Nathan Ferrandin-Schoffel
email: nathan.ferrandin-schoffel@mnhn.fr

Charlotte Martineau-Corcos

Camille Piovesan

Sabrina Paris- Lacombe

Odile Fichet

Anne-Laurence Dupont
email: anne-laurence.dupont@mnhn.fr

Stability of lignocellulosic papers strengthened and deacidified with aminoalkylalkoxysilanes

Keywords: Strengthening, Lignin, Newsprint paper, Alum-rosin, Mechanical properties, Acidity

The durability of paper documents treated with aminoalkylalkoxysilane copolymer (co-AAAS) flexible networks, as deacidifying and strengthenening agents, was studied. To this end, a 1922 lignocellulos ic newsprint paper was treated with co-AAASs and characterized. Physico-chemical properties such as pH, chromatism, opacity and mechanical strength were measured shortly upon treatment and several years after. In order to better understand the role of the initial degradation state of the paper and the impact of its constituents other than cellulose (lignin, alum-rosin sizing) on the treatment efficiency, several laboratory papers were added as models. It was shown that alum-rosin sizing contributed to a decrease in pH of the most degraded treated paper over the monitoring time, whereas the strengthening effect was mostly affected by the presence of lignin. In particular, for most papers, after several years, the gain in folding endurance was the lowest in the lignin-rich ones. The breaking length of all the papers increased post-treatment and over time, independently of the paper constituents. This was attributed to the slowly progressing aminoalkylalkoxysilanes polycondensation, on the scale of years, as evidenced by Cross Polarisation -Mass Angle Spinning 29 Si solid-state Nuclear Magnetic Resonance.

Introduction

A growing number of graphic documents in archives and libraries collections are at risk of rapid decay. Indeed, cellulose, the main biopolymer in paper, undergoes acid-catalyzed hydrolysis, which leads to its depolymerization, one consequence of which is the loss of the material's mechanical properties. Over time, some papers become so brittle that they do not withstand manual handling without material loss. Papers from the 19 th to mid-20 th containing a high proportion of groundwood pulp and sized with alum-rosin are often the most degraded [1]. Unbleached woodpulp contains significant amounts of lignin (up to 31%) [2], a biopolymer that releases colored and acidic compounds, mainly through oxidation reactions [3-5], which in turn favor cellulose hydrolysis. Alum-rosin sizing is also a major source of acids in paper [6][7][8]. In order to counter the acid-catalyzed reactions and retard the degradation, (mass) deacidification treatments have been developed in the second half of the 20 th century, some of which are still used in several major libraries worldwide [9,10]. The alkaline additives neutralize the acids and provide an alkaline reserve. The latter acts as a buffer and delays further acidcatalyzed hydrolysis. However, such treatments are not usually applied to very brittle paper records, which are in dire need of mechanical strengthening to revert a rapidly approaching end of lifetime. At present, no commercially available treatment can restore the mechanical properties of brittle papers at the mass scale, and workshop-scale solutions are scarce.

Since 2003 [11], one-pot treatments based on aminoalkylalkoxysilanes (AAASs) are investigated in the laboratory for the purpose of simultaneous deacidification and strengthening of acidic and brittle papers. AAAS monomers bear one or several amine groups (alkaline) that allow both the deacidification and the deposition of an alkaline reserve in the paper, through an optimal adsorption on the cellulosic fibers via hydrogen bonds [12][START_REF] Souguir | Chemical 15 and Physicochemical Investigation of an Aminoalkylalkoxysilane As Strengthening Agent for 16 Cellulosic Materials[END_REF][START_REF] Souguir | 19 Strengthening of degraded cellulosic material using a diamine alkylalkoxysilane[END_REF]. The mechanical strengthening is assumed to be driven by in situ polycondensation of the AAAS in the paper, which forms a polymer matrix intertwined to the paper fibrous network. The first step of the polymerization reaction is the hydrolysis of the AAAS alkoxy groups. This is achieved thanks to the residual water naturally present in the paper and the moisture in the air, with no need to adding water to the treatment solution. The second step is the condensation (hydrolytic and/or alcoholic [15]) of the silanol and alkoxysilane functions, which yields a poly-AAAS with some degree of polymerization (DP). The polycondensation being catalyzed in alkaline conditions [16], the amine functions of the AAAS accelerate this reaction step.

Treatment formulations based on binary mixtures of a trifunctional AAAS (three alkoxy groups), 3aminopropyltriethoxysilane (AP), and a bifunctional AAAS (two alkoxy groups), the latter being either 3-aminopropylmethyldiethoxysilane (AM) or N-(2-aminoethyl)-3-aminopropylmethyldimethoxysilane (DIA), were first developed [START_REF] Piovesan | Paper 29 strengthening by polyaminoalkylalkoxysilane copolymer networks applied by spray or immersion: 30 a model study[END_REF][18][19]. The purpose was to introduce a flexible copolymer network (co-AAAS) in the paper. The trifunctional monomer AP significantly improves the paper tensile strength, and acts as a cross-linker to the bifunctional monomers AM and DIA. The latter react to form linear molecules, which tend to increase the plasticity and folding endurance mostly (pliability). The treatment is thus versatile, as the monomers, their proportions and concentrations are easy to modify. Moreover, it is a mild treatment as for obvious reasons, when restoring cultural heritage artefacts, the use of harsh chemicals or severe treatment conditions has to be avoided. The application of the treatment can be performed in ambient conditions, either by spray or by immersion, with undiluted or diluted AAASs, preferentially in an organic solvent to avoid cellulose fibers swelling and dissolution of graphic media. Hexamethyldisiloxane (HMDSO) has been used because it is colorless and non-protic. Additionally, it is the solvent used in the mass scale Battelle process [10,20]. HMDSO is inert and non-toxic and it avoids fiber swelling and bleeding of water-soluble inks, dyes and binders.

Co-AAAS formulations were first applied to a simple model paper made of cotton linters (S2) [START_REF] Piovesan | Paper 29 strengthening by polyaminoalkylalkoxysilane copolymer networks applied by spray or immersion: 30 a model study[END_REF].

Both spray and immersion applications of the treatment were effective. The formation of co-AAAS networks was verified through solid-liquid extractions. An alkaline reserve was introduced, and the paper was considerably strengthened, with an increase in the number of double folds of at least 250% with AP-AM 50-50 (wt%). Co-AAAS treatments were subsequently applied to various lignocellulos ic model papers [18,19]. However, the same formulations yielded mixed outcomes, and it was proposed that several constituents in the paper could impact the strengthening. In particular, the presence of alumrosin sizing and groundwood pulp allegedly hampered the reinforcement, while the impact of the kaolin filler was deemed not noticeable. Co-AAAS formulations were later applied to model papers which had undergone artificial aging beforehand in order to reach a degradation level similar to that of century-old newsprint paper. The strengthening efficiency was lowest for the most severely degraded lignocellulos ic papers. An optimized co-AAAS formulation was then to an authentic newsprint paper published in 1922 and succeeded in improving its pH, folding endurance and zero-span tensile strength [19].

The co-AAAS treatments will have a better chance to be used by paper conservators once the strengthening limiting factors linked to paper constituents, and any adverse effect, are understood. For a sustainable application, it is a requisite that the treatment should also be durable. For instance, loss of opacity and yellowing should be limited and change as slowly as possible. To this purpose, the focus of the present work was to evaluate the stability of treated with co-AAAS several years ago, and to understand better the reasons affecting the treatment efficiency. The physico-chemical properties measured were pH, chromatism, opacity, co-axial tensile strength and folding endurance. Papers previously treated with various co-AAAS formulations, and which, by the time of the present study, had naturally aged in ambient conditions during three to seven years since the treatment, were examined. This natural aging period circumvented the need for artificial aging. The data were paralleled with those obtained immediately after the treatment.

Experimental

Chemicals

3-aminopropyltriethoxysilane (AP) (> 98%), 3-aminopropylmethyldiethoxysilane (AM) (97%), N-(2-aminoethyl)-3-aminopropylmethyldimethoxysilane (DIA) (97%) and hexamethyldisiloxane (HMDSO) (98%) were purchased from ABCR (Gelest, France) and used as received.

Paper samples

The paper samples included one newsprint paper dated 1922, and several machine-made modern laboratory papers as models. The model papers were used in their unaged form (undegraded) or were artificially aged (degraded) before the co-AAAS treatment. These untreated samples served as control samples and are henceforth referred to as (t 0 ) samples. The samples were examined immediately after co-AAAS treatment (called (t) samples) and several years (x) after the treatment (called (t + x) samples), period during which they were stored in stable conditions at 23 °C and 50% relative humidity in a climate chamber. The co-AAAS uptake and the value of x is indicated in Table S1 (Supporting Information file).

Naturally aged newsprint paper

A naturally aged lignocellulosic newsprint paper, "Journal des Fabricants de Sucre", issued September 2 nd 1922, was provided by the Bibliothèque Nationale de France. It was called J4. Its constituents and main physicochemical properties are reported in Table 1.

The pulp and fibers source of J4 were determined by microscopic examination and histological staining with Herzberg, Lofton-Merritt and Graff-C solutions [START_REF] Isenberg | Pulp and paper microscopy[END_REF]. Microchemical tests for rosin and aluminum enabled confirming the presence of alum-rosin sizing in the paper [START_REF] Browning | Analysis of Paper[END_REF]. Kaolin fillers were identified using Attenuated Total Reflectance -Fourier-transform infrared spectroscopy (ATR-FTIR) (Figure S1 in the Supporting Information file) by the two absorption bands around 3690 cm -1 and 3620 cm -1 (O-H stretch) [START_REF] Derrick | Infrared Spectroscopy in Conservation Science[END_REF] of kaolinite. The amount of kaolin was determined by measuring the ash content (TAPPI T211 om-02) (Table 1) (two repeat measurements). . The aging had been stopped when the average folding endurance fell under or was equal to 20 double folds, which was reached after different durations for each paper. The aged samples are called S2a, C3a and C4a thereafter to distinguish from their unaged counterparts S2ua, C3ua, C4ua.

Model papers

Paper treatment

As the physico-chemical data set at (t) was partial, paper sheets were treated following the same procedure as Piovesan et al. [18,19] with minor changes, as indicated below. In particular, for each sample added, care was taken to obtaining a closely similar uptake so as to ensure a relevant intra-sample comparison of the data. The average uptake of the papers at (t) and (t + x) is reported in Table S1 (Supporting Information file). The uptake UP was calculated as follows:

UP(wt%) = w a -w b w b × 100 (1)
where w b and w a are the paper weights before and after treatment, respectively. After treatment, each newly prepared sample was dried for at least 3 h at room temperature, conditioned at 23 °C and 50%

RH for 24 h at least (TAPPI T402 sp-03) and weighed.

Unaged model papers (S2ua, C1ua, C3ua and C4ua) were treated by immersion. They were cut in 12 cm × 25 cm sheets, rolled and inserted in glass tubes (Wheaton, 35 mm internal diameter × 147 mm, 144 mL). An immersion period of 30 min allowed obtaining uptakes between 4% and 8% for AP-AM 50-50 (wt/wt) (10 wt% in HMDSO) and AP-DIA 50-50 (wt/wt) (4 wt% in HMDSO).

The artificially aged model papers S2a, C3a and C4a, as well as the newsprint paper J4 (cut along folded lines in 13 cm × 28 cm samples) were treated by spray using an airbrush (Airbrush Hobby Kit, Silverline), either with AP-AM or with AP-DIA mixtures, in 50-50 or 5-95 wt% proportions, without dilution in HMDSO.

Physico-chemical characterizations

The cold extract pH was measured with a pH-meter (Mettler Toledo MA235) according to TAPPI T509 om-02 standard, adapted to a sample mass of 100 mg (same mass/volume ratio). Two repeat measurements were carried out per paper, and the average reported with the standard deviation.

