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

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.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.