摘要:

invests in longer term research for economic reasons, whereas the universities for understandable reasons continue to do the type of research they are good at—i.e., in a chemical context, petrochemistry. Of course, the above picture is black and white, and there are exceptions, but we advocate strongly the placement of much more emphasis on plant-based raw materials in research and in education. In this contribution the radical starting hypothesis has been that from 2040 onwards we will no longer use fossil organic raw materials, as a consequence of exhaustion and environmental considerations.The fundamental question is raised: ‘Is this technologically feasible while maintaining our present way of life in terms of food, organic materials and energy consumption?’.Situation analysis 1995 and prognoses for 2040 Energy In 1980, the world energy consumption amounted to about 350 EJ (E = exa = 1018) per annum with a world population of 5 billion, or, on average, 2200 W per capita. In the developed world this number varies from about 3400 W per capita in Japan to 10000 W per capita in the USA, whereas in the developing world this varies from 30 W per capita in Bangladesh to 620 W per capita in Brazil.In Table 1 some critical global data are given. The fundamental questions arising from these data are whether it is at all possible, without fossil resources, to produce sufficient food (1st priority), organic materials (2nd priority) and energy (3rd priority) to allow 9–10 billion people2 a decent life. The present-day commercial energy demands are mainly (85%) met by fossil fuels (oil ca. 40%, coal ca. 25%, gas ca. 20%). Nuclear energy contributes about 5%, the same figure applies for hydroelectric power. The remaining 5% is divided over renewable sources such as biomass, solar energy, wind, etc. Some 14% of the energy consumption consists of non-commercial energy, mainly in the developing world, from biomass (straw, sugar cane, etc.).Summary In the year 2040 we will have to feed 9–10 billion people and to provide them with energy and materials. We want them to live according to the requirements of a developed society, and we do not want to pollute the earth or change the climate. The question addressed in this contribution is whether this scenario—sustainable technological growth—is technologically possible on a world wide basis.In 1996, more than 80% of the world’s energy demands (350 EJ a21) originated from fossil resources (mineral oil, coal and gas) and the same holds for organic raw materials. In view of the exhaustion of fossil resources and environmental considerations, our radical hypothesis has been that we will no longer use fossil organic resources from, let’s say, 2040 onwards. Our natural resources are the sun, CO2 in the atmosphere and land and water on the earth.It will be shown that these natural resources and the appropriate conversion technologies will enable us to move from the present fossil resources based economy to a plant-based economy as far as organic raw materialsare concerned. The sun will be the major source of energy.The limiting factors in such a scenario are: 1 the amount of agricultural land available; 2 the ability of the world community to overcome the socio-economic and political problems in realising the scenario as formulated in the first paragraph of this abstract. It will be illustrated which technologies need to be developed further and fine-tuned in the coming decades.Examples will be given how present-day bulk organic chemicals and materials can be approached from biomass. Methanol is expected to develop as a key chemical (and hydrogen-carrier). Moreover, one may consider a partial substitution of materials of today by “green” materials, that are directly or in a few reaction steps available from biomass, such as starch and polylactate, respectively.One thing is certain: the plant will be the ‘plant’ of the future! Introduction The era of a chemical industry based on fossil resources (i.e., mineral oil, gas and coal) will gradually come to an end in the course of the next century. Two main reasons for this prediction are: (1) The stocks of fossil resources are finite. First of all, mineral oil stocks will be exhausted around 2050 if we continue our present way of life. The times that more oil was found each year than was consumed are over.The present low oil prices should not confuse us. See the recent convincing article of Campbell and Laherrère.1 Natural gas will last longer (some 75 years) and coal will last longest (> 200 years). (2) Environmental considerations. All kinds of pollution from global warming to acid rain, from smog to ground water pollution, have been linked to the use of fossil fuels.Hence we are really living on the threshold of a new era in chemistry—and