Metal-Organic Frameworks for CO2 Sequestration.

[hypervalent_iodine]

The concept of carbon capture and storage is something most people are aware of. It refers to a technology used to address problem of CO₂ emissions by some the world's largest offenders - namely the fossil fuel industry - by sequestering and storing large amounts of the gas. Over the years, many methods have been developed to help achieve this, all with varying degrees of success, as per the below flow chart:

 

[From here]

I'd like the focus of this thread to only be on one of these, specifically metal organic frameworks (MOF's). For those who do not know what these are, the wiki article on them is fairly informative. The major issues in developing a way to capture CO₂ from industrial exhausts is in being able to create something that is scalable, cost effective and of course, something that works. Typically, they operate in one of three modes: pre-combustion, oxy-fuel combustion or post-combustion. For the sake of brevity I'll only be talking about post-combustion methods, which involves the segregation of CO₂ from flue gas, the composition of which (for brown coal) is as follows:

 

• 74% N₂, 14.3% CO₂, 7.7% H₂O, 3% O₂ and trace amounts of other gases.

Using current technology, which is based off a patent from the 1930's, achieving separation of CO₂ from flue gas increases the power requirement of a brown coal plant by 40%. It works by utilsing an aqueous solution of alcoholic amines (typically 30% w/w) to sequester CO₂ by a process of chemisorption - i.e. by reacting the amines with CO₂ to generate the corresponding carbamates - followed by a process of chemical desorption to regenerate the amines.

 

This is where MOF's come in. Currently, MOF's used in post-combustion sequestration are effected either by physisorption or more recently, chemisorption. The latter certainly reduces the energy required to regenerate the solid material as opposed to the aqueous amines, however they are not particularly selective and have low capacity. Chemisorption of CO₂ using MOF's overcomes many of the issues with physisorption and is very similar to the current method used - it aims to adopt a process of chemisorption (generating carbamates by reaction with an amine) to separate the CO₂ followed by desorption. There has been some work done in this area over the past few years (see here), however I will only use one very recent example in this thread, which comes from a collaborative effort by the University of Sydney, Berkley and the Van't Hoff Institute for Molecular Sciences:

 

McDonald, T. M.; D'Alessandro, D. M.; Krishna, R.; Long, J. R., Chem. Sci., 2011, 2, 2022 - 2028.

 

Their idea looks like this:

 

[image from their paper, referenced above; BTTri = 1,3,5-tri(1H-1,2,3-triazol-4-yl)benzene; mmen = N,N'-dimethylethylenediamine]

 

Rather than using alcoholic amines, this work utilises the secondary alkylamine, N,N'-dimethylethylenediamine (mmen) and since MOF's work in solid phase, the need for solvent is abolished. I won't go too much into the paper, except to say that it shows a great deal of promise in selectively taking up CO₂ - in fact, it's been shown to work above and beyond any other MOF to date (for binary mixtures, at least).

 

My own personal opinion of the technology is that it comes across a bit of a waste of time (within this context, at least). To my mind, the problems surrounding CO₂ capture and storage and solutions thereto seem so obvious that I'm wondering if perhaps I'm just missing something. I know I'm not the only person who thinks this - one of the authors of the above paper gave a seminar last year at my university and the queries I'm raising here are in fact the same as those brought up by professors in the audience. Primarily, it boils down to two questions:

 

• This technology does not 'do away with' the gas, it simply captures it. The solution to 'what do we do with all of this gas' was presented by D'Allesandrio in a seminar I attended last year. She stated that current methods involved storing it underground and using it to push up oil to the surface; indeed, there are already facilities around the world that do this. This to me seems a little silly, since I can't see how you would keep it where you put it and because it seems like there would or should be a plethora of safety concerns to go with it. Does this method of storage actually work and are there in fact any imminent safety issues surrounding its use?*

• Perhaps most importantly, what I am failing to understand is why we would waste time and money developing and implementing this technology for CO2 sequestration and storage, when planting trees could do the same thing more efficiently? As far as I am aware, there are some places that are trying to solve the issue by planting trees around industrial plants (I'm not sure where this is being done, it is really a vague recollection) - is this not a ubiquitously viable option?

