Zeolite polymerization, ethylene

Platinum (metal)- and acid (oxide)-catalyzed processes were developed to convert petroleum to high-octane fuels. Hydrodesulfurization catalysis removed sulfur from the crude to prevent catalyst deactivation. The discovery of microporous crystalline alumina silicates (zeolites) provided more selective and active catalysts for many reactions, including cracking, hydrocracking, alkylation, isomerization, and oligomerization. Catalysts that polymerize ethylene, propylene, and other molecules were discovered. A new generation of bimetallic catalysts that were dispersed on high-surface-area (100-400 m /g) oxides was synthesized. 

Ethylene. The highly reactive H-Y and rare earth X-zeolites have been reported to polymerize ethylene proj e , hex-1-ene and 2,3-dimethyl-... 

The presence of methylenic bands shifted at higher frequency in the very early stages of the polymerization reaction has also been reported by Nishimura and Thomas [114]. A few years later, Spoto et al. [30,77] reported an ethylene polymerization study on a Cr/silicalite, the aluminum-free ZSM-5 molecular sieve. This system is characterized by localized nests of hydroxyls [26,27,115], which can act as grafting centers for chromium ions, thus showing a definite propensity for the formation of mononuclear chromium species. In these samples two types of chromium are present those located in the internal nests and those located on the external surface. Besides the doublet at 2920-2850 cm two additional broad bands at 2931 and 2860 cm are observed. Even in this favorable case no evidence of CH3 groups was obtained [30,77]. The first doublet is assigned to the CH2 stretching mode of the chains formed on the external surface of the zeolite. The bands at 2931 and... 

Molybdenum allyl complexes react with surface OH groups to produce catalysts active for olefin metathesis.34 35 Using silica as a support for the heterog-enization of Ti and Zr complexes for the polymerization of ethylene did not give clear results.36 In these cases, HY zeolite appeared to be a more suitable support. The comparable productivity of the zeolite-supported catalyst with... 

Chromium zeolites are recognised to possess, at least at the laboratory scale, notable catalytic properties like in ethylene polymerization, oxidation of hydrocarbons, cracking of cumene, disproportionation of n-heptane, and thermolysis of H20 [ 1 ]. Several factors may have an effect on the catalytic activity of the chromium catalysts, such as the oxidation state, the structure (amorphous or crystalline, mono/di-chromate or polychromates, oxides, etc.) and the interaction of the chromium species with the support which depends essentially on the catalysts preparation method. They are ruled principally by several parameters such as the metal loading, the support characteristics, and the nature of the post-treatment (calcination, reduction, etc.). The nature of metal precursor is a parameter which can affect the predominance of chromium species in zeolite. In the case of solid-state exchange, the exchange process initially takes place at the solid- solid interface between the precursor salt and zeolite grains, and the success of the exchange depends on the type of interactions developed [2]. The aim of this work is to study the effect of the chromium precursor on the physicochemical properties of chromium loaded ZSM-5 catalysts and their catalytic performance in ethylene ammoxidation to acetonitrile. 

A study of the OH vibrations and of the rate of the acid catalyzed ethylene polymerization on the parent and V2O5-P2O5 loaded HY, HZSM-11 zeolites, showed that for HY ans HZSM-5 both P and V in the V2O5-P2O5 phase interact with the... 

Yashima, T., Ushida, Y., Ehisawa, M. and Hara, N. Polymerization of ethylene over transition-metal exchanged Y-zeolites. J. Catal., 1975, 36, 320-326. 

A comparison of the reactions of ethylene on three kinds of Ru-Y zeolites shows that the self-hydrogenation rate has the order Ru-H-Y > Ru-Ca-Y > Ru-Na-Y. The hydrogenation rate depends on the amount of coadsorbed hydrogen to roughly first order. The presence of polymerization products beyond butane is not observed. The reactions of ethylene are affected by reaction temperature with some rearrangement of butane to isobutane at intermediate temperatures. Carbon-carbon bond cleavage to methane predominates above 573K. 

Ruthenium is known to catalyze a number of reactions, including the Fischer-Tropsch synthesis of hydrocarbons (7) and the polymerization of ethylene (2). The higher metal dispersions and the shape selectivity that a zeolite provides has led to the study of ruthenium containing zeolites as catalytic materials (3). A number of factors affect the product distribution in Fischer-Tropsch chemistry when zeolites containing ruthenium are used as the catalyst, including the location of the metal (4) and the method of introducing ruthenium into the zeolite (3). 

Cobalt-containing zeolites have been studied for polymerization of ethylene (155-157). The catalysts which were prepared by precipitating cobalt carbonate together with zeolites A, X, Y, and mordenite were not very selective, yielding large amounts of ethane as well as C3 and C4 hydrocarbons. 

Petrofin [Process enhancement through recovery of olefins] A process for recovering olefins (ethylene and propylene) from polymerization processes by adsorption on zeolite 4A. Developed by BOC and used at Montell s polypropylene plant at Lake Charles, LA. First demonstrated in 1997. 

