Fluorided Chromium Catalysts for Polyolefin Molecular Weight Control
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Solution Overview
Problem
Traditional chromium-based catalyst systems for producing polyolefins lack high catalytic activity and flexibility in achieving desired molecular weight distributions and long chain branching levels, limiting their application in various end-use scenarios.
Innovation Solution
The development of fluorided chromium catalysts through a process involving calcining a supported chromium catalyst and subsequent fluoriding at a lower temperature, which enhances catalyst activity and produces polymers with specific molecular weight and branching characteristics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If traditional chromium-based catalyst systems are used, then good extrusion processibility and polymer melt strength are achieved, but catalytic activity and flexibility in molecular weight distribution are limited
Solution Approach 1:
The patent applies parameter changes by modifying the chromium catalyst through fluoridation treatment and controlled calcination at specific temperatures (500-900°C). This chemical modification changes the catalyst's active site properties, enabling high catalytic activity while producing polymers with tailored molecular weight distributions and reduced long chain branching, thus resolving the contradiction between productivity and adaptability
2Manufacturing precision
If traditional chromium catalysts are used, then broad molecular weight distribution is achieved, but long chain branching levels remain high
Solution Approach 1:
The fluoridation treatment and controlled calcination parameters transform the catalyst's chemical properties, enabling precise control over polymerization kinetics. This results in broad molecular weight distribution while simultaneously suppressing long chain branching formation, achieving manufacturing precision without the harmful effects
3Quantity of substance
If fluorided chromium catalysts are prepared by conventional fluoriding methods, then fluorine content is achieved, but catalyst activity is reduced
Solution Approach 1:
The patent applies preliminary action by performing calcination before fluoridation treatment. This sequence prepares the chromium oxide surface in advance, creating optimal conditions for fluorine incorporation that maintains high catalytic activity. The preliminary thermal treatment activates the catalyst support, enabling subsequent fluoridation to proceed without compromising productivity
4Ease of manufacture
If peak fluoriding temperature is close to peak calcining temperature, then fluorine incorporation is efficient, but catalyst structure is damaged
Solution Approach 1:
The patent resolves this contradiction by optimizing the temperature parameters: calcination is performed at 500-900°C to activate the support, while fluoridation is conducted at a lower temperature (200-400°C) to incorporate fluorine without damaging the catalyst structure. This parameter differentiation maintains both manufacturing efficiency and structural reliability
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The fluorided chromium catalysts exhibit improved catalytic activity and produce polyolefins with broader molecular weight distributions and reduced long chain branching, offering better processibility and a range of melt indices.
Implementation Method 1
calcining a supported chromium catalyst at a peak calcining temperature to produce a calcined supported chromium catalyst
Implementation Method 2
contacting the calcined supported chromium catalyst at a peak fluoriding temperature with a vapor comprising a fluorine-containing compound to produce the fluorided chromium catalyst
Implementation Method 3
contacting a fluorided chromium catalyst and an optional co-catalyst with an olefin monomer and an optional olefin comonomer in a polymerization reactor system under polymerization conditions to produce an olefin polymer
Data Source
AI summary
Methods for preparing a fluorided chromium catalyst can include a step of calcining a supported chromium catalyst at a peak calcining temperature to produce a calcined supported chromium catalyst, followed by contacting the calcined supported chromium catalyst at a peak fluoriding temperature with a vapor comprising a fluorine-containing compound to produce the fluorided chromium catalyst. The peak fluoriding temperature can be at least 50° C. less, and often from 200° C. to 500° C. less, than the peak calcining temperature. Polymers produced using the fluorided chromium catalyst can have a beneficial combination of higher melt index, narrower molecular weight distribution, and lower long chain branch content.