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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.

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.

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 Bronsted 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.

While making fuels and chemicals via plastic upcycling has become an important research topic recently, integrating waste plastic into the drug supply chain is exceedingly rare. Here, starting from polyphenylene oxide plastic, the selective production of 3,5-dimethyl phenol as an important pharmaceutical intermediate was achieved over Ru/Nb2O5, which is the first example of integrating waste plastic into the drug supply chain. It is found that the small-sized Ru particles and the Bronsted acid sites play key roles in the selective production of 3,5-dimethyl phenol. One the one hand, the small-sized Ru particles on 0.05% Ru/Nb2O5 inhibit the saturation of the aromatic ring (no ring saturation products were detected) and the removal of the phenolic hydroxyl group, resulting in a high selectivity to phenolic products. On the other hand, the Bronsted acid sites in Ru/Nb2O5 can stabilize the carbocation intermediates in the C(o)–O cleavage process, affording a distinct selectivity to 3,5-dimethyl phenol. This catalyst was also capable of handling common polyphenylene oxide plastics to selectively prepare 3,5-dimethyl phenol, adding new options for the synthesis of 3,5-dimethyl phenol.

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.

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, NbO

Hydrodeoxygenation (HDO) is a crucial process for the synthesis of biofuels from renewable biomass. Here, several bimetallic Ni–M/Nb2O5 catalysts (M = Fe, Co, Cu) were synthesized and evaluated in the HDO of biofuel precursors (aldol adduct of furfural with acetone) to liquid alkane and it was found that Ni–Cu/Nb2O5 has the best performance (86.5% yield of octane and 5.1% yield of heptane). Various characterization techniques show that Ni–Cu alloy is formed over the Ni–Cu/Nb2O5 catalyst, which may have the main active metal sites. Moreover, the Ni–Cu alloy has more sites for the adsorption–activation of H2, leading to the high activity. During the HDO process, the ring-opening of the intermediate butyl-tetrahydrofuran (BTHF) and the conversion of octanol are two rate-determining steps. The kinetic study confirms that the ring-opening of BTHF over Ni–Cu/Nb2O5 is more favorable than that over Ni/Nb2O5. More importantly, the Ni–Cu alloy significantly restrained the undesirable decarbonylation due to the weak adsorption of carbonyl, which was key for the efficient production of Cn alkanes. This work provides an in-depth understanding of the role of Ni–Cu alloys and new insights into the design of non-noble metal catalysts for the HDO of biofuel precursors.

Selective synthesis of primary amines via reductive amination becomes an important research topic due to their wide applications. Various metal-based catalysts (Ru, Ir, Pt, Rh, etc.) have been developed; however, most systems suffer from low efficiency and poor stability. Here, we revealed that the hydride-like NH2δ- species, generated by the dissociation of NH3 over a core-shell structured Co@CoO catalyst is capable of accelerating the ammonolysis of Schiff bases, the reaction intermediates. This catalyst can handle various reductive aminations of aldehydes and ketones under mild conditions and run 21 times without deactivation. The combination of various spectroscopic measurements and computational modelling illustrated that this catalyst not only drives the dissociation of H2 to active Hδ- species, it also enables the homolytic and heterolytic cleavages of NH3 to NH2δ- species. D2 isotopic tracing experiment provided further evidence of the direct participation of hydride-like NH2δ- species in the ammonolysis of the Schiff bases. Theoretical calculations also verified the stable co-adsorption state of the Hδ- and NH2δ- species which allows the Schiff base to move freely on the surface of the CoO shell, resulting in the exceptional catalytic activity. This study demonstrates, for the first time, the potential of metal-oxide catalysts for the production of primary amines through reductive amination.

