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ZSM-5
123
Applied Catalysis A: General, 103 (1993) 123-134 Elsevier Science Publishers B.V., Amsterdam APCAT
A2573
Propane aromatization over Pt-Tl/ZSM-5 S. Scire and R. Maggiore Dipartimento di Scienze Chimiche, Universitk di Catania, Viale A. Doria 6,95127 Catania (Ztaly)
S. Galvagno Dipartimento di Chimica Zndustriale, Universitti di Messina, Gas. Post. 29,98166 Sant’Agata di Messina (Italy)
and C. Crisafulli and G. Toscano Dipartimento di Scienze Chimiche, Universitb di Catania, Viale A. Doria 6,95127 Catania (Italy) (Received 18 February 1993, revised manuscript received 24 May 1993)
Abstract Aromatization of propane has been studied on Pt-TI/ZSM-5 catalysts in the temperature range 350550°C. The effect of thallium precursor and Tl/Pt ratio has been also investigated. The best results have been obtained on the bimetallic samples prepared from TlsSO, and HxPtC!la. On these samples addition of thallium to Pt/ZSM-5 increases the selectivity to aromatics without significantly affecting the catalytic activity. A maximum aromatic yield of about 70% has been obtained on the sample with a Tl/Pt ratio of 1. It is suggested that thallium dilutes the platinum active sites thus reducing their crack-
ing activity. Key words: aromatization; bimetallic catalysts; platinum-thallium;
zeolites; ZSM-5
INTRODUCTION
The transformation of light alkanes, such as ethane and propane, into aromatics is a process of great industrial interest. Acid H-ZSM-5 zeolites combined with transition metals have shown good aromatic yields; Pt, Ga and Znbased catalysts have been reported to be the most active systems [l-6]. On H-ZSM-5 zeolite the activation of propane occurs only at temperatures higher than 5OO”C,while the aromatization of intermediate oligomers occurs between 200 and 250°C [ 1,7]. Addition of platinum to the zeolite greatly imCorrespondence to: Dr. R. Maggiore, Dipartimento di Scienxe Chimiche, Universitil di Catania, Viale A. Doria 6,95127 Catania, Italy. Tel. ( +39-95)33935, fax. ( + 39-95)580138.
0926-860X/93/$06.00
0 1993 Elsevier Science Publishers B.V.
All rights reserved.
124
5’. Scir6 et al./Appl. Catal. A 103 (1993) 123-134
proves the dehydrogenation of alkanes leading to higher aromatic yields [ 5,7,8]. The positive dehydrogenating action of platinum is, however, accompanied by cracking and hydrogenolysis reactions limiting the selectivity towards aromatics. The presence of a second metal can sometimes decrease cracking activity of platinum increasing aromatic yields [ 9,101. We have recently obtained on Pt-Ir/ZSM-5 an aromatic yield of 61.2% [ 111, comparable with that observed on the most efficient catalysts for propane aromatization. On the basis of an extensive study carried out on different bimetallic platinum-based catalysts supported on H-ZSM-5, in this paper we report and discuss the results obtained on Pt-Tl/ZSM-5, with particular attention to the influence of thallium precursor and experimental conditions in order to maximize aromatic yields. EXPERIMENTAL
The support used was a commercial H-ZSM-5 zeolite (Conteka) with a SiO,/ A&O3 ratio of 30 and a surface area of 400 m2/g. Pt/ZSM-5 and Tl/ZSM-5 (1 wt.-%) were prepared by incipient wetness impregnation of the zeolite with H2PtCls and Tl,SO, or T1N03, respectively. Bimetallic Pt-Tl catalysts were prepared: (a) by impregnation with aqueous solution of H2PtCls and subsequently with Tl,SO,; (b) by coimpregnation with H,PtCI, and TlNO,. The platinum content was constant in all catalysts (1 wt.-% ), whereas thallium content was changed to obtain the desired Tl/Pt atomic ratio. The catalyst, dried at 12O”C, was pressed into pellets, without any binder, crushed and the fraction 80-140 mesh was always used. Catalysts pretreatment included in situ heating in flowing air for 2 h at 450” C and, subsequently, in a stream of hydrogen for 3 h at 450’ C. Aromatization of propane was carried out in a flow system employing a tubular reactor at atmospheric pressure containing about 0.25 g of catalyst. The reactant feed was a purified mixture of propane/helium (50 vol.-% ) . Propane flow-rates were in the range 0.3-3.0 l/h. Reaction products were analyzed by a gas chromatograph (Carlo Erba model 4200 with flame ionization detector) connected directly to the flow system. The procedure used was as follows: the reactant mixture was passed over the catalyst for 15 min before sampling the products for analysis. The feed was then cut out and the catalyst was treated at 450” C in flowing hydrogen for 1 h before taking another experimental point. By using this procedure, conversions and selectivities were reproducible within 2-3%. Conversion, yields and selectivities were calculated on carbon number basis. The absence of diffusional limitations was verified by preliminary runs carried out at different catalyst loadings and different grain sizes. Hydrogen chemisorption was measured in a conventional pulse system op-
S. Scirk et al./Appl. Catal. A 103 (1993) 123-134
125
erating at room temperature. Negligible amounts of hydrogen were found to chemisorb on the support and on thallium. Temperature-programmed reduction (TPR) experiments were carried out in a typical gas chromatographic apparatus at a heating rate of 5’ C/min using 5 vol.-% Hz in Ar. RESULTS
Fig. 1 shows the conversion of propane as a function of reaction temperature (350-550°C) over samples prepared by subsequent impregnation of chloroplatinic acid and thallous sulphate at a contact time, t, of nearly 5 s. One can observe that addition of different amounts of thallium to Pt/ZSM-5 leaves the conversions obtained on the monometallic platinum sample virtually unchanged. This behaviour is also observed at contact times other than 5 s. It is noteworthy that Tl/ZSM-5 showed a moderate decrease in conversion and the same distribution of reaction products as the unloaded support. The influence of addition of thallium on Pt/ZSM-5 becomes evident when we consider the distribution of reaction products. Yields as a function of conversions at T = 500 ’ C are reported in Figs. 2-4; the amount of ethene is always negligible. It is possible to note that, on all samples, aromatic yields rise on increasing conversion levels reaching a maximum at a conversion of nearly 80%; then a sharp decrease can be observed (Fig. 2). The addition of thallium increases significantly the aromatization activity of Pt/ZSM-5. At this temperature an aromatic yield of 65% at a Tl/Pt ratio of 1 has been obtained. This yield is much higher than that observed on the monometallic Pt/ZSM-5 sample (44% ). Lower thallium contents give intermediate results. Data of products distribution (Figs. 2-4) indicate that higher aromatic yield
350
450
T (“C) Fig. 1. Propane conversion on Pt-Tl/ZSM-5 temperature. Contact time, r= 5 5.