 

 

*It's worth noting that by D'Alessandro's own admission, the safety of this set up has not been fully investigated. It's curious then, that this is in operation in other parts of the globe. Here in Australia we have one storage plant, situated in Otway, which is operational but only being used for testing as of last year.

[Xittenn]

This makes me think of hemoglobin but with the center carrying CO2. Preventing excess CO2 from entering the atmosphere can help make CO2 management a manageable problem. My thoughts on this would be to capture the CO2 at source and diffuse the excess through algae tanks for H2 production and other applications. Algae isn't necessarily the most readily accommodating material for realizing flue filtration.

 

I don't see research into technologies like these as ever being a waste of research, although they might be an unjustified use of resources. I'm sure something like this will find application somewhere. This sounds to me like a promising technology for spacecraft, submarine, or aircraft missions and the demands of closed space human respiration carbon capture.

[hypervalent_iodine]

This makes me think of hemoglobin but with the center carrying CO2. Preventing excess CO2 from entering the atmosphere can help make CO2 management a manageable problem. My thoughts on this would be to capture the CO2 at source and diffuse the excess through algae tanks for H2 production and other applications. Algae isn't necessarily the most readily accommodating material for realizing flue filtration.

 

The issue then is the energy required to maintain them and the space required to house the systems, etc. I think there has been some efforts in that area, but I don't now how successful or how viable it is. I kind of like the idea of algae as biofuel feedstocks, but that's another story.

 

I don't see research into technologies like these as ever being a waste of research, although they might be an unjustified use of resources. I'm sure something like this will find application somewhere. This sounds to me like a promising technology for spacecraft, submarine, or aircraft missions and the demands of closed space human respiration carbon capture.

 

Ah, yes, but you'll notice I stated that I found it pointless within this context. Of course it's not a waste of research, I just think that focusing it towards this particular application is a waste. There was mention of using MOF's as drug delivery systems as well as for other biomedical applications. The presentation I attended also went into research where they investigated their use as conductors by incorporating redox active ligands into a ruthenium based framework.

[Xittenn]

It's a very interesting line of research that I otherwise had never heard of. I'm very interested in how selectivity for CO2 at low pressures is achieved and how the CO2 is subsequently released. I will definitely read the article once my confirmation email comes through.

[hypervalent_iodine]

It's to do with the way it's sequestered. The reaction itself is a rather simple one (the formation of a zwitterionic carbamate in this case) and it is inherently selective.

 

 

 

The reaction is reversible and within this particular context, is reversed by using simple temperature swings.

 

So, by tethering a secondary or primary amine to their framework, they create a functionally dense, solid support with many available sites for reaction. In the paper I linked, they found that ~ 82% of the reactive sites had reacted with CO₂ with minimal uptake of N₂ from the binary mixture (close to 0 mmol/gram, in fact), which is a pretty decent result.

[mississippichem]

You know I've recently read some literature on sequestration of carbon dioxide by incorporating it into biodegradable gamma-cyclic polycarbonates. Somehow they accomplished this with a Sn(II) catalyst too directly from CO₂ gas (though it was a high pressure process). I'll have to dig and see if I can find it again as I believe it is relevant to this discussion.

 

The issue then is the energy required to maintain them and the space required to house the systems, etc. I think there has been some efforts in that area, but I don't now how successful or how viable it is. I ind of like the idea of algae as biofuel feedstocks, but that's another story.

 

I share your skepticism of this technology. There are reasons that our whole society runs on the oxidation (rapid combustion) of alkanes and not the reduction of carbonates and carbon dioxide. One of those reasons is that [math] \Delta H_{f} = -3.95 \ \mathrm{kJ} \cdot \mathrm{mol} ^{-1} [/math] for CO₂ at standard conditions. It's a tall order to find a way to economically synthesize your way out of that potential well!

