Designing for a green chemistry future – Science Magazine

Call for design and innovation in an integrated systems framework

Pursuing improved design for sustainability in complex systems is acutely challenging when using a traditional reductionist approach (6). In the chemical sector, a reductionist focus on function has resulted in pharmaceuticals that not only extend and enhance life and well-being but also persist in our water after their useful life, causing exposure to unintended populations (7); agricultural chemicals that increase crop yields while causing fish kills and degrading groundwater (8); and chemicals that make materials durable and impart long-term performance but bioaccumulate in our bodies and in the biosphere (9). Despite many examples of the failure modes of the reductionist approach, we often continue to use this framework to address sustainability challenges, as evidenced by the focus on isolated, individual metrics (e.g., greenhouse gas emissions, energy or water consumption, and ecotoxicity) instead of on reframing sustainability as a multidimensional problem of nested complex systems (10).

Incrementally improving current products and processes through efficiency measures is the focus of many sustainability efforts in the chemical industry, but this approach will not be sufficient. Instead, we need transformative, disruptive innovations to sustainably provide desired functions. Systems must be considered in their entirety to identify solutions that do not shift impacts or cause unintended consequences elsewhere. The traditional reductionist approach, then, must be coupled with integrative systems thinking to inform designs for a sustainable future (11). For example, knowledge about the functional performance of a molecule is only a minimal requirement; knowledge about the potential hazards of the molecule is also needed. Instead of solving a singular challenge in ways that may exacerbate other challenges (e.g., trading off bio-based fuels for land-use pressures or competition with food), there is now the opportunity for so-called “nexus solutions” (12), that is, solutions that synergistically advance multiple sustainability challenges—for example, an earth abundant metal catalyst that splits water using sunlight, yielding hydrogen for energy storage and producing water when that hydrogen is combusted for energy recovery (13). Designing future fuels that are produced in a carbon-neutral way to reduce local air pollution emissions and increase engine efficiency is another example (14). Although there are current discussions on cascading nonlinear problems (e.g., increased fossil energy generation→greater water stress→refugee migration→civil unrest and military conflict), systems thinking and design targeting nexus solutions allow for the possibility of cascading nonlinear solutions: discreet and specific actions that create benefits that are multiplied and magnified (e.g., CO₂ utilization shifts a waste to a feedstock→the use of toxic reagents, such as phosgene, is avoided→CO₂ emissions decrease→the rise of CO₂ levels slows→global climate change is mitigated).

Expanding the definition of performance beyond technical function to include sustainability

Realizing the necessary changes to the chemical sector requires a redefinition of how the concept of performance has historically been measured. Since commercial synthetic chemistry was started in the mid-19th century with the introduction of Perkin’s mauve dye, the criterion by which chemical products were judged was “performance.” Performance has been almost entirely defined as the ability to efficiently accomplish a narrowly defined function (e.g., color for dyes, adhesion for glues, or the ability to kill pests for pesticides). However, the unintended consequences of this singular focus demonstrate the imperative that our definition of performance must broaden to include all of the aspects that we care about beyond function, particularly sustainability.

This expanded definition of performance requires chemical product and process designers to know not only the mechanisms of technical functional performance but also the mechanisms of harm that the substances can induce. This expanded definition of performance implicitly requires that anyone designing, inventing, and intentionally making a chemical product or process must have a working knowledge of the molecular hazard, be it a global, physical, or toxicological hazard. After more than a century of incidents and accidents that have resulted in adverse consequences to human health and the environment, the training for chemists that integrates toxicology deeply into the curriculum remains the exception and not the rule. Enabling the consideration of chemical hazards in the same manner as chemical performance will require that our education programs, like our technologies, expand the definition of function to include sustainability attributes.

The redefinition of performance also directly affects the business model of the chemical sector, because one part of the strategy of diminishing adverse impact is to reduce the absolute quantity of material needed, thereby reducing harm potential across the life cycle. The “F-factor” (15) partially captures the concept of maximal functionality: The notion is to maximize function while minimizing chemical or material mass, which is akin to the Moore’s Law challenge for computing power (16). Of course, the notion of minimizing mass is intended as a proxy for limiting the amount of feedstock, processing and transport energy, manufacturing waste, end-of-life material for management, and, subsequently, the associated harm. This approach can be applied to whatever service is required—for example, color, lubrication, or cleaning—and will shift the profit imperative from selling as much material as possible to selling as much function as possible with a concomitant decrease in hazard. Such a paradigm shift is in accordance with the United Nations Industrial Development Organization’s emerging emphasis on “chemical leasing” (17), or selling the function, rather than a quantity, of the chemical. This reduces costs of material production and values efficiency and efficacy in ways that maximize profits in alignment with sustainability (Fig. 1, right).