samples prepared from TI,SOI. Effect of reaction
126
S. S&t? et al./Appl. Catal. A 103 (1993) 123-134 70
-e-
Pt TI/Pt
0.1
+
TI/Pt
05
+
TI/Pt
10
TI/Pt
125
@- -a+-
=-+ 4c-
td
h
Fig. 2. Aromatic yield as a function of conversion on Pt-Tl/ZSM-5 T=Wo”C.
*
Tl/R
0.1
TI/Ft
0.5
--c
TI/Pt
10
+
TI/Pt
125
4b-+
samples prepared from TlPS04.
33
2s
10 a I 40
0, M
I con:
5-
-s-
Pt
*
TI/Pt
0.1
TI/Pt
0.5
++ +
TI/Pt
10
+
Tlpt
125
I m (Z)
3-
l-
c-, 20
I
40
con:
-b
I
I 80 (z)
Fig. 3. Ethane (a) and methane (b) yields as a function of conversion on Pt-Tl/ZSM-5 prepared from T12S04. T=500”C.
samples
127
S. Scirk et al./Appl. Catal. A 103 (1993) 123-134 4
Pt
x
let
0.1
Fig. 4. Propene (a) and C, (b) yields as a function of conversion on Pt-TI/ZSM-5 pared from Tl,SOI. T= 500°C.
samples pre-
TABLE 1 BTX distribution on Pt-Tl/ZSM-5
samples prepared from T&SO,; T=500”C
Tl/Pt ratio
1.0
0.5
0.1
0
Conv. ( % ) Arom. sel. ( % )
76.2 81.9
80.3 76.2
76.3 65.3
76.8 56.0
8.2 36.7 55.1
9.0 43.3 47.7
14.7 44.8 40.5
21.8 45.4 32.8
Arom. Distribution Benzene Toluene Xylenes
(%):
128
5’.Scirt? et al./Appl. Catal. A 103 (1993) 123-134
Fig. 5. Selectivity to aromatics as a function of conversion at different temperatures (400-600” C) on a bimetallic Pt-Tl/ZSM-5 sample (Tl/Pt = 0.1) prepared from Tl,SOI.
can be correlated to a decrease of ethane and, to a less extent, of methane, while addition of thallium to Pt/ZSM-5 does not substantially modify the yields towards intermediate products (propene and C,) . The distribution, within the aromatic fraction, of benzene, toluene and xylenes (BTX) is also modified by adding thallium to Pt/ZSM-5. Table 1 shows the BTX molar ratios at conversion levels (80%) where highest are the aromatic yields. We can observe that on increasing Tl/Pt ratio, together with an increase of aromatic selectivities, BTX distribution shifts towards xylenes, with a corresponding decrease of toluene and, above all, of benzene. This last component is always the less abundant. An increase of selectivity to xylenes had been also observed on all samples at decreasing conversion levels. Only at conversions higher than 80% selectivities move rapidly from xylenes to toluene and then to benzene. This indicates advanced cracking and demethylation conditions, as shown by the intense ethane (Fig. 3a) and methane (Fig. 3b) formation. The above reported data apply to Pt-TI samples with an atomic ratio equal or less than 1; a further addition of thallium ( TljPt higher than 1) , even though this does not modify catalytic activity (Fig. 1 ), lowers aromatic yields, compared to sample with Tl/Pt = 1 (Fig. 2 ) . Aromatic yields have been found to depend on the reaction temperature. Fig. 5 shows the aromatic selectivities obtained on the sample with a Tl/Pt ratio of 0.1 in the range of temperature 400-600°C. Up to 55O”C, the higher the temperature, the higher is the aromatic selectivity. Any further increase of temperature does not improve catalytic performance. At each temperature selectivities increase on increasing conversion levels up to a certain value and then sharply decrease, this value shifting to higher conversions on increasing reaction temperature.
