Green Chemistry Principle #9: Catalysis

By Alex Waked, Member-At-Large for the GCI

9. Catalytic reagents (as selective as possible) are superior to stoichiometric reagents.

In Video #9, Lilin and Jamy discuss the advantages of catalytic reagents over stoichiometric reagents.


In stoichiometric reactions, the reaction can often be very slow, may require significant energy input in the form of heat, or may produce unwanted byproducts that could be harmful to the environment or cost lots of money to dispose of. Most chemical processes employing catalysts are able to bypass these drawbacks.

A catalyst is a reagent that participates in a chemical reaction, yet remains unchanged after the reaction is complete. The way they typically work is by lowering the energy barrier of a given reaction by interacting with specific locations on the reactants, as demonstrated in Figure 1 below. The reactants are represented by the red and blue objects, and the catalyst by the green one. Without catalyst, the reactants cannot react with each another to form the desired product. However, once the catalyst interacts with them, the reactants become compatible and can subsequently react together. The desired product is released and the catalyst is regenerated to continue interacting with the remaining reactants to produce more product.

Principle 9 Figure 1 - catalysis

Figure 1. Graphic of a catalyst’s function in a catalytic reaction. The catalyst is green, and the reactants are red and blue.

In other words, a catalyst can be thought of as a key that can unlock specific keyholes, where a keyhole represents a particular chemical reaction. One common example of a catalytic reaction that is taught in introductory organic chemistry is the hydrogenation of ketones (Scheme 1, also discussed in the video). The stoichiometric reaction involves the addition of sodium borohydride, followed by addition of water. In this reaction, borane (BH3) and sodium hydroxide are (formally) generated as waste. By simply employing palladium on carbon as catalyst, the ketone can react directly with H2 to generate the same desired product without producing any waste.

Principle 9 Scheme 1 - catalysis example

Scheme 1. Stoichiometric vs. catalytic reduction of a ketone.

While catalytic reagents appear to play an impactful role in the development of greener processes, there are always a couple points on the flip side of the coin to consider. For instance, a reaction employing a catalyst may not necessarily be “green”, since the “greenness” of the catalyst itself should be considered as well (ie. Is the catalyst itself toxic? Is it environmentally benign?). In addition, the lifetime of a catalyst matters; a catalyst can in theory perform a reaction an infinite number of times, but in practice it loses its effectiveness after a certain period of time.

Nevertheless, when these points are considered and addressed, the impact of catalytic reagents on green processes cannot be ignored. The production of fine chemicals and the pharmaceutical industries are just a couple areas where this impact is seen.[1] By focusing innovative research around the principle of catalysis, together with the other principles of Green Chemistry, we are moving in the right direction by paving the way to new sustainable processes.

Reference:
[1] Delidovich, I.; Palkovits, R. Green Chem. 2016, 18, 590-593.

Advertisements
A New Green Chemistry Metric: The Green Aspiration Level™

A New Green Chemistry Metric: The Green Aspiration Level™

By Samantha A. M. Smith, Member-at-Large for the GCI

Sam_blog 1

Figure 1. Process materials – green mass metric relationships

Green Chemistry Principle number two, Atom Economy, focuses on metrics used to compare the efficiency of a reaction.1 However, Atom Economy doesn’t take into account solvents, reagents such as catalysts, drying agents, energy, or recyclability of any of the materials. Is it reasonable for an industry such as pharma to use such a metric? What about E-factor, which is a measure of process waste and, if “complete” (cEF = complete E-factor), also recyclability of solvents and catalysts? It’s known that the pharmaceutical industry generally has the highest E-factor values compared to petrochemicals, bulk, and fine chemicals, indicating more waste generated per mass of desired product.2 But if you wanted to compare your technology to already implemented pharmaceutical processes, where would you find such information?

Roschangar, Sheldon, and Senanayake created a new metric for such a purpose: the Green Aspiration Level™.3,4 This new metric allows one to compare an ideal process with the average commercial process in terms of environmental impact for the production of a pharmaceutical. Say you have an alternative product to Viagra™ and want to know if its production is more or less impactful. You could apply any of the existing metrics (including yield, atom economy, E-factor, and more, summarized in Table 1 of reference 3), or you could use the Green Aspiration Level™ (GAL). To do so, you determine the waste (Complete Environmental Impact Factor (cEF) or Process Mass Intensity (PMI)) and assess the complexity of the process, and use those to calculate the GAL, and in turn the Relative Process Greenness (RPG). From there, you can consult Table 1 (below) to determine the greenness rating of such a process.4

Waste and Complexity

Waste refers to a simple metric such as cEF or PMI (with reactor cleaning and solvent recycling excluded). Complexity of the process refers to the number of steps with no concession transformations, that is those that do not directly contribute to the building of the target molecules’ skeleton.5 The waste and complexity metrics require that the process starting materials are less than $100 USD/mol for proper comparison.

Green Aspiration Level™

Roschangar and coworkers have collected data on many commercial processes to develop an appropriate metric, and they currently use 26 kg of waste per kg of product as a standard based on their findings. This value is known as the average GAL, or tGAL.3,4

GAL        = (tGAL) x Complexity

= 26 x Complexity

Relative Process Greenness

RPG       = GAL/cEF

This metric is used as the comparison point for processes. The comparison can be done at different stages of development, either early or late development, and then again for those processes that are commercialized. In Table 1, there are minimum RPG values that will associate the process with an appropriate greenness percentile.

RPI         = RPG(current) – RPG(early)

RPG can also be used to determine the improvement of a process. From early development, to late development, to commercialization, the difference in consecutive RPG values will give your Relative (Green) Process Improvement (RPI). In this case, the higher the number the better.

Sam_blog 2

Table 1. Rating Matrix for Relative Process Greenness (RPG) in Pharmaceutical Drug Manufacturing [3]

It turns out the current commercial process for Viagra™ is actually quite efficient and is currently in the 90th percentile, exceeding the commercial average by 143% (RPG). The full process of determining and using this new metric, the Green Aspiration Level™, is described by Roschangar and coworkers in two very in-depth articles.3,4

References

1 Anastas, P. T., Warner, J. C. “Principles of green chemistry.” Green chemistry: Theory and practice (1998): 29-56.

2 Sheldon, R. A., Catalysis and Pollution Prevention, Chem. Ind. (London), 1997, 12–15.

3 Roschangar, F., Sheldon, R. A., Senanayake, C. H., Green Chem. 2015, 17, 752. DOI: 10.1039/C4GC01563K

4 Roschangar, F., Colberg, J., Dunn, P. J., Gallou, F., Hayler, J. D., Koenig, S. G., Kopach, M. E., Leahy, D. K., Mergelsberg, I., Tucker, J. L., Sheldon, R. A., Senanayake, C. H., Green Chem. 2017, 19, 281. DOI: 10.1039/c6gc02901a 

5 Crow, J. M., “Stepping toward ideality”, Chemistry World, accessed July 13th, 2017. URL: https://www.chemistryworld.com/feature/stepping-toward-ideality/5190.article

Figures from Roschangar et al. 2015 reproduced with the permission of the Royal Society of Chemistry.

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!