89.1
iso-/n- ratio
~
~c
30.7 62.1 7.2
51.8 35.0 13.2
.
.
.
Selectivity (%) p-cymene m-cymene o-cymene
28.7 65.5 5.8
33.4 59.0 7.6
p-n-propyltoluene m-n-propyltoluene o-n-propyltoluene "Percent isopropanol transformed into C3-C 4 or aromatics.
(with simultaneous decrease of NPTs)
is o b s e r v e d , as f o r H - Z S M - 5 A
a n d B. T h e p a r a -
s e l e c t i v i t y is m o r e p r o n o u n c e d w i t h t h e b u l k i e r I P T s c o m p a r e d to N P T s . E v e n t h o u g h t h e " q u a s i s t e a d y - s t a t e " c o n v e r s i o n o f t o l u e n e r e a c h e s n e a r l y the s a m e v a l u e f o r H - Z S M - 5 A and B and H-(Fe)ZSM-5,
t h e i r s e l e c t i v i t y to p - I P T d i f f e r c o n s i d e r a b l y . T h e h i g h e s t a n d
Z ~ejka et al./Applied Catalysis A: General 108 (1994) 187-204
199
comparable para-selectivity, was found with H-ZSM-5B and H-(Fe)ZSM-5 (the latter exhibits a lower number and acid strength of OH groups).
3.2. H- Y zeolite The H-Y zeolite exhibits a higher total conversion of toluene compared to H-ZSM-5A (Tables 1 and 8), which results from the higher number of acid sites and which can also reflect the more open structure, although of a considerably lower acid strength. Over the whole temperature range (470-620 K), the toluene conversion, selectivity to higher aromatics, C 6 - C 9 and C~ + and expectably, the zeolite deactivation are higher compared to H-ZSM-5 (Tables 1, 2 and 8). A slight decrease in toluene conversion was found with the reaction TOS (Table 8). With increasing temperature the PTs selectivity decreases and the concentration o f C 6 - C 9 aromatics and propanol conversion to C 3 - C 4 alkenes increases. This is caused by a substantial increase in the rate of the disproportionation, dealkylation and transalkylation reactions, which can easily occur in an open Y zeolite structure. As expected, no para-isomer enrichment in IPTs was observed. The Y zeolite yields exclusively IPTs at 520 K, NPTs being absent in the products as has already been reported by Fraenkel and Levy [ 16]. However, a substantial concentration of NPTs (iso-/n- ratio about 0.8) was observed in the gaseous phase at temperatures above 550 K.
3.3. H-mordenite A high initial toluene conversion on H-mordenite, reflecting the presence of a high number of strong acid sites, is followed by a dramatic decrease with TOS at 520 and 620 K, due to a well known blocking of the one-dimensional mordenite structural channels (Table 9). Therefore, the dependence of the reaction behaviour on temperature can hardly be predicted. With increasing temperature a substantially higher concentration of C6-C 9 products appears in the initial reaction period, then at higher TOS values, PT selectivity increases considerably. The higher aromatics are not found in high concentration among the reaction products as they are probably retained in the zeolite channels. The IPTs are exclusively formed and no NPTs were found among the reaction products up to 620 K.
4. Discussion 4.1. The effect of time-on-stream and contact time The H-ZSM-5A and B and H-(Fe)ZSM-5 molecular sieves have been already investigated in the alkylation of toluene and ethylbenzene with ethylene [6,8]. In none of these reactions the conversion of toluene or ethylbenzene increased with TOS as is observed for toluene alkylation with isopropanol. However, the increase in toluene conversion as well as in IPT selectivity with TOS for H-(Fe)ZSM-5 is not so dramatic as for H-ZSM-5B (cf. Tables 2 and 7). A possible role of water molecules, evolved from isopropanol during the reaction, should not be overestimated because in the toluene alkylation with methanol and ethanol a high amount of water is usually formed and no increase in toluene conversion
200
J. (~ejka et al./Applied Catalysis A." General 108 (1994) 187-204
with TOS has been reported [ 3,21 ]. Such a high increase in toluene conversion with TOS for the propylation reaction was not observed with the more open H-Y and H-mordenite structures and, on the contrary, an expected decrease in toluene conversion, owing to zeolite deactivation, was found. These results clearly evidence that the overall alkylation reaction is controlled by the rate of transport/desorption of relatively bulky IPT molecules from the zeolite channel system, especially with molecular sieves having a MFI structure. This is likely also the reason why the toluene conversion increases with TOS over these molecular sieves even though some deactivation of the zeolite can be expected and was indicated by a decrease in concentration of high molecular weight C~j + and products of side reactions leading to C 6 - C 9 aromatics (Tables 1-5). This seems to be mainly connected with the low transport rate of the more bulky IPTs which remain for a longer time in the zeolite channel system (an increase of IPTs concentration on the account of NPT in the gas phase with TOS), while smaller linear NPTs diffuse easily [22]. This conclusion on the rate controlling step is also supported by the toluene conversion at different contact times. It should be mentioned here that while with molecular sieves having a MFI structure a higher concentration of the para-isomer is achieved for PTs (higher for IPT as compared to NPT) with H-Y and H-mordenite only an equilibrium composition of both PTs is found.
4.2. The effect of acidity; number and strength of acid sites Surprisingly, while the value of toluene conversion in toluene disproportionation and its alkylation with ethylene was found to be proportional to the overall molecular sieve acidity [ 18], in toluene alkylation with isopropanol the toluene conversion has no relation to the number and strength of acid sites. A different number of acid sites (cf.H-ZSM-5A and B) does not cause a difference in toluene conversion, moreover, with H-(Fe)ZSM-5, possessing both a lower number and a lower acid strength of bridging OH groups, the toluene conversion was higher. This again supports the conclusion that the transport/desorption of the PTs is the reaction rate controlling step. However, the lower acid strength and number of acid sites (cf. H-ZSM-5 and H- (Fe) ZSM-5) evidently suppress competitive reactions and, therefore, PTs are predominantly formed with a high selectivity. Similarly, the iso-/ n-alkyl ratio in PTs is higher with a molecular sieve of lower acidity (cf. Tables 1, 2 and 7). Within the MFI structural type of molecular sieves, an increased acidity (by number and/or strength of acid sites) or temperature, i.e. reactivity of active sites, yields a higher concentration of n-propyltoluenes (Table 6). However, it appears that H-mordenite, exhibiting the highest acidity (reflected also in toluene conversion) does not produce any NPT. This clearly indicates that for the formation of NPTs the molecular sieve structure plays a decisive role while the site activity is an additional, second order, factor affecting selectivity to NPTs.
4.3. Propyltoluene positional isomer composition; para-/meta-/ortho-selectivity It is generally accepted that during the alkylation reactions positional isomerization within dialkylbenzenes is taking place via both unimolecular and bimolecular mechanisms, depending on the reaction space given by the structural type of molecular sieves and temperature
J. (~ejka et al. / Applied Catalysis A: General 108 (1994) 187-204
201
[23-25 ]. The resulting dialkylbenzene p-, m-, o- isomer composition in molecular sieves with MFI structure is affected by the selectivity of the initial alkylation step, by the ratio of the rates of alkylation and isomerization reactions in comparison with the ratio of diffusion rates of individual isomers passing out of the inner molecular sieve volume. Eventually, the rate of the alkylation and isomerization reactions occurring on the surface non-hindered acid sites, where no steric constraints exist, can affect the resulting composition. While the contribution of diffusion processes of the individual dialkylbenzene isomers to the molecular sieve para-selectivity is without any doubt [13-15], the contribution of the individual mentioned reactions to the para-selectivity [26], taking place in the channel systems as well as on the outer crystal surface, is still controversial and widely discussed. Moreover, the effect of the acidity of the isomorphously substituted molecular sieves having a MFI structure (strength and number of sites) on the para-selectivity is not clear. Recently Raj et al. [27] reported a higherpara-selectivity for xylenes in toluene disproportionation and alkylation with methanol over ferrisilicates compared to aluminosilicates with MFI structure. On the other hand, we have found a lowerpara-selectivity for H-(Fe)ZSM-5 than H(AI) ZSM-5 [ 18 ] (of a comparable crystal size) in toluene alkylation with ethylene. In this study of toluene alkylation with isopropanol a comparable para-selectivity is obtained with molecular sieves of different number and acid strength (H-(Fe)ZSM-5 and H-ZSM-5B). However, it should be pointed out that extra-framework iron species, which are present in the ferrisilicates, can be assumed to contribute to a lower para-selectivity especially when located on the crystal surface [ 12,18]. Therefore, definite conclusions about the effect of molecular sieve acidity on para-selectivity can not be drawn.
