Anuji Abraham

Influence of framework silicon to aluminium ratio on aluminium coordination and distribution in zeolite Beta investigated by 27Al MAS and 27Al MQ MAS NMR

27Al magic-angle spinning (MAS) and triple quantum (3Q) MAS NMR spectroscopic techniques were used to characterise zeolite Beta samples with framework Si/Al ratios between 9 and 215, obtained by synthesis in fluoride medium. A carefully controlled stepwise calcination procedure was adopted to obtain H-Beta. A partial resolution of the T-sites was observed in the 27Al MAS NMR spectra, the resolution increasing with increasing the Si/Al ratios. The relative intensity of these peaks varied gradually, with Si/Al ratio showing that the relative occupancy of the crystallographic T-sites changes with Si/Al ratio. The tetraethylammonium cation, used as an organic structure-directing agent in Beta synthesis, affects the average chemical shift of aluminium atoms in different T-sites. In H-Beta, octahedrally coordinated framework-associated aluminium atoms that could be quantitatively reverted into tetrahedral coordination were observed. The amount of this octahedral aluminium species decreases with increasing Si/Al ratio and it was absent for the two high-silica H-Beta samples with Si/Al = 110 and 215. Specific framework tetrahedral T-sites tend to convert to framework-associated octahedral sites during calcination. It is suggested that two aluminium T-sites, which are adjacent or close to each other, obeying the Loewenstein’s rule (i.e., no Al–O–Al linkage), are required for the hydrolysis of a Si–O–Al bond for the formation of octahedrally coordinated aluminium. The distribution of aluminium in zeolite Beta is a function of the Si/Al ratio and is non-uniformly distributed over the crystallographic T-sites.

Evidence for Dynamics on a 100 ns Time Scale from Single-and Double-Quantum Nitrogen-14 NMR in Solid Peptides

The indirect detection of 14N spectra via protons in the manner of heteronuclear multiple-quantum correlation (HMQC) allows one to obtain single- (SQ) and double-quantum (DQ) 14N spectra in solids. A comparison of the SQ and DQ line widths as a function of temperature with simulations reveals motions in the tripeptide AAG with rates on the order of 107 s(-1) at 49 degrees C.

1H MAS NMR Study of Cysteine-Coated Gold Nanoparticles

(1)H MAS NMR experiments were performed on gold nanoparticles coated with l-cysteine. The experiments show that l-cysteine molecules are zwitterions and support a structural model of cysteine forming two layers. The inner layer is composed of cysteine molecules chemisorbed to the gold surface via the sulfur atom. The outer layer interacts with the chemisorbed layer. The (1)H NMR suggests that the cysteine in the outer layer exhibits large amplitude motion about specific carbon-carbon bonds.

TRAPDOR double-resonance and high-resolution MAS NMR for structural and template studies in zeolite ZSM-5

A combination of (27)Al magic-angle spinning (MAS)/multiple-quantum (MQ) MAS, and (27)Al-{(14)N} TRAnsfer of Population in DOuble-Resonance (TRAPDOR) nuclear magnetic resonance (NMR) was used to study aluminium environments in zeolite ZSM-5. (27)Al-{(14)N} TRAPDOR experiments, in combination with (14)N NMR were employed to show that the two tetrahedral peaks observed in the (27)Al MAS/3Q-MAS spectra of as-synthesized ZSM-5 are due to aluminium atoms occupying crystallographically inequivalent T-sites. A (13)C-{(27)Al} TRAPDOR experiment was used to study the template, tetrapropyl ammonium bromide (TPABr), in the three-dimensional pore system of ZSM-5. The inequivalency of the methyl groups of TPA was observed in the (13)C-{(27)Al} TRAPDOR spectra of as-synthesized ZSM-5 and the motion of the methyl end of the propyl chain appeared to be more restricted in the sinusoidal channel than in the straight channel.

Stability of zeolite MCM-22 with varying Si al ratios; A solid-state NMR investigation

Determination of the local environment of aluminum in zeolites after various post-synthesis treatments is important for understanding structure-performance relationships. MCM-22 zeolites, synthesized with different silicon-to-aluminum ratios, were treated under various conditions: direct calcination in air, calcination in ammonia and oxygen, and ammonia treatment after calcination. The aluminum co-ordination in a zeolitic framework is highly sensitive to the treatments. The aluminum distribution over crystallographic T-sites in the zeolitic framework was followed by 27 Al MAS and 27 Al MQ-MAS NMR experiments. Loss of tetrahedrally co-ordinated aluminum on several T-sites and appearance of octahedrally coordinated aluminum was observed after calcination. Higher Si/Al ratios result in less octahedrally co-ordinated aluminum. This partial structural collapse can be restored by treatment with a base such as ammonia. An increased disorder in the framework is found after the conversion of octahedrally co-ordinated aluminum to tetrahedrally co-ordinated aluminum compared to the starting material.

13-P-23-Characterization of acidic sites in zeolites by heteronuclear double resonance solid state NMR

Publisher Summary This chapter presents the characterization of acidic sites in zeolites by heteronuclear double resonance solid-state nuclear magnetic resonance (NMR). The structural characterization of Bronsted acid sites in zeolites can be investigated through heteronuclear double resonance NMR experiments under magic angle spinning (MAS). With these experiments, the 1 H- 27 Al heteronuclear dipolar interactions are recoupled by using the rotational echo adiabatic passage double resonance (REAPDOR) technique. The signal evolution is followed and monitored under MAS, and the REAPDOR fraction is experimentally measured. Its time evolution is shown to reflect the differences in Bronsted acidity of three well-known zeolites—namely, LTL, LTY, and MOR.