Chromium catalysts, heterogeneous

Chromium compounds decompose primary and secondary hydroperoxides to the corresponding carbonyl compounds, both homogeneously and heterogeneously (187—191). The mechanism of chromium catalyst interaction with hydroperoxides may involve generation of hexavalent chromium in the form of an alkyl chromate, which decomposes heterolyticaHy to give ketone (192). The oxidation of alcohol intermediates may also proceed through chromate ester intermediates (193). Therefore, chromium catalysis tends to increase the ketone alcohol ratio in the product (194,195). 

In 1951 Robert Banks and Paul Hogan of Phillips Petroleum discovered that ethylene could be polymerized under rather mild conditions of temperature and pressure to afford high molecular weight polyethylene using chromium trioxide as the catalyst. This invention laid the foundation for both the Phillips and Union Carbide processes for ethylene polymerization (both use heterogeneous chromium catalysts). 

Therefore, in such heterogeneous polymerizations, almost all industrial catalysts are supported, for example on silica, whereas the typical Ziegler s titanium catalysts are by definition supported on magnesium chloride. These catalysts are adsorbed at the surface or incorporated into the crystal structure of the support. Other catalysts, such as Phillips chromium catalysts, can be coupled at the support surface by a chemical bond. 

Due to their multi-sited nature, Ziegler-Natta and chromium catalysts produce structurally heterogeneous ethylene homo- and copolymers. This means that the polymers have broad MWD and broad composition (short-chain branching) distribution (Fig. 9). Catalyst active sites that produce lower molecular weights also have a tendency to incorporate more comonomer... 

Ziegler-Natta Catalysts (Heterogeneous). These systems consist of a combination of a transition metal compound from groups IV to VIII and an organometallic compound of a group I—III metal.23 The transition metal compound is called the catalyst and the organometallic compound the cocatalyst. Typically the catalyst is a halide or oxyhalide of titanium, chromium, vanadium, zirconium, or molybdenum. The cocatalyst is often an alkyl, aryl, or halide of aluminum, lithium, zinc, tin, cadmium, magnesium, or beryllium.24 One of the most important catalyst systems is the titanium trihalides or tetra-halides combined with a trialkylaluminum compound. 

Less than 15% of the ore is transformed into chromium compounds, principally chromates, dichromates, chromium(VI) oxide, chromium(III) oxide, and so on. Alkaline oxidative roasting of chromite in rotary kilns yields sodium chromate (see equation 1), which is leached out with water and typically converted into sodium dichromate with sulfimc acid (equation 2) or carbon dioxide (equations). Fiuther treatment of sodium dichromate with sulfuric acid yields chromium(VI) oxide ( chromic acid ), while its reduction (with carbon, sulfur, or anuuo-nium salts) produces chromium(III) oxide. Finally, basic chromium(III) salts, for example Cr(0H)S04, which are used as tanning agents for animal hides, also result from reduction of sodium dichromate. Heterogeneous chromium catalysts are used for the polymerization of ethylene. 

Similarly, the CrAPO-5- and chromium silicalite-1 (CrS-l)-catalyzed oxidation of aromatic side-chains with TBHP or O2 as the primary oxidant [27-31] almost certainly arises as a result of soluble chromium(VI) leached from the catalyst. The same probably applies to benzylic oxidations with TBHP catalyzed by chromium-pillared montmorillonite [32]. More recently, a chromium Schiff s base complex tethered to the mesoporous silica, MCM-41, was claimed [33] to be an active and stable catalyst for the autoxidation of alkylaromatic side-chains. It would seem unlikely, however, that Schiff s base ligands can survive autoxidation conditions. Indeed, on the basis of our experience with chromium-substituted molecular sieves we consider it unlikely that a heterogeneous chromium catalyst can be developed that is both active and stable to leaching under normal oxidizing conditions with O2 or RO2H in the liquid phase. Similarly, vanadium-substituted molecular sieves are also unstable towards leaching under oxidizing conditions in the liquid phase [6,34]. 

In 1920s, the studies on the catalysts for ammonia sjmthesis were performed sporadically in BASF, instead, the company mainly focused on the organic synthesis under high pressm-es and the new fields in heterogeneous catalysis. Dm-ing the development of ammonia synthesis catalysts, researchers provided valuable information about the dm-ability, thermal stability, sensitivity to poisons, and in particular to the concept of promoter. Mittasch smnmarized the roles of various additives as shown in Fig. 1.9. The hypothesis of successful catalyst is multi-component system proposed by Mittasch was confirmed to be very successful. Iron-chromium catalysts for water gas shift reaction, zinc hromium catalystfor methanol synthesis, bismuth iron catalysts for ammonia oxidation and iron/zinc/alkali catalysts for coal hydrogenation were successively developed in BASF laboratories. 

