The Future of Sustainability in the Younger Generations’ Hands

The Future of Sustainability in the Younger Generations’ Hands

By Alex Waked, Co-chair for the GCI

In the last couple decades, there has been an increasing focus on developing sustainable practices in society to reduce our environmental impact. Probably the most notable effort in this endeavour is the signing of the Paris Agreement within the United Nations Framework Convention on Climate Change, in which 194 states and the European Union have set goals to reduce the global carbon footprint to reasonable levels.

As we progress forward, there will be a need to propagate this mindset to the coming generations. Fortunately, I don’t think there will be too much difficulty in achieving this. A growing number of schools have been instituting environmental- and sustainability-related courses in their curricula. In my opinion, this strategy has been the most effective in conveying the importance of being conscious of any consequences of our actions and learning how to improve.

In the last few years, many of the chemistry courses at the University of Toronto have incorporated green chemistry and safety modules in both the laboratory and theory sections of the courses. The number of factors that we now consider when designing experiments is much larger than in the past. For instance, are the molecules we’re synthesizing going to be very toxic? Can they be safely disposed of? Do we use harmful substances or solvents during the experiment? How much chemical waste is produced?

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Figure 1. Graphic of the 12 Principles of Green Chemistry, which currently play an important role in designing safe and environmentally benign chemical processes.1

These are all questions that have traditionally been overlooked in the past. However, the description of the 12 Principles of Green Chemistry by Anastas and Warner in 19982 was a huge and essential step forward in the current direction we’re heading of thinking about chemistry from a sustainability and safety perspective. Many student-led groups and schools are now taking initiative in this endeavour.

The earlier and more the students are taught about these topics, the greater the chance it will have of the students developing genuine interests in them. In June of this year, the University of Toronto Schools held their first Sustainability Fair, in which grade 8-9 students participated in a science fair-like event where they worked on sustainability-related projects.

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Figure 2. Examples of posters at the University of Toronto Schools’ Sustainability Fair in June 2018.3

The GCI was invited to participate in listening to the students’ presentations describing their projects and to give any advice and encouragement to them; three of us, myself included, attended it. I would say there were at least 40 projects in total. These are just a few examples of some the projects:

  • Calculating how much water was saved by reducing shower time over a 2-week period
  • Collecting and recycling e-waste (any old electrical parts) that would traditionally be thrown away in the garbage
  • Calculating the reduction of carbon footprint by biking to work or school instead of driving

There were two things that really stood out to us: one being the range of topics (water reduction, carbon footprint reduction, recycling plastics and electronic waste, and minimizing food waste), and two being the genuine enthusiasm and interest of the students in their projects.

These are the students that will develop into people that will have important leadership roles in society in the future. The prospect of this is what excites me and gives me confidence that the future generations will continue to propel society forward in terms of being even more environmentally conscious and actually walk the walk, and not only talk the talk!

References:

  1. The Green Chemistry Initiative website. Accessed September 13, 2018. <http://greenchemuoft.ca/resources.php&gt;
  2. Anastas, P. T.; Warner, J. C. Green Chemistry: Theory and Practice, Oxford University Press: New York, 1998, p. 30.
  3. Obtained with permission of the University of Toronto Schools.

 

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Green Chemistry Principle #11: Real-Time Analysis for Pollution Prevention

Green Chemistry Principle #11: Real-Time Analysis for Pollution Prevention

By Alex Waked, Co-chair for the GCI

  1. Analytical methodologies need to be further developed to allow for real-time, in-process monitoring and control prior to the formation of hazardous substances.

In Video #11, Rachel and I discuss the importance of continuously monitoring chemical processes in real-time.

Most of us have driven a car before. Picture yourself driving down the highway in a car that doesn’t have any windows or rearview mirrors. I’d imagine it would be hard to not get into some sort of accident. Now add all the windows and the mirrors. It’d probably be safer to drive now, right?

