How well can renewable resources mimic commodity monomers and polymers?

Authors

  • Robert T. Mathers

    Corresponding author
    1. Department of Chemistry, Pennsylvania State University, New Kensington, Pennsylvania 15068
    • Department of Chemistry, Pennsylvania State University, New Kensington, Pennsylvania 15068
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Abstract

This highlight discusses the recent progress aimed at maximizing the potential of biomass for commodity monomers and polymers. These efforts are no longer solely academic issues. In recent years, a variety of alkene, diene, aromatic, and condensation type monomers have utilized renewable resources, such as cellulose, lignin, plant oils, starches, and monoterpenes in commercial polymers. Generally, these multifaceted efforts involve pretreatment of biomass with thermal, chemical, or physical methods followed by a catalyst sequence that entails a combination of acid-catalysis, bio-catalysis, or metal-based catalysis. In this regard, synthesis strategies for ethylene, propylene, α-olefins, methylmethacrylate, 1,3-butadiene, 1,3-cyclohexadiene, isoprene, 1,3-propanediol, 1,4-butanediol, and terephthalic acid are discussed as well as opportunities for other renewable-based monomers. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012

INTRODUCTION

In recent years, a great deal of research has been directed toward green polymerization methods.1 These multifaceted efforts have resulted in a variety of synthesis strategies to utilize renewable resources.1–3 Many of these systems are composed of entirely renewable materials, while others combine renewable components with petroleum-based monomers. Much of the impetus for utilizing renewable resources in polymer science has resulted from the field of green chemistry which advocates for the use of renewable feedstocks.4 Currently, the scope of renewable resources in polymer science has begun to permeate commodity monomers and polymers which have typically utilized petroleum, natural gas, and coal-based starting materials. Consequently, the use of renewable resources is no longer an academic curiosity. Due to recent advances, a review of the status of renewable resources within the context of commodity monomers would be worthwhile for academic, government, and industrial scientists.

The conversion of biomass to chemicals for commodity monomers is intertwined with catalysis. Although an understanding of the complexity of catalytic mechanisms is often challenging, catalysts reduce waste and provide access to interesting molecules through reactions that have a high degree of atom economy. Due to the importance of catalytic reactions in industrial and academic settings, catalysis is a common thread seen through many examples in this highlight.

Even though catalysis is viewed as the foundation of green chemistry,5 the complexity of synthesizing catalysts which are able to convert renewable resources into chemicals presents a nontrivial problem. In fact, what is becoming increasingly clear is that renewable resources often require a combination of several different types of catalysis. As a result, the shift away from petroleum-based chemicals towards renewable resources is challenging and often requires an interdisciplinary approach.6, 7 For example, biocatalysis with enzymes and microorganisms may precede catalytic transformations with solid acids. Then, this catalyst sequence may be followed by transition metal-based catalysts in an effort to either directly replicate petroleum-based monomers or produce analogues that mimic the physical properties of certain commodity polymers. On a molecular level, converting certain types of renewable resources into chemicals for the purpose of synthesizing commodity monomers often requires increasing the carbon:oxygen ratio. This translates into the progressive reduction in oxygen content so that the monomers will be compatible with certain transition metal polymerization catalysts. For some renewable resources, this means starting with water tolerant processes, such as fermentation, and ending with water intolerant processes.

Although renewable molecules and derivatives of biomass present many exciting possibilities for polymer science,8 the examples in this highlight will focus on monoterpenes, triglycerides, lignin, cellulose, and starch in the context of high-volume polyolefins and thermoplastics. Table 1 lists some of the most important commodity monomers discussed in this highlight. Ethylene is the leading petrochemical in the US and worldwide followed by propylene and vinyl chloride. During the following four-part discussion of alkene, diene, aromatic, and condensation type monomers, this highlight will:

  • 1Identify current commodity polymers with renewable components.
  • 2Suggest possibilities for how renewable resources might contribute to certain commodity polymers. Although this highlight will not access the economical considerations for replacing petroleum-based monomers with a renewable or sustainable equivalent, a direct replacement would be beneficial from a production standpoint.
  • 3Examine how renewable resources can mimic the structural features and physical properties of commodity polymers. Adapting renewable resources to produce analogues of common thermoplastics will allow similar processing conditions.
Table 1. Ranking and Quantities of Important Industrial Monomers and Polymers
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PART 1: PRETREATMENT OF BIOMASS

Generally, biomass will undergo a pretreatment to enhance processing, surface area, and reactivity. The complexity and wide variety of physical, thermal, or chemical pretreatment methods have been extensively reviewed.20–22 In this regard, the efficiency and success of subsequent catalytic reactions with biomass strongly depends on the choice of pretreatment. Although this highlight will focus on catalytic methods that allow renewable resources to mimic commodity monomers and polymers, a brief mention of common pretreatment techniques, such as steam explosion, liquid hot water, dilute acid (H2SO4), lime [Ca(OH)2], and ammonia demonstrate the interdisciplinary nature of utilizing renewable resources. The following four paragraphs illustrate select examples of how pretreatments fit into the overall picture of converting renewable resources into chemicals for commodity monomers.

