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Catalytic conversion of polyolefins to value-added products offers an alternative route to capture value from plastic waste. Here we initially examine reactions of a polyethylene model (hexatriacontane, C36H74) on a Pt/SiO2 catalyst under typical hydrogenolysis and hydrocracking temperatures, which leads to irreversibly adsorbed surface hydrocarbons identified after extraction of hexatriacontane with excess hot toluene. The IR spectra of these catalysts after extraction reveal only aliphatic C–H stretches. SiO2 alone leads similar hydrocarbon adsorption on the surface where extended extraction fails to fully remove the adsorbed hydrocarbons from neat silica. The amount of hydrocarbon irreversibly adsorbed increases nearly 10-fold when the reactant is changed from hexatriacontane to polyethylene (Mn = 4000 Da), but the adsorbed quantity is insensitive to reaction temperature (200–300 °C). These results demonstrate significant, nonextractable hydrocarbon deposition on catalyst support surfaces without dehydrogenation catalyst present at temperatures typical of catalytic deconstruction of polyolefin waste, which may limit catalyst turnover and impact the product distribution.
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Understanding liquid–liquid equilibria in binary mixtures of hydrocarbons with a thermally robust perarylphosphonium-based ionic liquid
Binary mixtures of hydrocarbons and a thermally robust ionic liquid (IL) incorporating a perarylphosphonium-based cation are investigated experimentally and computationally. Experimentally, it is seen that excess toluene added to the IL forms two distinct liquid phases, an “ion-rich” phase of fixed composition and a phase that is nearly pure toluene. Conversely, n -heptane is observed to be essentially immiscible in the neat IL. Molecular dynamics simulations capture both of these behaviours. Furthermore, the simulated composition of the toluene-rich IL phase is within 10% of the experimentally determined composition. Additional simulations are performed on the binary mixtures of the IL and ten other small hydrocarbons having mixed aromatic/aliphatic character. It is found that hydrocarbons with a predominant aliphatic character are largely immiscible with the IL, while those with a predominant aromatic character readily mix with the IL. A detailed analysis of the structure and energetic changes that occur on mixing reveals the nature of the ion-rich phase. The simulations show a bicontinuous phase with hydrocarbon uptake akin to absorption and swelling by a porous absorbent. Aromatic hydrocarbons are driven into the neat IL via dispersion forces with the IL cations and, to a lesser extent, the IL anions. The ion–ion network expands to accommodate the hydrocarbons, yet maintains a core connective structure. At a certain loading, this network becomes stretched to its limit. The energetic penalty associated with breaking the core connective network outweighs the gain from new hydrocarbon–IL interactions, leaving additional hydrocarbons in the neat phase. The spatially alternating charge of the expanded IL network is shown to interact favourably with the stacked aromatic subphase, something not possible for aliphatic hydrocarbons.
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- Award ID(s):
- 1800122
- PAR ID:
- 10356220
- Date Published:
- Journal Name:
- RSC Advances
- Volume:
- 11
- Issue:
- 50
- ISSN:
- 2046-2069
- Page Range / eLocation ID:
- 31328 to 31338
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D1RA06268A
