Supports zeolites

Hydrocracking is catalyzed by substances that promote cracking and hydrogenation together. In commercial use are Ni, Co, Cr, W, and V or their oxides, presulfided before use, on acid supports. Zeolites loaded with palladium also have been used. 

Although hollow fibers are thought to be an excellent candidate to be used as support-they are cheap and have a very high surface area to volume (>1000 m m ) - very few reports on hollow-fiber-supported zeolite membranes exist in the open literature. For zeohte membranes, ceramic hollow fibers are preferred because of their mechanical and thermal stability. Recently, Alshebani... 

Previously, we have developed several techniques for platinum supported zeolite catalysts to improve the benzene product purity, including on-line sulfiding [3], precoking [6], and dual-bed catalyst system [7]. We report herein an in-depth investigation on the synergism of proton zeolite and platinum supported ZSM-12 catalyst (Pt/Z12) in a cascade dual-catalyst system. 

A dual-bed catalyst system has been developed to tackle the key problems in benzene product impurity during heavy aromatics transalkylation processing over metal-supported zeolite catalysts. It was found that by introducing zeolite H-Beta as a complementary component to the conventional single-bed Pt/ZSM-12 catalyst, the cascaded dual-bed catalyst shows synergistic effect not only in catalytic stability but also in adjustments of benzene product purity and product yields and hence should represent a versatile catalyst system for heavy aromatics transalkylation. 

Stratakis, M. and Rabalakos, C. (2001). Chemoselective hydroperoxidation of alkenylarenes within thionin-supported zeolite Na-Y. Tetrahedron Lett. 42, 4545-4547... 

Recently, we reported that an Fe supported zeolite (FeHY-1) shows high activity for acidic reactions such as toluene disproportionation and resid hydrocracking in the presence of H2S [1,2]. Investigations using electron spin resonance (ESR), Fourier transform infrared spectroscopy (FT-IR), MiJssbauer and transmission electron microscopy (TEM) revealed that superfine ferric oxide cluster interacts with the zeolite framework in the super-cage of Y-type zeolites [3,4]. Furthermore, we reported change in physicochemical properties and catalytic activities for toluene disproportionation during the sample preparation period[5]. It was revealed that the activation of the catalyst was closely related with interaction between the iron cluster and the zeolite framework. In this work, we will report the effect of preparation conditions on the physicochemical properties and activity for toluene disproportionation in the presence of 82. ... 

Fig. 1 Correlation between Fe " " concentration of the preparation solution and physicochemical properties as well as catalytic properties of the Fe supported zeolite.

Three Fe supported zeolites were prepared by modifying NH Y with 0.25M Fe(N03)3 at various temperatures from 293K to 373K, Figure 2 shows the influence of preparation temperature on physicochemical properties and catalytic activity of the obtained catalysts. 

Thus moderate temperature as well as Fe concentration must be carefully selected for the preparation of the active Fe-supported zeolite. 

Systematic diffusion experiments were also conducted with self-supported zeolite wafers (7-14 mg cm ) which were activated at 10 Pa and 675 K for 1.5 h. Prior to contact with the sorbate, the IR cell was filled with dried helium as a carrier gas. Subsequently, one or two components (benzene or ethylbenzene), carried by helium bubbling through thermostatted saturators, could be admitted. A system of mass-flow controllers allowed for an independent change of the partial pressures while the total pressure could be kept constant [22]. The time required to pass the sorbate from the inlet valve to the place of the zeolite wafer was about 4 s. IR spectra were obtained in intervals as short as 0.37 s. 

TABLE 22. Photooxygenation of isobutenylarenes 20 and 137-146 within the thionin-supported zeolite Na-Y... 

The asymmetric photooxygenation of alkenes in chiral-supported zeolites was also reported [144,145] ... 

As an example of hybridization of zeolites with cellulose derivatives, self-supporting zeolite membranes with a sponge-like architecture and zeolite microtubes were prepared by using CA filter membranes as a template [154]. The hierarchical structure with sub-nanometer- to micrometer-sized pores is a characteristic of great promise for a wide range of applications such as catalysis, adsorption, and separation. There was also an attempt to prepare alginate membranes incorporated with zeolites, e.g., for pervaporation separation of water/acetic acid mixtures [155]. 

FTIR spectra of the self-supported zeolite discs were collected at a resolution of 2 cm 1 using a Nicolet Magna 550 FTIR spectrometer. Pyridine was used as a probe molecule to characterise Broensted and Lewis acid sites. Prior to all FTIR experiments the samples were activated under vacuum at 400°C for 16 h. The detailed experimental procedure is described elsewhere.12 The IR spectra of the reduced samples were collected at ambient temperature after reduction of the catalysts by hydrogen at 500°C (Ph2 = 50 torr, duration of treatment = 1 h). 

