Metal-oxide frameworks, MOF's:

https://phys.org/news/2018-11-mofs-industrial.html

I wonder if these MOF's can replace activated carbon used now in cryopumps, where they are cooled down to about 10 degrees Kelvin to turn the assembly into a vacuum pump? The whole idea is a gas molecule checks in but can't check out because it is frozen to a surface of an extremely porous substance, now activated carbon.

@sonhouse

MOF are much more expensive to make than activated carbon. As yet there is no method knwon to make them really cheap.

@Ponderable

I don't think that's a great issue in the proposed applications, especially filtering for lithium - provided the life time cost is lower, or the additional filtering capability offsets the higher price of the material.

@DeepThought

Lithiumfiltering yes
cryopumps no

@Ponderable

I don't know enough about cryopumps to make any kind of statement. I was going by what's in the phys.org article more than sonhouse's post. Besides, cryopumps aren't exactly consumer items, there might be some technical advantages that outweigh the additional cost even if there's no general advantage in cryopump applications - at least in research settings.

@DeepThought

In now Quote the conclusions of the original paper:

This report describes permanent, accessible and reversible porosity inherent to glasses derived from metal–organic frameworks. Two precursor crystalline ZIFs were designed, and their high temperature melting monitored in situ by a range of combined diffraction experiments. Notably, these glasses are distinct from those reported by Yaghi et al.,13 which are prepared via sol–gel methods, and in which Tg disappears after solvent evaporation. This means that sol–gel glasses cannot be formed by heating to high temperatures. The discovery of accessible porosity in glasses derived from MOFs may serve as the foundation for a new class of porous hybrid inorganic–organic materials. We expect that developments in this field will be enabled by (i) the large number of known MOF or coordination polymer structures that can serve as potential glass precursors32, (ii) the ability to combine the chemical diversity of MOFs with established techniques for handling and moulding glasses, (iii) the availability of several techniques for vitrifying crystalline frameworks12,33 and (iv) the use of post-synthetic techniques that are employed in other glass families to increase available surface areas34.

We envisage a plethora of potential applications will stem from porous MOF glasses, including membranes for chemical separations, catalysis, ion transport and conductivity35. These glasses should not however be placed into competition for the ultra-high surface areas heralded for crystalline MOFs, but seen in the light of ease of processing, mechanical stability and possible use in separations. Additional avenues for research may also arise from their comparison and contextualization with conventional glasses. In this light, MOF glasses may be geared towards applications in optics, where one of their principle advantages will lie in their softer nature and correspondingly lower processing temperatures. The combination of the stimuli responsivity of MOF chemistry36 with the glass domain will also lead to new, smart applications and a new era of glass technology37.

Sonhouse had another idea for an application which I perceived directly as a non-starter (and I have some colelagues working on MOF's) and which is explicitely excluded by the original paper.

Thanks for bringing the paper to my Attention. I wouldn't have read it without this 😉

@Ponderable

Besides cost, suppose cost came down, what specifically about MOF's make them unsuitable for cryopumps? The activated carbon is certainly cheap and has a huge surface area which at 10 degrees Kelvin, means a gas molecule checks in but loses most of its kinetic energy and thus sticks to the surface area of the carbon pellets,
It seems to me it is the large surface area that is the key to the usefulness of MOF's and carbon in this app, so what would inhibit this stuff from working on cryo's?

@sonhouse

Surface per$ is highest with activated carbon, because the raw material is abundand and cheap (think cocnut shells).
The linkers alone to make the MOF's are orders of magnitude more expensive than the activated carbon, and the metal precursors are also not really cheap.

Physically MOF's are well suited to absorb specific molecules, while activated carbon just takes more or less everything. So for a cryopump, where I just want to get rid of molecules I take something that just has a large surface is easy to handle and cheap, as opposed to something expensive with large surface and sometimes not so nice side effects (think of metall poweder.

In the paper they described glasslike oligolayers of MOF which retain their high surface. So they tackled the easy handling and they need the specifity. All well.

@Ponderable
So activated carbon has higher surface area than these MOF's? The only reason I brought up the link to cryopumps was I thought the piece said it had the highest surface area of anything but maybe they meant was compared to other MOF's.

I know activated carbon is dirt cheap, I was just thinking if there was a material with say twice the internal surface area I could get twice the pumping time before regen.

One thing that happens and I guess it would happen with MOF's is the carbon pellets fall off the second stage and just drop into the guts and therefore do not contribute to the pumping speed, if enough of them fall off it needs rebuild.
I think that has to do with the glue they use (epoxy, probably) to stick the carbon bits to the cold head, which is around 10 degrees Kelvin. Not sure, maybe if the surface area of the MOF's were equal to carbon, it may stick better to the cold head since I would assume the MOF is harder.
I am probably pissing in the wind here but just thinking of ways my cryo pumps can work more reliably.

For instance, if the cryo dumps because of a vacuum leak, a strong enough leak can send a shock wave into the cold head that rattles some of the pellets loose, which doesn't happen much but when I do a clean I always see the dam pellets that have fallen off into the gate valve area..