Gy. Onyestyák

Cyclohexane conversion over H-zeolite supported platinum

The catalytic conversion of cyclohexane over H-ZSM-5 in hydrogen or nitrogen and in hydrogen atmosphere over a series of H-zeolites loaded with platinum was studied using a flow-through microreactor/GC on-line system at pressures of 13 bar. The distribution of the products formed via hydrodecyclization was found to depend definitely on whether or not dimerization of primarily formed hexyl-carbenium ions is sterically hindered in the zeolitic pore system. Reaction routes are suggested which involve, as an essential step, the hydrogenation of intermediate carbenium ions, formed either primarily by reactant adsorption or cracking, by spill-over hydrogen activated on the metallic catalyst component. The basically different product distribution obtained by decyclization of cyclohexane in nitrogen atmosphere or over monofunctional H-ZSM-5 is attributed to disproportionation of primarily formed hexyl-carbenium ions and subsequent hydrogen transfer reactions which results in a broad spectrum of paraffins and aromatics in the effluent and in carbon-rich deposits on the catalyst surface causing fast catalyst deactivation. This process does not manifest itself in bifunctional catalysis due to the much faster hydrogenation and, hence, elimination of carbenium ions by hydrogenation.

An NH3-TPD and -FR study on the acidity of sulfated zirconia

Abstract Temperature-programmed desorption (TPD) and frequency response (FR) methods were used to study the dynamics of ammonia sorption over sulfated zirconia (SZ). The results were interpreted in comparison with results obtained for zirconia (Z) sulfated zirconia–titania (SZT) and zeolite H-ZSM-5 (HZ). Strong Lewis (L) and Bronsted acid (B) sites were detected on each of the samples. NH3 was found to bind to species formed from ammonia on the L and B sites (sites Ll and Lb) by H-bonds. The H-bound ammonia was released at lower temperatures than that directly bound to the B and L sites. FR resonance signals were ascribed to sorption processes over sites B, Lb, and Ll. The L sites were fully covered by NH3 up to the highest temperature of the FR measurement, therefore, these sites could not be detected. For SZ and SZT, the FR intensity was found independent of the temperature suggesting that these samples contain energetically heterogeneous sorption sites.

Equilibrium and dynamics of acetylene sorption in multiwalled carbon nanotubes

Abstract Multiwalled carbon nanotubes (MWCNT) were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), and by adsorption of N 2 and acetylene. The dynamics of acetylene sorption was studied by frequency response spectroscopy. The average tube dimension and the shell number were determined from a statistical evaluation of the TEM images. This value agreed with that obtained from XRD via the Scherrer equation only if a shape factor of 0.49 was used. The diffusion of acetylene in the nanotubes of the MWCNT sample was found to be the rate-controlling step of the sorption process. Relationships between the tube dimensions and the equilibrium and dynamic sorption properties were demonstrated.

The skeletal isomerization of n-butene over ferrierite catalysts

Abstract The catalytic properties of H-ferrierite zeolites, prepared by hydrothermal and dry-state synthesis, were compared in the skeletal isomerization of 1-butene. The activity was found to be strongly influenced by the presence of extra-framework Al (EFAl) species in the zeolite micropores and by the carbonaceous deposit formed during the reaction. The dynamics of mass transport in the zeolite crystallites were studied by the frequency-response (FR) method using acetylene, propane, n - and i -butane as probe molecules. The selectivity of n -butene conversion showed correlation with the dynamics of the n -butane sorption process. Either the diffusion or the sorption was the rate-controlling slowest step of n -butane mass transport, depending on the amount and the distribution of the EFAl species in the sample. In the catalyst sample, free of EFAl, the transport was found to be controlled by the rate of sorption. At an EFAl content of about 0.5–1.0 EFAl/unit cell the diffusion resistance of the micropores governed the n -butane transport rate.

Deactivation and Stabilization of Late Transition Metal Zeolite Catalysts for Acetylene Hydration

Abstract Hydration of acetylene to acetaldehyde proceeds with appropriate rates on Cu-, Ag-, Cd-, Zn-, Hg-forms of zeolites below 200°C at atmospheric pressure. Stabilities and selectivities were studied for two sets of catalysts: for Cd-forms of A-, X-, Y-zeolites, mordenite, clinopti1olite, and for Cu-, Ag-, Cd-, Zn-, Hg-, Pb-clinopti1olites. A-, X-, Y-, MOR zeolites deactivate because crotonic condensation followed by the formation of carbonaceous deposits and unsaturated gaseous compounds takes place. Cu 2+ , Ag + , Hg 2+ are reduced and the supported metals formed are inactive. The other forms of clinopti1olite are catalysts of high stability and selectivity. Activity increases in the order of Pb-