Biomass, which is water insoluble or difficult to process, often requires physical pretreatment, such as steam explosion or ball milling, before gasification, fast pyrolysis, or hydrocracking.20, 23 The breakdown of crystallinity in cellulose or destruction of the carbon-oxygen framework in lignin helps overcome the structural integrity of renewable resources.23 Often, gasification to produce syngas (CO/H2) is more efficient than directly combusting renewable resources. Additionally, gasification makes transport easier due to removal of water. Although, many finely-tuned industrial processes make syngas from coal, biomass-derived syngas may become more economically viable in the future. Currently, the reduction of tar formation during gasification and pyrolysis of biomass is an important issue.24 Reports of super-critical water gasification show promise for lower temperature processes.25

Chemical pretreatment of lignocellulosic biomass with dilute acids increase surface area, hydrolyze hemicelluloses to xylose, and alter the structure of lignin.21 Since hemicelluloses surround the crystalline cellulose fibers, pretreatment with dilute acids helps promote subsequent reactions on the cellulose. Due to the enormous quantity of commodity polymers produced each year, obtaining adequate amounts of chemicals from renewable resources will require biorefineries that simultaneously produce monomers and fuels to offset energy costs.20 Many of the DOE's top renewable chemicals are produced by via the biorefinery concept. For example, acid-catalyzed hydrolysis of biomass with dilute sulfuric acid produces furfural and levulinic acid.3 Levulinic acid has been utilized by DuPont for pryrrolidones and lactones.26

Certain types of biomass, such as starches obtained from corn, wheat, cassava, sorghum, and sago palm, need to first undergo a water-based enzymatic treatment with amylases to yield glucose before fermentation strategies.27 Lignocellulosic materials also need an initial degradation step to obtain substrates which can be fermented to ethanol.28 Fortunately, the high selectivity of many microorganisms is an excellent method for deoxygenating biomass and increasing the C:O ratio. For example, the C:O ratio for glucose could be increased from 1 to 2 when glucose is transformed to ethanol. Fermentation is often more selective than acid-catalyzed hydrolysis.

Recovery of plant oils necessitates grinding, pressing, or solvent extraction. Afterwards, the monounsaturated and polyunsaturated alkenes are excellent candidates for a variety of rearrangement reactions with metathesis catalysts29 or numerous addition-type reactions, such as those involving acrylates,30, 31 carboxylic acids,32 enones,33, 34 or epoxides.35–38 Plant oils are also well suited for polyurethanes.36, 39 Additionally, nonconjugated alkenes in plant oils are commonly isomerized to conjugated dienes for Diels-Alder reactions.40

PART 2: ALKENE MONOMERS

Ethylene-Based Monomers

As shown in Table 1, the production of ethylene dominates the petrochemical market. Due to the pervasive nature of polymers based on ethylene, a sustainable source of ethylene would be very valuable. However, to make a sizeable impact on the enormous quantity of ethylene consumed worldwide, large quantities of renewable materials are needed. One very promising development involves fermentation of biomass to obtain bio-ethanol.27

The worldwide production of bio-ethanol in 2007 was led by the United States (23 × 109 L) and Brazil (21 × 109 L).41 In Brazil, bio-ethanol is obtained by fermenting the residue of sugar cane, known as bagasse. In contrast to the fermentation of corn-based glucose in the United States, bagasse is primarily composed of cellulose, hemicelluloses, and lignin. Growing sugarcane in Brazil is less expensive than growing and harvesting corn in the United States. In actuality, the energy input:output ratio for fermenting glucose from corn (1:1.5) in the United States is reportedly six times lower than fermentation of sugarcane (1:9) in Brazil.41 Although life cycle analysis of renewable resources is beyond the scope of this highlight, further discussion on the viability of biomass strategies is available.42, 43

Alternatively, ethanol can be obtained from biomass using a gasification strategy followed by hydrogenation of CO (eq 1) or CO2 (eq 2).44 These exothermic hydrogenation reactions require temperatures below 350 °C at 30 bar and utilize Rh or modified Fischer-Tropsch catalysts. Depending on the reaction conditions and catalyst, methanol formation and methanation (eqs 3 and 4) lower the yield of ethanol.

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In Figure 1, the endothermic dehydration process for converting bio-ethanol to ethylene involves heterogeneous catalysts, such as zeolites (HZSM-5) or mesoporous silica (Ni-MCM-41).45, 46 A number of researchers have investigated the modification of HZSM-5 with alkaline-earth, rare-earth, or transition metals.46, 47 Depending on the reaction temperature for a particular catalyst, diethyl ether may be an intermediate in the formation of ethylene. The formation of diethyl ether is exothermic and would be favored at lower temperatures. Currently, the dehydration of bio-ethanol is conducted by Braskem in Brazil.

