On the role of N-methylmorpholine-N-oxide (NMMO) in the generation of elemental transition metal precipitates in cellulosic materials

Several literature reports describe the role of aqueous solutions of N-methylmorpholine-N-oxide monohydrate (NMMO) as a suitable medium for the generation of transition metal (nano)particles in or on cellulosic materials and further elaborate its role as a co-reactant of the transition metal salts that are reduced to the elemental metal. However, this would assign NMMO the role of a reductant, which is in contradiction of its obvious oxidative nature. In the present study, the exemplary cases of silver, gold, and platinum salts as the precursors of the respective metal (nano)particles in aqueous NMMO/cellulose mixtures were investigated. Naturally, NMMO did not act as a reducing agent in any case—this role was taken over by the frequently used NMMO stabilizer propyl gallate, or by cellulose itself, into which carbonyl and carboxyl groups were introduced. Also, hypochlorite—produced intermediately from chloride ions and subsequently undergoing disproportionation into chloride and chlorate—or transient N-methylene(morpholinium) ions generated from NMMO, which are in turn oxidized to formyl morpholide, can act as the corresponding reductants while the metal ions are reduced, depending on the reaction conditions. Apart from providing interesting mechanistic insights, the study points to the importance of a precise description of the composition of the chemical systems used, as well as the importance of seemingly inert auxiliaries, which turned out to be essential co-reactants in the metal (nano)particle generation.

Abstract Several literature reports describe the role of aqueous solutions of N-methylmorpholine-N-oxide monohydrate (NMMO) as a suitable medium for the generation of transition metal (nano)particles in or on cellulosic materials and further elaborate its role as a co-reactant of the transition metal salts that are reduced to the elemental metal. However, this would assign NMMO the role of a reductant, which is in contradiction of its obvious oxidative nature. In the present study, the exemplary cases of silver, gold, and platinum salts as the precursors of the respective metal (nano)particles in aqueous NMMO/cellulose mixtures were investigated. Naturally, NMMO did not act as a reducing agent in any case-this role was taken over by the frequently used NMMO stabilizer propyl gallate, or by cellulose itself, into which carbonyl and carboxyl groups were introduced. Also, hypochlorite-produced intermediately from chloride ions and subsequently undergoing disproportionation into chloride and chlorate-or transient N-methylene(morpholinium) ions generated from NMMO, which are in turn oxidized to formyl morpholide, can act as the corresponding reductants while the metal ions are reduced, depending on the reaction conditions. Apart from providing interesting mechanistic insights, the study points to the importance of a precise description of the composition of the chemical systems used, as well as the importance of seemingly inert auxiliaries, which turned out to be essential co-reactants in the metal (nano)particle generation.

Introduction
The use of N-methylmorpholine-N-oxide monohydrate (NMMO, 1) in the manufacture of cellulosic products according to the lyocell technology, mostly fibers, is well established on an industrial scale. While more dilute aqueous solutions of NMMO have a swelling effect on cellulosic pulp, the respective monohydrate is able to dissolve cellulose at elevated temperatures above 80°C so that a spinnable dope is obtained. In theory, this dissolution process is merely physical, but in practice quite many side reactions might occur (Rosenau et al. 2001), which, however, can efficiently be suppressed by the addition of appropriate stabilizers. Propyl gallate (PG, gallic acid propyl ester, 2) is the most common one of them (Rosenau et al. 2002a,b), which is even applied on an industrial scale, but also a non-acidic process medium contributes to minimizing degradation reactions of the solvent. Potential hazards of improper handling of NMMO in its different application fields (cellulose dissolution and fiber/film manufacture, biomass pretreatment, organic synthesis) have recently be summarized and pointed out (Rosenau and French 2021).
The lyocell system has very frequently been used also on a laboratory scale for research on novel cellulosic products, especially the embedding of particles and nanoparticles into cellulosic matricesin most cases fibers, but also films, beads, and pulp (Firgo et al. 1995;Marini and Bruneis 1996;Fink et al. 2001;Sayyed et al. 2019). The incorporation into fibers can be achieved either by (nano)particlecontaining dopes, which are then spun out, or by swelling cellulosic substrates in aqueous NMMO and precipitating the nanoparticles into/onto this matrix (Shah et al. 2013). The by far most dominant topic in this regard is the generation of transition metal precipitates, in particular from silver, because of its antimicrobial/antibacterial properties (Smiechowicz et al. 2011(Smiechowicz et al. , 2018(Smiechowicz et al. , 2020(Smiechowicz et al. ,2019Rac-Rumijowska et al. 2017). But also gold nanoparticles have been studied (Yokota et al. 2008;Kitaoka et al. 2011;Hatakeyama et al. 2019), as well as platinum and lanthanide equivalents Skwierczyńska et al. 2019). Luminescent cellulosic fibers have been obtained from an aqueous NMMO medium by incorporating europium-doped (Kulpinski et al. 2012) or terbium-doped ) particles into the matrix. In general, NMMO has been characterized as a co-reactant for the transition metal ions and swelling agent for cellulose, which provides a beneficial environment for both nanoparticle generation (instead of larger precipitates) and their incorporation into the cellulose matrix (instead of mere surface adsorption). While this medium has apparently proved to be quite reliable and thus has gained popularity for the generation of transition metal precipitates in cellulosic fibers, there is still a puzzling question about this system from a chemical point of view, which has not yet been pointed out so far.