Redesigning the chemical value chain with nonfossil feedstocks

Today’s chemical production relies almost exclusively on oil, gas, and coal as the sources of carbon. The petrochemical value chain that emerged in the second half of the 20th century forms a highly integrated network, sometimes referred to as the “petroleum tree” (Fig. 3). Petrochemical refineries produce less than a dozen building blocks, in particular, short-chain olefins and aromatics. Together with synthesis gas, which is a mixture of CO and H₂, these few compounds form the stem of carbon that constitutes the huge molecular diversity in branches, twigs, and leaves of more than 100,000 chemical substances in final products.

The value generation of petrochemistry largely results from the powerful arsenal of synthetic methods for the introduction of functional groups into molecules. Consequently, the availability of starting materials and the desired product functions have a direct feedback connection with the development of chemical production routes and processes. Improvements in the corresponding synthetic methods will undoubtedly remain a major field of research with direct positive impacts on the environmental footprint of chemical products (Fig. 3, top, green arrows). Owing to the inherent problems of the depletion of a finite resource, connection to climate change, and inextricable links to toxic congeners, petroleum is not a sustainable option, and ultimately, the design of new value chains based on nonfossil carbon sources and energy from renewable resources will pave the way to closed carbon cycles. This paradigm shift, marking nothing less than the next industrial (r)evolution in chemistry, has already started (Fig. 3, bottom, green arrows).

Major scientific breakthroughs and innovations will be necessary to make the utilization of renewable carbon sources increasingly competitive. Advanced hydrogen technologies (31) and electrochemical processes (32) are actively researched to harness the required “decarbonized” energy. Among carbon sources, lignocellulosic biomass, one of the most abundantly available raw materials on Earth (33), and CO₂ (34) are available in sufficient quantities to achieve a “defossilization” of the chemical value chain. Recycled plastic material is another potential large-volume carbon source in the framework of a circular economy (35). In contrast to fossil resources made up from carbon and hydrogen only, these feedstocks comprise mostly highly oxidized and “overfunctionalized” molecules. Their transformation, therefore, requires innovative synthetic concepts and methodologies for the removal or reorganization of functional groups.

One option to deal with the challenge of complexity is to remove it—for example, through intermediate syngas production combined with the Fischer-Tropsch technology to produce a petroleum proxy as a drop-in replacement (36). Thereby, additional roots can be added to the petroleum tree from almost any nonfossil feedstock. “Green carbon” is then transported through all existing branches and leaves. Although this approach allows for repurposing established knowledge and infrastructure, it destroys and reassembles valuable functionality.

An alternative strategy is to take advantage of the inherent higher complexity of renewable feedstocks to open molecular shortcuts to the desired functional groups. For example, the production of important platform chemicals such as 1,3-propanediol or succinic acid by microbial fermentation of (waste) glycerol or sugars near room temperature in water is increasingly rivaling the multistep petrochemical routes (37). Similar considerations apply to the use of CO₂ that can be integrated into the value chain through existing large-volume base chemicals, methanol being one of the most promising options (38). Tailored chemo- and biocatalysts that can be adapted to variations in feedstock quality and fluctuations in energy supply, together with the development of highly integrated and energy-efficient purification processes, will be important scientific motors behind this development.

Perhaps to even greater advantage, renewable feedstocks provide entirely new chemical building blocks (39) that can drive the realization of improved functionality without the historic adverse impacts to human health and the environment. One molecule that has recently garnered interest is monosaccharide-derived furane dicarboxylic acid (FDCA), a possible building block for new polyester products such as containers for carbonated liquids (40). The incorporation of CO₂ directly into polymer chains of consumer products is already being industrialized (21). A growing number of new synthetic methodologies based on selective coupling of CO₂ and H₂ with other substrates illustrates their broad potential for construction of functional groups at late stages of product synthesis (41).

The necessary integration of the molecular and engineering sciences with the systems thinking on product performance is obvious in this area. This will enable integration of energy and material fluxes at the chemistry-energy nexus (42) and defines new opportunities for sector coupling of chemistry with, for example, agriculture, steel, or cement industries. Deconstructing a highly complex target molecule through “retrosynthetic analysis” to plan its assembly from existing building blocks and synthetic methods is a central pillar of today’s synthetic organic chemistry (43). The same conceptual thinking can be transferred into a new design framework for synthetic pathways to improved target products from renewable feedstocks (44).