4.4. Formation of n-propyltoluenes; iso-/n-propyl selectivity Both IPTs and NPTs are formed with molecular sieves having a MFI structure in the alkylation of toluene with isopropanol. Fraenkel and Levy ] 16] and Parikh et al. [ 17] supposed NPTs to be secondary reaction products formed by a direct unimolecular isomerization ofp-IPT to p-NPT. However, we have found dramatic differences in iso-/n- ratio among MFI, Y and mordenite structures. While with molecular sieves having a MFI structure (both aluminium and iron analogues and at various temperatures), the iso-/n- is ranging from a value of 0.6 to 13 (for toluene-to-isopropanol molar ratio 9.6), mordenite yields exclusively IPTs, over the whole range of reaction temperatures and the selectivity of H-Y to NPTs is strongly influenced by the temperature; up to 520 K no NPTs are observed and above 570 K a comparable concentration of NPTs and IPTs is formed. It is evident that large differences in iso-/n-selectivity among MFI, Y and mordenite should be related to the geometry of the reaction space of individual molecular sieves. We believe that the reaction is taking place according to a reaction mechanism that is completely different from that of unimolecular skeletal isomerization of IPTs to NPTs. The primary adsorbed species from isopropanol should be an isopropoxy cation, which is moreover thermodynamically much more favourable than its linear isomer [28] and the formation of IPT molecules as primary products can be expected. Our detailed study [29] on the interaction of p-IPT with benzene on H-ZSM-5 (initiated by a first observation of IPT transformation to NPT in the presence of toluene by Beyer and Borb61y [ 30] ), revealed that a transfer of the isopropyl group from p-IPT to the benzene molecule is taking place
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J. ~ejka et al. / Applied Catalysis A: General 108 (1994) 187-204
leading to the formation of n-propylbenzene. During the preparation of the revised version of this paper, other evidence on the bimolecular mechanism of NPT formation in H-ZSM11 based on a ~3C nuclear magnetic resonance (NMR) study of cumene isomerization has been given by Ivanova et al. [ 31 ]. Such an isopropyl group transfer from IPT to benzene, including isomerization to a npropyl group, was not observed with H-Y zeolite up to temperatures of 520 K. However, above 550 K the concentration of n-propylbenzene over H-Y increased substantially. Thus, the reaction ofp-IPT with benzene revealed the same features in PTs isomer composition on H-ZSM-5 and H-Y as those for the alkylation of toluene with isopropanol. Therefore, NPTs should be formed as secondary products during toluene alkylation with isopropanol via a bimolecular reaction of IPT with toluene present in excess in the reaction volume, i.e. the molecular sieve channels. This conclusion is in line with the observation that the lower toluene-to-isopropanol molar ratio in the feed (i.e. the lower relative toluene concentration), yields higher selectivity to IPTs (cf. Tables 2 and 7). The bimolecular reaction between IPT and toluene in molecular sieves with MFI structure is clearly enhanced by the molecular sieve activity, i.e. by its acidity and increased temperature. Comparing the data for H-ZSM-5A and B and H-(Fe)ZSM-5 (Tables 1, 2 and 7) it is seen that the degree of NPT formation from IPT follows the number and acid strength of bridging OH groups; therefore, most likely the same sites as for the alkylation reaction (OH groups) take part in the bimolecular skeletal isomerization of IPTs to NPTs. However, for the procedure of this bimolecular reaction a specific reaction volume seems to be needed. It has been reported that the alkylation of toluene with isopropanol over Friedel-Crafts catalyst occurring in non-hindered space yields exclusively IPTs [32]. Similarly, with a more open structure of Y zeolite NPT is formed only at higher temperatures, while the unidimensional structure of mordenites does not allow the bimolecular reaction, leading to NPTs, to take place. Therefore, it is supposed that the special geometric arrangement of the channels of molecular sieves with MFI structure forces the toluene molecule to approach the adsorbed IPT molecule in such a way that the following bimolecular transition complex is formed, which can be further transformed into NPT.
It is supposed, similar to the bimolecular mechanism of xylene isomerization, according to Corma and Sastre [25], that the bimolecular transition complex cannot involve two Wheland complexes within close vicinity. More probably one Wheland complex (IPT) could interact with the other molecule (toluene), retained owing to a strong electric field present in the inner zeolite volume. It can be summarized that the molecular sieve structure, controlling the reaction space, enables via "structure-directed transition-state selectivity"
J. ~ejka et al. / Applied Catalysis A: General 108 (1994) 187-204
203
the bimolecular reaction between IPT and toluene, leading to skeletal isomerization of isoto n-propyltoluene.
5. Conclusion It is concluded that the alkylation of toluene with isopropanol is controlled by the desorption/transport of the bulky reaction products, propyltoluenes, out of the inner void volume of the molecular sieves especially those with MFI structure. The isopropyltoluenes as the primary products can be further transformed into n-propyltoluenes preferably in the three-dimensional channel structure of MFI and Y zeolites, n-Propyltoluene formation is taking place via a bimolecular reaction between IPT and toluene, present in an excess in the reaction mixture. This interaction is enabled to a large extent in the three-dimensional channel (cavities) arrangement of molecular sieves and it is hindered in uni-dimensional mordenites. The intersections of the MFI structure are the most convenient geometry among the investigated structural types, where toluene and IPT can approach each other in a convenient way. With the more open structure of H-Y, higher temperatures are needed to enhance the isomerization rate. This molecular sieve structural effect is explained in terms of the "structure-directed transition-state selectivity" and the specific approach of interacting molecules is likely a necessary condition for the bimolecular reaction to take place. However, the rate of this reaction is also affected by the reactivity of the molecular sieve, i.e. its acidity given by the number and strength of the bridging OH groups and reaction temperature. From a practical point of view, ifp-isopropyltoluene (p-cymene) is the desired product, molecular sieves with MFI structure and a lower acidity (Fe analogues), a temperature about 520 K and short contact times can be suggested for p-cymene synthesis. Moreover, the selectivity of such molecular sieve catalysts towards propyltoluenes is high and no side reactions, as transalkylation, methyltransfer and disproportionation, take place as with more open Y and mordenite zeolites. However, with molecular sieves having a MFI structure, undesired n-propyltoluenes are always formed. Their formation can be suppressed by using a low reaction temperature, short contact times and a low toluene-to-isopropanol ratio, which, in addition, affects the overall toluene conversion positively; however, attention should be paid to the easier deactivation of the molecular sieve with higher concentration of isopropanol in the feed.
Acknowledgement The work of J.Q. and B.W. was supported by a grant from the Academy of Sciences of the Czech Republic (No. 44003). The authors are indebted to Drs. G. Vorbeck and R. Fricke (Berlin) for providing a ferrisilicate sample.