The formation of transition metal particles from metal salts is evidently not just a physical/mechanical process that causes the transition from the macro-to the micro-or nano-scale-such as the formation of nano-fibrillated cellulose from cellulosic precursors. It is a true chemical reaction, more specifically a reduction of metal cations to elemental metals, i.e., from a positive oxidation state to oxidation state zero. Obviously, an isolated oxidation process does not exist-any oxidation is accompanied by a corresponding reduction, both reactions together constituting a redox pair. However, NMMO is a strong oxidant itself and certainly not capable of functioning as a direct reducing agent for the transition metal cations (Albini 1993;Pieterse 2013). This puts us in the somewhat paradoxical situation that a relatively commonly used reaction system exists that has been successfully applied to the synthesis of elaborate (nano)particle-containing cellulosic matrices, but its chemistry is apparently not at all well understood, and in some cases incorrectly portrayed in the literature. In short, NMMO's reported role as a coreactant and the nature of the reductant effecting generation of the transition metals from their salts remain unclear.
In this study, we have investigated the chemistry of different systems for the generation of transition metal precipitates on cellulosic substrates in aqueous NMMO in more detail, especially concerning the redox reactions taking place. When elemental metal species are formed by reduction, something else must, in turn, be oxidized. The identification of these redox processes was thus the major task.

General
All chemicals were obtained from Sigma-Aldrich (Schnelldorf, Germany). Thin-layer chromatography (TLC) was performed on silica gel 60 plates (5 9 10 cm, 0.25 mm) with fluorescence detection under UV light at 254 nm. Transition metal salts were of the highest purity commercially available. N-Methylmorpholine-N-oxide was recrystallized from dioxane, N-methylmorpholine-N-oxide monohydrate from acetone. Bidistilled water was used throughout. Propyl gallate was recrystallized from toluene until colorless and stored in the dark in a tightly closed vessel under Ar. The usual slightly yellow appearance of this compound points to trace impurities from ellagic acid and its bis(ortho-quinone). The silver ion concentration in aqueous solution was determined by means of an ion-selective electrode (ISE-3, Thomas Scientific, Swedesboro, New Jersey, USA) after adjustment to pH 6 with 0.1 M aqueous HNO 3 . By the addition of different NMMO and N-methylmorpholine (NMM) concentrations to a blank of 1 mM AgNO 3 solution, it was confirmed that the presence of the organic compounds did not influence the result of the measurement.
NMR experiments were carried out on a Bruker Avance II 400 instrument ( 1 H resonance at 400.13 MHz, 13 C resonance at 100.62 MHz), equipped with a 5 mm broadband probe head (BBFO) with z-gradient. Standard Bruker pulse programs were used. Data collection: 32 k data points, apodization with a Gaussian window function (GB = 0.3) prior to Fourier transformation. Acquisition time: 2.5 s, relaxation delay: 1 s. Bruker TopSpin 3.5 software was used for the acquisition and processing of the NMR data. Spectra were recorded in CDCl 3 in the case of isolated compounds (e.g. tocopherol derivative, Nformylmorpholine) or D 2 O (direct measurement of NMMO reaction mixtures). Chemical shifts are given in d ppm values relative to TMS as the internal standard, coupling constants are given in Hz. 13 C peak assignment was done through APT, HSQC, and HMBC spectra.
General procedure for reduction of transition metal ions to metal precipitates in NMMO solution A 50 wt% aqueous solution of (freshly recrystallized) N-methylmorpholine-N-oxide (50 mL) was blended under stirring with propyl gallate (0.50 g, 2 wt% rel. to anhydrous NMMO) and cellulosic pulp (air-dried, 2.50 g, 5% rel. to the NMMO solution) for 30 min at r.t. A transition metal salt (see ''Results and discussion'' section and Table 1, 500 or 1000 ppm rel. to cellulose, 1.2-10 mg) was dissolved in stirred bidistilled water (2.0 mL) at r.t. and NMMO (2.00 g) was added. This solution was added to the NMMO/PG/ pulp mixtures during 30 s under stirring. In variations of the procedure, either propyl gallate or cellulosic pulp were omitted or concentrations were changed (see ''Results and discussion'' section). The mixture was quickly heated to the desired reaction temperature (see ''Results and discussion'' section) by immersing the reaction vessel into a pre-heated oil bath. After certain reaction intervals, aliquots (3 mL) were taken and mixed with 20 mL of bidistilled water under stirring. The precipitated cellulose was collected by filtration and further analyzed by GPC. The liquid phase was used directly for analysis of ions (see below) or extracted with cyclohexane (3 9 5 mL). The organic extracts were washed with brine (2 9 5 mL), dried over anhydrous MgSO 4 , and analyzed by GC-MS or TLC. CAUTION! NMMO mixtures with transition metal ions might undergo violent, explosive reactions with splashing, strong smoke evolvement, charring, and possibly fire. Work only in a well-functioning fume hood and observe all necessary precautionary measures (safety shield, safety goggles, etc.)!

