Mechanistic Pathways for Olefin Hydroisomerization and

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Mechanistic Pathways for Olefin Hydroisomerization and Aromatization in Fluid Catalytic Cracking Gasoline Hydro-upgrading Yu Fan,†,‡ Jizhou Yin,‡ Gang Shi,‡ Haiyan Liu,‡ and Xiaojun Bao*,†,‡ State Key Laboratory of HeaVy Oil Processing, China UniVersity of Petroleum, Beijing 102249, P. R. China and The Key Laboratory of Catalysis, China National Petroleum Corporation (CNPC), China UniVersity of Petroleum, Beijing 102249, P. R. China ReceiVed January 16, 2009. ReVised Manuscript ReceiVed April 1, 2009

The reactivities of model hydrocarbons (n-paraffins, i-paraffins, olefins, naphthenes, and aromatics) typical for fluid catalytic cracking (FCC) gasoline were investigated over a Ni-Mo/modified HZSM-5 + Al2O3 catalyst. Olefins had the highest reactivity and were unidirectionally converted into molecules of the other groups, especially i-paraffins and aromatics. On the basis of the results, a reaction mechanism was proposed for olefin hydroisomerization and aromatization in the presence of excess hydrogen. During the olefin hydroisomerization and aromatization, the initial adsorption sites of the olefins are the acid sites rather than the metal sites of the bifunctional catalyst. The olefin hydroisomerization reaction is in accordance with the hydrogen spillover concept. The function of the metal sites is to dissociate hydrogen molecules into active H ions, and the acid sites are the reaction sites. The olefin aromatization in the presence of hydrogen occurs through diene and cyclo-olefin intermediates by hydrogen transfer/dehydrogenation and cyclization. The results obtained form the fundamental basis for the olefin conversion in the presence of hydrogen and shed a light on the correlation between the reactivity of model compounds and that of real gasoline with complex compositions.

1. Introduction Current and future regulations of clean fuels are pushing toward lower olefin levels in gasoline.1 Since fluid catalytic cracking (FCC) gasoline provides 90% of olefins in the total gasoline pool, this stream has become a primary target for olefin reduction. Unfortunately, most options to greatly reduce the olefin content of FCC gasoline (such as optimization of the FCC process,2 use of new FCC catalysts,2 selective hydrogenation3 and hydrocracking/isomerization4) typically result in substantial losses in gasoline octane number and product yield. In previous work, we developed a novel FCC gasoline hydro-upgrading catalyst using HZSM-5 zeolite modified by combined steaming and citric acid treatments.5 By converting olefins in FCC gasoline into i-paraffins and aromatics via hydroisomerization and hydroaromatization, this catalyst showed excellent octane number preservation, yield improvement abilities, and good stability while significantly reducing the olefin content in FCC gasoline. It is well-known that for the process development and commercial application of catalysts, the study of reaction mechanism is very important. For this purpose, we attempted to correlate the catalyst acidity with the olefin conversion * Corresponding author. E-mail: [email protected]; phone: +86(0) 10 89734836; fax: +86(0) 10 89734979. † State Key Laboratory of Heavy Oil Processing, China University of Petroleum. ‡ CNPC, China University of Petroleum. (1) Liu, H.; Yu, J.; Fan, Y.; Shi, G.; Bao, X. Petrol. Sci. 2008, 5, 285– 294. (2) Ouyang, F.; Weng, H. Petrol. Sci. Technol. 2007, 25, 399–409. (3) Fan, Y.; Lu, J.; Shi, G.; Liu, H.; Bao, X. Catal. Today 2007, 125, 220–228. (4) Babich, I. V.; Moulijn, J. A. Fuel 2003, 82, 607–631. (5) Fan, Y.; Lin, X.; Shi, G.; Liu, H.; Bao, X. Microporous Mesoporous Mater. 2007, 98, 174–181.

pathways in FCC gasoline hydro-upgrading.6,7 However, the correlation could not mechanistically explain the olefin conversion very well, due to the complex hydrocarbon composition of FCC gasoline. Generally, mechanistic studies on hydrocarbon conversion are based on model compounds in view of the simplicity of their components. At present, mechanistic studies on hydrocarbon isomerization are focused on the isomerization of n-paraffins (such as n-butane, n-heptane, and n-octane).8-10 According to the classical isomerization mechanism,11-13 n-paraffins are dehydrogenated on the catalyst metal sites, and the produced olefins are protonated on the catalyst acid sites to form the corresponding alkylcarbenium ions. Thereafter, these alkylcarbenium ions undergo skeletal rearrangement on the acid sites and return to the catalyst metal sites, where they are converted into the corresponding i-paraffins by hydrogenation. As to hydrocarbon aromatization, mechanistic studies are mainly related to the aromatization of light olefins14-16 and light (6) Fan, Y.; Bao, X.; Shi, G.; Wei, W.; Xu, J. Appl. Catal., A 2004, 275, 61–71. (7) Fan, Y.; Bao, X.; Shi, G. Catal. Lett. 2005, 105, 67–75. (8) Dorado, F.; Romero, R.; Can˜izares, P. Appl. Catal., A 2002, 236, 235–243. (9) Blomsma, E.; Martens, J. A.; Jacobs, P. A. J. Catal. 1996, 159, 323– 331. (10) Kuznetsov, P. N. J. Catal. 2003, 218, 12–23. (11) Mills, G. A.; Heinemann, H.; Milliken, T. H.; Oblad, A. G. Ind. Eng. Chem. 1953, 45, 134–137. (12) Park, K. C.; Ihm, S. K. Appl. Catal., A 2001, 203, 201–209. (13) Deldari, H. Appl. Catal., A 2005, 293, 1–10. (14) Lukyanov, D. B.; Gnep, N. S.; Guisnet, M. Ind. Eng. Chem. Res. 1994, 33, 223–234. (15) Song, Y.; Zhu, X.; Xie, S.; Wang, Q.; Xu, L. Catal. Lett. 2004, 97, 31–36. (16) Choudhary, V. R.; Panjala, D.; Banerjee, S. Appl. Catal., A 2002, 231, 243–251.

