6-11: Investigation of the porous structure of cellulosic biomass through confocal laser scanning microscopy (CLSM)

Mass transport limitations within the porous matrix of cellulosic materials restrict the ability of cell wall degrading enzymes to hydrolyze cellulose into fermentable sugars. The study of the diffusive behavior of probe molecules into and out of cellulosic substrates can help us assess the pore size distribution and the underlying mass transport limitations within these materials. In this study, fluorescently-labeled dextrans with molecular weights ranging from 20kDa to 150kDa were used as probes to assess diffusion and entrapment in the porous structure of filter paper. These probes have hydrodynamic radii that are comparable to the size of cellulases and can give insight into the diffusion hindrance and entrapment encountered as these enzymes hydrolyze cellulosic particles. As probes diffuse into biomass particles, they encounter a complex structure with a distribution of pore widths, which results in heterogeneous diffusion. Therefore, a mass transfer model involving pore grouping was developed to separate easy and hard to access pores and predict the diffusion coefficients of the probes in each type of pore. Modeling results show 74% of the accessible pore volume is from easily accessible pores and the rest from diffusion-hindered pores for 6 nm probes. Therefore, this method can be applied to study the porous structure inside biomass and help access the diffusion process for certain enzymes with known sizes.