Just Keep Flowing

Just Keep Flowing

By Nour Tanbouza (twitter @Nour_Tanbouza), PhD student, Laval University

Flow chemistry is a synthetic technique that enables chemical reactions to take place in a continuously flowing manner as opposed to running a reaction in a flask, sometimes termed batch chemistry. It has become incredibly mainstream and has been adopted by many chemical industries as a means to increase efficiency of large-scale reactions in highly controlled setups.1 What is flow chemistry, and why is it important? Furthermore, the main question, how does it contribute to sustainability?

Let us start by putting on a lab coat and safety goggles and strolling through a modern synthetic chemistry lab. Now, have a look around. What you will absolutely recognize and remember is a vast space of fume hoods and benches with different apparatus lying around like round bottom flasks, chromatography columns, stirrers, hot plates, etc. After that, take a browse through images of those same types of laboratories from the 1900s or even from the 1700s. Surely you will notice some improved safety features but what will strike you the most is how similar they are in terms of the equipment used then and now. We indeed currently have better stirring and heating equipment etc., but we still do reactions in round bottom flasks as batches.

Figure 1. on the left: 18th century laboratory used by Antoine Lavoisier (credits Sandstein / CC BY (https://creativecommons.org/licenses/by/3.0)); on the right: Modern synthetic chemistry laboratory ( credits Elrond / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)

In 2005, the catastrophic T2 laboratories explosion occurred after a thermal runaway and high-pressure build-up of their 2,500-gallon batch reactor producing MMT (methylcyclopentadienyl manganese tricarbonyl).2 These types of accidents pose a huge risk to human life and the environment in addition to the legal and financial troubles that the company could face. Thus, there is an exigent need for safe and practical technologies that enable an efficient scale up of chemical reactions. These pursuits explain the recent uptake of flow chemistry by many manufacturing companies, especially those in the pharmaceutical industry.

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Figure 2. Aerial view of T2 Laboratories explosion

Flow chemistry can be thought of as a bench chemist’s very own cherry tree. The raw material is fed into the roots (pumps). Roots are the heart of the tree, and the same holds for the pumps of a flow system, so it is pivotal that they are well taken care of and are in perfect shape. Those nice healthy roots then flow the raw material over a large surface area up into the stems and leaves where reaction conditions are highly controlled and in perfect balance to elute the desired product continuously. Thus, whether targeting a few milligrams or multi-kilograms of product, it is dependent on how much feed material is flowed into the reactor. A flow reactor can be as small as a chip and still produce the needed amount of product. In 2019, flow chemistry was announced by the IUPAC (International Union of Pure and Applied Chemistry) as one of the ten chemical innovations that will change our world.3 There has been a significant paradigm shift by many industries, especially the pharmaceutical industry, to adopt flow chemistry. It is a technology that promises on-demand drug production, which is vital primarily for developing countries to access drugs in a decentralized manner.1

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Figure 3. An academic flow system (equipped with a photoreactor)

Green chemistry principles and a chemical industry’s agenda align when it comes to large scale reactions. Thus, it is not so surprising to see a significant uptake of flow chemistry by many companies. This kind of endorsement has helped spark research in continuous flow which is beginning to become a dominating area of study. Among the UN Sustainability Goals is responsibility for consumption and production, which is achieved in flow because it minimizes the amount of material needed for screening and allows reactions to take place in highly concentrated media. Reaction conditions being highly controlled (such as temperatures, pressure, mixing, etc.), allow reactions to be more selective and thus decreases any by-products and increases productivity.4 Also, hazardous chemicals can be safely manipulated in flow because there is no significant build-up at any given time. It is very versatile and modular where multiple reactions can be installed in sequence to consume any reactive intermediates in situ, and purification systems can be added directly as well. A reaction can be run at extremely high temperatures that go above boiling points which can enable reactions to proceed faster while being inherently safer and consuming significantly less energy when compared to a batch reactor.

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Figure 4. Illustration of a flow chemistry setup

This type of “thinking outside the flask” means stepping outside of a long-standing comfort zone which is not always trivial. However, this type of venture and side-by-side work of engineers and chemists is what made flow chemistry possible, and it is changing our world. Flow chemistry is still in its early stages, yet so much innovation has already been introduced. Give it a few years, and when you walk back into that synthetic chemistry lab, prepare to be flabbergasted by a space that resembles nothing of the past.

