Effect Of Lignin Structure And Hydrogenolysis On Its Antioxidant Activity

Lignin is a renewable phenolic polymer with a certain degree of antioxidant activity.Compared to the synthetic antioxidant,lignin used as a natural antioxidant has the advantages of abundant resources,low toxicity and biodegradability.However,the large-scale application in industry of lignin as an antioxidant has been limited due to its lower antioxidant activity derived from complex structure and low degree of conjugation.To improve the antioxidant activity of lignin,the relationship between the structural characteristics of lignin and antioxidant activity was investigated.The effects of different substituent groups on the antioxidant activity were discussed in detail combining lignin model compound and DFT theory calculation.Based on the theoretical studies,a hydrogenolysis of lignin process was designed to prepare the lignin-based antioxidant.The prepared antioxidant was added into the vanish system to explore its potential application in the coating industry for the purpose of expanding the high-valued utilization of technical lignin.The antioxidant capacity of different kinds of lignin was evaluated,and it was found that phenolic hydroxyl was the main structural characteristic factor of effecting of antioxidant activity of lignin by combining the characteristic structure and model of lignin.According to the results of the partial correlation analysis,the structural descriptors governing the antioxidant activity were in decreasing order of Phenolic OH（0.80）>C=O in the side chain（-0.73）>Aliphatic OH（-0.61）>-CH₂-（0.55）>M_w（-0.51）>OCH₃（0.50）.The relationship equation between structure and activity was obtained on the basis of correlated multivariate component regression as:Y=–26.762X₁–23.884X₂–9.566X₃+22.580X₄+66.262X₅+21.383X₆+46.149,where Y was IC₅₀,mg?L^-1;X₁ was the content of phenolic OH,mmol·g^-1;,X₂ was the content of-OCH₃/100Ar,X₃ was the content of-CH₂-/100Ar,X₄ was the content of aliphatic OH,mmol·g^-1,X₅ was the content of C=O in the side chain/100Ar,and X₆ was the molecular weight,g·mol^-1,and all the variables were nondimensionalized.The influence of substituent groups on antioxidant capacity of phenolic compounds was explored by using density functional function（DFT）theory calculation.It was found that the types and locations of the substituted groups had the important impact on the antioxidant activity.The antioxidant activity of diphenolic compound was determined by the locations between the two phenolic hydroxyl,following in the order of o-benzenediol>p-benzenediol>m-benzenediol.This was because O-benzenediol free radical generated from H abstraction can be stabilized by the formation of intramolecular hydrogen bonds.The antioxidant activity of monophenolic compound containing methoxyl was influenced by the location of these two functional groups,following in decreasing order p-methoxyphenol>o-methoxyphenol>m-methoxyphenol.This was because lone pair electron of oxygen atom contributed to the greatest stabilizing effect to free radical when methoxyl existed in para position of phenolic OH.While methoxyl group was existed in ortho position of phenolic OH,the intramolecular hydrogen bonds could be formed to prevent H-abstraction,redecing the antioxidant.When two methoxyl were existed in ortho-positions of phenolic hydroxyl,the antioxidant activity could be enhanced,and it was because reduced bond energy of dehydrogenation resulted in the easy production of free radical,Moreover,intramolecular hydrogen bond which stabilized the free radical improved the antioxidant activity.When the alkyl substitution was existed in the phenolic compound,the delocalization of unpaired electrons in the radical was enlarged,which stabilized the radical,leading to that phenolic compound with para-alkyl had higher antioxidant activity than those without alkyl substitution.However,the antioxidant activity of the phenolic compounds gradually decreased with the increase of the alkyl chain length in para-position due to its lower solubility and migration rate in a polar solvent.Aliphatic hydoxyl on the side chain of phenolic compounds resulted in a lower stability of free radical due to the electron withdrawing effect,and therefore exhibited a lower antioxidant activity.However,the electron withdrawing of aliphatic hydroxyl decreased with the increase of alkyl chain length,resulting in the improvement of antioxidant activity.The stability of free radicals was reduced when a para-substitution withα-carbonyl was existed,decreasing the antioxidant activity.When the carbonyl was located at theβorγposition,the electron withdrawing effect decreased gradually with the increase of chain length,and then the antioxidant activity improved gradually.With Pd/C as catalyst,formic acid as hydrogen donor and ethanol/water as solvent,alkali lignin（AL）was hydrodepolymerized.Under the optimal process variations,the degradation degree of AL reached 85.0%,and the IC₅₀ value of hydrogenolysis product of alkali lignin（HPAL）reached 25.2±0.9 mg·L^-1 in terms of the DPPH·radical scavenging activity,contributing to a significant improvement of antioxidant activity,increasing by 60.3%.Moreover,the antioxidant activity of HPAL was even higher than that of BHT(38.7±1.0mg·L^-1)which was a commercial antioxidant.It was found that hydrogenolysis cleaved the linkages between structural units of lignin,forming more phenolic compounds with low molecular weight phenolic compounds.Consequently,the content of phenolic hydroxyl groups increased by 45.3%,and the content of carbonyl in the side chain and aliphatic hydroxyl decreased by 63.6%and 20.2%,respectively.Hydrogenolysis was one of effective ways to obtain lignin-based compound with high phenolic hydroxyl content and low molecular weight lignin.HPAL was added to varnish to investigate its improvement of antioxidant properties in the varnish.The antioxidatant activity of HPAL in varnish was better than that of AL.in which varnish with the addition 0.3 wt%of HPAL（0.3 wt%HPAL-varnish）exhibited good antioxidant performance after 350 h UV irradiation.Compared to blank varnish,the antioxidant performance of 0.3 wt%HPAL-varnish coating on pine board and medium density fiberboard were improved by 62.8%and 70.6%,respectively.Compared to 0.3 wt%AL-varnish,the antioxidant performance of 0.3 wt%HPAL-varnish coating on pine board and medium density fiberboard were improved by 31.4%and 45.9%,respectively.Compared with commercial antioxidant 1010,the antioxidant performance of 0.3 wt%HPAL-varnish coating on pine board and medium density fiberboard were improved by 52.1%and 58.6%,respectively.Moreover,0.2 wt%HPAL-varnish still exhibited good performance as UV-blocking after 350 h of UV irradiation,which indicated that HPAL was better in both antioxidatant and antiultraviolet than commercial antioxidant 1010.The addition of HPAL had no negative effect on the properties of varnish and its film.The results showed that the development of natural antioxidant based on lignin is a promising approach to high-valued utilizing lignin.