Polyolefins, the largest used commodity plastics in the world, find extensive application in many fields. However, most end up in landfills or incineration, leading to severe ecological crises, environmental pollution, and serious resource waste problems. As representatives on chemical upcycling of polyolefin plastics polyolefin waste to fuels and bulk/fine chemicals, polyolefin catalytic cracking and hydrocracking based on zeolite or metal/zeolite composite catalysts are considered the most effective paths due to their large capacity and strong adaptability to existing petrochemical equipment. After an overview of the reaction mechanisms of pyrolysis and catalytic cracking, this review aims to comprehensively discuss the influence of zeolite catalyst structure (acidity, pore structure, and morphology) on the catalytic activity, selectivity, and stability of polyolefin cracking, particularly emphasizing the importance for matching acidity and pore structure for target product formation. Subsequently, the structure–activity relationship between the metal site and zeolite’s acid site in polyolefin hydrocracking is also discussed. In the end, emerging opportunities and challenges are proposed to promote a more efficient way for polyolefin chemical upcycling. 

Understanding the Structure–Activity Relationships in Catalytic Conversion of Polyolefin Plastics by Zeolite-Based Catalysts: A Critical Review

Although being transition metals, the Fenton‐inactive group 3–4 elements (Sc, Y, La, Ti, Zr, and Hf) can easily lose all the outermost s and d electrons, leaving behind ionic sites with nearly empty outermost orbitals that are stable but inactive for oxygen involved catalysis. Here, it is demonstrated that the dynamic coordination network can turn these commonly inactive ionic sites into platinum‐like catalytic centers for the oxygen reduction reaction (ORR). Using density functional theory calculations, a macrocyclic ligand coordinated yttrium single‐atom (YN4) moiety is identified, which is originally ORR inactive because of the too strong binding of hydroxyl intermediate, while it can be activated by an axial ligand X through the covalency competition between YX and YOH bonds. Strikingly, it is also found that the binding force of the axially coordinated ligand is an effective descriptor, and the chlorine ligand is screened out with an optimal binding force that behaves self‐adaptively to facilitate each ORR intermediate steps by dynamically changing its YCl covalency. These experiments validate that the as‐designed YN4‐Cl moieties embedded within the carbon framework exhibit a high half‐wave potential (E1/2 = 0.85 V) in alkaline media, the same as that of the commercial Pt/C catalyst .

Coordination Chemistry of Large‐Sized Yttrium Single‐Atom Catalysts for Oxygen Reduction Reaction

The chemical conversion of CO2 into hydrocarbon fuels and chemicals using green hydrogen not only utilizes abundant CO2 as a carbon feedstock but also enables the storage of hydrogen. Herein, we investigate the direct hydrogenation of CO2 to gasoline and olefins over a series of bifunctional iron-zeolite tandem catalysts operated at high temperatures (> 300 ℃). This process may efficiently utilize CO2 discharged from industrial combustion and green H2 produced by solid oxide electrolytic cells (SOEC). The optimized FeMnK+H-ZSM-5 catalyst offers a 70% selectivity of C5−C11 range hydrocarbons together with a 17% selectivity of C2−C4 lower olefins at 320 °C. The CO2 conversion levels and the aromatics contents could be greatly enhanced as the temperature increases from 320 ℃ to 400 ℃. The hydrocarbon distribution is mainly determined by the micropore size of the zeolites. The dynamic evolution of bifunctional catalysts and its impact on bifunctional catalysis was systematically investigated. 

Direct conversion of carbon dioxide into liquid fuels and chemicals by coupling green hydrogen at high temperature

Metal promotion is the most widely adopted strategy for enhancing the hydrogenation functionality of an oxide catalyst. Typically, metal nanoparticles or dopants are located directly on the catalyst surface to create interfacial synergy with active sites on the oxide, but the enhancement effect may be compromised by insufficient hydrogen delivery to these sites. Here, we introduce a strategy to promote a ZnZrOx methanol synthesis catalyst by incorporating hydrogen activation and delivery functions through optimized integration of ZnZrOx and Pd supported on carbon nanotube (Pd/CNT). The CNT in the Pd/CNT + ZnZrOx system delivers hydrogen activated on Pd to a broad area on the ZnZrOx surface, with an enhancement factor of 10 compared to the conventional Pd-promoted ZnZrOx catalyst, which only transfers hydrogen to Pd-adjacent sites. In CO2 hydrogenation to methanol, Pd/CNT + ZnZrOx exhibits drastically boosted activity-the highest among reported ZnZrOx-based catalysts-and excellent stability over 600 h on stream test, showing potential for practical implementation. 

