Water Extract of Banana: The Tasty Fruit for Efficient Green Chemistry

Water Extract of Banana: The Tasty Fruit for Efficient Green Chemistry

By Matt Gradiski, Member-at-Large for the GCI

Bananas. They’re a fantastic healthy snack, delicious to bake into bread or flavour medicine, and even the choice speak-and-spell for singer Gwen Stefani. Now, thanks to two excellent reports in 2015, an efficient medium for two sophisticated organic transformations can be added to its list of uses.

Published in Green Chemistry in January 2015, researchers were able to perform Suzuki-Miyaura (SM) cross-coupling in a neat solution of water extract of banana (WEB).1 WEB is made by simply drying the peel of a banana, burning the dried remains, and extracting the ashes with water (Figure 1). What results is a brown-orange liquid holding tremendous catalytic capability.

Matt_blog_1

Figure 1. Preparation of WEB solution [1]

 Typically, SM coupling requires the addition of external ligands, base, or other reaction promoters that can often be very expensive. The reaction is known to be able to take place in aqueous media; however, organic solvents are usually the more common choice. While the SM reaction still requires a noble-metal palladium catalyst, using a WEB medium for this reaction completely replaces the use of external additives and organic solvents (Figure 2). The only thing better than being able to do your reaction in water, is to do your reaction in water quickly! The longest reported reaction time using this system was 20 minutes, with times as a low as 5 minutes, and yields as high as 99%, all being carried out at room temperature for 12 different products.

Matt_blog2_2

Figure 2. Example of Suzuki-Miyaura coupling in WEB

Extending the scope of WEB’s usefulness, a report in July of the same year in Green Chemistry showed that the medium can also be used effectively for the catalytic Dakin reaction.2 This reaction converts an ortho- or para-hydroxy aromatic aldehyde or ketone into its corresponding benzenediol through reaction with hydrogen peroxide in base (Figure 3).

Matt_blog3

Figure 3. Proposed Dakin oxidation mechanism catalyzed by WEB [2]

Similar to SM coupling, the Dakin reaction requires addition of an external base, typically sodium or potassium hydroxide. However, it was found in the study that no external base was required when the reaction was carried out in WEB. The WEB solution was effective enough to initiate the reaction via deprotonation of hydrogen peroxide, generating the nucleophilic hydroperoxide anion that is required for the reaction to take place. All 16 reactions screened in the study were carried out at room temperature with the use of no external additives or organic solvent. Reaction times were as long as 60 minutes, and isolated yields ranged from 90-98%!

But what makes WEB such an efficient medium for green chemistry? Although the exact identity of the active species is currently unknown, the two aforementioned studies gathered valuable information about what could be promoting their reactions from a report in 2007.3 It was identified that banana peels contain a large amount of potassium and sodium carbonate as well as sodium chloride and other trace elements. It was speculated that the high concentration of alkali metal carbonates in WEB was responsible for the acceleration of these organic transformations.

So, the next time you are finished having a banana, don’t monkey around and throw it away! Give it to a chemist in need, it may help them out more than you think!

 

References

1)         P. R. Boruah, A. A. Ali, B. Saikia and D. Sarma, Green Chem., 2015, 17, 1442–1445. DOI:10.1039/C4GC02522A

2)         B. Saikia, P. Borah and N. Chandra Barua, Green Chem., 2015, 17, 4533–4536. DOI:10.1039/C5GC01404B

3)         D. C. Deka and N. N. Talukdar, IJTK, 2007, 6 (1), 72-78.

 

Figures from Boruah et al. 2015 and Saikia et al. 2015 reproduced with the permission of the Royal Society of Chemistry.

Advertisements

Veggie (Scrap) Tales – Are plant-based polymers the answer to our plastic conundrum?

By Molly Sung, Secretary for the GCI

Plastic is one of the most ubiquitous materials on the planet. Everything from our toothbrushes, to pens, take-out containers, or parts used in the automotive or aeronautic industries are made from plastic. What started off as a convenient and cheap alternative to traditional materials has become a global reliance – and it’s taking its toll.

Traditional plastics are petroleum-based – and as we know, petroleum is a non-renewable resource and its extraction, processing, and use contributes to environmental pollution and climate change. When plastic bags were first gaining popularity in the 1950s and 60s, one of the selling points of using plastic bags was that they were more durable and long-lasting than paper,1 but that’s also exactly the problem. Plastic doesn’t degrade easily like paper does, so it starts to accumulate. This accumulation in landfills and, unfortunately, our waters has spurred research in the development of plastics that can break down over time.

