Ind. Eng. Chem. Res. 2002, 41, 5949-5951
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KINETICS, CATALYSIS, AND REACTION ENGINEERING Dehydrogenation of Neohexane to Neohexene on Platinum Polymetallic Catalysts S. B. Kogan and M. Herskowitz* Blechner Center for Industrial Catalysis and Process Development, Department of Chemical Engineering, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel
Selective dehydrogenation of neohexane to neohexene was carried out on platinum polymetallic catalysts at high dilution in steam to overcome severe chemical equilibrium limitations. The conversion at 550 °C, WHSV ) 1 h-1, and a dilution ratio of 20 was close to equilibrium values of 17%. Two catalysts, Pt-Sn-K/alumina and Pt-Sn-K-Fe/alumina, yielded similar initial performances of high selectivity of 83 mol %, significantly higher than those of supported Cr catalysts. However, fast coke formation is inherent to this process. Therefore, regeneration of the catalysts is a critical step in the process. Steam regeneration is the most practical method of regeneration if the catalyst performances can be retained for many cycles. Addition of Fe to the Pt-Sn-K/alumina catalyst yielded excellent long-term stability in steam regeneration compared with the poor behavior of the basic Pt catalyst. Introduction Neohexene (3,3-dimethylbutene-1) is an important intermediate.1 The commercial method for its production is metathesis of diisobutylene and ethylene on a WO3-SiO2 catalyst developed by Phillips Petroleum Co.1-3 The process started at 1600 tons/year and was expanded several times.3 This study presents an alternative route of producing neohexene from neohexane, produced by low-temperature isomerization of hexanes.4 Dehydrogenation of neohexane is described in some old patents5,6 and is not mentioned in recent reviews.7,8 The equilibrium constant is small at temperatures up to about 550 °C, yielding a low conversion of neohexane to neohexene of about 17% at a dilution factor of 20 in an inert gas. Dehydrogenation on a chromia-alumina catalyst with no diluent has been applied for light paraffins.9 However, it produces low selectivity in neohexane dehydrogenation, about 55 mol %.5 Using a higher temperature to increase the equilibrium conversion results in excessive coke on the catalyst, rendering its application impractical. Selective platinum catalysts normally operate in hydrogen. The scope of this study is the application of highperformance polymetallic platinum catalysts in the dehydrogenation of neohexane in a steam medium, demonstrated for light hydrocarbons.10-14 Experimental Section Experimental Procedure. The catalytic experiments were carried out in an isothermal fixed-bed, stainless steel reactor (i.d. ) 18 mm and L ) 240 mm) * To whom correspondence should be addressed. Tel: 9728-6461482. Fax: 972-8-6472902. E-mail: herskow@ bgumail.bgu.ac.il.
as described elsewhere.13 The operation pressure was 1 bar in all tests. Water and neohexane were both fed by peristaltic pumps. The product from the reactor flowed through a trap kept at 3 °C for steam and hydrocarbons C5+ condensation and through a trap at -78 °C (cooled by CO2) for light hydrocarbons condensation. The gaseous product from the second trap consisted of nitrogen (with negligible amounts of C1-C2) used as a carrier for the liquid feed and water. Nitrogen flow during the reaction cycle was 2 L/h. Sometimes the hot product flow was sampled by a syringe heated to 60 °C and analyzed. The hydrocarbons were analyzed by a Chrompack CP9001 GC equipped with a flame ionization detector and capillary column WCOT (fused silica) with fixed-phase CP-Sil-5 CB (L ) 10 m and i.d. ) 0.25 mm). Neohexane, neohexene, 2,3-dimethylbutene-1, and 2,3-dimethylbutane supplied by Aldrich (purity > 97%) were standards for product analysis. The neohexane feedstock was prepared by hydrogenation of neohexene (supplied by Philips Research Co.) on a Pd/C catalyst at 5 bar and 100 °C. The product contained 96-97 wt % neohexane, 2.3-2.8 wt % isohexanes, and 0.7-1.2 wt % pentanes. The total conversion of neohexane was calculated as the ratio of neohexane reacted to neohexane in the feed, while selectivity to neohexene was the ratio of neohexene in the product to neohexane reacted. Coke accumulated in the catalysts during the reaction was determined by burning in a closed vessel (∼0.5 L) at 500 °C, followed by CO2 analysis. Catalyst Preparation. All catalysts were prepared with θ-alumina as a support obtained by calcination of γ-alumina supplied by Engelhard Co. as described in ref 13. The θ structure of the support was confirmed by X-ray diffraction (XRD). The surface area was 120 m2/ g, and the pore volume was 0.45 cm3/g.
10.1021/ie0203680 CCC: $22.00 © 2002 American Chemical Society Published on Web 11/05/2002
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Ind. Eng. Chem. Res., Vol. 41, No. 24, 2002
Table 1. Neohexane Dehydrogenation on Platinum and Chromia Catalysts at 550 °C and 1 bar catalyst
a
composition (wt. %), H/Pt
Pt-1
Pt-0.5, Sn-2.1, K-1.6 (0.30)
Pt-2
Pt-0.5, Sn-1.8, Fe-2, K-1.4 (0.39)
Cr-1
Cr-18, K-1.4
conversiona (mol %)
WHSV (h-1)
molar ratio diluent/reactant
test
equilibrium
neohexene selectivity (mol %)
1.0b 1.0 1.0 1.0 1.0 0.5 0.5
H2O, 20 H2O, 10 H2O, 20 H2O, 20 H2, 3 N2, 10 H2O, 20
11.6 12.5 16.8 16.3 0.6 12 1
12 13 17 17 2 13 17
84 85 81 83 30 58 25
Neohexene/(neohexane + neohexene). b 520 °C.
Table 2. Compositions of the Feed and Product (%)a component
feed
lights 0.7 neohexene
