Optimization of a two‐step process comprising lipase catalysis and thermal cyclization improves the efficiency of synthesis of six‐membered cyclic carbonate from trimethylolpropane and dimethylcarbonate

Abstract

Six-membered cyclic carbonates are potential monomers for phosgene and/or isocyanate free polycarbonates and polyurethanes via ring-opening polymerization. A two-step process for their synthesis comprising lipase-catalyzed transesterification of a polyol, trimethylolpropane (TMP) with dimethylcarbonate (DMC) in a solvent-free system followed by thermal cyclization was optimized to improve process efficiency and selectivity. Using full factorial designed experiments and partial least squares (PLS) modeling for the reaction catalyzed by Novozym®435 (N435; immobilized Candida antarctica lipase B), the optimum conditions for obtaining either high proportion of monocarbonated TMP and TMP-cyclic-carbonate (3 and 4), or dicarbonated TMP and monocarbonated TMP-cyclic-carbonate (5 and 6) were found. The PLS model predicted that the reactions using 15%-20% (w/w) N435 at DMC:TMP molar ratio of 10-30 can reach about 65% total yield of 3 and 4 within 10 h, and 65%-70% total yield of 5 and 6 within 32-37 h, respectively. High consistency between the predicted results and empirical data was shown with 66.1% yield of 3 and 4 at 7 h and 67.4% yield of 5 and 6 at 35 h, using 18% (w/w) biocatalyst and DMC:TMP molar ratio of 20. Thermal cyclization of the product from 7 h reaction, at 110°C in the presence of acetonitrile increased the overall yield of cyclic carbonate 4 from about 2% to more than 75% within 24 h. N435 was reused for five consecutive batches, 10 h each, to give 3+4 with a yield of about 65% in each run.

Six‐membered cyclic carbonates from trimethylolpropane: Lipase‐mediated synthesis in a flow reactor and in silico evaluation of the reaction

“…DMC to TMP ratio was shown in the earlier work to have significant impact on the reaction rate and selectivity of the desirable products . Figure shows TMP conversion and total yield of 3 + 4 in the reactions run with different amounts of DMC.…”

“…The amount of starting TMP used was based on the methanol adsorption capacity [20% (w/w)] of the molecular sieves packed in the column. The dimensions of the system were adjusted in order to be able to use the desired DMC:TMP ratio and other reaction conditions such as the biocatalyst amount and the reaction temperature based on our previous studies …”

“…The stability and reusability of N435 were investigated in presence of molecular sieves by recycling the biocatalyst for consecutive batch reactions between TMP and DMC in previous report. Although the biocatalyst activity decreased gradually during the batches, N435 could be recycled to obtain 3 + 4 at an average yield of about 65% with high reproducibility upto five times recycling.…”

“…The product was collected in the reservoir and was concentrated by evaporation of the DMC. It was then dissolved in acetonitrile [1:20 (w/v) TMP carbonate:acetonitrile] and subjected to thermal treatment in the presence of molecular sieves [1:1 (w/w) with respect to TMP carbonate] at 110°C for cyclization . Aliquots were taken at different times during the reactions for analyses.…”

“…We have recently reported solvent‐free synthesis of six‐membered TMP‐cyclic carbonate using lipase‐mediated reaction between TMP and dimethylcarbonate (DMC) in a batch reactor followed by thermal cyclization . We observed that lipase catalyzed the formation of linear carbonated products predominantly, but even small amounts of cyclic carbonate, dimer and oligomer were formed from the first product, the monocarbonated TMP ( 3 ) having a reactive carbonate and two hydroxyl groups (Scheme ).…”