Production of 1,3-Propanediol From Glycerol

The production of 1,3-propanediol (PDO) from glycerol through fermentation has gained attention in recent years due to its sustainable and eco-friendly process, often leveraging biodiesel by-products and resulting in lower greenhouse gas emissions. PDO is an important chemical, a monomer, used in the synthesis of polymers such as polytrimethylene terephthalate (PTT),  polyesters, polyethers and polyurethanes and it has applications in cosmetics, detergents, and other sectors.

1. Background on 1,3-Propanediol and Glycerol

1,3-Propanediol (PDO) is a diol, meaning it has two hydroxyl (OH) groups attached to a three-carbon backbone. Traditionally, PDO is synthesized through petrochemical methods, which involve the hydration of acrolein or the hydroformylation of ethylene oxide. However, the bio-based production of PDO from glycerol provides a renewable alternative to these fossil fuel-based processes.

Glycerol, a by-product of biodiesel production, is a versatile and energy-rich feedstock that can be fermented by certain microorganisms to produce PDO. With the increased production of biodiesel in recent years, glycerol has become abundant, providing a low-cost, renewable substrate for PDO production.

2. Microbial Pathways for PDO Production

The microbial production of PDO from glycerol occurs in anaerobic conditions through two main biochemical pathways:

• Oxidative Pathway: This pathway involves the oxidation of glycerol to dihydroxyacetone, which is subsequently metabolized to pyruvate. Pyruvate can be used by cells to produce energy and intermediates but does not directly contribute to PDO formation.
• Reductive Pathway: The reductive pathway is crucial for PDO production. In this process, glycerol is reduced to 3-hydroxypropionaldehyde (3-HPA) and then further reduced to PDO by an NADH-dependent enzyme. This pathway is the main route for producing PDO and involves enzymes like glycerol dehydratase and 1,3-propanediol oxidoreductase.

In anaerobic conditions, the reductive pathway enables glycerol fermentation to yield PDO as a primary product, along with by-products such as acetate, butyrate, and ethanol, depending on the microorganism.

3. Microorganisms Used in PDO Production

Several microorganisms are capable of converting glycerol to PDO. Some of the most studied and industrially relevant include:

• Clostridium butyricum: A well-known anaerobic bacterium capable of efficiently converting glycerol to PDO. It requires specific growth conditions and is known for its high PDO yield but can also produce undesirable by-products.
• Klebsiella pneumoniae: This bacterium is a facultative anaerobe, meaning it can function in both aerobic and anaerobic environments. K. pneumoniae is robust and produces PDO with relatively high yield and productivity. However, it has biosafety concerns due to its pathogenicity.
• Escherichia coli: E. coli has been genetically engineered to express pathways for PDO production. While it does not naturally produce PDO, it is a highly adaptable organism that can be tailored for industrial-scale production through genetic modifications.
• Lactobacillus reuteri: Another potential PDO producer, L. reuteri naturally metabolizes glycerol and can produce PDO, although typically at lower yields.

4. Fermentation Process for PDO Production

Step 1: Preparation of Fermentation Medium

Glycerol, the primary carbon source, is combined with essential nutrients like nitrogen sources, trace elements, and vitamins to support microbial growth. Optimal pH and temperature conditions must be established for efficient PDO production.

Step 2: Inoculation and Fermentation

The prepared medium is inoculated with the chosen microorganism and then incubated under anaerobic conditions. Since oxygen inhibits the reductive pathway necessary for PDO production, the system must be oxygen-free, typically achieved through nitrogen gas sparging or a closed bioreactor.

Step 3: PDO Production and By-product Formation

Throughout fermentation, glycerol is metabolized by the microorganism, resulting in PDO and by-products. By-products like acetate and ethanol are inevitable but can be minimized through careful control of the fermentation conditions.

Step 4: Product Recovery and Purification

After fermentation, PDO is separated from the broth through a series of purification steps. Filtration removes microbial cells, while liquid-liquid extraction, distillation, and crystallization processes further purify PDO to the desired quality.

5. Optimization Strategies

Several strategies enhance PDO yield and productivity, such as:

• Metabolic Engineering: Genetic modifications in organisms like E. coli or K. pneumoniae can enhance PDO yields by redirecting metabolic fluxes toward the reductive pathway. Enzymes involved in by-product formation may be knocked out to favor PDO production.
• Process Parameters: Temperature, pH, glycerol concentration, and nutrient availability are carefully monitored and optimized. Typically, pH levels of around 6-7 and temperatures of 30–37°C favor PDO-producing microorganisms.
• Co-Fermentation and Substrate Feeding: In continuous or fed-batch fermentations, glycerol can be fed incrementally, which helps avoid substrate inhibition and prolongs the production phase.
• Immobilization of Cells: Immobilizing cells on a support material can stabilize the microbial population, improving PDO yields and enabling easier separation of the cells after fermentation.

6. Industrial and Environmental Benefits

The bioproduction of PDO from glycerol offers numerous industrial and environmental benefits:

• Reduced Carbon Footprint: Compared to petroleum-based PDO production, biotechnological production significantly lowers greenhouse gas emissions.
• Use of Renewable Feedstock: Utilizing waste glycerol from biodiesel production provides an added value to biodiesel industries, making the overall biodiesel production process more economically feasible and sustainable.
• Lower Energy Consumption: The fermentation process operates under moderate conditions (low temperature and ambient pressure), reducing energy needs compared to petrochemical synthesis.

7. Challenges in PDO Fermentation

Despite its advantages, PDO fermentation faces some challenges:

• Substrate Cost and Supply: Although glycerol is abundant, fluctuations in its availability or price can impact the overall economics of PDO production.
• Pathogenic Risks: Organisms like K. pneumoniae pose biosafety concerns, and alternatives must be developed or proper containment must be ensured.
• By-product Formation: By-products such as acetate or butyrate can reduce overall yield and necessitate additional purification steps, which increase operational costs.
• Scale-Up and Purification: While lab-scale PDO production is feasible, scaling up requires overcoming mass transfer limitations, maintaining anaerobic conditions, and designing effective downstream purification processes.

8. Advances and Future Prospects

Research in PDO production from glycerol is progressing rapidly, with promising advances such as:

• Synthetic Biology and Genome Editing: Techniques like CRISPR/Cas9 enable precise genetic modifications to optimize metabolic pathways and increase PDO yields.
• Strain Engineering and Adaptive Evolution: Developing more robust strains that tolerate higher glycerol concentrations or produce fewer by-products could enhance PDO productivity.
• Process Intensification: Innovations in bioreactor design, such as membrane bioreactors or continuous flow systems, could improve productivity and ease product recovery.

Producing 1,3-propanediol from glycerol through fermentation represents a sustainable alternative to petrochemical routes, leveraging renewable feedstocks and offering environmental benefits. Although challenges remain, advances in microbial engineering and fermentation technology continue to improve PDO yields, making this process increasingly viable for industrial-scale production. As glycerol supplies from biodiesel increase and technology advances, the bio-based production of PDO could play an essential role in creating more sustainable, bio-based chemicals in the future.