User:Jesikatrese

Topic: Metal-organic frameworks (MOFs)

Group members: Maude and Gustavo

We thought this would be a good topic to work on because MOFs and their applications in carbon capture are only covered briefly on Wikipedia. In articles that do talk about MOFs, there are places to expand, and some articles relevant to MOFs don't talk about them at all (e.g. the carbon capture and storage article doesn't mention MOFs at all)

Articles to add information about MOFs:
 * Carbon capture and storage
 * Metal-organic framework
 * Carbon dioxide scrubber

Peer-reviewed:

 * User talk:Minhtrang 92/sandbox
 * User talk:Sophia.chu007/sandbox
 * User talk:Yorukcan/sandbox
 * User talk:Achiu17/sandbox

The following are our additions to the three Wikipedia articles "Carbon capture and storage", "Metal-organic framework", and "Carbon dioxide scrubber".

= Metal-organic frameworks =

Carbon Capture
Main article: carbon capture and storage

Because of their small, tunable pore sizes and high void fractions, MOFs are a promising potential material for use as an adsorbent to capture CO2. MOFs could provide a more efficient alternative to traditional amine solvent-based methods in CO2 capture from coal-fired power plants.

MOFs could be employed in each of the main three carbon capture configurations for coal-fired power plants: pre-combustion, post-combusiton, and oxy-combustion. However, since the post-combustion configuration is the only one that can be retrofitted to existing plants, it garners the most interest and research. In post-combustion carbon capture, the flue gas from the power plant would be fed through a MOF in a packed-bed reactor setup. Flue gas is generally 40 to 60 °C with a partial pressure of CO2 at 0.13 - 0.16 bar. The main component CO2 must be separated from is nitrogen, but there are small amounts of other gasses as well. A typical flue gas composition is: 73-77% N2, 15-16% CO2, 5-7% H2O, 3-4% O2, 800 ppm SO2, 10 ppm SO3, 500 ppm NOx, 100 ppm HCl, 20 ppm CO, 10 ppm hydrocarbons, 1ppb Hg.

CO2 can bind to the MOF surface through either physisorption or chemisorption, where physisorption occurs through van der Waals interactions and chemisorption occurs through covalent bonds being formed between the CO2 and MOF surface. Once the MOF is saturated with CO2, the MOF would then be regenerated (CO2 is removed from the MOF to be transported elsewhere for sequestration or enhanced oil recovery) through either a temperature swing or a pressure swing. In a temperature swing, the MOF would be heated up until CO2 desorbs; temperature swing regeneration is usually used in post-combustion configurations where the heat is supplied from heat exchangers with the power plant. To achieve working capacities comparable to the amine solvent process, the MOF must be heated up to around 200 C. In a pressure swing, the pressure would be decreased until CO2 desorbs; this would usually be used in a pre-combustion carbon capture configuration.

The Yaghi group tested various MOFs for their performance in absorbing CO2 and found their highest CO2 capacity achieved of 33.5 mmol CO2 / g MOF at ambient temperature and 35 bar, using MOF-177. For comparison, zeolite 13X has a CO2 capacity of 7.4 mmol/g at 32 bar and ambient temperature, and MAXSORB has a capacity of 25 mmol/g at 35 bar and ambient temperature. MOF-177 has pore sizes of 11 and 17 Angstroms and is comprised of zinc complexes connected by benzene rings, with the repeating structure being Zn 4 O(1,3,5-benzenetribenzoate) 2.

In addition to their tunable selectivities for different molecules, another property of MOFs that makes them a good candidate for carbon capture is their low heat capacities. Monoethanolamine (MEA) solutions, the leading method for capturing CO2 from flue gas, have a heat capacity between 3-4 J/g K since they are mostly water. This is one of the main factors contributing to the energy penalty in the solvent regeneration step— when the absorbed CO2 is removed from the MEA solution so that it can be reused. MOF-177, on the other hand, has a heat capacity of 0.5 J/g K at ambient temperature. The low heat capacities of MOFs could significantly reduce the energy penalty of carbon capture, which is currently around 30% of plant power for MEA solution-based methods.

In a project sponsored by the DOE and operated by UOP LLC in collaboration with faculty of four different universities, MOFs were tested as possible carbon dioxide removal agents in post-combustion flue gas. They were able to separate 90% of the CO2 from the flue gas stream using a vacuum pressure swing process. From extensive investigation, they found out that the best MOF to be used was Mg(dobdc), which has a 21.7 wt% CO2 loading capacity. Estimations showed that, if a similar system would be applied to a large scale power plant, the cost of energy would increase by 65%, while a NETL baseline amine based system would cause an increase of 81% (the DOE goal is 35%). The cost of capturing CO2  would be $57 / ton CO2 captured, while for the amine system the cost is estimated to be $72 / ton CO2 captured. The project estimated that the total capital required to implement such project in a 580 MW power plant would be $354 million.

= Carbon capture and storage = Carbon Dioxide can be captured out of air or fossil flue power plant flue gas using absorption (or carbon scrubbing), membrane gas separation, or adsorption technologies. Amines are the leading carbon scrubbing technology.

Capture
Flue gas from the combustion of coal in oxygen has a large concentration of CO2, about 10-15% CO2 whereas natural gas power plant flue gas is about 5-10% CO2. Therefore, it is more energy and cost efficient to capture CO2 from coal-fired power plants.

