Priestley Medal Address
Green Factories Melvin Calvin, University of California, Berkeley
This article is based on the Priestley Medal Address presented by Dr. Calvin last week at the ACS national meeting in Anaheim, Calif. Calvin, head of the Laboratory of Chemical Biodynamics at the University of California, Berkeley, received the medal, ACS's highest honor, in recognition of his distinguished services to chemistry. The work he describes was supported, in part, by the solar energy division of the Department of Energy. In considering the subject of this address it seemed proper to examine the idea of how best to use the knowledge acquired through basic research over the past 30 years in our laboratory and elsewhere throughout the world to help solve the worldwide problem of diminishing energy supply and increasing energy needs. The "green factories" (that is, the green plants) are, in absolute terms, one of the most effective means we have available to us to restore a positive balance to our energy bank account. The way in which plants can convert the energy of sunshine into food, fuel, and fiber of useful type provides an example of how basic science can be used in the service of mankind. Sunshine and its conversion into chemical energy and
material through the pathways of carbon reduction and quantum conversion in the green plant is our ultimate energy source; it is relatively free of political control, universally available on an annually renewable basis, and environmentally clean. What we as scientists need to do is to take the basic knowledge acquired in the lab and devise synthetic means of achieving the same kinds of energy processes as the green plant does all by itself, with only input from sunlight. This effort may be considered as having two distinct parts: first, the development of new kinds of "petroleum plantation" agriculture, with a view toward using the actual substances of the plant for sources of fuel and material, and second, the creation of entirely new synthetic methods for reproducing in the lab (and ultimately on a much broader scale) the quantum conversion steps of natural photosynthesis with the resulting generation of hydrogen, one of the main energy sources of the future. The idea of using green plants as a source of hydrocarbons, as opposed to a source of carbohydrates or proteins, resulted from two types of events that came together rather remarkably about four years ago. When the Arab oil embargo of 1973 occurred, lines at gasoline stations were very long, and all of us spent a good deal of time waiting in these lines. During some of these moments it occurred to me that there must be plants available whose end products could be hydrocarbons and which could be grown and "harvested," so these hydrocarbonlike materials might be used as substitutes for some of the materials
Euphorbia tirucalli, a latex-bearing plant shown here In Brazil, also grows in southern California 30
C&EN March 20, 1978
that come to us from petroleum. The second event, even more personal, was the result of my wife's gardening activities on our ranch near Healdsburg in northern California. On our property there grows a plant called the "gopher plant" (Euphorbia iathyris) whose leaves produce a milky latex—latex, of course, being a hydrocarbon emulsion. The juxtaposition of the idea of using hydrocarbon-producing plants (with the latex emulsion) as a source of fuel or materials and the availability of at least one type of such a plant gave rise to an activity in our lab that is continuing to this day. Before discussing our efforts in developing "petroleum plantations" and growing "green factories" for the production of hydrocarbonlike materials, I would like to describe briefly some of the energy sources available (and their constraints), the concept of using plants for materials production in the U.S. and elsewhere in the world, and, finally, the creation in southern California of a controlled experiment in the growth and harvesting of particular plants for their oil content, an entirely new agricultural undertaking. Natural gas, coal, and oil (all fossilized photosynthetic products) provide a little more than 95% of the U.S. energy supply; the rest of our resources are very small in comparison. About half of the total energy contained in this supply is lost in a nonuseful fashion, called rejected energy, in which the heat or energy content becomes available at too low a temperature to be useful in any practical way. The largest fraction of useful energy, on the other hand, results from the fact that the significant energy uses are industrial, and commercial and residential heating; those uses represent relatively efficient means of energy use because the energy in terms of heat is not limited by the Carnot limitation, i.e., it is not used to create mechanical work. In 1976, we imported 15 quads of energy, whereas in 1973-74, before the institution of Project Independence, imports represented 12.9 quads. The backwards progress we have made between 1973 and 1976 shows that the problem of imported
energy cannot be solved by the application of ordinary efforts. Petroleum simply is not available for discovery, which is another reason energy supplies cannot be increased by merely increasing the rate of petroleum discovery. Also, raising the price of petroleum and natural gas does not increase production indefinitely. When you realize that oil and natural gas supplies are, for the most part, the fossilized remains of photosynthetic activity of several million years ago, it is easy to conceive that there is a limit to its availability. One of the more practical ways to estimate availability of petroleum is to measure the rate of discovery as a function of the number of feet of well drilled. For the period 1920 to 1950, the rate of oil discovery per unit well drilled was about the same; for every 100 million feet of well drilled there were about 20 billion bbl of oil discovered. By 1950, the rate of discovery per 100 million