Influence of the ageing of zeolite coke on composition and deactivating effect

Studies in Surface Science and Catalysis, Vol. 139 J.J. Spivey, G.W. Roberts and B.H. Davis (Editors) 9 2001 Elsevier Science B.V. All rights reserved.

29

Influence of the ageing of zeolite coke on composition and deactivating effect H.S Cerqueira ~#, A. Rabeharitsara', P. Ayrault ~, J. Datka b, P. Magnoux ~, M. Guisnet ~'* "Universit6 de Poitiers, UMR CNRS 6503, Catalyse en Chimie Organique, 40, avenue du recteur Pineau, 86022 Poitiers cedex- France b Jagiellonian University, Department of Chemistry, Ingardena 3, 30-060 Cracow- Poland *Corresponding author. Tel : 33 5 49 45 39 05, Fax: 33 5 49 45 37 79 E-mail: michel. ~uisnet~univ-ooitiers. fr v

.

v

ABSTRACT The ageing of coke formed during m-xylene transformation at 250~ over a USHY zeolite was investigated under nitrogen flow at the reaction temperature. This ageing treatment causes a decrease in the zeolite coke content and an increase in the size and aromaticity of coke molecules. Pyridine adsorption followed by IR spectroscopy, carried out on the coked samples before and after ageing, shows that ageing causes a significant decrease in the concentrations of protonic and Lewis acid sites able to retain pyridine adsorbed at 150~ The deactivating effect of coke is also more pronounced after than before ageing. A change in the mode of deactivation from site poisoning to pore blockage is proposed to explain these observations. 1. INTRODUCTION Acid zeolites are used as catalysts in many refining and petrochemical processes and should play in the near future a significant role in the synthesis of speciality chemicals. During all these processes, heavy secondary products are formed within the zeolite cages or channels and remain there because of adsorption, of steric blockage, etc. These heavy products generally called coke are responsible for catalyst deactivation either because they poison the acid sites or because they block their access [1-7]. Because of the narrow size of the zeolite micropores, coke is constituted by relatively simple and not very polyaromatic molecules which can migrate slowly through the pores and can undergo various reactions during stripping and accidental stopping. We show here that a deep change in the composition of coke and on its effect on the zeolite acidity and activity can occur even for treatment under nitrogen flow at relatively low temperatures (250~ A USHY zeolite was chosen for this study, coke being formed during m-xylene transformation through isomerization and disproportionation at 250~ # Present address : PETROBRAS/CENPES/FCC,CidadeUniversitaria~Q7, lllmdo Fund,~o,CEP: 219494)00. Rio de Janeiro - Brazil.

30 2. EXPERIMENTAL The USHY zeolite ( N a o . 4 I - I 2 9 . 6 A 1 3 0 5 i 1 6 2 0 3 8 4 with 22.7 extraframework aluminium atoms) was obtained by calcination of a NH4Y zeolite (CBV 500, from PQ) under air flow at 500~ for 12h. The porosity and acidity characteristics were previously reported as well as the conditions for m-xylene transformation (Pm-~yl~e= 0.1 bar, PN2 = 0.9 bar, T=250~ [8]. Acidity of the samples was determined from pyridine adsorption followed by IR spectroscopy [8]. The carbon content was measured by total burning at 1020~ under helium and oxygen with a Thermoquest NA 2100 analyser. The method of recovering the coke from coked zeolite samples was already described [4]. Nitrogen adsorption measurements were performed at-196~ with the gas adsorption system ASAP 2010 (Micromeritics). The coked catalyst was ageing under nitrogen flow (101.h 1) at the reaction temperature. 3. RESULTS AND DISCUSSION 3.1 Conversion of m-xylene, coke formation and coke composition

Figure 1 shows that there is a very rapid deactivation of the USHY zeolite: m-xylene conversion passes from 55% at 2 minutes to 3% at 75 minutes. This deactivation can be related to the retention of carbonaceous compounds on the zeolite ("coke"). The coke content of the zeolite first increases very rapidly with time-on-stream then more slowly. 60

,,

10

0

40

6 ~

c 0

o,..

