methods of preparation of Epichlorohydrin

Epichlorohydrin (ECH) is a key raw material used in the production of epoxy resins, synthetic glycerol, and other industrial chemicals. As a versatile organic compound, its demand has led to the development of several methods for its preparation. In this article, we will explore the main methods of preparation of epichlorohydrin, their mechanisms, advantages, and industrial relevance.

1. Chlorohydrin Method

The traditional method for the preparation of epichlorohydrin is the chlorohydrin process, which involves the reaction of propylene with chlorine. This process takes place in two major steps:

• Step 1: Formation of Propylene Chlorohydrin
  In the first step, propylene (C₃H₆) reacts with chlorine in the presence of water, forming a mixture of 1-chloro-2-propanol and 2-chloro-1-propanol, commonly referred to as propylene chlorohydrin. The reaction mechanism is as follows:

  [ C3H6 Cl2 H2O \rightarrow C3H7ClO (Chlorohydrin) ]

• Step 2: Dehydrochlorination to Epichlorohydrin
  The chlorohydrin then undergoes dehydrochlorination, typically using a strong base like sodium hydroxide (NaOH). This eliminates hydrogen chloride (HCl) and results in the formation of epichlorohydrin:

  [ C3H7ClO NaOH \rightarrow C3H5ClO (Epichlorohydrin) NaCl H_2O ]

This method of preparation of epichlorohydrin is well-established but has environmental drawbacks, primarily due to the formation of significant amounts of chlorinated by-products, including wastewater and HCl emissions. Nonetheless, it remains widely used in regions where infrastructure for waste handling is in place.

2. Glycerol-Based Method

In recent years, sustainability concerns have led to the development of greener methods for the preparation of epichlorohydrin. One such method is the glycerol-based process, which uses renewable raw materials. Glycerol, a by-product of biodiesel production, serves as the starting material, making this method highly attractive in terms of sustainability.

• Step 1: Conversion of Glycerol to Dichloropropanol
  Glycerol (C₃H₈O₃) is chlorinated using hydrogen chloride (HCl) or chlorine to form dichloropropanol (DCP). This is an intermediate compound required for further reactions:

  [ C3H8O3 2HCl \rightarrow C3H6Cl2O (Dichloropropanol) H_2O ]

• Step 2: Cyclization to Epichlorohydrin
  In the next step, dichloropropanol is dehydrochlorinated using a base (such as sodium hydroxide) to produce epichlorohydrin:

  [ C3H6Cl2O NaOH \rightarrow C3H5ClO (Epichlorohydrin) NaCl H2O ]

This method of preparation of epichlorohydrin has significant environmental benefits because it utilizes renewable feedstocks and generates less toxic waste. Moreover, it aligns with the growing global emphasis on green chemistry and sustainability, making it a preferred choice for many industries.

3. Direct Oxidation Method

Another innovative approach to the preparation of epichlorohydrin involves direct oxidation. This method eliminates the need for chlorine-based reagents by using hydrogen peroxide (H₂O₂) and a catalyst to oxidize allyl chloride (C₃H₅Cl) directly to epichlorohydrin.

• Step 1: Oxidation of Allyl Chloride
  Allyl chloride reacts with hydrogen peroxide in the presence of a titanium silicate catalyst (such as TS-1) under mild conditions to form epichlorohydrin. The reaction can be represented as follows:

  [ C3H5Cl H2O2 \rightarrow C3H5ClO (Epichlorohydrin) H_2O ]

This process is considered cleaner than the chlorohydrin method as it does not generate HCl as a by-product, thereby reducing corrosive emissions and effluents. Additionally, it offers higher selectivity, which improves the overall yield of epichlorohydrin. However, the cost and availability of hydrogen peroxide and the catalyst can be limiting factors for its wide-scale adoption.

4. Biotechnological Methods

With advancements in biotechnology, there is growing interest in the biotechnological preparation of epichlorohydrin. Enzymatic and microbial methods involve the use of engineered organisms or enzymes to convert bio-based precursors into epichlorohydrin. While still in the experimental phase, this method has the potential to revolutionize epichlorohydrin production by using bio-renewable sources and operating under milder conditions.

Although biotechnological methods are not yet commercially viable on a large scale, they hold promise for the future as the world shifts towards more sustainable chemical processes.

Conclusion

The methods of preparation of epichlorohydrin have evolved significantly from the traditional chlorohydrin process to more environmentally friendly and sustainable approaches like the glycerol-based method and direct oxidation. Each method has its own advantages and challenges, but with increasing demand for green chemistry, methods that reduce environmental impact are becoming more prevalent. As research and innovation continue, newer and more efficient methods for the preparation of epichlorohydrin are likely to emerge, meeting both industrial needs and environmental goals.