(634b) A Novel Transport-Reaction Model for the Estimation of Topochemical Changes during the Pretreatment of Plant Biomass Using Raman Spectroscopy

Porous materials have a complex network of pores and fibers which affect their structural and transport properties. In case of plant cell walls, the internal structure consisting of cellulose fibers enclosed in a lignocellulose matrix and pore spaces, has a significant influence on the efficiency of the biomass conversion process, the properties of the materials and their end-use applications. Since plant biomass is a renewable source and serves as a starting material for a wide range of bioproducts including biofuels, bioplastics, pulp and paper, etc., it is extremely important that we understand the effect of structure on conversion processes. The first step in the conversion process is the pretreatment step, which achieves the disruption of the recalcitrant structure of lignocellulose and increases the efficiency of the subsequent hydrolysis. One or more of the cell wall components are dissolved during this process which causes significant changes in the structure and topochemistry. These changes have to be studied in order to determine the efficiency of pretreatment strategies. In this regard, several 2-D and 3-D characterization techniques have been used. However, in order to get a true sense of the structure of plant cell walls, 3D characterization techniques such as X-ray computed tomography (X-ray CT) and TEM computed tomography (TEM-CT) have been very effective. The X-ray CT offers an insight into the micron scale structure of plant cell walls whereas the TEM-CT is a more powerful technique and visualizes the nanoscale architecture of these cell walls and hence, provides a complete understanding of the 3D structure. The changes in structural characteristics such as porosity, specific surface area and 3D pore size distribution during the pretreatment process can be determined from the processed TEM-CT images.

The structural information obtained from TEM-CT can be correlated with topochemical information from Raman spectroscopy to fully understand the physiochemical structure of plant cell walls. The pretreatment of plant biomass is treated as a diffusion-reaction process. A transport-reaction model based on a hybrid random walk and reaction is proposed to estimate the topochemical changes in the cell walls. A fixed number of particles of the reagent used for the pretreatment process, which is representative of its concentration, diffuse through the lumen and pore spaces of the cell wall and follow a random walk path until they encounter the cell wall interface. At this interface, they either react with one more cell wall components or diffuse through the cell wall or bounce back into pore space. This is decided by the reaction probability as well ratio of diffusivities of the pore and cell walls. The random path followed by these particles and the corresponding changes in spatial concentration of the cell wall components such as lignin, cellulose and hemicellulose are monitored throughout the pretreatment process. Thus, such a stochastic dynamic approach keeps track of both the structural changes and spatial concentration changes of the reactants. The transport-reaction model was tested initially for a model system consisting of a periodic array of spheres. This was extended to biomass images obtained from Raman Spectroscopy. A 3D stack of Raman images was obtained by duplicating a single Raman image giving rise to a cylindrical network of pores and cell wall fibers on which the model was tested. The model can also be applied to TEM-CT images after making changes to account for nanoscale transport. Thus, such an approach gives a complete understanding of the conversion process and is very useful in determining the efficiency of such processes.