# Novel MOF paper composites for the removal of airborne pollutants from the environment of cultural heritage artefacts

Pierre Tignol, Vanessa Pimenta, Anne-Laurence Dupont, Silvia Carvalho, Moisés Pinto, Christian Serre, Bertrand Lavédrine

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## ► To cite this version:

Pierre Tignol, Vanessa Pimenta, Anne-Laurence Dupont, Silvia Carvalho, Moisés Pinto, et al.. Novel MOF paper composites for the removal of airborne pollutants from the environment of cultural heritage artefacts. Journal of Cultural Heritage, 2025, 71, pp.265-273. 10.1016/j.culher.2024.12.004. hal-04858301

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# Novel MOF paper composites for the removal of airborne pollutants from the environment of cultural heritage artefacts

Pierre Tignol,¹,² Vanessa Pimenta,² Anne-Laurence Dupont,¹ Silvia Carvalho,³ Moisés L. Pinto,³

Christian Serre,² Bertrand Lavédrine¹

## Affiliations

Pierre Tignol, Anne-Laurence Dupont, Bertrand Lavédrine

¹ Centre de Recherche sur la Conservation, Museum National d'Histoire Naturelle, Ministère de la Culture, CNRS, 75005, Paris, France

Email: bertrand.lavedrine@mnhn.fr

Pierre Tignol, Vanessa Pimenta, Christian Serre

² Institut des Matériaux Poreux de Paris, ESPCI Paris, Ecole Normale Supérieure, CNRS, PSL University, 75005, Paris, France

Email: christian.serre@espci.psl.eu

Silvia Carvalho, Moisés L. Pinto

³CERENA, Departamento de Engenharia Química, Instituto Superior Técnico, Universidade de Lisboa, Campus Alameda, 1049-001 Lisboa, Portugal

**Keywords:** Metal Organic Frameworks, porous solids, cellulose, VOCs, VVOCs, air quality, preventive conservation.
Highlights

* Porous materials utilized as adsorbents for the removal of pollutants are typically in powder form, which is not always convenient nor practical in the proximity of cultural heritage (CH) artefacts.

* A novel shaping, as a paper sheet loaded to up to 75 wt% of porous material is proposed and tested in the laboratory.

* The paper composite can be loaded with Metal-Organic Frameworks (MOFs), activated carbon or NaY zeolite.

* The use of the MOF MIL-100(Fe) makes the paper composite remarkably effective in selectively capturing very volatile and volatile organic compounds without releasing them even when moisture is present

* A very conclusive test was conducted using the MOF paper sheet to trap acetic acid from a degraded cellulose acetate film.
**Abstract**

The quality of indoor air in Cultural Heritage (CH) institutions is a prime concern for the preservation of the artefacts in the collections. When stored in enclosures such as boxes and cabinets, or when exhibited in display cases or poorly ventilated rooms, a buildup of pollutants, in particular very volatile (VVOCs) and volatile organic compounds (VOCs), can occur and be detrimental to the objects. These airborne pollutants arise from the slow deterioration of the objects themselves as well as from the outgassing of storage, exhibition and cleaning products and materials (wood, plastics, varnishes, paints, cleaning products). Carboxylic acids are a category of airborne pollutants that are particularly harmful to cultural goods, while aldehydes might be hazardous to human health. Adsorbents are often used to trap these pollutants, and currently activated carbons and zeolites are the most common ones. Nonetheless they are not compound-selective, their efficiency and regeneration ability are limited and they can re-emit the pollutants. Metal-Organic Frameworks (MOFs) and, specifically, the benchmark mesoporous Fe(III) trimesate MOF denoted MIL-100(Fe), have been recently reported to selectively capture carboxylic acids, even in the presence of moisture. In this work, we studied three adsorbents, namely activated carbon, NaY zeolite and the MOF MIL-100(Fe), shaped as paper sheets, by mixing them with cellulose fibers and nanofibers. Their performance for the capture of various target substances notoriously emitted by CH artefacts, namely formic acid, acetic acid, acrylic acid, and furfural was compared. The tests were carried out in passive adsorption mode, in a purpose-made exposure laboratory bottle, in the presence of moisture and at pollutant concentrations of a few hundred ppm. The MOF paper composite showed better efficiency than the other two paper composites, being 15% to 30% more efficient than the activated carbon paper at target substance saturation level and reaching adsorption capacities between 200 and 300 $mg_{pollutant} \cdot g^{-1}_{adsorbent}$ under the experimental conditions. Moreover, only the MOF paper sheet did not release the pollutants. In a view of a practical application, the MIL-100(Fe) paper sheet was tested for the adsorption of acetic acid vapours emitted by cellulose acetate films, and confirmed the experimental results. This innovative solution could therefore be of use for air purification in heritage institutions to better protect airborne pollutant-sensitive cultural artefacts.