So what does this have to do with chemistry, or with green chemistry principle #11 in particular? Windows and rearview mirrors provide the driver with means to monitor their surroundings in real time and allows them to react and adjust. This is exactly the idea behind principle #11 – the design of analytical methodologies to monitor chemical reactions in real time and allow for adjustments. We can think of the windows and rearview mirrors as examples of such “analytical methodologies”.

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Figure 1. An NMR Spectrometer (left) and a TLC place under UV light (right) [1, 2].

As chemists, we conduct several experiments every day. Depending on the type of chemistry, the goal of these experiments can be to synthesize a novel target compound, design newer chemical processes, or simply study the properties and reactivity of a compound of interest. In a lot of these cases, it is necessary to use various analytical techniques to monitor the reaction. In the case of the simplest chemical reaction, reactants A and B react together to form a product C. How do we know when the reaction is complete? Typically, we can use techniques such as NMR or TLC (Figure 1) to see how far along the reaction has proceeded.

In many industrial settings, it’s crucial to have suitable analytical methods to monitor reactions in real-time. The scale of the reactions performed at these plants are big enough such that issues that we typically consider being only minor ones at the research lab scale can become very problematic.

An example of such a case is an exothermic reaction, in which energy is released as heat. At bench scale (grams), one can use a simple ice bath to cool down an exothermic reaction. And even if the solution’s temperature does end up rising, this usually doesn’t pose a great risk due to the small scale of the reaction.

If we now look at a similar exothermic reaction at an increased scale (kilograms), even a small increase in the solution’s temperature poses a much greater problem. The reaction rate increases at higher temperatures, further increasing the temperature as the reaction proceeds, and hence a rapid increase in the reaction rate. This is called a thermal runaway. At this point it’s nearly impossible to stop the cycle and can result in an explosion. One of the most notable examples is the Texas City disaster in 1947,3 in which a cargo ship containing more than 2000 tons of ammonium nitrate detonated, initiating a chain-reaction of additional fires and explosions in other nearby ships, killing more than 400 people (Figure 2).

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Figure 2. Aerial view of the Texas City disaster [4].

Suffice to say, there is currently a huge emphasis in industrial settings to monitor and control large-scale processes in real-time.4 Changes in temperature are monitored by internal thermometers, changes in pressure can be monitored by barometers, and changes in pH can be monitored by pH meters. With the help of these analytical tools, it’s easy to verify if a reaction’s conditions exceed the safe limits, and subsequently halt the process before anything gets out of hand.

 

References:

(1) http://researchservices.pitt.edu/facilities/nmr-spectroscopy-lab

(2) https://www.youtube.com/watch?v=HZzA9M0H40U

(3) “Texas City explosion of 1947”, Encyclopædia Britannica. April 9, 2018. Accessed May 2, 2018. <https://www.britannica.com/event/Texas-City-explosion-of-1947&gt;

(4) https://sputniknews.com/in_depth/201509011026442762/

(5) “Green Chemistry Principle #11: Real-time analysis for Pollution Prevention”, American Chemical Society. Accessed May 2, 2018. <https://www.acs.org/content/acs/en/greenchemistry/what-is-green-chemistry/principles/green-chemistry-principle–11.html&gt;

ACS Summer School on Green Chemistry and Sustainable Energy 2018

ACS Summer School on Green Chemistry and Sustainable Energy 2018

By Kevin Szkop and Rachel Hems

The Colorado School of Mines in Golden, CO is a wonderful campus with cutting-edge facilities and a great place to spend a week with 60 young scientists interested in green chemistry. This is where the ACS Summer School on Green Chemistry and Sustainable Energy was held from July 10 – 17. The group consisted of chemists and chemical engineers from North and South America, all with unique perspectives, experiences, and attitudes towards sustainability. Below is a photo of our awesome class!