129
S. Sir& et al./Appl. Catal. A 103 (1993) 123-134
T (“C> 70
+
TI
I
Fig. 6. Propane conversion (a) and aromatic selectivity (b) on Pt-Tl/ZSM-5 from TlN03. Effect of reaction temperature. Contact time, r=5 s.
samples prepared
The highest aromatic yield (70% with sel. = 83% and conv. = 85% ) was obtained at 550°C and 7= 1 s on the sample with a Tl/Pt ratio of 1. Addition of thallium does not modify the behaviour of Pt/ZSM-5 towards deactivation rate. On all samples the higher the reaction temperature is, the quicker is the decrease in total conversion with reaction time; at 400°C and 7= 2.5 s, for instance, the activity remains practically constant over about 10 h. At 550°C and 7=2.5 s, during the same 10 h, the conversion decreases from 90% to 25%; a drop in cracking and aromatic products is observed, while the yield of propene rises. When TlNO, is used as thallium precursor, catalytic behaviour (Fig. 6) is different from that reported above using Tl,SO,. In fact, the presence of thallium, at least up to a Tl/Pt ratio of 1, does not effectively modify propane conversion; aromatic selectivity undergoes a very slight increase. At higher Tl/
S. S&g et al./Appl. Catal. A 103 (1993) 123-134
130
Pt ratios (Tl/Pt = 1.25) a drastic decrease in conversion and aromatic selectivity is observed, especially at the higher temperatures. DISCUSSION
Aromatization of propane over Pt/ZSM-5 has been suggested to occur through the following reaction scheme [ 7,111:
1
2
5a a1iph.h
5b C6-C,
alicycl
__*
C,-C,
arom.
Scheme 1
Platinum catalyses the dehydrogenation of propane to propene and of CgC8 naphthenes to aromatics. This positive action is, however, accompanied by cracking and hydrogenolysis reactions at an extent that depends on the geometric and electronic state of the metal crystallites which can be modified by addition of a second metal [ 12-161. We have previously reported that when the addition of a second metal, such as tin or lead, increases the electron density of platinum, catalytic activity decreases together with cracking and hydrogenolysis products leading to higher aromatic selectivities [ 9,101; the reverse is obtained when the electron density of platinum is decreased by addition of rhenium or lanthanum [ 111. Strong dilution effects, which inhibit hydrogenolysis reactions, can lead to a different behaviour, as observed on Pt-Ir/ZSM5 which showed both higher activity and aromatic selectivities than Pt/ZSM5 [ 111. Platinum-support interactions can also modify the catalytic behaviour by an electron transfer from platinum to support [ 17-191. The acid sites of the ZSM-5 zeolite play an important role in the oligomerization of the propene intermediate (which already occurs at 200-250’ C ) and in the formation of aromatics through alicyclic compounds. These acid sites show also a cracking activity which is, however, lower than that observed on platinum. On H-ZSM-5 the activation of propane can occur only at high temperature with low yields and with a reaction pathway different from that observed on Pt/ZSM-5 [ 1,7]. It is noteworthy that Tl/ZSM-5 has an activity barely lower than the zeolitic support alone and the distribution of reaction products is practically the same. This is an evidence of the inactivity of thallium towards the reaction. The absence of dehydrogenating activity of thallium is also confirmed by some experiments performed on silica. In fact Tl/Si02, between 450 and 55O”C, is
S. Scirti et al./Appl. Catal. A 103 (1993) 123-134
131
completely inactive whereas Pt/SiOp, at the same conditions, exhibits high dehydrogenating and hydrogenolytic activity. The slight decrease in the catalytic activity observed on Tl/ZSM-5 compared to H-ZSM-5 (Fig. 1) can be attributed to a moderate reduction of the acidity of H-ZSM-5, by addition of thallium, as observed by ammonia temperature-programmed desorption (TPD ) experiments [ 201. In the absence of an active metal the acidity is, in fact, the main parameter responsible for alkane activation. No change in surface acidity by addition of thallium to Pt/ZSM-5 has been observed by TPD spectra of adsorbed ammonia [ 211. It should be noted that all platinum and Pt-Tl samples supported on the ZSM-5 zeolite used in our experiments did not practically chemisorb hydrogen at room temperature, therefore no turnover frequencies could be calculated. A low capability of Pt/ZSM-5 to chemisorb hydrogen has been previously observed by Folefoc and Dwyer [ 221 and attributed, among other factors, to the presence of electron-deficient platinum atoms. In any case, the interpretation of chemisorption data on metals supported on zeolites has not yet been fully clarified. In Fig. 7 are reported the TPR spectra of Pt-Tl/ZSM-5 samples prepared using Tl,SO, as thallium precursor. Pt-Tl bimetallic catalysts show spectra which are different from those obtained on the platinum and thallium monometallic samples. The shoulder observed at 90-llO”C, due to the platinum precursor (the maximum of the peak occurs at 150’ C), becomes more evideni on bimetallic samples on increasing the Tl/Pt ratio. Moreover the HJTl ratic (H, is the amount of hydrogen consumed during TPR) of 3.2, measured on the
I
0
100
200 T
300
400
500
800
I’CI
Fig. 7. TPR spectra of Pt-Tl/ZSM-5
samples prepared from T&30,.
