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Influence of zeolite structure on ethylbenzene transformation
Zeolites and Related Materials: Trends, Targets and Challenges Proceedings of 4th International FEZA Conference A. Gédéon, P. Massiani and F. Babonneau (Editors) © 2008 Elsevier B.V. All rights reserved.
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Influence of zeolite structure on ethylbenzene transformation P. Moreaua, N. S. Gnepa, P. Magnouxa, E.Guillonb, S. Lacombeb, M. Guisneta a
UMR 6503, Catalyse en Chimie Organique, Faculté des Sciences, 40, avenue du Recteur Pineau, 86022 Poitiers Cedex, France b IFP-Lyon, BP3-69390 Vernaison, France.
Abstract Ethylbenzene (EB) transformation was carried out on bifonctional catalysts based on 10MR zeolites (ZSM-5, Ferrierite, ZSM-22, EU-1) and compared to Mordenite based catalysts. This work shows that monodimensional (1D) 10MR channels or large cavities are highly selective towards isomerization. For 10MR(1D) zeolites, this selectivity is attributed to microporosity blockage suggesting a pore mouth catalysis. Keywords: ethylbenzene isomerization, zeolite
1. Introduction Isomerization of C8 aromatic cut resulting from naptha reforming or steam cracking is the main route of para-xylene production. Para-xylene is the major precursor for the manufacture of fibers and polyester films. The C8 aromatic cut contains xylenes but also ethylbenzene.The goal of the isomerization unit is to trasnform meta and orthoxylene into thermodynamic equilibrium mixture of xylenes, while transforming EB into valuable products, benzene through dealkylation or xylenes through isomerization. Whereas xylene isomerization occurs through acid catalysis, bifunctional metal-acid catalysis is required for EB isomerization [1,2]. Large differences in EB transformation onto zeolites could be partly explained by the various pore structure of zeolites, acid strength and steric limitation [2,3]. Recent work [4] showed that EU-1, an intermediate pore size (10MR) monodimensional zeolite, leads to very high isomerization selectivity during EB conversion. This results from the blockage by carbonaceous deposits of the access to the inner sites of micropores. The objective of this work is to determine the influence of the porous structure (size and shape) and acidity (number and strength of the acid sites) on isomerization selectivity during the conversion of ethylbenzene on bifunctional catalysts Pt-Al2O3/ 10 MR zeolite. The transformation of EB was carried out on intimate mixtures of Pt/Al2O3 (PtA) and 10 MR zeolites (ZSM-5, ZSM-22, Ferrierite, EU-1) catalysts and compared to Mordenite reference catalyst activity.
2. Experimental Four 10MR zeolites in H-form were studied: ZSM-5(MFI) (Alsipenta) (Si/Al=13.5), Ferrierite (FER) (Si/Al=10) from Zeolyst , ZSM-22(TON) (Si/Al=33) and EU1(EUO)(Si/Al=15) synthesised by IFP. One sample of Mordenite (MOR) (Si/Al=12) was also studied as 12MR reference catalyst. The bifunctional catalysts were obtained by milling a mixture of 1%(wt)Pt/Al2O3 (1PtA) with zeolites (90%wt PtA/ 10%zeolite) then pelletizing and sieving at 0.2-0.4mm. EB transformation was carried out in a fixed
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bed reactor under the following conditions: T=410°C, P=10 bars, Hydrogen pressure / Ethylbenzene pressure =4 and at various contact time. Reaction products are xylenes (isomerization), diethylbenzene (disproportionation), benzene and ethylene (dealkylation), naphtenes (hydrogenation) and cracking products (C3-C6). Selectivity is compared at the same level of EB conversion.
