Reaction Kinetics Of Microcrystalline Cellulose Oxidization And Adsorption Of Modified Products On Ag~+

Microcrystalline cellulose (MCC) has the property of a large specific surface area, high crystallinity and uniform width, but it has disadvantages of weak adsorption performance and poor solubility at the same time. The absorptive property and solubility of MCC could be improved by introducing carboxyl group through oxidation modification. Cellulose based on composites of silver have the advantages of mechanical properties of cellulose and antimicrobial properties of silver, so it has a wide application prospect in the fields of biological tissue, catalytic oxidation and dielectric materials. MGC was carboxylated modified by 2,2,6,6-tetramethylpiperidine-l-oxyl radical (TEMPO)/NaClO/NaClO2 system. The kinetics of the reaction process and the adsorption performance on Ag+of oxidized products was investigated. The research contents and the results were as follows.MCC as raw materials was carboxylated modified by TEMPO/NaClO/NaClO2 system under weak acidic conditions. The influence of reaction time and temperature, the amount of TEMPO, amount of NaCIO and NaClO2 on carboxyl content of fiber were studied. The suitable reaction conditions were reaction time 24h, temperature 50 ℃, NaClO 10mmol/g and NaClO2 20-25mmol/g. The peak of sodium carboxylate was observed on the oxidized MCC and it increased with oxidized time increased through Fourier transform infrared spectroscopy (FTIR) spectrum analysis.The experiments showed that MCC was introduced carboxyl group by oxidation reaction and reaction time has a certain impact on the formation of carboxyl on the oxidized product. Thermogravimetric analysis (TGA) showed the initial degradation temperature, the maximum temperature and the equilibrium temperature of degradation advanced, which indicated oxidation reaction reduced the thermal stability of MCC. Compared with MCC before oxidation, the residual quantity of oxidized MCC increased. It showed that oxidized products had sodium carboxylate. The solid-state C nuclear magnetic resonance (13C-NMR) spectra revealed that some primary hydroxyl on the C6 of oxidized MCC was oxidized to carboxyl.Considering the degradation of NaClO for MCC, the kinetics of TEMPO/NaClO/NaClO2 system oxidation on MCC was built and employed to fit experimental data. The goodness of the fitting was higher than 0.97. X-ray diffraction (XRD) analysis found the crystallinity indexes of MCC rose slightly, and the grain size of (002) crystal plane had an upward trend as the reaction progresses. That illustrated oxidation reaction had degradation effect on the amorphous region and the surface of crystallization area of MCC.The analysis of the determination of the degree of polymerization(DP) found that the DP of MCC declined faster at the beginning of the reaction, and later flattened. Furthermore, DP were influenced by the reaction temperature. The scanning electron microscope(SEM) figures showed super-molecular structure of MCC was destroyed in the oxidation process by TEMPO/NaClO/NaClO2 system.The influenced of solution pH and adsorption time on the adsorption quantity of modified MCC adsorption Ag+ was analyzed. The most appropriate adsorption condition was adsorption time 10 h and pH7.0. The pseudo first order kinetics and the pseudo secondary dynamic model were used to fit on the adsorption process, respectively. The results showed the pseudo secondary dynamic model could fit the experimental data well, which indicated the adsorption process of modified MCC on Ag+ was a chemical adsorption process. Through the analysis of the adsorption isotherm, the adsorption of modified MCC on Ag+fitted the Langmuir model better. Thermodynamic parameter calculation results indicated that the adsorption process was a spontaneous endothermic process(ΔG<0). Energy dispersive spectrometer(EDS) and FTIR figures showed that the adsorption process of modified MCC on Ag+ was the exchange process of the Na+ of sodium carboxylate on modified MCC between Ag+ in the solution. The TGA line showed silver elements played a catalytic role in the initial and equilibration stage of modified MCC thermal degradation, and played an obstructive role in the middle.