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The 2018 ACS Summer School on Green Chemistry and Sustainable Energy class

The program consisted of technical and professional development sessions. A highlight was a life cycle assessment group project and presentation, led by Prof. Philip Jessop from Queen’s University. During Professor Jessop’s lectures, we learned how to think about the “greenness” of a process, and how this often-nebulous concept is best used as a comparative tool. While every process likely has downfalls, using the green chemistry principles and metrics allowed us to think critically about which process has the least downfalls, and how to address these in our work. The assignment included a group project, during which groups of students had to evaluate the merits and drawbacks of 5 synthetic routes to the same product. In this context, we learned that it is not only the reagents that go into a flask, but everything that happens behind the scenes, including shipping of reagents, the type of waste generated, amount of energy consumed, and much, much more. As a synthetic chemist (Kevin), it really made me think about solvent consumption and work up techniques in my own work!

In addition to learning about green chemistry and sustainable energy, there were some great professional development lectures and activities. Dr. Nancy Jenson, the program manager for the Petroleum Research Fund at the ACS, gave an engaging talk on tips for writing research proposals and common mistakes that are made. While she gave examples from her experience at the Petroleum Research Fund, there were many lessons that could be applied to any type of proposal writing.

Another great professional development lecture was given by Joerg Schlatterer from the American Chemical Society. He gave an overview of the ACS’s many resources for young chemists, such as the Chem IDP website for career planning, workshops for prospective faculty organized by the Graduate & Postdoctoral Scholars Office, and the new Catalyzing Career Networking program at ACS National Meetings. As part of the career planning case study, we took some time to make some SMART goals for ourselves for the next two years. I (Rachel) found it’s really helpful to have others share their goals and give suggestions for yours to make them the SMARTest they can be!

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Rafting down Clear Creek

Of course, we also had time to have fun! On the Saturday (also Rachel’s birthday!) we went white water rafting on Clear Creek. The river is mountain fed, so it was very cold, but it was a beautiful warm and sunny day! We had a great time rafting down the river, with a quick stop to jump in for a swim. It was a great way to spend my birthday! Throughout the week-long summer school, there was a decent amount of free time to enjoy the sunshine and the sights around Golden. Some of the fun things we got to do were swim in and raft down the river that goes through ‘downtown’ Golden, an early morning hike up the South Table Mountain, tour the Coors Brewery, and get to know all the other awesome chemists!

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Kevin and Rachel enjoying the Golden nightlife after a long day of learning!

We highly recommend attending this summer school. It is a great opportunity to learn and to meet great people who care about sustainable chemistry! Read more about past GCI members that have attended the ACS Summer School in 2014 and  2017.

More information on the summer school and how to apply can be found online here.

The plastic problem – accumulation before alternatives

The plastic problem – accumulation before alternatives

By Karlee Bamford, Treasurer for the GCI

Plastics undoubtedly play a central role in our daily lives and played a pivotal role in the development of consumer societies across the globe for over a century. Concurrent with newfound materials and newfound possibilities, unprecedented environmental problems have emerged as a result of our reliance on plastics. The accumulation of plastics in allocated disposal sites (e.g. landfills) and in otherwise uninhabited spaces (e.g. beaches, open ocean) present threats to human health, water security, and food supply. These challenges now impact communities globally, irrespective of their actual contribution to the generation of plastic waste, and affect individuals of all economic backgrounds.

Figure 1. Examples of waste plastic accumulation in landfills and the environment. Images source: Pixabay.

Given the scale and significance of these challenges, is there anything that chemists can do to resolve this panhuman problem? A recent blog post from the Green Chemistry Initiative (https://greenchemuoft.wordpress.com/category/author/molly-sung/) highlighted the advances that have been made in synthetic and materials chemistry towards plant-derived and biodegradable plastics as alternatives to traditional petroleum-derived plastics. While this is undoubtedly a crucial area of research as humanity has become permanently dependent on plastics, the design of next generation plastics that are inherently sustainable will not mitigate the overwhelming impacts of existing plastic waste. Arguably, attenuating the problem of plastic waste is more important than finding alternatives to traditional plastics. Indeed, the decomposition time for products made from the top four families of commodity plastics (PP, PE, PVC, PET), produced on a 224.6 million tonne-scale alone in 2017,1 is estimated at 1 to 600 years in marine environments2 and considerably longer in landfills due to lack of moisture.4

