Chromium Catalyst Preparation for High Molecular Weight Polyethylene
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Solution Overview
Problem
Existing Phillips catalysts for polyethylene production have limited activity and productivity at low activation temperatures, resulting in polymers with low molecular weight and poor homogeneity, and require high calcination temperatures that lead to sintering and decreased activity.
Innovation Solution
A process for preparing a supported catalyst by forming a homogeneous solution of chromium and secondary dopants like zirconium or zinc with a protic or aprotic polar solvent, applying it to an inorganic support, and calcining under oxidative conditions at temperatures between 350 to 1050°C to achieve high molecular weight polyethylene with improved bulk density and homogeneity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If high calcination temperatures are used to increase catalyst activity, then catalyst activity improves, but support sintering occurs and activity decreases
Solution Approach 1:
The patent introduces secondary dopants (Zr, Zn, Sc, V, Nb, Ta) that modify the thermal stability and catalytic properties of chromium oxide. These dopants allow the catalyst to maintain high activity at lower calcination temperatures (350-950°C) while preventing support sintering. The dopants change the chemical and physical parameters of the catalyst system, enabling decoupling of activity enhancement from high temperature treatment.
Solution Approach 2:
The patent creates composite catalyst systems by combining chromium oxide with secondary dopants (forming Cr-xM mixed oxides where M = Zr, Zn, Sc, V, Nb, or Ta). These composite materials exhibit synergistic effects where the secondary dopant enhances both the thermal stability and catalytic activity, allowing the catalyst to function effectively at moderate temperatures without support degradation.
2Temperature
If low activation temperatures are used to preserve support structure, then support stability improves, but catalyst activity and productivity decrease
Solution Approach 1:
The secondary dopants fundamentally change the activation characteristics of chromium oxide. The presence of dopants like Zr, Zn, Sc, V, Nb, or Ta modifies the reduction behavior and electronic structure of chromium species, enabling high catalytic activity to be achieved at lower activation temperatures. This parameter change allows the system to bypass the traditional temperature-activity correlation.
Solution Approach 2:
The secondary dopants act as sacrificial components that facilitate chromium activation at lower temperatures. They undergo chemical changes during the activation process, enabling gentle treatment conditions while still achieving the necessary catalyst activation. The dopants essentially 'sacrifice' their stability to enable milder activation conditions.
3Manufacturing precision
If high molecular weight polyethylene is produced, then polymer quality improves, but catalyst activity decreases
Solution Approach 1:
The composite Cr-M oxide systems (where M = Zr, Zn, Sc, V, Nb, or Ta) create multiple active sites with different functionalities. Some sites favor high molecular weight polymer formation while others maintain high activity. The composite structure provides a distribution of active sites that simultaneously achieve both high polymer molecular weight and high catalytic productivity.
Solution Approach 2:
The secondary dopants create local variations in the catalyst structure and electronic properties. Different regions of the catalyst surface have different compositions and activities, with some areas optimized for chain growth (producing high molecular weight) and others for high turnover rates. This spatial differentiation of function resolves the contradiction between polymer quality and productivity.
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 process results in catalysts with high activity and productivity at low activation temperatures, producing polyethylene with high molecular weight and bulk density, and a narrower molar mass distribution, suitable for applications in blown film extrusion.
Implementation Method 1
calcining this at temperatures of from 350 to 950° C. The calcination converts chromium present in valences lower than six into the hexavalent state
Implementation Method 2
bringing the solution from a) into contact with a finely divided inorganic support
Data Source
AI summary
Process for preparing a supported catalyst for the polymerization and/or copolymerization of olefins which has a chromium content of from 0.01 to 5% by weight, based on the element, which comprises(a) preparing a homogeneous solution comprising an organic or inorganic chromium compound and at least one further organic or inorganic compound of elements selected from among Mg, Ca, Sr, B, Al, Si, P, Bi, Sc, V, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Ru, Rh, Pd, Hf, Ta, W in a protic or aprotic polar solvent,(b) bringing the solution from a) into contact with a finely divided inorganic support to form a catalyst precursor,(c) if appropriate, removing the solvent from the catalyst precursor and(d) calcining the catalyst precursor at temperatures of from 350 to 950° C., preferably 400 to 900° C., under oxidative conditions.