Regioselective palladium-catalysed aerobic oxidation of dextran and its use as a bio-based binder in paperboard coatings

The coatings industry is aiming to replace petrochemical-based binders in products such as paints and lacquers with bio-based alternatives. Native polysaccharide additives are already used, especially as adhesives, and here we show the use of oxidised dextran as a bio-based binder additive. Linear dextran with a molecular weight of 6 kDa was aerobically oxidised in water at the C3-position of its glucose units, catalysed by [(neocuproine)PdOAc]2(OTf)2. The resulting keto-dextran with different oxidation degrees was studied using adipic dihydrazide as a crosslinker in combination with the commercial petrochemical-based binder Joncryl®. Coating experiments show that part of the Joncryl® can be replaced by keto-dextran while maintaining the desired performance.

The aerobic oxidation of dextran with a varying molecular weight (lot number of 6 kDa dextran: BCCF2262, and of 70 kDa dextran: BCCD3043 from Merck) was carried out in 200 mL glass vessels of a H.E.L. PolyBLOCK ® reactor.The reactor is a fully automated synthesis platform (Fig. 3) with overhead agitation as well as an oxygen line.
Oxidation is reported in % of the number of residues.Thus, an oxidation degree of 30% means that 30% of the residues has been oxidized.For dextran with an MW of 6 kDa this corresponds to 12 residues out of 37 residues on average.For this reaction this means an approximate TON of the palladium catalyst of 80.The palladium was recovered by filtration after the reaction but the catalyst was not re-used.Recovery of the dextran was high but could not be determined accurately as it was difficult to remove residual water from the product.The starting material was dissolved in H 2 O/MeCN (in the reported ratio).Afterwards, the catalyst was added as a solid.An oxygen atmosphere was applied by attaching an oxygen cylinder to the system.After a reaction time of 20 h at 45 °C, charcoal was added to the reaction mixture, and the mixture was then filtered through five layers of Whatman glass fiber filters.The filtrate was then lyophilized overnight.Analysis was mainly done by qNMR occasionally supported by IR spectroscopy.The analysis of branched high molecular weight dextran became more challenging due to a broader NMR signal.This led to less accurate integration.[(Neocuproine)PdOAc] 2 OTf 2 )] was prepared following the literature procedure.Spectral data corresponded with literature. 3spite the optimizations that were done on the oxidation of dextran using palladium(II) neocuproine triflate as catalyst, the activity of the catalyst decreased over time due to the two ways of degradation described earlier by Waymouth et al. (Fig. 4) and was, therefore, removed and not reused.Equation of the calibration curve for calculating the oxidation degree with IR: y=0,0512x-0,0014.
Oxidation of positions other than the C3-OH in the glucose monomer unit was not observed in the NMR spectra but potentially occurred at very low levels.

Fragmentation studies of keto-dextran
To a solution of 6 kDa linear keto-dextran (400 mg, 0.067 mmol)) with an oxidation degree of 10% in 8 mL water, K 2 CO 3 (90 mg, 0.651 mmol, 9.77 equiv.) was added.The reaction mixture was heated to 40 °C and stirred overnight.Subsequently, 0.5 mL of the solution was filtered by cut-off centrifugal filters (Amicon ® Ultra-0.5 Centrifugal Filter for 5 kDa, 3 kDa, and 1 kDa).The filtrate was lyophilized, weighted (25 mg ± 0.5 mg remaining) and afterwards analyzed by qNMR in D 2 O as solvent.

Droplet coatings of keto-dextrans with ADH and their analysis by IR spectroscopy
The keto-dextrans were stored at -20 °C and taken as bio-based binders for cross-linking experiments.The solid content of the aqueous solution with the dextrans was 10 w% in order to match the rheological behavior of the commercial binder. 4The degree of oxidation varied up to 37%.

General procedure of droplet coatings (if not mentioned otherwise)
Several dextrans (each 100 mg) were dissolved in water (each 0.9 mL, 10 w% solid content) and ADH (0.5 equiv.per keto-moiety) was added as a solid.The reaction mixture was stirred for 30 min at r.t.Afterwards, 4 times 50 µL was applied as a droplet on a glass microscopy plate.