 

References:

  1. Malet-Sanz, L.; Susanne, F., Continuous flow synthesis. A pharma perspective. J. Med. Chem. 2012, 55, 4062-4098.
  2. http://www.csb.gov/UserFiles/file/T2%20Final%20Report.pdf
  3. Gomollón-Bel, F., Ten Chemical Innovations That Will Change Our World. Chemistry International 2019.
  4. Jensen, K. F.; Rogers, L., Continuous manufacturing – the Green Chemistry promise? Green Chemistry 2019, 21, 3481-3498.

The Great Step Backwards: Polymer to Monomer

By Hyungjun Cho, member-at-large for the GCI

There is a movement to develop a new type of product life system called ‘the circular economy’ [3]. Part of this movement aims to manufacture products from recycled or raw materials, and after its useful lifetime, re-introduce the product (now considered waste) as recycled material. The motivation for the introduction of the circular economy is to minimize the need for virgin raw material, especially when it originates from non-renewable resources. This effort is being spearheaded by the Ellen MacArthur Foundation with major industry partners like Google, Unilever, Solvay, and Philips, among others [3]. A critical component to the function of the circular economy is developing the capability to turn waste into a desirable product.

There are several methods of recycling all the different types of materials we use in every day life. This blog will discuss a niche in the ‘plastic to monomer’ field. Evidently, in April of 2019, IUPAC named ‘plastic to monomer’ as part of the 10 chemical innovations that could have high impact in society [6]. Before discussing ‘plastic to monomer’, I must clarify the term ‘plastic’. Generally, plastic is made up of many polymer chains that are physically entangled with one another. A macroscopic analogy is when many electrical wires (think of Christmas tree lights) become entangled: the wires are stuck to each other and the rigidity of the ball of wire is greater than the rigidity of a single wire.

Much like the type of wire influences the tangled ball it forms, the chemical structure of the polymer influences the material properties of the plastic. Examples of properties of plastics include rigidity, elasticity, malleability, gas permeability, friction to skin, transparency, and many others. The polymers that are used for commercial plastic products have been studied and developed for decades to be able perform a specific function. For example, polyvinyl chloride and polystyrene were initially discovered in the 1800s [2,8]. Thus, it would be ideal if the currently used polymers can be de-polymerized back into monomers for recycling purposes. This would be a major move by the plastics industry to become environmentally friendly.

The conventional method to turn polymers into monomer is thermal decomposition. Samples of polymer can be heated to high temperature (typically 220-500 °C) to break some of the bonds that hold the monomers together [10]. When this occurs, radicals can form at the site of the broken bond, which can lead to de-polymerization [10]. The required temperature and how much monomer is formed is dependent on the chemical structure of the monomers that are formed. Thermal decomposition to recover monomer is suitable only for a few types of polymers, such as poly(α-methylstyrene), which has ceiling temperature of 66 °C to propagate depolymerization; the monomer recovery after thermal decomposition of poly(α-methylstyrene) is excellent at 95% [11]. However, for polymers like polyethylene (PE, the most produced polymer) and polypropylene (PP, 2nd most produced polymer), the monomer recovery yield is poor (0.025-2%) [11]. In some cases such as polyvinylchloride (PVC, 3rd most produced polymer), thermal decomposition is even more problematic because PVC will release harmful hydrochloric acid and vinylenes upon heating [11]. Thus, the monomer recovery is poor (1 %) and the process is highly corrosive.

Therefore, one of the key challenges to address for ‘polymer to monomer’ is to perform de-polymerization at a low temperature. There are 4 recent publications that explore this challenge [5,7,9,12]. In general, the authors synthesized polymers using reversible-deactivation radical polymerization (RDRP) techniques and explored the de-polymerization reactions they encountered. Below is a brief highlight from the publications from the Haddleton group [9] and the Gramlich group [5].

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Scheme 1: De-polymerization of RAFT polymers with trithioester end-group [5]. Reproduced from ref. [5] with permission from The Royal Society of Chemistry.

Flanders et al. polymerized methacrylate monomers, including methylmethacrylate (MMA), using reversible addition-fragmentation chain-transfer (RAFT) polymerization with a trithioester chain-transfer agent (CTA) [5]. This type of polymerization places trithioester end-group at end of the polymer chain (Scheme 1). Typically, this end-group is used to re-start the polymerization at the trithioester end of the polymer. However, as we will see, it may have another function. The authors isolated the polymer, then re-dissolved the polymer in 1,4-dioxane at 70 °C (Scheme 1). This caused monomers to be released from the polymer chains at a temperature much less than the ceiling temperature of MMA, which is 227 °C [13]. Analysis of the polymer after partial de-polymerization demonstrated that the trithioester end-group was still attached to the polymer and the size dispersity (range of polymer ‘molecular weight’) was low, which suggested that the de-polymerization was moderated by the trithioester end-group. The authors observed 10-35% de-polymerization after heating at 70 °C for 12-60 hrs.

