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performed using sulfated zirconia catalysts with the addition of platinum, Pt/SO4 ... Zirconium hydroxide was obtained by precipitation from Zr(SO4)2 with an ...

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ScienceDirect Procedia Engineering 152 (2016) 116 – 121

International Conference on Oil and Gas Engineering, OGE-2016

Isomerization of n-hexane on Pd/SO42-/ZrO2/Al2O3 and mechanical mixtures Pd/Al2O3 (Pd/SiO2) + SO42-/ZrO2/Al2O3 Dzhikiya O.V.a *, Smolikov M.D.a,b, Zatolokina E.V.a, Kazantsev K.V.a, Belyi A.S.a,b a

Institute of Hydrocarbon Processing, 54 Neftezavodskaya St, Omsk 644040, Russian Federation b Omsk State Technical University, 11 Mira Pr., Omsk 644050, Russian Federation

Abstract Pd/Al2O3, Pd/SiO2 and their mechanical mixtures with SO42-/ZrO2/Al2O3 were prepared and tested in isomerization of nhexane. The best catalytic performance was reached on the alumina systems where a part of palladium atoms was in the charged state. © 2016 2016The TheAuthors. Authors.Published Published Elsevier © by by Elsevier Ltd.Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the Omsk State Technical University. Peer-review under responsibility of the Omsk State Technical University Keywords: palladium; sulfated zirconia; silica; alumina; n-hexane isomerization.

1. Introduction The current world trend for motor gasolines is the production of environmentally safe high-octane motor fuels for the improvement of ecological situation on the planet. The requirements to environmental safety of fuels are regularly revised to become more stringent. To meet the ecological requirements is a difficult task for oil refineries since the tough standards for the content of aromatic compounds and olefins in gasoline limit the use of reformed and catalytic cracking gasolines that are produced by the main oil refining processes [1]. Their fraction in the gasoline stock is compensated by the processes yielding high-octane di- and trimethyl-substituted isomers of normal alkanes, the content of which is not regulated. One of such processes is the isomerization of pentane-hexane fraction. In industry, isomerization of С5-С6 fraction is performed using sulfated zirconia catalysts with the addition of platinum, Pt/SO 42-/ZrO2/Al2O3. Sulfated zirconia SO42-/ZrO2 possesses a high acidity, which makes it possible to perform skeletal isomerization

* Corresponding author. Tel.: +7-3812-673-334. E-mail address: [email protected]

1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the Omsk State Technical University

doi:10.1016/j.proeng.2016.07.641

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of normal alkanes at low temperatures, 140-200°С, with a high selectivity; however, sulfated zirconia rapidly deactivates [2,3]. The deactivation may be caused by coking [4-6], a loss of surface sulfur compounds [7, 8], changes in the oxidation state of sulfur [9], and transformation of the catalytically active tetragonal modification of ZrO2 into inactive monoclinic species in the course of reaction [10]. It is known that the promotion of SO 42-/ZrO2 with platinum group metals increases activity and stability of the system in isomerization of С 5-С6 alkanes. The Pt/SO42-/ZrO2 systems have been studied in detail. From a practical standpoint, it is important to elucidate the role of other metals in the catalysts based on SO42-/ZrO2. Pd, Ni and Ir are also employed for the promotion of sulfated zirconia. However, such studies are quite scarce [12-17]. Our previous study [18] showed that the charge state of Pd atoms affects the performance of n-hexane isomerization. The present work is a continuation of our study on the role of palladium state in the catalysts for nhexane isomerization. The approach used to investigate the role of palladium state in Pd/SO 42-/ZrO2/Al2O3 was based on a comparison of the complex catalytic system with simpler and well characterized catalysts. Such catalysts were represented by mechanical mixtures consisting of the “metallic component” Pd/Al2O3 (or Pd/SiO2) and the “acidic component” SO42-/ZrO2/Al2O3. 2. Experimental Catalyst preparation PdCl/Al2O3, PdAc/Al2O3, PdCl/SiO2. The catalysts with Al2O3 were synthesized using powdered aluminum hydroxide. At the peptization step, the powder was supplemented with an aqueous solution of oxalic acid (7 wt.%). The obtained pulp was extruded and then dried at 120°С for 2 hours. The support was calcined in flowing dry air at 520°С for 1 hour. Textural characteristics of Al2O3: SBET = 250 m2/g, and Vpore = 0.46 cm3/g. PdCl/SiO2 was prepared with silica having the following characteristics of the pore structure: SBET = 340 m2/g, and Vpore = 0.98 cm3/g. Prior to palladium deposition, the synthesized supports were evacuated to remove air from the pores, and then they were impregnated with a palladium-containing solution. The impregnating solutions were prepared using PdCl2 and Pd(CH3COO)2 salts (JSC Krastsvetmet). After the impregnation, PdCl/Al2O3 and PdAc/Al2O3 catalysts were dried at 120°С for 2 hours and then calcined in flowing dry air at 450°С for 1 hour and reduced in a flow of purified H2 at 500°С for 1 hour. “PdCl” and “PdAc” in the catalyst names indicate the use of PdCl2 and Pd(CH3COO)2 salts, respectively (see Table 1). The PdCl/SiO2 catalyst was dried at 200°C for 2 hours and then reduced in a flow of purified H2 at 250°C for 1 hour. SO42-/ZrO2/Al2O3 (SZA). Zirconium hydroxide was obtained by precipitation from Zr(SO4)2 with an aqueous solution of ammonia at a constant pH 9-10. The resulting precipitate was filtered and washed with distilled water in order to bring pH to 7. After washing, zirconium hydroxide was reinforced with aluminum hydroxide at a ratio of 80%: 20% referred to ZrO2 and Al2O3, respectively. Zirconium hydroxide was sulfated at the step of plasticization of a mixture of zirconium hydroxide and aluminum hydroxide. 63% sulfuric acid served as the sulfating agent. The obtained pulp was extruded and then dried at 120°С for 2 hours and calcined in a muffle furnace at 650°С for 3 hours. The content of SO42- in SZA was 7 wt.%. Textural characteristics of SZA: Ssp = 114 2/g, and Vpore = 0.34 cm3/g. Table 1. Catalyst compositions and pretreatments. Catalyst

