(800d) 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(PODE_n, CH₃O(CH₂O)_nCH₃) are new concerned oxygenated compounds as green diesel fuel additives. Among the series of PODE_n, PODE_3-5are ideal because of the proper physical properties for diesel.

Dimethoxymethane and paraformaldehyde are firstly reported to be used as raw materials to synthesize the PODE_n compounds in this work, with cation exchange resins as catalysts. Compared with some reports on synthesis of PODE_n, 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 PODE_n near 100%, and the amount of PODE_3-5 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 PODE_n+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 PODE_n.