3. Results and discussion Initial activity (TOF) was measured on fresh catalysts and EB conversion was also followed with time on stream (table 1). ZSM-5 zeolite catalyst is respectively 40, 25 and 2 times initially more active for the EB conversion than the Ferrierite, ZSM-22 and EU-1 catalysts (table 1). Except for ZSM-5, high deactivation occurs on zeolite catalysts as shown by EB conversion drop at different contact time (table 1). Table 1. Initial activity for acid reactions and EB conversion (initial and after 8h) EU-1 Ferrierte ZSM-22 Mordenite ZSM-5 TOF (h-1) 750 25 135 440 1600 Initial EB conversion (%) (τ (s)) 1 EB conversion after 8h (%) (τ (s)) *
42 (415s)
38(4156s)
30(1660s)
27(240s)
62(415s)
29(415s)
22(4156s)
20(1660s)
17(240s)
68(415s)
* τ contact time (s)
On ferrierite, ZSM-22 and EU-1 zeolite catalysts, 10MR monodimensional zeolite structures (1D), the main reaction is the isomerization of ethylbenzene (figure 1a). ZSM-5, 10MR three-dimensional structure (3D) zeolite is very selective in dealkylation (90%) (figure 1b) and no deactivation was observed within 8 hours of reaction. This particular selectivity of the zeolite ZSM-5 can be partly explained by the presence of strong acid sites and its porous structure that on one hand promotes the containment of molecules in the pores (presence of 8-9Å cages at the intersection of channels) and on the other hand prevents the formation of coke and therefore pore blockage. 40
60 50
30
% désalkylation
% Isomérisation
35 25 20 15 10
40 30 20 10
5
0
0 0
20
40 X (%)
60
80
0
20
40
60
80
X (%)
Figure 1. Initial yields in (%) isomerization (a), dealkylation (b) towards EB conversion 1%PtA/EU-1 (), Ferrierite (), ZSM-22 (z) et ZSM-5 ().
As previously observed [4] on EU-1 catalyst deactivation leads to isomerization selectivity improvement (table 2) whereas dealkylation and disproportionation selectivity decreases. The same effect is observed for ferrierite and ZSM-22 catalysts to a lesser extent. Isomerization selectivity reach more than 70% for these catalysts after 8
Influence of zeolite structure on ethylbenzene transformation
1181
hours of reaction. For ZSM-5 and Mordenite zeolite catalysts, selectivity is unchanged with time on stream, respectively towards dealkylation (89%) for ZSM-5 and isomerization for Mordenite (68%) [5]. Table 2. Selectivity (%) for EB transformation on 1% Pt/Al2O3 (90%wt) / zeolite (10%wt)for fresh catalyst (and after 8 hours (into brackets), at EB conversion around 35%
Zeolite ZSM-5 Ferrierite ZSM-22 EU-1 Mordenite
Isomerization 1 (1) 66 (75) 68 (71) 54 (72) 69 (68)
Disproportionation 3 (3) 7 (5) 4 (4) 16 (12) 23 (25)
Dealkylation 89 (89) 19 (14) 12 (11) 15 (6) 5 (4)
Cracking 7 (7) 9 (6) 16 (14) 15 (10) 3 (3)
For 10MR zeolites (EU-1, Ferrierite, ZSM-22) the increased selectivity towards isomerization after deactivation is to be related to zeolite microporosity blockage (formation of coke) as confirmed by nitrogen adsorption experiments. To have better accuracy on microporous volume modification porosity quantification was realized over bifunctional catalyst containing a larger percentage of zeolite (45%) associated with a more hydrogenating component (4PtA) [6]. On stabilized catalyst it is showed that coke causes a significant decrease in porous volume at low nitrogen partial pressure assuming that coke deposits provoke a large blockage of the microporosity. These observations could be interpreted as a pore mouth catalysis. It was suggested that EU-1 fresh catalyst comprises two types of active sites, inner and external acid sites, the first ones which are non selective to isomerization and sensitive to deactivation, the second ones selective to isomerization but non sensitive to deactivation. Selectivity of inner acid sites could be estimated by difference between results obtained after 45 minutes and those obtained after 8 hours. These results are shown on table 3. Table 3. Initial selectivity (%) of inner acidic sites for zeolite catalysts (EB conversion ~35%)
Zeolite ZSM-5 Ferrierite ZSM-22 EU-1 Mordénite
Isomerization 2 18 40 17 68
Disproportionation 2 6 6 26 25
Dealkylation 81 48 23 31 4
Cracking 14 28 31 26 3
Initial inner acid sites isomerization selectivity is low for 10MR zeolites and high for Mordenite catalysts. This suggests that large 12MR channels of Mordenite are favorable to EB isomerization into xylenes in the zeolite microporosity. As for 10MR 1D zeolites, the isomerization selectivity improvement is correlated with the microporosity plugging, it is proposed that EB isomerization on these coked catalyst mainly occurs on the outer surface acid sites. For ZSM-5 10MR 3D zeolite catalyst, EB transformation occurs mainly inside zeolite microporosity and porosity remains unchanged during time on stream as well as the catalyst activity.
