Ionic Liquid Polymeric Membranes for the Separation of Carbon Dioxide From Nitrogen Streams.

[NNadir]

The paper I'll discuss in this post is: Imidazolium-Based Copoly(Ionic Liquid) Membranes for CO2/N2 Separation ( Isabel M. Marrucho et al Ind. Eng. Chem. Res., 2019, 58 (5), pp 2017–2026)

I believe that it is extremely urgent to phase out fossil fuels, not in some far off "by 2050" or "by 2100" putative so called "renewable energy" nirvana that will never come, but now. It is not ethical or just to expect our children, grandchildren and great grandchildren will be able to do what we ourselves cannot do or are too cheap to do, this after we have seriously destroyed most of the world's resources by surrendering them to entropy.

The technology to do this exists, but we must surrender our stupidity in order to use it. It's called "nuclear energy."

Early this week, several daily readings at the Mauna Loa carbon dioxide observatory recorded (momentarily) readings greater than 414 ppm. The weekly average, was 412.40 ppm, just shy (by 10 parts per billion) of the weekly average record set two weeks ago, 412.01 ppm. The unfortunate thing about this reading is that the consequences of the reading today will have effects years later, since the ocean is warming rapidly and because it also displays thermal inertia, it can only cool slowly.

Therefore it is urgent not only to stop dumping carbon dioxide and other dangerous fossil fuel wastes into the atmosphere will they kill directly as poisons as well as slowly as climate agents, but to begin, as quickly as possible to remove them from the air.

One avenue for capturing carbon dioxide is via the combustion of biomass, and capture of the exhaust.

This is best done by closed combustion in oxygen enriched atmospheres, a subject I discussed recently: On the combustion of biomass in oxygen enriched carbon dioxide atmospheres. (Well, it was interesting to write, in any case, even if it may have been unreadable.)

However, the production of oxyfuels requires a pure oxygen source, and energetically sustainable oxygen sources currently do not exist, since air separation is driven by electricity and in this country more and more electricity is being obtained by the combustion of dangerous fossil fuels than at any time in history. Oxygen cannot be made in a sustainable fashion unless it is made from the thermochemical splitting of water or carbon dioxide, and our ignorance has prevented us from moving forward with that, although the technology has been demonstrated at least on pilot scales.

The next best option is to capture carbon dioxide from the combustion of biomass in such a way as the serious pollutants, including but not limited to carbon dioxide are captured and put to use by reduction.

Hence my interest in the cited paper above, since a combustion stream is largely a mixture of carbon dioxide and nitrogen.

The paper concerns a class of materials that has been an intense area of research, "ionic liquids" which are salts of organic cations with organic anions (and sometimes inorganic ions) that can be liquid at room temperature but exhibit essentially no vapor pressure, and thus do not release fumes or easily catch fire.

In this case, materials featuring structures common to many ionic liquids are incorporated into polymers, this for the purpose of separating carbon dioxide.

From the introductory text of the paper:

The development of new materials for carbon dioxide (CO2) capture is becoming of vital importance as concerns about the growing concentration of anthropogenic CO2, associated with global warming and unpredictable climate changes, are being widely expressed. In particular, considerable attention has been paid to ionic liquids (ILs), due to the unique tunability of their properties1,2 and superior CO2 affinity in comparison with light gases, such as N2 and CH4.3,4 Several approaches have been proposed: from mixtures of ILs with other solvents, such as amines5 and glycols,6 to functionalization of ILs with diverse chemical groups,7?12 to other more sophisticated methodologies, as for instance ILs impregnation on different porous supports, such as polymeric membranes,13,14 zeolites,15 silica gel,16 and metal?organic frameworks.17

In what concerns the use of ILs in membranes, two strategies deserve special attention: the use of an inert porous membrane to support ILs and the incorporation of ILs in a polymeric matrix. Supported ionic liquid membranes (SILMs) have been widely studied for CO2 separation as they offer a facile preparation methodology.18 Results using a large number of ILs show that SILMs is a very promising strategy, since high CO2 permeabilities and attractive permselectivities can be obtained. However, SILMs present a major drawback: their low stability under high trans-membrane pressures and high temperatures, due to the weak capillary forces that hold ILs within the pores. On the other hand, the incorporation of ILs in polymeric membranes, in which the IL is entrapped in the tight spaces between the polymer chains, has proven to be a successful approach, providing membranes with increased mechanical strength compared to SILMs. A large variety of polymers has been used to prepare polymer?IL composites, 19?21 but the most successful approach lays on the use of poly(ionic liquid)s (PILs),22,23 since they allow the incorporation of higher amounts of ILs, due to the large degree of strong ionic interactions between the IL and the PIL components. On top of that, neat PIL membranes possess higher CO2 sorption capacities than their corresponding IL monomers24 and present CO2/CH4 and CO2/N2 similar or greater than those observed for SILMs.