132
S. ScirB et al. jApp1. Catal. A 103 (1993) 123-134
Tl/ZSM-5 sample, indicates a reduction of the sulphate to metallic thallium and T&S, in agreement with previously published results [ 231. Even though the TPR spectra of the bimetallic samples are not a simple combination of the monometallic catalysts, the presence and the nature of a Pt-Tl interaction cannot be clearly identified. Data of Fig. 1 show that addition of thallium to Pt/ZSM-5 does not effectively modify propane conversions. Also the initial rates of reaction, important sign of dehydrogenating activity of platinum, do not change by adding thallium. Moreover propene and C, hydrocarbon yields do not depend on the presence of thallium (Fig. 4), thus showing that platinum affinity towards intermediate alkenes is not altered. These results indicate that the presence of thallium does not effectively modify the electronic properties of platinum. However the considerable change in aromatic selectivities by addition of thallium, (Fig. 2) indicates the presence of another kind of interaction that improves dehydrocyclization of C&-C8aliphatic to aromatics. It is well known that thallium forms alloys with other metals, platinum included [ 241. A Pt-Tl interaction on alumina supported catalysts has also been reported in the literature in benzene hydrogenation, benzene exchange with deuterium and in cyclopentane hydrogenolysis [ 251. In a bimetallic system, the interaction between the metals has been mainly related to an electronic effect (ligand effect) or to a dilution effect (ensemble effect) [ 261. In our case we can suggest that the main parameter responsible for the observed catalytic behaviour is a dilution effect which causes a reduction in the size of the active metal aggregates thus decreasing the cracking activity of the platinum sites towards intermediate oligomers; consequently dehydrocyclization of these oligomers to aromatic becomes strongly favoured (Fig. 2). It is well known that cracking of C-C bonds needs active sites made of a number of adjacent surface atoms larger than that required for dehydrocyclization and dehydrogenation reactions [ 27,281. Aromatic selectivities and yields reach a maximum at a Tl/Pt ratio of 1 and then decrease for further addition of thallium (Fig. 2). This behaviour has also been observed by Volter et al. [29] on Pt-Pb/AlzOs and Pt-Sn/AlaO, in cyclohexane dehydrogenation and in n-heptane dehydrocyclization. It is likely that at higher concentration thallium tends to self-aggregate instead of diluting platinum sites. A similar effect of the concentration of the inactive metal has been previously observed on Ru-Cu/AlzO, which shows formation of separate ruthenium and copper crystallites at high Cu/Ru ratios [ 301. Dehydrogenation of propane and dehydrocyclization of the intermediates oligomers are the most important reactions in the conversion of propane to aromatics. More severe operative conditions can improve propane transformation with higher aromatic yields (Fig. 5 ) but in any case with some limitations at each temperature. These limitations are evidenced at high conversions by the decrease in the selectivity to aromatics and by the corresponding in-
S. Sciri! et al./Appl. Catal. A 103 (1993) 123-134
133
crease in the cracking products (Figs. 2-3 ) . At 2’~ 550’ C transformation of propane through steps (l-5) tends to reach maximum aromatic yield which approaches the thermodynamic equilibrium. The conversion of propane at which this maximum yield is established depends on the temperature and it is lower at lower temperature [ 71. When the formation of aromatic approaches the thermodynamic equilibrium, an increase of contact time increases the transformation of propane through step (7) and through the cracking of the other reaction products. Details on the effect of contact time and temperature on the hydrogenolysis activity of Pt/ZSM-5 have been reported elsewhere [ 311. With respect to the deactivating behaviour of these Pt-Tl bimetallic samples, it is noteworthy that addition of thallium does not modify the deactivation rate, which remains high, especially at temperatures higher than 500” C. This is probably due to the absence of electronic interactions between platinum and thallium. On Pt-Sn and Pt-Pb on A1203 an electronic interaction is responsible for the higher stability of these systems [ 29 1. The use of another thallium precursor or a different preparation method of Pt-Tl bimetallics did not improve the catalytic performance of the monometallic Pt/ZSM-5. In fact, in the case of samples prepared from TINOB (Fig. 6)) aromatic yields do not increase, compared to Pt/ZSM-5, up to a Tl/Pt ratio of 1. At higher ratios (Tl/Pt = 1.25) a strong decrease both in aromatic selectivity and in propane conversion has been observed. Probably at higher thallium content, larger aggregates of platinum, which favour cracking reactions, are formed. Moreover preliminary experiments carried out on a bimetallic catalyst prepared by cationic exchange of H-ZSM-5 with Pt ( NH3),C1, and subsequent impregnation with Tl,SO, have shown a strong decrease in conversion and aromatic selectivities if compared with the Pt-exchanged monometallic sample. This aspect, however, needs a further study and will be reported elsewhere. CONCLUSIONS
The above reported results have shown on the samples prepared by impregnation, using Tl,SO, as thallium precursor, that addition of thallium to Pt/ ZSM-5 increases aromatic selectivities without modifying propane conversion. An aromatic yield of 70% was obtained at 550” C and r= 1s on the sample with a Tl/Pt ratio of 1. The positive influence of thallium is displayed in the dehydrocyclization of intermediate oligomers with a dilution effect on platinum aggregates. ACKNOWLEDGEMENT
This work has been carried out with the financial support of C.N.R. (Progetto Finalizzato Chimica fine e secondaria II).
