Local Order of Amorphous Zeolite Precursors from ~(29)Si{~1H} CPMAS and ~(27)A1 and ~(23)Na MQMAS NMR and Evidence for the Nature of Medium-Range Order from Neutron Diffraction
Amorphous aluminum silicates,isolated from hydrothermal reactions used to form crystalline zeolite A,have been studied using a number of techniques.NMR studies using ~(29)Si{~1H} cross-polarization (CP) MAS and ~(27)A1 and ~(23)Na multiple-quantum (MQ) MAS methods provide information about the local atomic structure of the solids.Materials isolated at the early stages of reaction are silicon rich,and some of the aluminum is found in six-coordinate sites as in the amorphous alumina starting material.~(29)Si-{~1H} CPMAS NMR spectra show that a range of silicon environments are present in the amorphous solids.The line widths in the ~(27)A1 MQMAS NMR spectra of the amorphous materials suggest that a range of Al-O distances and/or Si-O-Al angles is present,rather than the well-defined units present in zeolite A.Although the average ~(23)Na isotropic chemical shift suggests a coordination number of close to six for all sodium throughout the crystallization,the sodium in the amorphous precursor phase can be distinguished from that in the crystalline zeolite.The neutron diffraction study of the amorphous aluminosilicates reveals information about atomic order over longer length scales.The first sharp diffraction in the neutron diffraction data indicates,for the first time,that the medium-range order of the amorphous precursors changes prior to crystallization.The shift in the T-O peak (T is a tetrahedral atom:Si or Al) in the radial distribution function is consistent with the incorporation of aluminum into the silicate network at the early stages of reaction,and this is accompanied by the growth of an Na-O peak,as more charge-balancing cations are required.The broadness of the T-T nonbonded correlation for the amorphous solid,compared with that seen for crystalline zeolite A,indicates that there are no well-defined secondary building units present before the formation of zeolite crystals.
展开▼
