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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 CO2 fixation reactions. Several renewable and sustainable catalytic transformations could be achieved via environmentally benign pathways through this alternative metal-free approach.

Solid acid catalysts occupy a special class in heterogeneous catalysis for their efficiency in eco-friendly conversion of biomass into demanding chemicals. We synthesized porphyrin containing porous organic polymers (PorPOPs) using colloidal silica as a support. Post-modification with chlorosulfonic acid enabled sulfonic acid functionalization, and the resulting material (PorPOPS) showed excellent activity and durability for the conversion of fructose to 5-hydroxymethyl furfural (HMF) in green solvent water. PorPOPS composite was characterized by N2 sorption, FTIR, TGA, CHNS, FESEM, TEM and XPS techniques, justifying the successful synthesis of organic networks and the grafting of sulfonic acid sites (5 wt%). Furthermore, a high surface area (260 m2/g) and the presence of distinct mesopores of ~15 nm were distinctly different from the porphyrin containing sulfonated porous organic polymer (FePOP-1S). Surprisingly the hybrid PorPOPS showed an excellent yield of HMF (85%) and high selectivity (>90%) in water as compared to microporous pristine-FePOP-1S (yield of HMF = 35%). This research demonstrates the requirement of organic modification on silica surfaces to tailor the activity and selectivity of the catalysts. We foresee that this research may inspire further applications of biomass conversion in water in future environmental research.

Atomically dispersed metal-single-atoms have become a frontier in solid catalysis due to their characteristics electronic properties. However, for biomass conversion employing metal-single-atoms as catalysts is rather challenging since it suffers...

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