S. ScirB et al. jApp1. Catal. A 103 (1993) 123-134
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Applied Catalysis A: General, 103 (1993) 123-134 Elsevier Science Publishers B.V., Amsterdam APCAT
A2573
Propane aromatization over Pt-Tl/ZSM-5 S. Scire and R. Maggiore Dipartimento di Scienze Chimiche, Universitk di Catania, Viale A. Doria 6,95127 Catania (Ztaly)
S. Galvagno Dipartimento di Chimica Zndustriale, Universitti di Messina, Gas. Post. 29,98166 Sant’Agata di Messina (Italy)
and C. Crisafulli and G. Toscano Dipartimento di Scienze Chimiche, Universitb di Catania, Viale A. Doria 6,95127 Catania (Italy) (Received 18 February 1993, revised manuscript received 24 May 1993)
Abstract Aromatization of propane has been studied on Pt-TI/ZSM-5 catalysts in the temperature range 350550°C. The effect of thallium precursor and Tl/Pt ratio has been also investigated. The best results have been obtained on the bimetallic samples prepared from TlsSO, and HxPtC!la. On these samples addition of thallium to Pt/ZSM-5 increases the selectivity to aromatics without significantly affecting the catalytic activity. A maximum aromatic yield of about 70% has been obtained on the sample with a Tl/Pt ratio of 1. It is suggested that thallium dilutes the platinum active sites thus reducing their crack-
ing activity. Key words: aromatization; bimetallic catalysts; platinum-thallium;
zeolites; ZSM-5
INTRODUCTION
The transformation of light alkanes, such as ethane and propane, into aromatics is a process of great industrial interest. Acid H-ZSM-5 zeolites combined with transition metals have shown good aromatic yields; Pt, Ga and Znbased catalysts have been reported to be the most active systems [l-6]. On H-ZSM-5 zeolite the activation of propane occurs only at temperatures higher than 5OO”C,while the aromatization of intermediate oligomers occurs between 200 and 250°C [ 1,7]. Addition of platinum to the zeolite greatly imCorrespondence to: Dr. R. Maggiore, Dipartimento di Scienxe Chimiche, Universitil di Catania, Viale A. Doria 6,95127 Catania, Italy. Tel. ( +39-95)33935, fax. ( + 39-95)580138.
0926-860X/93/$06.00
0 1993 Elsevier Science Publishers B.V.
All rights reserved.
124
5’. Scir6 et al./Appl. Catal. A 103 (1993) 123-134
proves the dehydrogenation of alkanes leading to higher aromatic yields [ 5,7,8]. The positive dehydrogenating action of platinum is, however, accompanied by cracking and hydrogenolysis reactions limiting the selectivity towards aromatics. The presence of a second metal can sometimes decrease cracking activity of platinum increasing aromatic yields [ 9,101. We have recently obtained on Pt-Ir/ZSM-5 an aromatic yield of 61.2% [ 111, comparable with that observed on the most efficient catalysts for propane aromatization. On the basis of an extensive study carried out on different bimetallic platinum-based catalysts supported on H-ZSM-5, in this paper we report and discuss the results obtained on Pt-Tl/ZSM-5, with particular attention to the influence of thallium precursor and experimental conditions in order to maximize aromatic yields. EXPERIMENTAL
The support used was a commercial H-ZSM-5 zeolite (Conteka) with a SiO,/ A&O3 ratio of 30 and a surface area of 400 m2/g. Pt/ZSM-5 and Tl/ZSM-5 (1 wt.-%) were prepared by incipient wetness impregnation of the zeolite with H2PtCls and Tl,SO, or T1N03, respectively. Bimetallic Pt-Tl catalysts were prepared: (a) by impregnation with aqueous solution of H2PtCls and subsequently with Tl,SO,; (b) by coimpregnation with H,PtCI, and TlNO,. The platinum content was constant in all catalysts (1 wt.-% ), whereas thallium content was changed to obtain the desired Tl/Pt atomic ratio. The catalyst, dried at 12O”C, was pressed into pellets, without any binder, crushed and the fraction 80-140 mesh was always used. Catalysts pretreatment included in situ heating in flowing air for 2 h at 450” C and, subsequently, in a stream of hydrogen for 3 h at 450’ C. Aromatization of propane was carried out in a flow system employing a tubular reactor at atmospheric pressure containing about 0.25 g of catalyst. The reactant feed was a purified mixture of propane/helium (50 vol.-% ) . Propane flow-rates were in the range 0.3-3.0 l/h. Reaction products were analyzed by a gas chromatograph (Carlo Erba model 4200 with flame ionization detector) connected directly to the flow system. The procedure used was as follows: the reactant mixture was passed over the catalyst for 15 min before sampling the products for analysis. The feed was then cut out and the catalyst was treated at 450” C in flowing hydrogen for 1 h before taking another experimental point. By using this procedure, conversions and selectivities were reproducible within 2-3%. Conversion, yields and selectivities were calculated on carbon number basis. The absence of diffusional limitations was verified by preliminary runs carried out at different catalyst loadings and different grain sizes. Hydrogen chemisorption was measured in a conventional pulse system op-
S. Scirk et al./Appl. Catal. A 103 (1993) 123-134
125
erating at room temperature. Negligible amounts of hydrogen were found to chemisorb on the support and on thallium. Temperature-programmed reduction (TPR) experiments were carried out in a typical gas chromatographic apparatus at a heating rate of 5’ C/min using 5 vol.-% Hz in Ar. RESULTS
Fig. 1 shows the conversion of propane as a function of reaction temperature (350-550°C) over samples prepared by subsequent impregnation of chloroplatinic acid and thallous sulphate at a contact time, t, of nearly 5 s. One can observe that addition of different amounts of thallium to Pt/ZSM-5 leaves the conversions obtained on the monometallic platinum sample virtually unchanged. This behaviour is also observed at contact times other than 5 s. It is noteworthy that Tl/ZSM-5 showed a moderate decrease in conversion and the same distribution of reaction products as the unloaded support. The influence of addition of thallium on Pt/ZSM-5 becomes evident when we consider the distribution of reaction products. Yields as a function of conversions at T = 500 ’ C are reported in Figs. 2-4; the amount of ethene is always negligible. It is possible to note that, on all samples, aromatic yields rise on increasing conversion levels reaching a maximum at a conversion of nearly 80%; then a sharp decrease can be observed (Fig. 2). The addition of thallium increases significantly the aromatization activity of Pt/ZSM-5. At this temperature an aromatic yield of 65% at a Tl/Pt ratio of 1 has been obtained. This yield is much higher than that observed on the monometallic Pt/ZSM-5 sample (44% ). Lower thallium contents give intermediate results. Data of products distribution (Figs. 2-4) indicate that higher aromatic yield
350
450
T (“C) Fig. 1. Propane conversion on Pt-Tl/ZSM-5 temperature. Contact time, r= 5 5.