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J. ~ejka et al. / Applied Catalysis A: General 108 (1994) 187-204
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
W.W. Kaeding, C. Chu, L.B. Young and S.A. Butter, J. Catal., 69 ( 1981 ) 392. W.W. Kaeding, L.B. Young and C. Chu, J. Catal., 89 (1984) 267. G. Paparatto, E. Moretti, G. Leofanti and F. Gatti, J. Catal., 105 (1987) 227. W.W. Kaeding, J. Catal., 95 (1985) 512. J.H. Kim, S. Namba and T. Yashima, Bull. Chem. Soc. Jpn., 61 (1988) 1051. J. (~ejka, B. Wichterlovfi and S. Bedn~i~ov~i,Appl. Catal., 79 ( 1991 ) 215. H. Vinek and J.A. Lercher, J. Mol. Catal., 64 ( 1991 ) 23. B. Wichterlovt~ and J. (~ejka, Catal. Lett., 16 (1992) 421. N.Y. Chen, W.W. Kaeding and F.G. Dwyer, J. Am. Chem. Soc., 101 (1979) 6783. N.Y. Chen, J. Catal., 114 (1988) 17. T. Hibino, M. Niwa and Y. Murakami, J. Catal., 128 ( 1991 ) 551. J. (~ejka, B. Wichterlov~i, J. Krtil, M. Kfivfmek and R. Fricke, Stud. Surf. Sci. Catal., 69 ( 1991 ) 347. J. Wei, J. Catal., 76 (1982) 433. S.F. Garcia and P.B. Weisz, J. Catal., 121 (1990) 294. G. Mirth, J. (~ejka and J.A. Lercher, J. Catal., 139 (1993) 24. D. Fraenkel and M. Levy, J. Catal., 118 (1989) 10. P.A. Parikh, N. Subrahmanyam, S.Y. Bhat and A.B. Halgeri, Appl. Catal. A, 90 (1992) 1. B. Wichterlov~i, G. Vorbeck, R. Fricke, J. Richter-Mendau and J. (~ejka, Coll. Czech. Chem. Commun., 57 (1992) 799. 19 H.G. Karge, M. Laniecki, M. Ziolek, G. Onyestyak, A. Kiss, P. Kleinschmit and M. Siray, Stud. Surf. Sci. Catal., 49 (1989) 1327. 20 J. Mikulec, S. Beran, B. Wichterlovfi and P. Jfrfi, Appl. Catal., 16 (1985) 389. 21 W.W. Kaeding, C. Chu, L.B. Young, B. Weinstein and S.A. Butter, J. Catal., 67 (1981) 159. 22 R.V. Choudhary and D.B. Apolekar, J. Catal., 117 (1989) 542. 23 S.M. Csicsery, J. Catal., 108 (1987) 433. 24 A. Corma and E. Sastre, J. Chem. Soc., Chem. Commun., 594 ( 1991 ). 25 A. Corma and E. Sastre, J. Catal., 129 ( 1991 ) 177. 26 D.H. Olson and W.O. Haag, Am. Chem. Soc., Symp. Ser., 248 (1984) 275. 27 A. Raj, K.R. Reddy, J.S. Reddy and R. Kumar, in L. Guczi, F. Solimosi and P. T6t6nyi (Editors), New Frontiers in Catalysis, Proceedings 10th International Congress on Catalysis, Budapest, 19-24 July 1992 (Studies in Surface Science and Catalysis, Vol. 75), Elsevier, Amsterdam, 1993, p. 1715. 28 P. Sykes, A Guidebook to Mechanism in Organic Chemistry, 5th ed., Longmans, Green, New York, 1981, p. 106. 29 B. Wichterlov~i and J. (~ejka, J. Catal., in press. 30 H.K. Beyer and G. Borb61y, in Y. Murakami, A. Iijima and J.W. Ward (Editors), New Developments in Zeolites Science and Technology, Proceedings 7th International Zeolites Conference, Tokyo, 17-22 August 1986, Kodansha-Elsevier, Tokyo-Amsterdam, 1986, p. 867. 31 I.I. Ivanova, D. Brunel, J.B. Nagy, G. Daelen and E.G. Derouane, Stud. Surf. Sci. Catal., 78 (1993) 319. 32 R.H. Allen and L.D. Yats, J. Am. Chem. Soc., 83 ( 1961 ) 2799.
Applied Catalysis A: General, 108 (1994) 187-204
Factors controlling iso-/n- and para-selectivity in the alkylation of toluene with isopropanol on molecular sieves Jiff Cejka, Gennadij A. Kapustin ~, Blanka Wichterlov~i* J. Heyrovsk3;Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejgkova 3, 182 23 Prague 8, Czech Republic (Received 7 July 1993, revised manuscript received 22 October 1993 )
Abstract Toluene alkylation with isopropanol was investigated over molecular sieves possessing different acidity (AI- and Fe-silicates) and structural type (Y, mordenite and MFI structure) to elucidate factors playing a decisive role in iso-/n- and para-selectivity in propyltoluenes. The desorption/ transport of bulky propyltoluenes from the zeolite channel systems was found to be the reaction rate controlling step. Of the propyltoluenes only cymenes were found with H-mordenite, and H-Y yielded n-propyltoluenes only at temperatures above 550 K, while up to 520 K cymenes were exclusively produced. On the other hand, with isomorphously substituted molecular sieves with MFI structure, having different acidities, a substantial concentration of n-propyltoluenes besides cymenes was found over the whole temperature range investigated (470~620 K). The data indicate that the dominating thctor for formation of n-propyltoluenes ( via a bimolecular mechanism involving the reaction between isopropyltoluene and toluene molecule) is the zeolite structural type. Further, the n-propyltoluene formation is enhanced by the acidic activity of the molecular sieves, controlled by the number and strength of the acid sites and by the reaction temperature, as well as by longer contact time. Thus, both the MFI structure and high acidity of the molecular sieves tend to produce n-propyltoluene. However, simultaneously this structure prefers an over-equilibrium concentration of para-alkyltoluenes. As a compromise between these factors, the highest yield of the desired product, p-cymene, can be found with molecular sieves having a MFI structure possessing a low number of bridging OH groups of a lower acid strength, (H- (Fe) ZSM-5 ), and by employing short contact times and a reaction temperature below 570 K.
Key words."cymenes; MFI; molecular sieves; mordenite; selectivity (para-) ; steric constraints; toluene alkylation; zeolites
*Corresponding author. /On leave from N.D. Zelinsky Institute of Organic Chemistry, Moscow, Russia.
SSDIO926-860X(93) E0220-7
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1. Introduction Selective synthesis of para-dialkylbenzenes, like xylene, ethyltoluene, diethylbenzene, via alkylation reactions catalyzed by zeolites have been thoroughly investigated over the last two decades [ 1-8]. It was concluded that molecular sieves with MFI structure and especially those with their surface modified by silica or basic oxides such as e.g. P205 and MgO can exhibit high para-selectivity yielding para-dialkylbenzenes with a purity higher than 95%, as is required for further processing [9-12]. The molecular sieve's para-selectivity has been ascribed mainly to substantial differences in transport rates of individual (p-, m-, o-) dialkylbenzene isomers through the MFI molecular sieve channel system, which possesses a diameter very close to the kinetic diameters of dialkylbenzene molecules [ 13-15]. Moreover, the distribution of strong acid sites between the inner channels and the crystal surface, where no geometrical constraints exist, can affect the resulting product isomer distribution [ 16]. Recently, the attention was focused on the alkylation of toluene with isopropanol over molecular sieves [ 16,17 ]. These studies were stimulated by the need for p-cymene, which is a source material for production of fungicides, pesticides, flavours as well as heat media. It has been shown that with large pore 12-membered ring H-Y and H-mordenite zeolites, a high selectivity to isopropyltoluenes (cymenes) can be obtained, in contrast to zeolites with MFI structure, which yield besides cymenes also n-propyltoluenes [ 16]. The isomorphous substitution of aluminium, gallium, iron in silicates with MFI structure resulted in a slight difference in iso-/n- and in para-selectivity. Higher selectivity to isopropyltoluenes was found with ferrisilicates, i.e. with molecular sieves of a lower acid strength compared to aluminium analogues [ 17 ]. The aim of this study was to determine and clear up the factors controlling iso-/n-alkyl and para-selectivity of molecular sieves in isopropylation of toluene. Molecular sieves with different geometry of the inner reaction volume (i.e. structural type) and number and strength of acid sites (tailored by isomorphous substitution of aluminium and iron for silicon) were employed. Simultaneously, the effect of feed composition (different tolueneto-propanol molar ratios), reaction temperature and contact time on the toluene conversion, product composition and time-on-stream behaviour of the molecular sieve catalysts was followed in detail.
2. Experimental H-ZSM-5 zeolites with Si/A1 molar ratio 13.6 (H-ZSM-5A 1.08 mmol O H / g ) and 22.5 (H-ZSM-5B 0.69 mmol O H / g ) and crystal size in the range from 0.5 to 2.5 /zm, H-Y zeolite with Si/A1 of 2.5 (4.3 mmol O H / g ) and H-mordenite with Si/A1 7.3 ( 1.48 mmol OH/g) were supplied by the Research Institute for Oil and Hydrocarbon Gases, Slovak Republic. The details of synthesis and characterization of H- (Fe) ZSM-5 (Si/Fe = 27.5, 0.46 mmol O H / g ) have been given elsewhere [ 18]. The number and character of the strong acid sites were determined from the high-
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189
temperature peak of the temperature-programmed desorption of ammonia (for details see refs. [6,12] ) and IR spectra of OH groups recorded on a FT-IR Nicolet MX-1E spectrometer, respectively. The molecular sieve crystallinity was checked by X-ray diffraction and IR spectra of skeletal vibrations. Toluene alkylation with isopropanol was performed in a vapour phase continuous glass down-flow microreactor at atmospheric pressure and a weight hourly space velocity (WHSV) in the range 2.5-10.0 h ~based on toluene. Nitrogen used as a carrier gas was saturated in separate streams with toluene at 335 K to a level of 18.5 vol.-% and with isopropanol at 308 K, giving a toluene-to-isopropanol molar ratio of 9.6 (for comparison a ratio of 5.3 was also used). Zeolite catalyst (0.40 g) in a granulated form (0.3-0.7 mm) was placed in a microreactor bed (inner diameter, 10 mm) and pretreated in an oxygen stream at 770 K (H-ZSM-5A,B), at 720 K (H-mordenite) and at 670 K (H-Y, H-(Fe)ZSM-5) for 1 h. Then the reactor was cooled down to a preset reaction temperature (420-620 K). The reaction products were analyzed by means of an "on-line" high-resolution capillary gas chromatograph (Hewlett-Packard 5890 II) equipped with flame ionization and mass spectrometric detectors (Hewlett-Packard 5971A). The first analysis was done after 15 min time-on-stream, followed by further sampling of the reaction product at 40 min intervals.