 

There is also a massive kinetic inertness problem that you get from a carbon surrounded by two electron withdrawing oxygen atoms and two cumulated pi-bonds. This of course leads one to the question "what do we do with all this carbamic acid or carbon dioxide we've collected!?"

Though I really appreciate the chemistry behind the MOF you have presented, and I think it's a valid avenue to pursue for chemistry's sake, I just don't see this ever making a difference in atmospheric CO₂ for engineering reasons.

 

On the other hand, the chemistry of metal organic frameworks is very rich and has recently piqued my interest with the possibility of some of then being good electron/electron hole/exciton semi-conductors.

 

Ah, yes, but you'll notice I stated that I found it pointless within this context. Of course it's not a waste of research, I just think that focusing it towards this particular application is a waste.

 

Agreed.

 

There was mention of using MOF's as drug delivery systems as well as for other biomedical applications. the presentation I attended also went into research where they investigated their use as conductors by incorporating redox active ligands into a ruthenium based framework.

 

Bold mine

 

I'm hip to this research as well and I think it shows promise just based on novel electronic properties alone. I recently saw a speaker who was making similar MOF's with a focus on molecular self assembly, i.e. crystal morphology control through varying reaction time.

 

Great Topic. I'll try and conjure something else up to contribute.

 

EDIT: the [imath] \eta ^{4} [/imath] binding mode chloride bridging four Cu centers in a plane freaks me out

Edited April 8, 2012 by mississippichem

[hypervalent_iodine]

I should also add to my above reply to Xitten that the overall selectivity of such a system is also generated by sterically inhibiting the potential for physisorption.

 

 

mississippi:

 

 

I share your skepticism of this technology. There are reasons that our whole society runs on the oxidation (rapid combustion) of alkanes and not the reduction of carbonates and carbon dioxide. One of those reasons is that [math] \Delta H_{f} = -3.95 \ \mathrm{kJ} \cdot \mathrm{mol} ^{-1} [/math] for CO₂ at standard conditions. It's a tall order to find a way to economically synthesize your way out of that potential well!

 

I'm glad you brought this up. The difference in energy requirement between the current technology and this example is noticeable, but as an old professor of mine pointed out, small enough that the difference could easily be made up on a warm summer's day. I forget what the numbers were exactly.

 

 

There is also a massive kinetic inertness problem that you get from a carbon surrounded by two electron withdrawing oxygen atoms and two cumulated pi-bonds. This of course leads one to the question "what do we do with all this carbamic acid or carbon dioxide we've collected!?"

 

The proposed solution is to pump it underground, use it to push up oil and then store it there. As I said in the OP, it seems a rather tenuous and risky thing to do, yet I am told it is in full operation in some parts of the world.

 

Though I really appreciate the chemistry behind the MOF you have presented, and I think it's a valid avenue to pursue for chemistry's sake, I just don't see this ever making a difference in atmospheric CO₂ for engineering reasons.

 

Oh yes, the chemistry is wonderful. As I said to Xitten, I'm not knocking MOF's as a whole, just their use in CO₂ sequestration. The solution they came up with is certainly eloquent and it is an improvement on what we currently use, but then so are trees. I suspect that scaling up may also be an issue, even if you ignore the obvious cost efficiency factor.

 

 

On the other hand, the chemistry of metal organic frameworks is very rich and has recently piqued my interest with the possibility of some of then being good electron/electron hole/exciton semi-conductors.

 

<...>

 

I'm hip to this research as well and I think it shows promise just based on novel electronic properties alone. I recently saw a speaker who was making similar MOF's with a focus on molecular self assembly, i.e. crystal morphology control through varying reaction time.

