F. Roessner

On the nature of spilt-over hydrogen

Various experimental results on hydrogen spillover which have been obtained since the first evidence for this phenomenon are discussed concerning the nature of the activated hydrogen species. It will be demonstrated that the physical nature of the spilt-over species, especially their charge, can only be described by considering their interaction with the solid. A new model is proposed which is based on the description of spilt-over hydrogen as electron donor located at the surface. Consequently, H+ ions and H atoms coexist on the surface of the catalyst as also stated experimentally. Their ratio is determined by the electronic properties of the adsorbate/solid system. Considering atomic and recombinative desorption of spilt-over hydrogen, the adsorption isotherms are calculated exemplary for titania. The model is applied to interpret some experimental results related to hydrogen spillover, especially the partial electron transfer from the hydrogen species to the solid.

Acylation of aromatic compounds on H-Beta zeolites

Abstract Friedel–Crafts acylation of anisole by acetic anhydride and the Fries rearrangement of phenyl acetate were investigated in the liquid phase using the H-form of various zeolites. Zeolite Beta was found to be the most active catalyst for acylation reactions compared to Y- and ZSM-5 zeolites. The reactions were found to be controlled by mass transfer limitations caused by coke deposition. Two types of coke (extractable and non-extractable) were identified. Combining liquid phase reaction with continuous extraction of the catalyst with refluxing reaction mixture in a Soxhlet-like reactor led to a higher conversion of phenyl acetate in the Fries rearrangement. The ratio of protonated to acylated surface centres strongly determines the selectivity. The formation of o -hydroxyacetophenone proceeds both via intramolecular rearrangement and intermolecular acylation. On the other hand, p -hydroxyacetophenone is formed entirely by intermolecular acylation.

Hydrogen spillover in bifunctional catalysis

A model is proposed to describe the mechanism of hydroconversions on bifunctional catalysts. This mechanism allows a better interpretation of experimental results. It incorporates the concept of hydrogen spillover, i.e. the surface diffusion of activated hydrogen species. The main consequence of the model is that all reaction steps can occur on one reaction site whereas the classical model requires three reaction steps on different reaction sites, Additionally, the catalytic properties of layered bifunctional catalysts (consisting of a macroscopical metal-containing and acidic part) depending on the geometry of such systems can be easily explained. The new model is based on the assumption that a dynamic equilibrium of two coexisting activated (spilt-over) hydrogen species is formed on the surface; one of them bring neutral (H atom) and the other electrically charged (H+ ion). The influence of the spilt-over hydrogen is illustrated and discussed in the case of the conversion of cyclohexane on different metal-containing and acidic catalysts.

Generation of active sites for ethane aromatization in ZSM-5 zeolites by a solid-state reaction of zinc metal with bronsted acid sites of the zeolite

A zinc-modified ZSM-5 catalyst was prepared by a solid-state reaction between zinc dust and H-ZSM-5. The resulting catalyst is as active for the aromatization of ethane as zinc-modified ZSM-5 prepared by impregnation.

The performance of Ti-MCM-41 in aqueous media and after mechanical treatment studied by in situ XANES, UV/Vis and test reactions

Abstract The influence of water on the epoxidation of cyclohexene with H 2 O 2 and tert -butyl hydroperoxide (tbhp) on Ti-MCM-41 molecular sieves prepared by post-synthetic modification of the support with titanium alkoxides was investigated. The catalytic performance depends on the hydrophilicity/hydrophobicity of the MCM-41 surface, but also on the interactions between titanium centers and oxidants. Using XANES, a higher coordination number and an irreversible transformation of isolated into oxide-like titanium species was observed when the reaction was performed with H 2 O 2 compared to tbhp. The mechanical treatment of Ti-MCM-41 in a ball mill or press did not influence the local environment around the titanium centers, whereas the MCM-41 structure was damaged causing a decrease of the conversion of cyclohexene.

