Ordered aluminosilicate mesostructures having a SiO 2 /Al 2 O 3 ratio near 5 were prepared using a surfactanttemplated synthesis. The solid structures were analyzed by scanning electron microscopy (SEM), nitrogen sorption to determine pore volume and pore sizes, powder X-ray diffraction (PXRD), thermal desorption of a chemisorbed base to determine strong acid site density, and magic-angle-spinning (MAS) NMR spectroscopy for the 27 Al and 29 Si nuclei. We were successful in synthesizing a mesoporous aluminosilicate having a SiO 2 / Al 2 O 3 ratio near 5 when the source of the aluminum ion was Al hydroxide and the surfactant was cetyltrimethylammonium cation (C 16 TMA + ). The uncalcined samples showed only tetrahedral aluminum as confirmed by 27 Al MAS NMR. When the pH of the synthesis gel was 10.5, the mesostructure developed a pore volume of 0.59 cm 3 /g with an average pore diameter of 2.2 nm. The pore volume was less at pH values other then 10.5 (e.g., for pH 12.6 and 8.1, the pore volumes were 0.50 and 0.43 cm 3 /g, respectively). The aluminosilicate mesostructure showed Brønsted and Lewis sites as confirmed by the IR vibrational spectroscopy of chemisorbed pyridine. The mesostructure Brønsted acid site density was 0.36 µmol H + /m 2 -solid, based on Brønsted acid site titration by isopropylamine temperature-programmed desorption. It was similar to that observed for an amorphous alumina-silica having a SiO 2 /Al 2 O 3 ratio near 6. However, the mesostructure Brønsted site density per unit mass was much lower than that measured for an acidic Y-faujasite with a SiO 2 /Al 2 O 3 ratio of 2.75.
Cardiovascular illness is most commonly caused by a constriction, called a stenosis. A non-linear mathematical model with a free moving boundary was introduced to study viscous flow in tapered elastic tubes with axisymmetric constrictions subject to a prescribed pressure drop and a uniform external pressure. An iterative numerical scheme using a boundary iteration method was developed to solve the model. Effects of stenosis severity and stiffness, pressure drop, external pressure and stiffness of the vessel wall on the flow and wall motion were evaluated. It was found that stenosis severity, pressure drop and external pressure played more dominant roles than tube wall stiffness and stenosis stiffness perturbation. Tubes with 71 and 78 per cent stenoses showed two areas of negative transmural pressure and complex contraction-expansion-contraction wall motion patterns. Two types of tube diameter contraction and negative transmural pressure were observed, one was just distal to the stenosis and the other was near the outlet of the tube. Experiments using stenotic silicone tubes were conducted to quantify the tube law and verify the predicted pressure-flow relationship. The agreement between the numerical results and experimental measurements is better than that from a previous model which assumed periodicity of the tube and imposed different pressure conditions.1998 John Wiley & Sons, Ltd.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.