unfortunately in most chemical research and education this fact is not sufficiently recognised. The background of this attitude lies undoubtedly in the fact that industry hardly The development of sustainable routes to chemicals has often been overlooked since the discovery of plentiful oil supplies.However, with an increasing population, and oil stocks being finite, we now face a situation where we must use sustainable raw materials if the chemical industry is to survive. That is the claim made by the authors of this provocative analysis of the current state of the chemical industry.The paper develops a scenario which investigates the feasibility of actually achieving a sustainable plant-based chemical industry which could meet the needs of the planet’s population in the middle of the next century, without impinging on agricultural land. DJM Green Context From fossil to green C. Okkerse and H. van Bekkum* Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands.Fax: +31 15 2784289; E-mail: H.vanBekkum@stm.tudelft.nl Received 7th December 1998 Green Chemistry April 1999 107 C GThe world reserves of economically exploitable fossil organic raw materials (oil, gas, coal) amounted to 35 700 EJ in 1991. Total reserves are speculated to be 260000 EJ. Hence, in 1991 there remained a stock of fossil fuel for about 100 years, based on the 1991 consumption (oil 50 years, gas 75 years, coal > 200 years).Of course, oil and gas exploration can prolong this period, but the fact remains that the stocks of fossil fuel are finite. Also, we would like to avoid an increase of CO2 emission. If we take into account that the expected growth of the world population is from 5 billion today to 9–10 billion in 2040 (cf.UN estimates) and that the world average energy consumption per capita will increase from 2200 W today to 3000 W in 2040 for a developed and more equalised society, then the energy needs in 2040 will be around 900 EJ or 2.5 times the present energy consumption. 3 The sustained growth scenario requires switching from fossil fuel to renewable forms of energy and organic raw materials.It is good to realise that until 1850 all organic consumer products and industrial raw materials were plant-based. Within the relatively short period of 150 years society changed from a mainly plant-based economy to an economy based on fossil fuels (coal until the end of the 19th century, until 1950 mineral oil, and now more and more natural gas).Wood supplied 70% of the fuel demand in 1870, in 1920 70% came from coal, in 1970 70% from mineral oil. Let’s return to renewable energy. For environmental and safety reasons we have abandoned nuclear energy in our scenario. Our nuclear reactor is the sun—at a safe distance from our planet. The sun applies advanced technology: nuclear fusion. The radiation of solar energy arriving on the earth is estimated to be 2.8 3 106 EJ a21, hence 3000 times the energy requirements in 2040.However, the amount that can be converted into useful energy is orders of magnitudes smaller. Renewable forms of energy are solar energy in various forms (different types of solar heating collectors (T <100°C), solar mirrors (T >100 °C) and photo-voltaic (PV) solar cells), energy based on wind-driven windmill parks, energy based on biomass, energy based on water power (hydroelectric, tidal, wave), geothermal energy, etc.In this contribution we will limit ourselves to solar energy and energy from biomass, because in the longer term they offer the best alternatives to fossil fuel. A key question is: ‘Is it possible to harvest sufficient biomass, after deduction of the food needs of a world population of 10 billion, to produce the required amount of organic raw materials?’ The answer to this question depends mainly on the yield of the photosynthesis and the amount of agricultural land available to produce biomass.Although the learning curve will be long, PV cells and similar technologies based on the same principle are key factors4,5 in the future large scale application of solar energy.The future of PV cells looks good, also economically: the EU expects6 the same price for PV electricity as for fossil-based electricity by the year 2010. The same holds for the required amount of land to place the solar modules equipped with PV cells. To give an impression: the Table 1 Situation analysis 1995 and prognosis for 2040 Critical global data: 1995 2040 Population 5 3 109 10 3 109 Energy consumption/EJ 350 900 Energy consumption per capita/W 2200 3000 Agricultural land/ha 3.4 3 109 2.8 3 109 Organic materials/ton 0.3 3 103 1.0 3 103 ha = hectare = 104 m2.world wide electricity consumption in 1990 could be produced on 10 million ha—this figure can be compared, e.g., with the 225 million ha required to produce the world wide wheat consumption.In