Studies in Surface Science and Catalysis, volume 158 J. t~ejka,N. Zilkov~iand P. Nachtigall (Editors) 9 2005 Elsevier B.V. All rights reserved.
1645
Dependence between the activity and selectivity of NaLaY and N aCeY catalysts in the catalytic disproportionation of toluene D. Nibou a and S. Amokrane-Niboub
a'Laboratoire de Science et de Gfnie des Matfriaux, Dfpartement des Sciences des Matfriaux, GMGP,Universit6 des Sciences et de la Technologie Houari Boumediene. BP.32, E1 Alia, Bab-Ezzouar, Algfrie. Email:
[email protected] bLaboratoire des phfnomfnes de transfert, Dfpartement de Gfnie des Procfdfs, FGMGP, Universit6 des Sciences et de la Technologie Houari Boumediene. BP. 32, El Alia, BabEzzouar, Algfrie. Email:
[email protected]
Y-faujasite previously synthesised by hydrothermal crystallisation and used in exchange ions by several elements as cerium, lanthanum, strontium and cobalt showed interesting catalytic activities in toluene disproportionation. Appreciable selectivities with respect to main reactions were also observed. The relationship between the catalytic activity and the selectivity of NaCeY and NaLaY catalysts is investigated. It was found that cerium cations improve a specific selectivity strongly depending on the catalyst activity.
1. INTRODUCTION An interesting low cost alternative in revalorizing loaded zeolites ion exchangers consists in their further use as catalysts. The good affinity of the aluminosilicates to heavy elements fixation could be of great interest for catalysis. Many chemical reactions involving organic compounds or hydrocarbons can be performed over exchanged zeolites [1-4]. Toluene disproportionation to benzene and xylenes over catalysts containing lanthanum and cerium cations is a good example to illustrate this. In such reactions, the zeolites with high silica-alumina ratios and improved shape selectivity present interesting catalytic performances, especially in p-alkylbenzenes formation. [5-9]. Nevertheless, large pore zeolites like faujasites [10-12] give rise to the most flexible catalyst which can be used in all hydrocarbons conversions. The main features of faujasitic aluminosilicates consist in their ability to fix high radius cations which enable them to perform specific selectivities and significant activities. Their selectivities can be oriented according to the nature of the introduced cation and the working conditions. In this context,
1646 the present work was carried out in order to define the relationship existing between the activity in the toluene disproportionation process and the selectivity of some catalysts containing lanthanum and cerium cations. The performances of such catalysts were explained by comparison with the activities and selectivities of zeolites containing cobalt and strontium cations.
2. EXPERIMENTAL
2.1. Preparation of catalysts Y-faujasite was prepared by hydrothermal method. The hydrothermal synthesis was carried out by heating a gel of molar composition 5 Na20 : 3 A1203 : 15 SiO 2 : 200 H20 under autogenously pressure at 80~ for 48 hours in Teflon-lined stainless steel autoclave. The obtained crystalline zeolite was separated from the mother solution by filtration, washed several times with distillate water until 7 pH value and then dried at 70~ The sources of silicon, sodium and aluminium were respectively Aeorosil 200 (99.8 wt.%), sodium hydroxide (99 % Prolabo) and metallic aluminium (99.9 % pur.). The obtained crystalline powder was extruded in order to get pellet form catalysts. These pellets present an average particle size of 1-2 x 1 mm. Many samples of such zeolite were contacted with aqueous solutions (1 M) containing nitrate salts of lanthanum, cerium, cobalt and strontium. Different sodium exchange rates were obtained by varying the contact time of the zeolite in the impregnation lanthanum and cerium solutions. 2.2. Characterization The zeolite was characterization by powder X-ray diffraction (Philips PW 1710 diffractometer, Cu K ~ radiation) and the exchanged samples were characterized by inductively coupled plasma (Hilger Analytical ICP 2500). The acidity was determined by desorption of butylamine. The samples (0.1 g) were first calcined in a glass reactor by heating under helium to 550~ by 50~ steps for 6 hours, cooled to room temperature and contacted with butyl amine (from Fluka, 99% pur.) prepared in benzene (0.05 N). The samples were then stirred for 15 hours to reach equilibrium adsorption of the base. The adsorbed samples were separated from the base liquor by filtration. This last was titrated using a perchloric acid solution (0.05). 2.3. Catalytic reaction The toluene disproportionation process was carried out in a tubular pyrex reactor containing a fixed bed of 5 g of pelleted catalyst. All the experiments were run in the same operating conditions i.e. weight hourly space velocity (WHSV) = 0.7 to 1 h -1 for reactor feed (with Merck toluene dried over metallic sodium), reaction pressure = 1 atm, fixed time on stream - 90 min, temperature range - 350 to 500~ Before each experiment, the catalysts
1647
were calcined at 500~ under nitrogen flow for 3 h. After each experiment, the catalyst was washed with acetone in order to remove the heavy hydrocarbons. The reaction products were analysed by gas chromatography using a Perkin Elmer instrument, a (Bentone 34 + dinonylphthalate) / Chromosorb WAWS column (length = 5 m and internal diameter = 2 mm) and a FID detector.
