(567e) Impact of Controlled Morphology and Defect Density on Gas Sorption in a MOF for Direct Air Capture of Carbon Dioxide

Net-zero emissions technologies have been identified by the IPCC as an important tool to achieve the goal of a less than 2 °C global average temperature rise over pre-industrial levels. Several companies are currently working to commercialize direct-air capture (DAC) technologies to remove CO₂ directly from the atmosphere, but significant strides need to be made in the development of sorbent materials to make this an economically viable approach. Sorbent materials need to exhibit high CO₂ capacity at low CO2 concentrations (i.e. at atmospheric concentrations of ~400 ppm), rapid sorption and desorption kinetics to facilitate capture and regeneration cycles, low regeneration energy to minimize operating cost and carbon footprint, and low material capital costs. Sorbent materials also need to exhibit high stability over numerous sorption/regeneration cycles to increase the lifetime of the materials.

We have studied a series of metal organic framework (MOF) materials with the aim of optimizing and scaling them up as solid sorbents for direct air capture of CO₂. Specifically, we are interested in tuning the particle size of the MOFs and the porosity of both the nano- and meso- porous structures present in a packed-bed geometry. A test series of controlled UiO-66 materials with well-characterized defect-levels and particles sizes was examined initially, and then we selected MOFs that have a high capacity (mmol/g) of CO₂ while also considering the isosteric heat of adsorption (Q_st). Herein we will report on results for Mg-MOF-74-en (Capacity = 1.51 mmol/g).

Modulated syntheses of Mg-MOF-74 using a variety of modulators (benzoic acid, formic acid, and salicylic acid) resulted in variable and controllable particle sizes and morphologies, indicating that Mg-MOF-74 is highly favorable for morphological and size control studies relating to synthesis and gas sorption performance. The resulting MOF samples were studied using isothermal gravimetric and volumetric sorption techniques. Based on these results, a subset of the MOFs were studied under multiple sorption/regeneration cycles in a packed bed sorption apparatus under DAC operating conditions.