feet of well drilled was much smaller, only about 7 billion. Data available indicate that the real measure of the difficulty of new discoveries is a measure of the exhaustion of supply. Almost identical information can be produced for natural gas in the U.S., and world oil and natural gas discoveries, including those in the U.S.S.R. and China as well as Eastern Europe, have followed the same trend since 1965. Another method of measuring oil supply can be found in the price. Price, however, reflects not only supply but political factors (in addition to the geological factors) that control supply. After staying almost constant for about 10 years, the price of oil started up in 1971, with a tremendous increase occurring in 1973. The price still is rising and undoubtedly will go even higher. Our supply of fossil hydrocarbons will gradually be exhausted, in spite of comments that if we spend more money, more oil will be discovered. There is a limit as to how far that argument can be carried, and there is no ambiguity about the evidence that fossilized photosynthetic carbon is being consumed at an accelerated rate. King Hubbert, a geologist for the U.S. Geological Survey, has been discussing this problem publicly for at least five years and has described how fuels, specifically coal and oil, come into and go out of use. If information on wood use had been included in his calculations, it would have shown that wood was the most important source about 1800, but consumption fell off as coal was discovered; then the more convenient oil took over in the early 20th century. By 1970 oil was the dominant source. However, by the year 2000 the prediction is that oil will have peaked out. King Hubbert is probably correct in his guesstimates and oil may even peak out before that time. There is, however, an enormous amount of coal, which is also fossilized carbon, available to us. This fact has induced the President to say we should devote our biggest effort to finding ways to use coal as an energy source in place of orl and natural gas. There are incentives now to induce industry to transform its power plants from oil and natural gas to coal; this is the reverse of the trend introduced 30 years ago when industry converted from coal to oil and natural gas. Now that the relatively clean fuels are being exhausted, the political planners of the U.S. are trying to reverse their use; I believe this actually will occur. However, there are constraints on coal that have only recently come to light, largely since people have begun to think about the expansion of coal as a source of energy. It is not that these effects were unknown; it is merely that they are being taken somewhat more seriously. These well-known effects are the environmental costs in terms of the land itself (for example, strip mining) and the health aspect,, which has been well established. One of the consequences of burning coal is, of course, the production of soot. In more scientifically designed combustion chambers less soot is produced, but the carbon dioxide problem (which I will discuss later) is not eliminated. The particulates (carcinogenic hydrocarbons) that come from the stacks can be reduced by better combustion processes or filtration, but one March 20, 1978 C&EN
31
thing that cannot be done is to eliminate the carbon dioxide resulting from the combustion of carbon. Although carcinogen production will be increased by extended use of coal, it is possible to prevent that, but at a cost. If coal is liquefied (for example, in a 25,000 ton-per-day plant), the liquid will contain 200,000 lb of polycyclic aromatic hydrocarbons and 10 lb of benzo[a]pyrene, one of the most carcinogenic of aromatic chemicals, whose mechanism of action we have some reason to believe we understand. This problem is not completely insoluble, because the benzo[a]pyrene can be removed from the oil, again at a cost, or its formation can be prevented. Natural petroleum contains 1 to 5 ppm of benzo[a]pyrene, whereas oil produced from coal contains 10 to 100 ppm, 10 to 20 times as much. We need coal plants for our future energy supplies, but they need to be cleaned up so as not to produce increased quantities of carcinogenic materials. Nationally and globally we need all of the energy sources— coal, nuclear, solar—that can be called upon to fulfill our needs. Our total use of energy in the U.S. in 1976 was about 70 quads and the projection is that we will be short 20 quads by 2000. To give you some idea of the significance of that shortage: The shortage in 1973-74 as a result of the oil embargo was 1 quad. A shortage of 20 quads will be very large indeed. The longer-term global effects of increased coal consumption should also be recognized. When fossil carbon of any kind is burned, the carbon from the stored pool in the ground is converted to carbon dioxide, which is released into the atmosphere. To close that cycle, the plants have to take carbon dioxide from the atmosphere and reduce it again. One sink for the atmospheric carbon dioxide is photosynthetic activity, and the other sink is the ocean in the form of a dissolved calcium salt. As carbon dioxide dissolves in the ocean, calcium carbonate falls to the bottom. Both of these sinks (and perhaps others are yet unknown) are not absorbing carbon dioxide at the rate at which it is entering the atmosphere. In the winter the CO2 level rises and every summer it falls, but only about half of the CO2 entering the atmosphere in the winter comes out in the summer. The net result is a constant and continuing rise of the CO2 concentration each season. For example, there has been a rise from 315 to 330 ppm of CO2 in the past 16 years, according to data from the Mauna Loa Observatory in Hawaii. Similar results have been obtained from stations in the Arctic and Antarctic. The cycles at the poles are not as sharp as at Mauna Loa, which is located at 20° north, but they show the same percentage rise. We must therefore try to extrapolate back, using carbon-14 measurement