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Related Experiment Videos

Synthesis, spectroscopy and catalysis of

Weckhuysen1, Ramachandra Rao R, Pelgrims

  • 1Centrum voor Oppervlaktechemie en Katalyse, Department Interfasechemie, K.U.Leuven, Heverlee, Belgium. bert.weckhuysen@agr.kuleuven.ac.be

Chemistry (Weinheim an Der Bergstrasse, Germany)
|September 19, 2000
PubMed
Summary

Chromium acetyl acetonate grafted onto MCM-41 materials catalyzes ethylene polymerization, forming high molecular weight polyethylene nanofibres. The Al-containing MCM-41 support and high-temperature pretreatment yield the most active catalyst.

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Area of Science:

  • Materials Science
  • Catalysis
  • Polymer Chemistry

Background:

  • Mesoporous silica materials like MCM-41 are versatile supports for catalysts.
  • Chromium complexes, such as chromium acetyl acetonate [Cr(acac)3], are known polymerization initiators.
  • Understanding the interaction between chromium complexes and mesoporous supports is crucial for catalyst design.

Purpose of the Study:

  • To graft chromium acetyl acetonate [Cr(acac)3] onto pure silica MCM-41 (SiMCM-41) and Al-containing MCM-41 (AlMCM-41).
  • To characterize the synthesized materials and investigate the interaction mechanisms.
  • To evaluate the catalytic activity of these materials in ethylene polymerization and understand the resulting polymer morphology.

Main Methods:

  • Grafting of [Cr(acac)3] onto SiMCM-41 and AlMCM-41 supports.

Related Experiment Videos

  • Material characterization using X-ray diffraction, N2 adsorption, thermogravimetric analysis, UV-Vis-NIR diffuse reflectance spectroscopy (DRS), electron spin resonance (ESR), and Fourier transform infrared spectroscopy.
  • Ethylene polymerization experiments in gas and slurry phases at 100°C, monitoring catalytic activity and polymer characteristics.

    • Main Results:

    • Two interaction mechanisms were identified: hydrogen bonding on SiMCM-41 and ligand exchange or hydrogen bonding on AlMCM-41.
    • As-synthesized Cr3+ species exhibited distorted pseudo-octahedral coordination, while calcination led to Cr6+ (dichromate/chromate) and Cr5+ species.
    • The most active catalyst, 1 wt% [Cr(acac)3]-AlMCM-41 pretreated at high temperatures, achieved a polymerization rate of 14000 g PE/(g Cr·h), producing high molecular weight polyethylene nanofibres.

    Conclusions:

    • The nature of the MCM-41 support (Si vs. Al content) and pretreatment conditions significantly influence catalyst activity and chromium speciation.
    • Ethylene polymerization over Cr-MCM-41 catalysts leads to the formation of unique polyethylene nanofibre structures within the mesopores.
    • Combined DRS-ESR spectroscopy is effective for monitoring chromium redox states and coordination environments during catalysis.