3. Results
The product composition is a result of several competing and subsequent reactions. The major reaction is toluene alkylation with isopropanol and propene, formed by isopropanol dehydration, leading to a mixture of (p-, m-, o- and iso-, n-) propyltoluenes. Simultaneously, propene oligomerization and oligomer cracking, alkylation of aromatics, toluene disproportionation, propyltoluene isomerization, dealkylation and transalkylation can be expected to proceed. As will be shown further on, the actual composition of gaseous products is substantially affected, besides the mentioned reactions, also by transport/desorption processes of bulkier propyltoluenes from the molecular sieve channel systems. The light gas fractions of the products consist exclusively of propene and a small amount of butenes (less than 10 vol.-% in the light gas fraction), while no aliphatics (including methane) were found. Higher aromatics (C~ ~+ ) consist mainly of di-methylpropylbenzenes and butyltoluenes, but contain also a small amount of dipropylmethylbenzenes. This means that propene oligomers are cracked to lower alkenes, which immediately alkylate toluene and to a lower extent alkylate the products of toluene alkylation with propanol. 3.1. Molecular sieves having a M F I structure
Tables 1 and 2 present the typical dependence of toluene conversion and product distribution on time-on-stream (TOS) for H-ZSM-5A and B. Besides toluene conversion the values of isopropanol conversion to alkylaromatics and to propene are given in all tables. In no case isopropanol was completely transformed into alkylaromatics. Over both types of H-ZSM-5 the toluene conversion with TOS increases to the "quasi steady-state" value of approx. 7% (at longer TOS the conversion values start to decline). The concentration of isopropyltoluenes (IPTs) (except for the o-isomer) increases substantially with TOS at the
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Table 1 TOS dependence of toluene conversion and product distribution in toluene alkylation with isopropanol over HZSM-5A (temperature 520 K, WHSV 10.0 h ~, toluene-to-isopropanol molar ratio 9.6) Time (min) Toluene conversion ( % ) Propanol conversion ( % )
15 5.8 100.0 17.2 82.8
55 6.4 100.0 12.5 87.5
95 6.8 100.0 10.5 89.5
Selectivity (vol.-%) Benzene Ethylbenzene p-xylene m-xylene Cumene o-xylene Propylbenzene p-ethyltoluene m-ethyltoluene o-ethyltoluene p-cymene m-cymene o-cymene p-n-propyltoluene m-n-propyltoluene o-n-propyltoluene Higher aromatics
2.7 0.9 0.7 0.9 0.8 1.4 9.0 22.0 12.5 29.0 4.8 15.2
1.2 0.5 0.2 0.4 0.5 0.5 13.4 29.7 12.4 24.9 3.8 12.5
0.7 0.3 0. I 0.2 0.4 0.3 17.6 32.0 11.6 20.8 3.3 12.6
Selectivity (voL-%) ,~ cymenes n-propyltoluenes ~' propyltoluenes
31.0 46.3 77.3
43.1 41.1 74.2
49.6 35.7 85.3
0.8
1.2
1.5
Selectivity (%) p-cymene m-cymene o-cymene
29.1 70.9 -
3 I. 1 68.9 -
35.5 64.5
p-n-propyltoluene m-n-propyltoluene o-n-propyltoluene
27.0 62.6 10.4
30.2 60.6 9.2
32.5 58.3 9.2
C3-C 4 (vol-,%) a
Aromatics (vol-.%)"
iso-/n- ratio
"Percent isopropanol transformed into C3 C4 or aromatics. Thermodynamic equilibrium composition at 498 K: isopropyltoluenes: para 28.2, meta 56.4, ortho 15.4; n-propyltoluenes: para 28.3, meta 56.6, ortho 15.1 [ 17]. The equilibrium ratio of IPTs/NPTs was estimated from the cumene/n-propylbenzene ratio to be 0.6 at 500 K [20].
e x p e n s e o f n - p r o p y l t o l u e n e s ( N P T s ) ; t h e s e l e c t i v i t y v a l u e s w i t h r e s p e c t to I P T s a n d N P T s are i n v e r s e l y p r o p o r t i o n a l e a c h to o t h e r ( T a b l e s 1 a n d 2 ) . T h e r e f o r e , t h e i s o - / n - p r o p y l t o l u e n e ratio i n c r e a s e s s i g n i f i c a n t l y w i t h t i m e - o n - s t r e a m , r e a c h i n g a " s t e a d y s t a t e " v a l u e at
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Table 2 TOS dependence of toluene conversion and product distribution in toluene alkylation with isopropanol over HZSM-5B (temperature 520 K, WHSV 10.0 h ~, toluene-to-isopropanol molar ratio 9.6) Time (min) Toluene conversion (%) Propanol conversion (%) C3-C4 ( vol.-% )" Aromatics ( vol.-% )"
15 4.4 100.0 17.7 82.3
55 6.2 100.0 14.8 85.2
95 6.6 100.0 17.9 82.1
175 6.7 100.0 17.4 82.6
-
-
+ -
-
-
-
Selectivity ( vol.- %) Benzene Ethylbenzene p-xylene m-xylene Cumene o-xylene Propylbenzene p-ethyltoluene m-ethyholuene o-ethyltoluene p-cymene m-cymene o-cymene p-n-propyltoluene m-n-propyholuene o-n-propyltoluene Higher aromatics
2.4 . 1.5 . . 1.1 1.8 0.3 . 26.7 8.3
0.4 .
.
.
+ . .
. .
. .
+ 0.2 .
.
-
. 72.7 I 1. I
25.8 16.4 3.4 12.3
63.1 1 1.0 _ 12.6 4.6 0.9 7.1
7.4 3.4 0.5 4.9
73.7 1I. 1 _ 7.3 3.4 0.3 4.2
35.0 45.6 80.6
74.1 18.1 92.2
83.8 11.3 95.1
84.8 11.0 95.8
0.8
4.3
7.4
7.7
p-cymene m-cymene o-cymene
76.3 23.7 -
85. I 14.9
86.8 13.2 -
86.9 13.1 -
p-n-propyltoluene m-n-propyltoluene o-n-propyltoluene
56.6 36.0 7.4
69.6 25.4 5.0
65.5 30.0 4.5
66.4 30.9 2.7
Selectivity (vol.-%) X cymenes n-propyltoluenes X propyltoluenes iso-/n- ratio
Selectivity (%)
"Percent isopropanol transformed into Ct-Ca or aromatics. + traces of product. a t i m e w h e n the t o l u e n e c o n v e r s i o n a l s o a p p r o a c h e s a c o n s t a n t v a l u e . S i m i l a r t r e n d s in toluene conversion and product distribution were observed by Fraenkel and Levy [16]. S i m u l t a n e o u s l y , d u r i n g the w h o l e r e a c t i o n r u n the o v e r a l l s e l e c t i v i t y to p r o p y l t o l u e n e s i n c r e a s e s a n d , o n the o t h e r h a n d , the s e l e c t i v i t y to b y - p r o d u c t s ( C 6 - C 9 a n d C ~~+ a r o m a t i c s ) s u b s t a n t i a l l y d e c r e a s e s . T h i s c l e a r l y i n d i c a t e s that a l t h o u g h the t o l u e n e c o n v e r s i o n i n c r e a s e s d u r i n g the first s t a g e s o f the r e a c t i o n , the z e o l i t e d e a c t i v a t e s w i t h T O S . T h e h i g h e r the
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Table 3 TOS dependence of toluene conversion and product distribution in toluene alkylation with isopropanol over HZSM-5B (temperature 520 K, WHSV 5.0 h ~, toluene-to-isopropanol molar ratio 9.6) Time (min) Toluene conversion ( % ) Propanol conversion ( % ) C~-C4 ( vol.-% )" Aromatics ( vol.-% )"
15 4.0 100.0 19.4 80.6
55 5.8 100.0 14.6 85.4
95 6.6 100.0 13.3 86.7
Seleetivity ( vol. - % ) Benzene Ethylbenzene p-xylene m-xylene Cumene o-xylene Propylbenzene p-ethyltoluene m-ethyltoluene o-ethyltoluene p-cymene m-cymene o-cymene p-n-propyltoluene m-n-propyltoluene o-n-propyltoluene Higher aromatics
4.9 0.4 2.8 0.3 + + 1.8 2.5 1.2 . 19.3 9.3 20.8 22.3 3.9 10.6
1.4 + 0.8 + 0.6 0.9 +
0.6 0.4 -
Selectivity ( vol.- % ) cymenes -Yn-propyltoluenes X propyltoluenes
.
