Hydrosilylation reaction refers to the addition reaction between organosilicon compounds containing silicon-hydrogen bonds and compounds containing unsaturated bonds under certain conditions. One of the important response types. The unsaturated bond may be a carbon-carbon double bond, a carbon-carbon triple bond, a carbon-oxygen double bond, a carbon-nitrogen double bond, and the like. Among them, the addition reaction of silicon-hydrogen bond and carbon-carbon double bond is the most common hydrosilylation reaction in the silicone industry, and is the core reaction of many addition-type silicone products.
For the hydrosilylation reaction, in addition to the silicon hydrogen compound and the unsaturated compound, the catalyst is an indispensable component, which plays the role of accelerating the reaction speed, shortening the reaction time and lowering the reaction temperature. Catalysts for hydrosilylation reactions are mainly compounds and complexes of the transition metal elements of group VIIIB in the periodic table of elements, such as platinum, palladium, nickel and rhodium, and their complexes, among which platinum catalysts have the highest catalytic activity and are most widely used.
Platinum catalysts used to catalyze hydrosilylation reactions are mainly divided into homogeneous catalysts and heterogeneous catalysts. Wherein, the homogeneous catalyst and the reactant are in the same phase, and there is no phase interface. In the hydrosilylation reaction, the homogeneous catalyst generally refers to a liquid catalyst.
There are three main types of homogeneous platinum catalysts: the first is to dissolve chloroplatinic acid (H2PtCl6 6H2O) in organic solvents such as ethanol, isopropanol, and tetrahydrofuran, and let them interact to form complexes, which are called "Speier catalysts". This kind of catalyst is easy to make and easy to use; the second type of platinum catalyst is a complex of platinum and vinyl double head, called Karstedt's catalyst (Karstedt's catalyst), which has high reactivity and can be stored stably , and has good compatibility with various types of polysiloxanes, and is the most widely used catalyst in addition-type silicone release agents; the third type of platinum catalyst is chloroplatinic acid in addition to forming with other unsaturated compounds Complexes, and also form complexes with ketones, cyclopentadiene, esters, alcohols, crown ethers, heteroatom-containing crown ethers, and polysiloxanes, and are used as catalysts.
The homogeneous platinum catalyst has high activity and reaction selectivity, but because it is in the same phase as the reactant, it is difficult to separate after the reaction and cannot be reused, so the cost is high, and it may cause heavy metal ion pollution. Therefore, researchers immobilize homogeneous catalysts to make heterogeneous catalysts. The so-called immobilization of homogeneous catalysts is to combine homogeneous catalysts with solid supports by physical or chemical methods to form a special catalyst. The heterogeneous platinum catalyst is in a different phase from the reactant, and the common form is that the solid catalyst catalyzes the hydrosilylation reaction of the liquid mixed reactant.
The heterogeneous catalysts for hydrosilylation reactions are mainly divided into two types, traditional heterogeneous platinum catalysts and polymer metal complex catalysts. Among them, traditional heterogeneous catalysts are formed by adsorbing transition metals on inorganic particles such as carbon black and alumina. This type of catalyst has high stability and can be recycled and reused, but its catalytic activity and selectivity are low. Higher temperature and pressure are required. The polymer metal complex catalyst consists of three parts: a polymer carrier, a ligand bonded to the carrier, and a transition metal. Generally speaking, although heterogeneous catalysts have the advantages of high stability and recyclable reuse, their application is limited due to their low activity and selectivity, complicated preparation process, and reduced catalytic activity when reused. limit.
There are many substances that can "poison" platinum catalysts, causing their catalytic activity to decrease or even lose their activity. For example, organic compounds containing elements such as nitrogen, phosphorus, and sulfur, and ionic compounds containing heavy metals such as tin, lead, mercury, bismuth, and arsenic. Therefore, special attention should be paid during the use of platinum catalysts to avoid contact of platinum catalysts with these substances that can poison them. There are many types and forms of platinum catalysts. Therefore, in the application process, it is necessary to select a suitable platinum catalyst according to the characteristics of the reaction system in order to obtain the best catalytic effect.
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