Effects Of Steam Explosion Combined With Hydrorefining On Biofilm Material Prepared By Straw Liquefaction And Nutrient Release Characteristics Of Coated Controlled-release Urea

The bio-based polyurethane coated fertilizers（BPCFs）are prepared by forming a coating on the surface of fertilizer particles by isocyanates reacted with polyol products produced by direct liquefaction of crop straws.However,the polyol products have the drawbacks of low hydroxyl value,complicated composition types,and many by-products,which causes bio-based polyurethane film with weak cross-linking and high hydrophilicity,resulting in a short nutrient release longevity of the bio-based polyurethane coated fertilizers.In this study,an efficient technology of instant steam explosion pretreatment of maize straw combined with hydrogenation and upgrading of liquefaction products in phosphoric acid-liquefied straw systems was innovated,in which magnetic nanoparticles made of silica-coated iron oxide（Fe₃O₄@Si O₂）as catalysts were used to catalyze the hydrogenation and upgrading of liquefaction products.Based on this technology,the small and medium-sized hemicellulose of maize straw,the ions influencing formation of the film,and the non-film by-products in the liquefaction products were efficiently removed.Finally,the quality of straw liquefaction polyols was improved,and the goal of extending the release longevity of BPCFs was achieved.The main research results are as follows:（1）The steam explosion process（2.0MPa,5min）hydrolyzed hemicellulose,effectively separated the cellulose and lignin components of the straw,exposed the cellulose components,reduced crystallinity of the cellulose,and significantly improved the fluidization efficiency of the phosphoric acid catalyzed liquefaction of maize straw system.The liquefaction residue rate reduced from 10.45%to 9.15%（P<0.05）.The hydroxyl value of the liquefaction products increased from 83.58mg KOH g^-1 to 121.06mg KOH g^-1（P<0.05）.At the same time,the steam explosion process destroyed the cell wall of maize straw,hydrolyzed hemicellulose,and significantly reduced the content of difficult-to-form-film ions(K⁺,Ca²⁺and Mg²⁺).The reaction between isocyanates and polyol products produced by direct liquefaction of crop straws was promoted,thereby improving the compactness and stability of bio-based polyurethane membrane materials.And the controlled release longevity of straw liquefied bio-based polyurethane coated urea was increased from 10 days to 39 days.（2）During the hydrogenation and upgrading processes added Fe₃O₄@Si O₂ after catalytic hydrogenation by the catalyst,the polyol product provided a hydrogen source for alcohols such as primary alcohols（such as 1,2-propanediol,etc.）that were not reacted and generated during the hydrogenation process,which hydrogenates the carbonyl compounds aldehydes,ethers,and ketones that were not conducive to polyurethane film formation into alcohols containing-OH.When reacting with isocyanates,more alcohols containing-OH in the polyol formed a biologically based polyurethane film with a high cross-linking degree and dense structure.Hydrogenation added Fe₃O₄@Si O₂ catalytic and upgrading of maize straw products resulted in an increase in the hydroxyl value of maize straw liquefaction polyols from 83.58mg KOH g^-1to 127.13mg KOH g^-1（P<0.05）,and the controlled release longevity for preparing bio-based polyurethane coated urea from maize straw liquefaction was increased from 10 days to 53 days.（3）The efficient technology of instant steam explosion pretreatment of maize straw combined with hydrogenation and upgrading of liquefaction products in phosphoric acid-liquefied straw systems combined the benefits of two important technologies,steam explosion pretreatment and catalytic hydrogenation,to improve the compactness and stability of the film forming material for the reaction of liquefied polyols and isocyanates.The hydroxyl value of maize straw liquefaction polyols prepared by two key technologies increased from 83.58mg KOH g^-1 to 197.20mg KOH g^-1,while the controlled release period for preparing bio-based polyurethane coated urea from maize straw liquefaction polyols reacted with isocyanates was increased from 10 days to 56 days.To sum up,the controlled release performance of bio-based polyol coated fertilizers were improved by using this efficient technology in which instant steam explosion pretreatment of maize straw combined with hydrogenation and upgrading of liquefaction products in phosphoric acid-liquefied straw systems.Furthermore,the theoretical foundation and technical support for the industrial production of bio-based polyurethane coated controlled release fertilizers and the use of biomass in sustainable agriculture were provided.