Zirconocene catalysts supported on modified and nonmodified silica
Polimery 2001, No 4, 257
The temperature-programmed desorption (TPD) technique was used to study, by mass spectrometry (MS), the desorbates released from SiO2-supported MAO (I) and SiO2-supported Cp2ZrCl2 (II) surfaces. With II, Cp groups were found to be copiously released at temperatures exceeding 200 °C. Hence, decomposition of Cp2ZrCl2 while the compound is being heterogenized, carmot explain the low activity of the catalyst II in which Cp2ZrCl2 is chemically bonded to the carrier surface through OH groups. The MS (intensity vs. temperature) curves recorded for the I desorbates exhibit maxima (Fig. 2) for m/z = 15-16 at about 200 °C and 400 °C. They represent the methyl groups liberated on thermal destruction of =Al-C- bonds in respectively AolO(Me)2 and AlO2Me. At about 400 °C, the MAO oligomer molecules we re fragmented to produce AlOMe (m/z = 58), AlOAl (m/z = 70), AlOMe2 (m/z = 73), AlO2Me (m/z = 74), Al3O3 rings (m/z = 131), and (Al3O3)-O- rings (m/ z = 147). Alternating adsorption (up to 8 times) of AlMe3 vapor followed by H2O vapor on SiO2 surface was tried to synthesize SiO2-supported MAO with no "free" AlMe3. As a result, surface structures of the type ≡Si-O-(AlMeO)n (n = 8) were produced. The MAO thus synthesized was showed by TPD studies to have a linear or a planar net structure.
Keywords: supported metallocene, methylaluminoxane (MAO), heterogenization, temperature-programmed desorption (TPD)