(18b) Controlled Morphology, Post-Synthetic Modification, and Scale-up of MOFs 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 observing the effect of synthesis scale-up and particle morphology on packed bed CO₂ sorption in DAC-relevant conditions. Previously, a test series of controlled UiO-66 materials with well-characterized defect-levels and particles sizes was examined, and additional MOFs were selected based on high capacity (mmol/g) of CO₂ while also considering the isosteric heat of adsorption (Q_st). Herein we will present on DAC using Mg-MOF-74 and MIL-101-Cr based sorbents.

Modulated syntheses of Mg-MOF-74 using different modulators (benzoic and salicylic acid) as well as a variety of reaction scales (100 mL, 250 mL, and 500 mL) resulted in variable and controllable particle morphologies, indicating that Mg-MOF-74 is highly favorable for morphological and size control studies relating to synthesis and gas sorption performance. MIL-101-Cr was synthesized in 25 mL, 265 mL and 600 mL reaction vessels using acetic acid as a modulator, resulting in negligible crystallographic or morphological difference between the samples. Additionally, a fraction of each synthesized MOF sample underwent post-synthetic ethylenediamine grafting to increase chemisorption site density. The resulting MOF samples were studied using isothermal gravimetric and volumetric sorption techniques. Based on these results, a subset of the MOFs was studied under multiple sorption/regeneration cycles in a packed bed sorption apparatus under DAC operating conditions.