What is under discussion are "PILS."

Here are some PILS referenced in the paper:




The caption:

Figure 1. Different PIL copolymers structures which have been used to develop CO2 separation membranes.(42?44)

The authors here make a new series of PILS

Here is some of the interesting organic chemistry by which the "PILS" in question are made:



The caption:

Scheme 1. Synthesis of CTA

"CTA" here refers to "Chain Transfer Agent," an agent designed to control the size of polymers by trapping the reactive mechanistic intermediate during the polymerization reaction. Here the potassium salt of of ethyl xanathate is reacted with ethyl 2-bromopropionate to give 2-ethoxythiocarbonylsulfanyl-propionic acid ethyl ester.

The use of chain transfer agents in polymerization reactions is called "reversible addition?fragmentation chain transfer" (RAFT) polymerization.

Here is the synthesis of the "ionic liquid" portion of the polymer:



The caption:

Scheme 2. Synthesis of Vinylimidazolium Monomers

The RAFT polymerization:



The caption:

Scheme 3. RAFT Polymerization of Vinylimidazolium Monomers

Assembly of the final co-polymers:



The caption:

Scheme 4. Synthesis of the Copoly(Ionic Liquid)s [ViRIm](Sty)X

Note that several different polymers are described in this graphic, not just a single polymer.

The differing polymers were then cast into membranes by dissolving them in a mixture of acetone and a commercially available ionic liquid called [C2mim][NTf2], and then spread into petri dishes, where, upon drying they gave membranes.

Not all of the polymers could form membranes however. Some were brittle and broke; others simply would not spread properly:



The caption:

Figure 2. Pictures of the PIL–IL membranes: (A) poly(ViPenIm)(Sty)NTf2 with 10 wt % IL, (B) poly(ViBenIm)(Sty)NTf2 with 25 wt % IL, (C) poly(ViPenIm)(Sty)NTf2 with 30 wt % IL, (D) poly(ViBenIm) NTf2 with 20 wt % IL, (E) poly(ViNapIm) NTf2 with 20 wt % IL.

The authors write:

Our attempts to prepare free-standing membranes with the two synthesized pure homo PILs, namely poly(ViBenIm)NTf2 and poly- (ViNapIm)NTf2, were unsuccessful, since they became very brittle and broke even before being peeled out of the Petri dishes. Although the prepared poly(vinylimidazolium)-polystyrene copolymers showed better film forming ability than the homo PILs, the obtained co-PIL membranes were not flexible enough to be peeled out of the Petri dishes and handled without breaking. Therefore, the incorporation of different amounts of free [C2mim][NTf2] IL was tested as a strategy to enhance the film forming ability of the homo and poly(vinylimidazolium)- polystyrene copolymers. Also, it is well documented in the literature that a large amount of IL incorporated into the PIL leads to higher CO2 permeability and diffusivity through the PIL?IL membrane.

Some of the physical chemistry of measuring the properties of gas diffusion:

Gas transport through the prepared PIL?IL dense membranes was assumed to follow a solution-diffusion mass transfer mechanism,48 where the permeability (P) is related to diffusivity (D) and solubility (S) as follows:

P = D × S (1)

The permeate flux of each gas (Ji) was determined experimentally using eq 2,49 where Vp is the permeate volume, ?pd is the variation of downstream pressure, A is the effective membrane surface area, t is the experimental time, R is the gas constant, and T is the temperature.

