(748c) Transport and Reaction Modeling of Biomass Pretreatment and Topochemical Evolution Using Actual 3D Structure of Plant Cell Walls

Plant biomass is a renewable raw material for a wide variety of bio-based products which include pulp and paper, bio-plastics, biofuels, wood plastic composites etc. The process of conversion of plant biomass to other useful products is heavily dependent on our understanding of the physio-chemical structure of plant cell walls. Plant cell walls are composed of a complex network of pore spaces and fibers of cell wall components that has a significant influence on the efficiency of the biomass conversion process, the properties of the bioproducts and their end-use applications. Hence it is important to characterize the 3D plant cell wall structure. 3D characterization techniques such as X-ray computed tomography (XRCT) and Transmission electron microscopy computed tomography (TEM-CT) have been popular. Although, the X-ray CT provides micron scale visualization, it is not sufficient to get a complete understanding of the 3D structure of plant cell walls. Hence, a more powerful technique of TEM-CT is used to probe the nanoscale 3D ultrastructure. The computed tomography technique is based on capturing a series of images by successive tilts around a single axis and reconstructing the 3D image by back projection. 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 first step in the conversion process is the pretreatment step, which disrupts 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. 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 from TEM-CT can be correlated with the topochemical distribution from Raman Spectroscopy to develop a transport-reaction model based on a hybrid random walk and reaction. This model involves a stochastic dynamic approach that keeps track of the change in structure and spatial concentration of the cell wall components simultaneously. A fixed number of particles of the reagent used for pretreatment, which is representative of its concentration, diffuse through the pore spaces of the plant cell walls till it encounters a pore-fiber interface. This path can be monitored by a stochastic hybrid random walk approach. At the interface, the reagent used for pretreatment can either react with one or more of the cell wall components or undergo reflection back into the pore space, based on the reaction probability. The, spatial concentration profile of the pretreating agent at different times can be determined by keeping track of the number of particles of the pretreating agent at each position as time proceeds. The spatial concentration profiles of the cell wall components and their evolution during biomass conversion can also be determined from the corresponding rate equations.

Thus, with such a model, a complete understanding of the 3D structure and the physicochemical distribution and its evolution can be understood. This provides a fundamental insight into biomass recalcitrance and how they can be addressed in developing efficient conversion strategies.