Plastic waste is a major environmental issue; converting it directly into valuable chemicals by using catalysts is a promising alternative to plastic recycling. Here, we report the selective catalytic cracking of polypropylene (PP), a typical commodity plastic, to high-value light olefins (C2–C5), below pyrolytic temperature (290 °C) and without external hydrogen supply, by using zeolite catalysts. Among the H+-form zeolites with different structures, HMFI showed the highest yields of light hydrocarbons (C2–C5), of which light olefins (C2–C5) were the major products. The HMFI-catalyzed PP conversion was applicable to the upcycling of a model PP waste, resulting in a 61.9% light hydrocarbon yield. The results of catalytic and in situ IR experiments using model HMFI catalysts with a small amount of external Brønsted acid sites suggested that the Brønsted acid sites on the external surface of HMFI are indispensable for the PP conversion and are posited to be the active sites for the cracking of PP into short-chain (oligomeric) hydrocarbon species as intermediate products. Density functional theory analyses were conducted to determine plausible reaction pathways by adopting 2,4-dimethylheptene as the shortest unit of the oligomeric species. The obtained results show that the β-scission of 2,4-dimethylheptene by Brønsted acid sites yields isobutene and propylene (or a propyl alkoxide group) via carbocation intermediates with an activation energy below 118 kJ mol–1. Operando UV–vis and IR experiments under the reaction conditions, combined with ex situ 1H NMR and 13C NMR analyses of the spent catalyst, show that some of the olefins are further converted to light or heavy aromatics (coke deposit), probably via carbenium ion species.@en

Significant efforts have been dedicated to the direct syngas conversion into ethanol, however, achieving a high ethanol yield remains a formidable task. In this study, we present the direct syngas-to-ethanol conversion over Li-promoted RhOx/MgO catalyst (RhOx/Li2O/MgO). The ethanol space-time yield (EtOH STY) and selectivity reached 12.2 mmol gcat–1 h–1 and 20%, respectively, at a 35% CO conversion over the RhOx/Li2O/MgO catalyst. The RhOx/Li2O/MgO catalyst demonstrated superior performance in terms of both ethanol selectivity and STY compared to Rh/Li2O catalysts on other support materials and Rh/MgO catalysts promoted with other alkali metals. In situ/operando spectroscopic techniques, combined with other characterisations and theoretical calculations, have elucidated the interactions between Li2O and Rh on the MgO surface. These interactions promote the formation of new active sites and weaken CO adsorption on the Rh surface, thereby enhancing ethanol production. This work provides a promising strategy for improving ethanol yield in syngas conversion processes.@en

Low temperature and high pressure are thermodynamically more favorable conditions to achieve high conversion and high methanol selectivity in CO₂ hydrogenation. However, low-temperature activity is generally very poor due to the sluggish kinetics, and thus, designing highly selective catalysts active below 200 °C is a great challenge in CO₂-to-methanol conversion. Recently, Re/TiO₂ has been reported as a promising catalyst. We show that Re/TiO₂ is indeed more active in continuous and high-pressure (56 and 331 bar) operations at 125-200 °C compared to an industrial Cu/ZnO/Al₂O₃ catalyst, which suffers from the formation of methyl formate and its decomposition to carbon monoxide. At lower temperatures, precise understanding and control over the active surface intermediates are crucial to boosting conversion kinetics. This work aims at elucidating the nature of active sites and active species by means of in situ/operando X-ray absorption spectroscopy, Raman spectroscopy, ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Transient operando DRIFTS studies uncover the activation of CO₂ to form active formate intermediates leading to methanol formation and also active rhenium carbonyl intermediates leading to methane over cationic Re single atoms characterized by rhenium tricarbonyl complexes. The transient techniques enable us to differentiate the active species from the spectator one on TiO₂ support, such as less reactive formate originating from spillover and methoxy from methanol adsorption. The AP-XPS supports the fact that metallic Re species act as H₂ activators, leading to H-spillover and importantly to hydrogenation of the active formate intermediate present over cationic Re species. The origin of the unique reactivity of Re/TiO₂ was suggested as the coexistence of cationic highly dispersed Re including single atoms, driving the formation of monodentate formate, and metallic Re clusters in the vicinity, activating the hydrogenation of the formate to methanol.

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