on tree rings, to calculate the approximate CO2 level for the previous 100 years. This method indicates that the CO2 level had risen another 15% between 1890 and 1958. It has increased another 5% in just the past 15 years. The rate of increase is much faster now than 100 years ago, and there is no ambiguity about the rate of injection of CO2 into the atmosphere being about twice as large as the rate of removal. The consequence of the increase in CO2 concentration is as yet theoretical because the 5% change in level is still too small to permit unambiguous attribution of climatic consequences. Nevertheless, we know enough about CO2 to be sure that it is transparent to visible light of the sun and opaque to the infrared light reflected from the earth. Therefore, the CO2 acts as a blanket over the earth's surface and one can expect that the heat load on the earth will increase. This is the so-called greenhouse effect. When sunlight strikes anywhere on the earth, a fraction is turned into infrared light, which is reabsorbed by the CO2 and reflected downward. One of the consequences of the rising CO2 leveWs an increase in the average global temperature. We have Reprints of this C&EN special report will be available at $2.00 per copy. For 10 or more copies, $1.25 per copy. Send requests to: C&EN Reprint Department, American Chemical Society, 1155—16th St., N.W., Washington, D.C. 20036. On orders of $20 or less, please send check or money order with request. 32
C&EN March 20, 1978
seen the concentration of CO2 increase over the past 15 years, but there has not been a clearly recognized corresponding change in global climatic conditions. The meteorological models currently available are not sophisticated enough to tell us what the climate should be, and what effect the CO2 increase has now or will have in the future. Also, the CO2 increase is inside the "noise level" of the annual climatic fluctuations, so far. However, if we wait until the concentration has increased to the point where it becomes a noticeable factor in climate change (using meteorological models), it will be too late to change the effect. The CO2 concentration increase could change the entire worldwide pattern of agricultural production on the earth's surface, and the pattern of living would be markedly different 100 years from now from what it is today. The methods I prefer to advocate to increase our energy prospects are those that use sunshine in some useful way that will allow us to "harvest" it as it comes to the earth's surface, with a minimum environmental problem: no CO2 problem, no carcinogen problem. Green plants can catch the sun and reduce the carbon, particularly on the equator where plants are the most productive. Green plants are able to fix 1 kg per m2 per year of carbon along the equatorial region where the weather is warm and there is plenty of water. Other areas of the world that are warm—South Africa, Chile, the U.S. Southwest, and North Africa—unlike the equatorial region often lack water. Therefore, in many of the semiarid areas of the world is it not possible by ordinary methods to use the green plant as a factory? The distribution of sunshine in the U.S. indicates that the Southwest is the area of choice, but it still has inadequate water supplies to support green plants, which are the best solar energy "factories" we have. We must learn how to use the green plant in the most efficient way possible, both for itself and for the model it provides for synthetic devices. Through the mechanism of the photosynthetic carbon cycle, the green plant captures the carbon dioxide from the atmosphere and, with the aid of sunshine, separates hydrogen from the water to reduce the carbon dioxide first to carbohydrate (such as sugar) in which there is only one oxygen atom on each carbon atom. Eventually, some plants can take the carbohydrate and reduce it all the way to hydrocarbons, with no oxygen at all on the carbon atoms. This is essentially what petroleum is. Most plants store their sunshine as half-reduced carbon in the form of carbohydrate (wood, cellulose, starch, or sugar). The sunshine enters the plant, is captured by the green chlorophyll, and the light-capturing step separates the charge on either side of a membrane. There is a positive charge on one side of the membrane and a negative charge on the other. The positive charge eventually shows up as molecular oxygen with the negative charge making active hydrogen that is used by the plant to reduce the carbon dioxide to carbohydrate. If there is no carbon dioxide available, some of the active hydrogen, in certain plants, may show up as molecular hydrogen. Whereas most green plants store the sun's energy as carbohydrates, some store their energy as hydrocarbons. It is the combination of capture and chemical storage of the sunshine in the green plant that I will discuss in further detail. First, let us talk about the majority of the· world's plants, which store most of their energy as carbohydrates. Classic examples of efficient plants that produce fermentable sugar directly are sugarcane and corn, both of which belong to the same family and have similar efficiencies. The preparation of sugarcane for harvest involves burning the dry leaves, leaving the stem (or stalk) of the sugarcane intact. The cane is cut and harvested within one day after the burning. The cut cane is then moved to the mill, where it is crushed with water; the water goes to the evaporator for extraction of sugar, and the cellulosic residue (bagasse) is burned in the boilers to produce steam needed to run the mill. Excess steam from this process is run through turbines to generate electricity. Thus the factory collects the cane and generates sugar from the juice, alcohol from the molasses, and
