| Gum rosin is a significant renewable resources in forest chemical industry, which is composed by several resin acids. Rosin is producted form gum rosin, and has a wide application. However, it is easy to be oxidized due to the conjugated bonds, the unstable molecular structure in rosin, which leads to its color fastness and limit the immediate application of rosin, and these are the main reason for its reduced cost. Hence, the oxdition reaction of rosin is a hot filed in both domestic and overseas. The aim of this study is to investigate the oxidation kenetic and process of abietic acid and levopimaric acid, the main ingredient of rosin and gum rosin, respectively. The results would be available to provide theoretically guidace to improve the antioxidant abilities of resin and the development of high quality resin product. The main research contents are:(1) The quantitative analysis methods of the oxidation process of abietic acid and levopimaric acid are established.Primarily, a gas-solid polyethylene film reactor is desiged to investigate the oxidation process of abietic acid and levopimaric acid, which can be used for micro-reaction and online detection probed by UV and IR sepctroscopy. Secondly, the HPLC condition is optimized, which can be used in the quantitative analysis of abietic acid, levopimaric acid and their oxides. The respective standard curves of abietic acid and levopimaric acid under the spectra of HPLC are:y=1.988×105nA+5.914×104、y=1.069×104nL+1.248×104.The oxidation behavior of abietic acid and levopimaric acid are monitored by FT-IR and UV spectroscopy, and the data is analyzed by 2D-IR.(2) The thermal oxidation kinetic of abietic acid is probed by HPLC.The thermal oxidation kinetic of abietic acid under the temperature range of 303-333 K is probed by HPLC. Under the wavelength located at 241 nm, the change of concentration of abietic acid are detected, and the oxidation kinetic equation is:ra=ca×1.12×106×exp(-49.51×103/RT),Ea=49.51kJ/mol, Aa=1.12 × 106min-1. Under the wavelength of 220 nm, a oxide of abietic acid is detected, and the standard curves is:y=5.097×104nAO+5.723 × 103. The kinetic of the oxide is calculated firstly:r2=cP × 6.09 X 105 × exp(-48.06 × 103/RT),E2=48.06kJ/mol,A2=6.09×105min-1. Two kinetic models, consecutive reaction and parallel reaction are used to fit the experimental data, and make the residual and goodness-of-fit analysis further. The results demonstrated that the oxidation of abietic acid is a consecutive reaction. The kinetic of the intermediate is calculated theoretically:r1=cO × 3.51 × 103 × exp(-58.96 × 103/RT),E1=58.96kJ/mol,A1=3.51 × 103min-1. Meanwhile, this step is the control step in the consecutive reaction.(3) The investigation of oxidation process of abietic acid by using two-dimensional infrared correlation spectroscopy.A novel polyethylene film reactor is designed for gas-solid reaction. It is suitable for monitoring oxidation by both FT-IR and UV spectroscopy.Useful and novel information about the timing of the structural transformations during oxidation process is obtained, on basis of the strong correlation peaks in the synchronous maps of PCMW2D spectra when the different bands regions (C=C region, C-O region, C-H region) arecalculated. The results imply that the transition time ranges of abietic acid isfrom approximately 100 min to 160 min.The 2D-IR correlation spectra in the three aforementioned MIR regions indicate the sequence of changes in the conjugated bond and hydroxyl groups and based on the kinetic, LC-MS and UV analysis, a process of oxidation of abietic acid is conclued:thehydroxylgroup was formed at C7 by the attack of oxygen,thereby inducing the isomerization of the conjugated band.Meanwhile, the methylene at C12 was converted by an oxygen atom to a hydroxyl intermediate. Hydrogen continuedto react with oxygen to form C=O and water. Finally, the conjugated band was converted into a peroxide before transforming into an oxidant.(4) The thermal oxidation kinetic of levopimaric acid is probed by HPLC.The thermal oxidation kinetic of levopimaric acid under the temperature range of 303-323 K is probed by HPLC. Under the wavelength located at 271 nm, the change of concentration of abietic acid are detected, and the oxidation kinetic equation is:rLA=cLA × 2.38 × 105 × exp(-44.96 × 103/RT), ELA=44.96kJ/mol, ALA=2.38×105min-1. Under the wavelength of 230 nm, two oxides of levopimaric acid are detected, and the standard curves are:y=1.956× 105nO1-1.096 × 103, y=1.276×105nO2-1-275×103. The kinetic of thesetwo oxidesare calculated firstly. The kinetic equation of oxide 1 is:rO1=cO1×5.38× 103×exp(-35.85×103/RT), EO1=35.85kJ/mol, AO1=5.38×103min-1. The kinetic equation of oxide 2 is:rO2=cO2 × 2.86 × 108 × exp(-67.09 × 103/RT), EO2=67.09kJ/mol, AO2=2.86 X 108min-1. Two kinetic models, consecutive reaction and parallel reaction are used to fit the experimental data, and make the residual and goodness-of-fit analysis further. The results reflected that the oxidation of levopimaric acid contain both consecutive reaction and parallel reaction. Under the relatively low temperature, the two oxides produced as parallel products, whereas under a high temperature, oxide 1 is the intermidiate.(5) The investigation of oxidation process of levopimaric acid by using two-dimensional infrared correlation spectroscopy.The auto-oxidation process of levopimaric acid is investigated by using 2D-IR and PCMW2D correlation. In view of PCMW information based on spectra, we can discover the intensities change as follows:The rapid oxidation process of C=C group takes place between 100-200 min while the C-O group has the time range of 120-220 min and share the same transition point at 150 min. We then utilize 2DIR method to obtain the changing sequence of the related bands, and based on the kinetic analysis, a process of oxdiation of levopimaric acid is conclued:The C=C in conjugated group added with oxygen, cause the formation of C-O in peroxide group and a new C=C on C13-C14. It is the typical intermediate of levopimaric acid, the 8,12-peroxy-△ 13(14)-dihydroabietic acid. The peroxide continue rearrangement reaction under tow process:One of the route is to produce 8,14;12,13-diepoxyabietic acid; Whereas the other route is to form two new hydroxyl groups on C8 and C12 respectively. And one of the hydroxyl (on C8) convert to epoxy group, as reported before. The other one hydroxyl (on C12) transform to keto form, due to the enol structure produced by the isomerism of C=C. |
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