Zeolite cavities, reactivity

SIMS (61,64,86), microscopy (65), XPS (56), electron microprobe techniques (14,66), electron paramagnetic resonance (EPR) (67) and luminescence experiments (68) have been successfully employed to probe and study V mobility and reactivity on a catalyst surface. TEM, STEM and energy dispersive X-ray emission (EDX) measurements have indicated that V interaction with REY-crystals induced vanadate clusters formation (65). Vanadium was also found capable of reacting with rare-earths outside the zeolite cavities to form LaVQ4... 

Another major application of Ti oxide species prepared within the zeolite cavities includes the photocatalytic decomposition of environmentally noxious substances into benign compounds. Examples include photodecomposition of chlor-ophenol [158] and of NO into N2 and O2 [159]. Ti02-loaded MCM-41 shows a much lower apparent activation energy for the photodegradation of acetophenone than that of Ti02-loaded zeolite-X, -Y and -A. The differences in reactivity with different frameworks was related to the efficiency of trapping of the photogenerated electrons and/or holes [160]. 

Borvomwattananont A, Bein T (1992) Reactivity of (tiimethylstannyl)pentacarbonylmanga-nese in zeolite cavities. J Phys Chem 96 9447WeU-Defined Metalhc and BimetaUic Clusters Supported on Oxides and Zeolites... 

Photoreactivity of androstenedione 116 (Scheme 1.26) was demonstrated to be dramatically affected by the electric field of zeolite. The epimerization of 116 to yield 117 is the major reaction in isotropic solution such as hexane, methylene chloride, methanol and cyclohexane. The reduction of the cyclohexenone A-ring can be observed in only propanol, with 118 and 119 being given in 14% combined yield. However, irradiation of 116 in NaY afforded exclusively the reduction products in >85% yield, and no product due to the reaction at the D-ring was observed. This reactivity change observed for the androstenedione included in zeolite cavity was ascribed to the lowering of the n,n excited state of the A-ring due to the electric field in zeolite cavity. 

In conclusion, although all aluminium (and silicon) atoms in zeolite Y crystals are equally accessible through the zeolite cavities, they are chemically not identical, and their reactivity towards dealumination reagents could be different. Unfortunately, no methods are actually available for determining directly the T atom environment in terms of NNN of an aluminium atom. Indeed, the different aluminium T sites are even not resolved in the 27 1 resonance envelope, measured with the high-resolution solid state magic angle... 

In the zeolite-promoted benzylation, however, it was observed that the reactivities of the alcohols decreased in the order 1-decanol > benzyl alcohol as cyclohexylmethanol a neopentyl alcohol 1-adamantylmethanol. The adsorption isotherms for the alcohols on zeolite KY in benzene show that even the most bulky 1-adamantylmethanol can be adsorbed inside the cavities of KY as readily as 1-decanol (Fig. 2). Hence the low conversion of 1-adamantylmethanol in the KY system can be ascribed to steric hindrance in the transition state the alcohol is so bulky that it is relatively difficult for the alcohol to be oriented in close proximity to a benzyl chloride molecule between acidic and basic sites of the zeolite. 

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