Exploiting Lignin: A Green Resource - ACS Symposium Series (ACS

Chapter 6

Exploiting Lignin: A Green Resource

Downloaded by UNIV OF FLORIDA on October 31, 2017 | http://pubs.acs.org Publication Date (Web): October 23, 2017 | doi: 10.1021/bk-2017-1268.ch006

Jianfeng Zhang1,2 and Michael A. Brook*,1 1Department

of Chemistry and Chemical Biology, McMaster University , 1280 Main St. W., Hamilton, ON, Canada L8S 4M1 2Northboro Research and Development Center, Saint Gobain Performance Plastics, 9 Goddard Rd., Northboro, Maine 01532, United States *E-mail: [email protected].

Lignin is a readily available bioresource that is currently underutilized. Techniques to extract lignin from its biological matrix are well established. The development of mild processes to modify lignin, particularly its polar surface, has been accelerating over the last two decades, in particular. It is, therefore, now possible to create high value products, especially biocomposites, from lignin in both developed and developing economies. This review seeks to provide an overview of processes that are particularly amenable to practice aound the planet and that take advantage of the many beneficial properties of lignin.

The ‘Greening’ of Products by the Incorporation of Naturally Occurring Polymers Such as Lignin Lignin is one of the most available biopolymers on the planet – the second most abundant organic polymer after cellulose. Our Canadian perspective is biased by the ready availability of both hardwood and softwood lignin, which constitutes an abundant resource. However, many other plants, including food crops, contain significant quantities of lignin – about 10-30% (1) – that is currently, at best, underutilized; it is estimated that less than 1% of lignin is used in industry, aside from as a source of fuel (2) (in pulp and paper plants, lignin is used as a fuel if its calorific content is cheaper than the price of oil) (3). Any source of biomass, particularly waste biomass, may contain accessible lignin that could be utilized to make higher value materials. For example, lignin has been extracted from rice hulls (4), sugar cane bagasse (5) and switchgrass (6), among many other plant © 2017 American Chemical Society Grosse; Mobilizing Chemistry Expertise To Solve Humanitarian Problems Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


sources. The industrial production of lignin for its own merit is increasing for applications ranging from surfactants, antioxidants, dispersants to adhesives (7); there is a long-standing market for vanillin derived from lignin (vanilla extract) in food (8). A sustainable world requires us to utilize natural materials responsibly, which is not a simple task. It implies a switch from non-renewable to renewable resources, and a significant decrease in the inputs needed to produce goods from those natural materials. In addition, a complete life cycle analysis must direct us to create products that (bio)degrade to non-toxic entities that themselves act as nutrients, not pollutants, with little or no energy input; the loop should be complete. In light of the humanitarian focus of this book, rather than a comprehensive review (1, 9–12), we have emphasized technologies for the exploitation of lignin that are sustainable (or could be made so), and which should be practicable around the world, including in (economically) resource challenged areas. By this, we mean areas where lignin is readily accessible, and where the current exploitation of lignin is inefficient or insufficient. We will briefly examine the exploitation of lignin in the ‘biorefinery’ to produce low molecular weight products, typically monomers, or other higher value materials. Our bias, however, is that the world will more readily benefit from exploitation of the intrinsic beneficial properties of lignin by incorporating it – possibly after some modification – into other polymeric materials; this will be the main focus of the chapter.

The Chemical Units and Linkages of Lignin Lignin is crosslinked polymer network of “infinite” molecular weight. Although there are no well-defined repeating units, lignin is a polymer derived from three different phenylpropanoid monomers: p-hydroxyphenylpropane, guaiacyl propane (G), and sinapyl propane (S) (Figure 1) (2, 13). The ratio of these monomers varies between plant species (13). In general, hardwood lignin contains almost equal amounts of guaiacyl (G) and sinapyl (S) groups; lignin obtained from grass and softwood has much higher fractions of G units (2, 13). The synthesis of lignin involves biomediated polymerization within the plant. This process, ‘lignification,’ is catalyzed by enzymes, produces a three-dimensional network from the conversion of the noted three monomers crosslinked with both ether and carbon-carbon bonds (13). The dominant linkage (β-O-4) is an ether bond condensed from the β alcohol and 4-phenol, which accounts for more than ~50% of the linkages (14). The 5-position of the guaiacyl propane (G) is very reactive (15, 16), resulting in various other ether and carbon-carbon moieties like 4-O-5, β-β, β-5, and 5-5 (Figure 1) (13, 16). As a consequence,of their higher G content, lignin in grass and softwood is a very highly branched and crosslinked structure (16, 17). The organofunctionality of lignin includes large amounts of methoxy, phenolic hydroxy, and some terminal aldehyde groups (13).

