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Ultrastructural characterization of the core-shell structure of a wide range of periodate oxidized cellulose from different native sources by solid state 13C CP-MAS NMR Julien Leguy, Yoshiharu Nishiyama, Bruno Jean, and Laurent Heux ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03772 • Publication Date (Web): 03 Nov 2018 Downloaded from http://pubs.acs.org on November 21, 2018

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ACS Sustainable Chemistry & Engineering

Ultrastructural characterization of the core-shell structure of a wide range of periodate oxidized cellulose from different native sources by solid state 13C CP-MAS NMR Julien Leguy, Yoshiharu Nishiyama*, Bruno Jean, Laurent Heux* Univ. Grenoble Alpes, CNRS, CERMAV, 38000 Grenoble, France Address for all authors: CERMAV-CNRS 601 rue de la Chimei 38 041 Grenoble Cedex 9, France [email protected] [email protected] [email protected] [email protected]

ABSTRACT The periodate oxidation of cellulose, which leads to the opening of the glucosyl rings to yieldtwo aldehyde groups in the so called dialdehyde cellulose, stands as one of the major and promising pathways for the derivatization of cellulose for the design of new biosourced materials. Despite the wide use of this method and the simplicity of its chemistry, the measurement of the degree of oxidation (DO) of the periodate oxidized cellulose is not straightforward due to the concomitant side reactions and heterogeneity of the oxidized products. Here, we present a new simple method based on solid state

13

C CP-MAS NMR

spectroscopy relying on the quantification of the intricate signals of the washed oxidized sample, using the sharp C1 signal of intact cellulose as an internal standard, allowing the determination of degree of substitution from 0 to 2. This new NMR method, applied first to

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microfibrillated cellulose, is compared to the current conventional methods, namely the UV absorbance monitoring of the periodate consumption and the oxime formation protocol. Agreement between the DONMR and DOUVvalues can be obtained if one assumes a full oxidation for the solubilized part of the product. Regarding the DO deduced from the oxime formation, it agrees with the present DONMR only if buffered conditions are used during the reaction to prevent hydrolysis. Solid-state NMR experiments also provided information on the heterogeneous character of the reaction in favor of a core-shell model of an oxidized cellulosic material skin surrounding an intact core of unmodified native cellulose. This method could be applied on a wide range of substrates with different morphology and crystallinity.

KEY WORDS Cellulose, Periodate oxidation, 13C CP-MAS NMR, Solid State NMR, Oxime titration, UV-Vis spectroscopy

Introduction Controlling and understanding the modification of natural polymers is a key bottleneck for the promotion of biosourced polymers, as most of the reaction are heterogeneous by nature. Cellulose modification is an emblematic example as this polymer is widely abundant and difficult to modify, due to its dense network of hydrogen bonds. The extended use of cellulose as a raw material for structural applications requires the widening of the versatility of this naturally ubiquitous polymer. The chemical modification of the cellulosic backbone is one of the most used strategies as it allows to add new functionalities and design the properties of the targeted materials. Among the broad variety of chemical pathways that can be envisaged, periodate oxidation stands as a


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unique reaction as it is able to drastically modify the cellulose backbone under mild conditions, while keeping the original cellulose stereoregularity. Unlike TEMPOOxidized Cellulose (TOC) that leaves the glucose ring intact, periodate oxidation selectively cleaves the cellulose glucosyl rings between the C2 and C3 carbons and converts the two associated alcohols moieties into two aldehyde groups, leading to what can be called Periodate Oxidized Cellulose or POC by analogy with the with the TEMPO oxidation (TOC), not to be confused with Dialdehyde Cellulose (or DAC) that should be in principle the fully modified cellulosic material. Initially, the periodate oxidation of polysaccharides was used for structural analysis purpose,1-2 but more recently it has been revived as a way to prepare a new class of cellulose-based products. For example, POC with intermediate degree of oxidation (DO) can be used as a component in the processing of all-cellulose thermosets with interesting mechanical properties,3 or to make transparent and strong films with high oxygen barrier properties in its almost completely oxidized form.4 As a reactive platform, POC is a potential intermediate for the creation of a large variety of bio-based products.5-19 When reduced with sodium borohydride, POC gets converted into an acyclic stereoregular polyalcohol of enhanced flexibility,5-6 which can be used to increase the ductility of paper7 or to make oxygenbarrier films.8 In other systems, the grafting of amines on the created aldehyde groups is a way to obtain flocculating agents,11-12,

