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Of molecules adsorbed on zeolites

For adsorption in zeolites, the biased Monte Carlo method as developed by Smit is an excellent method to determine the free energies of molecules adsorbed on zeolites [9bj. This method can be used to compute the concentration of molecules adsorbed on zeolites, as we discuss below. [Pg.16]

Nuclear Magnetic Resonance Studies of Molecules Adsorbed on Zeolites A, X, and Y... [Pg.430]

Ultraviolet Spectroscopic Investigation of Molecular Adsorption and Ionization of Molecules Adsorbed on Zeolites... [Pg.86]

A study was made of the ultraviolet spectra of benzene, alkyl-, amino-, and nitro-derivatives of benzene, diphenyl-amine, triphenylmethane, triphenylcarbinol, and anthra-quinone adsorbed on zeolites with alkali exchange cations, on Ca- and Cu-zeolites, and on decationized zeolites. The spectra of molecules adsorbed on zeolites totally cationized with alkali cations show only absorption bands caused by molecular adsorption. The spectra of aniline, pyridine, triphenylcarbinol, and anthraquinone adsorbed on decationized zeolite and Ca-zeolite are characterized by absorption of the corresponding compounds in the ionized state. The absorption bands of ionized benzene and cumene molecules appear only after uv-excitation of the adsorbed molecules. The concentration of carbonium ions produced during adsorption of triphenylcarbinol on Ca-zeolite and on the decationized zeolite depends on the degree of dehydroxyla-tion of the zeolite. [Pg.86]

The Role of Excitation in the Ionization of Molecules Adsorbed on Zeolites. Ultraviolet irradiation of benzene and cumene (10), adsorbed on Na-zeolite, did not lead to the appearance of absorption bands reflect-... [Pg.91]

The first mode of the high resolution C-NMR of adsorbed molecules was recently reviewed Q-3) and the NMR parameters were thoroughly discussed. In this work we emphasize the study of the state of adsorbed molecules, their mobility on the surface, the identification of the surface active sites in presence of adsorbed molecules and finally the study of catalytic transformations. As an illustration we report the study of 1- and 2-butene molecules adsorbed on zeolites and on mixed tin-antimony oxides (4>3). Another application of this technique consists in the in-situ identification of products when a complex reaction such as the conversion of methanol, of ethanol (6 7) or of ethylene (8) is run on a highly acidic and shape-selective zeolite. When the conversion of methanol-ethylene mixtures (9) is considered, isotopic labeling proves to be a powerful technique to discriminate between the possible reaction pathways of ethylene. [Pg.104]

Supramolecular concepts involved in the size- and shape-selective aspects of the channels and cavities of zeolites are used to control the selectivity of reactions of species produced by photoexcitation of molecules encapsulated within zeolites. The photochemistry of ketones in zeolites has been extensively studied. Photoexcitation of ketones adsorbed on zeolites at room temperature produces radical species by the Norrish type 1 reaction. A geminate (born together) radical pair is initially produced by photolysis of the ketone, and the control of the reaction products of such radicals is determined by the initial supramolecular structure... [Pg.217]

The spectrum of triphenylcarbinol, adsorbed on zeolites containing alkali cations, is characterized by the appearance of an absorption band at 350 m/x, which belongs neither to molecules in the state of molecular adsorption nor to the triphenylcarbonium ion. This indicates that there are adsorbed molecules in the same state of intermediate perturbation. The interpretation of this absorption band requires further investigation. [Pg.94]

For ethane, adsorption values of 4.04 wt% for DD3R were found while Na-A adsorbs 7.4 wt% at similar conditions (700 Torr, 25°C), so DD3R adsorbs 54.6% of the amount of ethane adsorbed on zeolite A. The accessible volume of DD3R is 0.48% compared with zeolite A, so based on the void volume, both materials show comparable adsorption of a non-polar molecule. The adsorption of ethylene amounts to 43.5 %, compared to zeolite A (3.65 wt% and 8.4 wt%, respectively). The slightly higher preference of zeolite A for an aUcene can be ejqrlained in terms of stronger interactions of an unsaturated hydrocarbon with the ionic nature of the zeolite A surfece. Calcium-exchanged type A zeolites were found to exhibit the same behaviour firr propane and propylene [38]. [Pg.427]

Spectral study of the donor-acceptor complexes of aromatic molecules adsorbed on zeolites. [Pg.614]

PFG (pulsed field gradient) NMR data determined the diffusion coefficients of methane in 3 MFI-type siliceous zeolite silicalite samples.640 The rotational motion of butane and pentane molecules adsorbed on zeolite ZK-5... [Pg.158]

We now report how theoretical methods can be used to provide information on the adsorption, diffusion, and reactivity of hydrocarbons within acidic zeolite catalysts. In Section A, dealing with adsorption, the physical chemistry of molecules adsorbed in zeolites is reviewed. Furthermore, in this section the results of hydrocarbon diffusion as these data are obtained from the use of the same theoretical methods are described. In Section B we summarize the capability of the quantum-chemical approaches. In this section, the contribution of the theoretical approaches to the understanding of physical chemistry of zeolite catalysis is reported. Finally, in Section C, using this information, we study the kinetics of a reaction catalyzed by acidic zeolite. This last section also illustrates the gaps that persist in the theoretical approaches to allow the investigation of a full catalytic cycle. [Pg.405]

