Carbon Dioxide Fixation in Two Plug Flow Reactors in Series Over a Copper Based Methanol Synthesis Catalyst in the First Reactor and a Cobalt Based Fischer-Tropsch Synthesis Catalyst in the Second Reactor

Utilization of CO₂ as a raw material is of interest not only as carbon source but also because of its impact on the environment through the greenhouse effect. There have been various attempts to fixate CO2, such as (1) Hydrogenation of CO₂ to methanol over methanol synthesis catalysts; (2) hydrogenation of CO₂ to hydrocarbon fuels over Fischer-Tropsch synthesis (FTS) catalysts.

Hydrogenation of CO₂ over cobalt based catalysts under FTS reaction conditions have been conducted by various researchers. Activity data clearly showed that CO₂ is readily hydrogenated over cobalt based catalysts. However, because the cobalt catalysts have low water gas shift (WGS) activity and high hydrogenation activity, the products for the FT CO₂ hydrogenation are methane rich short chain paraffins which are not commercially attractive. Based on our group research, if the cobalt catalyst is mixed with a WGS catalyst, the product distribution for FT CO₂ hydrogenation is beneficially changed.

Copper based catalysts are very active in both WGS and methanol synthesis reactions. For the production of methanol from synthesis gas, it appears that the pathway to methanol formation is through CO₂ as an intermediate. Hydrogenation of CO₂ to methanol with the by-product of carbon monoxide over copper containing catalyst is a widely studied subject. However, a series reactors configuration can be used for the hydrogenation of CO₂ to methanol and hydrocarbon fuels by combining methanol synthesis, reverse-WGS and FTS reactions.

In our study, CO₂ fixation over copper and cobalt based catalysts under different reaction conditions were carried out in two plug flow reactors. The two PFR reactors are combined in series: the copper based catalyst in the first reactor and the cobalt based catalyst in the second reactor. For CO₂ hydrogenation over the copper based catalyst, the main products are methanol and carbon monoxide. The chemical equilibrium for both the reverse-WGS and methanol synthesis reactions limits the CO₂ conversion at the reaction conditions. The product distribution, catalyst activity and selectivity of CO₂ hydrogenation between only one PFR reactor with cobalt catalyst and the two PFR reactors in series (first with copper catalyst and second with cobalt catalyst) are compared. Based on the experimental data and thermodynamic considerations, a potential optimized process design for CO₂ hydrogenation is suggested to fixate CO₂ to both methanol and hydrocarbon fuels.