Novel Functionalized Metal Organic Frameworks for Low Cost Carbon Capture

by Gregory Day & Carlos Ybanez of framergy, Inc.

Delivering internal emission reductions is one of the leading ways to achieve carbon neutrality. Adopting point source carbon dioxide capture technologies and their efficient implementation will be key advantages for corporations with carbon neutrality and footprint reduction goals.

Metal organic frameworks (MOFs) are highly porous materials notable for their uniform pore sizes and shapes and their high degree of tunability. These features have put them at the forefront of carbon capture technologies, as the small size of the carbon dioxide molecule requires a material with a high surface area and narrow pore widths to properly adsorb.

The tunability of MOFs has long been utilized to install further CO2 capture species, such as amines, into the MOF pores to achieve the highly selective capture of CO2 at low concentrations.  This will be needed for carbon control with natural gas and even direct air capture.

In the past, most MOF organizations have focused on installing amines within the pores of the material, attempting to synergize the effect of both the MOF pores and the amine functional group together to achieve efficient CO2 capture. However, the small, highly basic amines typically utilized are known to partially degrade many MOFs before they can effectively tether to the pores, limiting the number of species that this strategy can be utilized with.

As part of their recent work on carbon capture, the team at framergy has opted to turn this idea on its head, instead utilizing heavy amines to engage in surface adhesion to a microporous MOF.

framergy appended the amines tetraethylenepentamine (TEPA) and polyethylenimine (PEI) to the surface of the MOFs AYRSORB™ F250 and A250. These two materials correspond to the known PCN-250 structure utilizing either iron (F250) or aluminum (A250).

This structure is highly robust and stable due to its highly connected ligand and hard acid metal centers. This high degree of stability allows for the facile doping of amines in solvents such as ethanol. This is in stark contrast to more traditional amine doping, which typically needs to be performed in non-polar solvents to prevent base-induced degradation of the MOF.

The key distinction between the amine-doped F250 and A250 systems is that the amine is mainly doped on the surface of the material, due to the large size of the amines relative to the PCN-250 structure pores. Testing of these materials at the laboratory scale showed that this surface coverage is highly beneficial to the kinetic capture of CO2.

Using thermogravimetric analysis (TGA) equipped with a CO2 flow meter showed that the mass uptake is improved with the higher molecular weight amines TEPA and PEI compared to the low molecular weight amines ethylene-diamine and diethylenetriamine (DETA, Figure 1).

TGA CO2 adsorption curves for a) F250-amine and b) A250-amine.

Figure 1. TGA CO2 adsorption curves for a) F250-amine and b) A250-amine. Image Credit: framergy, Inc.

These kinetic CO2 capture results can also be utilized at the bench scale using kg quantities of MOFs. Taking advantage of framergy’s metric-ton scale synthesis of AYRSORB F250, the team was able to achieve complete capture of CO2 from a 10% CO2 stream for over 15 minutes. Even at elevated temperatures this material still showed high adsorption affinity, with a 5 minute breakthrough time observed for a 60 °C CO2 adsorption test (Figure 2a).

What is more, this material can be readily recovered, where many classic amine-modified MOFs require regeneration at temperatures above 150 °C, F250-amine shows complete regeneration with only 100 °C regeneration. In fact, the material shows little difference in release time as the temperature is increased, suggesting that even lower temperatures could be utilized for cyclic capture of CO2 (Figure 2b).

The material is also stable to the exposure of acid gases that are typically observed in industrial processes. At accelerated testing conditions with higher than typical acid gas concentrations, framergy’s MOFs achieved complete CO2 capture and performed with only a 14% drop in its uptake performance compared to the pristine material (after being exposed to 10 ppm SOx and NOx for 1 h).

It is also important to note that this drop is not additive, performing additional 1 hour exposures does not result in any more noticeable loss in performance, suggesting that only a small portion of the adsorption sites are susceptible to acid gas degradation (Figure 2c).

a) Breakthrough time for AYRSORB™ F250-amine as a function of adsorption temperature, and b) regeneration of F250-amine at different temperatures and times, showing minimal difference in CO2 release rate between conditions, c) CO2 uptake after SOx and NOx exposure.

Figure 2. a) Breakthrough time for AYRSORB™ F250-amine as a function of adsorption temperature, and b) regeneration of F250-amine at different temperatures and times, showing minimal difference in CO2 release rate between conditions, c) CO2 uptake after SOx and NOx exposure. Image Credit: framergy, Inc.​

This combination of features, the highly stable MOF structure, the facile synthesis of the parent MOF, the use of low-cost solvents and low reaction times, and the surface functionalization of the MOF by the amine, results in a material that can engage in the facile capture of CO2 from low concentration streams.

The low amine loading and the distinction between pore: CO2 interactions and amine: CO2 interactions result in a material that has highly beneficial regeneration conditions compared to the current MOF-amine composites. With a growing need for low-cost CO2 capture, the features of the amine functionalized AYRSORB™ materials are poised to deliver results in many applications.

This information has been sourced, reviewed and adapted from materials provided by framergy, Inc.

For more information on this source, please visit framergy, Inc.

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