Platinum-sulfated zirconia catalysts. Dependence of activity on sulfur addition method

~

AA PT PA LL E IY DSS C I A: GENERAL

ELSEVIER

Applied Catalysis A: General 144 (1996) 205-219

Platinum-sulfated zirconia catalysts. Dependence of activity on sulfur addition method Dennis E. Sparks, Robert A. Keogh, Burtron H. Davis * Center for Applied Energy Research, University of Kentucky, 3572 Iron Works Pike, Lexington, KY 4051 l, USA Received 20 November 1995; revised 5 March 1996; accepted 7 March 1996

Abstract Sulfur was added to hydrous zirconia using a number of methods. Sulfur was added using ammonium sulfate, H2S, SO 2 as well as H2SO 4. The impact of the method of sulfur addition on the properties of the catalysts is reported. The activity was evaluated for all catalysts prepared by measuring the conversion of n-hexadecane in batch reactors. The H 2 8 0 4 , ammonium sulfate and SO 2 methods of sulfur addition produced the most active catalysts. These catalysts were then studied in a trickle bed reactor. The hydrocracking and hydroisomerization yields were independent of the method of sulfur addition and strictly a function of conversion. One of the methods using SO 2 produced a catalyst which showed high activity for over 500 h on stream. Keywords: Zirconia; Sulfur addition; Catalytic cracking; Sulfated; Fe-Mn-ZrO 2, Pt-SO 2- -ZrO> Alkane isomerization; Hexadecane

1. I n t r o d u c t i o n

The preparation of zirconia by precipitation involves complex chemistry and may produce an 'amorphous' precursor that leads to either the tetragonal or monoclinic phase following calcination [1,2]. The stabilization of zirconia in the tetragonal or cubic phase is a complex subject and will not be described here [3,4]. Important for consideration for this manuscript is that the presence of sulfate on the surface will stabilize the tetragonal phase against transformation

[51. * Corresponding author. davis @alpha.caer.uky.edu.

Tel.:

( + 1-606)

2570251;

fax:

( + 1-606)

0926-860X/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S 0 9 2 6 - 8 6 0 X ( 9 6 ) 0 0 1 1 6 - 0

2570302;

e-mail:

206

(NH0~,.°>O4 + Metal Oxide

D.E. Sparks et al. /Applied Catalysis A: General 144 (1996) 205-219

0

Calcined at 723K Moisture

0

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evac.> 373K /

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nH~,O

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%'%

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Scheme 1.

A number of approaches have been offered for the addition of the sulfur species. Even when a common reagent is utilized, e.g., H2SO 4, a variety of procedures may be employed [6-9]. In spite of the variety of methods, it appears that the sulfuric acid adds by chemisorption (a chemical reaction) so that a similar structure should be formed by any impregnation method [8]. On the other hand, a sol-gel method may incorporate sulfate in the bulk as well as on the surface [10]. Several of the variety of techniques that have been utilized to prepare sulfated zirconia have been summarized by Yamaguchi [11] as shown in Scheme 1. It is implied by the common structure II in Ref. [11] that all preparative procedures lead to a material with the same catalytic properties. The present study utilized the conversion by isomerization a n d / o r cracking of n-hexadecane in an attempt to define the catalytic activities of zirconia catalysts in which sulfur is added by the approaches illustrated in Scheme 1.

2. Experimental One kilogram of zirconia was prepared by rapid precipitation from a 0.3 M aqueous solution, prepared from anhydrous zirconium tetrachloride, by adding while stirring vigorously sufficient 15 M ammonium hydroxide to produce a molar ratio of OH:Zr of about 10:1. The precipitate was washed four times with distilled water by repeated filtration and reslurrying cycles followed by four

