Comparison between leached metakaolin and leached diatomaceous earth as raw materials for the synthesis of ZSM-5
Inexpensive raw materials have been used to prepare ZSM-5 zeolites with SiO2/Al2O3 molar ratios in the range 20 – 40. Kaolin or Bolivian diatomaceous earth was used as aluminosilicate raw materials and sodium hydroxide and n- butylamine were used as mineralizing agents and template. Dealumination of the raw materials by acid leaching made it possible to reach appropriate SiO2/Al2O3 ratios and to reduce the amount of iron and other impurities. After mixing the components and aging, hydrothermal treatment was carried out and the products were recovered The results clearly show for the first time that well-crystallized ZSM-5 can be directly prepared from leached metakaolin or leached diatomaceous earth using sodium hydroxide and n-butylamine as mineralizing agents and template under appropriate synthesis conditions. A longer induction time prior to crystallization was observed for reaction mixtures prepared from leached diatomaceous earth, probably due to slower digestion of the fossilized diatom skeletons as compared with that for microporous leached metakaolin. The use of leached diatomaceous earth allowed higher yield of ZSM-5 crystals within comparable synthesis times. However, low amounts of Mordenite formed, which was related to the high calcium content of diatomaceous earth. Another considerable advantage of diatomaceous earth over kaolin is that diatomaceous earth does not require heat treatment at high temperature for metakaolinization.
Solution-mediated growth of NBA-ZSM-5 crystals retarded by gel entrapment
Abstract The synthesis of flat tablet-shaped ZSM-5 crystals from a gel using metakaolin as aluminosilicate source and n -butyl amine as structure directing agent was investigated. The evolution inside the solid phase was characterized by X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, thermogravimetry and mass spectrometry. A kinetic study indicated that the nucleation of the majority crystals occurred concurrently with the formation of the gel upon heating the starting liquid suspension. Microstructural evidences undeniably showed that the gel precipitated on ZSM-5 crystals and mineral impurities originating from kaolin. As a result, crystal growth was retarded by gel entrapment, as indicated by the configuration and morphology of the embedded crystals. The results presented herein are harmonized with a solution-mediated nucleation and growth mechanism. Our observations differ from the autocatalytic model that suggests that the nuclei rest inside the gel until released when the gel is consumed. Our results show instead that it is crystals that formed in an early stage before entrapment inside the gel that rest inside the gel until exposed at the gel surface. These results illustrate the limitation of the classical method used in the field to determine nucleation profiles when the crystals become trapped inside the gel.
The synthesis of ferrierite, ZSM-5, and Theta-1 in the presence of diethanolamine: Experimental
Abstract The synthesis of zeolites from gels containing silica, alumina, potassium and/or sodium hydroxide or chloride, diethanolamine, and water was studied. Zeolites Theta-1, ZSM-5, ferrierite, and mordenite, and the dense phase cristobalite, were produced by varying the SiO 2 Al 2 O 3 ratio, the alkalinity and the alkali metal cation. The effect of varying the SiO 2 Al 2 O 3 ratio was found to be of great importance. The ranges of SiO 2 Al 2 O 3 ratios over which each zeolite is formed have been established, and discrete upper and lower limits were found to exist in the cases of ZSM-5, Theta-1, and ferrierite. The formation of these materials was followed under a variety of conditions using X-ray diffraction, thermal analysis, MAS n.m.r. and AAS. The kinetics of formation of these zeolites were investigated, as were the compositions and characteristics of the solid phases collected during the syntheses. The location and charge of the diethanolamine present within the solid materials were found to be significant. Diethanolamine is considered to have a pore-filling role and also a charge-compensatory role when the amine group is protonated, and hence its overall effectiveness as a stabilizer depends upon the type of zeolite structure and the aluminum content.