Ind. Eng. Chem. Res. 1993,32, 1046-1052
1046
Selective Aromatization of C3 and C4 Paraffins over Modified Encilite Catalysts. 1. Qualitative Study A p u r b a K.Jana and Musti S. Rao' Department of Chemical Engineering, Indian Institute of Technology, Kanpur 208 016, U.P.,India
The catalytic activity of Zn-encilite in the aromatization of propane and n-butane has been studied by a comparative study of the yields of benzene, toluene, and xylenes (BTX) using propane and n-butane as feeds. The results showed that n-butane had better activity and selectivity to BTX than propane. The effects of reaction temperature, contact time, inlet concentration, poisoning with ammonia on H-encilite, and different degrees of ion exchange with Zn2+ion were studied in detail in order to obtain information on the reaction mechanism. The results indicated that the activity and the selectivity were dictated by the acidity and Zn loading on the catalyst. 1. Introduction
2. Experimental Section
The production of aromatic hydrocarbons such as benzene, toluene, and xylenes (BTX) from c2-C~light paraffins (Batchelder et al., 1985;Braginet al., 1986;Bragin et al., 1988; Bayense et al., 1991; Bayense and Van Hooff, 1991; Ehgelene et al., 1985; Inui et al., 1986a; Jianhua et al., 1990; Minachev et al., 1988a; Martindale et al., 1988; Mole et al., 1985; Ono et al., 1987; Smith et al., 1988; Wakushima et al., 1988)has been a topic of current interest by several researchers. These BTX compounds are consumed in large quantities as high octane blending components and also as raw materials in the petrochemical industry. Mowry et al. (1985) reported a cyclar process to produce aromatics from LPG. The first cyclar unit has been reported to be on stream at Grangemouth, Scotland (Doolanet al., 1989). Significant improvements have been made in the direct one-stage conversion of light alkanes, mainly propane and n-butane, to aromatics over Ga-, Zn-, and/or Pt-modified zeolites of the pentasil family (Yuan et al., 1988; Inui et al., 1988; Krupina et al., 1989; Furuya et al., 1988;Inui et al., 1987; Minachev et al., 1988b). Inui et al. (1987) and Krupina et al. (1989) studied the aromatization of propane and n-butane, respectively, over various transition metal ion-exchanged ZSM-5 catalysts. It has been reported previously that the yield of aromatics over Ga-ZSM-5 with propane feedstock was 56% at 550 "C (Kitagawa et al., 1986) and over gallium oxide containing particles with the ZSM-5 zeolite crystallites was 49% at54O"C (Jianhuaetal., 1990)forn-butane feedstock. Ga-silicates and Pt/Ga-silicates (Inui et al., 1987)and Pt/ Zn-silicates (Inui et al., 1986b)were also found to be active catalysts for the effective conversion of propane with a reasonable yield. Maggiore et al. (1991) have reported the influence of various rare earth elements over Pt-ZSM-5 catalysts on propane aromatization. Recently, Le Van Mao et al. (1991) studied the kinetics of n-butane aromatization on ZSM-5 and gallium-bearing ZSM-5 catalysts. However,further improvements of the catalysts and process conditions seem to be necessary to establish this process for the direct conversion of C3 and C4 paraffins to aromatics. The aim of the present study was (1) to conduct a detailed comparative study of the catalytic transformation undergone by the propane and n-butane using Znexchanged encilite catalysts, (2) to study the influence of systematic variations in process conditions, and (3) to obtain some insight into the mechanism of the reaction.
