Impregnating magnesium carbonate with polyethyleneimine to capture carbon dioxide.

[NNadir]

The paper I'll discuss in this thread is this one: Impregnation of PEI in Novel Porous MgCO3 for Carbon Dioxide Capture from Flue Gas (Xiao et al, Ind. Eng. Chem. Res., 2019, 58 (12), pp 4979–4987)

Despite the title of the paper I am discussing herein, I personally believe that the concept of the "flue" should be phased out as rapidly as possible. "Flues" are waste dumping devices; in almost every case, they are the equivalent of pipes dumping raw sewage into rivers and other bodies of water. Flues dump waste into what has become humanity's favorite waste dump, it's planetary atmosphere, which is rapidly being destroyed by indifference and/or the inexplicable popular enthusiasm for technologies which don't work very well; here, as usual, I'm referring to the multi-trillion dollar investment in wind and solar energy which has done nothing, absolutely nothing, to arrest the acceleration of climate change. We are now at around 412 ppm of CO2 in the atmosphere; at the end of March, 1998, we were at 369 ppm.

Elon Musk. Tesla electric car. Megawatts Solar. Megawatt wind.

We are oblivious.

As we are oblivious, it will fall to future generations, from the immediate through the end of human time, to clean up our mess, and do so after we have robbed them of important resources. The clean up of the mess we've made of the planetary atmosphere, is an unimaginable engineering challenge which will require the generation of vast amounts of energy while using zero fossil fuels, almost all of which will have been oxidized and dumped in the atmosphere as even more waste to clean up.

After much study, I consider that this task is just over the line of feasibility; it might be accomplished, but only with a massive concerted effort of all of humanity, such a concerted effort being the most improbable feature of the effort among all features, included the technical features.. We are making 1930's fascism look like small change, given the consequences of the environmental results of present day fascism (albeit disguised as "democracy." )

While I oppose flues, I do consider that combustion ironically represents a part of the path to removing carbon dioxide waste from the atmosphere, at least in the case where the carbon dioxide is generated in an atmosphere of pure oxygen (this generated by nuclear heat) with the combustion of waste biomass. Under these circumstances a pure stream of carbon oxides (monoxide and dioxide) are generated; where steam is present, hydrogen and carbon dioxide, a form of "syn gas" that can essentially replace all materials now obtained from dangerous petroleum, can be generated.) Similarly, "dry reforming" heating biomass to high temperatures under an atmosphere of pure carbon dioxide, can generate carbon monoxide, which can be disproportionated into various allotropes of carbon and more carbon dioxide.

For various reasons, including the increase of energy efficiency under certain rather obscure but real circumstances, carbon capture technologies are of interest, even if the idea of "carbon sequestration" in waste dumps is a Quixotic and useless exercise that will not work. Hence my interest in this paper.

My comments aside, the paper begins with a genuflection to the idea of "carbon capture & storage" "CCS" as opposed to what I believe to be essential in order to give these processes any remote chance of being useful, sustainable and economic, "carbon capture and utilization" "CCU." It also refers, as it comes from a Chinese institution, to coal, a fuel I oppose along with the allegedly "green" dangerous fossil fuel, dangerous natural gas, and of course, petroleum.

From the introduction:

Global warming and other consequential environmental problems resulting from the greenhouse effect have received a great deal of attention in recent years. Since CO2 is the major contributor to greenhouse gases, it is particularly important and urgent to reduce the amount of CO2 emitted into the atmosphere due to the utilization of fossil fuels.1 Considered to be a critical solution to global CO2 emission reduction, CO2 capture and storage (CCS) technology has been given an urgent requirement for its own development.2 Among the various CCS technologies, the chemical absorption using aqueous solutions of amines, such as monoethanolamine (MEA), methyldiethanolamine (MDEA), and diethanolamine (DEA), is the most mature and well-established one for CO2 capture.3,4 However, this process presents major drawbacks, such as high operating costs, evaporation of amine solution, and equipment corrosion,5 which lowered the production efficiency of coal-fired power plants by 10−12%.6 Thus, there is a growing demand on new energy-efficient CO2 capture techniques for CCS applications. The adsorption process with the use of solid adsorbents has been developed to overcome these drawbacks in chemical absorption and showed the advantages of high product purity, low energy consumption, low toxicity, and ease of adsorption and regeneration,7−9 which displayed a broad application prospect in adsorptive separation of CO2 from flue gas.10,11 During recent years, numerous studies have reported that the CO2 capture capacity of porous solid adsorbents could be greatly enhanced by amine modification.12,13 These amine-modified solid adsorbents can be simply obtained by physically impregnating the porous supports with amine,14 which showed a higher CO2 capture capacity and lower cost compared to the grafting methods.15 An excellent amine-modified adsorbent should have unobstructed pore structure for CO2 transfer16 and a high capture capacity of CO2...