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# 1. Introduction

In cultural heritage (CH) institutions, air quality plays a significant role in the preservation of the collections. For their protection against dust or pests or to provide passive environmental buffering, CH artefacts are generally stored in closed environments, either rooms, storage cabinets or showcases [1]. However, physical protection does not address the problem of airborne pollutants such as very volatile (VVOCs) and volatile organic compounds (VOCs), which, being locally emitted indoors can be harmful for the artefacts [2]. Wood [3], paints [4], adhesives [5], [6] and resins [7] have all been identified as VVOC and/or VOC emitters, in particular carboxylic acids which could be VVOCs (e.g. formic acid) or VOCs, acetic acid, acrylic acid). In confined spaces, significant concentrations of carboxylic acids can be reached, ranging from tens of ppb to a few ppm [8-13]. At these levels, damage is frequently observed on artefacts made of inorganic materials (metals [14-16], glass [17], ceramics [18,19]) and/or organic materials (natural [20,21] and synthetic polymers [22,23]), which indicates a high sensitivity of these materials to the airborne pollutants [24]. On the other hand, a VVOC such as formaldehyde, if not specifically reported in the literature as a potential risk for the artefacts, can be prejudicial to human health, and has in fact been categorized as carcinogen (category 1) and mutagen (category 2) by the European commission. Furfural (an aldehyde) and α-pinene (a terpene) are produced mainly from the degradation of cellulose-based polymer materials, wood products or cleaning products and are often present in the air of CH buildings [25-27]. The long-term preservation of collections therefore requires air purification whether through active or passive adsorption to eliminate or mitigate deleterious VOCs/VVOCs. VOCs and VVOCs have distinct physico-chemical properties and differentiate mostly by their boiling point and vapor pressure. Hence, all organic pollutants (VVOCs and VOCs) used in this work will henceforth be referred to as airborne pollutants or pollutants.

An increasing number of buildings are equipped with a heating-ventilation-air-conditioning (HVAC) system that allows air to circulate continuously into the rooms [28]. HVAC systems control the Relative Humidity (RH) around 50% and temperature (T) between 15 and 25°C, while providing efficient filtration of particulate matter [29]. However many artefacts are kept in enclosures with a very low air exchange rate, which favours the buildup of airborne pollutants [30]. To overcome this issue, passive filtration, which consists in introducing sorbent material in the enclosure, can be used [31]. Grontoft et al. applied this strategy in display cases at the Roman Fort Museum in Chesters (England) and found a 78% to 96% reduction in acetic acid
and formic acid concentrations, initially between 200 and 500 ppb [32]. This method therefore appears simple to implement and effective. However, efficiency depends on the nature of the sorbent [33].

The sorbent materials used in CH institutions are generally commercially available activated carbon (AC) and zeolites. AC is an amorphous, non-polar carbonaceous porous solid with a wide pore size distribution (micro to macropores ranging from less than 1 nm to more than 50 nm) and an average specific surface area of 1400 m².g⁻¹ [34,35]. Zeolites are hydrophilic, crystalline, microporous solids (pore size often between 0.3 and 1.3 nm) from the aluminosilicates family, with an average specific surface area of 800 m².g⁻¹ [36,37]. These materials are both produced in powder form and are commercially available in the form of beads, granules, and, for conservation purposes, clothes and/or papers [37]. The capacity of these different adsorbents to trap pollutants can be experimentally assessed. The experimental conditions for the tests need to mimic a reasonable array of real situations, namely: (1) medium relative humidity (30-60%), (2) atmospheric pressure, (3) relatively low pollutants concentrations (ppb – low ppm level) and (4) passive sorption in a closed container. Schieweck tested the performance of some shaped adsorbents (AC beads and fabrics, zeolite granules) for trapping airborne pollutants (formic acid, acetic acid, formaldehyde, toluene, α-pinene) in a 1 m³ exposure box at concentrations ranging from 0.5 to 1.5 ppm [38]. All sorbents exhibited a similar adsorption performance, close to 100% after 25-30 h exposure, but the full adsorption capacities were not provided. On the other hand, polar airborne pollutants might be released after adsorption under changed atmospheres. This can be ascribed to the low affinity of the AC towards these pollutants. In the case of (cationic) zeolites, the preferential adsorption of water over pollutants can be due to their strong hydrophilic character. In addition, the regeneration ability of AC was shown to be low, with 10% loss per regeneration cycle in the case of acetic acid [39]. It should be possible to overcome these shortcomings by using porous solids with higher selectivity and a low affinity for water, featuring specific adsorption sites targeting specific pollutants.

Metal-Organic Frameworks (MOFs) are hybrid, crystalline and micro-/mesoporous solids [40]. They are formed by the ion-covalent assembly of an organic linker (the ligand) and an inorganic building block, allowing for a tailored adjustment of the specific surface area, size and geometry of the pores, as well as chemical functionality [41,42]. Dedecker *et al.* demonstrated the excellent ability of Zr-based hydrophobic microporous MOFs (namely UiO-66-2CF₃ and MIL-140B) to selectively capture acetic acid (AcOH) in the presence of water in passive sorption conditions [43]. The authors built a pollutant test chamber (V = 2.9 dm³) with controlled RH
(40%) with a photoionisation detector (PID) for the continuous monitoring of VOCs at room temperature. The two MOFs exhibited a better performance than zeolite (NaY) and AC (RB4), with AcOH concentration dropping from 200 µg.dm⁻³ (or 80 ppm) to 4.2, 9.5, 20.2 and 50 µg. dm⁻³ (or 1.7, 3.8, 8.1 and 20 ppm), respectively. This result was assigned to a good compromise between their moderate affinity for the polar VOCs and their relative hydrophobic character. Very recently, the perfluorinated MIL-53(Al)-CF₃ was shown to be even more efficient for the capture of acetic acid under similar ambient conditions, due to both a unique combination of confinement and stronger host-guest interactions [44]. However, all these MOFs are either very expensive, mainly due to the cost of the perfluorinated ligand, or hardly scalable under green chemistry conditions, which likely rules out their use in art preservation.

