Heat and Mass Transport in Processing of Lignocellulosic Biomass for Fuels and Chemicals - Sustainable Biotechnology: Sources of Renewable Energy - page 8

w/w) and behave as a wet granular material [27]. This material is highly compress-
ible and the wet particles easily “stick” to each other and agglomerate. With no free
water in the system, the material becomes difficult to shear or uniformly mix.
At the ultrastructural scale of plant cell walls, catalysts must penetrate through
the nano-pore structure of the cell wall matrix to access the “buried” and inter-
meshed carbohydrate polymers. Based on reported average cell wall pore sizes of
5-25 nm [28-31], small chemical catalysts (<1 nm) may not face as significant
a penetration barrier as do enzymes (about 10 nm). The most dominant commer-
cial cellulase component enzyme, cellobiohydrolase I or Cel7A, has dimensions
of ~ 5

× ×

12 nm [32, 33] which is roughly the same size as smallest of these
reported nano-pores, likely restricting accessibility to primarily surface cellulose
chains. Once they have penetrated the cell wall matrix, these enzymes must locate
suitable substrates. For Cel7A, this implies that a region of cellulose microfibril has
been sufficiently unsheathed from lignin and hemicellulose to expose the cellulose
core (Fig. 1). This unsheathing process may be accomplished by the pretreatment or
as an ablative effect caused by the system of cellulase enzymes which can peel away
microfibrils from the surface layers. Lignin is also a major impediment to cellulase
action because it is difficult to remove uniformly or modify through pretreatment.
Furthermore, it is entirely unclear at this time if lignin can be effectively removed
from cell walls using enzymes.

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Fig. 1.1 Cartoon depiction
of cellular-scale ( a )and
molecular-scale ( b ) obstacles
to heat and mass transport in
lignocellulosic biomass