18

hydrophobized cellulose,13,

17

materials

reacting to light,9, 16 or oxygen-barrier films.14 Periodate oxidation of cellulose followed by post-oxidation leads to dicarboxylic acid cellulose (DCC), an highly charged anionic polymer that finds applications in ion exchange chromatography.10, 15, 19 Among all these applications, the extent of the reaction and the ultrastructure (i.e. morphology, crystallinity…) are key parameters that conditions the final properties of the considered material, due to the fact that periodate oxidation of cellulose is a heterogeneous reaction



by nature. Hence, the investigation of the ultrastructure as well as the measurement of the DO are a prerequisite for a rational use of these reactions and have a precise control of the amount of created aldehyde groups to properly identify the structure/properties relationship of the desired products. The aldehyde amount in a POC sample can be expressed in different ways: (i) the number of moles of aldehyde per gram of product : a fully oxidized sample (DAC) would reach 12.5 mmol/g), which is the most commonly used unit,20-21 (ii) the percentage of oxidation representing the ratio of oxidized units over the total number of anhydroglucose units (AGU) (a fully oxidized sample would have an oxidation percentage of 100%)22-23 and finally (iii) the degree of oxidation (DO) defined as the average number of aldehyde groups introduced per AGU (a fully oxidized sample would have a DO of 2 since two aldehyde groups are created when one AGU is oxidized).24-25 This last unit will be used in the present study. Different methods to measure the DO exist. In the first method, the aldehyde moieties can be further oxidized into carboxylic acid groups using acidified sodium chlorite and these groups can in turn be titrated with NaOH.2, 26-30 This method in the literature was used to detect low amounts of aldehydes groups (less than 8% oxidized units),2, 26 and for cellulose reducing-end characterization.27 When a low amount of aldehyde is introduced by periodate oxidation, good agreement between hydroxylamine and conductometric titration are found. 29-30 Samples with extended periodate oxidation (up to a DO of 1.2) were also characterized by conductometry28 but in this case, hydroxylamine titration was not carried out. Yield loss would be expected during the chlorite post-oxidation step for such high degree of oxidation that can bias the estimation of DO of the periodate oxidized product.


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The second method is to follow the periodate consumption. The DO can be deduced from monitoring of the periodate concentration in the reaction medium by the UV absorbance. The expected amount of aldehyde groups introduced on a cellulose substrate should be proportional to this consumption since one mole of periodate should lead to two moles of aldehyde groups.20, 22-23, 28-31 This method has been widely used since it is simple and straightforward. 19, 21, 22, 27-30 In a third method, the most popular for polysaccharide and especially for cellulose, POC is subjected to a reaction with hydroxylamine hydrochloride, leading to the conversion of aldehyde moieties into oximes, while liberating HCl molecules, which can be titrated by NaOH. The protocol, initially proposed by Zhao and Heindel, 32 involves a titration of the released HCl with NaOH back to an initial pH of 4 after the reaction between the POC and hydroxylamine.24 The net alkali consumption to recover the pH of 4 is expected to correspond to the aldehyde content. In similar protocols, Yang et al.33 used an initial pH of 3.5, while Larsson et al.,34 Cervin et al.35 and Lopez Duran et al.36 used a pH of 4 and Kim et al.,20, 37 and Lindh et al.,21 used a pH of 4.5. Nitrogen elemental analysis after oximation was also used to quantify the aldehyde contents.19, 26 Kim and Kuga38 found an average match between the results of different DO measurements methods including nitrogen elemental analysis after reaction with hydroxylamine-hydrochloride

or

the

periodate

consumption,

the

periodate

consumption giving always higher DO. With the exception of the UV technique, these methods may suffer from the limited accessibility of the analytical reagents toward the aldehyde groups located within the cellulosic matrix and the possible titration of other aldehyde groups e.g. those located at chain ends, resulting from over oxidation…. In parallel to chemical titration methods, solid-state

13C

NMR is a powerful tool to

quantify functional groups in bulk material, as the entire content of the material under



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investigation is probed and the method does not require any additional reaction. However, in the case of POC, deducing DO values from NMR data is not straightforward since the created aldehyde groups are never observed in the carbonyl region of the POC spectra. Indeed, in these bulk samples the concentration of hydroxyl groups is high and the newly formed highly reactive aldehyde groups quickly interact with these hydroxyls to form various covalent hemiacetals or hemialdals,21,

24-25, 33, 37, 39-40

whose carbon

resonance signals overlap with those of the intact glucosyl residues. So far, solid-state

13C

NMR has been mainly used for measuring the degree of

crystallinity of cellulose before and after periodate oxidation.21, 33, 37, 39, 41-42 However, as initially proposed by Guigo et al. in the study of the oxidation and benzylamination of microfibrillated cellulose,24 this technique should also allow measuring DOs. In the present work, we address the question of the accurate quantitative measurement of the DO of POC samples by an NMR technique. For this, using microfibrillated cellulose as a starting material in order to limit accessibility issues, we have devised various experimental conditions of periodate oxidation to obtain a series of POC products with DO values covering the entire achievable DO range (from 0 to 2). We have used these products to test a protocol leading to the calculation of DO values from 13C CP-MAS NMR data, and compared these values to those of conventional quantification methods. Furthermore, we extended this characterization method to various substrates including cotton fibers, cellulose nanocrystals and microcrystalline cellulose to check the generalization of the method to any cellulosic substrates. Materials and methods Materials A batch of paste-like never dried microfibrillated cellulose (MFC) prepared by a mechano-enzymatic treatment43-44 of spruce sulfite pulp, with a consistency of about