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Scheme 2: ATRP of NIPAM in carbonated water, followed by de-polymerization [9]. Reproduced from ref. [9] with permission from The Royal Society of Chemistry.

Lloyd et al. used an alkylbromide initiator, Cu-based catalyst system to polymerize N-isopropylacrylamide (NIPAM) in Highland Spring carbonated water at 0 °C (Scheme 2) [9]. This type of polymerization places a halide at the end of the polymer chain. The authors monitored the monomer conversion into polymer using 1H-NMR spectroscopy. They measured that ca. 99% of the monomer was converted into polymer chains within 10 min. Unexpectedly, in the next 50 min. the authors observed 50% de-polymerization. The authors attempted to optimize de-polymerization conditions by changing the pH, using dry ice in HPLC grade water instead of Highland Spring carbonated water, etc. which led to 34-71% de-polymerization after 0.5-24 hrs. Years later, the same group used a very similar polymerization condition to polymerize NIPAM [1]. This time, non-carbonated water was used as the solvent and they did not report any de-polymerization.

The reports on RDRP followed by de-polymerization highlighted here are not yet ready to make an impact to ‘plastic to monomer’. The authors admit that the mechanism of de-polymerization is unknown. However, these seem to be the first set of reports on de-polymerization occurring at low temperatures. Perhaps these publications could be the birth of the reversible-deactivation radical de-polymerization (RDRDe-P) field. This is especially intriguing because RDRP have already been studied for decades in academia and are being adopted by the polymer industry [4]. Companies like BASF, Solvay, DuPont, L’Oréal, Unilever, 3 M, Arkema, PPG Industries, etc. already claimed patents for technology and products based on RDRP [4]. Somewhat ironically, RDRP was also part of the IUPAC’s 10 chemical innovations for impact on society but not for its potential to recycle polymer [6].

The polymers of the future may not be made from monomers abundantly used today, but the polymers of the future may be degradable through a low energy process.

References

  1.  Alsubaie, F.; Liarou, E.; Nikolaou, V.; Wilson, P.; Haddleton, D. M. Thermoresponsive Viscosity of Polyacrylamide Block Copolymers Synthesised via Aqueous Cu-RDRP. European Polymer Journal 2019, 114, 326–331.
  2. Baumann, E. Ueber Einige Vinylverbindungen. Justus Liebigs Annalen der Chemie 1872, 163 (3), 308–322.
  3. Circular Economy – UK, USA, Europe, Asia & South America – The Ellen MacArthur Foundation https://www.ellenmacarthurfoundation.org/ (accessed Jan 5, 2020).
  4. Destarac, M. Industrial Development of Reversible-Deactivation Radical Polymerization: Is the Induction Period Over? Chem. 2018, 9 (40), 4947–4967.
  5. Flanders, M. J.; Gramlich, W. M. Reversible-Addition Fragmentation Chain Transfer (RAFT) Mediated Depolymerization of Brush Polymers. Chem. 2018, 9 (17), 2328–2335.
  6. Gomollón-Bel, F. Ten Chemical Innovations That Will Change Our World: IUPAC Identifies Emerging Technologies in Chemistry with Potential to Make Our Planet More Sustainable. Chemistry International 2019, 41 (2), 12–17.
  7. Li, L.; Shu, X.; Zhu, J. Low Temperature Depolymerization from a Copper-Based Aqueous Vinyl Polymerization System. Polymer 2012, 53 (22), 5010–5015.
  8. Liebig, J. Justus Liebig’s Annalen Der Chemie. Annalen der Chemie 1832, 1874-1978.
  9. Lloyd, D. J.; Nikolaou, V.; Collins, J.; Waldron, C.; Anastasaki, A.; Bassett, S. P.; Howdle, S. M.; Blanazs, A.; Wilson, P.; Kempe, K.; et al. Controlled Aqueous Polymerization of Acrylamides and Acrylates and “in Situ” Depolymerization in the Presence of Dissolved CO2. Commun. 2016, 52 (39), 6533–6536.
  10. Microwave-Assisted Polymer Synthesis. Springer eBooks 2016
  11. Moldoveanu, Șerban. Analytical Pyrolysis of Synthetic Organic Polymers; Techniques and instrumentation in analytical chemistry; Elsevier: Amsterdam ; Oxford, 2005.
  12. Sano, Y.; Konishi, T.; Sawamoto, M.; Ouchi, M. Controlled Radical Depolymerization of Chlorine-Capped PMMA via Reversible Activation of the Terminal Group by Ruthenium Catalyst. European Polymer Journal 2019, 120, 109181.
  13. SFPE Handbook of Fire Protection Engineering, 5th ed.; Hurley, M. J., Gottuk, D. T., Jr, J. R. H., Harada, K., Kuligowski, E. D., Puchovsky, M., Torero, J. L., Jr, J. M. W., Wieczorek, C. J., Eds.; Springer-Verlag: New York, 2016.
Canada Becomes a Leader in Carbon Capture

