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Relationship between zeolite structure and hydrogen-transfer reactions in naphthenes and paraffins cracking
J. Weilkamp, H.G. Karge, H . Pfeifcr and W. HGldcrich (Eds.) Zeolires and Relaied Microporous Maieriols: Siale of ihe Ari 1994 Studies in Surfacc Sciencc and Calalysis, Vol. 84 0 1994 Elscvicr Scicncc B . V . All righis rcscrvcd.
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Relationship between zeolite structure and hydrogen-transfer reactions in naphthenes and paraffins cracking E. Benazzi, Th. Chapus, T. Cheron, H. Cauffriez and Ch. Marcilly. lnstitut FranGais du PGtrole, 1 & 4 avenue de Bois-PrGau, BP 31 1, 92506 RueilMalmaison, Cedex, France.
SUMMARY The limitation of hydrogen-transfer reaction in FCC units, when zeolitic additives are used, is desired to maximize the olefins production and to limit aromatics content of gasoline. In this work, the influence of zeolite structure on H-transfer reactions which occur during the cracking of n-hexane and a light gasoline cut (70-140°C) from hydrocracking, composed of a mixture of naphthenes and paraffins, was studied. The results obtained show that there is a link between the zeolite structure and the importance of H-transfer reactions in naphthenes and paraffins cracking. Such reactions appear as being dominant when using Y zeolite, and minimized when using MFI zeolite. Beta and Mordenite zeolites are intermediate and H-transfer reactions are less important in the case of Mordenite in comparison with Beta zeolite. The influence of the Si/AI ratio on the H-transfer reaction is also discussed. 1. INTRODUCTION
The 1990 Clean Air Act and the new forthcoming laws represent a challenge for the refining industry. The reduction in aromatics contents (1.O vol.% maximun benzene content) and the necessity of introducing a minimum of oxygenates compounds in motor fuel (2.0 wt.% minimum oxygen content) [ l ] will dramatically change gasoline composition and will need the use of new catalysts and processing technologies. Consequently, growing demands for gasoline from alkylation and for oxygenates compounds as MTBE (methyl t-butyl ether) and TAME (t-amyl methyl ether) are expected. This situation will require the production of larger amounts of isobutane, isobutylene, n-butenes and amylenes and will make that one of the main objectives of the FCC units will be to produce optimized quantities of light olefins and more particularly the C4 olefins. In order to increase the production of olefins it has been established that the use of zeolitic additives could be a successful way. For instance, use of MFI as additive to FCC catalysts increases propylene and very little C4 olefins production and allows to improve gasoline research octane number. The observed effect when using zeolitic additives depends on the H-transfer reaction which plays a very important role in catalytic cracking. In FCC units, the limitation of H-transfer reactions is desired to maximize olefins production and to limit aromatics content of gasoline. In this paper we studied the influence of zeolite structure on H-transfer reactions which occur during the cracking of n-hexane and a light gasoline cut (70-140°C) from hydrocracking, composed of a mixture of naphthenes and paraffins.
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2. EXPERIMENTAL 2.1. n-hexane cracking This study was carried out in a fixed-bed reactor at 300°C under atmospheric pressure using diluted n-hexane as feed (N2/nC6 molar ratio = 4) and using WHSV [2 -45 h-11. Four H-MFI solids with Si/AI ratio varying from 10 and 48 (Conteka and PQ corporation), three H-Mordenite samples with Si/AI ratio equal to 5, 10 (Tosoh Corporation) and 37 (obtained by a direct acid leaching of the mordenite possessing a Si/AI=lO), two H-Beta samples (one with Si/AI=l 1 prepared according to [2] and another one with Si/AI=19 dealuminated according to [3]) and one H-Y sample (Si/AI=17 obtained by steaming the NH4 form and followed by an acid treatement), were tested under powder form in this part of the study. 2.2. Cracking of light gasoline cut from hydrocracking This study was performed using a Micro Activity Test (MAT) unit, comprising a fixed-bed reactor containing 5 g. catalyst, heated at temperatures between 530 and 600°C and under atmospheric pressure. WHSV was kept constant (40 h-1) and Cat./Feed ratio was varied in the range 6 - 10. Mordenite and Beta zeolites were tested after being embedded with silica (50 wt. %/50 wt. Yo). Three catalyst samples were tested: one commercial FCC catalyst, containing a rare-earth exchanged Y zeolite (3.4 wt. % RE203 on catalyst), one embedded mordenite sample (Si/AI=80) and one embedded Beta sample (Si/AI =110). The commercial FCC catalyst was steamed at 73OOC prior to testing, bringing Y zeolite unit cell size to 24.36 A, which corresponds to Si/AI framework ratio of about 13. Composition of light gasoline cut, expressed in terms of distribution in hydrocarbons families, is given in Table 1, which shows the predominance of naphthenes ( 56 wt. Yo)- mainly C7 to C9- and paraffins ( 40 wt. Yo) - mainly C7 to C9 isoparaffins.
