The Catalyst Review December 2019 - 12

SPECIAL FEATURE
compression without the use of additional binders. A
Medel'Pharm STYL'ONE Evolution tableting instrument
was used for shaping purpose. Prior to compression,
powders were deagglomerated by grinding. The die
was then filled with 200 mg of UiO-66 or 300 mg
of HKUST-1 powder. Punches diameter of 1.1 cm
were used, allowing a tableting surface of 1.00 cm².
The effect of tableting pressure on the final tablet
properties including density, specific surface area (SSA,
BET method) and hardness is presented in Table 1.

Table 1. Textural properties and resulting volumetric surface of optimized MOF tablets.

Source: Dhainaut et al. 2017

The study confirmed that the porous structure of MOFs degrades when compression pressure is applied at given threshold which is
MOF depending. For UiO-66, no loss of SSA has been observed until 18 MPa, resulting in a 2.5 increase of the density with respect
to the tap powder density. For HKUST-1, a good compromised was found at 121 MPa offering appropriate pellet hardness which a
minimum of loss of surface area. For all MOF studied (not all shown here), powders can be pressed without the use of additional
binders, yielding tablets of up to a two-fold their corresponding tap density at the expense of slightly reduced micropore volume and
surface area.
Compaction-granulation has been studied by Johnson
Table 2. Summary of data obtained from the recycling fines study.
Matthey for a range of MOFs including MIL-100(Fe), HKUST-1,
CPO-27(Ni). In large scale applications the fines, particles
below 400 µm, are recycled back into the forming method.
How this recycling would impact the performance of theses
MOFs was unknown. This is important since assuming a
steady state recycling of fines with 70% loss per compaction,
25% of the material constituting a pellet has been compacted
5 times or more. To investigate this an experiment looking
Source: Hindocha and Poulston 2017.
at how fine recycling would affect material porous structure
was completed. Samples were compacted and the fines
obtained re-compacted a total of five times. MOF powders were passed through a roll compactor set to a certain pressure (50 - 125
bar). The resulting ribbons were crushed and sieved to give particles with a 300-1000 mm size range (Table 2).
The powder XRD data of the samples shows that repeated compaction does
not have an impact on the MOF structures of MIL-100(Fe), CPO-27(Ni), UiO66-COOH and UiO-66-(COOH)2. However, this is not the case for HKUST-1
for which its structure decomposed during repeated compaction which is
accompanied by a major drop of its surface area. For CPO-27(Ni) a very small
reduction in surface area is observed whilst MIL-100(Fe) lost 44 % of its
surface area (Figure 8).
Extrusion process has been extensively studied at Fraunhofer IKTS for a range
of MOFs including UiO-66-COOH, CPO-27(Ni), Al(OH)-fumarate and HKUST-1
for the production of pellets and cylindrical samples. The paste contains the
MOF powder, a dispersing liquid and organic or inorganic substances that
acts as binder and auxiliary substances for processing. Water was used as
dispersing liquid for all MOFs except HKUST-1 which is unstable in water. The
extrusion was carried out using a laboratory piston extrusion equipment
with a nozzle diameter of 1.5mm. Lines with approximately 300mm length
are extruded and dried at room temperature. The dried lines were cut to
small cylinders (i.e., pellets) with a length of approximately 1.5-2.0mm. It was
noticed that extrusion process can significantly affect MOF porous structure.
However, good results were obtained for HKUST-1 and UiO-66 among others.
For HKUST-1, a specific surface area of 95% of the raw MOF material was
achieved under consideration that the pellets contain 20wt.-% of binder
(Figure 9). Larger cylindrical bodies of 9 mm of diameter were also produced
without structural damage.
12

Figure 8. Shaped MIL-100(Fe) made via compaction and
sieving. process steps in commodity chemical production.

Source: JohnsonMatthey.
Figure 9. HKUST-1 extruded cylindrical pellets of 1.5-2 mm
height and 1.5 mm diameter and a bulk density to 0.6 g/
cm³.

Source: Fraunhofer IKTS.

The Catalyst Review

December 2019

The Catalyst Review December 2019

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