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2%, was purchased from the Centre Technique du Papier (CTP), Grenoble, France. It contains less than 4% of hemicelluloses on a dry weight basis. Cotton cellulose was provided by Buckeye Cellulose Corporation and further hydrolyzed to Cellulose Nanocrystals (CNC) with sulfuric acid with 65% (w/v) during 30 min at 63°C following a protocol described by Elazzouzi-Hafraoui et al.45 The final CNC concentration was 2.8 wt%. Microcrystalline cellulose CC31 was purchased from Sigma Aldrich (France). Sodium metaperiodate with purity ≥ 99.8% was acquired from Carl Roth (France). Oxidation protocols In a typical experiment, about 50 g of the 2% MFC suspension (equivalent to 1 g of dry cellulose) were inserted into a flask equipped with magnetic stirring. Periodate, from 0.2 to 1.3 equivalent of the total amount AGUs was dissolved in 40 mL deionized water and added to the MFC suspension. About 50 mL deionized water was further added to adjust the cellulose concentration to 7.2 g/L. The flask was wrapped with aluminum foil to avoid UV-light degradation of sodium metaperiodate.46 The oxidation was carried out for reaction times ranging from 2 to 170 h at room temperature for various stoechiomletric ratios to cover a wide range of experimental conditions. At the end of the reaction, the mixture was centrifuged for 1 h at 20,000 g. The pellet was then redispersed in deionized water using an Ultra-Turrax double cylinder homogenizer. The centrifugation/redispersion process was repeated 3 to 4 times to remove any residual salts until the conductivity of the supernatant was below 30 μS/cm. The product was then freeze-dried and stored before use. DO measured before and after lyophilisation was the same within the precision of the measurement. For the oxidation of cotton CNCs, CC31, cotton linters or bisulfite fibers, the exact same protocol was used except the oxidation was performed at 29, 25, 17 and 8 g/L respectively, and only with 1.3 eq of periodate. The CC31 was purified like the MFC. The CNCs and the bisulfite fibers were



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dialyzed (14 kDa cutoff membrane) until the conductivity of the water bath was below 10 μS/cm after a night. The cotton linters were filtered on a 7 μm nylon filter and thoroughly washed with deionized water.

Measuring the degree of oxidation (DO) Periodate consumption monitored by UV-visible spectroscopy, yielding DOUV The periodate concentration in the supernatant was deduced from the measurement of the absorbance at 290 nm measured using a Varian CARY 50 BIO UV-visible spectrometer, as first described by Maekawa et al.22 This wavelength was preferred to the one corresponding to the maximum absorbance of periodate (223 nm) due to an absorbance linearity on a larger range of periodate concentration (0.02 – 0.2 mM, Supporting Information Figure S-1).28 Briefly, an aliquot of the supernatant resulting from the centrifugation at the end of the reaction was immediately filtered through a 0.4 µm syringe filter, and the absorbance at 290 nm was measured after dilution with distilled water to reach a sample concentration in the 0.02 – 0.2 mM range. As each molecule of periodate oxidizes one AGU to give two aldehyde groups, the degree of oxidation extracted from the periodate consumption, DOUV, can be obtained from: 𝐷𝑂 = 2

𝑉([𝑁𝑎𝐼𝑂4 ]𝑖 − [𝑁𝑎𝐼𝑂4 ]𝑓 )∙𝑀𝐴𝐺𝑈

(1)

𝑤

where [NaIO4]i (mol.L-1) is the periodate concentration at the beginning of the reaction, [NaIO4]f (mol.L-1) is the periodate concentration at the end of the reaction measured by UV, V (L) is the volume of the reaction solution, MAGU (160 g.mol-1) is the molar mass of an oxidized AGU, and w (g) is the weight of dried cellulose used in the reaction.


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Aldehyde amount deduced from the quantitative reaction with hydroxylamine hydrochloride Titration of released HCl (DOt-NaOH) In a typical experiment,32 a 100 mg POC sample was reacted with 25 mL of an aqueous 0.25 M hydroxylamine hydrochloride solution for 2 h at room temperature, which led to a minimum hydroxylamine/carbonyl ratio of 5 for fully oxidized samples. The pH of the hydroxylamine solution and the hydroxylamine solution with POC was initially around 3.2. After the 2 h reaction time, the pH of the whole sample decreased due to HCl release (sometimes below pH 1). The sample was then titrated with a NaOH solution (0.01 M) back to pH 3.2 and DOt-NaOH is calculated from: 𝐷𝑂𝑡−𝑁𝑎𝑂𝐻 =

𝑉𝑒𝑞 ∙[𝑁𝑎𝑂𝐻]∙𝑀𝐴𝐺𝑈

(2)

𝑤

where Veq (L) is the volume of NaOH added to reach pH 3.2, [NaOH] (mol.L-1, here 0.01 N) is the NaOH concentration.

