FIELD OF THE INVENTION

The present disclosure generally relates to chromium catalysts, methods for preparing the chromium catalysts, methods for using the chromium catalysts to polymerize olefins, the polymer resins produced using such chromium catalysts, and articles produced using these polymer resins. More particularly, the present disclosure relates to irradiating a supported hexavalent chromium catalyst with UV-visible light, and associated catalyst preparation systems, polymerization reactor systems, and olefin polymerization processes.

BACKGROUND OF THE INVENTION

Chromium catalysts are among the most common catalysts used in olefin polymerizations. Supported chromium catalysts often are prepared by impregnating chromium onto a solid support, e.g., a solid oxide, followed by a calcining step. Generally, calcining is conducted in an oxidizing atmosphere, such that the chromium species within the supported chromium catalyst can be converted to hexavalent chromium.

There are methods to reduce supported chromium catalysts to lower oxidations states, such as Cr(II), Cr(III), Cr(IV), and Cr(V), but such methods often require high temperature, reduce catalyst activity, and result in polymers with poor melt index potential. It would be beneficial to prepare reduced chromium catalysts that do not suffer from these deficiencies. Accordingly, it is to these ends that the present invention is generally directed.

SUMMARY OF THE INVENTION

This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description. This summary is not intended to identify required or essential features of the claimed subject matter. Nor is this summary intended to be used to limit the scope of the claimed subject matter.

One aspect of the present invention is directed to catalyst preparation systems that can comprise (a) a catalyst preparation vessel configured to irradiate a slurry of a supported chromium catalyst in a diluent with a light beam at a wavelength in the UV-visible spectrum, (b) a catalyst inlet configured to introduce the slurry of the supported chromium catalyst into the catalyst preparation vessel, wherein at least a portion of the chromium is in the hexavalent oxidation state, and (c) a reduced catalyst outlet configured to withdraw a slurry of a reduced chromium catalyst from the catalyst preparation vessel. Polymerization reactor systems incorporating the catalyst preparation systems also are disclosed herein.

Methods for reducing a chromium catalyst for olefin polymerization are provided in another aspect of the present invention. A representative method can comprise irradiating a reductant and a supported chromium catalyst comprising chromium in the hexavalent oxidation state with a light beam at a wavelength in the UV-visible spectrum to reduce at least a portion of the supported chromium catalyst to form a reduced chromium catalyst.

Olefin polymerization processes are provided in yet another aspect of this invention, and such processes can comprise (I) irradiating a reductant and a supported chromium catalyst comprising chromium in the hexavalent oxidation state with a light beam at a wavelength in the UV-visible spectrum to convert at least a portion of the supported chromium catalyst to form a reduced chromium catalyst, and (II) contacting the reduced chromium catalyst and an optional co-catalyst with an olefin monomer and an optional olefin comonomer in a polymerization reactor system under polymerization reaction conditions to produce an olefin polymer.

Reduced chromium catalysts and olefin polymers produced by the respective methods and processes also are encompassed herein. Unexpectedly, and beneficially, the reduced chromium catalysts can have a higher catalyst activity and/or productivity compared to chromium catalysts prepared without an irradiation step. Moreover, olefin polymers described herein often are characterized by higher melt flow properties and a narrower molecular weight distribution than olefin polymers produced by processes using reduced chromium catalysts prepared without an irradiation step.

Ethylene-based polymers having an unexpected and beneficial combination of properties also are disclosed herein. An ethylene polymer in one aspect of this invention can be characterized by a ratio of Mw/Mn in a range from about 30 to about 110, a ratio of Mz/Mw in a range from about 4 to about 10, and a CY-a parameter in a range from about 0.2 to about 0.4. An ethylene polymer in another aspect of this invention can be characterized by a ratio of Mw/Mn in a range from about 6 to about 15, a ratio of Mz/Mw in a range from about 30 to about 70, and a melt index in a range from about 0.5 to about 10 g/10 min.

Both the foregoing summary and the following detailed description provide examples and are explanatory only. Accordingly, the foregoing summary and the following detailed description should not be considered to be restrictive. Further, features or variations can be provided in addition to those set forth herein. For example, certain aspects can be directed to various feature combinations and sub-combinations described in the detailed description.

