Biocatalytic Conversion of Syngas to Liquid Fuels

Introduction

Syngas—a mixture of CO, CO₂, and H₂—is a valuable intermediate generated from gasification of biomass, municipal solid waste, or industrial flue gases. Traditionally, converting syngas into fuels relies on thermochemical methods like Fischer–Tropsch synthesis, which require high temperatures, pressures, and expensive metal catalysts.

However, biocatalytic conversion offers a gentler, greener route. It involves using microbial enzymes or whole-cell biocatalysts to convert syngas into liquid fuels such as ethanol, butanol, and longer-chain alcohols, under mild conditions. This bio-based process is gaining momentum due to its carbon efficiency, flexibility in gas composition, and integration with circular carbon strategies.

What Products Are Produced?

• Bioethanol – Primary fuel output from acetogenic pathways
• Butanol and Hexanol – Via chain elongation in secondary fermentation
• Acetone and Acetate – Co-products from clostridial fermentation
• Methanol – In hybrid chemo-biological routes
• Single-cell biomass – Protein-rich byproduct for feed

Pathways and Production Methods

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

• Used by acetogenic bacteria to convert CO/CO₂ + H₂ → Acetyl-CoA
• Acetyl-CoA is reduced to ethanol or acetic acid
• Efficient and linear, with high carbon fixation potential

2. Chain Elongation for Higher Alcohols

• Secondary biocatalysts convert acetate + ethanol → butyrate, caproate
• These acids can be hydrogenated or converted into butanol, hexanol

3. Enzyme-Assisted Conversion

• Isolated or immobilized enzymes (e.g., CO dehydrogenase, hydrogenases) used in bioreactors
• Still emerging but promising for selective conversions

4. Hybrid Systems

• Thermochemical gasification + biocatalytic conversion
• CO-rich gas → microbial fermentation → ethanol/butanol
• Integrated carbon recycling systems

Catalysts and Key Tools Used

• Whole-Cell Biocatalysts:

• Clostridium ljungdahlii, C. autoethanogenum – CO/CO₂ to ethanol
• Acetobacterium woodii – Efficient CO₂ reduction
• Clostridium carboxidivorans – Butanol and hexanol production
• Syntrophic co-cultures for chain elongation and redox balance

• Enzymes and Complexes:

• Carbon monoxide dehydrogenase (CODH) – Converts CO to CO₂
• Hydrogenase – Drives redox balance using H₂
• Acetyl-CoA synthase – Core step in fuel precursor formation

• Reactor Technologies:

• Bubble column bioreactors
• Pressurized gas-fed fermenters
• Immobilized enzyme reactors (under development)

Case Study: LanzaTech’s Biocatalytic Ethanol Platform

Highlights

• Uses engineered Clostridium autoethanogenum to ferment CO from steel mills
• Achieved >80% carbon utilization efficiency
• Produces ethanol and other chemicals like 2,3-butanediol
• Partnered with ArcelorMittal (Belgium), Tata Steel (India), and Zara for carbon-negative chemicals

Timeline

• 2005 – LanzaTech begins development
• 2013 – First industrial-scale pilot plant
• 2017 – Commercial plant in China with Shougang Steel
• 2022 – Integration with SAF (sustainable aviation fuel) production via downstream chemistry

Global and Indian Startups Working in This Area

Global

• LanzaTech (USA/New Zealand) – Syngas to ethanol via engineered biocatalysts
• INERATEC (Germany) – Modular gas-to-liquid hybrid systems
• Coskata (USA) – Previously focused on syngas to ethanol with engineered microbes
• Synata Bio (USA) – Chain-elongation-based fuel production from syngas

India

• Tata Steel x LanzaTech – CO-rich off-gas fermentation into ethanol
• CSIR-NEERI, NIIST, and IIP – Developing indigenous microbial consortia
• IIT Delhi & IISc Bangalore – Working on enzyme-assisted CO fixation
• TERI – Focus on rural-scale syngas fermentation and product valorization

Market and Demand

The biocatalytic syngas-to-fuel sector is part of the carbon recycling and advanced biofuel market, valued at USD 2.1 billion in 2023, expected to reach USD 6.8 billion by 2030, at a CAGR of ~18%.

Major End-Use Segments:

• Blended transportation fuels (ethanol, butanol)
• Aviation fuels (via ethanol to jet pathways)
• Industrial solvents and green chemicals
• Carbon-negative building blocks (e.g., acetate, 2,3-BDO)

Key Growth Drivers

• Carbon utilization from waste gases (steel, cement, biomass)
• Low energy requirement vs. thermochemical methods
• High selectivity of biocatalysts under mild conditions
• Rapid developments in synthetic biology and reactor design
• Favorable policies for CO₂ capture and circular economy fuels

Challenges to Address

• Low gas solubility and mass transfer limitations
• Product inhibition and slow microbial growth on CO
• Scaling enzyme systems remains cost-intensive
• Maintaining biocatalyst activity in real syngas (with impurities)
• Long fermentation durations compared to chemical catalysis

Progress Indicators

• 2006 – Wood–Ljungdahl engineered for ethanol in Clostridium
• 2012 – Pilot plants using steel mill gas initiated
• 2017 – First commercial ethanol plant from waste gas
• 2020 – Chain elongation for higher alcohols enters TRL 6
• 2023 – Integration with SAF and acetate-to-jet routes

Biocatalytic syngas-to-fuel systems are at TRL 7–8 globally, with TRL 6–7 in India, particularly in CO-to-ethanol conversion using microbial cell factories.

Conclusion

Biocatalytic conversion of syngas to liquid fuels offers a promising, low-carbon, and scalable solution to repurpose industrial emissions and biomass-derived gases into valuable renewable fuels. As biocatalysts become more efficient and systems integrate with carbon capture and utilization (CCU) strategies, this route can significantly contribute to a fossil-free fuel future.

With strong progress in India and abroad, these technologies stand poised to turn waste gases into wealth, making them central to the bioeconomy and climate action.

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