The activity and selectivity of bifunctional catalysts for n-C16 hydroisomerization are determined by only two parameters, the balance between the metal and acid functions and their degree of ...intimacy; both of them are easy to be quantified.
•n-C16 Transformation over three Pt-HBEA series differing by Pt–H+ intimacy.•Determining parameters: balance and intimacy between metal and acid functions.•Hydrogenating/acid balance quantified by Pt/H+ concentration ratio.•Pt/H+ intimacy expressed by nas, number of acid steps per converted n-C16 molecule.•nas Is easily drawn from the initial product distribution.
n-Hexadecane (n-C16) transformation was carried out at 220°C and 30bar over bifunctional catalysts with Pt and HBEA as hydro-dehydrogenating and acid components. Three series of catalysts were prepared, with different levels of proximity between Pt and acid sites: in series 1, Pt was essentially located on the outer surface of HBEA crystal agglomerates of ∼12.5μm, series 2 and 3 resulted from combination of Pt-Al2O3 and HBEA particles of ∼70μm and from mechanical mixture of Pt-Al2O3 and HBEA particles of ∼300μm, respectively. The rate and selectivity of n-C16 hydroisomerization was shown to be determined by only two parameters, the balance between the metal and acid functions and their degree of intimacy. Both of them were easy to be quantified: the first one by the CPt/CH+ ratio between the concentrations of accessible Pt and protonic sites, the second one by nas, the number of acid steps undergone by olefinic intermediates during their diffusion between two Pt sites which can be drawn from the initial product distribution.
The formation of coke resulting from propene transformation at 623 K on a faujasite zeolite occurs according to a product shape selectivity mechanism and yields to the formation of highly alkylated ...polyaromatic molecules such as naphthalene, pyrene, and coronene. Their main parts are trapped in the inner cavities (supercages), poison Brønsted acid sites, and plug micropores. With a common thermal regeneration process, coke burns at 800 K, whereas this study shows that a complete regeneration of zeolite (i.e., total recovery of the native acidity and microporosity) can be achieved at 293 K by using a nonthermal plasma with a low energy consumption in a fixed-bed dielectric barrier reactor: a geometry suitable for industrial scaling. The kinetic rates of coke oxidation and the recovery of acidity and microporosity are similar. The active species (e.g., O*, O2 +) are able to diffuse within the zeolite micropores and oxidize the light molecules, 36 times faster than the heavier ones. Thanks to a complete characterization of both the regenerated catalyst and the remaining coke molecules, a reaction scheme is proposed. We claim that catalyst regeneration assisted by nonthermal plasma is a real alternative to thermal combustion.
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•Pressure of propene influences kinetic of coke formation and its growth mechanism.•Nature of coke is function on the pressure of coking agent.•At 1bar coke grows by successive ...alkylation/cyclization/hydrogen transfer reactions.•At 0.1bar coke grows by condensation of coke molecules.
Mechanisms of coke formation and growth transformation on MOR zeolite were investigated using a microbalance at 623K for two propene pressures: 0.1 and 1bar. The low and high coking pressures correspond to laboratory and industrial conditions, respectively. Initial coke deposition is faster at high pressure, but the maximum uptake is higher at low pressure. The complete determination of chemical composition of coke has required the use of a lot of techniques (IRFT, GC–MS, MALDI-TOF). From the change of chemical composition of coke, it can be proposed by different mechanisms of growth of coke molecules trapped in the zeolite micropores. The coke grows under industrial conditions (high olefins pressure) by successive alkylation/cyclization/aromatization reactions, while under laboratory conditions (low olefins pressure) the growth proceeds preferentially by condensation of coke molecules.
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•Negative impact of the desilication of ZSM-5 on the propane aromatization.•Negative impact of the desilication of ZSM-5 on the cyclohexane dehydrogenation.•No impact of the ...desilication of ZSM-5 on the n-hexane cracking.•At higher gallium content, a steric retention of coke precursors in the mesopores.
