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Publications (6)20.28 Total impact

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    ABSTRACT: 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.
    No preview · Article · May 2009 · Solid State Nuclear Magnetic Resonance
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    ABSTRACT: A series of well-defined catalysts based on platinum nanoparticles supported on amorphous silica–alumina with varying composition was prepared by incipient wetness impregnation. Quantitative structural characterization of alumina and aluminosilicate domains by 27Al (3Q) MAS NMR spectroscopy showed that the concentration of Brønsted acid sites determined by pyridine adsorption is related solely to the concentration of the aluminosilicate domain. However, only a very small fraction of the tetrahedrally coordinated aluminum led to the formation of Brønsted acid sites, because the negative charge on most of the tetrahedral Al–O was balanced by cationic aluminum oxide clusters. The preparation method led to small, uniform (0.6–0.8 nm) Pt particles on all aluminum-containing supports.
    No preview · Article · Oct 2007 · Journal of Catalysis
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    ABSTRACT: A combination of 27Al magic-angle spinning (MAS)/multiple quantum (MQ)-MAS, 13C-1H CPMAS, and 13C-{27Al} transfer of population in double-resonance (TRAPDOR) nuclear magnetic resonance (NMR) were used for the structural elucidation of the aluminum alkoxides aluminum ethoxide, aluminum isopropoxide, and aluminum tertiarybutoxide. Aluminum alkoxides exist as oligomers with aluminum in different coordinations. High-resolution 27Al MAS NMR experiments with high-spinning speed distinguished the aluminum atoms in different environments. The 27Al MAS NMR spectrum gave well-resolved powder patterns with different coordinations. Z-filter MQ-MAS was performed to obtain the number and types of aluminum environments in the oligomeric structure. 13C-1H CPMAS chemical shifts resolved the different carbon species (-CH3, =CH2, =CH-, and =C=) in the structures. 13C-{27Al} TRAPDOR experiments were employed to obtain relative Al-C dipolar interactions and to distinguish between terminal and bridging alkoxides in the crystallographic structures. The complete characterization of selected aluminum alkoxides using advanced NMR methods has evidenced the tetrameric structure for aluminum isopropoxide and the dimeric structure for aluminum tertiary-butoxide, as reported in the literature, and proposed a polymeric structure for aluminum ethoxide.
    No preview · Article · May 2006 · The Journal of Physical Chemistry B
  • A. Abraham · S. B. Hong · R. Prins · J. A. van Bokhoven
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    ABSTRACT: 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 27Al MAS and 27Al MQ-MAS NMR experiments. Loss of tetrahedrally co-ordinated aluminum on several T-sites and appearance of octahedrally co-ordinated 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.
    No preview · Article · Dec 2005 · Studies in surface science and catalysis
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    ABSTRACT: 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.
    No preview · Article · Jun 2004 · Physical Chemistry Chemical Physics
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    ABSTRACT: Bifunctional, Lewis, and Brønsted acidic molecular sieves were prepared by stepwise zinc ion exchange of zeolite BEA. The relation between the location of the Zn2+ cations in the zeolite structure and the Lewis acidity of the metal cations was explored. Several techniques were used for the detailed characterization of the material, including IR spectroscopy, with pyridine and acetonitrile as probe molecules, temperature programmed desorption, with ammonia and 2-propylamine as probe molecules, X-ray absorption spectroscopy, and 27Al MAS and MQMAS NMR spectroscopy. At low zinc concentrations (<0.15 Zn/Al) the cations are preferentially incorporated in the vicinity of two framework aluminum atoms. With increasing zinc loading (0.15 ≤ Zn/Al ≤ 0.26), additional cation sites are created at nearby framework aluminum pairs with two zinc cations being bridged by an oxygen atom. At higher zinc loading (0.26 < Zn/Al < 0.77) zinc oxide is formed in addition to the other two Zn2+ species.
    Full-text · Article · Mar 2004 · The Journal of Physical Chemistry B