Variations of the general procedure
The general procedures were slightly varied (presence or concentration of components, reaction temperature, reaction time) as described in the main text, schemes, and Table 1. For trapping of intermediary HOCl/ HOBr, 3,4-dehydro-a-tocopheryl acetate (0.1 g) dissolved in 1,4-dioxane (1 mL) was added along with propyl gallate. For the determination of chlorate, the taken reaction aliquots (3 mL) diluted with bidistilled water (20 mL) were used. For the direct reaction of NMMO with transition metal ions, the 50 wt% aqueous NMMO solution was replaced with anhydrous NMMO.
Determination of chlorate in the NMMO solution (a) The aqueous solution (ca. 10 mL) obtained by dilution of the withdrawn aliquot was filtered to remove cellulose and 2 M aqueous HNO 3 was added (pH * 1). Aqueous AgNO 3 solution (0.1 M) was added dropwise into the acidified solution under stirring until no further precipitation occurred. The solution was briefly heated (ca. 50°C for 1 min), the white precipitate (AgCl) was collected by centrifugation and the clear supernatant separated. Aqueous AgNO 3 solution (0.1 M, 5 mL) and one drop of concentrated HNO 3 were added during stirring. The addition of a few drops of saturated aqueous KNO 2 solution caused the immediate formation of a white precipitate of AgCl, the chloride being generated in the reduction of chlorate to chloride by nitrite (ClO 3 The aqueous solution obtained by dilution of the withdrawn aliquot (ca. 20 mL) was filtered to remove cellulose, diluted to 50 mL with bidistilled water, mixed with anion exchange resin (2 g, Dowex, OH -form), and gently stirred for 15 min at r.t. The anion exchanger was collected by filtration, washed with warm ethanol (50 mL, to remove adsorbed NMMO) and bidistilled water (2 9 10 mL), and regenerated with 0.1 M aqueous NaOH solution (10 mL 3,4-Dehydro-a-tocopheryl acetate (0.1 g) dissolved in 1,4-dioxane (1 mL) was added together with propyl gallate, and the general protocol (see above) was followed. The washed and dried organic extracts were evaporated in vacuo and the mixture was analyzed by TLC. One analytically pure sample was obtained by flash chromatography on silica gel (eluent: toluene). The analytical data of the purified trapping products of HOCl and HOBr are given below.

Results and discussion
The initial expectation and hope that the redox processes in the transition metal/NMMO mixtures were always the same-i.e., independent of the metal/ metal salt used-has unfortunately not occurred: apparently, it is a rather dynamic and flexible reaction system. However, this points out all the more how important a detailed characterization of the underlying chemistry can become, not just in mechanistic organic chemistry, but also in material science work. The corresponding studies on the reaction mechanisms are presented below.

Elemental silver species on cellulosic substrates
Several procedures have been proposed to synthesize silver nanoparticles in aqueous NMMO and incorporate them into cellulosic fibers (Rac-Rumijowska et al. 2017Smiechowicz et al. 2011;Shah et al. 2013). These approaches used 50 wt% aqueous NMMO stabilized with 2% propyl gallate. The metal was introduced in the form of aqueous AgNO 3 (between 0.04 and 1 M), which was either added to the pulp or directly into the lyocell spinning dope to get a final Ag concentration of about 250-500 ppm. Blending and dissolution (with concomitant metal precipitate production) were done at 115°C for 10 min, followed by dry-wet spinning at the same temperature.
It was logical to assume that in return for the reduction of Ag ? ions to elemental silver, functionalities of cellulose would be oxidized, especially the hemiacetals of the reducing ends (which do not have their name for nothing). This would lead to carboxyl groups (gluconic acid moiety) at the end of the cellulose chains. Likewise, oxidation of the cellulosic OH groups along the chains was conceivable, which would lead to keto groups at C2 and C3, and aldehyde or carboxyl (glucuronic) functions at C6. Similar reactions between Ag ? and cellulose-albeit in the absence of NMMO-had already been used to label the reducing ends of celluloses to prepare silvercontaining cellulosics (Kato 1959;Gagnaire and Vincendon 1973;Kuga and Brown 1988).
The content of oxidized groups in cellulosics and their relation to the molecular weight distribution can be reliably and accurately determined by the CCOA method for carbonyls (Röhrling et al. 2002a, b;Potthast et al. 2003) and the FDAM method for carboxyls (Bohrn et al. 2006), two frequently applied approaches when accurate monitoring of oxidative processes of cellulosics becomes necessary (Potthast et al. 2005). Both rely on group-selective quantitative fluorescence labeling in combination with gel permeation chromatography (Potthast et al. 2015).