In parallel, Severino *et al.* have demonstrated the capacity of MIL-100(Fe) to be extremely selective for the capture of AcOH [45]. According to a recent study by Severino *et al.*, the production cost at industrial scale in the same experimental conditions as in the laboratory should be below $30 per kg of MIL-100(Fe) [46]. This easily scalable MOF is typically synthesized by mixing Fe³⁺ metal ions and trimesic acid (benzene-1,3,5-tricarboxylic acid) in water at atmospheric pressure [45,47]. The assembly of iron trimers and trimesate leads to the formation of a zeo-type porous network with two mesoporous cages (2.5 and 2.9 nm wide), accessible through microporous windows featuring Lewis acidic open metal sites (OMS) (Figure 1) [48]. These OMS are, under ambient conditions, occupied by water molecules due to a high interaction energy (-74.6 kJ.mol⁻¹). Remarkably, through a chemisorption mechanism, acetic acid molecules (adsorption enthalpy of -130.5 kJ.mol⁻¹) can easily replace water molecules, at any moisture content, thereby achieving a similar performance in passive sorption mode as the best perfluorinated MOFs. Recently, it was shown that, once placed into a box containing a cellulose acetate film, MIL-100(Fe) could selectively trap the AcOH emitted [49] with an approximate tenfold reduction over one month of exposure. Noteworthy, the real field conditions performance was close to the one observed in the laboratory. However, the tests were carried out using the powder MOF, placed in pouches, which, not being totally impractical, can lead to particles shading through. This is not advisable both in contact with the artefacts and for technical staff. However, MIL-100(Fe) is biocompatible [50] and has emerged as a suitable candidate for the preservation of cellulosic and photographic materials [49].
A more practical and safe shaping is thus advisable. MOF shaping should preferably involve a
simple, fast and green process, by using low toxicity precursors, and produce a functional
product with the highest possible MOF content, and properties as close as possible to those of
the powder. Beads and granules are commonly used, but they can lead to structural
modifications of the MOF, diffusion problems, poisoning of preferential adsorption sites (e.g.
OMS) and/or pore blockage. In our previous work, we reported a new, green and simple
method, that combines the use of wood pulp fibers and nanocellulose, to produce a paper sheet
loaded with a wide range of adsorbents (MOFs, zeolite, activated carbon) at a very high loading,
of at least 75 wt% [51]. In this procedure MOF particles are mixed with a few wt % of softwood
fibers (S) and nanofibrillated cellulose (NFC) in water at room temperature for 15 minutes prior
to filtration. The cellulosic composite formed is a fibres-MOF network with sufficient fibre
bonds, as S provides flexibility, while NFC acts as a microstructuring and mechanical
strengthening agent. The 75 wt% MIL-100(Fe) paper sheet exhibited good flexibility and
tensile strength properties, enabling an easy handling. Moreover, the AcOH adsorption
properties were totally unaffected and no significant penalty in terms gas diffusivity was
observed. With these characteristics, the MOF paper sheet outperforms classical mechanically-
shaped products (beads, granules) where a poisoning of the OMS was observed [51].

The aim of this work was to compare the performance of paper sheets with a very high loading of different porous solids, namely MIL-100(Fe), activated carbon and zeolite, for the capture of various airborne pollutants: polar ones such as formic acid (VVOC), acetic acid and acrylic acid (VOCs), as well as intermediate polarity furfural and the hydrophobic α-pinene (VOCs). The tests were carried out in passive adsorption mode in a purpose-made exposure laboratory bottle, and in the presence of moisture (RH = 35 – 55 %), by comparing the adsorption capacity and eventual release of pollutants. Finally, the AcOH adsorption capacity of MIL-100(Fe) paper composite was tested in the presence of cellulose acetate films, approaching real use conditions.

**2. Materials and methods**

Materials

All chemicals were used without further purification. 1,3,5-benzenetricarboxylic acid, 98%
(Alfa Aesar), 3,5-pyrazoledicarboxylic acid monohydrate (98%, Alfa Aesar), α-Pinene, 98%
(Sigma-Aldrich), Activated charcoal (Fisher Scientific), Glacial acetic acid, 100% (Supelco), Acrylic acid, 99% (Sigma-Aldrich), Aluminium(III) hydroxide hydrate (Alfa Aesar), Formic acid, >98% (Sigma-Aldrich), Ethanol absolute anhydrous (Carlo Erba reagents), Furfural, 99% (Sigma-Aldrich), Iron fine powder (Riedel-de Haën), Nanofibrillated cellulose (Celova, Weidmann), Nitric acid 65% (Carlo Erba reagents), Softwood pulp (HWBK, Canson), Zeolite Y, sodium 5.1 SiO₂:Al₂O₃ (Alfa Aesar).

### MOFs synthesis:

MIL-100(Fe) and MOF-303(Al) were both synthetised in water. Protocoles are described in SI.

### Paper composites preparation

All the 75 wt% porous solids loaded paper composites were prepared following conditions previously reported [51]. Protocoles are described in SI.