Post combustion capture is most popular in research because existing fossil fuel power plants can be retrofitted to include CCS technology in this configuration.

Capture technologies
Carbon dioxide can be separated out of air or flue gas with absorption, adsorption, or membrane gas separation technologies. Absorption, or carbon scrubbing, with amines is currently the dominant capture technology. Membrane and adsorption technologies are still in the developmental research stages, initiating primary pilot plants in the near future. Metal-Organic Frameworks (MOFs) are a novel, promising adsorption carbon capture technology. Carbon dioxide adsorbs to a MOF through physisorption or chemisorption based on the porosity and selectivity of the MOF leaving behind a Greenhouse gas poor gas stream that is more environmentally friendly. The carbon dioxide is then stripped off the MOF using temperature swing adsorption (TSA) or pressure swing adsorption (PSA) so the MOF can be reused. Adsorbents and absorbents require regeneration steps where the CO2 is removed from the sorbent or solution that collected it out of the flue gas in order for the sorbent or solution to be reused. Monoethanolamine (MEA) solutions, the leading amine for capturing CO2, have a heat capacity between 3-4 J/g K since they are mostly water. Higher heat capacities add to the energy penalty in the solvent regeneration step. Thus, to optimize a MOF for carbon capture, low heat capacities and heats of adsorption are desired. Additionally, high working capacity and high selectivity are selected for in order to capture as much CO2 as possible from the flue gas. However, there is an energy trade off with selectivity and energy expenditure. As the amount of CO2 captured increases, the energy, and therefore cost, required to regenerate increases. A large drawback of MOFs for CCS is their chemical and thermal stability. Current research is looking to optimize MOF properties for CCS, but it has proven difficult to find these optimizations that also result in a stable MOF. Metal reservoirs are also a limiting factor to the potential success of MOFs.

Capture is attributed to about two thirds of the total cost of CCS, making it limit the wide-scale deployment of CCS technologies. To optimize a CO2 capture process would significantly increase the feasibility of CCS since the transport and storage steps of CCS are rather mature technologies.

An alternate method under development is chemical looping combustion (CLC). Chemical looping uses a metal oxide as a solid oxygen carrier. Metal oxide particles react with a solid, liquid or gaseous fuel in a fluidized bed combustor, producing solid metal particles and a mixture of carbon dioxide and water vapor. The water vapor is condensed, leaving pure carbon dioxide, which can then be sequestered. The solid metal particles are circulated to another fluidized bed where they react with air, producing heat and regenerating metal oxide particles that are recirculated to the fluidized bed combustor. A variant of chemical looping is calcium looping, which uses the alternating carbonation and then calcination of a calcium oxide based carrier as a means of capturing CO2.

A few engineering proposals have been made for the more difficult task of removing CO2 from the atmosphere – a form of climate engineering – but work in this area is still in its infancy. Capture costs are estimated to be higher than from point sources, but may be feasible for dealing with emissions from diffuse sources such as automobiles and aircraft. The theoretically required energy for air capture is only slightly more than for capture from point sources. The additional costs come from the devices that use the natural air flow. Global Research Technologies demonstrated a pre-prototype of air capture technology in 2007.

= Carbon dioxide scrubber =

Metal-organic frameworks (MOFs)
Metal-organic framework s are one of the most promising new technologies for carbon dioxide capture and sequestration via adsorption. Although no large-scale commercial technology exists nowadays, several research findings have indicated the great potential that MOFs have for CO2 removal. Their characteristics such as pore structure and surface functions can be easily tuned to improve CO2 adsorption selectivity over other gases.

A MOF could be specifically designed as a CO2 removal agent in post-combustion power plants. In this scenario, the flue gas would pass through a bed packed with a MOF material, where CO2 would be adsorbed. After saturation is reached, CO2 could be desorbed by doing a pressure or temperature swing. Carbon dioxide could then be compressed to supercritical conditions in order to be stored underground or utilized in enhanced oil recovery processes. However, this is not possible in large scale yet due to several difficulties, one of those being the production of MOFs in great quantities.

Another problem is the availability of metals necessary to synthesize MOFs. In a hypothetical scenario where these materials are used to capture all CO2 needed to avoid global warming issues, such as temperature rise above 2oC from pre-industrial average temperature, we would need more metals than it's available on Earth. For example, to synthesize all MOFs using vanadium, we would need 1620% of 2010 global reserves. Even if using magnesium-based MOFs, which have demonstrated a great capacity to adsorb CO2, we would need 14% of 2010 global reserves, which is a considerable amount. Also, extensive mining would be necessary, leading to more potential environmental problems.

In a project sponsored by the DOE and operated by UOP LLC in collaboration with faculty of four different universities, MOFs were tested as possible carbon dioxide removal agents in post-combustion flue gas. They were able to separate 90% of the CO2 from the flue gas stream using a vacuum pressure swing process. From extensive investigation, they found out that the best MOF to be used was Mg/DOBDC, which has a 21.7 wt% CO2 loading capacity. Estimations showed that, if a similar system would be applied to a large scale power plant, the cost of energy would increase by 65%, while a NETL baseline amine based system would cause an increase of 81% (the DOE goal is 35%). Also, each ton of CO2 avoided would cost $57, while for the amine system this cost is estimated to be $72. The project ended in 2010,estimating that the total capital required to implement such project in a 580 MW power plant was 354 million dollars.

= References =