Titration of consumed base (DOt-HCl) Alternatively, in a method described by Green,47 the pH of the hydroxylamine hydrochloride solution can first be increased to pH 5.2 by a controlled addition of NaOH, and titrated to pH 3.2 by HCl addition after oxime formation, avoiding the strong acidic conditions of the previous protocol.47 In a typical experiment, a 100 mg sample was reacted with 25 mL of an aqueous solution of hydroxylamine hydrochloride (0.72 M) and NaOH (0.12 M) for 2 h at room temperature, which led to a minimum hydroxylamine/carbonyl ratio of 14 for fully oxidized samples. With this protocol, the pH never goes below 4.8 during the oxime formation and less HCl is needed to reach a pH of 3.2 when the oximation proceeds.



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The DOt-HCl is calculated from: 𝐷𝑂𝑡−𝐻𝐶𝑙 =

(𝑉𝑏𝑙𝑎𝑛𝑘 −𝑉𝑒𝑞 )∙[𝐻𝐶𝑙]∙𝑀𝐴𝐺𝑈

(3)

𝑤

where Vblank (L) is the volume of HCl needed to reach pH 3.2 on a blank sample without POC, Veq (L) is the volume of HCl needed to reach pH 3.2 with the POC sample, [HCl] (mol.L-1, here 0.1 N) is the HCl concentration.

Aldehyde amount deduced from 13C CP-MAS NMR spectroscopy Solid-state

13C

NMR spectra were acquired using a Bruker Avance III DSX 400 MHz

spectrometer operating at 100.6 MHz for

13C,

using the combination of the cross-

polarization, high-power proton decoupling and magic angle spinning (CP-MAS) method. The spinning speed was set at 12,000 Hz. The 1H radio-frequency field strength was set to give a 90° pulse duration of 2.5 µs. The 13C radio-frequency field strength was obtained by matching the Hartman-Hahn conditions at 60 kHz. At least 2,000 scans were integrated with a contact time of 2 ms using a ramp CP protocol and a recycle delay of 2 s. The acquisition time was 35 ms and the sweep width set at 29,400 Hz. The chemical shifts were calibrated with respect to the carbonyl peak of glycine (176.03 ppm).

DO estimation from NMR spectra (DONMR) After the reaction with periodate, the cellulose units exist in two forms: either oxidized, meaning that the glucosyl ring is open, or unmodified with intact sugar rings. This large difference in the chemical environment between the two forms makes the chemical shift of both C1 for intact sugar rings and C1 for oxidized moieties easily distinguishable. As a first approximation, it was assumed that the signal of the unmodified remaining


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cellulose after periodate treatment was the same as that in the starting material, and that the sensitivity of each carbon did not change after oxidation. The intensity of the C1 signal of cellulose at 105 ppm was then used to define the subspectra arising from the remaining intact cellulose (spectrum S0) in the POC sample (spectrum S) since it is the only region where the spectra of cellulose and the spectra of fully oxidized POC do not overlap, as shown in Supporting Information Figure S-2. As seen in this figure, the contribution due to the intact cellulose in the POC sample is assumed to be the S0 spectra, identical to that of the starting cellulose, but normalized by the intensity at 105 ppm. The fraction of carbon atoms belonging to the intact cellulose with respect to the total number of carbon atoms in the sample is obtained by dividing the integral of the normalized S0 sub-spectra with that of the whole S spectra, the integrations being done from 40 to 230 ppm. If we assume that the oxidation is completely selective, giving two aldehyde groups for each oxidized glucosyl ring, the degree of oxidation (DO), as defined by the number of aldehyde groups per glycosyl residue, will be obtained from: DO = 2

𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑜𝑥𝑖𝑑𝑖𝑧𝑒𝑑 𝐴𝐺𝑈

(4)

𝑡𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟𝑜𝑓 𝐴𝐺𝑈

=2

𝑡𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝐴𝐺𝑈−𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑢𝑛𝑜𝑥𝑖𝑑𝑖𝑧𝑒𝑑 𝐴𝐺𝑈

= 2 (1 −

𝑡𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝐴𝐺𝑈 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑢𝑛𝑜𝑥𝑖𝑑𝑖𝑧𝑒𝑑 𝐴𝐺𝑈 𝑡𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝐴𝐺𝑈

)

(5) (6)

and thus from the normalized spectra, DO𝑁𝑀𝑅 = 2(1 −

∫ 𝑆0 ) ∫𝑆

(7)

Where ∫ 𝑆0 and ∫ 𝑆 correspond to the integrated quantities of the S0 (initial cellulose) and S (analyzed POC sample) spectra, respectively.