92 Grosse; Mobilizing Chemistry Expertise To Solve Humanitarian Problems Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Extraction of Lignin A clear requirement for the exploitation of lignin is separation from its original biological matrix. While there are applications for which the complex biological lignin-containing matrix can be used “as is”, for example, mechanical pulp for newsprint (18), it must normally be extracted from the cellulose, hemicellulose and other plant constituents with which it is found in nature. This normally involves a pulping process (19, 20). The aromatic ether moieties render lignin soluble in some organic solvents (unlike the sugar-based materials from which it must be separated) including low molecular weight alcohols and ketones (21, 22), although more expensive solvents (23) or ionic liquids (24, 25) may do the extraction more efficiently. More commonly, the isolation of lignin requires chemical modification. Unlike the celluloses and hemi-celluloses with which it is normally combined, lignin contains acidic phenolic groups (the pKas of phenols ~ 10, while the pKas of saccharidic alcohols ~16-17). The water solubility of lignin is therefore enhanced after treatment with base to form phenolates (soda process) (10, 26), which can be accompanied by lignin degradation in the presence of Na2S (kraft process) (27). Acidic sulfite pulping (28) that forms sulfonate anions generated from SO2 also increases lignin solubility in water (Figure 2) (10). Note that these processes are normally followed by chemical bleaching to create bright papers from cellulose. Most of the processes require elevated temperatures (150-200 °C) for a few hours. The kraft pulping process is currently the most popular for large scale papermaking processes, but the older soda process is particularly practicable because of its simplicity. In some cases, prior to lignin separation, the biomass is subjected to an acid hydrolysis step that is used to facilitate the isolation of sugars such as xylose from the mixture, which can be sold or otherwise used. A direct comparison of the processes used to extract lignin (after the removal of xylose) permitted an assessment of their relative costs and environmental impact. The cost estimates (Table 1) were based on prices in the United States and will be affected by the scale of process, labor costs, energy costs…. Extrapolation of these data to other locales is not trivial. Nevertheless, it can be seen that the simplest processes to practice lignin extraction involve the organosolv process (provided there is access to the appropriate organic solvents) and the soda process, which needs only base and which operates very well on biomass from grasses, sugar cane bagasse, etc. An examination of Table 1 provides guidance on which processes could be most easily operated with limited resources. Direct soda extraction, in particular, uses relatively mild conditions, produces lignin at a lower cost, and has a low relatively low environmental impact (29). The other processes require more sophisticated equipment, with a corresponding higher cost. The organosolv process needs some further consideration. In certain locations, for example, Brazil, the cost and availability of sugar cane bagasse and ethanol will be lower than in many other areas. The high environmental index for this process is directly related to release of ethanol that constitutes a greenhouse gas. Better capture of these byproducts would lower the level of impact significantly. 93 Grosse; Mobilizing Chemistry Expertise To Solve Humanitarian Problems Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Figure 1. (A) The structure of monomers and their approximate abundance in lignin from different plants, (B) the major linkages in lignin connecting the propanes.