 

 

think if you look up D'Alessandro's name, you may find some other papers by her in the area of MOF's as conductors (Edit: I found this one). The first half of her seminar last year was all about that aspect of her research, though to be honest I only listened enough to take a few notes and I don't recall if what she presented had been published. Looking at my notes, the body of it seems to be focused around the incorporation of a redox active paddlewheel shaped ligand - (O₂CMe₄)(THF)₂ - onto a switchable Ru^{II, III} frame (reminds me of Prussian blue) to generate an interpenetrated BCC structure that has low porosity and is thermally stable up to 500^oC. Doesn't look like her group has managed to utilise this in any prototypes, but progress is progress.

 

I found this paper, which looks like a nice review of the possible applications for MOF's. I might add more about this when I have had a chance to read it.

[paulhorth]

I'm not a chemist, but I am a chemical engineer in the oil & gas industry, fairly familiar with amine solvent processes for removing CO2 and H2S from hydrocarbon gases (not from flue gas). So my comments start from ignorance of this interesting field.

The basic problem with recovering CO2 from flue gas is the low pressure and the low concentration, so the driving forces are poor. Better to recover CO2 at higher pressure from some kind of pressurised combustion process.

 

I would mention a couple of engineering problems with a solid absorbent for flue gas:

(1) Flue gas is dirty, full of all kinds of rogue components like ash and partly combusted stuff, which would foul the adsorbent

(2) How do you deliver the heat for regeneration? With a conventional amine solvent you boil it, with a solid bed you would need to pass heated clean gas through it. Generating this gas requires a whole additional system.

 

More general points: Converting the CO2 to trees changes the question to "what do we do with all this wood?" Answer: -burn it, and recycle to more trees? OK, but trees need two scarce resources, namely land and time.

 

Further question: Can this metallic compound be modified to adsorb SO2? or even H2S?

 

I'll follow with interest

Paul

[hypervalent_iodine]

Who said we need to get rid of the trees? Keep them there and let them continue to take up CO₂. I agree that the land thing is an issue. I'd be interested in seeing numbers regarding how many fully matured trees you'd need to effectively neutralise emissions from an industrial plant. I'd say it would be rather large, although considering that current methods don't remove anywhere close to all CO₂, you'd really only need a fraction of that number for it to be an improvement.

The time factor can be overcome by using a fast growing species like pine.

I'm not sure how MOFs would respond to H₂S or SO₂. I'm not sure how prevalent either of those are in flue gas so perhaps they won't be so much of an issue. How does the current system tolerate them (and the rest of the mess you mentioned)?

As for the heating issue - I'm not really sure. I think it was mentioned in the seminar, but I don't seem to have written it down. I might look into it when I get back into Brisbane and have a better connection.

[bulla]

ho a good read, how I would like to go to town on this subject, nice to read your views on the subject, but you know, I see mankind doing some very stupid and totaly ridiculous things and each and every time its to prop up mega profit, and haven't we attacked this earth enough, and already left an impossible legacy, for future generations to come, never a thought for the future just a fast quick buck fix, to get past the regulators and EPA regs

 

Here we are trying to stop gas mining and fraccing from poising our water and fabulous underground resources of water, then the coal miners want to dig the place up and dump there wastes into our rivers and then they want to pump further wastes in to the strata, ho criki will the forces of money and profit's never see the light of human survival

 

People in Australia are waking up, and we have just thrown one state government of Queensland out of power because they were not listening to the environment and its electors the people want sanity, not ten years of profit overseas, and leave us with smashed underground environment that will eventually finish on the the top and back in the environment

 

Co2 sequestration is not a sustainable environmental proposition, on any grounds, just add one 7.5 earth quake and its all over red rover

Edited April 11, 2012 by bulla

[hypervalent_iodine]

bulla, most of that has absolutely nothing to do with this thread, so I'm going to ignore it.

 

Co2 sequestration is not a sustainable environmental proposition, on any grounds, just add one 7.5 earth quake and its all over red rover

 

How is cutting down CO₂ emissions not environmentally sustainable? The whole point is that it is environmentally sustainable, so I'm rather confused by that comment.

[Mosheh Thezion]

I cannot help but wonder, what is the point???

our world is covered with forest fires.

