Timo Pääkkönen

Simultaneous preparation of cellulose nanocrystals and micron-sized porous colloidal particles of cellulose by TEMPO-mediated oxidation

TEMPO-mediated oxidation of commercially available microgranular cellulose resulted in particles of three different length scales: rod-like cellulose nanocrystals (∼100 nm in length), micron-sized particles (microns in length), and coarse particles (typically tens of microns in length). The particles in the two larger fractions were highly porous, consisting of aligned nanocrystals.

TEMPO-mediated oxidation of microcrystalline cellulose: limiting factors for cellulose nanocrystal yield

Cellulose nanocrystal (CNC) production suffers, among other problems, from low yields. The focus of this study was to investigate the universal effect of charge density, centrifugation, and mechanical treatment as limiting causes of yield. Microcrystalline cellulose (MCC) was used as the starting material in order to eliminate the relatively arbitrary yield losses caused by the hydrolysis conditions. To disintegrate MCC into nanocrystals, high surface charge in the form of carboxylic groups was introduced by TEMPO-mediated oxidation, after which the material was mechanically treated, and separated into fine and coarse fractions. The fine fraction collected as supernatant after separation by centrifugation had a yield of 17–20% independent of the mechanical treatment method or time used. The particle sizes of these fractions did not significantly differ from each other, which raises questions on the efficiency of the mechanical treatment (sonication) and centrifugation in traditional CNC production. The results imply that radically new approaches in preparation are needed for truly meaningful increases in the CNC yield.

Activation of TEMPO by ClO2 for oxidation of cellulose by hypochlorite — Fundamental and practical aspects of the catalytic system

Bromide-free TEMPO-catalyzed oxidation of the primary alcohols by sodium hypochlorite (NaOCl) does not proceed without a prior activation of the catalyst. Here were demonstrate an immediate in situ activation of the catalyst with an equimolar addition of chlorine dioxide (ClO2) relative to TEMPO. Sodium bromide (NaBr) had a similar role in activating the catalyst although NaBr was needed in excess and the activation took several minutes depending on the dosage of NaBr. The activation method, or the concentration of NaBr, did not affect the bulk oxidation rate. The selectivity of the ClO2 initiated oxidation remained high up to NaOCl addition of 3mol/kg bleached birch kraft pulp after which additional loss in yield and depolymerization of cellulose were emphasized with negligible increase in carboxylate content. A carboxylate content of 0.8-1mol/kg, sufficient for easy mechanical fibrillation of the pulp, was achieved under mild conditions with NaOCl addition of 2-2.5mol/kg pulp.

From vapour to gas: optimising cellulose degradation with gaseous HCl

A cellulose degradation technique utilizing a pressurized HCl gas (up to 100 kPa) device is introduced. High pressure HCl quickly degraded cellulose in purified cotton linters, reaching the so-called levelling-off degree of polymerisation (LODP) in less than 1.5 h. LODP marks the point where the disordered portions of cellulose microfibrils have been degraded and only the crystalline portions remain, generally signalling the end of cellulose degradation unless remarkably high concentrations are used. In the present high pressure system, however, continued hydrolysis following the LODP was detected by incremental release of sugars from the hydrolysate after its exposure to water, supposedly caused by erosion from the cellulose crystallite ends. With minimal water consumption and the ease of recycling the gaseous acid, the technique could be a potential candidate for pre-treatment considering the future production of cellulose nanomaterials, particularly cellulose nanocrystals.