Results and discussion Obtaining DOs from NaOH oxime titration (DOt-NaOH): A first series of samples was produced from the periodate oxidation of MFC for 65 h with increasing amounts of periodate, from 0.2 to 1.3 equivalent of AGU. At first, we followed Zhao and Heindel’s protocol32 where the reaction time with the hydroxylamine solution was set to 2 h. To probe the completion of oxime formation, this reaction time was then increased to 24, 65 and 96 h. The DOs extracted from oxime titration, noted DOt-NaOH, and corresponding to the various reaction times with hydroxylamine are shown in Figure 1 as a function of the periodate to AGU molar ratio.

Figure 1. DOt-NaOH of MFCs oxidized for 65 h measured by hydroxylamine titration as a function of the periodate to AGU molar ratio for variable reaction times with hydroxylamine. Hatched areas correspond to DOmax, the theoretical DO when all periodate input would have been consumed for oxidation. Figure 1 shows that the DOt-NaOH keeps increasing with the reaction time between the POC and the hydroxylamine, without any clear critical time or leveling off time. The oxime method to titrate the generated HCl by Zhao and Heindel32 was originally used for oxidized dextran, a water-soluble polymer, i.e. under homogeneous conditions where the accessibility is maximized. Under these conditions, a 2 h reaction time with


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hydroxylamine was probably sufficient to achieve a full completion of the reaction. However, in the case of oxidized MFCs, a heterogeneous reaction takes place and the accessibility toward the reagent is strongly hampered, thus requiring longer reaction times to achieve completion of the reaction. For 1.3 equivalent periodate added, DOt-NaOH even exceeds the theoretical maximum value of 2 after a 96 h reaction with hydroxylamine. The reason for this overestimation is probably due to the acid catalyzed hydrolysis of the sample as a result of the low pH. In fact, for highly oxidized samples, the pH decreases below 1 two hours after the mixing of the POC with the hydroxylamine solution. The extended reaction time in these highly acidic conditions can then lead to the hydrolysis of the POC as well as to that of unoxidized cellulose, thus liberating new reducing ends that also react with hydroxylamine. This contribution would be negligible as long as the degree of polymerization remains high, but when the number of short fragments increases, the contribution of the chain ends becomes significant, resulting in an overestimation of DOs that can even exceed the theoretical maximum. Additionally, heterogeneous conditions apparently led to slower oximation and the longer reaction time results in interference with acid hydrolysis.

Figure 2. Comparison of the DO of MFCs oxidized for 170 h with periodate measured by the two hydroxylamine titration methods (after 2 h of reaction between POC and



hydroxylamine), DOt-NaOH and DOt-HCl, and DOUV, as a function of the periodate to AGU molar ratio. Hatched areas correspond to the maximum theoretical DO, DOmax. Comparison of the titration protocols (DOt-NaOH and DOt-HCl): A second series of periodate oxidation was carried out for 170 h with periodate to AGU molar ratio ranging from 0.2 to 1.3 and the DOs of the resulting POC samples were measured by the two aforementioned oxime titration methods, yielding DOt-NaOH and DOt-HCl. Results are reported in Figure 2. The high discrepancy between these two titrations methods is most probably the consequence of the fact that DOt-NaOH is strongly affected by the sharp pH drop during the POC/hydroxylamine reaction. Compared to Zhao and Heindel protocol32 made for soluble dextran, the alternative consumed base titration protocol, initially proposed for cellulosic samples, involves NaOH addition to achieve more buffered conditions, avoiding the sharp pH drop and thus results in much higher DOt-HCl values for an identical reaction time (2 h) between the hydroxylamine and the POC. For DOt-HCl measurements, the kinetics of oxime formation is probably faster due to the higher hydroxylamine concentration compared to that of Zhao and Heindel’s protocol and the excessive acid hydrolysis is avoided, as the medium is closer to neutrality (pH always higher than 4.8). This protocol also leads to more stable DOs over a period of 24 h of reaction between POC and hydroxylamine.51

Obtaining DOs from titration with UV-Visible Spectroscopy (DOUV): The 170 h-oxidized samples were also characterized by UV-Visible spectroscopy (Figure 2). DOUV was always close to the theoretical maximum after 170 h of oxidation, but had about twice the value of DOt-NaOH. For the two samples with more than 1 equivalent of periodate, DOUV was higher than DOmax (for example DOUV with 1.3eq = 2.3 > DOmax= 2), which means that in these cases, periodate is at least partly consumed by side


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reactions.48 This extra consumption is obvious for samples with NaIO4 to AGU molar ratio larger than 1, and highly probable in the other cases. Thus, the UV method is likely to overestimate the dialdehyde production on the recovered products.

