Professor Masahiro Kamitani from the Department of Chemistry, Kitasato University, Japan, presents silicone materials and recent achievements in their production process with iron abundant earth
Silicone materials (not silicone) are polymers based on silicon in the form of oils, rubbers and resins; these are used in a wide range of applications, for example, cosmetics, kitchenware, contact lenses, etc. (Figure 1).
Compared to carbon-based polymers like polyethylene (one of the plastics), these silicon-based polymers have excellent properties, especially in terms of high resistance to heat and weathering. Nevertheless, the market size of silicones, as well as the range of their applications, is much smaller than that of carbon-based polymers mainly due to the high cost of silicone materials due to the problems associated with their production.
Current silicone production process
The majority of synthetic carbon-based polymers (plastics), such as polyethylene (PE), polypropylene (PP) and polystyrene (PS), are prepared mainly from limited fossil resources; on the other hand, the main resource of silicon is sand, which is very abundant on Earth. The chemical bonds in silicone materials are Si-O-Si bonds in the main chain and Si-C bonds in the side chain. Si-O-Si bonds are ubiquitous in natural materials, for example in quartz and sand such as silica (SiO2). Thus, the heat and weather resistance properties of silicones are derived from the sturdy main structure (Si-O-Si), which is similar to silica (the main component of glass).
The Si-C bond does not exist in nature and is prepared in industry and research laboratories. In other words, all of the silicone materials in the world are produced by man-made chemical processes. Hydrosilylation is one of the main chemical reactions used for the formation of Si-C bonds from raw materials (hydrosilane and alkene). Pt-based catalysts, called Karstedt catalysts, are extremely active for hydrosilylation and are used in industrial processes. However, the Pt is well dispersed in the silicone products after the reaction and for the most part becomes non-recyclable. The amount of non-recyclable Pt is estimated at around 3% of annual world production (AJ Holwell, Platinum Metal Review 2008, 52, 243-246), which increases the cost of silicone production.
Inexpensive iron catalysts alternating with platinum catalysts
Bolm called the flourishing research on the development of iron-based catalysts the “new iron age” (C. Bolm, Nat. Chem. 2009, 1, 420); therefore, iron has been actively studied as a synthetic catalyst to replace the more expensive precious metal catalysts (eg platinum) which are widely used for chemical processes. Iron is the most abundant transition metal on the planets and, unlike precious metals, it is relatively cheaper and less toxic.
The replacement of platinum catalysts for silicone production with an iron catalyst has been widely studied in recent decades. Although some iron catalysts with activities comparable to those of platinum catalysts have been reported to date, they are not suitable for practical use due to their sensitivity to air. Other iron-based catalysts were stable in air, but exhibited lower activity. Thus, there is a trade-off between the stability and activity of the catalyst, and it is very difficult to achieve high stability and activity simultaneously, which is important for designing an iron catalyst for practical use.
Stable but active iron catalysts
Recently, our research group has developed air-stable iron complexes for various catalytic reactions (Kamitani et al., Bull. Chem. Soc. Jpn. 2018, 91, 1429-1435) and one of them them is now on sale from a private company. The stability of these catalysts was confirmed by an air exposure test (Figure 3).
The series of iron complexes exhibited catalytic performance for hydrosilylation, i.e. its turnover number (TON = 480,000) and turnover frequency (TOF> 10,000 / h) are the values the highest reported for iron catalysts (Kamitani et al., Chem. Lett. 2019, 48, 1196-1198). We are currently carrying out new developments for more useful and practical applications. This study was funded by the New Energy and Industrial Technology Development Organization (NEDO, Japan) project (PL: Dr. Kazuhiko Sato, AIST, Japan). We also thank Dr Hiroshi Nakazawa (OCU, Japan) for his support of the NEDO project.
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