Chemistry: electrical and wave energy – Processes and products – Electrostatic field or electrical discharge
Patent
1987-08-18
1988-11-22
Andrews, R. L.
Chemistry: electrical and wave energy
Processes and products
Electrostatic field or electrical discharge
C25F 500
Patent
active
047863820
DESCRIPTION:
BRIEF SUMMARY
This invention relates to an electrochemical process for the electrochemical oxidative degradation of lignins and related substances, and to an electro-chemical cell in which the process may be performed.
Lignin is, after cellulose, the principal constituent of the woody structure of higher plants. About 25% of dry wood consists of lignin, in part deposited in the xylem cell walls and in part located in the intercellular spaces, where it may constitute as much as 70% of the solid materials present.
The exact chemical structure of lignin, either in wood, where it is usually bonded to plant polysaccharides, or when separated from other wood substances, is not fully known. Much is known however about the structure of certain isolated lignines. For example the lignin isolated from coniferous trees is though to be a polymer resulting from enzymically induced oxidation of coniferyl alcohol.
Lignins appear to be constructed of phenylpropane units, substituted princiaplly by methoxy and hydroxy groups, and joined in a polymeric structure by various types of linking groups.
The most common types of substituted phenylpropane units in both coniferous and deciduous lignins are hydroxyphenylpropane (i), syringylpropane (ii) and guaiacylpropane (iii) units: ##STR1##
The relative proportions of these three units vary between coniferous and deciduous lignins, eg coniferous lignin contains about 14% (i), 7% (ii) and 79% (iii), whereas deciduous lignins contain about 3 of (ii) to 2 of (iii). As well as the methoxy and hydroxy groups, smaller quantities of other minor functional groups may also be present on these units.
The phenylpropane units in lignin are linked mainly by carbon-carbon bonds and by ether linkages. Spectroscopic data suggest that about 25% of the units are linked as biphenyl linkages. The phenolic oxygen in about 66% of the units is present as an ether linkage.
Some examples of typical linkages are shown below together with the approximate percentages to which they occur in a typical lignin structure. ##STR2##
But a wide variety of other linkages probably also exist in lignins, particularly between the propyl chains to form cyclic species such as cyclic ethers, such as ix and x below: ##STR3##
By means of such linkages the phenylpropane units are linked into a large polymeric structure, probably randomly linked. Average molecular weights for coniferous lignin is over 10,000, whilst the average molecular weight of deciduous lignin probably does not exceed 5000.
A suggested structure for coniferous lignin incorporating such bonding is shown in Kirk-Othmer `Encyclopaedia of Chemical Technology` 2nd Edn, Vol 12 (1967) p 367.
Millions of tons of lignins are potentially available annually from industry, such as wood and bark wastes from the lumber industry, the match industry, and particularly from the wood pulp and paper industries.
In the pulp industry lignin is usually obtained as dissolved lignosulphonic acid or as lignosulphonate salts as a result of cooking wood chips under pressure in the presence of aqueous sulphurous acid or sulphites, which leaves the cellulose as a residue for example for paper making. From the solution the acid or salt may be obtained by drying.
From these lignosulphonates, alkali lignate salts may be prepared by hydrolysis using aqueous hydroxides, especially sodium and calcium hydroxides. Alkali lignates may also be prepared directly from wood chips by cooking them with sodium hydroxide, optionally with a little sodium sulphide present. These lignates are almost free from non-lignin organic constituents but may contain a little combined sulphur if they have been prepared from the sulphonates or if sodium sulphide has been used.
Another source of lignin which is likely to become of increasing importance is straw. Millions of tons of straw are wasted each year, eg by burning. Straw contains about 16% of lignin. Although straw lignin is built up of the units discussed above, it has a slightly different structure to wood lignin. Straw lignin may be extracted chemically eg by sodium hyd
REFERENCES:
patent: 775829 (1904-11-01), Kitsee
patent: 2828253 (1958-03-01), Kurz
Chemical Abstracts, vol. 88, 1978, E. I. Kovalenko et al., "Electrochemical Oxidation of the Natural Polymer Lignin", p. 111, No. 154596C.
Smith Carment Z.
Utley James H. P.
Andrews R. L.
The Minister of Agriculture Fisheries and Food in Her Britannic
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