Al2O3 Catalysts

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The most efficient catalyst for cyclohexane and n heptane isomerization and benzene hydroisomer ization is the platinum containing catalyst (0.3 wt % Pt) whose ...
ISSN 20700504, Catalysis in Industry, 2012, Vol. 4, No. 4, pp. 253–260. © Pleiades Publishing, Ltd., 2012. Original Russian Text © E.A. Belopukhov, A.S. Belyi, M.D. Smolikov, D.I. Kir’yanov, T.I. Gulyaeva, 2012, published in Kataliz v Promyshlennosti.

CATALYSIS IN OIL REFINING

Benzene Hydroisomerization over Pt/MOR/Al2O3 Catalysts E. A. Belopukhova, A. S. Belyia, b, M. D. Smolikova, b, D. I. Kir’yanova, and T. I. Gulyaevaa a

Institute of Hydrocarbons Processing, Siberian Branch, Russian Academy of Sciences, Omsk, 644040 Russia b Omsk State Technical University, Omsk, 644050 Russia

Abstract—Benzene hydroisomerization is among the promising processes converting benzene into methyl cyclopentane (MCP), which is an environmentally friendlier, octane boosting component of motor fuels. Benzene hydroisomerization into MCP over the Pt/MOR/Al2O3 (MOR = mordenite) catalytic system is reported here. The dependence of the yield of the target product on the acidic properties of the support and platinum precursor ([Pt(NH3)4]Cl2 or H2PtCl6) have been investigated in order to optimize the catalyst com position. The acidic properties of the surface have been altered by introducing 30–95 wt % alumina into the sup port. Catalytic activity has been measured in the hydroisomerization of cyclohexane and a benzene (20 wt %) + nheptane (80 wt %) mixture in a flow reactor at 250–350°C, 1.5 MPa, H2 : CH = 3 : 1, a cyclohexane LHSV of 6 h–1, a mixed feedstock LHSV of 2 h–1, a catalyst bed volume of 2 cm3, and catalyst pellet sizes of 0.25– 0.75 mm. The most efficient catalyst for cyclohexane and nheptane isomerization and benzene hydroisomer ization is the platinumcontaining catalyst (0.3 wt % Pt) whose support consists of 30 wt % MOR and 70 wt % Al2O3. The highest yield of the target products of isomerization in the presence of this catalyst is attained in the temperature range from 280 to 310°C, which is thermodynamically favorable for MCP formation from benzene. This indicates that this catalyst is promising for the hydroisomerization of benzenecontaining gas oline fractions. Use of H2PtCl6, a readily available chemical, as the platinum precursor is favorable for com mercialization of the catalyst and ensures price attractiveness in its industrialscale manufacturing. Keywords: mordenite, platinum catalysts, benzene hydroisomerization DOI: 10.1134/S2070050412040046

INTRODUCTION The presentday requirements imposed on the quality of motor gasoline concern not only its perfor mance characteristics (octane number, carbonform ing properties, etc.), but also its environmental appro priateness. The current environmental regulations on motor gasoline stringently restrict the concentration of aromatic hydrocarbons, including benzene, in commercial gasoline [1]. A promising process for converting benzene into methylcyclopentane (MCP) [2], an environmentally friendlier substance, is the hydroisomerization of ben zenecontaining gasoline fractions. In this process, benzene conversion is accompanied by further isomerization of the paraffins, which improves the octane characteristics of the product [2, 3]. Benzene hydroisomerization includes benzene hydrogenation to cyclohexane (CH) and the isomer ization of the latter to MCP, which has a larger octane number. Figure 1 presents data characterizing the composition of the equilibrium benzene + CH + MCP mixture [4]. If follows from these data that the optimum temperature range for MCP formation is 250–320°C, in which the maximum equilibrium MCP yield is 80–82% and the MCP/CH ratio, which characterizes the extent of equilibration, is 4.0–4.6.

The catalysts that are extensively studied in the hydroisomerization of benzene and its mixtures with alkanes and cycloalkanes are platinumcontaining bifunctional ones. Platinum is deposited onto porous solid acid supports, namely, zeolites [5, 6], sulfated zirconia [7, 8], and tungstatecontaining zirconia [9]. A drawback of the catalysts supported on sulfated and tungstatecontaining zirconia is that they cause ring opening and alkane hydrocracking—side reactions reducing the yield of MCP and other liquid prod ucts—because of the imbalance between the acidic and hydrogenating functions of the catalyst [7–9]. In the preparation of zeolitesupported catalysts in the laboratory, the most popular platinum precursor is the ammine complex [Pt(NH3)4]Cl2, which ensures rapid and strong platinum binding and a large amount of platinum adsorbed. At the same time, commercial batches of the catalyst are mainly produced using chloroplatinic acid H2PtCl6 as the platinum precursor. The purpose of this study was to optimize the acidic properties of the catalyst support via the addition of alumina as the binder to the zeolite and to elucidate the dependence of the benzene hydroisomerization performance of the catalyst on the platinum precursor ([Pt(NH3)4]Cl2 and H2PtCl6).

