Ind. Eng. Chem. Res. 2007, 46, 6899-6903
6899
Dehydration of Phenyl Ethanol to Styrene under Reactive Distillation Conditions: Understanding the Catalyst Deactivation Jean-Paul Lange* and Vincent Otten† Shell Global Solutions, Badhuisweg 3, 1031 CM Amsterdam, The Netherlands
Styrene is commercially produced by the dehydration of phenyl ethanol generated during the coproduction of propene oxide. This reaction can be carried out at mild temperature under reactive distillation conditions using a medium-pore zeolite such as H-ZSM-5 (Lange, J.-P.; Otten, V. J. Catal. 2006, 238, 6). We investigate here the effect of process parameters on the formation of heavy byproducts and on the resulting catalyst deactivation. In particular, we looked at the effect of cofeeding bulky nitrogen-bases or radical scavenger, diluting the feed and improving the stripping of styrene. All these observations indicate that the heavy byproducts are formed inside the zeolite pores, prior to styrene desorption. Their formation seems to be enhanced by diffusion resistance in the catalyst. The deactivating role of heavy products was further confirmed by a significant recovery of activity and selectivity after calcination. A conceptual process design is proposed, based on a slurry reactor with continuous product removal and continuous catalyst regeneration. Such a regeneration loop is uncommon in the chemical industry. 1. Introduction The major commercial route toward styrene is based on the catalytic dehydrogenation of ethylbenzene, which was discovered and developed by IG Farben in the 1930s.1,2 Although less important, manufacturing routes via the dehydration of 1-phenyl ethanol, Φ-CH(OH)-CH3, have been operated for some time. In the 1940s, Union Carbide Chemicals commercialized a styrene process based on the oxidation of ethylbenzene to acetophenone, Φ-C(O)-CH3, followed by selective hydrogenation to 1-phenyl ethanol and, finally, dehydration to styrene.2,3 In the mid 1970s, however, a route based on the coproduction of styrene and propylene oxide was developed by Halcon and improved by Shell.1 It involved (i) the oxidation of ethylbenzene with air to its hydroperoxide, (ii) a subsequent oxygen transfer between the hydroperoxide and propene to form 1-phenyl ethanol and propene oxide, and (iii) the dehydration of the 1-phenyl ethanol to styrene. The dehydration of 1-phenyl ethanol has been reported to utilize a titania- or alumina-based catalyst for dehydrating 1-phenyl ethanol in the gas phase at ∼300 °C.1,4,5 Catalysts with stronger acidity like zeolites are known to catalyze this reaction at a high rate but to deactivate it within hours under such conditions.6,7 Recently, we showed that zeolite catalysts also operate successfully under gas-phase conditions, when the diffusion characteristics of the catalyst particles are optimized.8,9 Still later, we exploited the high activity of the zeolites by operating them at a low temperature under reactive distillation conditions.10 The temperature was chosen to be low enough (