Investigations on hydrogen spillover. Part 1.—Electrical conductivity studies on titanium dioxide

The influence of hydrogen spillover on the conductivity of polycrystalline platinum-containing (in comparison to platinum-free) titania samples has been investigated. A special two-component sample geometry, consisting of a Pt/TiO2 part and a TiO2 part, was used to separate the conductivity effects after exposure to hydrogen which could arise owing to changes of the platinum/titania interface and to the action of the spilt-over hydrogen species.It was found that the electrical resistance of Pt/TiO2 samples was decreased by treatment with hydrogen at low temperatures (room temperature to 80 °C) and that, vice versa, the evacuation of hydrogen led to an increase in sample resistance. The electrical resistance of pure TiO2 was not affected by hydrogen.Using two-component (Pt/TiO2)–TiO2 samples the conductivity changed in both sample components after hydrogen adsorption. The decrease of resistance in the TiO2 component of the (Pt/TiO2)–TiO2 sample occurred with a delay due to the diffusion of the spilt-over hydrogen species from the Pt/TiO2 into the TiO2 component. The conductivity change in the TiO2 component can be correlated with the action of activated hydrogen species formed on platinum and having diffused onto the titania support in a spillover form. The spilt-over hydrogen species can be described as surface electron donors, i.e. as coexisting H atoms and H+ ions. Their chemisorption on titania corresponds to an electron transfer to the support and, thus, to an increase of the n-type conductivity.

Aromatization of ethane on platinum containing ZSM-5 zeolites

Abstract The conversion of ethane into aromatics on Pt/H-ZSM-5 zeolites starts at 670 K. The composition of the reaction products strongly depends on the temperature and gas space velocity (GHSV). Above 900 K the alkylaromatics formed preferentially at lower temperatures were dealkylated and disproportionated giving benzene and C 8+ aromatics as well. The reaction mechanism involves the primary dehydrogenation of ethane to ethene followed by oligomerization and aromatization. The influence of hydrogen on the reaction pathway is explained in terms of the concept of spillover.

Investigations on hydrogen spillover. Part 2.—Hydrocarbon conversion on bifunctional catalysts

The effect of hydrogen spillover on the cracking of n-hexane on erionite has been investigated. Changes in selectives have been found depending on the nature of the carrier gas and the presence of platinum. It has been demonstrated that platinum activates hydrogen that is able to migrate over large distances (spillover hydrogen). On acidic centres the activated hydrogen has been found to be involved in the hydrocarbon conversion. When hydrogen was substituted by nitrogen, Pt/H–erionite was found to exhibit only a monofunctional acidic property which has been characterized by a C3 : C4 ratio of almost 1 : 1 and the appearance of C5 compounds formed via secondary reactions of intermediate alkenes. Based on these results an extended model of the bifunctional catalysis has been proposed that includes activation, migration and consumption of spillover hydrogen. The product distribution was then satisfactorily explained, assuming that hydrogen dissociates into radicals on platinum and then spills over onto the support surface where a dynamic equilibrium is established between the hydrogen radicals and protons.

Conversion of Ethane into Aromatic Compounds on Zsm-5 Zeolites Modified by Zinc

Abstract H-ZSM-5 zeolites modified by zinc exhibit a high activity for the conversion of ethane into aromatics, even at the relatively low temperature of 773 K. The activity and selectivity of these catalysts are related to the zinc species introduced via various methods but, surprisingly, not to the acidic properties.

Impact of Organic Interlayer Anions in Hydrotalcite Precursor on the Catalytic Activity of Hydrotalcite-Derived Mixed Oxides

A series of hydrotalcite‐derived mixed oxides containing different organic interlayer anions in the hydrotalcite precursors have been synthesized and studied as catalysts in the transesterification of glycerol trioctanoate with methanol. There are significant differences in the catalytic activity of these mixed oxides, indicating a great impact of interlayer anions in hydrotalcite precursors. Generally, anions with high charge density and, therefore, high content within the interlayer of the hydrotalcites lead to higher catalytic activity in the derived mixed oxides in contrast to anions with low charge density. An influence of interlayer anions on the structural properties of the hydrotalcites, such as interlayer distance, which is determined by X‐ray diffraction analysis, and on the surface area and pore volume of the corresponding mixed oxides, which are determined by nitrogen adsorption, is obtained. Furthermore, interlayer anions had a significant effect on the thermal stability of the hydrotalcite structure as well as on the pore size distribution of derived mixed oxides.