Materials and methods
All solvents were AR grade and purchased from Xi’an Chemical Agent Co, and the 93# gasoline is commercially available. The phase stabilizing and pressure reducing tests were carried out on DFY-cryostat instrument (Xi'an Yuhui Instrument Co.Ltd.) and DSL-080 vapor pressure detector (Dalian the Ceon Electronic Equipment Co.Ltd.).
Synthesis of tartaric esters
The short-chain alcohols can co-dissolved with water, so the produced water can not be separated from the reactant, while long-chain alcohol can carry the produced water out under reflux. So different methods were used for the synthesis of tartaric esters.
Method A [12]: Tartaric acid (23 g), methanol (18 mL), and p-toluenesulfonic acid (TsOH) (0.6 g) were added in a 250 ml flask. Refluxing for 10 h, the mixture was cooled to room temperature. Methanol and methyl tartaric were distillated respectively. The synthesis of ethyl tartaric and propyl tartaric is similar to method above.
Method B [13, 14]: Tartaric acid (0.15 mol), n-butanol (0.45 mol), cyclohexane (30 mL), p-toluenesulfonic acid (0.5 g) were added in a 250 ml flask equipped with a water separator. Refluxing for 5 h, the mixture was cooled to room temperature, cyclohexane, n-butanol and n-butyl tartaric were separated by vacuum distillation. The synthesis of amyl tartaric, hexyl tartaric, hepyl tartaric, octyl tartaric, decyl tartaric is similar to method above.
Phase stability test
The fuel blends were prepared by blending 15, 30, 50 and 65 vol. % of methanol with base gasoline, and the gasoline blends were assigned as M15, M30, M50 and M65. The phase stabilizing tests were carried out according to Chinese National standards of GB 8017–87, GB/T 23799–2009, DB61/T 352–2004 and DB51/T 448–2004. First the test tube full of methanol-gasoline with different ratios was placed in a cryostat, and then the temperature was adjusted from 40°C to −25°C. At each degree, the tube was taken out and was shaken for two to three seconds, and the phase separation temperature was determined as the solution becomes cloudy [15, 16]. The tests were repeated until the separation temperature does not change for 3 times.
Vapor pressure test
Effect of tartaric esters on vapor pressure of methanol-gasoline was investigated according to Chinese standards of GB 8017–87. The methanol-gasoline was poured into the vapor pressure detector and put into the water bath of 37.8°C. The methanol-gasoline was intensive mixed by taking the detector from the water bath every 5 min and reversing violently. The operation was repeated until the pressure does not change for 3 times.