An example of a biodegradable plastic is polylactic acid (PLA). The starting material, lactic acid, can be obtained through fermentation of crops such as sugarcane or corn, which can undergo condensation to form short chains (oligomers). Next, these oligomers undergo depolymerization to form lactide, a cyclic ester, which is then polymerized with the help of a catalyst to give PLA, shown in Figure 1.2

Molly_blog2

Figure 1. Synthesis of polylactic acid (PLA), a biodegradable plastic, from lactic acid.

PLA performs comparably to the popular commercial plastic polyethylene terephthalate (PET, labelled with the “1” inside the recycling symbol). It is currently used in food packaging (such as disposable cups), as medical implants,2 and has also found renewed popularity as a common filament for 3D printing, but it’s not without its problems. The monomer, lactide, can have varying stereochemistry which influences the final polymer product and the mechanical properties of the plastic. Significant strides have been made in this area of research, but possibly the biggest barrier to using PLA is the competition with the food industry for the starting material. This is incidentally the same problem many first-generation biofuels ran into. But what if we could take food waste and turn it into usable plastics?

While there are some technologies being developed to use non-food materials like cellulose as a bioplastic, many of these methods require fairly harsh reactions. A gentler, water-based approach to make a cellulose-based plastic was recently reported by a research team from the Italian Institute of Technology and the University of Milano-Bicocca in the journal Green Chemistry.3

Molly_blog3

Figure 2. Image of the bioplastic films made from different vegetable powders: (A) carrot, (B) parsley, (C) radicchio, (D) cauliflower. Reproduced from Perotto et al. [3].

This new technique uses waste from the food-industry, including carrot, cauliflower, radicchio, or parsley waste. The vegetable matter must first be dried and ground into a micronized powder, but otherwise no further processing or purification is required to make the veggie waste usable in this process. To make the plastic films, the researchers simply mixed the vegetable powder with a weakly acidic solution (5 % HCl w/w) at 40 °C, then removed any residual acid through dialysis and let the suspension dry in a petri dish for 48 hours. This process has a 90 % conversion of the vegetable waste into bioplastic (by weight) and the product has very promising mechanical properties (Figure 2).

In particular, in measuring the elasticity and tensile strength of the bioplastic films, it was found that the carrot film had comparable properties to polypropylene (commonly used for rigid plastic containers – otherwise referred to as number “5” plastics).

The researchers also tested important factors for plastics being considered for food storage applications. First, they studied whether the films would interact with water. The parsley film was found to absorb water fairly readily. Conversely, the carrot filmed exhibited hydrophobic behaviour – an uncommon characteristic for vegetable-derived plastics. This hydrophobic behaviour means that the moisture from food is unlikely to soak through the plastic film or structurally damage it.

One very interesting property of the radicchio waste is that it is rich in anthocyanins. Anthocyanin is what gives radicchio, red cabbage, and beets their vibrant red colour. More importantly, anthocyanins are known anti-oxidants and materials rich in these anti-oxidants are currently being investigated as food-packaging materials that extend the shelf-life of food.4 Unfortunately, these vegetable films tested to be fairly permeable to oxygen, which would offset any benefit from the antioxidant-rich radicchio film. However, the researchers showed that if the vegetable waste was blended with polyvinyl alcohol (PVA), the oxygen permeability can be reduced significantly and was even an improvement on the pure PVA.

Lastly, and very importantly, the researchers tested for the biodegradability of the films. To test the rate of biodegradation, the researchers submerged the carrot film in seawater to measure the rate of oxygen consumption by the seawater organisms responsible for the biodegradation of the film. They found that the film decomposed fairly quickly in 15 days.

These scientists have now demonstrated a very mild process in the synthesis of bioplastics that have mechanical properties similar to one of the most common commercial plastics. They have also made a plastic that, because of the presence of anthocyanins, may have applications in food storage that can help reduce food-waste.

What is especially promising about these bioplastics is how little purification of the vegetable waste is required to make them; however, there are improvements to be made. A major obstacle these materials will face is their performance in wet or humid environments as well as scaling up to an industrial process. It is clear that we need more sustainable materials and these vegetable waste plastics present an exciting new avenue towards biodegradable bioplastics.

 

References

  1. Laskow. How the Plastic Bag Became So Popular. The Atlantic [Online] 2014. https://www.theatlantic.com/technology/archive/2014/10/how-the-plastic-bag-became-so-popular/381065
  2. Gupta et al., J. Prog. Polym. Sci. 2007, 32, 4, 455-482. DOI: 10.1016/j.progpolymsci.2007.01.005
  3. Perotto et al., Green Chemistry, 2018, 20, 804-902. DOI: 10.1039/C7GC03368K
  4. N. Tran, et al., Food Chemistry, 2017, 216, 324-333. DOI: 10.1016/j.foodchem.2016.08.055

 

Figure from Perotto et al. 2018 reproduced with the permission of the Royal Society of Chemistry.