Ethylene polymerization calcination

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. 

In using a similar approach, Thiine et al. [29] applied static SIMS to show that Cr/SiC>2 model catalysts which are active for ethylene polymerization contain only monochromates. Secondary ions with more than one Cr ion in the cluster, such as Cr2C>4 and Cr203, disappeared from the spectra after the catalysts had been calcined only CrSiOx ions remained. Aubriet et al. [30] studied the anchoring of chromium acetyl acetonate, Cr(acac)3 to a planar SiO2/Si(100) model support with static SIMS. Chromium polymerization catalysts are discussed further in Chapter 9. 

The original recipe involved the aqueous impregnation of chromic acid on silica, although nowadays less-poisonous chromium(III) salts are used. Over the years, a family of Phillips-type catalysts has emerged producing no less than 50 different types of polyethylene, and this versatility is the reason for the commercial success of the Phillips ethylene polymerization process. The properties of the desired polymer product can be tailored by varying parameters such as calcination temperature, polymerization temperature and pressure, by adding titania as... 

When the Cr02Cl2 adsorbed as chromate, such as on silica that had been calcined at 400 °C, normal polymerization activity was observed at 100 °C and a concentration of ethylene of 1.0 mol L-1 in isobutane. Indeed, the activity was nearly identical to that of Cr03/silica activated at 400 °C. The kinetics profile of the polymerization reaction was also the same, as shown in Figure 7. The polymer FILMI, MW, and MW breadth were also almost the same, as was the UV-vis reflectance spectrum. In contrast, the chlorochromate catalysts were not active for ethylene polymerization under these conditions. Thus, the monochromate species... 

TABLE 2 Ethylene Polymerization Activities of Cr/Silica Catalysts Calcined in Air at 850 °C, Then Reduced in CO Under Various Conditions... 

Now consider the case in which the secondary (chromium-attachment) step is conducted at a lower temperature than the primary (dehydroxylation) step. An example is shown by the data in Figure 124. Again, Cr/silica-titania (2.5 wt% Ti) was calcined in N2, CO, or CS2, but always at 871 °C. The temperature of the secondary step in air was varied from 300 to 900 °C. These catalysts were then tested for ethylene polymerization and the resultant polymers were analyzed. Instead of MI,... 

For example, data representing the kinetics of polymerization with the Cr(II) trimethylsilylmethyl compound are shown in Figure 183. The alumina support was first treated with fluoride and calcined at 600 °C, and then it was impregnated with three different loadings of the chromium compound, which actually exists as a tetramer, Cr4(TMSM)8 [660]. These catalysts were then tested for ethylene polymerization, and the observed activity was found to be roughly proportional to the chromium coverage. 

An example is shown in Figure 192. Two samples of an aluminophosphate support with a P/A1 atomic ratio of 0.4 were calcined at 300 and at 700 °C, then treated with dicumenechromium(O). These catalysts were tested for ethylene polymerization and the resultant polymers were analyzed. The low-MW peak was found to be larger when the support had been calcined at 700 °C. A third peak, indicative of very high-MW polymer, was also present when the support was calcined at only... 

An experiment is summarized in Table 56 that demonstrates that some even more unusual chromium catalysts can be made by this approach. Bis (f-butyl) chromate, (f-but-0)2CrC>2, is a chromate ester that is soluble in hexane and other hydrocarbons. When deposited onto silica calcined at 600 °C, it has no activity for ethylene polymerization. Dicumenechro-mium(O) is a compound that also dissolves in hydrocarbons, and exhibits no (or marginal) activity when deposited onto calcined silica. However, in the experiments referred to in Table 56, the two compounds were deposited sequentially onto silica, and considerable activity did develop from some unknown redox product formed from the two. In this example, the maximum activity seems to have been obtained when the catalyst contained about 40% Cr(VI) and 60% Cr(0). 

The calcined clay was treated with TiCl4. Attachment of Ti(IV) to the clay surface was suggested to occur at coordinatively unsaturated Mg ions (presumably formed by the removal of water during calcination) on the edges of the octahedral ribbons and to involve bridging chloride interactions, as in VII. Upon activation by AFBU3 (Al/Ti = 15), ethylene polymerization was conducted in n-hexane at atmospheric pressure and 40°C. 

Monoi T, Ikeda H, Sasaki Y, Matsumoto Y Ethylene polymerization with silica-supported CrrCH(SiMe3)2l3 catalyst. Effect of sihca calcination temperature and Cr content, Polvni J35(7) 608-611, 2003. 

The Phillips Cr/silica catalyst is prepared by impregnating a chromium compound (commonly chromic acid) onto a support material, most commonly a wide-pore silica, and then calcining in oxygen at 923 K. In the industrial process, the formation of the propagation centers takes place by reductive interaction of Cr(VI) with the monomer (ethylene) at about 423 K [4]. This feature makes the Phillips catalyst unique among all the olefin polymerization catalysts, but also the most controversial one [17]. 

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 ... 

However, when the silica has been calcined at 800°C or above only chlorochromate is formed from chromyl chloride. This mimics the behavior of other reagents such as chlorosilanes, TiCl4, or BC13, which have been used to determine the extent of hydroxyl pairing on silicas (20-23). The chlorochromate species does not polymerize ethylene. 

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