3. RESULTS AND DISCUSSION The X-ray diffraction spectrum of the synthesized Y-faujasite sample was reported in Fig. 1. It is in good agreement with the reported diffraction data [ 13]. It seems that the obtained solid appear to be pure Y-faujasite with respect FAU type-structure.
~ 2'750"
tt~ tg) uO
t'-:
~2250 1750
125o
~
o
0
250 o'
g
'
'
2's
'
3's
l:'2e]
Fig. 1. X-ray diffraction spectrum of the synthesized Y-faujasite The anhydrous chemical composition of Y-faujasite found by inductively coupled plasma is Na0.195 (Si0.575A10.230) O2 with Si/A1 molar ratio of 2.5. Generally, Y-faujasite shows a good behaviour with respect to cation exchange by heavy multivalent elements and does not present significant modifications neither of framework structure nor of Si/A1 ratio value [4-12]. Effectively, after ion exchange, the crystallinity of samples remains constant and is not altered when the Y-faujasite impregnation is not excessively repeated. A feature of such catalysts consists in their surface acidity. Sodium substitution by multivalent cations often improves this acidity. For all catalysts, the surface acidity increases as sodium exchange percent increases as shown in table 1. The rare earths cations like lanthanum and cerium improve essentially Br6nsted acidic sites. These sites promote the formation of protonic sites and favour consequently carbocationic mechanisms. In fact, it is well known that toluene disproportionation to benzene and xylenes (2 Toluene (T) --) Benzene (B) + Xylenes (X)) requires Br6nsted acid sites. Sidechain reactions such as isomerization (ortho-X --) meta-X para-X --) meta-Xylene) and
1648 xylenes disproportionation (2 X --) T + Trimethylbenzenes (TMB)) occur on similar active sites but with weaker acid strength. The selectivity to the main process is dependent only on the distribution of the acidity strength on the catalyst surface. This distribution in turn is strongly depending on the catalyst activity. Table 1 Acidity and sodium exchange percent of samples Samples Y-faujasite NaLaY(1) NaLaY(2) NaLaY(3) NaLaY(4) NaCeY(1) NaCeY(2) NaCeY(3) NaCeY(4) NaCoY NaSrY
Sodium exchange percent (Na + %) 0 54 71 73 74 56 70 73 75 73 73
Acidity (mmole buNH2/g) 0.18 1.56 1.94 2.11 2.52 1.59 1.87 1.99 2.61 1.30 1.41
The selectivity with respect to the main process is defined by the mole rations B/X or B/(X+2TMB), whereas the activity is represented by the consumption of a certain mole of toluene. One must take into account the amount of toluene yielded by other reactions unless these reactions present low contributions to the global equilibrium. The better the selectivity (to methyl transposition processes), the closer to unity will be these ratios. This limit corresponds to the stoechiometric reactions of toluene disproportionation (with or without xylene isomerization which produces as much benzene as xylenes) or to the extended process to xylenes disproportionation. 3.0
9 99 9
2.5
(2) (1) (3) I (4)
O
~2.0 Q
A
1.5 A
1.0
o.1
--
o'.2
vA ,
0.3
WA, 'I"
0.4
oi~
o'.8
Conversion (mole/mole)
Fig. 2. Dependence between toluene conversion and B/X mole ratio for NaLaY (with various sodium exchange percent (Na+ %)) catalysts. (1): 74 %; (2)" 73 %; (3): 71% and (4): 54 %. Operating conditions: WHSV = 1 h-~, time-on-stream: 90 min, temperature: 450~ T/Helium = .005 mole/mole.