The section on catalyst preparation (Chapters 8 and 9) is concerned with the preparation of catalyst supports, zeolites, and supported catalysts, with an emphasis on general principles and mechanistic aspects. For the supported catalysts the relation between the preparative method and the surface chemistry of the support is highlighted. The molecular approach is maintained throughout. 

The range of pore sizes important to good catalytic function are from around 1OA in the supported zeolite to perhaps 100,000 A (10 pm) in the zeolite/silica-alumina composite. The technique of gas adsorption is of little use beyond about 250A, so that by far the largest range of important pore sizes (and related interactions between pores which constitute the pore structure of the particle) are assessable only through the mercury porosimetry technique. Many practitioners in catalyst characterisation claim that... 

Supported metal oxide catalysts are a new class of catalytic materials that are excellent oxidation catalysts when redox surface sites are present. They are ideal catalysts for investigating catalytic molecular/electronic structure-activity selectivity relationships for oxidation reactions because (i) the number of catalytic active sites can be systematically controlled, which allows the determination of the number of participating catalytic active sites in the reaction, (ii) the TOP values for oxidation studies can be quantitatively determined since the number of exposed catalytic active sites can be easily determined, (iii) the oxide support can be varied to examine the effect of different types of ligand on the reaction kinetics, (iii) the molecular and electronic structures of the surface MOj, species can be spectroscopically determined under all environmental conditions for structure-activity determination and (iv) the redox surface sites can be combined with surface acid sites to examine the effect of surface Bronsted or Lewis acid sites. Such fundamental structure-activity information can provide insights and also guide the molecular engineering of advanced hydrocarbon oxidation metal oxide catalysts such as supported metal oxides, polyoxo metallates, metal oxide supported zeolites and molecular sieves, bulk mixed metal oxides and metal oxide supported clays. 

In these samples the electron deficiency appears clearly related to the proton concentration of the supporting zeolite. The results indicate a direct interaction of Pd with protons, in accordance with the metal-proton adduct model. 

Steam reforming was the primary reaction over these nickel catalysts. The presence of hydrocarbons (G2 to G5) which would indicate cracking reactions occurred to the extent of less than 10% in the reaction products. The presence of methane, which would indicate partial reforming, did not exceed 5% in the reaction products. There does not appear to be any significant difference in product selectivity for the n-hexane steam reforming reaction over nickel on the 2 quite different supports—zeolite vs. alumina. Carbonaceous residues accumulated in the case of all the nickel catalysts where reforming activity was sustained and the carbon deposition on the zeolite catalysts compared favorably with G56. 

It is quite challenging to rmderstand in what way the zeolite influences the metal compared to other supports. The electronic changes that could be induced by the pore system are quite subtle and metal particle size effects may overrule these changes [200]. hi comparison to metal-support interactions on macroporous oxides, the interaction between metal particles and the supporting zeolite matrix seems more pronounced. This may be because the metal particles interact with the zeolite lattice over a substantial fraction of their surfece. It has also been suggested that in addition to the intrinsic electronic effects due to the small size of the metal particles in the zeolite cage, a modification of the electronic structure of the metal by the acidic zeolite framework has to be considered [201,202]. 

In spite of the progress made in the field of supported zeolite membranes during the last decade, a number of points still need to be explored or studied further, including... 

One of the simplest ways of extending this single-pore model is to assume that variations in the size of pore spaces can be represented by a variable-diameter assembly of such pores, referred to as a parallel bundle of pores. An example is shown in Fig. 3, for a variation of the model applied to a supported zeolite cracking catalyst. In this example [10] the zeolite pores are simply configured along the pore walls, so that the parallel bundle represents the Si/Alumina-support pore spaces. 

Figure 3 Parallel-bundle model for a supported zeolite FCC catalyst. (From Ref. 12.)...

R. Mann and G. Thomson, Deactivation of a supported zeolite catalyst Diffusion, reaction and coke deposition in a parallel bundle, Chem. Eng. Sci. 42 555 (1987). 

Supported zeolite membranes have been prepared using numerous procedures [4] such as alignment of crystals in electrical fields, electroplating, self-assembly, growth on organic molecular layers, covalent linkages, hydrothermal synthesis (in situ and ex situ), hydrothermal method microwave heating assisted, dry gel method (vapor-phase transport method and steam-assisted crystallization), synthesis at the interface between two fluid phases, etc. 

Generally speaking, the single-gas flux through supported zeolite membranes, for a given temperature, depends on the sorption capacity of the gas on the zeolite pores and its equilibrium adsorption constant (Langmuir isotherm is often used to describe the relationship between the amount adsorbed and the gas-phase pressure), the gas diffusion coefficient, the thickness of the zeolite layer, the porosity of the support, and the pressure at the feed and permeate sides. 

Vroon ZAEP. Synthesis and transport studies of thin ceramic supported zeolite (MFI) membranes. PhD dissertation. Delft University, Delft, The Netherlands, 2000. 

Preparation of catalyst supports, zeolites and mesoporous materials... 

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