### Adsorption capacity of the porous solids-based papers

The exposure laboratory bottle set-up (Figure S1) was composed of a borosilicate glass bottle (Duran®) (V = 0.5 dm³) and a photoionisation detector (PID) (ppbRAE3000+, Honeywell) was developed in order to carry out adsorption tests for various airborne pollutants (formic acid, acrylic acid, acetic acid, furfural, α-pinene). A 10.6 eV lamp was used for the detection of acetic acid, acrylic acid and furfural and a 11.7 eV lamp was used for the detection of formic acid. The detection range of the PID is comprised between 0 and 10 000 ppm. The teflon lid of the laboratory bottle has three inlets each closed by a valve. The first inlet (blue circle) is connected to fresh air from outside, filtered through an activated carbon filter. The second inlet (green circle) connects the laboratory bottle to the PID. The third inlet (orange circle) is used for the VVOC or VOC injection with a glass syringe ($V_{injected} = 1 \mu L$).

A logger is placed into the laboratory bottle to monitor the relative humidity (RH) and the temperature (T). Between tests, the exposure laboratory bottle is cleaned by circulating filtered air until no pollutant is detected by the PID. The 3 valves are then closed followed by the injection into the laboratory bottle of a single VVOC (formic acid) or VOC (acrylic acid, acetic acid, furfural or α-pinene), which is left standing for 30 minutes to allow homogeneous dispersion. The exact airborne pollutant concentration in the laboratory bottle is measured by the PID. After the incubation period, the PID pumps the air out of the laboratory bottle at a rate of 0.5 L.min⁻¹, which is replaced by fresh air, resulting in a decrease in the concentration of
detected airborne pollutants until it reaches 0 ppm. The whole procedure is repeated three times for each VVOC or VOC.

To evaluate the adsorption capacity of the 75 wt% porous solid loaded paper sheets, measurements were performed in the presence of the adsorbent in the laboratory bottle: 50 mg of paper sheet (MIL-100(Fe), AC, zeolite NaY) is first inserted into the laboratory bottle (Figure 2), the following steps being identical to the procedure described above. The adsorption of a sulphite softwood pulp paper (100% cellulose, no additives) (12.5 mg) was measured as reference to draw the baseline. The baseline integrates of the concentration vs. time curve during the purge of the laboratory bottle allows to assess the full amount of airborne pollutant. This is calculated as follows:

$$V_R = \frac{D_{PID} \times \Delta_t \times C_{pollutant} \times \sum ppm}{\rho_{pollutant}}$$

Where $V_R$ is the volume of pollutant in the laboratory bottle (µL), $D_{PID}$ is the flow rate of the PID pump (L.min⁻¹), $\Delta t$ is the interval between each PID measurement (0.5 min), $C_{pollutant}$ is the mass concentration of the air pollutant (µg.dm⁻³), $\sum ppm$ is the integration of the air pollutant detected over a 0.5 min time interval (ppm). $D_{PID}$ (0.5 L.min⁻¹) and $\Delta t$ (0.5 min) are constant and $\rho_{pollutant}$ is the density of the pollutant (g.dm⁻³).

## Release tests

After the adsorption tests, the porous solids-based paper sheets were individually placed in a hermetically sealed glass vial (V = 0.5 L) in presence of a specific passive dositube for the detection of carboxylic acids (Gastec, No. 81D) or aldehydes (Gastec, No. 91D), depending on the pollutant adsorbed. Pollutant concentration in the vial was deduced after 24 hours at room temperature (25°C).

## Adsorption kinetics profile of AC and MIL-100(Fe) paper sheets

Adsorption tests were carried out in the same way as above, varying the exposure time of the MOF paper sheet to VVOC (formic acid) or VOCs (acetic acid, acrylic acid or furfural) in the laboratory bottle from 15, 30, 60 to 90 minutes. The AC paper sheet was exposed to the same pollutants for 30 and 90 minutes.

## Adsorption behavior of AC and MIL-100(Fe) paper at saturation
To achieve pollutant saturation of the composites, the exposure time of each adsorbent was increased to 1.5 hours. The mass of adsorbent was determined from the work by Severino *et al.* on MIL-100(Fe) and acetic acid powder [45]. According to their experiments, at a partial pressure of 0.022, MIL-100(Fe) powder can adsorb up to 6.5 mmol.g⁻¹ of AcOH. However, 1µL (1.05 mg) of acetic acid into a volume of 0.5 dm³ corresponds to a partial pressure of 0.027 (details of the calculation given in SI). Given the adsorption capacity of the MIL-100(Fe) powder (6.5 mmolₐcOH·g⁻¹ₘOF), this yields 2.6 mg of powder, corresponding to 3.5 mg of MOF composite paper sheet (containing 75wt% MIL-100(Fe)). The same mass of composite (3.5 mg) was used with the other pollutants as well.

## Practical use of MIL-100(Fe) paper sheet

To determine the adsorption capacity of MIL-100(Fe) paper sheet for pollutants emitted by a cellulose triacetate motion picture film, a breakthrough test (active sorption) was set-up (Figure S2). This system comprises a 1L laboratory bottle containing a reel of 35 mm cellulose triacetate film (m = 276 g) with an A-D strip® ranking between 2 and 3. Between the laboratory bottle and the PID, an inline filter holder was inserted. The pollutant concentration was measured until a stable continuous value was obtained. Then the 75 wt% MIL-100(Fe) paper sheet was placed in the filter holder (m = 275 mg). The pollutant measurement was performed until the concentration measured by the PID reached the same value as measured without the paper composite, indicating saturation.