Canada Becomes a Leader in Carbon Capture

By Karlee Bamford, Treasurer for the GCI

The attention of international media has been captured by the remarkable success in CO2 sequestration achieved by the Canadian company Carbon Engineering, located in Squamish, British Columbia. Sustainability-related, world-saving initiatives often have an easier sell in the media than, say, incremental advances reported by researchers on equally sustainable academic pursuits (rough, eh?). In this instance the craze over Carbon Engineering’s advances has been amplified by the news of their recent partnerships with household-name energy and oil giants, such as Chevron, BHP, and Occidental Petroleum, in the form of a CAD $68 million investment.  So, what is this incredible advance?

From the success of their pilot plant and the data they’ve accumulated thus far, Carbon Engineering implementation of their technology has achieved capture of The technology in question can be split into two major advances. Referred to as direct air capture, or DAC, the first process developed by Carbon Engineering involves the transfer of gaseous CO2 from ambient air to an absorber fluid, a strongly basic solution of sodium or potassium hydroxide. The transfer process is achieved using an air-liquid contactor, designed and described by the company in 2012,4 that involves an array of fans, pumps, cheap PVC piping and structure, and fluid distributors. These components are fundamentally no different than those commonly found in cooling towers used as heat exchangers for water cooling. However, the orthogonal geometry (Figure 1) of air (atmospheric, ~ 400 ppm CO2) and fluid (the absorber) flow differs significantly, making repurposing of existing cooling tower designs for DAC an inefficient and expensive strategy for CO2 capture.

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Figure 1. Commercial realization of air-fluid contactor designed by Carbon Engineering. M = Na or K. Image obtained from CanTech Letter and modified.5

The CO2 taken up by the alkaline absorber fluid is converted to carbonate (CO32-) salts and can be precipitated from the aqueous solution by treatment with calcium hydroxide to give calcium carbonate pellets. The captured CO2 can thus be stored as calcium carbonate or can be cleanly regenerated as pure CO2 gas, with elimination of a CaO , at high temperatures (650 °C) for commercial resale. The byproduct CaO may even repurposed by conversion back to Ca(OH)2 in a lime slaker, using water.1 Carbon Engineering has been piloting this process at their facility in Squamish since 2015, according to their website, after having tested a smaller prototype from 2010 and published the performance results in 2013.6 At the time of Carbon Engineering’s founding and until as recently as 2018, no commercial-scale air capture systems had been developed, which was a direct result of the anticipated inefficiency of CO2 capture using conventional cooling tower designs.4 Undeterred, Carbon Engineering has proven otherwise with their innovative use of cross-flow geometry.

The second break-through technology from Carbon Engineering is their patented Air To FuelsTM process, which they’ve been piloting since 2017. Taking the stored CO2 from their DAC process, Carbon Engineering has successfully produced a clean, sulfur-free, source of hydrocarbon fuel that requires no further modification for consumer consumption. The process involves passing the regenerated CO2 gas through a reactor containing hydrogen (H2) gas to generate synthesis gas (syn-gas), a mixture of CO and . The syn-gas is then passed through a Fischer-Tropsch reactor where the synthetic hydrocarbon fuel is thermally generated over a heterogenous base-metal catalyst (e.g. iron, cobalt, nickel).7

The technologies have been developed by the research groups of founder and U of T alumnus Prof. David Keith. Prof. Keith is currently faculty at Harvard University in the School of Engineering and Applied Sciences. To date, the company has filed 13 patents and produced numerous publications describing their innovations. According to media reports,8 recent multimillion-dollar investments will allow their and the company has already signed a memorandum of understanding with Squamish First Nations about their intentions.9

One of the most attractive aspects of the DAC and Air to FuelsTM technology is location. Plants could, hypothetically, be built anywhere, as CO2 is well mixed in the atmosphere and Carbon Engineering’s technology does not require that CO2 capture occur at the point of CO2 generation as in, for example, CO2-scrubbers used in exhaust systems.