While these methods showed a clear oxidative effect of pure NMMO monohydrate on cellulose (120-130°C, 2 h) accompanied by chain degradation (cf. also Rosenau et al. 2005a, b), no such effects were seen for the silver-containing system, which contained 50 wt% aqueous NMMO stabilized with 2% propyl gallate, 5% cellulosic pulp and 500 ppm Ag ? (rel. to the pulp, as 1 mM AgNO 3 ). Mixing of the reagents occurred at r.t. and the reaction at 50°C (see ''Materials and Methods'' section). Under these conditions, the C=O and COOH content of the cellulose as well as the molecular weight stayed largely unchanged, see Fig. 1. This evidently excluded cellulose as the main reaction partner (= reductant) of the silver ions. Also, the NMMO content remained nearly unchanged, with NMMO consumption by degradation to N-methylmorpholine being below 0.8%. In the absence of silver ions, under otherwise identical conditions, the results were the same as in their presence: almost no introduction of oxidized groups and rather little chain degradation was observed. Oxidation of cellulose was minor and became more pronounced only at temperatures [ 120°C and reaction times [ 1 h.
The actual co-reactant for the silver ions was found more or less by serendipity when we observed a blueblack discoloration at spots of scratching with a metal spatula, which brought propyl gallate (2) back to mind. This compound could not only be a stabilizer that stays in the background and is largely ignored, but it could be as well an active reagent and co-reactant. The observed discoloration was due to gallic acid-iron complexes, which are extremely color-intense. Such compounds have been the basis of inks for centuries, if not millennia, and countless-sometimes rather adventurous-recipes exist for their preparation (Melo et al. 2019;Díaz Hidalgo et al. 2018;Zervos and Alexopoulou 2015). However, all these historic protocols are based on gallic acid (from gall apples) and the addition of iron ions.
When we extracted the reaction mixture with toluene, we found a single reaction product of propyl gallate, namely ellagic acid (4), and a direct linear correlation of its formation with the consumption of Ag ? , which indicated a direct conversion and the absence of other parallel or side reactions. Propyl gallate and Ag ? were strictly stoichiometrically converted into ellagic acid and elemental Ag as the final products (Fig. 2, left). Propyl gallate (2% rel. to aqueous NMMO) was always present in large excess compared to the silver ions (500 ppm rel. to the 5 wt% dissolved cellulose) so that its concentration could be assumed to remain constant during the reaction. Reaction kinetics, which were recorded at r.t. to slow down the reaction, thus followed a pseudo-first-order with regard to the silver ion concentration and ellagic acid formation. In a reverse experiment-in which an aqueous solution without NMMO being present was used-propyl gallate (2) was added into a tenfold excess of silver ions, giving also a pseudo-first-order kinetics for propyl gallate consumption and ellagic acid (4) formation. This points to the fact that the ratedetermining step is a bimolecular reaction, with its reaction rate r = d[4]/dt = k[Ag ? ][2] being first-order with regard to both silver ion concentration and propyl gallate concentration, resulting in overall pseudo-firstorder when either Ag ? or PG are present in large excess, i.e., r = d[4]/dt = k[Ag ? ] = k[2], as shown in Fig. 2, right, with k, k and k being the respective kinetic rate constants.
In the first step of the overall redox process, silver ions are reduced by propyl gallate to elemental silver and the corresponding phenoxyl radicals (2a), which immediately recombine to the diphenic acid ester intermediate (3). Mainly entropically driven, this intermediate is immediately converted to ellagic acid and two equivalents of 1-propanol (Scheme 1). The   redox couple active in this system can thus be summarized with Eq. 1: with Prop-OGall = propyl gallate, Prop-OH = 1propanol, and EllA being the common abbreviation of ellagic acid.
The formation of ellagic acid (4) is a typical reaction of gallic acid and its derivatives (Aguilar-Zarate et al. 2018;Jourdes et al. 2012), known for more than a century (Bleuler and Perkin 1916). The analysis of propyl gallate and its reaction products is straightforward as both can be extracted quantitatively with small amounts of toluene (dichloromethane works equally well) followed by determination with GC/MS or HPTLC.
In the absence of propyl gallate, no silver reduction was observed under the applied conditions (50°C), only to some minor extent after extended reaction times of more than 2 h. At 80°C, however, the reaction was completed within 30 min also in the absence of propyl gallate. At the same time, oxidation of cellulose and some chain degradation were observed, which confirmed that cellulose acted as the co-reactant of the silver ions, taking over the usual role of propyl gallate, although requiring harsher conditions (longer reaction times or higher temperatures), see Fig. 3. When also the silver ions were left out (in addition to the absent propyl gallate), these oxidation effects almost vanished, proving that cellulose oxidation under the used conditions was due to the silver ions and not due to NMMO.

