Hermann K. Beyer

Properties of the end members in the Pentasil-family of zeolites: characterization as adsorbents

Domestically synthesized HZSM-5 and HSZM-11 zeolites were shown to have the same crystal morphology and unit cell dimensions as the Mobil materials. The zeolites were 100% crystalline and crystallographically pure. Sorption of N 2 , O 2 , H 2 S, CO 2 and of C 3 –C 10 n-paraffins, of i -C 4 , i -C 5 , neopentane, the monomethylnonanes and the xylene isomers was followed by thermogravimetric or volumetric techniques. Generally, the sorption capacity of HZSM-5 was always higher than HSZM-11. The sorption behaviour could be explained by simple models. When pore intersections of both zeolites are considered, the maximum sorption capacity corresponds to the closest possible packing at these intersections. Paraffin sorption occurs according to an end-to-end adsorption model. The concept of total adsorbate length was useful to rationalize the adsorption.

A New Method for the Dealumination of Faujasite-Type Zeolites

Publisher Summary This chapter presents a new method for the dealumination of faujasite-type zeolites. Y zeolites can be dealuminated by reaction with silicon tetrachloride without collapse of the crystal structure. X-ray diffraction pattern and mid-infrared spectra point to a high degree of ordering in the framework of the dealuminated products, and the adsorption of hydrocarbons reveals no significant differences in the pore system compared with Na–Y. In contrast to the described new dealumination method, dealumination under hydrothermal conditions changes the pore system and the structure of the framework to a certain degree. Y zeolites dealuminated by silicon tetrachloride may offer new application possibilities as hydrophobic adsorbents. It can be expected that the hydrogen form of partly dealuminated zeolite Y is an active catalyst for hydrocarbon reactions with excellent thermal and hydrothermal stability.

Shape-selectivity changes in high-silica zeolites

Y-type, ZSM-5 and ZSM-11 zeolites with the same chemical composition have been prepared. Peak indexation and microscopic examination were used to identify the phases and to examine the samples for degree of crystallinity. The acidity was measured by temperature-programmed desorption of NH3 and infrared examination of OH groups upon interaction with hydrogen-bond-acceptor molecules. These data were consistent with the results derived from total activity and selectivity measurements of the isomerization and hydrocracking of n-decane on the Pt-loaded zeolites.The distribution of the feed isomors served to distinguish between HZSM-5 and HZSM-11 type structures, while the cracked products could be predicted using simple reaction schemes. 2,2,4-Trimethylpentane was not able to enter the pores of the Pentasil zeolites, reacting near the pore mouth or entering the pores after dehydrocyclisation on Pt clusters external to the zeolite.

Introduction of Cations into Zeolites by Solid-State Reaction

Abstract A brief review is given of early observations of cation incorporation into zeolites through solid-state reaction. The general procedure and techniques suitable for investigation of solid-state ion exchange in zeolites are described. Results of systematic studies on introduction of alkaline, alkaline earth, rare earth, transition metal and noble metal cations into hydrogen, ammonium and sodium forms of zeolites are reported. In these studies, halides or oxides of the in-going cations are preferentially employed. Particular attention is paid to the stoichiometry of the solid-state ion exchange. It is shown that in several cases a one-step solid-state reaction leads to a 100% cation incorporation, whereas such a high degree of exchange is difficult to obtain by conventional methods. As is illustrated by a few selected examples, solid-state ion exchange might be an interesting way to prepare active acid and bifunctional catalysts.

PREPARATION OF HIGH-SILICA FAUJASITES BY TREATMENT WITH SILICON TETRACHLORIDE

NaY has been dealuminated in highly controlled conditions by SiCl4. The materials thus obtained, after subsequent washing and ammonium exchange, were characterized by chemical analysis, thermogravimetry, X-ray diffraction, infrared spectroscopy of the framework and surface hydroxyl groups and 29Si nuclear magnetic resonance spectroscopy.In order to obtain highly crystalline materials in a reproducible way, a very specific dealumination and washing procedure has to be followed. The mechanism of the dealumination is complex and always combines direct isomorphic substitution with acid leaching of aluminium. The materials obtained have continuously varying properties as far as unit-cell dimensions and framework vibrations are concerned. In contrast, the amount of extra-lattice aluminium retained in the zeolite reaches a maximum at intermediate degrees of dealumination.