Figure 1.

Synthesis of ethylene via the dehydration of bio-ethanol obtained from fermenting biomass.

As shown in Figure 2, ethylene can be used as a monomer for high-density polyethylene (HDPE), a comonomer for linear low density polyethylene (LLDPE), and a starting material for many other alkene-based monomers. Additionally, copolymers of ethylene with propylene, styrene, vinyl acetate, and acrylic acid have many desirable physical properties. From a commercial standpoint, petroleum-based ethylene is predominately utilized as the starting material in Figure 2. Although Braskem converts bio-ethylene into polyethylene, the full potential for incorporating ethanol-derived ethylene into other polymers in Figure 2 is not realized. Since alkenes are ubiquitous in polymer science, it is useful to note that the starting materials in Figure 2 are all relatively simple gases (oxygen, hydrogen, chlorine, carbon monoxide, ethylene, formaldehyde) and liquids (methanol, benzene, acetic acid), but cleverly assembled with selective catalytic reactions and high yield processes. As a result, renewable resources could partially contribute too many commodity monomers.

Figure 2.

Schematic representation of catalytic possibilities for producing alkene monomers and polymers using bio-based ethanol made from the fermentation of lignocelluloses or glucose.

The oligomerization of ethylene is a common method for synthesizing molecules with even numbers of carbon atoms, such as 1-butene, 1-hexene, and other higher α-olefins.48 A wide variety of catalysts, such as those based on nickel, chromium, iron, cobalt, and aluminum, produce a range of terminal alkenes.49, 50 The composition of the resulting α-olefins can be classified as a nonselective Shultz-Flory distribution or a selective Poisson distribution. The well-known shell higher olefins process (SHOP) with nickel (II) chloride based catalysts produces a Shultz-Flory distribution of α-olefins through coordination of ethylene followed by insertion as suggested by the Cossee mechanism (Fig. 3). After oligomerization of ethylene, the even numbered alkyl chains are detached from the metal center via β-hydride elimination. In contrast to the Cossee mechanism, a more promising mechanism in terms of selectivity involves metallacycle intermediates with chromium catalysts.51

Figure 3.

Cossee mechanism showing the formation of 1-butene via the dimerization of ethylene followed by β-hydride elimination. Depending on the catalyst (Ln = ligand) and rate of β-hydride elimination, other α-olefins are also possible, but omitted for clarity purposes.

Since not all α-olefins are equally desirable, a catalyst which produces a Poisson distribution provides higher yields of more economically viable α-olefins. For example, in Figure 4 the trimerization of ethylene results since the ring strain for the seven-membered metallacycle favors more β-hydride elimination compared to the stable five-membered metallacycle. The catalysts in Figure 4 may be heterogeneous Phillips catalysts or homogeneous analogues.52 Recent work with homogeneous chromium (III) chloride/methaluminoxane (MAO) catalysts indicates that transmetalation with dialkyl zinc reagents allow possibilities for functional oligomers of 1-hexene and 1-octene.53 The propensity for trimerization or tetramerization depends on the substituents attached to the PNP ligand [Ph2PN(R)Ph2].

Figure 4.

Synthesis of 1-hexene (n = 1) and 1-octene (n = 2) via metallacycle intermediates followed by β-hydride elimination and reductive elimination.

Propylene

The polymerization of propylene accounted for 64% of the propylene consumption in 2004.54 Since propylene is the second most common petrochemical (Table 1), converting biomass into propylene would alleviate some dependence on petroleum. Several strategies can be envisioned to produce propylene via renewable and sustainable pathways. As shown in Figure 5, these include starting materials based on fermentation methods (bio-ethanol, bio-butanol), gasification (syngas), and cracking of biomass to produce hydrocarbon mixtures. Currently, alcohols appear to be key intermediates in the synthesis of propylene for many of the potential pathways in Figure 5.

Figure 5.

Possibilities for synthesis of propylene from biomass using fermentation, gasification, or cracking strategies.

Braskem in Brazil, which currently makes polyethylene via a bio-ethylene platform that involves dehydration of sugar cane-based ethanol, is also planning to produce propylene from bio-ethanol.55 Additionally, Braskem and Novozymes, a Danish firm, have a partnership to develop a fermentation method to produce propylene. Ideally, a direct route to propylene through metabolic engineering would be more selective than cracking and possibly fewer synthetic steps than the 1-butene isomerization-metathesis sequence. Dow Chemical Company is also pursuing a renewable source of propylene which is not based on ethanol.56 The type of biomass will most likely depend on geographical location.