a scenario without fossil fuel, solar energy may also be the starting point of a hydrogen economy by electrolysis of water. Hydrogen gas is a clean and powerful fuel. Also, hydrogen is important in making NH 3, and reaction with CO (or CO2) yields CH3OH, a key chemical for the manufacture of other organic compounds. This part on energy is concluded with a scenario as described by the Shell company7,8 (Table 2).The Table speaks for itself. Biomass A plant is a ‘growth machine’: the above-ground part catches light and CO2 (0.03% in the atmosphere), the below-ground part absorbs water and mineral nutrients. The combined intake enables the plant to produce—along ingenious routes—biomass that can be invested in growth above, and below, ground or stored.By photosynthesis, according to the reaction nC O2 + nH2O ? ( C H2O )n + nO2, and by further biosynthesis numerous carbohydrates are formed, the composition of which depends on the type of plant. We are all familiar with monosaccharides (glucose and fructose), disaccharides (sucrose) and polysaccharides (e . g ., starch, cellulose, inulin).Moreover plants manufacture systems containing less oxygen, like triglycerides and terpenes, e t c . The maximum theoretical yield of photosynthesis, related to white light, is 6.6%. The maximum values reached in practice9 are 2.4 to 3.2% for C4 plants (sugar cane, sorghum, corn, miscanthus) and 1.7 to 1.9% for C3 plants (sugar beet, eucalyptus). On average, the yield of photosynthesis is around 0.5%.Nevertheless, under favourable circumstances, 30–70 tons of dry biomass can be harvested per hectare and per annum. It is anticipated (see Rabbinge10,11) that plant breeding and genetic modification of plants will further increase the yields of biomass per hectare (and the biomass composition), though the growth will be less fast than before.12 Thus global productivity increases (rises in cereal yields per hectare) have been reduced from 2.2% per year in the period 1967–1982 to 1.5% per year in 1982–1994.According to Eggersdorfer et al.13 the world biomass production amounts to 170 000 million tons per annum, of which, however, only 3% or 6000 million tons per annum are being cultivated, harvested and used (food and non-food).In Fig. 1, data about the quantities of renewable and fossil resources are given. Note that the total use figures (for 1992) for biomass and fossil resources are about the same. Table 2 Possible energy scenario 1990–20407,8 1990 2040 Energy Source EJ ± % EJ ± % Trad. biomass 55 16 60 6 Coal 70 20 180 18 Oil 130 38 160 17 Gas 55 16 140 14 Hydro 18 5 50 5 Nuclear 17 5 60 6 Wind — — 70 7 New biomass — — 120 13 Solar — — 130 14 Total 350 100 1000 100 108 Green Chemistry April 1999Fig. 1 Renewable and fossil resources (after Eggersdorder et al.13).More than 90% of fossil resources are used for energy generation and less than 10% for organic raw materials. Biomass production, apart from wood for paper (200 million tons per annum) and construction material, is mainly used for food.The medium-term prospects for the production of organic materials and chemicals from biomass have been reviewed by Morris and Ahmed14 under the provocative title ‘The Carbohydrate Economy—Making Chemical and Industrial Materials from Plant Matter’. The authors give many quantitative data as to how and for which products the transition from plant-based to fossil raw materials has taken place. They expect that in the next 50 years biomass will remain the main raw material for food production but will also become again a major raw material for the organo-chemical industry.Before discussing how the desired materials can be made from biomass, we first need to answer the question of how much biomass can be produced on planet earth to: a feed 9–10 billion people—first priority b provide the population with the required organic materials— second priority c make a contribution to the energy needs In 1980, Bruin8 reported extensive calculations on the energy flows in the agricultural/food system (see Fig. 2). Although the efficiency of photosynthesis is very low, the total result of photosynthesis is large.A flow of about 95 TW (1 TW = 31.7 EJ a21 is fixed in biomass, about 30 TW of which are in the form of poorly accessible phytoplankton in the oceans, about 45 TW are in forests and about 20 TW in other plants, 5 TW of the latter being contributed by agriculture. These flows are impressive indeed when compared to the present world energy consumption of about 9 TW. From Fig. 2 one may derive that the conversion yields from biomass to food and wood are low. Food consumption amounts to 0.52 TW (= 2700 kcal per capita per day, in 1995). Wood for human use amounts to 1 TW, fossil fuels input is 9 TW. For the 1.52 TW of consumed biomass, 70 TW solar energy was