>

w 30 L_

4)

4) > e"

-

o 20

4

o

0

o

10

0

20

40

60

80

T O S (min)

Fig. 1. m-Xylene conversion and coke formation at 250~ over USHY as a function of timeon-stream (TOS). The composition of coke was determined at different coke contents through the method developed in our laboratory [4], i.e., dissolution of the zeolite in a hydrofluoric acid solution then recovery and GC/MS analysis of the part of coke soluble in methylene chloride. In this case, all the coke components are soluble in methylene chloride. It should be emphasized that practically no coke could be recovered in this solvent through a direct

31 soxhlet treatment of the coked zeolite samples, which shows that practically all the coke molecules are located within the zeolite pores. The composition of coke depends very much on time-on-stream (TOS), hence on coke content. At short TOS, coke is mainly constituted by methyl aromatic compounds with one to three aromatic rings. At 75 minutes (C wt% = 7.3), polyaromatic compounds with 4 to 7 tings appear at the expense of the primary coke components (Table 1). Table 1. Main Coke components retained on USHY at T=250~ for different TOS Family

Formula

Coke distribution (wt%) TOS =2min [TOS =18min [TOS =60min [TOS =75min

Q

(cH,~

[••

(CH~

n= 1-4

10

10

10

10

n----0-3

10

15

5

5

~ " ~

(cH~

n=3-6

20

[•••

(CH3)a

n=l-7.

10

5

5

5

n=0-4

50

40

45

30

25

20

20

5

5

15

10

15

(CH3)n C

~

~ ~

(ca,~.

n= 1-5

(CH3)n

n=0-7

(CH,)n n=l-6

~

n=l-7

32 3.2 Ageing of coke

After m xylene transformation for different times, the coked zeolite samples were aged under nitrogen flow for 1 or 6 hours at the reaction temperature. This treatment causes a decrease in coke content, this decrease being more especially pronounced as the coke content is low (Table 2). Table 2. Influence on coke content (C wt%) of ageing under nitrogen flow of zeolite samples coked for different times (TOS). TOS (min) Ageing Time (h) C wt% % removal

2

0 2.6 /

60 1 5.2 21

18

1 0.4 85

0 4.6 /

1 2.9-3.6 20-35

6 2.5-2.7 40-45

0 6.6 /

6 3.4 48

Ageing causes a large change in the IR coke bands. The effect of ageing on the absorbance of the four main bands in the 1655-1350 cm -1 region is shown in figure 2 for values of reaction times between 2 and 60 minutes and of ageing times of 1 and 6 hours.

B

g 15

5 3

on

8

m o m ;

"to

0.

2

[]

]

I

i=1

m

5

0

2

4

1

6

0

i

D

,

2

4

6

C (wt'/.)

C (wt'/,)

D

i

8

v Itl

8

mO 1 2

6

o.8

,~ ;

;

9

10

1.6

'r

[

I M

i~ 0.4

2

0 0

2

4 C (wt'/,)

6

8

0

2

,

.

4

6

c (wty.)

Fig. 2. Intensity of different band of coke vs coke content. (~) after reaction, (Q) after ageing. A: band at 1600 cm -1, B: band at 1505 cm -1, C: band at 1450 crn-1, D: band at 1355 cm 1.

