Precise activation of C-C bonds for recycling and upcycling of plastics.

The rapid accumulation of plastic waste has led to a severe environmental crisis and a noticeable imbalance between manufacturing and recycling. Fortunately, chemical upgradation of plastic waste holds substantial promise for addressing these challenges posed by white pollution. During plastic upcycling and recycling, the key challenge is to activate and cleave the inert C-C bonds in plastic waste. Therefore, this perspective delves deeper into the upcycling and recycling of polyolefins from the angle of C-C activation-cleavage. We illustrate the importance of C-C bond activation in polyolefin depolymerization and integrate molecular-level catalysis, active site modulation, reaction networks and mechanisms to achieve precise activation-cleavage of C-C bonds. Notably, we draw potential inspiration from the accumulated wisdom of related fields, such as C-C bond activation in lignin chemistry, alkane dehydrogenation chemistry, C-Cl bond activation in CVOC removal, and C-H bond activation, to influence the landscape of plastic degradation through cross-disciplinary perspectives. Consequently, this perspective offers better insights into existing catalytic technologies and unveils new prospects for future advancements in recycling and upcycling of plastic.

Ameliorating Adsorption Performance for Poisonous Methylene Blue through the Amino Functionalization of Metal-Organic Frameworks.

Amino (-NH2)-functionalized metal-organic frameworks (MOFs) are widely applied to improve the properties of materials owing to the rich host-guest chemical properties of amino groups. In this work, the amino-functionalization strategy was thus employed to improve the sorption performance of methylene blue (MB). The introduction of -NH2 groups in AOBTC-Zn did not reduce the pore size of the framework but rather modulated and optimized the host-guest interactions of MOFs. The MB+ sorption result was significantly improved by the NH2-functionalized NH2-AOBTC-Zn. The results showed that the maximum sorption capacity of NH2-AOBTC-Zn is much higher (1623 mg/g) than that of AOBTC-Zn (204 mg/g), which was comparable with that of MIL-68(Al) (1666 mg/g). The adsorption kinetics and isothermal models indicated that the MB+ sorption processes of both MOFs were consistent with the Langmuir isothermal and pseudo-second-order kinetic models. The single-group and multicomponent sorption experiments showed that the sorption behavior was the result of π-π interaction, electrostatic interaction, hydrogen bonding interaction, and pore size interaction. In particular, NH2-AOBTC-Zn exhibits a higher adsorption capacity than AOBTC-Zn due to the additional hydrogen bonding interactions it provided. These may guide the design of porous MOFs with side group modification for liquid phase sorption/separation.

NbOx-Based Catalysts for the Activation of C-O and C-C Bonds in the Valorization of Waste Carbon Resources.

Escalating energy demand, the depletion of fossil fuels, and abnormal climate change are recognized as the key challenges in the 21st century. The valorization of biomass and plastic, representing the most abundant natural and man-made polymers, respectively, as alternatives to fossil fuel is one of the promising solutions to creating a carbon-neutral, waste-free society. Catalysis is an essential tool for manipulating energy transformations via bond-breaking and bond-forming principles. To producing chemicals and fuels via biomass valorization and plastic upcycling, the cleavage of C-O and C-C bonds is the major catalytic route, given that the two are mainly constructed by various interunit C-O and C-C linkages. In this work, a consensus concerning the catalytic mechanism is reached: the activities for the cleavage of C-O and C-C bonds highly depend on the catalyst ability to activate the C-O and C-C bonds. Among the catalysts reported, NbOx-based catalysts show a unique, superstrong ability to activate C-O and C-C bonds. While research on biomass valorization over NbOx-based catalysts maintains its momentum, plastic upcycling driven by an efficient NbOx-based catalyst capable of activating C-O and C-C bonds is quickly catching up. Therefore, deepening the understanding of NbOx-based catalysts for the activation of C-O and C-C bonds is of importance to further drive biomass valorization and plastic upcycling, even in many other related areas. Herein, we present progress on the activation of C-O and C-C bonds in waste carbon resources, with an emphasis on our own work in using NbOx-based catalysts. First, we introduce NbOx-based catalysts for the activation of C-O and C-C bonds in biomass with a special focus on explaining how NbOx-based catalysts activate C-O and C-C bonds and why NbOx-based catalysts can activate C-O and C-C bonds so efficiently. Then, unified descriptors to embody the abilities to extract O from oxygenated compounds and an adsorbed benzene ring, namely "oxygen affinity" and "benzene ring affinity", were defined to standardize C-O and Carom-Caliph activation chemistry. Furthermore, we highlight the emerging opportunities of NbOx-based catalysts for plastic upcycling by learning the wisdom accumulated from the activation of C-O and C-C bonds in biomass. Finally, our own insights into future recommendations in this promising field are provided.