D.E. Sparks et al. / Applied Catalysis A: General 144 (1996) 205-219

207

wash cycles using a 10-15% (by volume) ethanol/water mixture and a ninth cycle with a 90% ethanol/water. The filtrate for the final wash contained about 2 ppm chloride. The hydrous zirconia was then dried in air for 5 days at 110°C to obtain a constant weight. Glass wool plugs were used to situate 15 g portions of the dried zirconia near the center of a 1" diameter glass reactor, which had been wrapped with a coil of nichrome wire and insulation. Sulfur was added using two gas mixtures, one containing 5.08 mol-% hydrogen sulfide in nitrogen and the other containing 1.89 mol-% sulfur dioxide in nitrogen. The reactor was flushed with the sulfur-containing gas mixture before beginning heat-up. A septum fitted below the reactor allowed sampling of the exit gas, which was analyzed with a Varian 3700 packed column ( 6 ' × 0.125" Porapak Q) gas chromatograph. When the sulfur species began to appear in the exit gas, the flowrate was reduced to approximately 20 m l / m i n and heat-up commenced. Three sulfur addition runs at temperatures ranging from 20 to 400°C were made with each of the two gas mixtures. The exit gas SO 2 (or H2S) concentration quickly fell to zero as the zirconia was heated, indicating retention of the sulfur. At breakthrough (usually 1-4 h), the exit gas concentration rose quickly to approximately the feed gas concentration. The tests were allowed to continue overnight for a total of 18-22 h. Despite drying the zirconia to a constant weight, it was later discovered to contain small amounts of ethanol, resulting in the formation of CO 2 and water during the H2S runs, plus the formation of H2S during the SO 2 runs. Free sulfur was observed at the bottom of the reactor after several of the runs. Additional runs were performed with a drying step which preceded the sulfation. The hydrous zirconia was loaded in the reactor and then heated to either 200 or 300°C in ultra high purity (UHP) nitrogen. After cooling the zirconia to room temperature, one of the sulfur-containing gases was substituted for the nitrogen and the sulfur was added to the zirconia as before. This pretreatment eliminated the formation of free sulfur and substantially reduced the production of CO 2 and water. Two other samples of the hydrous zirconia were sulfated using 0.5 M aqueous sulfuric acid or 0.5 M aqueous ammonium sulfate. Using 15 ml of 0.5 M solution per gram of zirconia, the slurries were stirred for 6 h, then filtered and dried in air overnight at 110°C. Pt was added to each of the sulfated zirconia samples by the incipient wetness technique using aqueous chloroplatinic acid solutions sufficient to add 0.6 wt.-% platinum. The finished catalysts were dried overnight in air at 110°C and then ground to - 100 mesh. The catalysts are designated to reflect both the sulfur source and the sulfating conditions. Zr is used to designate the precipitated zirconia that had been dried at 110°C, while DZr is used to indicate a zirconia with additional drying at 200-300°C in UHP nitrogen immediately prior to sulfation. The zirconia

208

D.E. Sparks et al. / Applied Catalysis A: General 144 (1996) 205-219

designation is followed by the sulfur source, indicated by / S O 2 a n d / H 2 S for the two gases, by / A c i d for H2SO 4, and by / A S for ammonium sulfate. The final part of the designation is the highest temperature used in preparing the catalyst. For example, the catalyst designated D Z r / H z S / 3 0 0 was a zirconia with an additional drying step (at 300°C for 2 h in UHP nitrogen) that was sulfated with H2S at a temperature of 300°C. 2.1. Batch reactor experiments

The batch reactor runs were used to identify the most promising catalysts for further evaluation in a trickle bed reactor. The catalysts were activated using a temperature of 725°C in air for 2 h. The activated catalyst was placed directly into a dried batch reactor while still hot. Sulfur contents of the catalysts before and after activation were measured using Method C of ASTM D-4239 with a Leco Corporation SC-432 sulfur analyzer. The reactor was placed in a desiccator to cool to room temperature. Dried n-hexadecane (4 g) was loaded into the reactor, which was then purged of air by six cycles of pressurizing to 500 psig with UHP hydrogen and slowly venting. After a final 500 psig hydrogen charge, the reactor was plunged into a 150°C fluidized sand bath for 15 min. The reactor was shaken vertically in the sand batch, and after removing from the heated batch, it was plunged into a room temperature sand batch to cool it to ambient in less than 2 min. After slowly venting the pressure, the liquid reaction products were filtered to remove the catalyst and analyzed by capillary gas chromatography. Gas product samples were not collected and therefore no mass balances attempted, conversion being defined as 100 minus the GC area percent for n-hexadecane in the liquid product. Catalyst production conditions, surface areas, sulfur contents, and batch reactor activities are tabulated in Table 1. 2.2. Runs in continuous trickle bed reactor

