MAl_2O_4:Eu~(2+)���RE~(3+)������������������������������

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Solid acid-catalyzed dehydration/Beckmann rearrangement of aldoximes: towards high atom efficiency green processes

Rare earth metal ion exchanged (La3+, Ce3+, RE3+) KFAU-Y zeolites were prepared by simple ion-exchange methods and have been characterized using different physico-chemical techniques. In this paper a novel application of solid acid catalysts in the dehydration/ Beckmann rearrangement of aldoximes; benzaldoxime and 4-methoxybenzaldoxime is reported. Dehydration/Beckmann rearrangement reactions of benzaldoxime and 4-methoxybenzaldoxime is carried out in a continuous down flow reactor at 473K. 4-Methoxybenzaldoxime gave both Beckmann rearrangement product (4-methoxyphenylformamide) and dehydration product (4-methoxybenzonitrile) in high overall yields. The difference in behavior of the aldoximes is explained in terms of electronic effects. 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 studies show a fast decline in the activity of the catalyst due to neutralization of acid sites by the basic reactant and product molecules.

Acetalization of ketones on K-10 clay and rare earth exchanged HFAU-Y zeolites: A mild and facile procedure for the synthesis of dimethylacetals

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.

Synthesis, Characterization, Luminescence and Defect Centres in CaYAl(3)O(7):Eu(3+) Red Phosphor

CaYAl(3)O(7):Eu(3+) phosphor was prepared at furnace temperatures as low as 550A degrees C by a solution combustion method. The formation of crystalline CaYAl(3)O(7):Eu(3+) was confirmed by powder X-Ray diffraction pattern. The prepared phosphor was characterized by SEM, FT-IR and photoluminescence techniques. Photoluminescence measurements indicated that emission spectrum is dominated by the red peak located at 618 nm due to the (5)D(0)-(7)F(2) electric dipole transition of Eu(3+) ions. Electron Spin Resonance (ESR) studies were carried out to identify the centres responsible for the thermoluminescence (TL) peaks. Room temperature ESR spectrum of irradiated phosphor appears to be a superposition of two distinct centres. One of the centres (centre I) with principal g-value 2.0126 is identified as an O(-) ion while centre II with an isotropic g-factor 2.0060 is assigned to an F(+) centre (singly ionized oxygen vacancy). An additional defect centre is observed during thermal annealing experiments and this centre (assigned to F(+) centre) seems to originate from an F centre (oxygen vacancy with two electrons). The F(+) centre appears to correlate with the observed high temperature TL peak in CaYAl(3)O(7):Eu(3+) phosphor.

Influ��ncia da acidez da suspens��o coloidal para a gera����o de defeitos pontuais e emiss��o no infra vermelho em SnO2 dopado com Er

P��s-gradua����o em Ci��ncia e Tecnologia de Materiais - FC

Nanomateriais luminescentes de terras raras utilizando complexos de benzenotricarboxilatos como precursores

O material Y2O3:Eu3+ vem sendo usado comercialmente como lumin��foro vermelho desde da d��cada de 1960, em uma grande variedade de aplica����es devido ao seu elevado rendimento qu��ntico (pr��ximo de 100 %), elevada pureza de cor e boa estabilidade. Portanto, este trabalho prop��e um novo m��todo de s��ntese baseado nos complexos benzenotricarboxilatos (BTC) de terras raras trivalentes (RE3+) dopados com ��ons Eu3+. O objetivo principal �� produzir materiais luminescente RE2O3:Eu3+ a temperatura mais baixa (500 ��C) e em escala nanom��trica. Os complexos precursores [RE(BTC):Eu3+] e [RE(TLA)��n(H2O):Eu3+], onde RE3+: Y, Gd e Lu; BTC: ��cido trim��sico (TMA) e ��cido trimel��tico (TLA) foram calcinados em diferentes temperaturas de 500 a 1000 ��C, a fim de obter os materiais luminescentes RE2O3:Eu3+. Os complexos foram caracterizados por an��lise elementar de carbono e hidrog��nio, analise t��rmica (TG), espectroscopia de absor����o no infravermelho (FTIR), difra����o de raios-X - m��todo do p�� (XPD) e microscopia eletr��nica de varredura (SEM). Todos os complexos s��o cristalinos e termo est��veis at�� 460 ��C. Dados de fosforesc��ncia dos complexos de Y, Gd e Lu mostram que o n��vel T1 do an��on BTC3- tem energia acima do n��vel emissor 5D0 do ��on Eu3+, indicando que os ligantes podem atuar como sensibilizadores de energia intramolecular. O estudo das propriedades fotoluminescentes dos complexos dopados foi baseado nos espectros de excita����o e emiss��o e curvas de decaimento de luminesc��ncia. Ademais, foram determinados os par��metros de intensidades experimentais (Ωλ), tempos de vida (τ), taxas de decaimentos radiativo (Arad) e n��o-radiativo (Anrad). Os materiais luminescentes RE2O3:Eu3+ foram sintetizados de forma bem sucedida por meio do m��todo benzenotricarboxilatos calcinados a 500, 600, 700, 800, 900 e 1000 ��C, apresentando alta homogeneidade qu��mica e controle de tamanho de cristalito. Os nanomateriais foram caracterizados pelas t��cnicas de FTIR, XPD SEM e TEM revelando a obten����o dos materiais C-RE2O3:Eu3+ mesmo a 500 ��C. Os dados de XPD dos materiais confirmaram um aumento do tamanho dos cristalitos de 5 at�� 52 nm (equa����o de Scherrer) de em fun����o da temperatura de calcina����o de 500 a 1000 ��C, respectivamente, corroborados pelas t��cnicas de SEM e TEM. Os espectros de emiss��o de RE2O3:Eu3+ mostram uma banda larga atribu��da a transi����o interconfiguracional de transfer��ncia de carga ligante-metal (LMCT) em 260 nm, i.e. O2-(2p)&#8594Eu3+(4f6). Al��m disso, foram observadas linhas finas de absor����o devido as transi����es intraconfiguracionais 4f do ��on eur��pio (7F0,1&#85945LJ; J: 0, 1, 2, 3 e 4), como esperado. As propriedades fotoluminescentes dos lumin��foros foram baseadas nos espectros (excita����o e emiss��o) e curvas de decaimento luminescente. Os par��metros de intensidade experimental, tempos de vida, assim como as taxas de decaimentos radiativos e n��o radiativos foram calculados. As propriedades fot��nicas dos nanomateriais s��o consistentes com o s��tio de baixa simetria C2 ocupado pelo ��on Eu3+ no C-RE2O3:Eu3+, produzindo emiss��o vermelha dominada pela transi����o hipersens��vel 5D0&#85947F2 do ��on Eu3+ no sitio C2, ao inv��s do s��tio centrossim��trico S6. Al��m disso, os nanomateriais Y2O3:Eu3+ exibem caracter��sticas espectrosc��picas semelhantes e elevados valores de efici��ncia qu��ntica (η~91 %), compat��vel com os lumin��foros comerciais dispon��veis no mercado. Este novo m��todo pode ser utilizado para o desenvolvimento de novos nanomateriais contendo ��ons terras raras, assim como outros ��ons met��licos.