samples prepared from TI,SOI. Effect of reaction
126
S. S&t? et al./Appl. Catal. A 103 (1993) 123-134 70
-e-
Pt TI/Pt
0.1
+
TI/Pt
05
+
TI/Pt
10
TI/Pt
125
@- -a+-
=-+ 4c-
td
h
Fig. 2. Aromatic yield as a function of conversion on Pt-Tl/ZSM-5 T=Wo”C.
*
Tl/R
0.1
TI/Ft
0.5
--c
TI/Pt
10
+
TI/Pt
125
4b-+
samples prepared from TlPS04.
33
2s
10 a I 40
0, M
I con:
5-
-s-
Pt
*
TI/Pt
0.1
TI/Pt
0.5
++ +
TI/Pt
10
+
Tlpt
125
I m (Z)
3-
l-
c-, 20
I
40
con:
-b
I
I 80 (z)
Fig. 3. Ethane (a) and methane (b) yields as a function of conversion on Pt-Tl/ZSM-5 prepared from T12S04. T=500”C.
samples
127
S. Scirk et al./Appl. Catal. A 103 (1993) 123-134 4
Pt
x
let
0.1
Fig. 4. Propene (a) and C, (b) yields as a function of conversion on Pt-TI/ZSM-5 pared from Tl,SOI. T= 500°C.
samples pre-
TABLE 1 BTX distribution on Pt-Tl/ZSM-5
samples prepared from T&SO,; T=500”C
Tl/Pt ratio
1.0
0.5
0.1
0
Conv. ( % ) Arom. sel. ( % )
76.2 81.9
80.3 76.2
76.3 65.3
76.8 56.0
8.2 36.7 55.1
9.0 43.3 47.7
14.7 44.8 40.5
21.8 45.4 32.8
Arom. Distribution Benzene Toluene Xylenes
(%):
128
5’.Scirt? et al./Appl. Catal. A 103 (1993) 123-134
Fig. 5. Selectivity to aromatics as a function of conversion at different temperatures (400-600” C) on a bimetallic Pt-Tl/ZSM-5 sample (Tl/Pt = 0.1) prepared from Tl,SOI.
can be correlated to a decrease of ethane and, to a less extent, of methane, while addition of thallium to Pt/ZSM-5 does not substantially modify the yields towards intermediate products (propene and C,) . The distribution, within the aromatic fraction, of benzene, toluene and xylenes (BTX) is also modified by adding thallium to Pt/ZSM-5. Table 1 shows the BTX molar ratios at conversion levels (80%) where highest are the aromatic yields. We can observe that on increasing Tl/Pt ratio, together with an increase of aromatic selectivities, BTX distribution shifts towards xylenes, with a corresponding decrease of toluene and, above all, of benzene. This last component is always the less abundant. An increase of selectivity to xylenes had been also observed on all samples at decreasing conversion levels. Only at conversions higher than 80% selectivities move rapidly from xylenes to toluene and then to benzene. This indicates advanced cracking and demethylation conditions, as shown by the intense ethane (Fig. 3a) and methane (Fig. 3b) formation. The above reported data apply to Pt-TI samples with an atomic ratio equal or less than 1; a further addition of thallium ( TljPt higher than 1) , even though this does not modify catalytic activity (Fig. 1 ), lowers aromatic yields, compared to sample with Tl/Pt = 1 (Fig. 2 ) . Aromatic yields have been found to depend on the reaction temperature. Fig. 5 shows the aromatic selectivities obtained on the sample with a Tl/Pt ratio of 0.1 in the range of temperature 400-600°C. Up to 55O”C, the higher the temperature, the higher is the aromatic selectivity. Any further increase of temperature does not improve catalytic performance. At each temperature selectivities increase on increasing conversion levels up to a certain value and then sharply decrease, this value shifting to higher conversions on increasing reaction temperature.
