The selective epoxidation of conjugated olefins containing allylic substituents and epoxidation of propylene in the presence of butadiene

Abstract

The epoxidation of isoprene (2-methyl-1,3-butadiene) and piperylene (1,3-pentadiene), both conjugated olefins containing allylic methyl groups, has been conducted using conventional, CsCl-promoted, Ag/ α-Al 2O 3 catalysts. Selectivities to the allylic olefin epoxide isomers are over 20% and are much higher than expected due to the presence of the conjugated olefin structure. The epoxidation of propylene over the same catalyst under the same conditions is only 2.5%. The epoxidation of propylene in the presence of butadiene also yields PO in much higher selectivities. The presence of as little as 1% C 4H 6 in the reaction feedstream increases the selectivity to PO from 2.5% to over 40% at the expense of overall activity for C 3H 6 conversion. The upper limit of selectivity to PO in the presence of C 4H 6 appears to be approximately 50%, suggesting an upper limit for the effectiveness of this methodology. Epoxidation of C 4H 6 alone on similar Ag catalysts indicates that the consecutive reaction of EpB to CO 2/H 2O is strongly limited by the presence of excess C 4H 6 in the feedstream. In addition, the selectivity to EpB is directly proportional to the amount of C 4H 6 in the reaction feed stream. Selectivities >90% are obtained only when there is sufficient C 4H 6 in the reaction feedstream to control the concentration of the reactive Ag–O surface. For C 4H 6 epoxidation, all CO 2/H 2O is formed by a consecutive reaction pathway from EpB; there is no parallel pathway for the direct formation of CO 2/H 2O from C 4H 6. Using the selective epoxidation of C 4H 6 as the model for understanding the enhancement in selectivity for allylic olefin epoxide formation, the most likely reason for improved selectivities is that strongly adsorbed C 4H 6 (or other conjugated olefins) limits the ensemble size of contiguous Ag–O surface sites. These ensembles are too small for PO combustion, but not too small for PO formation.