33 The intensity of the band at 1600 cm ~ (the so-called coke band) which is considered as characteristic of polyaromatic species [3] increases slightly with ageing as well as those of the bands at 1450 cm1 and at 1355 crn-1 whereas the intensity of the band at 1505 crn1 which corresponds to less condensed aromatics [8] decreases. Therefore, in addition to the decrease in coke content there is an increase in the degree of aromaticity of coke. Similar observations have been previously made by various authors on zeolites [9-13] as well as on other acid catalysts [ 14-16]. However, the sweeping under inert gas flow of the coked samples was always carried out at temperatures much higher than in this work. This change in coke aromaticity was confirmed by comparing the coke composition before and after ageing of zeolite samples used for 2 and 18 minutes in m-xylene transformation. Coke formed at 2 minutes reaction (Table 1) contain approximately 50 wt% of poly-methyl-benzene, naphthalene, diphenylmethane and fluorene compounds (grouped in family A) and 50 wt% of anthracene or phenanthrene (family B). Ageing for 1 hour causes the complete disappeareance of compounds of family A except fluorene derivatives, the disappearance of 85% of compounds of family B and the formation of a small amount of polyaromatic compounds with 4 and 5 rings (family D). The atomic H/C ratio of coke, estimated from coke composition, is equal to 1.1 and 0.9 before and after ageing. The change in the composition of coke formed after 18 minutes reactions was also quantitatively estimated. Figure 3 shows the complete disappearance of B components, the increase in the amount of D compounds and the formation of very polyaromatic coke molecules insoluble in methylene chloride (I). Again, the atomic H/C ratio of coke is smaller after than before ageing (e.g. 0.75 after 6 hours ageing against 0.95).

1.5

3:

1

0.5

0

2

4

6

Time (h)

Fig.3. Percentage of the various coke families deposited after 18 minutes reaction at 250~ on the USHY zeolite as a function of the ageing time under nitrogen flow at the reaction temperature. (I) Insoluble part of coke. The change with ageing in coke amount and composition can be partly explained by the desorption of small and not very polar molecules of coke: e.g. compounds of family A or cracking products of coke components: e.g. C compounds can be easily cracked into A compounds:

34 H

~~*"'~~~

(CH3)n H+

(c)

(R+)

(a)

Furthermore, the formation of more polyaromatic compounds requires condensation within the pores between coke molecules or between coke molecules and molecules resulting from their cracking. Thus D compounds could result from the following scheme involving as first step the acid condensation of A and B compounds

~]'/+

R+

r

~

CH2

+

03)

-" R+'- RH ~

-H+

-'~ - U +

(D) The same type of reactions between the bulkiest coke molecules could be responsible for the formation of insoluble coke molecules. 3.3 Influence of ageing on the porosity and acidity of coked samples The effect of coke deposits on activity, acidity and porosity has been previously determined [17], the conclusion being that in the range of coke contents considered in this study (< 7wt%) deactivation was due to site poisoning. Adsorption experiments on the aged samples show that ageing does not significantly modify the effect of coke on the pore volume accessible to nitrogen. Therefore, it can be concluded that ageing does not cause blockage of the access of nitrogen molecules to the pore volume. However, a large effect of ageing on the concentrations of protonic and of Lewis acid sites able to retain pyridine adsorbed at 150~ can be observed (Table 3). Thus, for a sample coked for 18 minutes, despite the elimination of 45wt% of the coke, ageing for 6 hours causes a decrease of 45% of the Bronsted acidity and of 25% of the Lewis acidity. This means that the effect of coke content on both Bronsted and Lewis acidity is significantly more pronounced on aged than on coked samples. This is clearly shown in figure 4 representing the change in the concentration of Bronsted and Lewis acid sites as a function of nk, the number of coke molecules. An important remark is that, whereas before ageing, coke molecules have no effect on the Lewis acidity at low coke contents, they have a large effect with aged

35 samples. A decrease in Lewis acidity is also observed by ageing of samples coked for 60 minutes (Table 3). However, in this case a small increase in the Bronsted acidity is found, but here again the effect of coke content on the protonic activity is more pronounced with aged than with coked samples. Table 3. Acidity of coked zeolite samples before and after ageing. TOS: time on stream (min). AT: ageing time (h). Ca+, CL concentrations of Bronsted and of Lewis acid sites. Samples

C (wt%)

Acidity (lamol g-l) CH+

Coked TOS = 2 Coked TOS = 18 Aged AT = 1 AT = 6 Coked TOS = 60 Aged AT = 1 AT = 6

2.6

658

315

4.6

520

319

2.9 2.5

473 291

186 236

6.6

323

253

5.2 3.4

333 414

199 208

800

350

A

700

B 300

~. 600

~250

O

a

E 500 9

9

0

E

400

200

Q

--~ 150

-o :300 O o

CL

w ~

200

100

-J

50

100

0

,

,

200

400

nk 11~mol-g "1)

~

600

0

,

,

200

400

600

nk (IL mol.g "t)

Fig.4. Concentration of Bronsted acid sites (A) and Lewis acid sites (B) adsorbing pyridine at 150~ vs nk, the number of coke molecules. (~) after reaction, (121)after ageing.