Figure 2. Typical processes for extracting lignin (showing model reactions). 94 Grosse; Mobilizing Chemistry Expertise To Solve Humanitarian Problems Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

Table 1. Processing steps, production and environmental costs for lignin isolation (28) Soda process

Kraft process*

Sulfite*

Organosolve*

Temperature (°C)

90

170

120

170

Reaction time (h)

1.5

3.5

0.7

1

11.25% Na2S

NaHSO3 2%

EtOH 50% (v/v) 1:6 solid/liquid

2%

3.75%

11%

Sugar cane bagasse

5.10

6.30

7.80

16.00

Rice husk

3.50

4.30

5.30

10.20

Reagent/solvent % (w/v) Base % (w/v) NaOH


Lignin cost/kg US$

Potential Environmental Impact/kg

*

Sugar cane bagasse

0.02

0.09

0.03

0.25

Rice husk

0.02

0.09

0.03

0.25

Requires a pressure reactor.

Degradation to Higher Value, Small Molecules – The Biorefinery There is an old joke in the pulp and paper business that, “You can make anything from lignin except money!” However, that view is changing because of market driven interest in green chemistry, new commercial sources of lignin, lower oil prices that incentivize creation of value added products from lignin and, more importantly, the observation that many interesting, beneficial properties result from the incorporation/utilization of lignin in biocomposites. We start, however, with degradation to useful organic compounds. The rich aromatic composition of lignin has inspired scientists and engineers to consider it, once decomposed, as a supplemental green resource for petroleum-based feedstocks. Lignin has evolved to resist to enzymatic and chemical degradation (9). Biodegradation (30, 31) is too slow to be a practical as a source of chemicals. Although much effort has been expended over the years to develop degradation processes to overcome strong C-C and ether bonds, most processes remain too costly, too inefficient and, for the purposes of this book, too expensive thermally or in the costs of catalyst used. Currently, only a small portion of lignin it is utilized and converted for vanillin (32–34). A significant change can be expected in the near future with the numerous emerging technologies (3, 16, 35–41). Due to the limitation of space, only selected examples are presented that, in our opinion, show particular promise. Many very efficient routes to the decomposition of model lignin compounds have been reported. Few, however, have been practically applied to actual lignin (16, 42). To better understand the challenges to degradation that the polymer 95 Grosse; Mobilizing Chemistry Expertise To Solve Humanitarian Problems Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

provides, as opposed to individual constituents, we examine the complexity of network structures in both hardwood and softwood lignin.


Hardwood Lignin – Relative Linear Structure Hardwood contains a relatively higher concentration of sinapyl propane (S), where the 5-position is blocked by a methoxy group; the position is not available for condensation with other monomers (15, 16). Thus, hardwood lignin contains large linear polymeric fractions (16, 17) and is, therefore, relatively readily decomposed by cleavage of the β-O-4 linkage (43). To give some context on the difficulty of decomposing (hardwood) lignin, already in 1938 very high conversion (~70%) was achieved using Cu-CrO (44), but at 250-260 °C under 200-350 atmospheres of hydrogen for about 18 hours. The aromatics were degraded and reduced to cycloaliphatic derivatives such as 4-n-propylcyclohexanol, 4-n-propylcyclohexanediol, and 3-(4-hydroxycyclohexyl)-propanol. With active carbon-supported noble metal catalysts (45) under modest H2 pressure (4 MPa at RT), decomposition occurred at 200 °C over 4 h to give ~42% conversion to aromatic monomers (~33%) or dimers (~8%) with Pt/C as the catalyst. More recently, Ni-based catalysts, including Ni/C (46), and Ni-noble metal bimetallic catalysts (47, 48), were used for degradation of birch wood-derived lignin for propylguaiacol and propylsyringol. However, degradation using mild, efficient processes and inexpensive catalysts remains a challenge. The condensation of ethers, alcohols, and other oxygenated groups (49–52), catalyzed by tris(pentafluorophenyl)borane (B(C6F5)3), with hydrosilanes or silicone hydrides is a new chemistry route for structured and functionalized silicones (53). This reaction also efficiently converts alkoxyphenyl and phenol groups to aryl silyl ethers (53–55). The process could also be applied directly to both model compounds and actual hardwood lignin (Figure 3) (56, 57). The β-O-4 linkage of the model compound is cleaved efficiently (>90% conversion) using hydrosilanes (some of which are relatively inexpensive) in the presence of B(C6F5)3; other oxygenated groups like methoxy, phenol, and alcohols are also reduced to silyl ethers or alkanes. This reaction only requires mild sonication under ambient pressure at 50 °C for 3 h.