Table 1 Composition of light gasoline cut from hydrocracking (70 - 140°C), expressed in distribution by hydrocarbon family and carbon number (wt. Yo) C5 C6 C7 C8 C9 C10 C11 Total n Paraffins 0.17 2.18 3.13 2.48 0.83 0.10 8.89 i Paraffins 3.86 8.96 10.35 6.56 1.48 31.21 Naphthenes 0.08 5.43 17.17 21.63 9.98 1.56 55.85 Aromatics 0.29 1.14 1.62 0.88 0.09 0.03 4.05 Total 0.25 11.76 30.40 36.08 18.25 3.23 0.03 100.00 3. RESULTS AND DISCUSSION 3.1. Cracking of n-hexane It has been shown [4,5], that there are two modes of n-hexane cracking over zeolites, the monomolecular cracking route which involves a protolytic scission and the bimolecular classical route involving a P-scission. Their relative contribution depends on the reaction temperature, the aluminum content and the structure of the zeolites. Similarly, we can expect that the occurence of a secondary reaction like the H-transfer reaction will be strongly influenced by the pore structure.
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The results obtained (figure 1) show that, for a given time on stream and a nC6 conversion level, the paraffins/olefins ratio, which reflects in n-hexane cracking the importance of H-transfer reactions, reaches the highest values for H-Beta and H-Y zeolites, and the smallest ones for H-MFI samples, whereas intermediate values are obtained for H-mordenite samples. These observations can be related to differences in zeolites structure. Large pore sieves such as Y or Beta zeolites have a three-dimensional channels structure, that means enough space in order to form bulky bimolecular reaction intermediates involved in H-transfer reactions [6], whereas in MFI zeolite, which possesses a much smaller microporous volume, the formation of these species is possible but more difficult in comparison with the other zeolites. The case of the Mordenite is intermediate, since it is a large pore sieve (12 MR) but with only one-dimensional 12-rings channels and no cavity like Y zeolite. A similar evolution is observed if the iCUnC4 ratio is reported versus the zeolite structure. This ratio reflects, more particularly, the involvement of tertiary C4 carbenium ions in bimolecular H-transfer reactions.
figure 1 :
Evolution of the ParaffidOlefin ratio versus the zeolite structure and Si/A atomic ratio
The low level of H-transfer reaction observed for H-MFI samples, in comparison with the other zeolites, could be interpreted in terms of steric constraints. Indeed, the H-transfer reaction involves bulkier bimolecular transition state complexes which may problably form, in most cases, in the larger channels intersections [7]. Consequently, H-transfer reactions are limited for the MFI zeolite. Concerning the influence of the Si/AI ratio, the figure 1 evidences that whatever the aluminum content of the MFI solids (Si/AI between 10 and 48) the level of the H-transfer reactions remains constant and low, which is not in agreement with the hypothesis that H-transfer reaction should be favoured by two adjacent acid sites and so by a high lattice aluminum concentration [6]. These results suggest that in MFI type
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zeolite the factor which limits the H-transfer reaction is the steric constraint and that the H-transfer step would involve an adsorbed carbenium ion located at an acid centre and an incoming paraffin which gives the hydride ion to adsorbed carbenium ion (Eley-Rideal mechanism) [8]. In the case of the mordenite samples we can observe that the sample with the higher aluminum content (Si/AI=5) leads to the higher paraffin/olefin ratio indicating a high level of H-transfer reactions. This result would be agreed with the increase of this reaction with the aluminum content. But, the H-MOR sample possessing a Si/AI=37 presents a higher paraffidolefin ratio than the H-MOR (10). In the case of the H-MOR(37) sample, the presence of communications created between the main channels (along [OOl]) during the dealumination step of the H-MOR(10) sample by strong acid leaching could explain this result. Indeed, the defects created during the dealumination step could accomodate the bulky transition states of the H-transfer reactions. Nevertheless, the low aluminum content in the H-MOR(37) sample which would lead to a lower coke formation and so to weaker diffusional limitations could explain the higher H-transfer reactions level.