10.1021/ef900030h CCC: $40.75  2009 American Chemical Society Published on Web 04/23/2009

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Table 1. Carbon Number Distribution and Group Composition of a Typical FCC Gasoline carbon number 3 4 5 6 7 8 9 10 11 12 13 total (v %)

total n-paraffin i-paraffin olefin naphthene aromatics (v %) 0.0 0.1 0.9 0.9 0.8 0.9 0.2 0.3 0.2 0.1 0.0 4.3

0.0 0.0 7.7 7.0 3.2 2.5 1.6 1.1 1.4 0.6 0.1 25.0

0.0 1.0 13.3 10.2 8.3 5.2 3.5 1.0 0.3 0.0 0.0 42.7

0.0 0.0 0.2 0.2 3.9 1.8 1.1 0.3 0.2 0.1 0.0 7.8

0.0 0.0 0.0 0.5 2.3 5.2 5.8 4.5 1.8 0.0 0.0 20.2

0.0 1.1 22.0 18.8 18.4 15.6 12.2 7.2 3.8 0.7 0.1 100.0

Table 2. Properties of Catalyst SCZM item

value

unit

packing density micropore surface area external surface area micropore volume mesopore volume weak Lewis acidity weak Bro¨nsted acidity medium and strong Lewis acidity medium and strong Bro¨nsted acidity modified HZSM-5 zeolite alumina NiO MoO3

0.71 224 102 0.106 0.133 57.3 35.3 62.0 127.5 75 21 1.0 3.0

g/mL m2/g m2/g mL/g mL/g µmol/g µmol/g µmol/g µmol/g wt % wt % wt % wt %

Table 3. Reaction Results of Pentane and Octane

paraffins17,18 in the absence of H2. It is well accepted that the light olefins from the dehydrogenation of light paraffins are transformed into aromatics through oligomerization, cyclization, and dehydrogenation/hydrogen transfer. The dehydrogenation reaction is performed on the catalyst metal sites, whereas the oligomerization, cyclization, and hydrogen transfer reactions are performed on the catalyst acid sites. On the whole, the study of the mechanism of olefin hydroisomerization is hardly touched in open literature, and to the best of our knowledge no attention has been paid to the mechanism of olefin and paraffin aromatization in the presence of H2. The present investigation places emphasis on these two untouched aspects. To better understand the mechanism of olefin hydroisomerization and aromatization in the presence of H2, we herein report the conversion behavior of model hydrocarbons typical for FCC gasoline and the reactivity of real FCC gasoline over a modified HZSM-5 zeolite-based catalyst and present a detailed mechanistic explanation for the olefin conversion in FCC gasoline hydro-upgrading. 2. Experimental Section 2.1. Feedstocks and Catalyst. The detailed group composition of a representative FCC gasoline (Shandong Shenghua Refinery), obtained by the gas chromatograph method, is listed in Table 1. From Table 1, it can be seen that in the FCC gasoline the higher content of n-paraffins, i-paraffins, olefins, and aromatics are distributed on carbon numbers of 5-8, 5-6, 5-6, and 8-9, respectively. Therefore we selected pentane and octane, iso-hexane, pentene and hexene, and xylene as the representatives of n-paraffins, i-paraffins, olefins, and aromatics, respectively. In addition, cyclohexane was selected as the representative of naphthenes in view of its typical structure property. According to the above analysis, the following model compounds were used in the mechanistic study of olefin conversion: pentane (99%, Nanjing Guanghua Chemical Co., P. R. China), octane (98%, Beijing Dongfang Chemical Co., P. R. China), 2-methylpentane (99%, Beijing Hengyezhongyuan Chemical Co., P. R. China), iso-octane (99%, Beijing Yike Chemical Co., P. R. China), 1-pentene (99%, Acros), 1-hexene (99%, Acros), cyclohexane (99%, Beijing Dongfang Chemical Co., P. R. China), xylene mixture (28% o-xylene, 19% p-xylene, 41% m-xylene, 12% ethylbenzene, Beijing Jinxing Chemical Plant, P. R. China). The catalyst with steamed/citric acid modified HZSM-5 zeolite as support and Ni-Mo as active metals was prepared as described elsewhere and is denoted as catalyst SCZM.5 The main properties of catalyst SCZM are given in Table 2. Another catalyst with the same composition as that of catalyst SCZM was prepared by mechanically mixing Ni-Mo-impregnated (17) Guisnet, M.; Gnep, N. S. Appl. Catal., A 1996, 146, 33–64. (18) Viswanadham, N.; Pradhan, A. R.; Ray, N.; Vishnoi, S. C.; Shanker, U.; Prasada Rao, T. S. R. Appl. Catal., A 1996, 137, 225–233.

