Study On Functional Construction Of Al-loaded Hydrochar And Action Mechanisms In Glucose Isomerization And Nitrogen And Phosphorus Adsorption

Biochar has been applied in various fields due to its wide sources,adjustable pore structure and directional modification of surface functional groups,etc.Biochar can be divided into pyrochar and hydrochar according to the preparation method.Although pyrochar has been widely studied and applied,hydrochar has a lower preparation temperature and can transform high-moisture content biomass into hydrochar in a short time without drying pretreatment.Besides,hydrochar has abundant oxygen-and nitrogen-containing functional groups on its surface and more stable physical and chemical properties in the liquid phase.Therefore,from the perspectives of cost,environment and properties to overcome the disadvantages of existing technologies,open up the preparation and control process channel of hydrochar and study its functional advantages is of great significance for constructing different types of hydrochar to replace expensive,inefficient,complex or difficult to recover catalysts or adsorbents in catalysis and adsorption.The basic research on its preparation and mechanism regulation can provide theoretical basis and technical support for its industrial application.This study was based on the hydrothermal carbonization of three components of lignocellulose and corn stover,as well as the functional regulation of the hydrochar.The morphology,structure and characteristics of hydrochar were basically regulated by adjusting the reaction temperature,holding time,metal doping and other technological conditions.According to the application requirements,hydrochar was modified specifically,and then the synthesis mechanism of hydrochar based materials and its action mechanism in the process of glucose isomerism and nitrogen and phosphorus adsorption were explored.The main research contents and results are as follows:（1）Study on mechanism of hydrothermal carbonization of three components of lignocellulose and crystal control of aluminum.Hydrochar was prepared from corn stover and the three components under different hydrothermal temperature,holding time,reaction system and aluminum chloride solution concentration.It was found that the addition of aluminum chloride could accelerate the hydrothermal reaction process.The carbonization degree,morphology,the specific surface area and the crystal structure of hydrochar after the modification of aluminum chloride had obvious changes.At higher concentration or hydrothermal temperature,the amorphous aluminum could be transferred to complex polyaluminum hydrochloride crystal.The hydrothermal carbonization mechanism of corn stover and the three components were different.Cellulose and hemicellulose were hydrolyzed to form 5-hydroxymethylfurfural and furan soluble,which then bonded by continuous polymerization and condensation to form carbon microspheres.At higher hydrothermal temperature,lignin degraded to form phenolic oligomers.In corn stover,these phenolic oligomers reacted with the hydrolysates of cellulose and hemicellulose,and then formed complex hydrochar.（2）Study on the preparation of functional hydrochar based catalysts and their catalysis of glucose isomerization.In this study,mild hydrothermal treatment combined with limited oxygen calcination was developed to treat corn stover and three components of lignocellulose for directional preparation of hydrochar based catalysts.The prepared catalyst was synthesized by in-situ loading Al,which had excellent catalytic and recycling performance for glucose isomerization.The fructose yield of35.09%and selectivity of 77.42%were obtained at 160℃for 20 min by catalyst prepared by hydrothermal carbonization at 150℃and then calcination at 400℃with corn stover as raw material.The fructose yield was still more than 25%after the fourth recycling,which was comparable to the expensive industrial catalyst.The optimized hydrothermal and calcination steps were crucial to improving performance by regulating the active sites and catalyst structures,which could make aluminum and the carrier cooperate to catalyze glucose conversion efficiently.Three components in corn stover played distinct roles in the catalysis step.Cellulose played a leading role,and hemicellulose and lignin,as the efficient binding carrier of Al,played the assisting roles.The three components interacted with each other and played a synergistic role.（3）Study on the performance and mechanism of phosphate adsorption by Al-loaded hydrochar.In this study,the adsorbents prepared for phosphate adsorption by hydrothermal carbonization using corn stover as raw material.The results showed that when the concentration of Al Cl₃ solution was 2 mol/L,the hydrothermal temperature of 180-220℃was an important stage for transforming Al element from the amorphous state,which was less effective for phosphate adsorption,to the polyaluminum hydrochloride crystal with high adsorption performance.Al element that existed in polyaluminum hydrochloride crystal could be more stably loaded onto the hydrochar.The phosphate adsorption capacity by the adsorbent prepared at 260°C reached to114.40 mg/g.The phosphate adsorption capacity could be further improved by adjusting the amount of adsorbent added and the p H of the solution.The pseudo-second-order kinetic model and Langmuir isotherm model could better describe the phosphate adsorption process on Al-loaded hydrochar based adsorbent.Phosphate adsorption mainly depended on the precipitation reaction between Al crystal and phosphate,in which the hydrochar carrier played a synergistic role.（4）Preparation of multistage porous hydrochar based adsorbent and its adsorption of ammonia nitrogen.The preparation of Al-loaded hydrochar was acted in one pot with corn stover.Then using potassium bicarbonate as activator,the pore size and chemical structure of hydrochar were adjusted by impregnation combined with high temperature anoxic calcination,and then used for ammonia nitrogen adsorption.It was found that the developed pore structure could be obtained by combining the three steps of hydrothermal,impregnation and calcination.Each step was an important factor affecting the adsorption performance.The adsorption mechanism of multistage porous hydrochar based adsorbents was multilayer adsorption.With the increase of adsorbent addition amount,the adsorption amount increased.When the amount of adsorbent was0.2 g,the adsorption capacity of ammonia nitrogen could be increased to 41.55 mg/g,which was higher than the adsorption capacity of biochar in many other literatures.Ammonia nitrogen adsorption reaction was more suitable in neutral environment.With the increase of adsorption time,the adsorption amount of ammonium nitrogen decreased in a zigzag pattern.The surface adsorption capacity of the adsorbent is greater than the internal adsorption capacity.The process is easy to desorption and is more suitable for rapid adsorption and desorption of ammonium nitrogen.