Carbon dioxide capture by a novel material that mimics a plant enzyme

Could allow for dramatic reductions in the overall energy cost of carbon capture in power plants or the atmosphere
September 3, 2015

Atomic structure of the adsorbed* carbon dioxide (gray sphere bonded to two red spheres) inserted between the manganese (green sphere) and amine (blue sphere) groups within the novel metal-organic framework, forming a linear chain of ammonium carbamate (top). Some hydrogen atoms (white sphere) are omitted for clarity. (credit: Image courtesy of Thomas McDonald, Jarad Mason, and Jeffrey Long)

A novel porous material that achieves carbon dioxide (CO2) capture-and-release with only small shifts in temperature has been developed by a team of researchers at the Center for Gas Separations Relevant to Clean Energy Technologies, led by the University of California, Berkeley (a DOE Energy Frontier Research Center), and associates.

This metal-organic framework (MOF) structure, which adsorbs* CO2, closely resembles an enzyme found in plants known as RuBisCO, which captures CO2 from the atmosphere for conversion into nutrients.

The discovery* paves the way for designing more efficient materials that dramatically reduce overall energy cost of carbon capture. Such materials could be used for carbon capture from fossil-fuel-based power plants as well as from the atmosphere, mitigating the greenhouse effect.

The enhanced carbon capture efficiency of the new class of materials could allow for dramatic reductions in the overall energy cost of carbon capture in power plants or even from the atmosphere, according to the researchers.

* Adsorbed CO2 is captured on the surface of a material; absorbed CO2 is captured inside the material.

** The cooperative mechanism for carbon dioxide (CO2) adsorption in porous MOF materials:

First, a CO2 molecule gets inserted between a metal ion and an amine group within the cylindrical pore of the MOF. Interestingly, the chemical environment of the MOF with the adsorbed CO2  is very similar to that of plant enzyme RuBisCO with a bound CO2.

RuBisCO plays an essential role in biological carbon fixation by plants and conversion into nutrients. In the case of the newly synthesized diamine-appended MOFs, however, the inserted CO2 reorganizes the chemical environment at the adjacent metal ion site to be just right for the insertion of the next CO2.

As more CO2 enters the pore, a cooperative domino effect ensues that leads to the formation of linear chains of ammonium carbamate along the cylindrical pore surfaces of the MOF.

Gas adsorption measurements show the high selectivity of the material for CO2 from the typical composition of flue gas from fossil-fuel-based power plants that contains nitrogen, water, and CO2.

Furthermore, the material has large working capacities — the amount of CO2 adsorbed and desorbed for a given amount of material — that are enabled by only moderate temperature shifts for the adsorption and desorption processes.

Finally, the research points out that changing the strength of the metal-diamine bond through metal substitution allows for rational tuning of the adsorption and desorption properties.

Abstract of Cooperative insertion of CO2 in diamine-appended metal-organic frameworks

The process of carbon capture and sequestration has been proposed as a method of mitigating the build-up of greenhouse gases in the atmosphere. If implemented, the cost of electricity generated by a fossil fuel-burning power plant would rise substantially, owing to the expense of removing CO2 from the effluent stream. There is therefore an urgent need for more efficient gas separation technologies, such as those potentially offered by advanced solid adsorbents. Here we show that diamine-appended metal-organic frameworks can behave as ‘phase-change’ adsorbents, with unusual step-shaped CO2 adsorption isotherms that shift markedly with temperature. Results from spectroscopic, diffraction and computational studies show that the origin of the sharp adsorption step is an unprecedented cooperative process in which, above a metal-dependent threshold pressure, CO2 molecules insert into metal-amine bonds, inducing a reorganization of the amines into well-ordered chains of ammonium carbamate. As a consequence, large CO2 separation capacities can be achieved with small temperature swings, and regeneration energies appreciably lower than achievable with state-of-the-art aqueous amine solutions become feasible. The results provide a mechanistic framework for designing highly efficient adsorbents for removing CO2 from various gas mixtures, and yield insights into the conservation of Mg2+ within the ribulose-1,5-bisphosphate carboxylase/oxygenase family of enzymes.