Biomedical Engineering Reference
In-Depth Information
by ether linkages. Today we know that wood, depending on the tree species, part
of the tree and growing conditions, consists of roughly between 20 and 40% lignin
(Sj ostr om).
Perhaps the greatest difficulty in the study of naturally occurring lignin is that it is
not possible to comprehensively investigate this material in situ . As such, the majority
of advances in the understanding of this important biomacromolecule have come from
investigations into the isolated lignins from various natural sources as well as from syn-
thetically prepared phenylpropane based polymers. The elucidation of structural units
and types of bonding between monolignols has come from the analysis of lignin poly-
mer fragments using sophisticated techniques such as nuclear magnetic resonance, mass
spectrometry and spectrophotometric techniques to name a few. All of these techniques,
however, rely on some knowledge of the starting raw material (i.e. from where was the
lignin isolated) and also how the lignin fragments were isolated.
There are numerous techniques for isolating lignin on vastly different scales. For
example, small quantities of lignin with only relatively minor chemical changes can be
prepared by the ball milling of wood samples whereas high quantities of kraft lignin
and lignosulfonates are produced as a by-product during the chemical pulping of wood
fibres.
To characterize and make further use of lignin it is necessary to separate the lignin
from the wood material. It is generally considered that this can only be done by more
or less changing the lignin chemical structure to different degrees, often with significant
reductions in molecular weight and introduction of non-native chemical species. In
Payen's first experiment the carbohydrate matrix as well as the lignin was partly oxidized
and degraded by the nitric acid treatment. This gave a lignin of lower molecular mass
that was richer in carboxyl and phenolic groups than in the native state, and thereby
more easily dissolved during the following alkaline extraction. From those days and up
to now several different routes and industrial processes for the removal and isolation of
lignin from wood, pulp fibres and spent liquors have been developed. Some of these
different isolation methods along with the advantages and possibility of using these
techniques in the bio-refining of lignin are presented in the subsections below. Ideally,
any method that produces a well defined lignin material which can be constantly prepared
in a reproducible fashion will lend itself to further use in value added applications in the
future bio-refinery. This perhaps is a goal that might seem difficult to achieve when the
starting material is a branched, three-dimensional amorphous macromolecular structure
that appears randomly polymerized.
7.3.1
Isolation of Lignin from Wood and Pulp Fibres
Amongst the methods used for isolation of lignin from wood, the ones referred to below
are considered to give lignin substrates essentially unchanged in their chemical struc-
tures. The first method described involves extraction with organic solvents and was
suggested by Brauns (Brauns 1939) and gave very low yields. Due to this it is thus
doubtful if the Brauns lignin is fully representative for the lignin in wood. Bjorkman
introduced grinding as a pretreatment before toluene extraction to yield what today is
defined as milled wood lignin (MWL) (Bjorkman 1956, 1957, Bjorkman and Person
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