36 This difference in the effect of coke on the protonic acidity, found with coked samples, could be related, at least for a part, to the displacement by pyridine molecules of weakly basic coke molecules from the acid sites. Indeed, this displacement was shown to occur more significantly from coked [8] than from aged samples [18]. However, the more pronounced effect of aged coke on the acidity seems to be mainly due to a blockage by the bulky coke molecules formed by ageing, of the access of pyridine molecules to the acid sites. In agreement with this proposal, the conversion of m-xylene is, at isocoke content, lower on aged than on coked samples e.g. with the zeolite sample coked for 18 minutes and aged for 6 hours which has a coke content of 2.5wt%, the conversion of m-xylene is equal to 33.5% instead of 55% on a non aged sample with the same coke content. CONCLUSION The treatment under nitrogen flow at 250~ of a USHY zeolite coked during m-xylene transformation at the same temperature causes a decrease in coke content and a large increase in the size and aromaticity of coke molecules and in their effect on acidity and activity. This shows that a particular attention should be paid in the choice of operating conditions during stripping and accidental stopping of reactors operating with zeolite catalysts. ACKNOWLEDGMENT The financial support of the CAPES Foundation (Brazilian government) is gratefully acknowledged. REFERENCES

1. L.D. Rollmann, D.E. Walsh, Progress in Catalyst Deactivation, NATO ASI Series E, (1982) p.81. 2. E.G. Derouane, Stud. Surf. Sci. Catal., vol. 20 (1985) p.221. 3. H.G. Karge, Stud. Surf. SCI. Catal., vol. 58 (1991) p.531. 4. M. Guisnet, P. Magnoux, Appl. Catal., 54 (1989) 1. 5. M. Guisnet, P. Magnoux, D. Martin, Stud. Surf. Sci. Catal., vol. 111 (1997) p. 1. 6. G.F. Froment, Stud. Surf. Sci. Catal., vol. 68 (1991) p.53. 7. P. Magnoux, P. Cartraud, S. Mignard, M. Guisnet, J. Catal., 106 (1987) 242. 8. H.S. Cerqueira, P. Ayrault, J. Datka, M. Guisnet, Micropor. Mesopor. Mat., 38 (2000)197. 9. D.M. Bibby, G.D. McLellan, R.F. Howe, Stud. Surf. Sci. Catal., vol. 34 (1987) p.651. 10. D.M. Bibby, R:F. Howe, G.D. McLellan, Appl. Catal. A, 93 (1992) 1. 11. B. Dimon, P. Cartraud, P. Magnoux, M. Guisnet, Appl. Catal. A, 101 (1993) 351. 12. Y. Boucheffa, C. Thomazeau, P. Cartraud, P. Magnoux, M. Guisnet, S. Jullian, Ind. Eng. Chem. Res., 36, N ~ 8 (1997) 3198. 13. K. Moljord, P. Magnoux, M. Guisnet, Appl. Catal. A, 122 (1995) 21. 14. R.G. Haldeman, M.C. Botty, J. Phys. Chem., 63 (1959) 489. 15. A.G. Gayubo, J.M. Arandes, A.T. Aguayo, M. Olazar, J. Bilbao, Chem. Eng. Sci., 48 (1993)2741. 16. C. Royo, J.V. Ibarra, A. Monzon, J. Santamaria, Ind. Eng. Chem. Res., 33 (1994) 2563. 17. H.S. Cerqueira, P. Ayrault, J. Datka, P. Magnoux, M. Guisnet, J. Catal., 196 (2000)149. 18. H.S. Cerqueira, unpublished results.