Isobutane Alkylation

A clear example of the possible use of acid and/or superacid solids as catalysts is the alkylation of isobutane with butenes. Isobutane alkylation with low-molecular-weight olefins is one of the most important refining process for the production of high-octane number (RON and MON), low red vapor pressure (RVP) gasoline.32 Currently, the reaction is carried out using H2SO4 or HF (Table 13.1), although several catalytic systems have been studied in the last few

32-34

years.32 34

Adequate acid characteristics of solids are needed since the selectivity to tri-methylpentanes (TMP), and especially the 2,2,4-isomer, strongly depends on the nature and strength of the acid sites of the catalyst. Thus, excessively strong acidity will drive the reaction toward cracking reactions, while an acidity that is too low will mostly produce oligomerization of the olefins (Figure 13.5).9,34

TABLE 13.3 Isobutane/2-Butene Alkylation over Acidic Salts of 12-Tungstophosphoric Acids

Catalyst

H3PW12O40

(NH4)2HP W12O40

Cs2.5H0.5P W12O40

Cs2.5H0.5P W12O40

2-Butene

56.9

45.3

73.1

85.7

conversion, %

Yield of TMP, %

35.4

34.4

38.5

41.1

TMP/DMH

4.5

3.7

2.8

3.4

Source: After Ref. 36.

Source: After Ref. 36.

Moreover, the efficiency of these catalysts could be modified by tailoring the nature of the metal oxide support and/or reaction conditions (especially the reaction temperature). In this way, interesting conclusions can be obtained when comparing the isobutane/2-butene alkylation catalyzed on two of the most studied catalysts, that is, beta zeolite and sulfated zirconia, when operating at different reaction temperatures. (Table 13.2).19

The catalyst with lower acid strength (beta zeolite) presents the higher selectivity to (TMP) at high reaction temperatures, while the sulfated zirconia presents an opposite trend: the lower the reaction temperature, the higher the selectivity to TMP is. In the case of sulfated zirconia catalysts, cracking rather than alkylation is favored at high reaction temperatures, while oligomerization rather than alky-lation is favored on the beta zeolite at low reaction temperature.

Contrarily, the nature of the metal oxide in sulphate-based catalysts or the chemical composition of heteropolyoxometaltes also influences their catalytic

Zrd2 Sn02 Ti02 Fe203 Al203 Metal oxide support

Figure 13.6 Variation of both the conversion of 2-butene and the selectivity to trimethyl-pentanes (TMP), with the nature of the metal oxide support obtained during the isobutene/ 2-butene alkylation at 32°C over sulfated supported on different metal oxides. (After Ref. 35.)

Zrd2 Sn02 Ti02 Fe203 Al203 Metal oxide support

Figure 13.6 Variation of both the conversion of 2-butene and the selectivity to trimethyl-pentanes (TMP), with the nature of the metal oxide support obtained during the isobutene/ 2-butene alkylation at 32°C over sulfated supported on different metal oxides. (After Ref. 35.)

TABLE 13.4 Isobutane/2-Butene Alkylation over H3PWi2O40/ SO2"/ZrO2 Catalyst

2-Butene

Selectivity to

TMP Selectivity

Reaction Cycle

Conversion, %

Saturated C8, %

Isoctanes Fraction,a%

Fresh catalyst

93

95

76

First

92

55

69

Second

95

40

62

Third

91

91

70

Eleventh

83

78

84

Source: After Ref. 40. aTMP x 100/C8.

