Preparation method of low-propionyl-content cellulose acetate propionate mixed ester

The invention relates to a preparation method of low-propionyl-content cellulose acetate propionate mixed ester. Cellulose is prepared from, by weight, 650 g of wood pulp, 296 g of propionic acid, 2250 g of acetic acid, 118 g of acetic anhydride, 2070 g of propionic anhydride and 19.5-52 g of catalysts. The ratio of liquid acid catalysts to heteropolyacid catalyst is between 4 to 6 and 6 to 4. The preparation method includes the steps of conducting activating, the esterification reaction, mixing, magnesium acetate solution adding, hydrolysis and chromatography, sequentially conducting conventional filtering, washing and drying, and obtaining the finished product. The method has the advantages that the propionyl content of the obtained cellulose acetate propionate finished product is 15-20%, and the acetyl content is smaller than 22-30%; the mixture of liquid acid and heteropolyacid is selected for the catalysts, and therefore the sulfuric acid consumption is lowered, the corrosion to a technological device and the pollution to the environment can be reduced, and technological safety is improved; meanwhile, by adding heteropolyacid, the esterification reaction is accelerated, and the hydrolysis reaction is more precise and controllable.