The pKa values of AM, AP and DIA were determined at 22 °C by acid-base titrations. Each AAAS was diluted in distilled water (0.1 M) and titrated with a 0.1 M HCl aqueous solution (Honeywell). The titration curves (pH as a function of HCl volume) are reported in the Supporting Information file (Figures

S2, S3 and S4).

Color and opacity measurements were performed using a hand-held spectrophotometer CM-26dG (Konica Minolta) equipped with an integrating sphere. The configuration adopted was in reflectance mode (360 -740 nm spectral range, 10 nm steps), with the specular light component included (SPIN), (kN m -1 ) according to:

BL = 102 × T R ( 2
)
where R is the grammage of the paper (g m -2 ) (TAPPI T494 om-01 standard).

Folding Endurance (FE) was determined with a Tinius Olsen instrument (applied tension: 500 g) according to TAPPI T511 om-02 standard. FE is reported as the average number of double folds before breaking (N(FE)). For both TS and FE, ten measurements were done per sample in the machine direction of the sheets. Average values are reported with the standard deviations.

The static contact angle measurements were carried out in air at room temperature with a DSA-P instrument (Kruss, Germany). A drop of water (10 µL -Millipore ultrapure) was added on the surface of the paper. The static contact angles were measured from Young-Laplace drop profile fittings. The measurements were carried out in ten different locations. Average values are reported with standard deviations.

The 1 H 29 Si cross-polarization (CP) under magic-angle spinning (MAS) NMR spectra were recorded on a NEO Bruker 500 WB NMR spectrometer (11.7 T). The papers were cut in small pieces and introduced in 4 mm outer diameter zirconia rotors. A 4 mm double resonance probe was used, and all samples were spun at 10 kHz MAS frequency. For the CP transfer, the radiofrequency fields applied on 1 H and 29 Si channels were 50 kHz and 60 kHz, respectively. The contact time was set to 4.0 ms and the recycle delay to 4 s. 1 H decoupling was applied during signal acquisition. Between ca 22 000 and 140 000 transients were accumulated per sample (between 1 and 9 days experimental time). The 29 Si chemical shifts are externally referenced to TMS at 0 ppm. The spectrum deconvolutions were performed with the Dmfit software [24], using simple Lorentzian/Gaussian line shapes.

Results and discussion

Chromatism, opacity, pH and mechanical properties measurements were performed on J4 at (t) and

(t + 3). A comparison with the data obtained at (t) and (t + x) for the model papers treated with AP-AM and AP-DIA was made. The aim was to investigate which parameters (paper constituents and/or degradation state) are the limiting factors to the treatment efficiency after several years. Therefore, the impact of the papers' degradation state on the treatment efficiency was examined closely.

pH change

At (t), both AP-DIA 50-50 and AP-DIA 5-95 raised the pH of J4 to values above 7 (7.27 and 7.55, respectively) (Figure 1a). However, after 3 years, the pH decreased to 6.19 (AP-DIA 50-50) and 6.11

(AP-DIA 5-95). In order to investigate the cause of the pH decrease, the results were paralleled to those obtained for the unaged and aged model papers treated with AP-DIA 50-50 (Figure 1b). These papers show a similar natural aging after treatment (between 4 and 5 years) (Table S1 in the Supporting Information file). In contrast to J4, the pH of most model papers did not decrease with natural aging after treatment, to the exception of C3a, even though its pH did not fall below 7. For the other model papers, except for C3ua, the pH at (t + x) was slightly higher than the value measured at (t). Even though much care was taken in the sample preparation, these slight variations were attributed to the small disparity in the uptakes between (t) and (t + x) since, as explained above, some of the (t) samples were prepared de novo (Table S1 in the Supporting Information file). Incidentally, this observation also indicates that the pH is strongly correlated to the uptake, an issue that will be examined further on. S1).

Several years after treatment, in stable ambient conditions, none of the papers showed a pH below 6, which is a satisfactory outcome. With AP-DIA 50-50, the decrease in pH of C3a was similar to that of J4 (0.96 and 1.08 units, respectively), and this decrease occurred approximately over the same period (4 and 3 years, respectively). In addition, at (t 0 ), the pH of C3a (pH = 5.43) was close to the pH of C3ua (pH = 5.50) and the pH of C4a (pH = 5.49). Thus, the decrease in pH that occurred over time after treatment seems unrelated to the pH of the control papers. Furthermore, since the proportion of GWP in C4 is higher than in C3 (about 80% and 40%, respectively, see Table 1), a higher lignin content had most likely no major impact either on the post-treatment pH decrease. Additionally, because neither C4ua nor C4a showed a decline in pH, a high lignin content does not seem to consume the alkaline reserve over time, and hence cannot explain the fall in pH of J4 and C3a.

Another commonality between J4 and C3, also shared with C1, is the alum-rosin sizing. However, in contrast, no decrease of pH was observed for C1ua and C3ua at (t + x). Therefore, a probable cause could be the artificial aging underwent by C3a. The conditions used (temperature, RH) possibly affected the sizing and increased its acidification speed. As the rosin softening point is between 40 °C and 60 °C [START_REF] Cabaret | Study of rosin softening point through thermal 50 treatment for a better understanding of maritime pine exudation[END_REF], the rosin in the model papers aged at 90 °C for several weeks may have undergone physicochemical changes. Thus, the rheology of the sizing in paper could have changed during the artificial aging. As J4 has been stored for a century in unknown conditions of temperature and RH, the rosin may have been affected as well.

Alum and rosin are both sources of acidity (and degradation) in paper. Rosin favors the acid hydrolysis of cellulose as it contains acidic terpenoids, including abietic acid and dehydroabietic acid [26]. During the sizing process, rosin is first dissolved in an alkaline aqueous solution through an acidbase reaction with either sodium carbonate or sodium hydroxide [START_REF] Banik | Paper and Water: A Guide for Conservators[END_REF][START_REF] Gess | The sizing of paper with rosin and alum at acid pHs[END_REF], leading to the formation of amphiphilic salts. Alum (aluminum sulfate) is then used to precipitate the rosinate, which forms a film bonded to the cellulose fibers through electrostatic interactions [START_REF] Gess | The sizing of paper with rosin and alum at acid pHs[END_REF]. To ensure a good binding, the stoichiometry used usually favors an excess of alum, which then stays in the fibers. In the presence of water, aluminum sulfate undergoes hydrolytic decomposition, forming aquo complexes and releases protons [START_REF] Banik | Paper and Water: A Guide for Conservators[END_REF]. This occurs during natural aging but could also be accelerated during the artificial aging.

In conclusion, alum is thought to play a major role in the increase of the acidy of the most degraded papers that were treated with co-AAAS.

The absolute pH variation (ΔpH) as a function of the uptake for the model papers and J4 treated with AP-DIA 50-50 at (t) and (t + x) is shown in Figure 2a. As the alkalinity increases with the uptake [START_REF] Piovesan | Paper 29 strengthening by polyaminoalkylalkoxysilane copolymer networks applied by spray or immersion: 30 a model study[END_REF],

all the points at (t) (triangles) fall along a trendline. The data points at (t + x) (circles) follow this trendline as well, to the exception of J4 and C3a (circled data points in Figure 2a), which fall distinctly below. The pH decrease indicates that regardless of the small inter-sample variations in the uptake, the alkaline reserve introduced by the amines was consumed over time in a larger proportion for J4 and C3a than for the other papers, which supports the observations made above about the increase of the acidity.

The same was true for the papers treated with AP-AM 50-50 (Figure 2b). J4 and C3a are also distinctly below the trendline at (t). As mentioned above, both J4 and C3 contain groundwood pulp and are sized with alum-rosin. This supports the fact that, as foreseen, among the model papers, C3 is the most relevant to model J4. This is all the more applicable for the aged sample C3a, in particular because of its low FE value (N(FE) = 14), close to that of J4 (N(FE) = 17) (section 3.4), which indicates a closely matching mechanical degradation state. As pKa of AM is higher than both pKas of DIA, ΔpH at (t) of the model papers treated with AP-AM and AP-DIA were compared. It is then congruent that, when treated with AP-AM to similar uptakes (Table S1), S2ua, C1ua, C3ua, C4ua, S2a and C4a showed higher ΔpH values (ΔpH = 2.94, 2.68, 2.80, 3.19, 2.89 and 3.98, respectively) than when treated with AP-DIA (ΔpH = 2.21, 2.18, 2.52, 2.44, 2.64 and 3.58, respectively). In addition, pH at (t + x) of the papers which had the nearest uptakes was compared, namely S2ua (UP = 6% for AP-AM, UP = 8% for AP-DIA), C1ua (UP = 6% for AP-AM, UP = 7% for AP-DIA) and C3a (UP = 7% for AP-AM, 6% for AP-DIA). In each case, the pH was higher with AP-AM (pH = 9.46, 8.42 and 7.30, respectively) than with AP-DIA (pH = 9.19, 8.17 and 7.10, respectively), for equal and even slightly longer natural aging duration x (Table S1 in the Supporting Information file). This result indicates that at (t), AP-AM deacidifies more efficiently than AP-DIA, a benefit that lasts until (t + x). This result is counterintuitive if one considers that, due to the presence of two amine groups, DIA should be a better deacidifying agent. Nonetheless, the alkaline pH obtained with the two formulations indicates that both AM and DIA provide a sustainable deacidification.

Color and opacity changes

Deacidification treatments can induce some yellowing of the paper [10,29], and this could be considered as a drawback by conservators. For a sustainable application of the AAAS, yellowing, as well as loss of opacity or optical modification of the printing media, should be limited, if not absent. As a first observation, none of the treatments did modify the optical properties of the printing inks. No bleeding was observed after adding the AAAS, which confirms earlier findings [START_REF] Piovesan | 6 Application de traitements à base de polyaminoalkylalcoxysilanes pour la désacidification et le 7 renforcement simultanés de documents anciens sur papier[END_REF]. The values of b * and opacity after co-AAAS treatments were measured (Figures 3 and4). S1).

For J4 treated with AP-DIA (50-50 and 5-95), yellowing and loss of opacity were observed after treatment. With both formulations, at (t), Δb * ≈ + 4 (Figure 3a), whereas the opacity decreased by 3.0 points for AP-DIA 50-50 and by 4.7 points for AP-DIA 5-95 (Figure 3b). This loss of opacity is not trivial, yet not unacceptable, considering that the final values were still above 91%, indicating high opacity. The total chromaticity change ΔE * of the samples comparing (t) to (t 0 ) was 5.6 for AP-DIA 50-50, and 4.1 for AP-DIA 5-95. This chromatic change is perceptible with the naked eye, considering that the Just Noticeable Difference (JND) in the 1976 CIELAB color space has been defined as ΔE * ≈ 2

[31]. The values of b * and opacity of the samples treated with either formulation were stable or were not significantly different for the samples at (t + x). This indicates that in J4 the optical modifications fully occurred immediately after treatment (t).

Recently, the organic reactions likely occurring between the AAAS and the biopolymers in paper were studied using model compounds [START_REF] Ferrandin-Schoffel | Modeling the 11 Reactivity of Aged Paper with Aminoalkylalkoxysilanes as Strengthening and Deacidification 12 Agents[END_REF]. Among other reactions, the formation of yellow compounds such as Schiff bases and glycosylamine was identified. The immediate yellowing observed for the treated papers can therefore be explained by the fast reaction between the amine groups of AAAS and specific functional groups on cellulose, hemicelluloses and lignin, as well as their degradation products in the paper, leading to the formation of yellow compounds, among other compounds. S1).