Figure 2. Examples of the top five most-produced commodity polymers and their production scale in 2017.1,3

Traditional plastic-recycling methods are not equipped to resolve the issue of waste plastic accumulation either. Recycling can be broken down into three distinct varieties: primary, secondary, and tertiary.5 Primary recycling, which is equivalent to repurposing or reusing, is used limitedly for products such as plastic bottles, typically made of PET, which be directly reused following the necessary sterilization. Secondary recycling involves mechanical processing of plastics into new materials and frequently results in reduction of the plastics overall quality or durability due to the thermal or chemical processes involved. Primary and secondary recycling account for the majority of recycling efforts, however, as a consequence of poor consumer compliance (e.g. <10 % in the US and 30-40 % in the EU)6 and the deteriorating value of plastics with repeated secondary recycling, all plastics eventually become waste. The last and most underutilized form of recycling is tertiary recycling, the degradation or depolymerization of plastics into useful chemicals or materials. In the last year alone, numerous high profile editorial and review articles have appeared in Science7,8,9 and Nature6,10 emphasizing the incredible potential of chemical (tertiary) recycling as means of reducing plastic waste and as a new, sustainable chemical feedstock for the polymer (plastics) industry.

The challenge of chemical recycling is immediately evident: plastics have been expertly designed to be highly durable and chemically resistant, and thus, plastics cannot be easily transformed chemically. Ideally, polymers used in plastics could be depolymerized to monomer for subsequent repolymerization. For condensation polymers, such as polyethylene terephthalate (PET), the reverse of the polymerization reaction is the addition of a small molecule to the polymer to reform monomer. While completely reversible on paper or in theory, such depolymerization strategies have had limited success for PET.

Reacting the polymeric PET material with protic reagents (e.g. amines, alcohols) followed by hydrolysis to give monomers that can be repolymerized, if of sufficient purity (Figure 3), requires high temperature (250-300 °C) and high pressure (0.1-4 MPa) conditions unless additives, such as strong acids and bases or metal salts, are used.11 The action of many additives is not well understood, thus precluding rational improvement of the system. Hydrolysis of PET itself, especially at neutral pH, is the most challenging approach to PET chemical recycling as water is a relatively poor nucleophile. Hence stronger nucleophiles, such as ethylene glycol, are preferred.

Figure 3. Depolymerization of PET by glycolysis.

One practical problem in the chemical recycling of any plastic is its insolubility. Phase transfer catalysts –  species capable of transferring from one phase to another – have been used to address the insolubility of PET12 and have permitted the direct hydrolysis of PET at operating temperatures as low as 80 °C, as in the work of Karayannidis and coworkers (Figure 4). The phases in these systems are the insoluble PET polymer (the organic phase) and the basic solution (the aqueous phase) surrounding it.13

Figure 4. Phase transfer catalyzed hydrolysis of PET (catalyst shown in blue).

Addition polymers, such as polypropylene (PP) or polyethylene (PE), cannot be depolymerized to monomer form using the above strategies as their polymerization does not involve the loss of small molecules. Until very recently, the best end-of-life purpose for the majority of plastics has been energy recovery through incineration. The work of Huang and coworkers on the chemical degradation of PE plastics is a break-through for the field of plastic recycling. While previous studies have reported that thermolysis of PE yields poorly defined mixtures of hydrocarbons, these authors have found a remarkable, highly targeted method for converting PE to a narrow distribution of fuels (3 to 30 carbons in length) using a dehydrogenative metathesis strategy (Figure 5).14 The homogeneous iridium catalysts employed were previously reported in the literature for alkane dehydrogenation (step 1) and hydrogenation (step 3), but no such polymer substrates had apparently been attempted for main-chain dehydrogenation. Similarly, the authors used a previously-established rhenium oxide/aluminium oxide catalyst for olefin metathesis (step 2).