2.1. Catalyst Preparation. Encilite was provided by IPCL, Baroda, India. Encilite is a ZSM-5-type catalyst with a SiOdAl203 ratio of 76. The catalysts were detemplated by heating them in flowing air at 500 "C. The ammonium form was made by ion-exchanging 30 g of catalyst seven times with 1.0 L of a solution of 1 M ammonium nitrate under reflux conditions at 80 "C for 20 h each time. The exchanged material was filtered and washed thoroughly with distilled water and dried at 30 "C. Part of the "4-encilite was calcined in air at 500 OC for 24 h, and it was subsequently converted to the hydrogen form. The other part of the "4-encilite was exchanged with Zn2+ cation. The exchange was achieved by contacting 30 g of "4-encilite with 700 mL of an aqueous solution of 1 M Zn(N03)2*6H20under reflux conditions at 80 "C for 24 h. The catalyst was then filtered, washed thoroughly with distilled water, and dried at 80 "C. Znencilite catalyst was pressed and crushed, and the 10-20mesh fraction was separated by sieving. The reduction of the catalyst particle size had no influence on the reaction, showing that the reaction was not diffusion controlled. 2.2. Catalyst, Characterization. X-ray diffraction patterns of the samples were obtained with a Reich Seifert ISO-DEBYEFLEX 2002 powder diffractometer using Cu Ka radiation. The beam was filtered and monochromatized by a nickel filter and graphite single crystal curved monochromator, respectively. The scanning speed was 1.2 deg/min in terms of 20. The samples were scanned from 6" to 50'. Silicon was used as an internal standard to give the instrumental broadening factor. This was utilized to calculate the crystallite size. Secondary electron images of the samples were taken by using a JEOL JSM 840A scanning electron microscope. The surface areas were measured with a quantachrome Brunauer-EmmettTeller (BET) apparatus. Infrared spectroscopic measurements were performed on a Perkin-Elmer FT-IR series 1600spectrometer working in the 4000-400-crn-' region. The samples (40 mg) were pressed into self-supporting wafers and placed into stainless steel IR cells fitted with KBr windows. The IR cells with samples were heated under vacuum at 450 "C for 4 h before the measurements. In the IR measurements of pyridine-adsorbed zeolites, the samples were kept at 450 OC under vaccum for 6 h. The samples were then cooled to 150 OC, and dehydrated pyridine vapor was introduced to the IR cell. After 1.5 h of adsorption, the IR cell was evacuated at 150 "C for 2 h and outgassed at 500 O C for 2 h. All IR spectra were recorded at room temperature.
0888-5885/93/2632-lO46$04.00/0 0 1993 American Chemical Society
Ind. Eng. Chem. Res.,Vol. 32, No.6,1993 1047 TO EXHAUST
u
u
turn Figure 1. ExperimentaJ setup: 1, propaneh-butane cylinder; 2, nitrogen cylinder; 3, air m m p m r ; 4, soap bubble metsr;5. packed silica gel tower; 6, CaCh drying tube, 7, rotameter; 8. needle valve; 9,lO,misingcbnmbsrs; ll,preheater;l2,hunaee;13,tbe~oeouples; 14, quartz wool packing; 15, catalyst bsd; 16, stainlesssteel reactor; 17, gas sampling valves; 18,cwlercondenser; 19,condensatestorage bottle.
