(217e) Synthesis of Novel 3DOM-Prevoskites Redox System for CO2 based Oxidative Ethane Dehydrogenation with Integrated Carbon Capture

With growth in global energy demand and reliance on fossil fuels for energy, CO₂ emissions is projected to be at higher level in the future. To address this challenge, a viable CO₂ capture, storage and utilization technology is the need of the hour. Conventional technologies to convert highly stable CO₂ molecule generated form fossil fuel combustion into value added product requires addition of significant external energy to overcome low CO₂ conversion kinetics/thermodynamics. To lower the threshold of these hurdles, a two-step molten salt mediated oxidative dehydrogenation (MM-ODH, (Step 1-4)) approach which, addresses the key challenges for feasible CO₂ utilization is studied over the core-shell redox catalyst. The redox catalyst is composed of molten carbonate (Li₂CO₃) covering the solid perovskites substrate.

CO₂ capture step

CO₂ (in flue gas) + X₂O (alkali metal oxides in molten salt) → X₂CO₃ (1)

MeO_x-1 + ½ O₂ (in flue gas) → MeO_x (mixed oxide particles) (2)

CO₂ assisted ODH step

R-CH₂-CH₃ + X₂CO₃ → R-CH=CH₂ +CO+H₂O +X₂O (3)

R-CH₂-CH₃ +MeO_x → R-CH=CH₂ +H₂O + MeO _x-1 (4)

In CO₂ capture step, alkali metal oxide salt captures CO₂, and reduced mixed oxide particles reacts with O₂ to oxidize it to MeO_x. In ODH step, oxidized carbonate and mixed oxide particles reacts with alkane to produce lower olefins, CO and H₂O.

Use of commercial perovskites as substrate are limited by low surface area and pore volume. In the present study, an effort is made is to develop a 3-dimensional ordered macro-porous (3DOM) perovskite La_0.8 Sr_0.2 FeO₃(LSFO) for oxidative dehydrogenation (ODH). This strategy was proposed based on the following hypotheses: 1) the porous structure can provide large surface area and pore volume to accommodate the carbonate salt; and 2) ordered porous structure is expected to improve the overall efficiency of the reaction system by enhancing mass transport through creating controllable pathways for reactants and products. To determine the CO₂ capture performance of synthesized redox catalyst, Thermogravimetric analysis (TGA) was conducted on LSF substrate without and with molten carbonates (Figure 1). In reduction step, H₂ gas is flown through the catalyst for 5 minutes. Reduction and oxidation strep are separated by 20 minute purging using inert gas. TGA indicates a 0.37 wt.% and 1.15 wt.% increase in catalyst weight during the CO₂ injection for without and with carbonate covered LSF substrate respectively. These results are in line with our approach, which stated that using a two-step MM-ODH catalyst CO₂ can be captured and valorize to value added products on a single catalyst.

As-synthesized catalyst were assessed for Ethane ODH at 750 °C. 0.5 g of catalyst (0, 20, 30, 60, 80 and 100 wt.% Li₂CO₃@LSF) was packed tubular reactor followed by quartz wool on either side. Mass flow controller (MFCs) modulates the flow rates of inert (10 ml/min) and reactant gases. Valve A and B opens during the reduction (5 ml/min) and oxidation (5 ml/min) steps, respectively. Valve C for inert gas flow remains during 5-minute purge that follows both reduction and oxidation steps and during reduction and oxidation step, respectively. Figure 2 shows ethane conversion and CO₂ capture over different corresponding to 5^th cycle of C₂H₆:CO₂ injection protocol. A constant ethane conversion of 65% is observed for the as-synthesized catalyst. An increase in CO₂ capture is observed when the molten salt weight percentage is increased for 20 wt.% Li₂CO₃@LSF (32 % CO₂ capture) to 80 wt.% Li₂CO₃@LSF (50 % CO₂ capture) which can assert that molten salt still has property to capture the CO₂ after 5^th consecutive cycle.