Mar Brendan D, Kulik Heather J
Department of Chemical Engineering, Massachusetts Institute of Technology , Cambridge, Massachusetts 02139, United States.
J Phys Chem A. 2017 Jan 19;121(2):532-543. doi: 10.1021/acs.jpca.6b11414. Epub 2017 Jan 5.
Lignocellulosic biomass is an abundant, rich source of aromatic compounds, but direct utilization of raw lignin has been hampered by both the high heterogeneity and variability of linking bonds in this biopolymer. Ab initio steered molecular dynamics (AISMD) has emerged both as a fruitful direct computational screening approach to identify products that occur through mechanical depolymerization (i.e., in sonication or ball-milling) and as a sampling approach. By varying the direction of force and sampling over 750 AISMD trajectories, we identify numerous possible pathways through which lignin depolymerization may occur in pyrolysis or through catalytic depolymerization as well. Here, we present eight unique major depolymerization pathways discovered via AISMD for the recently characterized spirodienone lignin branching linkage that may comprise around 10% weight of all lignin in some softwoods. We extract representative trajectories from AISMD and carry out reaction pathway analysis to identify energetically favorable pathways for lignin depolymerization. Importantly, we identify dynamical effects that could not be observed through more traditional calculations of bond dissociation energies. Such effects include thermodynamically favorable recovery of aromaticity in the dienone ring that leads to near-barrierless subsequent ether cleavage and hydrogen-bonding effects that stabilize newly formed radicals. Some of the most stable spirodienone fragments that reside at most 1 eV above the reactant structure are formed with only 2 eV barriers for C-C bond cleavage, suggesting key targets for catalyst design to drive targeted depolymerization of lignin.
木质纤维素生物质是一种丰富的芳香族化合物来源，但由于这种生物聚合物中连接键的高度异质性和变异性，原生木质素的直接利用受到了阻碍。从头算引导分子动力学（AISMD）已成为一种卓有成效的直接计算筛选方法，用于识别通过机械解聚（即超声处理或球磨）产生的产物，同时也是一种采样方法。通过改变力的方向并对750多条AISMD轨迹进行采样，我们确定了木质素在热解或催化解聚过程中可能发生解聚的众多可能途径。在这里，我们展示了通过AISMD发现的八条独特的主要解聚途径，这些途径涉及最近表征出的螺二烯酮木质素支链连接，在某些软木中，这种连接可能占所有木质素重量的10%左右。我们从AISMD中提取代表性轨迹，并进行反应途径分析，以确定木质素解聚的能量有利途径。重要的是，我们确定了通过更传统的键解离能计算无法观察到的动力学效应。这些效应包括二烯酮环中芳香性的热力学有利恢复，这导致随后的醚键断裂几乎无势垒，以及稳定新形成自由基的氢键效应。一些最稳定的螺二烯酮片段位于反应物结构上方最多1电子伏特处，其形成时C-C键断裂的势垒仅为2电子伏特，这为驱动木质素靶向解聚的催化剂设计提供了关键靶点。
