György Onyestyák

Frequency-response study of micro- and macro-pore diffusion in manufactured zeolite pellets

Intra- and inter-crystalline diffusion of carbon dioxide and isobutane in different manufactured 5A and 13X pellets have been investigated by the frequency-response method. Despite three orders of magnitude difference in micropore diffusivities for carbon dioxide in 5A and 13X zeolite crystals, its macropore diffusivities in 5A and 13X pellets are nearly the same. The macropore diffusivities were found to be the controlling step in all pellets under study. The diffusivity of carbon dioxide was found to be about twice that of isobutane in the macropores of 13X pellets.

Dynamic and catalytic studies of H-ferrierites made by hydrothermal and dry state syntheses

Abstract The activity and selectivity of skeletal isomerization of 1-butene and disproportionation of ethylbenzene on H-ferrierite catalysts prepared by dry state recrystallization of magadiite and conventional hydrothermal synthesis from an alkaline SiO2/Al2O3 gel are compared. The isobutene yields of the samples were found to be similar even though the ferrierite crystals had quite different morphologies. Significant differences were observed in the isobutene selectivities which can be correlated with processes occurring inside the crystals. Ammonia, acetylene, methane and propane sorption and intracrystalline diffusion rate-spectra are reported using the frequency-response technique. The sorption properties of the acid sites and the diffusional resistances of the morphologically different ferrierite crystals are compared. It was found that by the dry state synthesis the quality of the ferrierite framework was similar to that obtained by the conventional hydrothermal synthesis. The channels of ferrierite recrystallized from magadiite, however, seem to have fewer barriers and, thus, show better kinetic properties, e.g., faster diffusion of propane and higher selectivity to the formation of isobutene.

An FR study of the n- and i-butane diffusion in theta-1, ferrierite and silicalite-1

Abstract In the one-, two-, and three-dimensional (1D, 2D, and 3D) micropore system of theta-1, ferrierite, and silicalite-1, respectively, diffusion to the sorption sites was found to control the rate of propane and butane sorption at 373 K and 133 Pa pressure. The diffusivity of propane (C 3 ) and n -butane ( n -C 4 ) was about the same and much faster than that of the i -butane ( i -C 4 ). In theta-1 and ferrierite, the i -C 4 transport was too slow to be characterized by the frequency–response (FR) method. Ten-ring apertures determine the critical pore dimension of each material. In ferrierite, however, the transport diffusivity of the n -alkanes was about 10 −15 m 2 s −1 , several orders of magnitude smaller than that in theta-1 and silicalite-1. Results suggest that normal alkane diffusion proceeds in the 3D pore system of silicalite-1 and single-file diffusion prevails in the micropores of ferrierite and theta-1.

Frequency-response NH3 sorption study of acidic sites in H-ZSM-5 catalysts

The frequency-response (FR) technique has been used to study the kinetics of ammonia adsorption–desorption processes and to characterise the acidic sites in two different, well characterized H-ZSM-5 catalysts. The FR method has been shown to be capable of distinguishing the different strengths and concentrations of acid sites using sorbate pressures close to those applied under real reaction conditions. The method, therefore, has advantages over techniques which require high vacuum or low probe molecule pressures to obtain relevant information. The FR observations were in agreement with the results of other examinations. Although in the range of this study the direct interaction between ammonia and the acidic sites can be detected most probably only above 623 K, the rate spectra of hydrogen-bonded ammonia sorption on ammonium ions seems to be characteristic for acidic catalysts. Adspecies detected by the FR technique must also play a role in any dynamic system of practical interest. Combination of the technique with other methods is suggested to obtain a more complete description of zeolitic acid sites.

Novel Cu and Cu2In/aluminosilicate type catalysts for the reduction of biomass-derived volatile fatty acids to alcohols

AbstractThis work relates to the consecutive reduction of short chain carboxylic acids (volatile fatty acids, VFAs) to alcohols as main products. Acetic acid (AA) was used as a reactant to model the VFAs that can be produced by either thermochemical or biological biomass degradation. The amorphised zeolite supported copper catalysts (Cu/SiAl), especially the In-modified CuIn/SiAl catalysts, showed high hydroconversion activity and selectivity for alcohol, ester and aldehyde. Catalysts containing dispersed copper particles in amorphous aluminosilicate were obtained by dehydrating and H2-reducing Cu-forms of low-silica synthetic zeolites (A, X, P). The activity of the highly destructed Cu-aluminosilicates was found to depend on the structure of the zeolite precursor. The formation of ethyl acetate could be suppressed by adding water to the AA feed and by modifying the catalyst, e.g. by In2O3 additive. In the catalysts modified by In2O3 additive formation of copper-indium alloy phase (Cu2In intermetallic compound) was detected resulting in a different selectivity than the one recorded for the Cu/SiAl.