3.2. Cracking of light gasoline cut from hydrocracking
Table 2 shows the yield structures obtained by MAT catalytic tests performed at 530°C (CatJfeed = 10) with three different zeolite structures. Table 2: Influence of zeolite structure on yield structure obtained by cracking of a light gasoline cut (70 - 140°C) from hydrocracking (wt. %) Zeolite Y (FCC Cat) Mordenite Beta (Si/AI atomic ratio) (13) (80) (1 10) Temperature ("C) 530 530 530 530 530 .~ Cat.ifeed 6 10 10 6 10 Conv (N+P) (wt. %) (*) 36.9 37.9 41.9 34.1 28.7 H2 0.07 0.14 0.14 0.05 0.05 c1 0.52 0.33 0.41 0.44 0.1 4 c2 0.34 0.1 1 0.45 0.30 0.53 c2= 1.14 1.38 1.24 2.46 3.20
Lwaas c3 c3=
ll2taLQ i C4 n C4 i C4= n C4=
lx!taLu EG
Gasoline 1C5-221"C1 ' LCO (221:350"C) Coke
M a 2 2 9
7.46 3.78 11 24 11.20 3.1 3 0.33 0.79
9.36 4.65 14.01 18.00 5.69 0.59 1.29
136 3.09 7.72
8.85 1.88 2.1 0 3.35
;138e28
10.24 7.99 18.23 14.77 5.36 1.99 2.88
11.94 9.55 21.49 17.05 7.03 2.56 3.75
lu525.57
XLL9
m2Q30.40
68.44 0.94 2.0
68.00 1.70 1.7
47.12 2.50 3.8
2Gm39.58 53.90 0.77 3.5
2LM
4.3235l.es 36.90 2.45 4.5
Results from Table 2 and Table 3 allow to compare FCC catalyst (Y zeolite) and Beta zeolite at same conversion and show that Beta zeolite produces more gas and less gasoline than Y (43.2 wt. % instead of 39.6 wt. % LPG yield at 37 wt. %
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conversion). Mordenite produces less gas than other zeolites at same operating conditions, which illustrates the relatively low ability of this zeolite to crack gasoline in LPG. Since H-transfer promotes the interaction of olefins with naphthenes to form more refractory paraffins and aromatics, relative importance of such reactions can be appreciated through evolution of several parameters:
- LPG Olefinicity is the result of a competition between cracking reactions, which are producing olefins, and H-transfer reactions, which are olefin-consuming. Table 3 shows that Beta zeolite gives, at same conversion, a higher LPG olefinicity than Y zeolite (0.30 instead of 0.17 for Y zeolite at 37 wt. % conversion). Data concerning Mordenite were only obtained at slightly lower conversion, but seem to exhibit a much higher LPG olefinicity. The overall hydrogen transfer is therefore lower on Beta than on Y zeolite, in agreement with Bonetto et al. [9], and with n-hexane cracking results as well, and the lowest on Mordenite. The differences between Beta, Mordenite and Y zeolites can be explained by the higher SVAI ratio of Beta and Mordenite and by higher geometrical restrictions for bimolecular reactions like H-transfer reactions, which can be expected to be more critical in the case of light gasoline cracking than for n-hexane cracking, due to higher average size of reactants. These geometrical restrictions are increasing according to the following order : Y c Beta c Mordenite. Table 3: Influence of zeolite structure on LPG olefinicity and gasoline composition and quality Zeolite Y lFCC Cat.) Mordenite Beta (Si/AI ratio) (13) (80) (1 10) Temperature ("C) 530 530 530 530 530 . . Cat.iFeed 6 10 10 6 10 37.9 41.9 36.9 28.7 Conv (N + P) (wt. %) (*) 34.1 0.44 0.71 0.44 0.33 C3 Olefinicity 0.34 0.1 9 0.21 ~4 Olefinicity 0.07 0.07 0.34 LPG Olefinicity 0.18 0.17 0.49 0.30 0.31 iC4= / Total C4= 0.29 0.31 0.39 0.41 0.41 iC4 / iC4= 33.9 30.5 4.2 7.4 6.7 Gasoline ComDosition n Paraffins 13.87 12.56 9.64 9.29 6.18 i Paraffins 32.12 25.28 31.05 29.68 25.29 Olefins 2.22 1.39 3.30 5.52 4.00 Naphthenes 12.70 10.80 37.09 17.54 16.34 Aro matics 39.08 49.97 18.92 37.97 48.18 RON MON
--
90.3
69.2
__
91.1
-81.8 64.8 -82.2 r)Global conversion of Naphthenes + Paraffins is defined as follows: Conv (N + P) = [(N + P)feed - (N + P)prod.] I (N + P )feed where (N + P)feed and (N + P)prod are respectively the contents in naphthenes and paraffins in the feed and in the products.
