and vehicle distances traveled grew 143% in that period, restrictions on nitrogen oxide emissions have achieved some success in controlling some of the growth in this pollutant, but not enough to prevent an overall increase.
Emissions of NO from electric power plants, their other major source, have recently been falling somewhat.
As a consequence of the increase in overall NO emissions, ground-level ozone concentrations increased in the southern (especially around Houston) and north-central regions of the United States in the 1990s. The latter effect can be seen in Figure 3-7, where high ozone levels center around the New York–Boston area and a few midwestern sites, and somewhat lesser levels cover most of the East Coast and Midwest, extending into southern Ontario. High ozone levels have been a problem in Southern California for many decades.
To help reduce the incidence of summertime smog in south-central Canada and the northeastern United States, the two nations have signed an Annex to their Air Quality Agreement. The United States was committed to reduce NOX emissions originating in northern and northeastern states by 35% by 2007, and also to reduce VOC emissions during summer months when most smog forms in this region. Canada agreed to reduce its NOX emis- sions from power plants in southern Ontario by 50% by the same date. The drastic lowering of the allowed sulfur levels in gasoline should assist in the reduction of NOX from vehicles. Emission standards for SUVs, trucks, and buses are also being tightened to make them more in line with those for regu- lar automobiles.
The long blackout of electrical power that occurred in August, 2003 in eastern North America yielded some interesting information concerning the contribution of power plants to air pollution in that region. Measurements over Pennsylvania taken 24 hours after the blackout began found that SO2 levels were down 90%, and ozone levels down about 50%, compared to a similar hot, sunny day a year earlier, and that visibility increased by about 40 km because haze from particulates had decreased by 70%.
PROBLEM 3-9
Green Chemistry: Strategies to Reduce VOCs Emanating from Organic Solvents 101
The Gothenburg Protocol, which controls the release of many pollutants in Europe, was expected to have reduced NOX emissions there by more than 40% by 2010, compared to 1990 levels. Great Britain—which saw its emis- sions decline by the late 1990s by almost 40% compared to their peak in the late 1980s—had to reduce their emissions by another third from 1998 levels by 2010 in order to meet these regulations. European VOC emissions were due to drop by 40% according to the protocol.
Green Chemistry: Strategies to Reduce VOCs Emanating from Organic Solvents
In addition to their role in paints, organic solvents are used in many different products and processes, in both commercial and household applications.
More than 15 billion kilograms of organic solvents are used worldwide each year in such areas as the electronics, cleaning, automotive, chemical, mining, food, and paper industries. These liquids include not only hydrocarbon sol- vents but also halogenated solvents. Both types of solvents contribute not only to air pollution as VOCs but also to water pollution (Chapter 11). Some halogenated solvents contribute to the depletion of the ozone layer, as seen in Chapter 2.
The following three sections give examples of green chemistry illustrat- ing different strategies for reducing VOCs and their emissions. The first illustrates a method for reducing the amount of VOCs in paints, while the second and third are examples of replacing organic solvents with solvents that do not produce VOCs.
3.15 Green Chemistry: A Nonvolatile, Reactive Coalescent for the Reduction of VOCs
in Latex Paints
Paints generally consist of three major components: pigment, binder, and solvent. The pigment gives the paint color and may be natural or synthetic, and may be organic or inorganic. The binder, also known as the vehicle or resin, provides adhesion, binds the pigment, and provides such proper- ties as toughness, durability, flexibility, and gloss. The primary function of the solvent is to act as the carrier for the nonvolatile components.
The solvent may be water (in latex or water-based paints) or organic (in oil-based or alkyd paints). During the course of drying, the solvent evapo- rates into the surrounding air and is the major source of VOCs from oil- based paints.
Although oil-based paints have been known for centuries, latex paints became commercially available only in the 1950s. The introduction of latex
Review Questions 8–12 are based on material in the above sections.
paints greatly reduced the amount of VOCs compared to oil-based paints.
Recent, more stringent environmental regulations at the federal and state levels have led to significant efforts to reduce VOCs even from latex paints in the United States.
One significant source of VOCs in latex paints is an additive that acts as a coalescent. The resin in latex paints is composed of very small particles of an organic polymer suspended in water. Coalescents are organic compounds that are absorbed by the resin particles and soften or plasticize these particles.
When the paint is spread, the water evaporates and these particles flow together to form a thin, uniform film that adheres to the surface of the mate- rial being painted. The paint hardens as the coalescent is slowly emitted into the atmosphere. Paint contains 2–3% coalescent by mass. Since more than 2 billion liters (600 million gallons) of latex paint are used in the United States each year, more than 50 million kilograms (120 million pounds) of coalescents are emitted into the atmosphere annually. Worldwide emissions of these VOCs are estimated to be more than three times this value.