The most active of the catalysts that had been sulfated with SO 2 and H2S as determined by the batch reactor studies, Z r / S O 2 / 3 0 0 and D Z r / H 2 S / 2 0 0 , were selected along with Z r / A s for testing in a trickle bed reactor. The stainless steel reactor (0.5" o.d. (0.035" wall thickness) × 16" long) was jacketed with a 1 inch o.d. aluminum sheath and positioned vertically in a single zone tube furnace with a 12" heated zone. The catalyst bed (typically 4" deep) was centered in the middle of the furnace and held in place by glass wool plugs. A coaxial 0.125" o.d. thermowell allowed a thermocouple to be positioned at the midpoint of the catalyst bed. Hydrogen delivery was regulated with a mass flow metering valve. A high pressure syringe pump was used for precise metering of liquid feedstock. The

209

D.E. Sparks et al. / Applied Catalysis A: General 144 (19961 205-219 Table I Catalyst characterization Catalyst

Zr/H~S/300 Z r / H 2S / 4 0 0 Zr/H2S/725 a Zr/SO 2/20 Zr/SO 2/300 Z r / S O z/20(I DZr/SO 2/200 b DZr/H2S/300~ D Z r / H 2 S / 2 0 0 ,1 Zr/Acid Zr/AS ~ b ~ d

Sulfur

Sulfating

Sulfur content (wt.-%) Surface area (m 2 / g )

source

temp. (°C)

conversion (c~) Before After Before After activation activation activation activation

H2S H ~S H2 S SO, SO, SO, SO, H.S H2 S H 2 SO4 (NH4)2SO 4

300 400 400 20 300 200 200 300 200 20 20

2.97 2.23 0.75 3.58 2.50 3.86 2.66 1.89 2.40 3.88 1.61

0.79 0.76 0.28 0.66 0.74 0.67 0.66 0.71 0.84 1.17 1.12

140 85 2{1 260 185 220 197 171 228 181 274

76

65 67

n - C 16

14.08 0.55 2.61 46.21 56.88 27.29 54.57 3.32 20.32 62.02 69.67

Z r / H 2 S / 7 2 5 calcined at 725°C for 2 h in air before sulfating. D Z r / S O z / 2 0 0 dried at 300°C for 2 h in N 2 before sulfating. D Z r / S O 2 / 3 0 0 dried at 300°C for 2 h in N 2 before sulfating. D Z r / H 2 S / 2 0 0 dried at 200°C overnight in N. before sulfating.

liquid product was collected in an ambient temperature receiver located below the reactor, with the exit gas flowing through a back pressure regulator before being vented. To reduce the opportunity for water contamination, 3 g of freshly activated catalyst was loaded while still hot into a reactor that had been preheated in a 70°C oven. The loaded reactor was then quickly installed in the furnace, connected to the feed lines and receiver, and purged with hydrogen. After pressure testing and establishing the desired reactor pressure and hydrogen flowrate, the reactor was slowly heated to 150°C before starting a flow of anhydrous n-hexadecane. The initial conditions for each run were 100 psig, 1.0 g / h n-hexadecane per gram of catalyst, and a hydrogen to n-hexadecane molar feed ratio of 3:1. Liquid product samples were collected at 1-3 h intervals. When the analysis of these samples indicated a constant conversion level, a gas sample was collected. The feed rates and/or reactor pressure were then adjusted to attain another conversion level, returning to the original conditions occasionally to check for catalyst aging. Typically, the initial steady-state condition was attained during 12-16 h, with subsequent steady-state conversion levels requiring somewhat less time, usually 6 - 1 0 h. Total hydrocarbon mass recoveries were typically greater than 98% for conversions up to about 70%, where about 95% of the product was still being captured as a liquid. Conversions were calculated using the mass recovery of n-hexadecane in the liquid product.