However, with the excitement surrounding Carbon Engineering’s projected ability to capture CO2 at low cost and high volume, controversy has inevitably been close to follow. The interest from large oil corporations in this technology may not be as principled in sustainability as it appears but driven in part by their need for large volumes of CO2 for so-called green fracking (hydraulic fracturing). Supporting further oil extraction in this way goes completely counter to the need for elimination of emissions that the 2018 Intergovernmental Panel on Climate Change (IPCC) report clearly indicates must accompany advances in carbon capture and storage.10 Still, perhaps the positives outweigh the negatives in this instance. This very week, Environment and Climate Change Canada reported that Canada is warming at twice the rate of the rest of the globe.11 The need for efficient technologies to address climate change has never been more immediate. Fortunately, Carbon Engineering is not alone: at least two other companies with commercial plans for CO2 capture have started in Switzerland (Climeworks)12 and the USA (Global Thermostat).13 Whether the Canadian solution is adapted worldwide will depend not only upon Carbon Engineering, but also upon how these alternative approaches evolve.  For once, it is probably best not to pick a team to cheer for but, instead, hope that each country’s company develop a complimentary capture strategy to address the international dilemma that is climate change.

References:

  1. Keith, D. W.; Holmes, G.; St. Angelo, D.; Heidel, K., Joule 2018, 2, 1573-1594.
  2. American Physical Society. Direct Air Capture of CO2 with Chemicals: A Technology Assessment for the APS Panel on Public Affairs. June 1, 2011 https://www.aps.org/policy/reports/assessments/upload/dac2011.pdf ; accessed April 24, 2019.
  3. Carbon Engineering, https://carbonengineering.com/ .
  4. Holmes, G.; Keith, D. W., Trans. R. Soc. A 2012, 370, 4380-403.
  5. Artist’s rendition of a commercial scale Carbon Engineering contactor, CanTech Letter. https://www.cantechletter.com/2016/10/squamish-b-c-s-carbon-engineering-begins-scale-co2-capture-new-deal/ ; accessed April 4, 2019.
  6. Holmes, K. Nold, T. Walsh, K. Heidel, M. A. Henderson, J. Ritchie, P. Klavins, A. Singh and D. W. Keith, Energy Procedia, 2013, 37, 6079-6095.
  7. Heidel, Keton et al. Method and system for synthesizing fuel from dilute carbon dioxide source. WO2018112654A1, 2017.
  8. BBC News, Matt McGrath. Climate change: ‘Magic bullet’ carbon solution takes big step. April 3, 2019 https://www.bbc.com/news/science-environment-47638586 ; accesed April 3, 2019.
  9. CBC News, Angela Sterritt. In fight to combat climate change, Squamish Nation joins forces to capture carbon. November 29, 2018. https://www.cbc.ca/news/canada/british-columbia/in-fight-to-combat-climate-change-squamish-nation-joins-forces-to-capture-carbon-1.4924017 ; accesesd April 4, 2019.
  10. Intergovernmental Panel on Climate Change 2018 Summary for Policy Makers, Global Warming of 1.5 °C. https://www.ipcc.ch/site/assets/uploads/sites/2/2018/07/SR15_SPM_version_stand_alone_LR.pdf ; accessed April 4, 2019.
  11. Environment and Climate Change Canada, Canada’s Changing Climate Report, April 1, 2019. https://www.nrcan.gc.ca/environment/impacts-adaptation/21177 ; accesed April 4, 2019.
  12. Climeworks, http://www.climeworks.com/
  13. Global Thermostat, https://globalthermostat.com/
How green is your bromination reaction?

How green is your bromination reaction?

By Diya Zhu: Symposium Coordinator for the GCI

Electrophilic bromination is a common type of reaction in undergraduate organic laboratories. In these experiments, we rarely use Br2 as a bromine source. Why? This dense brownish-red liquid is a pain in the butt for a few reasons. First of all, it fumes. Once you open the bottle, orange fumes start migrating everywhere. Without efficient ventilation, soon you will smell an offensive and suffocating odor. Second, bromine is corrosive to human tissue as a liquid and its vapours irritate the eyes and throat. Moreover, with inhalation, bromine vapours are very toxic. Third, bromine is very dense, with a density of 3.1 g/cm3, which makes it very difficult to measure and transfer.