used. When corrected for respiration losses, maintenance of terrestrial plants and forests, and for phytoplankton growth, a net energy consumption of about 37 TW yields 1.52 TW biomass (food and wood) for use by humans.Hence the yield of this conversion system is only 4%. In particular, the conversion of biomass to meat and milk via cattle has a very low yield. Scenario 2040 The assumptions for 2040 are summarised in Table 3. How much agricultural land will be required to feed 10 billion people with 2500 kcal day21? Bruin calculated15 this for a standard daily diet per capita consisting of 480 g grain, 750 g milk and 80 g meat.Required for grain (22 tons ha21 a21) : 159 3 106 ha Required for milk and meat : 495 3 106 ha Oils and fats, vegetables, sugar, fruit, coffee, tea, etc. : 833 3 106 ha Assuming that the agricultural system functions with 75% of the maximum yield, we arrive at a figure of 2.0 3 109 ha required for food production in 2040. In energy units, this agricultural system delivers: food 38 EJ a21 (= 4% of total energy requirement), straw 32 EJ a21, manure 16 EJ a21.Hence, for non-food biomass production 0.8 3 109 ha remain, which, at a maximum yield (estimate: 50 tons ha21 a21), may deliver 40 3109 tons a21 biomass.Without adverse ecological consequences, forests may deliver another 4 3 109 tons a21. Waste streams are expected to contribute another 5 3 109 tons a21. In conclusion, in 2040 there will be available for non-food application 50 3 109 tons a21 of biomass, or about 25 3109 tons of carbon for organics, which was our second priority after food. We estimate the demand for organic raw materials to be 1000 3 106 tons a21 in 2040 (presently 300 3 106 tons a21), which, with a conversion yield of 20%, could be produced from 5 3 109 ton biomass.The third priority is energy from biomass. There still remains 45 3 109 tons of biomass for this purpose, which could deliver in energy terms 45 3 109 3 15 3 109 3 0.3 or about 200 EJ a21. This provides a substantial contribution to the energy needs of 2040 (935 EJ); the remainder has to come, in a scenario without fossil fuel, from solar energy and other forms of renewable resources (water, wind, geothermal, etc.).It may be noted that our figure of 200 EJ energy contribution from biomass in 2040 corresponds reasonably well with the figures (Table 2) provided by Shell (180 EJ in 2040). From all that has been said so far it will be clear that our conclusions are very sensitive to: * maximum available agricultural land * water supply * yield of agricultural system * size of world population * energy consumption per capita * conversion yields of biomass to organic materials and energy Conditions There are a number of conditions underlying the biomass availability conclusion. 1. The conclusion is valid judged from a global perspective.At regional and national levels enormous differences may occur depending on the degree of technical development, climate, nature of the soil and socio-economic factors. 2. The scenario proposed requires a world-wide infrastructure, e.g., certain agricultural products especially where the yield is highest, PV cells particularly where most solar energy can be harvested, etc. This global infrastructure will be difficult to realise from a political and social point of view. 3. The conclusion holds only under the condition of continuing technological progress with a time horizon of more than 10 years. This is a heavy responsibility for national governments, the European Union and the United Nations. Green Chemistry April 1999 1094.There is no reason for pessimism regarding the required yield improvement in agriculture. Rabbinge10,11 pointed to the fact that, despite the growth in population, the food situation in the world has improved considerably during the last 100 years, even to the extent that Europe and the USA have difficulties in coping with the food surpluses as a consequence of the continuing productivity improvement in agriculture, e.g., from 2 tonnes wheat per hectare in 1900 to 8 tonnes wheat per hectare in Europe today.The authors also calculated that, if the agricultural land in Europe were optimally used, the total European harvest of agricultural products could be produced on 50 million ha as compared with 130 million ha today. 