# 3. Results and discussion

## 3.1 Adsorption performance of the porous solids-based papers

The initial airborne pollutant concentration in the air volume of the laboratory bottle was a few hundred ppm (125 - 600 ppm, depending on the pollutant (Table 1, Figure S3.a-S6.a), corresponding to the injected volume of 1 µL into the laboratory bottle (at 35% to 55% RH). The 75 wt% porous solids (activated carbon, NaY zeolite and MIL-100(Fe)) loaded paper sheets tested for pollutant capture are shown in Figure 2. The adsorption capacity of 75 wt % MIL-100(Fe), activated carbon and NaY zeolite loaded papers (Figure 3, Tables S1) was determined from testing a sample mass of 50 mg and an exposure time of 30 minutes to each pollutant (Figure S3-S6). The adsorption profiles for the carboxylic acids re similar, where the porous
solid based paper sheets have an adsorption capacity 4 to 6 times higher than the reference sulphite pulp paper (3.5 ± 0.8 to 5.8 ± 1.3 mgpollutant·g⁻¹adsorbent), indicating that cellulose plays a minor role in trapping the airborne pollutants. Activated carbon and MIL-100(Fe) paper sheets have almost identical performance, with adsorption capacities ranging from 19.2 ± 0.3 to 23.7 ± 0.1 mgpollutant·g⁻¹adsorbent. The zeolite paper sheet, has a lower adsorption capacity, between 17 ± 0.5 and 22 ± 0.4 mgpollutant·g⁻¹adsorbent.

The adsorption profile is slightly different with furfural, an aldehyde with intermediate polarity (Figure S6.b-d). The performance of the zeolite paper sheet is very poor, falling below that of the reference sulphite pulp paper. On the other hand, MIL-100(Fe) paper sheet shows improved efficiency compared to AC paper sheet, with adsorption capacities of 23 ± 0.03 and 20.5 ± 0.3 mgpollutant·g⁻¹adsorbent, respectively. In the literature, the capture of furfural using adsorbents is often carried out in aqueous solution [52]. For instance, the adsorption capacity of a zeolite (e.g. ZSM-5, SBET < 500 m²·g⁻¹), an activated carbon (SBET = 1100 m²·g⁻¹) and a MOF (e.g. MIL-53(Al), SBET = 1500 m²·g⁻¹) and have been reported to be about 60, 375 and 593 mgfurfural·g⁻¹adsorbent, respectively [53-55]. Our results follow the same trends and can be partially explained by the respective specific surface area of the adsorbents (SBET(zeolite) < SBET(AC) < SBET(MOF)). Furfural is also a fairly intermediate polarity VOC, but its miscibility in water is low, which could explain the poor performance of the zeolite, the latter being highly hydrophilic. It is interesting to note that with a non polar VOC (e.g. α-pinene), MIL-100(Fe) paper sheet was less efficient than AC paper sheet, with an adsorption capacity of about 9 ± 1 and 16.7 ± 0.05 mgpollutant·g⁻¹adsorbent, respectively (Figure S7, Table S2). This can be attributed to the favorured water sorption, [56], as MIL-100(Fe) exhibits an amphiphilic character while AC is hydrophobic (as shown by the water adsorption isotherms in Figure S8). In the presence of moisture during the capture tests (35% to 55% RH), this results in a lower adsorption capacity for the MOF paper composite compared to the AC paper sheet. For these reasons, the capture of α-pinene was not studied further.

In summary, these comparative tests showed an efficient adsorption of polar pollutants by MOF and AC paper sheets with similar adsorption capacities for carboxylic acids and a higher capacity of the MOF paper sheet for furfural. The zeolite paper sheet was slightly less efficient for carboxylic acids, and completely ineffective for furfural. It is noteworthy that at average room conditions, the adsorbate can be released from the adsorbent and become a source of pollution, which, in a CH environement can be problematic due to the low airborne pollutants
levels acceptance and/or low or no air renewal. To evaluate this aspect, release tests of porous solids-based paper sheets were carried out at room temperature.

## 3.2 Pollutant release tests

After the adsorption tests, the composite paper sheets loaded with 75 wt% porous solids were individually placed in a vessel together with a passive sampling tube, in order to measure the potential release of organic vapours **Table 2** shows the concentrations of the various pollutants measured. Remarkably, unlike activated carbon and zeolite paper sheets, the MOF paper composite did not release any pollutant after one day. This can be explained by the strong interaction between the pollutants and MIL-100(Fe), especially for carboxylic acids [44]. However, when it comes to furfural, the interaction energy with the Fe trimers of the MOF is lower. As suggested by Severino *et al*. the enthalpy of adsorption of and aldehyde (R-CHO) is close to that of water (-74.6 kJ.mol⁻¹) [45]. However, since the boiling point of furfural is high (162°C), this could explain why no release was observed from the MOF paper. On the other hand, AC has no such anchoring Lewis acid sites in its pores to capture the airborne pollutants [34] and cationic zeolites are very hydrophilic [57], suggesting a greater affinity for water molecules over pollutants. This would explain the more pronounced release of pollutants from zeolite than from AC. It has to be noted that the lower the pollutants’ boiling point (Bp,formic acid < Bp,acetic acid < Bp,acrylic acid < Bp,furfural), the higher the volatility and the more likely the re-emission of vapours from the porous solid. Both the MIL-100(Fe) and AC paper composites showed slightly similar adsorption capacities while the former was the only to exhibit a pollutant release-free behavior. Thus, these two samples were selected for further work.

