Title : Green and scalable approaches for synthesis and encapsulation of clean nanoclusters inside cerium MOFs for catalysis: Sustainable synthesis of exovinylene carbonates and glycerol carbonate from glycerol and primary propargylic alcohols and CO2
Abstract:
Exovinylene carbonates (ECs) and glycerol carbonate (GLC) are important cyclic carbonates with applications such as in pharmaceuticals, Li-ion batteries and biological systems. They are also building blocks or intermediates in synthesis of polycarbonates and polyhydroxyurethanes. Direct carboxylative cyclization of cheap primary propargylic alcohols (PPAs) and crude bio-glycerol (GL) by heterogeneous catalysis is the most sustainable approach to access these chemicals and to offset CO2 and waste bioGL. Efforts from various research groups in the recent past to engineer catalytic systems to access GLC emphasized purified GL and transesterification using costly organic carbonates, ignoring the need to develop routes using crude GL which is in excess and impeding the development of the biodiesel industry. For ECs, studies have narrowed on homogeneous catalysts to convert tertiary and secondary PAs due to their ease of cyclization. The cheap PPAs, because they are difficult to cyclize, have been ignored yet they are available on a large scale from the reaction of alkynes with aldehydes. Additionally toxic byproducts, unacceptably very low atom-economy and tedious purification steps are involved. These issues have hindered large-scale production of ECs and maintained relative high prices for GLC. Cu, Ag and Pd have been reported in various carboxylative cyclization reactions and have ability to adsorb GL, while CeO2 activates small molecules including CO2. Therefore, we synthesized nanoclusters (NCs) of Cu, Ag and Pd using alcohols and water only with PVP, a weakly binding, environmentally benign protecting agent. The CeO2-NCs where synthesized by decomposition of the cerium salt in pyridine at RT. The NCs were encapsulated in stable Ce organic frameworks (CeMOFs) using simple and green approaches and the NC-MOF composites employed in coupling of crude and pure GL with CO2 to afford the GLC. Cyclization of PPA was enabled using polymer anchored NC-MOF composite. To the best of our knowledge, we report the first successful and purely heterogeneous catalyst system for direct carboxylative cyclization of PPAs and the highest GLC yield (>21%) for direct carboxylation of crude GL using sustainable protocols. The protocols (with great attention paid to sustainability and recyclability) use cheap, green and/or commercially available chemicals and thus have great potential for industrial application.