Opacity and b * values for the model papers at (t 0 ) and at (t + x) treated with AP-DIA 50-50 are reported in Figure 4. Yellowing of all the treated papers was observed. Opacity loss was observed too, except for C3ua. Δb * values (difference in the data between (t 0 ) and (t + x)) for S2ua (+ 3.5, UP = 8%), C1ua (+ 7.5, UP = 7%), C3ua (+ 10.4, UP = 8%), C4ua (+ 8.6, UP = 9%), S2a (+ 2.7, UP = 9%), C3a (+ 4.2, UP = 6%) and C4a (+ 6.8, UP = 19%) were all above JND (Figure 4a). The slightest yellowing was observed for S2 (S2ua and S2a). As S2 is made of cotton linter (lignin-free pulp) and is unsized, the lignin in C4, the alum-rosin sizing in C1, and the presence of both in C3, were suspected as possible causes of the yellowing observed at (t + x). This can occur through reactions of these paper constituents with the AAASs, as proposed above. Indeed, rosin contains oxidized terpenoids [26], and lignin and its degradation products bear numerous carbonyl groups [3]. These oxidized groups can lead to the formation of Schiff bases through reaction with the AAAS amine moiety.

The loss of opacity in connection with lignin and alum-rosin was less straightforward (Figure 4b).

The decrease was the highest for C4a (loss of 7.5 points). However, this result was not attributed to the paper constituents, but mainly to the fact that C4a had the highest uptake (UP = 17%), inducing a larger modification of the physical interactions of the paper with light.

Paper's opacity is largely related to the amount of fillers. In addition to increasing the whiteness and brightness of paper, kaolin is also used to increase its opacity [START_REF] Bundy | Kaolin in paper filling and coating[END_REF]. It is thus congruent that J4 and C3, which have the highest amount of kaolin (ash content of 18.1 wt% and 13.1 wt%, respectively) are the most opaque papers, whereas S2 and C1, which have no fillers, have the lowest opacity values (Figure 4b). Nevertheless, the relation between the amount of kaolin and the decrease in opacity after treatment was unclear. For instance, at similar paper density (similar thickness and grammage), the opacity loss of C1ua and C3a treated with AP-DIA 50-50 to similar uptakes (UP = 7% and UP = 6%, respectively)

was the same (Δopacity = 2.8%), despite the fact that C3 contains kaolin and C1 does not. Moreover, as the ash content of C3ua and C3a was the same, no loss of kaolin occurs during aging. As the decrease in opacity occurred for C3a and not for C3ua, the variations could not be attributed to the amount of kaolin. Hence, the larger contributor to the opacity variations seems to be the treatment itself.

AP-AM 50-50 induced yellowing of all the model papers as well (Figure S6a in the Supporting Information file). Again, the smallest Δb * was obtained for S2 (Δb * = + 2.2 for S2ua and Δb * = + 0.8 for S2a), which supports the discussion above on the role of alum-rosin sizing and lignin in the color change.

S2ua and C3ua were the only papers to show no decrease in opacity, with even a slight increase for C3ua (Figure S6b in the Supporting Information file). Because of the slight variations in the uptake from one treated paper to the other, Δb * and Δopacity (absolute values) were plotted as a function of the uptake for both AP-AM 50-50 and AP-DIA 50-50 formulations (Figure S7 in the Supporting Information file).

A trend between these changes and the uptake appeared but less clearly, and whether AP-AM or AP-DIA led to larger variations was not obvious.

For J4 treated with AP-DIA 50-50, between (t 0 ) and (t + 3), Δb * = + 4.1, and the loss of opacity was of 2.5 points (Figure 3). These values were similar to those of C3a (Δb * = + 4.2, loss of opacity of 2.8%, Figure 4), for similar uptake (J4: UP = 5%, C3a: UP = 6%) and similar aging duration x (C3a: x = 4).

The fact that J4 and C3 were both alum-rosin sized and contained groundwood pulp corroborates the hypothesis formulated above about the impact of alum-rosin and lignin on the visual changes. On the other hand, as with AP-AM treatment, no connection between the amount of fillers and the decrease in opacity could be established.

As with pH, the variations in yellowing and loss of opacity over time of J4 and C3a were similar.

This confirms that, as reported above, the artificially aged C3 (C3a) appears yet as the most relevant paper to model the naturally aged lignocellulosic newsprint J4. In order to understand and confirm this result, the mechanical properties of the treated papers were characterized. Prior to this, as the mechanical properties of polymers and composites strongly depend on their degree of polymerization [34], the polycondensation state of the AAAS in the paper was studied.

Progress of the polycondensation

The polycondensation rate of DIA was closely examined. Previously, Souguir et al. used 29 Si CP-MAS NMR to estimate the number-average degree of polymerization (𝐷𝑃 ̅̅̅̅ n ) of the homopolymers AM and DIA in S2ua at (t) [START_REF] Souguir | Chemical 15 and Physicochemical Investigation of an Aminoalkylalkoxysilane As Strengthening Agent for 16 Cellulosic Materials[END_REF][START_REF] Souguir | 19 Strengthening of degraded cellulosic material using a diamine alkylalkoxysilane[END_REF]. In both cases, a low 𝐷𝑃 ̅̅̅̅ n (around 10) indicated that small oligomers were formed. Numerous silanol groups (-Si-OH) should then be present at the end positions, the latter being able to form hydrogen bonds with cellulose and water molecules [35,36]. The small 𝐷𝑃 ̅̅̅̅ n at (t) is confirmed by the gain in hydrophilic character of the treated papers. Indeed, water contact angle measurements could not be performed, as the water drop was absorbed in all the treated papers in a few seconds, including the alum-rosin sized ones which are otherwise hydrophobic (J4 (94.3° ± 8.6°), C1 and C3). Daher et al. made similar observations with polyurethane foam, which became more hydrophilic when treated with AM [START_REF] Daher | Consolidation of degraded polyurethane foams by means of 26 polysiloxane mixtures: Polycondensation study and application treatment[END_REF]. They proposed the formation of hydrogen bonds between water molecules and the amine functional groups on the AAAS.

Siloxane chains with higher 𝐷𝑃 ̅̅̅̅ n should decrease the number of silanol groups and lead to a progressive increase of the hydrophobic character of the paper. This assumption was confirmed at (t +

x) as the water drop remained on the surface of all the papers (unsized and sized). For J4 treated with AP-AM and AP-DIA, static water contact angle at (t + 3) was 102° ± 4 and 83° ± 8 for the 50-50 proportion, and 102° ± 4 and 90° ± 5 for the 5-95 proportion, respectively. Thus, the polycondensation would have progressed slowly with time. In order to confirm this hypothesis, 29 Si CP-MAS NMR was carried out on J4 treated with AP-DIA 5-95 at (t) and (t + 3) (Figure 5). Right: expansions of the area of interest, with the resonance deconvolutions. The red spectrum is the sum of the D 1 (green) and D 2 (blue) contributions.

On both spectra, the two peaks overlapping at about -16 ppm and -22 ppm were related to D 1 (silicon atom bonded to one -O-Si group) and D 2 (silicon atom bonded to two -O-Si groups), respectively [START_REF] Souguir | 19 Strengthening of degraded cellulosic material using a diamine alkylalkoxysilane[END_REF][START_REF] Pellizzi | 29 Reinforcement properties of 3-aminopropylmethyldiethoxysilane and N-(2-Aminoethyl)-3-30 aminopropylmethyldimethoxysilane on polyurethane ester foam[END_REF]. A third intense signal was visible at about -91 ppm, which corresponds to the 29 Si NMR signal of kaolinite (aluminosilicate) [START_REF] Carter | The Chemistry of Paper Preservation: Part 1. The Aging of Paper and Conservation 4 Techniques[END_REF] used as a filler in J4. There was no D 0 signal (characteristic of DIA monomer [40]) in the [-3; -7] ppm range, indicating that the free monomers all reacted to form oligomers, which confirmed the earlier results by Souguir et al. [START_REF] Souguir | Chemical 15 and Physicochemical Investigation of an Aminoalkylalkoxysilane As Strengthening Agent for 16 Cellulosic Materials[END_REF][START_REF] Souguir | 19 Strengthening of degraded cellulosic material using a diamine alkylalkoxysilane[END_REF]. The integration of both deconvoluted D 1 and D 2 peaks indicated a 𝐷𝑃 ̅̅̅̅ n of DIA in J4 treated with AP-DIA 5-95 about 14 at (t), which increased to 30 at (t + 3). T signals related to trifunctional AP in the [-40; -70] ppm range

[40] were almost not noticeable, which could be attributed to its very low proportion in the treatment formulation (5 wt%). To investigate whether the assumption of neglecting the contribution of AP in the copolymer 𝐷𝑃 ̅̅̅̅ n was correct, 29 Si CP-MAS NMR was carried out on C3a treated with DIA 100% (UP = 9 ± 3%, treatment by spray), at (t) and (t + 6). The integration of D 1 and D 2 yielded 𝐷𝑃 ̅̅̅̅ n about 13 at (t) and 36 at (t + 6), values which are in agreement with those calculated for J4 treated with AP -DIA 5-95.

Thus, the kinetics of polymerization of DIA in both papers was extremely slow, leading after several years to small oligomers. This confirms the hypothesis of very slow polycondensation kinetics, and is consistent with the increase of the hydrophobic character of the treated papers with time.

The average 𝐷𝑃 ̅̅̅̅ n of AM in J4 treated with AP-AM 5-95 was about 36 at (t + 4), as shown by the 29 Si CP-MAS NMR spectrum (Figure S8 in the Supporting Information file). Thus, for a similar duration, the oligomerization of AM and DIA leads to a similarly low 𝐷𝑃 ̅̅̅̅ n . These temporal modifications of the 𝐷𝑃 ̅̅̅̅ n of the AAAS in the paper may have an impact on its mechanical properties.

Mechanical properties changes

FE and TS measurements were carried out, which allowed obtaining N(FE), BL, EB and Y values for J4 and for the model papers. While TS measures the inter-and intra-fiber strength, FE is a wear test related to the flexibility of the paper, but also to its tensile strength, as the test specimen is under tension that eventually causes failure. FE is widely used for the evaluation of paper permanence as it is more sensitive to early modifications than tensile strength measurements, even though the data are less repeatable [41]. More than BL, EB provides information about the material's ductility, i.e. its ability to undergo plastic deformation before breaking. Young modulus Y is a measurement of the paper stiffness and elasticity. In other words, J4 had a higher BL but also became stiffer (higher Y). In addition, BL and Y kept increasing between (t) and (t + 3) (+ 13% and + 17%, respectively) with still no change in EB.

The data at (t) and (t + x) of the model papers treated with AP-DIA 50-50 are shown in Figure 8 and S1).

Confirming previous findings [18,19], the application of AP-DIA 50-50 to the unaged model papers S2ua, C1ua, C3ua and C4ua and to the aged S2a improved FE at (t) to various levels (Figure 8). Besides

C1ua, which showed a large increase of FE with time (+ 36%), the other papers showed FE at (t + x) that was either stable (S2ua and C3ua) or lower (C4ua, -38%, S2a, -38% and C4a, -59%). Nevertheless, for S2a, N(FE) decreased from 76 at (t) to 47 at (t + x). However, this is still 2.4 times the value at (t 0 )

(N(FE) = 20). This indicates that FE strengthening of S2a, if not stable, at least was durable. For C3a and C4a, the treatment did not improve FE at (t), and a slight decrease occurred over time for C4a.

N(FE) variations of the model papers treated with AP-AM 50-50 over time were very similar to those observed when treated with AP-DIA 50-50 (Figure S9 in the Supporting Information file).

BL of the model papers at (t) were all above the values at (t 0 ), if only slightly for C1ua (Figure 9a).

This indicates that the treatment AP-DIA 50-50 was efficient to strengthen all the papers. In addition, besides C4ua and S2a, which showed a stable BL at (t) and (t + x), the other model papers underwent an increase in BL with time. This is consistent with the slow progress of the polycondensation. No correlation could be established between the variations of BL and FE. S1).

