Lignocellulosic precursor

Mackay D.M. Roberts P.V (1982) The dependence of char and carbon yield lignocellulosic precursor composition. Carbon,20, 7. 

Table 5.2. Characteristics of chars obtained at 600 °C from different lignocellulosic precursors...

All these lignocellulosic precursors produce a char upon carbonization with a yield ranging from 20 to 30 wt%, this meaning that after therm activation the overall yield may be around 10 wt%. Woods, as sawdust, are favored for the manufacture of powdered activated carbons by the chemical activation method (addition of H3PO4). The precursor is crushed and sieved to a coarse powder before impregnation, and the mixture is then subjected to heat treatment. 

Production capacity of activated carbon in Japan is widely distributed among many companies, the largest producers being Takeda Chemicals Industries (23%), Kuraray Chemicals (18%), Futamura Chemical Industries (18%) and Mitsubishi Chemical Corporation (11%). Lignocellulosic precursors such as coconut shell, wood and sawdust are the more frequently used, with smaller proportion of coal, resinous pitch, etc. Regeneration capacity is estimated to be around 30,000 tonnes per year. Some of the activated carbon is produced outside Japan, mainly in Southeast Asia, where joint venture companies have been established. 

In addition to the conventional carbon vapor deposition method, our research group has used two additional procedures for the preparation of CMS i) controlled uncatalysed gasification of chars obtained from lignocellulosic precursors and ii) mild oxidation of a char and subsequent controlled removal of oxygen surface groups (this second procedure can also be applied to a previous CMS with wider micropore width, to reduce the pore width). 

In the first of these procedures, the lignocellulosic precursor (coconut shells or peach stones) was acid washed to eliminate mineral matter as far as possible and then slowly carbonized. The char was activated thermally) with carbon dioxide at 750 °C (to ensure a slow gasification) to controlled bum-offs. 

The macerals in lower rank coals, eg, lignite and subbiturninous coal, are more complex and have been given a special classification. The term huminite has been appUed to the macerals derived from the humification of lignocellulosic tissues. Huminite is the precursor to the vitrinite observed in... 

Activated carbons are obtained by physical (steam or C02 at 800°C-1000°C) or chemical (KOH at 700°C-800°C) treatment of carbons. The most usual carbon precursors are lignocellulosic derivatives, polymers, pitch, etc. Activated carbons are generally highly microporous (<2nm) with some amount of mesopores (2-50 nm) depending on the degree of activation, their BET SSA ranges from 1000 to 3500m2 g 1. 

Fig. 19 shows the adsorption capacity attained with other precursors such as a bituminous and a subbituminous coal, vs. the heat treatment temperature and compared with wood [46]. It is clear that phosphoric acid activation must only be applied to lignocellulosic materials since the coals hardly develop any porosity. 

Inhibitory furaldehydes in lignocellulose hydrolysates include 2-furaldehyde (furfural) and 5-hydroxymethyl-2-furaldehyde (HMF) (Fig. 2). The concentrations of furfural and HMF in lignocellulose hydrolysates are highly dependent on the raw material and on the conditions used for acid hydrolysis. Softwood acid hydrolysates contain low amounts of furfural compared with HMF [58]. Hardwood hydrolysates, which contain high concentrations of pentoses, the precursors to furfural, contain more similar amounts. Several recent investigations [102-105] deal with the effect of the furaldehydes on S. cerevisiae and the conversion of furfural to furfuryl alcohol and HMF to 5-hydroxymethyl-fur-furyl alcohol by S. cerevisiae. The presence of the furaldehydes causes lag phases in the formation of biomass and ethanol. 

Phosphoric acid and zinc chloride are activating agents usually used for the activation of lignocellulosic materials which of coal precursors or chars have not been previously carbonized. Contrarily, cilkaline metal compounds, usually KOH, are used for the activation (Table 8.5). 

In industry, activated carbons are essentially produced by carbonization (pyrolysis at temperatures up to 900°C under neutral atmosphere) of various precursors (lignocellulosic, polymers, anthracites,. ..), followed by physical and chemical activation [1], Physical activation is generally realized around 900°C through partial gasification of carbon, using CO2 or steam, according to Eqs. 12.1 and 12.2 ... 

Physical activation has traditionally included a controlled gasification of the carbonaceous material that has previously been carbonized, although occasionally the activation of the precursor can be done directly. Many different carbonaceous precursors have been employed for physical activation lignocellulosic materials, coals, woods, and materials of polymeric origin. The samples are typically treated to 800-1100°C with an oxidant gas, mainly CO2 or steam, so that carbon atoms are removed selectively. Although this process obviously involves a chemical reaction (and is not merely a physical process), it is known as physical activation. 

Otowa et al. [53], who mixed petroleum cokes with KOH, as well as those of Beguin and Setton [122], Mochida et al. [123,124], and Yamashita and Ouchi [125-127] related to reactions between aromatic hydrocarbons and other precursors with hydroxides, and those of Diez-Teran et al. [84], who mixed KOH with a lignocellulosic material. 

Activation results with KOH and NaOH are not necessarily the same. The relative effectiveness depends on the precursor used, especially on its structural order NaOH appears to be better for carbons without structural order (e.g., subbituminous coal, lignite, and lignocellulosic materials), whereas KOH appears to be better for those having some structural order (e.g., anthracite, heat-treated coals, MWCNT). 

Levulinic acid and its derivatives are key platform molecules opening promising pathways for novel bio-based polymers from lignocellulosic feedstock and could be potential precursors of synthetic mbbers or resins. Carbohydrate-based monomers suited for polycondensation have been primarily considered for developing a growing industrial sector, now starting to compete with conventional oil-based polymer production. 

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