An ammonia temperature-programmed desorption (TPD) study was carried out using a heating rate of 10 Wmin. Prior to TPD, 0.5 g of the sample was placed in a 10-mm-0.d. quartz reactor and calcined in air at 500 "C. The sample was then cooled to 50 OC. Ammonia (12%) inheliumusedastheadsorbatewaspassedoverthesample at 50 "C for 2 h. Helium was then passed over the sample at a flow rate of 40 cm3/min for 2 h to desorb weakly adsorbed ammonia, followed by the start of temperature programming. The TPD profiles were detected by a thermal conductivity detector. The exit gas was bubbled through a 0.05 N HzS04solution. 2.3. Experimental Setup. A schematic diagram of the experimental setup is given in Figure 1. Two soap bubble meters and a rotameter were used to meter n-butane/propane,nitrogen, and air,respectively. Needle valves were provided to control the flow rates of these various gases. Drying tubes containing self-indicating silicagelwereprovidedintheflowlooptoremovemoisture, if any, from the feed gases and soap bubble meters. The gaseswerethenmixedinamixiichamber,passedthrough a temperature-controlledpreheater, and finally fed to the reactor. All experiments were performed by using an integral, fixed bed, gas-phase reactor at atmospheric pressure. The reactor was a type 304 Stainless steel tube, with an outer diameter of 15 mm and a wall thickness of 2.5 mm. It was approximately 61 cm in length, which consisted of inlet, outlet, and reactor sections. Two chromel-alumel thermocouples, TC1 and TC2, in 2-mm0.d. stainless steel sheaths coaxial with the reactor tube, were used for measuring the temperatures at the middle and end of the catalyst bed. The reactor was placed in a 32-mm4.d. vertical furnace. The preheater and reaction temperatures were controlled using two Indotherm MPC 500 Model programmable temperature controllers of C-1200 'C range. A cooler condenser was used for the rapid cooling of the reactor effluents. The temperature of the condenser was kept at 5 "C, by passing refrigerated water through its jacket. A bottle with a B10 joint was used to separate the condensates, if any, from the reactor effluent stream. Two gas sampling valves were provided in order to inject a fixed volume of the product gas to the gas chromatograph for analysis. 2.4. Experimental Procedure. The reador tube was assembled, and the threaded joints in contact with high temperatures were sealed with high alumina/sodium silicate cement. The reactor tube was packed with sieved catalyst, sandwiched between 10 and 5 cm3of quartz wool packing at the inlet and exit sections, respectively. Prior to reaction the catalyst (0.6 g) was heated in a flowing air stream at a flow rate of 140 cm3/min for 3 h a t the reaction
Wm
(a)
(b)
Figure 2. Secondary electron images of (a) H-encilite and (b) ZnEncilite samplea (X15 0139).
50
LO
30 20 2 8 (degrees)
10 6
Figure 3. X-ray diffraction patterns of (a) H-encilite and (b) Znencilite samples.
temperature, and then in a flowing nitrogen stream for 1 h at the same temperature. Subsequently, n-butane/ propane and nitrogen were introduced at the desired flow rates into the reactor. After 1h, when steady state was obtained, as observed experimentally, 0.5 cm3 of the effluent stream was analyzed for aromatic hydrocarbons. The rest of the reactor effluent was passed through a condenser for removalofaromatic hydrocarbons,and then 1 cm3 of the noncondensed gases (mainly aliphatic hydrocarbons) was analyzed for C1-C4 hydrocarbons. The rest of the gases was vented to the atmosphere through a soap bubble meter. 2.5. Analytical Methods. The reactor effluents were analyzed periodically by two gas chromatographs (GC). For the analysis of aromatic hydrocarbons an HP 5890A gas chromatograph fitted with a flame ionization detector (FID) was used. A 2-m-long OV-101 on chromosorb W column separated the aromatic hydrocarbons. The aliphatic hydrocarbons were analyzed using a NUCON gas chromatograph. A 2-m-long parapack-Q column separated the aliphatic hydrocarbons. 3. Results and Discussions 3.1. Physical Properties of E-Encilite and ZnEncilite Catalysts. Scanning electron micrographs of both the catalystsare shown in Figure 2. Morphologically, it shows that the catalysts were comprised of highly intergrown crystals (probably aggregates) of an overall ellipsoidal shape and of 0.5-1.5-pm diameter with numerous facets and pores. Figure 3 showsthe superimposed X-ray diffraction pattern of H-encilite and Zn-encilite. The peak positions as well as the number of peaks of both the materials are the same, which shows that they have identical crystal structures. The X-ray peaks were rather broad due to the fine crystallite size. The crystallite size
1048 Ind. Eng. Chem. Res., Vol. 32,No. 6,1993
, o o l
Temperature
, 'C
Figure 6. Temperature-programmed desorption profiles of (a) H-encilite and (b) Zn-encilite samples. -
0 3800
3600 Wove number ( c n i ' )
1001
3400
Figure 4. Infrared spectra of (a) H-encilite and (b) Zn-encilite aamples.
