Coking-Resistant Polyethylene Upcycling Modulated by Zeolite Micropore Diffusion.

Although the mass production of synthetic plastics has transformed human lives, it has resulted in waste accumulation on the earth. Here, we report a low-temperature conversion of polyethylene into olefins. By mixing the polyethylene feed with rationally designed ZSM-5 zeolite nanosheets at 280 °C in flowing hydrogen as a carrier gas, light hydrocarbons (C1-C7) were produced with a yield of up to 74.6%, where 83.9% of these products were C3-C6 olefins with almost undetectable coke formation. The reaction proceeds in multiple steps, including polyethylene melting, flowing to access the zeolite surface, cracking on the zeolite surface, formation of intermediates to diffuse into the zeolite micropores, and cracking into small molecules in the zeolite micropores. The ZSM-5 zeolite nanosheets kinetically matched the cascade cracking steps on the zeolite external surface and within micropores by boosting the intermediate diffusion. This feature efficiently suppressed the intermediate accumulation on the zeolite surface to minimize coke formation. In addition, we found that hydrogen participation in the cracking process could hinder the formation of polycyclic species within zeolite micropores, which also contributes to the rapid molecule diffusion. The coking-resistant polyethylene upcycling process at a low temperature not only overturns the general viewpoint for facile coke formation in the catalytic cracking over the zeolites but also demonstrates how the polyethylene-based plastics can be upcycled to valuable chemicals. In addition to the model polyethylene, the reaction system worked efficiently for the depolymerization of multiple practically used polyethylene-rich plastics, enabling an industrially and economically viable path for dealing with plastic wastes.

A Physical Entangling Strategy for Simultaneous Interior and Exterior Modification of Metal-Organic Framework with Polymers.

A new strategy uses a common feature of metal-organic frameworks (MOFs), namely porosity rather than functionality, to achieve simultaneous interior and exterior modification of a MOF with polymers. We demonstrate that an anhydride-terminated polyimide oligomer can be covalently grafted to the amine-functionalized methacrylate polymer backbone residing underneath the MOF surface and physically entangled within the 3D nanochannels. The MOF particles were evenly coated with a thin layer of polyimide brushes on the surface thereby exhibiting increased dispersibility in solvent media as well as in polymer matrix. The MOF pores were decorated with aliphatic amine groups to endow the MOF with higher CO2 affinity at low pressure. The polyimide-grafted surface allowed MOF particles to interact favorably with the polyimide matrix, producing defect-free MMM with drastically improved CO2 permeability and maintaining the inherent CO2 /N2 and CO2 /CH4 selectivity of the neat polymeric membrane.

Interfacial Engineering in Metal-Organic Framework-Based Mixed Matrix Membranes Using Covalently Grafted Polyimide Brushes.

Good interfacial compatibility is the key to realize the full potential of metal-organic framework-based mix matrix membranes for gas separation. Here we report a new approach that uses polyimide brushes covalently grafted on the MOF surface to engineer the MOF-polymer interface. Benefiting from the strong brush-brush interaction, polyimide grafted MOF particles can form a stand-alone membrane at 88 wt % MOF loading without the addition of polymeric matrix. Compared to traditional mixed-matrix membranes, the modified membranes exhibit improved ductility up to 472%, reduced interfacial tearing phenomenon under shear force, decreased matrix chain mobility, and improved plasticization resistance against CO2. Most importantly, with increasing MOF loading, only the modified membranes exhibit simultaneous increase of selectivity and permeability for CO2/N2 and CO2/CH4 separation, following the trend predicted by the modified Maxwell model.

Natural Language to Visualization by Neural Machine Translation.

Supporting the translation from natural language (NL) query to visualization (NL2VIS) can simplify the creation of data visualizations because if successful, anyone can generate visualizations by their natural language from the tabular data. The state-of-the-art NL2VIS approaches (e.g., NL4DV and FlowSense) are based on semantic parsers and heuristic algorithms, which are not end-to-end and are not designed for supporting (possibly) complex data transformations. Deep neural network powered neural machine translation models have made great strides in many machine translation tasks, which suggests that they might be viable for NL2VIS as well. In this paper, we present ncNet, a Transformer-based sequence-to-sequence model for supporting NL2VIS, with several novel visualization-aware optimizations, including using attention-forcing to optimize the learning process, and visualization-aware rendering to produce better visualization results. To enhance the capability of machine to comprehend natural language queries, ncNet is also designed to take an optional chart template (e.g., a pie chart or a scatter plot) as an additional input, where the chart template will be served as a constraint to limit what could be visualized. We conducted both quantitative evaluation and user study, showing that ncNet achieves good accuracy in the nvBench benchmark and is easy-to-use.

Physical mixing of a catalyst and a hydrophobic polymer promotes CO hydrogenation through dehydration.

In many reactions restricted by water, selective removal of water from the reaction system is critical and usually requires a membrane reactor. We found that a simple physical mixture of hydrophobic poly(divinylbenzene) with cobalt-manganese carbide could modulate a local environment of catalysts for rapidly shipping water product in syngas conversion. We were able to shift the water-sorption equilibrium on the catalyst surface, leading to a greater proportion of free surface that in turn raised the rate of syngas conversion by nearly a factor of 2. The carbon monoxide conversion reached 63.5%, and 71.4% of the hydrocarbon products were light olefins at 250°C, outperforming poly(divinylbenzene)-free catalyst under equivalent reaction conditions. The physically mixed CoMn carbide/poly(divinylbenzene) catalyst was durable in the continuous test for 120 hours.

Fischer-Tropsch synthesis to olefins boosted by MFI zeolite nanosheets.

Catalytic reactions are severely restricted by the strong adsorption of product molecules on the catalyst surface, where promoting desorption of the product and hindering its re-adsorption benefit the formation of free sites on the catalyst surface for continuous substrate conversion1,2. A solution to this issue is constructing a robust nanochannel for the rapid escape of products. We demonstrate here that MFI zeolite crystals with a short b-axis of 90-110 nm and a finely controllable microporous environment can effectively boost the Fischer-Tropsch synthesis to olefins by shipping the olefin molecules. The ferric carbide catalyst (Na-FeCx) physically mixed with a zeolite promoter exhibited a CO conversion of 82.5% with an olefin selectivity of 72.0% at the low temperature of 260 °C. By contrast, Na-FeCx alone without the zeolite promoter is poorly active under equivalent conditions, and shows the significantly improved olefin productivity achieved through the zeolite promoter. These results show that the well-designed zeolite, as a promising promoter, significantly boosts Fischer-Tropsch synthesis to olefins by accelerating escape of the product from the catalyst surface.

Tracking and Visualization of Functional Domains in Stratified Metal-Organic Frameworks Using Gold Nanoparticles.

We report here a new technique for the identification and visualization of functional domains in stratified metal-organic frameworks (MOFs). The technique, namely, gold diffusion enabled domain identification, utilizes the diffusion of Au nanoparticles within MOF cavities to track and selectively stain the more Au-philic domain in an MOF particle thereby allowing direct observation of domains, determination of domain sequences, and, in certain cases, domain boundaries under transmission electron microscopy. This method is an excellent tool for studying MOF materials with complex domain hierarchy.