2902
Ind. Eng. Chem. Res. 2007, 46, 2902-2906
Selectivity Differences of Hexene Isomers in the Alkylation of Benzene over Solid Phosphoric Acid Reinier J. J. Nel and Arno de Klerk* Fischer-Tropsch Refinery Catalysis, Sasol Technology Research and DeVelopment, P.O. Box 1, Sasolburg 1947, South Africa
Alkylation of benzene with 1-hexene, 2-methylpentenes, 3-methylpentenes, and 2,3-dimethyl-2-butene has been investigated over a solid phosphoric acid (SPA) catalyst at 220 °C. It was found that on SPA linear olefins preferably alkylated, rather than dimerized, as opposed to branched olefins that preferably dimerized. The initial selectivity ratios of dimerization to alkylation for the hexene isomers are 0.16 (n-hexenes), 12 (3-methylpentenes), 21 (2-methylpentenes), and 33 (2,3-dimethylbutenes). SPA did not readily form heavy oligomers or dialkylated benzenes and had a high propensity for the skeletal isomerization of hexenes. The alkylbenzene product selectivities could not be used to confirm that benzene alkylation always proceeded through a protonated cyclopropane (PCP) intermediate over SPA, as was found during SPA catalyzed alkylation of benzene with 1-pentene. Introduction The possibility of using solid phosphoric acid (SPA) as a solid acid catalyst for the selective alkylation of benzene with linear C5 and heavier olefins has previously been explored using 1-pentene.1 It was found that SPA had a high alkylation and low dimerization selectivity for n-pentenes and that little dialkylation of benzene took place. It was also found that the alkylation selectivity of the linear olefins was eroded by skeletal isomerization. The branched olefins participated in both olefin dimerization and benzene alkylation, thereby reducing the overall olefin selectivity for alkylation. This pointed to a significant selectivity difference during reaction depending on the branching of the olefin, which seemed to parallel the reactivity difference reported for olefin dimerization on SPA.2 Two postulates made during the SPA alkylation of benzene with 1-pentene1 had to be verified, namely, that linear olefins preferably alkylated, while branched olefins preferably dimerized and that the alkylation of linear olefins proceeded via a protonated cyclopropane (PCP) intermediate, which is the same intermediate for skeletal isomerization.3 In addition to this, it was considered important to determine whether the position as well as degree of branching had an influence on the selectivity of alkylation compared to dimerization. To accomplish this, hexenes have been selected as the alkylating olefin. The present paper investigates the SPA catalyzed alkylation of benzene with 1-hexene, 2-methylpentenes, 3-methylpentenes, and 2,3-dimethylhexenes. The study focuses on the differences in product selectivity and reaction classes obtained with the different hexene isomers and how it relates to the mechanism. All experiments were done in batch reactors at 220 °C, 3.8 MPa, and with an equimolar benzene to olefin ratio. Experimental Section Materials. Benzene (99.9%), 1-hexene (97%), 2-methyl-1pentanol (99%), 2-ethyl-1-butanol (98%), 2,3-dimethyl-2-butene (98%), and tetradecane (+99%) were obtained from Aldrich and used without further purification. The mono-branched * To whom correspondence should be addressed. Tel.: +27 16 9602549. Fax: +27 11 522-3517. E-mail:
[email protected].
hexenes were prepared by dehydration of the corresponding alcohol over an η-alumina catalyst (Syndol, Scientific Design) at 380-400 °C and atmospheric pressure. The product was further purified by distillation. A mixture of 2-methyl-1-pentene (49%) and 2-methyl-2-pentene (43%) was produced from 2-methyl-1-pentanol, and a mixture of 2-ethyl-1-butene (38%), cis-3-methyl-2-pentene (23%), and trans-3-methyl-2-pentene (30%) was produced from 2-ethyl-1-butanol. These mixtures will be referred to as 2-methylpentenes (92%) and 3-methylpentenes (91%). The impurities were n-hexenes (6%) and other branched hexene isomers (2-3%). All experiments were done with a commercial C84/3 SPA catalyst obtained from Su¨d-Chemie Sasol, Sasolburg. The catalyst was manufactured using a Celite FB kieselguhr, and it is the same catalyst that was used for the alkylation of benzene with 1-pentene.1 Equipment and Procedure. The reactions were carried out in a stainless steel batch reactor with magnetic stirrer bar and external heater. A low catalyst to reactant ratio was used to keep conversion low and to allow initial selectivities to be observed. The SPA catalyst (0.3 g) was crushed (