The strengthening at (t) was observed with AP-AM 50-50 as well. Overall, at similar uptakes, BL values of the model papers were even higher than with AP-DIA 50-50 (Figure 10a). However, and except for C4ua, the opposite situation was observed at (t + x), where for each paper, BL was higher with AP-DIA (Figure 10b). This is due to a larger decrease with time in BL of the papers treated with AP-AM, to the exception of C3a (Figure S10a). Nevertheless, for both formulations, at (t + x), BL values were always higher than at (t 0 ), which indicates a durable strengthening, despite the fluctuating temporal trends. In addition, at (t), with both formulations, the highest increase in BL was obtained for the sample with the highest uptake (Figure 10a). However, at (t + x), the absolute variation ΔBL was not proportional to the uptake (Figure 10b). (Figure 9b). Between (t) and (t + x), EB evolved differently among the papers, and no correlation could be evidenced, neither with the constituents, nor with the increase in the 𝐷𝑃 ̅̅̅̅ n of the AAAS. Besides, for the same formulation, the highest values of Y were those at (t + x) in all cases, where Y was higher than at (t 0 ), and also higher than at (t) (Figure 9c), showing that Y increased as the polycondensation of the AAAS progressed. Only C4ua did not follow this trend, as Y was similar at (t 0 ) (Y = 2008 ± 106 MPa), at (t) (Y = 2157 ± 110 MPa) and at (t + x) (Y = 2082 ± 137 MPa). Therefore, the stiffness of the treated papers increased with time, except for C4ua where it remained almost unchanged. This can be paralleled to the increase in Y observed for J4 treated with AP-DIA 5-95 (Figure 7c).

EB and Y of the model papers treated with AP-AM 50-50 are in Figures S10b andS10c (Supporting Information file). The absolute variations ΔEB and ΔY as a function of the uptake were plotted. For each paper, at similar uptakes, the application of AP-DIA induced higher ΔEB values, at (t) but also at (t +

x) (Figure S11). In contrast, ΔY was overall higher at (t) and (t + x) with AP-AM 50-50 (Figure S12).

Thus, ultimately, at (t + x), AP-DIA 50-50 led to higher BL and EB, whereas, as reported above, both AP-AM and AP-DIA yielded similar N(FE) (except for J4). Subsequently, and in contrast to the deacidification efficiency, in the 50-50 proportion, AP-DIA allowed a more sustainable strengthening of the paper than AP-AM.

Unlike with BL, no clear correlation was found with EB and Y when plotted as a function of the uptake at (t) and (t + x) (Figure S13 in the Supporting Information file). The variation of N(FE) as a While TS was always improved after treatment, the variations of FE depended considerably on the type of paper. Different trends were observed at (t), but also between (t) and (t + x). Firstly, FE was affected by the papers' constituents, independently of the degradation level (aged or unaged). C4ua was strengthened at (t), but N(FE) values at (t 0 ) and (t + x) were almost equal, showing that the FE strengthening was not sustained after several years. The lowest ΔN were obtained for the two papers with the lowest initial values, i.e the most degraded (aged) ones, C3a and C4a. As C4 is unsized, and because ΔN for C1ua, which is sized, was very high (ΔN = + 340%), it seems that the presence of alumrosin was not a key factor of FE lack of strengthening. On the other hand, unaged alum-rosin differs chemically from aged alum-rosin. However, the alum-rosin sizing alone still does not explain why C4a (unsized) was not strengthened. The type of pulp and the fiber source seem more relevant. Among the most degraded papers, the lignin-rich groundwood pulp papers yielded ΔN values among the lowest.

For instance, N(FE) of C4ua decreased further with time (ΔN = -38%) than N(FE) of C3ua (ΔN = -12%), and this was paralleled to the fact that the proportion of groundwood pulp is larger in C4 than in C3 (see Table 1). As described above, it was shown that the AAAS can react with acidic and oxidized functions, which are abundant in lignin and its degradation products [START_REF] Ferrandin-Schoffel | Modeling the 11 Reactivity of Aged Paper with Aminoalkylalkoxysilanes as Strengthening and Deacidification 12 Agents[END_REF]. It was hypothesized that these side reactions could affect the strengthening efficiency, especially in the case of the aged papers, which are more strongly oxidized. Indeed, the lignin-rich and very brittle (degraded) newsprint paper was the only paper to show no FE improvement after the addition of AP-DIA 5-95. In research by Souguir et al.

where DIA was used to treat model papers and extremely brittle book sheets made of woodpulp [START_REF] Souguir | 19 Strengthening of degraded cellulosic material using a diamine alkylalkoxysilane[END_REF], lignin-rich groundwood pulp and the fiber oxidation state were put forth as the main factors affecting the strengthening efficiency. The results obtained in the present work fully corroborated this hypothesis.

Conclusion 1 2

A temporal analysis of the physico-chemical properties of several papers treated with co-AAAS was 3 undertaken. The first aim was to study the changes occurring with time, and hence the stability of the 4 treatment and that of the treated paper. The second aim was to better understand the limiting factors in 5 the durability of the deacidification and strengthening. To this purpose, the use of model papers was 6 fully relevant in order to discriminate the respective contributions of the paper constituents. Alum was 7 found to be the main possible cause of the progressive pH decrease of the most degraded treated papers 8 with time. An alkaline reserve was consistently introduced in all the papers regardless of their 9 composition and degradation state. This alkaline reserve started to be consumed within a few years after 10 treatment in the most degraded papers sized with alum-rosin, showing how fast-decaying these papers 11 are. Among the paper constituents, lignin was proven to affect the strengthening efficiency of the co-12 AAAS treatment the most, which corroborated previous observations and hypotheses. Upon treatment, 13 the yellowing was also more pronounced in the papers containing alum-rosin sizing and/or lignin. The 14 choice of the AAAS monomers was impactful as well. While AP-AM 50-50 was found more efficient for a durable deacidification, the use of DIA instead of AM in the binary mixture with AP ultimately 1 induced a more durable strengthening. It has to be noted that the time scale used in this temporal study 2 was much shorter than the ''archival'' time scale. In order to examine the longer-term durability of the 3 co-AAAS treatments, the monitoring should be continued in the future. As evidenced by 29 Si solid-state 4 NMR, in situ polycondensation of the AAAS in the papers was shown to progress at a very slow pace 5 in ambient conditions and evolved over the course of several years. Thus, the polymerization kinetics 6 are also a key factor affecting the strengthening. In the future, the use of catalysts to speed up the 7 polycondensation rate is conceivable in order to optimize the treatment formulation according to the 8 paper constituents.

using a 3

 3 mm diameter measurement aperture. The colorimetric coordinates values L * , a * and b * were calculated in the CIELAB 1976 color space, with the D65 Standard Illuminant and 10° Standard Observer (ISO 5631-2 standard). The opacity values were determined with the C Standard Illuminant and 10° Standard Observer (ISO 2471 standard). The measurements were repeated on five to ten different locations on the samples' surface. The averages are reported with the standard deviations. The color difference was calculated as ∆E * = √ ∆L * 2 + ∆a * 2 + ∆b * 2 .Prior to mechanical testing measurements, the samples were conditioned according to TAPPI T402 sp-03. The tensile strength measurement (TS) was carried out with an Adamel Lhomargy instrument (DY-20N). Tensile Breaking Length (BL), Elongation at Break (EB) and Young modulus (Y) were calculated. BL (km) is the limiting length of a strip of paper of uniform width, beyond which, if such a strip was suspended by one end, it would break of its own weight. It is related to the tensile strength T

Figure 1 .

 1 Figure 1. Cold-extract pH of (a) J4 treated with AP-DIA (50-50 and 5-95) and (b) the model papers (unaged and aged) treated with AP-DIA 50-50. Grey filled bar: untreated papers (t 0 ). Hatched bars: treated papers at (t). White bars: treated papers at (t + x) (x and UP are in TableS1).

Figure 2 .

 2 Figure 2. Absolute pH variation (ΔpH) as a function of the uptake (UP) for the papers treated with (a) AP-DIA 50-50 and (b) AP-AM 50-50. Triangles: treated papers at (t). Circles: treated papers at (t + x) (x is in TableS1).

Figure 3 .

 3 Figure 3. (a) b * and (b) opacity of J4 treated with AP-DIA (50-50 and 5-95). Grey filled bar: J4 untreated (t 0 ). Hatched bars: J4 treated at (t). White bars: J4 treated at (t + 3) (UP is in TableS1).

Figure 4 .

 4 Figure 4. (a) b * and (b) opacity of the model papers (unaged and aged) treated with AP-DIA 50-50. Grey filled bar: untreated papers (t 0 ). White bars: treated papers at (t + x) (x and UP are in TableS1).

Figure 5 .

 5 Figure 5. Left: 29 Si CP-MAS NMR spectra of J4 treated with AP-DIA 5-95 at (a) (t) and (b) (t + 3). Right: expansions of the area of interest, with the resonance deconvolutions. The red spectrum is the sum of the D 1 (green) and D 2 (blue) contributions.

Figure 6 .

 6 Figure 6. Number of double folds (N(FE)) of J4 treated with AP-AM and AP-DIA (50-50 and 5-95). Grey filled bar: J4 untreated (t 0 ). Hatched bars: J4 treated at (t). White bars: J4 treated at (t + 3) (UP is in TableS1).

Figure 7 .

 7 Figure 7. (a) Breaking Length (BL), (b) Elongation at Break (EB) and (c) Young modulus (Y) of J4 treated with AP-DIA 5-95. Grey filled bars: J4 untreated (t 0 ). Hatched bars: J4 treated at (t) (UP = 7 ± 1%). White bars: J4 treated at (t + 3) (UP = 6 ± 1%).

Figure 9 .

 9 Figure 9.

Figure 8 .

 8 Figure 8. Number of double folds (N(FE)) of the model papers treated with AP-DIA 50-50. (a) unaged papers and (b) aged papers. Grey filled bar: untreated papers (t 0 ). Hatched bars: treated papers at (t) (data from Piovesan et al. 2017 and 2018[START_REF] Souguir | 19 Strengthening of degraded cellulosic material using a diamine alkylalkoxysilane[END_REF]15]). White bars: treated papers at (t + x) (x and UP are in TableS1).

Figure 9 .

 9 Figure 9. (a) Breaking Length (BL), (b) Elongation at Break (EB) and (c) Young modulus (Y) of the model papers (unaged and aged) treated with AP-DIA 50-50. Grey filled bars: untreated papers (t 0 ). Hatched bars: treated papers at (t). White bars: treated papers at (t + x) (x and UP are in TableS1).

Figure 10 .

 10 Figure 10. Absolute variation of breaking length (ΔBL) as a function of the uptake (UP) for the treated model papers (unaged and aged) at (a) (t) and (b) (t + x) (x is in TableS1). Squares: AP-AM 50-50. Triangles: AP-DIA 50-50.

  supported by the Paris Seine Graduate School Humanities, Creation, Heritage, 13 Investissements d'Avenir ANR-17-EURE-0021 -Foundation for Cultural Heritage Science, and by the 14 Ile-de-France DIM RESPORE. The participation of the Bibliothèque Nationale de France is 15 acknowledged. Alice Gimat and Oulfa Belhadj (CRC), Isabelle Fabre-Francke and Sophie Cantin 16 (LPPI), Hervé Cheradame (Polytheragene), Mohamed Haouas and Nathalie Steunou (Institut Lavoisier 17 de Versailles) are warmly thanked for technical contribution and fruitful discussions. 18 19 A Supplementary Material file is available. It contains supporting information to the article, including 20 the values of UP and the aging duration x, ATR-FTIR spectrum (J4, (t 0 )) titration curves (pH = f(V)) of 21 AP, AM and DIA, 29 Si NMR spectrum (J4 treated with AP-AM 5-95, (t + 4)), and pH, b * , opacity, N(FE), 22 BL, EB and Y values of the treated papers. 23 24

Table 1 .

