Preparation Of Solid Superacid SO₄^2-/ZrO₂ Based Catalysts And Its Effect On Hydrogen Reduction Reaction Of Soda Lignin

Soda lignin is a type technical lignin from the pulping process.Currently,the common treatment methods for soda lignin were as follows:discharged to the environment,or burned after drying,which resulted in the environmental pollution and waste of resources.Low content of active functional groups and high degree of polymerization are major issues limiting the soda lignin's utilization.Hydrogenation reduction is one of the most effective methods to improve the soda lignin's reactivity.After the reaction,the content of hydroxyl value of soda lignin increased,and the polymerization degree decreased.In addition,the antioxidant activity of soda lignin can be improved during the reaction.In this study,we designed and prepared two type metal-solid acid bifunctional catalysts CuO/SO₄^2-/ZrO₂ and Pd/SO₄^2-/ZrO₂ using the solid super acid SO₄^2-/ZrO₂ as carrier.Then the catalysts to were used to catalyze the hydrogen reduction reaction of the soda lignin.The studies were as follows:?1?The solid acid catalyst SO₄^2-/ZrO₂ was prepared by cdipping and roasting mthod.The effect of catalyst preparation conditions,such as the roasting temperature and concentration of sulfuric acid eluent,on the catalytic reduction was studied.The acid strength of SO₄^2-/ZrO₂ catalysts was evaluated by the methods of Hammett indicator and NH3-TPD.The structure of SO₄^2-/ZrO₂ catalysts was characterized by the methods of XRD,BET,and SEM.The soda lignin,extracted from the wheat straw black liquor,was catalytically degraded by using the SO₄^2-/ZrO₂ catalysts.The optimum reaction conditions were determined by a single factor experiment using the total hydroxyl value and phenolic hydroxyl value as the index.The effects of reaction temperature,time,and amount of catalysts added on the reaction were studied.Chemical analysis,NMR techniques,FT-IR spectra,and GPC were utilized to identify the structural features of lignin before and after reactions.Based on the analysis results,the mechanism of lignin degradation was discussed.Results showed that the solid super acid catalyst SO₄^2-/ZrO₂ was prepared by eluting Zr?OH?4 with 1.0 mol·L-1 sulfuric acid solution for 30 min and roasting at 550 ? for 3.5 h.The acid strength of catalyst was H0<-13.16,and the BET surface area and micropore surface area of catalyst were 36.28 m2·g-1 and 2.26 m2·g-1,respectively.The reaction conditions were determined as follows:the soda lignin of 4.0 g was dissolved in the dioxane-water solution,the catalyst of 0.2 g was added to the mixture and then transferred to a reaction kettle.The system was heated at 100 ? for 4 hours under a hydrogen pressure of 3 MPa.After the reaction,the content of total hydroxyl groups of the soda lignin increased by 57.1%,the content of phenolic hydroxyl groups increased by 11.8%,the weight-average molecular weight decreased from 8200 g-mol-1 to 6200 g-mol-1,and the number-average molecular weight decreased from 3700 g-mol-1 to 2400 g·mor-1.In addition,approximately 25.0%of the ?-O-4 linkages were degraded in the reaction.The main effect of SO₄^2-/ZrO₂ was related to the cleavage of(3-0-4 linkages.The new formed structures of lignin-bound Hibbert ketones and lignin chains lead to an increase in the total hydroxyl groups and phenolic hydroxyl groups.?2?The metal-solid acid bifunctional catalyst CuO/SO₄^2-/ZrO₂ was prepared in two steps:the SO₄^2-/ZrO₂ carrier was first prepared,and the CuO was then loaded onto SO₄^2-/ZrO₂ carrier by dipping and roasting method.The structure of CuO/SO₄^2-/ZrO₂ catalysts was characterized by the methods of XRD,BET,and SEM.The effects of reaction temperature,time,and amount of catalysts added were studied.The structural features of lignin before and after reactions were characterized using the methods such as chemical analysis,NMR techniques,FT-IR spectra,and GPC.Then the reaction mechanism was discussed.Results shcwed that the preparation conditions of catalyst CuO/SO₄^2-/ZrO₂ were as follows:the roasting temperature was 350 ? and the roastirg time was 3 hours.The catalyst CuO/SO4-/Zr02 retained the super acidity of SO₄^2-/ZrO₂,the CuO loading