Untersuchung an oxidischen Katalysatoren. XXVI. Die Wirkung modifizierter Mordenite beim formselektiven Spalten von Paraffinen

Studies on Oxide Catalysts. XXVI. Effect of Modified Mordenites in the Shape Selective Cracking of Paraffins Shape selectivity of mordenite in the cracking of n-octane and iso-octane depends in a complex manner on the acidity of the solid, the equivalent content of exchanged cations, the distribution of active sites within the mordenites and on reaction conditions. All influencing factors are explained by slow diffusion of the branched hydrocarbon in the pore system of the mordenite.     

Nematic liquid crystals with a tetrafluoroethylene bridge in the mesogenic core structure

A dramatic increase of the clearing temperatures of liquid crystals based on bis(cyclohexyl)ethane 1 by 50 to 70 K can be achieved by the perfluorination of the central ethylene link. Conformational analysis indicates that this effect is due to the increased rigidity of the mesogenic core structure and to the suppression of conformers with a bent shape. Materials based on bis(cylohexyl)tetrafluoroethane 2 might play a crucial role as materials for the next generation of active matrix LCDs with reduced power consumption.     

27Al-MAS-NMR- und IR-spektroskopische Untersuchungen zur Aluminium-Koordination in röntgenamorphen SiO2 ?Al2O3-Festkörpern

²⁷Al MAS N.M.R. and I.R. Investigations on the Aluminium Coordination in Amorphous Silica-Alumina Gels Amorphous silica-alumina of different Si/Al ratios have been investigated by ²⁷Al magic-angle spinning n.m.r. (MAS n.m.r.) and i.r. spectroscopy. Aluminosilicate framework structures were found for the Na⁺- and NH₄⁺-exchanged forms of the investigated gels. The thermal deammoniation of the NH₄⁺-exchanged gels, producing the H⁺ forms, causes an irreversible damage of the aluminosilicate framework. This process is accompanied by a change of the coordination state of a part of the aluminium, converting AlO₄ into AlO₆ units. The progress of the solid-state reaction upon further thermal treatment of the H⁺ forms is reflected by a considerable broadening of the n.m.r. spectra and a decrease of the intensity. The possibility that well-known results, including those of nonframework aluminium, obtained for comparable zeolitic systems may also be valid for amorphous solids, is discussed.     

Hydroxidgruppen an Zeolithen. II. Zahl und Eigenschaften von Hydroxidgruppen an CeNaY- und HNaY-Zeolithen unterschiedlichen Austauschgrades

Hydroxide Groups on Zeolites. II. Number and Properties of Hydroxide Groups on CeNaY and HNaY Zeolites of Different Exchange Degree The number of hydroxide groups on CeNaY and HNaY zeolites was examined by D₂ exchange, and their properties in dependence of the cation exchange degrees were studied by IR spectroscopy. On CeNaY zeolites there exist six kinds and on HNaY zeolites at least seven kinds of hydroxide groups. On the CeNaY zeolites, the hydroxide groups are produced by dissociative chemisorption of water on Ce³⁺ ions. Their total number increases continuously with increasing exchange degree. Some of the hydroxide groups are acid BRÖNSTED centers whose number increases with increasing exchange degree and decreases with the temperature of preheating increasing to about 600°C. On the HNaY zeolites, the hydroxide groups are produced by thermal decomposition of the NH⁺₄ ions, by dealumination and interaction of the Al³⁺ ions produced in this way in the place of cations with water. Above the threshold value of 35% the total number of the hydroxide groups increases very rapidly with increasing exchange degree. One part of the hydroxide groups decreasing with increasing exchange degree acts as acid BRÖNSTED centers. The number of these centers does not decrease until at preheating temperatures above 450°C.     

Liquid crystals carrying stereodefined vicinal difluoro- and trifluoro- alkyl motifs

The synthesis of the stereospecifically fluorinated difluoro- and trifluoro- rac-3 and rac-4 is described where the fluorine atoms are positioned adjacent/vicinal to each other and the physical characteristics of these candidate liquid crystals including negative dielectric anisotropy are measured and rationalised.     

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Isomonodromic Deformations of Connections with Singularities of Parahoric Formal Type

In previous work, the authors have developed a geometric theory of fundamental strata to study connections on the projective line with irregular singularities of parahoric formal type. In this paper, the moduli space of connections that contain regular fundamental strata with fixed combinatorics at each singular point is constructed as a smooth Poisson reduction. The authors then explicitly compute the isomonodromy equations as an integrable system. This result generalizes work of Jimbo, Miwa, and Ueno to connections whose singularities have parahoric formal type.