 1 Main constituents, structure and physico-chemical characteristics of the papers.

		S2ua	C1ua	C3ua	C4ua	J4
	Fibers and pulp	Cotton linters	Cotton linters	GWP/BSP	GWP/BSP	GWP/UBSP/BSP,
				40/60,	80/20,	softwood, trace of
				softwood	softwood	straw
	Sizing	None	Alum-rosin	Alum-rosin	None	Alum-rosin
	Filler	None	None	Kaolin	Kaolin	Kaolin
	Ash content (wt%)	< 0.1	0.8  0.1	13.1  0.1	6.6  0.4	18.1  0.4
	Grammage (g m -2 )	76	80	80	52	58
	Thickness (µm)	150 ± 7	159 ± 3	152 ± 5	130 ± 2	61 ± 3
	pH	6.62 ± 0.08	5.86 ± 0.11	5.50 ± 0.08	6.21 ± 0.06	4.62 ± 0.01
	GWP: groundwood pulp. BSP: bleached sulfite pulp. UBSP: unbleached sulfite pulp.
	Four machine made model papers S2, C1, C3 and C4 were chosen based on their fiber constituents,
	and the presence or absence of kaolin fillers and alum-rosin sizing (Table 1). They had been treated

function of EB was investigated for AP-AM and AP-DIA. The relative variation of double folds ΔN was defined as follows:

where N(FE) a is the value after treatment (at (t) or (t + x)) and N(FE) b the value at (t 0 ). Figure 11a S1).


Chunzheng Wang
email: czwang@upc.edu.cn

Weisong Xu

Zhengxing Qin

Svetlana Mintova

Spontaneous galvanic deposition of nanoporous Pd on microfibrous-structured Al-fibers for CO oxidative coupling to dimethyl oxalate

Keywords: nanoporous palladium, galvanic deposition, microfibrous-structured catalyst, aluminium fiber, dimethyl oxalate

Spontaneous galvanic deposition of nanoporous Pd on microfibrous-structured Al-fibers for CO oxidative coupling to dimethyl oxalate

Chunzheng Wang, Weisong Xu, Zhengxing Qin, Svetlana

Introduction

Nanoporous palladium (np-Pd) is of great fundamental and technological importance for wide applications in catalysis, energy storage, sensors and fuel cells [1][2][3]. Various unsupported np-Pd samples have been successfully prepared by dealloying, consolidation, combustion, hydrothermal growth and template synthesis [4,5]. The fabricated np-Pd possesses high surface-to-volume ratio, interpenetrating nanostructure and large characteristic lengths in nanoligaments [1]. Owing to the special surface effect and resistance to sintering, the np-Pd exhibits good activity and stability in heterogeneous reactions [2,3]. Nevertheless, the high cost of the unsupported np-Pd hinders its practical application, and the as-made powder cannot satisfy the requirements of conventional fixed-bed reactors.

Aluminium metal is inexpensive, highly thermal-conductive, and is so soft that it could be easily made into different microstructured packings such as fibers and foams. These packings are opening avenues to "ideal" catalytic reactors with advantages of high throughput, low pressure drop and improved heat/mass transfer [6,7]. However, the real application of the microstructured Al metal as reactor packings is hampered by the inert surface and low surface area [8]. Recently, boehmite nanosheets were in-situ generated on microfibrous-structured Al-fibers by the hydrothermal oxidation (4 H2O + 2 Al  3 H2 + 2 AlOOH), and then incipiently impregnated with palladium precursor to obtain the Pd catalyst [9]. Therefore, it is important to find a direct and an effective way to functionalize aluminium metal.

The vapor phase carbonylation of methyl nitrite (MN) to dimethyl oxalate (DMO), a process converting inorganic C1 to organic C2 plays a key role from syngas to ethylene glycol (product of DMO hydrogenation) [10,11]. This process involves two main reactions: (1) Pd-catalyzed CO oxidative coupling: 2 CH3ONO + 2 CO  2 NO + CH3OCOCOOCH3; and (2) MN regeneration without catalyst: 2 NO + 2 CH3OH + 1/2 O2  2 CH3ONO + H2O. Although this process has been applied at industrial scale, the commercial Pd/-Al2O3 catalyst needs a high Pd content of 2 wt% (the-state-of-the-art) [12]. Decreasing the Pd content is an on-going effort which could be achieved by tuning the Pd shape/size and adding of a promoter on the CO oxidative coupling to DMO [13,14].

However, little attention is paid to the -Al2O3 support used, which suffers from poor mass/heat transfer and non-regular flow pattern for the strongly exothermic and fast reaction.

Herein, we report the preparation of a supported catalyst using nanoporous palladium embedded on microfibrous-structured Al-fibers. We also reveal the top-down, versatile and effective method of the catalyst fabrication through the spontaneous galvanic deposition between Pd 2+ and Al (2 Al + 3 Pd 2+  2 Al 3+ + 3 Pd). The np-Pd/Al-fiber catalyst combines the activity and stability of the np-Pd and improved heat/mass transfer, large permeability originated from the microfibrous-structured Al-fibers.

Experimental

A large thin-sheet fibrofelt (2.0 m length  1.0 m width  ~1.3 mm thickness) with Al-fibers (60 μm diameter, 99.9 wt% Al; Shanghai Xincai Network-microstructured Material Co. Ltd., China) was employed as pristine catalyst support [9]. Typically, circular shaped Al-fibers (16 mm diameter; Fig. S1) were tailored from the purchased fibrofelt and pre-treated with a NaOH solution (0.1 wt%) at 25 º C to remove the oxide layers (Al2O3) from the surface. After that, palladium was deposited onto the pre-treated Al-fibers with the aid of galvanic replacement between Pd 2+ and Al (2 Al + 3 Pd 2+  2 Al 3+ + 3 Pd). This reaction proceeded spontaneously at room temperature when the pre-treated Al-fibers were impregnated with an acetic acid (CH3COOH) solution containing appointed amount of palladium acetate (Pd(CH3COO)2; Sinopharm Chemical Reagent Co. Ltd., China). During the galvanic process, the orange solution gradually became clear. The resulting Al-fibers were dried at 100 º C for 2 h, followed by calcining at 300 º C (air) for 2 h to obtain the np-Pd/Al-fiber catalyst. The nominal Pd content in the np-Pd/Al-fiber catalysts was in the range of 0.1 -2.0 wt%. In addition, we prepared a reference catalyst of Pd/-Al2O3 (nominal Pd content of 0.3 wt%) using incipient-wetness impregnation with Pd(CH3COO)2 as a precursor and -Al2O3 (Alfa Aesar (China) Chemical Co. Ltd.) as a support, followed by drying at 100 º C for 2 h and calcination at 300 º C in air for 2 h. The details of catalyst characterizations and the calculations of the turnover frequency (TOF), conversion and selectivity are available in the Supporting Information (SI).

Results and discussion

The strategy for preparation of the microfibrous-structured np-Pd/Al-fiber catalyst is schematically presented in Fig. 1A. The Al-fibers with irregular three-dimensional network and 90 % void volume, are used as pristine catalyst support (Fig. 1A and Fig. S1). The original surface of the Al-fibers is smooth, as shown in the SEM images (Fig. S2). After the spontaneous deposition carried out at room temperature through the galvanic replacement between Pd 2+ and Al (2 Al + 3 Pd 2+  2 Al 3+ + 3 Pd), tree-like np-Pd is epitaxially formed on the Al-fibers surface (Fig. 1B). The circular shape of Al-fibers remains intact (Fig. S1). Both SEM and TEM images show that the np-Pd is composed of 10-30 nm ligaments surrounded by irregular nanopores having a wide size distribution (Fig. 1C,D). The EDX spectra of the np-Pd contain three peaks corresponding to Pd, Al and O with an average content of 90.8, 3.0 and 6.2 wt%, respectively (Fig. S3). The presence of Al and O is attributed to the Al substrate and the oxidation of the np-Pd and Al, respectively. The XRD patterns show that only the characteristic diffraction peaks of Al (JCPDS 04-0787) are identified in the mother Al-fibers support (Fig. S4). For the np-Pd/Al-fiber catalyst, the diffraction peaks of Pd(200) (JCPDS 01-1201) and PdO(101) (JCPDS 88-2434) are weak, possibly due to the low Pd content (Fig. S4). The PdO phase is formed unavoidably due to the oxidation of the np-Pd during the calcination (see Experimental section), which is consistent with the EDX elemental mapping results (Fig. S3 and Fig. S4). Such PdO can be completely reduced below 25 º C due to the undetectable H2 consumption above 25 º C studied by H2-temperature programmed reduction (Fig. S5). The nominal Pd content is 0.5 wt%, while the actual content is 0.28 wt% measured by ICP-AES analysis. The loss of Pd is occurred during the galvanic deposition; a visible black powder at the bottom of the galvanic solution was observed. The black powder has nanoporous structure similar to the np-Pd observed on the np-Pd/Al-fiber catalyst (Fig. 1C and Fig. S6). Medium dispersion (D) of Pd is measured to be 0.09 by CO pulse chemisorption measurements, and the particle size (d) is determined to be ~12 nm (d = 1.12 / D [15]), which is smaller than the size obtained by TEM (average particle size of 19 nm; Fig. 1D). As noted previously, the process of spontaneous galvanic deposition includes the following two steps: (1) Al  Al 3+ + 3 e -and (2) Pd 2+ +2 e - Pd. The galvanic deposition proceeds automatically at room temperature once the Al-fibers are impregnated with the CH3COOH solution of Pd(CH3COO)2. The driving force of this reaction is the large difference in electrode potential between Al 3+ /Al 0 (-1.66 V) and Pd 2+ /Pd 0 (0.91 V) pairs. Palladium is first nucleated and deposited at energy-favorable sites or dislocation centers on the Al-fibers surface [16]. With prolonged time of galvanic deposition, the palladium is epitaxially formed on the Al-fiber and assembled as np-Pd, which is confirmed by SEM-EDX analysis (Fig. S7).

The effect of the nominal Pd content on the catalytic activity of the np-Pd/Al-fiber in the CO oxidative coupling to DMO was studied (Fig. S8). Clearly, with the increase of Pd content from 0.1 to 0.5 wt%, at 150 º C, the CO conversion is increased from 31.3% to 55.6% and gradually increased to 65.5% with further rising of the Pd content up to 2.0 wt%. While the DMO selectivity (~95%) is almost independent of the Pd content. Based on the balance between the CO conversion and the catalyst cost, the optimal Pd content of 0.5 wt% was identified, while the actual one is 0.28 wt%.

As shown in Fig. 2A, the np-Pd/Al-fiber catalyst shows higher DMO selectivity than the conventional Pd/-Al2O3 catalyst. Both catalysts show a volcano-type variation of CO conversion by elevating the reaction temperature from 120 to 170 º C. The np-Pd/Al-fiber catalyst reaches a maximal CO conversion at 160 º C while the Pd/-Al2O3 catalyst at 140 º C, which indirectly indicates the effect of the structure for np-Pd on Al-fibers and Pd nanoparticles on -Al2O3. The actual Pd content and Pd oxidation state are similar for both catalysts, but Pd dispersion and surface Pd atoms of the np-Pd/Al-fiber catalyst are much lower than for the Pd/-Al2O3 (Table 1, XPS spectra in Fig. S9). However, the np-Pd/Al-fiber catalyst exhibits a TOF of 1.2 s -1 , which is four times higher than for the Pd/-Al2O3 (0.2 s -1 ), suggesting that the former possesses far larger intrinsic activity on the CO oxidative coupling to DMO. The np-Pd with well-connected network of nanoligaments, possesses nearly equivalent concave and convex surface (negative and positive curvature), thereby leading to lattice contraction and expansion based on the Laplace equation [17,18]. The HRTEM images show lattice fringes of 0.208-0.231 nm that correspond to Pd(111) facet on some disordered areas for the np-Pd/Al-fiber catalyst (Fig. S10); this result indicates that some lattices are expanded while some are contracted. Such lattice disorder of the np-Pd has a remarkable influence on the surface properties and adsorption performance, as confirmed by first-principles calculations [19,20].

It is thus inferred that the unique nanoporous structure of the np-Pd is possibly responsible for the enhanced intrinsic activity.

