The fast pyrolysis of lignocellulosic materials like wood typically yields crude bio-oil (up to 75 wt.%), gases (10-20 wt.%), and char (10-15 wt. %). However, it only converts between 10 and 20% of the native cellulose into levoglucosan; the rest is transformed into small molecules and charcoal with lower economic value. This low levoglucosan yield contrasts with yields close to 60% reported from the fast or vacuum pyrolysis of cellulose (Shafizadeh et al. 1979, 1982, Scott et al. 1995, 1989, Molton and Demmitt 1977). The use of pyrolysis for the production of levoglucosan can be traced back to the studies of Pictet and Sarisin in 1918. Under vacuum pyrolysis the authors reported yields of an aqueous phase close of 32 wt. % and a yellow pastry of 44 wt. %. The yellow fraction was associated to levoglucosan. It had been discovered in 1894 by Tanret (Lede 2012). The paper of Pictet and Sarisin (1918) was the first to point out the potential of cellulose pyrolysis as a method for the production of glucose and alcohol.
The causes for the low yield of levoglucosan typically obtained from the fast pyrolysis of lignocellulosic materials are still unknown, but several generations of researchers have studied this complex reaction (Golova and Krylova 1960, Halpen and Patai 1969, Golova 1975, Kilzer and Broido 1975, Broido 1976, Shafizadeh et al 1982, Evans and Milne 1987, Scott et al 1989, Pouwels et al 1989, Diebold 1994, Antal and Varheyi 1995, Milasavljevic and Subberg 1995, Di Blasi 2000, Teixeira et al 2011, Lede 2012). It is known that even tiny amounts of alkaline metals like sodium and potassium, naturally found in biomass, can decrease the yield of usable sugars by accelerating fragmentation reactions (Nowakowski and Jones 2008). The catalytic effect of alkalines can be eliminated by acid washing the feedstock (Oudenhoven et al 2012). Early on, researchers found that adding small amounts of sulfuric acid to lignocellulosic materials (cellulose with lignin) induces a two-fold increase in levoglucosan yield (Shafizadeh and Stevenson 1982). The group of Professor Brown at Iowa State found that in materials with alkalines, this phenomenon is due to the passivation of the catalytic effect of alkalines via the formation of stable salts (Kuzhiyil et al. 2012). More recently Zhou et al (2013) proved that after washing to remove the minerals, mild acid impregnation further increased the yields of levoglucosan, perhaps by mitigating the undesirable interactions between cellulose and the other constituents of the lignocellulosic matrix.
Table 1 highlights important pretreatment methods found in the literature. A combination of acid-wash and acid-impregnation seems to be the most viable approach to increasing levoglucosan yields, though it has not been fully explored. The major challenge facing the scientific community is to control cellulose primary depolymerization reactions and to mitigate levoglucosan secondary reactions to achieve yields close to those that are theoretically possible. A study recently conducted in a wire mesh reactor under very fast heating rates and a very high vacuum by Twente University suggest that up to 95 % of the cellulose can be converted in fermentable anhydrosugars via fast pyrolysis (Westerhof et al. 2014). New strategies have to be developed to achieve levoglucosan yields close to those that can be theoretically obtained by cellulose primary pyrolysis reactions.
Table 1: Important chemical pretreatments of cellulose and biomass to increase pyrolytic sugars
|Pretreatment method||Biomass; pyrolysis||LG yield (mass %)||Explanation provided||Ref.|
|Sulfuric acid wash (5%) 90°C 5.5 hr followed by water wash to pH 6.6||Cellulose (Avicel); fluidized bed (500°C)||26.9 w/o acid 38.41 w/ acid||Hydrolysis of cellulose breaks glycosidic bonds; ash removal||Piskorz et al. 1989|
|Sulfuric acid wash (5%) 90°C 5.5 hr followed by water wash to pH 6.6||Poplar wood (50% cellulose); fluidized bed (500°C)||3.04 w/o acid 30.42 w/ acid||Hydrolysis of cellulose breaks glycosidic bonds; ash removal||Piskorz et al. 1989|
|Organic extraction (8h toluene-ethanol) followed by hydrolysis (0.1 M H2SO4, >100oC), rinse w/ H2O, then 0.1% (feedstock) H2SO4 impregnation||Douglas fir (46% cellulose); tube furnace (400°C)||12 w/o acid 26 w/ acid 56 w/acid (on cellulose basis)||Extractives, hemicellulose, ash removal; suppressed lignin-cellulose reactions||Shafizadeh at al. 1982|
|Acid wash & H2O rinse, 0.1% (feedstock) H2SO4 impregnation||Douglas fir (46% cellulose); tube furnace (400°C)||9 w/o acid 19 w/ acid 41.3 w/acid (on cellulose basis)||Ash removal||Shafizadeh at al. 1982|
|Water wash, 0.1 % (feedstock) H2SO4 impregnation||Douglas fir (46% cellulose); tube furnace (400°C)||7 w/o acid 14 w/ acid 30 w/acid (on cellulose basis)||Suppressed lignin-cellulose reactions and inorganics||Shafizadeh at al. 1982|
|Acid wash, 0.1% H2SO4 impregnation||Cellulose; tube furnace (400°C)||36 w/o 35 w/||Shafizadeh at al. 1982|
|10% acetic acid, 3.75% acetone, 3.75%, 1.5% propionic acid, 1.5% guiacol; rinse with DI water||Pine; fluidized bed (530°C)||3.4 w/o 18 wash + rinse (39 on cellulose basis) 11.1 wash, no rinse (24 on cellulose basis)||Mineral removal||Oudenhoven et al. 2012|
|0.2 mmol/g various acids, 15 g Aq. w/ 5g biomass, dry||Switchgrass (33.3% cellulose); Py-GC/MS (500oC)||2 w/o acid (6 on cellulose basis) 1.5 acetic/formic acid (4.5 on cellulose basis) 3 nitric acid (9 on cellulose basis) 5 HCl (15 on cellulose basis) 12 H3PO4 (36 on cellulose basis) 16 sulfuric acid (48 on cellulose basis)||H2SO4>H3PO4>HCl>HNO3 Chlorides, phosphates, and sulfates form thermally stable salts (inorganic suppression)||Kuzhiyil et al. 2012|
|0.3% H2SO4, 2L/600g biomass followed by drying||Douglas fir (45% cellulose); augur reactor (500°C)||5.5 w/o (12 on cellulose basis) 12 w/ (26 on cellulose basis)||Alkaline inorganics suppression||Zhou et al. 2013|
|0.05% H2SO4, 2L/600g biomass followed by drying||Douglas fir (45% cellulose); fluidized bed reactor (500°C)||3 w/o (6.7 on cellulose basis) 5.5 w/ (12 on cellulose basis)||Alkaline inorganics suppression||Zhou et al. 2013|
As a dehydrated sugar monomer, levoglucosan can be separated and hydrolyzed (Bennett et al. 2009) and converted to a multitude of chemical species, including alcohols and lipids (Prosen et al. 1993, Lian et al. 2010, Jarboe et al 2011). More recently the direct fermentation of levoglucosan in organisms with the levoglucosan kinase enzyme has been proven (Layton et al 2011). The use of levoglucosan for the production of polymers was extensively studied in the 1980s and early 1990s (Witczak 1994) but today is receiving very little attention.
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Text courtesy of Manuel Garcia Perez of Washington State University.
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