Effect of cerium loading on structure and morphology of modified Ce-USY zeolites

Abstract

Este trabalho descreve detalhadamente o efeito da quantidade de cério na estrutura e morfologia da zeólita NH 4 USY. Ce-USY (2-25% m/m de CeO 2 ) foi obtido por impregnação úmida de CeCl 3 seguida de calcinação a 550 ºC por 8 h. Em quantidades baixas (2-10%), foi observado que as espécies de cério encontram-se nas posições de troca iônica na rede, enquanto em maiores teores (15-25%) pequenos agregados formaram-se na superfície da HUSY. Difratometria de raios X (XRD) mostrou apenas reflexões relacionadas à HUSY, confirmando a alta dispersão das espécies de cério, porém as análises por espectroscopia Raman com transformada de Fourier (FT-Raman) detectaram CeO x para os materiais acima de 10%. A reação do CeCl 3 com NH 4 USY produziu NH 4 Cl, o qual se decompõe em HCl, ocasionando a desaluminização da rede. Os materiais apresentaram um aumento da razão Lewis/Brønsted com o aumento da quantidade de cério, devido a interação do excesso de cério com os grupos OH da USY e consequente formação de espécies CeO x . This work describes comprehensibly the effect of cerium loading on the structure and morphology of NH 4 USY zeolite. The Ce-USY (2-25 wt.% of CeO 2 ) was obtained by wet impregnation of CeCl 3 followed by calcination at 550 ºC for 8 h. At low loadings (2-10%), cerium species are mainly located at ion exchange positions in the framework, whereas at higher loadings (15-25%), small aggregates were formed on the HUSY surface. X-ray diffractograms (XRD) exhibited only the reflections related to HUSY, demonstrating the high dispersion of cerium species, but Fourier transform Raman spectroscopy (FT-Raman) detected CeO x for the materials above 10%. Reaction of CeCl 3 with NH 4 USY produced NH 4 Cl, which decomposed to form HCl, leading to framework dealumination. The materials showed an increased Lewis/Brønsted ratio with increasing cerium loadings due to the interaction between the excess cerium and the OH groups of USY, and the consequent formation of CeO x species.Keywords: cerium USY zeolite, cerium oxide, ultra stable Y zeolite, solid-state ion exchange, 27 Al and 29 Si MAS NMR spectroscopy IntroductionFaujasite-type zeolites (X and Y) are largely applied in fluid catalytic cracking (FCC) of crude oil distillates and in hydrodesulfurization of fossil diesel oil and gasoline. 1 Y zeolites, either in their sodium or ammonium form, can be exchanged for rare earth ions to achieve the following main goals: (i) to prevent aluminum loss from the zeolite structure, which results in enhanced structural resistance to the severe hydrothermal conditions of the regeneration step of FCC, 2 and (ii) to increase the activity, because the introduction of rare earth species modifies the number of acidic protons through either hydrolysis or enhancement of ionic fields inside the zeolite structure. 3 Because rare earth cations modify the acidic properties of zeolites, their insertion has an effect on both thermal and mechanical stability, which are important for zeolite application to industrial processes such as pyrolysis, 4 oxidat...