Oxidation of cellulose in pressurized carbon dioxide
Graphical abstract
Introduction
Oxidized cellulose is a very interesting material for biomedical applications, because it is degradable in human body [1] and also exhibits haemostatic and antibacterial properties. Oxidized cellulose is thus used as a raw material for medical devices such as adhesion barriers, sutures, absorbable haemostats or scaffolds for tissue engineering. To obtain this product, nitrogen dioxide (NO2) is the most suitable oxidant because, in this case, oxidation does not affect secondary hydroxyl groups of the cellulose, and thus secondary reactions are avoided. In this case oxidized cellulose maintains acceptable mechanical properties, allowing its use as a biomedical device. The global reaction is illustrated by the following scheme:
The first industrial process for oxidation of cellulose was based on the use of gaseous nitrogen dioxide, where the gas was circulated across layers of fabrics [2]. But this process was abandoned because of the difficulty of handling a gas (especially when its atmospheric condensation point is 21 °C, as it is the case for NO2), as well as because mastering the exothermicity of the reaction in a gaseous system at industrial conditions is uneasy. Consequently, solvent based processes were proposed. In this case, nitrogen dioxide was dissolved in an organic solvent, and thus the oxidation took place in the liquid phase [3]. Nevertheless, the oxidant could cause some damage to the solvent, and solvent degradation products were often present. Therefore, the biomedical company Johnson and Johnson proposed the use of inert halogenated solvents, and patented the use of perfluorocarbons as solvents for this reaction [4].
Recently the use of supercritical carbon dioxide as a suitable inert solvent to perform oxidation of cellulose was proposed and patented [5]. Because of its peculiar physical properties and its specific “green” character, supercritical CO2 is now growing interest as a reaction medium for many chemical syntheses, such as hydrogenation, hydroformylation, oxidation or polymerization [6] for instance. Especially, as it is the result of complete oxidation of carbon, CO2 is a solvent of choice for oxidation reactions considering the absence of by-products possibly issued from degradation of the solvent by the oxidant. In the view of biomedical applications for oxidized cellulose, using CO2 has the major advantage to yield processed fabrics as “solvent residue free”. More widely, carbon dioxide is considered as a green non-expensive solvent, and, in this case, compares very favourably with perfluorocarbons.
This work focuses on the main aspects of oxidation of cellulose in high-pressure CO2 and using nitrogen dioxide as the oxidant.
Section snippets
Generalities on cellulose oxidation by nitrogen dioxide
Nitrogen dioxide does not exist as pure molecular specie, but is in equilibrium with its dimer, nitrogen tetroxide, as follows:N2O4 ⇆ 2NO2When temperature is less than 262.15 K, nitrogen dioxide is completely dimerized giving nitrogen tetroxide (N2O4) and this dimer dissociates into 2 mol of NO2 as temperature increases, proportion of each compound depending on conditions of temperature and pressure. Under atmospheric pressure, the mixture boils at 294.25 K, the liquid being yellowish brown and the
Cellulose
Regenerated cellulose, i.e., man-made cellulose fibres produced by the cuprammonium process [17] from Bemberg company, was used as the starting raw material for all this study. This regenerated cellulose, usually termed rayon, consists in fibres that are spun in such a way to form a continuous filament. Such continuous filaments can be knitted together to form simple braid or meshes.
Chemicals
NO2/N2O4 was supplied by Air Liquide (water content max 0.5%wt) and CO2 TP by Linde Gas.
Oxidation experimental set-up
Experiments were
Influence of the pressure
When considering phase equilibria of the CO2–NO2/N2O4 mixture at 313 K issued from [13] and presented in Fig. 3 we can deduce that, if reaction is run above 8.5 MPa, it takes place in a homogeneous reacting mixture, whatever its composition. Indeed, the biphasic zone where liquid and vapour are in equilibrium, has to be avoided in order to insure that the reaction is run in a homogeneous phase to obtain homogeneously oxidized samples. Under 8.5 MPa, depending on the composition, the reaction may
Conclusion
Results upon oxidation of cellulose with nitrogen dioxide in high-pressure CO2 medium are reported here for the first time. Kinetics of oxidation, influence of pressure, temperature and initial moisture content of cellulose were investigated. Although high-pressure CO2 matches all the requirements to be a green solvent for the reaction of oxidation with nitrogen dioxide, it indeed appears that its role is not as neutral as expected and the degree of oxidation of cellulose depends on the amount
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