Alkali zeolites, stability

Experimental phase equilibria studies by Campbell and Fyfe (1965) Thompson (1971)and Liou (1971a) indicate an approximate 180°C. lower stability for albite in the presence of quartz and analcite from 12 to 2000 atmospheres pressure. A calculated stability for analcite at 3Kb is about 120°C. (Campbell and Fyfe, 1965), conditions equivalent to rock pressures at 7.5Km depth. However, if water pressure is lower than total, litho-static pressure, the termal stability of a very hydrous, low-density mineral such as analcite can be significantly lowered (Greenwood, 1961). The experimental transformation of alkali zeolites to analcite at 100°C. and 2-3 atmospheres pressures was demonstrated by Boles (1971). The alumina content of the alkali zeolites used in this latter study was found to influence that of the analcite produced, and this independently of the amount of crystalline quartz added to the initial materials. 

Probably the only general statement which can be made about the experimental studies on zeolites is that the majority of published data is inapplicable directly to natural minerals. This is due either to the excessively high temperatures under which the experiments are performed, outside of the physical limits of zeolite stability, or to short time spans of observation which do not allow the silicates to come to equilibrium with the fluids of the experiments. Those studies designed to determine zeolite stability indicate that the most silica-poor alkali zeolite, analcite, is not stable above 180°C. More silica-rich species will be found below this temperature. However, the reasons for the crystallization of one or another of the silica-rich alkali zeolites are not yet elucidated. 

A consideration of natural occurrence and chemical composition of alkali zeolites allows a certain refinement of the zeolite facies concept previously proposed. The key factor is the grouping of the alkali zeolites into a continuous solid solution series. Other possible coexisting phases of similar composition are sodium and potassium feldspar, natrolite and analcite. The extent of solid solution decreases with temperature, possibly also with pressure. This effect allows the sequential series zeolite-K feldspar, zeolite-analcite-K feldspar, analcite-K feldspar-albite and eventually two feldspars to the exclusion of analcite, the alkali zeolite with the highest stability limits. 

A new concept of selective photo-oxidation in zeolite cages has been recently proposed [47], The hydrocarbon radical cation-Oz charge-transfer pair is generated inside the cavities of alkali zeolites, in which the large electrostatic field stabilizes the highly polar charge transfer states of hydrocarbon-Oz collisi-onal pair and allows to control the pathways of further transformation. 

Easily ionizable anthracene forms the cation-radical as a result of sorption within Li-ZSM-5. In case of other alkali cations, anthracene was sorbed within M-ZSM-5 as an intact molecule without ionization (Marquis et al. 2005). Among the counterbalancing alkali cations, only Li+ can induce sufficient polarization energy to initiate spontaneous ionization during the anthracene sorption. The lithium cation has the smallest ion radius and its distance to the oxygen net is the shortest. The ejected electron appears to be delocalized in a restricted space around Li+ ion and Al and Si atoms in the zeolite framework. The anthracene cation-radical appears to be in proximity to the space where the electron is delocalized. This opens a possibility for the anthracene cation-radical to be stabilized by the electron s negative field. In other words, a special driving force for one-electron transfer is formed, in case of Li-ZSM-5. 

A further possibility for side-chain alkylation of toluene is oxidative methylation with methane. Catalysts with occluded alkali metal oxides, prepared by impregnating zeolites with alkali metal hydroxides followed by calcination, usually exhibit better performance.441 Further enhancement was achieved by impregnating ion-exchanged zeolites 442 Significant improvements in stability and the yields of Cg hydrocarbons were also observed when NaX was impregnated with 13% MgO which was found to increase the amounts of active sites.443... 

It is proposed that in mixed organic base-alkali systems, the presence of the organic base changes the solid-liquid equilibrium and stabilizes larger sol-like aluminosilicate species ( 25 m/ ). The alkali ion affects agglomeration of the sol particles to larger amorphous precipitate particles from 100 to 500 min size which subsequently crystallize to zeolite. 

To improve process economics, further work is needed to improve catalyst lifetimes. A more stable system employed a noble metal-loaded potassium L-zeolite catalyst for the condensation of ethanol with methanol to produce a 1-propanol and 2-methyl-l-propanol (US patent no. 5,300,695) (18). However, yields were small compared with the large amounts of CO and C02 produced from the methanol. More recently, Exxon patented a noble metal-loaded alkali metal-doped mixed metal (Zr, Mn, Zn) oxide (US patent nos. 6,034,141 and 5,811,602) (19,20). The catalyst was used in a syngas atmosphere. As with other catalysts, the higher temperatures resulted in decomposition of methanol. Changes in catalyst composition were noted at higher temperatures, but the stability of the catalyst was not discussed. Recently, compositions including Ni, Rh, Ru, and Cu were investigated (21,22). 

However, the zeolite is not a unique substrate for this reaction, as is indicated in a recent patent (180), where it is shown that a Cu+-exchanged mont-morillonite clay and synthetic amorphous aluminosilicate will also catalyze butadiene cyclodimerization with high selectivities to VCH (>95%). Preexchange of these aluminosilicates with Cs+ ions was claimed to increase catalyst stability. This is most probably explained by a reduction in surface acidity resulting from the alkali metal ion exchange. 

The calcosilicate zeolite-like crystal material CAS-1 was hydrothermally synthesized and the thermal stability of the samples were investigated. The effects of composition of raw materials, reaction temperature and alkali metals on the synthesis of CAS-I were addressed. Cation exchange reactions and their influences on the thermal stability of CAS-I framework structure were also studied. The samples were characterized by XRD, TEM, SEM, DT-TGA, AAS and chemical analysis. The results showed that CAS-1 could be obtained from a wide range of composition of raw materials and reaction conditions. The cations have great influence on the thermal stability of the CAS-I framework structure. 

If empty zeolite LTA is exposed to cesium gas, and redox reaction with the exchangeable cations of the zeolite can occur, then the product Cs" " ions may seal the zeolite crystals. Everything within the zeolite may be encapsulated, including the product atoms, perhaps as neutral clusters. Well, nearly all cations, including Na" ", are reduced by Cs(g) only some of the other alkali-metal cations may resist reaction. Depending upon the mobility of the Cs" " ions at the reaction temperature, the surfaces of the crystals may be closed initially. If the reaction proceeds, the entire volume of the zeolite may be sealed. The ultimate outcome of the process will depend upon the mobilities of all the atoms and ions in the structure at the reaction temperature, and upon considerations of the stability and lability of any structural subunits which may form. 

We speak, therefore, of base catalyzed reactions, if the strength of the base sites is high enough to stabilize anionic or polarized species with a marked negative charge and if these species are part of the catalytic cycle. Interpretation of catalytic results with respect to the role of acid and base sites remains, however, always ambiguous as the stabilizing effect of the metal cation (for zeolites usually an alkali metal cation) is difficult to assess. 

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