Flexible Metal-Organic Frameworks for Adsorptive Separation of Liquid Hydrocarbons

ConspectusThe separation and purification of liquid hydrocarbons are vital for producing various petrochemical feedstocks. However, the structural and chemical similarities among these hydrocarbons make conventional separation methods such as distillation and extraction both challenging and energy intensive. Adsorptive separation using porous solid adsorbents presents a promising and energy-efficient alternative. Among these, flexible metal-organic frameworks (FMOFs) as a subgroup of MOFs have emerged as an unparalleled class of porous materials for highly selective adsorption-based separation and purification of liquid hydrocarbons. Having intrinsically dynamic structures, FMOFs exhibit very different adsorption behaviors compared to rigid MOFs (RMOFs), those that do not involve structure transformations upon activation, and molecular adsorption.In recent years, we have focused on developing high-performance FMOFs with different dimensionalities, spanning from one-dimensional (1D) chains to two-dimensional (2D) layers and to three-dimensional (3D) networks. Their structural flexibility arises from either local rotation and vibration of organic linkers or global structural changes, enabling stimuli-responsive molecular adsorption. By leveraging their distinct temperature- and adsorbate-dependent adsorption behaviors, we have achieved highly efficient separation of numerous important liquid hydrocarbons through molecular sieving, the mechanism that offers the highest selectivity. The unique dynamic structures and adsorption properties allow FMOFs to have high adsorption capacity, exceptional selectivity, and fast kinetics simultaneously, overcoming the trade-offs typically encountered by conventional adsorbents including RMOFs.In this Account, we summarize our recent advances using FMOFs for the adsorptive separations of three key groups of liquid hydrocarbons: C6 alkane isomers, C8 alkylaromatic isomers, and C6 cyclic hydrocarbons. We highlight the interplay between FMOF structures and sorbate properties, focusing on their unique temperature- and adsorbate-dependent adsorption behaviors and molecular sieving mechanism responsible for high selectivity. We also discuss their inverse size-selective adsorption phenomena stemmed from adsorbate-induced structural transformations─a phenomena not possible in conventional RMOFs, which typically exhibit size-exclusion selectivity. Representative FMOFs with superior separation performance are discussed, along with their underlying working principles. Finally, we address the existing challenges and propose potential strategies to enhance their performance aiming for applications in petrochemical industry. Overall, our studies not only unveil a new dimension of flexible porous crystals but also provide a strategic framework for their design and implementation in highly selective, sieving-based molecular separation. By deepening the understanding of structure-property relationships, our findings offer valuable insights that can inspire future advancements in adsorptive separation technologies, with significant implications for the petrochemical industry and beyond.