Obtaining DOs from 13C CP-MAS NMR spectroscopy (DONMR): As shown above, the two classical methods, namely oxime titration and UV-absorbance, do not provide clear and reproducible DO values due to the inherent heterogeneous conditions that are used and the concomitant side reactions. Here, a new DO measurement approach based on

13C

CP-MAS NMR is proposed. As a physical method,

solid-state NMR is not sensitive to the heterogeneous reactivity limitations encountered with the oxime titration, and the analysis is directly made on the freeze-dried recovered substrate. Figure 3 shows the

13C

CP-MAS NMR spectra of a series of 65 h-periodate

oxidized MFC samples, identical to those tested in Figure 1. The spectra are normalized to have a constant total integrated area for each sample, as we expect the number of carbons to be constant (6 carbons on each glucosyl ring).

Figure 3. 13C CP-MAS NMR spectra in the dry state of MFCs oxidized for 65 h while varying the NaIO4 to AGU molar ratio (from 0.2 to 1.3 eq). For each spectrum, the total integrated area was normalized to 1.



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As systematically reported, the absence of signals in the 160-200 ppm region indicates that the aldehyde groups do not exist in their free form but have readily recombined with neighboring hydroxyl groups to give various hemiacetal and/or hemialdal entities, which give rise to a broad signal in the 90-100 ppm region.3, 21, 24-25, 31, 33, 49 In Figure 3, the resonance at 105 ppm assigned to the C1 of native cellulose decreases as a function of the periodate concentration, while the hemiacetal signals are increasing, which is consistent with an increase of the DO with the NaIO4 to AGU molar ratio.24-25 As described in the experimental part, the intensity of the signal from the C1 of cellulose at 105 ppm was used as an internal standard for the remaining amount of un-oxidized cellulose. By normalization of the C1 of the initial MFCs with the C1 of the remaining intact cellulose in the POC samples, and by integrating the whole spectra, the calculation of a percentage of oxidation or DO can be quantitatively extracted from a

13C

CP-MAS

NMR spectrum of any POC sample, using equation 7. A series of periodate oxidation of MFC suspensions were also performed using 0.2 to 1.3 equivalent of periodate to AGU and increasing oxidation times from 2 to 170 h. The DONMR calculated from the spectral integration as a function of reaction time and the amount of periodate are shown in Figure 4. The corresponding series of spectra are available in Supporting Information Figures S-3 to S-6. For a given NaIO4 to AGU molar ratio, the DONMR clearly increases with the oxidation time until 65 h. In addition, for a given reaction time, the DONMR clearly increases with the NaIO4 to AGU molar ratio. These data show that a very precise control of the DO of oxidized MFC samples in the 01.9 range can be reached by varying the two reaction parameters, namely time and periodate concentration. Remarkably, solid-state NMR spectra indicate the presence of


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remaining native cellulose up to a DONMR of 1.8, indicating the heterogeneous character of the oxidation even at high degree of oxidation.

Figure 4. Evolution with reaction time of the DO measured by 13C CP-MAS NMR , DONMR, of periodate oxidized MFCs for variable amounts of NaIO4 to AGU molar ratio. To evaluate the relevance of the NMR methods, NMR spectra of POC were reconstructed by a linear combination of the initial NMR spectrum of MFC and of the NMR spectrum of the most oxidized cellulose (DO=2.0) (Figure S-7 to S-12). 𝐷𝐴𝐶 𝑠𝑝𝑒𝑐𝑡𝑟𝑢𝑚 =

𝐷0 𝐷𝑂 × (𝐷𝐴𝐶 𝐷𝑂 = 2 𝑠𝑝𝑒𝑐𝑡𝑟𝑢𝑚) + (1 − ) × (𝑀𝐹𝐶 𝑠𝑝𝑒𝑐𝑡𝑟𝑢𝑚) 2 2

The resulting calculated spectra are very close to the experimental ones, strongly supporting the idea that the oxidized samples are two-phase materials made of fully oxidized and unreacted native cellulose. It is worth noting that there is still a slight systematic difference (less than 5 %) between the original and the reconstructed spectra, in the C4 region where the reconstructed spectra for intermediate DOs are slightly overestimated compared to the experiment. To account for this observation, one has to remember that the reaction most probably progresses from the surface to the interior of the microfibrils, leading to a heterogeneous core-shell structure, as already



observed by solid state NMR for the gas phase esterification of nanocelluloses50-51 , or in heterogeneous esterification in the acetylation reaction57 or even higher esters.58 For sufficiently high degrees of oxidation, the microfibrils are then partially consumed by the reaction and their dimensions and hence crystallinity should decrease concomitantly, explaining changes in the ratio of crystalline (near 90 ppm) and disordered / surface (around 85 ppm) contributions. The slight differences between experimental and reconstructed spectra would reflect this change in proportion of crystalline and surface fractions. Conversely, a minor contribution at high magnetic field is also always present for the C1 carbon resonance in the experimental spectra for the intermediate products, which can be due to chemically intact glucose residues with a slightly modified chemical shift due to a different environment. Altogether, these differences that reflect the ultrastructural modifications, account for less than 5% of the total area, making the evaluation of the oxidation degree by this technique quite robust and reproducible.

