**1. Introduction**

Operating in a littoral and marine environment provides the U.S. Navy with unique access to a vast environmental resource of carbon. The world's oceans are the largest carbon reservoirs containing approximately 38,000 gigatons [1]. Carbon and hydrogen are the principal building blocks needed to synthesize hydrocarbons. It is envisioned that these hydrocarbons may one day be used to produce operational fuel. Synthesizing "drop-in" replacement fuel at or near the point of use, translates into "Freedom of Action for the Warfighter" and potential long-term cost savings and strategic advantages for the Department of Defense (DOD) [2–4]. If the energy required for the process is nuclear or renewable, the entire low carbon fuel process could be considered CO2 neutral [5–7].

The future capability of producing fuel from inorganic carbon (CO2) and H2 in seawater is dependent on the development of processes and technologies specifically designed for such applications. The U.S. Navy has recently patented a process and an apparatus for the simultaneous extraction

of CO2 and production of H2 from seawater [8–10]. However, the primary limitations in using the CO2 and H2 as building blocks for the synthesis of hydrocarbons are the high energy barrier for the redox and polymerization reactions necessary to synthesize longer chain molecules to be used as fuel [5–7,11]. While electrochemical and photochemical CO2 conversion processes in water continue to improve in efficiency, challenges remain for these fuel synthesis approaches. These challenges include low hydrocarbon yields, catalyst stability, and difficulty in scaling-up the processes [5,7,11]. Two-step thermochemical approaches are one of the few proven scalable methods for the production of liquid hydrocarbons ranging from C6–C17 from CO2 and H2 [6,12–14]. Step 1 involves the conversion of CO2 and H2 to intermediates (methanol, olefins, CO) [6,12–17]. Step 2 processes these intermediates to C6–C17 hydrocarbons. Commercially, methanol and CO intermediates have both been successfully utilized in this two-step thermochemical approach [13,14], whereas the synthesis of olefin intermediates has only been extensively studied and demonstrated at the laboratory scale [6,17]. In order to evaluate the feasibility of directly synthesizing olefin intermediates as the first step towards operational fuel production for military and commercial applications, the process has to be scaled-up and demonstrated in thermochemical reactor platforms that will be relevant to off-shore and remote synthetic fuel production applications [18,19]. An additional advantage to scaling the chemical conversion of CO2 to light olefin intermediates is that these intermediates serve as key building blocks in the chemical industry [6,12,17].

Commercial-scale, low-cost, modular fixed-bed reactors are being designed and evaluated for remote Fischer-Tropsch synthesis (FTS) processes that use natural gas as the starting material [18]. These advantages could also make commercial-scale fixed-bed reactors ideal for the scale-up of CO2 hydrogenation technologies. Since catalyst physical properties and the reactor type are known to influence the product selectivity, mass transfer, and conversion of hydrogenation reactions [17–21], they are important parameters to consider upon transitioning from the laboratory to commercial scale.

In previous work, highly active Fe-Mn-K/supported CO2 hydrogenation catalysts were characterized and evaluated at the laboratory scale in both a continuously stirred tank/thermal reactor (CSTR) [22] and a fixed-bed reactor [17]. The catalyst materials were demonstrated to be capable of functioning as an effective catalyst to convert CO2 to short-chain olefins. In the present work, the synthesis of Fe-Mn-K-based catalyst is scaled-up 300 times and operated in a commercial-scale prototype modular fixed-bed reactor that is 176 times larger by volume than previous laboratory scale studies. The findings of this paper show how catalyst and reactor scale-up along with recycling a portion of the product stream significantly enhance CO2 conversion efficiency and dramatically change product selectivity. These results are used to expand modeling efforts to bridge the gap between bench-scale research and the development and implementation of a commercial process.
