ELSEVIER

Conversion

Applied Catalysis A: General 114 ( 1994) 287-293

of n-heptane over Pt- and Pd/SAPO-1 1

catalysts

M.A. Chaar’, J.B. Butt* Department of Chemical Engineering, Northwestern University. Evanston, IL 60208, USA

(Received 27 January 1994, accepted 9 March 1994)

Abstract

The conversion of n-heptane over a set of Pt- and Pd-/SAPO-I 1 catalysts has been investigated over the temperature range 300-500°C with a feed ratio (H,/HC) of 15 at 1 atm. Two additional Pt- based catalysts, modified by Na addition (Na/Pt of 0.50 and 0.87) were also studied. Major products were. cracked species (C,-C,), i-heptane, n-heptene and toluene. Minor amounts of methylcyclo- hexane, -hexene and ethylcyclopentane were also observed with the sodium-containing catalysts. Deactivation by coke formation was observed primarily in the 400 to 500°C range, however the selectivities for isomerization and cracking were not affected by cracking. The Pt/SAPO-1 1 catalysts were superior to the Pd formulations for activity, activity maintenance and ring closure selectivity. Na is an effective promoter for toluene formation, particularly at higher temperatures. The overall reaction pathways for these catalysts are consistent with a scheme proposed previously for n-hexane [M. Hoffmeister and J.B. Butt, Appl. Catal. A, 82 (1992) 1691.

Keywords: SAPO; Hydrocarbons; Aromatics; Pt; Pd

1. Introduction

The use of SAPO-11 and metal-loaded SAPO-11 as catalysts for hydrocarbon conversion reactions has been reported for a number of examples. We have previ- ously reported significant selectivity for benzene formation from methylcyclopen- tane (MCP) at 400°C on either Pt- or Pd/SAPO-1 1 (ca. 1 wt.-% metal) prepared via wet impregnation (WI) from an aqueous solution of the nitrate [ 11. Benzene formation was not observed with n-hexane as the reactant for the pure-metal-loaded SAPO-11, but the addition of l-2 wt.-% Na from NaI$(aq) promoted benzene

*Corresponding author. Tel. ( + l-708)4917620, fax. ( + l-708)4671018. ‘Present address: Faculty of Chemical and Petroleum Engineering, Al Baath University, PO Box 77, Horns, Syria.

0926-860X/94/$07.00 @ 1994 Elsevier Science B.V. All rights reserved cQnrnn?r ornv,nr\nnn*Q 1,

288 M.A. Chuar, J.B. Butt/Applied Catalysis A: General 114 (1994) 287-293

selectivity to about half the level observed with MCP on the unpromoted catalysts. This is a significant change. Cracking selectivities followed an opposite pattern, decreasing by a factor of about 2 on the Na-modified catalysts.

The higher selectivity for benzene seen in ref. [ l] was shown to correspond to a lower level of protonic acidity [Lewis acidity was not affected [ 211. Thus, direct ring-closure of n-hexane can lead to either MCP or cyclohexane, with benzene from the latter via metal-catalyzed dehydrogenation.

The present work is to investigate specifically whether the pattern of aromatic selectivity, including the dependence on sodium content in the catalyst, is continued for the next molecule in the series, n-heptane. A primary interest here then is toluene formation.

2. Experimental

The catalysts were prepared according to the methods described in ref. [ 11, and the materials were the same. Metal-loaded catalysts were prepared by WI from the metal nitrate, as mentioned above. Catalysts containing sodium were prepared from WI samples by adding a concentrated NaN3 solution with Na+ at the level of 1 wt.- % to the point of incipient wetness. Further treatment was as in ref. [ 11. Two catalysts were also prepared via ion exchange from dilute solutions of the metal nitrates over a period of 95 h. Again, further treatment was as in [ 11. Properties of the catalysts are given in Table 1, although not all results with all catalysts are given here.