0.2 0.4 .
175 6.8 100.0 13.5 86.5
-
.
41.0 13.3 18.7 10.6 2. I 10.6
57.8 14.5 I 1.4 6.2 1. I 7.5
69.8 12.6 0.7 6.7 3.5 0.8 5.9
28.6 47.0 75.6
54.3 31.4 85.7
72.3 18.7 91.0
82.4 I 1.0 93.4
0.6
1.7
3.9
7.5
Seleetivity (%) p-cymene m-cymene o-cymene
67.5 32.5 -
75.4 24.6 -
80.0 20.0 -
84.1 15.1 0.8
p-n-propyltoluene m-n-propyltoluene o-n-propyltoluene
44.2 47.5 8.3
59.6 33.8 6.6
60.7 33.3 6.0
60.9 31.8 7.3
iso-/n-ratio
-
"Percent isopropanol translbrmed into C -Ca or aromatics. + traces of product. n u m b e r o f b r i d g i n g O H g r o u p s o f t h e z e o l i t e is, t h e l o w e r is the s e l e c t i v i t y to p r o p y l t o l u e n e s , to p - c y m e n e ( p - I P T ) a n d p - n - p r o p y l t o l u e n e ( p - N P T )
as w e l l as the l o w e r is t h e i s o - /
n - p r o p y l t o l u e n e ratio. W h e n v a r y i n g the W H S V v a l u e s ( r e c i p r o c a l c o n t a c t t i m e ) f o r H - Z S M - 5 B
(cf. Tables
2 - 4 ) , t h e " q u a s i s t e a d y - s t a t e " c o n v e r s i o n s (i.e. c o n s t a n t c o n v e r s i o n v a l u e s o b t a i n e d at T O S a b o v e 2 0 0 m i n ) r e a c h p r a c t i c a l l y the s a m e v a l u e s ( 6 . 2 , 6.8 a n d 6 . 7 ) at d i f f e r e n t
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193
Table 4 TOS dependence of toluene conversion and product distribution in toluene alkylation with isopropanol over HZSM-5B (temperature 520 K, W H S V 2.5 h - ~, toluene-to-isopropanol molar ratio 9.6) Time ( m i n ) Toluene conversion ( % ) Propanol conversion ( % ) C3-C4 (vol.-%)" Aromatics ( vol.-% )"
15 3.1 100.0 22.8 77.2
55 5.4 100.0 16.6 83.4
135 5.5 100.0 13.8 86.2
215 6.2 100.0 9.5 90.5
14.7 0.6 4.5 1.6 . . 2.4 2.4 2.7 . 8.2 12.5 . 1 1.0 29.8 4.0 5.6
2.9 + 1.6 + . . 1.2 1.2 0.5 . 20.5 17.1 . 18.8 20.6 3.6 12.0
1.9 1.1
0.5 0.5 -
0.8 0.9 0.2
0.3 -
27.6 16.0
51.0 18.4
20.3 15.8 3.0 12.2
10.5 7.8 1.7 9.4
Selectivity ( vol.- % ) Benzene Ethylbenzene p-xylene m-xylene Cumene o-xylene Propylbenzene p-ethyltoluene m-ethyltoluene o-ethyltoluene p-cymene m-cymene o-cymene p-n-propyltoluene m-n-propyltoluene o-n-propyltoluene Higher aromatics
. .
.
.
. .
.
.
Selectivity (vol.-%) S, cymenes n-propyltoluenes propyltoluenes
20.9 44.8 65.7
37.6 43.0 80.6
43.6 39. I 82.7
69.4 20.0 89.4
0.5
0.9
1.1
3.5
p-cymene m-cymene o-cymene
39.4 60.6 -
54.6 45.4
63.2 36.8 -
73.5 26.5 -
p-n-propyltoluene m-n-propyltoluene o-n-propyltoluene
24.6 66.5 8.9
43.8 47.9 8.3
52.0 40.4 7.6
52.4 38.9 8.7
iso-/n- ratio
Selectivi O' (%)
"Percent isopropanol transformed into C~-C4 or aromatics. + traces of the product.
W H S V s of 2.5, 5.0 and 10.0 h J, respectively, while the "initial" conversion is the lowest for the longest contact time. Simultaneously, the longest contact time ( W H S V 2.5 h ~) gives, as expected, a higher selectivity to C6-C9 and C~j+ by-products, especially at the beginning of the reaction, as the catalyst needs a longer reaction time to be deactivated to the same degree as that operated at higher W H S V values. As may be expected, with a lower toluene-to-isopropanol molar ratio in the feed (5.3), a higher conversion of toluene was
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Table 5 TOS dependence of toluene conversion and product distribution in toluene alkylation with isopropanol over HZSM-5B ( temperature 520 K, WHSV 10.0 h- ~, toluene-to-isopropanol molar ratio 5.3 ) Time (min) Toluene conversion ( % ) Propanol conversion ( % ) C3-C 4 ( vol,-% )" Aromatics (vol.-%)"
15 8.5 100.0 18.6 81.4
55 9.0 100.0 37.3 62.7
95 9.4 100.0 37.3 62.7
52.0 15.2 1.2 13.3 5.0 1.3 9.3
74.1 17.0 2.0 2.9
75.8 15.3 1.7 2.7
1.4
1.5
0.4 2.2
0.3 2.7
68.4 19.6 88.0
93. I 4.7 97.8
92.8 4.5 97.3
3.5
20.0
20.7
p-cymene m-cymene o-cymene
75.9 22.3 1.8
79.7 18.2 2.1
81.7 16.4 1.9
p-n-propyltoluene m-n-propyltoluene o-n-propyltoluene
67.7 25.6 6.8
61.4 31.2 7.4
59.4 33.4 7.2
Selectivi~ ( vol. - % )
Benzene Ethylbenzene p-xylene m-xylene Cumene o-xylene Propylbenzene p-ethyltoluene m-ethyltoluene o-ethyltoluene p-cymene m-cymene o-cymene p-n-propyltoluene m-n-propyltoluene o-n-propyltoluene Higher aromatics
0.8 0.6
0.4 0.8
Selectivity ( vol.- % )
v cymenes n-propyltoluenes ,~ propyltoluenes iso-/n- ratio Selectivity (%)
"Percent isopropanol transformed into C3-Ca or aromatics. o b s e r v e d ( T a b l e s 2 a n d 5 ) . M o r e o v e r , a h i g h e r s e l e c t i v i t y to p r o p y l t o l u e n e s a n d a c o n s i d erably h i g h e r i s o - / n - a l k y l i s o m e r ratio w a s f o u n d i n d i c a t i n g that side r e a c t i o n s o f t o l u e n e leading to o t h e r d i a l k y l b e n z e n e s are s u p p r e s s e d . It is r a t h e r difficult to e v a l u a t e the e f f e c t o f t e m p e r a t u r e o n the zeolite activity, e s p e c i a l l y in the initial stages o f the r e a c t i o n , i.e. after 15 , i n o f T O S , w h e n n o n - s t a t i o n a r y c o n v e r s i o n and p r o d u c t c o m p o s i t i o n is o b t a i n e d . A t a low t e m p e r a t u r e o f 4 7 0 K the t o l u e n e c o n v e r s i o n
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195
Table 6 Temperature dependence of toluene conversion and product distribution in toluene alkylation with isopropanol over H-ZSM-5B after 95 min ofTOS (WHSV 10.0 h - ~, toluene-to-isopropanol molar ratio 9.6) Temperature (K) Toluene conversion ( % ) Propanol conversion (%) C3-C4 ( vol.-% )" Aromatics (vol.-%)"
470 1.6 100.0 76.4 23.6
520 6.6 100.0 17.9 82.1
570 4.7 100.0 28.1 71.9
620 5.5 100.0 41.5 58.5
. 52.8 38.8 . 7.5 0.3 0.6
+ -
5.5 1.3 3.2 0.7 0.2 0.2 0.7 5.7 3.4
22.2 5.9 5.9 5.2 0.1 1.7 0.3 10.5 15.4
8.3 6.1
2. I 2.7
7.4 3.4 0.5 4.9
18.5 25.4 2.2 18.6
3.7 9. I 0.7 14.4
Selectivi~ (vol.-%) Benzene Ethylbenzene p-xylene m-xylene Cumene o-xylene Propylbenzene p-ethyltoluene m-ethyltoluene o-ethyltoluene p-cymene m-cymene o-cymene p-n-propyltoluene m-n-propyholuene o-n-propyltoluene Higher aromatics
.