-z v
1
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i
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0
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z
0 V
00 1700
1600
1500
Wove number
1400
(~6')
Figure 6. Infrared spectra of pyridine-adsorbed(a) H-encilite and (b) Zn-encilite samples.
calculated from the Schemer line broadening technique (Klug et al., 1975)using silicon as an internal standard was found to be 6.3nm. BET surface areas of 427 and 419 m2/g were obtained by the nitrogen adsorption technique for H-encilite and Zn-encilite, respectively. The IR spectra of H-encilite and Zn-encilite in the hydroxyl stretching region are shown in Figure 4. Both the samples show absorption bands at 3600and 3715cm-'. The bands at 3600 and 3715 cm-l are attributed to the framework A1-OH-Si groups and terminal Si-OH groups, respectively. These two groups are responsible for the Bronsted acid sites and are in agreement with previous data (Auroux et al., 1979;Yamagishi et al., 1990). The nature of the acid sites was investigated by using IR spectra of the pyridine-adsorbed samples. Figure 5 shows the IR spectra of pyridine-adsorbed H-encilite and Zn-encilite. They are almost similar in nature. The absorption bands were observed at 1542,1500,and 1453 cm-1. The band at 1542 cm-l, attributed to the C-C stretching vibration of the pyridinium ions, characterizes Bronsted acid sites, while the band at 1453 cm-l indicates the presence of Lewis acid sites which arises from the C-C stretch of a coordinatively bonded pyridine complex (Yamagishi et al., 1990;Vedrine et al., 1979). Figure 6 shows the TPD profiles of H-encilite and Znencilite catalysts. For H-encilite and Zn-encilite the lowtemperature peak was observed at 208 "C, whereas the high-temperature peaks were observed at 429 and 478 OC, respectively. The low-temperature peak represents weak acid sites where ammonia molecules were weakly adsorbed. Similarly,the high-temperature peak corresponds to strong acid sites present on the catalyst. The high-temperature peak of Zn-encilite occurs at a higher temperature compared to that of H-encilite, suggesting that Zn-encilite contains stronger strong acid sites than H-encilite. The entire amounts of ammonia which were desorbed between
/
I X
,
0 4 50
LBO
510
570
540
600
REACTION TEMPERATURE ("C)
Figure 7. Effect of reaction temperature on the propane and n-butane conversionsand their respective BTX selectivitiesand yields over Zn-encilite (feed gas, 15% propaneh-butane and the rest nitrogen; WIF = 2.2 g.h-mol-'): 0, propane conversion; A, BTX selectivity with propane aa feed; *, BTX yield with propane aa feed; 0 , n-butane conversion; X, BTX selectivity with n-butane aa feed; +, BTX yield with n-butane aa feed.
50 and 600O C were 1.46and 1.45mmol of NHJg of catalyst for H-encilite and Zn-encilite, respectively. The above result indicates that though the amount of acid sites remains unaffected the strength of the strong acid sites is enhanced by the incorporation of the Zn2+ ion in the H-encilite catalyst. 3.2. Effect of Reaction Temperature. The conversion, selectivity, and yield were calculated as follows (Kitagawa et al., 1986):
n, X 100 '76 conversion = nf % selectivity =
'76 yield =
.hghcp
n, x 3/4
nhcpNhcp
loo
nf x 3/4
The conversion of propane and n-butane to BTX was carried out over Zn-encilite at several reaction temperatures and is shown in Figure 7. The total conversion increased with temperature. The conversions were 98.3% at 575 "C and 61.4% at 600 "C for n-butane and propane, respectively. The conversion of propane was lower than that of n-butane at any temperature because propane was less reactive for dehydrogenation (from propane to propylene) than n-butane (from n-butane to butene). The BTX selectivities also increased with reaction temperature up to 550 OC using n-butane and 575 OC using propane as
Ind. Eng. Chem. Res., Vol. 32, No. 6, 1993 1049 3
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0.01.4 450
480 510 540 570 REACTION TEMPERATURE (OC)
600 ...