Reaction measurements were made in a conventional fixed-bed reactor, operated under low conversion conditions over the temperature range 300-500°C. For the

Table 1 Index of catalysts prepared

Prepared by wet impregnation (WIT

With Pt

1.2.0% Pt/SAPO-11 2. (2.0QPt + l.O%Na) /SAPO-11 3. (1.5%Pt+1.3%Na)/SAPO-11 4. 1.1% Pt/SAPO-11

With Pdb 1. 1.8%Pd/SAPO-11

Prepared by ion exchange (IE)

With Pt l.O.l4%Pt/SAPO-11

With Pdb 1.0.7%Pd/SAPO-11

“Metal loadings f 0.1%. ?kime catalysts as in ref. [ 11.

MA. Chaar, J.B. Butt/Applied Catalysis A: General 114 (1994) 287-293 289

experiments reported here a (HJHC) ratio of 15 was employed at 1 atm, with a space velocity (SC) of 200 cm3/g,, min. UHP hydrogen was used as supplied and the n-heptane (Aldrich, 99.7%) was stored over 4A sieves and used as required with no additional treatment. No conversion was observed in blank reactor runs at 400°C; there was less than 1% conversion over SAPO-11 with no metals at the same conditions, and this disappeared after 1 h on-stream.

A standardized pretreatment procedure was used. After introduction of the cat- alyst into the reactor a flow of oxygen, 20 cm3/min was established and the temperature ramped from 298 K to 723 K over 30 min, and then maintained at 723 K for 2 h. Oxygen flow was then terminated, followed by a helium purge at 100 cm3/min for 15 min at 723 K, after which hydrogen at 30 cm3/min was introduced and maintained for 2 h at 723 K. The reactor was then brought to reaction temper- ature over a period of O-30 min, depending on reaction temperature, in flowing hydrogen and n-heptane introduced into the feed stream. Additional details on operation and analytical methods are given in ref. [ 11.

3. Results

Typical results are shown in Table 2 for reaction at 400 and 500°C. Values quoted are for 3 h time-on-stream and represent essentially steady conversion levels. Replication of conversion/selectivity values in these experiments was typically within f 5% of the value shown, and all carbon balances were closed to within 97%.

Fig. 1 shows total conversion (wt.-%) on both the Pt/SAPO-11 and Na-added catalysts as a function of temperature. The addition of 1% Na to the 2% Pt/SAPO- 11 significantly promotes overall conversion [ (2%Pt+ l%Na) at 400°C is about the same as 2%Pt at 5OO”C], although the similarities in activity levels and in initial deactivation rates at the higher temperature suggest strong diffusional influence under those conditions.

Selectivity* patterns are informative. Cracking selectivities (C&J for Pt/ SAPO-11 and (Pt + Na) /SAPO-1 1 were constant with time-on-stream, and not very sensitive to temperature. This indicates little change in the acidic function of the catalyst with use, so the decay in activity pictured in Fig. 1 is the result of changes in the metallic function.

Toluene selectivity and yield follow a time-on-stream pattern at 400-500°C similar to that for conversion, with time scales for steady-state behavior also cor- responding. The toluene yield as a function of temperature is shown in Fig. 2 for examples of Pt and (Pt + Na) catalysts. This appears a rather moderate temperature dependence, since overall toluene yield is also governed by decreased conversion

*Selectivity is defined as (wt. i to j) /(wt. i reacted), and yield is (wt. i to j) / (initial wt. i), giving conversion BS (yield) /(selectivity).