item

pentane

octane

conversion (%) eC3 in product (v %)

24.2 7.4

94.3 25.4

n-paraffin i-paraffin olefin naphthene aromatics

Group Composition in Product (v %) 77.9 12.6 8.3 0.1 1.1

36.7 27.9 27.5 2.5 5.5

Al2O3 and the steaming/citric acid modified HZSM-5 zeolite. First, the precursor of Al2O3 (pseudoboehmite, Tianjin Hengmeilin Chemical Co., P. R. China) was impregnated successively with aqueous solutions of (NH4)6Mo7O24 and Ni(NO3)2 (Beijing Chemical Co., P. R. China) according to the required metal loadings. Then, the obtained solids were mechanically mixed with the steaming/ citric acid modified HZSM-5 zeolite and shaped by extrusion according to the desired ratio. Finally, the mixture was dried at 120 °C for 5 h and calcined at 500 °C for 4 h. The catalyst without active metals, denoted as catalyst ZM, was prepared as follows. The mixture of the modified HZSM-5 zeolite and Al2O3 was shaped, dried at 120 °C for 5 h and calcined at 500 °C for 4 h. The resulting catalyst contains 75 wt % modified HZSM-5 zeolite with the same ratio as that in catalyst SCZM. 2.2. Catalytic Experiments. The catalytic experiments of the FCC gasoline and the various hydrocarbons were carried out in a flow-type apparatus designed for continuous operation. This apparatus consists of a gas-feeding system controlled by a mass flowmeter, a syringe pump liquid feeding system, a fixed-bed reactor, and an online analysis system. The reactor, with an internal diameter of 10 mm, was loaded with a catalyst sample of ca. 2 g. First, the catalyst was sulfided at 230, 290, and 320 °C for 2 h at each temperature, by a stream containing 3 wt % CS2 in cyclopentane (6 mL/h) through the catalyst bed in the presence of pure H2. Then, a model compound or FCC gasoline was fed into the reactor at a predetermined flow rate after the temperature was increased to reaction temperature. Finally, the reaction was carried out at 380 °C, a liquid hourly space velocity (LHSV) of 1.5 h-1, a total pressure of 1.5 MPa, and a volumetric ratio of H2 to hydrocarbon/gasoline of 200. The hydrocarbon compositions of the feedstocks and products were determined online by an Agilent 1790 gas chromatograph with a flame ionization detector, a HP-PONA capillary column (50 m × 0.2 mm), and data processing software (GC 99, Beijing Research Institute of Petroleum Processing, SINOPEC, P. R. China).

3. Results 3.1. Pentane and Octane. The results of the reactions of pentane (99%) and octane (98%) are presented in Table 3. The conversion of pentane is much lower than that of octane and the content of light hydrocarbons (eC3) in the pentane conversion is lower than that in the octane conversion, indicating that

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Table 4. Carbon Number Distribution in the Reaction Product with Octane As Feedstock carbon number

n-paraffin (v %)

i-paraffin (v %)

olefin (v %)

naphthene (v %)

aromatics (v %)

1 2 3 4 5 6 7 8 9 10 11 12

0.0 1.4 0.0 20.1 7.2 1.6 0.4 5.6 0.0 0.1 0.1 0.3

0.0 0.0 0.0 13.4 7.8 3.7 1.5 1.2 0.3 0.0 0.0 0.0

0.0 0.0 24.0 0.8 0.5 0.2 0.1 0.8 0.7 0.4 0.1 0.0

0.0 0.0 0.0 0.0 0.1 0.0 1.6 0.7 0.1 0.0 0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.2 1.3 2.1 1.2 0.5 0.2 0.0

