Synthesis of Polyoxymethylene Dimethyl Ethers From Dimethoxymethane and Paraformaldehyde Catalyzed By Cation Exchange Resins

Synthesis of oxygenated compounds as diesel fuel additives from coal-based C1 chemicals is of great importance to solve the large surplus of C1 chemicals and the diesel supply shortage in China. When added to diesel , oxygenated compounds (methanol , dimethyl ether , dimethoxymethane , etc.) can significantly reduce smoke and engine emissions , which is closely correlated to the oxygen content of the compounds. However , methanol with low solubility in diesel , dimethyl ether with higher vapor pressure and lower viscosity than diesel and dimethoxymethane with a low cetane number are difficult for wide use as diesel fuel additives. As higher homologue of dimethoxymethane , polyoxymethylene dimethyl ethers(PODEn , CH3O(CH2O)nCH3) are new concerned oxygenated compounds as green diesel fuel additives. Among the series of PODEn , PODE3-5are ideal because of the proper physical properties for diesel. Dimethoxymethane and paraformaldehyde are firstly reported to be used as raw materials to synthesize the PODEn compounds in this work , with cation exchange resins as catalysts. Compared with some reports on synthesis of PODEn , no water is produced during this process , which is favorable for reducing side reactions and increasing the product yields. Seven different cation exchange resins were examined in this work. Among them , the NKC-9 resin was the most active catalyst. The characteristic results of scanning electron microscopy and nitrogen adsorption-desorption , together with the analysis of catalysts acid strength and the number of acid sites , were used to explain different catalytic performances. It was found that strong-acid cation exchange resin with large surface area , proper pore volume and pore diameter and high concentration of acid sites is favourable to give higher catalytic activity. The effects of reaction temperature , DMM/CH2O molar ratio , reaction time and catalyst loading on formaldehyde conversion and product distribution were studied. At optimized operating conditions , a formaldehyde conversion of 85.1% was obtained with the selectivity to PODEn near 100% , and the amount of PODE3-5 in products was 36.6wt%. A possible mechanism , which includes decomposition of a paraformaldehyde molecule to formaldehyde molecules , protonation of formaldehyde molecule at the carbonyl group , formation of the intermediate and finally formation of a PODEn+1 molecular , was proposed to give a step-by-step account of the bond reorganizations that take place in the course of the reactions. The catalyst characterizations and reaction data would lay a solid foundation for the further study on PODEn.