Since the conversion of methanol from natural gas to olefins is conducted industrially with Lurgi's methanol to propylene (MTP) process and the methanol to olefins (MTO) process developed by Norsk Hydro and UOP,12 biomass-derived syngas-to-methanol may become a promising feedstock for propylene. The gasification of plant oil and biomass could also be employed to convert syngas to methanol followed by Fischer-Tropsch chemistry. Conversely, another strategy might include fermentation of biomass to yield isopropyl alcohol followed by dehydration.

Certain alcohols, such as glycerol, can be catalytically hydrogenated with Pd or WO3/ZrO2 to produce propylene.57, 58 Glycerol is attractive since it can be obtained from biodiesel production and fermentation.59 Other hydrogenolysis reactions of glycerol with Ru/C, Pd/C, Ni/C, and Raney nickel produce propylene glycol.60

For the dehydration of bio-butanol, isomerization followed by metathesis with ethylene is necessary to obtain an odd number of carbon atoms. Although the fermentation of sugars with Clostridium acetobutylicum has been known for many years as the acetone-butanol-ethanol (ABE) process, most of the current bio-butanol technology is aimed at utilizing a more diverse array of biomass options with recombinant microorganisms. Globally, the development of fermentation technology to produce bio-butanol has been undertaken by ButylFuel, Gevo, Butamax, Butalco GmbH, Green Biologics, Cobalt Biofuels, and many others. Since bio-butanol serves as a gasoline replacement, it may become more important than bio-ethanol.61 Ideally, a direct fermentation pathway to propylene would minimize additional catalytic steps and simplify separation of propylene gas from the fermentation reactor.

In addition to the variety of alcohol-based feedstocks for propylene, catalytic cracking has potential to generate mixtures of hydrocarbons, such as propane, propylene, and butanes.20, 62 Other researchers have used hydrothermal treatments of biodiesel63 and agricultural64 waste products to obtain fuel mixtures. Based on data for catalytic cracking and hydrothermal reactions, these methods produce mixtures of hydrocarbon products which are very useful for fuels but provide lower selectivity compared to fermentation routes.

Methacrylate Monomers

Currently, a number of industrial processes produce over 4.5 × 109 lbs methyl methacrylate (MMA) each year.19 Some of these involve ethylene (BASF, Eastman, Lucite), propyne (Shell), acetone (ACH-L), isobutylene (Asahi), or isobutane (Arkema).19, 65 Figure 6(a) suggests that the ethylene based method for producing MMA could utilize renewable components such as bio-ethanol, biomass-derived syngas, and methanol. Alternatively, through a joint venture between Rohm & Haas and Ceres, another strategy for MMA is being funded by the U.S. Department of Agriculture.66, 67 As shown in Figure 6(b), this bio-synthesis procedure involves isobutric acid followed by a number of possible reaction pathways. Potentially, methacrylic acid, ethyl methylacrylate, and butyl methacrylate could also be produced from this platform. Given the emphasis on bio-ethanol and bio-butanol, ethyl and butyl methylacrylate will soon become important renewable monomers.

Figure 6.

Potential methods for utilizing renewable starting materials for methacrylate monomers based on (a) bio-ethanol from sugar cane or corn starch, (b) fermentation of lignocellulose, (c) conversion of carbon dioxide into monomer precursors via photosynthesis,70 and (d) polymerization of cyclic lactones which mimic MMA.

Carbon dioxide may also become a significant feedstock for certain moieties of methacrylate monomers. Already, CO2 has been reported as a comonomer for polymerization of polycarbonates containing limonene oxide.68 As summarized in Figure 6(c), researchers at UCLA have produced isobutyraldehyde and isobutanol with genetically engineered Synechococcus elongatus.69 In contrast to the fermentation of glucose into ethanol via glycolysis, S. elongatus converts CO2 into chemicals via a photosynthesis process. Another noteworthy method for utilizing CO2 in the synthesis of MMA involves the electrocatalytic reduction to formaldehyde or methanol.70

While this highlight does not seek to evaluate the economics for producing methacrylate monomers, renewable resources may begin to play a greater role in what has traditionally been petroleum, natural gas, and coal-based processes. A biorefinery could partially contribute to the necessary starting materials since all these processes use methanol, bio-ethanol, or bio-butanol. Additionally, several use syngas and byproducts of methanol, such as formaldehyde which can be obtained from the oxidation of methanol.

Combining a bio-ethanol platform to make ethylene with a biomass gasification process to produce syngas would provide most of the starting materials in Figure 6(a). Moreover, gasification improvements allow conversion of cellulosic biomass with Rh/CeO2/SiO2 (at 500–600 °C) into syngas without char formation.71 Other methods such as plasma technology72 and catalytic steam reforming73, 74 may provide other options for starting materials in Figure 6(a). In contrast, the fermentation methods in Figure 6(b,c) synthesize isobutric acid and isobutyraldehyde at much lower temperatures and may require fewer synthetic steps.