The np-Pd/Al-fiber catalyst exhibits a CO conversion of ~55%, and a DMO selectivity of ~95% for 150 hours test (Fig. 2B). The diffraction peak of Pd(200) (JCPDS 01-1201) is almost intact for the used catalyst after 150 hours test (XRD pattern in Fig. S11) suggesting the anti-sintering of the np-Pd. These results clearly indicate that the np-Pd/Al-fiber catalyst is very stable. Moreover, in comparison with the catalysts reported in the literature, the np-Pd/Al-fiber has low Pd content and medium catalytic performance (Table S1). However, the used support of our microfibrous-structured Al-fiber possesses enhanced heat/mass transfer and low pressure drop compared to the common -Al2O3 support. e TOF according to the CO conversion at 140 º C and surface Pd atoms.

Conclusions

We report the preparation of nanoporous palladium (np-Pd) catalyst on microfibrous-structured

Fig. 1 .

 1 Fig. 1. (A) Strategy applied for preparation of the np-Pd/Al-fiber catalyst by spontaneous galvanic

Fig. 2 .

 2 Fig. 2. (A) Conversion and selectivity as a function of reaction temperature (np-Pd/Al-fiber: 0.28

Table 1 2 a

 12 Catalyst characteristics of the np-Pd/Al-fiber and Pd/-Al2O3 catalysts a . Sample D b d (nm) b Pd (wt%) c Surface Pd (mol gcat. -Reaction conditions: 140  C, GHSV = 3000 mL g -1 h -1 , CO/CH3ONO/N2 =14/10/76 (volume), 1 bar. b Dispersion (D) of Pd determined through pulse chemisorption of 10 vol% CO with a stoichiometry of CO/Pd = 1 and nanoparticle size (d, nm) obtained by the formula (d = 1.12 / D) [15]. c Actual Pd content measured by ICP-AES analysis. d Surface Pd atoms estimated from actual Pd content and Pd dispersion (D).

Acknowledgements

This work is supported by the China Postdoctoral Science Foundation (2017M622311), Fundamental Research Funds for the Central Universities (18CX02148A), Postdoctoral Research Project of Qingdao (BY20170209) and the Sino-French LIA "Zeolites".

Al-fibers for the CO oxidative coupling to DMO. The microfibrous-structured np-Pd/Al-fiber catalyst from macro-to nanoscale is fabricated by a tailored spontaneous deposition via the galvanic replacement between Pd 2+ and Al (2 Al + 3 Pd 2+  2 Al 3+ + 3 Pd). For the catalyst with a low Pd content (0.28 wt%), a CO conversion of ~55% and a DMO selectivity of ~95% are obtained for a feedgas of CO/CH3ONO/N2 (14/10/7.6, volume) at GHSV of 3000 mL g -1 h -1 . Notably, the catalyst provides stable activity and selectivity through the 150 hours lifetime test. The np-Pd/Al-fiber catalyst shows four times larger intrinsic activity (i.e., turnover frequency) than the Pd/-Al2O3 catalyst, which is attributed to the structure difference between the np-Pd with nanoligaments and the Pd nanoparticles present in the Pd/-Al2O3 catalyst. Moreover, compared to the catalysts reported in the literature, our np-Pd/Al-fiber catalyst has the unique structure of np-Pd with improved intrinsic activity, and shows enhanced heat/mass transfer, large permeability originated from the microfibrous-structured Al-fibers.


Hailing Guo

Svetlana Mintova
email: mintova@ensicaen.fr

Keywords: Mixed matrix membranes, Sodalite, Gas separation, Hydrogen, Nitrogen Stability

Nanosized sodalite crystals (Nano-SOD) are used as active filler to prepare mixed matrix membranes (MMMs) for promoting the H2/N2 gas separation performance. The Nano-SOD with extremely small crystallites (40-50 nm) are synthesized from a colloidal suspension free of organic structural directing agent, and uniformly dispersed in the Polyetherimide (PEI) matrix. The Nano-SOD filler with a proper aperture size (2.8 Å) allow only hydrogen molecules passing through and rejecting the nitrogen thus improving the selectivity of the membranes. The high dispersion of nanosized sodalite crystals in the polymer matrix and the proper interactions between the inorganic and organic phases greatly improve the membrane separation performance and minimize interfacial holes. The MMMs showed a high H2 permeability (~7155.1 Barrer at 25°C under atmospheric pressure) and ideal H2/N2 selectivity factor of ~16.9 in a single gas test. Moreover, in a gas mixture (H2/N2, 25-100°C ), the selectivity factor increased significantly to ~30.9. The high stability of the MMMs consisting of highly dispersed nanosized SOD crystals in PEI matrix for H2/N2 separation (6 weeks continuous test), rendered them as important materials for ammonia synthesis application that requires and also releases a large amount of hydrogen.

Introduction

Ammonia is one of the most important industry material, which has an over 200 million tons annual production. [1] The ammonia synthesis is a high energy consumption industry, which consumes nearly 1.4% of all global energy, [2] mainly consumed by feed gas hydrogen production.

Nevertheless synthetic ammonia requires vast hydrogen, also releases large amounts of hydrogen. Its purge gas (around 40 billion Nm 3 annual) mainly contains 60-70 % hydrogen and 20-25 % nitrogen, leading to a serious air pollution and around 26 billion Nm 3 hydrogen waste. Therefore, recovering and reusing hydrogen from the purge gas and for synthetic ammonia process is environmental and economic. H2/N2 is one of the most difficult gas pairs to be separated, due to the following reasons:

(1) similar molecular size (H2 of 2.89 Å and N2 of 3.64 Å), [3] resulting in molecular sieving difficulty; (2) similar quadrupole moment (quadrupole moments of H2(0) and N2 (4.7 × 10 -40 Cm 2 )), [4][5][6] leading to similar sorption ability; and (3) relatively low Knudsen selectivity, which for H2/N2 is only 3.74. [7,8] Thus, the separation of hydrogen from nitrogen requires more attentions.

For gas separation, membrane technology is widely considered together with other methods including pressure swing adsorption (PSA) and cryogenic distillation. The advantages of membranes are low energy consumption, low investments and operation cost, small footprint, easy operation and being environmentally benign. [9][10][11] Among various membrane materials, the polymeric membranes have reasonably low cost, easy processing and thus widely used in industries. [12] However their performance are limited by a "trade-off" phenomenon between gas selectivity and permeability. This phenomenon is described by Robeson using an upper bound correlation between permeability and selectivity for specific gas pair, which has been used to evaluate polymeric or polymer mix matrix membrane. [13] The metallic membranes, especially Pd membranes, have an excellent H2 permeability and high H2 purity but suffer from hydrogen embrittlement below 300°C . This implies that high temperature is required which is connected with high-energy process. [10,14] Inorganic membranes using zeolite and MOFs materials have acted as potential separators due to their unique properties, including uniform pores, framework structures, molecular sieving and reasonable stability. [15,16] They exhibit super permselectivity at ambient conditions. While their difficulties in scale up and easy damage limit further industrial application. With the aim of enhancing membrane separation performance, mixed matrix membranes (MMMs) are attracted as they combine advantages of large scale applications of polymeric membranes and high separation performance of inorganic particles (fillers), which is proposed as an effective mean for polymer matrix membranes to break the "trade-off" [17] and obtain high permeability. [18,19] and 4A) were the first attempt for hydrogen separation. [20] The highest H2 permeability obtained was for PES with 50% 4A zeolite, which was 14.1 Barrer at 25°C , which was twice higher than for the pure PES membrane's permeability. The ideal selectivity of H2/N2 increased from 46 to 56 indicating the selectivity promoted by zeolite filler. Chaidou et al. prepared MMMs consisting of Polyimide (PI) and zeolites (13X, 4A and ZSM-5), and they were used in selected gas mixtures of He, H2, CO2 and N2 for systematic research. [21] Compared with the PI membrane, the maximal permeability of the PI/13X membrane increased by 178 % (26.34 Barrer), while the H2/N2, H2/CO2 and He/N2 selectivity of all MMMs with high zeolite loading decreased (up to -24.1%). Peydayesh et al. fabricated MMMs containing DDR type zeolite (pores of 3.6 × 4.4 Å) with polyimide for H2/CH4 separation. [22] These membranes achieved ideal selectivity of 375 and H2 permeability of 34.9 Barrer. Many micronsized zeolites including 13X, [20,21,23] 4A, [21,23,24] ZSM-5, [21] silicate-1, [25] and mesoporous SBA-15 [26] were used for preparation of membranes, however none of the above zeolites have appropriate pore opening for selective separation of H2. Additionally, the low external surface area and less accessible apertures of micronsized zeolite crystals cause high diffusion resistance. More importantly, the weak adhesion between the polymer and zeolite crystals causes a large number of uncontrollable voids, leading to low selectivity due to the polymer-zeolite interface. Therefore, to reduce or even eliminate interface voids or pore blocking, coupling agents or compatibilizers were used. Duval et al. modified silicalite-1 by sliane-coupling agents to promote the adhesion between the polymer and zeolite crystals. [26] The modification decreased the permeability but not affecting the selectivity of the membranes. Khan et al. studied the effect of coupling agent aminopropyltrimethoxysilane (APTMS) in Polusulfone (PSF)/zeolite 3A MMMs for H2/CO2 separation. [27] The selectivity improved from 1.7 to 3.57 with a five-fold permeability reduction. Yong et al. introduced 2, 4, 6-triaminopyrimidine (TAP) as a compatibilizer into PI/zeolite MMMs. [23] The CO2/N2 and O2/N2 selectivity of MMMs with TAP increased almost five time compared with the MMMs without compatibilizer. However the gas permeability for all decreased greatly which was mainly attributable to the blocked micropores of zeolites. Therefore, it is an important issue to control the properties of the MMMs on order to separate selectively hydrogen from nitrogen. Scheme 1. Schematic presentation of mixed matrix membrane (MMMs) prepared with nanosized sodalite filler (Nano-SOD) and polyetherimide (PEI)

In this paper, we present a new method for preparation of MMMs for promoting the H2/N2 gas separation performance. As shown in Scheme 1, firstly, green synthesis of nanosized sodalite (SOD) crystals without organic template was performed. The nanosized sodalite crystals have uniform particle size distribution and large external surface area, which are the basis for more accessible active sites and short diffusion pathways bringing higher permeance. The nanosized sodalite crystals were highly stable and well dispersed in the polyetherimide matrix. The high molecular sieving for H2/N2 with sodalite crystals was achieved. The SOD with six-member rings with an aperture size of 2.8 Å was chosen as filler, which only permitted small molecules like hydrogen to enter. [28,29] Wang et al. studied the performance of sodalite-polyimide membranes in hydrogen and nitrogen separation. [30] The results showed an improvement in the ideal selectivity, while the permeability of hydrogen did not change significantly, which may be due to the influence of the modifier on sodalite crystals. The hydrogen passed through the MMMs quickly because of both the short diffusion path in nanosized sodalite crystals and the openness of permeable apertures in MMMs' inner structure. While nitrogen permeation was restricted due to the molecular sieving property of nanosized sodalite. We expected the MMMs to achieve a simultaneous increase in both permeance and selectivity by utilizing nanosized sodalite crystals with the exact aperture size.

Experimental Section

Materials

Polyetherimide (PEI) was supplied by SABIC. Colloidal silica (LUDOX HS-30) and sodium aluminate (37-45 % Na2O and 50-60 % Al2O3, Technical, anhydrous) were purchased from Sigma-Aldrich and aluminium powder (-325 mesh, 99.5 %, metals basis) was obtained from Alfa Aesar. Sodium metasilicate pentahydrate (95 %) and 3-Aminopropyl-(diethoxy)methylsilane (APDEMS, 97 %) were purchased from Aladdin. Sodium hydroxide (NaOH) and dichloromethane (CH2Cl2, 99.5 %) were provided from Xilong Scientific. Ammonium chioride was purchased from Sinopharm Chemical Reagent Co. Ltd.