Table 5. Reaction Results of 2-Methylpentane and Iso-octane item

2-methylpentane

iso-octane


11.5 6.1

7.1 3.3



2.1 94.5 3.3 0.1 0.1

long-chain n-paraffins are easy to crack and convert into other hydrocarbons.19 As shown in Table 4, the majority of the hydrocarbons formed in the reaction of octane are C3 olefins and C4 paraffins. During the conversion of octane, it is mainly cracked into C4 olefins and C4 paraffins as well as C3 olefins and C5 paraffins. C4 olefins are subsequently converted into C4 paraffins by hydroisomerization reaction, so the content of C4 paraffins is high in the reaction product with octane as feedstock. A number of the produced C3 olefins come from the initial cracking of octane and the secondary cracking of the formed C5 paraffins.10 The naphthene and aromatics contents in the reaction product were much lower than those of the other three groups of hydrocarbons (Table 4), due to the fast cracking of octane. 3.2. 2-Methylpentane and Iso-octane. As shown in Table 5, iso-octane has a lower conversion and less cracking products than 2-methylpentane. Iso-octane (i.e., 2,2,4-trimethylpentane) is a multibranched isomer with stable structure, whereas 2-methylpentane is a single-branched isomer that is easy to crack.20 This difference in structure results in the different conversion behavior. On the whole, the conversion of i-paraffins was lower than that of n-paraffins (Tables 3 and 5). From the viewpoint of hydro-upgrading FCC gasoline, the low reactivity of i-paraffins is favorable for octane number preservation due to the relatively higher octane numbers of i-paraffins than of n-paraffins. 3.3. Xylene Mixture and Cyclohexane. The results of the reactions of the xylene mixture (28% o-xylene, 19% p-xylene, 41% m-xylene, and 12% ethylbenzene) and cyclohexane (99%) are listed in Table 6, and the carbon number distributions of the corresponding reaction products are shown in Tables 7 and 8, respectively. The conversion of the xylene mixture is lower than that of cyclohexane, and the content of light hydrocarbons (eC3) in the product of the former is lower than in that of the latter (Table 6), indicating a higher structure stability of aromatics than of naphthene. (19) Sa´nchez, P.; Dorado, F.; Ramos, M. J.; Romero, R.; Jime´nez, V.; Valverde, J. L. Appl. Catal., A 2006, 314, 248–255. (20) Tu, X.; Fang, X.; Zhao, L.; You, B. Ind. Catal. 2007, 15, 19–23.

Table 6. Reaction Results of Xylene Mixture and Cyclohexane item

xylene mixture

cyclohexane


9.5a 2.1

20.1 7.9



5.7 2.8 9.0 79.7 2.8

a The conversion was calculated from the variation in the total content of the xylene mixture.

Table 7. Carbon Number Distribution in the Product with the Xylene Mixture As Feedstock carbon number

n-paraffin (v %)

i-paraffin (v %)

olefin (v %)

naphthene (v %)

aromatics (v %)

1 2 3 4 5 6 7 8 9 10 11 12

0.0 1.2 0.0 0.2 0.1 0.0 0.0 0.0 0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.1 0.1 0.0 0.1 0.3 0.1 0.0 0.0 0.0

0.0 0.0 0.9 0.0 0.0 0.0 0.0 0.1 0.1 0.4 0.0 0.0

0.0 0.0 0.0 0.0 0.4 0.0 0.1 0.1 0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0 0.0 3.0 1.3 90.2 0.4 0.6 0.1 0.0

Table 8. Carbon Number Distribution in the Product with Cyclohexane As Feedstock carbon number

n-paraffin (v %)

i-paraffin (v %)

olefin (v %)

naphthene (v %)

aromatics (v %)

1 2 3 4 5 6 7 8 9 10 11 12

0.0 0.8 0.0 2.9 1.2 0.8 0.0 0.0 0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.8 0.5 1.2 0.0 0.1 0.0 0.2 0.0 0.0

0.0 0.0 7.1 0.1 0.1 0.1 1.3 0.1 0.2 0.0 0.1 0.0

0.0 0.0 0.0 0.0 0.0 79.5 0.0 0.2 0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0 1.0 1.1 0.5 0.1 0.1 0.0

By combining Tables 6 and 7, one can observe that ethane, benzene, and toluene are the main products of the xylene mixture conversion, indicating the predominant cracking of the phenylethyl and benzyl side-chains. In fact, saturation and ring-opening of monocyclic aromatics over non-noble metal catalysts are very difficult, even under much more severe reaction conditions than those used in the present study,21,22 so the side-chain cracking of aromatics is preponderant. By combining Tables 6 and 8, one can observe that the majority of the hydrocarbons formed in the reaction of cyclohexane are C4 paraffins and C3 olefins. During the conversion of cyclohexane, it is mainly converted into two C3 olefins as well as C4 olefins and C2 olefins by ring-opening and subsequent cracking.23 C4 olefins are hydrogenated into C4 paraffins, and C2 olefins can be converted into C4 paraffins by dimerization and subsequent hydrogenation.24 Thus, the contents of C3 olefins (21) Du, H.; Fairbridge, C.; Yang, H.; Ring, Z. Appl. Catal., A 2005, 294, 1–21. (22) Song, C.; Ma, X. Appl. Catal., B 2003, 41, 207–238. (23) Figueras, F.; Coq, B.; Walter, C.; Carriat, J. Y. J. Catal. 1997, 169, 103–113. (24) Onyestya´k, G.; Pa´l-Borbe´ly, G.; Beyer, H. K. Appl. Catal., A 2002, 229, 65–74.



