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.
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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.
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Abstract
We report on the results of an extensive campaign of optical and mechanical characterization of the ion-beam sputtered oxide layers (Ta
2
O
5
, TiO
2
, Ta
2
O
5
–TiO
2
, SiO
2
) within the ...high-reflection coatings of the Advanced LIGO, Advanced Virgo and KAGRA gravitational-wave detectors: refractive index, thickness, optical absorption, composition, density, internal friction and elastic constants have been measured; the impact of deposition rate and post-deposition annealing on coating internal friction has been assessed. For Ta
2
O
5
and SiO
2
layers, coating internal friction increases with the deposition rate, whereas the annealing treatment either erases or largely reduces the gap between samples with different deposition history. For Ta
2
O
5
–TiO
2
layers, the reduction of internal friction due to TiO
2
doping becomes effective only if coupled with annealing. All measured samples showed a weak dependence of internal friction on frequency
ϕ
c
(
f
) =
af
b
, with −0.208 <
b
< 0.140 depending on the coating material considered. SiO
2
films showed a mode-dependent loss branching, likely due to spurious losses at the coated edge of the samples. The reference loss values of the Advanced LIGO and Advanced Virgo input (ITM) and end (ETM) mirror HR coatings have been updated by using our estimated value of Young’s modulus of Ta
2
O
5
–TiO
2
layers (120 GPa) and are about 10% higher than previous estimations.
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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The catalytic fast pyrolysis of oak over two zeolites (microporous and hierarchical) was investigated in a microfluidized bed reactor (MFBR) at 500 °C and as a function of the biomass-to-catalyst ...ratio. A hierarchical zeolite was produced by desilication with a NaOH solution of a conventional HZSM-5 zeolite. The outlet of the MFBR was connected to a single photoionization mass spectrometer (SPI-MS) for the on-line analysis of volatiles. This on-line analysis method allows studying the dynamics of volatile formation (in real time) and the deactivation of 2 zeolites upon stepwise injections of wood particles. Strikingly, the selectivity of targeted mono-aromatic compounds (quantified by gas chromatography) is doubled after desilication of the zeolite. The coked zeolites were characterized by TEM-EDX, digestion in fluoric acid, MALDI-TOF MS, etc . Three different types of coke are evidenced: (1) coke trapped inside micropores, (2) external coke formed on the outer surface of the crystals and (3) coke precursors deposited in the mesopores. The latter two cokes are much less toxic than the microporous coke. The open mesopores produced after desilication can be seen as “highways” where big molecules (such as levoglucosan) can diffuse to more accessible pore mouths. Brønsted acid sites present on microporous mouths can be active for the conversion of bigger molecules to small fragments. These fragments diffuse and form aromatics in the micropores (shape selectivity). The mesopores also promote the evacuation of catalytic products, thus enhancing the selectivity of mono-aromatic hydrocarbons. Desilicated zeolites present higher selectivity to mono-aromatics and stability upon coke deposition than microporous zeolites.
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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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