Deuterium solid-state NMR study of the dynamics of molecules sorbed by zeolites

Eckman, R.; Vega, Alexander J.

doi:10.1021/j100410a044

Cited by 112 publications

(46 citation statements)

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“…This is probably because more limited molecular excursions are permitted for these larger molecules within the various zeolite media than for benzene, where free rotation is allowed. These conclusions are in accord with those drawn from 2 H-quadrupole-NMR measurements [145][146][147] . The larger mesitylene molecules can penetrate zeolite X and (more slowly) mordenite, but are excluded from the internal pores of ZSM5-silicalite.…”

Section: Cyclohexadienyl and 25-dimethylcyclohexadienyl Radicals In supporting

confidence: 90%

Muonium–the Second Radioisotope of Hydrogen: A Remarkable and Unique Radiotracer in the Chemical, Materials, Biological and Environmental Sciences

Rhodes

2012

Science Progress

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Muonium (Mu), may be regarded as a radioactive hydrogen atom with a positive muon as its nucleus, and is formed in a range of media which are irradiated with positive muons. This exotic atom can be considered as a second radioisotope of hydrogen, along with tritium. Addition of this light atom (with a mass 1/9th that of a normal hydrogen, protium, atom) to unsaturated organic molecules forms free radicals, in which the muon serves as a radioactive and magnetic probe of their kinetic and structural properties. Suitable examples are chosen to illustrate the very large functionality of organic radicals which have been measured using muons and various methods of μSR, where μ stands for muon, S for spin and R may refer to rotation, resonance or relaxation. The principal techniques illustrated are transverse-field muon spin rotation (TF-μSR), avoided level crossing muon spin resonance (ALC-μSR) and longitudinal-field muon spin relaxation (LF-μSRx). Structural studies of radicals, the determination of mechanisms for radical formation, the measurement of radical stabilisation energies, the determination of the kinetics of reactions of free muonium atoms and of free radicals have all been accomplished using TF-μSR methods. It is further shown that TF-μSR is most useful in measuring radical reaction rates in non-aqueous media, to provide information of relevance to cell membrane damage and repair. Muonium may further be used as a mechanistic probe since it determines a true pattern of H-atom reactivity in molecules, against which results from similar radiolysed materials may be compared. [In many solid materials that are exposed to ionising radiation, apparent H-atom adduct radicals are detected but which originate from charge-neutralisation of positive holes (radical cations) and ejected electrons, without free H-atoms being formed. DNA is the superlative example of this. Free H-atoms normally feature in the province of radiolysed aqueous media]. The applications of ALC-μSR and LF-μSRx in studying the reorientation of reactive radicals on reactive surfaces forms the substantive proportion of the review: considered specifically are radicals sorbed in zeolites, in clays and in porous silica, in porous carbons and on ice-surfaces, in connection with their role as intermediates in catalytic systems, particularly hydrocarbon cracking and oxidation processes, and in atmospheric aerosol chemistry. The formation of muonium and other muon species in cation-exchanged zeolite-X samples are also considered, according to the evidence of longitudinal field repolarisation measurements. Finally, mention is given of the use of μSR techniques for studying radicals in the gas-phase.

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Section: Cyclohexadienyl and 25-dimethylcyclohexadienyl Radicals In supporting

confidence: 90%

Muonium–the Second Radioisotope of Hydrogen: A Remarkable and Unique Radiotracer in the Chemical, Materials, Biological and Environmental Sciences

Rhodes

2012

Science Progress

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“…Although there are some able to these systems. disagreements, following highly unusual pure com Microscopic investigations such as NMR and XRD ponent equilibrium behavior were commonly ob (Eckman and Vega, 1986;Mentzen and Bosselet, served. (1) Isotherm shape changes from type-I to IV 1988;Reischman et al, 1988;etc. ) identify two types of with decreasing temperature ; Guo et sorbate molecules with different mobility in pen aI., 1989).…”

Section: Introductionmentioning

confidence: 99%

Adsorption equilibrium of benzene—p-xylene vapor mixture on silicalite

Talu

1994

Chemical Engineering Science

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Abstract-Ads()rpti()n equilibrium ()f benzene-p-xylene vapor mixture ()n silicalite is measured at 70¢C with a specially designed cyclic volumetric apparatus at pressure levels of 2.53 and 1.20 kPa. The isobaric isotherms are S-shaped, and selectivity curves at different pressures cross over; the p-xylene selectivity at 2.53 kPa is higher than that at 1.20 kPa over a certain c()mposition range. The heterogeneous ideal adsorbed solution (HIAS) model implemented on two patches qualitatively predicts these highly unusual behavior. The observed extraordinary phenomena are attributed to structural heterogeneity, a result of the light-fit of sorbate molecules in silicalite pores. The success of HIAS attests to tlte paramount importance of adequate representation of heterogeneity in adsorption models.

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“…It must be taken into account that the metal cations will create a surface heterogeneity which also can contribute to the line width via distribution over coupling constants or reorientation dynamics. This is supported by various NMR experiments on HZSM-5 with a benzene loading of two molecules per unit cell where a variety of different types of restricted motion within the zeolite framework was observed [9][10][11].…”

Section: Resultsmentioning

confidence: 66%