Selectivity isobutane alkylation

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/Si02 (NS-1), sulfated zirconia (SZ), and MCM-41-supported 12-tungstophosphoric acid (HPW/MCM). Experimental conditions T = 32 C TOS = 1 min molar ratio of 15.

The zeolite composition and structure, which can affect hydrogen transfer activity, are important parameters determining the activity, selectivity, and stability of the zeolite during isobutane alkylation. In the case of USY zeolites, a maximum initial 2-butene conversion was observed for a framework Si/Al ratio of about 6 (63). However, the TMP/DMH ratio, which can be taken as a measure of the alkylation/oligomerization ratio, continuously increased when decreasing the framework Si/Al ratio. On the other hand, the amount and nature of extraframework Al (EFAL) species also affected the alkylation properties of USY zeolites (64). 

The relative location of refinery and acid plant is one of the most important factors in the economic decision between sulfuric acid and anhydrous hydrogen fluoride as a catalyst for alkylation. Besides the distance, other factors such as regeneration of spent acid, energy costs, the nature of the feed and increasingly stringent regulatory constraints play an important role in the selection of alkylation catalyst. Sulfuric acid is selected for alkylation if feed is rich in pentenes or n-butene. HF is selected if the feed is rich in propenes or isobutane. 

One of these is the question of where does the reaction occur It is often assumed that alkylation proceeds in the bulk acid phase (12a), but one of the aims of this report is to show that alkylation must proceed in at least two phases and that the reaction occurring at the hydrocarbon-acid interface is by far the most important in controlling the quality or selectivity of alkylate i.e. the formation of a Cg fraction from isobutane plus butenes while minimizing the production of side products. The fact that alkylation does not occur uniformly throughout the acid has been recently suggested by Doshi and Albright (12b). 

Dealumination of Y zeolite by H4EDTA removes aluminium not only from the framework but also from the interstitial spaces resulting in samples completely free from extraframework aluminium. However, the degree of dealumination without structural collapse was rather low. Mildly dealuminated Y zeolites are found to be more active and selective for isobutane alkylation. 

An afterglow microwave plasma with stabilized pulse power was applied to the activation of zeolite catalysts for isobutane alkylation with butenes. It was found that the pretreatment of zeolite catalysts in a microwave plasma discharge affected their properties. The catalysts exhibited higher activity, stability in operation, and selectivity (the fraction of trimethylpentanes in the alkylate increased). The properties of catalysts after plasma activation depend on the treatment conditions such as plasma temperature and nonequilibrium character and depend only slightly on the initial activity of catalysts, which is primarily controlled by the catalyst preparation conditions. 

Aluminum chloride has been known for a long time to catalyze this reaction. H owever, its high acidity leads to low selectivity for alkylate. Acidic chloroaluminates proved to be interesting alternative catalysts and solvents [28] because it is possible to tune their Lewis acidity by adjusting their composition. The alkylation of ethylene or butene with isobutane has been performed in continuous-flow pilot plant operation at IFF. The feed, a mixture of olefin and isobutane, is pumped continuously into the well-stirred reactor, which contains the IL catalyst In the case of ethylene, which is less reactive than butene, [pyridine, HClj/AlClj (1 2 molar ratio) IL proved to be the best candidate. The reaction can be run at room temperature and provides good quality alkylate (2,3-dimethylbutanes is the major product) over a period of 300 h (MON = 90-94 RON = 98-101). 

On the other hand, for theUS Y-catalyzed alkylation of isobutane with frans-2-butene at high levels of conversion (100%), adding SCCO2 decreased the catalytic longevity and product selectivity. The alkylation of toluene with propylene over siUcon-modifled HZSM-5 zeolite using SCCO2 increased the yield of cymene and reduced the cracking of propylene compared with the reaction under atmospheric pressure (Scheme 41). ... 

Fluorinated silica and alumina have also been used for isobutane alkylation with olefins in a batch reactor at 0°C. The fluorinated alumina was active only when mixed with BF3 H2O, but the fluorinated silica was active by itself The selectivity to trimethylpentanes obtained with these catalysts is much lower than that obtained with H2SO4 as a catalyst (48). At 80°C, however, F/AI2O3 catalysts are active for isobutane/2-butene alkylation, though relatively low butene conversions (27% for the most active catalyst containing 1.3% F) are obtained (49). As expected, octenes are the predominant hydrocarbons in the Cg fraction at such a low conversions. The F/AI2O3 catalysts contained both Bronsted and Lewis acid sites in a proportion that depends on the F loading. A good correlation between... 

