No abstract

No abstract

No abstract

For thl: catalytic cracking of Cs to C9 hydrocarbons on ZSM-5, we demonstrate quantitatively the contributions of each of two mechanisms for molecular shape selectivity. Using crystallites of different sizes and activities, and classical methods for evaluating diffusion inhibition of the reaction rate, we separate the effects of mass-transport-induced selectivity from that created by steric inhibition by the size of a reaction complex. The selective cracking of n-paraffins compared to monomethyl paraffins (from Cs to C,) is due to a higher intrinsic rate constant of the n-paraffin, with diffusional mass transport playing no appreciable role. In contrast, dimethyl paraffin cracking is strongly diffusion-inhibited. The methyl paraffin/n-paraffin discrimination is a result of steric constraint on the sizeable methyl paraffin/carbonium ion reaction complex. This structural selectivity is shown to be absent for the corresponding olefins where such complexes do not arise. The diffusivities at reaction conditions have been determined. For the linear hydrocarbon, diffusivity notably exceeds that expected from the Knudsen model. This reminds us to review assumptions of conventional concepts of mass transport. The availability of zeolites now allows us to probe many basic phenomena in catalysis, molecular configuration and dynamics, including mass transport.

No abstract

No abstract

Many diseases are dominated by persistent growth of capillary blood vessels. Tumor growth is also angiogenesis-dependent. Safe and effective angiogenesis inhibitors are needed to determine whether control of angiogenesis would be therapeutic. Heparin and certain steroids, administered together, can inhibit angiogenesis in a synergistic manner. This "pair" effect suggested that specific hydrophilic cycloamyloses may be suitable heparin substitutes. beta-Cyclodextrin tetradecasulfate administered with a steroid inhibits angiogenesis at 100 to 1000 times the effectiveness of heparin in the chick embryo bioassay. This cyclic oligosaccharide also augments the anti-angiogenic effect of angiostatic steroids against corneal neovascularization in rabbits when beta-cyclodextrin tetradecasulfate and a steroid are inserted into the cornea or applied topically as eyedrops.

Shape-selective hydrocarbon catalysis, such as the conversion capability of zealite catalysts of the ZSM-5 type to produce high-grade gasoline from methanol, can be extended to produce a similar gasoline or aromatics from plant extracts. Examples are rubber latex, corn oil, and peanut oil. Novel mechanisms for shape-selective reaction sequences are demonstrated.