Dimerization presumably takes place on the transition metal-containing sites, and alkylation on the acidic sites of zeolltic surface. The sodium form of zeolite exchanged with transition metal cations Is capable of dimerization (and further polymerization), but does not practically exhibit alkylating capacity. This explains the composition of the product obtained from ethylene and Isobutane over this catalyst (Table V, column 3). 

The catalytic work on the zeolites has been carried out using the pulse microreactor technique (4) on the following reactions cracking of cumene, isomerization of 1-butene to 2-butene, polymerization of ethylene, equilibration of hydrogen-deuterium gas, and the ortho-para hydrogen conversion. These reactions were studied as a function of replacement of sodium by ammonium ion and subsequent heat treatment of the material (3). Furthermore, in some cases a surface titration of the catalytic sites was used to determine not only the number of sites but also the activity per site. Measurements at different temperatures permitted the determination of the absolute rate at each temperature with subsequent calculation of the activation energy and the entropy factor. For cumene cracking, the number of active sites was found to be equal to the number of sodium ions replaced in the catalyst synthesis by ammonium ions up to about 50% replacement. This proved that the active sites were either Bronsted or Lewis acid sites or both. Physical defects such as strains in the crystals were thus eliminated and the... 

Ethylene for polymerization to the most widely used polymer can be made by the dehydration of ethanol from fermentation (12.1).6 The ethanol used need not be anhydrous. Dehydration of 20% aqueous ethanol over HZSM-5 zeolite gave 76-83% ethylene, 2% ethane, 6.6% propylene, 2% propane, 4% butenes, and 3% /3-butane.7 Presumably, the paraffins could be dehydrogenated catalyti-cally after separation from the olefins.8 Ethylene can be dimerized to 1-butene with a nickel catalyst.9 It can be trimerized to 1-hexene with a chromium catalyst with 95% selectivity at 70% conversion.10 Ethylene is often copolymerized with 1-hexene to produce linear low-density polyethylene. Brookhart and co-workers have developed iron, cobalt, nickel, and palladium dimine catalysts that produce similar branched polyethylene from ethylene alone.11 Mixed higher olefins can be made by reaction of ethylene with triethylaluminum or by the Shell higher olefins process, which employs a nickel phosphine catalyst. 

The conversion of reactants is illustrated by the dehydration-polymerization of 2-isopropanol adsorbed on K- and Cs-ZSM-5 zeolites as followed by 1JC-NMR. On the other hand, the polymerization of ethylene shows a clear-cut difference between three-dimensional channel systems (ZSM-5 and ZSM-11) able to promote the molecular traffic of reactants and on one-dimensional channel systems (ZSM-48) where some unreacted ethylene is still detected after polymerization. 

Anderson et al. showed that the active sites involved in the conversion of methanol on zeolites are not Lewis acids. Wolthuizen et al., ° however, presented evidence that the presence of Lewis-acid sites enhances the polymerization of ethylene. This is in agreement with the results obtained with HY zeolites,where reaction of ethylene at 80 °C was observed only after dehydroxylation of the Bronsted acid sites into Lewis acid sites. At higher temperatures, ethylene is well-known to react on catalysts with strong Bronsted acid sites.Sayed and Cooney reported the involvement of aluminum Lewis sites in the formation of dimethylether. Haber and Szybalska observed that, when ethanol is converted on boron aluminum phosphates, only dehydration to ethylene takes place on the Bronsted acid sites, whereas, on Lewis acid-base centers, ethanol is mainly dehydrated to diethylether. 

Nickel oxide and nickel complexes supported on silica, silica-alumina, different zeolites, and polymeric materials have been reported to be active for ethylene dimerization. Yashima et reported that ethylene dimerization can... 

Cr-Y zeolite catalysts ate particularly active for the bulk polymerization of ethyfene resulting in the formation of polymers (m. p. 134—42 ) with crystalline and unbranched chains. The optimum evaluation temperature of catalyst activaticm is 350 C and the yield of pdyethjdene increases linearly with ethylene pressure (range 5—50 atia) and time. The active sites on the Cr-Y catalyst appear to be coiqrosed of Cr" " ions whkh remain supported on the zeolites. 

In order to avoid any polymerization of ethylene on acid sites experiments were conducted at 210 K on a series of dealuminated zeolites. The figure 9 shows the increase in Av as the framework A1 content decreases. For this series of materials prepared by steaming followed by acid leaching, the acid strength increases up to an A1 atomic fraction close to 0.05 (29). 

Likewise, monovalent and trivalent rhodium were known as good dimerization catalysts for ethylene (37,38) and indeed rhodium exchanged zeolite Y appeared as an efficient and selective catalyst in ethylene dimerization under mild conditions (O-20°C, 200 Torr of ethylene). By contrast HY was inactive under similar temperature and pressure conditions and at 200°C polymerization and cracking were observed (36). Thus again the dimerization is not acid-catalyzed. The active species is however uncertain but most probably not metallic, perhaps trivalent or monovalent (36,39). 

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