The Phillips catalyst, or the Phillips supported chromium catalyst, is the

catalyst Catalysis () is the increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quick ...

used to produce approximately half of the world's

polyethylene Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most commonly produced plastic. It is a polymer, primarily used for packaging (plastic bags, plastic films, geomembranes and containers including bott ...

. A heterogeneous catalyst, it consists of a

chromium Chromium is a chemical element; it has Symbol (chemistry), symbol Cr and atomic number 24. It is the first element in Group 6 element, group 6. It is a steely-grey, Luster (mineralogy), lustrous, hard, and brittle transition metal. Chromium ...

oxide An oxide () is a chemical compound containing at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion (anion bearing a net charge of −2) of oxygen, an O2− ion with oxygen in the oxidation st ...

supported on

silica gel Silica gel is an amorphous and porosity, porous form of silicon dioxide (silica), consisting of an irregular three-dimensional framework of alternating silicon and oxygen atoms with nanometer-scale voids and pores. The voids may contain wate ...

. Polyethylene, the most-produced synthetic polymer, is produced industrially by the

polymerization In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many fo ...

ethylene Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene (a hydrocarbon with carbon–carbon bond, carbon–carbon doub ...

: :n C₂H₄ → (C₂H₄)_n Although exergonic (i.e., thermodynamically favorable), the reaction requires catalysts. Three main catalysts are employed commercially: the Phillips catalyst,

Ziegler–Natta catalyst A Ziegler–Natta catalyst, named after Karl Ziegler and Giulio Natta, is a catalyst used in the synthesis of polymers of 1-alkenes ( alpha-olefins). Two broad classes of Ziegler–Natta catalysts are employed, distinguished by their solubility ...

s (based on titanium trichloride), and, for specialty polymers,

metallocene A metallocene is a compound typically consisting of two cyclopentadienyl anions (, abbreviated Cp) bound to a metallic element, metal center (M) in the oxidation state II, with the resulting general formula Closely related to the metallocenes are ...

-based catalysts.

Preparation and mechanism of action

The Phillips catalyst is prepared by impregnating high surface area silica gel with

chromium trioxide Chromium trioxide (also known as chromium(VI) oxide or chromic anhydride) is an inorganic compound with the formula . It is the acidic anhydride of chromic acid, and is sometimes marketed under the same name. This compound is a dark-purple solid ...

or related chromium compounds. The solid precatalyst is then calcined in air to give the active catalyst. Only a fraction of the chromium is catalytically active, a fact that interferes with elucidation of the catalytic mechanism. The active catalyst is often depicted as a chromate

ester In chemistry, an ester is a compound derived from an acid (either organic or inorganic) in which the hydrogen atom (H) of at least one acidic hydroxyl group () of that acid is replaced by an organyl group (R). These compounds contain a distin ...

bound to the silica surface. The mechanism for the polymerization process is the subject of much research, the central question being the structure of the active species, which is assumed to be an organochromium compound. Robert L. Banks and J. Paul Hogan, both at Phillips Petroleum, filed the first patents on the Phillips catalyst in 1953. Four years later, the process was commercialized.J.P. Hogan, R.L. Banks, U.S. Patent 2,825,721 to Phillips Petroleum Company, filed August, 1954 and issued March, 1958.

References

{{Chromates and dichromates Industrial processes Polymer chemistry Catalysts Coordination complexes Chromium(VI) compounds Chromium–oxygen compounds