253

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T °C optimum 3

Concentration, wt %

80

2

1 60

40

20

0

100

200 300 Temperature, °C

400

500

Fig. 1. (1) Cyclohexane, (2) MCP, and (3) benzene concentrations in the equilibrium mixture. PH 2 = 1.2 MPa, PB = 0.15 MPa.

EXPERIMENTAL The catalyst support was mordenite (MOR) with a silica modulus of SiO2/Al2O3 = 20 (Zeolyst Interna tional Co.). The zeolite was converted to Hform by calcination at 450°C. Next, platinum was deposited onto the zeolite from a [Pt(NH3)4]Cl2 solution. Before platinum deposition, the support was loaded with ammonia by incipientwetness impregnation with a 1 N ammonia solution with pH 10–11 (2% NH3 of the zeolite weight). The platinum ammine was adsorbed on to the zeolite from its solution at room temperature. The acidity of the catalysts was regulated by dilu ting the zeolite with alumina. This was done by adding pseudoboehmite (Sasol Germany GmbH) as the binder to the zeolite. Thereafter, the mixed support was calcined at 500°C in air. The mordenite content of the mixed support was varied between 5 and 70 wt %. The supports will be designated AMOR5–AMOR70, where A = alumina, MOR = mordenite, and the num ber is the zeolite (MOR) content of the support. For preparation of the PtA/MOR and PtA/AMOR 30 catalysts, platinum was deposited by impregnation of the support with a solution of the platinum ammine [Pt(NH3)4]Cl2 (subscript A—PtA). For preparation of the of the PtAc/AMOR5–70 and PtAc/Al2O3 catalysts, platinum was deposited from a solution of the H2PtCl6 acid (subscript Ac—PtAc). The platinum content of the catalysts was 0.3 wt %. The designations and com positions of the catalysts examined are listed in Table 1. After platinum deposition, all catalysts were airdried at 120°C, and the catalysts prepared using the plati

num ammine were additionally calcined for 3 h in flowing dry air at 450°C. Catalytic tests were carried out in an isothermal fixedbed flow reactor with a thermocouple well along its axis. The reactor was charged with 2 cm3 of a cata lyst (size fraction of 0.25–0.75 mm). After being loaded into the reactor, the catalyst was reduced with flowing purified hydrogen. The temperature was grad ually raised to 350°C (over 1 h) and was maintained at this level for 1 h. The hydroisomerization activity of the catalysts was measured for two types of feedstock, namely, cyclohexane and a benzene + nheptane mix ture (20 : 80 w/w). The composition of this mixture is the same as the typical composition of the benzene containing fraction of the reformate from the initial boiling point to 85°C. The tests were performed under the following conditions: T = 250–350°C, P = 1.5 MPa, H2 : CH = 3 : 1, cyclohexane LHSV of 6 h–1, mixed feedstock LHSV of 2 h–1. The products were analyzed on line by directing the vapor–gas mixture to Tsvet 800 chromatograph fitted with a PONA/PIONA cap illary column (J&W Scientific). The measure of the activity and selectivity of the catalysts was the feed stock conversion and the yields of the target products. The acidic properties of the catalysts were studied by temperatureprogrammed desorption (TPD) of ammonia on an AutoChem II 2920 Micromeritics chemisorption analyzer with a thermalconductivity detector. Before TPD measurements, all samples were calcined at 500°C for 2 h. CATALYSIS IN INDUSTRY

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Table 1. Catalyst compositions and designations Catalyst

Pt content, wt %

Support composition

MOR



HMordenite, 100%

PtA/MOR

0.3

HMordenite, 100%

PtA/AMOR30

0.3

Al2O3, 70% HMordenite, 30%

PtAc/AMOR70

0.3

Al2O3, 30% НMordenite, 70%

PtAc/AMOR30

0.3

Al2O3, 70 % HMordenite, 30%

PtAc/AMOR10

0.3

Al2O3, 90% HMordenite, 10%

PtAc/AMOR5

0.3

Al2O3, 95% HMordenite, 5%

PtAc/Al2O3

0.3

Al2O3, 100%

RESULTS AND DISCUSSION Cyclohexane Isomerization over the MOR and Pt/MOR Catalysts For estimating the isomerizing activity of the initial zeolite and the zeolite containing 0.3 wt % Pt, the MOR and Pt/MOR catalysts were tested in cyclohex ane isomerization. The data characterizing cyclohex ane conversion are presented in Table 2 and Fig. 2. It follows from these data that the introduction of platinum markedly enhances the activity of the cata lyst: the cyclohexane conversion at 250°C increases from 11 to 74%, while isomerization selectivity

