A comprehensive review of municipal solid waste chemistry relevant to hydrothermal liquefaction

Municipal solid waste (MSW) poses a major challenge due to its increased heterogeneity and complex chemical composition. Derived from varied household and industrial streams, MSW contains a broad mix of organic and inorganic materials with highly diverse chemical structures and reactivities. Grasping this chemical complexity is critical for developing efficient and sustainable waste valorization and recovery pathways. Yet, the highly variable nature of MSW means that its behavior in bioconversion processes like hydrothermal liquefaction (HTL) is still not well understood, resulting in difficulties in achieving consistent operation and predictable product quality. However, the existing HTL literature is highly fragmented, with most studies examining isolated fractions such as food waste, plastics, or lignocellulosic materials rather than real mixed MSW. As a result, a unified reaction-network framework integrating all MSW fractions is still lacking. While recent studies suggest synergistic interactions between plastics and food or paper fractions that affects product distribution, solvolysis, radical recombination, and liquefaction efficiency, these cross-fraction interactions remain largely unexplored and represent an emerging research frontier. This review therefore examines the detailed chemical composition of MSW and the fundamental chemical processes governing its conversion via HTL. Emphasis is placed on the interplay between key organic components, their intrinsic properties, and their transformation pathways under hydrothermal conditions. Key technical barriers limiting efficient MSW HTL are systematically analyzed from a structural chemistry perspective. By synthesizing existing literature and identifying knowledge gaps, this review proposes actionable directions to accelerate the development and deployment of HTL for efficient MSW valorization.