Syngas-to-Olefins: Evaluation of Reaction Kinetics and Optimization of Packed Beds of Mixed Catalysts

Packed beds containing physical mixtures of both Zn-Zr mixed oxide catalyst and SAPO-34 convert syngas directly into a mixture of C2-C5 olefins and paraffins in a single reaction vessel. Specifically, the mixed oxide catalyst is responsible for intermediate oxygenate synthesis from syngas while the molecular sieve catalyzes olefin synthesis from oxygenate intermediates generated on the mixed oxide. Unfortunately for targeted olefin synthesis, olefins are generated in presence of feed hydrogen, causing undesirable direct hydrogenation of the desired olefin product. Kinetic measurements with co-fed propylene over each catalyst, Zn-Zr mixed oxide and SAPO-34, independently confirm olefin hydrogenation activity over both components of the composite bed. While pure hydrogen and olefin feeds demonstrate the mixed oxide catalyst as more strongly catalyzing hydrogenation, the addition of CO to the feed of hydrogen and propylene drops the activity of propylene hydrogenation over the Zn-Zr oxide. Adding water co-feeds to the CO/propylene/H2 mixture further drops hydrogenation activity. In sum for the target syngas-to-olefins chemistry under reaction conditions of syngas feed and produced water, olefin hydrogenation predominantly occurs over the SAPO-34 catalyst, rather than over the catalyst responsible for hydrogenating CO into oxygenate intermediates. A developed kinetic model consistent with this conclusion describes measurements at differing feed compositions, temperatures, space velocities, and bed catalyst mixing ratios. The developed kinetic model is further extended via optimization to predict the desired catalyst mixing ratios for olefin yield of laboratory scale reactor beds.