The Technology Challenges

This section reviews the challenges faced by the exploitation of CO2, stranded methane, and biomass residues in an attempt to highlight areas where technology breakthroughs and research efforts are needed.

10.2.1 Exploitation of CO2

Phosgene is commercially obtained by passing carbon monoxide and chlorine over activated carbon.3 Around 7 million tons of phosgene are produced worldwide, 85% of which is utilized in the production of isocyanates for polyurethanes, 10% in the production of aromatic polycarbonate, and 5% in fine chemicals. Phosgene is a highly poisonous material that requires extreme handling precautions and that can generate corrosive by-products such as HCl. An advantageous alternative would be to replace phosgene with dimethylcarbonate (DMC), which

is a nontoxic, nonchlorine-containing molecule. Around 70,000 tons/year of DMC is currently produced by oxidative carbonylation of carbon monoxide4 in a process not economically viable for large-scale production. To overcome this problem, the direct synthesis of DMC from methanol and CO2 is being actively researched.5 More specifically, catalyst development and ways to remove the water created during the reaction are being investigated to find a way to shift the reaction equilibrium toward better DMC yield.

Beyond DMC and its utilization as methylating agent or as polycondensation building block,6 CO2 can be used as feedstock in a number of reactions that are highlighted in Figure 10.7. Most of these reactions need further research before CO2 can be utilized as a mass feedstock for the chemical industry.

10.2.2 Exploitation of Stranded Methane

Replacing one or several of the hydrogen atoms in methane by one or several other atoms than hydrogen automatically creates secondary or tertiary C-H bonds. Secondary and tertiary C-H bonds are more reactive than a primary C-H bond. During oxidation reactions, this leads to an easier oxidation of the reaction products than methane, and consequently to a low(er) reaction selectivity. Such reactions therefore produce complicated reactant mixtures that require costly purification operations to isolate the targeted end molecule. Chemists have designed two strategies to overcome this challenge. However, none of them have yielded economically viable processes so far.

• Full Oxidation Followed by Recombination (Two-step Route): This strategy is exemplified by the production of syngas (CO/H2) followed by Fisher -Tropsch conversion of syngas into higher alkane or into methanol, depending on reaction conditions.7 Fisher-Tropsch chemistry works by radical polymerization, which leads to product mixtures. Such mixtures, however, necessitate cleaning steps to produce chemical intermediates with high purity, which add cost to the process. Additionally, the current catalysts for methane to syngas conversion suffer from coking, further limiting the economic viability of the process. Novel catalysts need to be developed while keeping economies of scale.

• Selective Coupling in One Step: Novel catalysts, for example, for methane oxidative coupling, combined with the usage of novel process engineering technologies (microreactors for highly exothermic reaction, membrane reactors for early purification, etc.) are being actively developed.

A list of some of these on-going efforts and their current status follows:

• Methane to Methanol and/or Formaldehyde: Recent research indicates that a catalyst system in the presence of H2SO4 can convert methane directly into methanol. Homogeneous catalyst systems show promise. Also, heterogeneous Fe-ZSM-5 catalysts are reported to be attractive for this chemistry. Novel plasma reactors to generate hydroxyl radicals are also being investigated.

• Methane to Ethylene: One target is to achieve an ethylene selectively of 90% at a methane conversion level of 50% in a single pass. Additionally, design of novel recycle reactors or membrane systems (to remove the ethyl-ene produced) remain part of the active research.

• Methane to Benzene: Both oxidative and nonoxidative routes have been reported. Most attention has been directed at nonoxidative aromatization. In particular, Chinese workers are active in this field. Recently, attractive results have been reported for Mo-loaded HZSM-5 catalysts.

• Methane and Toluene to Styrene: Basic catalysts in the presence of oxygen and/or air are reported to be attractive catalysts for this reaction. Most research was performed in the late 1980s and early 1990s. The fundamentals resemble the oxidative coupling reaction of methane to ethylene.

These reactions require a lot more research to reach fruition, most specifically in the field of long-term catalyst stability and selectivity. Furthermore, in many instances the reaction mechanism and the active catalytic site are still poorly understood. Issues such as the importance of site isolation and phase cooperation

(in mixed oxide catalysts) inducing synergistic effects are increasingly receiving attention.

