At the molecular level, polymers are long chains in which the emergent material properties are dictated by the movement, arrangement, and interactions of these chains. Key factors that contribute to how the polymer chains move and rearrange are the molecular identity and arrangement, crystallinity, and molecular weight. Generally, the monomer identity influences the final application of the polymer by dictating many properties, such as the glass transition temperature (Tg). 12 As the Tg represents a softening of the material, it is a prime factor in determining the final polymer application. Flexible molecules in the backbone, which can relax faster, may result in low Tg materials with applications such as PE bags or rubber (i.e. polybutadiene). 13 Meanwhile, rigid molecules or molecules that result in stronger interchain interactions (and relax on longer timescales) can result in high Tg materials, ideal for reinforced applications. In general, when materials are at temperatures below the Tg, the polymer chains are kinetically arrested, exhibiting higher strengths. Even though monomer identity is often the largest contribution to Tg, it is not the only factor, as molecular weight, 14 tacticity, 15 and crystallinity 16 also contribute. While nearly all polymers exhibit a Tg characteristic of their amorphous region, semi-crystalline polymers will also exhibit concomitant melting behaviour crystalline in their crystalline regions, making them semi-crystalline. Crystallinity has a direct impact on polymer properties, as increases in crystallinity augment the strength of the final product and reduce the permeability of liquids and gases. Co-monomers (e.g., isophthalic acid in poly(ethylene terephthalate) (PET)) are often used to lower or completely remove crystallinity to make polymers easier to process or more transparent. 17 Finally, molecular weight, and the distributions of molecular weights, have some effect on the thermomechanical polymer properties (e.g., increasing molecular weight leads to higher Tg, modulii, etc.). However, over a critical molecular weight, nearly all thermomechanical polymer properties are constant. The exception to this generalization is the viscosity of a polymer melt, which scales with the molecular weight to the 3-3.5 power (η ~ MW 3-3.5 ) and also encapsulates properties such as diffusivity. These factors together contribute to polymer recalcitrance by limiting polymer mobility and accessibility to chemical linkages, posing a challenge for catalytic plastics deconstruction.

Mixed plastics waste represents an abundant and largely untapped feedstock for the production of valuable products. The chemical diversity and complexity of these materials, however, present major barriers to realizing this opportunity. In this work, we show that metal-catalyzed autoxidation depolymerizes comingled polymers into a mixture of oxygenated small molecules that are advantaged substrates for biological conversion. We engineer a robust soil bacterium, Pseudomonas putida , to funnel these oxygenated compounds into a single exemplary chemical product, either β-ketoadipate or polyhydroxyalkanoates. This hybrid process establishes a strategy for the selective conversion of mixed plastics waste into useful chemical products.

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The accumulation of plastic waste in the environment has prompted the development of new chemical recycling technologies. A recently reported approach employed homogeneous organometallic catalysts for tandem dehydrogenation and olefin cross metathesis to depolymerize polyethylene (PE) feedstocks to a mixture of alkane products. Here, we build on that prior work by developing a fully heterogeneous catalyst system using a physical mixture of SnPt/γ-Al2O3 and Re2O7/γ-Al2O3. This heterogeneous catalyst system produces a distribution of linear alkane products from a model, linear C20 alkane, n-eicosane, and from a linear PE substrate (which is representative of high-density polyethylene), both in an n-pentane solvent. For the PE substrate, a molecular weight decrease of 73% was observed at 200 °C in 15 h. This type of tandem chemistry is an example of an olefin-intermediate process, in which poorly reactive aliphatic substrates are first activated through dehydrogenation and then functionalized or cleaved by a highly-active olefin catalyst. Olefin-intermediate processes like that examined here offer both a selective and versatile means to depolymerize polyolefins at lower severity than traditional pyrolysis or cracking conditions.

Atomic layer deposition (ALD) was used to deposit Ni and Pt on alumina supports to form monometallic and bimetallic catalysts with initial particle sizes of 1 to 2.4 nm.The ALD catalysts were more active (per mass of metal) than catalysts prepared by incipient wetness (IW) for dry reforming of methane (DRM), and they did not form carbon whiskers during reaction due to their sufficiently small size. Catalysts modified by Pt ALD had higher rates of reaction per mass of metal and inhibited coking, whereas NiPt catalysts synthesized by IW still formed carbon whiskers. Temperatureprogrammed reduction of Ni catalysts modified by Pt ALD indicated the presence of bimetallic interaction. Density functional theory calculations suggested that under reaction conditions, the NiPt surfaces form Ni-terminated surfaces that are associated with higher DRM rates (due to their C and O adsorption energies, as well as the CO formation and CH 4 dissociation energies).

Reactivity of molecular catalysts can be controlled by organic ligands that regulate the steric and electronic properties of catalyst sites. This level of control has generally been unavailable for heterogeneous catalysts. We show that self-assembled monolayers (SAMs) on titania with tunable electronic properties provided fine control over surface reactivity. Controlling the identity of substituents on benzylphosphonic acid SAMs modulated the near-surface electrostatics, enabling regulation of the dehydration activity of 1-propanol and 1-butanol over a wide range, with activities and selectivities of the optimal catalyst far exceeding those of uncoated TiO2. The dipole moment of the adsorbed phosphonate was strongly correlated to the dehydration activity; kinetic measurements and computational modeling indicated that the interfacial electric field altered the transition-state structure and energy. Coating catalysts with SAMs having controllable charge distributions may provide a general approach to heterogeneous catalyst design analogous to the variation of ligands in molecular catalysts.

Cooperative catalysts containing a combination of noble metal hydrogenation sites and Brønsted acid sites are critical for many reactions, including the deoxygenation (DO) of biomass-derived oxygenates in the upgrading of pyrolysis oil. One route toward the design of cooperative catalysts is to tether two different catalytically active functions so that they are in close proximity while avoiding undesirable interactions that can block active sites. Here, we deposited carboxylic acid (CA)-functionalized organophosphonate monolayers onto Al2O3-supported Pd nanoparticle catalysts to prepare bifunctional catalysts containing both Brønsted acid and metal sites. Modification with phosphonic acids (PAs) improved activity and selectivity for gas-phase DO reactions, but the degree of improvement was highly sensitive to both the presence and positioning of the CA group, suggesting a significant contribution from both the PA and CA sites. Short spacer lengths of 1–2 methylene groups between the phosphonate head and CA tail were found to yield the best DO rates and selectivities, whereas longer chains performed similarly to self-assembled monolayers having alkyl tails. Results from a combination of density functional theory and Fourier transform infrared spectroscopy suggested that the enhanced catalyst performance on the optimally positioned CAs was due to the generation of strong acid sites on the Al2O3 support adjacent to the metal. Furthermore, the high activity of these sites was found to result from a hydrogen-bonded cyclic structure involving cooperativity between the phosphonate head group and CA tail function. More broadly, these results indicate that functional groups tethered to supports via organic ligands can influence catalytic chemistry on metal nanoparticles.