Figure 3. Silane reduction of a model hardwood lignin compound. 96 Grosse; Mobilizing Chemistry Expertise To Solve Humanitarian Problems Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

The efficiency of the process with model compounds could be extended to hardwood lignin decomposition (57, 58). The reduction of dominant β-O-4 linkage led to mixtures of monomers and low molecular weight oligomers in only 10 to 20 minutes of sonication at room temperature. In real samples, deactivation of the catalyst accompanied the process, such that catalyst loadings of over 5% were required to convert 90% of the lignin into organosoluble materials. Although the carbon-carbon and diphenyl ether linkages are resistant to reduction under these mild conditions; alternative Lewis acid catalysts may permit hydrosilanes to cleave the diphenyl ether linkers (59, 60).


Softwood Lignin – A 3D Cross-Linked Network The decomposition of softwood is generally more difficult due to its more highly cross-linked network structure. The higher content of linkages like 5-5, β-5, β-1, and 4-O-5 are resistant to degradation by most processes, preventing the fragmentation of such a branched network. For example, the B(C6F5)3-reduction with hydrosilanes did not result in satisfactory decomposition of softwood lignin (57); only 30% of lignin could be rendered organosoluble. Only very few other catalysts have been found that can destroy the softwood lignin structure (up to 60% conversion), however, they require very harsh conditions. For example, the solid acid (SiO2-Al2O3) is used to decompose different lignin into monomers at 250 °C for 30 min; and the catalyst to lignin weight ratio is at 1:1 ratio (61). In more recent research, only organosolv lignin could be effectively decomposed using basic conditions in dimethyl carbonate (62). Softwood lignin depolymerization remains a very potent technical challenge. Interest in this topic has experienced explosive growth during the last few years with various other emerging strategies, including microwave (63–66), ionic liquids (67–71), and supercritical fluids (72–75), etc. coming on stream. It is anticipated that methods using increasingly mild conditions (low temperature and pressure), and higher degradation efficiency (faster depolymerization rate/high conversion) and environmental efficiency (low energy input/pollution), will become viable in the next years.

Conversion of Lignin to Higher Value Products: Carbon Fibers Carbon fibers are increasingly used as low weight, high strength reinforcing agents. They are commonly produced from polyacrylonitrile (PAN). Several strategies have been adopted to utilize lignin in this application on its own (76), doped into PAN, or copolymerized with PAN (77) and then thermalized to give the carbon fiber. Fibres of diameters as low as 200 nm (78), solid or hollow, could be achieved using electrospinning without the need of binders. It is not clear that the quality of these fibers match those derived from PAN (1), but the ability to convert lignin into high value materials will encourage continued research in this area. 97 Grosse; Mobilizing Chemistry Expertise To Solve Humanitarian Problems Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


The Useful Properties of Lignin Itself To Be Exploited in Performance Materials Irrespective of the means of isolation (see above), the chemical features of lignin that are associated with its utility are linked (from a chemist’s perspective!) to the functional groups it presents. The highly aromatic constituents of lignin are excellent sources of energy; as they are less oxidized, the calorific content is much higher than cellulose. The high aromatic character provides lignin with a much higher refractive index, ranging from 1.55-1.65 depending on the wavelength of the light, than aliphatic polymers (79). Many of the properties of a given lignin depend on the specific species from which it was extracted and the method of extraction. One reference amusingly suggested that there is no consistency in the literature regarding lignin because every tiny variation in processing leads to changes in the resulting isolated products. For example, the molecular weight of lignin depends highly on the method of extraction. Organosolv lignin is the lowest molecular weight (MW) 500-3000 g/mol material (the higher molecular weight fractions do not readily dissolve); the MW of kraft lignin is higher ~1000-5000 g/mol (higher molecular weight fractions are more soluble because of free phenolates at higher pH); and the highest is 5000–400,000 g/mol for lignosulfonate-derived lignin (10, 80) (at lower pH (

Exploiting Lignin: A Green Resource - ACS Symposium Series (ACS

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