Metal precursor

CPd, wt. %

Тcal*, °С

Тred**, °С

PdCl/Al2O3

PdCl2

0.4

450

500

PdAc/Al2O3

Pd(CH3COO)2

0.6

450

500

PdCl/SiO2

PdCl2

0.3

-

250

SZА

-

-

650

-

PdCl/SZА

PdCl2

0.6

400

250

*Calcination temperature.

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O.V. Dzhikiya et al. / Procedia Engineering 152 (2016) 116 – 121 **Reduction temperature.

Pd/SO42-/ZrO2/Al2O3. Sulfated zirconia prepared by the technique described above was evacuated to remove air and then impregnated with a palladium-containing solution. The impregnating solution was prepared with the use of PdCl2 (JSC Krastsvetmet). After the impregnation, the catalyst was dried at 120°C for 2 hours and calcined in flowing dry air at 450°С for 1 hour. The synthesized catalysts and their treatments are listed in Table 1. Mechanical mixtures were prepared by mixing each of “metallic” components – PdCl/SiO2, PdCl/Al2O3, and PdAc/Al2O3 – with the “acidic” component – SO42-/ZrO2/Al2O3. Weight ratios of “metallic” and “acidic” components in the mixtures were varied in a series of 10:90, 25:75, 50:50, 75:25, 90:10. Experimental procedure The pore structure was examined using the nitrogen adsorption isotherms taken at 77 K on Sorptomatic 1900 instrument. In the calculation of specific surface area, the value of molecular nitrogen loading site in the filled monolayer was taken equal to 0.162 nm2. The total pore volume was found from the nitrogen adsorption at 77 K and P/Ps = 0.996. Molar volume of liquid nitrogen was taken equal to 34.68 cm3/mol. Isomerization of n-hexane was studied in a flow setup with the isothermal tubular reactor having a stationary catalyst bed in a temperature range of 140–500°С at a pressure of 1.5 MPa, liquid hourly space velocity of the raw material of 2 h–1, and H2/n-C6 = 3/1 (mol). The raw material – n-hexane (OOO Omskreaktiv) – was dried on a NaX molecular sieve. The reaction products were analyzed using a Tsvet-800 chromatograph with a flame-ionization detector on a capillary column Petrocol DH 50.

3. Results and discussion Fig. 1 illustrates results of the catalytic testing of (PdCl/Al2O3+SZA) catalysts having different ratio of components.

Fig. 1 The conversion of n-hexane (a) and the yield of hexane isomers (b) over mixed (PdCl/Al2O3+SZA) catalysts. Ratios of “metallic” and “acidic” components: 10:90, 25:75, 50:50, 75:25, 90:10.

One can see from the presented data that (Pd Cl/Al2O3+SZA) catalysts with the prevalence of “metal” (90:10) or “acid” (10:90) components are low active. In the case of the (PdCl/Al2O3+SZA) mixture with the component ratio of 90:10, the low activity can be attributed to insufficient number of acid sites that enable isomerization of n-hexane at low temperatures. The low initial activity of the (Pd Cl/Al2O3+SZA) catalyst with the 10:90 ratio may be caused by deactivation of the “acidic” component of SZA already at the reaction step due to insufficient amount of “metallic” component, which provides hydrogenation of the coke precursor. Since the reaction proceeds in the presence of

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hydrogen, a rise of temperature leads to partial regeneration of SZA, which enhances the catalyst activity. The best catalytic performance was reached over mixed (Pd Cl/Al2O3+SZA) catalysts with the component ratio of 50:50. Fig. 2 displays the results obtained for isomerization over “metallic” components (a) and mixed catalysts (b). The data obtained over the Pd/SO42-/ZrO2/Al2O3 catalyst are shown for comparison purposes.