- Among butenes, isobutylene is particularly sensitive to H-transfer reactions, which convert this olefin in isobutane. Relative importance of H-transfer reactions
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can be appreciated, in Table 3, through iC4/iC4= ratio, which is very much higher with Y zeolite than for Beta zeolite (30.5 and 7.4 respectively for Y and Beta). Lower value (4.2) is obtained with Mordenite at somewhat lower conversion (29 wt. % conversion).
- Gasoline composition is also affected by H-transfer reactions, which reduce the concentration of highly reactive olefins, and therefore substantially cut the number of secondary reactions, leading to a "stabilized" gasoline, which means lower olefin content and enriched in paraffins and aromatics. Comparison at same conversion (37 wt. Yo conversion) of Y and Beta zeolites in terms of gasoline composition (Table 3), show that Beta zeolite gives a more olefinic gasoline (5.5wt. % olefins instead of 1.4 wt. Yo)with less aromatics (38wt. % instead of 50 wt. Yo).Mordenite cannot be compared to other zeolites at same conversion. RON and MON have been calculated, using a simulation program developed by IFP [lo]. RON and MON of gasoline fraction are much lower in the case of Mordenite: this can be explained by the much lower concentration of high octane compounds (aromatics) and the higher concentration of low octane compounds (unconverted naphthenes), while no significant difference is observed concerning olefins and isoparaffins contents.
- No significant difference between Y and Beta zeolites is seen concerning coke yield (Table 2) at same conversion (3.5and 3.8 wt. % resp.). All these results concerning comparisons of Y, Mordenite and Beta zeolites, in light gasoline cracking, are in good agreement with those obtained for n-hexane and show, according to LPG olefinicity, that use of Mordenite zeolite allows to limit H-transfer reactions which can be explained by differences in zeolite structures.
CONCLUSION Data obtained in this study allow us to link zeolite structure and importance of Htransfer reactions in naphthenes and paraffins cracking. Such reactions appear as being dominant when using Y zeolite, and minimized when using MFI zeolite. Beta and Mordenite zeolites are intermediate and H-transfer reactions are less important in the case of Mordenite in comparison with Beta zeolite. Such results are interesting if we consider the interest to limit H-transfer reactions in FCC units, in order to promote olefins production. Presently, the only zeolite used at the industrial scale as a FCC additive is MFI zeolite, which is able to significantly lower H-transfer reactions, and therefore to increase olefins production (mainly propylene) and to improve gasoline octanes. Beta zeolite is potentially able to shift to C4's the gain in olefins production, due to its slightly bigger pore diameter, but use of this zeolite is faced to two important drawbacks: i) insufficient stability to regeneration conditions (which would necessitate use of relative high amounts of additive). ii) high price due to synthesis conditions. The main drawback of Mordenite zeolite as a FCC catalyst additive is its low ability to crack gasoline in olefinic LPG.
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REFERENCES 1. R.J. Schmidt, P.L. Bogdan and N.L. Gilsdorf, Chemtech, February (1993) 41. 2. J.A. Martens, J. Perez-Parienteand P.A. Jacobs, Ada Phys. Chem. 31 (1985) 487. 3. M. Maache, A. Janin, J.C. Lavalley, J.F. Joly and E. Benazzi, Zeolites, 13 (1993) 41 9. 4. W.O. Haag and Dessau R.M., Proc. 8th Int. Congr. Catal., Verlag Chemie, Vol. II, 305, Berlin, (1984). 5. A. Corma, J. Planelles, J. Sanchez-Marin and F. Thomas, J. Catal., 93 (1985) 30. 6. A.F.H. Wielers, M. Vaarkamp and M.F.M. Post, J. Cata1.,127 (1991) 51. 7.E.G. Derouane and J.C. Vedrine, J. Mol. Catal., 8 (1980) 479; P. Dejaifve, J.C. Vedrine, V. Bolis and E.G. Derouane, J. Catal., 63 (1980) 331. 8. W.O. Haag, R.M. Lago and P.B. Weisz, Faraday Disc., 72 (1982) 317. 9. L. Bonetto, A. Corma and E. Herrero, Proc. 9th Int. Zeolite Conf. (R. Von Ballmoos, J.B. Higgins and M.M.J. Treacy, Eds.), Butterworth-Heinemann, Vol. II, 639, Montreal, (1992). 10. J.P. Durand, 15th Int. Symp. on Capillary Chromatography (P. Sandra, Ed.), Vol. II, 1396, (1993).