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Figure 1. A bottle containing bromine.1

Instead, N-bromosuccinimide (NBS) is often used as a brominating and oxidizing agent in various electrophilic addition, radical addition, and electrophilic substitution reactions. Pure NBS is a white crystalline solid with a melting point of 175-180 oC. Even though it’s a solid and easier to handle, you still need to be careful when working with NBS. Due to the higher electronegativity of nitrogen, the Br atom is partially positively charged and thus electrophilic, ready to be attacked by a nucleophile (eg. an alkene).

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Figure 2. N-bromosuccinimide (NBS)

NBS will form bromonium ions with alkenes, and when an alcohol or water is added, it will attack the bromonium ion, which will generate bromohydrins. Importantly, the nucleophilic attack only happens on the face opposite the bromonium ion.

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Figure 3. Alkene reaction with NBS showing the bromonium ion and attack of water to form a racemic mixture.

Usually, when undergraduate students preform this experiment, we also emphasized the importance of Green Chemistry. Green chemistry and its 12 principles help to improve conventional reactions. For example, increasing the efficiency of synthetic methods, reducing the steps of synthesis, and minimizing toxic reagents and solvents. In the formation of bromohydrins, compared to using Br2, NBS is less hazardous.  Also, water or alcohol can be used as the solvent which eliminates the use of organic solvents, especially chlorinated solvents.

However, the use of NBS also creates by-products. For example, succinimide and the very strong hydrobromic acid. It also has a reduced atom economy, since only one Br atom of 8 atoms in a NBS molecule is used in bromination.

With all of this taken into consideration, can it be concluded that NBS is a greener alternative to Br2? What do you think, and which reagent will you be reaching for in your next bromination experiment?

References:

  1. https://en.wikipedia.org/wiki/Bromine

Green Chemistry Principle #12: Inherently Safer Chemistry for Accident Prevention

By Brian De La Franier, Member-at-large for the GCI

  1. Inherently Safer Chemistry for Accident Prevention: Substances and the form of a substance used in a chemical process should be chosen to minimize the potential for chemical accidents, including releases, explosions, and fires.

In the 12th and final video of the GCI series on the 12 principles of green chemistry, Gabby and Qusai investigate the 12th principle on inherently safer chemistry and note several common issues found in many labs.

The 12th principle is frequently called the safety principle and is often overlooked when considering green chemistry principles.  However, the broad nature of the 12th principle means it both incorporates many of the other principles and is almost impossible to achieve without considering all 12 of them, given that the overall goal of green chemistry is to reduce hazards and pollution.

In the Video #11 blog post, Alex wrote about how driving a car with no windows and mirrors would lead to accidents as there would be no real-time way to analyze your surroundings. The 12th principle is akin to having that car inspected before driving it.  It is insuring that all aspects of the car, from the engine, to the brakes, to the steering are all in working order so that the car is less likely to get into an accident.  With this principle we consider the ingredients of a reaction (the parts of the car), and make sure that they don’t pose excessive hazards.

An example mentioned in the Video #12 of a hazardous chemical that can be replaced in synthesis is methyl isocyanate, a molecule used in the synthesis of the insecticide carbaryl.  In 1984, this toxic compound was released into the air from a pesticide plant in Bhopal, India, immediately killing 3,800 people, and causing premature death in thousands more.1 This disaster could have been avoided had the plant instead used methylamine to carry out the reaction.2

GCI 12th principle blog photo 1

Figure 1. The remains of the pesticide plant that led to the Bhopal disaster. [3]

Although this principle is specifically about the avoidance of using or producing hazardous compounds, the idea of avoiding hazards can be extended to other areas of the lab.  Storing chemicals that are reactive together, such as oxidizers and flammable materials, leads to a risk of release and reaction.  If these compounds leak from their containers and react, they will create a large fire. This is a hazard that could be easily avoided by storing these chemical types separately.

Another hazard in the lab is liquid spills. Anything that has been spilled should be immediately cleaned up to prevent people from slipping on it or receiving chemical burns from an unknown substance.  If someone comes across an acid spill, but does not know what it is they could easily be burned in attempting to clean it up.  Returning to the car analogy, leaving an unknown spill would be like giving someone a damaged car to drive without telling them. The unfortunate driver could be injured as a result of faulty brakes, just as another lab member could be injured by your spill in the lab.

As with our car, the lab should be kept safe and in good repair. If there are damaged parts in a car you should always repair them before driving it, just as if there are hazardous chemicals or situations in our lab they should be replaced before performing reactions.

References:

  1. Broughton, E. (2005). The Bhopal disaster and its aftermath: a review. Environmental Health, 4(1), 6.
  2. Thomas A. Unger (1996). Pesticide Synthesis Handbook (Google Books excerpt). William Andrew. pp. 67–68.
  3. https://www.dnaindia.com/analysis/column-bhopal-gas-tragedy-will-the-suffering-ever-end-2040370

 

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”.

