(267c) New Insight into Oxidative Dehydrogenation of n-Butenes to Butadiene

1,3-Butadiene is an important monomer for rubber and plastics industries. It is primarily produced as a co-product of olefins by steam cracking of naphtha. With the increasing demand on butadiene and the future impact of shale gas, the on-purpose production of butadiene has attracted great interest in both academia and industry. We have developed a high performance catalyst suitable for commercial application. In this presentation, we report our work on the synthesis and performance of the catalyst, and the studies of the reaction mechanisms with an emphasis on impact of process conditions on catalyst performance under practical conditions. We thus propose a semi-empirical kinetic model for the reactor scale-up and design.

The Cr-free, Zn-Fe-O based mixed metal oxides catalysts were synthesized by co-precipitation, spray-drying, and solid state reaction methods. The catalysts were characterized by XRF for elemental analysis, XRD for structure identification, N₂ adsorption for BET surface area, pore volume and pore size distribution, Total Carbon Analyzer for coke formation. Typically, 3.0 mL of the catalyst was tested in a fixed-bed reactor at 330-380 °C, 1 atm, and C₄H₈ GHSV of 400 h^-1. For catalyst screening and stability testing, the feed typically contains C₄H₈ at 30 mL/min, O₂ at 20 mL/min, H₂O at 365 mL/min, and CH₄ (used as diluent and analytical internal standard) at 30 mL/min. The feed and products were analyzed by an online GC equipped with an FID and a TCD. The kinetic studies were carried in a similar reactor by varying process conditions.

The catalyst studied has a spinel structure, ZnFe­₂O₄, as verified by XRD. Promoters such as Ca, Mg, and Co in small amounts can significantly improve catalyst activity and stability. The XRD results did not show any oxide forms of Ca, Mg, and Co indicating they replaced Zn in the spinel structure. In addition to the spinel, the presence of Fe₂O₃ in a solid solution with the spinel improves performance in both catalyst activity and selectivity.

The catalyst synthesis involves many steps and conditions. Among them, the precise control of the pH value during the co-precipitation is so critical that any minor derivation could result in a significant impact on the catalyst performance. The pH value directly impacts the concentration of Zn in the catalyst. We believe the Zn concentration together the promoters incorporated into the spinel determine the catalyst performance. With the optimal compositions of the catalyst prepared under the well-controlled conditions, our catalyst showed stable performance up to 4,000 hours with a C₄H₈ conversion of over 70% and selectivity to C₄H₆ over 90% that meet the target of commercial operation.

To understand the impact of the process conditions on the catalyst performance, we conducted kinetic studies based on a reaction network of a typical selective oxidation reaction. The experiments were carried out in fixed bed reactors. Process variables studied include ratios of oxygen to butene, steam to butene, pressure, and temperature. We performed kinetic modeling of the experimental results. Key controlling parameters were identified and new reaction conditions were discovered which significantly improved the yield of butadiene up to 87%, while the literature reported a maximum yield of 75%. Based on the results, a semi-empirical kinetic model was proposed and modeled. The kinetic model provides insight into the reaction mechanisms and provides guidance on the design and operation of pilot plant for further assessment of the technology for commercial practice.

We developed a Cr-free ZnFe₂O₄ based catalyst promoted by multiple elements that can be scaled up for commercial use. We thoroughly studied catalyst synthesis and identified critical conditions that determine catalyst performance. Our kinetic studies evaluated the impact of the process conditions and identified new conditions that can enhance catalyst performance. Based on such studies, we proposed a semi-empirical kinetic model that can provide insight into the reaction mechanisms under practical conditions and guidance for reactor scale up and design for pilot plan studies and future commercial practice.