61st Annual Report on Research 2016

This project explored the hypothesis that combination of the intensified process efficiency of monoliths with the isomerization and cracking capacity of ZSM-5 can enhance FTS selectivity to gasoline range products. Monolithic Co catalysts coated with ZSM-5 showed high FTS selectivity to gasoline range products (C₅-C₁₂) at 230 °C and 12 bar. Gasoline selectivity was found to be as high as 60.9 wt.% of the total carbon products (including CO₂) with CO conversion as high as 78.7 %. The addition of ZSM-5 on the monolith catalyst not only improved the gasoline selectivity but also gasoline quality, in terms of olefin and isomer composition. Investigation of the temperature effect on catalyst performance showed that the liquid product selectivity shifted to hydrocarbons of lower carbon numbers with the increase of temperature. CO₂ selectivity increased sharply because of the enhancement of the water gas shift reaction. More isomers and olefins were produced over the ZSM-5-coated monoliths at high temperatures, but at the expense of the liquid product yield. Increasing reaction pressure led to higher selectivity to heavy hydrocarbons. Low pressure favored the production of isomers and olefins. High pressure was shown to introduce diffusion limitations to the ZSM-5 layer of the FTS catalysts synthesized. A moderate pressure of 12 bar was proposed to favor gasoline production. This ACS-PRF DNI project supported one graduate student and has served as an undergraduate research study for two undergraduate students of the Chemical and Biomolecular Department at UConn.

To prepare the structured catalysts, 200 cpsi cordierite monoliths were shaped to fit a 0.5’’ ID reactor. Alumina wash-coating solution was prepared, by adding 5 g Boehmite and 30 g deionized water into 25 g γ-Al₂O₃. The monoliths, pretreated at 120 °C, were immersed in the wash-coating solution for 1 min, and the excess solution was gently blown off with pressurized air. After wash-coating, the monoliths were dried at 120 °C for 4 h and calcined at 400 °C for 12 h. Wash-coating was repeated to tune the thickness of the Al₂O₃ layer. After wash-coating, the active material was deposited by immersing the monolith into a Co(NO₃)₂•6H₂O solution, prepared by dissolving 33.3 g Co(NO₃)₂•6H₂O in 25.6 ml DI water, for 1 min. The excess solution was blown off gently. The catalyst was then dried at 120 °C for 4 h and calcined at 400 °C for 12 h. The final ZSM-5 coating was applied by dip coating the monolith into a NH₄-ZSM-5 slurry, prepared by mixing 20 g NH₄-ZSM-5 with 31 ml DI water. The excess solution was again blown off, and the previously described drying and calcination protocols were repeated. Two wash-coatings of Al₂O₃ and a single Cobalt impregnation produced approximately 5 wt.% Co₃O₄ loading on the monolith. In the following, the notation for the catalyst represents the coating sequence.

The functionality of the ZSM-5 coating was analyzed in terms of its capacity to enhance cracking and isomerization reactions, as well as its capability to control the size of the hydrocarbons produced. FTS was performed using catalysts with and without ZSM-5 coating to test this hypothesis. Only results that were within 5 wt.% mass balance error were accepted. Experiments were repeated at least three times or as many required to meet the mass balance requirement. The ZSM-5 layer was responsible for decreased CO₂ selectivity. The CO₂ decrease could be explained by the strong hydrophilicity of the ZSM-5, which decreased the local composition of H₂O on the Co sites and limited the extent of the water gas shift reaction over Co. Fig.1 shows that the hydrocarbon selectivity of the C₅-C₁₂ range product increased significantly with the ZSM-5/Co/Al₂O₃/M catalyst. C₅-C₁₂ liquid product selectivity reached 60.9 wt.%. The reason for this selectivity improvement is the cracking, isomerization, and oligomerization reactions over the ZSM-5 catalyst membrane. Hydrocracking and isomerization reactions shifted the selectivity of heavy hydrocarbons (>C₁₂) to lighter products (C₅-C₁₂), while oligomerization reactions decreased the C₂-C₄ yields and favored products in the desired C₅-C₁₂ range. The Co/Al₂O₃/M catalyst was selective to heavy hydrocarbons up to C₂₈. There was a shift of selectivity from heavy hydrocarbons to light hydrocarbons when the ZSM-5 coated catalyst was used. The selectivity peak for the ZSM-5/Co/Al₂O₃/M catalyst was at C₈, which indicates a high octane rating and a high-quality gasoline product. The same conclusion can be drawn from Fig.1(b-c). The selectivity to isomers reached 49.8 wt.%, which is about 15 wt.% higher than previously reported values for FTS. Hydrocracking and isomerization over the ZSM-5 coated catalysts were very extensive. In summary, monolith catalysts coated with ZSM-5 membrane showed high activity and high selectivity towards gasoline range products.

Fig.1: Liquid hydrocarbon distribution. (a) Selectivity of different carbon number species for Co/Al₂O₃/M and ZSM-5/Co/Al₂O₃/M. (b) (c) Paraffin, isomer, olefin selectivity as a function of carbon number for Co/Al₂O₃/M and ZSM-5/Co/Al₂O₃/M.