1649
The dependence between toluene conversion and B/X mole ratio for NaLaY and NaCeY catalysts (with various sodium exchange percent (Na + %)) is shown in Fig. 2 and Fig. 3. B/X mole ratio increased as toluene conversion increased. For conversion smaller than 0.4 mole/mole, this ratio is close to unity and its dependence on toluene conversion can be neglected. Because of the inverse pseudo proportionality existing between the activity and the selectivity of a catalyst, the concept of selectivity does not present an interest for low conversions. 9 (1) (2) 9 (3) 9
2.5-
9 (4)
2.0 E ,_..,
9
~ 1.5 m
9 9 m
1.0
o.o
0.2
9
9
o'.4
o'.6
Conversion (mole/mole)
Fig. 3. Dependence between toluene conversion and B/X mole ratio for NaCeY (with various sodium exchange percent (Na + %)) catalysts. (1): 75 %; (2): 73 %; (3): 70 % and (4)" 56 %. Operating conditions: WHSV = 1 h -~, time-on-stream: 90 min, temperature: 450~ T/Helium = .005 mole/mole.
1.8
1.4 O ,_.., O
1.6
La ~o
1.3
Sr
9
1.2
Co
O
1.4~o
La
O
E c',l
Ce
+ 1.1
9
Na 1.0 l
o
"ql--"O Sr
~e 1.0
~
~
~
% wt, coke Fig. 4. Effect of coke yielding on mole ratios B/X and B/(X + 2 TMB). Operating cond.: see Fig. 2. Sodium exchange percent (Na + %) for all catalysts is equal to 73 % except that of NaCeY (70 %). The mole ratios B/X and B/(X + 2 TMB) depend primary on toluene conversion, but the amounts of coke depend on toluene conversions as well as selectivity. The obturing effect of coke deposits on the pore sizes may play a role in the selectivity and consequently in the values of these ratios.
1650 On the other hand, it is difficult to deal with selectivity for high conversions because of the preponderance of side reactions. For example, around the 0.6 mole/mole value a small change in toluene conversion results in an important increase of B/X value as shown in Fig. 2 and Fig. 3. Thus, the resulting compromise limits the activity-selectivity correlation to a field of moderate working conditions which permit to obtain conversions in the range from 0.40 to 0.55 mole/mole at mole ratio B/X not exceeding a value of 1.5. Conversions higher than 0.45-0.50 (obtained for temperatures exceeding 440~ involve dehydroxylation phenomena and Br/Snsted sites will considerably decrease, yielding new aprotonic centres. Consequently, increasing activation of the catalysts improves non-desired processes such as coke formation and affects the selectivity [14-16]. Thus, the process selectivity decreases as the catalytic activity increases. Nevertheless, excessive activity generally results in a significant coke deposit. The subsequent decrease of the pore size could play a non-negligible role in improving the shape selectivity. This observation is valid for all catalysts dealt with in the present work.
1.6
Co
[]
Sr 1.4.
1.2-
[]
[]
Ce
Na
1.0 1.0
111
112
113
B/(X + 2 TMB)
Fig. 5. Dependence between molar ratios B/X and B/(X + 2 TMB) for different catalysts. Operating conditions: see Fig. 2. Sodium exchange percent (Na+ %) for all catalysts is equal to 73 % except that of NaCeY (70 %). The dependence between B/X and B/(X + 2 TMB) indicates that TMB's result essentially from xylenes disproportionation and that benzene and xylenes are primary products of toluene disproportionation. This relation shows clearly the field corresponding to the maximal selectivity with respect to the main reaction. Another similar feature of such catalysts is that coke formation occurs even at B/X or B/(X + 2 TMB) ratios equal to unity (Fig. 4), i.e. before trimethylbenzenes (TMB) are yielded. This is consistent with the fact that only toluene and xylenes (benzene being relatively more stable) contribute effectively to coke deposit. TMB's are involved in pyrolysis process only for B/X mole ratio exceeding a value of 1.1 and for coke amount exceeding approximately 1 %. The B/(X + 2 TMB) ratio dramatically increases from 0 to 1.26 as the coke amount increases from 0.8-1.0 to 2.0-2.2 %. Simultaneously, the B/X ratio increased from 1.1 to 2.02.2, indicating that the contribution of xylenes and TMB's to secondary processes becomes appreciable. One can also conclude that for NaLaY and NaCeY catalysts, the selectivity is
