Structure and Dynamic Processes Involved in the Amorphization of Zeolite Y with Different Cations

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Student thesis: Doctoral ThesisDoctor of Philosophy

Original languageEnglish
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Award date2018
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Abstract

Zeolites, as promising raw materials for making glass, have been studied for many years. Their framework structures provide them with properties different from other mineral materials, such as low density, water absorption capacity and ion exchange property. Building from subunit cages, zeolites have a huge family with different properties and topologies. Sodium zeolite Y can be mass-produced and is easy to obtain which is therefore the studied material in this thesis. Although some researches have been done on glass formation from zeolites, many aspects in the amorphization process including thermodynamics, structural changes, dynamical changes need more study. Further more, study on glass formation from ion exchanged zeolite glass is now a new research topic.

In this thesis, ex situ temperature induced amorphization of sodium zeolite Y
(Na58Al58Si134O384·212H2O) has been studied. Multiple techniques were used to investigate different properties of it. Differential scanning calorimetry (DSC) was conducted to study the thermal properties of Na zeolite Y, finding that the structural collapse happens over a large temperature range covering 200K, in which the speed of collapse accelerates with higher temperature. Therefore, three target temperatures were chosen in this range to amorphize Na zeolite Y using different lengths of heating time to obtain samples with different degree of amorphization. These amorphous samples are then studied by X-ray powder diffraction (XRPD) and High resolution X-ray powder diffraction (HR-XRPD) to detect their structural changes during collapse. The data were analyzed by Rietveld Methods (TOPAS software), finding that the amorphization shrinks the cage topology, and makes the charge-compensating Na ion move to sodalite cages and double 6-fold rings (D6R), which are smaller than the super cages. Raman spectroscopy was also used to study the dynamics of Na zeolite Y and
its amorphization. The band of 4-fold and 6-fold rings which make up the cages can be clearly seen in the Raman spectra of crystalline zeolite Y, along with a small stretching mode peak feature. When amorphization take place, the peaks for 4- and 6-fold rings will first decrease then grow again suggesting the disappearance and regrowth of the rings. Some new rings like 5-fold rings appear after collapse similar to other dense alumina silicate glasses. Inelastic neutron scattering (INS) was also used to detect the dynamic properties of the Na zeolite Y, suggesting that the cage structure shrinks with amorphization.

Ion exchange can make a lot of changes to the properties of the Na zeolite Y before and after amorphization, therefore, Cu and Nd exchanged zeolite Y are also studied in this thesis. Studied by DSC, the ion exchanged zeolite Y shows higher collapse temperature and faster collapse within amorphization range. This can be caused by the higher field strength and different cation occupancy of the ion exchanged species. Samples with different degrees of amorphization were studied by XRPD and HR-XRPD similar to Na zeolite Y, finding that the exchanged ion shrinks the unit cell in proportion to their higher field strength and smaller ionic sizes in the order Na-Nd-Cu. However, the movement of the cation caused by the heat treatment during amorphization makes different influence on the Nd and Cu exchanged zeolite. The unit cell size of amorphous Cu zeolite Y becomes larger than crystalline Cu zeolite Y, but still smaller than the crystalline Na zeolite Y. This is due to high field strength of the Cu2+ cation and movement of the cations from supercage to sodalite cage. The change of the amorphous Nd zeolite Y is similar to that occurring in Na zeolite Y. Raman
spectroscopy for Cu zeolite Y showed quite different features to Na zeolite Y and Nd zeolite Y, the latter two being similar to one another. In Cu zeolite Y the 4-fold and 6-fold peaks diminish in intensity slightly and then grow, when new features like 5-fold rings appearing. The band for 6-fold rings dominates the features of 4- and 5-fold rings, indicating a change in topology for Cu exchanged zeolite Y.

This thesis includes suggestions for further work to confirm the findings. Taken
together, these findings contribute to the expanding literature of zeolite amorphization and may find commercialization applications.