Volcanoes spew as much Co2 as human kind....

and our Oceans obsorb it.. and convert it into rock, via... lime...

 

 

So.. whats the point... why fear Co2 as vented waste????

 

 

I would think the best option... is its collection and conversion via nuclear energy to create hyrdocarbons to burn as fuel. (Well established tech)

I.e.. if we collect CO2 from the air.. and make fuel from it... then we have a clean carbon cycle.

 

Sequestration does seem... nuts... in the long run... as it has the potential to kill entire valleys of people... as CO2 is heavy.. and if squezzed to the surface.. in abundance... could kill many.

 

I would not want to live anywhere near such a sequestration site... in fact... if i did.. i would move.

 

But its use... as a product to make other things, instead of let loose as waste, is perfectly reasonable, and should be encouraged.

 

 

-Mosheh Thezion

[hypervalent_iodine]

Okay, to bulla and Mosheh:

 

Read the title of this thread. It is not about whether carbon sequestration is good or bad and it is not about climate change. It is about the use of metal organic frameworks as a technology for carbon sequestration in industrial plants. Whether you like it or not, industrial plants exist and will continue to exist into the foreseeable future. I find it absurd that anyone would think that a commercially and environmentally viable way to make industrial output cleaner is not a good idea - but this is not the thread for that argument.

 

Also, Mosheh, you seem to be confusing sequestration with storage.

[At just flattened at!]

If plant-life does exactly what we need, can we not copy what the plants do? Build a organic machine of sorts.

[mississippichem]

Keep on topic please. I post this as a regular poster and not as staff.

 

We have a good thread going. Don't derail it!

[hypervalent_iodine]

If plant-life does exactly what we need, can we not copy what the plants do? Build a organic machine of sorts.

 

Because plants are terribly complicated and I cant imagine that coating the flue of a coal plant with some sort of synthetic leaf would be in sturdy enough to be viable. It would be dead before you could capture anything.

[bulla]

Well may you have a good thread, but I believe the hole issue of all carbon release damaging the environment is a load of bunk, but however, if someone can come up with a way, as postulated here, and use it as a benefit, well and good and more power to your arm, and in any case all climate is magnetic

[Phi for All]

Well may you have a good thread, but I believe the hole issue of all carbon release damaging the environment is a load of bunk, but however, if someone can come up with a way, as postulated here, and use it as a benefit, well and good and more power to your arm, and in any case all climate is magnetic

!

Moderator Note

bulla, this is off-topic for this thread and is considered thread hijacking, which is against the rules you agreed to when you joined. If you wish to discuss why you think carbon release doesn't damage the environment, search for a similar thread or start one of your own.

 

Please read the opening posts to determine what the topic is about before trying to respond. Do not further derail this thread by responding to this modnote.

[At just flattened at!]

That is if the carbon dioxide capturing device/coating had to be located in the flue, if it was taking CO2 out of the air at another location then it would do the same job overall. If we can make artificial skin I'm sure in time we could make a membrane that does the job of a plant. I'll leaf you to it I don't want to hijack the thread either.

[paulhorth]

Coming back to the question of absorption of H2S and SO2. Although off topic for CO2 sequestration, I am interested because, in the oil and gas industry, amine solvents are used largely for removal of H2S rather than CO2, so I thought that your solid amine compounds could also have this function.

It's true that in flue gas there isn't much H2S, all the sulphur in the fuel should be SO2, and this only in coal fired systems. However, the SO2 might take up capacity in your adsorbent and would need to be effectively removed on regeneration.

 

There is a lot of H2S produced worldwide with oil and gas, and it all has to be removed. It is generally oxidised catalytically to sulphur. Because the supply of sulphur far outstrips the demand, the sulphur accumulates in huge cast slabs in certain parts of the world. The sulphur blocks in the Tengiz oilfield in Kazakhstan can be seen from space (so I'm told...). Any new process for H2S removal offering advantages over amine solvents would find a great deal of interest.

 

Paul