Waste crab shells used as biomass to make valuable chemicals

Chitin is the second most widely-spread polymer on the planet after cellulose. As a chemist, I would describe a polymer as the repetition of an indefinite number of identical units called monomer. Chitin is constructed in a similar way and the basic monomer that makes this material is called N-acetyl glucosamine or NAG. For the curious ones, the chemical structure of NAG is a unit of glucose modified by the presence of acetamide.

Chitin s a strong and robust material and it’s the main component of crab and shrimp shells, insects and some mushrooms. Its abundance and the desire to use biomaterials as feedstock in alternative to fossil fuels made chitin from biomass an attractive way of producing valuable chemicals. In fact, a range of products including sugars, amino acids and other chemicals have been obtained from chitin or directly from waste shrimp shells in the recent years. [1, 2, 3]

NAG is one of the most valuable chemicals because it has a number of medical and cosmetic applications. It has been used to repair joint damages, for examples. Many clinical trials have been performed to treat patients suffering from joint disorders with preparations containing NAG. The results from these studies indicated that NAG improved the prevention of joint damage significantly. [4, 5] NAG is also used as an ingredient in cosmetics for improving skin wrinkles and colour. [6, 7] A more comprehensive review on production and applications of NAG can be found here.

A recent publication in the journal Green Chemistry featured a new and simple way of making NAG from chitin. [8] The author of this study found that using water and calcium chloride to treat chitin would produce the desired NAG product. The author tried a number of other commonly used salts such as sodium chloride (table salt) or magnesium chloride (the compound used to de-ice streets and highways in winter) but they observed no conversion of chitin into NAG. However, calcium chloride in the presence of small quantities of sulfuric acid and zinc bromide would convert chitin into NAG well.

All the chemical used in this procedure are cheap and widely available. From a sustainable point of view, the author mentioned that calcium is the 5th most abundant metal on earth and calcium chloride can be found in sea water. Zinc bromide is also used in rechargeable batteries and sulfuric acid is a naturally occurring acid which has a number of other applications such as cleaning agent, it’s used to make fertilisers, cleaning products and has application in the pharmaceutical industry.

To reach the best conditions for conversion of chitin into NAG, the researchers also tried to change temperature and time of the process. The temperatures were varied from 0 to 30 °C and the time of the process was tested from as little as 15 min up to 3 h. The best combinations temperature/time was obtained when the process was carried out at 20 °C for 60 min.

I found this study interesting and a nice chemical way to produce NAG from chitin using mild conditions and commercially available compounds. Other methods to produce NAG from chitin have been used in the past including enzymes or more aggressive chemical ways, and this stands out for its simplicity. If I had all compounds at home, I could do it myself in my kitchen. However, the authors don’t offer a way for the recollection of NAG after its formation from chitin. Additionally, they don’t really use any waste, such as crab or shrimp shells to test how it would perform when translated into real life applications outside the pure academic scope of the publication.

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[1] Chem Catal., 2022, 2, 2302–2311.

[2] ChemSusChem, 2013, 6, 2259–2262.

[3] Bioprocess Biosyst. Eng., 2019, 42, 611–619.

[4] Method for Treating Rheumatoid Arthritis. US Patent 2008003258, 2007

[5] Treatment of Joint Damage or Pain. GB Patent 2403405, 2005.

[6] Skin Treatment System. US Patent 5866142, 1999

[7] Cosmetics Composition Comprising Extract of Natural Materials for Improving Acne and Skin Wrinkles and Whitening Skin. KR Patent 20050004355 A, 2005

[8] Green Chem., 2023, 25, 2596–2607.

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