46 resultados para Rare-earth exchanged zeolite-Y

em Cochin University of Science


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The acidity of the various rare-earth exchanged zeolite-Y catalysts has been examined by titration method using Hammett indicators and is correlated with the catalytic activity of the samples in the benzylation of 0-xylene.

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Dimethylacetals of ketones; cyclohexanone, acetophenone, and benzophenone have been prepared by reacting ketones with methanol under mild reaction conditions. Large pore zeolites (H-Y and its rare earth metal, Ce3+, La3+, and RE3+ modified forms), and mesoporous clay (K-10 montmorillonite and its cerium exchanged counterpart) with regular pore structure, silica and silica-alumina have been used as catalysts. Clay catalysts are found to be much more active than zeolites, thanks to slightly bigger pore size. The nature of the pores of the solid acid catalysts determine acetalization efficiency of a particular catalyst. As evidenced by the reaction time studies, the catalyst decay is greater over the zeolites than over the clays. Carrying out the reaction with ketones of different molecular sizes it is shown that K-10 clays and rare earth exchanged H-Y zeolites are promising environmentally friendly catalysts for their use in the production fine chemicals.

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In this paper, a novel application of solid acid catalysts in the Beckmann rearrangement of E,E-cinnamaldoxime in the synthesis of an important heterocyclic compound; isoquinoline is reported. E,E-Cinnamaldoxime under ambient reaction conditions on zeolite catalysts underwent Beckmann rearrangement to produce isoquinoline in yields of ca. 86–95%. Cinnamonitrile and cinnamaldehyde were formed as by-products. LaH-Y zeolite produces maximum amount of the desired product (yield 95.6%). However, the catalysts are susceptible for deactivation due to the basic nature of the reactants and products, which neutralize the active sites. H-Y zeolite is more susceptible (22% deactivation in 10 h) for deactivation compared to the cerium-exchanged counterpart (18% deactivation in 10 h). Thus, the optimal protocol allows isoquinoline to be synthesised in excellent yields through the Beckmann rearrangement of cinnamaldoxime. The reaction is simple, effective, does not involve any other additives, and environmentally benign.

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The Lewis acidity of yttrium and dysprosium exchanged zeolite Y and ZSM-5 has been determined by titration method using Hammett indicators. The acidity of the Y form increases with increase in concentration of the rare earth cation in the Y zeolite. It is independent of the amount of the rare earth ion for ZSM-5. The data have been correlated with the activity of these zeolites for the esterification of butanol using acetic acid.

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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.

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The effect of residual cations in rare earth metal modified faujasite–Y zeolite has been monitored using magic angle spinning NMR spectral analysis and catalytic activity studies. The second metal ions being used are Na+, K+ and Mg+. From a comparison of the spectra of different samples, it is concluded that potassium and magnesium exchange causes a greater downfield shift in the 29Si NMR peaks. Also, lanthanum exchanged samples show migration behavior from large cages to small cages, which causes the redistribution of second counter cations. It is also observed that Mg2+ causes the most effective migration of lanthanum ions due to its greater charge. The prepared systems were effectively employed for the alkylation of benzene with 1-octene in the vapor phase. From the deactivation studies it is observed that the as-exchanged zeolites possess better stability towards reaction condition over the pure HFAU zeolite.

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The synthesis of dimethyl acetals of carbonyl compounds such as cyclohexanone, acetophenone, and benzophenone has successfully been carried out by the reaction between ketones and methanol using different solid acid catalysts. The strong influence of the textural properties of the catalysts such as acid amount and adsorption properties (surface area and pore volume) determine the catalytic activity. The molecular size of the reactants and products determine the acetalization ability of a particular ketone. The hydrophobicity of the various rare earth exchanged Mg–Y zeolites, K-10 montmorillonite clay, and cerium exchanged montmorillonite (which shows maximum activity) is more determinant than the number of active sites present on the catalyst. The optimum number of acidic sites as well as dehydrating ability of Ce3+-montmorillonite and K-10 montmorillonite clays and various rare earth exchanged Mg–Y zeolites seem to work well in shifting the equilibrium to the product side.

