Production of Dialdehyde Cellulose and Periodate Regeneration: Towards feasible oxidation processes

Sammanfattning: Cellulose is an attractive raw material that has lately become more interesting thanks to its degradability and renewability and the environmental awareness of our society. With the intention to find new material properties and applications, studies on cellulose derivatization have increased. Dialdehyde cellulose (DAC) is a derivative that is produced by selective cleavage of the C2-C3 bond in an anhydroglucose unit in the cellulose chain, utilizing sodium periodate (NaIO4) that works as a strong oxidant. At a fixed temperature, the reaction time as well as the amount of added periodate affect the resulting aldehyde content. DAC has shown to have promising properties, and by disintegrating the dialdehyde fibers into fibrils, thin films with extraordinary oxygen barrier at high humidity can be achieved. Normally, barrier properties of polysccharide films deteriorate at higher humidity due to their hygroscopic character. This DAC barrier could therefore be a potential environmentally-friendly replacement for aluminum which is utilized in many food packages today. The aim of this study was to investigate the possibilities to produce dialdehyde cellulose at an industrial level, where the regeneration of consumed periodate plays a significant role to obtain a feasible process. A screening of the periodate oxidation of cellulose containing seven experiments was conducted by employing the program MODDE for experimental design. The reaction time was varied between 2-8 hours and the ratio NaIO4 to fiber in was between 1-2 (w/w) for small-scale experiments (1 g fiber), which resulted in an aldehyde content between 14-80 %. An oxidation degree around 30 % was set as a goal, and the optimal point at a fixed temperature of 50°C was assessed to be a ratio of 1.5 and a reaction time of 2.5 h, including 30 min of cooling. Furthermore, the MODDE evaluation suggested that the time and quantity of added periodate equally effected the reaction. An up-scaling of the system with 22.5 g of NaIO4 and 15 g of cellulose fibers and a total reaction time of 3h, resulted in 39 % oxidation degree and a yield of 92 %. For the regeneration of periodate, Oxone® was tested, but too low yields were obtained. More studies are needed in order to understand and optimize this process. Better results where gained when utilizing a 10 % hypochlorite solution (NaOCl) that was refluxed with the filtrate from the periodate oxidation of cellulose. A spectrophotometric method was developed to be able to quantify the amount of periodate and thereby the amount of residual iodate (IO3-), i.e. the byproduct to oxidize back to IO4-. An optimization study was performed with eleven experiments with the time varying between 1-4 hours and the molar ratio of NaOCl to IO3- between 1-4. However, it was found that the residual periodate also consumed the hypochlorite, so the real molar ratio of NaOCl to IO3- and IO4- was only 0.38-1.52. The highest ratio of 1.52 with a reaction time of 4 h generated the highest regeneration of 81 %. From the MODDE evaluation it was suggested that the reaction time does not have as significant effect upon the process as the amount of added NaOCl has. By optimizing this reaction further, it should be possible to reach even more satisfying results. However, it was proved that the precipitated product was sodium paraperiodate, Na3H2IO6, and this regenerated product was successfully used to oxidize cellulose fibers to DAC. Surprisingly, the oxidation degree became much higher, 43 %, despite that the same condition was employed as before, but the reason for this can be the lower pH that was utilized. Even though there still are questions to be answered, this study has contributed to knowledge that could be utilized to take the oxidation process closer to industrialization.