Promoting And Deactivating Effects of Carbonaceous Deposits During Skeletal 1-Butene Isomerization Over Ferrierite
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Hebisch, Karoline L.
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Abstract
Most microporous solid acid catalysts deactivate during hydrocarbon conversion because of carbon depositions (“coke”) blocking pores and poisoning active centers. However, several cases of reaction enhancement have been reported. One reaction in which coke has a promoting effect is the skeletal isomerization of linear butene to iso-butene over the zeolite ZSM-35 (framework type FER, possessing perpendicular intersecting 8-R and 10-R channels). While consensus exists about the reaction mechanism during reaction startup, the reaction location and the reaction pathway at peak catalyst performance are still contested.
Time-resolved catalyst characterization data are collected to (1) understand the effects of high temperature (T=420 °C) and carbon deposition on the zeolite micropore structure and Brønsted acid site accessibility and (2) the location and chemical nature of carbon deposits. Three distinct reaction stages are identified: catalyst startup (0-24 h), optimal performance (50-300 h) and catalyst deactivation (>300 h). Most of the deposits (~5 wt%) form within the initial 24 h and are located inside the micropores, rendering them effectively inaccessible to probe molecules (e.g., N2 and Ar) and leading to an expansion of the crystal unit cell. Adsorption isotherms of several hydrocarbons revealed that only small molecules with a kinetic diameter of <4.7 Å can diffuse into the pores before they become obstructed by carbon deposits. Calculation of diffusion coefficients from transient adsorption data at reaction temperature shows that even small molecules are severely hindered in their diffusion, concluding that the reaction occurs at the pore entrances.
Operando reactivity quenching with basic probe molecules with different steric constraints shows that acid centers exist under reaction conditions for tens of hours, are located in the pore mouths, and can be reversibly poisoned. Because a combination of steric confinement, acidity, and carbonaceous deposits is needed to successfully facilitate skeletal isomerization, the active sites are concluded to be monoaromatic species, which form during catalyst startup in the pore mouths and are protonated by internal Brønsted acid sites. The positive charge is delocalized and communicated via a methyl group to the catalyst exterior, where skeletal butene isomerization is facilitated. The outstanding activity of ferrierite for this reaction is explained by FER’s ability to anchor the catalytically active deposits in the pores while preventing their premature deactivation by hindering side-chain growth and condensation reactions. Catalyst deactivation is explained by the formation of external, polyaromatic condensates preventing the reactants from accessing the active sites in the pore mouths.
With this information, the catalyst performance is optimized by selective oxidation of residual organic structure directing agent (OSDA). Three promoting effects of residual OSDA are identified. Residual OSDA selectively poisons the strongest Brønsted acid sites, thereby substantially suppressing side product formation while also improving catalyst lifetime. Carbonaceous fragments further act as a precursor for the active site, thereby shortening the unselective startup phase.
Lastly, two catalyst regeneration strategies are explored – oxidation and supercritical fluid extraction. Characterization data of oxidatively regenerated samples shows that coke combusts in a manner similar to a shrinking-core model. If done under milder conditions (500 °C in air flow) for 30 min – 1 h, a substantial amount of deactivating species can be removed while the internal deposits remain largely intact. In contrast, due to higher solubility and increased mobility of small aliphatic, olefinic, and monoaromatic species, internal deposits are preferentially removed during solvent extraction with supercritical CO2. Based on preliminary performance and catalyst characterization data, recommendations for future regeneration approaches are derived. Understanding the activating and deactivating effects on catalyst activity is crucial for prolonging catalyst lifetimes and increasing the efficiency of regeneration processes, which will help the transition from a fossil-resource-based economy to a bioeconomy.
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2024-07-30
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Dissertation