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Relationship between zeolite structure and hydrogen-transfer reactions in naphthenes and paraffins cracking E. Benazzi, Th. Chapus, T. Cheron, H. Cauffriez and Ch. Marcilly. lnstitut FranGais du PGtrole, 1 & 4 avenue de Bois-PrGau, BP 31 1, 92506 RueilMalmaison, Cedex, France.
SUMMARY The limitation of hydrogen-transfer reaction in FCC units, when zeolitic additives are used, is desired to maximize the olefins production and to limit aromatics content of gasoline. In this work, the influence of zeolite structure on H-transfer reactions which occur during the cracking of n-hexane and a light gasoline cut (70-140°C) from hydrocracking, composed of a mixture of naphthenes and paraffins, was studied. The results obtained show that there is a link between the zeolite structure and the importance of H-transfer reactions in naphthenes and paraffins cracking. Such reactions appear as being dominant when using Y zeolite, and minimized when using MFI zeolite. Beta and Mordenite zeolites are intermediate and H-transfer reactions are less important in the case of Mordenite in comparison with Beta zeolite. The influence of the Si/AI ratio on the H-transfer reaction is also discussed. 1. INTRODUCTION
The 1990 Clean Air Act and the new forthcoming laws represent a challenge for the refining industry. The reduction in aromatics contents (1.O vol.% maximun benzene content) and the necessity of introducing a minimum of oxygenates compounds in motor fuel (2.0 wt.% minimum oxygen content) [ l ] will dramatically change gasoline composition and will need the use of new catalysts and processing technologies. Consequently, growing demands for gasoline from alkylation and for oxygenates compounds as MTBE (methyl t-butyl ether) and TAME (t-amyl methyl ether) are expected. This situation will require the production of larger amounts of isobutane, isobutylene, n-butenes and amylenes and will make that one of the main objectives of the FCC units will be to produce optimized quantities of light olefins and more particularly the C4 olefins. In order to increase the production of olefins it has been established that the use of zeolitic additives could be a successful way. For instance, use of MFI as additive to FCC catalysts increases propylene and very little C4 olefins production and allows to improve gasoline research octane number. The observed effect when using zeolitic additives depends on the H-transfer reaction which plays a very important role in catalytic cracking. In FCC units, the limitation of H-transfer reactions is desired to maximize olefins production and to limit aromatics content of gasoline. In this paper we studied the influence of zeolite structure on H-transfer reactions which occur during the cracking of n-hexane and a light gasoline cut (70-140°C) from hydrocracking, composed of a mixture of naphthenes and paraffins.
1664
2. EXPERIMENTAL 2.1. n-hexane cracking This study was carried out in a fixed-bed reactor at 300°C under atmospheric pressure using diluted n-hexane as feed (N2/nC6 molar ratio = 4) and using WHSV [2 -45 h-11. Four H-MFI solids with Si/AI ratio varying from 10 and 48 (Conteka and PQ corporation), three H-Mordenite samples with Si/AI ratio equal to 5, 10 (Tosoh Corporation) and 37 (obtained by a direct acid leaching of the mordenite possessing a Si/AI=lO), two H-Beta samples (one with Si/AI=l 1 prepared according to [2] and another one with Si/AI=19 dealuminated according to [3]) and one H-Y sample (Si/AI=17 obtained by steaming the NH4 form and followed by an acid treatement), were tested under powder form in this part of the study. 2.2. Cracking of light gasoline cut from hydrocracking This study was performed using a Micro Activity Test (MAT) unit, comprising a fixed-bed reactor containing 5 g. catalyst, heated at temperatures between 530 and 600°C and under atmospheric pressure. WHSV was kept constant (40 h-1) and Cat./Feed ratio was varied in the range 6 - 10. Mordenite and Beta zeolites were tested after being embedded with silica (50 wt. %/50 wt. Yo). Three catalyst samples were tested: one commercial FCC catalyst, containing a rare-earth exchanged Y zeolite (3.4 wt. % RE203 on catalyst), one embedded mordenite sample (Si/AI=80) and one embedded Beta sample (Si/AI =110). The commercial FCC catalyst was steamed at 73OOC prior to testing, bringing Y zeolite unit cell size to 24.36 A, which corresponds to Si/AI framework ratio of about 13. Composition of light gasoline cut, expressed in terms of distribution in hydrocarbons families, is given in Table 1, which shows the predominance of naphthenes ( 56 wt. Yo)- mainly C7 to C9- and paraffins ( 40 wt. Yo) - mainly C7 to C9 isoparaffins.