Agreement between DONMR and DOt-HCl As aforementioned, DOt-HCl are always higher than DOt-NaOH after a conventional 2 h reaction time between POC and hydroxylamine. If we now compare these DO values with the results from NMR, the DOt-NaOH and DOt-HCl correspond to about 60 % and 90 % of the DONMR (see Supporting Information Figure S-13), respectively. The DOt-NaOH is probably underestimating the degree of oxidation due to slow kinetics of oximation competing with side reactions at low pH on solid substrate as explained in section 2.2. The DOt-HCl (Green’s titration protocol) on the other hand gives fairly good agreement with the proposed NMR quantification. The DOt-HCl measurmenent is not sensitive to


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oximation time52, and thus the two methods seem to be robust and reliable for wide DO range.

Comparison between solid state NMR and UV measurements of periodate consumption The oxidation mechanism implies that each periodate ion leads to a di-aldehyde unit and thus, the samples should reach a DO of 2 when 1 equivalent or more of periodate per AGU has been used. However, in this work, the final DONMR in the samples was systematically lower than expected. For example, a sample oxidized for 170 h with 1.0 eq periodate per AGU had a DONMR of only 1.7, with significant amount of intact native cellulose, whereas DOUV indicated that 99% of the introduced periodate had been consumed (i.e. DOUV=1.98). To reconcile these discrepancies, we should keep in mind that the characterization is performed on the solid pellet recovered after extensive centrifugation, the reaction medium being discarded. Interestingly, Figure 5 shows that the mass yield of the recovered POC decreases linearly as oxidation proceeds, whatever the time and periodate conditions used for the experiments, reaching a final value of about 60% for almost fully oxidized cellulose. The discarded soluble fraction most probably consists of strongly oxidized chains that were solubilized before forming hemiacetal linkages with neighboring chains. This mechanism would be favored by the dilute conditions used (0.7%) and is in agreement with previous reports indicating that highly oxidized dialdehyde cellulose was soluble in hot water and remained stable in solution after cooling.20, 40



Figure 5. Mass yield evolution with the degree of oxidation of samples recovered after successive centrifugations at 20,000 g followed by freeze-drying.

In all these experiments the solubilized cellulose fraction is not recovered, but its DO is expected to be higher than that of the recovered solid due to higher accessibility. Two extreme scenarios could then be envisaged: (i) the DO of the solubilized part is the same as that of the recovered solid or (ii) the solubilized part has a DO of 2. To test these hypotheses, various presumed periodate quantities were calculated from DOUV and DONMR data as well as from the mass yield after purification and freeze-drying. The UVVis measurements gives the total amount of periodate consumed during the oxidation reaction, NaIO4 consumed. This amount of periodate consumed to oxidize the unrecovered fraction can be calculated from the mass yield but depends on the DO of the solubilized chains, delimiting two different cases. On the one hand, if one assumes that the DO of these solubilized chains is the same as that of the recovered solid POC, the necessary consumed amount of periodate can be calculated and can be considered as the lowest reference NaIO4 calculated (partially oxidized). On the other hand, if the solubilized chains are considered to be fully oxidized, a consumed periodate amount, NaIO4 calculated (fully oxidized) can be extracted.


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Figure 6 compares the initial amount of periodate put in the reaction medium, NaIO4Initial, the total periodate consumption as measured by UV measurements, NaIO4 consumed, with NaIO4 calculated (partially oxidized) (Straight black line) and with NaIO4 calculated (fully oxidized) (Straight blue line). At low and intermediate degrees of oxidation, the experimental data points are in very good agreement with a consumption of NaI04 that take into account the removal of fully oxidized chains left in the supernatant. In this regime, the fully oxidized chains (supposed to be water soluble) are progressively peeled off the crystals, explaining both the continuous decrease of the yield and the need of a higher equivalent number of moles of NaIO4 to reach a given D0. For the highest degrees of oxidation (i.e. > 1.5), the experimental data strongly deviates from the predicted ones with an amount of consumed periodate well above both the measured and predicted degree of oxidation, indicating the presence of side reactions (mostly massive hydrolysis of chains and overoxidation) that occurs at the highest degrees of oxidation.52

Figure 6. Various amounts of periodate measured or calculated from DOUV, DONMR and mass yield data. See text for details.