Figure 8. Effect of reaction temperature on the product distribution using propane in the feed over Zn-encilite (feed gas, 15% propane and the rest nitrogen; W/F= 2.2 ph-mol-1).
1.9
2.0
2.3
3.2
3.7
WIF(grn h r l m o l )
Figure 10. Effect of contact time on the propane and n-butane conversionsand BTX selectivities and yields over Zn-enciliteat 660 "C (feed gas, 16% propane/n-butane and the rest nitrogen): 0 , propane conversion; 0 , BTX selectivity using propane as feed; A~ BTX yield with propane as feed; 0, n-butane conversion; BTX selectivity using n-butane as feed; B, BTX yield with n-butane as feed.
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480 510 ' REACTION TEMPERATURE ('C)
0
Figme 9. Meetof reactiontemperatures on the product dutribution using n-butane in the feed over Zn-encilite (feed gas, 16% n-butane and the rest nitrogen; WIF = 2.2 g-h-mol-1).
feeds, respectively, but at higher temperatures they decreased probably due to the cracking of the alkanes. Moreover, at higher temperatures, the BTX selectivity for propane was higher than for n-butane, reflecting the higher oligomerizationtendency of propylene than butene. The yields of BTX increased with reaction temperature and reached 45.1% at 600 OC and 66.8% at 575 OC for propane and n-butane, respectively. Figures 8 and 9 show the influence of the reaction temperature on the product distribution for propane and n-butane conversions,respectively. In the case of n-butane (Figure91, toluene and xylene fractionsdecreased,whereas the benzene fraction increased. This is probably due to the dealkylation of toluene and xylenes, forming benzene and methane. The selectivities of ethylene and ethane decreased with an increase in temperature. The selectivities of the olefins decreased with reaction temperature, reflecting the higher reactivity of these substances. 3.3. Effect of Contact Time. The dependence of the weight of the catalysts; F,flow rate contact time (WJF (W, of the feed)) on the conversion,BTX selectivity,and BTX yield is shown in Figure 10. The total conversion and BTX selectivity increased with contact time for both propane and n-butane. The BTX yield increased very sharply with contact time, and was 65.5% at a WIF ratio of 2.2 gh-mol-l and 44.0% at a WIF ratio of 2.8 g.h.mol-l for n-butane and propane, respectively. The total con-
WIF
(gm h r l m o l )
Figure 11. Effect of contact time on the product distribution using propane in the feed over Zn-encilite at 550 O C (feed gas, 16% propane and the rest nitrogen).
version, BTX selectivities, and BTX yields almost attain steady values at WIF values of 2.8 ghnmol-l or greater. Figures 11 and 12 show the product distributions of propane and n-butane conversions using Zn-encilite at various contact times. As the contact time increased, the fraction of paraffins and aromatics in the products increased, while the fractions of CzH4, CsHs, and C4H8 decreased. The aliphatic hydrocarbons with six or more carbon atoms were not found in the products, indicating that these hydrocarbons, if at all formed in the zeolite pores, were immediately converted to smaller molecules by cracking or to aromatic hydrocarbons by cyclization and subsequentdehydrogenation. However,the selectivity data show that the probability of the formation of aromatic hydrocarbons is favored. 3.4. Effect of Inlet Concentration of PropanelnButane. In Figure 13, the dependence of the total conversion, BTX yield, and BTX selectivity on the inlet concentration of propane and n-butane is shown for Znencilite Catalysts. The figure shows that the conversion of propane and n-butane increases slowly with inlet concentration. The total conversion of propane is lowered further than n-butane conversion, indicatingless reactivity
1050 Ind. Eng. Chem. Res., Vol. 32, No. 6, 1993 ~
~
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I
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Toluhne
-
-f
28,0Y d Benzene
I N L E T CONCENTRATION OF PROPANE
('/e)
Figure 14. Effect of inlet concentration of propane on the product distribution using propane in the feed over Zn-encilite at 660 "C (W/F= 2.2 g-h-mol-1). 40.0,
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20
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4;
00
5 10 15 20 25 30 INLET CONCENTRATION OF PROPANEln-BUTANE(%)
Figure 13. Effect of inlet concentration of propane and n-butane on their respective conversionsand BTX selectivities and yielde over Zn-encilitaat 550 "C (W/F = 2.2 g-h-mol-'): 0 ,propane conversion; 0,BTXselectivityusingpropaneasfeed;h,BTXyieldusingpropane as feed; a, n-butane conversion; A, BTX selectivity using n-butane as feed; W, BTX yield with n-butane as feed.
