Processive-Cleavage and Functionalization-Cleavage for Deconstruction of Polyolefins
Pr. Aaron D. Sadow
Department of Chemistry and Ames Laboratory, Iowa State University
Invited at LBM by Laurence Grimaud
The massive quantities of single-use plastics discarded each year requires new sustainable end-of-life solutions. Current technologies, such as melt-processing for recycling or incineration for partial energy recovery, are insufficient to deal with the crisis in its entirety. New methods involving chemical upcycling, by catalytic conversion of the used materials into higher value products, could provide molecules that make use of components of the existing catenated carbon starting materials. In addition, few existing catalytic methods can break carbon-carbon bonds in aliphatic hydrocarbons lacking directing groups and introduce new heteroatom functionality. The Institute for Cooperative Upcycling of Plastics (iCOUP), a Department of Energy-Basic Energy Sciences Energy Frontier Research Center (EFRC) is investigating catalytic principles governing transformations that break carbon-carbon bonds in polyolefins at regular intervals, in order to develop methods for conversion of long catenated polymer chains into narrow distributions of value-added products. A bio-inspired processive mechanism may be targeted by understanding nature of polymer-surface adsorption and dynamics, and then designing catalytic architectures that mimic key properties of processive enzymes. We have constructed and studied the first example of such a catalyst and will highlight benefits from this approach in hydrogenolysis of polyolefins into narrow distributions of shorter linear hydrocarbon chains.
A second approach considers approaches to break carbon-carbon bonds in polyolefins and concurrently introduce reactive sites that could enable the use of products as chemical synthons. Early transition metal complexes are capable of breaking carbon-carbon bonds via β-alkyl elimination to form shorter carbon chains in oligomerylmetal species. Transmetalation to a main group element could afford reactive species to access fatty alcohols and fatty acids by oxidation or carboxylation, for example. We have developed examples of such transformations, based on organozirconium single-site catalysts and hydride-generating organoaluminum reagents.