electricity from the excess steam. A sugar plantation, therefore, really is a self-contained "energy farm." No new energy has to enter the system except the sunshine itself. The sugarcane captures the sun, storing a very large fraction of fermentable sugar or cellulosic residue; the sugar mill uses both. One example of a better method to utilize the ability of plants to capture and store solar energy is Brazil's decision to use land which is capable of growing sugarcane in high yield. Sugar from cane on these lands is being converted into alcohol by fermentation. The weight is reduced by a factor of two, and very little of the energy is lost in that conversion. In November 1975, the Brazilians, realizing that they had no accessible petroleum source of naphtha for ethylene or, for that matter, for gasoline, decided that they would encourage both new plantations for sugarcane and new fermentation facilities to make alcohol directly from the sugarcane juice. To do this, the government provided loans at low interest. The Brazilians, who are the largest sugarcane growers in the world, produced approximately 7 million or 8 million tons of raw sugar in that year, which yielded 700 million liters of 95% alcohol by fermentation of the residual molasses. It is entirely possible that Brazil will attain its stated goal— 20% alcohol in liquid fuels—by 1982 because of the remarkable rate at which new sugarcane acreage is being introduced, especially in the San Francisco River region and other parts of northeastern Brazil. In fact, in January 1978 about 10% of the fuel requirements of the state of Rio de Janeiro was being met by alcohol. One of the main economic factors that has made this transformation of sugarcane from food to fuel possible in Brazil is the availability of large amounts of relatively inexpensive land that can be machine-cultivated to produce cane. The sugarcane-alcohol-cellulose-sugarcane cycle on the large, self-sufficient sugar plantations is efficient and cost-effective. It now appears that most of the industrial alcohol of Brazil can be made by fermentation of molasses that remains after the crystalline sugar has been removed. Sugarcane also can fulfill at least part of the need for chemicals and materials. The fermentation ethanol can be dehydrated over alumina to ethylene, which can be fed into the stream of petrochemicals. Many other materials can be made from the cane juice by different types of fermentation processes, and eventually these alternative processes may become more significant as a source of chemicals than they are presently. Sugarcane now has become one of the major sources of chemical raw materials in Brazil. In the U.S. we make practically no fermentation industrial alcohol at the present time. This method of making alcohol went out of style in about 1950, when it became possible to obtain industrial alcohol by adding a water molecule to ethylene, having obtained the ethylene from naphtha. It now appears that we might return to the earlier methods of making industrial alcohol because of our lessened petroleum reserves, and in some areas of the U.S. this kind of development is already occurring. You may recall that in the oil embargo days of 1973-74 there was a note in one of the U.S. newspapers that the legislature of Nebraska had passed a law reducing the state tax on gasoline if it contained as little as 10% fermentation alcohol. The reason for this legislation was that the storage facilities for corn in Nebraska were strained, and a substantial percentage of the corn crop spoiled that year and could not be used for food. The spoiled corn was used to make fermentation alcohol, which was added to gasoline; this material was called gasohol. This program was very successful, and Nebraska now has a program to show that the production of "gasohol" is an economic process. Chemical engineering professor William Scheller, who is in charge of this particular project at the University of Nebraska, has produced a chart showing the consequences of various uses of corn in Nebraska. If 100 bushels of corn grown on 1 acre of land is fed directly to cattle, a weight gain of about 480 lb results. If, instead, 20 of the 100 bushels are fermented to alcohol (60 gal approximately) with the remains
of the fermentation (the distilled dry yeast, about 300 lb) added to the 80 bushels of original unfermented corn and fed to the cattle, 520 lb of meat will result instead of 480 lb. Thus, 60 gal of fermentation alcohol and 50 lb of additional meat are created by diverting 20% of the corn to the fermenter. The creation of yeast protein during conversion of starch to alcohol is the single factor that makes the difference. I have mentioned previously that some green plants do indeed store their energy as hydrocarbons. The one that comes most immediately to mind is the Hevea rubber tree, which produces latex by reducing the carbon all the way to hydrocarbon. The idea that plants containing hydrocarbonlike materials, such as the latex from Hevea, could be used as a source of hydrocarbons gave rise to the work currently under way in our laboratory. We have separated the latex from Hevea brasiliensis into its component parts and have also done molecular weight analyses. It is important to know how to manipulate the hydrocarbonlike molecules from these plants in a manner that will ultimately give us the type of products necessary for materials and fuel substitutes. There are other latex-producing plants in addition to the Euphorbiaceae. There is a member of the Compositeae family known as guayule which grows in the northern deserts of Mexico and in the southwestern U.S. This desert shrub produces a high-molecular-weight hydrocarbon that could be used as natural rubber. During World War II, when the supply of natural rubber from Malaysia was cut off, efforts were made (especially in California and other areas of the western U.S.) to grow guayule as a source of rubber. Because of the concurrent development of synthetic rubber, however, it was not deemed feasible to continue the development of guayule for this purpose after the war and the efforts, at least in the U.S., ceased. However, the Mexican government recently has instituted once more the development