210

D.Iz: Sparks et al./Applied Catalysis A: General 144 (1996) 205-219

2.3. Product analysis Liquid and gas samples were analyzed using an HP 5890 gas chromatograph equipped with a 60 m DB5 capillary column and a flame ionization detector. Since this system is incapable of detecting hydrogen, the hydrogen content of the gas samples was determined using a Carle 311H gas chromatograph calibrated with a primary gas standard. The total gas composition was then calculated using this value and the hydrocarbon distribution reported by the HP 5890.

3. Results The influence of sulfate upon the retention of surface area is evident from the data in Fig. 1. For example, the surface area of a zirconia sulfated with H z S O 4 is about 4.5 times that of unsulfated zirconia when both samples are calcined at 600°C. The treatment of hydrous zirconia with either H2S or SO z at an elevated temperature produces a material that has a surface area corresponding to that of an unsulfated sample calcined at the same temperature. Thus, it appears that the loss of surface area is more rapid than the addition of sulfur or, most likely, that the added H2S or SO 2 is not present in a form that stabilizes the zirconia against loss of surface area. These data show that the various preparative procedures in Scheme 1 do not lead to a common catalyst.

350

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500

600

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Temperature (°C) Fig. I. The effect of sulfur addition and temperature of activation upon the surface area of Zr catalysts ( O , Z r / H 2 S ; z~, D Z r / H 2 S ; , , Z r / S O 2 ; O, D Z r / S O 2 / 2 0 0 ; A, unsulfated zirconia; A, Z r / A c i d ) .

D.E. Sparks et al./Applied Catalysis A: General 144 (1996) 205-219

21 I

As shown by the data in Table 1, there is little correlation between the sulfur held by the sample prior to calcination and the level of sulfur in the sample following calcination. 3.1. Batch reactor studies

An earlier series of experiments utilized portions of Z r / a c i d catalyst in which the activation times at 725°C were varied from 15 to 120 min. The sulfur content of these five samples ranged from a low of 0.73 wt.-% after 120 rain to 1.62 wt.-% at 15 min. These five samples showed a linear relationship between sulfur content and conversion of n-hexadecane (Fig. 2). The conversion obtained with various portions of a large batch of catalyst prepared by the addition of sulfuric acid is very reproducible. However, the optimum calcination temperature may vary from one batch of catalyst to the next, even though seemingly identical preparative procedures were utilized. Almost all of the catalysts produced with either H , S or SO 2 exhibited a sulfur content of 0.7-0.8 wt.-% after activation at 725°C for 2 h. Based on the earlier results (the straight line of Fig. 2), n-hexadecane conversions levels of about 70% were anticipated. However, the conversions for all of the catalysts prepared using H2S and SO 2 are lower than this expected value. While varying widely, all of the SO2-derived catalysts produced a higher conversion of n-hexadecane than those produced using H ~S. Catalysts Z r / A c i d 100 9o

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212

D.E. Sparks et al. /Applied Catalysis A: General 144 (1996) 205-219

and Z r / A S contained more sulfur after activation than the gas-sulfated catalysts, and exhibited slightly higher conversions than the best of the SO 2 catalysts (Fig. 2). The BET surface area (after 120 min at 725°C) of the most active SO2-treated catalyst was 76 m Z / g and the most active H2S-treated catalyst was 65 mZ/g, both higher than the 52 m 2 / g of the 120 min sample in the activation time test of the Z r / a c i d catalysts. Thus, the differences in activity should not be due to differences in the surface areas. In the screening studies, many portions of a common catalyst batch were activated separately, and a range ( + 15 m 2 / g ) of the measured surface areas were observed. 3.2. Trickle bed reactor studies

The conversions obtained during start-up under the same conditions for each of the four catalysts (Zr/acid, Z r / A S , Z r / S O 2 / 3 0 0 , D Z r / H 2 S / 3 0 0 ) increased for about 12 h before reaching a steady-state conversion level (Fig. 3). Comparing the data in Figs. 2 and 3 reveals what appears to be a contradiction since, in the batch reactor testing, Z r / a c i d was the most active catalyst, whereas in the trickle bed reactor Z r / S O 2 yielded the highest conversion. Prior experience using Z r / a c i d catalysts in the trickle bed reactor had been that the catalysts typically exhibited 30-40% conversion at the same operating conditions. Z r / A S and D Z r / H 2 S / 3 0 0 both achieved this same plateau, but surprisingly, the SO 2 catalyst achieved a 70% conversion at steady state.

