The hydrothermal transformation of solid geopolymers into zeolites

This study confirms the existence of several-nanometer ordered structures in amorphous geopolymers by HRTEM and SAED. To obtain low-ratio Si/Al geopolymers and to overcome the disadvantages posed by the use of impure raw materials, an amorphous Al2O3–SiO2 powder is first prepared using a sol–gel method in accordance with specific chemical compositions. The composition of the Al2O3–2SiO2–Na2O geopolymer is designed according to the NaA zeolite composition (Si/Al/Na atom ratio is 1:1:1), and the expected NaA zeolites are obtained from the Al2O3–2SiO2–Na2O geopolymer using a hydrothermal reaction. This process can be used to transform geopolymers into zeolites and provides a new preparation method for the self-supporting NaA zeolite membrane. The preliminary separation resulted in alcohol dehydration and shows that ethanol solutions can be powerfully dehydrated by pervaporation. A feed containing 90 wt.% ethanol at 20 °C and achieving a water flux of 2.67 kg m−2 h−1 with a separation factor H2O/EtOH ≈ 200 is found.

The results proved that the geopolymerization process could develop zeolite nucleation when the Si/Al ratio, water consumption and reaction conditions are appropriate. The most important finding is that zeolite crystals can grow in solid gels. The key evidence was obtained from 27Al and 29Si MAS NMR spectra test results. These results confirmed the same coordination structure of the Al2O3–2SiO2–Na2O geopolymers and the NaA zeolites and the existence of NaA zeolite seeds in the geopolymers, which can be grown into NaA zeolite crystals with the proper crystallization conditions. 27Al and 29Si MAS NMR spectra of the calcined powders, the geopolymers and the NaA zeolite: (a) Al2O3–SiO2 powders; (b) Al2O3–2SiO2–Na2O geopolymers; (c) NaA zeolites.Figure optionsDownload as PowerPoint slideHighlights
► This study confirmed the existence of nano-scale ordered structure in geopolymers.
► The Al2O3–2SiO2–Na2O geopolymer was changed to NaA zeolite by hydrothermal process.
► MAS NMR results proved that zeolite crystals can grow in solid gels.
► The paper provides new preparation method for self-supporting zeolite membrane.