129
S. Sir& et al./Appl. Catal. A 103 (1993) 123-134
T (“C> 70
+
TI
I
Fig. 6. Propane conversion (a) and aromatic selectivity (b) on Pt-Tl/ZSM-5 from TlN03. Effect of reaction temperature. Contact time, r=5 s.
samples prepared
The highest aromatic yield (70% with sel. = 83% and conv. = 85% ) was obtained at 550°C and 7= 1 s on the sample with a Tl/Pt ratio of 1. Addition of thallium does not modify the behaviour of Pt/ZSM-5 towards deactivation rate. On all samples the higher the reaction temperature is, the quicker is the decrease in total conversion with reaction time; at 400°C and 7= 2.5 s, for instance, the activity remains practically constant over about 10 h. At 550°C and 7=2.5 s, during the same 10 h, the conversion decreases from 90% to 25%; a drop in cracking and aromatic products is observed, while the yield of propene rises. When TlNO, is used as thallium precursor, catalytic behaviour (Fig. 6) is different from that reported above using Tl,SO,. In fact, the presence of thallium, at least up to a Tl/Pt ratio of 1, does not effectively modify propane conversion; aromatic selectivity undergoes a very slight increase. At higher Tl/
S. S&g et al./Appl. Catal. A 103 (1993) 123-134
130
Pt ratios (Tl/Pt = 1.25) a drastic decrease in conversion and aromatic selectivity is observed, especially at the higher temperatures. DISCUSSION
Aromatization of propane over Pt/ZSM-5 has been suggested to occur through the following reaction scheme [ 7,111:
1
2
5a a1iph.h
5b C6-C,
alicycl
__*
C,-C,
arom.
Scheme 1
Platinum catalyses the dehydrogenation of propane to propene and of CgC8 naphthenes to aromatics. This positive action is, however, accompanied by cracking and hydrogenolysis reactions at an extent that depends on the geometric and electronic state of the metal crystallites which can be modified by addition of a second metal [ 12-161. We have previously reported that when the addition of a second metal, such as tin or lead, increases the electron density of platinum, catalytic activity decreases together with cracking and hydrogenolysis products leading to higher aromatic selectivities [ 9,101; the reverse is obtained when the electron density of platinum is decreased by addition of rhenium or lanthanum [ 111. Strong dilution effects, which inhibit hydrogenolysis reactions, can lead to a different behaviour, as observed on Pt-Ir/ZSM5 which showed both higher activity and aromatic selectivities than Pt/ZSM5 [ 111. Platinum-support interactions can also modify the catalytic behaviour by an electron transfer from platinum to support [ 17-191. The acid sites of the ZSM-5 zeolite play an important role in the oligomerization of the propene intermediate (which already occurs at 200-250’ C ) and in the formation of aromatics through alicyclic compounds. These acid sites show also a cracking activity which is, however, lower than that observed on platinum. On H-ZSM-5 the activation of propane can occur only at high temperature with low yields and with a reaction pathway different from that observed on Pt/ZSM-5 [ 1,7]. It is noteworthy that Tl/ZSM-5 has an activity barely lower than the zeolitic support alone and the distribution of reaction products is practically the same. This is an evidence of the inactivity of thallium towards the reaction. The absence of dehydrogenating activity of thallium is also confirmed by some experiments performed on silica. In fact Tl/Si02, between 450 and 55O”C, is
S. Scirti et al./Appl. Catal. A 103 (1993) 123-134
131
completely inactive whereas Pt/SiOp, at the same conditions, exhibits high dehydrogenating and hydrogenolytic activity. The slight decrease in the catalytic activity observed on Tl/ZSM-5 compared to H-ZSM-5 (Fig. 1) can be attributed to a moderate reduction of the acidity of H-ZSM-5, by addition of thallium, as observed by ammonia temperature-programmed desorption (TPD ) experiments [ 201. In the absence of an active metal the acidity is, in fact, the main parameter responsible for alkane activation. No change in surface acidity by addition of thallium to Pt/ZSM-5 has been observed by TPD spectra of adsorbed ammonia [ 211. It should be noted that all platinum and Pt-Tl samples supported on the ZSM-5 zeolite used in our experiments did not practically chemisorb hydrogen at room temperature, therefore no turnover frequencies could be calculated. A low capability of Pt/ZSM-5 to chemisorb hydrogen has been previously observed by Folefoc and Dwyer [ 221 and attributed, among other factors, to the presence of electron-deficient platinum atoms. In any case, the interpretation of chemisorption data on metals supported on zeolites has not yet been fully clarified. In Fig. 7 are reported the TPR spectra of Pt-Tl/ZSM-5 samples prepared using Tl,SO, as thallium precursor. Pt-Tl bimetallic catalysts show spectra which are different from those obtained on the platinum and thallium monometallic samples. The shoulder observed at 90-llO”C, due to the platinum precursor (the maximum of the peak occurs at 150’ C), becomes more evideni on bimetallic samples on increasing the Tl/Pt ratio. Moreover the HJTl ratic (H, is the amount of hydrogen consumed during TPR) of 3.2, measured on the
I
0
100
200 T
300
400
500
800
I’CI
Fig. 7. TPR spectra of Pt-Tl/ZSM-5
samples prepared from T&30,.