Water resistance
After drying for 24 h at r.t., the water resistance of the droplet coatings was tested by putting the coated glass microscopy plates in a standing position and pouring 10 mL over each droplet.A graphical representation of smearing patterns can be found in Fig. 10.   6 Bar coatings of keto-dextrans with a commercial keto-functionalised Joncryl® binder and

Composition of mixtures
The BASF keto-functionalized Joncryl ® binder normally is a self-crosslinking acrylic emulsion with ADH already added.It is a binder for water-based inks used for surface printing on film substrates.
In order to analyze the co-cross-linking behavior with the oxidized dextran, this time the binder was prepared without the addition of ADH, the latter was added later in the desired amounts.The commercial binder consists of 5% diacetone acrylamide (DAAM) which is, hence, also the degree of functionalization (ketones).
The binder blends were prepared by mixing Joncryl ® (emulsion with a solid content of 43%) with the bio-polymer (solution with a solid content of 15%) in various ratios, and water, resulting in an overall binder's solid content of ~30 w%.After stirring this mixture for 15 min at r.t.(1000 rpm with an overhead agitator), ADH (as a solution with 8 w% solids) was added and stirring was continued for another 30 min at r.t.Subsequently, the emulsion was distributed over two vials and to one vial a blue pigment was added.Both emulsions were then taken for bar-coatings on an unsealed (semi-porous) Leneta N2A-test chart whose surface simulates wood or unsealed wallboard.Bio-polymer: Maltodextrin, linear dextran with a molecular weight of 6 kDa, linear keto-dextran with an oxidation degree of 10%, 21% or 27%.Addition: ADH was not used to calculate the bio-content because it can come from different sources (bio-based possible). 5paint applicator from RK print-coat instrument Ltd.England was used (Fig. 12 right) for applying the bar coatings.~1.5 mL of the clean mixture as well as the pigmented mixture were pipetted next to each other on the Leneta test chart (Fig. 12 left).The bar speed was set to 8. The chosen film thickness was 12 µm.The coatings were directly dried at 60 °C for 1 min and then at r.t. for 24 h.
The wire wound bar was cleaned after every run with tap water and a brush.

1Fig. 1 Fig. 2
Fig. 1 Molecular structure of linear low molecular weight dextran and branched high molecular weight dextran.

Fig. 4
Fig. 4 Schematic set-up plan of PolyBLOCK® reactor with 4 glass vessels of 200 mL, a direct oxygen supply, overhead stirring, and a cooling block with water supply.

Fig. 5
Fig. 5 Full mechanistic cycle of the complex during aerobic oxidation with two ways of catalyst degradation.Adapted from Waymouth et al.3

Fig. 8
Fig. 8 qNMR spectra red: native branched dextran with a molecular weight of 70 kDa, and petrol: branched ketodextran with a molecular mass of 70 kDa and an oxidation degree of 3%.

Fig. 10
Fig. 10 Calibration curve for oxidation degree of dextran.

Fig. 11
Fig. 11 Cross-linking reaction of linear low molecular weight dextran with a degree of oxidation of 40% with ADH as crosslinker in water.

Fig. 13 FTIR
Fig. 13 FTIR spectra of solid linear native dextran, of solid linear dextran with an oxidation degree of 21%, and of solid linear dextran with an oxidation degree of 21% with ADH ("coating"), as well as, only solid ADH as reference spectra.Y-axis was normalized at 1000 cm -1 .

Fig. 14
Fig. 14 Left: Leneta N2A unsealed test chart; left: Set-up of bar coating experiments at BASF using a paint applicator from RK print-coat instrument Ltd.England.

Fig. 15
Fig. 15 All three photos from left to right 5 w%, 10 w%, 15 w%, and 20 w% of keto-dextran with Joncryl® binder and ADH in water.Left photo with keto-dextran of an oxidation degree of 10%; middle photo with keto-dextran of an oxidation degree of 11%; and right photo with keto-dextran of an oxidation degree of 27%.

Fig. 19
Fig. 19 Final result of water resistance tests with 20w% bio-polymer and 80 w% Joncryl® binder and stoichiometric amounts of ADH.Degree of oxidation of keto-dextrans in title.

Fig. 20
Fig. 20 Final result of reference water resistance tests with 10 w% or 15 w% bio-polymer without ADH on Leneta 2A sealed test charts.Degree of oxidation of keto-dextrans in title.

Table 3 Summary of the water resistance tests of droplet coatings with different dextran polymers and ADH as crosslinker. Conditions Linear dextran 0% ox. degree Keto-dextran 5-10% ox. degree Keto-dextran 17- 21% ox. degree Keto-dextran 26- 37% ox. degree
Dextran in water (10 w% solid content) with 0.5 equiv.ADH per keto-moiety.For native dextran, the same amount of ADH was added as for an oxidation degree of 10%.Stirring for 30 min at r.t. of the reaction mixture before applying as droplet coating on glass microscopy plates and drying for 24 h at r.t.