Bioconversion of CO₂ to Acetate

Introduction

Acetate is a versatile two-carbon (C₂) molecule widely used in solvents, plastics, food preservatives, and as a chemical building block. Traditionally, it is synthesized from fossil-based methanol or petrochemical acetic acid pathways. However, with increasing CO₂ levels and the global push for decarbonization, the bioconversion of CO₂ to acetate presents a revolutionary opportunity to transform a greenhouse gas into a valuable commodity chemical.

This is achieved using autotrophic or acetogenic microorganisms that can fix CO₂ through specific metabolic pathways, notably the Wood–Ljungdahl pathway. These microbes convert CO₂ and electrons (from H₂ or electricity) into acetate, often using minimal inputs and operating under mild conditions—making the process attractive for carbon capture and utilization (CCU).

What Products Are Produced?

Acetic acid (Acetate):

• Food preservatives and additives
• Solvents (e.g., for paints, inks, cleaners)
• Precursor for vinyl acetate monomer (VAM) – used in adhesives and polymers
• Feedstock for bioplastics, biofuels, and industrial fermentation

Pathways and Production Methods

1. Wood–Ljungdahl Pathway (Reductive Acetyl-CoA Pathway)

• Most common microbial route for CO₂ to acetate
• CO₂ + 4H₂ → Acetate + 2H₂O
• Involves two branches: carbonyl and methyl, forming acetyl-CoA → acetate
• Microbes: Clostridium ljungdahlii, Sporomusa ovata, Acetobacterium woodii

2. Electrobiofermentation (Microbial Electrosynthesis)

• Electroactive microbes use electrons directly from electrodes or via H₂ evolution
• Converts CO₂ + electricity → acetate
• Often integrated with renewable electricity sources

3. Gas Fermentation (CO + CO₂ + H₂ Mix)

• Syngas fermentation using acetogens for improved yields
• Feedstock: industrial off-gases, biogas reformate, or steel mill emissions

Catalysts and Key Tools Used

Autotrophic Acetogens:

• Clostridium ljungdahlii, Moorella thermoacetica, Sporomusa ovata
• Anaerobic, strict CO₂/H₂ utilizers with robust carbon fixation

Electron Sources:

• Molecular H₂ from electrolyzers
• Electrodes in MES systems (carbon cloth, graphite, stainless steel)
• Redox mediators like neutral red, methyl viologen

Genetic Tools & Enhancements:

• CRISPR-based editing to boost acetate flux
• Knockout of competing pathways (e.g., ethanol, lactate)
• Synthetic control of redox cofactor ratios

Case Study: LanzaTech’s CO₂ Fermentation Platform

Highlights

• Modified Clostridium autoethanogenum for acetate production from CO₂ + H₂
• Demonstrated continuous production in 100-liter bioreactors
• Integrated process with renewable hydrogen and captured CO₂
• Tech licensed for bioplastics precursor streams

Timeline

• 2015 – CO₂-based acetate strain developed in lab
• 2018 – Pilot testing with renewable energy integration
• 2021 – Demonstrated acetate recovery for downstream polymer synthesis
• 2024 – Acetate units co-located with ethanol plants for CCU synergy

Global and Indian Startups Working in This Area

Global

• LanzaTech (USA) – Commercial-scale CO₂ fermentation platforms
• Twelve (USA) – CO₂-to-chemicals via hybrid electrobiological systems
• CarbonCycle Biotech (Germany) – Bioconversion of CO₂ to acetate in MES
• Electrochaea (EU) – Power-to-acetate from grid-connected bioreactors

India

• IIT Madras – MES research using Sporomusa strains
• IISc Bangalore – CO₂ biofixation reactors for acetate and formate
• Breathe Applied Sciences – Electro-bioreactors for acetate generation
• TERI – Developing acetate for circular bioplastic systems from CO₂

Market and Demand

The global acetic acid market reached USD 13.5 billion in 2023 and is projected to grow to USD 19.2 billion by 2030, with a CAGR of ~5.1%. Biobased and CO₂-derived acetate are increasingly preferred for sustainability-linked applications, including bio-PVA films, green solvents, and food additives.

Major Use Segments:

• Polymers (vinyl acetate monomer, cellulose acetate)
• Food industry (preservatives, flavor agents)
• Pharma and chemicals (reagents, intermediates)
• Bioplastics and packaging films

Key Growth Drivers

• Rising demand for carbon-negative platform chemicals
• Push for CO₂ capture and utilization by governments and industry
• High versatility of acetate as a drop-in and precursor molecule
• Integration with renewable H₂ production (electrolysis or biomass)
• Growing interest in biorefinery co-products

Challenges to Address

• Electron transfer limitations in MES-based systems
• pH imbalance and energy input costs during electrofermentation
• Ensuring high selectivity and titer for acetate over side-products
• Biofilm maintenance and contamination risks in long-term operation
• In India: Scaling MES or gas fermentation at low cost and variable power

Progress Indicators

• 2013–2016 – Lab-scale MES acetate systems developed globally
• 2017–2019 – H₂-driven acetate fermenters piloted
• 2021 – Continuous-flow acetate production from flue gas in EU
• 2023 – India’s first MES unit for acetate initiated at pilot scale
• 2024 – Acetate supply chains explored for bioplastics and adhesives

Globally, bioconversion of CO₂ to acetate is at TRL 6–7, with several pilot and demo plants integrated with renewable power. In India, it’s currently at TRL 4–5, with strong academic engagement and early commercialization efforts.

Conclusion

The bioconversion of CO₂ to acetate is more than a carbon capture strategy—it is a carbon valorization pathway. By transforming waste CO₂ into industrially useful acetate, this approach supports the circular carbon economy while producing a key platform molecule for chemicals, plastics, and fuels.

As India ramps up investments in CCU, hydrogen, and biomanufacturing, acetate could emerge as a flagship product linking CO₂ reduction with green economic growth.

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