Methylaluminoxane catalyst supports

Jones and co-workers976 have recently reported the use of catalyst 49 (Figure 5.15) with perfluorinated alkanesulfonic acid sites anchored to SBA-15 as a methylaluminoxane-free supported cocatalyst for ethylene polymerization. When catalyst 49 and trimethylaluminum were used in combination with Cp 2ZrMe2 as the metallocene precatalyst, productivities as high as 1000 kg polyethylene molZr-1 h 1 were obtained without experiencing reactor fouling. 

Zhang, Z., Guo, C.-Y., Cui, N., Ke, Y, and Hu, Y. 2004. Preparation of linear low density polyethylene by in situ copolymerization of ethylene with Zr supported on montmorillonite/ Fe/methylaluminoxane catalyst system. Journal of Applied Polymer Science 94 1690-1696. 

Today, less than 5 % of polypropylene is produced using metallocene catalysts. Metallocene catalysts are mainly ZrCh catalysts supported on silica in combination with co-catalysts like methylaluminoxane (MAO). These catalysts show very specific characteristics and may also be combined with Ziegler-Natta catalysts. These catalysts are mainly used to produce specific product ranges and they influence plant configurations. 

Living polymerization Modified methylaluminoxane Single-site catalyst Supported catalyst... 

Although some important aspects concerning the nature of metallocene supported catalysts are not clear at the present stage, the profile of the formation of catalysts (catalyst precursors) with MgCl2, A1203 and Si02 supports and with a methylaluminoxane-pretreated Si02 support can be represented as follows [201, 202] ... 

Most of the supported metallocene catalysts reported so far were devised to immobilise the metallocene on the surface of inorganic carriers, utilising the ionic interaction between the Cl ligands of the metallocene and the surface active site [schemes (19) to (21)]. Similarly, in the methylaluminoxane-pretreated catalyst, the metallocene is immobilised by an analogous ionic interaction [scheme (22)]. Therefore, it is obvious that catalyst precursors formed according to schemes (19) to (22) can be easily activated with common aluminium trialkyls. 

In the case of chlorine-free catalysts such as Mg(OH)2/Ti(OBu)4—[A1 (Me)0]x and Si02/Ti(0Bu)4—[Al(Me)0]x, heterogeneous species are assumed to promote the syndiospecific polymerisation of styrene [67,68]. In a polymerisation system with the latter catalyst, best results were achieved when treating the carrier with [AI(Mc)0]x prior to supporting Ti(OBu)4 (no further activation with methylaluminoxane was needed). The polymerisation rate reaches a maximum at an Al/Ti molar ratio of 20 this is much lower than the value of the Al/Ti molar ratio required to reach the maximum polymerisation rate in the respective homogeneous system, i.e. the system without a carrier [54]. 

As previously mentioned, one of the primary motivations for the development of site-isolated aminosilicas is to construct a better molecular-level understanding of immobilized catalysts through the use of a more uniformly reactive surface. Within the area of a-olefin polymerizations, another parameter that negatively affects the ability to study well-defined surfaces is the use of methylaluminoxane (MAO) as a catalyst activator. The exact structure of the MAO species has been postulated to exist in a number of different forms, which makes it difficult to elucidate the exact nature of its interactions with the surface [21]. To address this issue, a well-defined sulfonic acid organic/inorganic hybrid material was developed to serve as both a support and a catalyst activator for homogeneous a-olefin polymerization catalysts [22]. 

Soluble metallocene catalysts can be prepared by using hydrocarbon-soluble methylaluminoxane or organoborate activators, or their insoluble counterparts can be made by deposition of the metallocene onto silica or alumina solid acid activators [496]. In our experience, the differences in activity, and in the polymer, between soluble nonsupported and insoluble supported catalysts, are not great, which is another indication that under normal conditions the mass transport is not a major issue. 

The mechanism of monomer insertion and steric control in polymerizations of a-olefins by the metallocene catalysts received considerable attention [293-297]. There is no consensus on the mechanism of polymerization. Many studies of chain propagation tend to support the Cossee-Arleman mechanism [293-297]. An example is work by Miyake et al. [294] who synthesized unsymmetrical ansa-metallocenes and separated them into threo and erythro isomers. Both isomers coupled with methylaluminoxane polymerize propylene in toluene to highly isotactic polymers of = 105,000. The isotactic placement is greater that 99.6% and the polymer melting point is 161°C. 

Examples given below illustrate synthesis of alkene polymerization catalysts, but these catalysts are simpler than the supported metallocenes used in industry, because they lack the promoter methylaluminoxane (MAO), an ill-defined material that greatly complicates characterization. Other examples given below illustrate (a) details of the surface chemistry of conversion of an organometallic precursor into a supported catalyst (b) synthesis of metal clusters of various sizes and compositions on a family of supports from metal carbonyl precursors and (c) synthesis of supported bimetallic clusters with combinations of noble (e.g., Pt) and oxophilic (e.g., W) metals that give quite stable catalysts with extremely high metal dispersions. 

Soga, K., Monoi, T. Polymerization of styrene with Mg(OH)xCl2 (x = 0-2)-supported Ti(OBu)4 catalysts combined with methylaluminoxane. Macromolecules, 23,1558-1560 (1990). 

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