Hydration of acetylene on Cd2+ exchanged zeolites

The reaction takes place on Cd-containing Lewis sites, whereas the irreversible adsorption and condensation of product acetaldehyde on acidic OH groups of Cd-X, FAU catalyst results in deactivation. Preadsorbed ammonia moderates the deactivation, because it reacts with acetylene, and pyridine homologues are formed which block the accessible OH groups. Since in divalent cationic forms of clinoptilolite no OH groups are generated, Cd-clinoptilolite proved to be a catalyst of permanent activity.AbstractРеакция протекает на центрах Льюиса, содержащих Cd, в то время как необратимая адсорбция и конденсация продукта — ацетальдегида на кислых OH группах катализатора FAU, Cd-X. Преадсорбированный аммиак уменьшает дезактивацию, т.к. он реагирует с ацетиленом и образуются гомологи пиридина, которые блокируют доступные OH группы. Так как в биваленьных катионных формах клиноптилолита не образуются группы OH, то Cd-клиноптилолит оказывается катализатором постоянной активности.

Interactions of reactants in hydration of acetylene on Cd-zeolite catalysts

Abstract IR measurements were carried out with Cd-MFI and Cd-clinoptilolite at 25°C. Samples of different water content were contacted with acetylene at different pressures. Adsorbed acetaldehyde was formed from acetylene and water both adsorbed on similar Cd 2+ Z − 2 sites. The reaction between the two adsorbed reactants reveals the validity of Langmuir-Hinshelwood kinetics. Decrease of water content or increase of acetylene pressure facilitated the formation of carbonaceous deposits especially in the case of Cd-MFI. Deposits formed from acetylene displace adsorbed acetylene and acetaldehyde.

Upgraded biofuel from alcohol–acetone feedstocks over a two-stage flow-through catalytic system

Consecutive alkylation of acetone (A) with ethanol (E) in a ratio of 1 : 2 and hydrodeoxygenation (HDO) of the obtained Guerbet products were studied. A fixed bed flow-through reactor system was applied at total pressure of 21 bar and in the temperature range of 175–350 °C using inert He or/and reducing H2 carrier gases. In the alkylation process catalysts based on a hydrogen borrowing strategy were applied using neutral activated carbon (AC) or alkaline hydrotalcite (HT) supports, namely 5 wt% Pd/AC in presence of a 30 wt% KOH alkaline additive or 5 wt% Pd/HT. After self- and cross-condensation of the reactants the effluent product mixture was treated in a H2 stream over a second commercial hydrodeoxygenating (HDO) catalyst (NiMo/alumina) layer. All the catalysts were activated in H2 flow at 350 °C. In inert helium carrier gas over Pd-catalysts C5–C7 mono- or dialkylated ketones were obtained as main products however different C7–C11 mono- or dialkylated ketones were also formed due to Guerbet coupling of ethanol to butanol mimicking the acetone–butanol–ethanol (ABE) fermentation mixture as reactant. By replacing helium with hydrogen medium over Pd-catalyst systems the produced ketones could be reduced to alcohols. Finally, for hydrocarbon fuel production, the oxygen content of the alcohol–ketone mixture can be eliminated completely, by applying a second reactor section filled up with an industrial NiMo/alumina catalyst producing the parallel alkanes.

Comparison of Hydrosulfurization Zeolite Catalysts Prepared in Different Ways

Abstract Transition metal ions can be introduced into zeolites by solid state ion-exchange for preparing catalysts to be used in hydrosulfurization. Chlorides of Co, Mn, Zn, Cd, Ca were contacted with NH 4 -Y; CdCl 2 with NH 4 -MOR and NH 4 -X; and different compounds of Cd with NH 4 -Y. Ion-exchange was followed by i. r. spectroscopy: decrease of OH band intensity and bands generated by dissociative H 2 S adsorption were detected. The rearrangement of adsorbed bases after H 2 S adsorption observed in i. r. spectra indicated more extensive dissociative adsorption of H 2 S on Cd-faujasites prepared in solid state than in liquid phase ion-exchange. Dehydrosulfurization tests also revealed this recognition.

Acetylene hydration on different Cd-zeolites

Abstract Various Cd-zeolite preparations, containing adsorbed water in controlled amounts, were 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 detected by FTIR. Cd-derivatives of MOR, MFI, and clinoptilolite were prepared by conventional ion exchange and by a novel method which is based on redox reaction between zerovalent cadmium atoms and zeolitic lattice protons upon heating the mixtures of Cd dust and NH 4 Lrform of zeolites up to 350 °C. Samples partially ion exchanged in aqueous medium were more active in the formation of acetaldehyde than those which were obtained via solid-state reaction and contained more Cd2+ in the cationic sites of these zeolites. For preparation of Cd-FER, however, the latter method was found to be applicable more advantageously. Cd-X,FAU could not be prepared with Cd dust because of crystal collapse below the temperature of the solid-state redox reaction (in this case at low temperature the so called contact induced ion exchange method was applied using Cd-(NO 3 ) 2 ). The activities of Cd 2+ cations are quite different in different zeolites prepared similarly.