290 MA. Chaar, J.B. Butt/Applied Catalysis A: General I14 (1994) 287-293

Table 2 Conversion and selectivity of n-heptane over metal-loaded SAPO-11 catalysts

Metal Conversion Selectivity to products (wt.-%)

(wt.-%)

C-C, i-C, n-C; MCH ECP TOL RC

MCHE DMCP

T= 500°C. 3 h time-on-stream O.l5%Pt 8.7 0.7%Pd“ 8.3

1 .8%Pd" 10.8

l.l%Pt 2.9

(1.5%Pt+ 1.3%Na) 10.0

2%Pt 15.1

(2%Pt+ l%Na) 13.8

T= 4&W, 3 h time-on-stream

O.l5%R 20.8

0.7%Pd 38.9

1.88Pd 16.5

l.l%Pt 1.3

( l.S%Pt + 1.3QNa) 10.2

2%Pt 40.5

(2%Pt+ l%Na) 15.7

71.7 0 0 0 0 28.3 28.3 66.6 15.3 0 0 0 17.6 17.6 83.9 0 0 0 0 16.0 16.0 39.5 0 0 0 0 60.5 60.5

3.6 0 9.8 52.7 5.5 28.3 86.5 57.6 3.0 2.9 2.6 2.0 31.9 36.5

10.1 5.4 5.5 36.2 17.9 24.9 79.0

53.6 42.0 0 0 0 0.9 0.9

48.8 45.0 0 0 0 6.2 6.2

63.7 33.0 0 0 0 3.8 3.8 67.1 21.1 0 0 0 0.4 0.4

13.8 17.8 10.3 8.4 24.0 21.7 60.1

31.3 52.4 1.7 0 1.3 6.8 8.1

17.0 51.5 4.3 5.0 9.1 13.1 21.2

“At 450°C.

All catalysts on SAPO-11

MCH = methylcyclohexane

MCHE = methylcyclohexene

ECP = ethylcyclopentane

DMCP = dimethylcyclopentane

TOL = toluene

RC = ring closure

,O? t

40 -‘+- +-+-

; ;\

+ + +-

!!

1: -&Q

+~~3&_*_

10 -

0’ 0 2 4 6 0 10

Time. h

Fig. 1. Comparison of weight-% conversion and deactivation for 2%Pt/SAPO-11 and (2%Pt + l%Na) ISAPO-

11 at 400 and 500°C. Space velocity = 200 cm3 (total feed, SC) /g_, mitt; H,/HC = 15. ( + ) 2%Pt/SAPO-11,

400”C,(O)2%Pt/SAPO-ll,5000C,(A)(2%pt+l%Na)/SAPO-11,400”C,( +)(2%Pt+l%Na)/SAPO-11,

500°C.

M.A. Chaar, J.B. Butt/Applied Catalysis A: General 114 (1994) 287-293

6

5

1

291

0 200 300 400 !soo 600

Tenparatue. C

Fig. 2. Toluene yield at steady activity level (3 h) for 2WPt/SAPO-11 and (2%Pt+ l%Na)/SAPO-11 as a fonctionofreaction temperature. ( + ) 2%Pt/SAPO-11, (A) (2%Pt+ l%Na)/SAPO-11.

at higher temperatures. An apparent activation energy for toluene formation here is about 5.0 f 0.2 kcal/mol. This number reflects the combined influences of both catalyst deactivation and pore diffusion, and the temperature behavior is similar to that of benzene formation in studies with n-hexane [ 1,3 1.

The other reaction contributing to conversion is isomerization to i-heptane (2- and 3-methylhexane) for which, for example, the 2%F@t/SAPO-11 exhibits mod- erate selectivity at 400°C (50%, Table 2)) but which decreases markedly at higher temperatures. This selectivity is also quite sensitive to the presence of sodium, as seen upon comparison of the 2%R and the Na-containing R catalysts in Table 2. This isomerization is also primarily acid catalyzed, as evidenced by its sensitivity to sodium level and the fact that its selectivity is independent of the extent of deactivation.

3.1. Low-temperature behavior

The results described above pertain to 400 < T < 500°C. For the “low-tempera- ture” region, 300 < T < 4OO”C, there was no significant deactivation of any of the catalysts either in effects on conversion or selectivity. Moreover, the overall con- version in this temperature range increases with increasing temperature (apparent activation energy based on steady-state n-heptane conversion on 2%FWSAPO-11 of 5.3 + 0.2 kcal/mol) , compared to a decrease for T> 400°C. These two regions of differing temperature behavior provide a good example of the importance of the combined effects of configurational diffusion [ 41 and deactivation on the operation of such catalysts in alkane conversion reactions*.