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Copper(II) complexes of two biologically important ligands, viz., embelin (2,5-dihydroxy-3-undecyl-2,5-cyclohexadien 1,4-dione) and 2-aminobenzimidazole were entrapped in the cages of zeolite Y by the flexible ligand method. The capability of these compounds in catalyzing the reduction of oxygen (industrially known as deoxo reaction) was explored and the results indicate an enhancement of the catalytic properties from that of the simple copper ion exchanged zeolite. These point to the ability of the ligands in enhancing the oxygen binding capability of the metal ion. Elemental analyses, Fourier transform infrared (FTIR), diffuse reflectance and EPR spectral studies, magnetic susceptibility measurements, TG, surface area analyses and powder X-ray diffraction studies were used in understanding the presence, composition and structure of the complexes inside the cages. The study also reveals the increased thermal and mechanical stability of the complexes as a result of encapsulation.

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The objective of the present work is to improve the textural and structural properties of zeolite-Y through ion exchange with rare earth metals. We meant to obtain a comparative evaluation of the physicochemical properties and catalytic activity of rare earth modified H-Y, Na-Y, K-Y, and Mg-Y zeolites. Friedel-Crafts alkylations of benzene with higher 1- olefins such as 1-octene, 1-decene, and 1dodecene for the synthesis of linear alkylbenzene (LAB) have been selected for the present study. An attempt has also been directed towards the correlation of the enhancement in 2-phenylalkane formation to the improvement in the textural and structural properties upon rare earth modification for the zeolite-Y. The present method for LAB synthesis stands as an effective Green alternative for the existing hydrofluoric acid technology

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In this regard Schiff base complexes have attracted wide attention. Furthermore, such complexes are found to play important role in analytical chemistry, organic synthesis, metallurgy, refining of metals, electroplating and photography. Many Schiff base complexes are reported in literature. Their properties depend on the nature of the metal ion as well as on the nature of the ligand. By altering the ligands it is possible to obtain desired electronic environment around the metal ion. Thus there is a continuing interest in the synthesis of simple and zeolite encapsulated Schiff base complexes of metal ions. Zeolites have a number of striking structural similarities to the protein portion of natural enzymes. Zeolite based catalysts are known for their remarkable ability of mimicking the chemistry of biological systems. In view of the importance of catalysts in all the areas of modern chemical industries, an effort has been made to synthesize some simple Schiff base complexes, heterogenize them by encapsulating within the supercages of zeoliteY cavities and to study their applications. The thesis deals with studies on the synthesis and characterization of some simple and zeoliteY encapsulated Mn(II), Fe(III), Co(II), Ni(II) and Cu(II) complexes and on the catalytic activity of these complexes on some oxidation reactions. Simple complexes were prepared from the Schiff base ligands SBT derived from 2-aminobenzothiazole and salicylaldehyde and the ligand VBT derived from 2-aminobenzothiazole and vanillin (4-hydroxy-3- methoxybenzaldehyde). ZeoliteY encapsulated Mn(II), Fe(III), Co(II), Ni(II) and Cu(II) complexes of Schiff base ligands SBT and VBT and also of 2-aminobenzothiazole were synthesized. All the prepared complexes were characterized using the physico-chemical techniques such as chemical analysis (employing AAS and CHN analyses), magnetic moment studies, conductance measurements and electronic and FTIR spectra. EPR spectra of the Cu(II) complexes were also carried out to know the probable structures and nature of Cu(II) complexes. Thermogravimetric analyses were carried out to obtain the information regarding the thermal stability of various complexes. The successful encapsulations of the complexes within the cavities of zeoliteY were ascertained by XRD, surface area and pore volume analysis. Assignments of geometries of simple and zeoliteY encapsulated complexes are given in all the cases. Both simple and zeoliteY encapsulated complexes were screened for catalytic activity towards oxidation reactions such as decomposition of hydrogen peroxide, oxidation of benzaldehyde, benzyl alcohol, 1-propanol, 2-propanol and cyclohexanol.