The current scale of plastics production and the accompanying waste disposal problems represent a largely untapped opportunity for chemical upcycling. Tandem catalytic conversion by platinum supported on γ-alumina converts various polyethylene grades in high yields (up to 80 weight percent) to low-molecular-weight liquid/wax products, in the absence of added solvent or molecular hydrogen, with little production of light gases. The major components are valuable long-chain alkylaromatics and alkylnaphthenes (average ~C30, dispersity Ð = 1.1). Coupling exothermic hydrogenolysis with endothermic aromatization renders the overall transformation thermodynamically accessible despite the moderate reaction temperature of 280°C. This approach demonstrates how waste polyolefins can be a viable feedstock for the generation of molecular hydrocarbon products.
Inorganic oxides play a crucial role in the activation of atomically dispersed metal oxides for catalytic olefin transformations, but the inefficient activation processes remain poorly understood. Activation of methyltrioxorhenium (MTO) for propene metathesis via its deposition on the surface of γ-Al 2 O 3 typically results in <5% active sites, and these sites deactivate rapidly. Simple substitution of the support by a less crystalline (largely amorphous) alumina (a-Al 2 O 3 ) results in ca. 4× more activity and at least 10× more productivity. On both types of alumina, metathesis is initiated only at specific sites, whose availability limits the catalytic activity. While the two aluminas have similar total numbers of Lewis acid sites, the less crystalline support activates twice as many grafted MTO sites. Interestingly, a-Al 2 O 3 has nearly double the number of strong Lewis acid sites. However, the number of active sites is ca. 10× lower than the total number of strong Lewis acid sites, and metathesis proceeds even when most are occupied by pyridine. DQSQ and D-HMQC 1 H and 27 Al solid-state NMR reveal that many Lewis acid sites are co-located with surface hydroxyl groups, which prevent activation and/or cause rapid deactivation. Undercoordinated Al sites on dominant (110) facets, which retain hydroxyl groups under catalyst preparation conditions, are therefore unlikely to lead to stable active sites. In contrast, the minor (100) facets of γ-Al 2 O 3 , which are completely dehydroxylated, contain strongly Lewis-acidic five-coordinate Al sites that are necessarily remote from surface hydroxyl groups. Such sites, which are relatively more abundant on less well-crystallized aluminas, are inferred to be responsible for generating stable metathesis sites.
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.
A new family of supramolecular materials is exploited from waste thermosets via one-step retrosynthetic approach, which exhibits distinguished adhesion properties in dry/wet environments, good corrosion resistance and dynamic reversibility. This...
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