Table 1 Composition of light gasoline cut from hydrocracking (70 - 140°C), expressed in distribution by hydrocarbon family and carbon number (wt. Yo) C5 C6 C7 C8 C9 C10 C11 Total n Paraffins 0.17 2.18 3.13 2.48 0.83 0.10 8.89 i Paraffins 3.86 8.96 10.35 6.56 1.48 31.21 Naphthenes 0.08 5.43 17.17 21.63 9.98 1.56 55.85 Aromatics 0.29 1.14 1.62 0.88 0.09 0.03 4.05 Total 0.25 11.76 30.40 36.08 18.25 3.23 0.03 100.00 3. RESULTS AND DISCUSSION 3.1. Cracking of n-hexane It has been shown [4,5], that there are two modes of n-hexane cracking over zeolites, the monomolecular cracking route which involves a protolytic scission and the bimolecular classical route involving a P-scission. Their relative contribution depends on the reaction temperature, the aluminum content and the structure of the zeolites. Similarly, we can expect that the occurence of a secondary reaction like the H-transfer reaction will be strongly influenced by the pore structure.
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The results obtained (figure 1) show that, for a given time on stream and a nC6 conversion level, the paraffins/olefins ratio, which reflects in n-hexane cracking the importance of H-transfer reactions, reaches the highest values for H-Beta and H-Y zeolites, and the smallest ones for H-MFI samples, whereas intermediate values are obtained for H-mordenite samples. These observations can be related to differences in zeolites structure. Large pore sieves such as Y or Beta zeolites have a three-dimensional channels structure, that means enough space in order to form bulky bimolecular reaction intermediates involved in H-transfer reactions [6], whereas in MFI zeolite, which possesses a much smaller microporous volume, the formation of these species is possible but more difficult in comparison with the other zeolites. The case of the Mordenite is intermediate, since it is a large pore sieve (12 MR) but with only one-dimensional 12-rings channels and no cavity like Y zeolite. A similar evolution is observed if the iCUnC4 ratio is reported versus the zeolite structure. This ratio reflects, more particularly, the involvement of tertiary C4 carbenium ions in bimolecular H-transfer reactions.
figure 1 :
Evolution of the ParaffidOlefin ratio versus the zeolite structure and Si/A atomic ratio
The low level of H-transfer reaction observed for H-MFI samples, in comparison with the other zeolites, could be interpreted in terms of steric constraints. Indeed, the H-transfer reaction involves bulkier bimolecular transition state complexes which may problably form, in most cases, in the larger channels intersections [7]. Consequently, H-transfer reactions are limited for the MFI zeolite. Concerning the influence of the Si/AI ratio, the figure 1 evidences that whatever the aluminum content of the MFI solids (Si/AI between 10 and 48) the level of the H-transfer reactions remains constant and low, which is not in agreement with the hypothesis that H-transfer reaction should be favoured by two adjacent acid sites and so by a high lattice aluminum concentration [6]. These results suggest that in MFI type
1666
zeolite the factor which limits the H-transfer reaction is the steric constraint and that the H-transfer step would involve an adsorbed carbenium ion located at an acid centre and an incoming paraffin which gives the hydride ion to adsorbed carbenium ion (Eley-Rideal mechanism) [8]. In the case of the mordenite samples we can observe that the sample with the higher aluminum content (Si/AI=5) leads to the higher paraffin/olefin ratio indicating a high level of H-transfer reactions. This result would be agreed with the increase of this reaction with the aluminum content. But, the H-MOR sample possessing a Si/AI=37 presents a higher paraffidolefin ratio than the H-MOR (10). In the case of the H-MOR(37) sample, the presence of communications created between the main channels (along [OOl]) during the dealumination step of the H-MOR(10) sample by strong acid leaching could explain this result. Indeed, the defects created during the dealumination step could accomodate the bulky transition states of the H-transfer reactions. Nevertheless, the low aluminum content in the H-MOR(37) sample which would lead to a lower coke formation and so to weaker diffusional limitations could explain the higher H-transfer reactions level.
