Supported Chromium Catalysts

Slurry processes may use a combination of organo-aluminum and organo-magnesium compounds with titanium tetrachloride. Phillips-type catalysts (supported chromium compounds) are also used. Pressures and temperatures are moderate. 

This reaction is first conducted on a chromium-promoted iron oxide catalyst in the high temperature shift (HTS) reactor at about 370°C at the inlet. This catalyst is usually in the form of 6 x 6-mm or 9.5 x 9.5-mm tablets, SV about 4000 h . Converted gases are cooled outside of the HTS by producing steam or heating boiler feed water and are sent to the low temperature shift (LTS) converter at about 200—215°C to complete the water gas shift reaction. The LTS catalyst is a copper—zinc oxide catalyst supported on alumina. CO content of the effluent gas is usually 0.1—0.25% on a dry gas basis and has a 14°C approach to equihbrium, ie, an equihbrium temperature 14°C higher than actual, and SV about 4000 h . Operating at as low a temperature as possible is advantageous because of the more favorable equihbrium constants. The product gas from this section contains about 77% H2, 18% CO2, 0.30% CO, and 4.7% CH. ... 

Second, in the early 1950s, Hogan and Bank at Phillips Petroleum Company, discovered (3,4) that ethylene could be catalyticaHy polymerized into a sohd plastic under more moderate conditions at a pressure of 3—4 MPa (435—580 psi) and temperature of 70—100°C, with a catalyst containing chromium oxide supported on siUca (Phillips catalysts). PE resins prepared with these catalysts are linear, highly crystalline polymers of a much higher density of 0.960—0.970 g/cnr (as opposed to 0.920—0.930 g/cnf for LDPE). These resins, or HDPE, are currentiy produced on a large scale, (see Olefin polymers, HIGH DENSITY POLYETHYLENE). 

In catalytic toluene hydrodealkylation, toluene is mixed with a hydrogen stream and passed through a vessel packed with a catalyst, usually supported chromium or molybdenum oxides, platinum or platinum oxides, on siHca or alumina (50). The operating temperatures range from 500—595°C... 

These siUca-supported catalysts demonstrate the close connections between catalysis in solutions and catalysis on surfaces, but they are not industrial catalysts. However, siUca is used as a support for chromium complexes, formed either from chromocene or chromium salts, that are industrial catalysts for polymerization of a-olefins (64,65). Supported chromium complex catalysts are used on an enormous scale in the manufacture of linear polyethylene in the Unipol and Phillips processes (see Olefin polymers). The exact stmctures of the surface species are still not known, but it is evident that there is a close analogy linking soluble and supported metal complex catalysts for olefin polymerization. 

Union Carbide Corp. also uses a siUca-supported chromium catalyst in their extremely low cost Unipol gas-phase linear low density ethylene copolymer process, which revolutionized the industry when it was introduced in 1977 (86—88). The productivity of this catalyst is 10 —10 kg polymer/kg transition metal contained in the catalyst. By 1990, the capacity of Unipol linear low density polyethylene reactors was sufficient to supply 25% of the world s total demand for polyethylene. 

Fig. 1. Examples of the kinetic curves during ethylene polymerization by chromium oxide catalysts. Support—SiOs temperature—80°C polymerization at constant ethylene pressure in perfect mixing reactor. Curve 1—catalyst reduced by CO at 300°C. Curve 2— catalyst activated in vacuum (400°C) polymerization in the case of (1) and (2) in solvent (heptane) ethylene pressure 10 kg/cm2 02 content in ethylene 1 ppm, HsO 3 ppm. Curves 3, 4, 5, 6—catalyst activated in vacuum (400°C) polymerization without solvent ethylene pressure 19 (curve 3), 13 (curve 4), 4 (curve 5), and 2 (curve 6) kg/cm2 02 content in ethylene 1 ppm, HsO = 12 ppm.

In Hogan (69) it was supposed that in a highly active catalyst containing 0.01% of chromium all the chromium ions act as active centers. According to this it was calculated that in the catalyst containing 1% of chromium on silica the number of propagation centers reached 10% of the supported chromium. 

Table IV presents the results of the determination of polyethylene radioactivity after the decomposition of the active bonds in one-component catalysts by methanol, labeled in different positions. In the case of TiCU (169) and the catalyst Cr -CjHsU/SiCU (8, 140) in the initial state the insertion of tritium of the alcohol hydroxyl group into the polymer corresponds to the expected polarization of the metal-carbon bond determined by the difference in electronegativity of these elements. The decomposition of active bonds in this case seems to follow the scheme (25) (see Section V). But in the case of the chromium oxide catalyst and the catalyst obtained by hydrogen reduction of the supported chromium ir-allyl complexes (ir-allyl ligands being removed from the active center) (140) C14 of the...

Weckhuysen et al. [6,33] have recently published several UV-Vis DRS works devoted to investigate the surface chemistry of supported chromium catalysts as a function of the support composition. The same authors [34] have also tried... 

When a calcined Cr(VI)/Si02 catalyst is fed with ethylene at 373-423 K, an induction time is observed prior to the onset of the polymerization. This is attributed to a reduction phase, during which chromium is reduced and ethylene is oxidized [4]. Baker and Garrick obtained a conversion of 85-96% to Cr(II) for a catalyst exposed to ethylene at 400 K formaldehyde was the main by-product [44]. Water and other oxidation products have been also observed in the gas phase. These reduction products are very reactive and consequently can partially cover the surface. The same can occur for reduced chromium sites. Consequently, the state of sihca surface and of chromium after this reduction step is not well known. Besides the reduction with ethylene of Cr(Vl) precursors (adopted in the industrial process), four alternative approaches have been used to produce supported chromium in a reduced state ... 

