Hubert Koller

High silica zeolites with three-dimensional systems of large pore channels

Abstract Several high silica zeolites with three-dimensional systems of large pore channels have been prepared in aqueous fluoride media: Beta, ITQ-7, ITQ-10 and ITQ-14. Pure silica beta can crystallize using up to 17 organic cations considerably different in size, shape, rigidity and C/N ratios. There are clear differences in the powder X-ray diffraction patterns of calcined beta materials prepared with different cations, suggesting structural differences between them. However, this technique cannot be readily used to get further insights into these differences. 19 F MAS NMR spectroscopy shows the existence of fluoride occluded in double four ring cages in most of the different betas, thus demonstrating for the first time the real occurrence in these intergrown materials of polymorphs different from A and B. Double four ring cages are also present in ITQ-7 and in the new ITQ-10 and ITQ-14 zeolites.

ITQ-12: a new microporous silica polymorph potentially useful for light hydrocarbon separationsElectronic supplementary information (ESI) available: details of the structure solution, Rietveld refinements in space groups C2/m and Cm and energy minimisation calculations in C2/m, Cm and C2. See http://www.rsc.org/suppdata/cc/b3/b306440a/

The new synthetic form of microporous crystalline silica, denoted as ITQ-12, shows a high potential for the separation of propane and propene from its mixtures.

Cation-induced transformation of boron-coordination in zeolites

The coordination of boron atoms and their association with counterions in dehydrated zeolites B-ZSM-5, B-Beta, and B-SSZ-24 have been studied by solid state NMR methods. 11B MAS NMR spectra show that boron occurs in both trigonal (B[3]) and tetrahedral (B[4]) coordination in the zeolite framework. The isotropic 11B chemical shift (δcs) allows one to distinguish between trigonal extra-framework boron species (δcs=18–19 ppm), Bnf[3], and trigonal framework boron (δcs=9.8–10.7 ppm), Bf[3]. Extra-framework boron species can be avoided if the zeolites are completely dehydrated with care. Large counterions such as sodium, lithium and ammonium ions stabilise boron in tetrahedral coordination, whereas protons cause a transformation to trigonal boron in the framework. 11B{23Na} and 11B{1H} rotational echo double resonance (REDOR) NMR reveals that B[4] is selectively associated with sodium ions, and Bf[3] is associated with protons in mixed H,Na zeolites. 1H{11B} REDOR experiments allow one to distinguish between protons in surface or defect silanols (δ=1.7–1.8 ppm) and protons close to Bf[3] (δ=1.9–2.0, 2.2–2.4, 2.7–3.0 ppm).

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.

27Al quadrupole interaction in zeolites loaded with probe molecules—a quantum-chemical study of trends in electric field gradients and chemical bonds in clusters

The electric field gradient (EFG) has been calculated in zeolite clusters at the aluminium site surrounded by four SiO4 tetrahedra. Density functional theory (DFT) with the 6-31G** basis set has been employed. Formation of a Brønsted acid site by protonation of one oxygen atom of the AlO4 tetrahedron perturbs the coordination of aluminium, i.e., the corresponding Al-O bond is considerably weaker than in the unprotonated case. This leads to a large EFG, and the calculated quadrupole coupling constant (QCC) for 27Al is 18.2 MHz. Different probe molecules were adsorbed on the Brønsted site. The hydrogen bond formed between the acid proton and the probe molecule weakened the zeolitic O-H bond. For conservation of the overall bond order of the oxygen atom, its bonds to the neighboring tetrahedral atoms (Si, Al) become stronger. As a consequence, the perturbation of the AlO4 tetrahedron and the EFG at the aluminium position decrease depending on the strength of the hydrogen bond. Perturbation of an oxygen atom of the AlO4 tetrahedron by accepting a hydrogen bond from the base molecule also affects the corresponding Al-O bond order. A linear correlation is found between the calculated QCC constants for 27Al and the Al-O bond orders of the oxygen atoms which are perturbed by protonation or by hydrogen bonds. A geometrical shear strain parameter and a simple electrostatic point charge model are less successful at predicting the trends in EFG which clearly shows the importance of the chemical bonds.