3.2. Cracking of light gasoline cut from hydrocracking
Table 2 shows the yield structures obtained by MAT catalytic tests performed at 530°C (CatJfeed = 10) with three different zeolite structures. Table 2: Influence of zeolite structure on yield structure obtained by cracking of a light gasoline cut (70 - 140°C) from hydrocracking (wt. %) Zeolite Y (FCC Cat) Mordenite Beta (Si/AI atomic ratio) (13) (80) (1 10) Temperature ("C) 530 530 530 530 530 .~ Cat.ifeed 6 10 10 6 10 Conv (N+P) (wt. %) (*) 36.9 37.9 41.9 34.1 28.7 H2 0.07 0.14 0.14 0.05 0.05 c1 0.52 0.33 0.41 0.44 0.1 4 c2 0.34 0.1 1 0.45 0.30 0.53 c2= 1.14 1.38 1.24 2.46 3.20
Lwaas c3 c3=
ll2taLQ i C4 n C4 i C4= n C4=
lx!taLu EG
Gasoline 1C5-221"C1 ' LCO (221:350"C) Coke
M a 2 2 9
7.46 3.78 11 24 11.20 3.1 3 0.33 0.79
9.36 4.65 14.01 18.00 5.69 0.59 1.29
136 3.09 7.72
8.85 1.88 2.1 0 3.35
;138e28
10.24 7.99 18.23 14.77 5.36 1.99 2.88
11.94 9.55 21.49 17.05 7.03 2.56 3.75
lu525.57
XLL9
m2Q30.40
68.44 0.94 2.0
68.00 1.70 1.7
47.12 2.50 3.8
2Gm39.58 53.90 0.77 3.5
2LM
4.3235l.es 36.90 2.45 4.5
Results from Table 2 and Table 3 allow to compare FCC catalyst (Y zeolite) and Beta zeolite at same conversion and show that Beta zeolite produces more gas and less gasoline than Y (43.2 wt. % instead of 39.6 wt. % LPG yield at 37 wt. %
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conversion). Mordenite produces less gas than other zeolites at same operating conditions, which illustrates the relatively low ability of this zeolite to crack gasoline in LPG. Since H-transfer promotes the interaction of olefins with naphthenes to form more refractory paraffins and aromatics, relative importance of such reactions can be appreciated through evolution of several parameters:
- LPG Olefinicity is the result of a competition between cracking reactions, which are producing olefins, and H-transfer reactions, which are olefin-consuming. Table 3 shows that Beta zeolite gives, at same conversion, a higher LPG olefinicity than Y zeolite (0.30 instead of 0.17 for Y zeolite at 37 wt. % conversion). Data concerning Mordenite were only obtained at slightly lower conversion, but seem to exhibit a much higher LPG olefinicity. The overall hydrogen transfer is therefore lower on Beta than on Y zeolite, in agreement with Bonetto et al. [9], and with n-hexane cracking results as well, and the lowest on Mordenite. The differences between Beta, Mordenite and Y zeolites can be explained by the higher SVAI ratio of Beta and Mordenite and by higher geometrical restrictions for bimolecular reactions like H-transfer reactions, which can be expected to be more critical in the case of light gasoline cracking than for n-hexane cracking, due to higher average size of reactants. These geometrical restrictions are increasing according to the following order : Y c Beta c Mordenite. Table 3: Influence of zeolite structure on LPG olefinicity and gasoline composition and quality Zeolite Y lFCC Cat.) Mordenite Beta (Si/AI ratio) (13) (80) (1 10) Temperature ("C) 530 530 530 530 530 . . Cat.iFeed 6 10 10 6 10 37.9 41.9 36.9 28.7 Conv (N + P) (wt. %) (*) 34.1 0.44 0.71 0.44 0.33 C3 Olefinicity 0.34 0.1 9 0.21 ~4 Olefinicity 0.07 0.07 0.34 LPG Olefinicity 0.18 0.17 0.49 0.30 0.31 iC4= / Total C4= 0.29 0.31 0.39 0.41 0.41 iC4 / iC4= 33.9 30.5 4.2 7.4 6.7 Gasoline ComDosition n Paraffins 13.87 12.56 9.64 9.29 6.18 i Paraffins 32.12 25.28 31.05 29.68 25.29 Olefins 2.22 1.39 3.30 5.52 4.00 Naphthenes 12.70 10.80 37.09 17.54 16.34 Aro matics 39.08 49.97 18.92 37.97 48.18 RON MON
--
90.3
69.2
__
91.1
-81.8 64.8 -82.2 r)Global conversion of Naphthenes + Paraffins is defined as follows: Conv (N + P) = [(N + P)feed - (N + P)prod.] I (N + P )feed where (N + P)feed and (N + P)prod are respectively the contents in naphthenes and paraffins in the feed and in the products.