In 2005, the Archer Daniels Midland Company (ADM) won a Presidential Green Chemistry Challenge Award for the development of coalescents (Archer RCTM) that actually bind through covalent bonds to each other and to the resin, and thereby become part of the paint film; they are therefore not emitted as VOCs. Figure 3-11a shows the structure of TMB, a common coalescent, while Figure 3-11b is the structure of the coalescent developed by ADM. Both of these ester com- pounds have low polarities and are thus absorbed by the resin particles. However, (b) is an ester of propylene glycol and linoleic acid. Linoleic acid is a major component of linseed oil and, like linseed oil, it can undergo auto-oxidative cross- linking in the presence of oxygen due to the carbon–carbon double bonds present, as illustrated by the prototype reaction shown in Figure 3-12.
The oxidized, cross-linked polymer chains have low vapor pressure and furthermore are absorbed into and bonded to the resin, further decreasing their volatility and ability to be emitted.
O O OH
O O HO
(b) (a)
O OH
O O2
FIGURE 3-12 Auto-oxidation cross-linking of an unsaturated system.
FIGURE 3-11 Structures of (a) 2,2,4-trimethyl-1, 3-pentanediol monoisobutyrate (TMB) and (b) propylene glycol monoester of linoleic acid (Archer RCTM).
Green Chemistry: Strategies to Reduce VOCs Emanating from Organic Solvents 103
Traditional coalescents such as TMB are made from petroleum feed- stocks. Another green feature of the coalescent developed by ADM is that it can be produced from the renewable bio-feedstocks linoleic acid and propylene glycol. Linoleic acid is readily available from corn and sunflower oils, and although propylene glycol has traditionally been pro- duced from petroleum, it can be formed from glycerin, a by-product of the synthesis of biodiesel (as we will see in Chapter 7 in another example of green chemistry).
3.16 Green Chemistry: The Replacement of Organic Solvents with Supercritical and Liquid Carbon Dioxide; Development of Surfactants for This Compound
Discovering solvents with less environmental impact, and even designing processes that use no solvents at all, are the subjects of many green chemistry initiatives. Carbon dioxide, CO2, is one solvent that is receiving consider- able attention as a replacement for traditional organic solvents. Although carbon dioxide is a gas at room temperature and pressure, it can be liquefied easily by the application of pressure. In addition to liquid carbon dioxide, there is considerable interest in supercritical carbon dioxide (a discussion of supercritical fluids can be found in Box 3-2) as a solvent in the electronics industry. The decaffeination of coffee and tea with carbon dioxide is a well- known application of this solvent.
Liquid carbon dioxide is attractive as a solvent due to its low viscosity and polarity and its wetting ability. Because of its low polarity, carbon diox- ide is able to dissolve many small organic molecules. However, larger mole- cules including oils, polymers, waxes, greases, and proteins are generally insoluble in it. To increase the solubility of compounds in water, surfactants such as soaps and detergents have been developed which allow this very polar solvent to dissolve less polar materials such as oils and grease. In an analogous fashion, surfactants for carbon dioxide have been developed which increase the range of materials that will dissolve in it.
Joseph DeSimone, of the University of North Carolina and North Caro- lina State University, earned a Presidential Green Chemistry Challenge Award in 1997 for his preparation and development of polymeric surfactants for carbon dioxide. DeSimone is currently the director of the National Sci- ence Foundation Science and Technology Center for Environmentally Responsible Solvents and Processes. This center focuses on discovering ways to replace conventional organic solvents and water with carbon dioxide in a multitude of processes. An example of a surfactant developed by DeSimone
is the block copolymer shown in Figure 3-13a. This molecule has nonpolar regions, which are CO2-philic, and polar regions, which are CO2-phobic.
When dissolved in carbon dioxide, the CO2-philic regions orient themselves to interact with the surrounding carbon dioxide solvent, while the CO2- phobic regions aggregate with one another. The overall result is the forma- tion of a structure know as a micelle (Figure 3-13b). Polar substances that normally do not dissolve in carbon dioxide will dissolve in the center polar region of the micelle.