Figure 5. The transition-metal catalyzed degradation of PE to liquid fuels reported by Huang and Guan (catalysts shown in blue).14

The chemical recycling of PET by phase transfer catalysis and of PE by dehydrogenative-metathesis have very little in common with one another on a technical level. What unites these two strategies is the desire to transform the problematic, highly abundant and inexpensive resource that is waste plastic into useful commodities. Perhaps more importantly, these two examples both take revolutionary approaches to old problems through inspiration from fundamental research and parallels found in small molecule catalysis. Rethinking the plastic problem into a challenge for catalysis, rather than solely a call for clever materials design, is critical if we wish to reduce the threats that waste plastics pose to our health and our environment.

References:

  1. Tavazzi, L., et al., The Excellence of the Plastics Supply Chain in Relaunching Manufacturing in Italy and Europe, The European House, Ambrosetti, 2013 (as cited in Bühler‐Vidal, J. O. The Business of Polyethylene. In Handbook of Industrial Polyethylene and Technology; Spalding, M. A.; Chatterjee, A. M., Eds.; John Wiley & Sons: Hoboken, NJ, 2017; p. 1305).
  2. Mote Marine Laboratory Biodegradation Timeline; 1993. Available from: https://www.mass.gov/files/documents/2016/08/pq/pocket-guide-2003.pdf ; accessed July 10, 2018.
  3. Image sources: Image sources: (Plastic recycling symbols) http://naturalsociety.com/recycling-symbols-numbers-plastic-bottles-meaning/ ; (PP) https://www.screwfix.com/p/stranded-polypropylene-rope-blue-6mm-x-30m/98570 ; (LLDPE) https://www.polymersolutions.com/blog/differences-between-ldpe-and-hdpe/ ; (HDPE) https://chemglass.com/bottles-high-density-polyethylene-hdpe-wide-mouths ; (PVC) https://omnexus.specialchem.com/selection-guide/polyvinyl-chloride-pvc-plastic ; (PET) https://ecosumo.wordpress.com/2009/06/04/what-does-the-recycle-symbol-mean-part-2/
  4. Andrady, A. L. Journal of Macromolecular Science, Part C: Polymer Reviews, 1994, 34(1), 25-76.
  5. Hopewell, J.; Dvorak, R.; Kosior, E. Trans. R. Soc. B, 2009, 364, 2115–2126.
  6. Rahimi, A.; García, J. M. Nature Reviews Chemistry, 2017, 1, 0046.
  7. MacArthur, E. Science, 2017, 358 (6365), 843.
  8. García, J. M.; Robertson, M. L. Science, 2017, 358(6365), 870-872.
  9. Sardon, H.; Dove, A. P. Science, 2018, 360(6387), 380-381.
  10. The Future of Plastic. Nature Communications, 2018, 9, 2157.
  11. Venkatachalam, S.; Nayak, S. G.; Labde, J. V.; Gharal, P. R.; Rao, K.; Kelkar, A. K. Degradation and Recyclability of Poly (Ethylene Terephthalate). In Polyester; Saleh, H. E. M., Ed.; InTech: London, 2004; p. 78.
  12. Glatzer, H. J.; Doraiswamy, L. K. Eng. Sci. 2000, 55(21), 5149-5160.
  13. Kosmidis, V. A.; Achilias, D. S.; Karayannidis, G. P. Mater. Eng. 2001, 286(10), 640-647.
  14. Jia, X.; Qin, C.; Friedberger, T.; Guan, Z.; Huang, Z. Science Advances 2016, 2(6), e1501591.

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.

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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.

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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).

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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.

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

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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

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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.

Glycoside Hydrolases: A Doorway to Alternative Energy

Glycoside Hydrolases: A Doorway to Alternative Energy

By Namrata Jain, GreenChem UBC (Invited post!)

Biofuels, in particular bioethanol, are widely accepted as carbon-neutral fuels1, meaning they have no net greenhouse gas emissions; the amount of carbon dioxide produced during their combustion equals the amount fixed from the atmosphere while the plants grow. These fuels provide an alternative to the current outrageous usage rate of fossil fuels. Plant biomass, a renewable and abundantly available natural resource, is used as the main source for bioethanol production.

