The synthesis of difluoromethane mainly includes dichloromethane fluorination method, hydrochloroflurocarbon hydrogenolysis reduction method, formaldehyde fluorination method, trioxane method.
In dichloromethane fluorination method, dichloromethane react with HF or KF. When KF is used, ω-fluorine, chlorine, hydrogen-3-oxa-polyfluoroalkyl sulfonyl fluoride or sulfonyl chloride will be needed as a catalyst, and the synthesis will become lack of versatility. So HF is usually chosen to react with dichloromethane in the industry synthesis.
The main reaction formulas in dichloromethane fluorination method:
According to the phase where the reactions take place, difluoromethane fluorination method is divided into liquid phase fluorination, gas phase fluorination and sectionally continuous fluorination method.
Liquid phase method: The process parameters for the synthesis of difluoromethane in liquid phase are as follows: the reaction pressure is 1~10 kg/cm2 in the presence of hydrogen fluoride and fluorination catalyst, the reaction temperature is 50~150 ℃ (the selected temperature at which the hydrogen fluoride is not liquefied). It is characterized in that in the reactor, the HF is essentially in gaseous form and the CH2 Cl2 is in excess and mainly in the liquid phase. The catalyst used is SbClXFy (x + y = 5, y = 1 to 4, preferably y = 2 to 3). The fluorination catalyst is used in an amount such that the molar content of the catalyst in the liquid reaction mixture is from 10% to 90%. At 10% or less, the residence time of the reaction gas is long, the impurities of the R40 are generated in large amount and the yield is lowered. In the case of 90% or more, due to the small amount of organic substances in the reactor, and the amount of catalyst entrained in the atomization is large enough to jam the piping. Above all, the concentration is preferably 40 to 70%.
Gas phase fluorination method uses gaseous chloralkane (such as CH2Cl2, CF3CH2Cl and CH3CCl3, etc.) or olefins (Cl2C = CCl2 and Cl2C = CHCl, etc.) and anhydrous HF as raw materials in fixed bed reactor. Hydrofluoroalkane are prepared on the surface of the catalyst by fluorine-chlorine exchange reaction using solid particles of oxides, fluorides or oxyfluorides of Cr and other additives as catalysts.
Sectional continuous fluorination process: The first stage is a gas phase reaction in which HF reacts with difluoromethane in the presence of a composite catalyst consisting of chloride, hydroxide oxide or fluoride of Al and Cr. The second stage is a liquid phase reaction in which the product from the first stage is directly introduced into the liquid phase reactor in the presence of the chlorides of Sb or their mixture with Sn chlorides as catalyst. The method has a long process pipeline, and has the problems of low utilization rate of raw materials and high energy consumption.