The renewable cyclic monomer tulipanlin [Fig. 6(d), R = H], which is found in tulips, provides a renewable option for MMA. Polymerization of this lactone with ATRP provides polymers with much higher Tg values than those expected from polymethylmethacrylate.75 Alternatively, hydrogenation of levulinic acid converts the carboxylic acid to a cyclic ester (γ-valerolactone).26 The heterogeneous gas-phase reaction of γ-valerolactone with formaldehyde yields another cyclic analogue of MMA known as α-methylene-γ-valerolactone [Fig. 6(d), R = CH3].

Isobutylene

The renewable synthesis of isobutylene through biological and chemical catalysis is very promising. As mentioned during the discussion of methacrylate monomers, the isobutyraldehyde synthesis in Figure 6(c) can also produce isobutanol. Further dehydration with solid acids or another type of heterogeneous catalyst would lead to isobutylene. Additionally, nonfermentation pathways, which metabolize glucose with engineered E. coli, can also utilize 2-keto-acid decarboxylase and produce isobutanol as well as 1-propanol (precursor to propylene), 1-butanol, and 2-phenylethanol (precursor to styrene).76 Currently, Gevo is fermenting biomass and converting the resulting isobutanol to isobutylene.

Due to the availability of β-pinene, an isomerization-metathesis sequence in Figure 7 provides a renewable method for isobutylene. The intramolecular cyclization of myrcene in Figure 7 can be done neat77 or in dodecane.78 The cyclic diene has been polymerized under cationic and free-radical conditions,78 but so far the isobutylene from this method has not been utilized as a monomer. Potentially, the cyclic diene in Figure 7 could replace isoprene in the synthesis of butyl rubber.

Figure 7.

Thermal isomerization of β-pinene and subsequent intramolecular metathesis for the synthesis of isobutylene and an isoprene substitute.

PART 3: DIENE MONOMERS

Isoprene

Procedures for obtaining isoprene (C5) from renewable resources have a long standing history in the synthetic rubber industry. These methods include the pyrolysis of natural rubber (1860), turpentine (1884), and dl-limonene (1911) to obtain isoprene in low yields.79 However, by 1911, Staudinger had optimized the conversion of dl-limonene to isoprene using an electrically heated coil of wire known as an isoprene lamp to obtain isoprene in 67% yield.

More recently, a fermentation method has become a viable option for isoprene production through technology jointly developed by Goodyear and Genencor. The BioIsoprene™ strategy involves the fermentation of sugars, such as glucose, or other biomass, with engineered bacteria.80 This continuous process produces gaseous isoprene that is recovered and purified for polymerization.81 As shown in Figure 8, the production of BioIsoprene™ involves the metabolism of glucose with recombinant E. coli through two possible pathways. Both the mevalonate (MVA) and 5-methyl erythritol phosphate (MEP) pathways convert isopentenyl pyrophosphate (IPP) to 1,1-dimethylallyl pyrophosphate (DMAPP). The alkene isomerization of IPP to DMAPP during the synthesis of BioIsoprene™ mirrors natural rubber (NR) biosynthesis. Nonetheless, unlike NR biosynthesis, which involves propagation of additional IPP units, a plant-based isoprene synthase converts the DMAPP to isoprene.82 This process produces 2 g/L/h of isoprene.

Figure 8.

Synthesis of BioIsoprene from glucose using the MVA or MEP pathways. The synthetic scheme was adapted from reference 80.

An evaluation of natural rubber trees with the BioIsoprene™ process suggests that this fermentation process produces a significant quantity of isoprene. For example, the yearly production of natural rubber from clones of Hevea trees may be a high as 2500 kg/hectare83 with an average value of 5 kg/tree.84 The productivity varies with geographical location, environmental conditions, and management techniques, but for certain clones, such as RRIM 600, the optimal annual yield is 2110 kg/hectare.85 Considering a tree density of ∼400 to 530 trees per hectare, an individual RRIM 600 tree produces 11–15 g natural rubber per day. Hypothetically, a 15 L reactor could produce enough BioIsoprene™ in 30 min to match the daily production of NR from a Hevea tree.

The polymerization of BioIsoprene™ with heterogeneous TiCl4/(iso-butyl)3Al in hexane resulted in high cis-1,4 content (98.5%).80 The bio-based polyisoprene, referred to as BioNatsyn™ had Mooney viscosity (71) values and glass transition temperatures (Tg = −61 °C) which were very similar to petroleum-based polymers.

1,3-Butadiene

Historically, butadiene has been made from ethanol (eq 5) with a variety of heterogeneous catalysts17 such as those based on tantala/silica,86 alumina/ZnO,87 and MgO/silica.88 More recently, bimetallic (Zr:Zn) and trimetallic (Cu:Zr:Zn) catalysts supported on silica produced up to 67% 1,3-butadiene with the remainder being ethylene and acetone.89 The multistep mechanism initially involves the oxidation of ethanol to acetaldehyde followed by an Aldol reaction that produces acetaldol. Then, this four-carbon molecule undergoes a couple possible dehydration mechanisms before resulting in 1,3-butadiene. Since fermentation strategies will produce bio-ethanol, butadiene can potentially be considered a renewable-based monomer. Currently, a direct fermentation route to butadiene is not reported.