Table 9. Reaction Results of 1-Pentene and 1-Hexene item

1-pentene

1-hexene

conversion (%) eC3 in product (v %) benzene in product (v %)

100 19.1 0.4

100 18.5 0.6


Group Composition in Product (v%) 22.3 37.0 20.7 3.3 16.8

19.4 31.6 20.5 2.6 25.9

Table 10. Carbon Number Distribution in the Product with 1-Pentene As Feedstock carbon number

n-paraffin (v %)

i-paraffin (v %)

olefin (v %)

naphthene (v %)

aromatics (v %)

1 2 3 4 5 6 7 8 9 10 11 12

0.1 2.4 0.0 11.4 5.9 1.6 0.4 0.2 0.1 0.1 0.0 0.0

0.0 0.0 0.0 13.6 11.3 6.2 2.9 1.5 0.6 0.4 0.3 0.2

0.0 0.0 16.6 0.6 0.6 0.2 0.3 1.2 0.9 0.2 0.1 0.0

0.0 0.0 0.0 0.0 0.2 0.0 2.0 0.6 0.3 0.1 0.1 0.0

0.0 0.0 0.0 0.0 0.0 0.4 3.3 6.3 4.1 1.9 0.8 0.0

Table 11. Carbon Number Distribution in the Product with 1-Hexene As Feedstock carbon number

n-paraffin (v %)

i-paraffin (v %)

olefin (v %)

naphthene (v %)

aromatics (v %)

1 2 3 4 5 6 7 8 9 10 11 12

0.1 1.9 0.1 10.4 4.2 1.2 0.3 0.2 0.0 0.1 0.1 0.8

0.0 0.0 0.0 12.5 8.8 5.7 2.5 1.1 0.5 0.1 0.3 0.1

0.0 0.0 16.5 0.4 0.4 0.2 0.2 0.8 0.6 1.2 0.2 0.0

0.0 0.0 0.0 0.0 0.1 0.0 1.7 0.5 0.2 0.1 0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.6 4.6 13.2 4.1 2.4 1.0 0.0

and C4 paraffins are high in the reaction product with cyclohexane as feedstock. 3.4. 1-Pentene and 1-Hexene. The results of the reactions of 1-pentene (99%) and 1-hexene (99%) are presented in Table 9, and the carbon number distributions of the corresponding reaction products are shown in Tables 10 and 11, respectively. Both olefins have a conversion of 100% and are severely cracked with a C3- yield of about 19 v % (mainly light olefins). The high yields of i-paraffins and aromatics indicate the good hydroisomerization and aromatization reactivities of 1-pentene and 1-hexene. 1-Hexene has a lower hydroisomerization reactivity and a higher aromatization reactivity than 1-pentene, as evidenced by the i-paraffin and aromatics contents in Tables 9-11. 1-Hexene with six carbons is prone to form aromatics by dehydrogenation and cyclization, and this conversion limits the formation of i-paraffins, whereas 1-pentene forms aromatics by cracking, oligomerization, dehydrogenation, and cyclization, and part of the produced intermediates can be converted into i-paraffins. In the products of 1-pentene and 1-hexene, the very low benzene contents and the much higher contents of other aromatics (mainly alkyl benzene) demonstrate the occurrence of benzene alkylation, because benzene is the intermediate

Figure 1. Carbon number distributions of i-paraffins in the products of the different hydrocarbons.

Figure 2. Carbon number distributions of aromatics in the products of the different hydrocarbons.

during the conversion of olefins into alkyl benzene.25 The very low benzene yield makes the product quality meet the strict regulation on the benzene content in clean gasoline. 3.5. Comparison of Hydroisomerization and Aromatization Behavior of Different Hydrocarbons. Figure 1 shows that the main hydroisomerization product of pentane is isopentane with the same carbon number, whereas the main products of octane are the lighter isobutane and isopentane, indicating that the reaction of octane follows the initial cracking and subsequent hydroisomerization pathways. Among the five hydrocarbons studied, cyclohexane has the lowest i-paraffin yield, that is, a small quantity of C6 isomers, suggesting a low degree of isomerization. Both 1-pentene and 1-hexene have the higher content of i-paraffins mainly with carbon numbers of 4-6, indicating the simultaneity of the cracking/hydroisomerization pathway and the direct hydroisomerization pathway for the olefins. On the whole, the hydroisomerization reactivity of the above hydrocarbons decreases in the order of olefin > n-paraffin > naphthene. Figure 2 shows that in the presence of hydrogen, the carbon numbers of aromatics formed from the different hydrocarbons are mainly between 7 and 9, except for low aromatics formed from pentane. The increasing carbon number of n-paraffins is favorable for aromatization, as evidenced by the much higher aromatics yield of octane than of pentane. Compared with the other three hydrocarbons, 1-pentene and 1-hexene give a higher aromatics yield, and 1-hexene is more preponderant for aromatization than 1-pentene. Obviously, the olefins are the best reactants for aromatization among the above hydrocarbons. From Figure 2, it is noted that almost all of the hydrocarbons yield the highest C8 aromatics among their aromatic products. This can be reasonaly interpreted as follows: first, the different hydrocarbons are prone to be cracked into small molecules, as shown in Tables 4, 8, 10, and 11; then, the resulting C4 hydrocarbons are converted into C8 aromatics (xylene) by (25) Wang, L.; Yang, B.; Wang, Z. Chem. Eng. J. 2005, 109, 1–9.