Synthesis of nanosized sodalite crystals and post-synthesis treatment

A typical synthesis mixture with the following composition 18Na2O:5SiO2:1Al2O3:240H2O was prepared by dropwise addition of freshly prepared homogeneous aluminate solution (0.54 g aluminium powder, 5 g NaOH and 10 g H2O) into silicate solution (10 g colloidal silica, 9.368 g NaOH and 26.2 g H2O) under vigorous stirring in an ice-water bath. Then the mixture was stirred for 24 h at room temperature and the resulting suspension was transferred to a polypropylene bottle, and heated at 60°C under static conditions for 48 h. The products were washed by centrifugation (5000 rpm, 20 min), followed by re-dispersion in distilled water; this procedure was repeated several times until the pH of the decanted solution was 7.0. Finally the samples were dried using a freeze dryer for 60 h (vacuum of 5 Pa, -80°C ). The nanosized sodalite crystals will be further abbreviated as Nano-SOD.

Post-synthesis treatment: To further study the property of Nano-SOD, two post synthesis treatments were performed. Ammonium exchange [31] (supplementary information 1) was carried out to study the gas separation performance of SOD in proton form (sample Nano-SOD H+ ). Surface modification [32] (supplementary information 2) by coupling agent (3-Aminopropyl-(diethoxy)methylsilane, APDEMS) was applied to investigate the formation of voids or blocking of pores at the sodalite-polymer interface.

The modified sample will be further abbreviated as Nano-SOD mod in this paper.

Synthesis of micronsized sodalite crystals

The composition of the synthesis mixture for the micronsized sodalite crystals (Micro-SOD) was 50Na2O:5SiO2:1Al2O3:1005H2O. [33] The aluminate solution was prepared by dissolving sodium hydroxide (4.12 g) in deionized water (23.53 g) followed by adding sodium aluminate (0.22 g). Sodium hydroxide (5.59 g) was mixed with deionized water (23.53 g) and sodium metasilicate (1.66 g) to prepare the silicate solution. After adding aluminate solution slowly into the silicate solution under stirring at room temperature, the mixture was stirred vigorously for 0.5 h in order to be homogenous. The resulting mixture (gel) was poured in a polypropylene bottle, and heated at 90°C for 22 h. The products were purified following the procedure described above.

Preparation of Mixed Matrix Membranes (MMMs)

Polyetherimide (PEI) particles were dried at 120°C for 24 h. Then the dried PEI particles (5 wt. %) were added into dichloromethane (CH2Cl2), and subjected to stirring for 8 h until the solution became clear (this will be named PEI matrix). All samples, including Nano-SOD, Micro-SOD, Nano-SOD H+ , and Nano-SOD mod , were dispersed in the PEI matrix solution under sonication for 2 h; the solutions were stationed till all bubbles were removed. These solutions were deposited on precleaned glass plates at room temperature. Thus the mixed matrix membranes (MMMs) consisting of polyetherimide and sodalite crystals (fillers) were obtained under drying at room temperature for 24 h, followed by heating at 60°C for 48 h, and 120°C for 24 h. The thickness of the MMMs was between 30 and 49 μm. Membranes consisting of PEI and Nano-SOD with mass concentrations of 0%, 2%, 10%, 20%, 30%, will be further abbreviated as 0-nano-SOD-PEI, 2-nano-SOD-PEI, 10-nano-SOD-PEI, 20-nano-SOD-PEI, and 30-nano-SOD-PEI, respectively. Membranes formed with PEI and 10 wt. % Micro-SOD, 10 wt. % Nano-SOD H+ , 10 wt. % Nano-SOD mod , will be further abbreviated as 10-micro-SOD-PEI, 10-nano-SOD H+ -PEI, and 10-nano-SOD mod -PEI, respectively. Characterization X-ray diffraction (XRD) patterns of pure sodalite samples and mixed matrix membranes were recorded using a PANalytical empyrean model X-ray diffractometer with Cu Kα radiation (all samples were run in the range 5° to 75° 2). Fourier transform infrared spectroscopy (FTIR) spectra were recorded using a Bruker Vertex 70 spectrometer with an average of 64 scans in the range 400-4000 cm -1 . Thermal stability of nanosized sodalite crystals was evaluated by thermogravimetric analysis (TG) using a HETZSCH QMS 403C analyzer (40-800°C with heating rate of 10 °C• min -1 under air). Differential scanning calorimetry (DSC) study was performed on MMMs using a HETZSCH QMS 403C analyser. The measurement was made at a heating rate of 10 °C• min -1 in a nitrogen-filler chamber, and a first run was done from 20°C to 400°C followed by quenching and a second heating. The size and morphology of sodalite crystals and the surface and cross-section of mixed matrix membranes were measured by a JEOL JSM-7900F scanning electron microscope (SEM) and a JEOL JEM-2100UHR transmission electron microscope (TEM). The distribution of the sodalite crystals in the membranes was measured by EDS-SEM using JEOL-7900F SEM and Oxford Instruments X-MaxN Energy-Dispersive Spectroscopy. The hydrogen and nitrogen adsorption-desorption experiments were performed at 77K using Quantachrome autosorb iQ3 gas sorption analyzer. The external surface area was determined by the Brunauer-Emmett-Teller Method (BET).The 13 C-NMR spectra were recorded by a Bruker AVANCE III 400 MHz spectrometers.

Mixed Matrix Membranes (MMMs): permeability test

Single and mixed gas permeability tests of pure PEI membrane and PEI/sodalite mixed matrix membranes were evaluated for H2, N2, CO2, CH4 and H2/N2 using a permeation measurement equipment. The membranes, cut into squares with a dimension of 3.14 cm 2 , were placed in a stainless steel cell; both the temperatures (25°C to 100°C) and pressures (atmospheric pressure) in the cell were controlled. The membranes were perched continuously by argon (Ar) flow and either single gases (H2 or N2) or gas mixtures (H2/N2) were delivered. A flow meter was used to measure the gas fluctuation. The volume ration of the feed gas mixture (H2:N2) was regulated from 1:1 to 3:1. After 3 hours of permeation, the gases which penetrated the membrane were analysed by a gas chromatograph (SHIMADZU GC-2014C).

The gas permeability coefficient was calculated with the following equation:

𝑃 = 10 -10 𝑙𝑁𝑖 𝐴𝛥𝑝𝑖 (1)
where P (Barrer) is gas permeability coefficient, l (cm) is the membrane thickness, Ni

(cm 3 (STP)•s -1 ) is the permeate flow rate of component i, A (m 2 ) is the effective membrane area, Δpi (cmHg) is the trans-membrane pressure drop of i.(1Barrer =10 -10 cm 3 (STP)cm• m -2 •s -1 • cmHg - 1 )
The selectivity of membrane was evaluated by the separation factor (α) calculated with the following equation:

𝛼 𝑖/𝑗 = 𝑋𝑖/𝑋𝑗 𝑌𝑖/𝑌𝑗 (2)
where i, j represent the two components in the gas mixtures, and X and Y are the mole fractions in the permeance and feed solution respectively.

The gas permeability was also calculated using the Maxwell model, which describes the gas transport in mixed matrix membranes according to the following equation: [34] 𝑷 𝒊𝑴𝑴𝑴 = 𝑷 𝒊𝒑 [

𝑷 𝒊𝒔 +𝟐𝑷 𝒊𝒑 -𝟐𝜱 𝒊𝒔 (𝑷 𝒊𝒑 -𝑷 𝒊𝒔 ) 𝑷 𝒊𝒔 +𝟐𝑷 𝒊𝒑 +𝜱 𝒊𝒔 (𝑷 𝒊𝒑 -𝑷 𝒊𝒔 ) ] ( 3
)
where PiMMM is the gas permeability of penetrant i in the mixed matrix membranes; Pip and Pis correspond to the gas permeability of penetrant i in the polymer matrix and molecular sieve particles, respectively; Φs is the volume fraction of molecular sieve particles in the whole membrane.

Results and Discussion

Properties of MMMs and Nano-SOD

The optical photographs of mixed matrix membranes (0-nano-SOD-PEI, 10-nano-SOD-PEI and 30nano-SOD-PEI) are shown in Figure S1. The presence of sodalite crystals in the mixed matrix membranes (MMMs) is confirmed by XRD (Figure S2 andS3) and FT-IR (Figure S4) characterizations; a detailed discussion of the properties of the MMMs is provided in the SI. [28,35,36] The distribution of Nano-SOD in the MMMs is evaluated using the cross-section SEM images and energy dispersive spectroscopy analysis The average size of the sodalite crystals is 36 nm, which is consistent with the size determined by TEM and SEM (see Figure 1). The pure Nano-SOD crystals are subjected to H2 and N2 adsorption. As shown in Figure S8, the nanosized sodalite crystals exhibit high adsorption capacity towards hydrogen, while no nitrogen was adsorbed. The nanosized sodalite crystals have higher external surface area of (120.0 m 2 •g -1 ) than the micronsized sodalite (2.93 m 2 •g -1 ) (Figure S9). This suggests more accessible sites for hydrogen which is beneficial for enhancing the diffusion of hydrogen in the hybrid membranes. cm -1 to 459 cm -1 (Figure S10), suggesting the weak interaction (possibly hydrogen bonding) between the PEI and the Nano-SOD. This shift is observed in all MMMs spectra (Figure S4), illustrating that the C NMR spectroscopy; the spectra of pure PEI and 10-nano-SOD-PEI were recorded (Figure S11). The two peaks at ~167 ppm and ~153 ppm corresponding to C1 and C2 are present in the spectra. [37] These two peaks shifted to a lower field as highlighted by the arrows, pointing out the weak interaction (possibly hydrogen bonding) between the polymeric PEI and the SOD nanocrystals. [38] The high dispersion of the nanosized sodalite crystals in the polymer matrix strengthens the interaction between the PEI and sodalite, which is causing the formation of small-sized interfacial voids. The addition of aggregated sodalite crystals in the 30-nano-SOD-PEI membrane, leaded to the formation larger voids which was expected.

Gas separation performance

Following the successful preparation of the PEI/Nano-SOD MMMs, gas separation studies were performed. The PEI/Nano-SOD MMMs were mounted in a gas separation cell (Figure S12); single and mixed gases were delivered to the MMMs. The influence of the Nano-SOD loading in the MMMs on the gas separation was studied (Figure 2). As shown, the permeability for H2 increased with the increase of the amount of Nano-SOD loaded into PEI. Interestingly, when the amount of Nano-SOD reached 30 wt. %, the permeability of hydrogen dropped sharply. The MMM doped with 10 % Nano-SOD (10-nano-SOD-PEI) exhibits the best gas separation performance with the highest ideal selectivity factor (~16.9) and H2 permeability (~7155.1 Barrer). In contrast, the pure PEI membrane (0-nano-SOD-PEI) has ~13.5

Barrer hydrogen permeability and 4.33 ideal selectivity index. The high H2 separation performance of the 10-nano-SOD-PEI membrane is primarily due to the presence of highly dispersed SOD nanocrystals with uniform size (cages size of 2.8 Å) that allows only hydrogen to penetrate, while the nitrogen is rejected.

Compared to 10-nano-SOD-PEI, the 30-nano-SOD-PEI has poor gas separation performance due to the significant agglomeration of Nano-SOD crystals observed by SEM and EDS-SEM. The agglomeration leads to the inefficiency of molecular sieve by blocking the active cages. The SOD crystals in proton form are fully amorphous (see experimental section, Nano-SOD H+ ) and they were used as inactive filler to prove that the hydrogen is transported through the sodalite crystals.

The Nano-SOD H+ sample is amorphous as seen in Figure S13. b, i.e., the diffraction peaks disappear indicating the collapse of the crystalline structure. From the TEM pictures (Figure S13. c, d), one can see that the crystal size of Nano-SOD does not change after ammonium exchange. As shown in the Figure 2, compared to the 10-nano-SOD-PEI (crystalline), the hydrogen permeability of 10-nano-SOD H+ -PEI (amorphous) decreased sharply, as well as the ideal selectivity factor, chiefly attributing to the loss of crystallinity of SOD particles (collapse of the SOD cages) in the Nano-SOD H+ . The decrease of nitrogen permeability is related to the small voids formed between the amorphous particles (Nano-SOD H+ ) and PEI (Figure S14). Thus it is conformed that the crystalline Nano-SOD with the appropriate aperture size is responsible for the molecule sieving of hydrogen from nitrogen. The effect of small-sized interfacial voids on gas separation results of the membranes was also studied (Table 1). The increase of gas permeability of the 10-nano-SOD H+ -PEI membrane without the assistant of open-framework structure of Nano-SOD is primarily attributed to the presence of interfacial voids.

