Catalytic processes for bio-based polyester building blocks

Abstract

The development and introduction of synthetic plastics into the economy in the early 1900’s has significantly increased the standards of living worldwide. Many traditional materials such as wood, stone, metal and glass could suddenly be replaced by cheap, versatile polymers derived from copious petroleum reserves. Approximately one century later, scientists and researchers are now exploring more durable alternatives, such as bio-based and/or biodegradable plastics, in an effort to reduce the negative environmental impact that is associated to the production and consumption of oil-based plastics, such as contributing to climate change and pollution. The most well-established bio-based polymer on the market today is polylactic acid (PLA). Currently, this promising plastic represents less than 0.1% of the global amount of plastics produced. To increase PLA’s market share, there is a fueling demand for more cost-competitive production routes and functional (co-)polymers for high-value or high-performance applications. In the past decade, significant efforts have been devoted to develop new, catalytic routes towards PLA’s building block, lactic acid, as an alternative to its conventional, but expensive fermentative production. In contrast, less studies have focused on the conversion of lactic acid to lactide, the actual precursor for commodity PLA, despite that this step comprises approx. 30% of PLA’s total production cost. In this doctoral work, a new process was developed for lactide production by a catalytic gas-phase transesterification of alkyl lactates. Compared to the state of the art, this process is the first continuous process reported that does not require solvents or diluted lactic acid solutions, but merely a pure feedstock of alkyl lactates. In gas-phase processes, the use of alkyl lactates instead of lactic acid is advantageous, since the esters are more volatile than lactic acid, they do not polymerize spontaneously in concentrated solutions and they do not have an acid group which might catalyze unwanted side-reactions. Moreover, alkyl lactates are intermediates during lactic acid purification and can be produced by emerging catalytic routes. With this process, an unprecedented lactide productivity can be achieved compared to other, lactic acid-based gas-phase processes. Supported TiO2/SiO2 catalysts were selected from a broad screening as materials of choice for catalyzing the cyclization of alkyl lactates. The reaction conditions were optimized to maximize lactide yield and/or productivity, in accordance to the underlying thermodynamics. From extensive characterization of various Ti-Si catalysts, it was elucidated that covalent Ti-O-Si bonds between the Ti active sites and the Si-support are required in order to be catalytically active and selective towards lactide, irrespective of the catalyst structure, topology or morphology. Through DR UV-VIS spectroscopy, it was found that isolated tetrahedral TiO4 sites are more active than five-fold TiO5 or octahedral TiO6 sites, and the former culminate at high Ti-dispersions. Hence, lactide productivity was optimized by balancing the amount of active sites on the catalyst and the specific catalytic activity of each site. High-surface SiO2 supports (e.g. MCM-41) provided the largest surface area and hence, a greater Ti-dispersion, even at higher loadings of Ti. The highest lactide productivity was therefore obtained with a TiO2/MCM-41 catalyst. The catalytic production of lactide in gas-phase is flexible on the alkyl lactates used. No significant thermodynamic differences were observed, but the reactivity of the esters is correlated to the basicity of the leaving alcohol group, and generally decreases in the order of methyl > ethyl ≈ n-butyl > isopropyl lactate. Moreover, the same process can be expanded towards the production of other cyclic esters as well, such as glycolide from methyl glycolate. In a separate chapter of this doctoral thesis, we also explored the catalytic production of functional, lactic acid-like C4-esters, viz. methyl vinyl glycolate (MVG) and methyl-4-methoxy-2-hydroxybutanoate (MMHB) from tetrose sugars. Of these, MVG in particular is considered a suitable building block for functionalizable, high-value PLA polymers or a platform molecule for other high-end chemicals. The selectivity towards both products highly depends on the pore size of the heterogeneous Sn-catalyst and reaction temperature. Confinement effects inside the pores of Sn-zeolites lead to an increased selectivity towards MVG, especially at higher reaction temperatures. Development of such catalytic routes towards functionalizable polyester building blocks paves the way towards high-value PLA, whereas the new ester-based gas phase process for lactide production could reduce production costs of commodity PLA.nrpages: 167status: publishe