Strategies for extracting NMR parameters from Na-23 MAS, DOR and MQMAS spectra. A case study for Na4P2O7

The 23Na magic-angle spinning (MAS), double rotation (DOR) and multiple-quantum magic-angle spinning (MQMAS) NMR spectra of anhydrous sodium pyrophosphate, Na4P2O7, measured at five different Larmor frequencies (nuL) ranging from 105.8 MHz (corresponding to 400 MHz 1H frequency) to 211.6 MHz (800 MHz) are analysed and the complete set of NMR parameters (C(qcc), etaQ and delta(iso)) of the four crystallographically inequivalent sodium sites were determined with high accuracy. Different approaches of spectra evaluation are discussed and their results are compared. The most reliable results are obtained from a combined evaluation of five DOR and three MQMAS spectra but also from two DOR and one MAS spectra or even from a single MQMAS spectrum all data can be derived. It is shown that Na4P2O7 may serve as a useful reference material for experimental set-up and reliability tests of the various NMR experiments.

Evidence for Selective Association of Tetrahedral BO4 Units with Na+ and of Trigonal BO3 Units with H+ in Dehydrated Zeolite B‐ZSM‐5 from Solid‐State NMR Spectroscopy

Conclusive proof has now been obtained for the selective association of trigonal BO3 and tetrahedral BO4 units with H+ and Na+ ions, respectively, in B-ZSM-5 zeolites (the interactions are depicted in the picture). This was achieved with a combination of 11 B{23 Na}, 11 B{1 H}, and 1 H{11 B} rotational echo double resonance (REDOR) NMR spectroscopic experiments.

Solid State NMR of Porous Materials

Solid state NMR spectroscopy applied to the science of crystalline micro- and mesoporous silica materials over the past 10 years is reviewed. A survey is provided of framework structure and connectivity analyses from chemical shift effects of various elements in zeolites including heteroatom substitutions, framework defects and pentacoordinated silicon for zeolites containing fluoride ions. New developments in the field of NMR crystallography are included. Spatial host-guest ordering and confinement effects of zeolite-sorbate complexes are outlined, with special emphasis on NMR applications utilizing the heteronuclear dipolar interaction. The characterization of zeolite acid sites and in situ NMR on catalytic conversions is also included. Finally, the motion of extra-framework cations is investigated in two tutorial cases of sodium hopping in sodalite and cancrinite.

29Si NMR of Inorganic Solids

Since the first systematic study of silicates and aluminosilicates by high-resolution solid-state 29Si NMR was published in 1980 [1], the application of this method has grown rapidly and is fast becoming a powerful tool in the structural characterization of a wide range of silicates and other silicon-containing materials. The development is closely related to the great progress made in the last decade in the instrumentation, techniques and methods for the registration and evaluation of the 29Si NMR spectra and their structural interpretation. Nowadays, high magnetic field strengths and a wide selection of experimental techniques are available by modern solid-state NMR spectrometers. Since 29Si is a spin 1/2 nucleus, magic angle spinning (MAS) yields simple spectra with complete averaging of the chemical-shift anisotropy and is, therefore, the most important technique for the measurement of highly resolved 29Si NMR spectra of microcrystalline or amorphous solids. In proton containing samples, line broadening due to dipolar 29Si-1H interactions can be removed by high power proton decoupling, and cross-polarization (CP) may be used for signal enhancement and the detection of protons in close proximity to the silicon atom, e.g. in SiOH groupings. In addition, 2D 29Si MAS NMR techniques such as COSY and INADEQUATE have recently been applied to study three-dimensional framework connectivities.

The dynamics of hydrogen bonds and proton transfer in zeolites – joint vistas from solid-state NMR and quantum chemistry

The different aspects of zeolite Brønsted acid sites are reviewed from the perspective of solid-state NMR spectroscopy and quantum-chemical calculations. The strength of the combined use of these two methods is demonstrated. Special emphasis is dedicated to the structure and dynamics of hydrogen-bonded complexes of zeolites with H2O and CH3OH.