The sustainable use of materials, including the (re)design of products to maximize their sustainability, is an area of immense and rapidly growing environmental and economic importance. It also impacts the greater goal of enabling a much larger fraction of the world’s population to achieve the high standard of living that technology now confers upon the residents of developed nations, without exceeding the Earth’s capacity to supply the necessary raw materials and absorb the inevitable byproducts of activities required to generate technology.
The University of California, Santa Barbara has created an interdisciplinary initiative to integrate sustainability considerations into research in the chemical sciences and engineering, including assessment/minimization of environmental impact and assessment/optimization of economic feasibility, cultivation of public awareness, and social acceptance of sustainability goals. A team of researchers from chemistry and biochemistry, chemical engineering, environmental science, technology management, and communication is collaborating to improve the efficiency, reduce the environmental impact, and improve social acceptance of alternative chemical technologies.
As the focal point for this effort, the University has established a cluster of four endowed research Chairs: the Mellichamp Academic Initiative Professorships in Sustainability. The Chairholders will have overlapping and complementary interests in green chemistry, sustainable manufacturing, catalytic processing, and the economics of new technologies.
The US chemical industry is a cornerstone of American manufacturing, and the scale of chemical production is immense. The chemical industry produces commodities (large-volume, low-cost basic organic building blocks, such as ethylene, methanol, etc., as well as inorganics such as ammonia and sulfuric acid), fine and specialty chemicals (smaller volume, higher-cost products such as pharmaceuticals, agrochemicals, coatings, pigments, etc.) and a wide variety of structural and functional materials including polymers, glasses, ceramics, composites, and catalysts. Indeed, the chemical industry is involved in 96 % of all manufactured products. US chemical shipments total 850 million tons annually, or 2.7 tons per person per year. The development and production of chemicals contributes 25 % of our GDP; at 12 % of US exports, it is the largest exporting sector in the country. Nearly 800,000 Americans are directly employed in the chemical industry, and each of their chemistry jobs creates 7.6 chemistry-dependent jobs in sectors such as health care, durable and non-durable goods, construction, and mining. Furthermore, the average annual salary of US employees in the chemical industry exceeds the average US manufacturing wage by more than 40 %. Overall, the industry is responsible for 4.2 million American jobs, either directly or indirectly. California has over 76,000 chemistry jobs, representing $8.5 billion in wages (with an additional $250 billion in wages in chemistry-dependent industries).
A sustainable chemical enterprise must simultaneously ensure the reliable and sustainable sourcing of its raw materials; conduct efficient and highly selective chemical synthesis and separations; avoid hazardous reagents and intermediates wherever possible; understand the environmental impact of discarded products, and design for (chemical) recycling. Some strategies include optimizing chemical processing; discovering Earth-abundant metal replacements for rare metals used in catalysis; inventing catalysts to unlock the chemical constituents in biomass; designing materials that are recyclable and are not harmful when released into the environment; and acquiring tools to assess the relative sustainability of alternative pathways and materials. Although such an integrated approach has yet to be achieved, the motivation is large and growing.
The global scale of the problems involving sustainable chemistry and engineering, as well as the huge potential impact of effective solutions, are illustrated in the following examples:
Global food security
In the 20th century, the invention of the Haber-Bosch process for converting atmospheric nitrogen into ammonia, together with the Green Revolution that dramatically improved agricultural yields, eliminated natural limits on bioavailable nitrogen and enabled an expansion of world population from 1.6 billion to 7 billion. Globally, the amounts of chemically and biologically fixed nitrogen in our food supply are now comparable, meaning that chemically-fixed nitrogen feeds about half of the world’s population. However, when one limiting element becomes abundant, another becomes limiting: that element is now phosphorus. It is extracted from phosphate rock (phosphorite), a sedimentary deposit. The world’s largest producers, China and the US, are expected deplete their reserves in the next 50 years, assuming continued extraction at current rates (we note that this is an optimistic scenario, and does not account for further population expansion, increased dietary protein, or the demand for phosphate-based fertilizers from the emerging biofuels industry). The most productive of the US mines, located in Florida, will be depleted within 20 years,and the US recently became a phosphate importer. China now imposes seasonal tariffs exceeding 100 % on exported phosphate, in order to conserve its resources for domestic use. Globally, phosphate appears to be more geographically concentrated than crude oil; if industry estimates are accurate, a single country (Morocco) controls a higher fraction (70 %) of global reserves than the crude oil in all 12 member states of OPEC combined. This represents an important supply interruption risk, and a potential threat to global food security. We urgently need to develop sustainable chemistry to enable the recovery and reuse of phosphorus. Human consumption and excretion of phosphorus amounts to 3 million tons annually, turning our cities into valuable sources of this essential and completely non-substitutable element in human nutrition, via recovery during wastewater treatment. New phosphate recovery technologies, possibly including adsorption, ion exchange, and nano-filtration, followed by precipitation or crystallization, are urgently needed.
Reducing dependence on critical metals and minerals
The precious metals (Ru, Rh, Pd, Os, Ir, Pt) make highly effective catalysts in a wide range of chemical reactions, due to their readiness to change oxidation states and their reluctance to form recalcitrant oxides. Unfortunately, they are also some of the least abundant elements in the Earth’s crust. Over the course of the past century, many metals have been extracted from virgin ores at exponentially-increasing rates, to meet the infrastructure and industrial needs of rapidly industrializing societies in North America and Europe. Widespread implementation of new technologies, such as fuel cells, may be strongly limited by the availability of these metals. For example, the in-use stock of platinum (Pt) needed for a fleet of 0.5 billion H2 fuel-cell cars (i.e., approx. half of the world’s current passenger vehicle fleet) would exhaust our remaining lithospheric stock of Pt in just 15 years, even if there were no competing uses for Pt.