A variety of solid acids besides zeolites have been tested as alkylation catalysts. Sulfated zirconia and related materials have drawn considerable attention because of what was initially thought to be their superacidic nature and their well-demonstrated ability to isomerize short linear alkanes at temperatures below 423 K. Corma et al. (188) compared sulfated zirconia and zeolite BEA at reaction temperatures of 273 and 323 K in isobutane/2-butene alkylation. While BEA catalyzed mainly dimerization at 273 K, the sulfated zirconia exhibited a high selectivity to TMPs. At 323 K, on the other hand, zeolite BEA produced more TMPs than sulfated zirconia, which under these conditions produced mainly cracked products with 65 wt% selectivity. The TMP/DMH ratio was always higher for the sulfated zirconia sample. These distinctive differences in the product distribution were attributed to the much stronger acid sites in sulfated zirconia than in zeolite BEA, but today one would question this suggestion because of evidence that the sulfated zirconia catalyst is not strongly acidic, being active for alkane isomerization because of a combination of acidic character and redox properties that help initiate hydrocarbon conversions (189). The time-on-stream behavior was more favorable for BEA, which deactivated at a lower rate than sulfated zirconia. Whether differences in the adsorption of the feed and product molecules influenced the performance was not discussed. 

The 1-butene conversion and product distribution obtained at 25°C after 1 h of alkylation reaction of isobutane on JML-I50 and Beta catalysts are summarized in Table 6.1. The conversion (97%) with JML-I50 catalyst is higher than that (86%) with zeolite Beta. The primary products with the above catalysts are Cs compounds (59.9% with JML-I50 and 62% with Beta). The Cg products mainly consist of trimethylpentanes (TMPs), 58.7% for JML-I50 and 73% for zeolite Beta. The TMP/DMH (dimethylhexane) ratios are 13.5 for JLM-I50 and 4.1 for Beta, demonstrating that the selectivity of JML-I50 is higher than that of zeolite Beta. The yields of alkylate are 6.6 mL and 5.2 mL for JML-I50 and Beta zeolite, respectively. The weights of alkylate produced per weight of butene fed to the reactor are 1.13 and 0.95 for JML-I50 and zeolite Beta, respectively. 

Rorvik, T., Mostad, H.B., Karlsson, A., and Ellestad, O.H. (1997) Isobutane/ 2-butene alkylation on fresh and regenerated La-EMT-51 compared with H-EMT. The catalysts selectivity changes at high butene conversion in a slurry reactor. Appl. Catal. A, 156, 257-283. 

Studies with sulfated zirconia also show similar fast catalyst deactivation in the alkylation of isobutane with butenes. It was found, however, that original activities were easily restored by thermal treatment under air without the loss of selectivity to trimethylpentanes. Promoting metals such as Fe, Mn, and Pt did not have a marked effect on the reaction.362,363 Heteropoly acids supported on various oxides have the same characteristics as sulfated zirconia.364 Wells-Dawson heteropoly acids supported on silica show high selectivity for the formation of trimethylpentanes and can be regenerated with 03 at low temperature (125°C).365... 

Silica-supported triflic acid catalysts were prepared by various methods (treatment of silica with triflic acid at 150°C or adsorption of the acid from solutions in trifluoroacetic acid or Freon-113) and tested in the isobutane-1-butene alkylation.161 All catalysts showed high and stable activity (near-complete conversion at room temperature in a continuous flow reactor at 22 bar) and high selectivity to form saturated C8 isomers (up to 99%) and isomeric trimethylpentanes (up to 86%). Selectivities to saturated C8 isomers, however, decreased considerable with time-on-stream (79% and 80% after 24 h). 

The isobutane-1 -butene alkylation was studied in dense CO2 in both fixed-bed and slurry reactors.165-167 Both Nafion SAC-13 and Nation SAC-25 exhibited steady-state conversions and selectivities for 50 h. Enhanced Cg alkylate selectivity could be achieved at near total butene conversion. The maximum value attained, however, was only about 40%. The higher effective alkylation rate constant for SAC-25 compared to SAC-13 indicates improved accessibility of the acid sites. Nafion SAC-13 and SAC-25 applied in a study to test the effect of supercritical fluids on alkylation exhibited only modest activities.168... 

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