Pt precursor – [Pt(NH3)4]Cl2

H2PtCl6

remains at the 96–97% level. This is the temperature at which the highest MCP yield is obtained with the Pt/MOR catalyst, which is 71% (while the equilibrium yield is 75–76 %). For the platinumfree MOR cata lyst, the MCP yield at 350°C is 42% at an isomeriza tion selectivity of 81%. In the case of the Pt/MOR catalyst, raising the reaction temperature dramatically reduces the MCP yield, which is below 1% at 350°C and near100% cyclohexane conversion. The decrease in the cyclo hexanetoMCP isomerization selectivity in this case is explained by the acceleration of ring opening reac tions, which is due to the high acidity of the pure zeo

100

MCP yield, wt %

80

60

1 40

20

2 0 240

260

280 300 320 Temperature, °C

340

360

Fig. 2. Temperature dependence of the MCP yield in cyclohexane isomerization over the (1) MOR and (2) PtA/MOR catalysts. P = 1.5 MPa, LHSV = 6 h–1, H2 : CH = 3 mol/mol, catalyst volume of 2 cm3, size fraction of 0.25–0.75 mm. CATALYSIS IN INDUSTRY

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Table 2. Characteristics of the cyclohexane isomerization reaction

Catalyst

T, °C

CH conversion, X

CHtoMCP isomerization selectivity, Siso

Yield of stable catalysate MCP yield CH yield MCP : CH (C5+ hydrocarbons) wt %

MOR

250 280 300 320 350

11 24 35 43 52

97 92 89 86 80

99.9 99.0 98.1 97.1 95.2

10.6 22.5 30.9 36.5 41.8

89.1 75.6 65.3 57.4 48.0

0.1 0.3 0.5 0.6 0.9

PtA/MOR

250 280 300 320 350

74 81 88 96 100

96 85 61 22 0.3

99.4 95.8 85.5 60.1 16.9

71.6 69.1 53.0 20.9 0.3

25.8 18.5 12.5 4.4 0.1

2.8 3.7 4.2 4.7 3.1

PtA/AMOR30

250 280 300 320 350

29 72 81 86 95

99 96 89 73 26

99.9 99.2 96.4 89.9 63.2

28.2 69.0 72.4 62.6 24.9

71.4 28.4 18.6 14.0 5.2

0.4 2.4 3.9 4.5 4.8

PtAc/AMOR30

250 280 300 320 350

6 40 74 84 89

97 98 96 89 65

100 99.8 99.1 97.0 88.9

5.8 39.0 71.1 74.4 57.8

94.0 60.4 26.3 16.3 11.1

0.1 0.6 2.7 4.6 5.2

Note: Reaction conditions: P = 1.5 MPa, LHSV = 6 h–1, H2 : CH = 3 mol/mol, catalyst volume of 2 cm3, size fraction of 0.25–0.75 mm.

lite. At the same time, it is clear from Fig. 1 that the maximum equilibrium MCP yield is 80–82% and is attained at 290–310°C. In order to raise the MCP yield, we attempted to optimize the acidic properties of the catalyst by intro ducing alumina into the support. Table 3. Temperature ranges of ammonia desorption and total concentrations of acid sites for supports with differ ent alumina contents Desorption peak temperatures, °C

Total amount of ammonia, µmol/g

MOR

198, >500

1512

AMOR70

190, >500

1316

AMOR50

The same

955

AMOR30

187, >500

743

209

584

Support

Al2O3

TemperatureProgrammed Desorption of NH3 In the catalyst industry, alumina is often used as a potent binder imparting the necessary strength to cat alyst pellets and as a regulator of the acidic properties of the catalyst as a whole. It is also significant that alu mina is much less expensive than zeolites. The acidic properties of the support upon the intro duction of alumina into the Pt/MOR catalyst were estimated by the ammonia TPD method. We exam ined the MOR, AMOR70, AMOR30, and Al2O3 supports, as well as AMOR50, which was prepared for a more detailed investigation of how the acidic proper ties of the support change as its mordenite content is decreased. The TPD data are presented in Fig. 3 and Table 3. The TPD profiles (Fig. 3) show two temperature ranges of ammonia desorption. The first range, which is attributed to weak acid sites, is 100–250°C. The sec ond ammonia desorption region is above 480°C and characterizes strong acid sites [10]. It is clear from the TPD profiles that, as the zeolite is progressively CATALYSIS IN INDUSTRY