Fig. 2. The conversion of n-hexane over “metallic” components (a) and their mixtures with SZA (b) versus temperature. Designations: PdCl/Al2O3, PdAc/Al2O3, PdCl/SiO2, (PdCl/Al2O3 + SZA), (PdAc/Al2O3 + SZA), (PdCl/SiO2 + SZA) Pd/SO42/ZrO2/Al2O3.

It is seen that PdCl/Al2O3, PdAc/Al2O3, and PdCl/SiO2 catalysts are active in a high-temperature region of 400500°С. Due to the low acidity of silica, isomerization over the PdCl/SiO2 catalyst occurs on metallic palladium [18], similar to isomerization over Pt/SiO2 catalysts [19-20]. Alumina-containing samples are more active, which may be related to the state of palladium in these systems. Fig. 2b and Table 2 show data for mixed catalysts with the component ratio of 50:50. For all mechanical Table 2. Characteristics of mixed catalysts and Pdcl/SZA.

Catalyst

Pd/SiO2 + SZA

PdАс/Al2O3 + SZA

T, °C

Conversion X, wt.%

Selectivity S, %

Yield of

Yield of

i-C6, wt.%

2,2-DMB, wt.%

180

2.1

100

2.1

0

0

200

6.1

100

6.1

0

0

220

22.1

91

20.1

0

0

240

74.5

60

44.5

3.2

4.5

260

85.2

35

29.6

2.5

5.7

180

54.7

96

52.4

4.0

4.1

200

73.0

92

67.4

7.9

8.3

220

81.2

84

68.5

11.5

13.1

240

86.1

68

58.3

13.0

18.0

2,2-DMB/∑C6

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Pdcl/Al2O3 + SZA

Pdcl/SZA

180

68.8

93

63.7

7.1

7.5

200

79.2

88

69.7

10.7

11.8

220

84.6

77

64.9

13.4

16.8

240

89.4

55

48.9

13.3

22.4

140

89.6

93

83.6

34.8

37.0

160

89.8

84

75.8

30.3

35.3

180

88.5

81

72.1

24.9

29.8

200

89.1

70

62.1

20.3

27.9

220

92.4

46

42.1

14.5

29.2

mixtures, the temperature region of n-hexane isomerization is shifted toward lower temperatures. It is seen that the mechanical mixture (Pd/SiO2 + SZA), where palladium is in the metallic state, is low active in the low-temperature region: at 220°С the conversion of n-hexane reaches 22 wt.%. Similar to the case with “metallic” components, mixed alumina-containing catalysts are more active: at 220°С the conversion of n-hexane reaches 81-85 wt.% (see Table 2). This may be caused by a greater acidity of alumina, whose acid sites can also contribute to the total activity of these mixtures. It should be noted that the mixed catalysts with alumina contain not only the metallic atoms of Pd0 but also the charged palladium atoms, which are expected to contribute also to the activity of these systems, especially in the low-temperature region of 180-220°С. Thus, activity of the mixture (PdCl/Al2O3+SZA), where palladium is mostly in the charged state, is higher as compared to (PdAc/Al2O3+SZA), where the major part of palladium is in the metallic state. The conversion at 180°С is equal to 69 and 55 wt.%, selectivity is 93 and 96 wt.%, and the yield of hexane isomers is 64 and 52 wt.%, respectively, for these mixtures (see Table 2). The results obtained indicate that the best catalytic performance in isomerization of n-hexane was observed for the mixed catalysts in which a part of palladium atoms is in the charged state. 4. Conclusion Mixed alumina-supported catalysts efficiently catalyze isomerization of n-hexane. The catalytic performance of these mixtures in the temperature region of 180-220°С is close to that of palladium sulfated zirconium catalysts Pd/SO42-/ZrO2/Al2O3. Among the factors determining high catalytic performance of these systems is the presence of charged palladium atoms in their active sites along with metallic Pd 0 atoms. The results obtained on mechanical mixtures indicate that the performance of Pd/SO42-/ZrO2/Al2O3 catalysts can be improved by developing the synthesis techniques that ensure the presence of charged palladium atoms.

Acknowledgments Physicochemical properties of the catalysts were studied using facilities of the Omsk Research Collaboration Center SB RAS. The work was supported by the Russian Foundation for Basic Research (Project No. 16-43-550196 r_a).

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