Extension to other cellulosic substrates



In order to extend the accuracy of this methodology, we submitted four other types of celluloses to periodate oxidation with an excess of 1.3 eq/AGU for various times. Different sources commonly used in the literature namely cotton linters, cotton CNCs, commercial microcrystalline cellulose (CC31) and cellulose fibers from sulfite pulp were used. The series included samples from micrometer-sized sizes (linters and fibers) to nanometer-sized sizes (CC31 and CNC) from two different origins and with varying crystallinity series gather. The resulting spectra are shown in Figure 7 and display qualitatively the same behavior as the one observed for cellulose microfibrils, with a decrease in the intensity of the C1 signal of native cellulose with a concomitant increase of the contribution of the hemiacetals between 90 and 100 ppm.

Figure 7 Different cellulose sources oxidized with 1.3 eq of periodate / AGU for variable times: a) cotton linters, b) cotton CNCs, c) CC31 and d) Bisulfite fibers.


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The evaluation of the degree of oxidation degree of oxidation was performed using the same subtraction procedure described in the preceding sections. The results are indicated in Table 1, together with the one obtained for cellulose microfibrils. Table 1: DO obtained after time-controlled periodate oxidation (1.3 eq/AGU) on different cellulose substrates Cellulose

Cotton linters

Cotton CNCs CC31 Bisulfite Fibers MFC

Time of oxidation (h) 1 15 24 70 10 24 65 10 24 65 8 30 65 10 24 65

DO 0.0 0.6 0.5 1.5 0.4 1.2 1.8 1.0 1.2 1.7 0.5 1.2 1.7 1.0 1.4 1.8

From the inspection of the obtained values, it appears that the results are qualitatively similar indicating that the nature of the substrate is of secondary importance and that very high level of oxidation can be obtained even with more crystalline substrates. However, the kinetics is slightly different depending on the characteristics of the samples. For a given substrate, the oxidation proceeds faster with nanometer-sized sized samples (CNC compared to linters and CC31 and MFC compared to fibers), whereas crystallinity plays the same qualitative role, the less crystalline samples being the fastest oxidized.



Conclusions In this study, we have observed that in the

13C

CP-MAS NMR spectra of the periodate

oxidized cellulose, the resonances corresponding to the oxidized cellulose part always have the same shape, independently of the DO. These resonances do not overlap with the peak attributed to the C1 of underivatized cellulose. This peak can be used to quantify the residual cellulose fraction and to deduce the degree of oxidation from NMR data. The periodate consumption monitored by UV absorption was always higher compared to the DO measured by the NMR spectrum of the recovered product, while the method based on oxime formation at a pH around 5 gave values close to the NMR method. At lower pH the oxime formation is slow and induces cellulose hydrolysis, leading to unreliable values. The DO of the recovered product estimated by solid-state NMR agrees with the periodate consumption from UV absorbance at 290 nm if we consider that the solubilized fraction is fully oxidized. Thus when a partial dissolution occurs, the UV absorbance can be used to follow the oxidation reaction, while the degree of oxidation of the solid phase has to be determined separately by solid state NMR or oxime formation in buffered conditions. Moreover, solid-state NMR gives information on the ultrastructure of the product, especially on the inherent heterogeneous character of the periodate oxidation, leading to a two-phase product made of a fully oxidized cellulose skin surrounding an intact but partially consumed native cellulose core. We provide here an accurate, simple and reproducible 13C NMR method that allows both DO measurements after periodate oxidation of cellulose and importantly the monitoring of its physical state, which could be extended to any types of cellulose.


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ACKNOWLEDGMENT The authors acknowledge the French Agence Nationale de la Recherche (ANR ‐ 14 ‐

CE07‐0018‐04 in collaboration with the Solvay Company) the Glyco@Alps program (Investissements d’Avenir Grant No. ANR-15-IDEX-02) for financial support, Institut Carnot PolyNat (ANR-16-CARN-025-01) for scientific support. The authors are grateful to H. Chanzy for valuable help during the editing of the manuscript. We thank the NanoBio chemistry platform (ICMG FR 2607) for granting access to NMR spectrometers.

Supporting information Supporting Information contains a calibration curve of absorbance as function of the periodate concentration, the NMR spectra superposition protocol for DO calculation by 13C

CP-MAS NMR, the

reconstructed

13C

13C

CP-MAS NMR spectra of periodate oxidized MFC series,

CP-MAS NMR spectra of oxidized MFC, and the relationship between

DONMR and the DOtitrations.

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TOC

Periodate oxidized cellulose (POC) is a core-shell material that is a plate-form for further biosourced and sustainable material owing to the reactivity of the aldehyde shell.


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