## 3.3 Adsorption kinetics profile of AC and MIL-100(Fe) paper sheets

The adsorption kinetics of pollutants by the MOF paper sheet were first studied by varying the exposure time in the experimental laboratory bottle (from 15 to 90 min) (Figure 4, S9-12). After 15 minutes, 84.9 ± 1.5 to 95.2 ± 0.7 % of the 1 µL injected (depending on the pollutant) were adsorbed (Table S3). When the exposure time of the paper sheet to the pollutants was extended, adsorption continued. After 90 min, all the injected pollutants were adsorbed, suggesting that the MOF was not saturated yet. The same trend was observed for the AC paper sheet, with measurements in the laboratory bottle at the two different exposure time intervals (30 and 90 min) (Figure S13-S16, Table S4). These fast kinetics can be explained by the shaping
of the MOF into paper, which allows: (1) a large surface area which favours the exposure to the pollutants and (2) a high permeability through macropores that facilitate vapour diffusion [49]. To better illustrate the microstructure of the samples, computed tomography (CT-scan) was carried out. The samples revealed a homogeneous distribution of porous solid particles in the paper composite (Figure 2). In addition, the video of each sample (SI file) clearly shows the homogeneous spatial distribution of particles within the cellulose matrix where micron-sized macropores can be observed randomly throughout the volume. In further tests, in order to compare the adsorption capacity of activated carbon and MIL-100(Fe) paper sheets at saturation, the exposure time of 90 min was used and the mass of sample was lowered (details in the experimental section).

## 3.4 Pollutants adsorption behavior at saturation

The adsorption capacity of 75 wt % MIL-100(Fe) and AC loaded paper sheets (Figure 5.a, Tables S5) were measured for each pollutant (Figures S17-S20). MIL-100(Fe) paper composite exhibits high adsorption capacities of 283 ± 10.8; 245 ± 3.5; 273 ± 2 and 216 ± 1 mg$_{pollutant}$.g⁻¹$_{adsorbent}$ (or 6.15 ± 0.2; 4.07 ± 0.06; 3.79 ± 0.02 and 2.25 ± 0.05 mmol$_{pollutant}$.g⁻¹$_{adsorbent}$) for formic acid, acetic acid, acrylic acid and furfural, respectively, while those of AC paper sheet are slightly lower, i.e. of 192.5 ± 16.5; 206 ± 3.3; 241 ± 3 and 178 ± 1 mg$_{pollutant}$.g⁻¹$_{adsorbent}$ (or 4.15 ± 0.4; 3.43 ± 0.06; 3.35 ± 0.02 and 1.85 ± 0.05 mmol$_{pollutant}$.g⁻¹$_{adsorbent}$), respectively. MIL-100(Fe) thus outperforms AC by 15 to 30%, while maintaining its crystalline structure intact regardless of the pollutant adsorbed (Figure S21). For a given adsorbent (MIL-100(Fe) or AC), it is interesting to note that the higher the molar mass (M) of the pollutant (M$_{formic acid}$ < M$_{acetic acid}$ < M$_{acrylic acid}$ < M$_{furfural}$), the lower the molar adsorption capacity (expressed in mmol$_{pollutant}$.g⁻¹$_{adsorbent}$), while the mass adsorption capacities (expressed in mg$_{pollutant}$.g⁻¹$_{adsorbent}$) are close. As the volume injected for each pollutant is constant (1µL), this corresponds to decreasing molar quantities of the compounds (Table 1), and ultimately to proportionally lower molar adsorption capacities. When the pollutant exposure time of the MOF paper sheet was extended overnight, the adsorption capacities were the same, indicating that the saturation level had already been reached after 90 minutes (Figure S22, Table S6). The better performance of MIL-100(Fe) paper sheet compared to the AC paper sheet despite its slightly more hydrophilic character can be explained by its higher BET specific surface area (S$_{BET,MIL-100(Fe)}$ = 1540 ± 13 m².g⁻¹ vs S$_{BET,AC}$ = 1150 ± 2 m².g⁻¹) [29] as well as by the strong interactions with the pollutants, especially for carboxylic acids, due to the Lewis acidic sites (Figure S9.b) [45].
Interestingly, in the specific case of AcOH, the adsorption isotherms at 25°C of the MOF as powder and shaped as paper sheet were measured and showed similar performance (Figure 5.b). In the exposure laboratory bottle, the amount of AcOH adsorbed was 5.5 mmol<sub>AcOH.g<sup>-1</sup>MOF</sub> (or 245 mg<sub>AcOH.g<sup>-1</sup>MOF</sub>) at a partial pressure of 0.027 (details of the calculation in SI). This lines up with the corresponding value on the isotherm (Figure 5.b), where the adsorption capacity is about 6.8 mmol<sub>AcOH.g<sup>-1</sup>MOF</sub>. The slight difference might be ascribed to different detection methods: the indirect method used (PID) while the amount of AcOH adsorbed in the case of an adsorption isotherm is directly deduced from the microbalance values.

In the literature, other MOFs have been tested for AcOH capture in the presence of moisture. So *et al.* tested MIL-68(Al) [58], a MOF composed of corner sharing chains of metal octahedra [AlO₄(OH)₂] and 1,4-dibenzenedicarloxate ligand to form a 3D structure with trigonal and hexagonal 1D channels. Despite tests carried out at much higher acetic acid concentrations (1000 ppm, P/P₀ ~ 0.035), the authors report an adsorption capacity of 79.5 mg<sub>AcOH.g<sup>-1</sup>MOF</sub>, three times lower than MIL-100(Fe) paper sheet, despite the higher S<sub>BET</sub> (about 1778 m².g⁻¹ for MIL-68(Al)). These results confirm the outstanding performance of MIL-100(Fe) paper sheet for pollutants capture, which allows to envision its use for optimizing the preservation of cultural artefacts.