Principle11_1

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).

Principle11_2

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.

Green Chemistry at CSC2017 – The 100th Canadian Chemistry Conference and Exhibition

By Kevin Szkop and Alex Waked

This year, the GCI partnered with the Chemical Institute of Canada (CIC), the organizing body of the CSC2017, to be closely involved in various aspects of Canada’s largest chemistry meeting.

In collaboration with GreenCentre Canada and CIC, the GCI organized a Professional Development Workshop as part of the CSC2017 program. This event consisted of four components:

The green chemistry crash course, led by Dr. Laura Reyes. Laura is a founding member of the GCI, and is now working in marketing & communications with GreenCentre Canada.

A case study, led by Dr. Tim Clark, Technology Leader at GreenCentre Canada. The case study gave attendees a unique opportunity to learn about some projects that GreenCentre has been developing and in collaboration with peers, learn how to find applications for new intellectual property (IP) and how to make contacts within relevant companies.

Kevin CSC blog 1

Dr. Tim Clark leading the GreenCentre Canada Industry Case Study

Career panel discussion, sponsored by Gilead, featuring members of academia and industry.

A coffee mixer for an opportunity for informal networking.

 

Supplementary to the Professional Development Workshop, the GCI organized a technical session, co-hosted by the Inorganic, Environmental, and Industrial sections of the conference. This new symposium, entitled “Recent Advances in Sustainable Chemistry”, brought together students, professors, industry, and government speakers to showcase a diverse and engaging collection of new trends in green and sustainable chemistry practices across all sectors of chemistry. Highlighted talks included Dr. Martyn Poliakoff from the University of Nottingham, also a CSC2017 Plenary Lecturer, Dr. David Bergbreiter from Texas A&M University, and Dr. William Tolman from the University of Minnesota.

Kevin CSC blog 2

Dr. Martyn Poliakoff teaching the audience about NbOPO4 acid catalysts found in Brazilian mines

Dr. Bergbreiter’s lecture was an engaging one. His enthusiastic approach to the use of renewable and bio-derived polymers as green solvents was captivating to both industrial and academic chemists.

Dr. Martyn Poliakoff, a plenary speaker at the conference, gave a phenomenal talk during the first day of the symposium. His charismatic style complimented perfectly the cutting-edge research ongoing in his group at the University of Nottingham. Particularly interesting was the use of flow processes in tandem with photochemistry to yield large quantities of natural products useful in the drug industries.

Dr. Tolman’s talk was of interest to essentially anyone working in an academic environment, especially for student run groups, like the GCI, with both academic interests as well as safety awareness initiatives. In the first part of the talk, synthetic and mechanistic studies of renewable polymers were discussed. The second part shifted focus to student-led efforts to enhance the safety culture in academic labs, which stood out from most of the other talks in our symposium.

One highlight was a group of graduate students at the University of Minnesota organizing a tour of Dow Chemicals to observe the work and safety codes in an industrial setting, which they used as a lesson to bring back to their own research labs. This caught the eye of most of the GCI members, which inspired us to organize a similar day trip in the future.

In further efforts to make our symposium accessible to undergraduate and graduate students, the GCI partnered with GreenCentre Canada to award five Travel Scholarships to deserving students from across Canada to provide financial aid to participate in the conference.

We thank all of our speakers, both national and international, for their participation in the program. It was a great success!

 

Taking Concrete Steps to CO2 Sequestration

Taking Concrete Steps to CO2 Sequestration

By Annabelle Wong, Member-at-Large for the GCI

With heightened concerns on greenhouse gas (GHG) emissions in recent years, scientists and engineers have come up with some innovative solutions to mitigate carbon dioxide emissions. One solution is to utilize and covert CO2 to everyday products such as fuels and plastics. Recently I learned that CO2 is now being converted into cement on an industrial scale.

Concrete is the most common construction material for buildings, roads, and bridges. Cement is one of the components of concrete and acts as a glue to hold concrete together. However, cement manufacturing is an energy-intensive process and the cement/concrete industry is one of the biggest CO2 emitters. In fact, 5% of the global GHG emission stems from cement production.1–3 To understand why so much CO2 is released, let’s first take a look at how cement is produced.