Elemental platinum species on cellulosic substrates
Platinum nanoparticles have been generated from different platinum salts and precipitated onto cellulosic carriers (Shah et al. 2013), with different temperatures between room temperature and 40°C having been used, mainly to adjust the precipitation rate and nanoparticle size. In our experiments, we first used 50% aqueous NMMO stabilized with 2% propyl gallate and later the same medium without the PG stabilizer, along with 5% cellulosic pulp and 1000 ppm (rel. to the pulp) of Pt salt.
For all platinum salts tested (with the only exception of hexafluoroplatinate that did not react at all), the propyl gallate (2) present as a stabilizer of the NMMO acted as the reductant, being converted to ellagic acid (4), see Table 1, column 2. The basic redox reaction Scheme 1 Reduction of silver ions to elemental silver with the lyocell stabilizer propyl gallate (2) being oxidized in turn to ellagic acid (4) via the corresponding phenoxyl radical intermediates (2a) and diphenic acid bis(n-propyl) ester (3) was thus analogous to the silver case (see also Scheme 1). However, the reduction of the platinum ions proceeded much faster than that of Ag ? . It occurred already at room temperature, while for Ag ? temperatures around 80-100°C were required. Also the stoichiometry of the reaction was obviously different: while for silver a ratio of 1:1 between Ag ? and PG was required (corresponding to Ag: ellagic acid = 2:1, see Fig. 2, left), the stoichiometric ratio Pt:PG was 1:2 for Pt(II) and 1:4 for Pt(IV) salts, corresponding to Pt: ellagic acid = 1:1 for Pt(II) and 1:2 for Pt(IV), respectively. Differences in the reactivity of the salts at room temperature were not observed, and the reaction did not change at all when no cellulose was present.
With the same abbreviations as used above (Prop-OGall for propyl gallate, Prop-OH for n-propanol, and EllA for ellagic acid), the general redox system for Pt(II/IV) and propyl gallate can be summarized by Eqs. 2 and 3, respectively: In the absence of propyl gallate, the reduction to elemental platinum required significantly higher temperatures, a fact that has been overlooked so far. The precipitation of elemental Pt commenced around 60°C for the platinum(diammin)-complexes and Pt(IV) halides, and at 90°C for the tested Pt(II) halides, see Table 1, column 3. Similar to the PG-case, hexafluoroplatinate (IV), which is known to be an exceptionally stable platinum complex (Holleman and Wiberg 1985b), did not react at all, but also hexahydroxidoplatinate(IV), which reacted well with PG, showed no reaction in its absence (except at high temperature). Once more the results were the same independent of whether cellulose was present or not. Obviously, when the easy-to-oxidize PG was lacking, Changes of carbonyl group content, carboxyl group content, and M w of a cellulosic pulp (beech sulfite, 5% rel. to solvent) upon treatment in different NMMO systems. In the absence of propyl gallate, cellulose takes over the role of the reductant of silver ions, but more drastic conditions are required for the reduction to proceed. Values are changes relative to the starting material: C=O 18.3 lmol/g, COOH 8.6 lmol/g, M w 460 kg/mol (cf. Fig. 1) the system had to swerve and use a less ''convenient'' reductant. For all chloride-containing platinum salts, chlorate (ClO 3 -) was found as one reaction product, for which the chloride in the starting material was obviously the only source. The presence of chlorate was confirmed by ion chromatography and precipitation (see ''Materials and Methods'' section); other Clbased anions apart from chloride itself (chlorite and perchlorate), were absent. Since neither Pt(II) nor Pt(IV) oxidizes chloride to chlorate directly (otherwise the respective salts would not be stable), an involvement of NMMO was indicated, and indeed, the formation of equivalent amounts of N-methylmorpholine (NMM, 5), the deoxygenation product of NMMO, was observed, see Fig. 4 and Scheme 2. Therefore, three parallel processes seemed to be operative in the reaction system, the reduction of the platinum ions to elemental metal, oxidation of chloride, and reduction of NMMO (1) to NMM (5).
Such oxidation of halides by NMMO has been observed for the first time in the reaction of cyanuric chloride with NMMO (Rosenau et al. 2002a,b) and later also for gold complexes in the presence of NMMO monohydrate (Kitaoka et al. 2011). In both cases, chlorate was the final product, which was formed as a disproportionation product of initially generated hypochlorite (HOCl/ClO -). Therefore, we assumed that a similar mechanism was also operative in the present case of the Pt salts. Indeed, the addition of the HOCl-trapping agent 3,4-dehydro-a-tocopheryl acetate (6) to the reaction system (platinum salt, 50% aqueous NMMO, 60°C, no propyl gallate) afforded the trapping product of hypochlorous acid (HOCl), chlorohydrin 3-chloro-4-hydroxy-a-tocopheryl acetate (7), almost quantitatively (Scheme 2). The situation was analogous in the case of bromide salts (see Table 1), with intermediate hypobromite being trapped by trapping agent (6) as 3-bromo-4-hydroxya-tocopheryl acetate (8) in 64% yield. The advantage of this trapping reagent and its trapping products is their very high lipophilicity due to the isoprenoid C 16 H 33 side chain, which renders them extremely well extractable with n-hexane, also from very complex multi-component mixtures, and easily analyzable.