of propane. The BTX selectivity for propane and n-butane decreased with increasing inlet concentration. The effect of inlet concentrations of propane and n-butane on the product distribution is shown in Figures 14and 15,respectively. From both the figures it is obvious that though propane is less reactive than n-butane, their dehydrogenated producta have reverse reactivitiq. At a lower inlet concentration, propylene from propane formed in less amount than butene from n-butane. 3.5. Reactions over Zn-Encilite with Ammonia Poisoning. The conversions of propane and n-butane over Zn-encilite catalysta at 550 "C with ammonia as a poison are shown in Figure 16. In these experiments, a steady-state reaction was first established in the absence of poison, and the reactant conversion was measured. Then a small quantity of ammonia gas was injected along the feed stream, and the reactant conversions were measured. As shown in the figure, the steady-state conversion showed a sharp decline on introduction of the poison. The conversion subsequently recovered slowly, perhaps due to the desorption of ammonia. It was also found that the BTX selectivities almost remained unaffected due to poisoning of the catalyst with ammonia (not shown in the plot). 3.6. Effect of Zn Content in Zn-Encilite. Figure 17
Benzene
I-
L"
n
i
16Ot 0
a
0.0
5
10 15 20 25 I N L E T CONCENTRATION OF n-BUTANE('Io)
30
Figure 16. Effect of inlet concentration of n-butane on the product distribution using n-butane in the feed over Zn-encilite at 550 "C (W/F= 2.2 g.h-mol-').
'ooy;
O%
'
60 ' ' 120 ' TIME ( m i n )
'
1BO
Figure 16. Effectofammoniapoieoningonthepropaneandn-butane conversion over Zn-encilib at 550 "C (feed gas: 15% propane/nbutane and the rest nitrogen; W/F= 2.2 g-hemol-1): @, propane conversion; 0 , n-butane conversion.
shows the results of the conversion and BTX selectivities of propane and n-butane on H-encilite and Zn-encilite with different weight percenta of Zn at 550 OC. The conversions of propane and n-butane both increased very slowly with Zn content in Zn-encilite and attained an
Ind. Eng. Chem. Res., Vol. 32, No. 6, 1993 1061
BO
i 4
20c
0.01 0.0
I
'
0.16
1
'
002
1
1
0.48
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0.64
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i -I
0.80
Z n Content (WTY.)