Some unusual properties of activated and reduced AgNaA zeolites

Carbon monoxide, oxygen and hydrogen were found to be chemisorbed on dehydrated AgA zeolites. This was investigated in detail using volumetric sorption and temperature programmed desorption techniques. Also i.r. and mass spectrometry were used to characterize the solid and the desorbed molecules.It was found that as a result of an auto-reductive process, colour centres are created upon degassing of the zeolite. These centres sorb hydrogen and oxygen dissociatively, while one molecule of carbon monoxide was chemisorbed per Ag ion available in the supercage. It is proposed that linear Ag+3 clusters are formed upon activation, the ends of which constitute chemisorption sites for hydrogen and oxygen.

Solid-state ion exchange in zeolites: Part I. Alkaline chlorides/ZSM-5

Mixtures of solid alkaline chlorides MeCl (Me = Li, Na, K, Rb, Cs) and NH 4 -ZSM-5 or H-ZSM-5 react at elevated temperatures to evolve gaseous hydrogen chloride and form the respective cationic form of ZSM-5. The strong acidic OH groups are completely and the nonacidic ones partially removed, as evidenced by i.r. Thermogravimentic analysis, titration of the evolved gases, and temperatureprogrammed gas evolution monitored by mass spectrometry render the discrimination between a lowtemperature and a high-temperature exchange reaction possible. Evaluation of the analytical data shows good agreement between the aluminum content of the framework and the amount of neutralizing cations.

High-temperature interaction of solid Cu chlorides and Cu oxides in mixtures with H-forms of ZSM-5 and Y zeolites

A high-temperature solid-state ion exchange between separate phases of solid Cu chlorides and hydrogen forms of ZSM-5 and Y zeolites occurs above 570 K and reaches substantial degrees at 770 and 670 K, respectively, i.e. at temperatures at which considerable self-dehydroxylation of the particular zeolite does not take place. The process has been followed using IR, EPR and MS techniques through the evolution of hydrochloric acid into the gaseous phase, the consumption of the bridging OH groups in the zeolite as well as by the formation of isolated Cu ions that exhibit strong electron-accepting properties. The exchange rate is very fast in the initial stage, being proportional to the amount of copper salt in the mixture, and levels off to zero after several hours. Owing to the more difficult deaggregation of Cu oxides compared to Cu chlorides, the interaction of Cu oxides with H-ZSM-5 and H-Y at 770 K and 670 K, respectively, results in a very limited consumption of the bridging OH groups as well as in a decreased formation of isolated Cu ions and electron-acceptor sites compared with the corresponding Cu chloride mixtures. Besides the ability of the copper phases (chlorides or oxides) to deaggregate, a further striking factor affecting the exchange reaction is the mobility of the zeolite protons and exchanged cations, which is controlled by the structure type of the zeolite and by the temperature. This is clearly reflected by the positive effect of the temperature on the exchange process and the higher degree of exchange (at a given temperature) found for H-Y zeolites compared with H-ZSM-5.

Synthesis and characterization of a ferrierite made by recrystallization of an aluminium-containing hydrated magadiite

Abstract A new method for the synthesis of high-silica ferrierite (Si/Al=15–18) by recrystallization of aluminium-containing magadiite varieties in air-dry state and aqueous suspension with piperidine as template is reported. Details on removal and thermal stability of the template occluded during the synthesis (four molecules per unit cell) are given. Properties (e.g. thermal and hydrothermal stability, acidity) of ferrierites prepared by the dry synthesis technique are presented and compositional and morphological characteristics of the products are compared with those of ferrierite conventionally prepared from alkaline SiO 2 /Al 2 O 3 gel in aqueous medium.

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.