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This is really exciting news: a MOF (CALF-20) is going to be employed as CO2 sorbent at the industrial scale (cement production).
If, like me, you are wondering what MOF this is, why it works well for CO2 capture and how it can be deployed at such scale, here is a thread.
1/n https://twitter.com/RajendranArvind/status/1334950716964630528
If, like me, you are wondering what MOF this is, why it works well for CO2 capture and how it can be deployed at such scale, here is a thread.
1/n https://twitter.com/RajendranArvind/status/1334950716964630528
CALF-20 stands for CALgary Framework 20 and comes off the group of George Shimizu @UCalgary. Prof Shimizu is well known for his work on proton conducting MOFs and also in the field of metal phosphonates, having published some landmark papers on phosphonate-based MOFs.
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In 2009, the group first reported @ChemCommun a material, now named CALF-15, based on Zn(II) and two very simple organic linkers: 3-amino-1,2,4-triazole and oxalic acid.
Its crystal structure is made up of Zn-triazolate layers pillared by oxalates.
https://pubs.rsc.org/en/content/articlelanding/2009/cc/b911481e
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Its crystal structure is made up of Zn-triazolate layers pillared by oxalates.
https://pubs.rsc.org/en/content/articlelanding/2009/cc/b911481e
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This material features small pores (0.6 nm) and a surface area of 7 m2/g, according to N2 sorption at 77 K.
Yet, its CO2 adsorption behaviour is very promising: it adsorbs well at low partial pressure and its heat of adsorption is very stable around 40 kJ/mol at any loading.
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Yet, its CO2 adsorption behaviour is very promising: it adsorbs well at low partial pressure and its heat of adsorption is very stable around 40 kJ/mol at any loading.
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One year later, the same group published a paper @ScienceMagazine, providing an atomic level description of CO2 binding sites in CALF-15, confirming the physisorptive nature of the sorbent/CO2 interaction.
https://science.sciencemag.org/content/330/6004/650.abstract
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https://science.sciencemag.org/content/330/6004/650.abstract
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Adsorption of CO2 occurs with a cooperative mechanism between two distinct binding sites, mainly involving dispersion forces between CO2 molecules adsorbed on different sites. The proximity of these sites seems key to enable cooperation.
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Another couple of years, and there is this @angew_chem paper on a similar compound, where oxalate is replaced by phosphate, suggesting that the amine group might actually not be useful at all.
https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.201105109
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https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.201105109
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Come 2014 and a patent reporting synthesis and gas sorption behaviour of CALF-20 is filed: https://patents.google.com/patent/US9782745/en17
CALF-20 contains simple 1,2,4-triazole, the amino group is gone.
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CALF-20 contains simple 1,2,4-triazole, the amino group is gone.
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CALF-20, different from CALF-15, is porous to N2 at 77 K, allowing to measure a surface area of about 550 m2/g.
The water stability of the MOF is also revealed and attributed to the high degree of connectivity of the linkers: tridentate triazole, tetradentate oxalate.
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The water stability of the MOF is also revealed and attributed to the high degree of connectivity of the linkers: tridentate triazole, tetradentate oxalate.
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It is not clear whether the MOF is hydrophobic or has distinct adsorption sites for CO2 and H2O.
The fact that the as synthesised form (prepared in water/alcohol mixture) contains no solvent in the pores points to it being hydrophobic.
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The fact that the as synthesised form (prepared in water/alcohol mixture) contains no solvent in the pores points to it being hydrophobic.
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More recently, another patent came out describing an improved synthesis for CALF-20: first mix triazole and oxalic acid in ethanol/water to precipitate triazolium oxalate, then add a basic Zn salt (ZnCO3, Zn(OH)2, ZnO) and let it react for 18 h at RT.
https://patents.google.com/patent/WO2019204934A1/en
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https://patents.google.com/patent/WO2019204934A1/en
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It is claimed that the production cost of CALF-20 is 20-30 $/kg, which is quite impressive. How can it be so cheap? Two important things are cost of building blocks and synthesis conditions.
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8% of Zn annual production (11 million t) is used to make ZnO: http://www.chemmarket.info/en/home/article/1719/ Oxalic acid is produced in a volume of 120000 t per year: https://en.wikipedia.org/wiki/Oxalic_acid
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1,2,4-triazole is an intermediate for the synthesis of agrochemicals and pharmaceuticals, so it is already widely used in industry.
Just to give an idea, Sigma sells it for 40 eur/kg: https://www.sigmaaldrich.com/catalog/product/mm/808388
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Just to give an idea, Sigma sells it for 40 eur/kg: https://www.sigmaaldrich.com/catalog/product/mm/808388
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A synthesis done in water/ethanol (very cheap solvents) at RT requires no special equipment: no pressure is developed, the building blocks are not highly acidic/basic, they should not be corrosive either. It is basically the best scenario possible.
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The main bottleneck could actually be workup in this case, but I am sure it is no big issue for industry to deal with filtration of large amounts of powders.
I am eager to see if they can really manufacture this MOF at the ton scale.
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I am eager to see if they can really manufacture this MOF at the ton scale.
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Shimizu and a couple of former PhD students have set up a spin off company, Zoramat, which looks like it is focusing big time on the large scale production of CALF-20
https://zoramat.com/
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https://zoramat.com/
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Yesterday I was involved in a nice discussion on sorbent densification with, among others, @darren_broom, @RajendranArvind and @DavidFairen.
What about CALF-20? We don't know much, but some guesses can be made.
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What about CALF-20? We don't know much, but some guesses can be made.
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Crystal density is 1.6 cm3/g, relatively high for MOFs, but not surprising for ultramicroporous ones.
Powder likely has lower density, but compaction should increase it without too much harm to the framework: small pores, high connectivity usually good for mech stability.
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Powder likely has lower density, but compaction should increase it without too much harm to the framework: small pores, high connectivity usually good for mech stability.
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Other properties of interest for engineers (working capacity, kinetics, selectivity from gas mixtures, heat capacity, thermal conductivity, regeneration energy) are not known, or at least I could not find anything in this regard.
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Finally, cement kilns emit flue gases with relatively high CO2 concentration (20-25%), making adsorbents potentially more competitive with amine scrubbing, so it is not too surprising that the first large scale CO2 capture project with MOFs targets this industry.
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