Many other emerging technologies rely on uncommon elements. They may be relatively abundant but very highly distributed (e.g., the not-so-rare “rare earths” used in permanent magnets for electric vehicles and wind turbines). This results in high environmental costs for their extraction and purification. Alternately, their supply may be strongly limited by co-production, due to their occurrence as minor impurities in other metal-containing ores (e.g., In, Ga and Te used in photovoltaics and solid-state lighting). Rapid demand growth and geopolitical supply-security risks also pose impediments to large-scale deployment. Consequently, many are now widely considered to be critical elements, for which supply limitations and/or disruptions pose significant threats to our economic and/or social well-being. It is becoming urgent to minimize their use, replacing them wherever possible with Earth-abundant metals in large-scale applications. Sustainable design of processes and/or materials involving critical elements must take into account supply security, long-term availability and the environmental impact of production. How a material will be used, disposed, reused, or recycled is also key: the fate of a material through its use and end-of-life phases is largely determined during the design phase. Dissipative uses of critical elements can make material recovery prohibitively costly, and must be avoided.
Conserving energy and freshwater
Many chemical industry practices are highly energy- and water-intensive. For example, the Haber-Bosch process (described above), which produces 500 million tons of ammonia-based fertilizer annually, consumes 1-2 % of the entire world energy supply. Light olefins (ethylene and propylene) are versatile chemical building blocks and are produced in very large quantities (ca. 150 million tons/a). The main process is thermal (steam) cracking of hydrocarbons, conducted at ca. 900 °C. In 2000, this accounted for about 3 x 1018 J of primary energy use, or about 20 % of all the energy consumed by the global chemical industry, and nearly 200 million tons of CO2 emissions - about 30% of all CO2 emissions by the chemical industry. Some of this energy use is thermodynamically unavoidable, since chemical reactions that are thermodynamically unfavorable require energy input to proceed. However, innovative design of catalysts and reactors can result in huge energy savings. Separations are another energy-intensive component of the chemical and related industries. These essential purification steps generally account for 40 - 70% of plant operating costs. A large part of this cost is energy, which is required to drive thermodynamically unfavorable “un-mixing”. Energy consumption depends sensitively on how the separation devices are configured: sub-optimal sequences can result in energy penalties of 50% or more. The design of optimal separation systems with appropriate heat integration can lead to very large energy savings. Likewise, the chemical industry is a large user of freshwater; in regions where the chemical industry is concentrated, such as Texas, its fraction of industrial state freshwater use is ca. 45%. Process optimization to minimize the use of freshwater is another critical sustainability issue.
Using renewable raw materials
The vast majority of synthetic carbon-based materials are currently made from a handful of petroleum-derived building blocks, including ethylene, propylene, butenes, benzene, toluene, xylene and methanol. These components are converted, using chemistry, into polymers, coatings, adhesives, sealants, building materials, automotive components, packaging, pesticides, pharmaceuticals, insulation, medical equipment, optical components, cleaning agents, furnishings, and many others. The building blocks, or “feedstocks” have high energy content and are currently obtained from the same source (crude oil) as transportation and heating fuels. Recent concern about the sustainability of petroleum has inspired research into alternative feedstocks, including alternative fossil fuels such as coal and natural gas, and potentially renewable feedstocks derived from biomass. To avoid competing with food uses, the ultimate goal of this research is to mobilize the components of lignocellulosic biomass. The US Department of Energy estimates that at least a billion tons could be available annually in the US to meet future demand. The vision for the future involves replacing petroleum refineries with “bio-refineries”, situated in agricultural areas and surrounded by the land on which the feedstocks are grown. However, concerns about the challenges of transporting and storing perishable raw biomass, the enormous water requirements for processing, and land use changes incurred when arable areas are converted to fuel and chemical production, have raised important questions about how sustainable such scenarios truly are. New reactions and separations for continuous (not batch) processing will be required to make these processes much more efficient and versatile, especially in operations much smaller than typical world-scale petrochemical plants.
Reducing hazard and risk throughout the supply chain
Reduced risk of exposure and minimal environmental toxicity are important dimensions of sustainable chemistry. Cradle-to-grave life cycle assessments and fate and transport studies must be employed to quantify potential emissions of chemicals at different life cycle stages, pathways through the environment, and likely exposure concentrations. The safety of chemical processing can be increased through substitution of hazardous/toxic components by less dangerous ones, just-in-time production to minimize transport and storage of unsubstitutable reactants/intermediates, and targeted design based on knowledge of the solubility, mobility and bioavailability of products. Life-cycle thinking often reveals trade-offs between and sometimes even within environmental, social and economic objectives. For example, thin film photovoltaics reduce greenhouse gas (GHG) emissions while consuming some of the earth’s most exotic metal resources. Reducing fossil energy use and acid emissions by light-weighting motor vehicles, using H2 as an alternative fuel, and decreasing the sulfur content of petroleum, requires alloys and catalysts based on cobalt, whose extraction in Central Africa has fueled regional armed conflicts for decades. Understanding the trade-offs between various environmental, social and economic objectives is critical in the sustainable design of materials and products.
The importance of integrating physical science and engineering with social science
Replacement of conventional technologies by more sustainable versions is by no means automatic or rapid. New approaches must be cost-competitive; in manufacturing, this often means considering large existing capital investments, as well as supply risks. Changing or uncertain regulatory environments also play a role in the willingness of companies to invest in new technologies. A realistic understanding of the economic barriers facing new technologies is an essential part of successful commercialization strategy.