Natural zeolite utilisation in pollution control: A review of applications to metals' effluents

This paper assesses the potential of natural zeolite utilization as a low-cost ion-exchange and sorbent material in pollution control and recovery of metals. The improvement of the purity of natural zeolites coupled with their chemical modification capabilities in order to provide specific properties may provide a cost-effective alternative for the treatment of heavy metal contaminated soil and water. The application of natural zeolites in industrial pollution control is becoming important and the level of technical effort is increasingly expanding. The extent of application is unlikely to be related solely to their low cost, but also to improved properties and performance characteristics, which will advance the practical implementation of natural zeolites' technology.     

Methanol adsorption and dehydration on alkali metal exchanged NaY zeolites

FT-IR spectroscopy has been applied in a study of methanol adsorption on M^INaY zeolites (M=Li, K, Rb, Cs). Coordinatively and/or hydrogen bonded methanol was registered in the temperature range 300–473 K. Dehydration of methanol to dimethyl ether occurred at 573 and 623 K. It was found that the activity of the catalyst for methanol dehydration strongly depends on the kind of alkali metal cation, and well correlates the IR data relative to methanol adsorption.     

Pyrene oxidation as a model reaction for characterizing the mechanism of coke oxidation on Y zeolites

The same effect of temperature on the production of CO, CO₂ and H₂O was found during oxidative treatment of HY zeolites either impregnated with pyrene or coked. Therefore, pyrene oxidation can be used to specify the mechanism of coke oxidation. Three types of reactions can be distinguished: (a) condensation of polyaromatic molecules, (b) oxidation of polyaromatics into aldehydes, ketones, acids and anhydrides and (c) decarbonylation or decarboxylation of oxygenated compounds, all these reactions involving protonic sites.     

Change in the decomposition mode of 1-propanamine due to the presence of cations in MFI zeolite

The adsorption and thermal analysis of 1-propanamine has been compared over MFI zeolites which contain H, Ga, and In cations. In the case of H⁺-containing materials, NH₃ and propene are simultaneously desorbed above 600 K. This behavior, is well known and characteristic of the Hofmann elimination reaction. However, a distinctly different mode of reaction is observed in the case of Ga and In containing materials. NH₃ is released below 600 K, propene and other products are released above 600 K, and a stable residue remains above 800 K. It is suggested that such behavior results from Lewis acid interactions of Ga or In cations with propanamine.On leave from the Bulgarian Academy of Sciences, Institute of Organic Chemistry.     

The desorption of O2 during NO and N2O decomposition on Cu- and Fe-zeolites

The decomposition of NO and of N₂O over a CuZSM-5 zeolite and a Fe-mordenite, respectively, has been studied using tracer techniques. The results demonstrate the high mobility of the lattice oxygen ions in self-diffusion. They afford a possible explanation for the problem of how two extralattice oxygens located at positions remote from each other may combine to form the O₂ molecules which are spontaneously desorbed in these redox reactions. They show that a portion of the lattice oxygen mixes into the O₂ released on decomposition. The data also show that N¹⁸O and N₂ ¹⁸O undergo exchange with the catalyst oxygen under reaction conditions.On leave from Central Research Institute for Chemistry, Hungarian Academy of Sciences, H-1525 Budapest, Hungary.     

The origin of the band at 1462 cm−1 generally appearing upon desorption of pyridine from acidic solids. Steps towards a more comprehensive understanding

The pyridine adsorption-desorption method, followed by IR spectroscopy, is generally applied for acidity determination of solid catalysts. A peculiar band at 1462 cm^–1 was observed by several authors upon pyridine desorption at higher temperatures and was interpreted in various ways. A new complex between Lewis framework acid sites and pyridine is proposed and several evidences are reported.     

Dehydration of aldoximes on rare earth exchanged (La<Superscript>3+</Superscript>, Ce<Superscript>3+</Superscript>, RE<Superscript>3+</Superscript>, Sm<Superscript>3+</Superscript>) Na–Y zeolites: A facile route for the synthesis of nitriles

Rare earth exchanged Na–Y zeolites, H-mordenite, K-10 montmorillonite clay and amorphous silica-alumina were effectively employed for the continuous synthesis of nitriles. Dehydration of benzaldoxime and 4-methoxybenzaldoxime were carried out on these catalysts at 473 K. Benzonitrile (dehydration product) was obtained in near quantitative yield with benzaldoxime whereas; 4-methoxybenzaldoxime produces both Beckmann rearrangement (4-methoxyphenylformamide) as well as dehydration products (4-methoxy benzonitrile) in quantitative yields. The production of benzonitrile was near quantitative under heterogeneous reaction conditions. The optimal protocol allows nitriles to be synthesized in good yields through the dehydration of aldoximes. Time on stream (TOS) studies show decline in the activity of the catalysts due to neutralization of acid sites by the basic reactant and product molecules and water formed during the dehydration of aldoximes.     

Isobutane/2-butene alkylation on MCM-22 catalyst. Influence of zeolite structure and acidity on activity and selectivity

The catalytic behavior of the novel MCM-22 zeolite for the continuous alkylation of isobutane with 2-butene has been investigated at a temperature of 50°C, 2.5 MPa total pressure, and a variety of olefin space velocities. At high olefin conversions the MCM-22 zeolite showed a very high initial cracking activity attributable to strong Brønsted acid sites, as well as to the existence of strong diffusional restrictions of the TMP's (formed inside the zeolite) to exit through the channels. At short times on stream (TOS), TMP's account for ca. 40% of the C₈ fraction. The olefin conversion and the cracking activity rapidly decline with TOS, while the alkylate product became richer in dimethylhexenes, indicating a predominance of 2-butene dimerization and a loss of hydrogen transfer activity as the catalyst aged. Moreover, MCM-22 gives less TMP's than large-pore zeolites (USY, beta, mordenite), but more than the mediumpore ZSM-5 at similar 2-butene conversion. The latter catalyst was much more selective for olefin dimerization than for isobutane alkylation, presumably because formation of the bulkier TMP's was strongly impeded in its smaller pores.     

Aldol condensation in gaseous phase by zeolite catalysts

Vapour-phase reactions of, in situ, prepared formaldehyde with methyl propionate were studied using X, Y and ZSM-5 zeolite catalysts. Base properties of these zeolites were enhanced by KOH or NaN₃ treatment. The niobium and molybdenum ZSM-5 zeolite supported oxides were also tested for their catalytic activity. The results are discussed in terms of an ability of zeolite catalysts to synthesize methyl methacrylate.