was 5.02 wt%,the BET surface area and micropore surface area of catalyst were 23.60 m2·g-1 and 0.87 m2-g-1,respectively.The reaction conditions were determined as follows:the soda lignin of 4.0 g was dissolved in the dioxane-water solution,the catalyst of 0.2 g was added to the mixture and then transferred to a reaction kettle.The system was heated at 100 ? for 4 hours under a hydrogen pressure of 3 MPa.After the reaction,the reactivity of soda lignin was improved:the content of total hydroxyl groups increased by 62.9%,the content of phenolic hydroxyl groups increased by 29.4%,the content of methoxyl groups decreased by 10.2%,the weight-average molecular weight decreased from 8200 g·mol-1 to 5900 g·mol-1,and the number-average molecular weight decreased from 3700 g-mol-1 to 2300 g-mol-1.In addition,approximately 22.5%of the ?-O-4 linkages were degraded in the reaction.The SO₄^2-/ZrO₂ mainly cleaved the ?-O-4 linkages,whereas CuO cracked the methoxy group on the benzene ring.The new formed structures of lignin-bound Hibbert ketones and lignin chains lead to an increase in the total hydroxyl groups and phenolic hydroxyl groups.?3?The Pd/SO₄^2-/ZrO₂ catalyst was prepared in two steps:the SO₄^2-/ZrO₂ carrier was prepared by the dipping and roasting method,and the Pa was loaded onto the SO₄^2-/ZrO₂ carrier using the reduction method.The structure of Pd/SO₄^2-/ZrO₂ catalysts was characterized by the methods of NH3-TPD,XRD,BET,and SEM.The effect of reaction conditions,such as temperature,time,catalysts addition amount,and solvent,on the reaction was studied.The structure of lignin samples was characterized by using the analytical methods such as chemical analysis,NMR techniques,FT-IR spectra,and GPC.After that,the reaction mechanism of lignin degradation was discussed.Results showed that the catalyst Pd/SO₄^2-/ZrO₂ retained the super acidity of SO₄^2-/ZrO₂,the Pd loading was 1.42 wt%,the BET surface area and micropore surface area of catalyst were 32.90 m·g-1 and 1.86 m-g-1,respectively.The reaction conditions were determined as follows:the soda lignin of 4.0 g was dissolved in dioxane-water solution,the catalyst of 0.2 g was added to the mixture and then transferred to a reaction kettle.The system was heated at 100 ? for 4 hours under a hydrogen pressure of 3 MPa.After the reaction,the reactivity of soda lignin was improved:the content of total hydroxyl groups increased by 97.1%,the content of phenolic hydroxyl groups increased by 35.3%,the content of methoxyl groups decreased by 13.0%,the weight-average molecular weight decreased from 8200 g-mol-1 to 4900 g-mol-1,and the number-average molecular weight decreased from 3700 g-mol-to 2400 g-mol-1.Approximately 63.3%of the ?-O-4 linkages were degraded in the reaction.The Pd/SO₄^2-/ZrO₂ catalyst works by breaking the ?-0-4 linkages and degrading the methoxy groups of the soda lignin.The new formed structures of lignin-bound Hibbert ketones and lignin chains lead to an increase in the total hydroxyl groups and phenolic hydroxyl groups.Soda lignin undergoes a hydrolysis reaction in the acidic ionic liquid[Bmim]Cl.After the reaction,the contents of total hydroxyl groups and phenolic hydroxyl groups of soda lignin were increased by 51.4%and 11.8%,respectively.?4?The antioxidant activity of lignin samples was evaluated by the methods of DPPH radicals scavenging,ABTS+ radicals scavenging,and reducing power.Then the relationships between lignin type,content of total hydroxyl groups,content of phenolic hydroxyl groups,weight-average molecular weight,and antioxidant activity were discussed.Results showed that the catalyst Pd/SO₄^2-/ZrO₂ had the best effects on improving the antioxidant activity of soda lignin compared with the other catalysts.After the reaction,the ability of soda lignin on DPPH radical scavenging and ABTS+ radical scavenging were improved by 24.3%and 32.8%,respectively,and the reducing power was improved by 17.4%.The phenolic hydroxyl value of lignin is positively correlated with its antioxidant activity.Catalyzed soda lignin can be used to synthesize fine chemical products,prepare lignin-based polymer composites,and it can also be used as natural polymer antioxidants for development and utilization.