*This in spite of the fact that X-my and hydrogen chemisorption studies both indicate that the metals are located as crystallites on the surface of the SAPO-11 particles [ 1,3] and were thus relatively accessible (not within the intrinsic SAPO-11 pore network). The metal dispersion of several samples of 2%Pt- and l.WPd/SAPO-11 averaged about 50% exposed for both metals. X-ray diffraction from 2%Pt/SAPO- 11 gave a welldefined Pt,O, peak after calcination at 550°C 6 h.

292 M.A. Char. J.B. Butt/Applied Catalysis A: General 114 (1994) 287-293

50

(5) 40 - 1

s

6 3o ._

e

s

20 ; \

10 - +\+_

0 I

M

0.00 0.50 1.00 1.50

NdPt

Fig. 3. (a) Overall conversion at 400°C as a function of (Na/Pt) for Pt-based SAPO- 11 catalysts. (b) Selectivities as a function of (Na/Pt). (A) i-C,, ( + ) (C,-C,), (0) toluene, ( + ) n-heptane.

3.2. Platinum versus palladium

The discussion above has dealt primarily with Pt-based catalysts, since it would appear to be the metal of choice. Prior work with n-hexane indicated similar overall activity for the two metals on a per-weight basis, but with lower benzene selectivity for palladium. This pattern is continued with n-heptane. Palladium has lower selec- tivity for toluene, produces a much higher proportion of cracked products over the entire temperature range, and deactivates to a greater extent than platinum.

3.3. Influence of sodium

As mentioned above, there is a significant effect of sodium content on the performance of the Pt-based catalysts, revealed by comparison of 2%Pt-, ( 2%Pt + 1 %Na) - and ( 1 S%Pt + 1.3%Na) /SAPO- 11. These have weight ratios (Na/Pt) of 0,0.50 and 0.87, respectively. The conversion and selectivity patterns at 400-C (3 h) are shown in Figs. 3a and b, respectively, and seem well-correlated

MA. Chaar, J. B. Butt/Applied Catalysis A: General I14 (1994) 287-293 293

in terms of this ratio. Increasing sodium content is accompanied by decreases in both cracking and isomerization selectivity, and an increase in toluene selectivity. The decrease in cracking and isomerization selectivities here is more pronounced than that observed for n-hexane [ 11.

4. Conclusions

The reaction pathways on these SAPO-based catalysts are similar for n-hexane and n-heptane, with a pronounced promotion of aromatics formation upon the incorporation of sodium. This pattern would probably not be substantially different for higher n-alkanes, although the amount of incorporation into side chains might differ.

Some specific conclusions are: ( 1) The bifunctional nature of the catalysts is reflected in the difference in activity

decay vs. constant selectivity for cracking and isomerization. The behavior of toluene selectivity vs. time-on-stream is similar to that of overall conversion.

(2) Activity and selectivity comparisons among the catalysts investigated verify the preference for platinum over palladium both in terms of lower deactivation and higher aromatics selectivity.

(3) Product selectivities are significantly affected by the presence of sodium in P&based SAPO-11 catalysts.

(4) The combination of configurational diffusion and deactivation is a major factor in determination of the complex temperature behavior of these metal/SAPO- 11 catalysts.

Acknowledgement

We are indebted to Dr. J. Rabo for the SAPO-11 samples. Financial support was provided by the Alexander von Humboldt-Stiftung, Ministry of Higher Education, Syria, and Al Baath University.

References

] I] M. Hoffmeister and J.B. Butt, Appl. Catal. A, 82 ( 1992) 169. [2] S-J. Choungand J.B. Butt, Appl. Catal., 64 (1990) 173. [ 31 M.A. Chaar, Report MHC92, Dept. of Chemical Engineering, Northwestern University, 1992. [41 E.G. Derouane, J. Catal., 100 (1986) 541.