Isomerization and Dehydrocyclization of 1,3-Pentadiene

Piperylene concentrate is a complex mixture of 5-carbon unsaturated hydrocarbons obtained as a by-product when naphtha or gas oils are cracked. The major component in this mixture is 1,3-pentadiene. During the course of this study, a number of trials, utilizing liquid phase reaction conditions, were made to investigate the geometric isomerization of 1,3-pentadiene and its separation from the piperylene concentrate. Isomerization was accomplished employing catalytic amounts of iodine at temperatures ranging from 0°C to reflux. Using this method, the maximum amount of trans-1,3-pentadiene obtained was 70% as compared to 51% in the piperylene concentrate. Recovery of the product was 90%, with the remainder being diiodo compounds and polymer. Isomerizations employing catalytic amounts of potassium tert-butoxide were also investigated. Using this anionic isomerization, the theoretical amount of trans-1,3-pentadiene (84% @ 20°C) was obtained in the product. The greatest drawback with this technique was the low recover (50%), due to the extensive polymer formation. Successful separation of 1,3-pentadiene from the mixture was accomplished through cuprous chloride complexing. Utilizing this technique, 54% of the 1,3-pentadiene was recovered from the mixture, with the separated product being 99.9% pure 1,3-pentadiene. Separation of trans-1,3-pentadiene from the mixture was accomplished through a Diels-Alder reaction with maleic anhydride. Since this dienophile will react readily with trans-1,3-pentadiene but not cis-1,3-pentadiene, this method offered an easy and efficient means of removing the former isomer from the mixture. In attempting to reverse this Diels-Alder, via pyrolysis, many products were obtained; including those present in the original mixture. The gas phase dehydrocyclization of 1,3-pentadiene was investigated in a 316 stainless steel tubular flow reactor utilizing various heterogeneous and homogeneous catalysts. The selectivity to cyclopentadiene was greatest (60%) in the presence of a hydrogen sulfide promoter. For all other catalysts, the selectivity remained relatively constant (30%). This constant selectivity over a wide range of parameters indicates that a significant amount of competing side reactions are prevailing within the preheater section of the apparatus