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257

600 1

0.40

T 500

2 TCD signal

0.30

600

0.25

3

0.20

4

300 0.15

Temperature, °C

0.35

200 5

0.10

100 0.05 0

10

20

30

40 50 60 Time, min

70

80

90

0 100

Fig. 3. Temperatureprogrammed desorption of ammonia from supports containing different percentages of alumina: (1) MOR, (2) AMOR70, (3) AMOR50, (4) AMOR30, and (5) Al2O3.

100

MCP yield, wt %

80

3 60

40

2 20

1 0 240

260

280

320 300 340 Temperature, °C

360

Fig. 4. Temperature dependence of the MCP yield in cyclohexane isomerization over the (1) PtА/MOR, (2) PtA/AMOR30, and (3) PtAc/AMOR30 catalysts. P = 1.5 MPa, LHSV = 6 h–1, H2 : CH = 3 mol/mol, catalyst volume of 2 cm3, size fraction of 0.25– 0.75 mm.

diluted with alumina, the position of the lower tem perature peak remains unchanged and only the pro portions of strong and weak acid sites vary in the addi tive way.

prepared on the mixed support AMOR30 (30 wt % mordenite + 70 wt % Al2O3) by platinum deposition from platinum ammine and chloroplatinic acid solu tions. For comparison, we present the same data obtained for the aluminafree Pt/MOR catalyst.

Platinum Precursor Effect on Cyclohexane Isomerization Table 2 and Fig. 4 present the results of the investi gation of cyclohexane isomerization over the catalysts

For both catalysts on the mixed support, the MCP yield curves are shifted to higher temperatures relative to the same curve for the catalyst supported on pure zeolite. For the catalyst prepared using H2PtCl6, this

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Table 4. Hydroisomerization data for the benzene (20 wt %) + nheptane (80 wt %) mixture Catalyst PtAc/AMOR70

PtAc/AMOR30

PtAc/AMOR10

PtAc/AMOR5

PtAc/Al2O3

Temperature, °C

CH yield, MCP yield, MCP/CH % % ratio

nC7 conversion, %

iC7 yield, C5+ yield, % %

300

3.8

16.2

3.85

78

35.7

78.4

310

3.2

14.9

3.99

87

22.3

52.6

300

3.9

15.7

4.05

69

35.0

81.0

310

3.3

14.7

4.43

80

33.7

70.3

300

5.0

15.0

2.77

48

29.5

92.2

310

3.8

16.0

4.18

64

36.9

86.7

300

6.2

10.7

1.73

33

25.4

96.8

310

4.6

13.7

3.01

49

33.5

94.1

300

19.7

0.2

0.01

traces

0.9

99.6

310

19.5

0.3

0.01

traces

0.9

99.6

Note: Conditions: P = 1.5 MPa, LHSV = 2 h–1, H2/CH = 3 mol/mol, catalyst volume of 2 cm3, size fraction of 0.25–0.75 mm.

shift is approximately 20°C larger than for the catalyst prepared from [Pt(NH3)4]Cl2. The highest MCP yield with the PtA/AMOR30 catalyst is 72 wt % and is attained at 300°C, with MCP : CH = 3.9. For the PtAc/AMOR30 catalyst, the highest MCP yield is 74% and is reached 320°C, with MCP : CH = 4.6. Thus, the highest MCP yield is afforded by the PtAc/AMOR30 catalyst, which is pre pared using chloroplatinic acid. The introduction of 70 wt % alumina reduces the activity of the catalyst, but enhances its selectivity toward the target isomerization reaction. The observed slight increase in the MCP yield is due to the cyclo hexane isomerization reaction shifting to higher tem peratures, which are thermodynamically favorable for MCP formation. The equilibrium MCP : CH weight ratio is 1.7 at 250°C and 4.6 at 300°C [4]. The stron gest effect of alumina in respect of the shift of the reac tion to higher temperatures and approach to equilib rium values is observed for the catalysts prepared using H2PtCl6. Thus, by modifying the acidic properties of the support, it is possible to control the efficiency of the catalyst. Benzene Hydroisomerization For the benzene hydroisomerization catalyst, we optimized the composition of the zeolite support by introducing 30 to 95 wt % alumina. The changes in the acidic properties of the mixed support were estimated from the outcomes of the nheptane isomerization in the benzene (20 wt %) + nheptane (80 wt %) mixture. The results of the conversion of this model mixture are

