Preparation and synergistic mechanism of reed-based cellulose acetate catalyzed by La3+
Objective Aiming at the high value utilization of reed-based pulp,obtained from the rich reed resources in Dongting Lake area,this research explores the feasibility for the preparation of reed-based cellulose acetate under an efficient catalytic system and tests and analyzes the microstructure and performance of the thus-obtained reed-based cellulose acetate.Meanwhile,the co-catalytic mechanism of La3+in the process of acetylation was studied in detail.Method The reed-based cellulose acetate was prepared by low temperature acetylation.,where acetic acid is used as solvent,acetic anhydride as acylating agent,concentrated sulfuric acid and La3+as co-catalyst for the first time.The effects of acetic anhydride dosage,acetylation time,acetylation temperature and the addition amount of La3+on the degree of substitution and crystallinity of the acetylation products were systematically studied.The whiteness and polymerization degree of the prepared reed-based cellulose acetate were evaluated.The degree of substitution,crystallization property,morphology and structure of the thus-obtained sample were scrutinized,measured and analyzed.The density functional theory was used to analyze the synergistic catalytic mechanism of La3+during the acetylation of dissolved reed pulp.Results Under the conditions that the reaction time was 2 h,the reaction temperature was 70℃,and the solid-liquid ratio of acetic anhydride to acetic anhydride was 2:100:18,the corresponding acetylation products showed obvious acetyl functional group characteristic absorption peaks around 1 750,1 380 and 1 235 cm-1,regardless of whether La3+was added.However,it is worth noting that when the mass ratio of lanthanum nitrate to reed-based pulp was 22.5％,the acetyl functional group characteristics in the corresponding FT-IR spectrum illustrated obvious blue shift,which was due to the fact that the hydroxyl group in the dissolved pulp was replaced by a larger acetyl group during the acetylation reaction,resulting in breakage of the intermolecular and intramolecular hydrogen bond of the product.The maximum degree of substitution was 2.85,the whiteness of the thus-obtained cellulose acetate was 87.45 and the average degree of polymerization was 171.Scanning electron microscope images of the acetylation products under different conditions showed porous structures.The characteristic(210),(310),(021)and(012)crystal planes representing cellulose acetate showed diffraction peaks of acetylated products after the addition of different amounts of lanthanum nitrate.When the ratio of lanthanum nitrate to reed pulp was 22.5％,the characteristic diffraction peak was the strongest.In addition to the slight increase of grain size corresponding to(021)crystal plane,the grain size corresponding to other crystal planes was decreased significantly,indicating that under the synergistic catalysis of an appropriate amount of La3+,the acetylation reaction of dissolved reed pulp was easier to penetrate into the crystallization zone,so that the hydrogen bond in the fiber chain was continuously opened and then broken.The DFT calculation results supported and confirmed that with the addition of La3+,due to its unique electronic structure and coordination ability,La3+and acetic anhydride molecules interact to form an intermediate complex[La(Ac2O)2]3+,and the electron density in the complex is redistributed.This greatly reduces the energy barrier for the ionized electron donor HSO4-of sulfuric acid to react with it to form acetyl sulfuric acid,which in turn acetylates with cellulose in the dissolved reed pulp.Conclusion The reed-based cellulose acetate was successfully prepared by low temperature acetylation by using concentrated sulfuric acid and La3+as co-catalyst.Density functional theory(DFT)analysis shows that the addition of La3+can effectively improve the catalytic efficiency of acetylation.The successful preparation of reed-based cellulose acetate will provide a foundation for high-value utilization of natural renewable cellulose resources,which has important academic significance and obvious social and economic value.
dissolved reed pulpcellulose acetatecellulose acetylationsynergistic catalysisdegree of substitution
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