NaA zeolite synthesis from geopolymer precursor
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NaA zeolite synthesis from geopolymer precursor Cui Xuemin, He Yan, Liu Leping, and Chen Jinyu, Guangxi Key Lab of Petrochemical Resource Processing and Process Intensification Technology, School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, P.R. China Address all correspondence to Cui Xuemin at [email protected] (Received 1 July 2011; accepted 26 September 2011)
Abstract This study confirms the existence of several nanometer-ordered structures in amorphous Al2O3–2SiO2–Na2O–7H2O geopolymers by highresolution transmission electron microscopy and selected area electron diffraction. The composition ratio of the geopolymers was designed according to the NaA zeolite composition (Si/Al/Na atom ratio of 1:1:1), and the expected NaA molecular sieve was achieved by the Al2O3– 2SiO2–Na2O–7H2O geopolymers through a hydrothermal reaction. This design concept was used to convert geopolymers into zeolite molecular sieves and provides a new preparation method for zeolite membranes or zeolite bulk materials.
“Geopolymer,” which was first described by Joseph Davidovits in 1978, is a non-crystalline or quasicrystalline gel that has a three-dimensional network.[1] Geopolymers are prepared using natural minerals or solid waste from the polymerization of silicon–oxygen tetrahedra and aluminum–oxygen tetrahedra.[1–5] Geopolymers possess favorable characteristics, such as high strength, excellent construction performance, and good resistance to corrosion from hot water and high temperature, due to their unique, three-dimensional oxide network structure that comes from inorganic polycondensation reactions. Geopolymers have broad applications in transportation, emergency repairs, metallurgy, and nuclear waste disposal.[1–5] Currently, raw materials with complex compositions, such as natural kaolin, clay, granulated blast furnace slag, and fly ash, are used for geopolymerization.[1–5] The structure and morphology of geopolymers are poorly understood because they exhibit poor crystallinity. The end products of geopolymerization reactions have unclear chemical compositions and crystal structures. Geopolymers were thus described as zeolite-like structures in many studies.[2–5] To overcome the disadvantages of using impure raw materials for geopolymer preparation, amorphous Al2O3–SiO2 powders were first prepared from raw materials using a sol–gel method with simple chemical compositions. When reacted with alkali solutions, these powders underwent geopolymerization to form products that are easily studied and are similar to those made from natural materials.[6,7] In this study, pure Al2O3–SiO2 and Al2O3–2SiO2 powders were first synthesized by a sol–gel method from tetraethoxysilane and aluminum nitrate according to designed Si/Al molar ratios. The dried gel powders were then calcined at 800 °C for 2 h. During this process, a sodium silicate solution was used as an alkali activator (molar ratio of SiO2:Na2O was
1:1), and the molar ratio of Al2O3 in the synthetic powders to that Na2O in the sodium
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