.
.
72.7 11.1 .
.
.
Selectivity (vol.-%) v cymenes n-propyltoluenes propyltoluenes
91.6 7.8 99.4
83.8 1 1.3 95.1
14.4 46.1 60.5
4.8 13.5 18.3
iso-/n- ratio
I 1.7
7.4
0.3
0.3
p-cymene m-cymene o-cymene
57.6 42.4 .
86.8 13.2
57.5 42.5
42.7 57.3
p-n-propyholuene m-n-propyltoluene o-n-propyltoluene
96. l 3.9 -
40. I 55. I 4.8
27.4 67.4 5.2
Selectivity (%)
.
. 65.5 30.0 4.5
.
"Percent isopropanol transtbrmed into C C4 or aromatics. + traces of product. d e c r e a s e s s t e a d i l y w i t h T O S o w i n g to a v e r y s l o w d e s o r p t i o n o f t h e p r o d u c t s ( 3 . 9 % a f t e r 15 m i n , 1.6% a f t e r 95 m i n ) . T h i s is in a c c o r d w i t h ( i ) the o b s e r v e d so c a l l e d " l o w temperature coke"
f o r m e d at l o w t e m p e r a t u r e s b y a l k e n e o l i g o m e r s , d e s c r i b e d first b y
K a r g e et al. [ 1 9 ] , a n d ( i i ) s l o w d e s o r p t i o n rate o f d i e t h y l b e n z e n e s in the e t h y l b e n z e n e a l k y l a t i o n w i t h e t h y l e n e [ 4 ] . T h e r e f o r e , the " q u a s i s t e a d y s t a t e " o f the r e a c t i o n ( T O S a b o v e 95 m i n ) w a s c h o s e n to c o m p a r e the p r o d u c t c o m p o s i t i o n o v e r H - Z S M - 5 B at v a r i o u s
196
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Table 7 TOS dependence of toluene conversion and product distribution in toluene alkylation with isopropanol over H(Fe)ZSM-5 (WHSV 10.0 h - ~, temperature 520 K, toluene-to-isopropanol molar ratio 9.6) Time (min) Toluene conversion ( % ) Propanol conversion (%) C3-C4 (vol.-%)" Aromatics ( vol.-% )"
15 6.5 100.0 21.6 78.4
55 7.0 100.0 21.0 79.0
95 7.1 100.0 20.6 79,4
135 7.1 100.0 21.6 78.4
-
-
74.7 14.1
75.1 13.7
2,9 3.1 0.8 4.4
2.8 3.0 0.7 4.7
Selectivi~ ( vol.- % ) Benzene Ethylbenzene p-xylene m-xylene Cumene o-xylene Propylbenzene p-ethyltoluene m-ethyltoluene o-ethyltoluene p-cymene m-cymene o-cymene p-n-propyholuene m-n-propyltoluene o-n-propyltoluene Higher aromatics
. . . . . . 70.3 16.1 . 2.7 3.3 1.5 6.1
Selectivi~ (vol.- % ) cymenes n-propyltoluenes propyltoluenes
86.4 7.5 93.9
87.2 7.6 94.8
88.8 6.8 95.6
88.8 6.5 95.3
iso-/n- ratio
I 1.6
11.7
13.0
13.5
Selectivi~' (%) p-cymene m-cymene o-cymene
81.3 18.7 -
83.6 16.4 -
84.2 15.8
84.6 15.4 -
p-n-propyltoluene m-n-propyltoluene o-n-propyltoluene
35.8 44.8 19.4
40.9 46.5 12.6
42.4 45.5 12.1
43. l 46.2 10.7
. . .
. . .
. . .
. . .
. . . 72,9 14.3 . 3, I 3.5 1.0 5.2
.
. . .
.
"Percent isopropanol transformed into C3-C4 or aromatics. t e m p e r a t u r e s ( T a b l e 6 ) . W h i l e at 4 7 0 a n d 5 2 0 K a h i g h s e l e c t i v i t y to p r o p y l t o l u e n e s
is
r e a c h e d ( 9 9 a n d 9 5 % , r e s p e c t i v e l y ) , at 5 7 0 a n d 6 2 0 K a c o n s i d e r a b l e a m o u n t o f b y - p r o d u c t s (C6-C9 and C~+
aromatics)
was observed
d e c r e a s i n g t h i s s e l e c t i v i t y to 6 0 a n d 1 8 % ,
r e s p e c t i v e l y . S i m u l t a n e o u s l y , t h e N P T c o n c e n t r a t i o n is h i g h e r t h a n t h a t o f I P T . T h e r e f o r e , t h e o v e r a l l s e l e c t i v i t y to P T s d e c r e a s e s s h a r p l y w i t h i n c r e a s i n g t e m p e r a t u r e a n d t h e i s o - / n - r a t i o a l s o d e c r e a s e s , a p p r o a c h i n g a v a l u e c l o s e to t h a t o f a n e q u i l i b r i u m ( T a b l e 1 ) [ 1 7 , 2 0 ].
J. ~ejka et al. / Applied Catalysis A." General 108 (1994) 18 ~ 204
197
Table 8 Temperature dependence of toluene conversion and product distribution in toluene alkylation with isopropanol over H-Y zeolite after 15 and 95 min ofTOS (WHSV 10.0 h J, toluene-to-isopropanol molar ratio 9.6) Temperature (K) Time (rain) Toluene conversion ( % ) Propanol conversion ( % ) C3-C~ ( vol.-% )" Aromatics ( vol.-% )"
520 15 9.6 100.0 12.6 87.4
520 95 8.7 100.0 9.9 90. I
620 15 8.1 100.0 45.0 55.0
620 95 7.8 100.0 50.9 49.1
2.0 0.2 2.0 2.7 2.3 1.6 2.3 2.4 0.6 23.5 54.0 4.6 -
1.7 0.1 2.4 1.2 2.0 1.7 1.7 1.6 0.4 24.8 56.0 4.8 1.6
26.3 1.6 14.4 17.9 0.4 8.1 2.5 2.6 5.5 1.3 2.5 5.5 + 2.5 6.2 2.6 -
23.1 1.0 15.7 16.5 0.5 7.5 1.9 2.3 4.7 1.0 3.4 7.6 + 2.7 6.7 2.0 3.4
8.0 I 1.3 19.3
I 1.0 I 1.4 22.4
Selectivio, ( vol.- % ) Benzene Ethylbenzene p-xylene m-xylene Cumene o-xylene Propylbenzene p-ethyltoluene m-ethyltoluene o-ethyltoluene p-cymene m-cymene o-cymene p-n-propyltoluene m-n-propyltoluene o-n-propyltoluene Higher aromatics
1.7
Selectivity (wd.-%) Y, cymenes n-propyltoluenes ~' propyltoluenes
82.1 82. I
85.6
iso-/n- ratio
~
~c
28.6 65.8 5.6
29.0 65.4 5.6
3 I. I 68.9 -
31.1 68.9
-
-
22. I 54.9 23.0
23.7 58.8 17.5
85.6
0.71
0.96
Selectivio, (%) p-cymene m-cymene o-cymene p-n-propyltoluene m-n-propyltoluene o-n-propyltoluene
"Percent isopropanol transformed into C 3 - C 4 o r aromatics. + traces of product. F o r the H - ( F e ) Z S M - 5
c o n t a i n i n g b r i d g i n g O H g r o u p s o f l o w e r a c i d i t y c o m p a r e d to
aluminium analogues, nearly only PTs -products (no C6-C9 aromatics)
mainly IPTs --
are f o u n d a m o n g the r e a c t i o n
( T a b l e s 2 a n d 7 ) . H o w e v e r , C ~ t + a r o m a t i c s a p p e a r in
c o n c e n t r a t i o n s c o m p a r a b l e to t h o s e o b t a i n e d w i t h a l u m i n i u m a n a l o g u e s . A t a t e m p e r a t u r e o f 5 2 0 K an i n c r e a s e in t o l u e n e c o n v e r s i o n , a n d in I P T s a n d p - I P T c o n c e n t r a t i o n s w i t h T O S
198
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Table 9 Temperature dependence of toluene conversion and product distribution in toluene alkylation with isopropanol over H-Mordenite after 15 and 95 min ofTOS (WHSV 10.0 h - ~, toluene-to-isopropanol molar ratio 9.6) Temperature (K) Time (min) Toluene conversion (%) Propanol conversion (%) C3-C4 (vol.-%)" Aromatics ( vol.-%)"
520 15 7.7 100.0 24.5 75.5
520 95 2.4 100.0 62.1 37.9
620 15 10.1 100.0 51.4 48.6
620 95 1.8 100.0 77.2 22.8
51.3 34.7 13.1
46.9 0.8 10.9 ! 9.4 0.2 7.7 0.3 0.5 1.0 0.8 2.2 5.1 0.4
33.4 59.0 7.6
3.8
-
7.7
100.0
89.1
7.7
100.0
~
o~
Selectivi~ (vol.-%) Benzene Ethylbenzene p-xylene m-xylene Cumene o-xylene Propylbenzene p-ethyltoluene m-ethyltoluene o-ethyltoluene p-cymene m-cymene o-cymene p-n-propyltoluene m-n-propyltoluene o-n-propyltoluene Higher aromatics
8.7 0.2 5.9 3.5 1.9 1.5 0.4 0.5 0.6 23.4 47.3 5.5 . . . 0.6
. . .