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4. Conclusion This work shows that monodimensional (1D) 10MR or large cavities (12MR) zeolites are highly selective towards isomerization. For 10MR(1D) zeolites, this selectivity is attributed to microporosity blockage suggesting a pore mouth catalysis. Among 10MR 1D zeolite, EU-1 zeolite exhibits higher performances for EB isomerization (activity and selectivity for xylenes).
References 1] J.M.Silva, M.F.Ribeiro, F.Ramoa Ribeiro, E.Benazzi, M.Guisnet, Applied Catalysis A: General 125 (1995) 15 [2] A.Corma, Zeolite Microporous Solids: synthesis, structure and reactivity, NATO ASI Series C, 352 (1992) 373 [3] F.Alario, M.Guisnet, Zeolites for cleaner technologies, Catalytic science Series, Imperial college Press, London, 3 (2002) 189 [4] F.Moreau, P.Moreau, N.S.Gnep, P.Magnoux, S.Lacombe, M.Guisnet, Micro and mesoporous Materials 90 (2006) 327 [5] F.Moreau, N.S.Gnep, S.Lacombe, E.Merlen, M.Guisnet, Appl. Catal. 230 (2002) 253-262 [6] P.Moreau, PhD University of Poitiers, (2005)
1179
Influence of zeolite structure on ethylbenzene transformation P. Moreaua, N. S. Gnepa, P. Magnouxa, E.Guillonb, S. Lacombeb, M. Guisneta a
UMR 6503, Catalyse en Chimie Organique, Faculté des Sciences, 40, avenue du Recteur Pineau, 86022 Poitiers Cedex, France b IFP-Lyon, BP3-69390 Vernaison, France.
Abstract Ethylbenzene (EB) transformation was carried out on bifonctional catalysts based on 10MR zeolites (ZSM-5, Ferrierite, ZSM-22, EU-1) and compared to Mordenite based catalysts. This work shows that monodimensional (1D) 10MR channels or large cavities are highly selective towards isomerization. For 10MR(1D) zeolites, this selectivity is attributed to microporosity blockage suggesting a pore mouth catalysis. Keywords: ethylbenzene isomerization, zeolite
1. Introduction Isomerization of C8 aromatic cut resulting from naptha reforming or steam cracking is the main route of para-xylene production. Para-xylene is the major precursor for the manufacture of fibers and polyester films. The C8 aromatic cut contains xylenes but also ethylbenzene.The goal of the isomerization unit is to trasnform meta and orthoxylene into thermodynamic equilibrium mixture of xylenes, while transforming EB into valuable products, benzene through dealkylation or xylenes through isomerization. Whereas xylene isomerization occurs through acid catalysis, bifunctional metal-acid catalysis is required for EB isomerization [1,2]. Large differences in EB transformation onto zeolites could be partly explained by the various pore structure of zeolites, acid strength and steric limitation [2,3]. Recent work [4] showed that EU-1, an intermediate pore size (10MR) monodimensional zeolite, leads to very high isomerization selectivity during EB conversion. This results from the blockage by carbonaceous deposits of the access to the inner sites of micropores. The objective of this work is to determine the influence of the porous structure (size and shape) and acidity (number and strength of the acid sites) on isomerization selectivity during the conversion of ethylbenzene on bifunctional catalysts Pt-Al2O3/ 10 MR zeolite. The transformation of EB was carried out on intimate mixtures of Pt/Al2O3 (PtA) and 10 MR zeolites (ZSM-5, ZSM-22, Ferrierite, EU-1) catalysts and compared to Mordenite reference catalyst activity.