With the intermediacy of HOCl confirmed, the corresponding redox system can be characterized in a way that in the overall process Pt(II/IV) is reduced to Pt(0) while chloride is oxidized to chlorate. In detailed steps, Pt(II/IV) is reduced to elemental platinum and Clis oxidized, in turn, to ClO -(Scheme 2, red). At the same time, NMMO is reduced to NMM (Scheme 2, blue). This is followed immediately by the disproportionation of the formed hypochlorite into chloride and chlorate, which is a kinetically and thermodynamically highly favored process (Holleman and Wiberg 1985a). Note that the two reduction processes-that of Pt(II/IV) and that of NMMO-are not independent: neither Pt(II/IV) alone nor NMMO alone is reduced by chloride, but in combination, they are obviously sufficiently reactive. Coulombic interaction of the negative exo-oxygen in NMMO with transition metals is known to increase the length of the highly polar N-O bond and to render that bond less stable (Linton 1940). Thus, it represents the first step of transition metal ion-induced N-O bond cleavage (Rosenau et al. 2001). It is reasonable to assume that a similar NMMO-metal interaction is also present in the Pt-NMMO-Cl system. In any case, NMMO would be required to enter the coordination sphere of the Pt cations for the reaction to occur.
The overall redox reaction of Pt(II) salts can thus be described by Eqs. 4-6 and those of Pt(IV) salts by Eqs. 7-9, cf. Scheme 2. Equations 4 and 7 describe the actual redox process under the generation of hypochlorite, Eqs. 5 and 8 represent the immediate disproportionation of hypochlorite, and Eqs. 6 and 9 give the resulting net process (the starting salts are written in their non-dissociated form for clarity). With regard to all Pt reactions with chloride, the situation is fully analogous for the bromide case.
3 PtCl 2 þ 3 NMMO þ 3 H 2 O ! 3 Pt þ 3 NMM þ 2 HClO 3 þ 4 HCl ð6Þ 3 PtCl 4 þ 6 NMMO þ 6 H 2 O ! 3 Pt þ 6 NMM þ 4 HClO 3 þ 8 HCl ð9Þ Interestingly, there was a distinct reactivity difference between Pt salts that have chloro ligands, i.e. chloride in the coordination space sphere of Pt, and those salts in which chloride is the counter anion. The former reacted much more readily (see Table 1): at 60°C the reaction was complete within less than 30 min. In the case of [PtCl 6 ] 2as the most reactive complex salt tested (Table 1)  It is known that platinum complexes can undergo ligand isomerism (Kukushkin 1991). Either ligands change places within the coordination sphere or there is an exchange between ligands and counterions. Such isomerism is too unfavorable at room temperature but becomes more prominent at higher temperatures. In water both aqua-and hydroxido-complexes can occur depending on pH and concentrations. The fact that the exceptionally stable [PtF 6 ] 2-, which does not undergo ligand exchange at all, is not reduced in the NMMO system-even if excess chloride is added to the solution-might be an indication that only Clin the coordination sphere of the central metal ion, i.e., as a chloro ligand, can undergo a reaction according to Eqs. 6 or 9, but not chloride outside the ligand shell. If this is correct, the slower reactivity of some salts could be readily explained by the ligand-anion isomerism: the freely mobile chloride anions must first be immobilized and coordinated to the central cation before they become amenable to the oxidation process. This interesting detail of platinum salt reactivity is currently investigated in more detail, especially concerning the question of whether cis-and transcomplexes (Holleman and Wiberg 1985b) can be distinguished by reactivity differences towards Scheme 2 Reduction of platinum(IV) ions to elemental platinum. At the same time, chloride/bromide ions are being oxidized to hypochlorite/hypobromite, which was confirmed by trapping reactions with a tocopherol (vitamin E) derivative (6), and NMMO (1) is in turn reduced to NMM (5). The hypohalite undergoes immediate disproportionation into chloride and chlorate (bromide and bromate) which are found as stable halide-derived species after the reaction. The reactions of platinum(II) would be analogous NMMO or not, which is of interest in view of analytics of cisplatin-type chemotherapeutics. Table 1 seems to indicate that the chloro ligands in cis-Pt complexes reacted faster than in the trans-counterparts.
From the viewpoint of material science, a large number of suitable platinum salts are placed at disposal to generate the corresponding Pt particles, and the different reactivities could allow adjustments of kinetics and properties. Still, the mechanistic question remained how platinum salts would react that do not provide halide ions as co-reactants, neither in the coordination sphere nor as counteranions. We addressed this question by means of Pt(NO 3 ) 2 , [PtF 6 ] 2? and [Pt(OH) 6 ] 2? , both as the respective potassium salts (Table 1). While the fluoro-complex showed no reactivity at all, the hydroxido-complex and the platinum nitrate reacted slowly at 115°C and caused concomitant oxidation of cellulose parallel to Pt(0) generation, see Fig. 5. The reaction of Pt salts with cellulose in 50 wt% aqueous NMMO in the absence of chloride was thus analogous to that of silver salts in the absence of propyl gallate (see Fig. 1).

