Günter Engelhardt

High resolution 29Si n.m.r. of dealuminated and ultrastable Y-zeolites

The high resolution magic angle spinning 29 Si n.m.r. spectra of a number of dealuminated and ultrastable Y-zeolites have been studied. The well resolved spectra with narrow lines indicate a mainly regular structure and highly ordered Si-Al distribution of the zeolite framework. The quantitative distribution of the five building units Si(OSi) 4− n (OAl) n with n equal to 0−4 and the Si/Al ratio of the aluminosilicate lattice of the zeolites could be determined from the spectra. The degree of dealumination of the zeolite framework has been quantitatively estimated. By means of 1 H- 29 Si cross-polarization technique information on SiOH groups in defect centres of the dealuminated structure has been obtained.

High resolution 29Si n.m.r. of dealuminated Y-zeolites 1. The dependence of the extent of dealumination on the degree of ammonium exchange and the temperature and water vapour pressure of the thermochemical treatment

Abstract 29 Si n.m.r. spectra have been measured of a series of Y zeolites dealuminated by thermochemical treatment using deep-bed or steamed shallow-bed conditions. From spectra the relative contents of the Si(OSi) 4− n (OAI) n building units and the Si/Al ratio of the tetrahedral zeolite lattice have been estimated. It is shown that the extent of dealumination is limited by the degree of ammonium exchange of the starting material and depends on the temperature and partial pressure of steam used during the thermochemical treatment. Depending on the conditions of the treatment any desired Si/Al ratio within the limits of 2.5 to 8 can be obtained and no preferred compositions with definite Si/Al ratios have been observed.

High-resolution solid-state 29Si and 27Al n.m.r. of aluminosilicate intermediates in zeolite A synthesis

Abstract Solid aluminosilicate gels formed as intermediates in zeolite A synthesis were studied by 29Si and 27Al magic angle spinning n.m.r. The spectra reveal that the initial gel consists mainly of tetrahedral Si(OAl)4 and Al(OSi)4 building units which form an amorphous structure with alternating Si,Al ordering. With increasing time of heating of the reaction mixture the transformation of the amorphous gel phase into highly crystalline zeolite Na-A can be followed by characteristic changes of the n.m.r. spectra. By comparison with previous studies it is concluded that the composition and structural properties of the initially formed gel may be highly dependent on the specific mode of preparation.

The PI4+ cation has an extremely large negative 31P nuclear magnetic resonance chemical shift, due to spin–orbit coupling: A quantum-chemical prediction and its confirmation by solid-state nuclear magnetic resonance spectroscopy

We have used density-functional methods including explicit spin–orbit corrections, to calculate the 31P nuclear magnetic resonance (NMR) chemical shifts of the tetrahalophosphonium cations PX4+ (X=F, Cl, Br, I). The agreement between theory and experimental literature data for PF4+, PCl4+, and PBr4+ is good. For PI4+, the calculations predict an extremely negative (high-field) shift of approximately −520 ppm, due to particularly large spin–orbit contributions from the four heavy iodine substituents, transmitted to the phosphorus nucleus by a very effective Fermi-contact mechanism. No experimental data were available for PI4+. We have, therefore, prepared the compounds PI4AsF6, PI4SbF6, PI4AlI4, and PI4GaI4 and recorded their solid-state 31P NMR spectra, both with and without magic-angle spinning of the sample. Using the noncoordinating AsF6− and SbF6− anions, the measured isotropic shifts are −519 and −517 ppm, respectively, in good agreement with the predicted extreme value for the isolated cation. In co...

29Si MAS n.m.r. of aluminosilicate sodalites: Correlations between chemical shifts and structure parameters

Abstract The 29 Si n.m.r. chemical shifts of 16 sodalites of the general composition M 6+x [SiAlO 4 ] 6 A x ·nH 2 O (M = alkali metal cation, A = OH − , anion) cover a range of more than 20 ppm and show linear correlations with the SiOAl bond angles and the lattice constants. As a result of the different steric and coordination requirements of the various nonframework constituents, bond-angle variations between 126 and 160° occur, and these can be calculated directly from the chemical shift data. The 29 Si chemical shifts and the average SiOAl bond angles of Si(4Al) environments in several other zeolites also fit the same type of correlation that has been established here for the sodalites.

Synthesis, X-ray diffraction, and MAS n.m.r. characteristics of tetrahydroxoborate sodalite, Na8[AlSiO4]6[B(OH)4]2

Abstract Hydrothermal crystallization in the system Na2OSiO2Al2O3B2O3H2O reveals the dominant formation of tetrahydroxoborate sodalite, Na8[AlSiO4]6[B(OH)4]2, in the temperature range of 473–773 K at pressures of 100–150 MPa and concentrations of 17 Na2O:2 SiO2:Al2O3:xB2O3:yH2O (x = 0.1–10; y = 260–290). The high-temperature form (T ≥ 310 K) of tetrahydroxoborate sodalite shows cubic symmetry [space group P 4 3n, a 0 = 9.010(1) A ], whereas the superstructure diffraction pattern of the low-temperature form (T ≦ 270 K) has been indexed successfully by means of transformation of the cubic ∼ 9 A subcell to orthorhombic symmetry with a0 = 25.510(1), b0 = 12.750(1), and c 0 = 9.020(1) A . 29Si MAS n.m.r. shows complete ordering of the Si,Al atoms in the tetrahedral framework of tetrahydroxoborate sodalite. The observed single peak of the 11B MAS n.m.r. spectra correlates with the imbibition of [B(OH)4]− at the center of the sodalite cages. Substitution of B for Al in the tetrahedral sites of the sodalite framework and threefold coordinated boron within the sodalite cages or in the framework can be excluded.