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Cyclohexadiene

The polymerization of 1,3-cyclohexadiene (1,3-CHD) provides a polymer with robust properties due to the cyclic monomer. Anionic, free-radical, and transition metal catalyzed polymerizations are common methods for polymerizing 1,3-CHD.90–95 Anionic polymerization methods result in microstructures with 1,2 and 1,4-linkages whereas certain metal-based catalysts can produce predominately 1,4-linkages. The presence of 1,4-sequences in the polymer can serve as a precursor to polyparaphenylene.

The synthesis of 1,3-CHD often involves elimination reactions starting with halogenated compounds such as 1,2-dibromocyclohexane, 3-bromocyclohexene, or chlorocyclohexene.90, 96 Recently, a renewable method for the synthesis and polymerization of 1,3-CHD was reported.97 As shown in Figure 9, plant oils, such as soybean, corn, and canola oils, contain polyunsaturated triglycerides which will react with metathesis catalysts to produce 1,4-CHD. Monounsaturated triglycerides produced acyclic C12, C15, and C18 isomers. Isomerization of neat 1,4-CHD to 1,3-CHD was accomplished with a ruthenium hydride catalyst {RuH(CO)Cl[P(C6H5)3]3}. Polymerization of 1,3-CHD with a Ni(acac)2/MAO catalyst was investigated in neat monomer, hydrogenated d-limonene, and toluene. Investigation of the isomerization-polymerization sequence with a two-step multicatalytic method and a one-step cascade sequence both produced highly crystalline polyCHD.

Figure 9.

Synthesis and polymerization of 1,3-cyclohexadiene from plant oils using a metathesis and isomerization reactions. The synthetic scheme is adapted from reference 97.

Dicyclopentadiene

Since large quantities of dicyclopentadiene (DCPD) are produced by the petrochemical industry each year, DCPD is very economical. When heated, DCPD will decompose via a retro-Diels-Alder reaction to produce cyclopentadiene (CPD). Although renewable mimics for DCPD and CPD are not currently reported, a couple of possible options exist for these dienes. First, the 1,3-CHD in Figure 9 could serve as a replacement for CPD in Diels-Alder reactions or as a diene monomer in ethylene-propylene-diene (EPDM) polymerizations. Typically, EPDM polymerizations utilize small quantities of DCPD or 5-ethylidene-2-norbornene for cross-linking or vulcanizing. Second, integrating the polymerization of DCPD with renewable monoterpenes, such as limonene oxide or β-pinene, allows the synthesis of hyperbranched polymers98 or thermosets77 with tunable physical properties. Although ring opening metathesis polymerization (ROMP) of DCPD with ruthenium metathesis complexes provides a robust thermoset, incorporation of β-pinene results in chain transfer during the polymerization and controls cross-linking.

PART 4: AROMATIC MONOMERS

Lignin is a complex, amorphous polymer which serves as a matrix that surrounds the hemicelluloses and cellulose in plants and trees. After cellulose, lignin is the most abundant natural polymer. Generally, the amount of lignin varies among species and even depends on the method to extract the lignin, but decreases as follows: softwood > hardwood > grasses. Some sources report lignin comprises 20% of hardwood trees and 28% of softwood trees.99 As shown in Figure 10, lignin is primarily composed of three different phenypropane units.100 These three units, which are referred to as monolignols, are arranged in ether (β-O-4 and 4-O-5), biphenyl (5-5), cyclic ether (β-5 and dibenzodioxocin), bicyclic ether (β-β, and spiro (spirodienone) linkages. The β-O-4 units comprise ∼50% of linkages in softwood trees and 60% of hardwood linkages.101 Generally, the propensity for common covalent connections in Figure 10 decreases as follows: β-O-4 > 5-5 > β-5 > β-1 > 4-O-5 > β-β.

Figure 10.

The three primary building blocks of lignin and common covalent connections found in lignin. Dibenzodioxocin and spirodienone linkages are not shown. Structures adapted from reference 20.

The amount of forestlands in the United States (749 million acres) and Canada (993 million acres) is quite staggering.102 In the United States, the annual production of dry woody biomass from waste (14 million tons), mills (87 million tons), and harvest residues (64 million tons) is just a fraction of the total amount of lignin available.103 For example, a joint study by the US Departments of Energy and Agriculture estimate the maximum amount of dry woody biomass that can be annually produced in a sustainable manner from forestlands (368 million tons) and agricultural lands (998 million tons) exceeds 1 billion tons.104 Due to the projected availability of lignin, “woody biomass is anticipated to be an important component of any future renewable energy portfolio.”103

In the context of polymer science, the enormous quantity of available lignin presents excellent opportunities for synthesis of aromatic monomers. Currently, the full potential of lignin for commodity polymers is underutilized, but not due to lack of effort. In fact, research on lignin has a rich history that extends beyond combustion technologies to pyrolysis, gasification, and biochemical treatments. Generally, the temperatures for converting lignin via hydrocracking (350–450 °C) are lower than flash vacuum pyrolysis (400–650 °C)105 and gasification (1,095–1,490 °C).106 The difficulty in obtaining aromatic chemicals from lignin lies in the need for catalysts, either heterogeneous of homogeneous, which can break a complicated network of C[BOND]O and C[BOND]C bonds (see Fig. 10) under conditions that do not lead to substantial amounts of tar or gasification.