listed in Table 4 and Figs. 5 and 6. Clearly, the MCP and heptane isomer yields pass through a maximum in the 290–310°C range and then fall off. The decrease in the yield of the target products at higher tempera tures is due to the side reactions, namely, ring opening and heptanes hydrocracking, which result in the for mation of considerable amounts of C5 and C6 alkanes and C1–C4 gaseous hydrocarbons. As the zeolite content of the catalyst is decreased from 70 to 30%, the catalytic activity curve shifts to higher temperatures by 20°C. Nevertheless, the high est yield of of the target products remains unchanged because the reaction occurs in the thermodynamically favorable temperature range. The data presented in Table 4 indicate that, for the catalysts containing 30 and 70% zeolite at 300 and 310°C, the MCP : CH ratio is close to its equilibrium value (3.9–4.4), which is attained as the reaction tem perature is raised. However, for the sample containing 70 wt % zeolite, raising the temperature brings about heptane hydrocracking and ring opening reactions, which decrease the MCP yield and the total yield of liquid products. The decrease in the acidity of the mordenitesupported platinum catalyst because of the introduction of alumina (Fig. 3) and because of the decrease of the zeolite content to 30 wt % ensures the retention of the high MCP yield (15–16 wt %) owing to the shift of the reaction to higher temperatures and owing to the increase in the cyclohexane isomerization selectivity. Therefore, the optimum component ratio is MOR : Al2O3 = 30 : 70, which reduces the cost of the catalyst and increases its mechanical strength. CATALYSIS IN INDUSTRY

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25 PtAc/MOR70 PtAc/MOR30 PtAc/MOR10 PtAc/MOR5 PtAc/Al2O3

MCP yield, wt %

20

15

10

5

0 200

220

240 260 280 Temperature, °C

300

320

Fig. 5. Temperature dependence of the MCP yield in the hydroisomerization of the benzene (20 wt %) + nheptane (80 wt %) mixture over the PtAc/AMOR catalysts. P = 1.5 MPa, LHSV = 2 h–1, H2 : hydrocarbons = 3 mol/mol, catalyst volume of 2 cm3, size fraction of 0.25–0.75 mm.

50 45

PtAc/MOR70 PtAc/MOR30 PtAc/MOR10 PtAc/MOR5 PtAc/Al2O3

40 iC7 yield, wt %

35 30 25 20 15 10 5 0 2.0

220

240 260 280 Temperature, °C

300

320

Fig. 6. Temperature dependence of the yield of heptane isomers in the hydroisomerization of the benzene (20 wt %) + nheptane (80 wt %) mixture over the PtAc/AMOR catalysts. P = 1.5 MPa, LHSV = 2 h–1, H2 : hydrocarbons = 3 mol/mol, catalyst volume of 2 cm3, size fraction of 0.25–0.75 mm.

CONCLUSIONS It has been demonstrated that the introduction of Al2O3 into the zeolitic support is a way of regulating the acidity of the Pt/MOR/Al2O3 catalyst. The decrease in the acidity on passing to the modified cat alyst is due to the difference between the inherent acidities of the zeolite and alumina. The most efficient catalyst in cyclohexane and n heptane isomerization and benzene hydroisomeriza tion is the 0.3 wt % Pt catalyst whose support consists CATALYSIS IN INDUSTRY

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of 30 wt % mordenite and 70 wt % alumina. The high est yield of the target products of isomerization in the presence of this catalyst is attained in the temperature range from 280 to 310°C, which is thermodynamically favorable for MCP formation from benzene. There fore, this catalyst is promising for use in the hydroi somerization of benzenecontaining gasoline frac tions. Use of H2PtCl6, a readily available chemical, as the platinum precursor is favorable for commercialization

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of the catalyst and ensures its price attractiveness in its industrialscale production.

5. Jiménez, C., Romero, F.J., Roldán, R., Marias, J.M., and Gómez, J.P., Appl. Catal., A, 2003, vol. 249, p. 175.

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7. Miyaji, A. and Okuhara, T., Catal. Today, 2003, vol. 81, p. 43. 8. Lavrenov, A.V., Kazakov, M.O., Duplyakin, V.K., and Likholobov, V.A., Pet. Chem., 2009, vol. 49, no. 3, p. 218. 9. Benitez, V.M., Grau, J.M., Yori, J.C., Pieck, C.L., and Vera, C.R., Energy Fuels, 2006, vol. 20, p. 1791. 10. Hidalgo, C.V., Iton, H., Hattory, T., Niva, M., and Murakami, Y., J. Catal., 1984, p. 362.

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