Formaldehyde, which has the potential of being hazardous to human health, can also be found in CH environments [26]. Recently, Sadovnik *et al.* reported an aluminum pyrazole dicarboxylate MOF, denoted MOF-303, for the selective capture of formaldehyde through a chemisorption mechanism on the pyrazole group forming oxymethyl group [59]. This MOF exhibited a high adsorption capacity (i.e. several mmol<sub>pollutant.g<sup>-1</sup>MOF</sub>) and leak-free storage at room temperature. In that context, we performed a preliminary experiment with MOF-303 and successfully produced a 75 wt% loaded paper composite (Figure S23). The characterizations indicated similar features as the MIL-100(Fe) paper sheet, which hints a potential to be used in CH environments to limit the levels of formaldehyde. The capture of other noxious pollutants (e.g. H₂S, NOx), notoriously very harmful towards CH artefacts as well as for human health, could also be specifically targeted. Previous studies have shown that MIL-100(Fe) in its powdered form, was very efficient to capture (H₂S) or to convert (NOx) these gases [60,61], which holds the promise of expanding even further the potential of MIL-100(Fe) paper sheets for the preventive conservation of CH artefacts.

### 3.5 Example of a practical use of MIL-100(Fe) paper sheet
To approach real CH situations, the performance of MIL-100(Fe) paper sheet was evaluated in the presence of cellulose acetate based artefacts, which are known to release AcOH upon ageing, and especially motion picture films [62-64]. Recently, Al Mohtar et al. showed that MIL-100(Fe) paper sheet was compliant with the photo-activity test (ISO 18916:2007) and did not emit volatile compounds that would be detrimental to cellulose-based materials [49]. Kammer et al. analysed real cellulose acetate-based films emissions using proton-transfer-reaction mass spectrometry (PTR-MS) and observed a continuous release of AcOH (analysis for 5 minutes after equilibrium was reached in the laboratory bottle) [20]. A large number of other pollutants emitted by the films were also detected, arising from the additives used for the films manufacture. However, for the highly degraded films, they observed that the AcOH proportion was higher than 90% of the total airborne pollutant (TVOC) balance.

An acetate-based motion picture was placed in a laboratory bottle connected to the PID (more details on the set-up in the materials and methods section, Figure S2). A continuous concentration of 48 ppm AcOH in the laboratory bottle was monitored during 2-3h, corresponding to an emitted flux of 1070 ng.s⁻¹ (detailed calculation in SI file). To determine the experimental AcOH adsorption capacity of the MIL-100(Fe) paper sheet, a breakthrough study was carried out by forcing the airflow charged with airborne pollutants from the laboratory bottle through the MOF composite (Figure S24.a). The concentration of pollutant detected by the PID fell sharply, reaching almost 0 ppm and remained null for 2 h before rising again, indicating that the saturation of the composite was gradually reached. At the end of the test, the AcOH concentration detected in the laboratory bottle (~36 ppm) was lower than the initial one (~48 ppm), possibly due to the continuous purge of the film, leading to less emissions. A linear decrease in the concentration was taken into account when calculating the amount of AcOH adsorbed (Figure S24.b). The calculated adsorption capacity for MIL-100(Fe) was 137 mgAcOH.g⁻¹MOF (or 2.28 mmolAcOH.g⁻¹MOF, calculations given in SI). In other words, the concentration of AcOH vapors detected (36 < C < 48 ppm) corresponds to low partial pressure (0.0017 < P/P₀ < 0.0023), representing an adsorption capacity between 2.1 and 2.3 mmolAcOH.g⁻¹MOF according to the AcOH isotherm at 25°C for MIL-100(Fe) paper sheet (Figure 6). Thus, the AcOH adsorption capacity deduced from the breakthrough experiment and the pure AcOH isotherm at the same temperature and partial pressure were similar, confirming once again the high selectivity of MIL-100(Fe) paper for AcOH in a situation mimicking real in-field exposures. The latter encompasses being exposed simultaneously to a complex mixture of airborne pollutants in the presence of moisture. The new solution thus enables a limited use of
cold or cool storage in film archives, thereby leading to a notable reduction in their carbon footprint as suggested in [49].

