Oxidation of cellulose in pressurized carbon dioxide

Abstract

This work presents first results upon oxidation of type II cellulose by nitrogen dioxide dissolved in carbon dioxide at high pressure. This reaction leads to oxidized cellulose, a natural-based bioresorbable fabric used for biomedical applications. The oxidation reaction takes place in a heterogeneous fluid–solid system. Kinetics of oxidation is presented here and effects of operating conditions such as pressure, temperature and initial moisture content of cellulose are investigated. Results are presented in terms of degree of oxidation of cellulose and quality of the final oxidized cellulose, which has been characterized using liquid-state and solid-state ¹³C NMR. The experimental results show the existence of possible secondary reactions which may lead to oxidized cellulose with insufficient mechanical strength. An attempt is made to evidence and understand the role of CO₂ as a solvent in this system. Indeed, although supercritical CO₂ appears to be a suitable candidate as a solvent for oxidation reactions, some inhibiting effect on nitrogen dioxide activity are observed in this case.

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.