Ind. Eng. Chem. Res. 2006, 45, 7399-7408
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Alkylation of Benzene with 1-Pentene over Solid Phosphoric Acid Michele Cowley, Arno de Klerk,* Reinier J. J. Nel, and Johann D. Rademan Fischer-Tropsch Refinery Catalysis, Sasol Technology Research and DeVelopment, P.O. Box 1, Sasolburg 1947, South Africa
The solid phosphoric acid (SPA) catalyzed alkylation of benzene with 1-pentene was studied in the range 160-220 °C, 3.8 MPa, and with aromatic to olefin ratios in the range 6:1-1:6. It could be shown that SPA has the potential to be a selective alkylation catalyst for the production of linear alkyl benzenes (LABs) even at low aromatic to olefin ratios, provided that reaction pathways leading to skeletal isomerization can be suppressed. It was also found that steric effects were more important than electronic effects in governing the alkylation selectivity of SPA and that a protonated cyclopropane structure was the most likely transition-state for alkylation that followed an Eley-Rideal mechanism. Laboratory and pilot plant studies indicated that the carbon chain length of the product was limited to C15, with less than 5% di-alkylation of benzene taking place, even for reactions at 1:6 aromatic to olefin ratio. Introduction Solid phosphoric acid (SPA) catalysts were developed and commercialized for C2-C5-olefin oligomerization in the 1930s.1-3 Soon thereafter, it was realized that SPA is also active for the alkylation of aromatics with olefins, and until the 1990s, SPA was the main catalyst type used for the alkylation of benzene and toluene with propene to produce cumene and cymene, respectively. Since then, SPA has gradually been replaced by zeolites for the alkylation of aromatics with propene.4-6 Zeolites are not extensively used for the alkylation of benzene with higher olefins, and linear alkylbenzenes that are used in detergent applications are still mostly manufactured by liquid acid catalysts, with the Detal solid acid catalyst process licensed by UOP being the exception.7 Interest in SPA as an alkylation catalyst was piqued by noting that compared to C3- and C4-olefins, SPA was less active for the oligomerization of n-pentenes8 and higher linear olefins over the temperature range 150-220 °C.9 It was hoped that this attribute could be used to favor alkylation selectivity at low aromatic to olefin ratios, thereby reducing the reactor size and recycle cost associated with SPA-based alkylation processes. Aromatics also have a strong π-type interaction with acid sites that causes the aromatic ring to be adsorbed onto the catalyst by lying flat on the surface to act as diluent.10 The kinetic model followed by the alkylation reaction will influence the selectivity toward oligomerization and multiple alkylation. If an Eley-Rideal mechanism (where reaction occurs between an adsorbed olefin and benzene in the bulk phase) is followed, the aromatic molecules will effectively block some acid sites, thereby diluting the surface concentration of olefins and the opportunity for olefin dimerization to take place. This could help to lower the aromatic to olefin ratio attainable during commercial higher olefin alkylation. If a Langmuir-Hinshelwood mechanism (where reaction occurs between an adsorbed olefin and adsorbed aromatic) is operative, the probability for multiple alkylation will be high, unless steric effects caused by the free rotation of the alkyl group causes the alkylated aromatic to desorb after alkylation takes place. Alkylation is generally accepted to follow an Eley-Rideal mechanism when the pore structure is not constraining,11-13 as * To whom correspondence should be addressed. Tel.: +27 16 9602549. Fax: +27 11 522-3517. E-mail:
[email protected].
in SPA. Only in small and medium pore zeolites, which have a space constrained catalytic environment, does the alkylation mechanism obey Langmuir-Hinshelwood kinetics.11 It is known that SPA has a lower selectivity for multiple alkylation than zeolites during propene alkylation,6,14-15 supporting an EleyRideal mechanism. If this holds for higher olefins too, byproduct formation and the need for transalkylation will be reduced. This is especially important when dealing with longer alkyl groups, where transalkylation would be difficult to accomplish without significant associated cracking.16-17 The alkylation of benzene with 1-pentene was selected as a test reaction, because the alkylation products are described in the literature.18 It is also the first olefin in the series of higher olefins that shows lower oligomerization activity over SPA than C3- and C4-olefins, and from an industrial point of view, it is the most abundant naphtha range olefin produced by hightemperature Fischer-Tropsch.19 Laboratory scale batch reactors were used to probe selectivity profiles at different temperatures (160-220 °C) and aromatic to olefin ratios (6:1-1:6). The reaction was also studied in a pilot plant scale packed-bed flow reactor with uncrushed commercial SPA catalyst under similar operating conditions and using an aromatic to olefin ratio of 1:1. Experimental Materials. The tests were done with commercial C84/3 solid phosphoric acid catalyst obtained from Su¨d-Chemie Sasol Catalysts, Sasolburg, South Africa. The catalyst was manufactured with Celite FB type kieselguhr, and its properties are listed in Table 1. The catalyst is supplied in the form of extrudates, 6 mm in diameter and varying in length from 6 to 10 mm. The 1-pentene (+99%) was obtained from Sasol Alpha Olefins at the Synfuels refineries in Secunda, South Africa. The 1-pentene content was 99.3% as determined by gas chromatographic analysis, with 0.5% 2-methyl-1-butene being the main impurity. The 1-pentene was free of oxygenates and dienes and had a water content of less than 0.001%. The benzene (+99%) used in the batch reactor studies was obtained from NT Laboratory supplies and contained 99.7% benzene by analysis. The benzene used for the pilot plant study was HPLC-grade (+99.9%) obtained from Aldrich. The water content of the benzene was 0.02%. In all batch reactor experiments, ntetradecane (+99%) from Aldrich was used as solvent. Nitrogen
10.1021/ie060197p CCC: $33.50 © 2006 American Chemical Society Published on Web 10/04/2006
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Ind. Eng. Chem. Res., Vol. 45, No. 22, 2006
Table 1. Characterization of the Su1 d-Chemie Sasol C84/3 SPA Catalyst property
value
density (g‚cm-3) pore volume (cm3‚g-1) carbon content (%) total acid (%) free acid (%) ortho:pyro ratio
1.01 0.15 0.02 78 25 330:170
(99.999%) was obtained from Afrox. All chemicals were used without any further purification. Equipment and Procedure. Batch reactor tests were done in stainless steel autoclaves with a useful volume of 150 mL (Figure 1). The autoclave was preloaded with crushed catalyst (