5. Our conclusion is based on optimum agricultural yields.These can only be reached by growing plants in areas where the conditions for those plants are optimal (availability of water, nutrients, etc.). In optimising the world agricultural system, plant breeding, plant biotechnology, and expert computer systems will play an important role. Expert computer systems will enable the agrarian to control plant diseases, to reduce water consumption and to manage the various crops efficiently.Table 3 Assumptions scenario 2040 World population 10 3 109 Available agricultural land/ha 2.8 3 109 Tropical forest/ha 3.7 3 109 Forest in moderate climate/ha 1.7 3 109 Urban area/ha 1.0 3 109 Deserts, tundra etc./ha 2.0 3 109 Food per capita/W; kcal day21 120; 2500 Food total per annum/TW; EJ a21 1.2; 38 Energy per capita/W 3000 Energy total/TW; EJ a21 30; 935 Organic materials/tonnes a21 1000 3 106 6.Genetically-modified crops should be accepted world-wide. In view of the rapid developments in North America, the authors feel confident about this. 7. Overall, we cannot escape the conclusion that, in 2040, sustainable technology will also be able to deliver the required products, but that the real core of the sustainability problem is of socio-political nature (where to produce what).Biomass as raw material for organic materials and chemicals Biomass may be utilised in different ways to provide us with organic compounds and materials: a. Nature already produces the desired structures and isolation of these components mostly requires only physical methods.E x a m p l e s : polysaccharides (cellulose, starch, alginate, pectin, agar, chitin, inulin, etc.), disaccharides (sucrose and lactose), triglycerides, lecithin, natural rubber, gelatin, flavours and fragrances, etc. Some present-day production volumes are sucrose 115 3 106 t a21, triglycerides 85 3 106 t a21, natural rubber 5.5 3 106 t a21. Cotton, the natural cellulose fiber,16 is produced in a volume of 20 3 106 t a21, an amount which equals the sum17 of all synthetic fibers (volume order: polyester > polyamide > acrylic). The possibilities to produce organic chemicals directly by and from the plant by means of plant biotechnology will increase dramatically.The plant is the ‘plant’ of the future. b. One step (bio)chemical modification of naturally produced structures under a.Examples: cellulose and starch derivatives, glucose and fructose, glycerol, fatty acids; ethanol, citric acid, glutamic acid and lactic acid by fermentation. Lactulose, lactitol and lactobionic acid by isomerization, hydrogenation and oxidation, respectively, from lactose. Nature offers various fine starting materials for pharmaceuticals.Thus morphine is converted by one methylation step into the cough medicine codeine (200 t a21) (whereas one acetylation step leads to heroin). 110 Green Chemistry April 1999 Fig. 2 Energy flows in the agricultural/food system.8c. In several steps organic chemicals and organic materials are obtained from natural products. Examples include: ethanol can be converted to today’s No. 1 organic chemical, ethylene; sorbitol and mannitol by hydrogenation of glucose and sucrose, respectively; vitamin C in several steps from glucose; (S)-b-hydroxy-t-butyrolactone in two steps from lactose;18 (2)-menthol in six steps from bpinene; 19 fatty alcohols and amines from triglycerides; alkyl polyglucosides from glucose and fatty alcohols, etc.; succinic acid from glucose and CO2 (!).20 d.‘Back to C1-Chemistry’ by using biomass as the carbon and hydrogen source, converting it into small fragments (synthesis gas) and building it up again to the desired structures. In the above the focus has been on chemical structures and less on the product areas. For some large product groups it can be stated that the green label (renewables-based) is accepted as a selling advantage.We mention: flavours and fragrances; cosmetics; adhesives; lubricants; detergent formulations; agrochemicals. Primary conversion technologies of biomass In Fig. 3, an overview is given of the candidate primary conversion technologies of biomass, ranked according to water content. The three most important technologies will be dealt with in some detail. Fig. 3 Biomass conversion technologies. Gasification Biomass can be converted into power plant fuel by gasification21 with a high yield and in an environmentally friendly fashion. Also, in the longer term, the economics of this process look good, notably for energy crops. The gasification takes place with air, at temperatures of around 850 °C. The gas consists of 13% H2, 17% CO, 4% CH4, 12% CO2, 13% H2O and 40% N2 with a caloric value of 6 MJ m23.In the 2040 scenario, 80 EJ a21 could be produced from waste streams and 200 EJ a21 from energy crops, on a global scale. The removal of sulfur-containing components, tar, char and ash from the gas is critical for use in gas turbines and for methanol production. The technology is promising. Many pilot plants are in operation, large installations are in the planning phase.The gas could presumably also be used in Fischer–Tropsch synthesis. Hydrothermolysis During the period 1982–1993, the Royal Dutch Shell Laboratory developed a process to convert biomass into liquid fuel, so-called b i o - c r u d e .2 2 This process is called HTU (Hydro-Thermal