A unified view on catalytic conversion of biomass and waste plastics.

Originating from the desire to improve sustainability, producing fuels and chemicals from the conversion of biomass and waste plastic has become an important research topic in the twenty-first century. Although biomass is natural and plastic synthetic, the chemical nature of the two are not as distinct as they first appear. They share substantial structural similarities in terms of their polymeric nature and the types of bonds linking their monomeric units, resulting in close relationships between the two materials and their conversions. Previously, their transformations were mostly studied and reviewed separately in the literature. Here, we summarize the catalytic conversion of biomass and waste plastics, with a focus on bond activation chemistry and catalyst design. By tracking the historical and more recent developments, it becomes clear that biomass and plastic have not only evolved their unique conversion pathways but have also started to cross paths with each other, with each influencing the landscape of the other. As a result, this Review on the catalytic conversion of biomass and waste plastic in a unified angle offers improved insights into existing technologies, and more importantly, may enable new opportunities for future advances.

Recovery of Arenes from Polyethylene Terephthalate (PET) over a Co/TiO2 Catalyst.

Upcycling of spent plastics has become a more emergent topic than ever before due to the rapid generation of plastic waste associated with the change of lifestyles of the human society. Polyethylene terephthalate (PET) is a major aromatic plastic and herein, the conversion of PET back into arenes was demonstrated in a one-pot reaction combining depolymerization and hydrodeoxygenation (HDO) over a Co/TiO2 catalyst. The effectiveness of the Co/TiO2 catalyst in HDO and the underlining reaction pathway were established using the PET monomer terephthalic acid (TPA) as the substrate. Quantitative TPA conversion together with 75.2 mol% xylene and toluene selectivity under 30 bar initial H2 pressure at 340 °C was achieved after 4 h reaction. More encouragingly, the catalyst induced both depolymerization and HDO reaction via C-O bond cleavage when PET was used as a substrate. 78.9 mol% arenes (toluene and xylene) was obtained under optimized conditions.

H2 -free Plastic Conversion: Converting PET back to BTX by Unlocking Hidden Hydrogen.

The strong desire for a circular economy makes obtaining fuels and chemicals via plastic degradation an important research topic in the 21st century. Here, the first example of the H2 -free polyethylene terephthalate (PET) conversion to BTX (benzene, toluene and xylene) was achieved by unlocking hidden hydrogen in the ethylene glycol part over Ru/Nb2 O5 catalyst. Among the whole process (hydrolysis, reforming and hydrogenolysis/decarboxylation), the parallel hydrogenolysis and decarboxylation were competing and the rate-determining step. Ru/Nb2 O5 exhibited superior hydrogenolysis and poorer decarboxylation performance in direct comparison with Ru/NiAl2 O4 , accordingly contributing to the distinct selectivity to alkyl aromatics among BTX. Ru species on Nb2 O5 , unlike those on NiAl2 O4 , showed more Ruδ+ species owing to the strong interaction between Ru and Nb2 O5 , restricting the undesired decarboxylation. Along with NbOx species for C-O bond activation, excellent reactivity towards the H2 -free conversion of PET back to BTX with alkyl aromatics as dominant species was achieved. This H2 -free system was also capable of converting common real PET plastics back to BTX, adding new options in the circular economy of PET.