1651 inversely proportional to coke deposit and the pyrolysis essentially implies toluene, xylenes and to a lesser extent TMB's consumption. On the other hand, for NaCoY and NaSrY catalysts, this trend seems to be reserved, i.e. increase of coke deposit results in an increase in the selectivity. Indeed, a decrease of B/X and B/(X + 2 TMB) ratios is observed. Pyrolysis is more favoured by these catalysts. For instance, for similar values of B/X and B/(X + 2 TMB) ratios coke deposit is more important than on NaLaY and NaCeY. This means that on NaCoY and NaSrY catalysts, coke deposit results essentially from pyrolysis of heavier or more alkylated aromatic hydrocarbons. It is well known that the amount of coke might be proportional to the molecular weight of the cracked compounds. The heavy products are more retained in catalyst pores than ones and their further cracking is more probable. Another aspect of the present investigations concerns the dependence between B/X and B/(X + 2 TMB) ratios which may give an idea about TMB's appearance as shown in Fig. 5. Certain proportionality between both ratios is observed indicating that xylenes are also involved in TMB's formation, likely by disproportionation. The change of the slope of curve shows that this process becomes less preponderant for NaSrY, NaCoY and NaLaY catalysts and that the amount of TMB's is smaller than that of converted xylenes. Cracking and hydrodemethylation of xylenes and TMB's predominantly occur whereas NaCeY catalyst shows better selectivity to methyl rearrangement process. If one takes into account that hydrodealkylation easiness is somewhat proportional to the number of alkyl groups of the aromatic ring, it clearly appears that the reactions involving one methyl cleavage are the most probable [ 14]. For alkylbenzenes, the probability to lose one methyl group varies as follows: TMB > xylenes > toluene. Anyway, hydrodemethylation occurs only in the presence of large amounts of hydrogen yielded by pyrolysis and then both processes are depending one upon the other. 4. CONCLUSION On the basis of basis of these arguments, one may conclude that cracking of toluene, xylenes (more particularly ortho and meta isomers because of their steric effect) and TMB's cannot be avoided but only minimized by a suitable choice of compensating cations and of operating conditions. For instance, NaCeY catalyst may generate better selectivity and reduced coke deposit when used at moderate operating conditions, i.e. with moderate catalytic activity. Non desired reactions are involved only after reaching a certain value of toluene conversion over catalyst. Thus, cerium cations improve a specific selectivity strongly depending on the catalyst activity.
REFERENCES
[1] [2]
D. Nibou thesis, HCR, Algiers, 1990. E.G.Derouane in Zeolite science and technology, NATO ASI Series E, Appl. Catal. 80, R. Ribeiro et al. (Eds.), Martinus NijhoffPublishers, Hague, 1984. pp. 347.
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[3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [ 13] [ 14] [15] [16]
C.T. Chu and C.D. Chang, J. Phys. Chem. 89 (1985) 1569. D. Nibou, A. Azzouz, E. Dumitriu and V. Bilba, Rev. Roum. Chim., 39 (1994) 1099. R.N. Mesrham, J. Chem. Technol. Biotechnol. 37 (1987) 111. V.S. Nayak, L. Riekert,; Appl. Catal. 23 (1986) 403. C. Benzouharova, C. Dimitrov, V. Nenova, B. Spassov and H. Lechert, Appl. Catal. 21 (1986) 149. J. (~ejka, N. 7,ilkovfi, Z. Tvarfi2kovfi and B. Wichterlov~i in Stud. Surf. Sci. Catal. V. 97, L. Bonneviot and S. Kaliaguine (Eds.), Amsterdam; Oxford; Elsevier; 1995 M. Bolognini, F. Cavani, D. Scagliarini, C. Flego, C. Perego, M. Saba, Catal.Today, 75, (2002) 103. A. Azzouz, D. Nibou and B. Abbad, Appl. fatal. A, 79 (1991) 19. H.G. Karge, K. Hatada, Y. Zhang and R. Fiedorous, Zeolites, 3 (1983) 13. D. Nibou and S. Lebaili, Quim. Anal. 16 Sup.I (1997) 147. M.M.J. Treacy and J.B. Higgins in 'Collection of Simulated XRD Powder Patterns for Zeolites', Fourth Revised Edition, Elsevier, Amsterdam, London, New York, Paris (2001). S. Mikhail, S.M. Ayoub and Y. Barakat., Zeolites 7 (1987) 231. V. Hulea, N. Bilba, M. Lupascu, E. Dumitriu, D. Nibou, S. Lebaili and H. Kessler, Microporous Mater., 8 (1997) 201 S. Oprea, A. Azzouz, E. Dumitriu and M. Constantinescu, Bull. Soc. Chim. Belg. 92 (1983) 289
