A "Maxwell's Demon" For Carbon Dioxide Capture?

[NNadir]

Recently I had the pleasure of acquiring a wonderful monograph Molecules at Work, subtitled "Selfassembly, Nanomaterials, Molecular Machinery"

I can't tell you that I've actually read it yet, but it certainly promises to be interesting when and if I get around finally to reading it, or at least sections of it. The part I can't wait to read is the part about molecular machines which are exactly what they sound like, since molecules (or small groups) of molecules that perform machine like activities.

Probably the best known molecular machine occurs naturally, the enzyme ATP synthase, which is located in the mitochondria of eucaryotic cells. This molecule rotates in such a way as to "force" a phosphate moiety onto ADP to make ATP, the latter molecule functioning as the "energy currency" of the cells of all higher organisms. The driving force for this activity is a proton gradient, basically an electric field. Fluorescent labels have been attached ATP synthase and the motion of the enzyme has actually been filmed microscopically.

Before the actual discovery of molecular machines, the great physicist James Maxwell posited one in (what came to be known as) a "thought experiment." This was "Maxwell's Demon", the "Demon" being a tiny being on a molecular scale who could actually work to violate the 2nd Law of Thermodynamics (a law that has apparently always been unpopular with certain kinds of people like, um, say, Amory Lovins, and people who think that everything can be saved by ever more Rube Goldberg types of energy storage systems. Energy storage, because of the 2nd law, must always waste energy, but no matter trying to educate people on that score.)

The Demon was posited to examine a question in the then arising kinetic/statistical theory of gases which states that heat may be thought of as a function of the average speed of molecules in a gas. The hotter the gas, the faster the average speed. Of course since the speed is an average it follows that some of the molecules are moving fast and others slower but the temperature observed is the canonical ensemble of all the molecules. The way the "Demon" was imagined to violate the second law (as well as the "zeroth law&quot was to stand by a trap door in a perfectly insulated wall and open it whenever a molecule traveling faster than the average molecule was coming at it, and close it whenever molecules traveling slower than the average speed came. In this way, the fast molecules would accumulate on one side, the slow ones on the other, one side would become cold and the other hot.

The "problem" of the demon has now been solved by appeal to information theory, where the demon is required to consume energy to make the decisions, thus he is nothing more than a heat pump, a device (like a refrigerator or air conditioner) that consumes energy in order to produce a temperature gradient. Thus the 2nd law is "safe," and, as Einstein had it - it will always be safe - because it's a fundamental law of nature that cannot be overruled.

Today, nevertheless scientists are now actually building new kinds of molecular machines, these on the scale of the Maxwell's imagined "Demon." I came across a fun one in a journal I've added to my reading list, Chemistry of Materials. (My son is starting a Materials Science Engineering education, and I must admit I'm vicariously thrilled at the prospect of such an exciting intellectual pursuit.) This "demon" opens and shuts trap doors to let carbon dioxide molecules in and out of its nanoscale (yoctoliter) containers.

Here is a link to the paper, which may be behind a firewall, but can be accessed in a good university or college library:

Azobenzene Guest Molecules as Light-Switchable CO2 Valves in an Ultrathin UiO-67 Membrane (Knebel et al, Chem. Mater., 2017, 29 (7), pp 3111–3117)

In recent years there have been major advances in organometallic chemistry relevant to materials science, highly structured molecules with huge surface areas, known as "MOF's" or Metal Organic Frameworks. These, which are loosely related but not really the same as naturally occurring zeolites are useful as catalyst supports since they have a huge surface area per unit volume, and also for gas separations, gas storage, the removal of toxic elements and molecules from dilute solutions, etc.