- Among butenes, isobutylene is particularly sensitive to H-transfer reactions, which convert this olefin in isobutane. Relative importance of H-transfer reactions
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can be appreciated, in Table 3, through iC4/iC4= ratio, which is very much higher with Y zeolite than for Beta zeolite (30.5 and 7.4 respectively for Y and Beta). Lower value (4.2) is obtained with Mordenite at somewhat lower conversion (29 wt. % conversion).
- Gasoline composition is also affected by H-transfer reactions, which reduce the concentration of highly reactive olefins, and therefore substantially cut the number of secondary reactions, leading to a "stabilized" gasoline, which means lower olefin content and enriched in paraffins and aromatics. Comparison at same conversion (37 wt. Yo conversion) of Y and Beta zeolites in terms of gasoline composition (Table 3), show that Beta zeolite gives a more olefinic gasoline (5.5wt. % olefins instead of 1.4 wt. Yo)with less aromatics (38wt. % instead of 50 wt. Yo).Mordenite cannot be compared to other zeolites at same conversion. RON and MON have been calculated, using a simulation program developed by IFP [lo]. RON and MON of gasoline fraction are much lower in the case of Mordenite: this can be explained by the much lower concentration of high octane compounds (aromatics) and the higher concentration of low octane compounds (unconverted naphthenes), while no significant difference is observed concerning olefins and isoparaffins contents.
- No significant difference between Y and Beta zeolites is seen concerning coke yield (Table 2) at same conversion (3.5and 3.8 wt. % resp.). All these results concerning comparisons of Y, Mordenite and Beta zeolites, in light gasoline cracking, are in good agreement with those obtained for n-hexane and show, according to LPG olefinicity, that use of Mordenite zeolite allows to limit H-transfer reactions which can be explained by differences in zeolite structures.
CONCLUSION Data obtained in this study allow us to link zeolite structure and importance of Htransfer reactions in naphthenes and paraffins cracking. Such reactions appear as being dominant when using Y zeolite, and minimized when using MFI zeolite. Beta and Mordenite zeolites are intermediate and H-transfer reactions are less important in the case of Mordenite in comparison with Beta zeolite. Such results are interesting if we consider the interest to limit H-transfer reactions in FCC units, in order to promote olefins production. Presently, the only zeolite used at the industrial scale as a FCC additive is MFI zeolite, which is able to significantly lower H-transfer reactions, and therefore to increase olefins production (mainly propylene) and to improve gasoline octanes. Beta zeolite is potentially able to shift to C4's the gain in olefins production, due to its slightly bigger pore diameter, but use of this zeolite is faced to two important drawbacks: i) insufficient stability to regeneration conditions (which would necessitate use of relative high amounts of additive). ii) high price due to synthesis conditions. The main drawback of Mordenite zeolite as a FCC catalyst additive is its low ability to crack gasoline in olefinic LPG.
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REFERENCES 1. R.J. Schmidt, P.L. Bogdan and N.L. Gilsdorf, Chemtech, February (1993) 41. 2. J.A. Martens, J. Perez-Parienteand P.A. Jacobs, Ada Phys. Chem. 31 (1985) 487. 3. M. Maache, A. Janin, J.C. Lavalley, J.F. Joly and E. Benazzi, Zeolites, 13 (1993) 41 9. 4. W.O. Haag and Dessau R.M., Proc. 8th Int. Congr. Catal., Verlag Chemie, Vol. II, 305, Berlin, (1984). 5. A. Corma, J. Planelles, J. Sanchez-Marin and F. Thomas, J. Catal., 93 (1985) 30. 6. A.F.H. Wielers, M. Vaarkamp and M.F.M. Post, J. Cata1.,127 (1991) 51. 7.E.G. Derouane and J.C. Vedrine, J. Mol. Catal., 8 (1980) 479; P. Dejaifve, J.C. Vedrine, V. Bolis and E.G. Derouane, J. Catal., 63 (1980) 331. 8. W.O. Haag, R.M. Lago and P.B. Weisz, Faraday Disc., 72 (1982) 317. 9. L. Bonetto, A. Corma and E. Herrero, Proc. 9th Int. Zeolite Conf. (R. Von Ballmoos, J.B. Higgins and M.M.J. Treacy, Eds.), Butterworth-Heinemann, Vol. II, 639, Montreal, (1992). 10. J.P. Durand, 15th Int. Symp. on Capillary Chromatography (P. Sandra, Ed.), Vol. II, 1396, (1993).