Metal-organic frameworks’ (MOFs) adsorption potential is significantly reduced by turning the original powder into pellets or granules, a mandatory step for their use at industrial scale. Pelletization is commonly performed by mechanical compression, which often induces the amorphization or pressure-induced phase transformations. The objective of this work is the rigorous study of the impact of mechanical pressure (55.9, 111.8 and 186.3 MPa) onto three commercial materials (Basolite C300, F300 and A100). Phase transformations were determined by powder X-ray diffraction analysis, whereas morphological changes were followed by nitrogen physisorption. Methane adsorption was studied in an atmospheric fixed bed. Significant crystallinity losses were observed, even at low applied pressures (up to 69.9% for Basolite C300), whereas a structural change occurred to Basolite A100 from orthorhombic to monoclinic phases, with a high cell volume reduction (13.7%). Consequently, adsorption capacities for both methane and nitrogen were largely reduced (up to 53.6% for Basolite C300), being related to morphological changes (surface area losses). Likewise, the high concentration of metallic active centers (Basolite C300), the structural breathing (Basolite A100) and the mesopore-induced formation (Basolite F300) smooth the dramatic loss of capacity of these materials.
Cyclohexane is a representative of volatile organic compounds
(VOCs).
VOCs can cause serious health problems in case of continuous exposure;
therefore, it is essential to develop efficient personal protective
equipment. Historically, activated carbons are used as VOC adsorbents.
However, the emergence of promising novel adsorbents, such as metal–organic
frameworks, has pushed the research to study their behavior under
the same conditions. In this work, the use of the well-known HKUST-1
MOF of different particle sizes (20 μm, 300–600 μm,
and 1–1.18 mm) for the adsorption of low-grade (5000 ppm) cyclohexane
combined with different water concentrations (dry, 27 and 80% RH)
in a fixed bed is proposed. The results were compared under the same
conditions for a typically used activated carbon, PICACTIF TA 60.
HKUST-1 has higher affinity to cyclohexane than PICACTIF for the whole
pressure range studied, especially at low partial pressures. It begins
to adsorb much earlier (0.0025 kPa) than the activated carbon (0.01
kPa). However, a different adsorption behavior is evidenced for both
materials in the presence of water vapor since HKUST-1 is very hydrophilic
in the zone near to the copper open metal sites, whereas PICACTIF
is hydrophobic. After three consecutive cycles, good stability results
were obtained for the MOF, comparable to activated carbon, even in
the presence of water. As the main finding, although the unstability
of HKUST-1 is well established under high humid conditions, the kinetic
of degradation has not been established so far. Here, it is shown
that the time usage of HKUST-1 as the adsorbent for respiratory mask
(single pass) is not affected by the degradation of the structure,
which may occur on a longer time scale. Finally, shaping by tableting
provides good results since it is possible to increase the MOF density
by around 69% with minor loss of adsorption capacity. The best fraction
is 300–600 μm, reaching cyclohexane breakthrough times
around 85 min/cm3 at 80% RH, comparable with PICACTIF-activated
carbon and promising for practical applications.
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