Many of the well known examples of solid phased carbon dioxide capture agents are challenging to synthesize on an industrial scale, a point the authors make referring to silica base absorbents, including the well known MCM-41:

Although amine-modified mesoporous silica-based materials exhibit excellent CO2 adsorption properties, the preparation of mesoporous silica is not cost-effective due to the use of expensive silica sources and surfactants in the synthesis, leading to difficulties with large scale manufacturing.32 Besides, it is an essential step to remove the organic surfactants after the synthesis of silica materials, which indeed involves the use of high temperature and chemicals that could increase the cost and the environmental burden.33 Therefore, the easily synthesized and environmentally friendly porous materials with superior performance and desired economics urgently need to be developed as the support of amine-modified adsorbents. Moreover, in addition to N2 and CO2, the flue gas also contains water vapor, SO2, and NOX, which may affect the performance of amine-modified adsorbents during CO2 capture.

What the authors propose is to synthesize a mesoporous form of magnesium carbonate, having the interesting property that its preparation is a case of CCU, inasmuch as the synthesis utilizes carbon dioxide as a reactant:

2.2. Synthesis of Adsorbents. The porous MgCO3 was prepared as the procedure reported previously.34 Briefly, MgO was mixed with methanol, after stirring under 3 bar CO2 pressure at 50 °C for 3 h, the mixture reacted under 1 bar CO2 pressure at 25 °C, followed by drying at 70 °C for 3 days, the dried product was calcined at 250 °C for 3 h with a 3 h ramp time. On the basis of this method, in order to select the best support, the following 5 samples (M1 to M5) were synthesized under different experimental conditions, which are shown in Table 1, respectively.

Methanol is readily available from syn gas. Table 1 lists synthesis conditions. M4, which is the most discussed porous MgCO3 form is prepared with the methanol containing 33% toluene, toluene being a product of the dangerous petroleum industry which is, albeit not industrially, conceivable to obtain from certain forms of biomass, for example by the reaction of butadiene (from cellulose derived furan) or pentadiene (from methyl furan) with ethylene (from syn gas) or propylene (also from syn gas). M4 is prepared by stirring MgO in this solvent under a CO2 atmosphere for 4 days at room temperature.

Further aspects of the process are described, using ethanol, also available from syn gas, and, of course, albeit as questionably as is the case with other so called "renewable energy" schemes, from grain:

PEI-modified MgCO3 adsorbents were prepared via a wet impregnation method.35 The desired amount of PEI dissolved uniformly in ethanol was added to the sufficiently dried MgCO3. The resulting slurry was stirred and refluxed at 80 °C for 2 h. After completely evaporating the ethanol at 80 °C, the sample was dried at 100 °C for 2 h in an oven. The obtained adsorbent was denoted as xP-M, where x (x = 10, 20, 30) indicated the mass percentage of PEI. The synthetic process of porous MgCO3 and PEI-modified MgCO3 adsorbents is illustrated schematically in Figure 1.

The "x" in "xPM" carries through the paper, for example 20P-M, is 20% PEI and 80% MgCO3.

Beginning with Figure 1, let's now just look at the pictures, a useful way to get a feel for a full paper before reading it in detail.



The caption:

Figure 1. A schematic diagram of the synthesis of porous MgCO3 and the impregnation process of PEI.

