Zeolites single crystals

The title Spectroscopy in Catalysis is attractively compact but not quite precise. The book also introduces microscopy, diffraction and temperature programmed reaction methods, as these are important tools in the characterization of catalysts. As to applications, I have limited myself to supported metals, oxides, sulfides and metal single crystals. Zeolites, as well as techniques such as nuclear magnetic resonance and electron spin resonance have been left out, mainly because the author has little personal experience with these subjects. Catalysis in the year 2000 would not be what it is without surface science. Hence, techniques that are applicable to study the surfaces of single crystals or metal foils used to model catalytic surfaces, have been included. 

Scandella L, Binder G, Mezzacasa T, Gobrecht J, Berber R, Lang HP, Gerber CH, Gimsewski JK, Koegler JH, and Hansen JC. Combination of single crystal zeolites and microfabrication Two applications towards zeolite nanodevices. Micropor Mesopor Mater 1998 21(4-6) 403 09. 

A schematic picture of different t5q)es of pores is given in Fig. 9.1 and of main types of pore shapes in Fig. 9.2. In single crystal zeolites the pore characteristics are an intrinsic property of the crystalline lattice [3] but in zeolite membranes other pore types also occur. As can be seen from Fig. 9.1, isolated pores and dead ends do not contribute to the permeation under steady conditions. With adsorbing gases, dead end pores can contribute however in transient measurements [1,2,3]. Dead ends do also contribute to the porosity as measured by adsorption techniques but do not contribute to the effective porosity in permeation. Pore shapes are channel-like or slit-shaped. Pore constrictions are important for flow resistance, especially when capillary condensation and surface diffusion phenomena occur in systems with a relatively large internal surface area. 

A theoretical model describing gas transport in microporous (single crystal zeolite) membranes in regions a and bi of Fig. 9.20 was proposed by Barrer [85] and is schematically given in Fig. 9.21. 

Membrane Wicke Kallenbach Single crystal Zeolite membrane Catalyst effectiveness factor... 

Fodor, D., Pacosovd, L., Krumeich, F., and van Bokhoven, J-A. (2014) Facile synthesis of nano-sized hollow single crystal zeolites under mild conditions. Chem. Commun., 50, 76-78. 

In the as-synthesized MFI-crystals the tetrapropylammonium (TPA) ions are occupying the intersections between the straight (parallel) and the sinusoidal channels of the zeolite, thus providing an efficient pore filling. The detailed structure of as-synthesized MFI-TPA has been elucidated by X-ray single crystal analysis (ref. 3). Also the combination tetrabutyl-Ztetraethylammonium can be applied as template in MFI-synthesis. A 1 1 build-in is found then (Fig. 1). When only tetrabutylammonium is available as template, the MEL (ZSM-11) lattice is formed with another distance between the channel intersections. 

In order to get the pore system of zeolites available for adsorption and catalysis the template molecules have to be removed. This is generally done by calcination in air at temperatures up to 500 °C. A careful study (ref. 12) of the calcination of as-synthesized TPA-containing MFI-type single crystals by infrared spectroscopy and visible light microscopy showed that quat decomposition sets in around 350 °C. Sometimes special techniques are required, e.g. heating in an ammonia atmosphere (ref. 13) in the case of B-MFI (boron instead of aluminum present) to prevent loss of crystallinity of the zeolite during template quat removal. 

Surface science studies have generated much insight into how hydrocarbons react on the surfaces of platinum single crystals. We refer to Somorjai [G.A. Somor-jai. Introduction to Surface Chemistry and Catalysis (1994), Wiley, New York] for a detailed overview. Also, the reactions of hydrocarbons on acidic sites of alumina or on zeolites have been studied in great detail [H. van Bekkum, E.M. Flanigan and J.C. Jansen (Eds.), Introduction to Zeolite Science and Practice (1991), Elsevier, Amsterdam],... 