Pinewood char templated mordenite/carbon honeycomb composite

Biotemplating with carbonized wood is a novel way of zeolite adsorbent and catalyst production, with uniform, hierarchically ordered macro- and micropore structure, having low diffusional resistances against mass transport of adsorptives, reactants and products around the zeolite crystallites. Pinewood (Picea abies) cubes of 1-cm size edges were carbonized at 700 °C in nitrogen flow. The obtained charcoal cubes contain parallel channels of 20–30 μm diameter, separated by about 1–3 μm thick carbon walls. The hydrophobic character of the channel walls was effectively reduced using oxidative treatment with nitric acid. Following saturation with zeolite synthesis solution the carbon cubes were exposed to hydrothermal conditions. Mordenite/carbon composite materials were formed where Na-mordenite microcrystals coated the walls of the carbon macropores. The thickness and density of the zeolite layer was controlled by varying the synthesis conditions. Owing to reduced diffusional resistance, the preparations are favourable adsorbents and catalysts.

Acetylene-Hydration Kinetics on Cadmium-Exchanged Clinoptilolite Catalyst. Preliminary Communication

Hydration of acetylene under steady-state conditions around 450 K proceeds on Cd-clinoptilolite without catalyst deactivation and formation of by-products. Reaction rates were determined under steady-state conditions at different partial pressures of acetylene, water, and acetaldehyde. In relation with the results, rate equations for different kinetic models were evaluated. Langmuir-Hinshelwood kinetics was established. According to this model, acetylene and water must adsorb on similar sites, and the surface reaction between the adsorbed reactants is the rate-determining step, which is followed by equilibrated desorption of the produced acetaldehyde.

Acetone alkylation with ethanol over multifunctional catalysts by a borrowing hydrogen strategy

Step by step alkylation of acetone (A) with ethanol (E) in a ratio of 1:2 was investigated. A fixed bed flow-through reactor system was used at a total pressure of 21 bar and in the temperature range of 150–350 °C in inert He or a reducing H2 medium. Following the hydrogen borrowing methodology, two types of catalysts were prepared; using neutral activated carbon (AC) and alkaline hydrotalcite (HT) supports, namely 5 wt% Pd/AC in the presence of alkaline additives (10, 20 and 30 wt% KOH or 20% K3PO4); 9 wt% Cu/HT and 5 wt% Pd/HT. The catalysts were activated in a H2 flow at 350 °C. Different yields of mono- or dialkylated ketones were observed. In a hydrogen medium over the same catalyst systems the ketone products could be reduced to alcohols. In this study the Pd/HT catalyst seems to be the most promising for fuel production based on biomass fermentation.

Catalytic activity of transition metal zeolites

Abstract The isomerization of n-butenes proceeds on Ca-A and Co-A zeolites as well as on H- and Ni-clinoptilolites. The oligomerization of isobutene takes place similarly on both Ca-A and Co-A zeolites. On acidic and transition metal forms, however, the reaction mechanisms are different. The activity of different metal forms of zeolites for the addition of H2S to the CC double bond depends on the electronegativity of the cations introduced, and not on the acidity of these zeolites. Small pore zeolites are less active than large pore zeolites. Oligomerization followed by coke formation takes place on zeolites of high Si/Al ratio and on Co and Ni forms. Highest activity and selectivity can be attained with Cd- and Zn-X, FAU. Interconversion of thiols and thioethers can also be performed. Acetylene hydration is catalyzed only by late transition metal ions introduced into the zeolites. High stability is attained when no acid sites are generated by the cations and thus crotonic condensation is avoided; further, the active cations are not reduced during the reaction. The requirements are fulfilled by Cd-clinoptilolite.

Hydration of acetylene on Zn- and Cd-zeolites

Abstract Zn- and Cd-exchanged derivatives of two rhyolite tuffs of different clinoptilolite contents and Zn-MFI were prepared by conventional ion exchange in aqueous medium and by reacting metal dust with NH4-form zeolites. The sample wafer, containing adsorbed water, was contacted with acetylene in an IR cell at 0.03 bar. The activity of the zeolites was estimated from the formation rate of adsorbed acetaldehyde at 50 °C. The activities paralleled the Zn- or Cd-contents of the clinoptilolite-containing rocks. Cd and Zn metal dust reacted with the NH4-zeolites at about 300 and 440 °C, respectively. The amount of H2 evolved was stoichiometric with the amount of metal cations incorporated. The structure of the Cd- and Zn-clinoptilolite was damaged above 250 °C and completely destroyed above 450 °C. The Zn-MFI preparations were of higher thermal stability than the corresponding clinoptilolites.