Thermal reduction at 623 K by means of CO is a common method of producing reduced and catalytically active chromium centers. In this case the induction period in the successive ethylene polymerization is replaced by a very short delay consistent with initial adsorption of ethylene on reduce chromium centers and formation of active precursors. In the CO-reduced catalyst, CO2 in the gas phase is the only product and chromium is found to have an average oxidation number just above 2 [4,7,44,65,66], comprised of mainly Cr(II) and very small amount of Cr(III) species (presumably as Q -Cr203 [66]). Fubini et al. [47] reported that reduction in CO at 623 K of a diluted Cr(VI)/Si02 sample (1 wt. % Cr) yields 98% of the silica-supported chromium in the +2 oxidation state, as determined from oxygen uptake measurements. The remaining 2 wt. % of the metal was proposed to be clustered in a-chromia-like particles. As the oxidation product (CO2) is not adsorbed on the surface and CO is fully desorbed from Cr(II) at 623 K (reduction temperature), the resulting catalyst acquires a model character in fact, the siliceous part of the surface is the same of pure silica treated at the same temperature and the anchored chromium is all in the divalent state. 

In another example, a polymer-supported chromium porphyrin complex was supported on ArgoGel Cl and then employed for the ring-opening polymerization of 1,2-cyclohexene oxide and C02 [95], This complex showed higher activity than a C02-soluble equivalent, and the solid nature of the catalyst meant that recycling of the catalyst was much easier. 

Future challenges for polymerization model catalysts are to study the structure of polymers below their melting point in what is called the nascent morphology. Such work can be undertaken on silica-supported chromium catalysts as discussed above, or on so-called single-site catalysts, such as metallocenes, applied on flat silica supports. 

When controlled nitridation of surface layers is required, as for example in the modification of the chemical properties of the surface of a support, the atomic layer deposition (ALD) technique can be applied." This technique is based upon repeated separate saturating reactions of at least two different reactants with the surface, which leads to the controlled build-up of thin films via reaction of the second component with the chemisorbed residues of the first reactant. Aluminium nitride surfaces have been prepared on both alumina and silica supports by this method wherein reaction cycles of trimethylaluminium and ammonia have been performed with the respective supports, retaining their high surface areas." This method has been applied to the modification of the support composition for chromium catalysts supported on alumina." ... 

The supported chromium complex "CHRISS" was first reported by us in 1997 (Figure 6) [10]. It was shown to be stable to reaction conditions and an active catalyst for the oxidation of methylaromatics with air as the only consumable source of oxygen. The catalyst was prepared by first forming in solution a metal complex with pendant triethoxysilane groups which is then imprinted onto a silica gel surface. 

The polymerization behavior of Cr/alumina seems to reflect the higher hydroxyl population. More surface hydroxyls also means more sites available to support chromium, and alumina does stabilize about twice as much Cr(Vl) as silica. However, the higher chromium levels do not yield a more active catalyst. Cr/alumina is typically only one tenth as active as Cr/silica. Termination rates are also extremely depressed on Cr/alumina. Both effects could be attributed to the extra hydroxyls, which are thought to interfere with polymerization. 

Alumina supported chromium oxide catalyst is highly selective in paraxylene... 

A chromocene catalyst supported on silica has been studied (94), with ethylene adding to a Cr—H bond. It is remarkable that the chromium atom, as well as the migrating hydrogen atom, appears to be essentially neutral. For a discussion, see the original article (94). 

McDaniel, M. P., Supported Chromium Catalysts for Ethylene Polymerization , Adv. Catal., 33, 47-98 (1985). 

The use of supports in heterogeneous catalysis was well understood by 1950 and flourished with the discovery in 1954 by Hogan and Banks [220,221] of highly active chromium trioxide catalysts supported on silica, which could... 

Polymerization with Complex Catalysts. High density polyethylene reached a domestic production of 1.25 billion pounds in 1968. It is made either with a stereospecific Ziegler-Natta catalyst or on a supported chromium oxide catalyst. The latter forms a complex with the silica-alumina and is activated by treatment with air and steam at elevated temperature. The mechanism is such that electrons are donated to the catalyst in order to be returned under polymerizational-promoting conditions, consequently lowering the energy of the system ... 

Qince the discovery (6) of supported chromium oxide catalysts for polymerization and copolymerization of olefins, many fundamental studies of these systems have been reported. Early studies by Topchiev et al. (18) deal with the effects of catalyst and reaction variables on the over-all kinetics. More recent studies stress the nature of the catalytically active species (1, 2, 9,13, 14,16, 19). Using ESR techniques, evidence is developed which indicates that the active species are Cr ions in tetrahedral environment. Other recent work presents a more detailed look at the reaction kinetics. For example, Yermakov and co-workers (12) provide evidence which suggests that chain termination in the polymerization of ethylene on the catalyst surface takes place predominantly by transfer with monomer, and Clark and Bailey (3, 4) give evidence that chain growth occurs through a Langmuir-Hinshelwood mechanism. 

---