Figure 17. Dependence of propane and n-butane conversion and their respective BTX selectivitiesand yields on the degree of Zn2+ exchange at 550 "C (feed gas, 15% propane/n-butaneand the rest nitrogen; W/F= 2.2 ph-mol-1): XI propane conversion; 0 , BTX selectivity using propane as feed; 0 , n-butane conversion; +, BTX selectivity using n-butane as feed. Table I. Product Distribution for the Conversion of Propylene and Butene at a Reaction Temperature of 550 O C , W/F = 2.2 gphmol-l, and 15% Inlet Concentration of Propylene/Butene and the Rest NIas Diluent H-encilite Zn-encilite DroDylene butene Drowlene butene conversion (% ) 92.1 94.8 96.1 97.2 product distribution (wt % ) 2.2 1.7 methane 3.5 2.4 3.7 4.1 4.3 3.8 ethane ethylene 9.4 12.4 2.1 3.5 30.5 24.9 7.5 4.6 propane propylene 0.0 4.3 0.0 6.1 18.6 12.2 2.4 4.3 butane 8.7 14.6 21.5 benzene 18.4 toluene 12.1 18.2 35.4 34.1 m- and p-xylene 6.4 4.1 11.6 10.3 o-xylene 4.8 1.2 6.2 8.2 3.6 2.3 5.5 4.3 AS+
almost constant conversion at higher Zn content, whereas the BTX selectivities increased sharply with increasing Zn content. However, further improvement of activity was not achieved by increasing the amount of Zn in Znencilite catalysts. This may be because the number of ion exchange sites in H-encilite is limited by the H content in the catalysts. 3.7. Conversion of Propylene and Butene over H-Encilite and Zn-Encilite. Table I shows the conversion of propylene and butene over H-encilite and Znencilite under the conditions of 15% inlet concentrations of propylene and butene and WIF = 2.2 g-h-mol-' at a 550 OC reaction temperature. The results show that the aromatic selectivities over Zn-encilite were very much higher than those over H-encilite. This implies that the Zn component is more effective for the aromatization of the propylene and butene. 3.8. Catalyst Stability Study. The activity of the catalyst was also investigated on Zn-encilite for both propane and n-butane as reactants. During reaction, the activity of the catalyst remained more or less constant for 4-5 h depending on the reaction conditions. After this the activity decreased rapidly. After reaction, the catalyst was regenerated in situ with air at the reaction temperature of 550 "C for 4 h. Figure 18shows the effect of the number of reaction-regeneration
0.0 1.0 NO. OF CYCLES
2.0
3.0
OF REGENERATION
4.0
5.0
& REACTION
Figure 18. Effect of reaction-regeneration cycles on yield of BTX for propane and n-butane conversion: +, BTX yield using propane as feed; elBTX yield using n-butane as feed.
cycles on the yield of BTX. As shown, the activity of the catalyst was highly stable and also remained unaffected after several reaction-regeneration cycles. 4. Reaction Mechanism
The high activity of Zn-encilite for the conversion of propane and n-butane to aromatic hydrocarbons and the fact that the reactions were poisoned by ammonia leads to the conclusion that the reaction proceeds via the formation of carbenium ions. This implies that it is an acid-catalyzed reaction which relies on the strength of acid sites present in the catalyst. There is a possibility that the Zn loading in Zncontaining zeolite may enhance the propane and n-butane conversion. By the incorporation of Zn2+in the Zn-encilite catalysts, Figure 17 suggests that the total conversions of propane and n-butane were not improved by any significant amount. The marginal enhancement of conversion may be due to the presence of stronger strong acid sites in Zn-encilite, as shown in Figure 6. It was also observed that the nature of the acid sites (confirmed by the IR spectra of the pyridine-adsorbed zeolite samples, as shown in Figure 5) as well as the total amount of acid sites (obtained from the TPD of ammonia) almost remain unchanged by Zn2+loading in H-encilite. Therefore, it can be concluded that the sorbed C3 and C4 olefinic species were formed by hydride abstraction from propane and n-butane, respectively, and gave hydrogen directly.
