The Dissolution Of Cellulose In Lithium Hydrate And Preparation Of Functional Cellulose Materials

Concerns on dwindling petroleum reserves and environmental deterioration are the driving forces behind the current boom in biofuel and biochemical production from lignocellulose.Cellulose,as a natural polymer material,has attracted much attention due to its availability,biodegradability and renewability.However,the exist of strong hydrogen bonds between intra-molecular of cellulose keeps its chemical properties are stable and hinders economical and efficient processing and conversion of cellulose to desired products.So,the efficient dissolution of cellulose is an important prerequisite for its further functional application.In this paper,lithium hydrate was used as solvent to dissolve cellulose and the mechanism of dissolution was studied.At the same time,the preparation of functional cellulose gel beads and oxidized cellulose was realized by using the dissolution system.The specific research contents are as follows:Firstly,the effect of dissolving conditions on cellulose dissolution in lithium hydrate was discussed and the dissolution mechanism was presented.The results showed that the dissolution of cellulose was mainly affected by the concentration of lithium bromide,the p H value of the solution,the concentration of cellulose,the degree of cellulose polymerization and the reaction temperature.At 120?for 30 min,the solubility of MCC can reach as high as 8 wt%.The rapid dissolution of cellulose by lithium bromide hydrate is mainly attributed to two aspects:On one hand,the coordination between Li⁺and hydroxyl group in the solution destroys the hydrogen bond network in the cellulose structure;On the other hand,the hydrophilicity of cellulose chain was significantly improved by coordination with hydrated lithium ion,which could effectively offset the hydrophobic effect of cellulose caused by accumulation of cellulose chain.Moreover,the lower charge density of lithium anion,the less interaction with lithium ion.Then the coordination between hydroxyl group and lithium hydrate will be more effective,which will contribute by the effective solubility of cellulose.Secondly,cellulose/lithium hydrate solution was applied to prepare cellulose beads by dropping them into the coagulation bath.The results indicated that the concentration of cellulose,the polarity of coagulation bath and the temperature of solution have a great influence on morphology and performance of cellulose beads.As the concentration of cellulose solution is higher than 2.5%,the cellulose beads exhibited with a tail.However,it becomes flat as soon as the concentration is lower than 1.5 wt%.So,to obtain the cellulose beads with a round sphere,optimal concentration should be controlled in the range of 1.5-2.5 wt%.Polarity of the coagulation bath is crucial to forming cellulose beads with ideal shape and uniform size.The cellulose beads formed in the coagulation bath with polarity of 1.5 to 2.0 displayed round shape.The viscosity of cellulose solution is greatly relying on temperature.In a certain range,the lower viscosity of the solution at a higher temperature facilitated the formation of a better morphology for cellulose beads.During our experiment,we also tried to use liquid paraffin as coagulation bath and find cellulose beads present complete round shape as the concentration of cellulose was controlled at 3 wt%.The extrusion strength of cellulose beads could be up to 2.52×10⁵ MPa.Cellulose beads prepared with dissolution-regeneration method posses loose structure and high porosity.Finally,the homogeneous selective-oxidation of cellulose was developed using TEMPO/Na Cl O/Na Cl O₂ system in lithium bromide hydrates to produce oxidized cellulose with high substitution degree.During oxidizing,the optimum condition of 40?for 72 h was recommended.The oxidized product gained with this condition exhibited obvious absorption peak at 1730 cm^-1 through FTIR analysis,which proved that carboxyl group was successfully equipped on cellulose.By analysis of solid-state NMR spectroscopy,we found new characteristic peaks appeared at 176 ppm in the curves of oxidized cellulose,which further confirmed the existence of carboxyl group on the oxidized products.Both of those results manifested that TEMPO/Na Cl O/Na Cl O₂oxidation system can selectively oxidize the primary hydroxyl group in cellulose to carboxyl group,and oxidized cellulose with the substitution degree of 65.56%could be obtained.Compared with the heterogeneous reaction of oxidization,the homogeneous reaction can effectively improve the substitution degree of functional groups in the product.In addition,the initial pyrolysis temperature of the oxidation products derived from homogenous reaction is as low as 185?.