Title Optimization And Design Of The Directional Depolymerization Process Of Lignin In Lignocellulose

The continued existence of global climate change and the long-term dependence on petroleum resources have made human continuously explore effective ways to produce energy,chemicals and materials from renewable resources,which has also promoted the development and innovation of the refinery of lignocellulose.In the past,the biorefinery mainly focused on the conversion and utilization of sugar components,and underestimated the value of lignin components.Lignin is the only aromatic hydrocarbon renewable resource that exists in large quantities in nature.But in the traditional paper and pulp industry,a large amount of lignin is converted into heat and electricity in an inefficient way of direct combustion.In recent years,the emergence of the lignin-first depolymerization process has provided a feasible route for the realization of efficient lignin utilization.At the same time,the carbohydrate components remain in solid form,which can be upgraded and transformed.Nowadays,the research focus of lignin-first depolymerization process has begun to gradually move from batch process to semi-continuous process,and it is also promoting the realization of future industrial scale-up and continuous production of this process.However,due to a certain gap between the technical conditions of the two,there are still some problems that need to be resolved during the moving process.The research content of this paper is based on the problems existing in the moving process,including the retention of hemicellulose,the synergistic relationship of solvent extraction and reductive stabilisation,the construction of mild semi-continuous process and research of feedstock in pilot scale semi-continuous process and exploration of product application.The specific research contents are as follows:(1)The core of the lignin-first depolymerization process is not only to achieve efficient depolymerization of lignin,but also the retention of carbohydrates.The loss of carbohydrates mainly comes from hemicellulose.Through the development of highly selective catalyst,precise separation of lignin and carbohydrates is achieved.MoxC/CNT can extract and depolymerize lignin efficiently,and the yield of lignin oil is as high as 98.1 wt%.At the same time,the cellulose and hemicellulose components are well retained,and the yields of C6 and C5 sugars can reach 98%and 90%,respectively.The MoxC/CNT catalyst obtained under 700℃ carbothermal reduction conditions has the best performance.At this time,the MoxC particles are simultaneously distributed inside and outside the carbon nanotubes.(2)The solvent extraction and the reductive stabilization of the lignin-first depolymerization semi-continuous process need to be performed separately and the possibility can be verified through the online catalyst addition reactor.The main function of Ru/C catalyst is to realize the reductive stabilization of unsaturated lignin fragments rather than participate in the lignin extraction.Once the temperature is higher than 200℃ when the catalyst is added,the reductive stabilization from catalyst begins to weaken,and the yield of lignin monomers also begins to decrease.The depolymerized fragments of lignin undergo rapid repolymerization at around 250℃.The timeliness of catalytic reductive stabilization is closely related to lignin extraction temperature.In the delignification process under high temperature conditions,timely reductive stabilization of the catalyst is essential.However,when the lignin extraction temperature is lower than 200℃,the reduction stabilization can be delayed to a certain extent,and it is feasible to separate the lignin extraction and the reduction stabilization.(3)Reducing the operating pressure of lignin-first depolymerization semi-continuous process is the key to achieve the continuous process in the future.A semi-continuous process of efficient extraction and depolymerization of lignin can be achieved under mild reaction conditions of 190℃ and 0.8 MPa hydrogen pressure by using ethylene glycol which is a high-boiling point solvent.In the semi-continuous process with high catalyst content,the yield of lignin monomer can reach 19.7 wt%.The ratio of monomer to oil yield is 0.46,which is higher than the result of the corresponding batch process.Carbohydrates are also well retained,and the retention rates of C6 and C5 sugars reach 93.8%and 84.4%,respectively.Ru/C pellet catalyst has the same performance as the commercial Ru/C powder catalyst,which is more suitable for fixed-bed reaction.Low-boiling solvents include methanol and ethanol cannot achieve the lignin-first depolymerization semi-continuous process under mild conditions.Mild process condition is the basis for scale-up of semi-continuous process,and also provides the possibility for the use of continuous feeder to realize the real continuous process in the future.(4)Powder feedstock is usually used in lignin-first depolymerization batch process,but pellet feedstock is more suitable for pilot scale semi-continuous processes.The walnut shells feedstock with different particle sizes are selected to react in batch process to verify the applicability.The lignin content in walnut shell is high,and the monomer yield can reach 42.6 wt%.Appropriately increasing the particle size of feedstock will reduce the yield of lignin oil,but the yield of lignin monomers will not be greatly reduced.The feasibility of delignification on a pilot scale is verified using pellet feedstock,and the product analysis is not much different from the results of the laboratory test.In terms of product application,lignin products can be separated by extraction and carbohydrates can be uesd to prepare biochar with high specific surface area.The specific surface area of biochar prepared from carbohydrates obtained from 20-40 mesh walnut shell is as high as 1225.74 m2/g.