The efficient utilization of lignocellulosic biomass has tremendous potential to reduce the excessive dependence on fossil fuels. Here, we provide an overview on the recent achievements in the catalytic production of value-added chemicals and fuels. When targeting chemicals, a key objective is to maximize the product selectivity to favor the subsequent separation. This can be achieved through the design of catalysts and optimization of catalytic systems based on the deep understanding of the catalytic mechanism. For production of fuels, attention should be paid to the establishment of an energy-efficient process for high-quality fuels. This can be realized through the design of C–C coupling reactions and the development of multifunctional catalysts to minimize the reaction steps from lignocellulose to fuels. In addition, the full utilization of lignocellulose into biofuels and chemicals in a single process is separately introduced. Finally, several personal perspectives on the opportunities and challenges within this promising field are discussed.

Aldol condensation is of significant importance for the production of fuel precursors from biomass-derived chemicals and has received increasing attention. Here we report a Nb 2 O 5 catalyst with excellent activity and stability in the aldol condensation of biomass-derived carbonyl molecules. It is found that in the aldol condensation of furfural with 4-heptanone, Nb 2 O 5 has obviously superior activity, which is not only better than that of other common solid acid catalysts (ZrO 2 and Al 2 O 3 ), more importantly, but also better than that of solid base catalysts (MgO, CaO, and magnesium-aluminum hydrotalcite). The detailed characterizations by N 2 sorption/desorption, NH 3 -TPD, Py-FTIR and DRIFTS study of acetone adsorption reveal that Nb 2 O 5 has a strong ability to activate the C=O bond in carbonyl molecules, which helps to generate a metal enolate intermediate and undergo the nucleophilic addition to form a new C–C bond. Furthermore, the applicability of Nb 2 O 5 to aldol condensation is extended to other biomass-derived carbonyl molecules and high yields of target fuel precursors are obtained. Finally, a multifunctional Pd/Nb 2 O 5 catalyst is prepared and successfully used in the one-pot synthesis of liquid alkanes from biomass-derived carbonyl molecules by combining the aldol condensation with the sequential hydrodeoxygenation.

The efficient utilization of renewable glucose has enormous potential to reduce the excessive dependence on fossil resources, but remains challenging owing to the inevitable formation of undesirable humins and multistep processes. Reported here is a new 2,4-pentanedione/H2O–NaCl biphasic system for the direct one-step conversion of glucose to high-quality fuel precursors with a high utilization efficiency of glucose (90%). The formation of humins is markedly restrained with an entirely new pathway, i.e. glucose directly reacts with 2,4-pentanedione to afford fuel precursors via several chemical transformations, which is totally different from the traditional glucose-to-fructose-to-HMF way. Control experiments and DFT calculations confirm that this new pathway is thermodynamically favorable compared with the traditional way. This simple system is also capable of converting starch to fuel precursors, paving a new way for the efficient valorization of glucose-based natural sugars.

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 H

The upgrading of plastic waste is one of the grand challenges for the 21

Robinson annulation may be useful for converting biomass-derived compounds into jet-fuel-ranged alkylcyclohexanes, but it has not been fully developed until now. Herein, a strategy for the synthesis of highly branched C15 alkylcyclohexanes with lignocellulose-derived furfural and 2,4-pentanedione was developed with Robinson annulation as the key step. Low-freezing-point (−81 °C) and high-density (0.8139 g/mL) highly branched alkylcyclohexanes were produced by two steps, namely, a one-pot cyclization process including the aldol condensation of furfural with 2,4-pentanedione and the subsequent Robinson annulation of the aldol adducts with 2,4-pentanedione over CoCl2·6H2O catalyst, then following with the Pd/NbOPO4-catalyzed hydrodeoxygenation.

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*6H2O was found to be the most active for Michael addition of 2,4-pentanedione with FA (single aldol adduct of furfural and acetone, 4-(2-furanyl)-3-butene-2-one). Over CoCl2*6H2O, 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 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) and low freezing point (< 223 K) branched alkanes with 18, 23 carbons with lubricant range were also obtained over Pd/NbOPO4 catalyst. These highly branched alkanes can be directly used as transportation fuels or additives. This work opens a novel strategy for the synthesis of highly branched alkanes with low-freezing-point from renewable biomass.

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.

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.