Contributed by Michiel Dusselier, PhD, Dr. Joseph Breen Memorial Fellow, 2013. Postdoctoral researcher at Center for Surface Chemistry and Catalysis, Faculty of Bioscience engineering, KU Leuven, Belgium
The popularity of bioplastics - defined as being either partially biodegradable or renewable, or both - is on the rise not just from an academic perspective, but also from an industrial one. Numbers published last year predict that the global production of bioplastics is to grow 300% by 2018. Aside from drop-ins bio-polyethylene and bio-PET, polylactic acid, or PLA, is the major synthetic bioplastic out there, taking up about 11% of the global production capacities. PLA is 100% renewable and, given the right conditions, biodegradable. Next to the high potential of this thermoplastic in an impressive range of applications (packaging, textiles, fibers and foams), PLA is a promising alternative for polypropylene, polystyrene and even PET in certain markets. Moreover, the polymer is perfectly suited for 3D printing as well as for in vivo application due to its biocompatibility. In spite of the environmentally benign performance of PLA in life cycle assessments, the major bottleneck preventing a larger scale breakthrough is the high production cost.
One of the main cost factors at play is the synthesis of lactide. To transform lactic acid, derived from sugar fermentation, into lactide, a two-step process is followed: first, a low quality PLA plastic (pre-polymer) is made that, in a second step, is broken down again to yield the lactide building block (cfr. red route in the scheme). That building block is then polymerized to give a good quality PLA plastic. Part of the lactic acid feedstock is wasted in this process, as the process and its conditions (high temperatures and costly vacuum pressures) induce side-product formation and a degree of unwanted racemization. The side-products also infer the need for additional purification steps before the lactide building block is pure enough for polymerization to PLA. In essence, the current route presents a detour that could be more feedstock-, energy- and time-efficient.
With the principles of green chemistry in mind, we set out to introduce a re-usable, heterogeneous catalyst and to aim for the direct synthesis of lactide from lactic acid. We succeeded and developed a one-step catalytic process that converts lactic acid with a zeolite under solvent reflux with water-removal. The new process has many advantages: it allows a step to be skipped, runs at milder conditions (e.g. atmospheric pressure) and has minimal side-product formation and no racemization. The purity of the building block is very high, eliminating the need for excessive purification. The process is theoretically waste-free, as the few side-products formed can be easily recycled to the reactor and the zeolite is reusable. Although the introduction of a solvent could be seen as disadvantageous, it is a perfect bridge between the selectivity of the chemical reaction and the follow-up lactide isolation: a single liquid/liquid extraction with water on the reactor outlet yields a 98% pure lactide in the solvent that can be crystallized with solvent recycle.
The key to the invention - to the ‘shortcut’- is that we use the zeolite catalyst to speed up and guide the reaction in the right direction (away from side-products). Zeolites are microporous minerals, with active centers only accessible via a network of sub-nanometer pores. Lactic acid can easily get in, is transformed into the building block, which easily gets out again. The side products are larger and can simply not be formed in these pores. This concept of steering the reaction outcome with spatial restriction is known as ‘shape-selectivity’ and is used in refineries and petrochemical plants every day to make our daily fuels and chemicals. We have thus applied a petrochemical concept to bioplastics production.
Back in 2010, we delivered the first proof-of-concept of this catalytic process. After that, we faced a difficult question: to pursue commercial relevance and IP generation, or immediate valorization through publication? We decided in favor of the first, since after all, from an engineering point of view, you hope to actually change or improve something in real life and not just on paper. After a couple of years of embargo, during IP generation and its transfer to an interested company (studying up-scaling options), we were still able to go public with the process, and have since successfully published in Science.
These developments render me hopeful that they exemplify the value and applicability of green chemistry and catalysis research, here deployed in the field bio-based polymers. I hope that our invention will lead to a cheaper and greener PLA production, as well as inspire people to continue their efforts in green chemistry R&D.
The article, “Shape-selective zeolite catalysis for bioplastics production” was recently published by M. Dusselier, P. Van Wouwe, A. Dewaele, P. A. Jacobs and B. F. Sels, in Science 2015, vol. 349.
I was a Joseph Breen Memorial Fellow in 2013, and I attended the 17th Green Chemistry and Engineering conference in Bethesda, MD.
The work I submitted for this fellowship was carried out during the embargo for the lactide process. At that time, we looked into the synthesis of lactic acid and other alpha-hydroxy ester molecules with chemical catalysis from biomass sugars. Next to reporting on a bifunctional carbon-silica composite catalyst for lactate production from trioses (J. Am. Chem. Soc. 2012, vol. 134), I looked into the formation of less common, but intriguing lactic acid look-alike building blocks. In the end, we showed the pathways needed to form these molecules through cascade catalysis. For certain applications of PLA, it would be desirable if some side chains in the polymer would be accessible for further tailoring its properties. We proved exactly that, by creating a co-polymer of one of these new building blocks with lactic acid via polycondensation and modifying the hydrophobicity of a surface coated with these polyesters (ACS Catal. 2013, vol. 3).Interestingly, the novel shape-selective process does not only work for lactide, but also for certain of these other alpha-hydroxy acids. Henceforth, the process provides an essential link between polymer chemistry - where the cyclic dimers (such as lactide) are the ideal starting point for polymerization - and biomass valorization research - where the production of alpha-hydroxy acids (such as lactic acid) is targeted.
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