One of the most promising techniques for studying transition metal ions involves the use of zeolite single crystals. Such crystals offer a unique opportunity to carry out single crystal measurements on a large surface area material. Suitable crystals of the natural large pore zeolites are available, and fairly small crystals of the synthetic zeolites can be obtained. The spectra in the faujasite-type crystals will not be simple because of the magnetically inequivalent sites however, the lines should be sharp and symmetric. Work on Mn2+ in hydrated chabazite has indicated that there is only one symmetry axis in that material 173), and a current study in the author s laboratory on Cu2+ in partially dehydrated chabazite tends to confirm this observation. 

Tin incorporated mesoporous Sn-MFI catalysts with different Si/Sn ratio using microwave were synthesized with carbon as hard template. These tin MFI catalysts were characterized using various physicochemical techniques XRD reviled the formation of more crystalline MFI structures which was further supported by the SEM and TEM imaging which clearly showed well ordered zeolite single crystals with mesoporosity. The N2 sorption isothers reviled the formation of bimodal mesoporous zeolites and the presence of tin in tetrahedral site was confirmed by FTIR (970 cm 1) and XPS (3ds/2 and 3 dj 2 electronic states). The thus synthesized mesoporous Sn-MFI catalysts with different Si/Sn ratios were used in studying the catalytic Baeyer-Villiger Oxidation (BVO) of cyclic ketones... 

The water and A1 contents decrease while the amount of M cations per unit cell of zeolite increases. For Li and Na,the morphology consists of clusters of polycrystalline aggregates. Better outlined single crystals are observed for K and Rb and additional pronounced twinning appears for (Cs)ZSM-5 (Figure 10). 

McCusker, LB. (1991) Zeolite crystallography, structure determination in the absence of conventional single-crystal data. Acto Cryst., A47, 297-313. 

The location of p-xylene in a single crystal of zeolite H-ZSM-5 with a new, sorbate-induced, orthorhombic framework symmetry. Acta Crystal., B45, 423 31. 

In the case of Ox + in zeolite L, a half-cone angle a of 72° was obtained from quantitative measurements on single crystals [15]. The orientation of the transition moments with respect to the zeolite channels can be determined directly... 

In Fig. 12 in Ref 25, fluorescence microscopy images of different dye-loaded zeolite L single crystals are shown. Each line consists of three pictures of the same sample, but with different polarizations of the fluorescence observed. In the first one, the total fluorescence of the crystals is shown, and in the others, the fluorescence with the polarization direction indicated by the arrows is displayed. The zeolite was loaded with the following dyes (A) Py+, (B) PyGY", (C) PyB +, (D) POPOP (see Table 1). Most crystals show a typical sandwich structure with fluorescent dyes at the crystal ends and a dark zone in the middle. This situation can be observed when the diffusion of the dyes in the channels has not yet reached its equilibrium situation. It illustrates nicely how the molecules penetrate the crystals via the two openings on each side of the one-dimensional channels. 

In our laboratory we have obtained a number of inclusion compounds using cadmium(II) cyanide or isopolycyanocadmate(II) as the hosts and several organic molecules as the guests. The host structures so far determined by single crystal diffraction experiments have been classified into three groups in general, 3-cristobalite-like [1], clay-like, and zeolite-like [2]. 

The five years since last considering specifically recent developments in X-ray and neutron diffraction methods for zeolites [1] have witnessed substantial progress. Some techniques, such as high resolution powder X-ray diffraction using synchrotron X-rays, have blossomed from earliest demonstrations of feasibility to widespread and productive application. Others, such as neutron powder diffraction, have shown steady progress. For still others, notably microcrystal diffraction, a variety of circumstances have contributed to extended gestation periods. Additionally, opportunities scarcely considered earlier (such as single crystal Laue diffraction, and certain developments in computer simulations that complement diffraction work) now command broad attention and warrant the commitment of substantial further investment. 

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