of guayule plantations in the northern areas near Saltillo, and the production of guayule in this region has increased markedly. The interest in guayule in Mexico and elsewhere has increased to the point where several international conferences have recently brought together experts from various disciplines to focus on the problem of improved plants, increased production, and other aspects of a project of this type. Guayule cannot be tapped like Hevea because the rubber in guayule is not in tubules, as in the bark of Hevea. The rubber droplets are captured individually in individual cells, and every single cell has to be broken to extract the rubber. The plant grows to a height of about 3 feet and then is pulled out of the ground for harvesting the rubberlike material. In Mexico they are currently harvesting the wild plants. Guayule, of course, is not a source of hydrocarbon for us in the southwestern U.S. but of rubber. For example, Firestone has about 100,000 acres in west Texas that it intends to devote
Latex flows from a wound in the bark of Euphorbia lactea March 20, 1978 C&EN
33
Molecular weight distribution of polyisoprenes isolated from H. brasiliensis and E. tirucalli Relative abundance
150
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100
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50
0
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106 105 104 Molecular weight (polystyrene standards)
103
to guayule cultivation for rubber production. Guayule, already a commercial crop in Mexico, will become commercial soon in the U.S. as well, in the same type of semiarid area that we are exploring to use for the production of oil-producing plants. As mentioned earlier, we found on our ranch in northern California members of the Euphorbiaceae family that contain latex that is like that in Hevea, although chemically it is not quite the same. This turned our minds to an exploration of the whole family of Euphorbiaceae plants, to which Hevea belongs. Another genus of that family, Euphorbia, has about 2000 to 3000 different species. We then began our search for some species of Euphorbia that would grow in the dry, semiarid lands of various parts of the U.S. We found Euphorbia lathyris (go pher plant) to be an excellent candidate as a hydrocarbonproducing plant. This species ultimately became the one of choice for our petroleum plantation, for reasons that will be discussed later. Also, while my wife and I were in Brazil two years ago looking at the Hevea and sugarcane, we observed another plant around the sugarcane plantations as a hedge. This was Euphorbia ti rucalli, popularly called the African milk bush. It also is a latex-bearing plant which we have found growing in southern California; it probably would also grow well in Arizona and Texas. The latex from the E. tirucalli is similar to that of other Euphorbias. It is interesting to note that about 40 years ago a species of Euphorbia, E. resinifera, was grown in Morocco. Approximately 125,000 hectares of land were harvested, with a production of 10,000 liters of latex/hectare, from which 1700 kg of rubber (benzene extractables) and 2750 kg of gum resin (acetone extractables) were obtained. I do not know whether this effort was ever repeated, or whether it was done only once, nor do I know the exact purpose of the planting. This information does, however, reinforce the feasibility of growing various species of Euphorbia for the latex that they produce. We visited Puerto Rico in the spring of 1977 in connection with an energy conference at the University of Puerto Rico. The Puerto Ricans want to develop solar energy and are thinking of using sugarcane, an important crop there, to make fermen tation alcohol in a way similar to what has been done in Brazil. During that trip we searched the dry side of the island for other species of Euphorbia that might be candidates for hydrocarbon production. There we found Euphorbia lactea, which grows to a height of 10 to 15 feet. A knife inserted into the bark of the tree produces a flow of latex similar to that for Hevea, which we have examined chemically in the laboratory. At the present time we are becoming familiar with the chemical composition of latexes from various species of Euphorbia collected from all over the world. It is clear from these tests that there are at least two major families that are candidates for oil-producing plants, the Euphorbiaceae and the Asclepiaeceae, with individual plants in the family Sapotaceae and Moraceae as well. 34
C&EN March 20, 1978
I would now like to discuss what we have done in terms of cultivation of oil-producing plants. Some of the plants I have discussed as possible candidates (E. tirucalli and E. lactea) are trees that take several years to reach harvestable maturity. We finally chose the E. lathyris, which is an annual, for the first experiments with cultivation in southern California. The plants of E. lathyris grow to about 4 feet in height in seven months; they were planted from unselected seed in February 1977. We are able to obtain a yield plot for E. lathyris by cutting indi vidual plants, weighing them, and measuring their hydrocarbon content. Our planted area in southern California is the begin ning of a horticultural experiment in which wild plants are being cultivated under controlled conditions. Our intent is to grow the plants under different conditions—seasons, irrigation re quirements, etc., to obtain yield data. The yield rates for E. lathyris have been converted into equivalents of barrels of oil at 8% by weight. The plant is actu ally more like 10% by weight oil, but the figure of 8% has been used because it is unlikely that all of the oil can be extracted from the plant material. Using a figure of 8% yield by weight converts to about 10 bbl of oil per acre per year, which is the minimum yield. The oil from the E. lathyris or E. tirucalli extractions is an isoprenoid similar to rubber, the difference being that when water is taken away from the rubber