where H+(s)and CnH+2n+l(g)denote the acidic sites and sorbed olefinic species on the zeolite, respectively. Figures 7 and 10 show that the total conversion as well as aromatic hydrocarbon selectivities increased with temperature and contact time, respectively. This implies that the aromatic hydrocarbonswere formed as secondary products. At lower temperatures the amounts of methane and ethane were larger than those of the aromatic products which may be due to the cracking of propane and n-butane. At higher temperatures the source of the small amount of lower alkanes may be the cracking of higher aliphatic hydrocarbons (formed by the oligomerization from intermediate C3 and Cq olefinic species) and dealkylation of substituted aromatic or dicyclic compounds. The yield of aromatic compounds was higher with Znencilite than with H-encilite as shown in Figure 17. This
1052 Ind. Eng. Chem. Res., Vol. 32, No. 6, 1993
implies that the Zn component enhanced the aromatization step and suppressed the cracking tendency of the intermediates. The yield of aromatics from propylene and butene over H-encilite and Zn-encilite, as shown in Table I, also gives additional support that the Zn component is effective only in the steps of conversion of intermediates to aromatics. 5. Conclusion The Zn component loaded in H-encilite takes a significant role in the aromatization step. Although Zn-encilite is not very active for propane, it is highly active and selective for the conversion of n-butane to BTX. The acidic property of the H-encilite is not affected by Zn ion exchange.
Acknowledgment We would like to thank IPCL, Baroda, for providing the encilite samples. Nomenclature n, = number of moles of propaneln-butane consumed nf = number of moles of propaneln-butane fed n h q = number of moles of hydrocarbon produced Nhcp= number of carbon atoms in the molecular formula of the produced hydrocarbon W/F= contact time Literature Cited Auroux, A.; Bolis, V.; Wienchowski, P.; Gravelle, P. C.; Vedrine, J. C. Study of the Acidity of ZSM-5 Zeolite by Microcalorimetry and Infrared Spectroscopy. J. Chem. SOC., Faraday Tram. 11979,75, 2544-2555. Batchelder, R. F.; Pennline, H. W.; Schehl, R. R.;Finseth, D. H. Catalytic Conversionof CaCr Paraffii to Gasoline. Energy Res. Abstr. 1985, 10 (4), Abstr. No. 6549. Bayense, C. R.; Van der Pol, A. J. H. P.; Van Hooff, J. H. C. Aromatization of Propane over MFI-Gallosilicates. Appl. Catal. 1991, 72,81-98. Bayense, C. R.; Van Hooff, J. H. C. Aromatization of Propane Over Gallium-containing H-ZSM-5 Zeolites. Influence of the Preparation Method on the Product Selectivity and the Catalytic Stability. Appl. Catal. A General 1991, 79, 127-140. Bragin, 0. V.; Shpiro, E. S.; Preobrazhemkii, A. V.; h e v , S. A.; Vaeina, T. V.; Dyueenbina, B. B.; Antoshin, G. V.; Mmachev, Kh. M. The Stateof Platinum in High-Silica Zeolites and Ita Catalytic Activity in Ethane and Propane Aromatization. Appl. Catal. 1966, 27 (2), 219-31. Bragin, 0. V.; Vaeina, T. V.; Isaev, S. A.; Kudryavteeva, G. A.; Sitnik, V. P.; Preobrazhenskii,A. V.Aromatizationof Ethane and Propane on Modified Pentasils. Zzv. Akad. Nauk SSSR, Ser. Khim. 1988, 1,32-6. Doolan,P.C.;Puiado,P.R.Make AromaticsfromLPG. Hydrocarbon R O C ~ S1'S9.8% Sept, 72-76. Engelene, C. W. R.; Wolthuizen, J. P.; Van Hooff, J. H. C. Reactions of Proeane Over a Bifunctional Pt/H-ZSM-5 Catalyst. Appl. Catal. -1985, 19 (l), 153-163. Furuya, M.; Nakajima,H. SynthesizingAromatic Hydrocarbons from Lower Aliphatic Hydrocarbons in Presence of ZSM-5 Zeolites. Jpn. Pat. 6314,732, Jan 21, 1988. Inui, T.; Okazumi,F.; Makino, Y. Synthesisof Aromatic Hydrocarbone from Light Paraffins on Pt/H-ZSM-5 Catalyst. Chem. Express 1986a, 1 (l), 53-6.
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Received for review February 23, 1993 Accepted March 5,1993