2. Experimental Four 10MR zeolites in H-form were studied: ZSM-5(MFI) (Alsipenta) (Si/Al=13.5), Ferrierite (FER) (Si/Al=10) from Zeolyst , ZSM-22(TON) (Si/Al=33) and EU1(EUO)(Si/Al=15) synthesised by IFP. One sample of Mordenite (MOR) (Si/Al=12) was also studied as 12MR reference catalyst. The bifunctional catalysts were obtained by milling a mixture of 1%(wt)Pt/Al2O3 (1PtA) with zeolites (90%wt PtA/ 10%zeolite) then pelletizing and sieving at 0.2-0.4mm. EB transformation was carried out in a fixed
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P. Moreau et al.
bed reactor under the following conditions: T=410°C, P=10 bars, Hydrogen pressure / Ethylbenzene pressure =4 and at various contact time. Reaction products are xylenes (isomerization), diethylbenzene (disproportionation), benzene and ethylene (dealkylation), naphtenes (hydrogenation) and cracking products (C3-C6). Selectivity is compared at the same level of EB conversion.
3. Results and discussion Initial activity (TOF) was measured on fresh catalysts and EB conversion was also followed with time on stream (table 1). ZSM-5 zeolite catalyst is respectively 40, 25 and 2 times initially more active for the EB conversion than the Ferrierite, ZSM-22 and EU-1 catalysts (table 1). Except for ZSM-5, high deactivation occurs on zeolite catalysts as shown by EB conversion drop at different contact time (table 1). Table 1. Initial activity for acid reactions and EB conversion (initial and after 8h) EU-1 Ferrierte ZSM-22 Mordenite ZSM-5 TOF (h-1) 750 25 135 440 1600 Initial EB conversion (%) (τ (s)) 1 EB conversion after 8h (%) (τ (s)) *
42 (415s)
38(4156s)
30(1660s)
27(240s)
62(415s)
29(415s)
22(4156s)
20(1660s)
17(240s)
68(415s)
* τ contact time (s)
On ferrierite, ZSM-22 and EU-1 zeolite catalysts, 10MR monodimensional zeolite structures (1D), the main reaction is the isomerization of ethylbenzene (figure 1a). ZSM-5, 10MR three-dimensional structure (3D) zeolite is very selective in dealkylation (90%) (figure 1b) and no deactivation was observed within 8 hours of reaction. This particular selectivity of the zeolite ZSM-5 can be partly explained by the presence of strong acid sites and its porous structure that on one hand promotes the containment of molecules in the pores (presence of 8-9Å cages at the intersection of channels) and on the other hand prevents the formation of coke and therefore pore blockage. 40
60 50
30
% désalkylation
% Isomérisation
35 25 20 15 10
40 30 20 10
5
0
0 0
20
40 X (%)
60
80
0
20
40
60
80
X (%)
Figure 1. Initial yields in (%) isomerization (a), dealkylation (b) towards EB conversion 1%PtA/EU-1 (), Ferrierite (), ZSM-22 (z) et ZSM-5 ().