latex a solid remains. This is because the molecular weight of polyisoprene from Hevea brasiliensis is between 500,000 and 2 million. On the other hand, when water is removed from E. tirucalli latex, a viscous liquid remains, the molecular weight of the polyisoprene being much less than that for rubber. The molecular weight distri bution of polyisoprenes isolated from Hevea brasiliensis and E. tirucalli was obtained, using gel permeation chromatogra phy. The Hevea latex molecular weight distribution is bimodal for a particular clone, whereas the molecular weight for the E. tirucalli latex is about 20,000. All of the Euphorbias that have been described in the literature produce isoprenelike materials, and there are cyclic isoprenes as well, such as terpenes, diterpenes, and sterols. These represent about one half the hydro carbon in the emulsion. The chemical mechanism for the plant's process of converting carbohydrate into isoprenoid hydrocarbons has been elucidated. We believe we know all of the chemical steps involved in con structing these large molecules, arid we will eventually be able to modify the plants genetically so they will produce molecules "to order," so to speak, and not in the random natural fash ion. The photosynthetic carbon cycle shows that carbohydrate is first created by taking carbon dioxide through the cycle. The carbohydrate then produces pyruvic acid, essentially the first carbon fixation product of the plant, which has been modified to the same redox level as glyceric acid. The pyruvic acid then loses a carbon dioxide molecule to give acetyl CoA (two car bons), two of which condense to create acetoacetyl CoA. A third acetyl CoA goes on the carbonyl function to make a tertiary alcohol, leaving one of the acids free. This is then reduced via CoA to mevalonic acid, which then goes through a series of steps via phosphorylation on the primary alcohol group and finally dehydration and decarboxylation to isopentenyl pyrophosphate (IPP), a five-carbon compound from which the polysioprenes are constructed. The IPP is first isomerized to dimethylallyl pyrophosphate (DMAPP), producing an allylic pyrophosphate that can drop the pyrophosphate and become a carbonium ion and react with another IPP to create a diterpene. This reaction can continue, building up the chain. The reason for pointing out the mechanism of polyisoprene manufacture is to indicate that every one of the enzymes involved in going from carbon dioxide to hydrocarbon is known, probably not well enough to modify their action, but we have the beginning. The question arises as to what factors determine the length of the chain that is to be constructed and what factors make the chain elongation cease. My belief is that these events occur in an oil (rubber) droplet, actually as an emulsion polymerization.
Growth rate of Euphorbia lathyris 1
Bbl of oil per acre, 8% yield 43,500 plants 17,500 plants per acre per acre
Grams per plant, dry weight
300
8"*
260
7-
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6-
220
5-
180
2-
4-
140
3-
100
1~
2-
60
Ι
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0
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Feb
Apr
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Jun
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Oct
On the end of the chain is a pyrophosphate with an allylic double bond; the terminal of the long chain is made from the rubber (oil) droplet itself. There are also detergents in the oil emulsion that stabilize the droplet. The incipient carbonium ion then reacts with the electron pair of the IPP to form a car bon-carbon bond; then the proton is removed and the result is now a polyisoprene with a longer chain. A chain elongation cannot occur without a catalyst of some type, and not much is yet known about the catalyst. It probably is a transition metal complex of some kind whose character has yet to be identified or determined, but whose activity perhaps is dependent on the radius of curvature of the rubber (oil) droplet. As the droplet grows with the increasing molecular weight, the radius of curvature decreases, changing the activity of the catalyst. In other words, the catalytic action shuts down at a certain size. This may be the physical method by which the green plant determines the molecular weight. Each strain of the plant differs from others by either the detergent which is used to stabilize the micelle or in the nature of the catalyst, or both. The frontier of this work on the polymerization of isoprenes is now at the point where it is necessary to discover the nature of the catalyst, on the one hand, and the chemical composition of the detergent, on the other. We know that the molecular weight of the polymerization reaction is determined by the particular strain of plant, and we must now examine the catalyst and the detergents which are critical to understanding the molecular weight distribution of latexes from various plants. We can eventually modify the hydrocarbon molecular weight down into the few thousand region. We also want to reduce the degree of unsaturation, that is, put more hydrogen in, thus "engineer the cells" of the plant to have them create the desired chemical products. I suspect that our yields will not be limited to the unsaturated hydrocarbon, which is what the plants produce today in the natural state. The yield of desired products from the plants which will be cultivated could be raised sub stantially by selecting seeds properly, even without "cell engi neering." When I showed the yield data of E. lathyris to representa tives of people experienced in plant breeding, they predicted that with proper seed selection the yield of hydrocarbons from these plants could be raised to 20 bbl of oil per acre per year. There is no doubt that when any selection program is started on the wild plants that the yields will immediately double. I recently visited the petroleum plantation and observed that the plants are growing at a more rapid rate in some plots than in others. Because the growth rate is variable, it will be possible to select seed from the faster growing plants for continued cultivation. The type of exploration described above gives two practical approaches to renewable resources. First, we can use the hy drocarbon as it comes from the plant itself (2 to 10% by weight)