Supercritical Carbon Dioxide BOX 3-2
The supercritical fluid state of matter is pro- duced when gases or liquids are subjected to very high pressures and, in some cases, to elevated temperatures. At pressures and tem- peratures at or beyond the critical point, separate gaseous and liquid phases of a substance no longer exist. Under these conditions, only the supercritical state, with properties that lie between those of a gas and those of a liquid, exists. For carbon dioxide, the critical pressure is 72.9 atm and the critical temperature is only
31.3°C, as illustrated in the phase diagram in Figure 1. Depending upon exactly how much pressure is applied, the physical properties of the supercritical fluid vary between those of a gas (relatively lower pressures) and those of a liquid (higher pressures); the variation of prop- erties with P or T is particularly acute near the critical point. Thus the density of supercritical carbon dioxide varies over a considerable range, depending upon how much pressure (beyond 73 atm) is applied to it
Pressure (atm)
0 –40 –80
–120 40 80
Temperature (°C) 10,000
1000 Solid
Liquid
Supercritical fluid
Critical point 31°C at 72.8 atm
Gas 100
10 1 0.1 0.01 0.001
FIGURE 1 Phase diagram for carbon dioxide.
Green Chemistry: Strategies to Reduce VOCs Emanating from Organic Solvents 105
DeSimone was one of founders of a dry- cleaning chain that uses liquid carbon dioxide, along with surfactants that he developed, to clean clothes. The spent liquid carbon dioxide is drained from the clothes after the wash cycle (in much the same as the wash water in our washing machines at home is drained off after the wash cycle) and the carbon dioxide is allowed to evaporate by simply reducing the pressure. The carbon dioxide vapors are then captured, liquefied by increasing the pressure, and reused for another wash. Carbon dioxide is plentiful and inexpensive, since it can be recovered as a by-product from natural gas wells or ammonia production. Capture of car- bon dioxide from these processes puts to good use this compound which would normally be released to the atmosphere and contribute to global warming (see Chapter 6). By way of contrast, most dry cleaners in North America presently use perchloroethylene, Cl2CRCCl2, known as PERC, as the solvent. PERC is a VOC, since it has a high vapor pressure and readily escapes into the troposphere if not carefully controlled. PERC is also is a ground- water contaminant (see Chapter 11) and is a suspected human carcinogen.
3.17 Green Chemistry: Using Ionic Liquids to Replace Organic Solvents: Cellulose, a Naturally Occurring Polymer Replacement for Petroleum- Derived Polymers
Cellulose (Figure 3-14) is a polymer of glucose that makes up about 40% of all organic matter on Earth. About 700 billion tonnes of cellulose exist on Earth, with another 40 billion tonnes produced each year by plants as the major component of biomass from atmospheric carbon dioxide and water via photo- synthesis. This removal of carbon dioxide from the atmosphere helps to miti- gate some of the global warming caused by anthropogenic emissions of the gas.
Many polymers produced from crude oil are ubiquitous in our everyday lives, including polyethylene terephthalate (PET), which is found in bever- age bottles and polyester clothing, polyethylene, which is employed in mak- ing plastic bags and milk jugs, polyvinyl chloride, which is found as plastic
999CH29CH999CH29CH999 C"O
CO2-phobic chain segment
O CH2(CF2)6CF3
n m
CO2-philic chain segment (a)
Carbon dioxide solvent
CO2-philic chain segments CO2-phobic chain segments (b)
FIGURE 3-13 A copolymer surfactant for carbon dioxide.
(b) A micelle in liquid carbon dioxide. [Source: M. C. Cann and M. E.
Connelly, Real-World Cases in Green Chemistry (Washington, DC: American Chemical Society, 2000).]
pipes and shower curtains, and polystyrene, which we discussed in the green chemistry section in Chapter 2. Hundreds of millions of kilograms of these petrochemical-based polymers are produced each year, requiring as raw mate- rial approximately 700 million barrels of crude oil. As the price of conven- tional crude oil increases and the supply declines, a major focus of green chemistry is the production of organic chemicals, including polymers, from biomass (see the green chemistry section in Chapter 7). An even more intriguing opportunity is to use naturally occurring polymers such as cellulose to replace crude oil in these syntheses.
The use of cellulose is severely limited by its insolubility in water and in traditional organic solvents. The strong intra- and inter-chain hydrogen bonding between the numerous hydroxyl groups on the cellulose polymer are likely the reason for this insolubility, which results in very poor processability for cellulose. Consequently, only about 0.1 billion tonnes of cellulose has been used annually as a feedstock for further processing.
In the previous green chemistry section, the replacement of traditional organic solvents with supercritical and liquid carbon dioxide was discussed.
