Sustainable Raw Material Base and Green Chemistry New Raw Materials Create a New Set of Challenges for Green Chemistry
Ever since Paul Anastas wrote the standard textbook on green chemistry, both the term itself and the principles behind it have become well established in the world of chemistry. The author of this article, DECHEMA CEO Prof. Kurt Wagemann, looks at how green chemistry is impacting the transition in the raw material base.
When Paul Anastas published the book Green Chemistry: Theory and Practice in 1998, he put the aspirations for “soft chemistry” on a rational, pragmatic footing. At the time, Anastas was working at the US Environmental Protection Agency and he is currently responsible for the Agency’s R&D activities. The term green chemistry quickly gained widespread acceptance in the English-speaking world and other terms such as green solvents, green reagents, etc. soon followed. In Germany however, many people were more reluctant to embrace the term “green chemistry” because of its political connotations. “Sustainability” is currently the preferred term, although you now often hear the term green chemistry as well.
However that may be, the ten (later twelve) principles of green chemistry are well known and no chemist questions them. The list of rules includes the following:
- Use environment-benign solvents, preferably water, wherever possible
- In addition, atom economy is an essential criterion for assessing the quality and elegance of a reaction.
The principles are now part of the university curriculum. In many cases, lab course content has been reviewed to eliminate “non-green” reagents and reaction steps. The quantities of reagents/batch sizes have been reduced, and the concept of atom economy should be familiar to every chemistry student. The chemical industry is firmly committed to the vision of sustainability. Starting with the debate on the environmental compatibility of plastics, lifecycle assessment (LCA) has become an increasingly important tool, and it is now standard practice to use LCA as a basis for comparing various process and product options.
The 12 Principles of Green Engineering
The twelve principles of green engineering put forward in 2003 are far less well known due to the cryptic wording. However it is well worth taking a look at them:
- 1. Inherent Rather: Than Circumstantial Designers need to strive to ensure that all materials and energy inputs and outputs are as inherently nonhazardous as possible.
- 2. Prevention Instead of Treatment: It is better to prevent waste than to treat or clean up waste after it is formed.
- 3. Design for Separation: Separation and purification operations should be designed to minimize energy consumption and materials use.
- 4. Maximize Efficiency: Products, processes, and systems should be designed to maximize mass, energy, space, and time efficiency.
- 5. Output-Pulled Versus Input-Pushed Products, processes, and systems should be “output pulled" rather than "input pushed" through the use of energy and materials.
- 6. Conserve Complexity: Embedded entropy and complexity must be viewed as an investment when making design choices on recycle, reuse, or beneficial disposition.
- 7. Durability Rather Than Immortality: Targeted durability, not immortality, should be a design goal.
- 8. Meet Need, Minimize Excess: Design for unnecessary capacity or capability (e.g., “one size fits all”) solutions should be considered a design flaw.
- 9. Minimize Material Diversity: Material diversity in multicomponent products should be minimized to promote disassembly and value retention.
- 10. Integrate Material and Energy Flows: Design of products, processes, and systems must include integration and interconnectivity with available energy and materials flows.
- 11. Design for Commercial “Afterlife”: Products, processes, and systems should be designed for performance in a commercial "afterlife."
- 12. Renewable Rather Than Depleting: Material and energy inputs should be renewable rather than depleting.
Are the 12 Principles Still Valid and Which One Should Be Added?
It is now 20 / 10 years since these principles were first published. Given the huge challenges which the transformation of the energy and raw material base presents to the chemical industry and chemical engineering, it is reasonable to ask whether some of the principles should be reviewed and whether new principles should be added.
The principles of green chemistry and green engineering mandate the use of renewable raw materials. In the context of minimized resource consumption, engineers are currently discussing cascaded utilization of biomass and integrated production strategies for biorefineries. Utilization of residue streams from other industries, particularly food and cellulose production, is a major consideration.
Competing uses of biomass, in particular food and animal feed production and power generation, as well as water and nature conservation issues create significant conflict potential. The chemical industry will have to respond by developing highly efficient process designs and higher product value-added.
The transition in the raw material base extends beyond renewables. The role of natural gas, coal and even CO2 will be at least as important in the medium term. In the past, CO2 was regarded as totally irrelevant in the raw material mix due to the very low carbon energy level.
There is now considerable interest in CO2, particularly in applications where the target product has a structural element containing carbon and one or two oxygen atoms.
New Synthesis Techniques Required For Renewables
Since renewables and CO2 both contain too much oxygen in relation to the target end products, reduction has to take place during the conversion process.
Reduction processes and the use of hydrogen / energy are two inherent disadvantages. That is only the case however if the target compounds are the same as in petro-based chemistry.
The development of new intermediates which contain more oxygen can be an attractive alternative. Platform chemicals such as diamines, diols and dicarbonic acids for the preparation of polyester and polyamide are attracting an increasing level of interest.
Conversion to syngas is the other alternative, and it applies to renewables such as wood and straw as well as coal and methane. From this perspective, syngas is a universally accessible raw material and the task now is to develop a very broad spectrum of applications. Besides the familiar processes for producing hydrocarbons and methanol, there is a need to develop efficient techniques for a number of other intermediates such as alcohols and esters.
Catalysts Hold the Key to Sustainability
Catalysis is definitely the priority issue for achieving a sustainable raw material base. Catalysts hold the key to the avoidance of auxiliary substance utilization (e.g. “old timers” such as Friedel-Krafts alcylation). In addition, selective control of the reaction, high target product yields and minimum waste also reduce the effort needed for separation. The result is minimal energy consumption combined with maximum raw material efficiency. In conformance with the principles of green chemistry, the reactions should take place at low temperatures. Lower temperatures normally equate to higher selectivity, but it is important to keep in mind that particularly in integrated production environments waste heat can be utilized far more efficiently at higher temperatures.
Whatever the raw material base, recyclability and utilization cascading are essential aspects to consider during product development (this applies especially to plastics). Utilization of the tightly bound carbon in CO2 without using the energy content will probably restrict biodegradability to niche applications.
One key aspect is missing from the existing list of principles, namely water consumption. Water is vital for the production of plant-based biomass and for large-scale chemical production, but its availability is limited in many parts of the world. Water scarcity is a major factor which is limiting the expansion of coal-based chemical production in China's coal fields. The need to minimize water consumption and maximize water recycling will have to be added to chemical engineering principles 4, 5 and 12.