Elemental gold species on cellulosic substrates
The generation of elemental gold in aqueous NMMO follows the same reaction pathways as in the cases of Ag and Pt. In our work, the starting gold complex was tetrachloroaurate(III), in the form of chloroauric acid H[AuCl 4 ], similar to literature accounts (Yokota et al. 2008;Kitaoka et al. 2011). In 50 wt% aqueous NMMO stabilized by 2% propyl gallate (2), Au(III) was neatly reduced to elemental Au(0) at 50°C while ellagic acid (4) was formed in turn. This is analogous to the reaction as shown in Scheme 1 for Ag ? and propyl gallate. In the absence of propyl gallate, the system once again took the ''chloride pathway'' described above for the Pt case, and chloride was oxidized to intermediate hypochlorite, which immediately underwent disproportionation into chloride and chlorate, cf. Scheme 2. The reaction in 50% aqueous NMMO was completed within 5 min at 80°C, whereas the literature listed conditions of 80% aqueous NMMO at 100°C, under which the presence of HOCl as an intermediate had been detected by trapping (Yokota et al. 2008 Fig. 1) a repetition of this trapping experiment was not considered necessary in our case because of the only slightly differing conditions. The overall reaction can be described by Eqs. 10-12, in analogy to the above Pt case (Scheme 2).
In the case of Au(NO 3 ) 3 as the Au 3? source, i.e. Clbeing absent, no reaction was observed at 80°C, but it started at 115°C. This reaction behavior was thus completely analogous to the Pt case. It was somewhat faster, however, showing complete consumption already after approx. 10 min, while the Pt reduction needed about 30 min. With the absence of chloride as the co-reactant, cellulose became the reductant also in the Au case, and this process was once again monitored by following the increase of carbonyl and carboxyl moieties in the polysaccharide (Fig. 5). Also here, the reaction did not stop when the concentration of metal ions was higher than the approximate amount of reducing ends in cellulose, which means that evidently also cellulosic hydroxyl groups were oxidized. These hydroxyl groups are converted to carbonyl structures at C2, C3, or C6, while reducing ends (and some carbonyls at C6) afford carboxylic acid moieties upon oxidation.

Transition metal species from aqueous NMMO solutions in the absence of cellulose
From the previous experiments we have learned that different chemical species can act as reductants towards the transition metal ions under lyocell (aqueous NMMO) conditions: either propyl gallate, chloroligands or cellulose. But how does the system behave chemically when neither of these three co-reactants is available, i.e. if only the transition metal salt and aqueous NMMO are present? The answer could not be clearly extracted from the literature, since the description of the exact mixture of the composition is sometimes rather vague. After all, the methods described were aimed at the nanoparticles produced and their properties rather than at an accurate chemical and mechanistic description of the redox systems involved.
It is known that the interaction of transition metal ions and NMMO, also as an aqueous solution, can cause exothermicity and explosions due to the autocatalytic decay of NMMO if NMMO is present in excess (Rosenau et al. 1999). Transition metal ions can also be used to degrade equivalent amounts of amine N-oxides under controlled conditions, which can be used for synthetic purposes (Dasgupta and Donaldson 1998;Coleman et al. 2000;Gomes and Antunes 2001;Rosenau et al. 2004). We, therefore, used a concentrated aqueous solution of the transition metal salts (AgNO 3 , PtCl 2 , and H[AuCl 4 ]) in diglyme (2.5 wt%) to which NMMO in the same solvent was very slowly added at 120°C to take advantage of the dilution effect and render the reaction controllable. The reaction was immediately recognizable by a color change due to the precipitated (nano)particulate metals. The metal ions were always present in excess because the small amounts of added NMMO reacted immediately and could not accumulate. Analysis of the mixture showed-independent of the metal involved-only one single NMMO-derived reaction product, namely N-formylmorpholine (10), apart from less than 5% of N-methylmorpholine (5). The 1 H and 13 C NMR spectra of compound 10 are shown in Fig. 6. Note that the free rotation of the C-N amide bond is restricted at room temperature due to its partial double bond character, which renders the two ''sides'' of the morpholine ring magnetically inequivalent so that two 13 C resonances appear for the two N-CH 2 moieties and also two for the two O-methylene groups. Increasing the measurement temperature would diminish the shift difference, and at the coalescence temperature T c , when the C-N bond rotation becomes fast on the NMR time scale, the two separated peaks would coincide. This is similar to the cellulose solvent N,N-dimethylacetamide, which also shows restricted C-N bond rotation at room temperature, and the determination of the activation barrier of this bond rotation based on the NMR coalescence temperature (Chrapava et al. 2003).
