Studies On The Catalytic Synthesis Of Polyoxymethylene Dimethylethers

Polyoxymethylene dimethyl ethers (PODE) are a series of compounds with the formula CH3O(CH2O)nCH3, in which n is an integer equal to or greater than 1 (usually smaller than 10). These compounds are of high oxygen contents (42%-51%) and cetane numbers (>49). As diesel additives, they can improve the thermal efficiency and reduce emissions of particular materials (PM) and NOX. In addition, the boiling points, vapor pressures and solubility of PODE are in the range of diesel and thus they can be directly blended into diesel without the structural changes of diesel engines.The catalysts used for the synthesis of PODE mainly include the liquid acids and solid catalysts such as acidic resins, molecular sieves and solid superacids. The liquid acids are corrosive to reactors and difficult to separate from the reaction products. Some solid acid catalysts are expensive and difficult to synthesize. Thus, it is important to develop the solid acid catalysts that are cheap and easy to prepare and separate from the reaction products. Meanwhile, the presence of water in the system would lead to the accumulation of water when recycling the by-products for the synthesis of PODE, resulting in the significant decrease of yields of target products. Thus, the separation of water from the synthesis system of PODE is another important issue of this study.In this work, some solid acid catalysts were prepared and used for the synthesis of PODE. Some molecular sieves were tested for the separation of water from dimethoxymethane (DMM) that might mimic the PODE synthesis system. Some important results obtained in this work are summarized below.(1) Cheap raw materials with simple structures were selected to prepare the ionic liquids. Four ionic liquids, pyridine sulfate,2,4-dimethyl pyridine sulfate, quinoline sulfate and methane sulfonic acid pyridine were synthesized. Water in the synthesized ionic liquids was removed by washing with acetone, and solid ionic liquids at room temperature were obtained, which were expected to be easily separated from PODE by simple filtration. Pyridine sulfate and quinoline sulfate were selected as catalysts for the synthesis of PODE and PODE2-8 were successfully prepared with the similar yields to the case when sulfuric acid was used as a catalyst. After the reaction, the quinoline sulfate catalyst presented as a solid and was more easily separated from the products, as compared to the pyridine sulfate catalyst. However, the decomposition of quinoline sulfate catalyst up to 6% might occur during the synthesis of PODE, which might be unfavorable for the subsequent separation and purification of target products.(2) Ferric sulfate was tested as a solid acid catalyst for the synthesis of PODE and PODE2-8 were successfully obtained with the lower yields as compared to the case when sulfuric acid was used as a catalyst. The optimal mass of ferric sulfate was determined to be about 5% of total mass of reactants. When the mass of ferric sulfate was lower than that amount, ferric sulfate would be saturated by water formed during the reaction and even hydrolyzed, leading to the lower activity for the reactions. The ferric sulfate catalyst could be recycled for the synthesis of PODE at 150℃ through the simple filtration and heating (drying). However, some iron cations might be remained in the products although the contents were extremely low (-2 ppm), which could be further reduced by adsorption with MgO to guarantee the subsequent purification and application of desired PODE products.(3) Different amount of water was added into DMM to mimic the PODE synthesis system containing small amount of water. Molecular sieves 3A and 4A were tested for the separation of water from the mixture of DMM and water. Initially, the weights of molecular sieves could be increased up to 15% due to the adsorption of water.3A molecular sieve possessed the higher capacity than 4A for the absorption of water. After regeneration by heating, the capacities of these molecular sieves for the adsorption of water were decreased to about 11%(remained high). Thus, these molecular sieves could be re-used as effective agents to remove water from PODE synthesis system, although the specific parameters, such as flow rates and regeneration conditions should be further optimized.