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Fig. 3. n-Hexadecane conversions obtained for Zr catalysts ( n , Zr/AS; A, Zr/Acid).

©,

Zr/SO 2/300; I, DZr/H 2S/200;

D.E. Sparks et al./Applied Catalysis A: General 144 (1996) 205-219 80

213

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o & 2O 10

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i

i

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100 150 200 250 300 350 400 450 500 550 TOS (hr,)

Fig, 4. n-Hexadecane conversions obtained for the Z r / S O 2 / 3 0 0 catalyst.

In addition to the high initial conversion demonstrated by Z r / S O J 3 0 0 , this catalyst was also the most durable of the four tested. The performance of this catalyst when it was returned to the start-up conditions is indicated in Fig. 4. After studies at different feed rates, start-up conditions were repeated after 100 h, showing that the conversion had decreased to about 40%. Subsequent checks showed only a slow gradual decline to about 30% conversion after more than 500 h on stream. The catalyst also survived several shutdown/startup cycles with no loss of activity. The product distributions, in weight percent, are plotted against the conversion of n-hexadecane for the four catalysts in Fig. 5. Several process variables were altered to obtain the conversion levels shown. In the run with a catalyst Z r / a c i d the conversion was varied by altering the hydrogen to hexadecane molar feed ratio from 1:1 to 15:1 while maintaining a constant n-hexadecane feed rate. For the runs with the catalysts Z r / A S and D Z r / H 2 S / 2 0 0 , the range of conversions was obtained by altering both the n-hexadecane and hydrogen feed rates (n-hexadecane WHSV of 0.5 to 3.0 g / h per gram of catalyst) while keeping a constant hydrogen to n-hexadecane molar feed ratio of 3:1. For the catalyst D Z r / S O 2 / 3 0 0 , both the reactor pressure (100 to 735 psig) and the feed rates (n-hexadecane WHSV ranging from 1 to 6 with the hydrogen to nhexadecane ratio held constant at 3:1) were varied. The data (Fig. 5) show that the product distributions depend only on n-hexadecane conversion, and are independent of the type of sulfur addition or the process conditions studied. As n-hexadecane conversion increases, the amount of iso-hexadecanes increases to about 12 wt.-% of the total product, and then decreases at higher conversion

D.E. Sparks et al. / Applied Catalysis A: General 144 (1996) 205-219

214

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n-C~6 Conversion (wt. %) Fig. 5. T h e p r o d u c t d i s t r i b u t i o n s o b t a i n e d f o r Z r catalysts ( Q , DZr/H

Zr/Acid;

13, Z r / A S ;

O, Zr/SO 2/300;

II,

2S/200).

levels. Even at the highest conversion levels, neither methane nor ethane was detected in the gas product, and products with carbon numbers 14 and 15 were not present in significant quantities in the liquid product. With increasing conversion, the principal product gain was in the C 5 - C 9 carbon-number range, with the amount of both the C3-C 4 and C~0-C~3 carbon-number products increasing slowly. The carbon-number distributions of the cracked products (CI6 compounds excluded) were similar for all four catalysts (Fig. 6). The data in Fig. 6 were chosen from test periods during the four runs that had the same process conditions and exhibited similar conversion levels. The data for the four runs all show a maximum at C 7 - C 8 and all have a shoulder in the C10-C~3 carbonnumber region. The conversion level of n-hexadecane appears to exert only a minor, if any, influence on the i s o / n ratio of the C4-C13 products. The data in Fig. 7 show that the presence of water in the material that is sulfated with SO 2 plays a role in determining the amount of sulfur that is retained. A sample of material that has been dried in flowing nitrogen for 3 h at 300°C prior to sulfation does not retain as much sulfur as a material that is

D.E. Sparks et al. / Applied Catalysis A: General 144 (1996) 205-219 18

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Fig. 6. T h e c a r b o n n u m b e r d i s t r i b u t i o n s o f the c r a c k e d p r o d u c t s for Z r c a t a l y s t s ( O , Z r / A c i d ; Zr/SO 2/300; ,, DZr/H2S/300).