The value of ext is determined by the local crystal structure and it changes as the structure evolves. The INS spectrum of benzene adsorbed on zeolite is shown in Fig. 5.8 ( 7.5.4.1) [17]. Here the benzene sits on an uncongested surface and intermolecular forces restrain the molecule only weakly. The basic spectral features of the benzene molecule remain visible in Fig. 5.8. The benzene is thus physisorbed and has not reacted with the surface. [Pg.203]

Photochemistry provides a powerful and versatile means of probing the mobility of species adsorbed on surfaces (9,10)- The basic reason for this power is that the absorption of light can produce, instantaneously on the time scale of diffusion, reactive intermediates whose chemistry is totally determined by their mobility on the surface of the porous solid. With proper selection of the reactant species, information concerning the mobility of the precursor reactive intermediates can be. locked into the structure of the stable, isolable products. In such cases, (11-15) product analysis provides a simple, yet elegant method to obtain information on the dynamics of motion of molecules adsorbed on the zeolites. [Pg.204]

THOMAS, J. K., Physical Aspects of Photochemistry and Radiation-Chemistry of Molecules Adsorbed on Si02, Gamma-A Os, Zeolites, and Clays , Chem. Rev. 1993, 93, 301-320. [Pg.12]

In the following sections, we shall begin our presentation with a discussion of the computational method, method developments and details of the calculations. Sections 3 and 4 are devoted to adsorption complexes of small probe molecules on ionic metal oxide MgO and a-AlaOa surfaces as well as in zeolites, respectively. Deposited transition metal particles on metal oxide surfaces and in zeolite cavities and complexes with them are addressed in Sections 5 and 6. Complexes of molecules adsorbed on nanoclusters of transition metals are considered in Section 7. The chapter will close with a summary and our outlook. [Pg.370]

Tn most applications of zeolites, surface properties and reactivity are of major importance. Infrared spectroscopy can give useful information on the constitution and surface properties of zeolites and how these are modified by various treatments. Changes in the spectra of the zeolite and of molecules adsorbed on the surface can yield direct information about the surface, how adsorbed molecules interact, and where molecules adsorb. [Pg.386]

In the field of heterogeneous catalysis, pyridine is frequently employed as a molecule probe of the surface acid sites of zeolites and other oxide catalysts, with which the amount and the strength of the acid sites are determined, for instance by infrared measurements of the intensity and the peah position of pyridine adsorbed on zeolite surfaces [9]. Despite its importance in this respect, details of the structure and the molecule orientation within the adsorbed pyridine layer on these nonconductive substrates are not known, due to the lack of techniques to yield such data before the advent of AFM, The series of our present work constitutes the first report on the determination, by means of. A.FM. of the array structure of the pyridine base adlayers on zeolite surfaces, and the estimation of the molecular orientation within the adsorbed layer. [Pg.186]

As important examples, IR investigations on SOj, H2S and HjO molecules adsorbed on zeolites should be mentioned. All three of them proved to be non-hnear, exhibiting the fundamental modes displayed in Fig. 41 (cf. [645]). [Pg.124]

Electronic spectroscopy has significantly contributed to the characterization of zeolites and of molecules adsorbed on their internal surface. A comprehensive review of UV-VIS spectroscopic investigations on zeolitic systems is too wide a scope to cover in this article. Instead different topics of application will be surveyed illustrated by selected examples taking into special account more recent work. As in zeolite science it has become fashionable to speak of host/guest chemistry, so first the host and later the guest will be discussed. [Pg.373]

Considerable interest has been shown recently in the chemistry of species adsorbed on zeolite surfaces. For example, a Raman spectroscopic investigation of zeolites and the molecules adsorbed thereon has been undertaken. All the zeolites examined give weak Raman spectra in nearly all cases the samples gave rise to an excessive background, and in order to minimize the problem it was necessary to use high-purity materials. Although these spectra are not as informative as the i.r. framework frequencies, they still show differences between zeolite structures. The Raman spectra of adsorbed... [Pg.345]

Thomas, J. K., Physical aspect of photochemistry and radiation chemistry of molecules adsorbed on SiOj, y-AljO, zeolites and clays, Chem. Rev., 93, 301, 1993. [Pg.1099]

Deuterium NMR has recently been used to study molecular motion of organic adsorbates on alumina (1.) and in framework aluminosilicates (2). The advantage of NMR is that the quadrupole interaction dominates the spectrum. This intramolecular interaction depends on the average ordering and dynamics of the individual molecules. In the present work we describe NMR measurements of deuterated benzene in (Na)X and (Cs,Na)X zeolite. [Pg.485]

CoSx-MoSx/NaY exhibited doublet bands at 1867 and 1807 cm, accompanying a weak shoulder peak at ca. 1880 cm. These signals are apparently assigned to those of NO molecules adsorbed on Co sulfides. No peaks ascribable to e NO adsorption on Mo sulfide sites were detected at all. What is important in Fig.7 is that in CoSx-MoSx/NaY, coordinative unsaturation sites are present only on the Co sites in spite of the coexistence of the same amount of Mo sulfide species in the zeolite cavities. These results clearly support that the Co sites in CoSx-MoSx/NaY play major roles in the HYD and HDS reactions. [Pg.509]


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