## Conclusions

The results of this research carry substantial implications for the cultural heritage conservation field. Firstly, they underscore the feasibility of manufacturing paper sheets containing a very high level of adsorbent fillers, such as activated carbon, zeolites, or MOFs. Furthermore, MOF MIL-100 has demonstrated exceptional effectiveness in capturing carboxylic acids and a range of other VVOCs and VOCs. This not only enhances its value as a useful resource for preventive conservation of cultural heritage but also underscores its potential benefits for human health. The paper composites are made under green chemistry conditions with biosourced materials: cellulose fibers and nanofibers. The very high loading achieved of 75 wt% is unprecedented and applies not only to MOF (MIL-100(Fe)) but to other porous solids, such as activated carbon and NaY zeolite. The methodology is versatile and enables to envision a similar production of other types of cellulose-adsorbent composites to efficiently and selectively target various airborne chemical compounds in a sustainable way. These paper composites feature a very high specific surface area to maximize the exposure and the capture of pollutants, with a macroporous structure that favours fast diffusion. The adsorption tests were carried out in a purpose-built exposure laboratory bottle at medium RH (35 – 55 %) and airborne pollutants concentrations of several hundred ppm. Not only the MOF composite exhibited similar sorption capacities as the bare MOF powder, but it showed a significantly better performance than the zeolite and AC paper sheets, with higher pollutants adsorption capacities and a remarkable leak-free vapour storage. Finally, we demonstrated that the AcOH adsorption capacity of MIL-100(Fe) paper composite once exposed to AcOH vapours as well as to numerous other pollutants from a real old cellulose acetate film and in the presence of moisture, remains remarkably unchanged. More generally, it confirms the suitability of the MOF paper sheet to be used in museum and archives cabinets and conservation boxes, where CH artefacts are stored and maintain the adequate air quality that will considerably extend their lifetime. Due to the intrinsic properties of MIL-100(Fe), paper composites loaded with other MOFs can be useful also for the capture of other categories of noxious pollutants, such as NOx and H₂S. Lastly, the development of a colorimetric detector would be useful for monitoring the saturation level of the MOF paper sheet.
**Conflict of Interest**

The authors declare no conflict of interest.

**Acknowledgements**

This work was funded by the European Union Horizon 2020 research and innovation program under the grant agreement No 760801\Nemosine, and by Fundação para a Ciência e a Tecnologia (FCT) in the scope of the projects UIDB/04028/2020 & UIDP/04028/2020 (CERENA). Our thanks to Karine Janel, Nathalie Marlin and Gérard Mortha from INP Grenoble, for the access and training of the pulp handsheet machine at Pagora (LGP2, UMR5518), Pierre-Aimé Romulas and Sarah Pinto Gomes for their technical help in some of the adsorption tests during their internship, to Abeer Al Mohtar for measuring the AcOH isotherm of the MIL-100(Fe) paper sheet, to Weidmann for providing NFC, and to Georges Mouchaham for helping to draw porous solids structures on Diamond software at ESPCI (IMAP, UMR8004).

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<figure><img src="image_3.png" /><figcaption>**Figure 6.** AcOH adsorption isotherm at 25°C of the 75 wt % MIL-100(Fe) loaded paper sheet in the P/P₀ 0-0.005 range.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Figure 1.** Scheme illustrating the construction of MIL-100(Fe) (view of the small mesoporous cage). Iron polyhedras, carbons and oxygens are represented in orange, grey and red, respectively (hydrogen atoms not represented).</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Adsorption capacity of paper sheets (m = 50 mg) containing 75 wt % of porous solid (MIL-100(Fe), activated carbon or NaY zeolite) and cellulose (m = 12.5 mg) of several pollutants (formic acid, acetic acid, acrylic acid and furfural) in the exposure laboratory bottle (RH=35-55%) after an exposure time of 30 minutes.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Adsorption kinetics of 1 µL of different pollutants (formic acid, acetic acid, acrylic acid, furfural) by a 75 wt% MIL-100(Fe) loaded paper sheet (m = 50 mg).</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Figure 5.** a) Comparison to the adsorption capacity of paper sheets (m = 3.5mg) containing 75 wt % of MIL-100(Fe) or activated carbon for several pollutants (formic acid, acetic acid, acrylic acid and furfural) in the exposure laboratory bottle (RH=35-55%) after an exposure time of 90 minutes. b) AcOH isotherm at 25°C of MIL-100(Fe) powder and 75 wt % loaded MIL-100(Fe) paper sheet.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Figure 2.** Scheme of the exposure laboratory bottle for the adsorption tests of various airborne pollutants with adsorbents in the form of paper sheets loaded with 75% wt porous solids and their respective computed tomography scan.</figcaption></figure>
<table><caption>**Table 1.** Initial concentrations measured by the PID after injection of 1µL of pollutant (formic acid, acetic acid, acrylic acid or furfural) in the air volume of the exposure laboratory bottle.</caption><thead><tr><th>Pollutant</th><th>Initial concentration (ppm)</th><th>Quantity injected (mmol)</th></tr></thead><tbody><tr><td>Formic acid</td><td>260 ± 17</td><td>25.5</td></tr><tr><td>Acetic acid</td><td>570 ± 16</td><td>17.5</td></tr><tr><td>Acrylic acid</td><td>410 ± 13</td><td>14.5</td></tr><tr><td>α-pinene</td><td>125 ± 5</td><td>6</td></tr><tr><td>Furfural</td><td>180 ± 25</td><td>12</td></tr></tbody></table>
<table><caption>**Table 2.** Concentrations of pollutants (formic acid, acetic acid, acrylic acid, furfural) individually measured by a Gastec® passive tube in the presence of paper consisting of 75 wt% porous solids after adsorption tests.</caption><thead><tr><th>Paper composite</th><th>Formic acid (ppm)</th><th>Acetic acid (ppm)</th><th>Acrylic acid (ppm)</th><th>Furfural (ppm)</th></tr></thead><tbody><tr><td>MIL-100(Fe)</td><td>0</td><td>0</td><td>0</td><td>0</td></tr><tr><td>Activated Carbon</td><td>3.3</td><td>0.8</td><td>0.4</td><td>0.2</td></tr><tr><td>Zeolite NaY</td><td>4.2</td><td>5</td><td>3.4</td><td>-</td></tr></tbody></table>