Evaluating the Effect of Composition, Structure and Functionality on Atmospheric CO2 Adsorption in Porous Solid Sorbents

Abstract

Carbon dioxide (CO2) is the most abundant greenhouse gas in the atmosphere, and it is estimated to be responsible for approximately two thirds of the total global warming effect caused by anthropogenic gas emissions. Meeting the Paris Agreement objective of temperature increases well below 2°C above pre-industrial levels will require a widespread transition towards green energy generation, industrial decarbonization, and active removal of CO2 from the atmosphere through negative emotion technologies including direct air capture (DAC). The low concentration of CO2 in the atmosphere makes this separation more costly and energy intensive than other forms of carbon capture. For this reason, porous solid adsorbents have been proposed for DAC due to their low regeneration energy requirements relative to the current industry standard, aqueous amine-based absorption. In this work, we investigate the effects of synthesis scale, particle morphology, and chemical functionality on CO2 capture performance for solid adsorbents. Metal-organic frameworks (MOFs), activated carbon, and cellulose aerogels are investigated in both single-gas static conditions, and mixed-gas dynamic conditions. MIL-101-Cr is one MOF that has been studied for CO2 capture due to its high surface area and chemical stability. We studied the effect of synthesis scale-up and ethylenediamine functionalization on the CO2 adsorption performance of the MOF in single-component and DAC conditions in the presence of humidity. Due to the strong chemical interactions between CO2 and amine groups, their introduction to CO2 adsorbents tends to increase adsorption capacity significantly. MIL-101-Cr showed no adverse effects from synthesis 13x and 33x scale-up, retaining a BET surface area above 2900 m2/g and a CO2 adsorption capacity of over 6 mmol/g in single-gas conditions and 0.020 mmol/g in humid DAC conditions. Functionalization of all MIL-101-Cr scales with ethylenediamine resulted in a decrease in specific surface area and inconsistent effects in CO2 adsorption across reaction scales and adsorption conditions. The lack of adverse effects on CO2 adsorption behavior with increasing synthesis scale of this MOF make it a promising candidate for further scale-up studies for its use in large-scale DAC operations. Another MOF, Mg-MOF-74, has a high affinity for CO2 due to the presence of open metal sites in the structure, but it is also unstable in the presence of humidity. We investigated the effect of synthesis scale-up and particle morphology control on the CO2 adsorption capacity of the MOF in single-component and DAC conditions, as well as stability in humid DAC conditions in particular. Mg-MOF-74 suffered decreases of 29% and 64% in BET surface area as the synthesis reaction was scaled up 2.5x and 5x, from 2034 m2/g at 1x scale, resulting in losses of CO2 adsorption capacity from 9.25  0.11 mmol/g for the 1x scale to approximately 4 mmol/g for 2.5x and 5x in single-gas conditions, and from 0.036  0.002 mmol/g for 1x to 0.031  0.003 mmol/g for 2.5x and 0.025 mmol/g for 5x in humid DAC conditions. For this MOF, increasing the synthesis scale negatively impacts the surface area and CO2 adsorption capacity of the material at all tested conditions. The use of different modulators in the synthesis of Mg-MOF-74 resulted in two distinct particle morphologies, named needles and haystacks, which showed BET surface areas of 1750 m2/g and 980 m2/g and CO2 uptakes of 0.065  0.005 mmol/g and 0.033  0.002 mmol/g, respectively, in 60% RH DAC conditions. These morphologies were unstable and evolved from distinct needle and haystack particle shapes to rice-like rods over a period of two years, after which the CO2 uptake of the former haystacks increased to 0.068  0.002 mmol/g in 45% RH DAC conditions. The two morphologies showed CO2 adsorption behaviors that were inconsistent with their specific surface areas and varied depending on % RH in the feed, which shows particle morphology plays an important role in the adsorption behavior of this MOF. We also investigated the effect of functionalizing activated carbon (AC), a porous physisorbent that can be derived from wood, with guanidine carbonate (G2CO3), an amine-containing salt, on the CO2 adsorption capacity in single-component and DAC conditions. Similarly to ethylenediamine, the amine groups in G2CO3 are expected to improve CO2 adsorption via the introduction of a chemisorption mechanism. AC was functionalized with G2CO3 via wet impregnation, and we observe a progressive decrease in the BET surface area of the adsorbents as G2CO3 loading increases. In single-component adsorption at 20°C, G2CO3-containing adsorbents outperform AC by at least 44% in CO2 pressures below 10 kPa. In dry DAC conditions, G2CO3 increased the CO2 adsorption capacity of AC from 0.044  0.001 mmol/g to a maximum of 0.153  0.002 mmol/g for 20% nominal G2CO3 loading. In 70% RH DAC conditions, the adsorption capacity of AC decreases by 45% while that of all G2CO3-containing adsorbents increases by a minimum of 80%, to a maximum CO2 adsorption capacity of 0.284  0.010 mmol/g at a 20% nominal G2CO3 loading. Over 15 consecutive adsorption-desorption cycles, the 20% nominal G2CO3 adsorbent shows slight changes in adsorption capacity that can be correlated to the variability of CO2 concentration in the feed, indicating a lack of adsorbent degradation. Its promising CO2 adsorption behavior in humid conditions and facile synthesis make AC-supported G2CO3 adsorbents an attractive option for DAC operations. This work shows how various characteristics and modifications on porous materials affect their CO2 adsorption capacity and stability in different environments and how each of them can potentially be of use for larger DAC operations.