Preparation Of Molecular Sieves Supported Ionic Liquids And Their Catalytic Performance In Synthesis Of Polyoxymethylene Dimethyl Ethers

Polyoxymethylene dimethyl ethers (DMM3-8) as an oxygenated fuelcompound is regarded as the very promising and green diesel additivecomponent. When added to diesel fuels, it can not only largely reduce emissionsof NOxand particulate matter, but also increase cetane number of diesel fuel,and require engine without any modifications. However, present study ofpolyoxymethylene dimethyl ethers is still in the initial stage, and it does notbeen applied to large-scale industrialized production.The key to the preparation of DMM3-8is the development of high catalyticcatalysts. At present, there are many catalysts used in the synthesis ofpolyoxymethylene dimethyl ethers. Among them, ionic liquids as green andenvironmental catalyst have higher catalytic activity and better product yieldcompared with other catalysts, but there are also some problems, such as highcost, difficult separation after the reaction. Therefore, a series of molecularsieves supported ionic liquids catalysts were prepared so as to solve aboveproblems, and sometimes improve the yield of polyoxymethylene dimethylethers.Firstly, the functionalized N-(3-sulfonic acid) propyl pyridinium ionicliquid was immobilized on NaY molecular sieve with (3-chloropropyl)trimethoxysilane as coupling agent. The catalysts were characterized by X-raydiffraction (XRD), infrared spectrometer (FT-IR), N2adsorption/desorption, scanning electron microscope (SEM) and elemental analysis measurements. Theinfluences of material molar ratio (ratio of methanol/trioxane), reactiontemperature and reaction time on the conversion rate of trioxane and yield of theproduct were investigated. It was concluded that optimal reaction conditionswere methanol/trioxane mole ratio of1.5, reaction temperature of110°C,reaction time of3h, the catalyst amount of2.0wt%and the initial nitrogenpressure of2.0MPa. Under these conditions, four different kinds of pyridiniumionic liquids supported on NaY molecular sieve on the effect ofpolyoxymethylene dimethyl ethers were discussed. The results showed thatimmobilized N-(3-sulfonic acid) propyl pyridinium P-toluene sulfonic acid ionicliquid had the best catalytic activity, the yield of DMM3-8was up to68.9%. Thecatalyst could be reused for four times without obvious loss of its activity.Then, N-(4-sulfonic acid) butyl imidazole ionic liquids supported on NaYmolecular sieve catalysts were synthesized in the same way, and the structure ofthe catalysts were characterized by XRD, FT-IR, N2absorption/desorption andelemental analysis measurements. Under the optimized reaction conditions, thetype of anions and the effect of solvents were investigated. The results showedthat the solvent had a great influence on the reaction, in the ethanol solvent,N-(4-sulfonic acid) butyl imidazole fluoroform sulfonic acid ionic liquidsupported on NaY molecular sieve catalyst had the best catalytic activity, theyeild of DMM3-8was up to62.9%and the catalyst had good repeated useperformance.Finally, using the mesoporous SBA-15molecular sieve as carrier,N-(3-sulfonic acid) propyl pyridinium ionic liquid supported on SBA-15molecular sieve catalyst was prepared. The catalysts were characterized by XRD,FT-IR, N2adsorption/desorption and elemental analysis measurements. Theinfluence of the amount of ionic liquid on the yield of polyoxymethylenedimethyl ethers was investgated. The results showed that the single SBA-15had no catalytic activity on the reaction of polyoxymethylene dimethyl ether, whileN-(3-sulfonic acid) propyl pyridinium ionic liquid supported on SBA-15molecular sieve catalyst had obvious catalytic activity. When the mass ratio ofthe carrier and the ionic liquid was4:1, the yeild of DMM3-8was up to36.3%.The catalyst could be reused for at least three times and its activity had almostany loss.