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Table 12. Variation of Carbon Number Distributions between the Product and the FCC Gasoline Feedstocka carbon number

n-paraffin ∆(v %)

i-paraffin ∆(v %)

olefin ∆(v %)

naphthene ∆(v %)

aromatics ∆(v %)

3 4 5 6 7 8 9 10 11 12 13 total (v %)

0.0 0.3 1.2 1.2 0.1 0.3 0.1 0.3 0.0 0.1 0.0 3.7

0.0 1.2 3.5 7.1 1.1 1.3 0.2 0.2 0.3 0.2 0.2 15.2

0.0 -1.3 -11.3 -9.8 -3.7 -4.0 -1.4 -1.1 -0.5 0.0 0.0 -32.9

0.0 0.0 -0.1 0.1 1.7 1.2 0.4 0.0 0.1 0.0 0.0 3.4

0.0 0.0 0.0 0.0 1.1 1.4 2.8 3.6 1.8 0.0 0.0 10.6

a ∆, The differences between the product contents and the feedstock contents.

isomerization, dimerization, and cyclization; the heavier aromatics than C8 aromatics tend to be side-chain cracked; and toluene can be formed only by one pathway, that is, the interaction between C3 and C4 hydrocarbons. 3.6. Real FCC Gasoline. The results of the reactions of the single hydrocarbons have shown that olefins are the most active reactants among the five group hydrocarbons. Do these results also hold for real FCC gasoline? To answer this question, we selected FCC gasoline as feedstock and list the corresponding reaction results in Table 12. Clearly, only the olefin contents with different carbon numbers greatly decrease, while the contents of the other four groups increase to different degrees, implying the preferential and unidirectional conversion of olefins into the other groups. The long-chain n-paraffins have very high reactivity as shown in Section 3.1, but their low contents in FCC gasoline (Table 1) lead to a negligible effect on the olefin conversion. According to the results in Sections 3.2-3.4, i-paraffins, aromatics, and naphthenes have much lower reactivity than olefins, so they have no influence on the olefin conversion as evidenced by the results shown in Table 12. The reaction results of the single model compounds and the FCC gasoline lead to the conclusion that the influence of the presence of other hydrocarbons on the conversion pathways of olefins is negligible, and thus the mechanistic study of olefin conversion with single olefins as feedstocks holds for that with a real FCC gasoline as feedstock. 4. Discussion on the Mechanism of Olefin Conversion In view of the higher aromatics content in the 1-hexene product than that in the 1-pentene product (Table 9) and the higher octane number of aromatics compared to i-paraffins, 1-hexene was selected as an example for the following mechanistic discussion of olefin conversion. 4.1. Initial Adsorption Sites of 1-Hexene. Generally, bifunctional catalysts have two kinds of active sites: metal sites and acid sites. Catalyst SCZM studied in the present investigation has acid sites associated with the modified HZSM-5 zeolite and metal sulfide sites with the sulfided Ni-Mo. The sites on the surface of metal sulfides are known to have hydrogenation and dehydrogenation functions like metal sites.26 The preferential adsorption sites of 1-hexene molecules play an important role in determining the pathways of the subsequent hydroisomerization and aromatization reactions. If the preferential adsorption sites of 1-hexene molecules are the metal sites over catalyst (26) Yang, M.; Nakamura, I.; Fujimoto, K. Appl. Catal., A 1995, 127, 115–124.

Table 13. Reaction Results of 1-Hexene over the Different Catalysts

item

catalyst ZM

mechanically mixed catalyst

catalyst SCZM


100 24.7

100 19.2

100 18.5


Group Composition in Product (v %) 36.6 19.9 15.8 31.0 25.9 21.0 4.2 6.1 17.5 22.0

19.4 31.6 20.5 2.6 25.9

SCZM, hydrogenation or dehydrogenation of 1-hexene should take place first and hexane or diene should be formed. However, the formation of mono-olefinic intermediates is necessary for converting n-paraffins or dienes into i-paraffins,9 so hexane or diene must be converted back into original 1-hexene only by dehydrogenation or hydrogenation. Obviously, the conversion of 1-hexene into itself by the above cycling hydrogenation/ dehydrogenation is meaningless. Therefore, the preferential adsorption sites of 1-hexene should be the acid sites, as will be discussed below in detail. 4.2. Hydroisomerization and Aromatization Mechanisms of 1-Hexene. Mills et al. proposed the classical hydroisomerization mechanism of n-paraffins for bifunctional catalysts consisting of highly dispersed metal particles and an acidic support.11 This model involves a repeated gas phase diffusion of olefinic intermediates between the metal and acid sites. However, it was later recognized that the reactions involved could also occur on one reaction site via the so-called hydrogen spillover mechanism in the presence of activated hydrogen species.27-29 To clarify the hydroisomerization mechanism of olefins, the product distributions over the three catalysts prepared are compared in Table 13. It can be seen that the content of light hydrocarbons (eC3) over catalyst ZM without active metals is higher than those over the others with active metals, indicating the higher cracking activity of the former. The high cracking activity can reduce the formation of i-paraffins and aromatics from olefins,7 which is the reason why catalyst ZM gives the lowest contents of i-paraffins and aromatics among the three catalysts. The ratio of the paraffin content to the aromatics content over catalyst ZM is close to 3.0, suggesting that the reaction of 1-hexene over catalyst ZM is predominated by the hydrogen transfer between olefin molecules. This is in coincidence with the mechansim proposed for the situation in the absence of hydrogen.30 Thus, atmosphere hydrogen is not activated over catalyst ZM. On the contrary, the presence of active metals over the other two catalysts can activate atmosphere hydrogen and accelerate dehydrogenation of hydrocarbons,26 so the contents of i-paraffins and aromatics over them increase obviously compared to catalyst ZM. To elucidate whether and how the activated hydrogen species migrate, we compare reaction results of 1-hexene over catalyst SCZM and the catalyst with the same composition as that of catalyst SCZM by mechanically mixing Ni-Mo-impregnated Al2O3 and the modified HZSM-5 zeolite. It is clear that the (27) Roessner, F.; Roland, U. J. Mol. Catal. A: Chem. 1996, 112, 401– 412. (28) Roland, U.; Braunschweig, T.; Roessner, F. J. Mol. Catal. A: Chem. 1997, 127, 61–84. (29) Franke, M. E.; Simon, U.; Roessner, F.; Roland, U. Appl. Catal., A 2000, 202, 179–182. (30) Choudhary, V. R.; Devadas, P.; Banerjee, S.; Kinage, A. K. Microporous Mesoporous Mater. 2001, 47, 253–267.


