A Novel Carbon Dioxide Absorbent: Lithium Orthosilicate.

[NNadir]

Reactions for the capture of carbon dioxide have been practiced for many thousands of years, although in is only recently, in the last two centuries, that people realized that this, in fact, what was being done. I am, of course, referring to the concrete industry when I say this.

The preparation of concrete's starting material, lime, is made by heating limestone, which consists mostly of calcium carbonate. The reaction is CaCO₃ <-> CaO + CO₂. It is safe to say that almost nowhere on earth is this industrial process used as a source of carbon dioxide, or - in wishful thinking talk - to sequester carbon dioxide. Almost in every case the carbon dioxide generated is dumped in the atmosphere, that big dump in the sky, along with all of the dangerous fossil fuel waste generated to provide the heat.

(For the record, I regard all of the sequestration talk as pure nonsense, only marginally better than the dumb "solar PV will save us" talk.)

However, a lot of work has been done which is dedicated to looking at this reaction or capture carbon dioxide in a cyclic fashion. In theory this reaction could be used to recover carbon dioxide from air, possibly with a biomass intermediate, although almost certainly a higher concentration of CO₂. is available from dangerous fossil fuel waste dumping devices also known as "smokestacks."

The capture of a gas by a solid substance, and regenerating that gas, however, is generally a function of the surface area of the particles involved. A drawback to the ancient lime/limestone process, besides the high temperatures required and corrosion problems, is that after several cycles, the calcium oxide particles become rather large and inefficient owing to sintering. (The volume (and thus mass) of a particle varies as the cube or its radius, and the surface area with the square, and thus larger diameter particles have a lower surface area to mass ratio. This problem limits the energy efficiency of the process, which is always endothermic, since one is required to heat mass that will do no work.

Many modifications of this process are being evaluated, some of which are quite interesting and even promising. I keep an eye on these things for certain reasons that have nothing to do with carbon sequestration or other garbage talk, since I don't believe that the concept of garbage and dumping anything are particularly useful.

Anyway. In my library research on this topic I came across interesting chemistry that has nothing at all to do with calcium based catalytic processes for the recovery of carbon dioxide. It involves the interesting chemistry of lithium orthosilicate, Li₄SiO₄.

The paper I will reference is the work of Mexican scientists, A. Lopez Ortiz at Mexico's Center for Advanced Materials, in the Department of Chemicals and Materials, presumably a State Organization in Mexico.

The title of the article is "A new synthesis route to Li₄SiO₄
as CO₂ catalytic/sorbent"

The abstract is here: http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TFG-4H10053-3&_user=10&_coverDate=10%2F30%2F2005&_alid=1053082805&_rdoc=1&_fmt=high&_orig=search&_cdi=5226&_sort=r&_docanchor=&view=c&_ct=1&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=5e2d11696df4ada15f9fa28ba2a6b6b3">Catalysis Today 107–108 (2005) 863–867

Some remarks from the paper touching on what I said above:

In the present world, economy relies on burning fossil fuels to produce affordable energy. A consequence of the use of these fuels is the emission of important amounts of carbon dioxide (CO₂) to the atmosphere creating environmental problems such as global climate warming. In the near future, if CO₂ removal is mandatory by stringent regulations, industry will certainly require improved and novel technologies for the efficient removal of CO₂₂ emissions to the atmosphere...

...The use of solid catalytic/sorbents for CO₂ sequestration have been widely used in the past and started several years ago for the CO₂ separation in the coal gasification process <5>. Several investigators examined the effects of temperature, pressure and reactive gas composition of calcium oxide (CaO) based materials using thermo-gravimetric (TGA) reactor techniques <6,7>. However, degradation (sintering) of these materials was observed during multicycle tests. Calcined mineral dolomite (MgCO₃–CaCO₃) proved to be superior in multicycle performance to previous CaO based past materials <6>. However, high regeneration temperatures were needed (950 8C) using this acceptor of mineral nature. Bandi et al. <8> proposed the use of mineral huntite (Mg₃Ca(CO₃)₄) presenting good regeneration properties. However, high regeneration temperatures again were the drawback. Also of mineral nature hydrotalcites (Mg₆Al₂(CO₃)(OH)₁₆
4H₂O) were used by Hufton et al. <9> and Ding and Alpay <10>, at relatively high temperatures 400–550 8C in their experiments for CO₂ capture. However the sorption capacity of these hydrotalcites was low.

Now for the interesting part:

Within the past 4 years, a new generation of synthetic acceptors evolved from the work from Ohashi and Nakagawa <11>, which developed a novel CO₂ separation technique by using regenerable lithium zirconate (Li₂ZrO₃) as a solid CO₂ acceptor in the temperature range of 450–700 8C. Main advantages of this material was its lower regeneration temperature (750 8C)compared to current mineral origin based sorbents...

It appears that the rate of absorption was low however, limiting the utility. However the work lead to the investigation of other analogues in the class:

Kato <12> proposed other acceptors based on lithium such as lithium ferrite (LiFeO₂), lithium nickelite (LiNiO₂), lithium titanate (Li₂TiO₃), lithium metasilicate (Li₂SiO₃) and lithium orthosilicate (Li₄SiO₄) used at various temperatures. Certainly among all these, Li₄SiO₄ was found to present the highest capacity and sorption rate with CO₂. Kato <12>exposed Li₄SiO₄ to 50 sorption–regeneration cycles without loosing (sic) its sorption capacity.

The absorption reactions involve in situ generation of lithium carbonates in an orthosilicate/silicate equilibrium.

One problem addressed by the Mexican scientists is the the original work on lithium orthosilicate absorption allowed for only 70% of the theoretical carbon dioxide absorption. Thus Dr. Lopez Ortiz developed an alternate synthesis using lithium nitrate, silicon dioxide (sand) with silicate intermediates. The side products of the reaction are essentially air, just nitrogen gas and oxygen gas, meaning that the chemistry is very clean, although high temperates 1200C are required.

The new lithium orthosilicates were usable for up to 50 cycles and were able to recover 98.4% of the theoretical carbon dioxide.

Fun and interesting I think.