(2)


Ideal gas permeability (Pi) was then determined from the steady-state gas flux (Ji), the membrane thickness (l), and the trans-membrane pressure difference (?pi), as shown in eq 3.50

(3)


Gas diffusivity (Di) was determined according eq 4. The time lag parameter (? ) was calculated by extrapolating the slope of the linear portion of the pd vs t curve back to the time axis, where the intercept is equal to ?.50

(4)


After Pi and Di were known, the gas solubility (Si) was calculated using the relationship shown in eq 1. The ideal permeability selectivity (or permselectivity), ?i/j, was obtained by dividing the permeability of the more permeable specie i to the permeability of the less permeable species j. As shown in eq 5, the permselectivity can also be expressed as the product of the diffusivity selectivity and the solubility selectivity:

(5)

The unit for gas permeability is the "Barrer" which is incorporates units of mass transfer, area and pressure.

Here are some results represented graphically:




The caption:

Figure 3. Gas permeabilities (A), diffusivities (B), and solubilities (C) through the prepared imidazolium-based co-PIL–IL membranes. Error bars represent standard deviations based on three experimental replicas

The results of their membranes is compared with some literature values for other PILS



The caption:

Figure 4. CO2/N2 separation performance of the co-PIL–IL composite membranes obtained in this work. Data is plotted on a log–log scale, and the upper bound is adapted from the work of Robeson.(53) Literature values of other PIL–IL composite membranes previously reported are also illustrated for comparison.(23,30,32,38,40,47,54)

Some comments from the conclusion of the paper on the overall results:

RAFT derived imidazolium-based homo- and copoly(ionic liquid)s were successfully synthesized and anion metathesis reactions were performed. The prepared polymers were characterized by nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). The results showed that the synthesized poly(vinylimidazolium)-polystyrene copolymers have high thermal stabilities (up to 300 °C) and are thus suitable for postcombustion CO2 separation. The membrane forming ability of the prepared imidazolium-based homo- and copoly- (ionic liquid)s was evaluated using the solvent casting technique. It was found that the synthesized homo- and poly(vinylimidazolium)-polystyrene copolymers were unable to be processed into mechanically stable flat form membranes due to their brittle nature. Consequently, the homo- and poly(vinylimidazolium)-polystyrene copolymers were blended with different amounts (10, 20, 25, 30, 40, and 60 wt %) of free [C2mim][NTf2] IL. Only the random copolymers poly- (ViPIm)(Sty)NTf2, poly(ViBenIm)(Sty)NTf2, and poly- (ViNapIm)(Sty)NTf2, combined with 10, 25, and 30 wt % of [C2mim][NTf2], respectively, resulted in stable and homogeneous free-standing solid membranes. The membranes exhibited CO2 permeabilities ranging from 16.5 to 24.5 and CO2/N2 permselectivities from 31.7 to 34.4, thus falling in a region of the Robeson plot below the 2008 upper bound, where other PILs bearing similar amounts of IL also are. Curiously, and due to the small amount of IL incorporated in the co-PIL?IL composites, the effect of the polymer backbone structure in the gas diffusion and solubility can be observed.

Often, while daydreaming, I think of all sorts of Rube Goldberg approaches to carbon capture (involving biomass combustion or heat driven reformation with either water or captured carbon dioxide). One approach that might be available in the short term, given the immediate emergency before thermochemically produced oxygen is available, is the use of compressed air in Brayton type cycles, assuming one can overcome the corrosive properties of sulfur, nitrogen and volatilized alkali metals. (I can imagine these things, but they're Rube Goldberg approaches to be sure.) In the substitution of compressed air for pure oxygen or oxygen/carbon dioxide atmospheres for combustion, the separation of nitrogen from carbon dioxide might prove useful, and one can imagine it indeed being driven by pressure gradients also utilized to turn turbines.

Just some random thoughts, some speculations...

But we're as a race of beings running out of time, even faster than I am running out of time, and I am out of time.

I hope you had a wonderful weekend. I did.

[erronis]

1. Nuclear energy - fission or fusion will be necessary to provide power for this planet

(I'm just an interested citizen - please educate me if I'm wrong!)

Nuclear energy already provides all of the earth's energy through the sun's light and heat (fueled by hydrogen fusing into helium). Wind, waves, etc. are all a result of Sol being there for us.

We need to learn to harness this process and to deal with the radioactive byproducts. We must and we will.

Renewable energy (solar voltaic, wind, hydro, etc.) will only give us a small percentage (<10%?) when they are working well. And they all require large amounts of existing energy to produce the infrastructure and maintenance.