Hybrid MOF Solid Sorbent-Based CO2 Capture Process for Power and Industrial Applications

Mustapha Soukri^a_, Ignacio Luz Minguez^a, Samuel J. Thompson^a, and Marty Lail^a

^aRTI International, Post Office Box 12194, Research Triangle Park, NC 27709-2194

Keywords: CO₂, Carbon capture; Solid sorbents; Metal-organic frameworks (MOFs); Fluidizable sorbent; PEI

CO₂ capture technologies have undergone significant development in the last several years, and today large-scale Carbon Capture & Sequestration (CCS) demonstrations are underway. Several concepts for capturing CO₂ in industrial processes and power plants are being developed. Among these, metal-organic frameworks (MOFs) adsorbents have received a great deal of attention because they exhibit remarkable CO₂ adsorption capacity at high pressures owing to their extremely high surface areas and porosity. However, the CO₂ adsorption capacities of most MOFs are unsatisfying at low pressures. Some attempts have been made to enhance the CO₂ capture ability for MOFs at low pressure, such as developing MOFs with amine functionality crafted in the organic ligands. Apparently, the basic amine group is an ideal functionality to strongly interact with acidic CO₂ molecules. However, amine-functionalized MOFs have been shown to be not very successful in CO₂ capture. Alternatively, some recent studies on incorporation of diamine to the open metal sites of MOFs have been shown to dramatically enhance CO₂ capture at low pressures. The open metal sites play an important role to anchor one end of the diamine groups and leave the other end (alkylamines) available to capture CO₂. Recently, physical impregnation of low molecular-weight linear polyethyleneimine (PEI) into MOFs has been studied as sorbent for CO₂ capture at low pressure and showed excellent CO₂ adsorption capacity, rapid adsorption kinetics, together with very high CO₂/N₂ selectivity in flue gas. Overall, these studies have demonstrated the successful combination of polymeric amine and MOFs as sorbents for post-combustion CO₂ capture, and represent a breakthrough for this class of adsorbent, for higher CO₂ capacity.

 Given that the field of MOFs is still emerging, further research is needed to develop MOFs targeting key material properties for post-combustion CO₂ capture such as stability, mechanical strength of sorbent particles, fluidizability, high density, long-term stability, competitive sorption, and lower cost associated with the bulk of MOFs synthesis and preparation. RTI strategy to improve the performance of the current state-of-art solid CO₂ sorbents is by developing a hybrid MOF-based CO₂ adsorbent. Our approach is to develop a general method for the incorporation of MOFs into fluidized supports, which bring a synergistic feature rising from their combination such as hierarchical micro-/mesoporosity to host and disperse an active polyamine while providing them more stability via coordination at open metal sites and free functional groups. Our research efforts were focused on the synthesis and characterization of highly porous MOFs, growing these MOFs inside the pores of mesoporous support (silica) at different MOF-to-silica ratios, and wet impregnation of these hybrid materials (MOF-silica) with PEI at different ratios. The overall goals of this hybrid MOF sorbent synthesis effort were to develop sorbents having superior characteristics such as:

• High CO₂ capacity
• Excellent MOF dispersion and homogeneity
• Good water and air stability
• Good chemical and thermal stability
• Enhanced attrition resistance
• Good fluidizability and solids handling capability