The potential of lignin-functionalized metal catalysts - A systematic review

The deployment of lignin, one of the most naturally occurring biopolymers and sources of aromatics with plenty of reactive functional sites, in discovering and developing catalysts and catalytic processes is a vital part of maintaining future energy and ecosystem balance. Lignin is remarkably biodegradable and highly compatible, which makes it an ideal candidate as a versatile support for a variety of metal catalysts, such as Pt, Pd, Ag, Co and Cu, representing a significant shift away from fossil-based and resource-intensive catalysts. This shift has farreaching implications for industrial applications and research endeavors. This review provides an elaborated appraisal of lignin-based metal catalysts for a variety of applications, where the functionalized substrate and metal properties, activity, selectivity, regeneration potential, and their interactions have been systematically scrutinized. The synthetic survey conducted highlights a critical issue in metal catalysts, namely, the random aggregation of metal particles. This aggregation significantly limits the performance of such catalysts. To address this challenge, this work proposes the development of lignin-functionalized bimetallic systems. These systems exhibit distinct properties and higher activity compared to single-metal catalysts, promising previously unattainable results. However, despite the advantages of lignin-based metals, their practical application is still in its early stages. This is due to the complexity of lignin chemistry, stability concerns, and the potential for degradation at high temperatures. The combination of optimized experiences, theoretical approaches and simulated mechanisms of lignin-based metal catalysts are inspiring prospects and crucial concerns that require further study in future research for the beneficial integration of bio-based catalysts.