Varkey SP, Lobo RF, Theopold KH (2003) Zeolite MCM-22 supported heterogeneous chromium catalyst (ot ethylene polymerization. Catal Lett 88 227-229... 

Weckhuysen BM, Schoonheydt RA, Mabbs FE, Collison D. 1996. Electron paramagnetic resonance of heterogeneous chromium catalysts. J Chem Soc, Faraday Trans 92(13) 2431-2436. 

Model catalysts. Model catalysts usually feature with well-defined structures and could provide much clearer information of the active site to understand the PhiUips catalyst. Scheme 3.2 depicts some typical silica-supported heterogeneous model catalysts for the PhiUips chromium catalyst. S-2 catalyst prepared by wet impregnation of bis(triphenylsUyl) chromate onto thermaUy pretreated silica gel could be considered as a... 

Scheme 3.2 Heterogeneous model catalysts for Phillips chromium catalyst.

Molecular modeling. To simulate the behavior of the real heterogeneous catalyst, a reasonable molecular model must be first built to mimic the active sites anchored on the support. Figure 3.3 shows some typical molecular models for the active sites of the Phillips chromium catalyst. Espehd and Borve had done a series of systematic density functional theory (DFT) investigations on the active sites of the Phillips chromium... 

Heterogeneous vapor-phase fluorination of a chlorocarbon or chlorohydrocarbon with HP over a supported metal catalyst is an alternative to the hquid phase process. Salts of chromium, nickel, cobalt or iron on an A1P. support are considered viable catalysts in pellet or fluidized powder form. This process can be used to manufacture CPC-11 and CPC-12, but is hampered by the formation of over-fluorinated by-products with Httle to no commercial value. The most effective appHcation for vapor-phase fluorination is where all the halogens are to be replaced by fluorine, as in manufacture of 3,3,3-trifluoropropene [677-21 ] (14) for use in polyfluorosiHcones. 

Most catalysts for solution processes are either completely soluble or pseudo-homogeneous all their catalyst components are introduced into the reactor as Hquids but produce soHd catalysts when combined. The early Du Pont process employed a three-component catalyst consisting of titanium tetrachloride, vanadium oxytrichloride, and triisobutjlalurninum (80,81), whereas Dow used a mixture of titanium tetrachloride and triisobutylalurninum modified with ammonia (86,87). Because processes are intrinsically suitable for the use of soluble catalysts, they were the first to accommodate highly active metallocene catalysts. Other suitable catalyst systems include heterogeneous catalysts (such as chromium-based catalysts) as well as supported and unsupported Ziegler catalysts (88—90). 

The reaction is cataly2ed by all but the weakest acids. In the dehydration of ethanol over heterogeneous catalysts, such as alumina (342—346), ether is the main product below 260°C at higher temperatures both ether and ethylene are produced. Other catalysts used include siUca—alumina (347,348), copper sulfate, tin chloride, manganous chloride, aluminum chloride, chrome alum, and chromium sulfate (349,350). 

Low pressure (0.1 to 20 MPa) and temperatures of 50 to 300°C using heterogeneous catalysts such as molybdenum oxide or chromium oxide supported on inorganic carriers to produce high density polyethylene (HDPE), which is more linear in nature, with densities of 0.94 to 0.97 g/cm. ... 

The metal catalyzed production of polyolefins such as high density polyethylene (HDPE), linear low density polyethylene (LLDPE) and polypropylene (PP) has grown into an enormous industry. Heterogeneous transition metal catalysts are used for the vast majority of PE and all of the PP production. These catalysts fall generally within two broad classes. Most commercial PP is isotactic and is produced with a catalyst based on a combination of titanium chloride and alkylaluminum chlorides. HDPE and LLDPE are produced with either a titanium catalyst or one based on chromium supported on silica. Most commercial titanium-based PE catalysts are supported on MgCl2. 

This complex and structurally related molecules served as a functional homogeneous model system for commercially used heterogeneous catalysts based on chromium (e.g. Cp2Cr on silica - Union Carbide catalyst). The kinetics of the polymerization have been studied to elucidate mechanistic features of the catalysis and in order to characterize the potential energy surface of the catalytic reaction. 

The use of heterogeneous catalysts in the liquid phase offers several advantages compared with homogeneous counterparts, in that it facilitates ease of recovery and recycling. A chromium-containing medium-pore molecular sieve (Si Cr > 140 1), CrS-2, efficiently catalyzes the direct oxidation of various primary amines to the corresponding nitro compounds using 70% t-butylhydroperoxide (TBHP).110... 

Although chromium-based ethylene polymerization catalysts have already been developed commercially, these processes are based on a heterogeneous catalyst... 

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