Gallium containing ZSM-5 zeolites were prepared by ion exchange and mechanical mixture from the parent ZSM-5 and two desilicated zeolites to be tested in propane aromatization. The alkaline treatment was made by sodium hydroxide, with or without tetrabutylammonium hydroxide. The catalysts prepared from desilicated zeolites are less active in propane aromatization and cyclohexane transformation and less selective into aromatics than those prepared from the parent zeolite. The desilication did not affect the acid properties, especially the strength of acid sites as seen by CO adsorption at 77 K and n-hexane cracking.
Multilayers coating are needed for large optical components performances, but the thickness non-uniformities over the useful aperture can generate spatial and chromatic variations of the reflectance, ...the transmittance and the wavefront errors. Although these dependences can be measured, they are difficult to anticipate if the underlying thickness variations are unknown. We present a model to retrieve these variations from wavefront error measurements that enables the computation of any optical properties over the useful aperture at any wavelength, angle of incidence or polarization.
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•Propane aromatization and cyclohexane transformation were carried at 803 K.•Gallosilicates are less active than ion exchange and mechanical mixture samples.•Activation of propane is ...by an alkyl mechanism.•Higher coke content is formed on gallosilicates.•The coke is toxic and localized in the micropores.
Three series of gallium-containing MFI zeolite were prepared with different gallium content by weight, by hydrothermal synthesis Ga-MFI (0.2–4.7 wt% Ga), ion exchange Ga/P (0.5–6.8 wt% Ga) and mechanical mixture Ga+P (0.5–10 wt% Ga). Under hydrogen at 600 °C on the aluminosilicate doped with gallium, a reducing solid ionic exchange occurs between a mobile species (Ga2O) and the protonic sites of the zeolite. The proportion of Ga implied in this exchange depends on both the proximity between Ga2O3 and the zeolite and the initial gallium content. The solid reductive exchange remains limited, ranging from 25% to only a few percent; therefore the gallium species content located in the zeolite micropores is only 0.2–0.4 wt%. The aluminosilicates doped with Ga are more active in both cyclohexane dehydrogenation and propane aromatization than the gallosilicates: the gallium in the framework is much less active than gallium in the exchange position.
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► DCM disproportionates into CO, CH3Cl, and HCl over γ-alumina. ► Pt oxidizes CO and CH3Cl into CO2. ► DCM oxidation follows a bifunctional mechanism over Pt/Al2O3. ► Oxygen does not ...intervene in CO formation. ► Water is necessary for hydroxyl group regeneration and catalysts stability.
The catalytic oxidation of dichloromethane (DCM, 1000ppm) in wet air was carried out in a fixed bed reactor over a range of Pt/γ-Al2O3 catalysts. The dichloromethane oxidation occurs following a bifunctional mechanism. DCM is disproportionated over γ-Al2O3 into a 1/1/3molar mixture of CO, CH3Cl, and HCl; the DCM transformation is followed by CH3Cl and CO oxidation into CO2. A mechanism of DCM disproportionation is proposed involving, as intermediates, formate species as well as alumina hydroxyl groups formed by water adsorption.
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•Desilication of mordenite zeolite modifies both porosity and acidic properties.•Presence of mesopores favors coke accumulation.•Desilication leads to a change in the coking mode: ...from pore blocking to poisoning.•This change requires that the mordenite zeolite possess a high hierarchy factor.•A high hierarchy factor requires severe alkaline conditions.
The impact of the textural properties of mordenite zeolites on the growth and toxicity of coke formed at 350°C during propene transformation is investigated. The addition of intracrystal mesopores by alkaline treatment to a unidimensional zeolite is a simple tool for changing its classical deactivation mode, pore blocking, to a mode more usual on zeolites with a three-dimensional framework, poisoning. This change requires that the zeolite possess a high hierarchy factor, as a result of severe alkaline conditions generating a lot of intracrystal mesopores. Desilication treatment removes part of the Brønsted acid sites and generates extra-framework aluminum species that can block access to the protonic sites and/or enhance their strength, which increases the coking rate. A mild acid leaching is enough to dissolve them. The presence of a few mesopores is sufficient to enhance the access of oxygen to coke, which favor its combustion.
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