Reductive treatments and viscosity determination in cuen / cuoxam / cadoxen
In studies with aged papers as well as naturally and artificially aged (oxidized) cellulosic model pulps, reduction procedures with sodium borohydride have proven effective to preserve cellulose integrity and increase the stability of cellulose chains in alkaline media (Henniges and Potthast 2009; Tang 1986; Wang et al. 2015). Such treatments of cellulose and polysaccharides with NaBH4 are usually carried out in alkaline aqueous media in order to improve the stability of the reagent (Henniges et al. 2011b). Increasing acidity (= decreasing pH) causes consumption of the reagent by the disproportionation reaction with protons from the solvent (H+ + H– H2) and evolution of hydrogen. At room temperature, the kinetic rate constant for the reduction is less than one order of magnitude smaller than those of β-elimination reactions so that the reduction can well compete with alkali-triggered β-elimination (Potthast et al. 2009; Hosoya et al. 2018) if the conditions (temperature, alkalinity, concentration) are properly chosen. In general, pronounced molar mass losses due to β-elimination upon NaBH4 treatment occurs only in the case of highly oxidized celluloses (> 80 µmol/g), while it is tolerably low for conventional pulps with low degrees of oxidation (> 30 µmol/g).
In order to address the inherent error in the standard pulp viscosity protocols, we explored the efficacy of such reductive treatments before dissolving the pulp in the organometallic solvents. If the oxidized groups, namely aldehydes and ketones, were returned to their “hydroxyl state” before β-elimination sets in, the main error would be eliminated and the state of the pulp without chain shortening would be correctly reported. For optimization we used two cellulosic substrates, cotton linters and a bleached beached sulfite pulp, which had already been the test specimens in previous viscosity measurement studies. Both the original pulps and their counterparts with different degree of oxidative modification were used (see Materials and Methods section). As also the oxidant used in these modifications can influence the alkali-lability of the pulps, e.g., by introducing different ratios of carbonyl vs. carboxyl groups (Ahn et al. 2019b), we used either H2O2 or HOCl as in previous studies.
To determine the molar mass distribution of the pulps, gel permeation chromatography in the solvent/eluant DMAc/LiCl was employed which is known to be completely non-degrading if used according to the optimized protocol (Potthast et al. 2002; Chrapava et al. 2003; Potthast et al. 2015). In addition, molar mass-related profiles of carbonyl groups according to the CCOA method (Röhrling et al. 2002a, 2002b; Potthast et al. 2003) were used to directly monitor the dynamic changes in the carbonyl group contents. By dissolving the pulp directly in the GPC solvent, we can monitor the unchanged molar mass distributions of the pulps (black graphs in Figs. 1 and 2). Alternatively, the pulps were treated exactly as during viscometry measurements, i.e., dissolved and kept in solution for 30 min (cuen) or 60 min (cuoxam, cadoxen), then precipitated in water and dissolved in DMAc/LiCl for GPC measurement. Now, GPC shows the state of the pulp after all processes that might have occurred in the viscometry solvents (colored graphs in Figs. 1 and 2).
Figures 1 and 2 present clear proof that a reduction step prior to dissolution in the viscosity solvents is beneficial with regard to molar mass preservation. The experiments underlying these figures employed a bleached hardwood sulfite pulp (HBSP), into which oxidized functionalities were introduced beforehand, with oxidation by NaOCl in Fig. 1 (starting carbonyl content 52 µmol/g) and with H2O2 in Fig. 2 (starting carbonyl content 50 µmol/g). The reduction procedure itself is innocuous with regard to the molar mass distribution. The curves before and after reduction are nearly congruent (Fig. 1, black graphs). Under special circumstances, the reduction even has a beneficial effect on the direct dissolution in DMAC/LiCl which normally does not affect the molar mass distribution at all: in the special case of an H2O2-oxidized pulp, which contains many C-2/C-3 keto groups that are notoriously labile, the molar mass distribution is slightly shifted to higher values indicating the protective effect of the reduction (Fig. 2, black graph). Such minor degradative effects upon dissolution in DMAc/LiCl – like the one seen here, which is eliminated by the reduction treatment – usually occur in pulps with significantly increased contents of oxidized groups and/or with eluants that have not been especially purified and contain some traces of water and alkaline impurities (dissolved N,N-dimethylamine). This result suggests that for pulps with carbonyl contents greater than about 30 µmol/g, a reduction step can be considered even for direct GPC measurements in DMAc/LiCl. Besides the effect on the molar mass distribution, it was evident that the carbonyl groups were largely removed through the reduction, and the carbonyl contents were significantly reduced (from 52 to 14 µmol/g in Fig. 1 and from 50 to 12 µmol/g in Fig. 2, see also Table1). It should be noted that these carbonyl contents can be further lowered by excessively long reduction times, but these long times are not practical for standard protocols, and the very low contents that can be achieved are bought at the cost of molar mass losses caused by extended contact with the alkaline reduction medium.
The colored subgraph in Figs. 1 and 2, each containing three molar mass curves, refer to the viscosity solvents. The solid, grey distributions show the molar mass distribution without dissolution in the viscosity solvent, after direct dissolution in the GPC solvent. The solid lines give the molar mass distributions after dissolution in the viscosity solvent, and the dashed lines after reduction and subsequent dissolution in the viscosity solvent. As expected, the gray, colored distributions (no viscosity solvent) correspond to the highest molar mass and the solid lines (viscosity solvent, no reduction) to the lowest. The dashed lines lie between these two extremes, clearly showing that the molar mass loss caused by the viscosity solvents is diminished by the reduction treatment. The dashed curves are not congruent with the gray colored distributions (which would imply complete prevention of any degradation), but are reasonably close to them and lie at significantly higher molar mass values than the solid curves (without reduction treatment).