m, Zr/AS;

A,

heated to 200°C in the SO2/nitrogen mixture; both samples were exposed to SO2/nitrogen under similar conditions at 200°C. In addition, the moles of water eliminated from the sample was essentially the same as the moles of SO2

] SO 2 Feed Concentration / . . . . . . . . i m

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14

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Fig. 7. S O 2 c o n c e n t r a t i o n in the exit s t r e a m d u r i n g the s u l f u r a d d i t i o n o f an a s - r e c e i v e d h y d r o u s Z r ( O ) a n d a dried ( 3 0 0 ° C , 3 h) h y d r o u s Z r ( U ) catalyst.

216

D.E. Sparks et al. /Applied Catalysis A: General 144 (1996) 205-219

retained; this result suggests that SO 2 is nearly displacing an adsorbed water molecule. Thus, the sample dried at 300°C contains less molecular water and adsorbs less SO 2. The shape of breakthrough curves shown in Fig. 8 are typical for the adsorption of both H2S and SO 2. To confirm the apparent superiority of the SO2-derived catalyst, the test was repeated. A sample (25 g) from another 1 kg batch of precipitated zirconia was sulfated and had platinum added using the same procedures and conditions as used to make Z r / S O 2 / 3 0 0 . When tested in the batch reactor after activation for 2 h at 725°C, it exhibited a very poor conversion of n-hexadecane (4%). When this same catalyst was activated for 2 h at 650°C, it demonstrated a batch reactor conversion of 73%, some 16% higher than the conversion obtained with the first Z r / S O 2 / 3 0 0 sample. When tested in the trickle bed reactor, this second Z r / S O 2 / 3 0 0 catalyst achieved about 30% conversion of n-hexadecane when activated at 725°C, rising to the 70% conversion with the lower activation temperature (Fig. 8). Thus, the preparation in which sulfur is added as SO 2 introduces factors which have not been defined adequately in our studies. The data generated in the batch reactor are reproducible, and represents a minimum of duplicate measurements. These tests show the catalyst prepared with sulfuric acid to be the most active, with some of those samples prepared with SO 2 approaching the activity levels defined by the catalysts prepared with H2SO 4. However, in the trickle bed reactor the catalyst prepared from SO 2 appears to yield at times a material that is more active than those prepared by other procedures. Furthermore, it appears that the sample prepared using SO 2 retains its activity for a longer period. The preparation of a catalyst that 80 j

•

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mm|

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mm

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o 20 1o

10

15

20

25

30

TOS (hr.) Fig. 8. Activity of Z r / S O 2 / 3 0 0 when activated at 650°C ( O ) and 725°C ( • ) for 2 h.

D.E. Sparks et al. / Applied Catalysis A : General 144 (1996) 205-219

217

produces 20 to 40% conversion of n-hexadecane in the trickle bed reactor at the low pressure (100 psig) conditions used in this study can be accomplished using any of the preparative methods outlined in the experimental section. We have not, however, defined the reason for the samples prepared using SO 2 having, for some preparations, a substantially higher activity and a much greater resistance to catalyst aging.

4. Discussion The data clearly show that the surface area of unsulfated zirconia declines uniformly from about 300 mZ/g to less than 20 m 2 / g as the material is heated for 4 h at temperatures to 700°C. On the other hand, the surface area of a material sulfated with H2SO 4 is 2 to 7 times higher than that of the unsulfated zirconia for the same calcination temperature. The materials that were sulfated by SO 2 or HzS, when calcined, yield surface areas that fits the curve defined for the unsulfated zirconia. Thus, the sulfur added in this manner does not interact with the zirconia to stabilize the surface area. Nearly complete retention of HzS or SO 2 by the hydrous zirconia is observed until the material becomes saturated and breakthrough occurs after 2 - 4 h to attain the same sulfur level in the exit stream as was present in the feed. Moreover, the moles of water evolved roughly equals the moles of H 2S or SO 2 retained during the adsorption process indicating that either the HzS or SO. is displacing an adsorbed water molecule or that the gas is inducing a reaction such as the idealized ones depicted below: ZrO(OH)2 + SO 2 --~ ZrO(OSO2) + H 2 0