Furthermore, the low gas separation index (H2/N2=3.07) confirms that the process is controlled by Knudsen diffusion caused by the presence of small-sized interfacial gaps. The contribution of sodalite crystals to the hydrogen permeability is substantial (5875 Barrer). Over 80% of the total flux for the 10nano-SOD-PEI membrane is dominated by the nanosized sodalite crystals (Table 1). With the aim of further studying the effect of interfacial voids, coupling agents were used (see experimental section, Nano-SOD mod ). Herein, the morphology of the Nano-SOD mod is presented in Figure S15. The gas separation of this modified membrane is shown in Figure 4, exhibiting the sharp decrease of gas permeability and a small improvement in selectivity. In order to investigate the influence of the coupling agent on the permeability, the cross-section SEM images were measured and displayed in Figure S16.

The severe agglomeration of sodalite crystals is observed, due to the adhesion increased by the coupling agent. As mentioned above, the high permeability is based on the sodalite nanocrystals high dispersion in the membrane. The modifier abolished the uniform dispersion of the sodalite nanoparticles, thus leading to dramatic reduction in their permeation efficiency and loss of permeability. Therefore, the modification is not suitable for the membranes in this work, as it destroys the essential advantage of the active Nano-SOD filler (high permeability). The advantages of nanosized sodalite were demonstrated through contrasting with micronsized sodalite (Figure S17). The membranes with 10 wt. % nanosized sodalite (Nano-SOD) showed higher H2 permeability and permselectivity than the membranes prepared with micronsized counterpart (Micro-SOD) (Figure 4). This is explained by the more accessible sites (due to the larger external surface area), and shorter diffusion pathway (due to the smaller crystal size) of the nanosized SOD in comparison to the micronsized SOD crystals. In the 10-nano-SOD-PEI membrane, hydrogen molecules penetrated easily in the permeable SOD apertures following the low diffusion resistance. While, the nitrogen permeation was restricted due to the molecular sieving property of the SOD crystals.

To further study the properties of the 10-nano-SOD-PEI membrane, the permeability tests for CH4 and CO2 were performed (Figure 5). The gas molecules with bigger kinetic diameter than 0.3 nm have apparently lower permeability than hydrogen. The permeability follows the following trend of CH4> N2> CO2, thus revealing that the permeability is based on Knudsen diffusion. This suggests the existence of voids, which is consistent with the previous analysis of interfacial voids. The schematic presentations of the MMMs and the potential gas transport pathways are shown in Figure 6. The possible permeation pathways in the MMMs consisting of small voids and fillers (sodalite and amorphous particles) could be expressed as follow: (1) polymer matrix without any fillers (tm/Pm),

(2) polymer matrix containing permeable fillers with voids (tm/Pm, tf/Pf, and tv/Pv) and (3) polymer matrix containing impermeable fillers with voids (tm/Pm and tv/Pv). [39] Herein, the t/P ratio between the transport path length and the permeability is considered as the transport resistance.

In the membranes with dispersed sodalite crystals containing voids (Figure 6. a), the permeability for hydrogen is larger than in the pure polymer permeability. Therefore, tf /Pf + tv/Pv is much lower than tm/Pm.

Thus, the minimum total resistance is dominated by the area with fillers (sodalite crystals). While in the membranes with aggregated sodalite crystals (Figure 6. b), some of the apertures are blocked and the voids are connected resulting in increased resistance tf/Pf. The increased resistance lead to the negative effect on the selectivity of the membrane as well. Additionally, as most zeolites are agglomerated, the proportion of the areas without filler increases, leading to decline of permeability. While the presence of amorphous particles leads to a decrease of the Pf. So the membranes containing amorphous particles display lower hydrogen permeability and selectivity (Figure 6. c).

Additionally, to minimize the overall resistance, fillers with short transport path length (tf) and high gas permeability (Pf) have to be applied. The results explained the distinct difference in permeability between the membranes prepared with nanosized (Nano-SOD) and micronsized (Micro-SOD) sodalite fillers, as shown in Figure 4. In the nanosized SOD crystals, the diffusion of hydrogen is faster. [40] While, in the micronsized sodalite crystals, not all cages are accessible and the diffusion is slower and even some cages are fully blocked.

For big molecules such as N2, CO2 and CH4, the sodalite cages are not accessible. The minimum total resistance is controlled by the interfacial voids. As the sodalite crystals are disperse well in the 10-nano-SOD-PEI membrane, the voids are separated and not connected. The selectivity of the membranes towards hydrogen in the H2/N2, H2/CO2 and H2/CH4 systems still maintains much higher than the Knudsen selectivity due to the molecular sieve effect of the nanosized sodalite crystals.

The Maxwell model is used to evaluate the H2/N2 gas separation performance of the PEI/Nano-SOD mixed matrix membrane. The experimental and theoretical results for permeability of H2 and N2 are summarized in Table S2. As shown, the experimental values are higher than those predicted by the Maxell model. The discrepancies may be primarily attributed to the low permeability of referenced pure sodalite membrane, which is mainly limited by the size of the sodalite crystals with distinctly longer diffusion path and bigger diffusion resistance. The small size voids also influenced the precision of this ideal gas separation model. S1. The permeation flux of H2 (~5000 Barrer) was much higher than those of N2 in the mixture showing that the membrane had higher size selectivity preference for H2. In comparison to the results from single gas separation test (~16.9 ideal separation factor), in the mixed component experiment a higher H2/N2 separation index (~28.5) was achieved. This is due to the preferential adsorption of hydrogen thus preventing nitrogen from passing through the SOD cages. Therefore, with the increase of H2/N2 ratio, the separation factors for H2/N2 is increased sharply. As shown in Figure 7, the H2 permeation increased, while the N2 permeation decreased when the feed gas ratio increased. An experiment with a H2/N2 proportion of 3 was performed, which is similar to the condition of simulated ammonia production. The 10-nano-SOD-PEI membrane possessed both unusual high separation index (~380) and permeance (~7700 Barrer), which would be of great significance for further industrial application. was above 215°C, which is in accordance with other reports. [41] The variation of the temperature (25°C -100°C) on the performance of the 10-nano-SOD-PEI was measured. The H2 permeation flux increased while the selectivity remains unchanged (Figure 8). These results demonstrate the great advantage of the MMMs containing SOD nanocrystals especially for the gas separation processes carried out at high temperature. The reproducibility and durability of the membrane were also examined. As shown in Figure 9, the separation factors and permeations over a 6-week period remained constant for the 10-nano-SOD-PEI membrane. This membrane could be used continuously over a long period of time proving the significant stability and recyclability; the 10-nano-SOD-PEI membrane was subjected to test for 6 months continuously. Besides, the same membrane was tested at different temperatures (25-100C) after continuous 3-weeks experiments. An increase in H2 permeability was observed due to the expansion of polymer after high temperature treatment. The membrane shows the same high selectivity. No differences of the membrane's before and after temperature tests are seen in the SEM images (Figure S19) demonstrating the high stability. The MMM consisting of PEI solution with 10 % nanosized SOD crystals reveals great improvement in gas separation. As shown in Figure 10, the MMM prepared in this work exhibits the highest H2 permeability (10-100 times higher), compared with other mixed matrix membranes. Although it shows slightly less selectivity than other mixed matrix membranes, it is important to note that the current MMM was not subjected to any modification. Additionally the thickness of the membrane could be further optimized. The MMM presented here reveals high hydrogen permeability similar to MOF and zeolites, while a higher selectivity at the same time (Figure S20). Besides the high gas separation performance, the high stability and easy processing make the membranes suitable for industrial application and more specifically for synthetic ammonia that requires vast hydrogen, and also releases large amounts of hydrogen.

Figure 10. H2/N2 gases separation performance of 10-nano-SOD-PEI membrane compared with literature data (PES/4A, [20] PES/13X, [20] PBI/Pd, [42] PI/ZIF-L, [43] PI/ZIF-8 [43] ) using Robeson's upper bound

Conclusions

In the present study, we used nanosized, homogeneously dispersed sodalite crystals as filler in the preparation of mixed matrix membranes. The membranes show high H2 permeability and H2/N2 selectivity. This membrane will be of great interest especially in synthetic ammonia purge gas purification.

The advantages of using SOD nanocrystals for the MMMs are: (1) proper aperture size of sodalite cages allowing only hydrogen to penetrate while nitrogen to is rejected; (2) nanosized sodalite crystals have large external surface area and shorter diffusion pathways originated from the small particle size (40 nm); (3) highly stable nanosized sodalite crystals in polymer matrix leading to the maximum utilization of their unique properties and preventing cage blocking and agglomeration. Finally, the mixed matrix membranes with good continuity, high stability and reproducibility demonstrated high H2 flux and selectivity.

(

  Figure S5 to S7). The pristine PEI membrane (Figure S5. a) is dense. The MMMs are continuous, and no obvious voids are observed (Figure S5. b and Figure S6) due to the high dispersion and stability of SOD nanocrystals in the PEI by adding of an amount (≤ 10 %). While, when the loading of SOD nanocrystals increases to 20 % or 30 %, some big holes are clearly formed, which is due to the agglomeration of sodalite nanocrystals (Figure S5. c). The SEM-EDS mapping also reveals the homogenous sodalite distribution in sample 10-nano-SOD-PEI, as well as the agglomeration of the Nano-SOD crystals in sample 30-nano-SOD-PEI. The pure SOD crystals (Nano-SOD), used as a filler for the MMMs, have a mean size of 40-50 nm, which is exhibited in SEM and TEM images (Figure 1). The crystal size is also calculated by the Scherrer equation, using the two most intense Bragg peaks at 14.1 ° and 24.5 ° 2Theta (Figure S2. b).

Figure 1 .

 1 Figure 1. (a) TEM and (b) SEM images of nanosized sodalite crystals

Figure 2 .

 2 Figure 2. Single gas permeability (PH2 and PN2) and ideal selectivity factor (Ideal S.F.) of MMMs at

Figure 3 . 1 .

 31 Figure 3. Performance (permeability and selectivity) of 10-nano-SOD-PEI and 10-nano-SOD H+ -PEI

Figure 4 .

 4 Figure 4. Performance (permeability and selectivity) of 10-nano-SOD-PEI, 10-micro-SOD-PEI and 10nano-SOD mod -PEI MMMs in gas separation

Figure 5 .

 5 Figure 5. Single gas permeability tests of 10-nano-SOD-PEI membrane at 25℃

Figure 6 .

 6 Figure 6. Schematic presentation of (a) 10-nano-SOD-PEI, (b) 30-nano-SOD-PEI, and (c) 10-nano-SOD H+ -PEI membranes

Figure 7 .Figure 8 .

 78 Figure 7. Influence of feed gas ratio (H2/N2) on gas separation performance of 10-nano-SOD-PEI

Figure 9 .

 9 Figure 9. Reproducibility and durability of 10-nano-SOD-PEI membrane for H2/N2 gas separation for 6

  Ge Yang, Hailing Guo*, Zixi Kang, Lei Zhao, Shou Feng, Feng Jiao, Svetlana Mintova* Page No. -Page No.

Acknowledgements

The authors gratefully acknowledged funding from Thousand Talents Program for Foreign Experts (WQ20152100284), the French-China Science Foundation (FFCSA), the National Natural Science Foundation of China (Grant No. 21673291), and the State Key Laboratory of Advanced Technology for Materials Synthesis and Processing (Wuhan University of Technology) (No. 2018-KF-13) and the International Associated Laboratory (LIA)-Zeolites.