Source: After Ref. 40. aTMP x 100/C8.

behavior. Thus, during the isobutane/butenes alkylation over sulfated metal oxides at 309 K, both the conversion of 2-butene and the selectivity to TMP decrease with the lower acid character of the metal oxide support: SO2 /ZrO2 > SO4"/TiO2 > SO2 /SnO2 > SO4_/Fe2O3 > SO4_/Al2O3 (Figure 13.6).35 However, it has been observed that the last catalytic trend can be modified by changing the reaction temperature.35

In the case of heteropolyoxometalates, the compositions can also be tailored in order to achieve good catalytic properties. Table 13.3 shows the variation of the catalytic performance of acidic salts of 12-tungstophosphoric acids during the iso-butene/2-butene alkylation.36

It can be seen that the catalytic activity strongly depends on the number and type of the incorporated countercation, which determines the number and strength of acid sites. In addition to this, the existence of mesoporosity (which also depends on the countercation) is also a key factor in the catalytic behavior of these catalysts.37 In this way, SiO2_ or MCM-41-supported heteropolyacids also have been studied in order to increase catalytic activity, apparently without modifying the acid strength.38 Figure 13.7 shows an effective confrontation between the catalytic results observed on nafion/SiO2, sulfated zirconia, and 12-tungstophosphoric acid supported on MCM-41 (HPW/MCM) at a reaction temperature of 50°C.9 Under these conditions, the HPW/MCM catalyst presents the better yield of TMP. However, similar productivities to that could be obtained on nafion/SiO2,9 or sul-fated zirconia,19 at higher or lower temperatures, respectively, than those optimized for the HPW/MCM-41 catalyst.38

Moreover, the catalyst deactivation must also be considered in order to use these solid materials in industrial processes. Figure 13.8 shows the variation of catalytic activity (2-butene conversion) with the time on stream obtained under the same reaction conditions on different solid-acid catalysts. It can be seen how all the solid-acids catalysts studied generally suffer a relatively rapid catalyst deactivation, although both beta zeolite and nafion-silica presented the lower catalyst decays. Since the regeneration of beta zeolite is more easy than of nafion, beta zeolite was considered to be an interesting alternative.39

¡22 Cracking

□ TMP

^ c9+yc5+

Figure 13.7 Conversion of 2-butene and the selectivities to cracking products, TMP, and C9+ hydrocarbons during the isobutane alkylation at 50°C on nafion/SiO2 (NS-1), sulfated zirconia (SZ), and MCM-41-supported 12-tungstophosphoric acid (HPW/MCM). Experimental conditions: T = 32°C; TOS = 1 min; olefin WHSV = 1 h_1: isobutane/2-butene molar ratio of 15.

NS-1 SZ HPW/MCM

Catalyst

Figure 13.7 Conversion of 2-butene and the selectivities to cracking products, TMP, and C9+ hydrocarbons during the isobutane alkylation at 50°C on nafion/SiO2 (NS-1), sulfated zirconia (SZ), and MCM-41-supported 12-tungstophosphoric acid (HPW/MCM). Experimental conditions: T = 32°C; TOS = 1 min; olefin WHSV = 1 h_1: isobutane/2-butene molar ratio of 15.

Figure 13.8 Catalyst decay during the isobutane alkylation on nafion/SiO2, sulfated zirconia, beta-zeolite, and MCM-41-supported 12-tungstophosphoric acid.

The use of a polyfunctional catalyst could enhance the life of the catalyst. A clear example is the use of H3PW12O40-SO2~/ZrO2 mixtures for isobutane/ butenes alkylation (Table 13.4).40 However, modifications of the type of reactor could also favor extended catalyst longevity.41,42 During the last few years, other alternatives have been proposed that favor a better catalyst regeneration and/or lower catalyst deactivation: the use of supercritical isobutene regeneration43 or dense-CO2 enhanced the reaction media.44

Recently, Exelus Inc. has developed an innovative isobutene alkylation technology (ExxSact) as an economically viable alternative to the hydrofluoric acid or sulphuric acid processes.45 In that case, both unique solid-acid catalyst (in which the strength and distribution of the acid sites reduce olefin oligomerization and paraffin cracking) and a novel fixed-bed reactor (using an innovative fluid dynamics and an unusual reactor residence time distribution) promises significantly improved yields and selectivity minimizing waste by-products and disposal problems associated with spent catalyst and regeneration of large quantities of liquid acids.

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