Towards the Circular Economy: Converting Aromatic Plastic Waste Back to Arenes over a Ru/Nb2 O5 Catalyst.

The upgrading of plastic waste is one of the grand challenges for the 21st century owing to its disruptive impact on the environment. Here, we show the first example of the upgrading of various aromatic plastic wastes with C-O and/or C-C linkages to arenes (75-85 % yield) via catalytic hydrogenolysis over a Ru/Nb2 O5 catalyst. This catalyst not only allows the selective conversion of single-component aromatic plastic, and more importantly, enables the simultaneous conversion of a mixture of aromatic plastic to arenes. The excellent performance is attributed to unique features including: (1) the small sized Ru clusters on Nb2 O5 , which prevent the adsorption of aromatic ring and its hydrogenation; (2) the strong oxygen affinity of NbOx species for C-O bond activation and Brønsted acid sites for C-C bond activation. This study offers a catalytic path to integrate aromatic plastic waste back into the supply chain of plastic production under the context of circular economy.

Chemicals from Lignin: A Review of Catalytic Conversion Involving Hydrogen.

Lignin is the most abundant biopolymer with aromatic building blocks and its valorization to sustainable chemicals and fuels has extremely great potential to reduce the excessive dependence on fossil resources, although such conversions remain challenging. The purpose of this Review is to present an insight into the catalytic conversion of lignin involving hydrogen, including reductive depolymerization and the hydrodeoxygenation of lignin-derived monomers to arenes, cycloalkanes and phenols, with a main focus on the catalyst systems and reaction mechanisms. The roles of hydrogenation sites (Ru, Pt, Pd, Rh) and acid sites (Nb, Ti, Mo), as well as their interaction in selective hydrodeoxygenation reactions are emphasized. Furthermore, some inspirational strategies for the production of other value-added chemicals are mentioned. Finally, some personal perspectives are provided to highlight the opportunities within this attractive field.

Selective production of indane and its derivatives from lignin over a modified niobium-based catalyst.

Starting from native lignin, the first example of the efficient production of indane and its derivatives was achieved over a CH2Cl2-modified Ru/Nb2O5 catalyst. Intensive characterization reveals that CH2Cl2 weakens the hydrogenolysis ability of Ru particles through the strong interaction between CH2Cl2 and Ru, and promotes the intramolecular cyclization reaction.

Production of Low-Freezing-Point Highly Branched Alkanes through Michael Addition.

A new approach for the production of low-freezing-point, high-quality fuels from lignocellulose-derived molecules was developed with Michael addition as the key step. Among the investigated catalysts, CoCl2 ⋅6 H2 O was found most active for the Michael addition of 2,4-pentanedione with FA (single aldol adduct of furfural and acetone, 4-(2-furanyl)-3-butene-2-one). Over CoCl2 ⋅6 H2 O, a high carbon yield of C13 oxygenates (about 75 %) can be achieved under mild conditions (353 K, 20 h). After hydrodeoxygenation, low-freezing-point (<223 K) branched alkanes with 13 carbons within jet fuel ranges were obtained over a Pd/NbOPO4 catalyst. Furthermore, C18,23 fuel precursors could be easily synthesized through Michael addition of 2,4-pentanedione with DFA (double-condensation product of furfural and acetone) under mild conditions and the molar ratio of C18 /C23 is dependent on the reaction conditions of Michael addition. After hydrodeoxygenation, high density (0.8415 g mL-1 ) and low-freezing-point (<223 K) branched alkanes with 18, 23 carbons within lubricant range were also obtained over a Pd/NbOPO4 catalyst. These highly branched alkanes can be directly used as transportation fuels or additives. This work opens a new strategy for the synthesis of highly branched alkanes with low freezing point from renewable biomass.