Upgrading). First biomass is treated in an aqueous slurry at 200 °C and 30 bar, followed by a treatment at 330 °C and 200 bar.This process results in a bio-crude, an oil with low oxygen content, which can be further upgraded by a catalytic hydrodeoxygenation step to a high quality naphtha or diesel oil with very low oxygen, nitrogen and sulfur contents. The oil yield is about 40% based on the biomass feed stock. Wood, agricultural and domestic (green) waste streams were successfully applied as feed stocks.According to Shell, this HTU process is the cheapest route to liquid biofuels. Its cost price would be in the order of $20–40 per barrel,22 as compared with fossil crude oil today at about $12 per barrel. Hence, the process is not yet economical under the present tax regime. This HTU process and many variants of this process lead directly to bio-crude, from which the known transport fuels and petrochemicals can be manufactured, without the extra sulfur-removing steps, etc., which are necessary with fossil fuel.Fermentation to ethanol By fermentation of biomass (sugars, grain, cellulose, etc.) with yeast a 6.5–11% ethanol in water solution is formed,23 from which 95 or 100% ethanol can be obtained by distillation (or membrane-filtration, or distillation-adsorption). Depending on the feed stock, a chemical or enzymatic hydrolysis is sometimes required first, to convert the biomass into monosaccharides.Alcohol is a raw material for many organic chemicals among which, as was already mentioned, today’s No. 1 organic chemical, ethylene. In India over 400 000 t a21 of alcohol is used24 in making ‘alco-chemicals’ with acetic acid and ethylene glycol as the numbers 1 and 2.Moreover, in India and China aqueous alcohol is directly applied in aromatic ethylation (ethylbenzene, 1,4- diethylbenzene and 4-ethyltoluene). Ethanol can also be used directly as a liquid fuel. The technology is well developed and applied on a large scale in the USA (corn-based) and in Brazil (sugar cane-based).It is expected that, as a result of better enzymatic hydrolysis and ethanol processing together with rising fossil fuel prices, bioethanol prices will become competitive with gasoline in 2010. A hydrogen economy? Solar energy, by means of PV or similar cells, will be (in the authors’ opinion) a main source of energy in the future. This technology seems essential in creating sustainable technological growth without fossil resources.Assuming that: a. 1% of the sunlight received by our planet (2.8 3 106 EJ a21) is captured by solar cells; b. the efficiency of the conversion from solar energy to electricity amounts to 20%; c. the yield of electrolysis of water by solar electricity is 60%; an energy-equivalent of 3360 EJ a21 could be produced in the form of hydrogen, which amounts to more than three times the required energy in 2040! Many improvements of the technology are possible; the choice of the semi-conductor material, generally silicon, the fixation of this material on film, the lay-out of modules, the architecture of modules, the storage of energy in batteries and in accumulators, etc.A step to ‘artificial’ photosynthesis is the development of the Grätzel cell5 by adsorption of a ruthenium complex on nanocrys- Green Chemistry April 1999 111talline TiO2.Owing to the large surface area of the TiO2, the yield of these solar cells is higher than that of conventional PV cells. As stated before, solar energy can lead directly to H2 as a key chemical for many applications.H2 may be used as such (e.g., fuel in space shuttles) or stored in metals such as Pd. Hydrogen can also be stored chemically by reaction with, e.g., CO2 to CH3OH. In turn, CH3OH is a very good liquid fuel and a key chemical in the production of many organic chemicals.25 Fig. 4 shows the versatility of methanol as a starting compound. Fig. 4 Methanol as a key chemical.In the decades to come hydrogen may also play an important role as hydrocarbon-derived fuel in fuel-cell engined cars. In the Shell/Daimler/Ford/Ballard concept conventional transport fuels are converted in a number of catalytic steps to H2 and CO2 after which hydrogen acts as clean fuel. The art is to get the CO content down to a few ppm. In conclusion, H2 is a powerful energy carrier and, via methanol, an essential starting material for (bulk) chemicals.Therefore, we may witness the start of a hydrogen-based economy, together with the plant-based economy. Approach of the present top organic materials We have investigated how the current top eight organic materials could be made from biomass. These top eight (in volume, 1998) are: polyethylene (48 3 106 t), polypropylene (23 3 106 t), polyvinyl chloride (26 3 106 t), polyethylene terephthalate (13 3 106 t), polystyrene (14 3 106 t), butadiene/co-polymers (8 3 106 