Max P. McDaniel

An XPS study of the Phillips Cr/silica polymerization catalyst

Abstract The Phillips Cr/silica polymerization catalyst has been examined by means of X-ray photoelectron spectroscopy and chemical methods, and the results have been compared to activity measurements from a high-pressure autoclave. Only Cr(VI) was found after O 2 at 600–800 °C, but CO at 350 °C cleanly reduced this to Cr(II). On contact with ethylene, which increased the binding energy slightly, the divalent material became an active polymerization catalyst. Reducing Cr(VI) with ethylene produced similar results. This indicates Cr(II) as the active precursor and that its electron density is decreased by the ethylene.

The state of Cr(VI) on the Phillips polymerization catalyst: IV. Saturation coverage

Cr/silica catalysts were activated in the range 200–900 °C to determine the maximum amount of Cr(VI) which could be stabilized by the silica at each temperature. All of the Cr impregnated onto silica remained hexavalent up to a sharply defined saturation coverage, and any excess over this limit merely decomposed, mainly to Cr2O3. This saturation coverage declined with increasing temperature, paralleling the silica hydroxyl population, and common modifiers like alumina or fluoride, which increase or decrease the OH population, also enhanced or diminished the stabilization of Cr(VI). Above 500 °C moisture quickly destabilized Cr(VI), as did lack of oxygen. These findings have again been discussed in terms of three simple models of occupation.

The state of Cr(VI) on the Phillips polymerization catalyst: II. The reaction between silica and CrO2Cl2

Abstract The reaction between chromyl chloride vapor and the hydroxyl population on calcined silica was examined at 200 °C in a flow system. When the silica had been calcined at high temperatures (≥800 °C) the CrO 2 Cl 2 attached by losing one chloride per Cr. This indicates the formation of a surface-SiOCrO 2 Cl species which, upon contact with ethylene, failed to develop any polymerization activity. However, when the silica had been calcined at low temperatures (≤400 °C), most of the CrO 2 Cl 2 attached by losing two chlorides per Cr, indicating the formation of a surface chromate species by reaction with hydroxyl pairs. On contact with ethylene this species developed polymerization activity very much like the conventional CrO 3 /silica catalysts, suggesting that here too the active species could begin as a surface chromate.

A Review of the Phillips Supported Chromium Catalyst and Its Commercial Use for Ethylene Polymerization

Abstract The Phillips supported chromium catalyst is used to produce some 40–50% of the worlds high-density polyethylene. The chemistry of this important catalyst is reviewed from an industrial perspective, on the basis of more than half a century of commercial experience, as well as published research. Polymer characteristics are used as an analytical tool to probe and better understand the active-site population on the catalyst.

The activation of the phillips polymerization catalyst: III. Promotion by titania

Cr/silica polymerization catalysts are rendered much more active by the incorporation of a small amount of titania either in or on the support. The molecular weight of the resultant polymer is also affected. In this report the promotional influence of titania has been examined under different activation conditions. Results varied widely with the type of incorporation and the method of activation. Cr/sub B/ centers were more sensitive to titania than Cr/sub A/ centers.

The state of Cr(VI) on the Cr/Silica polymerization catalyst

Abstract A brief study has been made of the hydroxyl population on silica before and after promotion with CrO 3 . The measured Δ OH Cr ratios did not equal a constant 1 or 2, as has often been expected for a pure dichromate or chromate surface species. Instead, they decreased smoothly with increasing temperature even to values well below 1. However, an analysis of these data which assumes normal dehydroxylation of Cr/Silica indicates initial bonding as a chromate surface species. Anhydrous impregnation of Cr gave a more direct answer, again indicating initial bonding as chromate.

A comparison of Cr/SiO2 and Cr/AlPO4 polymerization catalysts. I: Kinetics

Abstract Ethylene polymerization over Cr/aluminophosphate has been examined and compared to that over Cr/silica. The two catalysts display quite different kinetics and produce important differences in the polymer as well, but this may result from slight variations in a common underlying mechanism of polymerization. The active site on both catalysts is seen as a transient species in a series of consecutive reactions, including reduction, alkylation, and decay. This model has been used to analyze and interpret the development and decline of activity under various polymerization conditions.

The structure of coprecipitated aluminophosphate catalyst supports

Porous aluminophosphates, which are useful as catalyst supports for polymerization, isomerization, or other hydrocarbon conversions, can be made by coprecipitation when an acidic solution of A13+ and PO43− ions is neutralized. When the PAl ratio in solution is equal to or greater than one, A1PO4 is obtained often as a crystalline material, leaving the excess phosphate in solution. However, when excess Al3+ is present in solution (PAl < 1) then it also precipitates and the resulting support retains a similar PAl ratio to that in solution. In this study the structure of such aluminophosphates has been examined by means of X-ray diffraction and high resolution solid state NMR spectroscopy using both 27Al and 31P nuclei. These materials are not simple coprecipitated mixtures of A12O3 and A1PO4. In fact, no evidence for the presence of either species was detected. Instead they appear to be amorphous structures in which the phosphate is randomly dispersed, and the aluminum exists in one octahedral and several different tetrahedral environments. Results from ethylene polymerization over these catalysts also support this view.

Long Chain Branching in Polyethylene from the Phillips Chromium Catalyst

Supported chromium oxide polymerization catalysts can impart significant levels of long chain branching (LCB) to polyethylene made in a slurry process. LCB often dominates the rheological behavior of the polymer, and is thus responsible for the performance of many HDPE grades during commercial molding operations. LCB is difficult to accurately measure but its presence can be inferred from the visco‐elastic character of the polymer, relative to its molecular weight (MW) and MW distribution. Depending on catalyst choice, LCB levels from chromium oxide catalysts can range from near zero, to very high, rivaling the tightly bridged metallocenes. The most common catalyst and reactor variables were investigated with respect to their effect on LCB formation. The simple “macromonomer insertion” view does not in itself account for many of these LCB responses. In fact, no overall correlation was found between LCB and the catalysts ability to incorporate large comonomers. Instead, variables that influenced LCB most also govern the active site density, including activation temperature, chromium loading, and the presence of cocatalysts or poisons in the reactor. The physical structure of the catalyst support also had a strong influence on the elastic behavior of the polymer, independent of any effect on MW or MW distribution. These and other variables are discussed. #An earlier version of this paper was presented at ECOREP II, 2nd European Conference on Reaction Engineering and Polyolefins, Lyon, France, July 1–4, 2002.

The state of Cr(VI) on the Phillips polymerization catalyst: III. The reaction between CrO3/silica and HCl

CrO3/silica catalysts calcined between 400 and 900 °C were treated with HCl gas, which stripped off the Cr(VI) as CrO2Cl2 vapor leaving one additional hydroxyl for each point of attachment to the surface. The increase in the hydroxyl population has been examined in terms of three simple models to gain insight into how Cr(VI) attaches to the surface and whether it exists as chromate or dichromate. These OH pairs formed by stripping off the Cr were then reacted with CrO2Cl2 vapor, which mostly lost two chlorides per Cr and developed polymerization activity similar to that of the original catalyst. This again confirms the activity of the chromate surface species.