This compound was evidently an oxidation product of the intermediate N-(methylene)morpholinium ions (11) and N-hydroxymethylmorpholine (12) (Scheme 3), which are in equilibrium with each other. They both are carriers of the autocatalytic NMMO degradation (Scheme 3, red). The fact that mixing the reagents in an opposite way-i.e. addition of the transition metal salt solution into excess NMMOcaused an immediate violent reaction under reboiling, accompanied by black discoloration of the mixture and complete NMMO consumption, but without significant metal ion reduction, is an indirect proof of their formation. N-Hydroxymethylmorpholine (12) represents a hemiaminal of formaldehyde, which is a very strong reducing species and is readily further oxidized to the carboxylic stage (formic acid) (Potthast et al. 2000). Under the conditions used, we observed neither further oxidation of the formyl moiety to CO 2 , which would leave the remaining morpholine part detectable, nor hydrolysis of Nformylmorpholine into morpholine and formic acid.
Thus, the direct redox reaction of transition metal ions with NMMO is possible, but not NMMO itself is the corresponding reductant, but NMMO-derived intermediates, which are formed through the interaction of NMMO with the metal ions. If carried out cautiously, the process is astonishingly neat, and Nformylmorpholine is formed nearly quantitatively as the sole NMMO-derived product. However, the reaction cannot be conducted under the usual conditions used for metal particle precipitation on celluloses, as only NMMO degradation and no metal ion reduction would occur. Under the quite drastic conditions required (no aqueous dilution, temperatures of 130°C) NMMO has always to be the deficient reagent with the metal ions present in excess, otherwise, the NMMO present will immediately be autocatalytically degraded without the metal ions being reduced.

Conclusions
Addressing the exemplary cases of elemental Ag, Pt, and Au particles generated from the corresponding salts in aqueous NMMO, we have observed that different chemical species were able to take the role of the co-reacting reductants. The strong oxidant NMMO itself, as introductorily mentioned, is evidently unable to act directly in this way. However, it can still be chemically involved in the conversion, not just being present as the reaction medium or as a swelling agent. Upon comparison of the reaction rates of the coreacting reductants for the metal ions, a reactivity order was obvious which was the same for all three transition metals tested: propyl gallate [ [ chloride [ cellulose. In the presence of the common NMMO stabilizer propyl gallate (PG, 1), the latter is readily oxidized to ellagic acid (4) and the metal ions are reduced (Scheme 1). Neither NMMO nor cellulose are affected in this process, which proceeded already at room temperature or temperatures below 50°C. If PG is absent, chloride ions will become part of the redox pair, being oxidized to HOCl, while both metal ions and NMMO are reduced (Scheme 2). This process requires temperatures between 60 and 80°C to be completed in less than 30 min. Also, here, cellulose integrity is not compromised. If also chloride is absent, cellulose itself can act as the co-reactant, and carbonyl functions are introduced while the metal ions are reduced. Compared to the above redox systems, the reaction requires relatively high temperatures of 115°C and above and is relatively slow with reaction times of 30 min up to several hours for complete metal ion reduction. Apart from the easily affected reducing ends also cellulosic hydroxyl groups are oxidized. Additionally, the basic medium-mainly because of the generated N-methylmorpholine-causes chain degradation according to beta-alkoxy-elimination pathways (Hosoya et al. 2018) starting from the introduced carbonyl groups (Figs. 1, 3 and 5).
If no other co-reactant is present, excess transition metal ions can oxidize intermediate N-(methylene)morpholinium ions and N-hydroxymethylmorpholine, the product of a metal ion-induced intramolecular rearrangement of NMMO, to N-formylmorpholine (10), with the metal ions being reduced to elemental metal, in turn (Scheme 3). This reaction, proceeding at 120°C and above, does not proceed in aqueous NMMO solutions, but requires NMMO or its monohydrate, which can, however, be diluted with inert organic solvents. Caution must be exercised that the NMMO is slowly and carefully added to an excess of metal ions since otherwise the N-(methylene)morpholinium species will react with NMMO instead of the metal ions, and trigger the non-controllable autocatalytic degradation of the amine N-oxide.
With the present study on the chemical reaction mechanism of transition metal (nano)particle generation on cellulosics in aqueous NMMO, we hope to Scheme 3 Direct reaction of transition metal ions (excess) with NMMO monohydrate (1) at elevated temperatures: the intermediate, NMMO-derived N-(methylene)morpholinium ions (11) react with water to N-hydroxymethylmorpholine (12), which is oxidized to N-formylmorpholine (10) as the final reaction product, while the transition metal ions are reduced to elemental metal. If present in excess, NMMO is degraded in a highly exothermic and uncontrollable autocatalytic process, triggered by the formed N-(methylene)morpholinium ions (11), without concomitant metal reduction contribute to a better understanding of the chemical aspects of this interesting system and these intriguing materials and to correct certain inconsistencies in the description of the role of NMMO and the underlying chemistry.