In order to produce bioethanol, polymeric plant carbohydrates (polysaccharides) must be broken down into the corresponding monosaccharides, followed by fermentation via yeasts. Typically, starch-rich crops such as corn and sugarcane are the most heavily used as carbohydrate sources.

However, since utilization of these starchy sugars in bioethanol production competes with their use as food crops, there has been a recent shift towards utilization of lignocellulosic biomass.1 Lignocellulosic biomass includes cellulose and hemicelluloses present in non-edible parts of plants, and hence reduces dependence on edible, starch-rich crops.

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Figure 1. Structure of a plant cell wall, highlighting xyloglucan, a particular hemicellulose of interest. [2]

Lignocelluloses form an important part of the plant cell wall (Figure 1) and are composed of cellulose, hemicelluloses (such as xyloglucan), and polyaromatics called lignin. These polymers are tough and more difficult to break down to release monosaccharides, as compared to starch. Nevertheless, lignocelluloses are the most abundant biological material on earth and are an untapped resource.1

The complete utilization of this biomass, however, is hindered by the structural complexity of plant cell walls, arising from the heavy crosslinking between hemicelluloses, celluloses, and lignin within, making it difficult to access the degradable polysaccharidic components. Hemicelluloses, such as xyloglucan (Figure 2A), can make up 15-50 % of these lignocellulosic materials and have been the focus of research for optimization to use as a biofuel.

To efficiently break down the lignocelluloses, many types of enzymes are needed. Glycoside hydrolases, one such group of carbohydrate active enzymes, have proven to be very efficient in the hydrolysis of many complex polysaccharides.3 However, more details about the chemical structure of the enzymes, as well as a reliable way of comparing the kinetic activity of various enzymes has been of interest to researchers in the field.

One of the ways of quantifying the kinetic details of such enzymes is by designing chemical probes such as one shown in Figure 2B. Such probes are chemically very similar in structure to the polysaccharide of interest (eg. Figure 2A), and hence can subtly fit into the active site of the enzyme and manipulate its rate of catalysis in a controlled and quantifiable way, making comparisons between enzymes’ kinetics possible.

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Figure 2. Structures of (A) xyloglucan; and (B) xyloglucan oligosaccharide based probe.

These probes can also assist in the crystal structure formation of the enzyme providing key details about the nature of interactions between the enzyme and corresponding polysaccharide and the specific amino acids responsible for its catalytic activities (Figure 3).

The Brumer group at the University of British Columbia4 has recently designed one such probe (Figure 2B) specific for xyloglucan active enzymes (xyloglucanases) by chemically modifying a xyloglucan-derived heptasaccharide. This probe was able to provide valuable information about the kinetics, specificity, as well as structural details of a newly discovered xyloglucanase PbGH5, which is secreted by a microbe residing in the intestinal system of ruminants such as cows.

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Figure 3. Crystal structure of the characterised endoxyloglucanase in complex with the inhibitor. [4]

As more research goes into the design and improvement of such probes, we would be able to develop novel enzyme cocktails that can make bioethanol production more economically and practically viable, leading to gradual decrease in our dependence on fossil fuels for our energy needs.

 

References:

  1. Scheffran J. The Global Demand for Biofuels: Technologies, Markets and Policies. In: Biomass to Biofuels: Strategies for Global Industries. Blackwell Publishing Ltd.; 2010:27-54. doi:10.1002/9780470750025.ch2.
  2. https://en.wikipedia.org/wiki/Cell_wall#Plant_cell_walls
  3. Henrissat B, Davies G. Structural and sequence-based classification of glycoside hydrolases. Curr Opin Struct Biol. 1997;7(5):637-644. doi:http://dx.doi.org/10.1016/S0959-440X(97)80072-3.
  4. McGregor N, Morar M, Fenger TH, et al. Structure-function analysis of a mixed-linkage β-glucanase/xyloglucanase from key ruminal Bacteroidetes Prevotella bryantii B14. J Biol Chem. 2015;291(3):1175-1197. doi:10.1074/jbc.M115.691659.