. . .
. . .
0.9
Selectivity (vol.- %) cymenes ,~ n-propyltoluenes ~' propyltoluenes
76.2 . 76.2
89.1
iso-/n- ratio
~
~c
30.7 62.1 7.2
51.8 35.0 13.2
.
.
.
Selectivity (%) p-cymene m-cymene o-cymene
28.7 65.5 5.8
33.4 59.0 7.6
p-n-propyltoluene m-n-propyltoluene o-n-propyltoluene "Percent isopropanol transformed into C3-C 4 or aromatics.
(with simultaneous decrease of NPTs)
is o b s e r v e d , as f o r H - Z S M - 5 A
a n d B. T h e p a r a -
s e l e c t i v i t y is m o r e p r o n o u n c e d w i t h t h e b u l k i e r I P T s c o m p a r e d to N P T s . E v e n t h o u g h t h e " q u a s i s t e a d y - s t a t e " c o n v e r s i o n o f t o l u e n e r e a c h e s n e a r l y the s a m e v a l u e f o r H - Z S M - 5 A and B and H-(Fe)ZSM-5,
t h e i r s e l e c t i v i t y to p - I P T d i f f e r c o n s i d e r a b l y . T h e h i g h e s t a n d
Z ~ejka et al./Applied Catalysis A: General 108 (1994) 187-204
199
comparable para-selectivity, was found with H-ZSM-5B and H-(Fe)ZSM-5 (the latter exhibits a lower number and acid strength of OH groups).
3.2. H- Y zeolite The H-Y zeolite exhibits a higher total conversion of toluene compared to H-ZSM-5A (Tables 1 and 8), which results from the higher number of acid sites and which can also reflect the more open structure, although of a considerably lower acid strength. Over the whole temperature range (470-620 K), the toluene conversion, selectivity to higher aromatics, C 6 - C 9 and C~ + and expectably, the zeolite deactivation are higher compared to H-ZSM-5 (Tables 1, 2 and 8). A slight decrease in toluene conversion was found with the reaction TOS (Table 8). With increasing temperature the PTs selectivity decreases and the concentration o f C 6 - C 9 aromatics and propanol conversion to C 3 - C 4 alkenes increases. This is caused by a substantial increase in the rate of the disproportionation, dealkylation and transalkylation reactions, which can easily occur in an open Y zeolite structure. As expected, no para-isomer enrichment in IPTs was observed. The Y zeolite yields exclusively IPTs at 520 K, NPTs being absent in the products as has already been reported by Fraenkel and Levy [ 16]. However, a substantial concentration of NPTs (iso-/n- ratio about 0.8) was observed in the gaseous phase at temperatures above 550 K.
3.3. H-mordenite A high initial toluene conversion on H-mordenite, reflecting the presence of a high number of strong acid sites, is followed by a dramatic decrease with TOS at 520 and 620 K, due to a well known blocking of the one-dimensional mordenite structural channels (Table 9). Therefore, the dependence of the reaction behaviour on temperature can hardly be predicted. With increasing temperature a substantially higher concentration of C6-C 9 products appears in the initial reaction period, then at higher TOS values, PT selectivity increases considerably. The higher aromatics are not found in high concentration among the reaction products as they are probably retained in the zeolite channels. The IPTs are exclusively formed and no NPTs were found among the reaction products up to 620 K.
4. Discussion 4.1. The effect of time-on-stream and contact time The H-ZSM-5A and B and H-(Fe)ZSM-5 molecular sieves have been already investigated in the alkylation of toluene and ethylbenzene with ethylene [6,8]. In none of these reactions the conversion of toluene or ethylbenzene increased with TOS as is observed for toluene alkylation with isopropanol. However, the increase in toluene conversion as well as in IPT selectivity with TOS for H-(Fe)ZSM-5 is not so dramatic as for H-ZSM-5B (cf. Tables 2 and 7). A possible role of water molecules, evolved from isopropanol during the reaction, should not be overestimated because in the toluene alkylation with methanol and ethanol a high amount of water is usually formed and no increase in toluene conversion
200
J. (~ejka et al./Applied Catalysis A." General 108 (1994) 187-204
with TOS has been reported [ 3,21 ]. Such a high increase in toluene conversion with TOS for the propylation reaction was not observed with the more open H-Y and H-mordenite structures and, on the contrary, an expected decrease in toluene conversion, owing to zeolite deactivation, was found. These results clearly evidence that the overall alkylation reaction is controlled by the rate of transport/desorption of relatively bulky IPT molecules from the zeolite channel system, especially with molecular sieves having a MFI structure. This is likely also the reason why the toluene conversion increases with TOS over these molecular sieves even though some deactivation of the zeolite can be expected and was indicated by a decrease in concentration of high molecular weight C~j + and products of side reactions leading to C 6 - C 9 aromatics (Tables 1-5). This seems to be mainly connected with the low transport rate of the more bulky IPTs which remain for a longer time in the zeolite channel system (an increase of IPTs concentration on the account of NPT in the gas phase with TOS), while smaller linear NPTs diffuse easily [22]. This conclusion on the rate controlling step is also supported by the toluene conversion at different contact times. It should be mentioned here that while with molecular sieves having a MFI structure a higher concentration of the para-isomer is achieved for PTs (higher for IPT as compared to NPT) with H-Y and H-mordenite only an equilibrium composition of both PTs is found.
4.2. The effect of acidity; number and strength of acid sites Surprisingly, while the value of toluene conversion in toluene disproportionation and its alkylation with ethylene was found to be proportional to the overall molecular sieve acidity [ 18], in toluene alkylation with isopropanol the toluene conversion has no relation to the number and strength of acid sites. A different number of acid sites (cf.H-ZSM-5A and B) does not cause a difference in toluene conversion, moreover, with H-(Fe)ZSM-5, possessing both a lower number and a lower acid strength of bridging OH groups, the toluene conversion was higher. This again supports the conclusion that the transport/desorption of the PTs is the reaction rate controlling step. However, the lower acid strength and number of acid sites (cf. H-ZSM-5 and H- (Fe) ZSM-5) evidently suppress competitive reactions and, therefore, PTs are predominantly formed with a high selectivity. Similarly, the iso-/ n-alkyl ratio in PTs is higher with a molecular sieve of lower acidity (cf. Tables 1, 2 and 7). Within the MFI structural type of molecular sieves, an increased acidity (by number and/or strength of acid sites) or temperature, i.e. reactivity of active sites, yields a higher concentration of n-propyltoluenes (Table 6). However, it appears that H-mordenite, exhibiting the highest acidity (reflected also in toluene conversion) does not produce any NPT. This clearly indicates that for the formation of NPTs the molecular sieve structure plays a decisive role while the site activity is an additional, second order, factor affecting selectivity to NPTs.