132
S. ScirB et al. jApp1. Catal. A 103 (1993) 123-134
Tl/ZSM-5 sample, indicates a reduction of the sulphate to metallic thallium and T&S, in agreement with previously published results [ 231. Even though the TPR spectra of the bimetallic samples are not a simple combination of the monometallic catalysts, the presence and the nature of a Pt-Tl interaction cannot be clearly identified. Data of Fig. 1 show that addition of thallium to Pt/ZSM-5 does not effectively modify propane conversions. Also the initial rates of reaction, important sign of dehydrogenating activity of platinum, do not change by adding thallium. Moreover propene and C, hydrocarbon yields do not depend on the presence of thallium (Fig. 4), thus showing that platinum affinity towards intermediate alkenes is not altered. These results indicate that the presence of thallium does not effectively modify the electronic properties of platinum. However the considerable change in aromatic selectivities by addition of thallium, (Fig. 2) indicates the presence of another kind of interaction that improves dehydrocyclization of C&-C8aliphatic to aromatics. It is well known that thallium forms alloys with other metals, platinum included [ 241. A Pt-Tl interaction on alumina supported catalysts has also been reported in the literature in benzene hydrogenation, benzene exchange with deuterium and in cyclopentane hydrogenolysis [ 251. In a bimetallic system, the interaction between the metals has been mainly related to an electronic effect (ligand effect) or to a dilution effect (ensemble effect) [ 261. In our case we can suggest that the main parameter responsible for the observed catalytic behaviour is a dilution effect which causes a reduction in the size of the active metal aggregates thus decreasing the cracking activity of the platinum sites towards intermediate oligomers; consequently dehydrocyclization of these oligomers to aromatic becomes strongly favoured (Fig. 2). It is well known that cracking of C-C bonds needs active sites made of a number of adjacent surface atoms larger than that required for dehydrocyclization and dehydrogenation reactions [ 27,281. Aromatic selectivities and yields reach a maximum at a Tl/Pt ratio of 1 and then decrease for further addition of thallium (Fig. 2). This behaviour has also been observed by Volter et al. [29] on Pt-Pb/AlzOs and Pt-Sn/AlaO, in cyclohexane dehydrogenation and in n-heptane dehydrocyclization. It is likely that at higher concentration thallium tends to self-aggregate instead of diluting platinum sites. A similar effect of the concentration of the inactive metal has been previously observed on Ru-Cu/AlzO, which shows formation of separate ruthenium and copper crystallites at high Cu/Ru ratios [ 301. Dehydrogenation of propane and dehydrocyclization of the intermediates oligomers are the most important reactions in the conversion of propane to aromatics. More severe operative conditions can improve propane transformation with higher aromatic yields (Fig. 5 ) but in any case with some limitations at each temperature. These limitations are evidenced at high conversions by the decrease in the selectivity to aromatics and by the corresponding in-
S. Sciri! et al./Appl. Catal. A 103 (1993) 123-134
133
crease in the cracking products (Figs. 2-3 ) . At 2’~ 550’ C transformation of propane through steps (l-5) tends to reach maximum aromatic yield which approaches the thermodynamic equilibrium. The conversion of propane at which this maximum yield is established depends on the temperature and it is lower at lower temperature [ 71. When the formation of aromatic approaches the thermodynamic equilibrium, an increase of contact time increases the transformation of propane through step (7) and through the cracking of the other reaction products. Details on the effect of contact time and temperature on the hydrogenolysis activity of Pt/ZSM-5 have been reported elsewhere [ 311. With respect to the deactivating behaviour of these Pt-Tl bimetallic samples, it is noteworthy that addition of thallium does not modify the deactivation rate, which remains high, especially at temperatures higher than 500” C. This is probably due to the absence of electronic interactions between platinum and thallium. On Pt-Sn and Pt-Pb on A1203 an electronic interaction is responsible for the higher stability of these systems [ 29 1. The use of another thallium precursor or a different preparation method of Pt-Tl bimetallics did not improve the catalytic performance of the monometallic Pt/ZSM-5. In fact, in the case of samples prepared from TINOB (Fig. 6)) aromatic yields do not increase, compared to Pt/ZSM-5, up to a Tl/Pt ratio of 1. At higher ratios (Tl/Pt = 1.25) a strong decrease both in aromatic selectivity and in propane conversion has been observed. Probably at higher thallium content, larger aggregates of platinum, which favour cracking reactions, are formed. Moreover preliminary experiments carried out on a bimetallic catalyst prepared by cationic exchange of H-ZSM-5 with Pt ( NH3),C1, and subsequent impregnation with Tl,SO, have shown a strong decrease in conversion and aromatic selectivities if compared with the Pt-exchanged monometallic sample. This aspect, however, needs a further study and will be reported elsewhere. CONCLUSIONS
The above reported results have shown on the samples prepared by impregnation, using Tl,SO, as thallium precursor, that addition of thallium to Pt/ ZSM-5 increases aromatic selectivities without modifying propane conversion. An aromatic yield of 70% was obtained at 550” C and r= 1s on the sample with a Tl/Pt ratio of 1. The positive influence of thallium is displayed in the dehydrocyclization of intermediate oligomers with a dilution effect on platinum aggregates. ACKNOWLEDGEMENT
This work has been carried out with the financial support of C.N.R. (Progetto Finalizzato Chimica fine e secondaria II).
S. ScirB et al. jApp1. Catal. A 103 (1993) 123-134
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