Cross-Linked Porous Polymers as Heterogeneous Organocatalysts for Task-Specific Applications in Biomass Transformations, CO 2 Fixation, and Asymmetric Reactions

Metal-free catalysis is particularly challenging in the context of green and sustainable chemistry. High toxicity associated with the leaching of the metals from the catalysts has notorious environmental impact. To surmount such an effect, homogeneous organocatalysis can provide a green and alternative protocol. However, it suffers the drawbacks of low activity and selectivity, because of the neighboring effect of the solvent, and it is devoid of recyclability for sustainable operations. To address such issues, solid-supported heterogeneous organocatalysts are developed, and these have been attracting increasing interest over the years. Multifunctional porous organic materials are very demanding in catalysis, because of their robustness in the structure involving strong covalent bonds between organic building blocks. Furthermore, their high specific surface area, topological diversity, and finely dispersed catalytic sites in nanoscale could make these porous organic materials as excellent scaffold for several task-specific applications. Lightweight and high inherent porosity, as well as structural stability with tenability of the active functional groups, have reinforced their tremendous potential as solid organocatalyst. In this Perspective, we highlight the latest advancements in metal-free cross-linked amorphous porous organic polymers (POPs) and crystalline covalent organic frameworks (COFs), their design principle to incorporate catalytic sites for major applications such as biomass conversion, biofuel synthesis, asymmetric organocatalysis, and CO 2 fixation reactions. Several renewable and sustainable catalytic transformations could be achieved via environmentally benign pathways through this alternative metal-free approach.