Production of polyol carbonates and their intercalation into Smectite clays

Abstract

In hyper-saline conditions, clays in geosynthetic clay liners contract and fail to form a hydraulic barrier due to removal of water from the interlayer spaces of smectite, which is the swelling mineral component of bentonites used in geosynthetic clay liners. Five-membered cyclic carbonates such as propylene carbonate have been reported to form stable intercalated complexes with hydrated Na-smectite, which maintain swollen states at 1M). Glycerol carbonate was selected as an alternative candidate as it possesses a hydroxyl functionality which may assist in retention of interlayer water and which also can be synthesized from inexpensive reactants. A green, solvent free approach was developed for scalable production of glycerol carbonate from glycerol and urea. Glycerol carbonate was obtained in 83% yield under optimal conditions of glycerolysis of urea using a novel zinc monoglycerolate (ZMG) catalyst. The isocyanate ligand coordinated to the metal glycerolate was believed to be the key intermediate in the catalytic cycle (Turney et al., 2013). This methodology was extended to the synthesis of other cyclic carbonates starting from urea and various 1,2-diols employing the ZMG catalyst. Transesterification using dimethyl carbonate (an expensive reagent) was shown to be an alternative way to synthesize cyclic carbonates. Comparative yields for the products via two methods were obtained. The intercalation of glycerol carbonate into the interlayer galleries of sodium smectite was studied. The main driving force for intercalation of these molecules was considered to be the solvation of interlayer sodium cations. FTIR showed the presence of interlayer water in Na-smectite. Model FTIR studies based on varying amounts of water and sodium chloride in glycerol carbonate showed that water, as well as sodium cations, were interacting with glycerol carbonate. Long-term stability studies of glycerol carbonate in saline solution (1M) indicated it to be stable. Intercalated complexes of other cyclic carbonates with Na-smectite were prepared and characterized. Long term exposure of cyclic carbonates may result in the leaching out of cyclic carbonates from the clay. This can be addressed by modifying cyclic carbonates into polymers with a pendant cyclic carbonate. The hydroxyl functionality of glycerol carbonate was used to prepare various cyclic carbonate derivatives (esters, tosyl and bromo) that would potentially be useful in preparation of intercalated complexes. O-alkylation of hydroxyl containing cyclic carbonate resulted in precursor monomers. Ring opening polymerization of an epoxy cyclic carbonate resulted in selective opening of the epoxide ring to yield a polyether with retention of the cyclic carbonate. Photopolymerization of a styrene monomer with a pendant cyclic carbonate group yielded polystyrene with pendant cyclic carbonates. Polyether nanocomposites prepared via solution intercalation and in-situ ring opening polymerization resulted in intercalated and intercalated-disordered nanocomposites, respectively. Partially intercalated-disordered polystyrene cyclic carbonate nanocomposites were obtained via in-situ polymerization and solution intercalation approaches as determined on the basis of XRD. The in-situ formed polymer was separated from the clay and was similar to pure polymer synthesized independently. NaCl (3M) treatment with polyether nanocomposites revealed it to be stable when compared with epoxy monomer intercalated.