The changes in molar mass and carbonyl content of the HBSP pulp caused by the reduction process are summarized in Fig. 3. In all cases, the total amount of carbonyl groups, i.e., the “predetermined breaking points” of the cellulose chain under alkaline conditions, decreased significantly, and the effect of the subsequent alkaline dissolution types of—loss of molar
mass—was thus considerably attenuated. For cotton linters, a similar positive effect of the sodium borohydride reduction step was observed (Fig. 4). There was no clear difference between the viscosity solvents, the NaBH4 reduction step was similarly effective for all three.
At the same time, it became evident that both type of the pulp and nature of oxidative damage had an influence on the reduction, which is, after all, carried out as a heterogeneous process. The influence of the pulp type is simply explained by the different accessibility and crystallinity of the pulps, as well as the influence of the pre-oxidation, since different oxidants have different selectivity (C-2, C-3, C-6) and cause different ratios between oxidized groups (keto/aldehyde vs. carboxylic acids). This is fully consistent with similar observations from preceding work on the effects of viscometry solvents on cellulose integrity (Ahn et al. 2019a, Zaccaron et al. 2020). The results also made clear that the reduction is not a panacea that can completely eliminate the negative effect of viscosity solvent alkalinity on cellulose integrity. The protective effect was obvious, but even with the additional reduction step, the molar masses measured in cuen/cuoxam/cadoxen were still lower than those determined directly (GPC in DMAc/LiCl). Thus, it is important to keep in mind that the reduction step is a significant improvement with respect error prevention, although viscosity determination still cannot compete with GPC in terms of maintaining cellulose integrity.
Optimization of the standard reduction protocol
The reduction protocol to be used as a step in the DP-determination by viscometry must meet certain requirements. Of course, it should reduce the molar mass loss due to the solvent-induced (alkali-induced) β-elimination reactions as much as possible. However, this must be reconciled with operational requirements in daily lab practice. Viscometric DP determination is a standard procedure in the pulp and paper industries. Any change or addition to the established protocol must be as short, easy and economic as possible to find acceptance. No complicated procedures, lengthy work-ups or harmful chemicals should be involved.
We have optimized the reduction procedure with respect to temperature, reagent concentration and ratio, pH of the aqueous medium and reaction time (Fig. 5). In order to make the measurements of different pulps comparable and allow consistent evaluation, the molar mass preservation without dissolution in the viscometry solvent (the minimum possible loss) was designated as 100% and the molar mass preservation in the viscometry solvent without reduction step was designated as 0% (the maximum “achievable” loss). The beneficial effect of the reduction then lies between 0 and 100% – the greater the number, the greater the molar mass-preserving effect of the reductive treatment. By taking these relative values, we could average the data over different pulps, similar to the later application of the procedure within viscosity measurements, which would also encompass all types of pulps samples.
The influence of the temperature (5 °C, 22 °C (r.t.), 40 °C) was very small. The results for 5 °C and 22 °C were the same, so that the additional effort to cool the mixture below room temperature did not seem justified. The reduction effect (decrease of carbonyl content) was somewhat higher at 40 °C, while the molar-mass-preserving effect – the decisive factor – was slightly lower than at the other two temperatures. At the same time, degradation of the reductant through reaction with protons from the medium became considerable at 40 °C so that this temperature was ruled out. The influence of the pH appeared to be large enough to justify the additional complication of using a pH buffer instead of water, in order to keep the alkali-induced degradation at bay. At pH 6, considerable degradation of the reductant occurred, evident from hydrogen evolution, but at the same time a very good conservation of Mw was achieved. At pH 7 and 8, both effects were slightly weaker. Thus, pH 8 was the best compromise. At pH 9 and above, very little degradation of NaBH4 occurred, but Mw loss increased greatly. The pH effects were tested with reductant concentrations up to 5 M (see below). In daily lab practice, pH can be adjusted very easily by a buffer system. Sørensen phosphate buffers are common, readily available (also as premixed solutions with different ionic strength / buffer capacity and easy to prepare even in labs not specially equipped for chemical work.
As for the borohydride concentration, the Mw was better preserved at lower concentrations than at higher concentrations, an effect that seemed paradoxical at first glance. However, it must be kept in mind that the reduction is a heterogeneous process. It is well-known from general cellulose chemistry that high concentrations of ionic reagents can be counterproductive in heterogeneous cellulose reactions. Ions are adsorbtively enriched at the surface and further access of other ions into the matrix and the pore system is hindered by electrostatic repulsion. Borohydride concentrations between 0.1 M and 1 M worked best, with the lower concentration of 0.1 M chosen to not overload the capacity of the buffer and to save chemicals. Stoichiometrically, the amount of carbonyl groups to be reduced is minute compared to the amount of reagent introduced, but the heterogeneous reaction impedes the process. Very low concentrations would mean extended reaction times, increasing the importance of the slower β-elimination reactions. A ratio of 1 g pulp per 100 mL of solvent was maintained throughout, which is also the usual proportion used in viscometry measurements. The Mw-preserving effect was nearly constant between reaction times of 30 min and 2 h. At longer times, the Mw loss increased. Shorter reaction times at higher reagent concentrations did not improve the outcome because the heterogeneous reaction is determined by accessibility (reagent diffusion). Therefore, a reaction time of 30 min was set.
As a result of optimization, the general reduction procedure was carried out using 0.1 M sodium borohydride in phosphate buffer pH = 8 for 30 min at r.t., which combines optimum Mw retention with maximum robustness and ease of use.