UiO-67 - the trivial name refers to its discovery at the University of Oslo in Norway - is an array of zirconium oxide complexes supported in a framework of 4,4' biphenyl dicarboxylates:



Some text from the paper's introduction:

"

Metal?organic frameworks (MOFs), consisting of metal or metal oxide nodes interconnected by organic linker molecules, exhibit extraordinary properties as porous materials for gas separation and purification and can also be utilized as smart and intelligent materials. With molecularly designed linker molecules or linkers with side-chain functionalities that react as a result of external stimuli, several highly special properties could be introduced into the frameworks.1?3 Complicated synthesis and noncommercial organic ligands are often used, mostly tailor-made on the lab scale at very low yields, to show certain smart (photoresponsive) functions, for example, photochromism4,5or photoinduced drug release.6 Accompanied by the synthetically difficult approach to molecularly engineered linkers, deposition of the MOF as a thin layer on functional surfaces is rather complicated, but necessary to achieve the custom-made function. The first developed porous and light switchable MOF was reported in 2011 by Modrow et al.7 For functional membranes, usually a layer-by-layer deposition, including several washing steps, is employed to form a surface-anchored metal?organic framework (SURMOF) with tailored functionality and a state-of-the-art thickness and flux.8, 9

"

A description of a gas separation experiment in which a mixture of carbon dioxide and hydrogen is separated by use of the synthetic molecular machine:

"

2.7. In Situ Mixed-Gas Permeation and Irradiation. For in situ gas permeation, a H₂/CO₂ mixed gas with a flow rate of 25 mL/min each was applied on the feed side of the membrane. On the sweep side, a 50 mL/min flow rate of N₂ was applied. The membrane was kept under ambient conditions. To irradiate the sample with ultraviolet?visible (UV?vis) light, a Prizmatix FC5-LED high-power, fiber-coupled LED system was used. For the controlled desorption of AZB in the gas flow, the temperature of the membrane was adjusted to80 °C and the increase in the level of CO₂ permeation with time was monitored. While the desorption of AZB took place, the membrane was constantly irradiated at ? = 365 nm. After a certain amount of desorbed AZB after some time, the increase in CO₂ permeance increased rather quickly compared to that for the desorption process. This was an indicator of the achievement of AZB switchability. To obtain a curve in a plot of permeance versus time, the membrane was then cooled to RT. Gas permeation was performed under in situ irradiation alternating with ? = 365 nm and ? = 465 nm at RT. When a saturation level of the permeance change was reached, the wavelength was changed, yielding a quasi-sinusoidal switching plot for the selectivity of the H₂/CO₂ mixture.

"

Note that this "demon" consumes energy to operate; it is not, of course, a kind of perpetual motion machine.

Biphenyls, as highly conjugated molecules are sometimes utilized in some electrically conducting organic polymers, and I suppose that some people, looking at this system could make all kinds of leaps about artificial photosynthesis, leading to all kinds of wishful thinking about a solar powered world.

The solar scheme for addressing climate change is, however, a miserable and expensive failure that will never be as clean, as safe, or as sustainable as nuclear energy. This said, it will fall to future generations to deal with the consequences of our tremendous failure to be responsible, and these kinds of molecular machines might well have practical applications for the incredible task with which we have left all future generations, that is, to clean up our mess. The separation of carbon dioxide from dilute matrices, air, or better, seawater, will be an essential tool for any hope they may have for success. It may not be this molecular machine, or any molecular machine at all, but it is wise to explore these avenues until their potential utility is understood.

Esoteric, I think, but interesting.

Have a nice day tomorrow.

[eppur_se_muova]

1. Any mention of Maxwell's Demon gets me thinking of the Hilsch vortex tube ...

Which I just learned from Wikipedia has been applied to isotope separation, a possibility I had long wondered about.

Interesting where a little curious reading can lead us.

[NNadir]

2. I wasn't aware of this process; it's the first I've heard of it. I'm not an isotope separation...

...kind of guy, actually, and I think the isolation of ²³⁵U from natural uranium is unnecessary and, in fact, undesirable.

In fact, I've argued that it is a good idea to make isotope separations for actinides more difficult, in a commentary I wrote here:
On Plutonium, Nuclear War, and Nuclear Peace

Incorporating some thorium into nuclear fuel cycles, along with proper plutonium utilization should make isotopic separation entirely unnecessary in order to exploit nuclear energy to its best advantage for many generations to come, should humanity come to its senses, not necessarily a good bet.

I note that in this case, the uranium and thorium already mined - the latter being a waste product from lanthanide isolation for electric cars and wind turbines - is sufficient to power all of humanity's energy needs for several centuries.

This said, the method to which you directed me is certainly interesting. I wasn't aware of it and it's cute, if impractical, applied physics.

Thanx!