10.2.3 Exploitation of the Biomass

The molecular structures of the main biomass constituents are given in Figures 10.8 and 10.9. These structures induce a physicochemical behavior that is markedly different from the behavior of the hydrocarbons contained in crude oil (see Table 10.2). Note that the relative thermal fragility of the biomass molecular structures encourages the chemist to prefer thermally mild and therefore low-energy-consuming conversion techniques such as fermentation or hydrother-molysis to exploit biomass (see Figure 10.10).

As a matter of fact, most of the processes currently developed to generate bio-chemicals out of biomass involve fermentation of starch originating from corn, wheat, or rice, for example. The various chemicals obtainable from theses processes and their end applications are listed in Table 10.3. A lot of these fermented biochemicals, however, are not yet economically competitive compared with their petrochemical equivalent, essentially due to the large capital investment in equipment and land needed to implement the fermentation process on an industrial scale. An additional disadvantage of this route is that it competes with feedstock needed by the food industry. More research to reduce the costs of fermentation technology is needed.

Agricultural residues (stem, leaves, etc.) currently left in the fields after harvesting are made of cellulose, hemicellulose, and lignin. They are not competing with the feedstock for the food industry.

Starch

Starch

Amylo pektin

Cellulose

Cellobiose-Einheit oh hoho ch20h

Cellobiose-Einheit oh ch20h

Figure 10.8 Bio-based feedstock constituents.

ch2oh

Hemicellulose: Xylose and arabinose

Figure 10.8 Bio-based feedstock constituents.

Figure 10.9 Bio-based feedstock constituent: lignin.

Water: 15%

> Burning

> Hydrothermolysis (250-400°C, Hz)

• Anaerobic digestion

Figure 10.10 Carbohydrate and lignin processing technologies.

TABLE 10.2 Physicochemical Behavior of Feedstock

Crude-Oil-Based Feedstock

Biomass-Based Feedstock

Low degree of oxygenation

High degree of oxygenation

High volatility

Low volatility

Thermally stable

Thermal fragility

Unlike starch, which is amorphous, cellulose is fermented with difficulty because of its semicrystalline structure. As a consequence, ethanol fermented from cellulose using the latest generation enzymes would still be more expensive

TABLE 10.3 Industrial Strach-Based Product Opportunities

Technology Platform Sugar fermentation

Sugar fermentation and ther-mochemical processing

Chemical Lactic acid

Polylactic acid Ethyllactate

1, 3-Propanediol

Succinic acid

Succinic acid derivatives Tetrahydroturan 1, 4-butanediol g-butyrolactone N-methylpytrolldone

3-Hydroxypropionic acid and Terrivales Acrylic acid Acrylonitrille Acrylamide N-Butanol Itaconic acid

Propylene glycol

Levullnic acid and derivatives

Me-Tetrahydroturan 8-amino levullinic acid

Application

Acidulant, electroplating additive, textile/leather auxiliary

Thermoplastic polymer Solvent, intermediate

Specially resins

Surfactant, food, pharma, antibiotics, amino acid and vitamin production Solvents, adhesives, paints, printing inks, tapes, plasti-cizer, emulsifier, deicing compound, herbicide

Acrylates, acrylic polymers, fibers, and resins

Solvent, Plasticizer, polymer, reactive comonomer

Solvent, chain extender in PU, antifreeze, plasticizer oxygenates for fuels biodegradable herbicide, bisphenol A alternative than ethanol fermented from starch, which is itself noncompetitive with petrochemical ethanol10 (when government subsidies are not taken into account).

Cellulose and hemicellulose can be converted to biochemicals via acid treatment. These treatments, however, generate large aqueous waste streams (acid neutralization), which makes them economically uncompetitive.11 Recently, a process involving cellulose and hemicellulose hydrolysis in supercritical water was patented.12 This represents a very promising route to exploit renewable resources and agricultural residues. Figure 10.11 shows the cascade of biochemicals that could be obtained from cellulose, hemicellulose, and lignin when combining the hydrolysis process in super critical water with fermentation and acid treatment.