t), phenol resins (5.5 3 106 t) and polyamides (4 3 106 t).26 The following remarks—by way of examples—can be made: Polyethylene An obvious route (cf.Fig. 5) is from carbohydrates to ethanol and to ethylene. Substantial mass decrease occurs.Another route to the monomers ethylene and propylene is the selective conversion of methanol over suitable zeolites to a package of C2, C3 and C4 olefins. It may be noted that the monomer ethylene (ca. 75 3 106 t a21) is also the starting compound for the bulk chemicals ethylene oxide, ethylene glycol, ethylbenzene and styrene. Fig. 5 Biomass-based routes to lower olefins.Polypropylene Here the methanol-to-olefins (MTO) process27 seems the appro - priate route of the future. The monomer (»45 3 106 t a21) also serves the bulk compounds propylene oxide, acrylonitrile, acrolein and acrylic acid as a starting chemical. Butadiene (co-)polymers Ethanol can be converted into butadiene by the reaction sequence dehydrogenation to acetaldehyde, aldol condensation, dehydration.This sequence can be carried out in one process step28 over a MgO–SiO2 catalyst (Lebedew process) which seems an interesting option. Polystyrene Diels–Alder cyclo-addition of butadiene over a Cu(i) zeolite gives vinylcyclohexene in excellent yield (Fig. 6).29 Recently the technology has been developed to convert vinylcyclohexene directly into styrene.Fig. 6 Biomass-based routes to butadiene and styrene. Polyalkylene terephthalates Here the most important and still fast growing material is based on ethylene glycol and dimethyl terephthalate. Fig. 7 shows two green routes to terephthalic acid. The two-step route based on the cheap terpene limonene, seems the method of choice. The sequence starting from fructose consists of more steps than the limonene route and involves one difficult step: the Diels–Alder addition of ethylene to 2,5-furandicarboxylic acid.It may be noted that nature rarely delivers aromatic structures. Exceptions are the complex wood constituent lignin and the natural crosslinking agent ferulic acid. The question arises whether 2,5- furandicarboxylic acid could develop towards a ‘bioterephthalic acid’ by acting as a monomer in preparing polyesters and polyamides.As to the diol component of the polyalkylene terephthalates, green routes are as follows: (i) ethylene glycol via ethanol, ethene, ethene oxide, 112 Green Chemistry April 1999Fig. 7 Green routes to terephthalic acid. (ii) 1,4-butanediol by hydrogenation of succinic acid (ex glucose/ carbon dioxide), and (iii) 1,3-propanediol by fermentation of glucose as recently announced by DuPont.The latter process will enter into competition with the conventional 1,3-propanediol process via acrolein (Degussa) and the modern ethene oxide/carbon monoxide process (Shell). Substituting petro-based materials by green materials Although novel synthetic routes need to be developed when starting from biomass rather than from fossil organic material to produce the ‘top eight’ bulk materials, it seems to be technologically feasible.On the other hand, when the raw material source shifts from fossil to green, new materials will be discovered and developed with the same or new properties. See for instance the possibilities (Fig. 8) offered by the abundant natural mono-unsaturated Fig. 8 Oleic acid as key chemical. C18-acid oleic acid. 30 Upon oxidation or metathesis C9- and C18- dicarboxylic acids, respectively, are obtained that may constitute valuable monomers for new polymeric materials. Moreover, we mention in this context: — processed starch (e.g. with some glycerol or sorbitol added) as a biodegradable substitute of non-biodegradable polystyrene foam, e.g.in packaging applications; — poly-l-lactate (PURAC, NL) as a high-value biodegradable material for surgery and at short notice technical poly-l-lactate (Dow Chemical/Cargill) will be manufactured for relatively low-cost applications in which biodegradability is an issue; — nonionic surfactants obtained by alkylation (or acylation) of mono- and disaccharides (Fig. 9). Fig. 9 Alkylated sugars, nonionic surfactants. Here two natural sources join forces: the triglycerides, which provide (by transesterification and hydrogenation) the fatty alco - hols, and the sugars which replace the petrochemical ethylene oxide as the hydrophilic part of the surfactant. Note that the sugar is able to supply much more structural variation than ethylene oxide. Alkylated glucose (APG, 70 000 t a21, Henkel AG) is already on the market.31 In our group the alkylation of fructose has been studied.32 Other commercial products include sorbitan esters, Nmethyl gluconamides and sucrose fatty acid esters.Life cycle inventory and analysis Of course a fair comparison between petrochemical- and renewable- based products requires an analysis of