New Route from Biomass-based Levulinic to Polyurethanes
Mochamad Chalid, Universitas Indonesia
Ligno-cellulosic biomass such as wood and agricultural residues has been identified as an attractive source for chemical products. As such, it is a promising raw material to substitute (part of) the current mainly petrochemical based bulk chemicals. Large research efforts are currently undertaken to convert ligno-cellulosic biomass to low molecular weight building blocks, which subsequently can serve as starting materials for a wide range of chemical products. A well-known example is levulinic acid, which may be obtained from ligno-cellulosic biomass in good yields.
Levulinic acid is a versatile multi-purpose building block due to the presence of two reactive functional groups, i.e. a ketone and a carboxylic group. LA may be converted to a number of derivatives with high application potential. An attractive derivative is -valerolactone (GVL). GVL has existing applications in the food industry and is used as a component in drug delivery systems, and as a precursors for long chain alkanes of fuels.
GVL is considered as a starting material for the synthesis of bio-based polymers. However, ring opening polymerization of GVL, a well-established polymerization methodology, is cumbersome due to the low reactivity of GVL and only low molecular weight products are obtained so far. An alternative approach involves the conversion of GVL to a more reactive, polymerizable monomer. It could be a very promising concept to convert GVL to (high) molecular weight bio-based polymers having good thermal and mechanical properties.
The first part of this study involves the hydrogenation of LA to GVL, which uses a biphasic hydrogenation concept adding a water soluble Ru-catalysts, made in situ from RuCl3.3H2O and tris(m-sulfonatophenyl)phosphine (TPPTS). The hydrogenations were performed at mild conditions in a batch hydrogenation reactor. The effects of process variables like substrate concentration, hydrogen pressure, temperature, pH and the catalyst to substrate ratio on the LA conversion and GVL yield were determined.
The ring opening of GVL under mild conditions by reactions with amines is studied by bulk reactions between model amine compounds (ammonia, 2-aminoethanol, 2-phenylethylamine and morpholine) and GVL showed that steric hindrance and charge density at the nitrogen atom have a significant effect on the reactivity of the amine compounds. Furthermore, GVL ring opening reactions with diamine model compounds (1,2-diaminoethane, piperazine, and 1,2-diaminopropane) were performed, demonstrating the versatility of the synthetic methodology. In addition, these experiments also implied that steric hindrance is the most important factor determining the amine reactivity. Process variables (like mole ratio of GVL to 1,2-diaminoethane, reaction temperature and the use of a Lewis catalyst) have a significant effect on the reaction and allowed optimization of the product yield.
Some of the GVL-derived diols like N,N'-1,2-ethanediylbis-(4-hydroxypentanamide) and 4-hydroxy-N-(2-hydroxyethyl)-pentanamide were used to synthesize polyurethanes through the poly-addition of di-isocyanates. Subsequent studies were performed with a range of di-isocyanates such as phenylene-di-isocyanate, 1,6-hexamethylene-di-isocyanate and 2,4-toluene-di-isocyanate. Process studies (solvent, catalyst, and temperature) and a structure-reactivity analysis (by varying the backbone structures of both aliphatic and aromatic di-isocyanates) were performed. One-pot polymerization procedures at 140 °C in the presence of TEA in DMA resulted in 97% yield and the product with the highest molecular weight (156 kDalton).
Intra- and inter-molecular interactions between the amide and urethane groups such as hydrogen bonding, as indicated by FTIR, determine key properties. XRD studies indicate that the polymers are amorphous. The novel polyurethanes have good thermal stabilities (TGA measurements). Study of thermal and mechanical properties indicates that the molecular architecture in the polymer products such as the polarity and size of side group, chain flexibility, and molecular weight has a significant effect on their glass transition temperature and modulus.
Background Review Article:
New Route from Biomass-based Levulinic Acid to Polyurethanes, Chalid, Mochamad.