| Traps-anethole is the main component of star anise oil.The study on the oxidation characteristics of traps-anethole had important theoretical and practical value for the production,storage and deep processing of star anise oil and trans-anethole.In this paper,oxidation characteristics of trans-anethole with oxygen and selectivity of anisaldehyde were investigated.The main research contents and results were as follows:(1)The temperature and pressure of trans-anethole oxidation with oxygen was determined by self-designed mini closed pressure vessel device(MCPVT).The process and oxidation characteristics of the oxidation reaction were reasearched.In nitrogen,pressure in the vessel was stable,thermal decomposition of trans-anethole didn’t occur.But in oxygen,trans-anethole absorbed oxygen.Oxygen was absorbed slowly in the range of 0.15 hour and 0.9 hour,then trans-anethole oxidation reaction became rapid(0.9～3.0h),finally,oxidation process went to slow(3.0～5.5h)in 60℃ for 5.5h.With the increase of reaction temperature,the initial reaction time decreased(1.0h(55℃)>0.5h(75℃)>0.25h(95℃)),but the consumption of oxygen increased(1.23×10-3mol(55℃)<4.08×10-3mol(75℃)<6.09×10-3mol(95℃)).(2)Peroxide was measured by iodimetry and thin-layer chromatography(TLC).The results showed that:In atmosphere,the optimal conditions for the pure traps-anethole with oxygen generated peroxides(peroxide concentration = 85.07mmol/kg)were:T=95℃,t=4h.There were at least three kinds of peroxides.Under high pressure,peroxide concentration increased sharply when temperature over 80℃.At the same time,peroxide concentration was effected by the change of reaction time and n(traps-anethole)/n(O2).(3)Constituents of oxidation products were detected by gas chromatography-mass spectrometry(GC-MS).Also,the selectivity of anisaldehyde was investigated.The oxidation products were anisaldehyde(14.33%),acetaldehyde(1.44%),anethole epoxide(4.72%),anethole diol(0.77%)and P-anisil(0.12%)at n(traps-anethole)/n(O2)=1,55℃ for 3h.The oxidation products were anisaldehyde(25.56%),acetaldehyde(1.22%),anethole epoxide(0.68%),4-Methoxyphenylacetone(22.59%),anethole diol(3.39%)and P-anisil(1.25%)at n(trans-anethole)/n(O2)＝1,210℃ for 3h.Compare with the products in low temperature,a large number of 4-Methoxyphenylacetone was generated in high temperature.So the possible oxidative pathway for traps-anethole was offered.The selectivity of anisaldehyde was higher in low temperature through calculation.Reaction time was optimized.The selectivity of anisaldehyde was 70.36%at 55 ℃ for 7h,which provided a new idea for the oxidation of trans-anethole to anisaldehyde.(4)MCPVT determined the pressure behavior,and then oxidation kinetics was calculated.The results showed that:The oxygen consumption kinetics was zero-order in gas phase,the equation was lnki=-3.97×103·T-1-3.09,the coefficient of correlation was 0.9964.The trans-anethole oxidation kinetics was first-order in liquid phase,lnk2=-5.04×103·T-1+8.60,R2=0.9990.With the increase of reaction temperature,macyoscopic mass transfer coefficient improved.It was benefited for oxidation reaction,because more oxygen dissolved in liquid phase. |
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