3. Comparison of Major CO₂-to-Succinic-Acid Pathways and Representative Experimental Scales.

pathway (references)	overall conceptual route	theoretical carbon yield to succinic acid	thermodynamic and kinetic feasibility	separation and conditioning burden	process complexity	practical promise
C4 route via maleic anhydride −	CO2 → C2H4/CO/H2 → C4H10 or C4H8 → maleic anhydride → succinic acid	high in principle if C2/C4 coupling and oxidation are fully selective	thermodynamically favorable after C4 formation; kinetically mature for n-butane oxidation and maleic anhydride hydrogenation	requires C2H4/CO/H2 ratio control, C4 separation, and maleic anhydride recovery	high, but many, steps are industrially familiar	highest near-term feasibility: the maleic anhydride → succinic acid is mature
C3 route via acrylic acid −	CO2 + C2H4 → acrylic acid derivative → acrylic acid → succinic anhydride → succinic acid	high in principle if C2H4, CO2, and CO are all derived from CO2 and no decarboxylation occurs	ethylene carboxylation with CO2 is fundamentally difficult because CO2 is stable and C–C carboxylation has kinetic barriers	requires relatively pure C2H4 and CO2; acrylic acid derivative hydrolysis and purification	high	promising chemically, but less mature than C4 route; sensitive to purification and catalyst recycling
C3 route via propionaldehyde ,−	CO2 → C2H4/CO/H2 → propionaldehyde → acrylic acid → succinic anhydride → succinic acid	potentially high, but not yet quantified at route level	feasible at the step level but integration-limited; hydroformylation is feasible	lower initial separation if mixed gas is directly fed; oxidation and carbonylation requires purification	very high	conceptually attractive, but limited by low C3 selectivity and multiple downstream steps
C2 route via acetylene, ethylene oxide, acetaldehyde −	CO2 → C2H4 → C2H2, C2H4O, or CH3CHO → acrylic acid or succinic anhydride → succinic acid	potentially high, but highly route-dependent	individual C2 transformations are known, but multistep coupling to succinic acid is not yet demonstrated as an integrated process	requires purified C2H4; O2 introduction creates safety and removal burdens	very high	lower priority unless a direct and selective C2-to-C4 process is demonstrated
liquid acetate and acetic acid coupling route ,−	CO2/CO → acetate and acetic acid → succinic acid via dehydrogenative coupling	theoretically high from two C2 units	kinetically challenging due to selective C–C coupling; competing decarboxylation likely	concentration and salt removal from dilute streams	low step count but low selectivity	promising as a green liquid-phase concept, but far from commercial relevance
gas–liquid oxidative carbonylation to succinic acid derivatives ,	CO2 → C2H4/CO + ROH → succinic acid derivatives → hydrolysis → succinic acid	high in principle; alcohol can be recycled after hydrolysis depending on process design	chemically feasible; catalyst lifetime and stability remain key unknows	CO/C2H4 control, alcohol management, hydrolysis, and catalyst recovery	medium	potentially strong integration route because C2H4, CO, and alcohols can all be electrolysis-derived

^aTheoretical carbon yield refers to the ideal retention of carbon atoms in the succinic acid based on the conceptual reaction sequence and not the experimentally demonstrated overall carbon efficiency.

^bThermodynamic feasibility, separation burden, process complexity, and practical promise are qualitative assessments based on the cited representative studies and the integration requirements discussed in the text.