| As a main component of black liquid from papermaking industries adopting alkaline pulping, the high-efficient utilization of alkali lignin focuses on how to improve the chemical reactivity caused by its plentiful methoxyl and few aliphatic hydroxyls.Deoxidization effects of H2 on alkali lignin were considered under microwave radiation or at ambient and high pressure, catalyzing by Pd/C catalyst, using H2 as the reagent for deoxidization. Better conditions for the deoxidization of alkali lignin were obtained by analyzing influences on function group changes of alkali lignin caused by various factors. Effect of Pd/C catalyst on activation of alkali lignin was compared separately at ambient or high pressure and applying general heating way or microwave radiation. Results show that high pressure is benefited for improving the reactivity of alkali lignin and ameliorating its reaction capability, and microwave radiation is good at increasing phenol hydroxyl, while not showing special effect on enhancing the reactivity of alkali lignin as a whole. 1H-NMR and UV analysis show that, after reaction, benzene ring in the structure unit of the alkali lignin is steady, carboxyl, carbonyl and methoxyl all have reduced in different extent, while aliphatic and phenol hydroxyl have increased. The relative molecular weight and polydispersity of alkali lignin were determined by Gel permeation chromatography, indicating that weight average molecular weight (Mw) and number average molecular weight (Mn) have reduced in various degree after hydrogenation and activation, while the Mn drops more obviously, inducing the relative molecular weight of the alkali lignin is distributed more widely and the polydispersity index (D) has increased. The element component of alkali lignin was determined by Element Analyzer (EA), and the classical expression for the structure unit of alkali lignin is presented according to the results from EA and function groups' measurement. Expanded empirical formulas suggest that, in structure unit of C9, the numbers of phenol and aliphatic hydroxyl both have increased, while the carboxyl and carbonyl have decreased after hydrogenation. In conclusion, after hydrogenation and activation with H2, the activated groups of alkali lignin have increased obviously, and the chemical bonds between the structures units have ruptured partly, splitting the alkali lignin macromolecule into pieces, so the polymerization degree has reduced. Finally, the reaction performance of alkali lignin has a great improvement as a whole.Results indicate that the optimum conditions for deoxidization of alkali lignin and H2 over Pd/C catalyst at the high pressure are as follows: Pd/C catalyst with a Pd loading of 3% and an amount of 10%, the reaction temperature and time are 100℃ and 5h respectively, at a hydrogen pressure of 4MPa. In that condition, the total hydroxyl, aliphatic and phenol hydroxyl are 15.69%, 11.45% and 4.24%, increasing 158.06%, 310.40% and 28.86%separately;while the methoxyl has reduced from 13.85% to 10.14%. Mw and Mn are 13943, 5196 respectively, decreasing 17.25% and 40.67%, causing the value of D increased from 1.92 to 2.68.Micro-configuration of Pd/C catalyst was characterized by scanning electron microscope (SEM). Recycle and reutilization of the deactivated Pd/C catalyst were carried out, showing that the yield is 94.55% and the purity of Pd is 98.85% at last. Reasons for the deactivation of Pd/C catalyst were discussed, concluding that the main reasons are: the loss of Pd, the growing up of Pd crystal, micropore blocking of activated carbon and poisoning of the catalyst. Reuse experiment of the catalyst indicates that the fresh Pd/C catalyst still can keep better catalysis activity after reusing for three times.
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