Study Of Catalytic Lignin Depolymerization By "Metal-Acid" Bifunctional Catalyst

With the increasing consumption of fossil energy and related environmental pollution,the alternatives to fossil fuels with low impact on overall carbon emissions has attracted global attention.Biomass has the characteristics of large output,wide distribution,carbon neutrality,etc,and is the only renewable carbon source in nature,which can be converted into liquid fuels and chemicals to relieve the pressure of fossil energy.Lignocellulose,the most abundant type of biomass on earth,is mainly composed of cellulose,hemicellulose and lignin.Currently,the biorefinery strategies in the industry have been committed to the value-added of carbohydrates(such as bio-ethanol from carbohydrates,pulp or paper mill),and lignin is usually regarded as a low-value by-product or cheap energy source.However,the economic viability and environmental sustainability of a biorefinery can be significantly increased if the valorisation of lignin,the only renewable source of aromatics in nature.Lignin,a stable,three-dimensional biopolymer,is resistant towards biological and chemical deconstruction and degradation.The conversion of lignin into well-defined products is hampered by some problems,such as low depolymerization efficiency,high activity of lignin deconstruction intermediates,and the complexity of products.To address the above problems,a series of "Metal-acid" bifunctional catalysts were designed and synthesized,and the catalytic effects of the "Metal-acid" bifunctional catalysts on Kraft lignin depolymerization were systematically studied.The lignin raw material used in this thesis is Kraft lignin,there are a wide variety of depolymerization products.Rather than targeting a single commodity chemical from Kraft lignin,a more practical solution could be to produce a blended feedstock that can be integrated and upgraded within existing petrochemical products.Therefore,we used the yield of petroleum ether soluble product(mainly lignin monomers,dimers and a small amount of trimers)as an important index to evaluate the depolymerization effect of lignin.Based on the analysis of the difficulties,it is concluded that the efficient adsorption and polarization fracture of C-O bonds and the stability of intermediates are the key factors to realize the efficient depolymerization of lignin.Firstly,ZnCoOx nanoplates prepared by a homogeneous precipitation method was used for lignin depolymerization.It was found that the amount of urea and the content of cobalt during the preparation process would affect the performance of ZnCoOx catalyst.When the urea/metal ratio was 1.0 and the molar content of cobalt was 10%,the prepared ZnCoOx-10 nanoplates exhibited the highest activity.Under the optimal reaction conditions(300?,24 h),the yield of liquid product and petroleum ether soluble product were 81.7 and 58.0 wt%,respectively.Among them,the monomers are mainly guaiacol and its alkyl substituents,isoeugenol,and homovanillic acid.Kraft lignin shows a broader molecular weight distribution with the number average molecular weight and the weight average molecular weight of 1286 g/mol and 4360 g/mol,respectively.After depolymerization,the petroleum ether soluble product has a narrow distribution with the number average molecular weight 137-148 g/mol and the weight average molecular weight 213-232 g/mol.The meticulous characterization and control experiments indicated that the catalytic activity of ZnCoOx benefits from the synergy of Zn and Co.According to the yield and distribution of depolymerization products under different reaction conditions,the plausible reaction pathways of lignin depolymerization over ZnCoOx are proposed.On the basis of the ZnCoOx nanoplates,the Co-Zn/Off-Al H-beta catalyst with hierarchical pores was developed,which greatly improved the depolymerization efficiency of lignin.In the presence of Co:Zn=1:3/Off-AlH-beta catalyst,the yields of liquid product(87.5 wt%)and petroleum ether soluble product(81.1 wt%)were obtained at 320? for 24 h.Under these conditions,the liquid fuel gave a higher heating value of 33.3 MJ/kg,which is a significant increase from 26.0 MJ/kg of Kraft lignin.Through the change in the yield of petroleum ether soluble product in the cyclic experiments and the characterizations for the recovered catalysts,it was found that the deactivation of Co:Zn=1:3/Off-Al H-beta is mainly caused by the deposition of bulky lignin and its degradation product on the surface and within pores of the catalyst,resulting in the substrate could not effectively contact with the active components.But after simple re-calcination to removal of sediment,the activity of the catalyst can be almost completely restored.Considering that H-beta used in Co-Zn/Off-Al H-beta catalyst has Lewis acid aluminum species,the design concept of "Metal-acid" bifunctional catalyst with hierarchical pores is simplified.Herein,we prepared a bifunctional Ni/DeAl-beta catalyst through a two-step post synthesis method composed of partial dealumination-desilicication and nickel incorporation.In the presence of 0.6 Ni/DeAl-beta catalyst,lignin reacted at 300? for 36 h to obtain a high liquid product yield of 88.6 wt%and petroleum ether soluble product yield of 65.0 wt%.Compared with Co-Zn/Off-Al H-beta catalyst,0.6 Ni/DeAl-beta catalyst prepared with the similar preparation method is not as active as Co:Zn=1:3/Off-Al H-beta in the lignin depolymerization.The result demonstrated that the synergistic effect of Zn and Co is more conducive to the depolymerization of lignin,wherein Lewis acid Zn can polarize the C-O bonds,and the hydrogenation reaction initiated by Co stabilizes the depolymerization intermediates.In the reaction of ZnCoOx,Co-Zn/Off-Al H-beta and 0.6 Ni/DeAl-beta catalysts for lignin depolymerization,the different activities lead to different yield of petroleum ether soluble product.The monomers are mainly guaiacol and its alkyl substituents,and the oxygen content in the liquid product is between 19.1-21.5 wt%.In order to improve the quality of liquid fuels,it is highly desirable to upgrade the lignin depolymerization products by hydrodeoxygenation.A hollow Ni-Fe(H-NiFe2O4)catalyst was synthesized,and the Lewis acid Fe and Ni components were derived from the capsule wall.At high temperature,the H-NiFe2O4 catalyst can activate the methoxy group of guaiacols resulting in demethoxylation and alkylation reactions,and finally a series of alkyl-phenols was formed.The liquid product yield reached 90 wt%with minimum coke yield of 3.4 wt%at 320? for 24 h.The yield of petroleum ether soluble product was 70 wt%,of which the oxygen content was reduced to 13.9 wt%,and the high heating value increased from 26.0 MJ/kg of lignin to 35.3 MJ/kg.These results indicated that the H-NiFe2O4 catalyst can efficiently convert lignin into liquid fuels by hydrodeoxygenation.In addition to devoting to the development of high-efficiency catalysts for depolymerization of lignin,the catalytic mechanism of the catalysts in the depolymerization of lignin has also been explored,which will further contribute to the design of high-efficiency catalysts.The activities of ZnCoOx,Co-Zn/Off-A1 H-beta,Ni/DeAl-beta and H-NiFe2O4 in lignin depolymerization were compared,and Co:Zn=1:3/Off-Al H-beta showed the best catalytic performance.We propose a detailed mechanistic study conducted with lignin model compounds(a-O-4 and ?-O-4 linkages)via a Co:Zn=1:3/Off-Al H-beta catalyst that obtained the highest yield of petroleum ether soluble product.The results showed that the conversion was readily completed using a Co:Zn=1:3/Off=Al H-beta catalyst,but 40%of a-O-4 was converted and ?-O-4 did not react in the absence of catalyst under the same conditions.Lewis acid Zn could polarize and break the ether bonds by adsorbing and activating the C? and C?hydroxyl groups on the side chains,combined with the hydrogenation and hydrogenolysis reactions initiated by Co,and finally leads to the degradation of the model compounds.This work provides a deep understanding of the activity of Co-Zn/Off-AlH-beta catalyst in lignin depolymerization.