A number of excellent reviews have examined the multifaceted complexity surrounding the conversion of lignin into chemicals.107 By and large, the vast number of permeations surrounding the various types of wood, choice of pretreatment, type of catalyst, experimental conditions (temperature, time, pressure) produces widely different results. This is not entirely surprising given the variety of covalent connections in Figure 10. As a result, many catalysts, such as FeS, Mo, Co-Mo, Mb-Mo, Ni-W produce aromatic mixtures which are primarily composed of benzene, toluene, phenols and xylenes.101, 108, 109 Virent Energy Systems currently produces aromatics through a BioForming® process. Phenolic molecules from lignin have been incorporated into phenolic resins, but often the reactivity is lower than pure phenol due to the substituents on the aromatic ring.110

As shown in Figure 11, the variety of chemical building blocks from lignin could be hypothetically assembled to synthesize alternatives to petroleum-based aromatic monomers such as styrene or terephthalic acid. Possibilities include conversion of butadiene to ethylbenzene with nanoporous nickel (II) phosphate (VSB-1).111 Recent progress with catalytic fast pyrolysis is a promising method for maximizing output of aromatics while reducing oxygenated compounds.112 This process employs a combination of high heating rates to initially pyrolyze lignin followed by catalytic conversion of oxygenated molecules with zeolite catalysts to form aromatics, CO2 and water.113 Additionally, the reaction only requires short resonance times (∼ 2 min). Other oxygenates, such as glycerol, will also form aromatics.114 The flash vacuum pyrolysis of model lignin compounds at 500 °C produces styrene and phenol,105 but currently most reactions with lignin produce aromatic mixtures rather than styrene.

Figure 11.

Potential synthetic routes to styrene and terephthalic acid via fermentation, catalytic pyrolysis, or gasification pathways.

PART 5: CONDENSATION-TYPE MONOMERS

Terephthalic Acid

Figures 11 and 12 detail some of the feasible methods for converting renewable resources into terephthalic acid. Polymerization of terephthalic acid with ethylene glycol results in poly(ethylene terephthalate) (PET). The US production of purified terephthalic acid in 2001 was $2.1 billion.115 For comparison, the market for purified terephthalic acid was similar to styrene production ($1.9 billion in 2001),115 but less than vinyl chloride ($19 billion in 2002).13 In 2011, PepsiCo announced plans to make a completely renewable PET bottle. Coca-cola is also promoting a plant-based bottle with renewable components.

Figure 12.

Synthesis of terephthalic acid from dl-limonene using (a) dehydrogenation of dl-limonene to yield aromatic p-cymene and (b) oxidation of p-cymene to obtain terephthalic acid.

In Figure 11, isomerization of meta-xylene and disproportionation of two toluene molecules into one benzene molecule and one para-xylene molecule is a well-known technology that is currently used for aromatic mixtures (BTX). The pore-sizes of HZSM-5 zeolites are well suited for shape of para-xylene. Other promising renewable sources include dl-limonene (Fig. 12) which can be aromatized to para-cymene and converted of terephthalic acid.116 Analogues of PET have recently been reported using lignin-based monomers and acetic acid.117 After removal of acetic acid in Figure 13, the resulting polyesters had similar thermal properties as PET.

Figure 13.

Synthesis of lignin-based PET analogues. The synthetic scheme is based on reference 117.

Diols

Diols are often utilized as monomers for polyesters, polycarbonates, polyether glycols (polyols), and polyacetals. Recently, DuPont has developed fermentation technology with Tate & Lyle to convert sustainable sugars into C3 and C4 analogues of ethylene glycol. In the case of 1,3-propanediol, more than 100 × 106 lbs are produced annually.118 DuPont has also developed a purification method for 1,3-propanediol that uses an acid catalyst to eliminate polymer discoloration.119 Previously, 1,3-propanediol was obtained from the Shell process using hydroformylation of ethylene oxide or by the Degussa-DuPont process with propylene. Other methods for synthesizing 1,3-propanediol include microbial conversion of glycerol with recombinant C. acetobutylicum.120 Although the reduction of succinic acid is one method for obtaining 1,4-butanediol, DuPont currently converts glucose into 1,4-butanediol with a fermentation approach. In addition to fermentation strategies for C3 and C4 diols, monounsaturated triglycerides in plant oils can also provide larger diols for a polymerization process that yields novel polyacetals.121