To make cement, limestone (calcium carbonate, CaCO3), silica (SiO2), clay (containing mostly Al2O3), and water are mixed and heated. This reaction produces a significant amount of CO2 and is called calcination. During calcination, at temperatures above 700 °C, limestone is decomposed to lime, or calcium oxide, and CO2 (Reaction 1). Then, lime reacts with SiO2 to form calcium silicates (C2S in simplified cement chemist notation, where C = CaO, S = SiO2) and tricalcium silicates (C3S) as the temperature ramps up to 1500 °C (Figure 1). The final product, called clinker, is then cooled and milled into a fine power. Afterwards, minerals such as gypsum (CaSO4) are added to make cement.4 A useful animation of cement making can be found here.5

CaCO3 (s) → CaO (s) + CO2↑ (g)                   (1)

annabelle_blog_feb2017-2

Figure 1. Raw materials are heated up to 1500 degrees C to synthesize clinker. The ratios of products yielded at various temperatures are shown. [4]

CO2 generated via calcination actually only consists of 50% of the total CO2 emission from cement production, while 40% comes from fuel combustion for heating the reaction and 10% comes from electricity usage and transportation.6,7

The idea of rendering the cement process more sustainable is to capture CO2 from a cement plant’s flue gas and convert it to the starting material of cement, CaCO3, creating a carbon neutral process. Scientists and engineers have been developing different technologies to achieve this goal. For example, at Calera, a company in California, CO2 is first converted to carbonic acid. Then, Ca(OH)2, which can be found in industrial waste streams, is added to convert carbonic acid to CaCO3 and water. The overall reaction is shown in Reaction 2.8

CO2 + Ca(OH)2 → CaCO3 +H2O                     (2)

Iizuka et al.9 suggested that the Ca(OH)2 and calcium silicates can be extracted from waste concrete, such as concrete from dismantled buildings, as a source of calcium ions. Their methodology is similar to Calera’s, but the carbonic acid is used for the extraction of calcium ions from waste cement (Figure 2).9 Furthermore, Vance et al. has shown that liquid and supercritical CO2 can accelerate the formation of CaCO3 from Ca(OH)2.1

annabelle_blog_feb2017-3

Figure 2. Recycling CO2 and concrete to make limestone, the starting material of cement. [9]

On the other hand, CarbonCure, a Canadian company, takes a slightly different approach in CO2 sequestration in the concrete industry. In their technology, CO2 is incorporated in the concrete production process, rather than the cement production process. CO2 is injected into the wet concrete mixture, where it is mixed with water to form carbonates (Reactions 1-3 in Figure 2). Then, the carbonates react with the existing Ca2+ in cement to form calcium carbonate nanoparticles, or limestone nanoparticles (Reaction 6 in Figure 2), which are well distributed in the concrete. This technique not only upcycles CO2, but also increases the compressive strength of the material due to these limestone nanoparticles.10

As mentioned above, fuel combustion and use of electricity also contribute to the CO2 emissions of cement production. In this way, other efforts to reduce CO2 emissions include recovering heat from the cooled clinker,5 utilization of alternative fuels, reduction of clinker in cement,3,11 and utilization of cement to absorb CO2.2

With innovative research, development, and commercialization of CO2 conversion technologies, I am optimistic that they will have a solid impact in the near future at the global scale. However, despite the current advances in CO2 conversion technology, a collaborative effort on both CO2 capture and utilization, along with the infrastructure to bridge these two technologies together, is essential to realize a carbon- neutral society.

References

(1)         Vance, K.; Falzone, G.; Pignatelli, I.; Bauchy, M.; Balonis, M.; Sant, G. 2015.

(2)         Torrice, B. M. Chemical and Engineering News. November 2016, p 8.

(3)         Crow, J. M. Chemistry World. 2008.

(4)         Maclaren, D. C.; White, M. A. J. Chem. Educ. 2003, 80 (6), 623–635.

(5)         Cement Making Process http://www2.cement.org/basics/images/flashtour.html.

(6)         Explore Cement http://www.wbcsdcement.org/index.php/about-csi/explore-cement?showall=&start=2.

(7)         Mason, S. UCLA scientists confirm: New technique could make cement manufacturing carbon-neutral http://newsroom.ucla.edu/releases/ucla-scientists-confirm:-new-technique-could-make-cement-manufacturing-carbon-neutral.

(8)         The Process http://www.calera.com/beneficial-reuse-of-co2/process.html.

(9)         Iizuka, A.; Fujii, M.; Yamasaki, A.; Yanagisawa, Y. Ind. Eng. Chem. Res. 2004, 43, 7880–7887.

(10)      Technology http://carboncure.com/technology/.

(11)      Cement Industry Energy and CO2 Performance: Getting the Numbers Right (GNR); 2016.