Erionite, dealumination

Reactions with acids. Hydrochloric acid was used in the dealumination of clinoptilolite (1), erionite (14) and mor-denite (2,3,15,92). In the case of Y zeolite, dealumination with mineral acids was successful only after conversion of the zeolite into the ultrastable form (vide infra). Barrer and Makki (1) were the first to propose a mechanism for the removal of aluminum from mordenite by mineral acids. It involves the extraction of aluminum in a soluble form and its replacement by a nest of four hydroxyl groups as follows ... 

Reactions with salts. This procedure is more limited and is illustrated by the use of chromium chloride solutions under reflux for partial dealumination of Y and X zeolites (19), as well as of erionite (20). It is assumed that in this case a partial substitution of chromium for aluminum takes place, leading to the formation of Si-O-Cr bonds in the framework (19). Up to 40 percent of aluminum was removed by this method. Zeolites can also be dealuminated with solutions of ammonium fluorosilicate (107). 

Zeolites are crystalline aluminosilicates with a regular pore structure. These materials have been used in major catalytic processes for a number of years. The application using the largest quantities of zeolites is FCC [102]. The zeolites with significant cracking activity are dealuminated Y zeolites that exhibit greatly increased hydrothermal stability, and are accordingly called ultrastable Y zeolites (USY), ZSM-5 (alternatively known as MFI), mordenite, offretite, and erionite [103]. 

Zeolite catalysts in many forms are used for important commercial processes. The studies were extended to L zeolites, mordenite, erionite, and dealuminated faujasites and mordenites. More attention is paid now to zeolites with univalent and multivalent cations and to multicomponent catalysts. Among these some important examples are the tellurium-containing catalyst for hydrocarbon dehydrocyclization (42), the difunctional Ni- and Pd-zeolite catalysts for benzene hydrodimerization to phenylcyclohexane (42), the catalyst for the hydrogenation of phenol cyclohexanol (44), the 4% Ni/NaY which forms butanol, 2-ethylhexanol, 2-ethylhexanal, and 2-ethylhexanol from a mixture of n-butyraldehyde and hydrogen. 

In a cooperative effort, Linde Research and Union Carbide Nuclear Co. prospected for and located deposits of natural zeolites in Western United States. No deposits of A, X, Y, or faujasite were found. Numerous and extensive deposits of other useful zeolites were located (chabazite, erionite, mordenite, clinop-tilolite), claimed and at a later date some were mined and sold for special uses. We learned how to dealuminate zeolites while maintaining crystal structure, opening the pore and increasing the silica/alumina from 10 to about 20 in mordenite. Procedures for synthesizing A, X, and Y from clays were discovered. 

The spectrum for the 600°C-calcined fluorine-treated erionite sample shows substantial shifts in band positions, but band sharpening is less obvious. The bands at 1082, 792, 578, 470 and 438 cm- are shifted to 1098, 814, 585, 477 and 444 cm , respectively, after fluorine treatment and 600°c calcination. The large shifts observed are evidence of dealumination. The splitting of the 1082 cm-1 band into a doublet located at 1098 and 1085 cm-1 and some degree of band sharpening imply structure stabilization for fluorine treated erionite. 

Usually, inorganic and organic acids can be used for framework dealumination of zeolites, and the acids include hydrochloric acid, nitric acid, formic acid, acetic acid, and so on. According to its acid-resistance ability, hydrochloric acid can be used for high-silica zeolites such as mordenite, clinoptilolite, erionite, etc. We will take mordenite as an example to describe this dealumination method (Table 6.4). The first step in the treatment of mordenite using hydrochloric acid is to convert the zeolite into H-type, and further acid treatment can then enlarge the pore diameter through dealumination. After partial dealumination, the Si/Al ratio of the zeolite is increased and the heat-resistance, water-resistance, and acid-resistance abilities are enhanced. 

Because the aforementioned technique shows excellent effects for dealumination and silicon-addition, this technique has been applied to the adjustment of Si/Al ratio for various zeolites such as erionite, zeolite L, clinoptilolite, and chabazite. 

Methanol Conversion to Olefins. - Chabazite, erionite, zeolite T, and ZK-5 have been used by Chang et al. for the conversion of methanol into olefins. The C2-C4 olefin concentration in the hydrocarbon fraction was always less than 60 wt% at 100% methanol conversion. It follows from Table 3 that the hydrocarbon fraction becomes richer in Cj-C olefins as the conversion of methanol decreases. That is because the conversion of olefins to paraffins is lower. Hydrocarbon fractions with more than 80 wt% of Cj-C olefins were attained with a dealuminated H-erionite, but the conversion of methanol was very low. 

High (ca. 10 to 00) ZSM-5 dealuminated erionite, mordenite Y relatively high stability of the lattice high stability in acids low stability in bases low concentration of acid groups of high strength... 

The framework Si/Al ratio of zeolite Q (synthetic mazzite) was increased by treatment with SiCl4 at 500 °C from 4.24 to 6.00 without significant loss in crystalHnity [177]. The dealumination reaction was accompanied by a sfight increase in the hexagonal lattice parameter a while c remained unaffected. This unusual phenomenon, i.e., cell expansion upon isomorphous substitution of silicon for aluminum, as well as AL MAS NMR spectroscopic results, pointed to a redistribution of aluminum on at least two crystallographically different framework T-sites. Si MAS NMR spectra of offretite, erionite and zeofite Q all dealuminated with SiCl4 were presented in [178]. 

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