Lipase-Mediated Syntheses of Trimethylolpropane-Based Biolubricant and Cyclic Carbonate

Abstract: Biocatalysis is considered as benign and efficient alternative to chemical catalysis for organic syntheses. Lipases are the most versatile biological catalysts implemented so far with great potential for production of different chemicals and materials in non-conventional reaction media. This thesis presents investigations on lipase catalyzed esterification and transesterification reactions in solvent-free media with a polyol (tri-ol), trimethylolpropane (TMP) to form TMP-trioleate and -cyclic carbonate for lubricant and polymer applications, respectively. Conventional lubricants are mineral oil based and lack biodegradability resulting in their accumulation in the environment. Synthetic esters of polyhydric alcohols and fatty acids are biodegradable and possess desirable technical properties for lubricant applications. Synthesis of TMP-trioleate from oleic acid and TMP catalyzed by commercial immobilized Candida antarctica lipase B, Novozym®435 (N435) was studied by varying reaction parameters. The product obtained possesses desirable pour point (-42 °C) for lubricant applications in sub-zero conditions. The biocatalyst was recycled in reactions at 70 °C for 7 batches, 24 h each, with a half-life of 94 h. The biocatalyst half-life was doubled by washing it with 2-propanol between the batches. A simplified kinetic model was developed for the lipase-catalyzed reaction in order to facilitate optimization and design of the process and minimize the amount of resources required for investigations of the process. The methodology used for the kinetic modeling is applicable for similar types of enzymatic reactions involving multi-substrate multi-product systems. Cyclic carbonates are potential monomers for phosgene-/isocyanate-free polycarbonates and polyurethanes that have wide range of applications. Six-membered cyclic carbonates can readily undergo ring-opening polymerization to form aliphatic polycarbonates and polyurethanes and their copolymers. Six-membered cyclic carbonate with hydroxyl functional group was obtained with 75% yield using a chemoenzymatic process involving lipase B catalyzed transesterification of dimethylcarbonate (DMC) with TMP in the presence of molecular sieve to form linear TMP carbonate followed by thermal cyclization. Performing the reaction in a recirculating flow reactor, higher conversion rates were obtained compared to the batch process, the product was recovered easily without extra separation steps, and the biocatalyst and molecular sieve remained intact for reuse. In silico evaluations of the reaction accompanied with empirical investigations confirmed that lipase B prefers DMC as acyl-donor while TMP and its derivatives, formed during the course of the reaction, serve as acyl acceptors. The formation of TMP carbonate oligomers hence found to be non-enzymatic and intensified by heat.