From lignin to market: a technical and economic perspective of reductive depolymerization approaches

Abstract

Lignin has grown into one of the main candidates to replace fossil-based resources as it is the largest renewable source of aromatic building blocks. The complex structure of polymeric lignin, however, requires depolymerization to simpler building blocks for the chemical industry. One of the most promising depolymerization approaches is reductive depolymerization of which two process configurations are currently studied in pilot scale installations for upscaling to industrial scale: (i) reductive catalytic fraction-ation (RCF), and (ii) reductive catalytic depolymerization (RCD). Both technical and techno-economic aspects will be covered within this review, discussing the advantages and challenges of both approaches regarding processing, production costs, product output, and applications. In this regard, RCF benefits from its decreased energy and solvent consumption linked with being a one-step process and delivers a product with a high monomer content (∼25-45 wt%). RCD, on the other hand, has the advantage of continuous processing and reduced catalyst fouling and delivers a product that mainly consists of oligomers (<10 wt% monomers). The complete overview of both processes presented here addresses their potential, and can guide future researchers, policy makers and companies to make thoughtful decisions on lignin valorization. Green foundation 1. The review discusses the recent advances in reductive lignin depolymerization to produce renewable aromatic building blocks for a variety of applications. The focus lies on the two main approaches of reductive lignin depolymerization: reductive catalytic fractionation (RCF), starting from wood, and reductive catalytic depolymerization (RCD), starting from isolated lignin; 2. The benefits and limitations of both processes are discussed from a technical, techno-economic, and application point of view. This clear comparison can be used by companies, policy makers, and investors to make thoughtful decisions on their sustainable strategies for the future; 3. The availability of a pilot installation for each process emphasizes the industrial potential of both processes. However, each process is still faced with certain limitations, critically assessed within this review, that should be targeted in future research.