As previously observed [4] on EU-1 catalyst deactivation leads to isomerization selectivity improvement (table 2) whereas dealkylation and disproportionation selectivity decreases. The same effect is observed for ferrierite and ZSM-22 catalysts to a lesser extent. Isomerization selectivity reach more than 70% for these catalysts after 8
Influence of zeolite structure on ethylbenzene transformation
1181
hours of reaction. For ZSM-5 and Mordenite zeolite catalysts, selectivity is unchanged with time on stream, respectively towards dealkylation (89%) for ZSM-5 and isomerization for Mordenite (68%) [5]. Table 2. Selectivity (%) for EB transformation on 1% Pt/Al2O3 (90%wt) / zeolite (10%wt)for fresh catalyst (and after 8 hours (into brackets), at EB conversion around 35%
Zeolite ZSM-5 Ferrierite ZSM-22 EU-1 Mordenite
Isomerization 1 (1) 66 (75) 68 (71) 54 (72) 69 (68)
Disproportionation 3 (3) 7 (5) 4 (4) 16 (12) 23 (25)
Dealkylation 89 (89) 19 (14) 12 (11) 15 (6) 5 (4)
Cracking 7 (7) 9 (6) 16 (14) 15 (10) 3 (3)
For 10MR zeolites (EU-1, Ferrierite, ZSM-22) the increased selectivity towards isomerization after deactivation is to be related to zeolite microporosity blockage (formation of coke) as confirmed by nitrogen adsorption experiments. To have better accuracy on microporous volume modification porosity quantification was realized over bifunctional catalyst containing a larger percentage of zeolite (45%) associated with a more hydrogenating component (4PtA) [6]. On stabilized catalyst it is showed that coke causes a significant decrease in porous volume at low nitrogen partial pressure assuming that coke deposits provoke a large blockage of the microporosity. These observations could be interpreted as a pore mouth catalysis. It was suggested that EU-1 fresh catalyst comprises two types of active sites, inner and external acid sites, the first ones which are non selective to isomerization and sensitive to deactivation, the second ones selective to isomerization but non sensitive to deactivation. Selectivity of inner acid sites could be estimated by difference between results obtained after 45 minutes and those obtained after 8 hours. These results are shown on table 3. Table 3. Initial selectivity (%) of inner acidic sites for zeolite catalysts (EB conversion ~35%)
Zeolite ZSM-5 Ferrierite ZSM-22 EU-1 Mordénite
Isomerization 2 18 40 17 68
Disproportionation 2 6 6 26 25
Dealkylation 81 48 23 31 4
Cracking 14 28 31 26 3
Initial inner acid sites isomerization selectivity is low for 10MR zeolites and high for Mordenite catalysts. This suggests that large 12MR channels of Mordenite are favorable to EB isomerization into xylenes in the zeolite microporosity. As for 10MR 1D zeolites, the isomerization selectivity improvement is correlated with the microporosity plugging, it is proposed that EB isomerization on these coked catalyst mainly occurs on the outer surface acid sites. For ZSM-5 10MR 3D zeolite catalyst, EB transformation occurs mainly inside zeolite microporosity and porosity remains unchanged during time on stream as well as the catalyst activity.
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P. Moreau et al.
4. Conclusion This work shows that monodimensional (1D) 10MR or large cavities (12MR) zeolites are highly selective towards isomerization. For 10MR(1D) zeolites, this selectivity is attributed to microporosity blockage suggesting a pore mouth catalysis. Among 10MR 1D zeolite, EU-1 zeolite exhibits higher performances for EB isomerization (activity and selectivity for xylenes).
References 1] J.M.Silva, M.F.Ribeiro, F.Ramoa Ribeiro, E.Benazzi, M.Guisnet, Applied Catalysis A: General 125 (1995) 15 [2] A.Corma, Zeolite Microporous Solids: synthesis, structure and reactivity, NATO ASI Series C, 352 (1992) 373 [3] F.Alario, M.Guisnet, Zeolites for cleaner technologies, Catalytic science Series, Imperial college Press, London, 3 (2002) 189 [4] F.Moreau, P.Moreau, N.S.Gnep, P.Magnoux, S.Lacombe, M.Guisnet, Micro and mesoporous Materials 90 (2006) 327 [5] F.Moreau, N.S.Gnep, S.Lacombe, E.Merlen, M.Guisnet, Appl. Catal. 230 (2002) 253-262 [6] P.Moreau, PhD University of Poitiers, (2005)