all inputs and emissions involved.A life cycle inventory (LCI) covers the growing or mining of the raw materials, all energy components involved during transport and processing, emissions to air and water, etc., up to the end-product. An LCI is part of the complete life cycle analysis (LCA). LCIs have become popularly known as “cradle to factory gate studies” whereas LCAs are named ‘cradle to grave studies’.LCIs for conventional as well as for fully renewable-based surfactants have been made by Henkel workers.33 In a recent UK project34 LCAs were made for several surfactants and lubricants and point to environmental advantages of oleochemical over petrochemical feedstocks. Some conclusions * Scenario 2040 (almost doubling of world population, higher average standard of living, reduction of environmental pollution) seems technologically possible without the utilisation of fossil resources.* This technological approach requires, amongst other things, the massive development and application of solar energy and biomass for food, organic raw materials, and energy. Green Chemistry April 1999 113* The plant will be the ‘plant’ of the future.* Ultimately the limit to global growth will be the amount of agricultural land on earth. * Biomass conversion to organic raw materials is able to replace the existing organic chemistry which is mainly based on fossil resources while adding structurally new materials. * Altogether, much creativity is required of mankind. References 1 C. J. Campbell and J. H. Laherrère, The End of Cheap Oil, in Sci.Am., March 1998, p. 60. 2 UN estimates, medium fertility, see National Geographic, October 1998, Millennium supplement: population. NB If the average woman’s fertility is 2.6 then the population will be 27 billion in 2150; if it is 1.6 the total will drop to 3.6 billion in the year 2150. If fertility stabilizes at 2.0 the number will be 10.8 billion. 3 C. Okkerse and H.van Bekkum, in Starch 96, ed. H. van Doren and N. van Swaay, Zestec bv/Carbohydrate Research Foundation, The Hague, 1997, p. 1. 4 D. T. N. Kimman, Chemisch Magazine, October 1995, 431. 5 A. Hagfeldt and M. Gratzel, Chem. Rev., 1995, 95, 49. 6 The European Renewable Energy Study (TERES). ISBN 92- 826-6450-3 (volumes 1 to 4) Brussels–Luxembourg, 1994. 7 Shell Briefing Service, November 1994, Renewable Energy. 8 Shell Selected Paper, Peter Kassler, November 1994. 9 J. A. Barsham, Soil and Crop Science Society of Florida, Proceedings, 1983, 42, 2. 10 R. Rabbinge, Simulation Monographs, Pudoc, Wageningen, 1990, vol. 34. 11 R. Rabbinge, Chemisch Magazine, October 1995, p. 427. 12 C. C. Mann, Crop Scientists seek a New Revolution, in Science, 1999, 283, 310. 13 M. Eggersdorfer, J. Meijer and P. Eckes, FEMS Microbiol. Rev., 1992, 103, 355. 1 4 D. Morris and I. Ahmed, The Carbohydrate Economy, Institute for Local Self-Reliance, Washington DC, August 1992. 15 S. Bruin, Lecture: Biomass as a source of energy, presented at 13th International TNO Conference, Rotterdam, 27 and 28 March, 1980. 16 H.-G. Fritz et al., Study on production of thermoplastics and fibres based mainly on biological materials, Directorate- General XII, European Commission, 1994. 17 AKZO-Nobel brochure: Man-made fibers, in 1994. 18 Chem. Eng. News, July 13, 1998, p. 66. 19 R. A. Sheldon, Chirotechnology, Marcel Dekker, Inc., New York, 1993, p. 304. 20 M. J. van der Werf, M. V. Guettlez, M. K. Jain and J. G. Zeikus, Arch Microbiol., 1997, 167, 332. 21 C. D. Ouwens, Chemisch Magazine, October 1995, p. 442. 22 F. Goudriaan, Chemisch Weekblad, 1995, 34, 3 and personal communication. 23 B. Mouris, Chem. Ind., 18 June 1984, p. 435. 24 A. M. Kadakia, Chemical Weekly, August 5, 1997, p. 137. 25 J. R. Crocco, Hydrocarbon Processing, May 1994, 66 C. 26 W. C. Kuhlke, Hydrocarbon Processing, May 1994, p. 57. 27 I. E. Maxwell and W. H. J. Stork, Stud. Surf. Sci. Catal., 1991, 58, 616. 28 K. Weissermel and H.-J. Arpe, Industrial Organic Chemistry, 2nd edn., VCH, Weinheim, 1993. 29 I. E. Maxwell, R. S. Downing, J. J. de Boer and S. A. van Langen, J. Catal., 1980, 61, 485; 493. 30 S. Warwel, P. Bavaj, M. Rüsch and B. Wolff, in Perspektiven Nachwachsender Rohstoffe in der Chemie, ed. H. Eierdanz, VCH, Weinheim, 1996, p. 119. 31 APG News 1/95, Publication of Henkel Oleochemie. 3 2 A. M. van der Heijden, Thesis, Delft University of Technology, 1999. 33 F. Hirsinger et al., Tenside Surf. Det., 1995, 32, 128; 171; 193; 398; 420. 34 P. S. Wightman, R. M. Eavis, S. E. Batchelor, K. C. Walker and S. P. Carruthers, Cost Benefit Assessment including Life Cycle Assessment, of oils produced from UK-grown crops compared with mineral oils. Cf. Green Chemistry, February 1999, G6/G7. Paper 8/09539F 114 Green Chemistry April 1999

ISSN:1463-9262    

DOI:10.1039/a809539f

出版商:RSC    

年代:1999

数据来源: RSC