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Engineering nanoscale H supply chain to accelerate methanol synthesis on ZnZrOx

Unpredictable properties of industrial HY zeolite for tetralin hydrocracking

Maximizing the utilization of noble metals is crucial for applications such as catalysis. We found that the minimum loading of platinum for optimal performance in the hydroconversion of n-alkanes for industrially relevant bifunctional catalysts could be reduced by a factor of 10 or more through the rational arranging of functional sites at the nanoscale. Intentionally depositing traces of platinum nanoparticles on the alumina binder or the outer surface of zeolite crystals, instead of inside the zeolite crystals, enhanced isomer selectivity without compromising activity. Separation between platinum and zeolite acid sites preserved the metal and acid functions by limiting micropore blockage by metal clusters and enhancing access to metal sites. Reduced platinum nanoparticles were more active than platinum single atoms strongly bonded to the alumina binder. Description Better on the outside One way to maximize the activity of scarce and costly noble metals in catalysis is to use small cluster or even single atoms on metal oxide supports. Bifunctional platinum and zeolite catalysts are used industrially in alkane hydroconversion. Cheng et al. show that the amount of platinum used could be reduced and isomer selectivity improved when the catalyst preparation created platinum clusters on the surface of zeolite or its alumina binder, rather than in the interior of the zeolite channels. Preparations that would have formed single-atom catalysts were less active because the platinum ions adsorbed on the alumina surface were more difficult to reduce. —PDS Isomer selectivity in alkane hydroconversion improves when platinum clusters are located on rather than in zeolites.

Maximizing noble metal utilization in solid catalysts by control of nanoparticle location

Developing visible approaches to real-time monitor kinetic traps within the thermodynamic equilibrium process is of great importance for the fundamental research of dynamic supramolecular chemistry. Herein, we designed a dynamically...

A Time-Resolved and Visualized Host–Guest Self-assembly Behavior Controlled through Kinetic Trapping

Controlling cobalt Fischer–Tropsch stability and selectivity through manganese titanate formation

Methyl carboxylate esters have been shown to be potent promoters of low-temperature methanol dehydration to dimethyl ether (DME) using various zeolite catalysts. In the present work, catalytic kinetic studies, in-situ Fourier-transform infrared spectroscopy (FT-IR) and solid-state nuclear magnetic resonance spectroscopy (NMR) techniques were used to elucidate the promotional mechanism of methyl carboxylate esters on methanol dehydration to DME, using the medium pore zeolite H-ZSM-5 (MFI) as the catalyst. Kinetic studies were performed using the very potent methyl n-hexanoate promoter. The DME yield was dependent on both the methanol and methyl n-hexanoate partial pressures across the temperature ranges used in this study (110 to 130 °C). This is consistent with the promoted reaction being a bimolecular reaction between methanol and ester species adsorbed at the catalyst active sites, via an SN2 type reaction, as previously postulated. The in-situ FT-IR studies reveal that the Brønsted acid (BA) sites on H-ZSM-5 were very rapidly titrated by ester carbonyl group adsorption and bonded more strongly with esters than with methanol. Upon methanol addition, an even lower DME formation temperature (30 °C) was observed with methyl n-hexanoate pretreated H-ZSM-5 samples in the in-situ NMR studies, further confirming the strong promotion of this methyl ester on methanol dehydration to DME. The adsorption and reactivity of different methyl esters on H-ZSM-5 indicates that while methyl formate more easily dissociates into a surface methoxy species, [Si(OMe)Al], and carboxylic acid, it is a less potent promoter than alkyl-chain-containing methyl esters in methanol dehydration to DME, which in turn did not show this dissociative behavior in the low-temperature NMR studies. This indicates that methyl alkyl carboxylates do not need to be dissociated to a surface methoxy species to promote the methanol dehydration reaction and that a bimolecular associative mechanism plays an important role in promoting DME formation.

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Zhuoran Xu

Papers

Maximizing noble metal utilization in solid catalysts by control of nanoparticle location

A Time-Resolved and Visualized Host–Guest Self-assembly Behavior Controlled through Kinetic Trapping

Controlling cobalt Fischer–Tropsch stability and selectivity through manganese titanate formation

Kinetic and Spectroscopic Studies of Methyl Ester Promoted Methanol Dehydration to Dimethyl Ether on ZSM-5 Zeolite

Aromatic aldehydes as tuneable and ppm level potent promoters for zeolite catalysed methanol dehydration to DME