Zeolites various

So far, fewer than 10 types of carbenium ions have been reported to be persistent species formed upon adsorption of olefins or alcohols on acidic zeolites. Instead, surface alkoxy (alkoxide) species with carbenium-ion-like properties are suggested to act, most likely, as catalytic intermediates in reactions catalyzed by acidic zeolites. Various groups have observed that, upon adsorption of olefins or alcohols on acidic zeolites, alkoxy species are formed the observations are based on both in situ and ex situ A MAS NMR spectroscopy (49,50,71-80). 

The use of probe molecules that will sorb on specific acid sites has also been used to differentiate Brpnsted and Lewis sites. Studies on H-ZSM-5 and H-ZSM-8 suggest that pyridine cannot access the Lewis sites and thus essentially measures Brpnsted acidity 2,6-Dimethylpyridine has been reported to sorb preferentially on the Brpnsted sites of a variety of zeolites. Various amines have also been used to measure surface acidity. Since amines are weaker bases than either ammonia or pyridine, they preferentially sorb on the stronger acid sites. Both n-butyl amine and t-butyl amine have been used as adsorbates in TPD studies of zeolites. Comparison of the TPD spectra with butene isomerization has shown that butyl amines measure only the strong acid sites. 

In order to compare the costs involved in the TSA method and other conventional methods (viz., one step and two steps) for synthesis of the ash zeolites, various parameters (viz., type of the reagent used, duration of the steps, cost of the equipment and energy consumed) were considered and the details are presented in Table 6.6. 

The zeolite frameworks are built up of SiO tetraliedra, which are neutral, and AlO tetraliedra, which have a charge of-1. The charge of tire AlO tetraliedra is balanced by tire charges of cations located at various crystallographically defined positions in the zeolite, many of tliem exposed at tire internal surface. The cations are... 

Besides stmctural variety, chemical diversity has also increased. Pure silicon fonns of zeolite ZSM-5 and ZSM-11, designated silicalite-l [19] and silicahte-2 [20], have been synthesised. A number of other pure silicon analogues of zeolites, called porosils, are known [21]. Various chemical elements other than silicon or aluminium have been incoriDorated into zeolite lattice stmctures [22, 23]. Most important among those from an applications point of view are the incoriDoration of titanium, cobalt, and iron for oxidation catalysts, boron for acid strength variation, and gallium for dehydrogenation/aromatization reactions. In some cases it remains questionable, however, whether incoriDoration into the zeolite lattice stmcture has really occurred. 

I have carried out widespread studies on the application of a sensitive and selective preconcentration method for the determination of trace a mounts of nickel by atomic absorption spectrometry. The method is based on soi ption of Cu(II) ions on natural Analcime Zeolit column modified with a new Schiff base 5-((4-hexaoxyphenylazo)-N-(n-hexyl-aminophenyl)) Salicylaldimine and then eluted with O.IM EDTA and determination by EAAS. Various parameters such as the effect of pH, flow rate, type and minimum amount of stripping and the effects of various cationic interferences on the recovery of ions were studied in the present work. 

The powders of zeolites of various trademarks are used to produce petroleum-refining catalysts. In this connection, it is very important to have complete information concerning not only chemical composition and distribution of impurity elements, but also shape, surface, stmcture and sizes of particles. It allows a more detailed analysis of the physical-chemical characteristics of catalysts, affecting their activity at different stages of technological process. One prospective for solving these tasks is X-ray microanalysis with an electron probe (EPMA). 

The EPMA technique is developed for individual particles of fine-dispersed zeolite powder of various trademarks ZSM-5, ZSM-12, MOR, BEA. The phase and chemical composition of zeolite powder, the surface, shape of particles, stmcture and their distribution in terms of the size were studied using Superprobe-733 device. 

Dissolved Solids None Dissolved solids is measure of total amount of dissolved matter, determined by evaporation high concentrations of dissolved solids are objectionable because of process interference and as a cause of foaming in boilers Various softening processes, such as lime softening and cation exchange by hydrogen zeolite, will reduce dissolved, solids demineralization distillation reverse osmosis electrodialysis... 

The structural complexity of the 3D framework aluminosilicates precludes a detailed treatment here, but many of the minerals are of paramount importance. The group includes the feldspars (which are the most abundant of all minerals, and comprise 60% of the earth s crust), the zeolites (which find major applications as molecular sieves, desiccants, ion exchangers and water softeners), and the ultramarines which, as their name implies, often have an intense blue colour. All are constructed from Si04 units in which each O atom is shared by 2 tetrahedra (as in the various forms of Si02 itself), but up to one-half of the Si... 

The important property of ZSM-5 and similar zeolites is the intercrystalline catalyst sites, which allow one type of reactant molecule to diffuse, while denying diffusion to others. This property, which is based on the shape and size of the reactant molecules as well as the pore sizes of the catalyst, is called shape selectivity. Chen and Garwood document investigations regarding the various aspects of ZSM-5 shape selectivity in relation to its intercrystalline and pore structure. 

Methanol conversion to hydrocarbons over various zeolites (370X, 1 atm, 1 LHSV)... 

Various analytical tests determine zeolite properties. These tests supply information about the strength, type, number, and distribution of acid sites. Additional tests can also provide information about surface area and pore size distribution. The three most common parameters governing zeolite behavior are as follows ... 

Engelhard s in-situ FCC catalyst technology is mainly based on growing zeolite within the kaolin-based particles as shown in Figure 3-9A. The aqueous solution of various kaolins is spray dried to form micR)spheres. The microspheres are hardened in a high-temperature l,3f)(TF/704°C) calcination process. The NaY zeolite is produced by digestion of the microspheres, which contain metakaolin, and mullite with caustic or sodium silicate. Simultaneously, an active matrix is formed with the microspheres. The crystallized microspheres are filtered and washed prior to ion exchange and any final treatment. 

The most significant changes associated with adsorption which have been observed to date were the displacements (45) of Raman fundamentals of ethyne on adsorption on zeolite 4A (see Table IX). Such changes constitute a useful monitor of adsorbate-adsorbent interaction for various adsorbents. The appearance of the Raman spectrum of ethyne on zeolites A suggests an... 

The ability of zeolites to control the regioselectivity of Diels-Alder reaction has been investigated for the cycloaddition of isoprene with various dienophiles [20bj. Some results are reported in Table 4.6. All the zeolites tested afforded high regioselectivity but the reaction yield was generally low and depended on the zeolite as well as on the dienophile. 

In an early paper by our group (ref. 19) halobenzenes are brominated at 298 K in the liquid phase over various Y-zeolites (Fig. 7). Improved p/o-ratios with respect to conventional (FeBr3) catalysis are observed. 

Finally we mention in this section the non-catalytic selective bromination of aniline by the application of a zeolite pre-loaded with Bt2 as a slow release reagent (ref. 27). Aniline, dissolved in CCI4 was treated with Br2 adsorbed onto various zeolites and zeolite CaA was found to be most selective for monosubstitution (92%). The addition of organic bases improved the performance, probably due to scavenging of HBr. Also the toluidines could be monobrominated with this system with >95% selectivity. 

Various Cu-exchanged zeolites have been examined in the nucleophilic substitution of bromo- and chlorobenzene towards aminated and oxygenated compounds (ref. 30). In amination a consecutive reaction to diphenylamine and reduction to benzene are the side-reactions (Fig. 10). 

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