The caption:

The testing apparatus for measuring its performance as an absorbent:





The caption:

Figure 2. Diagram of experimental apparatus for CO2 adsorption.

Note that the authors are imagining this material to capture carbon dioxide from the flue gas from the combustion of dangerous coal. In contracts to the combustion of biomass in a pure oxygen atmosphere, the air fueled combustion of coal will contain considerable amounts of nitrogen. Hence the effect of nitrogen is considered important by the authors:




The caption:

Figure 3. N2 adsorption/desorption isotherms (a) and pore size distribution curves (b) of the prepared groups of MgCO3

It seems that the PEI loadings have a fairly large effect on gas availability in the pores, related to the extent to which pores in the magnesium carbonate are obstructed by the polymer.




The caption:

Figure 4. N2 adsorption/desorption isotherms (a) and pore size distribution curves (b) of M4 and adsorbents with different PEI loadings.

The caption:

Figure 5. FTIR spectra of M4 and adsorbents with different PEI loadings.

The caption:

Figure 6. SEM images of M4 (a), and adsorbents with different PEI loadings:10P-M (b), 20P-M (c), and 30P-M (d).

"Breakthrough" below refers to the point at which CO2 is detected after the flow has passed over the absorbent.

The caption:

Figure 7. Breakthrough curves of CO2 of M4 and adsorbents with different PEI loadings at 25 °C (a), 40 °C (b), 60 °C (c), and 75 °C (d).

20P-M can capture carbon dioxide at fairly high temperatures:




The caption:

Figure 8. Effect of adsorption temperature on the CO2 capture
capacities of M4 and adsorbents with different PEI loadings.

The effect of trace gases on the absorption:




The caption:

Figure 9. Effects of H2O, NO, and SO2 on the breakthrough curves (a), (c), (e), and CO2 capture capacity (b),(d), and (f) of 20P-M at 75 °C.

It is important to note here that even in pure oxygen, combusted biomass will contain limited amounts of these impurities because biomass will contain nitrogen (in proteins and nucleic acids) and sulfur, (from the amino acids cysteine and methionine, and molecules for which they are biological precursors.

The material shows excellent recyclability when the carbon dioxide is removed at approximately 100 C.



The caption:

Figure 10. CO2 capture capacity of 20P-M during 10 cycles of CO2 adsorption/desorption in dry and 10 vol % H2O contained flue gas.

An excerpt from the conclusion of the paper:

A variety of MgCO3 with different porous structures were successfully synthesized and characterized. The synthesis of MgCO3 was based on a facile and template-free method and utilized CO2 as reactant, allowing the porous MgCO3 to be new and promising CO2-storage materials. Meanwhile, the synthesis strategy developed is also beneficial to the potential utilization of CO2. Among those as-prepared MgCO3 materials, M4 with the optimal morphology was selected as support for CO2 adsorbent. A series of adsorbents with different PEI loadings were prepared by effective impregnation while the microstructure of the adsorbents was well maintained afterward. The capacity of CO2 capture in PEI-modified adsorbents was significantly increased, particularly for the adsorbent with 20% PEI loading (4 times higher than the one without PEI at 75 °C, up to 1.07 mmol/g). At low temperature (25 and 40 °C), because of the sterically hindered effect, adsorbents with relatively low PEI loading performed better than the highly loaded ones. On the contrary, the high PEIloaded adsorbents were advantageous at higher temperature (60 and 75 °C) where the diffusion resistance was reduced.

Whether we know it or not, whether we spend our time obliviously picking lint out of our navels glibly waxing enthusiastic for Elon Musk's stupid car and/or the endless series of "renewable energy breakthroughs" decade after decade, this while these "breakthroughs" fail to even slow the rise in the use of dangerous fossil fuels and the contamination of the atmosphere, or whether we recognize the need to change our attitudes and face the true magnitude of the problem, we are in very, very, very bad shape with respect to the environment on which all life depends.

Papers like this one allow, nevertheless, for a sliver of hope.

I trust you're having a pleasant Sunday afternoon.