A very interesting and relatively unknown group of compounds that is of H H
H H OH
H
O
O
OH OH
HO H H
H H OH
O
OH H
H H H
H OH
H
O
OH HO
H H H
H OH
H
O
OH O
O
n HO
HO
HO
FIGURE 3-14 Structure of cellulose.
Green Chemistry: Strategies to Reduce VOCs Emanating from Organic Solvents 107
growing interest as replacements for traditional organic solvents are called room temperature ionic liquids, or just ionic liquids (ILs). Most ionic com- pounds have characteristically high melting points due to their strong net- work of ionic bonding. For example, sodium chloride (table salt) has a melt- ing point of 801°C. In contrast, a few ionic compounds have melting points below or moderately above (100°C) room temperature; such compounds are known as (room temperature) ionic liq- uids. ILs are generally composed of bulky ions that have dispersed rather than localized charges and large nonpo- lar groups (Figure 3-15). As a conse- quence, their oppositely charged ions have only weak attractive interactions with one another, which results in the low melting points of these compounds.
One very attractive characteristic of ILs is their very low vapor pressure, in contrast to most organic solvents, which because of their significant vapor pres- sures are VOCs and contribute to tropo- spheric pollution. Because they are ionic, many ILs are nonvolatile and thus their potential to replace VOCs is of significant interest. Ionic liquids may
also be purified and recycled, thereby adding to the green characteristics of these solvents. In addition, they are nonflammable, and many are stable up to 300°C, making them attractive for reactions and processes which require high temperatures.
Another strong interest of the green chemistry community is the use of microwave ovens to facilitate chemical processes and reactions. Conven- tional heat sources—such as heating mantles, Bunsen burners, and oil baths—heat materials from the outside in, transferring energy (in turn) from the heat source to the bottom of the reaction vessel, to the solvent inside the beaker, and finally to the dissolved reactants. In each step, heat energy is lost to the surroundings as it is transferred. A microwave-absorbing reactant or solvent, however, can be targeted by microwaves and therefore can be directly heated by irradiation in a microwave oven or reactor. Thus with microwave heating the contents may be heated directly without heating the vessel. Most people have experienced this phenomenon when heating a cup
N N Cl
AlCl4 N
P O
O H3C(H2C)12
H3C(H2C)5
(CH2)5CH3 (CH2)5CH3 P
N
F3C
F2C CF2
CF2 O S
O O
FIGURE 3-15 Ion pairs in four typical ionic liquids.
of water in a household microwave oven. The water heats quite quickly while the cup remains relatively cool. Chemists have found that many reactions and processes can be accelerated in a microwave oven, whose efficiency of heating has the potential to reduce energy requirements.
In order to heat effectively via a microwave source, a substance must be polar and/or ionic. ILs heat up very quickly in a microwave since by nature they are ionic. They can reach temperatures as high as 300°C in 15 sec of microwave heating.
Robin Rogers and his group at The University of Alabama won a Presi- dential Green Chemistry Challenge Award in 2005 for their discovery that certain ILs readily dissolve cellulose when heated with a microwave oven.
The process they developed involves the use of gentle, pulsed microwave heating in a domestic microwave oven to expedite the dissolution of cellulose in ILs. Their studies indicate that with the IL 1-butyl-3-methylimidazolium chloride, they can produce solutions with up to 25% (by mass) cellulose.
CH3 H3C
N N
Cl
There is evidence that the chloride ion in this compound disrupts the internal hydrogen bonding in the cellulose, thereby leading to dissolution.
The addition of small amounts of water solvates the chloride ions, allowing the hydrogen bonding of the cellulose to resume. The cellulose then pre- cipitates from the solution, and can then be deposited as films, membranes, and fibers.
By dispersing additives in the IL either before or after the dissolution of cellulose, composite or encapsulated cellulose-based materials can be formed when the polymer is regenerated. For example, laccase, an enzyme found in fungi that degrades polyphenolic compounds, has been encapsulated in a cellulose support without loss of its activity. The enzyme, when supported on a cellulose film, can be immersed in an aqueous reaction environment and easily removed at the end of the reaction by removing the film, which can then be reused.
The Rogers group has also successfully suspended many other materials in cellulose. These include dyes that can be used to detect metals such as mercury, and magnetite (Fe3O4), which produces a composite with uniform magnetic properties. Using this method, cellulose can be combined with other polymers to produce blends. When mixed with polypropylene, a com- posite that has excellent tear properties is formed. The use of this material for packaging offers significant promise. Encapsulation of medically active compounds along with magnetic materials has the potential to produce microcapsules that can be directed to specific parts of the body. In addition,