Several options for polymerizing renewable-based diols are shown in Figure 14. The acid-catalyzed polymerization of 1,3-propanediol with tetrafluoroethane sulfonic acid forms polyols through a condensation mechanism.118 The resulting polytrimethylene ether glycol is currently marketed by DuPont as Cerenol™. The copolymerization of diols with purified terephthalic acid (PTA) provides alternatives to poly(ethylene terephthalate). Since 2007, several polyesters with renewable diols, such as DuPont Sorona® and Hytrel®, have been available. Sorona® is a polyester that contains 1,3-propanediol made from corn sugar (glucose) whereas Hytrel® contains 1,4-butanediol. Other important polyesters with C4 repeating units include poly(butylene succinate) which can be derived from furfural,122 or succinic acid.123 BioAmber received the 2011 Presidential Green Chemistry Challenge Award for successful commercialization of a bio-based method for producing succinic acids.

Figure 14.

Fermentation possibilities for the synthesis of renewable diols for polyester polymerizations. PTA = purified terephthalic acid.

Nylon

A number of possibilities exist for integrating renewable resources into aliphatic polyamides. Currently, Arkema commercializes Rilsan® PA 11, which is a Nylon 11 resin based on plant oils derived from castor beans. Due to the presence of alkenes in plant oils, metathesis strategies are able to mimic many different types of step-growth polymerizations.29 For example, acyclic diene metathesis (ADMET) polymerizations with derivatives of 10-undecenoic acid in Figure 15 resulted in polyamides.29 Recent advances with cross metathesis reactions of fatty esters and allyl chloride (Fig. 16) have potential to serve as Nylon-11 precursors.124

Figure 15.

Synthesis of Nylon analogues using ADMET polymerizations as described by reference 29.

Figure 16.

Synthesis of Nylon 11 precursors from reference 125 using cross metathesis of allyl chloride and methyl oleate.

The development of benign oxidation methods is an alternative to metathesis reactions. For example, oxidation of cyclohexene with 30% hydrogen peroxide allows the synthesize of adipic acid and other monomers.125 Since benzene could be obtained from pyrolysis of lignin (Fig. 11) and hydrogenated to cyclohexene, this benign oxidation with hydrogen peroxide may have future potential.

CONCLUSIONS AND OUTLOOK

The transformation of renewable resources to monomers for anionic (isoprene), cationic (isobutylene), condensation (diols, Nylon 11), free-radical (α-methylene-γ-valerolactone), and transition metal (ethylene) polymerizations has advanced to very sophisticated levels. Nonetheless, many opportunities still exist for making renewable versions of polyvinylchloride, poly(ethylene oxide), polystyrene, and polybutadiene. In this regard, future research aimed at a direct fermentation route to butadiene, propylene, vinyl chloride, or styrene from biomass would eliminate many synthetic steps. Currently, fermentation methods with recombinant microorganisms provide higher selectivity than cracking or pyrolysis methods. However, given the large quantity of available lignin, a catalytic-pyrolysis method for selectively converting lignin to styrene or divinylbenzene would be very practical.

Based on the content of this highlight, several observations can be made regarding the future of renewable resources in polymer science. First, the development of catalyst sequences based on the combination of biocatalysis, acid catalysts, and transition metal catalysts is making research on renewable resources more interdisciplinary. Second, the most promising renewable resources are nonfood sources, such as cellulose, lignin, and plant oils. The success and economic viability of biorefineries may strongly depend on geographical location which can dictate the availability of certain types of renewable resources. Thirdly, the commercialization of renewable-based polyols and polyesters by DuPont as well as ethanol-based polyethylene by Braskem illustrates that renewable monomers are not just an academic curiosity. In this regard, the current pace of academic and industrial research is anticipated to further mimic many more commodity monomers and polymers with green polymerization methods.

Biographical Information

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Robert T Mathers was graduated from North Carolina State University with a B.S. in chemistry. In 2002, he obtained a Ph.D. in Polymer Science at The University of Akron working with Professor Roderic P. Quirk. After two years of postdoctoral research at Cornell University with Professor Geoffrey W. Coates in the Department of Chemistry and Chemical Biology, Robert joined Pennsylvania State University. Currently, he is an Associate Professor of Chemistry at the New Kensington campus. His research interests focus on integrating renewable resources, such as plant oils and monoterpenes, with catalysis for the synthesis of monomers and polymers. At present, he is taking a one-year sabbatical in the laboratory of Professor Krzysztof Matyjaszewski at Carnegie Mellon University. Robert has served as coeditor for Green Polymerization Methods: Renewable Starting Materials, Catalysis, and Waste Reduction (Wiley-VCH) and the Handbook of Transition Metal Polymerization Catalysts (Wiley).

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