High-resolution solid-state 29Si and 27Al n.m.r. of aluminosilicate intermediates in the synthesis of zeolite A. Part II

Abstract By means of 29Si and 27Al n.m.r. and chemical analysis it is shown that the properties of the aluminosilicate gels formed as intermediates from reactant mixtures of equal overall composition in zeolite NaA synthesis depend highly on the chemical composition of the starting silicate and aluminate solutions. Using a sodium silicate solution of cSio2=1.7 M and molar Na/Si ratio of 2 an aluminosilicate gel of Si Al =1 precipitates, whereas a silica-rich initial aluminosilicate gel of Si Al = 2.4 is formed from a silicate solution of cSio2 = 1.7 M and Na Si = 0.6 . Including 29Si n.m.r. studies of the starting silicate solutions, the main reasons for the formation of different initial gels and the consequences for the zeolite A crystallization are discussed.

Location of Na+ and Cs+ cations in CsNaY zeolites studied by 23Na and 133Cs magic-angle spinning nuclear magnetic resonance spectroscopy combined with X-ray structure analysis by Rietveld refinement

Solid-state 23Na and 133Cs nuclear magnetic resonance (NMR) spectroscopy was used in conjunction with X-ray powder diffraction (XRD) to study the location of Na+ and Cs+ cations in dehydrated CsNaY zeolites. Rietveld refinement of the structure of a 72% cesium-exchanged CsNaY zeolite dehydrated in vacuo at 623 K revealed sodium cations at SI and SII′ sites while cesium cations were found at SI, SI′ SII, and SIII sites. The results of the Rietveld refinement agree with the cation sitings derived from the 23Na and 133Cs magic-angle spinning (MAS) NMR spectra of this sample. Four lines identified in the 133Cs MAS NMR spectra of CsNaY zeolites were attributed to the distinct cation sites. In addition, the potential of 133Cs MAS NMR for the quantitative determination of site populations is demonstrated. 23Na and 133Cs MAS NMR is used to locate sodium and cesium cations in a series of CsNaY zeolites with different degrees of cesium exchange and dehydrated at different temperatures. With increasing Cs+ content, the Na+ cations at SI′/SII sites are exchanged first for Cs+. In addition, Na+ cations at SI sites are replaced by Cs+ cations at high cesium exchange levels. Upon increasing the calcination temperature of the 72% cesium-exchanged sample from 473 K to 773 K, further migration of Cs+ and Na+ to SI sites is observed. Calcination and rehydration of a 72% cesium-exchanged CsNaY zeolite leads to a partial migration of sodium cations from the sodalite cages into the supercages making them accessible to further ion exchange. Cesium exchange levels up to 90% were achieved by a three-fold exchange-calcination cycle of the 72% cesium-exchanged sample without damage to the zeolite structure.

High resolution 29Si n.m.r. of dealuminated Y-zeolites. 2. Silicon, aluminium ordering in the tetrahedral zeolite lattice

Abstract The Si, Al orderings in the tetrahedral framework of dealuminated Y zeolites covering Si/Al ratios in the range 2.5–5.8 have been investigated using relative populations of Si(OSi)4−n−(OAl)n[=Si(nAl)] building units determined from 29Si n.m.r. spectra. It is concluded that the Si, Al ordering is predominantly determined by the final Si/Al ratio reached after dealumination and is for a given Si/Al ratio independent of the type (thermochemical treatment under deep-bed or steamed shallow-bed conditions, Al extraction by organic acids) and external conditions (temperature, steam pressure) of the dealumination process used. The Si(nAl) populations derived from the n.m.r. spectra of zeolites with increasing Si/Al ratios are compatible with Si, AI ordering schemes generated by combined replacement of Al against Si and mutual exchange of the locations of a pair of adjacent Si and Al atoms. Relations of the proposed Si, Al orderings to the acidic properties of dealuminated Y zeolites are shortly discussed.

Solid-state nuclear magnetic resonance investigation of cation siting in LiNaLSX zeolites

Abstract The location of Li + and Na + cations in a series of dehydrated low-silica LiNaX zeolites (LiNaLSX, framework Si/Al ratio=1.0) were characterized by 7 Li and 23 Na magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. Depending on the Li + content, up to three lines were observed in the 7 Li MAS NMR spectra attributed to Li + cations on SI′, SII and SIII sites. The 23 Na MAS NMR spectra of the pure sodium form NaLSX and of LiNaLSX samples with low Li + contents contain up to five lines belong to Na + cations located on SI, SI′, SII, and two different SIII′ sites. LiNaLSX zeolites containing more than 40% of Li + show only a single narrow line in the 23 Na MAS NMR spectra attributed to mobile sodium cations. The populations of the different cation sites were determined from the relative line intensities of the MAS NMR spectra. Below about 70% Li + exchange, lithium cations are located only on sites SI′ and SII. Between 70% and 100% Li + content these sites are fully occupied by Li + , and the population of site SIII by Li + increases. It was found that the nitrogen-adsorption capacity correlates well with the occupation of Li + at site SIII.