as a crude oil, refine it, rescue the sterols which it contains, crack the rest of the compounds to ethylene, propylene, etc., and then reconstruct the desired chemicals from those products; I feel that this particular approach can be developed immediately. Later, we may learn how the molecular weight is controlled and manipulate the plant to construct materials of the desired molecular weight for whatever purpose is desired. This ap proach, which will be longer and more complex, will use the plant as the collecting and constructing vehicle. Hydrocarbon-producing plants can be grown on land that is today nonproductive. There is a great deal of land of this type in the U.S. and other areas of the world. Cultivating hydrocar bon-producing plants on land of this type can be started almost immediately, even without genetic improvement of the plants, using the plants (seeds) as they are in the wild condition. Pro fessional horticulturists know nothing about the methods of large-scale cultivation of wild plants such as Euphorbias be cause they have never before been planted as a "crop." It is as though we were back in the Stone Age learning how to domes ticate wild plants as we did with the cultivation of sugarcane, wheat, and corn several thousand years ago. We are trying to introduce entirely new plants as a com mercial crop. This requires a great deal of thinking of a type somewhat outside the purview of ordinary agriculture, whose general role is to improve the crops already available (such as wheat, corn, rice, or sugar). It is difficult to assimilate the con cept of an entirely new crop. Professional agriculturalists are bothered by such factors as the type of fertilizer to be used, yields per acre, amount of water for irrigation, type of soil, and amount of sun needed. The answers to questions of this type are generally not well known for wild plants. One of the pur poses of the "gasoline tree plantation" in southern California is to get some initial answers to some of the practical questions concerning cultivation of these plants. What does the production of hydrocarbons from plants mean in terms of the present energy situation? How much can the production of hydrocarbons from a new source help fulfill our needs? The price of oil today is an artificial one of not less than $14 per bbl, and this price surely will rise. What would be the costs of producing crude oil from plants? We are trying to use land that does not produce anything today in the sense that agricultural land is productive. We have selected plants that will grow in dry, semiarid areas of the U.S. and the world. My agricultural colleagues have informed me that an acre of this type of plant could be grown for about $100. Our extraction method, which I have described to my chemical engineering colleagues, projects a cost of about $10 per bbl. If it costs $100 to grow the material for 10 bbl of oil and $10 per bbl to extract and produce the oil, then the cost of this type of hydrocarbon from plants would be $20 per bbl today. If the yield could be improved by a factor of two or three and the extraction process itself also could be improved, the price of this material would be on the edge of being practical. If the costs of petroleum rise another 10 to 20%, the use of hydrocarbon-producing plants, especially for feedstock materials, will become practical from an economic point of view. The development of the concept of the "petroleum plantation" and the cultivation of "green fac tories" is only one type of development for improvement of the world's energy picture. Our estimate of the hydrocarbon content of E. lathyris has been done in terms of organic extractables from the dried plant. This gives a figure of 8 to 10% of the dry weight of the whole plant for the hydrocarbonlike materials, which are mixtures of polyisoprene, sterols, glycerides, diterpenes, and others. The caloric value, estimated from a carbon-hydrogen-nitrogen analysis, is about 17,000 Btu per lb. It thus appears that the first and immediate possibility for the development of an economically useful solar energy and materials system is an outgrowth and, in a sense, a return to an older system—the use of the best existing solar energy-cap turing device we know, the green plant, by selecting and mod ifying it to produce the materials we would like to have, namely, March 20, 1978 C&EN
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hydrocarbons of suitable molecular weight and structure. The choice of the particular plants with which to begin such a large-scale development has yet to be made and will depend on growth rates and habits, hydrocarbon productivity and harvest adaptability. We are already, of course, capable of growing a carbohydrate-storing plant (sugarcane) and converting the fermentables into convenient form (alcohol). I think we have thus demonstrated one of the essential physical-chemical principles which are required to successfully simulate the photosynthetic transfer of electrons up an energy gradient from a low-lying donor system to a higher-lying ac ceptor system. Of course, in the green plant the donor system is ultimately water, which will thus generate oxygen after electrons have been removed from it. This is presumably ac complished in the green plant by some manganese complex. Here we come to one of the earliest and most important of the discoveries of Joseph Priestley. In 1772 he published in the Philosophical Transactions some of his studies of earlier years on different kinds of air. At that time he recognized that the breathing of air vitiated it for further use, whether by living organisms or as a combustion supporter in the burning of a candle. Exactly the same effect could have been achieved by first burning the candle in air. Priestley's most important ob servation, however, was the succeeding one in which he recog nized that this very same vitiated air could be restored by growing green plants in it. He thus detected the capability of plants to photochemically produce oxygen, which he later ob tained as the pure gas by heating red mercuric oxide. A second point of contact with the work described here was Priestley's imaginative invention of the name for the hydro carbon produced by the hevea tree. He discovered that the solid material resulting upon coagulation of the latex would rub out