distance between a metal site and an acid site over catalyst SCZM is much shorter than that over the mechanically mixed catalyst. The effect of the distance between a metal site and an acid site on the reaction performance of 1-hexene is shown in Table 13. Over both catalysts, the contents of n-paraffins, i-paraffins, and olefins in the products are almost the same, but the contents of naphthenes and aromatics are different. The almost identical i-paraffin content obtained over the two catalysts demonstrates that the hydroisomerization reaction of 1-hexene does not depend upon the metal site-acid site distance, verifying the validity of the hydrogen spillover concept for the olefin hydroisomerization, because the hydrocarbon intermediates involved in this concept do not need to move between the two kinds of catalytic sites. The hydrogen spillover mechanism of olefin hydroisomerization excludes the possibility of metal sites as the preferential adsorption sites of 1-hexene. In fact, if 1-hexene molecules preferentially adsorb on metal sites, their migration to the acid sites is necessary for forming carbenium ions. In the case of a long distance between metal and acid sites, part of the hydrocarbon intermediates desorbing from the metal sites go to the gas phase during migration, so the number of hydrocarbon intermediates that can reach the acid sites is reduced, leading to a decreased i-paraffin content in the product. However, this is in contradiction to the fact that the increased distance hardly influences the i-paraffin content in the product (Table 13). Therefore, we conclude that the preferential adsorption sites of 1-hexene are the acid sites rather than the metal sites over the catalyst. On the basis of the above discussion, we propose the following mechanism for olefin hydroisomerization: first, hydrogen molecules are dissociated into H+ and H- ions on the metal sites of the bifunctional catalyst, and then these ions go on the acid sites (Z) by hydrogen spillover (eqs 1-3); then, n-olefins adsorbed on the acid sites are converted into the secondary-carbenium ions (eq 4), and the latter are further converted into the tertiary-carbenium ions by the methyl transfer and subsequent hydrogen transfer (eqs 5 and 6); finally, i-paraffins are formed via the interaction between the H- ions and the tertiary-carbenium ions on acid sites (eq 7).


Figure 3. Effect of space velocity on the 1-hexene conversion and the light hydrocarbon (eC3) content in the product.

Figure 4. Effect of space velocity on the olefin content in the product of 1-hexene conversion.

Figure 5. Effect of space velocity on the product group composition of 1-hexene conversion.

The low contents of metal and alumina in catalyst SCZM and the mechanically mixed catalyst and the present reaction conditions lead to the great difficulty in hydrogenating monocyclic aromatics to naphthenes,31 so the difference between the contents of naphthenes and aromatics obtained over the two catalysts in Table 13 cannot be attributed to the hydrogenation (31) Cooper, B. H.; Donnis, B. B. L. Appl. Catal., A 1996, 137, 203– 223.

of monocyclic aromatics. This difference can be rationally explained as follows: 1-hexene molecules are first adsorbed on the acid sites, and then part of them are converted into cyclocarbenium ions that can be converted into aromatics by hydrogen transfer or dehydrogenation.16 The ratio of the paraffin content to the aromatics content over the two catalysts is less than 3.0, at which the formation of aromatics is controlled only by the hydrogen transfer reaction,32 so part of the aromatics formed over the two catalysts should come from the dehydrogenation of cyclo-carbeniums. Because the dehydrogenation reaction must be performed on the metal sites of bifunctional catalysts, the migration of cyclo-carbenium ions from the acid sites to the metal sites is necessary for aromatics formation. Thus, the increased distance between the metal sites and the acid sites results in the combination of cyclo-carbenium ions with spiltover hydrogen during the migration of cyclo-carbenium ions between the two catalytic sites, so the naphthene content increases and the aromatics content decreases over the mechanically mixed catalyst compared to those over catalyst SCZM. (32) Kitagawa, H.; Sendoda, Y.; Ono, Y. J. Catal. 1986, 101, 12–18.