It is thought that this cascade of products could be manufactured in a biorefin-ery by integrating all the unit operations needed to convert biomass into chemicals. A lot more research is needed, however, to develop the hydrothermolysis process and high-efficiency/low-cost fermentation and acid treatments. An efficient exploitation of lignin (the only natural source of aromatic rings) has also to be developed.

LCF-Bloreflnery

(Ligno-cellulosic feedstock biorefinery)

Technical llgnln

Alkaline hydrolysis oxidation

Vanlllne and derlv.

• Syringa aldehyde

Reductive hydrothermolysis

Phenols and Deriv.

• Phenols, benzene

Lignin Polymers and Resins for:

• Bonding material

• Artificial resin

• Resinous exchanger

Ligno-cellulosic feedstock (LCF)

Fractionation (e.g. "aquasolv"-process)

Cellulose oligomers (a) polymerization

Hemicellulose/polyoses

Cellulose

Hydrolysis acid treatment

Furfural

Furan and deriv.

• THF-butanediol

• THF-polyurethane

Hydrolysis /ferm.

Hydrothermolysis t

Polyalcohols

Reductive amination

Acid treatment

and derivatives

Levullnlc Acid (LEVA)

• Diisocyanates (Polyurethanes)

LEVA-esters f • Ethyl levulinate

Higher esters (Fuel additive, solvents)

Angelica lactones

(Solvents)

Poly-(a-angellca laetone)(Polymers, plasticizers)_

Polycarbonate

Figure 10.11 Example of a biorefinery. A ligno-cellulosic biorefinery.

10.2.4 Exploitation of Vegetable Oils

Vegetable oils represent only 5% of the renewable resources available. Today, vegetable oils currently provide a marginal carbon feedstock contribution to the chemical industry in such applications as solvents, surfactants, and lubricants. Vegetable oils may, however, play a much more important role in the future. They are mixtures of fatty acid trigclycerides whose typical molecular structures are given in Figure 10.12.

Unlike carbohydrates, which are highly functionalized with an average of one hydroxyl group per carbon, and therefore require cumbersome protection/depro-tection protocols to ensure high selectivity during chemical modification, fatty acids are bifunctional. Fatty acids can therefore be directly converted in a small number of steps (i.e., less cost intensive than carbohydrates) into a number of building blocks suitable for a variety of specialty applications, such as engineering thermoplastics (nylon, polyester) and thermoset resins (epoxies and poly-urethanes). Some of the chemistries available are shown in Figure 10.13. Among the chemistries, vegetable oil metathesis (see Figure 10.14) is of particular interest, as it allows a reduction in the fatty acid chain length. This chain-length reduction greatly enhances the chemical resistance and the mechanical performances of polymers based on such intermediates.13 Metathesis catalysts that are not poisoned by the polar functions present in vegetable oils have been developed,14 but are not yet advanced enough to permit an economically viable process.

Soybean oil is a statistical mixture of glycerol esters of palmitic acid (10%), stearic acid (3%), oleic acid (23%), linoleic acid (55%), and linolenic acid (9%).

Soybean oil is a statistical mixture of glycerol esters of palmitic acid (10%), stearic acid (3%), oleic acid (23%), linoleic acid (55%), and linolenic acid (9%).

Linolenic acid

Figure 10.12 Example of seed oil chemical composition (soybean oil).

Linolenic acid

Figure 10.12 Example of seed oil chemical composition (soybean oil).

Further research is needed in that area. It is thought that vegetable oil biorefi-neries working independently, or integrated with lignocellulosic biorefineries, will provide a diversified portfolio of chemicals, as petrochemical refineries do today. Beyond developing an economically viable metathesis, one of the main challenge in the successful development of an oleo-refinery is to find valuable outlets for glycerin.

Figure 10.13 Examples of chemistries available for vegetable oils.

Figure 10.14

biorefinery.

Cross-metathesis of ethylene and methyl oleate (ethenolysis) and associated

Figure 10.14

biorefinery.

Cross-metathesis of ethylene and methyl oleate (ethenolysis) and associated

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