pencil marks on paper. He thus gave the name "rubber" to this material. Finally, and perhaps most important today, we should rec ognize that Priestley embodied the pursuit of knowledge in the uninhibited form required to give rise to new discoveries and human progress from them. This freedom of thinking led him into troubles with his own social environment. Although he was born in 1733 into a family of Calvinists, he passed from the Calvinism of his own family to a rational Unitarianism in the later stages of his life. In keeping with his habit of free thinking, Priestley regularly took part in meetings of Birmingham's Lunar Society, which during the years 1766 to 1791 was actively promoting science and its applications to industry and crafts. "During these years Priestley was widely known as the defender of the principles of the French Revolu tion and an ardent advocate of civil and religious liberty. He angered the antirevolutionary populace by publicly disagreeing with the 'Reflections on the Revolution in France' (1790), written by the British statesman Edmund Burke, who opposed the revolution. On July 14,1791, on the second anniversary of the fall of the Bastille in Paris, an outbreak of mob violence occurred in Birmingham, in the course of which Priestley's house, library, and laboratory were destroyed. He was driven from Birmingham, never to return," much to the benefit of the colonies in North America after Priestley settled in Northum berland, Pa. The requirement for freedom is an essential component in the progress of science and it is as true today as it ever was. Freedom alone is no longer enough. For society to have the benefits of new thought, particularly in modern science, it must not only tolerate freedom of thought but provide the means, the climate, and the support for those who can do it. Π
Suggestions for additional reading
General references 1. Hall, D. O., Solar Energy Conversion through Biology—Is It a Practical Energy Source?, J. Inst. Fuel, in press. 2. Stanford Research Institute, "Effective Utilization of Solar Energy to Produce Clean Fuel," Report June 1974. 3. SRI International, "A Comparative Evaluation of Solar Alternatives: Implications for Federal R&D," Vol. I and Vol. II. January 1978. 4. Wilson, C. L. (ed), "Energy: Global Prospects 1985-2000," McGraw-Hill, New York (1977). 5. Carr, D. E., "Energy and the Earth Machine," W. W. Norton, New York (1977). 6. Halacy, D. S., Jr., "Earth, Water, Wind and Sun: Our Energy Alterna tives," Harper & Row, New York (1977); "The Coming Age of Solar En ergy," Avon Publishers, New York (1973). 7. Brinkworth, J., "Solar Energy for Man," John Wiley & Sons, New York (1976). 8. Buvet, R., et al. (eds), "Lving Systems as Energy Converters," Elsevier Publishing Co., New York (1977).
2. Hall, H. M., Long, F. L, "Rubber Content of North American Plants," Carnegie Institution of Washington, Washington, D.C. (1921). 3. Vanderbilt, Β. Μ., Rubber from Goldenrod. In "Thomas Edison, Chemist" (Chapter 9), American Chemical Society, Washington, D.C. (1971). 4. Proceedings of the International Rubber Conference, Kuala Lumpur, Malaysia, October 1975. Guayule 1. National Academy of Sciences, "Guayule: An Alternative Source of Natural Rubber," Washington, D.C. (1976). 2. McGinnies, W. G., Haase, E. F. (eds), International Conference on Utilization of Guayule, University of Arizona, Tucson (1975). 3. Campos-Lopez, E. (ed), Proceedings of the International Guayule Conference, Saltillo, Mexico, 1977; in press. 4. Yokoyama, H., Hayman, E. P., Hsu, W. J., Poling, S. M., Bauman, A. J., "Chemical Bioinduction of Rubber in Guayule Plant," Science, 197, 1076(1977).
Specific references in areas discussed are given below:
Petroleum plantations
Effect of increased C0 2 concentration in the atmosphere
1. Calvin, M., "Hydrocarbons Via Photosynthesis," Energy Res., 1, 299 (1977). 2. Calvin, M., Energy and Materials via Photosynthesis. In "Living Systems as Energy Converters," p. 231, R. Buvet et al., eds, Elsevier Publishing Co., New York (1977). 3. Calvin, M., "The Sunny Side of the Future," Chem. Tech., 7, 352 (1977). 4. Calvin, M., "Photosynthesis as a Resource for Energy and Materials," Photochem. Photobiol., 23, 425 (1976). 5. Nielsen, P. E., Nishimura, H., Otvos, J. W., Calvin, M., "Plant Crops as a Source of Fuel and Hydrocarbon-like Materials," Science, 198,942 (1977). 6. Calvin, M., "Chemistry, Population, Resources," Interdisciplin. Sci. Rev., in press. 7. Lipinsky, E., "Fuels from Biomass: Integration with Food and Materials Systems," Science, 199, 644(1978).
1. Stuiver, M., "Atmospheric Carbon Dioxide and Carbon Reservoir Changes," Science, 199, 253 (1978). 2. Woodwell, G. M., Whittaker, R. H., Reiners, W. Α., Likens, G. E., Delwiche, C. C, Botkin, D. B., "The Biota and the World Carbon Budget," Science, 199, 141 (1978). 3. Mercer, J. H., "West Antarctic Ice Sheet and C0 2 Greenhouse Effect: A Threat of Disaster," Nature, 271, 321 (1978). 4. Siegenthaler, U., Oeschger, H., "Predicting Future Atmospheric Carbon Dioxide Levels," Science, 199, 388 (1978). Rubber 1. Agricultural Research Service, U.S. Department of Agriculture, "Plants Collected and Tested by Thomas A. Edison as Possible Sources of Do mestic Rubber," Report ARS-34-74, July 1967.
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C&EN March 20, 1978