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Fan et al.

Figure 6. Pathways of the olefin aromatization over HZSM-5 and Ga/HZSM-5 catalysts in the absence of hydrogen.14,15

Figure 7. Reaction pathways for the olefin hydroisomerization and aromatization over Ni-Mo/modified HZSM-5 + Al2O3 catalyst in the presence of hydrogen.

To determine the intermediates of olefin hydroisomerization and aromatization, we investigated the effect of the space velocity on the reaction performance of 1-hexene over catalyst SCZM. The results in Figure 3 show that the conversion of 1-hexene is always 100% when the space velocity is lower than 6 h-1, but decreases with increasing space velocity when the space velocity is higher than 6 h-1. This indicates that the high space velocity can restrain the conversion of 1-hexene, even if olefins have the highest reactivity among the five groups of hydrocarbons. As shown in Figure 3, the light hydrocarbon (eC3) content in the product decreases with increasing space velocity, indicating a lower cracking of 1-hexene. The effects of the space velocity on the olefin content in the product of the 1-hexene conversion and on the group composition are presented in Figures 4 and 5, respectively. Increasing space velocity reduces the residence time of 1-hexene on the catalytically active sites and thus limits the further conversion of the intermediates and increases their concentration (Figure 4). The changes in the n-paraffin and naphthene contents are small with increasing space velocity, whereas the changes in the i-paraffin, olefin and aromatics contents are pronounced (Figure 5). Because i-olefins are the precursors of i-paraffins,33 the increase in the i-olefin content will lead to a decrease in the i-paraffin content (Figure 5). The trends of the i-olefin and i-paraffin contents in Figures 4 and 5 are consistent with the above analysis. Increasing space velocity also leads to an increasing cycloolefin content (Figure 4) and a decreasing aromatics content (Figure 5), suggesting that the cyclo-olefins are the intermediates of olefin aromatization in the presence of hydrogen. According to the proposed mechanism of olefin aromatization in the absence of hydrogen (Figure 6),14,15 dienes are the initial intermediates and cyclo-olefins are the subsequent intermediates, but in our experiments no diene is detected in the presence of hydrogen, even if the space velocity is increased up to 15 h-1 (not presented here). Because the conversion of dienes into cyclo-olefins is more reasonable (33) Alvarez, F.; Ribeiro, F. R.; Pe´rot, G.; Thomazeau, C.; Guisnet, M. J. Catal. 1996, 162, 179–189.

than the direct conversion of n-olefins into cyclo-olefins from the viewpoint of the structure of the hydrocarbons investigated, this phenomenon suggests that the conversion of dienes into cyclo-olefins is too fast to be tracked. In fact, diene intermediates in olefin aromatization have, to the best of our knowledge, never been reported in literature. In summary, the above discussion suggests the reaction pathways shown in Figure 7 for the olefin hydroisomerization and aromatization in the presence of excess hydrogen as encountered in FCC gasoline hydro-upgrading. Olefin molecules are first adsorbed on the acid sites of the bifunctional catalyst, and part of them are converted into carbenium ions by combination with H+ ions spilt over from the metal sites, while the residual olefins are converted into dienes by hydrogen transfer on the acid sites. Subsequently, the carbenium ions formed are transformed into i-paraffins by rearrangement, isomerization, and hydrogen spillover, while the dienes are successively converted into cyclo-olefins and aromatics by hydrogen transfer, cyclization, and hydrogen transfer/dehydrogenation. 5. Conclusions To better explain the conversion behavior of olefins in hydroupgrading FCC gasoline, the reaction performances of model hydrocarbons of different chemical structure and real FCC gasoline were investigated in the presence of hydrogen, and a mechanism of olefin hydroisomerization and aromatization was proposed. The reaction results of the model hydrocarbons showed that among the five groups of hydrocarbons, i-paraffins and aromatics had the lowest reactivities, and olefins presented the highest reactivity. The reactivities of n-paraffins and naphthenes were much lower than that of olefins and were relatively higher than those of i-paraffins and aromatics. The hydro-upgrading results of real FCC gasoline further confirmed the preferential and unidirectional conversion of olefins into molecules of the other groups, especially i-paraffins and aromatics. The results of the olefin reaction mechanism in the presence of hydrogen showed that some olefin molecules adsorbing on


the acid sites of the bifunctional catalyst are converted into carbenium ions by combination with the spilt-over hydrogen from the metal sites and successively into i-paraffins by rearrangement, while the others are successively converted into dienes, cyclo-olefins, and aromatics by hydrogen transfer, cyclization, and hydrogen transfer/dehydrogenation.

Energy & Fuels, Vol. 23, 2009 3023 Acknowledgment. This work was supported by the National Basic Research Program of China (Grant No. 2004CB217807), the Natural Science Foundation of China (Grant No. 20606037 and Grant No. 20825621), and the New Star Plan of Beijing (Grant No. 2007B073). EF900030H

Mechanistic Pathways for Olefin Hydroisomerization and

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