<i>In-situ </i>small-angle neutron scattering (SANS) and extensive molecular dynamics (MD) computer simulation were used in a combined approach to examine real-time breakdown of biomass and the temperature dependence of lignin structure and dynamics. Lignin, a major polymeric component of plant cell walls, forms aggregates in vivo during pretreatment of lignocellulosic biomass for ethanol production. The aggregates are thought to reduce ethanol yields by inhibiting enzymatic hydrolysis of cellulose. Here, we report on real-time SANS experiments during dilute acid pretreatment of biomass using a temperature-pressure reaction cell.  The temperature of the cell was ramped-up from 20 to 180 °C, then maintained at 180°C for 60 min and finally the cell was cooled down to room temperature. A clear characteristic structural feature first appeared at 120°C representative of a particle size. This structural feature with increasing temperature and residence time at 180°C, progressively increased in size. We have identified the particle that appears at 120°C as lignin aggregates. To understand variation in the lignin association process, native switchgrass, native poplar as well as lignin mutant switchgrass samples were investigated in real-time during dilute acid pretreatment.  Real-time measurements showed significantly higher increase in lignin aggregate size compared to ex-situ results for native poplar, radius of gyration of &gt; 600 Å to ~90 Å, respectively. On the other hand, native switchgrass undergoes a smaller difference, ~250 Å for real-time to ~150 Å for ex-situ measurements. Furthermore, the lignin aggregate characteristic feature for mutant switchgrass with lower lignin content understandably was absent for the entire duration of the dilute acid pretreatment. To understand the temperature-dependent structure and dynamics of individual lignin polymers in aqueous solution we performed extensive (17<i>μs</i>) MD simulations. Between 150 and 210 °C, the lignin transitions from a glassy, compact state to a mobile, extended state. In the low temperature states studied, the blobs are inter-permeable. In contrast, the particles at high temperatures become spatially separated, leading to a fractally crumpled globular form. Specifically, our combined approach indicates that lignin aggregation occurs rapidly during heating – not during the cool-down, as was previously assumed.

Real-time visualization of lignin structure during dilute acid pretreatment of native and lignin mutants of switchgrass and poplar

255th National Meeting (2018)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.