Aperture, ring

Common name Ring size, number of atoms Free aperture, nm Pore a Stmcture CAS Registry Number Formula... 

There are also apertures with eight-membered oxygen rings in this zeoHte. 

We will use a specihc example to demonstrate the problems likely to be encountered when testing. Assume we have an extremely large telescope with around 1000 - 1 m hex segments whose prime focal length is 50 m, or vertex radius is 100 m. The inner ring of segments will have their centers at an aperture radius h of 3.5 m and the outermost ring at 14.5 m. 

The exact nature of the zeolite is determined by the reaction conditions, the silica to alumina ratio and the base used. For example zeolite /3, a class of zeolites with relatively large pores, in the range of 0.7 nm, of which mordenite is an example, are usually made using tetraethylammonium hydroxide as the base. This acts as a template for the formation of 12-membered ring apertures (Figure 4.3). 

Figure 4.3 Mordenite showing 12-membered ring aperture...

Since their development in 1974 ZSM-5 zeolites have had considerable commercial success. ZSM-5 has a 10-membered ring-pore aperture of 0.55 nm (hence the 5 in ZSM-5), which is an ideal dimension for carrying out selective transformations on small aromatic substrates. Being the feedstock for PET, / -xylene is the most useful of the xylene isomers. The Bronsted acid form of ZSM-5, H-ZSM-5, is used to produce p-xylene selectively through toluene alkylation with methanol, xylene isomerization and toluene disproportionation (Figure 4.4). This is an example of a product selective reaction in which the reactant (toluene) is small enough to enter the pore but some of the initial products formed (o and w-xylene) are too large to diffuse rapidly out of the pore. /7-Xylene can, however. 

MCM-22 zeolite (IZA code MWW) presents a novel and original structure which is composed of two independent pore systems a 2-dimensional, sinusoidal, 10-ring intralayer channels system and an interlayer system with 12-ring supercages, accessible through 10-ring apertures [1], This unique pore structure of the aluminosilicate MCM-22 zeolites makes them interesting catalysts for a wide variety of acid-catalyzed reactions, e.g. alkylation of aromatics [2],... 

Fig. 5. HRTEM image of MAPO-36 showing well-defined large apertures. The inset shows the computed image where the outline of the 12-rings is clearly visible (top and bottom are with and without taking beam damage into account) (5,43).

Dark-field illumination is classified into three types. The first one is for a microscope equipped with low numerical aperture (NA) objective lenses (see Fig. 1). To cast a shadow at the objective lens, a ring-slit as shown in Fig. IB is inserted into the light path. The second is for highNA (>0.5) objective lenses. Special, ready-made dark-field condensers or lenses are used for dark-field illumination. The third is independent... 

Fig. 11.10. Diagram illustrating the inner surfaces of the primary components of a Paul (3D) quadrupole ion trap. Ions generated by an external source are injected into the trap through an aperture in one of the end caps. Scan functions for isolating ions in the trap, exciting the mass selected ions to induce unimolecular dissociation, and ejecting ions from the trap (for detection) are implemented through the application of DC and RF voltages to the ring electrode.

Alkylation over the MWW Zeolite. The MWW (or MCM-22) zeolite developed by Mobil as catalyst for ethylbenzene and cumene production deserves particular attention. Indeed, this zeolite presents unique structural features (Figure 12.5). Its structure is constituted of three independent pore systems " large supercages (inner diameter of 7.1 A dehned by a 12-member-ring [12-MR], height 18.2 A) each connected to six others through 10-MR apertures... 

T-atoms in ring Maximum free aperture (nm) Typical free apertures (nm) ... 

The superiority of the flat flames over the simple round ones, explains an observation which has been made with regard to the argand burners. When the apertures in it are placed so far apart as to form a circle of distinct jets, the effect is one-third weaker, -with the same current of gas, than when the jets— of one-sixth, to one-eighth of an inch—unite into a single flat ring. 

---