4.3. Propyltoluene positional isomer composition; para-/meta-/ortho-selectivity It is generally accepted that during the alkylation reactions positional isomerization within dialkylbenzenes is taking place via both unimolecular and bimolecular mechanisms, depending on the reaction space given by the structural type of molecular sieves and temperature
J. (~ejka et al. / Applied Catalysis A: General 108 (1994) 187-204
201
[23-25 ]. The resulting dialkylbenzene p-, m-, o- isomer composition in molecular sieves with MFI structure is affected by the selectivity of the initial alkylation step, by the ratio of the rates of alkylation and isomerization reactions in comparison with the ratio of diffusion rates of individual isomers passing out of the inner molecular sieve volume. Eventually, the rate of the alkylation and isomerization reactions occurring on the surface non-hindered acid sites, where no steric constraints exist, can affect the resulting composition. While the contribution of diffusion processes of the individual dialkylbenzene isomers to the molecular sieve para-selectivity is without any doubt [13-15], the contribution of the individual mentioned reactions to the para-selectivity [26], taking place in the channel systems as well as on the outer crystal surface, is still controversial and widely discussed. Moreover, the effect of the acidity of the isomorphously substituted molecular sieves having a MFI structure (strength and number of sites) on the para-selectivity is not clear. Recently Raj et al. [27] reported a higherpara-selectivity for xylenes in toluene disproportionation and alkylation with methanol over ferrisilicates compared to aluminosilicates with MFI structure. On the other hand, we have found a lowerpara-selectivity for H-(Fe)ZSM-5 than H(AI) ZSM-5 [ 18 ] (of a comparable crystal size) in toluene alkylation with ethylene. In this study of toluene alkylation with isopropanol a comparable para-selectivity is obtained with molecular sieves of different number and acid strength (H-(Fe)ZSM-5 and H-ZSM-5B). However, it should be pointed out that extra-framework iron species, which are present in the ferrisilicates, can be assumed to contribute to a lower para-selectivity especially when located on the crystal surface [ 12,18]. Therefore, definite conclusions about the effect of molecular sieve acidity on para-selectivity can not be drawn.
4.4. Formation of n-propyltoluenes; iso-/n-propyl selectivity Both IPTs and NPTs are formed with molecular sieves having a MFI structure in the alkylation of toluene with isopropanol. Fraenkel and Levy ] 16] and Parikh et al. [ 17] supposed NPTs to be secondary reaction products formed by a direct unimolecular isomerization ofp-IPT to p-NPT. However, we have found dramatic differences in iso-/n- ratio among MFI, Y and mordenite structures. While with molecular sieves having a MFI structure (both aluminium and iron analogues and at various temperatures), the iso-/n- is ranging from a value of 0.6 to 13 (for toluene-to-isopropanol molar ratio 9.6), mordenite yields exclusively IPTs, over the whole range of reaction temperatures and the selectivity of H-Y to NPTs is strongly influenced by the temperature; up to 520 K no NPTs are observed and above 570 K a comparable concentration of NPTs and IPTs is formed. It is evident that large differences in iso-/n-selectivity among MFI, Y and mordenite should be related to the geometry of the reaction space of individual molecular sieves. We believe that the reaction is taking place according to a reaction mechanism that is completely different from that of unimolecular skeletal isomerization of IPTs to NPTs. The primary adsorbed species from isopropanol should be an isopropoxy cation, which is moreover thermodynamically much more favourable than its linear isomer [28] and the formation of IPT molecules as primary products can be expected. Our detailed study [29] on the interaction of p-IPT with benzene on H-ZSM-5 (initiated by a first observation of IPT transformation to NPT in the presence of toluene by Beyer and Borb61y [ 30] ), revealed that a transfer of the isopropyl group from p-IPT to the benzene molecule is taking place
202
J. ~ejka et al. / Applied Catalysis A: General 108 (1994) 187-204
leading to the formation of n-propylbenzene. During the preparation of the revised version of this paper, other evidence on the bimolecular mechanism of NPT formation in H-ZSM11 based on a ~3C nuclear magnetic resonance (NMR) study of cumene isomerization has been given by Ivanova et al. [ 31 ]. Such an isopropyl group transfer from IPT to benzene, including isomerization to a npropyl group, was not observed with H-Y zeolite up to temperatures of 520 K. However, above 550 K the concentration of n-propylbenzene over H-Y increased substantially. Thus, the reaction ofp-IPT with benzene revealed the same features in PTs isomer composition on H-ZSM-5 and H-Y as those for the alkylation of toluene with isopropanol. Therefore, NPTs should be formed as secondary products during toluene alkylation with isopropanol via a bimolecular reaction of IPT with toluene present in excess in the reaction volume, i.e. the molecular sieve channels. This conclusion is in line with the observation that the lower toluene-to-isopropanol molar ratio in the feed (i.e. the lower relative toluene concentration), yields higher selectivity to IPTs (cf. Tables 2 and 7). The bimolecular reaction between IPT and toluene in molecular sieves with MFI structure is clearly enhanced by the molecular sieve activity, i.e. by its acidity and increased temperature. Comparing the data for H-ZSM-5A and B and H-(Fe)ZSM-5 (Tables 1, 2 and 7) it is seen that the degree of NPT formation from IPT follows the number and acid strength of bridging OH groups; therefore, most likely the same sites as for the alkylation reaction (OH groups) take part in the bimolecular skeletal isomerization of IPTs to NPTs. However, for the procedure of this bimolecular reaction a specific reaction volume seems to be needed. It has been reported that the alkylation of toluene with isopropanol over Friedel-Crafts catalyst occurring in non-hindered space yields exclusively IPTs [32]. Similarly, with a more open structure of Y zeolite NPT is formed only at higher temperatures, while the unidimensional structure of mordenites does not allow the bimolecular reaction, leading to NPTs, to take place. Therefore, it is supposed that the special geometric arrangement of the channels of molecular sieves with MFI structure forces the toluene molecule to approach the adsorbed IPT molecule in such a way that the following bimolecular transition complex is formed, which can be further transformed into NPT.
It is supposed, similar to the bimolecular mechanism of xylene isomerization, according to Corma and Sastre [25], that the bimolecular transition complex cannot involve two Wheland complexes within close vicinity. More probably one Wheland complex (IPT) could interact with the other molecule (toluene), retained owing to a strong electric field present in the inner zeolite volume. It can be summarized that the molecular sieve structure, controlling the reaction space, enables via "structure-directed transition-state selectivity"
J. ~ejka et al. / Applied Catalysis A: General 108 (1994) 187-204
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the bimolecular reaction between IPT and toluene, leading to skeletal isomerization of isoto n-propyltoluene.
5. Conclusion It is concluded that the alkylation of toluene with isopropanol is controlled by the desorption/transport of the bulky reaction products, propyltoluenes, out of the inner void volume of the molecular sieves especially those with MFI structure. The isopropyltoluenes as the primary products can be further transformed into n-propyltoluenes preferably in the three-dimensional channel structure of MFI and Y zeolites, n-Propyltoluene formation is taking place via a bimolecular reaction between IPT and toluene, present in an excess in the reaction mixture. This interaction is enabled to a large extent in the three-dimensional channel (cavities) arrangement of molecular sieves and it is hindered in uni-dimensional mordenites. The intersections of the MFI structure are the most convenient geometry among the investigated structural types, where toluene and IPT can approach each other in a convenient way. With the more open structure of H-Y, higher temperatures are needed to enhance the isomerization rate. This molecular sieve structural effect is explained in terms of the "structure-directed transition-state selectivity" and the specific approach of interacting molecules is likely a necessary condition for the bimolecular reaction to take place. However, the rate of this reaction is also affected by the reactivity of the molecular sieve, i.e. its acidity given by the number and strength of the bridging OH groups and reaction temperature. From a practical point of view, ifp-isopropyltoluene (p-cymene) is the desired product, molecular sieves with MFI structure and a lower acidity (Fe analogues), a temperature about 520 K and short contact times can be suggested for p-cymene synthesis. Moreover, the selectivity of such molecular sieve catalysts towards propyltoluenes is high and no side reactions, as transalkylation, methyltransfer and disproportionation, take place as with more open Y and mordenite zeolites. However, with molecular sieves having a MFI structure, undesired n-propyltoluenes are always formed. Their formation can be suppressed by using a low reaction temperature, short contact times and a low toluene-to-isopropanol ratio, which, in addition, affects the overall toluene conversion positively; however, attention should be paid to the easier deactivation of the molecular sieve with higher concentration of isopropanol in the feed.
Acknowledgement The work of J.Q. and B.W. was supported by a grant from the Academy of Sciences of the Czech Republic (No. 44003). The authors are indebted to Drs. G. Vorbeck and R. Fricke (Berlin) for providing a ferrisilicate sample.
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