(1)

ZrO(OH)2 + 2H2S ~ ZrO(SH)2 + 2 H 2 0

(2)

Presumably the structures such as idealized in reactions (1) and (2) would undergo further oxidation to produce the sulfate group during the calcination procedure. It was reported [8] that sulfuric acid reacts with the hydrous zirconia to release water; however, the moles of water lost per mole of sulfur added decreases with increasing loading of sulfur. Presumably, the acidity of the sulfur acid solution is sufficient to catalyze dehydration of the hydrous zirconia. It is surprising that the materials calcined at 650 or 725°C retain a similar amount of sulfur; thus, the amount of sulfur retained is not directly related to the surface area of the calcined material. For the catalyst prepared using sulfuric acid, it appears that 15 min calcination produces an active catalyst. Furthermore, longer calcination times up to 2 h results in a decline in the sulfur content; the conversion of n-hexadecane and the sulfur content is related linearly. This latter observation implies that all, or a

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constant fraction, of the sulfur present in the catalyst contributes to n-hexadecane conversion. The catalyst prepared by adsorbing SO 2 had low activity compared to the same material that had been activated at 650°C in air. Thus, a high temperature calcination appears to be necessary for producing the 'superactive' catalyst. Invariably, the material prepared by adding sulfur as H2S produced a catalyst that exhibited lower activity in the batch reactor testing. When the catalysts were tested in a trickle bed reactor a gradual increase in activity with time was observed during the initial start-up. There appears to be an induction period of about 10 h for the material to attain the highest activity. Following the induction period most catalysts exhibited a period of constant activity that could extend up to 600 h. In some few instances, as illustrated in Fig. 4, a few catalysts exhibited a period of 'superactivity' that could last for up to 50-100 h. Following this period of superactivity the activity declined to a level of stable activity that resembled that which was observed with nearly all catalysts tested in the trickle bed reactors (approximately 40% n-hexadecane conversion for the 'standard' conditions). Assuming that all sulfur atoms present in the catalyst provide active sites of equal activity, this corresponds to the conversion of 177 molecules of n - h e x a d e c a n e / S / h (2.9 s-~). This activity level is about l 0 3 times that reported for F e / M n / S O 4 / Z r O 2 catalysts for the conversion of n-butane [12]. The selectivity, defined by: (1) the fraction of isomerized C ~6 in the conversion of n-C 16, (2) the carbon number distribution of the cracked products, or (3) the i / n ratio of the cracked products, did not depend upon the method of sulfur addition. Thus, each preparative procedure appears to provide similar catalytic sites in the calcined material. It appears that the conversion follows first order kinetics, at least for conversions in the range 0 to 50% conversion. At higher conversion levels, the data deviate from first order kinetics, and this is expected for a heterogeneous reaction that involves chemisorption.

5. Conclusions The amount of sulfur retained by Pt-containing zirconia catalysts depends upon the temperature that the material is dried prior to sulfur addition and the form of the added sulfur. Adding sulfur as sulfuric acid appears to result in an interaction that inhibits surface area loss to a greater extent than samples prepared by the addition of SO 2, HzS o r N H 4 S O 4. The sulfur added by use of the latter three compounds does not stabilize the surface during the initial periods of the thermal activation process; thus, the surface area of materials prepared using these three reagents was the same as an unsulfated zirconia that had been subjected to the same heat treatment. In spite of the differences in the

D.E. Sparks et al. /Applied Catalysis A: General 144 11996) 205-219

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surface area of the activated materials, they contained a similar amount of sulfur following activation. Furthermore, each preparative procedure appears to provide similar catalytic sites in the activated material. A catalyst prepared using SO 2 or H2S had to be activated in air at a high temperature in order to have an active catalyst. Some of the catalysts exhibited 'superactivity' for up to 100 h or longer. Assuming all sulfur atoms are on the surface, the conversion rate at 150°C for n-hexadecane was 2.9 s-I.

Acknowledgements This work was supported by the DOE contract #DE-AC22-90PC90049 and the Commonwealth of Kentucky.

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